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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 OSTI2020

First Principle Investigation of Electronic, Transport, and Bulk Properties of Zinc-Blende Magnesium Sulfide

We have studied electronic, structural, and transport properties of zinc-blende magnesium sulfide (zb-MgS). We employed a local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO) method. Our computational method is able to reach the ground state of a material, as dictated by the second theorem of density functional theory (DFT). Consequently, our findings have the physical content of DFT and agree with available, corresponding experimental ones. The calculated band gap of zb-MgS, a direct gap equal to 4.43 eV, obtained at the experimental lattice constant of 5.620 Å, completely agrees with the experimental band gap of 4.45 ± 0.2 eV. We also report total (DOS) and partial (pDOS) densities of states, electron and hole effective masses, the equilibrium lattice constant, and the bulk modulus. The calculated pDOS also agree with the experiment for the description of the states at the top and the bottom of the valence and conduction bands, respectively.

42 ENGINEERING↗
NASA NTRS2019

A Communication Channel Density Estimating Generative Adversarial Network

Autoencoder-based communication systems use neural network channel models to backwardly propagate message reconstruction error gradients across an approximation of the physical communication channel. In this work, we develop and test a new generative adversarial network (GAN) architecture for the purpose of training a stochastic channel approximating neural network. In previous research, investigators have focused on additive white Gaussian noise (AWGN) channels and/or simplified Rayleigh fading channels, both of which are linear and have well defined analytic solutions. Given that training a neural network is computationally expensive, channel approximation networks— and more generally the autoencoder systems—should be evaluated in communication environments that are traditionally difficult. To that end, our investigation focuses on channels that contain a combination of non-linear amplifier distortion, pulse shape filtering, intersymbol interference, frequency-dependent group delay, multipath, and non-Gaussian statistics. Each of our models are trained without any prior knowledge of the channel. We show that the trained models have learned to generalize over an arbitrary amplifier drive level and constellation alphabet. We demonstrate the versatility of our GAN architecture by comparing the marginal probability density function of several channel simulations with that of their corresponding neural network approximations

Smith, Aaron↗
DOE OSTI2015

On the influence of polarization effects in predicting the interfacial structure and capacitance of graphene-like electrodes in ionic liquids

The electric double layer (C{sub D}) and electrode quantum (C{sub Q}) capacitances of graphene-based supercapacitors are investigated using a combined molecular dynamics and density functional theory approach. In particular, we compare an approach that includes electronic polarization to one that is polarization-free by evaluating both C{sub D} and C{sub Q} using [EMIM][BF{sub 4}] ionic liquid as a model electrolyte. Our results indicate that the inclusion of polarization effects can yield higher C{sub D} values—in this study by up to 40% around ±2 V—which we attribute primarily to the presence of charge smearing at the electrode-electrolyte interface. On the other hand, we find that the polarization-induced distortion of the electronic structure of graphene does not noticeably alter the predicted C{sub Q}. Our analysis suggests that an accurate description of the spatial charge distribution at the graphene interface due to polarization is necessary to improve our predictive capabilities, though more notably for C{sub D}. However, the conventional polarization-free approximation can serve as an efficient tool to study trends associated with both the C{sub Q} and C{sub D} at the interface of various graphene-like materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Does DFT+U mimic hybrid density functionals?

Here, this work examines the question of how a Hubbard U correction to a local exchange–correlation functional compares with adding Hartree–Fock exchange to a local functional for both solid-state and molecular properties. We compute a solid-state property, namely the band gap, and thermochemical molecular properties, in particular main-group bond energies, transition-metal–ligand bond energies, and barrier heights, to elucidate whether the DFT+U method mimics hybrid DFT. We find that a calculation with a Hubbard U correction may or may not mimic a hybrid functional – depending on the atom, the subshell, and the property to which it is applied. For band gaps, we find that adding a Hubbard U correction to the valence d orbitals of transition metals increases the band gap, which thereby gets closer to the experimental value, while adding a Hubbard U correction to valence s or p orbitals of main group elements need not always increase the band gap. For molecular thermochemistry, we find that adding a Hubbard U correction to a local density functional need not have the same effect as adding Hartree–Fock exchange to a local density functional. For example when compared to a local DFT, hybrid DFT increases the barrier height in all cases, but DFT+U does not always increase the barrier height. For the band gaps of transition metal monoxides, the Hubbard-corrected results lowered the mean errors significantly and were comparable to what could be achieved with a much more expensive hybrid functional, but for reaction barrier heights and bond energies the Hubbard correction can lower the mean error by approximately a kcal/mol. As part of the analysis, we also compare VASP and Gaussian 09 calculations for the same density functional.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Correlations in rare-earth transition-metal permanent magnets

