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Sample records for phase formation mechanism

  1. Amorphous phase formation in mechanically alloyed iron-based systems

    NASA Astrophysics Data System (ADS)

    Sharma, Satyajeet

    Bulk metallic glasses have interesting combination of physical, chemical, mechanical, and magnetic properties which make them attractive for a variety of applications. Consequently there has been a lot of interest in understanding the structure and properties of these materials. More varied applications can be sought if one understands the reasons for glass formation and the methods to control them. The glass-forming ability (GFA) of alloys can be substantially increased by a proper selection of alloying elements and the chemical composition of the alloy. High GFA will enable in obtaining large section thickness of amorphous alloys. Ability to produce glassy alloys in larger section thicknesses enables exploitation of these advanced materials for a variety of different applications. The technique of mechanical alloying (MA) is a powerful non-equilibrium processing technique and is known to produce glassy (or amorphous) alloys in several alloy systems. Metallic amorphous alloys have been produced by MA starting from either blended elemental metal powders or pre-alloyed powders. Subsequently, these amorphous alloy powders could be consolidated to full density in the temperature range between the glass transition and crystallization temperatures, where the amorphous phase has a very low viscosity. This Dissertation focuses on identifying the various Fe-based multicomponent alloy systems that can be amorphized using the MA technique, studying the GFA of alloys with emphasis on improving it, and also on analyzing the effect of extended milling time on the constitution of the amorphous alloy powder produced at earlier times. The Dissertation contains seven chapters, where the lead chapter deals with the background, history and introduction to bulk metallic glasses. The following four chapters are the published/to be published work, where the criterion for predicting glass formation, effect of Niobium addition on glass-forming ability (GFA), lattice contraction on

  2. Mechanism of Phase Formation in the Batch Mixtures for Slag-Bearing Glass Ceramics - 12207

    SciTech Connect

    Stefanovsky, Sergey V.; Stefanovsky, Olga I.; Malinina, Galina A.

    2012-07-01

    Slag surrogate was produced from chemicals by heating to 900 deg. C and keeping at this temperature for 1 hr. The product obtained was intermixed with either sodium di-silicate (75 wt.% waste loading) or borax (85 wt.% slag loading). The mixtures were heat-treated within a temperature range of 25 to 1300 deg. C. The products were examined by X-ray diffraction and infrared spectroscopy. The products prepared at temperatures of up to 1000 deg. C contained both phase typical of the source slag and intermediate phases as well as phases typical of the materials melted at 1350 deg. C such as nepheline, britholite, magnetite and matrix vitreous phase. Vitrification process in batch mixtures consisting of slag surrogate and either sodium di-silicate or sodium tetraborate runs through formation of intermediate phases mainly silico-phosphates capable to incorporate Sm as trivalent actinides surrogate. Reactions in the batch mixtures are in the whole completed by ∼1000 deg. C but higher temperatures are required to homogenize the products. If in the borate-based system the mechanism is close to simple dissolution of slag constituents in the low viscous borate melt, then in the silicate-based system the mechanism was found to be much complicated and includes re-crystallization during melting with segregation of newly-formed nepheline type phase. (authors)

  3. Phenomenological thermodynamics and the structure formation mechanism of the CuTi₂S₄ rhombohedral phase.

    PubMed

    Talanov, Michail V; Shirokov, Vladimir B; Talanov, Valery M

    2016-04-21

    The theory of structural phase transition in CuTi2S4 is proposed. The symmetry of order parameters, thermodynamics and the mechanism of the atomic structure formation of the rhombohedral Cu-Ti-thiospinel have been studied. The critical order parameter inducing the phase transition has been found. Within the Landau theory of phase transitions, it is shown that the phase state may change from the high-symmetry cubic disordered Fd3[combining macron]m phase to the low-symmetry ordered rhombohedral R3[combining macron]m phase as a result of phase transition of the first order close to the second order. It is shown that the rhombohedral structure of CuTi2S4 is formed as a result of the displacements of all types of atoms and the ordering of Cu-atoms (1 : 1 order type in tetrahedral spinel sites), Ti-atoms (1 : 1 : 6 order type in octahedral spinel sites), and S-atoms (1 : 1 : 3 : 3 order type). The Cu- and Ti-atoms form metal nanoclusters which are named a "bunch" of dimers. The "bunch" of dimers in CuTi2S4 is a new type of self-organization of atoms in frustrated spinel-like structures. It is shown that Ti-atoms also form other types of metal nanoclusters: trimers and tetrahedra. PMID:27035866

  4. Phenomenological thermodynamics and the structure formation mechanism of the CuTi₂S₄ rhombohedral phase.

    PubMed

    Talanov, Michail V; Shirokov, Vladimir B; Talanov, Valery M

    2016-04-21

    The theory of structural phase transition in CuTi2S4 is proposed. The symmetry of order parameters, thermodynamics and the mechanism of the atomic structure formation of the rhombohedral Cu-Ti-thiospinel have been studied. The critical order parameter inducing the phase transition has been found. Within the Landau theory of phase transitions, it is shown that the phase state may change from the high-symmetry cubic disordered Fd3[combining macron]m phase to the low-symmetry ordered rhombohedral R3[combining macron]m phase as a result of phase transition of the first order close to the second order. It is shown that the rhombohedral structure of CuTi2S4 is formed as a result of the displacements of all types of atoms and the ordering of Cu-atoms (1 : 1 order type in tetrahedral spinel sites), Ti-atoms (1 : 1 : 6 order type in octahedral spinel sites), and S-atoms (1 : 1 : 3 : 3 order type). The Cu- and Ti-atoms form metal nanoclusters which are named a "bunch" of dimers. The "bunch" of dimers in CuTi2S4 is a new type of self-organization of atoms in frustrated spinel-like structures. It is shown that Ti-atoms also form other types of metal nanoclusters: trimers and tetrahedra.

  5. A new approach for mechanisms of ferroelectric crystalline phase formation in PVDF nanocomposites.

    PubMed

    Abolhasani, Mohammad Mahdi; Naebe, Minoo; Guo, Qipeng

    2014-06-14

    This paper proposes a new mechanism for ferroelectric polymorph formation of poly(vinylidene fluoride) (PVDF) nanocomposites. Utilizing time-resolved Fourier transform infrared spectroscopy (FTIR), the real-time investigation of the conformational changes of the PVDF chain segment during crystallization of neat PVDF and the corresponding nanocomposite was performed. Whilst PVDF-clay nanocomposites exhibited mainly the β crystal phase coexisting with the γ phase at low Tc (Tc < 155 °C), the coexistence of γ and β crystalline phases was found at a high Tc temperature range (Tc > 155 °C). Experimental results were compared with predictions of the Lauritzen and Hoffman (LH) model and discrepancies were observed between model predictions and experiments. We then recalled the Brochard-de Gennes (BD) model and proposed that different crystalline polymorph formation should be inferred as a transition in the reeling-in rate dependence of the friction coefficient on nanocomposites rather than as a change in the relative rates of secondary nucleation and substrate completion. Combining LH and BD models we proposed a new mechanism to answer the contradictory questions associated with nanocomposite polymorphism. The coexistence of different polymorphs in nanocomposites was proposed to be associated with the coexistence of fast and slow moving chains, which were recognized as the free and adsorbed chains by nanofillers. PMID:24756089

  6. Formation mechanism of gas bubble superlattice in UMo metal fuels: Phase-field modeling investigation

    NASA Astrophysics Data System (ADS)

    Hu, Shenyang; Burkes, Douglas E.; Lavender, Curt A.; Senor, David J.; Setyawan, Wahyu; Xu, Zhijie

    2016-10-01

    Nano-gas bubble superlattices are often observed in irradiated UMo nuclear fuels. However, the formation mechanism of gas bubble superlattices is not well understood. A number of physical processes may affect the gas bubble nucleation and growth; hence, the morphology of gas bubble microstructures including size and spatial distributions. In this work, a phase-field model integrating a first-passage Monte Carlo method to investigate the formation mechanism of gas bubble superlattices was developed. Six physical processes are taken into account in the model: 1) heterogeneous generation of gas atoms, vacancies, and interstitials informed from atomistic simulations; 2) one-dimensional (1-D) migration of interstitials; 3) irradiation-induced dissolution of gas atoms; 4) recombination between vacancies and interstitials; 5) elastic interaction; and 6) heterogeneous nucleation of gas bubbles. We found that the elastic interaction doesn't cause the gas bubble alignment, and fast 1-D migration of interstitials along <110> directions in the body-centered cubic U matrix causes the gas bubble alignment along <110> directions. It implies that 1-D interstitial migration along [110] direction should be the primary mechanism of a fcc gas bubble superlattice which is observed in bcc UMo alloys. Simulations also show that fission rates, saturated gas concentration, and elastic interaction all affect the morphology of gas bubble microstructures.

  7. Mechanisms for the formation of secondary organic aerosol components from the gas-phase ozonolysis of alpha-pinene.

    PubMed

    Ma, Yan; Russell, Andrew T; Marston, George

    2008-08-01

    Gas-phase ozonolysis of alpha-pinene was studied in static chamber experiments under 'OH-free' conditions. A range of multifunctional products-in particular low-volatility carboxylic acids-were identified in the condensed phase using gas chromatography coupled to mass spectrometry after derivatisation. The dependence of product yields on reaction conditions (humidity, choice of OH radical scavengers, added Criegee intermediate scavengers, NO(2)etc.) was investigated to probe the mechanisms of formation of these products; additional information was obtained by studying the ozonolysis of an enal and an enone derived from alpha-pinene. On the basis of experimental findings, previously suggested mechanisms were evaluated and detailed gas-phase mechanisms were developed to explain the observed product formation. Atmospheric implications of this work are discussed.

  8. Creation and formation mechanism of new carbon phases constructed by amorphous carbon

    NASA Astrophysics Data System (ADS)

    Yao, Mingguang; Cui, Wen; Liu, Bingbing

    Our recent effort is focusing on the creation of new hard/superhard carbon phases constructed by disordered carbons or amorphous carbon clusters under high pressure. We showed that the pressure-induced amorphous hard carbon clusters from collapsed fullerenes can be used as building blocks (BBs) for constructing novel carbon structures. This new strategy has been verified by compressing a series of intercalated fullerides, pre-designed by selecting various dopants with special features. We demonstrate that the boundaries of the amorphous BBs are mediated by intercalated dopants and several new superhard materials have been prepared. We also found that the dopant-mediated BBs can be arranged in either ordered or disordered structures, both of which can be hard enough to indent the diamond anvils. The hardening mechanisms of the new phases have also been discussed. For the glassy carbon (GC) constructructed by disordered fullerene-like nanosized fragments, we also found that these disordered fragments can bond and the compressed GC transformed into a transparent superhard phase. Such pressure-induced transformation has been discovered to be driven by a novel mechanism (unpublished). By understanding the mechanisms we can clarify the controversial results on glassy carbon reported recently. The authors would like to thank the financial support from the National Natural Science Foundation of China (No. 11474121, 51320105007).

  9. Identification of Abnormal Phase and its Formation Mechanism in Synthesizing Chalcogenide Films

    NASA Astrophysics Data System (ADS)

    Liu, Kegao; Ji, Nianjing; Xu, Yong; Liu, Hong

    2016-09-01

    Chalcogenide films can be used in thin-film solar cells due to their high photoelectric conversion efficiencies. It was difficult to identify one abnormal phase with high X-ray diffraction (XRD) intensity and preferred orientation in the samples for preparing chalcogenide films by spin-coating and co-reduction on soda-lime glass (Na2OṡCaOṡ6SiO2) substrates. The raw materials and reductant are metal chlorides and hydrazine hydrate respectively. In order to identify this phase, a series of experiments were done under different conditions. The phases of obtained products were analyzed by XRD and the size and morphology were characterized by scanning electron microscope (SEM) and atomic force microscopy (AFM). From the experimental results, first it was proved that the abnormal phase was water-soluble by water immersion experiment, then it was identified as NaCl crystal through XRD, energy dispersive spectrometer (EDS) and SEM. The cubic NaCl crystals have high crystallinity with size lengths of about 0.5-2μm and show a <100> preferred orientation. The reaction mechanism of NaCl crystal was proposed as follows: The NaCl crystal was formed by reaction of Na2O and HCl in a certain experimental conditions.

  10. Study on the Formation and Precipitation Mechanism of Mn5Si3 Phase in the MBA-2 Brass Alloy

    NASA Astrophysics Data System (ADS)

    Li, Hang; Jie, Jinchuan; Zhang, Pengchao; Jia, Chunxu; Wang, Tongmin; Li, Tingju

    2016-06-01

    Mn5Si3 is an attractive dispersion in the special brass, owing to its high hardness and high wear resistance. In the present study, synchrotron X-ray radiography and rapid cooling were applied to investigate the formation mechanism of Mn5Si3 phase in the MBA-2 brass alloy. The primary Mn5Si3 phase is proved to exist stably in the alloy melt and nucleate from the melt at temperatures above 1373 K (1100 °C). In addition, the precipitation mechanism of Mn5Si3 phase is addressed systematically by the isothermal heat treatment. The Mn5Si3 particles are observed to precipitate from the matrix at temperatures above 1023 K (750 °C), and a crystallographic orientation relationship is found between the precipitated Mn5Si3 particle and β phase: (110)_{β } //(1overline{1} 00)_{{{{Mn}}5 {{Si}}3 }} and [overline{1} 11]_{β } //[11overline{2} overline{2} ]_{{{{Mn}}5 {{Si}}3 }} . However, the precipitation of Mn5Si3 phase is thermodynamically inhibited at lower temperatures, which can be ascribed to the increase in the Gibbs free energy of formation of Mn5Si3 with decreasing the temperature.

  11. Synthesis and formation mechanism of pinnoite by the phase transition process

    NASA Astrophysics Data System (ADS)

    Lin, Feng; Dong, Yaping; Peng, Jiaoyu; Wang, Liping; Li, Wu

    2016-06-01

    Pinnoite (MgB2O(OH)6) for the first time was synthesized using the solid-liquid-solid conversion method. The effects of reaction time, pH value and concentrations of magnesium and borate were investigated. Pinnoite was synthesized under the optimum condition of 8 mmol hungtsaoite and 1% boric acid solution at 80 °C. The products were determined using X-ray diffraction, Fourier-transform infrared spectroscopy, TG-DSC and a UV-vis spectrometer. The change processes of the surface morphology of pinnoite were investigated using scanning electron microscopy. In addition, the formation mechanism of pinnoite was discussed according to the changes in the content of precipitation and pH value.

  12. Colloidal nanocrystals of orthorhombic Cu2ZnGeS4: phase-controlled synthesis, formation mechanism and photocatalytic behavior

    NASA Astrophysics Data System (ADS)

    Fan, Cong-Min; Regulacio, Michelle D.; Ye, Chen; Lim, Suo Hon; Lua, Shun Kuang; Xu, Qing-Hua; Dong, Zhili; Xu, An-Wu; Han, Ming-Yong

    2015-02-01

    The orthorhombic polymorph of Cu2ZnGeS4 (CZGS) is a metastable wurtzite-derived phase that can only be prepared in the bulk form by extensive heating at high temperatures (>=790 °C) when using the conventional solid-state reaction route. By employing a facile solution-based synthetic strategy, we were able to obtain phase-pure orthorhombic CZGS in nanocrystalline form at a much lower reaction temperature. Prior to this work, the colloidal synthesis of single-phase orthorhombic CZGS on the nanoscale has never been reported. We find that the use of an appropriate combination of coordinating solvents and precursors is crucial to the sole formation of this metastable phase in solution. A possible formation mechanism is proposed on the basis of our experimental results. Because CZGS consists of environmentally benign metal components, it is regarded as a promising alternative material to the technologically useful yet toxic cadmium-containing semiconductors. The orthorhombic CZGS nanocrystals display strong photon absorption in the visible spectrum and are photocatalytically active in dye degradation under visible-light illumination.The orthorhombic polymorph of Cu2ZnGeS4 (CZGS) is a metastable wurtzite-derived phase that can only be prepared in the bulk form by extensive heating at high temperatures (>=790 °C) when using the conventional solid-state reaction route. By employing a facile solution-based synthetic strategy, we were able to obtain phase-pure orthorhombic CZGS in nanocrystalline form at a much lower reaction temperature. Prior to this work, the colloidal synthesis of single-phase orthorhombic CZGS on the nanoscale has never been reported. We find that the use of an appropriate combination of coordinating solvents and precursors is crucial to the sole formation of this metastable phase in solution. A possible formation mechanism is proposed on the basis of our experimental results. Because CZGS consists of environmentally benign metal components, it is

  13. Colloidal nanocrystals of orthorhombic Cu2ZnGeS4: phase-controlled synthesis, formation mechanism and photocatalytic behavior.

    PubMed

    Fan, Cong-Min; Regulacio, Michelle D; Ye, Chen; Lim, Suo Hon; Lua, Shun Kuang; Xu, Qing-Hua; Dong, Zhili; Xu, An-Wu; Han, Ming-Yong

    2015-02-21

    The orthorhombic polymorph of Cu2ZnGeS4 (CZGS) is a metastable wurtzite-derived phase that can only be prepared in the bulk form by extensive heating at high temperatures (≥790 °C) when using the conventional solid-state reaction route. By employing a facile solution-based synthetic strategy, we were able to obtain phase-pure orthorhombic CZGS in nanocrystalline form at a much lower reaction temperature. Prior to this work, the colloidal synthesis of single-phase orthorhombic CZGS on the nanoscale has never been reported. We find that the use of an appropriate combination of coordinating solvents and precursors is crucial to the sole formation of this metastable phase in solution. A possible formation mechanism is proposed on the basis of our experimental results. Because CZGS consists of environmentally benign metal components, it is regarded as a promising alternative material to the technologically useful yet toxic cadmium-containing semiconductors. The orthorhombic CZGS nanocrystals display strong photon absorption in the visible spectrum and are photocatalytically active in dye degradation under visible-light illumination.

  14. MECHANISMS OF PHASE FORMATION IN THE VITRIFICATION OF HIGH-FERROUS SAVANNAH RIVER SITE SB2 HLW SLUDGE SURROGATE - 9300

    SciTech Connect

    Marra, J

    2008-08-27

    Phase formation mechanisms associated with the vitrification of high-ferrous Savannah River Site (SRS) Sludge Batch 2 (SB2) high level waste surrogate were studied by infrared spectroscopy (IRS) and X-ray diffraction (XRD). Two mixtures at 50 wt% waste loading with commercially available Frit 320 (Li{sub 2}O - 8 wt %, B{sub 2}O{sub 3} - 8 wt %, Na{sub 2}O - 12 wt %, SiO{sub 2} - 72 wt %) and batch chemicals (LiOH {center_dot} H{sub 2}O, H{sub 3}BO{sub 3}, NaNO{sub 3}, SiO{sub 2}) to represent the frit formulation were prepared as slurries with a water content of {approx}50 wt%. The mixtures were air-dried at a temperature of 115 C and heat-treated at 500, 700, 900, 1000, 1100, 1200, and 1300 C for 1 hr at each temperature. Infrared spectra and XRD patterns of the products produced at each temperature were recorded. In both mixtures prepared using frit and batch chemicals to represent the frit, phase formation reactions were completed within the temperature range between 900 and 1000 C. However, residual quartz was still present in glass produced from the mixture with batch chemicals even at 1100 C. Although, the phase composition and structure of the glassy products obtained from both mixtures at temperatures over 1000 C were similar, the products obtained from the mixture using actual frit were more homogeneous than those from the mixture with batch chemicals. Thus, the use of frit rather than batch chemicals reduced the temperature range of phase formation and provided for production of higher quality glass.

  15. Formation mechanism of primary phases and eutectic structures within undercooled Pb-Sb-Sn ternary alloys

    NASA Astrophysics Data System (ADS)

    Wang, Weili; Dai, Fuping; Wei, Bingbo

    2007-08-01

    The solidification characteristics of three types of Pb-Sb-Sn ternary alloys with different primary phases were studied under substantial undercooling conditions. The experimental results show that primary (Pb) and SbSn phases grow in the dendritic mode, whereas primary (Sb) phase exhibits faceted growth in the form of polygonal blocks and long strips. (Pb) solid solution phase displays strong affinity with SbSn intermetallic compound so that they produce various morphologies of pseudobinary eutectics, but it can only grow in the divorced eutectic mode together with (Sb) phase. Although (Sb) solid solution phase and SbSn intermetallic compound may grow cooperatively within ternary eutectic microstructures, they seldom form pseudobinary eutectics independently. The (Pb)+(Sb)+SbSn ternary eutectic structure usually shows lamellar morphology, but appears as anomalous eutectic when its volume fraction becomes small. EDS analyses reveal that all of the three primary (Pb), (Sb) and SbSn phases exhibit conspicuous solute trapping effect during rapid solidification, which results in the remarkable extension of solute solubility.

  16. A solid-phase mechanism of shock-wave formation of dust particles of heavy metals

    NASA Astrophysics Data System (ADS)

    Lin, E. E.; Mikhailov, A. L.; Khvorostin, V. N.

    2016-08-01

    The possibility of formation of dust particles in solid as a result of shock-wave destruction of the initial crystalline material structure and subsequent coalescence of atomic clusters (nanoparticles), which leads to the aggregation of mesocrystalline particles (grains) in the shocked layer, is discussed.

  17. Design and characterization of a smog chamber for studying gas-phase chemical mechanisms and aerosol formation

    NASA Astrophysics Data System (ADS)

    Wang, X.; Liu, T.; Bernard, F.; Ding, X.; Wen, S.; Zhang, Y.; Zhang, Z.; He, Q.; Lü, S.; Chen, J.; Saunders, S.; Yu, J.

    2014-01-01

    We describe here characterization of a new state-of-the-art smog chamber facility for studying atmospheric gas-phase and aerosol chemistry. The chamber consists of a 30 m3 fluorinated ethylene propylene (FEP) Teflon film reactor housed in a temperature-controlled enclosure equipped with black lamps as the light source. Temperature can be set in the range from -10 to 40 °C at accuracy of ±1 °C as measured by eight temperature sensors inside the enclosure and one just inside the reactor. Matrix air can be purified with non-methane hydrocarbons (NMHCs) < 0.5 ppb, NOx/O3/carbonyls < 1 ppb and particles < 1 cm-3. The photolysis rate of NO2 is adjustable between 0 and 0.49 min-1. At 298 K under dry conditions, the average wall loss rates of NO, NO2 and O3 were measured to be 1.41 × 10-4 min-1, 1.39 × 10-4 min-1 and 1.31 × 10-4 min-1, respectively, and the particle number wall loss rate was measured to be 0.17 h-1. Auxiliary mechanisms of this chamber are determined and included in the Master Chemical Mechanism to evaluate and model propene-NOx-air irradiation experiments. The results indicate that this new smog chamber can provide high-quality data for mechanism evaluation. Results of α-pinene dark ozonolysis experiments revealed secondary organic aerosol (SOA) yields comparable to those from other chamber studies, and the two-product model gives a good fit for the yield data obtained in this work. Characterization experiments demonstrate that our Guangzhou Institute of Geochemistry, Chinese Academy Sciences (GIG-CAS), smog chamber facility can be used to provide valuable data for gas-phase chemistry and secondary aerosol formation.

  18. Diagenetic Microcrystalline Opal Varieties from the Monterey Formation, CA: HRTEM Study of Structures and Phase Transformation Mechanisms

    NASA Technical Reports Server (NTRS)

    Cady, Sherry L.; Wenk, H.-R.; DeVincenzi, Don (Technical Monitor)

    1994-01-01

    Microcrystalline opal varieties form as intermediary precipitates during the diagenetic transformation of biogenically precipitated non-crystalline opal (opal-A) to microquartz. With regard to the Monterey Formation of California, X-ray powder diffraction studies have shown that a decrease in the primary d-spacing of opal-CT toward that of cristobalite occurs with increasing diagenesis. The initial timing of opal-CT/quartz formation and the value of the primary opal-CT d-spacing, are influenced by the sediment. lithology. Transmission electron microscopy methods (CTEM/HRTEM) were used to investigate the structure of the diagenetic phases and establish transformation mechanisms between the varieties of microcrystalline opals in charts and porcelanites from the Monterey Formation. HRTEM images revealed that the most common fibrous varieties of microcrystalline opals contain varying amounts of structural disorder. Finite lamellar units of cristobalite-and tridymite-type. layer sequences were found to be randomly stacked in a direction perpendicular to the fiber axis. Disordered and ordered fibers were found to have coprecipitated within the same radial fiber bundles that formed within the matrix of the Most siliceous samples. HRTEM images, which reveal that the fibers within radial and lepispheric fiber bundles branch non-crystallographically, support an earlier proposal that microspheres in chert grow via a spherulitic growth mechanism. A less common variety of opal-CT was found to be characterized by non-parallel (low-angle) stacking sequences that often contain twinned lamellae. Tabular-shaped crystals of orthorhombic tridymite (PO-2) were also identified in the porcelanite samples. A shift in the primary d-spacing of opal-CT has been interpreted as an indication of solid-state ordering g toward a predominantly cristobalite structure, (opal-C). Domains of opal-C were identified as topotactically-oriented overgrowths on discrete Sections of opal-CT fibers and as

  19. Control of Y₂O₃ phase and its nanostructure formation through a very high energy mechanical milling

    SciTech Connect

    Lee, M.K.; Park, E.K.; Park, J.J.; Rhee, C.K.

    2013-05-01

    The formation behavior of Y₂O₃ ceramic particles was studied by employing a very high energy ball milling (milling energy: ~165 kJ/g·hit, milling speed: 1000 rpm). Both the XRD and HRTEM studies revealed that the high impact strain energy generated during the milling caused a drastic phase transition from the original C-type cubic (space group Ia3, a=10.58 Å) to the metastable B-type monoclinic (space group C2/m, a=13.89 Å), finally followed by a partial solid-state amorphization. The cubic phase was difficult to be reduced down to smaller than 10 nm, while the monoclinic phase was stabilized at sizes smaller than 10 nm with a mean crystallite size of 7.57 nm. Consequently, the existence of Y₂O₃ at a nanoscale smaller than 10 nm is possible by forming metastable monoclinic crystals, which are strain-induced. - Graphical abstract: The fig shows the solid-state phase formation of Y₂O₃ by very high energy input into the particles during milling: ordered body-centered cubic phase (space group Ia3, a=10.58 Å) nanocrystalline monoclinic phase (space group C2/m, a=13.89 Å) disordered monoclinic phase partial amorphous phase. The formation of Y₂O₃ smaller than 10 nm was strongly dependent on whether the phase transition from cubic to monoclinic occurred. Highlights: • This paper analyses very high energy milling behavior of coarse Y₂O₃ particles. • A drastic phase transition from cubic to monoclinic occurred with a partial amorphization. • An existence of Y₂O₃ smaller than 10 nm is possible by forming strain-induced monoclinic crystals.

  20. Release of gas-phase halogens by photolytic generation of OH in frozen halide-nitrate solutions: an active halogen formation mechanism?

    PubMed

    Abbatt, J; Oldridge, N; Symington, A; Chukalovskiy, V; McWhinney, R D; Sjostedt, S; Cox, R A

    2010-06-17

    To better define the mechanisms by which condensed-phase halides may be oxidized to form gas-phase halogens under polar conditions, experiments have been conducted whereby frozen solutions containing chloride (1 M), bromide (1.6 x 10(-3) to 5 x 10(-2) M), iodide (<1 x 10(-5) M), and nitrate (0.01 to 1 M) have been illuminated by ultraviolet light in a continually flushed cell. Gas-phase products are quantified using chemical ionization mass spectrometry, and experiments were conducted at both 248 and 263 K. Br(2) was the dominant product, along with smaller yields of IBr and trace BrCl and I(2). The Br(2) yields were largely independent of the Br(-)/Cl(-) ratio of the frozen solution, down to seawater composition. However, the yields of halogens were strongly dependent on the levels of NO(3)(-) and acidity in solution, consistent with a mechanism whereby NO(3)(-) photolysis yields OH that oxidizes the condensed-phase halides. In support, we observed the formation of gas-phase NO(2), formed simultaneously with OH. Gas-phase HONO was also observed, suggesting that halide oxidation by HONO in the condensed phase may also occur to some degree. By measuring the production rate of condensed-phase OH, using benzoic acid as a radical trap, we determine that the molar yield of Br(2) formation relative to OH generation is 0.6, consistent with each OH being involved in halide oxidation. These studies suggest that gas-phase halogen formation should occur simultaneously with NO(x) release from frozen sea ice and snow surfaces that contain sufficient halides and deposited nitrate.

  1. Isoprene and terpene gas-phase mechanisms and their effect on ozone formation over the regional scale

    SciTech Connect

    Stockwell, W.R.; Kuhn, M.; Seefeld, S.; Kirchner, F.

    1997-12-31

    Ozone is produced through the photo-oxidation of nitrogen oxides and volatile organic compounds. Biogenic emissions are an important source of reactive organic compounds such as isoprene and terpenes. Their reactions contribute to the production of ozone and aerosol particles. The photo-oxidation of isoprene and terpene affect the atmosphere`s nitrogen budget through the formation of nitrates and peroxyacetyl nitrates (PAN). Biogenic compounds also affect hydroperoxide formation rates. The authors have developed new oxidation mechanisms for isoprene, a-pinene and d-limonene based upon recent laboratory results. However, many unknowns remain in the experimental data and it was necessary to complete the a-pinene and d-limonene mechanism by using reactions that were analogous to known reactions for alkenes of lower carbon number. The new mechanism for isoprene, a-pinene and d-limonene was successfully tested against smog chamber runs. The authors will present the new biogenic mechanisms, compare simulations with environmental chamber runs and show results of simulations for typical urban, rural and remote conditions. They will show evaluations of the influence of biogenic emissions on the concentrations of ozone, nitrates, hydroperoxides and peroxynitrates over the regional scale.

  2. Formation mechanism of superconducting phase and its three-dimensional architecture in pseudo-single-crystal KxFe2-ySe2

    DOE PAGESBeta

    Liu, Yong; Xing, Qingfeng; Straszheim, Warren E.; Marshman, Jeff; Pedersen, Pal; McLaughlin, Richard; Lograsso, Thomas A.

    2016-02-11

    Here, we report how the superconducting phase forms in pseudo-single-crystal KxFe2-ySe2. In situ scanning electron microscopy (SEM) observation reveals that, as an order-disorder transition occurs, on cooling, most of the high-temperature iron-vacancy-disordered phase gradually changes into the iron-vacancy-ordered phase whereas a small quantity of the high-temperature phase retains its structure and aggregates to the stripes with more iron concentration but less potassium concentration compared to the iron-vacancy-ordered phase. The stripes that are generally recognized as the superconducting phase are actually formed as a remnant of the high-temperature phase with a compositional change after an “imperfect” order-disorder transition. It should bemore » emphasized that the phase separation in pseudo-single-crystal KxFe2-ySe2 is caused by the iron-vacancy order-disorder transition. The shrinkage of the high-temperature phase and the expansion of the newly created iron-vacancy-ordered phase during the phase separation rule out the mechanism of spinodal decomposition proposed in an early report [Wang et al, Phys. Rev. B 91, 064513 (2015)]. Since the formation of the superconducting phase relies on the occurrence of the iron-vacancy order-disorder transition, it is impossible to synthesize a pure superconducting phase by a conventional solid state reaction or melt growth. By focused ion beam-scanning electron microscopy, we further demonstrate that the superconducting phase forms a contiguous three-dimensional architecture composed of parallelepipeds that have a coherent orientation relationship with the iron-vacancy-ordered phase.« less

  3. Formation mechanism of superconducting phase and its three-dimensional architecture in pseudo-single-crystal KxF e2 -yS e2

    NASA Astrophysics Data System (ADS)

    Liu, Yong; Xing, Qingfeng; Straszheim, Warren E.; Marshman, Jeff; Pedersen, Pal; McLaughlin, Richard; Lograsso, Thomas A.

    2016-02-01

    We report how the superconducting phase forms in pseudo-single-crystal KxF e2 -yS e2 . In situ scanning electron microscopy (SEM) observation reveals that, as an order-disorder transition occurs, on cooling, most of the high-temperature iron-vacancy-disordered phase gradually changes into the iron-vacancy-ordered phase, whereas a small quantity of the high-temperature phase retains its structure and aggregates to the stripes with more iron concentration but less potassium concentration compared to the iron-vacancy-ordered phase. The stripes that are generally recognized as the superconducting phase are actually formed as a remnant of the high-temperature phase with a compositional change after an "imperfect" order-disorder transition. It should be emphasized that the phase separation in pseudo-single-crystal KxF e2 -yS e2 is caused by the iron-vacancy order-disorder transition. The shrinkage of the high-temperature phase and the expansion of the newly created iron-vacancy-ordered phase during the phase separation rule out the mechanism of spinodal decomposition proposed in an early report [Z. Wang et al., Phys. Rev. B 91, 064513 (2015), 10.1103/PhysRevB.91.064513]. Since the formation of the superconducting phase relies on the occurrence of the iron-vacancy order-disorder transition, it is impossible to synthesize a pure superconducting phase by a conventional solid state reaction or melt growth. By focused ion beam scanning electron microscopy, we further demonstrate that the superconducting phase forms a contiguous three-dimensional architecture composed of parallelepipeds that have a coherent orientation relationship with the iron-vacancy-ordered phase.

  4. Phase space quantum mechanics

    NASA Astrophysics Data System (ADS)

    Błaszak, Maciej; Domański, Ziemowit

    2012-02-01

    This paper develops an alternative formulation of quantum mechanics known as the phase space quantum mechanics or deformation quantization. It is shown that the quantization naturally arises as an appropriate deformation of the classical Hamiltonian mechanics. More precisely, the deformation of the point-wise product of observables to an appropriate noncommutative ⋆-product and the deformation of the Poisson bracket to an appropriate Lie bracket are the key elements in introducing the quantization of classical Hamiltonian systems. The formalism of the phase space quantum mechanics is presented in a very systematic way for the case of any smooth Hamiltonian function and for a very wide class of deformations. The considered class of deformations and the corresponding ⋆-products contains as a special case all deformations which can be found in the literature devoted to the subject of the phase space quantum mechanics. Fundamental properties of ⋆-products of observables, associated with the considered deformations are presented as well. Moreover, a space of states containing all admissible states is introduced, where the admissible states are appropriate pseudo-probability distributions defined on the phase space. It is proved that the space of states is endowed with a structure of a Hilbert algebra with respect to the ⋆-multiplication. The most important result of the paper shows that developed formalism is more fundamental than the axiomatic ordinary quantum mechanics which appears in the presented approach as the intrinsic element of the general formalism. The equivalence of two formulations of quantum mechanics is proved by observing that the Wigner-Moyal transform has all properties of the tensor product. This observation allows writing many previous results found in the literature in a transparent way, from which the equivalence of the two formulations of quantum mechanics follows naturally. In addition, examples of a free particle and a simple harmonic

  5. Phase Field Fracture Mechanics.

    SciTech Connect

    Robertson, Brett Anthony

    2015-11-01

    For this assignment, a newer technique of fracture mechanics using a phase field approach, will be examined and compared with experimental data for a bend test and a tension test. The software being used is Sierra Solid Mechanics, an implicit/explicit finite element code developed at Sandia National Labs in Albuquerque, New Mexico. The bend test experimental data was also obtained at Sandia Labs while the tension test data was found in a report online from Purdue University.

  6. Mechanism of GEMS formation

    SciTech Connect

    Bradley, J P; Dai, Z R

    2004-03-10

    GEMS (glass with embedded metal and sulfides) in interplanetary dust particles (IDPs) were examined using 200 keV analytical transmission electron microscopy. The morphologies and crystallography of embedded relict grains reveal that GEMS are pseudomorphs formed by irradiation processing of crystals free-floating in space. Some GEMS retain a compositional and morphological ''memory'' of the crystal from which they formed. Pseudomorphism rules out condensation, annealing, flash heating, or shock melting as alternative mechanisms of GEMS formation. A significant and often dominant fraction of the atoms in GEMS were sputtered deposited from other grains. Therefore, a normal (solar) isotopic composition is not a reliable indicator of whether GEMS formed in the solar system or in presolar interstellar or circumstellar environments.

  7. Mechanistic Studies Of Combustion And Structure Formation During Combustion Synthesis Of Advanced Materials: Phase Separation Mechanism For Bio-Alloys

    NASA Technical Reports Server (NTRS)

    Varma, A.; Lau, C.; Mukasyan, A.

    2003-01-01

    Among all implant materials, Co-Cr-Mo alloys demonstrate perhaps the most useful balance of resistance to corrosion, fatigue and wear, along with strength and biocompatibility [1]. Currently, these widely used alloys are produced by conventional furnace technology. Owing to high melting points of the main alloy elements (e.g. Tm.p.(Co) 1768 K), high-temperature furnaces and long process times (several hours) are required. Therefore, attempts to develop more efficient and flexible methods for production of such alloys with superior properties are of great interest. The synthesis of materials using combustion phenomena is an advanced approach in powder metallurgy [2]. The process is characterized by unique conditions involving extremely fast heating rates (up to 10(exp 6 K/s), high temperatures (up to 3500 K), and short reaction times (on the order of seconds). As a result, combustion synthesis (CS) offers several attractive advantages over conventional metallurgical processing and alloy development technologies. The foremost is that solely the heat of chemical reaction (instead of an external source) supplies the energy for the synthesis. Also, simple equipment, rather than energy-intensive high-temperature furnaces, is sufficient. This work was devoted to experiments on CS of Co-based alloys by utilizing thermite (metal oxide-reducing metal) reactions, where phase separation subsequently produces materials with tailored compositions and properties. Owing to high reaction exothermicity, the CS process results in a significant increase of temperature (up to 3000 C), which is higher than melting points of all products. Since the products differ in density, phase separation may be a gravitydriven process: the heavy (metallic phase) settles while the light (slag) phase floats. The goal was to determine if buoyancy is indeed the major mechanism that controls phase segregation.

  8. Ion yields for some salts in MALDI: mechanism for the gas-phase ion formation from preformed ions.

    PubMed

    Moon, Jeong Hee; Shin, Young Sik; Bae, Yong Jin; Kim, Myung Soo

    2012-01-01

    Preformed ion emission is the main assumption in one of the prevailing theories for peptide and protein ion formation in matrix-assisted laser desorption ionization (MALDI). Since salts are in preformed ion forms in the matrix-analyte mixture, they are ideal systems to study the characteristics of preformed ion emission. In this work, a reliable method to measure the ion yield (IY) in MALDI was developed and used for a solid salt benzyltriphenylphosphonium chloride and two room-temperature ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate. IY for the matrix (α-cyano-4-hydroxycinnamic acid, CHCA) was also measured. Taking 1 pmol salts in 25 nmol CHCA as examples, IYs for three salts were similar, (4-8) × 10(-4), and those for CHCA were (0.8-1.2) × 10(-7). Even though IYs for the salts and CHCA remained virtually constant at low analyte concentration, they decreased as the salt concentrations increased. Two models, Model 1 and Model 2, were proposed to explain low IYs for the salts and the concentration dependences. Both models are based on the fact that the ion-pair formation equilibrium is highly shifted toward the neutral ion pair. In Model 1, the gas-phase analyte cations were proposed to originate from the same cations in the solid that were dielectrically screened from counter anions by matrix neutrals. In Model 2, preformed ions were assumed to be released from the solid sample in the form of neutral ion pairs and the anions in the ion pairs were assumed to be eliminated via reactions with matrix-derived cations.

  9. Deformation-phase transformation coupling mechanism of white layer formation in high speed machining of FGH95 Ni-based superalloy

    NASA Astrophysics Data System (ADS)

    Du, Jin; Liu, zhanqiang; Lv, Shaoyu

    2014-02-01

    Ni-based superalloy represents a significant metal portion of the aircraft critical structural and engine components. When these critical structural components in aerospace industry are manufactured with the objective to reach high reliability levels and excellent service performance, surface integrity is one of the most relevant parameter used for evaluating the quality of finish machined surfaces. In the study of surface integrity, the formation white layer is a very important research topic. The formation of white layer on the Ni-based superalloy machined surface will reduce the machined parts service performance and fatigue life. This paper was conducted to determine the effects of cutting speed on white layer formation in high speed machining of FGH95 Ni-based superalloy. Optical microscope, scanning electron microscope and X-ray diffraction were employed to analyze the elements and microstructures of white layer and bulk materials. The statistical analysis for grain numbers was executed to study the influence of cutting speed on the grain refinement in the machined surface. The investigation results showed that white layer exhibits significantly different microstructures with the bulk materials. It shows densification, no obvious structural features characteristic. The microstructure and phase of Ni-based solid solution changed during cutting process. The increase of cutting speed causes the increase of white layer thickness when the cutting speed is less than 2000 m/min. However, white layer thickness reduces with the cutting speed further increase. The higher the cutting speed, the more serious grains refinement in machined surface. 2-D FEM for machining FGH95 were carried out to simulate the cutting process and obtained the cutting temperature field, cutting strain field and strain rate field. The impact mechanisms of cutting temperature, cutting strain and strain rates on white layer formation were analyzed. At last, deformation-phase transformation

  10. Effect of Calendula officinalis Flower Extract on Acute Phase Proteins, Antioxidant Defense Mechanism and Granuloma Formation During Thermal Burns.

    PubMed

    Chandran, Preethi K; Kuttan, Ramadasan

    2008-09-01

    Effect of Calendula officinalis flower extract was investigated against experimentally induced thermal burns in rats. Burn injury was made on the shaven back of the rats under anesthesia and the animals were treated orally with different doses of the flower extract (20 mg, 100 mg and 200 mg/kg body weight). The animals treated with the extract showed significant improvement in healing when compared with the control untreated animals. The indicators of the wound healing such as collagen-hydroxyproline and hexosamine contents were significantly increased in the treated group indicating accelerated wound healing in the treated animals. The acute phase proteins-haptoglobin and orosomucoid which were increased due to burn injury were found to be decreased significantly in 200 mg/kg body weight extract treated animals. The antioxidant defense mechanism, which was decreased in the liver during burn injury, was found to be enhanced in treated animals. The lipid peroxidation was significantly lowered in the treated group when compared to control animals. Tissue damage marker enzymes- alkaline phosphatase, alanine and aspartate transaminases were significantly lowered in the treated groups in a dose dependant manner. The histopathological analyses of skin tissue also give the evidence of the increased healing potential of the extract after burn injury.

  11. Effect of Calendula officinalis Flower Extract on Acute Phase Proteins, Antioxidant Defense Mechanism and Granuloma Formation During Thermal Burns

    PubMed Central

    Chandran, Preethi K.; Kuttan, Ramadasan

    2008-01-01

    Effect of Calendula officinalis flower extract was investigated against experimentally induced thermal burns in rats. Burn injury was made on the shaven back of the rats under anesthesia and the animals were treated orally with different doses of the flower extract (20 mg, 100 mg and 200 mg/kg body weight). The animals treated with the extract showed significant improvement in healing when compared with the control untreated animals. The indicators of the wound healing such as collagen-hydroxyproline and hexosamine contents were significantly increased in the treated group indicating accelerated wound healing in the treated animals. The acute phase proteins—haptoglobin and orosomucoid which were increased due to burn injury were found to be decreased significantly in 200 mg/kg body weight extract treated animals. The antioxidant defense mechanism, which was decreased in the liver during burn injury, was found to be enhanced in treated animals. The lipid peroxidation was significantly lowered in the treated group when compared to control animals. Tissue damage marker enzymes- alkaline phosphatase, alanine and aspartate transaminases were significantly lowered in the treated groups in a dose dependant manner. The histopathological analyses of skin tissue also give the evidence of the increased healing potential of the extract after burn injury. PMID:18818737

  12. Formation mechanism of calcium hexaluminate

    NASA Astrophysics Data System (ADS)

    Chen, Jun-hong; Chen, Hai-yang; Yan, Ming-wei; Cao, Zheng; Mi, Wen-jun

    2016-10-01

    To investigate the formation mechanism of calcium hexaluminate (CaAl12O19, CA6), the analytically pure alumina and calcia used as raw materials were mixed in CaO/Al2O3 ratio of 12.57:137.43 by mass. The raw materials were ball-milled and shaped into green specimens, and fired at 1300-1600°C. Then, the phase composition and microstructure evolution of the fired specimen were studied, and a first principle calculation was performed. The results show that in the reaction system of CaO and Al2O3, a small amount of CA6 forms at 1300°C, and greater amounts are formed at 1400°C and higher temperatures. The reaction is as follows: CaO·2Al2O3 (CA2) + 4Al2O3 → CA6. The diffusions of Ca2+ in CA2 towards Al2O3 and Al3+ in Al2O3 towards CA2 change the structures in different degrees of difficulty. Compared with the difficulty of structural change and the corresponding lattice energy change, it is deduced that the main formation mechanism is the diffusion of Ca2+ in CA2 towards Al2O3.

  13. Influence of water on the homogeneous gas-phase formation mechanism of polyhalogenated dioxins/furans from chlorinated/brominated phenols as precursors.

    PubMed

    Shi, Xiangli; Zhang, Ruiming; Zhang, Haijun; Xu, Fei; Zhang, Qingzhu; Wang, Wenxing

    2015-10-01

    Water is of great chemical importance due to its ability to form hydrogen bond. Polyhalogenated dibenzo-p-dioxin/benzofurans (PHDD/Fs) are notorious due to their persistence, bioaccumulation and extremely high toxicity. Water is ubiquitous, and a deep knowledge of its influence on the formation mechanism of PHDD/Fs is necessary. This work investigated the influence of water on the homogeneous gas-phase formation of PHDD/Fs from halogenated phenols (HPs) as precursors by using quantum chemical calculations with the aid of the MPWB1K theoretical approach in connection with the 6-31+G(d,p) and 6-311+G(3df,2p) basis sets. The schematic energy profile in the presence of water was constructed and compared with the situation without water. This study reveals for the first time that the introduction of water promotes the formation of halogenated phenoxy radicals (HPRs) from the H abstraction reactions of HPs with atomic H and OH radicals by lowering the reaction energy barriers and opening new low-energy pathways. Another intriguing finding of this work is that the inclusion of a water molecule produces a catalytic effect on the H-shift step involved in the formation of PHDFs and thus their formation potential is enhanced.

  14. Kinetics and Mechanisms of Cr(VI) Formation via the Oxidation of Cr(III) Solid Phases by Chlorine in Drinking Water.

    PubMed

    Chebeir, Michelle; Liu, Haizhou

    2016-01-19

    Hexavalent chromium Cr(VI), typically existing as the oxyanion form of CrO4(2-), is being considered for more stringent drinking water standards by regulatory agencies. Cr(VI) can be inadvertently produced via the oxidation of trivalent chromium Cr(III) solids. This study investigated the kinetics and mechanisms of Cr(III) solids oxidation by chlorine in drinking water and associated Cr(VI) formation. Batch experiments were carried out with three Cr(III) solids of environmental relevance, i.e., chromium hydroxide Cr(OH)3(s), chromium oxide Cr2O3(s), and copper chromite Cu2Cr2O5(s). Impacts of water chemical parameters including pH (6.0-8.5) and bromide concentration (0-5 mg/L) were examined. Results showed that the rapid oxidation of Cr(III) solid phases by chlorine was accompanied by Cr(VI) formation and an unexpected production of dissolved oxygen. Analysis of reaction stoichiometry indicated the existence of Cr intermediate species that promoted the autocatalytic decay of chlorine. An increase in pH modestly enhanced Cr(VI) formation due to changes of reactive Cr(III) surface hydroxo species. Bromide, a trace chemical constituent in source waters, exhibited a catalytic effect on Cr(VI) formation due to an electron shuttle mechanism between Cr(III) and chlorine and the bypass of Cr intermediate formation. The kinetics data obtained from this study suggest that the oxidation of Cr(III) solids by chlorine in water distribution systems can contribute to Cr(VI) occurrence in tap water, especially in the presence of a trace level of bromide. PMID:26647114

  15. Kinetics and Mechanisms of Cr(VI) Formation via the Oxidation of Cr(III) Solid Phases by Chlorine in Drinking Water.

    PubMed

    Chebeir, Michelle; Liu, Haizhou

    2016-01-19

    Hexavalent chromium Cr(VI), typically existing as the oxyanion form of CrO4(2-), is being considered for more stringent drinking water standards by regulatory agencies. Cr(VI) can be inadvertently produced via the oxidation of trivalent chromium Cr(III) solids. This study investigated the kinetics and mechanisms of Cr(III) solids oxidation by chlorine in drinking water and associated Cr(VI) formation. Batch experiments were carried out with three Cr(III) solids of environmental relevance, i.e., chromium hydroxide Cr(OH)3(s), chromium oxide Cr2O3(s), and copper chromite Cu2Cr2O5(s). Impacts of water chemical parameters including pH (6.0-8.5) and bromide concentration (0-5 mg/L) were examined. Results showed that the rapid oxidation of Cr(III) solid phases by chlorine was accompanied by Cr(VI) formation and an unexpected production of dissolved oxygen. Analysis of reaction stoichiometry indicated the existence of Cr intermediate species that promoted the autocatalytic decay of chlorine. An increase in pH modestly enhanced Cr(VI) formation due to changes of reactive Cr(III) surface hydroxo species. Bromide, a trace chemical constituent in source waters, exhibited a catalytic effect on Cr(VI) formation due to an electron shuttle mechanism between Cr(III) and chlorine and the bypass of Cr intermediate formation. The kinetics data obtained from this study suggest that the oxidation of Cr(III) solids by chlorine in water distribution systems can contribute to Cr(VI) occurrence in tap water, especially in the presence of a trace level of bromide.

  16. Comparison of experimental approaches to study selective properties of thick phase-amplitude holograms recorded in materials with diffusion-based formation mechanisms

    NASA Astrophysics Data System (ADS)

    Borisov, Vladimir; Klepinina, Mariia; Veniaminov, Andrey; Angervaks, Aleksandr; Shcheulin, Aleksandr; Ryskin, Aleksandr

    2016-04-01

    Volume holographic gratings, both transmission and reflection-type, may be employed as one-dimensional pho- tonic crystals. More complex two- and three-dimensional holographic photonic-crystalline structures can be recorded using several properly organized beams. As compared to colloidal photonic crystals, their holographic counterparts let minimize distortions caused by multiple inner boundaries of the media. Unfortunately, it's still hard to analyze spectral response of holographic structures. This work presents the results of thick holographic gratings analysis based on spectral-angular selectivity contours approximation. The gratings were recorded in an additively colored fluorite crystal and a glassy polymer doped with phenanthrenequinone (PQ-PMMA). The two materials known as promising candidates for 3D diffraction optics including photonic crystals, employ diffusion-based mechanisms of grating formation. The surfaces of spectral-angular selectivity were obtained in a single scan using a white-light LED, rotable table and a matrix spectrometer. The data expressed as 3D plots make apparent visual estimation of the grating phase/amplitude nature, noninearity of recording, etc., and provide sufficient information for numerical analysis. The grating recorded in the crystal was found to be a mixed phase-amplitude one, with different contributions of refractive index and absorbance modulation at different wavelengths, and demonstrated three diffraction orders corresponding to its three spatial harmonics originating from intrinsically nonlinear diffusion-drift recording mechanism. Contrastingly, the grating in the polymeric medium appeared purely phase and linearly recorded.

  17. Analysis of interface formation mechanism in GaN double-polarity selective-area growth by metalorganic vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Kuze, Kenta; Osumi, Noriyuki; Fujita, Yohei; Inoue, Yoku; Nakano, Takayuki

    2016-05-01

    The fabrication of quasi-phase-matching (QPM) crystals by selective-area growth on the two asymmetrically polar surfaces of GaN is examined. We attempted the fabrication of GaN-QPM crystals by one-time growth using a carbon mask. For GaN double-polarity selective-area growth (DP-SAG), we investigated the effect of varied nitriding times of the Al2O3 templates patterned with the carbon mask. We optimized the nitriding conditions for the DP-SAG process, and evaluated the substrate fabricated by the optimized DP-SAG process. In addition, we examined the interface formation mechanism of DP-GaN fabricated by GaN DP-SAG process. We determined that it is possible to fabricate DP-GaN with a sharp interface by optimizing the growth conditions.

  18. Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the southeast United States and co-benefit of SO2 emission controls

    NASA Astrophysics Data System (ADS)

    Marais, E. A.; Jacob, D. J.; Jimenez, J. L.; Campuzano-Jost, P.; Day, D. A.; Hu, W.; Krechmer, J.; Zhu, L.; Kim, P. S.; Miller, C. C.; Fisher, J. A.; Travis, K.; Yu, K.; Hanisco, T. F.; Wolfe, G. M.; Arkinson, H. L.; Pye, H. O. T.; Froyd, K. D.; Liao, J.; McNeill, V. F.

    2016-02-01

    Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for isoprene SOA formation coupled to a detailed gas-phase isoprene oxidation scheme. The mechanism is based on aerosol reactive uptake coefficients (γ) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC4RS) and ground-based (SOAS) observations over the southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NOx ≡ NO + NO2) over the southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO2) react significantly with both NO (high-NOx pathway) and HO2 (low-NOx pathway), leading to different suites of isoprene SOA precursors. We find a mean SOA mass yield of 3.3 % from isoprene oxidation, consistent with the observed relationship of total fine organic aerosol (OA) and formaldehyde (a product of isoprene oxidation). Isoprene SOA production is mainly contributed by two immediate gas-phase precursors, isoprene epoxydiols (IEPOX, 58 % of isoprene SOA) from the low-NOx pathway and glyoxal (28 %) from both low- and high-NOx pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC4RS. Observations show a strong relationship between IEPOX SOA and sulfate aerosol that we explain as due to the effect of sulfate on aerosol acidity and volume. Isoprene SOA concentrations increase as NOx emissions decrease (favoring the low-NOx pathway for isoprene oxidation), but decrease more strongly as SO2 emissions decrease (due to the effect of sulfate on aerosol acidity and volume). The US Environmental Protection Agency (EPA) projects 2013-2025 decreases in anthropogenic emissions of

  19. Impact of solid second phases on deformation mechanisms of naturally deformed salt rocks (Kuh-e-Namak, Dashti, Iran) and rheological stratification of the Hormuz Salt Formation

    NASA Astrophysics Data System (ADS)

    Závada, P.; Desbois, G.; Urai, J. L.; Schulmann, K.; Rahmati, M.; Lexa, O.; Wollenberg, U.

    2015-05-01

    Viscosity contrasts displayed in flow structures of a mountain namakier (Kuh-e-Namak - Dashti), between 'weak' second phase bearing rock salt and 'strong' pure rock salt types are studied for deformation mechanisms using detailed quantitative microstructural study. While the solid inclusions rich ("dirty") rock salts contain disaggregated siltstone and dolomite interlayers, "clean" salts reveal microscopic hematite and remnants of abundant fluid inclusions in non-recrystallized cores of porphyroclasts. Although the flow in both, the recrystallized "dirty" and "clean" salt types is accommodated by combined mechanisms of pressure-solution creep (PS), grain boundary sliding (GBS), transgranular microcracking and dislocation creep accommodated grain boundary migration (GBM), their viscosity contrasts observed in the field outcrops are explained by: 1) enhanced ductility of "dirty" salts due to increased diffusion rates along the solid inclusion-halite contacts than along halite-halite contacts, and 2) slow rates of intergranular diffusion due to dissolved iron and inhibited dislocation creep due to hematite inclusions for "clean" salt types Rheological contrasts inferred by microstructural analysis between both salt rock classes apply in general for the "dirty" salt forming Lower Hormuz and the "clean" salt forming the Upper Hormuz of the Hormuz Formation and imply strain rate gradients or decoupling along horizons of mobilized salt types of different composition and microstructure.

  20. Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the southeast United States and co-benefit of SO2 emission controls

    EPA Science Inventory

    Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for...

  1. Mechanism of polymer nanoparticle formation by nanoprecipitation

    NASA Astrophysics Data System (ADS)

    Zhao, Chen; Li, Tingting; van Keuren, Edward

    Nanoprecipitation method is a simple and convenient way to produce nanoparticles from polymers in solution. The control of nanoparticle size and size distribution plays a pivotal role in the use of nanoprecipitation for drug delivery. We investigated various factors and initial conditions that affect the particle size, such as the initial solute concentration, solvent/non-solvent ratio and the molecular weight of the polymer samples. The results shed light on the mechanisms of particle formation and phase separation which occurs in nanoprecipitation. Spinodal decomposition, which takes place as a result of a quench to a sufficiently high supersaturation, is believed to be the main mechanism that governs the phase separation. In addition, the glass transition of the polymers will also be a key factor that contributes to the thermodynamics and kinetics of the phase separation and the resulting particle size and morphology.

  2. Geometric phase in Bohmian mechanics

    SciTech Connect

    Chou, Chia-Chun; Wyatt, Robert E.

    2010-10-15

    Using the quantum kinematic approach of Mukunda and Simon, we propose a geometric phase in Bohmian mechanics. A reparametrization and gauge invariant geometric phase is derived along an arbitrary path in configuration space. The single valuedness of the wave function implies that the geometric phase along a path must be equal to an integer multiple of 2{pi}. The nonzero geometric phase indicates that we go through the branch cut of the action function from one Riemann sheet to another when we locally travel along the path. For stationary states, quantum vortices exhibiting the quantized circulation integral can be regarded as a manifestation of the geometric phase. The bound-state Aharonov-Bohm effect demonstrates that the geometric phase along a closed path contains not only the circulation integral term but also an additional term associated with the magnetic flux. In addition, it is shown that the geometric phase proposed previously from the ensemble theory is not gauge invariant.

  3. Structure and mechanism of the formation of core-shell nanoparticles obtained through a one-step gas-phase synthesis by electron beam evaporation.

    PubMed

    Nomoev, Andrey V; Bardakhanov, Sergey P; Schreiber, Makoto; Bazarova, Dashima G; Romanov, Nikolai A; Baldanov, Boris B; Radnaev, Bair R; Syzrantsev, Viacheslav V

    2015-01-01

    The structure of core-shell Cu@silica and Ag@Si nanoparticles obtained in one-step through evaporation of elemental precursors by a high-powered electron beam are investigated. The structure of the core and shell of the particles are investigated in order to elucidate their mechanisms of formation and factors affecting the synthesis. It is proposed that the formation of Cu@silica particles is mainly driven by surface tension differences between Cu and Si while the formation of Ag@Si particles is mainly driven by differences in the vapour concentration of the two components.

  4. Possible mechanisms of macrolayer formation

    SciTech Connect

    Sadasivan, P.; Chappidi, P.R.; Unal, C.; Nelson, R.A.

    1992-01-01

    The high heat flux nucleate boiling region, also called the vapor mushroom region, has been shown to have a thin liquid layer on the heater surface under the large mushroom-shaped vapor bubbles that grow from the heater surface. The name given to this liquid layer is the macrolayer to differentiate it from the microlayer that exists under the discrete bubbles found at lower heat fluxes in the nucleate boiling region. Typical thicknesses of this macrolayer range from 50 to 500 {mu}m for water on a flat horizontal boiling surface and depend upon the heat flux. Thus, the macrolayer is thicker than the wedge-shaped microlayers, found under discrete bubbles, which range in thickness from 1 to 10 {mu}m. Although the mechanism of microlayer formation and its evaporation is conceptually simple that of the macrolayer is still not understood. This paper critically compares the potential mechanisms proposed for macrolayer formation. These mechanisms include the Helmholtz instability applied to the vapor stem above active nucleation sites, liquid trapped by lateral coalescence of discrete bubbles that initially form during the mushroom bubble's waiting period, and the limitation of liquid resupply at mushroom departure as a result of vapor flow from the active nucleation sites.

  5. Possible mechanisms of macrolayer formation

    SciTech Connect

    Sadasivan, P.; Chappidi, P.R.; Unal, C.; Nelson, R.A.

    1992-05-01

    The high heat flux nucleate boiling region, also called the vapor mushroom region, has been shown to have a thin liquid layer on the heater surface under the large mushroom-shaped vapor bubbles that grow from the heater surface. The name given to this liquid layer is the macrolayer to differentiate it from the microlayer that exists under the discrete bubbles found at lower heat fluxes in the nucleate boiling region. Typical thicknesses of this macrolayer range from 50 to 500 {mu}m for water on a flat horizontal boiling surface and depend upon the heat flux. Thus, the macrolayer is thicker than the wedge-shaped microlayers, found under discrete bubbles, which range in thickness from 1 to 10 {mu}m. Although the mechanism of microlayer formation and its evaporation is conceptually simple that of the macrolayer is still not understood. This paper critically compares the potential mechanisms proposed for macrolayer formation. These mechanisms include the Helmholtz instability applied to the vapor stem above active nucleation sites, liquid trapped by lateral coalescence of discrete bubbles that initially form during the mushroom bubble`s waiting period, and the limitation of liquid resupply at mushroom departure as a result of vapor flow from the active nucleation sites.

  6. Effect of silicon on the phase formation in mechanically activated systems based on Fe(75)C(25): Temperature-induced transformations in mechanosynthesized composites

    NASA Astrophysics Data System (ADS)

    Volkov, V. A.; Chulkina, A. A.; Ul'yanov, A. I.; Elsukov, E. P.

    2012-04-01

    Transformations realized in mechanosynthesized amorphous-nanocrystalline Fe(75)C(25 - x)Si( x) (0 ≤ x ≤ 10 at %) alloys during heating have been studied using dynamic magnetic susceptibility measurements, X-ray diffraction, and metallography. In contrast to mechanosynthesized alloys consisting of α-Fe, Fe3C, and amorphous phases, the annealed alloys with x > 5 at % were found to exhibit the formation of an additional phase such as Fe5SiC. After heating to 700 and 800°C, the powder particles of alloys contain a large amount of uniformly distributed graphite particles of ˜0.5 μm in size. The formation of particles results from the cementite decomposition, which is accelerated at the expense of partial silicon dissolution in cementite and in the presence of α-Fe nanograins as well.

  7. Mechanism of formation of. gamma. -bismuth molybdate

    SciTech Connect

    Kustova, G.N.; Yurchenko, E.N.; Odegova, G.V.; Lazarenko, T.P.; Tarasova, D.V.; Aleshina, G.I.

    1986-04-01

    Infrared and Raman spectroscopic methods were used to study the mechanism of the formation of a Bi-Mo catalyst. X-ray-amorphous Bi/sub 2/MoO/sub 6/ is formed upon precipitation from solutions of bismuth nitrate and ammonium heptamolybdate at pH 2, which, after removal of the NH/sub 4/NO/sub 3/ side product by filtration and heating, crystallizes to the ..gamma..-phase. The presence of NH/sub 4/NO/sub 3/ causes a change in the nature of the solid-phase transformations upon heat treatment leading to decomposition of Bi/sub 2/MoO/sub 6/ into the ..cap alpha..-molybdate and a basic bismuth salt which react upon a further increase in temperature to form the ..gamma..-phase. This difference in the generation process leads to a change in the state of the catalyst surface.

  8. Formation mechanisms of metal colloids

    NASA Astrophysics Data System (ADS)

    Halaciuga, Ionel

    Highly dispersed uniform metallic particles are widely used in various areas of technology and medicine and are likely to be incorporated into many other applications in the future. It is commonly accepted that size, shape and composition of the particles represent critical factors in most applications. Thus, understanding the mechanisms of formation of metal particles and the ways to control the physical (e.g. shape, size) and chemical (e.g. composition) properties is of great importance. In the current research, the formation of uniform silver spheres is investigated experimentally. The parameters that influence the formation of silver particles when concentrated iso-ascorbic acid and silver-polyamine complex solutions are rapidly mixed were studied in the absence of dispersants. We found that by varying the nature of the amine, temperature, concentration of reactants, silver/amine molar ratio, and the nature of the silver salt, the size of the resulting silver particles can be varied in a wide range (0.08--1.5 microm). The silver particles were formed by aggregation of nanosize subunits as substantiated by both electron microscopy and X-ray diffraction techniques and by the vivid rapid color changes during the chemical precipitation process. From the practical standpoint, the goal of this research was to prepare well dispersed spherical silver particles having a relatively smooth surface and a diameter of about 1 microm to satisfy the demands of the current electronic materials market. A two stage particle growth model previously developed to explain the narrow size distribution occurring in synthesis of gold spheres was applied to the present experimental system, and the parameters that control the size distribution characteristics were identified. The kinetic parameter required to match the final particle size was found to be in agreement with the one used previously in modeling formation of gold spheres, suggesting that similar kinetics governs the

  9. Mechanism of formation of hydantoins

    SciTech Connect

    Worman, J.J.; Uhrich, K.; Olson, E.; Diehl, J.; Farnum, S.; Hawthorne, S.

    1985-10-01

    Hydantoins, (I), are biologically active compounds which occur in nature and have been isolated from such sources as sugar beets and butterfly wings. Synthetic analogs have found widespread use as anticonvulsant drugs, bacteriocides, stabilizers in photographic film, and in the preparation of high temperature epoxy resins. Uses, preparation, and reactions of hydantoins have been extensively studied and are reported elsewhere. Recently a number of hydantoin isomers have been detected and identified in coal gasification condensate water from the gasification of Indian Head (ND) lignite in a slagging fixed bed gasifier. The hydantoins constitute a major portion of the organics in the condensate water. In generation the concentrations of hydantoins found in the condensate water from an ash gasifier are smaller than those found from the slagging process. Lignite coal reacting in a slagging gasifier gives the largest concentration of hydantoins in the condensate water. The major isomer in the coal gasification condensate water is 5,5-dimethylhydantoin (DMH). DMH is formed from individual species (acetone, cyanide, ammonia, and carbonate) in the condensate water and does not form directly in the gasifier. The rate of formation of DMH was first order in all of the reactants as expressed by the equation: Rate of formation of DMH = (Acetone) (HCN) (NH/sub 3/) (CO/sub 2/). This kinetic data is valuable in predicting the rate of formation of DMH in coal gasification condensate water, provided the model is applicable. A mechanism consistent with the kinetic data and partially verified by others is shown in Scheme 1. 13 refs.

  10. Early phases of star formation

    NASA Astrophysics Data System (ADS)

    Bok, B. J.

    1981-04-01

    Five broad areas of potential star formation in our galaxy and the Magellanic Clouds are presented. The role of gravitational collapse in concentrating matter into eventual stars is examined briefly. The five areas of research are: (1) giant molecular clouds with dimensions of 50 to 100 parsecs and masses equivalent to 100,000 or more suns; (2) the proximity of an H II emission nebula with an embedded or attached cluster of association of O and B stars to a large molecular cloud; (3) the larger so-called globules, notably the roundish and often isolated dark nebulae called Barnard objects, of which 200 or so have been identified within 500 parsecs of the sun; (4) close passage or collisions between interstellar clouds; and (5) supernova explosions. The Large Magellanic Clouds are also examined as an example of an area of potential star formation without the protection of a cosmic dust cloud. Finally, the likelihood that many new stars might possess planets and perhaps even life is discussed.

  11. Mineralogical signatures of stone formation mechanisms.

    PubMed

    Gower, Laurie B; Amos, Fairland F; Khan, Saeed R

    2010-08-01

    The mechanisms involved in biomineralization are modulated through interactions with organic matrix. In the case of stone formation, the role of the organic macromolecules in the complex urinary environment is not clear, but the presence of mineralogical 'signatures' suggests that some aspects of stone formation may result from a non-classical crystallization process that is induced by acidic proteins. An amorphous precursor has been detected in many biologically controlled mineralization reactions, which is thought to be regulated by non-specific interactions between soluble acidic proteins and mineral ions. Using in vitro model systems, we find that a liquid-phase amorphous mineral precursor induced by acidic polypeptides can lead to crystal textures that resemble those found in Randall's plaque and kidney stones. This polymer-induced liquid-precursor process leads to agglomerates of coalesced mineral spherules, dense-packed spherulites with concentric laminations, mineral coatings and 'cements', and collagen-associated mineralization. Through the use of in vitro model systems, the mechanisms involved in the formation of these crystallographic features may be resolved, enhancing our understanding of the potential role(s) that proteins play in stone formation.

  12. Gas-Phase Infrared; JCAMP Format

    National Institute of Standards and Technology Data Gateway

    SRD 35 NIST/EPA Gas-Phase Infrared; JCAMP Format (PC database for purchase)   This data collection contains 5,228 infrared spectra in the JCAMP-DX (Joint Committee for Atomic and Molecular Physical Data "Data Exchange") format.

  13. Mechanisms and Geochemical Models of Core Formation

    NASA Astrophysics Data System (ADS)

    David Rubie; Seth Andrew Jacobson

    2016-03-01

    The formation of the Earth's core is a consequence of planetary accretion and processes in the Earth's interior. The mechanical process of planetary differentiation is likely to occur in large, if not global, magma oceans created by the collisions of planetary embryos. Metal-silicate segregation in magma oceans occurs rapidly and efficiently unlike grain scale percolation according to laboratory experiments and calculations. Geochemical models of the core formation process as planetary accretion proceeds are becoming increasingly realistic. Single stage and continuous core formation models have evolved into multi-stage models that are couple to the output of dynamical models of the giant impact phase of planet formation. The models that are most successful in matching the chemical composition of the Earth's mantle, based on experimentally-derived element partition coefficients, show that the temperature and pressure of metal-silicate equilibration must increase as a function of time and mass accreted and so must the oxygen fugacity of the equilibrating material. The latter can occur if silicon partitions into the core and through the late delivery of oxidized material. Coupled dynamical accretion and multi-stage core formation models predict the evolving mantle and core compositions of all the terrestrial planets simultaneously and also place strong constraints on the bulk compositions and oxidation states of primitive bodies in the protoplanetary disk.

  14. Study of Formation Mechanisms of Gas Hydrate

    NASA Astrophysics Data System (ADS)

    Yang, Jia-Sheng; Wu, Cheng-Yueh; Hsieh, Bieng-Zih

    2015-04-01

    Gas hydrates, which had been found in subsurface geological environments of deep-sea sediments and permafrost regions, are solid crystalline compounds of gas molecules and water. The estimated energy resources of hydrates are at least twice of that of the conventional fossil fuel in the world. Gas hydrates have a great opportunity to become a dominating future energy. In the past years, many laboratory experiments had been conducted to study chemical and thermodynamic characteristics of gas hydrates in order to investigate the formation and dissociation mechanisms of hydrates. However, it is difficult to observe the formation and dissociation of hydrates in a porous media from a physical experiment directly. The purpose of this study was to model the dynamic formation mechanisms of gas hydrate in porous media by reservoir simulation. Two models were designed for this study: 1) a closed-system static model with separated gas and water zones; this model was a hydrate equilibrium model to investigate the behavior of the formation of hydrates near the initial gas-water contact; and 2) an open-system dynamic model with a continuous bottom-up gas flow; this model simulated the behavior of gas migration and studied the formation of hydrates from flowed gas and static formation water in porous media. A phase behavior module was developed in this study for reservoir simulator to model the pressure-volume-temperature (PVT) behavior of hydrates. The thermodynamic equilibriums and chemical reactions were coupled with the phase behavior module to have functions modelling the formation and dissociation of hydrates from/to water and gas. The simulation models used in this study were validated from the code-comparison project proposed by the NETL. According to the modelling results of the closed-system static model, we found that predominated location for the formation of hydrates was below the gas-water contact (or at the top of water zone). The maximum hydrate saturation

  15. Formation of metastable phases by spinodal decomposition

    NASA Astrophysics Data System (ADS)

    Alert, Ricard; Tierno, Pietro; Casademunt, Jaume

    2016-10-01

    Metastable phases may be spontaneously formed from other metastable phases through nucleation. Here we demonstrate the spontaneous formation of a metastable phase from an unstable equilibrium by spinodal decomposition, which leads to a transient coexistence of stable and metastable phases. This phenomenon is generic within the recently introduced scenario of the landscape-inversion phase transitions, which we experimentally realize as a structural transition in a colloidal crystal. This transition exhibits a rich repertoire of new phase-ordering phenomena, including the coexistence of two equilibrium phases connected by two physically different interfaces. In addition, this scenario enables the control of sizes and lifetimes of metastable domains. Our findings open a new setting that broadens the fundamental understanding of phase-ordering kinetics, and yield new prospects of applications in materials science.

  16. Formation of metastable phases by spinodal decomposition

    PubMed Central

    Alert, Ricard; Tierno, Pietro; Casademunt, Jaume

    2016-01-01

    Metastable phases may be spontaneously formed from other metastable phases through nucleation. Here we demonstrate the spontaneous formation of a metastable phase from an unstable equilibrium by spinodal decomposition, which leads to a transient coexistence of stable and metastable phases. This phenomenon is generic within the recently introduced scenario of the landscape-inversion phase transitions, which we experimentally realize as a structural transition in a colloidal crystal. This transition exhibits a rich repertoire of new phase-ordering phenomena, including the coexistence of two equilibrium phases connected by two physically different interfaces. In addition, this scenario enables the control of sizes and lifetimes of metastable domains. Our findings open a new setting that broadens the fundamental understanding of phase-ordering kinetics, and yield new prospects of applications in materials science. PMID:27713406

  17. Effect of alteration phase formation on the glass dissolution rate

    SciTech Connect

    Ebert, W.L.

    1997-10-01

    The dissolution rates of many glasses have been observed to increase upon the formation of certain alteration phases. It is important to understand the mechanism by which alteration phases affect glass corrosion behavior and the glass dissolution rate to reliably predict whether or not similar effects will occur in a disposal environment and the impact of phase formation on the long-term performance of waste glass. While solid state transformation of a glass to thermodynamically more stable phases in kinetically prohibitive, contact by water provides an energetically favorable pathway for this transformation to occur by a dissolution-reprecipitation mechanism. The kinetics of the transformation depends on the dissolution kinetics of the glass and the precipitation kinetics of the alteration phases. The rates of these two processes are linked primarily through the solution activity of orthosilicic acid (and perhaps also that of an aluminum-bearing species).

  18. Magma ocean: Mechanisms of formation

    NASA Technical Reports Server (NTRS)

    Kaula, W. M.

    1992-01-01

    The thermal state of the Earth at the time relevant to formation of a magma ocean was dominated by the great impact that created the Moon. As shown in computer experiments, the iron in the impacting bodies quickly sank to the core of the proto-Earth, while a significant fraction of silicates was pushed far enough out beyond the geosynchronous limit to constitute the main material of the Moon. Most of any atmosphere would have been pushed aside, rather than being expelled in the impact. However, the energy remaining in the material not going to the core or expelled was still sufficient to raise its temperature some 1000's of degrees, enough to vaporize silicates and to generate a strong 'planetary wind': a hydrodynamic expansion carrying with it virtually all volatiles plus appreciable silicates. This expansion was violent and uneven in its most energetic stage, but probably the resulting magma ocean was global. The duration, until cooling, was sufficient for silicates to condense to melt and the duration was probably short. Comparison of the Earth and Venus indicates that the great impact was extraordinarily effective in removing volatiles from the proto-Earth; in particular, the enormous differences in primordial inert gases between the planets demand a catastrophic difference in origin circumstances. On the other hand, the comparison limits the amount of silicates lost by the Earth to a rather minor fraction; most of that expelled in the wind must have condensed soon enough for the silicate to fall back to Earth or be swept up by the proto-Moon. So the Earth was left with a magma ocean. The question is whether sufficient water was retained to constitute a steam atmosphere. Probably not, but unknowns affecting this question are the efficiencies of outgassing in great impacts and in subsequent convective churnings deep in the mantle. During the stage when mantle convection is turbulent, an appreciable fraction of volatiles were also retained at depth, perhaps in

  19. Phase separation and the formation of cellular bodies

    NASA Astrophysics Data System (ADS)

    Xu, Bin; Broedersz, Chase P.; Meir, Yigal; Wingreen, Ned S.

