CheS-Mapper 2.0 for visual validation of (Q)SAR models

PubMed Central

2014-01-01

Background Sound statistical validation is important to evaluate and compare the overall performance of (Q)SAR models. However, classical validation does not support the user in better understanding the properties of the model or the underlying data. Even though, a number of visualization tools for analyzing (Q)SAR information in small molecule datasets exist, integrated visualization methods that allow the investigation of model validation results are still lacking. Results We propose visual validation, as an approach for the graphical inspection of (Q)SAR model validation results. The approach applies the 3D viewer CheS-Mapper, an open-source application for the exploration of small molecules in virtual 3D space. The present work describes the new functionalities in CheS-Mapper 2.0, that facilitate the analysis of (Q)SAR information and allows the visual validation of (Q)SAR models. The tool enables the comparison of model predictions to the actual activity in feature space. The approach is generic: It is model-independent and can handle physico-chemical and structural input features as well as quantitative and qualitative endpoints. Conclusions Visual validation with CheS-Mapper enables analyzing (Q)SAR information in the data and indicates how this information is employed by the (Q)SAR model. It reveals, if the endpoint is modeled too specific or too generic and highlights common properties of misclassified compounds. Moreover, the researcher can use CheS-Mapper to inspect how the (Q)SAR model predicts activity cliffs. The CheS-Mapper software is freely available at http://ches-mapper.org. Graphical abstract Comparing actual and predicted activity values with CheS-Mapper.

A fixed-time diffusion analysis method determines that the three cheV genes of Helicobacter pylori differentially affect motility

PubMed Central

Lowenthal, Andrew C.; Simon, Christopher; Fair, Amber S.; Mehmood, Khalid; Terry, Karianne; Anastasia, Stephanie; Ottemann, Karen M.

2009-01-01

Helicobacter pylori is a chemotactic bacterium that has three CheV proteins in its predicted chemotaxis signal transduction system. CheV proteins contain both CheW- and response-regulator-like domains. To determine the function of these proteins, we developed a fixed-time diffusion method that would quantify bacterial direction change without needing to define particular behaviours, to deal with the many behaviours that swimming H. pylori exhibit. We then analysed mutants that had each cheV gene deleted individually and found that the behaviour of each mutant differed substantially from wild-type and the other mutants. cheV1 and cheV2 mutants displayed smooth swimming behaviour, consistent with decreased cellular CheY-P, similar to a cheW mutant. In contrast, the cheV3 mutation had the opposite effect and the mutant cells appeared to change direction frequently. Additional analysis showed that the cheV mutants displayed aberrant behaviour as compared to the wild-type in the soft-agar chemotaxis assay. The soft-agar assay phenotype was less extreme compared to that seen in the fixed-time diffusion model, suggesting that the cheV mutants are able to partially compensate for their defects under some conditions. Each cheV mutant furthermore had defects in mouse colonization that ranged from severe to modest, consistent with a role in chemotaxis. These studies thus show that the H. pylori CheV proteins each differently affect swimming behaviour. PMID:19332820

The Emotional Communication in Hearing Questionnaire (EMO-CHeQ): Development and Evaluation.

PubMed

Singh, Gurjit; Liskovoi, Lisa; Launer, Stefan; Russo, Frank

2018-06-11

The objectives of this research were to develop and evaluate a self-report questionnaire (the Emotional Communication in Hearing Questionnaire or EMO-CHeQ) designed to assess experiences of hearing and handicap when listening to signals that contain vocal emotion information. Study 1 involved internet-based administration of a 42-item version of the EMO-CHeQ to 586 adult participants (243 with self-reported normal hearing [NH], 193 with self-reported hearing impairment but no reported use of hearing aids [HI], and 150 with self-reported hearing impairment and use of hearing aids [HA]). To better understand the factor structure of the EMO-CHeQ and eliminate redundant items, an exploratory factor analysis was conducted. Study 2 involved laboratory-based administration of a 16-item version of the EMO-CHeQ to 32 adult participants (12 normal hearing/near normal hearing (NH/nNH), 10 HI, and 10 HA). In addition, participants completed an emotion-identification task under audio and audiovisual conditions. In study 1, the exploratory factor analysis yielded an interpretable solution with four factors emerging that explained a total of 66.3% of the variance in performance the EMO-CHeQ. Item deletion resulted in construction of the 16-item EMO-CHeQ. In study 1, both the HI and HA group reported greater vocal emotion communication handicap on the EMO-CHeQ than on the NH group, but differences in handicap were not observed between the HI and HA group. In study 2, the same pattern of reported handicap was observed in individuals with audiometrically verified hearing as was found in study 1. On the emotion-identification task, no group differences in performance were observed in the audiovisual condition, but group differences were observed in the audio alone condition. Although the HI and HA group exhibited similar emotion-identification performance, both groups performed worse than the NH/nNH group, thus suggesting the presence of behavioral deficits that parallel self

Flagellar dynamics reveal the distribution of chemotactic signaling molecule CheY-P in E. coli

NASA Astrophysics Data System (ADS)

Bano, Roshni; Mears, Patrick; Chemla, Yann; Golding, Ido

E. colicells swim in a random walk consisting of ''runs'' - during which the flagella that propel the cell rotate counter-clockwise (CCW) - and ''tumbles''- during which one or more flagella rotate clockwise (CW). The tumbling frequency is modulated by the phosphorylation state of the signaling molecule CheY, which depends on the cell's environment. Phosphorylated CheY (CheY-P) binds to a flagellar motor and engenders a change in rotation state from CCW to CW. Despite advances in methods used to observe chemotactic signaling, it remains a challenge to measure the CheY-P level in cells directly. Here, we used an optical trap assay coupled with fluorescence microscopy to observe the dynamics of fluorescently labelled flagella in individual cells. By measuring the distribution of flagellar states in multi-flagellated cells and using our recent finding that each flagellar motor independently measures the cellular CheY-P concentration, we are able to extract the probability distribution of the CheY-P level in the cell. This analysis reveals the magnitude of fluctuations in chemotactic signaling in the live cell. We further investigate how this CheY-P distribution changes when cells encounter chemical gradients and perform chemotaxis. This work was supported by the National Science Foundation (NSF) through the Centre for Physics of Living Cells (CPLC).

Space Station Freedom CHeCS overview. [Crew Health Care System

NASA Technical Reports Server (NTRS)

Boyce, Joey B.

1990-01-01

The current status, progress, and future plans for development of the Crew Health Care System (CHeCS) for the International Space Station Freedom are presented. Essential operational biomedical support requirements for the astronauts, including medical care, environmental habitat monitoring, and countermeasures for the potentially maladaptive physiological effects of space flight will be provided by the CHeCS. Three integral parts will make up the system: a health maintenance facility, an environmental health system, and the exercise countermeasures facility. Details of each of the major systems and their subsystems are presented.

MSL Chemistry and Mineralogy X-Ray Diffraction X-Ray Fluorescence (CheMin) Instrument

NASA Technical Reports Server (NTRS)

Zimmerman, Wayne; Blake, Dave; Harris, William; Morookian, John Michael; Randall, Dave; Reder, Leonard J.; Sarrazin, Phillipe

2013-01-01

This paper provides an overview of the Mars Science Laboratory (MSL) Chemistry and Mineralogy Xray Diffraction (XRD), X-ray Fluorescence (XRF) (CheMin) Instrument, an element of the landed Curiosity rover payload, which landed on Mars in August of 2012. The scientific goal of the MSL mission is to explore and quantitatively assess regions in Gale Crater as a potential habitat for life - past or present. The CheMin instrument will receive Martian rock and soil samples from the MSL Sample Acquisition/Sample Processing and Handling (SA/SPaH) system, and process it utilizing X-Ray spectroscopy methods to determine mineral composition. The Chemin instrument will analyze Martian soil and rocks to enable scientists to investigate geophysical processes occurring on Mars. The CheMin science objectives and proposed surface operations are described along with the CheMin hardware with an emphasis on the system engineering challenges associated with developing such a complex instrument.

COMPARISON BETWEEN ELLMAN AND RADIOMETRIC METHODS FOR ASSESSING CHOLINESTERASE (CHE) INHIBITION IN RATS TREATED WITH N-METHYL CARBAMATE INSECTICIDES.

EPA Science Inventory

Carbamylated ChE is unstable and readily reactivates. This reactivation, promoted by increasing temperature and dilution, could have an impact on ex vivo ChE assays by decreasing apparent ChE inhibition. To assess the best method for measuring ChE inhibition in brain and RBCs f...

Interactions of the chemotaxis signal protein CheY with bacterial flagellar motors visualized by evanescent wave microscopy.

PubMed

Khan, S; Pierce, D; Vale, R D

The chemotaxis signal protein CheY of enteric bacteria shuttles between transmembrane methyl-accepting chemotaxis protein (MCP) receptor complexes and flagellar basal bodies [1]. The basal body C-rings, composed of the FliM, FliG and FliN proteins, form the rotor of the flagellar motor [2]. Phosphorylated CheY binds to isolated FliM [3] and may also interact with FliG [4], but its binding to basal bodies has not been measured. Using the chemorepellent acetate to phosphorylate and acetylate CheY [5], we have measured the covalent-modification-dependent binding of a green fluorescent protein-CheY fusion (GFP-CheY) to motor assemblies in bacteria lacking MCP complexes by evanescent wave microscopy [6]. At acetate concentrations that cause solely clockwise rotation, GFP-CheY molecules bound to native basal bodies or to overproduced rotor complexes with a stoichiometry comparable to the number of C-ring subunits. GFP-CheY did not bind to rotors lacking FIiM/FliN, showing that these subunits are essential for the association. This assay provides a new means of monitoring protein-protein interactions in signal transduction pathways in living cells.

Role of CheW protein in coupling membrane receptors to the intracellular signaling system of bacterial chemotaxis.

PubMed Central

Liu, J D; Parkinson, J S

1989-01-01

Chemotactic behavior in Escherichia coli is mediated by membrane-associated chemoreceptors that transmit sensory signals to the flagellar motors through an intracellular signaling system, which appears to involve a protein phosphorylation cascade. This study concerns the role of CheW, a cytoplasmic protein, in coupling methyl-accepting chemotaxis proteins (MCPs), the major class of membrane receptors, to the intracellular signaling system. Steady-state flagellar rotation behavior was examined in a series of strains with different combinations and relative amounts of CheW, MCPs, and other signaling components. At normal expression levels, CheW stimulated clockwise rotation, and receptors appeared to enhance this stimulatory effect. At high expression levels, MCPs inhibited clockwise rotation, and CheW appeared to augment this inhibitory effect. Since overexpression of CheW or MCP molecules had the same behavioral effect as their absence, chemoreceptors probably use CheW to modulate two distinct signals, one that stimulates and one that inhibits the intracellular phosphorylation cascade. Images PMID:2682657

Evolutionary genomics suggests that CheV is an additional adaptor for accommodating specific chemoreceptors within the chemotaxis signaling complex

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

Ortega, Davi R.; Zhulin, Igor B.; Punta, Marco

Escherichia coli and Salmonella enterica are models for many experiments in molecular biology including chemotaxis, and most of the results obtained with one organism have been generalized to another. While most components of the chemotaxis pathway are strongly conserved between the two species, Salmonella genomes contain some chemoreceptors and an additional protein, CheV, that are not found in E. coli. The role of CheV was examined in distantly related species Bacillus subtilis and Helicobacter pylori, but its role in bacterial chemotaxis is still not well understood. We tested a hypothesis that in enterobacteria CheV functions as an additional adaptor linkingmore » the CheA kinase to certain types of chemoreceptors that cannot be effectively accommodated by the universal adaptor CheW. Phylogenetic profiling, genomic context and comparative protein sequence analyses suggested that CheV interacts with specific domains of CheA and chemoreceptors from an orthologous group exemplified by the Salmonella McpC protein. Structural consideration of the conservation patterns suggests that CheV and CheW share the same binding spot on the chemoreceptor structure, but have some affinity bias towards chemoreceptors from different orthologous groups. Finally, published experimental results and data newly obtained via comparative genomics support the idea that CheV functions as a "phosphate sink" possibly to off-set the over-stimulation of the kinase by certain types of chemoreceptors. Altogether, our results strongly suggest that CheV is an additional adaptor for accommodating specific chemoreceptors within the chemotaxis signaling complex.« less

Evolutionary genomics suggests that CheV is an additional adaptor for accommodating specific chemoreceptors within the chemotaxis signaling complex

DOE PAGES

Ortega, Davi R.; Zhulin, Igor B.; Punta, Marco

2016-02-04

Escherichia coli and Salmonella enterica are models for many experiments in molecular biology including chemotaxis, and most of the results obtained with one organism have been generalized to another. While most components of the chemotaxis pathway are strongly conserved between the two species, Salmonella genomes contain some chemoreceptors and an additional protein, CheV, that are not found in E. coli. The role of CheV was examined in distantly related species Bacillus subtilis and Helicobacter pylori, but its role in bacterial chemotaxis is still not well understood. We tested a hypothesis that in enterobacteria CheV functions as an additional adaptor linkingmore » the CheA kinase to certain types of chemoreceptors that cannot be effectively accommodated by the universal adaptor CheW. Phylogenetic profiling, genomic context and comparative protein sequence analyses suggested that CheV interacts with specific domains of CheA and chemoreceptors from an orthologous group exemplified by the Salmonella McpC protein. Structural consideration of the conservation patterns suggests that CheV and CheW share the same binding spot on the chemoreceptor structure, but have some affinity bias towards chemoreceptors from different orthologous groups. Finally, published experimental results and data newly obtained via comparative genomics support the idea that CheV functions as a "phosphate sink" possibly to off-set the over-stimulation of the kinase by certain types of chemoreceptors. Altogether, our results strongly suggest that CheV is an additional adaptor for accommodating specific chemoreceptors within the chemotaxis signaling complex.« less

A CheR/CheB fusion protein is involved in cyst cell development and chemotaxis in Azospirillum brasilense Sp7.

PubMed

Wu, Lixian; Cui, Yanhua; Hong, Yuanyuan; Chen, Sanfeng

2011-12-20

We here report the sequence and functional analysis of cstB of Azospirillum brasilense Sp7. The predicted cstB contains C-terminal two PAS domains and N-terminal part which has similarity with CheB-CheR fusion protein. cstB mutants had reduced swarming ability compared to that of A. brasilense wild-type strain, implying that cstB was involved in chemotaxis in A. brasilense. A microscopic analysis revealed that cstB mutants developed mature cyst cells more quickly than wild type, indicating that cstB is involved in cyst formation. cstB mutants were affected in colony morphology and the production of exopolysaccharides (EPS) which are essential for A. brasilense cells to differentiate into cyst-like forms. These observations suggested that cstB was a multi-effector involved in cyst development and chemotaxis in A. brasilense. Copyright © 2010 Elsevier GmbH. All rights reserved.

A cheZ-Like Gene in Azorhizobium caulinodans Is a Key Gene in the Control of Chemotaxis and Colonization of the Host Plant.

PubMed

Liu, Xiaolin; Liu, Wei; Sun, Yu; Xia, Chunlei; Elmerich, Claudine; Xie, Zhihong

2018-02-01

Chemotaxis can provide bacteria with competitive advantages for survival in complex environments. The CheZ chemotaxis protein is a phosphatase, affecting the flagellar motor in Escherichia coli by dephosphorylating the response regulator phosphorylated CheY protein (CheY∼P) responsible for clockwise rotation. A cheZ gene has been found in Azorhizobium caulinodans ORS571, in contrast to other rhizobial species studied so far. The CheZ protein in strain ORS571 has a conserved motif similar to that corresponding to the phosphatase active site in E. coli The construction of a cheZ deletion mutant strain and of cheZ mutant strains carrying a mutation in residues of the putative phosphatase active site showed that strain ORS571 participates in chemotaxis and motility, causing a hyperreversal behavior. In addition, the properties of the cheZ deletion mutant revealed that ORS571 CheZ is involved in other physiological processes, since it displayed increased flocculation, biofilm formation, exopolysaccharide (EPS) production, and host root colonization. In particular, it was observed that the expression of several exp genes, involved in EPS synthesis, was upregulated in the cheZ mutant compared to that in the wild type, suggesting that CheZ negatively controls exp gene expression through an unknown mechanism. It is proposed that CheZ influences the Azorhizobium -plant association by negatively regulating early colonization via the regulation of EPS production. This report established that CheZ in A. caulinodans plays roles in chemotaxis and the symbiotic association with the host plant. IMPORTANCE Chemotaxis allows bacteria to swim toward plant roots and is beneficial to the establishment of various plant-microbe associations. The level of CheY phosphorylation (CheY∼P) is central to the chemotaxis signal transduction. The mechanism of the signal termination of CheY∼P remains poorly characterized among Alphaproteobacteria , except for Sinorhizobium meliloti , which

Distinct Domains of CheA Confer Unique Functions in Chemotaxis and Cell Length in Azospirillum brasilense Sp7.

PubMed

Gullett, Jessica M; Bible, Amber; Alexandre, Gladys

2017-07-01

Chemotaxis is the movement of cells in response to gradients of diverse chemical cues. Motile bacteria utilize a conserved chemotaxis signal transduction system to bias their motility and navigate through a gradient. A central regulator of chemotaxis is the histidine kinase CheA. This cytoplasmic protein interacts with membrane-bound receptors, which assemble into large polar arrays, to propagate the signal. In the alphaproteobacterium Azospirillum brasilense , Che1 controls transient increases in swimming speed during chemotaxis, but it also biases the cell length at division. However, the exact underlying molecular mechanisms for Che1-dependent control of multiple cellular behaviors are not known. Here, we identify specific domains of the CheA1 histidine kinase implicated in modulating each of these functions. We show that CheA1 is produced in two isoforms: a membrane-anchored isoform produced as a fusion with a conserved seven-transmembrane domain of unknown function (TMX) at the N terminus and a soluble isoform similar to prototypical CheA. Site-directed and deletion mutagenesis combined with behavioral assays confirm the role of CheA1 in chemotaxis and implicate the TMX domain in mediating changes in cell length. Fluorescence microscopy further reveals that the membrane-anchored isoform is distributed around the cell surface while the soluble isoform localizes at the cell poles. Together, the data provide a mechanism for the role of Che1 in controlling multiple unrelated cellular behaviors via acquisition of a new domain in CheA1 and production of distinct functional isoforms. IMPORTANCE Chemotaxis provides a significant competitive advantage to bacteria in the environment, and this function has been transferred laterally multiple times, with evidence of functional divergence in different genomic contexts. The molecular principles that underlie functional diversification of chemotaxis in various genomic contexts are unknown. Here, we provide a molecular

Distinct Domains of CheA Confer Unique Functions in Chemotaxis and Cell Length in Azospirillum brasilense Sp7

PubMed Central

Gullett, Jessica M.

2017-01-01

ABSTRACT Chemotaxis is the movement of cells in response to gradients of diverse chemical cues. Motile bacteria utilize a conserved chemotaxis signal transduction system to bias their motility and navigate through a gradient. A central regulator of chemotaxis is the histidine kinase CheA. This cytoplasmic protein interacts with membrane-bound receptors, which assemble into large polar arrays, to propagate the signal. In the alphaproteobacterium Azospirillum brasilense, Che1 controls transient increases in swimming speed during chemotaxis, but it also biases the cell length at division. However, the exact underlying molecular mechanisms for Che1-dependent control of multiple cellular behaviors are not known. Here, we identify specific domains of the CheA1 histidine kinase implicated in modulating each of these functions. We show that CheA1 is produced in two isoforms: a membrane-anchored isoform produced as a fusion with a conserved seven-transmembrane domain of unknown function (TMX) at the N terminus and a soluble isoform similar to prototypical CheA. Site-directed and deletion mutagenesis combined with behavioral assays confirm the role of CheA1 in chemotaxis and implicate the TMX domain in mediating changes in cell length. Fluorescence microscopy further reveals that the membrane-anchored isoform is distributed around the cell surface while the soluble isoform localizes at the cell poles. Together, the data provide a mechanism for the role of Che1 in controlling multiple unrelated cellular behaviors via acquisition of a new domain in CheA1 and production of distinct functional isoforms. IMPORTANCE Chemotaxis provides a significant competitive advantage to bacteria in the environment, and this function has been transferred laterally multiple times, with evidence of functional divergence in different genomic contexts. The molecular principles that underlie functional diversification of chemotaxis in various genomic contexts are unknown. Here, we provide a

Borrelia burgdorferi CheY2 Is Dispensable for Chemotaxis or Motility but Crucial for the Infectious Life Cycle of the Spirochete

PubMed Central

Xu, Hui; Sultan, Syed; Yerke, Aaron; Moon, Ki Hwan; Wooten, R. Mark

2016-01-01

ABSTRACT The requirements for bacterial chemotaxis and motility range from dispensable to crucial for host colonization. Even though more than 50% of all sequenced prokaryotic genomes possess at least one chemotaxis signaling system, many of those genomes contain multiple copies of a chemotaxis gene. However, the functions of most of those additional genes are unknown. Most motile bacteria possess at least one CheY response regulator that is typically dedicated to the control of motility and which is usually essential for virulence. Borrelia burgdorferi appears to be notably different, in that it has three cheY genes, and our current studies on cheY2 suggests that it has varied effects on different aspects of the natural infection cycle. Mutants deficient in this protein exhibit normal motility and chemotaxis in vitro but show reduced virulence in mice. Specifically, the cheY2 mutants were severely attenuated in murine infection and dissemination to distant tissues after needle inoculation. Moreover, while ΔcheY2 spirochetes are able to survive normally in the Ixodes ticks, mice fed upon by the ΔcheY2-infected ticks did not develop a persistent infection in the murine host. Our data suggest that CheY2, despite resembling a typical response regulator, functions distinctively from most other chemotaxis CheY proteins. We propose that CheY2 serves as a regulator for a B. burgdorferi virulence determinant that is required for productive infection within vertebrate, but not tick, hosts. PMID:27799336

CHeCS: International Space Station Medical Hardware Catalog

NASA Technical Reports Server (NTRS)

2008-01-01

The purpose of this catalog is to provide a detailed description of each piece of hardware in the Crew Health Care System (CHeCS), including subpacks associated with the hardware, and to briefly describe the interfaces between the hardware and the ISS. The primary user of this document is the Space Medicine/Medical Operations ISS Biomedical Flight Controllers (ISS BMEs).

The C. elegans che-1 gene encodes a zinc finger transcription factor required for specification of the ASE chemosensory neurons.

PubMed

Uchida, Okiko; Nakano, Hiroyuki; Koga, Makoto; Ohshima, Yasumi

2003-04-01

Chemotaxis to water-soluble chemicals such as NaCl is an important behavior of C. elegans when seeking food. ASE chemosensory neurons have a major role in this behavior. We show that che-1, defined by chemotaxis defects, encodes a zinc-finger protein similar to the GLASS transcription factor required for photoreceptor cell differentiation in Drosophila, and that che-1 is essential for specification and function of ASE neurons. Expression of a che-1::gfp fusion construct was predominant in ASE. In che-1 mutants, expression of genes characterizing ASE such as seven-transmembrane receptors, guanylate cyclases and a cyclic-nucleotide gated channel is lost. Ectopic expression of che-1 cDNA induced expression of ASE-specific marker genes, a dye-filling defect in neurons other than ASE and dauer formation.

Borrelia burgdorferi CheY2 Is Dispensable for Chemotaxis or Motility but Crucial for the Infectious Life Cycle of the Spirochete.

PubMed

Xu, Hui; Sultan, Syed; Yerke, Aaron; Moon, Ki Hwan; Wooten, R Mark; Motaleb, M A

2017-01-01

The requirements for bacterial chemotaxis and motility range from dispensable to crucial for host colonization. Even though more than 50% of all sequenced prokaryotic genomes possess at least one chemotaxis signaling system, many of those genomes contain multiple copies of a chemotaxis gene. However, the functions of most of those additional genes are unknown. Most motile bacteria possess at least one CheY response regulator that is typically dedicated to the control of motility and which is usually essential for virulence. Borrelia burgdorferi appears to be notably different, in that it has three cheY genes, and our current studies on cheY2 suggests that it has varied effects on different aspects of the natural infection cycle. Mutants deficient in this protein exhibit normal motility and chemotaxis in vitro but show reduced virulence in mice. Specifically, the cheY2 mutants were severely attenuated in murine infection and dissemination to distant tissues after needle inoculation. Moreover, while ΔcheY2 spirochetes are able to survive normally in the Ixodes ticks, mice fed upon by the ΔcheY2-infected ticks did not develop a persistent infection in the murine host. Our data suggest that CheY2, despite resembling a typical response regulator, functions distinctively from most other chemotaxis CheY proteins. We propose that CheY2 serves as a regulator for a B. burgdorferi virulence determinant that is required for productive infection within vertebrate, but not tick, hosts. Copyright © 2016 American Society for Microbiology.

Efficient Computation of Sparse Matrix Functions for Large-Scale Electronic Structure Calculations: The CheSS Library.

PubMed

Mohr, Stephan; Dawson, William; Wagner, Michael; Caliste, Damien; Nakajima, Takahito; Genovese, Luigi

2017-10-10

We present CheSS, the "Chebyshev Sparse Solvers" library, which has been designed to solve typical problems arising in large-scale electronic structure calculations using localized basis sets. The library is based on a flexible and efficient expansion in terms of Chebyshev polynomials and presently features the calculation of the density matrix, the calculation of matrix powers for arbitrary powers, and the extraction of eigenvalues in a selected interval. CheSS is able to exploit the sparsity of the matrices and scales linearly with respect to the number of nonzero entries, making it well-suited for large-scale calculations. The approach is particularly adapted for setups leading to small spectral widths of the involved matrices and outperforms alternative methods in this regime. By coupling CheSS to the DFT code BigDFT, we show that such a favorable setup is indeed possible in practice. In addition, the approach based on Chebyshev polynomials can be massively parallelized, and CheSS exhibits excellent scaling up to thousands of cores even for relatively small matrix sizes.

Demonstrating the Open Data Repository's Data Publisher: The CheMin Database

NASA Astrophysics Data System (ADS)

Stone, N.; Lafuente, B.; Bristow, T.; Pires, A.; Keller, R. M.; Downs, R. T.; Blake, D.; Dateo, C. E.; Fonda, M.

2018-04-01

The Open Data Repository's Data Publisher aims to provide an easy-to-use software tool that will allow researchers to create and publish database templates and related data. The CheMin Database developed using this framework is shown as an example.

Organophosphate and carbamate insecticides in agricultural waters and cholinesterase (ChE) inhibition in common carp (Cyprinus carpio)

USGS Publications Warehouse

Gruber, S.J.; Munn, M.D.

1998-01-01

Cholinesterase (ChE) activity was used as a biomarker for assessing exposure of common carp (Cyprinus carpio) to organophosphate and carbamate insecticides from irrigated agricultural waters. Carp were collected from a lake (Royal Lake) that receives most of its water from irrigation return flows and from a reference lake (Billy Clapp Lake) outside of the irrigation system. Results indicated that the mean whole-brain ChE activity of carp from Royal Lake (3.47 μmol/min/g tissue) was 34.2% less than that of carp from Billy Clapp Lake (5.27 μmol/min/g tissue) (p = 0.003). The depressed ChE activity in brain tissue of Royal Lake carp was in response to ChE-inhibiting insecticides detected in water samples in the weeks prior to tissue sampling; the most frequently detected insecticides included chlorpyrifos, azinphos-methyl, carbaryl, and ethoprop. Neither sex nor size appears to be a covariable in the analysis; ChE activity was not correlated with fish length or weight in either lake and there was no significant difference in ChE activity between the two sexes within each lake. Although organophosphate and carbamate insecticides can break down rapidly in the environment, this study suggests that in agricultural regions where insecticides are applied for extended periods of the year, nontarget aquatic biota may be exposed to high levels of ChE-inhibiting insecticides for a period of several months.

Performing Mineral Hydration Experiments in the CheMin Diffractometer on Mars

NASA Technical Reports Server (NTRS)

Vaniman, D. T.; Yen, A. S.; Rampe, E. B.; Blake, D. F.; Chipera, S. J.; Morookian, J. M.; Ming, D. W.; Bristow, T. F.; Morris, R. V.; Geller, R.;

Molecular Interaction and Cellular Location of RecA and CheW Proteins in Salmonella enterica during SOS Response and Their Implication in Swarming.

PubMed

Irazoki, Oihane; Aranda, Jesús; Zimmermann, Timo; Campoy, Susana; Barbé, Jordi

2016-01-01

In addition to its role in DNA damage repair and recombination, the RecA protein, through its interaction with CheW, is involved in swarming motility, a form of flagella-dependent movement across surfaces. In order to better understand how SOS response modulates swarming, in this work the location of RecA and CheW proteins within the swarming cells has been studied by using super-resolution microscopy. Further, and after in silico docking studies, the specific RecA and CheW regions associated with the RecA-CheW interaction have also been confirmed by site-directed mutagenesis and immunoprecipitation techniques. Our results point out that the CheW distribution changes, from the cell poles to foci distributed in a helical pattern along the cell axis when SOS response is activated or RecA protein is overexpressed. In this situation, the CheW presents the same subcellular location as that of RecA, pointing out that the previously described RecA storage structures may be modulators of swarming motility. Data reported herein not only confirmed that the RecA-CheW pair is essential for swarming motility but it is directly involved in the CheW distribution change associated to SOS response activation. A model explaining not only the mechanism by which DNA damage modulates swarming but also how both the lack and the excess of RecA protein impair this motility is proposed.

Pragmatic Idealism: Ernesto Che Guevaras Strategic Choice for Bolivia

DTIC Science & Technology

2017-06-01

that the guerrilla foco could be used as an inspiration to the masses, and that, by its very existence, people would be encouraged to support and join...Paperwork Reduction Project (0704-0188) Washington DC 20503. 1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE June 2017 3. REPORT TYPE AND...strategic, and psychological factors meant to achieve a specific end. 14. SUBJECT TERMS Che, Guevara, guerrilla warfare, Cuba, Bolivia

McArthur rotates the CHeCS Rack during Expedition 12

NASA Image and Video Library

2005-12-09

ISS012-E-10806 (9 December 2005) --- Astronaut William S. (Bill) McArthur Jr., Expedition 12 commander and NASA space station science officer, rotates the Crew Health Care System (CHeCS) rack in order to access the Avionics Air Assembly (AAA) air ducts during in-flight maintenance (IFM) in the Destiny laboratory of the International Space Station.

Quantum-mechanics-derived 13Cα chemical shift server (CheShift) for protein structure validation

PubMed Central

Vila, Jorge A.; Arnautova, Yelena A.; Martin, Osvaldo A.; Scheraga, Harold A.

2009-01-01

A server (CheShift) has been developed to predict 13Cα chemical shifts of protein structures. It is based on the generation of 696,916 conformations as a function of the φ, ψ, ω, χ1 and χ2 torsional angles for all 20 naturally occurring amino acids. Their 13Cα chemical shifts were computed at the DFT level of theory with a small basis set and extrapolated, with an empirically-determined linear regression formula, to reproduce the values obtained with a larger basis set. Analysis of the accuracy and sensitivity of the CheShift predictions, in terms of both the correlation coefficient R and the conformational-averaged rmsd between the observed and predicted 13Cα chemical shifts, was carried out for 3 sets of conformations: (i) 36 x-ray-derived protein structures solved at 2.3 Å or better resolution, for which sets of 13Cα chemical shifts were available; (ii) 15 pairs of x-ray and NMR-derived sets of protein conformations; and (iii) a set of decoys for 3 proteins showing an rmsd with respect to the x-ray structure from which they were derived of up to 3 Å. Comparative analysis carried out with 4 popular servers, namely SHIFTS, SHIFTX, SPARTA, and PROSHIFT, for these 3 sets of conformations demonstrated that CheShift is the most sensitive server with which to detect subtle differences between protein models and, hence, to validate protein structures determined by either x-ray or NMR methods, if the observed 13Cα chemical shifts are available. CheShift is available as a web server. PMID:19805131

X-Ray Diffraction on Mars: Scientific Discoveries Made by the CheMin Instrument

NASA Technical Reports Server (NTRS)

Rampe, E. B.; Blake, D. F.; Ming, D. W.; Bristow, T. F.

2017-01-01

The Mars Science Laboratory Curiosity landed in Gale crater in August 2012 with the goal to identify and characterize habitable environments on Mars. Curiosity has been studying a series of sedimentary rocks primarily deposited in fluviolacustrine environments approximately 3.5 Ga. Minerals in the rocks and soils on Mars can help place further constraints on these ancient aqueous environments, including pH, salinity, and relative duration of liquid water. The Chemistry and Mineralogy (CheMin) X-ray diffraction and X-ray fluorescence instrument on Curiosity uses a Co X-ray source and charge-coupled device detector in transmission geometry to collect 2D Debye-Scherrer ring patterns of the less than 150 micron size fraction of drilled rock powders or scooped sediments. With an angular range of approximately 2.52deg 20 and a 20 resolution of approximately 0.3deg, mineral abundances can be quantified with a detection limit of approximately 1-2 wt. %. CheMin has returned quantitative mineral abundances from 16 mudstone, sandstone, and aeolian sand samples so far. The mineralogy of these samples is incredibly diverse, suggesting a variety of depositional and diagenetic environments and different source regions for the sediments. Results from CheMin have been essential for reconstructing the geologic history of Gale crater and addressing the question of habitability on ancient Mars.

Fan filter cleaning on the CHeCS AAA in the US Lab

NASA Image and Video Library

2009-05-05

ISS019-E-013710 (5 May 2009) --- Japan Aerospace Exploration Agency (JAXA) astronaut Koichi Wakata, Expedition 19/20 flight engineer, cleans a fan filter on the Crew Health Care System Avionics Air Assembly (CHeCS AAA) in the Destiny laboratory of the International Space Station.

Crew Health Care System (CHeCS) Design Research, Documentations, and Evaluations

NASA Technical Reports Server (NTRS)

CLement, Bethany M.

2011-01-01

The Crew Health Care System (CHeCS) is a group within the Space Life Science Directorate (SLSD) that focuses on the overall health of astronauts by reinforcing the three divisions - the Environmental Maintenance System (EMS), the Countermeasures System (CMS), and the Health Maintenance System (HMS). This internship provided opportunity to gain knowledge, experience, and skills in CHeCS engineering and operations tasks. Various and differing tasks allowed for occasions to work independently, network to get things done, and show leadership abilities. Specific exercises included reviewing hardware certification, operations, and documentation within the ongoing Med Kit Redesign (MKR) project, and learning, writing, and working various common pieces of paperwork used in the engineering and design process. Another project focused on the distribution of various pieces of hardware to off-site research facilities with an interest in space flight health care. The main focus of this internship, though, was on a broad and encompassing understanding of the engineering process as time was spent looking at each individual step in a variety of settings and tasks.

CheS-Mapper - Chemical Space Mapping and Visualization in 3D.

PubMed

Gütlein, Martin; Karwath, Andreas; Kramer, Stefan

2012-03-17

Analyzing chemical datasets is a challenging task for scientific researchers in the field of chemoinformatics. It is important, yet difficult to understand the relationship between the structure of chemical compounds, their physico-chemical properties, and biological or toxic effects. To that respect, visualization tools can help to better comprehend the underlying correlations. Our recently developed 3D molecular viewer CheS-Mapper (Chemical Space Mapper) divides large datasets into clusters of similar compounds and consequently arranges them in 3D space, such that their spatial proximity reflects their similarity. The user can indirectly determine similarity, by selecting which features to employ in the process. The tool can use and calculate different kind of features, like structural fragments as well as quantitative chemical descriptors. These features can be highlighted within CheS-Mapper, which aids the chemist to better understand patterns and regularities and relate the observations to established scientific knowledge. As a final function, the tool can also be used to select and export specific subsets of a given dataset for further analysis.

CheS-Mapper - Chemical Space Mapping and Visualization in 3D

PubMed Central

2012-01-01

Analyzing chemical datasets is a challenging task for scientific researchers in the field of chemoinformatics. It is important, yet difficult to understand the relationship between the structure of chemical compounds, their physico-chemical properties, and biological or toxic effects. To that respect, visualization tools can help to better comprehend the underlying correlations. Our recently developed 3D molecular viewer CheS-Mapper (Chemical Space Mapper) divides large datasets into clusters of similar compounds and consequently arranges them in 3D space, such that their spatial proximity reflects their similarity. The user can indirectly determine similarity, by selecting which features to employ in the process. The tool can use and calculate different kind of features, like structural fragments as well as quantitative chemical descriptors. These features can be highlighted within CheS-Mapper, which aids the chemist to better understand patterns and regularities and relate the observations to established scientific knowledge. As a final function, the tool can also be used to select and export specific subsets of a given dataset for further analysis. PMID:22424447

Application of a Curriculum Hierarchy Evaluation (CHE) Model to Sequentially Arranged Tasks.

ERIC Educational Resources Information Center

O'Malley, J. Michael

A curriculum hierarchy evaluation (CHE) model was developed by combining a transfer paradigm with an aptitude-treatment-task interaction (ATTI) paradigm. Positive transfer was predicted between sequentially arranged tasks, and a programed or nonprogramed treatment was predicted to interact with aptitude and with tasks. Eighteen four and five…

How to Improve Students' Comprehension Concerning the Major Terms of Functional Groups?--In the Experiment of OrCheTaboo Game

ERIC Educational Resources Information Center

Akkuzu, Nalan; Uyulgan, Melis Arzu

2016-01-01

In this research, for an effective learning of concepts in the scope of functional groups in organic chemistry, it has been developed as "Organic Chemistry Taboo (OrCheTaboo)" which is an educational activity by the researchers. The aim of this study is to analyze the effect of the game OrCheTaboo on learning of concepts related to…

On the Application of Simple Experiments to the Teaching of ChE Thermodynamics

ERIC Educational Resources Information Center

McNeil, Kenneth M.

1978-01-01

This article describes the undergraduate ChE thermodynamics course at Drexel University and the reasons for incorporating a series of laboratory experiments into the course. Included is a list of lecture topics and a description of each of the experiments. (BB)

CheNER: a tool for the identification of chemical entities and their classes in biomedical literature.

PubMed

Usié, Anabel; Cruz, Joaquim; Comas, Jorge; Solsona, Francesc; Alves, Rui

2015-01-01

Small chemical molecules regulate biological processes at the molecular level. Those molecules are often involved in causing or treating pathological states. Automatically identifying such molecules in biomedical text is difficult due to both, the diverse morphology of chemical names and the alternative types of nomenclature that are simultaneously used to describe them. To address these issues, the last BioCreAtIvE challenge proposed a CHEMDNER task, which is a Named Entity Recognition (NER) challenge that aims at labelling different types of chemical names in biomedical text. To address this challenge we tested various approaches to recognizing chemical entities in biomedical documents. These approaches range from linear Conditional Random Fields (CRFs) to a combination of CRFs with regular expression and dictionary matching, followed by a post-processing step to tag those chemical names in a corpus of Medline abstracts. We named our best performing systems CheNER. We evaluate the performance of the various approaches using the F-score statistics. Higher F-scores indicate better performance. The highest F-score we obtain in identifying unique chemical entities is 72.88%. The highest F-score we obtain in identifying all chemical entities is 73.07%. We also evaluate the F-Score of combining our system with ChemSpot, and find an increase from 72.88% to 73.83%. CheNER presents a valid alternative for automated annotation of chemical entities in biomedical documents. In addition, CheNER may be used to derive new features to train newer methods for tagging chemical entities. CheNER can be downloaded from http://metres.udl.cat and included in text annotation pipelines.

The chemotaxis-like Che1 pathway has an indirect role in adhesive cell properties of Azospirillum brasilense.

PubMed

Siuti, Piro; Green, Calvin; Edwards, Amanda Nicole; Doktycz, Mitchel J; Alexandre, Gladys

2011-10-01

The Azospirillum brasilense chemotaxis-like Che1 signal transduction pathway was recently shown to modulate changes in adhesive cell surface properties that, in turn, affect cell-to-cell aggregation and flocculation behaviors rather than flagellar-mediated chemotaxis. Attachment to surfaces and root colonization may be functions related to flocculation. Here, the conditions under which A. brasilense wild-type Sp7 and che1 mutant strains attach to abiotic and biotic surfaces were examined using in vitro attachment and biofilm assays combined with atomic force microscopy and confocal microscopy. The nitrogen source available for growth is found to be a major modulator of surface attachment by A. brasilense and could be promoted in vitro by lectins, suggesting that it depends on interaction with surface-exposed residues within the extracellular matrix of cells. However, Che1-dependent signaling is shown to contribute indirectly to surface attachment, indicating that distinct mechanisms are likely underlying flocculation and attachment to surfaces in A. brasilense. © 2011 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.

Single Particle Jumps in Sheared SiO2

NASA Astrophysics Data System (ADS)

McMahon, Sean; Vollmayr-Lee, Katharina; Cookmeyer, Jonathan; Horbach, Juergen

We study the dynamics of a sheared glass via molecular dynamics simulations. Using the BKS potential we simulate the strong glass former SiO2. The system is initially well equilibrated at a high temperature, then quenched to a temperature below the glass transition, and, after a waiting time at the desired low temperature, sheared with constant strain rate. We present preliminary results of an analysis of single particle trajectories of the sheared glass. We acknowledge the support via NSF REU Grant #PHY-1156964, DoD ASSURE program, and NSF-MRI CHE-1229354 as part of the MERCURY high-performance computer consortium. We thank G.P. Shrivastav, Ch. Scherer and B. Temelso.

Reading Strategies: Issues in the Computerization of Machiavelli's "Il demonio che prese moglie".

ERIC Educational Resources Information Center

Morgan, Leslie Zarker

1994-01-01

The ideal computer-based foreign language reading program must include cognitive background, a learning taxonomy, sound computer design, and knowledge of what is needed for the specific language. Machiavelli's "Il demonia che prese moglie" is chosen for study due to its historical interest. (63 references) (CK)

Astrobiological Significance of Definitive Mineralogical Analysis of Martian Surface Samples Using the CheMin XRD/XRF Instrument

NASA Technical Reports Server (NTRS)

Feldman, S. M.; Blake, D. F.; Sarrazin, P.; Bish, D. L.; Chipera, S. J.; Vaniman, D. T.; Collins, S.

2004-01-01

The search for evidence of habitability, or of extant or extinct life on Mars, will initially be a search for evidence of past or present conditions supportive of life. The three key requirements for the emergence of life are thought to be liquid water; a suitable energy source; and chemical building blocks. CheMin is a miniaturized XRD/XRF (X-Ray diffraction / X-ray fluorescence) instrument which has been developed for definitive mineralogic analysis of soils and rocks on the Martian surface. The CheMin instrument can provide information that is highly relevant to each of these habitability requirements as summarized below.

Writing "Che" Writing: Apocryphal Diaries and the Deconstruction of Guevara's Myth

ERIC Educational Resources Information Center

Weiser, Frans

2013-01-01

Ernesto "Che" Guevara's iconic photograph has taken on mythical proportions since his death. Though his image has ironically been exploited by the apparatus of capitalism against which he fought, his translation into a symbol has assured that his foothold within popular culture remains largely unassailable. While recent films and…

14th EuCheMS International Conference on Chemistry and the Environment ICCE 2013: IEC-WTC, Barcelona, Spain, 25-28 June 2013.

PubMed

Luis, Santiago V; Jover, Eric

2014-10-01

The 14th European Association of Chemical and Molecular Sciences (EuCheMS) International Conference on Chemistry and the Environment (ICCE 2013) took place on 25-28 June 2013 at the IEC and the WTC in Barcelona, Spain. The ICCE is a well-established biannual conference organized by the Division of Chemistry and the EuCheMS.

Phyllosilicate analysis capabilities of the CheMin mineralogical instrument on the Mars Science Laboratory (MSL '11) Curiosity Rover

NASA Astrophysics Data System (ADS)

Blake, D. F.; Bish, D. L.; Vaniman, D. T.; Chipera, S.; Bristow, T. F.; Sarrazin, P.

2011-12-01

The CheMin mineralogical instrument on the MSL '11 Curiosity rover will return quantitative X-ray diffraction data (XRD) from scooped soil samples and drilled rock powders collected from the Mars surface. Samples of 45-65 mm3 from material sieved to <150 μm will be delivered through a funnel to one of 27 reuseable sample cells (five additional cells on the sample wheel contain diffraction or fluorescence standards). Sample cells are 8-mm diameter discs with 7-μm thick Mylar or Kapton windows spaced 170 μm apart. Within this volume, the sample is shaken by piezoelectric vibration at sonic frequencies, causing the powder to flow past a narrow, collimated X-ray beam in random orientations over the course of an analysis. In this way, diffraction patterns exhibiting little to no preferred orientation can be obtained even from minerals normally exhibiting strong preferred orientation such as phyllosilicates. Individual analyses will require several hours over one or more Mars sols. For typical well-ordered minerals, CheMin has a Minimum Detection Limit (MDL) of <3% by mass, an accuracy of better than 15% and a precision of better than 10% for phases present in concentrations >4X MDL (12%). The resolution of the diffraction patterns is 0.30 degrees 2θ, and the angular measurement range is 4-55 degrees 2θ. With this performance, CheMin can identify and distinguish a number of clay minerals. For example, discrimination between 1:1 phyllosilicates (such as the kaolin minerals), with repeat distances of ~7Å, and smectites (e.g., montmorillonite, nontronite, saponite), with repeat distances from 10-15Å, is straightforward. However, it is important to note that the variety of treatments used in terrestrial laboratories to aid in discrimination of clay minerals will not be accessible on Mars (e.g., saturation with ethylene glycol vapor, heat treatments). Although these treatments will not be available on Mars, dehydration within the CheMin instrument could be used to

Bacterial Energy Sensor Aer Modulates the Activity of the Chemotaxis Kinase CheA Based on the Redox State of the Flavin Cofactor.

PubMed

Samanta, Dipanjan; Widom, Joanne; Borbat, Peter P; Freed, Jack H; Crane, Brian R

2016-12-09

Flagellated bacteria modulate their swimming behavior in response to environmental cues through the CheA/CheY signaling pathway. In addition to responding to external chemicals, bacteria also monitor internal conditions that reflect the availability of oxygen, light, and reducing equivalents, in a process termed "energy taxis." In Escherichia coli, the transmembrane receptor Aer is the primary energy sensor for motility. Genetic and physiological data suggest that Aer monitors the electron transport chain through the redox state of its FAD cofactor. However, direct biochemical data correlating FAD redox chemistry with CheA kinase activity have been lacking. Here, we test this hypothesis via functional reconstitution of Aer into nanodiscs. As purified, Aer contains fully oxidized FAD, which can be chemically reduced to the anionic semiquinone (ASQ). Oxidized Aer activates CheA, whereas ASQ Aer reversibly inhibits CheA. Under these conditions, Aer cannot be further reduced to the hydroquinone, in contrast to the proposed Aer signaling model. Pulse ESR spectroscopy of the ASQ corroborates a potential mechanism for signaling in that the resulting distance between the two flavin-binding PAS (Per-Arnt-Sim) domains implies that they tightly sandwich the signal-transducing HAMP domain in the kinase-off state. Aer appears to follow oligomerization patterns observed for related chemoreceptors, as higher loading of Aer dimers into nanodiscs increases kinase activity. These results provide a new methodological platform to study Aer function along with new mechanistic details into its signal transduction process. © 2016 by The American Society for Biochemistry and Molecular Biology, Inc.

A technician monitors the CHeX, a USMP-4 experiment which will be flown on STS-87, in the SSPF

NASA Technical Reports Server (NTRS)

1997-01-01

United States Microgravity Payload-4 (USMP-4) experiments are prepared to be flown on Space Shuttle mission STS-87 in the Space Station Processing Facility at Kennedy Space Center (KSC). Here, a technician is monitoring the Confined Helium Experiment, or CHeX, that will use microgravity to study one of the basic influences on the behavior and properties of materials by using liquid helium confined between silicon disks. CHeX and several other experiments are scheduled for launch aboard STS-87 on Nov. 19 from KSC.

NASA Ames Science Instrument Launches Aboard New Mars Rover (CheMin)

NASA Image and Video Library

2011-11-23

When NASA's Mars Science Laboratory lands in a region known as Gale Crater in August of 2012, it will be poised to carry out the most sophisticated chemical analysis of the Martian surface to date. One of the 10 instruments on board the rover Curiosity will be CheMin - short for chemistry and mineralogy. Developed by Ames researcher David Blake and his team, it will use new technology to analyze and identify minerals in the Martian rocks and soil. Youtube: NASA Ames Scientists Develop MSL Science Instrument

Incorporation of Data Analysis throughout the ChE Curriculum Made Easy with DataFit

ERIC Educational Resources Information Center

Brenner, James R.

2007-01-01

At Florida Tech, we have incorporated DataFit from Oakdale Engineering throughout the entire curriculum, beginning with ChE 1102, an eight-week, one-day-per-week, two-hour, one-credit-hour, second-semester Introduction to Chemical Engineering course in a hands-on computer classroom. Our experience is that students retain data analysis concepts…

Novitskiy participates in a CHeCS medical contingency drill in the U.S. Laboratory

NASA Image and Video Library

2012-11-26

ISS034-E-005260 (26 Nov. 2012) --- Russian cosmonaut Oleg Novitskiy, Expedition 34 flight engineer, participates in a Crew Health Care System (CHeCS) medical contingency drill in the Destiny laboratory of the International Space Station. This drill gives crew members the opportunity to work as a team in resolving a simulated medical emergency onboard the space station.

Shkaplerov participates in a CHeCS Medical Contingency Drill in the U.S. Laboratory

NASA Image and Video Library

2011-12-16

ISS030-E-012600 (16 Dec. 2011) --- Russian cosmonaut Anton Shkaplerov, Expedition 30 flight engineer, participates in a Crew Health Care System (CHeCS) medical contingency drill in the Destiny laboratory of the International Space Station. This drill gives crew members the opportunity to work as a team in resolving a simulated medical emergency onboard the space station.

Novitskiy participates in a CHeCS medical contingency drill in the U.S. Laboratory

NASA Image and Video Library

2012-11-26

ISS034-E-005266 (26 Nov. 2012) --- Russian cosmonaut Oleg Novitskiy, Expedition 34 flight engineer, participates in a Crew Health Care System (CHeCS) medical contingency drill in the Destiny laboratory of the International Space Station. This drill gives crew members the opportunity to work as a team in resolving a simulated medical emergency onboard the space station.

Buckskin Drill Hole and CheMin X-ray Diffraction

NASA Image and Video Library

2015-12-17

The graph at right presents information from the NASA Curiosity Mars rover's onboard analysis of rock powder drilled from the "Buckskin" target location, shown at left. X-ray diffraction analysis of the Buckskin sample inside the rover's Chemistry and Mineralogy (CheMin) instrument revealed the presence of a silica-containing mineral named tridymite. This is the first detection of tridymite on Mars. Peaks in the X-ray diffraction pattern are from minerals in the sample, and every mineral has a diagnostic set of peaks that allows identification. The image of Buckskin at left was taken by the rover's Mars Hand Lens Imager (MAHLI) camera on July 30, 2015, and is also available at PIA19804. http://photojournal.jpl.nasa.gov/catalog/PIA20271

McArthur rotates the CHeCS rack back into position after cleaning the AAA fan

NASA Image and Video Library

2005-12-01

ISS012-E-09931 (1 December 2005) --- Astronaut William S. (Bill) McArthur Jr., Expedition 12 commander and NASA space station science officer, rotates the Crew Health Care System (CHeCS) rack back into position after cleaning the Avionics Air Assembly fan in the Destiny laboratory of the International Space Station.

Ivanishin participates in a CHeCS Medical Contingency Drill in the U.S. Laboratory

NASA Image and Video Library

2011-12-16

ISS030-E-012604 (16 Dec. 2011) --- Russian cosmonauts Anatoly Ivanishin (foreground) and Anton Shkaplerov, both Expedition 30 flight engineers, participate in a Crew Health Care System (CHeCS) medical contingency drill in the Destiny laboratory of the International Space Station. This drill gives crew members the opportunity to work as a team in resolving a simulated medical emergency onboard the space station.

Communication: The formation of CHe{sup 2+} by radiative association

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

Zicler, E.; Pauzat, F.; Chaquin, P.

2016-03-21

The detection of ArH{sup +} has revived the interest in the search for noble gas containing species. Despite helium being the second most abundant element in the universe (He/H ∼ 1/10), it has never been observed in any other form than that of a neutral/ionized atom in the interstellar medium. Because He is the “most noble” gas, its non-observation as part of neutral molecular systems is understandable. It is more surprising for charged species, especially HeH{sup +} whose spectral signatures are well documented in the laboratory. The purpose of this work was to find a simple positive ion containing He,more » and likely to be observed as an alternative to undetected HeH{sup +}. Among the HeX{sup 2+} diatomics formed with first row atoms, we focused on X = C because of both its relative abundance and the magnitude of its ionization potentials with respect to He. The formation of CHe{sup 2+} by radiative association is the center of this study. The question was addressed by means of numerical simulations using high level ab initio calculations of the CHe{sup 2+} potential surface, followed by a quantum chemical determination of the rate coefficients for the corresponding radiative association in the range of 10 to 1000K. The radiative association path shows a potential well deep enough to accommodate 20 vibrational levels, and no barrier to oppose the reaction. The rate coefficient varies from ∼4.5 × 10{sup −20} cm{sup 3}s{sup −1} to ∼2.5 × 10{sup −22} cm{sup 3}s{sup −1} for the temperatures considered. The present study suggests that the existence of this species has to be searched for mainly in highly irradiated regions.« less

Burbank participates in a CHeCS Medical Contingency Drill in the U.S. Laboratory

NASA Image and Video Library

2011-12-16

ISS030-E-012613 (16 Dec. 2011) --- NASA astronaut Dan Burbank (foreground), Expedition 30 commander, and Russian cosmonaut Anton Shkaplerov, flight engineer, participate in a Crew Health Care System (CHeCS) medical contingency drill in the Destiny laboratory of the International Space Station. This drill gives crew members the opportunity to work as a team in resolving a simulated medical emergency onboard the space station.

