Coupled grain boundary motion in aluminium: the effect of structural multiplicity

NASA Astrophysics Data System (ADS)

Cheng, Kuiyu; Zhang, Liang; Lu, Cheng; Tieu, Kiet

2016-05-01

The shear-induced coupled grain boundary motion plays an important role in the deformation of nanocrystalline (NC) materials. It has been known that the atomic structure of the grain boundary (GB) is not necessarily unique for a given set of misorientation and inclination of the boundary plane. However, the effect of the structural multiplicity of the GB on its coupled motion has not been reported. In the present study we investigated the structural multiplicity of the symmetric tilt Σ5(310) boundary in aluminium and its influence on the GB behaviour at a temperature range of 300 K-600 K using molecular dynamic simulations. Two starting atomic configurations were adopted in the simulations which resulted in three different GB structures at different temperatures. Under the applied shear deformation each GB structure exhibited its unique GB behaviour. A dual GB behaviour, namely the transformation of one GB behaviour to another during deformation, was observed for the second starting configuration at a temperature of 500 K. The atomistic mechanisms responsible for these behaviour were analysed in detail. The result of this study implicates a strong relationship between GB structures and their behaviour, and provides a further information of the grain boundary mediated plasticity in nanocrystalline materials.

Dopant distributions in n-MOSFET structure observed by atom probe tomography.

PubMed

Inoue, K; Yano, F; Nishida, A; Takamizawa, H; Tsunomura, T; Nagai, Y; Hasegawa, M

2009-11-01

The dopant distributions in an n-type metal-oxide-semiconductor field effect transistor (MOSFET) structure were analyzed by atom probe tomography. The dopant distributions of As, P, and B atoms in a MOSFET structure (gate, gate oxide, channel, source/drain extension, and halo) were obtained. P atoms were segregated at the interface between the poly-Si gate and the gate oxide, and on the grain boundaries of the poly-Si gate, which had an elongated grain structure along the gate height direction. The concentration of B atoms was enriched near the edge of the source/drain extension where the As atoms were implanted.

Atomic scale structure and chemistry of interfaces by Z-contrast imaging and electron energy loss spectroscopy in the STEM

DOE Office of Scientific and Technical Information (OSTI.GOV)

McGibbon, M.M.; Browning, N.D.; Chisholm, M.F.

The macroscopic properties of many materials are controlled by the structure and chemistry at the grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. The high-resolution Z-contrast imaging technique in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition can be interpreted intuitively. This direct image allows the electron probe to be positioned over individual atomic columns for parallel detection electron energy loss spectroscopy (PEELS) at a spatial resolution approaching 0.22nm. The bonding information which can bemore » obtained from the fine structure within the PEELS edges can then be used in conjunction with the Z-contrast images to determine the structure at the grain boundary. In this paper we present 3 examples of correlations between the structural, chemical and electronic properties at materials interfaces in metal-semiconductor systems, superconducting and ferroelectric materials.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Yongsoo; Chen, Chien-Chun; Scott, M. C.

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling ‘real’ materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily onmore » average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. The work presented here combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure–property relationships at the fundamental level.« less

Deciphering chemical order/disorder and material properties at the single-atom level

DOE PAGES

Yang, Yongsoo; Chen, Chien-Chun; Scott, M. C.; ...

2017-02-01

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling ‘real’ materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily onmore » average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. The work presented here combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure–property relationships at the fundamental level.« less

Chevron Defect at the Intersection of Grain Boundaries with Free Surfaces in Au

NASA Astrophysics Data System (ADS)

Radetic, T.; Lançon, F.; Dahmen, U.

2002-08-01

We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90° <110> tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.

Coercivity degradation caused by inhomogeneous grain boundaries in sintered Nd-Fe-B permanent magnets

NASA Astrophysics Data System (ADS)

Chen, Hansheng; Yun, Fan; Qu, Jiangtao; Li, Yingfei; Cheng, Zhenxiang; Fang, Ruhao; Ye, Zhixiao; Ringer, Simon P.; Zheng, Rongkun

2018-05-01

Quantitative correlation between intrinsic coercivity and grain boundaries in three dimensions is critical to further improve the performance of sintered Nd-Fe-B permanent magnets. Here, we quantitatively reveal the local composition variation across and especially along grain boundaries using the powerful atomic-scale analysis technique known as atom probe tomography. We also estimate the saturation magnetization, magnetocrystalline anisotropy constant, and exchange stiffness of the grain boundaries on the basis of the experimentally determined structure and composition. Finally, using micromagnetic simulations, we quantify the intrinsic coercivity degradation caused by inhomogeneous grain boundaries. This approach can be applied to other magnetic materials for the analysis and optimization of magnetic properties.

Influence of Al grain boundaries segregations and La-doping on embrittlement of intermetallic NiAl

NASA Astrophysics Data System (ADS)

Kovalev, Anatoly I.; Wainstein, Dmitry L.; Rashkovskiy, Alexander Yu.

2015-11-01

The microscopic nature of intergranular fracture of NiAl was experimentally investigated by the set of electron spectroscopy techniques. The paper demonstrates that embrittlement of NiAl intermetallic compound is caused by ordering of atomic structure that leads to formation of structural aluminum segregations at grain boundaries (GB). Such segregations contain high number of brittle covalent interatomic bonds. The alloying by La increases the ductility of material avoiding Al GB enrichment and disordering GB atomic structure. The influence of La alloying on NiAl mechanical properties was investigated. GB chemical composition, atomic and electronic structure transformations after La doping were investigated by AES, XPS and EELFS techniques. To qualify the interatomic bonds metallicity the Fermi level (EF) position and electrons density (neff) in conduction band were determined in both undoped and doped NiAl. Basing on experimental results the physical model of GB brittleness formation was proposed.

Dislocation structures of Σ3 {112} twin boundaries in face centered cubic metals

NASA Astrophysics Data System (ADS)

Wang, J.; Anderoglu, O.; Hirth, J. P.; Misra, A.; Zhang, X.

2009-07-01

High resolution transmission electron microscopy of nanotwinned Cu films revealed Σ3 {112} incoherent twin boundaries (ITBs), with a repeatable pattern involving units of three {111} atomic planes. Topological analysis shows that Σ3 {112} ITBs adopt two types of atomic structure with differing arrangements of Shockley partial dislocations. Atomistic simulations were performed for Cu and Al. These studies revealed the structure of the two types of ITBs, the formation mechanism and stability of the associated 9R phase, and the influence of stacking fault energies on them. The results suggest that Σ3 {112} ITBs may migrate through the collective glide of partial dislocations.

How grain boundaries affect the efficiency of poly-CdTe solar-cells: A fundamental atomic-scale study of grain boundary dislocation cores using CdTe bi-crystal thin films.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Klie, Robert

It is now widely accepted that grain boundaries in poly-crystalline CdTe thin film devices have a detrimental effect on the minority carrier lifetimes, the open circuit voltage and therefore the overall solar-cell performance. The goal of this project was to develop a fundamental understanding of the role of grain boundaries in CdTe on the carrier life-time, open-circuit voltage, Voc, and the diffusion of impurities. To achieve this goal, i) CdTe bi-crystals were fabricated with various misorientation angels, ii) the atomic- and electronic structures of the grain boundaries were characterized using scanning transmission electron microscopy (STEM), and iii) first-principles density functionalmore » theory modeling was performed on the structures determined by STEM to predict the grain boundary potential. The transport properties and minority carrier lifetimes of the bi-crystal grain boundaries were measured using a variety of approaches, including TRPL, and provided feedback to the characterization and modeling effort about the effectiveness of the proposed models.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Hao; Lee, H. S.; Sarahan, M. C.

Grain boundaries (GBs) in complex oxides such as perovskites have been shown to readily accommodate nonstoichiometry changing the electrostatic potential at the boundary plane and effectively controlling material properties such as capacitance, magnetoresistance and superconductivity. Understanding and quantifying exactly how variations in atomic scale nonstoichiometry at the boundary plane extend to the practical mesoscale operating length of the system is therefore critical for improving the overall properties. Bicrystals of SrTiO 3 were fabricated to provide the model GB model structures that are analysed in this paper. We show that statistical analysis of aberration-corrected scanning transmission electron microscope images acquired frommore » a large area of GB is an effective routine to understanding the variation in boundary structure that occurs to accommodate nonstoichiometry. In the case of the SrTiO 3 22.6° Σ13 (510)/[100] GB analysed here, the symmetric atomic structures observed from a micron-long GB can be categorized as two different competing structural arrangements, with and without a rigid-body translation along the boundary plane. How this quantified experimental approach can provide direct insights into the GB energetics is further confirmed from the first principles density functional theory, and the effect of nonstoichiometry in determining the GB energies is quantified.« less

Electrical characterization of grain boundaries of CZTS thin films using conductive atomic force microscopy techniques

DOE Office of Scientific and Technical Information (OSTI.GOV)

Muhunthan, N.; Singh, Om Pal; Toutam, Vijaykumar, E-mail: toutamvk@nplindia.org

2015-10-15

Graphical abstract: Experimental setup for conducting AFM (C-AFM). - Highlights: • Cu{sub 2}ZnSnS{sub 4} (CZTS) thin film was grown by reactive co-sputtering. • The electronic properties were probed using conducting atomic force microscope, scanning Kelvin probe microscopy and scanning capacitance microscopy. • C-AFM current flow mainly through grain boundaries rather than grain interiors. • SKPM indicated higher potential along the GBs compared to grain interiors. • The SCM explains that charge separation takes place at the interface of grain and grain boundary. - Abstract: Electrical characterization of grain boundaries (GB) of Cu-deficient CZTS (Copper Zinc Tin Sulfide) thin films wasmore » done using atomic force microscopic (AFM) techniques like Conductive atomic force microscopy (CAFM), Kelvin probe force microscopy (KPFM) and scanning capacitance microscopy (SCM). Absorbance spectroscopy was done for optical band gap calculations and Raman, XRD and EDS for structural and compositional characterization. Hall measurements were done for estimation of carrier mobility. CAFM and KPFM measurements showed that the currents flow mainly through grain boundaries (GB) rather than grain interiors. SCM results showed that charge separation mainly occurs at the interface of grain and grain boundaries and not all along the grain boundaries.« less

Structure and transport at grain boundaries in polycrystalline olivine: An atomic-scale perspective

NASA Astrophysics Data System (ADS)

Mantisi, Boris; Sator, Nicolas; Guillot, Bertrand

2017-12-01

Structure and transport properties at grain boundaries in polycrystalline olivine have been investigated at the atomic scale by molecular dynamics simulation (MD) using an empirical ionocovalent interaction potential. On the time scale of the simulation (a few tens of nanoseconds for a system size of ∼650,000 atoms) grain boundaries and grain interior were identified by mapping the atomic displacements along the simulation run. In the investigated temperature range (1300-1700 K) the mean thickness of the grain boundary phase is evaluated between 0.5 and 2 nm, a value which depends on temperature and grain size. The structure of the grain boundary phase is found to be disordered (amorphous-like) and is different from the one exhibited by the supercooled liquid. The self-diffusion coefficients of major elements in the intergranular region range from ∼10-13 to 10-10 m2/s between 1300 and 1700 K (with DSigb < DOgb < DFegb < DMggb) and are only one order of magnitude smaller than those evaluated in the supercooled melt. In using a newly derived expression for the bulk self-diffusion coefficient it is concluded that the latter one is driven by the grain boundary contribution as long as the grain size is smaller than a centimeter. In assuming that the electrical conduction at grain boundaries is purely ionic, the macroscopic grain boundary conductivity is found to be two orders of magnitude lower than in molten olivine, and one order of magnitude higher than the lattice conductivity. A consequence is that the conductivity of the olivine polycrystal is dominated by the grain interior contribution as soon as the grain size is larger than a micrometer or so. The grain boundary viscosity has been evaluated from the Green-Kubo relation expressing the viscosity as function of the stress tensor time correlation function. In spite of a slow convergence of the calculation by MD, the grain boundary viscosity was estimated about ∼105 Pa s at 1500 K, a value in agreement with high-temperature viscoelastic relaxation data. An interesting information gained from MD is that sliding at grain boundaries is essentially controlled by the internal friction between the intergranular phase and the grain edges.

Structure and electronic properties of grain boundaries in earth-abundant photovoltaic absorber Cu2ZnSnSe4.

PubMed

Li, Junwen; Mitzi, David B; Shenoy, Vivek B

2011-11-22

We have studied the atomic and electronic structure of Cu(2)ZnSnSe(4) and CuInSe(2) grain boundaries using first-principles calculations. We find that the constituent atoms at the grain boundary in Cu(2)ZnSnSe(4) create localized defect states that promote the recombination of photon-excited electron and hole carriers. In distinct contrast, significantly lower density of defect states is found at the grain boundaries in CuInSe(2), which is consistent with the experimental observation that CuInSe(2) solar cells exhibit high conversion efficiency without the need for deliberate passivation. Our investigations suggest that it is essential to effectively remove these defect states in order to improve the conversion efficiency of solar cells with Cu(2)ZnSnSe(4) as photovoltaic absorber materials. © 2011 American Chemical Society

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Hak-Sung, E-mail: hslee@kims.re.kr; Park, Chanbum; Oh, Chang-Seok

Highlights: • We model the sample grain boundary of LiCoO2, one of important Li cathode materials. • Rigid body translation was found the asymmetric GB is more stable than symmetric GB. • The vacancy formation energy of Li and O was estimated with first principles calculations. • This model boundary can help to find a new dopant to improve Li diffusions. - Abstract: An atomic structure of LiCoO{sub 2} model grain boundary, Σ2 [1120](1102), is introduced and grain boundary energies with rigid body translations are investigated systematically to find the most stable interface structures. It is found that the coordinatedmore » structures of Co and O in the vicinity of grain boundary are strongly related to grain boundary energy. Examining nonstoichiometry at grain boundary, the defect energetics of Li and O site at grain boundary are estimated. In addition, the effect of grain boundary on Li diffusion is investigated to calculate Li diffusion across grain boundary.« less

Size-Dependent Grain-Boundary Structure with Improved Conductive and Mechanical Stabilities in Sub-10-nm Gold Crystals

NASA Astrophysics Data System (ADS)

Wang, Chunyang; Du, Kui; Song, Kepeng; Ye, Xinglong; Qi, Lu; He, Suyun; Tang, Daiming; Lu, Ning; Jin, Haijun; Li, Feng; Ye, Hengqiang

2018-05-01

Low-angle grain boundaries generally exist in the form of dislocation arrays, while high-angle grain boundaries (misorientation angle >15 ° ) exist in the form of structural units in bulk metals. Here, through in situ atomic resolution aberration corrected electron microscopy observations, we report size-dependent grain-boundary structures improving both stabilities of electrical conductivity and mechanical properties in sub-10-nm-sized gold crystals. With the diameter of a nanocrystal decreasing below 10 nm, the high-angle grain boundary in the crystal exists as an array of dislocations. This size effect may be of importance to a new generation of interconnects applications.

On the interaction of solutes with grain boundaries

DOE PAGES

Dingreville, Remi Philippe Michel; Berbenni, Stephane

2015-11-01

Solute segregation to grain boundaries is considered by modeling solute atoms as misfitting inclusions within a disclination structural unit model describing the grain boundary structure and its intrinsic stress field. The solute distribution around grain boundaries is described through Fermi–Dirac statistics of site occupancy. The susceptibility of hydrogen segregation to symmetric tilt grain boundaries is discussed in terms of the misorientation angle, the defect type characteristics at the grain boundary, temperature, and the prescribed bulk hydrogen fraction of occupied sites. Through this formalism, it is found that hydrogen trapping on grain boundaries clearly correlates with the grain boundary structure (i.e.more » type of structural unit composing the grain boundary), and the associated grain boundary misorientation. Specifically, for symmetric tilt grain boundaries about the [001] axis, grain boundaries composed of both B and C structural units show a lower segregation susceptibility than other grain boundaries. A direct correlation between the segregation susceptibility and the intrinsic net defect density is provided through the Frank–Bilby formalism. Moreover, the present formulation could prove to be a simple and useful model to identify classes of grain boundaries relevant to grain boundary engineering.« less

On the interaction of solutes with grain boundaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dingreville, Remi Philippe Michel; Berbenni, Stephane

Solute segregation to grain boundaries is considered by modeling solute atoms as misfitting inclusions within a disclination structural unit model describing the grain boundary structure and its intrinsic stress field. The solute distribution around grain boundaries is described through Fermi–Dirac statistics of site occupancy. The susceptibility of hydrogen segregation to symmetric tilt grain boundaries is discussed in terms of the misorientation angle, the defect type characteristics at the grain boundary, temperature, and the prescribed bulk hydrogen fraction of occupied sites. Through this formalism, it is found that hydrogen trapping on grain boundaries clearly correlates with the grain boundary structure (i.e.more » type of structural unit composing the grain boundary), and the associated grain boundary misorientation. Specifically, for symmetric tilt grain boundaries about the [001] axis, grain boundaries composed of both B and C structural units show a lower segregation susceptibility than other grain boundaries. A direct correlation between the segregation susceptibility and the intrinsic net defect density is provided through the Frank–Bilby formalism. Moreover, the present formulation could prove to be a simple and useful model to identify classes of grain boundaries relevant to grain boundary engineering.« less

The chromium doping of Ni{sub 3}Fe alloy and restructuring of grain boundary ensemble at the phase transition A1→L1{sub 2}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perevalova, Olga; Konovalova, Elena, E-mail: knv123@yandex.ru; Koneva, Nina

2016-01-15

The grain boundary structure of the Ni{sub 3}(Fe,Cr) alloy is studied in states with a short and long-range order formed at the phase transition A1→L1{sub 2}. It is found that the new boundaries of general and special types are formed during an ordering annealing, wherein the special boundaries share increases. The spectrum of special boundaries is changed due to decreasing of ∑3 boundary share. It leads to weakening of the texture in the alloy with atomic long-range order. The features of change of the special boundaries spectrum at the phase transition A1→L1{sub 2} in the Ni{sub 3}(Fe,Cr) alloy are determinedmore » by decreasing of the stacking fault energy and the atomic mean square displacement at the chromium doping.« less

Atomic-level structural correlations across the morphotropic phase boundary of a ferroelectric solid solution: xBiMg 1/2Ti 1/2O 3-(1$-$x)PbTiO 3

DOE PAGES

Datta, Kaustuv; Neder, Reinhard B.; Chen, Jun; ...

2017-03-28

Revelation of unequivocal structural information at the atomic level for complex systems is uniquely important for deeper and generic understanding of the structure property connections and a key challenge in materials science. Here in this paper we report an experimental study of the local structure by applying total elastic scattering and Raman scattering analyses to an important non-relaxor ferroelectric solid solution exhibiting the so-called composition-induced morphotropic phase boundary (MPB), where concomitant enhancement of physical properties have been detected. The powerful combination of static and dynamic structural probes enabled us to derive direct correspondence between the atomic-level structural correlations and reportedmore » properties. The atomic pair distribution functions obtained from the neutron total scattering experiments were analysed through big-box atom-modelling implementing reverse Monte Carlo method, from which distributions of magnitudes and directions of off-centred cationic displacements were extracted. We found that an enhanced randomness of the displacement-directions for all ferroelectrically active cations combined with a strong dynamical coupling between the A- and B-site cations of the perovskite structure, can explain the abrupt amplification of piezoelectric response of the system near MPB. Finally, altogether this provides a more fundamental basis in inferring structure-property connections in similar systems including important implications in designing novel and bespoke materials.« less

Graded Interface Models for more accurate Determination of van der Waals-London Dispersion Interactions across Grain Boundaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

van Benthem, Klaus; Tan, Guolong; French, Roger H

2006-01-01

Attractive van der Waals V London dispersion interactions between two half crystals arise from local physical property gradients within the interface layer separating the crystals. Hamaker coefficients and London dispersion energies were quantitatively determined for 5 and near- 13 grain boundaries in SrTiO3 by analysis of spatially resolved valence electron energy-loss spectroscopy (VEELS) data. From the experimental data, local complex dielectric functions were determined, from which optical properties can be locally analysed. Both local electronic structures and optical properties revealed gradients within the grain boundary cores of both investigated interfaces. The obtained results show that even in the presence ofmore » atomically structured grain boundary cores with widths of less than 1 nm, optical properties have to be represented with gradual changes across the grain boundary structures to quantitatively reproduce accurate van der Waals V London dispersion interactions. London dispersion energies of the order of 10% of the apparent interface energies of SrTiO3 were observed, demonstrating their significance in the grain boundary formation process. The application of different models to represent optical property gradients shows that long-range van der Waals V London dispersion interactions scale significantly with local, i.e atomic length scale property variations.« less

Electronic structure and switching behavior of the metastable silicene domain boundary

NASA Astrophysics Data System (ADS)

Oh, Youngtek; Cho, Yeonchoo; Kwon, Hyeokshin; Lee, Junsu; Jeon, Insu; Ko, Wonhee; Kim, Hyo Won; Ku, JiYeon; Kim, Gunn; Suh, Hwansoo; Hwang, Sung Woo

2017-06-01

Silicene, a silicon allotrope with a buckled honeycomb lattice, has been extensively studied in the search for materials with graphene-like properties. Here, we study the domain boundaries of a silicene 4 × 4 superstructure on an Ag(111) surface at the atomic resolution using scanning tunneling microscopy (STM) and spectroscopy (STS) along with density functional theory calculations. The silicene domain boundaries (β-phases) are formed at the interface between misaligned domains (α-phases) and show a bias dependence, forming protrusions or depressions as the sample bias changes. In particular, the STM topographs of the silicene-substrate system at a bias of ˜2.0 V show brightly protruding domain boundaries, which can be explained by an energy state originating from the Si 3s and 3pz orbitals. In addition, the topographs depicting the vicinity of the domain boundaries show that the structure does not follow the buckled geometry of the atomic ball-and-stick model. Inside the domain, STS data showed a step-up at ˜0.4 V, which originated from the Si 3p orbitals. We found this step-up to have shifted, which may be attributed to the strain effect at the interface regions between silver and silicene and between the domain and its boundary upon performing spatially resolved STS measurements. The metastable characteristic of the domain boundary (β-phase) causes changes, such as creation or annihilation, in the buckling structures (switching behavior). The observed low activation energy for the buckling change between distinct states may find applications in the electronic control of properties related to domain boundary structures in silicene.

Atomic scale study of grain boundary segregation before carbide nucleation in Ni-Cr-Fe Alloys

NASA Astrophysics Data System (ADS)

Li, Hui; Xia, Shuang; Liu, Wenqing; Liu, Tingguang; Zhou, Bangxin

2013-08-01

Three dimensional chemical information concerning grain boundary segregation before carbide nucleation was characterized by atom probe tomography in two Ni-Cr-Fe alloys which were aged at 500 °C for 0.5 h after homogenizing treatment. B, C and Si atoms segregation at grain boundary in Alloy 690 was observed. B, C, N and P atoms segregation at grain boundary in 304 austenitic stainless steel was observed. C atoms co-segregation with Cr atoms at the grain boundaries both in Alloy 690 and 304 austenitic stainless steel was found, and its effect on the carbide nucleation was discussed. The amount of each segregated element at grain boundaries in the two Ni-Cr-Fe alloys were analyzed quantitatively. Comparison of the grain boundary segregation features of the two Ni-Cr-Fe alloys were carried out based on the experimental results. The impurity and solute atoms segregate inhomogeneously in the same grain boundary both in 304 SS and Alloy 690. The grain boundary segregation tendencies (Sav) are B (11.8 ± 1.4) > P (5.4 ± 1.4) > N (4.7 ± 0.3) > C (3.7 ± 0.4) in 304 SS, and B (6.9 ± 0.9) > C (6.7 ± 0.4) > Si (1.5 ± 0.2) in Alloy 690. Cr atoms may co-segregate with C atoms at grain boundaries before carbide nucleation at the grain boundaries both in 304 SS and Alloy 690. Ni atoms generally deplete at grain boundary both in 304 SS and Alloy 690. The literature shows that the Ni atoms may co-segregate with P atoms at grain boundaries [28], but the P atoms segregation do not leads to Ni segregation in the current study. In the current study, Fe atoms may segregate or deplete at grain boundary in Alloy 690. But Fe atoms generally deplete at grain boundary in 304 SS. B atoms have the strongest grain boundary segregation tendency both in 304 SS and Alloy 690. The grain boundary segregation tendency and Gibbs free energy of B in 304 SS is higher than in Alloy 690. C atoms are easy to segregate at grain boundaries both in 304 SS and Alloy 690. The grain boundary segregation tendency and Gibbs free energy of C in Alloy 690 is higher than in 304 SS, due to the higher bulk C concentration and the site competition of P atoms which segregate at grain boundary [29,30]. It is imply that the segregation tendency is influenced by the bulk concentration of the segregates. Si atoms slightly segregate at grain boundaries in Alloy 690, but do not segregate at grain boundaries in 304 SS. N and P atoms segregate at grain boundary in 304 SS, and their segregation Gibbs free energy are similar. N atoms may be exhausted by the TiN precipitated in the matrix and can not be observed in the grain boundary of Alloy 690 [19]. Mn atoms deplete at grain boundary in 304 SS. This phenomenon is similar to that of proton irradiation induced segregation in 304 SS [32]. B, C, N, P segregation Gibbs energies are similar both in 304 SS and Alloy 690. B and C atoms segregate at grain boundary both in Alloy 690 and 304 SS, P and N segregate at grain boundary in 304 SS. Si atoms segregate at grain boundary in Alloy 690, but do not segregate at grain boundary in 304 SS. Cr enriches at grain boundary both in Alloy 690 and 304 SS, although carbide does not nucleate. Ni and Fe may segregate, deplete or homogeneously distribute at grain boundary in Alloy 690, but they deplete at grain boundary in 304 SS. C and Cr atoms co-segregate at grain boundaries before carbide nucleation in Alloy 690 and 304 SS. Combination with other results in literatures, the evolution of Cr concentration at grain boundary should be enrichment at grain boundary before carbide nucleation, depletion at grain boundary after carbide precipitation, and healing after obvious growth of carbide. After aging treatment at 500 °C for 0.5 h, the total reduction of grain boundary free energy due to segregation is 27.489 kJ/mol for Alloy 690 and 45.207 kJ/mol for 304.

Atomistic simulation of the trapping capability of He-vacancy defects at Ni {\\sum}^{}3\\left(1\\bar{1}2\\right)[110] grain boundary

NASA Astrophysics Data System (ADS)

Gong, Hengfeng; Wang, Chengbin; Zhang, Wei; Huai, Ping; Lu, Wei; Zhu, Zhiyuan

2016-12-01

He atoms tend to cluster and precipitate into bubbles that prefer to grow in the grain boundaries, resulting in high temperature He embrittlement with significantly degraded material properties. This is a major bottleneck in employing Ni-based alloys for applications such as molten salt reactors (MSRs). This paper focuses on understanding how the local grain boundary structure interacts with He atoms and how the local atomistic environment in the grain boundary influences the binding energy of He defects. Using molecular dynamics simulations, we have investigated the trapping capability of the Ni {\\sum}3≤ft(1 \\bar{1} 2\\right)≤ft[1 1 0\\right] grain boundary to He defects (He N ) and to He-vacancy defects (He N V M ). The two defects in the Ni grain boundary exhibit geometries with high symmetry. The binding energy of an interstitial He atom to He N V M defects is found to be generally larger in pure Ni than that in the grain boundary. We compared the binding energy of He N defects to the Ni vacancy and to the Ni grain boundary, finding that the Ni vacancy possesses a higher trapping strength to He N . We also found that the binding strength of He N to the grain boundary is stronger than that of He N V M to the grain boundary. The He-vacancy ratio in He N V M defects does not significantly affect the binding energy in the grain boundary plane. The current work will provide insight in understanding the experimentally observed He bubble formation in Ni-based alloys and bridge atomic scale events and damage with macroscopic failure.

Analysis of the Atomic-Scale Defect Chemistry at Interfaces in Fluorite Structured Oxides by Electron Energy Loss Spectroscopy

DTIC Science & Technology

2001-11-01

electronic properties, i.e. oxygen coordination and cation valence at grain boundaries of the fluorite structured Gdo]2Ceo.gO 2_x ceramic membrane material...required to obtain a detailed understanding of the atomic scale phenomena in ceramics, as the polycrystalline nature of Gdo.2Ceo.802- ceramic membrane material

The Formation, Transport Properties and Microstructure of 45 Degrees (001) Tilt Grain Boundaries in Yttrium BARIUM(2) COPPER(3) OXYGEN(7-X) Thin Films

NASA Astrophysics Data System (ADS)

Vuchic, Boris Vukan

1995-01-01

Most high angle grain boundaries in high-T _{c} superconductors exhibit weak link behavior. The Josephson-like properties of these grain boundaries can be used for many device applications such as superconducting quantum interference devices (SQUIDs). The structure-property relationship of different types of 45 ^circ (001) YBa_2 Cu_3O_{7-x} thin film grain boundary junctions are examined to study their weak link nature. A technique, termed sputter-induced epitaxy, is developed to form 45^circ (001) tilt grain boundaries in YBa_2Cu _3O_{7-x} thin films on (100) MgO substrates. A low voltage ion bombardment pre-growth substrate treatment is used to modify the epitaxial orientation relationship between the thin film and the substrate in selected regions. By modifying the orientation of the thin film, grain boundary junctions can be placed in any configuration on the substrate. A variety of pre-growth sputtering conditions in conjunction with atomic force microscopy and Rutherford backscatter spectrometry are used to determine the role of the ions in modifying the substrate surface. Sputter-induced epitaxy is extended to a multilayer MgO/LaAlO_3 substrate, allowing integration of the sputter -induced epitaxy junctions into multilayer structures. The low temperature transport properties of the sputter-induced epitaxy junctions and a set of bi-epitaxial grain boundaries are studied. Individual grain boundaries are isolated and characterized for resistance vs. temperature, current vs. voltage as a function of temperature and magnetic field behavior. Resistive and superconducting grain boundaries are compared. Microstructural analysis is performed using scanning electron microscopy, transmission electron microscopy and high resolution electron microscopy (HREM). Marked differences are observed in the microstructure of resistive and superconducting grain boundaries. HREM studies suggest the importance of the local atomic scale structure of the grain boundary in transport properties. A phenomenological grain boundary model is proposed to describe the structure -property relationship of the boundaries.

Atomistic simulations of dislocation pileup: Grain boundaries interaction

DOE PAGES

Wang, Jian

2015-05-27

Here, using molecular dynamics (MD) simulations, we studied the dislocation pileup–grain boundary (GB) interactions. Two Σ11 asymmetrical tilt grain boundaries in Al are studied to explore the influence of orientation relationship and interface structure on dislocation activities at grain boundaries. To mimic the reality of a dislocation pileup in a coarse-grained polycrystalline, we optimized the dislocation population in MD simulations and developed a predict-correct method to create a dislocation pileup in MD simulations. MD simulations explored several kinetic processes of dislocations–GB reactions: grain boundary sliding, grain boundary migration, slip transmission, dislocation reflection, reconstruction of grain boundary, and the correlation ofmore » these kinetic processes with the available slip systems across the GB and atomic structures of the GB.« less

Domain pinning near a single-grain boundary in tetragonal and rhombohedral lead zirconate titanate films

DOE PAGES

Marincel, Dan M.; Zhang, H. R.; Briston, J.; ...

2015-04-27

The interaction of grain boundaries with ferroelectric domain walls strongly influences the extrinsic contribution to piezoelectric activity in Pb(Zr,Ti)O 3 (PZT), ubiquitous in modern transducers and actuators. However, the fundamental understanding of these phenomena has been limited by complex mechanisms originating from the interplay of atomic-level domain wall pinning, collective domain wall dynamics, and emergent mesoscopic behavior. This contribution utilizes engineered grain boundaries created by depositing epitaxial PZT films with various Zr:Ti ratio onto 24º SrTiO 3 tilt bicrystals. The nonlinear piezoelectric response and surface domain structure across the boundary are investigated using piezoresponse force microscopy whilst cross section domainmore » structure is studied using transmission electron microscopy. The grain boundary reduces domain wall motion over a width of 800±70 nm for PZT 45:55 and 450±30 nm for PZT 52:48. Phase field modeling provides an understanding of the elastic and electric fields associated with the grain boundary and local domain configurations. In conclusion, this study demonstrates that complex mesoscopic behaviors can be explored to complement atomic-level pictures of the material system.« less

Thermodynamics of inversion-domain boundaries in aluminum nitride: Interplay between interface energy and electric dipole potential energy

NASA Astrophysics Data System (ADS)

Zhang, J. Y.; Xie, Y. P.; Guo, H. B.; Chen, Y. G.

2018-05-01

Aluminum nitride (AlN) has a polar crystal structure that is susceptible to electric dipolar interactions. The inversion domains in AlN, similar to those in GaN and other wurtzite-structure materials, decrease the energy associated with the electric dipolar interactions at the expense of inversion-domain boundaries, whose interface energy has not been quantified. We study the atomic structures of six different inversion-domain boundaries in AlN, and compare their interface energies from density functional theory calculations. The low-energy interfaces have atomic structures with similar bonding geometry as those in the bulk phase, while the high-energy interfaces contain N-N wrong bonds. We calculate the formation energy of an inversion domain using the interface energy and dipoles' electric-field energy, and find that the distribution of the inversion domains is an important parameter for the microstructures of AlN films. Using this thermodynamic model, it is possible to control the polarity and microstructure of AlN films by tuning the distribution of an inversion-domain nucleus and by selecting the low-energy synthesis methods.

Characterizing the boundary lateral to the shear direction of deformation twins in magnesium

DOE PAGES

Liu, Y.; Li, N.; Shao, S.; ...

2016-06-01

The three-dimensional nature of twins, especially the atomic structures and motion mechanisms of the boundary lateral to the shear direction of the twin, has never been characterized at the atomic level, because such boundary is, in principle, crystallographically unobservable. We thus refer to it here as the dark side of the twin. Here, using high-resolution transmission electron microscopy and atomistic simulations, we characterize the dark side of {101 ⁻2} deformation twins in magnesium. It is found that the dark side is serrated and comprised of {101 ⁻2} coherent twin boundaries and semi-coherent twist prismatic–prismatic {211 ⁻0} boundaries that control twinmore » growth. The conclusions we find in this work apply to the same twin mode in other hexagonal close-packed materials, and the conceptual ideas discussed here should hold for all twin modes in crystalline materials.« less

Multiscale Modeling of Grain Boundaries in ZrB2: Structure, Energetics, and Thermal Resistance

NASA Technical Reports Server (NTRS)

Lawson, John W.; Daw, Murray S.; Squire, Thomas H.; Bauschlicher, Charles W., Jr.

2012-01-01

A combination of ab initio, atomistic and finite element methods (FEM) were used to investigate the structures, energetics and lattice thermal conductance of grain boundaries for the ultra high temperature ceramic ZrB2. Atomic models of idealized boundaries were relaxed using density functional theory. Information about bonding across the interfaces was determined from the electron localization function. The Kapitza conductance of larger scale versions of the boundary models were computed using non-equilibrium molecular dynamics. The interfacial thermal parameters together with single crystal thermal conductivities were used as parameters in microstructural computations. FEM meshes were constructed on top of microstructural images. From these computations, the effective thermal conductivity of the polycrystalline structure was determined.

The relationship between grain boundary structure, defect mobility, and grain boundary sink efficiency

PubMed Central

Uberuaga, Blas Pedro; Vernon, Louis J.; Martinez, Enrique; Voter, Arthur F.

2015-01-01

Nanocrystalline materials have received great attention due to their potential for improved functionality and have been proposed for extreme environments where the interfaces are expected to promote radiation tolerance. However, the precise role of the interfaces in modifying defect behavior is unclear. Using long-time simulations methods, we determine the mobility of defects and defect clusters at grain boundaries in Cu. We find that mobilities vary significantly with boundary structure and cluster size, with larger clusters exhibiting reduced mobility, and that interface sink efficiency depends on the kinetics of defects within the interface via the in-boundary annihilation rate of defects. Thus, sink efficiency is a strong function of defect mobility, which depends on boundary structure, a property that evolves with time. Further, defect mobility at boundaries can be slower than in the bulk, which has general implications for the properties of polycrystalline materials. Finally, we correlate defect energetics with the volumes of atomic sites at the boundary. PMID:25766999

The relationship between grain boundary structure, defect mobility, and grain boundary sink efficiency

DOE PAGES

Uberuaga, Blas Pedro; Vernon, Louis J.; Martinez, Enrique; ...

2015-03-13

Nanocrystalline materials have received great attention due to their potential for improved functionality and have been proposed for extreme environments where the interfaces are expected to promote radiation tolerance. However, the precise role of the interfaces in modifying defect behavior is unclear. Using long-time simulations methods, we determine the mobility of defects and defect clusters at grain boundaries in Cu. We find that mobilities vary significantly with boundary structure and cluster size, with larger clusters exhibiting reduced mobility, and that interface sink efficiency depends on the kinetics of defects within the interface via the in-boundary annihilation rate of defects. Thus,more » sink efficiency is a strong function of defect mobility, which depends on boundary structure, a property that evolves with time. Further, defect mobility at boundaries can be slower than in the bulk, which has general implications for the properties of polycrystalline materials. Finally, we correlate defect energetics with the volumes of atomic sites at the boundary.« less

Creation and Analysis of Atomic Structures for CdTe Bi-crystal Interfaces by the Grain Boundary Genie

DOE Office of Scientific and Technical Information (OSTI.GOV)

Buurma, Christopher; Sen, Fatih G.; Paulauskas, Tadas

2015-01-01

Grain boundaries (GB) in poly-CdTe solar cells play an important role in species diffusion, segregation, defect formation, and carrier recombination. While the creation of specific high-symmetry interfaces can be straight forward, the creation of general GB structures in many material systems is difficult if periodic boundary conditions are to be enforced. Here we describe a novel algorithm and implementation to generate initial general GB structures for CdTe in an automated way, and we investigate some of these structures using density functional theory (DFT). Example structures include those with bi-crystals already fabricated for comparison, and those planning to be investigated inmore » the future.« less

STEM and APT characterization of scale formation on a La,Hf,Ti-doped NiCrAl model alloy.

PubMed

Unocic, Kinga A; Chen, Yimeng; Shin, Dongwon; Pint, Bruce A; Marquis, Emmanuelle A

2018-06-01

A thermally grown scale formed on a cast NiCrAl model alloy doped with lanthanum, hafnium, and titanium was examined after isothermal exposure at 1100 °C for 100 h in dry flowing O 2 to understand the dopant segregation along scale grain boundaries. The complex scale formed on the alloy surface was composed of two types of substrates: phase-dependent, thin (<250 nm) outer layers and a columnar-grained ∼3.5 μm inner alumina layer. Two types of oxides formed between the inner and outer scale layers: small (3-15 nm) La 2 O 3 and larger (≤50 nm) HfO 2 oxide precipitates. Nonuniform distributions of the hafnium, lanthanum, and titanium dopants were observed along the inner scale grain boundaries, with hafnium dominating in most of the grain boundaries of α-Al 2 O 3. The concentration of reactive elements (RE) seemed to strongly depend on the grain boundary structure. The level of titanium grain boundary segregation in the inner scale decreased toward the model alloy (substrate), confirming the fast outward diffusion of titanium. Hafnium was also observed at the metal-scale interface and in the γ' (Ni 3 Al) phase of the alloy. High-resolution scanning transmission electron microscopy (STEM) confirmed the substitution of REs for aluminum atoms at the scale grain boundaries, consistent with both the semiconducting band structure and the site-blocking models. Both STEM and atom probe tomography allowed quantification of REs along the scale grain boundaries across the scale thickness. Analysis of the scale morphology after isothermal exposure in flowing oxygen revealed a myriad of new precipitate phases, RE segregation dependence on grain boundary type, and atomic arrangement along scale grain boundaries, which is expected to influence the scale growth rate, stability, and mechanical properties. Copyright © 2018 Elsevier Ltd. All rights reserved.

Grain boundaries structures and wetting in doped silicon, nickel and copper

NASA Astrophysics Data System (ADS)

Meshinchi Asl, Kaveh

This thesis reports a series of fundamental investigations of grain boundary wetting, adsorption and structural (phases) transitions in doped Ni, Cu and Si with technological relevance to liquid metal embrittlement, liquid metal corrosion and device applications. First, intrinsically ductile metals are prone to catastrophic failure when exposed to certain liquid metals, but the atomic level mechanism for this effect is not fully understood. A nickel sample infused with bismuth atoms was characterized and a bilayer interfacial phase that is the underlying cause of embrittlement was observed. In a second related study, we showed that addition of minor impurities can significantly enhance the intergranular penetration of bismuth based liquids in polycrystalline nickel and copper, thereby increasing the liquid metal corrosion rates. Furthermore, we extended a concept that was initially proposed in the Rice-Wang model for grain boundary embrittlement to explain our observations of the impurity-enhanced intergranular penetration of liquid metals. Finally, a grain-boundary transition from a bilayer to an intrinsic is observed in the Si-Au system. This observation directly shows that a grain boundary can exhibit a first-order "phase" transition, which often implies abrupt changes in properties.

Gold-promoted structurally ordered intermetallic palladium cobalt nanoparticles for the oxygen reduction reaction.

PubMed

Kuttiyiel, Kurian A; Sasaki, Kotaro; Su, Dong; Wu, Lijun; Zhu, Yimei; Adzic, Radoslav R

2014-11-06

Considerable efforts to make palladium and palladium alloys active catalysts and a possible replacement for platinum have had a marginal success. Here we report on a structurally ordered Au10Pd₄₀Co₅₀ catalyst that exhibits comparable activity to conventional platinum catalysts in both acid and alkaline media. Electron microscopic techniques demonstrate that, at elevated temperatures, palladium cobalt nanoparticles undergo an atomic structural transition from core-shell to a rare intermetallic ordered structure with twin boundaries forming stable {111}, {110} and {100} facets via addition of gold atoms. The superior stability of this catalyst compared with platinum after 10,000 potential cycles in alkaline media is attributed to the atomic structural order of PdCo nanoparticles along with protective effect of clusters of gold atoms on the surface. This strategy of making ordered palladium intermetallic alloy nanoparticles can be used in diverse heterogeneous catalysis where particle size and structural stability matter.

Atomic electron tomography: 3D structures without crystals

DOE PAGES

Miao, Jianwei; Ercius, Peter; Billinge, S. J. L.

2016-09-23

Crystallography has been fundamental to the development of many fields of science over the last century. However, much of our modern science and technology relies on materials with defects and disorders, and their three-dimensional (3D) atomic structures are not accessible to crystallography. One method capable of addressing this major challenge is atomic electron tomography. By combining advanced electron microscopes and detectors with powerful data analysis and tomographic reconstruction algorithms, it is now possible to determine the 3D atomic structure of crystal defects such as grain boundaries, stacking faults, dislocations, and point defects, as well as to precisely localize the 3Dmore » coordinates of individual atoms in materials without assuming crystallinity. In this work, we review the recent advances and the interdisciplinary science enabled by this methodology. We also outline further research needed for atomic electron tomography to address long-standing unresolved problems in the physical sciences.« less

Charging effect at grain boundaries of MoS2

NASA Astrophysics Data System (ADS)

Yan, Chenhui; Dong, Xi; Li, Connie H.; Li, Lian

2018-05-01

Grain boundaries (GBs) are inherent extended defects in chemical vapor deposited (CVD) transition metal dichalcogenide (TMD) films. Characterization of the atomic structure and electronic properties of these GBs is crucial for understanding and controlling the properties of TMDs via defect engineering. Here, we report the atomic and electronic structure of GBs in CVD grown MoS2 on epitaxial graphene/SiC(0001). Using scanning tunneling microscopy/spectroscopy, we find that GBs mostly consist of arrays of dislocation cores, where the presence of mid-gap states shifts both conduction and valence band edges by up to 1 eV. Our findings demonstrate the first charging effect near GBs in CVD grown MoS2, providing insights into the significant impact GBs can have on materials properties.

Grain-Boundary Resistance in Copper Interconnects: From an Atomistic Model to a Neural Network

NASA Astrophysics Data System (ADS)

Valencia, Daniel; Wilson, Evan; Jiang, Zhengping; Valencia-Zapata, Gustavo A.; Wang, Kuang-Chung; Klimeck, Gerhard; Povolotskyi, Michael

2018-04-01

Orientation effects on the specific resistance of copper grain boundaries are studied systematically with two different atomistic tight-binding methods. A methodology is developed to model the specific resistance of grain boundaries in the ballistic limit using the embedded atom model, tight- binding methods, and nonequilibrium Green's functions. The methodology is validated against first-principles calculations for thin films with a single coincident grain boundary, with 6.4% deviation in the specific resistance. A statistical ensemble of 600 large, random structures with grains is studied. For structures with three grains, it is found that the distribution of specific resistances is close to normal. Finally, a compact model for grain-boundary-specific resistance is constructed based on a neural network.

Surface properties of atomically flat poly-crystalline SrTiO3

PubMed Central

Woo, Sungmin; Jeong, Hoidong; Lee, Sang A.; Seo, Hosung; Lacotte, Morgane; David, Adrian; Kim, Hyun You; Prellier, Wilfrid; Kim, Yunseok; Choi, Woo Seok

2015-01-01

Comparison between single- and the poly-crystalline structures provides essential information on the role of long-range translational symmetry and grain boundaries. In particular, by comparing single- and poly-crystalline transition metal oxides (TMOs), one can study intriguing physical phenomena such as electronic and ionic conduction at the grain boundaries, phonon propagation, and various domain properties. In order to make an accurate comparison, however, both single- and poly-crystalline samples should have the same quality, e.g., stoichiometry, crystallinity, thickness, etc. Here, by studying the surface properties of atomically flat poly-crystalline SrTiO3 (STO), we propose an approach to simultaneously fabricate both single- and poly-crystalline epitaxial TMO thin films on STO substrates. In order to grow TMOs epitaxially with atomic precision, an atomically flat, single-terminated surface of the substrate is a prerequisite. We first examined (100), (110), and (111) oriented single-crystalline STO surfaces, which required different annealing conditions to achieve atomically flat surfaces, depending on the surface energy. A poly-crystalline STO surface was then prepared at the optimum condition for which all the domains with different crystallographic orientations could be successfully flattened. Based on our atomically flat poly-crystalline STO substrates, we envision expansion of the studies regarding the TMO domains and grain boundaries. PMID:25744275

Molecular microelectrostatic view on electronic states near pentacene grain boundaries

NASA Astrophysics Data System (ADS)

Verlaak, Stijn; Heremans, Paul

2007-03-01

Grain boundaries are the most inevitable and pronounced structural defects in pentacene films. To study the effect of those structural defects on the electronic state distribution, the energy levels of a hole on molecules at and near the defect have been calculated using a submolecular self-consistent-polarization-field approach in combination with atomic charge-quadrupole interaction energy calculations. This method has been benchmarked prior to application on four idealized grain boundaries: a grain boundary void, a void with molecules squeezed in between two grains, a boundary between two grains with different crystallographic orientations, and a grain boundary void in which a permanent dipole (e.g., a water molecule) has nested. While idealized, those views highlight different aspects of real grain boundaries. Implications on macroscopic charge transport models are discussed, as well as some relation between growth conditions and the formation of the grain boundary.

The helium effect at grain boundaries in Fe-Cr alloys: A first-principles study

NASA Astrophysics Data System (ADS)

Zemła, M. R.; Wróbel, J. S.; Wejrzanowski, T.; Nguyen-Manh, D.; Kurzydłowski, K. J.

2017-02-01

Helium is produced in the structural materials in nuclear power plants by nuclear transmutation following neutron irradiation. Since the solubility of helium in all metals is extremely low, helium tends to be trapped at defects such as vacancies, dislocations and grain boundaries, which cause material embrittlement. Density functional theory (DFT) calculations were performed in order to investigate the helium effect at grain boundaries (GBs) in iron-chromium alloys. Both cohesive energy and magnetic properties at symmetric Σ3(1 1 1) and Σ5(2 1 0) tilt Fe GBs are studied in the presence of Cr and He atoms. It is found that the presence of Cr atoms increases cohesive energy, at different He concentrations, and strongly influences magnetic properties at the GBs. The effect of the segregation energy of helium atom as a function of the different positions of Cr atoms located inside/outside a GB has been considered. Results of the present first-principles study enable one to clarify the role of Cr in understanding the helium effect in Fe-Cr-based alloys.

Quantum metrology of phase for accelerated two-level atom coupled with electromagnetic field with and without boundary

NASA Astrophysics Data System (ADS)

Yang, Ying; Liu, Xiaobao; Wang, Jieci; Jing, Jiliang

2018-03-01

We study how to improve the precision of the quantum estimation of phase for an uniformly accelerated atom in fluctuating electromagnetic field by reflecting boundaries. We find that the precision decreases with increases of the acceleration without the boundary. With the presence of a reflecting boundary, the precision depends on the atomic polarization, position and acceleration, which can be effectively enhanced compared to the case without boundary if we choose the appropriate conditions. In particular, with the presence of two parallel reflecting boundaries, we obtain the optimal precision for atomic parallel polarization and the special distance between two boundaries, as if the atom were shielded from the fluctuation.

Atom probe study of grain boundary segregation in technically pure molybdenum

DOE Office of Scientific and Technical Information (OSTI.GOV)

Babinsky, K., E-mail: katharina.babinsky@stud.unileoben.ac.at; Weidow, J., E-mail: jonathan.weidow@chalmers.se; Knabl, W., E-mail: wolfram.knabl@plansee.com

2014-01-15

Molybdenum, a metal with excellent physical, chemical and high-temperature properties, is an interesting material for applications in lighting-technology, high performance electronics, high temperature furnace construction and coating technology. However, its applicability as a structural material is limited because of the poor oxidation resistance at high temperatures and a brittle-to-ductile transition around room temperature, which is influenced by the grain size and the content of interstitial impurities at the grain boundaries. Due to the progress of the powder metallurgical production during the last decades, the amount of impurities in the current quality of molybdenum has become so small that surface sensitivemore » techniques are not applicable anymore. Therefore, the atom probe, which allows the detection of small amounts of impurities as well as their location, seems to be a more suitable technique. However, a site-specific specimen preparation procedure for grain boundaries in refractory metals with a dual focused ion beam/scanning electron microscope is still required. The present investigation describes the development and successful application of such a site-specific preparation technique for grain boundaries in molybdenum, which is significantly improved by a combination with transmission electron microscopy. This complimentary technique helps to improve the visibility of grain boundaries during the last preparation steps and to evidence the presence of grain and subgrain boundaries without segregants in atom probe specimens. Furthermore, in industrially processed and recrystallized molybdenum sheets grain boundary segregation of oxygen, nitrogen and potassium is successfully detected close to segregated regions which are believed to be former sinter pores. - Highlights: • First study of grain boundary segregation in molybdenum by atom probe • Site-specific preparation technique by FIB and TEM successfully developed • Grain boundary segregation of oxygen, nitrogen and potassium found • Segregation in former sinter-pores detected • Presence of grain boundaries without segregation evidenced.« less

Misoriented grain boundaries vicinal to the ? twin in Nickel part II: thermodynamics of hydrogen segregation

NASA Astrophysics Data System (ADS)

O'Brien, Christopher J.; Foiles, Stephen M.

2016-05-01

Grain boundary engineered materials are of immense interest for their resistance to hydrogen embrittlement. This work builds on the work undertaken in Part I on the thermodynamic stability and structure of misoriented grain boundaries vicinal to the ?? (coherent-twin) boundary to examine hydrogen segregation to those boundaries. The segregation of hydrogen reflects the asymmetry of the boundary structure with the sense of rotation of the grains about the coherent-twin boundary, and the temperature-dependent structural transition present in one sense of misorientation. This work also finds that the presence of hydrogen affects a change in structure of the boundaries with increasing concentration. The structural change effects only one sense of misorientation and results in the reduction in length of the emitted stacking faults. Moreover, the structural change results in the generation of occupied sites populated by more strongly bound hydrogen. The improved understanding of misoriented twin grain boundary structure and the effect on hydrogen segregation resulting from this work is relevant to higher length-scale models. To that end, we examine commonly used metrics such as free volume and atomic stress at the boundary. Free volume is found not to be useful as a surrogate for predicting the degree of hydrogen segregation, whereas the volumetric virial stress reliably predicts the locations of hydrogen segregation and exclusion at concentrations below saturation or the point where structural changes are induced by increasing hydrogen concentration.

Atomic migration of carbon in hard turned layers of carburized bearing steel

DOE PAGES

Bedekar, Vikram; Poplawsky, Jonathan D.; Guo, Wei; ...

2016-01-01

In grain finement and non-equilibrium there is carbon segregation within grain boundaries alters the mechanical performance of hard turning layers in carburized bearing steel. Moreover, an atom probe tomography (APT) study on the nanostructured hard turning layers reveals carbon migration to grain boundaries as a result of carbide decomposition during severe plastic deformation. In addition, samples exposed to different cutting speeds show that the carbon migration rate increases with the cutting speed. For these two effects lead to an ultrafine carbon network structure resulting in increased hardness and thermal stability in the severely deformed surface layer.

Strain solitons and topological defects in bilayer graphene

PubMed Central

Alden, Jonathan S.; Tsen, Adam W.; Huang, Pinshane Y.; Hovden, Robert; Brown, Lola; Park, Jiwoong; Muller, David A.; McEuen, Paul L.

2013-01-01

Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151–157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6–11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene. PMID:23798395

Electrical transport across grain boundaries in graphene monolayers on SiC(0 0 0 \\bar{1} )

NASA Astrophysics Data System (ADS)

Zhou, Xiaodong; Ji, Shuai-Hua; Chockalingam, S. P.; Hannon, J. B.; Tromp, R. M.; Heinz, T. F.; Pasupathy, A. N.; Ross, F. M.

2018-07-01

We measure the role of structural defects, including grain boundaries and step edges, in determining the electrical transport characteristics of polycrystalline graphene monolayers synthesized on C-face SiC(0 0 0 ) by thermal decomposition. A combination of multi-probe scanning tunneling microscopy/potentiometry and low-energy electron microscopy allows the transport properties of individual grain boundaries to be correlated with their misorientation and atomic-level structure, without any device fabrication. We find that different types of grain boundary show dramatically different transport properties, and that boundaries can change structure and resistivity along their length. Boundary regions made up of dislocation superlattices separated by continuous graphene exhibit relatively low resistivity which is comparable to the resistivity of the graphene sheet itself. Other grain boundaries display trench structures with a resistivity 1–2 orders of magnitude greater and sufficient to dominate transport through the polycrystalline sheet. We also measure the transport properties of step edges and monolayer-bilayer boundaries on C-face graphene and compare them to Si-face graphene. Such measurements offer a guideline for optimizing graphene growth on SiC to improve its electronic properties.

Structure identification methods for atomistic simulations of crystalline materials

DOE PAGES

Stukowski, Alexander

2012-05-28

Here, we discuss existing and new computational analysis techniques to classify local atomic arrangements in large-scale atomistic computer simulations of crystalline solids. This article includes a performance comparison of typical analysis algorithms such as common neighbor analysis (CNA), centrosymmetry analysis, bond angle analysis, bond order analysis and Voronoi analysis. In addition we propose a simple extension to the CNA method that makes it suitable for multi-phase systems. Finally, we introduce a new structure identification algorithm, the neighbor distance analysis, which is designed to identify atomic structure units in grain boundaries.

The interaction between atomic displacement cascades and tilt symmetrical grain boundaries in α-zirconium

NASA Astrophysics Data System (ADS)

Kapustin, P.; Svetukhin, V.; Tikhonchev, M.

2017-06-01

The atomic displacement cascade simulations near symmetric tilt grain boundaries (GBs) in hexagonal close packed-Zirconium were considered in this paper. Further defect structure analysis was conducted. Four symmetrical tilt GBs -∑14?, ∑14? with the axis of rotation [0 0 0 1] and ∑32?, ∑32? with the axis of rotation ? - were considered. The molecular dynamics method was used for atomic displacement cascades' simulation. A tendency of the point defects produced in the cascade to accumulate near the GB plane, which was an obstacle to the spread of the cascade, was discovered. The results of the point defects' clustering produced in the cascade were obtained. The clusters of both types were represented mainly by single point defects. At the same time, vacancies formed clusters of a large size (more than 20 vacancies per cluster), while self-interstitial atom clusters were small-sized.

Projected Hybrid Orbitals: A General QM/MM Method

PubMed Central

2015-01-01

A projected hybrid orbital (PHO) method was described to model the covalent boundary in a hybrid quantum mechanical and molecular mechanical (QM/MM) system. The PHO approach can be used in ab initio wave function theory and in density functional theory with any basis set without introducing system-dependent parameters. In this method, a secondary basis set on the boundary atom is introduced to formulate a set of hybrid atomic orbtials. The primary basis set on the boundary atom used for the QM subsystem is projected onto the secondary basis to yield a representation that provides a good approximation to the electron-withdrawing power of the primary basis set to balance electronic interactions between QM and MM subsystems. The PHO method has been tested on a range of molecules and properties. Comparison with results obtained from QM calculations on the entire system shows that the present PHO method is a robust and balanced QM/MM scheme that preserves the structural and electronic properties of the QM region. PMID:25317748

Determination of gaseous fission product behavior near the cerium dioxide Σ 3 (111)/[11 bar0] tilt grain boundary via first-principles study

NASA Astrophysics Data System (ADS)

Xi, Jianqi; Liu, Bin; Xu, Haixuan; Zhang, Yanwen; Weber, William J.

2018-02-01

Grain boundaries (GBs) are the most abundant structural defects in nanostructured nuclear fuels and play an important role in determining fission product behavior, which further affects the performance of nuclear fuels. In this work, cerium dioxide (CeO2) is used as a surrogate material for mixed oxide fuels to understand gaseous fission product behavior, specifically Xe. First-principles calculations are employed to comprehensively study the behavior of Xe and trap sites for Xe near the Σ 3 (111)/[11 bar0] grain boundary in CeO2, which will provide guidance on overall trends for Xe stability and diffusion at grain boundaries vs in the bulk. Significant segregation behavior of trap sites, regardless of charge states, is observed near the GB. This is mainly ascribed to the local atomic structure near the GB, which results in weaker bond strength and more negative segregation energies. For Xe, however, the segregation profile near the GB is different. Our calculations show that, as the size of trap sites increases, the segregation propensity of Xe is reduced. In addition, under hyper-stoichiometric conditions, the solubility of Xe trapped at the GB is significantly higher than that in the bulk, suggesting higher Xe concentration than that in the bulk. The results of this work demonstrate that the diffusion mechanism of Xe in CeO2 is comparable to that in UO2. The diffusion activation energies of Xe atoms in the Σ 3 GB are lower than that in the bulk CeO2. These results suggest that the diffusivity of Xe atoms is higher along the GB than that in the bulk, which enhances the aggregation of Xe atoms near the GB.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Field, Kevin G.; Yang, Ying; Busby, Jeremy T.

Radiation induced segregation (RIS) is a well-studied phenomena which occurs in many structurally relevant nuclear materials including austenitic stainless steels. RIS occurs due to solute atoms preferentially coupling to mobile point defect fluxes that migrate and interact with defect sinks. Here, a 304 stainless steel was neutron irradiated up to 47.1 dpa at 320 °C. Investigations into the RIS response at specific grain boundary types were utilized to determine the sink characteristics of different boundary types as a function of irradiation dose. A rate theory model built on the foundation of the modified inverse Kirkendall (MIK) model is proposed andmore » benchmarked to the experimental results. This model, termed the GiMIK model, includes alterations in the boundary conditions based on grain boundary structure and includes expressions for interstitial binding. This investigation, through experiment and modeling, found specific grain boundary structures exhibit unique defect sink characteristics depending on their local structure. Furthermore, such interactions were found to be consistent across all doses investigated and had larger global implications including precipitation of Ni-Si clusters near different grain boundary types.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Field, Kevin G.; Yang, Ying; Allen, Todd R.

Radiation induced segregation (RIS) is a well-studied phenomena which occurs in many structurally relevant nuclear materials including austenitic stainless steels. RIS occurs due to solute atoms preferentially coupling to mobile point defect fluxes that migrate and interact with defect sinks. Here, a 304 stainless steel was neutron irradiated up to 47.1 dpa at 320 °C. Investigations into the RIS response at specific grain boundary types were utilized to determine the sink characteristics of different boundary types as a function of irradiation dose. A rate theory model built on the foundation of the modified inverse Kirkendall (MIK) model is proposed andmore » benchmarked to the experimental results. This model, termed the GiMIK model, includes alterations in the boundary conditions based on grain boundary structure and includes expressions for interstitial binding. This investigation, through experiment and modeling, found specific grain boundary structures exhibit unique defect sink characteristics depending on their local structure. Such interactions were found to be consistent across all doses investigated and had larger global implications including precipitation of Ni-Si clusters near different grain boundary types.« less

Gold–promoted structurally ordered intermetallic palladium cobalt nanoparticles for the oxygen reduction reaction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kuttiyiel, Kurian A.; Sasaki, Kotaro; Su, Dong

2014-11-06

Considerable efforts to make palladium and palladium alloys active catalysts and a possible replacement for platinum have had a marginal success. Here, we report on a structurally ordered Au₁₀Pd₄₀Co₅₀ catalyst that exhibits comparable activity to conventional platinum catalysts in both acid and alkaline media. Electron microscopic techniques demonstrate that via addition of gold atoms PdCo nanoparticles undergo at elevated temperatures an atomic structural transition from core-shell to a rare intermetallic ordered structure with twin boundaries forming stable {111}, {110} and {100} facets. The superior stability of this catalyst compared to platinum after 10,000 potential cycles in alkaline media is attributedmore » to the atomic structural order of PdCo nanoparticles along with protective effect of clusters of gold atoms on the surface. This strategy of making ordered palladium intermetallic alloy nanoparticles can be used in diverse heterogeneous catalysis where particle size and structural stability matters.« less

Boundaries of intergrowths between mineral individuals: A zone of secondary mineral formation in aggregates

NASA Astrophysics Data System (ADS)

Brodskaya, R. L.; Bil'Skaya, I. V.; Lyakhnitskaya, V. D.; Markovsky, B. A.; Sidorov, E. G.

2007-12-01

Intergrowth boundaries between mineral individuals in dunite of the Gal’moenan massif in Koryakia was studied in terms of crystal morphology, crystal optics, and ontogenesis. The results obtained allowed us to trace the staged formation of olivine and chromite and four generations of these minerals. Micro-and nanotopography of boundary surfaces between intergrown mineral individuals of different generations was examined with optic, electron, and atomic force microscopes. The boundaries between mineral individuals of different generations are distinguished by their microsculpture for both olivine and chromite grains. Both minerals demonstrate a compositional trend toward refinement from older to younger generations. The decrease in the iron mole fraction in olivine and chromite is accompanied by the crystallization of magnetite along weakened zones in olivine of the first generation and as outer rims around the chromite grains of the second generation observable under optic and electronic microscopes. The subsequent refinement of chromite results in the release of PGE from its lattice, as established by atomic power microscopy. The newly formed PGM are localized at the boundaries between mineral individuals and, thus, mark a special stage in the ontogenetic evolution of mineral aggregates. Further recrystallization is expressed in the spatial redistribution of grain boundaries and the formation of monomineralic intergrowth boundaries, i.e., the glomerogranular structure of rock and substructures of PGM, chromite, and olivine grains as intermediate types of organization of the granular assemblies in the form of reticulate, chain, and cellular structures and substructures of aggregates.

Evidence of van Hove singularities in ordered grain boundaries of graphene.

PubMed

Ma, Chuanxu; Sun, Haifeng; Zhao, Yeliang; Li, Bin; Li, Qunxiang; Zhao, Aidi; Wang, Xiaoping; Luo, Yi; Yang, Jinlong; Wang, Bing; Hou, J G

2014-06-06

It has long been under debate whether the electron transport performance of graphene could be enhanced by the possible occurrence of van Hove singularities in grain boundaries. Here, we provide direct experimental evidence to confirm the existence of van Hove singularity states close to the Fermi energy in certain ordered grain boundaries using scanning tunneling microscopy. The intrinsic atomic and electronic structures of two ordered grain boundaries, one with alternative pentagon and heptagon rings and the other with alternative pentagon pair and octagon rings, are determined. It is firmly verified that the carrier concentration and, thus, the conductance around ordered grain boundaries can be significantly enhanced by the van Hove singularity states. This finding strongly suggests that a graphene nanoribbon with a properly embedded ordered grain boundary can be a promising structure to improve the performance of graphene-based electronic devices.

Atomic resolution study of the interfacial bonding at Si3N4/CeO2-δ grain boundaries

NASA Astrophysics Data System (ADS)

Walkosz, W.; Klie, R. F.; Öǧüt, S.; Borisevich, A.; Becher, P. F.; Pennycook, S. J.; Idrobo, J. C.

2008-08-01

Using a combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope, we examine the atomic and electronic structures at the interface between Si3N4 (101¯0) and CeO2-d intergranular film (IGF). Ce atoms are observed to segregate to the interface in a two-layer periodic arrangement, which is significantly different from the structure observed in a previous study. Our EELS experiments show (i) oxygen in direct contact with the terminating Si3N4 open-ring structures, (ii) a change in the Ce valence from a nominal oxidation state of +3 to almost +4 moving from the interface into the IGF, and (iii) a uniform concentration of Si in the film.

The shear response of copper bicrystals with Σ11 symmetric and asymmetric tilt grain boundaries by molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Zhang, Liang; Lu, Cheng; Tieu, Kiet; Zhao, Xing; Pei, Linqing

2015-04-01

Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11<1 1 0> tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination along the boundary plane. A non-planar structure with dissociated intrinsic stacking faults was prevalent in Σ11 asymmetric GBs of Cu. This type of structure can significantly increase the ductility of bicrystal models under shear deformation. A grain boundary can be a source of dislocation and migrate itself at different stress levels. The intrinsic free volume involved in the grain boundary area was correlated with dislocation nucleation and GB sliding, while the dislocation nucleation mechanism can be different for a grain boundary due to its different equilibrium structures.Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11<1 1 0> tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination along the boundary plane. A non-planar structure with dissociated intrinsic stacking faults was prevalent in Σ11 asymmetric GBs of Cu. This type of structure can significantly increase the ductility of bicrystal models under shear deformation. A grain boundary can be a source of dislocation and migrate itself at different stress levels. The intrinsic free volume involved in the grain boundary area was correlated with dislocation nucleation and GB sliding, while the dislocation nucleation mechanism can be different for a grain boundary due to its different equilibrium structures. Electronic supplementary information (ESI) available: Movies show the evolution of different grain boundaries under shear deformation: S-0, S-54.74, S-70.53-A, S-70.53-B, S-90. See DOI: 10.1039/c4nr07496c

Defect sink characteristics of specific grain boundary types in 304 stainless steels under high dose neutron environments

DOE PAGES

Field, Kevin G.; Yang, Ying; Busby, Jeremy T.; ...

2015-03-09

Radiation induced segregation (RIS) is a well-studied phenomena which occurs in many structurally relevant nuclear materials including austenitic stainless steels. RIS occurs due to solute atoms preferentially coupling to mobile point defect fluxes that migrate and interact with defect sinks. Here, a 304 stainless steel was neutron irradiated up to 47.1 dpa at 320 °C. Investigations into the RIS response at specific grain boundary types were utilized to determine the sink characteristics of different boundary types as a function of irradiation dose. A rate theory model built on the foundation of the modified inverse Kirkendall (MIK) model is proposed andmore » benchmarked to the experimental results. This model, termed the GiMIK model, includes alterations in the boundary conditions based on grain boundary structure and includes expressions for interstitial binding. This investigation, through experiment and modeling, found specific grain boundary structures exhibit unique defect sink characteristics depending on their local structure. Furthermore, such interactions were found to be consistent across all doses investigated and had larger global implications including precipitation of Ni-Si clusters near different grain boundary types.« less

Impact of Wide-Ranging Nanoscale Chemistry on Band Structure at Cu(In, Ga)Se 2 Grain Boundaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stokes, Adam; Al-Jassim, Mowafak; Diercks, David

The relative chemistry from grain interiors to grain boundaries help explain why grain boundaries may be beneficial, detrimental or benign towards device performance. 3D Nanoscale chemical analysis extracted from atom probe tomography (APT) (10’s of parts-per-million chemical sensitivity and sub-nanometer spatial resolution) of twenty grain boundaries in a high-efficiency Cu(In, Ga)Se 2 solar cell shows the matrix and alkali concentrations are wide-ranging. The concentration profiles are then related to band structure which provide a unique insight into grain boundary electrical performance. Fluctuating Cu, In and Ga concentrations result in a wide distribution of potential barriers at the valence band maximummore » (VBM) (-10 to -160 meV) and the conduction band minimum (CBM) (-20 to -70 meV). Furthermore, Na and K segregation is not correlated to hampering donors, (In, Ga) Cu and V Se, contrary to what has been previously reported. In addition, Na and K are predicted to be n-type dopants at grain boundaries. An overall band structure at grain boundaries is presented.« less

Impact of Wide-Ranging Nanoscale Chemistry on Band Structure at Cu(In, Ga)Se 2 Grain Boundaries

DOE PAGES

Stokes, Adam; Al-Jassim, Mowafak; Diercks, David; ...

2017-10-26

The relative chemistry from grain interiors to grain boundaries help explain why grain boundaries may be beneficial, detrimental or benign towards device performance. 3D Nanoscale chemical analysis extracted from atom probe tomography (APT) (10’s of parts-per-million chemical sensitivity and sub-nanometer spatial resolution) of twenty grain boundaries in a high-efficiency Cu(In, Ga)Se 2 solar cell shows the matrix and alkali concentrations are wide-ranging. The concentration profiles are then related to band structure which provide a unique insight into grain boundary electrical performance. Fluctuating Cu, In and Ga concentrations result in a wide distribution of potential barriers at the valence band maximummore » (VBM) (-10 to -160 meV) and the conduction band minimum (CBM) (-20 to -70 meV). Furthermore, Na and K segregation is not correlated to hampering donors, (In, Ga) Cu and V Se, contrary to what has been previously reported. In addition, Na and K are predicted to be n-type dopants at grain boundaries. An overall band structure at grain boundaries is presented.« less

In Situ Atomic-Scale Observation of Electrochemical Delithiation Induced Structure Evolution of LiCoO2 Cathode in a Working All-Solid-State Battery.

PubMed

Gong, Yue; Zhang, Jienan; Jiang, Liwei; Shi, Jin-An; Zhang, Qinghua; Yang, Zhenzhong; Zou, Dongli; Wang, Jiangyong; Yu, Xiqian; Xiao, Ruijuan; Hu, Yong-Sheng; Gu, Lin; Li, Hong; Chen, Liquan

2017-03-29

We report a method for in situ atomic-scale observation of electrochemical delithiation in a working all-solid-state battery using a state-of-the-art chip based in situ transmission electron microscopy (TEM) holder and focused ion beam milling to prepare an all-solid-state lithium-ion battery sample. A battery consisting of LiCoO 2 cathode, LLZO solid state electrolyte and gold anode was constructed, delithiated and observed in an aberration corrected scanning transmission electron microscope at atomic scale. We found that the pristine single crystal LiCoO 2 became nanosized polycrystal connected by coherent twin boundaries and antiphase domain boundaries after high voltage delithiation. This is different from liquid electrolyte batteries, where a series of phase transitions take place at LiCoO 2 cathode during delithiation. Both grain boundaries become more energy favorable along with extraction of lithium ions through theoretical calculation. We also proposed a lithium migration pathway before and after polycrystallization. This new methodology could stimulate atomic scale in situ scanning/TEM studies of battery materials and provide important mechanistic insight for designing better all-solid-state battery.

Misoriented grain boundaries vicinal to the (111) <11¯0> twin in nickel part II: Thermodynamics of hydrogen segregation

DOE PAGES

O’Brien, Christopher J.; Foiles, Stephen M.

2016-04-19

Grain boundary engineered materials are of immense interest for their resistance to hydrogen embrittlement. This work builds on the work undertaken in Part I on the thermodynamic stability and structure of misoriented grain boundaries vicinal to the Σ3 (111) <11¯0> (coherent-twin) boundary to examine hydrogen segregation to those boundaries. The segregation of hydrogen reflects the asymmetry of the boundary structure with the sense of rotation of the grains about the coherent-twin boundary, and the temperature-dependent structural transition present in one sense of misorientation. This work also finds that the presence of hydrogen affects a change in structure of the boundariesmore » with increasing concentration. The structural change effects only one sense of misorientation and results in the reduction in length of the emitted stacking faults. Moreover, the structural change results in the generation of occupied sites populated by more strongly bound hydrogen. The improved understanding of misoriented twin grain boundary structure and the effect on hydrogen segregation resulting from this work is relevant to higher length-scale models. To that end, we examine commonly used metrics such as free volume and atomic stress at the boundary. In conclusion, free volume is found not to be useful as a surrogate for predicting the degree of hydrogen segregation, whereas the volumetric virial stress reliably predicts the locations of hydrogen segregation and exclusion at concentrations below saturation or the point where structural changes are induced by increasing hydrogen concentration.« less

Single-domain epitaxial silicene on diboride thin films

DOE PAGES

Fleurence, A.; Gill, T. G.; Friedlein, R.; ...

2016-04-12

Epitaxial silicene, which forms spontaneously on ZrB 2(0001) thin films grown on Si(111) wafers, has a periodic stripe domain structure. By adsorbing additional Si atoms on this surface, we find that the domain boundaries vanish, and a single-domain silicene sheet can be prepared without altering its buckled honeycomb structure. The amount of Si required to induce this change suggests that the domain boundaries are made of a local distortion of the silicene honeycomb lattice. LastlThe realization of a single domain sheet with structural and electronic properties close to those of the original striped state demonstrates the high structural flexibility ofmore » silicene.« less

Single-domain epitaxial silicene on diboride thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fleurence, A., E-mail: antoine@jaist.ac.jp; Friedlein, R.; Aoyagi, K.

