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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Tuning nanoparticle size for enhanced functionality in perovskite thin films deposited by metal organic deposition

Because of pressing global environmental challenges, focus has been placed on materials for efficient energy use, and this has triggered the search for nanostructural modification methods to improve performance. Achieving a high density of tunable-sized second-phase nanoparticles while ensuring the matrix remains intact is a long-sought goal. In this paper, we present an effective, scalable method to achieve this goal using metal organic deposition in a perovskite system REBa 2 Cu 3 O 7 (rare earth (RE)) that enhances the superconducting properties to surpass that of previous achievements. We present two industrially compatible routes to tune the nanoparticle size by controlling diffusion during the nanoparticle formation stage by selecting the second-phase material and modulating the precursor composition spatially. Combining these routes leads to an extremely high density (8 × 10 22 m -3 ) of small nanoparticles (7 nm) that increase critical currents and reduce detrimental effects of thermal fluctuations at all magnetic field strengths and temperatures. This method can be directly applied to other perovskite materials where nanoparticle addition is beneficial.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Analytic techniques for solving the transport equations in electroweak baryogenesis

We develop an efficient method for solving transport equations, particularly in the context of electroweak baryogenesis. It provides fully-analytical results under mild approximations and can also test semi-analytical results, which are applicable in more general cases. Key elements of our method include the reduction of the second-order differential equations to first order, representing the set of coupled equations as a block matrix of the particle densities and their derivatives, identification of zero modes, and block decomposition of the matrix. We apply our method to calculate the baryon asymmetry of the Universe (BAU) in a Standard Model effective field theory framework of complex Yukawa couplings to determine the sensitivity of the resulting BAU to modifications of various model parameters and rates, and to estimate the effect of the commonly-used thin-wall approximation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Hybrid gausslet/Gaussian basis sets

We introduce hybrid gausslet/Gaussian basis sets, where a standard Gaussian basis is added to a gausslet basis in order to increase accuracy near the nuclei while keeping the spacing of the grid of gausslets relatively large. The Gaussians are orthogonalized to the gausslets, which are already orthonormal, and approximations are introduced to maintain the diagonal property of the two electron part of the Hamiltonian so that it continues to scale as the second power of the number of basis functions rather than the fourth. We introduce several corrections to the Hamiltonian designed to enforce certain exact properties, such as the values of certain two-electron integrals. We also introduce a simple universal energy correction that compensates for the incompleteness of the basis stemming from the electron–electron cusps based on the measured double occupancy of each basis function. We perform a number of Hartree Fock and full configuration interaction (full-CI) test calculations on two electron systems and Hartree Fock on a ten-atom hydrogen chain to benchmark these techniques. The inclusion of the cusp correction allows us to obtain complete basis set full-CI results for the two electron cases at the level of several micro-Hartrees, and we see similar apparent accuracy for Hartree Fock on the ten-atom hydrogen chain.

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

Path-integral Monte Carlo study of particles obeying quantum mechanics and classical statistics

Ultracold atomic systems have been of great research interest in the past, with more recent attention being paid to systems of mixed species. In this work, we carry out nonperturbative path-integral Monte Carlo (PIMC) simulations of $N$ distinguishable particles at finite temperature, which can be thought of as an ultracold atomic system containing $N$ distinct species. We use the PIMC approach to calculate thermodynamic properties of particles interacting via hard-sphere and hard-cavity potentials. The first problem we study is a two-particle system interacting via a hard-sphere and hard-cavity interaction in order to test the effectiveness of two approximations for the thermal density matrix corresponding to these potentials. We then apply the PIMC method to a system of many hard-sphere particles under periodic boundary conditions at varying temperature in order to calculate the energy per particle, pressure, and specific heat of the system. We examine how finite-size effects impact the results of PIMC simulations of hard-sphere particles and when the thermodynamic limit has been reached. Finally, our results provide microscopic benchmarks for a system containing distinguishable particles, which can be thought of as a limiting case for ultracold atomic systems of mixed species.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Computing the QRPA level density with the finite amplitude method

