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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 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Finite-size subthermal regime in disordered SU ( N ) -symmetric Heisenberg chains

SU⁡(N) symmetry is incompatible with the many-body localized (MBL) phase, even when strong disorder is present. However, recent studies have shown that finite-size SU⁡(2) systems exhibit nonergodic, subthermal behavior, characterized by the breakdown of the eigenstate thermalization hypothesis, and by the excited eigenstates entanglement entropy that is intermediate between area and volume law. In this paper, we extend previous studies of the SU⁡(2)-symmetric disordered Heisenberg model to larger systems, using the time-dependent density matrix renormalization group (tDMRG) method. We simulate quench dynamics from weakly entangled initial states up to long times, finding robust subthermal behavior at stronger disorder. Although we find an increased tendency towards thermalization at larger system sizes, the subthermal regime persists at intermediate time scales, nevertheless, and therefore should be accessible experimentally. At weaker disorder, we observe signatures of thermalization; however, entanglement entropy exhibits slow sublinear growth, in contrast to conventional thermalizing systems. Furthermore, we study dynamics of the SU⁡(3)-symmetric disordered Heisenberg model. Similarly, strong disorder drives the system into subthermal regime, albeit thermalizing phase is broader compared to the SU⁡(2) case. Finally, our findings demonstrate the robustness of the subthermal regime in spin chains with non-Abelian continuous symmetry, and are consistent with eventual thermalization at large system sizes and long time scales, suggested by previous studies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Gas–Phase Synthesis of 3–Vinylcyclopropene via the Crossed Beam Reaction of the Methylidyne Radical (CH; X 2 Π) with 1,3–Butadiene (CH 2 CHCHCH 2 ; X 1 Ag)

The crossed molecular beam reactions of the methylidyne radical (CH; X 2 Π) with 1,3-butadiene (CH 2 CHCHCH 2 ; X 1 Ag) along with their (partially) deuterated counterparts were performed at collision energies of 20.8 kJ mol -1 under single collision conditions. Combining our laboratory data with ab initio calculations, we reveal that the methylidyne radical may add barrierlessly to the terminal carbon atom and/or carbon-carbon double bond of 1,3-butadiene, leading to doublet C 5 H 7 intermediates with life times longer than the rotation periods. Furthermore, these collision complexes undergo non-statistical unimolecular decomposition through hydrogen atom emission yielding the cyclic cis- and trans-3-vinyl-cyclopropene products with reaction exoergicities of 119±42 kJ mol -1 . Since this reaction is barrierless, exoergic, and all transition states are located below the energy of the separated reactants, these cyclic C 5 H 6 products are predicted to be accessed even in low-temperature environments, such as in hydrocarbon-rich atmospheres of planets and cold molecular clouds such as TMC-1.

3-vinylcyclopropene↗
DOE OSTI2018

A detailed examination of laser-ion acceleration mechanisms in the relativistic transparency regime using tracers

Here we present a particle-in-cell study of linearly polarized laser-ion acceleration systems, in which we use both two-dimensional (2D) and three-dimensional (3D) simulations to characterize the ion acceleration mechanisms in targets which become transparent to the laser pulse during irradiation. First, we perform a target length scan to optimize the peak ion energies in both 2D and 3D, and the predictive capabilities of 2D simulations are discussed. Tracer analysis allows us to isolate the acceleration into stages of target normal sheath acceleration (TNSA), hole boring (HB), and break-out afterburner (BOA) acceleration, which vary in effectiveness based on the simulation parameters. The thinnest targets reveal that enhanced TNSA is responsible for accelerating the most energetic ions, whereas the thickest targets have ions undergoing successive phases of HB and TNSA (in 2D) or BOA and TNSA (in 3D); HB is not observed to be a dominant acceleration mechanism in the 3D simulations. It is in the intermediate optimal regime, both when the laser breaks through the target with appreciable amplitude and when there is enough plasma to form a sustained high density flow, that BOA is most effective and is responsible for the most energetic ions. Eliminating the transverse laser spot size effects by performing a plane wave simulation, we can isolate with greater confidence the underlying physics behind the ion dynamics we observe. Specifically, supplemented by wavelet and FFT analyses, we match the post-transparency BOA acceleration with a wave-particle resonance with a high-amplitude low-frequency electrostatic wave of increasing phase velocity, consistent with that predicted by the Buneman instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1982