Burbank participates in a CHeCS Medical Contingency Drill in the U.S. Laboratory

NASA Image and Video Library

2011-12-16

ISS030-E-012609 (16 Dec. 2011) --- NASA astronaut Dan Burbank (foreground), Expedition 30 commander, and Russian cosmonaut Anton Shkaplerov, flight engineer, participate in a Crew Health Care System (CHeCS) medical contingency drill in the Destiny laboratory of the International Space Station. This drill gives crew members the opportunity to work as a team in resolving a simulated medical emergency onboard the space station.

McArthur removes AAA clamps and ducts inside the CHeCS Rack during Expedition 12

NASA Image and Video Library

2005-12-09

ISS012-E-10817 (9 December 2005) --- Astronaut William S. (Bill) McArthur Jr., Expedition 12 commander and NASA space station science officer, opens the back panel of the Crew Health Care System (CHeCS) rack and removes the Avionics Air Assembly (AAA) air ducts during in-flight maintenance (IFM) in the Destiny laboratory of the International Space Station.

X-Ray Diffraction Reference Intensity Ratios of Amorphous and Poorly Crystalline Phases: Implications for CheMin on the Mars Science Laboratory

NASA Technical Reports Server (NTRS)

Morris, R. V.; Achilles, C. N.; Chipera, S. J.; Ming, D. W.; Rampe, E. B.

2013-01-01

The CheMin instrument on the Mars Science Laboratory (MSL) rover Curiosity is an X-ray diffraction (XRD) and X-ray fluorescence (XRF) instrument capable of providing the mineralogical and chemical compositions of rocks and soils on the surface of Mars. CheMin uses a microfocus X-ray tube with a Co target, transmission geometry, and an energy-discriminating X-ray sensitive CCD to produce simultaneous 2-D XRD patterns and energy-dispersive X-ray histograms from powdered samples. Piezoelectric vibration of the cell is used to randomize the sample to reduce preferred orientation effects. Instrument details are provided in [1, 2, 3]. Analyses of rock and soil samples by the Mars Exploration Rovers (MER) show nanophase ferric oxide (npOx) is a significant component of the Martian global soil [4] and is thought to be one of the major contributing phases that the Curiosity rover will encounter if a soil sample is analyzed in Gale Crater. Because of the nature of this material, npOx will likely contribute to an X-ray amorphous or short-order component of a XRD pattern measured by the CheMin instrument.

Effects of Kapton Sample Cell Windows on the Detection Limit of Smectite: Implications for CheMin on the Mars Science Laboratory Mission

NASA Technical Reports Server (NTRS)

Achilles, C. N.; Ming, Douglas W.; Morris, R. V.; Blake, D. F.

2012-01-01

The CheMin instrument on the Mars Science Laboratory (MSL) rover Curiosity is an X-ray diffraction (XRD) and X-ray fluorescence (XRF) instrument capable of providing the mineralogical and chemical compositions of rocks and soils on the surface of Mars. CheMin uses a microfocus X-ray tube with a Co target, transmission geometry, and an energy-discriminating X-ray sensitive CCD to produce simultaneous 2-D XRD patterns and energy-dispersive X-ray histograms from powdered samples. CheMin has two different window materials used for sample cells -- Mylar and Kapton. Instrument details are provided elsewhere. Fe/Mg-smectite (e.g., nontronite) has been identified in Gale Crater, the MSL future landing site, by CRISM spectra. While large quantities of phyllosilicate minerals will be easily detected by CheMin, it is important to establish detection limits of such phases to understand capabilities and limitations of the instrument. A previous study indicated that the (001) peak of smectite at 15 Ang was detectable in a mixture of 1 wt.% smectite with olivine when Mylar is the window material for the sample cell. Complications arise when Kapton is the window material because Kapton itself also has a diffraction peak near 15 Ang (6.8 deg 2 Theta). This study presents results of mineral mixtures of smectite and olivine to determine smectite detection limits for Kapton sample cells. Because the intensity and position of the smectite (001) peak depends on the hydration state, we also analyzed mixtures with "hydrated" and "dehydrated"h smectite to examine the effects of hydration state on detection limits.

Detecting Nanophase Weathering Products with CheMin: Reference Intensity Ratios of Allophane, Aluminosilicate Gel, and Ferrihydrite

NASA Technical Reports Server (NTRS)

Rampe, E. B.; Bish, D. L.; Chipera, S. J.; Morris, R. V.; Achilles, C. N.; Ming, D W.; Blake, D. F.; Anderson, R. C.; Bristow, T. F.; Crisp, A.;

[I direttori delle riviste vanno avanti: attenersi ai principi accademici, adottare trasparenza e rispetto e applicare le regole.

PubMed

Teixeira da Silva, Jaime A; Dobránszki, Judit

2018-05-01

Nonostante i cambiamenti in corso nell'ambito delle pubblicazioni scientifiche (STEM - scienza, tecnologia, ingegneria e medicina), è indubbio che i direttori delle riviste restano i più importanti custodi del processo editoriale. In questo articolo vengono analizzati i motivi per cui i direttori delle riviste devono continuare ad attenersi ai principi editoriali fondamentali e a utilizzare le nuove tecnologie digitali solo per dare forza al contenuto, ma non per sostituirlo nella sua qualità. Per raggiungere questo obiettivo, in un'epoca caratterizzata da controlli ed equilibri, i direttori delle riviste devono essere attentamente sorvegliati e ritenuti responsabili tanto quanto gli autori che essi stessi valutano. Qualunque scorciatoia potrebbe avere conseguenze negative e inaspettate per quella rivista che non si attenesse a tali regole, che vedrebbe intaccata la propria reputazione e che dovrebbe faticare per recuperare la fiducia dei propri lettori. Quindi, a rischio di sembrare scontati, è meglio prevenire il danno alla reputazione piuttosto che curarlo. L'unico modo per evitarlo è che i direttori delle riviste si adattino a un crescente movimento sempre più critico, che richiede maggiore trasparenza e responsabilità da parte di chi contribuisce a costruire la base delle conoscenze scientifiche e dalla società, specialmente laddove la ricerca è finanziata pubblicamente.

CHeCS (Crew Health Care Systems): International Space Station (ISS) Medical Hardware Catalog. Version 10.0

NASA Technical Reports Server (NTRS)

2011-01-01

The purpose of this catalog is to provide a detailed description of each piece of hardware in the Crew Health Care System (CHeCS), including subpacks associated with the hardware, and to briefly describe the interfaces between the hardware and the ISS. The primary user of this document is the Space Medicine/Medical Operations ISS Biomedical Flight Controllers (ISS BMEs).

Cyclic di-GMP differentially tunes a bacterial flagellar motor through a novel class of CheY-like regulators.

PubMed

Nesper, Jutta; Hug, Isabelle; Kato, Setsu; Hee, Chee-Seng; Habazettl, Judith Maria; Manfredi, Pablo; Grzesiek, Stephan; Schirmer, Tilman; Emonet, Thierry; Jenal, Urs

2017-11-01

The flagellar motor is a sophisticated rotary machine facilitating locomotion and signal transduction. Owing to its important role in bacterial behavior, its assembly and activity are tightly regulated. For example, chemotaxis relies on a sensory pathway coupling chemical information to rotational bias of the motor through phosphorylation of the motor switch protein CheY. Using a chemical proteomics approach, we identified a novel family of CheY-like (Cle) proteins in Caulobacter crescentus , which tune flagellar activity in response to binding of the second messenger c-di-GMP to a C-terminal extension. In their c-di-GMP bound conformation Cle proteins interact with the flagellar switch to control motor activity. We show that individual Cle proteins have adopted discrete cellular functions by interfering with chemotaxis and by promoting rapid surface attachment of motile cells. This study broadens the regulatory versatility of bacterial motors and unfolds mechanisms that tie motor activity to mechanical cues and bacterial surface adaptation.

Fincke unbolts the front panel of the CHeCS Rack for inspection and cleaning during Expedition 9

NASA Image and Video Library

2004-09-16

ISS009-E-23061 (16 September 2004) --- Astronaut Edward M. (Mike) Fincke, Expedition 9 NASA ISS science officer and flight engineer, uses a drill to unfasten a panel on the CHeCS Rack in the Destiny laboratory of the International Space Station (ISS). Fincke was about to perform an inspection of the Avionics Air Assembly.

Travel as a Ritual toward Transformative Consciousness: Juxtaposing Che Guevara's Biography and Teacher Candidates' Narratives

ERIC Educational Resources Information Center

Lea, YiShan

2013-01-01

This article discusses the development of critical consciousness by examining the biographical-narratives in relationship to the experiential accounts on travel. Biographical narratives are important cultural texts filled with history and cultural nuances. The biography of Ernesto Che Guevara has resonated with readers and viewers from around the…

Ford and Novitskiy participate in a CHeCS Medical Contingency Drill in the U.S. Laboratory

NASA Image and Video Library

2012-11-26

ISS034-E-005268 (26 Nov. 2012) --- NASA astronaut Kevin Ford (background), Expedition 34 commander; and Russian cosmonaut Oleg Novitskiy, flight engineer, participate in a Crew Health Care System (CHeCS) medical contingency drill in the Destiny laboratory of the International Space Station. This drill gives crew members the opportunity to work as a team in resolving a simulated medical emergency onboard the space station.

Laboratory Evolved Gas Analyses of Si-rich Amorphous Materials: Implications for Analyses of Si-rich Amorphous Material in Gale Crater by the Mars Science Laboratory Sample Analysis at Mars Instrument

NASA Astrophysics Data System (ADS)

McAdam, A.; Knudson, C. A.; Sutter, B.; Andrejkovicova, S. C.; Archer, P. D., Jr.; Franz, H. B.; Eigenbrode, J. L.; Morris, R. V.; Ming, D. W.; Sun, V. Z.; Milliken, R.; Wilhelm, M. B.; Mahaffy, P. R.; Navarro-Gonzalez, R.

2016-12-01

The Chemistry and Mineralogy (CheMin) instrument onboard the Mars Science Laboratory (MSL) rover detected Si-rich amorphous or poorly ordered materials in several samples from Murray Formation mudstones and Stimson Formation sandstones. High-SiO2 amorphous materials such as opal-A or rhyolitic glass are candidate phases, but CheMin data cannot be used to distinguish between these possibilities. In the Buckskin (BS) sample from the upper Murray Formation, and the Big Sky (BY) and Greenhorn (GH) samples from the Stimson Formation, evolved gas analyses by the Sample Analysis at Mars (SAM) instrument showed very broad H2O evolutions during sample heating at temperatures >450-500°C, which had not been observed from previous samples. BS also had a significant broad evolution <450-500°C. We have undertaken a laboratory study targeted at understanding if the data from SAM analyses can be used to place constraints on the nature of the amorphous phases. SAM-like evolved gas analyses have been performed on several opal and rhyolitic glass samples. Opal-A samples exhibited wide <500°C H2O evolutions, with lesser H2O evolved above 500°C. H2O evolution traces from rhyolitic glasses varied, having either two broad H2O peaks, <300°C and >500°C, or a broad peak centered around 400°C. For samples that produced two evolutions, the lower temperature peak was more intense than the higher temperature peak, a trend also exhibited by opal-A. This trend is consistent with data from BS, but does not seem consistent with data from BY and GH which evolved most of their H2O >500°C. It may be that dehydration of opal-A and/or rhyolitic glass can result in some preferential loss of lower temperature H2O, to produce traces that more closely resemble BY and GH. This is currently under investigation and results will be reported.

Novitskiy and Tarelkin both participate in a CHeCS medical contingency drill in the U.S. Laboratory

NASA Image and Video Library

2012-11-26

ISS034-E-005261 (26 Nov. 2012) --- Russian cosmonauts Oleg Novitskiy (left) and Evgeny Tarelkin, both Expedition 34 flight engineers, participate in a Crew Health Care System (CHeCS) medical contingency drill in the Destiny laboratory of the International Space Station. This drill gives crew members the opportunity to work as a team in resolving a simulated medical emergency onboard the space station.

Coupling cosmogenic dating and magnetostratigraphy to constrain the chronological evolution of peri-Mediterranean karsts during the Messinian and the Pliocene: Example of Ardèche Valley, Southern France

NASA Astrophysics Data System (ADS)

Tassy, Aurélie; Mocochain, Ludovic; Bellier, Olivier; Braucher, Régis; Gattacceca, Jérôme; Bourlès, Didier

2013-05-01

The Ardèche River entrenches a deep canyon in the Saint Remèze plateau from Vallon-Pont-d'Arc to its confluence with the Rhône. This plateau is part of the Ardèche Cretaceous limestone plateau located at the edge of the Mid Rhône valley. It is characterized by dense multi-level cave systems, such as Saint-Marcel Cave (50 km of mapped passages) and Chauvet Cave, famous for its paleolithic paintings. Until now, and despite the absence of absolute dating, stepping of the Saint Remèze cave levels has been interpreted as the result of the Messinian salinity crisis. To clarify this interpretation, fluvial sediments of cave systems have been absolutely dated, while cave sediments have been demonstrated to be ideal for "burial dating" based on the different radioactive decay rates of the in situ-produced cosmogenic nuclides 10Be and 26Al. Combined with magnetostratigraphy and constrained by the Lower Ardèche base-level curve, this contribution provides an absolute dating for each cave level. The obtained results are consistent with the stepping per ascensum model of both surface landforms and caves for the Messinian-Pliocene eustatic cycle. Finally, this study provides evidence for a rise of the Ardèche river level to 40 m above the Pliocene abandonment surface. The second active period of the Chauvet Cave is evidenced between 2.96 and 2.18 Ma (cave filling). An absolute dating for the Pliocene abandonment surface between 1.94 and 1.77 Ma is also obtained, which brings new understandings to the geodynamic evolution of the area. The Lower Ardèche has been uplifted after the Pliocene, with a rate of 0.03 mm/year since 1.77 Ma.

High-Temperature, Perhaps Silicic, Volcanism on Mars Evidenced by Tridymite Detection in High-SiO2 Sedimentary Rock at Gale Crater, Mars

NASA Technical Reports Server (NTRS)

Morris, R. V.; Vaniman, D. T.; Blake, D. F.; Gellert, R.; Chipera, S. J.; Rampe, E. B.; Ming, D. W.; Morrison, S. M.; Downs, R. T.; Treiman, A. H.;

Lessons Learned from the Crew Health Care System (CHeCS) Rack 1 Environmental Control and Life Support (ECLS) Design

NASA Technical Reports Server (NTRS)

Williams, David E.

2006-01-01

This paper will provide an overview of the International Space Station (ISS) Environmental Control and Life Support (ECLS) design of the Crew Health Care System (CHeCS) Rack 1 and it will document some of the lessons that have been learned to date for the ECLS equipment in this rack.

Indirect cannibalism by crèche-aged American White Pelican (Pelecanus erythrorhynchos) chicks

USGS Publications Warehouse

Bartos, Alisa J.; Sovada, Marsha A.; Igl, Lawrence D.; Pietz, Pamela J.

2013-01-01

At nesting colonies of American White Pelicans (Pelecanus erythrorhynchos), many chicks die from siblicide, severe weather, and disease; this results in carcasses available for scavenging by conspecifics (i.e., indirect cannibalism). Indirect cannibalism has not been reported previously for this species. We describe five cases of crèche-aged American White Pelican chicks consuming or attempting to consume dead younger chicks at two nesting colonies in the northern plains of North America. Cannibalism in the American White Pelican appears to be rare and likely plays no role in the species’ population ecology or dynamics; however, it might be an important survival strategy of individual chicks when food resources are limited.

The effect of river dynamics induced by the Messinian Salinity Crisis on karst landscape and caves: Example of the Lower Ardèche river (mid Rhône valley)

NASA Astrophysics Data System (ADS)

Mocochain, Ludovic; Audra, Philippe; Clauzon, Georges; Bellier, Olivier; Bigot, Jean-Yves; Parize, Olivier; Monteil, Philippe

2009-05-01

The karstic canyon of Lower Ardèche is located in the Middle Rhône valley, which is directly tributary to the Mediterranean Sea. The Rhône River is emblematic of the Messinian Salinity Crisis (MSC) impact on landscape morphology. Along the edge of the Saint-Remèze Plateau, the Rhône valley displays four benchmark levels generated by the MSC: the Pre-evaporitic abandonment surface (1), the Messinian erosional surface (2), the Marine/non-marine surface of the Pliocene ria (3) and the Pliocene abandonment surface (4). The study of these benchmark levels allows us to reconstruct the evolution of the regional base level over the last 6 Ma. We obtain a curve for base-level evolution that provides a geodynamic reference, which is used to investigate the morphogenesis of the Saint-Remèze karstic plateau. The Ardèche River downcuts the Saint-Remèze Plateau in a deep canyon, from Vallon-Pont-d'Arc to the West, to its confluence with the Rhône to the East. Several abandoned valleys are present along the western edge of the Saint-Remèze Plateau at the inlet of the Ardèche canyon. In these abandoned valleys, the fluvial deposits are related to several periods, from the Pliocene onwards. They provide important insights into the fluvial dynamics: a 160 m-thick aggradation sequence infilled the Ardèche canyon during the Pliocene. This aggrading river caused the first lateral shifting, as an aggradation epigenesis. This first infilling shows that the Ardèche canyon already existed before the Pliocene. Secondly, it has been demonstrated that the Ardèche Canyon is downcut into the Pre-evaporitic surface of the Saint-Remèze Plateau, dated to 5.45 Ma [Martini, J., 2005. Etude des paléokarsts des environs de Saint-Remèze (Ardèche, France): mise en évidence d'une rivière souterraine fossilisée durant la crise de salinité messinienne. Karstologia 45-46, 1-18]. Consequently, the canyon downcutting is entirely due to the MSC, and occurred during a time span of only 100

Cherenkov and scintillation light separation on the CheSS experiment

NASA Astrophysics Data System (ADS)

Caravaca, Javier; Land, Benjamin; Descamps, Freija; Orebi Gann, Gabriel D.

2016-09-01

Separation of the scintillation and Cherenkov light produced in liquid scintillators enables outstanding capabilities for future particle detectors, the most relevant being: particle directionality information in a low energy threshold detector and improved particle identification. The CheSS experiment uses an array of small, fast photomultipliers (PMTs) and state-of-the-art electronics to demonstrate the reconstruction of a Cherenkov ring in liquid scintillator using two techniques: based on the photon density and using the photon hit time information. A charged particle ionizing a scintillation medium produces a prompt Cherenkov cone and late isotropic scintillation light, typically delayed by several ns. The fast response of our PMTs and DAQ provides a precision well below the ns level, making possible the time separation. Furthermore, the usage of the new developed water-based liquid scintillators (WbLS) enhances the separation since it allows tuning of the Cherenkov/Scintillation ratio. Latest results on the separation for pure liquid scintillators and WbLS will be presented.

Curiosity Rover's CheMin Instrument Investigates Mineralogy of Gale Crater and Implications for Diagenesis

NASA Astrophysics Data System (ADS)

Fendrich, Kim; Rampe, Elizabeth; Vaniman, David; Bish, David; Blake, David; Treiman, Allan; Ming, Doug; Morris, Richard; Bristow, Tom; Cavanagh, Patrick; Downs, Robert; Morrison, Shaunna; Chipera, Steve; Achilles, Cherie; Farmer, Jack; Sarrazin, Philippe; Crisp, Joy; Morookian, John Michael; Yen, Albert; Gellert, Ralf

2015-04-01

The Mars Science Laboratory rover Curiosity employs a suite of instruments to investigate past or present habitability of Mars, as observed at Gale crater and particularly in the lower strata of the crater's central mound, informally named Mount Sharp. The X-ray diffractometer on board, CheMin, is used to assess the quantitative mineralogy of scooped soil samples and drilled rock powders. Methods of modeling diffraction peak positions and intensities to evaluate the abundances of minerals include Rietveld refinement and FULLPAT (full-pattern fitting). Each of the samples analyzed by CheMin contains X-ray amorphous material. The amorphous component chemistry is resolved by subtracting the chemistry of the crystalline composition, as determined by X-ray diffraction data, from the bulk sample chemistry, as determined by the Alpha Particle X-ray Spectrometer (APXS). Diffraction results have been obtained on five samples thus far to include Rocknest, John Klein, Cumberland, Windjana and Confidence Hills. Soil samples collected at Rocknest, an aeolian bedform in Gale crater, were the first to be analyzed in situ by CheMin. The Rocknest mineral assemblage is basaltic (plagioclase, Fe-forsterite, augite, pigeonite) and contains amorphous material that is compositionally similar to palagonitic volcanic soils found on Earth, with the addition of sulfur and chlorine. The four drill analyses are characteristic of deposition in a variety of fluvio-lacustrine environments and exhibit evidence of low-temperature diagenesis. Both John Klein and Cumberland are part of the Sheepbed mudstone at Yellowknife Bay, where the first drilled samples were acquired as well as the first evidence of a habitable environment on Mars. Drilled three meters apart from each other, the two samples reveal basaltic minerals similar to those at Rocknest, as well as phyllosilicates, Fe-oxides/hydroxides, Ca-sulfates, Fe-sulfides, and amorphous materials. The nature and hydration of interlayer cations

CheD: chemical database compilation tool, Internet server, and client for SQL servers.

PubMed

Trepalin, S V; Yarkov, A V

2001-01-01

An efficient program, which runs on a personal computer, for the storage, retrieval, and processing of chemical information, is presented, The program can work both as a stand-alone application or in conjunction with a specifically written Web server application or with some standard SQL servers, e.g., Oracle, Interbase, and MS SQL. New types of data fields are introduced, e.g., arrays for spectral information storage, HTML and database links, and user-defined functions. CheD has an open architecture; thus, custom data types, controls, and services may be added. A WWW server application for chemical data retrieval features an easy and user-friendly installation on Windows NT or 95 platforms.

Sequence comparison of phoR, gyrB, groEL, and cheA genes as phylogenetic markers for distinguishing Bacillus amyloliquefaciens and B. subtilis and for identifying Bacillus strain B29.

PubMed

Yu, C; Jin, J; Meng, L-Q; Xia, H-H; Yuan, H-F; Wang, J; Yu, D-S; Zhao, X-Y; Sha, C-Q

2017-05-20

Given the close genetic relationship between Bacillus amyloliquefaciens and B. subtilis, distinguishing the two solely based on their physiological and biochemical characteristics and 16S rRNA sequences is difficult. Molecular identification was used to discover suitable genes for distinguishing the two bacteria, and to identify the bio-controlling strain B29, due to molecular identification has been paid more and more attention. The similarity of four genes, cheA, gyrB, groEL and phoR, of the two species was compared by the software BLASTN and MAGA, and phylogenetic tree was constructed. The B29 strain was re-identified by using the screened genes. The similarities of the four genes, gyrB, groEL, cheA and phoR, of the two species were 93-95%, 82-84%, 76-78% and 76-77%, respectively. The homologies of the four genes of the strain B29 and the strains of B. amyloliquefaciens strains were more than 95%. We determined how well the phoR and cheA genes could be used to differentiate B. amyloliquefacien and B. subtilis. The previously isolated biological control strain B29, initially classified as B. subtilis, was re-classified as B. amyloliquefaciens. Our data indicate that other than the phoR gene, the cheA gene might be a useful phylogenetic marker for differentiating B. subtilis and B. amyloliquefaciens.

A predictive computational model of the kinetic mechanism of stimulus-induced transducer methylation and feedback regulation through CheY in archaeal phototaxis and chemotaxis.

PubMed

Streif, Stefan; Oesterhelt, Dieter; Marwan, Wolfgang

2010-03-18

Photo- and chemotaxis of the archaeon Halobacterium salinarum is based on the control of flagellar motor switching through stimulus-specific methyl-accepting transducer proteins that relay the sensory input signal to a two-component system. Certain members of the transducer family function as receptor proteins by directly sensing specific chemical or physical stimuli. Others interact with specific receptor proteins like the phototaxis photoreceptors sensory rhodopsin I and II, or require specific binding proteins as for example some chemotaxis transducers. Receptor activation by light or a change in receptor occupancy by chemical stimuli results in reversible methylation of glutamate residues of the transducer proteins. Both, methylation and demethylation reactions are involved in sensory adaptation and are modulated by the response regulator CheY. By mathematical modeling we infer the kinetic mechanisms of stimulus-induced transducer methylation and adaptation. The model (deterministic and in the form of ordinary differential equations) correctly predicts experimentally observed transducer demethylation (as detected by released methanol) in response to attractant and repellent stimuli of wildtype cells, a cheY deletion mutant, and a mutant in which the stimulated transducer species is methylation-deficient. We provide a kinetic model for signal processing in photo- and chemotaxis in the archaeon H. salinarum suggesting an essential role of receptor cooperativity, antagonistic reversible methylation, and a CheY-dependent feedback on transducer demethylation.

Adélie penguin foraging behaviour and krill abundance along the Wilkes and Adélie land coasts, Antarctica

NASA Astrophysics Data System (ADS)

Wienecke, B. C.; Lawless, R.; Rodary, D.; Bost, C.-A.; Thomson, R.; Pauly, T.; Robertson, G.; Kerry, K. R.; LeMaho, Y.

2000-08-01

The foraging behaviour of Adélie penguins Pygoscelis adeliae was studied simultaneously at Shirley Island (SI, 110°E) and at Petrel Island (PI, 140°E) in approximate conjunction with the ship-based krill survey conducted on board the RSV Aurora Australis. Acoustic and trawl data were collected near both study sites, albeit at the end of the penguins' breeding season. The distances travelled by Adélie penguins from Shirley Island were significantly greater than those travelled by penguins from Petrel Island (SI 31-144 km; PI 6-79 km). Mean foraging trip durations and mean maximal distances travelled were also significantly different between colonies (duration: SI guard 55±32 h, crèche 113±17 h; PI guard 32±9 h, crèche 25±7 h; distance: SI guard 182±135 km, crèche 353±93 km; PI guard 100±42 km, crèche 86±28 km). All penguins foraged over the continental shelf or the shelf break and not in oceanic waters. The percentage distribution of dive depths was similar at both colonies; nearly 70% of all dives were to <35 m. Trawls from the ship contained krill Euphausia superba and E. crystallorophias near SI but only E. superba near PI. Biomass measurements showed that near SI 61% of krill biomass occurred at 63-97 m but the penguins dived to this depth range only 12% of their time; near PI 83% of the biomass was found from 43 to 63 m and 20% of dives reached these depths. The diet of the SI penguins consisted mainly of E. crystallorophias (51-53% by mass), while penguins from PI ingested large amounts of both euphausiids (27-38% E. superba, 22-39% E. crystallorophias). At SI, the remainder of the diet consisted of fish, mainly Pleuragramma antarcticum (26-30%), and amphipods (<1%). Similarly, at PI, fish contributed 19-37% to the penguins' diet and amphipods constituted 1-3%.

A 36,000-Year-Old Volcanic Eruption Depicted in the Chauvet-Pont d'Arc Cave (Ardèche, France)?

PubMed

Nomade, Sébastien; Genty, Dominique; Sasco, Romain; Scao, Vincent; Féruglio, Valérie; Baffier, Dominique; Guillou, Hervé; Bourdier, Camille; Valladas, Hélène; Reigner, Edouard; Debard, Evelyne; Pastre, Jean-François; Geneste, Jean-Michel

2016-01-01

Among the paintings and engravings found in the Chauvet-Pont d'Arc cave (Ardèche, France), several peculiar spray-shape signs have been previously described in the Megaloceros Gallery. Here we document the occurrence of strombolian volcanic activity located 35 km northwest of the cave, and visible from the hills above the cave entrance. The volcanic eruptions were dated, using 40Ar/39Ar, between 29 ± 10 ka and 35 ± 8 ka (2σ), which overlaps with the 14C AMS and thermoluminescence ages of the first Aurignacian occupations of the cave in the Megaloceros Gallery. Our work provides the first evidence of an intense volcanic activity between 40 and 30 ka in the Bas-Vivarais region, and it is very likely that Humans living in the Ardèche river area witnessed one or several eruptions. We propose that the spray-shape signs found in the Chauvet-Pont d'Arc cave could be the oldest known depiction of a volcanic eruption, predating by more than 34 ka the description by Pliny the Younger of the Vesuvius eruption (AD 79) and by 28 ka the Çatalhöyük mural discovered in central Turkey.

Lista dei caratteri macroscopici per l’identificazione dei legni

Treesearch

Flavio Ruffinatto; Alan Crivellaro; Alex C. Wiedenhoeft

2016-01-01

L’identificazione dei legni costituisce il primo passo per ottenere conoscenze indispensabili in molti ambiti di studio. Ad esempio, prima di eseguire il restauro di un manufatto di interesse culturale è indispensabile conoscere il nome dei legni che lo compongono. Oppure, nel caso in cui si sospetti che un legno protetto da convenzioni internazionali sia stato...

Big Sky and Greenhorn Drill Holes and CheMin X-ray Diffraction

NASA Image and Video Library

2015-12-17

The graph at right presents information from the NASA Curiosity Mars rover's onboard analysis of rock powder drilled from the "Big Sky" and "Greenhorn" target locations, shown at left. X-ray diffraction analysis of the Greenhorn sample inside the rover's Chemistry and Mineralogy (CheMin) instrument revealed an abundance of silica in the form of noncrystalline opal. The broad hump in the background of the X-ray diffraction pattern for Greenhorn, compared to Big Sky, is diagnostic of opal. The image of Big Sky at upper left was taken by the rover's Mars Hand Lens Imager (MAHLI) camera the day the hole was drilled, Sept. 29, 2015, during the mission's 1,119th Martian day, or sol. The Greenhorn hole was drilled, and the MAHLI image at lower left was taken, on Oct. 18, 2015 (Sol 1137). http://photojournal.jpl.nasa.gov/catalog/PIA20272

TIME COURSE AND DOSE RESPONSE ASSESSMENT OF CHOLINESTERASE (CHE) INHIBITION IN ADULT RATS TREATED ACUTELY WITH CARBARYL, METHOMYL, METHIOCARB, OXAMYL, OR PROPOXUR.

EPA Science Inventory

To compare the toxicity of 5 N-methyl carbamates, the time course and dose response profiles for ChE inhibition were established for each. For the time course comparison, adult male Long Evans rats (n=5 dose group) were dosed orally with either carbaryl (CB; 30 mg/kg in corn oi...

Negotiating and Appropriating the "One Person, One Language" Policy within the Complex Reality of a Multilingual Crèche in Strasbourg

ERIC Educational Resources Information Center

Caporal-Ebersold, Eloise; Young, Andrea

2016-01-01

The aim of this article is to analyse the early childhood education and care (ECEC) language policy in the city of Strasbourg, focusing on an ethnographic case study of a newly established bilingual English-French crèche in the city. In France, establishing an early childhood education structure--more specifically, a day care centre catering to…

Calorimetria nucleare in collisioni tra ioni pesanti ad energie intermedie

NASA Astrophysics Data System (ADS)

Spezzi, L.

2002-07-01

Nel presente lavoro di tesi ci si e' occupati di uno studio sistematico di misure di temperatura ed energia di eccitazione in sistemi nucleari formati in reazioni tra ioni pesanti ad energie intermedie. La temperatura a stata misurata tramite due diversi metodi: rapporto tra le rese di isotopi che differiscono di uno stesso numero di nucleoni ( d/t-3He/4He , 3He/4He -6Li/7Li) - temperatura isotopica -, e rapporto tra le popolazioni degli stati eccitati in frammenti instabili (5Li e 6Li) che decadono in particelle leggere in coincidenza - temperatura di emissione. Le misure in esame sono state effettuate presso i Laboratori Nazionali del Sud per i sistemi 93Nb+93Nb, 93Nb+116,124Sn, 116,120Sn+116,124Sn e 116Sn+93Nb ad energie comprese tra 17 e 40 A MeV. L'apparato di rivelazione era costituito da un odoscopio ad alta granularite (96 telescopi Si (50micron)-Si(300micron)-CsI(Tl) ) e basse soglie di rivelazione, utilizzato sia per identificare risonanze in frammenti eccitati che per misure di rese di isotopi di frammenti con 2<=Z<=6, e dal multirivelatore FIASCO che, grazie alla misura contemporanea di posizione e tempo di volo dei frammenti provenienti dal proiettile (PLF - Projectile Like Fragments) e dal bersaglio (TLF - Target Like Fragment), e' stato utilizzato per misurare direttamente l'energia di eccitazione del sistema in studio. Analisi precedenti, condotte nell'ambito di varie collaborazioni, hanno messo in evidenza la possible esistenza di una transizione di fase liquido-gas del primo ordine nella materia nucleare. In tale contesto, ci siamo occupati di una verifica della curva calorica nucleare, ovvero della correlazione esistente fra temperatura ed energia di eccitazione del sistema, indagando, nel contempo, sulla sua dipendenza dalla massa del sistema che si frammenta. Si e' effettuato, inoltre, un confronto fra i due diversi metodi di misura della temperatura (temperatura isotopica e di emissione): l'andamento plateau-like della curva calorica e' stato

SiGe nano-heteroepitaxy on Si and SiGe nano-pillars.

PubMed

Mastari, M; Charles, M; Bogumilowicz, Y; Thai, Q M; Pimenta-Barros, P; Argoud, M; Papon, A M; Gergaud, P; Landru, D; Kim, Y; Hartmann, J M

2018-07-06

In this paper, SiGe nano-heteroepitaxy on Si and SiGe nano-pillars was investigated in a 300 mm industrial reduced pressure-chemical vapour deposition tool. An integration scheme based on diblock copolymer patterning was used to fabricate nanometre-sized templates for the epitaxy of Si and SiGe nano-pillars. Results showed highly selective and uniform processes for the epitaxial growth of Si and SiGe nano-pillars. 200 nm thick SiGe layers were grown on Si and SiGe nano-pillars and characterised by atomic force microscopy, x-ray diffraction and transmission electron microscopy. Smooth SiGe surfaces and full strain relaxation were obtained in the 650 °C-700 °C range for 2D SiGe layers grown either on Si or SiGe nano-pillars.

A 36,000-Year-Old Volcanic Eruption Depicted in the Chauvet-Pont d’Arc Cave (Ardèche, France)?

PubMed Central

Nomade, Sébastien; Genty, Dominique; Sasco, Romain; Scao, Vincent; Féruglio, Valérie; Baffier, Dominique; Guillou, Hervé; Bourdier, Camille; Valladas, Hélène; Reigner, Edouard; Debard, Evelyne; Pastre, Jean–François; Geneste, Jean-Michel

2016-01-01

Among the paintings and engravings found in the Chauvet-Pont d’Arc cave (Ardèche, France), several peculiar spray-shape signs have been previously described in the Megaloceros Gallery. Here we document the occurrence of strombolian volcanic activity located 35 km northwest of the cave, and visible from the hills above the cave entrance. The volcanic eruptions were dated, using 40Ar/39Ar, between 29 ± 10 ka and 35 ± 8 ka (2σ), which overlaps with the 14C AMS and thermoluminescence ages of the first Aurignacian occupations of the cave in the Megaloceros Gallery. Our work provides the first evidence of an intense volcanic activity between 40 and 30 ka in the Bas-Vivarais region, and it is very likely that Humans living in the Ardèche river area witnessed one or several eruptions. We propose that the spray-shape signs found in the Chauvet-Pont d’Arc cave could be the oldest known depiction of a volcanic eruption, predating by more than 34 ka the description by Pliny the Younger of the Vesuvius eruption (AD 79) and by 28 ka the Çatalhöyük mural discovered in central Turkey. PMID:26745626

Serum cholinesterase polymorphism (CHE1 and CHE2 loci) among several Indian groups from Amazon region of Brazil, and segregation of the C5 variant in families.

PubMed

Guerreiro, J F; Santos, S E; Aguiar, G F

1989-04-01

Eight Indian tribes from the Amazon region of Brazil (Araweté, Arara, Yamamadi, Kararaô, Karitiana, Waiampi, Surui and Cinta Larga) were studied for the distribution of the atypical and C5 variants of serum cholinesterase. None of them presented the CHE1*A allele, but the C5 variant was found in the Araweté (20.4%), Kararaô (15.6%), Karitiana (50.5%), Surui (12.3%) and Cinta Larga (19.6%) tribes. The frequency of the C5+ phenotype in the Karitiana is the highest reported thus far in human populations. Segregation studies considering the C5 variant were made among the Karitiana, and also among the Urubu-Kaapor and Munduruku tribes previously studied by Guerreiro et al [1987a, 1987b]. Most of the data were in agreement with the genetic hypothesis, but they also revealed a significant lack of the C5+ phenotype in offspring from C5+ X C5+ matings, as well as the occurrence of two C5+ children from C5- X C5- unions, in the Urubu-Kaapor tribe.

Comparative molecular dynamics simulation studies for determining factors contributing to the thermostability of chemotaxis protein "CheY".

PubMed

Paul, Manish; Hazra, Mousumi; Barman, Arghya; Hazra, Saugata

2014-01-01

Comparative molecular dynamics simulations of chemotaxis protein "CheY" from thermophilic origin Thermotoga maritima and its mesophilic counterpart Salmonella enterica have been performed for 10 ns each at 300 and 350 K, and 20 ns each at 400 and 450 K. The trajectories were analyzed in terms of different factors like root-mean-square deviation, root-mean-square fluctuation, radius of gyration, solvent accessible surface area, H-bonds, salt bridge content, and protein-solvent interactions which indicate distinct differences between the two of them. The two proteins also follow dissimilar unfolding pathways. The overall flexibility calculated by the trace of the diagonalized covariance matrix displays similar flexibility of both the proteins near their optimum growth temperatures. However, at higher temperatures mesophilic protein shows increased overall flexibility than its thermophilic counterpart. Principal component analysis also indicates that the essential subspaces explored by the simulations of two proteins at different temperatures are nonoverlapping and they show significantly different directions of motion. However, there are significant overlaps within the trajectories and similar direction of motions are observed for both proteins at 300 K. Overall, the mesophilic protein leads to increased conformational sampling of the phase space than its thermophilic counterpart. This is the first ever study of thermostability of CheY protein homologs by using protein dynamism as a main impact. Our study might be used as a model for studying the molecular basis of thermostability of two homologous proteins from two organisms living at different temperatures with less visible differences.

[Un processo che può gettare luce sulla cosiddetta 'paura di sé' nel disturbo ossessivo-compulsivo: l'Identificazione Retrospettiva delle Motivazioni e delle Inclinazioni].

PubMed

Mannino, Gherardo; Guerini, Rossella

2018-01-01

RIASSUNTO. Al fine di comprendere meglio la psicopatologia del disturbo ossessivo-compulsivo (DOC), in anni recenti è stata prestata una crescente attenzione alla cosiddetta "paura di sé" (fear of self), vale a dire la paura che le persone con DOC nutrono di ospitare dentro di sé aspetti inaccettabili. Tuttavia, l'esatta dinamica di questo fenomeno ancora non è chiara: di conseguenza, allo scopo di delucidarla meglio, sarà qui descritto uno specifico processo psicologico denominato Identificazione Retrospettiva delle Motivazioni e delle Inclinazioni (IRMI). Quando un paziente con DOC valuta in qualche modo come inaccettabile una propria esperienza interna (pensieri, emozioni, immagini mentali, ecc.) o un proprio comportamento, identificherà retrospettivamente le presunte motivazioni/inclinazioni negative che sarebbero state all'origine di quell'esperienza o comportamento. L'IRMI non solo aiuta a comprendere meglio il fenomeno della "paura di sé", ma possiede anche importanti implicazioni per la terapia.

Scienza dei servizi

NASA Astrophysics Data System (ADS)

Cavenago, Dario; Mezzanzanica, Mario

I servizi sono oramai centrali nella vita sociale di ogni Paese. L'aspettativa di ogni attore sociale, sia esso cittadino/utente e/o istituzione, è quella di ricevere da uno scambio che ha alla base un servizio un contributo che si caratterizza per una qualità intrinseca. Il mantenimento di tale promessa richiede all'azienda erogatrice un forte coinvolgimento, nel disegno del servizio, degli utenti ed una capacità di gestione della conoscenza; ciò è particolarmente significativo per quei servizi che si caratterizzano per una forte personalizzazione o complessità progettuale. Disegnare un servizio, metterlo in produzione e gestirne l'erogazione ha una complessità variabile in base al grado di coinvolgimento dei destinati ed alle dimensioni critiche che esprimono le condizioni di fattibilità di un servizio di qualità appropriato alle attese degli utilizzatori. Ugualmente la globalizzazione dell'economia e la scarsità delle risorse ha portato l'attenzione, ai fini della sostenibilità delle condizioni sopra richiamate, di una ricerca di modelli e strumenti che permettano una valutazione e una standardizzazione dei processi di produzione ed erogazione secondo condizioni di economicità. Tali primi e sintetici elementi hanno condotto diversi attori istituzionali, grandi imprese ed università attraverso i loro centri di ricerca, ad una azione di riflessione sul grado di "ingegnerizzazione" dei processi sottesi alla erogazione dei servizi ed in particolare ai servizi che attuano le politiche dei grandi settori di base dell'economia di una Nazione: educazione, sanità, sociale, sviluppo infrastrutture ecc.

IL TRAPIANTO ORTOTOPICO DEL FEGATO

PubMed Central

STARZL, THOMAS E.

2010-01-01

E’ormai noto che esiste la possibilità rivoluzionaria di utilizzare il fegato per il trattamento della stadio terminale delle epatopatie. Nel gennaio 1980 si celebra il decimo anna di sopravvivenza con fegato trapiantato (la più lunga della letteratura) di un paziente da noi trattato. Si tratta di uno dei 12 malati sottoposti a trapianto e seguiti per più di 5 anni. La nota positiva di questa tipo di trattamento è rappresentata dall’eccellente tenore di vita che i pazienti conducono e dalla riabilitazione sociale e professionale. La nota negativa è data, invece, dal fatto che i buoni risultati non vengono raggiunti con regolarità e non possono essere previsti con esattezza. In questa breve rassegna considereremo la esperienza da noi fatta presso l’Università di Denver nel Colorado, mettendo in risalto le cause dell’elevata mortalità precoce e le prospettive future di questa mezzo terapeutico. PMID:21572898

Anisotropic selective etching between SiGe and Si

NASA Astrophysics Data System (ADS)

Ishii, Yohei; Scott-McCabe, Ritchie; Yu, Alex; Okuma, Kazumasa; Maeda, Kenji; Sebastian, Joseph; Manos, Jim

2018-06-01

In Si/SiGe dual-channel FinFETs, it is necessary to simultaneously control the etched amounts of SiGe and Si. However, the SiGe etch rate is higher than the Si etch rate in not only halogen plasmas but also physical sputtering. In this study, we found that hydrogen plasma selectively etches Si over SiGe. The result shows that the selectivity of Si over SiGe can be up to 38 with increasing Ge concentration in SiGe. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) results indicate that hydrogen selectively bonds with Si rather than with Ge in SiGe. During the etching, hydrogen-induced Si surface segregation is also observed. It is also observed that the difference in etched amount between SiGe and Si can be controlled from positive to negative values even in Si/SiGe dual-channel fin patterning while maintaining the vertical profiles. Furthermore, no plasma-induced lattice damage was observed by transmission electron microscopy for both Si and SiGe fin sidewalls.

Nanocrystalline Si pathway induced unipolar resistive switching behavior from annealed Si-rich SiNx/SiNy multilayers

NASA Astrophysics Data System (ADS)

Jiang, Xiaofan; Ma, Zhongyuan; Yang, Huafeng; Yu, Jie; Wang, Wen; Zhang, Wenping; Li, Wei; Xu, Jun; Xu, Ling; Chen, Kunji; Huang, Xinfan; Feng, Duan

2014-09-01

Adding a resistive switching functionality to a silicon microelectronic chip is a new challenge in materials research. Here, we demonstrate that unipolar and electrode-independent resistive switching effects can be realized in the annealed Si-rich SiNx/SiNy multilayers with high on/off ratio of 109. High resolution transmission electron microscopy reveals that for the high resistance state broken pathways composed of discrete nanocrystalline silicon (nc-Si) exist in the Si nitride multilayers. While for the low resistance state the discrete nc-Si regions is connected, forming continuous nc-Si pathways. Based on the analysis of the temperature dependent I-V characteristics and HRTEM photos, we found that the break-and-bridge evolution of nc-Si pathway is the origin of resistive switching memory behavior. Our findings provide insights into the mechanism of the resistive switching behavior in nc-Si films, opening a way for it to be utilized as a material in Si-based memories.

Development of Si/SiGe heterostructures

NASA Astrophysics Data System (ADS)

Hauenstein, R. J.; Veteran, J. L.; Young, M. H.

1991-01-01

New molecular beam epitaxy (MBE) materials growth and doping processes were developed for the fabrication of Si/SiGe heterostructure devices. These new materials processes are applied to the demonstration of cryogenic n-p-n Si/Si 1-x Gex/Si heterojunction bipolar transistors (HBT). This application has special significance as an enabling DoD technology for fast low noise, high performance readout and signal processing circuits for IR focal systems. Reliable, versatile methods were developed to grow very high quality Si/SiGe strained layer heterostructures and multilayers. In connection with this program methods were developed to dope the Si and SiGe with B, Sb and Ga. B and Sb were found to be the preferred dopants for p and n regions respectively, of the HBT devices. The test devices clearly displayed gain enhancement due to the heterojunction and provided useful gains from room temperature down to 10 K.

MSL SAM-Like Evolved Gas Analyses of Si-rich Amorphous Materials

NASA Technical Reports Server (NTRS)

McAdam, Amy; Knudson, Christine; Sutter, Brad; Andrejkovicova, Slavka; Archer, P. Douglas; Franz, Heather; Eigenbrode, Jennifer; Morris, Richard; Ming, Douglas; Sun, Vivian;

Zur chemie der marsoberfläche

USGS Publications Warehouse

Keil, Klaus; Clark, Benton C.; Baird, A.K.; Toulmin, Priestley; Rose, Harry J.

1978-01-01

Analyses of 13 samples of Martian surface materials with the Viking X-ray fluorescence spectrometers show SiO2 similar to that of terrestrial mafic rocks, whereas Fe2O3, Cl, and S are higher and Al2O3, K2O, Rb, Sr, Y, and Zr are lower. Low totals suggest presence of CO2, H2O, and Na2O. Duricrust fragments are higher in S than fines, but samples from both landing sites are surprisingly similar. We suggest that Martian surface materials are aeolian deposits of complex mixtures of weathering products of maficultramafic rocks, possibly consisting of iron-rich clays, sulfates, iron oxides, carbonates, and chlorides.

Processing and Properties of SiC/MoSi2-SiC Composites Fabricated by Melt Infiltration

NASA Technical Reports Server (NTRS)

Bhatt, Ramakrishna T.; Hebsur, Mohan G.

2000-01-01

Hi-Nicalon SiC fiber reinforced MoSi2-SiC matrix composites (SiC/MoSi2-SiC) have been fabricated by the melt infiltration approach. The composite consists of approximately 60 vol%, 2-D woven BN/SiC coated Hi-Nicalon SiC fibers and approximately 40 vol% MoSi2-SiC matrix. The room temperature tensile properties and thermal conductivity of the SiC/MoSi2-SiC composites were measured and compared with those of the melt infiltrated SiC/SiC composites. The influence oi fiber architecture on tensile properties was also evaluated. Results indicate that the primary modulus, stress corresponding to deviation from linearity, and transverse thermal conductivity values for the SiC/MoSi2-SiC composites are significantly lower than those for the SiC/SiC composites. Microcracking of the matrix due to the large difference in thermal expansion between MoSi2 and SiC appears to be the reason for the lower matrix dominated properties of SiC/MoSi2-SiC composites.

Nitriding kinetics of Si-SiC powder mixtures as simulations of reaction bonded Si3N4-SiC composites

NASA Technical Reports Server (NTRS)

Lightfoot, A.; Sheldon, B. W.; Flint, J. H.; Haggerty, J. S.

1989-01-01

The nitriding kinetics of Si and Si plus SiC powder mixtures were studied to simulate the fabrication of RBSN-SiC ceramic matrix composites. Very clean, assynthesized, and solvent-exposed powders were studied; C-rich and Si-rich SiC 0.04-0.05 micron diameter powders were mixed in varying concentrations with SiH4-derived 0.2-0.3 micron diameter Si powder. Complete nitridation is achieved with C-rich SiC powders in 140 min at 1250 C, and in the centers of Si-rich SiC powders in 15 min. The effects on the incubation periods, fast reaction periods, and slow reaction periods that characterize these nitriding processes were studied to explain unusual reverse reaction gradients and other effects of contamination.

Epitaxial growth and characterization of Si/NiSi 2/Si(111) heterostructures

NASA Astrophysics Data System (ADS)

Rizzi, Angela; Förster, A.; Lüth, H.; Slijkerman, W.

1989-04-01

Si/NiSi 2/Si(111) heterostructures are grown under UHV conditions. The well known "template" method is used to produce the epitaxial NiSi 2 interlayer. On top of the suicide, the silicon epitaxial growth is obtained by means of gas phase reaction of SiH 4 at a surface temperature of 500° C. The Si growth rate is strongly enhanced by predissociation of SiH 4 using a hot tungsten filament in the vicinity of the surface. The single steps of the growth are followed in-situ by means of AES, HREELS and LEED analysis. Ex-situ high resolution RBS analysis is also applied for characterization.

Sub-barrier fusion of Si+Si systems

NASA Astrophysics Data System (ADS)

Colucci, G.; Montagnoli, G.; Stefanini, A. M.; Bourgin, D.; Čolović, P.; Corradi, L.; Courtin, S.; Faggian, M.; Fioretto, E.; Galtarossa, F.; Goasduff, A.; Haas, F.; Mazzocco, M.; Scarlassara, F.; Stefanini, C.; Strano, E.; Urbani, M.; Szilner, S.; Zhang, G. L.

2017-11-01

The near- and sub-barrier fusion excitation function has been measured for the system 30Si+30Si at the Laboratori Nazionali di Legnaro of INFN, using the 30Si beam of the XTU Tandem accelerator in the energy range 47 - 90 MeV. A set-up based on a beam electrostatic deflector was used for detecting fusion evaporation residues. The measured cross sections have been compared to previous data on 28Si+28Si and Coupled Channels (CC) calculations have been performed using M3Y+repulsion and Woods-Saxon potentials, where the lowlying 2+ and 3- excitations have been included. A weak imaginary potential was found to be necessary to reproduce the low energy 28Si+28Si data. This probably simulates the effect of the oblate deformation of this nucleus. On the contrary, 30Si is a spherical nucleus, 30Si+30Si is nicely fit by CC calculations and no imaginary potential is needed. For this system, no maximum shows up for the astrophysical S-factor so that we have no evidence for hindrance, as confirmed by the comparison with CC calculations. The logarithmic derivative of the two symmetric systems highlights their different low energy trend. A difference can also be noted in the two barrier distributions, where the high-energy peak present in 28Si+28Si is not observed for 30Si+30Si, probably due to the weaker couplings in last case.

Growth of amorphous and epitaxial ZnSiP 2–Si alloys on Si

DOE PAGES

Martinez, Aaron D.; Miller, Elisa M.; Norman, Andrew G.; ...

2018-01-30

ZnSiP 2is a wide band gap material lattice matched with Si, with potential for Si-based optoelectronics. Here, amorphous ZnSiP 2–Si alloys are grown with tunable composition. Films with Si-rich compositions can be crystallized into epitaxial films.

A multilayered approach of Si/SiO to promote carrier transport in electroluminescence of Si nanocrystals

PubMed Central

2012-01-01

The electroluminescence (EL) and photoluminescence of Si nanocrystals (Si-nc) from multilayered samples of Si/SiO are investigated. Si-nc are formed within Si and SiO layers after furnace annealing. It is found that the presence of Si interlayers creates extra carrier paths for EL emission. A comparative study is further performed on a multilayered Si/SiO sample and a single-layered one with Si and SiO homogeneously mixed. Both samples have the same ratio of Si to O and the same contents of Si and O. The multilayered sample is found to have higher EL intensity, less turn-on voltage, lower resistance, and higher current efficiency than the single-layered one. The results indicate that Si interlayers in Si/SiO may act as carrier channels, which promote carrier transport and enhance the EL emission of Si-nc. PMID:22448989

The localization and crystallographic dependence of Si suboxide species at the SiO2/Si interface

NASA Technical Reports Server (NTRS)

Grunthaner, P. J.; Hecht, M. H.; Grunthaner, F. J.; Johnson, N. M.

1987-01-01

X-ray photoemission spectroscopy has been used to examine the localization and crystallographic dependence of Si(+), Si(2+), and Si(3+) suboxide states at the SiO2/Si interface for (100)and (111)-oriented substrates with gate oxide quality thermal oxides. The Si(+) and Si(2+) states are localized within 6-10 A of the interface while the Si(3+) state extends about 30 A into the bulk SiO2. The distribution of Si(+) and Si(2+) states shows a strong crystallographic dependence with Si(2+) dominating on (100) substrates and Si(+) dominating on (111) substrates. This crystallographic dependence is anticipated from consideration of ideal unreconstructed (100) and (111) Si surfaces, suggesting that (1) the Si(+) and Si(2+) states are localized immediately within the first monolayer at the interface and (2) the first few monolayers of substrate Si atoms are not significantly displaced from the bulk. The total number of suboxide states observed at the SiO2/Si interface corresponds to 94 and 83 percent of a monolayer for these (100) and (111) substrates, respectively.

