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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.

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At least 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Interaction of gas molecules with crystalline polymer separation membranes: Atomic-scale modeling and first-principles calculations

Carbon dioxide (CO2)-induced plasticization can significantly decrease the gas separation performance of membranes in high-temperature or high pressure conditions, such as industrial methane (CH4) separations. In this paper, we investigated the crystalline phase of three polymers (polybenzimidazole (PBI), Bis(isobutylcarboxy)polybenzimidazole (PBI-Butyl), and KaptonTM) and interactions between gas molecules (CO2 and N2) and these polymers. A novel, molecular dynamics (MD) based, computational technique was employed to find unknown crystalline structures of these polymer materials. The interaction of CO2 and N2 gases with these crystals was studied by first-principles calculations and by classical MD simulations. The results showed that the packing structure and the interlayer coupling in polymer crystals determine the permeability and diffusivity of gas molecules. This methodology also allows prediction of plastic swelling in these materials caused by gas molecules absorbed in the polymer matrix.

03 NATURAL GAS↗
DOE OSTI2017

First-principles prediction of high-entropy-alloy stability

High entropy alloys (HEAs) are multicomponent compounds whose high configurational entropy allows them to solidify into a single phase, with a simple crystal lattice structure. Some HEAs exhibit desirable properties, such as high specific strength, ductility, and corrosion resistance, while challenging the scientist to make confident predictions in the face of multiple competing phases. We demonstrate phase stability in the multicomponent alloy system of Cr–Mo–Nb–V, for which some of its binary subsystems are subject to phase separation and complex intermetallic-phase formation. Our first-principles calculation of free energy predicts that the configurational entropy stabilizes a single body-centered cubic (BCC) phase from T = 1700 K up to melting, while precipitation of a complex intermetallic is favored at lower temperatures. We form the compound experimentally and confirm that it develops as a single BCC phase from the melt, but that it transforms reversibly at lower temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Thermodynamic and mechanical stabilities of α- and β-Ta{sub 4}AlC{sub 3} via first-principles investigations

Recently, it has been predicted that Ta{sub 4}AlC{sub 3} is likely to exhibit an (α–β) polymorphic transformation at temperatures above 1873 K. However, recent X-ray diffraction and transmission electron microscopy experiments suggest on the other hand that the α phase remains stable up to temperatures close to the limit of experimental capabilities and no transition has yet been observed. While the matter has already been settled experimentally, in this work, we re-investigate the phase stability problem in Ta{sub 4}AlC{sub 3} by using first-principles methods. The study was carried out by considering both thermodynamic and mechanical stabilities of the Ta{sub 4}AlC{sub 3} polymorphs. Particularly, finite-temperature Gibbs free energies and elastic properties of the polymorphs were calculated using density functional theory. Calculation results reveal that the α phase continue to be stable even at temperatures exceeding 1875 K, which is in agreement with experimental results reported in literature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Electronic Properties of Ultra‐Wide Bandgap B x Al 1− x N Computed from First‐Principles Simulations

Abstract Ultra‐wide bandgap (UWBG) materials such as AlN and BN hold great promise for future power electronics due to their exceptional properties. They exhibit large bandgaps, high breakdown fields, high thermal conductivity, and high mechanical strengths. AlN and BN have been extensively researched, however, their alloys, B x Al 1− x N, are much less studied despite their ability to offer tunable properties by adjusting x . In this article, the electronic properties of 17 recently predicted ground states of B x Al 1− x N in the x = 0 − 1 range are predicted using first‐principles density functional theory and many‐body perturbation theory within GW approximation. All the B x Al 1− x N structures are found to be UWBG materials and have bandgaps that vary linearly from that of wurtzite‐phase ( w ) AlN (6.19 eV) to that of w ‐BN (7.47 eV). The bandstructures of B x Al 1− x N show that a direct‐to‐indirect bandgap crossover occurs near x = 0.25. Furthermore, it is found that B x Al 1− x N alloys have much larger dielectric constants than the constituent bulk materials (AlN = 9.3 ɛ 0 or BN = 7.3 ɛ 0 ), with values reaching as high as 12.1 ɛ 0 . These alloys are found to exhibit large dielectric breakdown fields in the range 9–35 MV cm −1 with a linear dependence on x . This work provides the much needed advancement in the understanding of the properties of B x Al 1− x N to aid their application in next‐generation devices.