    Cellular bodies in eukaryotic cells spontaneously assemble to form cellular compartments. Among other functions, these bodies carry out essential biochemical reactions. Cellular bodies form micron-sized structures, which, unlike canonical cell organelles, are not surrounded by membranes. A recent in vitro experiment has shown that phase separation of polymers in solution can explain the formation of cellular bodies. We constructed a lattice-polymer model to capture the essential mechanism leading to this phase separation. We used both analytical and numerical tools to predict the phase diagram of a system of two interacting polymers, including the concentration of each polymer type in the condensed and dilute phase.

  20. Magnetohydrodynamic mechanism for pedestal formation.

    PubMed

    Guazzotto, L; Betti, R

    2011-09-16

    Time-dependent two-dimensional magnetohydrodynamic simulations are carried out for tokamak plasmas with edge poloidal flow. Differently from conventional equilibrium theory, a density pedestal all around the edge is obtained when the poloidal velocity exceeds the poloidal sound speed. The outboard pedestal is induced by the transonic discontinuity, the inboard one by mass redistribution. The density pedestal follows the formation of a highly sheared flow at the transonic surface. These results may be relevant to the L-H transition and pedestal formation in high performance tokamak plasmas.

  1. Formation of phase space holes and clumps.

    PubMed

    Lilley, M K; Nyqvist, R M

    2014-04-18

    It is shown that the formation of phase space holes and clumps in kinetically driven, dissipative systems is not restricted to the near threshold regime, as previously reported and widely believed. Specifically, we observe hole-clump generation from the edges of an unmodulated phase space plateau, created via excitation, phase mixing and subsequent dissipative decay of a linearly unstable bulk plasma mode in the electrostatic bump-on-tail model. This has now allowed us to elucidate the underlying physics of the hole-clump formation process for the first time. Holes and clumps develop from negative energy waves that arise due to the sharp gradients at the interface between the plateau and the nearly unperturbed, ambient distribution and destabilize in the presence of dissipation in the bulk plasma. We confirm this picture by demonstrating that the formation of such nonlinear structures in general does not rely on a "seed" wave, only on the ability of the system to generate a plateau. In addition, we observe repetitive cycles of plateau generation and erosion, the latter due to hole-clump formation and detachment, which appear to be insensitive to initial conditions and can persist for a long time. We present an intuitive discussion of why this continual regeneration occurs. PMID:24785043

  2. Interplay between micelle formation and waterlike phase transitions

    NASA Astrophysics Data System (ADS)

    Heinzelmann, G.; Figueiredo, W.; Girardi, M.

    2010-02-01

    A lattice model for amphiphilic aggregation in the presence of a structured waterlike solvent is studied through Monte Carlo simulations. We investigate the interplay between the micelle formation and the solvent phase transition in two different regions of temperature-density phase diagram of pure water. A second order phase transition between the gaseous (G) and high density liquid (HDL) phases that occurs at very high temperatures, and a first order phase transition between the low density liquid (LDL) and (HDL) phases that takes place at lower temperatures. In both cases, we find the aggregate size distribution curve and the critical micellar concentration as a function of the solvent density across the transitions. We show that micelle formation drives the LDL-HDL first order phase transition to lower solvent densities, while the transition G-HDL is driven to higher densities, which can be explained by the markedly different degrees of micellization in both cases. The diffusion coefficient of surfactants was also calculated in the LDL and HDL phases, changing abruptly its behavior due to the restructuring of waterlike solvent when we cross the first order LDL-HDL phase transition. To understand such behavior, we calculate the solvent density and the number of hydrogen bonds per water molecule close to micelles. The curves of the interfacial solvent density and the number of hydrogen bonds per water molecule in the first hydration signal a local phase change of the interfacial water, clarifying the diffusion mechanism of free surfactants in the solvent.

  3. Phase Formation Behavior in Ultrathin Iron Oxide.

    PubMed

    Jõgi, Indrek; Jacobsson, T Jesper; Fondell, Mattis; Wätjen, Timo; Carlsson, Jan-Otto; Boman, Mats; Edvinsson, Tomas

    2015-11-17

    Nanostructured iron oxides, and especially hematite, are interesting for a wide range of applications ranging from gas sensors to renewable solar hydrogen production. A promising method for deposition of low-dimensional films is atomic layer deposition (ALD). Although a potent technique, ALD of ultrathin films is critically sensitive to the substrate and temperature conditions where initial formation of islands and crystallites influences the properties of the films. In this work, deposition at the border of the ALD window forming a hybrid ALD/pulsed CVD (pCVD) deposition is utilized to obtain a deposition less sensitive to the substrate. A thorough analysis of iron oxide phases formation on two different substrates, Si(100) and SiO2, was performed. Films between 3 and 50 nm were deposited and analyzed with diffraction techniques, high-resolution Raman spectroscopy, and optical spectroscopy. Below 10 nm nominal film thickness, island formation and phase dependent particle crystallization impose constraints for deposition of phase pure iron oxides on non-lattice-matching substrates. Films between 10 and 20 nm thickness on SiO2 could effectively be recrystallized into hematite whereas for the corresponding films on Si(100), no recrystallization occurred. For films thicker than 20 nm, phase pure hematite can be formed directly with ALD/pCVD with very low influence of the substrate on either Si or SiO2. For more lattice matched substrates such as SnO2:F, Raman spectroscopy indicated formation of the hematite phase already for films with 3 nm nominal thickness and clearly for 6 nm films. Analysis of the optical properties corroborated the analysis and showed a quantum confined blue-shift of the absorption edge for the thinnest films. PMID:26506091

  4. Magnetic phase formation in irradiated austenitic alloys

    SciTech Connect

    Gussev, Maxim N; Busby, Jeremy T; Tan, Lizhen; Garner, Francis A.

    2014-01-01

    Austenitic alloys are often observed to develop magnetic properties during irradiation, possibly associated with radiation-induced acceleration of the ferrite phase. Some of the parametric sensitivities of this phenomenon have been addressed using a series of alloys irradiated in the BOR-60 reactor at 593K. The rate of development of magnetic phase appears to be sensitive to alloy composition. To the first order, the largest sensitivities to accelerate ferrite formation, as explored in this experiment, are associated with silicon, carbon and manganese and chromium. Si, C, and Mn are thought to influence diffusion rates of point defects while Cr plays a prominent role in defining the chromium equivalent and therefore the amount of ferrite at equilibrium. Pre-irradiation cold working was found to accelerate ferrite formation, but it can play many roles including an effect on diffusion, but on the basis of these results the dominant role or roles of cold-work cannot be identified. Based on the data available, ferrite formation is most probably associated with diffusion.

  5. Mechanism of formation of curved Kikuchi lines

    NASA Astrophysics Data System (ADS)

    Karakhanyan, R. K.; Karakhanyan, K. R.

    2008-07-01

    The mechanism of formation of curved Kikuchi lines, observed at displacement of point reflections from their normal positions, is proposed. Curving of Kikuchi lines is explained taking into account the participation of diffracted electron beams in the formation of Kikuchi electron diffraction patterns.

  6. Nonlinear dynamic theory for photorefractive phase hologram formation

    NASA Technical Reports Server (NTRS)

    Kim, D. M.; Shah, R. R.; Rabson, T. A.; Tittle, F. K.

    1976-01-01

    A nonlinear dynamic theory is developed for the formation of photorefractive volume phase holograms. A feedback mechanism existing between the photogenerated field and free-electron density, treated explicitly, yields the growth and saturation of the space-charge field in a time scale characterized by the coupling strength between them. The expression for the field reduces in the short-time limit to previous theories and approaches in the long-time limit the internal or photovoltaic field. Additionally, the phase of the space charge field is shown to be time-dependent.

  7. Hexagonal phase WO{sub 3} nanorods: Hydrothermal preparation, formation mechanism and its photocatalytic O{sub 2} production under visible-light irradiation

    SciTech Connect

    Peng, Tianyou; Ke, Dingning; Xiao, Jiangrong; Wang, Li; Hu, Jun; Zan, Ling

    2012-10-15

    WO{sub 3} with various morphologies and crystal phases were prepared via a hydrothermal process in the presence of different Na{sub 2}SO{sub 4} concentrations. Experimental results indicated that the preferred growth face ((200) crystal plane) of hexagonal WO{sub 3} could adsorb abundant Na{sup +}, which induced the WO{sub 3} growing along [001] crystal direction to form nanorods, and hexagonal phase WO{sub 3} nanorods with diameter of 30-150 nm and length of 0.5-5 {mu}m were obtained in the presence of 0.25 M Na{sub 2}SO{sub 4}, while excessive Na{sub 2}SO{sub 4} led to a mixed crystal consisting of cubic H{sub 2}W{sub 2}O{sub 7} and hexagonal WO{sub 3}. The photoactivities for O{sub 2} evolution for various products were investigated by using Fe(NO{sub 3}){sub 3} as a sacrificial reagent under visible-light irradiation, and the hexagonal phase WO{sub 3} nanorods show a better photoactivity than the other products with different morphologies and crystal phases due to its fast three-dimensional photogenerated carrier transfer along its special rod-shaped structure. - Graphical abstract: As a preferred growth plane, (200) plane of h-WO{sub 3} can adsorb abundant of Na{sup +}, which induces the WO{sub 3} growing along [001] direction to form nanorods with high length-diameter ratio. Highlights: Black-Right-Pointing-Pointer WO{sub 3} with various morphologies were hydrothermally prepared. Black-Right-Pointing-Pointer Hexagonal WO{sub 3} nanorods with diameter of 30-150 nm can be obtained from 0.25 M Na{sub 2}SO{sub 4} solution. Black-Right-Pointing-Pointer Hexagonal WO{sub 3} nanorods show a better photoactivity than the other products. Black-Right-Pointing-Pointer The good photoactivity can be due to its fast carrier transfer along its rod-shaped structure.

  8. Molecular mechanisms of bone formation in spondyloarthritis.

    PubMed

    González-Chávez, Susana Aideé; Quiñonez-Flores, Celia María; Pacheco-Tena, César

    2016-07-01

    Spondyloarthritis comprise a group of inflammatory rheumatic diseases characterized by its association to HLA-B27 and the presence of arthritis and enthesitis. The pathogenesis involves both an inflammatory process and new bone formation, which eventually lead to ankylosis of the spine. To date, the intrinsic mechanisms of the pathogenic process have not been fully elucidated, and our progress is remarkable in the identification of therapeutic targets to achieve the control of the inflammatory process, yet our ability to inhibit the excessive bone formation is still insufficient. The study of new bone formation in spondyloarthritis has been mostly conducted in animal models of the disease and only few experiments have been done using human biopsies. The deregulation and overexpression of molecules involved in the osteogenesis process have been observed in bone cells, mesenchymal cells, and fibroblasts. The signaling associated to the excessive bone formation is congruent with those involved in the physiological processes of bone remodeling. Bone morphogenetic proteins and Wnt pathways have been found deregulated in this disease; however, the cause for uncontrolled stimulation remains unknown. Mechanical stress appears to play an important role in the pathological osteogenesis process; nevertheless, the association of other important factors, such as the presence of HLA-B27 and environmental factors, remains uncertain. The present review summarizes the experimental findings that describe the signaling pathways involved in the new bone formation process in spondyloarthritis in animal models and in human biopsies. The role of mechanical stress as the trigger of these pathways is also reviewed. PMID:26838262

  9. Mechanism of K-phase growth under magnesium combustion

    SciTech Connect

    Florko, A.V.; Golovko, V.V.; Kondrat`ev, E.N.

    1995-09-01

    A model is proposed for magnesia crystal growth during the combustion of single magnesium particles and in the front of laminar diffusion two-phase flame. It is shown that the basic mechanism limiting the condensation rate is the formation of Schottky defects. The energy of their formation has been determined. The results of dispersion analysis of combustion products at air pressures of (0.1-1) {sm_bullet} 10{sup 5} Pa are in good agreement with the calculation data.

  10. Histopathology Predicts the Mechanism of Stone Formation

    NASA Astrophysics Data System (ADS)

    Evan, Andrew P.

    2007-04-01

    About 5% of American women and 12% of men will develop a kidney stone at some time in their life and these numbers appear to be on the rise. Despite years of scientific research into the mechanisms of stone formation and growth, limited advances have been made until recently. Randall's original observations and thoughts on the mechanisms for kidney stone formation have been validated for idiopathic calcium oxalate stone formers (ICSF) but not for most other stone forming groups. Our current studies on selected groups of human stone formers using intraoperative papillary biopsies has shown overwhelming evidence for the presence of Randall's plaque in ICSF and that stone formation and growth are exclusively linked to its availability to urinary ions and proteins. Intense investigation of the plaque-stone junction is needed if we are to understand the factors leading to the overgrowth process on exposed regions of plaque. Such information should allow the development of treatment strategies to block stone formation in ICSF patients. Patients who form brushite stones, or who form apatite stones because of distal renal tubular acidosis (dRTA), or patients with calcium oxalate stones due to obesity bypass procedures, or patients with cystinuria, get plugged inner medullary collecting ducts (IMCD) which leads to total destruction of the lining cells and focal sites of interstitial fibrosis. These stone formers have plaque but at levels equal to or below non-stone formers, which would suggest that they form stones by a different mechanism than do ICSF patients.

  11. Cellular and molecular mechanisms underlying presynapse formation

    PubMed Central

    Chia, Poh Hui; Li, Pengpeng

    2013-01-01

    Synapse formation is a highly regulated process that requires the coordination of many cell biological events. Decades of research have identified a long list of molecular components involved in assembling a functioning synapse. Yet how the various steps, from transporting synaptic components to adhering synaptic partners and assembling the synaptic structure, are regulated and precisely executed during development and maintenance is still unclear. With the improvement of imaging and molecular tools, recent work in vertebrate and invertebrate systems has provided important insight into various aspects of presynaptic development, maintenance, and trans-synaptic signals, thereby increasing our understanding of how extrinsic organizers and intracellular mechanisms contribute to presynapse formation. PMID:24127213

  12. Investigating materials formation with liquid-phase and cryogenic TEM

    NASA Astrophysics Data System (ADS)

    de Yoreo, J. J.; N. A. J. M., Sommerdijk

    2016-08-01

    The recent advent of liquid-phase transmission electron microscopy (TEM) and advances in cryogenic TEM are transforming our understanding of the physical and chemical mechanisms underlying the formation of materials in synthetic, biological and geochemical systems. These techniques have been applied to study the dynamic processes of nucleation, self-assembly, crystal growth and coarsening for metallic and semiconductor nanoparticles, (bio)minerals, electrochemical systems, macromolecular complexes, and organic and inorganic self-assembling systems. New instrumentation and methodologies that are currently on the horizon promise new opportunities for advancing the science of materials synthesis.

  13. Investigation of formation mechanisms of chips in orthogonal cutting process

    NASA Astrophysics Data System (ADS)

    Ma, W.

    2012-08-01

    This work investigates the formation mechanisms of chips in orthogonal cutting of mild steel and the transformation conditions between various morphology chips. It is supposed that the modeling material follows the Johnson-Cook constitutive model. In orthogonal cutting process, both the plastic flow and the instability behaviors of chip materials are caused by the plane strain loadings. Therefore, the general instability behaviors of materials in plane strain state are first analyzed with linear perturbation method and a universal instability criterion is established. Based on the analytical results, the formation mechanisms of chips and the transformation conditions between continuous and serrated chips are further studied by instability phase diagram method. The results show that the chip formation strongly depends on the intensity ratios between shear and normal stresses. The ratios of dissipative rates of plastic work done by compression and shear stresses govern the transformation from continuous to serrated chips. These results are verified by the numerical simulations on the orthogonal cutting process.

  14. Structural mechanism of the formation of mineral Na-tveitite-a new type of phase with a fluorite-derivative structure-in the NaF-CaF{sub 2}-(Y,Ln)F{sub 3} natural system

    SciTech Connect

    Golubev, A. M. Otroshchenko, L. P.; Sobolev, B. P.

    2012-03-15

    Relationships between the chemical compositions and structures of the mineral tveitite from the southern Norway pegmatites (with the idealized formula Ca{sub 14}Y{sub 5}F{sub 43}) and Na-tveitite from the Rov mountain (Keivy, Kola Peninsula) Na{sub 2.5}Ca{sub 10}Ln{sub 1.5}Y{sub 5}F{sub 42} are considered. According to the structural mechanism of its formation, Na-tveitite is a nanocomposite crystal based on the crystalline matrix Ca{sub 14}Y{sub 5}F{sub 43} with the ordered arrangement of {l_brace}Ca{sub 8}[CaY{sub 5}]F{sub 69}{r_brace} clusters which contain anionic {l_brace}F{sub 13}{r_brace} cuboctahedra with F{sup 1-} at the center. When Na-tveitite is formed, 29% of these clusters are statistically replaced by Na-'Y' clusters {l_brace}[Na{sub 0.5}(Y,Ln){sub 0.5}]{sub 14}F{sub 64}{r_brace} with {l_brace}F{sub 8}{r_brace} cubes at the center (analogs of matrix fluorite groups {l_brace}Ca{sub 14}F{sub 64}{r_brace}). This replacement gives rise to composition-imperfect (Na, Ca, 'Y') cationic positions and occupancy-deficient F positions, which correspond to {l_brace}F{sub 13}{r_brace} cuboctahedra and the {l_brace}F{sub 8}{r_brace} cubes that replace them. The difference between Na-tveitite and fluorite phases M{sub 1-x}R{sub x}F{sub 2+x} is as follows: its matrix is the structure of the ordered phase (tveitite) into which Na-containing rare earth fragments of fluorite-type structure are incorporated instead of ordered-phase structural blocks (clusters).

  15. Formation mechanisms of magnetotail dipolarization fronts

    NASA Astrophysics Data System (ADS)

    Fu, Huishan

    2016-07-01

    Dipolarization fronts (DFs) are frequently detected in the Earth's magnetotail. How these DFs are formed is still poorly understood. Three possible mechanisms have been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: transient reconnection can produce DFs. We suggest that the DFs detected in the near-Earth region (XGSM > -10 RE) are primarily attributed to jet braking, while the DFs detected in the mid- or far-tail region (XGSM < -15 RE) are primarily attributed to transient reconnection or spontaneous formation. In the jet-braking mechanism, the high-speed flow 'pushes' the pre-existing plasmas to produce the DF, so that there is causality between high-speed flow and DF. In the transient-reconnection mechanism, there is no causality between high-speed flow and DF, because the frozen-in condition is violated.

  16. Mechanisms of scaling in pattern formation

    PubMed Central

    Umulis, David M.; Othmer, Hans G.

    2013-01-01

    Many organisms and their constituent tissues and organs vary substantially in size but differ little in morphology; they appear to be scaled versions of a common template or pattern. Such scaling involves adjusting the intrinsic scale of spatial patterns of gene expression that are set up during development to the size of the system. Identifying the mechanisms that regulate scaling of patterns at the tissue, organ and organism level during development is a longstanding challenge in biology, but recent molecular-level data and mathematical modeling have shed light on scaling mechanisms in several systems, including Drosophila and Xenopus. Here, we investigate the underlying principles needed for understanding the mechanisms that can produce scale invariance in spatial pattern formation and discuss examples of systems that scale during development. PMID:24301464

  17. Peptide Bond Formation Mechanism Catalyzed by Ribosome

    PubMed Central

    Świderek, Katarzyna; Marti, Sergio; Tuñón, Iñaki; Moliner, Vicent; Bertran, Juan

    2015-01-01

    In this paper we present a study of the peptide bond formation reaction catalyzed by ribosome. Different mechanistic proposals have been explored by means of Free Energy Perturbation methods within hybrid QM/MM potentials, where the chemical system has been described by the M06-2X functional and the environment by means of the AMBER force field. According to our results, the most favourable mechanism in the ribosome would proceed through an eight-membered ring transition state, involving a proton shuttle mechanism through the hydroxyl group of the sugar and a water molecule. This transition state is similar to that described for the reaction in solution (J. Am. Chem. Soc. 2013, 135, 8708–8719) but the reaction mechanisms are noticeable different. Our simulations reproduce the experimentally determined catalytic effect of ribosome that can be explained by the different behaviour of the two environments. While the solvent reorganizes during the chemical process involving an entropic penalty, the ribosome is preorganized in the formation of the Michaelis complex and does not suffer important changes along the reaction, dampening the charge redistribution of the chemical system. PMID:26325003

  18. Space Shuttle Pinhole Formation Mechanism Studies

    NASA Technical Reports Server (NTRS)

    Jacobson, Nathan S.

    1998-01-01

    Pinholes have been observed to form on the wing leading edge of the space shuttle after about 10-15 flights. In this report we expand upon previous observations by Christensen (1) that these pinholes often form along cracks and are associated with a locally zinc-rich area. The zinc appears to come from weathering and peeling paint on the launch structure. Three types of experimental examinations are performed to understand this issue further: (A) Detailed microstructural examination of actual shuttle pinholes (B) Mass spectrometric studies of coupons containing, actual shuttle pinholes and (C) Laboratory furnace studies of ZnO/SiC reactions and ZnO/SiC protected carbon/carbon reaction. On basis of these observations we present a detailed mechanism of pinhole formation due to formation of a corrosive ZnO-Na-2-O-SiO2 ternary glass, which flows into existing cracks and enlarges them.

  19. Deformation mechanisms in a Laves phase

    SciTech Connect

    Liu, Yaping; Allen, S.M.; Livingston, J.D.

    1992-12-31

    The stress-induced phase transformation between C36 and C15 structures in Fe{sub 2}Zr is studied by electron microscopy. Nucleus of transformation is believed to be pre-existing C15 layers in C36 particles. Microstructural evidence for three mechanisms of growth of a new phase were found: Fault accumulation and rearrangement, moving of a individual partial dislocations between two phases, and the migration of microscopic ledges composed of a series of Shockley partials between C36 and C15. Plastic deformation by slip on non-basal planes of C36 caused by indentation is studied.

  20. Mechanisms of lamellar collagen formation in connective tissues.

    PubMed

    Ghazanfari, Samaneh; Khademhosseini, Ali; Smit, Theodoor H

    2016-08-01

    The objective of tissue engineering is to regenerate functional tissues. Engineering functional tissues requires an understanding of the mechanisms that guide the formation and evolution of structure in the extracellular matrix (ECM). In particular, the three-dimensional (3D) collagen fiber arrangement is important as it is the key structural determinant that provides mechanical integrity and biological function. In this review, we survey the current knowledge on collagen organization mechanisms that can be applied to create well-structured functional lamellar tissues and in particular intervertebral disc and cornea. Thus far, the mechanisms behind the formation of cross-aligned collagen fibers in the lamellar structures is not fully understood. We start with cell-induced collagen alignment and strain-stabilization behavior mechanisms which can explain a single anisotropically aligned collagen fiber layer. These mechanisms may explain why there is anisotropy in a single layer in the first place. However, they cannot explain why a consecutive collagen layer is laid down with an alternating alignment. Therefore, we explored another mechanism, called liquid crystal phasing. While dense concentrations of collagen show such behavior, there is little evidence that the conditions for liquid crystal phasing are actually met in vivo. Instead, lysyl aldehyde-derived collagen cross-links have been found essential for correct lamellar matrix deposition. Furthermore, we suggest that supra-cellular (tissue-level) shear stress may be instrumental in the alignment of collagen fibers. Understanding the potential mechanisms behind the lamellar collagen structure in connective tissues will lead to further improvement of the regeneration strategies of functional complex lamellar tissues.

  1. Metastable phase formation in undercooled liquid lead alloys

    NASA Technical Reports Server (NTRS)

    Fecht, Hans J.

    1991-01-01

    During non-equilibrium processes metastable phases are formed instead of stable phases due to the operation of various kinetic or structural constraints. By removing the most effective nucleation sites for the stable phase using emulsified droplet samples, stable phase formation can be prohibited in a broad range of undercooling and the phase space available in the metastable regime can be mapped out. With this method the thermodynamic properties of the undercooled liquid and the metastable phase boundaries corresponding to reversible metastable (solid + liquid) phase equilibria are examined from experimental data. The analysis reveals important implications for the nucleation kinetics and the choice of the kinetically most favored solidification path.

  2. The free energy of mechanically unstable phases

    PubMed Central

    van de Walle, A.; Hong, Q.; Kadkhodaei, S.; Sun, R.

    2015-01-01

    Phase diagrams provide ‘roadmaps' to the possible states of matter. Their determination traditionally rests on the assumption that all phases, even unstable ones, have well-defined free energies under all conditions. However, this assumption is commonly violated in condensed phases due to mechanical instabilities. This long-standing problem impedes thermodynamic database development, as pragmatic attempts at solving this problem involve delicate extrapolations that are highly nonunique and that lack an underlying theoretical justification. Here we propose an efficient computational solution to this problem that has a simple interpretation, both as a topological partitioning of atomic configuration space and as a minimally constrained physical system. Our natural scheme smoothly extends the free energy of stable phases, without relying on extrapolation, thus enabling a formal assessment of widely used extrapolation schemes. PMID:26130613

  3. String Mechanism for Relativistic Jet Formation

    NASA Astrophysics Data System (ADS)

    Dyadechkin, S. A.; Semenov, V. S.; Punsly, B.; Biernat, H. K.

    Here we present our latest studies of relativistic jet formation in the vicinity of a rotating black hole where the reconnection process has been taken into account. In order to simplify the problem, we use Lagrangian formalism and develop a method which enables us to consider a magnetized plasma as a set of magnetic flux tubes [5,6]. Within the limits of the Lagrangian approach, we perform numerical simulations of the flux tube (nonlinear string) behavior which clearly demonstrates the process of relativistic jet formation in the form of outgoing torsional nonlinear aves. It turns out that the jet is produced deep inside the ergosphere where the flux tube takes away spinning energy from the black hole due to the nonlocal Penrose process [2]. This is similar to the Blandford-Znajek (BZ) mechanism to some extent [8], however, the string mechanism is essentially time dependent. It is shown that the leading part of the accreting tube gains negative energy and therefore has to stay in the ergosphere forever. Simultaneously, another part of the tube propagates along the spinning axis away from the hole with nearly the speed of light. As a result, the tube is continuously stretching and our mechanism is essentially time dependent. Obviously, such process cannot last infinitely long and we have to take into account the reconnection process. Due to reconnection, the topology of the flux tube is changed and it gives rise to a plasmoid creation which propagates along spin axis of the hole with relativistic speed carrying off the energy and angular momentum away from the black hole.

  4. Phase diagram of hopping conduction mechanisms in polymer nanofiber network

    SciTech Connect

    Li, Jeng-Ting; Lu, Yu-Cheng; Jiang, Shiau-Bin; Zhong, Yuan-Liang; Yeh, Jui-Ming

    2015-12-07

    Network formation by nanofiber crosslinking is usually in polymer materials as application in organic semiconductor devices. Electron hopping transport mechanisms depend on polymer morphology in network. Conducting polymers morphology in a random network structure is modeled by a quasi-one-dimensional system coupled of chains or fibers. We observe the varying hopping conduction mechanisms in the polyaniline nanofibers of the random network structure. The average diameter d of the nanofibers is varied from approximately 10 to 100 nm. The different dominant hopping mechanisms including Efros-Shklovskii variable-range hopping (VRH), Mott VRH, and nearest-neighbor hopping are dependent on temperature range and d in crossover changes. The result of this study is first presented in a phase diagram of hopping conduction mechanisms based on the theories of the random network model. The hopping conduction mechanism is unlike in normal semiconductor materials.

  5. Phase diagram of hopping conduction mechanisms in polymer nanofiber network

    NASA Astrophysics Data System (ADS)

    Li, Jeng-Ting; Lu, Yu-Cheng; Jiang, Shiau-Bin; Zhong, Yuan-Liang; Yeh, Jui-Ming

    2015-12-01

    Network formation by nanofiber crosslinking is usually in polymer materials as application in organic semiconductor devices. Electron hopping transport mechanisms depend on polymer morphology in network. Conducting polymers morphology in a random network structure is modeled by a quasi-one-dimensional system coupled of chains or fibers. We observe the varying hopping conduction mechanisms in the polyaniline nanofibers of the random network structure. The average diameter d of the nanofibers is varied from approximately 10 to 100 nm. The different dominant hopping mechanisms including Efros-Shklovskii variable-range hopping (VRH), Mott VRH, and nearest-neighbor hopping are dependent on temperature range and d in crossover changes. The result of this study is first presented in a phase diagram of hopping conduction mechanisms based on the theories of the random network model. The hopping conduction mechanism is unlike in normal semiconductor materials.

  6. O2 on ganymede: Spectral characteristics and plasma formation mechanisms

    USGS Publications Warehouse

    Calvin, W.M.; Johnson, R.E.; Spencer, J.R.

    1996-01-01

    Weak absorption features in the visible reflectance spectrum of Jupiter's satellite Ganymede have been correlated to those observed in the spectrum of molecular oxygen. We examine the spectral characteristics of these absorption features in all phases of O2 and conclude that the molecular oxygen is most likely present at densities similar to the liquid or solid ??-phase. The contribution of O2 to spectral features observed on Ganymede in the near-infrared wavelength region affects the previous estimates of photon pathlength in ice. The concentration of the visible absorption features on the trailing hemisphere of Ganymede suggests an origin due to bombardment by magneto-spheric ions. We derive an approximate O2 formation rate from this mechanism and consider the state of O2 within the surface.

  7. The formation phase of the solar nebula

    NASA Technical Reports Server (NTRS)

    Bodenheimer, Peter; Yorke, Harold W.; Rozyczka, Michal; Tohline, Joel E.

    1990-01-01

    Hydrodynamical calculations of the collapse of an axisymmetric, rotating protostellar cloud, with radiation transport and without magnetic fields, are presented. The collapse is assumed to start from a centrally condensed sphere of radius 5 x 10 to the 15th cm, a mean density of 4 x 10 to the -15th g/cu cm, a total mass of 1 solar mass, and a total angular momentum of 10 to the 53rd g sq cm per sec. The numerical grid is chosen to resolve the region of disk formation between 1 and 60 AU from the center. Frequency-dependent radiative transfer calculations show how the emergent spectrum of the structure depends upon viewing angle with respect to the rotation axis and how the observed isophotal contours should depend on wavelength and viewing angle. The central part of the protostar, interior to 1 AU, is not resolved numerically but is modeled approximately. At the end of the calculation, this region is found to have a mass of 0.6 solar mass and a ratio of rotational to gravitational energy of about 0.4, sufficiently large to be unstable to nonaxisymmetric perturbations. Although the disk is gravitationally stable according to the local Toomre criterion, the nonaxisymmetric structure in the center is likely to lead to angular momentum transport.

  8. The mechanics of endothelial gap formation

    NASA Astrophysics Data System (ADS)

    Chattoraj, Joyjit; Del Gado, Emanuela; Hardin, C. Corey; Krishnan, Ramaswamy

    The vascular endothelium is a layer of specialized cells, referred to as endothelial cells (EC) that line the internal surfaces of blood vessels and are largely responsible for regulating the transit of fluids, solutes and immune system cells from the circulation, across the vessel wall, and into the tissues. We investigate the physics of the mechanical events that may proceed and eventually lead to dramatic increase of its permeability, leading to serious illness. In combination with experiments measuring local stresses and gap formation in EC in different conditions, we devise a minimal model based on an amorphous assembly of adhesive particles, subjected to an imposed tension. Numerical simulations of the model show that, as a function of the rate at which the tension is imposed, the system goes from an elastic regime in which small gaps increase in number to a ``plastic'' one, where pre-existing gaps increase in size, and internal stresses display large hetereogeneities and long range correlations. This second regime bears intriguing similarities with the experimental finding in EC monolayers.

  9. Dynamical phase transitions in quantum mechanics

    NASA Astrophysics Data System (ADS)

    Rotter, Ingrid

    2012-02-01

    The nucleus is described as an open many-body quantum system with a non-Hermitian Hamilton operator the eigenvalues of which are complex, in general. The eigenvalues may cross in the complex plane (exceptional points), the phases of the eigenfunctions are not rigid in approaching the crossing points and the widths bifurcate. By varying only one parameter, the eigenvalue trajectories usually avoid crossing and width bifurcation occurs at the critical value of avoided crossing. An analog spectroscopic redistribution takes place for discrete states below the particle decay threshold. By this means, a dynamical phase transition occurs in the many-level system starting at a critical value of the level density. Hence the properties of the low-lying nuclear states (described well by the shell model) and those of highly excited nuclear states (described by random ensembles) differ fundamentally from one another. The statement of Niels Bohr on the collective features of compound nucleus states at high level density is therefore not in contradiction to the shell-model description of nuclear (and atomic) states at low level density. Dynamical phase transitions are observed experimentally in different quantum mechanical systems by varying one or two parameters.

  10. Modeling liquid-liquid phase transitions and quasicrystal formation

    NASA Astrophysics Data System (ADS)

    Skibinsky, Anna

    In this thesis, studies which concern two different subjects related to phase transitions in fluids and crystalline solids are presented. Condensed matter formation, structure, and phase transitions are modeled using molecular dynamics simulations of simple discontinuous potentials with attractive and repulsive interactions. Novel phase diagrams are proposed for quasicrystals, crystals, and liquids. In the first part of the thesis, the formation of a quasicrystal in a two dimensional monodisperse system is investigated using molecular dynamics simulations of hard sphere particles interacting via a two-dimensional square-well potential. It is found that for certain values of the square-well parameters more than one stable crystalline phase can form. By quenching the liquid phase at a very low temperature, an amorphous phase is obtained. When this the amorphous phase is heated, a quasicrystalline structure with five-fold symmetry forms. From estimations of the Helmholtz potentials of the stable crystalline phases and of the quasicrystal, it is concluded that within a specific temperature range, the observed quasicrystal phase can be the stable phase. The second part of the thesis concerns a study of the liquid-liquid phase transition for a single-component system in three dimensions, interacting via an isotropic potential with a repulsive soft-core shoulder at short distance and an attractive well at an intermediate distance. The potential is similar to potentials used to describe such liquid systems as colloids, protein solutions, or liquid metals. It is shown that the phase diagram for such a potential can have two lines of first-order fluid-fluid phase transitions: one separating a gas and a low-density liquid (LDL), and another between the LDL and a high-density liquid (HDL). Both phase transition lines end in a critical point, a gas-LDL critical point and, depending on the potential parameters, either a gas-HDL critical point or a LDL-HDL critical point. A

  11. Synchronization, phase locking, and metachronal wave formation in ciliary chains.

    PubMed

    Niedermayer, Thomas; Eckhardt, Bruno; Lenz, Peter

    2008-09-01

    Synchronization and wave formation in one-dimensional ciliary arrays are studied analytically and numerically. We develop a simple model for ciliary motion that is complex enough to describe well the behavior of beating cilia but simple enough to study collective effects analytically. Beating cilia are described as phase oscillators moving on circular trajectories with a variable radius. This radial degree of freedom turns out to be essential for the occurrence of hydrodynamically induced synchronization of ciliary beating between neighboring cilia. The transitions to the synchronized and phase-locked state of two cilia and the formation of metachronal waves in ciliary chains with different boundary conditions are discussed.

  12. Synchronization, phase locking, and metachronal wave formation in ciliary chains

    NASA Astrophysics Data System (ADS)

    Niedermayer, Thomas; Eckhardt, Bruno; Lenz, Peter

    2008-09-01

    Synchronization and wave formation in one-dimensional ciliary arrays are studied analytically and numerically. We develop a simple model for ciliary motion that is complex enough to describe well the behavior of beating cilia but simple enough to study collective effects analytically. Beating cilia are described as phase oscillators moving on circular trajectories with a variable radius. This radial degree of freedom turns out to be essential for the occurrence of hydrodynamically induced synchronization of ciliary beating between neighboring cilia. The transitions to the synchronized and phase-locked state of two cilia and the formation of metachronal waves in ciliary chains with different boundary conditions are discussed.

  13. Omega phase formation in titanium and titanium alloys

    SciTech Connect

    Gray, G.T. III; Morris, C.E.; Lawson, A.C.

    1992-05-01

    Although the response of titanium alloys to dynamic loading is receiving increased attention in the literature (particularly in the area of shear-band formation), a more limited experimental database exists concerning the detailed structure/property relationships of titanium alloys subjected to shock loading. In this study, preliminary results concerning the influence of alloy chemistry on the property of omega-phase formation and its structure in three titanium alloys are presented. The influence of shock-wave deformation on the phase stability and substructure evolution of high-purity (low-interstitial) titanium, A-70 (3700 ppm oxygen) titanium, and Ti-6Al-4V were probed utilizing real-time velocity interferometry (VISAR) and ``soft`` shock-recovery techniques. VISAR wave profiles of shock-loaded high-purity titanium revealed the omega-phase pressure-induced transition to occur at approximately 10.4 GPa. Wave profile measurements on A-70 Ti shocked to pressures up to 35 GPa and Ti-6Al-4V shocked to pressures up to 25 GPa exhibited no evidence of a three-wave structure indicative of a pressure-induced phase transition. Neutron and X-ray diffractometry and TEM analysis confirmed the presence of retained {omega}-phase in the electrolytic-Ti and the absence of {omega}-phase in the shock-recovered A-70 Ti and Ti-6Al-4V. Suppression of the {alpha}-{omega} phase transition in A-70 Ti, containing a high interstitial oxygen content, is seen to simultaneously correlate with suppression of deformation twinning. Neutron diffraction was used to measure the in-situ bulk lattice constants and volume fraction of the {alpha} and {omega} phases in the recovered high-purity titanium samples that were shock loaded. The influence of alloy content on the kinetics of formation/retention of {omega}-phase and substructure evolution is discussed and contrasted in light of previous literature studies.

  14. The phase evolution mechanism in Fe(Se, Te) system

    NASA Astrophysics Data System (ADS)

    Liu, Jixing; Li, Chengshan; Zhang, Shengnan; Feng, Jianqing; Zhang, Pingxiang; Zhou, Lian

    2016-08-01

    The phase evolution mechanism in Fe(Se, Te) system during sintering was investigated with step-by-step heat treatment process. It was noticed that the diffusion processes between Fe and Se (Te) as well as that between Se and Te were both very important to the formation of superconducting Fe(Se, Te) phase with very uniform chemical composition. During heat treatment, solid solutions of (Se, Te)ss with different chemical composition were formed with the diffusion of Se atoms into Te solids and Te atoms into Se melts, simultaneously. Then with the increasing temperature, Fe atoms diffused into (Se, Te)ss, Fe(Se, Te)2 and Fe(Se, Te) phases were formed in sequence with the increasing Fe content. The chemical composition in melts became more and more uniform with the further increasing of sintering temperature and dwell time. Therefore, it was suggested that in order to achieve Fe(Se, Te) phase with high superconducting properties, it was necessary to enhance the diffusion process during sintering. The critical temperature of the sample, which was sintered at 700 °C for 12 h with slow cooling process and an O2-annealing process for 24 h, was above 14.0 K. This Tc value proved that a good superconducting β phase could be obtained under this sintering condition.

  15. Statistical mechanics of soft-boson phase transitions

    NASA Technical Reports Server (NTRS)

    Gupta, Arun K.; Hill, Christopher T.; Holman, Richard; Kolb, Edward W.

    1991-01-01

    The existence of structure on large (100 Mpc) scales, and limits to anisotropies in the cosmic microwave background radiation (CMBR), have imperiled models of structure formation based solely upon the standard cold dark matter scenario. Novel scenarios, which may be compatible with large scale structure and small CMBR anisotropies, invoke nonlinear fluctuations in the density appearing after recombination, accomplished via the use of late time phase transitions involving ultralow mass scalar bosons. Herein, the statistical mechanics are studied of such phase transitions in several models involving naturally ultralow mass pseudo-Nambu-Goldstone bosons (pNGB's). These models can exhibit several interesting effects at high temperature, which is believed to be the most general possibilities for pNGB's.

  16. A GAS-PHASE FORMATION ROUTE TO INTERSTELLAR TRANS-METHYL FORMATE

    SciTech Connect

    Cole, Callie A.; Wehres, Nadine; Yang Zhibo; Thomsen, Ditte L.; Bierbaum, Veronica M.; Snow, Theodore P. E-mail: Nadine.Wehres@colorado.edu E-mail: Veronica.Bierbaum@colorado.edu E-mail: dlt@chem.ku.dk

    2012-07-20

    The abundance of methyl formate in the interstellar medium has previously been underpredicted by chemical models. Additionally, grain surface chemistry cannot account for the relative abundance of the cis- and trans-conformers of methyl formate, and the trans-conformer is not even formed at detectable abundance on these surfaces. This highlights the importance of studying formation pathways to methyl formate in the gas phase. The rate constant and branching fractions are reported for the gas-phase reaction between protonated methanol and formic acid to form protonated trans-methyl formate and water as well as adduct ion: Rate constants were experimentally determined using a flowing afterglow-selected ion flow tube apparatus at 300 K and a pressure of 530 mTorr helium. The results indicate a moderate overall rate constant of (3.19 {+-} 0.39) Multiplication-Sign 10{sup -10} cm{sup 3} s{sup -1} ({+-} 1{sigma}) and an average branching fraction of 0.05 {+-} 0.04 for protonated trans-methyl formate and 0.95 {+-} 0.04 for the adduct ion. These experimental results are reinforced by ab initio calculations at the MP2(full)/aug-cc-pVTZ level of theory to examine the reaction coordinate and complement previous density functional theory calculations. This study underscores the need for continued observational studies of trans-methyl formate and for the exploration of other gas-phase formation routes to complex organic molecules.

  17. A Gas-phase Formation Route to Interstellar Trans-methyl Formate

    NASA Astrophysics Data System (ADS)

    Cole, Callie A.; Wehres, Nadine; Yang, Zhibo; Thomsen, Ditte L.; Snow, Theodore P.; Bierbaum, Veronica M.

    2012-07-01

    The abundance of methyl formate in the interstellar medium has previously been underpredicted by chemical models. Additionally, grain surface chemistry cannot account for the relative abundance of the cis- and trans-conformers of methyl formate, and the trans-conformer is not even formed at detectable abundance on these surfaces. This highlights the importance of studying formation pathways to methyl formate in the gas phase. The rate constant and branching fractions are reported for the gas-phase reaction between protonated methanol and formic acid to form protonated trans-methyl formate and water as well as adduct ion: Rate constants were experimentally determined using a flowing afterglow-selected ion flow tube apparatus at 300 K and a pressure of 530 mTorr helium. The results indicate a moderate overall rate constant of (3.19 ± 0.39) × 10-10 cm3 s-1 (± 1σ) and an average branching fraction of 0.05 ± 0.04 for protonated trans-methyl formate and 0.95 ± 0.04 for the adduct ion. These experimental results are reinforced by ab initio calculations at the MP2(full)/aug-cc-pVTZ level of theory to examine the reaction coordinate and complement previous density functional theory calculations. This study underscores the need for continued observational studies of trans-methyl formate and for the exploration of other gas-phase formation routes to complex organic molecules.

  18. [The determination of the sequence of formation of mechanical injuries].

    PubMed

    Avkhodiev, G I; Belomestnova, O V; Reshetnikova, S S

    2012-01-01

    The objective of the present work was to highlight the problem of the sequence of formation of mechanical injuries and estimate the prospects for the application of selected modes of their solution. Various approaches to the assessment of a combination of morphological features studied in the course of forensic medical examinations are considered in order to determine the sequence of formation of mechanical injuries.

  19. Effect of electron irradiation exposure on phase formation, microstructure and mechanical strength of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} superconductor prepared via co-precipitation method

    SciTech Connect

    Mohiju, Zaahidah 'Atiqah; Hamid, Nasri A. Kannan, V.; Abdullah, Yusof

    2015-04-29

    In this work the effect of electron irradiation on the mechanical properties of Bi2Sr2CaCu2O8 (Bi-2212) superconductor was studied by exposing the Bi-2212 superconductor with different doses of electron irradiation. Bi-2212 samples were prepared by using co-precipitation method. Irradiation was performed with irradiation dose of 100 kGray and 200 kGray, respectively. Characterization of the samples was performed by using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Instron Universal Testing machine was used to measure the strength of the samples. The XRD patterns for the non-irradiated and irradiated samples show well-defined peaks of which could be indexed on the basis of a Bi-2212 phase structure. XRD patterns also indicate that electron irradiation did not affect the Bi-2212 superconducting phase. SEM micrographs show disorientation in the texture of the microstructure for irradiated samples. Sample exposed to 200 kGray electron irradiation dose shows enhancement of grain size. Their grain growth and texture improved slightly compared to other sample. The results also show that enlargement of grain size resulted in higher mechanical strength.

  20. Formation of Frank-Kasper σ-phase from polydisperse diblock copolymers

    NASA Astrophysics Data System (ADS)

    Liu, Meijiao; Li, Weihua; Shi, An-Chang

    Recent experimental and theoretical studies have revealed a number of complex spherical phases including the complex Frank-Kasper σ-phase, which consists of 30 spheres in a unit cell. It is desirable to understand the mechanisms for the formation of the complex spherical phases such as the A15-phase and the Frank-Kasper σ-phase in block copolymers. Based on the observation that the A15-phase and the Frank-Kasper σ-phase are composed of spherical domains with different sizes, we hypothesize that polydispersity of the block copolymers could be used to obtain these complex phases. We tested this hypothesis by carrying out self-consistent field theory for polydisperse AB diblock copolymers. Specially we studied the relative stability of various spherical phases, including the fcc, bcc, A15 and Frank_Kasper σ-phase, in binary blends composed of AB block copolymers different lengths of the A-blocks. Our results revealed that the Frank-Kasper σ-phase could be stabilized by tailoring the length ratio as well as the compositions of the two diblock copolymers. The distribution of the diblocks in the system indicates that copolymer segregation is the origin of the formation of spherical domains with different sizes.

  1. Kinetic Study to Predict Sigma Phase Formation in Duplex Stainless Steels

    NASA Astrophysics Data System (ADS)

    dos Santos, Daniella Caluscio; Magnabosco, Rodrigo

    2016-04-01

    This work presents an improved kinetic study of sigma phase formation during isothermal aging between 973 K and 1223 K (700 °C and 950 °C), based on Kolmogorov-Johnson-Mehl-Avrami (K-J-M-A) model, established from volume fraction of sigma phase determined in backscattered electron images over polished surfaces of aged samples. The kinetic study shows a change in the main mechanism of sigma formation between 973 K and 1173 K (700 °C and 900 °C), from a nucleation-governed stage to a diffusion-controlled growth-coarsening stage, confirmed by a double inclination in K-J-M-A plots and microstructural observations. A single inclination in K-J-M-A plots was observed for the 1223 K (950 °C) aging temperature, showing that kinetic behavior in this temperature is only related to diffusion-controlled growth of sigma phase. The estimated activation energies for the nucleation of sigma phase are close to the molybdenum diffusion in ferrite, probably the controlling mechanism of sigma phase nucleation. The proposed time-temperature-transformation (TTT) diagram shows a "double c curve" configuration, probably associated to the presence of chi-phase formed between 973 K and 1073 K (700 °C and 800 °C), which acts as heterogeneous nuclei for sigma phase formation in low aging temperatures.

  2. Formation of vortices in first order phase transitions

    NASA Astrophysics Data System (ADS)

    Melfo, Alejandra; Perivolaropoulos, Leandros

    1995-07-01

    Using a toy model Langrangian we investigate the formation of vortices in first order phase transitions. The evolution and interactions of vacuum bubbles are also studied using both analytical approximations and a numerical simulation of scalar field dynamics. A long-lived bubble wall bound state is discovered and its existence is justified by using a simplified potential for the bubble wall interaction. The conditions that need to be satisfied for vortex formation by bubble collisions are also studied with particular emphasis placed on geometrical considerations. These conditions are then implemented in a Monte Carlo simulation for the study of the probability of defect formation. It is shown that the probability of vortex formation by the collision of relativistically expanding bubbles gets reduced by about 10% due to the above-mentioned geometric effects.

  3. Formation of vortices in first order phase transitions

    SciTech Connect

    Melfo, A. |; Perivolaropoulos, L. |

    1995-07-15

    Using a toy model Langrangian we investigate the formation of vortices in first order phase transitions. The evolution and interactions of vacuum bubbles are also studied using both analytical approximations and a numerical simulation of scalar field dynamics. A long-lived bubble wall bound state is discovered and its existence is justified by using a simplified potential for the bubble wall interaction. The conditions that need to be satisfied for vortex formation by bubble collisions are also studied with particular emphasis placed on geometrical considerations. These conditions are then implemented in a Monte Carlo simulation for the study of the probability of defect formation. It is shown that the probability of vortex formation by the collision of relativistically expanding bubbles gets reduced by about 10% due to the above-mentioned geometric effects.

  4. Stalk Phase Formation: Effects of Dehydration and Saddle Splay Modulus

    PubMed Central

    Kozlovsky, Yonathan; Efrat, Avishay; Siegel, David A.; Kozlov, Michael M.

    2004-01-01

    One of the earliest lipid intermediates forming in the course of membrane fusion is the lipid stalk. Although many aspects of the stalk hypothesis were elaborated theoretically and confirmed by experiments it remained unresolved whether stalk formation is always an energy consuming process or if there are conditions where the stalks are energetically favorable and form spontaneously resulting in an equilibrium stalk phase. Motivated by a recent breakthrough experiments we analyze the physical factors determining the spontaneous stalk formation. We show that this process can be driven by interplay between two factors: the elastic energy of lipid monolayers including a contribution of the saddle splay deformation and the energy of hydration repulsion acting between apposing membranes. We analyze the dependence of stalk formation on the saddle splay (Gaussian) modulus of the lipid monolayers and estimate the values of this modulus based on the experimentally established phase boundary between the lamellar and the stalk phases. We suggest that fusion proteins can induce stalk formation just by bringing the membranes into close contact, and accumulating, at least locally, a sufficiently large energy of the hydration repulsion. PMID:15454446

  5. Star formation and gas phase history of the cosmic web

    NASA Astrophysics Data System (ADS)

    Snedden, Ali; Coughlin, Jared; Phillips, Lara Arielle; Mathews, Grant; Suh, In-Saeng

    2016-01-01

    We present a new method of tracking and characterizing the environment in which galaxies and their associated circumgalactic medium evolve. We have developed a structure finding algorithm that uses the rate of change of the density gradient to self-consistently parse and follow the evolution of groups/clusters, filaments and voids in large-scale structure simulations. We use this to trace the complete evolution of the baryons in the gas phase and the star formation history within each structure in our simulated volume. We vary the structure measure threshold to probe the complex inner structure of star-forming regions in poor clusters, filaments and voids. We find that the majority of star formation occurs in cold, condensed gas in filaments at intermediate redshifts (z ˜ 3). We also show that much of the star formation above a redshift z = 3 occurs in low-contrast regions of filaments, but as the density contrast increases at lower redshift, star formation switches to the high-contrast regions, or inner parts, of filaments. Since filaments bridge the void and cluster regions, it suggests that the majority of star formation occurs in galaxies in intermediate density regions prior to the accretion on to groups/clusters. We find that both filaments and poor clusters are multiphase environments distinguishing themselves by different distributions of gas phases.

  6. Shallow Methane Hydrates: Rates, Mechanisms of Formation and Environmental Significance.

    NASA Astrophysics Data System (ADS)

    Torres, M. E.; Trehu, A. M.

    2005-05-01

    Shallow gas hydrates have been identified at more than 20 locations worldwide, and are commonly associated with observations of bubble discharge at the seafloor. These deposits are host to active chemosynthetic communities and are likely to play a predominant role in energy, climate and carbon cycle issues associated with hydrate processes. Because seafloor gas hydrates are not in equilibrium with seawater, these deposits require a constant supply of methane to replace loss by continuous diffusion to bottom water. We will summarize evidence documenting that at the shallow deposits on Hydrate Ridge (OR) methane must be delivered in the free gas phase and present simple models used to infer formation rates, which are orders of magnitude higher than those for hydrates formed deeper in the sediment column (Torres et al., 2004). At Hydrate Ridge, methane gas is channeled from deep accretionary margin sequences to the gas hydrate stability zone (GHSZ) through a permeable layer that has been mapped seismically (Horizon A). High gas pressure in this horizon can drive gas through the GHSZ to the seafloor (Trehu et al., 2004). We will review current ideas that address mechanisms whereby gas migrates from Horizon A to the seafloor, including inhibition by capillary effects and the development of a high salinity front that can shift the hydrate stability field enough to allow for methane transport as a gas phase.

  7. The Mechanics of Pseudotachylite Formation in Impacts

    NASA Astrophysics Data System (ADS)

    Melosh, H. J.

    2003-12-01

    Geologist James Shand first recognized pseudotachylites near the Vredefort structure in 1916. They appear to be black, glassy veins that often contain broken fragments of country rock. The veins range from millimeters thick to masses many meters in extent. Since this discovery they have puzzled several generations of geologists. Pseudotachylites are evidently due to rapid melting of rock in place and seem to be associated with environments, such as faulting, landslides and impacts, where rock is put into rapid motion. I examine the basic constraints controlling the formation of pseudotachylites in the rapidly sheared rocks in the vicinity of a large meteorite impact. The prevailing opinion among many geologists is that pseudotachylites are formed by friction melting of rock. The principal mystery of pseudotachylite formation is not that friction can cause melting, but that it seems to form thick masses of it. Yet such thick masses ought to preclude melting by reducing the friction between sliding rock masses. I propose that a solution to this conundrum is that the melt produced by sliding on narrow shear zones is extruded into the adjacent country rock, thus keeping the sliding surfaces narrow while thick masses of melt accumulate in pockets opened by slip on faults oriented at large angles to the sliding plane.

  8. Simulating the Phases of the Moon Shortly After Its Formation

    NASA Astrophysics Data System (ADS)

    Noordeh, Emil; Hall, Patrick; Cuk, Matija

    2014-04-01

    The leading theory for the origin of the Moon is the giant impact hypothesis, in which the Moon was formed out of the debris left over from the collision of a Marssized body with the Earth. Soon after its formation, the orbit of the Moon may have been very different than it is today. We have simulated the phases of the Moon in a model for its formation wherein the Moon develops a highly elliptical orbit with its major axis tangential to the Earth's orbit. This note describes these simulations and their pedagogical value.

  9. Dynamic phases, pinning, and pattern formation for driven dislocation assemblies

    DOE PAGESBeta

    Zhou, Caizhi; Reichhardt, Charles; Olson Reichhardt, Cynthia J.; Beyerlein, Irene J.

    2015-01-23

    We examine driven dislocation assemblies and show that they can exhibit a set of dynamical phases remarkably similar to those of driven systems with quenched disorder such as vortices in superconductors, magnetic domain walls, and charge density wave materials. These phases include pinned-jammed, fluctuating, and dynamically ordered states, and each produces distinct dislocation patterns as well as specific features in the noise fluctuations and transport properties. Lastly, our work suggests that many of the results established for systems with quenched disorder undergoing plastic depinning transitions can be applied to dislocation systems, providing a new approach for understanding pattern formation andmore » dynamics in these systems.« less

  10. Dynamic phases, pinning, and pattern formation for driven dislocation assemblies

    SciTech Connect

    Zhou, Caizhi; Reichhardt, Charles; Olson Reichhardt, Cynthia J.; Beyerlein, Irene J.

    2015-01-23

    We examine driven dislocation assemblies and show that they can exhibit a set of dynamical phases remarkably similar to those of driven systems with quenched disorder such as vortices in superconductors, magnetic domain walls, and charge density wave materials. These phases include pinned-jammed, fluctuating, and dynamically ordered states, and each produces distinct dislocation patterns as well as specific features in the noise fluctuations and transport properties. Lastly, our work suggests that many of the results established for systems with quenched disorder undergoing plastic depinning transitions can be applied to dislocation systems, providing a new approach for understanding pattern formation and dynamics in these systems.

  11. Fluid Mechanics of Blood Clot Formation

    PubMed Central

    Fogelson, Aaron L.; Neeves, Keith B.

    2015-01-01

    Intravascular blood clots form in an environment in which hydrodynamic forces dominate and in which fluid-mediated transport is the primary means of moving material. The clotting system has evolved to exploit fluid dynamic mechanisms and to overcome fluid dynamic challenges to ensure that clots that preserve vascular integrity can form over the wide range of flow conditions found in the circulation. Fluid-mediated interactions between the many large deformable red blood cells and the few small rigid platelets lead to high platelet concentrations near vessel walls where platelets contribute to clotting. Receptor-ligand pairs with diverse kinetic and mechanical characteristics work synergistically to arrest rapidly flowing cells on an injured vessel. Variations in hydrodynamic stresses switch on and off the function of key clotting polymers. Protein transport to, from, and within a developing clot determines whether and how fast it grows. We review ongoing experimental and modeling research to understand these and related phenomena. PMID:26236058

  12. Fluid Mechanics of Blood Clot Formation

    NASA Astrophysics Data System (ADS)

    Fogelson, Aaron L.; Neeves, Keith B.

    2015-01-01

    Intravascular blood clots form in an environment in which hydrodynamic forces dominate and in which fluid-mediated transport is the primary means of moving material. The clotting system has evolved to exploit fluid dynamic mechanisms and to overcome fluid dynamic challenges to ensure that clots that preserve vascular integrity can form over the wide range of flow conditions found in the circulation. Fluid-mediated interactions between the many large deformable red blood cells and the few small rigid platelets lead to high platelet concentrations near vessel walls where platelets contribute to clotting. Receptor-ligand pairs with diverse kinetic and mechanical characteristics work synergistically to arrest rapidly flowing cells on an injured vessel. Variations in hydrodynamic stresses switch on and off the function of key clotting polymers. Protein transport to, from, and within a developing clot determines whether and how fast it grows. We review ongoing experimental and modeling research to understand these and related phenomena.

  13. Ar + induced interfacial mixing and phase formation in the Al/Cr system

    NASA Astrophysics Data System (ADS)

    Kim, H. K.; Kim, S. O.; Song, J. H.; Kim, K. W.; Woo, J. J.; Whang, C. N.; Smith, R. J.

    1991-07-01

    Evaporated Al/Cr bilayer thin films were irradiated by 80 keV Ar + at doses in the range from 1 × 10 15 to 2 × 10 16 Ar +/cm 2 at room temperature in order to investigate the Ar + induced interfacial mixing behavior and the phase formation and transition by Ar + bombardment. Ion bombardment induces intermixing across the Al/Cr interface and mixing variance increases with increasing ion dose. Cascade and thermal spike models are found to be not adequate for the ion beam mixing mechanism at room temperature in this system. The Al 13Cr 2 phase is formed as an initial phase by ion beam mixing and then transforms into the Al 11Cr 2 or Al 4Cr phases at subsequent ion bombardment. This result is discussed in terms of the enhanced atomic mobility and the thermodynamical driving force by introducing the concept of an effective heat of formation.

  14. Enabling Technologies for Direct Detection Optical Phase Modulation Formats

    NASA Astrophysics Data System (ADS)

    Xu, Xian

    Phase modulation formats are believed to be one of the key enabling techniques for next generation high speed long haul fiber-optic communication systems due to the following main advantages: (1) with a balanced detection, a better receiver sensitivity over conventional intensity modulation formats, e.g., a ˜3-dB sensitivity improvement using differential phase shift keying (DPSK) and a ˜1.3-dB sensitivity improvement using differential quadrature phase shift keying (DQPSK); (2) excellent robustness against fiber nonlinearities; (3) high spectrum efficiency when using multilevel phase modulation formats, such as DQPSK. As the information is encoded in the phase of the optical field, the phase modulation formats are sensitive to the phase-related impairments and the deterioration induced in the phase-intensity conversion. This consequently creates new challenging issues. The research objective of this thesis is to depict some of the challenging issues and provide possible solutions. The first challenge is the cross-phase modulation (XPM) penalty for the phase modulated channels co-propagating with the intensity modulated channels. The penalty comes from the pattern dependent intensity fluctuations of the neighboring intensity modulated channels being converted into phase noise in the phase modulation channels. We propose a model to theoretically analyze the XPM penalty dependence on the walk off effect. From this model, we suggest that using fibers with large local dispersion or intentionally introducing some residual dispersion per span would help mitigate the XPM penalty. The second challenge is the polarization dependent frequency shift (PDf) induced penalty during the phase-intensity conversion. The direct detection DPSK is usually demodulated in a Mach-Zehnder delay interferometer (DI). The polarization dependence of DI introduces a PDf causing a frequency offset between the laser's frequency and the transmissivity peak of DI, degrading the demodulated DPSK

  15. Simulating the Phases of the Moon Shortly after Its Formation

    ERIC Educational Resources Information Center

    Noordeh, Emil; Hall, Patrick; Cuk, Matija

    2014-01-01

    The leading theory for the origin of the Moon is the giant impact hypothesis, in which the Moon was formed out of the debris left over from the collision of a Mars sized body with the Earth. Soon after its formation, the orbit of the Moon may have been very different than it is today. We have simulated the phases of the Moon in a model for its…

  16. Formation of vortex loops (strings) in continuous phase transitions.

    PubMed

    Bowick, Mark J; Cacciuto, Angelo; Travesset, Alex

    2002-02-01

    The formation of vortex loops (global cosmic strings) in an O(2) linear sigma model in three spatial dimensions is analyzed numerically. For over-damped Langevin dynamics we find that defect production is suppressed by an interaction between correlated domains that reduces the effective spatial variation of the phase of the order field. The degree of suppression is sensitive to the quench rate. A detailed description of the numerical methods used to analyze the model is also reported. PMID:11863613

  17. The fluid mechanics of thrombus formation

    NASA Technical Reports Server (NTRS)

    1972-01-01

    Experimental data are presented for the growth of thrombi (blood clots) in a stagnation point flow of fresh blood. Thrombus shape, size and structure are shown to depend on local flow conditions. The evolution of a thrombus is described in terms of a physical model that includes platelet diffusion, a platelet aggregation mechanism, and diffusion and convection of the chemical species responsible for aggregation. Diffusion-controlled and convection-controlled regimes are defined by flow parameters and thrombus location, and the characteristic growth pattern in each regime is explained. Quantitative comparisons with an approximate theoretical model are presented, and a more general model is formulated.

  18. Solid-state graphene formation via a nickel carbide intermediate phase [Nickel carbide (Ni3C) as an intermediate phase for graphene formation

    SciTech Connect

    Xiong, W; Zhou, Yunshen; Hou, Wenjia; Guillemet, Thomas; Silvain, Jean-François; Lahaye, Michel; Lebraud, Eric; Xu, Shen; Wang, Xinwei; Cullen, David A; More, Karren Leslie; Lu, Yong Feng

    2015-01-01

    Direct formation of graphene with controlled number of graphitic layers on dielectric surfaces is highly desired for practical applications. Despite significant progress achieved in understanding the formation of graphene on metallic surfaces through chemical vapor deposition (CVD) of hydrocarbons, very limited research is available elucidating the graphene formation process via rapid thermal processing (RTP) of solid-state amorphous carbon, through which graphene is formed directly on dielectric surfaces accompanied by autonomous nickel evaporation. It is suggested that a metastable hexagonal nickel carbide (Ni3C) intermediate phase plays a critical role in transforming amorphous carbon to 2D crystalline graphene and contributing to the autonomous Ni evaporation. Temperature resolved carbon and nickel evolution in the RTP process is investigated using Auger electron spectroscopic (AES) depth profiling and glancing-angle X-ray diffraction (GAXRD). Formation, migration and decomposition of the hexagonal Ni3C are confirmed to be responsible for the formation of graphene and the evaporation of Ni at 1100 °C. The Ni3C-assisted graphene formation mechanism expands the understanding of Ni-catalyzed graphene formation, and provides insightful guidance for controlled growth of graphene through the solid-state transformation process.

  19. Solid-state graphene formation via a nickel carbide intermediate phase [Nickel carbide (Ni3C) as an intermediate phase for graphene formation

    DOE PAGESBeta

    Xiong, W; Zhou, Yunshen; Hou, Wenjia; Guillemet, Thomas; Silvain, Jean-François; Lahaye, Michel; Lebraud, Eric; Xu, Shen; Wang, Xinwei; Cullen, David A; et al

    2015-01-01

    Direct formation of graphene with controlled number of graphitic layers on dielectric surfaces is highly desired for practical applications. Despite significant progress achieved in understanding the formation of graphene on metallic surfaces through chemical vapor deposition (CVD) of hydrocarbons, very limited research is available elucidating the graphene formation process via rapid thermal processing (RTP) of solid-state amorphous carbon, through which graphene is formed directly on dielectric surfaces accompanied by autonomous nickel evaporation. It is suggested that a metastable hexagonal nickel carbide (Ni3C) intermediate phase plays a critical role in transforming amorphous carbon to 2D crystalline graphene and contributing to themore » autonomous Ni evaporation. Temperature resolved carbon and nickel evolution in the RTP process is investigated using Auger electron spectroscopic (AES) depth profiling and glancing-angle X-ray diffraction (GAXRD). Formation, migration and decomposition of the hexagonal Ni3C are confirmed to be responsible for the formation of graphene and the evaporation of Ni at 1100 °C. The Ni3C-assisted graphene formation mechanism expands the understanding of Ni-catalyzed graphene formation, and provides insightful guidance for controlled growth of graphene through the solid-state transformation process.« less

  20. Causal mechanisms in airfoil-circulation formation

    NASA Astrophysics Data System (ADS)

    Zhu, J. Y.; Liu, T. S.; Liu, L. Q.; Zou, S. F.; Wu, J. Z.

    2015-12-01

    In this paper, we trace the dynamic origin, rather than any kinematic interpretations, of lift in two-dimensional flow to the physical root of airfoil circulation. We show that the key causal process is the vorticity creation by tangent pressure gradient at the airfoil surface via no-slip condition, of which the theoretical basis has been given by Lighthill ["Introduction: Boundary layer theory," in Laminar Boundary Layers, edited by L. Rosenhead (Clarendon Press, 1963), pp. 46-113], which we further elaborate. This mechanism can be clearly revealed in terms of vorticity formulation but is hidden in conventional momentum formulation, and hence has long been missing in the history of one's efforts to understand lift. By a careful numerical simulation of the flow around a NACA-0012 airfoil, and using both Eulerian and Lagrangian descriptions, we illustrate the detailed transient process by which the airfoil gains its circulation and demonstrate the dominating role of relevant dynamical causal mechanisms at the boundary. In so doing, we find that the various statements for the establishment of Kutta condition in steady inviscid flow actually correspond to a sequence of events in unsteady viscous flow.

  1. Phase separation dynamics during Myxococcus xanthus fruiting body formation

    NASA Astrophysics Data System (ADS)

    Liu, Guannan; Bahar, Fatmagul; Patch, Adam; Thutupalli, Shashi; Yllanes, David; Marchetti, M. Cristina; Welch, Roy; Shaevitz, Joshua

    Many living systems take advantage of collective behavior for group survival. We use the soil-dwelling bacterium Myxococcus xanthus as a model to study out-of-equilibrium phase separation during fruiting body formation. M. xanthus cells have the ability to glide on solid surfaces and reverse their direction periodically. When starved, M. xanthus cells aggregate together and form structures called fruiting bodies, inside of which cells sporulate to survive stressful conditions. We show that at high cell density the formation of fruiting bodies is a phase separation process. From experimental data that combines single-cell tracking, population-scale imaging, mutants, and drug applications, we construct the phase diagram of M. xanthus in the space of Péclet number and cell density. When wild type cells are starved, we find that they actively increase their Péclet number by modulating gliding speed and reversal frequency which induces a phase separation from a gas-like state to an aggregated fruiting body state.

  2. Cluster formation and phase separation in heteronuclear Janus dumbbells

    NASA Astrophysics Data System (ADS)

    Munaò, G.; O'Toole, P.; Hudson, T. S.; Costa, D.; Caccamo, C.; Sciortino, F.; Giacometti, A.

    2015-06-01

    We have recently investigated the phase behavior of model colloidal dumbbells constituted by two identical tangent hard spheres, with the first being surrounded by an attractive square-well interaction (Janus dumbbells, Munaó et al 2014 Soft Matter 10 5269). Here we extend our previous analysis by introducing in the model the size asymmetry of the hard-core diameters and study the enriched phase scenario thereby obtained. By employing standard Monte Carlo simulations we show that in such ‘heteronuclear Janus dumbbells’ a larger hard-sphere site promotes the formation of clusters, whereas in the opposite condition a gas-liquid phase separation takes place, with a narrow interval of intermediate asymmetries wherein the two phase behaviors may compete. In addition, some peculiar geometrical arrangements, such as lamellæ, are observed only around the perfectly symmetric case. A qualitative agreement is found with recent experimental results, where it is shown that the roughness of molecular surfaces in heterogeneous dimers leads to the formation of colloidal micelles.

  3. Anomalous surface phase formation on Pt sub 3 Sn <110>

    SciTech Connect

    Haner, A.N.; Ross, P.N. ); Bardi, U. . Dipt. di Chimica)

    1990-06-01

    LEED analysis of the clean annealed surface of a {l angle}110{r angle} oriented Pt{sub 3}Sn single crystal surface indicates the formation of a multilayer surface phase which does not have the L1{sub 2} bulk structure. LEISS analysis indicates a surface stoichiometry of ca. 1:1 with Sn atoms displaced ca. 1.4 above the plane of Pt atoms. The surface phase is hypothesized to be a rhombic distortion of the {l angle}0001{r angle} plant of PtSn, which has a B8{sub 1} (NiAs-type) bulk structure. It is not clear whether the phase forms by precipitation of PtSn due to a slight (0.5%) stoichiometric excess of Sn in the bulk, or due to multilayer reconstruction driven by surface segregation. 20 refs., 3 figs.

  4. Nanostructures formation in ferroelectrics in the process of phase transformation

    NASA Astrophysics Data System (ADS)

    Ishchuk, V.; Spiridonov, N.; Sobolev, V.

    2014-11-01

    Inhomogeneous states caused by the coexistence of the ferroelectric (FE) and antiferroelectric (AFE) phases in lead-zirconate-titanate based solid solutions have been investigated. It has been found that the domains of the FE and AFE phases with sizes of the order of 20 nm to 30 nm coexist in the bulk of the samples due to a small difference in the free energies of these phases. The coherent character of the interphase boundaries (IPBs) leads to the concentration of the elastic stresses along these boundaries. These elastic stresses cause the local decomposition of the solid solution and formation of segregates near the IPBS due to the condition that equivalent positions of the crystal lattice are occupied by the ions with different sizes. The sizes of the segregates formed in this way are of the order 8 nm to 15 nm. Some physical effects caused by the presence of these segregate nanostructures are analyzed and discussed.

  5. Distinct phases of Wnt/β-catenin signaling direct cardiomyocyte formation in zebrafish

    PubMed Central

    Dohn, Tracy E.; Waxman, Joshua S.

    2011-01-01

    Normal heart formation requires reiterative phases of canonical Wnt/β-catenin (Wnt) signaling. Understanding the mechanisms by which Wnt signaling directs cardiomyocyte (CM) formation in vivo is critical to being able to precisely direct differentiated CMs from stem cells in vitro. Here, we investigate the roles of Wnt signaling in zebrafish CM formation using heat-shock inducible transgenes that increase and decrease Wnt signaling. We find that there are three phases during which CM formation is sensitive to modulation of Wnt signaling through the first 24 hours of development. In addition to the previously recognized roles for Wnt signaling during mesoderm specification and in the pre-cardiac mesoderm, we find a previously unrecognized role during CM differentiation where Wnt signaling is necessary and sufficient to promote the differentiation of additional atrial cells. We also extend the previous studies of the roles of Wnt signaling during mesoderm specification and in pre-cardiac mesoderm. Importantly, in pre-cardiac mesoderm we define a new mechanism where Wnt signaling is sufficient to prevent CM differentiation, in contrast to a proposed role in inhibiting cardiac progenitor (CP) specification. The inability of the CPs to differentiate appears to lead to cell death through a p53/Caspase-3 independent mechanism. Together with a report for an even later role for Wnt signaling in restricting proliferation of differentiated ventricular CMs, our results indicate that during the first 3 days of development in zebrafish there are four distinct phases during which CMs are sensitive to Wnt signaling. PMID:22094017

  6. Study of the formation and effects of sigma phase in 21-6-9 stainless steel

    SciTech Connect

    Packard, C.L.; Mataya, M.C.; Edstrom, C.M.

    1981-11-07

    Work performed to date on the study of the formation and effects of sigma phase in 21-6-9 stainless steel is summarized in this report. Sigma phase was identified in forgings and as-rolled plate by color etching and microprobe analysis. In as-rolled plate sigma was found to start transforming from delta ferrite within 30 minutes at 1500/sup 0/F, with almost complete transformation after 24 hours at 1500/sup 0/F. The effect of sigma phase on room temperature mechanical properties was evaluated by tensile testing, Charpy impact testing, and impact shear testing. Sigma phase was found to severely reduce transverse ductility and longtudinal and transverse impact resistance. The greater the amount of sigma present, the greater was its effect on mechanical properties. Vendor contacts indicated that controlling delta ferrite to a minimum in the as-rolled plate is both possible and practical.

  7. Isopropylammonium Formate as a Mobile Phase Modifier for Liquid Chromatography

    PubMed Central

    Collins, Matthew P.; Zhou, Ling; Camp, Suzanne E.; Danielson, Neil D.

    2012-01-01

    Isopropylammonium formate (IPAF), a new alkylammonium formate (AAF) room temperature ionic liquid, has been synthesized from isopropylamine and formic acid and characterized as an organic solvent mobile phase replacement for reversed-phase liquid chromatography (LC). Characterization of IPAF solvent properties in water such as pH, conductivity, and viscosity, as well as its synthesis, is described. The LC polarity (P′) and the solvent strength (S) parameters are determined to be 6.0 and 2.4, respectively, similar to those same parameters for methanol and acetonitrile. Application of this RTIL is demonstrated as an organic solvent replacement for reversed-phase LC to separate a test mixture of niacinamide, acetophenone and p-nitroaniline. The van Deemter plot profile for several columns of different dimensions, particle size, pore size and stationary phase are compared using an IPAF–water mobile phase. At flow rates above 2 mL/min, on-line mixing of the viscous IPAF with water appears not to be uniform. A flattening of the van Deemter profile is noted for particularly short (50 mm) wide bore (4.6 mm) columns packed with larger particles (10 µm). Small particle longer columns likely facilitated mixing at the beginning of the column generating typical linearly increasing van Deemeter curves. IPAF has been further shown as a function of temperature to be a non-denaturing modifier solvent for the separation of the protein cytochrome c from tryptophan compared to methanol. This is important to show, because the semi-preparative separation of native proteins using AAF mobile phases is the long-term goal of this research program. PMID:22718743

  8. PLETHORA gradient formation mechanism separates auxin responses

    PubMed Central

    Siligato, Riccardo; Smetana, Ondřej; Díaz-Triviño, Sara; Salojärvi, Jarkko; Wachsman, Guy; Prasad, Kalika; Heidstra, Renze; Scheres, Ben

    2015-01-01

    During plant growth, dividing cells in meristems must coordinate transitions from division to expansion and differentiation, thus generating three distinct developmental zones: the meristem, elongation zone and differentiation zone1. Simultaneously, plants display tropisms, rapid adjustments of their direction of growth to adapt to environmental conditions. It is unclear how stable zonation is maintained during transient adjustments in growth direction. In Arabidopsis roots, many aspects of zonation are controlled by the phytohormone auxin and auxin-induced PLETHORA (PLT) transcription factors, both of which display a graded distribution with a maximum near the root tip2-12. In addition, auxin is also pivotal for tropic responses13,14. Here, using an iterative experimental and computational approach, we show how an interplay between auxin and PLTs controls zonation and gravitropism. We find that the PLT gradient is not a direct, proportionate readout of the auxin gradient. Rather, prolonged high auxin levels generate a narrow PLT transcription domain from which a gradient of PLT protein is subsequently generated through slow growth dilution and cell-to-cell movement. The resulting PLT levels define the location of developmental zones. In addition to slowly promoting PLT transcription, auxin also rapidly influences division, expansion and differentiation rates. We demonstrate how this specific regulatory design in which auxin cooperates with PLTs through different mechanisms and on different timescales enables both the fast tropic environmental responses and stable zonation dynamics necessary for coordinated cell differentiation. PMID:25156253

  9. The formation of topological defects in phase transitions

    NASA Technical Reports Server (NTRS)

    Hodges, Hardy M.