Here, we describe a new algorithm to calculate the vibrational nuclear level density of an atomic nucleus. Fictitious perturbation operators that probe the response of the system are generated by drawing their matrix elements from some probability distribution function. We use the Finite Amplitude Method to explicitly compute the response for each such sample. With the help of the Kernel Polynomial Method, we build an estimator of the vibrational level density and provide the upper bound of the relative error in the limit of infinitely many random samples. The new algorithm can give accurate estimates of the vibrational level density. Since it is based on drawing multiple samples of perturbation operators, its computational implementation is naturally parallel and scales like the number of available processing units.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Synthesis and characterization of aluminium–alumina micro- and nano-composites by spark plasma sintering

Graphical abstract: The evolution of microstructure by varying the particle size of reinforcement in the matrix employing spark plasma sintering has been demonstrated here in Al–Al{sub 2}O{sub 3} system. An emphasis has been laid on varying the reinforcement particle size and evaluating the microstructural morphologies and their implications on mechanical performance of the composites. Nanocomposites of 0.5, 1, 3, 5, 7 volume % alumina (average size < 50 nm) reinforced in aluminium matrix were fabricated by powder metallurgy route using spark plasma sintering technique technique at a temperature of 773 K and pressure of 50 MPa. Another set of specimens having composition 1, 5, 20 vol.% of alumina (average size ∼ 10 μm) had been fabricated to compare the physical as well as mechanical attributes of the microcomposite as well as the nanocomposites. These micro- and nano-composites have been characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy followed by density, microhardness and nanoindentation measurements. The alumina nanoparticles revealed an interface showing appreciable physical intimacy with the aluminium matrix compared to that of the alumina microparticles. The interfacial integrity in case of nanocomposites is better than in the microcomposite which has been studied using microscopic techniques. Spark plasma sintering imparts enhanced densification as well as matrix-reinforcement proximity which has been corroborated with the experimental results. - Highlights: • The Al–Al{sub 2}O{sub 3} micro- and nano-composites fabricated by spark plasma sintering. • Better matrix-reinforcement integrity in nanocomposites than microcomposites. • Spark plasma sintering method results in higher density and hardness values. • High density and hardness values of nanocomposites than microcomposites. • High dislocation density in spark plasma sintered Al–Al{sub 2}O{sub 3} composites. - Abstract: In the present study, an emphasis has been laid on evaluation of the microstructural morphologies and their implications on mechanical performance of the composites by varying the reinforcement particle size. Nanocomposites of 0.5, 1, 3, 5, 7 volume % alumina (average size < 50 nm) and microcomposites of 1, 5, 20 volume % of alumina (average size ∼ 10 μm) reinforced in aluminium matrix were fabricated by spark plasma sintering technique at a temperature of 773 K and pressure of 50 MPa. These micro- and nano-composites have been characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy followed by density, microhardness and nanoindentation hardness measurements. The alumina nanoparticles revealed appreciable physical intimacy with the aluminium matrix than that of alumina microparticles. The highest nanohardness recorded 0.85 GPa and 99% densification for 7 and 1 vol.% Al–Al{sub 2}O{sub 3} nancomposites respectively. Spark plasma sintering imparts enhanced densification and matrix-reinforcement proximity which have been corroborated with the experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Spin-State Ordering in Metal-Based Compounds Using the Localized Active Space Self-Consistent Field Method

Quantitatively accurate calculations for spin-state ordering in transition-metal complexes typically demand a robust multiconfigurational treatment. The poor scaling of such methods with increasing size makes them impractical for large, strongly correlated systems. Density matrix embedding theory (DMET) is a fragmentation approach that can be used to specifically address this challenge. The single-determinantal bath framework of DMET is applicable in many situations, but it has been shown to perform poorly for molecules characterized by strong correlation when a multiconfigurational self-consistent field solver is used. To ameliorate this problem, the localized active space self-consistent field (LASSCF) method was recently described. Here in this work, LASSCF is applied to predict spin-state energetics in mono- and di-iron systems, and we show that the model offers an accuracy equivalent to that of CASSCF but at a substantially lower computational cost. Performance as a function of basis set and active space is also examined.

