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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

First-principles study on the effect of alloying elements on the elastic deformation response in β-titanium alloys

Theoretical deformation response of hypothetical β-titanium alloys was investigated using first-principles calculation technique under periodic boundary conditions. Simulation was carried out on hypothetical 54-atom supercell of Ti–X (X = Cr, Mn, Fe, Zr, Nb, Mo, Al, and Sn) binary alloys. The results showed that the strength of Ti increases by alloying, except for Cr. The most effective alloying elements are Nb, Zr, and Mo in the current simulation. The mechanism of bond breaking was revealed by studying the local structure around the alloying element atom with respect to volume change. Moreover, the effect of alloying elements on bulk modulus and admissible strain was investigated. It was found that Zr, Nb, and Mo have a significant effect to enhance the admissible strain of Ti without change in bulk modulus.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

First-principles assessment of thermoelectric properties of CuFeS 2

Composed of inexpensive and naturally abundant elements, the chalcopyrite mineral CuFeS 2 has received attention as a potentially useful thermoelectric. We use first-principles electronic structure and Boltzmann transport theory calculations to investigate thermoelectric properties of n -doped CuFeS 2 . We find that energy-dependent carrier lifetimes that are inversely proportional to the electronic density-of-states are crucial for reproducing experimental data on the transport properties, including the measured values of the Seebeck coefficient, α. The heavy-effective-mass conduction band promotes high values of α, but it also leads to low mobility due to strong electron-acoustic-phonon scattering. Low mobility forces one to rely on high carrier concentration to achieve high conductivity, which decreases α and limits the achievable power factor. Our calculations predict that ideally doped CuFeS 2 that has been nanostructured to an average grain size d≈20nm can attain thermoelectric figures of merit z T=0.25 to 0.8 for T=300 to 700 K, respectively, due to a reduction in the lattice thermal conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2015

First-principles investigation of hydrous post-perovskite

A stable, hydrogen-defect structure of post-perovskite (hy-ppv, Mg 1–x SiH 2x O 3 ) has been determined by first-principles calculations of the vibrational and elastic properties up to 150 GPa. Among three potential hy-ppv structures analyzed, one was found to be stable at pressures relevant to the lower-mantle D" region. Hydrogen has a pronounced effect on the elastic properties of post-perovskite due to magnesium defects associated with hydration, including a reduction of the zero-pressure bulk (K 0 ) and shear (G 0 ) moduli by 5% and 8%, respectively, for a structure containing ~1 wt.% H 2 O. However, with increasing pressure the moduli of hy-ppv increase significantly relative to ppv, resulting in a structure that is only 1% slower in bulk compressional velocity and 2.5% slower in shear-wave velocity than ppv at 120 GPa. In contrast, the reduction of certain anisotropic elastic constants (C ij ) in hy-ppv increases with pressure (notably, C 55 , C 66 , and C 23 ), indicating that hydration generally increases elastic anisotropy in hy-ppv at D" pressures. Calculated infrared absorption spectra show two O–H stretching bands at ~3500 cm –1 that shift with pressure to lower wavenumber by about 2 cm –1 /GPa. At 120 GPa the hydrogen bonds in hy-ppv are still asymmetric. Furthermore, the stability of a hy-ppv structure containing 1–2 wt.% H 2 O at D" pressures implies that post-perovskite may be a host for recycled or primordial hydrogen near the Earth’s core-mantle boundary.

58 GEOSCIENCES↗
DOE OSTI2015

Effect of sulfur doping on thermoelectric properties of tin selenide – A first principles study

In this work we present the thermoelectric properties of tin selenide (SnSe) and sulfur doped tin selenide(SnSe{sub (1-x)}S{sub x}, x= 0.125 and 0.25) obtained using first principles calculations. We investigated the electronic band structure using the FP-LAPW method within the sphere of the density functional theory. Thermoelectric properties were calculated using BOLTZTRAP code using the constant relaxation time approximation at three different temperatures 300, 600 and 800 K. Seebeck coefficient (S) was found to decrease with increasing temperature, electrical conductivity (σ/τ) was almost constant in the entire temperature range and thermal conductivity (κ/τ) increased with increasing temperature for all samples. Sulfur doped samples showed enhanced seebeck coefficient, decreased thermal conductivity and decreased electrical conductivity at all temperatures. At 300 K, S increased from 1500 µV/K(SnSe) to 1720μV/K(SnSe{sub 0.75}S{sub 0.25}), thermal conductivity decreased from 5 × 10{sup 15} W/mKs(SnSe) to 3 × 10{sup 15} W/mKs(SnSe{sub 0.75}S{sub 0.25}), electrical conductivity decreased from 7 × 10{sup 20}/Ωms(SnSe) to 5 × 10{sup 20} /Ωms(SnSe{sub 0.75}S{sub 0.25}). These calculations show that sulfur doped tin selenide exhibit better thermoelectric properties than undoped tin selenide.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