    1989-01-01

    It was argued, and fought through numerical work that the results of non-dynamical Monte Carlo computer simulations cannot be applied to describe the formation of topological defects when the correlation length at the Ginzburg temperature is significantly smaller than the horizon size. To test the current hypothesis that infinite strings at formation are essentially described by Brownian walks of size the correlation length at the Ginzburg temperature, fields at the Ginzburg temperature were equilibrated. Infinite structure do not exist in equilibrium for reasonable definitions of the Ginzburg temperature, and horizons must be included in a proper treatment. A phase transition, from small-scale to large-scale string or domain wall structure, is found to occur very close to the Ginzburg temperature, in agreement with recent work. The formation process of domain walls and global strings were investigated through the breaking of initially ordered states. To mimic conditions in the early Universe, cooling times are chosen so that horizons exist in the sample volume when topological structure formation occurs. The classical fields are evolved in real-time by the numerical solution of Langevin equations of motion on a three dimensional spatial lattice. The results indicate that it is possible for most of the string energy to be in small loops, rather than in long strings, at formation.

  10. Role of Nucleation and Growth in Two-Phase Microstructure Formation

    SciTech Connect

    Shin, Jong Ho

    2007-01-01

    During the directional solidification of peritectic alloys, a rich variety of two-phase microstructures develop, and the selection process of a specific microstructure is complicated due to the following two considerations. (1) In contrast to many single phase and eutectic microstructures that grow under steady state conditions, two-phase microstructures in a peritectic system often evolve under non-steady-state conditions that can lead to oscillatory microstructures, and (2) the microstructure is often governed by both the nucleation and the competitive growth of the two phases in which repeated nucleation can occur due to the change in the local conditions during growth. In this research, experimental studies in the Sn-Cd system were designed to isolate the effects of nucleation and competitive growth on the dynamics of complex microstructure formation. Experiments were carried out in capillary samples to obtain diffusive growth conditions so that the results can be analyzed quantitatively. At high thermal gradient and low velocity, oscillatory microstructures were observed in which repeated nucleation of the two phases was observed at the wall-solid-liquid junction. Quantitative measurements of nucleation undercooling were obtained for both the primary and the peritectic phase nucleation, and three different ampoule materials were used to examine the effect of different contact angles at the wall on nucleation undercooling. Nucleation undercooling for each phase was found to be very small, and the experimental undercooling values were orders of magnitude smaller than that predicted by the classical theory of nucleation. A new nucleation mechanism is proposed in which the clusters of atoms at the wall ahead of the interface can become a critical nucleus when the cluster encounters the triple junction. Once the nucleation of a new phase occurs, the microstructure is found to be controlled by the relative growth of the two phases that give rise to different

  11. Physiological and molecular biochemical mechanisms of bile formation

    PubMed Central

    Reshetnyak, Vasiliy Ivanovich

    2013-01-01

    This review considers the physiological and molecular biochemical mechanisms of bile formation. The composition of bile and structure of a bile canaliculus, biosynthesis and conjugation of bile acids, bile phospholipids, formation of bile micellar structures, and enterohepatic circulation of bile acids are described. In general, the review focuses on the molecular physiology of the transporting systems of the hepatocyte sinusoidal and apical membranes. Knowledge of physiological and biochemical basis of bile formation has implications for understanding the mechanisms of development of pathological processes, associated with diseases of the liver and biliary tract. PMID:24259965

  12. Impact of Gas-Phase Mechanisms on Weather Research Forecasting Model with Chemistry (WRF/Chem) Predictions: Mechanism Implementation and Comparative Evaluation

    EPA Science Inventory

    Gas-phase mechanisms provide important oxidant and gaseous precursors for secondary aerosol formation. Different gas-phase mechanisms may lead to different predictions of gases, aerosols, and aerosol direct and indirect effects. In this study, WRF/Chem-MADRID simulations are cond...

  13. A FIB induced boiling mechanism for rapid nanopore formation

    PubMed Central

    Das, K; Freund, J B; Johnson, H T

    2015-01-01

    Focused ion beam (FIB) technology is widely used to fabricate nanopores in solid-state membranes. These nanopores have desirable thermomechanical properties for applications such as high-throughput DNA sequencing. Using large scale molecular dynamics simulations of the FIB nanopore formation process, we show that there is a threshold ion delivery rate above which the mechanism underlying nanopore formation changes. At low rates nanopore formation is slow, with the rate proportional to the ion flux and therefore limited by the sputter rate of the target material. However, at higher fluxes nanopores form via a thermally dominated process, consistent with an explosive boiling mechanism. In this case, mass is rapidly rearranged via bubble growth and coalescence, much more quickly than would occur during sputtering. This mechanism has the potential to greatly speed up nanopore formation. PMID:24356374

  14. Breakdown of phase D by formation of aluminous bridgmanite

    NASA Astrophysics Data System (ADS)

    Katsura, T.; Ishii, T.; Miyajima, N.; Petitgirard, S.; McCammon, C. A.

    2015-12-01

    A certain amount of water is considered to be transported into the deep mantle through subduction in the forms of hydrous minerals. Although the major constituent minerals can host significant amounts of water in the upper mantle, bridgmantite (Brm) and ferropericlase (fPc) can incorporate only a limited amount of water. Therefore, if water is transported into the lower mantle through subduction, it must be hosted by dense hydrous magnesium silicates (DHMSs) such as phase D. However, it is not clear whether the DHMSs can exist in the lower mantle. Brm contains significant amounts of Fe3+ (Fe3+/ΣFe > 0.1) even in coexistence with metallic iron. The amounts of Fe3+ increase with increasing Al2O3 content in Brm, and the Fe3+/ΣFe ratio exceeds 0.5 in the lower-mantle Brm. Since the Fe3+/ΣFe ratios are only 0.01-0.03 and 0.10-0.14 in the upper-mantle peridotite and MORB, respectively, formation of Brm would take oxygen out of other minerals. Hence, we expect breakdown of DHMS by reduction of OH into H2 through the Brm formation. To test this hypothesis, we conducted the following experiments. The starting material consisted of an assemblage of a pre-synthesized phase D together with an natural olivine single crystal or a pyroxene aggregate with bulk composition of pyrolite minus fPc (Pyr-fPc) synthesized at an PO2 ≈ IW, T = 1100 K and ambient pressure. The two-phase samples were pressurized to 28 GPa and heated to 1100 K for 1 hour. First SEM observation of recovered samples shows that a 2-µm thick SiO2-rich fine-grained layer has formed on the boundary between phase D and Pyr-fPc Brm indicating the breakdown of phase D, while no evidence of breakdown of phase D was observed in the experiments with the olivine single crystal. The present result suggests that, although water can be transported into the top of the lower mantle in a form of phase D through subduction, phase D will decompose by reduction at the depth to 720 km. The produced H2 should be re

  15. Multifunctional acid formation from the gas-phase ozonolysis of beta-pinene.

    PubMed

    Ma, Yan; Marston, George

    2008-10-28

    The gas-phase ozonolysis of beta-pinene was studied in static chamber experiments, using gas chromatography coupled to mass spectrometric and flame ionisation detection to separate and detect products. A range of multifunctional organic acids-including pinic acid, norpinic acid, pinalic-3-acid, pinalic-4-acid, norpinalic acid and OH-pinalic acid-were identified in the condensed phase after derivatisation. Formation yields for these products under systematically varying reaction conditions (by adding different OH radical scavengers and Criegee intermediate scavengers) were investigated and compared with those observed from alpha-pinene ozonolysis, allowing detailed information on product formation mechanisms to be elucidated. In addition, branching ratios for the initial steps of the reaction were inferred from quantitative measurements of primary carbonyl formation. Atmospheric implications of this work are discussed.

  16. Domain wall formation in late-time phase transitions

    NASA Technical Reports Server (NTRS)

    Kolb, Edward W.; Wang, Yun

    1992-01-01

    We examine domain wall formulation in late time phase transitions. We find that in the invisible axion domain wall phenomenon, thermal effects alone are insufficient to drive different parts of the disconnected vacuum manifold. This suggests that domain walls do not form unless either there is some supplemental (but perhaps not unreasonable) dynamics to localize the scalar field responsible for the phase transition to the low temperature maximum (to an extraordinary precision) before the onset of the phase transition, or there is some non-thermal mechanism to produce large fluctuations in the scalar field. The fact that domain wall production is not a robust prediction of late time transitions may suggest future directions in model building.

  17. Relative Navigation Algorithms for Phase 1 of the MMS Formation

    NASA Technical Reports Server (NTRS)

    Kelbel, David; Lee, Taesul; Long, Anne; Carpenter, Russell; Gramling, Cheryl

    2003-01-01

    This paper evaluates several navigation approaches for the first phase of the Magnetospheric Multiscale (MMS) mission, which consists of a tetrahedral formation of four satellites in highly eccentric Earth orbits of approximately 1.2 by 12 Earth radii at an inclination of 10 degrees. The inter-satellite separation is approximately 10 kilometers near apogees. Navigation approaches were studied using ground station m g e =d two-way Doppler measurements, Global Positioning System (GPS) pseudorange measurements, crosslink range measurements among the members flying in formation, and various combinations of these measurement types. An absolute position accuracy of 10 kilometers or better can be achieved with most of the approaches studied and a relative position accuracy of 100 meters or better can be achieved at apogee in some cases. Among the various approaches studied, the approaches that use a combination of GPS and crosslink measurements were found to be more reliable in terms of absolute and relative navigation accuracies and operational flexibility.

  18. A comparison of methods to predict solid phase heats of formation of molecular energetic salts.

    PubMed

    Byrd, Edward F C; Rice, Betsy M

    2009-01-01

    In this study a variety of methods were used to compute the energies for lattice enthalpies and gas phase heats of formation of the ionic constituents used in Born-Fajans-Haber cycles to produce solid phase heats of formation of molecular ionic energetic crystals. Several quantum mechanically based or empirical approaches to calculate either the heat of formation of the ionic constituents in the gas phase (deltaH(o)f(g)) or the lattice enthalpy (deltaH(o)Lattice) were evaluated. Solid phase heats of formation calculated from combinations of deltaH(o)f(g) and deltaH(o)Lattice determined through various approaches are compared with experimental values for a series of molecular energetic salts with 1:1, 2:1 and 2:2 charge ratios. Recommendations for combinations of deltaH(o)f(g) and deltaH(o)Lattice to produce best agreement with experiment are given, along with suggestions for improvements of the methods.

  19. Mechanism of vacancy formation induced by hydrogen in tungsten

    SciTech Connect

    Liu, Yi-Nan; Ahlgren, T.; Bukonte, L.; Nordlund, K.; Shu, Xiaolin; Yu, Yi; Lu, Guang-Hong; Li, Xiao-Chun

    2013-12-15

    We report a hydrogen induced vacancy formation mechanism in tungsten based on classical molecular dynamics simulations. We demonstrate the vacancy formation in tungsten due to the presence of hydrogen associated directly with a stable hexagonal self-interstitial cluster as well as a linear crowdion. The stability of different self-interstitial structures has been further studied and it is particularly shown that hydrogen plays a crucial role in determining the configuration of SIAs, in which the hexagonal cluster structure is preferred. Energetic analysis has been carried out to prove that the formation of SIA clusters facilitates the formation of vacancies. Such a mechanism contributes to the understanding of the early stage of the hydrogen blistering in tungsten under a fusion reactor environment.

  20. Droplets Formation and Merging in Two-Phase Flow Microfluidics

    PubMed Central

    Gu, Hao; Duits, Michel H. G.; Mugele, Frieder

    2011-01-01

    Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed. PMID:21731459

  1. Droplets formation and merging in two-phase flow microfluidics.

    PubMed

    Gu, Hao; Duits, Michel H G; Mugele, Frieder

    2011-01-01

    Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed.

  2. Fibril Formation and Phase Separation in Aqueous Cellulose Ethers

    NASA Astrophysics Data System (ADS)

    Maxwell, Amanda; Schmidt, Peter; McAllister, John; Lott, Joseph; Bates, Frank; Lodge, Timothy

    Aqueous solutions of many cellulose ethers are known to undergo thermoreversible gelation and phase separation upon heating to form turbid hydrogels, but the mechanism and resulting structures have not been well understood. Turbidity, light scattering and small-angle neutron scattering (SANS) are used to show that hydroxypropyl methylcellulose (HPMC) chains are dissolved in water below 50 °C and undergo phase separation at higher temperatures. At 70 °C, at sufficiently high concentrations in water, HPMC orders into fibrillar structures with a well-defined radius of 18 +/- 2 nm, as characterized by cryogenic transmission electron microscopy and SANS. The HPMC fibril structure is independent of concentration and heating rate. However, HPMC fibrils do not form a percolating network as readily as is seen in methylcellulose, resulting in a lower hot-gel modulus, as demonstrated by rheology.

  3. Metastable bcc phase formation in the Nb-Cr system

    SciTech Connect

    Thoma, D.J.; Schwarz, R.B.; Perepezko, J.H.; Plantz, D.H.

    1993-08-01

    Extended metastable bcc solid solutions of Nb-Xat.%Cr (X = 35, 50, 57, 77, 82, and 94) were synthesized by two-anvil splat-quenching. In addition, bcc (Nb-67at.%Cr) was prepared by mechanically alloying mixtures of niobium and chromium powders. The lattice parameters were measured by X-ray diffraction and the Young`s moduli were measured by low-load microindentation. The composition dependence of the lattice parameters and elastic moduli show a positive deviation with respect to a rule of mixtures. During continuous heating at 15C/min., the metastable precursor bcc phases decomposed at temperatures above 750C to uniformly refined microstructures.

  4. Methylammonium formate as a mobile phase modifier for reversed-phase liquid chromatography

    PubMed Central

    Grossman, Shau; Danielson, Neil D.

    2009-01-01

    Although alkylammonium ionic liquids such as ethylammonium nitrate and ethylammonium formate have been used as mobile phase “solvents” for liquid chromatography (LC), we have shown that methylammonium formate (MAF), in part because of its lower viscosity, can be an effective replacement for methanol (MeOH) in reversed-phase LC. Plots of log retention factor versus the fraction of MeOH and MAF in the mobile phase indicate quite comparable solvent strength slope values of 2.50 and 2.05, respectively. Using a polar endcapped C18 column, furazolidone and nitrofurantoin using 20% MAF-80% water could be separated in 22 min but no baseline separation was possible using MeOH as the modifier, even down to 10%. Suppression of silanol peak broadening effects by MAF is important permitting a baseline separation of pyridoxine, thiamine, and nicotinamide using 5% MAF-95% water at 0.7 mL/min. Using 5% MeOH-95% water, severe peak broadening for thiamine is evident. The compatibility of MAF as a mobile phase modifer for LC with mass spectrometry detection of water soluble vitamins is also shown. PMID:18849044

  5. Mechanics of Metals with Phase Changes

    SciTech Connect

    Lashley, J.C.

    2001-01-01

    New experimental data is presented on some exotic metals that exhibit phase changes at cryogenic temperatures. The types of phase changes that were detected in the specific heat data range from martensitic (diffusion less) transitions to superconducting transitions. In addition, the charge density wave (CDW) state in uranium metal was detected in the specific heat. Specific-heat measurements were made in zero-magnetic field using an apparatus capable of obtaining temperatures as low as 0.4 K. Calibration performed on this apparatus, using a single-crystal copper sample, show its accuracy to be 0.50%, while the resolution was better than 0.1%. Our measurements demonstrate that similar high precision and accurate specific-heat measurements can be obtained on milligram-scale samples. In Chapters 2 and 3, specific-heat measurements are presented for the B2 (CsCl structure) alloy AuZn and for {alpha}-uranium (orthorhombic symmetry). The AuZn alloy exhibits a continuous transition at 64.75 K and an entropy of transition of ({Delta}S{sub tr}) 2.02 J K{sup {minus}1} mol{sup {minus}1}. Calculation of the Debye temperature, by extrapolating of the high temperature phase elastic constants to T = 0 K yields a value of 207 K ({+-}2 K), in favorable agreement with the calorimetric value of 219 K ({+-}0.50 K), despite the intervening martensitic transition. Reported results for single-crystal {alpha}-U show a low-temperature limiting {Theta}{sub D} of 256 K ({+-}0.50 K) and four low-temperature anomalies: a superconducting transition below 1 K, an electronic transition at 22 K, and two anomalies at 38 K and at 42 K indicative of the CDW state. In order to continue the study of the actinide series of elements, a program was initiated to first purify and then grow single crystals of plutonium. Accordingly, the focus of Chapters 4 through 6 will be a description of plutonium sample preparation. In this program plutonium metal was purified via zone refining, using a levitated molten

  6. Phase Separation and Pattern Formation in a Binary Bose-Einstein Condensate

    SciTech Connect

    Sabbatini, Jacopo; Davis, Matthew J.; Zurek, Wojciech H.

    2011-12-02

    The miscibility-immiscibility phase transition in binary Bose-Einstein condensates (BECs) can be controlled by a coupling between the two components. Here we propose a new scheme that uses coupling-induced pattern formation to test the Kibble-Zurek mechanism (KZM) of topological-defect formation in a quantum phase transition. For a binary BEC in a ring trap we find that the number of domains forming the pattern scales as a function of the coupling quench rate with an exponent as predicted by the KZM. For a binary BEC in an elongated harmonic trap we find a different scaling law due to the transition being spatially inhomogeneous. We perform a ''quantum simulation'' of the harmonically trapped system in a ring trap to verify the scaling exponent.

  7. Phase formation in BZT (10/90) ceramic

    NASA Astrophysics Data System (ADS)

    Batra, Punita; Singh, Sangeeta

    2014-04-01

    BaZr0.10Ti0.90O3 ferroelectric ceramic composition was synthesized by conventional solid state route. The ceramic sample was calcined at 1000°C for 4 hours. The sample was then subjected to XRD analysis to confirm the phase formation. Some extra peaks with low intensity, corresponding to unreacted BaO and ZrO2 were observed. The powder sample was then pressed into circular pellets and sintered at 1200°C, 1250°C, 1300°C and 1350°C for 4 hours. The extra peaks in XRD were found to diminish in case of sample sintered at 1350°C.

  8. Characterising the acceleration phase of blast wave formation

    SciTech Connect

    Fox, T. E. Pasley, J.; Robinson, A. P. L.; Schmitz, H.

    2014-10-15

    Intensely heated, localised regions in uniform fluids will rapidly expand and generate an outwardly propagating blast wave. The Sedov-Taylor self-similar solution for such blast waves has long been studied and applied to a variety of scenarios. A characteristic time for their formation has also long been identified using dimensional analysis, which by its very nature, can offer several interpretations. We propose that, rather than simply being a characteristic time, it may be interpreted as the definitive time taken for a blast wave resulting from an intense explosion in a uniform media to contain its maximum kinetic energy. A scaling relation for this measure of the acceleration phase, preceding the establishment of the blast wave, is presented and confirmed using a 1D planar hydrodynamic model.

  9. [Mechanism and risk factors of oral biofilm formation].

    PubMed

    Pasich, Ewa; Walczewska, Maria; Pasich, Adam; Marcinkiewicz, Janusz

    2013-08-02

    Recent microbiological investigations completely changed our understanding of the role of biofilm in the formation of the mucosal immune barrier and in pathogenesis of chronic inflammation of bacterial etiology. It is now clear that formation of bacterial biofilm on dental surfaces is characteristic for existence of oral microbial communities. It has also been proved that uncontrolled biofilms on dental tissues, as well as on different biomaterials (e.g. orthodontic appliances), are the main cause of dental diseases such as dental caries and periodontitis. The aim of this paper is to explain mechanisms and consequences of orthodontic biofilm formation. We will discuss current opinions on the influence of different biomaterials employed for orthodontic treatment in biofilm formation and new strategies employed in prevention and elimination of oral biofilm ("dental plaque").

  10. Molecular Mechanisms of Floral Boundary Formation in Arabidopsis

    PubMed Central

    Yu, Hongyang; Huang, Tengbo

    2016-01-01

    Boundary formation is a crucial developmental process in plant organogenesis. Boundaries separate cells with distinct identities and act as organizing centers to control the development of adjacent organs. In flower development, initiation of floral primordia requires the formation of the meristem-to-organ (M–O) boundaries and floral organ development depends on the establishment of organ-to-organ (O–O) boundaries. Studies in this field have revealed a suite of genes and regulatory pathways controlling floral boundary formation. Many of these genes are transcription factors that interact with phytohormone pathways. This review will focus on the functions and interactions of the genes that play important roles in the floral boundaries and discuss the molecular mechanisms that integrate these regulatory pathways to control the floral boundary formation. PMID:26950117

  11. Homogeneous gas-phase formation of polychlorinated naphthalene from dimerization of 4-chlorophenoxy radicals and cross-condensation of phenoxy radical with 4-chlorophenoxy radical: Mechanism and kinetics study

    NASA Astrophysics Data System (ADS)

    Xu, Fei; Zhang, Ruiming; Li, Yunfeng; Zhang, Qingzhu

    2015-10-01

    A direct density functional theory (DFT) calculation was performed for the formation of polychlorinated naphthalenes (PCNs) from dimerization of 4-chlorophenoxy radicals (4-CPRs) and cross-condensation of phenoxy radical (PhR) with 4-CPR, respectively. Several energetically feasible formation routes were proposed. The rate constants were computed by the canonical variational transition-state theory (CVT) with the small curvature tunneling (SCT) contribution over temperature range of 600-1200 K. This study shows that PCN productions from the dimerization of 4-CPRs just contain DCNs. All the monochlorinated naphthalene (MCN) detected in the experiment from 4-chlorophenol (4-CP) as precursor are formed form the cross-condensation of PhR with 4-CPR.

  12. Quasi-Hermitian quantum mechanics in phase space

    SciTech Connect

    Curtright, Thomas; Veitia, Andrzej

    2007-10-15

    We investigate quasi-Hermitian quantum mechanics in phase space using standard deformation quantization methods: Groenewold star products and Wigner transforms. We focus on imaginary Liouville theory as a representative example where exact results are easily obtained. We emphasize spatially periodic solutions, compute various distribution functions and phase-space metrics, and explore the relationships between them.

  13. Molecular mechanisms responsible for formation of Golgi ribbon.

    PubMed

    Mironov, Alexander A; Beznoussenko, Galina V

    2011-01-01

    The formation of the Golgi ribbon takes place in protists and metazoans. It is especially prominent in mammalian cells during interphase. Golgi ribbon formation represents an orchestrated sequence of events based not only on different molecular mechanisms but also on discrete cellular functions. Mechanisms responsible for the generation of the Golgi ribbon include Golgi centralization, cis- and trans-Golgins, molecular machines responsible for the fusion of cargo domains with cisternal rims, and several other less studied factors. Here, we substantiate the hypothesis that cis-Golgins function mostly not as tethering factors, but are responsible for the attachment of the cis-most cisternae to the medial Golgi stacks, whereas trans-Golgins are responsible for the attachment of the trans-most cisterna to the medial Golgi stacks. This hypothesis is tested analyzing predictions derived from it and related to molecular mechanisms responsible for mitotic fragmentation of Golgi stacks.

  14. Formation of asymmetrical structured silica controlled by a phase separation process and implication for biosilicification.

    PubMed

    Shi, Jia-Yuan; Yao, Qi-Zhi; Li, Xi-Ming; Zhou, Gen-Tao; Fu, Sheng-Quan

    2013-01-01

    Biogenetic silica displays intricate patterns assembling from nano- to microsize level and interesting non-spherical structures differentiating in specific directions. Several model systems have been proposed to explain the formation of biosilica nanostructures. Of them, phase separation based on the physicochemical properties of organic amines was considered to be responsible for the pattern formation of biosilica. In this paper, using tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4) as silica precursor, phospholipid (PL) and dodecylamine (DA) were introduced to initiate phase separation of organic components and influence silica precipitation. Morphology, structure and composition of the mineralized products were characterized using a range of techniques including field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), infrared spectra (IR), and nitrogen physisorption. The results demonstrate that the phase separation process of the organic components leads to the formation of asymmetrically non-spherical silica structures, and the aspect ratios of the asymmetrical structures can be well controlled by varying the concentration of PL and DA. On the basis of the time-dependent experiments, a tentative mechanism is also proposed to illustrate the asymmetrical morphogenesis. Therefore, our results imply that in addition to explaining the hierarchical porous nanopatterning of biosilica, the phase separation process may also be responsible for the growth differentiation of siliceous structures in specific directions. Because organic amine (e.g., long-chair polyamines), phospholipids (e.g., silicalemma) and the phase separation process are associated with the biosilicification of diatoms, our results may provide a new insight into the mechanism of biosilicification.

  15. Formation of Asymmetrical Structured Silica Controlled by a Phase Separation Process and Implication for Biosilicification

    PubMed Central

    Shi, Jia-Yuan; Yao, Qi-Zhi; Li, Xi-Ming; Zhou, Gen-Tao; Fu, Sheng-Quan

    2013-01-01

    Biogenetic silica displays intricate patterns assembling from nano- to microsize level and interesting non-spherical structures differentiating in specific directions. Several model systems have been proposed to explain the formation of biosilica nanostructures. Of them, phase separation based on the physicochemical properties of organic amines was considered to be responsible for the pattern formation of biosilica. In this paper, using tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4) as silica precursor, phospholipid (PL) and dodecylamine (DA) were introduced to initiate phase separation of organic components and influence silica precipitation. Morphology, structure and composition of the mineralized products were characterized using a range of techniques including field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), infrared spectra (IR), and nitrogen physisorption. The results demonstrate that the phase separation process of the organic components leads to the formation of asymmetrically non-spherical silica structures, and the aspect ratios of the asymmetrical structures can be well controlled by varying the concentration of PL and DA. On the basis of the time-dependent experiments, a tentative mechanism is also proposed to illustrate the asymmetrical morphogenesis. Therefore, our results imply that in addition to explaining the hierarchical porous nanopatterning of biosilica, the phase separation process may also be responsible for the growth differentiation of siliceous structures in specific directions. Because organic amine (e.g., long-chair polyamines), phospholipids (e.g., silicalemma) and the phase separation process are associated with the biosilicification of diatoms, our results may provide a new insight into the mechanism of biosilicification. PMID:23585878

  16. New understanding of microstructure formation of the rubber phase in thermoplastic vulcanizates (TPV).

    PubMed

    Wu, Hanguang; Tian, Ming; Zhang, Liqun; Tian, Hongchi; Wu, Youping; Ning, Nanying

    2014-03-21

    The breakup of the rubber phase in an ethylene-propylene-diene monomer (EPDM)/polypropylene (PP) blend at the early stage of dynamic vulcanization is similar to that in an unvulcanized EPDM/PP blend because of the low crosslink density of the EPDM phase. In this work, the minimum size of the rubber phase in the unvulcanized EPDM/PP blend was first calculated by using the critical breakup law of viscoelastic droplets in a matrix. The calculated results showed that the minimum size of the rubber phase in the unvulcanized blend was in the nanometer scale (25-46 nm), not the micrometer scale as reported in many works. Meanwhile, the actual size of the rubber phase in the thermoplastic vulcanizate (TPV) at both the early stage and the final stage of dynamic vulcanization was observed by using peak force tapping atomic force microscopy (PF-AFM). The results indicated that the EPDM phase indeed broke up into nanoparticles at the early stage of dynamic vulcanization, in good agreement with the calculated results. More interestingly, we first revealed that the micrometer-sized rubber particles commonly observed in TPV were actually the agglomerates of rubber nanoparticles with diameters between 40 and 60 nm. The mechanism for the formation of rubber nanoparticles and their agglomerates during dynamic vulcanization was then discussed. Our work provides guidance to control the microstructure of the rubber phase in TPV to prepare high performance TPV products for a wide range of applications in the automobile and electronic industries.

  17. A directional nucleation-zipping mechanism for triple helix formation.

    PubMed

    Alberti, Patrizia; Arimondo, Paola B; Mergny, Jean-Louis; Garestier, Thérèse; Hélène, Claude; Sun, Jian-Sheng

    2002-12-15

    A detailed kinetic study of triple helix formation was performed by surface plasmon resonance. Three systems were investigated involving 15mer pyrimidine oligonucleotides as third strands. Rate constants and activation energies were validated by comparison with thermodynamic values calculated from UV-melting analysis. Replacement of a T.A base pair by a C.G pair at either the 5' or the 3' end of the target sequence allowed us to assess mismatch effects and to delineate the mechanism of triple helix formation. Our data show that the association rate constant is governed by the sequence of base triplets on the 5' side of the triplex (referred to as the 5' side of the target oligopurine strand) and provides evidence that the reaction pathway for triple helix formation in the pyrimidine motif proceeds from the 5' end to the 3' end of the triplex according to the nucleation-zipping model. It seems that this is a general feature for all triple helices formation, probably due to the right-handedness of the DNA double helix that provides a stronger base stacking at the 5' than at the 3' duplex-triplex junction. Understanding the mechanism of triple helix formation is not only of fundamental interest, but may also help in designing better triple helix-forming oligonucleotides for gene targeting and control of gene expression.

  18. Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation.

    PubMed

    Donaldson, D James; Kroll, Jay A; Vaida, Veronica

    2016-07-15

    Sulfur chemistry is of great interest to the atmospheric chemistry of several planets. In the presence of water, oxidized sulfur can lead to new particle formation, influencing climate in significant ways. Observations of sulfur compounds in planetary atmospheres when compared with model results suggest that there are missing chemical mechanisms. Here we propose a novel mechanism for the formation of sulfurous acid, which may act as a seed for new particle formation. In this proposed mechanism, the lowest triplet state of SO2 ((3)B1), which may be accessed by near-UV solar excitation of SO2 to its excited (1)B1 state followed by rapid intersystem crossing, reacts directly with water to form H2SO3 in the gas phase. For ground state SO2, this reaction is endothermic and has a very high activation barrier; our quantum chemical calculations point to a facile reaction being possible in the triplet state of SO2. This hygroscopic H2SO3 molecule may act as a condensation nucleus for water, giving rise to facile new particle formation (NPF).

  19. Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation

    NASA Astrophysics Data System (ADS)

    Donaldson, D. James; Kroll, Jay A.; Vaida, Veronica

    2016-07-01

    Sulfur chemistry is of great interest to the atmospheric chemistry of several planets. In the presence of water, oxidized sulfur can lead to new particle formation, influencing climate in significant ways. Observations of sulfur compounds in planetary atmospheres when compared with model results suggest that there are missing chemical mechanisms. Here we propose a novel mechanism for the formation of sulfurous acid, which may act as a seed for new particle formation. In this proposed mechanism, the lowest triplet state of SO2 (3B1), which may be accessed by near-UV solar excitation of SO2 to its excited 1B1 state followed by rapid intersystem crossing, reacts directly with water to form H2SO3 in the gas phase. For ground state SO2, this reaction is endothermic and has a very high activation barrier; our quantum chemical calculations point to a facile reaction being possible in the triplet state of SO2. This hygroscopic H2SO3 molecule may act as a condensation nucleus for water, giving rise to facile new particle formation (NPF).

  20. Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation

    PubMed Central

    Donaldson, D. James; Kroll, Jay A.; Vaida, Veronica

    2016-01-01

    Sulfur chemistry is of great interest to the atmospheric chemistry of several planets. In the presence of water, oxidized sulfur can lead to new particle formation, influencing climate in significant ways. Observations of sulfur compounds in planetary atmospheres when compared with model results suggest that there are missing chemical mechanisms. Here we propose a novel mechanism for the formation of sulfurous acid, which may act as a seed for new particle formation. In this proposed mechanism, the lowest triplet state of SO2 (3B1), which may be accessed by near-UV solar excitation of SO2 to its excited 1B1 state followed by rapid intersystem crossing, reacts directly with water to form H2SO3 in the gas phase. For ground state SO2, this reaction is endothermic and has a very high activation barrier; our quantum chemical calculations point to a facile reaction being possible in the triplet state of SO2. This hygroscopic H2SO3 molecule may act as a condensation nucleus for water, giving rise to facile new particle formation (NPF). PMID:27417675

  1. Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation.

    PubMed

    Donaldson, D James; Kroll, Jay A; Vaida, Veronica

    2016-01-01

    Sulfur chemistry is of great interest to the atmospheric chemistry of several planets. In the presence of water, oxidized sulfur can lead to new particle formation, influencing climate in significant ways. Observations of sulfur compounds in planetary atmospheres when compared with model results suggest that there are missing chemical mechanisms. Here we propose a novel mechanism for the formation of sulfurous acid, which may act as a seed for new particle formation. In this proposed mechanism, the lowest triplet state of SO2 ((3)B1), which may be accessed by near-UV solar excitation of SO2 to its excited (1)B1 state followed by rapid intersystem crossing, reacts directly with water to form H2SO3 in the gas phase. For ground state SO2, this reaction is endothermic and has a very high activation barrier; our quantum chemical calculations point to a facile reaction being possible in the triplet state of SO2. This hygroscopic H2SO3 molecule may act as a condensation nucleus for water, giving rise to facile new particle formation (NPF). PMID:27417675

  2. Halo formation in three-dimensional bunches with various phase space distributions

    NASA Astrophysics Data System (ADS)

    Fedotov, A. V.; Gluckstern, R. L.; Kurennoy, S. S.; Ryne, R. D.

    1999-01-01

    A realistic treatment of halo formation must take into account 3D beam bunches and 6D phase space distributions. We recently constructed, analytically and numerically, a new class of self-consistent 6D phase space stationary distributions, which allowed us to study the halo development mechanism without being obscured by the effect of beam redistribution. In this paper we consider nonstationary distributions and study how the halo characteristics compare with those obtained using the stationary distribution. We then discuss the effect of redistribution on the halo development mechanism. In contrast to bunches with a large aspect ratio, we find that the effect of coupling between the r and z planes is especially important as the bunch shape becomes more spherical.

  3. Mechanism of the process formation; podocytes vs. neurons.

    PubMed

    Kobayashi, Naoto

    2002-05-15

    In this review article we discuss the common mechanism for cellular process formation. Besides the podocyte, the mechanism of process formation, including cytoskeletal organization and signal transduction, etc., has been studied using neurons and glias as model systems. There has been an accumulation of data showing common cell biological features of the podocyte and the neuron: 1) Both cells possess long and short cell processes equipped with highly organized cytoskeletal systems; 2) Both show cytoskeletal segregation; microtubules (MTs) and intermediate filaments (IFs) in podocyte primary processes and in neurites, while actin filaments (AFs) are abundant in podocyte foot processes in neuronal synaptic regions; 3) In both cells, process formation is mechanically dependent on MTs, whose assembly is regulated by various microtubule- associated proteins (MAPs); 4) In both cells, process formation is positively regulated by PP2A, a Ser/Thr protein phosphatase; 5) In both cells, process formation is accelerated by laminin, an extracellular matrix protein. In addition, recent data from our and other laboratories have shown that podocyte processes share many features with neuronal dendrites: 1) Podocyte processes and neuronal dendrites possess MTs with mixed polarity, namely, plus-end-distal and minus-end-distal MTs coexist in these processes; 2) To establish the mixed polarity of MTs, both express CHO1/MKLP1, a kinesin-related motor protein, and when its expression is inhibited formation of both podocyte processes and neuronal dendrites is abolished; 3) The elongation of both podocyte processes and neuronal dendrites is supported by rab8-regulated basolateral-type membrane transport; 4) Both podocyte processes and neuronal dendrites express synaptopodin, an actin-associated protein, in a development-dependent manner; interestingly, in both cells, synaptopodin is localized not in the main shaft of processes but in thin short projections from the main shaft. We propose

  4. Structural phase transitions in trigonal Selenium induce the formation of a disordered phase

    NASA Astrophysics Data System (ADS)

    Pal, Anirban; Gohil, Smita; Sengupta, Surajit; Poswal, H. K.; Sharma, Surinder M.; Ghosh, Shankar; Ayyub, Pushan

    2015-10-01

    Arguments based on the Mermin-Wagner theorem suggest that the quasi-1D trigonal phase of Se should be unstable against long wavelength perturbations. Consisting of parallel Se-Se chains, this essentially fragile solid undergoes a partial transition to a monoclinic structure (consisting of 8-membered rings) at low temperatures (≈50 K), and to a distorted trigonal phase at moderate pressures (≈3GPa). Experimental investigations on sub-millimeter-sized single crystals provide clear evidence that these transitions occur via a novel and counter-intuitive route. This involves the reversible formation of an intermediate, disordered structure that appears as a minority phase with increasing pressure as well as with decreasing temperature. The formation of the disordered state is indicated by: (a) a ‘Boson-peak’ that appears at low temperatures in the specific heat and resonance Raman data, and (b) a decrease in the intensity of Raman lines over a relatively narrow pressure range. We complement the experimental results with a phenomenological model that illustrates how a first order structural transition may lead to disorder. Interestingly, nanocrystals of trigonal Se do not undergo any structural transition in the parameter space studied; neither do they exhibit signs of disorder, further underlining the role of disorder in this type of structural transition.

  5. Hebbian and neuromodulatory mechanisms interact to trigger associative memory formation

    PubMed Central

    Johansen, Joshua P.; Diaz-Mataix, Lorenzo; Hamanaka, Hiroki; Ozawa, Takaaki; Ycu, Edgar; Koivumaa, Jenny; Kumar, Ashwani; Hou, Mian; Deisseroth, Karl; Boyden, Edward S.; LeDoux, Joseph E.

    2014-01-01

    A long-standing hypothesis termed “Hebbian plasticity” suggests that memories are formed through strengthening of synaptic connections between neurons with correlated activity. In contrast, other theories propose that coactivation of Hebbian and neuromodulatory processes produce the synaptic strengthening that underlies memory formation. Using optogenetics we directly tested whether Hebbian plasticity alone is both necessary and sufficient to produce physiological changes mediating actual memory formation in behaving animals. Our previous work with this method suggested that Hebbian mechanisms are sufficient to produce aversive associative learning under artificial conditions involving strong, iterative training. Here we systematically tested whether Hebbian mechanisms are necessary and sufficient to produce associative learning under more moderate training conditions that are similar to those that occur in daily life. We measured neural plasticity in the lateral amygdala, a brain region important for associative memory storage about danger. Our findings provide evidence that Hebbian mechanisms are necessary to produce neural plasticity in the lateral amygdala and behavioral memory formation. However, under these conditions Hebbian mechanisms alone were not sufficient to produce these physiological and behavioral effects unless neuromodulatory systems were coactivated. These results provide insight into how aversive experiences trigger memories and suggest that combined Hebbian and neuromodulatory processes interact to engage associative aversive learning. PMID:25489081

  6. COMPETING MECHANISMS OF MOLECULAR HYDROGEN FORMATION IN CONDITIONS RELEVANT TO THE INTERSTELLAR MEDIUM

    SciTech Connect

    Lemaire, J. L.; Vidali, G.; Baouche, S.; Chehrouri, M.; Chaabouni, H.; Mokrane, H.

    2010-12-20

    The most efficient mechanism of the formation of molecular hydrogen in the current universe is by association of hydrogen atoms on the surface of interstellar dust grains. The details of the processes of its formation and release from the grain are of great importance in the physical and chemical evolution of the space environments where it takes place. The main puzzle is still the fate of the 4.5 eV released in H{sub 2} formation and whether it goes into internal energy (rovibrational excitation), translational kinetic energy, or heating of the grain. The modality of the release of this energy affects the dynamics of the ISM and its evolution toward star formation. We present results of the detection of the rovibrational states of the just-formed H{sub 2} as it leaves the surface of a silicate. We find that rovibrationally excited molecules are ejected into the gas phase immediately after formation over a much wider range of grain temperatures than anticipated. Our results can be explained by the presence of two mechanisms of molecule formation that operate in partially overlapping ranges of grain temperature. A preliminary analysis of the relative importance of these two mechanisms is given. These unexpected findings, which will be complemented with experiments on the influence of factors such as silicate morphology, should be of great interest to the astrophysics and astrochemistry communities.

  7. Void lattice formation as a nonequilibrium phase transition

    SciTech Connect

    Semenov, A. A.; Woo, C. H.

    2006-07-01

    The evolution of a void ensemble in the presence of one-dimensionally migrating self-interstitials is considered, consistently taking into account the nucleation of voids via the stochastic accumulation of vacancies. Including the stochastic fluctuations of the fluxes of mobile defects caused by the random nature of diffusion jumps and cascade initiation, the evolution of the void ensemble is treated using the Fokker-Planck equation approach. A system instability signaling a nonequilibrium phase transition is found to occur when the mean free path of the one-dimensionally moving self-interstitials becomes comparable with the average distance between the voids at a sufficiently high void-number density. Due to the exponential dependence of the void nucleation probability on the net vacancy flux, the nucleation of voids is much more favored at the void lattice positions. Simultaneously, voids initially nucleated at positions where neighboring voids are nonaligned will also shrink away. These two processes leave the aligned voids to form a regular lattice. The shrinkage of nonaligned voids is not a usual thermodynamic effect, but is a kinetic effect caused entirely by the stochastic fluctuations in point-defect fluxes received by the voids. It is shown that the shrinkage of the nonaligned voids, and thus the formation of the void lattice, occurs only if the effective fraction of one-dimensional interstitials is small, less than about 1%. The formation of the void lattice in this way can be accomplished at a void swelling of below 1%, in agreement with experimental observation. The dominance of void nucleation at void-lattice positions practically nullifies the effect of void coalescence induced by the one-dimensional self-interstitial transport.

  8. Mechanical behaviour and formation process of silkworm silk gut.

    PubMed

    Cenis, José L; Madurga, Rodrigo; Aznar-Cervantes, Salvador D; Lozano-Pérez, A Abel; Marí-Buyé, Núria; Meseguer-Olmo, Luis; Plaza, Gustavo R; Guinea, Gustavo V; Elices, Manuel; Del Pozo, Francisco; Pérez-Rigueiro, José

    2015-12-14

    High performance silk fibers were produced directly from the silk glands of silkworms (Bombyx mori) following an alternative route to natural spinning. This route is based on a traditional procedure that consists of soaking the silk glands in a vinegar solution and stretching them by hand leading to the so called silkworm guts. Here we present, to the authors' best knowledge, the first comprehensive study on the formation, properties and microstructure of silkworm gut fibers. Comparison of the tensile properties and microstructural organization of the silkworm guts with those of naturally spun fibers allows gain of a deeper insight into the mechanisms that lead to the formation of the fiber, as well as the relationship between the microstructure and properties of these materials. In this regard, it is proved that an acidic environment and subsequent application of tensile stress in the range of 1000 kPa are sufficient conditions for the formation of a silk fiber.

  9. Physical Mechanisms of Pattern Formation in the Early Chick Embryo

    NASA Astrophysics Data System (ADS)

    Balter, Ariel; Glazier, James; Zaitlen, Benji; Chaplain, Mark; Weijer, Cornelis

    2007-03-01

    Gastrulation marks a critical step in early embryogenesis when the first recognizable patterns are laid down. Although the genome maintains ultimate responsibility for this pattern formation, it cannot actually control the organization of individual cells. The robustness of embryogenic pattern formation suggests that a few simple, physical mechanisms are unleashed and that self-organization results. We perform numerical simulations of early chick gastrulation using an agent based method in which individual cells interact via a handful of behaviors including adhesivity, secretion and chemotaxis. Through these simulations we have identified certain behaviors as being important for various stages and morphological events. For instance, experimental results on primitive streak formation are best reproduced by a model in which the Kohler's Sickle secretes a chemo repellant for streak tip cells, and cell polarization appears to be important for initiating polonaise motion during streak elongation.

  10. Diffusion mechanism of exchange bias formation in permalloy-manganese nanostructures at thermo-magnetic treatment.

    PubMed

    Blinov, I V; Krinitsina, T P; Matveev, S A; Milyaev, M A; Sedova, P N; Popov, V V; Ustinov, V V

    2012-09-01

    A mechanism of unidirectional exchange anisotropy formation at thermo-magnetic treatment of permalloy-manganese bilayers has been studied. A shift of hysteresis loops appears at annealing beginning from 230 degrees C. The maximal exchange field of 155 Oe is reached after the 250 degrees C annealing for 2 h. As demonstrated by transmission electron microscopy, the exchange bias and the coercivity growth result from an ordered anti-ferromagnetic NiFeMn phase formation due to the diffusion interaction of permalloy and manganese at annealing. PMID:23035517

  11. Formation of Intermediate Carbon Phases in Hydrothermal Abiotic Organic Synthesis

    NASA Astrophysics Data System (ADS)

    Fu, Q.; Foustoukos, D. I.; Seyfried, W. E.

    2005-12-01

    With high dissolved concentrations of methane and other hydrocarbon species revealed at the Rainbow and Logatchev vent systems on the Mid-Atlantic Ridge, it is essential to better understand reaction pathways of abiotic organic synthesis in hydrothermal systems. Thus, we performed a hydrothermal carbon reduction experiment with 13C labeled carbon source at temperature and pressure conditions that approximate those inferred for ultramafic-hosted hydrothermal systems. Pentlandite, a common alteration mineral phase in subseafloor reaction zones, acted as a potential catalyst. Surface analysis techniques (XPS and ToF-SIMS) were used to characterize intermediate carbon species within this process. Time series dissolved H2 and H2S concentrations indicated thermodynamic equilibrium. Dissolved H2 and H2S concentrations of 13 and 2 mmol/kg, respectively, are approximately equivalent to measured values in Rainbow and Logatchev hydrothermal systems. Isotopically pure 13C methane and other alkane species (C2H6 and C3H8) were observed throughout the experiment, and attained steady state conditions. XPS analysis on mineral product surface indicated carbon enrichment on mineral surface following reaction. The majority of surface carbon involves species containing C-C or C-H bonds, such as alkyl or methylene groups. Alcohol and carboxyl groups in fewer amounts were also observed. ToF-SIMS analysis, which can offer isotope identification with high mass resolution, showed that most of these carbon species were 13C-labeled. Unlike gas phase Fischer-Tropsch synthesis, no carbide was observed on mineral product surface during the experiment. Therefore, a reaction pathway is proposed for formation of dissolved linear alkane species in hydrothermal abiotic organic synthesis, where oxygen-bearing organic compounds are expected to form in aqueous products by way of alcohol and carboxyl groups on mineral catalyst surface.

  12. Porosity/bubble formation mechanism in laser surface enamelling

    NASA Astrophysics Data System (ADS)

    Akhter, R.; Li, L.; Edwards, R. E.; Gale, A. W.

    2003-03-01

    The grouts between commercial tiles applied to walls and floors can be contaminated over time and normally have to be removed by manual or mechanical processes. To overcome the contamination problem, a specially developed tile grout was used to fill the voids between the tiles. The base filler was overlaid with an enamel surface that glazes after laser irradiation. One problem discovered in this work is that bubbles and porosities were formed after laser treatment. The use of water glass (sodium silicate) as a binder has been undertaken in this study. This paper investigates the mechanism of bubble formation and its effects on the enamel surface. It has been found that the CO 2 gas released during the reaction causes bubble formation. The results and the technique for the removal of bubbles/porosities are presented in the paper.

  13. Wine protein haze: mechanisms of formation and advances in prevention.

    PubMed

    Van Sluyter, Steven C; McRae, Jacqui M; Falconer, Robert J; Smith, Paul A; Bacic, Antony; Waters, Elizabeth J; Marangon, Matteo

    2015-04-29

    Protein haze is an aesthetic problem in white wines that can be prevented by removing the grape proteins that have survived the winemaking process. The haze-forming proteins are grape pathogenesis-related proteins that are highly stable during winemaking, but some of them precipitate over time and with elevated temperatures. Protein removal is currently achieved by bentonite addition, an inefficient process that can lead to higher costs and quality losses in winemaking. The development of more efficient processes for protein removal and haze prevention requires understanding the mechanisms such as the main drivers of protein instability and the impacts of various wine matrix components on haze formation. This review covers recent developments in wine protein instability and removal and proposes a revised mechanism of protein haze formation.

  14. Neural and Cellular Mechanisms of Fear and Extinction Memory Formation

    PubMed Central

    Orsini, Caitlin A.; Maren, Stephen

    2012-01-01

    Over the course of natural history, countless animal species have evolved adaptive behavioral systems to cope with dangerous situations and promote survival. Emotional memories are central to these defense systems because they are rapidly acquired and prepare organisms for future threat. Unfortunately, the persistence and intrusion of memories of fearful experiences are quite common and can lead to pathogenic conditions, such as anxiety and phobias. Over the course of the last thirty years, neuroscientists and psychologists alike have attempted to understand the mechanisms by which the brain encodes and maintains these aversive memories. Of equal interest, though, is the neurobiology of extinction memory formation as this may shape current therapeutic techniques. Here we review the extant literature on the neurobiology of fear and extinction memory formation, with a strong focus on the cellular and molecular mechanisms underlying these processes. PMID:22230704

  15. Resolving the molecular mechanism of cadherin catch bond formation

    SciTech Connect

    Manibog, Kristine; Li, Hui; Rakshit, Sabyasachi; Sivasankar, Sanjeevi

    2014-06-02

    Classical cadherin Ca(2+)-dependent cell-cell adhesion proteins play key roles in embryogenesis and in maintaining tissue integrity. Cadherins mediate robust adhesion by binding in multiple conformations. One of these adhesive states, called an X-dimer, forms catch bonds that strengthen and become longer lived in the presence of mechanical force. Here we use single-molecule force-clamp spectroscopy with an atomic force microscope along with molecular dynamics and steered molecular dynamics simulations to resolve the molecular mechanisms underlying catch bond formation and the role of Ca(2+) ions in this process. Our data suggest that tensile force bends the cadherin extracellular region such that they form long-lived, force-induced hydrogen bonds that lock X-dimers into tighter contact. When Ca(2+) concentration is decreased, fewer de novo hydrogen bonds are formed and catch bond formation is eliminated

  16. Defect formation mechanism during PECVD of a-Si:H

    SciTech Connect

    Maeda, Keiji; Umezu, Ikurou

    1997-07-01

    Defect formation mechanism in a-Si:H during PECVD at substrate temperature below 250 C is considered to be breaking of weak bonds in the Urbach tail. To break weak bonds, an extra energy is necessary. This energy is supplied by the reaction energy of SiH{sub 3} precursor at the growing surface incorporating SiH{sub 2} into the network. The defect density is experimentally shown to be proportional to a product of the energy supply frequency, i.e., SiH{sub 2} density, and the weak bond density which is obtained by the Urbach energy. By analysis using the configurational coordinate diagram the energy level of the broken weak bond is determined to be 0.2 eV above the valence band mobility edge. There is similarity of the defect formation mechanism during deposition to that of the Staebler-Wronski effect.

  17. Nucleation mechanism for the direct graphite-to-diamond phase transition.

    PubMed

    Khaliullin, Rustam Z; Eshet, Hagai; Kühne, Thomas D; Behler, Jörg; Parrinello, Michele

    2011-07-24

    Graphite and diamond have comparable free energies, yet forming diamond from graphite in the absence of a catalyst requires pressures that are significantly higher than those at equilibrium coexistence. At lower temperatures, the formation of the metastable hexagonal polymorph of diamond is favoured instead of the more stable cubic diamond. These phenomena cannot be explained by the concerted mechanism suggested in previous theoretical studies. Using an ab initio quality neural-network potential, we carried out a large-scale study of the graphite-to-diamond transition assuming that it occurs through nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrate that the large lattice distortions that accompany the formation of diamond nuclei inhibit the phase transition at low pressure, and direct it towards the hexagonal diamond phase at higher pressure. The proposed nucleation mechanism should improve our understanding of structural transformations in a wide range of carbon-based materials.

  18. Nucleation mechanism for the direct graphite-to-diamond phase transition.

    PubMed

    Khaliullin, Rustam Z; Eshet, Hagai; Kühne, Thomas D; Behler, Jörg; Parrinello, Michele

    2011-09-01

    Graphite and diamond have comparable free energies, yet forming diamond from graphite in the absence of a catalyst requires pressures that are significantly higher than those at equilibrium coexistence. At lower temperatures, the formation of the metastable hexagonal polymorph of diamond is favoured instead of the more stable cubic diamond. These phenomena cannot be explained by the concerted mechanism suggested in previous theoretical studies. Using an ab initio quality neural-network potential, we carried out a large-scale study of the graphite-to-diamond transition assuming that it occurs through nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrate that the large lattice distortions that accompany the formation of diamond nuclei inhibit the phase transition at low pressure, and direct it towards the hexagonal diamond phase at higher pressure. The proposed nucleation mechanism should improve our understanding of structural transformations in a wide range of carbon-based materials. PMID:21785417

  19. Formation of binary phase gratings in photopolymer-liquid crystal composites by a surface-controlled anisotropic phase separation

    SciTech Connect

    Park, Jae-Hong; Khoo, Iam Choon; Yu, Chang-Jae; Jung, Min-Sik; Lee, Sin-Doo

    2005-01-10

    We report on formation of binary phase gratings in photopolymer-liquid crystal (PLC) composites using a surface-controlled phase separation method. The binary nature of the PLC phase gratings is produced by employing a single step photo-ablation through an amplitude photomask which precisely controls the interfacial interactions between the LC and the photopolymer on the alignment layer. A subsequent illumination of the ultraviolet light onto the whole PLC promotes an anisotropic phase separation resulting in the formation of distinct binary patterns for the PLC structure. The electrically tunable diffraction properties of the binary phase gratings are presented.

  20. Mechanisms of formation damage in matrix-permeability geothermal wells

    SciTech Connect

    Bergosh, J.L.; Wiggins, R.B.; Enniss, D.O.

    1982-04-01

    Tests were conducted to determine mechanisms of formation damage that can occur in matrix permeability geothermal wells. Two types of cores were used in the testing, actual cores from the East Mesa Well 78-30RD and cores from a fairly uniform generic sandstone formation. Three different types of tests were run. The East Mesa cores were used in the testing of the sensitivity of core to filtrate chemistry. The tests began with the cores exposed to simulated East Mesa brine and then different filtrates were introduced and the effects of the fluid contrast on core permeability were measured. The East Mesa cores were also used in the second series of tests which tested formation sandstone cores were used in the third test series which investigated the effects of different sizes of entrained particles in the fluid. Tests were run with both single-particle sizes and distributions of particle mixes. In addition to the testing, core preparation techniques for simulating fracture permeability were evaluated. Three different fracture formation mechanisms were identified and compared. Measurement techniques for measuring fracture size and permeability were also developed.

  1. [Current concepts of the mechanisms of formation of gunshot fractures].

    PubMed

    Pigolkin, Iu I; Dubrovin, I A; Leonov, S V; Mikhaĭlenko, A V; Dubrovin, A I; Zotkin, D A

    2013-01-01

    The mechanisms of formation of gunshot fractures in flat bones inflicted by a semispherical bullet were investigated using expert and experimental materials. The process of crack formation was considered in terms of the Hertzian contact problem and the Hill-Johnson model. It was shown that the fracture develops as a result of combination of stresses and strains in the bone tissue leading to the formation of a hydrostatic nucleus prior to tissue fragmentation. Dynamic fluctuations (waves) generated in the zone of hydrostatic compression resulting from the gunshot injury propagate with the velocity of sound from the nucleus in the direction of the bullet movement. According to the Hill-Johnson model, the waves propagate in the direction of the impact within a parabolically expanding space; this accounts for the mechanism of formation of parabolic cracks and the specific shape of the defect that the bullet produces in the flat bones. The dynamic load applied by means of an indentor forms at a higher rate than the velocity of sound in the affected material. It gives reason to consider the effect of a bullet moving with the speed of 250 m/s as quasi-static loading. The results obtained in this study make a contribution to the theory of impact effect of a bullet and provide a deeper insight into the physical nature of the direct and sideway action of a gunshot projectile. Moreover, they explain the cause behind the widening of the outlet part of the perforating fracture in the flat bones.

  2. Peptide Formation Mechanism on Montmorillonite Under Thermal Conditions

    NASA Astrophysics Data System (ADS)

    Fuchida, Shigeshi; Masuda, Harue; Shinoda, Keiji

    2014-02-01

    The oligomerization of amino acids is an essential process in the chemical evolution of proteins, which are precursors to life on Earth. Although some researchers have observed peptide formation on clay mineral surfaces, the mechanism of peptide bond formation on the clay mineral surface has not been clarified. In this study, the thermal behavior of glycine (Gly) adsorbed on montmorillonite was observed during heating experiments conducted at 150 °C for 336 h under dry, wet, and dry-wet conditions to clarify the mechanism. Approximately 13.9 % of the Gly monomers became peptides on montmorillonite under dry conditions, with diketopiperazine (cyclic dimer) being the main product. On the other hand, peptides were not synthesized in the absence of montmorillonite. Results of IR analysis showed that the Gly monomer was mainly adsorbed via hydrogen bonding between the positively charged amino groups and negatively charged surface sites (i.e., Lewis base sites) on the montmorillonite surface, indicating that the Lewis base site acts as a catalyst for peptide formation. In contrast, peptides were not detected on montmorillonite heated under wet conditions, since excess water shifted the equilibrium towards hydrolysis of the peptides. The presence of water is likely to control thermodynamic peptide production, and clay minerals, especially those with electrophilic defect sites, seem to act as a kinetic catalyst for the peptide formation reaction.

  3. Exchange anisotropy as mechanism for spin-stripe formation in frustrated spin chains

    NASA Astrophysics Data System (ADS)

    Pregelj, M.; Zaharko, O.; Herak, M.; Gomilšek, M.; Zorko, A.; Chapon, L. C.; Bourdarot, F.; Berger, H.; Arčon, D.

    2016-08-01

    We investigate the spin-stripe mechanism responsible for the peculiar nanometer modulation of the incommensurate magnetic order that emerges between the vector-chiral and the spin-density-wave phase in the frustrated zigzag spin-1/2 chain compound β -TeVO4 . A combination of magnetic-torque, neutron-diffraction, and spherical-neutron-polarimetry measurements is employed to determine the complex magnetic structures of all three ordered phases. Based on these results, we develop a simple phenomenological model, which exposes the exchange anisotropy as the key ingredient for the spin-stripe formation in frustrated spin systems.

  4. Importance of Silicon and Mechanisms of Biosilica Formation in Plants

    PubMed Central

    Siti Nor Akmar, Abdullah; Rafii, Mohd Y.; Tengoua, F. F.; Nurul Mayzaitul Azwa, Jamaludin; Shabanimofrad, M.

    2015-01-01

    Silicon (Si) is one of the most prevalent macroelements, performing an essential function in healing plants in response to environmental stresses. The purpose of using Si is to induce resistance to distinct stresses, diseases, and pathogens. Additionally, Si can improve the condition of soils, which contain toxic levels of heavy metals along with other chemical elements. Silicon minimizes toxicity of Fe, Al, and Mn, increases the availability of P, and enhances drought along with salt tolerance in plants through the formation of silicified tissues in plants. However, the concentration of Si depends on the plants genotype and organisms. Hence, the physiological mechanisms and metabolic activities of plants may be affected by Si application. Peptides as well as amino acids can effectively create polysilicic species through interactions with different species of silicate inside solution. The carboxylic acid and the alcohol groups of serine and asparagine tend not to engage in any significant role in polysilicates formation, but the hydroxyl group side chain can be involved in the formation of hydrogen bond with Si(OH)4. The mechanisms and trend of Si absorption are different between plant species. Furthermore, the transportation of Si requires an energy mechanism; thus, low temperatures and metabolic repressors inhibit Si transportation. PMID:25685787

  5. Importance of silicon and mechanisms of biosilica formation in plants.

    PubMed

    Sahebi, Mahbod; Hanafi, Mohamed M; Siti Nor Akmar, Abdullah; Rafii, Mohd Y; Azizi, Parisa; Tengoua, F F; Nurul Mayzaitul Azwa, Jamaludin; Shabanimofrad, M

    2015-01-01

    Silicon (Si) is one of the most prevalent macroelements, performing an essential function in healing plants in response to environmental stresses. The purpose of using Si is to induce resistance to distinct stresses, diseases, and pathogens. Additionally, Si can improve the condition of soils, which contain toxic levels of heavy metals along with other chemical elements. Silicon minimizes toxicity of Fe, Al, and Mn, increases the availability of P, and enhances drought along with salt tolerance in plants through the formation of silicified tissues in plants. However, the concentration of Si depends on the plants genotype and organisms. Hence, the physiological mechanisms and metabolic activities of plants may be affected by Si application. Peptides as well as amino acids can effectively create polysilicic species through interactions with different species of silicate inside solution. The carboxylic acid and the alcohol groups of serine and asparagine tend not to engage in any significant role in polysilicates formation, but the hydroxyl group side chain can be involved in the formation of hydrogen bond with Si(OH)4. The mechanisms and trend of Si absorption are different between plant species. Furthermore, the transportation of Si requires an energy mechanism; thus, low temperatures and metabolic repressors inhibit Si transportation.

  6. Physical Mechanisms of Glaze Ice Scallop Formations on Swept Wings

    NASA Technical Reports Server (NTRS)

    Vargas, Mario; Reshotko, Eli

    1998-01-01

    An experiment was conducted to understand the physical mechanisms that lead to the formation of scallops on swept wings. Icing runs were performed on a NACA 0012 swept wing tip at 45 deg, 30 deg, and 15 deg sweep angles. A baseline case was chosen and direct measurements of scallop height and spacing, castings, video data and close-up photographic data were obtained. The results showed the scallops are made of glaze ice feathers that grow from roughness elements that have reached a minimum height and are located beyond a given distance from the attachment line. This distance depends on tunnel conditions and sweep angle, and is the critical parameter in the formation of scallops. It determines if complete scallops, incomplete scallops or no scallops are going to be formed. The mechanisms of growth for complete and incomplete scallops were identified. The effect of velocity, temperature and LWC on scallop formation was studied. The possibility that cross flow instability may be the physical mechanism that triggers the growth of roughness elements into glaze ice feathers is examined.

  7. Project ARGO: Gas phase formation in simulated microgravity

    NASA Technical Reports Server (NTRS)

    Powell, Michael R.; Waligora, James M.; Norfleet, William T.; Kumar, K. Vasantha

    1993-01-01

    The ARGO study investigated the reduced incidence of joint pain decompression sickness (DCS) encountered in microgravity as compared with an expected incidence of joint pain DCS experienced by test subjects in Earth-based laboratories (unit gravity) with similar protocols. Individuals who are decompressed from saturated conditions usually acquire joint pain DCS in the lower extremities. Our hypothesis is that the incidence of joint pain DCS can be limited by a significant reduction in the tissue gas micronuclei formed by stress-assisted nucleation. Reductions in dynamic and kinetic stresses in vivo are linked to hypokinetic and adynamic conditions of individuals in zero g. We employed the Doppler ultrasound bubble detection technique in simulated microgravity studies to determine quantitatively the degree of gas phase formation in the upper and lower extremities of test subjects during decompression. We found no evidence of right-to-left shunting through pulmonary vasculature. The volume of gas bubble following decompression was examined and compared with the number following saline contrast injection. From this, we predict a reduced incidence of DCS on orbit, although the incidence of predicted mild DCS still remains larger than that encountered on orbit.

  8. Morphogengineering roots: comparing mechanisms of morphogen gradient formation

    PubMed Central

    2012-01-01

    Background In developmental biology, there has been a recent focus on the robustness of morphogen gradients as possible providers of positional information. It was shown that functional morphogen gradients present strong biophysical constraints and lack of robustness to noise. Here we explore how the details of the mechanism which underlies the generation of a morphogen gradient can influence those properties. Results We contrast three gradient-generating mechanisms, (i) a source-decay mechanism; and (ii) a unidirectional transport mechanism; and (iii) a so-called reflux-loop mechanism. Focusing on the dynamics of the phytohormone auxin in the root, we show that only the reflux-loop mechanism can generate a gradient that would be adequate to supply functional positional information for the Arabidopsis root, for biophysically reasonable kinetic parameters. Conclusions We argue that traits that differ in spatial and temporal time-scales can impose complex selective pressures on the mechanism of morphogen gradient formation used for the development of the particular organism. PMID:22583698

  9. Effects of Strain Rates on Mechanical Properties and Fracture Mechanism of DP780 Dual Phase Steel

    NASA Astrophysics Data System (ADS)

    Li, Shengci; Kang, Yonglin; Zhu, Guoming; Kuang, Shuang

    2015-06-01

    The mechanical properties of DP780 dual phase steel were measured by quasi-static and high-speed tensile tests at strain rates between 0.001 and 1000 s-1 at room temperature. The deformation and fracture mechanisms were analyzed by observation of the tensile fracture and microstructure near the fracture. Dynamic factor and feret ratio quantitative methods were applied to study the effect of strain rate on the microstructure and properties of DP780 steel. The constitutive relation was described by a modified Johnson-Cook and Zerilli-Armstrong model. The results showed that the strain rate sensitivity of yield strength is bigger than that of ultimate tensile strength; as strain rate increased, the formation of microcracks and voids at the ferrite/martensite interface can be alleviated; the strain rate effect is unevenly distributed in the plastic deformation region. Moreover, both models can effectively describe the experimental results, while the modified Zerilli-Armstrong model is more accurate because the strain-hardening rate of this model is independent of strain rate.

  10. A mechanism of raft formation on both plasma membrane layers

    NASA Astrophysics Data System (ADS)

    Sornbundit, Kan; Modchang, Charin; Triampo, Wannapong; Triampo, Darapond; Nuttavut, Narin

    2013-10-01

    A double-layered membrane model is proposed to explain raft formation and induction on extracellular (outer) and cytoplasmic (inner) leaflets of plasma membranes in a situation where only the outer layer has a tendency to phase-separate. In the model, lipid exchange with the surrounding medium is allowed on both layers, but lipid exchange between layers is not allowed. Simulations display domain stabilization on both layers. The effect of the lipid recycling frequencies on stationary domain sizes is also investigated. It is found that stationary domain sizes decrease when lipid recycling frequencies are stronger. Linear stability analysis is used to verify the results.

  11. Mechanical regulation of vascular network formation in engineered matrices.

    PubMed

    Lesman, Ayelet; Rosenfeld, Dekel; Landau, Shira; Levenberg, Shulamit

    2016-01-15

    Generation of vessel networks within engineered tissues is critical for integration and perfusion of the implanted tissue in vivo. The effect of mechanical cues in guiding and stabilizing the vessels has begun to attract marked interest. This review surveys the impact of mechanical cues on formation of vascular networks in 2D and 3D gel matrices. We give less emphasis to regulation of endothelial monolayers and single endothelial cells. Several vascularization models have consistently found that the stress generated in the gel, and encountered by embedded cells, control various aspects of vascular network formation, including sprouting, branching, alignment, and vessel maturation. This internal stress is generated by cell contractile forces, and is balanced by gel stiffness and boundary constrains imposed on the gel. Actin and myosin II are key molecular players in controlling initiation of vessel sprouting and branching morphogenesis. Additionally, the impact of external mechanical cues on tissue vascularization, and studies supporting the notion that mechanical forces regulate vascularization in the live animal are reviewed.

  12. A mechanical device to study geometric phases and curvatures

    NASA Astrophysics Data System (ADS)

    Gil, Salvador

    2010-04-01

    A simple mechanical device is introduced that can be used to illustrate the parallel transport of a vector along a curved surface and the geometric phase shift that occurs when a vector is carried along a loop on a curved surface. Its connection with the Foucault pendulum and Berry phases is discussed. The experimental results are in close agreement with the theoretical expectations. The experiment is inexpensive and conceptually easy to understand and perform.

  13. Formation Mechanism of Barrier Layer in the Subtropical Pacific

    NASA Astrophysics Data System (ADS)

    Katsura, S.; Oka, E.; Sato, K.

    2014-12-01

    Formation mechanism of barrier layers (BLs) in the subtropical Pacific was investigated by using Argo profiling float data and shipboard hydrographic section data. In this region, BLs were formed mainly in winter in association with the sea surface salinity (SSS) front, which was located on the equator side of the SSS maximum region. While BLs from gridded Argo data were broadly distributed, their distribution from raw Argo profiles was patchy and their temporal scale was shorter than 10 days. Formation mechanism of BLs was attributed to two processes: freshening near the sea surface and salinification in the subsurface. As for the former process, poleward Ekman advection of fresher water from the tropics across the SSS front was dominant, while the effect of precipitation was small. As for the latter process, inflow of high salinity water into mixed layers associated with the SSS front can contribute to the BL formation, but actually the core of subducted Tropical Water was too deep to affect salinity structure in the mixed layers across the seasonal thermocline. These features strongly suggest that tilting of the SSS front is essentially important for the BL formation in the subtropical Pacific. This tilting process can only occur where the contribution of horizontal SSS gradient to the horizontal density gradient is strong, and explains why spatial distribution of BLs corresponded to the SSS front. Seasonal variation of BLs corresponded well to that of mixed layer depth, indicating that the deeper mixed layers are before the BL formation, the thicker BLs are formed when the SSS front is tilted.

  14. Phase distribution and flow mechanism in an amphibolite facies ultramylonite

    NASA Astrophysics Data System (ADS)

    Kilian, Rüdiger

    2014-05-01

    Rocks deforming by diffusion creep, are usually characterized by a small grain size, a weak or no crystallographic preferred orientation and an anti-correlated phase distribution of which the latter gives the most revealing insight into the active deformation mechanism. The present study focuses on the phase distribution in an amphibolite facies ultramylonite from a several meters wide shear zone within the Nordmannvik Nappe of the Norwegian Caledonides. In the shear zone, a granulite facies protolith is transformed to a fine grained matrix of quartz (50%), biotite (20%), white mica (20%), oligoclase (7%) and ilmenite/titanite with grain sizes below 10 μm (eq. diameter). Large grains of garnet, white mica and plagioclase form porphyroclasts. At high matrix proportions white mica and plagioclase porphyroclasts are less abundant. The matrix shows a homogeneous fabric and shows a strong anti-correlation of phases. Quartz forms single grains or clusters, which are at most a few grains thick, with a long axis inclined at 30 - 60° to the foliation, antithetic to the sense of shear. Quartz clusters have a regular spacing of ~30 μm, separated by biotite-stacks and oligoclase. White mica forms parallel to the foliation and replaces longer biotite grains (during shearing of the mica). Concurrently new biotite grows at those quartz grain boundaries, which are oriented at a high angle to the foliation. Only adjacent to porphyroclasts, the matrix homogeneity is disturbed. Biotite and plagioclase are depleted in the compressional sector and grow in the extensional sector. Correspondingly, garnet porphyroclasts have newly grown Ca-rich rims in compressional sectors and signs of dissolution in extensional ones. Thermodynamic modeling suggests that the modal composition of the matrix and the Ca-rich garnet rims form the stable assemblage. The microstructural positions of the phases can be related to the kinematics of granular flow. The alignment of quartz grains into clusters

  15. Mantle-slab interaction and redox mechanism of diamond formation.

    PubMed

    Palyanov, Yuri N; Bataleva, Yuliya V; Sokol, Alexander G; Borzdov, Yuri M; Kupriyanov, Igor N; Reutsky, Vadim N; Sobolev, Nikolai V

    2013-12-17

    Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle-slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle-slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content. PMID:24297876

  16. Mantle–slab interaction and redox mechanism of diamond formation

    PubMed Central

    Palyanov, Yuri N.; Bataleva, Yuliya V.; Sokol, Alexander G.; Borzdov, Yuri M.; Kupriyanov, Igor N.; Reutsky, Vadim N.; Sobolev, Nikolai V.

    2013-01-01

    Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle–slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle–slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content. PMID:24297876

  17. Formation of Nonclassical Ordered Phases of A B -Type Multiarm Block Copolymers

    NASA Astrophysics Data System (ADS)

    Gao, Ya; Deng, Hanlin; Li, Weihua; Qiu, Feng; Shi, An-Chang

    2016-02-01

    The formation of ordered phases from block copolymers is driven by a delicate balance between the monomer-monomer interaction and chain configurational entropy. The configurational entropy can be regulated by designed chain architecture, resulting in a new entropy-driven mechanism to control the self-assembly of ordered phases from block copolymers. An effective routine to regulate the configurational entropy is to utilize multiarm architecture, in which the entropic contribution to the free energy could be qualitatively controlled by the fraction of bridging configurations. As an illustration of this mechanism, the phase behavior of two A B -type multiarm block copolymers, B0-(Bi-Ai) m and (B1-Ai-B2) m where the minority A blocks form cylindrical or spherical domains, are examined using the self-consistent field theory (SCFT). The SCFT results demonstrate that the packing symmetry of the cylinders or spheres can be controlled by the length of the bridging B blocks. Several nonclassical ordered phases, including a novel square array cylinder with p 4 m m symmetry, are predicted to form from the A B -type multiarm block copolymers.

  18. Mechanism for direct graphite-to-diamond phase transition

    PubMed Central

    Xie, Hongxian; Yin, Fuxing; Yu, Tao; Wang, Jian-Tao; Liang, Chunyong

    2014-01-01

    Using classical molecular dynamics with a more reliable reactive LCBOPII potential, we have performed a detailed study on the direct graphite-to-diamond phase transition. Our results reveal a new so-called “wave-like buckling and slipping” mechanism, which controls the transformation from hexagonal graphite to cubic diamond. Based on this mechanism, we have explained how polycrystalline cubic diamond is converted from hexagonal graphite, and demonstrated that the initial interlayer distance of compressed hexagonal graphite play a key role to determine the grain size of cubic diamond. These results can broaden our understanding of the high pressure graphite-to-diamond phase transition. PMID:25088720

  19. Mechanism for direct graphite-to-diamond phase transition.

    PubMed

    Xie, Hongxian; Yin, Fuxing; Yu, Tao; Wang, Jian-Tao; Liang, Chunyong

    2014-08-04

    Using classical molecular dynamics with a more reliable reactive LCBOPII potential, we have performed a detailed study on the direct graphite-to-diamond phase transition. Our results reveal a new so-called "wave-like buckling and slipping" mechanism, which controls the transformation from hexagonal graphite to cubic diamond. Based on this mechanism, we have explained how polycrystalline cubic diamond is converted from hexagonal graphite, and demonstrated that the initial interlayer distance of compressed hexagonal graphite play a key role to determine the grain size of cubic diamond. These results can broaden our understanding of the high pressure graphite-to-diamond phase transition.