It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition-metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo{sub 5}. On an independent-electron level, the use of a single Slater determinant with broken spin symmetry introduces Hund's rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDA + U approximations to DFT. By contrast, rare-earth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. Analyzing DFT as a Legendre transformation and using Bethe's crystal-field theory, we show that the corresponding density functionals are very different from familiar LSDA-type expressions and outline the effect of spin-charge separation on the magnetocrystalline anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Quantum chemical study of the structures and dynamic behavior of tricarbonyl complexes of Group 6 metals (Cr, Mo, W) with polyaromatic hydrocarbons using the density functional theory

The quantum chemical study of the mechanism was performed for tricarbonyl η{sup 6}-complexes of coronene I-M and kekulene II-M (M = Cr, Mo, W) by the density functional method. The activation barriers of η{sup 6},η{sup 6}-interring haptotropic rearrangements (IHR), being the migration of the metaltricarbonyl group M(CO){sub 3} from one six-membered aromatic ring to another, were determined. The processes of η{sup 6},η{sup 6}-IHR in the metal tricarbonyl complexes with relatively high polycyclic aromatic hydrocarbons (PAH) I and II occur with close energy barriers (ΔG{sup ≠} ≈ 20—25 kcal mol{sup –1}), which are lower than the barriers (ΔG{sup ≠} ~ 30 kcal mol{sup –1}) of similar transformations measured or calculated earlier for the chromium tricarbonyl complexes of naphthalene and its derivatives and other PAH. For the molybdenum tricarbonyl complexes the activation barriers of η{sup 6},η{sup 6}-IHR decrease additionally by ~ 5 kcal mol{sup –1} compared to those for the chromium tricarbonyl complexes, whereas for the tungsten tricarbonyl complexes they increase again and become approximately equal to the activation barriers of similar chromium tricarbonyl complexes. All stationary states on the potential energy surface determining the mechanism of η{sup 6},η{sup 6}-IHR are characterized by a decrease in hapticity compared to the initial and final complexes.

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

Density functional theory of material design: fundamentals and applications—II

Abstract This is the second and the final part of the review on density functional theory (DFT), referred to as DFT-II. In the first review, DFT-I, we have discussed wavefunction-based methods, their complexity, and basics of density functional theory. In DFT-II, we focus on fundamentals of DFT and their implications for the betterment of the theory. We start our presentation with the exact DFT results followed by the concept of exchange-correlation (xc) or Fermi-Coulomb hole and its relationship with xc energy functional. We also provide the exact conditions for the xc-hole, xc-energy and xc-potential along with their physical interpretation. Next, we describe the extension of DFT for non-integer number of electrons, the piecewise linearity of total energy and discontinuity of chemical potential at integer particle numbers, and derivative discontinuity of the xc potential, which has consequences on fundamental gap of solids. After that, we present how one obtains more accurate xc energy functionals by going beyond the LDA. We discuss the gradient expansion approximation (GEA), generalized gradient approximation (GGA), and hybrid functional approaches to designing better xc energy functionals that give accurate total energies. However, these functionals fail to predict properties like the ionization potential and the band gap. Thus, we next describe different methods of modelling these potentials and results of their application for calculation of the band gaps of different solids to highlight accuracy of different xc potentials. Finally, we conclude with a glimpse on orbital-free density functional theory and the machine learning approach.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Applicability of the Strongly Constrained and Appropriately Normed Density Functional to Transition-Metal Magnetism