2016-04-11

Epitaxial silicene, which forms spontaneously on ZrB{sub 2}(0001) thin films grown on Si(111) wafers, has a periodic stripe domain structure. By adsorbing additional Si atoms on this surface, we find that the domain boundaries vanish, and a single-domain silicene sheet can be prepared without altering its buckled honeycomb structure. The amount of Si required to induce this change suggests that the domain boundaries are made of a local distortion of the silicene honeycomb lattice. The realization of a single domain sheet with structural and electronic properties close to those of the original striped state demonstrates the high structural flexibility ofmore » silicene.« less

Grain boundary engineering to control the discontinuous precipitation in multicomponent U10Mo alloy

DOE PAGES

Devaraj, Arun; Kovarik, Libor; Kautz, Elizabeth; ...

2018-03-30

Here, we demonstrate here that locally stabilized structure and compositional segregation at grain boundaries in a complex multicomponent alloy can be modified using high temperature homogenization treatment to influence the kinetics of phase transformations initiating from grain boundaries during subsequent low temperature annealing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography of a model multicomponent metallic alloy —uranium-10 wt% molybdenum (U-10Mo) a nuclear fuel, that is highly relevant to worldwide nuclear non-proliferation efforts, we demonstrate the ability to change the structure and compositional segregation at grain boundary, which then controls the subsequent discontinuous precipitation kinetics during sub-eutectoid annealing.more » A change in grain boundary from one characterized by segregation of Mo and impurities at grain boundary to a phase boundary with a distinct U 2MoSi 2C wetting phase precipitates introducing Ni and Al rich interphase complexions caused a pronounced reduction in area fraction of subsequent discontinuous precipitation. The broader implication of this work is in highlighting the role of grain boundary structure and composition in metallic alloys on dictating the fate of grain boundary initiated phase transformations like discontinuous precipitation or cellular transformation. This work highlights a new pathway to tune the grain boundary structure and composition to tailor the final microstructure of multicomponent metallic alloys.« less

Grain boundary engineering to control the discontinuous precipitation in multicomponent U10Mo alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Devaraj, Arun; Kovarik, Libor; Kautz, Elizabeth

Here, we demonstrate here that locally stabilized structure and compositional segregation at grain boundaries in a complex multicomponent alloy can be modified using high temperature homogenization treatment to influence the kinetics of phase transformations initiating from grain boundaries during subsequent low temperature annealing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography of a model multicomponent metallic alloy —uranium-10 wt% molybdenum (U-10Mo) a nuclear fuel, that is highly relevant to worldwide nuclear non-proliferation efforts, we demonstrate the ability to change the structure and compositional segregation at grain boundary, which then controls the subsequent discontinuous precipitation kinetics during sub-eutectoid annealing.more » A change in grain boundary from one characterized by segregation of Mo and impurities at grain boundary to a phase boundary with a distinct U 2MoSi 2C wetting phase precipitates introducing Ni and Al rich interphase complexions caused a pronounced reduction in area fraction of subsequent discontinuous precipitation. The broader implication of this work is in highlighting the role of grain boundary structure and composition in metallic alloys on dictating the fate of grain boundary initiated phase transformations like discontinuous precipitation or cellular transformation. This work highlights a new pathway to tune the grain boundary structure and composition to tailor the final microstructure of multicomponent metallic alloys.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhai, Hua; Zhang, Jialin, E-mail: jialinzhang@hunnu.edu.cn; Yu, Hongwei, E-mail: hwyu@hunnu.edu.cn

We study the geometric phase of a uniformly accelerated two-level atom coupled with vacuum fluctuations of electromagnetic fields in the presence of a perfectly reflecting plane. We find that the geometric phase difference between the accelerated and inertial atoms which can be observed by atom interferometry crucially depends on the polarizability of the atom and the distance to the boundary and it can be dramatically manipulated with anisotropically polarizable atoms. In particular, extremely close to the boundary, the phase difference can be increased by two times as compared to the case without any boundary. So, the detectability of the effectsmore » associated with acceleration using an atom interferometer can be significantly increased by the presence of a boundary using atoms with anisotropic polarizability.« less

Molecular statics simulation of CdTe grain boundary structures and energetics using a bond-order potential

NASA Astrophysics Data System (ADS)

Stechmann, Guillaume; Zaefferer, Stefan; Raabe, Dierk

2018-06-01

The structure and energetics of coincidence site lattice grain boundaries (GB) in CdTe are investigated by mean of molecular statics simulations, using the Cd–Zn–Te bond-order potential (second iteration) developed by Ward et al (2012 Phys. Rev. B 86 245203; 2013 J. Mol. Modelling 19 5469–77). The effects of misorientation (Σ value) and interface plane are treated separately, complying with the critical need for full five-parameter characterization of GB. In addition, stoichiometric shifts, occurring between the inner interfaces and their adjacent atomic layers, are also predicted, revealing the energetic preference of Te-rich boundaries, opening opportunities for crystallography-based intrinsic interface doping. Our results also suggest that the intuitive assumption that Σ3 boundaries with low-indexed planes are more energetically favorable is often unfounded, except for coherent twins developing on {111} boundary planes. Therefore, Σ5, 7 or 9 boundaries, with lower interface energy than that of twin boundaries lying on different facets, are frequently encountered.

Research Update: Spatially resolved mapping of electronic structure on atomic level by multivariate statistical analysis

NASA Astrophysics Data System (ADS)

Belianinov, Alex; Ganesh, Panchapakesan; Lin, Wenzhi; Sales, Brian C.; Sefat, Athena S.; Jesse, Stephen; Pan, Minghu; Kalinin, Sergei V.

2014-12-01

Atomic level spatial variability of electronic structure in Fe-based superconductor FeTe0.55Se0.45 (Tc = 15 K) is explored using current-imaging tunneling-spectroscopy. Multivariate statistical analysis of the data differentiates regions of dissimilar electronic behavior that can be identified with the segregation of chalcogen atoms, as well as boundaries between terminations and near neighbor interactions. Subsequent clustering analysis allows identification of the spatial localization of these dissimilar regions. Similar statistical analysis of modeled calculated density of states of chemically inhomogeneous FeTe1-xSex structures further confirms that the two types of chalcogens, i.e., Te and Se, can be identified by their electronic signature and differentiated by their local chemical environment. This approach allows detailed chemical discrimination of the scanning tunneling microscopy data including separation of atomic identities, proximity, and local configuration effects and can be universally applicable to chemically and electronically inhomogeneous surfaces.

Design principles for high efficiency small-grain polysilicon solar cells, with supporting experimental studies

NASA Technical Reports Server (NTRS)

Lindholm, F. A.; Neugroschel, A.; Sah, C. T.

1982-01-01

Design principles suggested here aim toward high conversion efficiency (greater than 15 percent) in polysilicon cells. The principles seek to decrease the liabilities of both intragranular and grain-boundary-surface defects. The advantages of a phosphorus atom concentration gradient in a thin (less than 50 microns) base of a p(+)/n(x)/n(+) drift-field solar cell, which produces favorable gradients in chemical potential, minority-carrier mobility and diffusivity, and recombination lifetime (via phosphorus gettering) are suggested. The degrading effects of grain boundaries are reduced by these three gradients and by substituting atoms (P, H, F or Li) for vacancies on the grain-boundary surface. From recent experiments comes support for the benefits of P diffusion down grain boundaries and, for quasi-grain-boundary-free and related structures. New analytic solutions for the n(x)-base include the effect of a power-law dependence between P concentration and lifetime. These provide an upper-bound estimate on the open circuit voltage. Finite-difference numerical solutions of the six Shockley equations furnish complete information about all solar-cell parameters and add insight concerning design.

Ab initio prediction of stable nanotwin double layers and 4O structure in Ni2MnGa

NASA Astrophysics Data System (ADS)

Zelený, Martin; Straka, Ladislav; Sozinov, Alexei; Heczko, Oleg

2016-12-01

The ab initio electronic structure calculations of the Ni2MnGa alloy indicate that the orthorhombic 4O structure exhibits the lowest energy compared to all known martensitic structures. The 4O structure is formed by nanotwin double layers, i.e., oppositely oriented nanotwins consisting of two (101) lattice planes of nonmodulated martensitic structure. It exhibits the lowest occupation of density of states at the Fermi level. The total energy 1.98 meV/atom below the energy of nonmodulated martensite is achieved within structural relaxation by shifting Mn and Ga atoms at the nanotwin boundaries. The same atomic shift can also be found in other martensitic nanotwinned or modulated structures such as 10M and 14M, which indicates the importance of the nanotwin double layer for the stability of these structures. Our discovery shows that the nanotwinning or modulation is a natural property of low-temperature martensitic phases in Ni-Mn-Ga alloys.

Charge density wave order in 1D mirror twin boundaries of single-layer MoSe 2

DOE PAGES

Barja, Sara; Wickenburg, Sebastian; Liu, Zhen-Fei; ...

2016-04-18

Here, We provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries (MTBs) of single-layer semiconducting MoSe 2. Such MTBs have been previously observed by transmission electron microscopy and have been predicted to be metallic in MoSe 2 and MoS 2. Our low-temperature scanning tunnelling microscopy/spectroscopy measurements revealed a substantial bandgap of 100 meV opening at the Fermi energy in the otherwise metallic one-dimensional structures. We found a periodic modulation in the density of states along the MTB, with a wavelength of approximately three lattice constants. In addition to mapping the energy-dependent densitymore » of states, we determined the atomic structure and bonding of the MTB through simultaneous high-resolution non-contact atomic force microscopy. Density functional theory calculations based on the observed structure reproduced both the gap opening and the spatially resolved density of states.« less

Atomic-Scale Imaging of Surfaces and Interfaces. Materials Research Society Symposium Proceedings Held in Boston, Massachusetts on November 30-December 2, 1992. Volume 295

DTIC Science & Technology

1992-01-01

basic reference structure, changes to which can be studied as a function of doping and/or processing parameters . and correlated to electrical and...MICROSCOPY CHARACTERIZATION OF EPITAXIAL GROWTH OF Ag DEPOSITED ON MgO MICROCUBES 127 J. Liu, M. Pan, and GE. Spinnler REAL-TIME VIEWING OF DYNAMIC...IMAGING OF GRAIN BOUNDARIES IN Pr- DOPED ZnO CERAMICS 189 I.G. Solorzano, J.B. VanDer Sande, K.K. Baek, and H.L. Tuller ATOMIC STRUCTURES AND DEFECTS OF

Atomic structure of a metal-supported two-dimensional germania film

NASA Astrophysics Data System (ADS)

Lewandowski, Adrián Leandro; Schlexer, Philomena; Büchner, Christin; Davis, Earl M.; Burrall, Hannah; Burson, Kristen M.; Schneider, Wolf-Dieter; Heyde, Markus; Pacchioni, Gianfranco; Freund, Hans-Joachim

2018-03-01

The growth and microscopic characterization of two-dimensional germania films is presented. Germanium oxide monolayer films were grown on Ru(0001) by physical vapor deposition and subsequent annealing in oxygen. We obtain a comprehensive image of the germania film structure by combining intensity-voltage low-energy electron diffraction (I/V-LEED) and ab initio density functional theory (DFT) analysis with atomic-resolution scanning tunneling microscopy (STM) imaging. For benchmarking purposes, the bare Ru(0001) substrate and the (2 ×2 )3 O covered Ru(0001) were analyzed with I/V-LEED with respect to previous reports. STM topographic images of the germania film reveal a hexagonal network where the oxygen and germanium atom positions appear in different imaging contrasts. For quantitative LEED, the best agreement has been achieved with DFT structures where the germanium atoms are located preferentially on the top and fcc hollow sites of the Ru(0001) substrate. Moreover, in these atomically flat germania films, local site geometries, i.e., tetrahedral building blocks, ring structures, and domain boundaries, have been identified, indicating possible pathways towards two-dimensional amorphous networks.

Self-consistent average-atom scheme for electronic structure of hot and dense plasmas of mixture.

PubMed

Yuan, Jianmin

2002-10-01

An average-atom model is proposed to treat the electronic structures of hot and dense plasmas of mixture. It is assumed that the electron density consists of two parts. The first one is a uniform distribution with a constant value, which is equal to the electron density at the boundaries between the atoms. The second one is the total electron density minus the first constant distribution. The volume of each kind of atom is proportional to the sum of the charges of the second electron part and of the nucleus within each atomic sphere. By this way, one can make sure that electrical neutrality is satisfied within each atomic sphere. Because the integration of the electron charge within each atom needs the size of that atom in advance, the calculation is carried out in a usual self-consistent way. The occupation numbers of electron on the orbitals of each kind of atom are determined by the Fermi-Dirac distribution with the same chemical potential for all kinds of atoms. The wave functions and the orbital energies are calculated with the Dirac-Slater equations. As examples, the electronic structures of the mixture of Au and Cd, water (H2O), and CO2 at a few temperatures and densities are presented.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miao, Jianwei; Ercius, Peter; Billinge, S. J. L.

Crystallography has been fundamental to the development of many fields of science over the last century. However, much of our modern science and technology relies on materials with defects and disorders, and their three-dimensional (3D) atomic structures are not accessible to crystallography. One method capable of addressing this major challenge is atomic electron tomography. By combining advanced electron microscopes and detectors with powerful data analysis and tomographic reconstruction algorithms, it is now possible to determine the 3D atomic structure of crystal defects such as grain boundaries, stacking faults, dislocations, and point defects, as well as to precisely localize the 3Dmore » coordinates of individual atoms in materials without assuming crystallinity. In this work, we review the recent advances and the interdisciplinary science enabled by this methodology. We also outline further research needed for atomic electron tomography to address long-standing unresolved problems in the physical sciences.« less

Vacancy-Induced Formation and Growth of Inversion Domains in Transition-Metal Dichalcogenide Monolayer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, Junhao; Pantelides, Sokrates T.; Zhou, Wu

2015-04-23

Sixty degree grain boundaries in semiconducting transition-metal dichalcogenide (TMDC) monolayers have been shown to act as conductive channels that have profound influence on both the transport properties and exciton behavior of the monolayers. We show that annealing TMDC monolayers at high temperature induces the formation of large-scale inversion domains surrounded by such 60° grain boundaries. To study the formation mechanism of such inversion domains, we use the electron beam in a scanning transmission electron microscope to activate the dynamic process within pristine TMDC monolayers. Moreover, the electron beam acts to generate chalcogen vacancies in TMDC monolayers and provide energy formore » them to undergo structural evolution. We directly visualize the nucleation and growth of such inversion domains and their 60° grain boundaries atom-by-atom within a MoSe 2 monolayer and explore their formation mechanism. Combined with density functional theory, we conclude that the nucleation of the inversion domains and migration of their 60° grain boundaries are driven by the collective evolution of Se vacancies and subsequent displacement of Mo atoms, where such a dynamical process reduces the vacancy-induced lattice shrinkage and stabilizes the system. Our results can help to understand the performance of such materials under severe conditions (e.g., high temperature).« less

Impact of internal crystalline boundaries on lattice thermal conductivity: Importance of boundary structure and spacing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aghababaei, Ramin, E-mail: ramin.aghababaei@epfl.ch; Anciaux, Guillaume; Molinari, Jean-François

2014-11-10

The low thermal conductivity of nano-crystalline materials is commonly explained via diffusive scattering of phonons by internal boundaries. In this study, we have quantitatively studied phonon-crystalline boundaries scattering and its effect on the overall lattice thermal conductivity of crystalline bodies. Various types of crystalline boundaries such as stacking faults, twins, and grain boundaries have been considered in FCC crystalline structures. Accordingly, the specularity coefficient has been determined for different boundaries as the probability of the specular scattering across boundaries. Our results show that in the presence of internal boundaries, the lattice thermal conductivity can be characterized by two parameters: (1)more » boundary spacing and (2) boundary excess free volume. We show that the inverse of the lattice thermal conductivity depends linearly on a non-dimensional quantity which is the ratio of boundary excess free volume over boundary spacing. This shows that phonon scattering across crystalline boundaries is mainly a geometrically favorable process rather than an energetic one. Using the kinetic theory of phonon transport, we present a simple analytical model which can be used to evaluate the lattice thermal conductivity of nano-crystalline materials where the ratio can be considered as an average density of excess free volume. While this study is focused on FCC crystalline materials, where inter-atomic potentials and corresponding defect structures have been well studied in the past, the results would be quantitatively applicable for semiconductors in which heat transport is mainly due to phonon transport.« less

Exploration of the mechanisms of temperature-dependent grain boundary mobility: Search for the common origin of ultrafast grain boundary motion

DOE PAGES

O’Brien, C. J.; Foiles, S. M.

2016-04-19

The temperature dependence of grain boundary mobility is complex, varied, and rarely fits ideal Arrhenius behavior. This work presents a series of case studies of planar grain boundaries in a model FCC system that were previously demonstrated to exhibit a variety of temperature-dependent mobility behaviors. It is demonstrated that characterization of the mobility versus temperature plots is not sufficient to predict the atomic motion mechanism of the grain boundaries. Herein, the temperature-dependent motion and atomistic motion mechanisms of planar grain boundaries are driven by a synthetic, orientation-dependent, driving force. The systems studied include CSL boundaries with Σ values of 5,more » 7, and 15, including both symmetric and asymmetric boundaries. These boundaries represent a range of temperature-dependent trends including thermally activated, antithermal, and roughening behaviors. Examining the atomic-level motion mechanisms of the thermally activated boundaries reveals that each involves a complex shuffle, and at least one atom that changes the plane it resides on. The motion mechanism of the antithermal boundary is qualitatively different and involves an in-plane coordinated shuffle that rotates atoms about a fixed atom lying on a point in the coincident site lattice. Furthermore, this provides a mechanistic reason for the observed high mobility, even at low temperatures, which is due to the low activation energy needed for such motion. However, it will be demonstrated that this mechanism is not universal, or even common, to other boundaries exhibiting non-thermally activated motion. This work concludes that no single atomic motion mechanism is sufficient to explain the existence of non-thermally activated boundary motion.« less

Influence of initial seed distribution on the pattern formation of the phase field crystals

NASA Astrophysics Data System (ADS)

Starodumov, Ilya; Galenko, Peter; Kropotin, Nikolai; Alexandrov, Dmitri V.

2017-11-01

The process of crystal growth can be expressed as a transition of atomic structure to a finally stable state or to a metastable state. In the Phase Field Crystal Model (PFC-model) these states are described by regular distributions of the atomic density. Getting the system into any metastable condition may be caused by the peculiarities of the computational domain, initial and boundary conditions. However, an important factor in the formation of the crystal structure can be the initial disturbance. In the report we show how different types of initial disturbance can change the finally stable state of crystal structure in equilibrium.

Electric field imaging of single atoms

PubMed Central

Shibata, Naoya; Seki, Takehito; Sánchez-Santolino, Gabriel; Findlay, Scott D.; Kohno, Yuji; Matsumoto, Takao; Ishikawa, Ryo; Ikuhara, Yuichi

2017-01-01

In scanning transmission electron microscopy (STEM), single atoms can be imaged by detecting electrons scattered through high angles using post-specimen, annular-type detectors. Recently, it has been shown that the atomic-scale electric field of both the positive atomic nuclei and the surrounding negative electrons within crystalline materials can be probed by atomic-resolution differential phase contrast STEM. Here we demonstrate the real-space imaging of the (projected) atomic electric field distribution inside single Au atoms, using sub-Å spatial resolution STEM combined with a high-speed segmented detector. We directly visualize that the electric field distribution (blurred by the sub-Å size electron probe) drastically changes within the single Au atom in a shape that relates to the spatial variation of total charge density within the atom. Atomic-resolution electric field mapping with single-atom sensitivity enables us to examine their detailed internal and boundary structures. PMID:28555629

Point Defects and Grain Boundaries in Rotationally Commensurate MoS 2 on Epitaxial Graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Xiaolong; Balla, Itamar; Bergeron, Hadallia

2016-03-28

With reduced degrees of freedom, structural defects are expected to play a greater role in two-dimensional materials in comparison to their bulk counterparts. In particular, mechanical strength, electronic properties, and chemical reactivity are strongly affected by crystal imperfections in the atomically thin limit. Here, ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) are employed to interrogate point and line defects in monolayer MoS2 grown on epitaxial graphene (EG) at the atomic scale. Five types of point defects are observed with the majority species showing apparent structures that are consistent with vacancy and interstitial models. The total defect densitymore » is observed to be lower than MoS2 grown on other substrates and is likely attributed to the van der Waals epitaxy of MoS2 on EG. Grain boundaries (GBs) with 30° and 60° tilt angles resulting from the rotational commensurability of MoS2 on EG are more easily resolved by STM than atomic force microscopy at similar scales due to the enhanced contrast from their distinct electronic states. For example, band gap reduction to ~0.8 and ~0.5 eV is observed with STS for 30° and 60° GBs, respectively. In addition, atomic resolution STM images of these GBs are found to agree well with proposed structure models. This work offers quantitative insight into the structure and properties of common defects in MoS2 and suggests pathways for tailoring the performance of MoS2/graphene heterostructures via defect engineering.« less

Determination of gaseous fission product behavior near the cerium dioxide Σ 3 (111)/[1 1 ¯ 0] tilt grain boundary via first-principles study

DOE PAGES

Xi, Jianqi; Liu, Bin; Xu, Haixuan; ...

2017-12-02

We presenmore » t that grain boundaries (GBs) are the most abundant structural defects in nanostructured nuclear fuels and play an important role in determining fission product behavior, which further affects the performance of nuclear fuels. In this work, cerium dioxide (CeO 2) is used as a surrogate material for mixed oxide fuels to understand gaseous fission product behavior, specifically Xe. First-principles calculations are employed to comprehensively study the behavior of Xe and trap sites for Xe near the Σ 3 (111)/[1 1 ¯ 0] grain boundary in CeO 2, which will provide guidance on overall trends for Xe stability and diffusion at grain boundaries vs in the bulk. Significant segregation behavior of trap sites, regardless of charge states, is observed near the GB. This is mainly ascribed to the local atomic structure near the GB, which results in weaker bond strength and more negative segregation energies. For Xe, however, the segregation profile near the GB is different. Our calculations show that, as the size of trap sites increases, the segregation propensity of Xe is reduced. In addition, under hyper-stoichiometric conditions, the solubility of Xe trapped at the GB is significantly higher than that in the bulk, suggesting higher Xe concentration than that in the bulk. The results of this work demonstrate that the diffusion mechanism of Xe in CeO 2 is comparable to that in UO 2. The diffusion activation energies of Xe atoms in the Σ3GB are lower than that in the bulk CeO 2. Lastly, these results suggest that the diffusivity of Xe atoms is higher along the GB than that in the bulk, which enhances the aggregation of Xe atoms near the GB.« less

Determination of gaseous fission product behavior near the cerium dioxide Σ 3 (111)/[1 1 ¯ 0] tilt grain boundary via first-principles study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xi, Jianqi; Liu, Bin; Xu, Haixuan

We presenmore » t that grain boundaries (GBs) are the most abundant structural defects in nanostructured nuclear fuels and play an important role in determining fission product behavior, which further affects the performance of nuclear fuels. In this work, cerium dioxide (CeO 2) is used as a surrogate material for mixed oxide fuels to understand gaseous fission product behavior, specifically Xe. First-principles calculations are employed to comprehensively study the behavior of Xe and trap sites for Xe near the Σ 3 (111)/[1 1 ¯ 0] grain boundary in CeO 2, which will provide guidance on overall trends for Xe stability and diffusion at grain boundaries vs in the bulk. Significant segregation behavior of trap sites, regardless of charge states, is observed near the GB. This is mainly ascribed to the local atomic structure near the GB, which results in weaker bond strength and more negative segregation energies. For Xe, however, the segregation profile near the GB is different. Our calculations show that, as the size of trap sites increases, the segregation propensity of Xe is reduced. In addition, under hyper-stoichiometric conditions, the solubility of Xe trapped at the GB is significantly higher than that in the bulk, suggesting higher Xe concentration than that in the bulk. The results of this work demonstrate that the diffusion mechanism of Xe in CeO 2 is comparable to that in UO 2. The diffusion activation energies of Xe atoms in the Σ3GB are lower than that in the bulk CeO 2. Lastly, these results suggest that the diffusivity of Xe atoms is higher along the GB than that in the bulk, which enhances the aggregation of Xe atoms near the GB.« less

Phase-field model with plastic flow for grain growth in nanocrystalline material

NASA Astrophysics Data System (ADS)

Steinbach, Ingo; Song, Xiaoyan; Hartmaier, Alexander

2010-01-01

A phase-field model is presented which considers the accumulation of structural defects in grain boundaries by an isotropic eigenstrain associated with the grain boundaries. It is demonstrated that the elastic energy caused by dilatation of the grain boundary with respect to the bulk crystal contributes largely to the grain boundary energy. The sign of this contribution can be both positive and negative dependent on the local stress state in the grain boundary. Self-diffusion of atoms is taken into account to relax the stress caused by the dilatation of the grain boundary. Application of the model to discontinuous grain growth in pure nanocrystalline cobalt material is presented. Linear grain growth is found in the nanocrystalline state, which is explained by the interpretation of grain boundary motion as a diffusive process defining an upper limit of the grain boundary velocity independent of the grain boundary curvature but dependent on temperature. The transition to regular grain growth at a critical temperature, as observed experimentally, is explained by the drop of theoretical grain boundary velocity due to its mean curvature during coarsening of the nanograin structure below the maximum velocity.

Mechanical properties and crack growth behavior of polycrystalline copper using molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Qiu, Ren-Zheng; Li, Chi-Chen; Fang, Te-Hua

2017-08-01

This study investigated the mechanical properties and crack propagation behavior of polycrystalline copper using a molecular dynamics simulation. The effects of temperature, grain size, and crack length were evaluated in terms of atomic trajectories, slip vectors, common neighbor analysis, the material’s stress-strain diagram and Young’s modulus. The simulation results show that the grain boundary of the material is more easily damaged at high temperatures and that grain boundaries will combine at the crack tip. From the stress-strain diagram, it was observed that the maximum stress increased as the temperature decreased. In contrast, the maximum stress was reduced by increasing the temperature. With regard to the effect of the grain size, when the grain size was too small, the structure of the sample deformed due to the effect of atomic interactions, which caused the grain boundary structure to be disordered in general. However, when the grain size was larger, dislocations appeared and began to move from the tip of the crack, which led to a new dislocation phenomenon. With regards to the effect of the crack length, the tip of the crack did not affect the sample’s material when the crack length was less than 5 nm. However, when the crack length was above 7.5 nm, the grain boundary was damaged, and twinning structures and dislocations appeared on both sides of the crack tip. This is because the tip of the crack was blunt at first before sharpening due to the dislocation effect.

Concurrent atomistic and continuum simulation of bi-crystal strontium titanate with tilt grain boundary

PubMed Central

Yang, Shengfeng; Chen, Youping

2015-01-01

In this paper, we present the development of a concurrent atomistic–continuum (CAC) methodology for simulation of the grain boundary (GB) structures and their interaction with other defects in ionic materials. Simulation results show that the CAC simulation allows a smooth passage of cracks through the atomistic–continuum interface without the need for additional constitutive rules or special numerical treatment; both the atomic-scale structures and the energies of the four different [001] tilt GBs in bi-crystal strontium titanate obtained by CAC compare well with those obtained by existing experiments and density function theory calculations. Although 98.4% of the degrees of freedom of the simulated atomistic system have been eliminated in a coarsely meshed finite-element region, the CAC results, including the stress–strain responses, the GB–crack interaction mechanisms and the effect of the interaction on the fracture strength, are comparable with that of all-atom molecular dynamics simulation results. In addition, CAC simulation results show that the GB–crack interaction has a significant effect on the fracture behaviour of bi-crystal strontium titanate; not only the misorientation angle but also the atomic-level details of the GB structure influence the effect of the GB on impeding crack propagation. PMID:25792957

Concurrent atomistic and continuum simulation of bi-crystal strontium titanate with tilt grain boundary.

PubMed

Yang, Shengfeng; Chen, Youping

2015-03-08

In this paper, we present the development of a concurrent atomistic-continuum (CAC) methodology for simulation of the grain boundary (GB) structures and their interaction with other defects in ionic materials. Simulation results show that the CAC simulation allows a smooth passage of cracks through the atomistic-continuum interface without the need for additional constitutive rules or special numerical treatment; both the atomic-scale structures and the energies of the four different [001] tilt GBs in bi-crystal strontium titanate obtained by CAC compare well with those obtained by existing experiments and density function theory calculations. Although 98.4% of the degrees of freedom of the simulated atomistic system have been eliminated in a coarsely meshed finite-element region, the CAC results, including the stress-strain responses, the GB-crack interaction mechanisms and the effect of the interaction on the fracture strength, are comparable with that of all-atom molecular dynamics simulation results. In addition, CAC simulation results show that the GB-crack interaction has a significant effect on the fracture behaviour of bi-crystal strontium titanate; not only the misorientation angle but also the atomic-level details of the GB structure influence the effect of the GB on impeding crack propagation.

VoroTop: Voronoi cell topology visualization and analysis toolkit

NASA Astrophysics Data System (ADS)

Lazar, Emanuel A.

2018-01-01

This paper introduces a new open-source software program called VoroTop, which uses Voronoi topology to analyze local structure in atomic systems. Strengths of this approach include its abilities to analyze high-temperature systems and to characterize complex structure such as grain boundaries. This approach enables the automated analysis of systems and mechanisms previously not possible.

Equilibrium composition of interphase boundaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wynblatt, P.

1990-01-01

Two modeling approaches have been used to investigate segregation effects at interphase boundaries. The first approach is based on the nearest neighbor bond model, used in conjunction with the regular solution approximation, and is an extension of an earlier framework developed to address segregation phenomena at free surfaces. In order to model a semicoherent interphase boundary, we have employed a second modeling approach, based on Monte Carol simulation, in conjunction with the embedded atom method (EAM). The EAM is a powerful new method for describing interatomic interactions in metallic systems. It includes certain many-body interactions that depend on the localmore » environment of an atom. The Monte Carol approach has been applied to semicoherent interphase boundaries in Cu-Ag-Au alloys dilute in Au. These alloys consist of coexisting Cu-rich and Ag-rich phases, which differ in lattice constant by about 12%, such that good matching across in interface occurs when nine structural units of the Cu-rich phase are opposed to eight structural units of the Ag-rich phase. Thus far, interfaces with two different orientations have been studied: {l brace}001{r brace}-Cu//{l brace}001{r brace}-Ag, {l angle}110{r angle}-Cu//{l angle}110{r angle}-Ag; and {l brace}111{r brace}-Cu//{l brace}111{r brace}-Ag, {l angle}110{r angle}-Cu//{l angle}110{r angle}-Ag. These two interfaces will be referred to as the (001) and (111) interphase boundaries, for short. 18 refs.« less

Multi-scale characterization by FIB-SEM/TEM/3DAP.

PubMed

Ohkubo, T; Sepehri-Amin, H; Sasaki, T T; Hono, K

2014-11-01

In order to improve properties of functional materials, it is important to understand the relation between the structure and the properties since the structure has large effect to the properties. This can be done by using multi-scale microstructure analysis from macro-scale to nano and atomic scale. Scanning electron microscope (SEM) equipped with focused ion beam (FIB), transmission electron microscope (TEM) and 3D atom probe (3DAP) are complementary analysis tools making it possible to know the structure and the chemistry from micron to atomic resolution. SEM gives us overall microstructural and chemical information by various kinds of detectors such as secondary electron, backscattered electron, EDS and EBSD detectors. Also, it is possible to analyze 3D structure and chemistry via FIB serial sectioning. In addition, using TEM we can focus on desired region to get more complementary information from HRTEM/STEM/Lorentz images, SAED/NBD patterns and EDS/EELS to see the detail micro or nano-structure and chemistry. Especially, combination of probe Cs corrector and split EDS detectors with large detector size enable us to analyze the atomic scale elemental distribution. Furthermore, if the specimen has a complicated 3D nanostructure, or we need to analyze light elements such as hydrogen, lithium or boron, 3DAP can be used as the only technique which can visualize and analyze distribution of all constituent atoms of our materials within a few hundreds nm area. Hence, site-specific sample preparation using FIB/SEM is necessary to get desired information from region of interest. Therefore, this complementary analysis combination works very well to understand the detail of materials.In this presentation, we will show the analysis results obtained from some of functional materials by Carl Zeiss CrossBeam 1540EsB FIB/SEM, FEI Tecnai G(2) F30, Titan G2 80-200 TEMs and locally build laser assisted 3DAP. As the one of the example, result of multi-scale characterization for ultra-fine grain Nd-Fe-B permanent magnet will be shown [1]. In order to improve the magnetic properties, especially to increase the coercivity (resistance against magnetization reversal) of the magnet, decreasing the grain size and isolating each grain by non-ferromagnetic grain boundary phase are quite important since the nucleation of magnetic reversal from grain boundary phase can be suppressed and pinning force of magnetic domain wall at the grain boundary phase can be strengthened. Therefore, micro and nano structure and chemistry analysis can shed a light do grain boundary engineering.Figure 1(a,b) shows SEM BSE images of ultrafine grain Nd-Fe-B sintered magnet and the reconstructed 3D tomography of Nd-rich phases obtained by FIB/SEM serial sectioning. This data can provide us information about the distribution of Nd-rich phase and its volume fraction. Moreover, the HRTEM image from the grain boundary phase, the 3DAP maps and the concentration depth profiles are shown in Fig. 1(c,d,e). This magnet shows high coercivity (1517kA/m), and by comparing these results with the microstructures of low coercivity specimen, importance of grain boundary formation was confirmed and it gives us hint to improve the coercivity further. We will show the detail and results from other materials.jmicro;63/suppl_1/i6/DFU046F1F1DFU046F1Fig. 1.(a) SEM BSE images of ultrafine grain Nd-Fe-B sintered magnet. (b) 3D FIB/SEM tomography of Nd-rich phases. (c) HRTEM image from the grain boundary phase. (d) 3DAP maps of Nd, Cu and Al. (e) Concentration depth profiles for Fe, Nd+Pr, B, Co, Cu and Al, determined from the selected box in (d)[1]. © The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

EBIC/TEM investigations of defects in solar silicon ribbon materials

NASA Technical Reports Server (NTRS)

Ast, D. G.

1981-01-01

Transmission electron microscopy was used to investigate the defect structure of edge defined film growth (EFG) material, web dentritic ribbons (WEB), and ribbon to ribbon recrystallized material (RTR). The most common defects in all these materials are coherent first order twin boundaries. These coherent twins can be very thin, a few atomic layers. Bundles of the twins which contain odd numbers of twins will in optical images appear as a seemingly single first twin boundary. First-order coherent twin boundaries are not electrically active, except at locations where they contain intrinsic (grain boundary) dislocations. These dislocations take up small deviations from the ideal twin relation and play the same role in twin boundaries as conventional and play the some role in twin boundaries as conventional edge and screw dislocations in small angle tilt and twist boundaries.

Atom Probe Tomography of Phase and Grain Boundaries in Experimentally-Deformed and Hot-Pressed Wehrlite

NASA Astrophysics Data System (ADS)

Cukjati, J.; Parman, S. W.; Cooper, R. F.; Zhao, N.

2017-12-01

Atom probe tomography (APT) was used to characterize the chemistry of three grain boundaries: an olivine-olivine (ol-ol) and olivine-clinopyroxene (ol-cpx) boundary in fine-grained experimentally-deformed wehrlite and an ol-cpx boundary in a fine-grained, hot-pressed wehrlite. Grain boundaries were extracted and formed into APT tips using a focused ion beam (FIB). The tips were analyzed in a reflectron-equipped LEAP4000HR (Harvard University) at 1% or 0.5% detection rate, 5pJ laser energy and 100kHz pulse rate. Total ion counts are between 40 and 100 million per tip. Examination of grain and phase boundaries in wehrlite are of interest since slow-diffusing and olivine-incompatible cations present in cpx (e.g. Ca and Al) may control diffusion-accommodated grain boundary sliding and affect mantle rheology (Sundberg & Cooper, 2008). At steady state, ol-cpx aggregates are weaker than either ol or cpx end member, the results of which are not currently well-explained. We investigate grain boundary widths to understand the transport of olivine-incompatible elements. Widths of grain/phase boundary chemical segregation are between 3nm and 6nm for deformed ol-ol and ol-cpx samples; minimally-deformed (hot-pressed) samples having slightly wider chemical segregation widths. Chemical segregation widths were determined from profiles of Na, Al, P, Cl, K, Ca, or Ni, although not all listed elements can be used for all samples (e.g. Na, K segregation profiles can only be observed for ol-ol sample). These estimates are consistent with prior estimates of grain boundary segregation by atom probe tomography on ol-ol and opx-opx samples (Bachhav et al., 2015) and are less than ol-ol interface widths analyzed by STEM/EDX (Hiraga, Anderson, & Kohlstedt, 2007). STEM/EDX will be performed on deformed wehrlite to investigate chemical profile as a function of applied stress orientation and at length scales between those observable by APT and EPMA. Determination of phase boundary chemistry and structure allows for better modeling of the rheology of multiphase aggregates and better understanding of diffusive transport and storage of incompatible elements along grain boundaries.

Atomic level characterization in corrosion studies

NASA Astrophysics Data System (ADS)

Marcus, Philippe; Maurice, Vincent

2017-06-01

Atomic level characterization brings fundamental insight into the mechanisms of self-protection against corrosion of metals and alloys by oxide passive films and into how localized corrosion is initiated on passivated metal surfaces. This is illustrated in this overview with selected data obtained at the subnanometre, i.e. atomic or molecular, scale and also at the nanometre scale on single-crystal copper, nickel, chromium and stainless steel surfaces passivated in well-controlled conditions and analysed in situ and/or ex situ by scanning tunnelling microscopy/spectroscopy and atomic force microscopy. A selected example of corrosion modelling by ab initio density functional theory is also presented. The discussed aspects include the surface reconstruction induced by hydroxide adsorption and formation of two-dimensional (hydr)oxide precursors, the atomic structure, orientation and surface hydroxylation of three-dimensional ultrathin oxide passive films, the effect of grain boundaries in polycrystalline passive films acting as preferential sites of passivity breakdown, the differences in local electronic properties measured at grain boundaries of passive films and the role of step edges at the exposed surface of oxide grains on the dissolution of the passive film. This article is part of the themed issue 'The challenges of hydrogen and metals'.

Research Update: Spatially resolved mapping of electronic structure on atomic level by multivariate statistical analysis

DOE PAGES

Belianinov, Alex; Panchapakesan, G.; Lin, Wenzhi; ...

2014-12-02

Atomic level spatial variability of electronic structure in Fe-based superconductor FeTe0.55Se0.45 (Tc = 15 K) is explored using current-imaging tunneling-spectroscopy. Multivariate statistical analysis of the data differentiates regions of dissimilar electronic behavior that can be identified with the segregation of chalcogen atoms, as well as boundaries between terminations and near neighbor interactions. Subsequent clustering analysis allows identification of the spatial localization of these dissimilar regions. Similar statistical analysis of modeled calculated density of states of chemically inhomogeneous FeTe1 x Sex structures further confirms that the two types of chalcogens, i.e., Te and Se, can be identified by their electronic signaturemore » and differentiated by their local chemical environment. This approach allows detailed chemical discrimination of the scanning tunneling microscopy data including separation of atomic identities, proximity, and local configuration effects and can be universally applicable to chemically and electronically inhomogeneous surfaces.« less

Research Update: Spatially resolved mapping of electronic structure on atomic level by multivariate statistical analysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Belianinov, Alex, E-mail: belianinova@ornl.gov; Ganesh, Panchapakesan; Lin, Wenzhi

2014-12-01

Atomic level spatial variability of electronic structure in Fe-based superconductor FeTe{sub 0.55}Se{sub 0.45} (T{sub c} = 15 K) is explored using current-imaging tunneling-spectroscopy. Multivariate statistical analysis of the data differentiates regions of dissimilar electronic behavior that can be identified with the segregation of chalcogen atoms, as well as boundaries between terminations and near neighbor interactions. Subsequent clustering analysis allows identification of the spatial localization of these dissimilar regions. Similar statistical analysis of modeled calculated density of states of chemically inhomogeneous FeTe{sub 1−x}Se{sub x} structures further confirms that the two types of chalcogens, i.e., Te and Se, can be identified bymore » their electronic signature and differentiated by their local chemical environment. This approach allows detailed chemical discrimination of the scanning tunneling microscopy data including separation of atomic identities, proximity, and local configuration effects and can be universally applicable to chemically and electronically inhomogeneous surfaces.« less

Effects of solutes on dislocation nucleation from grain boundaries

DOE PAGES

Borovikov, Valery; Mendelev, Mikhail I.; King, Alexander H.

2016-12-27

When grain sizes are reduced to the nanoscale, grain boundaries (GB) become the dominant sources of the dislocations that enable plastic deformation. Here, we present the first molecular dynamics (MD) study of the effect of substitutional solutes on the dislocation nucleation process from GBs during uniaxial tensile deformation. A simple bi-crystal geometry is utilized in which the nucleation and propagation of dislocations away from a GB is the only active mechanism of plastic deformation. Solutes with atomic radii both larger and smaller than the solvent atomic radius were considered. Although the segregation sites are different for the two cases, bothmore » produce increases in the stress required to nucleate a dislocation. MD simulations at room temperature revealed that this increase in the nucleation stress is associated with changes of the GB structure at the emission site caused by dislocation emission, leading to increases in the heats of segregation of the solute atoms, which cannot diffuse to lower-energy sites on the timescale of the nucleation event. These results contribute directly to understanding the strength of nanocrystalline materials, and suggest suitable directions for nanocrystalline alloy design leading toward structural applications.« less

New atom probe approaches to studying segregation in nanocrystalline materials.

PubMed

Samudrala, S K; Felfer, P J; Araullo-Peters, V J; Cao, Y; Liao, X Z; Cairney, J M

2013-09-01

Atom probe is a technique that is highly suited to the study of nanocrystalline materials. It can provide accurate atomic-scale information about the composition of grain boundaries in three dimensions. In this paper we have analysed the microstructure of a nanocrystalline super-duplex stainless steel prepared by high pressure torsion (HPT). Not all of the grain boundaries in this alloy display obvious segregation, making visualisation of the microstructure challenging. In addition, the grain boundaries present in the atom probe data acquired from this alloy have complex shapes that are curved at the scale of the dataset and the interfacial excess varies considerably over the boundaries, making the accurate characterisation of the distribution of solute challenging using existing analysis techniques. In this paper we present two new data treatment methods that allow the visualisation of boundaries with little or no segregation, the delineation of boundaries for further analysis and the quantitative analysis of Gibbsian interfacial excess at boundaries, including the capability of excess mapping. Copyright © 2013 Elsevier B.V. All rights reserved.

Investigation of twin-twin interaction in deformed magnesium alloy

NASA Astrophysics Data System (ADS)

Sun, Qi; Ostapovets, Andriy; Zhang, Xiyan; Tan, Li; Liu, Qing

2018-03-01

Using transmission electron microscopy, we characterised the structures of the boundary caused by the interactions between different ? twin variants that share the same ? zone axis in a deformed magnesium alloy. We found that the twin-twin boundaries can adopt the habit planes that are parallel to the (0 0 0 2) basal plane or the ? prismatic plane or the ? twinning plane of the interacting twins. To investigate the formation mechanism of various twin-twin boundaries, we also performed atomic simulations. The results indicate that the formation of a twin-twin boundary may be related to the reaction of twinning disconnections that glide on the basal-prismatic planes of the interacting twins.

Atomic Resolution Study of the Interfacial Bonding at Si3N4/CeO2-δ Grain Boundaries

NASA Astrophysics Data System (ADS)

Klie, Robert F.; Walkosz, Weronika; Ogut, Serdar; Borisevich, A.; Becher, Paul F.; Pennycook, Steve J.; Idrobo, Juan C.

2008-03-01

Using a combination of atomic resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope, we examine the atomic and electronic structures at the interface between Si3N4 (10 10) and CeO2-δ inter-granular film (IGF). Ce atoms are observed to segregate to the interface in a two-layer periodic arrangement, which is significantly different compared to the structure observed in a previous study. Our EELS experiments show that (i) oxygen is present at the interface in direct contact with the terminating Si3N4 open-ring structures, (ii) the Ce valence state changes from +3 to +4 in going from the interface into the IGF, and (iii) while the N concentration decreases away from the Si3N4 grains into the IGF, the Si concentration remains uniform across the whole width of the IGF. Possible reasons for these observed structural and electronic variations at the interface and their implications for future studies on Si3N4/rare-earth oxide interfaces are briefly discussed.

Structural stability and energetics of grain boundary triple junctions in face centered cubic materials

NASA Astrophysics Data System (ADS)

Adlakha, I.; Solanki, K. N.

2015-03-01

We present a systematic study to elucidate the role of triple junctions (TJs) and their constituent grain boundaries on the structural stability of nanocrystalline materials. Using atomistic simulations along with the nudge elastic band calculations, we explored the atomic structural and thermodynamic properties of TJs in three different fcc materials. We found that the magnitude of excess energy at a TJ was directly related to the atomic density of the metal. Further, the vacancy binding and migration energetics in the vicinity of the TJ were examined as they play a crucial role in the structural stability of NC materials. The resolved line tension which takes into account the stress buildup at the TJ was found to be a good measure in predicting the vacancy binding tendency near the TJ. The activation energy for vacancy migration along the TJ was directly correlated with the measured excess energy. Finally, we show that the resistance for vacancy diffusion increased for TJs with larger excess stored energy and the defect mobility at some TJs is slower than their constituent GBs. Hence, our results have general implications on the diffusional process in NC materials and provide new insight into stabilizing NC materials with tailored TJs.

Ab initio and Molecular Dynamic models of displacement damage in crystalline and turbostratic graphite

NASA Astrophysics Data System (ADS)

McKenna, Alice

One of the functions of graphite is as a moderator in several nuclear reactor designs, including the Advanced Gas-cooled Reactor (AGR). In the reactor graphite is used to thermalise the neutrons produced in the fission reaction thus allowing a self-sustained reaction to occur. The graphite blocks, acting as the moderator, are constantly irradiated and consequently suffer damage. This thesis examines the types of damage caused using molecular dynamic (MD) simulations and ab intio calculations. Neutron damage starts with a primary knock-on atom (PKA), which is travelling so fast that it creates damage through electronic and thermal excitation (this is addressed with thermal spike simulations). When the PKA has lost energy the subsequent cascade is based on ballistic atomic displacement. These two types of simulations were performed on single crystal graphite and other carbon structures such as diamond and amorphous carbon as a comparison. The thermal spike in single crystal graphite produced results which varied from no defects to a small number of permanent defects in the structure. It is only at the high energy range that more damage is seen but these energies are less likely to occur in the nuclear reactor. The thermal spike does not create damage but it is possible that it can heal damaged sections of the graphite, which can be demonstrated with the motion of the defects when a thermal spike is applied. The cascade simulations create more damage than the thermal spike even though less energy is applied to the system. A new damage function is found with a threshold region that varies with the square root of energy in excess of the energy threshold. This is further broken down in to contributions from primary and subsequent knock-on atoms. The threshold displacement energy (TDE) is found to be Ed=25eV at 300K. In both these types of simulation graphite acts very differently to the other carbon structures. There are two types of polycrystalline graphite structures which simulations have been performed on. The difference between the two is at the grain boundaries with one having dangling bonds and the other one being bonded. The cascade showed the grain boundaries acting as a trap for the knock-on atoms which produces more damage compared with the single crystal. Finally the effects of turbostratic disorder on damage is considered. Density functional theory (DFT) was used to look at interstitials in (002) twist boundaries and how they act compared to AB stacked graphite. The results of these calculations show that the spiro interstitial is more stable in these grain boundaries, so at temperatures where the interstitial can migrate along the c direction they will segregate to (002) twist boundaries.

Structural Evolution of Supercritical CO2 across the Frenkel Line.

PubMed

Bolmatov, Dima; Zav'yalov, D; Gao, M; Zhernenkov, Mikhail

2014-08-21

Here, we study structural properties of the supercritical carbon dioxide and discover the existence of persistent medium-range order correlations, which make supercritical carbon dioxide nonuniform and heterogeneous on an intermediate length scale. We report on the CO2 heterogeneity shell structure where, in the first shell, both carbon and oxygen atoms experience gas-like-type interactions with short-range order correlations while within the second shell, oxygen atoms essentially exhibit a liquid-like type of interactions due to localization of transverse-like phonon packets. Importantly, we highlight a catalytic role of atoms inside of the nearest-neighbor heterogeneity shell in providing a mechanism for diffusion and proving the existence of an additional thermodynamic boundary in the supercritical carbon dioxide on an intermediate length scale. Finally, we discuss important implications for answering the intriguing question whether Venus may have had CO2 oceans and urge for an experimental detection of this persistent local-order heterogeneity.

Continuous modeling of a grain boundary in MgO and its disclination induced grain-boundary migration mechanism

NASA Astrophysics Data System (ADS)

Cordier, P.; Sun, X.; Taupin, V.; Fressengeas, C.

2016-12-01

Grain boundaries (GBs) are thin material layers where the lattice rotates from one orientation to the next one within a few nanometers. Because they treat these layers as infinitely thin interfaces, large-scale polycrystalline representations fail to describe their structure. Conversely, atomistic representations provide a detailed description of the GBs, but their character remains discrete and not prone to coarse-graining procedures. Continuum descriptions based on kinematic and crystal defect fields defined at interatomic scale are appealing because they can provide smooth and thorough descriptions of GBs, recovering in some sense the atomistic description and potentially serving as a basis for coarse-grained polycrystalline representations. In this work, a crossover between atomistic description and continuous representation of a MgO tilt boundary in polycrystals is set-up to model the periodic arrays of structural units by using dislocation and disclination dipole arrays along GBs. The strain, rotation, curvature, disclination and dislocation density fields are determined in the boundary area by using the discrete atomic positions generated by molecular dynamics simulations. Then, this continuous disclination/dislocation model is used as part of the initial conditions in elasto-plastic continuum mechanics simulations to investigate the shear-coupled boundary migration of tilt boundaries. The present study leads to better understanding of the structure and mechanical architecture of grain boundaries.

Introducing Overlapping Grain Boundaries in Chemical Vapor Deposited Hexagonal Boron Nitride Monolayer Films

PubMed Central

2017-01-01

We demonstrate the growth of overlapping grain boundaries in continuous, polycrystalline hexagonal boron nitride (h-BN) monolayer films via scalable catalytic chemical vapor deposition. Unlike the commonly reported atomically stitched grain boundaries, these overlapping grain boundaries do not consist of defect lines within the monolayer films but are composed of self-sealing bilayer regions of limited width. We characterize this overlapping h-BN grain boundary structure in detail by complementary (scanning) transmission electron microscopy techniques and propose a catalytic growth mechanism linked to the subsurface/bulk of the process catalyst and its boron and nitrogen solubilities. Our data suggest that the overlapping grain boundaries are comparatively resilient against deleterious pinhole formation associated with grain boundary defect lines and thus may reduce detrimental breakdown effects when polycrystalline h-BN monolayer films are used as ultrathin dielectrics, barrier layers, or separation membranes. PMID:28410557

Effect of irradiation on mechanical properties of symmetrical grain boundaries investigated by atomic simulations

NASA Astrophysics Data System (ADS)

Wang, X. Y.; Gao, N.; Setyawan, W.; Xu, B.; Liu, W.; Wang, Z. G.

2017-08-01

Tensile response of irradiated symmetric grain boundaries to the externally applied strain has been studied using atomic simulation methods. The absorption of irradiation induced defects by grain boundaries has been confirmed to degrade the mechanical properties of grain boundaries through the change of its undertaken deformation mechanism. Atomic rearrangement, the formations of a stress accumulation region and vacancy-rich zone and the nucleation and movement of dislocations under stress effect have been observed after the displacement cascades in grain boundaries, which are considered as main reasons to induce above degradation. These results suggest the necessity of considering both trapping efficiency to defects and the mechanical property change of irradiated grain boundaries for further development of radiation resistant materials.

Simultaneous increase in the strength, plasticity, and corrosion resistance of an ultrafine-grained Ti-4Al-2V pseudo-alpha-titanium alloy

NASA Astrophysics Data System (ADS)

Chuvil'deev, V. N.; Kopylov, V. I.; Nokhrin, A. V.; Bakhmet'ev, A. M.; Sandler, N. G.; Kozlova, N. A.; Tryaev, P. V.; Tabachkova, N. Yu.; Mikhailov, A. S.; Ershova, A. V.; Gryaznov, M. Yu.; Chegurov, M. K.; Sysoev, A. N.; Smirnova, E. S.

2017-05-01

The influence of severe plastic deformation on the structural-phase state of grain boundaries in a Ti-4Al-2V (commercial PT3V grade) pseudo-alpha-titanium alloy has been studied. It is established that increase in the strength, plasticity, and corrosion resistance of this alloy is related to the formation of an ultrafine- grained structure. In particular, it is shown that an increase in the resistance to hot-salt intergranular corrosion is due to diffusion-controlled redistribution of aluminum and vanadium atoms at the grain boundaries of titanium formed during thermal severe plastic deformation.

Iron state in iron nanoparticles with and without zirconium

NASA Astrophysics Data System (ADS)

Filippov, V. P.; Khasanov, A. M.; Lauer, Yu. A.

2017-11-01

Mössbauer and X-ray methods are used for investigations of structure, stability and characteristics of pure-iron grain and two iron-zirconium alloys such as Fe + 5 wt.% Zr and Fe + 10 wt.% Zr. The used powder was ground for 24 h in a SPEX Model 8000 mill shaker. Complex nanoparticles are found, which change their properties under milling. Mössbauer spectral parameters are obtained for investigated materials. Milling results in formation of nanosized particles with two states of iron atoms: one main part is pure α-Fe and another part of iron atoms displaced in grain boundaries or defective zones in which hyperfine magnetic splitting decrease to ˜ 30.0 T. In alloys with Zr three iron states are formed in each alloy, main part of iron is in the form of α-Fe and another two states depend on the concentration of Zr and represent iron in grain boundaries with Zr atoms in nearest neighbor. The changing of iron states is discussed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Datta, Kaustuv; Neder, Reinhard B.; Chen, Jun

Revelation of unequivocal structural information at the atomic level for complex systems is uniquely important for deeper and generic understanding of the structure property connections and a key challenge in materials science. Here in this paper we report an experimental study of the local structure by applying total elastic scattering and Raman scattering analyses to an important non-relaxor ferroelectric solid solution exhibiting the so-called composition-induced morphotropic phase boundary (MPB), where concomitant enhancement of physical properties have been detected. The powerful combination of static and dynamic structural probes enabled us to derive direct correspondence between the atomic-level structural correlations and reportedmore » properties. The atomic pair distribution functions obtained from the neutron total scattering experiments were analysed through big-box atom-modelling implementing reverse Monte Carlo method, from which distributions of magnitudes and directions of off-centred cationic displacements were extracted. We found that an enhanced randomness of the displacement-directions for all ferroelectrically active cations combined with a strong dynamical coupling between the A- and B-site cations of the perovskite structure, can explain the abrupt amplification of piezoelectric response of the system near MPB. Finally, altogether this provides a more fundamental basis in inferring structure-property connections in similar systems including important implications in designing novel and bespoke materials.« less

Heterocrystal and bicrystal structures of ZnS nanowires synthesized by plasma enhanced chemical vapour deposition

NASA Astrophysics Data System (ADS)

Jie, J. S.; Zhang, W. J.; Jiang, Y.; Meng, X. M.; Zapien, J. A.; Shao, M. W.; Lee, S. T.

2006-06-01

ZnS nanowires with heterocrystal and bicrystal structures were successfully synthesized using the DC-plasma chemical vapour deposition (CVD) method. The heterocrystalline ZnS nanowires have the zinc blende (ZB) and wurtzite (WZ) zones aligned alternately in the transverse direction but without an obvious period. The bicrystal ZnS nanowires are composed of two ZB fractions separated by a clear grain boundary along the length. Significantly, the grain boundaries in both the heterocrystal and bicrystal structures are atomically sharp without any visible lattice distortion. The effects of plasma species, ion bombardment, and silicon impurities in the formation of these distinctive structures are discussed. A defect-induced red-shift and broadening of the band-gap emission are revealed in photoluminescence (PL) and cathodoluminescence (CL) measurements.

Structural refinement of vitreous silica bilayers

NASA Astrophysics Data System (ADS)

Sadjadi, Mahdi; Wilson, Mark; Thorpe, M. F.

The importance of glasses resides not only in their applications but in fundamental questions that they put forth. The continuous random network model can successfully describe the glass structure, but determining details, like ring statistics, has always been difficult using only diffraction data. But recent atomic images of 2D vitreous silica bilayers can offer valuable new insights which are hard to be observed directly in 3D silica models/experiments (for references see). However, the experimental results are prone to uncertainty in atomic positions, systematic errors, and being finite. We employ special boundary conditions developed for such networks to refine the experimental structures. We show the best structure can be found by using various potentials to maximize information gained from the experimental samples. We find a range of densities, the so-called flexibility window, in which tetrahedra are perfect. We compare results from simulations using harmonic potentials, MD with atomic polarizabilities included and DFT. We should thank David Drabold and Bishal Bhattarai for useful discussions. Support through NSF Grant # DMS 1564468 is gratefully acknowledged.

Crystal structure refinement of ReSi1.75 with an ordered arrangement of silicon vacancies

NASA Astrophysics Data System (ADS)

Harada, Shunta; Hoshikawa, Hiroaki; Kuwabara, Kosuke; Tanaka, Katsushi; Okunishi, Eiji; Inui, Haruyuki

2011-08-01

The crystal structure and microstructure of ReSi1.75 were investigated by synchrotron X-ray diffraction combined with scanning transmission electron microscopy. ReSi1.75 contains an ordered arrangement of vacancies in Si sites in the underlying tetragonal C11b lattice of the MoSi2-type and the crystal structure is monoclinic with the space group Cm. Atomic positions of Si atoms near vacancies are considerably displaced from the corresponding positions in the parent C11b structure, and they exhibit anomalously large local thermal vibration accompanied by large values of atomic displacement parameter. There are four differently-oriented domains with two of them being related to each other by the 90° rotation about the c-axis of the underlying C11b lattice and the other two being their respective twins. The habit planes for domain boundaries observed experimentally are consistent with those predicted with ferroelastic theory.

Photo ion spectrometer

DOEpatents

Gruen, Dieter M.; Young, Charles E.; Pellin, Michael J.

1989-01-01

A charged particle spectrometer for performing ultrasensitive quantitative analysis of selected atomic components removed from a sample. Significant improvements in performing energy and angular refocusing spectroscopy are accomplished by means of a two dimensional structure for generating predetermined electromagnetic field boundary conditions. Both resonance and non-resonance ionization of selected neutral atomic components allow accumulation of increased chemical information. A multiplexed operation between a SIMS mode and a neutral atomic component ionization mode with EARTOF analysis enables comparison of chemical information from secondary ions and neutral atomic components removed from the sample. An electronic system is described for switching high level signals, such as SIMS signals, directly to a transient recorder and through a charge amplifier to the transient recorder for a low level signal pulse counting mode, such as for a neutral atomic component ionization mode.

Defect character at grain boundary facet junctions: Analysis of an asymmetric Σ = 5 grain boundary in Fe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Medlin, D. L.; Hattar, K.; Zimmerman, J. A.

Grain boundaries often develop faceted morphologies in systems for which the interfacial free energy depends on the boundary inclination. Although the mesoscale thermodynamic basis for such morphological evolution has been extensively studied, the influence of line defects, such as secondary grain boundary dislocations, on the facet configurations has not been thoroughly explored. In this paper, through a combination of atomistic simulations and electron microscopic observations, we examine in detail the structure of an asymmetric Σ = 5 [001] grain boundary in well-annealed, body-centered cubic (BCC) Fe. The observed boundary forms with a hill-and-valley morphology composed of nanoscale {310} and {210}more » facets. Our analysis clarifies the atomic structure of the {310}/{210} facet junctions and identifies the presence of an array of secondary grain boundary dislocations that are localized to these junctions. Analysis of the Burgers vectors of the grain boundary dislocations, which are of type (1/5)<310> and (1/5)<120>, shows that the defect density is consistent with that required to accommodate a small observed angular deviation from the exact Σ = 5 orientation relationship. As a result, these observations and analysis suggest a crucial role for secondary grain boundary dislocations in dictating the length-scale of grain boundary facets, a consideration which has not been included in prior analyses of facet evolution and equilibrium facet length.« less

Defect character at grain boundary facet junctions: Analysis of an asymmetric Σ = 5 grain boundary in Fe

DOE PAGES

Medlin, D. L.; Hattar, K.; Zimmerman, J. A.; ...

2016-11-16

Grain boundaries often develop faceted morphologies in systems for which the interfacial free energy depends on the boundary inclination. Although the mesoscale thermodynamic basis for such morphological evolution has been extensively studied, the influence of line defects, such as secondary grain boundary dislocations, on the facet configurations has not been thoroughly explored. In this paper, through a combination of atomistic simulations and electron microscopic observations, we examine in detail the structure of an asymmetric Σ = 5 [001] grain boundary in well-annealed, body-centered cubic (BCC) Fe. The observed boundary forms with a hill-and-valley morphology composed of nanoscale {310} and {210}more » facets. Our analysis clarifies the atomic structure of the {310}/{210} facet junctions and identifies the presence of an array of secondary grain boundary dislocations that are localized to these junctions. Analysis of the Burgers vectors of the grain boundary dislocations, which are of type (1/5)<310> and (1/5)<120>, shows that the defect density is consistent with that required to accommodate a small observed angular deviation from the exact Σ = 5 orientation relationship. As a result, these observations and analysis suggest a crucial role for secondary grain boundary dislocations in dictating the length-scale of grain boundary facets, a consideration which has not been included in prior analyses of facet evolution and equilibrium facet length.« less

Defect structures in ordered intermetallics; grain boundaries and surfaces in FeAl, NiAl, CoAl and TiAl

NASA Astrophysics Data System (ADS)

Mutasa, Batsirai Manyara

Ordered intermetallics based on transition metal aluminides have been proposed as structural materials for advanced aerospace applications. The development of these materials, which have the advantages of low density and high operating temperatures, have been focused on the aluminides of titanium, nickel and iron. Though these materials exhibit attractive properties at elevated temperatures, their utilization is limited due to their propensity for low temperature fracture and susceptibility to decreased ductility due to environmental effects. A major embrittlement mechanism at ambient temperatures in these aluminides has been by the loss of cohesive strength at the interfaces (intergranular failure). This study focuses on this mechanism of failure, by undertaking a systematic study of the energies and structures of specific grain boundaries in some of these compounds. The relaxed atomistic grain boundary structures in B2 aluminides, FeAl, NiAl and CoAl and L10 gamma-TiAl were investigated using molecular statics and embedded atom potentials in order to explore general trends for a series of B2 compounds as well as TiAl. The potentials used correctly predict the proper mechanism of compositional disorder of these compounds. Using these potentials, point defects, free surface energies and various grain boundary structures of similar energies in three B2 compounds, FeAl, NiAl and CoAl were studied. These B2 alloys exhibited increasing anti-phase boundary energies respectively. The misorientations chosen for detailed study correspond to the Sigma5(310) and Sigma5(210) boundaries. These boundaries were investigated with consideration given to possible variations in the local chemical composition. The effects of both boundary stoichiometry and bulk stoichiometry on grain boundary energetics were also considered. Defect energies were calculated for boundaries contained in both stoichiometric and off-stoichiometric bulk. The surface energies for these aluminides were also calculated so that trends concerning the cohesive energy of the boundaries could be studied. The implications of stoichiometry, the multiplicity of the boundary structures and possible transformations between them for grain boundary brittleness are also discussed.

C-Cr segregation at grain boundary before the carbide nucleation in Alloy 690

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li Hui, E-mail: huili@shu.edu.cn; Laboratory for Microstructures, Shanghai University, Shanghai, 200444; Xia Shuang

2012-04-15

The grain boundary segregation in Alloy 690 was investigated by atom probe tomography. B, C and Si segregated at the grain boundary. The high concentration regions for each segregation element form a set of straight arrays that are parallel to each other in the grain boundary plane. The concentration fluctuation has a periodicity of about 7 nm in the grain boundary plane. Before the Cr{sub 23}C{sub 6} nucleation at grain boundaries, the C-Cr co-segregate on one side of the grain boundaries while not the exact grain boundary core regions have been detected. The reasons why grain boundary carbides have coherentmore » orientation relationship only with one side of nearby grain which grain boundary is located at high index crystal plane were discussed. - Highlights: Black-Right-Pointing-Pointer Grain boundary segregation in Alloy 690 was investigated by atom probe tomography. Black-Right-Pointing-Pointer B, C and Si segregate at the grain boundary. Black-Right-Pointing-Pointer Concentration of segregated atoms periodicity fluctuated in the grain boundary plane. Black-Right-Pointing-Pointer C and Cr co-segregate on one side of the grain boundary before carbide nucleation.« less

Effect of irradiation on mechanical properties of symmetrical grain boundaries investigated by atomic simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, X. Y.; Gao, N.; Setyawan, W.

Tensile response of irradiated symmetric grain boundaries to externally applied strain has been studied using atomic simulation methods. The absorption of irradiation induced defects by grain boundaries has been confirmed to degrade the mechanical properties of grain boundaries through the change of its under- taken deformation mechanism. Atomic rearrangement, the formations of a stress accumulation region and vacancy-rich zone and the nucleation and movement of dislocations under stress effect have been observed after the displacement cascades in grain boundaries, which are considered as main reasons to induce above degradation. These results suggest the necessity of considering both trap- ping efficiencymore » to defects and the mechanical property change of irradiated grain boundaries for further development of radiation resistant materials.« less

Growth model and structure evolution of Ag layers deposited on Ge films.

PubMed

Ciesielski, Arkadiusz; Skowronski, Lukasz; Górecka, Ewa; Kierdaszuk, Jakub; Szoplik, Tomasz

2018-01-01

We investigated the crystallinity and optical parameters of silver layers of 10-35 nm thickness as a function 2-10 nm thick Ge wetting films deposited on SiO 2 substrates. X-ray reflectometry (XRR) and X-ray diffraction (XRD) measurements proved that segregation of germanium into the surface of the silver film is a result of the gradient growth of silver crystals. The free energy of Ge atoms is reduced by their migration from boundaries of larger grains at the Ag/SiO 2 interface to boundaries of smaller grains near the Ag surface. Annealing at different temperatures and various durations allowed for a controlled distribution of crystal dimensions, thus influencing the segregation rate. Furthermore, using ellipsometric and optical transmission measurements we determined the time-dependent evolution of the film structure. If stored under ambient conditions for the first week after deposition, the changes in the transmission spectra are smaller than the measurement accuracy. Over the course of the following three weeks, the segregation-induced effects result in considerably modified transmission spectra. Two months after deposition, the slope of the silver layer density profile derived from the XRR spectra was found to be inverted due to the completed segregation process, and the optical transmission spectra increased uniformly due to the roughened surfaces, corrosion of silver and ongoing recrystallization. The Raman spectra of the Ge wetted Ag films were measured immediately after deposition and ten days later and demonstrated that the Ge atoms at the Ag grain boundaries form clusters of a few atoms where the Ge-Ge bonds are still present.

A novel Gaussian-Sinc mixed basis set for electronic structure calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jerke, Jonathan L.; Lee, Young; Tymczak, C. J.

2015-08-14

A Gaussian-Sinc basis set methodology is presented for the calculation of the electronic structure of atoms and molecules at the Hartree–Fock level of theory. This methodology has several advantages over previous methods. The all-electron electronic structure in a Gaussian-Sinc mixed basis spans both the “localized” and “delocalized” regions. A basis set for each region is combined to make a new basis methodology—a lattice of orthonormal sinc functions is used to represent the “delocalized” regions and the atom-centered Gaussian functions are used to represent the “localized” regions to any desired accuracy. For this mixed basis, all the Coulomb integrals are definablemore » and can be computed in a dimensional separated methodology. Additionally, the Sinc basis is translationally invariant, which allows for the Coulomb singularity to be placed anywhere including on lattice sites. Finally, boundary conditions are always satisfied with this basis. To demonstrate the utility of this method, we calculated the ground state Hartree–Fock energies for atoms up to neon, the diatomic systems H{sub 2}, O{sub 2}, and N{sub 2}, and the multi-atom system benzene. Together, it is shown that the Gaussian-Sinc mixed basis set is a flexible and accurate method for solving the electronic structure of atomic and molecular species.« less

The Pt site reactivity of the molecular graphs of Au6Pt isomers

NASA Astrophysics Data System (ADS)

Xu, Tianlv; Jenkins, Samantha; Xiao, Chen-Xia; Maza, Julio R.; Kirk, Steven R.

2013-12-01

Within the framework of the theory of atoms in molecules (QTAIM), in an exploratory study we propose a new measure of site reactivity equivalent to the atomic coordination number based purely on the electronic structure. It was found that the number of ring critical points (NNRCPs) positioned on the boundary of the atomic basin of the dopant (Pt) nucleus correlated very well with the relative zero point energy (ZPE) corrected energies. A weaker condition (i.e. than the number of associated bond paths) for the association of the dopant Pt nucleus with the Au6Pt molecular graph is found for NNRCP = 0.

Atomisti modeling of the microstructure and transport properties of lead-free solder alloys

NASA Astrophysics Data System (ADS)

Sellers, Michael S.

Damage mechanics models of lead-free solder joints in nanoelectronics continue to improve, and in doing so begin to utilize quantitative values describing processes at the atomic level, governing phenomena like electromigration and thermomigration. In particular, knowledge of the transport properties of specific microstructures helps continuum level models fully describe these larger-scale damage phenomena via multi-scale analysis. For example, diffusivities for different types of grain boundaries (fast diffusion paths for solvent and solute atoms, and vacancies), and a description of the boundary structure as a function of temperature, are critical in modeling solder microstructure evolution and, consequently, joint behavior under extreme temperature and electric current. Moreover, for damage that develops at larger length scales, surface energies and diffusivities play important roles in characterizing void stability and morphology. Unfortunately, experiments that investigate these kind of damage phenomena in the atomistic realm are often inconsistent or unable to directly quantify important parameters. One case is the particular transport and structural properties of grain boundaries in Sn (the main component in lead-free solder alloys) and their behavior in the presence of Ag and Cu impurities. This information is crucial in determining accurate diffusivity values for the common SnAgCu (SAC) type solder. Although an average grain boundary diffusivity has been reported for polycrystalline Sn in several works, the value for grain boundary width is estimated and specific diffusivities for boundaries known to occur in Sn have not been reported, to say nothing of solute effects on Sn diffusivity and grain boundary structure. Similarly, transport properties of Sn surfaces remain relatively uninvestigated as well. These gaps and inconsistencies in atomistic data must be remedied for micro- and macro-scale modeling to improve. As a complement to experimental work and possessing the ability to fill in the gaps, molecular simulation serves to reinforce experimental predictions and provide insight into the atomistic processes that govern studied phenomena. In the present body of work, we employ molecular statics and dynamics simulations in the characterization and computation of betaSn surface energies and surface diffusivities, the determination of diffusivities and structural properties of specific betaSn grain boundaries, and the investigation of Cu and Ag solute effects on betaSn grain boundaries. In our study of betaSn surfaces, energies for low number Miller index surfaces are computed and the (100) plane is found to have the lowest un-relaxed energy. We then find that two simple hopping mechanisms dominate adatom diffusion transitions on this surface. For each, we determine hopping rates of the adatom and compute its tracer diffusivity. Our work on grain boundaries investigates the self-diffusion properties and structure of several betaSn symmetric tilt grain boundaries using molecular dynamics simulations. We find that larger diffusive widths are exhibited by higher excess potential energy grain boundaries. Diffusivities in the directions parallel to the interface plane are also computed and activation energies are found with the Arrhenius relation. These are shown to agree well with experimental data. Finally, we examine the effect that solute atoms of Ag and Cu have on the microstructure of betaSn. Excess energies of the (101) symmetric tilt betaSn grain boundary are computed as a function of solute concentration at the interface, and we show that Ag lowers the energy at a greater rate than Cu. We also quantify segregation enthalpies and critical solute concentrations (where the excess energy of the boundary is reduced to zero). The effect of solute type on shear stress is also examined, and we show that solute has a strong effect on the stabilization of higher energy grain boundaries under shear stress. We then look at the self-diffusivity of Sn in the (101) symmetric tilt betaSn grain boundary and show that adding both Ag or Cu decrease the grain boundary self-diffusivity of Sn as solute amount in the interface increases. Effects of larger concentrations of Cu in particular are also investigated.

Multivariate statistical characterization of charged and uncharged domain walls in multiferroic hexagonal YMnO3 single crystal visualized by a spherical aberration-corrected STEM.