  20. Quark CP-phase and Froggatt-Nielsen mechanism

    NASA Astrophysics Data System (ADS)

    Hattori, Chuichiro; Matsuda, Masahisa; Matsunaga, Mamoru; Matsuoka, Takeo

    2016-08-01

    On the basis of the Froggatt-Nielsen mechanism, we study quark flavor mixings in the SU (6) × SU (2)R model. The characteristic structure of the CKM matrix is attributed to the hierarchical effective Yukawa couplings due to the Froggatt-Nielsen mechanism and also to the state-mixings beyond the MSSM. We elucidate the detailed form of the CKM matrix elements and find interesting relations between the CP violating phase and three mixing angles. Taking the existing data of three mixing angles, we estimate the quark CP-phase at δ = (75 ± 3) °. This result is in accord with observations.

  1. Rheological controls on the terrestrial core formation mechanism

    NASA Astrophysics Data System (ADS)

    Golabek, G. J.; Gerya, T. V.; Ziethe, R.; Kaus, B. J. P.; Tackley, P. J.

    2009-04-01

    Knowledge about the terrestrial core formation mechanism is still very limited. The fracturing mechanism was proposed for cold planetary interiors surrounded by an iron layer [Stevenson, 1981], which develops from an overlying magma ocean. In this case the cold central region is displaced by a degree one mode from the centre of the accreting planet and fractured due to the large stresses. In contrast the consideration of short-lived radioactive heating may result in warmer central regions and the preference of higher mode iron diapirism as core formation mechanism [e.g. Rubie et al., 2007; Ziethe and Spohn, 2007]. Until now most numerical models of core formation via diapirism were limited to the simulation of the sinking of a single diapir. We perform 2D cylindrical simulations using the code I2ELVIS applying the newly developed "spherical-Cartesian" methodology [Gerya and Yuen, 2007]. It combines finite differences on a fully staggered rectangular Eulerian grid and Lagrangian marker-in-cell technique for solving momentum, continuity and temperature equations as well as the Poisson equation for gravity potential in a self-gravitating planetary body. In the model the planet is surrounded by a low viscosity, massless fluid ("sticky air") to simulate a free surface [Schmeling et al., 2008]. We apply a temperature- and stress-dependent viscoplastic rheology inside Mars- and Earth-sized planets and include heat release due to radioactive decay, shear and adiabatic heating. As initial condition we use randomly distributed iron diapirs with random sizes in the range 50 to 100 km radius inside the accreting planet, which represent the iron delivered by predifferentiated impactors. A systematic investigation of the diapir behaviour for different activation volumes and Peierls stresses is being performed, and results are being compared to the isotopic time scale of core formation on terrestrial planets. We show that the rheology controls which formation mechanism becomes

  2. Mechanisms of amyloid formation revealed by solution NMR

    PubMed Central

    Karamanos, Theodoros K.; Kalverda, Arnout P.; Thompson, Gary S.; Radford, Sheena E.

    2015-01-01

    Amyloid fibrils are proteinaceous elongated aggregates involved in more than fifty human diseases. Recent advances in electron microscopy and solid state NMR have allowed the characterization of fibril structures to different extents of refinement. However, structural details about the mechanism of fibril formation remain relatively poorly defined. This is mainly due to the complex, heterogeneous and transient nature of the species responsible for assembly; properties that make them difficult to detect and characterize in structural detail using biophysical techniques. The ability of solution NMR spectroscopy to investigate exchange between multiple protein states, to characterize transient and low-population species, and to study high molecular weight assemblies, render NMR an invaluable technique for studies of amyloid assembly. In this article we review state-of-the-art solution NMR methods for investigations of: (a) protein dynamics that lead to the formation of aggregation-prone species; (b) amyloidogenic intrinsically disordered proteins; and (c) protein–protein interactions on pathway to fibril formation. Together, these topics highlight the power and potential of NMR to provide atomic level information about the molecular mechanisms of one of the most fascinating problems in structural biology. PMID:26282197

  3. Intermetallic Phase Formation in Explosively Welded Al/Cu Bimetals

    NASA Astrophysics Data System (ADS)

    Amani, H.; Soltanieh, M.

    2016-08-01

    Diffusion couples of aluminum and copper were fabricated by explosive welding process. The interface evolution caused by annealing at different temperatures and time durations was investigated by means of optical microscopy, scanning electron microscopy equipped with energy dispersive spectroscopy, and x-ray diffraction. Annealing in the temperature range of 573 K to 773 K (300 °C to 500 °C) up to 408 hours showed that four types of intermetallic layers have been formed at the interface, namely Al2Cu, AlCu, Al3Cu4, and Al4Cu9. Moreover, it was observed that iron trace in aluminum caused the formation of Fe-bearing intermetallics in Al, which is near the interface of the Al-Cu intermetallic layers. Finally, the activation energies for the growth of Al2Cu, AlCu + Al3Cu4, Al4Cu9, and the total intermetallic layer were calculated to be about 83.3, 112.8, 121.6, and 109.4 kJ/mol, respectively. Considering common welding methods ( i.e., explosive welding, cold rolling, and friction welding), although there is a great difference in welding mechanism, it is found that the total activation energy is approximately the same.

  4. Mechanisms of the water-gas-shift reaction by iron pentacarbonyl in the gas phase.

    PubMed

    Rozanska, Xavier; Vuilleumier, Rodolphe

    2008-10-01

    We analyzed the mechanisms of the water-gas-shift reaction catalyzed by Fe(CO) 5/OH (-) in the gas phase using DFT methods. The systematic analysis of the accessible reaction mechanisms and the consideration of the Gibbs free energies allows for different reaction routes than previously suggested. In the dominant catalytic cycle, the hydride [FeH(CO) 4]- is the important intermediate. Associative reaction mechanisms are not favorable under moderate and low pressures. At high pressure, a side reaction takes over and prevents the conversion of H 2O and CO to H 2 and CO 2 and leads to the formation of HCOOH.

  5. Linear volcanic chains in oceans: Possible formation mechanisms

    NASA Astrophysics Data System (ADS)

    Peive, A. A.

    2007-07-01

    Possible formation mechanisms of linear volcanic chains in oceans are considered with particular emphasis placed on tectonic processes in the lithosphere. Nonparallel patterns of volcanic chains, as well as irregular variations in volcanism ages, may be due to the formation of sigmoid fractures that appear in certain stress fields. The tectonic stress may control the dimensions of volcanic chains, their lengths, and the volcanism intensity. At the same time, certain assumptions are necessary. For example, to explain shallow magmatism, it must be assumed that the temperature of the asthenosphere is close to the melting point of mantle material, although the asthenosphere may be highly variable in the degree of enrichment. Hence, even insignificant variations in the temperature, volatile contents, or bulk composition may provoke large-volume melting. It is shown that the rotation of the Earth causes additional displacements of plates relative to the underlying mantle. While a fertile fragment exists in the mantle, such an inhomogeneity remains stationary relative to the moving plate and the melting of this inhomogeneity may result in the growth of volcanic uplift. The global stress field determined by plate boundaries and an intraplate factor controls the distribution of the stress fields, which are responsible for the formation of volcanic chains. It is concluded that the available data on the age progressions and character of linear volcanic chains within oceanic plates provide no grounds for any single hypothesis explaining the formation of these chains. The most universal hypothesis seems to be the explanation based on shallow tectonic processes. The localization and formation mechanism of volcanic chains are determined by the stress field in the lithosphere, thermal compression and expansion, the specific features of the plate structure, melt dynamics, and the occurrence of fertile material in the mantle rather than by temperature. The volume of volcanic

  6. The probabilistic mechanism of formation of block structures

    NASA Astrophysics Data System (ADS)

    Ivanov, V. I.

    2012-03-01

    Questions on the formation of block structures are considered. It is shown that the block structure is characteristic of bodies in a wide range of scales from microscopic to astronomic and from the bodies of nonliving nature to living organisms and communities. A scheme of the mechanism of the probabilistic formation of block structures is suggested. The characteristics general for structures of all scales are revealed. Evidence is presented that the hierarchical pattern of element sizes is characteristic of natural structures in which the ratio of linear sizes of elements neighboring by hierarchy is 2-5, while the characteristic scale coefficient is √ N , where N is the total number of elements of which the system is formed. The block-probabilistic approach ensures knowledge of rare catastrophic events, including earthquakes, market crashes, floods, and industrial catastrophes, or creative events such as the formation of hypercomplex systems similar to organisms and communities. The statistics of rare events follows the power distribution (the distribution with a "heavy tail") rather than the exponential one and especially the Poisson distribution, the Gaussian distribution, or the distributions with "light tails" close to them. The expression for the factor of increasing the formation probability of the systems, which is of many orders of magnitude even for the simplest systems, is acquired.

  7. Formation of organic acids from the gas-phase ozonolysis of terpinolene.

    PubMed

    Ma, Yan; Marston, George

    2009-06-01

    Gas-phase ozonolysis of terpinolene was studied in static chamber experiments using gas chromatography coupled to mass spectrometric and flame ionisation detection to separate and detect products. Two isomers of C(7)-diacids and three isomers of C(7)-aldehydic acids were identified in the condensed phase after derivatisation. Possible mechanisms of formation of these acids were investigated using different OH radical scavengers and relative humidities, and were compared to those reported earlier for the ozonolysis of beta-pinene. In addition, branching ratios for some of the individual reaction steps, e.g. the branching ratio between the two hydroperoxide channels of the C(7)-CI, were deduced from the quantitative product yield data. Branching ratios for POZ decomposition and the stabilisation/decomposition of the C(7-)CI were also obtained from measurements of the C(7) primary carbonyl product. PMID:19458821

  8. Molecular length distribution and the formation of smectic phases

    PubMed Central

    Kapernaum, Nadia; Hartley, C Scott; Roberts, Jeffrey C; Lemieux, Robert P

    2009-01-01

    Summary The phase diagrams of two mixtures of chemically similar smectogenic mesogens strongly differing in molecular length were investigated. In these mixtures the nematic phase present in the pure short mesogen disappeared rapidly on the addition of the longer mesogen, while the smectic state was preserved. In the smectic state the smectic A phase was the much more stable phase as the smectic C phase disappeared quite rapidly as well. In these compounds the loss of the smectic C phase is accompanied by a decrease in smectic translational order and very small tilt angles. This leads to a concentration induced smectic C to smectic A transition. Thus smectic A seems to be the most stable phase to accommodate mesogenic molecules of substantially different length. These surprising results are of general interest for the understanding of the structure and dynamics of smectic phases, as the structure of these bidisperse smectics is signified by extensive out-of-layer fluctuations. PMID:20300469

  9. Molecular length distribution and the formation of smectic phases.

    PubMed

    Kapernaum, Nadia; Hartley, C Scott; Roberts, Jeffrey C; Lemieux, Robert P; Giesselmann, Frank

    2009-11-13

    The phase diagrams of two mixtures of chemically similar smectogenic mesogens strongly differing in molecular length were investigated. In these mixtures the nematic phase present in the pure short mesogen disappeared rapidly on the addition of the longer mesogen, while the smectic state was preserved. In the smectic state the smectic A phase was the much more stable phase as the smectic C phase disappeared quite rapidly as well. In these compounds the loss of the smectic C phase is accompanied by a decrease in smectic translational order and very small tilt angles. This leads to a concentration induced smectic C to smectic A transition. Thus smectic A seems to be the most stable phase to accommodate mesogenic molecules of substantially different length. These surprising results are of general interest for the understanding of the structure and dynamics of smectic phases, as the structure of these bidisperse smectics is signified by extensive out-of-layer fluctuations.

  10. Sensitivity analysis of a mixed-phase chemical mechanism using automatic differentiation

    SciTech Connect

    Zhang, Y.; Easter, R.C.

    1998-08-01

    A sensitivity analysis of a comprehensive mixed-phase chemical mechanism is conducted under a variety of atmospheric conditions. The local sensitivities of gas and aqueous phase species concentrations with respect to a variety of model parameters are calculated using the novel automatic differentiation ADIFOR tool. The main chemical reaction pathways in all phases, interfacial mass transfer processes, and ambient physical parameters that affect tropospheric O{sub 3} formation and O{sub 3}-precursor relations under all modeled conditions are identified and analyzed. The results show that the presence of clouds not only reduces many gas phase species concentrations and the total oxidizing capacity but alters O{sub 3}-precursor relations. Decreases in gas phase concentrations and photochemical formation rates of O{sub 3} can be up to 9{percent} and 100{percent}, respectively, depending on the preexisting atmospheric conditions. The decrease in O{sub 3} formation is primarily caused by the aqueous phase reactions of O{sub 2}{sup {minus}} with dissolved HO{sub 2} and O{sub 3} under most cloudy conditions. {copyright} 1998 American Geophysical Union

  11. Clarifying the dominant sources and mechanisms of cirrus cloud formation.

    PubMed

    Cziczo, Daniel J; Froyd, Karl D; Hoose, Corinna; Jensen, Eric J; Diao, Minghui; Zondlo, Mark A; Smith, Jessica B; Twohy, Cynthia H; Murphy, Daniel M

    2013-06-14

    Formation of cirrus clouds depends on the availability of ice nuclei to begin condensation of atmospheric water vapor. Although it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective have not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent. Further, composition analysis combined with relative humidity measurements suggests that heterogeneous freezing was the dominant formation mechanism of these clouds.

  12. Clarifying the dominant sources and mechanisms of cirrus cloud formation.

    PubMed

    Cziczo, Daniel J; Froyd, Karl D; Hoose, Corinna; Jensen, Eric J; Diao, Minghui; Zondlo, Mark A; Smith, Jessica B; Twohy, Cynthia H; Murphy, Daniel M

    2013-06-14

    Formation of cirrus clouds depends on the availability of ice nuclei to begin condensation of atmospheric water vapor. Although it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective have not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent. Further, composition analysis combined with relative humidity measurements suggests that heterogeneous freezing was the dominant formation mechanism of these clouds. PMID:23661645

  13. Multiple repressive mechanisms in the hippocampus during memory formation.

    PubMed

    Cho, Jun; Yu, Nam-Kyung; Choi, Jun-Hyeok; Sim, Su-Eon; Kang, SukJae Joshua; Kwak, Chuljung; Lee, Seung-Woo; Kim, Ji-il; Choi, Dong Il; Kim, V Narry; Kaang, Bong-Kiun

    2015-10-01

    Memory stabilization after learning requires translational and transcriptional regulations in the brain, yet the temporal molecular changes that occur after learning have not been explored at the genomic scale. We used ribosome profiling and RNA sequencing to quantify the translational status and transcript levels in the mouse hippocampus after contextual fear conditioning. We revealed three types of repressive regulations: translational suppression of ribosomal protein-coding genes in the hippocampus, learning-induced early translational repression of specific genes, and late persistent suppression of a subset of genes via inhibition of estrogen receptor 1 (ESR1/ERα) signaling. In behavioral analyses, overexpressing Nrsn1, one of the newly identified genes undergoing rapid translational repression, or activating ESR1 in the hippocampus impaired memory formation. Collectively, this study unveils the yet-unappreciated importance of gene repression mechanisms for memory formation.

  14. Pore formation mechanism of porous poly(DL-lactic acid) matrix membrane.

    PubMed

    Phaechamud, Thawatchai; Chitrattha, Sasiprapa

    2016-04-01

    Porous PLA structure has been widely used in cell transplantation, drug carrier and wound dressing. The porous structure can be controlled depending on the choice of the polymer, solvent, nonsolvent and preparation parameters. In this study, the porous PLA matrix membranes were prepared by adding PEG 400 in PLA solution using dichloromethane (DCM) as solvent prior to casting. The influence of other liquids as co-solvent on pore formation and the structural change during membrane formation were evaluated. The co-solvents affected both porous topography and mechanical properties of PLA membrane. The porous matrix were produced when the non-solvent of PLA was used as co-solvent. Cryo-SEM micrographs revealed that PEG 400 still remained in the PLA porous matrix membrane. From the tracking of the structural change during film formation, the PLA-PEG solution changed into porous structure by liquid liquid phase separation and solidification processes, respectively. Thermogravimetric analysis revealed that PLA-PEG in DCM solution exhibited the two-step of weight loss, the first step occurred from DCM evaporation and the second step occurred from the degradation of PLA-PEG matrix. The liquid-liquid phase separation and solidification started when the amount of DCM was higher than PEG 400 for 2.67 folds and DCM amount was equal to that of PEG 400, respectively. These results could clarify the pore formation mechanism of porous PLA membrane and will be useful for the further investigation and application. PMID:26838905

  15. Motionless electromagnetic phase stepping versus mechanical phase stepping in x-ray phase-contrast imaging with a compact source.

    PubMed

    Harmon, Katherine J; Miao, Houxun; Gomella, Andrew A; Bennett, Eric E; Foster, Barbara A; Bhandarkar, Priya; Wen, Han

    2015-04-21

    X-ray phase contrast imaging based on grating interferometers detects the refractive index distribution of an object without relying on radiation attenuation, thereby having the potential for reduced radiation absorption. These techniques belong to the broader category of optical wavefront measurement, which requires stepping the phase of the interference pattern to obtain a pixel-wise map of the phase distortion of the wavefront. While phase stepping traditionally involves mechanical scanning of a grating or mirror, we developed electromagnetic phase stepping (EPS) for imaging with compact sources to obviate the need for mechanical movement. In EPS a solenoid coil is placed outside the x-ray tube to shift its focal spot with a magnetic field, causing a relative movement between the projection of the sample and the interference pattern in the image. Here we present two embodiments of this method. We verified experimentally that electromagnetic and mechanical phase stepping give the same results and attain the same signal-to-noise ratios under the same radiation dose. We found that the relative changes of interference fringe visibility were within 3.0% when the x-ray focal spot was shifted by up to 1.0 mm in either direction. We conclude that when using x-ray tube sources, EPS is an effective means of phase stepping without the need for mechanical movement.

  16. Mechanical compaction directly modulates the dynamics of bile canaliculi formation.

    PubMed

    Wang, Yan; Toh, Yi-Chin; Li, Qiushi; Nugraha, Bramasta; Zheng, Baixue; Lu, Thong Beng; Gao, Yi; Ng, Mary Mah Lee; Yu, Hanry

    2013-02-01

    Homeostatic pressure-driven compaction is a ubiquitous mechanical force in multicellular organisms and is proposed to be important in the maintenance of multicellular tissue integrity and function. Previous cell-free biochemical models have demonstrated that there are cross-talks between compaction forces and tissue structural functions, such as cell-cell adhesion. However, its involvement in physiological tissue function has yet to be directly demonstrated. Here, we use the bile canaliculus (BC) as a physiological example of a multicellular functional structure in the liver, and employ a novel 3D microfluidic hepatocyte culture system to provide an unprecedented opportunity to experimentally modulate the compaction states of primary hepatocyte aggregates in a 3D physiological-mimicking environment. Mechanical compaction alters the physical attributes of the hepatocyte aggregates, including cell shape, cell packing density and cell-cell contact area, but does not impair the hepatocytes' remodeling and functional capabilities. Characterization of structural and functional polarity shows that BC formation in compact hepatocyte aggregates is accelerated to as early as 12 hours post-seeding; whereas non-compact control requires 48 hours for functional BC formation. Further dynamic immunofluorescence imaging and gene expression profiling reveal that compaction accelerated BC formation is accompanied by changes in actin cytoskeleton remodeling dynamics and transcriptional levels of hepatic nuclear factor 4α and Annexin A2. Our report not only provides a novel strategy of modeling BC formation for in vitro hepatology research, but also shows a first instance that homeostatic pressure-driven compaction force is directly coupled to the higher-order multicellular functions. PMID:23233209

  17. [Current concepts of the mechanisms of formation of gunshot fractures].

    PubMed

    Pigolkin, Iu I; Dubrovin, I A; Leonov, S V; Mikhaĭlenko, A V; Dubrovin, A I; Zotkin, D A

    2013-01-01

    The mechanisms of formation of gunshot fractures in flat bones inflicted by a semispherical bullet were investigated using expert and experimental materials. The process of crack formation was considered in terms of the Hertzian contact problem and the Hill-Johnson model. It was shown that the fracture develops as a result of combination of stresses and strains in the bone tissue leading to the formation of a hydrostatic nucleus prior to tissue fragmentation. Dynamic fluctuations (waves) generated in the zone of hydrostatic compression resulting from the gunshot injury propagate with the velocity of sound from the nucleus in the direction of the bullet movement. According to the Hill-Johnson model, the waves propagate in the direction of the impact within a parabolically expanding space; this accounts for the mechanism of formation of parabolic cracks and the specific shape of the defect that the bullet produces in the flat bones. The dynamic load applied by means of an indentor forms at a higher rate than the velocity of sound in the affected material. It gives reason to consider the effect of a bullet moving with the speed of 250 m/s as quasi-static loading. The results obtained in this study make a contribution to the theory of impact effect of a bullet and provide a deeper insight into the physical nature of the direct and sideway action of a gunshot projectile. Moreover, they explain the cause behind the widening of the outlet part of the perforating fracture in the flat bones. PMID:25474911

  18. Formation of Secondary Particulate Matter by Reactions of Gas Phase Hexanal with Sulfate Aerosol Particles

    NASA Astrophysics Data System (ADS)

    Zhang, J.

    2003-12-01

    The formation of secondary particulate matter from the atmospheric oxidation of organic compounds can significantly contribute to the particulate burden, but the formation of organic secondary particulate matter is poorly understood. One way of producing organic secondary particulate matter is the oxidation of hydrocarbons with seven or more carbon atoms to get products with low vapor pressure. However, several recent reports suggest that relatively low molecular weight carbonyls can enter the particle phase by undergoing heterogeneous reactions. This may be a very important mechanism for the formation of organic secondary particulate matter. Atmospheric aldehydes are important carbonyls in the gas phase, which form via the oxidation of hydrocarbons emitted from anthropogenic and biogenic sources. In this poster, we report the results on particle growth by the heterogeneous reactions of hexanal. A 5 L Continuous Stirred Tank Reactor (CSTR) is set up to conduct the reactions in the presence of seed aerosol particles of deliquesced ammonia bisulfate. Hexanal is added into CSTR by syringe pump, meanwhile the concentrations of hexanal are monitored with High Pressure Liquid Chromatograph (HPLC 1050). A differential Mobility Analyzer (TSI 3071) set to an appropriate voltage is employed to obtain monodisperse aerosols, and another DMA associated with a Condensation Nuclear Counter (TSI 7610) is used to measure the secondary particle size distribution by the reaction in CSTR. This permits the sensitive determination of particle growth due to the heterogeneous reaction, very little growth occurs when hexanal added alone. Results for the simultaneous addition of hexanal and alcohols will also be presented.

  19. Nanoscale self-assembly of starch: Phase relations, formation, and structure

    NASA Astrophysics Data System (ADS)

    Creek, John A.

    This project has been undertaken to develop a fundamental understanding of the spherulitic self-assembly of starch polymers from aqueous solution, both as a model for starch granule initiation in vivo and as a biologically-inspired material with applications in the food and pharmaceutical industries. Botanical starches were observed to form semi-crystalline spherulites from aqueous solution when cooled after a high temperature treatment, and the processes resulting in spherulite formation were investigated. Based on the influence of cooling rate on spherulite formation from a botanical starch, liquid-liquid demixing in competition with crystallization was proposed as the mechanism leading to spherulite formation (summarized in a hypothetical phase diagram). Study of amylose and amylopectin self-assembly demonstrated that the linear polymer plays the primary role in forming spherulites. As a result, the roles of degree of polymerization, concentration, and thermal processing conditions on amylose self-assembly were explored. Thermal properties, final system morphology, and crystalline allomorph were characterized. In all cases the experimental findings supported the proposed phase diagram. Finally, the crystalline nanostructure of the spherulites was probed using atomic force microscopy (AFM), revealing a seemingly universal level of structure in crystalline starch materials. This was compared to an existing model of crystallization for synthetic polymers involving a transitional liquid crystalline-like ordering---a comparison that makes sense in light of the known helical structure of starch.

  20. Formation mechanism of the protective layer in a blast furnace hearth

    NASA Astrophysics Data System (ADS)

    Jiao, Ke-xin; Zhang, Jian-liang; Liu, Zheng-jian; Xu, Meng; Liu, Feng

    2015-10-01

    A variety of techniques, such as chemical analysis, scanning electron microscopy-energy dispersive spectroscopy, and X-ray diffraction, were applied to characterize the adhesion protective layer formed below the blast furnace taphole level when a certain amount of titanium- bearing burden was used. Samples of the protective layer were extracted to identify the chemical composition, phase assemblage, and distribution. Furthermore, the formation mechanism of the protective layer was determined after clarifying the source of each component. Finally, a technical strategy was proposed for achieving a stable protective layer in the hearth. The results show that the protective layer mainly exists in a bilayer form in the sidewall, namely, a titanium-bearing layer and a graphite layer. Both the layers contain the slag phase whose major crystalline phase is magnesium melilite (Ca2MgSi2O7) and the main source of the slag phase is coke ash. It is clearly determined that solid particles such as graphite, Ti(C,N) and MgAl2O4 play an important role in the formation of the protective layer, and the key factor for promoting the formation of a stable protective layer is reasonable control of the evolution behavior of coke.

  1. Mechanisms of nascent fiber formation during avian skeletal muscle hypertrophy

    NASA Technical Reports Server (NTRS)

    McCormick, K. M.; Schultz, E.

    1992-01-01

    This study examined two putative mechanisms of new fiber formation in postnatal skeletal muscle, namely longitudinal fragmentation of existing fibers and de novo formation. The relative contributions of these two mechanisms to fiber formation in hypertrophying anterior latissimus dorsi (ALD) muscle were assessed by quantitative analysis of their nuclear populations. Muscle hypertrophy was induced by wing-weighting for 1 week. All nuclei formed during the weighting period were labeled by continuous infusion of 5-bromo-2'-deoxyuridine (BrdU), a thymidine analog, and embryonic-like fibers were identified using an antibody to ventricular-like embryonic (V-EMB) myosin. The number of BrdU-labeled and unlabeled nuclei in V-EMB-positive fibers were counted. Wing-weighting resulted in significant muscle enlargement and the appearance of many V-EMB+ fibers. The majority of V-EMB+ fibers were completely independent of mature fibers and had a nuclear density characteristics of developing fibers. Furthermore, nearly 100% of the nuclei in independent V-EMB+ fibers were labeled. These findings strongly suggest that most V-EMB+ fibers were nascent fibers formed de novo during the weighting period by satellite cell activation and fusion. Nascent fibers were found primarily in the space between fascicles where they formed a complex anastomosing network of fibers running at angles to one another. Although wing-weighting induced an increase in the number of branched fibers, there was no evidence that V-EMB+ fibers were formed by longitudinal fragmentation. The location of newly formed fibers in wing-weighted and regenerating ALD muscle was compared to determine whether satellite cells in the ALD muscle were unusual in that, if stimulated to divide, they would form fibers in the inter- and intrafascicular space. In contrast to wing-weighted muscle, nascent fibers were always found closely associated with necrotic fibers. These results suggest that wing-weighting is not simply another

  2. Phase Transition in Postsynaptic Densities Underlies Formation of Synaptic Complexes and Synaptic Plasticity.

    PubMed

    Zeng, Menglong; Shang, Yuan; Araki, Yoichi; Guo, Tingfeng; Huganir, Richard L; Zhang, Mingjie

    2016-08-25

    Postsynaptic densities (PSDs) are membrane semi-enclosed, submicron protein-enriched cellular compartments beneath postsynaptic membranes, which constantly exchange their components with bulk aqueous cytoplasm in synaptic spines. Formation and activity-dependent modulation of PSDs is considered as one of the most basic molecular events governing synaptic plasticity in the nervous system. In this study, we discover that SynGAP, one of the most abundant PSD proteins and a Ras/Rap GTPase activator, forms a homo-trimer and binds to multiple copies of PSD-95. Binding of SynGAP to PSD-95 induces phase separation of the complex, forming highly concentrated liquid-like droplets reminiscent of the PSD. The multivalent nature of the SynGAP/PSD-95 complex is critical for the phase separation to occur and for proper activity-dependent SynGAP dispersions from the PSD. In addition to revealing a dynamic anchoring mechanism of SynGAP at the PSD, our results also suggest a model for phase-transition-mediated formation of PSD. PMID:27565345

  3. Auto Mechanics. Pre-Apprenticeship Phase 1 Training. Instructor's Guide.

    ERIC Educational Resources Information Center

    Lane Community Coll., Eugene, OR.

    This instructor's guide accompanies the self-paced student training modules on auto mechanics, one of which is available separately as CE 032 867. Introductory materials include an introduction to pre-apprenticeship and its three phases of training, a recommended preocedure for conducting pre-apprenticeship training, and a course outline. Teaching…

  4. Lipid Microdomains: Structural Correlations, Fluctuations, and Formation Mechanisms

    NASA Astrophysics Data System (ADS)

    Fan, Jun; Sammalkorpi, Maria; Haataja, Mikko

    2010-03-01

    Compositional lipid microdomains (“lipid rafts”) in mammalian plasma membranes are believed to facilitate many important cellular processes. While several physically distinct scenarios predicting the presence of finite-sized microdomains in vivo have been proposed in the past, direct experimental verification or falsification of model predictions has remained elusive. Herein, we demonstrate that the combination of the spatial correlation and temporal fluctuation spectra of the lipid domains can be employed to unambiguously differentiate between the existing theoretical scenarios. Furthermore, the differentiation of the raft formation mechanisms using this methodology can be achieved by collecting data at physiologically relevant conditions without the need to tune control parameters.

  5. Cementite Formation from Hematite-Graphite Mixture by Simultaneous Thermal-Mechanical Activation

    NASA Astrophysics Data System (ADS)

    Ashrafzadeh, Milad; Soleymani, Amir Peyman; Panjepour, Masoud; Shamanian, Morteza

    2015-04-01

    In this study, the effect of simultaneous thermal-mechanical activation (STMA) process on carbothermic reduction of hematite and also on iron carbide formation has been investigated. For this purpose, the STMA process was performed for 3 and 6 hours at 973 K (700 °C) and 1073 K (800 °C) on hematite-graphite powder mixtures (with 22 wt pct C) in argon atmosphere. The XRD patterns showed that by performing this process at 973 K (700 °C), the initial hematite reduction led to the formation of wüstite in the presence of graphite. Metallic iron phase was also formed along with wüstite phase at 1073 K (800 °C) for 3 hours, and by increasing the process time to 6 hours, in addition to the metallic iron, iron carbide was also formed. The SEM images and EDS analysis obtained at 1073 K (800 °C)were also indicative of the formation of pearlite structure along with proeutectoid cementite phase and free carbon in the form of graphite in the structure of the samples. According to the results of the image analysis, the percentage of the carbon content was more than 2.22 wt pct in this process lasting for 6 hours at 1073 K (800 °C). Also, DTA results showed that the sample hot milled for 6 hours at 1073 K (800 °C) contained more than 2.1 wt pct carbon. The mechanism of metallic iron and cementite formation from hematite was proposed. Therefore, the STMA process led to an increase in the rate of carbothermic reduction of hematite to metallic iron and reduced its starting temperature relative to the non-simultaneous application of each of the thermal and mechanical activation. Finally, this process can be brought up as a new method for the production of iron carbide from iron oxides.

  6. The mechanics of ringed basin formation. [lunar impact craters

    NASA Technical Reports Server (NTRS)

    Melosh, H. J.; Mckinnon, W. B.

    1978-01-01

    The study investigates the mechanics of ring formation in lunar basins by collapse or slumping. Two classes of models are presented in order to broaden the range of plausible mechanisms involved. Extrusion flow models which assume a weak layer underlying a strong layer as well as plastic flow models which assume a continuous decrease of cohesion strength with depth are considered. In both classes, strength or resistance to deformation decreases with depth. If this decrease is rapid enough, in either set of models, at least one concentric fault scarp forms. The ratio between the scarp radius and the crater radius varies between 1.2 and about 2.7 depending on the model, and is thus in the range of observed radius ratios.

  7. A unifying picture of gas-phase formation and growth of PAH (Polycyclic Aromatic Hydrocarbons), soot, diamond and graphite

    NASA Technical Reports Server (NTRS)

    Frenklach, Michael

    1990-01-01

    A variety of seemingly different carbon formation processes -- polycyclic aromatic hydrocarbons and diamond in the interstellar medium, soot in hydrocarbon flames, graphite and diamond in plasma-assisted-chemical vapor deposition reactors -- may all have closely related underlying chemical reaction mechanisms. Two distinct mechanisms for gas-phase carbon growth are discussed. At high temperatures it proceeds via the formation of carbon clusters. At lower temperatures it follows a polymerization-type kinetic sequence of chemical reactions of acetylene addition to a radical, and reactivation of the resultant species through H-abstraction by a hydrogen atom.

  8. Reexamination of CO formation during formic acid decomposition on the Pt(1 1 1) surface in the gas phase

    NASA Astrophysics Data System (ADS)

    Wang, Yingying; Zhang, Dongju; Liu, Peng; Liu, Chengbu

    2016-08-01

    Existing theoretical results for formic acid (HCOOH) decomposition on Pt(1 1 1) cannot rationalize the easy CO poisoning of the catalysts in the gas phase. The present work reexamined HCOOH decomposition on Pt(1 1 1) by considering the effect of the initial adsorption structure of the reactant on the reactivity. Our calculations present a new adsorption configuration of HCOOH on Pt(1 1 1), from which the formation of CO is found to be competing with the formation of CO2. The newly proposed mechanism improves our understanding for the mechanism of HCOOH decomposition catalyzed by Pt-based catalysts.

  9. Np Incorporation into Uranyl Alteration Phases: A Quantum Mechanical Approach

    SciTech Connect

    Shuller, Lindsay; Ewing, Rodney C. |; Becker, Udo

    2007-07-01

    Neptunium is a major contributor to the long-term radioactivity in a geologic repository for spent nuclear fuel (SNF) due to its long half-life (2.1 million years). The mobility of Np may be decreased by its incorporation into the U{sup 6+} phases that form during the corrosion of SNF. The ionic radii of Np{sup 5+} (0.089 nm) and U{sup 6+} (0.087 nm) are similar, as is their chemistry. Experimental studies have shown that Np can be incorporated into uranyl phases up to concentrations on the order of 100 ppm. The low concentration of Np in the uranyl phases complicates experimental detection and presents a significant challenge for determining the incorporation mechanism. Therefore, we have used quantum mechanical calculations to investigate incorporation mechanisms and evaluate the energetics of Np substituting for U. CASTEP, a density functional theory based code that uses plane waves to model the behavior of the valence electrons and pseudo-potentials to model the behavior of the core and inner valence electrons, was used to calculate optimal H positions, relaxed geometry, and the energy of different uranyl phases. The incorporation energy for Np into uranyl alteration phases was calculated for studtite, [(UO{sub 2})O{sub 2}(H{sub 2}O){sub 2}](H{sub 2}O){sub 2}, and boltwoodite, HK(UO{sub 2})(SiO{sub 4}) 1.5(H{sub 2}O). Studtite is the rare case of a stable, naturally-occurring peroxide mineral, in this case a uranyl hydroxyl peroxide that forms in the presence of H{sub 2}O{sub 2} from the radiolysis of H{sub 2}O. For studtite, two incorporation mechanisms were evaluated: (1) charge-balanced substitution of Np{sup 5+} and H{sup +} for one U{sup 6+}, and (2) direct substitution of Np{sup 6+} for U{sup 6+}. For boltwoodite, the H atomic positions prior to Np incorporation were determined, as well as the Np incorporation mechanisms and the corresponding substitution energies. The preferential incorporation of Np into different structure types of U{sup 6+} minerals

  10. Noise and disorder: Phase transitions and universality in a model of opinion formation

    NASA Astrophysics Data System (ADS)

    Crokidakis, Nuno

    2016-02-01

    In this work, we study a three-state opinion formation model considering two distinct mechanisms, namely independence and conviction. Independence is introduced in the model as a noise by means of a probability of occurrence q. On the other hand, conviction acts as a disorder in the system, and it is introduced by two discrete probability distributions. We analyze the effects of such two mechanisms on the phase transitions of the model, and we found that the critical exponents are universal over the order-disorder frontier, presenting the same universality class of the mean-field Ising model. In addition, for one of the probability distributions, the transition may be eliminated for a wide range of the parameters.

  11. New understanding of microstructure formation of the rubber phase in thermoplastic vulcanizates (TPV).

    PubMed

    Wu, Hanguang; Tian, Ming; Zhang, Liqun; Tian, Hongchi; Wu, Youping; Ning, Nanying

    2014-03-21

    The breakup of the rubber phase in an ethylene-propylene-diene monomer (EPDM)/polypropylene (PP) blend at the early stage of dynamic vulcanization is similar to that in an unvulcanized EPDM/PP blend because of the low crosslink density of the EPDM phase. In this work, the minimum size of the rubber phase in the unvulcanized EPDM/PP blend was first calculated by using the critical breakup law of viscoelastic droplets in a matrix. The calculated results showed that the minimum size of the rubber phase in the unvulcanized blend was in the nanometer scale (25-46 nm), not the micrometer scale as reported in many works. Meanwhile, the actual size of the rubber phase in the thermoplastic vulcanizate (TPV) at both the early stage and the final stage of dynamic vulcanization was observed by using peak force tapping atomic force microscopy (PF-AFM). The results indicated that the EPDM phase indeed broke up into nanoparticles at the early stage of dynamic vulcanization, in good agreement with the calculated results. More interestingly, we first revealed that the micrometer-sized rubber particles commonly observed in TPV were actually the agglomerates of rubber nanoparticles with diameters between 40 and 60 nm. The mechanism for the formation of rubber nanoparticles and their agglomerates during dynamic vulcanization was then discussed. Our work provides guidance to control the microstructure of the rubber phase in TPV to prepare high performance TPV products for a wide range of applications in the automobile and electronic industries. PMID:24652229

  12. Formation mechanism and ordered patterns in Cu films deposited on silicone oil surfaces

    NASA Astrophysics Data System (ADS)

    Chen, Miao-Gen; Xie, Jian-Ping; Ye, Gao-Xiang

    2006-12-01

    A copper (Cu) film system, deposited on silicone oil surfaces by vapor phase deposition method, has been fabricated and its formation mechanism as well as ordered patterns has been studied. The formation mechanism of the films obeys the two-stage growth model. The ordered patterns, which are composed of a large number of parallel keys with different width w but nearly uniform length L, are observed in the continuous Cu films. It is noted that, if the nominal film thickness d=120.0 nm, the value of α=L/(4w) reaches its maximum at the deposition rate f=0.05 nm/s. The experiment indicates that the ordered patterns mainly result from the ordered material aggregation, which depends closely on the internal stress in the nearly free sustained Cu film system.

  13. Mechanical modulation method for ultrasensitive phase measurements in photonics biosensing.

    PubMed

    Patskovsky, S; Maisonneuve, M; Meunier, M; Kabashin, A V

    2008-12-22

    A novel polarimetry methodology for phase-sensitive measurements in single reflection geometry is proposed for applications in optical transduction-based biological sensing. The methodology uses altering step-like chopper-based mechanical phase modulation for orthogonal s- and p- polarizations of light reflected from the sensing interface and the extraction of phase information at different harmonics of the modulation. We show that even under a relatively simple experimental arrangement, the methodology provides the resolution of phase measurements as low as 0.007 deg. We also examine the proposed approach using Total Internal Reflection (TIR) and Surface Plasmon Resonance (SPR) geometries. For TIR geometry, the response appears to be strongly dependent on the prism material with the best values for high refractive index Si. The detection limit for Si-based TIR is estimated as 10(-5) in terms Refractive Index Units (RIU) change. SPR geometry offers much stronger phase response due to a much sharper phase characteristics. With the detection limit of 3.2*10(-7) RIU, the proposed methodology provides one of best sensitivities for phase-sensitive SPR devices. Advantages of the proposed method include high sensitivity, simplicity of experimental setup and noise immunity as a result of a high stability modulation.

  14. A new mechanism for dendritic pattern formation in dense systems

    PubMed Central

    Oikawa, Noriko; Kurita, Rei

    2016-01-01

    Patterns are often formed when particles cluster: Since patterns reflect the connectivity of different types of material, the emergence of patterns affects the physical and chemical properties of systems and shares a close relationship to their macroscopic functions. A radial dendritic pattern (RDP) is observed in many systems such as snow crystals, polymer crystals and biological systems. Although most of these systems are considered as dense particle suspensions, the mechanism of RDP formation in dense particle systems is not yet understood. It should be noted that the diffusion limited aggregation model is not applicable to RDP formation in dense systems, but in dilute particle systems. Here, we propose a simple model that exhibits RDP formation in a dense particle system. The model potential for the inter-particle interaction is composed of two parts, a repulsive and an attractive force. The repulsive force is applied to all the particles all the time and the attractive force is exerted only among particles inside a circular domain, which expands at a certain speed as a wave front propagating from a preselected centre. It is found that an RDP is formed if the velocity of the wave front that triggers the attractive interaction is of the same order of magnitude as the time scale defined by the aggregation speed. PMID:27353447

  15. A new mechanism for dendritic pattern formation in dense systems.

    PubMed

    Oikawa, Noriko; Kurita, Rei

    2016-01-01

    Patterns are often formed when particles cluster: Since patterns reflect the connectivity of different types of material, the emergence of patterns affects the physical and chemical properties of systems and shares a close relationship to their macroscopic functions. A radial dendritic pattern (RDP) is observed in many systems such as snow crystals, polymer crystals and biological systems. Although most of these systems are considered as dense particle suspensions, the mechanism of RDP formation in dense particle systems is not yet understood. It should be noted that the diffusion limited aggregation model is not applicable to RDP formation in dense systems, but in dilute particle systems. Here, we propose a simple model that exhibits RDP formation in a dense particle system. The model potential for the inter-particle interaction is composed of two parts, a repulsive and an attractive force. The repulsive force is applied to all the particles all the time and the attractive force is exerted only among particles inside a circular domain, which expands at a certain speed as a wave front propagating from a preselected centre. It is found that an RDP is formed if the velocity of the wave front that triggers the attractive interaction is of the same order of magnitude as the time scale defined by the aggregation speed. PMID:27353447

  16. Formation mechanism of shock-induced particle jetting.

    PubMed

    Xue, K; Sun, L; Bai, C

    2016-08-01

    The shock dissemination of granular rings or shells is characterized by the formation of coherent particle jets that have different dimensions from those associated with the constituent grains. In order to identify the mechanisms governing the formation of particle jets, we carry out the simulations of the shock dispersal of quasi-two-dimensional particle rings based on the discrete-element method. The evolution of the particle velocities and contact forces on the time scales ranging from microseconds to milliseconds reveals a two-stage development of particle jets before they are expelled from the outer surface. Much effort is made to understand the particle agglomeration around the inner surface that initiates the jet formation. The shock interaction with the innermost particle layers generates a heterogeneous network of force chains with clusters of strong contacts regularly spaced around the inner surface. Momentum alongside the stresses is primarily transmitted along the strong force chains. Therefore, the clustering of strong force chains renders the agglomeration of fast-moving particles connected by strong force chains. The fast-moving particle clusters subsequently evolve into the incipient particle jets. The following competition among the incipient jets that undergo unbalanced growth leads to substantial elimination of the minor jets and the significant multiplication of the major jets, the number of jets thus varying with time. Moreover, the number of jets is found to increase with the strength of the shock loading due to an increased number of jets surviving the retarding effect of major jets. PMID:27627376

  17. Formation mechanism of shock-induced particle jetting

    NASA Astrophysics Data System (ADS)

    Xue, K.; Sun, L.; Bai, C.

    2016-08-01

    The shock dissemination of granular rings or shells is characterized by the formation of coherent particle jets that have different dimensions from those associated with the constituent grains. In order to identify the mechanisms governing the formation of particle jets, we carry out the simulations of the shock dispersal of quasi-two-dimensional particle rings based on the discrete-element method. The evolution of the particle velocities and contact forces on the time scales ranging from microseconds to milliseconds reveals a two-stage development of particle jets before they are expelled from the outer surface. Much effort is made to understand the particle agglomeration around the inner surface that initiates the jet formation. The shock interaction with the innermost particle layers generates a heterogeneous network of force chains with clusters of strong contacts regularly spaced around the inner surface. Momentum alongside the stresses is primarily transmitted along the strong force chains. Therefore, the clustering of strong force chains renders the agglomeration of fast-moving particles connected by strong force chains. The fast-moving particle clusters subsequently evolve into the incipient particle jets. The following competition among the incipient jets that undergo unbalanced growth leads to substantial elimination of the minor jets and the significant multiplication of the major jets, the number of jets thus varying with time. Moreover, the number of jets is found to increase with the strength of the shock loading due to an increased number of jets surviving the retarding effect of major jets.

  18. A new mechanism for dendritic pattern formation in dense systems

    NASA Astrophysics Data System (ADS)

    Oikawa, Noriko; Kurita, Rei

    2016-06-01

    Patterns are often formed when particles cluster: Since patterns reflect the connectivity of different types of material, the emergence of patterns affects the physical and chemical properties of systems and shares a close relationship to their macroscopic functions. A radial dendritic pattern (RDP) is observed in many systems such as snow crystals, polymer crystals and biological systems. Although most of these systems are considered as dense particle suspensions, the mechanism of RDP formation in dense particle systems is not yet understood. It should be noted that the diffusion limited aggregation model is not applicable to RDP formation in dense systems, but in dilute particle systems. Here, we propose a simple model that exhibits RDP formation in a dense particle system. The model potential for the inter-particle interaction is composed of two parts, a repulsive and an attractive force. The repulsive force is applied to all the particles all the time and the attractive force is exerted only among particles inside a circular domain, which expands at a certain speed as a wave front propagating from a preselected centre. It is found that an RDP is formed if the velocity of the wave front that triggers the attractive interaction is of the same order of magnitude as the time scale defined by the aggregation speed.

  19. Molecular Mechanisms of the Formation and Progression of Intracranial Aneurysms

    PubMed Central

    KATAOKA, Hiroharu

    2015-01-01

    Until recently, only a little was understood about molecular mechanisms of the development of an intracranial aneurysm (IA). Recent advancements over the last decade in the field of genetics and molecular biology have provided us a wide variety of evidences supporting the notion that chronic inflammation is closely associated with the pathogenesis of IA development. In the field of genetics, large-scale Genome-wide association studies (GWAS) has identified some IA susceptible loci and genes related to cell cycle and endothelial function. Researches in molecular biology using human samples and animal models have revealed the common pathway of the initiation, progression, and rupture of IAs. IA formation begins with endothelial dysfunction followed by pathological remodeling with degenerative changes of vascular walls. Medical treatments inhibiting inflammatory cascades in IA development are likely to prevent IA progression and rupture. Statins and aspirin are expected to suppress IA progression by their anti-inflammatory effects. Decoy oligodeoxynucleotides (ODNs) inhibiting inflammatory transcription factors such as nuclear factor kappa-B (NF-κB) and Ets-1 are the other promising choice of the prevention of IA development. Further clarification of molecular mechanisms of the formation and progression of IAs will shed light to the pathogenesis of IA development and provide insight into novel diagnostic and therapeutic strategies for IAs. PMID:25761423

  20. Boron nitride hollow nanospheres: Synthesis, formation mechanism and dielectric property

    SciTech Connect

    Zhong, B.; Tang, X.H.; Huang, X.X.; Xia, L.; Zhang, X.D.; Wang, C.J.; Wen, G.W.

    2015-04-15

    Highlights: • BN hollow nanospheres are fabricated in large scale via a new CVD method. • Morphology and structure are elucidated by complementary analytical techniques. • Formation mechanism is proposed based on experimental observations. • Dielectric properties are investigated in the X-band microwave frequencies. • BN hollow nanospheres show lower dielectric loss than regular BN powders. - Abstract: Boron nitride (BN) hollow nanospheres have been successfully fabricated by pyrolyzing vapors decomposed from ammonia borane (NH{sub 3}BH{sub 3}) at 1300 °C. The final products have been extensively characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The BN hollow nanospheres were ranging from 100 to 300 nm in diameter and around 30–100 nm in thickness. The internal structure of the products was found dependent on the reaction temperatures. A possible formation mechanism of the BN hollow nanospheres was proposed on the basis of the experimental observations. Dielectric measurements in the X-band microwave frequencies (8–12 GHz) showed that the dielectric loss of the paraffin filled by the BN hollow nanospheres was lower than that filled by regular BN powders, which indicated that the BN hollow nanospheres could be potentially used as low-density fillers for microwave radomes.

  1. Analysis of Europan Cycloid Morphology and Implications for Formation Mechanisms

    NASA Technical Reports Server (NTRS)

    Marshall, S. T.; Kattenhorn, S. A.

    2004-01-01

    Europa's highly fractured crust has been shown to contain features with a range of differing morphologies. Most lineaments on Europa are believed to have initiated as cracks, although the type of cracking (e.g. tensile vs. shear) remains unclear and may vary for different morphologies. Arcuate lineaments, called cycloids or flexi, have been observed in nearly all imaged regions of Europa and have been modeled as tensile fractures that were initiated in response to diurnal variations in tides. Despite this hypothesis about the formation mechanism, there have been no detailed analyses of the variable morphologies of cycloids. We have examined Galileo images of numerous locations on Europa to develop a catalog of the different morphologies of cycloids. This study focuses on variations in morphology along individual cycloid segments and differences in cusp styles between segments, while illustrating how morphologic evidence can help unravel formation mechanisms. In so doing, we present evidence for cycloid cusps forming due to secondary fracturing during strike-slip sliding on pre-existing cycloid segments.

  2. THE FORMATION MECHANISM OF GAS GIANTS ON WIDE ORBITS

    SciTech Connect

    Dodson-Robinson, Sarah E.; Veras, Dimitri; Ford, Eric B.; Beichman, C. A.

    2009-12-10

    The recent discoveries of massive planets on ultra-wide orbits of HR 8799 and Fomalhaut present a new challenge for planet formation theorists. Our goal is to figure out which of three giant planet formation mechanisms-core accretion (with or without migration), scattering from the inner disk, or gravitational instability-could be responsible for Fomalhaut b, HR 8799 b, c and d, and similar planets discovered in the future. This paper presents the results of numerical experiments comparing the long-period planet formation efficiency of each possible mechanism in model A star, G star, and M star disks. First, a simple core accretion simulation shows that planet cores forming beyond 35 AU cannot reach critical mass, even under the most favorable conditions one can construct. Second, a set of N-body simulations demonstrates that planet-planet scattering does not create stable, wide-orbit systems such as HR 8799. Finally, a linear stability analysis verifies previous work showing that global spiral instabilities naturally arise in high-mass disks. We conclude that massive gas giants on stable orbits with semimajor axes a approx> 35 AU form by gravitational instability in the disk. We recommend that observers examine the planet detection rate as a function of stellar age, controlling for the planets' dimming with time. Any age trend would indicate that planets on wide orbits are transient relics of scattering from the inner disk. If planet detection rate is found to be independent of stellar age, it would confirm our prediction that gravitational instability is the dominant mode of producing detectable planets on wide orbits. We also predict that the occurrence ratio of long-period to short-period gas giants should be highest for M dwarfs due to the inefficiency of core accretion and the expected small fragment mass (approx10 M {sub Jup}) in their disks.

  3. Influence of Powder Metallurgical Processing Routes on Phase Formations in a Multicomponent NbSi-Alloy

    NASA Astrophysics Data System (ADS)

    Seemüller, C.; Hartwig, T.; Mulser, M.; Adkins, N.; Wickins, M.; Heilmaier, M.

    2014-09-01

    Refractory metal silicide composites on the basis of Nbss-Nb5Si3 have been investigated as potential alternatives for nickel-base superalloys for years because of their low densities and good high-temperature strengths. NbSi-based composites are typically produced by arc-melting or casting. Samples in this study, however, were produced by powder metallurgy because of the potential for near net-shape component fabrication with very homogeneous microstructures. Either gas atomized powder or high-energy mechanically alloyed elemental powders were compacted by powder injection molding or hot isostatic pressing. Heat treatments were applied for phase stability evaluation. Slight compositional changes (oxygen, nitrogen, or iron) introduced by the processing route, i.e., powder production and consolidation, can affect phase formations and phase transitions during the process. Special focus is put on the distinction between different silicides (Nb5Si3 and Nb3Si) and silicide modifications (α-, β-, and γ-Nb5Si3), respectively. These were evaluated by x-ray diffraction and energy-dispersive spectroscopy measurements with the additional inclusion of thermodynamic calculations using the calculated phase diagram method.

  4. Formation and Maturation of Phase-Separated Liquid Droplets by RNA-Binding Proteins.

    PubMed

    Lin, Yuan; Protter, David S W; Rosen, Michael K; Parker, Roy

    2015-10-15

    Eukaryotic cells possess numerous dynamic membrane-less organelles, RNP granules, enriched in RNA and RNA-binding proteins containing disordered regions. We demonstrate that the disordered regions of key RNP granule components and the full-length granule protein hnRNPA1 can phase separate in vitro, producing dynamic liquid droplets. Phase separation is promoted by low salt concentrations or RNA. Over time, the droplets mature to more stable states, as assessed by slowed fluorescence recovery after photobleaching and resistance to salt. Maturation often coincides with formation of fibrous structures. Different disordered domains can co-assemble into phase-separated droplets. These biophysical properties demonstrate a plausible mechanism by which interactions between disordered regions, coupled with RNA binding, could contribute to RNP granule assembly in vivo through promoting phase separation. Progression from dynamic liquids to stable fibers may be regulated to produce cellular structures with diverse physiochemical properties and functions. Misregulation could contribute to diseases involving aberrant RNA granules. PMID:26412307

  5. Secondary Aerosol: Precursors and Formation Mechanisms. Technical Report on Grant

    SciTech Connect

    Weinstein-Lloyd, Judith B

    2009-05-04

    This project focused on studying trace gases that participate in chemical reactions that form atmospheric aerosols. Ammonium sulfate is a major constituent of these tiny particles, and one important pathway to sulfate formation is oxidation of dissolved sulfur dioxide by hydrogen peroxide in cloud, fog and rainwater. Sulfate aerosols influence the number and size of cloud droplets, and since these factors determine cloud radiative properties, sulfate aerosols also influence climate. Peroxide measurements, in conjunction with those of other gaseous species, can used to distinguish the contribution of in-cloud reaction to new sulfate aerosol formation from gas-phase nucleation reactions. This will lead to more reliable global climate models. We constructed and tested a new 4-channel fluorescence detector for airborne detection of peroxides. We integrated the instrument on the G-1 in January, 2006 and took a test flight in anticipation of the MAX-Mex field program, where we planned to fly under pressurized conditions for the first time. We participated in the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) - Megacity Aerosol EXperiment Mexico City (MAX-Mex) field measurement campaign. Peroxide instrumentation was deployed on the DOE G-1 research aircraft based in Veracruz, and at the surface site at Tecamac University.

  6. The effect of selective desorption mechanisms during interstellar ice formation

    NASA Astrophysics Data System (ADS)

    Kalvāns, J.

    2015-04-01

    Major components of ices on interstellar grains in molecular clouds—water and carbon oxides—occur at various optical depths. This implies that selective desorption mechanisms are at work. An astrochemical model of a contracting low-mass molecular cloud core is presented. Ice was treated as consisting of the surface and three subsurface layers (i.e., sublayers). Photodesorption, reactive desorption, and indirect reactive desorption were investigated. The latter manifests itself through desorption from H+H reaction on grains. Desorption of shallow subsurface species was also included. Modeling results suggest the existence of a “photon-dominated ice” during the early phases of core contraction. Subsurface ice is chemically processed by interstellar photons, which produces complex organic molecules (COMs). Desorption from the subsurface layer results in high COM gas-phase abundances at AV = 2.4-10 mag. This may contribute toward an explanation for COM observations in dark cores. It was found that photodesorption mostly governs the onset of ice accumulation onto grains. Reaction-specific reactive desorption is efficient for small molecules that form via highly exothermic atom-addition reactions. Higher reactive desorption efficiency results in lower gas-phase abundances of COMs. Indirect reactive desorption allows for closely reproducing the observed H2O:CO:CO2 ratio toward a number of background stars. Presumably, this can be done by any mechanism whose efficiency fits with the sequence CO≥slant C{{O}2}\\gg {{H}2}O. After the freeze-out has ended, the three sublayers represent chemically distinct parts of the mantle. The likely AV threshold for the appearance of CO ice is 8-10.5 mag. The lower value is supported by observations.

  7. Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation

    NASA Astrophysics Data System (ADS)

    Shiraiwa, M.; Yee, L. D.; Schilling, K.; Loza, C. L.; Craven, J. S.; Zuend, A.; Ziemann, P. J.; Seinfeld, J.

    2013-12-01

    Organic aerosols are ubiquitous in the atmosphere and play a central role in climate, air quality and public health. The aerosol size distribution is key in determining its optical properties and cloud condensation nucleus activity. The dominant portion of organic aerosol is formed through gas-phase oxidation of volatile organic compounds, so-called secondary organic aerosol (SOA). Typical experimental measurements of SOA formation include total SOA mass and atomic oxygen-to-carbon ratio. These measurements, alone, are generally insufficient to reveal the extent to which condensed-phase reactions occur in conjunction with the multi-generation gas-phase photooxidation. Combining laboratory chamber experiments and kinetic gas-particle modeling for the dodecane SOA system, here we show that the presence of particle-phase chemistry is reflected in the evolution of the SOA size distribution as well as its mass concentration. Particle-phase reactions are predicted to occur mainly at the particle surface and the reaction products contribute more than half of the SOA mass. Chamber photooxidation with a mid-experiment aldehyde injection confirms that heterogeneous reaction of aldehydes with organic hydroperoxides forming peroxyhemiacetals can lead to a large increase in SOA mass. The results of the current work have a number of implications for SOA models. While the dynamics of an aerosol size distribution reflects the mechanism of growth, we demonstrate here that it provides a key constraint in interpreting laboratory and ambient SOA formation. This work, although carried out specifically for the long chain alkane, dodecane, is expected to be widely applicable to other major classes of SOA precursors. SOA consists of a myriad of organic compounds containing various functional groups, which can generally undergo heterogeneous/multiphase reactions forming low-volatility products such as oligomers and other high molecular mass compounds. If particle-phase chemistry is indeed

  8. [Mechanisms of Forespore Formation during Polysporogenesis of an Anaerobic Bacterium Anaerobacter polyendosporus PST(T)].

    PubMed

    Duda, V I; Suzina, N E

    2015-01-01

    Forespore formation in the anaerobic bacterium Anaerobacterpolyendosporus PS-1(T) was studied by phase contrast, fluorescence, and electron microscopy. It is concluded that in this bacterium the formation of all forespores in multispore sporangia occurs via the same mechanism as that operating in all known bacilli and clostridia during the single-spore variant of endogenous sporogenesis. Its cytological indicators are as follows: (1) formation of the forespore septum, (2) engulfment of the smaller prespore cell by the larger mother cell, (3) cortex synthesis, (4) assembly of the spore coats, (5) exosporium formation, and (6) lysis of the mother cell. Polysporogenesis in strain PS-1(T) is characterized by synchronous formation of all spores (siblings) in a given sporangium and by the absence of any indication of forespore division within the mother cell. These data suggest that multiple spores within a single PS-1(T) cell result not from division of the first forespores developing at one or two cell poles, as it was reported for another polysporogenic bacterium, "Metabacterium polyspora", but rather from simultaneous independent formation of several prespores in a single mother cell in the course of modified cell division. PMID:27169242

  9. Shocked cobbles in Lower Cretaceous Duwon Formation, South Korea: their classification and possible formation mechanisms

    NASA Astrophysics Data System (ADS)

    Lim, Hyoun Soo; Chae, Yong-Un; Kim, Kyung Soo; Kim, Cheng-Bin; Huh, Min

    2016-04-01

    Shocked cobbles are the cobbles having shock-induced deformation structures on the surfaces. The most distinctive macroscopic features are the subparallel fractures and the pervasive surface craters, with or without radial fractures. Until now, these shocked cobbles have been reported mainly in Europe, America, and Africa, but never been found or reported in Korea. Shocked cobbles have recently found in the Lower Cretaceous Duwon Formation in South Korea, which was the second report in Asia. The Duwon Formation consists mainly of conglomerates, gravelly sandstones and intercalated mudstone and shale layers. The shocked cobbles are commonly found in the lowermost clast-supported conglomerate layers, and they show various deformation features, such as pockmarked (circular or elliptical) cobbles, cratered (Hertzian or bowl-shaped) cobbles with or without radial fractures, cobbles showing subparallel fractures, and strongly squashed or heavily dissected cobbles. In general, these deformation structures are considered to have resulted from pressure dissolution by overburden, tectonic compression, and seismic or meteorite impacts. However, the exact formation mechanism is not clearly understood, and still in debate. The shocked cobbles found in the Duwon Formation have similar features to those of previously reported shocked cobbles, especially to Triassic Buntsandstein conglomerates in northeastern Spain. Based on the degree of deformation, the Duwon shocked cobbles can be divided into four types, which are (1) faint contact marks, (2) pitted marks without any fractures, (3) pitted marks with radial or sub-parallel fractures affected by pits, and (4) intensive fractures and heavily dissected fragments. The possible mechanisms for the Duwon shocked cobbles are thought to be crushing process by shear stress and pressure solution.

  10. Bridging the gap: disk formation in the Class 0 phase with ambipolar diffusion and Ohmic dissipation

    NASA Astrophysics Data System (ADS)

    Dapp, Wolf B.; Basu, Shantanu; Kunz, Matthew W.

    2012-05-01

    Context. Ideal magnetohydrodynamical (MHD) simulations have revealed catastrophic magnetic braking in the protostellar phase, which prevents the formation of a centrifugal disk around a nascent protostar. Aims: We determine if non-ideal MHD, including the effects of ambipolar diffusion and Ohmic dissipation determined from a detailed chemical network model, will allow for disk formation at the earliest stages of star formation. Methods: We employ the axisymmetric thin-disk approximation in order to resolve a dynamic range of 9 orders of magnitude in length and 16 orders of magnitude in density, while also calculating partial ionization using up to 19 species in a detailed chemical equilibrium model. Magnetic braking is applied to the rotation using a steady-state approximation, and a barotropic relation is used to capture the thermal evolution. Results: We resolve the formation of the first and second cores, with expansion waves at the periphery of each, a magnetic diffusion shock, and prestellar infall profiles at larger radii. Power-law profiles in each region can be understood analytically. After the formation of the second core, the centrifugal support rises rapidly and a low-mass disk of radius ≈ 10 R⊙ is formed at the earliest stage of star formation, when the second core has mass ~10-3 M⊙. The mass-to-flux ratio is ~104 times the critical value in the central region. Conclusions: A small centrifugal disk can form in the earliest stage of star formation, due to a shut-off of magnetic braking caused by magnetic field dissipation in the first core region. There is enough angular momentum loss to allow the second collapse to occur directly, and a low-mass stellar core to form with a surrounding disk. The disk mass and size will depend upon how the angular momentum transport mechanisms within the disk can keep up with mass infall onto the disk. Accounting only for direct infall, we estimate that the disk will remain ≲10 AU, undetectable even by ALMA, for

  11. Effect of temperature on the reaction pathway of calcium carbonate formation via precursor phases

    NASA Astrophysics Data System (ADS)

    Purgstaller, Bettina; Mavromatis, Vasileios; Konrad, Florian; Dietzel, Martin

    2016-04-01

    It has been earlier postulated that some biogenic and sedimentary calcium carbonate (CaCO3) minerals (e.g. calcite and aragonite) are secondary in origin and have originally formed via a metastable calcium carbonate precursor phase (e.g. amorphous CaCO3, [1-2]). Such formation pathways are likely affected by various physicochemical parameters including aqueous Mg and temperature. In an effort to improve our understanding on the formation mechanism of CaCO3 minerals, precipitation experiments were carried out by the addition of a 0.6 M (Ca,Mg)Cl2 solution at distinct Mg/Ca ratios (1/4 and 1/8) into a 1 M NaHCO3 solution under constant pH conditions(8.3 ±0.1). The formation of CaCO3 was systematically examined as a function of temperature (6, 12, 18 and 25 ±0.3° C). During the experimental runs mineral precipitation was monitored by in situ Raman spectroscopy as well as by continuous sampling and analyzing of precipitates and reactive solutions. The results revealed two pathways of CaCO3 formation depending on the initial Mg/Ca ratio and temperature: (i) In experiments with a Mg/Ca ratio of 1/4 at ≤ 12° C as well as in experiments with a Mg/Ca ratio of 1/8 at ≤ 18° C, ikaite (CaCO3 6H2O) acts as a precursor phase for aragonite formation. (ii) In contrast higher temperatures induced the formation of Mg-rich amorphous CaCO3 (Mg-ACC) which was subsequently transformed to Mg-rich calcite. In situ Raman spectra showed that the transformation of Mg-ACC to Mg-calcite occurs at a higher rate (˜ 8 min) compared to that of ikaite to aragonite (> 2 h). Thus, the formation of aragonite rather than of Mg-calcite occurs due to the slower release of Ca2+and CO32- ions into the Mg-rich reactive solution during retarded ikaite dissolution. This behavior is generally consistent with the observation that calcite precipitation is inhibited at elevated aqueous Mg/Ca ratios. [1] Addadi L., Raz S. and Weiner S. (2003) Advanced Materials 15, 959-970. [2] Rodriguez-Blanco J. D

  12. Laws of formation of polar smectic phases under a frustrated interaction

    SciTech Connect

    Dolganov, P. V.; Zhilin, V. M.; Kats, E. I.

    2012-12-15

    The Landau theory of phase transitions with a two-component order parameter is used to systematically calculate the structures and phase diagrams of polar liquid crystals. Commensurate and incommensurate structures with a layer-type period form as a result of a frustration interaction. Phase diagrams are calculated when various short- and long-range interlayer interactions are sequentially introduced. As a result, the nature of formation of various structures is revealed. The calculated phase diagrams explain the formation of various phases, their temperature sequence (including so-called 'unusual' sequence of phases), and the nature of forces responsible for the formation of a certain structure and the shape of a phase diagram.

  13. Surface mechanics mediate pattern formation in the developing retina.

    PubMed

    Hayashi, Takashi; Carthew, Richard W

    2004-10-01

    Pattern formation of biological structures involves organizing different types of cells into a spatial configuration. In this study, we investigate the physical basis of biological patterning of the Drosophila retina in vivo. We demonstrate that E- and N-cadherins mediate apical adhesion between retina epithelial cells. Differential expression of N-cadherin within a sub-group of retinal cells (cone cells) causes them to form an overall shape that minimizes their surface contact with surrounding cells. The cells within this group, in both normal and experimentally manipulated conditions, pack together in the same way as soap bubbles do. The shaping of the cone cell group and packing of its components precisely imitate the physical tendency for surfaces to be minimized. Thus, simple patterned expression of N-cadherin results in a complex spatial pattern of cells owing to cellular surface mechanics. PMID:15470418

  14. An interfacial mechanism for cloud droplet formation on organic aerosols

    NASA Astrophysics Data System (ADS)

    Ruehl, Christopher R.; Davies, James F.; Wilson, Kevin R.

    2016-03-01

    Accurate predictions of aerosol/cloud interactions require simple, physically accurate parameterizations of the cloud condensation nuclei (CCN) activity of aerosols. Current models assume that organic aerosol species contribute to CCN activity by lowering water activity. We measured droplet diameters at the point of CCN activation for particles composed of dicarboxylic acids or secondary organic aerosol and ammonium sulfate. Droplet activation diameters were 40 to 60% larger than predicted if the organic was assumed to be dissolved within the bulk droplet, suggesting that a new mechanism is needed to explain cloud droplet formation. A compressed film model explains how surface tension depression by interfacial organic molecules can alter the relationship between water vapor supersaturation and droplet size (i.e., the Köhler curve), leading to the larger diameters observed at activation.

  15. Circuit Mechanisms Underlying Motor Memory Formation in the Cerebellum

    PubMed Central

    Lee, Ka Hung; Mathews, Paul J.; Reeves, Alexander M.B.; Choe, Katrina Y.; Jami, Shekib A.; Serrano, Raul E.; Otis, Thomas S.

    2015-01-01

    SUMMARY The cerebellum stores associative motor memories essential for properly timed movement; however, the mechanisms by which these memories form and are acted upon remain unclear. To determine how cerebellar activity relates to movement and motor learning, we used optogenetics to manipulate spontaneously firing Purkinje neurons (PNs) in mouse simplex lobe. Using high-speed videography and motion tracking, we found that altering PN activity produced rapid forelimb movement. PN inhibition drove movements time-locked to stimulus onset, whereas PN excitation drove delayed movements time-locked to stimulus offset. Pairing either PN inhibition or excitation with sensory stimuli triggered the formation of robust, associative motor memories; however, PN excitation led to learned movements whose timing more closely matched training intervals. These findings implicate inhibition of PNs as a teaching signal, consistent with a model whereby learning leads first to reductions in PN firing that subsequently instruct circuit changes in the cerebellar nucleus. PMID:25843404

  16. Axion field and the quark nugget's formation at the QCD phase transition

    NASA Astrophysics Data System (ADS)

    Liang, Xunyu; Zhitnitsky, Ariel

    2016-10-01

    We study a testable dark-matter (DM) model outside of the standard weakly interacting massive particle paradigm in which the observed ratio Ωdark≃Ωvisible for visible and dark-matter densities finds its natural explanation as a result of their common QCD origin when both types of matter (DM and visible) are formed at the QCD phase transition and both are proportional to ΛQCD. Instead of the conventional "baryogenesis" mechanism, we advocate a paradigm when the "baryogenesis" is actually a charge separation process which always occurs in the presence of the C P odd axion field a (x ). In this scenario, the global baryon number of the Universe remains zero, while the unobserved antibaryon charge is hidden in the form of heavy nuggets, similar to Witten's strangelets and compromise the DM of the Universe. In the present work, we study in great detail a possible formation mechanism of such macroscopically large heavy objects. We argue that the nuggets will be inevitably produced during the QCD phase transition as a result of Kibble-Zurek mechanism on formation of the topological defects during a phase transition. Relevant topological defects in our scenario are the closed bubbles made of the NDW=1 axion domain walls. These bubbles, in general, accrete the baryon (or antibaryon) charge, which eventually results in the formation of the nuggets and antinuggets carrying a huge baryon (antibaryon) charge. A typical size and the baryon charge of these macroscopically large objects are mainly determined by the axion mass ma. However, the main consequence of the model, Ωdark≈Ωvisible, is insensitive to the axion mass which may assume any value within the observationally allowed window 10-6 eV ≲ma≲10-3 eV . We also estimate the baryon-to-entropy ratio η ≡nB/nγ˜10-10 within this scenario. Finally, we comment on implications of these results to the axion search experiments, including the microwave cavity and the Orpheus experiments.

  17. Directed networks' different link formation mechanisms causing degree distribution distinction

    NASA Astrophysics Data System (ADS)

    Behfar, Stefan Kambiz; Turkina, Ekaterina; Cohendet, Patrick; Burger-Helmchen, Thierry

    2016-11-01

    Within undirected networks, scientists have shown much interest in presenting power-law features. For instance, Barabási and Albert (1999) claimed that a common property of many large networks is that vertex connectivity follows scale-free power-law distribution, and in another study Barabási et al. (2002) showed power law evolution in the social network of scientific collaboration. At the same time, Jiang et al. (2011) discussed deviation from power-law distribution; others indicated that size effect (Bagrow et al., 2008), information filtering mechanism (Mossa et al., 2002), and birth and death process (Shi et al., 2005) could account for this deviation. Within directed networks, many authors have considered that outlinks follow a similar mechanism of creation as inlinks' (Faloutsos et al., 1999; Krapivsky et al., 2001; Tanimoto, 2009) with link creation rate being the linear function of node degree, resulting in a power-law shape for both indegree and outdegree distribution. Some other authors have made an assumption that directed networks, such as scientific collaboration or citation, behave as undirected, resulting in a power-law degree distribution accordingly (Barabási et al., 2002). At the same time, we claim (1) Outlinks feature different degree distributions than inlinks; where different link formation mechanisms cause the distribution distinctions, (2) in/outdegree distribution distinction holds for different levels of system decomposition; therefore this distribution distinction is a property of directed networks. First, we emphasize in/outlink formation mechanisms as causal factors for distinction between indegree and outdegree distributions (where this distinction has already been noticed in Barker et al. (2010) and Baxter et al. (2006)) within a sample network of OSS projects as well as Java software corpus as a network. Second, we analyze whether this distribution distinction holds for different levels of system decomposition: open

  18. Formation mechanism for a hybrid supramolecular network involving cooperative interactions.

    PubMed

    Mura, Manuela; Silly, Fabien; Burlakov, Victor; Castell, Martin R; Briggs, G Andrew D; Kantorovich, Lev N

    2012-04-27

    A novel mechanism of hybrid assembly of molecules on surfaces is proposed stemming from interactions between molecules and on-surface metal atoms which eventually got trapped inside the network pores. Based on state-of-the-art theoretical calculations, we find that the new mechanism relies on formation of molecule-metal atom pairs which, together with molecules themselves, participate in the assembly growth. Most remarkably, the dissociation of pairs is facilitated by a cooperative interaction involving many molecules. This new mechanism is illustrated on a low coverage Melamine hexagonal network on the Au(111) surface where multiple events of gold atoms trapping via a set of so-called "gate" transitions are found by kinetic Monte Carlo simulations based on transition rates obtained using ab initio density functional theory calculations and the nudged elastic band method. Simulated STM images of gold atoms trapped in the pores of the Melamine network predict that the atoms should appear as bright spots inside Melamine hexagons. No trapping was found at large Melamine coverages, however. These predictions have been supported by preliminary STM experiments which show bright spots inside Melamine hexagons at low Melamine coverages, while empty pores are mostly observed at large coverages. Therefore, we suggest that bright spots sometimes observed in the pores of molecular assemblies on metal surfaces may be attributed to trapped substrate metal atoms. We believe that this type of mechanism could be used for delivering adatom species of desired functionality (e.g., magnetic) into the pores of hydrogen-bonded networks serving as templates for their capture. PMID:22680886

  19. Wavelength preserved phase erasure and PSK to conventional OOK data format conversion based on phase sensitive amplification

    NASA Astrophysics Data System (ADS)

    Yu, Kan; Yang, Weili; Yu, Yu

    2016-10-01

    In this paper, a phase erasure and format conversion of phase-shift keying (PSK) to conventional on-off keying (OOK) is proposed and demonstrated theoretically and experimentally. Using a single-pump nondegenerate phase sensitive amplification process in a highly nonlinear fiber, the 0 and 1-bits of the PSK signal obtain different gains through amplification and de-amplification. As a result, the modulation information is transferred onto the amplitude. With an optimized input power difference between the signal and idler, the signal phase information is erased with wavelength preservation after the PSA. The output constellation and eye diagrams show an effective phase erasure and format conversion of PSK to conventional OOK. The error vector magnitude is utilized to evaluate the scheme performance. The proposed scheme provides the flexibility and resiliency for future photonic networks.

