Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Structure modelling”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Relation between the Electronic Properties and Structure of InAs/GaAs Quantum Dots Grown by Vapor-Phase Epitaxy

On the basis of a comprehensive study of the structural and electronic properties of InAs/GaAs quantum dots grown by vapor-phase epitaxy at atmospheric pressure, a structural model of the quantum dots as three truncated pyramids adjoined at their bases is chosen. The model takes into account the diffusive smearing of the composition near the base and lateral surface as well as the segregation of indium near the vertex. It is established that the wave functions of charge carriers in the ground state are localized in a comparatively small region of the quantum dot near its vertex.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Crystal chemistry of the Pmnb polymorph of Li{sub 2}MnSiO{sub 4}

The crystal structure of the Pmnb polymorph of Li{sub 2}MnSiO{sub 4} (prepared by solid-state synthesis in argon at 900 Degree-Sign C) is characterized by Rietveld refinement of structural models using high resolution synchrotron X-ray and neutron powder diffraction data. The crystal structure is confirmed to be isostructural with Li{sub 2}CdSiO{sub 4} with lattice parameters a=6.30694(3), b=10.75355(4), and c=5.00863(2) A, which are in good agreement with previously published data. No evidence was found for mixed lithium/manganese sites. Testing of the material as a cathode in a lithium cell shows that 1.3 lithium ions per formula unit can be extracted on the first charge cycle but very little lithium can be re-inserted. These results are compared with those of other phase-pure Li{sub 2}MnSiO{sub 4} polymorphs. - Graphical abstract: Polyhedral representation of the crystal structure of Li{sub 2}MnSiO{sub 4} in the Pmnb space group. LiO{sub 4}, MnO{sub 4} and SiO{sub 4} tetrahedra are shown in green, purple and blue, respectively. Highlights: Black-Right-Pointing-Pointer A phase-pure sample of the Pmnb polymorph of Li{sub 2}MnSiO{sub 4} was synthesized. Black-Right-Pointing-Pointer Characterized by Rietveld refinement using high resolution synchrotron X-ray and neutron powder diffraction data. Black-Right-Pointing-Pointer Results confirm the proposed Li{sub 2}CdSiO{sub 4}-type model with highly distorted Mn tetrahedra.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Molecular-scale structure of uranium(VI) immobilized with goethite and phosphate

The molecular-scale immobilization mechanisms of uranium uptake in the presence of phosphate and goethite were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Wet chemistry data from U(VI)-equilibrated goethite suspensions at pH 4-7 in the presence of {approx}100 {micro}M total phosphate indicated changes in U(VI) uptake mechanisms from adsorption to precipitation with increasing total uranium concentrations and with increasing pH. EXAFS analysis revealed that the precipitated U(VI) had a structure consistent with the meta-autunite group of solids. The adsorbed U(VI), in the absence of phosphate at pH 4-7, formed bidentate edge-sharing, {triple_bond}Fe(OH){sub 2}UO{sub 2}, and bidentate corner-sharing, ({triple_bond}FeOH){sub 2}UO{sub 2}, surface complexes with respective U-Fe coordination distances of {approx}3.45 and {approx}4.3 {angstrom}. In the presence of phosphate and goethite, the relative amounts of precipitated and adsorbed U(VI) were quantified using linear combinations of the EXAFS spectra of precipitated U(VI) and phosphate-free adsorbed U(VI). A U(VI)-phosphate-Fe(III) oxide ternary surface complex is suggested as the dominant species at pH 4 and total U(VI) of 10 {micro}M or less on the basis of the linear combination fitting, a P shell indicated by EXAFS, and the simultaneous enhancement of U(VI) and phosphate uptake on goethite. A structural model for the ternary surface complex was proposed that included a single phosphate shell at 3.6 {angstrom} (U-P) and a single iron shell at {approx}4.3 {angstrom} (U-Fe). While the data can be explained by a U-bridging ternary surface complex, ({triple_bond}FeO){sub 2}UO{sub 2}PO{sub 4}, it is not possible to statistically distinguish this scenario from one with P-bridging complexes also present.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2011

Synthesis, Structural and Electrical properties of Bi{sub 4}Ti{sub 3}O{sub 12} and Bi{sub 3.5}La{sub 0.5}Ti{sub 3}O{sub 12} Ferroelectric ceramics

