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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

First-Principles Investigation of Mechanical and Thermodynamic Properties of Nickel Silicides at Finite Temperature

First-principles calculations are performed to investigate lattice parameters, elastic constants and 3D directional Young’s modulus E of nickel silicides (i.e., β-Ni{sub 3}Si, δ-Ni{sub 2}Si, θ-Ni{sub 2}Si, ε-NiSi, and θ-Ni{sub 2}Si), and thermodynamic properties, such as the Debye temperature, heat capacity, volumetric thermal expansion coefficient, at finite temperature are also explored in combination with the quasi-harmonic Debye model. The calculated results are in a good agreement with available experimental and theoretical values. The five compounds demonstrate elastic anisotropy. The dependence on the direction of stiffness is the greatest for δ-Ni{sub 2}Si and θ-Ni{sub 2}Si, when the stress is applied, while that for β-Ni{sub 3}Si is minimal. The bulk modulus B reduces with increasing temperature, implying that the resistance to volume deformation will weaken with temperature, and the capacity gradually descend for the compound sequence of β-Ni{sub 3}Si > δ-Ni{sub 2}Si > θ-Ni{sub 2}Si > ε-NiSi > θ-Ni{sub 2}Si. The temperature dependence of the Debye temperature ΘD is related to the change of lattice parameters, and ΘD gradually decreases for the compound sequence of ε-NiSi > β-Ni{sub 3}Si > δ-Ni{sub 2}Si > θ-Ni{sub 2}Si > θ-Ni{sub 2}Si. The volumetric thermal expansion coefficient αV, isochoric heat capacity and isobaric heat capacity C{sub p} of nickel silicides are proportional to T{sup 3} at low temperature, subsequently, αV and C{sub p} show modest linear change at high temperature, whereas C{sub v} obeys the Dulong-Petit limit. In addition, β-Ni{sub 3}Si has the largest capability to store or release heat at high temperature. From the perspective of solid state physics, the thermodynamic properties at finite temperature can be used to guide further experimental works and design of novel nickel–silicon alloys.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Structural and Chemical Evolution of the Layered Li-Excess Li x MnO 3 as a Function of Li Content from First-Principles Calculations

Li 2 MnO 3 is a critical component in the family of "Li-excess" materials, which are attracting attention as advanced cathode materials for Li-ion batteries. Here, first-principle calculations are presented to investigate the electrochemical activity and structural stability of stoichiometric Li x MnO 3 (0 ≤ x ≤ 2) as a function of Li content. The Li 2 MnO 3 structure is electrochemically activated above 4.5 V on delithiation and charge neutrality in the bulk of the material is mainly maintained by the oxidization of a portion of the oxygen ions from O 2- to O 1- . While oxygen vacancy formation is found to be thermodynamically favorable for x < 1, the activation barriers for O 2- and O 1- migration remain high throughout the Li composition range, impeding oxygen release from the bulk of the compound. Defect layered structures become thermodynamically favorable at lower Li content (x < 1), indicating a tendency towards the spinel-like structure transformation. A critical phase transformation path for forming nuclei of spinel-like domains within the matrix of the original layered structure is proposed. Formation of defect layered structures during the first charge is shown to manifest in a depression of the voltage profile on the first discharge, providing one possible explanation for the observed voltage fade of the Li-excess materials.

25 ENERGY STORAGE↗
DOE OSTI2017

First-principles study of fission gas incorporation and migration in zirconium nitride

To evaluate the effectiveness of ZrN as a diffusion barrier against fission gases, we investigate in this paper the incorporation and migration of fission gas atoms, with a focus on Xe, in ZrN by first-principles calculations. The formations of point defects in ZrN, including vacancies, interstitials, divacancies, Frenkel pairs, and Schottky defects, are first studied. Among all the defects, the Schottky defect with two vacancies as first nearest neighbor is predicted to be the most favorable incorporation site for fission gas Xe in ZrN. The migration of Xe gas atom in ZrN is investigated through two diffusion mechanisms, i.e., interstitial and vacancy-assisted diffusions. The migration barrier of Xe gas atom through the intrinsic interstitials in ZrN is considerably lower than that through vacancies. Finally, therefore, at low temperatures fission gas Xe atoms diffuse mainly through interstitials in single crystal ZrN, whereas at high temperatures Xe may diffuse in ZrN assisted by vacancies.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Insight into ideal shear strength of Ni-based dilute alloys using first-principles calculations and correlational analysis