  20. Phase behavior, formation, and rheology of cubic phase and related gel emulsion in Tween 80/water/oil systems.

    PubMed

    Alam, Mohammad Mydul; Ushiyama, Kousuke; Aramaki, Kenji

    2009-01-01

    We investigated the phase behavior, formation, and rheology of the cubic phase (I(1)) and related O/I(1) gel emulsion in water/Tween 80/oil systems using squalane, liquid paraffin (LP), and decane as oil components. In the phase behavior study, the phase sequences were similar for squalane and LP systems, while a lamellar liquid crystal (L(alpha)) was observed for decane system. In all the systems the addition of oil to W(m) or H(1) phase induced the I(1) phase, which can solubilize some amounts of oil followed by the appearance of I(1)+O phase. The formation of the O/I(1) gel emulsion has been studied at a fixed w/s (50/50) and we found that 30 wt% decane, 70 wt% squalane, and 60 wt% LP can form the gel emulsion. The water/Tween 80/squalane system has been taken as a model system to study viscoelastic properties of the I(1) phase and O/I(1) gel emulsion. The I(1) phase shows a typical hard gel cubic structure under the frequency and the values of the complex viscosity, /eta*/ and the elastic modulus, G ' increase with the addition of squalane, which could be due to the neighboring micellar interaction. On the other hand, the decreasing values of the viscoelastic parameters in the O/I(1) gel emulsion simply relate to the volume fraction of the I(1) phase in the system.

  1. Seal Formation Mechanism Beneath Animal Waste Holding Ponds

    NASA Astrophysics Data System (ADS)

    Cihan, A.; Tyner, J. S.; Wright, W. C.

    2005-12-01

    Infiltration of animal waste from holding ponds can cause contamination of groundwater. Typically, the initial flux from a pond decreases rapidly as a seal of animal waste particulates is deposited at the base of the pond. The purpose of this study was to investigate the mechanism of the seal formation. Twenty-four soil columns (10-cm diameter by 43-cm long) were hand-packed with sand, silty loam or clay soils. A 2.3 m column of dairy or swine waste was applied to the top of the each column. The leakage rate from each column was measured with respect to time to analyze the effect of seal formation on different soil textures and animal waste types. We tested our hypothesis that seal growth and the subsequent decrease of leachate production adheres to a filter cake growth model. Said model predicts that the cumulative leakage rate is proportional to the square root of time and to the square root of the height of the waste.

  2. [Molecular mechanisms underlying the formation of neuromuscular junction].

    PubMed

    Higuchi, Osamu; Yamanashi, Yuji

    2011-07-01

    The neuromuscular junction (NMJ) is a synapse between a motor neuron and skeletal muscle. The contraction of skeletal muscle is controlled by the neurotransmitter acetylcholine (ACh), which is released from the motor nerve terminal. To achieve efficient neuromuscular transmission, acetylcholine receptors (AChRs) must be densely clustered on the muscle membrane of the NMJ. Failure of AChR clustering is associated with disorders of neuromuscular transmission such as congenital myasthenic syndromes (CMS) and myasthenia gravis (MG). Motoneuronal agrin and muscle-specific receptor tyrosine kinase (MuSK) are known to play essential roles in the formation and maintenance of NMJs in the central region of each muscle. However, it had been unclear how agrin activates MuSK. Recent studies have elucidated the roles of several key molecules, including the cytoplasmic adaptor protein Dok-7 and LDL receptor-related protein 4 (Lrp4), in agrin-induced MuSK activation. Moreover, new evidence indicates that cyclin-dependent kinase 5 (Cdk5) regulates postsynaptic differentiation. In this review, we summarize the latest developments in molecular mechanisms underlying NMJ formation in vertebrates. PMID:21747134

  3. Air at hydrophobic surfaces and kinetics of three phase contact formation.

    PubMed

    Krasowska, M; Zawala, J; Malysa, K

    2009-01-01

    This review focuses on the importance of air presence at hydrophobic solid surfaces for wetting film rupture and kinetics of three phase contact formation. Affinity to air is a typical feature of hydrophobic surfaces, but it has been often either overlooked or not taken into consideration. When the hydrophobic surface, contacted earlier with air, is immersed into water then air can stay attached to the surface. The origin of long range hydrophobic forces and data showing that these interactions were due to the bridging of nanobubbles attached to the hydrophobic surfaces are discussed. A major part of the review is devoted to the description and analysis of data showing that air (nano-, micro-bubbles and/or air film) present at a hydrophobic surface facilitated rupture of the liquid film and three phase contact formation during bubble collisions with flat Teflon plates of different surface roughness. Although all Teflon plates were highly hydrophobic (contact angles ca. 100 degrees -130 degrees ) the time of the three phase contact (TPC) formation and attachment of the colliding bubble was strongly affected by the plate surface roughness. The time of the TPC formation was shortened from over 80 down to 2-3 ms when the roughness was increased from below 1 microm to over 50 microm. Higher surface roughness means that larger amounts of air was entrapped during the Teflon plates' immersion in water. Additional experimental evidence is given, showing that facilitation of the TPC formation and the bubble attachment was due to air presence and re-distribution over the Teflon surfaces: i) prolonging the plate immersion time resulted in quicker attachment; ii) irregular and disappearing air pockets were recorded at a Teflon surface; iii) a satellite bubble left at a Teflon surface during the first collision facilitated the attachment; iv) attachment always occurred during the first collision in the case of a very rough "Teflon V" surface, but in highly concentrated n

  4. Self-Gravitating Relativistic Fluids: The Formation of a Free Phase Boundary in the Phase Transition from Hard to Soft

    NASA Astrophysics Data System (ADS)

    Christodoulou, Demetrios; Lisibach, André

    2016-11-01

    In the 1990s Christodoulou introduced an idealized fluid model intended to capture some of the features of the gravitational collapse of a massive star to form a neutron star or a black hole. This was the two-phase model introduced in `Self-gravitating relativistic fluids: a two phase model' (Demeterios, Arch Ration Mech Anal 130:343-400, 1995). The present work deals with the formation of a free phase boundary in the phase transition from hard to soft in this model. In this case the phase boundary has corners at the null points; the points which separate the timelike and spacelike components of the interface between the two phases. We prove the existence and uniqueness of a free phase boundary. Also the local form of the shock near the null point is established.

  5. Self-Gravitating Relativistic Fluids: The Formation of a Free Phase Boundary in the Phase Transition from Hard to Soft

    NASA Astrophysics Data System (ADS)

    Christodoulou, Demetrios; Lisibach, André

    2016-06-01

    In the 1990s Christodoulou introduced an idealized fluid model intended to capture some of the features of the gravitational collapse of a massive star to form a neutron star or a black hole. This was the two-phase model introduced in `Self-gravitating relativistic fluids: a two phase model' (Demeterios, Arch Ration Mech Anal 130:343-400, 1995). The present work deals with the formation of a free phase boundary in the phase transition from hard to soft in this model. In this case the phase boundary has corners at the null points; the points which separate the timelike and spacelike components of the interface between the two phases. We prove the existence and uniqueness of a free phase boundary. Also the local form of the shock near the null point is established.

  6. The early phases of galaxy formation and evolution

    NASA Astrophysics Data System (ADS)

    Dickinson, Mark

    2016-05-01

    I review observations of galaxy formation and evolution from multiwavelength deep field surveys. These programs are designed to detect and study galaxies over a large range of cosmic history, with observations and many different wavelengths, each of which is important for measuring different physical processes at work in evolving galaxies. I begin with some background motivating interest in multiwavelength deep fields, and a short review of energetic processes that result in emission from galaxies in different wavelength/energy ranges. I then give a short summary of major observing facilities that are currently used for multiwavelength deep surveys, and then offer short descriptions of some of the major surveys that have produced much of the data used today by astronomers studying distant galaxy evolution. I continue with an overview of different methods that are commonly used to identify various types of galaxies at high redshift in deep field data sets. Then, for the remainder of the review, I focus on measurements of star formation rates and stellar masses in galaxies, and how they are used to evaluate the history of galaxy growth and evolution. I discuss the ways in which data at different wavelengths are used to infer star formation rates and stellar masses, concentrating on the complementarity of different techniques, as well as their limitations. I then discuss application of these methods for deriving the global history of star formation in the universe. These measurement now push back over more than 13 billion years of cosmic history and portray the rapid early growth of galaxies, and then the long, slow decline of star formation rates to the present era. I discuss recent results on the nature of star formation in galaxies at the peak epoch of galaxy growth, particularly around redshifts of 2, and their implications about the dominant physical processes that shaped the growth of most galaxies from the earliest times to the present day.

  7. Investigation of the mechanism of impurity assisted nanoripple formation on Si induced by low energy ion beam erosion

    SciTech Connect

    Koyiloth Vayalil, Sarathlal; Gupta, Ajay; Roth, Stephan V.; Ganesan, V.

    2015-01-14

    A detailed mechanism of the nanoripple pattern formation on Si substrates generated by the simultaneous incorporation of pure Fe impurities at low energy (1 keV) ion beam erosion has been studied. To understand and clarify the mechanism of the pattern formation, a comparative analysis of the samples prepared for various ion fluence values using two complimentary methods for nanostructure analysis, atomic force microscopy, and grazing incidence small angle x-ray scattering has been done. We observed that phase separation of the metal silicide formed during the erosion does not precede the ripple formation. It rather concurrently develops along with the ripple structure. Our work is able to differentiate among various models existing in the literature and provides an insight into the mechanism of pattern formation under ion beam erosion with impurity incorporation.

  8. A THREE-PHASE CHEMICAL MODEL OF HOT CORES: THE FORMATION OF GLYCINE

    SciTech Connect

    Garrod, Robin T.

    2013-03-01

    A new chemical model is presented that simulates fully coupled gas-phase, grain-surface, and bulk-ice chemistry in hot cores. Glycine (NH{sub 2}CH{sub 2}COOH), the simplest amino acid, and related molecules such as glycinal, propionic acid, and propanal, are included in the chemical network. Glycine is found to form in moderate abundance within and upon dust-grain ices via three radical-addition mechanisms, with no single mechanism strongly dominant. Glycine production in the ice occurs over temperatures {approx}40-120 K. Peak gas-phase glycine fractional abundances lie in the range 8 Multiplication-Sign 10{sup -11}-8 Multiplication-Sign 10{sup -9}, occurring at {approx}200 K, the evaporation temperature of glycine. A gas-phase mechanism for glycine production is tested and found insignificant, even under optimal conditions. A new spectroscopic radiative-transfer model is used, allowing the translation and comparison of the chemical-model results with observations of specific sources. Comparison with the nearby hot-core source NGC 6334 IRS1 shows excellent agreement with integrated line intensities of observed species, including methyl formate. The results for glycine are consistent with the current lack of a detection of this molecule toward other sources; the high evaporation temperature of glycine renders the emission region extremely compact. Glycine detection with ALMA is predicted to be highly plausible, for bright, nearby sources with narrow emission lines. Photodissociation of water and subsequent hydrogen abstraction from organic molecules by OH, and NH{sub 2}, are crucial to the buildup of complex organic species in the ice. The inclusion of alternative branches within the network of radical-addition reactions appears important to the abundances of hot-core molecules; less favorable branching ratios may remedy the anomalously high abundance of glycolaldehyde predicted by this and previous models.

  9. Retention mechanism for polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography with monomeric stationary phases.

    PubMed

    Rafferty, Jake L; Siepmann, J Ilja; Schure, Mark R

    2011-12-23

    Reversed-phase liquid chromatography (RPLC) is the foremost technique for the separation of analytes that have very similar chemical functionalities, but differ only in their molecular shape. This ability is crucial in the analysis of various mixtures with environmental and biological importance including polycyclic aromatic hydrocarbons (PAHs) and steroids. A large amount of effort has been devoted to studying this phenomenon experimentally, but a detailed molecular-level description remains lacking. To provide some insight on the mechanism of shape selectivity in RPLC, particle-based simulations were carried out for stationary phases and chromatographic parameters that closely mimic those in an experimental study by Sentell and Dorsey [J. Chromatogr. 461 (1989) 193]. The retention of aromatic hydrocarbons ranging in size from benzene to the isomeric PAHs of the formula C(18)H(12) was examined for model RPLC systems consisting of monomeric dimethyl octadecylsilane (ODS) stationary phases with surface coverages ranging from 1.6 to 4.2 μmol/m(2) (i.e., stationary phases yielding low to intermediate shape selectivity) in contact with a 67/33 mol% acetonitrile/water mobile phase. The simulations show that the stationary phase acts as a very heterogeneous environment where analytes with different shapes prefer different spatial regions with specific local bonding environments of the ODS chains. However, these favorable retentive regions cannot be described as pre-existing cavities because the chain conformation in these local stationary phase regions adapts to accommodate the analytes.

  10. Amyloid β Oligomeric Species Present in the Lag Phase of Amyloid Formation

    PubMed Central

    Wolff, Martin; Unuchek, Dmitry; Zhang, Bo; Gordeliy, Valentin; Willbold, Dieter; Nagel-Steger, Luitgard

    2015-01-01

    Alzheimer’s disease (AD)-associated amyloid β peptide (Aβ) is one of the main actors in AD pathogenesis. Aβ is characterized by its high tendency to self-associate, leading to the generation of oligomers and amyloid fibrils. The elucidation of pathways and intermediates is crucial for the understanding of protein assembly mechanisms in general and in conjunction with neurodegenerative diseases, e.g., for the identification of new therapeutic targets. Our study focused on Aβ42 and its oligomeric assemblies in the lag phase of amyloid formation, as studied by sedimentation velocity (SV) centrifugation. The assembly state of Aβ during the lag phase, the time required by an Aβ solution to reach the exponential growth phase of aggregation, was characterized by a dominant monomer fraction below 1 S and a population of oligomeric species between 4 and 16 S. From the oligomer population, two major species close to a 12-mer and an 18-mer with a globular shape were identified. The recurrence of these two species at different initial concentrations and experimental conditions as the smallest assemblies present in solution supports the existence of distinct, energetically favored assemblies in solution. The sizes of the two species suggest an Aβ42 aggregation pathway that is based on a basic hexameric building block. The study demonstrates the potential of SV analysis for the evaluation of protein aggregation pathways. PMID:26024352

  11. Formation of microporous NiTi by transient liquid phase sintering of elemental powders.

    PubMed

    Ismail, Muhammad Hussain; Goodall, Russell; Davies, Hywel A; Todd, Iain

    2012-08-01

    Porous metallic structures are attractive for biomedical implant applications as their open porosity simultaneously improves the degree of fixation and decreases the mismatch in stiffness between bone and implant, improving bonding and reducing stress-shielding effects respectively. NiTi alloys exhibit both the shape memory effect and pseudoelasticity, and are of particular interest, though they pose substantial problems in their processing. This is because the shape memory and pseudoelastic behaviours are exceptionally sensitive to the presence of oxygen, and other minor changes in alloy chemistry. Thus in processing careful control of composition and contamination is vital. In this communication, we investigate these issues in a novel technique for producing porous NiTi parts via transient liquid phase sintering following metal injection moulding (MIM) of elemental Ni and Ti powders, and report a new mechanism for pore formation in the powder processing of metallic materials from elemental powders.

  12. Laser-driven formation of a high-pressure phase in amorphous silica

    SciTech Connect

    Salleo, Alberto; Taylor, Seth T.; Martin, Michael C.; Panero, Wendy R.; Jeanloz, Raymond; Genin, Francois Y.; Sands, Timothy

    2002-05-31

    A combination of electron diffraction and infrared reflectance measurements shows that synthetic silica transforms partially into stishovite under high-intensity (GW/cm2) laser irradiation, probably by the formation of a dense ionized plasma above the silica surface. During the transformation the silicon coordination changes from four-fold to six-fold and the silicon-oxygen bond changes from mostly covalent to mostly ionic, such that optical properties of the transformed material differ significantly from those of the original glass. This phase transformation offers one suitable mechanism by which laser-induced damage grows catastrophically once initiated, thereby dramatically shortening the service lifetime of optics used for high-power photonics applications such as inertial confinement fusion.

  13. Laboratory Investigations of Titan Haze Formation: Characterization of Gas Phase and Particle Phase Nitrogen

    NASA Astrophysics Data System (ADS)

    Horst, Sarah; Yoon, Heidi; Li, Rui; deGouw, Joost; Tolbert, Margaret

    2014-11-01

    Prior to the arrival of the Cassini-Huygens spacecraft, aerosol production in Titan’s atmosphere was believed to begin in the stratosphere where chemical processes are predominantly initiated by far ultraviolet (FUV) radiation. However, the discovery of very heavy ions, coupled with Cassini Ultraviolet Imaging Spectrograph (UVIS) occultation measurements that show haze absorption up to 1000 km altitude (Liang et al., 2007), indicates that haze formation initiates in the thermosphere. The energy environment of the thermosphere is significantly different from the stratosphere; in particular there is a greater flux of extreme ultraviolet (EUV) photons and energetic particles available to initiate chemical reactions, including the destruction of N2, in the upper atmosphere. The discovery of previously unpredicted nitrogen species in measurements of Titan’s atmosphere by the Cassini Ion and Neutral Mass Spectrometer (INMS) indicates that nitrogen participates in the chemistry to a much greater extent than was appreciated before Cassini (Vuitton et al., 2007). Additionally, measurements obtained by the Aerosol Collector Pyrolyzer (ACP) carried by Huygens to Titan’s surface may indicate that Titan’s aerosols contain significant amounts of nitrogen (Israël et al., 2005, 2006). The degree of nitrogen incorporation in the haze particles is important for understanding the diversity of molecules that may be present in Titan’s atmosphere and on its surface. We have conducted a series of Titan atmosphere simulation experiments using either spark discharge (tesla coil) or FUV photons (deuterium lamp) to initiate chemistry in CH4/N2 gas mixtures ranging from 0.01% CH4/99.99% N2 to 10% CH4/90% N2. We obtained in situ measurements using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) to measure the particle composition as a function of particle size and a proton-transfer ion-trap mass spectrometer (PIT-MS) to measure the composition of gas phase

  14. VO{sub 2} (A): Reinvestigation of crystal structure, phase transition and crystal growth mechanisms

    SciTech Connect

    Rao Popuri, Srinivasa; Artemenko, Alla; Labrugere, Christine; Miclau, Marinela; Villesuzanne, Antoine; Pollet, Michaël

    2014-05-01

    Well crystallized VO{sub 2} (A) microrods were grown via a single step hydrothermal reaction in the presence of V{sub 2}O{sub 5} and oxalic acid. With the advantage of high crystalline samples, we propose P4/ncc as an appropriate space group at room temperature. From morphological studies, we found that the oriented attachment and layer by layer growth mechanisms are responsible for the formation of VO{sub 2} (A) micro rods. The structural and electronic transitions in VO{sub 2} (A) are strongly first order in nature, and a marked difference between the structural transition temperatures and electronic transitions temperature was evidenced. The reversible intra- (LTP-A to HTP-A) and irreversible inter- (HTP-A to VO{sub 2} (M1)) structural phase transformations were studied by in-situ powder X-ray diffraction. Attempts to increase the size of the VO{sub 2} (A) microrods are presented and the possible formation steps for the flower-like morphologies of VO{sub 2} (M1) are described. - Graphical abstract: Using a single step and template free hydrothermal synthesis, well crystallized VO{sub 2} (A) microrods were prepared and the P4/ncc space group was assigned to the room temperature crystal structure. Reversible and irreversible phase transitions among different VO{sub 2} polymorphs were identified and their progressive nature was highlighted. Attempts to increase the microrods size, involving layer by layer formation mechanisms, are presented. - Highlights: • Highly crystallized VO{sub 2} (A) microrods were grown via a single step hydrothermal process. • The P4/ncc space group was determined for VO{sub 2} (A) at room temperature. • The electronic structure and progressive nature of the structural phase transition were investigated. • A weak coupling between structural and electronic phase transitions was identified. • Different crystallite morphologies were discussed in relation with growth mechanisms.

  15. Phase-field elasticity model based on mechanical jump conditions

    NASA Astrophysics Data System (ADS)

    Schneider, Daniel; Tschukin, Oleg; Choudhury, Abhik; Selzer, Michael; Böhlke, Thomas; Nestler, Britta

    2015-05-01

    Computational models based on the phase-field method typically operate on a mesoscopic length scale and resolve structural changes of the material and furthermore provide valuable information about microstructure and mechanical property relations. An accurate calculation of the stresses and mechanical energy at the transition region is therefore indispensable. We derive a quantitative phase-field elasticity model based on force balance and Hadamard jump conditions at the interface. Comparing the simulated stress profiles calculated with Voigt/Taylor (Annalen der Physik 274(12):573, 1889), Reuss/Sachs (Z Angew Math Mech 9:49, 1929) and the proposed model with the theoretically predicted stress fields in a plate with a round inclusion under hydrostatic tension, we show the quantitative characteristics of the model. In order to validate the elastic contribution to the driving force for phase transition, we demonstrate the absence of excess energy, calculated by Durga et al. (Model Simul Mater Sci Eng 21(5):055018, 2013), in a one-dimensional equilibrium condition of serial and parallel material chains. To validate the driving force for systems with curved transition regions, we relate simulations to the Gibbs-Thompson equilibrium condition (Johnson and Alexander, J Appl Phys 59(8):2735, 1986).

  16. A float mechanism of retention in reversed-phase chromatography

    NASA Astrophysics Data System (ADS)

    Deineka, V. I.; Deineka, L. A.; Saenko, I. I.; Chulkov, A. N.

    2015-07-01

    A float mechanism of retention in reversed-phase HPLC is proposed as an alternative to the known mechanisms of the distribution and hydrophobic expulsion of sorbate to the surface of a sorbent. Experimental data that the sorption of a flavylium structure is poorly sensitive to the position of OH groups, and that the retention of anthocyanins depends on the length of bonded alkyl radicals of reversed phase, form the basis of the proposed hypothesis. It is noted that the retention of anthocyanins depends on the orientation of hydroxyl groups in carbohydrate radicals, due to which the chromatographic behavior of anthocyanins is different for glucosides and galactosides, for arabinosides and xylosides, and so on. In other words, retention is a reliable indicator of the composition of a carbohydrate fragment. It is concluded that carbohydrate radicals serve as unique floats, while flat flavilic ions penetrate into the bonded phase. The existence of floats is the main reason for the lower efficiency (of the number of theoretical plates) of the peaks of anthocyanins. It is shown that if two carbohydrate radicals are present at different sites of aglycone (a two-float sorbate), the peaks of the substance are characterized by substantial additional broadening.

  17. Phase formation, liquid structure, and physical properties of amorphous and quasicrystal-forming alloys

    NASA Astrophysics Data System (ADS)

    Wessels, Victor Medgar

    2009-12-01

    Since the discovery of quasicrystals in 1985 and the development of commercially viable bulk metallic glasses (BMGs) in the mid 1990's a great deal of attention has been given to the characterization of new alloys with desirable properties, such as larger amorphous casting thickness, higher mechanical strength, or hydrogen storage capacity. Here, the results of a number of investigations into the structures and properties of some noncrystalline solid alloys will be presented and analyzed. Beamline electrostatic levitation (BESL), a method for determining supercooled liquid structure and phase formation in-situ, was used. Using BESL, the development of structural and chemical inhomogeneity was observed in supercooled liquid Cu46Zr 54 (a BMG when cast) with an onset at 845 +/- 5°C, providing experimental support for structural changes determined from molecular dynamics (MD) simulations of these liquids. Differing segregation of Hf and Zr atoms was observed in solidified Ti45Zr(38-x)HfxNi 17 using scanning electron microscopy (SEM), and correlated to a previously observed, sharp boundary in phase formation near x = 19 that was further investigated using BESL. In addition to the BESL studies, results will be presented and discussed on changes in microstructure and devitrification mechanisms with the addition of Ag in Mg65Cu(25-x)AgxGd 10 BMGs, interesting for their light weight and resistance to oxygen during casting, using transmission electron microscopy (TEM), SEM, and differential scanning calorimetry (DSC). Previous, preliminary results on the hydrogen storage capacity of icosahedral quasicrystal Ti45Zr38Ni 17 were re-examined, using an improved apparatus and analysis method developed as part of this work, and the previous results found to be in error.

  18. Phase equilibria in systems of formates with isobutyl alcohol

    SciTech Connect

    Seselkin, I.V.; Garber, Y.N.; Mironenko, V.F.

    1985-09-01

    The borate method, based on esterification with boric acid, was proposed for isolation of isobutyl alchohol. The method inv olves formation and hydrolysis of alkyl borate esters; in particular, formation and hydrolysis of triisobutyl borate. Esterification of isobutyl alcohol with boric acid is a reversible equilibrium reaction, and therefore in order to obtain high yields the water formed in the reaction must be removed. The presence of other organic compounds, which do not react with boric acid, in the mixture does not affect esterification of the alcohol. The reaction proceeds at 95-100/sup 0/ under atmospheric pressure. It was found that up to 97% of the isobutyl alcohol combines with boric acid. The resultant triisobutyl borate is isolated by ordinary distillation and then hydrolyzed to form boric acid and isobutyl alcohol.

  19. Mechanism of phase transition, from vapor to solid: Transient liquid phase is between the two

    NASA Astrophysics Data System (ADS)

    Mahapatra, A. K.; Wang, Junyong; Zhang, Hongwei; Han, Min

    2016-08-01

    The mechanism of phase transition, from vapor to solid, is studied by producing non-stoichiometric ZnO and CdS nanoclusters (NCs) by low-energy cluster beam deposition technique, and examining their morphological and compositional evolution over a long span of time. It is concluded that the transition of vapor to solid goes through a transient liquid phase: coagulation of a large number of atomic clusters first forms liquid NCs which then solidify. The nature of the material and the experimental conditions determine crystallinity and shape of the NCs during the solidification process.

  20. The formation mechanism of the Bonin high in August

    NASA Astrophysics Data System (ADS)

    Enomoto, Takeshi; Hoskins, Brian J.; Matsuda, Yoshihisa

    2003-01-01

    The Bonin high is a subtropical anticyclone that is predominant near Japan in the summer. This anticyclone is associated with an equivalent-barotropic structure, often extending throughout the entire troposphere. Although the equivalent-barotropic structure of the Bonin high has been known for years among synopticians because of its importance to the summer climate in east Asia, there are few dynamical explanations for such a structure. The present paper attempts to provide a formation mechanism for the deep ridge near Japan. We propose a new hypothesis that this equivalent-barotropic ridge near Japan is formed as a result of the propagation of stationary Rossby waves along the Asian jet in the upper troposphere ('the Silk Road pattern'). First, the monthly mean climatology is examined in order to demonstrate this hypothesis. It is shown that the enhanced Asian jet in August is favourable for the propagation of stationary Rossby waves and that the regions of descent over the eastern Mediterranean Sea and the Aral Sea act as two major wave sources. Second, a primitive-equation model is used to simulate the climatology of August. The model successfully simulates the Bonin high with an equivalent-barotropic structure. The upper-tropospheric ridge is found to be enhanced by a height anomaly of more than 80 m at 200 hPa, when a wave packet arrives. Sensitivity experiments are conducted to show that the removal of the diabatic cooling over the Asian jet suppresses the Silk Road pattern and formation of an equivalent-barotropic ridge near Japan, while the removal of the diabatic heating in the western Pacific does not.

  1. Formation of single-phase BaO nanoclusters

    SciTech Connect

    Du, Yingge; Kim, Dong Jun; Varga, Tamas; Wang, Zhitao; Szanyi, Janos; Lyubinetsky, Igor

    2011-06-01

    Combined analysis employing in-situ X-rays photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and ex-situ X-ray diffraction (XRD) has demonstrated the feasibility to deposit single-phase barium oxide overlayers by a direct sublimation of BaO material from an effusion cell. Furthermore, in situ scanning tunneling microscopy (STM) has confirmed that the BaO grows as nanoclusters with dimensions down to ~2 nm.

  2. Mechanism of pyranopterin ring formation in molybdenum cofactor biosynthesis.

    PubMed

    Hover, Bradley M; Tonthat, Nam K; Schumacher, Maria A; Yokoyama, Kenichi

    2015-05-19

    The molybdenum cofactor (Moco) is essential for all kingdoms of life, plays central roles in various biological processes, and must be biosynthesized de novo. During Moco biosynthesis, the characteristic pyranopterin ring is constructed by a complex rearrangement of guanosine 5'-triphosphate (GTP) into cyclic pyranopterin (cPMP) through the action of two enzymes, MoaA and MoaC (molybdenum cofactor biosynthesis protein A and C, respectively). Conventionally, MoaA was considered to catalyze the majority of this transformation, with MoaC playing little or no role in the pyranopterin formation. Recently, this view was challenged by the isolation of 3',8-cyclo-7,8-dihydro-guanosine 5'-triphosphate (3',8-cH2GTP) as the product of in vitro MoaA reactions. To elucidate the mechanism of formation of Moco pyranopterin backbone, we performed biochemical characterization of 3',8-cH2GTP and functional and X-ray crystallographic characterizations of MoaC. These studies revealed that 3',8-cH2GTP is the only product of MoaA that can be converted to cPMP by MoaC. Our structural studies captured the specific binding of 3',8-cH2GTP in the active site of MoaC. These observations provided strong evidence that the physiological function of MoaA is the conversion of GTP to 3',8-cH2GTP (GTP 3',8-cyclase), and that of MoaC is to catalyze the rearrangement of 3',8-cH2GTP into cPMP (cPMP synthase). Furthermore, our structure-guided studies suggest that MoaC catalysis involves the dynamic motions of enzyme active-site loops as a way to control the timing of interaction between the reaction intermediates and catalytically essential amino acid residues. Thus, these results reveal the previously unidentified mechanism behind Moco biosynthesis and provide mechanistic and structural insights into how enzymes catalyze complex rearrangement reactions.

  3. Campholenic aldehyde ozonolysis: a possible mechanism for the formation of specific biogenic secondary organic aerosol constituents

    NASA Astrophysics Data System (ADS)

    Kahnt, A.; Iinuma, Y.; Mutzel, A.; Böge, O.; Claeys, M.; Herrmann, H.

    2013-08-01

    In the present study, campholenic aldehyde ozonolysis was performed to investigate pathways leading to specific biogenic secondary organic aerosol (SOA) marker compounds. Campholenic aldehyde, a known α-pinene oxidation product, is suggested to be a key intermediate in the formation of terpenylic acid upon α-pinene ozonolysis. It was reacted with ozone in the presence and absence of an OH radical scavenger leading to SOA formation with a yield of 0.75 and 0.8, respectively. The resulting oxidation products in the gas and particle phases were investigated employing a denuder/filter sampling combination. Gas-phase oxidation products bearing a carbonyl group, which were collected by the denuder, were derivatised with 2,4-dinitrophenylhydrazine (DNPH) followed by Liquid Chromatography/negative ion Electrospray Ionisation Time-of-Flight Mass Spectrometry analysis and were compared to the gas-phase compounds detected by online Proton-Transfer-Reaction Mass Spectrometry. Particle-phase products were also analysed, directly or after DNPH derivatisation, to derive information about specific compounds leading to SOA formation. Among the detected compounds, the aldehydic precursor of terpenylic acid was identified and its presence was confirmed in ambient aerosol samples from the DNPH derivatisation, accurate mass data, and MS2 and MS3 fragmentation studies. Furthermore, the present investigation sheds light on a reaction pathway leading to the formation of terpenylic acid, involving α-pinene, α-pinene oxide, campholenic aldehyde, and terpenylic aldehyde. Additionally, the formation of diaterpenylic acid acetate could be connected to campholenic aldehyde oxidation. The present study also provides insights into the source of other highly functionalised oxidation products (e.g. m/z 201, C9H14O5 and m/z 215, C10H16O5), which have been observed in ambient aerosol samples and smog chamber-generated monoterpene SOA. The m/z 201 and 215 compounds were tentatively identified as a

  4. Mechanism of IAPP amyloid fibril formation involves an intermediate with a transient β-sheet.

    PubMed

    Buchanan, Lauren E; Dunkelberger, Emily B; Tran, Huong Q; Cheng, Pin-Nan; Chiu, Chi-Cheng; Cao, Ping; Raleigh, Daniel P; de Pablo, Juan J; Nowick, James S; Zanni, Martin T

    2013-11-26

    Amyloid formation is implicated in more than 20 human diseases, yet the mechanism by which fibrils form is not well understood. We use 2D infrared spectroscopy and isotope labeling to monitor the kinetics of fibril formation by human islet amyloid polypeptide (hIAPP or amylin) that is associated with type 2 diabetes. We find that an oligomeric intermediate forms during the lag phase with parallel β-sheet structure in a region that is ultimately a partially disordered loop in the fibril. We confirm the presence of this intermediate, using a set of homologous macrocyclic peptides designed to recognize β-sheets. Mutations and molecular dynamics simulations indicate that the intermediate is on pathway. Disrupting the oligomeric β-sheet to form the partially disordered loop of the fibrils creates a free energy barrier that is the origin of the lag phase during aggregation. These results help rationalize a wide range of previous fragment and mutation studies including mutations in other species that prevent the formation of amyloid plaques.

  5. Formation mechanism of the graphite-rich protective layer in blast furnace hearths

    NASA Astrophysics Data System (ADS)

    Jiao, Ke-xin; Zhang, Jian-liang; Liu, Zheng-jian; Liu, Feng; Liang, Li-sheng

    2016-01-01

    A long campaign life of blast furnaces is heavily linked to the existence of a protective layer in their hearths. In this work, we conducted dissection studies and investigated damage in blast furnace hearths to estimate the formation mechanism of the protective layer. The results illustrate that a significant amount of graphite phase was trapped within the hearth protective layer. Furthermore, on the basis of the thermodynamic and kinetic calculations of the graphite precipitation process, a precipitation potential index related to the formation of the graphite-rich protective layer was proposed to characterize the formation ability of this layer. We determined that, under normal operating conditions, the precipitation of graphite phase from hot metal was thermodynamically possible. Among elements that exist in hot metal, C, Si, and P favor graphite precipitation, whereas Mn and Cr inhibit this process. Moreover, at the same hot-face temperature, an increase of carbon concentration in hot metal can shorten the precipitation time. Finally, the results suggest that measures such as reducing the hot-face temperature and increasing the degree of carbon saturation in hot metal are critically important to improve the precipitation potential index.

  6. Titanium-based icosahedral quasicrystals and approximants: Phase formation, cluster structure, and hydrogenation

    NASA Astrophysics Data System (ADS)

    Majzoub, Eric Hish

    Equilibrium phase formation is reported for ternary Ti-Zr-Ni alloys near the icosahedral phase (i-phase) forming composition. The i-phase forms over a small compositional range from a high-temperature equilibrium phase mixture of the Laves and alpha(Ti/Zr) solid solution phases. Additions of small amounts of Pb, 1--2 at. %, are demonstrated to substantially effect the equilibrium phase formation and extend the stability of the i-phase to nearly 700°C. An electrochemical method was used to hydrogenate Ti-based quasicrystals and their crystal approximants. This technique gives a consistently high hydrogen to metal atom ratio of 1.9, without crystal hydride formation in the quasicrystal. Nuclear Magnetic Resonance (NMR) measurements of the hydrogen dipole-dipole interaction were made using hydrided i-phase samples. Comparisons with simulations based on hydrogen filling of the approximant-phase tetrahedral interstitials reveals that any filling order is consistent with the experimental data. Studies of the atomic structure of hydrided and unhydrided i-TiZrNi quasicrystal and its approximant are reported. We construct constrained icosahedral glass models using Bergman and Mackay clusters to describe the i-phase in Ti-Zr-Ni and Ti-TM-Si-O. A comparison of simulated and experimental diffraction reveals that, the Bergman and Mackay clusters are the fundamental clusters in i-TiZrNi and i-TiMnSiO, respectively.

  7. In situ characterization of formation and growth of high-pressure phases in single-crystal silicon during nanoindentation

    NASA Astrophysics Data System (ADS)

    Huang, Hu; Yan, Jiwang

    2016-04-01

    Pressure-induced intermediate phases of silicon exhibit unique characteristics in mechanics, chemistry, optics, and electrics. Clarifying the formation and growth processes of these new phases is essential for the preparation and application of them. For in situ characterization of the formation and growth of high-pressure phases in single-crystal silicon, a quantitative parameter, namely displacement change of indenter (Δ h) during the unloading holding process in nanoindentation, was proposed. Nanoindentation experiments under various unloading holding loads and loading/unloading rates were performed to investigate their effects on Δ h. Results indicate that Δ h varies significantly before and after the occurrence of pop-out; for the same maximum indentation load, it tends to increase with the decrease in the holding load and to increase with the increase in the loading/unloading rate. Thus, the value of Δ h can be regarded as an indicator that reflects the formation and growth processes of the high-pressure phases. Using Δ h, the initial position for the nucleation of the high-pressure phases, their growth, and their correlation to the loading/unloading rate were predictable.

  8. Statistical mechanics of topological phase transitions in networks

    NASA Astrophysics Data System (ADS)

    Palla, Gergely; Derényi, Imre; Farkas, Illés; Vicsek, Tamás

    2004-04-01

    We provide a phenomenological theory for topological transitions in restructuring networks. In this statistical mechanical approach energy is assigned to the different network topologies and temperature is used as a quantity referring to the level of noise during the rewiring of the edges. The associated microscopic dynamics satisfies the detailed balance condition and is equivalent to a lattice gas model on the edge-dual graph of a fully connected network. In our studies—based on an exact enumeration method, Monte Carlo simulations, and theoretical considerations—we find a rich variety of topological phase transitions when the temperature is varied. These transitions signal singular changes in the essential features of the global structure of the network. Depending on the energy function chosen, the observed transitions can be best monitored using the order parameters Φs=smax/M, i.e., the size of the largest connected component divided by the number of edges, or Φk=kmax/M, the largest degree in the network divided by the number of edges. If, for example, the energy is chosen to be E=-smax, the observed transition is analogous to the percolation phase transition of random graphs. For this choice of the energy, the phase diagram in the (,T) plane is constructed. Single-vertex energies of the form E=∑if(ki), where ki is the degree of vertex i, are also studied. Depending on the form of f(ki), first-order and continuous phase transitions can be observed. In case of f(ki)=-(ki+α)ln(ki), the transition is continuous, and at the critical temperature scale-free graphs can be recovered. Finally, by abruptly decreasing the temperature, nonequilibrium processes (e.g., nucleation and growth of particular topological phases) can also be interpreted by the present approach.

  9. Phase formation at bonded vanadium and stainless steel interfaces

    SciTech Connect

    Summers, T.S.E.

    1992-01-01

    The interface between vanadium bonded to stainless steel was studies to determine whether a brittle phase formed during three joining operations. Inertia friction welds between V and 21-6-9 stainless steel were examined using TEM. In the as-welded condition, a continuous, polygranular intermetallic layer about 0.25 {mu}m thick was present at the interface. This layer grew to about 50 {mu}m thick during heat treatment at 1000{degrees}C for two hours. Analysis of electron diffraction patterns confirmed that this intermetallic was the {omega} phase. The interface between vanadium and type 304, SANDVIK SAF 2205, and 21-6-9 stainless steel bonded by a co-extrusion process had intermetallic particles at the interface in the as-extruded condition. Heat treatment at 1000{degrees}C for two hours caused these particles to grow into continuous layers in all three cases. Based on the appearance, composition and hardness of this interfacial intermetallic, it was also concluded to be {omega} phase. Bonding V to type 430 stainless steel by co-extrusion caused V-rich carbides to form at the interface due to the higher concentration of C in the type 430 than in the other stainless steels investigated. The carbide particles initially present grew into a continuous layer during a two-hour heat treatment at 1000{degrees}C. Co-hipping 21-6-9 stainless steel tubing with V rod resulted in slightly more concentric specimens than the co-extruded ones, but a continuous layer of the {omega} phase formed during the hipping operation. This brittle layer could initiate failure during subsequent forming operations. The vanadium near the stainless steel interface in the co-extruded and co-hipped tubing in some cases was harder than before heat treatment. It was concluded that this hardening was due to thermal straining during cooling following heat treatment and that thermal strains might present a greater problem than seen here when longer tubes are used in actual applications.

  10. Formation of regular structures in the process of phase separation.

    PubMed

    Krekhov, Alexei

    2009-03-01

    Phase separation under directional quenching has been studied in a Cahn-Hilliard model. In distinct contrast to the disordered patterns which develop under a homogeneous quench, periodic stripe patterns are generated behind the quench front. Their wavelength is uniquely defined by the velocity of the quench interface in a wide range. Numerical simulations match perfectly analytical results obtained in the limit of small and large velocities of the quench interface. Additional periodic modulation of the quench interface may lead to cellular patterns. The quenching protocols analyzed are expected to be an effective tool in technological applications to design nanostructured materials. PMID:19392008

  11. Molecular mechanisms in the initiation phase of Wallerian degeneration.

    PubMed

    Chang, Biao; Quan, Qi; Lu, Shibi; Wang, Yu; Peng, Jiang

    2016-08-01

    Axonal degeneration is an early hallmark of nerve injury and many neurodegenerative diseases. The discovery of the Wallerian degeneration slow mutant mouse, in which axonal degeneration is delayed, revealed that Wallerian degeneration is an active progress and thereby illuminated the mechanisms underlying axonal degeneration. Nicotinamide mononucleotide adenylyltransferase 2 and sterile alpha and armadillo motif-containing protein 1 play essential roles in the maintenance of axon integrity by regulating the level of nicotinamide adenine dinucleotide, which seems to be the key molecule involved in the maintenance of axonal health. However, the function of nicotinamide mononucleotide remains debatable, and we discuss two apparently conflicting roles of nicotinamide mononucleotide in Wallerian degeneration. In this article, we focus on the roles of these molecules in the initiation phase of Wallerian degeneration to improve our understanding of the mechanisms underpinning this phenomenon. PMID:27062141

  12. Mechanism of voltage-gated channel formation in lipid membranes.

    PubMed

    Guidelli, Rolando; Becucci, Lucia

    2016-04-01

    Although several molecular models for voltage-gated ion channels in lipid membranes have been proposed, a detailed mechanism accounting for the salient features of experimental data is lacking. A general treatment accounting for peptide dipole orientation in the electric field and their nucleation and growth kinetics with ion channel formation is provided. This is the first treatment that explains all the main features of the experimental current-voltage curves of peptides forming voltage-gated channels available in the literature. It predicts a regime of weakly voltage-dependent conductance, followed by one of strong voltage-dependent conductance at higher voltages. It also predicts values of the parameters expressing the exponential dependence of conductance upon voltage and peptide bulk concentration for both regimes, in good agreement with those reported in the literature. Most importantly, the only two adjustable parameters involved in the kinetics of nucleation and growth of ion channels can be varied over broad ranges without affecting the above predictions to a significant extent. Thus, the fitting of experimental current-voltage curves stems naturally from the treatment and depends only slightly upon the choice of the kinetic parameters. PMID:26768224

  13. Mechanism of the formation for thoracic impedance change.

    PubMed

    Kuang, Ming-Xing; Xiao, Qiu-Jin; Cui, Chao-Ying; Kuang, Nan-Zhen; Hong, Wen-Qin; Hu, Ai-Rong

    2010-03-01

    The purpose of this study is to investigate the mechanism of the formation for thoracic impedance change. On the basis of Ohm's law and the electrical field distribution in the cylindrical volume conductor, the formula about the thoracic impedance change are deduced, and they are demonstrated with the model experiment. The results indicate that the thoracic impedance change caused by single blood vessel is directly proportional to the ratio of the impedance change to the basal impedance of the blood vessel itself, to the length of the blood vessel appearing between the current electrodes, and to the basal impedance between two detective electrodes on the chest surface, while it is inversely proportional to the distance between the blood vessel and the line joining two detective electrodes. The thoracic impedance change caused by multiple blood vessels together is equal to the algebraic addition of all thoracic impedance changes resulting from the individual blood vessels. That is, the impedance changes obey the principle of adding scalars in the measurement of the electrical impedance graph. The present study can offer the theoretical basis for the waveform reconstruction of Impedance cardiography (ICG).

  14. Numerical simulations of a siphon mechanism for quiescent prominence formation

    NASA Technical Reports Server (NTRS)

    Poland, A. I.; Mariska, J. T.; Klimchuk, J. A.

    1986-01-01

    Quiescent prominences represent a significant challenge to our understanding of the flow of mass and energy in the outer layers of the solar atmosphere. A small number of quiescent prominences contain as much mass as the entire corona (Athay, 1976). The problem then is how to get that much material into the relatively small volume of a prominence and maintain it at a temperature of 10,000 K in close proximity to material at one million K. The thermal insulation to conduction provided by the magnetic field explains the disparate temperatures. The mass source problem is less well understood. One method for supplying mass to the prominence is to siphon it from the chromosphere. The siphon mechanism begins with a magnetic loop that evolves into a configuration with a gravitational well, such as that described by Kippenhahn and Schluter (1957). This could be formed, for example, by a twist in the magnetic field. A gravitational well could also be formed by a condensation induced sag in the field. This could further enhance the condensation process. Once this well has formed, or as it is forming, the material in the well area of the loop must cool and condense to the point where radiative losses exceed any heat input. Additional material must also flow into the well from the underlying chromosphere to supply the mass required to form the prominence. One example from a series of numerical simulations that were performed to study the formation of quiescent prominences is presented.

  15. Planetary Embryo Bow Shocks as a Mechanism for Chondrule Formation

    NASA Astrophysics Data System (ADS)

    Mann, Christopher; Boley, Aaron C.; Morris, Melissa A.

    2015-01-01

    We investigate the plausibility of a planetary embryo bow shock as a mechanism for chondrule formation in the early solar system. A Mars-size planetary embryo traveling on a moderately excited orbit through the dusty early environment of the solar system will experience supersonic velocities relative to the circularly orbiting gas and dust. The resulting bow shock can thermally process solids that pass through it, with a wide range of possible conditions depending on impact radius. Volatile outgassing by the embryo along with some gas capture from the surrounding nebula can produce temporary atmospheres. We use radiation hydrodynamics simulations with direct particle integration to model the consequences of solids that encounter a bow shock produced by a 3000 km embryo with relative speeds to the gas of 5, 6, and 7 km/s. The embryos are envisaged to be surrounded by low- and high-mass atmospheres (0.75 and 6.25 Martian-mass atmospheres, respectively), and we explore different opacities for the gas. We find that a high-mass atmosphere and low dust opacity can produce peak temperatures and cooling rates that are most consistent with constraints set by chondrule furnace studies for plausible shock speeds.

  16. Mechanism of formation of supramolecular DNA-templated polymer nanowires.

    PubMed

    Watson, Scott M D; Galindo, Miguel A; Horrocks, Benjamin R; Houlton, Andrew

    2014-05-01

    Details of the mechanism of formation of supramolecular polymer nanowires by templating on DNA are revealed for the first time using AFM. Overall these data reveal that the smooth, regular, structures produced are rendered by highly dynamic supramolecular transformations occurring over the micrometre scale. In the initial stages of the process a low density of conducting polymer (CP) binds to the DNA as, essentially, spherical particles. Further reaction time produces DNA strands which are more densely packed with particles giving a beads-on-a-string appearance. The particles subsequently undergo dynamic reconfiguration so as to elongate along the template axis and merge to yield the highly regular, smooth morphology of the final nanowire. MD simulations illustrate the early stages of the process showing the binding of globular CP to duplex DNA, while the latter stages can be modeled effectively by a linear thermodynamic description based on the balance between the line energy, which accounts for adhesion of the material to the template, and its surface tension. This model accounts for the phenomena observed in the AFM studies: the relative success of DNA templating of polymers compared to metals; the slow approach to equilibrium; and the observed thinning and 'necking' phenomena as the structures transform from beads-on-a-string to smooth nanowire. PMID:24712548

  17. Mechanics of salt tongue formation with examples from Louisiana Slope

    SciTech Connect

    D'Onfro, P.

    1988-01-01

    Salt tongues up to several thousand feet thick and a few tens of miles long appear to intrude sediments along the Sigsbee Scarp and in the Mississippi fan in the Gulf of Mexico. Because salt tongues are impermeable and cover large areas of sediment, they have the potential to trap tremendous volumes of hydrocarbons. Field observations, laboratory experiments, and in-situ measurements in salt mines indicate that salt behaves like a viscous fluid over geologic time. Consequently, the same mechanical principles used to analyze igneous dike and sill formation can be applied to salt intrusions. Evidence suggests that salt tongues, like igneous sills, intrude sedimentary strata in which both horizontal principal compressive stresses exceed the overburden stress. This stress state exists in areas of regional tectonic or localized horizontal compression (e.g., in active thrust and foldbelts, in the toe regions of active growth fault systems, and around the flanks of intruding diapirs). This model puts constraints on both the timing of emplacement and the location of salt tongues.

  18. Luster decoration of ceramics: mechanisms of metallic luster formation

    NASA Astrophysics Data System (ADS)

    Pradell, T.; Molera, J.; Bayés, C.; Roura, P.

    2006-05-01

    Luster is a metallic decoration produced since early Islamic times (9th century AD in Iraq). Different studies have shown that medieval lusters are a metal-glass nanocomposite (metal nanoparticles embodied in a silica glassy matrix) obtained from the reaction of a copper and/or silver containing paint with a glaze. The mechanisms of formation of these metallic-like layers are investigated by laboratory reproductions of Medieval luster. Copper and silver lusters are obtained based on different thermal paths and atmospheres, and by using different glaze compositions. The ionic exchange between Cu+ and Ag+ ions from the luster paint with Na+ and K+ of the glaze, is demonstrated in either oxidizing or inert atmospheres and at firing temperatures between 500 °C and 600 °C. The reduction of copper and silver to their metallic state is obtained by introducing a reducing gas afterwards. The lusters are non-metallic red ruby copper or green with brown spots silver when developed over alkaline glazes, while they appear coppery and golden metallic when developed over mixed alkaline-lead glazes. SR-XRD, optical absorption and microprobe chemical analysis of the lusters indicate that the total amount of copper and silver, and the nature and size of the nanoparticles, are similar in both cases. Further work is needed to clarify the origin of these differences.

  19. A Mechanism for the Aqueous Phase Production of Alkyl Nitrates

    NASA Astrophysics Data System (ADS)

    Dahl, E. E.; Saltzman, E. S.; DeBruyn, W. J.

    2002-05-01

    Measurements of alkyl nitrates in the surface ocean and marine boundary layer indicate that there is an oceanic source of alkyl nitrates to the marine troposphere. Alkyl nitrates make up a portion of the total reactive nitrogen in the troposphere. They can contribute significantly to the NOx budget in the remote marine atmosphere, affecting regional ozone formation. The origin of the alkyl nitrate in the surface ocean is unknown. One possible mechanism for aqueous alkyl nitrate formation is the reaction of alkyl peroxy radicals with NO (ROO + NO -> RONO2). Peroxy radicals and NO have been observed in seawater at levels that make this a viable reaction (Blough 1997) (Zafiriou and McFarland 1981). In this project, steady state irradiations of nitrite and alkane solutions were used to determine the yield of alkyl nitrates from this reaction. The yield for ethyl nitrate has been determined to be 101+/-12% and 102+/-8% total yield for propyl nitrates (n-propyl and iso-propyl) with no evident temperature dependence between 5 and 30° C. Alkyl nitrates were also generated by the irradiation of natural seawater and nitrite-spiked seawater. These results indicate that the proposed mechanism may be a viable source of alkyl nitrates in surface waters.

  20. Mechanism of O((3)P) formation from a hydroxyl radical pair in aqueous solution.

    PubMed

    Codorniu-Hernández, Edelsys; Hall, Kyle Wm; Boese, A Daniel; Ziemianowicz, Daniel; Carpendale, Sheelagh; Kusalik, Peter G

    2015-10-13

    The reaction mechanism for the rapid formation of a triplet oxygen atom, O((3)P), from a pair of triplet-state hydroxyl radicals in liquid water is explored utilizing extensive Car-Parrinello MD simulations and advanced visualization techniques. The local solvation structures, the evolution of atomic charges, atomic separations, spin densities, electron localization functions, and frontier molecular orbitals, as well as free energy profiles, evidence that the reaction proceeds through a hybrid (hydrogen atom transfer and electron-proton transfer) and hemibond-assisted reaction mechanism. A benchmarking study utilizing high-level ab initio calculations to examine the interactions of a hydroxyl radical pair in the gas phase and the influence of a hemibonded water is also provided. The results presented here should serve as a foundation for further experimental and theoretical studies aimed at better understanding the role and potential applications of the triplet oxygen atom as a potent reactive oxygen species. PMID:26574263

  1. Phase dependent restoration mechanisms of TC8 titanium alloy during hot compression in the two phase region

    NASA Astrophysics Data System (ADS)

    Wang, K.; Luan, B. F.; Li, M. Q.; Liu, Q.

    2015-08-01

    The TC8 titanium alloy was isothermally compressed at 1133 K and 1213 K in the (α+β) two phase region. The microstructural evolution and restoration mechanism in the α and β phases were characterized by optical microscopy and transmission electron microscopy. The results show a significant effect of phase content on the microstructural evolution and restoration mechanism. The grain refinement occurs in the α phase at both temperatures, but in the β phase only at the higher temperature of 1213 K. This difference in microstructural evolution is attributed to the different temperature dependence of restoration mechanisms in the two phases. A significant increase in the volume fraction of β phase makes the restoration mechanism in the β phase change from dynamic recovery (DRV) to dynamic recrystallization (DRX), which subsequently induces the β grain refinement.

  2. Mechanisms of the early phases of plant gravitropism

    NASA Technical Reports Server (NTRS)

    Kiss, J. Z.

    2000-01-01

    Gravitropism is directed growth of a plant or plant organ in response to gravity and can be divided into the following temporal sequence: perception, transduction, and response. This article is a review of the research on the early events of gravitropism (i.e., phenomena associated with the perception and transduction phases). The two major hypotheses for graviperception are the protoplast-pressure and starch-statolith models. While most researchers support the concept of statoliths, there are suggestions that plants have multiple mechanisms of perception. Evidence supports the hypothesis that the actin cytoskeleton is involved in graviperception/transduction, but the details of these mechanisms remain elusive. A number of recent developments, such as increased use of the molecular genetic approach, magnetophoresis, and laser ablation, have facilitated research in graviperception and have allowed for refinement of the current models. In addition, the entire continuum of acceleration forces from hypo- to hyper-gravity have been useful in studying perception mechanisms. Future interdisciplinary molecular approaches and the availability of sophisticated laboratories on the International Space Station should help to develop new insights into mechanisms of gravitropism in plants.

  3. Modification of linear prepolymers to tailor heterogeneous network formation through photo-initiated Polymerization-Induced Phase Separation

    PubMed Central

    Szczepanski, Caroline R.; Stansbury, Jeffrey W.

    2015-01-01

    Polymerization-induced phase separation (PIPS) was studied in ambient photopolymerizations of triethylene glycol dimethacrylate (TEGDMA) modified by poly(methyl methacrylate) (PMMA). The molecular weight of PMMA and the rate of network formation (through incident UV-irradiation) were varied to influence both the promotion of phase separation through increases in overall free energy, as well as the extent to which phase development occurs during polymerization through diffusion prior to network gelation. The overall free energy of the polymerizing system increases with PMMA molecular weight, such that PIPS is promoted thermodynamically at low loading levels (5 wt%) of a higher molecular weight PMMA (120 kDa), while a higher loading level (20 wt%) is needed to induce PIPS with lower PMMA molecular weight (11 kDa), and phase separation was not promoted at any loading level tested of the lowest molecular weight PMMA (1 kDa). Due to these differences in overall free energy, systems modified by PMMA (11 kDa) underwent phase separation via Nucleation and Growth, and systems modified by PMMA (120 kDa), followed the Spinodal Decomposition mechanism. Despite differences in phase structure, all materials form a continuous phase rich in TEGDMA homopolymer. At high irradiation intensity (Io=20mW/cm2), the rate of network formation prohibited significant phase separation, even when thermodynamically preferred. A staged curing approach, which utilizes low intensity irradiation (Io=300µW/cm2) for the first ~50% of reaction to allow phase separation via diffusion, followed by a high intensity flood-cure to achieve a high degree of conversion, was employed to form phase-separated networks with reduced polymerization stress yet equivalent final conversion and modulus. PMID:26190865

  4. Mechanical parameters and flight phase characteristics in aquatic plyometric jumping.

    PubMed

    Louder, Talin J; Searle, Cade J; Bressel, Eadric

    2016-09-01

    Plyometric jumping is a commonly prescribed method of training focused on the development of reactive strength and high-velocity concentric power. Literature suggests that aquatic plyometric training may be a low-impact, effective supplement to land-based training. The purpose of the present study was to quantify acute, biomechanical characteristics of the take-off and flight phase for plyometric movements performed in the water. Kinetic force platform data from 12 young, male adults were collected for counter-movement jumps performed on land and in water at two different immersion depths. The specificity of jumps between environmental conditions was assessed using kinetic measures, temporal characteristics, and an assessment of the statistical relationship between take-off velocity and time in the air. Greater peak mechanical power was observed for jumps performed in the water, and was influenced by immersion depth. Additionally, the data suggest that, in the water, the statistical relationship between take-off velocity and time in air is quadratic. Results highlight the potential application of aquatic plyometric training as a cross-training tool for improving mechanical power and suggest that water immersion depth and fluid drag play key roles in the specificity of the take-off phase for jumping movements performed in the water. PMID:27125295

  5. Surfactants from the gas phase may promote cloud droplet formation.

    PubMed

    Sareen, Neha; Schwier, Allison N; Lathem, Terry L; Nenes, Athanasios; McNeill, V Faye

    2013-02-19

    Clouds, a key component of the climate system, form when water vapor condenses upon atmospheric particulates termed cloud condensation nuclei (CCN). Variations in CCN concentrations can profoundly impact cloud properties, with important effects on local and global climate. Organic matter constitutes a significant fraction of tropospheric aerosol mass, and can influence CCN activity by depressing surface tension, contributing solute, and influencing droplet activation kinetics by forming a barrier to water uptake. We present direct evidence that two ubiquitous atmospheric trace gases, methylglyoxal (MG) and acetaldehyde, known to be surface-active, can enhance aerosol CCN activity upon uptake. This effect is demonstrated by exposing acidified ammonium sulfate particles to 250 parts per billion (ppb) or 8 ppb gas-phase MG and/or acetaldehyde in an aerosol reaction chamber for up to 5 h. For the more atmospherically relevant experiments, i.e., the 8-ppb organic precursor concentrations, significant enhancements in CCN activity, up to 7.5% reduction in critical dry diameter for activation, are observed over a timescale of hours, without any detectable limitation in activation kinetics. This reduction in critical diameter enhances the apparent particle hygroscopicity up to 26%, which for ambient aerosol would lead to cloud droplet number concentration increases of 8-10% on average. The observed enhancements exceed what would be expected based on Köhler theory and bulk properties. Therefore, the effect may be attributed to the adsorption of MG and acetaldehyde to the gas-aerosol interface, leading to surface tension depression of the aerosol. We conclude that gas-phase surfactants may enhance CCN activity in the atmosphere.

  6. Surfactants from the gas phase may promote cloud droplet formation.

    PubMed

    Sareen, Neha; Schwier, Allison N; Lathem, Terry L; Nenes, Athanasios; McNeill, V Faye

    2013-02-19

    Clouds, a key component of the climate system, form when water vapor condenses upon atmospheric particulates termed cloud condensation nuclei (CCN). Variations in CCN concentrations can profoundly impact cloud properties, with important effects on local and global climate. Organic matter constitutes a significant fraction of tropospheric aerosol mass, and can influence CCN activity by depressing surface tension, contributing solute, and influencing droplet activation kinetics by forming a barrier to water uptake. We present direct evidence that two ubiquitous atmospheric trace gases, methylglyoxal (MG) and acetaldehyde, known to be surface-active, can enhance aerosol CCN activity upon uptake. This effect is demonstrated by exposing acidified ammonium sulfate particles to 250 parts per billion (ppb) or 8 ppb gas-phase MG and/or acetaldehyde in an aerosol reaction chamber for up to 5 h. For the more atmospherically relevant experiments, i.e., the 8-ppb organic precursor concentrations, significant enhancements in CCN activity, up to 7.5% reduction in critical dry diameter for activation, are observed over a timescale of hours, without any detectable limitation in activation kinetics. This reduction in critical diameter enhances the apparent particle hygroscopicity up to 26%, which for ambient aerosol would lead to cloud droplet number concentration increases of 8-10% on average. The observed enhancements exceed what would be expected based on Köhler theory and bulk properties. Therefore, the effect may be attributed to the adsorption of MG and acetaldehyde to the gas-aerosol interface, leading to surface tension depression of the aerosol. We conclude that gas-phase surfactants may enhance CCN activity in the atmosphere. PMID:23382211

  7. Local structure, composition, and crystallization mechanism of a model two-phase "composite nanoglass"

    NASA Astrophysics Data System (ADS)

    Chattopadhyay, Soma; Kelly, S. D.; Shibata, Tomohiro; Balasubramanian, M.; Srinivasan, S. G.; Du, Jincheng; Banerjee, Rajarshi; Ayyub, Pushan

    2016-02-01

    We report a detailed study of the local composition and structure of a model, bi-phasic nanoglass with nominal stoichiometry Cu55Nb45. Three dimensional atom probe data suggest a nanoscale-phase-separated glassy structure having well defined Cu-rich and Nb-rich regions with a characteristic length scale of ≈3 nm. However, extended x-ray absorption fine structure analysis indicates subtle differences in the local environments of Cu and Nb. While the Cu atoms displayed a strong tendency to cluster and negligible structural order beyond the first coordination shell, the Nb atoms had a larger fraction of unlike neighbors (higher chemical order) and a distinctly better-ordered structural environment (higher topological order). This provides the first experimental indication that metallic glass formation may occur due to frustration arising from the competition between chemical ordering and clustering. These observations are complemented by classical as well as ab initio molecular dynamics simulations. Our study indicates that these nanoscale phase-separated glasses are quite distinct from the single phase nanoglasses (studied by Gleiter and others) in the following three respects: (i) they contain at least two structurally and compositionally distinct, nanodispersed, glassy phases, (ii) these phases are separated by comparatively sharp inter-phase boundaries, and (iii) thermally induced crystallization occurs via a complex, multi-step mechanism. Such materials, therefore, appear to constitute a new class of disordered systems that may be called a composite nanoglass.

  8. Local structure, composition, and crystallization mechanism of a model two-phase "composite nanoglass".

    PubMed

    Chattopadhyay, Soma; Kelly, S D; Shibata, Tomohiro; Balasubramanian, M; Srinivasan, S G; Du, Jincheng; Banerjee, Rajarshi; Ayyub, Pushan

    2016-02-14

    We report a detailed study of the local composition and structure of a model, bi-phasic nanoglass with nominal stoichiometry Cu55Nb45. Three dimensional atom probe data suggest a nanoscale-phase-separated glassy structure having well defined Cu-rich and Nb-rich regions with a characteristic length scale of ≈ 3 nm. However, extended x-ray absorption fine structure analysis indicates subtle differences in the local environments of Cu and Nb. While the Cu atoms displayed a strong tendency to cluster and negligible structural order beyond the first coordination shell, the Nb atoms had a larger fraction of unlike neighbors (higher chemical order) and a distinctly better-ordered structural environment (higher topological order). This provides the first experimental indication that metallic glass formation may occur due to frustration arising from the competition between chemical ordering and clustering. These observations are complemented by classical as well as ab initio molecular dynamics simulations. Our study indicates that these nanoscale phase-separated glasses are quite distinct from the single phase nanoglasses (studied by Gleiter and others) in the following three respects: (i) they contain at least two structurally and compositionally distinct, nanodispersed, glassy phases, (ii) these phases are separated by comparatively sharp inter-phase boundaries, and (iii) thermally induced crystallization occurs via a complex, multi-step mechanism. Such materials, therefore, appear to constitute a new class of disordered systems that may be called a composite nanoglass.

  9. Formation of aqueous-phase α-hydroxyhydroperoxides (α-HHP): potential atmospheric impacts

    NASA Astrophysics Data System (ADS)

    Zhao, R.; Lee, A. K. Y.; Soong, R.; Simpson, A. J.; Abbatt, J. P. D.

    2013-06-01

    The focus of this work is on quantifying the degree of the aqueous-phase formation of α-hydroxyhydroperoxides (α-HHPs) via reversible nucleophilic addition of H2O2 to aldehydes. Formation of this class of highly oxygenated organic hydroperoxides represents a poorly characterized aqueous-phase processing pathway that may lead to enhanced SOA formation and aerosol toxicity. Specifically, the equilibrium constants of α-HHP formation have been determined using proton nuclear-magnetic-resonance (1H NMR) spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). Significant α-HHP formation was observed from formaldehyde, acetaldehyde, propionaldehyde, glycolaldehyde, glyoxylic acid, and methylglyoxal, but not from methacrolein and ketones. Low temperatures enhanced the formation of α-HHPs but slowed their formation rates. High inorganic salt concentrations shifted the equilibria toward the hydrated form of the aldehydes and slightly suppressed α-HHP formation. Using the experimental equilibrium constants, we predict the equilibrium concentration of α-HHPs to be in the μM level in cloud water, but it may also be present in the mM level in aerosol liquid water (ALW), where the concentrations of H2O2 and aldehydes can be high. Formation of α-HHPs in ALW may significantly affect the effective Henry's law constants of H2O2 and aldehydes but may not affect their gas-phase levels. The photochemistry and reactivity of this class of atmospheric species have not been studied.

  10. Formation of aqueous-phase α-hydroxyhydroperoxides (α-HHP): potential atmospheric impacts

    NASA Astrophysics Data System (ADS)

    Zhao, R.; Lee, A. K. Y.; Soong, R.; Simpson, A. J.; Abbatt, J. P. D.

    2013-02-01

    The focus of this work is on quantifying the degree of the aqueous-phase formation of α-hydroxyhydroperoxides (α-HHPs) via reversible nucleophilic addition of H2O2 to aldehydes. Formation of this class of highly oxygenated organic hydroperoxides represents a poorly characterized aqueous-phase processing pathway that may lead to enhanced SOA formation and aerosol toxicity. Specifically, the equilibrium constants of α-HHP formation have been determined using proton nuclear resonance (1H NMR) spectroscopy and proton transfer reaction mass spectrometry (PTR-MS). Significant α-HHP formation was observed from formaldehyde, acetaldehyde, propionaldehyde, glycolaldehyde, glyoxylic acid, methylglyoxal, but not from methacrolein and ketones. Low temperatures enhanced the formation of α-HHPs but slowed their formation rates. High inorganic salt concentrations shifted the equilibria toward the hydrated form of the aldehydes and slightly suppressed α-HHP formation. Using the experimental equilibrium constants, we predict the equilibrium concentration of α-HHPs to be in the μM level in cloud water but may be present in the mM level in aerosol liquid water (ALW), where the concentrations of H2O2 and aldehydes can be high. Formation of α-HHPs in ALW may significantly affect the effective Henry's law constants of H2O2 and aldehydes but may not affect their gas-phase levels. The photochemistry and reactivity of this class of atmospheric species have not been studied.

  11. The initial phase of fracture healing is specifically sensitive to mechanical conditions.

    PubMed

    Klein, Petra; Schell, Hanna; Streitparth, Florian; Heller, Markus; Kassi, Jean-Pierre; Kandziora, Frank; Bragulla, Hermann; Haas, Norbert P; Duda, Georg N

    2003-07-01

    Interfragmentary movements affect the quality and quantity of callus formation. The mounting plane of monolateral external fixators may give direction to those movements. Therefore, the aim of this study was to determine the influence of the fixator mounting plane on the process of fracture healing. Identically configured fixators were mounted either medially or anteromedially on the tibiae of sheep. Interfragmentary movements and ground reaction forces were evaluated in vivo during a nine week period. Histomorphological and biomechanical parameters described the bone healing processes. Changing only the mounting plane led to a modification of interfragmentary movements in the initial healing phase. The difference in interfragmentary movements between the groups was only significant during the first post-operative period. However, these initial differences in mechanical conditions influenced callus tissue formation significantly. The group with the anteromedially mounted fixator, initially showing significantly more interfragmentary movements, ended up with a significantly smaller callus diameter and a significantly higher callus stiffness as a result of advanced fracture healing. This demonstrates that the initial phase of healing is sensitive to mechanical conditions and influences the course of healing. Therefore, initial mechanical stability of an osteosynthesis should be considered an important factor in clinical fracture treatment.

  12. The provenance and formation of reduced carbon phases on Mars from the study of Martian meteorites.

    NASA Astrophysics Data System (ADS)

    Steele, A.; McCubbin, F. M.; Fries, M.

    2015-12-01

    Organic carbon compounds are essential building blocks of terrestrial life, so the occurrence and origin (biotic or abiotic) of organic compounds on Mars is of great significance. Indeed, the question of Martian organic matter is among the highest priority targets for robotic spacecraft missions in the next decade includ- ing the Mars Science Laboratory and Mars 2020. Sev- eral Martian meteorites contain organic carbon (i.e., macromolecular reduced carbon-rich material, not nec- essarily related to biota), but there is little agreement on its origins. Initial hypotheses for the origin of this organic carbon included: terrestrial contamination; chondritic meteoritic input; thermal decomposition of Martian carbonate minerals; direct precipitation from cooling aqueous fluids; and the remains of ancient Martian biota. We report on results from the analysis of 14 martian meteorites and show the distribution of organic phases throughout the samples analyzed. We will present formation scearios for each of the types of organic matter discovered. These studies when combined show 4 possible pools of reduced carbon on Mars. 1) impact generated graphite in the Tissint meteorite, 2) secondary hydrothermal generated graphite in ALH 84001, 3) primary igneous reduced carbon in 12 Martian meteorites associated with spinel inclusions in olivine and pyroxene 4) and potentially primary hydrothermally formed organic carbon / nitrogen containing organic species in the maskelynite phases of the Tissint meteorite. These studies show that Mars has produced reduced carbon / organic carbon via several mechanisms and reveal that the building blocks of life, if not life itself, are present on Mars.

  13. Stochastic simulation of fluctuation stage of phase transfer on solid surface during thin film formation

    SciTech Connect

    Bondareva, A.L.; Zmievskaya, G.I.

    2005-05-16

    Ions irradiation leads to nano-scale islands of thin cover formation. Influence on a solid surface of the following processes: fluctuation stage of phase transition (new phase island formation), its migration under long-range potentials of interaction which leads to brownian motion of islands on the surface, inelastic collisions of islands has been simulated by stochastic analogue method. The calculated time evolution of islands kinetic distribution function from islands square sizes and coordinates of its masses centres on the surface is calculated taking into account non-linear Gibbs energy of islands formation, which depends on lattice elastic responses as well on dislocation influence.

  14. Ultrasound Microbubble Treatment Enhances Clathrin-Mediated Endocytosis and Fluid-Phase Uptake through Distinct Mechanisms.

    PubMed

    Fekri, Farnaz; Delos Santos, Ralph Christian; Karshafian, Raffi; Antonescu, Costin N

    2016-01-01

    Drug delivery to tumors is limited by several factors, including drug permeability of the target cell plasma membrane. Ultrasound in combination with microbubbles (USMB) is a promising strategy to overcome these limitations. USMB treatment elicits enhanced cellular uptake of materials such as drugs, in part as a result of sheer stress and formation of transient membrane pores. Pores formed upon USMB treatment are rapidly resealed, suggesting that other processes such as enhanced endocytosis may contribute to the enhanced material uptake by cells upon USMB treatment. How USMB regulates endocytic processes remains incompletely understood. Cells constitutively utilize several distinct mechanisms of endocytosis, including clathrin-mediated endocytosis (CME) for the internalization of receptor-bound macromolecules such as Transferrin Receptor (TfR), and distinct mechanism(s) that mediate the majority of fluid-phase endocytosis. Tracking the abundance of TfR on the cell surface and the internalization of its ligand transferrin revealed that USMB acutely enhances the rate of CME. Total internal reflection fluorescence microscopy experiments revealed that USMB treatment altered the assembly of clathrin-coated pits, the basic structural units of CME. In addition, the rate of fluid-phase endocytosis was enhanced, but with delayed onset upon USMB treatment relative to the enhancement of CME, suggesting that the two processes are distinctly regulated by USMB. Indeed, vacuolin-1 or desipramine treatment prevented the enhancement of CME but not of fluid phase endocytosis upon USMB, suggesting that lysosome exocytosis and acid sphingomyelinase, respectively, are required for the regulation of CME but not fluid phase endocytosis upon USMB treatment. These results indicate that USMB enhances both CME and fluid phase endocytosis through distinct signaling mechanisms, and suggest that strategies for potentiating the enhancement of endocytosis upon USMB treatment may improve targeted

  15. Ultrasound Microbubble Treatment Enhances Clathrin-Mediated Endocytosis and Fluid-Phase Uptake through Distinct Mechanisms

    PubMed Central

    Fekri, Farnaz; Delos Santos, Ralph Christian; Karshafian, Raffi

    2016-01-01

    Drug delivery to tumors is limited by several factors, including drug permeability of the target cell plasma membrane. Ultrasound in combination with microbubbles (USMB) is a promising strategy to overcome these limitations. USMB treatment elicits enhanced cellular uptake of materials such as drugs, in part as a result of sheer stress and formation of transient membrane pores. Pores formed upon USMB treatment are rapidly resealed, suggesting that other processes such as enhanced endocytosis may contribute to the enhanced material uptake by cells upon USMB treatment. How USMB regulates endocytic processes remains incompletely understood. Cells constitutively utilize several distinct mechanisms of endocytosis, including clathrin-mediated endocytosis (CME) for the internalization of receptor-bound macromolecules such as Transferrin Receptor (TfR), and distinct mechanism(s) that mediate the majority of fluid-phase endocytosis. Tracking the abundance of TfR on the cell surface and the internalization of its ligand transferrin revealed that USMB acutely enhances the rate of CME. Total internal reflection fluorescence microscopy experiments revealed that USMB treatment altered the assembly of clathrin-coated pits, the basic structural units of CME. In addition, the rate of fluid-phase endocytosis was enhanced, but with delayed onset upon USMB treatment relative to the enhancement of CME, suggesting that the two processes are distinctly regulated by USMB. Indeed, vacuolin-1 or desipramine treatment prevented the enhancement of CME but not of fluid phase endocytosis upon USMB, suggesting that lysosome exocytosis and acid sphingomyelinase, respectively, are required for the regulation of CME but not fluid phase endocytosis upon USMB treatment. These results indicate that USMB enhances both CME and fluid phase endocytosis through distinct signaling mechanisms, and suggest that strategies for potentiating the enhancement of endocytosis upon USMB treatment may improve targeted

  16. Untangling the web: mechanisms underlying ER network formation.

    PubMed

    Goyal, Uma; Blackstone, Craig

    2013-11-01

    The ER is a continuous membrane system consisting of the nuclear envelope, flat sheets often studded with ribosomes, and a polygonal network of highly-curved tubules extending throughout the cell. Although protein and lipid biosynthesis, protein modification, vesicular transport, Ca(2+)dynamics, and protein quality control have been investigated in great detail, mechanisms that generate the distinctive architecture of the ER have been uncovered only recently. Several protein families including the reticulons and REEPs/DP1/Yop1p harbor hydrophobic hairpin domains that shape high-curvature ER tubules and mediate intramembrane protein interactions. Members of the atlastin/RHD3/Sey1p family of dynamin-related GTPases interact with the ER-shaping proteins and mediate the formation of three-way junctions responsible for the polygonal structure of the tubular ER network, with Lunapark proteins acting antagonistically. Additional classes of tubular ER proteins including some REEPs and the M1 spastin ATPase interact with the microtubule cytoskeleton. Flat ER sheets possess a different complement of proteins such as p180, CLIMP-63 and kinectin implicated in shaping, cisternal stacking and cytoskeletal interactions. The ER is also in constant motion, and numerous signaling pathways as well as interactions among cytoskeletal elements, the plasma membrane, and organelles cooperate to position and shape the ER dynamically. Finally, many proteins involved in shaping the ER network are mutated in the most common forms of hereditary spastic paraplegia, indicating a particular importance for proper ER morphology and distribution in large, highly-polarized cells such as neurons. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum. PMID:23602970

  17. Formation of quark phases in compact stars and SN explosion

    SciTech Connect

    Drago, A.; Pagliara, G.; Pagliaroli, G.; Villante, F. L.; Vissani, F.