Milne, Cody L.↗
DOE OSTI2017

First-principles identification of novel double perovskites for water-splitting applications

Identification of new materials for photo-electrochemical conversion of water into hydrogen and oxygen using visible solar light is one of the grand challenges of our times. Toward this goal, here we employ a hierarchy of down-selection steps based on structural constraints, thermodynamic stability, constraints on bandgap and band-edge positions to identify potential candidates residing in a target double perovskite chemical space. The adopted screening strategy results in four new promising candidate materials, which were studied in greater detail using first-principles computations for their thermodynamic stability, electronic structure and octahedral structural distortions. In conclusion, our theoretical investigation is expected to serve as a motivation for future experimental efforts targeted toward realizing these identified promising materials.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electronic, structural and magnetic studies of niobium borides of group 8 transition metals, Nb{sub 2}MB{sub 2} (M=Fe, Ru, Os) from first principles calculations

The Nb{sub 2}FeB{sub 2} phase (U{sub 3}Si{sub 2}-type, space group P4/mbm, no. 127) is known for almost 50 years, but until now its magnetic properties have not been investigated. While the synthesis of Nb{sub 2}OsB{sub 2} (space group P4/mnc, no. 128, a twofold superstructure of U{sub 3}Si{sub 2}-type) with distorted Nb-layers and Os{sub 2}-dumbbells was recently achieved, “Nb{sub 2}RuB{sub 2}” is still not synthesized and its crystal structure is yet to be revealed. Our first principles density functional theory (DFT) calculations have confirmed not only the experimental structures of Nb{sub 2}FeB{sub 2} and Nb{sub 2}OsB{sub 2}, but also predict “Nb{sub 2}RuB{sub 2}” to crystalize with the Nb{sub 2}OsB{sub 2} structure type. According to chemical bonding analysis, the homoatomic B–B interactions are optimized and very strong, but relatively strong heteroatomic M–B, B–Nb and M–Nb bonds (M=Fe, Ru, Os) are also found. These interactions, which together build a three-dimensional network, are mainly responsible for the structural stability of these ternary borides. The density-of-states at the Fermi level predicts metallic behavior, as expected, from metal-rich borides. Analysis of possible magnetic structures concluded preferred antiferromagnetic ordering for Nb{sub 2}FeB{sub 2}, originating from ferromagnetic interactions within iron chains and antiferromagnetic exchange interactions between them. -- Graphical abstract: Nb{sub 2}FeB{sub 2} (U{sub 3}Si{sub 2} structure type, space group P4/mbm, no. 127) is predicted to order antiferromagnetically, due to the presence of iron chains which show ferromagnetic interactions in the chains and antiferromagnetic interactions between them. “Nb{sub 2}RuB{sub 2}” is predicted to crystallize with the recently discovered Nb{sub 2}OsB{sub 2} twofold superstructure (space group P4/mnc, no. 128) of U{sub 3}Si{sub 2} structure type. The building of ruthenium dumbbells instead of chains along [001] is found to be responsible for the stabilization of this superstructure. Highlights: • Nb{sub 2}FeB{sub 2} is predicted to order antiferromagnetically. • Ferromagnetic interactions found in iron chains and antiferromagnetic ones between them. • Unknown “Nb{sub 2}RuB{sub 2}” predicted to crystallize with a twofold U{sub 3}Si{sub 2} superstructure. • Puckering of Nb-layer and Ru-dumbbell formation responsible for superstructure occurrence.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

First-principles elucidation of the effects of Al-doping on Li-ion diffusion in LiCoO 2

Al-doped garnet Li 7 La 3 Zr 2 O 12 solid-electrolyte and LiCoO 2 cathode are promising choices as catholyte materials in all solid-state Li batteries, however, interdiffusion of Al is commonly evident during high-temperature processing and electrochemical cycling. Furthermore, to address the impact of Al interdiffusion on Li + transport properties in LiCoO 2 , we carried out a systematic evaluation of Al doping on Li + diffusion barriers in LiCoO 2 using first-principles based methods. Following the monovacancy diffusion mechanism, Al-doping (primarily at the Co site) is found to improve Li diffusion kinetics in the LiCoO 2 lattice due to favorable CoO 6 octahedral distortion experienced at the transition states. However, when considering the previously established dominant divacancy diffusion mechanism, slower Li diffusion is generally expected. In addition, a broad variation of Li diffusion barriers is observed upon Al doping, which suggests the system may suffer from non-uniform Li incorporation and diffusion that adversely affects its rate capacity during cycling. In summary, this work highlights, for the rational design of catholyte of all solid-state batteries, special attention may need to be paid to address the potential impact of non-intentional doping induced during processing on the overall electrochemical performance of the catholyte.