Comparison of low frequency charge noise in identically patterned Si/SiO{sub 2} and Si/SiGe quantum dots

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

Freeman, Blake M.; Schoenfield, Joshua S.; Jiang, HongWen

We investigate and compare the charge noise in Si/SiO{sub 2} and Si/SiGe gate defined quantum dots with identically patterned gates by measuring the low frequency 1/f current noise through the biased quantum dots in the coulomb blockade regime. The current noise is normalized and used to extract a measurement of the potential energy noise in the system. Additionally, the temperature dependence of this noise is investigated. The measured charge noise in Si/SiO{sub 2} compares favorably with that of the SiGe device as well as previous measurements made on other substrates suggesting Si/SiO{sub 2} is a potential candidate for spin basedmore » quantum computing.« less

Microstructure and Mechanical Property of SiCf/SiC and Cf/SiC Composites

NASA Astrophysics Data System (ADS)

Lee, S. P.; Cho, K. S.; Lee, H. U.; Lee, J. K.; Bae, D. S.; Byun, J. H.

2011-10-01

The mechanical properties of SiC based composites reinforced with different types of fabrics have been investigated, in conjunction with the detailed analyses of their microstructures. The thermal shock properties of SiCf/SiC composites were also examined. All composites showed a dense morphology in the matrix region. Carbon coated PW-SiCf/SiC composites had a good fracture energy, even if their strength was lower than that of PW-Cf/SiC composites. SiCf/SiC composites represented a great reduction of flexural strength at the thermal shock temperature difference of 300 °C.

Kapitza resistance of Si/SiO2 interface

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

Bowen Deng; Aleksandr Chenatynskiy; Marat Khafizov

2014-02-01

A phonon wave packet dynamics method is used to characterize the Kapitza resistance of a Si/SiO2 interface in a Si/SiO2/Si heterostructure. By varying the thickness of SiO2 layer sandwiched between two Si layers, we determine the Kapitza resistance for the Si/SiO2 interface from both wave packet dynamics and a direct, non-equilibrium molecular dynamics approach. The good agreement between the two methods indicates that they have each captured the anharmonic phonon scatterings at the interface. Moreover, detailed analysis provides insights as to how individual phonon mode scatters at the interface and their contribution to the Kapitza resistance.

p-BaSi2/n-Si heterojunction solar cells on Si(001) with conversion efficiency approaching 10%: comparison with Si(111)

NASA Astrophysics Data System (ADS)

Deng, Tianguo; Sato, Takuma; Xu, Zhihao; Takabe, Ryota; Yachi, Suguru; Yamashita, Yudai; Toko, Kaoru; Suemasu, Takashi

2018-06-01

B-doped p-BaSi2 epitaxial layers with a hole concentration of 1.1 × 1018 cm‑3 were grown on n-Si(001) using molecular beam epitaxy to fabricate p-BaSi2/n-Si solar cells. The thickness (d) of the p-BaSi2 layer was varied from 20 to 60 nm to investigate its effect on the solar cell performance. The conversion efficiency under an AM1.5 illumination increased with d reaching a maximum of 9.8% at d = 40 nm, which is nearly equal to the highest efficiency (9.9%) for p-BaSi2/n-Si solar cells on Si(111). This study indicated that Si(001) substrates are promising for use in BaSi2 solar cells.

Central radio galaxies in groups: cavities, bubbles and the history of AGN heating

NASA Astrophysics Data System (ADS)

Giacintucci, S.; Venturi, T.; Raychaudhury, S.; Vrtilek, J.

2008-10-01

E' noto che le regioni centrali degli ammassi e gruppi di galassie costituiscono un ambiente in cui gas caldo e plasma radioemittente proveniente dalle galassie dominanti interagiscono tra loro. In particolare, si pensa che la radioemissione dell'AGN centrale ed i suoi possibili cicli di attivita', siano strettamente legati alla presenza di cavita' e "bubbles" nel gas intergalattico. Si presentera' lo status di un progetto osservativo effettuato con il Giant Metrewave Radio Telescope (GMRT, India) su di un campione di 18 gruppi di galassie, osservati a tre frequenze radio (235 MHz, 325 MHz e 610 MHz). Lo studio della morfologia radio degli AGN centrali e la relativa analisi spettrale permettono di ottenere stime sull'eta' di questi oggetti, e sulla loro energia totale, che a loro volta sono in relazione con le proprieta' X dei gruppi stessi. Per tutti gli oggetti del campione sono disponibili osservazioni Chandra di proprieta'. Per due oggetti del campione, 4C+24.36 (al centro di AWM04) e NGC741 (al centro di RSOG17) verra' presentato uno studio dettagliato.

[La diagnosi del disturbo da uso di alcol dal punto di vista psicologico].

PubMed

Coriale, Giovanna; Fiorentino, Daniela; Porrari, Raffaella; Battagliese, Gemma; Capriglione, Ida; Cereatti, Federica; Iannuzzi, Silvia; Mauri, Benilde; Galli, Domenica; Fiore, Marco; Attilia, Maria Luisa; Ceccanti, Mauro

2018-01-01

RIASSUNTO. Il disturbo da uso di alcol (DUA) è uno dei disturbi psichiatrici più comuni nella popolazione generale. Il DUA è caratterizzato da un pattern di bere eccessivo, che si mantiene nonostante gli effetti negativi che l'alcol ha sul funzionamento lavorativo, sulla salute, sulle problematiche legali, sull'educazione e sulla vita sociale. Attualmente, il modello bio-psico-sociale è quello che spiega meglio il DUA. Infatti, molte ricerche hanno fornito evidenze su come il DUA sia una patologia multidimensionale. Variabili biologiche, psicologiche e socio-culturali entrano in gioco nell'eziologia, nella natura, nel mantenimento e nel cambiamento nel tempo del disturbo. La fase diagnostica è un momento importante del processo di cura, perché il successo del trattamento dipende in larga misura dall'esattezza e dall'adeguatezza della diagnosi. La diagnosi clinica si basa su una valutazione globale del funzionamento del paziente e utilizza il colloquio e gli strumenti psicometrici come mezzo di raccolta di informazioni. Questo articolo fornirà una panoramica delle dimensioni psicologiche più importanti da valutare e sui migliori strumenti psicometrici da usare per una diagnosi adeguata.

[Atmospheric non-methane hydrocarbons near plants of crude oil first treatment].

PubMed

Bustaffa, Elisa; De Marinis Loiotile, Annamaria; Farella, Genoveffa; Petraccone, Stefania; De Gennaro, Gianluigi; Bianchi, Fabrizio

2016-01-01

La continua espansione delle attività di perforazione ai fini dell'estrazione petrolifera in prossimità di aree abitate ha fatto sì che negli ultimi anni l'attenzione si focalizzasse sull'impatto di questo processo fortemente industrializzato sulla salute pubblica. Le comunità che vivono nei dintorni di impianti di questo tipo devono, infatti, fronteggiare diversi problemi, quali l'inquinamento atmosferico e acustico, la contaminazione del suolo e delle acque sotterranee, il traffico dei camion da e verso il sito, incidenti e malfunzionamenti all'interno dell'impianto. In questo contesto, la valutazione del rischio per la salute è ostacolata dal fatto che l'esposizione alle sostanze chimiche presenti non può essere valutata in via definitiva, poiché non sempre si è a conoscenza di tutti i composti immessi nell'ambiente né delle loro concentrazioni, per non parlare del problema delle coesposizioni ad altri inquinanti. Nonostante l'oramai conclamato e vasto interesse generato da questo argomento, ad oggi esistono pochi studi basati su popolazioni riguardanti gli effetti sulla salute delle comunità che vivono in prossimità dei siti di perforazione ed estrazione; ciò genera la necessità di condurre campagne di monitoraggio mirate e studi epidemiologici che verifichino l'eventuale esistenza e natura di pattern di malattie associati a tali attività. La presente rassegna bibliografica individua, quindi, i principali inquinanti atmosferici presenti in prossimità di un impianto di primo trattamento del greggio e cerca di fornire un quadro generale delle loro potenziali sorgenti e caratteristiche.

Thin-film formation of Si clathrates on Si wafers

NASA Astrophysics Data System (ADS)

Ohashi, Fumitaka; Iwai, Yoshiki; Noguchi, Akihiro; Sugiyama, Tomoya; Hattori, Masashi; Ogura, Takuya; Himeno, Roto; Kume, Tetsuji; Ban, Takayuki; Nonomura, Shuichi

2014-04-01

In this study, we prepared Si clathrate films (Na8Si46 and NaxSi136) using a single-crystalline Si substrate. Highly oriented film growth of Zintl-phase sodium silicide, which is a precursor of Si clathrate, was achieved by exposing Na vapour to Si substrates under an Ar atmosphere. Subsequent heat treatment of the NaSi film at 400 °C (3 h) under vacuum (<10-2 Pa) resulted in a film of Si clathrates having a thickness of several micrometres. Furthermore, this technique enabled the selective growth of Na8Si46 and NaxSi136 using the appropriate crystalline orientation of Si substrates.

SiC Nanoparticles Toughened-SiC/MoSi2-SiC Multilayer Functionally Graded Oxidation Protective Coating for Carbon Materials at High Temperatures

NASA Astrophysics Data System (ADS)

Abdollahi, Alireza; Ehsani, Naser; Valefi, Zia; Khalifesoltani, Ali

2017-05-01

A SiC nanoparticle toughened-SiC/MoSi2-SiC functionally graded oxidation protective coating on graphite was prepared by reactive melt infiltration (RMI) at 1773 and 1873 K under argon atmosphere. The phase composition and anti-oxidation behavior of the coatings were investigated. The results show that the coating was composed of MoSi2, α-SiC and β-SiC. By the variations of Gibbs free energy (calculated by HSC Chemistry 6.0 software), it could be suggested that the SiC coating formed at low temperatures by solution-reprecipitation mechanism and at high temperatures by gas-phase reactions and solution-reprecipitation mechanisms simultaneously. SiC nanoparticles could improve the oxidation resistance of SiC/MoSi2-SiC multiphase coating. Addition of SiC nanoparticles increases toughness of the coating and prevents spreading of the oxygen diffusion channels in the coating during the oxidation test. The mass loss and oxidation rate of the SiC nanoparticle toughened-SiC/MoSi2-SiC-coated sample after 10-h oxidation at 1773 K were only 1.76% and 0.32 × 10-2 g/cm3/h, respectively.

a-Si:H/SiNW shell/core for SiNW solar cell applications

PubMed Central

2013-01-01

Vertically aligned silicon nanowires have been synthesized by the chemical etching of silicon wafers. The influence of a hydrogenated amorphous silicon (a-Si:H) layer (shell) on top of a silicon nanowire (SiNW) solar cell has been investigated. The optical properties of a-Si:H/SiNWs and SiNWs are examined in terms of optical reflection and absorption properties. In the presence of the a-Si:H shell, 5.2% reflection ratio in the spectral range (250 to 1,000 nm) is achieved with a superior absorption property with an average over 87% of the incident light. In addition, the characteristics of the solar cell have been significantly improved, which exhibits higher open-circuit voltage, short-circuit current, and efficiency by more than 15%, 12%, and 37%, respectively, compared with planar SiNW solar cells. Based on the current–voltage measurements and morphology results, we show that the a-Si:H shell can passivate the defects generated by wet etching processes. PMID:24195734

Fabrication of Si heterojunction solar cells using P-doped Si nanocrystals embedded in SiNx films as emitters

PubMed Central

2013-01-01

Si heterojunction solar cells were fabricated on p-type single-crystal Si (sc-Si) substrates using phosphorus-doped Si nanocrystals (Si-NCs) embedded in SiNx (Si-NCs/SiNx) films as emitters. The Si-NCs were formed by post-annealing of silicon-rich silicon nitride films deposited by electron cyclotron resonance chemical vapor deposition. We investigate the influence of the N/Si ratio in the Si-NCs/SiNx films on their electrical and optical properties, as well as the photovoltaic properties of the fabricated heterojunction devices. Increasing the nitrogen content enhances the optical gap E04 while deteriorating the electrical conductivity of the Si-NCs/SiNx film, leading to an increased short-circuit current density and a decreased fill factor of the heterojunction device. These trends could be interpreted by a bi-phase model which describes the Si-NCs/SiNx film as a mixture of a high-transparency SiNx phase and a low-resistivity Si-NC phase. A preliminary efficiency of 8.6% is achieved for the Si-NCs/sc-Si heterojunction solar cell. PMID:24188725

Nanocrystalline Si pathway induced unipolar resistive switching behavior from annealed Si-rich SiN{sub x}/SiN{sub y} multilayers

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

Jiang, Xiaofan; Ma, Zhongyuan, E-mail: zyma@nju.edu.cn; Yang, Huafeng

2014-09-28

Adding a resistive switching functionality to a silicon microelectronic chip is a new challenge in materials research. Here, we demonstrate that unipolar and electrode-independent resistive switching effects can be realized in the annealed Si-rich SiN{sub x}/SiN{sub y} multilayers with high on/off ratio of 10{sup 9}. High resolution transmission electron microscopy reveals that for the high resistance state broken pathways composed of discrete nanocrystalline silicon (nc-Si) exist in the Si nitride multilayers. While for the low resistance state the discrete nc-Si regions is connected, forming continuous nc-Si pathways. Based on the analysis of the temperature dependent I-V characteristics and HRTEM photos,more » we found that the break-and-bridge evolution of nc-Si pathway is the origin of resistive switching memory behavior. Our findings provide insights into the mechanism of the resistive switching behavior in nc-Si films, opening a way for it to be utilized as a material in Si-based memories.« less

Mudstone Mineralogy from Curiosity CheMin, 2013 to 2016

NASA Image and Video Library

2016-12-13

This series of pie charts shows similarities and differences in the mineral compositions of mudstones at 10 sites where NASA's Curiosity Mars rover collected rock-powder samples and analyzed them with the rover's Chemistry and Mineralogy (CheMin) instrument. The charts are arrayed in chronological order, with an indication of relative elevation as the rover first sampled two sites on the floor of Gale Crater in 2013 and later began climbing the crater's central mound, Mount Sharp. The pie chart farthest to the right and uphill shows composition at the "Sebina" target, sampled in October 2016. Five non-mudstone rock targets that the rover drilled and analyzed within this time frame are not included. The mineralogical variations in these mudstones may be due to differences in any or all of these factors: the source materials deposited by water that entered lakes, the processes of sedimentation and rock forming, and how the rocks were later altered. One trend that stands out is that the mineral jarosite -- shown in purple -- was more prominent in the "Pahrump Hills" area of lower Mount Sharp than at sites examined either earlier or later. Jarosite is an indicator of acidic water. Mudstone layers uphill from Pahrump Hills have barely detectable amounts of jarosite, indicating a shift away from acidic conditions in these overlying -- thus younger -- layers. Clay minerals, shown as green, declined in abundance at sites midway through this series, then came back as the rover climbed higher. Each drilled-and-analyzed target is identified with a two-letter abbreviation: JK for "John Klein," CB for "Cumberland." CH for "Confidence Hills," MJ for "Mojave," TP for "Telegraph Peak," BK for "Buckskin," OD for "Oudam," MB for "Marimba," QL for "Quela," and SB for Sebina. http://photojournal.jpl.nasa.gov/catalog/PIA21146

Reaction mechanisms at 4H-SiC/SiO2 interface during wet SiC oxidation

NASA Astrophysics Data System (ADS)

Akiyama, Toru; Hori, Shinsuke; Nakamura, Kohji; Ito, Tomonori; Kageshima, Hiroyuki; Uematsu, Masashi; Shiraishi, Kenji

2018-04-01

The reaction processes at the interface between SiC with 4H structure (4H-SiC) and SiO2 during wet oxidation are investigated by electronic structure calculations within the density functional theory. Our calculations for 4H-SiC/SiO2 interfaces with various orientations demonstrate characteristic features of the reaction depending on the crystal orientation of SiC: On the Si-face, the H2O molecule is stable in SiO2 and hardly reacts with the SiC substrate, while the O atom of H2O can form Si-O bonds at the C-face interface. Two OH groups are found to be at least necessary for forming new Si-O bonds at the Si-face interface, indicating that the oxidation rate on the Si-face is very low compared with that on the C-face. On the other hand, both the H2O molecule and the OH group are incorporated into the C-face interface, and the energy barrier for OH is similar to that for H2O. By comparing the calculated energy barriers for these reactants with the activation energies of oxide growth rate, we suggest the orientation-dependent rate-limiting processes during wet SiC oxidation.

Brazing SiC/SiC Composites to Metals

NASA Technical Reports Server (NTRS)

Steffier, Wayne S.

2004-01-01

Experiments have shown that active brazing alloys (ABAs) can be used to join SiC/SiC composite materials to metals, with bond strengths sufficient for some structural applications. The SiC/SiC composite coupons used in the experiments were made from polymerbased SiC fiber preforms that were chemical-vapor-infiltrated with SiC to form SiC matrices. Some of the metal coupons used in the experiments were made from 304 stainless steel; others were made from oxygen-free, high-conductivity copper. Three ABAs were chosen for the experiments: two were chosen randomly from among a number of ABAs that were on hand at the time; the third ABA was chosen because its titanium content (1.25 percent) is less than those of the other two ABAs (1.75 and 4.5 percent, respectively) and it was desired to evaluate the effect of reducing the titanium content, as described below. The characteristics of ABAs that are considered to be beneficial for the purpose of joining SiC/SiC to metal include wettability, reactivity, and adhesion to SiC-based ceramics. Prior to further development, it was verified that the three chosen ABAs have these characteristics. For each ABA, suitable vacuum brazing process conditions were established empirically by producing a series of (SiC/SiC)/ABA wetting samples. These samples were then sectioned and subjected to scanning electron microscopy (SEM) and energy-dispersive x-ray spectrometry (EDS) for analysis of their microstructures and compositions. Specimens for destructive mechanical tests were fabricated by brazing of lap joints between SiC/SiC coupons 1/8-in. (.3.2- mm) thick and, variously, stainless steel or copper tabs. The results of destructive mechanical tests and the SEM/EDS analysis were used to guide the development of a viable method of brazing the affected materials.

Nanocatalytic growth of Si nanowires from Ni silicate coated SiC nanoparticles on Si solar cell.

PubMed

Parida, Bhaskar; Choi, Jaeho; Ji, Hyung Yong; Park, Seungil; Lim, Gyoungho; Kim, Keunjoo

2013-09-01

We investigated the nanocatalytic growth of Si nanowires on the microtextured surface of crystalline Si solar cell. 3C-SiC nanoparticles have been used as the base for formation of Ni silicate layer in a catalytic reaction with the Si melt under H2 atmosphere at an annealing temperature of 1100 degrees C. The 10-nm thick Ni film was deposited after the SiC nanoparticles were coated on the microtextured surface of the Si solar cell by electron-beam evaporation. SiC nanoparticles form a eutectic alloy surface of Ni silicate and provide the base for Si supersaturation as well as the Ni-Si alloy layer on Si substrate surface. This bottom reaction mode for the solid-liquid-solid growth mechanism using a SiC nanoparticle base provides more stable growth of nanowires than the top reaction mode growth mechanism in the absence of SiC nanoparticles. Thermally excited Ni nanoparticle forms the eutectic alloy and provides collectively excited electrons at the alloy surface, which reduces the activation energy of the nanocatalytic reaction for formation of nanowires.

Oxide Structure Dependence of SiO2/SiOx/3C-SiC/n-Type Si Nonvolatile Resistive Memory on Memory Operation Characteristics

NASA Astrophysics Data System (ADS)

Yamaguchi, Yuichiro; Shouji, Masatsugu; Suda, Yoshiyuki

2012-11-01

We have investigated the dependence of the oxide layer structure of our previously proposed metal/SiO2/SiOx/3C-SiC/n-Si/metal metal-insulator-semiconductor (MIS) resistive memory device on the memory operation characteristics. The current-voltage (I-V) measurement and X-ray photoemission spectroscopy results suggest that SiOx defect states mainly caused by the oxidation of 3C-SiC at temperatures below 1000 °C are related to the hysteresis memory behavior in the I-V curve. By restricting the SiOx interface region, the number of switching cycles and the on/off current ratio are more enhanced. Compared with a memory device formed by one-step or two-step oxidation of 3C-SiC, a memory device formed by one-step oxidation of Si/3C-SiC exhibits a more restrictive SiOx interface with a more definitive SiO2 layer and higher memory performances for both the endurance switching cycle and on/off current ratio.

Formation of Si grains from a NaSi melt prepared by reaction of SiO2 and Na

NASA Astrophysics Data System (ADS)

Yamane, Hisanori; Morito, Haruhiko; Uchikoshi, Masahito

2013-08-01

A mixture of Na2SiO3 and NaSi was found to be formed by reaction of SiO2 and Na at 650 °C as follows: 5Na+3SiO2→2Na2SiO3+NaSi. Single crystals of NaSi were grown by cooling the mixture of Na2SiO3 and NaSi with an excess amount of Na from 850 °C, and polycrystalline Si was obtained by vaporization of Na from the crystals. Coarse grains of Si were also crystallized by Na evaporation after the formation of Na2SiO3 and Si-dissolved liquid Na at 830 °C. The Si grains were collected by washing the product with water. The yield of the Si grains was 85% of the ideal amount expected from the reaction.

High-dose MeV electron irradiation of Si-SiO2 structures implanted with high doses Si+

NASA Astrophysics Data System (ADS)

Kaschieva, S.; Angelov, Ch; Dmitriev, S. N.

2018-03-01

The influence was studied of 22-MeV electron irradiation on Si-SiO2 structures implanted with high-fluence Si+ ions. Our earlier works demonstrated that Si redistribution is observed in Si+-ion-implanted Si-SiO2 structures (after MeV electron irradiation) only in the case when ion implantation is carried out with a higher fluence (1016 cm-2). We focused our attention on the interaction of high-dose MeV electron irradiation (6.0×1016 cm-2) with n-Si-SiO2 structures implanted with Si+ ions (fluence 5.4×1016 cm-2 of the same order magnitude). The redistribution of both oxygen and silicon atoms in the implanted Si-SiO2 samples after MeV electron irradiation was studied by Rutherford back-scattering (RBS) spectroscopy in combination with a channeling technique (RBS/C). Our results demonstrated that the redistribution of oxygen and silicon atoms in the implanted samples reaches saturation after these high doses of MeV electron irradiation. The transformation of amorphous SiO2 surface into crystalline Si nanostructures (after MeV electron irradiation) was evidenced by atomic force microscopy (AFM). Silicon nanocrystals are formed on the SiO2 surface after MeV electron irradiation. The shape and number of the Si nanocrystals on the SiO2 surface depend on the MeV electron irradiation, while their size increases with the dose. The mean Si nanocrystals height is 16-20 nm after irradiation with MeV electrons at the dose of 6.0×1016 cm-2.

Revealing heterogeneous nucleation of primary Si and eutectic Si by AlP in hypereutectic Al-Si alloys.

PubMed

Li, Jiehua; Hage, Fredrik S; Liu, Xiangfa; Ramasse, Quentin; Schumacher, Peter

2016-04-28

The heterogeneous nucleation of primary Si and eutectic Si can be attributed to the presence of AlP. Although P, in the form of AlP particles, is usually observed in the centre of primary Si, there is still a lack of detailed investigations on the distribution of P within primary Si and eutectic Si in hypereutectic Al-Si alloys at the atomic scale. Here, we report an atomic-scale experimental investigation on the distribution of P in hypereutectic Al-Si alloys. P, in the form of AlP particles, was observed in the centre of primary Si. However, no significant amount of P was detected within primary Si, eutectic Si and the Al matrix. Instead, P was observed at the interface between the Al matrix and eutectic Si, strongly indicating that P, in the form of AlP particles (or AlP 'patch' dependent on the P concentration), may have nucleated on the surface of the Al matrix and thereby enhanced the heterogeneous nucleation of eutectic Si. The present investigation reveals some novel insights into heterogeneous nucleation of primary Si and eutectic Si by AlP in hypereutectic Al-Si alloys and can be used to further develop heterogeneous nucleation mechanisms based on adsorption.

A stable silicon(0) compound with a Si=Si double bond.

PubMed

Wang, Yuzhong; Xie, Yaoming; Wei, Pingrong; King, R Bruce; Schaefer, Henry F; von R Schleyer, Paul; Robinson, Gregory H

2008-08-22

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si-Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 +/- 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si-Si(Cl):L and L:Si=Si:L.

Mechanism for Si-Si Bond Rupture in Single Molecule Junctions.

PubMed

Li, Haixing; Kim, Nathaniel T; Su, Timothy A; Steigerwald, Michael L; Nuckolls, Colin; Darancet, Pierre; Leighton, James L; Venkataraman, Latha

2016-12-14

The stability of chemical bonds can be studied experimentally by rupturing single molecule junctions under applied voltage. Here, we compare voltage-induced bond rupture in two Si-Si backbones: one has no alternate conductive pathway whereas the other contains an additional naphthyl pathway in parallel to the Si-Si bond. We show that in contrast to the first system, the second can conduct through the naphthyl group when the Si-Si bond is ruptured using an applied voltage. We investigate this voltage induced Si-Si bond rupture by ab initio density functional theory calculations and molecular dynamics simulations that ultimately demonstrate that the excitation of molecular vibrational modes by tunneling electrons leads to homolytic Si-Si bond rupture.

Synchrotron x-ray thermal diffuse scattering probes for phonons in Si/SiGe/Si trilayer nanomembranes

DOE PAGES

McElhinny, Kyle M.; Gopalakrishnan, Gokul; Savage, Donald E.; ...

2016-05-17

Nanostructures offer the opportunity to control the vibrational properties of via the scattering of phonons due to boundaries and mass disorder as well as through changes in the phonon dispersion due to spatial confinement. Advances in understanding these effects have the potential to lead to thermoelectrics with an improved figure of merit by lowering the thermal conductivity and to provide insight into electron-phonon scattering rates in nanoelectronics. However, characterizing the phonon population in nanomaterials has been challenging because of their small volume and because optical techniques probe only a small fraction of reciprocal space. Recent developments in x-ray scattering nowmore » allow the phonon population to be evaluated across all of reciprocal space in samples with volumes as small as several cubic micrometers. We apply this approach, synchrotron x-ray thermal diffuse scattering (TDS), to probe the population of phonons within a Si/SiGe/Si trilayer nanomembrane. The distributions of scattered intensity from Si/SiGe/Si trilayer nanomembranes and Si nanomembranes with uniform composition are qualitatively similar, with features arising from the elastic anisotropy of the diamond structure. The TDS signal for the Si/SiGe/Si nanomembrane, however, has higher intensity than the Si membrane of the same total thickness by approximately 3.75%. Possible origins of the enhancement in scattering from SiGe in comparison with Si include the larger atomic scattering factor of Ge atoms within the SiGe layer or reduced phonon frequencies due to alloying.« less

SiC/SiC Cladding Materials Properties Handbook

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

Snead, Mary A.; Katoh, Yutai; Koyanagi, Takaaki

When a new class of material is considered for a nuclear core structure, the in-pile performance is usually assessed based on multi-physics modeling in coordination with experiments. This report aims to provide data for the mechanical and physical properties and environmental resistance of silicon carbide (SiC) fiber–reinforced SiC matrix (SiC/SiC) composites for use in modeling for their application as accidenttolerant fuel cladding for light water reactors (LWRs). The properties are specific for tube geometry, although many properties can be predicted from planar specimen data. This report presents various properties, including mechanical properties, thermal properties, chemical stability under normal and offnormalmore » operation conditions, hermeticity, and irradiation resistance. Table S.1 summarizes those properties mainly for nuclear-grade SiC/SiC composites fabricated via chemical vapor infiltration (CVI). While most of the important properties are available, this work found that data for the in-pile hydrothermal corrosion resistance of SiC materials and for thermal properties of tube materials are lacking for evaluation of SiC-based cladding for LWR applications.« less

SiGe/Si Monolithically Integrated Amplifier Circuits

NASA Technical Reports Server (NTRS)

Katehi, Linda P. B.; Bhattacharya, Pallab

1998-01-01

With recent advance in the epitaxial growth of silicon-germanium heterojunction, Si/SiGe HBTs with high f(sub max) and f(sub T) have received great attention in MMIC applications. In the past year, technologies for mesa-type Si/SiGe HBTs and other lumped passive components with high resonant frequencies have been developed and well characterized for circuit applications. By integrating the micromachined lumped passive elements into HBT fabrication, multi-stage amplifiers operating at 20 GHz have been designed and fabricated.

Exceptional cracking behavior in H-implanted Si/B-doped Si0.70Ge0.30/Si heterostructures

NASA Astrophysics Data System (ADS)

Chen, Da; Wang, Dadi; Chang, Yongwei; Li, Ya; Ding, Rui; Li, Jiurong; Chen, Xiao; Wang, Gang; Guo, Qinglei

2018-01-01

The cracking behavior in H-implanted Si/B-doped Si0.70Ge0.30/Si structures after thermal annealing was investigated. The crack formation position is found to closely correlate with the thickness of the buried Si0.70Ge0.30 layer. For H-implanted Si containing a buried 3-nm-thick B-doped Si0.70Ge0.30 layer, localized continuous cracking occurs at the interfaces on both sides of the Si0.70Ge0.30 interlayer. Once the thickness of the buried Si0.70Ge0.30 layer increases to 15 and 70 nm, however, a continuous sharp crack is individually observed along the interface between the Si substrate and the B-doped Si0.70Ge0.30 interlayer. We attribute this exceptional cracking behavior to the existence of shear stress on both sides of the buried Si0.70Ge0.30 layer and the subsequent trapping of hydrogen, which leads to a crack in a well-controlled manner. This work may pave the way for high-quality Si or SiGe membrane transfer in a feasible manner, thus expediting its potential applications to ultrathin silicon-on-insulator (SOI) or silicon-germanium-on-insulator (SGOI) production.

Challenge of Si/SiGe technology to optoelectronics

NASA Astrophysics Data System (ADS)

Chang, C. Y.; Jung, J. G.

1993-01-01

Low temperature epitaxy (LTE) of Si and SiGecanbe performed at a temperature of 550 C or lower. Very promising applications can be opened. Such as high speed/high frequency operations at 90GHZ by constructing heterojunction bipolar transistors. High performance FET'slikepseudomorphic p-channel orn-channel high mobility field effect transistors are presented which canbe composed to perform CMOS operations. Optoelectronic devices such as IRdetectors (1-12um), mutiple quantum well (MOW), disordered superlattice (d-SL) which are the potential candidatesof IR detector and optical sources (e.q. LED, LD etc.) Various physical insights regarding to SiGe heterostructures are presented which includeswave function filter, mass filter as well as band mixing are introduced. Researchesat National Nano Device Laboratory (NDL) which processes the capability of 0.3um Si ULSI technologies and SiGe works as well as lll-V, a-Si/SiGe lines are also presented.

Sintering Behavior of Spark Plasma Sintered SiC with Si-SiC Composite Nanoparticles Prepared by Thermal DC Plasma Process

NASA Astrophysics Data System (ADS)

Yu, Yeon-Tae; Naik, Gautam Kumar; Lim, Young-Bin; Yoon, Jeong-Mo

2017-11-01

The Si-coated SiC (Si-SiC) composite nanoparticle was prepared by non-transferred arc thermal plasma processing of solid-state synthesized SiC powder and was used as a sintering additive for SiC ceramic formation. Sintered SiC pellet was prepared by spark plasma sintering (SPS) process, and the effect of nano-sized Si-SiC composite particles on the sintering behavior of micron-sized SiC powder was investigated. The mixing ratio of Si-SiC composite nanoparticle to micron-sized SiC was optimized to 10 wt%. Vicker's hardness and relative density was increased with increasing sintering temperature and holding time. The relative density and Vicker's hardness was further increased by reaction bonding using additional activated carbon to the mixture of micron-sized SiC and nano-sized Si-SiC. The maximum relative density (97.1%) and Vicker's hardness (31.4 GPa) were recorded at 1800 °C sintering temperature for 1 min holding time, when 0.2 wt% additional activated carbon was added to the mixture of SiC/Si-SiC.

High temperature compounds for turbine vanes. [of SiC, Si3N4, and Si composites

NASA Technical Reports Server (NTRS)

Rhodes, W. H.; Cannon, R. M., Jr.

1974-01-01

Fabrication and microstructure control studies were conducted on SiC, Si3N and composites based on Si3N. Charpy mode impact testing to 2400 F established that Si3N4/Mo composites have excellent potential. Attempts to fabricate composites of Si3N4 with superalloys, both by hot pressing and infiltration were largely unsuccessful in comparison to using Mo, Re, and Ta which are less reactive. Modest improvements in impact strength were realized for monolithic Si3N4; however, SiC strengths increased by a factor of six and now equal values achieved for Si3N4. Correlations of impact strength with material properties are discussed. Reduced MgO densification aid additions to Si3N4 were found to decrease densification kinetics, increase final porosity, decrease room temperature bend strength, increase high temperature bend strength, and decrease bend stress rupture properties. The decrease in bend strength at high temperature for fine grain size SiC suggested that a slightly larger grain size material with a nearly constant strength-temperature relation may prove desirable in the creep and stress rupture mode.

Abrupt GaP/Si hetero-interface using bistepped Si buffer

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

Ping Wang, Y., E-mail: yanping.wang@insa-rennes.fr; Kuyyalil, J.; Nguyen Thanh, T.

We evidence the influence of the quality of the starting Si surface on the III-V/Si interface abruptness and on the formation of defects during the growth of III-V/Si heterogeneous crystal, using high resolution transmission electron microscopy and scanning transmission electron microscopy. GaP layers were grown by molecular beam epitaxy on vicinal Si (001). The strong effect of the Si substrate chemical preparation is first demonstrated by studying structural properties of both Si homoepitaxial layer and GaP/Si heterostructure. It is then shown that choosing adequate chemical preparation conditions and subsequent III-V regrowth conditions enables the quasi-suppression of micro-twins in the epilayer.more » Finally, the abruptness of GaP/Si interface is found to be very sensitive to the Si chemical preparation and is improved by the use of a bistepped Si buffer prior to III-V overgrowth.« less

Sintering Behavior of Spark Plasma Sintered SiC with Si-SiC Composite Nanoparticles Prepared by Thermal DC Plasma Process.

PubMed

Yu, Yeon-Tae; Naik, Gautam Kumar; Lim, Young-Bin; Yoon, Jeong-Mo

2017-11-25

The Si-coated SiC (Si-SiC) composite nanoparticle was prepared by non-transferred arc thermal plasma processing of solid-state synthesized SiC powder and was used as a sintering additive for SiC ceramic formation. Sintered SiC pellet was prepared by spark plasma sintering (SPS) process, and the effect of nano-sized Si-SiC composite particles on the sintering behavior of micron-sized SiC powder was investigated. The mixing ratio of Si-SiC composite nanoparticle to micron-sized SiC was optimized to 10 wt%. Vicker's hardness and relative density was increased with increasing sintering temperature and holding time. The relative density and Vicker's hardness was further increased by reaction bonding using additional activated carbon to the mixture of micron-sized SiC and nano-sized Si-SiC. The maximum relative density (97.1%) and Vicker's hardness (31.4 GPa) were recorded at 1800 °C sintering temperature for 1 min holding time, when 0.2 wt% additional activated carbon was added to the mixture of SiC/Si-SiC.

Ni-silicide growth kinetics in Si and Si/SiO2 core/shell nanowires.

PubMed

Ogata, K; Sutter, E; Zhu, X; Hofmann, S

2011-09-07

A systematic study of the kinetics of axial Ni silicidation of as-grown and oxidized Si nanowires (SiNWs) with different crystallographic orientations and core diameters ranging from ∼ 10 to 100 nm is presented. For temperatures between 300 and 440 °C the length of the total axial silicide intrusion varies with the square root of time, which provides clear evidence that the rate limiting step is diffusion of Ni through the growing silicide phase(s). A retardation of Ni-silicide formation for oxidized SiNWs is found, indicative of a stress induced lowering of the diffusion coefficients. Extrapolated growth constants indicate that the Ni flux through the silicided NW is dominated by surface diffusion, which is consistent with an inverse square root dependence of the silicide length on the NW diameter as observed for (111) orientated SiNWs. In situ TEM silicidation experiments show that NiSi(2) is the first forming phase for as-grown and oxidized SiNWs. The silicide-SiNW interface is thereby atomically abrupt and typically planar. Ni-rich silicide phases subsequently nucleate close to the Ni reservoir, which for as-grown SiNWs can lead to a complete channel break-off for prolonged silicidation due to significant volume expansion and morphological changes.

Ni-Silicide Growth Kinetics in Si and Si/SiO2 Core/Shell Nanowires

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

Hofmann, S.; Sutter, E.; Ogata, K.

A systematic study of the kinetics of axial Ni silicidation of as-grown and oxidized Si nanowires (SiNWs) with different crystallographic orientations and core diameters ranging from {approx} 10 to 100 nm is presented. For temperatures between 300 and 440 C the length of the total axial silicide intrusion varies with the square root of time, which provides clear evidence that the rate limiting step is diffusion of Ni through the growing silicide phase(s). A retardation of Ni-silicide formation for oxidized SiNWs is found, indicative of a stress induced lowering of the diffusion coefficients. Extrapolated growth constants indicate that the Nimore » flux through the silicided NW is dominated by surface diffusion, which is consistent with an inverse square root dependence of the silicide length on the NW diameter as observed for <111> orientated SiNWs. In situ TEM silicidation experiments show that NiSi{sub 2} is the first forming phase for as-grown and oxidized SiNWs. The silicide-SiNW interface is thereby atomically abrupt and typically planar. Ni-rich silicide phases subsequently nucleate close to the Ni reservoir, which for as-grown SiNWs can lead to a complete channel break-off for prolonged silicidation due to significant volume expansion and morphological changes.« less

Si/SiGe heterointerfaces in one-, two-, and three-dimensional nanostructures: their impact on SiGe light emission

NASA Astrophysics Data System (ADS)

Lockwood, David; Wu, Xiaohua; Baribeau, Jean-Marc; Mala, Selina; Wang, Xialou; Tsybeskov, Leonid

2016-03-01

Fast optical interconnects together with an associated light emitter that are both compatible with conventional Si-based complementary metal-oxide- semiconductor (CMOS) integrated circuit technology is an unavoidable requirement for the next-generation microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures, which can emit light at wavelengths within the important optical communication wavelength range of 1.3 - 1.55 μm, are already compatible with standard CMOS practices. However, the expected long carrier radiative lifetimes observed to date in Si and Si/Si1-xGex nanostructures have prevented the attainment of efficient light-emitting devices including the desired lasers. Thus, the engineering of Si/Si1-xGex heterostructures having a controlled composition and sharp interfaces is crucial for producing the requisite fast and efficient photoluminescence (PL) at energies in the range 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe nanostructures and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in three dimensions (corresponding to the case of quantum dots), two dimensions (corresponding to quantum wires), and one dimension (corresponding to quantum wells). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for nanostructure confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 - 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex nanostructures incorporated into a Si0.6Ge0.4 wavy

Synthesis and structural property of Si nanosheets connected to Si nanowires using MnCl2/Si powder source

NASA Astrophysics Data System (ADS)

Meng, Erchao; Ueki, Akiko; Meng, Xiang; Suzuki, Hiroaki; Itahara, Hiroshi; Tatsuoka, Hirokazu

2016-08-01

Si nanosheets connected to Si nanowires were synthesized using a MnCl2/Si powder source with an Au catalyst. The synthesis method has benefits in terms of avoiding conventionally used air-sensitive SiH4 or SiCl4. The existence of the Si nanosheets connected to the Si<111> nanowires, like sprouts or leaves with petioles, was observed, and the surface of the nanosheets was Si{111}. The nanosheets were grown in the growth direction of <211> perpendicular to that of the Si nanowires. It was evident from these structural features of the nanosheets that the nanosheets were formed by the twin-plane reentrant-edge mechanism. The feature of the observed lattice fringes, which do not appear for Si bulk crystals, of the Si(111) nanosheets obtained by high resolution transmission electron microscopy was clearly explained due to the extra diffraction spots that arose by the reciprocal lattice streaking effect.

Growth of strained Si/relaxed SiGe heterostructures on Si(110) substrates using solid-source molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Arimoto, Keisuke; Nakazawa, Hiroki; Mitsui, Shohei; Utsuyama, Naoto; Yamanaka, Junji; Hara, Kosuke O.; Usami, Noritaka; Nakagawa, Kiyokazu

2017-11-01

A strained Si/relaxed SiGe heterostructure grown on Si(110) substrate is attractive as a platform for high-hole-mobility Si-based electronic devices. To improve the electrical property, a smoother surface is desirable. In this study, we investigated surface morphology and microstructural aspects of strained Si/relaxed SiGe/Si(110) heterostructures grown by solid-source (SS) molecular beam epitaxy (MBE). It was revealed that SSMBE provides a way to grow strained Si/relaxed SiGe heterostructures with smooth surfaces. In addition, it was found that the strain in the SiGe layer of the SSMBE-grown sample is highly anisotropic whereas that of the GSMBE-grown sample is almost biaxially relaxed. Along with the surface morphology, the symmetry in degree of strain relaxation has implications for the electrical property. Results of a calculation shows that anisotropic strain is preferable for device application since it confines holes solely in the strained Si layer where hole mobility is enhanced.

Comparison of interfaces for (Ba,Sr)TiO3 films deposited on Si and SiO2/Si substrates

NASA Astrophysics Data System (ADS)

Suvorova, N. A.; Lopez, C. M.; Irene, E. A.; Suvorova, A. A.; Saunders, M.

2004-03-01

(Ba,Sr)TiO3(BST) thin films were deposited by ion sputtering on both bare and oxidized Si. Spectroscopic ellipsometry results have shown that a SiO2 underlayer of nearly the same thickness (2.6 nm in average) is found at the Si interface for BST sputter depositions onto nominally bare Si, 1 nm SiO2 on Si or 3.5 nm SiO2 on Si. This result was confirmed by high-resolution electron microscopy analysis of the films, and it is believed to be due to simultaneous subcutaneous oxidation of Si and reaction of the BST layer with SiO2. Using the conductance method, capacitance-voltage measurements show a decrease in the interface trap density Dit of an order of magnitude for oxidized Si substrates with a thicker SiO2 underlayer. Further reduction of Dit was achieved for the capacitors grown on oxidized Si and annealed in forming gas after metallization.

Ab initio and kinetic Monte Carlo study of lithium diffusion in LiSi, Li12Si7, Li13Si5 and Li15Si4

NASA Astrophysics Data System (ADS)

Moon, Janghyuk; Lee, Byeongchan; Cho, Maenghyo; Cho, Kyeongjae

2016-10-01

The kinetics of lithium atoms in various Li-Si binary compounds are investigated using density functional theory calculations and kinetic Monte Carlo calculations. The values of the Li migration energy barriers are identified by NEB calculations with vacancy-mediated, interstitial and exchange migration mechanisms in crystalline LiSi, Li12Si7, Li13Si4, and Li15Si4. A comparison of these NEB results shows that the vacancy-mediated Li migration is identified as the dominant diffusion mechanisms in Li-Si compounds. The diffusion coefficients of Li in Li-Si compounds at room temperature are determined by KMC simulation. From the KMC results, the recalculated migration energy barriers in LiSi, Li12Si7, Li13Si4, and Li15Si4 correspond to 0.306, 0.301, 0.367 and 0.320 eV, respectively. Compared to the Li migration energy barrier of 0.6 eV in crystalline Si, the drastic reduction in the Li migration energy barriers in the lithiated silicon indicates that the initial lithiation of the Si anode is the rate-limiting step. Furthermore, it is also found that Si migration is possible in Li-rich configurations. On the basis of these findings, the underlying mechanisms of kinetics on the atomic scale details are elucidated.

Strain-Engineered Nanomembrane Substrates for Si/SiGe Heterostructures

NASA Astrophysics Data System (ADS)

Sookchoo, Pornsatit

For Group IV materials, including silicon, germanium, and their alloys, although they are most widely used in the electronics industry, the development of photonic devices is hindered by indirect band gaps and large lattice mismatches. Thus, any heterostructures involving Si and Ge (4.17% lattice mismatch) are subject to plastic relaxation by dislocation formation in the heterolayers. These defects make many devices impossible and at minimum degrade the performance of those that are possible. Fabrication using elastic strain engineering in Si/SiGe nanomembranes (NMs) is an approach that is showing promise to overcome this limitation. A key advantage of such NM substrates over conventional bulk substrates is that they are relaxed elastically and therefore free of dislocations that occur in the conventional fabrication of SiGe substrates, which are transferred to the epilayers and roughen film interfaces. In this thesis, I use the strain engineering of NMs or NM stacks to fabricate substrates for the epitaxial growth of many repeating units of Si/SiGe heterostructure, known as a 'superlattice', by the elastic strain sharing of a few periods of the repeating unit of Si/SiGe heterolayers or a Si/SiGe/Si tri-layer structure. In both cases, the process begins with the epitaxial growth of Si/SiGe heterolayers on silicon-on-insulator (SOI), where each layer thickness is designed to stay below its kinetic critical thickness for the formation of dislocations. The heterostructure NMs are then released by etching of the SiO2 sacrificial layer in hydrofluoric acid. The resulting freestanding NMs are elastically relaxed by the sharing of strain between the heterolayers. The NMs can be bonded in-place to their host substrate or transferred to another host substrate for the subsequent growth of many periods of superlattice film. The magnitude of strain sharing in these freestanding NMs is influenced by their layer thicknesses and layer compositions. As illustrated in this

High Temperature Si-doped BN Interphases for Woven SiC/SiC Composites

NASA Technical Reports Server (NTRS)

Morscher, Gregory N.; Hurwitz, Frances; Yun, Hee Mann; Gray, Hugh R. (Technical Monitor)

2002-01-01

The hydrolytic stability of high-temperature deposited Si-doped BN has been shown in the past to be superior in comparison to "pure" BN processed at similar or even higher temperatures. This type of material would be very desirable as a SiC/SiC composite interphase that is formed by chemical infiltration into multi-ply woven preform. However, due to rapid deposition on the preform outer surface at the high processing temperature, this has proven very difficult. To overcome this issue, single plies of woven fabric were infiltrated with Si-doped BN. Three composite panels of different SiC fiber types were fabricated with Si-doped BN interphases including Sylramic, Hi-Nicalon Type S and Sylramic-iBN fiber-types. The latter fiber-type possesses a thin in-situ grown BN layer on the fiber surface. High Si contents (approx. 7 to 10 a/o) and low oxygen contents (less than 1 a/o) were achieved. All three composite systems demonstrated reasonable debonding and sliding properties. The coated Sylramic fabric and composites were weak due to fiber degradation apparently caused during interphase processing by the formation of TiN crystals on the fiber surface. The Hi-Nicalon Type S composites with Si-doped BN interphase were only slightly weaker than Hi-Nicalon Type S composites with conventional BN when the strength on the load-bearing fibers at failure was compared. On the other hand, the Sylramic-iBN fabric and composites with Si-doped BN showed excellent composite and intermediate temperature stress-rupture properties. Most impressive was the lack of any significant interphase oxidation on the fracture surface of stress-ruptured specimens tested well above matrix cracking at 815C.

Synthesis and characterization of laminated Si/SiC composites.

PubMed

Naga, Salma M; Kenawy, Sayed H; Awaad, Mohamed; Abd El-Wahab, Hamada S; Greil, Peter; Abadir, Magdi F

2013-01-01

Laminated Si/SiC ceramics were synthesized from porous preforms of biogenous carbon impregnated with Si slurry at a temperature of 1500 °C for 2 h. Due to the capillarity infiltration with Si, both intrinsic micro- and macrostructure in the carbon preform were retained within the final ceramics. The SEM micrographs indicate that the final material exhibits a distinguished laminar structure with successive Si/SiC layers. The produced composites show weight gain of ≈5% after heat treatment in air at 1300 °C for 50 h. The produced bodies could be used as high temperature gas filters as indicated from the permeability results.

Proton trapping in SiO 2 layers thermally grown on Si and SiC

NASA Astrophysics Data System (ADS)

Afanas'ev, V. V.; Ciobanu, F.; Pensl, G.; Stesmans, A.

2002-11-01

Positive charging of thermal SiO 2 layers on (1 0 0)Si and (0 0 0 1)6H-, 4H-SiC related to trapping of protons is studied using low-energy proton implantation into the oxide, and compared to the trapping of holes generated by 10-eV photons. Proton trapping has an initial probability close to 100% and shows little sensitivity to the annealing-induced oxygen deficiency of SiO 2. In contrast to protons, hole trapping in as-grown SiO 2 shows a much lower efficiency which increases upon oxide annealing, in qualitative correlation with the higher density of O 3Si• defects (E' centers) detected by electron spin resonance after hole injection. Despite these differences, the neutralization of positive charges induced by holes and protons has the same cross-section, and in both cases is accompanied by liberation of atomic H suggesting that protons account for positive charge in both cases. The rupture of Si-O bonds in the oxide observed upon proton injection suggests, as a first basic step, the bonding of a proton to a bridging oxygen atom in SiO 2 network.

Isotopic effects in sub-barrier fusion of Si + Si systems

NASA Astrophysics Data System (ADS)

Colucci, G.; Montagnoli, G.; Stefanini, A. M.; Esbensen, H.; Bourgin, D.; Čolović, P.; Corradi, L.; Faggian, M.; Fioretto, E.; Galtarossa, F.; Goasduff, A.; Grebosz, J.; Haas, F.; Mazzocco, M.; Scarlassara, F.; Stefanini, C.; Strano, E.; Szilner, S.; Urbani, M.; Zhang, G. L.

2018-04-01

Background: Recent measurements of fusion cross sections for the 28Si+28Si system revealed a rather unsystematic behavior; i.e., they drop faster near the barrier than at lower energies. This was tentatively attributed to the large oblate deformation of 28Si because coupled-channels (CC) calculations largely underestimate the 28Si+28Si cross sections at low energies, unless a weak imaginary potential is applied, probably simulating the deformation. 30Si has no permanent deformation and its low-energy excitations are of a vibrational nature. Previous measurements of this system reached only 4 mb, which is not sufficient to obtain information on effects that should show up at lower energies. Purpose: The aim of the present experiment was twofold: (i) to clarify the underlying fusion dynamics by measuring the symmetric case 30Si+30Si in an energy range from around the Coulomb barrier to deep sub-barrier energies, and (ii) to compare the results with the behavior of 28Si+28Si involving two deformed nuclei. Methods: 30Si beams from the XTU tandem accelerator of the Laboratori Nazionali di Legnaro of the Istituto Nazionale di Fisica Nucleare were used, bombarding thin metallic 30Si targets (50 μ g /cm2) enriched to 99.64 % in mass 30. An electrostatic beam deflector allowed the detection of fusion evaporation residues (ERs) at very forward angles, and angular distributions of ERs were measured. Results: The excitation function of 30Si+30Si was measured down to the level of a few microbarns. It has a regular shape, at variance with the unusual trend of 28Si+28Si . The extracted logarithmic derivative does not reach the LCS limit at low energies, so that no maximum of the S factor shows up. CC calculations were performed including the low-lying 2+ and 3- excitations. Conclusions: Using a Woods-Saxon potential the experimental cross sections at low energies are overpredicted, and this is a clear sign of hindrance, while the calculations performed with a M3Y + repulsion

Advances in SiC/SiC Composites for Aero-Propulsion

NASA Technical Reports Server (NTRS)

DiCarlo, James A.

2013-01-01

In the last decade, considerable progress has been made in the development and application of ceramic matrix composites consisting of silicon carbide (SiC) based matrices reinforced by small-diameter continuous-length SiC-based fibers. For example, these SiC/SiC composites are now in the early stages of implementation into hot-section components of civil aero-propulsion gas turbine engines, where in comparison to current metallic components they offer multiple advantages due to their lighter weight and higher temperature structural capability. For current production-ready SiC/SiC, this temperature capability for long time structural applications is 1250 degC, which is better than 1100 degC for the best metallic superalloys. Foreseeing that even higher structural reliability and temperature capability would continue to increase the advantages of SiC/SiC composites, progress in recent years has also been made at NASA toward improving the properties of SiC/SiC composites by optimizing the various constituent materials and geometries within composite microstructures. The primary objective of this chapter is to detail this latter progress, both fundamentally and practically, with particular emphasis on recent advancements in the materials and processes for the fiber, fiber coating, fiber architecture, and matrix, and in the design methods for incorporating these constituents into SiC/SiC microstructures with improved thermo-structural performance.

Modelling the influence of high currents on the cutoff frequency in Si/SiGe/Si heterojunction transistors

NASA Astrophysics Data System (ADS)

Briggs, P. J.; Walker, A. B.; Herbert, D. C.

1998-05-01

A one-dimensional self-consistent bipolar Monte Carlo simulation code has been used to model carrier mobilities in strained doped SiGe and the base-collector region of Si/SiGe/Si and SiC/Si heterojunction bipolar transistors (HBTs) with wide collectors, to study the variation of the cutoff frequency 0268-1242/13/5/005/img6 with collector current density 0268-1242/13/5/005/img7. Our results show that while the presence of strain enhances the electron mobility, the scattering from alloy disorder and from ionized impurities reduces the electron mobility so much that it is less than that of Si at the same doping level, leading to larger base transit times 0268-1242/13/5/005/img8 and hence poorer 0268-1242/13/5/005/img6 performance for large 0268-1242/13/5/005/img7 for an Si/SiGe/Si HBT than for an SiC/Si HBT. At high values of 0268-1242/13/5/005/img7, we demonstrate the formation of a parasitic electron barrier at the base-collector interface which causes a sharp increase in 0268-1242/13/5/005/img8 and hence a dramatic reduction in 0268-1242/13/5/005/img6. Based on a comparison of the height of this parasitic barrier with estimates from an analytical model, we suggest a physical mechanism for base pushout after barrier formation that differs somewhat from that given for the analytical model.