    2008-10-13

    We describe possible scenarios of quark deconfinement in compact stars and we analyze their astrophysical implications. The quark deconfinement process can proceed rapidly, as a strong deflagration, releasing a huge amount of energy in a short time and generating an extra neutrino burst. If energy is tranferred efficiently to the surface, like e.g. in the presence of convective instabilities, this burst could contribute to revitalize a partially failed SN explosion. We discuss how the neutrino observations from SN1987A would fit in this scenario. Finally, we focus on the fate of massive and rapidly rotating progenitors, discussing possible time separations between the moment of the core collapse and the moment of quark deconfinement. This mechanism can be at the basis of the interpretation of gamma ray bursts in which lines associated with heavy elements are present in the spectrum.

  18. Structure and fracture mechanism of a two-phase chromium-nickel alloy during high-temperature deformation

    NASA Astrophysics Data System (ADS)

    Mironenko, V. N.; Aronin, A. S.; Vasenev, V. V.; Aristova, I. M.; Shmyt'ko, I. M.; Trushnikova, A. S.

    2016-09-01

    The structure and mechanical properties of a two-phase Kh65N33V2FT alloy has been studied after tests at room and high temperatures. The morphology of the main phases, namely, solid solutions of nickel in chromium (α) and chromium in nickel (γ), is changed depending on temperature. The lattice parameters of the main phases have been determined. The main mechanism of deformation for this alloy is shown to be grain-boundary sliding. Bulk and grain-boundary diffusion creep and self-regulating diffusion-viscous flow is possible in the γ phase during high-temperature deformation. The heat resistance of this alloy is restricted to 1000°C because of the formation of a γ-phase percolation cluster.

  19. Gas-Phase Anionic σ-Adduct (Trans)formations in Heteroaromatic Systems1

    NASA Astrophysics Data System (ADS)

    Zimnicka, Magdalena; Danikiewicz, Witold

    2015-07-01

    Anions of nitroderivatives of thiophene and furan were subjected to the reactions with selected C-H acids in the gas phase. Various structures and reaction pathways were proposed for the observed ionic products. In general, the reactions of heteroaromatic anions with C-H acids may be divided into three groups, depending on the proton affinity difference between C-H acid's conjugate base and heteroaromatic anion (ΔPA). The proton transfer from C-H acid to heteroaromatic anion is a dominant process in the reactions for which ΔPA < 0 kcal mol-1, whereas the reactions with high ΔPA (ΔPA > 16 kcal mol-1) do not lead to any ionic products. The formation of σ-adducts and products of their further transformations according to the VNS, SNAr, cine, and tele substitution mechanisms have been proposed for reactions with moderate ΔPA. The other possible mechanisms as SN2 reaction, nucleophilic addition to the cyano group, ring-opening pathway, and halogenophilic reaction have also been discussed to contribute in the reactions between heteroaromatic anions and C-H acids.

  20. Structure and Mechanism of a Pentameric Formate Channel

    SciTech Connect

    Waight, A.; Love, J; Wang, D

    2010-01-01

    Formate transport across the inner membrane is a critical step in anaerobic bacterial respiration. Members of the formate/nitrite transport protein family function to shuttle substrate across the cytoplasmic membrane. In bacterial pathogens, the nitrite transport protein is involved in protecting bacteria from peroxynitrite released by host macrophages. We have determined the 2.13-{angstrom} structure of the formate channel FocA from Vibrio cholerae, which reveals a pentamer in which each monomer possesses its own substrate translocation pore. Unexpectedly, the fold of the FocA monomer resembles that found in water and glycerol channels. The selectivity filter in FocA consists of a cytoplasmic slit and a central constriction ring. A 2.5-{angstrom} high-formate structure shows two formate ions bound to the cytoplasmic slit via both hydrogen bonding and van der Waals interactions, providing a structural basis for the substrate selectivity of the channel.

  1. Theoretical Mechanistic and Kinetic Studies on Homogeneous Gas-Phase Formation of Polychlorinated Naphthalene from 2-Chlorophenol as Forerunner.

    PubMed

    Xu, Fei; Zhang, Ruiming; Li, Yunfeng; Zhang, Qingzhu; Wang, Wenxing

    2015-01-01

    Polychlorinated naphthalenes (PCNs) are dioxins-like compounds and are formed along with polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in thermal and combustion procedures. Chlorophenols (CPs) are the most important forerunners of PCNs. A comprehensive comprehension of PCN formation procedure from CPs is a precondition for reducing the discharge of PCNs. Experiments on the formation of PCNs from CPs have been hindered by PCN toxicity and short of precise detection methods for active intermediate radicals. In this work, PCN formation mechanism in gas-phase condition from 2-chlorophenol (2-CP) as forerunner was studied by quantum chemistry calculations. Numbers of energetically advantaged formation routes were proposed. The rate constants of key elementary steps were calculated over 600-1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT) method. This study illustrates formation of PCNs with one chlorine atom loss from 2-CP is preferred over that without chlorine atom loss. In comparison with formation of PCDFs from 2-CP, PCN products are less chlorinated and have lower formation potential.

  2. Theoretical Mechanistic and Kinetic Studies on Homogeneous Gas-Phase Formation of Polychlorinated Naphthalene from 2-Chlorophenol as Forerunner

    PubMed Central

    Xu, Fei; Zhang, Ruiming; Li, Yunfeng; Zhang, Qingzhu; Wang, Wenxing

    2015-01-01

    Polychlorinated naphthalenes (PCNs) are dioxins-like compounds and are formed along with polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in thermal and combustion procedures. Chlorophenols (CPs) are the most important forerunners of PCNs. A comprehensive comprehension of PCN formation procedure from CPs is a precondition for reducing the discharge of PCNs. Experiments on the formation of PCNs from CPs have been hindered by PCN toxicity and short of precise detection methods for active intermediate radicals. In this work, PCN formation mechanism in gas-phase condition from 2-chlorophenol (2-CP) as forerunner was studied by quantum chemistry calculations. Numbers of energetically advantaged formation routes were proposed. The rate constants of key elementary steps were calculated over 600–1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT) method. This study illustrates formation of PCNs with one chlorine atom loss from 2-CP is preferred over that without chlorine atom loss. In comparison with formation of PCDFs from 2-CP, PCN products are less chlorinated and have lower formation potential. PMID:26516839

  3. An HPLC chromatographic framework to analyze the β-cyclodextrin/solute complexation mechanism using a carbon nanotube stationary phase.

    PubMed

    Aljhni, Rania; Andre, Claire; Lethier, Lydie; Guillaume, Yves Claude

    2015-11-01

    A carbon nanotube (CNT) stationary phase was used for the first time to study the β-cyclodextrin (β-CD) solute complexation mechanism using high performance liquid chromatography (HPLC). For this, the β-CD was added at various concentrations in the mobile phase and the effect of column temperature was studied on both the retention of a series of aniline and benzoic acid derivatives with the CNT stationary phase and their complexation mechanism with β-CD. A decrease in the solute retention factor was observed for all the studied molecules without change in the retention order. The apparent formation constant KF of the inclusion complex β-CD/solute was determined at various temperatures. Our results showed that the interaction of β-CD with both the mobile phase and the stationary phase interfered in the complex formation. The enthalpy and entropy of the complex formation (ΔHF and ΔSF) between the solute molecule and CD were determined using a thermodynamic approach. Negative enthalpies and entropies indicated that the inclusion process of the studied molecule in the CD cavity was enthalpically driven and that the hydrogen bonds between carboxylic or aniline groups and the functional groups on the β-CD rim play an important role in the complex formation. PMID:26452814

  4. Mass transfer mechanism in chiral reversed phase liquid chromatography.

    PubMed

    Gritti, Fabrice; Guiochon, Georges

    2014-03-01

    The mechanism of mass transfer in chiral chromatography was investigated using an experimental protocol already applied in RPLC and HILIC chromatography. The different contributions to the reduced height equivalent to a theoretical plate (HETP) include the longitudinal diffusion HETP term, the solid-liquid mass transfer resistance HETP term, the short-range eddy dispersion HETP term, and the long-range eddy dispersion HETP term. Their accurate measurement permits the determination of the adsorption rate constant kads of trans-stilbene enantiomers on a column packed with Lux 5 μm Cellulose-1 particles. The experimental results demonstrate that the number of adsorption-desorption steps per unit time of chiral compounds on polysaccharide-based chiral stationary phases is four orders of magnitude smaller than that of achiral compounds.

  5. Lag phase and hydrolysis mechanisms of triacylglycerol film lipolysis.

    PubMed

    Snabe, Torben; Petersen, Steffen Bjørn

    2003-09-01

    We here present novel insights into the dynamic changes of a nanosized lipid film during enzymatic degradation. When adding an aqueous solution containing a triacylglycerol lipase to an approximately 100nm thin triolein film, which is supported on a hard surface, the film thickness, elasticity, viscosity, and chemical composition were obtained continuously. Both a mechanical technique (quartz crystal microbalance with dissipation monitoring) and a spectroscopic technique (attenuated total reflection Fourier transform infrared spectroscopy) were utilised for this study. Detailed data revealed the effects of pH, Ca(2+), and catalytic rate on lipolysis, including product release from the film. It was found that under basic conditions and without Ca(2+), the lipolytic activity commence instantaneously upon addition of enzyme, whereas product release from the substrate film awaits conditions that favours release. A model for removal of degradation products from the film is introduced, including a novel interpretation of the lag phase phenomenon.

  6. Thermoseparating aqueous two-phase systems: Recent trends and mechanisms.

    PubMed

    Leong, Yoong Kit; Lan, John Chi-Wei; Loh, Hwei-San; Ling, Tau Chuan; Ooi, Chien Wei; Show, Pau Loke

    2016-02-01

    Having the benefits of being environmentally friendly, providing a mild environment for bioseparation, and scalability, aqueous two-phase systems (ATPSs) have increasingly caught the attention of industry and researchers for their application in the isolation and recovery of bioproducts. The limitations of conventional ATPSs give rise to the development of temperature-induced ATPSs that have distinctive thermoseparating properties and easy recyclability. This review starts with a brief introduction to thermoseparating ATPSs, including its history, unique characteristics and advantages, and lastly, key factors that influence partitioning. The underlying mechanism of temperature-induced ATPSs is covered together with a summary of recent applications. Thermoseparating ATPSs have been proven as a solution to the demand for economically favorable and environmentally friendly industrial-scale bioextraction and purification techniques. PMID:26447739

  7. Effect of annealing process on the phase formation in poly(vinylidene fluoride) thin films

    SciTech Connect

    Abdullah, Ibtisam Yahya; Yahaya, Muhammad; Jumali, Mohd Hafizuddin Haji; Shanshool, Haider Mohammed

    2014-09-03

    This work reports the initial study on the effect of annealing process on the crystalline phase of poly(vinylidene fluoride) (PVDF) thin film. PVDF powder was dissolved in N,N-dimethylformamide before spin-coated onto a glass substrate to form a film. The films were annealed at 30°C, 90°C and 110°C for 5 hrs. The crystalline phase of the powder PVDF as received was investigated by using XRD and FTIR techniques. Moreover, the crystalline phases of thin films after annealing were investigated by using the same techniques. XRD analysis showed that in powder form PVDF exists in α-phase. Each annealed PVDF thin films exhibited identical formation of three-phases material namely γ (as major phase) while α and β phases as the minor phases. The FTIR analysis showed that the powder form of PVDF exists in α and β phases. FTIR measurement further confirmed the XRD results implying that the annealing process has no significant effect on the phase formation in PVDF films.

  8. Effect of annealing process on the phase formation in poly(vinylidene fluoride) thin films

    NASA Astrophysics Data System (ADS)

    Abdullah, Ibtisam Yahya; Yahaya, Muhammad; Jumali, Mohd Hafizuddin Haji; Shanshool, Haider Mohammed

    2014-09-01

    This work reports the initial study on the effect of annealing process on the crystalline phase of poly(vinylidene fluoride) (PVDF) thin film. PVDF powder was dissolved in N,N-dimethylformamide before spin-coated onto a glass substrate to form a film. The films were annealed at 30°C, 90°C and 110°C for 5 hrs. The crystalline phase of the powder PVDF as received was investigated by using XRD and FTIR techniques. Moreover, the crystalline phases of thin films after annealing were investigated by using the same techniques. XRD analysis showed that in powder form PVDF exists in α-phase. Each annealed PVDF thin films exhibited identical formation of three-phases material namely γ (as major phase) while α and β phases as the minor phases. The FTIR analysis showed that the powder form of PVDF exists in α and β phases. FTIR measurement further confirmed the XRD results implying that the annealing process has no significant effect on the phase formation in PVDF films.

  9. The PLATO opto-mechanical unit prototyping and AIV phase

    NASA Astrophysics Data System (ADS)

    Farinato, Jacopo; Viotto, Valentina; Gentile, Giorgia; Dima, Marco; Magrin, Demetrio; Piazza, Daniele; Ragazzoni, Roberto; Piotto, Giampaolo; Pagano, Isabella; Arcidiacono, Carmelo; Basso, Stefano; Benz, Willy; Gambicorti, Lisa; Ghigo, Mauro; Munari, Matteo; Pace, Emanuele; Scuderi, Salvatore; Catala, Claude

    2010-07-01

    PLATO is the acronym of PLAnetary Transits and Oscillations of stars, and it is a mission proposed for the ESA Cosmic Vision program in the Medium size program, with the target to detect and characterize exoplanets by the means of their transit on a bright star. The instrumental overall layout proposed by the Plato Payload Consortium consists in a multitelescope concept instrument, composed by several tens of telescope units, for which we are developing an all refractive optical solution. These devices are characterized by a very large Field of View (more than 20 degrees on one side) with an optical quality that fits most of the energy into a single CCD pixel. Such a goal can be achieved in a variety of solutions, some including aspheric elements as well. A complete prototype of one telescope unit is foreseen to be built initially (during phase B1) to show the alignment feasibility and, only in a second moment (Phase B2), to perform full environmental and functional test. The aim of this article is to describe the alignment, integration and verification strategy of the opto-mechanics of the prototype. Both the approaches of testing the telescope at the target working temperature or to test it at ambient temperature around a displaced zero point, taking into account the effects of thermal deformations, are considered and briefly sketched in this work.

  10. The mechanical properties of phase separated protein droplets

    NASA Astrophysics Data System (ADS)

    Jawerth, Louise; Ijavi, Mahdiye; Patel, Avinash; Saha, Shambaditya; Jülicher, Frank; Hyman, Anthony

    In vivo, numerous proteins associate into liquid compartments by de-mixing from the surrounding solution, similar to oil molecules in water. Many of these proteins and their corresponding liquid compartments play a crucial role in important biological processes, for instance germ line specification in C. elegans or in neurodegenerative diseases such as Amyotrophic lateral sclerosis (ALS). However, despite their importance, very little is known about the physical properties of the resulting droplets as well as the physical mechanisms that control their phase separation from solution. To gain a deeper understanding of these aspects, we study a few such proteins in vitro. When these proteins are purified and added to a physiological buffer, they phase separate into droplets ranging in size from a few to tens of microns with liquid-like behavior similar to their physiological counterparts. By attaching small beads to the surface of the droplets, we can deform the droplets by manipulating the beads directly using optical tweezers. By measuring the force required to deform the droplets we determine their surface tension, elasticity and viscosity as well as the frequency response of these properties. We also measure these properties using passive micro-rheology.

  11. A new mechanism for the formation of regolith on asteroids

    NASA Astrophysics Data System (ADS)

    Delbo, M.; Libourel, G.; Wilkerson, J.; Murdoch, N.; Michel, P.; Ramesh, K. T.; Ganino, C.; Verati, C.; Marchi, S.

    2014-12-01

    The soil of asteroids, like that of the Moon, Mars, and other rocky bodies in the Solar System, is made of a layer of pebbles, sand, and dust called regolith. Previous studies suggested that the regolith of asteroids is made from material ejected from impacts and re-accumulated on the surface and from boulders that are comminuted by micrometeoroid impacts. However, this classical scenario of regolith formation has problems to explain the regolith on km-sized and smaller asteroids: laboratory experiments and impact modelling have shown that the impact fragments can reach escape velocities and breaks free from the gravitational pull of these small asteroids, indicating the impact mechanism is not the dominant process for regolith creation. Other studies also reveal that there is too much regolith on small asteroids' surfaces to have been deposited there solely through impacts over the millions of years of asteroids' evolution. We discovered that another process is capable of gently breaking rocks at the surface of asteroids: thermal fatigue by temperature cycling. As asteroids spin about their rotation axes, their surfaces plunge in and out of shadow resulting in large surface temperature variations. The rapid heating and cooling creates thermal expansion and contraction in the asteroid material, initiating cracking and propagating existing cracks. As the process is repeated over and over, the crack damage increases with time, leading eventually to rock fragmentation (and production of new regolith). To study this process, in the laboratory, we subjected meteorites, used as asteroid material analogs, to 37 days of thermal cycles similar to those occurring on asteroids. We measured cracks widening at an average rate of 0.5 mm/y. Some fragments were also produced, indicating meteorite fragmentation. To scale our results to asteroid lifetime, we incorporated our measurements into a fracture model and we deduced that thermal cycling is more efficient than micrometeorite

  12. The mechanism of vapor phase hydration of calcium oxide: implications for CO2 capture.

    PubMed

    Kudłacz, Krzysztof; Rodriguez-Navarro, Carlos

    2014-10-21

    Lime-based sorbents are used for fuel- and flue-gas capture, thereby representing an economic and effective way to reduce CO2 emissions. Their use involves cyclic carbonation/calcination which results in a significant conversion reduction with increasing number of cycles. To reactivate spent CaO, vapor phase hydration is typically performed. However, little is known about the ultimate mechanism of such a hydration process. Here, we show that the vapor phase hydration of CaO formed after calcination of calcite (CaCO3) single crystals is a pseudomorphic, topotactic process, which progresses via an intermediate disordered phase prior to the final formation of oriented Ca(OH)2 nanocrystals. The strong structural control during this solid-state phase transition implies that the microstructural features of the CaO parent phase predetermine the final structural and physicochemical (reactivity and attrition) features of the product hydroxide. The higher molar volume of the product can create an impervious shell around unreacted CaO, thereby limiting the efficiency of the reactivation process. However, in the case of compact, sintered CaO structures, volume expansion cannot be accommodated in the reduced pore volume, and stress generation leads to pervasive cracking. This favors complete hydration but also detrimental attrition. Implications of these results in carbon capture and storage (CCS) are discussed.

  13. New Pathways for the Formation of Complex Organics and Prebiotic Synthesis in the Gas Phase

    NASA Astrophysics Data System (ADS)

    El-Shall, M. S.

    2010-04-01

    We study the formation mechanisms of complex organics that are present in interstellar clouds. The reaction of acetylene ion with water produces vinyl alcohol while the reaction of benzene ion with acetylene produces naphthalene-type ion.

  14. Reversible work of formation of an embryo of a new phase within a uniform macroscopic mother phase

    SciTech Connect

    Debenedetti, P.G.; Reiss, H. |

    1998-04-01

    A thermodynamically consistent formalism is derived for calculating the reversible work needed to form a small amount of a new phase (embryo) within a uniform macroscopic mother phase. The treatment goes beyond the classic work of Gibbs, who solved the problem for the particular case in which the embryo is in equilibrium with the mother phase, constituting a so-called critical nucleus. The formalism results in a new expression for the reversible work of embryo formation, the extrema conditions for which yield the correct conditions of equilibrium between the critical nucleus and the mother phase, as well as Gibbs{close_quote} result for the reversible work needed to form the critical nucleus. The new expression for the work of embryo formation differs from the one commonly used in the nucleation literature. In order to extend the Gibbsian formalism to noncritical nuclei, it is necessary to introduce a constraint that prevents the free transfer of matter between the embryo and the mother phase. The present approach is valid in the limit in which curvature contributions to the interfacial energy can be neglected. {copyright} {ital 1998 American Institute of Physics.}

  15. Phase Transformations in CuAu: Morphologies and Kinetics from Quantum Mechanics

    NASA Astrophysics Data System (ADS)

    Elder, Ken; Chakraborty, Bulbul; Goldenfeld, Nigel

    1996-03-01

    The existence of a modulated superlattice at intermediate temperatures in CuAu gives rise to a rich and complex set of phase transformations. For example, quenches from high and low temperatures into the modulated region leads respectively to the nucleation of labyrinth and bullseye patterns. In this alloy even the formation of the ordered superlattice from a disordered phase is dramatically different than that which occurs in standard disorder/order transformations. This transformation is strongly influenced by the presence of small metastable droplets that are the size of the modulated wavelength. A theoretical description(Bulbul Chakraborty, Ken Elder and Nigel Goldenfeld, Physica A, in press (1995)) of these kinetics and morphologies will be presented and compared to experiment. This description is based on a time-dependent Ginzburg-Landau equation that was derived from a quantum mechanical Hamiltonian using the embedded atom method(Bulbul Chakraborty and Zhigang Xi, Phys. Rev. Lett 68), 2039 (1992).

  16. On the electrostrictive mechanism of nanosecond-pulsed breakdown in liquid phase

    NASA Astrophysics Data System (ADS)

    Seepersad, Yohan; Pekker, Mikhail; Shneider, Mikhail N.; Dobrynin, Danil; Fridman, Alexander

    2013-04-01

    In this study we have studied the initial stage of the nanosecond-pulsed discharge development in liquid phase. Modelling predicts that in the case of fast rising strong nonhomogeneous electric fields in the vicinity of high-voltage pin electrode a region saturated with nanoscale non-uniformities may be developed. This phenomenon is attributed to the electrostriction mechanisms and may be used to explain development of breakdown in liquid phase. In this work, schlieren method was used in order to demonstrate formation of negative pressure region in liquids with different dielectric permittivity constants: water, ethanol and ethanol-water mixture. It is shown that this density perturbation, formed at the raising edge of the high-voltage pulse, is followed by a generation of a shock wave propagating with the speed of sound away from the electrode, with negative pressure behind it.

  17. Mechanical properties of irradiated multi-phase polycrystalline BCC materials

    NASA Astrophysics Data System (ADS)

    Song, Dingkun; Xiao, Xiazi; Xue, Jianming; Chu, Haijian; Duan, Huiling

    2015-04-01

    Structure materials under severe irradiations in nuclear environments are known to degrade because of irradiation hardening and loss of ductility, resulting from irradiation-induced defects such as vacancies, interstitials and dislocation loops, etc. In this paper, we develop an elastic-viscoplastic model for irradiated multi-phase polycrystalline BCC materials in which the mechanical behaviors of individual grains and polycrystalline aggregates are both explored. At the microscopic grain scale, we use the internal variable model and propose a new tensorial damage descriptor to represent the geometry character of the defect loop, which facilitates the analysis of the defect loop evolutions and dislocation-defect interactions. At the macroscopic polycrystal scale, the self-consistent scheme is extended to consider the multiphase problem and used to bridge the individual grain behavior to polycrystal properties. Based on the proposed model, we found that the work-hardening coefficient decreases with the increase of irradiation-induced defect loops, and the orientation/loading dependence of mechanical properties is mainly attributed to the different Schmid factors. At the polycrystalline scale, numerical results for pure Fe match well with the irradiation experiment data. The model is further extended to predict the hardening effect of dispersoids in oxide-dispersed strengthened steels by the considering the Orowan bowing. The influences of grain size and irradiation are found to compete to dominate the strengthening behaviors of materials.

  18. EXPERIMENTAL AND COMPUTATIONAL STUDIES OF THE FORMATION MECHANISM OF PROTONATED INTERSTELLAR DIAZINES

    SciTech Connect

    Wang, Zhe-Chen; Cole, Callie A.; Bierbaum, Veronica M.; Snow, Theodore P.

    2015-01-10

    Studies of interstellar chemistry have grown in number and complexity by both observations and laboratory measurements, and nitrogen-containing aromatics have been implicated as important interstellar molecules. In this paper, the gas-phase collision induced dissociation (CID) processes of protonated pyridazine (1,2-diazine), pyrimidine (1,3-diazine), and pyrazine (1,4-diazine) cations (C{sub 4}H{sub 5}N{sub 2} {sup +}) are investigated in detail both experimentally and theoretically. The major neutral loss for all three CID processes is HCN, leading to the formation of C{sub 3}H{sub 4}N{sup +} isomers; our density functional theory (DFT) calculations support and elucidate our experimental results. The formation of C{sub 3}H{sub 4}N{sup +} isomers from the reaction of abundant interstellar acrylonitrile (CH{sub 2}CHCN) and H{sup +}is also studied employing DFT calculations. Our results lead to a novel mechanism for interstellar protonated diazine formation from the consecutive reactions of CH{sub 2}CHCN+ H{sup +} + HCN. Moreover, our results motivate the continuing search for interstellar C{sub 3}H{sub 4}N{sup +} isomers as well as polycyclic aromatic N-containing hydrocarbons (PANHs)

  19. Modelling non-equilibrium secondary organic aerosol formation and evaporation with the aerosol dynamics, gas- and particle-phase chemistry kinetic multilayer model ADCHAM

    SciTech Connect

    Roldin, P.; Eriksson, A. C.; Nordin, E. Z.; Hermansson, E.; Mogensen, Ditte; Rusanen, A.; Boy, Michael; Swietlicki, E.; Svenningsson, Birgitta; Zelenyuk, Alla; Pagels, J.

    2014-08-11

    We have developed the novel Aerosol Dynamics, gas- and particle- phase chemistry model for laboratory CHAMber studies (ADCHAM). The model combines the detailed gas phase Master Chemical Mechanism version 3.2, an aerosol dynamics and particle phase chemistry module (which considers acid catalysed oligomerization, heterogeneous oxidation reactions in the particle phase and non-ideal interactions between organic compounds, water and inorganic ions) and a kinetic multilayer module for diffusion limited transport of compounds between the gas phase, particle surface and particle bulk phase. In this article we describe and use ADCHAM to study: 1) the mass transfer limited uptake of ammonia (NH3) and formation of organic salts between ammonium (NH4+) and carboxylic acids (RCOOH), 2) the slow and almost particle size independent evaporation of α-pinene secondary organic aerosol (SOA) particles, and 3) the influence of chamber wall effects on the observed SOA formation in smog chambers.

  20. Formation of prismatic loops from C15 Laves phase interstitial clusters in body-centered cubic iron

    SciTech Connect

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

    2015-03-01

    This Letter reports the transition of C15 phase self-interstitial clusters to loops in body-centered-cubic Iron. Molecular dynamics simulations are performed to evaluate the relative stabilities of difference interstitial cluster configurations including C15 phase structure and <100> and <111>/2 loops. Within a certain size range, C15 cluster are found more stable than loops, and the relative stabilities are reversed beyond that range. In accordance to the crossover in relative stabilities, C15 clusters may grow by absorbing individual interstitials at small sizes and transitions into loops eventually. The transition takes place by nucleation and reaction of <111>/2 loop segments. These observations explain the absence of C15 phase interstitial clusters predicted by density-functional-theory calculations in previous experimental observations. More importantly, the current results provide a new formation mechanism of <100> loops which requires no interaction of loops.

  1. Microscopic mechanism of nanocrystal formation from solution by cluster aggregation and coalescence

    PubMed Central

    Hassan, Sergio A.

    2011-01-01

    Solute-cluster aggregation and particle fusion have recently been suggested as alternative routes to the classical mechanism of nucleation from solution. The role of both processes in the crystallization of an aqueous electrolyte under controlled salt addition is here elucidated by molecular dynamics simulation. The time scale of the simulation allows direct observation of the entire crystallization pathway, from early events in the prenucleation stage to the formation of a nanocrystal in equilibrium with concentrated solution. The precursor originates in a small amorphous aggregate stabilized by hydration forces. The core of the nucleus becomes crystalline over time and grows by coalescence of the amorphous phase deposited at the surface. Imperfections of ion packing during coalescence promote growth of two conjoint crystallites. A parameter of order and calculated cohesive energies reflect the increasing crystalline order and stress relief at the grain boundary. Cluster aggregation plays a major role both in the formation of the nucleus and in the early stages of postnucleation growth. The mechanism identified shares common features with nucleation of solids from the melt and of liquid droplets from the vapor. PMID:21428633

  2. The Role of Possible Feedback Mechanisms in the Effects of Altered Gravity on Formation and Function of Gravireceptors of Mollusks and Fish

    NASA Technical Reports Server (NTRS)

    Kondrachuk, Alexander V.; Boyle, Richard D.

    2005-01-01

    The variety of the effects of altered gravity (AG) on development and function of gravireceptors cannot be explained by simple feedback mechanism that correlates gravity level and weight of test mass. The reaction of organisms to the change of gravity depends on the phase of their development. To predict this reaction we need to know the details of the mechanisms of gravireceptor formation

  3. Phase formation and texture of thin nickel germanides on Ge(001) and Ge(111)

    NASA Astrophysics Data System (ADS)

    De Schutter, B.; Van Stiphout, K.; Santos, N. M.; Bladt, E.; Jordan-Sweet, J.; Bals, S.; Lavoie, C.; Comrie, C. M.; Vantomme, A.; Detavernier, C.

    2016-04-01

    We studied the solid-phase reaction between a thin Ni film and a single crystal Ge(001) or Ge(111) substrate during a ramp anneal. The phase formation sequence was determined using in situ X-ray diffraction and in situ Rutherford backscattering spectrometry (RBS), while the nature and the texture of the phases were studied using X-ray pole figures and transmission electron microscopy. The phase sequence is characterized by the formation of a single transient phase before NiGe forms as the final and stable phase. X-ray pole figures were used to unambiguously identify the transient phase as the ɛ-phase, a non-stoichiometric Ni-rich germanide with a hexagonal crystal structure that can exist for Ge concentrations between 34% and 48% and which forms with a different epitaxial texture on both substrate orientations. The complementary information gained from both RBS and X-ray pole figure measurements revealed a simultaneous growth of both the ɛ-phase and NiGe over a small temperature window on both substrate orientations.

  4. Mechanisms of formation of chromosomal aberrations: insights from studies with DNA repair-deficient cells.

    PubMed

    Palitti, F

    2004-01-01

    In order to understand the mechanisms of formation of chromosomal aberrations, studies performed on human syndromes with genomic instability can be fruitful. In this report, the results from studies in our laboratory on the importance of the transcription-coupled repair (TCR) pathway on the induction of chromosomal damage and apoptosis by ultraviolet light (UV) are discussed. UV61 cells (hamster homologue of human Cockayne's syndrome group B) deficient in TCR showed a dramatic increase in the induction of chromosomal aberrations and apoptosis following UV treatment. At relatively low UV doses, the induction of chromosomal aberrations preceded the apoptotic process. Chromosomal aberrations probably lead to apoptosis and most of the cells had gone through an S phase after the UV treatment before entering apoptosis. At higher doses of UV, the cells could go into apoptosis already in the G1 phase of the cell cycle. Abolition of TCR by treatment with alpha-amanitin (an inhibitor of RNA polymerase II) in the parental cell line AA8 also resulted in the induction of elevated chromosomal damage and apoptotic response similar to the one observed in UV61 cells treated with UV alone. This suggests that the lack of TCR is responsible for the increased frequencies of chromosomal aberrations and apoptosis in UV61 cells. Hypersensitivity to the induction of chromosomal damage by inhibitors of antitopoisomerases I and II in Werner's syndrome cells is also discussed in relation to the compromised G2 phase processes involving the Werner protein. PMID:15162020

  5. Formation Mechanism of Titanium Carbide Crystal in Laser Synthesized Metal-Ceramic Composite Coating

    NASA Astrophysics Data System (ADS)

    Du, Baoshuai; Zhang, Zhongwen; Wang, Xinhong; Zou, Zengda

    In situ titanium carbide reinforced iron-based composite coating was deposited on mild carbon steel using laser surface engineering (LSE) with ferrotitanium and graphite as precursor. The microstructure and phase constituents of the deposited coating were characterized. Formation mechanism of titanium carbide crystal in the composite coating was elucidated by correlating the morphology of titanium carbide and the thermal cycle experienced by the precursor during the laser treatment. It was demonstrated that titanium carbide was formed in situ as a result of the metallurgical reaction between ferrotitanium and graphite following a liquid-precipitation route. Different morphologies of titanium carbide crystal (dendrite and fishbone) correspond to the primary and eutectic titanium carbide respectively.

  6. The Mechanism of First Raindrops Formation in Deep Convective Clouds

    SciTech Connect

    Khain, Alexander; Prabha, Thara; Benmoshe, Nir; Pandithurai, G.; Ovchinnikov, Mikhail

    2013-08-22

    The formation of first raindrops in deep convective clouds is investigated. A combination of observational data analysis and 2-D and 3-D numerical bin microphysical simulations of deep convective clouds suggests that the first raindrops form at the top of undiluted or slightly diluted cores. It is shown that droplet size distributions in these regions are wider and contain more large droplets than in diluted volumes. The results of the study indicate that the initial raindrop formation is determined by the basic microphysical processes within ascending adiabatic volumes. It allows one to predict the height of the formation of first raindrops considering the processes of nucleation, diffusion growth and collisions. The results obtained in the study explain observational results reported by Freud and Rosenfeld (2012) according to which the height of first raindrop formation depends linearly on the droplet number concentration at cloud base. The results also explain why a simple adiabatic parcel model can reproduce this dependence. The present study provides a physical basis for retrieval algorithms of cloud microphysical properties and aerosol properties using satellites proposed by Rosenfeld et al. ( 2012). The study indicates that the role of mixing and entrainment in the formation of the first raindrops is not of crucial importance. It is also shown that low variability of effective and mean volume radii along horizontal traverses, as regularly observed by in situ measurements, can be simulated by high-resolution cloud models, in which mixing is parameterized by a traditional 1.5 order turbulence closure scheme.

  7. Simulating the SOA formation of isoprene from partitioning and aerosol phase reactions in the presence of inorganics

    NASA Astrophysics Data System (ADS)

    Beardsley, Ross L.; Jang, Myoseon

    2016-05-01

    The secondary organic aerosol (SOA) produced by the photooxidation of isoprene with and without inorganic seed is simulated using the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model. Recent work has found the SOA formation of isoprene to be sensitive to both aerosol acidity ([H+], mol L-1) and aerosol liquid water content (LWC) with the presence of either leading to significant aerosol phase organic mass generation and large growth in SOA yields (YSOA). Classical partitioning models alone are insufficient to predict isoprene SOA formation due to the high volatility of photooxidation products and sensitivity of their mass yields to variations in inorganic aerosol composition. UNIPAR utilizes the chemical structures provided by a near-explicit chemical mechanism to estimate the thermodynamic properties of the gas phase products, which are lumped based on their calculated vapor pressure (eight groups) and aerosol phase reactivity (six groups). UNIPAR then determines the SOA formation of each lumping group from both partitioning and aerosol phase reactions (oligomerization, acid-catalyzed reactions and organosulfate formation) assuming a single homogeneously mixed organic-inorganic phase as a function of inorganic composition and VOC / NOx (VOC - volatile organic compound). The model is validated using isoprene photooxidation experiments performed in the dual, outdoor University of Florida Atmospheric PHotochemical Outdoor Reactor (UF APHOR) chambers. UNIPAR is able to predict the experimental SOA formation of isoprene without seed, with H2SO4 seed gradually titrated by ammonia, and with the acidic seed generated by SO2 oxidation. Oligomeric mass is predicted to account for more than 65 % of the total organic mass formed in all cases and over 85 % in the presence of strongly acidic seed. The model is run to determine the sensitivity of YSOA to [H+], LWC and VOC / NOx, and it is determined that the SOA formation of isoprene is most strongly related to [H

  8. Simulating the SOA formation of isoprene from partitioning and aerosol phase reactions in the presence of inorganics

    NASA Astrophysics Data System (ADS)

    Beardsley, R. L.; Jang, M.

    2015-11-01

    The secondary organic aerosol (SOA) produced by the photooxidation of isoprene with and without inorganic seed is simulated using the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model. Recent work has found the SOA formation of isoprene to be sensitive to both aerosol acidity ([H+]) and aerosol liquid water content (LWC) with the presence of either leading to significant aerosol phase organic mass generation and large growth in SOA yields (YSOA). Classical partitioning models alone are insufficient to predict isoprene SOA formation due to the high volatility of the photooxidation products and the sensitivity of their mass yields to variations in inorganic aerosol composition. UNIPAR utilizes the chemical structures provided by a near-explicit chemical mechanism to estimate the thermodynamic properties of the gas phase products, which are lumped based on their calculated vapor pressure (8 groups) and aerosol phase reactivity (6 groups). UNIPAR then determines the SOA formation of each lumping group from both partitioning and aerosol phase reactions (oligomerization, acid catalyzed reactions, and organosulfate formation) assuming a single homogeneously mixed organic-inorganic phase as a function of inorganic composition and VOC / NOx. The model is validated using isoprene photooxidation experiments performed in the dual, outdoor UF APHOR chambers. UNIPAR is able to predict the experimental SOA formation of isoprene without seed, with H2SO4 seed gradually titrated by ammonia, and with the acidic seed generated by SO2 oxidation. Oligomeric mass is predicted to account for more than 65 % of the total OM formed in all cases and over 85 % in the presence of strongly acidic seed. The model is run to determine the sensitivity of YSOA to [H+], LWC, and VOC / NOx, and it is determined that the SOA formation of isoprene is most strongly related to [H+] but is dynamically related to all three parameters. For VOC / NOx > 10, with increasing NOx both experimental and

  9. What is the mechanism of formation of hydroxyaluminosilicates?

    PubMed Central

    Beardmore, James; Lopez, Xabier; Mujika, Jon I.; Exley, Christopher

    2016-01-01

    The formation of hydroxyaluminosilicates is integral to the biogeochemical cycles of aluminium and silicon. The unique inorganic chemistry which underlies their formation explains the non-essentiality in biota of both of these elements. However, the first steps in the formation of hydroxyaluminosilicates were hitherto only theoretical and plausibly only accessible in silico. Herein we have used computational chemistry to identify and define for the first time these unique and ultimately critically important reaction steps. We have used density-functional theory combined with solvent continuum models to confirm first, the nature of the reactants, an aluminium hydroxide dimer and silicic acid, second, the reaction products, two distinct hydroxyaluminosilicates A and B and finally, how these are the precursors to highly insoluble hydroxyaluminosilicates the role of which has been and continues to be to keep inimical aluminium out of biota. PMID:27477995

  10. What is the mechanism of formation of hydroxyaluminosilicates?

    NASA Astrophysics Data System (ADS)

    Beardmore, James; Lopez, Xabier; Mujika, Jon I.; Exley, Christopher

    2016-08-01

    The formation of hydroxyaluminosilicates is integral to the biogeochemical cycles of aluminium and silicon. The unique inorganic chemistry which underlies their formation explains the non-essentiality in biota of both of these elements. However, the first steps in the formation of hydroxyaluminosilicates were hitherto only theoretical and plausibly only accessible in silico. Herein we have used computational chemistry to identify and define for the first time these unique and ultimately critically important reaction steps. We have used density-functional theory combined with solvent continuum models to confirm first, the nature of the reactants, an aluminium hydroxide dimer and silicic acid, second, the reaction products, two distinct hydroxyaluminosilicates A and B and finally, how these are the precursors to highly insoluble hydroxyaluminosilicates the role of which has been and continues to be to keep inimical aluminium out of biota.

  11. The formation, structure, and hydrogen storage properties of titanium/zironcium/hafnium based quasicrystals and related phases

    NASA Astrophysics Data System (ADS)

    Huett, Van Thi

    Previous studies of hydrogen storage in Ti-Zr-Ni quasicrystals are reviewed and expanded. Based on the new results presented in this thesis hydrogen may be used as a probe of local structure if elastic and electronic interactions of the hydrogen atom with the lattice and other hydrogen atoms in the lattice are better understood. As-cast and as-quenched phases found in the Ti-Hf-Ni ternary system are surveyed. A high order (3/2) rational approximant to the quasicrystal and a metallic glass were some of the phases found in the as-quenched survey. These two phases were further studied showing that the 3/2 rational approximant is metastable, transforming to a stable Ti2Ni-type phase upon heating. The metallic glass found in this system has a 65°C separation between the glass transition and the first crystallization event to a nano-domained icosahedrally ordered phase. Hydrogenation studies of the 3/2 rational approximant and the metallic glass in Ti-Hf-Ni were also performed. These studies show that the structures around the hydrogen atoms are similar in both phases. The structures are also similar to the Ti-Zr-Ni quasicrystals. Other hydrogen storage methods were explored. Mechanically alloyed Ti-Zr-Ni quasicrystals and metallic glasses have a potential to be used as hydrogen storage materials. These mechanically alloyed phases do not need to have their surfaces prepared or activated for hydrogenation to occur, and show little formation of the irreversible hydride phase. Pressure composition isotherms at higher pressures than have been performed before on these alloys were examined. This required a new high-pressure Sievart's type apparatus to be built. The initial results from pressure composition isotherm measurements show that there is another pressure plateau, above the one previously found at low pressures. This higher pressure plateau is a region of pressure and temperature suitable for hydrogen storage applications.

  12. The role of fluid flow and intermetallic phases in the formation of the primary Al-phase in AlSi alloys

    NASA Astrophysics Data System (ADS)

    Mikołajczak, P.; Ratke, L.

    2012-01-01

    In secondary AlSi alloys, the presence of small amounts of Fe causes the formation of intermetallic phases, which have a negative effect on mechanical and physical properties of castings. To understand the effect of fluid flow on the microstructure and intermetallic phases, Al-5/7/9 wt pet Si 0.2/0.5/1.0 wt pet Fe alloys have been directionally solidified under defined thermal (gradient 3 K/mm, solidification velocity 0.04 mm/s) and fluid flow (rotating magnetic field 6 mT) conditions. The primary α-Al phase and intermetallic phases were studied using light microscopy and SEM with EDX. The influence of fluid flow and intermetallic phases (β-Al5FeSi) on microstructure was characterized by changes of primary and secondary dendrite arm spacing and specific surface area of the dendrites. We observe a pronounced effect of flow on the length of the intermetallic precipitates, a macro-segregation Fe and Si and even small amounts of iron and thus intermetallics reduce possible effects of flow on microstructural parameters.

  13. High-temperature- and high-pressure-induced formation of the Laves-phase compound XeS2

    NASA Astrophysics Data System (ADS)

    Yan, Xiaozhen; Chen, Yangmei; Xiang, Shikai; Kuang, Xiaoyu; Bi, Yan; Chen, Haiyan

    2016-06-01

    We explore the reactivity of xenon with sulfur under high pressure, using unbiased structure searching techniques combined with first-principles calculations, which identify a stable XeS2 compound crystallized in a Laves phase with hypercoordinated (16-fold) Xe at 191 GPa and 0 K. Taking the thermal effects into account, we find that increasing the temperature could further stabilize it. The formation of XeS2 is a consequence of pressure-induced charge transfer from Xe to S atoms and the delocalization of Xe 5 p and S 3 p electrons. Meanwhile, the stabilization into a Laves phase of XeS2 is the result of delocalized chemical bonding and the need for optimum structure packing. The present discussion of the formation mechanism in XeS2 is general, and conclusions can be used to understand the formation of other Laves-phase compounds and the Xe chemistry that allows closed-shell Xe to participate in chemical reactions.

  14. Identification of the formation phases of filamentary damage induced by nanosecond laser pulses in bulk fused silica

    SciTech Connect

    Shen, Chao; Xu, Zhongjie; Chambonneau, Maxime E-mail: jiangtian198611@163.com; Cheng, Xiang'ai; Jiang, Tian E-mail: jiangtian198611@163.com

    2015-09-14

    Employing a pump-probe polarization-based two-frame shadowgraphy setup, the formation of filamentary damage induced in bulk fused silica by a nanosecond pulse at 1064 nm is investigated with a picosecond probe. Three different phases are exhibited in the damage experiments. The first phase is the formation of a micrometric plasma channel along the laser direction during the beginning of the pulse likely caused by multi-photon ionization. This channel exhibits growth during ∼400 ps, and the newly grown plasma is discrete. Then, during the end of the pulse, this channel evolves into a tadpole-like morphology showing an elliptical head upstream the laser flux followed by a thin tail. This observed asymmetry is attributed to shielding effects caused by both the plasma and hot modified silica. Once the damage shows its almost final morphology, a last phase consists in the launch of a pressure wave enlarging it after the laser pulse. The physical mechanisms that might be involved in the formation of plasma channels are discussed. The experimental data are first confronted to the moving breakdown model which overestimates the filamentary damage length. Finally, taking into account the temporal shape of the laser pulses, the coupling between Kerr-induced self-focusing and stimulated Brillouin scattering is discussed to interpret the observations.

  15. Development of theory-based health messages: three-phase programme of formative research.

    PubMed

    Epton, Tracy; Norman, Paul; Harris, Peter; Webb, Thomas; Snowsill, F Alexandra; Sheeran, Paschal

    2015-09-01

    Online health behaviour interventions have great potential but their effectiveness may be hindered by a lack of formative and theoretical work. This paper describes the process of formative research to develop theoretically and empirically based health messages that are culturally relevant and can be used in an online intervention to promote healthy lifestyle behaviours among new university students. Drawing on the Theory of Planned Behaviour, a three-phase programme of formative research was conducted with prospective and current undergraduate students to identify (i) modal salient beliefs (the most commonly held beliefs) about fruit and vegetable intake, physical activity, binge drinking and smoking, (ii) which beliefs predicted intentions/behaviour and (iii) reasons underlying each of the beliefs that could be targeted in health messages. Phase 1, conducted with 96 pre-university college students, elicited 56 beliefs about the behaviours. Phase 2, conducted with 3026 incoming university students, identified 32 of these beliefs that predicted intentions/behaviour. Phase 3, conducted with 627 current university students, elicited 102 reasons underlying the 32 beliefs to be used to construct health messages to bolster or challenge these beliefs. The three-phase programme of formative research provides researchers with an example of how to develop health messages with a strong theoretical- and empirical base for use in health behaviour change interventions. PMID:24504361

  16. Development of theory-based health messages: three-phase programme of formative research.

    PubMed

    Epton, Tracy; Norman, Paul; Harris, Peter; Webb, Thomas; Snowsill, F Alexandra; Sheeran, Paschal

    2015-09-01

    Online health behaviour interventions have great potential but their effectiveness may be hindered by a lack of formative and theoretical work. This paper describes the process of formative research to develop theoretically and empirically based health messages that are culturally relevant and can be used in an online intervention to promote healthy lifestyle behaviours among new university students. Drawing on the Theory of Planned Behaviour, a three-phase programme of formative research was conducted with prospective and current undergraduate students to identify (i) modal salient beliefs (the most commonly held beliefs) about fruit and vegetable intake, physical activity, binge drinking and smoking, (ii) which beliefs predicted intentions/behaviour and (iii) reasons underlying each of the beliefs that could be targeted in health messages. Phase 1, conducted with 96 pre-university college students, elicited 56 beliefs about the behaviours. Phase 2, conducted with 3026 incoming university students, identified 32 of these beliefs that predicted intentions/behaviour. Phase 3, conducted with 627 current university students, elicited 102 reasons underlying the 32 beliefs to be used to construct health messages to bolster or challenge these beliefs. The three-phase programme of formative research provides researchers with an example of how to develop health messages with a strong theoretical- and empirical base for use in health behaviour change interventions.

  17. Development of theory-based health messages: three-phase programme of formative research

    PubMed Central

    Epton, Tracy; Norman, Paul; Harris, Peter; Webb, Thomas; Snowsill, F. Alexandra; Sheeran, Paschal

    2015-01-01

    Online health behaviour interventions have great potential but their effectiveness may be hindered by a lack of formative and theoretical work. This paper describes the process of formative research to develop theoretically and empirically based health messages that are culturally relevant and can be used in an online intervention to promote healthy lifestyle behaviours among new university students. Drawing on the Theory of Planned Behaviour, a three-phase programme of formative research was conducted with prospective and current undergraduate students to identify (i) modal salient beliefs (the most commonly held beliefs) about fruit and vegetable intake, physical activity, binge drinking and smoking, (ii) which beliefs predicted intentions/behaviour and (iii) reasons underlying each of the beliefs that could be targeted in health messages. Phase 1, conducted with 96 pre-university college students, elicited 56 beliefs about the behaviours. Phase 2, conducted with 3026 incoming university students, identified 32 of these beliefs that predicted intentions/behaviour. Phase 3, conducted with 627 current university students, elicited 102 reasons underlying the 32 beliefs to be used to construct health messages to bolster or challenge these beliefs. The three-phase programme of formative research provides researchers with an example of how to develop health messages with a strong theoretical- and empirical base for use in health behaviour change interventions. PMID:24504361

  18. [HYPOTHESIS OF FORMATION DURING THE EVOLUTION OF MECHANISM OF SENESCENCE].

    PubMed

    Makrushin, A V

    2015-01-01

    A hypothesis is in the evolution origin of the mechanism of senescence--is consequence of growth of individual integrity and thus loss ability to asexual reproduction. Evolutionary precursor of senescence mechanism was probably morphogenetic adaptation of potentially immortal precambrian Metazoa with which they adapted to the changing environment. PMID:26390606

  19. MECHANISMS OF NITROUS OXIDE FORMATION IN COAL FLAMES

    EPA Science Inventory

    The paper gives results of a study, using both detailed kinetic modeling and plug-flow simulator experiments, to investigate an unknown mechanism by which N2O is formed in coal flames. This mechanism has considerable importance in determining the influence of common and advanced ...

  20. Microstructure and Mechanical Properties of Laves Phase-strengthened Fe-Cr-Zr Alloys

    DOE PAGESBeta

    Tan, Lizhen; Yang, Ying

    2014-12-05

    Laves phase-reinforced alloys have shown some preliminary promising performance at room temperatures. This paper aims at evaluating mechanical properties of Laves phase-strengthened alloys at elevated temperatures. Three Fe-Cr-Zr alloys were designed to favor the formation of eutectic microstructures containing Laves and body-centered cubic phases with the aid of thermodynamic calculations. Microstructural characterization was carried out on the alloys in as-processed and aged states using optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The effect of thermal aging and alloy composition on microstructure has been discussed based on microstructural characterization results. Mechanical properties have been evaluated by meansmore » of Vickers microhardness measurements, tensile testing at temperatures up to 973.15 K (700.15 °C), and creep testing at 873.15 K (600.15 °C) and 260 MPa. Alloys close to the eutectic composition show significantly superior strength and creep resistance compared to P92. Finally, however, their low tensile ductility may limit their applications at relatively low temperatures.« less

  1. Microstructure and Mechanical Properties of Laves Phase-strengthened Fe-Cr-Zr Alloys

    SciTech Connect

    Tan, Lizhen; Yang, Ying

    2014-12-05

    Laves phase-reinforced alloys have shown some preliminary promising performance at room temperatures. This paper aims at evaluating mechanical properties of Laves phase-strengthened alloys at elevated temperatures. Three Fe-Cr-Zr alloys were designed to favor the formation of eutectic microstructures containing Laves and body-centered cubic phases with the aid of thermodynamic calculations. Microstructural characterization was carried out on the alloys in as-processed and aged states using optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The effect of thermal aging and alloy composition on microstructure has been discussed based on microstructural characterization results. Mechanical properties have been evaluated by means of Vickers microhardness measurements, tensile testing at temperatures up to 973.15 K (700.15 °C), and creep testing at 873.15 K (600.15 °C) and 260 MPa. Alloys close to the eutectic composition show significantly superior strength and creep resistance compared to P92. Finally, however, their low tensile ductility may limit their applications at relatively low temperatures.

  2. Mechanisms of vascular dysfunction in acute phase of Trypanosoma cruzi infection in mice.

    PubMed

    Silva, Josiane F; Capettini, Luciano S A; da Silva, José F P; Sales-Junior, Policarpo; Cruz, Jader Santos; Cortes, Steyner F; Lemos, Virginia S

    2016-07-01

    Vascular disorders have a direct link to mortality in the acute phase of Trypanosoma cruzi infection. However, the underlying mechanisms of vascular dysfunction in this phase are largely unknown. We hypothesize that T. cruzi invades endothelial cells causing dysfunction in contractility and relaxation of the mouse aorta. Immunodetection of T. cruzi antigen TcRBP28 was observed in endothelial cells. There was a decreased endothelial nitric oxide synthase (eNOS)-derived NO-dependent vascular relaxation, and increased vascular contractility accompanied by augmented superoxide anions production. Endothelial removal, inhibition of cyclooxygenase 2 (COX-2), blockade of thromboxane A2 (TXA2) TP receptors, and scavenger of superoxide normalized the contractile response. COX-2, thromboxane synthase, inducible nitric oxide synthase (iNOS), p65 NFκB subunit and p22(phox) of NAD(P)H oxidase (NOX) subunit expressions were increased in vessels of chagasic animals. Serum TNF-α was augmented. Basal NO production, and nitrotyrosine residue expression were increased. It is concluded that T. cruzi invades mice aorta endothelial cells and increases TXA2/TP receptor/NOX-derived superoxide formation. Alongside, T. cruzi promotes systemic TNF-α increase, which stimulates iNOS expression in vessels and nitrosative stress. In light of the heart failure that develops in the chronic phase of the disease, to understand the mechanism involved in the increased contractility of the aorta is crucial.

  3. Delta ferrite-containing austenitic stainless steel resistant to the formation of undesirable phases upon aging

    DOEpatents

    Leitnaker, James M.

    1981-01-01

    Austenitic stainless steel alloys containing delta ferrite, such as are used as weld deposits, are protected against the transformation of delta ferrite to sigma phase during aging by the presence of carbon plus nitrogen in a weight percent 0.015-0.030 times the volume percent ferrite present in the alloy. The formation of chi phase upon aging is controlled by controlling the Mo content.

  4. Delta ferrite-containing austenitic stainless steel resistant to the formation of undesirable phases upon aging

    DOEpatents

    Leitnaker, J.M.

    Austenitic stainless steel alloys containing delta ferrite, such as are used as weld deposits, are protected against the transformation of delta ferrite to sigma phase during aging by the presence of carbon plus nitrogen in a weight percent 0.015 to 0.030 times the volume percent ferrite present in the alloy. The formation of chi phase upon aging is controlled by controlling the Mo content.

  5. Phase Transformations and Formation of Ultra-Fine Microstructure During Hydrogen Sintering and Phase Transformation (HSPT) Processing of Ti-6Al-4V

    NASA Astrophysics Data System (ADS)

    Sun, Pei; Fang, Zhigang Zak; Koopman, Mark; Xia, Yang; Paramore, James; Ravi Chandran, K. S.; Ren, Yang; Lu, Jun

    2015-12-01

    The hydrogen sintering and phase transformation (HSPT) process is a novel powder metallurgy method for producing Ti alloys, particularly the Ti-6Al-4V alloy, with ultra-fine microstructure in the as-sintered state. The ultra-fine microstructure is obtained as a direct result of the use of H2 gas during sintering. The refinement of the microstructure during HSPT is similar to that of thermal hydrogen processing (THP) of bulk Ti alloys. For both THP and HSPT of Ti-6Al-4V alloy, the mechanisms of the grain refinement depend on the phase equilibria and phase transformations in the presence of hydrogen, which are surprisingly still not well established to date and are still subjected to research and debate. In recent work by the present authors, a pseudo-binary phase diagram of (Ti-6Al-4V)-H has been determined by using in situ synchrotron XRD and TGA/DSC techniques. Aided by this phase diagram, the current paper focuses on the series of phase transformations during sintering and cooling of Ti-6Al-4V in a hydrogen atmosphere and the mechanisms for the formation of the ultra-fine microstructures obtained. Using experimental techniques, including in situ synchrotron XRD, SEM, EBSD, and TEM, the microstructural refinement was found to be the result of (1) the precipitation of ultra-fine α/α2 within coarse β grains during an isothermal hold at intermediate temperatures, and (2) the eutectoid transformation of β → α + δ at approximately 473 K (200 °C).

  6. Triggering star formation by both radiative and mechanical AGN feedback

    NASA Astrophysics Data System (ADS)

    Liu, Chao; Gan, Zhao-Ming; Xie, Fu-Guo

    2013-08-01

    We perform two dimensional hydrodynamic numerical simulations to study the positive active galactic nucleus (AGN) feedback which triggers, rather than suppresses, star formation. Recently, it was shown by Nayakshin et al. and Ishibashi et al. that star formation occurs when the cold interstellar medium (ISM) is squeezed by the impact of mass outflow or radiation pressure, respectively. Mass outflow is ubiquitous in this astrophysical context, and radiation pressure is also important if the AGN is luminous. For the first time in this subject, we incorporate both mass outflow feedback and radiative feedback into our model. Consequently, the ISM is shocked into shells by the AGN feedback, and these shells soon fragment into clumps and filaments because of Rayleigh-Taylor and thermal instabilities. We have two major findings: (1) the star formation rate can indeed be very large in the clumps and filaments. However, the resultant star formation rate density is too large compared with previous works, which is mainly because we ignore the fact that most of the stars that are formed would be disrupted when they move away from the galactic center. (2) Although radiation pressure feedback has a limited effect, when mass outflow feedback is also included, they reinforce each other. Specifically, in the gas-poor case, mass outflow is always the dominant contributor to feedback.

  7. Exploring Technology Supported Collaborative and Cooperative Group Formation Mechanisms

    ERIC Educational Resources Information Center

    Carapina, Mia; Boticki, Ivica

    2015-01-01

    This paper reflects on the systematic literature review paper (in progress), which analyzes technology enhanced collaborative and cooperative learning in elementary education worldwide from 2004 to 2015, focusing on the exploration of technology mediated group formation. The review paper reports on only a few cases of technology supported methods…

  8. Kinetics and mechanism of soot formation in hydrocarbon combustion

    NASA Technical Reports Server (NTRS)

    Frenklach, Michael

    1990-01-01

    The focus of this work was on kinetic modeling. The specific objectives were: detailed modeling of soot formation in premixed flames, elucidation of the effects of fuel structure on the pathway to soot, and the development of a numerical technique for accurate modeling of soot particle coagulation and surface growth. Those tasks were successfully completed and are briefly summarized.

  9. Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation.

    PubMed

    Shiraiwa, Manabu; Yee, Lindsay D; Schilling, Katherine A; Loza, Christine L; Craven, Jill S; Zuend, Andreas; Ziemann, Paul J; Seinfeld, John H

    2013-07-16

    Organic aerosols are ubiquitous in the atmosphere and play a central role in climate, air quality, and public health. The aerosol size distribution is key in determining its optical properties and cloud condensation nucleus activity. The dominant portion of organic aerosol is formed through gas-phase oxidation of volatile organic compounds, so-called secondary organic aerosols (SOAs). Typical experimental measurements of SOA formation include total SOA mass and atomic oxygen-to-carbon ratio. These measurements, alone, are generally insufficient to reveal the extent to which condensed-phase reactions occur in conjunction with the multigeneration gas-phase photooxidation. Combining laboratory chamber experiments and kinetic gas-particle modeling for the dodecane SOA system, here we show that the presence of particle-phase chemistry is reflected in the evolution of the SOA size distribution as well as its mass concentration. Particle-phase reactions are predicted to occur mainly at the particle surface, and the reaction products contribute more than half of the SOA mass. Chamber photooxidation with a midexperiment aldehyde injection confirms that heterogeneous reaction of aldehydes with organic hydroperoxides forming peroxyhemiacetals can lead to a large increase in SOA mass. Although experiments need to be conducted with other SOA precursor hydrocarbons, current results demonstrate coupling between particle-phase chemistry and size distribution dynamics in the formation of SOAs, thereby opening up an avenue for analysis of the SOA formation process.

  10. Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation.

    PubMed

    Shiraiwa, Manabu; Yee, Lindsay D; Schilling, Katherine A; Loza, Christine L; Craven, Jill S; Zuend, Andreas; Ziemann, Paul J; Seinfeld, John H

    2013-07-16

    Organic aerosols are ubiquitous in the atmosphere and play a central role in climate, air quality, and public health. The aerosol size distribution is key in determining its optical properties and cloud condensation nucleus activity. The dominant portion of organic aerosol is formed through gas-phase oxidation of volatile organic compounds, so-called secondary organic aerosols (SOAs). Typical experimental measurements of SOA formation include total SOA mass and atomic oxygen-to-carbon ratio. These measurements, alone, are generally insufficient to reveal the extent to which condensed-phase reactions occur in conjunction with the multigeneration gas-phase photooxidation. Combining laboratory chamber experiments and kinetic gas-particle modeling for the dodecane SOA system, here we show that the presence of particle-phase chemistry is reflected in the evolution of the SOA size distribution as well as its mass concentration. Particle-phase reactions are predicted to occur mainly at the particle surface, and the reaction products contribute more than half of the SOA mass. Chamber photooxidation with a midexperiment aldehyde injection confirms that heterogeneous reaction of aldehydes with organic hydroperoxides forming peroxyhemiacetals can lead to a large increase in SOA mass. Although experiments need to be conducted with other SOA precursor hydrocarbons, current results demonstrate coupling between particle-phase chemistry and size distribution dynamics in the formation of SOAs, thereby opening up an avenue for analysis of the SOA formation process. PMID:23818634

  11. Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation

    PubMed Central

    Shiraiwa, Manabu; Yee, Lindsay D.; Schilling, Katherine A.; Loza, Christine L.; Craven, Jill S.; Zuend, Andreas; Ziemann, Paul J.; Seinfeld, John H.

    2013-01-01

    Organic aerosols are ubiquitous in the atmosphere and play a central role in climate, air quality, and public health. The aerosol size distribution is key in determining its optical properties and cloud condensation nucleus activity. The dominant portion of organic aerosol is formed through gas-phase oxidation of volatile organic compounds, so-called secondary organic aerosols (SOAs). Typical experimental measurements of SOA formation include total SOA mass and atomic oxygen-to-carbon ratio. These measurements, alone, are generally insufficient to reveal the extent to which condensed-phase reactions occur in conjunction with the multigeneration gas-phase photooxidation. Combining laboratory chamber experiments and kinetic gas-particle modeling for the dodecane SOA system, here we show that the presence of particle-phase chemistry is reflected in the evolution of the SOA size distribution as well as its mass concentration. Particle-phase reactions are predicted to occur mainly at the particle surface, and the reaction products contribute more than half of the SOA mass. Chamber photooxidation with a midexperiment aldehyde injection confirms that heterogeneous reaction of aldehydes with organic hydroperoxides forming peroxyhemiacetals can lead to a large increase in SOA mass. Although experiments need to be conducted with other SOA precursor hydrocarbons, current results demonstrate coupling between particle-phase chemistry and size distribution dynamics in the formation of SOAs, thereby opening up an avenue for analysis of the SOA formation process. PMID:23818634

  12. Phase Formation and Transformations in Transmutation Fuel Materials for the LIFE Engine Part I - Path Forward

    SciTech Connect

    Turchi, P E; Kaufman, L; Fluss, M J

    2008-11-10

    The current specifications of the LLNL fusion-fission hybrid proposal, namely LIFE, impose severe constraints on materials, and in particular on the nuclear fissile or fertile nuclear fuel and its immediate environment. This constitutes the focus of the present report with special emphasis on phase formation and phase transformations of the transmutation fuel and their consequences on particle and pebble thermal, chemical and mechanical integrities. We first review the work that has been done in recent years to improve materials properties under the Gen-IV project, and with in particular applications to HTGR and MSR, and also under GNEP and AFCI in the USA. Our goal is to assess the nuclear fuel options that currently exist together with their issues. Among the options, it is worth mentioning TRISO, IMF, and molten salts. The later option will not be discussed in details since an entire report is dedicated to it. Then, in a second part, with the specific LIFE specifications in mind, the various fuel options with their most critical issues are revisited with a path forward for each of them in terms of research, both experimental and theoretical. Since LIFE is applicable to very high burn-up of various fuels, distinctions will be made depending on the mission, i.e., energy production or incineration. Finally a few conclusions are drawn in terms of the specific needs for integrated materials modeling and the in depth knowledge on time-evolution thermochemistry that controls and drastically affects the performance of the nuclear materials and their immediate environment. Although LIFE demands materials that very likely have not yet been fully optimized, the challenge are not insurmountable and a well concerted experimental-modeling effort should lead to dramatic advances that should well serve other fission programs such as Gen-IV, GNEP, AFCI as well as the international fusion program, ITER.

  13. Gramicidin-induced hexagonal H/sub II/ phase formation in erythrocyte membranes

    SciTech Connect

    Tournois, H.; Leunissen-Bijvelt, J.; Haest, C.W.M.; de Gier, J.; Kruij, B.

    1987-10-20

    Using /sup 31/P nuclear magnetic resonance (NMR), small-angle X-ray scattering (SAXS), and freeze-fracture electron microscopic (FFEM) techniques, it is shown that gramicidin induces a hexagonal H/sub II/ phase not only in liposomes prepared from total lipids extracted from human erythrocytes but also in isolated human erythrocyte membranes (white ghosts). A 37/sup 0/C, H/sub II/ phase formation is detected at a gramicidin to phospholipid molar ratio exceeding 1:80. The gramicidin-induced H/sub II/ phase exhibits a very small /sup 31/P chemical shift anisotropy indicating decreased head-group order, and it displays a temperature-dependent increase in tube diameter from 60.2 A at 4/sup 0/C to 64.2 A at 37/sup 0/C in ghosts and from 62.8 to 69.4 A at 37/sup 0/C in total lipid extracts, both in the presence of 1 mol of gramicidin/10 mol of phospholipid. /sup 31/P NMR data indicate that the H/sub II/ phase formation by gramicidin is temperature dependent and show the gradual disappearance of the H/sub II/ phase at low temperatures resulting in a bilayer type of /sup 31/P NMR line shape at 4/sup 0/C, whereas SAXS and FFEM data suggest equal amounts of H/sub II/ phases at all temperatures. The induction of the H/sub II/ phase by gramicidin is specific in that N-formylation of the four tryptophan residues of gramicidin completely blocks the hexagonal (H/sub II/) phase inducing ability of the peptide. The striking parallel between hexagonal H/sub II/ phase induction and the enhancement of lysophosphatidylcholine and palmitoylcarnitine transbilayer reorientation by gramicidin as well the lack of effect of the formylated gramicidin strongly suggests the H/sub II/ phase formation and flip enhancement are mechanistically related phenomena. It is suggested that the formation of gramicidin aggregates of specific structure which are intermediates in H/sub II/ phase formation leads to enhancement of transbilayer reorientation of phospholipids.

  14. Three-phase heaters with common overburden sections for heating subsurface formations

    SciTech Connect

    Vinegar, Harold J.

    2012-02-14

    A heating system for a subsurface formation is described. The heating system includes three substantially u-shaped heaters with first end portions of the heaters being electrically coupled to a single, three-phase wye transformer and second end portions of the heaters being electrically coupled to each other and/or to ground. The three heaters may enter the formation through a first common wellbore and exit the formation through a second common wellbore so that the magnetic fields of the three heaters at least partially cancel out in the common wellbores.

  15. Aging behavior and mechanical properties of maraging steels in the presence of submicrocrystalline Laves phase particles

    SciTech Connect

    Mahmoudi, A.; Ghavidel, M.R. Zamanzad; Nedjad, S. Hossein; Heidarzadeh, A.; Ahmadabadi, M. Nili

    2011-10-15

    Cold rolling and annealing of homogenized Fe-Ni-Mn-Mo-Ti-Cr maraging steels resulted in the formation of submicrocrystalline Fe{sub 2}(Mo,Ti) Laves phase particles. Optical and scanning electron microscopy, X-ray diffraction, tensile and hardness tests were used to study the microstructure, aging behavior and mechanical properties of the annealed steels. The annealed microstructures showed age hardenability during subsequent isothermal aging at 753 K. Ultrahigh fracture stress but poor tensile ductility was obtained after substantial age hardening in the specimens with 2% and 4% chromium. Increasing chromium addition up to 6% toughened the aged microstructure at the expense of the fracture stress by increasing the volume fraction of retained austenite. The Laves phase particles acted as crack nucleation sites during tensile deformation. - Highlights: {yields} Laves phases dispersed in a BCC iron matrix by annealing of cold rolled samples. {yields} The samples showed age hardenability during subsequent isothermal aging at 753 K. {yields} Ultrahigh fracture stress but poor ductility was obtained after age hardening. {yields} Increasing chromium addition toughened the aged microstructure. {yields} Laves phase particles acting as crack nucleation sites during tensile deformation.

  16. Santilli’s hadronic mechanics of formation of deuteron

    SciTech Connect

    Dhondge, Sudhakar S.

    2015-03-10

    In the present communication a brief review of the structure of deuteron proposed by Professor Santilli [1, 2] and its physical properties have been presented. Although Deuteron is a simple molecule, quantum mechanics has been unable to explain its different properties like the spin, magnetic moment, binding energy, stability, charge radius, dipole moment, etc. However, the Hadronic Mechanics developed by Santilli and applied by him [1, 2] to deuteron has succeeded in explaining the above properties to the scientific satisfaction. Santilli proposed Deuteron as a three body system which could take care of all the insufficiencies of quantum mechanics.

  17. Structural basis of Staphylococcus epidermidis biofilm formation: mechanisms and molecular interactions.

    PubMed

    Büttner, Henning; Mack, Dietrich; Rohde, Holger

    2015-01-01

    Staphylococcus epidermidis is a usually harmless commensal bacterium highly abundant on the human skin. Under defined predisposing conditions, most importantly implantation of a medical device, S. epidermidis, however, can switch from a colonizing to an invasive life style. The emergence of S. epidermidis as an opportunistic pathogen is closely linked to the biofilm forming capability of the species. During the past decades, tremendous advance regarding our understanding of molecular mechanisms contributing to surface colonization has been made, and detailed information is available for several factors active during the primary attachment, accumulative or dispersal phase of biofilm formation. A picture evolved in which distinct factors, though appearing to be redundantly organized, take over specific and exclusive functions during biofilm development. In this review, these mechanisms are described in molecular detail, with a highlight on recent insights into multi-functional S. epidermidis cell surface proteins contributing to surface adherence and intercellular adhesion. The integration of distinct biofilm-promoting factors into regulatory networks is summarized, with an emphasis on mechanism that could allow S. epidermidis to flexibly adapt to changing environmental conditions present during colonizing or invasive life-styles.

  18. Structural basis of Staphylococcus epidermidis biofilm formation: mechanisms and molecular interactions

    PubMed Central

    Büttner, Henning; Mack, Dietrich; Rohde, Holger

    2015-01-01

    Staphylococcus epidermidis is a usually harmless commensal bacterium highly abundant on the human skin. Under defined predisposing conditions, most importantly implantation of a medical device, S. epidermidis, however, can switch from a colonizing to an invasive life style. The emergence of S. epidermidis as an opportunistic pathogen is closely linked to the biofilm forming capability of the species. During the past decades, tremendous advance regarding our understanding of molecular mechanisms contributing to surface colonization has been made, and detailed information is available for several factors active during the primary attachment, accumulative or dispersal phase of biofilm formation. A picture evolved in which distinct factors, though appearing to be redundantly organized, take over specific and exclusive functions during biofilm development. In this review, these mechanisms are described in molecular detail, with a highlight on recent insights into multi-functional S. epidermidis cell surface proteins contributing to surface adherence and intercellular adhesion. The integration of distinct biofilm-promoting factors into regulatory networks is summarized, with an emphasis on mechanism that could allow S. epidermidis to flexibly adapt to changing environmental conditions present during colonizing or invasive life-styles. PMID:25741476

  19. Studies of the underlying mechanisms for optical nonlinearities of blue phase liquid crystals (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Chen, Chun-Wei; Khoo, Iam Choon; Zhao, Shuo; Lin, Tsung-Hsien; Ho, Tsung-Jui

    2015-10-01

    We have investigated the mechanisms responsible for nonlinear optical processes occurring in azobenzene-doped blue phase liquid crystals (BPLC), which exhibit two thermodynamically stable BPs: BPI and BPII. In coherent two wave-mixing experiments, a slow (minutes) and a fast (few milliseconds) side diffractions are observed. The underlying mechanisms were disclosed by monitoring the dynamics of grating formation and relaxation as well as by some supplementary experiments. We found the photothermal indexing and dye/LC intermolecular torque leading to lattice distortion to be the dominant mechanisms for the observed nonlinear response in BPLC. Moreover, the response time of the nonlinear optical process varied with operating phase. The rise time of the thermal indexing process was in good agreement with our findings on the temperature dependence of BP refractive index: τ(ISO) > τ(BPI) > τ(BPII). The relaxation time of the torque-induced lattice distortion was analogue to its electrostriction counterpart: τ'(BPI) > τ'(BPII). In a separate experiment, lattice swelling with selective reflection of <110> direction changed from green to red was also observed. This was attributable to the isomerization-induced change in cholesteric pitch, which directly affects the lattice spacing. The phenomenon was confirmed by measuring the optical rotatory power of the BPLC.

  20. On the formation mechanisms, spatial resolution and intensity of backscatter Kikuchi patterns.

    PubMed

    Zaefferer, S

    2007-01-01

    The present paper is divided into two main sections. In the first, the formation mechanisms of backscatter Kikuchi patterns (BKP) are discussed on the basis of measurements on the sharpness of Kikuchi lines and on the spatial, that means the lateral and depth resolution of the technique. We propose that thermal diffuse scattering is the important incoherent scattering mechanism involved in pattern formation. This mechanism is not considered in the classical description of the origin of backscattered electrons in the scanning electron microscope (SEM) which is why there is in some important points no agreement between classical Monte-Carlo-type electron trajectory simulations and experimental results. We assume that the energy spectrum of the backscattered electrons shows, similar to the spectra in transmission electron microscopy, a sharp zero-loss peak. In the second section, we discuss the intensity of Kikuchi bands in BKP. It is shown that the kinematical theory gives-of course-not the correct intensities, but that these intensities are, on the other hand, not too far off the experimental ones. We subsequently introduce a simple intensity correction procedure that is based on the two-beam dynamical theory, originally proposed by Blackman for transmission electron diffraction patterns. It is shown by examples of diffraction patterns of niobium and silicon that this procedure leads to satisfying results, once two unknown variables (a universal constant and the exit depth of the electrons) have been empirically fit. It is assumed that in the future, this correction will improve the possibilities of phase identification by backscatter Kikuchi diffraction patterns.