Al-doped LiCoO2↗
DOE OSTI2025

First-Principles Simulations Correlating X-ray Absorption Spectroscopy Features to Point Defects in h -BN

Hexagonal boron nitride (h-BN) is a promising material for a range of emerging applications in electronics, quantum information technology, and energy storage. Soft X-ray absorption spectroscopy (XAS) is powerful to reveal atomic details of BN, especially in the presence of defects. However, correlating XAS spectral features with specific defect types remains elusive. In this Letter, we report B K-edge XAS measurements of sputter-deposited turbostratic h-BN films and use a combination of first-principles spectroscopic simulations and analysis of detailed electronic structure and local charge transfer characteristics to elucidate their unique spectroscopic features. Our results show that the two main defect-related peaks, between the main π* resonances of h-BN and B2O3, as typically observed in BN films deposited by energetic condensation or bombarded with energetic ions, are associated with electronic states of H-passivated B atoms bonded to one and two oxygen impurity atoms, respectively. These conclusions hold significant implications for applications relying on defect-mediated properties of h-BN.

chemical structure↗
NASA NTRS2022

First-Principles Theory of the Rate of Magnetic Reconnection in Magnetospheric and Solar Plasmas

The rate of magnetic reconnection is of the utmost importance in a variety of processes because it controls, for example, the rate energy is released in solar flares, the speed of the Dungey convection cycle in Earth’s magnetosphere, and the energy release rate in harmful geomagnetic substorms. It is known from numerical simulations and satellite observations that the rate is approximately 0.1 in normalized units, but despite years of effort, a full theoretical prediction has not been obtained. Here, we present a first-principles theory for the reconnection rate in non-relativistic electron-ion collisionless plasmas, and show that the same prediction explains why Sweet-Parker reconnection is considerably slower. The key consideration of this analysis is the pressure at the reconnection site (i.e., the x-line). We show that the Hall electromagnetic fields in antiparallel reconnection cause an energy void, equivalently a pressure depletion, at the x-line, so the reconnection exhaust opens out, enabling the fast rate of 0.1. If the energy can reach the x-line to replenish the pressure, the exhaust does not open out. In addition to heliospheric applications, these results are expected to impact reconnection studies in planetary magnetospheres, magnetically confined fusion devices, and astrophysical plasmas.

Yi-Hsin Liu↗
DOE OSTI2018

Structure and Interaction of Ionic Liquid Monolayer on Graphite from First-Principles

The ionic-liquid/graphite interface is important in understanding the behaviors of the ionic-liquid electrolyte in carbon-based supercapacitors and Li-ion batteries. A row-like cation–anion structure of the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMP][TFSA]) ionic liquid (a popular choice for interfacial studies) on the graphite surface has been suggested on the basis of scanning tunneling microscopy images, but it is unclear if this structure is the most stable one. Herein, we explore alternative models of the [BMP][TFSA] monolayer on graphite basal plane with first-principles density functional theory. We find that the checkerboard type of structure is in fact more stable than the row-like structure. We further use simulated annealing based on classical molecular dynamics simulations to obtain more stable conformations of ions on the surface. Both charge-density-difference and noncovalent-interaction analyses show that the higher stability of the checkerboard structure than the row-like structure is due to the stronger anion–surface interactions in the former. Furthermore, our work therefore suggests a new structure of the [BMP][TFSA] monolayer on the graphite basal plane.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

First-principles theory of the rate of magnetic reconnection in magnetospheric and solar plasmas

Abstract The rate of magnetic reconnection is of the utmost importance in a variety of processes because it controls, for example, the rate energy is released in solar flares, the speed of the Dungey convection cycle in Earth’s magnetosphere, and the energy release rate in harmful geomagnetic substorms. It is known from numerical simulations and satellite observations that the rate is approximately 0.1 in normalized units, but despite years of effort, a full theoretical prediction has not been obtained. Here, we present a first-principles theory for the reconnection rate in non-relativistic electron-ion collisionless plasmas, and show that the same prediction explains why Sweet-Parker reconnection is considerably slower. The key consideration of this analysis is the pressure at the reconnection site (i.e., the x-line). We show that the Hall electromagnetic fields in antiparallel reconnection cause an energy void, equivalently a pressure depletion, at the x-line, so the reconnection exhaust opens out, enabling the fast rate of 0.1. If the energy can reach the x-line to replenish the pressure, the exhaust does not open out. In addition to heliospheric applications, these results are expected to impact reconnection studies in planetary magnetospheres, magnetically confined fusion devices, and astrophysical plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Thermal fluctuations and carrier localization induced by dynamic disorder in MAPbI 3 described by first-principles based tight-binding model