Here the present work examines the effect of alloying elements (denoted X) on the ideal shear strength for 26 dilute Ni-based alloys, Ni11X, as determined by first-principles calculations of pure alias shear deformations. The variations in ideal shear strength are quantitatively explored with correlational analysis techniques, showing the importance of atomic properties such as size and electronegativity. The shear moduli of the alloys are affirmed to show a strong linear relationship with their ideal shear strengths, while the shear moduli of the individual alloying elements were not indicative of alloy shear strength. Through combination with available ideal shear strength data on Mg alloys, a potential application of the Ni alloy data is demonstrated in the search for a set of atomic features suitable for machine learning applications to mechanical properties. As another illustration, the calculated Ni ideal shear strengths play a key role in a predictive multiscale framework for deformation behavior of single crystal alloys at large strains, as shown by simulated stress–strain curves.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Atomically thin interlayer phase from first principles enables defect-free incommensurate SnO2/CdTe interface

Advancing optoelectronic and emerging technologies increasingly requires control and design of interfaces between dissimilar materials. However, incommensurate interfaces are notoriously defective and rarely benefit from first-principles predictions, because no explicit atomic-structure models exist. Here, we adopt a bulk crystal structure prediction method to the interface geometry and apply it to SnO2/CdTe heterojunctions without and with the addition of CdCl2, a ubiquitous and beneficial, but abstruse processing step in CdTe photovoltaics. Whereas the direct SnO2/CdTe interface is highly defective, we discover a unique two-dimensional CdCl2 interphase, unrelated to the respective bulk structure. It facilitates a seamless transition from the rutile to zincblende lattices and removes defect-states from the interface bandgap. Implementing the predicted interface electronic structure in device simulations, we demonstrate the theoretical feasibility of bufferless oxide-CdTe heterojunction solar cells approaching the Shockley–Queisser limit. Our results highlight the broader potential of designing atomically thin interlayers to enable defect-free incommensurate interfaces.

14 SOLAR ENERGY↗
DOE OSTI2022

Optical and magneto-optical properties of ferromagnetic monolayer CrBr 3 : A first-principles G W and G W plus Bethe-Salpeter equation study

The discovery of atomically thin two-dimensional (2D) magnetic semiconductors has triggered enormous research interest recently. In this paper, we use first-principles many-body perturbation theory to study a prototypical 2D ferromagnetic semiconductor, monolayer chromium tribromide (CrBr 3 ). With broken time-reversal symmetry, spin-orbit coupling, and excitonic effects included through the full-spinor GW and GW plus Bethe-Salpeter equation (GW-BSE) methods, we compute the frequency-dependent layer polarizability tensor and dielectric function tensor that govern the optical and magneto-optical (MO) properties. In addition, we provide a detailed theoretical formalism for simulating magnetic circular dichroism, MO Kerr effect, and Faraday effect, demonstrating the approach with monolayer CrBr 3 . Due to reduced dielectric screening in 2D and the localized nature of the Cr d orbitals, we find strong self-energy effects on the quasiparticle band structure of monolayer CrBr 3 that give a 3.8 eV indirect bandgap. Also, excitonic effects dominate the low-energy optical and MO responses in monolayer CrBr 3 where a large exciton binding energy of 2.3 eV is found for the lowest bright exciton state with excitation energy at 1.5 eV. We further find that the MO signals demonstrate strong dependence on the excitation frequency and substrate refractive index. Furthermore, our theoretical framework for modeling optical and MO effects could serve as a powerful theoretical tool for future study of optoelectronic and spintronics devices consisting of van der Waals 2D magnets.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Carbon trimer as a 2 eV single-photon emitter candidate in hexagonal boron nitride: A first-principles study