  1. Planetary Embryo Bow Shocks as a Mechanism for Chondrule Formation

    NASA Astrophysics Data System (ADS)

    Mann, Christopher R.; Boley, Aaron C.; Morris, Melissa A.

    2016-02-01

    We use radiation hydrodynamics with direct particle integration to explore the feasibility of chondrule formation in planetary embryo bow shocks. The calculations presented here are used to explore the consequences of a Mars-size planetary embryo traveling on a moderately excited orbit through the dusty, early environment of the solar system. The embryo’s eccentric orbit produces a range of supersonic relative velocities between the embryo and the circularly orbiting gas and dust, prompting the formation of bow shocks. Temporary atmospheres around these embryos, which can be created via volatile outgassing and gas capture from the surrounding nebula, can non-trivially affect thermal profiles of solids entering the shock. We explore the thermal environment of solids that traverse the bow shock at different impact radii, the effects that planetoid atmospheres have on shock morphologies, and the stripping efficiency of planetoidal atmospheres in the presence of high relative winds. Simulations are run using adiabatic and radiative conditions, with multiple treatments for the local opacities. Shock speeds of 5, 6, and 7 km s-1 are explored. We find that a high-mass atmosphere and inefficient radiative conditions can produce peak temperatures and cooling rates that are consistent with the constraints set by chondrule furnace studies. For most conditions, the derived cooling rates are potentially too high to be consistent with chondrule formation.

  2. Formation mechanisms and optimization of trap-based positron beams

    NASA Astrophysics Data System (ADS)

    Natisin, M. R.; Danielson, J. R.; Surko, C. M.

    2016-02-01

    Described here are simulations of pulsed, magnetically guided positron beams formed by ejection from Penning-Malmberg-style traps. In a previous paper [M. R. Natisin et al., Phys. Plasmas 22, 033501 (2015)], simulations were developed and used to describe the operation of an existing trap-based beam system and provided good agreement with experimental measurements. These techniques are used here to study the processes underlying beam formation in more detail and under more general conditions, therefore further optimizing system design. The focus is on low-energy beams (˜eV) with the lowest possible spread in energies (<10 meV), while maintaining microsecond pulse durations. The simulations begin with positrons trapped within a potential well and subsequently ejected by raising the bottom of the trapping well, forcing the particles over an end-gate potential barrier. Under typical conditions, the beam formation process is intrinsically dynamical, with the positron dynamics near the well lip, just before ejection, particularly crucial to setting beam quality. In addition to an investigation of the effects of beam formation on beam quality under typical conditions, two other regimes are discussed; one occurring at low positron temperatures in which significantly lower energy and temporal spreads may be obtained, and a second in cases where the positrons are ejected on time scales significantly faster than the axial bounce time, which results in the ejection process being essentially non-dynamical.

  3. Theory of the formation of P4132(P4332)-phase spinels

    NASA Astrophysics Data System (ADS)

    Talanov, V. M.; Talanov, M. V.; Shirokov, V. B.

    2016-03-01

    A group-theoretical, thermodynamic, and structural study of the formation of P4132( P4332) spinel modification has been performed. In particular, the occurrence of unique hyper-kagome atomic order is analyzed. The critical order parameter inducing a phase transition is established. It is shown that the calculated structure of the low-symmetry P4132( P4332) phase is formed as a result of displacements of atoms of all types and due to the cation and anion ordering. The problem of the occurrence of unique hyper-kagome atomic order in the structures of P4132( P4332) spinel modifications is considered theoretically. It is proven within the Landau theory of phase transitions that the P4132( P4332) phase can be formed from the high-symmetry Fd3 m phase with an ideal spinel structure only as a result of first-order phase transition. Therefore, the formation of hyper-kagome sublattice in the P4132( P4332) phase is accompanied by a significant transformation of the spinel structure.

  4. Rapid reversible formation of a metastable subgel phase in saturated diacylphosphatidylcholines.

    PubMed Central

    Koynova, R; Tenchov, B G; Todinova, S; Quinn, P J

    1995-01-01

    Formation of well ordered lamellar subgel (SGII) phase in aqueous dispersions of L-dipalmitoylphosphatidylcholine upon cooling from the lamellar gel phase, without low-temperature equilibration, is observed in real time using synchrotron x-ray diffraction. It has the same lamellar repeat period as the gel phase from which it was formed but differs in its wide-angle diffraction pattern. The SGII phase forms at about 7 degrees C upon cooling at 2 degrees C/min. In temperature jump experiments at 1 degree C/s from 50 to -5 degrees C, the relaxation time of the lamellar gel-SGII transition is found to be approximately 15 s. The conversion between the lamellar gel and SGII phase is cooperative and rapidly reversible. Upon heating, it coincides in temperature with an endothermic event with a calorimetric enthalpy of 0.35 kcal/mol, the so-called sub-subtransition. Similar sub-subtransitions are also observed calorimetrically at temperatures approximately 10 degrees C below the subtransition, without low-temperature storage, in aqueous dispersions of L-dimyristoylphosphatidylcholine and L-distearoylphosphatidylcholine, but not in racemic DL-dipalmitoylphosphatidylcholine. The formation of the equilibrium lamellar crystalline Lc phase appears to take place only from within the SGII phase. PMID:7647241

  5. Decrease in spermidine content during logarithmic phase of cell growth delays spore formation of Bacillus subtilis.

    PubMed

    Ishii, I; Takada, H; Terao, K; Kakegawa, T; Igarashi, K; Hirose, S

    1994-11-01

    Bacillus subtilis 168M contained a large amount of spermidine during the logarithmic phase of growth, but the amount decreased drastically during the stationary phase. The extracts, prepared from B. subtilis cells harvested in the logarithmic phase, contained activity of arginine decarboxylase (ADC) rather than the activity of ornithine decarboxylase. In the presence of alpha-difluoromethylarginine (DFMA), a specific and irreversible inhibitor of ADC, the amount of spermidine in B. subtilis during the logarithmic phase decreased to about 25% of the control cells. Under these conditions, spore formation of B. subtilis 168M delayed greatly without significant inhibition of cell growth. The decrease in spermidine content in the logarithmic phase rather than in the stationary phase was involved in the delay of sporulation. Electron microscopy of cells at 24 hrs. of culture confirmed the delay of spore formation by the decrease of spermidine content. Furthermore, the delay of sporulation was negated by the addition of spermidine. These data suggest that a large amount of spermidine existing during the logarithmic phase plays an important role in the sporulation of B. subtilis.

  6. Reducing bit-error rate with optical phase regeneration in multilevel modulation formats.

    PubMed

    Hesketh, Graham; Horak, Peter

    2013-12-15

    We investigate theoretically the benefits of using all-optical phase regeneration in a long-haul fiber optic link. We also introduce a design for a device capable of phase regeneration without phase-to-amplitude noise conversion. We simulate numerically the bit-error rate of a wavelength division multiplexed optical communication system over many fiber spans with periodic reamplification and compare the results obtained with and without phase regeneration at half the transmission distance when using the new design or an existing design. Depending on the modulation format, our results suggest that all-optical phase regeneration can reduce the bit-error rate by up to two orders of magnitude and that the amplitude preserving design offers a 50% reduction in bit-error rate relative to existing technology.

  7. Palladium-modified aluminide coatings: Mechanisms of formation

    SciTech Connect

    Lamesle, P.; Steinmetz, P.; Steinmetz, J.; Alperine, S.

    1995-02-01

    The need to increase the efficiency of turbo engines has led manufacturers to increase the temperature of gases at the exhaust of the combustion chamber. Another limiting factor for the lifetime of blades or vanes used in gas turbines is hot corrosion due to the condensation of alkaline sulfate produced by the oxidation of sulfur contained in kerosene or fuels. To overcome these problems, the use of protective coatings has come into general use. A systematic investigation of the influence of Pd-Ni predeposit alloys on the microstructure and composition of aluminum diffusion coatings has been conducted on Ni base superalloys (mainly IN738). Their metallurgical structure has been studied with a special emphasis on the nature of the phases and distribution of the various elements throughout the coating section. A two-layer structure similar to that formed on simple aluminide coatings is observed whatever the type of aluminizing treatment (low and high aluminum activity, pack of vapor-phase coating). The superficial layer is, however, very different from that observed in simple aluminide coatings, since it is constituted with a ternary PdNi aluminide. Palladium concentration profiles, which significantly differ when using low or high activity cements, and the results of a study of the ternary Ni-Pd-Al phase diagram, provide qualitative indications concerning the coatings` growth processes. Palladium, if present at a sufficient level, enhances Al diffusion in the beta phase. In the case of low activity processes, a consequence of this increase of Al diffusion flux is the location of an NiAl reaction zone inside the coating.

  8. Learning Mechanism for Column Formation in the Olfactory Bulb

    PubMed Central

    Migliore, M.; Inzirillo, Carlo; Shepherd, Gordon M.

    2007-01-01

    Sensory discrimination requires distributed arrays of processing units. In the olfactory bulb, the processing units for odor discrimination are believed to involve dendrodendritic synaptic interactions between mitral and granule cells. There is increasing anatomical evidence that these cells are organized in columns, and that the columns processing a given odor are arranged in widely distributed arrays. Experimental evidence is lacking on the underlying learning mechanisms for how these columns and arrays are formed. To gain insight into these mechanisms, we have used a simplified realistic circuit model to test the hypothesis that distributed connectivity can self-organize through an activity-dependent dendrodendritic synaptic mechanism. The results point to action potentials propagating in the mitral cell lateral dendrites as playing a critical role in this mechanism. The model predicts that columns emerge from the interaction between the local temporal dynamics of the action potentials and the synapses that they activate during dendritic propagation. The results suggest a novel and robust learning mechanism for the development of distributed processing units in a cortical structure. PMID:18958240

  9. KEEPING A STEP AHEAD - FORMATIVE PHASE OF A WORKPLACE INTERVENTION TRIAL TO PREVENT OBESITY

    PubMed Central

    Zapka, Jane; Lemon, Stephenie C.; Estabrook, Barbara B.; Jolicoeur, Denise G.

    2008-01-01

    Background Ecological interventions hold promise for promoting overweight and obesity prevention in worksites. Given the paucity of evaluative research in the hospital worksite setting, considerable formative work is required for successful implementation and evaluation. Purpose This paper describes the formative phases of Step Ahead, a site-randomized controlled trial of a multi-level intervention that promotes physical activity and healthy eating in 6 hospitals in central Massachusetts. The purpose of the formative research phase was to increase the feasibility, effectiveness and likelihood of sustainability of the intervention. Design and Procedures The Step Ahead ecological intervention approach targets change at the organization, the interpersonal work environment and the individual levels. The intervention was developed using fundamental steps of intervention mapping and important tenets of participatory research. Formative research methods were used to engage leadership support and assistance and to develop an intervention plan that is both theoretically and practically grounded. This report uses observational data, program minutes and reports, and process tracking data. Developmental Strategies and Observations Leadership involvement (key informant interviews and advisory boards), employee focus groups and advisory boards, and quantitative environmental assessments cultivated participation and support. Determining multiple foci of change and designing measurable objectives and generic assessment tools to document progress are complex challenges encountered in planning phases. Lessons Learned Multi-level trials in diverse organizations require flexibility and balance of theory application and practice-based perspectives to affect impact and outcome objectives. Formative research is an essential component. PMID:18073339

  10. Formation mechanism of steep wave front in magnetized plasmas

    SciTech Connect

    Sasaki, M. Kasuya, N.; Itoh, S.-I.; Kobayashi, T.; Arakawa, H.; Itoh, K.; Fukunaga, K.; Yamada, T.; Yagi, M.

    2015-03-15

    Bifurcation from a streamer to a solitary drift wave is obtained in three dimensional simulation of resistive drift waves in cylindrical plasmas. The solitary drift wave is observed in the regime where the collisional transport is important as well as fluctuation induced transport. The solitary drift wave forms a steep wave front in the azimuthal direction. The phase of higher harmonic modes are locked to that of the fundamental mode, so that the steep wave front is sustained for a long time compared to the typical time scale of the drift wave oscillation. The phase entrainment between the fundamental and second harmonic modes is studied, and the azimuthal structure of the stationary solution is found to be characterized by a parameter which is determined by the deviation of the fluctuations from the Boltzmann relation. There are two solutions of the azimuthal structures, which have steep wave front facing forward and backward in the wave propagation direction, respectively. The selection criterion of these solutions is derived theoretically from the stability of the phase entrainment. The simulation result and experimental observations are found to be consistent with the theoretical prediction.

  11. Fundamental thermochemical properties of amino acids: gas-phase and aqueous acidities and gas-phase heats of formation.

    PubMed

    Stover, Michele L; Jackson, Virgil E; Matus, Myrna H; Adams, Margaret A; Cassady, Carolyn J; Dixon, David A

    2012-03-01

    The gas-phase acidities of the 20 L-amino acids have been predicted at the composite G3(MP2) level. A broad range of structures of the neutral and anion were studied to determine the lowest energy conformer. Excellent agreement is found with the available experimental gas-phase deprotonation enthalpies, and the calculated values are within experimental error. We predict that tyrosine is deprotonated at the CO(2)H site. Cysteine is predicted to be deprotonated at the SH but the proton on the CO(2)H is shared with the S(-) site. Self-consistent reaction field (SCRF) calculations with the COSMO parametrization were used to predict the pK(a)'s of the non-zwitterion form in aqueous solution. The differences in the non-zwitterion pK(a) values were used to estimate the free energy difference between the zwitterion and nonzwitterion forms in solution. The heats of formation of the neutral compounds were calculated from atomization energies and isodesmic reactions to provide the first reliable set of these values in the gas phase. Further calculations were performed on five rare amino acids to predict their heats of formation, acidities, and pK(a) values.

  12. Theory of the formation of the ordered LiZn{sub 0.5}Mn{sub 1.5}O{sub 4} phase

    SciTech Connect

    Talanov, V. M.; Shirokov, V. B.

    2013-03-15

    A theory of the structural phase transition in LiZn{sub 0.5}Mn{sub 1.5}O{sub 4} cathode material is proposed. The symmetry of the order parameter, thermodynamics, and mechanisms of formation of the atomic structure of low-symmetry ordered cubic lithium-zinc-manganese oxide spinel have been studied. The critical order parameter inducing the phase transition has been found. It is shown that the calculated LiZn{sub 0.5}Mn{sub 1.5}O{sub 4} structure is formed due to displacements and orderings of lithium, zinc, manganese, and oxygen atoms. Within the Landau theory of phase transitions, it is shown that the phase states may change from high-symmetry cubic disordered Fd3m phase to the low-symmetry ordered cubic P2{sub 1}3 phase as a result of first-order phase transitions.

  13. Formation of free radicals during mechanical degradation of elastomers.

    NASA Technical Reports Server (NTRS)

    Devries, K. L.; Williams, M. L.; Roylance, D. K.

    1971-01-01

    Solithane 113 (an amorphous polyurethane elastomer) was prepared by curing equal proportions of castor oil and trifunctional isocyanate for 6 hr 45 min at 170 F. The sample material was mechanically degraded by grinding below and above its glass transition point at liquid nitrogen and room temperatures. The EPR spectra of ground samples were recorded and the number of free radicals were determined by a computer double-integration of the recorded spectra and by a comparison of the values with those of a standard material. Curves of EPR spectra suggest that different molecular mechanisms may be active in degradation of this material below and above its glass transition temperature.

  14. A study of suppressed formation of low-conductivity phases in doped Li7La3Zr2O12 garnets by in situ neutron diffraction

    DOE PAGESBeta

    Chen, Yan; Rangasamy, Ezhiylmurugan; dela Cruz, Clarina R.; Liang, Chengdu; An, Ke

    2015-09-28

    Doped Li7La3Zr2O12 garnets, oxide-based solids with good Li+ conductivity and compatibility, show great potential as leading electrolyte material candidates for all-solid-state lithium ion batteries. Still yet, the conductive bulk usually suffers from the presence of secondary phases and the transition towards a low-conductivity tetragonal phase during synthesis. Dopants are designed to stabilize the high-conductive cubic phase and suppress the formation of the low-conductivity phases. In situ neutron diffraction enables a direct observation of the doping effects by monitoring the phase evolutions during garnet synthesis. It reveals the reaction mechanism involving the temporary presence of intermediate phases. The off-stoichiometry due tomore » the liquid Li2CO3 evaporation leads to the residual of the low-conductivity intermediate phase in the as-synthesized bulk. Appropriate doping of an active element may alter the component of the intermediate phases and promote the completion of the reaction. While the dopants aid to stabilize most of the cubic phase, a small amount of tetragonal phase tends to form under a diffusion process. Lastly, the in situ observations provide the guideline of process optimization to suppress the formation of unwanted low-conductivity phases.« less

  15. Impacts of relative permeability on CO2 phase behavior, phase distribution, and trapping mechanisms

    NASA Astrophysics Data System (ADS)

    Moodie, N.; McPherson, B. J. O. L.; Pan, F.

    2015-12-01

    A critical aspect of geologic carbon storage, a carbon-emissions reduction method under extensive review and testing, is effective multiphase CO2 flow and transport simulation. Relative permeability is a flow parameter particularly critical for accurate forecasting of multiphase behavior of CO2 in the subsurface. The relative per­meability relationship assumed and especially the irreducible saturation of the gas phase greatly impacts predicted CO2 trapping mechanisms and long-term plume migration behavior. A primary goal of this study was to evaluate the impact of relative permeability on efficacy of regional-scale CO2 sequestration models. To accomplish this we built a 2-D vertical cross-section of the San Rafael Swell area of East-central Utah. This model simulated injection of CO2 into a brine aquifer for 30 years. The well was then shut-in and the CO2 plume behavior monitored for another 970 years. We evaluated five different relative permeability relationships to quantify their relative impacts on forecasted flow results of the model, with all other parameters maintained uniform and constant. Results of this analysis suggest that CO2 plume movement and behavior are significantly dependent on the specific relative permeability formulation assigned, including the assumed irreducible saturation values of CO2 and brine. More specifically, different relative permea­bility relationships translate to significant differences in CO2 plume behavior and corresponding trapping mechanisms.

  16. Analysis of diarylmethylamine compounds using electrospray mass spectrometry: formation mechanisms of radical ions and dehydro cations.

    PubMed

    Cai, Tian; Xu, Xiao-Ying; Wu, Zhi-Jun

    2015-12-01

    A series of diarylmethylamine compounds were analyzed using electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS). [M](+)˙ and [M - H](+) were both observed, but showed different abundances. A possible mechanism for the formation of [M](+)˙ and [M - H](+) was proposed to explicate the rule for the ratio change of I([M](+)˙)/I([M-H](+)). The [M](+)˙ has two structures, which can interconvert into each other in the gas phase. The substituted groups on the benzene rings play a crucial role in the transfer between the two structures. Electron withdrawing groups can prevent the formation of carbocations, thus nitro-containing diarylmethylamines remained mainly as structure I and were detected as [M](+)˙. On the contrary, electron donating groups help to stabilize carbocations. This makes structure I transfer to structure II, and structure II prefers to further generate [M - H](+) by loss of an H radical. Nuclear magnetic resonance and D-labelled MS experiments indicate that the 1-C-H bond has strong activity.

  17. Nanobacteria: An alternative mechanism for pathogenic intra- and extracellular calcification and stone formation

    PubMed Central

    Kajander, E. Olavi; Çiftçioglu, Neva

    1998-01-01

    Calcium phosphate is deposited in many diseases, but formation mechanisms remain speculative. Nanobacteria are the smallest cell-walled bacteria, only recently discovered in human and cow blood and commercial cell culture serum. In this study, we identified with energy-dispersive x-ray microanalysis and chemical analysis that all growth phases of nanobacteria produce biogenic apatite on their cell envelope. Fourier transform IR spectroscopy revealed the mineral as carbonate apatite. The biomineralization in cell culture media resulted in biofilms and mineral aggregates closely resembling those found in tissue calcification and kidney stones. In nanobacteria-infected fibroblasts, electron microscopy revealed intra- and extracellular acicular crystal deposits, stainable with von Kossa staining and resembling calcospherules found in pathological calcification. Previous models for stone formation have led to an hypothesis that elevated pH due to urease and/or alkaline phosphatase activity is a lithogenic factor. Our results indicate that carbonate apatite can be formed without these factors at pH 7.4, at physiological phosphate and calcium concentrations. Nanobacteria can produce apatite in media mimicking tissue fluids and glomerular filtrate and provide a unique model for in vitro studies on calcification. PMID:9653177

  18. Mechanisms of endoderm formation in a cartilaginous fish reveal ancestral and homoplastic traits in jawed vertebrates

    PubMed Central

    Godard, Benoit G.; Coolen, Marion; Le Panse, Sophie; Gombault, Aurélie; Ferreiro-Galve, Susana; Laguerre, Laurent; Lagadec, Ronan; Wincker, Patrick; Poulain, Julie; Da Silva, Corinne; Kuraku, Shigehiro; Carre, Wilfrid; Boutet, Agnès; Mazan, Sylvie

    2014-01-01

    ABSTRACT In order to gain insight into the impact of yolk increase on endoderm development, we have analyzed the mechanisms of endoderm formation in the catshark S. canicula, a species exhibiting telolecithal eggs and a distinct yolk sac. We show that in this species, endoderm markers are expressed in two distinct tissues, the deep mesenchyme, a mesenchymal population of deep blastomeres lying beneath the epithelial-like superficial layer, already specified at early blastula stages, and the involuting mesendoderm layer, which appears at the blastoderm posterior margin at the onset of gastrulation. Formation of the deep mesenchyme involves cell internalizations from the superficial layer prior to gastrulation, by a movement suggestive of ingressions. These cell movements were observed not only at the posterior margin, where massive internalizations take place prior to the start of involution, but also in the center of the blastoderm, where internalizations of single cells prevail. Like the adjacent involuting mesendoderm, the posterior deep mesenchyme expresses anterior mesendoderm markers under the control of Nodal/activin signaling. Comparisons across vertebrates support the conclusion that endoderm is specified in two distinct temporal phases in the catshark as in all major osteichthyan lineages, in line with an ancient origin of a biphasic mode of endoderm specification in gnathostomes. They also highlight unexpected similarities with amniotes, such as the occurrence of cell ingressions from the superficial layer prior to gastrulation. These similarities may correspond to homoplastic traits fixed separately in amniotes and chondrichthyans and related to the increase in egg yolk mass. PMID:25361580

  19. New thresholds for Primordial Black Hole formation during the QCD phase transition

    NASA Astrophysics Data System (ADS)

    Sobrinho, J. L. G.; Augusto, P.; Gonçalves, A. L.

    2016-08-01

    Primordial Black Holes (PBHs) might have formed in the early Universe as a consequence of the collapse of density fluctuations with an amplitude above a critical value δc: the formation threshold. Although for a radiation-dominated Universe δc remains constant, if the Universe experiences some dust-like phases (e.g. phase transitions) δc might decrease, improving the chances of PBH formation. We studied the evolution of δc during the QCD phase transition epoch within three different models: Bag Model (BM), Lattice Fit Model (LFM), and Crossover Model (CM). We found that the reduction on the background value of δc can be as high as 77% (BM), which might imply a ˜10-10 probability of PBHs forming at the QCD epoch.

  20. Formation of Fourier phase shifts in the solar Ni I 6768 A line

    NASA Technical Reports Server (NTRS)

    Jones, Harrison P.

    1989-01-01

    A formalism is developed to understand better how Doppler shifts of spectrum lines as inferred from phase shifts in the Fourier transforms of line profiles are related to the underlying velocity structures which they are intended to measure. With a standard model atmosphere and a simplified, quasi-LTE treatment of line formation, the formalism is applied to the Ni I 6768 A line, which has been selected for use with a network of imaging interferometers under development by the Global Oscillations Network Group for research in helioseismology. Fourier phase shifts are found to be a remarkably linear measure of velocity even in the presence of gradients and unresolved lateral variations in the assumed velocity field. An assumed outward increase in amplitude of a model oscillatory velocity is noticeably reflected in the center-to-limb behavior of the simulated velocity measure, and a sample model of solar granulation is found to have a strong influence on the formation of the Fourier phase.

  1. Mechanisms underlying structural variant formation in genomic disorders

    PubMed Central

    Carvalho, Claudia M. B.; Lupski, James R.

    2016-01-01

    With the recent burst of technological developments in genomics, and the clinical implementation of genome-wide assays, our understanding of the molecular basis of genomic disorders, specifically the contribution of structural variation to disease burden, is evolving quickly. Ongoing studies have revealed a ubiquitous role for genome architecture in the formation of structural variants at a given locus, both in DNA recombination-based processes and in replication-based processes. These reports showcase the influence of repeat sequences on genomic stability and structural variant complexity and also highlight the tremendous plasticity and dynamic nature of our genome in evolution, health and disease susceptibility. PMID:26924765

  2. Electrical characteristics and formation mechanism of atmospheric pressure plasma jet

    SciTech Connect

    Liu, Lijuan; Zhang, Yu; Tian, Weijing; Meng, Ying; Ouyang, Jiting

    2014-06-16

    The behavior of atmospheric pressure plasma jet produced by a coplanar dielectric barrier discharge in helium in external electrostatic and magnetic field is investigated. Net negative charges in the plasma jet outside the tube were detected. The deflection of the plume in the external field was observed. The plasma jet is suggested to be formed by the electron beam from the temporal cathode which is accelerated by a longitudinal field induced by the surface charges on the dielectric tube or interface between the helium and ambient air. The helium flow is necessary for the jet formation in the surrounding air.

  3. Kinetics and mechanism of the electrochemical formation of iron oxidation products on steel immersed in sour acid media.

    PubMed

    Hernández-Espejel, Antonio; Palomar-Pardavé, Manuel; Cabrera-Sierra, Román; Romero-Romo, Mario; Ramírez-Silva, María Teresa; Arce-Estrada, Elsa M

    2011-03-01

    From electrochemical techniques (cyclic voltammetry, potential steps, and EIS), XRD, and SEM-EDX, the kinetics and mechanism of anodic film formation applying anodic potential steps on steel immersed in sour acid media was determined. It was found, from a thermodynamic analysis, based on equilibrium phase diagrams of the system considered in this work, that iron oxidation may produce different new solid phases, depending on the applied potential, the first being the iron oxidation associated with formation of FeS((c)) species, which in turn can be reoxidized to FeS(2(c)) or even to Fe(2)O(3(c)) at higher potential values. From analysis of the corresponding experimental potentiostatic current density transients, it was concluded that the electrochemical anodic film formation involves an E(1)CE(2) mechanism, whereby the first of the two simultaneous processes were the Fe electrochemical oxidation (E(1)) followed by FeS precipitation (C) that occurs by 3D nucleation and growth limited by mass transfer reaction and FeS oxidation (E(2)) forming a mix of different stoichiometry iron sulphides and oxides. From EIS measurements, it was revealed that the anodic film's charge transfer resistance diminishes as the potential applied for its formation becomes more anodic, thus behaving poorly against corrosion.

  4. Mechanisms of Nitrogen Oxide Formation During Ensiling of Dairy Feeds

    EPA Science Inventory

    This product is the abstract of a proposed paper. The emission of NOx has been observed during the ensiling process (Peterson et al. 1958, Maw et al. 2002). Substantial NOx is not inherently present in corn. It is generated by an unknown mechanism during the early days of the ens...

  5. Distributional Language Learning: Mechanisms and Models of Category Formation

    ERIC Educational Resources Information Center

    Aslin, Richard N.; Newport, Elissa L.

    2014-01-01

    In the past 15 years, a substantial body of evidence has confirmed that a powerful distributional learning mechanism is present in infants, children, adults and (at least to some degree) in nonhuman animals as well. The present article briefly reviews this literature and then examines some of the fundamental questions that must be addressed for…

  6. Distinct Neural Mechanisms Mediate Olfactory Memory Formation at Different Timescales

    ERIC Educational Resources Information Center

    McNamara, Ann Marie; Magidson, Phillip D.; Linster, Christiane; Wilson, Donald A.; Cleland, Thomas A.

    2008-01-01

    Habituation is one of the oldest forms of learning, broadly expressed across sensory systems and taxa. Here, we demonstrate that olfactory habituation induced at different timescales (comprising different odor exposure and intertrial interval durations) is mediated by different neural mechanisms. First, the persistence of habituation memory is…

  7. [The Kupershtokh-Medvedev electrostrictive instability as possible mechanism of initiation of phase transitions, domains and pores in lipid membranes and influence of microwave irradiation on cell].

    PubMed

    Zakhvataev, V E; Khlebopros, R G

    2012-01-01

    One of the possible mechanisms of initiation of local phase transitions and formation of nonuniform structure of biological and model lipid membranes is suggested. It is based on anisotropic electrohydrodynamic instability of Kupershtokh and Medvedev in strong electric field relative to density perturbations. This mechanism may clarify initial stages of formation of membrane domains and pores, some aspects of cell signalization and influence of microwave irradiation of nonthermal intensity on living organisms. PMID:22567911

  8. Geo-mechanical consequences of large scale fluid storage in the Utsira formation in the North Sea

    NASA Astrophysics Data System (ADS)

    Wangen, Magnus; Gasda, Sarah; Bjørnarå, Tore

    2016-04-01

    The Utsira formation in the North Sea is a more than 400 km long formation of Pliocene sand with a thickness in the range from 50m to 350m. The porosity of the Utsira formation is estimated to 35% and the permeability is of the order Darcy (1e-12 m2). This formation is being considered as a possible site for large scale CO2 storage, because of its large storage capacity and good permeability. In this work we look at the geo-mechanical implications of injecting "large'' volumes of fluid in the Utsira formation. Our modelling is based on Biot's poro-elasticity in combination with one-phase and two phase flow. We compare the pressure build-up from injection of brine with the pressure build-up from from injection of supercritical CO2. Well placement and near well modelling is not a part of the modelling. We study four different injection scenarios over 25 years, which have injection rates: 1 Mt/year, 10 Mt/year, 100 Mt/year and 1000 Mt/year. We observe that the pressure plume scales with the injection rate, which is the same behaviour as with Theis solution for pressure build-up. A particular concern is the mechanical properties of the Utsira sand and the cap rock. The cap rock is a Quaternary shale with a Young's modulus measured to 0.25 GPa and a Poisson ratio 0.25. A Young's modulus for the Utsira sand, which can be used to model expansion of the formation, is not measured. It is reasonable to assume that the loose sand has a low Young's modulus. We have tested low values of the Young's modulus for the sand and we get considerable mechanical expansion of the Utsira formation, even in the case of low pressure build-up from the fluid injection. Almost all the surface (seabed) uplift is linked to mechanical expansion of the sand. The strain of the Utsira formation and related surface uplift can be estimated with simple 1D models. Vertical 1D models apply because of the large lateral extent of the pressure plume compared to the thickness of the formation. The limits of

  9. Annealing-induced change in quantum dot chain formation mechanism

    NASA Astrophysics Data System (ADS)

    Park, Tyler D.; Colton, John S.; Farrer, Jeffrey K.; Yang, Haeyeon; Kim, Dong Jun

    2014-12-01

    Self-assembled InGaAs quantum dot chains were grown using a modified Stranski-Krastanov method in which the InGaAs layer is deposited under a low growth temperature and high arsenic overpressure, which suppresses the formation of dots until a later annealing process. The dots are capped with a 100 nm GaAs layer. Three samples, having three different annealing temperatures of 460°C, 480°C, and 500°C, were studied by transmission electron microscopy. Results indicate two distinct types of dot formation processes: dots in the 460°C and 480°C samples form from platelet precursors in a one-to-one ratio whereas the dots in the sample annealed at 500°C form through the strain-driven self-assembly process, and then grow larger via an additional Ostwald ripening process whereby dots grow into larger dots at the expense of smaller seed islands. There are consequently significant morphological differences between the two types of dots, which explain many of the previously-reported differences in optical properties. Moreover, we also report evidence of indium segregation within the dots, with little or no indium intermixing between the dots and the surrounding GaAs barrier.

  10. [Molecular mechanisms of cartilage formation and chondrocyte maturation].

    PubMed

    Tamamura, Yoshihiro; Iwamoto, Masahiro

    2004-07-01

    Cartilage plays multiple roles in vertebrate animals. In an embryonic stage and early postnatal life, cartilage is important not only as a structural support of early embryo but also as a template of endochondral bone. In a later postnatal life, cartilage provides smooth joint movement and tissue elasticity. A number of critical signaling molecules that regulate cartilage formation and chondrocytes maturation in endochondral bone formation have been identified to date. The interplay of those important molecules is also actively studied. However, several fundamental questions still remain unsolved. What signal initiates mesenchymal cell condensation? Does condensation enough to make cells competent for BMP-induced chondrogenesis? Is there chondrocyte stem cell in cartilage? Likewise, it is not known which factor triggers chondrocytes maturation. In this review article, we summarized the action of several key factors including BMP, hedgehog, PTHrP, and Wnt in condensation, chondrogenenic differentiation and maturation of chondrocytes. Towards further understanding of above fundamental questions, this review article also tried to propose future direction of cartilage biology research. PMID:15577071

  11. On the bar formation mechanism in galaxies with cuspy bulges

    NASA Astrophysics Data System (ADS)

    Polyachenko, E. V.; Berczik, P.; Just, A.

    2016-08-01

    We show by numerical simulations that a purely stellar dynamical model composed of an exponential disc, a cuspy bulge, and an NFW halo with parameters relevant to the Milky Way Galaxy is subject to bar formation. Taking into account the finite disc thickness, the bar formation can be explained by the usual bar instability, in spite of the presence of an inner Lindblad resonance, that is believed to damp any global modes. The effect of replacing the live halo and bulge by a fixed external axisymmetric potential (rigid models) is studied. It is shown that while the e-folding time of bar instability increases significantly (from 250 to 500 Myr), the bar pattern speed remains almost the same. For the latter, our average value of 55 km/s/kpc agrees with the assumption that the Hercules stream in the solar neighbourhood is an imprint of the bar-disc interaction at the outer Lindblad resonance of the bar. Vertical averaging of the radial force in the central disc region comparable to the characteristic scale length allows us to reproduce the bar pattern speed and the growth rate of the rigid models, using normal mode analysis of linear perturbation theory in a razor thin disc. The strong increase of the e-folding time with decreasing disc mass predicted by the mode analysis suggests that bars in galaxies similar to the Milky Way have formed only recently.

  12. Annealing-induced change in quantum dot chain formation mechanism

    SciTech Connect

    Park, Tyler D.; Colton, John S.; Farrer, Jeffrey K.; Yang, Haeyeon; Kim, Dong Jun

    2014-12-15

    Self-assembled InGaAs quantum dot chains were grown using a modified Stranski-Krastanov method in which the InGaAs layer is deposited under a low growth temperature and high arsenic overpressure, which suppresses the formation of dots until a later annealing process. The dots are capped with a 100 nm GaAs layer. Three samples, having three different annealing temperatures of 460°C, 480°C, and 500°C, were studied by transmission electron microscopy. Results indicate two distinct types of dot formation processes: dots in the 460°C and 480°C samples form from platelet precursors in a one-to-one ratio whereas the dots in the sample annealed at 500°C form through the strain-driven self-assembly process, and then grow larger via an additional Ostwald ripening process whereby dots grow into larger dots at the expense of smaller seed islands. There are consequently significant morphological differences between the two types of dots, which explain many of the previously-reported differences in optical properties. Moreover, we also report evidence of indium segregation within the dots, with little or no indium intermixing between the dots and the surrounding GaAs barrier.

  13. On the bar formation mechanism in galaxies with cuspy bulges

    NASA Astrophysics Data System (ADS)

    Polyachenko, E. V.; Berczik, P.; Just, A.

    2016-11-01

    We show by numerical simulations that a purely stellar dynamical model composed of an exponential disc, a cuspy bulge, and a Navarro-Frenk-White halo with parameters relevant to the Milky Way is subject to bar formation. Taking into account the finite disc thickness, the bar formation can be explained by the usual bar instability, in spite of the presence of an inner Lindblad resonance, that is believed to damp any global modes. The effect of replacing the live halo and bulge by a fixed external axisymmetric potential (rigid models) is studied. It is shown that while the e-folding time of bar instability increases significantly (from 250 to 500 Myr), the bar pattern speed remains almost the same. For the latter, our average value of 55 km s-1 kpc-1 agrees with the assumption that the Hercules stream in the solar neighbourhood is an imprint of the bar-disc interaction at the outer Lindblad resonance of the bar. Vertical averaging of the radial force in the central disc region comparable to the characteristic scale length allows us to reproduce the bar pattern speed and the growth rate of the rigid models, using normal mode analysis of linear perturbation theory in a razor-thin disc. The strong increase of the e-folding time with decreasing disc mass predicted by the mode analysis suggests that bars in galaxies similar to the Milky Way have formed only recently.

  14. Biomolecular Mechanisms of Pseudomonas aeruginosa and Escherichia coli Biofilm Formation

    PubMed Central

    Laverty, Garry; Gorman, Sean P.; Gilmore, Brendan F.

    2014-01-01

    Pseudomonas aeruginosa and Escherichia coli are the most prevalent Gram-negative biofilm forming medical device associated pathogens, particularly with respect to catheter associated urinary tract infections. In a similar manner to Gram-positive bacteria, Gram-negative biofilm formation is fundamentally determined by a series of steps outlined more fully in this review, namely adhesion, cellular aggregation, and the production of an extracellular polymeric matrix. More specifically this review will explore the biosynthesis and role of pili and flagella in Gram-negative adhesion and accumulation on surfaces in Pseudomonas aeruginosa and Escherichia coli. The process of biofilm maturation is compared and contrasted in both species, namely the production of the exopolysaccharides via the polysaccharide synthesis locus (Psl), pellicle Formation (Pel) and alginic acid synthesis in Pseudomonas aeruginosa, and UDP-4-amino-4-deoxy-l-arabinose and colonic acid synthesis in Escherichia coli. An emphasis is placed on the importance of the LuxR homologue sdiA; the luxS/autoinducer-II; an autoinducer-III/epinephrine/norepinephrine and indole mediated Quorum sensing systems in enabling Gram-negative bacteria to adapt to their environments. The majority of Gram-negative biofilms consist of polysaccharides of a simple sugar structure (either homo- or heteropolysaccharides) that provide an optimum environment for the survival and maturation of bacteria, allowing them to display increased resistance to antibiotics and predation. PMID:25438014

  15. Commitment and exploration as mechanisms of identity formation.

    PubMed

    Meeus, W; Iedema, J; Maassen, G H

    2002-06-01

    We report a two-wave longitudinal study of 1,571 Dutch adolescents concerning the role of commitment and exploration in identity development. We used the Utrecht-Groningen Identity Development Scale to measure commitment and exploration in the domains of relational and societal identity. Our results can be summarized in three points. (1) Commitment and exploration are related processes in the development of identity. Adolescents with strong commitments also frequently explore them, and adolescents with low exploration in general have weak commitments. (2) The longitudinal stability of commitment and exploration has a medium effect size. For relational identity the stability of commitment is greater than that of exploration, but this is not the case for societal identity. The explanation we give for the lack of this difference in stability between commitment and exploration in societal identity is that the formative period for societal identity comes primarily at the end of adolescence. In that connection, we conclude that for present-day Dutch adolescents the formation of relational identity probably precedes that of societal identity. (3) In neither identity domain is commitment predictive of exploration three years later, nor is the reverse the case. We conclude that no long-term developmental sequentiality of commitment and exploration was found, but the results do not rule out the possibility of short-term developmental sequentiality.

  16. Molecular mechanisms of compounds affecting bacterial biofilm formation and dispersal.

    PubMed

    Landini, Paolo; Antoniani, Davide; Burgess, J Grant; Nijland, Reindert

    2010-04-01

    Bacteria can switch between planktonic forms (single cells) and biofilms, i.e., bacterial communities growing on solid surfaces and embedded in a matrix of extracellular polymeric substance. Biofilm formation by pathogenic bacteria often results in lower susceptibility to antibiotic treatments and in the development of chronic infections; thus, biofilm formation can be considered an important virulence factor. In recent years, much attention has been directed towards understanding the biology of biofilms and towards searching for inhibitors of biofilm development and of biofilm-related cellular processes. In this report, we review selected examples of target-based screening for anti-biofilm agents: We focus on inhibitors of quorum sensing, possibly the most characterized target for molecules with anti-biofilm activity, and on compounds interfering with the metabolism of the signal molecule cyclic di-GMP metabolism and on inhibitors of DNA and nucleotide biosynthesis, which represent a novel and promising class of biofilm inhibitors. Finally, we discuss the activation of biofilm dispersal as a novel mode of action for anti-biofilm compounds. PMID:20165945

  17. Biomolecular Mechanisms of Pseudomonas aeruginosa and Escherichia coli Biofilm Formation.

    PubMed

    Laverty, Garry; Gorman, Sean P; Gilmore, Brendan F

    2014-07-18

    Pseudomonas aeruginosa and Escherichia coli are the most prevalent Gram-negative biofilm forming medical device associated pathogens, particularly with respect to catheter associated urinary tract infections. In a similar manner to Gram-positive bacteria, Gram-negative biofilm formation is fundamentally determined by a series of steps outlined more fully in this review, namely adhesion, cellular aggregation, and the production of an extracellular polymeric matrix. More specifically this review will explore the biosynthesis and role of pili and flagella in Gram-negative adhesion and accumulation on surfaces in Pseudomonas aeruginosa and Escherichia coli. The process of biofilm maturation is compared and contrasted in both species, namely the production of the exopolysaccharides via the polysaccharide synthesis locus (Psl), pellicle Formation (Pel) and alginic acid synthesis in Pseudomonas aeruginosa, and UDP-4-amino-4-deoxy-l-arabinose and colonic acid synthesis in Escherichia coli. An emphasis is placed on the importance of the LuxR homologue sdiA; the luxS/autoinducer-II; an autoinducer-III/epinephrine/norepinephrine and indole mediated Quorum sensing systems in enabling Gram-negative bacteria to adapt to their environments. The majority of Gram-negative biofilms consist of polysaccharides of a simple sugar structure (either homo- or heteropolysaccharides) that provide an optimum environment for the survival and maturation of bacteria, allowing them to display increased resistance to antibiotics and predation.

  18. Acute formation of lumbar discal cyst: what is the mechanism?

    PubMed

    Aydin, S; Kucukyuruk, B; Yildirim, H; Abuzayed, B; Bozkus, H; Vural, M

    2010-12-01

    Lumbar discal cysts are extremely rare pathologies, with only few reports describing these lesions in the literature. Moreover, their definite pathogenesis is still unknown, with proposed theories based on radialogic and histologic findings. In this report, the authors present an acute formation of a discal cyst, which is reported for the first time. Also, we center our case on the discussion of the possible pathogenesis. Also, this is the first case of discal cyst reported in Turkey. A 67-year-old woman, whose complaints, and clinical and radiological findings demonstrated lumbar disc herniation with acute Modic 1 degererative changes of the adjacent end plates of L3-4 level. After medical and physical therapies, follow-up lumbar MRI has been taken to demonstrate a discal cyst formation on the adjacent intervertebral disc, showed cranially migrated cyst superior posterior on herniated disc, in 2 weeks period. The patient was treated by microsurgical resection of the cyst, and her complaints resolved completely. PMID:21423085

  19. GAS PHASE SYNTHESIS OF (ISO)QUINOLINE AND ITS ROLE IN THE FORMATION OF NUCLEOBASES IN THE INTERSTELLAR MEDIUM

    SciTech Connect

    Parker, Dorian S. N.; Kaiser, Ralf I.; Kostko, Oleg; Troy, Tyler P.; Ahmed, Musahid; Mebel, Alexander M.; Tielens, Alexander G. G. M.

    2015-04-20

    Nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) have been proposed to play a key role in the astrochemical evolution of the interstellar medium, yet the formation mechanisms of even their simplest prototypes—quinoline and isoquinoline—remain elusive. Here, we reveal a novel concept that under high temperature conditions representing circumstellar envelopes of carbon stars, (iso)quinoline can be synthesized via the reaction of pyridyl radicals with two acetylene molecules. The facile gas phase formation of (iso)quinoline in circumstellar envelopes defines a hitherto elusive reaction class synthesizing aromatic structures with embedded nitrogen atoms that are essential building blocks in contemporary biological-structural motifs. Once ejected from circumstellar shells and incorporated into icy interstellar grains in cold molecular clouds, these NPAHs can be functionalized by photo processing forming nucleobase-type structures as sampled in the Murchison meteorite.

  20. Effect of integral proteins in the phase stability of a lipid bilayer: Application to raft formation in cell membranes

    NASA Astrophysics Data System (ADS)

    Gómez, Jordi; Sagués, Francesc; Reigada, Ramon

    2010-04-01

    The existence of lipid rafts is a controversial issue. The affinity of cholesterol for saturated lipids is manifested in macroscopic phase separation in model membranes, and is believed to be the thermodynamic driving force for raft formation. However, there is no clear reason to explain the small (nanometric) size of raft domains in cell membranes. In a recent paper Yethiraj and Weisshaar [Biophys. J. 93, 3113 (2007)] proposed that the effect of neutral integral membrane proteins may prevent from the formation of large lipid domains. In this paper we extend this approach by studying the effect of the protein size, as well as the lipid-protein interaction. Depending on these factors, two different mechanisms for nanodomain stabilization are shown to be possible for static proteins. The application of these results to a biological context is discussed.

  1. Metastable bcc phase formation in the Nb-Cr-Ti system

    SciTech Connect

    Thoma, D.J.; Perepezko, J.H.

    1994-08-01

    Metastable disordered bcc phases have been formed from the melt in the Nb-Cr-Ti system where primary Laves phases would develop under equilibrium solidification conditions. Three vertical temperature-composition sections in the ternary system incorporating NbCr, were evaluated: the Nb-Cr binary, the TiCr{sub 2}-NbCr{sub 2} isoplethal section, and the NbCr{sub 2}-Ti plethal section. In the rapid solidification of NbCr{sub 2}, metastable bcc phase formation was not observed, but deviations from NbCr{sub 2} stoichiometry or alloying with Ti was found to promote bcc phase formation by decreasing the required liquid undercooling to reach the metastable bcc liquidus and solidus. The metastable phases were characterized through x-ray diffraction (XRD), and systematic deviations from Vegard`s Rule have been defined in the three plethal sections. The metastable bcc phases decompose at temperatures >800{degrees}C to uniformly refined microstructures. As a result, novel microstructural tailoring schemes are possible through the metastable precursor microstructures.

  2. Atomic scale observation of phase separation and formation of silicon clusters in Hf higk-κ silicates

    NASA Astrophysics Data System (ADS)

    Talbot, E.; Roussel, M.; Genevois, C.; Pareige, P.; Khomenkova, L.; Portier, X.; Gourbilleau, F.

    2012-05-01

    Hafnium silicate films were fabricated by RF reactive magnetron sputtering technique. Fine microstructural analyses of the films were performed by means of high-resolution transmission electron microscopy and atom probe tomography. A thermal treatment of as-grown homogeneous films leads to a phase separation process. The formation of SiO2 and HfO2 phases as well as pure Si one was revealed. This latter was found to be amorphous Si nanoclusters, distributed uniformly in the film volume. Their mean diameter and density were estimated to be about 2.8 nm and (2.9 ± 0.4) × 1017 Si-ncs/cm3, respectively. The mechanism of the decomposition process was proposed. The obtained results pave the way for future microelectronic and photonic applications of Hf-based high-κ dielectrics with embedded Si nanoclusters.

  3. Atomic scale observation of phase separation and formation of silicon clusters in Hf higk-{kappa} silicates

    SciTech Connect

    Talbot, E.; Roussel, M.; Genevois, C.; Pareige, P.; Khomenkova, L.; Portier, X.; Gourbilleau, F.

    2012-05-15

    Hafnium silicate films were fabricated by RF reactive magnetron sputtering technique. Fine microstructural analyses of the films were performed by means of high-resolution transmission electron microscopy and atom probe tomography. A thermal treatment of as-grown homogeneous films leads to a phase separation process. The formation of SiO{sub 2} and HfO{sub 2} phases as well as pure Si one was revealed. This latter was found to be amorphous Si nanoclusters, distributed uniformly in the film volume. Their mean diameter and density were estimated to be about 2.8 nm and (2.9 {+-} 0.4) x 10{sup 17} Si-ncs/cm{sup 3}, respectively. The mechanism of the decomposition process was proposed. The obtained results pave the way for future microelectronic and photonic applications of Hf-based high-{kappa} dielectrics with embedded Si nanoclusters.

  4. A possible oriented attachment growth mechanism for silver nanowire formation

    SciTech Connect

    Murph, Simona E. Hunyadi; Murphy, Catherine J.; Leach, Austin; Gall, Kenneth

    2015-04-06

    Electron microscopy studies suggest that silver nanowires prepared by an approach reported earlier by us (Caswell, K. K., Bender, C. M., Murphy, C. J. Nano Lett.,2003, 3, 667–669) form through a coarsening process via an oriented attachment mechanism. Initially, silver nucleation centers were produced by chemical reduction of silver ions in boiling water, with sodium citrate and sodium hydroxide as additives in solution. These nucleation centers, with a twinned crystallographic orientation, ultimately merge into fully grown silver nanowires. This is a completely different mechanism from the seed-mediated growth approach, which has also been used to produce silver nanowires. Furthermore, companion molecular dynamics performed with the embedded atom method are in agreement with our experimental data.

  5. A possible oriented attachment growth mechanism for silver nanowire formation

    DOE PAGESBeta

    Murph, Simona E. Hunyadi; Murphy, Catherine J.; Leach, Austin; Gall, Kenneth

    2015-04-06

    Electron microscopy studies suggest that silver nanowires prepared by an approach reported earlier by us (Caswell, K. K., Bender, C. M., Murphy, C. J. Nano Lett.,2003, 3, 667–669) form through a coarsening process via an oriented attachment mechanism. Initially, silver nucleation centers were produced by chemical reduction of silver ions in boiling water, with sodium citrate and sodium hydroxide as additives in solution. These nucleation centers, with a twinned crystallographic orientation, ultimately merge into fully grown silver nanowires. This is a completely different mechanism from the seed-mediated growth approach, which has also been used to produce silver nanowires. Furthermore, companionmore » molecular dynamics performed with the embedded atom method are in agreement with our experimental data.« less

  6. Binary Formation Mechanisms: Constraints from the Companion Mass Ratio Distribution

    NASA Astrophysics Data System (ADS)

    Reggiani, Maddalena M.; Meyer, Michael R.

    2011-09-01

    We present a statistical comparison of the mass ratio distribution of companions, as observed in different multiplicity surveys, to the most recent estimate of the single-object mass function. The main goal of our analysis is to test whether or not the observed companion mass ratio distribution (CMRD) as a function of primary star mass and star formation environment is consistent with having been drawn from the field star initial mass function (IMF). We consider samples of companions for M dwarfs, solar-type stars, and intermediate-mass stars, both in the field as well as clusters or associations, and compare them with populations of binaries generated by random pairing from the assumed IMF for a fixed primary mass. With regard to the field we can reject the hypothesis that the CMRD was drawn from the IMF for different primary mass ranges: the observed CMRDs show a larger number of equal-mass systems than predicted by the IMF. This is in agreement with fragmentation theories of binary formation. For the open clusters α Persei and the Pleiades we also reject the IMF random-pairing hypothesis. Concerning young star-forming regions, currently we can rule out a connection between the CMRD and the field IMF in Taurus but not in Chamaeleon I. Larger and different samples are needed to better constrain the result as a function of the environment. We also consider other companion mass functions and we compare them with observations. Moreover the CMRD both in the field and clusters or associations appears to be independent of separation in the range covered by the observations. Combining therefore the CMRDs of M (1-2400 AU) and G (28-1590 AU) primaries in the field and intermediate-mass primary binaries in Sco OB2 (29-1612 AU) for mass ratios, q = M 2/M 1, from 0.2 to 1, we find that the best chi-square fit follows a power law dN/dqvpropq β, with β = -0.50 ± 0.29, consistent with previous results. Finally, we note that the Kolmogorov-Smirnov test gives a ~1% probability

  7. Benzyne-related mechanisms in the gas phase ion/molecule reactions of haloarenes

    NASA Astrophysics Data System (ADS)

    Linnert, Harrald V.; Riveros, José M.

    1994-12-01

    The low pressure ion/molecule reactions of NH-2, OHt- and MeO- with bromobenzene have been studied by Fourier transform ion cyclotron resonance to elucidate gas phase benzyne mechanisms. For OHt- and MeO-, the main reaction consists of benzyne elimination initiated by abstraction of an ortho proton and the subsequent formation of a solvated halide ion. Experiments with monodeuterated bromobenzenes suggest that reactions with OHt- are the result of long-lived complexes with extensive scrambling of hydrogen and deuterium. For NH-2, formation of all the isomeric bromophenide ions occurs without hydrogen-deuterium scrambling, revealing weak building in the collision complexes. The o-bromophenide ions are shown to react with other substrates by bromide transfer rather than by proton transfer, and evidence is presented that the meta and para isomers undergo isomerization to the ortho upon reaction with substrates of similar acidities. The [Delta]H0acid of bromobenzene is estimated to be 384.4 kcal mol- from bracketing experiments. An extension of these reactions to 1- and 2-bromonaphthalene provides an estimate for the heats of formation of 1,2-dehydronaphthalene (122 kcal mol-) and the 2,3-dehydronaphthalene (126 kcal mol-).

  8. Towards understanding of shape formation mechanism of mesoporous silica particles.

    PubMed

    Volkov, Dmytro O; Benson, James; Kievsky, Yaroslav Y; Sokolov, Igor

    2010-01-14

    Growth of even simple crystals is a rather hard problem to describe because of the non-equilibrium nature of the process. Meso(nano)porous silica particles, which are self-assembled in a sol-gel template synthesis, demonstrate an example of shapes of high complexity, similar to those observed in the biological world. Despite such complexity, here we present the evidence that at least a part of the formation of these shapes is an equilibrium process. We demonstrate it for an example of mesoporous fibers, one of the abundant shapes. We present a quantitative proof that the fiber free energy is described by the Boltzmann distribution, which is predicted by the equilibrium thermodynamics. This finding may open up new ground for a quantitative description of the morphogenesis of complex self-assembled shapes, including biological hierarchy.

  9. Molecular Mechanisms for Vascular Development and Secondary Cell Wall Formation.

    PubMed

    Yang, Jung Hyun; Wang, Huanzhong

    2016-01-01

    Vascular tissues are important for transporting water and nutrients throughout the plant and as physical support of upright growth. The primary constituents of vascular tissues, xylem, and phloem, are derived from the meristematic vascular procambium and cambium. Xylem cells develop secondary cell walls (SCWs) that form the largest part of plant lignocellulosic biomass that serve as a renewable feedstock for biofuel production. For the last decade, research on vascular development and SCW biosynthesis has seen rapid progress due to the importance of these processes to plant biology and to the biofuel industry. Plant hormones, transcriptional regulators and peptide signaling regulate procambium/cambium proliferation, vascular patterning, and xylem differentiation. Transcriptional regulatory pathways play a pivot role in SCW biosynthesis. Although most of these discoveries are derived from research in Arabidopsis, many genes have shown conserved functions in biofuel feedstock species. Here, we review the recent advances in our understanding of vascular development and SCW formation and discuss potential biotechnological uses. PMID:27047525

  10. Molecular Mechanisms for Vascular Development and Secondary Cell Wall Formation

    PubMed Central

    Yang, Jung Hyun; Wang, Huanzhong

    2016-01-01

    Vascular tissues are important for transporting water and nutrients throughout the plant and as physical support of upright growth. The primary constituents of vascular tissues, xylem, and phloem, are derived from the meristematic vascular procambium and cambium. Xylem cells develop secondary cell walls (SCWs) that form the largest part of plant lignocellulosic biomass that serve as a renewable feedstock for biofuel production. For the last decade, research on vascular development and SCW biosynthesis has seen rapid progress due to the importance of these processes to plant biology and to the biofuel industry. Plant hormones, transcriptional regulators and peptide signaling regulate procambium/cambium proliferation, vascular patterning, and xylem differentiation. Transcriptional regulatory pathways play a pivot role in SCW biosynthesis. Although most of these discoveries are derived from research in Arabidopsis, many genes have shown conserved functions in biofuel feedstock species. Here, we review the recent advances in our understanding of vascular development and SCW formation and discuss potential biotechnological uses. PMID:27047525

  11. The effect of dynamic recrystallization and LPO formation on deformation mechanisms in experimentally deformed plagioclase aggregates

    NASA Astrophysics Data System (ADS)

    Meyers, C. D.; Hirth, G.; Cross, A. J.; Prior, D. J.

    2013-12-01

    degrees antithetic to the shear plane, defined by a concentration of recrystallized grains. We believe that these shear bands are weak relative to the surrounding relict grains. The microstructural data and mechanical behavior indicate that this is the result of a transition to a larger contribution of deformation by diffusion creep in the newly formed fine grains. These results lend support to the field boundary hypothesis with regards to deformation of plagioclase under our experimental conditions. This has implications for mechanical behavior of the lower crust and illustrates a possible role of grain size reduction in the formation of weakened shear zones. This also provides a possible mechanism for strain localization in a single phase rock.

  12. Insights into the Formation and Evolution of Individual Compounds in the Particulate Phase during Aromatic Photo-Oxidation.

    PubMed

    Pereira, Kelly L; Hamilton, Jacqueline F; Rickard, Andrew R; Bloss, William J; Alam, Mohammed S; Camredon, Marie; Ward, Martyn W; Wyche, Kevin P; Muñoz, Amalia; Vera, Teresa; Vázquez, Mónica; Borrás, Esther; Ródenas, Milagros

    2015-11-17

    Secondary organic aerosol (SOA) is well-known to have adverse effects on air quality and human health. However, the dynamic mechanisms occurring during SOA formation and evolution are poorly understood. The time-resolved SOA composition formed during the photo-oxidation of three aromatic compounds, methyl chavicol, toluene and 4-methyl catechol, were investigated at the European Photoreactor. SOA was collected using a particle into liquid sampler and analyzed offline using state-of-the-art mass spectrometry to produce temporal profiles of individual photo-oxidation products. In the photo-oxidation of methyl chavicol, 70 individual compounds were characterized and three distinctive temporal profile shapes were observed. The calculated mass fraction (Ci,aer/COA) of the individual SOA compounds showed either a linear trend (increasing/decreasing) or exponential decay with time. Substituted nitrophenols showed an exponential decay, with the nitro-group on the aromatic ring found to control the formation and loss of these species in the aerosol phase. Nitrophenols from both methyl chavicol and toluene photo-oxidation experiments showed a strong relationship with the NO2/NO (ppbv/ppbv) ratio and were observed during initial SOA growth. The location of the nitrophenol aromatic substitutions was found to be critically important, with the nitrophenol in the photo-oxidation of 4-methyl catechol not partitioning into the aerosol phase until irradiation had stopped; highlighting the importance of studying SOA formation and evolution at a molecular level.

  13. Direct Observations of Sigma Phase Formation in Duplex Stainless Steels Using In Situ Synchrotron X-Ray Diffraction

    SciTech Connect

    Elmer, J. W.; Palmer, T. A.; Specht, Eliot D

    2007-01-01

    The formation and growth of sigma ( ) phase in 2205 duplex stainless steel (DSS) was observed and measured in real time using synchrotron radiation during 10 hour isothermal heat treatments at temperatures between 700 C and 850 C. Sigma formed in near-equilibrium quantities during the isothermal holds, starting from a microstructure which contained a balanced mixture of metastable ferrite and austenite. In-situ synchrotron diffraction continuously monitored the transformation, and these results were compared to those predicted by thermodynamic calculations. The data were further analyzed using a modified Johnson-Mehl-Avrami-Kolmogrov (JMAK) approach to determine kinetic parameters for sigma formation over this temperature range. The initial JMAK exponent, n, at low fractions of sigma was found to be approximately 7.0; however, toward the end of the transformation, n decreased to values of approximately 0.75. The change in the JMAK exponent was attributed to a change in the transformation mechanism from discontinuous precipitation with increasing nucleation rate, to growth of the existing sigma phase after nucleation site saturation occurred. Because of this change in mechanism, it was not possible to determine reliable values for the activation energy and pre-exponential terms for the JMAK equation. While cooling back to room temperature, the partial transformation of austenite resulted in a substantial increase in the ferrite content, but sigma retained its high-temperature value to room temperature.

  14. Direct Observations of Sigma Phase Formation in Duplex Stainless Steels using In Situ Synchrotron X-Ray Diffraction

    SciTech Connect

    Elmer, J W; Palmer, T A; Specht, E D

    2006-07-03

    The formation and growth of sigma phase in 2205 duplex stainless steel was observed and measured in real time using synchrotron radiation during 10 hr isothermal heat treatments at temperatures between 700 C and 850 C. Sigma formed in near-equilibrium quantities during the isothermal holds, starting from a microstructure which contained a balanced mixture of metastable ferrite and austenite. In situ synchrotron diffraction continuously monitored the transformation, and these results were compared to those predicted by thermodynamic calculations. Differences between the calculated and measured amounts of sigma, ferrite and austenite suggest that the thermodynamic calculations underpredict the sigma dissolution temperature by approximately 50 C. The data were further analyzed using a modified Johnson-Mehl-Avrami (JMA) approach to determine kinetic parameters for sigma formation over this temperature range. The initial JMA exponent, n, at low fractions of sigma was found to be approximately 7.0, however, towards the end of the transformation, n decreased to values of approximately 0.75. The change in the JMA exponent was attributed to a change in the transformation mechanism from discontinuous precipitation with increasing nucleation rate, to growth of the existing sigma phase after nucleation site saturation occurred. Because of this change in mechanism, it was not possible to determine reliable values for the activation energy and pre-exponential terms for the JMA equation. While cooling back to room temperature, the partial transformation of austenite resulted in a substantial increase in the ferrite content, but sigma retained its high temperature value to room temperature.

  15. Unexpected δ-Phase Formation in Additive-Manufactured Ni-Based Superalloy

    NASA Astrophysics Data System (ADS)

    Idell, Y.; Levine, L. E.; Allen, A. J.; Zhang, F.; Campbell, C. E.; Olson, G. B.; Gong, J.; Snyder, D. R.; Deutchman, H. Z.

    2016-03-01

    An as-built and solutionized Ni-based superalloy built by additive manufacturing through a direct metal laser sintering technique is characterized to understand the microstructural differences as compared to the as-wrought alloy. Initially, each layer undergoes rapid solidification as it is melted by the laser; however, as the part is built, the underlying layers experience a variety of heating and cooling cycles that produce significant microsegregation of niobium which allows for the formation of the deleterious δ-phase. The as-built microstructure was characterized through Vickers hardness, optical microscopy, scanning and transmission electron microscopy, electron back-scattering diffraction, x-ray diffraction, and synchrotron x-ray microLaue diffraction. The isothermal formation and growth of the δ-phase were characterized using synchrotron-based in situ small angle and wide angle x-ray scattering experiments. These experimental results are compared with multicomponent diffusion simulations that predict the phase fraction and composition. The high residual stresses and unexpected formation of the δ-phase will require further annealing treatments to be designed so as to remove these deficiencies and obtain an optimized microstructure.

  16. The Inflow Signature toward Different Evolutionary Phases of Massive Star Formation

    NASA Astrophysics Data System (ADS)

    Jin, Mihwa; Lee, Jeong-Eun; Kim, Kee-Tae; Evans, Neal J., II

    2016-08-01

    We analyze both HCN J = 1-0 and HNC J = 1-0 line profiles to study the inflow motions in different evolutionary stages of massive star formation: 54 infrared dark clouds (IRDCs), 69 high-mass protostellar objects (HMPOs), and 54 ultra-compact H ii regions (UCHIIs). Inflow asymmetry in the HCN spectra seems to be prevalent throughout all the three evolutionary phases, with IRDCs showing the largest excess in the blue profile. In the case of the HNC spectra, the prevalence of blue sources does not appear, apart from for IRDCs. We suggest that this line is not appropriate to trace the inflow motion in the evolved stages of massive star formation, because the abundance of HNC decreases at high temperatures. This result highlights the importance of considering chemistry in dynamics studies of massive star-forming regions. The fact that the IRDCs show the highest blue excess in both transitions indicates that the most active inflow occurs in the early phase of star formation, i.e., in the IRDC phase rather than in the later phases. However, mass is still inflowing onto some UCHIIs. We also find that the absorption dips of the HNC spectra in six out of seven blue sources are redshifted relative to their systemic velocities. These redshifted absorption dips may indicate global collapse candidates, although mapping observations with better resolution are needed to examine this feature in more detail.

  17. The Inflow Signature toward Different Evolutionary Phases of Massive Star Formation

    NASA Astrophysics Data System (ADS)

    Jin, Mihwa; Lee, Jeong-Eun; Kim, Kee-Tae; Evans, Neal J., II

    2016-08-01

    We analyze both HCN J = 1–0 and HNC J = 1–0 line profiles to study the inflow motions in different evolutionary stages of massive star formation: 54 infrared dark clouds (IRDCs), 69 high-mass protostellar objects (HMPOs), and 54 ultra-compact H ii regions (UCHIIs). Inflow asymmetry in the HCN spectra seems to be prevalent throughout all the three evolutionary phases, with IRDCs showing the largest excess in the blue profile. In the case of the HNC spectra, the prevalence of blue sources does not appear, apart from for IRDCs. We suggest that this line is not appropriate to trace the inflow motion in the evolved stages of massive star formation, because the abundance of HNC decreases at high temperatures. This result highlights the importance of considering chemistry in dynamics studies of massive star-forming regions. The fact that the IRDCs show the highest blue excess in both transitions indicates that the most active inflow occurs in the early phase of star formation, i.e., in the IRDC phase rather than in the later phases. However, mass is still inflowing onto some UCHIIs. We also find that the absorption dips of the HNC spectra in six out of seven blue sources are redshifted relative to their systemic velocities. These redshifted absorption dips may indicate global collapse candidates, although mapping observations with better resolution are needed to examine this feature in more detail.

  18. Low temperature phase formation of Tl-based superconducting thin films in reduced oxygen atmosphere

    NASA Technical Reports Server (NTRS)

    Wu, C. Y.; Foong, F.; Liou, S. H.; Ho, J. C.

    1993-01-01

    Tl-Ba-Cu-Cu-O superconducting thin films were prepared by magnetron sputtering with postannealing in a reduced oxygen atmosphere. Single-phase Tl2Ba2Ca2Cu3O(x) can form on the MgO substrate at 800 C under P(O2) about 0.1 atm. However the phase formation temperature can be affected by the starting composition of the film. Tl1Ba2Ca2Cu3O(x) phase can be formed by simply lowering the Tl2O pressure. The thermal stability of Tl1Ba2Ca2Cu3O(x) phase was studied by resistivity measurements at high temperatures.

  19. Optically transparent, mechanically durable, nanostructured superhydrophobic surfaces enabled by spinodally phase-separated glass thin films.

    PubMed

    Aytug, Tolga; Simpson, John T; Lupini, Andrew R; Trejo, Rosa M; Jellison, Gerald E; Ivanov, Ilia N; Pennycook, Stephen J; Hillesheim, Daniel A; Winter, Kyle O; Christen, David K; Hunter, Scott R; Haynes, J Allen

    2013-08-01

    We describe the formation and properties of atomically bonded, optical quality, nanostructured thin glass film coatings on glass plates, utilizing phase separation by spinodal decomposition in a sodium borosilicate glass system. Following deposition via magnetron sputtering, thermal processing and differential etching, these coatings are structurally superhydrophilic (i.e., display anti-fogging functionality) and demonstrate robust mechanical properties and superior abrasion resistance. After appropriate chemical surface modification, the surfaces display a stable, non-wetting Cassie-Baxter state and exhibit exceptional superhydrophobic performance, with water droplet contact angles as large as 172°. As an added benefit, in both superhydrophobic and superhydrophilic states these nanostructured surfaces can block ultraviolet radiation and can be engineered to be anti-reflective with broadband and omnidirectional transparency. Thus, the present approach could be tailored toward distinct coatings for numerous markets, such as residential windows, windshields, specialty optics, goggles, electronic and photovoltaic cover glasses, and optical components used throughout the US military.

  20. Optically transparent, mechanically durable, nanostructured superhydrophobic surfaces enabled by spinodally phase-separated glass thin films

    NASA Astrophysics Data System (ADS)

    Aytug, Tolga; Simpson, John T.; Lupini, Andrew R.; Trejo, Rosa M.; Jellison, Gerald E.; Ivanov, Ilia N.; Pennycook, Stephen J.; Hillesheim, Daniel A.; Winter, Kyle O.; Christen, David K.; Hunter, Scott R.; Haynes, J. Allen

    2013-08-01

    We describe the formation and properties of atomically bonded, optical quality, nanostructured thin glass film coatings on glass plates, utilizing phase separation by spinodal decomposition in a sodium borosilicate glass system. Following deposition via magnetron sputtering, thermal processing and differential etching, these coatings are structurally superhydrophilic (i.e., display anti-fogging functionality) and demonstrate robust mechanical properties and superior abrasion resistance. After appropriate chemical surface modification, the surfaces display a stable, non-wetting Cassie-Baxter state and exhibit exceptional superhydrophobic performance, with water droplet contact angles as large as 172°. As an added benefit, in both superhydrophobic and superhydrophilic states these nanostructured surfaces can block ultraviolet radiation and can be engineered to be anti-reflective with broadband and omnidirectional transparency. Thus, the present approach could be tailored toward distinct coatings for numerous markets, such as residential windows, windshields, specialty optics, goggles, electronic and photovoltaic cover glasses, and optical components used throughout the US military.

  1. Properties of highly refractory corundum ceramics containing glass phase, and their influence on the surface roughness after mechanical treatment

    SciTech Connect

    Degtyareva, E.V.; Barannik, Y.P.; Kabakova, I.I.; Skorodumova, E.B.

    1985-09-01

    Systematic investigations were made of the effect of the content of silica additives in the form of aluminosilicate raw materials, synthetic glasses, (aluminoborosilicate), and also pure silicon dioxide and mixtures of it with magnesium, calcium, yttrium, zirconium, chromium oxides, and zircon concentrate on the formation of glass phase and the content in it of SiO/sub 2/, the mechanical strength and impact strength of the ceramics and their abrasion resistance. A study was also made of the relationship between these properties and the state of the surface finish of the ceramics after mechanical treatment. The study showed that the amount of glass phase formed after firing and the content in the glass phase of silica are determined by the concentration of silicon oxide contained in the additive.

  2. Formation of lead-aluminate ceramics: Reaction mechanisms in immobilizing the simulated lead sludge.

    PubMed

    Lu, Xingwen; Shih, Kaimin

    2015-11-01

    We investigated a strategy of blending lead-laden sludge and an aluminum-rich precursor to reduce the release of hazardous lead from the stabilized end products. To quantify lead transformation and determine its incorporation behavior, PbO was used to simulate the lead-laden sludge fired with γ-Al2O3 by Pb/Al molar ratios of 1/2 and 1/12 at 600-1000 °C for 0.25-10 h. The sintered products were identified and quantified using Rietveld refinement analysis of X-ray diffraction data from the products generated under different conditions. The results indicated that the different crystallochemical incorporations of hazardous lead occurred through the formation of PbAl2O4 and PbAl12O19 in systems with Pb/Al ratios of 1/2 and 1/12, respectively. PbAl2O4 was observed as the only product phase at temperature of 950 °C for 3h heating in Pb/Al of 1/2 system. For Pb/Al of 1/12 system, significant growth of the PbAl12O19 phase clearly occurred at 1000 °C for 3 h sintering. Different product microstructures were found in the sintered products between the systems with the Pb/Al ratios 1/2 and 1/12. The leaching performances of the PbO, Pb9Al8O21, PbAl2O4 and PbAl12O19 phases were compared using a constant pH 4.9 leaching test over 92 h. The leachability data indicated that the incorporation of lead into PbAl12O19 crystal is a preferred stabilization mechanism in aluminate-ceramics.

  3. Mechanisms of Formation and Transformation of Ni-Fe Hydroxycarbonates

    SciTech Connect

    Refait, Ph.; Jeannin, M.; Reffass, M.; Drissi, S.H.; Abdelmoula, M.; Genin, J.-M.R.

    2005-04-26

    The mechanisms of the transformation of (Ni,Fe)(OH)2 precipitates in carbonated aqueous solutions were studied. The reactions were monitored by measuring the redox potential of the aqueous suspension, and end products were studied by Moessbauer spectroscopy, X-ray diffraction and Raman spectroscopy. The oxidation processes were compared to those occurring without Ni, that is when the initial hydroxide is Fe(OH)2. Schematically, the oxidation of Fe(OH)2 involves two intermediate compounds, the carbonated GR of formula Fe{sup II}{sub 4}Fe{sup III}{sub 2}(OH){sub 12}CO{sub 3} {center_dot} 2H{sub 2}O, and ferrihydrite, before to lead finally to goethite {alpha}-FeOOH. It proved possible to prepare Ni(II)-Fe(III) hydroxycarbonates with ratios Fe/Ni from 1/6 to 1/3. When the Fe/Ni ratio is larger than 1/3, a two stage oxidation process takes place. The first stage leads to a Ni(II)-Fe(II)-Fe(III) hydroxycarbonate. The second stage corresponds to the oxidation of the Fe(II) remaining inside the hydroxycarbonate and leads to a mixture of Ni(II)-Fe(III) hydroxycarbonate with ferrihydrite. The main effect of Ni is then to stop the reaction at an intermediate stage, as Ni(II) is not oxidised by O2, leaving unchanged the main features of the mechanisms of transformation.