Halide perovskites are strongly influenced by large amplitude anharmonic lattice fluctuations at room temperature. We develop a tight binding model for dynamically disordered MAPbI$_3$ based on density functional theory (DFT) calculations to calculate electronic structure for finite temperature crystal structures at the length scale of thermal disorder and carrier localization. The model predicts individual Hamiltonian matrix elements and band structures with high accuracy, owing to the inclusion of additional matrix elements and descriptors for non-Coulombic interactions. We apply this model to electronic structure at length and time scales inaccessible to first principles methods, finding an increase in band gap, carrier mass, and the sub-picosecond fluctuations in these quantities with increasing temperature as well as the onset of carrier localization in large supercells induced by thermal disorder at 300 K. Finally, we identify the length scale $L^*= 5$ nm as the onset of localization in the electronic structure, associated with associated with decreasing band edge fluctuations, increasing carrier mass, and Rashba splitting approaching zero.

36 MATERIALS SCIENCE↗
NASA NTRS2017

A First-Principles Model Based on Saturation of the Electron Cyclotron Drift Instability for Electron Transport in Hydrodynamics Simulations of Hall Thruster Plasmas

We present a computational model based on the hypothesis that the higher-than-expected electron mobility across magnetic field lines observed in Hall thrusters is due to the growth and eventual saturation of the electron cyclotron drift instability. The key improvement of this model with respect to previous work is that it decouples the saturation of the instability from the correlation that exists between mobility and magnitude of the wave perturbations in linear theory, thereby enabling first-principles simulations. The model has been incorporated in the 2-D (r-z) multi-fluids code Hall2De and simulations of the H6 thruster at 300 V, 20 A are numerically stable and achieve a steady-state solution at a computational cost that is not significantly higher than previous Hall2De simulations. The computed location of the acceleration region is found to be within 10% of the length of the acceleration channel compared to that inferred by experiments. The simulations also capture well the plasma gradients along the channel centerline of this thruster. To further establish the validity of this model, we plan to conduct simulations of other thrusters and/or operating conditions.

Lopez Ortega, Alejandro↗
DOE OSTI2024

First-principles study of hydrogen- and oxygen-related complexes in ScN

Scandium nitride (ScN) is an attractive material for electronic applications due to its high n-type conductivity. Native defects and unintentional impurities may limit its electron concentration and reduce its mobility; therefore, it is important to control their formation and incorporation. Hydrogen and oxygen are unintentional impurities that are commonly present during growth and processing. They act as shallow donors in ScN and hence may be regarded as harmless or even favorable to achieving n-type conductivity. Here we show, using state-of-the-art first-principles calculations, that these impurities can be detrimental because they readily form complexes with scandium vacancies (V Sc ). Isolated V Sc have relatively high formation energies and thus have low concentrations and little impact on electronic properties. However, complexes between V Sc and either hydrogen or oxygen form more readily than the pristine vacancy and will act as both compensating and scattering centers. Our results point to the importance of controlling the incorporation of hydrogen and oxygen in ScN (and AlScN alloys) to avoid degradation of the electronic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Elastic knowledge base of bcc Ti alloys from first-principles calculations and CALPHAD-based modeling

Here, titanium alloys are being investigated as suitable materials for load-bearing implants because of their biocompatibility and mechanical properties. Stress shielding, a common issue with the current load-bearing implant materials, occurs due to a Young’s modulus (E) mismatch between bone (~10–40 GPa) and implants (such as Ti-6Al-4V ~ 110 GPa), which leads to bone dying around the implant and ultimately implant failure. Reducing the Young’s modulus of Ti alloys may overcome the issues of stress shielding and improve implant materials. In the present work, first-principles calculations have been used to predict the single crystal elastic stiffness coefficients (c ij ’s) for the Ti-containing ternary alloys Ti-X-Y(X ≠ Y = Mo, Nb, Sn, Ta, Zr) in the bcc lattice. It is found that the ternary Ti-X-Y (X ≠ Y = Mo, Nb, Ta) alloys behave similarly; so do the ternary Ti-X-Sn (X = Mo, Nb, Ta) alloys and the Ti-X-Zr (X = Mo, Nb, Ta) alloys. This is expected due to the similarity between the Mo, Nb and Ta elements. The results also show that the Ti-Zr-X alloys stabilized the bcc phase at lower alloying concentrations. The polycrystalline aggregate properties are also estimated from the c ij ’s, including bulk modulus, shear modulus and Young’s modulus. The results show that Ti-alloys with compositions close to the bcc stability limit have the lowest E. In combination with previous predictions, a complete elastic database has been established using theCALPHAD (CALculation of PHAse Diagram) based modeling approach. The database results are compared with the E of higher order Ti alloys and shown to be able to predict the E accurately. This complete data-base forms a foundation to tailor Ti alloys for desired elastic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2015