The generation of single-photon emitters in hexagonal boron nitride around 2 eV emission is experimentally well recognized; however, the atomic nature of these emitters is unknown. In this Letter, we use first-principles calculations to demonstrate that carbon trimer substitutional defect (C 2 C N ) is a possible source of 2 eV single-photon emitter in hBN. Here, we showcase the calculations of a complete set of static and dynamical properties related to quantum defects, including exciton-defect couplings and electron-phonon interactions, from both density functional theory and many-body perturbation theory. In particular, we show that it is critical to consider both radiative and nonradiative processes when comparing with experimental lifetime for known 2 eV emitters. We find that C 2 C N has several key physical properties matching the ones of experimentally observed single-photon emitters. These include the zero-phonon line (2.13 eV), Huang-Rhys factor (1.35), photoluminescence lifetime (2.19 ns), phonon-sideband energy (180 meV), and photoluminescence spectrum. The identification of defect candidates for 2 eV emission paves the way for controllable single-photon emission generation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

First principles study on 2H–1T' transition in MoS 2 with copper

In this paper, the electronic properties of MoS 2 are strongly controlled by the structure, providing a route to their modulation. We report, based on first principles calculations, that the adsorption of metal atom Cu on the surface can induce the phase transition of MoS 2 from the semiconducting 2H to the metallic 1T' phase. Cu adsorption results in effective n-type doping of MoS 2 by charge transfer from Cu in the case of the 1T' phase. This is distinct from the behavior in the 2H phase, where Cu does not donate any charge, and it is also distinct from alkali metal adsorption, where charge is donated to both 2H and 1T' MoS 2 . Charge donation to the 1T' phase by Cu stabilizes it with respect to the 2H structure and importantly, it also reduces the energy barrier between the 2H and 1T' structures. This difference reflects the higher electronegativity of Cu, which also indicates that Cu-modified MoS 2 can be expected to be less chemically reactive than MoS 2 with alkali metal adatoms. The main atomic mechanism of the structural transition is the gliding of S atoms on the upper surface. Finally, we report the energetics of the 2H to 1T' transition with several other adatoms, Ag, Au, Ni, Pt and Pd, but none of them are as effective as Cu in inducing the transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

First-principles analysis of the Al-rich corner of Al-Li-Cu phase diagram

The phase diagram of Al-Li-Cu system in the Al-rich region was determined by means of first-principles calculations and statistical mechanics. The mixing enthalpies of many configurations for different lattices in the whole Al-Li-Cu system were determined by density functional theory simulations to find the stable phases in the convex hull. They were fitted with a cluster expansion to calculate the free energy of the configurations with different compositions as a function of temperature in the Al-rich region (Al content > 40 at.%) by means of Monte Carlo simulations. It was found that the ground state phases in the Al-rich part of the Al-Li-Cu phase diagram were α-Al, θ' (Al 2 Cu), $δ$' (Al 3 Li), $δ$ (AlLi) and T 1 (Al 6 Cu 4 Li 3 ), while θ'' (Al 3 Cu), T 1' (Al 2 CuLi) and Al 3 Cu 2 Li were found on the lowest mixing enthalpy surfaces of their lattices and were metastable. α-Al, $δ$ and T 1 are stable phases in the whole temperature range while $δ$' becomes metastable at very low temperature and θ (Al 2 Cu) replaces θ' as the stable phase at approximately 550 K due to the vibrational entropic contribution. In addition, the phase diagram in the Al-rich region was built and it was shown in isothermal sections from 100 to 900 K. They were in good agreement with the limited experimental data in the literature and provided new information regarding the stability, solubility and stoichiometry of the different phases. This information is important to understand the precipitation mechanisms during high temperature aging.