Polycrystalline ceramic samples of Bi{sub 4}Ti{sub 3}O{sub 12} and the La-doped Bi{sub 3.5}La{sub 0.5}Ti{sub 3}O{sub 12} have been synthesized by standard high temperature solid state reaction method using high purity oxides and carbonates. The effect of lanthanum doping on the structure of Bi{sub 4}Ti{sub 3}O{sub 12} powders was investigated by X-ray diffraction. A better agreement between the observed and calculated X-Ray diffraction pattern was obtained by performing the Rietveld refinement with a structural model using the non centrosymmetric space group Fmmm. Rietveld analysis revealed that with the partial substitution of La on the Bi site increases the a and b lattice parameters and decreases the c parameter. The activation energies calculated from dc conductivities are 1.033 eV and 2.244 eV which shows that La doping increases the resistivity of the material useful for dielectric devices.

36 MATERIALS SCIENCE↗
DOE OSTI2012

First-In-Class Small Molecule Inhibitors of the Single-Strand DNA Cytosine Deaminase APOBEC3G

APOBEC3G is a single-stranded DNA cytosine deaminase that comprises part of the innate immune response to viruses and transposons. Although APOBEC3G is the prototype for understanding the larger mammalian polynucleotide deaminase family, no specific chemical inhibitors exist to modulate its activity. High-throughput screening identified 34 compounds that inhibit APOBEC3G catalytic activity. Twenty of 34 small molecules contained catechol moieties, which are known to be sulfhydryl reactive following oxidation to the orthoquinone. Located proximal to the active site, C321 was identified as the binding site for the inhibitors by a combination of mutational screening, structural analysis, and mass spectrometry. Bulkier substitutions C321-to-L, F, Y, or W mimicked chemical inhibition. A strong specificity for APOBEC3G was evident, as most compounds failed to inhibit the related APOBEC3A enzyme or the unrelated enzymes E. coli uracil DNA glycosylase, HIV-1 RNase H, or HIV-1 integrase. Partial, but not complete, sensitivity could be conferred to APOBEC3A by introducing the entire C321 loop from APOBEC3G. Thus, a structural model is presented in which the mechanism of inhibition is both specific and competitive, by binding a pocket adjacent to the APOBEC3G active site, reacting with C321, and blocking access to substrate DNA cytosines.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2013

General Schema for [001] Tilt Grain Boundaries in Dense Packing Cubic Crystals

Atomic resolution Z-contrast images from a series of CeO2 [001] tilt grain boundaries at coincident site lattice (CSL) or near-CSL misorientations can all be explained within a structural unit model. These structural units (which cover all boundaries from 0-90 degrees) show striking similarities to comparable CSL boundaries observed in cubic crystal structures that are also derived from dense packing (fcc metal, rocksalt, perovskite etc). A general model for the structure of grain boundaries in such similarly structured materials systems has been developed that is based on the crystallography of the parent structures. Changes away from these predicted grain boundary symmetries can be interpreted as showing the frustration of symmetry caused by the incorporation of point defects (vacancies and impurities). This general model for grain boundary structures can, in principle, provide a means to infer the structure-property relationships in broad classes of materials.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2012

Structural trends for celestite (SrSO[subscript 4]), anglesite (PbSO[subscript 4]), and barite (BaSO[subscript 4]): Confirmation of expected variations within the SO[subscript 4] groups

The crystal structures of the isostructural orthorhombic sulfates celestite (SrSO{sub 4}), anglesite (PbSO{sub 4}), and barite (BaSO{sub 4}) were refined by Rietveld methods using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Their structural model was refined in space group Pbnm. The unit-cell parameters are a = 6.87032(3), b = 8.36030(5), c = 5.34732(1) {angstrom}, and V = 307.139(3) {angstrom}{sup 3} for SrSO{sub 4}; a = 6.95802(1), b = 8.48024(3), c = 5.39754(1) {angstrom}, and V = 318.486(1) {angstrom}{sup 3} for PbSO{sub 4}; and a = 7.15505(1), b = 8.88101(3), c = 5.45447(1) {angstrom}, and V = 346.599(1) {angstrom}{sup 3} for BaSO{sub 4}. The average <M-O> [12] distances are 2.827(1), 2.865(1), and 2.953(1) {angstrom} for SrSO{sub 4}, PbSO{sub 4}, and BaSO{sub 4}, respectively, and their corresponding average <S-O> [4] distances are 1.480(1), 1.477(3), and 1.471(1) {angstrom}. The geometrical features of the SO{sub 4} and MO{sub 12} polyhedra become more symmetrical from SrSO{sub 4} to BaSO{sub 4}. Across the series, the a, b, and c parameters vary non-linearly with increasing V. The radii of the M{sup 2+} cations, rM, <M-O> [12], and <S-O> [4] distances vary linearly with V. These structural trends arise from the effective size of the M{sup 2+} cation (rM: Sr < Pb < Ba) that is coordinated to 12 O atoms.