PubMed

Matsumoto, Takao; Ishikawa, Ryo; Tohei, Tetsuya; Kimura, Hideo; Yao, Qiwen; Zhao, Hongyang; Wang, Xiaolin; Chen, Dapeng; Cheng, Zhenxiang; Shibata, Naoya; Ikuhara, Yuichi

2013-10-09

A state-of-the-art spherical aberration-corrected STEM was fully utilized to directly visualize the multiferroic domain structure in a hexagonal YMnO3 single crystal at atomic scale. With the aid of multivariate statistical analysis (MSA), we obtained unbiased and quantitative maps of ferroelectric domain structures with atomic resolution. Such a statistical image analysis of the transition region between opposite polarizations has confirmed atomically sharp transitions of ferroelectric polarization both in antiparallel (uncharged) and tail-to-tail 180° (charged) domain boundaries. Through the analysis, a correlated subatomic image shift of Mn-O layers with that of Y layers, exhibiting a double-arc shape of reversed curvatures, have been elucidated. The amount of image shift in Mn-O layers along the c-axis is statistically significant as small as 0.016 nm, roughly one-third of the evident image shift of 0.048 nm in Y layers. Interestingly, a careful analysis has shown that such a subatomic image shift in Mn-O layers vanishes at the tail-to-tail 180° domain boundaries. Furthermore, taking advantage of the annular bright field (ABF) imaging technique combined with MSA, the tilting of MnO5 bipyramids, the very core mechanism of multiferroicity of the material, is evaluated.

Grain boundary phase transformations in PtAu and relevance to thermal stabilization of bulk nanocrystalline metals

DOE Office of Scientific and Technical Information (OSTI.GOV)

O’Brien, C. J.; Barr, C. M.; Price, P. M.

There has recently been a great deal of interest in employing immiscible solutes to stabilize nanocrystalline microstructures. Existing modeling efforts largely rely on mesoscale Monte Carlo approaches that employ a simplified model of the microstructure and result in highly homogeneous segregation to grain boundaries. However, there is ample evidence from experimental and modeling studies that demonstrates segregation to grain boundaries is highly non-uniform and sensitive to boundary character. This work employs a realistic nanocrystalline microstructure with experimentally relevant global solute concentrations to illustrate inhomogeneous boundary segregation. Furthermore, experiments quantifying segregation in thin films are reported that corroborate the prediction thatmore » grain boundary segregation is highly inhomogeneous. In addition to grain boundary structure modifying the degree of segregation, the existence of a phase transformation between low and high solute content grain boundaries is predicted. In order to conduct this study, new embedded atom method interatomic potentials are developed for Pt, Au, and the PtAu binary alloy.« less

Grain boundary phase transformations in PtAu and relevance to thermal stabilization of bulk nanocrystalline metals

DOE PAGES

O’Brien, C. J.; Barr, C. M.; Price, P. M.; ...

2017-10-31

There has recently been a great deal of interest in employing immiscible solutes to stabilize nanocrystalline microstructures. Existing modeling efforts largely rely on mesoscale Monte Carlo approaches that employ a simplified model of the microstructure and result in highly homogeneous segregation to grain boundaries. However, there is ample evidence from experimental and modeling studies that demonstrates segregation to grain boundaries is highly non-uniform and sensitive to boundary character. This work employs a realistic nanocrystalline microstructure with experimentally relevant global solute concentrations to illustrate inhomogeneous boundary segregation. Furthermore, experiments quantifying segregation in thin films are reported that corroborate the prediction thatmore » grain boundary segregation is highly inhomogeneous. In addition to grain boundary structure modifying the degree of segregation, the existence of a phase transformation between low and high solute content grain boundaries is predicted. In order to conduct this study, new embedded atom method interatomic potentials are developed for Pt, Au, and the PtAu binary alloy.« less

First-principles study of twin grain boundaries in epitaxial BaSi{sub 2} on Si(111)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baba, Masakazu; Suemasu, Takashi, E-mail: suemasu@bk.tsukuba.ac.jp; Kohyama, Masanori

2016-08-28

Epitaxial films of BaSi{sub 2} on Si(111) for solar cell applications possess three epitaxial variants and exhibit a minority carrier diffusion length (ca. 9.4 μm) much larger than the domain size (ca. 0.2 μm); thus, the domain boundaries (DBs) between the variants do not act as carrier recombination centers. In this work, transmission electron microscopy (TEM) was used to observe the atomic arrangements around the DBs in BaSi{sub 2} epitaxial films on Si(111), and the most stable atomic configuration was determined by first-principles calculations based on density functional theory to provide possible interface models. Bright-field TEM along the a-axis of BaSi{sub 2}more » revealed that each DB was a twin boundary between two different epitaxial variants, and that Ba{sup (II)} atoms form hexagons containing central Ba{sup (I)} atoms in both the bulk and DB regions. Four possible interface models containing Ba{sup (I)}-atom interface layers were constructed, each consistent with TEM observations and distinguished by the relationship between the Si tetrahedron arrays in the two domains adjacent across the interface. This study assessed the structural relaxation of initial interface models constructed from surface slabs terminated by Ba{sup (I)} atoms or from zigzag surface slabs terminated by Si tetrahedra and Ba{sup (II)} atoms. In these models, the interactions or relative positions between Si tetrahedra appear to dominate the relaxation behavior and DB energies. One of the four interface models whose relationship between first-neighboring Si tetrahedra across the interface was the same as that in the bulk was particularly stable, with a DB energy of 95 mJ/m{sup 2}. There were no significant differences in the partial densities of states and band gaps between the bulk and DB regions, and it was therefore concluded that such DBs do not affect the minority carrier properties of BaSi{sub 2}.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Valderrama, Billy; He, Lingfeng; Henderson, Hunter B.

Fission products, such as krypton (Kr), are known to be insoluble within UO 2, segregating towards grain boundaries, eventually leading to a lowering of the thermal conductivity and fuel swelling. Recent computational studies have identified that differences in grain boundary structure have a significant effect on the segregation behavior of fission products. However, experimental work supporting these simulations is lacking. Atom probe tomography was used to measure the Kr distribution across grain boundaries in UO 2. Polycrystalline depleted-UO 2 samples was irradiated with 0.7 and 1.8 MeV Kr-ions and annealed to 1000ºC, 1300ºC, and 1600°C for 1 hour to producemore » a Kr-bubble dominated microstructure. The results of this work indicate a strong dependence of Kr concentration as a function of grain boundary structure. Temperature also influences grain boundary chemistry with greater Kr concentration evident at higher temperatures, resulting in a reduced Kr concentration in the bulk. While Kr migration is active at elevated temperatures, no changes in grain size or texture were observed in the irradiated UO 2 samples.« less

Σ 3 (111 ) grain boundary of body-centered cubic Ti-Mo and Ti-V alloys: First-principles and model calculations

NASA Astrophysics Data System (ADS)

Yan, Jia-Yi; Ehteshami, Hossein; Korzhavyi, Pavel A.; Borgenstam, Annika

2017-07-01

The energetics and atomic structures of Σ 3 [1 1 ¯0 ] (111 ) grain boundary (GB) of body-centered cubic (bcc) Ti-Mo and Ti-V alloys are investigated using density-functional-theory calculations and virtual crystal approximation. The electron density in bcc structure and the atomic displacements and excess energy of the GB are correlated to bcc-ω phase stability. Model calculations based on pairwise interplanar interactions successfully reproduce the chemical part of GB energy. The chemical GB energy can be expressed as a sum of excess pairwise interactions between bcc (111) layers, which are obtained from Gaussian elimination of the total energies of a number of periodic structures. The energy associated with the relaxation near the GB is solved by numerical minimization using the derivatives of the excess interactions. Anharmonic interlayer interactions are necessary for obtaining accurate relaxation energy and excess GB volume from model calculations. The effect of GB on vibrational spectrum is also investigated. Segregation energies of B and Y to a substitutional site on the GB plane are calculated. Preliminary results suggest that Y tends to segregate, while B tends to antisegregate.

Unexpected Huge Dimerization Ratio in One-Dimensional Carbon Atomic Chains.

PubMed

Lin, Yung-Chang; Morishita, Shigeyuki; Koshino, Masanori; Yeh, Chao-Hui; Teng, Po-Yuan; Chiu, Po-Wen; Sawada, Hidetaka; Suenaga, Kazutomo

2017-01-11

Peierls theory predicted atomic distortion in one-dimensional (1D) crystal due to its intrinsic instability in 1930. Free-standing carbon atomic chains created in situ in transmission electron microscope (TEM)1-3 are an ideal example to experimentally observe the dimerization behavior of carbon atomic chain within a finite length. We report here a surprisingly huge distortion found in the free-standing carbon atomic chains at 773 K, which is 10 times larger than the value expected in the system. Such an abnormally distorted phase only dominates at the elevated temperatures, while two distinct phases, distorted and undistorted, coexist at lower or ambient temperatures. Atom-by-atom spectroscopy indeed shows considerable variations in the carbon 1s spectra at each atomic site but commonly observes a slightly downshifted π* peak, which proves its sp 1 bonding feature. These results suggest that the simple model, relaxed and straight, is not fully adequate to describe the realistic 1D structure, which is extremely sensitive to perturbations such as external force or boundary conditions.

Photo ion spectrometer

DOEpatents

Gruen, D.M.; Young, C.E.; Pellin, M.J.

1989-12-26

A charged particle spectrometer is described for performing ultrasensitive quantitative analysis of selected atomic components removed from a sample. Significant improvements in performing energy and angular refocusing spectroscopy are accomplished by means of a two dimensional structure for generating predetermined electromagnetic field boundary conditions. Both resonance and non-resonance ionization of selected neutral atomic components allow accumulation of increased chemical information. A multiplexed operation between a SIMS mode and a neutral atomic component ionization mode with EARTOF analysis enables comparison of chemical information from secondary ions and neutral atomic components removed from the sample. An electronic system is described for switching high level signals, such as SIMS signals, directly to a transient recorder and through a charge amplifier to the transient recorder for a low level signal pulse counting mode, such as for a neutral atomic component ionization mode. 12 figs.

Protecting quantum coherence of two-level atoms from vacuum fluctuations of electromagnetic field

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Xiaobao; Tian, Zehua; Wang, Jieci

In the framework of open quantum systems, we study the dynamics of a static polarizable two-level atom interacting with a bath of fluctuating vacuum electromagnetic field and explore under which conditions the coherence of the open quantum system is unaffected by the environment. For both a single-qubit and two-qubit systems, we find that the quantum coherence cannot be protected from noise when the atom interacts with a non-boundary electromagnetic field. However, with the presence of a boundary, the dynamical conditions for the insusceptible of quantum coherence are fulfilled only when the atom is close to the boundary and is transverselymore » polarizable. Otherwise, the quantum coherence can only be protected in some degree in other polarizable direction. -- Highlights: •We study the dynamics of a two-level atom interacting with a bath of fluctuating vacuum electromagnetic field. •For both a single and two-qubit systems, the quantum coherence cannot be protected from noise without a boundary. •The insusceptible of the quantum coherence can be fulfilled only when the atom is close to the boundary and is transversely polarizable. •Otherwise, the quantum coherence can only be protected in some degree in other polarizable direction.« less

Imaging and manipulation of adatoms on an alumina surface by noncontact atomic force microscopy

NASA Astrophysics Data System (ADS)

Simon, G. H.; Heyde, M.; Freund, H.-J.

2012-02-01

Noncontact atomic force microscopy (NC-AFM) has been performed on an aluminum oxide film grown on NiAl(110) in ultrahigh vacuum (UHV) at low temperature (5 K). Results reproduce the topography of the structural model, unlike scanning tunnelling microscopy (STM) images. Equipped with this extraordinary contrast the network of extended defects, which stems from domain boundaries intersecting the film surface, can be analysed in atomic detail. The knowledge of occurring surface structures opens up the opportunity to determine adsorption sites of individual adsorbates on the alumina film. The level of difficulty for such imaging depends on the imaging characteristics of the substrate and the interaction which can be maintained above the adsorbate. Positions of single adsorbed gold atoms within the unit cell have been determined despite their easy removal at slightly higher interaction strength. Preliminary manipulation experiments indicate a pick-up process for the vanishing of the gold adatoms from the film surface.

Atomic density effects on temperature characteristics and thermal transport at grain boundaries through a proper bin size selection

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vo, Truong Quoc; Kim, BoHung, E-mail: muratbarisik@iyte.edu.tr, E-mail: bohungk@ulsan.ac.kr; Barisik, Murat, E-mail: muratbarisik@iyte.edu.tr, E-mail: bohungk@ulsan.ac.kr

2016-05-21

This study focuses on the proper characterization of temperature profiles across grain boundaries (GBs) in order to calculate the correct interfacial thermal resistance (ITR) and reveal the influence of GB geometries onto thermal transport. The solid-solid interfaces resulting from the orientation difference between the (001), (011), and (111) copper surfaces were investigated. Temperature discontinuities were observed at the boundary of grains due to the phonon mismatch, phonon backscattering, and atomic forces between dissimilar structures at the GBs. We observed that the temperature decreases gradually in the GB area rather than a sharp drop at the interface. As a result, threemore » distinct temperature gradients developed at the GB which were different than the one observed in the bulk solid. This behavior extends a couple molecular diameters into both sides of the interface where we defined a thickness at GB based on the measured temperature profiles for characterization. Results showed dependence on the selection of the bin size used to average the temperature data from the molecular dynamics system. The bin size on the order of the crystal layer spacing was found to present an accurate temperature profile through the GB. We further calculated the GB thickness of various cases by using potential energy (PE) distributions which showed agreement with direct measurements from the temperature profile and validated the proper binning. The variation of grain crystal orientation developed different molecular densities which were characterized by the average atomic surface density (ASD) definition. Our results revealed that the ASD is the primary factor affecting the structural disorders and heat transfer at the solid-solid interfaces. Using a system in which the planes are highly close-packed can enhance the probability of interactions and the degree of overlap between vibrational density of states (VDOS) of atoms forming at interfaces, leading to a reduced ITR. Thus, an accurate understanding of thermal characteristics at the GB can be formulated by selecting a proper bin size.« less

Atomic density effects on temperature characteristics and thermal transport at grain boundaries through a proper bin size selection

NASA Astrophysics Data System (ADS)

Vo, Truong Quoc; Barisik, Murat; Kim, BoHung

2016-05-01

This study focuses on the proper characterization of temperature profiles across grain boundaries (GBs) in order to calculate the correct interfacial thermal resistance (ITR) and reveal the influence of GB geometries onto thermal transport. The solid-solid interfaces resulting from the orientation difference between the (001), (011), and (111) copper surfaces were investigated. Temperature discontinuities were observed at the boundary of grains due to the phonon mismatch, phonon backscattering, and atomic forces between dissimilar structures at the GBs. We observed that the temperature decreases gradually in the GB area rather than a sharp drop at the interface. As a result, three distinct temperature gradients developed at the GB which were different than the one observed in the bulk solid. This behavior extends a couple molecular diameters into both sides of the interface where we defined a thickness at GB based on the measured temperature profiles for characterization. Results showed dependence on the selection of the bin size used to average the temperature data from the molecular dynamics system. The bin size on the order of the crystal layer spacing was found to present an accurate temperature profile through the GB. We further calculated the GB thickness of various cases by using potential energy (PE) distributions which showed agreement with direct measurements from the temperature profile and validated the proper binning. The variation of grain crystal orientation developed different molecular densities which were characterized by the average atomic surface density (ASD) definition. Our results revealed that the ASD is the primary factor affecting the structural disorders and heat transfer at the solid-solid interfaces. Using a system in which the planes are highly close-packed can enhance the probability of interactions and the degree of overlap between vibrational density of states (VDOS) of atoms forming at interfaces, leading to a reduced ITR. Thus, an accurate understanding of thermal characteristics at the GB can be formulated by selecting a proper bin size.

Shock-induced Plasticity and Brittle Cracks in Aluminum Nitride

NASA Astrophysics Data System (ADS)

Branicio, Paulo; Kalia, Rajiv

2005-03-01

Two hundred and nine million atom molecular-dynamics simulation of hypervelocity projectile impact in aluminum nitride reveals strong interplay between shock-induced structural phase transformation, plastic deformation and brittle cracks. The shock wave splits into an elastic precursor and a wurtzite-to-rocksalt structural transformation wave. When the elastic wave reflected from the boundary of the sample interacts with the transformation wave front, nanocavities are generated along the penetration path of the projectile and dislocations in adjacent regions. The nanocavities coalesce to form mode I brittle cracks while dislocations generate kink bands that give rise to mode II cracks. These simulations provide a microscopic view of defects associated with simultaneous tensile and shear cracking at the structural phase transformation boundary due to shock impact in high-strength ceramics.

Favorable Concurrence of Static and Dynamic Phenomena at the Morphotropic Phase Boundary of x BiNi 0.5 Zr 0.5 O 3 - ( 1 - x ) PbTiO 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Datta, Kaustuv; Neder, Reinhard; Chen, J.

We demore » monstrate that the morphotropic phase boundary of the lead-free ferroelectric system x BiNi 0.5 Zr 0.5 O 3 - ( 1 - x ) PbTiO 3 (NBT-xBT) can be uniquely distinguished by a reduced polarizationstate along with a non-uniform attening of the local potential function of A- and B-site cationsof the perovskite-type structure. This establishes a robust structure-property connections based onthe atomic-level structural correlations which elucidates the long-standing question: why the MPBof NBT-xBT does not excel as much as Pb-based materials do; and further help to develop strategyin designing ecient lead-free ferroelectric systems.« less

Favorable Concurrence of Static and Dynamic Phenomena at the Morphotropic Phase Boundary of x BiNi 0.5 Zr 0.5 O 3 - ( 1 - x ) PbTiO 3

DOE PAGES

Datta, Kaustuv; Neder, Reinhard; Chen, J.; ...

2017-11-16

We demore » monstrate that the morphotropic phase boundary of the lead-free ferroelectric system x BiNi 0.5 Zr 0.5 O 3 - ( 1 - x ) PbTiO 3 (NBT-xBT) can be uniquely distinguished by a reduced polarizationstate along with a non-uniform attening of the local potential function of A- and B-site cationsof the perovskite-type structure. This establishes a robust structure-property connections based onthe atomic-level structural correlations which elucidates the long-standing question: why the MPBof NBT-xBT does not excel as much as Pb-based materials do; and further help to develop strategyin designing ecient lead-free ferroelectric systems.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bedekar, Vikram; Poplawsky, Jonathan D.; Guo, Wei

In grain finement and non-equilibrium there is carbon segregation within grain boundaries alters the mechanical performance of hard turning layers in carburized bearing steel. Moreover, an atom probe tomography (APT) study on the nanostructured hard turning layers reveals carbon migration to grain boundaries as a result of carbide decomposition during severe plastic deformation. In addition, samples exposed to different cutting speeds show that the carbon migration rate increases with the cutting speed. For these two effects lead to an ultrafine carbon network structure resulting in increased hardness and thermal stability in the severely deformed surface layer.

Structural and electronic properties of double-walled boron nitride nanocones

NASA Astrophysics Data System (ADS)

Brito, E.; Silva, T. S.; Guerra, T.; Leite, L.; Azevedo, S.; Freitas, A.; Kaschny, J. R.

2018-01-01

First principles calculations were applied to study the structural and electronic properties of different configurations of double-walled boron nitride nanocones with a disclination angle of 60°. The analysis includes different rotation angles, distance between apexes, as well as distinct types of antiphase boundaries. The calculations indicate that the non-rotated configuration of double-walled nanocone with a defective line composed by C and N atoms, forming C-N bonds, is the most stable configuration. It was found that the yam angle, apexes distance and defective line composition present significant influence on the electronic properties of such structures. Moreover, analyzing the spin charge density, for the electronic states near the Fermi level, it was also found that the configuration with a defective line containing C atoms presents a net magnetic moment.

Atomic scale characterization of white etching area and its adjacent matrix in a martensitic 100Cr6 bearing steel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Y.J., E-mail: y.li@mpie.de

Atom probe tomography was employed to characterize the microstructure and C distribution in the white etching area (WEA) of a martensitic 100Cr6 bearing steel subjected to rolling contact fatigue. Different from its surrounding matrix where a plate-like martensitic structure prevails, the WEA exhibits equiaxed grains with a uniform grain size of about 10 nm. Significant C grain boundary enrichment (>7.5at.%) and an overall higher C concentration than the nominal value are observed in the WEA. These results suggest that the formation of WEA results from severe local plastic deformation that causes dissolution of carbides and the redistribution of C. -more » Highlights: •APT has been applied to characterize the microstructure of white etching area (WEA). •Quantitative analyses of C distribution indicate that carbides are dissolved on the WEA. •WEA contains equiaxed grains with a uniform grain size of 10 nm. •C segregation at grain boundaries stabilizes the nanosized grain structure. •Formation of WEA is explained by severe local plastic deformation introduced by cyclic contact loading.« less

3D simulation of LISM oxygen flux with PUIs inside of heliosphere

DOE PAGES

Kawamura, Akito D.; Heerikhuisen, Jacob; Pogorelov, Nikolai V.; ...

2012-11-20

The structure of the heliospheric interface has attracted increasing attention with continual improvements in modelling and observations, during the last half decade. The Interstellar Boundary Explore (IBEX) spacecraft is returning important data that require a theoretical model of Heliosphere to ensure proper interpretation. Furthermore, we develop a framework for understanding the measurements of heavier-than-hydrogen atoms by IBEX in terms of a 3D MHD-neutral numerical solution of the sun's interaction with the interstellar medium, combined with a test particle approach for heavy atoms and ions.

Study of strain boundary conditions and GaAs buffer sizes in InGaAs quantum dots

NASA Technical Reports Server (NTRS)

Oyafuso, F.; Klimeck, G.; Boykin, T. B.; Bowen, R. C.; Allmen, P. von

2003-01-01

NEMO 3-D has been developed for the simulation of electronic structure in self-assembled InGaAs quantum dots on GaAs substrates. Typical self-assembled quantum dots in that material system contain about 0.5 to 1 million atoms. Effects of strain by the surrounding GaAs buffer modify the electronic structure inside the quantum dot significantly and a large GaAs buffer must be included in the strain and electronic structure.

Connection of the solar wind with the interstellar medium through numerical modeling

DOE PAGES

Heerikhuisen, J.; Zirnstein, E.; Kawamura, A. D.; ...

2013-06-13

In this article we investigate the interaction between the solar wind (SW) and the local interstellar medium (LISM) using spacecraft data and numerical simulations. In particular, we focus on neutral atom results from NASA's Interstellar Boundary EXplorer (IBEX) mission, and compare these with implementations of our neutral atom models that look at both the energetic neutral atoms (ENAs) which are created as hydrogen of LISM origin interacts with the heliosphere, as well as the transmission of interstellar Oxygen through the heliospheric interface. Lastly, the goal of this work is to better understand the global structure of the heliosphere and itsmore » interaction with the galaxy.« less

Atomic study of effects of crystal structure and temperature on structural evolution of Au nanowires under torsion

NASA Astrophysics Data System (ADS)

Wu, Cheng-Da; Tsai, Hsing-Wei

2018-06-01

The effect of temperature on the structural evolution of nanocrystalline (NC) and single-crystalline (SC) Au nanowires (NWs) under torsional deformation is studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effect is investigated using common neighbor analysis and discussed in terms of shear strain distribution and atomic flow field. The simulation results show that deformation for NC NWs is mainly driven by the nucleation and propagation of dislocations and the gliding of grain boundaries (GBs) and that for SC NWs is mainly driven by dislocations and the formation of disordered structures. Dislocations for NC and SC NWs easily nucleate at GBs and free surfaces, respectively. For NC NWs, torsional buckling occurs easily at GBs with large gliding. SC NWs have a more uniform and larger elastic deformation under torsion compared to that for NC NWs due to the former's lack of grains. SC NWs have a long period of elastic deformation transforming into plastic deformation. Increasing temperature facilitates stress transmission throughout NWs.

Thomas-Fermi model electron density with correct boundary conditions: Application to atoms and ions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Patil, S.H.

1999-01-01

The author proposes an electron density in atoms and ions, which has the Thomas-Fermi-Dirac form in the intermediate region of r, satisfies the Kato condition for small r, and has the correct asymptotic behavior at large values of r, where r is the distance from the nucleus. He also analyzes the perturbation in the density produced by multipolar fields. He uses these densities in the Poisson equation to deduce average values of r{sup m}, multipolar polarizabilities, and dispersion coefficients of atoms and ions. The predictions are in good agreement with experimental and other theoretical values, generally within about 20%. Hemore » tabulates here the coefficient A in the asymptotic density; radial expectation values (r{sup m}) for m = 2, 4, 6; multipolar polarizabilities {alpha}{sub 1}, {alpha}{sub 2}, {alpha}{sub 3}; expectation values {l_angle}r{sup 0}{r_angle} and {l_angle}r{sup 2}{r_angle} of the asymptotic electron density; and the van der Waals coefficient C{sub 6} for atoms and ions with 2 {le} Z {le} 92. Many of the results, particularly the multipolar polarizabilities and the higher order dispersion coefficients, are the only ones available in the literature. The variation of these properties also provides interesting insight into the shell structure of atoms and ions. Overall, the Thomas-Fermi-Dirac model with the correct boundary conditions provides a good global description of atoms and ions.« less

Identification of O-rich structures on platinum(111)-supported ultrathin iron oxide films

DOE PAGES

Merte, Lindsay R.; Bai, Yunhai; Zeuthen, Helene; ...

2016-01-06

Using high-resolution scanning tunneling microscopy (STM) we have studied the oxidation of ultrathin FeO films grown on Pt(111). At the initial stage of the FeO film oxidation by atomic oxygen exposure, we identified three distinct types of line defects, all of which form boundaries between FeO domains of opposite orientation. Two types of line defects appearing bright ( type-i) and dark ( type-ii) in the STM images at typical scanning parameters are “metallic”, whereas the third line defect exhibits nonmetallic behavior ( type-iii). Atomic-scale structure models of these line defects are proposed, with type-i defects exhibiting 4-fold coordinated Fe atoms,more » type-ii exhibiting 2-fold coordinated O atoms, and type-iii exhibiting tetrahedrally-coordinated Fe atoms. In addition, FeO 2 trilayer islands are formed upon oxidation, which appear at FCC-type domains of the moiré structure. At high scanning bias, distinct protrusions on the trilayer islands are observed over surface O ions, which are assigned to H adatoms. The experimental data are supported by density functional theory (DFT) calculations, in which bare and hydroxylated FeO 2 trilayer islands are compared. Finally, we compare the formation of O-rich features on continuous FeO films using atomic oxygen with the oxidation of Pt(111)-supported FeO islands accomplished by O 2 exposure.« less

Interaction between a Mg17Al12 precipitate and { 10\\bar 12} < 10\\bar 12 > twin boundary in magnesium alloys

NASA Astrophysics Data System (ADS)

Li, B.; Mathaudhu, S. N.

Interactions between Mg17Al12 precipitates and { 10\\bar 12} < 10\\bar 1\\bar 1 > twin boundaries (TBs) in magnesium were studied by molecular dynamics simulations. The results obtained agree well with experimental observations in which precipitates can be entirely engulfed by { 10\\bar 12} < 10\\bar 1\\bar 1 > twins without being sheared. Structural analysis of the TBs in the atomic scale shows that the TBs are extremely incoherent during twin growth and highly deviate from the { 10\\bar 12} twinning plane as previously observed in a number of experiments. The simulation studies indicate that { 10\\bar 12} < 10\\bar 1\\bar 1 > twinning was accomplished solely by atomic shuffling that converts the parent lattice to the twin lattice without involving twinning dislocations, resulting in zero shear strain at the TBs.

Pyramidal defects in highly Mg-doped GaN: atomic structure and influence on optoelectronic properties

NASA Astrophysics Data System (ADS)

Leroux, M.; Vennéguès, P.; Dalmasso, S.; de Mierry, P.; Lorenzini, P.; Damilano, B.; Beaumont, B.; Gibart, P.; Massies, J.

2004-07-01

A detailed transmission electron microscopy study is performed on the pyramidal inversion domains that appear in highly Mg-doped GaN grown by metalorganics vapor phase epitaxy or by the high-pressure, high-temperature method. From a comparison between high resolution images of the inversion domain boundaries and simulations using different atomic models, we conclude that both basal and inclined domain boundaries are likely formed of a monomolecular layer of the definite compound Mg{3}N{2}. We show that, due to their high concentration, the formation of these defects may account for auto-compensation in Mg-doped GaN. We also show that the local band bending induced by the polarity inversion due to these defects can be at the origin of the blue luminescence of highly Mg-doped GaN, always observed when nanometric pyramidal inversion domains are also present.

Atomic model of anti-phase boundaries in a face-centred icosahedral Zn Mg Dy quasicrystal

NASA Astrophysics Data System (ADS)

Wang, Jianbo; Yang, Wenge; Wang, Renhui

2003-03-01

An atomic model in the physical space for an anti-phase boundary (APB) in the ordered face-centred icosahedral Zn-Mg-Dy quasicrystal phase is presented, based on a six-dimensional model suggested by Ishimasa and Shimizu (2000 Mater. Sci. Eng. A 294-296 232, Ishimasa 2001 private communication). The physical space atomic positions of the defected structure were used for the calculation of the corresponding exit-plane wavefunction and high-resolution transmission electron microscopy images. The analysis of the defect by inverse Fourier transformation reveals that when superstructure reflection spots are used for back-transformation, then at the APB, bright lattice fringes are found to turn into dark ones, and vice versa. When fundamental reflections are used, the APB is not visible. This phenomenon is the same as the corresponding experimental study recently published by Heggen et al(2001a Phys. Rev. B 64 014202). Based on this atomic model it is found that the APB perpendicular to a fivefold axis A5 (APB-A5) is a non-conservative boundary, while the APB perpendicular to a pseudo-twofold axis A2P (APB-A2P) is a conservative one. This fact is consistent with the experimental observation (Heggen et al2002 J. Alloys Compounds 342 330) that the frequency of occurrence of APB-A5 is 90% in the heat-treated samples compared with that in the deformed samples (45%), while the frequency of occurrence of APB-A2P is 34% in the deformed samples compared with that in the heat-treated samples.

Direct imaging of thermally-activated grain-boundary diffusion in Cu/Co/IrMn/Pt exchange-bias structures using atom-probe tomography

NASA Astrophysics Data System (ADS)

Letellier, F.; Lechevallier, L.; Lardé, R.; Le Breton, J.-M.; Akmaldinov, K.; Auffret, S.; Dieny, B.; Baltz, V.

2014-11-01

Magnetic devices are often subject to thermal processing steps, such as field cooling to set exchange bias and annealing to crystallize amorphous magnetic electrodes. These processing steps may result in interdiffusion and the subsequent deterioration of magnetic properties. In this study, we investigated thermally-activated diffusion in Cu/Co/IrMn/Pt exchange biased polycrystalline thin-film structures using atom probe tomography. Images taken after annealing at 400 °C for 60 min revealed Mn diffusion into Co grains at the Co/IrMn interface and along Pt grain boundaries for the IrMn/Pt stack, i.e., a Harrison type C regime. Annealing at 500 °C showed further Mn diffusion into Co grains. At the IrMn/Pt interface, annealing at 500 °C led to a type B behavior since Mn diffusion was detected both along Pt grain boundaries and also into Pt grains. The deterioration of the films' exchange bias properties upon annealing was correlated to the observed diffusion. In particular, the topmost Pt capping layer thickness turned out to be crucial since a faster deterioration of the exchange bias properties for thicker caps was observed. This is consistent with the idea that Pt acts as a getter for Mn, drawing Mn out of the IrMn layer.

The Influence of Milling and Spark Plasma Sintering on the Microstructure and Properties of the Al7075 Alloy

PubMed Central

Málek, Přemysl; Minárik, Peter; Novák, Pavel; Průša, Filip

2018-01-01

The compact samples of an Al7075 alloy were prepared by a combination of gas atomization, high energy milling, and spark plasma sintering. The predominantly cellular morphology observed in gas atomized powder particles was completely changed by mechanical milling. The continuous-like intermetallic phases present along intercellular boundaries were destroyed; nevertheless, a small amount of Mg(Zn,Cu,Al)2 phase was observed also in the milled powder. Milling resulted in a severe plastic deformation of the material and led to a reduction of grain size from several µm into the nanocrystalline region. The combination of these microstructural characteristics resulted in abnormally high microhardness values exceeding 300 HV. Consolidation through spark plasma sintering (SPS) resulted in bulk samples with negligible porosity. The heat exposition during SPS led to precipitation of intermetallic phases from the non-equilibrium microstructure of both gas atomized and milled powders. SPS of the milled powder resulted in a recrystallization of the severely deformed structure. An ultra-fine grained structure (grain size close to 500 nm) with grains divided primarily by high-angle boundaries was formed. A simultaneous release of stored deformation energy and an increase in the grain size caused a drop of microhardness to values close to 150 HV. This value was retained even after annealing at 425 °C. PMID:29614046

Paving the way to nanoionics: atomic origin of barriers for ionic transport through interfaces.

PubMed

Frechero, M A; Rocci, M; Sánchez-Santolino, G; Kumar, Amit; Salafranca, J; Schmidt, Rainer; Díaz-Guillén, M R; Durá, O J; Rivera-Calzada, A; Mishra, R; Jesse, Stephen; Pantelides, S T; Kalinin, Sergei V; Varela, M; Pennycook, S J; Santamaria, J; Leon, C

2015-12-17

The blocking of ion transport at interfaces strongly limits the performance of electrochemical nanodevices for energy applications. The barrier is believed to arise from space-charge regions generated by mobile ions by analogy to semiconductor junctions. Here we show that something different is at play by studying ion transport in a bicrystal of yttria (9% mol) stabilized zirconia (YSZ), an emblematic oxide ion conductor. Aberration-corrected scanning transmission electron microscopy (STEM) provides structure and composition at atomic resolution, with the sensitivity to directly reveal the oxygen ion profile. We find that Y segregates to the grain boundary at Zr sites, together with a depletion of oxygen that is confined to a small length scale of around 0.5 nm. Contrary to the main thesis of the space-charge model, there exists no evidence of a long-range O vacancy depletion layer. Combining ion transport measurements across a single grain boundary by nanoscale electrochemical strain microscopy (ESM), broadband dielectric spectroscopy measurements, and density functional calculations, we show that grain-boundary-induced electronic states act as acceptors, resulting in a negatively charged core. Besides the possible effect of the modified chemical bonding, this negative charge gives rise to an additional barrier for ion transport at the grain boundary.

Paving the way to nanoionics: Atomic origin of barriers for ionic transport through interfaces

DOE PAGES

Frechero, M. A.; Rocci, M.; Sanchez-Santolino, G.; ...

2015-12-17

The blocking of ion transport at interfaces strongly limits the performance of electrochemical nanodevices for energy applications. The barrier is believed to arise from space-charge regions generated by mobile ions by analogy to semiconductor junctions. Here we show that something different is at play by studying ion transport in a bicrystal of yttria (9% mol) stabilized zirconia (YSZ), an emblematic oxide ion conductor. Aberration-corrected scanning transmission electron microscopy (STEM) provides structure and composition at atomic resolution, with the sensitivity to directly reveal the oxygen ion profile. We find that Y segregates to the grain boundary at Zr sites, together withmore » a depletion of oxygen that is confined to a small length scale of around 0.5 nm. Contrary to the main thesis of the space-charge model, there exists no evidence of a long-range O vacancy depletion layer. Combining ion transport measurements across a single grain boundary by nanoscale electrochemical strain microscopy (ESM), broadband dielectric spectroscopy measurements, and density functional calculations, we show that grain-boundary-induced electronic states act as acceptors, resulting in a negatively charged core. In conclusion, besides the possible effect of the modified chemical bonding, this negative charge gives rise to an additional barrier for ion transport at the grain boundary.« less

Two phase titanium aluminide alloy

DOEpatents

Deevi, Seetharama C.; Liu, C. T.

2001-01-01

A two-phase titanic aluminide alloy having a lamellar microstructure with little intercolony structures. The alloy can include fine particles such as boride particles at colony boundaries and/or grain boundary equiaxed structures. The alloy can include alloying additions such as .ltoreq.10 at % W, Nb and/or Mo. The alloy can be free of Cr, V, Mn, Cu and/or Ni and can include, in atomic %, 45 to 55% Ti, 40 to 50% Al, 1 to 5% Nb, 0.3 to 2% W, up to 1% Mo and 0.1 to 0.3% B. In weight %, the alloy can include 57 to 60% Ti, 30 to 32% Al, 4 to 9% Nb, up to 2% Mo, 2 to 8% W and 0.02 to 0.08% B.

Structure, chemistry, and stress corrosion cracking of grain boundaries in alloys 600 and 690

NASA Astrophysics Data System (ADS)

Stiller, Krystyna; Nilsson, Jan-Olof; Norring, Kjell

1996-02-01

The microstructure in six commercial batches of alloys 600 and 690 has been investigated using scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), atom probe field ion microscopy (APFIM), and secondary ion mass spectroscopy (SIMS). The materials were also tested with respect to their resistance to intergranular stress corrosion cracking (IGSCC) in high-purity water at 365 °. Applied microanalytical techniques allowed direct measurement of carbon concentration in the matrix together with determination of grain boundary micro structure and microchemistry in all material conditions. The distribution of oxygen near a crack in material tested with respect to IGSCC was also investigated. The role of carbon and chromium and intergranular precipitates on IGSCC is discussed.

Adsorption and dynamics of Si atoms at the monolayer Pb/Si(111) surface

NASA Astrophysics Data System (ADS)

Kumar, Rakesh; Fang, Chuang-Kai; Lee, Chih-Hao; Hwang, Ing-Shouh

2017-06-01

In this work, we studied the adsorption behavior of deposited Si atoms along with their diffusion and other dynamic processes on a Pb monolayer-covered Si(111) surface from 125 to 230 K using a variable-temperature scanning tunneling microscope. The Pb-covered Si(111) surface forms a low-symmetry rowlike (√{7 }×√{3 } ) structure in this temperature range and the Si atoms bind favorably to two specific on-top sites (T1 A and T1 B) on the trimer row after deposition at the sample temperature of ˜125 K . The Si atoms were immobile at low temperatures and started to switch between the two neighboring T1 A and T1 B sites within the same trimer when the temperature was raised to ˜150 K . When the temperature was raised above ˜160 K , the adsorbed Si atoms could hop to other trimers along the same trimer row. Below ˜170 K , short hops to adjacent trimers dominated, but long hops dominated at temperatures above ˜170 K . The activation energy and prefactor for the Si atoms diffusion were derived through analysis of continuous-time imaging at temperatures from 160 to 174 K. In addition, irreversible aggregation of single Si atoms into Si clusters started to occur at the phase boundaries or defective sites at temperatures above ˜170 K . At temperature above ˜180 K , nearly all Si atoms aggregated into clusters, which may have important implications for the atomic mechanism of epitaxial growth of Si on the Pb-covered Si(111) surface. In addition, our study provides strong evidence for breaking in the mirror symmetry in the (√{7 }×√{3 } )-Pb structure, which has implications for the atomic model of this controversial structure.

Crystal water dynamics of guanosine dihydrate: analysis of atomic displacement parameters, time profile of hydrogen-bonding probability, and translocation of water by MD simulation.

PubMed

Yoneda, Shigetaka; Sugawara, Yoko; Urabe, Hisako

2005-01-27

The dynamics of crystal water molecules of guanosine dihydrate are investigated in detail by molecular dynamics (MD) simulation. A 2 ns simulation is performed using a periodic boundary box composed of 4 x 5 x 8 crystallographic unit cells and using the particle-mesh Ewald method for calculation of electrostatic energy. The simulated average atomic positions and atomic displacement parameters are remarkably coincident with the experimental values determined by X-ray analysis, confirming the high accuracy of this simulation. The dynamics of crystal water are analyzed in terms of atomic displacement parameters, orientation vectors, order parameters, self-correlation functions of the orientation vectors, time profiles of hydrogen-bonding probability, and translocations. The simulation clarifies that the average structure is composed of various stable and transient structures of the molecules. The simulated guanosine crystal forms a layered structure, with four water sites per asymmetric unit, classified as either interlayer water or intralayer water. From a detailed analysis of the translocations of water molecules in the simulation, columns of intralayer water molecules along the c axis appear to represent a pathway for hydration and dehydration by a kind of molecular valve mechanism.

Thermal conductivity at the nanoscale: A molecular dynamics study

NASA Astrophysics Data System (ADS)

Lyver, John W., IV

With the growing use of nanotechnology and nanodevices in many fields of engineering and science, a need for understanding the thermal properties of such devices has increased. The ability for nanomaterials to conduct heat is highly dependent on the purity of the material, internal boundaries due to material changes and the structure of the material itself. Experimentally measuring the heat transport at the nanoscale is extremely difficult and can only be done as a macro output from the device. Computational methods such as various Monte Carlo (MC) and molecular dynamics (MD) techniques for studying the contribution of atomic vibrations associated with heat transport properties are very useful. The Green--Kubo method in conjunction with Fourier's law for calculating the thermal conductivity, kappa, has been used in this study and has shown promise as one approach well adapted for understanding nanosystems. Investigations were made of the thermal conductivity using noble gases, modeled with Lennard-Jones (LJ) interactions, in solid face-centered cubic (FCC) structures. MC and MD simulations were done to study homogeneous monatomic and binary materials as well as slabs of these materials possessing internal boundaries. Additionally, MD simulations were done on silicon carbide nanowires, nanotubes, and nanofilaments using a potential containing two-body and three-body terms. The results of the MC and MD simulations were matched against available experimental and other simulations and showed that both methods can accurately simulate real materials in a fraction of the time and effort. The results of the study show that in compositionally disordered materials the selection of atomic components by their mass, hard-core atomic diameter, well depth, and relative concentration can change the kappa by as much as an order of magnitude. It was found that a 60% increase in mass produces a 25% decrease in kappa. A 50% increase in interatomic strength produces a 25% increase in kappa, while as little as a 10% change in the hard core radius can almost totally suppress a materials ability to conduct heat. Additionally, for two LJ materials sharing an interface, the atomic vibrations altering the heat energy depend on the type of internal boundary in the material. Mass increases across the interfacial boundary enhance excitation of the very low frequency (ballistic) vibrational modes, while the opposite effect is seen as increases in hard core radius and interatomic strength enhance excitation of higher frequency vibrational modes. Additionally, it was found that this effect was diminished for higher temperatures around half the Debye temperatures. In nanodevices and nanomachines, there is an additional factor that degrades heat transport at the boundary. In fact, the interface induces a temperature jump consistent with a thermal resistance created by the boundary. It was found that the temperature jump, which is due to a boundary resistance, was significant in boundaries involving small mass changes, lesser in changes in hard core radii changes and even lesser for interatomic strength changes. The study of SiC nanowires and nanotubes showed that the structural changes produced vastly different kappa. The kappa in closely packed structures like nanowires and nanofilaments approximated that of the bulk SiC, yet were less sensitive to temperature than the 1/T relationship traditionally found in bulk systems. The more open nanostructures, like nanotubes, had vastly lower kappa values and are almost totally insensitive to temperature variation. The results of this study can be used in the design of nano-machines where heat generation and transport is a concern. Additionally, the design of nano-machines which transport heat like nano-refrigerators or nano-heaters may be possible due to a better selection of materials with the understanding of how the materials affect their nanothermal properties at the nano scale.

Role of Y-Al Oxides During Extended Recovery Process of a Ferritic ODS Alloy

NASA Astrophysics Data System (ADS)

Capdevila, C.; Pimentel, G.; Aranda, M. M.; Rementeria, R.; Dawson, K.; Urones-Garrote, E.; Tatlock, G. J.; Miller, M. K.

2015-08-01

The microstructural stability of Y-Al oxides during the recrystallization of Fe-Cr-Al oxide dispersion strengthened alloy is studied in this work. The goal is to determine the specific distribution pattern of oxides depending where they are located: in the matrix or at the grain boundaries. It was concluded that those located at the grain boundaries yielded a faster coarsening than the ones in the matrix, although no significant differences in composition and/or crystal structure were observed. However, the recrystallization heat treatment leads to the dissolution of the Y2O3 and its combination with Al to form the YAlO3 perovskite oxide particles process, mainly located at the grain boundaries. Finally, atom probe tomography analysis revealed a significant Ti build-up at the grain boundaries that might affect subsequent migration during recrystallization.

Interfaces in Heterogeneous Catalysts: Advancing Mechanistic Understanding through Atomic-Scale Measurements.

PubMed

Gao, Wenpei; Hood, Zachary D; Chi, Miaofang

2017-04-18

Developing novel catalysts with high efficiency and selectivity is critical for enabling future clean energy conversion technologies. Interfaces in catalyst systems have long been considered the most critical factor in controlling catalytic reaction mechanisms. Interfaces include not only the catalyst surface but also interfaces within catalyst particles and those formed by constructing heterogeneous catalysts. The atomic and electronic structures of catalytic surfaces govern the kinetics of binding and release of reactant molecules from surface atoms. Interfaces within catalysts are introduced to enhance the intrinsic activity and stability of the catalyst by tuning the surface atomic and chemical structures. Examples include interfaces between the core and shell, twin or domain boundaries, or phase boundaries within single catalyst particles. In supported catalyst nanoparticles (NPs), the interface between the metallic NP and support serves as a critical tuning factor for enhancing catalytic activity. Surface electronic structure can be indirectly tuned and catalytically active sites can be increased through the use of supporting oxides. Tuning interfaces in catalyst systems has been identified as an important strategy in the design of novel catalysts. However, the governing principle of how interfaces contribute to catalyst behavior, especially in terms of interactions with intermediates and their stability during electrochemical operation, are largely unknown. This is mainly due to the evolving nature of such interfaces. Small changes in the structural and chemical configuration of these interfaces may result in altering the catalytic performance. These interfacial arrangements evolve continuously during synthesis, processing, use, and even static operation. A technique that can probe the local atomic and electronic interfacial structures with high precision while monitoring the dynamic interfacial behavior in situ is essential for elucidating the role of interfaces and providing deeper insight for fine-tuning and optimizing catalyst properties. Scanning transmission electron microscopy (STEM) has long been a primary characterization technique used for studying nanomaterials because of its exceptional imaging resolution and simultaneous chemical analysis. Over the past decade, advances in STEM, that is, the commercialization of both aberration correctors and monochromators, have significantly improved the spatial and energy resolution. Imaging atomic structures with subangstrom resolution and identifying chemical species with single-atom sensitivity are now routine for STEM. These advancements have greatly benefitted catalytic research. For example, the roles of lattice strain and surface elemental distribution and their effect on catalytic stability and reactivity have been well documented in bimetallic catalysts. In addition, three-dimensional atomic structures revealed by STEM tomography have been integrated in theoretical modeling for predictive catalyst NP design. Recent developments in stable electronic and mechanical devices have opened opportunities to monitor the evolution of catalysts in operando under synthesis and reaction conditions; high-speed direct electron detectors have achieved sub-millisecond time resolutions and allow for rapid structural and chemical changes to be captured. Investigations of catalysts using these latest microscopy techniques have provided new insights into atomic-level catalytic mechanisms. Further integration of new microscopy methods is expected to provide multidimensional descriptions of interfaces under relevant synthesis and reaction conditions. In this Account, we discuss recent insights on understanding catalyst activity, selectivity, and stability using advanced STEM techniques, with an emphasis on how critical interfaces dictate the performance of precious metal-based heterogeneous catalysts. The role of extended interfacial structures, including those between core and shell, between separate phases and twinned grains, between the catalyst surface and gas, and between metal and support are discussed. We also provide an outlook on how emerging electron microscopy techniques, such as vibrational spectroscopy and electron ptychography, will impact future catalysis research.

Implementation of Shifted Periodic Boundary Conditions in the Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) Software

DTIC Science & Technology

2015-08-01

Atomic/Molecular Massively Parallel Simulator ( LAMMPS ) Software by N Scott Weingarten and James P Larentzos Approved for...Massively Parallel Simulator ( LAMMPS ) Software by N Scott Weingarten Weapons and Materials Research Directorate, ARL James P Larentzos Engility...Shifted Periodic Boundary Conditions in the Large-Scale Atomic/Molecular Massively Parallel Simulator ( LAMMPS ) Software 5a. CONTRACT NUMBER 5b

Multiscale modeling of thermal conductivity of high burnup structures in UO 2 fuels

DOE PAGES

Bai, Xian -Ming; Tonks, Michael R.; Zhang, Yongfeng; ...

2015-12-22

The high burnup structure forming at the rim region in UO 2 based nuclear fuel pellets has interesting physical properties such as improved thermal conductivity, even though it contains a high density of grain boundaries and micron-size gas bubbles. To understand this counterintuitive phenomenon, mesoscale heat conduction simulations with inputs from atomistic simulations and experiments were conducted to study the thermal conductivities of a small-grain high burnup microstructure and two large-grain unrestructured microstructures. We concluded that the phonon scattering effects caused by small point defects such as dispersed Xe atoms in the grain interior must be included in order tomore » correctly predict the thermal transport properties of these microstructures. In extreme cases, even a small concentration of dispersed Xe atoms such as 10 -5 can result in a lower thermal conductivity in the large-grain unrestructured microstructures than in the small-grain high burnup structure. The high-density grain boundaries in a high burnup structure act as defect sinks and can reduce the concentration of point defects in its grain interior and improve its thermal conductivity in comparison with its large-grain counterparts. Furthermore, an analytical model was developed to describe the thermal conductivity at different concentrations of dispersed Xe, bubble porosities, and grain sizes. Upon calibration, the model is robust and agrees well with independent heat conduction modeling over a wide range of microstructural parameters.« less

An assessment of memristor intrinsic fluctuations: a measurement of single atomic motion

NASA Astrophysics Data System (ADS)

Borghetti, Julien; Yang, J. Joshua; Medeiros-Ribeiro, Gilberto; Williams, R. Stanley

2010-03-01

Memristors provides electrically tunable resistance for upcoming non-volatile memory and future neuromorphic computing. One of the key benefits of such a device is its scalability, which can be demonstrated from an architectural perspective as well as from a fundamental physics limit. 4D addressing schemes utilizing cross bar structures that can be stacked several layers high above the chip embodies unlimited addressing space. On the other limit, the basic operating principles of memristive devices allow one to reach storage of information in a single atom. In this report of nanoscale (sub 50nm) devices, we detect single atom fluctuations, which would then represent the ultimate limit for noise sources thus delineating the boundary conditions for circuit design. We show that electrically induced individual atom migrations do not affect the overall device atomic configuration until a critical bias where a single local fluctuation triggers a general atomic reconfiguration. This instability illustrates the robustness of the device non-volatility upon small electrical stress.

Parametrization of Combined Quantum Mechanical and Molecular Mechanical Methods: Bond-Tuned Link Atoms.

PubMed

Wu, Xin-Ping; Gagliardi, Laura; Truhlar, Donald G

2018-05-30

Combined quantum mechanical and molecular mechanical (QM/MM) methods are the most powerful available methods for high-level treatments of subsystems of very large systems. The treatment of the QM-MM boundary strongly affects the accuracy of QM/MM calculations. For QM/MM calculations having covalent bonds cut by the QM-MM boundary, it has been proposed previously to use a scheme with system-specific tuned fluorine link atoms. Here, we propose a broadly parametrized scheme where the parameters of the tuned F link atoms depend only on the type of bond being cut. In the proposed new scheme, the F link atom is tuned for systems with a certain type of cut bond at the QM-MM boundary instead of for a specific target system, and the resulting link atoms are call bond-tuned link atoms. In principle, the bond-tuned link atoms can be as convenient as the popular H link atoms, and they are especially well adapted for high-throughput and accurate QM/MM calculations. Here, we present the parameters for several kinds of cut bonds along with a set of validation calculations that confirm that the proposed bond-tuned link-atom scheme can be as accurate as the system-specific tuned F link-atom scheme.

Thermodynamics, Diffusion, and Structure of Mg2SiO4 forsterite grain boundaries from atomistic modeling

NASA Astrophysics Data System (ADS)

Adjaoud, O.; Marquardt, K.; Jahn, S.

2011-12-01

Most materials are not single crystals but consist of crystalline grains of various sizes, misorientated with respect to each other and joint by grain boundaries. The latter influence many of the material properties. For instance, grain boundaries are short circuits for diffusion and thus they strongly influence transport properties of materials such as electrical conductivity, or mineral growth rates, creep, or phase transform. Olivine is a major component of the Earth's upper mantle and therefore it is of considerable importance to study its physical and thermodynamic polycrystalline properties. In the present study, we have used molecular dynamics simulations to model thermodynamics, self-diffusion and structure of a series of [100] symmetric tilt grain boundaries in forsterite. The interactions between the atoms are modeled by an advanced ionic interaction potential (Jahn and Madden, 2007). The parameters of the potential are fitted to ab initio results. The model was optimized for the Ca-Mg-Al-Si-O system and shows good transferability in a wide range of pressures, temperatures, and compositions. Thermodynamics and structure were simulated at ambient conditions, and self-diffusion coefficients were determined at ambient pressure and temperatures of 1250, 1500, 1750, and 2000 K. We find that the energy and excess free volume of the grain boundaries in forsterite depend significantly on the misorientation angle of the grain boundary. One of our modeled structures is compared with an high-resolution transmission electron micrograph (HRTEM) (Heinemann et al., 2005). We relate our findings to previous studies of grain boundaries in ionic materials and in metals. For small misorientation angles (up to 22.1°), grain boundary structures consist of an array of c-edge dislocations as suggested by Heinemann et al. (2005) and their energies can be readily fit with the Read-Shockley dislocation model for grain boundaries. For high misorientation angles (32.1° and 60.8°), the cores of dislocations overlap and form repeated structural units. Similar to energies and excess free volumes, the self-diffusion coefficients of Mg and O depend significantly on the misorientation angle of the grain boundaries and they are well fitted with Arrhenius law. We compare our results to MgO grain boundary diffusion in forsterite derived from reaction rim growth experiments (Gardés and Heinrich, 2010).

Defects in oxide surfaces studied by atomic force and scanning tunneling microscopy

PubMed Central

König, Thomas; Simon, Georg H; Heinke, Lars; Lichtenstein, Leonid

2011-01-01

Summary Surfaces of thin oxide films were investigated by means of a dual mode NC-AFM/STM. Apart from imaging the surface termination by NC-AFM with atomic resolution, point defects in magnesium oxide on Ag(001) and line defects in aluminum oxide on NiAl(110), respectively, were thoroughly studied. The contact potential was determined by Kelvin probe force microscopy (KPFM) and the electronic structure by scanning tunneling spectroscopy (STS). On magnesium oxide, different color centers, i.e., F0, F+, F2+ and divacancies, have different effects on the contact potential. These differences enabled classification and unambiguous differentiation by KPFM. True atomic resolution shows the topography at line defects in aluminum oxide. At these domain boundaries, STS and KPFM verify F2+-like centers, which have been predicted by density functional theory calculations. Thus, by determining the contact potential and the electronic structure with a spatial resolution in the nanometer range, NC-AFM and STM can be successfully applied on thin oxide films beyond imaging the topography of the surface atoms. PMID:21977410

Segregation and Migration of the Oxygen Vacancies in the 3 (111) Tilt Grain Boundaries of Ceria

DOE PAGES

Yuan, Fenglin; Liu, Bin; Zhang, Yanwen; ...

2016-03-01

In nanocrystalline materials, defect-grain boundary (GB) interaction plays a key role in determining the structure stability, as well as size-dependent ionic, electronic, magnetic and chemical properties. In this study, we systematically investigated using density functional theory segregation and migration of oxygen vacancies at the Σ3 [110] / (111) grain boundary of ceria. Three oxygen layers near the GB are predicted to be segregation sites for oxygen vacancies. Moreover, the presence of oxygen vacancies stabilizes this tilt GB at a low Fermi level and/or oxygen poor conditions. An atomic strain model was proposed to rationalize layer dependency of the relaxation energymore » for +2 charged oxygen vacancy. The structural origin of large relaxation energies at layers 1 and 2 was determined to be free-volume space that induces ion relaxation towards the GB. Our results not only pave the way for improving the oxygen transport near GBs of ceria, but also provide important insights into engineering the GB structure for better ionic, magnetic and chemical properties of nanocrystalline ceria.« less

Microstructural characterization of a Zr-Ti-Ni-Mn-V-Cr based AB 2-type battery alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Zhan

1999-01-01

Transmission Electron Microscopy (TEM), combined with X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) was employed to investigate a proprietary and multicomponent AB 2 type Nickel-Metal Hydride (Ni-MH) battery alloy. This material was prepared by High Pressure Gas Atomization (HPGA) and examined in both the as-atomized and heat treated condition. TEM examination showed a heavily faulted dendritic growth structure in as-atomized powder. Selected Area Diffraction (SAD) showed that this region consisted of both a cubic C15 structure with lattice constant a=7.03 and a hexagonal C14 structure with lattice parameter a=4.97 Å, c=8.11 Å. The Orientation Relationship (OR) between the C14 and C15 structures was determined to be (111)[1more » $$\\bar{1}$$0] C15//(0001)[11$$\\bar{2}$$0] C14. An interdendritic phase possessing the C14 structure was also seen. There was also a very fine grain region consisting of the C14 structure. Upon heat treatment, the faulted structure became more defined and appeared as intercalation layers within the grains. Spherical particles rich in Zr and Ni appeared scattered at the grain boundaries instead of the C14 interdendritic phase. The polycrystalline region also changed to a mixture of C14 and C15 structures. These results as well as phase stability of the C15 and C14 structures based on a consideration of atomic size factor and the average electron concentration are discussed.« less

Three-dimensional evaluation of gettering ability for oxygen atoms at small-angle tilt boundaries in Czochralski-grown silicon crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ohno, Yutaka, E-mail: yutakaohno@imr.tohoku.ac.jp; Inoue, Kaihei; Fujiwara, Kozo

2015-06-22

Three-dimensional distribution of oxygen atoms at small-angle tilt boundaries (SATBs) in Czochralski-grown p-type silicon ingots was investigated by atom probe tomography combined with transmission electron microscopy. Oxygen gettering along edge dislocations composing SATBs, post crystal growth, was observed. The gettering ability of SATBs would depend both on the dislocation strain and on the dislocation density. Oxygen atoms would agglomerate in the atomic sites under the tensile hydrostatic stress larger than about 2.0 GPa induced by the dislocations. It was suggested that the density of the atomic sites, depending on the tilt angle of SATBs, determined the gettering ability of SATBs.

Grain boundaries at the surface of consolidated MgO nanocrystals and acid-base functionality.

PubMed

Vingurt, Dima; Fuks, David; Landau, Miron V; Vidruk, Roxana; Herskowitz, Moti

2013-09-21

The increase of the surface basicity-acidity of MgO material by factors of 1.8-3.0 due to consolidation of its nanocrystals was demonstrated by the indicator titration. It was shown that the parallel increase of surface acidity and basicity is attributed to the formation of grain boundaries (GB) after MgO aerogel densification. A simple model predicting the increase of surface acidity-basicity of MgO that correlates with the results of direct measurements was proposed. The model is based on the study of the fine atomic structure at GB surface areas in consolidated MgO nanocrystals in the framework of Density Functional Theory. It is found that the displacements of coordinatively unsaturated surface ions near the GB are significant at the distances ~3-4 atomic layers from the geometrical contact plane between nanocrystals. The detailed analysis of atomic positions inside GB demonstrated the coordination deficiency of surface atoms at the GB areas leading to the formation of stretched bonds and to creation of low coordinated surface ions due to splitting of coordination numbers of surface atoms belonging to GB areas. Density of states for electrons shows the existence of additional states in the band gap close to the bottom of the conduction band. The adsorption energy of CO2 molecules atop oxygen atoms exposed at surface GB areas is of the same order of magnitude as that reported for oxygen atoms at crystallographic edges and corners of MgO crystals. It provides additional options for bonding of molecules at the surface of nanocrystalline MgO increasing the adsorption capacity and catalytic activity.

Effects of dopant induced defects on structural, multiferroic and optical properties of Bi1-x Pb x FeO3 (0 ≤ x ≤ 0.3) ceramics

NASA Astrophysics Data System (ADS)

Hassnain Jaffari, G.; Aftab, M.; Samad, Abdus; Mumtaz, Fiza; Awan, M. S.; Shah, S. Ismat

2018-01-01

Bi1-x Pb x FeO3 (0 ≤ x ≤ 0.3) has been characterized in detail with an aim to identify role of defect such as dopant, various vacancies, grain boundaries etc, and their effect on structural, optical and multiferroic properties. Structural analysis revealed that Pb substitution transforms the rhombohedral phase of BiFeO3 to the pseudocubic phase for x ≥ 0.15, consistently all vibrational Raman modes associated with the rhombohedral phase are found disappeared. Optical response revealed weakening of the d-d transitions with Pb addition indicating change in the Fe atoms environment consistent with the transition from non-centrosymmetric to the centrosymmetric structure. Transport and dielectric responses are explained in terms of hopping due to the presence of defects like oxygen vacancies and grain boundary conduction. In the high temperature regime, grain boundary conduction led to decrease in resistivity with the presence of a hump that is associated with hopping conduction. Extrinsic contributions in the transport properties correlate well with dielectric response. Magnetic and ferroelectric responses are also presented where role of oxygen vacancies defects has been clearly identified.

Au-induced deep groove nanowire structure on the Ge(001) surface: DFT calculations

NASA Astrophysics Data System (ADS)

Tsay, Shiow-Fon

2016-09-01

The atomic geometry, stability, and electronic properties of self-organized Au induced nanowires on the Ge(001) surface are investigated based on the density-functional theory in GGA and the stoichiometry of Au. A giant Ge zigzag chain structure is suggested for 0.75 ML Au coverage, which displays c(8 × 2) deep groove zigzag nanowire structure simulated STM images. The top layer Ge and Au atomic disorder introduces the chevron units into the zigzag nanowire structure STM image as per the experimental observations. The zigzag Ge nanowire exhibits a semi-metallic characteristic, and the electric transport occurs in between the Ge zigzag nanowire and the subsurface. The system exhibits obvious electronic correlations among the Ge nanowire, the nano-facet Au trimers and the deeper layer Ge atoms, that play an important role in the electronic structure. At surface Brillouin zone boundaries, an anisotropic two-dimensional upward parabolic surface-state band is consistent with the ARPES spectra reported by Nakatsuji et al. [Phys. Rev. B 80, 081406(R) (2009); Phys. Rev. B 84, 115411 (2011)]; this electronic structure is different from the quasi-one-dimensional energy trough reported by Schäfer et al. [Phys. Rev. Lett. 101, 236802 (2008); Phys. Rev. B 83, 121411(R) (2011)].

High-temperature studies of grain boundaries in ultrafine grained alloys by means of positron lifetime

NASA Astrophysics Data System (ADS)

Würschum, R.; Shapiro, E.; Dittmar, R.; Schaefer, H.-E.

2000-11-01

Atomic free volumes and vacancies in the ultrafine grained alloys Pd84Zr16, Cu 0.1 wt % ZrO2, and Fe91Zr9 were studied by means of positron lifetime. The thermally stable microstructures serve as a novel type of model system for studying positron trapping and annihilation as well as the thermal behavior of vacancy-sized free volumes over a wide temperature range up to ca. 1200 K by making use of a metallic 58Co positron source. In ultrafine grained Cu the thermal formation of lattice vacancies could be observed. In Pd84Zr16 an increase of the specific positron trapping rate of nanovoids and, in addition, detrapping of positrons from free volumes with a mean size slightly smaller than one missing atom in the grain boundaries contributes to a reversible increase of the positron lifetime of more than 60 ps with measuring temperature. In Fe91Zr9 similar linear high-temperature increases of the positron lifetime are observed in the nanocrystalline and the amorphous state. The question of thermal vacancy formation in grain boundaries is addressed taking into account the different types of interface structures of the present alloys.

Investigation of MHD flow structure and fluctuations by potassium lineshape fitting

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bauman, L.E.

1993-12-31

Multiple Potassium D-line emission absorption spectra from a high temperature, coal-fired flow have been fit to a radiative transfer, boundary layer flow model. The results of fitting spectra from the aerodynamic duct of the Department of Energy Coal-Fired Flow Facility provide information about the thickness and shape of the thermal boundary layer and the bulk potassium seed atom density in a simulated magnetohydrodynamic channel flow. Probability distribution functions for the entire set of more than six thousand spectra clearly indicate the typical values and magnitude of fluctuations for the flow: core temperature of 2538 {plus_minus} 20 K, near wall temperaturemore » of 1945 {plus_minus} 135 K, boundary layer width of about 1 cm, and potassium seed atom density of (5.1 {plus_minus} 0.8)x 10{sup 22}/m{sup 3}. Probability distribution functions for selected times during the eight hours of measurements indicate occasional periods of unstable combustion. In addition, broadband particle parameters during the unstable start of the test may be related to differing particle and gas temperatures. The results clearly demonstrate the ability of lineshape fitting to provide valuable data for diagnosing the high speed turbulent flow.« less

Surface diffusivity measurements on 8mol.% yttrium oxide-zirconia bicrystals via grain boundary grooving experiments

NASA Astrophysics Data System (ADS)

Nassirou, Maissarath

Thermal grooving at grain boundaries (GBs) is a capillary-driven evolution of surface topography in the region where the grain boundary emerges at a free surface. The study of these topographic changes can provide insight into surface energetics, and in our particular case, the measurement of surface diffusivity. We have measured the surface diffusion coefficient of 8mol% Y 2O3-ZrO2 by studying the formation of thermal grooves. We studied a total of five bicrystals, with well defined orientation relationships; random [110] -60°, random [001] -30°, Sigma13 [001]/{510}, Sigma13 [001]/{320}, Sigma5 [001]/{210}. Our calculations employed the Herring relation (1951), in which the variation in the chemical potential is related to changes in topography. The samples were annealed at 1300°C and 1400°C for various period of time. Atomic Force Microscopy was used to determine the exact geometry of the thermal grooves. A first approach consisted of estimating the diffusion coefficient by using Mullins' equation. yx=0= dsDs1/ 4gb2g s12G 5/4( WkTgs) 1/4t 1/4 Where y(x =0) is the groove depth at the GB triple junction, O is the atomic volume, gs is the surface tension, gb is the grain boundary surface energy, ds is the thickness of the diffusion layer, t is the annealing time, and Ds is the surface diffusion coefficient. In Mullins' derivation, the atomic structure of the surface was ignored and it was assumed that the surface energy is independent of crystallographic orientation. In the case of zirconia, the surface energy is anisotropic. We will describe in this work a new approach to measuring surface diffusivity which accounts for the surface energy anisotropy. The study of these bicrystals will emphasize the effect of grain boundary structure on the surface diffusion coefficient, and it is for that purpose that we selected bicrystals with different tilt axes and angles. The results obtained using the equation set we have developed will be compared to those obtained by Mullins, and we show that the anisotropic groove evolution, even when perfectly symmetrical, is much slower than the corresponding isotropic case.

Role of Y-Al oxides during extended recovery process of a ferritic ODS alloy

DOE PAGES

Capdevila, C.; Pimentel, G.; Aranda, M. M.; ...

2015-08-04

The microstructural stability of Y-Al oxides during the recrystallization of Fe-Cr-Al oxide dispersion strengthened alloy is studied in this work. The goal is to determine the specific distribution pattern of oxides depending where they are located: in the matrix or at the grain boundaries. It was concluded that those located at the grain boundaries yielded a faster coarsening than the ones in the matrix, although no significant differences in composition and/or crystal structure were observed. However, the recrystallization heat treatment leads to the dissolution of the Y 2O 3 and its combination with Al to form the YAlO 3 perovskitemore » oxide particles process, mainly located at the grain boundaries. Lastly, atom probe tomography analysis revealed a significant Ti build-up at the grain boundaries that might affect subsequent migration during recrystallization.« less

A molecular dynamics study of the effect of thermal boundary conductance on thermal transport of ideal crystal of n-alkanes with different number of carbon atoms

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rastgarkafshgarkolaei, Rouzbeh; Zeng, Yi; Khodadadi, J. M., E-mail: khodajm@auburn.edu

2016-05-28

Phase change materials such as n-alkanes that exhibit desirable characteristics such as high latent heat, chemical stability, and negligible supercooling are widely used in thermal energy storage applications. However, n-alkanes have the drawback of low thermal conductivity values. The low thermal conductivity of n-alkanes is linked to formation of randomly oriented nano-domains of molecules in their solid structure that is responsible for excessive phonon scattering at the grain boundaries. Thus, understanding the thermal boundary conductance at the grain boundaries can be crucial for improving the effectiveness of thermal storage systems. The concept of the ideal crystal is proposed in thismore » paper, which describes a simplified model such that all the nano-domains of long-chain n-alkanes are artificially aligned perfectly in one direction. In order to study thermal transport of the ideal crystal of long-chain n-alkanes, four (4) systems (C{sub 20}H{sub 42}, C{sub 24}H{sub 50}, C{sub 26}H{sub 54}, and C{sub 30}H{sub 62}) are investigated by the molecular dynamics simulations. Thermal boundary conductance between the layers of ideal crystals is determined using both non-equilibrium molecular dynamics (NEMD) and equilibrium molecular dynamics (EMD) simulations. Both NEMD and EMD simulations exhibit no significant change in thermal conductance with the molecular length. However, the values obtained from the EMD simulations are less than the values from NEMD simulations with the ratio being nearly three (3) in most cases. This difference is due to the nature of EMD simulations where all the phonons are assumed to be in equilibrium at the interface. Thermal conductivity of the n-alkanes in three structures including liquid, solid, and ideal crystal is investigated utilizing NEMD simulations. Our results exhibit a very slight rise in thermal conductivity values as the number of carbon atoms of the chain increases. The key understanding is that thermal transport can be significantly altered by how the molecules and the nano-domains are oriented in the structure rather than by the length of the n-alkane molecules.« less

Molecular Modeling for Calculation of Mechanical Properties of Epoxies with Moisture Ingress

NASA Technical Reports Server (NTRS)

Clancy, Thomas C.; Frankland, Sarah J.; Hinkley, J. A.; Gates, T. S.

2009-01-01

Atomistic models of epoxy structures were built in order to assess the effect of crosslink degree, moisture content and temperature on the calculated properties of a typical representative generic epoxy. Each atomistic model had approximately 7000 atoms and was contained within a periodic boundary condition cell with edge lengths of about 4 nm. Four atomistic models were built with a range of crosslink degree and moisture content. Each of these structures was simulated at three temperatures: 300 K, 350 K, and 400 K. Elastic constants were calculated for these structures by monitoring the stress tensor as a function of applied strain deformations to the periodic boundary conditions. The mechanical properties showed reasonably consistent behavior with respect to these parameters. The moduli decreased with decreasing crosslink degree with increasing temperature. The moduli generally decreased with increasing moisture content, although this effect was not as consistent as that seen for temperature and crosslink degree.

Enhanced magnetic and thermoelectric properties in epitaxial polycrystalline SrRuO3 thin films.

PubMed

Woo, Sungmin; Lee, Sang A; Mun, Hyeona; Choi, Young Gwan; Zhung, Chan June; Shin, Soohyeon; Lacotte, Morgane; David, Adrian; Prellier, Wilfrid; Park, Tuson; Kang, Won Nam; Lee, Jong Seok; Kim, Sung Wng; Choi, Woo Seok

2018-03-01

Transition metal oxide thin films show versatile electric, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO 3 thin films using pulsed laser epitaxy. Using the epitaxial stabilization technique with an atomically flat polycrystalline SrTiO 3 substrate, an epitaxial polycrystalline SrRuO 3 thin film with the crystalline quality of each grain comparable to that of its single-crystalline counterpart is realized. In particular, alleviated compressive strain near the grain boundaries due to coalescence is evidenced structurally, which induced the enhancement of ferromagnetic ordering of the polycrystalline epitaxial thin film. The structural variations associated with the grain boundaries further reduce the thermal conductivity without deteriorating the electronic transport, and lead to an enhanced thermoelectric efficiency in the epitaxial polycrystalline thin films, compared with their single-crystalline counterpart.

Surface structure of MgO underlayer with Ti diffusion for (002) oriented L10 FePt-based heat assisted magnetic recording media

NASA Astrophysics Data System (ADS)

Hinata, Sintaro; Jo, Shin; Saito, Shin

2018-05-01

Surface morphology of the MgO layer and magnetic properties of FePt-C layer deposited on the MgO were investigated for the FePt-based heat assisted magnetic recording media. Stacking structure of the underlayer for the FePt-C layer was MgO (0-5 nm)/Cr80Mn20 (0-30 nm)/Cr50Ti50 (0-50 nm)/glass sub.. Surface observation result for the MgO film by using an atomic force microscope revealed the existence of nodules with a height of about 2 nm and a network-like convex structure with a height difference of about sub nm (boundary wall, BW) on the MgO crystal grain boundary. Density of the nodules largely depends on the surface roughness of the CrTi layer, RaCrTi and it is suppressed from 10 to 2/0.5 μm2 by reducing RaCrTi from 420 to 260 pm. Height of the BW depends on thickness of the MgO layer, tMgO and it can be suppressed by reducing tMgO to less than 4 nm. From the cross-sectional energy dispersive x-ray mapping, it is clarified that the BW is formed by atomic diffusion of Ti atoms from CrTi layer due to the substrate heating process, and a compound consists of Mg, Ti and O atoms. This BW can be used as a template to magnetically isolate the FePt column in the FePt-based granular film, such as FePt-SiO2, if the size of the BW is reduced to less than 10 nm. M-H loop of the FePt-C granular film deposited on the underlayer showed that the nodule and BW induce oxidation of the FePt grains, and reduction of intergranular exchange coupling.

Heat-transport mechanisms in molecular building blocks of inorganic/organic hybrid superlattices

NASA Astrophysics Data System (ADS)

Giri, Ashutosh; Niemelä, Janne-Petteri; Tynell, Tommi; Gaskins, John T.; Donovan, Brian F.; Karppinen, Maarit; Hopkins, Patrick E.