36 MATERIALS SCIENCE↗
DOE OSTI2019

First-Principle Studies of Ferrimagnetic Double Perovskite Ca{sub 2}FeMoO{sub 6} Compound

Using first-principle calculations, the structural, electronic, and magnetic properties of the Ca{sub 2}FeMoO{sub 6} double perovskite compound is investigated. Different spin-ordering: ferrimagnetic (FiM), ferromagnetic (FM), and anti-ferromagnetic (AFM1 and AFM2) using the generalized gradient approximation (GGA) and GGA + U (Hubbard Coulomb onsite correction) are evaluated to determine the theoretical ground state. The value of the Hubbard Coulomb U parameter is varied from 1 to 4 eV. The ground state is found to be a FM spin-ordering within the GGA approach and FiM spin-ordering within the GGA + U approach (where U ≥ 3 eV) which is the experimental preferred configuration. We obtain the FiM spin-ordered half semiconducting state within the GGA + U approach for the Ca{sub 2}FeMoO{sub 6} compound. Within the GGA + U (where U ≥ 3 eV) approach, the FM phase maintains a half-metallic character with a net magnetic moment of 4 0 μ{sub B}, whereas the FiM phase have a spin gapless semiconducting (SGS) behavior at U = 3 eV, and an insulating character at U = 4 eV, with a net magnetic moment of 4 0 μ{sub B}. The main features found in the density of states profile show that the hybridization of the Fe and Mo d orbitals play an important role in determining the electronic and magnetic character of this compound.

36 MATERIALS SCIENCE↗
DOE OSTI2023

First-principles study of interfaces in Al/SiC metal-matrix composite system

In this work, first-principles calculations were performed on the interfaces between Al and SiC, which is a widely used strengthening agent in aluminum metal-matrix-composites (Al/MMC). C-terminated interfaces have much larger work of adhesion than Si-terminated interfaces, indicating that the former has much stronger interfacial bonding. The electron localization function shows that the chemical bonding between Al and C has a strong covalent character, while the bonding between Al and Si is largely metallic. As a result of the vastly different chemical bonding, the work of adhesion for C-terminated interfaces increases with the number of dangling bonds at the interface, while the opposite trend was observed for Si-terminated interfaces. Additionally, the interface energy for Si-terminated interfaces is comparable to that for C-terminated interfaces, suggesting both types of terminations can coexist in the Al/SiC system.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Pressure-induced magnetic moment abnormal increase in Mn 2 FeAl and non-continuing decrease in Fe 2 MnAl via first principles

The magnetism of Fe 2 MnAl and Mn 2 FeAl compounds are studied by first principles. Evolutions of magnetic moment of Fe 2 MnAl display distinct variation trends under pressure, showing three different slopes at different pressure intervals, 0~100GPa, 100~250GPa, 250–400GPa, respectively, and the moment collapses finally at 450GPa. The magnetic moment of Mn 2 FeAl shows an increasing tendency below 40GPa and decreases subsequently with pressure, and collapses ultimately at about 175GPa. Such non-continuing decrease of Fe 2 MnAl originates from the unusual charge transfer of Fe and Mn and bond populations rearrangement of Fe-Fe and Mn-Fe, whereas the distinct moment evolution of Mn 2 FeAl is attributed to the complicated distributions of bond populations. The half-metallicity of the compounds can be maintained at low pressure, below about 100GPa in Fe 2 MnAl and 50GPa in Mn 2 FeAl. The magnetic moment collapse process didn’t induce volume and bond length anomalies in the two compounds, the unique anomaly is the elastic softening behaviour in elastic constant c44 and shear (G) and Young’s (E) moduli of Fe 2 MnAl at 270GPa, where the second moment collapse occurs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Exchange interaction in hexagonal MnRhP from first-principles studies

Electronic structure and magnetic properties for MnRhP have been studied from a first-principles density functional calculation. The calculated lattice constants, a = 6.228 Å and c = 3.571 Å, are in good agreement with the experimental values of a = 6.223 Å and c = 3.585 Å. The calculated moment of Mn is 3.1 μ{sub B}/atom, resulting in a total moment of 3.0 μ{sub B}/atom due to small moments induced at Rh and P sites. The magnetic moment of Mn decreases with unit cell size. The exchange interactions are dominated by positive Mn-Mn exchange coupling (J{sub Mn−Mn}), implying a stable ferromagnetic ordering in Mn sublattice. In particular, J{sub Mn−Mn} shows a maximum value (1.5 mRy) at the the optimized unit cell size. The structural distortion or unit cell size change will affect J{sub Mn−Mn}, which is intimately related to the magneto-elastic and magneto-caloric effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A first principle study for the comparison of phonon dispersion of armchair carbon and silicon nanotubes