58 GEOSCIENCES↗
DOE OSTI2014

Structural stability and electronic properties of low-index surfaces of SnS

Thin film photovoltaic cells are increasingly important for cost-effective solar energy harvesting. Layered SnS is a promising absorber material due to its high optical absorption in the visible and good doping characteristics. We use first-principles calculations based on density functional theory to study structures of low-index surfaces of SnS using stoichiometric and oxygen-containing structural models, in order to elucidate their possible effect on the efficiency of the photovoltaic device. We find that the surface energy is minimized for the surface with orientation parallel to the layer stacking direction. Compared to stoichiometric surfaces, the oxygen-containing surfaces exhibit fewer electronic states near the band gap. This reduction of near-gap surface states by oxygen should reduce recombination losses at grain boundaries and interfaces of the SnS absorber, and should be beneficial to the efficiency of the solar cell.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Group III Acceptors with Shallow and Deep Levels in Silicon Carbide: ESR and ENDOR Studies

Results of investigations of Group III acceptors (B, Al, and Ga) in crystals of silicon carbide using the most informative electron spin resonance and electron nuclear double resonance methods are presented. Structural models of the acceptors with shallow and deep levels are considered. In addition to the data obtained earlier, studies using high-frequency magnetic resonance were obtained, which allowed revealing orthorhombic deviations from the axial symmetry for the deep acceptors; theoretical analysis explains experimentally found shifts of g factors for the deep acceptors arising due to the orthorhombic deviations, which appear probably due to the Jahn–Teller effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Characteristics of Amorphous As{sub 2}S{sub 3} Semiconductor Films Obtained via Spin Coating

Centrifugation is used in fabricating, e.g., films with large areas and/or thicknesses of several micrometers. However, it has yet to be widely employed for chalcogenide compounds, due to their relatively weak solubility in most solvents. Determining the optimum conditions for preparing solutions of chalcogenide compounds and obtaining films via centrifugation is therefore of great interest. Specific features of amorphous arsenic sulfide (As{sub 2}S{sub 3}) films prepared via the centrifugation of solutions in n-butylamine have been studied. These films were characterized by means of X-ray diffraction analysis, IR spectroscopy, atomic-force microscopy and Raman spectroscopy. It was shown that amorphous As{sub 2}S{sub 3} films have a greater elasticity modulus than those of analogous composition produced via thermal evaporation in vacuum, or As{sub 2}S{sub 3} glass. A structural model based on arsenic sulfide clusters whose surfaces are bound by negatively and positively charged ions is used to explain the experimental results obtained in this work. DC measurements show that the amorphous films exhibit semiconductor-type conductivity. Their room temperature conductivity is ~10{sup −15} S/cm, which indicates good dielectric properties. The films are optically transparent starting from the yellow spectral range, making them promising functional materials for engineering applications in optics and photonics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Distribution of Vacancies in a Hybrid M{sub (5–11/18)}X{sub (5–11/18)} Superstructure of a High-Temperature Ordered β-TiO Phase

A new structural model of the high-temperature ordered β-TiO phase of titanium monoxide with vacancies in metallic and nonmetallic sublattices has been proposed. The model suggests that the vacancies are simultaneously distributed over sites of two partially disordered superstructures: monoclinic M{sub 5}X{sub 5(mon)} (space group C2/m (A2/m)) and cubic M{sub 5}X{sub 5(cub)} (space group Pm3̄m). The hybrid superstructure belongs to space group P1m1 of the monoclinic system. The proposed notation M{sub (5–11/18)}X{sub (5–11/18)} of the hybrid superstructure takes into account an expansion of the vacancy sublattice due to the superposition of initial M{sub 5}X{sub 5{sup −}} type superstructures. It is shown that the arrangements of the superstructure reflections in the diffraction spectra of the M{sub (5–11/18)}X{sub (5–11/18)} and M{sub 5}X{sub 5(cub)} superstructures are identical.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Surface defects characterization in a quantum wire by coherent phonons scattering