2016-03-01

Nanomaterial interfaces and concomitant thermal resistances are generally considered as atomic-scale planes that scatter the fundamental energy carriers. Given that the nanoscale structural and chemical properties of solid interfaces can strongly influence this thermal boundary conductance, the ballistic and diffusive nature of phonon transport along with the corresponding phonon wavelengths can affect how energy is scattered and transmitted across an interfacial region between two materials. In hybrid composites composed of atomic layer building blocks of inorganic and organic constituents, the varying interaction between the phononic spectrum in the inorganic crystals and vibronic modes in the molecular films can provide a new avenue to manipulate the energy exchange between the fundamental vibrational energy carriers across interfaces. Here, we systematically study the heat transfer mechanisms in hybrid superlattices of atomic- and molecular-layer-grown zinc oxide and hydroquinone with varying thicknesses of the inorganic and organic layers in the superlattices. We demonstrate ballistic energy transfer of phonons in the zinc oxide that is limited by scattering at the zinc oxide/hydroquinone interface for superlattices with a single monolayer of hydroquinone separating the thicker inorganic layers. The concomitant thermal boundary conductance across the zinc oxide interfacial region approaches the maximal thermal boundary conductance of a zinc oxide phonon flux, indicative of the contribution of long wavelength vibrations across the aromatic molecular monolayers in transmitting energy across the interface. This transmission of energy across the molecular interface decreases considerably as the thickness of the organic layers are increased.

Electronic structure, charge transfer, and intrinsic luminescence of gadolinium oxide nanoparticles: Experiment and theory

NASA Astrophysics Data System (ADS)

Zatsepin, D. A.; Boukhvalov, D. W.; Zatsepin, A. F.; Kuznetsova, Yu. A.; Mashkovtsev, M. A.; Rychkov, V. N.; Shur, V. Ya.; Esin, A. A.; Kurmaev, E. Z.

2018-04-01

The cubic (c) and monoclinic (m) polymorphs of Gd2O3 were studied using the combined analysis of several materials science techniques - X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. Density functional theory (DFT) based calculations for the samples under study were performed as well. The cubic phase of gadolinium oxide (c-Gd2O3) synthesized using a precipitation method exhibits spheroidal-like nanoclusters with well-defined edges assembled from primary nanoparticles with an average size of 50 nm, whereas the monoclinic phase of gadolinium oxide (m-Gd2O3) deposited using explosive pyrolysis has a denser structure compared with natural gadolinia. This phase also has a structure composed of three-dimensional complex agglomerates without clear-edged boundaries that are ∼21 nm in size plus a cubic phase admixture of only 2 at.% composed of primary edge-boundary nanoparticles ∼15 nm in size. These atomic features appear in the electronic structure as different defects ([Gd…Osbnd OH] and [Gd…Osbnd O]) and have dissimilar contributions to the charge-transfer processes among the appropriate electronic states with ambiguous contributions in the Gd 5р - O 2s core-like levels in the valence band structures. The origin of [Gd…Osbnd OH] defects found by XPS was well-supported by PL analysis. The electronic and atomic structures of the synthesized gadolinias calculated using DFT were compared and discussed on the basis of the well-known joint OKT-van der Laan model, and good agreement was established.

Local Structure of Sb in Cretaceous-Tertiary Boundary Clays from Stevns Klint By the XAFS Method

NASA Astrophysics Data System (ADS)

Hongu, H.; Yoshiasa, A.; Tobase, T.; Hiratoko, T.; Isobe, H.; Arima, H.; Sugiyama, K.; Okube, M.

2014-12-01

The Cretaceous-Tertiary (K-T) mass extinctions has been thought to be due to the asteroid impact since Ir anomalies was found by Alvarez et al. (1980) . The boundary clay is also enriched in Cr, Co, Ni, Cu, Zn, As and Sb. Especially concentrations of Sb and As are unusually large. However, the origins and concentration processes of Sb are unknown. In this study, local structure around antimony atoms in K-T boundary clay from Stevns Klint, Denmark, was determined by Sb K-edge XAFS spectroscopy. The XAFS analyses give the information about the chemical state and coordination environment around Sb atoms and help identify of the concentration phase, and also may provide various kinds of information about the asteroid impact and mass extinction. The XAFS measurements were performed at the BL-NW10A beamline at the Photon Factory in KEK, Tsukuba, Japan. The XANES spectra and radial structure function (RSF) showed that Sb in K-T boundary clays is high oxidation state Sb5+ and occupies a SbO6 octahedral site. The Sb-O interatomic distance in K-T clay sample is 2.08(1) A. It is known that Sb5+ is stable form in soil and soil water under an equilibrium situation within the Earth's surface environment. Antimony belongs to group 15 in the periodic table below arsenic, and the chemical behavior of Sb5+ is similar to that of As5+. Because there is a close correlation on co-precipitation between As and Fe (Ebihara and Miura, 1996; Sakai et al., 2007) , it is considered that Sb also correlates closely with Fe compounds (e.g., ferric hydroxides). Abundant ferric hydroxides occur in K-T boundary clays. It is considered that one of the reasons of abnormal high concentrations of Sb and As in K-T boundary clays is a lot of dust from impact ejecta falls with iron ions and deposits on surface of the Earth for a short period of time after the asteroid impact. ReferencesL. W. Alvarez, Science, 208, 1095-1108 (1980) M. Ebihara and T. Miura, Geochimica et Cosmochimica Acta, 60, 5133-5144 (1996) S. Sakai et al., The American Institute of Physics, Conference Proceeding, 882, 274-276 (2007)

System for conversion between the boundary representation model and a constructive solid geometry model of an object

DOEpatents

Christensen, Noel C.; Emery, James D.; Smith, Maurice L.

1988-04-05

A system converts from the boundary representation of an object to the constructive solid geometry representation thereof. The system converts the boundary representation of the object into elemental atomic geometrical units or I-bodies which are in the shape of stock primitives or regularized intersections of stock primitives. These elemental atomic geometrical units are then represented in symbolic form. The symbolic representations of the elemental atomic geometrical units are then assembled heuristically to form a constructive solid geometry representation of the object usable for manufacturing thereof. Artificial intelligence is used to determine the best constructive solid geometry representation from the boundary representation of the object. Heuristic criteria are adapted to the manufacturing environment for which the device is to be utilized. The surface finish, tolerance, and other information associated with each surface of the boundary representation of the object are mapped onto the constructive solid geometry representation of the object to produce an enhanced solid geometry representation, particularly useful for computer-aided manufacture of the object.

Atom probe tomography analysis of high dose MA957 at selected irradiation temperatures

NASA Astrophysics Data System (ADS)

Bailey, Nathan A.; Stergar, Erich; Toloczko, Mychailo; Hosemann, Peter

2015-04-01

Oxide dispersion strengthened (ODS) alloys are meritable structural materials for nuclear reactor systems due to the exemplary resistance to radiation damage and high temperature creep. Summarized in this work are atom probe tomography (APT) investigations on a heat of MA957 that underwent irradiation in the form of in-reactor creep specimens in the Fast Flux Test Facility-Materials Open Test Assembly (FFTF-MOTA) for the Liquid Metal Fast Breeder Reactor (LMFBR) program. The oxide precipitates appear stable under irradiation at elevated temperature over extended periods of time. Nominally, the precipitate chemistry is unchanged by the accumulated dose; although, evidence suggests that ballistic dissolution and reformation processes are occurring at all irradiation temperatures. At 412 °C-109 dpa, chromium enrichments - consistent with the α‧ phase - appear between the oxide precipitates, indicating radiation induced segregation. Grain boundaries, enriched with several elements including nickel and titanium, are observed at all irradiation conditions. At 412 °C-109 dpa, the grain boundaries are also enriched in molecular titanium oxide (TiO).

Local rectification of heat flux

NASA Astrophysics Data System (ADS)

Pons, M.; Cui, Y. Y.; Ruschhaupt, A.; Simón, M. A.; Muga, J. G.

2017-09-01

We present a chain-of-atoms model where heat is rectified, with different fluxes from the hot to the cold baths located at the chain boundaries when the temperature bias is reversed. The chain is homogeneous except for boundary effects and a local modification of the interactions at one site, the “impurity”. The rectification mechanism is due here to the localized impurity, the only asymmetrical element of the structure, apart from the externally imposed temperature bias, and does not rely on putting in contact different materials or other known mechanisms such as grading or long-range interactions. The effect survives if all interaction forces are linear except the ones for the impurity.

The generation and accumulation of interstitial atoms and vacancies in alloys with L1{sub 2} superstructure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pantyukhova, Olga, E-mail: Pantyukhova@list.ru; Starenchenko, Vladimir, E-mail: star@tsuab.ru; Starenchenko, Svetlana, E-mail: sve-starenchenko@yandex.ru

2016-01-15

The dependences of the point defect concentration (interstitial atoms and vacancies) on the deformation degree were calculated for the L1{sub 2} alloys with the high and low antiphase boundaries (APB) energy in terms of the mathematical model of the work and thermal strengthening of the alloys with the L1{sub 2} structure; the concentration of the point defects generated and annihilated in the process of deformation was estimated. It was found that the main part of the point defects generating during plastic deformation annihilates, the residual density of the deformation point defects does not exceed 10{sup −5}.

Phonon-Induced Topological Transition to a Type-II Weyl Semimetal

NASA Astrophysics Data System (ADS)

Wang, Lin-Lin; Jo, Na Hyun; Wu, Yun; Kaminski, Adam; Canfield, Paul C.; Johnson, Duane D.

The emergence of topological quantum states requires certain combinations of crystalline symmetry with or without time reversal symmetry. Without restricting to searches for crystal structures with non-symmorphic symmetry operations in the space groups, we have studied the interplay between crystal symmetry, atomic displacements (lattice vibration), band degeneracy and topology. For a system with a full gap opening between the two band manifolds near the Fermi energy, we show that small atomic displacements (accessible via optical phonons near room temperature) can lower the symmetry to induce type-II Weyl points at the boundary between a pair of closely-lying electron and hole pockets. DOE Ames Laboratory LDRD.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marquis, Emmanuelle; Wirth, Brian; Was, Gary

Ferritic/martensitic (FM) steels such as HT-9, T-91 and NF12 with chromium concentrations in the range of 9-12 at.% Cr and high Cr ferritic steels (oxide dispersion strengthened steels with 12-18% Cr) are receiving increasing attention for advanced nuclear applications, e.g. cladding and duct materials for sodium fast reactors, pressure vessels in Generation IV reactors and first wall structures in fusion reactors, thanks to their advantages over austenitic alloys. Predicting the behavior of these alloys under radiation is an essential step towards the use of these alloys. Several radiation-induced phenomena need to be taken into account, including phase separation, solute clustering,more » and radiation-induced segregation or depletion (RIS) to point defect sinks. RIS at grain boundaries has raised significant interest because of its role in irradiation assisted stress corrosion cracking (IASCC) and corrosion of structural materials. Numerous observations of RIS have been reported on austenitic stainless steels where it is generally found that Cr depletes at grain boundaries, consistently with Cr atoms being oversized in the fcc Fe matrix. While FM and ferritic steels are also subject to RIS at grain boundaries, unlike austenitic steels, the behavior of Cr is less clear with significant scatter and no clear dependency on irradiation condition or alloy type. In addition to the lack of conclusive experimental evidence regarding RIS in F-M alloys, there have been relatively few efforts at modeling RIS behavior in these alloys. The need for predictability of materials behavior and mitigation routes for IASCC requires elucidating the origin of the variable Cr behavior. A systematic detailed high-resolution structural and chemical characterization approach was applied to ion-implanted and neutron-irradiated model Fe-Cr alloys containing from 3 to 18 at.% Cr. Atom probe tomography analyses of the microstructures revealed slight Cr clustering and segregation to dislocations and grain boundaries in the ion-irradiated alloys. More significant segregation was observed in the neutron irradiated alloys. For the more concentrated alloys, irradiation did not affect existing Cr segregation to grain boundaries and segregation to dislocation loops was not observed perhaps due to a change in the dislocation loop structure with increasing Cr concentration. Precipitation of α’ was observed in the neutron irradiated alloys containing over 9 at.% Cr. However ion irradiation appears to suppress the precipitation process. Initial low dose ion irradiation experiments strongly suggest a cascade recoil effect. The systematic analysis of grain boundary orientation on Cr segregation was significantly challenged by carbon contamination during ion irradiation or by existing carbon and therefore carbide formation at grain boundaries (sensitization). The combination of the proposed systematic experimental approach with atomistic modeling of diffusion processes directly addresses the programmatic need for developing and benchmarking predictive models for material degradation taking into account atomistic kinetics parameters« less

Observation of thermally etched grain boundaries with the FIB/TEM technique

DOE Office of Scientific and Technical Information (OSTI.GOV)

Palizdar, Y., E-mail: y.palizdar@merc.ac.ir; San Martin, D.; Ward, M.

2013-10-15

Thermal etching is a method which is able to reveal and characterize grain boundaries, twins or dislocation structures and determine parameters such as grain boundary energies, surface diffusivities or study phase transformations in steels, intermetallics or ceramic materials. This method relies on the preferential transfer of matter away from grain boundaries on a polished sample during heating at high temperatures in an inert/vacuum atmosphere. The evaporation/diffusion of atoms at high temperatures results in the formation of grooves at the intersections of the planes of grain/twin boundaries with the polished surface. This work describes how the combined use of Focussed Ionmore » Beam and Transmission Electron Microscopy can be used to characterize not only the grooves and their profile with the surface, but also the grain boundary line below the groove, this method being complementary to the commonly used scanning probe techniques. - Highlights: • Thermally etched low-carbon steel samples have been characterized by FIB/TEM • Grain boundary (GB) lines below the groove have been characterized in this way • Absence of ghost traces and large θ angle suggests that GB are not stationary but mobile • Observations correlate well with previous works and Mullins' investigations [22].« less

Strong anharmonic phonon scattering induced giant reduction of thermal conductivity in PbTe nanotwin boundary

NASA Astrophysics Data System (ADS)

Zhou, Yanguang; Yang, Jia-Yue; Cheng, Long; Hu, Ming

2018-02-01

Lead telluride (PbTe) is a renowned thermoelectric material with high energy conversion efficiency in medium to high temperature range. However, the performance of PbTe at room temperature is poor due to its relatively high lattice thermal conductivity, which is difficult to be engineered due to its intrinsic very short phonon mean-free path. By performing systematic first-principles and molecular-dynamics simulations, we report that the room-temperature lattice thermal conductivity of PbTe can be reduced by almost one order of magnitude (86%) using the recent experimentally observed nanotwin structure. The mechanism responsible for the dramatic decrease of thermal conductivity strongly depends on the type and mass of atoms at the twin boundary. For PbTe nanotwinned structures with Te at the twin boundary, phonon transport is dominated by the phonon confinement effect and phonon-twin boundary scattering, and the thermal conductivity converges to the bulk value when half of the periodic length is larger than the dominant phonon mean-free path. The same phenomenon is found in another comparison system of KCl nanotwinned structures. However, when Pb is present at the twin boundary, a scattering mechanism occurs: anharmonicity induced by the twin boundary. Due to the mass difference between Pb and Te, the thermal resistance for Pb residing at the twin boundary is found to be one order of magnitude larger than the case with Te at the twin boundary, which results in much stronger phonon-twin boundary scattering. Consequently, the lowest thermal conductivity of such PbTe nanotwinned structure is only 0.4 W/mK, which is reduced by about sevenfold compared to the bulk value of 2.85 W/mK; finally, the converged thermal conductivity cannot restore the bulk value even when half of the periodic length is much larger than the dominant mean-free path. These results offer useful guidance for the development of PbTe-based thermoelectrics and also suggest that nanotwins are excellent building blocks for enhancing the performance of existing thermoelectrics.

Multimillion to billion atom simulations of nanosystems under extreme conditions

NASA Astrophysics Data System (ADS)

Vashishta, P.

2008-12-01

Advanced materials and devices with nanometer grain/feature sizes are being developed to achieve higher strength and toughness in ceramic materials and greater speeds in electronic devices. Below 100 nm, however, continuum description of materials and devices must be supplemented by atomistic descriptions. Current state of the art atomistic simulations involve 10 million - 1 billion atoms. We investigate initiation, growth and healing of wing cracks in confined silica glass by multimillion atom molecular dynamics (MD) simulations. Under dynamic compression, frictional sliding of pre-crack surfaces nucleates nanovoids, which evolve into nanocrack columns at the pre-crack tip. Nanocrack columns merge to form a wing crack, which grows via coalescence with nanovoids in the direction of maximum compression. Lateral confinement arrests the growth and partially heals the wing crack. Growth and arrest of the wing crack occur repeatedly, as observed in dynamic compression experiments on brittle solids under lateral confinement. MD simulation of hypervelocity projectile impact in aluminum nitride and alumina has also been studied. The simulations reveal strong interplay between shock- induced structural phase transformation, plastic deformation and brittle cracks. The shock wave splits into an elastic precursor and a wurtzite-to-rocksalt structural transformation wave. When the elastic wave reflected from the boundary of the sample interacts with the transformation wave front, nanocavities are generated along the penetration path of the projectile and dislocations in adjacent regions. The nanocavities coalesce to form mode I brittle cracks while dislocations generate kink bands that give rise to mode II cracks. These simulations provide a microscopic view of defects associated with simultaneous tensile and shear cracking at the structural phase transformation boundary due to shock impact in high-strength ceramics. Initiation of chemical reactions at shock fronts prior to detonation and dynamic transition in the shock structure of an energetic material (RDX) and reaction of aluminium nanoparticles in oxygen atmosphere followed by explosive burning is also discussed.

Transfer Learning to Accelerate Interface Structure Searches

NASA Astrophysics Data System (ADS)

Oda, Hiromi; Kiyohara, Shin; Tsuda, Koji; Mizoguchi, Teruyasu

2017-12-01

Interfaces have atomic structures that are significantly different from those in the bulk, and play crucial roles in material properties. The central structures at the interfaces that provide properties have been extensively investigated. However, determination of even one interface structure requires searching for the stable configuration among many thousands of candidates. Here, a powerful combination of machine learning techniques based on kriging and transfer learning (TL) is proposed as a method for unveiling the interface structures. Using the kriging+TL method, thirty-three grain boundaries were systematically determined from 1,650,660 candidates in only 462 calculations, representing an increase in efficiency over conventional all-candidate calculation methods, by a factor of approximately 3,600.

Effect of Interface Structure on the Microstructural Evolution of Ceramics

DTIC Science & Technology

2007-11-06

because almost all the material properties are de - pendent upon their internal microstructures. Therefore, the microstructural evolution during the...growing interface de - pends upon the density of kinks on that interface. It fol- lows that the atomically smooth interface, which is char- acterized by...grain, and its de - tailed coarsening process has been treated elsewhere.139 During liquid-phase sintering, the formation of grain boundaries between

STRUCTURE OF THE INTERSTELLAR BOUNDARY EXPLORER RIBBON FROM SECONDARY CHARGE-EXCHANGE AT THE SOLAR–INTERSTELLAR INTERFACE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zirnstein, E. J.; McComas, D. J.; Heerikhuisen, J., E-mail: ezirnstein@swri.edu, E-mail: dmccomas@swri.edu, E-mail: jacob.heerikhuisen@uah.edu

2015-05-01

In 2009, the Interstellar Boundary Explorer discovered a bright “ribbon” of energetic neutral atom (ENA) flux in the energy range ≤0.4–6 keV, encircling a large portion of the sky. This observation was not previously predicted by any models or theories, and since its discovery, it has been the subject of numerous studies of its origin and properties. One of the most studied mechanisms for its creation is the “secondary ENA” process. Here, solar wind ions, neutralized by charge-exchange with interstellar atoms, propagate outside the heliopause; experience two charge-exchange events in the dense outer heliosheath; and then propagate back inside themore » heliosphere, preferentially in the direction perpendicular to the local interstellar magnetic field. This process has been extensively analyzed using state-of-the-art modeling and simulation techniques, but it has been difficult to visualize. In this Letter, we show the three-dimensional structure of the source of the ribbon, providing a physical picture of the spatial and energy scales over which the secondary ENA process occurs. These results help us understand how the ribbon is generated and further supports a secondary ENA process as the leading ribbon source mechanism.« less

System for conversion between the boundary representation model and a constructive solid geometry model of an object

DOEpatents

Christensen, N.C.; Emery, J.D.; Smith, M.L.

1985-04-29

A system converts from the boundary representation of an object to the constructive solid geometry representation thereof. The system converts the boundary representation of the object into elemental atomic geometrical units or I-bodies which are in the shape of stock primitives or regularized intersections of stock primitives. These elemental atomic geometrical units are then represented in symbolic form. The symbolic representations of the elemental atomic geometrical units are then assembled heuristically to form a constructive solid geometry representation of the object usable for manufacturing thereof. Artificial intelligence is used to determine the best constructive solid geometry representation from the boundary representation of the object. Heuristic criteria are adapted to the manufacturing environment for which the device is to be utilized. The surface finish, tolerance, and other information associated with each surface of the boundary representation of the object are mapped onto the constructive solid geometry representation of the object to produce an enhanced solid geometry representation, particularly useful for computer-aided manufacture of the object. 19 figs.

Study of crystallization mechanisms of Fe nanoparticle

NASA Astrophysics Data System (ADS)

Kien, P. H.; Trang, G. T. T.; Hung, P. K.

2017-06-01

In this paper, the nanoparticle (NP) Fe was investigated by means of molecular dynamics simulation. The crystallization mechanism was studied through the time evolution of crystal cluster and potential energies of different atom types. The simulation shows that the NP was crystallized into bcc crystal structure when it was annealed at 900 K for long times. At early stage of the annealing, small nuclei form in different places of NP and dissolve for short times. After long times some nuclei form and gather nearby which create the stable clusters in the core of NP. After that the crystal clusters grow in the direction to cover the core and then to spread into the surface of NP. Analyzing the energies of different type atoms, we found that the crystal growth is originated from specific atomic arrangement in the boundary region of crystal clusters.

Electron Microscopy Characterizations and Atom Probe Tomography of Intergranular Attack in Alloy 600 Exposed to PWR Primary Water

NASA Astrophysics Data System (ADS)

Olszta, Matthew J.; Schreiber, Daniel K.; Thomas, Larry E.; Bruemmer, Stephen M.

Detailed examinations of intergranular attack (IGA) in alloy 600 were performed after exposure to simulated PWR primary water at 325°C for 500 h. High-resolution analyses of IGA characteristics were conducted on specimens with either a 1 µm diamond or 1200-grit SiC surface finish using scanning electron microscopy, transmission electron microscopy and atom probe tomography techniques. The diamond-polish finish with very little preexisting subsurface damage revealed attack of high-energy grain boundaries that intersected the exposed surface to depths approaching 2 µm. In all cases, IGA from the surface is localized oxidation consisting of porous, nanocrystalline MO-structure and spinel particles along with regions of faceted wall oxidation. Surprisingly, this continuous IG oxidation transitions to discontinuous, discrete Cr-rich sulfide particles up to 50 nm in diameter. In the vicinity of the sulfides, the grain boundaries were severely Cr depleted (to <1 at%) and enriched in S. The 1200 grit SiC finish surface exhibited a preexisting highly strained recrystallized layer of elongated nanocrystalline matrix grains. Similar IG oxidation and leading sulfide particles were found, but the IGA depth was typically confined to the near-surface ( 400 nm) recrystallized region. Difference in IGA for the two surface finishes indicates that the formation of grain boundary sulfides occurs during the exposure to PWR primary water. The source of S remains unclear, however it is not present as sulfides in the bulk alloy nor is it segregated to bulk grain boundaries.

Atomistic modeling of grain boundary behavior under shear conditions in magnesium and magnesium-based binary alloys

NASA Astrophysics Data System (ADS)

Nahhas, M. K.; Groh, S.

2018-02-01

In this study, the structure, the energetic, and the strength of a { 10 1 bar 1 } < 11 2 bar 0 > symmetric tilt grain boundary in magnesium and magnesium binary alloys were analyzed in the framework of (semi-)empirical potentials. Following a systematic investigation of the transferability and accuracy of the interatomic potentials, atomistic calculations of the grain boundary energy, the grain boundary sliding energy, and the grain boundary strength were performed in pure magnesium and in binary MgX alloys (X = Al, Ca, Gd, Li, Sn, Y, Ag, Nd, and Pb). The data gained in this study were analyzed to identify the most critical material parameters controlling the strength of the grain boundary, and their consequence on atomic shuffling motions occurring at the grain boundary. From the methodology perspective, the role of in-plane and out-of plane relaxation on the grain boundary sliding energy curves was investigated. In pure magnesium, the results showed that in-plane relaxation is critical in activating b2{ 10 1 bar 1 } twinning dislocation resulting in grain boundary migration. In the alloy systems, however, grain boundary migration was disabled as a consequence of the pinning of the grain boundary by segregated elements. Finally, while the grain boundary energy, the shape of the grain boundary sliding energy curves, and the grain boundary sliding energy are critical parameters controlling the grain boundary strength in pure magnesium, only the grain boundary energy and the segregation energy of the alloying elements at the grain boundary were identified as critical material parameters in the alloys system.

Enhanced adsorption of Co atoms on grain boundary of boron nitride

NASA Astrophysics Data System (ADS)

Zhang, Tingting; Chen, Guibin; Zhu, Liyan

2017-11-01

Structural, energetic, electronic, and magnetic properties of Co monomer, dimer, and trimer adsorbed on a single-layer boron nitride (BN) with a grain boundary (GB) consisting of tetragons and octagons ( 4|8) are theoretically explored via density functional calculations. Due to the presence of 4|8 GB, the adsorption energies (EAs) of small Co clusters are generally enhanced by 10% as compared with those adsorbed on pristine BN, e.g., the EA of Co monomer, and dimer increase by 0.1 eV on a global amount of 0.87 eV, and 0.2 eV for the case of Co trimer. Most interestingly, the increase in adsorption energy exhibits a strong correlation to the number of atoms directly bonded to the substrate. The enhanced binding of Co adatom on the BN with 4|8 GBs ( BN 48 ) is due to the strong hybridization of d orbitals of Co adatom and the localized defect states at the 4|8 GBs. However, the GBs have negligible influence on the electronic and magnetic properties of adsorbates. Hence, the two-dimensional (2D) nanosheets with linear GBs might be a better candidate for anchoring the transition metal atoms than pristine BN. Such a strategy may also be applied to other 2D materials, e.g., MoS2 and phosphorene, to enhance the binding of adatom on them, or to utilize them as 1D templates to assemble transition metal atoms into nanowires.

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary-Condition Engineering.

PubMed

Xie, Lin; Li, Linze; Heikes, Colin A; Zhang, Yi; Hong, Zijian; Gao, Peng; Nelson, Christopher T; Xue, Fei; Kioupakis, Emmanouil; Chen, Longqing; Schlom, Darrel G; Wang, Peng; Pan, Xiaoqing

2017-08-01

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO 3 ferroelectric films is reported. Using aberration-corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm-thick ultrathin BiFeO 3 film is abnormally increased up to ≈90-100 µC cm -2 in the out-of-plane direction and a peculiar rumpled nanodomain structure with very large variation in c/a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase-field calculations, it is shown that it is the unique single atomic Bi 2 O 3 - x layer at the surface that leads to the enhanced polarization and appearance of the MPB-like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Grain boundaries in bcc-Fe: a density-functional theory and tight-binding study

NASA Astrophysics Data System (ADS)

Wang, Jingliang; Madsen, Georg K. H.; Drautz, Ralf

2018-02-01

Grain boundaries (GBs) have a significant influence on material properties. In the present paper, we calculate the energies of eleven low-Σ ({{Σ }}≤slant 13) symmetrical tilt GBs and two twist GBs in ferromagnetic bcc iron using first-principles density functional theory (DFT) calculations. The results demonstrate the importance of a sufficient sampling of initial rigid body translations in all three directions. We show that the relative GB energies can be explained by the miscoordination of atoms at the GB region. While the main features of the studied GB structures were captured by previous empirical interatomic potential calculations, it is shown that the absolute values of GB energies calculated were substantially underestimated. Based on DFT-calculated GB structures and energies, we construct a new d-band orthogonal tight-binding (TB) model for bcc iron. The TB model is validated by its predictive power on all the studied GBs. We apply the TB model to block boundaries in lath martensite and demonstrate that the experimentally observed GB character distribution can be explained from the viewpoint of interface energy.

Geometric shape control of thin film ferroelectrics and resulting structures

DOEpatents

McKee, Rodney A.; Walker, Frederick J.

2000-01-01

A monolithic crystalline structure and a method of making involves a semiconductor substrate, such as silicon, and a ferroelectric film, such as BaTiO.sub.3, overlying the surface of the substrate wherein the atomic layers of the ferroelectric film directly overlie the surface of the substrate. By controlling the geometry of the ferroelectric thin film, either during build-up of the thin film or through appropriate treatment of the thin film adjacent the boundary thereof, the in-plane tensile strain within the ferroelectric film is relieved to the extent necessary to permit the ferroelectric film to be poled out-of-plane, thereby effecting in-plane switching of the polarization of the underlying substrate material. The method of the invention includes the steps involved in effecting a discontinuity of the mechanical restraint at the boundary of the ferroelectric film atop the semiconductor substrate by, for example, either removing material from a ferroelectric film which has already been built upon the substrate, building up a ferroelectric film upon the substrate in a mesa-shaped geometry or inducing the discontinuity at the boundary by ion beam deposition techniques.

Motion of 1/3⟨111⟩ dislocations on Σ3 {112} twin boundaries in nanotwinned copper

NASA Astrophysics Data System (ADS)

Lu, N.; Du, K.; Lu, L.; Ye, H. Q.

2014-01-01

The atomic structure of Σ3 {112} ITBs in nanotwinned Cu is investigated by using aberration-corrected high resolution transmission electron microscopy (HRTEM) and in situ HRTEM observations. The Σ3 {112} ITBs are consisted of periodically repeated three partial dislocations. The in situ HRTEM results show that 1/3[111] partial dislocation moves on the Σ3 {112} incoherent twin boundary (ITB), which was accompanied by a migration of the ITB. A dislocation reaction mechanism is proposed for the motion of 1/3[111] Frank partial dislocation, in which the 1/3[111] partial dislocation exchanges its position with twin boundary dislocations in sequence. In this way, the 1/3[111] dislocation can move on the incoherent twin boundary in metals with low stacking fault energy. Meanwhile, the ITB will migrate in its normal direction accordingly. These results provide insight into the reaction mechanism of 1/3[111] dislocations and ITBs and the associated migration of ITBs.

Molecular dynamics study of the interaction between nanoscale interstitial dislocation loops and grain boundaries in BCC iron

NASA Astrophysics Data System (ADS)

Gao, N.; Perez, D.; Lu, G. H.; Wang, Z. G.

2018-01-01

Atomic simulations are used to investigate the interaction between nanoscale interstitial dislocation loops and grain boundaries (GBs), the subsequent evolution of the GBs' structures, and the resulting impact on mechanical properties, in BCC iron. The interaction between loops and GBs - Σ 3 { 111 } and Σ 3 { 112 } - is affected by the angle (θ) between the Burgers vector and the normal to the GB plane, as well as by the distribution of free volume (FV) and stress. Loops can be totally absorbed by Σ 3 { 111 } boundaries, while the interaction with Σ 3 { 112 } boundaries is found to change the Burgers vector and habit plane after absorption, but to otherwise leave the loop intact, resulting in selective absorption. When θ =90o , no absorption occurs in Σ 3 { 112 } . The stress accumulation induced by the absorption affects the local mechanical properties of GBs. In nanocrystalline iron sample, a similar phenomenon is also observed, resulting in rearrangement of GBs and grain growth.

An Atomistic View of the Incipient Growth of Zinc Oxide Nanolayers

DOE PAGES

Chu, Manh Hung; Tian, Liang; Chaker, Ahmad; ...

2016-08-09

The growth of zinc oxide thin films by atomic layer deposition is believed to proceed through an embryonic step in which three-dimensional nanoislands form and then coalesce to trigger a layer-by-layer growth mode. This transient initial state is characterized by a poorly ordered atomic structure, which may be inaccessible by X-ray diffraction techniques. Here in this work, we apply X-ray absorption spectroscopy in situ to address the local structure of Zn after each atomic layer deposition cycle, using a custom-built reactor mounted at a synchrotron beamline, and we shed light on the atomistic mechanisms taking place during the first stagesmore » of the growth. We find that such mechanisms are surprisingly different for zinc oxide growth on amorphous (silica) and crystalline (sapphire) substrate. Ab initio simulations and quantitative data analysis allow the formulation of a comprehensive growth model, based on the different effects of surface atoms and grain boundaries in the nanoscale islands, and the consequent induced local disorder. From a comparison of these spectroscopy results with those from X-ray diffraction reported recently, we observe that the final structure of the zinc oxide nanolayers depends strongly on the mechanisms taking place during the initial stages of growth. Finally, the approach followed here for the case of zinc oxide will be of general interest for characterizing and optimizing the growth and properties of more complex nanostructures.« less

An Atomistic View of the Incipient Growth of Zinc Oxide Nanolayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chu, Manh Hung; Tian, Liang; Chaker, Ahmad

The growth of zinc oxide thin films by atomic layer deposition is believed to proceed through an embryonic step in which three-dimensional nanoislands form and then coalesce to trigger a layer-by-layer growth mode. This transient initial state is characterized by a poorly ordered atomic structure, which may be inaccessible by X-ray diffraction techniques. Here in this work, we apply X-ray absorption spectroscopy in situ to address the local structure of Zn after each atomic layer deposition cycle, using a custom-built reactor mounted at a synchrotron beamline, and we shed light on the atomistic mechanisms taking place during the first stagesmore » of the growth. We find that such mechanisms are surprisingly different for zinc oxide growth on amorphous (silica) and crystalline (sapphire) substrate. Ab initio simulations and quantitative data analysis allow the formulation of a comprehensive growth model, based on the different effects of surface atoms and grain boundaries in the nanoscale islands, and the consequent induced local disorder. From a comparison of these spectroscopy results with those from X-ray diffraction reported recently, we observe that the final structure of the zinc oxide nanolayers depends strongly on the mechanisms taking place during the initial stages of growth. Finally, the approach followed here for the case of zinc oxide will be of general interest for characterizing and optimizing the growth and properties of more complex nanostructures.« less

Ergodic properties of the multidimensional rayleigh gas with a semipermeable barrier

NASA Astrophysics Data System (ADS)

Erdős, L.; Tuyen, D. Q.

1990-06-01

We consider a multidimensional system consisting of a particle of mass M and radius r (molecule), surrounded by an infinite ideal gas of point particles of mass m (atoms). The molecule is confined to the unit ball and interacts with its boundary ( barrier) via elastic collision, while the atoms are not affected by the boundary. We obtain convergence to equilibrium for the molecule from almost every initial distribution on its position and velocity. Furthermore, we prove that the infinite composite system of the molecule and the atoms is Bernoulli.

Effects of star-shape poly(alkyl methacrylate) arm uniformity on lubricant properties

DOE PAGES

Robinson, Joshua W.; Qu, Jun; Erck, Robert; ...

2016-03-29

Star-shaped poly(alkyl methacrylate)s (PAMAs) were prepared and blended into an additive-free engine oil to assess the structure property relationship between macromolecular structure and lubricant performance. These additives were designed with a comparable number of repeating units per arm and the number of arms was varied between 3 and 6. Well-defined star-shaped PAMAs were synthesized by atom transfer radical polymerization (ATRP) via a core-first strategy from multi-functional headgroups. Observations of the polymer-oil blends suggest that stars with less than four arms are favorable as a viscosity index improver (VII), and molecular weight dominates viscosity-related effects over other structural features. Star-shaped PAMAs,more » as oil additives, effectively reduce the friction coefficient in both mixed and boundary lubrication regime. Several analogs outperformed commercial VIIs in both viscosity and friction performance. Furthermore, increased wear rates were observed for these star-shaped PAMAs in the boundary lubrication regime suggesting pressure-sensitive conformations may exist.« less

Strain localization in thin films of Bi(Fe,Mn)O 3 due to the formation of stepped Mn 4+-rich antiphase boundaries

DOE PAGES

MacLaren, I.; Sala, B.; Andersson, S. M. L.; ...

2015-10-17

Here, the atomic structure and chemistry of thin films of Bi(Fe,Mn)O 3 (BFMO) films with a target composition of Bi 2FeMnO 6 on SrTiO 3 are studied using scanning transmission electron microscopy imaging and electron energy loss spectroscopy. It is shown that Mn4+-rich antiphase boundaries are locally nucleated right at the film substrate and then form stepped structures that are approximately pyramidal in three dimensions. These have the effect of confining the material below the pyramids in a highly strained state with an out-of-plane lattice parameter close to 4.1 Å. Outside the area enclosed by the antiphase boundaries, the out-of-planemore » lattice parameter is much closer to bulk values for BFMO. This suggests that to improve the crystallographic perfection of the films whilst retaining the strain state through as much of the film as possible, ways need to be found to prevent nucleation of the antiphase boundaries. Since the antiphase boundaries seem to form from the interaction of Mn with the Ti in the substrate, one route to perform this would be to grow a thin buffer layer of pure BiFeO 3 on the SrTiO 3 substrate to minimise any Mn-Ti interactions.« less

InSe monolayer: synthesis, structure and ultra-high second-harmonic generation

NASA Astrophysics Data System (ADS)

Zhou, Jiadong; Shi, Jia; Zeng, Qingsheng; Chen, Yu; Niu, Lin; Liu, Fucai; Yu, Ting; Suenaga, Kazu; Liu, Xinfeng; Lin, Junhao; Liu, Zheng

2018-04-01

III–IV layered materials such as indium selenide have excellent photoelectronic properties. However, synthesis of materials in such group, especially with a controlled thickness down to monolayer, still remains challenging. Herein, we demonstrate the successful synthesis of monolayer InSe by physical vapor deposition (PVD) method. The high quality of the sample was confirmed by complementary characterization techniques such as Raman spectroscopy, atomic force microscopy (AFM) and high resolution annular dark field scanning transmission electron microscopy (ADF-STEM). We found the co-existence of different stacking sequence (β- and γ-InSe) in the same flake with a sharp grain boundary in few-layered InSe. Edge reconstruction is also observed in monolayer InSe, which has a distinct atomic structure from the bulk lattice. Moreover, we discovered that the second-harmonic generation (SHG) signal from monolayer InSe shows large optical second-order susceptibility that is 1–2 orders of magnitude higher than MoS2, and even 3 times of the largest value reported in monolayer GaSe. These results make atom-thin InSe a promising candidate for optoelectronic and photosensitive device applications.

Edge-spin-derived magnetism in few-layer MoS2 nanomeshes

NASA Astrophysics Data System (ADS)

Kondo, G.; Yokoyama, N.; Yamada, S.; Hashimoto, Y.; Ohata, C.; Katsumoto, S.; Haruyama, J.

2017-12-01

Magnetism arising from edge spins is highly interesting, particularly in 2D atomically thin materials in which the influence of edges becomes more significant. Among such materials, molybdenum disulfide (MoS2; one of the transition metal dichalcogenide (TMD) family) is attracting significant attention. The causes for magnetism observed in the TMD family, including in MoS2, have been discussed by considering various aspects, such as pure zigzag atomic-structure edges, grain boundaries, and vacancies. Here, we report the observation of ferromagnetism (FM) in few-layer MoS2 nanomeshes (NMs; honeycomb-like array of hexagonal nanopores with low-contamination and low-defect pore edges), which have been created by a specific non-lithographic method. We confirm robust FM arising from pore edges in oxygen(O)-terminated MoS2-NMs at room temperature, while it disappears in hydrogen(H)-terminated samples. The observed high-sensitivity of FM to NM structures and critical annealing temperatures suggest a possibility that the Mo-atom dangling bond in pore edge is a dominant factor for the FM.

Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions.

PubMed

Shibata, Naoya; Pennycook, Stephen J; Gosnell, Tim R; Painter, Gayle S; Shelton, William A; Becher, Paul F

2004-04-15

Silicon nitride (Si3N4) ceramics are used in numerous applications because of their superior mechanical properties. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives. Understanding the origin of dopant effects, central to the design of high-performance Si3N4 ceramics, has been sought for many years. Here we show direct images of dopant atoms (La) within the nanometre-scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La for the crystalline surfaces, which is essential for forming elongated grains and a toughened microstructure. Whereas principles of micrometre-scale structural design are currently used to improve the mechanical properties of ceramics, this work represents a step towards the atomic-level structural engineering required for the next generation of ceramics.

Allotropic forms of carbon in the Invar Fe-Ni-C alloy before and after plastic deformation by upsetting

NASA Astrophysics Data System (ADS)

Nadutov, V. M.; Vashchuk, D. L.; Karbivskii, V. L.; Volosevich, P. Yu.; Davydenko, O. A.

2018-04-01

The effect of cold plastic deformation by upsetting (e = 1.13) on structure and hybridised bonds of carbon in the fcc Invar Fe-30.9%Ni-1.23% C alloy was studied by means of X-ray phase analysis and X-ray photoelectron spectroscopy. Carbon precipitates along grain boundaries and inside of grains in the alloy after annealing and plastic deformation were revealed. The presence of mainly sp2- and sp3-hybridised C-C bonds attributing to graphite and amorphous carbon as well as the carbon bonds with impurity atoms and metallic Fe and Ni atoms in austenitic phase were revealed in the annealed and deformed alloy. It was shown for the first time that plastic deformation of the alloy results in partial destruction of the graphite crystal structure, increasing the relative part of amorphous carbon, and redistribution of carbon between structural elements as well as in a solid solution of austenitic phase.

Simulation of fundamental atomization mechanisms in fuel sprays

NASA Technical Reports Server (NTRS)

Childs, Robert, E.; Mansour, Nagi N.

1988-01-01

Growth of instabilities on the liquid/gas interface in the initial region of fuel sprays is studied by means of numerical simulations. The simulations are based on solutions of the variable-density incompressible Navier-Stokes equations, which are obtained with a new numerical algorithm. The simulations give good agreement with analytical results for the instabilities on a liquid cylinder induced by surface tension and wind-induced instabilities. The effects of boundary layers on the wind-induced instabilities are investigated. It is found that a boundary layer reduces the growth rate for a single interface, and a comparison with inviscid theory suggests that boundary layer effects may be significantly more important than surface tension effects. The results yield a better estimate than inviscid theory for the drop sizes as reported for diesel sprays. Results for the planar jet show that boundary layer effects hasten the growth of Squire's 'symmetric' mode, which is responsible for jet disintegration. This result helps explain the rapid atomization which occurs in swirl and air-blast atomizers.

Complementary Characterization of Cu(In,Ga)Se₂ Thin-Film Photovoltaic Cells Using Secondary Ion Mass Spectrometry, Auger Electron Spectroscopy, and Atom Probe Tomography.

PubMed

Jang, Yun Jung; Lee, Jihye; Jeong, Jeung-Hyun; Lee, Kang-Bong; Kim, Donghwan; Lee, Yeonhee

2018-05-01

To enhance the conversion performance of solar cells, a quantitative and depth-resolved elemental analysis of photovoltaic thin films is required. In this study, we determined the average concentration of the major elements (Cu, In, Ga, and Se) in fabricated Cu(In,Ga)Se2 (CIGS) thin films, using inductively coupled plasma atomic emission spectroscopy, X-ray fluorescence, and wavelengthdispersive electron probe microanalysis. Depth profiling results for CIGS thin films with different cell efficiencies were obtained using secondary ion mass spectrometry and Auger electron spectroscopy to compare the atomic concentrations. Atom probe tomography, a characterization technique with sub-nanometer resolution, was used to obtain three-dimensional elemental mapping and the compositional distribution at the grain boundaries (GBs). GBs are identified by Na increment accompanied by Cu depletion and In enrichment. Segregation of Na atoms along the GB had a beneficial effect on cell performance. Comparative analyses of different CIGS absorber layers using various analytical techniques provide us with understanding of the compositional distributions and structures of high efficiency CIGS thin films in solar cells.

Roles of Vacancy/Interstitial Diffusion and Segregation in the Microchemistry at Grain Boundaries of Irradiated Fe-Cr-Ni alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Ying; Field, Kevin G.; Allen, Todd R.

2016-02-23

A detailed analysis of the diffusion fluxes near and at grain boundaries of irradiated Fe–Cr–Ni alloys, induced by preferential atom-vacancy and atom-interstitial coupling, is presented. The diffusion flux equations were based on the Perks model formulated through the linear theory of the thermodynamics of irreversible processes. The preferential atom-vacancy coupling was described by the mobility model, whereas the preferential atom-interstitial coupling was described by the interstitial binding model. The composition dependence of the thermodynamic factor was modeled using the CALPHAD approach. The calculated fluxes up to 10 dpa suggested the dominant diffusion mechanism for chromium and iron is via vacancy,more » while that for nickel can swing from the vacancy to the interstitial dominant mechanism. The diffusion flux in the vicinity of a grain boundary was found to be greatly modified by the segregation induced by irradiation, leading to the oscillatory behavior of alloy compositions in this region.« less

Deformable M-Reps for 3D Medical Image Segmentation.

PubMed

Pizer, Stephen M; Fletcher, P Thomas; Joshi, Sarang; Thall, Andrew; Chen, James Z; Fridman, Yonatan; Fritsch, Daniel S; Gash, Graham; Glotzer, John M; Jiroutek, Michael R; Lu, Conglin; Muller, Keith E; Tracton, Gregg; Yushkevich, Paul; Chaney, Edward L

2003-11-01

M-reps (formerly called DSLs) are a multiscale medial means for modeling and rendering 3D solid geometry. They are particularly well suited to model anatomic objects and in particular to capture prior geometric information effectively in deformable models segmentation approaches. The representation is based on figural models , which define objects at coarse scale by a hierarchy of figures - each figure generally a slab representing a solid region and its boundary simultaneously. This paper focuses on the use of single figure models to segment objects of relatively simple structure. A single figure is a sheet of medial atoms, which is interpolated from the model formed by a net, i.e., a mesh or chain, of medial atoms (hence the name m-reps ), each atom modeling a solid region via not only a position and a width but also a local figural frame giving figural directions and an object angle between opposing, corresponding positions on the boundary implied by the m-rep. The special capability of an m-rep is to provide spatial and orientational correspondence between an object in two different states of deformation. This ability is central to effective measurement of both geometric typicality and geometry to image match, the two terms of the objective function optimized in segmentation by deformable models. The other ability of m-reps central to effective segmentation is their ability to support segmentation at multiple levels of scale, with successively finer precision. Objects modeled by single figures are segmented first by a similarity transform augmented by object elongation, then by adjustment of each medial atom, and finally by displacing a dense sampling of the m-rep implied boundary. While these models and approaches also exist in 2D, we focus on 3D objects. The segmentation of the kidney from CT and the hippocampus from MRI serve as the major examples in this paper. The accuracy of segmentation as compared to manual, slice-by-slice segmentation is reported.

Deformable M-Reps for 3D Medical Image Segmentation

PubMed Central

Pizer, Stephen M.; Fletcher, P. Thomas; Joshi, Sarang; Thall, Andrew; Chen, James Z.; Fridman, Yonatan; Fritsch, Daniel S.; Gash, Graham; Glotzer, John M.; Jiroutek, Michael R.; Lu, Conglin; Muller, Keith E.; Tracton, Gregg; Yushkevich, Paul; Chaney, Edward L.

2013-01-01

M-reps (formerly called DSLs) are a multiscale medial means for modeling and rendering 3D solid geometry. They are particularly well suited to model anatomic objects and in particular to capture prior geometric information effectively in deformable models segmentation approaches. The representation is based on figural models, which define objects at coarse scale by a hierarchy of figures – each figure generally a slab representing a solid region and its boundary simultaneously. This paper focuses on the use of single figure models to segment objects of relatively simple structure. A single figure is a sheet of medial atoms, which is interpolated from the model formed by a net, i.e., a mesh or chain, of medial atoms (hence the name m-reps), each atom modeling a solid region via not only a position and a width but also a local figural frame giving figural directions and an object angle between opposing, corresponding positions on the boundary implied by the m-rep. The special capability of an m-rep is to provide spatial and orientational correspondence between an object in two different states of deformation. This ability is central to effective measurement of both geometric typicality and geometry to image match, the two terms of the objective function optimized in segmentation by deformable models. The other ability of m-reps central to effective segmentation is their ability to support segmentation at multiple levels of scale, with successively finer precision. Objects modeled by single figures are segmented first by a similarity transform augmented by object elongation, then by adjustment of each medial atom, and finally by displacing a dense sampling of the m-rep implied boundary. While these models and approaches also exist in 2D, we focus on 3D objects. The segmentation of the kidney from CT and the hippocampus from MRI serve as the major examples in this paper. The accuracy of segmentation as compared to manual, slice-by-slice segmentation is reported. PMID:23825898

Using complimentary microscopy methods to examine Ni-Mn-Si-precipitates in highly-irradiated reactor pressure vessel steels

DOE PAGES

Edmondson, P. D.; Parish, C. M.; Nanstad, R. K.

2017-05-29

Nano-scale Ni-Mn-Si-rich precipitates formed in a reactor pressure vessel steel under high neutron fluence have been characterized using highly complimentary atom probe tomography (APT) and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) combined with STEM-EDS modeling. Using these techniques in a synergistic manner to overcome the well-known trajectory aberrations in APT data, the average upper limit Fe concentration within the precipitates was found to be ~6 at.%. Using this knowledge, accurate compositions of the precipitates was determined and it was found that the spread of precipitate compositions was large, but mostly centered around the Γ2-and G-phases. The usemore » of STEM-EDS also allowed for larger areas to be examined, and segregation of minor solutes was observed to occur on grain boundaries, along with Ni-Mn-Si-rich precipitates that were smaller in size than those in the matrix. Solute segregation at the grain boundaries is proposed to occur through a radiation induced segregation or radiation enhanced diffusion mechanism due to the presence of a denuded zone about the grain boundary. It is also proposed that the reduced precipitate size at the grain boundaries is due to the structure of the grain boundary. The lack of Ni-Mn-Si precipitates observed in larger Mo-rich precipitates is also discussed, and the absence of the minor solutes required to form the Ni-Mn-Si precipitates results in the lack of nucleation. This is in contrast to cementite phases in which Ni-Mn-Si precipitates have been seen to be formed. It was also determined through this work that the exclusion of all the Fe ions during atom probe analysis is a reasonable approximation.« less

Understanding individual defects in CdTe thin-film solar cells via STEM: From atomic structure to electrical activity

DOE PAGES

Li, Chen; Poplawsky, Jonathan; Yan, Yanfa; ...

2017-07-01

Here in this paper we review a systematic study of the structure-property correlations of a series of defects in CdTe solar cells. A variety of experimental methods, including aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and electron-beam-induced current have been combined with density-functional theory. The research traces the connections between the structures and electrical activities of individual defects including intra-grain partial dislocations, grain boundaries and the CdTe/CdS interface. The interpretations of the physical origin underlying the structure-property correlation provide insights that should further the development of future CdTe solar cells.

Understanding individual defects in CdTe thin-film solar cells via STEM: From atomic structure to electrical activity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Chen; Poplawsky, Jonathan; Yan, Yanfa

Here in this paper we review a systematic study of the structure-property correlations of a series of defects in CdTe solar cells. A variety of experimental methods, including aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and electron-beam-induced current have been combined with density-functional theory. The research traces the connections between the structures and electrical activities of individual defects including intra-grain partial dislocations, grain boundaries and the CdTe/CdS interface. The interpretations of the physical origin underlying the structure-property correlation provide insights that should further the development of future CdTe solar cells.

Vacuum fluctuations and radiation reaction contributions to the resonance dipole-dipole interaction between two atoms near a reflecting boundary

NASA Astrophysics Data System (ADS)

Zhou, Wenting; Rizzuto, Lucia; Passante, Roberto

2018-04-01

We investigate the resonance dipole-dipole interaction energy between two identical atoms, one in the ground state and the other in the excited state, interacting with the electromagnetic field in the presence of a perfectly reflecting plane boundary. The atoms are prepared in a correlated (symmetric or antisymmetric) Bell-type state. Following a procedure due to Dalibard et al. [J. Dalibard et al., J. Phys. (Paris) 43, 1617 (1982);, 10.1051/jphys:0198200430110161700 J. Phys. (Paris) 45, 637 (1984), 10.1051/jphys:01984004504063700], we separate the contributions of vacuum fluctuations and radiation reaction (source) field to the resonance interaction energy between the two atoms and show that only the source field contributes to the interatomic interaction, while vacuum field fluctuations do not. By considering specific geometric configurations of the two-atom system with respect to the mirror and specific choices of dipole orientations, we show that the presence of the mirror significantly affects the resonance interaction energy and that different features appear with respect to the case of atoms in free space, for example, a change in the spatial dependence of the interaction. Our findings also suggest that the presence of a boundary can be exploited to tailor and control the resonance interaction between two atoms, as well as the related energy transfer process. The possibility of observing these phenomena is also discussed.

Role of string-like collective atomic motion on diffusion and structural relaxation in glass forming Cu-Zr alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Hao; Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4; Zhong, Cheng

2015-04-28

We investigate Cu-Zr liquid alloys using molecular dynamics simulation and well-accepted embedded atom method potentials over a wide range of chemical composition and temperature as model metallic glass-forming (GF) liquids. As with other types of GF materials, the dynamics of these complex liquids are characterized by “dynamic heterogeneity” in the form of transient polymeric clusters of highly mobile atoms that are composed in turn of atomic clusters exhibiting string-like cooperative motion. In accordance with the string model of relaxation, an extension of the Adam-Gibbs (AG) model, changes in the activation free energy ΔG{sub a} with temperature of both the Cumore » and Zr diffusion coefficients D, and the alpha structural relaxation time τ{sub α} can be described to a good approximation by changes in the average string length, L. In particular, we confirm that the strings are a concrete realization of the abstract “cooperatively rearranging regions” of AG. We also find coexisting clusters of relatively “immobile” atoms that exhibit predominantly icosahedral local packing rather than the low symmetry packing of “mobile” atoms. These two distinct types of dynamic heterogeneity are then associated with different fluid structural states. Glass-forming liquids are thus analogous to polycrystalline materials where the icosahedrally packed regions correspond to crystal grains, and the strings reside in the relatively disordered grain boundary-like regions exterior to these locally well-ordered regions. A dynamic equilibrium between localized (“immobile”) and wandering (“mobile”) particles exists in the liquid so that the dynamic heterogeneity can be considered to be type of self-assembly process. We also characterize changes in the local atomic free volume in the course of string-like atomic motion to better understand the initiation and propagation of these fluid excitations.« less

Damage Tolerance and Mechanics of Interfaces in Nanostructured Metals

NASA Astrophysics Data System (ADS)

Foley, Daniel J.

The concept of interface driven properties in crystalline metals has been one of the most intensely discussed topics in materials science for decades. Since the 1980s researchers have been exploring the concept of grain boundary engineering as route for tuning properties such as fracture toughness and irradiation resistance. This is especially true in ultra-fine grained and nanocrystalline materials where grain boundary mediated properties become dominant. More recently, materials composed of hierarchical nanostructures, such as amorphous-crystalline nanolaminates, have attracted considerable attention due to their favorable properties, ease of manufacture and highly tunable microstructure. While both grain boundary engineering and hierarchical nanostructures have shown promise there are still questions remaining regarding the role of specific attributes of the microstructure (such as grain boundaries, grain/layer size and inter/intralayer morphology) in determining material properties. This thesis attempts to address these questions by using atomistic simulations to perform deformation and damage loading studies on a series of nanolaminate and bicrystalline structures. During the course of this thesis the roles of layer thickness, interlayer structure and interlayer chemistry on the mechanical properties of Ni-NiX amorphous-crystalline nanolaminates were explored using atomistic simulations. This thesis found that layer thickness/thickness ratio and amorphous layer chemistry play a crucial role in yield strength and Young's modulus. Analysis of the deformation mechanisms at the atomic scale revealed that structures containing single crystalline, crystalline layers undergo plastic deformation when shear transformation zones form in the amorphous layer and impinge on the amorphous-crystalline interface, leading to dislocation emission. However, structures containing nanocrystalline, crystalline layers (both equiaxed and columnar nanocrystalline) undergo plastic deformation through a combination of grain boundary sliding and grain boundary mediated dislocation nucleation. Since grain boundaries were found to play a critical role as sources and sinks for dislocations in amorphous-crystalline nanolaminates a follow-up study on the effect of grain boundary character on damage accumulation/accommodation in copper symmetric tilt grain boundaries was performed. This study found that grain boundaries will become saturated with damage, a state where grain boundary energy and grain boundary free volume oscillate about a plateau during continuous defect loading (vacancy, interstitial and frenkel pair loading were all considered). Further, grain boundary character (specifically equilibrium grain boundary energy) was strongly correlated to the damage accommodation behavior of grain boundaries in copper. Finally, a study that attempted to link grain boundary damage saturation behavior to variations in grain boundary mechanical properties was performed. This study found no direct relationships between grain boundary damage saturation behavior and variations in grain boundary properties. The results of this thesis provide researchers with several strategies for tuning the properties of amorphous-crystalline nanolaminates. These strategies include manipulated bulk attributes such as layer thickness and morphology as well as manipulation of microscale attributes such as grain boundary engineering. Finally, this thesis provides valuable insight into the damage loading/accommodation behavior of FCC symmetric tilt grain boundaries.

Imaging the Hydrogen Absorption Dynamics of Individual Grains in Polycrystalline Palladium Thin Films in 3D

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yau, Allison; Harder, Ross J.; Kanan, Matthew W.

Defects such as dislocations and grain boundaries often control the properties of polycrystalline materials. In nanocrystalline materials, investigating this structure-function relationship while preserving the sample remains challenging because of the short length scales and buried interfaces involved. Here we use Bragg coherent diffractive imaging to investigate the role of structural inhomogeneity on the hydriding phase transformation dynamics of individual Pd grains in polycrystalline films in three-dimensional detail. In contrast to previous reports on single- and polycrystalline nanoparticles, we observe no evidence of a hydrogen-rich surface layer and consequently no size dependence in the hydriding phase transformation pressure over a 125-325more » nm size range. We do observe interesting grain boundary dynamics, including reversible rotations of grain lattices while the material remains in the hydrogen-poor phase. The mobility of the grain boundaries, combined with the lack of a hydrogen-rich surface layer, suggests that the grain boundaries are acting as fast diffusion sites for the hydrogen atoms. Such hydrogen-enhanced plasticity in the hydrogen poor phase provides insight into the switch from the size-dependent behavior of single-crystal nanoparticles to the lower transformation pressures of polycrystalline materials and may play a role in hydrogen embrittlement.« less

A Filtering Method to Reveal Crystalline Patterns from Atom Probe Microscopy Desorption Maps

DTIC Science & Technology

2016-03-26

Gault, S.P. Ringer, J.M. Cairney, Atom probe crystallography : characterization of grain boundary orientation relationships in nanocrystalline...J.M. Cairney, Atom probe crystallography : atomic- scale 3-D orientation mapping, Scr. Mater. 66 (11) (2012) 907. L. Yao /MethodsX 3 (2016) 268–273 273

VISIONS: Remote Observations of a Spatially-Structured Filamentary Source of Energetic Neutral Atoms near the Polar Cap Boundary During an Auroral Substorm

NASA Technical Reports Server (NTRS)

Collier, Michael R.; Chornay, D.; Clemmons, J.; Keller, J. W.; Klenzing, J.; Kujawski, J.; McLain, J.; Pfaff, R.; Rowland, D.; Zettergren, M.

2015-01-01

We report initial results from the VISualizing Ion Outflow via Neutral atom imaging during a Substorm (VISIONS) rocket that flew through and near several regions of enhanced auroral activity and also sensed regions of ion outflow both remotely and directly. The observed neutral atom fluxes were largest at the lower energies and generally higher in the auroral zone than in the polar cap. In this paper, we focus on data from the latter half of the VISIONS trajectory when the rocket traversed the polar cap region. During this period, many of the energetic neutral atom spectra show a peak at 100 electronvolts. Spectra with peaks around 100 electronvolts are also observed in the Electrostatic Ion Analyzer (EIA) data consistent with these ions comprising the source population for the energetic neutral atoms. The EIA observations of this low energy population extend only over a few tens of kilometers. Furthermore, the directionality of the arriving energetic neutral atoms is consistent with either this spatially localized source of energetic ions extending from as low as about 300 kilometers up to above 600 kilometers or a larger source of energetic ions to the southwest.

Grain Boundary Character Distributions In Isostructural Materials

NASA Astrophysics Data System (ADS)

Ratanaphan, Sutatch

Anisotropic grain boundary character distributions (GBCDs), which influence macroscopic materials properties, are thought to be controlled by the grain boundary energy anisotropy. Structurally, grain boundary could be viewed as two free surfaces joined together. Grain boundary energy could be simply defined by the total excess energy for creating two free surfaces minus the energy gained when new bonds are formed between these surfaces. This implies that different crystal structure should have different GBEDs and GBCDs. It was recently discovered that grain boundary energy distributions (GBED) in isostructural materials, a class of materials that share the same crystal structure, are directly related to one another. This suggests that GBCDs in isostructural materials might also be related in a similar way. To test this hypothesis, electron backscatter diffraction (EBSD) was used to map grain orientations in Ag, Au, Cu, Fe, and Mo. The GBCDs were determined from the stereological interpretation of EBSD maps containing on the order of 100,000 grains. It was found that the GBCDs of face-centered cubic (FCC) metals are statistically correlated, while the GBCDs of body-centered cubic (BCC) Fe and Mo are not correlated to the GBCD of FCC metals. The degree of the correlations among the FCC metals is weaker if there are significant differences in grain shape or texture. For example, Ag has the weakest correlation to the other FCC materials and also has quantitatively different grain shapes and texture. The relationship between the populations and energies of grain boundaries was also studied. By comparing the GBCDs of Al, Au, Cu, and Ni to the energies of 388 grain boundaries previously calculated by the Embedded Atom Method (EAM), we observed a moderately inverse correlation between the relative areas of grain boundaries and their energies. Interestingly, there are strong inverse correlations between the energies and populations of the most common grain boundaries (Sigma3, Sigma9, and Sigma27). Because the enhancement of twin related boundaries due to the prevalence Sigma3 boundaries results in a decrease in the grain boundary populations for the other boundary types, this inverse correlation is influenced by the crystallographic constraints at triple junctions. In other words, having an anisotropic misorientation distribution with strong maxima for certain boundaries biases the inverse correlation between grain boundary population and energy for other boundaries and causes different slopes at each misorientation. Interestingly, the inverse correlation at each misorientation is consistent with the Boltzmann distribution. Based on our results, it is possible to predict the GBCDs and GBEDs in isostructural polycrystalline materials by using a single GBCD and GBED. This principle is demonstrated by predicting the GBCD and GBED of Actinium (Ac). To investigate the GBED in the isostructural BCC metals, the energies of 408 grain boundaries in Fe and Mo were computed using atomistic simulations based on the embedded-atom method (EAM) potential. We found that the calculated boundary energies in Fe and Mo were strongly correlated and scaled with the ratio of the cohesive energy divided by the square of the lattice constant (Ecoh/a02). We would expect that the GBCD of Fe and Mo might be correlated in a similar manner to that of FCC metals. To test this hypothesis, we compared the GBCDs of Fe and Mo. We found that the GBCDs of Fe and Mo are moderately and strongly correlated when all boundary types and only Sigma3 boundaries were considered, respectively. In this thesis, the results demonstrated that the GBCDs of isostructural materials are correlated with one another and the magnitudes of correlation coefficients varied. Reduced correlations were observed when there were differences in the microstructure and crystallographic texture. The inverse relationship between grain boundary population and energy is more strongly correlated at each misorientation than over the entire five macroscopic parameters of grain boundary, especially when there is significant misorientation texture. This relationship leads to GBCDs of isostructural materials that are also more strongly correlated at each misorientation than over the entire grain boundary space.

Scanning probe microscopy and field emission schemes for studying electron emission from polycrystalline diamond

NASA Astrophysics Data System (ADS)

Chubenko, Oksana; Baturin, Stanislav S.; Baryshev, Sergey V.

2016-09-01

The letter introduces a diagram that rationalizes tunneling atomic force microscopy (TUNA) observations of electron emission from polycrystalline diamonds as described in the recent publications [Chatterjee et al., Appl. Phys. Lett. 104, 171907 (2014); Harniman et al., Carbon 94, 386 (2015)]. The direct observations of electron emission from the grain boundary sites by TUNA could indeed be the evidence of electrons originating from grain boundaries under external electric fields. At the same time, from the diagram, it follows that TUNA and field emission schemes are complimentary rather than equivalent for results interpretation. It is further proposed that TUNA could provide better insights into emission mechanisms by measuring the detailed structure of the potential barrier on the surface of polycrystalline diamonds.

Cohesion enhancing effect of magnesium in aluminum grain boundary: A first-principles determination

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang Shengjun; Freeman, Arthur J.; Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208

2012-06-04

The effect of magnesium on grain boundary cohesion in aluminum was investigated by means of first-principles calculations using the Rice-Wang model [Rice and Wang, Mater. Sci. Eng. A 107, 23 (1989)]. It is demonstrated that magnesium is a cohesion enhancer with a potency of -0.11 eV/atom. It is further determined through electronic structure and bonding character analysis that the cohesion enhancing property of magnesium is due to a charge transfer mechanism which is unusually strong and overcomes the negative result of the size effect mechanism. Consistent with experimental results, this work clarifies the controversy and establishes that Mg segregation doesmore » not contribute to stress corrosion cracking in Al alloys.« less

Ion irradiation of the native oxide/silicon surface increases the thermal boundary conductance across aluminum/silicon interfaces

NASA Astrophysics Data System (ADS)

Gorham, Caroline S.; Hattar, Khalid; Cheaito, Ramez; Duda, John C.; Gaskins, John T.; Beechem, Thomas E.; Ihlefeld, Jon F.; Biedermann, Laura B.; Piekos, Edward S.; Medlin, Douglas L.; Hopkins, Patrick E.

2014-07-01

The thermal boundary conductance across solid-solid interfaces can be affected by the physical properties of the solid boundary. Atomic composition, disorder, and bonding between materials can result in large deviations in the phonon scattering mechanisms contributing to thermal boundary conductance. Theoretical and computational studies have suggested that the mixing of atoms around an interface can lead to an increase in thermal boundary conductance by creating a region with an average vibrational spectra of the two materials forming the interface. In this paper, we experimentally demonstrate that ion irradiation and subsequent modification of atoms at solid surfaces can increase the thermal boundary conductance across solid interfaces due to a change in the acoustic impedance of the surface. We measure the thermal boundary conductance between thin aluminum films and silicon substrates with native silicon dioxide layers that have been subjected to proton irradiation and post-irradiation surface cleaning procedures. The thermal boundary conductance across the Al/native oxide/Si interfacial region increases with an increase in proton dose. Supported with statistical simulations, we hypothesize that ion beam mixing of the native oxide and silicon substrate within ˜2.2nm of the silicon surface results in the observed increase in thermal boundary conductance. This ion mixing leads to the spatial gradation of the silicon native oxide into the silicon substrate, which alters the acoustic impedance and vibrational characteristics at the interface of the aluminum film and native oxide/silicon substrate. We confirm this assertion with picosecond acoustic analyses. Our results demonstrate that under specific conditions, a "more disordered and defected" interfacial region can have a lower resistance than a more "perfect" interface.

Electronic Structure Calculation of Permanent Magnets using the KKR Green's Function Method

NASA Astrophysics Data System (ADS)

Doi, Shotaro; Akai, Hisazumi

2014-03-01

Electronic structure and magnetic properties of permanent magnetic materials, especially Nd2Fe14B, are investigated theoretically using the KKR Green's function method. Important physical quantities in magnetism, such as magnetic moment, Curie temperature, and anisotropy constant, which are obtained from electronics structure calculations in both cases of atomic-sphere-approximation and full-potential treatment, are compared with past band structure calculations and experiments. The site preference of heavy rare-earth impurities are also evaluated through the calculation of formation energy with the use of coherent potential approximations. Further, the development of electronic structure calculation code using the screened KKR for large super-cells, which is aimed at studying the electronic structure of realistic microstructures (e.g. grain boundary phase), is introduced with some test calculations.

Atomic force microscopic imaging of Acanthamoeba castellanii and Balamuthia mandrillaris trophozoites and cysts.

PubMed

Aqeel, Yousuf; Siddiqui, Ruqaiyyah; Ateeq, Muhammad; Raza Shah, Muhammad; Kulsoom, Huma; Khan, Naveed Ahmed

2015-01-01

Light microscopy and electron microscopy have been successfully used in the study of microbes, as well as free-living protists. Unlike light microscopy, which enables us to observe living organisms or the electron microscope which provides a two-dimensional image, atomic force microscopy provides a three-dimensional surface profile. Here, we observed two free-living amoebae, Acanthamoeba castellanii and Balamuthia mandrillaris under the phase contrast inverted microscope, transmission electron microscope and atomic force microscope. Although light microscopy was of lower magnification, it revealed functional biology of live amoebae such as motility and osmoregulation using contractile vacuoles of the trophozoite stage, but it is of limited value in defining the cyst stage. In contrast, transmission electron microscopy showed significantly greater magnification and resolution to reveal the ultra-structural features of trophozoites and cysts including intracellular organelles and cyst wall characteristics but it only produced a snapshot in time of a dead amoeba cell. Atomic force microscopy produced three-dimensional images providing detailed topographic description of shape and surface, phase imaging measuring boundary stiffness, and amplitude measurements including width, height and length of A. castellanii and B. mandrillaris trophozoites and cysts. These results demonstrate the importance of the application of various microscopic methods in the biological and structural characterization of the whole cell, ultra-structural features, as well as surface components and cytoskeleton of protist pathogens. © 2014 The Author(s) Journal of Eukaryotic Microbiology © 2014 International Society of Protistologists.

Diffusion induced atomic islands on the surface of Ni/Cu nanolayers

NASA Astrophysics Data System (ADS)

Takáts, Viktor; Csik, Attila; Hakl, József; Vad, Kálmán

2018-05-01

Surface islands formed by grain-boundary diffusion has been studied in Ni/Cu nanolayers by in-situ low energy ion scattering spectroscopy, X-ray photoelectron spectroscopy, scanning probe microscopy and ex-situ depth profiling based on ion sputtering. In this paper a new experimental approach of measurement of grain-boundary diffusion coefficients is presented. Appearing time of copper atoms diffused through a few nanometer thick nickel layer has been detected by low energy ion scattering spectroscopy with high sensitivity. The grain-boundary diffusion coefficient can be directly calculated from this appearing time without using segregation factors in calculations. The temperature range of 423-463 K insures the pure C-type diffusion kinetic regime. The most important result is that surface coverage of Ni layer by Cu atoms reaches a maximum during annealing and stays constant if the annealing procedure is continued. Scanning probe microscopy measurements show a Volmer-Weber type layer growth of Cu layer on the Ni surface in the form of Cu atomic islands. Depth distribution of Cu in Ni layer has been determined by depth profile analysis.

Evolution of displacement cascades in Fe-Cr structures with different [001] tilt grain boundaries

NASA Astrophysics Data System (ADS)

Abu-Shams, M.; Haider, W.; Shabib, I.

2017-06-01

Reduced-activation ferritic/martensitic steels of Cr concentration between 2.25 and 12 wt% are candidate structural materials for next-generation nuclear reactors. In this study, molecular dynamics (MD) simulation is used to generate the displacement cascades in Fe-Cr structures with different Cr concentrations by using different primary knock-on atom (PKA) energies between 2 and 10 keV. A concentration-dependent model potential has been used to describe the interactions between Fe and Cr. Single crystals (SCs) of three different coordinate bases (e.g. [310], [510], and [530]) and bi-crystal (BC) structures with three different [001] tilt grain boundaries (GBs) (e.g. Σ5, Σ13, and Σ17) have been simulated. The Wigner-Seitz cell criterion has been used to identify the produced Frenkel pairs. The results show a marked difference between collisions observed in SCs and those in BC structures. The numbers of vacancies and interstitials are found to be significantly higher in BC structures than those found in SCs. The number of point defects exhibits a power relationship with the PKA energies; however, the Cr concentration does not seem to have any influence on the number of survived point defects. In BC models, a large fraction of the total survived point defects (between 59% and 93%) tends accumulate at the GBs, which seem to trap the generated point defects. The BC structure with Σ17 GB is found to trap more defects than Σ5 and Σ13 GBs. The defect trapping is found to be dictated by the crystallographic parameters of the GBs. For all studied GBs, self-interstitial atoms (SIAs) are easily trapped within the GB region than vacancies. An analysis of defect composition reveals an enrichment of Cr in SIAs, and in BC cases, more than half of the Cr-SIAs are found to be located within the GB region.

On the small angle twist sub-grain boundaries in Ti3AlC2.

PubMed

Zhang, Hui; Zhang, Chao; Hu, Tao; Zhan, Xun; Wang, Xiaohui; Zhou, Yanchun

2016-04-01

Tilt-dominated grain boundaries have been investigated in depth in the deformation of MAX phases. In stark contrast, another important type of grain boundaries, twist grain boundaries, have long been overlooked. Here, we report on the observation of small angle twist sub-grain boundaries in a typical MAX phase Ti3AlC2 compressed at 1200 °C, which comprise hexagonal screw dislocation networks formed by basal dislocation reactions. By first-principles investigations on atomic-scale deformation and general stacking fault energy landscapes, it is unequivocally demonstrated that the twist sub-grain boundaries are most likely located between Al and Ti4f (Ti located at the 4f Wyckoff sites of P63/mmc) layers, with breaking of the weakly bonded Al-Ti4f. The twist angle increases with the increase of deformation and is estimated to be around 0.5° for a deformation of 26%. This work may shed light on sub-grain boundaries of MAX phases, and provide fundamental information for future atomic-scale simulations.