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

Information depth in backscattered electron microscopy of nanoparticles within a solid matrix

Highlights: • Backscattered electron microscopy was used for reliable determination of nanoparticle density and sizes within Al alloys • Backscattered electron information depth was determined as a function of nanoparticle size from Monte Carlo simulations • Unlike transmission electron microscopy, the method is non-destructive and allows analyzing large sample areas • Good agreement with transmission electron microscopy data shows that our method is suitable for nanocomposite materials - Abstract: Measuring the dimensions and number density of nanoparticles dispersed in a solid matrix is usually accomplished via transmission electron microscopy (TEM) which suffers from high cost, low throughput, and small analytical volume. In comparison, scanning backscattered electron microscopy is inexpensive, requires little sample preparation, and allows for the analysis of large sample areas. However, the information depth is usually not known precisely and depends on several factors such as the composition of the nanoparticles and the matrix as well as the size of the nanoparticles, hindering the reconstruction of the actual size distribution and three-dimensional number density. Here we present a method to estimate the information depth for spherical nanoparticles of different sizes in order to accurately determine size distribution and number density. The approach is based on Monte Carlo simulation of electron trajectories in the material and analysis of the obtained backscattered electron signal-to-noise-ratio. Our experimental results are compared to those obtained via TEM and good agreement is demonstrated; this shows that TEM can be replaced by scanning electron microscopy for studying nanocomposites in many cases.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Tunable Magnetic and Optical Anisotropy in ZrO 2 ‐Co Vertically Aligned Nanocomposites

Abstract Metamaterials have gained great research interest in recent years owing to their potential for property tunability, multifunctionality, and property coupling. As a new group of self‐assembled thin films, vertically aligned nanocomposite (VAN)‐based hybrid metamaterials have been demonstrated with significant anisotropic physical properties and a broad range of property tailorability, such as optical anisotropy, magnetic anisotropy, hyperbolic dispersion, and enhanced second harmonic generation properties. Herein, self‐assembled ZrO 2 ‐Co nanocomposite films, with high epitaxial quality and ultra‐fine vertically aligned Co nanopillars (with an average diameter of ≈2 nm) embedded in a ZrO 2 matrix, are fabricated using a pulsed laser deposition (PLD) method. The Co pillar density can be effectively tuned by varying the Co concentration in the target, which results in tunable optical properties and magnetic properties. Specifically, a high saturation magnetization of 100 emu cm −3 , strong out‐of‐plane magnetic anisotropy and tailorable magnetization properties are achieved via tuning the Co nanopillar density. Coupled with hyperbolic dispersion of dielectric constant from 950 to 1500 nm in wavelength, plasmonic Co metal nanopillars, and the unique dielectric ZrO 2 matrix, this new nanoscale hybrid metamaterial shows great potential for future integrated optical and magnetic device designs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Mixed-State Entanglement from Local Randomized Measurements

In this work, we propose a method for detecting bipartite entanglement in a many-body mixed state based on estimating moments of the partially transposed density matrix. The estimates are obtained by performing local random measurements on the state, followed by postprocessing using the classical shadows framework. Our method can be applied to any quantum system with single-qubit control. We provide a detailed analysis of the required number of experimental runs, and demonstrate the protocol using existing experimental data [Brydges et al., Science 364, 260 (2019)].

1-dimensional spin chains↗
DOE OSTI2015

Shift-and-invert parallel spectral transformation eigensolver: Massively parallel performance for density-functional based tight-binding

The Shift-and-invert parallel spectral transformations (SIPs), a computational approach to solve sparse eigenvalue problems, is developed for massively parallel architectures with exceptional parallel scalability and robustness. The capabilities of SIPs are demonstrated by diagonalization of density-functional based tight-binding (DFTB) Hamiltonian and overlap matrices for single-wall metallic carbon nanotubes, diamond nanowires, and bulk diamond crystals. The largest (smallest) example studied is a 128,000 (2000) atom nanotube for which ~330,000 (~5600) eigenvalues and eigenfunctions are obtained in ~190 (~5) seconds when parallelized over 266,144 (16,384) Blue Gene/Q cores. Weak scaling and strong scaling of SIPs are analyzed and the performance of SIPs is compared with other novel methods. Different matrix ordering methods are investigated to reduce the cost of the factorization step, which dominates the time-to-solution at the strong scaling limit. As a result, a parallel implementation of assembling the density matrix from the distributed eigenvectors is demonstrated.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Slides for MS&T2022: Carbon-Carbon Composites from 3D Printed Preforms Graphitized by Electric Field Assisted Sintering