Here we find that the recently developed self-consistent and appropriately normed meta-generalized gradient approximation, which has been found to provide highly accurate results for many materials, is, however, not able to describe the stability and properties of phases of Fe important for steel. Finally, this is due to an overestimated tendency toward magnetism and exaggeration of magnetic energies, which we also find in other transition metals.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A New Fokker–Planck Approach for the Relaxation-driven Evolution of Galactic Nuclei

We present an approach for simulating the collisional evolution of spherical isotropic stellar systems based on the one-dimensional Fokker–Planck equation. A novel aspect is that we use the phase volume as the argument of the distribution function instead of the traditionally used energy, which facilitates the solution. The publicly available code PhaseFlow implements a high-accuracy finite-element method for the Fokker–Planck equation, and can handle multiple-component systems, optionally with the central black hole and taking into account loss-cone effects and star formation. We discuss the energy balance in the general setting, and in application to the Bahcall–Wolf cusp around a central black hole, for which we derive a perturbative solution. We stress that the cusp is not a steady-state structure, but rather evolves in amplitude while retaining an approximately ρ∝r{sup −7/4} density profile. Finally, we apply the method to the nuclear star cluster of the milky Way, and illustrate a possible evolutionary scenario in which a two-component system of lighter main-sequence stars and stellar-mass black holes develops a Bahcall–Wolf cusp in the heavier component and a weaker ρ∝r{sup −3/2} cusp in the lighter, visible component, over the period of several Gyr. The present-day density profile is consistent with the recently detected mild cusp inside the central parsec, and is weakly sensitive to initial conditions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Effective destruction of CO by cosmic rays: Implications for tracing H{sub 2} gas in the universe

We report on the effects of cosmic rays (CRs) on the abundance of CO in H{sub 2} clouds under conditions typical for star-forming galaxies in the universe. We discover that this most important molecule for tracing H{sub 2} gas is very effectively destroyed in ISM environments with CR energy densities U{sub CR}∼(50−10{sup 3})×U{sub CR,Gal}, a range expected in numerous star-forming systems throughout the universe. This density-dependent effect operates volumetrically rather than only on molecular cloud surfaces (i.e., unlike FUV radiation that also destroys CO), and is facilitated by (a) the direct destruction of CO by CRs and (b) a reaction channel activated by CR-produced He{sup +}. The effect we uncover is strong enough to render Milky-Way-type Giant Molecular Clouds very CO-poor (and thus CO-untraceable), even in ISM environments with rather modestly enhanced average CR energy densities of U{sub CR}∼(10−50)×U{sub CR,Gal}. We conclude that the CR-induced destruction of CO in molecular clouds, unhindered by dust absorption, is perhaps the single most important factor controlling the CO-visibility of molecular gas in vigorously star-forming galaxies. We anticipate that a second-order effect of this CO destruction mechanism will be to make the H{sub 2} distribution in the gas-rich disks of such galaxies appear much clumpier in CO J = 1–0, 2–1 line emission than it actually is. Finally we give an analytical approximation of the CO/H{sub 2} abundance ratio as a function of gas density and CR energy density for use in galaxy-size or cosmological hydrodynamical simulations, and propose some key observational tests.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Electronic, structural, and substrate effect properties of single-layer covalent organic frameworks

Recently synthesized two-dimensional covalent organic frameworks (COFs) exhibit high surface area, large pore size, and unique structural architectures, making them promising materials for various energy applications. Here, a total of nine COFs structures, including two deposited on a hexagonal boron nitride substrate, are investigated using density functional theory, quasi-particle many-body theory within the GW approximation, and an image charge model. The structures considered belong to two major families (thiophene-based COF-n (T-COF-n) and tetrakis (4-aminophenyl) porphyrin-x (TAPP-x)) differing from the presence of B—O or C=N linkers. While T-COF-n structures are shown to constitute planar networks, TAPP-x systems can display non-negligible corrugation due to the out-of-plane rotation of phenyl rings. We find that the electronic properties do not differ significantly when altering the chain molecules within each family. Many-body effects are shown to lead to large band-gap increase while the presence of the substrate yields appreciable reductions of the gaps, due to substrate polarization effects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Growth mechanism of atomic layer deposition of zinc oxide: A density functional theory approach