Formation mechanism of Ruddlesden-Popper-type antiphase boundaries during the kinetically limited growth of Sr rich SrTiO3 thin films

PubMed Central

Xu, Chencheng; Du, Hongchu; van der Torren, Alexander J. H.; Aarts, Jan; Jia, Chun-Lin; Dittmann, Regina

2016-01-01

We elucidated the formation process for Ruddlesden-Popper-type defects during pulsed laser deposition of Sr rich SrTiO3 thin films by a combined analysis of in-situ atomic force microscopy, low energy electron diffraction and high resolution scanning transmission electron microscopy. At the early growth stage of 1.5 unit cells, the excess Sr results in the formation of SrO on the surface, resulting in a local termination change from TiO2 to SrO, thereby forming a Sr rich (2 × 2) surface reconstruction. With progressive SrTiO3 growth, islands with thermodynamically stable SrO rock-salt structure are formed, coexisting with TiO2 terminated islands. During the overgrowth of these thermodynamically stable islands, both lateral as well as vertical Ruddlesden-Popper-type anti-phase boundaries are formed, accommodating the Sr excess of the SrTiO3 film. We suggest the formation of thermodynamically stable SrO rock-salt structures as origin for the formation of Ruddlesden-Popper-type antiphase boundaries, which are as a result of kinetic limitations confined to certain regions on the surface. PMID:27922069

Formation mechanism of Ruddlesden-Popper-type antiphase boundaries during the kinetically limited growth of Sr rich SrTiO3 thin films

NASA Astrophysics Data System (ADS)

Xu, Chencheng; Du, Hongchu; van der Torren, Alexander J. H.; Aarts, Jan; Jia, Chun-Lin; Dittmann, Regina

2016-12-01

We elucidated the formation process for Ruddlesden-Popper-type defects during pulsed laser deposition of Sr rich SrTiO3 thin films by a combined analysis of in-situ atomic force microscopy, low energy electron diffraction and high resolution scanning transmission electron microscopy. At the early growth stage of 1.5 unit cells, the excess Sr results in the formation of SrO on the surface, resulting in a local termination change from TiO2 to SrO, thereby forming a Sr rich (2 × 2) surface reconstruction. With progressive SrTiO3 growth, islands with thermodynamically stable SrO rock-salt structure are formed, coexisting with TiO2 terminated islands. During the overgrowth of these thermodynamically stable islands, both lateral as well as vertical Ruddlesden-Popper-type anti-phase boundaries are formed, accommodating the Sr excess of the SrTiO3 film. We suggest the formation of thermodynamically stable SrO rock-salt structures as origin for the formation of Ruddlesden-Popper-type antiphase boundaries, which are as a result of kinetic limitations confined to certain regions on the surface.

Hydrogen behaviour at twist {110} grain boundaries in α-Fe

NASA Astrophysics Data System (ADS)

McEniry, Eunan J.; Hickel, Tilmann; Neugebauer, Jörg

2017-06-01

The behaviour of hydrogen at structural defects such as grain boundaries plays a critical role in the phenomenon of hydrogen embrittlement. However, characterization of the energetics and diffusion of hydrogen in the vicinity of such extended defects using conventional ab initio techniques is challenging due to the relatively large system sizes required when dealing with realistic grain boundary geometries. In order to be able to access the required system sizes, as well as high-throughput testing of a large number of configurations, while remaining within a quantum-mechanical framework, an environmental tight-binding model for the iron-hydrogen system has been developed. The resulting model is applied to study the behaviour of hydrogen at a class of low-energy {110}-terminated twist grain boundaries in α-Fe. We find that, for particular Σ values within the coincidence site lattice description, the atomic geometry at the interface plane provides extremely favourable trap sites for H, which also possess high escape barriers for diffusion. By contrast, via simulated tensile testing, weakly trapped hydrogen at the interface plane of the bulk-like Σ3 boundary acts as a `glue' for the boundary, increasing both the energetic barrier and the elongation to rupture. This article is part of the themed issue 'The challenges of hydrogen and metals'.

Hydrogen behaviour at twist {110} grain boundaries in α-Fe.

PubMed

McEniry, Eunan J; Hickel, Tilmann; Neugebauer, Jörg

2017-07-28

The behaviour of hydrogen at structural defects such as grain boundaries plays a critical role in the phenomenon of hydrogen embrittlement. However, characterization of the energetics and diffusion of hydrogen in the vicinity of such extended defects using conventional ab initio techniques is challenging due to the relatively large system sizes required when dealing with realistic grain boundary geometries. In order to be able to access the required system sizes, as well as high-throughput testing of a large number of configurations, while remaining within a quantum-mechanical framework, an environmental tight-binding model for the iron-hydrogen system has been developed. The resulting model is applied to study the behaviour of hydrogen at a class of low-energy {110}-terminated twist grain boundaries in α -Fe. We find that, for particular Σ values within the coincidence site lattice description, the atomic geometry at the interface plane provides extremely favourable trap sites for H, which also possess high escape barriers for diffusion. By contrast, via simulated tensile testing, weakly trapped hydrogen at the interface plane of the bulk-like Σ3 boundary acts as a 'glue' for the boundary, increasing both the energetic barrier and the elongation to rupture.This article is part of the themed issue 'The challenges of hydrogen and metals'. © 2017 The Author(s).

Probing nanocrystalline grain dynamics in nanodevices

PubMed Central

Yeh, Sheng-Shiuan; Chang, Wen-Yao; Lin, Juhn-Jong

2017-01-01

Dynamical structural defects exist naturally in a wide variety of solids. They fluctuate temporally and hence can deteriorate the performance of many electronic devices. Thus far, the entities of these dynamic objects have been identified to be individual atoms. On the other hand, it is a long-standing question whether a nanocrystalline grain constituted of a large number of atoms can switch, as a whole, reversibly like a dynamical atomic defect (that is, a two-level system). This is an emergent issue considering the current development of nanodevices with ultralow electrical noise, qubits with long quantum coherence time, and nanoelectromechanical system sensors with ultrahigh resolution. We demonstrate experimental observations of dynamic nanocrystalline grains that repeatedly switch between two or more metastable coordinate states. We study temporal resistance fluctuations in thin ruthenium dioxide (RuO2) metal nanowires and extract microscopic parameters, including relaxation time scales, mobile grain sizes, and the bonding strengths of nanograin boundaries. These material parameters are not obtainable by other experimental approaches. When combined with previous in situ high-resolution transmission electron microscopy, our electrical method can be used to infer rich information about the structural dynamics of a wide variety of nanodevices and new two-dimensional materials. PMID:28691094

Free vibration analysis of single-walled boron nitride nanotubes based on a computational mechanics framework

NASA Astrophysics Data System (ADS)

Yan, J. W.; Tong, L. H.; Xiang, Ping

2017-12-01

Free vibration behaviors of single-walled boron nitride nanotubes are investigated using a computational mechanics approach. Tersoff-Brenner potential is used to reflect atomic interaction between boron and nitrogen atoms. The higher-order Cauchy-Born rule is employed to establish the constitutive relationship for single-walled boron nitride nanotubes on the basis of higher-order gradient continuum theory. It bridges the gaps between the nanoscale lattice structures with a continuum body. A mesh-free modeling framework is constructed, using the moving Kriging interpolation which automatically satisfies the higher-order continuity, to implement numerical simulation in order to match the higher-order constitutive model. In comparison with conventional atomistic simulation methods, the established atomistic-continuum multi-scale approach possesses advantages in tackling atomic structures with high-accuracy and high-efficiency. Free vibration characteristics of single-walled boron nitride nanotubes with different boundary conditions, tube chiralities, lengths and radii are examined in case studies. In this research, it is pointed out that a critical radius exists for the evaluation of fundamental vibration frequencies of boron nitride nanotubes; opposite trends can be observed prior to and beyond the critical radius. Simulation results are presented and discussed.

Spatially-resolved studies on the role of defects and boundaries in electronic behavior of 2D materials

NASA Astrophysics Data System (ADS)

Hus, Saban M.; Li, An-Ping

2017-08-01

Two-dimensional (2D) materials are intrinsically heterogeneous. Both localized defects, such as vacancies and dopants, and mesoscopic boundaries, such as surfaces and interfaces, give rise to compositional or structural heterogeneities. The presence of defects and boundaries can break lattice symmetry, modify the energy landscape, and create quantum confinement, leading to fascinating electronic properties different from the ;ideal; 2D sheets. This review summarizes recent progress in understanding the roles of defects and boundaries in electronic, magnetic, thermoelectric, and transport properties of 2D layered materials. The focus is on the understanding of correlation of atomic-scale structural information with electronic functions by interrogating heterogeneities individually. The materials concerned are graphene, transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), and topological insulators (TIs). The experimental investigations benefit from new methodologies and techniques in scanning tunneling microscopy (STM), including spin-polarized STM, scanning tunneling potentiometry (STP), scanning tunneling thermopower microscopy, and multi-probe STM. The experimental effort is complemented by the computational and theoretical approaches, capable of discriminating between closely competing states and achieving the length scales necessary to bridge across features such as local defects and complex heterostructures. The goal is to provide a general view of current understanding and challenges in studying the heterogeneities in 2D materials and to evaluate the potential of controlling and exploiting these heterogeneities for novel functionalities and electron devices.

Modeling off-resonant nonlinear-optical cascading in mesoscopic thin films and guest-host molecular systems

NASA Astrophysics Data System (ADS)

Dawson, Nathan J.; Andrews, James H.; Crescimanno, Michael

2013-12-01

A model for off-resonant microscopic cascading of (hyper)polarizabilities is developed using a self-consistent field approach to study mesoscopic systems of nonlinear polarizable atoms and molecules. We find enhancements in the higher-order susceptibilities resulting from geometrical and boundary orientation effects. We include an example of the dependence on excitation beam cross sectional structure and a simplified derivation of the microscopic cascading of the nonlinear-optical response in guest-host systems.

Features of structural changes in the near-surface aluminum layer under various schemes of ion implantation

NASA Astrophysics Data System (ADS)

Kryzhevich, Dmitrij S.; Zolnikov, Konstantin P.; Korchuganov, Aleksandr V.

2017-10-01

The molecular dynamics simulation of structural rearrangements in the surface layer of aluminum samples under ion implantation of various intensities was carried out. The features of the internal structure and the crystallographic orientation of the irradiated crystallite were taken into account. To describe the interatomic interaction many-body potentials obtained in the framework of the embedded atom method were used. Irradiation of the {100} surface results in much less number of formed defects than irradiation of the {110} and {111} ones. When irradiating surfaces with beams of relatively low energy grains remain unchanged in the surface region and the formation of stacking faults was not observed. At a high intensity of irradiation, the near-surface layer of the crystallite melts. In the absence of heat removal, the centers of crystallization become grains lying on the boundary of the solid and liquid phases. Those grains increase due to the adjustment of the atoms of the liquid phase to their lattice. As a result, the grain size in the near-surface region increases.

Comparison between layering NbSe2 and rod characteristic of MgB2 by investigation of elastic constants

NASA Astrophysics Data System (ADS)

Shokri, Asiye; Yazdani, Ahmad; Barakati, Behrad

2018-03-01

The delicate balancing of strong anisotropy on strength of hybridisation resulted to CDW- order “TCDW=33K” and finally emerging superconductivity at “Tc = 7.2K” are the most intriguing question in characteristic behaviour of NbSe2. On other hand, the original mechanism of MgB2 old superconductor, which has unlike the cuprates a lower anisotropy on strength hybridisation is still unknown. We believe this could result to bond exchange and larger coherence length of the grain boundary to current. Since the cause and the mechanism of band strengths of two original layering and rod structures are consequence of bond- rupturing-atomic displacement, here the stability of crystalline structure of inter atomic potential through the elasticity-compressibility is investigated. Consequently, in order to clear out the strong difference between the layering NbSe2 and domination of rod-character of MgB2 the stability of both crystal structures through the cohesive energy c/a, czz and c33 are investigated. The proposed investigations are more evident on different characteristic behaviour of calculated parameters.

Origin of High Electronic Quality in Solar Cell Absorber CH3NH3PbI3

NASA Astrophysics Data System (ADS)

Yin, Wanjian; Shi, Tingting; Wei, Suhua; Yan, Yanfa

Thin-film solar cells based on CH3NH3PbI3 halide perovskites have recently shown remarkable performance. First-principle calculations and molecular dynamic simulations show that the structure of pristine CH3NH3PbI3 is much more disordered than the inorganic archetypal thin-film semiconductor CdTe. However, the structural disorders from thermal fluctuation, point defects and grain boundaries introduce rare deep defect states within the bandgaps; therefore, the material has high electronic quality. We have further shown that this unusually high electronic quality is attributed to the unique electronic structures of halide perovskite: the strong coupling between cation lone-pair Pb s orbitals and anion p orbitals and the large atomic size of constitute cation atoms. We further found that although CH3NH3PbI3 GBs do not introduce a deep gap state, the defect level close to the VBM can still act as a shallow hole trap state. Cl and O can spontaneously segregate into GBs and passivate those defect levels and deactivate the trap state.

Spatial Concentrations of Silicon Atoms in RF Discharges of Silane.

DTIC Science & Technology

1985-02-18

regions. These profiles were much more sensitive to plasma chemistry changes than profiles obtained from plasma emission. Experiments with nitrogen...addition demonstrated significant changes in the silicon atom profiles near the sheath boundary. Originator supplied keywords include: rf discharge, silane, plasma chemistry , silicon atom, laser-induced fluorescence.

The Effect of Water and Confinement on Self-Assembly of Imidazolium Based Ionic Liquids at Mica Interfaces

PubMed Central

Cheng, H.-W.; Dienemann, J.-N.; Stock, P.; Merola, C.; Chen, Y.-J.; Valtiner, M.

2016-01-01

Tuning chemical structure and molecular layering of ionic liquids (IL) at solid interfaces offers leverage to tailor performance of ILs in applications such as super-capacitors, catalysis or lubrication. Recent experimental interpretations suggest that ILs containing cations with long hydrophobic tails form well-ordered bilayers at interfaces. Here we demonstrate that interfacial bilayer formation is not an intrinsic quality of hydrophobic ILs. In contrast, bilayer formation is triggered by boundary conditions including confinement, surface charging and humidity present in the IL. Therefore, we performed force versus distance profiles using atomic force microscopy and the surface forces apparatus. Our results support models of disperse low-density bilayer formation in confined situations, at high surface charging and/or in the presence of water. Conversely, interfacial structuring of long-chain ILs in dry environments and at low surface charging is disordered and dominated by bulk structuring. Our results demonstrate that boundary conditions such as charging, confinement and doping by impurities have decisive influence on structure formation of ILs at interfaces. As such, these results have important implications for understanding the behavior of solid/IL interfaces as they significantly extend previous interpretations. PMID:27452615

The Effect of Water and Confinement on Self-Assembly of Imidazolium Based Ionic Liquids at Mica Interfaces.

PubMed

Cheng, H-W; Dienemann, J-N; Stock, P; Merola, C; Chen, Y-J; Valtiner, M

2016-07-25

Tuning chemical structure and molecular layering of ionic liquids (IL) at solid interfaces offers leverage to tailor performance of ILs in applications such as super-capacitors, catalysis or lubrication. Recent experimental interpretations suggest that ILs containing cations with long hydrophobic tails form well-ordered bilayers at interfaces. Here we demonstrate that interfacial bilayer formation is not an intrinsic quality of hydrophobic ILs. In contrast, bilayer formation is triggered by boundary conditions including confinement, surface charging and humidity present in the IL. Therefore, we performed force versus distance profiles using atomic force microscopy and the surface forces apparatus. Our results support models of disperse low-density bilayer formation in confined situations, at high surface charging and/or in the presence of water. Conversely, interfacial structuring of long-chain ILs in dry environments and at low surface charging is disordered and dominated by bulk structuring. Our results demonstrate that boundary conditions such as charging, confinement and doping by impurities have decisive influence on structure formation of ILs at interfaces. As such, these results have important implications for understanding the behavior of solid/IL interfaces as they significantly extend previous interpretations.

Analysis of the ceramic layer microstructure influence on plasma spray thermal barrier coating performance

NASA Astrophysics Data System (ADS)

Bogdanovich, V. I.; Giorbelidze, M. G.

2017-12-01

This paper outlines the results of analysis and describes the structure of the thermal protection coatings formed by atomic ion stream deposition in vacuum, and plasma thermal spraying method. Crystallite structure features are considered along with the crystallite dimensions, spatial orientation, and position of the boundaries between separate crystallites. Discontinuity, volume, and morphology of the pores has been evaluated. Experimental studies have been accomplished using various fractions of the powder-like material ZrO2 - 8%Y2O3. The influence of the coating microstructure on the coating performance has been analyzed, such as adhesive strength, thermal stability, and thermal conductivity.

27 CFR 9.69 - Yakima Valley.

Code of Federal Regulations, 2014 CFR

2014-04-01

... Bennett Ranch; (2) Then due east approximately 0.2 mile to the boundary of the Hanford Atomic Energy Commission Works; (3) Then southeast following the boundary of the Hanford AEC Works along the Rattlesnake...

27 CFR 9.69 - Yakima Valley.

Code of Federal Regulations, 2013 CFR

2013-04-01

... Bennett Ranch; (2) Then due east approximately 0.2 mile to the boundary of the Hanford Atomic Energy Commission Works; (3) Then southeast following the boundary of the Hanford AEC Works along the Rattlesnake...

27 CFR 9.69 - Yakima Valley.

Code of Federal Regulations, 2012 CFR

2012-04-01

... Bennett Ranch; (2) Then due east approximately 0.2 mile to the boundary of the Hanford Atomic Energy Commission Works; (3) Then southeast following the boundary of the Hanford AEC Works along the Rattlesnake...

Grain-boundary physics in polycrystalline CuInSe2 revisited: experiment and theory.

PubMed

Yan, Yanfa; Noufi, R; Al-Jassim, M M

2006-05-26

Current studies have attributed the remarkable performance of polycrystalline CuInSe2 (CIS) to anomalous grain-boundary (GB) physics in CIS. The recent theory predicts that GBs in CIS are hole barriers, which prevent GB electrons from recombining. We examine the atomic structure and chemical composition of (112) GBs in Cu(In,Ga)Se2 (CIGS) using high-resolution Z-contrast imaging and nanoprobe x-ray energy-dispersive spectroscopy. We show that the theoretically predicted Cu-vacancy rows are not observed in (112) GBs in CIGS. Our first-principles modeling further reveals that the (112) GBs in CIS do not act as hole barriers. Our results suggest that the superior performance of polycrystalline CIS should not be explained solely by the GB behaviors.

The stability boundary of group-III transition metal diboride ScB 2 (0 0 0 1) surfaces

NASA Astrophysics Data System (ADS)

Zhao, Hui; Qin, Na

2012-01-01

Experimental observations and theoretical investigations exhibit that a group-IV(V) transition metal diboride (0 0 0 1) surface is terminated with a 1 × 1 TM(B) layer. As to a group-III transition metal diboride, we have investigated the stability boundary of ScB2 (0 0 0 1) surfaces using first principles total energy plane-wave pseudopotential method based on density functional theory. The Mulliken charge population analysis shows that Sc atoms in the second layer cannot provide B atoms in the first layer with sufficient electrons to form a complete graphene-like boron layer. We also found that the charge transfer between the first and the second layer for the B-terminated surface is more than that for Sc-terminated surface. It elucidates the reason that the outermost interlayer spacing contract more strongly in the B-terminated surface than in the Sc-terminated surface. The surface energies of both terminated ScB2 (0 0 0 1) surfaces as a function of the chemical potential of B are also calculated to check the relative stability of the two surface structures.

Peculiar atomic dynamics in liquid GeTe with asymmetrical bonding: Observation by inelastic x-ray scattering

NASA Astrophysics Data System (ADS)

Inui, M.; Koura, A.; Kajihara, Y.; Hosokawa, S.; Chiba, A.; Kimura, K.; Shimojo, F.; Tsutsui, S.; Baron, A. Q. R.

2018-05-01

Collective dynamics in liquid GeTe was investigated by inelastic x-ray scattering at 2 ≤Q ≤31 nm-1 . The dynamic structure factor shows clear inelastic excitations. The excitation energies at low Q disperse with increasing Q , consistent with the behavior of a longitudinal-acoustic excitation. The dispersion curve has a flat-topped region around the pseudo-Brillouin-zone boundary, similar to what is observed in liquid Bi [Inui et al., Phys. Rev. B 92, 054206 (2015), 10.1103/PhysRevB.92.054206]. The dynamic structure factor shows a low-frequency excitation, and its coupling with the longitudinal-acoustic mode plays an important role for a flat-topped dispersion. From these results, it is inferred that atomic dynamics in liquid GeTe is strongly affected by a Peierls distortion similar to liquid Bi. By comparing the momentum transfer dependence of the excitation energy and quasielastic linewidth to partial structure factors obtained by our own ab initio molecular dynamics simulation for liquid GeTe, the quasielastic and inelastic components were found to be correlated with Te-Te and Ge-(Ge,Te) partial structure factors, respectively.

The effect of grain boundaries on the resistivity of polycrystalline silicon. Ph.D. Thesis - Va. Univ.

NASA Technical Reports Server (NTRS)

Fripp, A. L., Jr.

1974-01-01

The electrical resistivity of polycrystalline silicon films was investigated. The films were grown by the chemical vapor decomposition of silane on oxidized silicon wafers. The resistivity was found to be independent of dopant atom concentration in the lightly doped regions but was a strong function of dopant levels in the more heavily doped regions. A model, based on high dopant atom segregation in the grain boundaries, is proposed to explain the results.

Spectroscopic signatures of different symmetries of the superconducting order parameter in metal-decorated graphene

NASA Astrophysics Data System (ADS)

Saari, Timo; Nieminen, Jouko; Bansil, Arun

2017-06-01

Motivated by the recent experiments indicating superconductivity in metal-decorated graphene sheets, we investigate their quasi-particle structure within the framework of an effective tight-binding Hamiltonian augmented by appropriate BCS-like pairing terms for p-type order parameter. The normal state band structure of graphene is modified not only through interaction with adsorbed metal atoms, but also due to the folding of bands at Brillouin zone boundaries resulting from a \\sqrt{3}× \\sqrt{3}R{{30}\\circ} reconstruction. Several different types of pairing symmetries are analyzed utilizing Nambu-Gorkov Green’s function techniques to show that p+\\text{i}p -symmetric nearest-neighbor pairing yields the most enhanced superconducting gap. The character of the order parameter depends on the nature of the atomic orbitals involved in the pairing process and exhibits interesting angular and radial asymmetries. Finally, we suggest a method to distinguish between singlet and triplet type superconductivity in the presence of magnetic substitutional impurities using scanning tunneling spectroscopy.

Mathematical Modeling of Resonant Processes in Confined Geometry of Atomic and Atom-Ion Traps

NASA Astrophysics Data System (ADS)

Melezhik, Vladimir S.

2018-02-01

We discuss computational aspects of the developed mathematical models for resonant processes in confined geometry of atomic and atom-ion traps. The main attention is paid to formulation in the nondirect product discrete-variable representation (npDVR) of the multichannel scattering problem with nonseparable angular part in confining traps as the boundary-value problem. Computational efficiency of this approach is demonstrated in application to atomic and atom-ion confinement-induced resonances we predicted recently.

Interface structures and twinning mechanisms of {1¯012} twins in hexagonal metals

DOE PAGES

Gong, Mingyu; Hirth, John P.; Liu, Yue; ...

2017-06-07

In this paper, a controversy concerning the description of {1¯012} {1¯012} twinning, whether it is shear-shuffle or pure glide-shuffle or pure shuffle, has developed. There is disagreement about the interpretation of transmission electron microscopic observations, atomistic simulations and theories for twin growth. In this article, we highlight the atomic-level, characteristic, equilibrium and non-equilibrium boundaries and corresponding boundary defects associated with the three-dimensional ‘normal’, ‘forward’ and ‘lateral’ propagation of {1¯011} growth/annealing and deformation twins. Although deformation twin boundaries (TBs) after recovery exhibit some similarity to growth/annealing TBs because of the plastic accommodation of stress fields, there are important distinctions among them.more » These distinctions distinguish among the mechanisms of twin growth and resolve the controversy. In addition, a new type of disconnection, a glide disclination, is described for twinning. Synchroshear, seldom considered, is shown to be a likely mechanism for {1¯012} twinning.« less

ON THE GEOMETRY OF THE IBEX RIBBON

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sylla, Adama; Fichtner, Horst

2015-10-01

The Energetic Neutral Atom (ENA) full-sky maps obtained with the Interstellar Boundary Explorer (IBEX) show an unexpected bright narrow band of increased intensity. This so-called ENA ribbon results from charge exchange of interstellar neutral atoms with protons in the outer heliosphere or beyond. Among other hypotheses it has been argued that this ribbon may be related to a neutral density enhancement, or H-wave, in the local interstellar medium. Here we quantitatively demonstrate, on the basis of an analytical model of the principal large-scale heliospheric structure, that this scenario for the ribbon formation leads to results that are fully consistent withmore » the observed location of the ribbon in the full-sky maps at all energies detected with high-energy sensor IBEX-Hi.« less

Interphase boundary misorientation in mantle rocks

NASA Astrophysics Data System (ADS)

Morales, L. F.; Mainprice, D.; Boudier, F. I.

2017-12-01

Interphase boundaries are planar defects that separate two different phases, which may have different compositions and/or crystalline structures. Depending on the degree of atomic structure matching between the two adjacent phases, the interphase boundaries can be classified in coherent, semicoherent and incoherent phase boundaries. Here we present the recent developments of interphase misorientation boundary analyses calculated from EBSD data in an olivine-antigorite schist from the Val Malenco (Italy) and a spinel lherzolite from the Horoman peridotite complex (Japan). The antigorite schist is strongly foliated and contains about 78% antigorite and 22% olivine, with minor amounts (<1%) of magnetite and chlorite. The antigorite CPO is characterized by a point maxima of poles to (100) parallel to lineation and poles to (001) to the foliation normal. Phase transformation relationships between olivine and antigorite are evident in phase boundary misorientation analysis, (100)ol||(001)atg being more frequent than [001]ol||[010]atg. From the interphase misorientation analyses, we have described two new phase transformation relationships between olivine and antigorite. The studied lherzolite contain 70% olivine, 15% enstatite, 13% diopside and 2% spinel. It has a porphyroclastic texture materialized by enstatite and olivine in a matrix of olivine. Both enstatite, diopside and spinel occur along discontinuous bands parallel to the foliation of the sample. Olivine bulk CPO can be described as a fibre-[100], while both enstatite and diopside show a (001) fibre texture. Interphase misorientation angle distribution between olivine-enstatite and olivine-diopside follow approximately the distribution expected for uniform texture, with some minor (but important) differences at high angle phase boundaries, particularly for olivine-diopside. The pair angle-misorientation axes for the olivine-enstatite show a relatively uniform distribution for different misorientation angle intervals. On the other hand there is a clear concentration of misorientation axes parallel to [010] of olivine in the case of olivine-diopside phase boundaries, possibly related to melt percolation. These differences demonstrate the potential use of interphase misorientation for the study of material processes in rocks.

Local probe microscopic studies on Al-doped ZnO: Pseudoferroelectricity and band bending at grain boundaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kumar, Mohit; Basu, Tanmoy; Som, Tapobrata, E-mail: tsom@iopb.res.in

2016-01-07

In this paper, based on piezoforce measurements, we show the presence of opposite polarization at grains and grain boundaries of Al-doped ZnO (AZO). The polarization can be flipped by 180° in phase by switching the polarity of the applied electric field, revealing the existence of nanoscale pseudoferroelectricity in AZO grown on Pt/TiO{sub 2}/SiO{sub 2}/Si substrate. We also demonstrate an experimental evidence on local band bending at grain boundaries of AZO films using conductive atomic force microscopy and Kelvin probe force microscopy. The presence of an opposite polarization at grains and grain boundaries gives rise to a polarization-driven barrier formation atmore » grain boundaries. With the help of conductive atomic force microscopy, we show that the polarization-driven barrier along with the defect-induced electrostatic potential barrier account for the measured local band bending at grain boundaries. The present study opens a new avenue to understand the charge transport in light of both polarization and electrostatic effects.« less

Adsorption of metal atoms at a buckled graphene grain boundary using model potentials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Helgee, Edit E.; Isacsson, Andreas

Two model potentials have been evaluated with regard to their ability to model adsorption of single metal atoms on a buckled graphene grain boundary. One of the potentials is a Lennard-Jones potential parametrized for gold and carbon, while the other is a bond-order potential parametrized for the interaction between carbon and platinum. Metals are expected to adsorb more strongly to grain boundaries than to pristine graphene due to their enhanced adsorption at point defects resembling those that constitute the grain boundary. Of the two potentials considered here, only the bond-order potential reproduces this behavior and predicts the energy of themore » adsorbate to be about 0.8 eV lower at the grain boundary than on pristine graphene. The Lennard-Jones potential predicts no significant difference in energy between adsorbates at the boundary and on pristine graphene. These results indicate that the Lennard-Jones potential is not suitable for studies of metal adsorption on defects in graphene, and that bond-order potentials are preferable.« less

Role of out-of-plane dielectric thickness in the electrostatic simulation of atomically thin lateral junctions

NASA Astrophysics Data System (ADS)

Nipane, Ankur; Zhang, Yefei; Teherani, James T.

2018-06-01

Two-dimensional materials enable novel electronic and optoelectronic devices due to their unique properties. Device modeling plays a fundamental role in developing these novel devices by providing insights into the underlying physics. In this work, we present the dramatic impact of the simulated out-of-plane dielectric thickness on the electrostatics of lateral junctions formed from atomically thin materials. We show that unlike bulk junctions, the boundary conditions on the edges of the simulation region significantly affect the electrostatics of two-dimensional (2D) lateral junctions by modifying the out-of-plane electric field. We also present an intuitive understanding of the Neumann boundary conditions imposed on the boundaries of the simulation region. The Neumann boundary conditions alter the intended simulation by generating reflections of the device across the boundaries. Finally, we derive a minimal dielectric thickness for a symmetrically doped 2D lateral p-n junction, above which the out-of-plane simulation region boundaries minimally affect the simulated electric field, electrostatic potential, and depletion width of the junction.

Multiscale simulation of xenon diffusion and grain boundary segregation in UO₂

DOE PAGES

Andersson, David A.; Tonks, Michael R.; Casillas, Luis; ...

2015-07-01

In light water reactor fuel, gaseous fission products segregate to grain boundaries, resulting in the nucleation and growth of large intergranular fission gas bubbles. The segregation rate is controlled by diffusion of fission gas atoms through the grains and interaction with the boundaries. Based on the mechanisms established from earlier density functional theory (DFT) and empirical potential calculations, diffusion models for xenon (Xe), uranium (U) vacancies and U interstitials in UO₂ have been derived for both intrinsic (no irradiation) and irradiation conditions. Segregation of Xe to grain boundaries is described by combining the bulk diffusion model with a model formore » the interaction between Xe atoms and three different grain boundaries in UO₂ (Σ5 tilt, Σ5 twist and a high angle random boundary), as derived from atomistic calculations. The present model does not attempt to capture nucleation or growth of fission gas bubbles at the grain boundaries. The point defect and Xe diffusion and segregation models are implemented in the MARMOT phase field code, which is used to calculate effective Xe and U diffusivities as well as to simulate Xe redistribution for a few simple microstructures.« less

Electric field dependent local structure of (KxNa1-x) 0.5B i0.5Ti O3

NASA Astrophysics Data System (ADS)

Goetzee-Barral, A. J.; Usher, T.-M.; Stevenson, T. J.; Jones, J. L.; Levin, I.; Brown, A. P.; Bell, A. J.

2017-07-01

The in situ x-ray pair-distribution function (PDF) characterization technique has been used to study the behavior of (KxNa1-x) 0.5B i0.5Ti O3 , as a function of electric field. As opposed to conventional x-ray Bragg diffraction techniques, PDF is sensitive to local atomic displacements, detecting local structural changes at the angstrom to nanometer scale. Several field-dependent ordering mechanisms can be observed in x =0.15 , 0.18 and at the morphotropic phase boundary composition x =0.20 . X-ray total scattering shows suppression of diffuse scattering with increasing electric-field amplitude, indicative of an increase in structural ordering. Analysis of PDF peaks in the 3-4-Å range shows ordering of Bi-Ti distances parallel to the applied electric field, illustrated by peak amplitude redistribution parallel and perpendicular to the electric-field vector. A transition from <110 > to <112 > -type off-center displacements of Bi relative to the neighboring Ti atoms is observable with increasing x . Analysis of PDF peak shift with electric field shows the effects of Bi-Ti redistribution and onset of piezoelectric lattice strain. The combination of these field-induced ordering mechanisms is consistent with local redistribution of Bi-Ti distances associated with domain reorientation and an overall increase in order of atomic displacements.

Electric field dependent local structure of ( K x N a 1 - x ) 0.5 B i 0.5 Ti O 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goetzee-Barral, A. J.; Usher, T. -M.; Stevenson, T. J.

The in situ x-ray pair-distribution function (PDF) characterization technique has been used to study the behavior of (K xNa 1–x) 0.5Bi 0.5TiO 3, as a function of electric field. As opposed to conventional x-ray Bragg diffraction techniques, PDF is sensitive to local atomic displacements, detecting local structural changes at the angstrom to nanometer scale. Several field-dependent ordering mechanisms can be observed in x = 0.15, 0.18 and at the morphotropic phase boundary composition x = 0.20. X-ray total scattering shows suppression of diffuse scattering with increasing electric-field amplitude, indicative of an increase in structural ordering. Analysis of PDF peaks inmore » the 3–4-Å range shows ordering of Bi-Ti distances parallel to the applied electric field, illustrated by peak amplitude redistribution parallel and perpendicular to the electric-field vector. A transition from < 110 > to < 112 >-type off-center displacements of Bi relative to the neighboring Ti atoms is observable with increasing x. Analysis of PDF peak shift with electric field shows the effects of Bi-Ti redistribution and onset of piezoelectric lattice strain. Furthermore, the combination of these field-induced ordering mechanisms is consistent with local redistribution of Bi-Ti distances associated with domain reorientation and an overall increase in order of atomic displacements.« less

Electric field dependent local structure of ( K x N a 1 - x ) 0.5 B i 0.5 Ti O 3

DOE PAGES

Goetzee-Barral, A. J.; Usher, T. -M.; Stevenson, T. J.; ...

2017-07-31

The in situ x-ray pair-distribution function (PDF) characterization technique has been used to study the behavior of (K xNa 1–x) 0.5Bi 0.5TiO 3, as a function of electric field. As opposed to conventional x-ray Bragg diffraction techniques, PDF is sensitive to local atomic displacements, detecting local structural changes at the angstrom to nanometer scale. Several field-dependent ordering mechanisms can be observed in x = 0.15, 0.18 and at the morphotropic phase boundary composition x = 0.20. X-ray total scattering shows suppression of diffuse scattering with increasing electric-field amplitude, indicative of an increase in structural ordering. Analysis of PDF peaks inmore » the 3–4-Å range shows ordering of Bi-Ti distances parallel to the applied electric field, illustrated by peak amplitude redistribution parallel and perpendicular to the electric-field vector. A transition from < 110 > to < 112 >-type off-center displacements of Bi relative to the neighboring Ti atoms is observable with increasing x. Analysis of PDF peak shift with electric field shows the effects of Bi-Ti redistribution and onset of piezoelectric lattice strain. Furthermore, the combination of these field-induced ordering mechanisms is consistent with local redistribution of Bi-Ti distances associated with domain reorientation and an overall increase in order of atomic displacements.« less

Effect of the interface on the mechanical properties and thermal conductivity of bismuth telluride films

NASA Astrophysics Data System (ADS)

Lai, Tang-Yu; Wang, Kuan-Yu; Fang, Te-Hua; Huang, Chao-Chun

2018-02-01

Bismuth telluride (Bi2Te3) is a type of thermoelectric material used for energy generation that does not cause pollution. Increasing the thermoelectric conversion efficiency (ZT) is one of the most important steps in the development of thermoelectric components. In this study, we use molecular dynamics to investigate the mechanical properties and thermal conductivity of quintuple layers of Bi2Te3 nanofilms with different atomic arrangements at the interface and study the effects of varying layers, angles, and grain boundaries. The results indicate that the Bi2Te3 nanofilm perfect substrate has the ideal Young’s modulus and thermal conductivity, and the maximum yield stress is observed for a thickness of ∼90 Å. As the interface changed, the structural disorder of atomic arrangement affected the mechanical properties; moreover, the phonons encounter lattice disordered atomic region will produce scattering reduce heat conduction. The results of this investigation are helpful for the application of Bi2Te3 nanofilms as thermoelectric materials.

Automatic software correction of residual aberrations in reconstructed HRTEM exit waves of crystalline samples

DOE PAGES

Ophus, Colin; Rasool, Haider I.; Linck, Martin; ...

2016-11-30

We develop an automatic and objective method to measure and correct residual aberrations in atomic-resolution HRTEM complex exit waves for crystalline samples aligned along a low-index zone axis. Our method uses the approximate rotational point symmetry of a column of atoms or single atom to iteratively calculate a best-fit numerical phase plate for this symmetry condition, and does not require information about the sample thickness or precise structure. We apply our method to two experimental focal series reconstructions, imaging a β-Si 3N 4 wedge with O and N doping, and a single-layer graphene grain boundary. We use peak and latticemore » fitting to evaluate the precision of the corrected exit waves. We also apply our method to the exit wave of a Si wedge retrieved by off-axis electron holography. In all cases, the software correction of the residual aberration function improves the accuracy of the measured exit waves.« less

Big Data in Reciprocal Space: Sliding Fast Fourier Transforms for Determining Periodicity

DOE PAGES

Vasudevan, Rama K.; Belianinov, Alex; Gianfrancesco, Anthony G.; ...

2015-03-03

Significant advances in atomically resolved imaging of crystals and surfaces have occurred in the last decade allowing unprecedented insight into local crystal structures and periodicity. Yet, the analysis of the long-range periodicity from the local imaging data, critical to correlation of functional properties and chemistry to the local crystallography, remains a challenge. Here, we introduce a Sliding Fast Fourier Transform (FFT) filter to analyze atomically resolved images of in-situ grown La5/8Ca3/8MnO3 films. We demonstrate the ability of sliding FFT algorithm to differentiate two sub-lattices, resulting from a mixed-terminated surface. Principal Component Analysis (PCA) and Independent Component Analysis (ICA) of themore » Sliding FFT dataset reveal the distinct changes in crystallography, step edges and boundaries between the multiple sub-lattices. The method is universal for images with any periodicity, and is especially amenable to atomically resolved probe and electron-microscopy data for rapid identification of the sub-lattices present.« less

Big Data in Reciprocal Space: Sliding Fast Fourier Transforms for Determining Periodicity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vasudevan, Rama K.; Belianinov, Alex; Gianfrancesco, Anthony G.

Significant advances in atomically resolved imaging of crystals and surfaces have occurred in the last decade allowing unprecedented insight into local crystal structures and periodicity. Yet, the analysis of the long-range periodicity from the local imaging data, critical to correlation of functional properties and chemistry to the local crystallography, remains a challenge. Here, we introduce a Sliding Fast Fourier Transform (FFT) filter to analyze atomically resolved images of in-situ grown La5/8Ca3/8MnO3 films. We demonstrate the ability of sliding FFT algorithm to differentiate two sub-lattices, resulting from a mixed-terminated surface. Principal Component Analysis (PCA) and Independent Component Analysis (ICA) of themore » Sliding FFT dataset reveal the distinct changes in crystallography, step edges and boundaries between the multiple sub-lattices. The method is universal for images with any periodicity, and is especially amenable to atomically resolved probe and electron-microscopy data for rapid identification of the sub-lattices present.« less

Automatic software correction of residual aberrations in reconstructed HRTEM exit waves of crystalline samples

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ophus, Colin; Rasool, Haider I.; Linck, Martin

We develop an automatic and objective method to measure and correct residual aberrations in atomic-resolution HRTEM complex exit waves for crystalline samples aligned along a low-index zone axis. Our method uses the approximate rotational point symmetry of a column of atoms or single atom to iteratively calculate a best-fit numerical phase plate for this symmetry condition, and does not require information about the sample thickness or precise structure. We apply our method to two experimental focal series reconstructions, imaging a β-Si 3N 4 wedge with O and N doping, and a single-layer graphene grain boundary. We use peak and latticemore » fitting to evaluate the precision of the corrected exit waves. We also apply our method to the exit wave of a Si wedge retrieved by off-axis electron holography. In all cases, the software correction of the residual aberration function improves the accuracy of the measured exit waves.« less

Assessment of the Density Functional Tight Binding Method for Protic Ionic Liquids

PubMed Central

2015-01-01

Density functional tight binding (DFTB), which is ∼100–1000 times faster than full density functional theory (DFT), has been used to simulate the structure and properties of protic ionic liquid (IL) ions, clusters of ions and the bulk liquid. Proton affinities for a wide range of IL cations and anions determined using DFTB generally reproduce G3B3 values to within 5–10 kcal/mol. The structures and thermodynamic stabilities of n-alkyl ammonium nitrate clusters (up to 450 quantum chemical atoms) predicted with DFTB are in excellent agreement with those determined using DFT. The IL bulk structure simulated using DFTB with periodic boundary conditions is in excellent agreement with published neutron diffraction data. PMID:25328497

Chemical and Bandgap Engineering in Monolayer Hexagonal Boron Nitride

PubMed Central

Ba, Kun; Jiang, Wei; Cheng, Jingxin; Bao, Jingxian; Xuan, Ningning; Sun, Yangye; Liu, Bing; Xie, Aozhen; Wu, Shiwei; Sun, Zhengzong

2017-01-01

Monolayer hexagonal boron nitride (h-BN) possesses a wide bandgap of ~6 eV. Trimming down the bandgap is technically attractive, yet poses remarkable challenges in chemistry. One strategy is to topological reform the h-BN’s hexagonal structure, which involves defects or grain boundaries (GBs) engineering in the basal plane. The other way is to invite foreign atoms, such as carbon, to forge bizarre hybrid structures like hetero-junctions or semiconducting h-BNC materials. Here we successfully developed a general chemical method to synthesize these different h-BN derivatives, showcasing how the chemical structure can be manipulated with or without a graphene precursor, and the bandgap be tuned to ~2 eV, only one third of the pristine one’s. PMID:28367992

Chemical Quantification of Atomic-Scale EDS Maps under Thin Specimen Conditions

DOE PAGES

Lu, Ping; Romero, Eric; Lee, Shinbuhm; ...

2014-10-13

We report our effort to quantify atomic-scale chemical maps obtained by collecting energy-dispersive X-ray spectra (EDS) using scanning transmission electron microscopy (STEM) (STEM-EDS). Under a thin specimen condition and when the EDS scattering potential is localized, the X-ray counts from atomic columns can be properly counted by fitting Gaussian peaks at the atomic columns, and can then be used for site-by-site chemical quantification. The effects of specimen thickness and X-ray energy on the Gaussian peak-width are investigated by using SrTiO 3 (STO) as a model specimen. The relationship between the peak-width and spatial-resolution of an EDS map is also studied.more » Furthermore, the method developed by this work is applied to study a Sm-doped STO thin film and antiphase boundaries present within the STO film. We find that Sm atoms occupy both Sr and Ti sites but preferably the Sr sites, and Sm atoms are relatively depleted at the antiphase boundaries likely due to the effect of strain.« less

Effects of wettability and interfacial nanobubbles on flow through structured nanochannels: an investigation of molecular dynamics

NASA Astrophysics Data System (ADS)

Yen, Tsu-Hsu

2015-12-01

Solid-fluid boundary conditions are strongly influenced by a number of factors, including the intrinsic properties of the solid/fluid materials, surface roughness, wettability, and the presence of interfacial nanobubbles (INBs). The interconnected nature of these factors means that they should be considered jointly. This paper employs molecular dynamics (MD) simulation in a series of studies aimed at elucidating the influence of wettability in boundary behaviour and the accumulation of interfacial gas. Specifically, we examined the relationship between effective slip length, the morphology of nanobubbles, and wettability. Two methods were employed for the promotion of hydrophobicity between two structured substrates with similar intrinsic contact angles. We also compared anisotropic and isotropic atomic arrangements in the form of graphite and Si(100), respectively. A physical method was employed to deal with variations in surface roughness, whereas a chemical method was used to adjust the wall-fluid interaction energy (ɛwf). We first compared the characteristic properties of wettability, including contact angle and fluid density within the cavity. We then investigated the means by which variations in solid-fluid interfacial wettability affect interfacial gas molecules. Our results reveal that the morphology of INB on a patterned substrate is determined by wettability as well as the methods employed for the promotion of hydrophobicity. The present study also illustrates the means by which the multiple effects of the atomic arrangement of solids, surface roughness, wettability and INB influence effective slip length.

Proximity effects in cold gases of multiply charged atoms (Review)

NASA Astrophysics Data System (ADS)

Chikina, I.; Shikin, V.

2016-07-01

Possible proximity effects in gases of cold, multiply charged atoms are discussed. Here we deal with rarefied gases with densities nd of multiply charged (Z ≫ 1) atoms at low temperatures in the well-known Thomas-Fermi (TF) approximation, which can be used to evaluate the statistical properties of single atoms. In order to retain the advantages of the TF formalism, which is successful for symmetric problems, the external boundary conditions accounting for the finiteness of the density of atoms (donors), nd ≠ 0, are also symmetrized (using a spherical Wigner-Seitz cell) and formulated in a standard way that conserves the total charge within the cell. The model shows that at zero temperature in a rarefied gas of multiply charged atoms there is an effective long-range interaction Eproxi(nd), the sign of which depends on the properties of the outer shells of individual atoms. The long-range character of the interaction Eproxi is evaluated by comparing it with the properties of the well-known London dispersive attraction ELond(nd) < 0, which is regarded as a long-range interaction in gases. For the noble gases argon, krypton, and xenon Eproxi>0 and for the alkali and alkaline-earth elements Eproxi < 0. At finite temperatures, TF statistics manifests a new, anomalously large proximity effect, which reflects the tendency of electrons localized at Coulomb centers to escape into the continuum spectrum. The properties of thermal decay are interesting in themselves as they determine the important phenomenon of dissociation of neutral complexes into charged fragments. This phenomenon appears consistently in the TF theory through the temperature dependence of the different versions of Eproxi. The anomaly in the thermal proximity effect shows up in the following way: for T ≠ 0 there is no equilibrium solution of TS statistics for single multiply charged atoms in a vacuum when the effect is present. Instability is suppressed in a Wigner-Seitz model under the assumption that there are no electron fluxes through the outer boundary R3 ∝ n-1d of a Wigner-Seitz cell. Eproxi corresponds to the definition of the correlation energy in a gas of interacting particles. This review is written so as to enable comparison of the results of the TF formalism with the standard assumptions of the correlation theory for classical plasmas. The classic example from work on weak solutions (including charged solutions)—the use of semi-impermeable membranes for studies of osmotic pressure—is highly appropriate for problems involving Eproxi. Here we are speaking of one or more sharp boundaries formed by the ionic component of a many-particle problem. These may be a metal-vacuum boundary in a standard Casimir cell in a study of the vacuum properties in the 2l gap between conducting media of different kinds or different layered systems (quantum wells) in semiconductors, etc. As the mobile part of the equilibrium near a sharp boundary, electrons can (should) escape beyond the confines of the ion core into a gap 2l with a probability that depends, among other factors, on the properties of Eproxi for the electron cloud inside the conducting walls of the Casimir cell (quantum well). The analog of the Casimir sandwich in semiconductors is the widely used multilayer heterostructures referred to as quantum wells of width 2l with sides made of suitable doped materials, which ensure statistical equilibrium exchange of electrons between the layers of the multilayer structure. The thermal component of the proximity effects in semiconducting quantum wells provides an idea of many features of the dissociation process in doped semiconductors. In particular, a positive Eproxi > 0 (relative to the bottom of the conduction band) indicates that TF donors with a finite density nd ≠ 0 form a degenerate, semiconducting state in the semiconductor. At zero temperature, there is a finite density of free carriers which increases with a power-law dependence on T.

Electronic and atomic structures of Ti{sub 1-x}Al{sub x}N thin films related to their damage behavior

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tuilier, M.-H.; Pac, M.-J.; Girleanu, M.

2008-04-15

Ti and Al K-edge x-ray absorption spectroscopy is used to investigate the electronic structure of Ti{sub 1-x}Al{sub x}N thin films deposited by reactive magnetron sputtering. The experimental near edge spectra of TiN and AlN are interpreted in the light of unoccupied density of state band structure calculations. The comparison of the structural parameters derived from x-ray absorption fine structure and x-ray diffraction reveals segregation between Al-rich and Ti-rich domains within the Ti{sub 1-x}Al{sub x}N films. Whereas x-ray diffraction probes only the crystallized domains, the structural information derived from extended x-ray absorption fine structure analysis turns on both crystalline and grainmore » boundaries. The results are discussed by considering the damage behavior of the films depending on the composition.« less

Transformation of topologically close-packed β-W to body-centered cubic α-W: Comparison of experiments and computations.

PubMed

Barmak, Katayun; Liu, Jiaxing; Harlan, Liam; Xiao, Penghao; Duncan, Juliana; Henkelman, Graeme

2017-10-21

The enthalpy and activation energy for the transformation of the metastable form of tungsten, β-W, which has the topologically close-packed A15 structure (space group Pm3¯n), to equilibrium α-W, which is body-centered cubic (A2, space group Im3¯m), was measured using differential scanning calorimetry. The β-W films were 1 μm-thick and were prepared by sputter deposition in argon with a small amount of nitrogen. The transformation enthalpy was measured as -8.3 ± 0.4 kJ/mol (-86 ± 4 meV/atom) and the transformation activation energy as 2.2 ± 0.1 eV. The measured enthalpy was found to agree well with the difference in energies of α and β tungsten computed using density functional theory, which gave a value of -82 meV/atom for the transformation enthalpy. A calculated concerted transformation mechanism with a barrier of 0.4 eV/atom, in which all the atoms in an A15 unit cell transform into A2, was found to be inconsistent with the experimentally measured activation energy for any critical nucleus larger than two A2 unit cells. Larger calculations of eight A15 unit cells spontaneously relax to a mechanism in which part of the supercell first transforms from A15 to A2, creating a phase boundary, before the remaining A15 transforms into the A2 phase. Both calculations indicate that a nucleation and growth mechanism is favored over a concerted transformation. More consistent with the experimental activation energy was that of a calculated local transformation mechanism at the A15-A2 phase boundary, computed as 1.7 eV using molecular dynamics simulations. This calculated phase transformation mechanism involves collective rearrangements of W atoms in the disordered interface separating the A15 and A2 phases.

Theoretical investigations of plasma processes in the ion bombardment thruster

NASA Technical Reports Server (NTRS)

Wilhelm, H. E.

1975-01-01

A physical model for a thruster discharge was developed, consisting of a spatially diverging plasma sustained electrically between a small ring cathode and a larger ring anode in a cylindrical chamber with an axial magnetic field. The associated boundary-value problem for the coupled partial differential equations with mixed boundary conditions, which describe the electric potential and the plasma velocity fields, was solved in closed form. By means of quantum-mechanical perturbation theory, a formula for the number S(E) of atoms sputtered on the average by an ion of energy E was derived from first principles. The boundary-value problem describing the diffusion of the sputtered atoms through the surrounding rarefied electron-ion plasma to the system surfaces of ion propulsion systems was formulated and treated analytically. It is shown that outer boundary-value problems of this type lead to a complex integral equation, which requires numerical resolution.

Grain boundary microstructure, chemistry, and IGSCC in Alloy 600 and Alloy 690

DOE Office of Scientific and Technical Information (OSTI.GOV)

Norring, K.; Stiller, K.; Nilsson, J.O.

1992-12-31

The resistance to intergranular stress corrosion cracking of six different Alloy 600 and Alloy 690 steam generator tubes has been investigated. The composition of the materials at grain boundaries has been investigated using analytical transmission electron microscopy and atom probe field ion microscopy techniques. The depletion of chromium at the grain boundaries has been related to the type of grain boundary precipitates. Segregation of carbon and boron to the grain boundaries has been observed and quantified.

Highly responsive ground state of PbTaSe 2 : Structural phase transition and evolution of superconductivity under pressure

DOE PAGES

Kaluarachchi, Udhara S.; Deng, Yuhang; Besser, Matthew F.; ...

2017-06-09

Transport and magnetic studies of PbTaSe 2 under pressure suggest the existence of two superconducting phases with the low temperature phase boundary at ~ 0.25 GPa that is defined by a very sharp, first order, phase transition. The first order phase transition line can be followed via pressure dependent resistivity measurements, and is found to be near 0.12 GPa near room temperature. Transmission electron microscopy and x-ray diffraction at elevated temperatures confirm that this first order phase transition is structural and occurs at ambient pressure near ~ 425 K. The new, high temperature/high pressure phase has a similar crystal structuremore » and slightly lower unit cell volume relative to the ambient pressure, room temperature structure. Based on first-principles calculations this structure is suggested to be obtained by shifting the Pb atoms from the 1 a to 1 e Wyckoff position without changing the positions of Ta and Se atoms. PbTaSe 2 has an exceptionally pressure sensitive, structural phase transition with Δ T s / Δ P ≈ -1400 K/GPa near room temperature, and ≈ -1700 K/GPa near 4 K. This first order transition causes a ~ 1 K (~ 25 % ) steplike decrease in T c as pressure is increased through 0.25 GPa.« less

Self-organized semiconductor nano-network on graphene

NASA Astrophysics Data System (ADS)

Son, Dabin; Kim, Sang Jin; Lee, Seungmin; Bae, Sukang; Kim, Tae-Wook; Kang, Jae-Wook; Lee, Sang Hyun

2017-04-01

A network structure consisting of nanomaterials with a stable structural support and charge path on a large area is desirable for various electronic and optoelectronic devices. Generally, network structures have been fabricated via two main strategies: (1) assembly of pre-grown nanostructures onto a desired substrate and (2) direct growth of nanomaterials onto a desired substrate. In this study, we utilized the surface defects of graphene to form a nano-network of ZnO via atomic layer deposition (ALD). The surface of pure and structurally perfect graphene is chemically inert. However, various types of point and line defects, including vacancies/adatoms, grain boundaries, and ripples in graphene are generated by growth, chemical or physical treatments. The defective sites enhance the chemical reactivity with foreign atoms. ZnO nanoparticles formed by ALD were predominantly deposited at the line defects and agglomerated with increasing ALD cycles. Due to the formation of the ZnO nano-network, the photocurrent between two electrodes was clearly changed under UV irradiation as a result of the charge transport between ZnO and graphene. The line patterned ZnO/graphene (ZnO/G) nano-network devices exhibit sensitivities greater than ten times those of non-patterned structures. We also confirmed the superior operation of a fabricated flexible photodetector based on the line patterned ZnO/G nano-network.

Structural evolution dynamics in fusion of sumanenes and corannulenes: defects formation and self-healing mechanism

NASA Astrophysics Data System (ADS)

Sorkin, Anastassia; Su, Haibin

2018-06-01

The fusion processes of structures consisting of various combinations between sumanene and corannulene, leading to the formation of graphene nanoribbons (GNRs) under heating are simulated by density-functional-based tight-binding molecular dynamics. Distinct stages are unraveled in the course of GNR formation. Firstly, the carbon fragments coalescence into highly strained framework. Secondly, structural reconstruction invokes breaking most strained bonds to form a GNR structure containing numerous defects. Lastly, defects are remedied by the delicate ‘edge-facilitated self-healing’ process through two synergized edge-related effects: elevated mobility of defects and promoted structure reconstructions owing to the remarkable dynamics associated with edges. Importantly, detailed dynamics in the course of forming GNRs with defects and grain boundaries simulated in this work is valuable to provide better understanding at the atomistic scale of defect formation as well as self-healing in the context of the sp2 carbon network. In particular, edges play important roles in not only generating Stone–Wales (SW), 5-8-5 types of defects, 8-5-5-8 and pentagon–heptagon grain boundaries. In addition, our simulations predict the existence of one novel defect, coined as the Inverse SW defect, which is to be confirmed in future experimental studies. This study of dynamic structural evolution reveals that edges are prone to intrinsic and extrinsic modifications such as atomic-scale defects, structural distortions and inhomogeneity.

Grain boundary engineering to control the discontinuous precipitation in multicomponent U10Mo alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Devaraj, Arun; Kovarik, Libor; Kautz, Elizabeth

Grain boundaries in metallic alloys often play a crucial role, not only in determining the mechanical properties or thermal stability of alloys, but also in dictating the phase transformation kinetics during thermomechanical processing. We demonstrate that locally stabilized structure and compositional segregation at grain boundaries—“grain boundary complexions”—in a complex multicomponent alloy can be modified to influence the kinetics of cellular transformation during subsequent thermomechanical processing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography analysis of a metallic nuclear fuel highly relevant to worldwide nuclear non-proliferation efforts —uranium-10 wt% molybdenum (U-10Mo) alloy, new evidence for the existence of grainmore » boundary complexion is provided. We then modified the concentration of impurities dissolved in Υ-UMo grain interiors and/or segregated to Υ-UMo grain boundaries by changing the homogenization treatment, and these effects were used used to retard the kinetics of cellular transformation during subsequent sub-eutectoid annealing in this U-10-Mo alloy during sub-eutectoid annealing. Thus, this work provided insights on tailoring the final microstructure of the U-10Mo alloy, which can potentially improve the irradiation performance of this important class of alloy fuels.« less

Spontaneous spherical symmetry breaking in atomic confinement

NASA Astrophysics Data System (ADS)

Sveshnikov, Konstantin; Tolokonnikov, Andrey

2017-07-01

The effect of spontaneous breaking of initial SO(3) symmetry is shown to be possible for an H-like atom in the ground state, when it is confined in a spherical box under general boundary conditions of "not going out" through the box surface (i.e. third kind or Robin's ones), for a wide range of physically reasonable values of system parameters. The most novel and nontrivial result, which has not been reported previously, is that such an effect takes place not only for attractive, but also for repulsive interactions of atomic electrons with the cavity environment. Moreover, in the limit of a large box size R ≫ aB the regime of an atom, soaring over a plane with boundary condition of "not going out", is reproduced, rather than a spherically symmetric configuration, which would be expected on the basis of the initial SO(3) symmetry of the problem.

Scattering of particles in the presence of harmonic confinement perturbed by a complex absorbing potential

NASA Astrophysics Data System (ADS)

Maghari, A.; Kermani, M. M.

2018-04-01

A system of two interacting atoms confined in 1D harmonic trap and perturbed by an absorbing boundary potential is studied using the Lippmann-Schwinger formalism. The atom-atom interaction potential was considered as a nonlocal separable model. The perturbed absorbing boundary potential was also assumed in the form of Scarf II complex absorbing potential. The model is used for the study of 1D optical lattices that support the trapping of a pair atom within a unit cell. Moreover, it allows to describe the scattering particles in a tight smooth trapping surface and to analyze the bound and resonance states. The analytical expressions for wavefunctions and transition matrix as well as the absorption probabilities are calculated. A demonstration of how the complex absorbing potential affecting the bound states and resonances of particles confined in a harmonic trap is described.

The surface-induced spatial-temporal structures in confined binary alloys

NASA Astrophysics Data System (ADS)

Krasnyuk, Igor B.; Taranets, Roman M.; Chugunova, Marina

2014-12-01

This paper examines surface-induced ordering in confined binary alloys. The hyperbolic initial boundary value problem (IBVP) is used to describe a scenario of spatiotemporal ordering in a disordered phase for concentration of one component of binary alloy and order parameter with non-linear dynamic boundary conditions. This hyperbolic model consists of two coupled second order differential equations for order parameter and concentration. It also takes into account effects of the “memory” on the ordering of atoms and their densities in the alloy. The boundary conditions characterize surface velocities of order parameter and concentration changing which is due to surface (super)cooling on walls confining the binary alloy. It is shown that for large times there are three classes of dynamic non-linear boundary conditions which lead to three different types of attractor’s elements for the IBVP. Namely, the elements of attractor are the limit periodic simple shock waves with fronts of “discontinuities” Γ. If Γ is finite, then the attractor contains spatiotemporal functions of relaxation type. If Γ is infinite and countable then we observe the functions of pre-turbulent type. If Γ is infinite and uncountable then we obtain the functions of turbulent type.

Ab initio local-energy and local-stress analysis of tensile behaviours of tilt grain boundaries in Al and Cu

NASA Astrophysics Data System (ADS)

Wang, Hao; Kohyama, Masanori; Tanaka, Shingo; Shiihara, Yoshinori

2017-01-01

Tensile deformation and failure of Σ9 tilt grain boundaries (GBs) in Al and Cu have been examined by first-principles tensile tests (FPTTs). Local-energy and local-stress schemes were applied to clarify the variations of local energies and local hydrostatic stresses for all atoms during the deformation process. The GBs in Al and Cu exhibited quite different tensile behaviours in the FPTTs, despite their similar initial configurations. For the Al GB, there are two stages of deformation before failure. In the first stage, the back bonds of the interfacial bonds are mainly stretched, due to special high strength of the interfacial reconstructed bonds. In the second stage, the interfacial bonds begin to be significantly stretched due to high concentrated stresses, while stretching of the back bonds is suppressed. The atoms at the interfacial, back and bulk bonds have very different variations of local energies and local stresses during each stage, because the behaviour of each atom is significantly dependent on each local structural change due to the high sensitivity of sp electrons to the local environment in Al. The Cu GB has much higher tensile strength, and a natural introduction of stacking faults (SFs) occurs via the {111}< 112> shear slip in the bulk regions between the interfaces before the maximum stress is reached. This is caused by the smaller SF energy and lower ideal shear strength of Cu than Al, and is triggered by highly accumulated local energies and stress at the interface atoms. The local-energy distribution around the SF is consistent with the previous theoretical estimation. After the introduction of the SF, the local energies and stresses of all the atoms in the Cu GB supercell tend to become similar to each other during the tensile process, in contrast to the inhomogeneity in the Al GB. The origins of the different tensile behaviours observed for Al and Cu GBs are discussed with respect to the different bonding natures of Al and Cu, which are dominated by three sp valence electrons per atom for Al and by fully occupied d bands and s electrons for Cu.

Correlating structural dynamics and catalytic activity of AgAu nanoparticles with ultrafast spectroscopy and all-atom molecular dynamics simulations.

PubMed

Ferbonink, G F; Rodrigues, T S; Dos Santos, D P; Camargo, P H C; Albuquerque, R Q; Nome, R A

2018-05-29

In this study, we investigated hollow AgAu nanoparticles with the goal of improving our understanding of the composition-dependent catalytic activity of these nanoparticles. AgAu nanoparticles were synthesized via the galvanic replacement method with controlled size and nanoparticle compositions. We studied extinction spectra with UV-Vis spectroscopy and simulations based on Mie theory and the boundary element method, and ultrafast spectroscopy measurements to characterize decay constants and the overall energy transfer dynamics as a function of AgAu composition. Electron-phonon coupling times for each composition were obtained from pump-power dependent pump-probe transients. These spectroscopic studies showed how nanoscale surface segregation, hollow interiors and porosity affect the surface plasmon resonance wavelength and fundamental electron-phonon coupling times. Analysis of the spectroscopic data was used to correlate electron-phonon coupling times to AgAu composition, and thus to surface segregation and catalytic activity. We have performed all-atom molecular dynamics simulations of model hollow AgAu core-shell nanoparticles to characterize nanoparticle stability and equilibrium structures, besides providing atomic level views of nanoparticle surface segregation. Overall, the basic atomistic and electron-lattice dynamics of core-shell AgAu nanoparticles characterized here thus aid the mechanistic understanding and performance optimization of AgAu nanoparticle catalysts.

Structural contribution to the ferroelectric fatigue in lead zirconate titanate ceramics

NASA Astrophysics Data System (ADS)

Hinterstein, M.; Rouquette, J.; Haines, J.; Papet, Ph.; Glaum, J.; Knapp, M.; Eckert, J.; Hoffman, M.

2014-09-01

Many ferroelectric devices are based on doped lead zirconate titanate (PZT) ceramics with compositions near the morphotropic phase boundary (MPB), at which the relevant material's properties approach their maximum. Based on a synchrotron x-ray diffraction study of MPB PZT, bulk fatigue is unambiguously found to arise from a less effective field induced tetragonal-to-monoclinic transformation, at which the degradation of the polarization flipping is detected by a less intense and more diffuse anomaly in the atomic displacement parameter of lead. The time dependence of the ferroelectric response on a structural level down to 250 μs confirms this interpretation in the time scale of the piezolectric strain response.

UHV-TEM-REM Studies of Si(111) Surfaces

NASA Astrophysics Data System (ADS)

Yagi, K.; Yamanaka, A.; Sato, H.; Shima, M.; Ohse, H.; Ozawa, S.; Tanishiro, Y.

Recent progresses of ultra-high vacuum transmission and reflection electron microscope studies of clean Si(111) surfaces are described. Anisotropy of surface atomic steps such as step energy, bunching of steps, are studied. Out of phase boundaries are observed in transmission mode and its energy relative to the step energy is studied. The phase transition between the 1 × 1 and the 7 × 7 structures around 830°C, studied previously is re-examined under various conditions. Contraction strains of the 7 × 7 structure and adatom density on terraces play important role during the transition. Diffuse scattering observed by LEED and RHEED above the transition temperature is not observed in teh TED pattern from a thin film.

First-Principles Molecular Dynamics Studies of Organometallic Complexes and Homogeneous Catalytic Processes.

PubMed

Vidossich, Pietro; Lledós, Agustí; Ujaque, Gregori

2016-06-21

Computational chemistry is a valuable aid to complement experimental studies of organometallic systems and their reactivity. It allows probing mechanistic hypotheses and investigating molecular structures, shedding light on the behavior and properties of molecular assemblies at the atomic scale. When approaching a chemical problem, the computational chemist has to decide on the theoretical approach needed to describe electron/nuclear interactions and the composition of the model used to approximate the actual system. Both factors determine the reliability of the modeling study. The community dedicated much effort to developing and improving the performance and accuracy of theoretical approaches for electronic structure calculations, on which the description of (inter)atomic interactions rely. Here, the importance of the model system used in computational studies is highlighted through examples from our recent research focused on organometallic systems and homogeneous catalytic processes. We show how the inclusion of explicit solvent allows the characterization of molecular events that would otherwise not be accessible in reduced model systems (clusters). These include the stabilization of nascent charged fragments via microscopic solvation (notably, hydrogen bonding), transfer of charge (protons) between distant fragments mediated by solvent molecules, and solvent coordination to unsaturated metal centers. Furthermore, when weak interactions are involved, we show how conformational and solvation properties of organometallic complexes are also affected by the explicit inclusion of solvent molecules. Such extended model systems may be treated under periodic boundary conditions, thus removing the cluster/continuum (or vacuum) boundary, and require a statistical mechanics simulation technique to sample the accessible configurational space. First-principles molecular dynamics, in which atomic forces are computed from electronic structure calculations (namely, density functional theory), is certainly the technique of choice to investigate chemical events in solution. This methodology is well established and thanks to advances in both algorithms and computational resources simulation times required for the modeling of chemical events are nowadays accessible, though the computational requirements use to be high. Specific applications reviewed here include mechanistic studies of the Shilov and Wacker processes, speciation in Pd chemistry, hydrogen bonding to metal centers, and the dynamics of agostic interactions.

The competitive growth of cubic domains in Ti(1-x)AlxN films studied by diffraction anomalous near-edge structure spectroscopy.

PubMed

Pinot, Y; Tuilier, M-H; Pac, M-J; Rousselot, C; Thiaudière, D

2015-11-01

Titanium and aluminium nitride films deposited by magnetron sputtering generally grow as columnar domains made of oriented nanocrystallites with cubic or hexagonal symmetry depending on Al content, which are embedded in more disordered grain boundaries. The substitution of Al atoms for Ti in the cubic lattice of the films improves their resistance to wear and oxidation, allowing their use as protective coatings. Ti K-edge X-ray absorption spectroscopy, which probes both crystallized and more disordered grain boundaries, and X-ray diffraction anomalous fine structure, which is sensitive to short- and long-range order within a given crystallized domain, are carried out on a set of Ti(1-x)AlxN films deposited by magnetron sputtering on Si substrates. Attention is paid to the shape of the pre-edge region, which is sensitive to the symmetry of the site occupied by Ti atoms, either octahedral in face-centred-cubic Ti-rich (TiN, Ti0.54Al0.46N) samples or tetrahedral in hexagonal-close-packed Al-rich (Ti0.32Al0.68N) films. In order to obain information on the titanium environment in the well crystallized areas, subtraction of the smooth part of the energy-dependent structure factor for the Bragg reflections is applied to the pre-edge region of the diffraction anomalous data in order to restore their spectroscopic appearance. A flat pre-edge is related to the typical octahedral environment of Ti atoms for cubic reflections. The difference observed between pre-edge spectra associated with face-centred-cubic 200 and 111 Bragg reflections of Ti0.54Al0.46N is assigned to Ti enrichment of 111 large well ordered domains compared with the more disordered 200 ones. The sharp peak observed in the spectrum recorded from the hexagonal 002 peak of Ti0.32Al0.68N can be regarded as a standard for the pure tetrahedral Ti environment in hexagonal-close-packed nitride.

Study of alloy disorder in quantum dots through multi-million atom simulations

NASA Technical Reports Server (NTRS)

Kilmeck, Gerhard; Oyafuso, Fabiano; Boykin, T. B.; Bowen, R. C.; von Allmen, Paul A.

2003-01-01

A tight binding model which includes s, p, d, s orbitals is used to examine the electronic structures of an ensemble of dome-shaped In0.6 Ga0.4 As quantum dots. Given ensembles of identically sized quantum dots, variations in composition and configuration yield a linewidth broadening of less than 0.35 meV, much smaller than the total broadening determined from photoluminescence experiments. It is also found that the computed disorder-induced broadening is very sensitive to the applied boundary conditions, so that care must be taken to ensure proper convergence of the numerical results. Examination of local eigenenergies as functions of position shows similar convergence problems and indicates that an inaccurate resolution of the equilibrium atomic positions due to truncation of the simulation domain may be the source of the slow ground state convergence.

Subgrain boundary analyses in deformed orthopyroxene by TEM/STEM with EBSD-FIB sample preparation technique

NASA Astrophysics Data System (ADS)

Kogure, Toshihiro; Raimbourg, Hugues; Kumamoto, Akihito; Fujii, Eiko; Ikuhara, Yuichi

2014-12-01

High-resolution structure analyses using electron beam techniques have been performed for the investigation of subgrain boundaries (SGBs) in deformed orthopyroxene (Opx) in mylonite from Hidaka Metamorphic Belt, Hokkaido, Japan, to understand ductile deformation mechanism of silicate minerals in shear zones. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analysis of Opx porphyroclasts in the mylonitic rock indicated that the crystal orientation inside the Opx crystals gradually changes by rotation about the b-axis by SGBs and crystal folding. In order to observe the SGBs along the b-axis by transmission electron microscopy (TEM) or scanning TEM (STEM), the following sample preparation protocol was adopted. First, petrographic thin sections were slightly etched with hydrofluoric acid to identify SGBs in SEM. The Opx crystals whose b-axes were oriented close to the normal of the surface were identified by EBSD, and the areas containing SGBs were picked and thinned for (S) TEM analysis with a focused ion beam instrument with micro-sampling system. High-resolution TEM imaging of the SGBs in Opx revealed various boundary structures from a periodic array of dissociated (100) [001] edge dislocations to partially or completely incoherent crystals, depending on the misorientation angle. Atomic-resolution STEM imaging clearly confirmed the formation of clinopyroxene (Cpx) structure between the dissociated partial dislocations. Moreover, X-ray microanalysis in STEM revealed that the Cpx contains a considerable amount of calcium replacing iron. Such chemical inhomogeneity may limit glide motion of the dislocation and eventually the plastic deformation of the Opx porphyroclasts at a low temperature. Chemical profiles across the high-angle incoherent SGB also showed an enrichment of the latter in calcium at the boundary, suggesting that SGBs are an efficient diffusion pathway of calcium out of host Opx grain during cooling.

Energetic neutral atom and interstellar flow observations with IBEX: Implications for the global heliosphere

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schwadron, N. A., E-mail: nschwadron@unh.edu; Southwest Research Institute, San Antonio, TX, 78238; McComas, D. J.

2016-03-25

Since launch in Oct. 2008, IBEX, with its two energetic neutral atom (ENA) cameras, has provided humankind with the first-ever global images of the complex boundary separating the heliosphere from the local interstellar medium (LISM). IBEX’s energy-resolved all-sky maps, collected every six months, are yielding remarkable new insights into the heliospheres structure as it is shaped by the combined forces of the local interstellar flow, the local interstellar magnetic field (LISMF), and the evolving solar wind. IBEX has also acquired the first images of ENAs backscattered from the surface of the moon as well as global images of the magnetosphericmore » response to solar wind disturbances. IBEX thus addresses all three Heliophysics science objectives set forth in the 2014 Science Plan for NASAs Science Mission Directorate (SMD) as well as the goals in the recent Solar and Space Physics Decadal Survey (NRC 2012). In addition, with the information it provides on the properties of the LISM and the LISMF, IBEX represents a unique bridge between heliophysics and astrophysics, and fills in critical knowledge for understanding the habitability of exoplanetary systems and the future habitability of Earth and the solar system. Because of the few-year time lag due to solar wind and ENA transport, IBEX observed the solar wind/ LISM interaction characteristic of declining phase/solar minimum conditions. In the continuing mission, IBEX captures the response of the interstellar boundaries to the changing structure of the solar wind in its transition toward the “mini” solar maximum and possibly the decline into the next solar minimum. The continuing IBEX mission affords never-to-be-repeated opportunities to coordinate global imaging of the heliospheric boundary with in-situ measurements by the Voyagers as they pass beyond the heliopause and start to directly sample the LISM.« less

Large-scale production of (GeTe) x (AgSbTe 2) 100$-$x (x=75, 80, 85, 90) with enhanced thermoelectric properties via gas-atomization and spark plasma sintering

DOE PAGES

Kim, Hyo-Seob; Ames Lab., Ames, IA; Dharmaiah, Peyala; ...