Formation of Carbon-Carbon (C-C) composite materials requires putting a carbon fiber preform through a cycle of densification, carbonization, and graphitization. This cycle is then repeated until the desired properties are achieved. The graphitization phase is particularly energy demanding as it involves heating the components to in excess of 2000°C. In traditional furnaces this is also quite time consuming as typical heating rates are low, and the actual heat treatment may be limited by component dimensions, as heat is conducted throughout the part primarily by thermal diffusion. Joule heating by means of Electric Field Assisted Sintering (EFAS) systems enables high heating rates and causes the part to be heated from the inside, which eliminates the thermal diffusion time constraint. In our process, C-C composites are formed by pyrolyzation of 3D-printed carbon fiber preforms, vacuum infiltration of naphthalene based mesophase pitch, and carbonization. The part is then graphitized in the EFAS system. By this method graphitization of the composite matrix was achieved in less than an hour. A part density of greater than 1.6g/cc (up to 1.9g/cc demonstrated) was achieved after only two rounds of infiltration and a single graphitization. Overall the process took less than a week which is significantly faster than traditional methods. Optical microscopy and XRD results are discussed to elucidate the internal porosity and characterize the degree of graphitization. Testing was performed showing the impact on fiber direction to electrical and thermal properties of the final part.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Thermalization and light cones in a model with weak integrability breaking

Here, we employ equation-of-motion techniques to study the nonequilibrium dynamics in a lattice model of weakly interacting spinless fermions. Our model provides a simple setting for analyzing the effects of weak integrability-breaking perturbations on the time evolution after a quantum quench. We establish the accuracy of the method by comparing results at short and intermediate times to time-dependent density matrix renormalization group computations. For sufficiently weak integrability-breaking interactions we always observe prethermalization plateaus , where local observables relax to nonthermal values at intermediate time scales. At later times a crossover towards thermal behavior sets in. We determine the associated time scale, which depends on the initial state, the band structure of the noninteracting theory, and the strength of the integrability-breaking perturbation. Our method allows us to analyze in some detail the spreading of correlations and in particular the structure of the associated light cones in our model. We find that the interior and exterior of the light cone are separated by an intermediate region, the temporal width of which appears to scale with a universal power law t 1/3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The effect of hydrogen bonding on torsional dynamics: A combined far-infrared jet and matrix isolation study of methanol dimer

The effect of strong intermolecular hydrogen bonding on torsional degrees of freedom is investigated by far-infrared absorption spectroscopy for different methanol dimer isotopologues isolated in supersonic jet expansions or embedded in inert neon matrices at low temperatures. For the vacuum-isolated and Ne-embedded methanol dimer, the hydrogen bond OH librational mode of the donor subunit is finally observed at ∼560 cm{sup −1}, blue-shifted by more than 300 cm{sup −1} relative to the OH torsional fundamental of the free methanol monomer. The OH torsional mode of the acceptor embedded in neon is observed at ∼286 cm{sup −1}. The experimental findings are held against harmonic predictions from local coupled-cluster methods with single and double excitations and a perturbative treatment of triple excitations [LCCSD(T)] and anharmonic. VPT2 corrections at canonical MP2 and density functional theory (DFT) levels in order to quantify the contribution of vibrational anharmonicity for this important class of intermolecular hydrogen bond vibrational motion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Self-consistent determination of the many-body state of ultracold bosonic atoms in a one-dimensional harmonic trap

We study zero-temperature quantum fluctuations in harmonically trapped one-dimensional interacting Bose gases,using the self-consistent multiconfigurational time-dependent Hartree method. We define phase fluctuations from the full single-particle density matrix by the spatial decay exponent of off-diagonal long-range order. In a regime of mesoscopic particle numbers and moderate contact couplings, we derive the spatial dependence of the amplitude of phase fluctuations, determined from the self-consistently derived shape of the field operator orbitals and Fock space orbital occupation amplitudes. It is shown that the phase fluctuations display a peak, which in turn corresponds to a dip of the first-order correlations in position space, akin to what has previously been obtained in the Tonks–Girardeau limit of very large interactions and low densities.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2018