A spectral energetics analysis of atmospheric blocking

The spectral energetics of two blocking case studies from the winter of 1978-79 are calculated. One case occurred over the North Atlantic and the other over the North Pacific. The temporal evolution and geographical distribution of the energetics of the planetary-scale (zonal harmonic wavenumbers 1-4) and the intermediate-scale (wavenumbers 5-10) wave ensembles are examined to identify the physical mechanisms important in the blocking process. The Atlantic block was forced by the nonlinear interaction of intense baroclinic cyclone-scale waves with barotropic, ultralong waves. The Pacific blocking resulted from the baroclinic amplification of planetary-scale waves. In addition, at least two necessary conditions are required for the development of blocking. First, one of the forcing mechanisms must be in operation, and second, the phase of the antecedent planetary-scale waves must be such that the forcing mechanism can amplify them into a blocking pattern. In each of the cases studied, the blocking developed downstream of intense intermediate- or large-scale cyclogenesis.

Hansen, A. R.↗
DOE OSTI2022

Chiral spin symmetry and the QCD phase diagram

Lattice QCD simulations with chirally symmetric quarks have recently established approximate SU(2) CS and SU(2N F ) symmetries of the quantum effective action in a temperature range above the chiral crossover T ch , in which color-electric interactions between quarks dominate the dynamics. We show that such an intermediate temperature range between the chirally broken and plasma regimes is fully consistent with published screening mass spectra, which demonstrate the breakdown of thermal perturbation theory at the crossover between the partonic and the chiral spin symmetric regime at T s ~(2–3)T ch . From the known behavior of screening masses with baryon chemical potential, we deduce qualitatively how this chiral spin symmetric band extends into the QCD phase diagram. In the cold and dense region, we propose parity doubled baryons as possible candidates for chiral spin symmetric matter. This represents a special case of quarkyonic matter with confinement and restored chiral symmetry, and can smoothly transform to quark matter at sufficiently high densities. Finally, we discuss the potential of dilepton spectra to identify such matter forms.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Magnetic phase diagram of A 2 [FeCl 5 (H 2 O)] ( A = K, Rb, NH 4 )

Here, erythrosiderites with the formula A 2 FeX 5 ∙H 2 O, where A = Rb, K, and (NH 4 ) and X = Cl and Br are intriguing systems that possess various magnetic and electric phases, as well as multiferroic phases in which magnetism and ferroelectricity are coupled. In this report, we study the magnetic phase diagram of erythrosiderites as a function of superexchange interactions and magnetic anisotropies. To this end, we perform classical Monte Carlo simulations on magnetic Hamiltonians that contain five different superexchange interactions with single-ion anisotropies. Our phase diagram contains all magnetic ground states that have been experimentally observed in these materials. We argue that the ground states can be explained by varying the ratio of $\frac{J_4}{J_2}$. For $\frac{J_4}{J_2}$ > 0.95 a cycloidal spins structure is stabilized as observed in (NH 4 ) 2 FeCl 5 ∙H 2 O and otherwise a collinear spin structure is stabilized as observed in (K,Rb) 2 FeCl 5 ∙H 2 O. We also show that the difference in the single-ion anisotropy along a- and c- axes is essential to stabilize the intermediate state observed in (NH) 2 FeCl 5 ∙H 2 O.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural Phase Transformation in Strained Monolayer MoWSe 2 Alloy