First-principles study of the stability of free-standing germanene in oxygen atmosphere

The O{sub 2} dissociation and O atoms adsorption on free-standing germanene are studied by using first-principles calculations in this paper. Compared with the extremely active silicene in oxygen atmosphere, germanene is found to be less active due to an energy barrier for dissociation of about 0.57 eV. Moreover, the dissociated oxygen atom follows two opposite migration pathways on the germanene surface, which is quite different from the case of silicene. Furthermore, the migration and desorption of O atoms at room temperature are relatively difficult due to the strong Ge-O bonding, resulting in the formation of germanium oxides. Our results reveal the interplay between germanene and O{sub 2} and suggest the enhanced stability of germanene in oxygen atmosphere compared with silicene.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Phase transitions in HfO2 probed by first-principles computations

Ever since ferroelectricity was discovered in HfO2, the question of its origin remains controversial. Here, we probe this question using a combination of Landau theory of phase transitions and first-principles computations. In such an approach, the energy landscape associated with the phase transition between cubic and different experimentally demonstrated phases of HfO2 (tetragonal, monoclinic, orthorhombic Pbca, orthorhombic Pnma, and orthorhombic Pca21) is explored using density functional theory calculations. Computations revealed that stabilization of all but orthorhombic Pbca phase is driven by a single unstable zone-boundary antipolar mode X2−. When coupled with zone-center modes (Γ1+ and Γ3+), it stabilizes the tetragonal phase. Coupling with four additional modes (Γ5+, X3−, X5−, X5+) results in the monoclinic phase, which is the ground state of the material. If, however, Γ5+ mode is replaced with Γ4− mode, orthorhombic polar phase Pca21 is stabilized. The application of this framework to examine the effect of electric field on the ferroelectric phase of hafnia reveals that the field of 5 MV/cm is capable of stabilizing ferroelectric phase over the monoclinic one at 0 K.

Physics↗
DOE OSTI2019

Precise radiative lifetimes in bulk crystals from first principles: the case of wurtzite gallium nitride

Gallium nitride (GaN) is a key semiconductor for solid-state lighting, but its radiative processes are not fully understood. Here we show a first-principles approach to accurately compute the radiative lifetimes in bulk uniaxial crystals, focusing on wurtzite GaN. Our computed radiative lifetimes are in very good agreement with experiment up to 100 K. We show that taking into account excitons (through the Bethe–Salpeter equation) and spin–orbit coupling is essential for computing accurate radiative lifetimes. A model for exciton dissociation into free carriers allows us to compute the radiative lifetimes up to room temperature. Our work enables precise radiative lifetime calculations in III-nitrides and other anisotropic solid-state emitters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Two-dimensional lateral GaN/SiC heterostructures: First-principles studies of electronic and magnetic properties

We study the electronic and magnetic structures of quasi-one-dimensional interfaces along the zigzag direction in two-dimensional GaN/SiC heterostructures using first-principles calculations. Four representative heterostructures with six inequivalent interfaces are discussed in detail. Our results indicate that a bulk electric field will develop only when both interfaces feature no gap states and the total net charge at interfaces are of opposite sign. All the geometries studied exhibit an intriguing quasi-one-dimensional conductor character, of which three show finite nonzero magnetic moment. Furthermore, the magnetic moment in one of the systems can be tuned by applying an electric field along the normal direction of monolayer indicating a strong magnetoelectric coupling. Our analysis shows that the magnetization at the interfaces is closely related to the density of states at the Fermi level due to the Stoner instability, and the ribbon-width-dependent magnetization for different geometries implies the existence of an in-bulk electric field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Structure and electronic tunability of acene alkylamine based layered hybrid organic-inorganic perovskites from first principles

Twelve layered hybrid organic-inorganic perovskites combining oligoacene derivatives [phenylmethylammonium (PMA), naphthylmethylammonium (NMA), anthrylmethylammonium (AMA), and tetrylmethylammonium (TMA)] and lead halides (Cl - , Br - , and I - anions) are investigated by first-principles density functional theory (DFT), showing broad, rational tunability of band gap, quantum well type, and spin-dependent energy band properties. Six compounds are known from previous syntheses and are used to devise a computational search space for likely low-energy structures. Among the six known compounds, a refined structure is identified for (NMA) 2 ⁢ PbCl 4 and a new, lower-energy structure is suggested for (AMA) 2 ⁢PbCl 4 . The DFT based search methodology is next applied to predict the likely structures for the six unknown compounds. Computationally predicted energy levels for all 12 compounds from spin-orbit coupled hybrid DFT reveal tunable type I quantum well alignments, with frontier orbitals located on the inorganic component, on the organic component, or on both, as controlled by cation and anion selection. In conclusion, several structures spontaneously break local and global inversion symmetry, causing strong spin splitting of the conduction bands [up to 0.12 eV for (PMA) 2 ⁢PbCl 4 ] and showing potential for spin-dependent transport properties and future spintronic and tunable chiroptical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2024