The influence of surface defects on the scattering properties of elastic waves in a quasi-planar crystallographic waveguide is studied in the harmonic approximation using the matching method formalism. The structural model is based on three infinite atomic chains forming a perfect lattice surmounted by an atomic surface defect. Following the Landauer approach, we solve directly the Newton dynamical equation with scattering boundary conditions and taking into account the next nearest neighbour’s interaction. A detailed study of the defect-induced fluctuations in the transmission spectra is presented for different adatom masses. As in the electronic case, the presence of localized defect-induced states leads to Fano-like resonances. In the language of mechanical vibrations, these are called continuum resonances. Numerical results reveal the intimate relation between transmission spectra and localized defect states and provide a basis for the understanding of conductance spectroscopy experiments in disordered mesoscopic systems. The results could be useful for the design of phononic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Direct imaging of InSb (110)-(1x1) surface grown by molecular beam epitaxy

High-resolution transmission electron microscopy under a profile imaging condition (HR-profile TEM) was employed to determine the structural model for the InSb(110)-(1x1) relaxation surface grown by molecular beam epitaxy (MBE). HR-profile TEM analyses indicate that the chevron model, which is widely accepted for zinc-blende-type III-V(110)-(1x1) surfaces prepared by cleavage, is also applicable to the InSb(110)-(1x1) surface prepared under an Sb-rich MBE condition. The assignment of atomic species (In or Sb) of InSb(110)-(1x1) surfaces was confirmed based on a HR-profile TEM image that captures the connected facets of InSb(110)-(1x1) and InSb(111)B-(2x2). On the basis of the well-known atomic species of InSb(111)B-(2x2), the atomic species of the InSb(110)-(1x1) surface were deduced straightforwardly: the atoms shifted upward and downward at the topmost layer of the InSb(110)-(1x1) surface are Sb and In, respectively. The atomic arrangements of the InSb(110)-(1x1)-InSb(111)B-(2x2) facet determined by HR-profile TEM may represent the atomic arrangements of zinc-blende-type III-V(331)B surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2012

China's Building Energy Use: A Long-Term Perspective based on a Detailed Assessment

We present here a detailed, service-based model of China's building energy use, nested in the GCAM (Global Change Assessment Model) integrated assessment framework. Using the model, we explore long-term pathways of China's building energy use and identify opportunities of reducing greenhouse gas emissions. The inclusion of a structural model of building energy demands within an integrated assessment framework represents a major methodological advance. It allows for a structural understanding of the drivers of building energy consumption while simultaneously considering the other human and natural system interactions that influence changes in the global energy system and climate. We also explore a range of different scenarios to gain insights into how China's building sector might evolve and what the implications might be for improved building energy technology and carbon policies. The analysis suggests that China's building energy growth will not wane anytime soon, although technology improvement will put downward pressure on this growth. Also, regardless of the scenarios represented, the growth will involve the continued, rapid electrification of the buildings sector throughout the century, and this transition will be accelerated by the implementation of carbon policy.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2013

On atomic structure of Ge huts growing on the Ge/Si(001) wetting layer

Structural models of growing Ge hut clusters—pyramids and wedges—are proposed on the basis of data of recent STM investigations of nucleation and growth of Ge huts on the Si(001) surface in the process of molecular beam epitaxy. It is shown that extension of a hut base along <110> directions goes non-uniformly during the cluster growth regardless of its shape. Growing pyramids, starting from the second monolayer, pass through cyclic formation of slightly asymmetrical and symmetrical clusters, with symmetrical ones appearing after addition of every fourth monolayer. We suppose that pyramids of symmetrical configurations composed by 2, 6, 10, etc., monolayers over the wetting layer are more stable than asymmetrical ones. This might explain less stability of pyramids in comparison with wedges in dense arrays forming at low temperatures of Ge deposition. Possible nucleation processes of pyramids and wedges on wetting layer patches from identical embryos composed by 8 dimers through formation of 1 monolayer high 16-dimer nuclei different only in their symmetry is discussed. Schematics of these processes are presented. It is concluded from precise STM measurements that top layers of wetting layer patches are relaxed when huts nucleate on them.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Crystal structure and shape selection in the growth of 3D metallic crystallites on layered materials: Fe on MoS 2