2017-01-30

(GeTe) x(AgSbTe 2) 100$-$x: TAGS thermoelectrics are an attractive class of materials due to their combination of non-toxicity and good conversion efficiency at mid-temperature ranges. Here in the present work, we have utilized energy and time efficient high-pressure gas atomization and spark-plasma sintering techniques for large-scale preparation of samples with varying composition (i.e., (GeTe) x(AgSbTe 2) 100$-$x where x = 75, 80, 85, and 90). High-temperature x-ray diffraction was used to understand the phase transformation mechanism of the as-atomized powders. Detailed high-resolution transmission electron microscopy of the sintered samples revealed the presence of nanoscale precipitates, antiphase, and twin boundaries. Themore » nanoscale twins and antiphase boundaries serve as phonon scattering centers, leading to the reduction of total thermal conductivity in TAGS-80 and 90 samples. The maximum ZT obtained was 1.56 at 623 K for TAGS-90, which was ~94% improvement compared to values previously reported. The presence of the twin boundaries also resulted in a high fracture toughness (K IC) of the TAGS-90 sample due to inhibition of dislocation movement at the twin boundary.« less

Influence of grain boundary characteristics on thermal stability in nanotwinned copper

DOE PAGES

Niu, Rongmei; Han, Ke; Su, Yi-feng; ...

2016-08-12

High density grain boundaries provide high strength, but may introduce undesirable features, such as high Fermi levels and instability. We investigated the kinetics of recovery and recrystallization of Cu that was manufactured to include both nanotwins (NT) and high-angle columnar boundaries. We used the isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model to estimate activation energy values for recovery and recrystallization and compared those to values derived using the non-isothermal Kissinger equation. The JMAK model hinges on an exponent that expresses the growth mechanism of a material. The exponent for this Cu was close to 0.5, indicating low-dimensional microstructure evolution, which is associated withmore » anisotropic twin coarsening, heterogeneous recrystallization, and high stability. Since this Cu was of high purity, there was a negligible impurity-drag-effect on boundaries. The twin coarsening and heterogeneous recrystallization resulted from migration of high-angle columnar boundaries with their triple junctions in one direction, assisted by the presence of high concentration vacancies at boundaries. Analyses performed by electron energy loss spectroscopy of atomic columns at twin boundaries (TBs) and in the interior showed similar plasma peak shapes and L3 edge positions. As a result, this implies that values for conductivity and Fermi level are equal for atoms at TBs and in the interior.« less

Influence of grain boundary characteristics on thermal stability in nanotwinned copper

PubMed Central

Niu, Rongmei; Han, Ke; Su, Yi-feng; Besara, Tiglet; Siegrist, Theo M.; Zuo, Xiaowei

2016-01-01

High density grain boundaries provide high strength, but may introduce undesirable features, such as high Fermi levels and instability. We investigated the kinetics of recovery and recrystallization of Cu that was manufactured to include both nanotwins (NT) and high-angle columnar boundaries. We used the isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model to estimate activation energy values for recovery and recrystallization and compared those to values derived using the non-isothermal Kissinger equation. The JMAK model hinges on an exponent that expresses the growth mechanism of a material. The exponent for this Cu was close to 0.5, indicating low-dimensional microstructure evolution, which is associated with anisotropic twin coarsening, heterogeneous recrystallization, and high stability. Since this Cu was of high purity, there was a negligible impurity-drag-effect on boundaries. The twin coarsening and heterogeneous recrystallization resulted from migration of high-angle columnar boundaries with their triple junctions in one direction, assisted by the presence of high concentration vacancies at boundaries. Analyses performed by electron energy loss spectroscopy of atomic columns at twin boundaries (TBs) and in the interior showed similar plasma peak shapes and L3 edge positions. This implies that values for conductivity and Fermi level are equal for atoms at TBs and in the interior. PMID:27514474

Close contacts at the interface: Experimental-computational synergies for solving complexity problems

NASA Astrophysics Data System (ADS)

Torras, Juan; Zanuy, David; Bertran, Oscar; Alemán, Carlos; Puiggalí, Jordi; Turón, Pau; Revilla-López, Guillem

2018-02-01

The study of material science has been long devoted to the disentanglement of bulk structures which mainly entails finding the inner structure of materials. That structure is accountable for a major portion of materials' properties. Yet, as our knowledge of these "backbones" enlarged so did the interest for the materials' boundaries properties which means the properties at the frontier with the surrounding environment that is called interface. The interface is thus to be understood as the sum of the material's surface plus the surrounding environment be it in solid, liquid or gas phase. The study of phenomena at this interface requires both the use of experimental and theoretical techniques and, above all, a wise combination of them in order to shed light over the most intimate details at atomic, molecular and mesostructure levels. Here, we report several cases to be used as proof of concept of the results achieved when studying interface phenomena by combining a myriad of experimental and theoretical tools to overcome the usual limitation regardind atomic detail, size and time scales and systems of complex composition. Real world examples of the combined experimental-theoretical work and new tools, software, is offered to the readers.

Optical and electrical properties of polycrystalline and amorphous Al-Ti thin films

NASA Astrophysics Data System (ADS)

Canulescu, S.; Borca, C. N.; Rechendorff, K.; Davidsdóttir, S.; Pagh Almtoft, K.; Nielsen, L. P.; Schou, J.

2016-04-01

The structural, optical, and transport properties of sputter-deposited Al-Ti thin films have been investigated as a function of Ti alloying with a concentration ranging from 2% to 46%. The optical reflectivity of Al-Ti films at visible and near-infrared wavelengths decreases with increasing Ti content. X-ray absorption fine structure measurements reveal that the atomic ordering around Ti atoms increases with increasing Ti content up to 20% and then decreases as a result of a transition from a polycrystalline to amorphous structure. The transport properties of the Al-Ti films are influenced by electron scattering at the grain boundaries in the case of polycrystalline films and static defects, such as anti-site effects and vacancies in the case of the amorphous alloys. The combination of Ti having a real refractive index (n) comparable with the extinction coefficient (k) and Al with n much smaller than k allows us to explore the parameter space for the free-electron behavior in transition metal-Al alloys. The free electron model, applied for the polycrystalline Al-Ti films with Ti content up to 20%, leads to an optical reflectance at near infrared wavelengths that scales linearly with the square root of the electrical resistivity.

Accurate force field for molybdenum by machine learning large materials data

NASA Astrophysics Data System (ADS)

Chen, Chi; Deng, Zhi; Tran, Richard; Tang, Hanmei; Chu, Iek-Heng; Ong, Shyue Ping

2017-09-01

In this work, we present a highly accurate spectral neighbor analysis potential (SNAP) model for molybdenum (Mo) developed through the rigorous application of machine learning techniques on large materials data sets. Despite Mo's importance as a structural metal, existing force fields for Mo based on the embedded atom and modified embedded atom methods do not provide satisfactory accuracy on many properties. We will show that by fitting to the energies, forces, and stress tensors of a large density functional theory (DFT)-computed dataset on a diverse set of Mo structures, a Mo SNAP model can be developed that achieves close to DFT accuracy in the prediction of a broad range of properties, including elastic constants, melting point, phonon spectra, surface energies, grain boundary energies, etc. We will outline a systematic model development process, which includes a rigorous approach to structural selection based on principal component analysis, as well as a differential evolution algorithm for optimizing the hyperparameters in the model fitting so that both the model error and the property prediction error can be simultaneously lowered. We expect that this newly developed Mo SNAP model will find broad applications in large and long-time scale simulations.

Evidence of the no-slip boundary condition of water flow between hydrophilic surfaces using atomic force microscopy.

PubMed

Maali, Abdelhamid; Wang, Yuliang; Bhushan, Bharat

2009-10-20

In this study we present measurements of the hydrodynamic force exerted on a glass sphere glued to an atomic force microscopy (AFM) cantilever approaching a mica surface in water. A large sphere was used to reduce the impact of the cantilever beam on the measurement. An AFM cantilever with large stiffness was used to accurately determine the actual contact position between the sphere and the sample surface. The measured hydrodynamic force with different approach velocities is in good agreement with the Taylor force calculated in the lubrication theory with the no-slip boundary conditions, which verifies that there is no boundary slip on the glass and mica surfaces. Moreover, a detailed procedure of how to subtract the electrostatic double-layer force is presented.

Phason space analysis and structure modelling of 100 Å-scale dodecagonal quasicrystal in Mn-based alloy

NASA Astrophysics Data System (ADS)

Ishimasa, Tsutomu; Iwami, Shuhei; Sakaguchi, Norihito; Oota, Ryo; Mihalkovič, Marek

2015-11-01

The dodecagonal quasicrystal classified into the five-dimensional space group P126/mmc, recently discovered in a Mn-Cr-Ni-Si alloy, has been analysed using atomic-resolution spherical aberration-corrected electron microscopy, i.e. high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and conventional transmission electron microscopy. By observing along the 12-fold axis, non-periodic tiling consisting of an equilateral triangle and a square has been revealed, of which common edge length is a = 4.560 Å. These tiles tend to form a network of dodecagons of which size is ?a ≈ 17 Å in diameter. The tiling was interpreted as an aggregate of 100 Å-scale oriented domains of high- and low-quality quasicrystals with small crystallites appearing at their boundaries. The quasicrystal domains exhibited a densely filled circular acceptance region in the phason space. This is the first observation of the acceptance region in an actual dodecagonal quasicrystal. Atomic structure model consistent with the electron microscopy images is a standard Frank-Kasper decoration of the triangle and square tiles that can be inferred from the crystal structures of Zr4Al3 and Cr3Si. Four kinds of layers located at z = 0, ±1/4 and 1/2 are stacked periodically along the 12-fold axis, and the atoms at z = 0 and 1/2 form hexagonal anti-prisms consistently with the 126-screw axis. The validity of this structure model was examined by means of powder X-ray diffraction.

Secondary Interstellar Oxygen in the Heliosphere: Numerical Modeling and Comparison with IBEX-Lo Data

NASA Astrophysics Data System (ADS)

Baliukin, I. I.; Izmodenov, V. V.; Möbius, E.; Alexashov, D. B.; Katushkina, O. A.; Kucharek, H.

2017-12-01

Quantitative analysis of the interstellar heavy (oxygen and neon) atom fluxes obtained by the Interstellar Boundary Explorer (IBEX) suggests the existence of the secondary interstellar oxygen component. This component is formed near the heliopause due to charge exchange of interstellar oxygen ions with hydrogen atoms, as was predicted theoretically. A detailed quantitative analysis of the fluxes of interstellar heavy atoms is only possible with a model that takes into account both the filtration of primary and the production of secondary interstellar oxygen in the boundary region of the heliosphere as well as a detailed simulation of the motion of interstellar atoms inside the heliosphere. This simulation must take into account photoionization, charge exchange with the protons of the solar wind and solar gravitational attraction. This paper presents the results of modeling interstellar oxygen and neon atoms through the heliospheric interface and inside the heliosphere based on a three-dimensional kinetic-MHD model of the solar wind interaction with the local interstellar medium and a comparison of these results with the data obtained on the IBEX spacecraft.

On the effect of irradiation-induced resolution in modelling fission gas release in UO2 LWR fuel

NASA Astrophysics Data System (ADS)

Lösönen, Pekka

2017-12-01

Irradiation resolution of gas atoms and vacancies from intra- and intergranular bubbles in sintered UO2 fuel was studied by comparing macroscopic models with a more mechanistic approach. The applied macroscopic models imply the resolution rate of gas atoms to be proportional to gas concentration in intragranular bubbles and at grain boundary (including intergranular bubbles). A relation was established between the macroscopic models and a single encounter of an energetic fission fragment with a bubble. The effect of bubble size on resolution was quantified. The number of resoluted gas atoms per encounter of a fission fragment per bubble was of the same order of magnitude for intra- and intergranular bubbles. However, the resulting macroscopic resolution rate of gas atoms was about two orders of magnitude larger from intragranular bubbles. The number of vacancies resoluted from a grain face bubble by a passing fission fragment was calculated. The obtained correlations for resolution of gas atoms from intragranular bubbles and grain boundaries and for resolution of vacancies from grain face bubbles were used to demonstrate the effect of irradiation resolution on fission gas release.

Large scale motions of multiple limit-cycle high Reynolds number annular and toroidal rotor/stator cavities

NASA Astrophysics Data System (ADS)

Bridel-Bertomeu, Thibault; Gicquel, L. Y. M.; Staffelbach, G.

2017-06-01

Rotating cavity flows are essential components of industrial applications but their dynamics are still not fully understood when it comes to the relation between the fluid organization and monitored pressure fluctuations. From computer hard-drives to turbo-pumps of space launchers, designed devices often produce flow oscillations that can either destroy the component prematurely or produce too much noise. In such a context, large scale dynamics of high Reynolds number rotor/stator cavities need better understanding especially at the flow limit-cycle or associated statistically stationary state. In particular, the influence of curvature as well as cavity aspect ratio on the large scale organization and flow stability at a fixed rotating disc Reynolds number is fundamental. To probe such flows, wall-resolved large eddy simulation is applied to two different rotor/stator cylindrical cavities and one annular cavity. Validation of the predictions proves the method to be suited and to capture the disc boundary layer patterns reported in the literature. It is then shown that in complement to these disc boundary layer analyses, at the limit-cycle the rotating flows exhibit characteristic patterns at mid-height in the homogeneous core pointing the importance of large scale features. Indeed, dynamic modal decomposition reveals that the entire flow dynamics are driven by only a handful of atomic modes whose combination links the oscillatory patterns observed in the boundary layers as well as in the core of the cavity. These fluctuations form macro-structures, born in the unstable stator boundary layer and extending through the homogeneous inviscid core to the rotating disc boundary layer, causing its instability under some conditions. More importantly, the macro-structures significantly differ depending on the configuration pointing the need for deeper understanding of the influence of geometrical parameters as well as operating conditions.

Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study

DOE PAGES

Xu, Shuozhi; Xiong, Liming; Chen, Youping; ...

2016-01-29

Sequential slip transfer across grain boundaries (GB) has an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall–Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalised in terms of dislocation pile-ups against GBs. In spite of extensive studies in modelling individual phases and grains using atomistic simulations, well-accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long-range fieldsmore » of the dislocation pile-up come into play. In this work, concurrent atomistic-continuum simulations are performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on Σ3 coherent twin boundary (CTB) in Cu and Al, with dominant leading screw character at the site of interaction. A Frank-Read source is employed to nucleate dislocations continuously. It is found that subject to a shear stress of 1.2 GPa, screw dislocations transfer into the twinned grain in Cu, but glide on the twin boundary plane in Al. Moreover, four dislocation/CTB interaction modes are identified in Al, which are affected by (1) applied shear stress, (2) dislocation line length, and (3) dislocation line curvature. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-GB reactions and highlight the importance of directly modeling sequential dislocation slip transfer reactions using fully 3D models.« less

Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Shuozhi; Xiong, Liming; Chen, Youping

Sequential slip transfer across grain boundaries (GB) has an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall–Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalised in terms of dislocation pile-ups against GBs. In spite of extensive studies in modelling individual phases and grains using atomistic simulations, well-accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long-range fieldsmore » of the dislocation pile-up come into play. In this work, concurrent atomistic-continuum simulations are performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on Σ3 coherent twin boundary (CTB) in Cu and Al, with dominant leading screw character at the site of interaction. A Frank-Read source is employed to nucleate dislocations continuously. It is found that subject to a shear stress of 1.2 GPa, screw dislocations transfer into the twinned grain in Cu, but glide on the twin boundary plane in Al. Moreover, four dislocation/CTB interaction modes are identified in Al, which are affected by (1) applied shear stress, (2) dislocation line length, and (3) dislocation line curvature. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-GB reactions and highlight the importance of directly modeling sequential dislocation slip transfer reactions using fully 3D models.« less

Single walled boron nitride nanotube-based biosensor: an atomistic finite element modelling approach.

PubMed

Panchal, Mitesh B; Upadhyay, Sanjay H

2014-09-01

The unprecedented dynamic characteristics of nanoelectromechanical systems make them suitable for nanoscale mass sensing applications. Owing to superior biocompatibility, boron nitride nanotubes (BNNTs) are being increasingly used for such applications. In this study, the feasibility of single walled BNNT (SWBNNT)-based bio-sensor has been explored. Molecular structural mechanics-based finite element (FE) modelling approach has been used to analyse the dynamic behaviour of SWBNNT-based biosensors. The application of an SWBNNT-based mass sensing for zeptogram level of mass has been reported. Also, the effect of size of the nanotube in terms of length as well as different chiral atomic structures of SWBNNT has been analysed for their sensitivity analysis. The vibrational behaviour of SWBNNT has been analysed for higher-order modes of vibrations to identify the intermediate landing position of biological object of zeptogram scale. The present molecular structural mechanics-based FE modelling approach is found to be very effectual to incorporate different chiralities of the atomic structures. Also, different boundary conditions can be effectively simulated using the present approach to analyse the dynamic behaviour of the SWBNNT-based mass sensor. The presented study has explored the potential of SWBNNT, as a nanobiosensor having the capability of zeptogram level mass sensing.

Recombination activity of nickel, copper, and oxygen atoms segregating at grain boundaries in mono-like silicon crystals

NASA Astrophysics Data System (ADS)

Ohno, Yutaka; Kutsukake, Kentaro; Deura, Momoko; Yonenaga, Ichiro; Shimizu, Yasuo; Ebisawa, Naoki; Inoue, Koji; Nagai, Yasuyoshi; Yoshida, Hideto; Takeda, Seiji

2016-10-01

Three-dimensional distribution of impurity atoms was determined at functional Σ5{013} and small-angle grain boundaries (GBs) in as-grown mono-like silicon crystals by atom probe tomography combined with transmission electron microscopy, and it was correlated with the recombination activity of those GBs, CGB, revealed by photoluminescence imaging. Nickel (Ni), copper (Cu), and oxygen atoms preferentially segregated at the GBs on which arrays of dislocations existed, while those atoms scarcely segregated at Σ5{013} GBs free from dislocations. Silicides containing Ni and Cu about 5 nm in size and oxides about 1 nm in size were formed along the dislocation arrays on those GBs. The number of segregating impurity atoms per unit GB area for Ni and that for Cu, NNi and NCu, were in a trade-off correlation with that for oxygen, NO, as a function of CGB, while the sum of those numbers was almost constant irrespective of the GB character, CGB, and the dislocation density on GBs. CGB would be explained as a linear combination of those numbers: CGB (in %) ˜400(0.38NO + NNi + NCu) (in atoms/nm2). The GB segregation of oxygen atoms would be better for solar cells, rather than that of metal impurities, from a viewpoint of the conversion efficiency of solar cells.

Nanoscopic analysis of oxygen segregation at tilt boundaries in silicon ingots using atom probe tomography combined with TEM and ab initio calculations.

PubMed

Ohno, Y; Inoue, K; Fujiwara, K; Kutsukake, K; Deura, M; Yonenaga, I; Ebisawa, N; Shimizu, Y; Inoue, K; Nagai, Y; Yoshida, H; Takeda, S; Tanaka, S; Kohyama, M

2017-12-01

We have developed an analytical method to determine the segregation levels on the same tilt boundaries (TBs) at the same nanoscopic location by a joint use of atom probe tomography and scanning transmission electron microscopy, and discussed the mechanism of oxygen segregation at TBs in silicon ingots in terms of bond distortions around the TBs. The three-dimensional distribution of oxygen atoms was determined at the typical small- and large-angle TBs by atom probe tomography with a low impurity detection limit (0.01 at.% on a TB plane) simultaneously with high spatial resolution (about 0.4 nm). The three-dimensional distribution was correlated with the atomic stress around the TBs; the stress at large-angle TBs was estimated by ab initio calculations based on atomic resolution scanning transmission electron microscopy data and that at small-angle TBs were calculated with the elastic theory based on dark-field transmission electron microscopy data. Oxygen atoms would segregate at bond-centred sites under tensile stress above about 2 GPa, so as to attain a more stable bonding network by reducing the local stress. The number of oxygen atoms segregating in a unit TB area N GB (in atoms nm -2 ) was determined to be proportional to both the number of the atomic sites under tensile stress in a unit TB area n bc and the average concentration of oxygen atoms around the TB [O i ] (in at.%) with N GB ∼ 50 n bc [O i ]. © 2017 The Authors Journal of Microscopy © 2017 Royal Microscopical Society.

Superconducting critical fields of alkali and alkaline-earth intercalates of MoS2

NASA Technical Reports Server (NTRS)

Woollam, J. A.; Somoano, R. B.

1976-01-01

Results are reported for measurements of the critical-field anisotropy and temperature dependence of group-VIB semiconductor MoS2 intercalated with the alkali and alkaline-earth metals Na, K, Rb, Cs, and Sr. The temperature dependences are compared with present theories on the relation between critical field and transition temperature in the clean and dirty limits over the reduced-temperature range from 1 to 0.1. The critical-field anisotropy data are compared with predictions based on coupled-layers and thin-film ('independent-layers') models. It is found that the critical-field boundaries are steep in all cases, that the fields are greater than theoretical predictions at low temperatures, and that an unusual positive curvature in the temperature dependence appears which may be related to the high anisotropy of the layer structure. The results show that materials with the largest ionic intercalate atom diameters and hexagonal structures (K, Rb, and Cs compounds) have the highest critical temperatures, critical fields, and critical-boundary slopes; the critical fields of these materials are observed to exceed the paramagnetic limiting fields.

A theoretical study of the omega-phase transformation in metals

NASA Astrophysics Data System (ADS)

Sanati, Mahdi

I have studied the formation of o-phase from electronic and mesoscopic (domain wall) points of view. To study the formation of domain walls, I have extended the Landau model of Cook for the o-phase transition by including a spatial gradient (Ginzburg) term of the scalar order parameter. In general, the Landau free energy is an asymmetric double-well potential. From the variational derivative of the total free energy I obtained a static equilibrium condition. By solving this equation for different physical parameters and boundary conditions, I obtained different quasi-one-dimensional soliton-like solutions. These solutions correspond to three different types of domain walls between the o-phase and the beta-matrix. These results are used to model the formation of the o-phase in bcc Ti. Canonical band model and first principles calculations confirmed the instability of the bcc-phase of group III and IV transition metals with respect to the o-phase transformation. I showed that the d-electron density is the controlling parameter for this type of the transformation. Also the possibility of formation of the o-phase for rare earth metals is discussed. First-principles full-potential linear muffin-tin orbital method (FPLMTO) calculations are performed for o-type displacement of the atoms to study the formation of the o-phase in TiAl and Ti 3Al2Nb alloys. The results of my calculations showed an instability in ordered B2 TiAl structure with respect to the o-phase when one third of the Al atoms are replaced by Nb atoms. These phenomena are explained, first by symmetry arguments; then a pair potential model is used to illustrate this instability based on interactions between different pair of atoms derived from the electronic structure. In addition, importance of the atomic arrangements on the structural stability of the Ti3Al2 Nb system is discussed.

Spatial Heterogeneities and Onset of Passivation Breakdown at Lithium Anode Interfaces

DOE PAGES

Leung, Kevin; Jungjohann, Katherine L.

2017-09-08

Effective passivation of lithium metal surfaces, and prevention of battery-shorting lithium dendrite growth, are critical for implementing lithium metal anodes for batteries with increased power densities. Nanoscale surface heterogeneities can be “hot spots” where anode passivation breaks down. Motivated by the observation of lithium dendrites in pores and grain boundaries in all-solid batteries, we examine lithium metal surfaces covered with Li 2O and/or LiF thin films with grain boundaries in them. Electronic structure calculations show that at >0.25 V computed equilibrium overpotential Li 2O grain boundaries with sufficiently large pores can accommodate Li0 atoms which aid e– leakage and passivationmore » breakdown. Strain often accompanies Li insertion; applying an ~1.7% strain already lowers the computed overpotential to 0.1 V. Lithium metal nanostructures as thin as 12 Å are thermodynamically favored inside cracks in Li 2O films, becoming “incipient lithium filaments”. LiF films are more resistant to lithium metal growth. Finally, the models used herein should in turn inform passivating strategies in all-solid-state batteries.« less

Characterization of grain boundary conductivity of spin-sprayed ferrites using scanning microwave microscope

DOE Office of Scientific and Technical Information (OSTI.GOV)

Myers, J.; Nicodemus, T.; Zhuang, Y., E-mail: yan.zhuang@wright.edu

2014-05-07

Grain boundary electrical conductivity of ferrite materials has been characterized using scanning microwave microscope. Structural, electrical, and magnetic properties of Fe{sub 3}O{sub 4} spin-sprayed thin films onto glass substrates for different length of growth times were investigated using a scanning microwave microscope, an atomic force microscope, a four-point probe measurement, and a made in house transmission line based magnetic permeameter. The real part of the magnetic permeability shows almost constant between 10 and 300 MHz. As the Fe{sub 3}O{sub 4} film thickness increases, the grain size becomes larger, leading to a higher DC conductivity. However, the loss in the Fe{sub 3}O{submore » 4} films at high frequency does not increase correspondingly. By measuring the reflection coefficient s{sub 11} from the scanning microwave microscope, it turns out that the grain boundaries of the Fe{sub 3}O{sub 4} films exhibit higher electric conductivity than the grains, which contributes loss at radio frequencies. This result will provide guidance for further improvement of low loss ferrite materials for high frequency applications.« less

Spatial Heterogeneities and Onset of Passivation Breakdown at Lithium Anode Interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Leung, Kevin; Jungjohann, Katherine L.

Effective passivation of lithium metal surfaces, and prevention of battery-shorting lithium dendrite growth, are critical for implementing lithium metal anodes for batteries with increased power densities. Nanoscale surface heterogeneities can be “hot spots” where anode passivation breaks down. Motivated by the observation of lithium dendrites in pores and grain boundaries in all-solid batteries, we examine lithium metal surfaces covered with Li 2O and/or LiF thin films with grain boundaries in them. Electronic structure calculations show that at >0.25 V computed equilibrium overpotential Li 2O grain boundaries with sufficiently large pores can accommodate Li0 atoms which aid e– leakage and passivationmore » breakdown. Strain often accompanies Li insertion; applying an ~1.7% strain already lowers the computed overpotential to 0.1 V. Lithium metal nanostructures as thin as 12 Å are thermodynamically favored inside cracks in Li 2O films, becoming “incipient lithium filaments”. LiF films are more resistant to lithium metal growth. Finally, the models used herein should in turn inform passivating strategies in all-solid-state batteries.« less

Quantification by aberration corrected (S)TEM of boundaries formed by symmetry breaking phase transformations.

PubMed

Schryvers, D; Salje, E K H; Nishida, M; De Backer, A; Idrissi, H; Van Aert, S

2017-05-01

The present contribution gives a review of recent quantification work of atom displacements, atom site occupations and level of crystallinity in various systems and based on aberration corrected HR(S)TEM images. Depending on the case studied, picometer range precisions for individual distances can be obtained, boundary widths at the unit cell level determined or statistical evolutions of fractions of the ordered areas calculated. In all of these cases, these quantitative measures imply new routes for the applications of the respective materials. Copyright © 2017 Elsevier B.V. All rights reserved.

The linearly scaling 3D fragment method for large scale electronic structure calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, Zhengji; Meza, Juan; Lee, Byounghak

2009-07-28

The Linearly Scaling three-dimensional fragment (LS3DF) method is an O(N) ab initio electronic structure method for large-scale nano material simulations. It is a divide-and-conquer approach with a novel patching scheme that effectively cancels out the artificial boundary effects, which exist in all divide-and-conquer schemes. This method has made ab initio simulations of thousand-atom nanosystems feasible in a couple of hours, while retaining essentially the same accuracy as the direct calculation methods. The LS3DF method won the 2008 ACM Gordon Bell Prize for algorithm innovation. Our code has reached 442 Tflop/s running on 147,456 processors on the Cray XT5 (Jaguar) atmore » OLCF, and has been run on 163,840 processors on the Blue Gene/P (Intrepid) at ALCF, and has been applied to a system containing 36,000 atoms. In this paper, we will present the recent parallel performance results of this code, and will apply the method to asymmetric CdSe/CdS core/shell nanorods, which have potential applications in electronic devices and solar cells.« less

The Linearly Scaling 3D Fragment Method for Large Scale Electronic Structure Calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, Zhengji; Meza, Juan; Lee, Byounghak

2009-06-26

The Linearly Scaling three-dimensional fragment (LS3DF) method is an O(N) ab initio electronic structure method for large-scale nano material simulations. It is a divide-and-conquer approach with a novel patching scheme that effectively cancels out the artificial boundary effects, which exist in all divide-and-conquer schemes. This method has made ab initio simulations of thousand-atom nanosystems feasible in a couple of hours, while retaining essentially the same accuracy as the direct calculation methods. The LS3DF method won the 2008 ACM Gordon Bell Prize for algorithm innovation. Our code has reached 442 Tflop/s running on 147,456 processors on the Cray XT5 (Jaguar) atmore » OLCF, and has been run on 163,840 processors on the Blue Gene/P (Intrepid) at ALCF, and has been applied to a system containing 36,000 atoms. In this paper, we will present the recent parallel performance results of this code, and will apply the method to asymmetric CdSe/CdS core/shell nanorods, which have potential applications in electronic devices and solar cells.« less

On the fine structure of meteoritical taenite/tetrataenite and its interpretation

NASA Astrophysics Data System (ADS)

Albertsen, J. F.; Nielsen, H. P.; Buchwald, V. F.

1983-04-01

TEM, electron microprobe, and Moessbauer spectroscopy are used in investigating taenite fields from several meteorites. A delicate pattern of antiphase domains is revealed in the tetrataenite, as is the presence of low-Ni taenite at the antiphase boundaries in what was hitherto believed to be pure tetrataenite. The observations suggest that the 'cloudy taenite' (cloudy zone II) was formed by a magnetically induced spinodal decomposition of the metastable taenite during slow cooling below 400 C. It is thought likely that decompositin occurs when the Curie temperature of the alloy changes rapidly with composition, as it does in f.c.c. iron-nickel alloys containing approximately 28-43 percent Ni (wt pct). The large contribution to Gibbs free energy from magnetic ordering leads to inflections in the Gibbs free energy curve, making the alloy unstable with regard to decomposition, in this case into a magnetically and atomically ordered Ni-rich alloy plus a magnetically and atomically disordered Ni-poor alloy. The model accounts well for the structure and composition of the two phases in the cloudy taenite.

Topological interface physics in spinor Bose-Einstein condensates

NASA Astrophysics Data System (ADS)

Borgh, Magnus; Ruostekoski, Janne

2013-05-01

We present an experimentally viable scheme whereby the physics of coherent interfaces between topologically distinct regions can be studied in an atomic quantum gas. The interface engineering is achieved using the internal spin structures of atoms together with local control over interaction strengths. We consider a coherent interface between polar and ferromagnetic regions of a spin-1 Bose-Einstein condensate and show that defects representing different topologies can connect continuously across the boundary. We show that energy minimization leads to nontrivial interface-crossing defect structures, demonstrating how the method can be used to study stability properties of field-theoretical solitons. We demonstrate, e.g., the formation of a half-quantum vortex arch, an Alice arch, on the interface, exhibiting the topological charge of a point defect. We also demonstrate an energetically stable connection of a coreless vortex to two half-quantum vortices. Our method can be extended to study interface physics in spin-2 and spin-3 BECs with richer phenomenology, or in strongly correlated optical-lattice systems. We acknowledge financial support from the Leverhulme Trust.

Three-dimensional printing and deformation behavior of low-density target structures by two-photon polymerization

NASA Astrophysics Data System (ADS)

Liu, Ying; Stein, Ori; Campbell, John H.; Jiang, Lijia; Petta, Nicole; Lu, Yongfeng

2017-08-01

Two-photon polymerization (2PP), a 3D nano to microscale additive manufacturing process, is being used for the first time to fabricate small custom experimental packages ("targets") to support laser-driven high-energy-density (HED) physics research. Of particular interest is the use of 2PP to deterministically print low-density, low atomic-number (CHO) polymer matrices ("foams") at millimeter scale with sub-micrometer resolution. Deformation during development and drying of the foam structures remains a challenge when using certain commercial photo-resins; here we compare use of acrylic resins IP-S and IP-Dip. The mechanical strength of polymeric beam and foam structures is examined particularly the degree of deformation that occurs during the development and drying processes. The magnitude of the shrinkage in the two resins in quantified by printing sample structures and by use of FEA to simulate the deformation. Capillary drying forces are shown to be small and likely below the elastic limit of the core foam structure. In contrast the substantial shrinkage in IP-Dip ( 5-10%) cause large shear stresses and associated plastic deformation particularly near constrained boundaries such as the substrate and locations with sharp density variation. The inherent weakness of stitching boundaries is also evident and in certain cases can lead to delamination. Use of IP-S shows marked reduction in deformation with a minor loss of print resolution

DOE Office of Scientific and Technical Information (OSTI.GOV)

Danaie, Mohsen; Kepaptsoglou, Demie; Ramasse, Quentin M.

The vacancy ordering behavior of an A-site deficient perovskite system, Ca 1–xLa 2x/3TiO 3, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), adopting Pbnm symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown tomore » be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La 3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La 3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. In conclusion, the occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems.« less

Enhanced ferroelectric polarization and possible morphotrophic phase boundary in PZT-based alloys

DOE PAGES

Parker, David S.; Singh, David; McGuire, Michael A.; ...

2016-05-16

We present a combined theoretical and experimental study of alloys of the high performance piezoelectric PZT (PbZr 0.5Ti 0.5O 3) with BZnT (BiZn 0.5Ti 0.5O 3) and BZnZr (BiZn 0.5Zr 0.5O 3), focusing on atomic displacements, ferroelectric polarization, and elastic stability. From theory we find that the 75-25 PZT-BZnT alloy has substantially larger cation displacements, and hence ferroelectric polarization than the PZT base material, on the tetragonal side of the phase diagram. We also find a possible morphotrophic phase boundary in this system by comparing displacement patterns and optimized c/a ratios. Elastic stability calculations find the structures to be essentiallymore » stable. Lastly, experiments indicate the feasibility of sample synthesis within this alloy system, although measurements do not find significant polarization, probably due to a large coercive field.« less

Observations of molecular and atomic gas in photodissociation regions. [interstellar chemistry

NASA Technical Reports Server (NTRS)

Jaffe, D. T.; Howe, J. E.

1989-01-01

Dense gas at the ionized/neutral boundaries of molecular clouds illuminated by far-UV photons plays an important role in the appearance of the neutral interstellar medium. It also is a laboratory for the study of UV-photochemistry and of a number of heating and cooling phenomena not seen elsewhere. Fine structure lines of neutral and low ionization potential species dominate the cooling in the outer part of the photodissociation regions. Observations of these lines show that the regions are dense and highly clumped. Observations of H2 and CO show that heating by UV photons plays a significant role in the excitation of molecular lines near the H II/neutral boundary. Warm CO is more abundant in these regions than predicted by the standard theoretical models. Optical reflection nebulas provide an ideal laboratory for the study of photodissocciation region phenomena.

Atomic and Molecular Spectroscopic Studies of the DIII-D Neutral Beam Ion Source and Neutralizer

NASA Astrophysics Data System (ADS)

Crowley, B.; Rauch, J.; Scoville, J. T.; Sharma, S. K.; Choksi, B.

2015-11-01

The neutral beam system is interesting in that it comprises two distinct low temperature plasmas. Firstly, the ion source is typically a filament or RF driven plasma from which ions are extracted by a high voltage accelerator grid system. Secondly the neutralizer is essentially a low temperature plasma system with the beam serving as the primary ionization source and the neutralizer walls serving as conducting boundaries. Atomic spectroscopy of Doppler shifted D-alpha light emanating from the fast atoms is studied to determine the composition of the source and the divergence of the beam. Molecular spectroscopy involves measuring fine structure in electron-vibrational rotational bands. The technique has applications in low temperature plasmas and here it is used to determine gas temperature in the neutralizer. We describe the experimental set-up and the physics model used to relate the spectroscopic data to the plasma parameters and we present results of recent experiments exploring how to increase neutralization efficiency. Supported by the US DOE under DE-FC02-04ER54698.

Big data in reciprocal space: Sliding fast Fourier transforms for determining periodicity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vasudevan, Rama K., E-mail: rvv@ornl.gov; Belianinov, Alex; Baddorf, Arthur P.

Significant advances in atomically resolved imaging of crystals and surfaces have occurred in the last decade allowing unprecedented insight into local crystal structures and periodicity. Yet, the analysis of the long-range periodicity from the local imaging data, critical to correlation of functional properties and chemistry to the local crystallography, remains a challenge. Here, we introduce a Sliding Fast Fourier Transform (FFT) filter to analyze atomically resolved images of in-situ grown La{sub 5/8}Ca{sub 3/8}MnO{sub 3} (LCMO) films. We demonstrate the ability of sliding FFT algorithm to differentiate two sub-lattices, resulting from a mixed-terminated surface. Principal Component Analysis and Independent Component Analysismore » of the Sliding FFT dataset reveal the distinct changes in crystallography, step edges, and boundaries between the multiple sub-lattices. The implications for the LCMO system are discussed. The method is universal for images with any periodicity, and is especially amenable to atomically resolved probe and electron-microscopy data for rapid identification of the sub-lattices present.« less

Heterogeneity in homogeneous nucleation from billion-atom molecular dynamics simulation of solidification of pure metal.

PubMed

Shibuta, Yasushi; Sakane, Shinji; Miyoshi, Eisuke; Okita, Shin; Takaki, Tomohiro; Ohno, Munekazu

2017-04-05

Can completely homogeneous nucleation occur? Large scale molecular dynamics simulations performed on a graphics-processing-unit rich supercomputer can shed light on this long-standing issue. Here, a billion-atom molecular dynamics simulation of homogeneous nucleation from an undercooled iron melt reveals that some satellite-like small grains surrounding previously formed large grains exist in the middle of the nucleation process, which are not distributed uniformly. At the same time, grains with a twin boundary are formed by heterogeneous nucleation from the surface of the previously formed grains. The local heterogeneity in the distribution of grains is caused by the local accumulation of the icosahedral structure in the undercooled melt near the previously formed grains. This insight is mainly attributable to the multi-graphics processing unit parallel computation combined with the rapid progress in high-performance computational environments.Nucleation is a fundamental physical process, however it is a long-standing issue whether completely homogeneous nucleation can occur. Here the authors reveal, via a billion-atom molecular dynamics simulation, that local heterogeneity exists during homogeneous nucleation in an undercooled iron melt.

Semantic modeling of the structural and process entities during plastic deformation of crystals and rocks

NASA Astrophysics Data System (ADS)

Babaie, Hassan; Davarpanah, Armita

2016-04-01

We are semantically modeling the structural and dynamic process components of the plastic deformation of minerals and rocks in the Plastic Deformation Ontology (PDO). Applying the Ontology of Physics in Biology, the PDO classifies the spatial entities that participate in the diverse processes of plastic deformation into the Physical_Plastic_Deformation_Entity and Nonphysical_Plastic_Deformation_Entity classes. The Material_Physical_Plastic_Deformation_Entity class includes things such as microstructures, lattice defects, atoms, liquid, and grain boundaries, and the Immaterial_Physical_Plastic_Deformation_Entity class includes vacancies in crystals and voids along mineral grain boundaries. The objects under the many subclasses of these classes (e.g., crystal, lattice defect, layering) have spatial parts that are related to each other through taxonomic (e.g., Line_Defect isA Lattice_Defect), structural (mereological, e.g., Twin_Plane partOf Twin), spatial-topological (e.g., Vacancy adjacentTo Atom, Fluid locatedAlong Grain_Boundary), and domain specific (e.g., displaces, Fluid crystallizes Dissolved_Ion, Void existsAlong Grain_Boundary) relationships. The dynamic aspect of the plastic deformation is modeled under the dynamical Process_Entity class that subsumes classes such as Recrystallization and Pressure_Solution that define the flow of energy amongst the physical entities. The values of the dynamical state properties of the physical entities (e.g., Chemical_Potential, Temperature, Particle_Velocity) change while they take part in the deformational processes such as Diffusion and Dislocation_Glide. The process entities have temporal parts (phases) that are related to each other through temporal relations such as precedes, isSubprocessOf, and overlaps. The properties of the physical entities, defined under the Physical_Property class, change as they participate in the plastic deformational processes. The properties are categorized into dynamical, constitutive, spatial, temporal, statistical, and thermodynamical. The dynamical properties, categorized under the Dynamical_Rate_Property and Dynamical_State_Property classes, subsume different classes of properties (e.g., Fluid_Flow_Rate, Temperature, Chemical_Potential, Displacement, Electrical_Charge) based on the physical domain (e.g., fluid, heat, chemical, solid, electrical). The properties are related to the objects under the Physical_Entity class through diverse object type (e.g., physicalPropertyOf) and data type (e.g., Fluid_Pressure unit 'MPa') properties. The changes of the dynamical properties of the physical entities, described by the empirical laws (equations) modeled by experimental structural geologists, are modeled through the Physical_Property_Dependency class that subsumes the more specialized constitutive, kinetic, and thermodynamic expressions of the relationships among the dynamic properties. Annotation based on the PDO will make it possible to integrate and reuse experimental plastic deformation data, knowledge, and simulation models, and conduct semantic-based search of the source data originating from different rock testing laboratories.

Atom probe tomography investigation of lath boundary segregation and precipitation in a maraging stainless steel.

PubMed

Thuvander, Mattias; Andersson, Marcus; Stiller, Krystyna

2013-09-01

Lath boundaries in a maraging stainless steel of composition 13Cr-8Ni-2Mo-2Cu-1Ti-0.7Al-0.3Mn-0.2Si-0.03C (at%) have been investigated using atom probe tomography following aging at 475 °C for up to 100 h. Segregation of Mo, Si and P to the lath boundaries was observed already after 5 min of aging, and the amount of segregation increases with aging time. At lath boundaries also precipitation of η-Ni₃(Ti, Al) and Cu-rich 9R, in contact with each other, takes place. These co-precipitates grow with time and because of coarsening the area number density decreases. After 100 h of aging a ∼5 nm thick film-like precipitation of a Mo-rich phase was observed at the lath boundaries. From the composition of the film it is suggested that the phase in question is the quasicrystalline R' phase. The film is perforated with Cu-rich 9R and η-Ni₃(Ti, Al) co-precipitates. Not all precipitate types present in the matrix do precipitate at the lath boundaries; the Si-containing G phase and γ'-Ni₃(Ti, Al, Si) and the Cr-rich α' phase were not observed at the lath boundaries. Copyright © 2012 Elsevier B.V. All rights reserved.

Measurements in liquid fuel sprays

NASA Technical Reports Server (NTRS)

Chigier, N.; Mao, C. P.

1985-01-01

A ground test facility is being established at NASA Lewis Research Center to simulate the environmental and flight conditions needed to study adverse weather effects. One of the most important components is the water spray system which consists of many nozzles fitted on spray bars. Water is injected through air-assisted atomizers to generate uniform size drops to simulate icing in clouds. The primary objective is to provide experimental data on drop size distribution over a wide range of operating conditions. Correlation equations for mean drop size and initial injection parameters are being determined to assist in the design and modification of the Altitude Wind Tunnel. Special emphasis is being placed on the study of the aerodynamic structure of the air-assisted atomizer sprays. Detailed measurements of the variation of drop size distribution and velocity as a function of time and space are being made. Accurate initial and boundary conditions are being provided for computer model evaluation.

Crystal-Phase Quantum Wires: One-Dimensional Heterostructures with Atomically Flat Interfaces.

PubMed

Corfdir, Pierre; Li, Hong; Marquardt, Oliver; Gao, Guanhui; Molas, Maciej R; Zettler, Johannes K; van Treeck, David; Flissikowski, Timur; Potemski, Marek; Draxl, Claudia; Trampert, Achim; Fernández-Garrido, Sergio; Grahn, Holger T; Brandt, Oliver

2018-01-10

In semiconductor quantum-wire heterostructures, interface roughness leads to exciton localization and to a radiative decay rate much smaller than that expected for structures with flat interfaces. Here, we uncover the electronic and optical properties of the one-dimensional extended defects that form at the intersection between stacking faults and inversion domain boundaries in GaN nanowires. We show that they act as crystal-phase quantum wires, a novel one-dimensional quantum system with atomically flat interfaces. These quantum wires efficiently capture excitons whose radiative decay gives rise to an optical doublet at 3.36 eV at 4.2 K. The binding energy of excitons confined in crystal-phase quantum wires is measured to be more than twice larger than that of the bulk. As a result of their unprecedented interface quality, these crystal-phase quantum wires constitute a model system for the study of one-dimensional excitons.

Direct growth and patterning of multilayer graphene onto a targeted substrate without an external carbon source.

PubMed

Kang, Dongseok; Kim, Won-Jun; Lim, Jung Ah; Song, Yong-Won

2012-07-25

Using only a simple tube furnace, we demonstrate the synthesis of patterned graphene directly on a designed substrate without the need for an external carbon source. Carbon atoms are absorbed onto Ni evaporator sources as impurities, and incorporated into catalyst layers during the deposition. Heat treatment conditions were optimized so that the atoms diffused out along the grain boundaries to form nanocrystals at the catalyst-substrate interfaces. Graphene patterns were obtained under patterned catalysts, which restricted graphene formation to within patterned areas. The resultant multilayer graphene was characterized by Raman spectroscopy and transmission electron microscopy to verify the high crystallinity and two-dimensional nanomorphology. Finally, a metal-semiconductor diode with a catalyst-graphene contact structure were fabricated and characterized to assess the semiconducting properties of the graphene sheets with respect to the display of asymmetric current-voltage behavior.

Embedded-cluster calculations in a numeric atomic orbital density-functional theory framework.

PubMed

Berger, Daniel; Logsdail, Andrew J; Oberhofer, Harald; Farrow, Matthew R; Catlow, C Richard A; Sherwood, Paul; Sokol, Alexey A; Blum, Volker; Reuter, Karsten

2014-07-14

We integrate the all-electron electronic structure code FHI-aims into the general ChemShell package for solid-state embedding quantum and molecular mechanical (QM/MM) calculations. A major undertaking in this integration is the implementation of pseudopotential functionality into FHI-aims to describe cations at the QM/MM boundary through effective core potentials and therewith prevent spurious overpolarization of the electronic density. Based on numeric atomic orbital basis sets, FHI-aims offers particularly efficient access to exact exchange and second order perturbation theory, rendering the established QM/MM setup an ideal tool for hybrid and double-hybrid level density functional theory calculations of solid systems. We illustrate this capability by calculating the reduction potential of Fe in the Fe-substituted ZSM-5 zeolitic framework and the reaction energy profile for (photo-)catalytic water oxidation at TiO2(110).

Embedded-cluster calculations in a numeric atomic orbital density-functional theory framework

DOE Office of Scientific and Technical Information (OSTI.GOV)

Berger, Daniel, E-mail: daniel.berger@ch.tum.de; Oberhofer, Harald; Reuter, Karsten

2014-07-14

We integrate the all-electron electronic structure code FHI-aims into the general ChemShell package for solid-state embedding quantum and molecular mechanical (QM/MM) calculations. A major undertaking in this integration is the implementation of pseudopotential functionality into FHI-aims to describe cations at the QM/MM boundary through effective core potentials and therewith prevent spurious overpolarization of the electronic density. Based on numeric atomic orbital basis sets, FHI-aims offers particularly efficient access to exact exchange and second order perturbation theory, rendering the established QM/MM setup an ideal tool for hybrid and double-hybrid level density functional theory calculations of solid systems. We illustrate this capabilitymore » by calculating the reduction potential of Fe in the Fe-substituted ZSM-5 zeolitic framework and the reaction energy profile for (photo-)catalytic water oxidation at TiO{sub 2}(110)« less

Global Anisotropies in TeV Cosmic Rays Related to the Sun's Local Galactic Environment from IBEX

NASA Technical Reports Server (NTRS)

Schwadron, N. A.; Adams, F. C.; Christian, E. R.; Desiati, P.; Frisch, P.; Funsten, H. O.; Jokipii, J. R.; McComas, D. J.; Moebius, E.; Zank, G. P.

2014-01-01

Observations with the Interstellar Boundary Explorer (IBEX) have shown enhanced energetic neutral atom (ENA) emission from a narrow, circular ribbon likely centered on the direction of the local interstellar medium (LISM) magnetic field. Here, we show that recent determinations of the local interstellar velocity, based on interstellar atom measurements with IBEX, are consistent with the interstellar modulation of high-energy (tera-electron volts, TeV) cosmic rays and diffusive propagation from supernova sources revealed in global anisotropy maps of ground-based high-energy cosmic-ray observatories (Milagro, Asg, and IceCube). Establishing a consistent local interstellar magnetic field direction using IBEX ENAs at hundreds to thousands of eV and galactic cosmic rays at tens of TeV has wide-ranging implications for the structure of our heliosphere and its interactions with the LISM, which is particularly important at the time when the Voyager spacecraft are leaving our heliosphere.

Transmission electron microscopy investigation of interfaces in a two-phase TiAl alloy

NASA Astrophysics Data System (ADS)

Mahon, G. J.; Howe, J. M.

1990-06-01

The atomic structures of the γ/α2 and γ/γT interfaces in a TiAl alloy were investigated using conventional and high-resolution transmission electron microscopy (TEM) in order to understand the growth mechanisms and deformation behavior of the two-phase alloy. The results show that the α2 plates grow from the γ phase by the migration of a/6<112> partial dislocation ledges across the faces and that the γ/α2 interface usually contains closely spaced arrays of interfacial dislocations. Deformation twins cut through both γ twin boundaries and α2 plates during deformation, although slip of twinning c slocations through α2 appears to be a difficult process. Both the γ/α2 and γ/γT interfaces can be imaged and modeled at the atomic level, although slight crystal and/or beam tilt can complicate image interpretation.

Designing pinhole vacancies in graphene towards functionalization: Effects on critical buckling load

NASA Astrophysics Data System (ADS)

Georgantzinos, S. K.; Markolefas, S.; Giannopoulos, G. I.; Katsareas, D. E.; Anifantis, N. K.

2017-03-01

The effect of size and placement of pinhole-type atom vacancies on Euler's critical load on free-standing, monolayer graphene, is investigated. The graphene is modeled by a structural spring-based finite element approach, in which every interatomic interaction is approached as a linear spring. The geometry of graphene and the pinhole size lead to the assembly of the stiffness matrix of the nanostructure. Definition of the boundary conditions of the problem leads to the solution of the eigenvalue problem and consequently to the critical buckling load. Comparison to results found in the literature illustrates the validity and accuracy of the proposed method. Parametric analysis regarding the placement and size of the pinhole-type vacancy, as well as the graphene geometry, depicts the effects on critical buckling load. Non-linear regression analysis leads to empirical-analytical equations for predicting the buckling behavior of graphene, with engineered pinhole-type atom vacancies.

Global anisotropies in TeV cosmic rays related to the Sun's local galactic environment from IBEX.

PubMed

Schwadron, N A; Adams, F C; Christian, E R; Desiati, P; Frisch, P; Funsten, H O; Jokipii, J R; McComas, D J; Moebius, E; Zank, G P

2014-02-28

Observations with the Interstellar Boundary Explorer (IBEX) have shown enhanced energetic neutral atom (ENA) emission from a narrow, circular ribbon likely centered on the direction of the local interstellar medium (LISM) magnetic field. Here, we show that recent determinations of the local interstellar velocity, based on interstellar atom measurements with IBEX, are consistent with the interstellar modulation of high-energy (tera-electron volts, TeV) cosmic rays and diffusive propagation from supernova sources revealed in global anisotropy maps of ground-based high-energy cosmic-ray observatories (Milagro, Asγ, and IceCube). Establishing a consistent local interstellar magnetic field direction using IBEX ENAs at hundreds to thousands of eV and galactic cosmic rays at tens of TeV has wide-ranging implications for the structure of our heliosphere and its interactions with the LISM, which is particularly important at the time when the Voyager spacecraft are leaving our heliosphere.

Controlling the opto-electronic properties of nc-SiOx:H films by promotion of 〈220〉 orientation in the growth of ultra-nanocrystallites at the grain boundary

NASA Astrophysics Data System (ADS)

Das, Debajyoti; Samanta, Subhashis

2018-01-01

A systematic development of undoped nc-SiOx:H thin films from (SiH4 + CO2) plasma diluted by a combination of H2 and He has been investigated through structural, optical and electrical characterization and correlation. Gradual inclusion of O into a highly crystalline silicon network progressively produces a two-phase structure where Si-nanocrystals (Si-nc) are embedded into the a-SiOx:H matrix. However, at the intermediate grain boundary region the growth of ultra-nanocrystallites controls the effectiveness of the material. The ultra-nanocrystallites are the part and portion of crystallinity accommodating the dominant fraction of thermodynamically preferred 〈220〉 crystallographic orientation, most favourable for stacked layer device performance. Atomic H plays a dominant role in maintaining an improved nanocrystalliny in the network even during O inclusion, while He in its excited state (He*) maintains a good energy balance at the grain boundary and produces a significant fraction of ultra-nanocrystalline component which has been demonstrated to organize the energetically favourable 〈220〉 crystallographic orientation in the network. The nc-SiOx:H films, maintaining proportionally good electrical conductivity over an wide range of optical band gap, remarkably low microstructure factor and simultaneous high crystalline volume fraction dominantly populated by ultra-nanocrystallites of 〈220〉 crystallographic orientation mostly at the grain boundary, have been obtained in technologically most popular 13.56 MHz PECVD SiH4 plasma even at a low substrate temperature ∼250 °C, convenient for device fabrication.

Mechanisms of Texture Development in Lead-Free Piezoelectric Ceramics with Perovskite Structure Made by the Templated Grain Growth Process

PubMed Central

Kimura, Toshio; Yi, Yuan; Sakurai, Fumito

2010-01-01

The mechanisms of texture development were examined for BaTiO3 and a (K,Na,Li)(Nb,Ta)O3 solid solution made by the templated grain growth method, and compared with the mechanism in Bi0.5(Na,K)0.5TiO3. The dominant mechanism was different in each material; grain boundary migration in BaTiO3, solid state spreading in Bi0.5(Na,K)0.5TiO3, and abnormal grain growth in the (K,Na,Li)(Nb,Ta)O3 solid solution. The factor determining the dominant mechanism is the degree of smoothness of surface structure at an atomic level. PMID:28883364

Oxygen content modulation by nanoscale chemical and electrical patterning in epitaxial SrCoO3-δ (0 < δ ≤ 0.5) thin films.

PubMed

Hu, S; Seidel, J

2016-08-12

Fast controllable redox reactions in solid materials at room temperature are a promising strategy for enhancing the overall performance and lifetime of many energy technology materials and devices. Easy control of oxygen content is a key concept for the realisation of fast catalysis and bulk diffusion at room temperature. Here, high quality epitaxial brownmillerite SrCoO2.5 thin films have been oxidised to perovskite (P) SrCoO3 with NaClO. X-ray diffraction, scanning probe microscopy and x-ray photoelectron spectroscopy measurements were performed to investigate the structural and electronic changes of the material. The oxidised thin films were found to exhibit distinct morphological changes from an atomically flat terrace structure to forming small nanosized islands with boundaries preferentially in [100] or [010] directions all over the surface, relaxing the in-plane strain imposed by the substrate. The conductivity, or oxygen content, of each single island is confined by these textures, which can be locally patterned even further with electric poling. The high charging level at the island boundaries indicates a magnified electric capacity of SCO thin films, which could be exploited in future device geometries. This finding represents a new way of oxygen modulation with associated self-assembled charge confinement to nanoscale boundaries, offering interesting prospects in nanotechnology applications.

Oxygen content modulation by nanoscale chemical and electrical patterning in epitaxial SrCoO3-δ (0 < δ ≤ 0.5) thin films

NASA Astrophysics Data System (ADS)

Hu, S.; Seidel, J.

2016-08-01

Fast controllable redox reactions in solid materials at room temperature are a promising strategy for enhancing the overall performance and lifetime of many energy technology materials and devices. Easy control of oxygen content is a key concept for the realisation of fast catalysis and bulk diffusion at room temperature. Here, high quality epitaxial brownmillerite SrCoO2.5 thin films have been oxidised to perovskite (P) SrCoO3 with NaClO. X-ray diffraction, scanning probe microscopy and x-ray photoelectron spectroscopy measurements were performed to investigate the structural and electronic changes of the material. The oxidised thin films were found to exhibit distinct morphological changes from an atomically flat terrace structure to forming small nanosized islands with boundaries preferentially in [100] or [010] directions all over the surface, relaxing the in-plane strain imposed by the substrate. The conductivity, or oxygen content, of each single island is confined by these textures, which can be locally patterned even further with electric poling. The high charging level at the island boundaries indicates a magnified electric capacity of SCO thin films, which could be exploited in future device geometries. This finding represents a new way of oxygen modulation with associated self-assembled charge confinement to nanoscale boundaries, offering interesting prospects in nanotechnology applications.

Atomistic modeling of BN nanofillers for mechanical and thermal properties: a review.

PubMed

Kumar, Rajesh; Parashar, Avinash

2016-01-07

Due to their exceptional mechanical properties, thermal conductivity and a wide band gap (5-6 eV), boron nitride nanotubes and nanosheets have promising applications in the field of engineering and biomedical science. Accurate modeling of failure or fracture in a nanomaterial inherently involves coupling of atomic domains of cracks and voids as well as a deformation mechanism originating from grain boundaries. This review highlights the recent progress made in the atomistic modeling of boron nitride nanofillers. Continuous improvements in computational power have made it possible to study the structural properties of these nanofillers at the atomistic scale.

BetaCavityWeb: a webserver for molecular voids and channels

PubMed Central

Kim, Jae-Kwan; Cho, Youngsong; Lee, Mokwon; Laskowski, Roman A.; Ryu, Seong Eon; Sugihara, Kokichi; Kim, Deok-Soo

2015-01-01

Molecular cavities, which include voids and channels, are critical for molecular function. We present a webserver, BetaCavityWeb, which computes these cavities for a given molecular structure and a given spherical probe, and reports their geometrical properties: volume, boundary area, buried area, etc. The server's algorithms are based on the Voronoi diagram of atoms and its derivative construct: the beta-complex. The correctness of the computed result and computational efficiency are both mathematically guaranteed. BetaCavityWeb is freely accessible at the Voronoi Diagram Research Center (VDRC) (http://voronoi.hanyang.ac.kr/betacavityweb). PMID:25904629

First-principles study of the Kondo physics of a single Pu impurity in a Th host

DOE PAGES

Zhu, Jian -Xin; Albers, R. C.; Haule, K.; ...

2015-04-23

Based on its condensed-matter properties, crystal structure, and metallurgy, which includes a phase diagram with six allotropic phases, plutonium is one of the most complicated pure elements in its solid state. Its anomalous properties, which are indicative of a very strongly correlated state, are related to its special position in the periodic table, which is at the boundary between the light actinides that have itinerant 5f electrons and the heavy actinides that have localized 5f electrons. As a foundational study to probe the role of local electronic correlations in Pu, we use the local-density approximation together with a continuous-time quantummore » Monte Carlo simulation to investigate the electronic structure of a single Pu atom that is either substitutionally embedded in the bulk and or adsorbed on the surface of a Th host. This is a simpler case than the solid phases of Pu metal. With the Pu impurity atom we have found a Kondo resonance peak, which is an important signature of electronic correlations, in the local density of states around the Fermi energy. We show that the peak width of this resonance is narrower for Pu atoms at the surface of Th than for those in the bulk due to a weakened Pu - 5f hybridization with the ligands at the surface.« less

Atomic-scale studies on the effect of boundary coherency on stability in twinned Cu

NASA Astrophysics Data System (ADS)

Niu, Rongmei; Han, Ke; Su, Yi-Feng; Salters, Vincent J.

2014-01-01

The stored energy and hardness of nanotwinned (NT) Cu are related to interaction between dislocations and {111}-twin boundaries (TBs) studied at atomic scales by high-angle annular dark-field scanning transmission electron microscope. Lack of mobile dislocations at coherent TBs (CTBs) provides as-deposited NT Cu a rare combination of stability and hardness. The introduction of numerous incoherent TBs (ITBs) reduces both the stability and hardness. While storing more energy in their ITBs than in the CTBs, deformed NT Cu also exhibits high dislocation density and TB mobility and therefore has increased the driving force for recovery, coarsening, and recrystallization.

Three-Dimensional Microphase Separation and Synergistic Permeability in Stacked Lipid–Polymer Hybrid Membranes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kang, Minjee; Lee, Byeongdu; Leal, Cecilia

Here, we present new structures of soft-material thin films that augment the functionality of substrate-mediated delivery systems. A hybrid material composed of phospholipids and block copolymers adopts a multilayered membrane structure supported on a solid surface. The hybrid films comprise intentional intramembrane heterogeneities that register across multilayers. These stacked domains convey unprecedented enhancement and control of permeability of solutes across micrometer-thick films. Using grazing incidence X-ray scattering, phase contrast atomic force microscopy, and confocal microscopy, we observed that in each lamella, lipid and polymers partition unevenly within the membrane plane segregating into lipid- or polymer-rich domains. Interestingly, we found evidencemore » that like-domains align in registry across multilayers, thereby making phase separation three-dimensional. Phase boundaries exist over extended length scales to compensate the height mismatch between lipid and polymer molecules. We show that microphase separation in hybrid films can be exploited to augment the capability of drug-eluting substrates. Lipid–polymer hybrid films loaded with paclitaxel show synergistic permeability of drug compared to single-component counterparts. We present a thorough structural study of stacked lipid–polymer hybrid membranes and propose that the presence of registered domains and domain boundaries impart enhanced drug release functionality. This work offers new perspectives in designing thin films for controlled delivery applications« less

Three-Dimensional Microphase Separation and Synergistic Permeability in Stacked Lipid–Polymer Hybrid Membranes

DOE PAGES

Kang, Minjee; Lee, Byeongdu; Leal, Cecilia

2017-10-20

Here, we present new structures of soft-material thin films that augment the functionality of substrate-mediated delivery systems. A hybrid material composed of phospholipids and block copolymers adopts a multilayered membrane structure supported on a solid surface. The hybrid films comprise intentional intramembrane heterogeneities that register across multilayers. These stacked domains convey unprecedented enhancement and control of permeability of solutes across micrometer-thick films. Using grazing incidence X-ray scattering, phase contrast atomic force microscopy, and confocal microscopy, we observed that in each lamella, lipid and polymers partition unevenly within the membrane plane segregating into lipid- or polymer-rich domains. Interestingly, we found evidencemore » that like-domains align in registry across multilayers, thereby making phase separation three-dimensional. Phase boundaries exist over extended length scales to compensate the height mismatch between lipid and polymer molecules. We show that microphase separation in hybrid films can be exploited to augment the capability of drug-eluting substrates. Lipid–polymer hybrid films loaded with paclitaxel show synergistic permeability of drug compared to single-component counterparts. We present a thorough structural study of stacked lipid–polymer hybrid membranes and propose that the presence of registered domains and domain boundaries impart enhanced drug release functionality. This work offers new perspectives in designing thin films for controlled delivery applications« less

FIM/atom probe analysis of a heat treated 7150 aluminum alloy

NASA Astrophysics Data System (ADS)

Brenner, S. S.; Kowalik, J.; Hua Ming-Jian

1991-04-01

The stress corrosion cracking resistance of high strength aluminum alloys can be substantially altered by heat treatment. In addition to microstructural changes, the alloys may also undergo chemical changes as a result of the heat treatment which may affect the stress corrosion properties. The FIM/atom probe has been used to detect such changes. The compositions of the matrix, matrix precipitates, precipitate-free zone (PFZ) and grain boundary precipitates in a heat-treated 7150 Al alloy tempered to peak strength have been quantitatively measured. A substantial increase in the concentrations of Mg, Zn and Cu were found in the PFZ. The average compositions of the precipitates in the matrix and at the sub-boundaries were shown not to differ significantly. The coarser precipitates at high-angle boundaries, which may have a more important effect on stress corrosion, were difficult to analyze because of their low number density and the large grain size of the material.

Quantum mechanical fragment methods based on partitioning atoms or partitioning coordinates.

PubMed

Wang, Bo; Yang, Ke R; Xu, Xuefei; Isegawa, Miho; Leverentz, Hannah R; Truhlar, Donald G

2014-09-16

Conspectus The development of more efficient and more accurate ways to represent reactive potential energy surfaces is a requirement for extending the simulation of large systems to more complex systems, longer-time dynamical processes, and more complete statistical mechanical sampling. One way to treat large systems is by direct dynamics fragment methods. Another way is by fitting system-specific analytic potential energy functions with methods adapted to large systems. Here we consider both approaches. First we consider three fragment methods that allow a given monomer to appear in more than one fragment. The first two approaches are the electrostatically embedded many-body (EE-MB) expansion and the electrostatically embedded many-body expansion of the correlation energy (EE-MB-CE), which we have shown to yield quite accurate results even when one restricts the calculations to include only electrostatically embedded dimers. The third fragment method is the electrostatically embedded molecular tailoring approach (EE-MTA), which is more flexible than EE-MB and EE-MB-CE. We show that electrostatic embedding greatly improves the accuracy of these approaches compared with the original unembedded approaches. Quantum mechanical fragment methods share with combined quantum mechanical/molecular mechanical (QM/MM) methods the need to treat a quantum mechanical fragment in the presence of the rest of the system, which is especially challenging for those parts of the rest of the system that are close to the boundary of the quantum mechanical fragment. This is a delicate matter even for fragments that are not covalently bonded to the rest of the system, but it becomes even more difficult when the boundary of the quantum mechanical fragment cuts a bond. We have developed a suite of methods for more realistically treating interactions across such boundaries. These methods include redistributing and balancing the external partial atomic charges and the use of tuned fluorine atoms for capping dangling bonds, and we have shown that they can greatly improve the accuracy. Finally we present a new approach that goes beyond QM/MM by combining the convenience of molecular mechanics with the accuracy of fitting a potential function to electronic structure calculations on a specific system. To make the latter practical for systems with a large number of degrees of freedom, we developed a method to interpolate between local internal-coordinate fits to the potential energy. A key issue for the application to large systems is that rather than assigning the atoms or monomers to fragments, we assign the internal coordinates to reaction, secondary, and tertiary sets. Thus, we make a partition in coordinate space rather than atom space. Fits to the local dependence of the potential energy on tertiary coordinates are arrayed along a preselected reaction coordinate at a sequence of geometries called anchor points; the potential energy function is called an anchor points reactive potential. Electrostatically embedded fragment methods and the anchor points reactive potential, because they are based on treating an entire system by quantum mechanical electronic structure methods but are affordable for large and complex systems, have the potential to open new areas for accurate simulations where combined QM/MM methods are inadequate.

Structures Self-Assembled Through Directional Solidification

NASA Technical Reports Server (NTRS)

Dynys, Frederick W.; Sayir, Ali

2005-01-01

Nanotechnology has created a demand for new fabrication methods with an emphasis on simple, low-cost techniques. Directional solidification of eutectics (DSE) is an unconventional approach in comparison to low-temperature biomimetic approaches. A technical challenge for DSE is producing microstructural architectures on the nanometer scale. In both processes, the driving force is the minimization of Gibb's free energy. Selfassembly by biomimetic approaches depends on weak interaction forces between organic molecules to define the architectural structure. The architectural structure for solidification depends on strong chemical bonding between atoms. Constituents partition into atomic-level arrangements at the liquid-solid interface to form polyphase structures, and this atomic-level arrangement at the liquid-solid interface is controlled by atomic diffusion and total undercooling due to composition (diffusion), kinetics, and curvature of the boundary phases. Judicious selection of the materials system and control of the total undercooling are the keys to producing structures on the nanometer scale. The silicon-titanium silicide (Si-TiSi2) eutectic forms a rod structure under isothermal cooling conditions. At the NASA Glenn Research Center, directional solidification was employed along with a thermal gradient to promote uniform rods oriented with the thermal gradient. The preceding photomicrograph shows the typical transverse microstructure of a solidified Si-TiSi2 eutectic composition. The dark and light gray regions are Si and TiSi2, respectively. Preferred rod orientation along the thermal gradient was poor. The ordered TiSi2 rods have a narrow distribution in diameter of 2 to 3 m, as shown. The rod diameter showed a weak dependence on process conditions. Anisotropic etch behavior between different phases provides the opportunity to fabricate structures with high aspect ratios. The photomicrographs show the resulting microstructure after a wet chemical etch and a dry plasma etch. The wet chemical etches the silicon away, exposing the TiSi2 rods, whereas plasma etching preferentially etches the Si-TiSi2 interface to form a crater. The porous architectures are applicable to fabricating microdevices or creating templates for part fabrication. The porous rod structure can serve as a platform for fabricating microplasma devices for propulsion or microheat exchangers and for fabricating microfilters for miniatured chemical reactors. Although more work is required, self-assembly from DSE can have a role in microdevice fabrication.

Effect of Sr substitution on the room temperature electrical properties of La1-xSrxFeO3 nano-crystalline materials

NASA Astrophysics Data System (ADS)

Kafa, C. A.; Triyono, D.; Laysandra, H.

2017-07-01

LaFeO3 is a material with Perovskite structure which electrical properties got investigated a lot, because as a p-type semiconductor it showed good gas sensing behavior through resistivity comparison. Sr doping on LaFeO3 is able to improve the electrical conductivity through structural modification. Using the Sr atoms doping concentration (x) from 0.1 to 0.4, La1-xSrxFeO3 nanocrystal pellets were synthesized using sol-gel method, followed by gradual heat treatment and uniaxial compaction. Structural analysis from XRD characterization shows that the structure of the materials is Orthorhombic Perovskite. The topography of the sample by SEM reveals grain and grain boundary existence with emerging agglomeration. The electrical properties of the material, as functions of frequency, were measured by Impedance Spectroscopy method using RLC meter. Through the Nyquist plot and Bode plot, the electrical conductivity of La1-xSrxFeO3 is contributed by grain and grain boundaries. It is reported that La0.6Sr0.4FeO3 sample has the most superior electrical conductivity of all samples, and the electrical permittivity of both La0.8Sr0.2FeO3 and La0.7Sr0.3FeO3 are the most stable.

Magnetism out of antisite disorder in the J =0 compound Ba2YIrO6

NASA Astrophysics Data System (ADS)

Chen, Q.; Svoboda, C.; Zheng, Q.; Sales, B. C.; Mandrus, D. G.; Zhou, H. D.; Zhou, J.-S.; McComb, D.; Randeria, M.; Trivedi, N.; Yan, J.-Q.

2017-10-01

We systematically investigate the magnetic properties and local structure of Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase boundary or clusters of antisite disorder affect the magnetism observed in this 5 d4 compound. The experimental investigation involved comparison of the magnetic properties and atomic imaging of (1) a slow-cooled crystal, (2) a crystal quenched from 900°C after growth, and (3) a crystal grown using a faster cooling rate during growth than the slow-cooled one. Atomic-scale imaging by scanning transmission electron microscopy (STEM) shows that quenching from 900°C introduces Ir-rich antiphase boundaries in the crystals, and a faster cooling rate during crystal growth leads to clusters of Y and Ir antisite disorder. Compared to the slow-cooled crystals, Ba2YIrO6 crystals with clusters of antisite defects have a larger effective moment and a larger saturation moment, while quenched crystals with Ir-rich antiphase boundary show a slightly suppressed moment. Our DFT and model magnetic Hamiltonian calculations suggest magnetic condensation is unlikely, as the energy to be gained from superexchange is small compared to the spin-orbit gap. However, once Y is replaced by Ir in the antisite disordered region, the picture of local nonmagnetic singlets breaks down and magnetism can be induced. This is because of (a) enhanced interactions due to increased orbital overlap and (b) increased number of orbitals mediating the interactions. Our work highlights the importance of lattice defects in understanding the experimentally observed magnetism in Ba2YIrO6 and other J =0 systems.

Thermodynamic properties of small aggregates of rare-gas atoms

NASA Technical Reports Server (NTRS)

Etters, R. D.; Kaelberer, J.

1975-01-01

The present work reports on the equilibrium thermodynamic properties of small clusters of xenon, krypton, and argon atoms, determined from a biased random-walk Monte Carlo procedure. Cluster sizes ranged from 3 to 13 atoms. Each cluster was found to have an abrupt liquid-gas phase transition at a temperature much less than for the bulk material. An abrupt solid-liquid transition is observed for thirteen- and eleven-particle clusters. For cluster sizes smaller than 11, a gradual transition from solid to liquid occurred over a fairly broad range of temperatures. Distribution of number of bond lengths as a function of bond length was calculated for several systems at various temperatures. The effects of box boundary conditions are discussed. Results show the importance of a correct description of boundary conditions. A surprising result is the slow rate at which system properties approach bulk behavior as cluster size is increased.

Analysis of atomic force microscopy data for surface characterization using fuzzy logic

DOE Office of Scientific and Technical Information (OSTI.GOV)

Al-Mousa, Amjed, E-mail: aalmousa@vt.edu; Niemann, Darrell L.; Niemann, Devin J.