  4. Formation of gold nanorods by a stochastic "popcorn" mechanism.

    PubMed

    Edgar, Jonathan A; McDonagh, Andrew M; Cortie, Michael B

    2012-02-28

    Gold nanorods have significant technological potential and are of broad interest to the nanotechnology community. The discovery of the seeded, wet-chemical synthetic process to produce them may be regarded as a landmark in the control of metal nanoparticle shape. However, the mechanism by which the initial spherical gold seeds acquire anisotropy is a critical, yet poorly understood, factor. Here we examine the very early stages of rod growth using a combination of techniques including cryogenic transmission electron microscopy, optical spectroscopy, and computational modeling. Reconciliation of the available experimental observations can only be achieved by invoking a stochastic, "popcorn"-like mechanism of growth, in which individual seeds lie quiescent for some time before suddenly and rapidly growing into rods. This is quite different from the steady, concurrent growth of nanorods that has been previously generally assumed. Furthermore we propose that the shape is controlled by the ratio of surface energy of rod sides to rod ends, with values of this quantity in the range of 0.3-0.8 indicated for typical growth solutions. PMID:22301937

  5. In Situ Study of the Formation of Silicide Phases in Amorphous Co–Si Mixed Layers

    SciTech Connect

    Van Bockstael, C.; De Keyser, K; Demeulemeester, J; Vantomme, A; Van Meirhaeghe, R; Detavernier, C; Jordan-Sweet, J; Lavoie, C

    2010-01-01

    We investigate Co silicide phase formation when extra Si is added within an as deposited 50 nm Co film. The addition of Si is investigated for both the Co/SiO{sub 2} and Co/Si(1 0 0) system. A series of 10 Co-Si mixed films with a Si content varying from 21 to 59 at.% was prepared and investigated during annealing with in situ X-ray diffraction. The oxide system is used as reference system to identify phases that initially crystallize in an amorphous mixture of a given composition. Multiple phases can nucleate, and the temperature of crystallization depends on the Co-Si atomic ratio. Upon heating of the Co(Si)/Si system, the first reaction is a similar crystallization reaction of the Co(Si) mixture. Once the first phase is formed, one has the normal system of a silicide phase in contact with an unlimited amount of Si from the substrate, and the sequential phase formation towards CoSi{sub 2} is established. For deposited layers of composition ranging from 48%Si to 52%Si, the CoSi is the first phase to form and increasing the amount of Si leads to a remarkable improvement of the thermal stability of CoSi on Si(1 0 0). CoSi{sub 2} nucleation was extensively delayed by 150 C compared to the reaction observed from a pure Co film on Si(1 0 0). Electron backscatter diffraction measurements reveal that in this range, the gradual Si increase systematically leads to bigger CoSi grains (up to 20 {micro}m). This shows that the grain size of the CoSi precursor strongly affects the nucleation of the following CoSi{sub 2} phase. Laser-light scattering measurements suggest that adding more than 42%Si reduces the roughness of the CoSi{sub 2} layer.

  6. Single-phase and two-phase flow properties of mesaverde tight sandstone formation; random-network modeling approach

    NASA Astrophysics Data System (ADS)

    Bashtani, Farzad; Maini, Brij; Kantzas, Apostolos

    2016-08-01

    3D random networks are constructed in order to represent the tight Mesaverde formation which is located in north Wyoming, USA. The porous-space is represented by pore bodies of different shapes and sizes which are connected to each other by pore throats of varying length and diameter. Pore bodies are randomly distributed in space and their connectivity varies based on the connectivity number distribution which is used in order to generate the network. Network representations are then validated using publicly available mercury porosimetry experiments. The network modeling software solves the fundamental equations of two-phase immiscible flow incorporating wettability and contact angle variability. Quasi-static displacement is assumed. Single phase macroscopic properties (porosity, permeability) are calculated and whenever possible are compared to experimental data. Using this information drainage and imbibition capillary pressure, and relative permeability curves are predicted and (whenever possible) compared to experimental data. The calculated information is grouped and compared to available literature information on typical behavior of tight formations. Capillary pressure curve for primary drainage process is predicted and compared to experimental mercury porosimetry in order to validate the virtual porous media by history matching. Relative permeability curves are also calculated and presented.

  7. Phase stability, mechanical properties, hardness, and possible reactive routing of chromium triboride from first-principle investigations

    SciTech Connect

    Zhong, Ming-Min; Kuang, Xiao-Yu Wang, Zhen-Hua; Shao, Peng; Ding, Li-Ping; Huang, Xiao-Fen

    2013-12-21

    The first-principles calculations are employed to provide a fundamental understanding of the structural features and relative stability, mechanical and electronic properties, and possible reactive route for chromium triboride. The predicted new phase of CrB{sub 3} belongs to the rhombohedral phase with R-3m symmetry and it transforms into a hexagonal phase with P-6m2 symmetry at 64 GPa. The mechanical and thermodynamic stabilities of CrB{sub 3} are verified by the calculated elastic constants and formation enthalpies. Also, the full phonon dispersion calculations confirm the dynamic stability of predicted CrB{sub 3}. Considering the role of metallic contributions, the calculated hardness values from our semiempirical method for rhombohedral and hexagonal phases are 23.8 GPa and 22.1 GPa, respectively. In addition, the large shear moduli, Young's moduli, low Poisson's ratios, and small B/G ratios indicate that they are potential hard materials. Relative enthalpy calculations with respect to possible constituents are also investigated to assess the prospects for phase formation and an attempt at high-pressure synthesis is suggested to obtain chromium triboride.

  8. The formation mechanism of aluminum oxide tunnel barriers.

    SciTech Connect

    Cerezo, A.; Petford-Long, A. K.; Larson, D. J.; Pinitsoontorn, S.; Singleton, E. W.; Materials Science Division; Univ. Oxford; Seagate Tech.

    2006-01-01

    The functional properties of magnetic tunnel junctions are critically dependant on the nanoscale morphology of the insulating barrier (usually only a few atomic layers thick) that separates the two ferromagnetic layers. Three-dimensional atom probe analysis has been used to study the chemistry of a magnetic tunnel junction structure comprising an aluminium oxide barrier formed by in situ oxidation, both in the under-oxidized and fully oxidized states and before and after annealing. Low oxidation times result in discrete oxide islands. Further oxidation leads to a more continuous, but still non-stoichiometric, barrier with evidence that oxidation proceeds along the top of grain boundaries in the underlying CoFe layer. Post-deposition annealing leads to an increase in the barrier area, but only in the case of the fully oxidized and annealed structure is a continuous planar layer formed, which is close to the stoichiometric Al:O ratio of 2:3. These results are surprising, in that the planar layers are usually considered unstable with respect to breaking up into separate islands. Analysis of the various driving forces suggests that the formation of a continuous layer requires a combination of factors, including the strain energy resulting from the expansion of the oxide during internal oxidation on annealing.

  9. Barrier formation: potential molecular mechanism of enamel fluorosis.

    PubMed

    Lyaruu, D M; Medina, J F; Sarvide, S; Bervoets, T J M; Everts, V; Denbesten, P; Smith, C E; Bronckers, A L J J

    2014-01-01

    Enamel fluorosis is an irreversible structural enamel defect following exposure to supraoptimal levels of fluoride during amelogenesis. We hypothesized that fluorosis is associated with excess release of protons during formation of hypermineralized lines in the mineralizing enamel matrix. We tested this concept by analyzing fluorotic enamel defects in wild-type mice and mice deficient in anion exchanger-2a,b (Ae2a,b), a transmembrane protein in maturation ameloblasts that exchanges extracellular Cl(-) for bicarbonate. Defects were more pronounced in fluorotic Ae2a,b (-/-) mice than in fluorotic heterozygous or wild-type mice. Phenotypes included a hypermineralized surface, extensive subsurface hypomineralization, and multiple hypermineralized lines in deeper enamel. Mineral content decreased in all fluoride-exposed and Ae2a,b(-/-) mice and was strongly correlated with Cl(-). Exposure of enamel surfaces underlying maturation-stage ameloblasts to pH indicator dyes suggested the presence of diffusion barriers in fluorotic enamel. These results support the concept that fluoride stimulates hypermineralization at the mineralization front. This causes increased release of protons, which ameloblasts respond to by secreting more bicarbonates at the expense of Cl(-) levels in enamel. The fluoride-induced hypermineralized lines may form barriers that impede diffusion of proteins and mineral ions into the subsurface layers, thereby delaying biomineralization and causing retention of enamel matrix proteins.

  10. Characterization of water formation mechanisms on hydrophobic radome materials

    SciTech Connect

    Roberts, J.C.; Wienhold, P.D.; Nicholas, A.A.; Garner, P.N.

    1994-03-01

    The degree of hydrophobicity and contact angle of water on different radome coating materials was measured. The ability of these hydrophobic materials to bond to a radome substrate was also observed. The materials tested were; Raydell M-26-OS, Raydell M-15-OS, RA 7947, RA 7943, Teflon/Tedlar and Vellox-140. Each material was bonded to G-10 fiberglass panels and evaluated under conditions simulating dew formation, driving rain, freezing rain. Raydell M-26-OS had a relatively high contact angle, demonstrated excellent boding to a radome material, and had fair hydrophobic properties. RA 7947 and RA 7943 had relatively high contact angles, demonstrated poor and fair bonding, respectively, to a radome material, and exhibited good hydrophobic properties. The Teflon/Tedlar material had a low contact angle, demonstrated good bonding to a radome material, but showed fair to poor hydrophobic properties. The Vellox-140 and Raydell M-15-OS had relatively high to high contact angles, demonstrated excellent and possibly excellent bonding, respectively, to a radome material, and showed good to excellent, respectively, hydrophobic properties. These results were consistent whether the test temperature was 22{degrees}C (72{degrees}F) or -13{degrees}C (8{degrees}F) and whether the water was in the form of a mist or a concentrated stream. Long term tests and the effects of salt spray and dirt and the hydrophobic materials were not examined. 17 refs., 7 figs., 2 tabs.

  11. Secondary organic aerosol (trans)formation through aqueous phase guaiacol photonitration: a kinetic study

    NASA Astrophysics Data System (ADS)

    Kroflič, Ana; Grgić, Irena

    2014-05-01

    It is well known that atmospheric aerosols play a crucial role in the Earth's climate and public health (Pöschl 2005). Despite a great effort invested in the studies of secondary organic aerosol (SOA) budget, composition, and its formation mechanisms, there is still a gap between field observations and atmospheric model predictions (Heald et al. 2005, Hallquist et al. 2009, and Lim et al. 2010). The insisting uncertainties surrounding SOA formation and aging thus gained an increasing interest in atmospheric aqueous phase chemistry; they call for more complex and time consuming studies at the environmentally relevant conditions allowing confident extrapolation to desired ambient conditions. In addition to the adverse health effects of atmospheric particulate matter (PM) as such, toxicity is also attributed to nitro-aromatic and other organic compounds which have already been detected in real aerosol samples (Traversi et al. 2009). Moreover, low-volatility aromatic derivatives are believed to form at least partly in the aerosol aqueous phase and not only in the gas phase from where they partition into water droplets (Ervens et al. 2011). Two nitro derivatives of biomass burning tracer guaiacol have recently been found in winter PM10 samples from the city of Ljubljana, Slovenia, and aqueous photonitration reaction was proposed as their possible production pathway (Kitanovski et al. 2012). In this study the kinetics of guaiacol nitration in aqueous solution was investigated in the presence of H2O2 and NO2¯ upon simulated solar irradiation (Xenon lamp, 300 W). During the experiment the DURAN® flask with the reaction mixture was held in the thermostated bath and thoroughly mixed. The reaction was monitored for 44 hours at different temperatures. Guaiacol and its main nitro-products (4-nitroguaiacol, 4-NG; 6-nitroguaiacol, 6-NG; and 4,6-dinitroguaiacol, 4,6-DNG) were quantified in every aliquot, taken from the reaction mixture, by use of high pressure liquid

  12. Modeling Multiple Time Scales during Glass Formation with Phase-Field Crystals

    SciTech Connect

    Berry, Joel; Grant, Martin

    2011-04-29

    The dynamics of glass formation in monatomic and binary liquids are studied numerically using a microscopic field theory for the evolution of the time-averaged atomic number density. A stochastic framework combining phase-field crystal free energies and dynamic density functional theory is shown to successfully describe several aspects of glass formation over multiple time scales. Agreement with mode coupling theory is demonstrated for underdamped liquids at moderate supercoolings, and a rapidly growing dynamic correlation length is found to be associated with fragile behavior.

  13. A kinetic study on the potential of a hybrid reaction mechanism for prediction of NOx formation in biomass grate furnaces

    NASA Astrophysics Data System (ADS)

    Zahirović, Selma; Scharler, Robert; Kilpinen, Pia; Obernberger, Ingwald

    2011-10-01

    This paper presents the verification of a hybrid reaction mechanism (28 species, 104 reactions) by means of a kinetic study with a view to its application for the CFD-based prediction of gas phase combustion and NOx formation in biomass grate furnaces. The mechanism is based on a skeletal kinetic scheme that includes the subsets for H2, CO, NH3 and HCN oxidation derived from the detailed Kilpinen 97 reaction mechanism. To account for the CH4 breakdown two related reactions from the 4-step global mechanism for hydrocarbons oxidation by Jones and Lindstedt were adopted. The hybrid mechanism was compared to the global mechanism and validated against the detailed Kilpinen 97 mechanism. For that purpose plug flow reactor simulations at conditions relevant to biomass combustion (atmospheric pressure, 1200-1600 K) for approximations of the flue gases in a grate furnace at fuel lean and fuel rich conditions were carried out. The hybrid reaction mechanism outperformed the global one at all conditions investigated. The most striking differences obtained in predictions by the hybrid and the detailed mechanism at the residence times prior to ignition were attributed to the simplified description of the CH4 oxidation in the case of the former. The overall agreement regarding both combustion and NOx chemistry between the hybrid and the detailed mechanism was better at fuel lean conditions than at fuel rich conditions. However, also at fuel rich conditions, the agreement was improving with increasing temperature. Moreover, it was shown that an improvement in the prediction of NOx formation by the N-subset of the hybrid reaction mechanism can be achieved by replacing its C-H-O subset with that of the detailed one.

  14. Study of formation mechanism of incipient melting in thixo-cast Al–Si–Cu–Mg alloys

    SciTech Connect

    Du, Kang Zhu, Qiang Li, Daquan Zhang, Fan

    2015-08-15

    Mechanical properties of thixo-cast Al–Si–Cu–Mg alloys can be enhanced by T61 heat treatment. Copper and magnesium atoms in aluminum matrix can form homogeneously distributed precipitations after solution and aging treatment which harden the alloys. However, microsegregation of these alloying elements could form numerous tiny multi-compound phases during solidification. These phases could cause incipient melting defects in subsequent heat treatment process and degrade the macro-mechanical properties of productions. This study is to present heterogeneous distribution of Cu, Si, and Mg elements and formation of incipient melting defects (pores). In this study, incipient melting pores that occurred during solution treatment at various temperatures, even lower than common melting points of various intermetallic phases, were identified, in terms of a method of investigating the same surface area in the samples before and after solution treatment in a vacuum environment. The results also show that the incipient melting mostly originates at the clusters with fine intermetallic particles while also some at the edge of block-like Al{sub 2}Cu. The fine particles were determined being Al{sub 2}Cu, Al{sub 5}Cu{sub 2}Mg{sub 8}Si{sub 6} and Al{sub 8}Mg{sub 3}FeSi{sub 2}. Tendency of the incipient melting decreases with decreases of the width of the clusters. The formation mechanism of incipient melting pores in solution treatment process was discussed using both the Fick law and the LSW theory. Finally, a criterion of solution treatment to avoid incipient melting pores for the thixo-cast alloys is proposed. - Highlights: • In-situ comparison technique was used to analysis the change of eutectic phases. • The ralationship between eutectic phase size and incipient melting was studied. • Teat treatment criterion for higher incipient melting resistance was proposed.

  15. On the Mechanical Stability of Austenite Matrix After Martensite Formation in a Medium Mn Steel

    NASA Astrophysics Data System (ADS)

    He, B. B.; Huang, M. X.

    2016-07-01

    The present work employs the nanoindentation technique to investigate the effect of prior martensite formation on the mechanical stability of a retained austenite matrix. It is found that the small austenite grains that were surrounded by martensite laths have higher mechanical stability than the large austenite grains that were free of martensite laths. The higher mechanical stability of small austenite grains is due to its higher amount of defects resulting from the prior martensite formation. These defects act as barriers for the later martensite formation and therefore contribute to the higher mechanical stability of small austenite grains. As a result, the present work suggests that the formation of martensite tends to stabilize the surrounding austenite matrix. Therefore, it may explain the lower transformed amount of martensite after quenching as compared to the theoretical calculation using the Koistinen and Marburger (K-M) equation.

  16. Isostructural solid-solid phase transition in monolayers of soft core-shell particles at fluid interfaces: structure and mechanics.

    PubMed

    Rey, Marcel; Fernández-Rodríguez, Miguel Ángel; Steinacher, Mathias; Scheidegger, Laura; Geisel, Karen; Richtering, Walter; Squires, Todd M; Isa, Lucio

    2016-04-21

    We have studied the complete two-dimensional phase diagram of a core-shell microgel-laden fluid interface by synchronizing its compression with the deposition of the interfacial monolayer. Applying a new protocol, different positions on the substrate correspond to different values of the monolayer surface pressure and specific area. Analyzing the microstructure of the deposited monolayers, we discovered an isostructural solid-solid phase transition between two crystalline phases with the same hexagonal symmetry, but with two different lattice constants. The two phases corresponded to shell-shell and core-core inter-particle contacts, respectively; with increasing surface pressure the former mechanically failed enabling the particle cores to come into contact. In the phase-transition region, clusters of particles in core-core contacts nucleate, melting the surrounding shell-shell crystal, until the whole monolayer moves into the second phase. We furthermore measured the interfacial rheology of the monolayers as a function of the surface pressure using an interfacial microdisk rheometer. The interfaces always showed a strong elastic response, with a dip in the shear elastic modulus in correspondence with the melting of the shell-shell phase, followed by a steep increase upon the formation of a percolating network of the core-core contacts. These results demonstrate that the core-shell nature of the particles leads to a rich mechanical and structural behavior that can be externally tuned by compressing the interface, indicating new routes for applications, e.g. in surface patterning or emulsion stabilization.

  17. Multishelled Metal Oxide Hollow Spheres: Easy Synthesis and Formation Mechanism.

    PubMed

    Wu, Hongjing; Wu, Guanglei; Ren, Yanyan; Li, Xinghua; Wang, Liuding

    2016-06-20

    Uniform multishelled NiO, Co3 O4 , ZnO, and Au@NiO hollow spheres were synthesized (NiO and Co3 O4 hollow spheres for the first time) by a simple shell-by-shell self-assembly allowing for tuning of the the size, thickness and shell numbers by controlling the heat treatment, glucose/metal salt molar ratio, and hydrothermal reaction time. These findings further the development of synthetic methodologies for multishelled hollow structures and could open up new opportunities for deeper understanding of the mechanisms of shell-by-shell self-assembly. Moreover, the double-shelled NiO hollow sphere exhibits a higher photocatalytic activity for degradation of methyl orange than its morphological counterparts. PMID:27164001

  18. Mechanism of Hot Finger Formation in Mantle Wedge

    NASA Astrophysics Data System (ADS)

    Matsuo, M. Y.; Tamura, Y.; Sakaguchi, H.

    2013-12-01

    Processes of mantle melting and volcanic eruptions along subduction zones are often illustrated by the use of two-dimensional cross-section models of convergent margins. However, Quaternary volcanoes in the NE Japan arc could be grouped into ten volcano clusters striking transverse to the arc; these have an average width of ~ 50 km, and are separated by parallel gaps 30-75 km wide (Tamura et al., 2002). Moreover, the structure of the mantle wedge and arc crust beneath the NE Japan arc and the Izu-Bonin-Mariana arc, respectively, suggest that the third dimension, lying along the strike of the arc, is necessary to understand the actual production of magmas in subduction zones (e.g., Nakajima et al., 2001; Hasegawa & Nakajima, 2004; Kodaira et al., 2007; Kodaira et al., 2008). Common periodic structural variations, having wavelengths of 80-100 km, can be observed in both areas. This grouping of volcanoes and the structural variations may be related to locally developed hot regions within the mantle wedge that have the form of inclined, 50 km-wide fingers (hot fingers). The 'hot fingers' models (Tamura et al., 2002) may play an important role in linking the 3D structures within the mantle wedge and overlying arc crust to volcanic eruptions at the surface. To explore a physical and mathematical mechanism to produce a hot finger pattern, we develop a hydrodynamic model of mantle convection in mantle wedge. A hypothesis incorporated in our model is a double diffusive mechanism of mantle materials; diffusion of composition of mantle materials is much weaker than temperature diffusion. We show that our model shows a spatiotemporal pattern in a mantle material composition, temperature, and velocity that are similar to the spatiotemporal patterns observed in the NE Japan arc.

  19. Atmospheric fate of OH initiated oxidation of terpenes. Reaction mechanism of alpha-pinene degradation and secondary organic aerosol formation.

    PubMed

    Librando, Vito; Tringali, Giuseppe

    2005-05-01

    This paper studies the reaction products of alpha-pinene, beta-pinene, sabinene, 3-carene and limonene with OH radicals and of alpha-pinene with ozone using FT-IR spectroscopy for measuring gas phase products and HPLC-MS-MS to measure products in the aerosol phase. These techniques were used to investigate the secondary organic aerosol (SOA) formation from the terpenes. The gas phase reaction products were all quantified using reference compounds. At low terpene concentrations (0.9-2.1 ppm), the molar yields of gas phase reaction products were: HCHO 16-92%, HCOOH 10-54% (OH source: H2O2, 6-25 ppm); HCHO 127-148%, HCOOH 4-6% (OH source: CH3ONO, 5-8 ppm). At high terpene concentrations (4.1-13.2 ppm) the results were: HCHO 9-27%, HCOOH 15-23%, CH3(CO)CH3 0-14%, CH3COOH 0-5%, nopinone 24% (only from beta-pinene oxidation), limona ketone 61% (only from limonene oxidation), pinonaldehyde was identified during alpha-pinene degradation (OH source H2O2, 23-30 ppm); HCHO 76-183%, HCOOH 12-15%, CH3(CO)CH3 0-12%, nopinone 17% (from beta-pinene oxidation), limona ketone 48% (from limonene oxidation), pinonaldehyde was identified during alpha-pinene degradation (OH source CH3ONO, 14-16 ppm). Pinic acid, pinonic acid, limonic acid, limoninic acid, 3-caric acid, 3-caronic acid and sabinic acid were identified in the aerosol phase. On the basis of these results, we propose a formation mechanism for pinonic and pinic acid in the aerosol phase explaining how degradation products could influence SOA formation and growth in the troposphere.

  20. Study of Hind Limb Tissue Gas Phase Formation in Response to Suspended Adynamia and Hypokinesia

    NASA Technical Reports Server (NTRS)

    Butler, Bruce D.

    1996-01-01

    The purpose of this study was to investigate the hypothesis that reduced joint/muscle activity (hypo kinesia) as well as reduced or null loading of limbs (adynamia) in gravity would result in reduced decompression-induced gas phase and symptoms of decompression sickness (DCS). Finding a correlation between the two phenomena would correspond to the proposed reduction in tissue gas phase formation in astronauts undergoing decompression during extravehicular activity (EVA) in microgravity. The observation may further explain the reported low incidence of DCS in space.

  1. Atmospheric chemistry of gas-phase polycyclic aromatic hydrocarbons: formation of atmospheric mutagens.

    PubMed Central

    Atkinson, R; Arey, J

    1994-01-01

    The atmospheric chemistry of the 2- to 4-ring polycyclic aromatic hydrocarbons (PAH), which exist mainly in the gas phase in the atmosphere, is discussed. The dominant loss process for the gas-phase PAH is by reaction with the hydroxyl radical, resulting in calculated lifetimes in the atmosphere of generally less than one day. The hydroxyl (OH) radical-initiated reactions and nitrate (NO3) radical-initiated reactions often lead to the formation of mutagenic nitro-PAH and other nitropolycyclic aromatic compounds, including nitrodibenzopyranones. These atmospheric reactions have a significant effect on ambient mutagenic activity, indicating that health risk assessments of combustion emissions should include atmospheric transformation products. PMID:7821285

  2. Target mass dependence of 4ΛH formation mechanism from K- absorption at rest

    NASA Astrophysics Data System (ADS)

    Nara, Y.; Ohnishi, A.; Harada, T.

    1995-02-01

    Formation probabilities of Λ4H from K- absorption at rest on light nuclear targets are investigated employing antisymmetrized molecular dynamics combined with multi-step binary statistical decay. Calculated results show that as the target mass number decreases from 16O, 12C to 9Be, the main formation mechanism varies from statistical decay, followed by dynamical fragmentation, to direct formation in nuclear environment.

  3. Asian summer monsoon onset barrier and its formation mechanism

    NASA Astrophysics Data System (ADS)

    Liu, Boqi; Liu, Yimin; Wu, Guoxiong; Yan, Jinghui; He, Jinhai; Ren, Suling

    2015-08-01

    The onset process of Asian summer monsoon (ASM) is investigated based on diagnostic analysis of observations of precipitation and synoptic circulation. Results show that after the ASM commences over the eastern Bay of Bengal (BOB) around early May, the onset can propagate eastwards towards the South China Sea and western Pacific but is blocked on its westward propagation along the eastern coast of India. This blocking, termed the "monsoon onset barrier (MOB)", presents a Gill-type circulation response to the latent heating released by BOB monsoon convection. This convective condensation heating generates summertime (wintertime) vertical easterly (westerly) shear to its east (west) and facilitates air ascent (descent). The convection then propagates eastward but gets trapped on its westward path. To the east of the central BOB, the surface air temperature (SAT) cools faster than the underlying sea surface temperature (SST) due to monsoon onset. Thus more sensible heat flux supports the onset convection to propagate eastward. To the west of the central BOB, however, the land surface sensible heating over the Indian Peninsula is strengthened by the enhanced anticyclone circulation and air descent induced by the BOB monsoon heating. The strengthened upstream warm horizontal advection then produces a warm SAT center over the MOB region, which together with the in situ cooled SST reduces the surface sensible heating and atmospheric available potential energy to prevent the occurrence of free convection. Therefore, it is the change in both large-scale circulation and air-sea interaction due to BOB summer monsoon onset that contributes to the MOB formation.

  4. Mechanisms of iron-silica aqueous interaction and the genesis of Precambrian iron formation

    NASA Astrophysics Data System (ADS)

    Chemtob, S. M.; Catalano, J. G.; Moynier, F.; Pringle, E. A.

    2015-12-01

    Iron formations (IFs), Fe- and Si-rich chemical sediments common in Precambrian successions, preserve key information about the compositional, biological, and oxidative evolution of the Precambrian ocean. Stable Si isotopes (δ30Si) of IF have been used to infer paleo-oceanic composition, and secular variations in δ30Si may reflect ancient biogeochemical cycles. The δ30Si of primary Fe-Si precipitates that formed IF depends not only on the δ30Si of aqueous silica but also on the precipitation mechanism. Multiple formation mechanisms for these primary precipitates are plausible. Aqueous Si may have adsorbed on newly precipitated iron oxyhydroxide surfaces; alternatively, Fe and Si may have coprecipitated as a single phase. Here we explore variations in the Si isotope fractionation factor (ɛ) with Fe-Si aqueous interaction mechanism (adsorption vs. coprecipitation). In adsorption experiments, sodium silicate solutions (pH 8.1, 125-2000 µM Si) were reacted with iron oxide particles (hematite, ferrihydrite, goethite, and magnetite) for 24 to 72 hours. Resultant solutions had δ30Si between 0 and +6‰. Calculated ɛ varied significantly with oxide mineralogy and morphology. For ferrihydrite, ɛ = -1.7‰; for hematite, ɛ = -2 to -5‰, depending on particle morphology. Apparent ɛ decreased upon surface site saturation, implying a smaller isotope effect for polymeric Si adsorption than monomeric adsorption. In coprecipitation experiments, solutions of Na-silicate and Fe(II) chloride (0.4-10 mM) were prepared anaerobically, then air-oxidized for 3 days to induce precipitation. At low Si concentrations, magnetite formed; near silica saturation, lepidocrocite and ferrihydrite formed. The Si isotope fractionation factor for coprecipitation was within the range of ɛ observed for adsorption (ɛ = -2.3 ± 1.0‰). These results indicate that the mechanism of Fe-Si interaction affects ɛ, presumably due to varying silicate coordination environments. These isotopic

  5. A hydrogen-based oxidation mechanism relevant to planetary formation

    NASA Astrophysics Data System (ADS)

    Sharp, Zachary D.; McCubbin, Francis M.; Shearer, Charles K.

    2013-10-01

    It is now generally accepted that the Moon formed by collision of a Mars-sized impactor with the Earth (Hartmann and Davis, 1975), a process termed "the Giant Impact". The oxygen isotope compositions of both bodies are indistinguishable (Wiechert et al., 2001), a result that would require extensive chemical homogenization. Recent simulations have been developed that predict intimate mixing immediately following the impact (Canup, 2012; Ćuk and Stewart, 2012; Pahlevan and Stevenson, 2007), minimizing the oxygen isotope problem. Nevertheless, striking chemical differences remain. We propose that the process of hydrogen degassing and loss to space during magma cooling can explain differences in the water content, oxygen fugacity (f(O2)), and isotopic composition of Cl and H in the Earth-Moon system. At low f(O2), H2 gas is the stable O-H phase in basalts. The low solubility and rapid diffusivity of H2 explain the presently dry character of most lunar samples. Many of the apparently discrepant observations regarding the hydrous character of the Moon are reconciled by H2 degassing. Early H2 degassing also explains the high f(O2) of Earth's mantle. A loss of only 1/3 'ocean equivalent' water from Earth by hydrodynamic escape of H2 would shift the f(O2) of the upper mantle from the very low oxidation state equivalent to the Moon and other primitive differentiated bodies to its present oxidized state (near the fayalite-magnetite-quartz buffer). No other processes, such as late addition of material to Earth or injection of Fe3+-rich deep mantle materials to the upper mantle are required to explain the early elevated oxidation state of Earth's upper mantle.

  6. Polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) isomer patterns from municipal waste combustion: formation mechanism fingerprints.

    PubMed

    Ryu, Jae-Yong; Choi, Kum-Chan; Mulholland, James A

    2006-11-01

    Polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) byproducts can be formed in combustion systems by a variety of mechanisms. While total PCDD/F emissions and, to a lesser extent, homologue distributions from incinerators have been found to vary widely depending on combustion conditions, PCDD/F isomer distributions do not. Formation mechanisms can be grouped into two general categories: condensation of precursors, such as chlorinated phenols, and formation from particulate carbon, termed de novo synthesis. In addition to these mechanisms, chlorination and dechlorination reactions may affect isomer patterns. In this work, isomer patterns from field and laboratory municipal waste combustion samples are compared with computed thermodynamic distributions and those from the following experimental investigations: both gas-phase and metal-catalyzed condensation of chlorinated phenols, chlorination of dibenzo-p-dioxin and dibenzofuran, and dechlorination of octachlorodibenzo-p-dioxin and octachlorodibenzofuran. PCDD/F isomer patterns produced by different formation mechanisms in controlled experiments are distinct and robust, largely unaffected by combustion conditions. PCDD isomer patterns from municipal waste combustion are most similar to those produced by CuCl(2)-catalyzed phenol condensation from 10 chlorinated phenols. PCDF isomer patterns are most similar to those produced by chlorination and dechlorination.

  7. Mechanisms for axial band formation in a rotating drum of granular material

    NASA Astrophysics Data System (ADS)

    Newey, Michael K.

    We study granular particles, like sand or glass beads, that are mixed in a partially filled, horizontal, cylindrical drum. When the drum is rotated, it is observed that there is a flowing layer of grains on the free surface of the granular medium. In addition, if the particles have different sizes, spatial segregation of the particles by size is observed. This segregation occurs in two phases. During the first phase, called radial segregation, the smaller particles form a radial core. In the second, called axial segregation, particles segregate into alternating bands along the axis of the drum. We perform a detailed study of the characteristics of the flow to determine the physical mechanisms driving axial segregation. We characterize the top surface of the flowing layer by tracking particles using a high speed camera. We then extract average quantities such as velocity and diffusion. The average velocities show surprising behavior: Particles in small particle bands have a higher downhill flow velocity than particles in large particle bands. We also observe that there is a pattern of sideways velocity as a function of position down the flow. Particles flow into small particle bands in the middle of the flow but flow out of small particle bands at the bottom. We present the framework for a new model based on our experimentally observed results. We explain the axial band formation in terms of the observed surface flow patterns. We show how two physical processes could contribute to the band formation: (A) Accelerating granular material does not necessarily collide while decelerating granular material requires collisions. (B) Different size particles flow at different velocities. Our framework connects differences in flow velocities on the surface of the drum with the radial segregation in the bulk of the drum. We compare these results to current models, including models by Savage, Zik, Aranson, and Elperin. We test the general model assumption that the particles

  8. Mass-independent oxygen isotopic partitioning during gas-phase SiO2 formation.

    PubMed

    Chakraborty, Subrata; Yanchulova, Petia; Thiemens, Mark H

    2013-10-25

    Meteorites contain a wide range of oxygen isotopic compositions that are interpreted as heterogeneity in solar nebula. The anomalous oxygen isotopic compositions of refractory mineral phases may reflect a chemical fractionation process in the nebula, but there are no experiments to demonstrate this isotope effect during particle formation through gas-phase reactions. We report experimental results of gas-to-particle conversion during oxidation of silicon monoxide that define a mass-independent line (slope one) in oxygen three-isotope space of (18)O/(16)O versus (17)O/(16)O. This mass-independent chemical reaction is a potentially initiating step in nebular meteorite formation, which would be capable of producing silicate reservoirs with anomalous oxygen isotopic compositions. PMID:24159043

  9. Mechanisms of esophageal adenocarcinoma formation and approaches to chemopreventive intervention.

    PubMed

    Richmond, Ellen; Umar, Asad

    2016-02-01

    The incidence of esophageal adenocarcinoma (EAC), a debilitating and highly lethal malignancy, has risen dramatically over the past 40 years in the United States and other Western countries. To reverse this trend, EAC prevention and early detection efforts by clinicians, academic researchers and endoscope manufacturers have targeted Barrett's esophagus (BE), the widely accepted EAC precursor lesion. Data from surgical, endoscopic and pre-clinical investigations strongly support the malignant potential of BE. For patients with BE, the risk of developing EAC has been estimated at 11- to 125-fold greater than that of the individual at average risk. Nevertheless, screening for BE in symptomatic patients (ie, with symptoms of reflux) and surveillance in patients diagnosed with BE have not had a substantial impact on the incidence, morbidity or mortality of EAC; the overwhelming majority of EAC patients are diagnosed without a pre-operative diagnosis of BE. This article will discuss the current state of the science of esophageal adenocarcinoma prevention, including ideas about carcinogenesis and its underlying genomic and molecular level mechanisms, and suggest strategies for a systems approach to targeted preventive management. PMID:26970126

  10. Signaling dependent and independent mechanisms in pemphigus vulgaris blister formation.

    PubMed

    Saito, Masataka; Stahley, Sara N; Caughman, Christopher Y; Mao, Xuming; Tucker, Dana K; Payne, Aimee S; Amagai, Masayuki; Kowalczyk, Andrew P

    2012-01-01

    Pemphigus vulgaris (PV) is an autoimmune epidermal blistering disease caused by autoantibodies directed against the desmosomal cadherin desmoglein-3 (Dsg3). Significant advances in our understanding of pemphigus pathomechanisms have been derived from the generation of pathogenic monoclonal Dsg3 antibodies. However, conflicting models for pemphigus pathogenicity have arisen from studies using either polyclonal PV patient IgG or monoclonal Dsg3 antibodies. In the present study, the pathogenic mechanisms of polyclonal PV IgG and monoclonal Dsg3 antibodies were directly compared. Polyclonal PV IgG cause extensive clustering and endocytosis of keratinocyte cell surface Dsg3, whereas pathogenic mouse monoclonal antibodies compromise cell-cell adhesion strength without causing these alterations in Dsg3 trafficking. Furthermore, tyrosine kinase or p38 MAPK inhibition prevents loss of keratinocyte adhesion in response to polyclonal PV IgG. In contrast, disruption of adhesion by pathogenic monoclonal antibodies is not prevented by these inhibitors either in vitro or in human skin explants. Our results reveal that the pathogenic activity of polyclonal PV IgG can be attributed to p38 MAPK-dependent clustering and endocytosis of Dsg3, whereas pathogenic monoclonal Dsg3 antibodies can function independently of this pathway. These findings have important implications for understanding pemphigus pathophysiology, and for the design of pemphigus model systems and therapeutic interventions.

  11. Mechanisms of jet formation on the giant planets

    NASA Astrophysics Data System (ADS)

    Liu, J.; Schneider, T.

    2009-12-01

    The giant planet atmospheres exhibit alternating prograde (eastward) and retrograde (westward) jets of different speeds and widths, with an equatorial jet that is prograde on Jupiter and Saturn and retrograde on Uranus and Neptune (Porco et al. 2003, Sanchez-Lavega et al. 2003, Sanchez-Lavega et al. 2007, Hammel et al. 2001, Sromovsky et al. 2001). The jets are variously thought to be driven by differential radiative heating of the upper atmosphere or by intrinsic heat fluxes emanating from the deep interior (Williams 2003, Busse 1976, Heimpel et al. 2005, Aurnou et al. 2007). But existing models cannot account for the different flow configurations on the giant planets in an energetically consistent manner (Heimpel and Aurnou 2007, Aurnou et al. 2007). Here we use simulations with a three-dimensional general circulation model to show that the different flow configurations can be reproduced by mechanisms universal across the giant planets if differences in their radiative heating and intrinsic heat fluxes are taken into account. Whether the equatorial jet is prograde or retrograde depends on whether the deep intrinsic heat fluxes are strong enough that convection penetrates into the upper atmosphere and excites strong equatorial Rossby waves there. The different speeds and widths of the off-equatorial jets depend, among other factors, on the differential radiative heating of the atmosphere and the altitude of the jets. The simulations make predictions about as-yet unobserved aspects of the flow and temperature structures of the giant planets.

  12. Formation Mechanism of Flattened Top HFBI Domical Droplets.

    PubMed

    Yamasaki, Ryota; Haruyama, Tetsuya

    2016-04-21

    A water droplet assumes a spherical shape because of its own surface tension. However, water droplets containing dissolved hydrophobin (HFBI) have flat surfaces. In our previous study, the mechanism of this unique phenomenon was revealed. HFBI forms a self-organized membrane that has a densely packed and honeycomb-like structure. Furthermore, the buckling strength of the membrane is higher than the surface tension of the HFBI droplet. Therefore, an HFBI domical droplet has a flat surface. However, it was not clear why only the top of the domical droplet was flattened while other areas such as the side face were not. In this study, we observed HFBI domical droplets to investigate this phenomenon. The flat top area (self-organized HFBI membrane) remained parallel to the ground even if the substrate was tilted. Therefore, buoyancy was thought to be a factor affecting the HFBI membrane. In addition, the side face of the HFBI domical droplet was analyzed by atomic force microscopy and electrochemical impedance spectroscopy, and it was found that the sides of the HFBI droplet were not composed of densely packed HFBI membranes. PMID:27041119

  13. Third phase formation in the extraction of phosphotungstic acid by TBP in n-octane.

    SciTech Connect

    Antonio, M. R.; Chiarizia, R.; Jaffrennou, F.

    2010-08-30

    The solvent extraction of 12-phosphotungstic acid, also known as 12-tungstophosphoric acid-H{sub 3}PW{sub 12}O{sub 40}, the so-called Keggin heteropolyacid - by 0.73 M (20%v/v) tri-n-butyl phosphate (TBP) in n-octane under conditions comparable to those used previously for the extraction of conventional inorganic mineral acids is described. A simplified phase diagram for the pentanary system comprised of H{sub 3}PW{sub 12}O{sub 40}, HNO{sub 3}, H{sub 2}O, TBP, and n-octane reveals an extremely low initial concentration of H{sub 3}PW{sub 12}O{sub 40} (1.1 mM) at the LOC (limiting organic concentration) condition, far lower than the most effective third-phase-forming inorganic acid, namely HClO{sub 4}. The results from small-angle neutron scattering (SANS) indicate that the interparticle attraction energy - U(r) calculated through application of the Baxter sticky sphere model to the SANS data at the LOC condition - does not approach the -2 k{sub B} T value associated with phase splitting in previous studies of TBP third-phase formation. The third-phase formation model based on attractive interactions between polar cores of reverse micelles, successfully developed for TBP and other extraction systems does not apply to the extraction of H{sub 3}PW{sub 12}O{sub 40}. Rather, the separation of a third-phase from the TBP organic phase stems from the limited solubility of the heavy and highly polar H{sub 3}PW{sub 12}O{sub 40}-TBP species in the alkane diluent.

  14. Lignin peroxidase compound III. Mechanism of formation and decomposition

    SciTech Connect

    Wariishi, Hiroyuki; Gold, M.H. )

    1990-02-05

    Lignin peroxidase compound III (LiPIII) was prepared via three procedures: (a) ferrous LiP + O{sub 2} (LiPIIIa), (b) ferric LiP + O{sub 2}{sup {bar {sm bullet}}} (LiPIIIb), and (c) LiP compound II + excess H{sub 2}O{sub 2} followed by treatment with catalase (LiPIIIc). LiPIIIa, b, and c each have a Soret maximum at {approximately}414 nm and visible bands at 543 and 578 nm. LiPIIIa, b, and c each slowly reverted to native ferric LiP, releasing stoichiometric amounts of O{sub 2}{sup {bar {sm bullet}}} in the process. The LiPIII reversion reactions obeyed first-order kinetics with rate constants of {approximately}1.0 {times} 10{sup {minus}3} s{sup {minus}1}. In the presence of excess peroxide, at pH 3.0, native LiP, LiPII, and LiPIIIa, b, and c are all converted to a unique oxidized species (LiPIII*) with a spectrum displaying visible bands at 543 and 578 nm, but with a Soret maximum at 419 nm, red-shifted 5 nm from that of LiPIII. LiPIII* is bleached and inactivated in the presence of excess H{sub 2}O{sub 2} via a biphasic process. The fast first phase of this bleaching reaction obeys second-order kinetics, with a rate constant of 1.7 {times} 10{sup 1} M{sup {minus}1} s{sup {minus}1}. Addition of veratryl alcohol to LiPIII* results in its rapid reversion to the native enzyme, via an apparent one-step reaction that obeys second-order kinetics, with a rate constant of 3.5 {times} 10{sup 1} M{sup {minus}1} s{sup {minus}1}. Stoichiometric amounts of O{sub 2}{sup {bar {sm bullet}}} are released during this reaction. When this reaction was run under conditions that prevented further reactions, HPLC analysis of the products demonstrated that veratryl alcohol was not oxidized. These results suggest that the binding of veratryl alcohol to LiPIII* displaces O{sub 2}{sup {bar {sm bullet}}}, thus returning the enzyme to its native state. In contrast, the addition of veratryl alcohol to LiPIII did not affect the rate of spontaneous reversion of LiPIII to the native enzyme.

  15. Numerical simulation of mark formation in dual-stack phase-change recording

    NASA Astrophysics Data System (ADS)

    Meinders, E. R.; Borg, H. J.; Lankhorst, M. H. R.; Hellmig, J.; Mijiritskii, A. V.

    2002-06-01

    Dual-stack phase-change recording is an option to further increase the data capacity of rewritable optical disks. Such disks consist of two recording stacks that are both recorded and read from the same side of the disk. Consequently, the first recording stack needs therefore to be semitransparent to allow recording in the second recording stack. Thick nontransparent metal layers can therefore not be used in the first recording stack, which makes the first recording stack the most challenging stack from a thermal point of view. A numerical model based on crystal growth was developed to study formation and erasure of amorphous marks in phase-change stacks that are based on fast-growth doped SbTe phase-change materials. The validity of the model was demonstrated from transmission electron microscopy analyses of recorded marks that showed a good correspondence with the calculated mark shapes in a conventional single-stack recording stack. The model was subsequently applied to analyze formation and erasure of marks in slow-cooling phase-change stacks for digital versatile disk, (DVD) and digital video recording (DVR) recording conditions. The effect of the recording velocity, the erase power, and the crystal growth velocity on the erasability of amorphous marks was simulated. The calculated phenomena are in good agreement with the phenomena observed from DVD and DVR erasability measurements. Mark formation in slow-cooling recording stacks is characterized by severe recrystallization during writing. Two possible solutions are indicated in this article, aiming at reducing the heat accumulation and the resulting recrystallization during writing of amorphous marks. Additional semitransparent heat sinks improve the mark formation considerably but also require higher write powers. Another solution is the application of modified write strategies. Modeling and recorder results are discussed for both approaches.

  16. A two-phase composite in simple shear: Effective mechanical anisotropy development and localization potential

    NASA Astrophysics Data System (ADS)

    Dabrowski, M.; Schmid, D. W.; Podladchikov, Y. Y.

    2012-08-01

    We present a combined shape and mechanical anisotropy evolution model for a two-phase inclusion-bearing rock subject to large deformation. A single elliptical inclusion embedded in a homogeneous but anisotropic matrix is used to represent a simplified shape evolution enforced on all inclusions. The mechanical anisotropy develops due to the alignment of elongated inclusions. The effective anisotropy is quantified using the differential effective medium (DEM) approach. The model can be run for any deformation path and an arbitrary viscosity ratio between the inclusion and host phase. We focus on the case of simple shear and weak inclusions. The shape evolution of the representative inclusion is largely insensitive to the anisotropy development and to parameter variations in the studied range. An initial hardening stage is observed up to a shear strain of γ = 1 irrespective of the inclusion fraction. The hardening is followed by a softening stage related to the developing anisotropy and its progressive rotation toward the shear direction. The traction needed to maintain a constant shear rate exhibits a fivefold drop at γ = 5 in the limiting case of an inviscid inclusion. Numerical simulations show that our analytical model provides a good approximation to the actual evolution of a two-phase inclusion-host composite. However, the inclusions develop complex sigmoidal shapes resulting in the formation of an S-C fabric. We attribute the observed drop in the effective normal viscosity to this structural development. We study the localization potential in a rock column bearing varying fraction of inclusions. In the inviscid inclusion case, a strain jump from γ = 3 to γ = 100 is observed for a change of the inclusion fraction from 20% to 33%.

  17. Mechanism of formation of concentrically laminated spherules: Implication to Randall’s plaque and stone formation

    PubMed Central

    Amos, Fairland F.; Dai, Lijun; Kumar, Rajendra; Khan, Saeed R.; Gower, Laurie B.

    2013-01-01

    We report on the formation of calcium phosphate multi-laminated spherules via a polymer-induced liquid-like precursor (PILP) process. In this non-classical crystallization route, the precipitation of liquid-like amorphous calcium phosphate (ACP) particles is promoted using anionic polypeptide additives, and these droplets coalesce to form globules that later crystallize into spherulites. During crystallization of the amorphous globules, the polymer additive, as well as the waters of hydration, is excluded ahead of the crystallization front, but some polymer becomes entrapped within diffusion-limited zones. This results in the formation of concentric laminations with layers of variable density from organic-rich inclusions. The striking resemblance of these spherules with the crystals of the Randall’s plaque and other laminated stones suggests that such biological structures may form via an amorphous precursor process as well. Given the organic-rich environment present in the urinary tract, one might expect a large amount of organic materials to become entrapped within the stratified zones of a forming stone during this type of solidification and transformation process. PMID:19066874

  18. Biological pattern formation: from basic mechanisms to complex structures

    NASA Astrophysics Data System (ADS)

    Koch, A. J.; Meinhardt, H.

    1994-10-01

    The reliable development of highly complex organisms is an intriguing and fascinating problem. The genetic material is, as a rule, the same in each cell of an organism. How then do cells, under the influence of their common genes, produce spatial patterns? Simple models are discussed that describe the generation of patterns out of an initially nearly homogeneous state. They are based on nonlinear interactions of at least two chemicals and on their diffusion. The concepts of local autocatalysis and of long-range inhibition play a fundamental role. Numerical simulations show that the models account for many basic biological observations such as the regeneration of a pattern after excision of tissue or the production of regular (or nearly regular) arrays of organs during (or after) completion of growth. Very complex patterns can be generated in a reproducible way by hierarchical coupling of several such elementary reactions. Applications to animal coats and to the generation of polygonally shaped patterns are provided. It is further shown how to generate a strictly periodic pattern of units that themselves exhibit a complex and polar fine structure. This is illustrated by two examples: the assembly of photoreceptor cells in the eye of Drosophila and the positioning of leaves and axillary buds in a growing shoot. In both cases, the substructures have to achieve an internal polarity under the influence of some primary pattern-forming system existing in the fly's eye or in the plant. The fact that similar models can describe essential steps in organisms as distantly related as animals and plants suggests that they reveal some universal mechanisms.

  19. Biological pattern formation: from basic mechanisms to complex structures

    SciTech Connect

    Koch, A.J.; Meinhardt, H. )

    1994-10-01

    The reliable development of highly complex organisms is an intriguing and fascinating problem. The genetic material is, as a rule, the same in each cell of an organism. How then do cells, under the influence of their common genes, produce spatial patterns Simple models are discussed that describe the generation of patterns out of an initially nearly homogeneous state. They are based on nonlinear interactions of at least two chemicals and on their diffusion. The concepts of local autocatalysis and of long-range inhibition play a fundamental role. Numerical simulations show that the models account for many basic biological observations such as the regeneration of a pattern after excision of tissue or the production of regular (or nearly regular) arrays of organs during (or after) completion of growth. Very complex patterns can be generated in a reproducible way by hierarchical coupling of several such elementary reactions. Applications to animal coats and to the generation of polygonally shaped patterns are provided. It is further shown how to generate a strictly periodic pattern of units that themselves exhibit a complex and polar fine structure. This is illustrated by two examples: the assembly of photoreceptor cells in the eye of [ital Drosophila] and the positioning of leaves and axillary buds in a growing shoot. In both cases, the substructures have to achieve an internal polarity under the influence of some primary pattern-forming system existing in the fly's eye or in the plant. The fact that similar models can describe essential steps in organisms as distantly related as animals and plants suggests that they reveal some universal mechanisms.

  20. The kinetics and mechanism of an aqueous phase isoprene reaction with hydroxyl radical

    NASA Astrophysics Data System (ADS)

    Huang, D.; Zhang, X.; Chen, Z. M.; Zhao, Y.; Shen, X. L.

    2011-08-01

    Aqueous phase chemical processes of organic compounds in the atmosphere have received increasing attention, partly due to their potential contribution to the formation of secondary organic aerosol (SOA). Here, we analyzed the aqueous OH-initiated oxidation of isoprene and its reaction products including carbonyl compounds and organic acids, regarding the acidity and temperature as in-cloudy conditions. We also performed a laboratory simulation to improve our understanding of the kinetics and mechanisms for the products of aqueous isoprene oxidation that are significant precursors of SOA; these included methacrolein (MACR), methyl vinyl ketone (MVK), methyl glyoxal (MG), and glyoxal (GL). We used a novel chemical titration method to monitor the concentration of isoprene in the aqueous phase. We used a box model to interpret the mechanistic differences between aqueous and gas phase OH radical-initiated isoprene oxidations. Our results were the first demonstration of the rate constant for the reaction between isoprene and OH radical in water, 1.2 ± 0.4) × 1010 M-1 s-1 at 283 K. Molar yields were determined based on consumed isoprene. Of note, the ratio of the yields of MVK (24.1 ± 0.8 %) to MACR (10.9 ± 1.1%) in the aqueous phase isoprene oxidation was approximately double that observed for the corresponding gas phase reaction. We hypothesized that this might be explained by a water-induced enhancement in the self-reaction of a hydroxy isoprene peroxyl radical (HOCH2C(CH3)(O2)CH = CH2) produced in the aqueous reaction. The observed yields for MG and GL were 11.4 ± 0.3 % and 3.8 ± 0.1 %, respectively. Model simulations indicated that several potential pathways may contribute to the formation of MG and GL. Finally, oxalic acid increased steadily throughout the course of the study, even after isoprene was consumed completely. The observed yield of oxalic acid was 26.2 ± 0.8 % at 6 h. The observed carbon balance accounted for ~50 % of the consumed isoprene. The

  1. The kinetics and mechanism of an aqueous phase isoprene reaction with hydroxy radical

    NASA Astrophysics Data System (ADS)

    Huang, D.; Zhang, X.; Chen, Z. M.; Zhao, Y.; Shen, X. L.

    2011-03-01

    Aqueous phase chemical processes of organic compounds in the atmosphere have received increasing attention, partly due to their potential contribution to the formation of secondary organic aerosol (SOA). Here, we analyzed the aqueous oxidation of isoprene in clouds and its reaction products, including carbonyl compounds and organic acids. We also performed a laboratory simulation to improve our understanding of the kinetics and mechanisms for the products of aqueous isoprene oxidation that are significant precursors of SOA; these included methacrolein (MACR), methyl vinyl ketone (MVK), methyl glyoxal (MG), and glyoxal (GL). We used a novel chemical titration method to monitor the concentration of isoprene in the aqueous phase. We used a box model to interpret the mechanistic differences between aqueous- and gas-phase OH radical-initiated isoprene oxidations. Our results were the first demonstration of the rate constant for the reaction between isoprene and OH radical in water, 3.50 (± 0.98) × 109 M-1 s-1 at 283 K. Molar yields were determined based on consumed isoprene. Of note, the ratio of the yields of MVK (18.9 ± 0.8%) to MACR (9.0 ± 1.1%) in the aqueous phase isoprene oxidation was approximately double that observed for the corresponding gas phase reaction. We hypothesized that this might be explained by a water-induced enhancement in the self-reaction of a hydroxy isoprene peroxyl radical (HOCH2C(CH3)(O2)CH = CH2) produced in the aqueous reaction. The observed yields for MG and GL were 11.4 ± 0.3% and 3.8 ± 0.1%, respectively. Model simulations indicated that several potential pathways may contribute to the formation of MG and GL. Finally, oxalic acid increased steadily throughout the course of the study, even after isoprene was consumed completely. The observed yield of oxalic acid was 26.2 ± 0.8% at 6 h. The observed carbon balance accounted for ~50% of the consumed isoprene. The presence of high-molecular-weight compounds may have accounted for a

  2. SURVIVAL OF INTERSTELLAR MOLECULES TO PRESTELLAR DENSE CORE COLLAPSE AND EARLY PHASES OF DISK FORMATION

    SciTech Connect

    Hincelin, U.; Wakelam, V.; Hersant, F.; Guilloteau, S.; Commerçon, B.

    2013-09-20

    An outstanding question of astrobiology is the link between the chemical composition of planets, comets, and other solar system bodies and the molecules formed in the interstellar medium. Understanding the chemical and physical evolution of the matter leading to the formation of protoplanetary disks is an important step for this. We provide some new clues to this long-standing problem using three-dimensional chemical simulations of the early phases of disk formation: we interfaced the full gas-grain chemical model Nautilus with the radiation-magnetohydrodynamic model RAMSES, for different configurations and intensities of the magnetic field. Our results show that the chemical content (gas and ices) is globally conserved during the collapsing process, from the parent molecular cloud to the young disk surrounding the first Larson core. A qualitative comparison with cometary composition suggests that comets are constituted of different phases, some molecules being direct tracers of interstellar chemistry, while others, including complex molecules, seem to have been formed in disks, where higher densities and temperatures allow for an active grain surface chemistry. The latter phase, and its connection with the formation of the first Larson core, remains to be modeled.

  3. Instability of a Lamellar Phase under Shear Flow: Formation of Multilamellar Vesicles

    NASA Astrophysics Data System (ADS)

    Courbin, L.; Delville, J. P.; Rouch, J.; Panizza, P.

    2002-09-01

    The formation of closed-compact multilamellar vesicles (referred to in the literature as the ``onion texture'') obtained upon shearing lamellar phases is studied using small-angle light scattering and cross-polarized microscopy. By varying the shear rate γ ˙, the gap cell D, and the smectic distance d, we show that: (i)the formation of this structure occurs homogeneously in the cell at a well-defined wave vector qi, via a strain-controlled process, and (ii)the value of qi varies as (dγ ˙/D)1/3. These results strongly suggest that formation of multilamellar vesicles may be monitored by an undulation (buckling) instability of the membranes, as expected from theory.

  4. De novo formation of centrosomes in vertebrate cells arrested during S phase.

    PubMed

    Khodjakov, Alexey; Rieder, Conly L; Sluder, Greenfield; Cassels, Grisel; Sibon, Ody; Wang, Chuo-Lung

    2002-09-30

    The centrosome usually replicates in a semiconservative fashion, i.e., new centrioles form in association with preexisting "maternal" centrioles. De novo formation of centrioles has been reported for a few highly specialized cell types but it has not been seen in vertebrate somatic cells. We find that when centrosomes are completely destroyed by laser microsurgery in CHO cells arrested in S phase by hydroxyurea, new centrosomes form by de novo assembly. Formation of new centrosomes occurs in two steps: approximately 5-8 h after ablation, clouds of pericentriolar material (PCM) containing gamma-tubulin and pericentrin appear in the cell. By 24 h, centrioles have formed inside of already well-developed PCM clouds. This de novo pathway leads to the formation of a random number of centrioles (2-14 per cell). Although clouds of PCM consistently form even when microtubules are completely disassembled by nocodazole, the centrioles are not assembled under these conditions.

  5. Hydride-phase formation and its influence on fatigue crack propagationbehavior in a Zircaloy-4 alloy

    SciTech Connect

    Garlea, Elena; Choo, H.; Wang, G Y; Liaw, Peter K; Clausen, B; Brown, D. W.; Park, Jae-Sung; Rack, P. D.; Kenik, Edward A

    2010-01-01

    The hydride-phase formation and its influence on the fatigue behavior of a Zircaloy-4 alloy charged with hydrogen gas are investigated. First, the microstructure and fatigue crack propagation rate of the alloy in the as-received condition are studied. Second, the formation and homogeneous distribution of delta zirconium hydride ( -ZrH2) in the bulk, and its effect on the fatigue crack propagation rate are presented. The results show that in the presence of hydrides the zirconium alloy exhibits reduced toughness and enhanced crack growth rates. Finally, the influence of a pre-existing fatigue crack in the specimen and the subsequent hydride formation were investigated. The residual lattice strain profile around the fatigue crack tip was measured using neutron diffraction. The combined effects of residual strains and hydride precipitation on the fatigue behavior are discussed.

  6. Skeletal repair by in situ formation of the mineral phase of bone

    SciTech Connect

    Constantz, B.R.; Ison, I.C.; Fulmer, M.T.; Poser, R.D.; Smith, S.T.; VanWagoner, M.; Ross, J.; Goldstein, S.A.; Jupiter, J.B.; Rosenthal, D.I.

    1995-03-24

    A process has been developed for the in situ formation of the mineral phase of bone. Inorganic calcium and phosphate sources are combined to form a paste that is surgically implanted by injection. Under physiological conditions, the material hardens in minutes concurrent with the formation of dahllite. After 12 hours, dahllite formation was nearly complete, and an ultimate compressive strength of 55 megapascals was achieved. The composition and crystal morphology of the dahllite formed are similar to those of bone. Animal studies provide evidence that the material is remodeled in vivo. A novel approach to skeletal repair is being tested in human trials for various applications; in one of the trials the new biomaterial is being percutaneously placed into acute fractures. After hardening, it serves as internal fixation to maintain proper alignment while healing occurs. 33 refs., 6 figs., 1 tab.

  7. A substrate energy dissipation mechanism in in-phase and anti-phase micromachined z-axis vibratory gyroscopes

    NASA Astrophysics Data System (ADS)

    Trusov, Alexander A.; Schofield, Adam R.; Shkel, Andrei M.

    2008-09-01

    This paper analyzes energy dissipation mechanisms in vacuum-operated in-phase and anti-phase actuated micromachined z-axis vibratory gyroscopes. The type of actuation is experimentally identified as the key factor to energy dissipation. For in-phase devices, dissipation through the die substrate is the dominant energy loss mechanism. This damping mechanism depends strongly on the die attachment method; rigid die attachment minimizes the loss of energy at the cost of reduced vibrational and stress isolation. For anti-phase actuated devices, dissipation through the substrate is suppressed and immunity to external vibrations is provided. However, even in anti-phase actuated devices fabrication imperfections introduce structural non-symmetry, enabling dissipation of energy through the die substrate due to momentum imbalance. Based on the experimental investigation, an analytical model for energy dissipation through the die substrate is proposed and used to study the effects of the actuation type, die attachment and fabrication imperfections. The limiting Q-factor for in-phase devices is generally below 20 × 103 while Q-factors much higher than 100 × 103 can be achieved with balanced anti-phase actuated gyroscopes.

  8. Gas hydrate formation rates from dissolved-phase methane in porous laboratory specimens

    USGS Publications Warehouse

    Waite, William F.; Spangenberg, E.K.

    2013-01-01

    Marine sands highly saturated with gas hydrates are potential energy resources, likely forming from methane dissolved in pore water. Laboratory fabrication of gas hydrate-bearing sands formed from dissolved-phase methane usually requires 1–2 months to attain the high hydrate saturations characteristic of naturally occurring energy resource targets. A series of gas hydrate formation tests, in which methane-supersaturated water circulates through 100, 240, and 200,000 cm3 vessels containing glass beads or unconsolidated sand, show that the rate-limiting step is dissolving gaseous-phase methane into the circulating water to form methane-supersaturated fluid. This implies that laboratory and natural hydrate formation rates are primarily limited by methane availability. Developing effective techniques for dissolving gaseous methane into water will increase formation rates above our observed (1 ± 0.5) × 10−7 mol of methane consumed for hydrate formation per minute per cubic centimeter of pore space, which corresponds to a hydrate saturation increase of 2 ± 1% per day, regardless of specimen size.

  9. Mechanism of PdO thin film formation during the oxidation of Pd(1 1 1)

    NASA Astrophysics Data System (ADS)

    Kan, Heywood H.; Weaver, Jason F.

    2009-09-01

    We investigated the mechanism by which a surface oxide layer on Pd(1 1 1) transforms to a PdO(1 0 1) thin film during oxidation with gaseous oxygen atoms in ultrahigh vacuum. Our results provide evidence that the precursor to bulk PdO formation is a distinct oxide phase that forms as small particles, referred to as PdO seeds, after the surface oxide saturates. With increasing oxygen coverage, the PdO seeds grow in size and eventually transform to more stable particles that agglomerate to yield a PdO film. Oxidation effectively ceases when the surface oxide layer is completely replaced by the bulk PdO film, demonstrating that the surface oxide is needed for PdO formation at the conditions studied. Both the kinetics of PdO formation and the final thickness of the PdO thin film depend strongly on the thermal stability of the PdO seeds. Below the decomposition temperature of the seeds (˜600 K), oxidation follows kinetics similar to Langmuirian adsorption and appears to be limited only by the rate of oxygen adsorption onto the surface oxide. In contrast, PdO formation above 600 K initially exhibits acceleratory kinetics, with the rates starting low but increasing steadily during the initial growth of PdO. We also observe a significant decrease in PdO(1 0 1) film thickness and improved crystallinity when oxidation is conducted below 600 K. We show that the trends observed in the oxidation kinetics and film thickness can be qualitatively explained within the context of a model in which the thermodynamic stability of PdO particles increases with increasing particle size and PdO seeds/particles coexist with a two-dimensional (2D) gas of oxygen atoms adsorbed on the surface oxide layer. This model suggests that the PdO particle-2D gas coexistence relation gives rise to three distinct growth regimes, namely, stable seed nucleation, metastable seed nucleation and oxygen dissolution into the subsurface where the latter is established at 2D gas coverages below the stability

  10. Deformation Mechanisms is a-Phase Silicon Nitride Ceramics

    SciTech Connect

    J. A. Schneider; A. K. Mukherjee

    1998-08-12

    Changes of phase composition and morphology were investigated in Si{sub 3}N{sub 4} both before and after compressive deformation testing. Si{sub 3}N{sub 4} specimens, with 5 wt% Y{sub 2}O{sub 3} and 5 wt% MgAl{sub 2}O{sub 4} additives, were rapidly consolidated to preserve the initial, metastable {alpha}-phase present in the Si{sub 3}N{sub 4} starting powders. Constant strain rate compression tests were used to evaluate the strain rate dependency of the flow stress. At 1723 K, a flow stress dependency value of n = 2 was observed.

  11. Liquid-Liquid Phase Separation in Supersaturated Lysozyme Solutions and Associated Precipitate Formation/Crystallization

    NASA Technical Reports Server (NTRS)

    Muschol, Martin; Rosenberger, Franz

    1997-01-01

    Using cloud point determinations, the phase boundaries (binodals) for metastable liquid-liquid (L-L) separation in supersaturated hen egg white lysozyme solutions with 3%, 5%, and 7% (wlv) NaCl at pH= 4.5 and protein concentrations c between 40 and 400 mg/ml were determined. The critical temperature for the binodal increased approximately linearly with salt concentration. The coexisting liquid phases both remained supersaturated but differed widely in protein concentration. No salt repartitioning was observed between the initial and the two separated liquid phases. After the L-L separation, due to the presence of the high protein concentration phase, crystallization occurred much more rapidly than in the initial solution. At high initial protein concentrations, a metastable gel phase formed at temperatures above the liquid binodal. Both crystal nucleation and gel formation were accelerated in samples that had been cycled through the binodal. Solutions in the gel and L-L regions yielded various types of precipitates. Based on theoretical considerations, previous observations with other proteins, and our experimental results with lysozyme, a generic phase diagram for globular proteins is put forth. A limited region in the (T,c) plane favorable for the growth of protein single crystals is delineated.

  12. Thermodynamics of phase formation in the quantum critical metal Sr3Ru2O7

    PubMed Central

    Rost, A. W.; Grigera, S. A.; Bruin, J. A. N.; Perry, R. S.; Tian, D.; Raghu, S.; Kivelson, Steven Allan; Mackenzie, A. P.

    2011-01-01

    The behavior of matter near zero temperature continuous phase transitions, or “quantum critical points” is a central topic of study in condensed matter physics. In fermionic systems, fundamental questions remain unanswered: the nature of the quantum critical regime is unclear because of the apparent breakdown of the concept of the quasiparticle, a cornerstone of existing theories of strongly interacting metals. Even less is known experimentally about the formation of ordered phases from such a quantum critical “soup.” Here, we report a study of the specific heat across the phase diagram of the model system Sr3Ru2O7, which features an anomalous phase whose transport properties are consistent with those of an electronic nematic. We show that this phase, which exists at low temperatures in a narrow range of magnetic fields, forms directly from a quantum critical state, and contains more entropy than mean-field calculations predict. Our results suggest that this extra entropy is due to remnant degrees of freedom from the highly entropic state above Tc. The associated quantum critical point, which is “concealed” by the nematic phase, separates two Fermi liquids, neither of which has an identifiable spontaneously broken symmetry, but which likely differ in the topology of their Fermi surfaces. PMID:21933961

  13. Thermodynamics of phase formation in the quantum critical metal Sr3Ru2O7.

    PubMed

    Rost, A W; Grigera, S A; Bruin, J A N; Perry, R S; Tian, D; Raghu, S; Kivelson, Steven Allan; Mackenzie, A P

    2011-10-01

    The behavior of matter near zero temperature continuous phase transitions, or "quantum critical points" is a central topic of study in condensed matter physics. In fermionic systems, fundamental questions remain unanswered: the nature of the quantum critical regime is unclear because of the apparent breakdown of the concept of the quasiparticle, a cornerstone of existing theories of strongly interacting metals. Even less is known experimentally about the formation of ordered phases from such a quantum critical "soup." Here, we report a study of the specific heat across the phase diagram of the model system Sr(3)Ru(2)O(7), which features an anomalous phase whose transport properties are consistent with those of an electronic nematic. We show that this phase, which exists at low temperatures in a narrow range of magnetic fields, forms directly from a quantum critical state, and contains more entropy than mean-field calculations predict. Our results suggest that this extra entropy is due to remnant degrees of freedom from the highly entropic state above T(c). The associated quantum critical point, which is "concealed" by the nematic phase, separates two Fermi liquids, neither of which has an identifiable spontaneously broken symmetry, but which likely differ in the topology of their Fermi surfaces.

  14. Formation of (Bi,Pb)-2223 phase during solidifying and sintering

    SciTech Connect

    Lu, X.Y.; Nagata, A.; Kamada, S.

    1997-06-01

    High-temperature microstructures and the formation of (Bi,Pb)-2223 phase were examined in Bi{sub 1.84}Pb{sub 0.34}Sr{sub 1.91}Ca{sub 2.03}Cu{sub 3.06}O{sub x} during solidifying and sintering by high temperature optical microscope, XRD, SEM, EDS and AC susceptibility measurement. During solidification, firstly, the needle-like phase (Sr{sub x}Ca{sub 1{minus}x}){sub 2}CuO{sub 3} began to crystallize at about 930{degrees}C, secondly, the (Sr{sub x}Ca{sub 1{minus}x}) CuO{sub 2} with rectangular shape began to crystallize around 890 {degrees}C, and finally the plate-like 2212 phase began to form by the peritectic reaction of the two (Sr,Ca)-Cu-O compounds and liquid phase below 870{degrees}C. The 2223 phase and Sr{sub 6}Pb{sub 2}O{sub 9} were transformed slowly by the solid state reaction of the 2212 phase and residual (Sr,Ca)-Cu-O compounds in a long time sitering (> 18 h) at 840{degrees} C after solidification.

  15. Protein kinases paralleling late-phase LTP formation in dorsal hippocampus in the rat.

    PubMed

    Li, Lin; Wan, Jia; Sase, Sunetra; Gröger, Marion; Pollak, Arnold; Korz, Volker; Lubec, Gert

    2014-10-01

    Hippocampal long term potentiation (LTP), representing a cellular model for learning and memory formation, can be dissociated into at least two phases: a protein-synthesis-independent early phase, lasting about 4h and a protein-synthesis-dependent late phase LTP lasting 6h or longer, or even days. A large series of protein kinases have been shown to be involved and herein, a distinct set of protein kinases proposed to be involved in memory retrieval in previous work was tested in dorsal hippocampus of the rat following induction of late-phase LTP. A bipolar stimulation electrode was chronically implanted into the perforant path, while two monopolar recording electrodes were implanted into the dentate gyrus of the dorsal hippocampus. The recording electrode was measuring extracellular excitatory postsynaptic potentials, while the other one measured population spikes. Protein kinases were determined by immunoblotting and immunoflourescence on hippocampal areas showed the distribution pattern of protein kinases PKN1 and NEK7. Induction of LTP was proven, elevated levels for protein kinases PKN1, RPS6KB1, STK4, CDC42BPB, PRKG, TLK, BMX and decreased levels for NEK7, MAK14 and PLK1 were observed. A remarkable overlap of protein kinases observed in spatial memory processes with those proposed in LTP formation was demonstrated. The findings may be relevant for design of future studies on protein kinases and for the interpretation of previous work. PMID:24911953

  16. Rate of Formation of Trigonal Phase in Blends of Homogeneous Propylene 1-Hexene Copolymers

    NASA Astrophysics Data System (ADS)

    Janani, Hamed; Trujillo, Gabriel; Alamo, Rufina

    2013-03-01

    Blends of polyolefins such as polyethylenes, polypropylenes or their copolymers are often used to balance the strengths of each component towards improving processing, physical properties and performance. The final properties depend on the semicrystalline state acquired upon melt-solidification which is highly impacted by the state of melt miscibility. In this work we have assessed the critical composition difference for melt miscibility of propylene 1-hexene copolymers (PH) and selected a miscible pair with 11 and 21 mol% of 1-hexene respectively. PH21 crystallizes in a trigonal packing, while PH11 develops monoclinic crystallites (at low undercooling) or the mesomorphic form (at high undercooling). In situ DSC and WAXD analysis indicate that while the content of trigonal phase decreases with increasing PH11, the rate of formation of trigonal phase in the whole range of undercooling increases with addition of PH11, which as a pure component does not form trigonal phase. The unexpected enhanced kinetics of formation of trigonal phase with blending is attributed to the increasing composition of 1-hexene in the melt during isothermal crystallization of the blend.

  17. Asymptotic and uncertainty analyses of a phase field model for void formation under irradiation

    SciTech Connect

    Nan Wang; Srujan Rokkam; Thomas Hochrainer; Michael Pernice; Anter El-Azab

    2014-06-01

    We perform asymptotic analysis and uncertainty quantification of a phase field model for void formation and evolution in materials subject to irradiation. The parameters of the phase field model are obtained in terms of the underlying material specific quantities by matching the sharp interface limit of the phase field model with the corresponding sharp interface theory for void growth. To evaluate the sensitivity of phase field simulations to uncertainties in input parameters we quantify the predictions using the stochastic collocation method. Uncertainties arising from material parameters are investigated based on available experimental and atomic scale data. The results of our analysis suggest that the uncertainty in the formation and migration energies of vacancies are found to have a strong influence on the void volume fraction (or porosity). In contrast, the uncertainty resulting from the void surface energy has minimal affect. The analysis also shows that the model is consistent in the sense that its predictions do not drastically change as a result of small variations of the model input parameters.

  18. Crystal structures and phase formation thermodynamics of iron-gold nanoclusters

    NASA Astrophysics Data System (ADS)

    Mukherjee, Pinaki

    Alloy nanoparticles are being increasingly used in wide variety of applications (catalysis, contrast enhancement in magnetic resonance imaging, etc.). Knowledge of crystal structure and phase formation of the alloy nanoparticles is critical for these applications. Anomalous thermodynamic behavior and unusual phase formation have been reported for nanoparticles with size below 10 nm. In the present work, inert gas condensation (IGC) has been used to produce Au-Fe nanoclusters of varied compositions with a mean size between 5-10 nm. Here, the nanoclusters below 10 nm display complete solubility between Fe and Au in Fex Au1-x (0.3 < x < 0.8) alloys. The as-deposited clusters are single-crystalline but crystallinity is poor with significant disorder. New structural phases have been obtained for as-deposited clusters that are inconsistent with the equilibrium phase diagram. The as-deposited structures were either bcc for x > 0.65 and fcc for x < 0.65, with lattice parameters expanded relative to elemental and rule-of mixtures values, suggesting the presence of self-interstitials. The as-deposited clusters were ferromagnetic at room temperature. Heat treatment at 600°C for 15 minutes followed by furnace cooling resulted in the size-dependent transformation of the clusters into additional, non-equilibrium structures that depended on cluster composition, while larger clusters followed bulk behavior. At about 65 atom % Fe, clusters transformed to a well-ordered, single fcc phase with a lattice parameter of 0.363 nm, whereas the phase diagram predicted two-phase equilibrium. The stabilization of a single fcc phase was explained by a thermodynamic analysis. This analysis suggests that the single phase stability in the Fe-Au nanoparticles arises from the fact that the introduction of a phase boundary is energetically opposed. Heat treatment of as-deposited particles results in L12 and L1 0ordered structures near 1:3, 3:1 and 1:1 (Fe:Au) stoichiometries respectively

  19. Pattern formation in multiphase flow through porous media: continuum models and phase diagrams

    NASA Astrophysics Data System (ADS)

    Cueto-Felgueroso, L.; Juanes, R.

    2009-12-01

    Carbon capture and geologic storage, dissociation of methane hydrates in permafrost, infiltration of water in soil, and enhanced oil recovery, are some relevant examples of multiphase flow in porous media. While flow instabilities and pattern formation play a central role in these processes, our ability to describe them using mathematical models has been hampered by the lack of a macroscopic theory that explains the patterns observed in experimental and field conditions. We propose a new approach —phase-field modeling— to advance our fundamental understanding of multiphase porous media flow. The basic tenet, with origins in the mathematical description of solidification processes, is that the energy of the system is a function of the inhomogeneous distribution of fluid phases in the pore space, and should account for the presence of macroscopic interfaces. We present numerical simulations and compare our predictions with experimental observations. Numerical simulation of viscous fingering in a Hele-Shaw cell using the proposed phase-field modeling approach

  20. Formation and Stabilization of Single-Crystalline Metastable AuGe Phases in Ge Nanowires

    SciTech Connect

    Sutter, E.; Sutter, P.