Modeling Creep Effects within SiC/SiC Turbine Components

NASA Technical Reports Server (NTRS)

DiCarlo, J. A.; Lang, J.

2008-01-01

Anticipating the implementation of advanced SiC/SiC ceramic composites into the hot section components of future gas turbine engines, the primary objective of this on-going study is to develop physics-based analytical and finite-element modeling tools to predict the effects of constituent creep on SiC/SiC component service life. A second objective is to understand how to possibly select and manipulate constituent materials, processes, and geometries in order to minimize these effects. In initial studies aimed at SiC/SiC components experiencing through-thickness stress gradients, creep models were developed that allowed an understanding of detrimental residual stress effects that can develop globally within the component walls. It was assumed that the SiC/SiC composites behaved as isotropic visco-elastic materials with temperature-dependent creep behavior as experimentally measured in-plane in the fiber direction of advanced thin-walled 2D SiC/SiC panels. The creep models and their key results are discussed assuming state-of-the-art SiC/SiC materials within a simple cylindrical thin-walled tubular structure, which is currently being employed to model creep-related effects for turbine airfoil leading edges subjected to through-thickness thermal stress gradients. Improvements in the creep models are also presented which focus on constituent behavior with more realistic non-linear stress dependencies in order to predict such key creep-related SiC/SiC properties as time-dependent matrix stress, constituent creep and content effects on composite creep rates and rupture times, and stresses on fiber and matrix during and after creep.

Synthesis and characterization of laminated Si/SiC composites

PubMed Central

Naga, Salma M.; Kenawy, Sayed H.; Awaad, Mohamed; Abd El-Wahab, Hamada S.; Greil, Peter; Abadir, Magdi F.

2012-01-01

Laminated Si/SiC ceramics were synthesized from porous preforms of biogenous carbon impregnated with Si slurry at a temperature of 1500 °C for 2 h. Due to the capillarity infiltration with Si, both intrinsic micro- and macrostructure in the carbon preform were retained within the final ceramics. The SEM micrographs indicate that the final material exhibits a distinguished laminar structure with successive Si/SiC layers. The produced composites show weight gain of ≈5% after heat treatment in air at 1300 °C for 50 h. The produced bodies could be used as high temperature gas filters as indicated from the permeability results. PMID:25685404

Propagation of misfit dislocations from buffer/Si interface into Si

DOEpatents

Liliental-Weber, Zuzanna [El Sobrante, CA; Maltez, Rogerio Luis [Porto Alegre, BR; Morkoc, Hadis [Richmond, VA; Xie, Jinqiao [Raleigh, VA

2011-08-30

Misfit dislocations are redirected from the buffer/Si interface and propagated to the Si substrate due to the formation of bubbles in the substrate. The buffer layer growth process is generally a thermal process that also accomplishes annealing of the Si substrate so that bubbles of the implanted ion species are formed in the Si at an appropriate distance from the buffer/Si interface so that the bubbles will not migrate to the Si surface during annealing, but are close enough to the interface so that a strain field around the bubbles will be sensed by dislocations at the buffer/Si interface and dislocations are attracted by the strain field caused by the bubbles and move into the Si substrate instead of into the buffer epi-layer. Fabrication of improved integrated devices based on GaN and Si, such as continuous wave (CW) lasers and light emitting diodes, at reduced cost is thereby enabled.

Uniform Si nano-dot fabrication using reconstructed structure of Si(110)

NASA Astrophysics Data System (ADS)

Yano, Masahiro; Uozumi, Yuki; Yasuda, Satoshi; Asaoka, Hidehito

2018-06-01

Si nano-dot (ND) formation on Si(110) is observed by means of a scanning tunneling microscope (STM). The initial Si-NDs are Si crystals that are continuous from the substrate and grow during the oxide layer desorption. The NDs fabricated on the flat surface of Si(110)-1 × 1 are surrounded by four types of facets with almost identical appearance probabilities. An increase in the size of the NDs increases the variety of its morphology. In contrast, most Si-NDs fabricated on straight-stepped surface of Si(110)-16 × 2 reconstructed structure are surrounded by only a single type of facet, namely the \\text{Si}(17,15,1)-2 × 1 plane. An appearance probability of the facet in which the base line is along the step of Si(110)-16 × 2 exceeds 75%. This finding provides a fabrication technique of uniformed structural Si-NDs by using the reconstructed structure of Si(110).

Effect of the SiCl₄ Flow Rate on SiBN Deposition Kinetics in SiCl₄-BCl₃-NH₃-H₂-Ar Environment.

PubMed

Li, Jianping; Qin, Hailong; Liu, Yongsheng; Ye, Fang; Li, Zan; Cheng, Laifei; Zhang, Litong

2017-06-07

To improve the thermal and mechanical stability of SiC f /SiC or C/SiC composites with SiBN interphase, SiBN coating was deposited by low pressure chemical vapor deposition (LPCVD) using SiCl₄-BCl₃-NH₃-H₂-Ar gas system. The effect of the SiCl₄ flow rate on deposition kinetics was investigated. Results show that deposition rate increases at first and then decreases with the increase of the SiCl₄ flow rate. The surface of the coating is a uniform cauliflower-like structure at the SiCl₄ flow rate of 10 mL/min and 20 mL/min. The surface is covered with small spherical particles when the flow rate is 30 mL/min. The coatings deposited at various SiCl₄ flow rates are all X-ray amorphous and contain Si, B, N, and O elements. The main bonding states are B-N, Si-N, and N-O. B element and B-N bonding decrease with the increase of SiCl₄ flow rate, while Si element and Si-N bonding increase. The main deposition mechanism refers to two parallel reactions of BCl₃+NH₃ and SiCl₄+NH₃. The deposition process is mainly controlled by the reaction of BCl₃+NH₃.

Comparison of Cyclic Hysteresis Behavior between Cross-Ply C/SiC and SiC/SiC Ceramic-Matrix Composites

PubMed Central

Li, Longbiao

2016-01-01

In this paper, the comparison of cyclic hysteresis behavior between cross-ply C/SiC and SiC/SiC ceramic-matrix composites (CMCs) has been investigated. The interface slip between fibers and the matrix existed in the matrix cracking mode 3 and mode 5, in which matrix cracking and interface debonding occurred in the 0° plies are considered as the major reason for hysteresis loops of cross-ply CMCs. The hysteresis loops of cross-ply C/SiC and SiC/SiC composites corresponding to different peak stresses have been predicted using present analysis. The damage parameter, i.e., the proportion of matrix cracking mode 3 in the entire matrix cracking modes of the composite, and the hysteresis dissipated energy increase with increasing peak stress. The damage parameter and hysteresis dissipated energy of C/SiC composite under low peak stress are higher than that of SiC/SiC composite; However, at high peak stress, the damage extent inside of cross-ply SiC/SiC composite is higher than that of C/SiC composite as more transverse cracks and matrix cracks connect together. PMID:28787861

Comparison of Cyclic Hysteresis Behavior between Cross-Ply C/SiC and SiC/SiC Ceramic-Matrix Composites.

PubMed

Li, Longbiao

2016-01-19

In this paper, the comparison of cyclic hysteresis behavior between cross-ply C/SiC and SiC/SiC ceramic-matrix composites (CMCs) has been investigated. The interface slip between fibers and the matrix existed in the matrix cracking mode 3 and mode 5, in which matrix cracking and interface debonding occurred in the 0° plies are considered as the major reason for hysteresis loops of cross-ply CMCs. The hysteresis loops of cross-ply C/SiC and SiC/SiC composites corresponding to different peak stresses have been predicted using present analysis. The damage parameter, i.e. , the proportion of matrix cracking mode 3 in the entire matrix cracking modes of the composite, and the hysteresis dissipated energy increase with increasing peak stress. The damage parameter and hysteresis dissipated energy of C/SiC composite under low peak stress are higher than that of SiC/SiC composite; However, at high peak stress, the damage extent inside of cross-ply SiC/SiC composite is higher than that of C/SiC composite as more transverse cracks and matrix cracks connect together.

Low-temperature magnetotransport in Si/SiGe heterostructures on 300 mm Si wafers

NASA Astrophysics Data System (ADS)

Scappucci, Giordano; Yeoh, L.; Sabbagh, D.; Sammak, A.; Boter, J.; Droulers, G.; Kalhor, N.; Brousse, D.; Veldhorst, M.; Vandersypen, L. M. K.; Thomas, N.; Roberts, J.; Pillarisetty, R.; Amin, P.; George, H. C.; Singh, K. J.; Clarke, J. S.

Undoped Si/SiGe heterostructures are a promising material stack for the development of spin qubits in silicon. To deploy a qubit into high volume manufacturing in a quantum computer requires stringent control over substrate uniformity and quality. Electron mobility and valley splitting are two key electrical metrics of substrate quality relevant for qubits. Here we present low-temperature magnetotransport measurements of strained Si quantum wells with mobilities in excess of 100000 cm2/Vs fabricated on 300 mm wafers within the framework of advanced semiconductor manufacturing. These results are benchmarked against the results obtained in Si quantum wells deposited on 100 mm Si wafers in an academic research environment. To ensure rapid progress in quantum wells quality we have implemented fast feedback loops from materials growth, to heterostructure FET fabrication, and low temperature characterisation. On this topic we will present recent progress in developing a cryogenic platform for high-throughput magnetotransport measurements.

Controlled formation of GeSi nanostructures on pillar-patterned Si substrate

NASA Astrophysics Data System (ADS)

Zhou, Tong; Zeng, Ceng; Fan, Yongliang; Jiang, Zuimin; Xia, Jinsong; Zhong, Zhenyang; Fudan University Team; Huazhong University of Science; Technology Collaboration

2015-03-01

GeSi quantum nanostructures (QNs) have potential applications in optoelectronic devices due to their unique properties and compatibility with the sophisticated Si technology. However, the disadvantages of poor quantum efficiency of the GeSi QNs on flat Si (001) substrates hinder their optoelectronic applications. Today, numerous growth strategies have been proposed to control the formation of GeSi QNs in hope of improving the optoelectronic performances. One of the ways is to fabricate GeSi QNs on patterned substrates, where the GeSi QNs can be greatly manipulated in aspects of size, shape, composition, orientation and arrangement. Here, self-assembled GeSi QNs on periodic Si (001) sub-micro pillars (SPMs) are systematically studied. By controlling the growth conditions and the diameters of the SPMs, different GeSi QNs, including circularly arranged quantum dots (QDs), quantum rings (QRs), and quantum dot molecules (QDMs), are realized at the top edge of SMPs. Meanwhile, fourfold symmetric GeSi QDMs can be also obtained at the base edges of the SPMs. The promising features of self-assembled GeSi QNs are explained in terms of the surface chemical potential, which disclose the critical effect of surface morphology on the diffusion and the aggregation of Ge adatoms.

siRNAmod: A database of experimentally validated chemically modified siRNAs.

PubMed

Dar, Showkat Ahmad; Thakur, Anamika; Qureshi, Abid; Kumar, Manoj

2016-01-28

Small interfering RNA (siRNA) technology has vast potential for functional genomics and development of therapeutics. However, it faces many obstacles predominantly instability of siRNAs due to nuclease digestion and subsequently biologically short half-life. Chemical modifications in siRNAs provide means to overcome these shortcomings and improve their stability and potency. Despite enormous utility bioinformatics resource of these chemically modified siRNAs (cm-siRNAs) is lacking. Therefore, we have developed siRNAmod, a specialized databank for chemically modified siRNAs. Currently, our repository contains a total of 4894 chemically modified-siRNA sequences, comprising 128 unique chemical modifications on different positions with various permutations and combinations. It incorporates important information on siRNA sequence, chemical modification, their number and respective position, structure, simplified molecular input line entry system canonical (SMILES), efficacy of modified siRNA, target gene, cell line, experimental methods, reference etc. It is developed and hosted using Linux Apache MySQL PHP (LAMP) software bundle. Standard user-friendly browse, search facility and analysis tools are also integrated. It would assist in understanding the effect of chemical modifications and further development of stable and efficacious siRNAs for research as well as therapeutics. siRNAmod is freely available at: http://crdd.osdd.net/servers/sirnamod.

Comparative study of SiC- and Si-based photovoltaic inverters

NASA Astrophysics Data System (ADS)

Ando, Yuji; Oku, Takeo; Yasuda, Masashi; Shirahata, Yasuhiro; Ushijima, Kazufumi; Murozono, Mikio

2017-01-01

This article reports comparative study of 150-300 W class photovoltaic inverters (Si inverter, SiC inverter 1, and SiC inverter 2). In these sub-kW class inverters, the ON-resistance was considered to have little influence on the efficiency. The developed SiC inverters, however, have exhibited an approximately 3% higher direct current (DC)-alternating current (AC) conversion efficiency as compared to the Si inverter. Power loss analysis indicated a reduction in the switching and reverse recovery losses of SiC metal-oxide-semiconductor field-effect transistors used for the DC-AC converter is responsible for this improvement. In the SiC inverter 2, an increase of the switching frequency up to 100 kHz achieved a state-of-the-art combination of the weight (1.25 kg) and the volume (1260 cm3) as a 150-250 W class inverter. Even though the increased switching frequency should cause the increase of the switching losses, the SiC inverter 2 exhibited an efficiency comparable to the SiC inverter 1 with a switching frequency of 20 kHz. The power loss analysis also indicated a decreased loss of the DC-DC converter built with SiC Schottky barrier diodes led to the high efficiency for its increased switching frequency. These results clearly indicated feasibility of SiC devices even for sub-kW photovoltaic inverters, which will be available for the applications where compactness and efficiency are of tremendous importance.

Influence of CO annealing in metal-oxide-semiconductor capacitors with SiO2 films thermally grown on Si and on SiC

NASA Astrophysics Data System (ADS)

Pitthan, E.; dos Reis, R.; Corrêa, S. A.; Schmeisser, D.; Boudinov, H. I.; Stedile, F. C.

2016-01-01

Understanding the influence of SiC reaction with CO, a by-product of SiC thermal oxidation, is a key point to elucidate the origin of electrical defects in SiC metal-oxide-semiconductor (MOS) devices. In this work, the effects on electrical, structural, and chemical properties of SiO2/Si and SiO2/SiC structures submitted to CO annealing were investigated. It was observed that long annealing times resulted in the incorporation of carbon from CO in the Si substrate, followed by deterioration of the SiO2/Si interface, and its crystallization as SiC. Besides, this incorporated carbon remained in the Si surface (previous SiO2/Si region) after removal of the silicon dioxide film by HF etching. In the SiC case, an even more defective surface region was observed due to the CO interaction. All MOS capacitors formed using both semiconductor materials presented higher leakage current and generation of positive effective charge after CO annealings. Such results suggest that the negative fixed charge, typically observed in SiO2/SiC structures, is not originated from the interaction of the CO by-product, formed during SiC oxidation, with the SiO2/SiC interfacial region.

Powder metallurgy of Ge, Si, and Ge-Si

NASA Astrophysics Data System (ADS)

Schilz, Jürgen; Langenbach, Marion

1993-03-01

id="ab1"Planetary ball-milling and pressing behaviour of Ge, Si and Ge-Si powder mixtures are investigated. Scanning and transmission electron microscopy observations revealed the different microstructure of the two elements after milling: Ge remains in a microcrystalline state, whereas Si can be comminuted into grains consisting of nanocrystalline regions. Planetary milling of the two elements together, using agate balls and vial, did not reveal any compound formation. By hot-isostatic pressing, pure Ge and Ge-Si mixtures were densified to a higher value than pure Si. This denotes a plastic flow of the Ge component at a process temperature of 800°C. The microhardness of hot-pressed Ge reaches the bulk value; hot-pressed Si is very soft. Energy dispersive X-ray analysis and X-ray diffraction did not detect any impurity contamination from vial and milling media wear. Moreover, by electrical transport measurements it turned out that the net carrier concentration density resulting from electrical active impurities introduced by the milling and pressing process is below 2 x 1016 cm 3 at room temperature.

Tunable Synthesis of SiC/SiO2 Heterojunctions via Temperature Modulation

PubMed Central

Li, Wei; Yang, Daoyuan; Liu, Xinhong

2018-01-01

A large-scale production of necklace-like SiC/SiO2 heterojunctions was obtained by a molten salt-mediated chemical vapor reaction technique without a metallic catalyst or flowing gas. The effect of the firing temperature on the evolution of the phase composition, microstructure, and morphology of the SiC/SiO2 heterojunctions was studied. The necklace-like SiC/SiO2 nanochains, several centimeters in length, were composed of SiC/SiO2 core-shell chains and amorphous SiO2 beans. The morphologies of the as-prepared products could be tuned by adjusting the firing temperature. In fact, the diameter of the SiO2 beans decreased, whereas the diameter of the SiC fibers and the thickness of the SiO2 shell increased as the temperature increased. The growth mechanism of the necklace-like structure was controlled by the vapor-solid growth procedure and the modulation procedure via a molten salt-mediated chemical vapor reaction process. PMID:29748482

(113) Facets of Si-Ge/Si Islands; Atomic Scale Simulation

NASA Astrophysics Data System (ADS)

Kassem, Hassan

We have studied, by computer simulation, some static and vibrationnal proprieties of SiGe/Si islands. We have used a Valence Force Field combined to Monte Carlo technique to study the growth of Ge and SiGe on (001)Si substrates. We have focalised on the case of large pyramidal islands presenting (113) facets on the free (001)Si surface with various non uniform composition inside the islands. The deformation inside the islands and Raman spectroscopy are discussed.

Si1-yCy/Si(001) gas-source molecular beam epitaxy from Si2H6 and CH3SiH3: Surface reaction paths and growth kinetics

NASA Astrophysics Data System (ADS)

Foo, Y. L.; Bratland, K. A.; Cho, B.; Desjardins, P.; Greene, J. E.

2003-04-01

In situ surface probes and postdeposition analyses were used to follow surface reaction paths and growth kinetics of Si1-yCy alloys grown on Si(001) by gas-source molecular-beam epitaxy from Si2H6/CH3SiH3 mixtures as a function of C concentration y (0-2.6 at %) and temperature Ts (500-600 °C). High-resolution x-ray diffraction reciprocal lattice maps show that all layers are in tension and fully coherent with their substrates. Film growth rates R decrease with both y and Ts, and the rate of decrease in R as a function of y increases rapidly with Ts. In situ isotopically tagged D2 temperature-programmed desorption (TPD) measurements reveal that C segregation during steady-state Si1-yCy(001) growth results in charge transfer from Si surface dangling bonds to second-layer C atoms, which have a higher electronegativity than Si. From the TPD results, we obtain the coverage θSi*(y,Ts) of Si* surface sites with C backbonds as well as H2 desorption energies Ed from both Si and Si* surface sites. θSi* increases with increasing y and Ts in the kinetically limited segregation regime while Ed decreases from 2.52 eV for H2 desorption from Si surface sites with Si back bonds to 2.22 eV from Si* surface sites. This leads to an increase in the H2 desorption rate, and hence should yield higher film deposition rates, with increasing y and/or Ts during Si1-yCy(001) growth. The effect, however, is more than offset by the decrease in Si2H6 reactive sticking probabilities at Si* surface sites. Film growth rates R(Ts,JSi2H6,JCH3SiH3) calculated using a simple transition-state kinetic model, together with measured kinetic parameters, were found to be in excellent agreement with the experimental data.

Structural transformation of Si-rich SiNx film on Si via swift heavy ions irradiation

NASA Astrophysics Data System (ADS)

Murzalinov, D.; Akilbekov, A.; Dauletbekova, A.; Vlasukova, L.; Makhavikov, M.; Zdorovets, M.

2018-03-01

The effects of 200 MeV-Xe+ irradiation with fluencies of (109–1014) cm‑2 on the phase-structural transformation of Si-rich SiNx film deposited on Si substrate by low-pressure chemical vapor deposition have been reported. It has been shown from Raman scattering data that the swift heavy ions irradiation results in the dissolution of amorphous Si nanoclusters in nitride matrix. It has been shown, too, that the swift heavy ion irradiation leads to quenching a visual photoluminescence from nitride films.

The reduction of critical H implantation dose for ion cut by incorporating B-doped SiGe/Si superlattice into Si substrate

NASA Astrophysics Data System (ADS)

Xue, Zhongying; Chen, Da; Jia, Pengfei; Wei, Xing; Di, Zengfeng; Zhang, Miao

2016-11-01

An approach to achieve Si or SiGe film exfoliation with as low as 3 × 1016/cm2 H implantation dose was investigated. Two intrinsic Si0.75Ge0.25/Si samples, merged with B-doped Si0.75Ge0.25 layer and B-doped Si0.75Ge0.25/Si superlattice (SL) layer respectively, were used to study the formation of crack after 3 × 1016/cm2 H implantation and annealing. For the sample into which B doped Si0.75Ge0.25 layer is incorporated, only few discrete cracks are observed along both sides of the B doped Si0.75Ge0.25 layer; on the contrary, a continuous (100) oriented crack is formed in the B-doped Si0.75Ge0.25/Si SL layer, which means ion cut can be achieved using this material with 3 × 1016/cm2 H implantation. As the SIMS profiles confirm that hydrogen tends to be trapped at B-doped SiGe/Si interface, the formation of continuous crack in SL layer can be ascribed to the more efficient hydrogen trapping by the multiple B-doped SiGe/Si interfaces.

Electron mobility enhancement in epitaxial multilayer Si-Si/1-x/Ge/x/ alloy films on /100/Si

NASA Technical Reports Server (NTRS)

Manasevit, H. M.; Gergis, I. S.; Jones, A. B.

1982-01-01

Enhanced Hall-effect mobilities have been measured in epitaxial (100)-oriented multilayer n-type Si/Si(1-x)Ge(x) films grown on single-crystal Si substrates by chemical vapor deposition. Mobilities from 20 to 40% higher than that of epitaxial Si layers and about 100% higher than that of epitaxial SiGe layers on Si were measured for the doping range 8 x 10 to the 15th to 10 to the 17th/cu cm. No mobility enhancement was observed in multilayer p-type (100) films and n-type (111)-oriented films. Experimental studies included the effects upon film properties of layer composition, total film thickness, doping concentrations, layer thickness, and growth temperature.

Elettrodinamica classica

NASA Astrophysics Data System (ADS)

Lechner, Kurt

Nella scoperta della Relatività Ristretta l'Elettrodinamica, rappresentando una teoria relativistica per eccellenza, ha giocato un ruolo fondamentale. Il principio di relativit` a einsteiniana, che afferma che tutte le leggi della fisica devono avere la stessa forma in tutti i sistemi di riferimento inerziali, è emerso con forza da questa teoria ed è andato consolidandosi sempre di più, man mano che le nostre conoscenze del mondo microscopico sono diventate più complete: tutte le interazioni fondamentali rispettano infatti tale principio. Il modo più semplice ed elegante per implementarlo — difatti l'unico di un'utilità concreta — è rappresentato dal paradigma della covarianza a vista nell'ambito del calcolo tensoriale. Questo paradigma è stato applicato con successo a tutte le teorie di carattere fondamentale, come le teorie che descrivono le quattro interazioni fondamentali e le più speculative teorie di superstringa, e mantiene la sua piena efficacia anche in teoria quantistica. La nostra esposizione dell'Elettrodinamica classica si baserà dunque a ragione su questo paradigma.

An Isotope Study of Hydrogenation of poly-Si/SiOx Passivated Contacts for Si Solar Cells: Preprint

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

Schnabel, Manuel; Nemeth, William; van de Loo, Bas, W.H.

2017-06-26

For many years, the record Si solar cell efficiency stood at 25.0%. Only recently have several companies and institutes managed to produce more efficient cells, using passivated contacts of made doped poly-Si or a-Si:H and a passivating intrinsic interlayer in all cases. Common to these designs is the need to passivate the layer stack with hydrogen. In this contribution, we perform a systematic study of passivated contact passivation by hydrogen, using poly-Si/SiOx passivated contacts on n-Cz-Si, and ALD Al2O3 followed by a forming gas anneal (FGA) as the hydrogen source. We study p-type and n-type passivated contacts with implied Vocmore » exceeding 690 and 720 mV, respectively, and perform either the ALD step or the FGA with deuterium instead of hydrogen in order to separate the two processes via SIMS. By examining the deuterium concentration at the SiOx in both types of samples, we demonstrate that the FGA supplies negligible hydrogen species to the SiOx, regardless of whether the FGA is hydrogenated or deuterated. Instead, it supplies the thermal energy needed for hydrogen species in the Al2O3 to diffuse there. Furthermore, the concentration of hydrogen species at the SiOx can saturate while implied Voc continues to increase, showing that the energy from the FGA is also required for hydrogen species already at the SiOx to find recombination-active defects to passivate.« less

Pseudomorphic GeSiSn, SiSn and Ge layers in strained heterostructures

NASA Astrophysics Data System (ADS)

Timofeev, V. A.; Nikiforov, A. I.; Tuktamyshev, A. R.; Mashanov, V. I.; Loshkarev, I. D.; Bloshkin, A. A.; Gutakovskii, A. K.

2018-04-01

The GeSiSn, SiSn layer growth mechanisms on Si(100) were investigated and the kinetic diagrams of the morphological GeSiSn, SiSn film states in the temperature range of 150 °C-450 °C at the tin content from 0% to 35% were built. The phase diagram of the superstructural change on the surface of Sn grown on Si(100) in the annealing temperature range of 0 °C-850 °C was established. The specular beam oscillations were first obtained during the SiSn film growth from 150 °C to 300 °C at the Sn content up to 35%. The transmission electron microscopy and x-ray diffractometry data confirm the crystal perfection and the pseudomorphic GeSiSn, SiSn film state, and also the presence of smooth heterointerfaces between GeSiSn or SiSn and Si. The photoluminescence for the multilayer periodic GeSiSn/Si structures in the range of 0.6-0.8 eV was detected. The blue shift with the excitation power increase is observed suggesting the presence of a type II heterostructure. The creation of tensile strained Ge films, which are pseudomorphic to the underlying GeSn layer, is confirmed by the results of the formation and analysis of the reciprocal space map in the x-ray diffractometry. The tensile strain in the Ge films reached the value in the range of 0.86%-1.5%. The GeSn buffer layer growth in the Sn content range from 8% to 12% was studied. The band structure of heterosystems based on pseudomorphic GeSiSn, SiSn and Ge layers was calculated and the valence and conduction band subband position dependences on the Sn content were built. Based on the calculation, the Sn content range in the GeSiSn, SiSn, and GeSn layers, which corresponds to the direct bandgap GeSiSn, SiSn, and Ge material, was obtained.

Density-functional theory molecular dynamics simulations of a-HfO2/a-SiO2/SiGe and a-HfO2/a-SiO2/Ge with a-SiO2 and a-SiO suboxide interfacial layers

NASA Astrophysics Data System (ADS)

Chagarov, Evgueni A.; Kavrik, Mahmut S.; Fang, Ziwei; Tsai, Wilman; Kummel, Andrew C.

2018-06-01

Comprehensive Density-Functional Theory (DFT) Molecular Dynamics (MD) simulations were performed to investigate interfaces between a-HfO2 and SiGe or Ge semiconductors with fully-stoichiometric a-SiO2 or sub-oxide SiO interlayers. The electronic structure of the selected stacks was calculated with a HSE06 hybrid functional. Simulations were performed before and after hydrogen passivation of residual interlayer defects. For the SiGe substrate with Ge termination prior to H passivation, the stacks with a-SiO suboxide interlayer (a-HfO2/a-SiO/SiGe) demonstrate superior electronic properties and wider band-gaps than the stacks with fully coordinated a-SiO2 interlayers (a-HfO2/a-SiO2/SiGe). After H passivation, most of the a-HfO2/a-SiO2/SiGe defects are passivated. To investigate effect of random placement of Si and Ge atoms additional simulations with a randomized SiGe slab were performed demonstrating improvement of electronic structure. For Ge substrates, before H passivation, the stacks with a SiO suboxide interlayer (a-HfO2/a-SiO/Ge) also demonstrate wider band-gaps than the stacks with fully coordinated a-SiO2 interlayers (a-HfO2/a-SiO2/Ge). However, even for a-HfO2/a-SiO/Ge, the Fermi level is shifted close to the conduction band edge (CBM) consistent with Fermi level pinning. Again, after H passivation, most of the a-HfO2/a-SiO2/Ge defects are passivated. The stacks with fully coordinated a-SiO2 interlayers have much stronger deformation and irregularity in the semiconductor (SiGe or Ge) upper layers leading to multiple under-coordinated atoms which create band-edge states and decrease the band-gap prior to H passivation.

In vitro Cytotoxicity and Anti-herpes Simplex Virus Type 1 Activity of Hydroethanolic Extract, Fractions, and Isolated Compounds from Stem Bark of Schinus terebinthifolius Raddi.

PubMed

Nocchi, Samara Requena; de Moura-Costa, Gislaine Franco; Novello, Claudio Roberto; Rodrigues, Juliana; Longhini, Renata; de Mello, João Carlos Palazzo; Filho, Benedito Prado Dias; Nakamura, Celso Vataru; Ueda-Nakamura, Tânia

2016-01-01

Herpes simplex virus type 1 (HSV-1) is associated with orofacial infections and is transmitted by direct contact with infected secretions. Several efforts have been expended in the search for drugs to the treatment for herpes. Schinus terebinthifolius is used in several illnesses and among them, for the topical treatment of skin wounds, especially wounds of mucous membranes, whether infected or not. To evaluate the cytotoxicity and anti-HSV-1 activity of the crude hydroethanolic extract (CHE) from the stem bark of S. terebinthifolius, as well as its fractions and isolated compounds. The CHE was subjected to bioguided fractionation. The anti-HSV-1 activity and the cytotoxicity of the CHE, its fractions, and isolated compounds were evaluated in vitro by SRB method. A preliminar investigation of the action of CHE in the virus-host interaction was conducted by the same assay. CHE presented flavan-3-ols and showed anti-HSV-1 activity, better than its fractions and isolated compounds. The class of substances found in CHE can bind to proteins to form unstable complexes and enveloped viruses, as HSV-1 may be vulnerable to this action. Our results suggest that the CHE interfered with virion envelope structures, masking viral receptors that are necessary for adsorption or entry into host cells. The plant investigated exhibited potential for future development treatment against HSV-1, but further tests are necessary, especially to elucidate the mechanism of action of CHE, as well as preclinical and clinical studies to confirm its safety and efficacy. Crude hydroethanolic extract (CHE) presents promising activity against herpes simplex virus type 1 (HSV 1), with selectivity index (SI) = 22.50CHE has flavan-3-ols in its composition, such as catechin and gallocatechinThe fractions and isolated compounds obtained from CHE by bioguided fractionation are less active than the CHE against HSV-1CHE interferes with viral entry process in the host cell and acts directly on the viral

In vitro Cytotoxicity and Anti-herpes Simplex Virus Type 1 Activity of Hydroethanolic Extract, Fractions, and Isolated Compounds from Stem Bark of Schinus terebinthifolius Raddi

PubMed Central

Nocchi, Samara Requena; de Moura-Costa, Gislaine Franco; Novello, Claudio Roberto; Rodrigues, Juliana; Longhini, Renata; de Mello, João Carlos Palazzo; Filho, Benedito Prado Dias; Nakamura, Celso Vataru; Ueda-Nakamura, Tânia

2016-01-01

Background: Herpes simplex virus type 1 (HSV-1) is associated with orofacial infections and is transmitted by direct contact with infected secretions. Several efforts have been expended in the search for drugs to the treatment for herpes. Schinus terebinthifolius is used in several illnesses and among them, for the topical treatment of skin wounds, especially wounds of mucous membranes, whether infected or not. Objective: To evaluate the cytotoxicity and anti-HSV-1 activity of the crude hydroethanolic extract (CHE) from the stem bark of S. terebinthifolius, as well as its fractions and isolated compounds. Materials and Methods: The CHE was subjected to bioguided fractionation. The anti-HSV-1 activity and the cytotoxicity of the CHE, its fractions, and isolated compounds were evaluated in vitro by SRB method. A preliminar investigation of the action of CHE in the virus–host interaction was conducted by the same assay. Results: CHE presented flavan-3-ols and showed anti-HSV-1 activity, better than its fractions and isolated compounds. The class of substances found in CHE can bind to proteins to form unstable complexes and enveloped viruses, as HSV-1 may be vulnerable to this action. Our results suggest that the CHE interfered with virion envelope structures, masking viral receptors that are necessary for adsorption or entry into host cells. Conclusion: The plant investigated exhibited potential for future development treatment against HSV-1, but further tests are necessary, especially to elucidate the mechanism of action of CHE, as well as preclinical and clinical studies to confirm its safety and efficacy. SUMMARY Crude hydroethanolic extract (CHE) presents promising activity against herpes simplex virus type 1 (HSV 1), with selectivity index (SI) = 22.50CHE has flavan-3-ols in its composition, such as catechin and gallocatechinThe fractions and isolated compounds obtained from CHE by bioguided fractionation are less active than the CHE against HSV-1CHE interferes

SiGe layer thickness effect on the structural and optical properties of well-organized SiGe/SiO2 multilayers

NASA Astrophysics Data System (ADS)

Vieira, E. M. F.; Toudert, J.; Rolo, A. G.; Parisini, A.; Leitão, J. P.; Correia, M. R.; Franco, N.; Alves, E.; Chahboun, A.; Martín-Sánchez, J.; Serna, R.; Gomes, M. J. M.

2017-08-01

In this work, we report on the production of regular (SiGe/SiO2)20 multilayer structures by conventional RF-magnetron sputtering, at 350 °C. Transmission electron microscopy, scanning transmission electron microscopy, raman spectroscopy, and x-ray reflectometry measurements revealed that annealing at a temperature of 1000 °C leads to the formation of SiGe nanocrystals between SiO2 thin layers with good multilayer stability. Reducing the nominal SiGe layer thickness (t SiGe) from 3.5-2 nm results in a transition from continuous SiGe crystalline layer (t SiGe ˜ 3.5 nm) to layers consisting of isolated nanocrystals (t SiGe ˜ 2 nm). Namely, in the latter case, the presence of SiGe nanocrystals ˜3-8 nm in size, is observed. Spectroscopic ellipsometry was applied to determine the evolution of the onset in the effective optical absorption, as well as the dielectric function, in SiGe multilayers as a function of the SiGe thickness. A clear blue-shift in the optical absorption is observed for t SiGe ˜ 2 nm multilayer, as a consequence of the presence of isolated nanocrystals. Furthermore, the observed near infrared values of n = 2.8 and k = 1.5 are lower than those of bulk SiGe compounds, suggesting the presence of electronic confinement effects in the nanocrystals. The low temperature (70 K) photoluminescence measurements performed on annealed SiGe/SiO2 nanostructures show an emission band located between 0.7-0.9 eV associated with the development of interface states between the formed nanocrystals and surrounding amorphous matrix.

Disilane-based cyclic deposition/etch of Si, Si:P and Si1-yCy:P layers: I. The elementary process steps

NASA Astrophysics Data System (ADS)

Hartmann, J. M.; Benevent, V.; Barnes, J. P.; Veillerot, M.; Deguet, C.

2013-02-01

We have benchmarked the 550 °C, 20 Torr growth of Si:P and Si1-yCy:P using SiH4 and Si2H6. P segregation has prevented us from reaching P+ ion concentrations in Si higher than a few 1019 cm-3 using SiH4; the resulting surface ‘poisoning’ led to a severe growth rate reduction. Meanwhile, [P+] increased linearly with the phosphine flow when using Si2H6 as the Si precursor; values as high as 1.7 × 1020 cm-3 were obtained. The Si:P growth rate using Si2H6 was initially stable then increased as the PH3 flow increased. Mono-methylsilane flows 6.5-10 times higher were needed with Si2H6 than with SiH4 to reach the same substitutional C concentrations in intrinsic Si1-yCy layers ([C]subst. up to 1.9%). Growth rates were approximately six times higher with Si2H6 than with SiH4, however. 30 nm thick Si1-yCy layers became rough as [C]subst. exceeded 1.6% (formation of increasing numbers of islands). We have also studied the structural and electrical properties of ‘low’ and ‘high’ C content Si1-yCy:P layers (˜ 1.5 and 1.8%, respectively) grown with Si2H6. Adding significant amounts of PH3 led to a reduction of the tensile strain in the films. This was due to the incorporation of P atoms (at the expense of C atoms) in the substitutional sites of the Si matrix. Si1-yCy:P layers otherwise became rough as the PH3 flow increased. Resistivities lower than 1 mΩ cm were nevertheless associated with those Si1-yCy:P layers, with P atomic concentrations at most 3.9 × 1020 cm-3. Finally, we have quantified the beneficial impact of adding GeH4 to HCl for the low-temperature etching of Si. Etch rates 12-36 times higher with HCl + GeH4 than with pure HCl were achieved at 20 Torr. Workable etch rates close to 1 nm min-1 were obtained at 600 °C (versus 750 °C for pure HCl), enabling low-temperature cyclic deposition/etch strategies for the selective epitaxial growth of Si, Si:P and Si1-yCy:P layers on patterned wafers.

Hydrogen passivation of poly-Si/SiOx contacts for Si solar cells using Al2O3 studied with deuterium

NASA Astrophysics Data System (ADS)

Schnabel, Manuel; van de Loo, Bas W. H.; Nemeth, William; Macco, Bart; Stradins, Paul; Kessels, W. M. M.; Young, David L.

2018-05-01

The interplay between hydrogenation and passivation of poly-Si/SiOx contacts to n-type Si wafers is studied using atomic layer deposited Al2O3 and anneals in forming gas and nitrogen. The poly-Si/SiOx stacks are prepared by thermal oxidation followed by thermal crystallization of a-Si:H films deposited by plasma-enhanced chemical vapor deposition. Implied open-circuit voltages as high as 710 mV are achieved for p-type poly-Si/SiOx contacts to n-type Si after hydrogenation. Correlating minority carrier lifetime data and secondary ion mass spectrometry profiles reveals that the main benefit of Al2O3 is derived from its role as a hydrogen source for chemically passivating defects at SiOx; Al2O3 layers are found to hydrogenate poly-Si/SiOx much better than a forming gas anneal. By labelling Al2O3 and the subsequent anneal with different hydrogen isotopes, it is found that Al2O3 exchanges most of its hydrogen with the ambient upon annealing at 400 °C for 1 h even though there is no significant net change in its total hydrogen content.

A comparative study of three-terminal Hanle signals in CoFe/SiO{sub 2}/n{sup +}-Si and Cu/SiO{sub 2}/n{sup +}-Si tunnel junctions

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

Lee, Jeong-Hyeon; Cho, B. K., E-mail: chobk@gist.ac.kr; Grünberg Center for Magnetic Nanomaterials, Gwangju Institute of Science and Technology

We performed three-terminal (3T) Hanle measurement for two types of sample series, CoFe/SiO{sub 2}/n{sup +}-Si and Cu/SiO{sub 2}/n{sup +}-Si, with various tunnel resistances. Clear Hanle signal and anomalous scaling between spin resistance-area product and tunnel resistance-area product were observed in CoFe/SiO{sub 2}/n{sup +}-Si devices. In order to explore the origin of the Hanle signal and the impurity-assisted tunneling effect on the Hanle signal in our devices, Hanle measurement in Cu/SiO{sub 2}/n{sup +}-Si devices was performed as well. However, no detectable Hanle signal was observed in Cu/SiO{sub 2}/n{sup +}-Si, even though a lot of samples with various tunnel resistances were studiedmore » in wide temperature and bias voltage ranges. Through a comparative study, it is found that the impurity-assisted tunneling magnetoresistance mechanism would not play a dominant role in the 3T Hanle signal in CoFe/SiO{sub 2}/n{sup +}-Si tunnel junctions, where the SiO{sub 2} was formed by plasma oxidation to minimize impurities.« less

General Synthetic Method for Si-Fluoresceins and Si-Rhodamines

PubMed Central

2017-01-01

The century-old fluoresceins and rhodamines persist as flexible scaffolds for fluorescent and fluorogenic compounds. Extensive exploration of these xanthene dyes has yielded general structure–activity relationships where the development of new probes is limited only by imagination and organic chemistry. In particular, replacement of the xanthene oxygen with silicon has resulted in new red-shifted Si-fluoresceins and Si-rhodamines, whose high brightness and photostability enable advanced imaging experiments. Nevertheless, efforts to tune the chemical and spectral properties of these dyes have been hindered by difficult synthetic routes. Here, we report a general strategy for the efficient preparation of Si-fluoresceins and Si-rhodamines from readily synthesized bis(2-bromophenyl)silane intermediates. These dibromides undergo metal/bromide exchange to give bis-aryllithium or bis(aryl Grignard) intermediates, which can then add to anhydride or ester electrophiles to afford a variety of Si-xanthenes. This strategy enabled efficient (3–5 step) syntheses of known and novel Si-fluoresceins, Si-rhodamines, and related dye structures. In particular, we discovered that previously inaccessible tetrafluorination of the bottom aryl ring of the Si-rhodamines resulted in dyes with improved visible absorbance in solution, and a convenient derivatization through fluoride-thiol substitution. This modular, divergent synthetic method will expand the palette of accessible xanthenoid dyes across the visible spectrum, thereby pushing further the frontiers of biological imaging. PMID:28979939

Si

NASA Astrophysics Data System (ADS)

Fiameni, S.; Famengo, A.; Agresti, F.; Boldrini, S.; Battiston, S.; Saleemi, M.; Johnsson, M.; Toprak, M. S.; Fabrizio, M.

2014-06-01

Magnesium silicide (Mg2Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion in the middle-high temperature range. The detrimental effect of the presence of MgO on the TE properties of Mg2Si based materials is widely known. For this reason, the conditions used for synthesis and sintering were optimized to limit oxygen contamination. The effect of Bi doping on the TE performance of dense Mg2Si materials was also investigated. Synthesis was performed by ball milling in an inert atmosphere starting from commercial Mg2Si powder and Bi powder. The samples were consolidated, by spark plasma sintering, to a density >95%. The morphology, and the composition and crystal structure of samples were characterized by field-emission scanning electronic microscopy and x-ray diffraction, respectively. Moreover, determination of Seebeck coefficients and measurement of electrical and thermal conductivity were performed for all the samples. Mg2Si with 0.1 mol% Bi doping had a ZT value of 0.81, indicative of the potential of this method for fabrication of n-type bulk material with good TE performance.

p-n Junction Diodes Fabricated on Si-Si/Ge Heteroepitaxial Films

NASA Technical Reports Server (NTRS)

Das, K.; Mazumder, M. D. A.; Hall, H.; Alterovitz, Samuel A. (Technical Monitor)

2000-01-01

A set of photolithographic masks was designed for the fabrication of diodes in the Si-Si/Ge material system. Fabrication was performed on samples obtained from two different wafers: (1) a complete HBT structure with an n (Si emitter), p (Si/Ge base), and an n/n+ (Si collector/sub-collector) deposited epitaxially (MBE) on a high resistivity p-Si substrate, (2) an HBT structure where epitaxial growth was terminated after the p-type base (Si/Ge) layer deposition. Two different process runs were attempted for the fabrication of Si-Si/Ge (n-p) and Si/Ge-Si (p-n) junction diodes formed between the emitter-base and base-collector layers, respectively, of the Si-Si/Ge-Si HBT structure. One of the processes employed a plasma etching step to expose the p-layer in the structure (1) and to expose the e-layer in structure (2). The Contact metallization used for these diodes was a Cu-based metallization scheme that was developed during the first year of the grant. The plasma-etched base-collector diodes on structure (2) exhibited well-behaved diode-like characteristics. However, the plasma-etched emitter-base diodes demonstrated back-to-back diode characteristics. These back-to back characteristics were probably due to complete etching of the base-layer, yielding a p-n-p diode. The deep implantation process yielded rectifying diodes with asymmetric forward and reverse characteristics. The ideality factor of these diodes were between 1.6 -2.1, indicating that the quality of the MBE grown epitaxial films was not sufficiently high, and also incomplete annealing of the implantation damage. Further study will be conducted on CVD grown films, which are expected to have higher epitaxial quality.

Effect of germanium concentrations on tunnelling current calculation of Si/Si1-xGex/Si heterojunction bipolar transistor

NASA Astrophysics Data System (ADS)

Hasanah, L.; Suhendi, E.; Khairrurijal

2018-05-01

Tunelling current calculation on Si/Si1-xGex/Si heterojunction bipolar transistor was carried out by including the coupling between transversal and longitudinal components of electron motion. The calculation results indicated that the coupling between kinetic energy in parallel and perpendicular to S1-xGex barrier surface affected tunneling current significantly when electron velocity was faster than 1x105 m/s. This analytical tunneling current model was then used to study how the germanium concentration in base to Si/Si1-xGex/Si heterojunction bipolar transistor influenced the tunneling current. It is obtained that tunneling current increased as the germanium concentration given in base decreased.

Novel activation domain derived from Che-1 cofactor coupled with the artificial protein Jazz drives utrophin upregulation.

PubMed

Desantis, Agata; Onori, Annalisa; Di Certo, Maria Grazia; Mattei, Elisabetta; Fanciulli, Maurizio; Passananti, Claudio; Corbi, Nicoletta

2009-02-01

Our aim is to upregulate the expression level of the dystrophin related gene utrophin in Duchenne muscular dystrophy, thus complementing the lack of dystrophin functions. To this end, we have engineered synthetic zinc finger based transcription factors. We have previously shown that the artificial three-zinc finger protein named Jazz fused with the Vp16 activation domain, is able to bind utrophin promoter A and to increase the endogenous level of utrophin in transgenic mice. Here, we report on an innovative artificial protein, named CJ7, that consists of Jazz DNA binding domain fused to a novel activation domain derived from the regulatory multivalent adaptor protein Che-1/AATF. This transcriptional activation domain is 100 amino acids in size and it is very powerful as compared to the Vp16 activation domain. We show that CJ7 protein efficiently promotes transcription and accumulation of the acetylated form of histone H3 on the genomic utrophin promoter locus.

Leakage current and capacitance characteristics of Si/SiO2/Si single-barrier varactor

NASA Astrophysics Data System (ADS)

Mamor, M.; Fu, Y.; Nur, O.; Willander, M.; Bengtsson, S.

We investigate, both experimentally and theoretically, current and capacitance (I-V/C-V) characteristics and the device performance of Si/SiO2/Si single-barrier varactor diodes (SBVs). Two diodes were fabricated with different SiO2 layer thicknesses using the state-of-the-art wafer bonding technique. The devices have very low leakage currents (about 5×10-2 and 1.8×10-2 mA/mm2) and intrinsic capacitance levels of typically 1.5 and 50 nF/mm2 for diodes with 5-nm and 20-nm oxide layers, respectively. With the present device physical parameters (25-mm2 device area, 760-μm modulation layer thickness and 1015-cm-3 doping level), the estimated cut-off frequency is about 5×107 Hz. With the physical parameters of the present existing III-V triplers, the cut-off frequency of our Si-based SBV can be as high as 0.5 THz.

Positron annihilation at the Si/SiO2 interface

NASA Astrophysics Data System (ADS)

Leung, T. C.; Weinberg, Z. A.; Asoka-Kumar, P.; Nielsen, B.; Rubloff, G. W.; Lynn, K. G.

1992-01-01

Variable-energy positron annihilation depth-profiling has been applied to the study of the Si/SiO2 interface in Al-gate metal-oxide-semiconductor (MOS) structures. For both n- and p-type silicon under conditions of negative gate bias, the positron annihilation S-factor characteristic of the interface (Sint) is substantially modified. Temperature and annealing behavior, combined with known MOS physics, suggest strongly that Sint depends directly on holes at interface states or traps at the Si/SiO2 interface.

Optical and Structural Properties of Si Nanocrystals in SiO2 Films.

PubMed

Nikitin, Timur; Khriachtchev, Leonid

2015-04-22

Optical and structural properties of Si nanocrystals (Si-nc) in silica films are described. For the SiOx (x < 2) films annealed above 1000 °C, the Raman signal of Si-nc and the absorption coefficient are proportional to the amount of elemental Si detected by X-ray photoelectron spectroscopy. A good agreement is found between the measured refractive index and the value estimated by using the effective-medium approximation. The extinction coefficient of elemental Si is found to be between the values of crystalline and amorphous Si. Thermal annealing increases the degree of Si crystallization; however, the crystallization and the Si-SiO2 phase separation are not complete after annealing at 1200 °C. The 1.5-eV PL quantum yield increases as the amount of elemental Si decreases; thus, this PL is probably not directly from Si-nc responsible for absorption and detected by Raman spectroscopy. Continuous-wave laser light can produce very high temperatures in the free-standing films, which changes their structural and optical properties. For relatively large laser spots, the center of the laser-annealed area is very transparent and consists of amorphous SiO2. Large Si-nc (up to ∼300 nm in diameter) are observed in the ring around the central region. These Si-nc lead to high absorption and they are typically under compressive stress, which is connected with their formation from the liquid phase. By using strongly focused laser beams, the structural changes in the free-standing films can be made in submicron areas.

Light-emitting Si nanostructures formed by swift heavy ions in a-Si:H/SiO2 multilayer heterostructures

NASA Astrophysics Data System (ADS)

Cherkova, S. G.; Volodin, V. A.; Cherkov, A. G.; Antonenko, A. Kh; Kamaev, G. N.; Skuratov, V. A.

2017-08-01

Light-emitting nanoclusters were formed in Si/SiO2 multilayer structures irradiated with 167 MeV Xe ions to the doses of 1011-3  ×  1014 cm-2 and annealed in the forming-gas at 500 °C and in nitrogen at 800-1100 °C, 30 min. The thicknesses were ~4 nm or ~7-8 for the Si, and ~10 nm for the SiO2 layers. The structures were studied using photoluminescence (PL), Raman spectroscopy, and the cross-sectional high resolution transmission electron microscopy (HRTEM). As-irradiated samples showed the PL, correlating with the growth of the ion doses. HRTEM found the layers to be partly disintegrated. The thickness of the amorphous Si layer was crucial. For 4 nm thick Si layers the PL was peaking at ~490 nm, and quenched by the annealing. It was ascribed to the structural imperfections. For the thicker Si layers the PL was peaking at ~600 nm and was attributed to the Si-rich nanoclusters in silicon oxide. The annealing increases the PL intensity and shifts the band to ~790 nm, typical of Si nanocrystals. Its intensity was proportional to the dose. Raman spectra confirmed the nanocrystals formation. All the results obtained evidence the material melting in the tracks for 10-11-10-10 s providing thereby fast diffusivities of the atoms. The thicker Si layers provide more excess Si to create the nanoclusters via a molten state diffusion.

Subsurface Growth of CoSi2 by Deposition of Co on Si-Capped CoSi2 Seed Regions

NASA Technical Reports Server (NTRS)

Fathauer, R. W.; George, T.; Pike, W. T.

1991-01-01

At a growth temperature of 800 C, Co deposited on Si(111) diffuses through a Si cap and exhibits oriented growth on buried CoSi2 grains, a process referred to as endotaxy. This occurs preferentially to surface nucleation of CoSi2 provided the thickness of the Si cap is less than a critical value between 100 and 200 nm for a deposition rate of 0.01 nm/s. Steady-state endotaxy is modeled under the assumption that the process is controlled by Co diffusion.

Effect of PECVD SiNx/SiOyNx-Si interface property on surface passivation of silicon wafer

NASA Astrophysics Data System (ADS)

Jia, Xiao-Jie; Zhou, Chun-Lan; Zhu, Jun-Jie; Zhou, Su; Wang, Wen-Jing

2016-12-01

It is studied in this paper that the electrical characteristics of the interface between SiOyNx/SiNx stack and silicon wafer affect silicon surface passivation. The effects of precursor flow ratio and deposition temperature of the SiOyNx layer on interface parameters, such as interface state density Dit and fixed charge Qf, and the surface passivation quality of silicon are observed. Capacitance-voltage measurements reveal that inserting a thin SiOyNx layer between the SiNx and the silicon wafer can suppress Qf in the film and Dit at the interface. The positive Qf and Dit and a high surface recombination velocity in stacks are observed to increase with the introduced oxygen and minimal hydrogen in the SiOyNx film increasing. Prepared by deposition at a low temperature and a low ratio of N2O/SiH4 flow rate, the SiOyNx/SiNx stacks result in a low effective surface recombination velocity (Seff) of 6 cm/s on a p-type 1 Ω·cm-5 Ω·cm FZ silicon wafer. The positive relationship between Seff and Dit suggests that the saturation of the interface defect is the main passivation mechanism although the field-effect passivation provided by the fixed charges also make a contribution to it. Project supported by the National High Technology Research and Development Program of China (Grant No. 2015AA050302) and the National Natural Science Foundation of China (Grant No. 61306076).

Enhancing elastic stress relaxation in SiGe/Si heterostructures by Si pillar necking

NASA Astrophysics Data System (ADS)

Isa, F.; Salvalaglio, M.; Arroyo Rojas Dasilva, Y.; Jung, A.; Isella, G.; Erni, R.; Timotijevic, B.; Niedermann, P.; Gröning, P.; Montalenti, F.; von Känel, H.

2016-10-01

We demonstrate that the elastic stress relaxation mechanism in micrometre-sized, highly mismatched heterostructures may be enhanced by employing patterned substrates in the form of necked pillars, resulting in a significant reduction of the dislocation density. Compositionally graded Si1-xGex crystals were grown by low energy plasma enhanced chemical vapour deposition, resulting in tens of micrometres tall, three-dimensional heterostructures. The patterned Si(001) substrates consist of micrometre-sized Si pillars either with the vertical {110} or isotropically under-etched sidewalls resulting in narrow necks. The structural properties of these heterostructures were investigated by defect etching and transmission electron microscopy. We show that the dislocation density, and hence the competition between elastic and plastic stress relaxation, is highly influenced by the shape of the substrate necks and their proximity to the mismatched epitaxial material. The SiGe dislocation density increases monotonically with the crystal width but is significantly reduced by the substrate under-etching. The drop in dislocation density is interpreted as a direct effect of the enhanced compliance of the under-etched Si pillars, as confirmed by the three-dimensional finite element method simulations of the elastic energy distribution.