First-Principles Design and Analysis of an Efficient, Pb-Free Ferroelectric Photovoltaic Absorber Derived from ZnSnO 3

Ferroelectric oxides, which exhibit a spontaneous and reversible electric polarization, have recently gained interest for photovoltaic applications, because this polarization can potentially facilitate exciton separation and carrier extraction while also generating open-circuit voltages orders-of-magnitude larger than the band gap. However, photovoltaic efficiencies in these materials are often limited by large band gaps and high hole effective masses. Developing a means to simultaneously reduce both without destroying the ferroelectric polarization could revolutionize photovoltaic technologies. In this work, we use first-principles computations to describe how chemical substitution of oxygen in the recently characterized ferroelectric ZnSnO 3 with sulfur to form ZnSnS 3 reduces the band gap to a near-optimal 1.3 eV while leaving the polarization virtually unchanged. Furthermore, we show that other key photovoltaic materials characteristics, such as hole effective mass, dielectric constant, and absorption coefficient, are also dramatically improved by sulfur substitution. Finally, we demonstrate that the well-known semiconductor GaN provides an excellent substrate on which to grow ZnSnS 3 , forming a strongly bound interface that is free of intrinsic midgap states and exhibits an ideal band alignment for efficient carrier extraction. Finally, this work advances the search for an Earth-abundant, nontoxic, ferroelectric material exhibiting strong solar absorption, and may enable the development of solar cells with both high photovoltage and photocurrent.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

First-Principles Study of the Voltage Profile and Mobility of Mg Intercalation in a Chromium Oxide Spinel

The development of Mg batteries, which can potentially achieve higher energy densities than Li-ion systems, is in need of cathodes that can reversibly intercalate Mg 2+ and exhibit a higher energy density than the state-of-the-art Chevrel and thio-spinel cathodes. Recent theoretical and experimental studies indicate that the oxide spinel family presents a set of promising Mg cathodes. Here specifically, in this work, we investigate Mg intercalation into the spinel-MgxCr 2 O 4 system. Using first-principles calculations in combination with a cluster expansion model and the nudged elastic band theory, we calculate the voltage curve for Mg insertion at room temperature and the activation barriers for Mg diffusion, respectively, at different Mg concentrations in the Cr 2 O 4 structure. Our results identify a potential limitation to Mg intercalation in the form of stable Mg-vacancy orderings in the Cr 2 O 4 lattice, which exhibit high migration barriers for Mg diffusion in addition to a steep voltage change. Additionally, we propose cation substitution as a potential mechanism that can be used to suppress the formation of the stable Mg-vacancy ordering, which can eventually enable the practical usage of Cr 2 O 4 as a Mg-cathode.

25 ENERGY STORAGE↗
DOE OSTI2012

A first-principles theoretical approach to heterogeneous nanocatalysis

A theoretical approach to heterogeneous catalysis by sub-nanometre supported metal clusters and alloys is presented and discussed. Its goal is to perform a computational sampling of the reaction paths in nanocatalysis via a global search in the phase space of structures and stoichiometry combined with filtering which takes into account the given experimental conditions (catalytically relevant temperature and reactant pressure), and corresponds to an incremental exploration of the disconnectivity diagram of the system. The approach is implemented and applied to the study of propylene partial oxidation by Ag3 supported on MgO(100). First-principles density-functional theory calculations coupled with a Reactive Global Optimization algorithm are performed, finding that: (1) the presence of an oxide support drastically changes the potential energy landscape of the system with respect to the gas phase, favoring configurations which interact positively with the electrostatic field generated by the surface; (2) the reaction energy barriers for the various mechanisms are crucial in the competition between thermodynamically and kinetically favored reaction products; (3) a topological database of structures and saddle points is produced which has general validity and can serve for future studies or for deriving general trends; (4) the MgO(100) surface captures some major features of the effect of an oxide support and appears to be a good model of a simple oxide substrate; (5) strong cooperative effects are found in the co-adsorption of O2 and other ligands on small metal clusters. The proposed approach appears as a viable route to advance the role of predictive computational science in the field of heterogeneous nanocatalysis.

Chemistry↗