2011-07-15

In this paper we present a methodology to characterize surface nanostructures of thin films. The methodology identifies and isolates nanostructures using Atomic Force Microscopy (AFM) data and extracts quantitative information, such as their size and shape. The fuzzy logic based methodology relies on a Fuzzy Inference Engine (FIE) to classify the data points as being top, bottom, uphill, or downhill. The resulting data sets are then further processed to extract quantitative information about the nanostructures. In the present work we introduce a mechanism which can consistently distinguish crowded surfaces from those with sparsely distributed structures and present an omni-directional searchmore » technique to improve the structural recognition accuracy. In order to demonstrate the effectiveness of our approach we present a case study which uses our approach to quantitatively identify particle sizes of two specimens each with a unique gold nanoparticle size distribution. - Research Highlights: {yields} A Fuzzy logic analysis technique capable of characterizing AFM images of thin films. {yields} The technique is applicable to different surfaces regardless of their densities. {yields} Fuzzy logic technique does not require manual adjustment of the algorithm parameters. {yields} The technique can quantitatively capture differences between surfaces. {yields} This technique yields more realistic structure boundaries compared to other methods.« less

Effects of stuffing on the atomic and electronic structure of the pyrochlore Yb2Ti2O7

NASA Astrophysics Data System (ADS)

Ghosh, Soham S.; Manousakis, Efstratios

2018-06-01

There are reasons to believe that the ground state of the magnetic rare-earth pyrochlore Yb2Ti2O7 is on the boundary between competing ground states. We have carried out ab initio density functional calculations to determine the most stable chemical formula as a function of the oxygen chemical potential and the likely location of the oxygen atoms in the unit cell of the "stuffed" system. We find that it is energetically favorable in the stuffed crystal (with an Yb replacement on a Ti site) to contain oxygen vacancies which dope the Yb 4 f orbitals and qualitatively change the electronic properties of the system. In addition, with the inclusion of the contribution of spin-orbit coupling (SOC) on top of the GGA + U approach, we investigated the electronic structure and the magnetic moments of the most stable stuffed system. In our determined stuffed structure the valence bands as compared to those of the pure system are pushed down and a change in hybridization between the O 2 p orbitals and the metal ion states is found. Our first-principle findings should form a foundation for effective models describing the low-temperature properties of this material whose true ground state remains controversial.

Unoccupied Electron States and the Formation of Interface between Films of Dimethyl-Substituted Thiophene-Phenylene Coolygomers and Oxidized Silicon Surface

NASA Astrophysics Data System (ADS)

Komolov, A. S.; Lazneva, E. F.; Gerasimova, N. B.; Panina, Yu. A.; Zashikhin, G. D.; Pshenichnyuk, S. A.; Borshchev, O. V.; Ponomarenko, S. A.; Handke, B.

2018-05-01

The unoccupied electron states and the boundary potential barrier during deposition of ultrathin films of dimethyl-substituted thiophene-phenylene coolygomers of the type of CH3-phenylene-thiophene-thiophene-phenylene-CH3 (CH3-PTTP-CH3) on an oxidized silicon surface have been studied. The electronic characteristics have been measured in the energy range from 5 to 20 eV above the Fermi level using total current spectroscopy (TCS). The structure of the CH3-PTTP-CH3 film surfaces has been studied by atomic force microscopy (AFM), and the atomic compositions of the films have been studied by X-ray photoelectron spectroscopy (XPS). The changes in the maximum intensities measured by the TCS method obtained from the deposited CH3-PTTP-CH3 film and from the substrate during increasing in the organic coating thickness to 6 nm is discussed. The formation of the boundary potential barrier in the n-Si/SiO2/CH3-PTTP-CH3 is accompanied by the decrease in the surface work function from 4.2 ± 0.1 to 4.0 ± 0.1 eV as the organic coating thickness increases to 3 nm. The ratio of atomic concentrations C: S in the CH3-PTTP-CH3 films well corresponds to the chemical formula of CH3-PTTP-CH3 molecules. The roughness of the CH3-PTTP-CH3 coating surface was not higher than 10 nm on the 10 × 10 μm areas as the total CH3-PTTP-CH3-layer thickness was about 100 nm.

Dynamics of Nanoscale Grain-Boundary Decohesion in Aluminum by Molecular-Dynamics Simulation

NASA Technical Reports Server (NTRS)

Yamakov, V.; Saether, E.; Phillips, D. R.; Glaessegen, E. H.

2007-01-01

The dynamics and energetics of intergranular crack growth along a flat grain boundary in aluminum is studied by a molecular-dynamics simulation model for crack propagation under steady-state conditions. Using the ability of the molecular-dynamics simulation to identify atoms involved in different atomistic mechanisms, it was possible to identify the energy contribution of different processes taking place during crack growth. The energy contributions were divided as: elastic energy, defined as the potential energy of the atoms in fcc crystallographic state; and plastically stored energy, the energy of stacking faults and twin boundaries; grain-boundary and surface energy. In addition, monitoring the amount of heat exchange with the molecular-dynamics thermostat gives the energy dissipated as heat in the system. The energetic analysis indicates that the majority of energy in a fast growing crack is dissipated as heat. This dissipation increases linearly at low speed, and faster than linear at speeds approaching 1/3 the Rayleigh wave speed when the crack tip becomes dynamically unstable producing periodic dislocation bursts until the crack is blunted.

3-D Observation of dopant distribution at NAND flash memory floating gate using Atom probe tomography

NASA Astrophysics Data System (ADS)

Lee, Ji-hyun; Chae, Byeong-Kyu; Kim, Joong-Jeong; Lee, Sun Young; Park, Chan Gyung

2015-01-01

Dopant control becomes more difficult and critical as silicon devices become smaller. We observed the dopant distribution in a thermally annealed polysilicon gate using Transmission Electron Microscopy (TEM) and Atom probe tomography (APT). Phosphorus was doped at the silicon-nitride-diffusion-barrier-layer-covered polycrystalline silicon gate. Carbon also incorporated at the gate for the enhancement of operation uniformity. The impurity distribution was observed using atom probe tomography. The carbon atoms had segregated at grain boundaries and suppressed silicon grain growth. Phosphorus atoms, on the other hand, tended to pile-up at the interface. A 1-nm-thick diffusion barrier effectively blocked P atom out-diffusion. [Figure not available: see fulltext.

Molecular dynamics study of the role of symmetric tilt grain boundaries on the helium distribution in nickel

NASA Astrophysics Data System (ADS)

Torres, E.; Pencer, J.

2018-04-01

Helium impurities, from either direct implantation or transmutation reactions, have been associated with embrittlement in nickel-based alloys. Helium has very low solubility in nickel, and has been found to aggregate at lattice defects such as vacancies, dislocations, and grain boundaries. The retention and precipitation of helium in nickel-based alloys have deleterious effects on the material mechanical properties. However, the underlying mechanisms that lead to helium effects in the host metal are not fully understood. In the present work, we investigate the role of symmetric tilt grain boundary (STGB) structures on the distribution of helium in nickel using molecular dynamics simulations. We investigate the family of STGBs specific to the 〈 110 〉 tilt axis. The present results indicate that accumulation of helium at the grain boundary may be modulated by details of grain boundary geometry. A plausible correlation between the grain boundary energy and misorientation with the accumulation and mobility of helium is proposed. Small clusters with up to 6 helium atoms show significant interstitial mobility in the nickel bulk, but also become sites for nucleation and grow of more stable helium clusters. High-energy GBs are found mainly populated with small helium clusters. The high mobility of small clusters along the GBs indicates the role of these GBs as fast two-dimensional channels for diffusion. In contrast, the accumulation of helium in large helium clusters at low-energy STGB creates a favorable environment for the formation of large helium bubbles, indicating a potential role for low-energy STGB in promoting helium-induced GB embrittlement.

The effect of new interstellar medium parameters on the heliosphere and energetic neutral atoms from the interstellar boundary

DOE PAGES

Heerikhuisen, J.; Zirnstein, E. J.; Funsten, H. O.; ...

2014-03-05

Here we present new results from three-dimensional simulations of the solar wind interaction with the local interstellar medium (LISM) using recent observations by NASA's Interstellar Boundary EXplorer (IBEX) mission estimates of the velocity and temperature of the LISM. We investigate four strengths of the LISM magnetic field, from 1 to 4 μG, and adjust the LISM proton and hydrogen densities so that the distance to the termination shock (TS) in the directions of the Voyager spacecraft is just below 90 AU, and the density of hydrogen at the TS is close to 0.09 cm-3 in the nose direction. The orientationmore » of the magnetic field is chosen to point toward the center of the ribbon of enhanced energetic neutral atom (ENA) flux seen in the IBEX data. Our simulations show that the plasma and neutral properties in the outer heliosheath vary considerably as a function of the LISM magnetic field strength. We also show that the heliotail points downwind in all cases, though its structure is strongly affected by the external magnetic field. Lastly, comparison and consistency between the simulated ENA flux and the circularity of the ribbon as measured by IBEX are most consistent with a LISM magnetic field strength aligned with the center of the ribbon and a magnitude in the range 2.5-3 μG.« less

CIRCULARITY OF THE INTERSTELLAR BOUNDARY EXPLORER RIBBON OF ENHANCED ENERGETIC NEUTRAL ATOM (ENA) FLUX

DOE Office of Scientific and Technical Information (OSTI.GOV)

Funsten, H. O.; Higdon, D. M.; Larsen, B. A.

2013-10-10

As a sharp feature in the sky, the ribbon of enhanced energetic neutral atom (ENA) flux observed by the Interstellar Boundary Explorer (IBEX) mission is a key signature for understanding the interaction of the heliosphere and the interstellar medium through which we are moving. Over five nominal IBEX energy passbands (0.7, 1.1, 1.7, 2.7, and 4.3 keV), the ribbon is extraordinarily circular, with a peak location centered at ecliptic (λ{sub RC}, β{sub RC}) = (219.°2 ± 1.°3, 39.°9 ± 2.°3) and a half cone angle of φ{sub C} = 74.°5 ± 2.°0. A slight elongation of the ribbon, generally perpendicularmore » to the ribbon center-heliospheric nose vector and with eccentricity ∼0.3, is observed over all energies. At 4.3 keV, the ribbon is slightly larger and displaced relative to lower energies. For all ENA energies, a slice of the ribbon flux peak perpendicular to the circular arc is asymmetric and systematically skewed toward the ribbon center. We derive a spatial coherence parameter δ{sub C} ≤ 0.014 that characterizes the spatial uniformity of the ribbon over its extent in the sky and is a key constraint for understanding the underlying processes and structure governing the ribbon ENA emission.« less

The relationship between structure and magnetic properties in ultra-fine grained/nanostructured FePd alloys

NASA Astrophysics Data System (ADS)

Okumura, Hideyuki

In this study, the magnetic behavior including coercivity and the magnetic phase transition (ferromagnetic ↔ paramagnetic) and related phenomena were qualitatively and quantitatively investigated in ultra-fine grained/nanostructured FePd permanent magnet alloys, in relation to the microstructure and defect structure, and the results were compared with bulk FePd. Most of the alloy specimens investigated were in the form of epoxybonded magnets or isostatically-pressed pellets, formed from powders which were produced with high energy ball milling. Some results of thin films and ribbons produced with sputtering and melt-spinning, respectively, are also included in this thesis. Characterization of the materials was performed by using X-ray diffraction techniques with texture measurement, transmission electron microscopy with Lorentz microscopy, scanning electron microscopy with EDS analysis, optical microscopy and vibrating sample magnetometry. X-ray line broadening analysis was utilized for the quantitative characterization of the nanoscale microstructure, and it was found that the Cauchy-Gaussian profile assumption best describes the broadening data. Enhanced coercivities ˜10 times those of the bulk FePd obtained using conventional heat treatments were explained as the result of statistical (stochastic) unpinning of interaction domain walls out of the potential well at the grain boundary, and there is also an additional effect ascribed to an increase of the magnetocrystalline anisotropy, which is mainly due to the metastable c/a ratio of the nanostructured ordered phase and possibly to stress anisotropy. At the same time, there is also a decrease of the coercivity for smaller grain sizes because of the "magnetically soft" grain boundary phase. A semi-quantitative theoretical model is proposed, which includes the effect of exchange coupling between the ordered grains. The so-called Kronmuller analysis based on the wall pinning model was self-consistent, supporting the notion that wall pinning by grain boundary is the dominant mechanism controlling the coercivity in the nanostructured aggregates in which the magnetic structure is comprised of interaction domains. Furthermore, conventionally structure-insensitive, intrinsic properties such as the saturation magnetization and Curie temperature were found to become structure-sensitive in these materials. The results were semi-quantitatively explained by consideration of the extraordinary microstructure and defect structure involving the high and complex strain fields, metastable tetragonalities, nonequilibrium grain boundaries, extremely high surface-to-volume ratios and perturbed coordination spheres. The possible change in the atomic bond character particularly around grain boundaries is also briefly discussed. It seems that there is a significant fluctuation in exchange couplings at the grain boundary volume, causing the variation of the saturation magnetization, while for the variation of the Curie temperature the powder surface instead of the grain boundary is more important. A modified localized moment model and thus Hund's rules seem applicable to the FePd alloy systems, and the spin density fluctuations seem small in the FePd alloys.

Method for anisotropic etching in the manufacture of semiconductor devices

NASA Technical Reports Server (NTRS)

Koontz, Steven L. (Inventor); Cross, Jon B. (Inventor)

1993-01-01

Hydrocarbon polymer coatings used in microelectronic manufacturing processes are anisotropically etched by hyperthermal atomic oxygen beams (translational energies of 0.2 to 20 eV, preferably 1 to 10 eV). Etching with hyperthermal oxygen atom species obtains highly anisotropic etching with sharp boundaries between etched and mask protected areas.

Method for anisotropic etching in the manufacture of semiconductor devices

DOEpatents

Koontz, Steven L.; Cross, Jon B.

1993-01-01

Hydrocarbon polymer coatings used in microelectronic manufacturing processes are anisotropically etched by atomic oxygen beams (translational energies of 0.2-20 eV, preferably 1-10 eV). Etching with hyperthermal (kinetic energy>1 eV) oxygen atom species obtains highly anisotropic etching with sharp boundaries between etched and mask-protected areas.

Quantum parameter estimation in the Unruh–DeWitt detector model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hao, Xiang, E-mail: xhao@phas.ubc.ca; Pacific Institute of Theoretical Physics, Department of Physics and Astronomy, University of British Columbia, 6224 Agriculture Rd., Vancouver B.C., Canada V6T 1Z1; Wu, Yinzhong

2016-09-15

Relativistic effects on the precision of quantum metrology for particle detectors, such as two-level atoms are studied. The quantum Fisher information is used to estimate the phase sensitivity of atoms in non-inertial motions or in gravitational fields. The Unruh–DeWitt model is applicable to the investigation of the dynamics of a uniformly accelerated atom weakly coupled to a massless scalar vacuum field. When a measuring device is in the same relativistic motion as the atom, the dynamical behavior of quantum Fisher information as a function of Rindler proper time is obtained. It is found out that monotonic decrease in phase sensitivitymore » is characteristic of dynamics of relativistic quantum estimation. The origin of the decay of quantum Fisher information is the thermal bath that the accelerated detector finds itself in due to the Unruh effect. To improve relativistic quantum metrology, we reasonably take into account two reflecting plane boundaries perpendicular to each other. The presence of the reflecting boundary can shield the detector from the thermal bath in some sense.« less

A Statistical Approach for the Concurrent Coupling of Molecular Dynamics and Finite Element Methods

NASA Technical Reports Server (NTRS)

Saether, E.; Yamakov, V.; Glaessgen, E.

2007-01-01

Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental processes of material behavior at the atomistic level. However, increasing the size of the MD domain quickly presents intractable computational demands. A robust approach to surmount this computational limitation has been to unite continuum modeling procedures such as the finite element method (FEM) with MD analyses thereby reducing the region of atomic scale refinement. The challenging problem is to seamlessly connect the two inherently different simulation techniques at their interface. In the present work, a new approach to MD-FEM coupling is developed based on a restatement of the typical boundary value problem used to define a coupled domain. The method uses statistical averaging of the atomistic MD domain to provide displacement interface boundary conditions to the surrounding continuum FEM region, which, in return, generates interface reaction forces applied as piecewise constant traction boundary conditions to the MD domain. The two systems are computationally disconnected and communicate only through a continuous update of their boundary conditions. With the use of statistical averages of the atomistic quantities to couple the two computational schemes, the developed approach is referred to as an embedded statistical coupling method (ESCM) as opposed to a direct coupling method where interface atoms and FEM nodes are individually related. The methodology is inherently applicable to three-dimensional domains, avoids discretization of the continuum model down to atomic scales, and permits arbitrary temperatures to be applied.

Low Ozone in the Marine Boundary Layer of the Tropical Pacific Ocean

NASA Technical Reports Server (NTRS)

Singh, Hanwant B.; Gregory, G. L.; Andesrson, B.; Browell, E.; Sachse, G. W.; Davis, D. D.; Crawford, J.; Bradshaw, J. D.; Talbot, R.; Blake, D. R.;

Melting of Fe and Fe0.9Ni0.1 alloy at high pressures

NASA Astrophysics Data System (ADS)

Zhang, D.; Jackson, J. M.; Zhao, J.; Sturhahn, W.; Alp, E. E.; Hu, M. Y.; Toellner, T.

2014-12-01

Cosmochemical studies suggest that the cores of terrestrial planets are primarily composed of Fe alloyed with about 5 to 10 wt% Ni, plus some light elements (e.g., McDonough and Sun 1995). Thus, the high pressure melting curve of Fe0.9Ni0.1 is considered to be an important reference for characterizing the cores of terrestrial planets. We have determined the melting points of fcc-structured Fe and Fe0.9Ni0.1 up to 86 GPa using an in-situ method that monitors the atomic dynamics of the Fe atoms in the sample, synchrotron Mössbauer spectroscopy (Jackson et al. 2013). A laser heated diamond anvil cell is used to provide the high pressure-high temperature environmental conditions, and in-situ X-ray diffraction is used to constrain the pressure of the sample. To eliminate the influence of temperature fluctuations experienced by the sample on the determination of melting, we develop a Fast Temperature Readout (FasTeR) spectrometer. The FasTeR spectrometer features a fast reading rate (>100 Hz), a high sensitivity, a large dynamic range and a well-constrained focus. By combining the melting curve of fcc-structured Fe0.9Ni0.1 alloy determined in our study and the fcc-hcp phase boundary from Komabayashi et al. (2012), we calculate the fcc-hcp-liquid triple point of Fe0.9Ni0.1. Using this triple point and the thermophysical parameters from a nuclear resonant inelastic X-ray scattering study on hcp-Fe (Murphy et al. 2011), we compute the melting curve of hcp-structured Fe0.9Ni0.1. We will discuss our new experimental results with implications for the cores of Venus, Earth and Mars. Select references: McDonough & Sun (1995): The composition of the Earth. Chem. Geol. 120, 223-253. Jackson et al. (2013): Melting of compressed iron by monitoring atomic dynamics, EPSL, 362, 143-150. Komabayashi et al. (2012): In situ X-ray diffraction measurements of the fcc-hcp phase transition boundary of an Fe-Ni alloy in an internally heated diamond anvil cell, PCM, 39, 329-338. Murphy et al. (2011): Melting and thermal pressure of hcp-Fe from the phonon density of states, PEPI, 188, 114-120.

Surface structure and structural point defects of liquid and amorphous aluminosilicate nanoparticles.

PubMed

Linh, Nguyen Ngoc; Hoang, Vo Van

2008-07-02

The surface structure of liquid and amorphous aluminosilicate nanoparticles of composition Al(2)O(3)·2SiO(2) has been investigated in a model of different sizes ranging from 2.0 to 5.0 nm with the Born-Mayer type pair potential under non-periodic boundary conditions. Models have been obtained by cooling from the melts at a constant density of 2.6 g cm(-3) via molecular dynamics (MD) simulation. The surface structure has been investigated via the coordination number, bond-angle distributions and structural point defects. Calculations show that surface effects on surface static and thermodynamic properties of models are significant according to the change in the number of Al atoms in the surface layers. Evolution of the local environment of oxygen in the surface shell of nanoparticles upon cooling from the melt toward the glassy state was also found and discussed. In addition, the nanosize dependence of the glass transition temperature was presented.

Surface structure and structural point defects of liquid and amorphous aluminosilicate nanoparticles

NASA Astrophysics Data System (ADS)

Linh, Nguyen Ngoc; Van Hoang, Vo

2008-07-01

The surface structure of liquid and amorphous aluminosilicate nanoparticles of composition Al2O3·2SiO2 has been investigated in a model of different sizes ranging from 2.0 to 5.0 nm with the Born-Mayer type pair potential under non-periodic boundary conditions. Models have been obtained by cooling from the melts at a constant density of 2.6 g cm-3 via molecular dynamics (MD) simulation. The surface structure has been investigated via the coordination number, bond-angle distributions and structural point defects. Calculations show that surface effects on surface static and thermodynamic properties of models are significant according to the change in the number of Al atoms in the surface layers. Evolution of the local environment of oxygen in the surface shell of nanoparticles upon cooling from the melt toward the glassy state was also found and discussed. In addition, the nanosize dependence of the glass transition temperature was presented.

Detecting Hydrogen Chloride (HCl) in the Polluted Marine Boundary Layer Using Cavity Ring-Down Spectroscopy (CRDS)

NASA Astrophysics Data System (ADS)

Furlani, T.; Dawe, K.; VandenBoer, T. C.; Young, C.

2017-12-01

Oxidation initiated with chlorine atoms yields more ozone than oxidation initiated with hydroxyl radicals. Reasons for this are not fully understood, but the implications for mechanisms of oxidation chemistry are significant.1,2 Chlorine atoms have not been directly measured to date in the atmosphere and its abundance is usually inferred through steady-state approximations from all known formation and loss processes. A major reservoir for chlorine in the troposphere is by proton abstraction of organic compounds to form HCl.3 HCl can also be formed heterogeneously via acid displacement reactions with ubiquitously-found sodium chloride (NaCl) on solid surfaces with nitric acid (HNO3). The majority of the available chloride in the marine boundary layer comes from the sea salt in and around marine derived sea-spray aerosols. HCl is not a perfect sink and can react with hydroxyl radicals or be photolyzed to form chlorine atoms. The balance between loss and formation processes of chlorine atoms from HCl is highly dependent on many external factors, such as the wet and dry deposition rate of HCl. Measuring HCl in the gas and aerosol phase is important to the understanding of chlorine chemistry in the polluted marine boundary layer. HCl levels in the polluted marine boundary layer are typically between 100pptv-1ppbv,3 requiring the sensitive and selective detection capabilities of cavity ring-down spectroscopy (CRDS).4 We measured HCl using a Picarro CRDS in the polluted marine boundary layer for the first time. Measurements were conducted during April and May of 2017 in St. John's, Newfoundland and Labrador. The performance of the instrument will be discussed, as well as observations of HCl in the context of local conditions. References1Osthoff, H. D. et al. Nat. Geosci 1, 324-328 (2008). 2Young, C. J. et al. Atmos. Chem. Phys. 14, 3427-3440 (2014). 3Crisp, T. a et al. J. Geophys. Res. Atmos. 6897-6915 (2014). 4Hagen, C. L. et al. Atmos. Meas. Tech. 7, 345-357 (2014).

The atomic structure and polarization of strained SrTiO3/Si

NASA Astrophysics Data System (ADS)

Kumah, D. P.; Reiner, J. W.; Segal, Y.; Kolpak, A. M.; Zhang, Z.; Su, D.; Zhu, Y.; Sawicki, M. S.; Broadbridge, C. C.; Ahn, C. H.; Walker, F. J.

2010-12-01

For thin film devices based on coupling ferroelectric polarization to charge carriers in semiconductors, the role of the interface is critical. To elucidate this role, we use synchrotron x-ray diffraction to determine the interface structure of epitaxial SrTiO3 grown on the (001) surface of Si. The average displacement of the O octahedral sublattice relative to the Sr sublattice determines the film polarization and is measured to be about 0.05 nm toward the Si, with Ti off-center displacements 0.009 nm away from the substrate. Measurements of films with different boundary conditions on the top of the SrTiO3 show that the polarization at the SrTiO3/Si interface is dominated by oxide-Si chemical interactions.

Structural dynamics of the lac repressor-DNA complex revealed by a multiscale simulation.

PubMed

Villa, Elizabeth; Balaeff, Alexander; Schulten, Klaus

2005-05-10

A multiscale simulation of a complex between the lac repressor protein (LacI) and a 107-bp-long DNA segment is reported. The complex between the repressor and two operator DNA segments is described by all-atom molecular dynamics; the size of the simulated system comprises either 226,000 or 314,000 atoms. The DNA loop connecting the operators is modeled as a continuous elastic ribbon, described mathematically by the nonlinear Kirchhoff differential equations with boundary conditions obtained from the coordinates of the terminal base pairs of each operator. The forces stemming from the looped DNA are included in the molecular dynamics simulations; the loop structure and the forces are continuously recomputed because the protein motions during the simulations shift the operators and the presumed termini of the loop. The simulations reveal the structural dynamics of the LacI-DNA complex in unprecedented detail. The multiple domains of LacI exhibit remarkable structural stability during the simulation, moving much like rigid bodies. LacI is shown to absorb the strain from the looped DNA mainly through its mobile DNA-binding head groups. Even with large fluctuating forces applied, the head groups tilt strongly and keep their grip on the operator DNA, while the remainder of the protein retains its V-shaped structure. A simulated opening of the cleft of LacI by 500-pN forces revealed the interactions responsible for locking LacI in the V-conformation.

Creep deformation of grain boundary in a highly crystalline SiC fibre.

PubMed

Shibayama, Tamaki; Yoshida, Yutaka; Yano, Yasuhide; Takahashi, Heishichiro

2003-01-01

Silicon carbide (SiC) matrix composites reinforced by SiC fibres (SiC/SiC composites) are currently being considered as alternative materials in high Ni alloys for high-temperature applications, such as aerospace components, gas-turbine energy-conversion systems and nuclear fusion reactors, because of their high specific strength and fracture toughness at elevated temperatures compared with monolithic SiC ceramics. It is important to evaluate the creep properties of SiC fibres under tensile loading in order to determine their usefulness as structural components. However, it would be hard to evaluate creep properties by monoaxial tensile properties when we have little knowledge on the microstructure of crept specimens, especially at the grain boundary. Recently, a simple fibre bend stress relaxation (BSR) test was introduced by Morscher and DiCarlo to address this problem. Interpretation of the fracture mechanism at the grain boundary is also essential to allow improvement of the mechanical properties. In this paper, effects of stress applied by BSR test on microstructural evolution in advanced SiC fibres, such as Tyranno-SA including small amounts of Al, are described and discussed along with the results of microstructure analysis on an atomic scale by using advanced microscopy.

Effect of stress nonhomogeneity on the shear melting of a thin boundary lubrication layer.

PubMed

Lyashenko, Iakov A; Filippov, Alexander E; Popov, Mikhail; Popov, Valentin L

2016-11-01

We consider the dynamical properties of boundary lubrication in contact between two atomically smooth solid surfaces separated by an ultrathin layer of lubricant. In contrast to previous works on this topic, we explicitly consider the heterogeneity of tangential stresses, which arises in a contact of elastic bodies that are moved tangentially relative to each other. To describe phase transitions between structural states of the lubricant we use an approach based on the field theory of phase transitions. It is assumed that the lubricant layer, when stressed, can undergo a shear-melting transition of first or second order. While solutions for the homogeneous system can be easily obtained analytically, the kinetics of the phase transitions in the spatially heterogeneous system can only be studied numerically. In our numerical experiments melting of the lubricant layer starts from the outer boundary of contact and propagates to its center. The melting wave is followed by a wave of solidification. This process repeats itself periodically, following the stick-slip pattern that is characteristic of such systems. Depending on the thermodynamic and kinetic parameters of the model, different modes of sliding with almost complete or only partial intermediate solidification are possible.

Intrinsic Compressive Stress in Polycrystalline Films is Localized at Edges of the Grain Boundaries.

PubMed

Vasco, Enrique; Polop, Celia

2017-12-22

The intrinsic compression that arises in polycrystalline thin films under high atomic mobility conditions has been attributed to the insertion or trapping of adatoms inside grain boundaries. This compression is a consequence of the stress field resulting from imperfections in the solid and causes the thermomechanical fatigue that is estimated to be responsible for 90% of mechanical failures in current devices. We directly measure the local distribution of residual intrinsic stress in polycrystalline thin films on nanometer scales, using a pioneering method based on atomic force microscopy. Our results demonstrate that, at odds with expectations, compression is not generated inside grain boundaries but at the edges of gaps where the boundaries intercept the surface. We describe a model wherein this compressive stress is caused by Mullins-type surface diffusion towards the boundaries, generating a kinetic surface profile different from the mechanical equilibrium profile by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The Srolovitz-type surface diffusion that results from the stress counters the Mullins-type diffusion and stabilizes the kinetic surface profile, giving rise to a steady compression regime. The proposed mechanism of competition between surface diffusions would explain the flux and time dependency of compressive stress in polycrystalline thin films.

Intrinsic Compressive Stress in Polycrystalline Films is Localized at Edges of the Grain Boundaries

NASA Astrophysics Data System (ADS)

Vasco, Enrique; Polop, Celia

2017-12-01

The intrinsic compression that arises in polycrystalline thin films under high atomic mobility conditions has been attributed to the insertion or trapping of adatoms inside grain boundaries. This compression is a consequence of the stress field resulting from imperfections in the solid and causes the thermomechanical fatigue that is estimated to be responsible for 90% of mechanical failures in current devices. We directly measure the local distribution of residual intrinsic stress in polycrystalline thin films on nanometer scales, using a pioneering method based on atomic force microscopy. Our results demonstrate that, at odds with expectations, compression is not generated inside grain boundaries but at the edges of gaps where the boundaries intercept the surface. We describe a model wherein this compressive stress is caused by Mullins-type surface diffusion towards the boundaries, generating a kinetic surface profile different from the mechanical equilibrium profile by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The Srolovitz-type surface diffusion that results from the stress counters the Mullins-type diffusion and stabilizes the kinetic surface profile, giving rise to a steady compression regime. The proposed mechanism of competition between surface diffusions would explain the flux and time dependency of compressive stress in polycrystalline thin films.

The Observation of the Structure of M23C6/ γ Coherent Interface in the 100Mn13 High Carbon High Manganese Steel

NASA Astrophysics Data System (ADS)

Xu, Zhenfeng; Ding, Zhimin; Liang, Bo

2018-03-01

The M23C6 carbides precipitate along the austenite grain boundary in the 100Mn13 high carbon high manganese steel after 1323 K (1050 °C) solution treatment and subsequent 748 K (475 °C) aging treatment. The grain boundary M23C6 carbides not only spread along the grain boundary and into the incoherent austenite grain, but also grow slowly into the coherent austenite grain. On the basis of the research with optical microscope, a further investigation for the M23C6/ γ coherent interface was carried out by transmission electron microscope (TEM). The results show that the grain boundary M23C6 carbides have orientation relationships with only one of the adjacent austenite grains in the same planes: (\\bar{1}1\\bar{1})_{{{M}_{ 2 3} {C}_{ 6} }} //(\\bar{1}1\\bar{1})_{γ } , (\\bar{1}11)_{{{M}_{ 2 3} {C}_{ 6} }} //(\\bar{1}11)_{γ } ,[ 1 10]_{{{M}_{ 2 3} {C}_{ 6} }} //[ 1 10]_{γ } . The flat M23C6/ γ coherent interface lies on the low indexed crystal planes {111}. Moreover, in M23C6/ γ coherent interface, there are embossments which stretch into the coherent austenite grain γ. Dislocations distribute in the embossments and coherent interface frontier. According to the experimental observation, the paper suggests that the embossments can promote the M23C6/ γ coherent interface move. Besides, the present work has analyzed chemical composition of experimental material and the crystal structures of austenite and M23C6, which indicates that the transformation can be completed through a little diffusion for C atoms and a simple variant for austenite unit cell.

Defect-Induced Hedgehog Polarization States in Multiferroics

NASA Astrophysics Data System (ADS)

Li, Linze; Cheng, Xiaoxing; Jokisaari, Jacob R.; Gao, Peng; Britson, Jason; Adamo, Carolina; Heikes, Colin; Schlom, Darrell G.; Chen, Long-Qing; Pan, Xiaoqing

2018-03-01

Continuous developments in nanotechnology require new approaches to materials synthesis that can produce novel functional structures. Here, we show that nanoscale defects, such as nonstoichiometric nanoregions (NSNRs), can act as nano-building blocks for creating complex electrical polarization structures in the prototypical multiferroic BiFeO3 . An array of charged NSNRs are produced in BiFeO3 thin films by tuning the substrate temperature during film growth. Atomic-scale scanning transmission electron microscopy imaging reveals exotic polarization rotation patterns around these NSNRs. These polarization patterns resemble hedgehog or vortex topologies and can cause local changes in lattice symmetries leading to mixed-phase structures resembling the morphotropic phase boundary with high piezoelectricity. Phase-field simulations indicate that the observed polarization configurations are mainly induced by charged states at the NSNRs. Engineering defects thus may provide a new route for developing ferroelectric- or multiferroic-based nanodevices.

Deconstructing Temperature Gradients across Fluid Interfaces: The Structural Origin of the Thermal Resistance of Liquid-Vapor Interfaces

NASA Astrophysics Data System (ADS)

Muscatello, Jordan; Chacón, Enrique; Tarazona, Pedro; Bresme, Fernando

2017-07-01

The interfacial thermal resistance determines condensation-evaporation processes and thermal transport across material-fluid interfaces. Despite its importance in transport processes, the interfacial structure responsible for the thermal resistance is still unknown. By combining nonequilibrium molecular dynamics simulations and interfacial analyses that remove the interfacial thermal fluctuations we show that the thermal resistance of liquid-vapor interfaces is connected to a low density fluid layer that is adsorbed at the liquid surface. This thermal resistance layer (TRL) defines the boundary where the thermal transport mechanism changes from that of gases (ballistic) to that characteristic of dense liquids, dominated by frequent particle collisions involving very short mean free paths. We show that the thermal conductance is proportional to the number of atoms adsorbed in the TRL, and hence we explain the structural origin of the thermal resistance in liquid-vapor interfaces.

Defect-Induced Hedgehog Polarization States in Multiferroics.

PubMed

Li, Linze; Cheng, Xiaoxing; Jokisaari, Jacob R; Gao, Peng; Britson, Jason; Adamo, Carolina; Heikes, Colin; Schlom, Darrell G; Chen, Long-Qing; Pan, Xiaoqing

2018-03-30

Continuous developments in nanotechnology require new approaches to materials synthesis that can produce novel functional structures. Here, we show that nanoscale defects, such as nonstoichiometric nanoregions (NSNRs), can act as nano-building blocks for creating complex electrical polarization structures in the prototypical multiferroic BiFeO_{3}. An array of charged NSNRs are produced in BiFeO_{3} thin films by tuning the substrate temperature during film growth. Atomic-scale scanning transmission electron microscopy imaging reveals exotic polarization rotation patterns around these NSNRs. These polarization patterns resemble hedgehog or vortex topologies and can cause local changes in lattice symmetries leading to mixed-phase structures resembling the morphotropic phase boundary with high piezoelectricity. Phase-field simulations indicate that the observed polarization configurations are mainly induced by charged states at the NSNRs. Engineering defects thus may provide a new route for developing ferroelectric- or multiferroic-based nanodevices.

Inner-shell chemistry under high pressure

NASA Astrophysics Data System (ADS)

Miao, Maosheng; Botana, Jorge; Pravica, Michael; Sneed, Daniel; Park, Changyong

2017-05-01

Chemistry at ambient conditions has implicit boundaries rooted in the atomic shell structure: the inner-shell electrons and the unoccupied outer-shell orbitals do not contribute as the major component to chemical reactions and in chemical bonds. These general rules govern our understanding of chemical structures and reactions. We review the recent progresses in high-pressure chemistry demonstrating that the above rules can be violated under extreme conditions. Using a first principles computation method and crystal structure search algorithm, we demonstrate that stable compounds involving inner shell electrons such as CsF3, CsF5, HgF3, and HgF4 can form under high external pressure and may present exotic properties. We also discuss experimental studies that have sought to confirm these predictions. Employing our recently developed hard X-ray photochemistry methods in a diamond anvil cell, we show promising early results toward realizing inner shell chemistry experimentally.

A model for including thermal conduction in molecular dynamics simulations

NASA Technical Reports Server (NTRS)

Wu, Yue; Friauf, Robert J.

1989-01-01

A technique is introduced for including thermal conduction in molecular dynamics simulations for solids. A model is developed to allow energy flow between the computational cell and the bulk of the solid when periodic boundary conditions cannot be used. Thermal conduction is achieved by scaling the velocities of atoms in a transitional boundary layer. The scaling factor is obtained from the thermal diffusivity, and the results show good agreement with the solution for a continuous medium at long times. The effects of different temperature and size of the system, and of variations in strength parameter, atomic mass, and thermal diffusivity were investigated. In all cases, no significant change in simulation results has been found.

The Structure of the Heliosphere as Seen from In Situ and Remote Observations

NASA Astrophysics Data System (ADS)

Pogorelov, N. V.; Heerikhuisen, J.; Kim, T. K.; Zhang, M.

2017-12-01

The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW-LISM interaction. By performing three-dimensional, MHD plasma / kinetic neutral atom simulations, we describe the structure of the outer heliosheath (OHS) - the LISM plasma region affected by the presence of the heliosphere - and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. Numerical results are presented that reproduce shocks that pass by Voyager 1 in the OHS. We demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The effect of pickup ions is discussed in the context of in situ measurements. We also show that multi-TeV cosmic ray anisotropy can serve as an imager of the heliosphere due to its effect on the LISM properties. In particular, both the bow wave and the heliotail reveal themselves as characteristic features in the TeV flux images. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by IBEX. In particular, the position of the IBEX ribbon on the celestial sphere is strongly dependent on the choice of the plane that contains the vectors of velocity and magnetic field in the unperturbed LISM.

Monte Carlo Simulation Study of Atomic Structure of alnico Permanent Magnets

NASA Astrophysics Data System (ADS)

Nguyen, Manh Cuong; Wang, Cai-Zhuang; Ho, Kai-Ming

Lattice Monte Carlo simulation based on quinternary cluster expansion energy model is used to investigate nano-scale structure of alnico alloy, which is considered as a candidate material for rare-earth free high performance permanent magnets, especially for high or elevated temperature applications such as electric motor for vehicles. We observe phase decomposition of the master alnico alloy into FeCo-rich magnetic (α1) and NiAl-rich matrix (α2) phases. Concentrations of Fe and Co in α1 phase and Ni and Al in α2 phase are higher for lower annealing temperature. Ti is residing mostly in the α2 phase. The phase boundary between α1 and α2 phases are quite sharp with only few atomic layers. The α1 phase is in B2 ordering with Fe and Al occupying the α-site and Ni and Co occupying the β-site. The α2 phase is in L21 ordering with Al occupying the 4a-site. The phase composition profile again annealing temperature suggests that lower annealing temperature would improve the magnetism of α2 and diminish the magnetism of α2 phase, hence improve shape anisotropy of α1 phase rods and that of alnico.

Efficient FEM simulation of static and free vibration behavior of single walled boron nitride nanotubes

NASA Astrophysics Data System (ADS)

Giannopoulos, Georgios I.; Kontoni, Denise-Penelope N.; Georgantzinos, Stylianos K.

2016-08-01

This paper describes the static and free vibration behavior of single walled boron nitride nanotubes using a structural mechanics based finite element method. First, depending on the type of nanotube under investigation, its three dimensional nanostructure is developed according to the well-known corresponding positions of boron and nitride atoms as well as boron nitride bonds. Then, appropriate point masses are assigned to the atomic positions of the developed space frame. Next, these point masses are suitably interconnected with two-noded, linear, spring-like, finite elements. In order to simulate effectively the interactions observed between boron and nitride atoms within the nanotube, appropriate potential energy functions are introduced for these finite elements. In this manner, various atomistic models for both armchair and zigzag nanotubes with different aspect ratios are numerically analyzed and their effective elastic modulus as well as their natural frequencies and corresponding mode shapes are obtained. Regarding the free vibration analysis, the computed results reveal bending, breathing and axial modes of vibration depending on the nanotube size and chirality as well as the applied boundary support conditions. The longitudinal stiffness of the boron nitride nanotubes is found also sensitive to their geometric characteristics.

Revealing Grain Boundary Sliding from Textures of a Deformed Nanocrystalline Pd–Au Alloy

PubMed Central

Skrotzki, Werner; Zhao, Yajun; Pukenas, Aurimas; Birringer, Rainer

2018-01-01

Employing a recent modeling scheme for grain boundary sliding [Zhao et al. Adv. Eng. Mater. 2017, doi:10.1002/adem.201700212], crystallographic textures were simulated for nanocrystalline fcc metals deformed in shear compression. It is shown that, as grain boundary sliding increases, the texture strength decreases while the signature of the texture type remains the same. Grain boundary sliding affects the texture components differently with respect to intensity and angular position. A comparison of a simulation and an experiment on a Pd–10 atom % Au alloy with a 15 nm grain size reveals that, at room temperature, the predominant deformation mode is grain boundary sliding contributing to strain by about 60%. PMID:29370130

Effects of oxide distributed in grain boundaries on microstructure stability of nanocrystalline metals

NASA Astrophysics Data System (ADS)

Zhou, Kai; Li, Hui; Biao Pang, Jin; Wang, Zhu

2013-06-01

Nanocrystalline copper and zinc prepared by high-pressure compaction method have been studied by positron lifetime spectroscopy associated with X-ray diffraction. For nanocrystalline Cu, mean grain sizes of the samples decrease after being annealed at 900 °C and increase during aging at 180 °C, revealing that the atoms exchange between the two regions. The positron lifetime results indicate that the vacancy clusters formed in the annealing process are unstable and decomposed at the aging time below 6 hours. In addition, the partially oxidized surfaces of the nanoparticles hinder the grain growth during the ageing at 180 °C, and the vacancy clusters inside the disorder regions which are related to Cu2O need longer aging time to decompose. In the case of nanocrystalline Zn, the open volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ1). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τav) during the annealing, which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. For both nanocrystalline copper and zinc, the oxides in grain boundaries enhance the thermal stability of the microstucture, in spite of their different crystal structures. This effect is very important for the nanocrystalline materials using as radiation resistant materials.

Grain wall boundaries in centimeter-scale continuous monolayer WS2 film grown by chemical vapor deposition.

PubMed

Jia, Zhiyan; Hu, Wentao; Xiang, Jianyong; Wen, Fusheng; Nie, Anmin; Mu, Congpu; Zhao, Zhisheng; Xu, Bo; Tian, Yongjun; Liu, Zhongyuan

2018-06-22

Centimeter-scale continuous monolayer WS 2 film with large tensile strain has been successfully grown on oxidized silicon substrate by chemical vapor deposition, in which monolayer grains can be more than 200 μm in size. Monolayer WS 2 grains are observed to merge together via not only traditional grain boundaries but also non-traditional ones, which are named as grain walls (GWs) due to their nanometer-scale widths. The GWs are revealed to consist of two or three layers. Though not a monolayer, the GWs exhibit significantly enhanced fluorescence and photoluminescence. This enhancement may be attributed to abundant structural defects such as stacking faults and partial dislocations in the GWs, which are clearly observable in atomically resolved high resolution transmission electron microscopy and scanning transmission electron microscopy images. Moreover, GW-based phototransistor is found to deliver higher photocurrent than that based on monolayer film. These features of GWs provide a clue to microstructure engineering of monolayer WS 2 for specific applications in (opto)electronics.

Disconnections kinks and competing modes in shear-coupled grain boundary migration

NASA Astrophysics Data System (ADS)

Combe, N.; Mompiou, F.; Legros, M.

2016-01-01

The response of small-grained metals to mechanical stress is investigated by a theoretical study of the elementary mechanisms occurring during the shear-coupled migration of grain boundaries (GB). Investigating a model Σ 17 (410 ) GB in a copper bicrystal, both <110 > and <100 > GB migration modes are studied focusing on both the structural and energetic characteristics. The minimum energy paths of these shear-coupled GB migrations are computed using the nudge elastic band method. For both modes, the GB migration occurs through the nucleation and motion of disconnections. However, the atomic mechanisms of both modes qualitatively differ: While the <110 > mode presents no metastable state, the <100 > mode shows multiple metastable states, some of them evidencing some kinks along the disconnection lines. Disconnection kinks nucleation and motion activation energies are evaluated. Besides, the activation energies of the <100 > mode are smaller than those of the <110 > one except for very high stresses. These results significantly improve our knowledge of the GB migration mechanisms and the conditions under which they occur.

Multiscale Characterization of Microstructure in Near-Surface Regions of a 16MnCr5 Gear Wheel After Cyclic Loading

NASA Astrophysics Data System (ADS)

Medghalchi, Setareh; Jamebozorgi, Vahid; Bala Krishnan, Arjun; Vincent, Smobin; Salomon, Steffen; Basir Parsa, Alireza; Pfetzing, Janine; Kostka, Aleksander; Li, Yujiao; Eggeler, Gunther; Li, Tong

2018-05-01

The dependence of the microstructure on the degree of deformation in near-surface regions of a 16MnCr5 gear wheel after 2.1 × 106 loading cycles has been investigated by x-ray diffraction analysis, transmission electron microscopy, and atom probe tomography. Retained austenite and large martensite plates, along with elongated lamella-like cementite, were present in a less deformed region. Comparatively, the heavily deformed region consisted of a nanocrystalline structure with carbon segregation up to 2 at.% at grain boundaries. Spheroid-shaped cementite, formed at the grain boundaries and triple junctions of the nanosized grains, was enriched with Cr and Mn but depleted with Si. Such partitioning of Cr, Mn, and Si was not observed in the elongated cementite formed in the less deformed zone. This implies that rolling contact loading induced severe plastic deformation as well as a pronounced annealing effect in the active contact region of the toothed gear during cyclic loading.

Grain wall boundaries in centimeter-scale continuous monolayer WS2 film grown by chemical vapor deposition

NASA Astrophysics Data System (ADS)

Jia, Zhiyan; Hu, Wentao; Xiang, Jianyong; Wen, Fusheng; Nie, Anmin; Mu, Congpu; Zhao, Zhisheng; Xu, Bo; Tian, Yongjun; Liu, Zhongyuan

2018-06-01

Centimeter-scale continuous monolayer WS2 film with large tensile strain has been successfully grown on oxidized silicon substrate by chemical vapor deposition, in which monolayer grains can be more than 200 μm in size. Monolayer WS2 grains are observed to merge together via not only traditional grain boundaries but also non-traditional ones, which are named as grain walls (GWs) due to their nanometer-scale widths. The GWs are revealed to consist of two or three layers. Though not a monolayer, the GWs exhibit significantly enhanced fluorescence and photoluminescence. This enhancement may be attributed to abundant structural defects such as stacking faults and partial dislocations in the GWs, which are clearly observable in atomically resolved high resolution transmission electron microscopy and scanning transmission electron microscopy images. Moreover, GW-based phototransistor is found to deliver higher photocurrent than that based on monolayer film. These features of GWs provide a clue to microstructure engineering of monolayer WS2 for specific applications in (opto)electronics.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Altay, A.; Carter, C.B., E-mail: cbcarter@engr.uconn.ed; Rulis, P.

Calcium aluminates, compounds in the CaO-Al{sub 2}O{sub 3} phase system, are used in high-temperature cements and refractory oxides and have wide range of potential technological applications due to their interesting optical, electrical, thermal, and mechanical properties. They are used in both crystalline and glassy form; the glass is an isotropic material while the crystalline materials may be highly anisotropic. This paper will consider two particular crystalline materials, CA{sub 2} and CA{sub 6}, but the results should be applicable to all calcium aluminates. Although CA{sub 2} and CA{sub 6} crystals contain the same chemical species, Ca, Al, and O, the coordinationmore » and local environments of these species are different in the two structures and hence they show very different energy-loss near-edge structures (ELNES) when examined by electron energy-loss spectroscopy (EELS) in the TEM. The data obtained using ELNES can effectively provide a fingerprint for each compound and a map for their electronic structure. Once such fingerprints are obtained, they can be used to identify nano-sized particles/grains or material at interfaces and grain boundaries. In the present study, the local symmetry fingerprints for CA{sub 2} and CA{sub 6} structures are reported combining experimental spectra with electronic-structure calculations that allow the different features in the spectra to be interpreted. Al-L{sub 2,3} and O-K edge core-loss spectra from CA{sub 2} and CA{sub 6} were measured experimentally using electron energy-loss spectroscopy in a monochromated scanning transmission electron microscope. The near-edge structures were calculated for the different phases using the orthogonalized linear combination of atomic-orbitals method, and took account of core-hole interactions. It is shown that CA{sub 2} and CA{sub 6} structures exhibit distinctive experimental ELNES fingerprints so that these two phases can be separately identified even when present in small volumes. - Graphical abstract: Projection of the CA{sub 2} structure viewed in the [010] direction. Green atoms represent the Ca{sup 2+} ions and blue atoms represent the Al{sup 3+} ions. Two unit cells are outlined..« less

Quantitative Scanning Transmission Electron Microscopy of Electronic and Nanostructured Materials

NASA Astrophysics Data System (ADS)

Yankovich, Andrew B.

Electronic and nanostructured materials have been investigated using advanced scanning transmission electron microscopy (STEM) techniques. The first topic is the microstructure of Ga and Sb-doped ZnO. Ga-doped ZnO is a candidate transparent conducting oxide material. The microstructure of GZO thin films grown by MBE under different growth conditions and different substrates were examined using various electron microscopy (EM) techniques. The microstructure, prevalent defects, and polarity in these films strongly depend on the growth conditions and substrate. Sb-doped ZnO nanowires have been shown to be the first route to stable p-type ZnO. Using Z-contrast STEM, I have showed that an unusual microstructure of Sb-decorated head-to-head inversion domain boundaries and internal voids contain all the Sb in the nanowires and cause the p-type conduction. InGaN thin films and InGaN / GaN quantum wells (QW) for light emitting diodes are the second topic. Low-dose Z-contrast STEM, PACBED, and EDS on InGaN QW LED structures grown by MOCVD show no evidence for nanoscale composition variations, contradicting previous reports. In addition, a new extended defect in GaN and InGaN was discovered. The defect consists of a faceted pyramid-shaped void that produces a threading dislocation along the [0001] growth direction, and is likely caused by carbon contamination during growth. Non-rigid registration (NRR) and high-precision STEM of nanoparticles is the final topic. NRR is a new image processing technique that corrects distortions arising from the serial nature of STEM acquisition that previously limited the precision of locating atomic columns and counting the number of atoms in images. NRR was used to demonstrate sub-picometer precision in STEM images of single crystal Si and GaN, the best achieved in EM. NRR was used to measure the atomic surface structure of Pt nanoacatalysts and Au nanoparticles, which revealed new bond length variation phenomenon of surface atoms. In addition, NRR allowed for measuring the 3D atomic structure of the nanoparticles with less than 1 atom uncertainty, a long-standing problem in EM. Finally, NRR was adapted to EDS spectrum images, significantly enhancing the signal to noise ratio and resolution of an EDS spectrum image of Ca-doped NdTiO3 compared to conventional methods.

Flow Visualization in Supersonic Turbulent Boundary Layers.

NASA Astrophysics Data System (ADS)

Smith, Michael Wayne

This thesis is a collection of novel flow visualizations of two different flat-plate, zero pressure gradient, supersonic, turbulent boundary layers (M = 2.8, Re _theta ~ 82,000, and M = 2.5, Re_ theta ~ 25,000, respectively). The physics of supersonic shear flows has recently drawn increasing attention with the renewed interest in flight at super and hypersonic speeds. This work was driven by the belief that the study of organized, Reynolds -stress producing turbulence structures will lead to improved techniques for the modelling and control of high-speed boundary layers. Although flow-visualization is often thought of as a tool for providing qualitative information about complex flow fields, in this thesis an emphasis is placed on deriving quantitative results from image data whenever possible. Three visualization techniques were applied--'selective cut-off' schlieren, droplet seeding, and Rayleigh scattering. Two experiments employed 'selective cut-off' schlieren. In the first, high-speed movies (40,000 fps) were made of strong density gradient fronts leaning downstream at between 30^circ and 60^ circ and travelling at about 0.9U _infty. In the second experiment, the same fronts were detected with hot-wires and imaged in real time, thus allowing the examination of the density gradient fronts and their associated single-point mass -flux signals. Two experiments employed droplet seeding. In both experiments, the boundary layer was seeded by injecting a stream of acetone through a single point in the wall. The acetone is atomized by the high shear at the wall into a 'fog' of tiny (~3.5mu m) droplets. In the first droplet experiment, the fog was illuminated with copper-vapor laser sheets of various orientations. The copper vapor laser pulses 'froze' the fog motion, revealing a variety of organized turbulence structures, some with characteristic downstream inclinations, others with large-scale roll-up on the scale of delta. In the second droplet experiment, high-speed movies were made of the fog under general illumination, thus providing information about the streamwise evolution of the structures seen in the planar stills. Rayleigh scattering from a laser sheet was used to create instantaneous density cross-sections in the M = 2.5 boundary layer. The Rayleigh scattering experiment represents the first measurement of the instantaneous 2-D field of an intrinsic fluid property in any boundary layer. Imaged by an intensified UV camera, scattering from the Argon-Fluoride laser (193 nm) revealed density structures with sharp interfaces between high and low-density fluid. These pictures were also used to generated quantitative turbulence information. Density pdf profiles, intermittency values, density correlations, and structure shape data were derived with standard digital image-processing techniques.

Mechanics of low-dimensional carbon nanostructures: Atomistic, continuum, and multi-scale approaches

NASA Astrophysics Data System (ADS)

Mahdavi, Arash

A new multiscale modeling technique called the Consistent Atomic-scale Finite Element (CAFE) method is introduced. Unlike traditional approaches for linking the atomic structure to its equivalent continuum, this method directly connects the atomic degrees of freedom to a reduced set of finite element degrees of freedom without passing through an intermediate homogenized continuum. As a result, there is no need to introduce stress and strain measures at the atomic level. The Tersoff-Brenner interatomic potential is used to calculate the consistent tangent stiffness matrix of the structure. In this finite element formulation, all local and non-local interactions between carbon atoms are taken into account using overlapping finite elements. In addition, a consistent hierarchical finite element modeling technique is developed for adaptively coarsening and refining the mesh over different parts of the model. This process is consistent with the underlying atomic structure and, by refining the mesh to the scale of atomic spacing, molecular dynamic results can be recovered. This method is valid across the scales and can be used to concurrently model atomistic and continuum phenomena so, in contrast with most other multi-scale methods, there is no need to introduce artificial boundaries for coupling atomistic and continuum regions. Effect of the length scale of the nanostructure is also included in the model by building the hierarchy of elements from bottom up using a finite size atom cluster as the building block. To be consistent with the bravais multi-lattice structure of sp2-bonded carbon, two independent displacement fields are used for reducing the order of the model. Sparse structure of the stiffness matrix of these nanostructures is exploited to reduce the memory requirement and to speed up the formation of the system matrices and solution of the equilibrium equations. Applicability of the method is shown with several examples of the nonlinear mechanics of carbon nanotubes and carbon nanocones subject to different loadings and boundary conditions. This finite element technique is also used to study the natural frequencies of low-dimensional carbon nanostructures and comparing the results with those of a homogenized isotropic continuum shell. Conclusion is that, replacing the atomic lattice with an isotropic continuum shell for a graphene sheet does not significantly affect the vibration frequencies while in the case of carbon nanotubes and carbon nanocones there is a significant difference between the natural frequencies of the atomistic model and its continuum counterpart. In the case of the carbon nanotube, continuum model successfully captures the beam bending vibration modes while overestimating frequencies of the modes in which the cross-section undergoes significant deformation. Furthermore, in the case of carbon nanotubes, the continuum shell exhibits a torsional mode which appears to be an artifact resulting from the small nominal thickness typically used in the continuum shell approximation of these nanostructures. Results of this study indicate that isotropic continuum shell models, while simple and useful in static analysis, cannot accurately predict the vibration frequencies of these nanostructures. We have studied the bistable nature of single-walled carbon nanotubes by investigating the change in the tube's energy as it is compressed between flat rigid indenters of various widths. Assuming the nanotube deformed uniformly along its length and modeling the cross-section as an inextensible, non-linear beam we found that tubes with a radius greater than 12 A are bistable and that tubes with a radius greater than 25 A have a lower energy in the collapsed state than in the inflated state. The difference in energy between the collapsed and inflated states decreases nearly linearly with increasing tube radius. While the inflated state remains stable for tubes of all diameters, the energy barrier keeping the tube from collapsing approaches zero as the tube radius increases. We also demonstrate why collapse with a wide indenter may be difficult to observe in narrow tubes. A reduced-order model is developed for the dynamics of the carbon nanotube atomic force microscope probes. Bending behavior of the nanotube probe is modeled using Euler's elastica. A nonlinear moment-curvature relationship is implemeneted to account for the ovalization of the cross section of the nanotube during bending. Van der Waal forces acting between tube and the substrate is integrated over the surface of the tube and used as distributed follower forces acting on the equivalent elastica. Approximating the behavior of the nanotube with an elastica proved to be a very effiecient technique for modeling these nanostructures.

What We've Learned from IBEX ENA Imaging of the Heliosheath

NASA Astrophysics Data System (ADS)

Zirnstein, E.

2017-12-01

The Interstellar Boundary Explorer (IBEX) is an Earth-orbiting spacecraft equipped with two single-pixel cameras that detect neutral atoms produced by the interaction of the solar wind (SW) with the very local interstellar medium (VLISM), as well as neutral atoms flowing in from the VLISM itself. Since its launch in 2009, IBEX has provided us with over half a solar cycle of measurements of energetic neutral atom (ENA) fluxes produced in the heliosphere. In particular, a large number of these ENAs originate in the inner heliosheath, a region of nearly incompressible SW plasma that is slowed, compressed, and heated at the termination shock. As the heated SW plasma flows through the heliosheath and down the heliotail, charge-exchange with cold neutral atoms from the VLISM generates ENAs that travel on ballistic trajectories. Some of these ENAs travel to Earth and are detected by IBEX. IBEX all-sky observations of these ENAs show a unique energy and spatial dependence, revealing the structural and plasma properties of the heliosheath. One of the prime scientific objectives of the Interstellar Mapping and Acceleration Probe (IMAP) mission is to understand the SW-VLISM interaction, including the properties of the heliosheath and heliotail. This talk will review key IBEX observations of the inner heliosheath environment, the way they have shaped our understanding of the SW-VLISM interaction, and their implications for the upcoming IMAP mission.

Mechanical behavior enhancement of defective graphene sheet employing boron nitride coating via atomistic study

NASA Astrophysics Data System (ADS)

Setoodeh, A. R.; Badjian, H.

2017-12-01

The most stable form of boron nitride polymorph naming hexagonal boron nitride sheet has recently been widely concerned like graphite due to its interesting features such as electrical insulation and high thermal conductivity. In this study, the molecular dynamic simulations are implemented to investigate the mechanical properties of single-layer graphene sheets under tensile and compressive loadings in the absence and presence of boron-nitride coating layers. In this introduced hybrid nanostructure, the benefit of combining both individual interesting features of graphene and boron-nitride sheets such as exceptional mechanical and electrical properties can be simultaneously achieved for future potential application in nano devices. The influences of chiral indices, boundary conditions and presence of mono-atomic vacancy defects as well as coating dimension on the mechanical behavior of the resulted hybrid structure are reported. The interatomic forces between the atoms are modeled by employing the AIREBO and Tersoff-Brenner potentials for carbon-carbon and boron-nitrogen atoms in each layer, respectively. Furthermore, the van der Waal interlayer forces of carbon-boron and carbon-nitrogen are estimated by the Lennard-Jones potential field. Besides the potential improvement in electrical and physical properties of the nanostructure, it is demonstrated that the buckling load capacity of the fully coated graphene sheet with 3% concentration of mono-atomic vacancy defects noticeably enhances by amounts of 24.1%.

Atomic-Scale Mechanisms of Defect-Induced Retention Failure in Ferroelectrics.

PubMed

Li, Linze; Zhang, Yi; Xie, Lin; Jokisaari, Jacob R; Beekman, Christianne; Yang, Jan-Chi; Chu, Ying-Hao; Christen, Hans M; Pan, Xiaoqing

2017-06-14

The ability to switch the ferroelectric polarization using an electric field makes ferroelectrics attractive for application in nanodevices such as high-density memories. One of the major challenges impeding this application, however, has been known as "retention failure", which is a spontaneous process of polarization back-switching that can lead to data loss. This process is generally thought to be caused by the domain instability arising from interface boundary conditions and countered by defects, which can pin the domain wall and impede the back-switching. Here, using in situ transmission electron microscopy and atomic-scale scanning transmission electron microscopy, we show that the polarization retention failure can be induced by commonly observed nanoscale impurity defects in BiFeO 3 thin films. The interaction between polarization and the defects can also lead to the stabilization of novel functional nanodomains with mixed-phase structures and head-to-head polarization configurations. Thus, defect engineering provides a new route for tuning properties of ferroelectric nanosystems.

Phonon-induced topological transition to a type-II Weyl semimetal

DOE PAGES

Wang, Lin-Lin; Jo, Na Hyun; Wu, Yun; ...

2017-04-11

Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied here, as exemplified in NbNiTe 2, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For the NbNiTe 2 structure in space-group 53 (Pmna)$-$ having an inversion center arising from two glide planes and one mirror plane with a two-fold rotation and screw axis$-$a full gap opening exists between two band manifolds near the Fermi energy. Upon atomic displacements by optical phonons, the symmetry lowers to space-group 28 (Pma2), eliminating one glide plane along c, the associated rotation andmore » screw axis, and the inversion center. As a result, 20 Weyl points emerge, including four type-IIWeyl points in the Γ-X direction at the boundary between a pair of adjacent electron and hole bands. Thus, optical phonons may offer control of the transition to a Weyl fermion state.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Hyo-Seob; Ames Lab., Ames, IA; Dharmaiah, Peyala

(GeTe) x(AgSbTe 2) 100$-$x: TAGS thermoelectrics are an attractive class of materials due to their combination of non-toxicity and good conversion efficiency at mid-temperature ranges. Here in the present work, we have utilized energy and time efficient high-pressure gas atomization and spark-plasma sintering techniques for large-scale preparation of samples with varying composition (i.e., (GeTe) x(AgSbTe 2) 100$-$x where x = 75, 80, 85, and 90). High-temperature x-ray diffraction was used to understand the phase transformation mechanism of the as-atomized powders. Detailed high-resolution transmission electron microscopy of the sintered samples revealed the presence of nanoscale precipitates, antiphase, and twin boundaries. Themore » nanoscale twins and antiphase boundaries serve as phonon scattering centers, leading to the reduction of total thermal conductivity in TAGS-80 and 90 samples. The maximum ZT obtained was 1.56 at 623 K for TAGS-90, which was ~94% improvement compared to values previously reported. The presence of the twin boundaries also resulted in a high fracture toughness (K IC) of the TAGS-90 sample due to inhibition of dislocation movement at the twin boundary.« less

Autonomous Repair Mechanism of Creep Damage in Fe-Au and Fe-Au-B-N Alloys

NASA Astrophysics Data System (ADS)

Zhang, S.; Kwakernaak, C.; Tichelaar, F. D.; Sloof, W. G.; Kuzmina, M.; Herbig, M.; Raabe, D.; Brück, E.; van der Zwaag, S.; van Dijk, N. H.

2015-12-01

The autonomous repair mechanism of creep cavitation during high-temperature deformation has been investigated in Fe-Au and Fe-Au-B-N alloys. Combined electron-microscopy techniques and atom probe tomography reveal how the improved creep properties result from Au precipitation within the creep cavities, preferentially formed on grain boundaries oriented perpendicular to the applied stress. The selective precipitation of Au atoms at the free creep cavity surface results in pore filling, and thereby, autonomous repair of the creep damage. The large difference in atomic size between the Au and Fe strongly hampers the nucleation of precipitates in the matrix. As a result, the matrix acts as a reservoir for the supersaturated solute until damage occurs. Grain boundaries and dislocations are found to act as fast transport routes for solute gold from the matrix to the creep cavities. The mechanism responsible for the self-healing can be characterized by a simple model for cavity growth and cavity filling.

Solving Fluid Structure Interaction Problems with an Immersed Boundary Method

NASA Technical Reports Server (NTRS)

Barad, Michael F.; Brehm, Christoph; Kiris, Cetin C.

2016-01-01

An immersed boundary method for the compressible Navier-Stokes equations can be used for moving boundary problems as well as fully coupled fluid-structure interaction is presented. The underlying Cartesian immersed boundary method of the Launch Ascent and Vehicle Aerodynamics (LAVA) framework, based on the locally stabilized immersed boundary method previously presented by the authors, is extended to account for unsteady boundary motion and coupled to linear and geometrically nonlinear structural finite element solvers. The approach is validated for moving boundary problems with prescribed body motion and fully coupled fluid structure interaction problems. Keywords: Immersed Boundary Method, Higher-Order Finite Difference Method, Fluid Structure Interaction.

Mechanisms by which oxygen acts as a surfactant in giant magnetoresistance film growth

NASA Astrophysics Data System (ADS)

Larson, D. J.; Petford-Long, A. K.; Cerezo, A.; Bozeman, S. P.; Morrone, A.; Ma, Y. Q.; Georgalakis, A.; Clifton, P. H.

2003-04-01

The mechanisms by which oxygen acts as a surfactant in giant magnetoresistance multilayers have been elucidated for the first time. Three-dimensional atom probe analysis of Cu/CoFe multilayers reveals the elemental distributions at the atomic level. Interfacial intermixing and oxygen impurity levels have been quantified for the first time. Both with and without oxygen the intermixing is greater at the CoFe-on-Cu interface than at the Cu-on-CoFe one and for both interfaces, oxygen reduced the intermixing. The oxygen largely floats to the growing surface and is incorporated at grain boundaries. The oxygen also reduces conformal roughness and grain boundary grooving, indicating a reduction in long-range surface diffusion.

Synthesis of Al₂Ca Dispersoids by Powder Metallurgy Using a Mg-Al Alloy and CaO Particles.

PubMed

Fujita, Junji; Umeda, Junko; Kondoh, Katsuyoshi

2017-06-28

The elemental mixture of Mg-6 wt %Al-1 wt %Zn-0.3 wt %Mn (AZ61B) alloy powder and CaO particles was consolidated by an equal-channel angular bulk mechanical alloying (ECABMA) process to form a composite precursor. Subsequently, the precursor was subjected to a heat treatment to synthesize fine Al₂Ca particles via a solid-state reaction between the Mg-Al matrix and CaO additives. Scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS) and electron probe micro-analysis on the precursor indicated that 4.7-at % Al atoms formed a supersaturated solid solution in the α-Mg matrix. Transmission electron microscopy-EDS and X-ray diffraction analyses on the AZ61B composite precursor with 10-vol % CaO particles obtained by heat treatment confirmed that CaO additives were thermally decomposed in the Mg-Al alloy, and the solid-soluted Ca atoms diffused along the α-Mg grain boundaries. Al atoms also diffused to the grain boundaries because of attraction to the Ca atoms resulting from a strong reactivity between Al and Ca. As a result, needle-like (Mg,Al)₂Ca intermetallics were formed as intermediate precipitates in the initial reaction stage during the heat treatment. Finally, the precipitates were transformed into spherical Al₂Ca particles by the substitution of Al atoms for Mg atoms in (Mg,Al)₂Ca after a long heat treatment.

Investigation of olivine and orthopyroxene grain boundaries by atom probe tomography

NASA Astrophysics Data System (ADS)

Krawczynski, M.; Skemer, P. A.; Bachhav, M.; Dong, Y.; Marquis, E. A.

2016-12-01

Accurate chemical analysis at grain boundaries is challenging by traditional microscopic techniques, especially for poor conducting geological samples. Atom probe tomography (APT) is a unique technique that can elucidate chemistry and 3-D distribution of elements within a sample volume at the sub-nanometer length scale. With advances in laser and sample preparation techniques in the last decade, APT is now successfully applied to a wide range of poor conducting materials like metal oxides, ceramics, and biological minerals. In this study, we apply the APT technique to investigate the grain boundary chemistry of orthopyroxene (opx) and olivine. These minerals are the most abundant in the upper mantle and their grain boundaries may be important geochemical reservoirs in Earth. Moreover, physical properties such as grain boundary diffusivity, conductivity, and mobility, are likely influenced by the presence or absence of impurities. Single crystals of opx and olivine grains, separated from a San Carlos xenolith, were deformed at 1 GPa and 1500 K. Plastic deformation promoted dynamic recrystallization, creating new grain boundaries within a chemically homogeneous medium. Needle shaped specimens of opx-opx and olivine-olivine grain boundaries were prepared using standard lift out techniques and a dual beam focused ion beam (FIB). APT analyses were performed in laser mode with laser energy of 50 pJ/pulse, repetition rate of 200 kHz, and detection rate of 1%. A 3-D distribution of elements was reconstructed and 1-D profiles across the grain boundary have been calculated. Fe, Al, and Ca show enrichments at the grain boundaries for both phases, consistent with previous studies that used STEM/EDX or EPMA techniques. Although qualitatively similar, the spatial resolution of the APT method is significantly better than other methods, and our data show that the grain-boundary enrichment of minor elements in both olivine and pyroxene compositions is limited to a region no greater than 2-4 nm thick. These new data place constraints on the thickness of the grain boundary zone with unparalleled precision, allowing more accurate calculation of partition coefficients as well as diffusion coefficients from experimental studies of grain boundary diffusion.

Polarity-driven oxygen vacancy formation in ultrathin LaNiO 3 films on SrTiO 3

DOE PAGES

Tung, I-Cheng; Luo, Guangfu; Lee, June Hyuk; ...

2017-10-18

Oxide heterostructures offer a pathway to control emergent phases in complex oxides, but their creation often leads to boundaries that have a polar discontinuity. In order to fabricate atomic-scale arrangements of dissimilar materials, we need a clear understanding of the pathways by which materials resolve polarity issues. By examining the real-time lattice structure in-situ during growth for the case of polar LaNiO 3 synthesized on non-polar SrTiO 3 (001), we demonstrate how films in ultra-thin limit form as LaNiO 2.5 and then evolve into LaNiO 3 as the thickness increases. Theory explains how the polar energetics drives the formation ofmore » oxygen vacancies and the stability of these phases with thickness and structure.« less

A New Technique to Produce Clean and Thin Silicon Films In Situ in a UHV Electron Microscope for TEM-TED Studies of Surfaces

NASA Astrophysics Data System (ADS)

Ozawa, Soh-ichiro; Yamanaka, Akira; Kobayashi, Kunio; Tanishiro, Yasumasa; Yagi, Katsumichi

1990-04-01

A new technique of in situ oxygen gas reaction thinning of Si films at around 750-800°C in an ultrahigh-vacuum electron microscope was developed. The technique produced films as thin as 10 to 20 nm. Such a thin film allows us to observe surface atomic steps, out-of-phase boundaries and {1/7 0}, {1/7 1/7} and {2/7 0} spots from the Si(111)7× 7 surface. These spots were not observed in previous studies, having been masked by strong inelastic scattering. The technique is useful not only for detecting clear diffraction spots of kinematical intensity for surface structure analysis but also for observation of high-resolution plan-view structure images of clean and adsorbed surfaces.

Characterization of ordering in A-site deficient perovskite Ca 1–xLa 2x/3TiO 3 using STEM/EELS

DOE PAGES

Danaie, Mohsen; Kepaptsoglou, Demie; Ramasse, Quentin M.; ...

2016-09-15

The vacancy ordering behavior of an A-site deficient perovskite system, Ca 1–xLa 2x/3TiO 3, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), adopting Pbnm symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown tomore » be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La 3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La 3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. In conclusion, the occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems.« less

Theoretical study of the elasticity, mechanical behavior, electronic structure, interatomic bonding, and dielectric function of an intergranular glassy film model in prismatic β-Si3N4

NASA Astrophysics Data System (ADS)

Ching, W. Y.; Rulis, Paul; Ouyang, Lizhi; Aryal, Sitaram; Misra, Anil.

2010-06-01

Microstructures such as intergranular glassy films (IGFs) are ubiquitous in many structural ceramics. They control many of the important physical properties of polycrystalline ceramics and can be influenced during processing to modify the performance of devices that contain them. In recent years, there has been intense research, both experimentally and computationally, on the structure and properties of IGFs. Unlike grain boundaries or dislocations with well-defined crystalline planes, the atomic scale structure of IGFs, their fundamental electronic interactions, and their bonding characteristics are far more complicated and not well known. In this paper, we present the results of theoretical simulations using ab initio methods on an IGF model in β-Si3N4 with prismatic crystalline planes. The 907-atom model has a dimension of 14.533Å×15.225Å×47.420Å . The IGF layer is perpendicular to the z axis, 16.4Å wide, and contains 72 Si, 32 N, and 124 O atoms. Based on this model, the mechanical and elastic properties, the electronic structure, the interatomic bonding, the localization of defective states, the distribution of electrostatic potential, and the optical dielectric function are evaluated and compared with crystalline β-Si3N4 . We have also performed a theoretical tensile experiment on this model by incrementally extending the structure in the direction perpendicular to the IGF plane until the model fully separated. It is shown that fracture occurs at a strain of 9.42% with a maximum stress of 13.9 GPa. The fractured segments show plastic behavior and the formation of surfacial films on the β-Si3N4 . These results are very different from those of a previously studied basal plane model [J. Chen , Phys. Rev. Lett. 95, 256103 (2005)10.1103/PhysRevLett.95.256103] and add insights to the structure and behavior of IGFs in polycrystalline ceramics. The implications of these results and the need for further investigations are discussed.