Microstructural analysis of W-SiCf/SiC composite

NASA Astrophysics Data System (ADS)

Yoon, Hanki; Oh, Jeongseok; Kim, Gonho; Kim, Hyunsu; Takahashi, Heishichiro; Kohyama, Akira

2015-03-01

Continuous silicon carbide fiber-reinforced silicon carbide (SiCf/SiC) composites are promising structure candidates for future fusion power systems such as gas coolant fast channels, extreme high temperature reactor and fusion reactors, because of their intrinsic properties such as excellent mechanical properties, high thermal conductivity, good thermal-shock resistance as well as excellent physical and chemical stability in various environments under elevated temperature conditions. In this study, bonding of tungsten and SiCf/SiC was produced by hot-press method. Microstructure analyses were performed using SEM and TEM.

Computer simulation of CaSiO3 glass under compression: correlation between Si-Si pair radial distribution function and intermediate range order structure

NASA Astrophysics Data System (ADS)

Lan, Mai Thi; Thuy Duong, Tran; Iitaka, Toshiaki; Van Hong, Nguyen

2017-06-01

The structural organization of CaSiO3 glass at 600 K and under pressure of 0-100 GPa is investigated by molecular dynamics simulation (MDS). Results show that the atomic structure of CaSiO3 comprises SiO n and CaO m units considered as basic structural polyhedra. At low pressure, most of the basic structural polyhedra are SiO4, CaO5, CaO6 and CaO7. At high pressure most of the basic structural polyhedra are SiO5, SiO6 and CaO9, CaO10 and CaO11. The distribution of basic structural polyhedra is not uniform resulting in formation of Ca-rich and Si-rich regions. The distribution of SiO4, SiO5 and SiO6 polyhedra is also not uniform, but it tends to form SiO4-, SiO5-, and SiO6-clusters. For the Si-O network, under compression there is a gradual transition from the tetrahedral network (SiO4) to the octahedral network (SiO6) via SiO5 polyhedra. The SiO5-clusters are the same as immediate-phase in the transformation process. The size and shape of SiO4 tetrahedra change strongly under compression. While the size of SiO5 and SiO6 has also changed significantly, but the shape is almost unchanged under compression. The SiO n polyhedra can connect to each other via one common oxygen ion (corner-sharing bond), two common oxygen ions (edge-sharing bond) or three common oxygen ions (face-sharing bond). The Si-Si bond length in corner-sharing bonds is much longer than the ones in edge-sharing and face-sharing bonds. The change of intermediate range order (IRO) structure under compression relating to edge- and face-sharing bonds amongst SiO n at high pressure is the origin of the first peak splitting of the radial distribution functions of Si-Si pair. Under compression, the number of non-bridging oxygen (NBO) decreases. This makes the Si-O network more polymerized. At low pressure, most of the Ca2+ ions incorporate into the Si-O network via NBOs. At high pressure, the amount of NBO decreases, Ca2+ ions mainly incorporate into the Si-O network via bridging oxygen (BO) that

Optical and Structural Properties of Si Nanocrystals in SiO2 Films

PubMed Central

Nikitin, Timur; Khriachtchev, Leonid

2015-01-01

Optical and structural properties of Si nanocrystals (Si-nc) in silica films are described. For the SiOx (x < 2) films annealed above 1000 °C, the Raman signal of Si-nc and the absorption coefficient are proportional to the amount of elemental Si detected by X-ray photoelectron spectroscopy. A good agreement is found between the measured refractive index and the value estimated by using the effective-medium approximation. The extinction coefficient of elemental Si is found to be between the values of crystalline and amorphous Si. Thermal annealing increases the degree of Si crystallization; however, the crystallization and the Si–SiO2 phase separation are not complete after annealing at 1200 °C. The 1.5-eV PL quantum yield increases as the amount of elemental Si decreases; thus, this PL is probably not directly from Si-nc responsible for absorption and detected by Raman spectroscopy. Continuous-wave laser light can produce very high temperatures in the free-standing films, which changes their structural and optical properties. For relatively large laser spots, the center of the laser-annealed area is very transparent and consists of amorphous SiO2. Large Si-nc (up to ~300 nm in diameter) are observed in the ring around the central region. These Si-nc lead to high absorption and they are typically under compressive stress, which is connected with their formation from the liquid phase. By using strongly focused laser beams, the structural changes in the free-standing films can be made in submicron areas. PMID:28347028

Monolithically Integrated SiGe/Si PIN-HBT Front-End Transimpedance Photoreceivers

NASA Technical Reports Server (NTRS)

Rieh, J.-S.; Qasaimeh, O.; Klotzkin, D.; Lu, L.-H.; Katehi, L. P. B.; Yang, K.; Bhattacharya, P.; Croke, E. T.

1997-01-01

The demand for monolithically integrated photoreceivers based on Si-based technology keeps increasing as low cost and high reliability products are required for the expanding commercial market. Higher speed and wider operating frequency range are expected when SiGe/Si heterojunction is introduced to the circuit design. In this paper, a monolithic SiGe/Si PIN-HBT front-end transimpedance photoreceiver is demonstrated for the first time. For this purpose, mesa-type SiGe/Si PIN-HBT technology was developed. Fabricated HBTs exhibit f(sub max) of 34 GHz with DC gain of 25. SiGe/Si PIN photodiodes, which share base and collector layers of HBTs, demonstrate responsivity of 0.3 A/W at lambda=850 nm and bandwidth of 450 MHz. Based on these devices, single- and dual-feedback transimpedance amplifiers were fabricated and they exhibited the bandwidth of 3.2 GHz and 3.3 GHz with the transimpedance gain of 45.2 dB(Omega) and 47.4 dB(Omega) respectively. Monolithically integrated single-feedback PIN-HBT photoreceivers were implemented and the bandwidth was measured to be approx. 0.5 GHz, which is limited by the bandwidth of PIN photodiodes.

Development and Characterization of SiC)/ MoSi2-Si3N4(p) Hybrid Composites

NASA Technical Reports Server (NTRS)

Hebsur, Mohan G.

1998-01-01

Intermetallic compound MoSi2 has long been known as a high temperature material that has excellent oxidation resistance and electrical/thermal conductivity. Also its low cost, high melting point (2023 C), relatively low density (6.2 g/cu cm versus 9 g/cu cm for current engine materials), and ease of machining, make it an attractive structural material. However, the use of MoSi2 has been hindered due to its poor toughness at low temperatures, poor creep resistance at high temperatures, and accelerated oxidation (also known as 'pest' oxidation) at temperatures between approximately 450 and 550 C. Continuous fiber reinforcing is very effective means of improving both toughness and strength. Unfortunately, MoSi2 has a relatively high coefficient of thermal expansion (CTE) compared to potential reinforcing fibers such as SiC. The large CTE mismatch between the fiber and the matrix resulted in severe matrix cracking during thermal cycling. Addition of about 30 to 50 vol % of Si3N4 particulate to MoSi2 improved resistance to low temperature accelerated oxidation by forming a Si2ON2 protective scale and thereby eliminating catastrophic 'pest failure'. The Si3N4 addition also improved the high temperature creep strength by nearly five orders of magnitude, doubled the room temperature toughness and significantly lowered the CTE of the MoSi2 and eliminated matrix cracking in SCS-6 reinforced composites even after thermal cycling. The SCS-6 fiber reinforcement improved the room temperature fracture toughness by seven times and impact resistance by five times. The composite exhibited excellent strength and toughness improvement up to 1400 C. More recently, tape casting was adopted as the preferred processing of MoSi2-base composites for improved fiber spacing, ability to use small diameter fibers, and for lower cost. Good strength and toughness values were also obtained with fine diameter Hi-Nicalon tow fibers. This hybrid composite remains competitive with ceramic matrix

Fabrication of lightweight Si/SiC LIDAR mirrors

NASA Technical Reports Server (NTRS)

Goela, Jitendra S.; Taylor, Raymond L.

1991-01-01

A new, chemical vapor deposition (CVD) process was developed for fabricating lightweight, polycrystalline silicon/silicon-carbide (Si/SiC) mirrors. The process involves three CVD steps: (1) to produce the mirror faceplate; (2) to form the lightweight backstructure, which is deposited integral to the faceplate; and (3) to deposit a layer of optical-grade material, e.g., Si, onto the front surface of the faceplate. The mirror figure and finish are fabricated into the faceplate.

Mushroom-free selective epitaxial growth of Si, SiGe and SiGe:B raised sources and drains

NASA Astrophysics Data System (ADS)

Hartmann, J. M.; Benevent, V.; Barnes, J. P.; Veillerot, M.; Lafond, D.; Damlencourt, J. F.; Morvan, S.; Prévitali, B.; Andrieu, F.; Loubet, N.; Dutartre, D.

2013-05-01

We have evaluated various Cyclic Selective Epitaxial Growth/Etch (CSEGE) processes in order to grow "mushroom-free" Si and SiGe:B Raised Sources and Drains (RSDs) on each side of ultra-short gate length Extra-Thin Silicon-On-Insulator (ET-SOI) transistors. The 750 °C, 20 Torr Si CSEGE process we have developed (5 chlorinated growth steps with four HCl etch steps in-between) yielded excellent crystalline quality, typically 18 nm thick Si RSDs. Growth was conformal along the Si3N4 sidewall spacers, without any poly-Si mushrooms on top of unprotected gates. We have then evaluated on blanket 300 mm Si(001) wafers the feasibility of a 650 °C, 20 Torr SiGe:B CSEGE process (5 chlorinated growth steps with four HCl etch steps in-between, as for Si). As expected, the deposited thickness decreased as the total HCl etch time increased. This came hands in hands with unforeseen (i) decrease of the mean Ge concentration (from 30% down to 26%) and (ii) increase of the substitutional B concentration (from 2 × 1020 cm-3 up to 3 × 1020 cm-3). They were due to fluctuations of the Ge concentration and of the atomic B concentration [B] in such layers (drop of the Ge% and increase of [B] at etch step locations). Such blanket layers were a bit rougher than layers grown using a single epitaxy step, but nevertheless of excellent crystalline quality. Transposition of our CSEGE process on patterned ET-SOI wafers did not yield the expected results. HCl etch steps indeed helped in partly or totally removing the poly-SiGe:B mushrooms on top of the gates. This was however at the expense of the crystalline quality and 2D nature of the ˜45 nm thick Si0.7Ge0.3:B recessed sources and drains selectively grown on each side of the imperfectly protected poly-Si gates. The only solution we have so far identified that yields a lesser amount of mushrooms while preserving the quality of the S/D is to increase the HCl flow during growth steps.

Low Temperature Ohmic Contact Formation of Ni2Si on N-type 4H-SiC and 6H-SiC

NASA Technical Reports Server (NTRS)

Elsamadicy, A. M.; Ila, D.; Zimmerman, R.; Muntele, C.; Evelyn, L.; Muntele, I.; Poker, D. B.; Hensley, D.; Hirvonen, J. K.; Demaree, J. D.;

Synthesis, characterization and luminescence of europium perchlorate with MABA-Si complex and coating structure SiO2 @Eu(MABA-Si) luminescence nanoparticles.

PubMed

Fu, Zhi-Fang; Li, Wen-Xian; Bai, Juan; Bao, Jin-Rong; Cao, Xiao-Fang; Zheng, Yu-Shan

2017-05-01

This article reports a novel category of coating structure SiO 2 @Eu(MABA-Si) luminescence nanoparticles (NPs) consisting of a unique organic shell, composed of perchlorate europium(III) complex, and an inorganic core, composed of silica. The binary complex Eu(MABA-Si) 3 ·(ClO 4 ) 3 ·5H 2 O was synthesized using HOOCC 6 H 4 N(CONH(CH 2 ) 3 Si(OCH 2 CH 3 ) 3 ) 2 (MABA-Si) and was used as a ligand. Furthermore, the as-prepared silica NPs were successfully coated with the -Si(OCH 2 CH 3 ) 3 group of MABA-Si to form Si-O-Si chemical bonds by means of the hydrolyzation of MABA-Si. The binary complexes were characterized by elemental analysis, molar conductivity and coordination titration analysis. The results indicated that the composition of the binary complex was Eu(MABA-Si) 3 ·(ClO 4 ) 3 ·5H 2 O. Coating structure SiO 2 @Eu(MABA-Si) NPs were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and infrared (IR) spectra. Based on the SEM and TEM measurements, the diameter of core-SiO 2 particles was ~400 and 600 nm, and the thickness of the cladding layer Eu(MABA-Si) was ~20 nm. In the binary complex Eu(MABA-Si) 3 ·(ClO 4 ) 3 ·5H 2 O, the fluorescence spectra illustrated that the energy of the ligand MABA-Si transferred to the energy level for the excitation state of europium(III) ion. Coating structure SiO 2 @Eu(MABA-Si) NPs exhibited intense red luminescence compared with the binary complex. The fluorescence lifetime and fluorescence quantum efficiency of the binary complex and of the coating structure NPs were also calculated. The way in which the size of core-SiO 2 spheres influences the luminescence was also studied. Moreover, the luminescent mechanisms of the complex were studied and explained. Copyright © 2016 John Wiley & Sons, Ltd.

The impact resistance of SiC and other mechanical properties of SiC and Si3N4

NASA Technical Reports Server (NTRS)

Bradt, R. C.

1984-01-01

Studies focused on the impact and mechanical behavior of SiC and Si3N4 at high temperatures are summarized. Instrumented Charpy impact testing is analyzed by a compliance method and related to strength; slow crack growth is related to processing, and creep is discussed. The transient nature of flaw populations during oxidation under load is emphasized for both SiC and Si3N4.

In situ observation of melting and crystallization of Si on porous Si3N4 substrate that repels Si melt

NASA Astrophysics Data System (ADS)

Itoh, Hironori; Okamura, Hideyuki; Asanoma, Susumu; Ikemura, Kouhei; Nakayama, Masaharu; Komatsu, Ryuichi

2014-09-01

High temperature in situ observation of melting and crystallization of spherical Si droplets on a substrate with a porous surface was carried out for the first time using an original in situ observation apparatus. The contact angle between the Si melt and the substrate was measured to be 160°, with the Si melt forming spherical droplets on the substrate. During crystallization, a ring-like pattern was observed on the surface of the spherical Si melt droplets due to crystal growth at low levels of supercooling. The solidified spherical Si crystals consisted of single or twin grains. This demonstrates that high-quality spherical Si crystals can be prepared easily and stably by using a Si melt-repelling substrate.

Hydrogen passivation of poly-Si/SiO x contacts for Si solar cells using Al 2O 3 studied with deuterium

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

Schnabel, Manuel; van de Loo, Bas W. H.; Nemeth, William

Here, the interplay between hydrogenation and passivation of poly-Si/SiO x contacts to n-type Si wafers is studied using atomic layer deposited Al 2O 3 and anneals in forming gas and nitrogen. The poly-Si/SiO x stacks are prepared by thermal oxidation followed by thermal crystallization of a-Si:H films deposited by plasma-enhanced chemical vapor deposition. Implied open-circuit voltages as high as 710 mV are achieved for p-type poly-Si/SiO x contacts to n-type Si after hydrogenation. Correlating minority carrier lifetime data and secondary ion mass spectrometry profiles reveals that the main benefit of Al 2O 3 is derived from its role as amore » hydrogen source for chemically passivating defects at SiO x; Al 2O 3 layers are found to hydrogenate poly-Si/SiO x much better than a forming gas anneal. By labelling Al 2O 3 and the subsequent anneal with different hydrogen isotopes, it is found that Al 2O 3 exchanges most of its hydrogen with the ambient upon annealing at 400 °C for 1 h even though there is no significant net change in its total hydrogen content.« less

Hydrogen passivation of poly-Si/SiO x contacts for Si solar cells using Al 2O 3 studied with deuterium

DOE PAGES

Schnabel, Manuel; van de Loo, Bas W. H.; Nemeth, William; ...

2018-05-14

Here, the interplay between hydrogenation and passivation of poly-Si/SiO x contacts to n-type Si wafers is studied using atomic layer deposited Al 2O 3 and anneals in forming gas and nitrogen. The poly-Si/SiO x stacks are prepared by thermal oxidation followed by thermal crystallization of a-Si:H films deposited by plasma-enhanced chemical vapor deposition. Implied open-circuit voltages as high as 710 mV are achieved for p-type poly-Si/SiO x contacts to n-type Si after hydrogenation. Correlating minority carrier lifetime data and secondary ion mass spectrometry profiles reveals that the main benefit of Al 2O 3 is derived from its role as amore » hydrogen source for chemically passivating defects at SiO x; Al 2O 3 layers are found to hydrogenate poly-Si/SiO x much better than a forming gas anneal. By labelling Al 2O 3 and the subsequent anneal with different hydrogen isotopes, it is found that Al 2O 3 exchanges most of its hydrogen with the ambient upon annealing at 400 °C for 1 h even though there is no significant net change in its total hydrogen content.« less

High rectifying behavior in Al/Si nanocrystal-embedded SiOxNy/p-Si heterojunctions

NASA Astrophysics Data System (ADS)

Jacques, E.; Pichon, L.; Debieu, O.; Gourbilleau, F.; Coulon, N.

2011-05-01

We examine the electrical properties of MIS devices made of Al/Si nanocrystal-SiOxNy/p-Si. The J-V characteristics of the devices present a high rectifying behavior. Temperature measurements show that the forward current is thermally activated following the thermal diffusion model of carriers. At low reverse bias, the current is governed by thermal emission amplified by the Poole-Frenkel effect of carriers from defects located at the silicon nanocrystals/SiOxNy interfaces, whereas tunnel conduction in silicon oxynitride matrix dominates at high reverse bias. The devices exhibit a rectification ratio >104 for the current measured at V = ± 1 V. Study reveals that thermal annealing in forming gas (H2/N2) improves the electrical properties of the devices due to the passivation of defects.

Interfacial stability of CoSi2/Si structures grown by molecular beam epitaxy

NASA Technical Reports Server (NTRS)

George, T.; Fathauer, R. W.

1992-01-01

The stability of CoSi2/Si interfaces was examined in this study using columnar silicide structures grown on (111) Si substrates. In the first set of experiments, Co and Si were codeposited using MBE at 800 C and the resulting columnar silicide layer was capped by epitaxial Si. Deposition of Co on the surface of the Si capping layer at 800 C results in the growth of the buried silicide columns. The buried columns grow by subsurface diffusion of the deposited Co, suppressing the formation of surface islands of CoSi2. The column sidewalls appear to be less stable than the top and bottom interfaces, resulting in preferential lateral growth and ultimately in the coalescence of the columns to form a continuous buried CoSi2 layer. In the second set of experiments, annealing of a 250 nm-thick buried columnar layer at 1000 C under a 100 nm-thick Si capping layer results in the formation of a surface layer of CoSi2 with a reduction in the sizes of the CoSi2 columns. For a sample having a thicker Si capping layer the annealing leads to Ostwald ripening producing buried equiaxed columns. The high CoSi2/Si interfacial strain could provide the driving force for the observed behavior of the buried columns under high-temperature annealing.

Quasi free-standing epitaxial graphene fabrication on 3C-SiC/Si(111)

NASA Astrophysics Data System (ADS)

Amjadipour, Mojtaba; Tadich, Anton; Boeckl, John J.; Lipton-Duffin, Josh; MacLeod, Jennifer; Iacopi, Francesca; Motta, Nunzio

2018-04-01

Growing graphene on SiC thin films on Si is a cheaper alternative to the growth on bulk SiC, and for this reason it has been recently intensively investigated. Here we study the effect of hydrogen intercalation on epitaxial graphene obtained by high temperature annealing on 3C-SiC/Si(111) in ultra-high vacuum. By using a combination of core-level photoelectron spectroscopy, low energy electron diffraction, and near-edge x-ray absorption fine structure (NEXAFS) we find that hydrogen saturates the Si atoms at the topmost layer of the substrate, leading to free-standing graphene on 3C-SiC/Si(111). The intercalated hydrogen fully desorbs after heating the sample at 850 °C and the buffer layer appears again, similar to what has been reported for bulk SiC. However, the NEXAFS analysis sheds new light on the effect of hydrogen intercalation, showing an improvement of graphene’s flatness after annealing in atomic H at 600 °C. These results provide new insight into free-standing graphene fabrication on SiC/Si thin films.

Condition of Si crystal formation by vaporizing Na from NaSi

NASA Astrophysics Data System (ADS)

Morito, Haruhiko; Karahashi, Taiki; Yamane, Hisanori

2012-09-01

NaSi was heated at various Na vapor pressures (pNa 0.1-1.2 atm) and temperatures (973-1173 K) to investigate the condition of Si crystal formation from NaSi by Na evaporation. Silicon single crystals 1-3 mm in diameter were grown by evaporation of Na from Na-Si melt at 1173 K and pNa=0.74 atm.

From Si wafers to cheap and efficient Si electrodes for Li-ion batteries

NASA Astrophysics Data System (ADS)

Gauthier, Magali; Reyter, David; Mazouzi, Driss; Moreau, Philippe; Guyomard, Dominique; Lestriez, Bernard; Roué, Lionel

2014-06-01

High-energy ball milling is used to recycle Si wafers to produce Si powders for negative electrodes of Li-ion batteries. The resulting Si powder consists in micrometric Si agglomerates made of cold-welded submicrometric nanocrystalline Si particles. Silicon-based composite electrodes prepared with ball-milled Si wafer can achieve more than 900 cycles with a capacity of 1200 mAh g-1 of Si (880 mAh g-1 of electrode) and a coulombic efficiency higher than 99%. This excellent electrochemical performance lies in the use of nanostructured Si produced by ball milling, the electrode formulation in a pH 3 buffer solution with CMC as binder and the use of FEC/VC additives in the electrolyte. This work opens the way to an economically attractive recycling of Si wastes.

Al+Si Interface Optical Properties Obtained in the Si Solar Cell Configuration

DOE PAGES

Subedi, Indra; Silverman, Timothy J.; Deceglie, Michael G.; ...

2017-10-18

Al is a commonly used material for rear side metallization in commercial silicon (Si) wafer solar cells. In this study, through-the-silicon spectroscopic ellipsometry is used in a test sample to measure Al+Si interface optical properties like those in Si wafer solar cells. Two different spectroscopic ellipsometers are used for measurement of Al+Si interface optical properties over the 1128-2500 nm wavelength range. For validation, the measured interface optical properties are used in a ray tracing simulation over the 300-2500 nm wavelength range for an encapsulated Si solar cell having random pyramidal texture. The ray tracing model matches well with the measuredmore » total reflectance at normal incidence of a commercially available Si module. The Al+Si optical properties presented here enable quantitative assessment of major irradiance/current flux losses arising from reflection and parasitic absorption in encapsulated Si solar cells.« less

Al+Si Interface Optical Properties Obtained in the Si Solar Cell Configuration

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

Subedi, Indra; Silverman, Timothy J.; Deceglie, Michael G.

Al is a commonly used material for rear side metallization in commercial silicon (Si) wafer solar cells. In this study, through-the-silicon spectroscopic ellipsometry is used in a test sample to measure Al+Si interface optical properties like those in Si wafer solar cells. Two different spectroscopic ellipsometers are used for measurement of Al+Si interface optical properties over the 1128-2500 nm wavelength range. For validation, the measured interface optical properties are used in a ray tracing simulation over the 300-2500 nm wavelength range for an encapsulated Si solar cell having random pyramidal texture. The ray tracing model matches well with the measuredmore » total reflectance at normal incidence of a commercially available Si module. The Al+Si optical properties presented here enable quantitative assessment of major irradiance/current flux losses arising from reflection and parasitic absorption in encapsulated Si solar cells.« less

Abiologic silicon isotope fractionation between aqueous Si and Fe(III)-Si gel in simulated Archean seawater: Implications for Si isotope records in Precambrian sedimentary rocks

NASA Astrophysics Data System (ADS)

Zheng, Xin-Yuan; Beard, Brian L.; Reddy, Thiruchelvi R.; Roden, Eric E.; Johnson, Clark M.

2016-08-01

Precambrian Si-rich sedimentary rocks, including cherts and banded iron formations (BIFs), record a >7‰ spread in 30Si/28Si ratios (δ30Si values), yet interpretation of this large variability has been hindered by the paucity of data on Si isotope exchange kinetics and equilibrium fractionation factors in systems that are pertinent to Precambrian marine conditions. Using the three-isotope method and an enriched 29Si tracer, a series of experiments were conducted to constrain Si isotope exchange kinetics and fractionation factors between amorphous Fe(III)-Si gel, a likely precursor to Precambrian jaspers and BIFs, and aqueous Si in artificial Archean seawater under anoxic conditions. Experiments were conducted at room temperature, and in the presence and absence of aqueous Fe(II) (Fe(II)aq). Results of this study demonstrate that Si solubility is significantly lower for Fe-Si gel than that of amorphous Si, indicating that seawater Si concentrations in the Precambrian may have been lower than previous estimates. The experiments reached ∼70-90% Si isotope exchange after a period of 53-126 days, and the highest extents of exchange were obtained where Fe(II)aq was present, suggesting that Fe(II)-Fe(III) electron-transfer and atom-exchange reactions catalyze Si isotope exchange through breakage of Fe-Si bonds. All experiments except one showed little change in the instantaneous solid-aqueous Si isotope fractionation factor with time, allowing extraction of equilibrium Si isotope fractionation factors through extrapolation to 100% isotope exchange. The equilibrium 30Si/28Si fractionation between Fe(III)-Si gel and aqueous Si (Δ30Sigel-aqueous) is -2.30 ± 0.25‰ (2σ) in the absence of Fe(II)aq. In the case where Fe(II)aq was present, which resulted in addition of ∼10% Fe(II) in the final solid, creating a mixed Fe(II)-Fe(III) Si gel, the equilibrium fractionation between Fe(II)-Fe(III)-Si gel and aqueous Si (Δ30Sigel-aqueous) is -3.23 ± 0.37‰ (2�

The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust.

PubMed

Massalkhi, Sarah; Agúndez, M; Cernicharo, J; Velilla Prieto, L; Goicoechea, J R; Quintana-Lacaci, G; Fonfría, J P; Alcolea, J; Bujarrabal, V

2018-03-01

Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si-C bond detected in C-rich AGB stars are SiC 2 , SiC, and Si 2 C. To date, the ring molecule SiC 2 has been observed in a handful of evolved stars, while SiC and Si 2 C have only been detected in the C-star envelope IRC +10216. We aim to study how widespread and abundant SiC 2 , SiC, and Si 2 C are in envelopes around C-rich AGB stars and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars. We carried out sensitive observations with the IRAM 30m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC 2 , SiC, and Si 2 C in the λ 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC 2 and to derive SiC 2 fractional abundances in the observed envelopes. We detect SiC 2 in most of the sources, SiC in about half of them, and do not detect Si 2 C in any source, at the exception of IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC 2 line emission, which suggests that both species are chemically linked, the SiC radical probably being the photodissociation product of SiC 2 in the external layer of the envelope. We find a clear trend in which the denser the envelope, the less abundant SiC 2 is. The observed trend is interpreted as an evidence of efficient incorporation of SiC 2 onto dust grains, a process which is favored at high densities owing to the higher rate at which collisions between particles take place. The observed behavior of a decline in the SiC 2 abundance with increasing density strongly suggests that SiC 2 is an important gas

Improving Thermomechanical Properties of SiC/SiC Composites

NASA Technical Reports Server (NTRS)

DiCarlo, James A.; Bhatt, Ramakrishna T.

2006-01-01

Today, a major thrust toward improving the thermomechanical properties of engine components lies in the development of fiber-reinforced silicon carbide matrix composite materials, including SiC-fiber/SiC-matrix composites. These materials are lighter in weight and capable of withstanding higher temperatures, relative to state-of-the-art metallic alloys and oxide-matrix composites for which maximum use temperatures are in the vicinity of 1,100 C. In addition, the toughness or damage tolerance of the SiC-matrix composites is significantly greater than that of unreinforced silicon-based monolithic ceramics. For successful application in advanced engine systems, the SiC-matrix composites should be able to withstand component service stresses and temperatures for the desired component lifetimes. Inasmuch as the high-temperature structural lives of ceramic materials are typically limited by creep-induced growth of flaws, a key property required of such composite materials is high resistance to creep under conditions of use. Also, the thermal conductivity of the materials should be as high as possible so as to minimize component thermal gradients and thermal stresses. A state-of-the-art SiC-matrix composite is typically fabricated in a three-step process: (1) fabrication of a component-shaped architectural preform reinforced by thermally stable high-performance fibers, (2) chemical-vapor infiltration (CVI) of a fiber-coating material such as boron nitride (BN) into the preform, and (3) infiltration of an SiC-based matrix into the remaining porosity in the preform. Generally, the matrices of the highest-performing composites are fabricated by initial use of a CVI SiC matrix component that is typically more thermally stable and denser than matrix components formed by processes other than CVI. As such, the initial SiC matrix component made by CVI provides better environmental protection to the coated fibers embedded within it. Also, the denser CVI SiC imparts to the

Deep ultraviolet photodetectors based on p-Si/ i-SiC/ n-Ga2O3 heterojunction by inserting thin SiC barrier layer

NASA Astrophysics Data System (ADS)

An, Yuehua; Zhi, Yusong; Wu, Zhenping; Cui, Wei; Zhao, Xiaolong; Guo, Daoyou; Li, Peigang; Tang, Weihua

2016-12-01

Deep ultraviolet photodetectors based on p-Si/ n-Ga2O3 and p-Si/ i-SiC/ n-Ga2O3 heterojunctions were fabricated by laser molecular beam epitaxial (L-MBE), respectively. In compare with p-Si/ n-Ga2O3 heterostructure-based photodetector, the dark current of p-Si/ i-SiC/ n-Ga2O3-based photodetector decreased by three orders of magnitude, and the rectifying behavior was tuned from reverse to forward. In order to improve the quality of the photodetector, we reduced the oxygen vacancies of p-Si/ i-SiC/ n-Ga2O3 heterostructures by changing the oxygen pressure during annealing. As a result, the rectification ratio ( I F/ I R) of the fabricated photodetectors was 36 at 4.5 V and the photosensitivity was 5.4 × 105% under the 254 nm light illumination at -4.5 V. The energy band structure of p-Si/ n-Ga2O3 and p-Si/ i-SiC/ n-Ga2O3 heterostructures was schematic drawn to explain the physic mechanism of enhancement of the performance of p-Si/ i-SiC/ n-Ga2O3 heterostructure-based deep UV photodetector by introduction of SiC layer.

Comparison of Fatigue Life Between C/SiC and SiC/SiC Ceramic-Matrix Composites at Room and Elevated Temperatures

NASA Astrophysics Data System (ADS)

Longbiao, Li

2016-10-01

In this paper, the comparison of fatigue life between C/SiC and SiC/SiC ceramic-matrix composites (CMCs) at room and elevated temperatures has been investigated. An effective coefficient of the fiber volume fraction along the loading direction (ECFL) was introduced to describe the fiber architecture of preforms. Under cyclic fatigue loading, the fibers broken fraction was determined by combining the interface wear model and fibers statistical failure model at room temperature, and interface/fibers oxidation model, interface wear model and fibers statistical failure model at elevated temperatures in the oxidative environments. When the broken fibers fraction approaches to the critical value, the composites fatigue fracture. The fatigue life S-N curves and fatigue limits of cross-ply, 2D and 3D C/SiC and SiC/SiC composites at room temperature, 550 °C in air, 750 °C in dry and humid condition, 800 °C in air, 1000 °C in argon and air, 1100 °C, 1300 °C and 1500 °C in vacuum, have been predicted. At room temperature, the fatigue limit of 2D C/SiC composite with ECFL of 20 % lies between 0.78 and 0.8 tensile strength; and the fatigue limit of 2D SiC/SiC composite with ECFL of 20 % lies between 0.75 and 0.85 tensile strength. The fatigue limit of 2D C/SiC composite increases to 0.83 tensile strength with ECFL increasing from 20 to 22.5 %, and the fatigue limit of 3D C/SiC composite is 0.85 tensile strength with ECFL of 37 %. The fatigue performance of 2D SiC/SiC composite is better than that of 2D C/SiC composite at elevated temperatures in oxidative environment.

Dependence of Morphology of SiOx Nanowires on the Supersaturation of Au-Si Alloy Liquid Droplets Formed on the Au-Coated Si Substrate

NASA Astrophysics Data System (ADS)

Zhang, Han; Li, Ji-Xue; Jin, Ai-Zi; Zhang, Ze

2001-11-01

A thermodynamic theory about the dependence of morphology of SiOx nanowires on the super-saturation of alloy liquid droplets has been proposed on the basis of the vapour-liquid-solid growth mechanism and has been supported experimentally. By changing the Si concentration in the Au-Si liquid droplets formed on the Au-coated Si substrate, firework-, tulip- and bud-shaped SiOx nanowires were synthesized by a thermal evaporation method and distributed concentrically around some void defects in the Si substrate. Voids were formed underneath the surface of the Si substrate during the thermal evaporation at 850°C and resulted in the Si-concentration deficient thus different saturation of Au-Si droplets. Electron microscopy analysis showed that the nanowires had an amorphous structure and were terminated by Au-Si particles.

Isotropic plasma etching of Ge Si and SiN x films

DOE PAGES

Henry, Michael David; Douglas, Erica Ann

2016-08-31

This study reports on selective isotropic dry etching of chemically vapor deposited (CVD) Ge thin film, release layers using a Shibaura chemical downstream etcher (CDE) with NF 3 and Ar based plasma chemistry. Relative etch rates between Ge, Si and SiN x are described with etch rate reductions achieved by adjusting plasma chemistry with O 2. Formation of oxides reducing etch rates were measured for both Ge and Si, but nitrides or oxy-nitrides created using direct injection of NO into the process chamber were measured to increase Si and SiN x etch rates while retarding Ge etching.

Possibilities for LWIR detectors using MBE-grown Si(/Si(1-x)Ge(x) structures

NASA Technical Reports Server (NTRS)

Hauenstein, Robert J.; Miles, Richard H.; Young, Mary H.

1990-01-01

Traditionally, long wavelength infrared (LWIR) detection in Si-based structures has involved either extrinsic Si or Si/metal Schottky barrier devices. Molecular beam epitaxially (MBE) grown Si and Si/Si(1-x)Ge(x) heterostructures offer new possibilities for LWIR detection, including sensors based on intersubband transitions as well as improved conventional devices. The improvement in doping profile control of MBE in comparison with conventional chemical vapor deposited (CVD) Si films has resulted in the successful growth of extrinsic Si:Ga, blocked impurity-band conduction detectors. These structures exhibit a highly abrupt step change in dopant profile between detecting and blocking layers which is extremely difficult or impossible to achieve through conventional epitaxial growth techniques. Through alloying Si with Ge, Schottky barrier infrared detectors are possible, with barrier height values between those involving pure Si or Ge semiconducting materials alone. For both n-type and p-type structures, strain effects can split the band edges, thereby splitting the Schottky threshold and altering the spectral response. Measurements of photoresponse of n-type Au/Si(1-x)Ge(x) Schottky barriers demonstrate this effect. For intersubband multiquntum well (MQW) LWIR detection, Si(1-x)Ge(x)/Si detectors grown on Si substrates promise comparable absorption coefficients to that of the Ga(Al)As system while in addition offering the fundamental advantage of response to normally incident light as well as the practical advantage of Si-compatibility. Researchers grew Si(1-x)Ge(x)/Si MQW structures aimed at sensitivity to IR in the 8 to 12 micron region and longer, guided by recent theoretical work. Preliminary measurements of n- and p-type Si(1-x)Ge(x)/Si MQW structures are given.

Effects of SiC on Properties of Cu-SiC Metal Matrix Composites

NASA Astrophysics Data System (ADS)

Efe, G. Celebi; Altinsoy, I.; Ipek, M.; Zeytin, S.; Bindal, C.

2011-12-01

This paper was focused on the effects of particle size and distribution on some properties of the SiC particle reinforced Cu composites. Copper powder produced by cementation method was reinforced with SiC particles having 1 and 30 μm particle size and sintered at 700 °C. SEM studies showed that SiC particles dispersed in copper matrix homogenously. The presence of Cu and SiC components in composites were verified by XRD analysis technique. The relative densities of Cu-SiC composites determined by Archimedes' principle are ranged from 96.2% to 90.9% for SiC with 1 μm particle size, 97.0 to 95.0 for SiC with 30 μm particle size. Measured hardness of sintered compacts varied from 130 to 155 HVN for SiC having 1 μm particle size, 188 to 229 HVN for SiC having 1 μm particle size. Maximum electrical conductivity of test materials was obtained as 80.0% IACS (International annealed copper standard) for SiC with 1 μm particle size and 83.0% IACS for SiC with 30 μm particle size.

C incorporation and segregation during Si 1- yC y/Si( 0 0 1 ) gas-source molecular beam epitaxy from Si 2H 6 and CH 3SiH 3

NASA Astrophysics Data System (ADS)

Foo, Y. L.; Bratland, K. A.; Cho, B.; Soares, J. A. N. T.; Desjardins, P.; Greene, J. E.

2002-08-01

We have used in situ D 2 temperature-programmed desorption (TPD) to probe C incorporation and surface segregation kinetics, as well as hydrogen desorption pathways, during Si 1- yC y(0 0 1) gas-source molecular beam epitaxy from Si 2H 6/CH 3SiH 3 mixtures at temperatures Ts between 500 and 650 °C. Parallel D 2 TPD results from C-adsorbed Si(0 0 1) wafers exposed to varying CH 3SiH 3 doses serve as reference data. Si 1- yC y(0 0 1) layer spectra consist of three peaks: first-order β 1 at 515 °C and second-order β 2 at 405 °C, due to D 2 desorption from Si monodeuteride and dideuteride phases, as well as a new second-order C-induced γ 1 peak at 480 °C. C-adsorbed Si(0 0 1) samples with very high CH 3SiH 3 exposures yielded a higher-temperature TPD feature, corresponding to D 2 desorption from surface C atoms, which was never observed in Si 1- yC y(0 0 1) layer spectra. The Si 1- yC y(0 0 1) γ 1 peak arises due to desorption from Si monodeuteride species with C backbonds. γ 1 occurs at a lower temperature than β 1 reflecting the lower D-Si * bond strength, where Si * represents surface Si atoms bonded to second-layer C atoms, as a result of charge transfer from dangling bonds. The total integrated monohydride (β 1+γ 1) intensity, and hence the dangling bond density, remains constant with y indicating that C does not deactivate surface dangling bonds as it segregates to the second-layer during Si 1- yC y(0 0 1) growth. Si * coverages increase with y at constant Ts and with Ts at constant y. The positive Ts-dependence shows that C segregation is kinetically limited at Ts⩽650 °C. D 2 desorption activation energies from β 1, γ 1 and β 2 sites are 2.52, 2.22 and 1.88 eV.

Metastability of a-SiOx:H thin films for c-Si surface passivation

NASA Astrophysics Data System (ADS)

Serenelli, L.; Martini, L.; Imbimbo, L.; Asquini, R.; Menchini, F.; Izzi, M.; Tucci, M.

2017-01-01

The adoption of a-SiOx:H films obtained by PECVD in heterojunction solar cells is a key to further increase their efficiency, because of its transparency in the UV with respect to the commonly used a-Si:H. At the same time this layer must guarantee high surface passivation of the c-Si to be suitable in high efficiency solar cell manufacturing. On the other hand the application of amorphous materials like a-Si:H and SiNx on the cell frontside expose them to the mostly energetic part of the sun spectrum, leading to a metastability of their passivation properties. Moreover as for amorphous silicon, thermal annealing procedures are considered as valuable steps to enhance and stabilize thin film properties, when performed at opportune temperature. In this work we explored the reliability of a-SiOx:H thin film layers surface passivation on c-Si substrates under UV exposition, in combination with thermal annealing steps. Both p- and n-type doped c-Si substrates were considered. To understand the effect of UV light soaking we monitored the minority carriers lifetime and Sisbnd H and Sisbnd O bonding, by FTIR spectra, after different exposure times to light coming from a deuterium lamp, filtered to UV-A region, and focused on the sample to obtain a power density of 50 μW/cm2. We found a certain lifetime decrease after UV light soaking in both p- and n-type c-Si passivated wafers according to a a-SiOx:H/c-Si/a-SiOx:H structure. The role of a thermal annealing, which usually enhances the as-deposited SiOx passivation properties, was furthermore considered. In particular we monitored the UV light soaking effect on c-Si wafers after a-SiOx:H coating by PECVD and after a thermal annealing treatment at 300 °C for 30 min, having selected these conditions on the basis of the study of the effect due to different temperatures and durations. We correlated the lifetime evolution and the metastability effect of thermal annealing to the a-SiOx:H/c-Si interface considering the evolution

Molecular Structures of Al/Si and Fe/Si Coprecipitates and the Implication for Selenite Removal

PubMed Central

Chan, Ya-Ting; Kuan, Wen-Hui; Tzou, Yu-Min; Chen, Tsan-Yao; Liu, Yu-Ting; Wang, Ming-Kuang; Teah, Heng-Yi

2016-01-01

Aluminum and iron oxides have been often used in the coagulation processes during water purification due to their unique surface properties toward anions. In the presence of silica, the coprecipitation of Al/Si or Fe/Si might decrease the efficiency of wastewater purification and reuse. In this study, surface properties and molecular structures of Al/Si and Fe/Si coprecipitates were characterized using spectroscopic techniques. Also, the selenite removal efficiency of Al/Si and Fe/Si coprecipitates in relation to their surface and structural properties was investigated. While dissolved silicate increased with increasing pH from Fe/Si coprecipitates, less than 7% of silicate was discernible from Al/Si samples over the range from acidic to alkaline conditions. Our spectroscopic results showed that the associations between Al and Si were relatively stronger than that between Fe and Si in coprecipitates. In Al/Si coprecipitates, core-shell structures were developed with AlO6/AlO4 domains as the shells and Si frameworks polymerized from the SiO2 as the cores. However, Si framework remained relatively unchanged upon coprecipitation with Fe hydroxides in Fe/Si samples. The Si core with Al shell structure of Al/Si coprecipitates shielded the negative charges from SiO2 and thereby resulted in a higher adsorption capacity of selenite than Fe/Si coprecipitates. PMID:27095071

SiC and Si3N4 Recession Due to SiO2 Scale Volatility Under Combustor Conditions

NASA Technical Reports Server (NTRS)

Smialek, James L.; Robinson, R. Craig; Opila, Elizabeth J.; Fox, Dennis S.; Jacobson, Nathan S.

1999-01-01

SiC and Si3N4 materials were tested under various turbine engine combustion environments, chosen to represent either conventional fuel-lean or fuel-rich mixtures proposed for high speed aircraft. Representative CVD, sintered, and composite materials were evaluated in both furnace and high pressure burner rig exposure. While protective SiO2 scales form in all cases, evidence is presented to support paralinear growth kinetics, i.e. parabolic growth moderated simultaneously by linear volatilization. The volatility rate is dependent on temperature, moisture content, system pressure, and gas velocity. The burner tests were used to map SiO2 volatility (and SiC recession) over a range of temperature, pressure, and velocity. The functional dependency of material recession (volatility) that emerged followed the form: exp(-QIRT) * P(exp x) * v(exp y). These empirical relations were compared to rates predicted from the thermodynamics of volatile SiO and SiO(sub x)H(sub Y) reaction products and a kinetic model of diffusion through a moving, boundary layer. For typical combustion conditions, recession of 0.2 to 2 micron/h is predicted at 1200- 1400C, far in excess of acceptable long term limits.

Phase transformation in SiOx/SiO₂ multilayers for optoelectronics and microelectronics applications.

PubMed

Roussel, M; Talbot, E; Pratibha Nalini, R; Gourbilleau, F; Pareige, P

2013-09-01

Due to the quantum confinement, silicon nanoclusters (Si-ncs) embedded in a dielectric matrix are of prime interest for new optoelectronics and microelectronics applications. In this context, SiO(x)/SiO₂ multilayers have been prepared by magnetron sputtering and subsequently annealed to induce phase separation and Si clusters growth. The aim of this paper is to study phase separation processes and formation of nanoclusters in SiO(x)/SiO₂ multilayers by atom probe tomography. Influences of the silicon supersaturation, annealing temperature and SiO(x) and SiO₂ layer thicknesses on the final microstructure have been investigated. It is shown that supersaturation directly determines phase separation regime between nucleation/classical growth and spinodal decomposition. Annealing temperature controls size of the particles and interface with the surrounding matrix. Layer thicknesses directly control Si-nc shapes from spherical to spinodal-like structures. Copyright © 2012 Elsevier B.V. All rights reserved.

Processing and Structural Advantages of the Sylramic-iBN SiC Fiber for SiC/SiC Components

NASA Technical Reports Server (NTRS)

Yun, H. M.; Dicarlo, J. A.; Bhatt, R. T.; Hurst, J. B.

2008-01-01

The successful high-temperature application of complex-shaped SiC/SiC components will depend on achieving as high a fraction of the as-produced fiber strength as possible during component fabrication and service. Key issues center on a variety of component architecture, processing, and service-related factors that can reduce fiber strength, such as fiber-fiber abrasion during architecture shaping, surface chemical attack during interphase deposition and service, and intrinsic flaw growth during high-temperature matrix formation and composite creep. The objective of this paper is to show that the NASA-developed Sylramic-iBN SiC fiber minimizes many of these issues for state-of-the-art melt-infiltrated (MI) SiC/BN/SiC composites. To accomplish this, data from various mechanical tests are presented that compare how different high performance SiC fiber types retain strength during formation of complex architectures, during processing of BN interphases and MI matrices, and during simulated composite service at high temperatures.

Electroplating chromium on CVD SiC and SiCf-SiC advanced cladding via PyC compatibility coating

NASA Astrophysics Data System (ADS)

Ang, Caen; Kemery, Craig; Katoh, Yutai

2018-05-01

Electroplating Cr on SiC using a pyrolytic carbon (PyC) bond coat is demonstrated as an innovative concept for coating of advanced fuel cladding. The quantification of coating stress, SEM morphology, XRD phase analysis, and debonding test of the coating on CVD SiC and SiCf-SiC is shown. The residual tensile stress (by ASTM B975) of electroplated Cr is > 1 GPa prior to stress relaxation by microcracking. The stress can remove the PyC/Cr layer from SiC. Surface etching of ∼20 μm and roughening to Ra > 2 μm (by SEM observation) was necessary for successful adhesion. The debonding strength (by ASTM D4541) of the coating on SiC slightly improved from 3.6 ± 1.4 MPa to 5.9 ± 0.8 MPa after surface etching or machining. However, this improvement is limited due to the absence of an interphase, and integrated CVI processing may be required for further advancement.

Further Developments in Modeling Creep Effects Within Structural SiC/SiC Components

NASA Technical Reports Server (NTRS)

Lang, Jerry; DiCarlo, James A.

2008-01-01

Anticipating the implementation of advanced SiC/SiC composites into turbine section components of future aero-propulsion engines, the primary objective of this on-going study is to develop physics-based analytical and finite-element modeling tools to predict the effects of constituent creep on SiC/SiC component service life. A second objective is to understand how to possibly manipulate constituent materials and processes in order to minimize these effects. Focusing on SiC/SiC components experiencing through-thickness stress gradients (e.g., airfoil leading edge), prior NASA creep modeling studies showed that detrimental residual stress effects can develop globally within the component walls which can increase the risk of matrix cracking. These studies assumed that the SiC/SiC composites behaved as isotropic viscoelastic continuum materials with creep behavior that was linear and symmetric with stress and that the creep parameters could be obtained from creep data as experimentally measured in-plane in the fiber direction of advanced thin-walled 2D SiC/SiC panels. The present study expands on those prior efforts by including constituent behavior with non-linear stress dependencies in order to predict such key creep-related SiC/SiC properties as time-dependent matrix stress, constituent creep and content effects on composite creep rates and rupture times, and stresses on fiber and matrix during and after creep.

Silicon isotope fractionations in pure Si and Fe-Si systems and their geological implications

NASA Astrophysics Data System (ADS)

Zheng, X. Y.; Beard, B. L.; Reddy, T. R.; Roden, E. E.; Johnson, C.

2016-12-01

Amorphous Si or Si-bearing materials are ubiquitous in nature, and are likely precursors to various rock types, such as cherts and banded iron formations (BIFs). Si isotope exchange kinetics and fractionation factors between these materials and aqueous Si, however, are poorly constrained, preventing a mechanistic or quantitative understanding of geological δ30Si records. A series of laboratory experiments were conducted to provide better estimates on Si isotope exchange kinetics and fractionation factors. Equilibrium Si isotope fractionation factors between Fe(III)-Si gel and aqueous Si (Δ30Sigel-aq) in artificial Archean seawater (AAS), determined by a three-isotope method with a 29Si tracer, are -2.3‰ where Fe2+ is absent from the solution, and -3.2‰ where Fe2+ is present in the solution[1]. Aqueous Fe2+ catalyzes Si isotope exchange, and causes larger Si isotope fractionation due to incorporation into the solid that may have changed Si bonding. In contrast, our preliminary results show that Δ30Sigel-aq between pure Si gel and aqueous Si at equilibrium is -0.13‰. Ongoing experiments are intended to approach the isotope equilibrium from multiple directions to resolve potential kinetic effects, and to explore temperature dependence. Nonetheless, the contrast in Δ30Sigel-aq between Fe-Si and pure Si systems highlights a significant impact of Fe on Si isotope fractionations. These results have important implications for Si isotopes in Precambrian cherts and BIFs, as well as in weathering systems in general. Silicon isotope fractionation was also studied in experiments that involved dissimilatory iron reduction of Fe(III)-Si gel by Desulfuromonas acetoxidans in AAS[2], and was found to become larger with progression of Fe reduction. A Δ30Sigel-aq of -3.5‰ was observed at 32% reduction of Fe3+. This result explains lower δ30Si values in magnetite-associated quartz that those in hematite-associated quartz in some BIFs. The large Si isotope fractionation

The Abundance of SiC2 in Carbon Star Envelopes⋆: Evidence that SiC2 is a gas-phase precursor of SiC dust

PubMed Central

Massalkhi, Sarah; Agúndez, M.; Cernicharo, J.; Velilla Prieto, L.; Goicoechea, J. R.; Quintana-Lacaci, G.; Fonfría, J. P.; Alcolea, J.; Bujarrabal, V.

2017-01-01

Context Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si–C bond detected in C-rich AGB stars are SiC2, SiC, and Si2C. To date, the ring molecule SiC2 has been observed in a handful of evolved stars, while SiC and Si2C have only been detected in the C-star envelope IRC +10216. Aims We aim to study how widespread and abundant SiC2, SiC, and Si2C are in envelopes around C-rich AGB stars and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars. Methods We carried out sensitive observations with the IRAM 30m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC, and Si2C in the λ 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC2 and to derive SiC2 fractional abundances in the observed envelopes. Results We detect SiC2 in most of the sources, SiC in about half of them, and do not detect Si2C in any source, at the exception of IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC2 line emission, which suggests that both species are chemically linked, the SiC radical probably being the photodissociation product of SiC2 in the external layer of the envelope. We find a clear trend in which the denser the envelope, the less abundant SiC2 is. The observed trend is interpreted as an evidence of efficient incorporation of SiC2 onto dust grains, a process which is favored at high densities owing to the higher rate at which collisions between particles take place. Conclusions The observed behavior of a decline in the SiC2 abundance with increasing density strongly suggests that SiC2 is an important

Disegno dello studio genomico, ambientale, microbiomico e metabolomico sulla celiachia: un approccio al futuro della prevenzione personalizzata della celiachia

PubMed Central

SERENA, GLORIA; LEONARD, MAUREEN M.; CAMHI, STEPHANIE; HUEDO-MEDINA, TANIA B.; FASANO, ALESSIO

2017-01-01

Riassunto Negli ultimi anni abbiamo assistito a un fiorire di novità cliniche e scientifiche sulla celiachia (CE), ma forse la novità più importante che influenzerà il futuro della ricerca e della clinica in questo campo riguarda la storia naturale della malattia. Per molti anni si è creduto che la predisposizione genetica e l’esposizione al glutine fossero necessarie e sufficienti allo sviluppo della CE. Studi recenti, però suggeriscono che la perdita di tolleranza al glutine possa apparire in qualsiasi momento della vita a seguito di altri elementi. Inoltre, diversi fattori ambientali conosciuti per il loro ruolo nell’influenzare la composizione della microflora intestinale sono anche stati considerati legati allo sviluppo della CE. Tra questi fattori sono inclusi la modalità di parto, la dieta dell’infante e l’uso di antibiotici. A tutt’oggi, nessuno studio longitudinale di ampia scala ha determinato se e come la composizione del microbioma e il suo profilo metabolomico possano influenzare la perdita di tolleranza al glutine e il successivo sviluppo della CE in soggetti geneticamente predisposti. In questo articolo descriviamo uno studio prospettico, multicentrico e longitudinale su infanti a rischio per la CE che utilizzerà diverse tecniche per approfondire il ruolo che il microbioma intestinale ha durante i primi passaggi dello sviluppo della malattia autoimmune. PMID:27362724

Thermogravimetric and microscopic analysis of SiC/SiC materials with advanced interfaces

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

Windisch, C.F. Jr.; Jones, R.H.; Snead, L.L.

1997-04-01

The chemical stability of SiC/SiC composites with fiber/matrix interfaces consisting of multilayers of SiC/SiC and porous SiC have been evaluated using a thermal gravimetric analyzer (TGA). Previous evaluations of SiC/SiC composites with carbon interfacial layers demonstrated the layers are not chemically stable at goal use temperatures of 800-1100{degrees}C and O{sub 2} concentrations greater than about 1 ppm. No measureable mass change was observed for multilayer and porous SiC interfaces at 800-1100{degrees}C and O{sub 2} concentrations of 100 ppm to air; however, the total amount of oxidizable carbon is on the order of the sensitivity of the TGA. Further studies aremore » in progress to evaluate the stability of these materials.« less

Vaporization of SiO2 and MgSiO3

NASA Astrophysics Data System (ADS)

Stixrude, L. P.; Xiao, B.