Atomic layer deposition of zinc oxide (ZnO) using diethylzinc (DEZ) and water is studied using density functional theory. The reaction pathways between the precursors and ZnO surface sites are discussed. Both reactions proceed by the formation of intermediate complexes on the surface. The Gibbs free energy of the formation of these complexes is positive at temperatures above ∼120 °C and ∼200 °C for DEZ and water half-reactions, respectively. Spectroscopic ellipsometry results show that the growth per cycle changes at approximately the same temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2012

First-principles study on stability, energy gaps, optical phonon and related parameters of In{sub 1-x-y}Al{sub x}Ga{sub y}As alloys

Based on the density functional theory as implemented in the Abinit code under the virtual crystal approximation, the lattice constant, bulk modulus, elastic constants, gap energies, electron effective mass, the dielectric constants and born effective charge in In{sub 1-x-y}Al{sub x}Ga{sub y}As have been calculated with both GGA and LDA in the range 0{<=}y{<=}0.9801. The optical and acoustical phonon frequencies, Froehlich coupling parameter, deformation energy and polaron effective mass are calculated and their dependence on the Ga content is examined. For AlAs, our results are in reasonable agreement with the known data in the literature; while for other contents our treatments are predictions. Highlights: Black-Right-Pointing-Pointer We predict phonon frequencies, Froehlich coupling and polaron parameters. Black-Right-Pointing-Pointer Elastic constants, gap energies, the dielectric constants and born effective charge are prediction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Investigating the role of water on CO2-Utica Shale interactions for carbon storage and shale gas extraction activities – Evidence for pore scale alterations

In this work, we probed the physical and chemical alteration of the carbonate-rich Utica Shale following CO 2 exposure when thin films of water were present at the shale surface. All reaction conditions were examined at 40 °C and CO 2 pressures up to 10.3 MPa for 14 days. CO 2 dissolution in the water layer at 2342 cm -1 and carbonate dissolution and precipitation at 1424, 874, and 712 cm -1 were discerned with in-situ Fourier Transform infrared spectroscopy (FT-IR). Significant etching and pitting from exposure to CO 2 and water were observed with feature relocation scanning electron microscopy (SEM). Application of the density function theory (DFT) for pore size analysis showed that micropores between approximately 0.9 and 2 nm disappeared while the mesopore volume increased after dissolution of carbonate. These results may provide new insights that carbonate rich shales such as the Utica may have a greater potential for alterations of pore space due to the reactivity of CO 2 which may affect 1) how CO 2 storage in shale formations plays a role in Carbon Capture and Storage (CCS) activities, 2) if CO 2 can be utilized as a potential fracturing agent, and 3) whether CO 2 is an effective fluid for enhanced hydrocarbon extraction.

04 OIL SHALES AND TAR SANDS↗
DOE OSTI2025

Why Are 5-Thioglycopyranosyl Donors More Axially Selective than their Glycopyranosyl Counterparts? A Low and Variable Temperature NMR Spectroscopy and Computational Study