An Adsorptive Membrane Platform for Precision Ion Separation: Membrane Design and First‐Principles Studies

One of the key challenges in separation science is the lack of precise ion separation methods and mechanistic understanding crucial for efficiently recovering critical materials from complex aqueous matrices. Herein, first-principles electronic structure calculations and in situ Raman spectroscopy are studied to elucidate the factors governing ion discrimination in an adsorptive membrane specifically designed for transition metal ion separation. Density functional theory calculations and in situ Raman data jointly reveal the thermodynamically favorable binding preferences and detailed adsorption mechanisms for competing ions. How membrane binding preferences correlate with the electronic properties of ligands is explored, such as orbital hybridization and electron localization. The findings underscore the importance of the phenolate group in oxime ligands for achieving high selectivity among competing transition metal ions. In-depth understanding on which specific atomistic site within the microenvironment of metal-ligand binding pockets governs the ion discrimination behaviors of the host will build a solid foundation to guide the rational design of next-generation materials for precision separation essential for energy technologies and environment remediation. In tandem, synthetic controllability is demonstrated to transform 3D micrometer-scale crystals to a 2D crystalline selective layer in membranes, paving the way for more precise and sustainable advances in separation science.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Magnetic and Electronic Properties of BN Nanosheets with Different Nonmagnetic Metal Dopants: a First-Principles Study

The structural, electronic, and magnetic properties of monolayer boron nitride (BN) doped with a series of nonmagnetic metal atoms, including Ga, Li, Mg, and Na, are systematically investigated by the first-principles method. Both different dopants and doping sites are considered. Magnetic behavior is observed in the systems doped with Li, Mg, and Na atoms. On the other hand, owing to the stable structures of the BN monolayers with Li and Mg dopants among the three magnetic systems, we study the magnetic interaction in these two systems. Interestingly, a long-range ferromagnetic (FM) coupling is observed in the BN monolayer with two Li atoms doped as the Li–Li distance increases. Our results show that the FM interaction originates from a p–d exchange-like s–p coupling interaction. Moreover, nonmagnetic state appears in the system with two Mg dopants.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Structural, Electronic, and Magnetic Properties of Binary Heusler Alloys ZCl{sub 3} (Z = Li, Na, K, Rb) with DO{sub 3}-Type Structure from First Principle Calculations

The structural, electronic, and magnetic properties of binary Heusler alloys ZCl{sub 3} (Z = Li, Na, K, Rb) with DO{sub 3}-type structure have been studied by means of the first principle calculations. The calculated results reveal that all four alloys are half-metallic (HM) materials at the stable ferromagnetic ground state. The total magnetic moments (M{sub t}) of LiCl{sub 3}, NaCl{sub 3}, KCl{sub 3,} and RbCl{sub 3} alloys are all 2.000 μ{sub B}/f.u., following the Slater-Pauling rule M{sub t} = 24 − Z{sub t} for the former two alloys, while satisfying the Slater-Pauling rule M{sub t} = 30 − Z{sub t} for the latter two alloys. The spin-up band gaps of the binary Heusler alloys ZCl{sub 3} (Z = Li, Na, K, Rb) are 5.598, 5.687, 6.744, and 6.745 eV at equilibrium lattice constants 7.076, 7.237, 7.588, and 7.819 Å, respectively. The positive cohesion energy and negative formation energy indicate that they are thermodynamically stable. Moreover, wide HM regions for binary Heusler alloys ZCl{sub 3} (Z = Li, Na, K, Rb) show their HM characters are robust when the lattices are expanded or compressed under uniform and tetragonal strains. Finally, the total charge density, magnetic charge density, and charge density difference of the (110) plane for LiCl{sub 3} alloy are analyzed in detail.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