Nucleation and growth of supported 3D metal clusters or crystallites during deposition on MoS 2 , or on other weakly-adhering layered materials, can potentially produce diverse growth shapes, and even crystal structures differing from the bulk metal. For Fe deposition on MoS 2 , SEM and AFM observations reveal three distinct crystallite shapes. By comparison with atomistic structure models incorporating realistic Fe-MoS 2 interface structures, here we conclude that these are: triangular fcc(111) pyramids with sloped {100} side facets; bcc(110) A-frame tents with sloped {100} side facets; and bcc(110) mesas with vertical {100} and {110} side facets. The following picture is proposed for the competitive formation of clusters and crystallites with different structures: (i) small nanoclusters formed at the onset of deposition exhibit facile fluxional dynamics allowing sampling of different crystal structures and shapes; (ii) sufficient fluxionality implies a Boltzmann distribution of sampled structures, and thus coexistence of different structures follows from the demonstrated similar energies for those structures; (iii) growing clusters reach a threshold size above which the characteristic time scale for restructuring exceeds that for cluster growth. Thereafter, clusters are locked-in to a specific crystal structure and shape as revealed by imaging of larger crystallites. Despite a penalty for fcc(111) over bcc(111) pyramids based on bulk energetics, favorable surface and interface energies makes them preferable for smaller sizes.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Characterization of a novel domain ‘GATE’ in the ABC protein DrrA and its role in drug efflux by the DrrAB complex

A novel domain, GATE (Glycine-loop And Transducer Element), is identified in the ABC protein DrrA. This domain shows sequence and structural conservation among close homologs of DrrA as well as distantly-related ABC proteins. Among the highly conserved residues in this domain are three glycines, G215, G221 and G231, of which G215 was found to be critical for stable expression of the DrrAB complex. Other conserved residues, including E201, G221, K227 and G231, were found to be critical for the catalytic and transport functions of the DrrAB transporter. Structural analysis of both the previously published crystal structure of the DrrA homolog MalK and the modeled structure of DrrA showed that G215 makes close contacts with residues in and around the Walker A motif, suggesting that these interactions may be critical for maintaining the integrity of the ATP binding pocket as well as the complex. It is also shown that G215A or K227R mutation diminishes some of the atomic interactions essential for ATP catalysis and overall transport function. Therefore, based on both the biochemical and structural analyses, it is proposed that the GATE domain, located outside of the previously identified ATP binding and hydrolysis motifs, is an additional element involved in ATP catalysis. - Highlights: • A novel domain ‘GATE’ is identified in the ABC protein DrrA. • GATE shows high sequence and structural conservation among diverse ABC proteins. • GATE is located outside of the previously studied ATP binding and hydrolysis motifs. • Conserved GATE residues are critical for stability of DrrAB and for ATP catalysis.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2022

Woven ceramic matrix composite surrogate model based on physics-informed recurrent neural network

A recurrent neural network (RNN) based surrogate model is developed to emulate the nonlinear constitutive behavior of woven ceramic matrix composites (CMCs) driven by matrix damage at multiple length scales. Physics-informed constraints are introduced into the surrogate model through regularization to ground the prediction in physics and improve its predictive capabilities. Training data is generated using the multiscale generalized method of cells (MSGMC) approach coupled with a matrix damage model. This coupling permits simulating the nonlinear behavior of woven CMCs based on constituent response at the micro-, meso-, and macroscales. The multiscale repeating unit cell is loaded under non-monotonic conditions including multiple load / unload cycles and tension / compression. The fiber volume fraction as well as the intra- and intertow void volume fractions are also varied in the generation of training data. Therefore, the RNN-based surrogate model is tasked with predicting, as a function of variable input strain sequence and fiber and void volume fractions, the resulting stress versus strain response while satisfying physical constraints such as positive semi-definiteness of the tangent stiffness matrix and linear elastic unloading. Further, the trained surrogate model effectively matches the stress versus strain response and successfully predicts the tangent modulus throughout the loading regime. Neural network based surrogate models can offer efficient alternatives to running computationally intensive multiscale material models to simulate the nonlinear response of large structural models. Therefore the presented work provides evidence towards the feasibility of developing, training, and running such models for CMCs with complex architectures, nonlinear multiaxial material response, and under non-monotonic loading conditions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