2016-12-01

Vaporization of SiO2 and MgSiO3B Xiaoa and L Stixrude*a, a Department of Earth Sciences, University College London, WC1E 6BT London, UK *presenting author, email: l.stixrude@ucl.ac.uk Vaporization is an important process in Earth's earliest evolution during which giant impacts are thought to have produced a transient silicate atmosphere. As experimental data are very limited, little is known of the near-critical vaporization of Earth's major oxide components: MgO and SiO2. We have performed novel ab initio molecular dynamics simulations of vapor-liquid coexistence in the SiO2 and MgSiO3 systems. The simulations, based on density functional theory using the VASP code, begin with a suitably prepared liquid slab embedded in a vacuum. During the dynamical trajectory in the canonical ensemble, we see spontaneous vaporization, leading eventually to a steady-state chemical equilibrium between the two coexisting phases. We locate the liquid-vapor critical point at 6600 K and 0.40 g/cm3 for MgSiO3 and 5300 K and 0.43 g/cm3 for SiO2. By carefully examining the trajectories, we determine the composition and speciation of the vapor. For MgSiO3, We find that the vapor is significantly richer in Mg, O, and atomic (non-molecular) species than extrapolation of low-temperature experimental data has suggested. These results will have important implications for our understanding of the initial chemistry of the Earth and Moon and the initial thermal state of Earth.

A New Ordered Si/SiO2 phase: Infrared Spectroscopy Analysis and Modeling

NASA Astrophysics Data System (ADS)

Bradley, J.; Herbots, N.; Shaw, J.; Atluri, V.; Queeney, K. T.; Chabal, Y. J.

2003-10-01

A new ordered Si/SiO2 phase is grown by conventional oxidation on ordered, OH-terminated (1x1)Si(100) surfaces formed at room temperature in ambient using a wet chemical cleaning method [1, 2] combined with conventional oxidation. Si atoms within 1-2.5 nm thick SiO2 are found to be in registry with respect to Si atoms in the Si(100). The degree of ordering is characterized by combining ion channeling with nuclear resonance analysis, as well as Reflective High Energy Electron Diffraction (RHEED), and High Resolution Transmission Electron Microscopy (HRTRM) and is found to be confined to a 2nm region in the SiO2[1]. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Elastic Recoil Deflection (ERD) were used to profile silicon, oxygen, carbon, and hydrogen coverage within the ordered interphase. Most recently, infrared spectroscopy [2] was employed to investigate the bonding at the ordered Si/SiO2 interface and compare the suboxides region to conventional thermal oxides. Infrared spectroscopy shows that the TO red-shift due to SiOx cross-bonding at the Si/SiO2 interface is 50 % smaller and occurs more abruptly than in conventional thermal oxides. This indicates a more homogeneous bonding environment between Si and SiO2, which is consistent with the presence of an ordered phase. Using these results, we are modeling the structure of the 2 nm interphase with 3DSTRING [3]. This Monte Carlo Simulation enables us to compare the channeling spectra with the experimental data for the possible phase configuration in ordered SiOx on Si. [1] N. Herbots, V. Atluri, J. D. Bradley, J. Xiang, S. Banerjee, Q.Hurst, US Patent #6,613,677, Granted 9/2/2003 [2] N. Herbots, J. M. Shaw, Q. B. Hurst, M. P. Grams, R. J. Culbertson, D. J. Smith, V. Atluri, P. Zimmerman, and K. T. Queeney, Mat. Sci. Eng. B B87, 303-316 (2001). [3] K. T. Queeney, N. Herbots, Justin, M. Shaw, V. Atluri, Y. J. Chabal (to be published)

Low Activation Joining of SiC/SiC Composites for Fusion Applications: Modeling Thermal and Irradiation-induced Swelling Effects on Integrity of Ti3SiC2/SiC Joint

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

Nguyen, Ba Nghiep; Henager, Charles H.; Kurtz, Richard J.

This work developed a continuum damage mechanics model that incorporates thermal expansion combined with irradiation-induced swelling effects to study the origin of cracking observed in recent irradiation experiments. Micromechanical modeling using an Eshelby-Mori-Tanaka approach was used to compute the thermoelastic properties of the Ti3SiC2/SiC joint needed for the model. In addition, a microstructural dual-phase Ti3SiC2/SiC model was developed to determine irradiation-induced swelling of the composite joint at a given temperature resulting from differential swelling of SiC and the Ti3SiC2 MAX phase. Three cases for the miniature torsion hourglass (THG) specimens containing a Ti3SiC2/SiC joint were analyzed corresponding to three irradiationmore » temperatures: 800oC, 500oC, and 400oC.« less

Growth and characterizaton of 3C-SiC and 6H-SiC films on 6H-SiC wafers

NASA Technical Reports Server (NTRS)

Powell, J. A.; Petit, J. B.; Matus, L. G.; Lempner, S. E.

1992-01-01

Single crystal SiC films were grown by CVD on vicinal (0001) SiC wafers cut from boules produced by the modified sublimation method. Wafers with tilt angles less than 0.5 deg yielded 3C-SiC; tilt angles of 3 to 4 deg resulted in 6H-SiC films. The surface morphology of these films, up to 24 microns thick, were investigated as a function of growth parameters such as the Si/C ratio in the input gases and the presence of dopant materials such as nitrogen and trimethylaluminum.

Advanced Environmental Barrier Coatings Developed for SiC/SiC Composite Vanes

NASA Technical Reports Server (NTRS)

Lee, Kang N.; Fox, Dennis S.; Eldridge, Jeffrey I.; Zhu, Dongming; Bansal, Narottam P.; Miller, Robert A.

2003-01-01

Ceramic components exhibit superior high-temperature strength and durability over conventional component materials in use today, signifying the potential to revolutionize gas turbine engine component technology. Silicon-carbide fiber-reinforced silicon carbide ceramic matrix composites (SiC/SiC CMCs) are prime candidates for the ceramic hotsection components of next-generation gas turbine engines. A key barrier to the realization of SiC/SiC CMC hot-section components is the environmental degradation of SiC/SiC CMCs in combustion environments. This is in the form of surface recession due to the volatilization of silica scale by water vapor. An external environmental barrier coating (EBC) is a logical approach to achieve protection and long-term durability.

Chemical compatibility issues associated with use of SiC/SiC in advanced reactor concepts

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

Wilson, Dane F.

2015-09-01

Silicon carbide/silicon carbide (SiC/SiC) composites are of interest for components that will experience high radiation fields in the High Temperature Gas Cooled Reactor (HTGR), the Very High Temperature Reactor (VHTR), the Sodium Fast Reactor (SFR), or the Fluoride-cooled High-temperature Reactor (FHR). In all of the reactor systems considered, reactions of SiC/SiC composites with the constituents of the coolant determine suitability of materials of construction. The material of interest is nuclear grade SiC/SiC composites, which consist of a SiC matrix [high-purity, chemical vapor deposition (CVD) SiC or liquid phase-sintered SiC that is crystalline beta-phase SiC containing small amounts of alumina-yttria impurity],more » a pyrolytic carbon interphase, and somewhat impure yet crystalline beta-phase SiC fibers. The interphase and fiber components may or may not be exposed, at least initially, to the reactor coolant. The chemical compatibility of SiC/SiC composites in the three reactor environments is highly dependent on thermodynamic stability with the pure coolant, and on reactions with impurities present in the environment including any ingress of oxygen and moisture. In general, there is a dearth of information on the performance of SiC in these environments. While there is little to no excess Si present in the new SiC/SiC composites, the reaction of Si with O 2 cannot be ignored, especially for the FHR, in which environment the product, SiO 2, can be readily removed by the fluoride salt. In all systems, reaction of the carbon interphase layer with oxygen is possible especially under abnormal conditions such as loss of coolant (resulting in increased temperature), and air and/ or steam ingress. A global outline of an approach to resolving SiC/SiC chemical compatibility concerns with the environments of the three reactors is presented along with ideas to quickly determine the baseline compatibility performance of SiC/SiC.« less

Transformation of sludge Si to nano-Si/SiOx structure by oxygen inward diffusion as precursor for high performance anodes in lithium ion batteries

NASA Astrophysics Data System (ADS)

Hua, Qiqi; Dai, Dongyang; Zhang, Chengzhi; Han, Fei; Lv, Tiezheng; Li, Xiaoshan; Wang, Shijie; Zhu, Rui; Liao, Haojie; Zhang, Shiguo

2018-05-01

Although several Si/C composite structures have been proposed for high-performance lithium-ion batteries (LIBs), they have still suffered from expensive and complex processes of nano-Si production. Herein, a simple, controllable oxygen inward diffusion was utilized to transform Si sludge obtained from the photovoltaic (PV) industry into the nano-Si/SiOx structure as a result of the high diffusion efficiency of O inside Si and high surface area of the sludge. After further process, a yolk/shell Si/C structure was obtained as an anode material for LIBs. This composite demonstrated an excellent cycling stability, with a high reversible capacity (˜ 1250 mAh/g for 500 cycles), by void space originally left by the SiOx accommodate inner Si expansion. We believe this is a rather simple way to convert the waste Si into a valuable nano-Si for LIB applications.

MoSi2-Base Composites

NASA Technical Reports Server (NTRS)

Hebsur, Mohan G.

2003-01-01

Addition of 30 to 50 vol% of Si3N4 particulate to MoSi2 eliminated its low temperature catastrophic failure, improved room temperature fracture toughness and the creep resistance. The hybrid composite SCS-6/MoSi2-Si3N4 did not show any matrix cracking and exhibited excellent mechanical and environmental properties. Hi-Nicalon continuous fiber reinforced MoSi2-Si3N4 also showed good strength and toughness. A new MoSi2-base composite containing in-situ whisker-type (Beta)Si3N4 grains in a MoSi2 matrix is also described.

SiC and Si3N4 Recession Due to SiO2 Scale Volatility Under Combustor Conditions

NASA Technical Reports Server (NTRS)

Smialek, James L.; Robinson, Raymond C.; Opila, Elizabeth J.; Fox, Dennis S.; Jacobson, Nathan S.

1999-01-01

Silicon carbide (SiC) and Si3N4 materials were tested in various turbine engine combustion environments chosen to represent either conventional fuel-lean or fuel-rich mixtures proposed for high-speed aircraft. Representative chemical vapor-deposited (CVD), sintered, and composite materials were evaluated by furnace and high-pressure burner rig exposures. Although protective SiO2 scales formed in all cases, the evidence presented supports a model based on paralinear growth kinetics (i.e., parabolic growth moderated simultaneously by linear volatilization). The volatility rate is dependent on temperature, moisture content, system pressure, and gas velocity. The burner tests were thus used to map SiO2 volatility (and SiC recession) over a range of temperatures, pressures, and velocities. The functional dependency of material recession (volatility) that emerged followed the form A[exp(-Q / RT)](P(sup x)v(sup y). These empirical relations were compared with rates predicted from the thermodynamics of volatile SiO and SiOxHy reaction products and a kinetic model of diffusion through a moving boundary layer. For typical combustion conditions, recession of 0.2 to 2 micrometers/hr is predicted at 1200 to 1400 C, far in excess of acceptable long-term limits.

Measurement of the absorption cross sections of SiCl4, SiCl3, SiCl2 and Cl at H Lyman-α wavelength

NASA Astrophysics Data System (ADS)

Mével, R.; Catoire, L.; Fikri, M.; Roth, P.

2013-03-01

Atomic resonance absorption spectroscopy coupled with a shock tube is a powerful technique for studying high temperature dynamics of reactive systems. Presently, high temperature pyrolysis of SiCl4-Ar mixtures has been studied behind reflected shock waves. Using time-resolved absorption profiles at 121.6 nm and a detailed reaction model, the absorption cross sections of SiCl, SiCl, SiCl and Cl have been measured. Results agree well with available data for SiCl and constitute, to our knowledge, the first measurements for SiCl, SiCl and Cl at the Lyman-α wavelength. These data are relevant to silica particle production from SiCl-oxidant mixtures combustion synthesis.

NIMROD Modeling of HIT-SI and HIT-SI3

NASA Astrophysics Data System (ADS)

Morgan, Kyle; Jarboe, Tom; Hossack, Aaron

2016-10-01

The HIT-SI and HIT-SI3 devices are spheromaks formed and sustained via a set of Steady Inductive Helicity Injectors (SIHI) that are operated in AC. The experiment explores the formation and sustain of stable spheromaks with a variety of perturbation mode structures. The HIT-SI device consisted of two injectors with primarily n = 1 toroidal symmetry while the HIT-SI3 device has three injectors capable of a mixture of n = 1 and n = 2 perturbations or a primarily n = 3 perturbation, depending on the relative phase of the injectors. Using the NIMROD code to model these devices, we are able to validate with experimental results (previously only done on HIT-SI) and examine the interaction between the injectors and the spheromak. Simulations are performed with both finite and zero- β models to gain an understanding of the thermal properties of the device. Additionally, a set of extrapolation simulations has been performed illustrating the spontaneous formation of closed flux surfaces at high current amplification. Work supported by the US DOE.

Diffusion Bonding Technology of Tungsten and SiC/SiC Composites for Nuclear Applications

NASA Astrophysics Data System (ADS)

Kishimoto, Hirotatsu; Shibayama, Tamaki; Abe, Takahiro; Shimoda, Kazuya; Kawamura, Satoshi; Kohyama, Akira

2011-10-01

Silicon carbide (SiC) is a candidate for the structural material in the next generation nuclear plants. Use of SiC/SiC composites is expected to increase the operation temperature of system over 1000 °C. For the high temperature system, refractory metals are planned to be used for several components. Tungsten is a candidate of armor on the divertor component in fusion, and is planned to be used for an upper-end plug of SiC/SiC fuel pin in a Gas cooled Fast Reactor (GFR). Joining technique of the SiC/SiC composites and tungsten is an important issue for nuclear systems in future. Nano-Infiltration and Transient Eutectoid (NITE) method is able to provide dense stable and high strength SiC/SiC composites having high resistance against pressure at elevated temperature, a diffusion bonding technique is usable to join the materials. Present research produces a NITE-SiC/SiC composite and tungsten as the similar dimension as a projected cladding tube of fuel pin for GFR using diffusion bonding, and investigated microstructure and mechanical properties.

Evaluation of CVI SiC/SiC Composites for High Temperature Applications

NASA Technical Reports Server (NTRS)

Kiser, D.; Almansour, A.; Smith, C.; Gorican, D.; Phillips, R.; Bhatt, R.; McCue, T.

2017-01-01

Silicon carbide fiber reinforced silicon carbide (SiC/SiC) composites are candidate materials for various high temperature turbine engine applications because of their high specific strength and good creep resistance at temperatures of 1400 C (2552 F) and higher. Chemical vapor infiltration (CVI) SiC/SiC ceramic matrix composites (CMC) incorporating Sylramic-iBN SiC fiber were evaluated via fast fracture tensile tests (acoustic emission damage characterization to assess cracking behavior), tensile creep testing, and microscopy. The results of this testing and observed material behavior degradation mechanisms are reviewed.

Nonisovalent Si-III-V and Si-II-VI alloys: Covalent, ionic, and mixed phases

DOE PAGES

Kang, Joongoo; Park, Ji -Sang; Stradins, Pauls; ...

2017-07-13

In this paper, nonequilibrium growth of Si-III-V or Si-II-VI alloys is a promising approach to obtaining optically more active Si-based materials. We propose a new class of nonisovalent Si 2AlP (or Si 2ZnS) alloys in which the Al-P (or Zn-S) atomic chains are as densely packed as possible in the host Si matrix. As a hybrid of the lattice-matched parent phases, Si2AlP (or Si2ZnS) provides an ideal material system with tunable local chemical orders around Si atoms within the same composition and structural motif. Here, using first-principles hybrid functional calculations, we discuss how the local chemical orders affect the electronicmore » and optical properties of the nonisovalent alloys.« less

Fabrication of n-type Si nanostructures by direct nanoimprinting with liquid-Si ink

NASA Astrophysics Data System (ADS)

Takagishi, Hideyuki; Masuda, Takashi; Yamazaki, Ken; Shimoda, Tatsuya

2018-01-01

Nanostructures of n-type amorphous silicon (a-Si) and polycrystalline silicon (poly-Si) with a height of 270 nm and line widths of 110-165 nm were fabricated directly onto a substrate through a simple imprinting process that does not require vacuum conditions or photolithography. The n-type Liquid-Si ink was synthesized via photopolymerization of cyclopentasilane (Si5H10) and white phosphorus (P4). By raising the temperature from 160 °C to 200 °C during the nanoimprinting process, well-defined angular patterns were fabricated without any cracking, peeling, or deflections. After the nanoimprinting process, a-Si was produced by heating the nanostructures at 400°C-700 °C, and poly-Si was produced by heating at 800 °C. The dopant P diffuses uniformly in the Si films, and its concentration can be controlled by varying the concentration of P4 in the ink. The specific resistance of the n-type poly-Si pattern was 7.0 × 10-3Ω ṡ cm, which is comparable to the specific resistance of flat n-type poly-Si films.

Location and Electronic Nature of Phosphorus in the Si Nanocrystal − SiO2 System

PubMed Central

König, Dirk; Gutsch, Sebastian; Gnaser, Hubert; Wahl, Michael; Kopnarski, Michael; Göttlicher, Jörg; Steininger, Ralph; Zacharias, Margit; Hiller, Daniel

2015-01-01

Up to now, no consensus exists about the electronic nature of phosphorus (P) as donor for SiO2-embedded silicon nanocrystals (SiNCs). Here, we report on hybrid density functional theory (h-DFT) calculations of P in the SiNC/SiO2 system matching our experimental findings. Relevant P configurations within SiNCs, at SiNC surfaces, within the sub-oxide interface shell and in the SiO2 matrix were evaluated. Atom probe tomography (APT) and its statistical evaluation provide detailed spatial P distributions. For the first time, we obtain ionisation states of P atoms in the SiNC/SiO2 system at room temperature using X-ray absorption near edge structure (XANES) spectroscopy, eliminating structural artefacts due to sputtering as occurring in XPS. K energies of P in SiO2 and SiNC/SiO2 superlattices (SLs) were calibrated with non-degenerate P-doped Si wafers. results confirm measured core level energies, connecting and explaining XANES spectra with h-DFT electronic structures. While P can diffuse into SiNCs and predominantly resides on interstitial sites, its ionization probability is extremely low, rendering P unsuitable for introducing electrons into SiNCs embedded in SiO2. Increased sample conductivity and photoluminescence (PL) quenching previously assigned to ionized P donors originate from deep defect levels due to P. PMID:25997696

SEMICONDUCTOR TECHNOLOGY: SBH adjustment characteristic of the dopant segregation process for NiSi/n-Si SJDs

NASA Astrophysics Data System (ADS)

Haiping, Shang; Qiuxia, Xu

2010-05-01

By means of analyzing the I-V characteristic curve of NiSi/n-Si Schottky junction diodes (NiSi/n-Si SJDs), abstracting the effective Schottky barrier height (varphiB, eff) and the ideal factor of NiSi/n-Si SJDs and measuring the sheet resistance of NiSi films (RNiSi), we study the effects of different dopant segregation process parameters, including impurity implantation dose, segregation annealing temperature and segregation annealing time, on the varphiB, eff of NiSi/n-Si SJDs and the resistance characteristic of NiSi films. In addition, the changing rules of varphiB, eff and RNiSi are discussed.

Construction and characterization of spherical Si solar cells combined with SiC electric power inverter

NASA Astrophysics Data System (ADS)

Oku, Takeo; Matsumoto, Taisuke; Hiramatsu, Kouichi; Yasuda, Masashi; Shimono, Akio; Takeda, Yoshikazu; Murozono, Mikio

2015-02-01

Spherical silicon (Si) photovoltaic solar cell systems combined with an electric power inverter using silicon carbide (SiC) field-effect transistor (FET) were constructed and characterized, which were compared with an ordinary Si-based converter. The SiC-FET devices were introduced in the direct current-alternating current (DC-AC) converter, which was connected with the solar panels. The spherical Si solar cells were used as the power sources, and the spherical Si panels are lighter and more flexible compared with the ordinary flat Si solar panels. Conversion efficiencies of the spherical Si solar cells were improved by using the SiC-FET.

High-pressure transitions of diopside and wollastonite: phase equilibria and thermochemistry of CaMgSi 2O 6, CaSiO 3 and CaSi 2O 5-CaTiSiO 5 system

NASA Astrophysics Data System (ADS)

Akaogi, M.; Yano, M.; Tejima, Y.; Iijima, M.; Kojitani, H.

2004-06-01

Phase transitions of CaMgSi 2O 6 diopside and CaSiO 3 wollastonite were examined at pressures to 23 GPa and temperatures to 2000 °C, using a Kawai-type multiavil apparatus. Enthalpies of high-pressure phases in CaSiO 3 and in the CaSi 2O 5-CaTiSiO 5 system were also measured by high-temperature calorimetry. At 17-18 GPa, diopside dissociates to CaSiO 3-rich perovskite + Mg-rich (Mg,Ca)SiO 3 tetragonal garnet (Gt) above about 1400 °C. The solubilities of CaSiO 3 in garnet and MgSiO 3 in perovskite increase with temperature. At 17-18 GPa below about 1400 °C, diopside dissociates to Ca-perovskite + β-Mg 2SiO 4 + stishovite. The Mg, Si-phases coexisting with Ca-perovskite change to γ-Mg 2SiO 4 + stishovite, to ilmenite, and finally to Mg-perovskite with increasing pressure. CaSiO 3 wollastonite transforms to the walstromite structure, and further dissociates to Ca 2SiO 4 larnite + CaSi 2O 5 titanite. The latter transition occurs at 9-11 GPa with a positive Clapeyron slope. At 1600 °C, larnite + titanite transform to CaSiO 3 perovskite at 14.6±0.6 GPa, calibrated against the α-β transition pressure of Mg 2SiO 4. The enthalpies of formation of CaSiO 3 walstromite and CaSi 2O 5 titanite from the mixture of CaO and SiO 2 quartz at 298 K have been determined as -76.1±2.8, and -27.8±2.1 kJ/mol, respectively. The latter was estimated from enthalpy measurements of titanite solid solutions in the system CaSi 2O 5-CaTiSiO 5, because CaSi 2O 5 titanite transforms to a triclinic phase upon decompression. The enthalpy difference between titanite and the triclinic phase is only 1.5±4.8 kJ/mol. Using these enthalpies of formation and those of larnite and CaSiO 3 perovskite, the transition boundaries in CaSiO 3 have been calculated. The calculated boundaries for the wollastonite-walstromite-larnite + titanite transitions are consistent with the experimental determinations within the errors. The calculated boundary between larnite + titanite and Ca-perovskite has a slope of

Effect of the SiCl4 Flow Rate on SiBN Deposition Kinetics in SiCl4-BCl3-NH3-H2-Ar Environment

PubMed Central

Li, Jianping; Qin, Hailong; Liu, Yongsheng; Ye, Fang; Li, Zan; Cheng, Laifei; Zhang, Litong

2017-01-01

To improve the thermal and mechanical stability of SiCf/SiC or C/SiC composites with SiBN interphase, SiBN coating was deposited by low pressure chemical vapor deposition (LPCVD) using SiCl4-BCl3-NH3-H2-Ar gas system. The effect of the SiCl4 flow rate on deposition kinetics was investigated. Results show that deposition rate increases at first and then decreases with the increase of the SiCl4 flow rate. The surface of the coating is a uniform cauliflower-like structure at the SiCl4 flow rate of 10 mL/min and 20 mL/min. The surface is covered with small spherical particles when the flow rate is 30 mL/min. The coatings deposited at various SiCl4 flow rates are all X-ray amorphous and contain Si, B, N, and O elements. The main bonding states are B-N, Si-N, and N-O. B element and B-N bonding decrease with the increase of SiCl4 flow rate, while Si element and Si-N bonding increase. The main deposition mechanism refers to two parallel reactions of BCl3+NH3 and SiCl4+NH3. The deposition process is mainly controlled by the reaction of BCl3+NH3. PMID:28772986

Active Oxidation of SiC

NASA Technical Reports Server (NTRS)

Jacobson, Nathan S.; Myers,Dwight L.; Harder, Bryan J.

2011-01-01

The high temperature oxidation of silicon carbide occurs in either a passive or active mode, depending on temperature and oxygen potential. Passive oxidation forms a protective oxide film which limits attack of the SiC:SiC(s) + 3/2 O2(g) = SiO2(s) + CO(g.) Active oxidation forms a volatile oxide and leads to extensive attack of the SiC: SiC(s) + O2(g) = SiO(g) + CO(g). The transition points and rates of active oxidation are a major issue. Previous studies are reviewed and the leading theories of passive/active transitions summarized. Comparisons are made to the active/passive transitions in pure Si, which are relatively well-understood. Critical questions remain about the difference between the active-to-passive transition and passive-to-active transition. For Si, Wagner [2] points out that the active-to-passive transition is governed by the criterion for a stable Si/SiO2 equilibria and the passive-to-active transition is governed by the decomposition of the SiO2 film. This suggests a significant oxygen potential difference between these two transitions and our experiments confirm this. For Si, the initial stages of active oxidation are characterized by the formation of SiO(g) and further oxidation to SiO2(s) as micron-sized rods, with a distinctive morphology. SiC shows significant differences. The active-to-passive and the passive-to-active transitions are close. The SiO2 rods only appear as the passive film breaks down. These differences are explained in terms of the reactions at the SiC/SiO2 interface. In order to understand the breakdown of the passive film, pre-oxidation experiments are conducted. These involve forming dense protective scales of 0.5, 1, and 2 microns and then subjecting the samples with these scales to a known active oxidation environment. Microstructural studies show that SiC/SiO2 interfacial reactions lead to a breakdown of the scale with a distinct morphology.

Static and dynamic behavior of a Si/Si0.8Ge0.2/Si heterojunction bipolar transistor using Monte Carlo simulation

NASA Astrophysics Data System (ADS)

Galdin, Sylvie; Dollfus, Philippe; Hesto, Patrice

1994-03-01

A theoretical study of a Si/Si1-xGex/Si heterojunction bipolar transistor using Monte Carlo simulations is reported. The geometry and composition of the emitter-base junction are optimized using one-dimensional simulations with a view to improving electron transport in the base. It is proposed to introduce a thin Si-P spacer layer, between the Si-N emitter and the SiGe-P base, which allows launching hot electrons into the base despite the lack of natural conduction-band discontinuity between Si and strain SiGe. The high-frequency behavior of the complete transistor is then studied using 2D modeling. A method of microwave analysis using small signal Monte Carlo simulations that consists of expanding the terminal currents in Fourier series is presented. A cutoff frequency fT of 68 GHz has been extracted. Finally, the occurrence of a parasitic electron barrier at the collector-base junction is responsible for the fT fall-off at high collector current density. This parasitic barrier is lowered through the influence of the collector potential.

Boron doped Si rich oxide/SiO{sub 2} and silicon rich nitride/SiN{sub x} bilayers on molybdenum-fused silica substrates for vertically structured Si quantum dot solar cells

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

Lin, Ziyun, E-mail: z.lin@unsw.edu.au; Wu, Lingfeng; Jia, Xuguang

2015-07-28

Vertically structured Si quantum dots (QDs) solar cells with molybdenum (Mo) interlayer on quartz substrates would overcome current crowding effects found in mesa-structured cells. This study investigates the compatibility between boron (B) doped Si QDs bilayers and Mo-fused silica substrate. Both Si/SiO{sub 2} and Si/SiN{sub x} based QDs bilayers were studied. The material compatibility under high temperature treatment was assessed by examining Si crystallinity, microstress, thin film adhesion, and Mo oxidation. It was observed that the presence of Mo interlayer enhanced the Si QDs size confinement, crystalline fraction, and QDs size uniformity. The use of B doping was preferred comparedmore » to phosphine (PH{sub 3}) doping studied previously in terms of better surface and interface properties by reducing oxidized spots on the film. Though crack formation due to thermal mismatch after annealing remained, methods to overcome this problem were proposed in this paper. Schematic diagram to fabricate full vertical structured Si QDs solar cells was also suggested.« less

Bending and buckling of rolled-up SiGe /Si microtubes using nanorobotic manipulation

NASA Astrophysics Data System (ADS)

Zhang, Li; Dong, Lixin; Nelson, Bradley J.

2008-06-01

Mechanical properties of individual rolled-up SiGe /Si microtubes are investigated experimentally using nanorobotic manipulation. By applying bending loads, individual SiGe /Si microtubes demonstrate various deformation modes with increasing bending angle. Remarkably, the tested microtubes resist fracture even when bent back onto themselves (180° bending angle). Axial compression tests of microtubes with different turns are also performed. Among those tubes, 1.6-turn rolled-up SiGe /Si microtubes show typical Euler buckling behavior when the load is larger than a critical load, which can be estimated by the Euler formula for columns.

Elastic and inelastic properties of SiC/Si biomorphic composites and biomorphic SiC based on oak and eucalyptus

NASA Astrophysics Data System (ADS)

Kardashev, B. K.; Nefagin, A. S.; Smirnov, B. I.; de Arellano-Lopez, A. R.; Martinez-Fernandez, J.; Sepulveda, R.

2006-09-01

This paper reports on the results of a comparative investigation into the elastic and microplastic properties of biomorphic SiC/Si composites and biomorphic SiC prepared by pyrolysis of oak and eucalyptus with subsequent infiltration of molten silicon into a carbon matrix and additional chemical treatment to remove excess silicon. The acoustic studies were performed by the composite oscillator technique using resonant longitudinal vibrations at frequencies of about 100 kHz. It is shown that, in biomorphic SiC (as in biomorphic SiC/Si) at small-amplitude strains ɛ, adsorption and desorption of the environmental (air) molecules determine to a considerable extent the Young’s modulus E and the internal friction (decrement of acoustic vibrations δ) and that the changes in E and δ at these amplitudes are irreversible. The stress-microplastic strain curves are constructed from the acoustic data for the materials under study at temperatures of 100 and 290 K.

Fracture Mechanisms For SiC Fibers And SiC/SiC Composites Under Stress-Rupture Conditions at High Temperatures

NASA Technical Reports Server (NTRS)

DiCarlo, James A.; Yun, Hee Mann; Hurst, Janet B.; Viterna, L. (Technical Monitor)

2002-01-01

The successful application of SiC/SiC ceramic matrix composites as high-temperature structural materials depends strongly on maximizing the fracture or rupture life of the load-bearing fiber and matrix constituents. Using high-temperature data measured under stress-rupture test conditions, this study examines in a mechanistic manner the effects of various intrinsic and extrinsic factors on the creep and fracture behavior of a variety of SiC fiber types. It is shown that although some fiber types fracture during a large primary creep stage, the fiber creep rate just prior to fracture plays a key role in determining fiber rupture time (Monkman-Grant theory). If it is assumed that SiC matrices rupture in a similar manner as fibers with the same microstructures, one can develop simple mechanistic models to analyze and optimize the stress-rupture behavior of SiC/SiC composites for applied stresses that are initially below matrix cracking.

Wafer-scale high-throughput ordered arrays of Si and coaxial Si/Si(1-x)Ge(x) wires: fabrication, characterization, and photovoltaic application.

PubMed

Pan, Caofeng; Luo, Zhixiang; Xu, Chen; Luo, Jun; Liang, Renrong; Zhu, Guang; Wu, Wenzhuo; Guo, Wenxi; Yan, Xingxu; Xu, Jun; Wang, Zhong Lin; Zhu, Jing

2011-08-23

We have developed a method combining lithography and catalytic etching to fabricate large-area (uniform coverage over an entire 5-in. wafer) arrays of vertically aligned single-crystal Si nanowires with high throughput. Coaxial n-Si/p-SiGe wire arrays are also fabricated by further coating single-crystal epitaxial SiGe layers on the Si wires using ultrahigh vacuum chemical vapor deposition (UHVCVD). This method allows precise control over the diameter, length, density, spacing, orientation, shape, pattern and location of the Si and Si/SiGe nanowire arrays, making it possible to fabricate an array of devices based on rationally designed nanowire arrays. A proposed fabrication mechanism of the etching process is presented. Inspired by the excellent antireflection properties of the Si/SiGe wire arrays, we built solar cells based on the arrays of these wires containing radial junctions, an example of which exhibits an open circuit voltage (V(oc)) of 650 mV, a short-circuit current density (J(sc)) of 8.38 mA/cm(2), a fill factor of 0.60, and an energy conversion efficiency (η) of 3.26%. Such a p-n radial structure will have a great potential application for cost-efficient photovoltaic (PV) solar energy conversion. © 2011 American Chemical Society

Reduction in interface defect density in p-BaSi2/n-Si heterojunction solar cells by a modified pretreatment of the Si substrate

NASA Astrophysics Data System (ADS)

Yamashita, Yudai; Yachi, Suguru; Takabe, Ryota; Sato, Takuma; Emha Bayu, Miftahullatif; Toko, Kaoru; Suemasu, Takashi

2018-02-01

We have investigated defects that occurred at the interface of p-BaSi2/n-Si heterojunction solar cells that were fabricated by molecular beam epitaxy. X-ray diffraction measurements indicated that BaSi2 (a-axis-oriented) was subjected to in-plane compressive strain, which relaxed when the thickness of the p-BaSi2 layer exceeded 50 nm. Additionally, transmission electron microscopy revealed defects in the Si layer near steps that were present on the Si(111) substrate. Deep level transient spectroscopy revealed two different electron traps in the n-Si layer that were located at 0.33 eV (E1) and 0.19 eV (E2) below the conduction band edge. The densities of E1 and E2 levels in the region close to the heterointerface were approximately 1014 cm-3. The density of these electron traps decreased below the limits of detection following Si pretreatment to remove the oxide layers from the n-Si substrate, which involved heating the substrate to 800 °C for 30 min under ultrahigh vacuum while depositing a layer of Si (1 nm). The remaining traps in the n-Si layer were hole traps located at 0.65 eV (H1) and 0.38 eV (H2) above the valence band edge. Their densities were as low as 1010 cm-3. Following pretreatment, the current versus voltage characteristics of the p-BaSi2/n-Si solar cells under AM1.5 illumination were reproducible with conversion efficiencies beyond 5% when using a p-BaSi2 layer thickness of 100 nm. The origin of the H2 level is discussed.

Friction and Wear Properties of CrSiCN/SiC Tribopairs in Water Lubrication

NASA Astrophysics Data System (ADS)

Wu, Zhiwei; Zhou, Fei; Wang, Qianzhi

2018-05-01

CrSiCN coatings (3.4 at.% Si) were prepared on 316L stainless steels using unbalanced magnetron sputtering. According to the analysis results of x-ray diffractometer and x-ray photoelectrons spectroscopy, silicon in CrSiCN coatings mainly presented in the amorphous forms of a-SiN, a-SiCN and a-SiC. The hardness and Young's modulus of CrSiN coatings were 19.4 ± 0.6 and 306.1 ± 5.9 GPa, respectively. In addition, the ball-on-disk sliding tests of CrSiCN/SiC tribopairs were performed in distilled water at varying velocities (0.1-0.5 m/s) and loads (2-12 N). The friction coefficient of tribopairs presented a decreasing trend with respect to velocity at low applied loads (≤ 4 N). To be specific, the low friction coefficient of 0.05-0.14 accompanied with polished wear scar was obtained at high velocities and low loads. Finally, the wear mechanism map of CrSiCN/SiC tribopairs was proposed based on a combination of friction coefficient, wear scar morphology and wear rates of tribopairs.

Sintering activation energy MoSi2-WSi2-Si3N4 ceramic

NASA Astrophysics Data System (ADS)

Titov, D. D.; Lysenkov, A. S.; Kargin, Yu F.; Frolova, M. G.; Gorshkov, V. A.; Perevislov, S. N.

2018-04-01

The activation energy of sintering process was calculated based on dilatometric studies of shrinkage processes (Mo,W)Si2 + Si3N4 composite ceramic. (Mo,W)Si2 powders was obtained by solid-phase solutions of 70 wt% MoSi2 and 30 wt% WSi2 by SHS in the ISMAN RAS. The concentration rate Si3N4 was from 1 to 15 wt.%. The sintering was carried out to 1850°C in Ar atmosphere the heating rate of 5, 10, 12 and 15°C/min by the way of dilatometer tests. Based on the differential kinetic analysis method (Friedman’s method), the sintering process activation energy of (Mo,W)Si2 + Si3N4 were calculated. The two-stage sintering process and the dependence of the activation energy on the Si3N4 content was shown. Average value of 370 kJ/mol for Q was obtained.

High Mobility Transport Layer Structures for Rhombohedral Si/Ge/SiGe Devices

NASA Technical Reports Server (NTRS)

Choi, Sang Hyouk (Inventor); Park, Yeonjoon (Inventor); King, Glen C. (Inventor); Kim, Hyun-Jung (Inventor); Lee, Kunik (Inventor)

2017-01-01

An electronic device includes a trigonal crystal substrate defining a (0001) C-plane. The substrate may comprise Sapphire or other suitable material. A plurality of rhombohedrally aligned SiGe (111)-oriented crystals are disposed on the (0001) C-plane of the crystal substrate. A first region of material is disposed on the rhombohedrally aligned SiGe layer. The first region comprises an intrinsic or doped Si, Ge, or SiGe layer. The first region can be layered between two secondary regions comprising n+doped SiGe or n+doped Ge, whereby the first region collects electrons from the two secondary regions.

SiC/Si{sub 3}N{sub 4} nanotubes from peanut shells

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

Qadri, S. B.; Rath, B. B.; Gorzkowski, E. P.

2016-06-15

Nanotubes and nanoparticles of SiC and Si{sub 3}N{sub 4} were produced from the thermal treatment of peanut shells in argon and nitrogen atmospheres respectively, at temperatures in excess of 1350°C. Using x-ray diffraction, Raman spectroscopy and transmission electron microscopy analysis, the processed samples in argon atmosphere were shown to consist of 2H and 3C polytypes of SiC nanoparticles and nanotubes. Whereas the samples prepared in nitrogen atmosphere consisted of α-phase of Si{sub 3}N{sub 4}. Nanostructures formed by a single direct reaction provide a sustainable synthesis route for nanostructured SiC and Si{sub 3}N{sub 4}, for potential engineering applications due to theirmore » exceptional mechanical and electro-optic properties.« less

Quantitative analysis of hydrogen in SiO{sub 2}/SiN/SiO{sub 2} stacks using atom probe tomography

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

Kunimune, Yorinobu, E-mail: yorinobu.kunimune.vz@renesas.com; Shimada, Yasuhiro; Sakurai, Yusuke

2016-04-15

We have demonstrated that it is possible to reproducibly quantify hydrogen concentration in the SiN layer of a SiO{sub 2}/SiN/SiO{sub 2} (ONO) stack structure using ultraviolet laser-assisted atom probe tomography (APT). The concentration of hydrogen atoms detected using APT increased gradually during the analysis, which could be explained by the effect of hydrogen adsorption from residual gas in the vacuum chamber onto the specimen surface. The amount of adsorbed hydrogen in the SiN layer was estimated by analyzing another SiN layer with an extremely low hydrogen concentration (<0.2 at. %). Thus, by subtracting the concentration of adsorbed hydrogen, the actualmore » hydrogen concentration in the SiN layer was quantified as approximately 1.0 at. %. This result was consistent with that obtained by elastic recoil detection analysis (ERDA), which confirmed the accuracy of the APT quantification. The present results indicate that APT enables the imaging of the three-dimensional distribution of hydrogen atoms in actual devices at a sub-nanometer scale.« less

Modulating the Surface State of SiC to Control Carrier Transport in Graphene/SiC.

PubMed

Jia, Yuping; Sun, Xiaojuan; Shi, Zhiming; Jiang, Ke; Liu, Henan; Ben, Jianwei; Li, Dabing

2018-05-28

Silicon carbide (SiC) with epitaxial graphene (EG/SiC) shows a great potential in the applications of electronic and photoelectric devices. The performance of devices is primarily dependent on the interfacial heterojunction between graphene and SiC. Here, the band structure of the EG/SiC heterojunction is experimentally investigated by Kelvin probe force microscopy. The dependence of the barrier height at the EG/SiC heterojunction to the initial surface state of SiC is revealed. Both the barrier height and band bending tendency of the heterojunction can be modulated by controlling the surface state of SiC, leading to the tuned carrier transport behavior at the EG/SiC interface. The barrier height at the EG/SiC(000-1) interface is almost ten times that of the EG/SiC(0001) interface. As a result, the amount of carrier transport at the EG/SiC(000-1) interface is about ten times that of the EG/SiC(0001) interface. These results offer insights into the carrier transport behavior at the EG/SiC heterojunction by controlling the initial surface state of SiC, and this strategy can be extended in all devices with graphene as the top layer. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

NIMROD Simulations of the HIT-SI and HIT-SI3 Devices

NASA Astrophysics Data System (ADS)

Morgan, Kyle; Jarboe, Tom; Hossack, Aaron; Chandra, Rian; Everson, Chris

2017-10-01

The Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI) experiment uses a set of inductively driven helicity injectors to apply non-axisymmetric current drive on the edge of the plasma, driving an axisymmetric spheromak equilibrium in a central confinement volume. Significant improvements have been made to extended MHD modeling of HIT-SI, with both the resolution of disagreement at high injector frequencies in HIT-SI in addition to successes with the new upgraded HIT-SI3 device. Previous numerical studies of HIT-SI, using a zero-beta eMHD model, focused on operations with a drive frequency of 14.5 kHz, and found reduced agreement with both the magnetic profile and current amplification at higher frequencies (30-70 kHz). HIT-SI3 has three helicity injectors which are able to operate with different mode structures of perturbations through the different relative temporal phasing of the injectors. Simulations that allow for pressure gradients have been performed in the parameter regimes of both devices using the NIMROD code and show improved agreement with experimental results, most notably capturing the observed Shafranov-shift due to increased beta observed at higher finj in HIT-SI and the variety of toroidal perturbation spectra available in HIT-SI3. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences under Award Number DE-FG02- 96ER54361.

Non-switching to switching transferring mechanism investigation for Ag/SiO x /p-Si structure with SiO x deposited by HWCVD

NASA Astrophysics Data System (ADS)

Liu, Yanhong; Wang, Ruoying; Li, Zhongyue; Wang, Song; Huang, Yang; Peng, Wei

2018-04-01

We proposed and fabricated an Ag/SiO x /p-Si sandwich structure, in which amorphous SiO x films were deposited through hot wire chemical vapor deposition (HWCVD) using tetraethylorthosilicate (TEOS) as Si and O precursor. Experimental results indicate that the I–V properties of this structure transfer from non-switching to switching operation as the SiO x deposition temperature increased. The device with SiO x deposited at high deposition temperature exhibits typical bipolar switching properties, which can be potentially used in resistive switching random accessible memory (RRAM). The transferring mechanism from non-switching to switching can be ascribed to the change of structural and electronic properties of SiO x active layer deposited at different temperatures, as evidenced by analyzing FTIR spectrum and fitting its I–V characteristics curves. This work demonstrates a safe and practicable low-temperature device-grade SiO x film deposition technology by conducting HWCVD from TEOS.

SI Notes.

ERIC Educational Resources Information Center

Nelson, Robert A.

1983-01-01

Discusses legislation related to SI (International Systems of Units) in the United States. Indicates that although SI metric units have been officially recognized by law in the United States, U.S. Customary Units have never received a statutory basis. (JN)

Polarized luminescence of nc-Si-SiO x nanostructures on silicon substrates with patterned surface

NASA Astrophysics Data System (ADS)

Michailovska, Katerina; Mynko, Viktor; Indutnyi, Ivan; Shepeliavyi, Petro

2018-05-01

Polarization characteristics and spectra of photoluminescence (PL) of nc-Si-SiO x structures formed on the patterned and plane c-Si substrates are studied. The interference lithography with vacuum chalcogenide photoresist and anisotropic wet etching are used to form a periodic relief (diffraction grating) on the surface of the substrates. The studied nc-Si-SiO x structures were produced by oblique-angle deposition of Si monoxide in vacuum and the subsequent high-temperature annealing. The linear polarization memory (PM) effect in PL of studied structure on plane substrate is manifested only after the treatment of the structures in HF and is explained by the presence of elongated Si nanoparticles in the SiO x nanocolumns. But the PL output from the nc-Si-SiO x structure on the patterned substrate depends on how this radiation is polarized with respect to the grating grooves and is much less dependent on the polarization of the exciting light. The measured reflection spectra of nc-Si-SiO x structure on the patterned c-Si substrate confirmed the influence of pattern on the extraction of polarized PL.

Enhanced photovoltaic property by forming p-i-n structures containing Si quantum dots/SiC multilayers

PubMed Central

2014-01-01

Si quantum dots (Si QDs)/SiC multilayers were fabricated by annealing hydrogenated amorphous Si/SiC multilayers prepared in a plasma-enhanced chemical vapor deposition system. The thickness of amorphous Si layer was designed to be 4 nm, and the thickness of amorphous SiC layer was kept at 2 nm. Transmission electron microscopy observation revealed the formation of Si QDs after 900°C annealing. The optical properties of the Si QDs/SiC multilayers were studied, and the optical band gap deduced from the optical absorption coefficient result is 1.48 eV. Moreover, the p-i-n structure with n-a-Si/i-(Si QDs/SiC multilayers)/p-Si was fabricated, and the carrier transportation mechanism was investigated. The p-i-n structure was used in a solar cell device. The cell had the open circuit voltage of 532 mV and the power conversion efficiency (PCE) of 6.28%. PACS 81.07.Ta; 78.67.Pt; 88.40.jj PMID:25489285

The Degradation Behavior of SiCf/SiO2 Composites in High-Temperature Environment

NASA Astrophysics Data System (ADS)

Yang, Xiang; Cao, Feng; Qing, Wang; Peng, Zhi-hang; Wang, Yi

2018-04-01

SiCf/SiO2 composites had been fabricated efficiently by Sol-Gel method. The oxidation behavior, thermal shock property and ablation behavior of SiCf/SiO2 composites was investigated. SiCf/SiO2 composites showed higher oxidation resistance in oxidation atmosphere, the flexural strength retention ratio was larger than 90.00%. After 1300 °C thermal shock, the mass retention ratio was 97.00%, and the flexural strength retention ratio was 92.60%, while after 1500 °C thermal shock, the mass retention ratio was 95.37%, and the flexural strength retention ratio was 83.34%. After 15 s ablation, the mass loss rate was 0.049 g/s and recession loss rate was 0.067 mm/s. The SiO2 matrix was melted in priority and becomes loosen and porous. With the ablation going on, the oxides were washed away by the shearing action of the oxyacetylene flame. The evaporation of SiO2 took away large amount of heat, which is also beneficial to the protection for SiCf/SiO2 composites.

Porous nC-Si/SiOx nanostructured layer on Si substrate with tunable photoluminescent properties fabricated by direct, precursor-free microplasma irradiation in air

NASA Astrophysics Data System (ADS)

Wang, Tao; Hu, Mingshan; Yang, Bin; Wang, Xiaolin; Liu, Jingquan

2018-03-01

Porous nC-Si/SiOx photoluminescent nanostructured layer is fabricated by direct, precursor-free microplasma irradiation on Si substrate in air. It is confirmed that the deposited layer has porous and cluster-like structures by scanning electron microscopy (SEM) and profile scanning. Fourier transform infrared transmission (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS) results indicate the produced layer is actually composed of nanocrystalline silicon (nC-Si) embedded in SiOx matrix. Transmission electron microscopy (TEM) and Raman results show the mean particle size of nC-Si is mainly between 2 and 4 nm and the highest crystalline volume fraction reaches 86.9%. The photoluminescence (PL) measurement of nC-Si/SiOx layer exhibited a broad band centered at 1.7-1.9 eV, ranging from 1.2-2.4 eV, and could be tuned by varying the applied voltage. The synthetical mechanisms are discussed to explain the PL properties of the layers. We propose that the energetic ions bombing induced by high compressed electric field near the Si surface is the main reason for porous nC-Si/SiOx formation. Maskless deposition of the line pattern of nC-Si/SiOx layer was also successfully fabricated. This simple, maskless, vacuum-free and precursor-free technique could be used in various potential optoelectronics and biological applications in the future.

Formation of crystalline heteroepitaxial SiC films on Si by carbonization of polyimide Langmuir-Blodgett films

NASA Astrophysics Data System (ADS)

Luchinin, Viktor V.; Goloudina, Svetlana I.; Pasyuta, Vyacheslav M.; Panov, Mikhail F.; Smirnov, Alexander N.; Kirilenko, Demid A.; Semenova, Tatyana F.; Sklizkova, Valentina P.; Gofman, Iosif V.; Svetlichnyi, Valentin M.; Kudryavtsev, Vladislav V.

2017-06-01

High-quality crystalline nano-thin SiC films on Si substrates were prepared by carbonization of polyimide (PI) Langmuir-Blodgett (LB) films. The obtained films were characterized by Fourier transform-infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, Raman spectroscopy, transmission electon microscopy (TEM), transmission electron diffraction (TED), and scanning electron microscopy (SEM). We demonstrated that the carbonization of a PI film on a Si substrate at 1000 °C leads to the formation of a carbon film and SiC nanocrystals on the Si substrate. It was found that five planes in the 3C-SiC(111) film are aligned with four Si(111) planes. As a result of repeated annealing of PI films containing 121 layers at 1200 °C crystalline SiC films were formed on the Si substrate. It was shown that the SiC films (35 nm) grown on Si(111) at 1200 °C have a mainly cubic 3C-SiC structure with small amount of hexagonal polytypes. Only 3C-SiC films (30 nm) were formed on the Si(100) substrate at the same temperature. It was shown that the SiC films (30-35 nm) can cover the voids with size up to 10 µm in the Si substrate. The current-voltage (I-V) characteristics of the n-Si/n-SiC heterostructure were obtained by conductive atomic force microscopy.

Oxidation of TaSi2-Containing ZrB2-SiC Ultra-High Temperature Materials

NASA Technical Reports Server (NTRS)

Opila, Elizabeth J.; Smith, Jim; Levine, Stanley R.; Lorincz, Jonathan; Reigel, Marissa

2010-01-01

Hot pressed coupons of composition ZrB2-20 v% SiC-5 v% TaSi2 and ZrB2-20 v% SiC-20 v% TaSi2 were oxidized in stagnant air at temperatures of 1627 and 1927C for one, five and ten 10-minute cycles. The oxidation reactions were characterized by weight change kinetics, x-ray diffraction, and SEM/EDS. Detailed WDS/microprobe quantitative analyses of the oxidation products were conducted for the ZrB2-20 v% SiC-20 v% TaSi2 sample oxidized for five 10-minute cycles at 1927C. Oxidation kinetics and product formation were compared to ZrB2-20 v% SiC with no TaSi2 additions. It was found that the 20 v% TaSi2 composition exhibited improved oxidation resistance relative to the material with no TaSi2 additions at 1627C. However, for exposures at 1927C less oxidation resistance and extensive liquid phase formation were observed compared to the material with no TaSi2 additions. Attempts to limit the liquid phase formation by reducing the TaSi2 content to 5 v% were unsuccessful. In addition, the enhanced oxidation resistance at 1627C due to 20 v% TaSi2 additions was not achieved at the 5 v% addition level. The observed oxidation product evolution is discussed in terms of thermodynamics and phase equilibria for the TaSi2-containing ZrB2-SiC material system. TaSi2-additions to ZrB2-SiC at any level are not recommended for ultra-high temperature (>1900C) applications due to excessive liquid phase formation.

Comments on "Determination and Analysis of the Theoretical Production of a Bucket Wheel Excavator" / Uwagi I Komentarze Do Pracy: "Określanie I Analiza Teoretycznej Wydajności Pracy Koparki Wielonaczyniowej Kołowej"

NASA Astrophysics Data System (ADS)

Bošnjak, Srđan M.

2015-03-01

This paper comments on the recently published work dealing with the problem in the determination of the theoretical output of the bucket wheel excavator. It also includes remarks on the inadequacy in the problem approach and highlights the mistakes in the mathematical model. This work emphasizes the demand for a much wider and deeper approach to the problem of determining the output of the bucket wheel excavator. Opublikowany niedawno artykuł autorstwa Che i Chena (2014) poświecony jest ważnej kwestii jaką jest określenie teoretycznej wydajności koparki kołowej wielonaczyniowej. W załączonym przeglądzie literatury Che i Chen (2014) nie umieścili pozycji odnoszących się do metod urabiania, parametrów pracy koparki oraz teoretycznej wydajności wydobycia i być może to właśnie jest przyczyną pewnych niedokładności powstałych w trakcie rozwiązywania problemu. Intencją autora obecnej publikacji było: • Odniesienie się do krytycyzmu wyrażonego przez Che i Chena (2014) dotyczącego procedury obliczania prędkości w ruchu łukowym podanej w cytowanej literaturze przedmiotu (Pajer i in., 1971; Vetrov, 1971; Rasper, 1975; Durst i Vogt, 1988); • Zbadanie różnic pomiędzy procedurą określania teoretycznej wydajności zaproponowaną w pracy Che i Chena (2014) a odpowiednimi rozwiązaniami podanymi w literaturze; • Określenie prawidłowości i stosowalności teorii zaprezentowanej w pracy Che i Chena (2014) poprzez porównanie wyników uzyskanych z wykorzystaniem ich teorii oraz teorii podanych w wymienionych pozycjach literatury. W rozdziale 2 pracy (Che i Chen 2014) zatytułowanym " Uprzednio stosowane metody określania teoretycznej wydajności pracy koparki kołowej wielonaczyniowej" autorzy nie podali głównych odniesień literaturowych z których zaczerpnięte zostały równania (1)-(6)**. Ponadto, nie podali charakterystyki modelu działania koparki, na podstawie którego wyprowadzone zostały rzeczone r

The Current SI Seen From the Perspective of the Proposed New SI

PubMed Central

Taylor, Barry N.