5-Thioglycopyranosyl donors differ in reactivity and selectivity from simple glycopyranosyl donors. An extensive study has been conducted on the nature and stability of the reactive intermediates generated on the activation of per-O-acetyl and per-O-methyl 5-thioglucopyranosyl donors and the corresponding glucopyranosyl donors. Variable temperature nuclear magnetic resonance (NMR) studies with per-O-methylated or per-O-acetyl glycosyl sulfoxides and trichloroacetimidates on activation with trifluoromethanesulfonic anhydride or trimethylsilyl triflate are reported. These show that following initial adduct formation with the promoter conversion of the 5-thioglucopyranosyl donors to the 5-thioglucopyranosyl triflates requires higher temperatures than that of the glucopyranosyl donors to the glucopyranosyl triflates. It is demonstrated that neighboring group participation is a less important phenomenon for the peracetylated thioglucosyl donors than for the peracetylated glucosyl donors. A simple thiocarbenium ion was generated by protonation of 2,3-dihydro-4H-thiopyran at low temperature and characterized by NMR spectroscopy. However, the corresponding 5-thioglucopyranosyl thenium ions were not observed in any of the NMR studies of the 5-thiopyranosyl donors: the electron-withdrawing C–O bonds around the thiopyranoside core discourage thiocarbenium ion formation, just as they discourage oxocarbenium ion formation. Density functional theory (DFT) calculations reveal the tetrahydrothiopyranyl thiocarbenium ion to be approximately 2.5 kcal/mol lower in energy than the corresponding tetrahydropyranyl oxocarbenium ion relative to the corresponding covalent triflates. However, the computations reveal a 5.8 kcal/mol activation barrier for conversion of the tetrahydrothiopyranyl triflate to the thiocarbenium ion, while formation of the oxocarbenium ion–triflate ion pair from tetrahydropyranyl triflate requires only 2.6 kcal·mol –1 . Overall, the greater axial selectivity of 5-thioglycopyranosyl donors compared to analogous glycopyranosyl donors derives from (i) the lower kinetic reactivity necessitating higher reaction temperatures, (ii) the greater stability of the thiocarbenium ion over the oxocarbenium ion facilitating equilibration under thermodynamic conditions, (iii) the greater magnitude of the anomeric effect in the 5-thiosugars, and (iv) decreased neighboring group participation in the per-esterified 5-thiosugars.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Determination of a Density Functional Tight Binding Model with an Extended Basis Set and Three-Body Repulsion for Carbon Under Extreme Pressures and Temperatures

In this paper, we report here on development of a density functional tight binding (DFTB) simulation approach for carbon under extreme pressures and temperatures that includes an expanded basis set and an environmentally dependent repulsive energy. We find that including d-orbital interactions in the DFTB Hamiltonian improves determination of the electronic states at high pressure–temperature conditions, compared to standard DFTB implementations that utilize s- and p-orbitals only for carbon. We then determine a three-body repulsive energy through fitting to diamond, BC8, and simple cubic cold compression curve data, as well pressures from metallic liquid configurations from density functional theory (DFT) simulations. Our new model (DFTB-p3b) yields approximately 2 orders of magnitude increase in computational efficiency over standard DFT while retaining its accuracy for condensed phases of carbon under a wide range of conditions, including the metallic liquid phase at conditions up to 2000 GPa and 30 000 K. Finally, our results provide a straightforward method by which DFTB can be extended to studies of covalently bonded materials under extremely high pressures and temperatures such as the interiors of planets and other large celestial bodies.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Re-examining the silicon self-interstitial charge states and defect levels: A density functional theory and bounds analysis study

The self-interstitial atom (SIA) is one of two fundamental point defects in bulk Si. Isolated Si SIAs are extremely difficult to observe experimentally. Even at very low temperatures, they anneal before typical experiments can be performed. Given the challenges associated with experimental characterization, accurate theoretical calculations provide valuable information necessary to elucidate the properties of these defects. Previous studies have applied Kohn–Sham density functional theory (DFT) to the Si SIA, using either the local density approximation or the generalized gradient approximation to the exchange-correlation (XC) energy. The consensus of these studies indicates that a Si SIA may exist in five charge states ranging from -2 to +2 with the defect structure being dependent on the charge state. This study aims to re-examine the existence of these charge states in light of recently derived “approximate bounds” on the defect levels obtained from finite-size supercell calculations and new DFT calculations using both semi-local and hybrid XC approximations. We conclude that only the neutral and +2 charge states are directly supported by DFT as localized charge states of the Si SIA. Within the current accuracy of DFT, our results indicate that the +1 charge state likely consists of an electron in a conduction-band-like state that is coulombically bound to a +2 SIA. Furthermore, the -1 and -2 charge states likely consist of a neutral SIA with one and two additional electrons in the conduction band, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Comparison of GW band structure to semiempirical approach for an FeSe monolayer

We present the G 0 W 0 band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn-Sham density functional theory (DFT) with the Perdew, Burke, and Ernzerhof functional. We find the GW correction increases the bandwidth of the states forming the M pocket near the Fermi energy, while leaving the Γ pocket roughly unchanged. We then compare the G 0 W 0 quasiparticle band energies with the band structure from a simple empirical + A approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory. We show that this empirical correction succeeds in approximating the GW nonlocal and dynamical self-energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).

36 MATERIALS SCIENCE↗