First-principles study of the effect of phosphorus on nickel grain boundary

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Wenguan; Ren, Cuilan; Han, Han, E-mail: hanhan@sinap.ac.cn, E-mail: xuhongjie@sinap.ac.cn

2014-01-28

Based on first-principles quantum-mechanical calculations, the impurity-dopant effects of phosphorus on Σ5(012) symmetrical tilt grain boundary in nickel have been studied. The calculated binding energy suggests that phosphorus has a strong tendency to segregate to the grain boundary. Phosphorus forms strong and covalent-like bonding with nickel, which is beneficial to the grain boundary cohesion. However, a too high phosphorus content can result in a thin and fragile zone in the grain boundary, due to the repulsion between phosphorus atoms. As the concentration of phosphorus increases, the strength of the grain boundary increases first and then decreases. Obviously, there exists anmore » optimum concentration for phosphorus segregation, which is consistent with observed segregation behaviors of phosphorus in the grain boundary of nickel. This work is very helpful to understand the comprehensive effects of phosphorus.« less

Testing the limits of the Maxwell distribution of velocities for atoms flying nearly parallel to the walls of a thin cell.

PubMed

Todorov, Petko; Bloch, Daniel

2017-11-21

For a gas at thermal equilibrium, it is usually assumed that the velocity distribution follows an isotropic 3-dimensional Maxwell-Boltzmann (M-B) law. This assumption classically implies the assumption of a "cos θ" law for the flux of atoms leaving the surface. Actually, such a law has no grounds in surface physics, and experimental tests of this assumption have remained very few. In a variety of recently developed sub-Doppler laser spectroscopy techniques for gases one-dimensionally confined in a thin cell, the specific contribution of atoms moving nearly parallel to the boundary of the vapor container becomes essential. We report here on the implementation of an experiment to probe effectively the distribution of atomic velocities parallel to the windows for a thin (60 μm) Cs vapor cell. The principle of the setup relies on a spatially separated pump-probe experiment, where the variations of the signal amplitude with the pump-probe separation provide the information on the velocity distribution. The experiment is performed in a sapphire cell on the Cs resonance line, which benefits from a long-lived hyperfine optical pumping. Presently, we can analyze specifically the density of atoms with slow normal velocities ∼5-20 m/s, already corresponding to unusual grazing flight-at ∼85°-88.5° from the normal to the surface-and no deviation from the M-B law is found within the limits of our elementary setup. Finally we suggest tracks to explore more parallel velocities, when surface details-roughness or structure-and the atom-surface interaction should play a key role to restrict the applicability of an M-B-type distribution.

Multimillion atom simulations of dynamics of oxidation of an aluminum nanoparticle and nanoindentation on ceramics.

PubMed

Vashishta, Priya; Kalia, Rajiv K; Nakano, Aiichiro

2006-03-02

We have developed a first-principles-based hierarchical simulation framework, which seamlessly integrates (1) a quantum mechanical description based on the density functional theory (DFT), (2) multilevel molecular dynamics (MD) simulations based on a reactive force field (ReaxFF) that describes chemical reactions and polarization, a nonreactive force field that employs dynamic atomic charges, and an effective force field (EFF), and (3) an atomistically informed continuum model to reach macroscopic length scales. For scalable hierarchical simulations, we have developed parallel linear-scaling algorithms for (1) DFT calculation based on a divide-and-conquer algorithm on adaptive multigrids, (2) chemically reactive MD based on a fast ReaxFF (F-ReaxFF) algorithm, and (3) EFF-MD based on a space-time multiresolution MD (MRMD) algorithm. On 1920 Intel Itanium2 processors, we have demonstrated 1.4 million atom (0.12 trillion grid points) DFT, 0.56 billion atom F-ReaxFF, and 18.9 billion atom MRMD calculations, with parallel efficiency as high as 0.953. Through the use of these algorithms, multimillion atom MD simulations have been performed to study the oxidation of an aluminum nanoparticle. Structural and dynamic correlations in the oxide region are calculated as well as the evolution of charges, surface oxide thickness, diffusivities of atoms, and local stresses. In the microcanonical ensemble, the oxidizing reaction becomes explosive in both molecular and atomic oxygen environments, due to the enormous energy release associated with Al-O bonding. In the canonical ensemble, an amorphous oxide layer of a thickness of approximately 40 angstroms is formed after 466 ps, in good agreement with experiments. Simulations have been performed to study nanoindentation on crystalline, amorphous, and nanocrystalline silicon nitride and silicon carbide. Simulation on nanocrystalline silicon carbide reveals unusual deformation mechanisms in brittle nanophase materials, due to coexistence of brittle grains and soft amorphous-like grain boundary phases. Simulations predict a crossover from intergranular continuous deformation to intragrain discrete deformation at a critical indentation depth.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Guo, Rui; Gao, Liming, E-mail: liming.gao@sjtu.edu.cn; Li, Ming, E-mail: mingli90@sjtu.edu.cn

As the continuous shrinkage of the interconnect line width in microelectronics devices, there is a growing concern about the electromigration (EM) failure of bonding wire. In addition, an innovative Ag–8Au–3Pd alloy wire has shown promise as an economical substitute for gold wire interconnects due to the cost pressure of gold in the last decade. In present study of the Ag–8Au–3Pd alloy wire, the surface diffusion occupied the dominant position during EM failure, and the activation energy was found to be 0.61 eV. In order to reveal the failure mechanism, the cross-sections of the Ag–8Au–3Pd alloy wire during EM were preparedmore » by focused ion beam (FIB) micro-machining for electron backscatter diffraction (EBSD) analysis. The microstructure evolution of the Ag–8Au–3Pd alloy wire was characterized by the grain size and grain boundary. As a result, the EM failure originates in the atom transportation, which causes grain size increasing and atom diffusion on the wire surface. - Highlights: • The activation energy of Ag–8Au–3Pd alloy wire was obtained as 0.61 eV. • During EM, the silver atoms diffused from negative to the positive terminal on the wire surface. • The microstructure (grain size and grain boundary) was characterized by FIB-EBSD. • During EM, the atom transportation was found to cause grain size growth and atom diffusion on the wire surface.« less

The MOLDY short-range molecular dynamics package

NASA Astrophysics Data System (ADS)

Ackland, G. J.; D'Mellow, K.; Daraszewicz, S. L.; Hepburn, D. J.; Uhrin, M.; Stratford, K.

2011-12-01

We describe a parallelised version of the MOLDY molecular dynamics program. This Fortran code is aimed at systems which may be described by short-range potentials and specifically those which may be addressed with the embedded atom method. This includes a wide range of transition metals and alloys. MOLDY provides a range of options in terms of the molecular dynamics ensemble used and the boundary conditions which may be applied. A number of standard potentials are provided, and the modular structure of the code allows new potentials to be added easily. The code is parallelised using OpenMP and can therefore be run on shared memory systems, including modern multicore processors. Particular attention is paid to the updates required in the main force loop, where synchronisation is often required in OpenMP implementations of molecular dynamics. We examine the performance of the parallel code in detail and give some examples of applications to realistic problems, including the dynamic compression of copper and carbon migration in an iron-carbon alloy. Program summaryProgram title: MOLDY Catalogue identifier: AEJU_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJU_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU General Public License version 2 No. of lines in distributed program, including test data, etc.: 382 881 No. of bytes in distributed program, including test data, etc.: 6 705 242 Distribution format: tar.gz Programming language: Fortran 95/OpenMP Computer: Any Operating system: Any Has the code been vectorised or parallelized?: Yes. OpenMP is required for parallel execution RAM: 100 MB or more Classification: 7.7 Nature of problem: Moldy addresses the problem of many atoms (of order 10 6) interacting via a classical interatomic potential on a timescale of microseconds. It is designed for problems where statistics must be gathered over a number of equivalent runs, such as measuring thermodynamic properities, diffusion, radiation damage, fracture, twinning deformation, nucleation and growth of phase transitions, sputtering etc. In the vast majority of materials, the interactions are non-pairwise, and the code must be able to deal with many-body forces. Solution method: Molecular dynamics involves integrating Newton's equations of motion. MOLDY uses verlet (for good energy conservation) or predictor-corrector (for accurate trajectories) algorithms. It is parallelised using open MP. It also includes a static minimisation routine to find the lowest energy structure. Boundary conditions for surfaces, clusters, grain boundaries, thermostat (Nose), barostat (Parrinello-Rahman), and externally applied strain are provided. The initial configuration can be either a repeated unit cell or have all atoms given explictly. Initial velocities are generated internally, but it is also possible to specify the velocity of a particular atom. A wide range of interatomic force models are implemented, including embedded atom, Morse or Lennard-Jones. Thus the program is especially well suited to calculations of metals. Restrictions: The code is designed for short-ranged potentials, and there is no Ewald sum. Thus for long range interactions where all particles interact with all others, the order- N scaling will fail. Different interatomic potential forms require recompilation of the code. Additional comments: There is a set of associated open-source analysis software for postprocessing and visualisation. This includes local crystal structure recognition and identification of topological defects. Running time: A set of test modules for running time are provided. The code scales as order N. The parallelisation shows near-linear scaling with number of processors in a shared memory environment. A typical run of a few tens of nanometers for a few nanoseconds will run on a timescale of days on a multiprocessor desktop.

Dislocation imaging for orthopyroxene using an atom-resolved scanning transmission electron microscopy.

PubMed

Kumamoto, Akihito; Kogure, Toshihiro; Raimbourg, Hugues; Ikuhara, Yuichi

2014-11-01

Dislocations, one-dimensional lattice defects, appear as a microscopic phenomenon while they are formed in silicate minerals by macroscopic dynamics of the earth crust such as shear stress. To understand ductile deformation mechanisms of silicates, atomic structures of the dislocations have been examined using transmission electron microscopy (TEM). Among them, it has been proposed that {100}<001> primary slip system of orthopyroxene (Opx) is dissociated into partial dislocations, and a stacking fault with the clinopyroxene (Cpx) structure is formed between the dislocations. This model, however, has not been determined completely due to the complex structures of silicates. Scanning transmission electron microscopy (STEM) has a potential to determine the structure of dislocations with single-atomic column sensitivity, particularly by using high-angle annular dark field (HAADF) and annular bright field (ABF) imaging with a probing aberration corrector.[1] Furthermore, successive analyses from light microscopy to atom-resolved STEM have been achieved by focused ion beam (FIB) sampling techniques.[2] In this study, we examined dislocation arrays at a low-angle grain boundary of ∼1° rotation about the b-axis in natural deformed Opx using a simultaneous acquisition of HAADF/ABF (JEM-ARM200F, JEOL) equipped with 100 mm2 silicon drift detector (SDD) for energy dispersive X-ray spectroscopy (EDS). Figure 1 shows averaged STEM images viewed along the b- axis of Opx extracted from repeating units. HAADF provides the cation-site arrangement, and ABF distinguishes the difference of slightly rotated SiO4 tetrahedron around the a- axis. This is useful to distinguish the change of stacking sequence between the partial dislocations. Two types of stacking faults with Cpx and protopyroxene (Ppx) structures were identified between three partial dislocations. Furthermore, Ca accumulation in M2 (Fe) site around the stacking faults was detected by STEM-EDS. Interestingly, Ca is distributed not only in these stacking faults but also Opx matrix around the faults. jmicro;63/suppl_1/i17/DFU063F1F1DFU063F1Fig. 1. (a) HAADF and (b) ABF of Opx view of [010] direction with inset simulation images and models of its unit cell (a = 0.52, c = 1.83 nm). © The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Inversion domain boundaries in ZnO with additions of Fe2O3 studied by high-resolution ADF imaging.

PubMed

Wolf, Frank; Freitag, Bert H; Mader, Werner

2007-01-01

Columns of metal atoms in the polytypoid compound Fe2O3(ZnO)15 could be resolved by high angle annular dark field imaging in a transmission electron microscopy (TEM)/STEM electron microscope--a result which could not be realized by high-resolution bright field imaging due to inherent strain from inversion domains and inversion domain boundaries (IDBs) in the crystals. The basal plane IDB was imaged in [11 00] yielding the spacing of the two adjacent ZnO domains, while imaging in [21 1 0] yields the position of single metal ions. The images allow the construction of the entire domain structure including the stacking sequence and positions of the oxygen ions. The IDB consists of a single layer of octahedrally co-ordinated Fe3+ ions, and the inverted ZnO domains are related by point symmetry at the iron position. The FeO6 octahedrons are compressed along the ZnO c-axis resulting in a FeO bond length of 0.208 nm which is in the range of FeO distances in iron containing oxides. The model of the basal plane boundary resembles that of the IDB in polytypoid ZnO-In2O3 compounds.

2D or not 2D? The impact of nanoscale roughness and substrate interactions on the tribological properties of graphene and MoS2

NASA Astrophysics Data System (ADS)

Elinski, Meagan B.; Liu, Zhuotong; Spear, Jessica C.; Batteas, James D.

2017-03-01

The use of 2D nanomaterials for controlling friction and wear at interfaces has received increased attention over the past few years due to their unique structural, thermal, electrical and mechanical properties. These materials proffer potential critical solutions to challenges in boundary lubrication across numerous platforms ranging from engines, to biomedical implants and micro- and nano-scaled machines that will play a major role in the Internet of Things. There has been significant work on a range of 2D nanomaterials, such as graphene and molybdenum disulfide (MoS2). From these studies, their frictional properties have been shown to be highly dependent on numerous factors, such as substrate structure, strain, and competing chemical interactions between the interfaces in sliding contact. Moreover, when considering real contacts in machined interfaces, these surfaces are often composed of nanoscaled asperities, whose intermittent contact dominates the tribochemical processes that result in wear. In this review we aim to capture recent work on the tribological properties of graphene and MoS2 and to discuss the impacts of surface roughness (from the atomic scale to the nanoscale) and chemical interactions at interfaces on their frictional properties, and their use in designing advanced boundary lubrication schemes.

Liquid atomization in supersonic flows

NASA Astrophysics Data System (ADS)

Missoum, Azzedine

An experimental investigation of the atomization of a round liquid jet by coaxial, costream injection into a supersonic, Mach 1.5 air flow is reported. Extensive flow visualization was conducted using schlieren/shadowgraph, flash photography, and short duration (ns) laser imaging. The finer details of the jet were revealed when viewed under high magnification with the help of a microscope. The liquid and air pressures were varied individually. Photographic evidence indicates the presence of three regions within the liquid jet: a primary region enclosed by the first shock cell where the primary breakup occurs, a secondary region in which the jet is totally broken because of its interaction with the supersonic wave structure, and a third, subsonic region further downstream. It was found that the breakup mechanism of liquid jets in supersonic airstreams is quite complex. The breakup seems to be initiated by the growth of the turbulent structure on the liquid surface and the subsequent detachment of the three-dimensional structure as fine droplets by the intense shear at the liquid-gas interface. This seems to confirm the boundary layer stripping mechanism. The liquid jet expands into a bubble like formation as it interacts with the first set of waves. Higher liquid injection pressures resulted in higher initial spray angles. The liquid jet displayed a geometry strongly dependent on the pressure distribution resulting from the wave structure present in the supersonic jet. Droplet size and velocity distributions were measured by the P/DPA (Phase/Doppler Particle Analyzer) system. The Sauter Mean Diameter (SMD) was measured at several axial and radial locations at various liquid and air pressures. The SMD shows a decrease with increase in both the air-to-liquid mass flow ratio and the Weber number. The drop size decreased towards the outer edges of the jet. The results lead one to conclude that the coaxial, coflowing configuration is very attractive for atomizing scramjet liquid fuels.

Effect of autoclave heat treatments on the mechanical properties of the prealloyed powder cobalt-base alloy HS-31

NASA Technical Reports Server (NTRS)

Freche, J. C.; Ashbrook, R. L.

1973-01-01

The cobalt-base alloy HS-31 was atomized into powder and then consolidated by extrusion or by hot isostatic pressing (HIP) in an autoclave over a range of temperatures spanning the solidus, approximately 2340 F. Extrusions were subsequently autoclaved at the same conditions. Extrusions autoclaved at 2420 F had a life of 300 hours at 1200 F and 30 hours at 1800 F at stresses that result in a 10-hour life with cast HS-31. Superior stress rupture lives of autoclaved material are probably related to the solidification structure at the grain boundaries as well as to the increased grain size.

Craters and nanostructures on BaF2 sample induced by a focused 46.9nm laser

NASA Astrophysics Data System (ADS)

Cui, Huaiyu; Zhang, Shuqing; Li, Jingjun; Lu, Haiqiang; Zhao, Yongpeng

2017-08-01

We successfully damaged BaF2 samples by a 46.9nm capillary discharge laser of 100μJ focused by a toroidal mirror at a grazing incidence. Ablation craters with clear boundaries were detected by optical microscope and atomic force microscope (AFM). Laser-induced nanostructures with a period of ˜1μm were observed in the ablation area under single pulse irradiation and multiple pulses irradiation. The surface behavior was compared and analyzed with that induced by the laser of 50μJ. The nanostructures were supposed to be attributed to the thermoelastic effect and the period of the structures was effected by the energy of the laser.

Three-dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Heliospheric Magnetic Field. V. The Bow Wave, Heliospheric Boundary Layer, Instabilities, and Magnetic Reconnection

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pogorelov, N. V.; Heerikhuisen, J.; Roytershteyn, V.

The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW–LISM interaction. By performing three-dimensional, MHD plasma/kinetic neutral atom simulations, we determine the width of the outer heliosheath—the LISM plasma region affected by the presence of the heliosphere—and analyze quantitatively the distributions in front of the heliopause. Itmore » is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by Interstellar Boundary Explorer .« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Field, Kevin G.; Univ. of Wisconsin, Madison, WI; Miller, Brandon D.

Ferritic/Martensitic (F/M) steels with high Cr content posses the high temperature strength and low swelling rates required for advanced nuclear reactor designs. Radiation induced segregation (RIS) occurs in F/M steels due to solute atoms preferentially coupling to point defect fluxes which migrate to defect sinks, such as grain boundaries (GBs). The RIS response of F/M steels and austenitic steels has been shown to be dependent on the local structure of GBs where low energy structures have suppressed RIS responses. This relationship between local GB structure and RIS has been demonstrated primarily in ion-irradiated specimens. A 9 wt.% Cr model alloymore » steel was irradiated to 3 dpa using neutrons at the Advanced Test Reactor (ATR) to determine the effect of a neutron radiation environment on the RIS response at different GB structures. This investigation found the relationship between GB structure and RIS is also active for F/M steels irradiated using neutrons. The data generated from the neutron irradiation is also compared to RIS data generated using proton irradiations on the same heat of model alloy.« less

Influence of Yb{sub 2}O{sub 3} on electrical and microstructural characteristics of CaCu{sub 3}Ti{sub 4}O{sub 12} ceramics

DOE Office of Scientific and Technical Information (OSTI.GOV)

He, Kai; State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054; Luo, Yun

2015-09-15

Graphical abstract: Some Yb atoms entered in the lattice of CCTO substituted the Ca sites, the rest of Yb atoms concentrated at grain boundaries decreased the grain size. The dielectric constant was decreased by Yb doping. The dielectric loss of the CCTO could be greatly reduced at low frequency. - Highlights: • Yb atoms may take the place of Ca sites and concentrate at grain boundaries. • Tiny second phase corresponding to Yb may decrease the grain size. • Decrease of the grain size leads to the decrease of dielectric constant. • Yb doping could decrease the dielectric loss ofmore » CCTO. - Abstract: This paper focuses on the remarkable effects of Yb{sub 2}O{sub 3} doping on the microstructure and dielectric characteristics of CaCu{sub 3}Ti{sub 4}O{sub 12} (CCTO). Samples were prepared by the solid phase reaction method and sintered in air at 1030 °C for 12 h. X-ray diffraction and X-ray photoelectron spectroscopy studies confirm that the primary phase is CCTO. Some Yb{sup 3+} ions may substitute into the Ca site at the center or zenith sites of the CCTO lattice hexahedron, while the rest of the Yb atoms may concentrate at grain boundaries. The grain size of Yb{sub 2}O{sub 3}-doped CCTO ceramics were examined by scanning electron microscopy and demonstrate sharp grain size reduction with Yb{sub 2}O{sub 3} doping. From dielectric property measurements, the Yb{sub 2}O{sub 3} doping reduces the dielectric constant of CCTO, and the dielectric loss is also reduced.« less

Method for large-scale fabrication of atomic-scale structures on material surfaces using surface vacancies

DOEpatents

Lim, Chong Wee; Ohmori, Kenji; Petrov, Ivan Georgiev; Greene, Joseph E.

2004-07-13

A method for forming atomic-scale structures on a surface of a substrate on a large-scale includes creating a predetermined amount of surface vacancies on the surface of the substrate by removing an amount of atoms on the surface of the material corresponding to the predetermined amount of the surface vacancies. Once the surface vacancies have been created, atoms of a desired structure material are deposited on the surface of the substrate to enable the surface vacancies and the atoms of the structure material to interact. The interaction causes the atoms of the structure material to form the atomic-scale structures.

Chemically sensitive free-volume study of amorphization of Cu60Zr40 induced by cold rolling and folding

NASA Astrophysics Data System (ADS)

Puff, Werner; Rabitsch, Herbert; Wilde, Gerhard; Dinda, Guru P.; Würschum, Roland

2007-06-01

With the aim to contribute to a microscopical understanding of the processes of solid-state amorphization, the chemically sensitive technique of background—reduced Doppler broadening of positron-electron annihilation radiation in combination with positron lifetime spectroscopy and microstructural characterization is applied to a free volume study of the amorphization of Cu60Zr40 induced by consecutive folding and rolling. Starting from the constituent pure metal foils, a nanosale multilayer structure of elemental layers and amorphous interlayers develops in an intermediate state of folding and rolling, where free volumes with a Zr-rich environment occur presumably located in the hetero-interfaces between the various layers or in grain boundaries of the Cu layers. After complete intermixing and amorphization, the local chemical environment of the free volumes reflects the average chemical alloy composition. In contrast to other processes of amorphization, free volumes of the size of few missing atoms occur in the rolling-induced amorphous state. Self-consistent results from three different methods for analyzing the Doppler broadening spectra, i.e., S-W-parameter correlation, multicomponent fit, and the shape of ratio curves, demonstrate the potential of the background-reduced Doppler technique for chemically sensitive characterization of structurally complex materials on an atomic scale.

Optimization of Monte Carlo dose calculations: The interface problem

NASA Astrophysics Data System (ADS)

Soudentas, Edward

1998-05-01

High energy photon beams are widely used for radiation treatment of deep-seated tumors. The human body contains many types of interfaces between dissimilar materials that affect dose distribution in radiation therapy. Experimentally, significant radiation dose perturbations has been observed at such interfaces. The EGS4 Monte Carlo code was used to calculate dose perturbations at boundaries between dissimilar materials (such as bone/water) for 60Co and 6 MeV linear accelerator beams using a UNIX workstation. A simple test of the reliability of a random number generator was also developed. A systematic study of the adjustable parameters in EGS4 was performed in order to minimize calculational artifacts at boundaries. Calculations of dose perturbations at boundaries between different materials showed that there is a 12% increase in dose at water/bone interface, and a 44% increase in dose at water/copper interface. with the increase mainly due to electrons produced in water and backscattered from the high atomic number material. The dependence of the dose increase on the atomic number was also investigated. The clinically important case of using two parallel opposed beams for radiation therapy was investigated where increased doses at boundaries has been observed. The Monte Carlo calculations can provide accurate dosimetry data under conditions of electronic non-equilibrium at tissue interfaces.

Modeling elasticity in crystal growth.

PubMed

Elder, K R; Katakowski, Mark; Haataja, Mikko; Grant, Martin

2002-06-17

A new model of crystal growth is presented that describes the phenomena on atomic length and diffusive time scales. The former incorporates elastic and plastic deformation in a natural manner, and the latter enables access to time scales much larger than conventional atomic methods. The model is shown to be consistent with the predictions of Read and Shockley for grain boundary energy, and Matthews and Blakeslee for misfit dislocations in epitaxial growth.

Method for forming monolayer graphene-boron nitride heterostructures

DOEpatents

Sutter, Peter Werner; Sutter, Eli Anguelova

2016-08-09

A method for fabricating monolayer graphene-boron nitride heterostructures in a single atomically thin membrane that limits intermixing at boundaries between graphene and h-BN, so as to achieve atomically sharp interfaces between these materials. In one embodiment, the method comprises exposing a ruthenium substrate to ethylene, exposing the ruthenium substrate to oxygen after exposure to ethylene and exposing the ruthenium substrate to borazine after exposure to oxygen.

Observation of oscillatory radiation induced segregation profiles at grain boundaries in neutron irradiated 316 stainless steel using atom probe tomography

NASA Astrophysics Data System (ADS)

Barr, Christopher M.; Felfer, Peter J.; Cole, James I.; Taheri, Mitra L.

2018-06-01

Radiation induced segregation in austenitic Fe-Ni-Cr stainless steels is a key detrimental microstructural modification experienced in the current generation of light water reactors. In particular, Cr depletion at grain boundaries can be a significant factor in irradiation-assisted stress corrosion cracking. Therefore, having a complete knowledge and mechanistic understanding of radiation induced segregation at high dose and after a long thermal history is desired for continued sustainability of existing reactors. Here, we examine a 12% cold worked AISI 316 stainless steel hexagonal duct exposed in the lower dose, outer blanket region of the EBR-II reactor, by using advanced characterization and analysis techniques including atom probe tomography and analytical scanning transmission electron microscopy. Contrary to existing literature, we observe an oscillatory w-shape Cr and M-shape Ni concentration profile at 31 dpa. The presence and characterization through advanced atom probe tomography analysis of the w-shape Cr RIS profile is discussed in the context of the localized GB plane interfacial excess of the other major and minor alloying elements. The key finding of a co-segregation phenomena coupling Cr, Mo, and C is discussed in the context of the existing solute segregation literature under irradiation with emphasis on improved spatial and chemical resolution of atom probe tomography.

Enhanced structural stability of nanoporous zirconia under irradiation of He

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Tengfei; Huang, Xuejun; Wang, Chenxu

2012-01-01

This work reports a greatly enhanced tolerance for He irradiation-induced swelling in nanocrystalline zirconia film with interconnected nanoporous structure (hereinafter referred as to NC-C). Compared to bulk yttria-stabilized zirconia (YSZ) and another nanocrystalline zirconia film only with discrete nano voids (hereinafter referred as to NC-V), the NC-C film reveals good tolerance for irradiation of high-fluence He. No appreciable surface blistering can be found even at the highest fluence of 6 1017 cm2 in NCC film. From TEM analysis of as-irradiated samples, the enhanced tolerance for volume swelling in NCC film is attributed to the enhanced diffusion mechanism of deposited Hemore » via widely distributed nano channels. Furthermore, the growth of grain size is quite small for both nanocrystalline zirconia films after irradiation, which is ascribed to the decreasing of area of grain boundary due to loose structure and low energy of primary knock-on atoms for He ions.« less

Thermal conductivity and thermal boundary resistance of atomic layer deposited high-k dielectric aluminum oxide, hafnium oxide, and titanium oxide thin films on silicon

NASA Astrophysics Data System (ADS)

Scott, Ethan A.; Gaskins, John T.; King, Sean W.; Hopkins, Patrick E.

2018-05-01

The need for increased control of layer thickness and uniformity as device dimensions shrink has spurred increased use of atomic layer deposition (ALD) for thin film growth. The ability to deposit high dielectric constant (high-k) films via ALD has allowed for their widespread use in a swath of optical, optoelectronic, and electronic devices, including integration into CMOS compatible platforms. As the thickness of these dielectric layers is reduced, the interfacial thermal resistance can dictate the overall thermal resistance of the material stack compared to the resistance due to the finite dielectric layer thickness. Time domain thermoreflectance is used to interrogate both the thermal conductivity and the thermal boundary resistance of aluminum oxide, hafnium oxide, and titanium oxide films on silicon. We calculate a representative design map of effective thermal resistances, including those of the dielectric layers and boundary resistances, as a function of dielectric layer thickness, which will be of great importance in predicting the thermal resistances of current and future devices.

Implementation of the infinite-range exterior complex scaling to the time-dependent complete-active-space self-consistent-field method

NASA Astrophysics Data System (ADS)

Orimo, Yuki; Sato, Takeshi; Scrinzi, Armin; Ishikawa, Kenichi L.

2018-02-01

We present a numerical implementation of the infinite-range exterior complex scaling [Scrinzi, Phys. Rev. A 81, 053845 (2010), 10.1103/PhysRevA.81.053845] as an efficient absorbing boundary to the time-dependent complete-active-space self-consistent field method [Sato, Ishikawa, Březinová, Lackner, Nagele, and Burgdörfer, Phys. Rev. A 94, 023405 (2016), 10.1103/PhysRevA.94.023405] for multielectron atoms subject to an intense laser pulse. We introduce Gauss-Laguerre-Radau quadrature points to construct discrete variable representation basis functions in the last radial finite element extending to infinity. This implementation is applied to strong-field ionization and high-harmonic generation in He, Be, and Ne atoms. It efficiently prevents unphysical reflection of photoelectron wave packets at the simulation boundary, enabling accurate simulations with substantially reduced computational cost, even under significant (≈50 % ) double ionization. For the case of a simulation of high-harmonic generation from Ne, for example, 80% cost reduction is achieved, compared to a mask-function absorption boundary.

Twinning in fcc lattice creates low-coordinated catalytically active sites in porous gold

DOE Office of Scientific and Technical Information (OSTI.GOV)

Krajčí, Marian; Kameoka, Satoshi; Tsai, An-Pang

We describe a new mechanism for creation of catalytically active sites in porous gold. Samples of porous gold prepared by de-alloying Al{sub 2}Au exhibit a clear correlation between the catalytic reactivity towards CO oxidation and structural defects in the fcc lattice of Au. We have found that on the stepped (211) surfaces quite common twin boundary defects in the bulk structure of porous gold can form long close-packed rows of atoms with the coordination number CN = 6. DFT calculations confirm that on these low-coordinated Au sites dioxygen chemisorbs and CO oxidation can proceed via the Langmuir–Hinshelwood mechanism with themore » activation energy of 37 kJ/mol or via the CO–OO intermediate with the energy barrier of 19 kJ/mol. The existence of the twins in porous gold is stabilized by the surface energy.« less

A new incorporation mechanism for trivalent actinides into bioapatite: a TRLFS and EXAFS study.

PubMed

Holliday, Kiel; Handley-Sidhu, Stephanie; Dardenne, Kathy; Renshaw, Joanna; Macaskie, Lynne; Walther, Clemens; Stumpf, Thorsten

2012-02-28

One of the most toxic byproducts of nuclear power and weapons production is the transuranics, which have a high radiotoxicity and long biological half-life due to their tendency to accumulate in the skeletal system. This accumulation is inhomogeneous and has been associated with the chemical properties and structure of the bone material rather than its location or function. This suggests a chemical driving force to incorporation and requires an atomic scale mechanistic understanding of the incorporation process. Here we propose a new incorporation mechanism for trivalent actinides and lanthanides into synthetic and biologically produced hydroxyapatite. Time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure have been used to demonstrate that trivalent actinides and lanthanides incorporate into the amorphous grain boundaries of apatite. This incorporation site can be used to explain patterns in uptake and distribution of radionuclides in the mammalian skeletal system. © 2012 American Chemical Society

Ab initio calculations for industrial materials engineering: successes and challenges.

PubMed

Wimmer, Erich; Najafabadi, Reza; Young, George A; Ballard, Jake D; Angeliu, Thomas M; Vollmer, James; Chambers, James J; Niimi, Hiroaki; Shaw, Judy B; Freeman, Clive; Christensen, Mikael; Wolf, Walter; Saxe, Paul

2010-09-29

Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO(2) junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.

Dynamics of defect-loaded grain boundary under shear deformation in alpha iron

NASA Astrophysics Data System (ADS)

Yang, L.; Zhou, H. L.; Liu, H.; Gao, F.; Zu, X. T.; Peng, S. M.; Long, X. G.; Zhou, X. S.

2018-02-01

Two symmetric tilt grain boundaries (GBs) (Σ3〈110〉{112} and Σ11〈110〉{332}) in alpha iron were performed to investigate the dynamics of defect-loaded GBs under shear deformation. The results show that the loaded self-interstitial atoms (SIAs) reduce the critical stress of the coupled GB motion in the Σ3 GB, but increase the critical stress in the Σ11 GB. The loaded SIAs in the Σ3 GB easily form 〈111〉 clusters and remain in the bulk when the GB moves away. However, the SIAs move along with the Σ11 GB and combine with the vacancies in the bulk, leading to the defect self-healing. The helium (He) atoms loaded into the GBs significantly affect the coupled GB motion. Once He clusters emit interstitials, the Σ11 GB carries those interstitials away but the Σ3 does not. The loaded He atoms reduce the critical stress of the Σ3 GB, but increase the critical stress of the Σ11 GB.

Single layer of Ge quantum dots in HfO2 for floating gate memory capacitors.

PubMed

Lepadatu, A M; Palade, C; Slav, A; Maraloiu, A V; Lazanu, S; Stoica, T; Logofatu, C; Teodorescu, V S; Ciurea, M L

2017-04-28

High performance trilayer memory capacitors with a floating gate of a single layer of Ge quantum dots (QDs) in HfO 2 were fabricated using magnetron sputtering followed by rapid thermal annealing (RTA). The layer sequence of the capacitors is gate HfO 2 /floating gate of single layer of Ge QDs in HfO 2 /tunnel HfO 2 /p-Si wafers. Both Ge and HfO 2 are nanostructured by RTA at moderate temperatures of 600-700 °C. By nanostructuring at 600 °C, the formation of a single layer of well separated Ge QDs with diameters of 2-3 nm at a density of 4-5 × 10 15 m -2 is achieved in the floating gate (intermediate layer). The Ge QDs inside the intermediate layer are arranged in a single layer and are separated from each other by HfO 2 nanocrystals (NCs) about 8 nm in diameter with a tetragonal/orthorhombic structure. The Ge QDs in the single layer are located at the crossing of the HfO 2 NCs boundaries. In the intermediate layer, besides Ge QDs, a part of the Ge atoms is segregated by RTA at the HfO 2 NCs boundaries, while another part of the Ge atoms is present inside the HfO 2 lattice stabilizing the tetragonal/orthorhombic structure. The fabricated capacitors show a memory window of 3.8 ± 0.5 V and a capacitance-time characteristic with 14% capacitance decay in the first 3000-4000 s followed by a very slow capacitance decrease extrapolated to 50% after 10 years. This high performance is mainly due to the floating gate of a single layer of well separated Ge QDs in HfO 2 , distanced from the Si substrate by the tunnel oxide layer with a precise thickness.

The evolution of interaction between grain boundary and irradiation-induced point defects: Symmetric tilt GB in tungsten

NASA Astrophysics Data System (ADS)

Li, Hong; Qin, Yuan; Yang, Yingying; Yao, Man; Wang, Xudong; Xu, Haixuan; Phillpot, Simon R.

2018-03-01

Molecular dynamics method is used and scheme of calculational tests is designed. The atomic evolution view of the interaction between grain boundary (GB) and irradiation-induced point defects is given in six symmetric tilt GB structures of bcc tungsten with the energy of the primary knock-on atom (PKA) EPKA of 3 and 5 keV and the simulated temperature of 300 K. During the collision cascade with GB structure there are synergistic mechanisms to reduce the number of point defects: one is vacancies recombine with interstitials, and another is interstitials diffuse towards the GB with vacancies almost not move. The larger the ratio of the peak defect zone of the cascades overlaps with the GB region, the statistically relative smaller the number of surviving point defects in the grain interior (GI); and when the two almost do not overlap, vacancy-intensive area generally exists nearby GBs, and has a tendency to move toward GB with the increase of EPKA. In contrast, the distribution of interstitials is relatively uniform nearby GBs and is affected by the EPKA far less than the vacancy. The GB has a bias-absorption effect on the interstitials compared with vacancies. It shows that the number of surviving vacancies statistically has increasing trend with the increase of the distance between PKA and GB. While the number of surviving interstitials does not change much, and is less than the number of interstitials in the single crystal at the same conditions. The number of surviving vacancies in the GI is always larger than that of interstitials. The GB local extension after irradiation is observed for which the interstitials absorbed by the GB may be responsible. The designed scheme of calculational tests in the paper is completely applicable to the investigation of the interaction between other types of GBs and irradiation-induced point defects.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang Lin, E-mail: zhanglincsu@163.com; Liu Hengsan, E-mail: lhsj63@sohu.com; He Xinbo, E-mail: xb_he@163.com

The characteristics of rapidly solidified FGH96 superalloy powder and the thermal evolution behavior of carbides and {gamma} Prime precipitates within powder particles were investigated. It was observed that the reduction of powder size and the increase of cooling rate had transformed the solidification morphologies of atomized powder from dendrite in major to cellular structure. The secondary dendritic spacing was measured to be 1.02-2.55 {mu}m and the corresponding cooling rates were estimated to be in the range of 1.4 Multiplication-Sign 10{sup 4}-4.7 Multiplication-Sign 10{sup 5} K{center_dot}s{sup -1}. An increase in the annealing temperature had rendered the phase transformation of carbides evolvingmore » from non-equilibrium MC Prime carbides to intermediate transition stage of M{sub 23}C{sub 6} carbides, and finally to thermodynamically stable MC carbides. The superfine {gamma} Prime precipitates were formed at the dendritic boundaries of rapidly solidified superalloy powder. The coalescence, growth, and homogenization of {gamma}' precipitates occurred with increasing annealing temperature. With decreasing cooling rate from 650 Degree-Sign C{center_dot}K{sup -1} to 5 Degree-Sign C{center_dot}K{sup -1}, the morphological development of {gamma} Prime precipitates had been shown to proceed from spheroidal to cuboidal and finally to solid state dendrites. Meanwhile, a shift had been observed from dendritic morphology to recrystallized structure between 900 Degree-Sign C and 1050 Degree-Sign C. Moreover, accelerated evolution of carbides and {gamma}' precipitates had been facilitated by the formation of new grain boundaries which provide fast diffusion path for atomic elements. - Highlights: Black-Right-Pointing-Pointer Microstructural characteristic of FGH96 superalloy powder was investigated. Black-Right-Pointing-Pointer The relation between microstructure, particle size, and cooling rate was studied. Black-Right-Pointing-Pointer Thermal evolution behavior of {gamma} Prime and carbides in loose FGH96 powder was studied.« less

Heliospheric Neutral Atom Spectra Between 0.01 and 6 keV fom IBEX

NASA Technical Reports Server (NTRS)

Fuselier, S. A.; Allegrini, F.; Bzowski, M.; Funsten, H. O.; Ghielmetti, A. G.; Gloeckler, G.; Heirtzler, D.; Janzen, P.; Kubiak, M.; Kucharek, H.;

Simulating functional magnetic materials on supercomputers.

PubMed

Gruner, Markus Ernst; Entel, Peter

2009-07-22

The recent passing of the petaflop per second landmark by the Roadrunner project at the Los Alamos National Laboratory marks a preliminary peak of an impressive world-wide development in the high-performance scientific computing sector. Also, purely academic state-of-the-art supercomputers such as the IBM Blue Gene/P at Forschungszentrum Jülich allow us nowadays to investigate large systems of the order of 10(3) spin polarized transition metal atoms by means of density functional theory. Three applications will be presented where large-scale ab initio calculations contribute to the understanding of key properties emerging from a close interrelation between structure and magnetism. The first two examples discuss the size dependent evolution of equilibrium structural motifs in elementary iron and binary Fe-Pt and Co-Pt transition metal nanoparticles, which are currently discussed as promising candidates for ultra-high-density magnetic data storage media. However, the preference for multiply twinned morphologies at smaller cluster sizes counteracts the formation of a single-crystalline L1(0) phase, which alone provides the required hard magnetic properties. The third application is concerned with the magnetic shape memory effect in the Ni-Mn-Ga Heusler alloy, which is a technologically relevant candidate for magnetomechanical actuators and sensors. In this material strains of up to 10% can be induced by external magnetic fields due to the field induced shifting of martensitic twin boundaries, requiring an extremely high mobility of the martensitic twin boundaries, but also the selection of the appropriate martensitic structure from the rich phase diagram.

Contributions of phase and structural transformations in multicomponent Al-Mg alloys to the linear and nonlinear mechanisms of anelasticity

NASA Astrophysics Data System (ADS)

Golovin, I. S.; Bychkov, A. S.; Mikhailovskaya, A. V.; Dobatkin, S. V.

2014-02-01

The effects of the processes of severe plastic deformation (SPD), recrystallization, and precipitation of the β phase in multicomponent alloys of the Al-5Mg-Mn-Cr and Al-(4-5%)Mg-Mn-Zn-Sc systems on the mechanisms of grain-boundary relaxation and dislocation-induced microplasticity have been studied in some detail. To stabilize the ultrafine-grained structure and prevent grain growth, dispersed Al-transition-metal particles, such as Al3Zr, Al6Mn, Al7Cr, Al6(Mn,Cr), Al18Cr2Mg3 have been used. We have special interest in alloys with additions of scandium, which forms compounds of the Al3Sc type and favors the precipitation of finer particles compared to the aluminides of other transition metals. After SPD, Al-(4-5%)Mg-Mn-Zr-Sc alloys exhibit an enhanced recrystallization temperature. The general features of the dislocation and grain-boundary anelasticity that have been established for the binary Al-Mg alloys are retained; i.e., (1) the decrease in the dislocation density in the process of recrystallization of cold-worked alloys leads to the formation of a pseudo-peak in the curves of the temperature dependences of internal friction (TDIF) and to a decrease in the critical amplitude of deformation corresponding to the onset of dislocation motion in a stress field; (2) the precipitation of the β phase suppresses the grain-boundary relaxation; (3) the dissolution of the β phase, the passage of the magnesium atoms into the solid solution, and the precipitation of the β' phase upon heating hinder the motion of dislocations; (4) the coarsening of the highly dispersed particles containing Zr and Sc increases the dislocation mobility. The grain-boundary relaxation and dislocation-impurity interaction and their temperature dependences, as well as processes of the additional alloying of the binary alloys by Mn, Cr, Zr, and Sc, have been estimated quantitatively.

Improvement in plasma illumination properties of ultrananocrystalline diamond films by grain boundary engineering

NASA Astrophysics Data System (ADS)

Sankaran, K. J.; Srinivasu, K.; Chen, H. C.; Dong, C. L.; Leou, K. C.; Lee, C. Y.; Tai, N. H.; Lin, I. N.

2013-08-01

Microstructural evolution of ultrananocrystalline diamond (UNCD) films as a function of substrate temperature (TS) and/or by introducing H2 in Ar/CH4 plasma is investigated. Variation of the sp2 and sp3 carbon content is analyzed using UV-Raman and near-edge X-ray absorption fine structure spectra. Morphological and microstructural studies confirm that films deposited using Ar/CH4 plasma at low TS consist of a random distribution of spherically shaped ultra-nano diamond grains with distinct sp2-bonded grain boundaries, which are attributed to the adherence of CH radicals to the nano-sized diamond clusters. By increasing TS, adhering efficiency of CH radicals to the diamond lattice drops and trans-polyacetylene (t-PA) encapsulating the nano-sized diamond grains break, whereas the addition of 1.5% H2 in Ar/CH4 plasma at low TS induces atomic hydrogen that preferentially etches out the t-PA attached to ultra-nano diamond grains. Both cases make the sp3-diamond phase less passivated. This leads to C2 radicals attaching to the diamond lattice promoting elongated clustered grains along with a complicated defect structure. Such a grain growth model is highly correlated to explain the technologically important functional property, namely, plasma illumination (PI) of UNCD films. Superior PI properties, viz. low threshold field of 0.21 V/μm with a high PI current density of 4.10 mA/cm2 (at an applied field of 0.25 V/μm) and high γ-coefficient (0.2604) are observed for the UNCD films possessing ultra-nano grains with a large fraction of grain boundary phases. The grain boundary component consists of a large amount of sp2-carbon phases that possibly form interconnected paths for facilitating the transport of electrons and the electron field emission process that markedly enhance PI properties.

Investigation of Material Problems for High Temperature, High Power Space Energy-Conversion Systems.

DTIC Science & Technology

1986-07-01

Microplastic Deformation in Molybdenum with Grain-Boundary Segregations of Nickel, Phys. Met. Metall. 54, 180, 1982. 76. C. Lea, B. Muddle and D...8217-- *. ~a.-. ~ ~ ~ .. . ~ . ~ . .*.. -- ~ .. 25 93. T. Noda, T. Kainuma and M. Okada: Oxygen Desorption from Grain Boundaries of Molybdenum During Vacuu...behavior of this work function climb was explained by the desorption of atoms adsorbed or, the surface. The test result of the present W-Re samples showed

Conductive paths through polycrystalline BaTiO{sub 3}: Scanning probe microscopy study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ayvazian, Talin; Bersuker, Gennadi; Lingley, Zachary R.

2016-08-15

The microstructural features determining the leakage current through polycrystalline BaTiO{sub 3} films are investigated using Conductive Atomic Force Microscopy. Grain boundaries are found to be the dominant conductive paths compared to the conduction through the grains. Grain boundary currents are observed to reversibly rise with the increase of the applied DC voltages, indicating that the current is controlled by a field-activated charge transport process.

The Low‐Energy Neutral Imager (LENI)

PubMed Central

Mitchell, D. G.; Brandt, P. C.‐son.; Andrews, B. G.; Clark, G.

2016-01-01

Abstract To achieve breakthroughs in the areas of heliospheric and magnetospheric energetic neutral atom (ENA) imaging, a new class of instruments is required. We present a high angular resolution ENA imager concept aimed at the suprathermal plasma populations with energies between 0.5 and 20 keV. This instrument is intended for understanding the spatial and temporal structure of the heliospheric boundary recently revealed by Interstellar Boundary Explorer instrumentation and the Cassini Ion and Neutral Camera. The instrument is also well suited to characterize magnetospheric ENA emissions from low‐altitude ENA emissions produced by precipitation of magnetospheric ions into the terrestrial upper atmosphere, or from the magnetosheath where solar wind protons are neutralized by charge exchange, or from portions of the ring current region. We present a new technique utilizing ultrathin carbon foils, 2‐D collimation, and a novel electron optical design to produce high angular resolution (≤2°) and high‐sensitivity (≥10−3 cm2 sr/pixel) ENA imaging in the 0.5–20 keV energy range. PMID:27867800

Fe-Cr-Mo based ODS alloys via spark plasma sintering: A combinational characterization study by TEM and APT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Y. Q. Wu; K. N. Allahar; J. Burns

2013-08-01

Nanoscale oxides play an important role in oxide dispersion strengthened (ODS) alloys for improved high temperature creep resistance and enhanced radiation damage tolerance. In this study, transmission electron microscopy (TEM) and atom probe tomography (APT) were combined to investigate two novel Fe-16Cr-3Mo (wt.%) based ODS alloys. Spark plasma sintering (SPS) was used to consolidate the ODS alloys from powders that were milled with 0.5 wt.% Y2O3 powder only or with Y2O3 powder and 1 wt.% Ti. TEM characterization revealed that both alloys have a bimodal structure of nanometer-size (~ 100 – 500 nm) and micron-size grains with nanostructured oxide precipitatesmore » formed along and close to grain boundaries with diameters ranging from five to tens of nanometers. APT provides further quantitative analyses of the oxide precipitates, and also reveals Mo segregation at grain boundaries next to oxide precipitates. The alloys with and without Ti are compared based on their microstructures.« less

Grain boundary mobility in anion doped MgO

NASA Technical Reports Server (NTRS)

Kapadia, C. M.; Leipold, M. H.

1973-01-01

Certain anions OH(-), F(-) and Gl(-) are shown to enhance grain growth in MgO. The magnitude of their effect decreases in the order in which the anions are listed and depends on their location (solid-solution, second phase) in the MgO lattice. As most anions exhibit relatively high vapor pressures at sintering temperatures, they retard densification and invariably promote residual porosity. The role of anions on grain growth rates was studied in relation to their effect on pore mobility and pore removal; the atomic process controlling the actual rates was determined from observed kinetics in conjunction with the microstructural features. With respect to controlling mechanisms, the effects of all anions are not the same. OH(-) and F(-) control behavior through creation of a defect structure and a grain boundary liquid phase while Cl(-) promotes matter transport within pores by evaporation-condensation. Studies on an additional anion, S to the minus 2nd power gave results which were no different from undoped MgO, possibly because of evaporative losses during hot pressing. Hence, the effect of sulphur is negligible or undetermined.

Prevention of weld-decay in austenitic stainless steel by using surface mechanical attrition treatment

NASA Astrophysics Data System (ADS)

Laleh, Majid; Kargar, Farzad; Rouhaghdam, Alireza Sabour

2012-11-01

Surface mechanical attrition treatment (SMAT) was applied to the samples of a type AISI 304 stainless steel in order to induce grain refinement as well as formation of twins. Transmission electron microscopy and X-ray diffraction analysis results showed that the average grain size at the surface of the SMATed sample was about 10 nm. The untreated and SMATed samples were then welded using a one-pass gas tungsten arc procedure. The heat-affected zone (HAZ) of the samples was examined by optical microscopy and corrosion tests. Results of the double loop electrochemical potentiokinetic reactivation tests showed that the degree of sensitization in the HAZ for the SMATed sample was very low as compared to that of the untreated one. The pre-SMATed sample was resistant to intergranular corrosion. This is mainly due to the formation of high density of twins which are not prone to carbide precipitation because of their regular and coherent atomic structure and extreme low grain boundary energy as compared with those of other grain boundaries.

Gate-tunable memristive phenomena mediated by grain boundaries in single-layer MoS2

NASA Astrophysics Data System (ADS)

Sangwan, Vinod K.; Jariwala, Deep; Kim, In Soo; Chen, Kan-Sheng; Marks, Tobin J.; Lauhon, Lincoln J.; Hersam, Mark C.

2015-05-01

Continued progress in high-speed computing depends on breakthroughs in both materials synthesis and device architectures. The performance of logic and memory can be enhanced significantly by introducing a memristor, a two-terminal device with internal resistance that depends on the history of the external bias voltage. State-of-the-art memristors, based on metal-insulator-metal (MIM) structures with insulating oxides, such as TiO2, are limited by a lack of control over the filament formation and external control of the switching voltage. Here, we report a class of memristors based on grain boundaries (GBs) in single-layer MoS2 devices. Specifically, the resistance of GBs emerging from contacts can be easily and repeatedly modulated, with switching ratios up to ˜103 and a dynamic negative differential resistance (NDR). Furthermore, the atomically thin nature of MoS2 enables tuning of the set voltage by a third gate terminal in a field-effect geometry, which provides new functionality that is not observed in other known memristive devices.

Suzuki segregation in a binary Cu-Si alloy.

PubMed

Mendis, Budhika G; Jones, Ian P; Smallman, Raymond E

2004-01-01

Suzuki segregation to stacking faults and coherent twin boundaries has been investigated in a Cu-7.15 at.% Si alloy, heat-treated at temperatures of 275, 400 and 550 degrees C, using field-emission gun transmission electron microscopy. Silicon enrichment was observed at the stacking fault plane and decreased monotonically with increasing annealing temperature. This increase in the concentration of solute at the fault is due to the stacking fault energy being lowered at higher values of the electron-to-atom ratio of the alloy. From a McLean isotherm, the binding energy for segregation was calculated to be -0.021 +/- 0.019 eV atom(-1). Hardly any segregation was observed to coherent twin boundaries in the same alloy. This is because a twin has a lower interfacial energy than a stacking fault, so that the driving force for segregation is diminished.

Investigation of phonon transport and thermal boundary conductance at the interface of functionalized SWCNT and poly (ether-ketone)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, Haoxiang; Kumar, Satish, E-mail: satish.kumar@me.gatech.edu; Chen, Liang

2016-09-07

Carbon nanostructures such as carbon nanotube (CNT), graphene, and carbon fibers can be used as fillers in amorphous polymers to improve their thermal properties. In this study, the effect of covalent bonding of CNT with poly(ether ketone) (PEK) on interfacial thermal interactions is investigated using non-equilibrium molecular dynamics simulations. The number of covalent bonds between (20, 20) CNT and PEK is varied in the range of 0–80 (0%–6.25%), and the thermal boundary conductance is computed. The analysis reveals that covalent functionalization of CNT atoms can enhance the thermal boundary conductance by an order of magnitude compared to the non-functionalized CNT-PEKmore » interface at a high degree of CNT functionalization. Besides strengthening the thermal coupling, covalent functionalization is also shown to modify the phonon spectra of CNT. The transient spectral energy analysis shows that the crosslinks cause faster energy exchange from CNT to PEK in different frequency bands. The oxygen atom of hydroxyl group of PEK contributes energy transfer in the low frequency band, while aromatic and carbonyl carbon atoms play a more significant role in high frequency bands. In addition, by analyzing the relaxation time of the spectral temperature of different frequency bands of CNT, it is revealed that with increasing number of bonds, both lower frequency vibrational modes and higher frequency modes efficiently couple across the CNT-PEK interface and contribute in thermal energy transfer from CNT to the matrix.« less

Atomic resolved phase map of monolayer MoS2 retrieved by spherical aberration-corrected transport of intensity equation.

PubMed

Zhang, Xiaobin; Oshima, Yoshifumi

2016-10-01

An atomic resolution phase map, which enables us to observe charge distribution or magnetic properties at an atomic scale, has been pointed out to be retrieved by transport of intensity equation (TIE) when taking two atomic-resolved transmission electron microscope (TEM) images of small defocus difference. In this work, we firstly obtained the atomic-resolved phase maps of an exfoliated molybdenum disulfide sheet using spherical aberration-corrected transmission electron microscope. We successfully observed 60° grain boundary of mechanically exfoliated monolayer molybdenum disulfide sheet. The relative phase shift of a single molybdenum atomic column to the column consisting of two sulfur atoms was obtained to be about 0.01 rad on average, which was about half lower than the simulated TIE phase map, indicating that the individual atomic sites can be distinguished qualitatively. The appropriate condition for retrieving atomic-resolved TIE phase maps was briefly discussed. © The Author 2016. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Cross-Sectional Imaging of Boundary Lubrication Layer Formed by Fatty Acid by Means of Frequency-Modulation Atomic Force Microscopy.

PubMed

Hirayama, Tomoko; Kawamura, Ryota; Fujino, Keita; Matsuoka, Takashi; Komiya, Hiroshi; Onishi, Hiroshi

2017-10-10

To observe in situ the adsorption of fatty acid onto metal surfaces, cross-sectional images of the adsorption layer were acquired by frequency-modulation atomic force microscopy (FM-AFM). Hexadecane and palmitic acid were used as the base oil and typical fatty acid, respectively. A Cu-coated silicon wafer was prepared as the target substrate. The solvation structure formed by hexadecane molecules at the interface between the Cu substrate and the hexadecane was observed, and the layer pitch was found to be about 0.6 nm, which corresponds to the height of hexadecane molecules. This demonstrates that hexadecane molecules physically adsorbed onto the surface due to van der Waals forces with lying orientation because hexadecane is a nonpolar hydrocarbon. When hexadecane with palmitic acid was put on the Cu substrate instead of pure hexadecane, an adsorption layer of palmitic acid was observed at the interface. The layer pitch was about 2.5-2.8 nm, which matches the chain length of palmitic acid molecules well. This indicates that the original adsorption layer was monolayer or single bilayer in the local area. In addition, a cross-sectional image captured 1 h after observation started to reveal that the adsorbed additive layer gradually grew up to be thicker than about 20 nm due to an external stimulus, such as cantilever oscillation. This is the first report of in situ observation of an adsorbed layer by FM-AFM in the tribology field and demonstrates that FM-AFM is useful for clarifying the actual boundary lubrication mechanism.

Structural adaptations of proteins to different biological membranes

PubMed Central

Pogozheva, Irina D.; Tristram-Nagle, Stephanie; Mosberg, Henry I.; Lomize, Andrei L.

2013-01-01

To gain insight into adaptations of proteins to their membranes, intrinsic hydrophobic thicknesses, distributions of different chemical groups and profiles of hydrogen-bonding capacities (α and β) and the dipolarity/polarizability parameter (π*) were calculated for lipid-facing surfaces of 460 integral α-helical, β-barrel and peripheral proteins from eight types of biomembranes. For comparison, polarity profiles were also calculated for ten artificial lipid bilayers that have been previously studied by neutron and X-ray scattering. Estimated hydrophobic thicknesses are 30-31 Å for proteins from endoplasmic reticulum, thylakoid, and various bacterial plasma membranes, but differ for proteins from outer bacterial, inner mitochondrial and eukaryotic plasma membranes (23.9, 28.6 and 33.5 Å, respectively). Protein and lipid polarity parameters abruptly change in the lipid carbonyl zone that matches the calculated hydrophobic boundaries. Maxima of positively charged protein groups correspond to the location of lipid phosphates at 20-22 Å distances from the membrane center. Locations of Tyr atoms coincide with hydrophobic boundaries, while distributions maxima of Trp rings are shifted by 3-4 Å toward the membrane center. Distributions of Trp atoms indicate the presence of two 5-8 Å-wide midpolar regions with intermediate π* values within the hydrocarbon core, whose size and symmetry depend on the lipid composition of membrane leaflets. Midpolar regions are especially asymmetric in outer bacterial membranes and cell membranes of mesophilic but not hyperthermophilic archaebacteria, indicating the larger width of the central nonpolar region in the later case. In artificial lipid bilayers, midpolar regions are observed up to the level of acyl chain double bonds. PMID:23811361

Adaptive strain prompting a pseudo-morphotropic phase boundary in ferroelectric (1-x ) Na0.5Bi0.5TiO3-x BaTiO3

NASA Astrophysics Data System (ADS)

Datta, K.; Neder, R. B.; Richter, A.; Göbbels, M.; Neuefeind, J. C.; Mihailova, B.

2018-05-01

The understanding of the atomistic origin of the morphotropic phase boundary (MPB) occurring in composition-temperature phase diagrams of ferroelectric solid solutions is a key topic in material science because materials often exhibit anomalous properties at the MPB. Here we reveal mesoscopic-scale structural correlations for a leading Pb-free ferroelectric system, (1 -x ) Na0.5Bi0.5TiO3-x BaTiO3 (NBT-x BT ), by examining atomic pair distribution functions and Raman scattering data at ambient conditions. We demonstrate that the amplification of the piezoelectric properties of NBT-x BT at the MPB are predominantly driven by an easy switchability resulting from a progressive decoupling between strain and polarization as the Ba content increases from zero to the critical MPB composition. It was observed that as Ba content increases towards MPB, competing local correlations, such as A-site chemical order, antiferrodistortive correlations of correlated BO6 tilts, and antipolar Bi shifts, are reduced, which in turn renders favorable conditions for easy switching of local dipoles under external fields. In addition, the evolving characteristics of the atomic dynamics as a function of composition suggest that the local potential functions of the cations are not completely flat at the MPB. Altogether, our results reveal atomistic mechanisms responsible for the observed elevated MPB properties in the case of NBT-x BT which imply that the so-called MPB of NBT-x BT should not be categorized as originally introduced for Pb-containing solid solutions.

Effect of Surface Termination on the Electonic Properties of LaNiO₃ Films

DOE PAGES

Kumah, Divine P.; Malashevich, Andrei; Disa, Ankit S.; ...

2014-11-06

The electronic and structural properties of thin LaNiO₃ films grown by using molecular beam epitaxy are studied as a function of the net ionic charge of the surface terminating layer. We demonstrate that electronic transport in nickelate heterostructures can be manipulated through changes in the surface termination due to a strong coupling of the surface electrostatic properties to the structural properties of the Ni—O bonds that govern electronic conduction. We observe experimentally and from first-principles theory an asymmetric response of the structural properties of the films to the sign of the surface charge, which results from a strong interplay betweenmore » electrostatic and mechanical boundary conditions governing the system. The structural response results in ionic buckling in the near-surface NiO₂ planes for films terminated with negatively charged NiO₂ and bulklike NiO₂ planes for films terminated with positively charged LaO planes. The ability to modify transport properties by the deposition of a single atomic layer can be used as a guiding principle for nanoscale device fabrication.« less

Structure-function relations in the NTPase domain of the antiviral tRNA ribotoxin Escherichia coli PrrC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meineke, Birthe; Shuman, Stewart, E-mail: s-shuman@ski.mskcc.org

2012-06-05

Breakage of tRNA by Escherichia coli anticodon nuclease PrrC (EcoPrrC) underlies a host antiviral response to phage T4 infection. Expression of EcoPrrC is cytocidal in yeast, signifying that PrrC ribotoxicity crosses phylogenetic domain boundaries. EcoPrrC consists of an N-terminal NTPase module that resembles ABC transporters and a C-terminal nuclease module that is sui generis. PrrC homologs are prevalent in many other bacteria. Here we report that Haemophilus influenzae PrrC is toxic in E. coli and yeast. To illuminate structure-activity relations, we conducted a new round of mutational analysis of EcoPrrC guided by primary structure conservation among toxic PrrC homologs. Wemore » indentify 17 candidate active site residues in the NTPase module that are essential for toxicity in yeast when EcoPrrC is expressed at high gene dosage. Their functions could be educed by integrating mutational data with the atomic structure of the transition-state complex of a homologous ABC protein.« less

Optics & Opto-Electronic Systems

DTIC Science & Technology

1988-06-01

its reflection by the 13 cavity boundaries, and its reabsorption by the atom. Multimode corrections to the single-mode Jaynes - Cummings model are...walls. Transients in the Micromaser C. R. Stroud, Jr. The Jaynes - Cummings model of a single two-lev3l atom interacting with a single field mode of a...increasing laser intensity and to be as large as 22 bits/sec. A standard model of self- pumped phase conjugation due to four- wave mixing has been

Comments on the Parameters and Processes that Affect the Preservation Potential and Style of Oblique-Divergent Plate Boundaries

NASA Astrophysics Data System (ADS)

Umhoefer, P. J.

2014-12-01

Oblique-divergent or transtensional zones present particular challenges in ancient belts because of the poor preservation potential of the thinned continental crust and young oceanic crust. Many oblique belts will preferentially preserve their boundary zones that lie within continents rather than the main plate boundary zone, which will be at a much lower elevation and composed of denser crust. Zones of tectonic escape or strike-slip overprinting of arcs or plateaus deform continental crust and may be better preserved. Here I highlight parameters and processes that have major effects on oblique divergent belts. Strain partitioning is common, but not ubiquitous, along and across oblique boundaries; the causes of partitioning are not always clear and make this especially vexing for work in ancient belts. Partitioning causes complexity in the patterns of structures at all scales. Inherited structures commonly determine the orientation and style of structures along oblique boundaries and can control the pattern of faults across transtensional belts. Regionally, inherited trends of arcs or other 1000-km-scale features can control boundary structures. Experiments and natural examples suggest that oblique boundary zones contain less of a record of strike-slip faulting and more extensional structures. The obliquity of divergence produces predictable families of structures that typify (i) strike-slip dominated zones (obliquity <~20°), (ii) mixed zones (~20° - ~35°), and (iii) extension dominated zones (>~35°). The combination of partitioning and mixed structures in oblique zones means that the boundaries of belts with large-magnitude strike-slip faulting will commonly preserve little of no record of that faulting history. Plate boundaries localize strain onto the main plate boundary structures from the broader plate boundary and therefore the boundary zones commonly preserve the earlier structures more than later structures, a major problem in interpreting ancient belts. Sediment input is critical in some oblique plate boundaries because these belts become more pronounced sediment sinks over time. The evolving topography of oblique boundaries means that they have great variability of sediment flux into differing parts of the system; large rivers enter these belts only in special circumstances.

Electronic structure and bonding of intergranular glassy films in polycrystalline Si3 N4 : Ab initio studies and classical molecular dynamics simulations

NASA Astrophysics Data System (ADS)

Rulis, P.; Chen, J.; Ouyang, L.; Ching, W.-Y.; Su, X.; Garofalini, S. H.

2005-06-01

The electronic structure and bonding of a realistic model of an intergranular glassy film (IGF) was studied with multiple computational methods. The model has a Si-O-N glassy region sandwiched between crystalline basal planes of β-Si3N4 and contains a total of 798 atoms. It was constructed with periodic boundary conditions via classical molecular dynamics (MD) techniques using an accurate multibody atomic potential. The model was then further relaxed by the VASP (Vienna ab initio simulation package) program. It is shown that the VASP-relaxed structure reduces the total energy from the MD-relaxed structure by only 47.38eV , validating the accuracy of the multiatom potential used. The calculated electronic structure shows the IGF model to be an insulator with a sizable gap of almost 3eV . Quasidefectlike states can be identified near the band edges arising from the more strained Si-N and Si-O bonds at the interface. Calculation of the Mulliken effective charge and bond order values indicates that the bonds in the glassy region and at the interface can be enhanced and weakened by distortions in the bond length and bond angle. The states at the top of the valence band are derived mostly from the crystalline part of the Si-N bonding while the states at the bottom of the conduction band are dominated by the Si-O bonding in the glassy region. Calculation of the electrostatic potential across the interface shows an average band offset of about 1.5eV between the crystalline β-Si3N4 and the glassy Si-O-N region which could be related to the space charge model for IGF.

119Sn Mössbauer spectroscopy for assessing the local stress and defect state towards the tuning of Ni-Mn-Sn alloys

NASA Astrophysics Data System (ADS)

Unzueta, I.; López-García, J.; Sánchez-Alarcos, V.; Recarte, V.; Pérez-Landazábal, J. I.; Rodríguez-Velamazán, J. A.; Garitaonandia, J. S.; García, J. A.; Plazaola, F.

2017-05-01

The influence of defects and local stresses on the magnetic properties and martensitic transformation in Ni50Mn35Sn15 is studied at macroscopic and atomic scale levels. We show that both the structural and magnetic properties of the alloy are very sensitive to slight microstructural distortions. Even though no atomic disorder is induced by milling, the antiphase boundaries linked to dislocations promote the antiferromagnetic coupling of Mn, resulting in a significant decrease in the saturation magnetization. On the other hand, the temperature range of the transformation is considerably affected by the mechanically induced local stresses, which in turn does not affect the equilibrium temperature between the austenitic and martensitic phases. Finally, we demonstrate that the recovery of the martensitic transformation is directly related to the intensity of the non-magnetic component revealed by 119Sn Mössbauer spectroscopy. This result opens the possibility of quantifying the whole contribution of defects and the local stresses on the martensitic transformation in Ni-Mn-Sn alloys.

Periodic boundary conditions for QM/MM calculations: Ewald summation for extended Gaussian basis sets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Holden, Zachary C.; Richard, Ryan M.; Herbert, John M., E-mail: herbert@chemistry.ohio-state.edu

2013-12-28

An implementation of Ewald summation for use in mixed quantum mechanics/molecular mechanics (QM/MM) calculations is presented, which builds upon previous work by others that was limited to semi-empirical electronic structure for the QM region. Unlike previous work, our implementation describes the wave function's periodic images using “ChElPG” atomic charges, which are determined by fitting to the QM electrostatic potential evaluated on a real-space grid. This implementation is stable even for large Gaussian basis sets with diffuse exponents, and is thus appropriate when the QM region is described by a correlated wave function. Derivatives of the ChElPG charges with respect tomore » the QM density matrix are a potentially serious bottleneck in this approach, so we introduce a ChElPG algorithm based on atom-centered Lebedev grids. The ChElPG charges thus obtained exhibit good rotational invariance even for sparse grids, enabling significant cost savings. Detailed analysis of the optimal choice of user-selected Ewald parameters, as well as timing breakdowns, is presented.« less

Solitonic Excitations in Fermionic Superfluids and Progress towards Fermi Gas in Uniform Potential

NASA Astrophysics Data System (ADS)

Ku, Mark; Mukherjee, Biswaroop; Guardado-Sanchez, Elmer; Yan, Zhenjie; Patel, Parth; Yefsah, Tarik; Struck, Julian; Zwierlein, Martin

2015-05-01

We follow the evolution of a superfluid Fermi gas of 6Li atoms following a one-sided π phase imprint. Via tomographic imaging, we observe the formation of a planar dark soliton, and its subsequent snaking and decay into a vortex ring. The latter eventually breaks at the boundary of the superfluid, finally leaving behind a single, remnant solitonic vortex. The nodal surface is directly imaged and reveals its decay into a vortex ring via a puncture of the initial soliton plane. At intermediate stages we find evidence for more exotic structures resembling Φ-solitons. The observed evolution of the nodal surface represents dynamics that occurs at the length scale of the interparticle spacing, thus providing new experimental input for microscopic theories of strongly correlated fermions. We also report on the trapping of fermionic atoms of 6Li in a quasi-homogenous all-optical potential, and discuss progress towards directly observing the momentum distribution of the fermions in a box. This new tool offers the possibility to quantitatively study Fermi gases at finite temperature and in the presence of spin-imbalance, with unprecedented accuracy.

Phonon Transport at the Interfaces of Vertically Stacked Graphene and Hexagonal Boron Nitride Heterostructures

DOE PAGES

Yan, Zhequan; Chen, Liang; Yoon, Mina; ...

2016-01-12

Hexagonal boron nitride (h-BN) is a substrate for graphene based nano-electronic devices. We investigate the ballistic phonon transport at the interface of vertically stacked graphene and h-BN heterostructures using first principles density functional theory and atomistic Green's function simulations considering the influence of lattice stacking. We compute the frequency and wave-vector dependent transmission function and observe distinct stacking-dependent phonon transmission features for the h-BN/graphene/h-BN sandwiched systems. We find that the in-plane acoustic modes have the dominant contributions to the phonon transmission and thermal boundary conductance (TBC) for the interfaces with the carbon atom located directly on top of the boronmore » atom (C–B matched) because of low interfacial spacing. The low interfacial spacing is a consequence of the differences in the effective atomic volume of N and B and the difference in the local electron density around N and B. For the structures with the carbon atom directly on top of the nitrogen atom (C–N matched), the spatial distance increases and the contribution of in-plane modes to the TBC decreases leading to higher contributions by out-of-plane acoustic modes. We find that the C–B matched interfaces have stronger phonon–phonon coupling than the C–N matched interfaces, which results in significantly higher TBC (more than 50%) in the C–B matched interface. The findings in this study will provide insights to understand the mechanism of phonon transport at h-BN/graphene/h-BN interfaces, to better explain the experimental observations and to engineer these interfaces to enhance heat dissipation in graphene based electronic devices.« less

Related Structure Characters and Stability of Structural Defects in a Metallic Glass

PubMed Central

Niu, Xiaofeng; Feng, Shidong; Pan, Shaopeng

2018-01-01

Structural defects were investigated by a recently proposed structural parameter, quasi-nearest atom (QNA), in a modeled Zr50Cu50 metallic glass through molecular dynamics simulations. More QNAs around an atom usually means that more defects are located near the atom. Structural analysis reveals that the spatial distribution of the numbers of QNAs displays to be clearly heterogeneous. Furthermore, QNA is closely correlated with cluster connections, especially four-atom cluster connections. Atoms with larger coordination numbers usually have less QNAs. When two atoms have the same coordination number, the atom with larger five-fold symmetry has less QNAs. The number of QNAs around an atom changes rather frequently and the change of QNAs might be correlated with the fast relaxation metallic glasses. PMID:29565298

Diagnosing Students’ conception on atomic structure using open ended questions

NASA Astrophysics Data System (ADS)

Fitriza, Z.; Gazali, F.

2018-05-01

This study aims to diagnose students’ conception on atomic structure concepts using open ended questions. For this reason, a 7 items of assay test was administered to 135 senior high school students from different schools in West Sumatera. The data were collected using a an open ended test which is covering the concept used in the topic Atomic Structure. The open ended test of students’ conceptual was developed to identify the alternative conceptions that student might have regarding the concepts in Atomic Structure, to measure the level of students’ conceptions, and the way of students’ thinking concerning the concepts. The results showed that students find difficulties about some concepts of Atomic structure such as atom, atomic model, electron configuration, period and group.The result of this study illuminated the concepts to be underlined in developing teaching and learning approach concerning the topic of Atomic Structure.

The effect of Mn and B on the magnetic and structural properties of nanostructured Fe60Al40 alloys produced by mechanical alloying.

PubMed

Rico, M M; Alcázar, G A Pérez; Zamora, L E; González, C; Greneche, J M

2008-06-01

The effect of Mn and B on the magnetic and structural properties of nanostructured samples of the Fe60Al40 system, prepared by mechanical alloying, was studied by 57Fe Mössbauer spectrometry, X-ray diffraction and magnetic measurements. In the case of the Fe(60-x)Mn(x)Al40 system, 24 h milling time is required to achieve the BCC ternary phase. Different magnetic structures are observed according to the temperature and the Mn content for alloys milled during 48 h: ferromagnetic, antiferromagnetic, spin-glass, reentrant spin-glass and superparamagnetic behavior. They result from the bond randomness behaviour induced by the atomic disorder introduced by the MA process and from the competitive interactions of the Fe-Fe ferromagnetic interactions and the Mn-Mn and Fe-Mn antiferromagnetic interactions and finally the presence of Al atoms acting as dilutors. When B is added in the Fe60Al40 alloy and milled for 12 and 24 hours, two crystalline phases were found: a prevailing FeAl BCC phase and a Fe2B phase type. In addition, one observes an additional contribution attributed to grain boundaries which increases when both milling time and boron composition increase. Finally Mn and B were added to samples of the Fe60Al40 system prepared by mechanical alloying during 12 and 24 hours. Mn content was fixed to 10 at.% and B content varied between 0 and 20 at.%, substituting Al. X-ray patterns show two crystalline phases, the ternary FeMnAl BCC phase, and a (Fe,Mn)2B phase type. The relative proportion of the last phase increases when the B content increases, in addition to changes of the grain size and the lattice parameter. Such behavior was observed for both milling periods. On the other hand, the magnetic hyperfine field distributions show that both phases exhibit chemical disorder, and that the contribution attributed to the grain boundaries is less important when the B content increases. Coercive field values of about 10(2) Oe slightly increase with boron content. Comparison with previous results on FeAIB alloys shows that Mn promotes the structural stability of the nanostructured powders.