2011-01-01

A revised International System of Units (SI) proposed by the International Committee for Weights and Measures is under consideration by the General Conference on Weights and Measures for eventual adoption. Widely recognized as a significant advance for both metrology and science, it is defined via statements that explicitly fix the numerical values of a selected set of seven reference constants when the values of these constants are expressed in certain specified units. At first sight this approach to defining a system of units appears to be quite different from that used to define the current SI. However, by showing how the definitions of the seven base units of the current SI also fix the numerical values of a set of seven reference constants (broadly interpreted) when the values of these constants are expressed in their coherent SI units, and how the definition of the current SI can be recast into the same form as that of the revised SI under consideration, we show that the revision is not as radical a departure from the current SI as it might initially seem. PMID:26989600

Thermodynamic design of a high temperature chemical vapor deposition process to synthesize α-SiC in Si-C-H and Si-C-H-Cl systems

NASA Astrophysics Data System (ADS)

Kang, Yura; Yoo, Chang-Hyoung; Nam, Deok-Hui; Lee, Myung-Hyun; Seo, Won-Seon; Hong, Suklyun; Jeong, Seong-Min

2018-03-01

In this study, we thermodynamically reviewed the suitable growth process conditions of α-SiC in the Si-C-H system using tetramethylsilane (TMS) and in the Si-C-H-Cl system using methyltrichlorosilane (MTS). In the Si-C-H-Cl system, pure solid SiC was obtained at high temperatures under a larger range of hydrogen dilution ratios than that tolerated in the Si-C-H system. X-ray diffraction and micro-Raman analysis of the products obtained at 1900, 2000, and 2100 °C showed that the α-SiC becomes more dominant with increasing temperature in the Si-C-H-Cl system. While TMS was unsuitable for high temperature processing due to its high C/Si ratio, MTS was found to be appropriate for growing α-SiC crystals at high temperatures under a range of conditions. These results indicate that a novel method to grow α-SiC single crystals through HTCVD using MTS as a precursor could be established.

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

NASA Astrophysics Data System (ADS)

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

2017-06-01

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

Antioxidant migration resistance of SiOx layer in SiOx/PLA coated film.

PubMed

Huang, Chongxing; Zhao, Yuan; Su, Hongxia; Bei, Ronghua

2018-02-01

As novel materials for food contact packaging, inorganic silicon oxide (SiO x ) films are high barrier property materials that have been developed rapidly and have attracted the attention of many manufacturers. For the safe use of SiO x films for food packaging it is vital to study the interaction between SiO x layers and food contaminants, as well as the function of a SiO x barrier layer in antioxidant migration resistance. In this study, we deposited a SiO x layer on polylactic acid (PLA)-based films to prepare SiO x /PLA coated films by plasma-enhanced chemical vapour deposition. Additionally, we compared PLA-based films and SiO x /PLA coated films in terms of the migration of different antioxidants (e.g. t-butylhydroquinone [TBHQ], butylated hydroxyanisole [BHA], and butylated hydroxytoluene [BHT]) via specific migration experiments and then investigated the effects of a SiO x layer on antioxidant migration under different conditions. The results indicate that antioxidant migration from SiO x /PLA coated films is similar to that for PLA-based films: with increase of temperature, decrease of food simulant polarity, and increase of single-sided contact time, the antioxidant migration rate and amount in SiO x /PLA coated films increase. The SiO x barrier layer significantly reduced the amount of migration of antioxidants with small and similar molecular weights and similar physical and chemical properties, while the degree of migration blocking was not significantly different among the studied antioxidants. However, the migration was affected by temperature and food simulant. Depending on the food simulants considered, the migration amount in SiO x /PLA coated films was reduced compared with that in PLA-based films by 42-46%, 44-47%, and 44-46% for TBHQ, BHA, and BHT, respectively.

Carbon-Fibre-Reinforced SiC Composite (C/SiSiC) as an Alternative Material for Endoprosthesis: Fabrication, Mechanical and In-Vitro Biological Properties

PubMed Central

Reichert, Aline; Gadow, Rainer; Mayr, Hermann O.; Suedkamp, Norbert P.; Weichand, Partick; Bernstein, Anke

2018-01-01

Particle-induced periprosthetic osteolysis and subsequent aseptic implant loosening are a major cause of compromising the long-term results of total joint replacements. To date, no implant has been able to mirror radically the tribological factors (friction/lubrication/wear) of in vivo tribological pairings. Carbon-Fibre Reinforced SiC-Composites (C/SiSiC), a material primarily developed for brake technology, has the opportunity to fulfil this requirement. Until now, the material itself has not been used in medicine. The aim of this investigation was to test the suitability of C/SiSiC ceramics as a new material for bearing couples in endoprosthetics. After the preparation of the composites flexural strength was determined as well as the Young’s-modulus and the coefficient of friction. To investigate in vitro biological properties, MG 63 and primary human osteoblasts were cultured on C/SiSiC composites. To review the proliferation, the cytotoxicity standardized tests were used. The cell morphology was observed by light microscopy, ESEM, confocal and 3D-laserscanning microscopy. C/SiSiC possesses a high resistance to wear. Cells exhibited no significant alterations in morphology. Vitality was not impaired by contact with the ceramic composite. There was no higher cytotoxicity to observe. Regarding these results, C/SiSiC ceramics seem to be biologically and mechanically appropriate for orthopaedic applications. PMID:29470416

The corrosion behavior of CVI SiC matrix in SiCf/SiC composites under molten fluoride salt environment

NASA Astrophysics Data System (ADS)

Wang, Hongda; Feng, Qian; Wang, Zhen; Zhou, Haijun; Kan, Yanmei; Hu, Jianbao; Dong, Shaoming

2017-04-01

High temperature corrosion behavior and microstructural evolution of designed chemical-vapor-infiltrated SiC matrix in SiC fiber reinforced SiC ceramic matrix composites in 46.5LiF-11.5NaF-42.0KF (mol. %) eutectic salt at 800 °C for various corrosion time was studied. Worse damage was observed as extending the exposure time, with the mass loss ratio increasing from 0.716 wt. % for 50 h to 5.914 wt. % for 500 h. The mass loss rate showed a trend of first decrease and then increase with the extended corrosion exposure. Compared with the near-stoichiometric SiC matrix layers, the O-contained boundaries between deposited matrix layers and the designed Si-rich SiC matrix layers were much less corrosion resistant and preferentially corroded. Liner relationship between the mass loss ratio and the corrosion time obtained from 50 h to 300 h indicated that the corrosion action was reaction-control process. Further corrosion would lead to matrix layer exfoliation and higher mass loss ratio.

Thermoelectric properties of Si/CoSi2 sub-micrometer composites prepared by melt-spinning technique

NASA Astrophysics Data System (ADS)

Xie, Jun; Ohishi, Yuji; Ichikawa, Satoshi; Muta, Hiroaki; Kurosaki, Ken; Yamanaka, Shinsuke

2017-05-01

We here report on the influence of CoSi2 precipitates on the thermoelectric properties of heavily doped p-type Si. A simple self-assembly process using a melt-spinning technique followed by spark plasma sintering is introduced to prepare bulk Si/CoSi2 composites with a nominal composition of (Si0.99B0.01)95Co5. Scanning and transmission electron microscopy observations present clear evidence of a sub-micrometer CoSi2 phase with a size ranging from 50 to 500 nm. These sub-micrometer precipitates resulted in a retention of the high electrical performance of heavily doped Si, while simultaneously reducing thermal conductivity by over 20% compared to a coarse CoSi2 phase (1-10 μm) in a comparative sample prepared by arc melting and spark plasma sintering. As a result, a figure of merit ZT value of 0.21 at 1073 K was achieved in the sub-micrometer Si/CoSi2, an increase of 16% compared with the ZT value for homogeneous p-type Si with a similar carrier concentration. This suggests that the self-assembled sub-micrometer inclusions effectively enhanced the thermoelectric performance of Si-based thermoelectric materials.

Sr-Al-Si co-segregated regions in eutectic Si phase of Sr-modified Al-10Si alloy.

PubMed

Timpel, M; Wanderka, N; Schlesiger, R; Yamamoto, T; Isheim, D; Schmitz, G; Matsumura, S; Banhart, J

2013-09-01

The addition of 200 ppm strontium to an Al-10 wt% Si casting alloy changes the morphology of the eutectic silicon phase from coarse plate-like to fine fibrous networks. In order to clarify this modification mechanism the location of Sr within the eutectic Si phase has been investigated by a combination of high-resolution methods. Whereas three-dimensional atom probe tomography allows us to visualise the distribution of Sr on the atomic scale and to analyse its local enrichment, transmission electron microscopy yields information about the crystallographic nature of segregated regions. Segregations with two kinds of morphologies were found at the intersections of Si twin lamellae: Sr-Al-Si co-segregations of rod-like morphology and Al-rich regions of spherical morphology. Both are responsible for the formation of a high density of multiple twins and promote the anisotropic growth of the eutectic Si phase in specific crystallographic directions during solidification. The experimental findings are related to the previously postulated mechanism of "impurity induced twinning". Copyright © 2012 Elsevier B.V. All rights reserved.

Electrical characteristics and thermal stability of n+ polycrystalline- Si/ZrO2/SiO2/Si metal-oxide-semiconductor capacitors

NASA Astrophysics Data System (ADS)

Lim, Kwan-Yong; Park, Dae-Gyu; Cho, Heung-Jae; Kim, Joong-Jung; Yang, Jun-Mo; Ii, Choi-Sang; Yeo, In-Seok; Park, Jin Won

2002-01-01

We have investigated the thermal stability of n+ polycrystalline-Si(poly-Si)/ZrO2(50-140 Å)/SiO2(7 Å)/p-Si metal-oxide-semiconductor (MOS) capacitors via electrical and material characterization. The ZrO2 gate dielectric was prepared by atomic layer chemical vapor deposition using ZrCl4 and H2O vapor. Capacitance-voltage hysteresis as small as ˜12 mV with the flatband voltage of -0.5 V and the interface trap density of ˜5×1010cm-2 eV-1 were attained with activation anneal at 750 °C. A high level of gate leakage current was observed at the activation temperatures over 750 °C and attributed to the interfacial reaction of poly-Si and ZrO2 during the poly-Si deposition and the following high temperature anneal. Because of this, the ZrO2 gate dielectric is incompatible with the conventional poly-Si gate process. In the MOS capacitors having a smaller active area (<50×50 μm2), fortunately, the electrical degradation by further severe silicidation does not occur up to an 800 °C anneal in N2 for 30 min.

SHS synthesis of Si-SiC composite powders using Mg and reactants from industrial waste

NASA Astrophysics Data System (ADS)

Chanadee, Tawat

2017-11-01

Si-SiC composite powders were synthesized by self-propagating high-temperature synthesis (SHS) using reactants of fly ash-based silica, sawdust-based activated carbon, and magnesium. Fly ash-based silica and sawdust-based activated carbon were prepared from coal mining fly ash and Para rubber-wood sawdust, respectively. The work investigated the effects of the synthesis atmosphere (air and Ar) on the phase and morphology of the SHS products. The SHS product was leached by a two-step acid leaching processes, to obtain the Si-SiC composite powder. The SHS product and SHS product after leaching were characterized by X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray spectrometry. The results indicated that the SHS product synthesized in air consisted of Si, SiC, MgO, and intermediate phases (SiO2, Mg, Mg2SiO4, Mg2Si), whereas the SHS product synthesized in Ar consisted of Si, SiC, MgO and a little Mg2SiO4. The SiC content in the leached-SHS product was higher when Ar was used as the synthesis atmosphere. As well as affecting the purity, the synthesis atmospheres also affected the average crystalline sizes of the products. The crystalline size of the product synthesized in Ar was smaller than that of the product synthesized in air. All of the results showed that fly ash and sawdust could be effective waste-material reactants for the synthesis of Si-SiC composite powders.

On the wettability diversity of C/SiC surface: Comparison of the ground C/SiC surface and ablated C/SiC surface from three aspects

NASA Astrophysics Data System (ADS)

Wu, M. L.; Ren, C. Z.; Xu, H. Z.

2016-11-01

The coefficient of thermal conductivity was influenced by the wetting state of material. The wetting state usually depends on the surface wettability. C/SiC is a promising ceramic composites with multi-components. The wettability of C/SiC composites is hard to resort to the classical wetting theory directly. So far, few investigations focused on C/SiC surface wettability diversity after different material removal processes. In this investigation, comparative studies of surface wettability of ground C/SiC surface and laser-ablated C/SiC surface were carried out through apparent contact angle (APCA) measurements. The results showed that water droplets easily reached stable state on ground C/SiC surface; while the water droplets rappidly penetrated into the laser-ablated C/SiC surface. In order to find out the reason for wettability distinctions between the ground C/SiC surface and the laser-ablated C/SiC surface, comparative studies on the surface micro-structure, surface C-O-Si distribution, and surface C-O-Si weight percentage were carried out. The results showed that (1) A large number of micro cracks in the fuzzy pattern layer over laser-ablated C/SiC surfaces easily destoried the surface tension of water droplets, while only a few cracks existed over the ground C/SiC surfaces. (2) Chemical components (C, O, Si) were non-uniformly distributed on ground C/SiC surfaces, while the chemical components (C, O, Si) were uniformly distributed on laser-ablated C/SiC surfaces. (3) The carbon weight percentage on ground C/SiC surfaces were higher than that on laser-ablated C/SiC surfaces. All these made an essential contribution to the surface wettability diversity of C/SiC surface. Although more investigations about the quantitative influence of surface topography and surface chemical composition on composites wettability are still needed, the conslusion can be used in application: the wettability of C/SiC surface can be controlled by different material removal process

Spot-size converter with a SiO(2) spacer layer between tapered Si and SiON waveguides for fiber-to-chip coupling.

PubMed

Maegami, Yuriko; Takei, Ryohei; Omoda, Emiko; Amano, Takeru; Okano, Makoto; Mori, Masahiko; Kamei, Toshihiro; Sakakibara, Youichi

2015-08-10

We experimentally demonstrate low-loss and polarization-insensitive fiber-to-chip coupling spot-size converters (SSCs) comprised of a three dimensionally tapered Si wire waveguide, a SiON secondary waveguide, and a SiO(2) spacer inserted between them. Fabricated SSCs with the SiO(2) spacer exhibit fiber-to-chip coupling loss of 1.5 dB/facet for both the quasi-TE and TM modes and a small wavelength dependence in the C- and L-band regions. The SiON secondary waveguide is present only around the SSC region, which significantly suppresses the influence of the well-known N-H absorption of plasma-deposited SiON at around 1510 nm.

First-principles calculations of orientation dependence of Si thermal oxidation based on Si emission model

NASA Astrophysics Data System (ADS)

Nagura, Takuya; Kawachi, Shingo; Chokawa, Kenta; Shirakawa, Hiroki; Araidai, Masaaki; Kageshima, Hiroyuki; Endoh, Tetsuo; Shiraishi, Kenji

2018-04-01

It is expected that the off-state leakage current of MOSFETs can be reduced by employing vertical body channel MOSFETs (V-MOSFETs). However, in fabricating these devices, the structure of the Si pillars sometimes cannot be maintained during oxidation, since Si atoms sometimes disappear from the Si/oxide interface (Si missing). Thus, in this study, we used first-principles calculations based on the density functional theory, and investigated the Si emission behavior at the various interfaces on the basis of the Si emission model including its atomistic structure and dependence on Si crystal orientation. The results show that the order in which Si atoms are more likely to be emitted during thermal oxidation is (111) > (110) > (310) > (100). Moreover, the emission of Si atoms is enhanced as the compressive strain increases. Therefore, the emission of Si atoms occurs more easily in V-MOSFETs than in planar MOSFETs. To reduce Si missing in V-MOSFETs, oxidation processes that induce less strain, such as wet or pyrogenic oxidation, are necessary.

Introduction of Si/SiO{sub 2} interface states by annealing Ge-implanted films

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

Marstein, E.S.; Gunnaes, A.E.; Olsen, A.

2004-10-15

Nanocrystals embedded in SiO{sub 2} films are the subject of a number of recent works, mainly because of their potential usefulness in the fabrication of optoelectronic devices and nanocrystal memory structures. One interesting method for the fabrication of such nanocrystals is the ion implantation of segregating species into SiO{sub 2} films followed by heat treatment in order to induce nanocrystal formation. This method is both relatively simple and also compatible with the current MOS (metal-oxide-semiconductor) device technology. An unintentional effect can occur during the fabrication of nanocrystals using this method, namely a significant diffusion of the implanted species during annealing,more » away from the regions with the highest concentration. The Si/SiO{sub 2} interface can be exposed to this diffusion flux. This can result in an altered interface and have a significant influence on electronic devices. Here, we report on ion implantation of Ge into SiO{sub 2} on Si followed by annealing under conditions, resulting in Ge accumulation at the Si/SiO{sub 2} interface as determined by secondary-ion mass spectroscopy analysis, transmission electron microscopy with energy dispersive analysis of x-rays, and Rutherford backscattering spectrometry. The accumulation of Ge at the Si/SiO{sub 2} interface has also been reported before. The resulting effect on the electronic structure of the interface is a priori unknown. We have fabricated MOS capacitors on the sample structures and their capacitance-voltage characteristics were measured and analyzed. We measure an interface state density around 1x10{sup 12} cm{sup -2}, which is high compared to standard Si MOS devices. We discuss the results in terms of the previous electrical measurements on Ge-oxide interfaces and SiGe interfaces, which also can yield a high interface state density. The specific conditions we report result in a sufficiently low Ge concentration that nanocrystals are not segregated in the Si

SiC Technology

NASA Technical Reports Server (NTRS)

Neudeck, Philip G.

1998-01-01

Silicon carbide (SiC)-based semiconductor electronic devices and circuits are presently being developed for use in high-temperature, high-power, and/or high-radiation conditions under which conventional semiconductors cannot adequately perform. Silicon carbide's ability to function under such extreme conditions is expected to enable significant improvements to a far-ranging variety of applications and systems. These range from greatly improved high-voltage switching [1- 4] for energy savings in public electric power distribution and electric motor drives to more powerful microwave electronics for radar and communications [5-7] to sensors and controls for cleaner-burning more fuel-efficient jet aircraft and automobile engines. In the particular area of power devices, theoretical appraisals have indicated that SiC power MOSFET's and diode rectifiers would operate over higher voltage and temperature ranges, have superior switching characteristics, and yet have die sizes nearly 20 times smaller than correspondingly rated silicon-based devices [8]. However, these tremendous theoretical advantages have yet to be realized in experimental SiC devices, primarily due to the fact that SiC's relatively immature crystal growth and device fabrication technologies are not yet sufficiently developed to the degree required for reliable incorporation into most electronic systems [9]. This chapter briefly surveys the SiC semiconductor electronics technology. In particular, the differences (both good and bad) between SiC electronics technology and well-known silicon VLSI technology are highlighted. Projected performance benefits of SiC electronics are highlighted for several large-scale applications. Key crystal growth and device-fabrication issues that presently limit the performance and capability of high temperature and/or high power SiC electronics are identified.

MeV Si ion modifications on the thermoelectric generators from Si/Si + Ge superlattice nano-layered films

NASA Astrophysics Data System (ADS)

Budak, S.; Heidary, K.; Johnson, R. B.; Colon, T.; Muntele, C.; Ila, D.

2014-08-01

The performance of thermoelectric materials and devices is characterized by a dimensionless figure of merit, ZT = S2σT/K, where, S and σ denote, respectively, the Seebeck coefficient and electrical conductivity, T is the absolute temperature in Kelvin and K represents the thermal conductivity. The figure of merit may be improved by means of raising either S or σ or by lowering K. In our laboratory, we have fabricated and characterized the performance of a large variety of thermoelectric generators (TEG). Two TEG groups comprised of 50 and 100 alternating layers of Si/Si + Ge multi-nanolayered superlattice films have been fabricated and thoroughly characterized. Ion beam assisted deposition (IBAD) was utilized to assemble the alternating sandwiched layers, resulting in total thickness of 300 nm and 317 nm for 50 and 100 layer devices, respectively. Rutherford Backscattering Spectroscopy (RBS) was employed in order to monitor the precise quantity of Si and Ge utilized in the construction of specific multilayer thin films. The material layers were subsequently impregnated with quantum dots and/or quantum clusters, in order to concurrently reduce the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and raise the cross plane electrical conductivity. The quantum dots/clusters were implanted via the 5 MeV Si ion bombardment which was performed using a Pelletron high energy ion beam accelerator. We have achieved remarkable results for the thermoelectric and optical properties of the Si/Si + Ge multilayer thin film TEG systems. We have demonstrated that with optimal setting of the 5 MeV Si ion beam bombardment fluences, one can fabricate TEG systems with figures of merits substantially higher than the values previously reported.

Mass Spectrometric Investigation of Silicon Extremely Enriched in (28)Si: From (28)SiF4 (Gas Phase IRMS) to (28)Si Crystals (MC-ICP-MS).

PubMed

Pramann, Axel; Rienitz, Olaf

2016-06-07

A new generation of silicon crystals even further enriched in (28)Si (x((28)Si) > 0.999 98 mol/mol), recently produced by companies and institutes in Russia within the framework of a project initiated by PTB, were investigated with respect to their isotopic composition and molar mass M(Si). A modified isotope dilution mass spectrometric (IDMS) method treating the silicon as the matrix containing a so-called virtual element (VE) existing of the isotopes (29)Si and (30)Si solely and high resolution multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) were applied in combination. This method succeeds also when examining the new materials holding merely trace amounts of (29)Si (x((29)Si) ≈ 5 × 10(-6) mol/mol) and (30)Si (x((30)Si) ≈ 7 × 10(-7) mol/mol) extremely difficult to detect with lowest uncertainty. However, there is a need for validating the enrichment in (28)Si already in the precursor material of the final crystals, silicon tetrafluoride (SiF4) gas prior to crystal production. For that purpose, the isotopic composition of selected SiF4 samples was determined using a multicollector magnetic sector field gas-phase isotope ratio mass spectrometer. Contaminations of SiF4 by natural silicon due to storing and during the isotope ratio mass spectrometry (IRMS) measurements were observed and quantified. The respective MC-ICP-MS measurements of the corresponding crystal samples show-in contrast-several advantages compared to gas phase IRMS. M(Si) of the new crystals were determined to some extent with uncertainties urel(M) < 1 × 10(-9). This study presents a clear dependence of the uncertainty urel(M(Si)) on the degree of enrichment in (28)Si. This leads to a reduction of urel(M(Si)) during the past decade by almost 3 orders of magnitude and thus further reduces the uncertainty of the Avogadro constant NA which is one of the preconditions for the redefinition of the SI unit kilogram.

Positron annihilation in SiO 2-Si studied by a pulsed slow positron beam

NASA Astrophysics Data System (ADS)

Suzuki, R.; Ohdaira, T.; Uedono, A.; Kobayashi, Y.

2002-06-01

Positron and positronium (Ps) behavior in SiO 2-Si have been studied by means of positron annihilation lifetime spectroscopy (PALS) and age-momentum correlation (AMOC) spectroscopy with a pulsed slow positron beam. The PALS study of SiO 2-Si samples, which were prepared by a dry-oxygen thermal process, revealed that the positrons implanted in the Si substrate and diffused back to the interface do not contribute to the ortho-Ps long-lived component, and the lifetime spectrum of the interface has at least two components. From the AMOC study, the momentum distribution of the ortho-Ps pick-off annihilation in SiO 2, which shows broader momentum distribution than that of crystalline Si, was found to be almost the same as that of free positron annihilation in SiO 2. A varied interface model was proposed to interpret the results of the metal-oxide-semiconductor (MOS) experiments. The narrow momentum distribution found in the n-type MOS with a negative gate bias voltage could be attributed to Ps formation and rapid spin exchange in the SiO 2-Si interface. We have developed a two-dimensional positron lifetime technique, which measures annihilation time and pulse height of the scintillation gamma-ray detector for each event. Using this technique, the positronium behavior in a porous SiO 2 film, grown by a sputtering method, has been studied.

Chemical reactivity of SiC fibre-reinforced SiC with beryllium and lithium ceramic breeder materials

NASA Astrophysics Data System (ADS)

Kleykamp, H.

2000-12-01

SiC fibre-reinforced SiC fabrics (f-SiC/SiC) are considered for structural materials of advanced fusion blanket concepts. Priority tasks are compatibility studies of SiC with Li breeder ceramics and the Be neutron multiplier. Isothermal and anisothermal powder reactions by DTA up to 1220°C were examined between Li 4SiO 4, Li 2ZrO 3 and Li 2TiO 3, respectively, and SiC and SiC/SiO 2 mixtures, respectively. The SiC/SiO 2 mixture simulated the chemical state of Nicalon fibres. Solid state reactions between SiC and Be pellets were studied by capsule experiments. The reaction products Be 2C and Si were observed between the initial phases after annealing at 800°C and 900°C. A parabolic time law with a chemical diffusion coefficient D˜=2.6×10 -15 m 2/s of Be in the products was deduced at 900°C. Additional oxygen released from SiO 2 as a component of the simulated fibres oxidised the reaction products via the gas phase by formation of a Be 2SiO 4 layer. All reactions are kinetically hindered below 700°C.

Oxidation of SiC

NASA Astrophysics Data System (ADS)

Cooper, James A.

1997-03-01

SiC is a wide band gap hexagonal anisotropic semiconductor which is attractive for use in high voltage, high temperature, or high power applications. SiC is also the only compound semiconductor that can be thermally oxidized to form SiO_2, making it possible to construct many conventional MOS devices in this material. The electrical quality of the SiO_2/SiC interface is far from ideal, however, and considerable research is presently directed to understanding and improving this interface. Electrical characterization of the SiC MOS interface is complicated by the wide band gap, since most interface states are energetically too far removed from the conduction or valence bands to respond to electrical stimulation at room temperature. Moreover, very little information is yet available on the properties of the MOS interface on the 4H polytype of SiC (preferred because of it's higher bulk electron mobility) or on interfaces on crystalline surfaces perpendicular to the basal plane (where an equal number of Si and C atoms are present). Finally, electron mobilities in inversion layers on 4H-SiC reported to date are anomolously low, especially in consideration of the relatively high bulk mobilities in this polytype. In this talk we will discuss MOS characterization techniques for wide band gap semiconductors and review the current understanding of the physics of the MOS interface on thermally oxidized SiC.

β-FeSi2 films prepared on 6H-SiC substrates by magnetron sputtering

NASA Astrophysics Data System (ADS)

Hong, Li; Hongbin, Pu; Chunlei, Zheng; Zhiming, Chen

2015-06-01

β-FeSi2 thin films have been successfully prepared by magnetron sputtering and post rapid thermal annealing method on 6H-SiC (0001) substrates using a FeSi2 target and a Si target. X-ray diffraction (XRD) and Raman spectroscopy are applied to analyze the formation of β-FeSi2 films. XRD spectra reveal that the amorphous FeSi2 films are transformed to β-FeSi2 phase as the annealing temperature is increased from 500 to 900 °C for 5 min and the optimal annealing temperature is 900 °C. The formation of β-FeSi2 is also confirmed by Raman spectroscopy. Scanning electron microscope (SEM) observations indicate that the film is flat, relatively compact and the interface between β-FeSi2 and 6H-SiC is clear. Atomic force microscope (AFM) measurements demonstrate that the surface roughness confirmed by the root mean square (RMS) of the β-FeSi2 film is 0.87 nm. Near-infrared spectrophotometer observation shows that the absorption coefficient is of the order of 105 cm-1 and the optical band-gap of the β-FeSi2 film is 0.88 eV. The β-FeSi2 film with high crystal quality is fabricated by co-sputtering a FeSi2 target and a Si target for 60 min and annealing at 900 °C for 5 min. Project supported by the National Natural Science Foundation of China (No. 51177134) and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2015JM6286).

X-ray absorption spectroscopy study on SiC-side interface structure of SiO2–SiC formed by thermal oxidation in dry oxygen

NASA Astrophysics Data System (ADS)

Isomura, Noritake; Kosaka, Satoru; Kataoka, Keita; Watanabe, Yukihiko; Kimoto, Yasuji

2018-06-01

Extended X-ray absorption fine structure (EXAFS) spectroscopy is demonstrated to measure the fine atomic structure of SiO2–SiC interfaces. The SiC-side of the interface can be measured by fabricating thin SiO2 films and using SiC-selective EXAFS measurements. Fourier transforms of the oscillations of the EXAFS spectra correspond to radial-structure functions and reveal a new peak of the first nearest neighbor of Si for m-face SiC, which does not appear in measurements of the Si-face. This finding suggests that the m-face interface could include a structure with shorter Si–C distances. Numerical calculations provide additional support for this finding.

The roles of Eu during the growth of eutectic Si in Al-Si alloys

PubMed Central

Li, Jiehua; Hage, Fredrik; Wiessner, Manfred; Romaner, Lorenz; Scheiber, Daniel; Sartory, Bernhard; Ramasse, Quentin; Schumacher, Peter

2015-01-01

Controlling the growth of eutectic Si and thereby modifying the eutectic Si from flake-like to fibrous is a key factor in improving the properties of Al-Si alloys. To date, it is generally accepted that the impurity-induced twinning (IIT) mechanism and the twin plane re-entrant edge (TPRE) mechanism as well as poisoning of the TPRE mechanism are valid under certain conditions. However, IIT, TPRE or poisoning of the TPRE mechanism cannot be used to interpret all observations. Here, we report an atomic-scale experimental and theoretical investigation on the roles of Eu during the growth of eutectic Si in Al-Si alloys. Both experimental and theoretical investigations reveal three different roles: (i) the adsorption at the intersection of Si facets, inducing IIT mechanism, (ii) the adsorption at the twin plane re-entrant edge, inducing TPRE mechanism or poisoning of the TPRE mechanism, and (iii) the segregation ahead of the growing Si twins, inducing a solute entrainment within eutectic Si. This investigation not only demonstrates a direct experimental support to the well-accepted poisoning of the TPRE and IIT mechanisms, but also provides a full picture about the roles of Eu atoms during the growth of eutectic Si, including the solute entrainment within eutectic Si. PMID:26328541

The roles of Eu during the growth of eutectic Si in Al-Si alloys.

PubMed

Li, Jiehua; Hage, Fredrik; Wiessner, Manfred; Romaner, Lorenz; Scheiber, Daniel; Sartory, Bernhard; Ramasse, Quentin; Schumacher, Peter

2015-09-02

Controlling the growth of eutectic Si and thereby modifying the eutectic Si from flake-like to fibrous is a key factor in improving the properties of Al-Si alloys. To date, it is generally accepted that the impurity-induced twinning (IIT) mechanism and the twin plane re-entrant edge (TPRE) mechanism as well as poisoning of the TPRE mechanism are valid under certain conditions. However, IIT, TPRE or poisoning of the TPRE mechanism cannot be used to interpret all observations. Here, we report an atomic-scale experimental and theoretical investigation on the roles of Eu during the growth of eutectic Si in Al-Si alloys. Both experimental and theoretical investigations reveal three different roles: (i) the adsorption at the intersection of Si facets, inducing IIT mechanism, (ii) the adsorption at the twin plane re-entrant edge, inducing TPRE mechanism or poisoning of the TPRE mechanism, and (iii) the segregation ahead of the growing Si twins, inducing a solute entrainment within eutectic Si. This investigation not only demonstrates a direct experimental support to the well-accepted poisoning of the TPRE and IIT mechanisms, but also provides a full picture about the roles of Eu atoms during the growth of eutectic Si, including the solute entrainment within eutectic Si.

The roles of Eu during the growth of eutectic Si in Al-Si alloys

NASA Astrophysics Data System (ADS)

Li, Jiehua; Hage, Fredrik; Wiessner, Manfred; Romaner, Lorenz; Scheiber, Daniel; Sartory, Bernhard; Ramasse, Quentin; Schumacher, Peter

2015-09-01

Controlling the growth of eutectic Si and thereby modifying the eutectic Si from flake-like to fibrous is a key factor in improving the properties of Al-Si alloys. To date, it is generally accepted that the impurity-induced twinning (IIT) mechanism and the twin plane re-entrant edge (TPRE) mechanism as well as poisoning of the TPRE mechanism are valid under certain conditions. However, IIT, TPRE or poisoning of the TPRE mechanism cannot be used to interpret all observations. Here, we report an atomic-scale experimental and theoretical investigation on the roles of Eu during the growth of eutectic Si in Al-Si alloys. Both experimental and theoretical investigations reveal three different roles: (i) the adsorption at the intersection of Si facets, inducing IIT mechanism, (ii) the adsorption at the twin plane re-entrant edge, inducing TPRE mechanism or poisoning of the TPRE mechanism, and (iii) the segregation ahead of the growing Si twins, inducing a solute entrainment within eutectic Si. This investigation not only demonstrates a direct experimental support to the well-accepted poisoning of the TPRE and IIT mechanisms, but also provides a full picture about the roles of Eu atoms during the growth of eutectic Si, including the solute entrainment within eutectic Si.

Effect of High Si Content on U3Si2 Fuel Microstructure

NASA Astrophysics Data System (ADS)

Rosales, Jhonathan; van Rooyen, Isabella J.; Meher, Subhashish; Hoggan, Rita; Parga, Clemente; Harp, Jason

2018-02-01

The development of U3Si2 as an accident-tolerant nuclear fuel has gained research interest because of its promising high uranium density and improved thermal properties. In the present study, three samples of U3Si2 fuel with varying silicon content have been fabricated by a conventional powder metallurgical route. Microstructural characterization via scanning and transmission electron microscopy reveals the presence of other stoichiometry of uranium silicide such as USi and UO2 in both samples. The detailed phase analysis by x-ray diffraction shows the presence of secondary phases, such as USi, U3Si, and UO2. The samples with higher concentrations of silicon content of 7.5 wt.% display additional elemental Si. These samples also possess an increased amount of the USi phase as compared to that in the conventional sample with 7.3 wt.% silicon. The optimization of U3Si2 fuel performance through the understanding of the role of Si content on its microstructure has been discussed.

Si cycling in a forest biogeosystem - the importance of transient state biogenic Si pools

NASA Astrophysics Data System (ADS)

Sommer, M.; Jochheim, H.; Höhn, A.; Breuer, J.; Zagorski, Z.; Busse, J.; Barkusky, D.; Meier, K.; Puppe, D.; Wanner, M.; Kaczorek, D.

2013-07-01

The relevance of biological Si cycling for dissolved silica (DSi) export from terrestrial biogeosystems is still in debate. Even in systems showing a high content of weatherable minerals, like Cambisols on volcanic tuff, biogenic Si (BSi) might contribute > 50% to DSi (Gerard et al., 2008). However, the number of biogeosystem studies is rather limited for generalized conclusions. To cover one end of controlling factors on DSi, i.e., weatherable minerals content, we studied a forested site with absolute quartz dominance (> 95%). Here we hypothesise minimal effects of chemical weathering of silicates on DSi. During a four year observation period (05/2007-04/2011), we quantified (i) internal and external Si fluxes of a temperate-humid biogeosystem (beech, 120 yr) by BIOME-BGC (version ZALF), (ii) related Si budgets, and (iii) Si pools in soil and beech, chemically as well as by SEM-EDX. For the first time two compartments of biogenic Si in soils were analysed, i.e., phytogenic and zoogenic Si pool (testate amoebae). We quantified an average Si plant uptake of 35 kg Si ha-1 yr-1 - most of which is recycled to the soil by litterfall - and calculated an annual biosilicification from idiosomic testate amoebae of 17 kg Si ha-1. The comparatively high DSi concentrations (6 mg L-1) and DSi exports (12 kg Si ha-1 yr-1) could not be explained by chemical weathering of feldspars or quartz dissolution. Instead, dissolution of a relictic, phytogenic Si pool seems to be the main process for the DSi observed. We identified canopy closure accompanied by a disappearance of grasses as well as the selective extraction of pine trees 30 yr ago as the most probable control for the phenomena observed. From our results we concluded the biogeosystem to be in a transient state in terms of Si cycling.

Material Analysis of Coated Siliconized Silicon Carbide (SiSiC) Honeycomb Structures for Thermochemical Hydrogen Production

PubMed Central

Neises-von Puttkamer, Martina; Simon, Heike; Schmücker, Martin; Roeb, Martin; Sattler, Christian; Pitz-Paal, Robert

2013-01-01

In the present work, thermochemical water splitting with siliconized silicon carbide (SiSiC) honeycombs coated with a zinc ferrite redox material was investigated. The small scale coated monoliths were tested in a laboratory test-rig and characterized by X-ray diffractometry (XRD) and Scanning Electron Microscopy (SEM) with corresponding micro analysis after testing in order to characterize the changes in morphology and composition. Comparison of several treated monoliths revealed the formation of various reaction products such as SiO2, zircon (ZrSiO4), iron silicide (FeSi) and hercynite (FeAl2O4) indicating the occurrence of various side reactions between the different phases of the coating as well as between the coating and the SiSiC substrate. The investigations showed that the ferrite is mainly reduced through reaction with silicon (Si), which is present in the SiSiC matrix, and silicon carbide (SiC). These results led to the formulation of a new redox mechanism for this system in which Zn-ferrite is reduced through Si forming silicon dioxide (SiO2) and through SiC forming SiO2 and carbon monoxide. A decline of hydrogen production within the first 20 cycles is suggested to be due to the growth of a silicon dioxide and zircon layer which acts as a diffusion barrier for the reacting specie. PMID:28809316

Material Analysis of Coated Siliconized Silicon Carbide (SiSiC) Honeycomb Structures for Thermochemical Hydrogen Production.

PubMed

Neises-von Puttkamer, Martina; Simon, Heike; Schmücker, Martin; Roeb, Martin; Sattler, Christian; Pitz-Paal, Robert

2013-01-31

In the present work, thermochemical water splitting with siliconized silicon carbide (SiSiC) honeycombs coated with a zinc ferrite redox material was investigated. The small scale coated monoliths were tested in a laboratory test-rig and characterized by X-ray diffractometry (XRD) and Scanning Electron Microscopy (SEM) with corresponding micro analysis after testing in order to characterize the changes in morphology and composition. Comparison of several treated monoliths revealed the formation of various reaction products such as SiO₂, zircon (ZrSiO₄), iron silicide (FeSi) and hercynite (FeAl₂O₄) indicating the occurrence of various side reactions between the different phases of the coating as well as between the coating and the SiSiC substrate. The investigations showed that the ferrite is mainly reduced through reaction with silicon (Si), which is present in the SiSiC matrix, and silicon carbide (SiC). These results led to the formulation of a new redox mechanism for this system in which Zn-ferrite is reduced through Si forming silicon dioxide (SiO₂) and through SiC forming SiO₂ and carbon monoxide. A decline of hydrogen production within the first 20 cycles is suggested to be due to the growth of a silicon dioxide and zircon layer which acts as a diffusion barrier for the reacting specie.

ZrO2 film interfaces with Si and SiO2

NASA Astrophysics Data System (ADS)

Lopez, C. M.; Suvorova, N. A.; Irene, E. A.; Suvorova, A. A.; Saunders, M.

2005-08-01

The interface formed by the thermal oxidation of sputter-deposited Zr metal onto Si(100)- and SiO2-coated Si(100) wafers was studied in situ and in real time using spectroscopic ellipsometry (SE) in the 1.5-4.5 photon energy range and mass spectrometry of recoiled ions (MSRI). SE yielded optical properties for the film and interface and MSRI yielded film and interface composition. An optical model was developed and verified using transmission electron microscopy. Interfacial reaction of the ZrO2 was observed for both substrates, with more interaction for Si substrates. Equivalent oxide thicknesses and interface trap levels were determined on capacitors with lower trap levels found on samples with a thicker SiO2 underlayer. In addition to the optical properties for the intermixed interface layer, the optical properties for Zr metal and unreacted ZrO2 are also reported.

Si-H induced synthesis of Si/Cu2O nanowire arrays for photoelectrochemical water splitting

NASA Astrophysics Data System (ADS)

Zhang, Shaoyang; She, Guangwei; Li, Shengyang; Mu, Lixuan; Shi, Wensheng

2018-01-01

We report a facile and low-cost method to synthesize Si/Cu2O heterojunction nanowire arrays, without SiOx, at the Si/Cu2O interface. The reductive Si-H bonds on the surface of Si nanowires plays a key role in situ by reducing Cu(II) ions to Cu2O nanocubes and avoiding the SiOx interface layer. Different pH values would vary the electrochemical potential of reactions and as a result, different products would be formed. Utilized as a photoanode for water splitting, Si/Cu2O nanowire arrays exhibit good photoelectrochemical performance.

High Si-H local mode overtones in SiHD/sub 3/

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

Bernheim, R.A.; Lampe, F.W.; O'Keefe, J.F.

1984-01-01

Spectra for SiHD/sub 3/ obtained using a nonresonant photoacoustic cell mounted within the cavity of a CR490 tunable CW laser are reported herein. The symmetric top spectra exhibit partial rotational resolution. A relation for determining the Si-H bond distance is reported, and the Si-D bond distance is taken to be the same as the Si-H distance in the ground vibrational state. The bond angle is assumed to remain tetrahedral in both situations. The noted spectral vibrational band widths arise only from rotational structure with contributions from fast vibrational relaxation not being evident. 10 references, 2 figures, 1 table.

Studies of the Si/SiO2 interface using synchrotron radiation

NASA Technical Reports Server (NTRS)

Hecht, M. H.; Grunthaner, F. J.

1985-01-01

Synchrotron radiation photoemission spectroscopy (SRPS) in the 1-4 KeV photon energy range is a useful tool for interface characterization. Results are presented of a series of studies of the near-interface region of Si/SiO2 which confirm that a bond strain gradient exists in the oxide as a result of lattice mismatch. These experiments include measurement of photoemission lineshape changes as a function of photon energy, corresponding changes in the electron escape depth near the interface, and surface extended X-ray absorption fine structure (SEXAFS) measurements directly indicating the shortening of the Si-Si second nearest neighbor distance in the near-interface region of the oxide.

Proton conductivity and methanol permeability of Nafion-SiO2/SiWA composite membranes

NASA Astrophysics Data System (ADS)

Thiam, Hui San; Chia, Min Yan; Cheah, Qiao Rou; Koo, Charlene Chai Hoon; Lai, Soon Onn; Chong, Kok Chung

2017-04-01

Proton exchange membranes for a direct methanol fuel cell (DMFC) were prepared by incorporating silica/silicotungstic acid (SiO2/SiWA) inorganic composite into a Nafion polymer. The effects of SiO2/SiWA content on proton conductivity of membranes were investigated by using a four-probe conductivity cell. Methanol permeability of composite membrane was also determined by using a homemade diffusion cell and gas chromatography technique. It was found that proton conductivity of the composite membranes decreased with SiO2/SiWA content, however the highest proton conductivity achieved was 11% greater than the pure recast Nafion membrane. The methanol permeability of composite membrane was much lower than that of pure recast Nafion, in a reduction of 58% which indicated a better resistance to fuel crossover. Nafion-SiO2/SiWA composite membrane showed promising advantages over pure Nafion on electrochemical properties such as proton conductivity and fuel crossover and it is potentially attractive for use in DMFC.

Thermo-Mechanical Properties of SiC/SiC Composites with Hybrid CVI-PIP Matrices

NASA Technical Reports Server (NTRS)

Bhatt, R. T.; DiCarlo, J. A.

2004-01-01

For long term structural service, the upper temperature capability for slurry-cast melt infiltrated (MI) SiC/SiC composites is limited to approx. 1315 C because of silicon reaction with the SiC fibers. For applications requiring material temperatures in excess of 1315 C, alternate methods of manufacturing the SiC matrices without silicon are being investigated, such as a hybrid combination of CVI and PIP. In this study, stacked fabric plies of Sylramic i-BN SiC fibers were coated with a CVI BN interface layer followed by a partial CVI SiC matrix. The remaining porosity in the SiC/SiC preforms was then infiltrated with silicon carbide matrix by PIP. Thermo-mechanical property measurements indicate that these composites are stable to 1700 C in inert environments under no load conditions for 100 h and under load conditions to 1450 C in air for 300 h. The advantages, disadvantages, and potential of this composite system for high temperature applications will be discussed.

Hot Corrosion Behavior of Stainless Steel with Al-Si/Al-Si-Cr Coating

NASA Astrophysics Data System (ADS)

Fu, Guangyan; Wu, Yongzhao; Liu, Qun; Li, Rongguang; Su, Yong

2017-03-01

The 1Cr18Ni9Ti stainless steel with Al-Si/Al-Si-Cr coatings is prepared by slurry process and vacuum diffusion, and the hot corrosion behavior of the stainless steel with/without the coatings is studied under the condition of Na2SO4 film at 950 °C in air. Results show that the corrosion kinetics of stainless steel, the stainless steel with Al-Si coating and the stainless steel with Al-Si-Cr coating follow parabolic laws in several segments. After 24 h corrosion, the sequence of the mass gain for the three alloys is the stainless steel with Al-Si-Cr coating < the stainless steel with Al-Si coating < the stainless steel without any coating. The corrosion products of the three alloys are layered. Thereinto, the corrosion products of stainless steel without coating are divided into two layers, where the outside layer contains a composite of Fe2O3 and FeO, and the inner layer is Cr2O3. The corrosion products of the stainless steel with Al-Si coating are also divided into two layers, of which the outside layer mainly consists of Cr2O3, and the inner layer is mainly SiO2. The corrosion film of the stainless steel with Al-Si-Cr coating is thin and dense, which combines well with substrate. Thereinto, the outside layer is mainly Cr2O3, and the inside layer is Al2O3. In the matrix of all of the three alloys, there exist small amount of sulfides. Continuous and protective films of Cr2O3, SiO2 and Al2O3 form on the surface of the stainless steel with Al-Si and Al-Si-Cr coatings, which prevent further oxidation or sulfide corrosion of matrix metals, and this is the main reason for the much smaller mass gain of the two alloys than that of the stainless steel without any coatings in the 24 h hot corrosion process.

Effect of Ga Addition on Morphology and Recovery of Primary Si During Al-Si Alloy Solidification Refining

NASA Astrophysics Data System (ADS)

Li, Jingwei; Bai, Xiaolong; Li, Yanlei; Ban, Boyuan; Chen, Jian

2015-12-01

The effect of Ga addition on alloy macrostructure, morphology and recovery rate of primary Si during the Al-Si-Ga alloy solvent refining process of silicon was studied in this work. The addition of Ga to Al-Si alloy could change the morphology of the primary Si. The average plate thickness of the primary Si increases with increase of Ga content. With the increase of Ga content, the average plate length of the primary Si crystals becomes larger when the Ga content is less than 5% in the Al-30%Si-xGa alloy, but becomes smaller when the Ga content exceeds 5%. Al-Si-Ga alloys consist of three types, primary Si, GaxAl1-x, (α-Al+Si+β-Ga) eutectic. (111) is the preferred growth surface of the plate-like primary Si. The recovery rate of the primary Si increases with the increase of Ga content. When the Ga content increased to 20% in Al-30%Si-xGa alloy, the relative recovery rate of the primary Si increased to 50.41% than that in Al-30%Si alloy.

Modeling Creep Effects in Advanced SiC/SiC Composites

NASA Technical Reports Server (NTRS)

Lang, Jerry; DiCarlo, James

2006-01-01

Because advanced SiC/SiC composites are projected to be used for aerospace components with large thermal gradients at high temperatures, efforts are on-going at NASA Glenn to develop approaches for modeling the anticipated creep behavior of these materials and its subsequent effects on such key composite properties as internal residual stress, proportional limit stress, ultimate tensile strength, and rupture life. Based primarily on in-plane creep data for 2D panels, this presentation describes initial modeling progress at applied composite stresses below matrix cracking for some high performance SiC/SiC composite systems recently developed at NASA. Studies are described to develop creep and rupture models using empirical, mechanical analog, and mechanistic approaches, and to implement them into finite element codes for improved component design and life modeling

Synthesis of ternary Si clathrates in the A-Al-Si (A = Na and K) system

NASA Astrophysics Data System (ADS)

Imai, Motoharu; Singh, Shiva Kumar; Nishio, Mitsuaki; Yamada, Takahiro; Yamane, Hisanori

2015-07-01

With the aim of producing functional materials based on earth-abundant elements, we examined the synthesis of the ternary type-I clathrates A8AlxSi46-x (A = Na and K). The type-I Si clathrate K7.9(1)Al7.1(1)Si38.9(4), having a lattice parameter of 10.434(1) Å, was successfully synthesized via the direct reaction of K, Al, and Si by optimization of both the synthesis temperature and the molar ratios among the raw ingredients. K8Al7Si39 exhibited metallic conduction: its electrical resistivity increased with increasing temperature. The high pressure synthesis of Na8AlxSi46-x was also examined, using a belt-type apparatus and employing a mixture of NaSi, Al, and Si as the reagents. In this manner, the type-I Si clathrate Na8.7(9)Al0.5(1)Si45(2), having a lattice parameter of 10.211(1) Å, was synthesized at 5.5 GPa and 1570 K.

Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting.

PubMed

Naffouti, Meher; David, Thomas; Benkouider, Abdelmalek; Favre, Luc; Ronda, Antoine; Berbezier, Isabelle; Bidault, Sebastien; Bonod, Nicolas; Abbarchi, Marco

2016-02-07

We report the fabrication of Si-based dielectric Mie resonators via a low cost process based on solid-state dewetting of ultra-thin amorphous Si on SiO2. We investigate the dewetting dynamics of a few nanometer sized layers annealed at high temperature to form submicrometric Si-particles. Morphological and structural characterization reveal the polycrystalline nature of the semiconductor matrix as well as rather irregular morphologies of the dewetted islands. Optical dark field imaging and spectroscopy measurements of the single islands reveal pronounced resonant scattering at visible frequencies. The linewidth of the low-order modes can be ∼20 nm in full width at half maximum, leading to a quality factor Q exceeding 25. These values reach the state-of-the-art ones obtained for monocrystalline Mie resonators. The simplicity of the dewetting process and its cost-effectiveness opens the route to exploiting it over large scales for applications in silicon-based photonics.

Antireflective hydrophobic si subwavelength structures using thermally dewetted Ni/SiO2 nanomask patterns.

PubMed

Joo, Dong Hyuk; Leem, Jung Woo; Yu, Jae Su

2011-11-01

We report the disordered silicon (Si) subwavelength structures (SWSs), which are fabricated with the use of inductively coupled plasma (ICP) etching in SiCl4 gas using nickel/silicon dioxide (Ni/SiO2) nanopattens as the etch mask, on Si substrates by varying the etching parameters for broadband antireflective and self-cleaning surfaces. For the fabricated Si SWSs, the antireflection characteristics are experimentally investigated and a theoretical analysis is made based on the rigorous coupled-wave analysis method. The desirable dot-like Ni nanoparticles on SiO2/Si substrates are formed by the thermal dewetting process of Ni films at 900 degrees C. The truncated cone shaped Si SWS with a high average height of 790 +/- 23 nm, which is fabricated by ICP etching with 5 sccm SiCl4 at 50 W RF power with additional 200 W ICP power under 10 mTorr process pressure, exhibits a low average reflectance of approximately 5% over a wide wavelength range of 450-1050 nm. The water contact angle of 110 degrees is obtained, indicating a hydrophobic surface. The calculated reflectance results are also reasonably consistent with the experimental data.

Preparation of siRNA encapsulated nanoliposomes suitable for siRNA delivery by simply discontinuous mixing.

PubMed

Mokhtarieh, Amir Abbas; Lee, Jieun; Kim, Semi; Lee, Myung Kyu

2018-06-01

Previously a scalable and extrusion-free method has been developed for efficient liposomal encapsulation of DNA by twice stepwise mixing of lipids in ethanol and DNA solution using T-shape mixing chamber. In this study, we prepared nanoliposomes encapsulating siRNA by simply discontinuous mixing of lipids in ethanol/ether/water mixture and acidic siRNA solution without use of special equipment. The simple mixing siRNA/liposomal particles (siRNA/SMLs) prepared using ethanol/ether/water (3:1:1) mixture showed 120.4 ± 20.2 nm particle size, 0.174 ± 0.033 polydispersity and 86.5 ± 2.76% siRNA encapsulation rate. In addition, the SMLs almost completely protected the encapsulated siRNA from RNase A digestion. Coupling of anti-human epidermal growth factor receptor (EGFR) Fab' to siRNA/SMLs enhanced EGFR-specific cell penetration of SMLs and induced siRNA dependent gene silencing. Unexpectedly, the Cy5.5-labeled Fab' showed almost no in vivo targeting to the xenografted A549 tumors in SCID-NOD mice. However, multiple injection of the unmodified siRNA/SMLs accumulated in the tumors and induced siRNA-dependent in vivo gene silencing. These results demonstrate that the siRNA/SMLs can be used as a siRNA delivery tool for gene therapy. Copyright © 2018 Elsevier B.V. All rights reserved.

Dimensional stability and anisotropy of SiC and SiC-based composites in transition swelling regime

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

Katoh, Yutai; Koyanagi, Takaaki; McDuffee, Joel L.