First-principles study on the ferrimagnetic half-metallic Mn{sub 2}FeAs alloy

Mn-based full-Heusler alloys are kinds of promising candidates for new half-metallic materials. Basing on first principles, the electronic structures and magnetic properties of the Mn{sub 2}FeAs full-Heusler alloy have been investigated in detail. The Hg{sub 2}CuTi-type Mn{sub 2}FeAs compound obeys the Slater-Pauling rule, while the anti-parallel alignment atomic magnetic moments of Mn locating at different sites indicate it a ferrimagnetic alloy. The calculated spin-down bands behave half-metallic character, exhibiting a direct gap of 0.46 eV with a 100% spin polarization at the Fermi level. More studies show the compound would maintain half-metallic nature in a large range of variational lattice constants. We expect that our calculated results may trigger Mn{sub 2}FeAs applying in the future spintronics field. - Graphical abstract: The d orbitals of Mn and Fe atoms split into multi-degenerated levels which create new bonding and nonbonding states. These exchange splitting shift the Fermi level to origin band gap.▪ - Highlights: • The electronic structure and magnetic properties of Mn{sub 2}FeAs full-Heusler alloy were studied. • A total magnetic moment of 3μ{sub B} was obtained for Mn{sub 2}FeAs alloy, following the SP rule M{sub t}=Z{sub t}−24. • The origin of ferrimagnetism and half-metallic character in Mn{sub 2}FeAs were discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Acetylene Semi-Hydrogenation on a Perovskite Oxyhydride Surface: Insights from First Principles and Microkinetic Modeling

Although perovskite oxyhydrides (POHs) have shown promise as a support for selective hydrogenation, it is unknown if they themselves can catalyze alkyne semi-hydrogenation. Here, we use first-principles density functional theory, coupled with microkinetic modeling, to investigate acetylene semi-hydrogenation on a prototypical POH, cubic BaTiO 2.5 H 0.5 (BTOH). Two different mechanisms are examined on a representative surface of BTOH under the reaction conditions: both are based on the Horiuti–Polanyi mechanism, but the way of H 2 dissociation is different. In mechanism 1, a lattice hydride H atom and then a surface adsorbed H atom sequentially hydrogenate the adsorbed acetylene. In mechanism 2, two lattice hydride H atoms from the BTOH sequentially hydrogenate the adsorbed acetylene. In both mechanisms, the H atoms are replenished from gas phase H 2 dissociation. Using density functional theory (DFT), we have calculated pathways for these two mechanisms in hydrogenation of acetylene to ethylene, as well as further to ethane. Microkinetic modeling based on the DFT energetics indicates that at 523 K, hydrogenation to ethylene and ethane via mechanism 1 occurs at a rate up to two orders of magnitude faster than via mechanism 2. A selectivity analysis for the temperature range of 373–673 K shows that the product observed is essentially only ethylene within the more active mechanism 1. This is because at the steady state, there is a significant amount of surface anion vacancies that facilitate heterolytic H 2 dissociation and help stabilize a vinyl intermediate. In conclusion, the present findings suggest that POHs are capable of selective hydrogenation, thanks to their rich hydrogen surface-redox chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Efficient calculation of carrier scattering rates from first principles

Abstract The electronic transport behaviour of materials determines their suitability for technological applications. We develop a computationally efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 23 semiconductors, including the large 48 atom CH 3 NH 3 PbI 3 hybrid perovskite, and comparing the results against experimental measurements and more detailed scattering simulations. The Spearman rank coefficient of mobility against experiment ( r s = 0.93) improves significantly on results obtained using a constant relaxation time approximation ( r s = 0.52). We find our approach offers similar accuracy to state-of-the art methods at approximately 1/500th the computational cost, thus enabling its use in high-throughput computational workflows for the accurate screening of carrier mobilities, lifetimes, and thermoelectric power.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2014

A genetic algorithm for first principles global structure optimization of supported nano structures

We present a newly developed publicly available genetic algorithm (GA) for global structure optimisation within atomic scale modeling. The GA is focused on optimizations using first principles calculations, but it works equally well with empirical potentials. The implementation is described and benchmarked through a detailed statistical analysis employing averages across many independent runs of the GA. This analysis focuses on the practical use of GA’s with a description of optimal parameters to use. New results for the adsorption of M{sub 8} clusters (M = Ru, Rh, Pd, Ag, Pt, Au) on the stoichiometric rutile TiO{sub 2}(110) surface are presented showing the power of automated structure prediction and highlighting the diversity of metal cluster geometries at the atomic scale.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Magnetic anisotropy in Ta/CoFeB/MgO investigated by x-ray magnetic circular dichroism and first-principles calculation

We study the spin and orbital magnetic moments in Ta/Co{sub 0.4}Fe{sub 0.4}B{sub 0.2}/MgO by x-ray magnetic circular dichroism measurements as well as first-principles calculations, in order to clarify the origin of the perpendicular magnetic anisotropy. Both experimental and theoretical results show that orbital magnetic moment of Fe is more anisotropic than that of Co with respect to the magnetization direction. The anisotropy is larger for thinner CoFeB, indicating that Fe atoms at the interface with MgO contribute more than Co to the observed perpendicular magnetic anisotropy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