Using first principles density functional theoretical calculations, the present paper reports a systematic study of phonon dispersion curves in pristine carbon (CNT) and silicon nanotubes (SiNT) having chirality (6,6) in the armchair configuration. Some of the phonon modes are found to have negative frequencies which leads to instability of the systems under study. The number of phonon branches has been found to be thrice as much as the number of atoms. The frequency of the higher optical bands varies from 1690 to 1957 cm{sup −1} for CNT(6,6) while it is 596 to 658 cm{sup −1} for SiNT.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

First-principles calculations of lattice dynamics and thermodynamic properties for Yb 14 MnSb 11

Systematic first-principles calculations were performed to study the lattice dynamics of Yb 14 MnSb 11 and hence to obtain a wide range of its thermodynamic properties at high temperatures. Here, the calculated results were analyzed in terms of the lattice contribution and the electronic contribution, together with a comparison with a collection of experimental thermochemical data. At 0 K, the electronic density of states showed the typical feature of a p-type semiconductor—a small amount of unoccupied electronic states exclusively made of the major spin by a range of ~0.6 eV above the Fermi energy. It showed that the Mn atom had a ferromagnetic spin moment of ~4 μ B . Lastly, as a semiconductor, it was found that the electronic contribution to the heat capacity was substantial, with an electronic heat capacity coefficient of ~0.0006 J/mole-atom/K 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2018

First-Principles Study of Phase Stability of bcc XZn (X = Cu, Ag, and Au) Alloys

First-principles density-functional theory is used to study the phase stability/instability and anomalies in the formation of the high-temperature bcc phases of X Zn (X = Cu, Ag, and Au) alloys. Although from perhaps a naive point of view, their properties are expected to monotonically depend on the noble-metal (X) column position in the periodic table, this is not the case. For example, the middle-column AgZn alloy has a lower bcc order-disorder (critical) temperature than the CuZn and AuZn alloys above and below in the column. It is shown that this and other nonmonotonic behaviors can be explained in terms of a competition between atomic-size effects and X-atom d-orbital spatial extent. For example, charge-density studies and pair-potential modeling of X Zn alloys show that the effective Ag-Zn bond is significantly weaker than either the Cu-Zn or Au-Zn bond at their respective equilibrium lattice constants. Furthermore, we find that an increased atomic-core size effect initially weakens the X-Zn bonding as one goes from CuZn to AgZn, but then the larger d-orbital spatial extent for higher principal quantum numbers becomes a more dominant effect and increases the bonding from AgZn to AuZn. In conclusion, this study is focused on the highly symmetric cubic high-temperature phases, where relative bond-strength magnitudes should be far more important than any bond-directionality effects; the lattice parameters, bulk moduli, elastic constants, Debye temperatures, heats of formation, and order-disorder temperatures for the bcc phases of the three X Zn alloys are calculated and compared with experiment.

36 MATERIALS SCIENCE↗
DOE OSTI2019

First-principles study of mechanical and electronic properties of bent monolayer transition metal dichalcogenides

The mechanical and electronic properties of transition metal dichalcogenide (TMD) monolayers corresponding to transition groups IV, VI, and X are explored under mechanical bending from first principles calculations using the strongly constrained and appropriately normed (SCAN) meta-GGA. SCAN provides an accurate description of the phase stability of the TMD monolayers. Our calculated lattice parameters and other structural parameters agree well with experiment. We find that bending stiffness (or flexural rigidity) increases as the transition metal group goes from IV to X to VI, with the exception of PdTe 2 . Variation in mechanical properties (local strain, physical thickness) and electronic properties (local charge density, band structure) with bending curvature is discussed. The local strain profile of these TMD monolayers under mechanical bending is highly nonuniform. The mechanical bending tunes not only the thickness of the TMD monolayers, but also the local charge distribution as well as the band structures, adding more functionalization options to these materials.

36 MATERIALS SCIENCE↗