Swelling, or volumetric expansion, is an inevitable consequence of the atomic displacement damage in crystalline silicon carbide (SiC) caused by energetic neutron irradiation. Because of its steep temperature and dose dependence, understanding swelling is essential for designing SiC-based components for nuclear applications. Here in this study, swelling behaviors of monolithic CVD SiC and nuclear grade SiC fiber – SiC matrix (SiC/SiC) composites were accurately determined, supported by the irradiation temperature determination for individual samples, following neutron irradiation within the lower transition swelling temperature regime. Slightly anisotropic swelling behaviors were found for the SiC/SiC samples and attributed primarily to the combinedmore » effects of the pre-existing microcracking, fiber architecture, and specimen dimension. A semi-empirical model of SiC swelling was calibrated and presented. Finally, implications of the refined model to selected swelling-related issues for SiC-based nuclar reactor components are discussed.« less

Dimensional stability and anisotropy of SiC and SiC-based composites in transition swelling regime

DOE PAGES

Katoh, Yutai; Koyanagi, Takaaki; McDuffee, Joel L.; ...

2017-12-08

Swelling, or volumetric expansion, is an inevitable consequence of the atomic displacement damage in crystalline silicon carbide (SiC) caused by energetic neutron irradiation. Because of its steep temperature and dose dependence, understanding swelling is essential for designing SiC-based components for nuclear applications. Here in this study, swelling behaviors of monolithic CVD SiC and nuclear grade SiC fiber – SiC matrix (SiC/SiC) composites were accurately determined, supported by the irradiation temperature determination for individual samples, following neutron irradiation within the lower transition swelling temperature regime. Slightly anisotropic swelling behaviors were found for the SiC/SiC samples and attributed primarily to the combinedmore » effects of the pre-existing microcracking, fiber architecture, and specimen dimension. A semi-empirical model of SiC swelling was calibrated and presented. Finally, implications of the refined model to selected swelling-related issues for SiC-based nuclar reactor components are discussed.« less

[Cyberstalking among Italian nurses: a large multicentric study].

PubMed

Comparcini, Dania; Simonetti, Valentina; Lupo, Roberto; Galli, Francesco; Bocij, Paul; Cicolini, Giancarlo

2016-01-01

Determinare la prevalenza di cyberstalking tra gli infermieri italiani e valutare il livello di ansia e depressione nelle vittime. Da aprile a settembre 2014 è stato condotto uno studio multicentrico trasversale in un campione di infermieri italiani (n=997) utilizzando il questionario "Cyberstalking" per analizzare il fenomeno del cyberstalking. Ai partecipanti, che si sono autodefinite vittime di cyberstalking, è stato chiesto anche di compilare gli strumenti "Beck Depression Inventory" e "State-Trait Anxiety Inventory" per valutare, rispettivamente, i livelli di depressione ed ansia. La prevalenza di cyberstalking tra gli infermieri italiani è risultata pari al 23.3%. Il 42.7% ha dovuto cambiare il proprio stile di vita e lavorativo. Il cyberstalker era prevalentemente di sesso maschile (52%) e, nel 49% dei casi, era un paziente. Le vittime hanno riferito moderati livelli di ansia (media=28.4, SD=23.2) e depressione (media=92.7, SD=18.3); i risultati hanno mostrato un aumento dei livelli di depressione negli infermieri esperti nell'utilizzo del computer, gestori di siti web o blog, e una correlazione negativa tra il livello di ansia e gli infermieri esperti (r = -0.264). Il cyberstalking è un fenomeno che si riscontra frequentemente tra gli infermieri. Il rapporto infermiere - paziente gioca un ruolo centrale nello sviluppo del fenomeno e le vittime hanno riferito disordini correlati allo stress che influenzano la vita lavorativa. Questi risultati preliminari potrebbero sensibilizzare, i dirigenti ospedalieri, la politica e i centri antiviolenza al fine di sviluppare strategie risolutiv.

Kinetics and equilibrium adsorption study of selenium oxyanions onto Al/Si and Fe/Si coprecipitates.

PubMed

Chan, Y T; Liu, Y T; Tzou, Y M; Kuan, W H; Chang, R R; Wang, M K

2018-05-01

Inappropriate treatments for the effluents from semiconductor plants might cause the releases and wide distributions of selenium (Se) into the ecosystems. In this study, Al/Si and Fe/Si coprecipitates were selected as model adsorbents as they often formed during the wastewater coagulation process, and the removal efficiency of selenite (SeO 3 ) and selenate (SeO 4 ) onto the coprecipitates were systematically examined. The removal efficiency of SeO 3 and SeO 4 was highly related to surface properties of Al/Si and Fe/Si coprecipitates. The surface-attached Al shell of Al/Si coprecipitates shielded a portion of negative charges from the core SiO 2 , resulting in a higher point of zero charge than that of Fe/Si coprecipitates. Thus, adsorption of SeO 3 /SeO 4 was favorable on the Al/Si coprecipitates. Adsorptions of both SeO 3 and SeO 4 on Al/Si coprecipitates were exothermic reactions. On Fe/Si coprecipitates, while SeO 3 adsorption also showed the exothermic behavior, SeO 4 adsorption occurred as an endothermic reaction. The kinetic adsorption data of SeO 3 /SeO 4 on Al/Si and Fe/Si coprecipitates were described well by the pseudo-second-order kinetic model. SeO 4 and SeO 3 adsorption on Fe/Si or Al/Si were greatly inhibited by the strong PO 4 ligand, whereas the weak ligand such as SO 4 only significantly affected SeO 4 adsorption. The weakest complex between SeO 4 and Al was implied by the essentially SeO 4 desorption as SeO 4 /PO 4 molar ratios decreased from 0.5 to 0.2. These results were further confirmed by the less SeO 4 desorption (41%) from Fe/Si coprecipitates than that from Al/Si coprecipitates (78%) while PO 4 was added sequentially. Copyright © 2018 Elsevier Ltd. All rights reserved.

Large-scale synthesis of ear-like Si{sub 3}N{sub 4} dendrites from SiO{sub 2}/Fe composites and Si powders

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

Wang Feng; Graduate School of the Chinese Academy of Sciences, Beijing 100039; Jin Guoqiang

2008-07-01

Large-scale ear-like Si{sub 3}N{sub 4} dendrites were prepared by the reaction of SiO{sub 2}/Fe composites and Si powders in N{sub 2} atmosphere. The product was characterized by field emission scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results reveal that the product mainly consists of ear-like Si{sub 3}N{sub 4} dendrites with crystal structures, which have a length of several microns and a diameter of 100-200 nm. Nanosized ladder-like Si{sub 3}N{sub 4} was also obtained when changing the Fe content in the SiO{sub 2}/Fe composites. The Si{sub 3}N{sub 4} nanoladders have a length of hundreds nanometers to several micronsmore » and a width of 100-300 nm. The ear-like Si{sub 3}N{sub 4} dendrites are formed from a two-step growth process, the formation of inner stem structures followed by the epitaxial growth of secondary branches.« less

New Insights into Understanding Irreversible and Reversible Lithium Storage within SiOC and SiCN Ceramics

PubMed Central

Graczyk-Zajac, Magdalena; Reinold, Lukas Mirko; Kaspar, Jan; Sasikumar, Pradeep Vallachira Warriam; Soraru, Gian-Domenico; Riedel, Ralf

2015-01-01

Within this work we define structural properties of the silicon carbonitride (SiCN) and silicon oxycarbide (SiOC) ceramics which determine the reversible and irreversible lithium storage capacities, long cycling stability and define the major differences in the lithium storage in SiCN and SiOC. For both ceramics, we correlate the first cycle lithiation or delithiation capacity and cycling stability with the amount of SiCN/SiOC matrix or free carbon phase, respectively. The first cycle lithiation and delithiation capacities of SiOC materials do not depend on the amount of free carbon, while for SiCN the capacity increases with the amount of carbon to reach a threshold value at ~50% of carbon phase. Replacing oxygen with nitrogen renders the mixed bond Si-tetrahedra unable to sequester lithium. Lithium is more attracted by oxygen in the SiOC network due to the more ionic character of Si-O bonds. This brings about very high initial lithiation capacities, even at low carbon content. If oxygen is replaced by nitrogen, the ceramic network becomes less attractive for lithium ions due to the more covalent character of Si-N bonds and lower electron density on the nitrogen atom. This explains the significant difference in electrochemical behavior which is observed for carbon-poor SiCN and SiOC materials. PMID:28347008

Amorphization of nanocrystalline 3C-SiC irradiated with Si+ ions

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

Jiang, Weilin; Wang, H.; Zhang, Yanwen

2010-01-01

Irradiation induced amorphization in nanocrystalline and single crystal 3C-SiC has been studied using 1 MeV Si+ ions under identical irradiation conditions at room temperature and 400 K. The disordering behavior has been characterized using in-situ ion channeling and ex-situ x-ray diffraction methods. The results show that, compared to single crystal 3C-SiC, full amorphization of small 3C-SiC grains (~3.8 nm in size) at room temperature occurs at a slightly lower dose. Grain size decreases with increasing dose until a fully amorphized state is attained. The amorphization dose increases at 400 K relative to room temperature. However, at 400 K, the dosemore » for amorphization for 2.0 nm grains is about a factor of 4 and 8 smaller than for 3.0 nm grains and bulk single crystal 3C-SiC, respectively. The behavior is attributed to the dominance of defect-stimulated interfacial amorphization.« less

Opto-electronic properties of P-doped nc-Si-QD/a-SiC:H thin films as foundation layer for all-Si solar cells in superstrate configuration

NASA Astrophysics Data System (ADS)

Kar, Debjit; Das, Debajyoti

2016-07-01

With the advent of nc-Si solar cells having improved stability, the efficient growth of nc-Si i-layer of the top cell of an efficient all-Si solar cell in the superstrate configuration prefers nc-Si n-layer as its substrate. Accordingly, a wide band gap and high conducting nc-Si alloy material is a basic requirement at the n-layer. Present investigation deals with the development of phosphorous doped n-type nanocrystalline silicon quantum dots embedded in hydrogenated amorphous silicon carbide (nc-Si-QD/a-SiC:H) hetero-structure films, wherein the optical band gap can be widened by the presence of Si-C bonds in the amorphous matrix and the embedded high density tiny nc-Si-QDs could provide high electrical conductivity, particularly in P-doped condition. The nc-Si-QDs simultaneously facilitate further widening of the optical band gap by virtue of the associated quantum confinement effect. A complete investigation has been made on the electrical transport phenomena involving charge transfer by tunneling and thermionic emission prevailing in n-type nc-Si-QD/a-SiC:H thin films. Their correlation with different phases of the specific heterostructure has been carried out for detailed understanding of the material, in order to improve its device applicability. The n-type nc-Si-QD/a-SiC:H films exhibit a thermally activated electrical transport above room temperature and multi-phonon hopping (MPH) below room temperature, involving defects in the amorphous phase and the grain-boundary region. The n-type nc-Si-QD/a-SiC:H films grown at ˜300 °C, demonstrating wide optical gap ˜1.86-1.96 eV and corresponding high electrical conductivity ˜4.5 × 10-1-1.4 × 10-2 S cm-1, deserve to be an effective foundation layer for the top nc-Si sub-cell of all-Si solar cells in n-i-p structure with superstrate configuration.

On the interplay between Si(110) epilayer atomic roughness and subsequent 3C-SiC growth direction

NASA Astrophysics Data System (ADS)

Khazaka, Rami; Michaud, Jean-François; Vennéguès, Philippe; Nguyen, Luan; Alquier, Daniel; Portail, Marc

2016-11-01

In this contribution, we performed the growth of a 3C-SiC/Si/3C-SiC layer stack on a Si(001) substrate by means of chemical vapor deposition. We show that, by tuning the growth conditions, the 3C-SiC epilayer can be grown along either the [111] direction or the [110] direction. The key parameter for the growth of the desired 3C-SiC orientation on the Si(110)/3C-SiC(001)/Si(001) heterostructure is highlighted and is linked to the Si epilayer surface morphology. The epitaxial relation between the layers has been identified using X-ray diffraction and transmission electron microscopy (TEM). We showed that, regardless of the top 3C-SiC epilayer orientation, domains rotated by 90° around the growth direction are present in the epilayer. Furthermore, the difference between the two 3C-SiC orientations was investigated by means of high magnification TEM. The results indicate that the faceted Si(110) epilayer surface morphology results in a (110)-oriented 3C-SiC epilayer, whereas a flat hetero-interface has been observed between 3C-SiC(111) and Si(110). The control of the top 3C-SiC growth direction can be advantageous for the development of new micro-electro-mechanical systems.

Composite films of highly ordered Si nanowires embedded in SiGe0.3 for thermoelectric applications

NASA Astrophysics Data System (ADS)

Kikuchi, Akiou; Yao, Akifumi; Mori, Isamu; Ono, Takahito; Samukawa, Seiji

2017-10-01

We fabricated a high-density array of silicon nanowires (SiNWs) with a diameter of 10 nm embedded in silicon germanium (SiGe0.3) to give a composite thin film for thermoelectric device applications. The SiNW array was first fabricated by bio-template mask and neutral beam etching techniques. The SiNW array was then embedded in SiGe0.3 by thermal chemical vapor deposition. The cross-plane thermal conductivity of the SiNW-SiGe0.3 composite film with a thickness of 100 nm was 3.5 ± 0.3 W/mK in the temperature range of 300-350 K. Moreover, the temperature dependences of the in-plane electrical conductivity and in-plane Seebeck coefficient of the SiNW-SiGe0.3 composite were evaluated. The fabricated SiNW-SiGe0.3 composite film displayed a maximum power factor of 1 × 103 W/m K2 (a Seebeck coefficient of 4.8 × 103 μV/K and an electrical conductivity of 4.4 × 103 S/m) at 873 K. The present high-density SiNW array structure represents a new route to realize practical thermoelectric devices using mature Si processes without any rare metals.

Fabrication and Characteristics of an nc-Si/c-Si Heterojunction MOSFETs Pressure Sensor

PubMed Central

Zhao, Xiaofeng; Wen, Dianzhong; Li, Gang

2012-01-01

A novel nc-Si/c-Si heterojunction MOSFETs pressure sensor is proposed in this paper, with four p-MOSFETs with nc-Si/c-Si heterojunction as source and drain. The four p-MOSFETs are designed and fabricated on a square silicon membrane by CMOS process and MEMS technology where channel resistances of the four nc-Si/c-Si heterojunction MOSFETs form a Wheatstone bridge. When the additional pressure is P, the nc-Si/c-Si heterojunction MOSFETs pressure sensor can measure this additional pressure P. The experimental results show that when the supply voltage is 3 V, length-width (L:W) ratio is 2:1, and the silicon membrane thickness is 75 μm, the full scale output voltage of the pressure sensor is 15.50 mV at room temperature, and pressure sensitivity is 0.097 mV/kPa. When the supply voltage and L:W ratio are the same as the above, and the silicon membrane thickness is 45 μm, the full scale output voltage is 43.05 mV, and pressure sensitivity is 2.153 mV/kPa. Therefore, the sensor has higher sensitivity and good temperature characteristics compared to the traditional piezoresistive pressure sensor. PMID:22778646

Diffusion reaction of oxygen in HfO2/SiO2/Si stacks.

PubMed

Ferrari, S; Fanciulli, M

2006-08-03

We study the oxidation mechanism of silicon in the presence of a thin HfO2 layer. We performed a set of annealing in 18O2 atmosphere on HfO2/SiO2/Si stacks observing the 18O distribution in the SiO2 layer with time-of-flight secondary ion mass spectrometry (ToF-SIMS). The 18O distribution in HfO2/SiO2/Si stacks upon 18O2 annealing suggests that what is responsible for SiO2 growth is the molecular O2, whereas no contribution is found of the atomic oxygen to the oxidation. By studying the dependence of the oxidation velocity from oxygen partial pressure and annealing temperature, we demonstrate that the rate-determining step of the oxidation is the oxygen exchange at the HfO2/SiO2 interface. When moisture is chemisorbed in HfO2 films, the oxidation of the underlying silicon substrate becomes extremely fast and its kinetics can be described as a wet silicon oxidation process. The silicon oxidation during O2 annealing of the atomic layer deposited HfO2/Si is fast in its early stage due to chemisorbed moisture and becomes slow after the first 10 s.

Phosphorus doping of Si and Si1 - xGex grown by ultrahigh vacuum chemical vapor deposition using Si2H6 and GeH4

NASA Astrophysics Data System (ADS)

Chen, L. P.; Huang, G. W.; Chang, C. Y.

1996-03-01

100 ppm PH3 diluted in hydrogen is used as the n-type dopant gas in Si and Si1-xGex epilayers grown by ultrahigh vacuum chemical vapor deposition (UHVCVD) using Si2H6 and GeH4. The phosphorus concentration in Si increases linearly at a small PH3 flow rate and becomes nearly saturated at higher flow rates, while the phosphorus concentration in Si1-xGex only shows a nearly linear behavior with PH3 flow rate. The growth rates of Si and Si1-xGex epilayers decrease seriously (˜50%) and slightly (˜10%) with the increase of PH3 flow rate, respectively. These results can be explained by a model based on the enhancement of hydrogen desorption rate at smaller PH3 flow rates and different levels of the effects of phosphorus blocking of surface-activated sites between Si and Si1-xGex epilayers at higher PH3 flow rates.

High Mobility SiGe/Si Transistor Structures on Sapphire Substrates Using Ion Implantation

NASA Technical Reports Server (NTRS)

Alterovitz, S. A.; Mueller, C. H.; Croke, E. T.

2003-01-01

High mobility n-type SiGe/Si transistor structures have been fabricated on sapphire substrates by ion implanting phosphorus ions into strained 100 Angstrom thick silicon channels for the first time. The strained Si channels were sandwiched between Si(sub 0.7)Ge(sub 0.3) layers, which, in turn, were deposited on Si(sub 0.7)Ge(sub 0.3) virtual substrates and graded SiGe buffer layers. After the molecular beam epitaxy (MBE) film growth process was completed, ion thick silicon channels implantation and post-annealing were used to introduce donors. The phosphorous ions were preferentially located in the Si channel at a peak concentration of approximately 1x10(exp 18)/cu cm. Room temperature electron mobilities exceeding 750 sq cm/V-sec at carrier densities of 1x10(exp 12)/sq cm were measured. Electron concentration appears to be the key factor that determines mobility, with the highest mobility observed for electron densities in the 1 - 2x10(exp 12)/sq cm range.

Effect of SiC buffer layer on GaN growth on Si via PA-MBE

NASA Astrophysics Data System (ADS)

Kukushkin, S. A.; Mizerov, A. M.; Osipov, A. V.; Redkov, A. V.; Telyatnik, R. S.; Timoshnev, S. N.

2017-11-01

The study is devoted to comparison of GaN thin films grown on SiC/Si substrates made by the method of atoms substitution with the films grown directly on Si substrates. The growth was performed in a single process via plasma assisted molecular beam epitaxy. The samples were studied via optical microscopy, Raman spectroscopy, ellipsometry, and a comparison of their characteristics was made. Using chemical etching in KOH, the polarity of GaN films grown on SiC/Si and Si substrates was determined.

XRD and 29Si MAS-NMR spectroscopy across the β-Lu 2Si 2O 7- β-Y 2Si 2O 7 solid solution

NASA Astrophysics Data System (ADS)

Becerro, Ana I.; Escudero, Alberto

2005-01-01

Samples in the system Lu 2-xY xSi 2O 7 (0⩽ x⩽2) have been synthesized following the sol-gel method and calcined to 1300 °C, a temperature at which the β-polymorph is known to be the stable phase for the end-members Lu 2Si 2O 7 and Y 2Si 2O 7. The XRD patterns of all the compositions studied are compatible with the structure of the β-polymorph. Unit cell parameters are calculated as a function of composition from XRD patterns. They show a linear change with increasing Y content, which indicates a solid solubility of β-Y 2Si 2O 7 in β-Lu 2Si 2O 7 at 1300 °C. 29Si MAS NMR spectra of the different members of the system agree with the XRD results, showing a linear decrease of the 29Si chemical shift with increasing Y content. Finally, a correlation reported in the literature to predict 29Si chemical shifts in silicates is applied here to obtain the theoretical variation in 29Si chemical shift values in the system Lu 2Si 2O 7-Y 2Si 2O 7 and the results compare favorably with the values obtained experimentally.

Measurements of Local Strain Variation in Si(1-x)Ge(x)/Si Heterostructures

NASA Technical Reports Server (NTRS)

Bell, L. D.; Kaiser, W. J.; Manion, S. J.; Milliken, S. J.; Pike, W. T.; Fathauer, R. W.

1995-01-01

The energy splitting of the conduction-band minimum of Si(1-x), Ge(x), due to strain has been directly measured by the application of ballistic-electron-emission microscope (BEEM) spectroscopy to Ag/Si(1-x), Ge(x) structures. Experimental values for this conduction-band splitting agree well with calculations. For Au/Si(1-x), Ge(x), however, heterogeneity in the strain of the Si(1-x), Ge(x) layer is introduced by deposition of the Au. This variation is attributed to species interdiffusion, which produces a rough Si(1-x)Ge(x) surface. Preliminary modeling indicates that the observed roughness is consistent with the strain variation measured by BEEM.

Si-FeSi2/C nanocomposite anode materials produced by two-stage high-energy mechanical milling

NASA Astrophysics Data System (ADS)

Yang, Yun Mo; Loka, Chadrasekhar; Kim, Dong Phil; Joo, Sin Yong; Moon, Sung Whan; Choi, Yi Sik; Park, Jung Han; Lee, Kee-Sun

2017-05-01

High capacity retention Silicon-based nanocomposite anode materials have been extensively explored for use in lithium-ion rechargeable batteries. Here we report the preparation of Si-FeSi2/C nanocomposite through scalable a two-stage high-energy mechanical milling process, in which nano-scale Si-FeSi2 powders are besieged by the carbon (graphite/amorphous phase) layer; and investigation of their structure, morphology and electrochemical performance. Raman analysis revealed that the carbon layer structure comprised of graphitic and amorphous phase rather than a single amorphous phase. Anodes fabricated with the Si-FeSi2/C showed excellent electrochemical behavior such as a first discharge capacity of 1082 mAh g-1 and a high capacity retention until the 30th cycle. A remarkable coulombic efficiency of 99.5% was achieved within a few cycles. Differential capacity plots of the Si-FeSi2/C anodes revealed a stable lithium reaction with Si for lithiation/delithiation. The enhanced electrochemical properties of the Si-FeSi2/C nanocomposite are mainly attributed to the nano-size Si and stable solid electrolyte interface formation and highly conductive path driven by the carbon layer.

SiC/SiC Composites for 1200 C and Above

NASA Technical Reports Server (NTRS)

DiCarlo, J. A.; Yun, H.-M.; Morscher, G. N.; Bhatt, R. T.

2004-01-01

The successful replacement of metal alloys by ceramic matrix composites (CMC) in high-temperature engine components will require the development of constituent materials and processes that can provide CMC systems with enhanced thermal capability along with the key thermostructural properties required for long-term component service. This chapter presents information concerning processes and properties for five silicon carbide (SiC) fiber-reinforced SiC matrix composite systems recently developed by NASA that can operate under mechanical loading and oxidizing conditions for hundreds of hours at 1204, 1315, and 1427 C, temperatures well above current metal capability. This advanced capability stems in large part from specific NASA-developed processes that significantly improve the creep-rupture and environmental resistance of the SiC fiber as well as the thermal conductivity, creep resistance, and intrinsic thermal stability of the SiC matrices.

Planetary and meteoritic Mg/Si and δ30 Si variations inherited from solar nebula chemistry

NASA Astrophysics Data System (ADS)

Dauphas, Nicolas; Poitrasson, Franck; Burkhardt, Christoph; Kobayashi, Hiroshi; Kurosawa, Kosuke

2015-10-01

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth's mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe-Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at ∼1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of ∼1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth's core of 3.6 (+ 6.0 / - 3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.

SI units.

PubMed

Lehmann, H P

1979-01-01

The development of the International System of Units (Systeme International d'Unites--SE Units), based on seven fundamental quantities--length, mass, time, electric current, thermodynamic temperature, luminous intensity, and amount of substance is described. Units (coherent and noncoherent) for other measurable quantities that are derived from the seven basic quantities are reviewed. The rationale for the use of SE units in medicine, primarily as applied to clinical laboratory data, is discussed, and arguments are presented for the rigid adoption of SI units in medicine and for exceptions. Tables are given for the basic and derived SI units used in medicine and for conversion factors from the quantities and units in current use to those in SI units.

Diodes of nanocrystalline SiC on n-/n+-type epitaxial crystalline 6H-SiC

NASA Astrophysics Data System (ADS)

Zheng, Junding; Wei, Wensheng; Zhang, Chunxi; He, Mingchang; Li, Chang

2018-03-01

The diodes of nanocrystalline SiC on epitaxial crystalline (n-/n+)6H-SiC wafers were investigated, where the (n+)6H-SiC layer was treated as cathode. For the first unit, a heavily boron doped SiC film as anode was directly deposited by plasma enhanced chemical vapor deposition method on the wafer. As to the second one, an intrinsic SiC film was fabricated to insert between the wafer and the SiC anode. The third one included the SiC anode, an intrinsic SiC layer and a lightly phosphorus doped SiC film besides the wafer. Nanocrystallization in the yielded films was illustrated by means of X-ray diffraction, transmission electronic microscope and Raman spectrum respectively. Current vs. voltage traces of the obtained devices were checked to show as rectifying behaviors of semiconductor diodes, the conduction mechanisms were studied. Reverse recovery current waveforms were detected to analyze the recovery performance. The nanocrystalline SiC films in base region of the fabricated diodes are demonstrated as local regions for lifetime control of minority carriers to improve the reverse recovery properties.

Two-Dimensional Porous Sandwich-Like C/Si-Graphene-Si/C Nanosheets for Superior Lithium Storage.

PubMed

Yao, Weiqi; Chen, Jie; Zhan, Liang; Wang, Yanli; Yang, Shubin

2017-11-15

A novel two-dimensional porous sandwich-like Si/carbon nanosheet is designed and successfully fabricated as an anode for superior lithium storage, where a porous Si nanofilm grows on the two sides of reduced graphene oxide (rGO) and is then coated with a carbon layer (denoted as C/Si-rGO-Si/C). The coexistence of micropores and mesopores in C/Si-rGO-Si/C nanosheets offers a rapid Li + diffusion rate, and the porous Si provides a short pathway for electric transportation. Meanwhile, the coated carbon layer not only can promote to form a stable SEI layer, but also can improve the electric conductivity of nanoscale Si coupled with rGO. Thus, the unique nanostructures offer the resultant C/Si-rGO-Si/C electrode with high reversible capacity (1187 mA h g -1 after 200 cycles at 0.2 A g -1 ), excellent cycle stability (894 mA h g -1 after 1000 cycles at 1 A g -1 ), and high rate capability (694 mA h g -1 at 5 A g -1 , 447 mA h g -1 at 10 A g -1 ).

Cell-penetrating peptide-siRNA conjugate loaded YSA-modified nanobubbles for ultrasound triggered siRNA delivery.

PubMed

Xie, Xiangyang; Yang, Yanfang; Lin, Wen; Liu, Hui; Liu, Hong; Yang, Yang; Chen, Ying; Fu, Xudong; Deng, Jianping

2015-12-01

Due to the absence of effective in vivo delivery systems, the employment of small interference RNA (siRNA) in the clinic has been hindered. In this paper, a new siRNA targeting system for EphA2-positive tumors was developed, based on ultrasound-sensitive nanobubbles (NBs) and cell-permeable peptides (CPPs). Here, a CPP-siRNA conjugate (CPP-siRNA) was entrapped in an ephrin mimetic peptide (YSA peptide)-modified NB (CPP-siRNA/YSA-NB) and the penetration of the CPP-siRNA was temporally masked; local ultrasound stimulation triggered the release of CPP-siRNA from the NBs and activated its penetration. Subsequent research demonstrated that the CPP-siRNA/YSA-NBs had particle sizes of approximately 200 nm and a siRNA entrapment efficiency of more than 85%. The in vitro release results showed that over 90% of the encapsulated CPP-siRNA released from the NBs in the presence of ultrasound, while less than 1.5% of that (30 min) released without ultrasound. Cell experiments showed a the higher CPP-siRNA cellular uptake of CPP-siRNA/YSA-NB among the various formulations in human breast adenocarcinoma cells (MCF-7, EphA2 positive cells). Additionally, after systemic administration in mice, CPP-siRNA/YSA-NB accumulated in the tumor, augmented c-Myc silencing and delayed tumor progression. In conclusion, the application of CPP-siRNA/YSA-NB with ultrasound may provide a strategy for the selective and efficient delivery of siRNA. Copyright © 2015 Elsevier B.V. All rights reserved.

Structural, electronic, elastic, and thermodynamic properties of CaSi, Ca2Si, and CaSi2 phases from first-principles calculations

NASA Astrophysics Data System (ADS)

Li, X. D.; Li, K.; Wei, C. H.; Han, W. D.; Zhou, N. G.

2018-06-01

The structural, electronic, elastic, and thermodynamic properties of CaSi, Ca2Si, and CaSi2 are systematically investigated by using first-principles calculations method based on density functional theory (DFT). The calculated formation enthalpies and cohesive energies show that CaSi2 possesses the greatest structural stability and CaSi has the strongest alloying ability. The structural stability of the three phases is compared according to electronic structures. Further analysis on electronic structures indicates that the bonding of these phases exhibits the combinations of metallic, covalent, and ionic bonds. The elastic constants are calculated, and the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and anisotropy factor of polycrystalline materials are deduced. Additionally, the thermodynamic properties were theoretically predicted and discussed.

Theoretical mechanistic study on the ion-molecule reaction of SiCN+/SiNC+ with H2O.

PubMed

Wang, Jian; Ding, Yi-hong; Sun, Chia-chung

2005-02-15

The gas-phase ion-molecule reactions play very important roles in interstellar and in plasma chemistry. Motivated by recent astrophysical detection of the SiCN/SiNC radicals and laboratory characterization of some SiCN-containing species, we carried out a detailed potential energy survey on the SiCN+/SiNC(+) + H2O reaction at the Becke's three-parameter Lee-Yang-Parr-B3LYP/6-311G(d,p) and coupled cluster with single, double, and triple excitations-CCSD(T)/6-311 + G(2df,p) (single-point) levels as an attempt towards understanding the SiCN+/SiNC+ reaction mechanisms. In contrast to the carbene-featured analogous CCN+/CNC(+) + H2X (X=O,S) reactions, the title reaction SiCN+/SiNC(+) + H2O are not associated with any competitive silylene-insertion characters. Moreover, the -CN <--> -NC interconversion has a low barrier and plays an important role in determining the final product distributions. This is also in marked difference from the CCN+/CNC+ reaction. It is shown that the isomeric sila-cations SiCN+ and SiNC+ can both react with H2O to barrierlessly generate the major product P1 HOSi(+) + HCN and the minor one P3 HOSi(+) + HNC, whereas other low-lying products such as P2 SiNCO(+) + H2, and P(0) H2NSi(+) + CO are kinetically unfeasible. The high efficiency of the SiCN+/SiNC+ reaction towards H2O and the potential importance of SiCN+/SiNC+ ion chemistry in interstellar and SiCN-based microelectric and photoelectric processes strongly appeals for future laboratory investigations on the SiCN+/SiNC+ chemical reactivity.

Polymorphism in the Sc 2Si 2O 7-Y 2Si 2O 7 system

NASA Astrophysics Data System (ADS)

Escudero, Alberto; Alba, María D.; Becerro, Ana. I.

2007-04-01

This paper examines the structural changes with temperature and composition in the Sc 2Si 2O 7-Y 2Si 2O 7 system; members of this system are expected to form in the intergranular region of Si 3N 4 and SiC structural ceramics when sintered with the aid of Y 2O 3 and Sc 2O 3 mixtures. A set of different compositions have been synthesized using the sol-gel method to obtain a xerogel, which has been calcined at temperatures between 1300 and 1750 °C during different times. The temperature-composition diagram of the system, obtained from powder XRD data, is dominated by the β- RE2Si 2O 7 polymorph, with γ- RE2Si 2O 7 and δ- RE2Si 2O 7 showing very reduced stability fields. Isotherms at 1300 and 1600 °C have been analysed in detail to evaluate the solid solubility of the components. Although, the XRD data show a complete solid solubility of β-Sc 2Si 2O 7 in β-Y 2Si 2O 7 at 1300 °C, the 29Si MAS-NMR spectra indicate a local structural change at x ca. 1.15 (Sc 2-xY xSi 2O 7) related to the configuration of the Si tetrahedron, which does not affect the long-range order of the β- RE2Si 2O 7 structure. Finally, it is interesting to note that, although Sc 2Si 2O 7 shows a unique stable polymorph ( β), Sc 3+ is able to replace Y 3+ in γ-Y 2Si 2O 7 in the compositional range 1.86⩽ x⩽2 (where x is Sc 2-xY xSi 2O 7) as well as in δ-Y 2Si 2O 7 for compositions much closer to the pure Y 2Si 2O 7.

Recrystallization in Si upon ion irradiation at room temperature in Co/Si(111) thin film systems

NASA Astrophysics Data System (ADS)

Banu, Nasrin; Satpati, B.; Dev, B. N.

2018-04-01

After several decades of research it was concluded that for a constant flux recrystallization in Si upon ion irradiation is possible only at high temperature. At low temperature or at room temperature only amorphization can take place. However we have observed recrystallization in Si upon ion irradiation at room temperature in a Co/Si thin film system. The Co/Si sample was prepared by deposition of 25 nm Co on clean Si(111) substrate. An oxide layer (˜ 2nm) of cobalt at the top of the film due to air exposure. The ion irradiation was done at room temperature under high vacuum with 1MeV Si+ ion with low beam current < 400 nA. Earlier we have shown similar ion induced recrystallization in Si(100) substrate which had a sandwich Si/Ni/Si structure. This system had an epitaxial buffer Si layer on Si substrate. This study also shows that the phenomenon is independent of substrate orientation and buffer layer. We have used transmission electron microscopy (TEM) to study the recrystallization behavior.

An inert marker study for palladium silicide formation - Si moves in polycrystalline Pd2Si

NASA Technical Reports Server (NTRS)

Ho, K. T.; Lien, C.-D.; Shreter, U.; Nicolet, M.-A.

1985-01-01

A novel use of Ti marker is introduced to investigate the moving species during Pd2Si formation on 111 and 100 line-type Si substrates. Silicide formed from amorphous Si is also studied using a W marker. Although these markers are observed to alter the silicide formation in the initial stage, the moving species can be identified once a normal growth rate is resumed. It is found that Si is the dominant moving species for all three types of Si crystallinity. However, Pd will participate in mass transport when Si motion becomes obstructed.

Formation mechanisms of Si3N4 and Si2N2O in silicon powder nitridation

NASA Astrophysics Data System (ADS)

Yao, Guisheng; Li, Yong; Jiang, Peng; Jin, Xiuming; Long, Menglong; Qin, Haixia; Kumar, R. Vasant

2017-04-01

Commercial silicon powders are nitrided at constant temperatures (1453 K; 1513 K; 1633 K; 1693 K). The X-ray diffraction results show that small amounts of Si3N4 and Si2N2O are formed as the nitridation products in the samples. Fibroid and short columnar Si3N4 are detected in the samples. The formation mechanisms of Si3N4 and Si2N2O are analyzed. During the initial stage of silicon powder nitridation, Si on the outside of sample captures slight amount of O2 in N2 atmosphere, forming a thin film of SiO2 on the surface which seals the residual silicon inside. And the oxygen partial pressure between the SiO2 film and free silicon is decreasing gradually, so passive oxidation transforms to active oxidation and metastable SiO(g) is produced. When the SiO(g) partial pressure is high enough, the SiO2 film will crack, and N2 is infiltrated into the central section of the sample through cracks, generating Si2N2O and short columnar Si3N4 in situ. At the same time, metastable SiO(g) reacts with N2 and form fibroid Si3N4. In the regions where the oxygen partial pressure is high, Si3N4 is oxidized into Si2N2O.

Absolute Calibration of Si iRMs used for Measurements of Si Paleo-nutrient proxies

NASA Astrophysics Data System (ADS)

Vocke, R. D., Jr.; Rabb, S. A.

2016-12-01

Silicon isotope variations (reported as δ30Si and δ29Si, relative to NBS28) in silicic acid dissolved in ocean waters, in biogenic silica and in diatoms are extremely informative paleo-nutrient proxies. The resolution and comparability of such measurements depend on the quality of the isotopic Reference Materials (iRMs) defining the delta scale. We report new absolute Si isotopic measurements on the iRMs NBS28 (RM 8546 - Silica Sand), Diatomite, and Big Batch using the Avogadro measurement approach and comparing them with prior assessments of these iRMs. The Avogadro Si measurement technique was developed by the German Physikalish-Technische Bundesanstalt (PTB) to provide a precise and highly accurate method to measure absolute isotopic ratios in highly enriched 28Si (99.996%) material. These measurements are part of an international effort to redefine the kg and mole based on the Planck constant h and the Avogadro constant NA, respectively (Vocke et al., 2014 Metrologia 51, 361, Azuma et al., 2015 Metrologia 52 360). This approach produces absolute Si isotope ratio data with lower levels of uncertainty when compared to the traditional "Atomic Weights" method of absolute isotope ratio measurement calibration. This is illustrated in Fig. 1 where absolute Si isotopic measurements on SRM 990, separated by 40+ years of advances in instrumentation, are compared. The availability of this new technique does not say that absolute Si isotopic ratios are or ever will be better for normal Si isotopic measurements when seeking isotopic variations in nature, because they are not. However, by determining the absolute isotopic ratios of all the Si iRM scale artifacts, such iRMs become traceable to the metric system (SI); thereby automatically conferring on all the artifact-based δ30Si and δ29Si measurements traceability to the base SI unit, the mole. Such traceability should help reduce the potential of bias between different iRMs and facilitate the replacement of delta

Metal Induced Growth of Si Thin Films and NiSi Nanowires

DTIC Science & Technology

2010-02-25

Zinc Oxide Over MIG Silicon- We have been studying the formation of ZnO films by RF sputtering. Part of this study deals with...about 50 nm. 15. SUBJECT TERMS Thin film silicon, solar cells, thin film transistors , nanowires, metal induced growth 16. SECURITY CLASSIFICATION...to achieve, µc-Si is more desirable than a-Si due to its increased mobility. Thin film µc-Si is also a popular material for thin film transistors

Thermo-Oxidative Degradation Of SiC/Si3N4 Composites

NASA Technical Reports Server (NTRS)

Baaklini, George Y.; Batt, Ramakrishna T.; Rokhlin, Stanislav I.

1995-01-01

Experimental study conducted on thermo-oxidative degradation of composite-material specimens made of silicon carbide fibers in matrices of reaction-bonded silicon nitride. In SiC/Si3N4 composites of study, interphase is 3-micrometers-thick carbon-rich coat on surface of each SiC fiber. Thermo-oxidative degradation of these composites involves diffusion of oxygen through pores of composites to interphases damaged by oxidation. Nondestructive tests reveal critical exposure times.

Potential energy landscape of an interstitial O2 molecule in a SiO2 film near the SiO2/Si(001) interface

NASA Astrophysics Data System (ADS)

Ohta, Hiromichi; Watanabe, Takanobu; Ohdomari, Iwao

2008-10-01

Potential energy distribution of interstitial O2 molecule in the vicinity of SiO2/Si(001) interface is investigated by means of classical molecular simulation. A 4-nm-thick SiO2 film model is built by oxidizing a Si(001) substrate, and the potential energy of an O2 molecule is calculated at Cartesian grid points with an interval of 0.05 nm in the SiO2 film region. The result shows that the potential energy of the interstitial site gradually rises with approaching the interface. The potential gradient is localized in the region within about 1 nm from the interface, which coincides with the experimental thickness of the interfacial strained layer. The potential energy is increased by about 0.62 eV at the SiO2/Si interface. The result agrees with a recently proposed kinetic model for dry oxidation of silicon [Phys. Rev. Lett. 96, 196102 (2006)], which argues that the oxidation rate is fully limited by the oxidant diffusion.

Process-Induced Carbon and Sub-Layer in SiC/BN/SiC Composites: Characterization and Consequences

NASA Technical Reports Server (NTRS)

Ogbuji, L. U. J. T; Wheeler, D. R.; McCue, T. R.

2001-01-01

Following our detection of films of elemental carbon in the Hi-Nicalon TM/BN/SiC composite and its deleterious effect on oxidative durability, we have examined other SiC/BN/SiC systems. The problem is pervasive, and significant residues of free carbon are confirmed in Sylramic /BN/SiC materials. Effective techniques for routine detection and characterization of adventitious carbon in SiC/BN/SiC composites are discussed.

Correlated Si isotope anomalies and large C-13 enrichments in a family of exotic SiC grains

NASA Technical Reports Server (NTRS)

Stone, J.; Hutcheon, I. D.; Epstein, S.; Wasserburg, G. J.

1991-01-01

A hypothesis is presented to the effect that the distinctive morphological characteristics and comparatively simple Si isotope systematics identify the platy SiC crystals as a genetically related family, formed around a single isotopically heterogeneous presolar star on an association of related stars. The enrichments in C-13 and the Si isotope systematics of the platy SiC are broadly consistent with theoretical models of nucleosynthesis in low-mass, carbon stars on the ASG. The Si isotope array most plausibly reflects mixing between (Si-28)-rich material, inherited from a previous generation of stars, and material enriched in Si-29 and Si-30, produced in intershell regions by neutron capture during He-burning. The absence of a correlation between the Si and C isotopic compositions of the SiC suggests either episodic condensation of SiC, extending over several thermal pulses, in the atmosphere of a single star, or the derivation of the SiC from several stars characterized by different rates of C-13 production.

Preparation of SiO2-Protecting Metallic Fe Nanoparticle/SiO2 Composite Spheres for Biomedical Application

PubMed Central

Hsieh, Pin-Wei; Tseng, Ching-Li; Kuo, Dong-Hau

2015-01-01

Functionalized Fe nanoparticles (NPs) have played an important role in biomedical applications. In this study, metallic Fe NPs were deposited on SiO2 spheres to form a Fe/SiO2 composite. To protect the Fe from oxidation, a thin SiO2 layer was coated on the Fe/SiO2 spheres thereafter. The size and morphology of the SiO2@Fe/SiO2 composite spheres were examined by transmission electron microscopy (TEM). The iron form and its content and magnetic properties were examined by X-ray diffraction (XRD), inductively-coupled plasma mass spectrometry (ICP-MS) and a superconducting quantum interference device (SQUID). The biocompatibility of the SiO2@Fe/SiO2 composite spheres was examined by Cell Counting Kit-8 (CCK-8) and lactate dehydrogenase (LDH) tests. The intracellular distribution of the SiO2@Fe/SiO2 composite spheres was observed using TEM. XRD analysis revealed the formation of metallic iron on the surface of the SiO2 spheres. According to the ICP-MS and SQUID results, using 0.375 M FeCl3·6H2O for Fe NPs synthesis resulted in the highest iron content and magnetization of the SiO2@Fe/SiO2 spheres. Using a dye loading experiment, a slow release of a fluorescence dye from SiO2@Fe/SiO2 composite spheres was confirmed. The SiO2@Fe/SiO2 composite spheres co-cultured with L929 cells exhibit biocompatibility at concentrations <16.25 µg/mL. The TEM images show that the SiO2@Fe/SiO2 composite spheres were uptaken into the cytoplasm and retained in the endosome. The above results demonstrate that the SiO2@Fe/SiO2 composite spheres could be used as a multi-functional agent, such as a magnetic resonance imaging (MRI) contrast agent or drug carriers in biomedical applications.

Thermomechanical Performance of C and SiC Multilayer, Fiber-Reinforced, CVI SiC Matrix Composites

NASA Technical Reports Server (NTRS)

Morscher, Gregory N.; Singh, Mrityunjay

2004-01-01

Hybrid fiber approaches have been attempted in the past to alloy desirable properties of different fiber-types for mechanical properties, thermal stress management, and oxidation resistance. Such an approach has potential for the CrSiC and SiCrSiC composite systems. SiC matrix composites with different stacking sequences of woven C fiber (T300) layers and woven Sic fiber (Hi-NicalonTM) layers were fabricated using the standard CVI process. Delamination occurred to some extent due to thermal mismatch for all of the composites. However, for the composites with a more uniform stacking sequence, minimal delamination occurred, enabling tensile properties to be determined at room temperature and elevated temperatures (stress-rupture in air). Composites were seal-coated with a CVI SiC layer as well as a proprietary C-B-Si (CBS) layer. Definite improvement in rupture behavior was observed in air for composites with increasing SiC fiber content and a CBS layer. The results will be compared to standard C fiber reinforced CVI SiC matrix and Hi-Nicalon reinforced CVI SiC matrix composites.

Resistive switching behaviors of Au/pentacene/Si-nanowire arrays/heavily doped n-type Si devices for memory applications

NASA Astrophysics Data System (ADS)

Tsao, Hou-Yen; Lin, Yow-Jon

2014-02-01

The fabrication of memory devices based on the Au/pentacene/heavily doped n-type Si (n+-Si), Au/pentacene/Si nanowires (SiNWs)/n+-Si, and Au/pentacene/H2O2-treated SiNWs/n+-Si structures and their resistive switching characteristics were reported. A pentacene memory structure using SiNW arrays as charge storage nodes was demonstrated. The Au/pentacene/SiNWs/n+-Si devices show hysteresis behavior. H2O2 treatment may lead to the hysteresis degradation. However, no hysteresis-type current-voltage characteristics were observed for Au/pentacene/n+-Si devices, indicating that the resistive switching characteristic is sensitive to SiNWs and the charge trapping effect originates from SiNWs. The concept of nanowires within the organic layer opens a promising direction for organic memory devices.

Magnetotransport studies of mobility limiting mechanisms in undoped Si/SiGe heterostructures

NASA Astrophysics Data System (ADS)

Mi, X.; Hazard, T. M.; Payette, C.; Wang, K.; Zajac, D. M.; Cady, J. V.; Petta, J. R.

2015-07-01

We perform detailed magnetotransport studies on two-dimensional electron gases (2DEGs) formed in undoped Si/SiGe heterostructures in order to identify the electron mobility limiting mechanisms. By analyzing data from 26 different heterostructures, we observe a strong correlation between the background oxygen concentration in the Si quantum well and the maximum mobility. The highest-quality wafer supports a 2DEG with mobility μ =160 000 cm 2/Vs at a density n =2.17 ×1011 /cm 2 and exhibits a metal-to-insulator transition at a critical density nc=0.46 ×1011 /cm 2. We extract a valley splitting Δv˜150 μ eV at a magnetic field B =1.8 T. These results provide evidence that undoped Si/SiGe heterostructures are suitable for the fabrication of few-electron quantum dots.

Investigations of 3C-SiC inclusions in 4H-SiC epilayers on 4H-SiC single crystal substrates

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

Si, W.; Dudley, M.; Kong, H.S.

1997-03-01

Synchrotron white beam x-ray topography (SWBXT) and Nomarski optical microscopy (NOM) have been used to characterize 4H-SiC epilayers and to study the character of triangular inclusions therein. 4H-SiC substrates misoriented by a range of angles from (0001), as well as (1 1{bar 0}0) and (11 2{bar 0}) oriented substrates were used. No evidence was found for the nucleation of 3C-SiC inclusions at superscrew dislocations (along the [0001] axis) in the 4H-SiC substrates. Increasing the off-axis angle of the substrates from 3.5 to 6.5{degree} was found to greatly suppress the formation of the triangular inclusions. In the case of substrates misorientedmore » by 8.0{degree} from (0001) toward [112{bar 0}], the triangular inclusions were virtually eliminated. The crystalline quality of 4H-SiC epilayers grown on the substrates misoriented by 8.0{degree} from (0001) was very good. For the (11{bar 0}0) and (112{bar 0}) samples, there is no indication of 3C-SiC inclusions in the epilayers. Possible formation mechanisms and the morphology of 3C-SiC inclusions are discussed. 17 refs., 13 figs.« less

Growth and surface analysis of SiO2 on 4H-SiC for MOS devices

NASA Astrophysics Data System (ADS)

Kodigala, Subba Ramaiah; Chattopadhyay, Somnath; Overton, Charles; Ardoin, Ira; Gordon, B. J.; Johnstone, D.; Roy, D.; Barone, D.

2015-03-01

The SiO2 layers have been grown onto C-face and Si-face 4H-SiC substrates by two different techniques such as wet thermal oxidize process and sputtering. The deposition recipes of these techniques are carefully optimized by trails and error method. The growth effects of SiO2 on the C-face and Si-face 4H-SiC substrates are thoroughly investigated by AFM analysis. The growth mechanism of different species involved in the growth process of SiO2 by wet thermal oxide is now proposed by adopting two body classical projectile scattering. This mechanism drives to determine growth of secondary phases such as α-CH nano-islands in the grown SiO2 layer. The effect of HF etchings on the SiO2 layers grown by both techniques and on both the C-face and Si-face substrates are legitimately studied. The thicknesses of the layers determined by AFM and ellipsometry techniques are widely promulgated. The MOS capacitors are made on the Si-face 4H-SiC wafers by wet oxidation and sputtering processes, which are studied by capacitance versus voltage (CV) technique. From CV measurements, the density of trap states with variation of trap level for MOS devices is estimated.

Interfacial Thickness Guidelines for SiC(Fiber)/SiC(Matrix) Composites

NASA Technical Reports Server (NTRS)

Hurst, Janet B.

1998-01-01

Researchers at the NASA Lewis Research Center have developed a guideline for the interface thickness necessary for SiC(Fiber)/SiC(Matrix) composites to demonstrate good composite properties. These composite materials have potential commercial applications for high-temperature structural components such as engine hot sections. Several samples of each were composed from three different small-diameter (less than 20 mm), polymer-derived SiC fibers that were woven into two-dimensional cloths and laid up as preforms. The preforms were treated with a chemical-vapor-infiltrated boron nitride layer as an interfacial coating on the fiber surfaces to provide the necessary debonding characteristics for successful composite behavior. Then, the preforms were filled with additional SiC as a matrix phase.

Self-assembled patches in PtSi/n-Si (111) diodes

NASA Astrophysics Data System (ADS)

Afandiyeva, I. M.; Altιndal, Ş.; Abdullayeva, L. K.; Bayramova, A. İ.

2018-05-01

Using the effect of the temperature on the capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics of PtSi/n-Si (111) Schottky diodes the profile of apparent doping concentration (N Dapp), the potential difference between the Fermi energy level and the bottom of the conduction band (V n), apparent barrier height (Φ Bapp), series resistance (R s) and the interface state density N ss have been investigated. From the temperature dependence of (C–V) it was found that these parameters are non-uniformly changed with increasing temperature in a wide temperature range of 79–360 K. The voltage and temperature dependences of apparent carrier distribution we attributed to the existence of self-assembled patches similar the quantum wells, which formed due to the process of PtSi formation on semiconductor and the presence of hexagonal voids of Si (111).

Photo-induced intersubband absorption in {Si}/{SiGe} quantum wells

NASA Astrophysics Data System (ADS)

Boucaud, P.; Gao, L.; Visocekas, F.; Moussa, Z.; Lourtioz, J.-M.; Julien, F. H.; Sagnes, I.; Campidelli, Y.; Badoz, P.-A.; Vagos, P.

1995-12-01

We have investigated photo-induced intersubband absorption in the valence band of {Si}/{SiGe} quantum wells. Carriers are optically generated in the quantum wells using an argon ion laser. The resulting infrared absorption is probed with a step-scan Fourier transform infrared spectrometer. The photo-induced infrared absorption in SiGe quantum wells is dominated by two contributions: the free carrier absorption, which is similar to bulk absorption in a uniformly doped SiGe layer, and the valence subband absorption in the quantum wells. Both p- and s-polarized intersubband absorptions are measured. We have observed that the photo-induced intersubband absorption in doped samples is shifted to lower energy as compared to direct intersubband absorption. This absorption process is attributed to carriers away from the Brillouin zone center. We show that the photo-induced technique is appropriate to study valence band mixing effects and their influence on intersubband absorption.

Efficient siRNA Delivery Using Novel Cell-Penetrating Peptide-siRNA Conjugate-Loaded Nanobubbles and Ultrasound.

PubMed

Xie, Xiangyang; Lin, Wen; Li, Mingyuan; Yang, Yang; Deng, Jianping; Liu, Hui; Chen, Ying; Fu, Xudong; Liu, Hong; Yang, Yanfang

2016-06-01

Because of the absence of tolerable and effective carriers for in vivo delivery, the applications of small interfering RNA (siRNA) in the clinic for therapeutic purposes have been limited. In this study, development of a novel siRNA delivery system based on ultrasound-sensitive nanobubbles (NBs, nano-sized echogenic liposomes) and cell-permeable peptides (CPPs) is described. A CPP-siRNA conjugate was entrapped in an NB, (CPP-siRNA)-NB, and the penetration of CPP-siRNA was temporally masked; local ultrasound stimulation triggered the release of CPP-siRNA from the NBs and activated its penetration. Subsequent research revealed that the (CPP-siRNA)-NBs had a mean particle size of 201 ± 2.05 nm and a siRNA entrapment efficiency >85%. In vitro release results indicated that >90% of the encapsulated CPP-siRNA was released from NBs in the presence of ultrasound, whereas <1.5% (30 min) was released in the absence of ultrasound. Cell experiments indicated higher cellular CPP-siRNA uptake of (CPP-siRNA)-NBs with ultrasound among the various formulations in human breast adenocarcinoma cells (HT-1080). Additionally, after systemic administration in mice, (CPP-siRNA)-NBs accumulated in the tumor, augmented c-myc silencing and delayed tumor progression. In conclusion, the application of (CPP-siRNA)-NBs with ultrasound may constitute an approach to selective targeted delivery of siRNA. Copyright © 2016 World Federation for Ultrasound in Medicine & Biology. Published by Elsevier Inc. All rights reserved.

Precautions toward XTEM of Si3N4/SiO2

NASA Technical Reports Server (NTRS)