Mars Optimal Aerobrake Maneuver Estimation

Mars science satellites often perform orbit changes to obtain different measurements, ground tracks or relay operations. Large reductions in semi-major axis and eccentricity can be done efficiently using the atmospheric drag, a.k.a aerobrake. Aerobraking is one of the most challenging planetary orbit maneuvers in terms of planning and operations. The most important consideration for aerobraking is maintaining the spacecraft's periapsis within an allocated atmospheric density corridor, which is accomplished by raising or lowering periapsis through one or a series of very small and short maneuvers. These maneuvers must be performed as efficiently as possible due to propellant constraints. Work herein details a fast and accurate method to calculate the required impulsive velocity changes in the orbit to guarantee that the spacecraft remains in a prescribed density corridor. The method makes use of the orbit's state transition matrix to map the solution space around the reference orbit. It evaluates the most efficient maneuver epochs to target a given periapsis change with a linear optimal control for single or multiple maneuvers. A fast calculation of the maneuver allows for a more comprehensive evaluation of the trade space, and the selected maneuver may be re-targeted later with a higher- fidelity model. Comparisons against fully propagated models and direct method optimizations demonstrate the new method's performance.

Sarli, Bruno V.↗
DOE OSTI2022

Electronic structure of strongly correlated systems: recent developments in multiconfiguration pair-density functional theory and multiconfiguration nonclassical-energy functional theory

Strong electron correlation plays an important role in transition-metal and heavy-metal chemistry, magnetic molecules, bond breaking, biradicals, excited states, and many functional materials, but it provides a significant challenge for modern electronic structure theory. The treatment of strongly correlated systems usually requires a multireference method to adequately describe spin densities and near-degeneracy correlation. However, quantitative computation of dynamic correlation with multireference wave functions is often difficult or impractical. Multiconfiguration pair-density functional theory (MC-PDFT) provides a way to blend multiconfiguration wave function theory and density functional theory to quantitatively treat both near-degeneracy correlation and dynamic correlation in strongly correlated systems; it is more affordable than multireference perturbation theory, multireference configuration interaction, or multireference coupled cluster theory and more accurate for many properties than Kohn–Sham density functional theory. This perspective article provides a brief introduction to strongly correlated systems and previously reviewed progress on MC-PDFT followed by a discussion of several recent developments and applications of MC-PDFT and related methods, including localized-active-space MC-PDFT, generalized active-space MC-PDFT, density-matrix-renormalization-group MC-PDFT, hybrid MC-PDFT, multistate MC-PDFT, spin–orbit coupling, analytic gradients, and dipole moments. We also review the more recently introduced multiconfiguration nonclassical-energy functional theory (MC-NEFT), which is like MC-PDFT but allows for other ingredients in the nonclassical-energy functional. We discuss two new kinds of MC-NEFT methods, namely multiconfiguration density coherence functional theory and machine-learned functionals.

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

Evaluation of two-particle properties within finite-temperature self-consistent one-particle Green’s function methods: Theory and application to GW and GF2

One-particle Green’s function methods can model molecular and solid spectra at zero or non-zero temperatures. One-particle Green’s functions directly provide electronic energies and one-particle properties, such as dipole moment. However, the evaluation of two-particle properties, such as $\langle$S 2 $\rangle$ and $\langle$N 2 $\rangle$, can be challenging because they require a solution of the computationally expensive Bethe–Salpeter equation to find two-particle Green’s functions. We demonstrate that the solution of the Bethe–Salpeter equation can be completely avoided. Applying the thermodynamic Hellmann–Feynman theorem to self-consistent one-particle Green’s function methods, we derive expressions for two-particle density matrices in a general case and provide explicit expressions for GF2 and GW methods. Such density matrices can be decomposed into an antisymmetrized product of correlated one-electron density matrices and the two-particle electronic cumulant of the density matrix. Cumulant expressions reveal a deviation from ensemble representability for GW, explaining its known deficiencies. We analyze the temperature dependence of $\langle$S 2 $\rangle$ and $\langle$N 2 $\rangle$ for a set of small closed-shell systems. Interestingly, both GF2 and GW show a non-zero spin contamination and a non-zero fluctuation of the number of particles for closed-shell systems at the zero-temperature limit.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