Two-dimensional (2D) materials exhibit different mechanical properties from their bulk counterparts owing to their monolayer atomic thickness. In this paper, we have examined the mechanical behavior of 2D molybdenum tungsten diselenide (MoWSe 2 ) precipitation alloy grown using chemical vapor deposition and composed of numerous nanoscopic MoSe 2 and WSe 2 regions. Applying a bending strain blue-shifted the MoSe 2 and WSe 2 A 1g Raman modes with the stress concentrated near the precipitate interfaces predominantly affecting the WSe 2 modes. In situ local Raman measurements suggested that the crack propagated primarily thorough MoSe 2 -rich regions in the monolayer alloy. Molecular dynamics (MD) simulations were performed to study crack propagation in an MoSe 2 monolayer containing nanoscopic WSe 2 regions akin to the experiment. Raman spectra calculated from MD trajectories of crack propagation confirmed the emergence of intermediate peaks in the strained monolayer alloy, mirroring experimental results. The simulations revealed that the stress buildup around the crack tip caused an irreversible structural transformation from the 2H to 1T phase both in the MoSe 2 matrix and WSe 2 patches. This was corroborated by high-angle annular dark-field images. Finally, crack branching and subsequent healing of a crack branch were also observed in WSe 2 , indicating the increased toughness and crack propagation resistance of the alloyed 2D MoWSe 2 over the unalloyed counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2015

High‐resolution experiments on chemical oxidation of DNAPL in variable‐aperture fractures

Abstract Chemical oxidation of dense nonaqueous‐phase liquids (DNAPLs) by permanganate has emerged as an effective remediation strategy in fractured rock. We present high‐resolution experimental investigations in transparent analog variable‐aperture fractures to improve understanding of chemical oxidation of residual entrapped trichloroethylene (TCE) in fractures. Four experiments were performed with different permanganate concentrations, flow rates, and initial TCE phase geometry. The initial aperture field and evolving entrapped‐phase geometry were quantified for each experiment. The integrated mass transfer rate from the TCE phase for all experiments exhibited three time regimes: an early‐time regime with slower mass transfer rates limited by low specific interfacial area; an intermediate‐time regime with higher mass transfer rates resulting from breakup of large TCE blobs, which greatly increases specific interfacial area; and a late‐time regime with low mass transfer rates due to the deposition of MnO 2 precipitates. In two experiments, mass balance analyses suggested that TCE mass removal rates exceeded the maximum upper bound mass removal rates derived by assuming that oxidation and dissolution are the only mechanisms for TCE mass removal. We propose incomplete oxidation by permanganate and TCE solubility enhancement by intermediate reaction products as potential mechanisms to explain this behavior. We also speculate that some intermediate reaction products with surfactant‐like properties may play a role in lowering the TCE‐water interfacial tension, thus causing breakup of large TCE blobs. Our quantitative experimental measurements will be useful in the context of developing accurate computational models for chemical oxidation of TCE in fractures.

Arshadi, Masoud↗
DOE OSTI2012

Mechanistic Investigation of Ethanol SCR of NOx over Ag/Al2O3

A 2 wt.% Ag/{gamma}-Al{sub 2}O{sub 3} catalyst was studied for the ethanol selective catalytic reduction of NO{sub x} from 200 to 550 C and space velocities between 30,000 h{sup -1} and 140,000 h{sup -1}. Peak NO{sub x} conversions reached 85% at 400 C, and an activation energy was determined to be 57 kJ/mol with a feed of ethanol to NO{sub x} or HC{sub 1}/NO{sub x} = 3. Up to 80% of the NO is oxidized to NO{sub 2} at 250 C, but overall NO{sub x} conversion is only 15%. Interestingly, ethanol oxidation occurs at much lower temperatures than NO{sub x} reduction; at 250 C, ethanol oxidation is 80% when flowing ethanol + NO + O{sub 2}. This increased reactivity, compared to only 15% when flowing only ethanol + O{sub 2}, combined with the observation that NO is not oxidized to NO{sub 2} in the absence of ethanol illustrates a synergistic relationship between the reactants. To further investigate this chemistry, a series of DRIFTS experiments were performed. To form nitrates/nitrites on the catalysts it was necessary to include ethanol in the feed with NO. These nitrates/nitrites were readily formed when flowing NO{sub 2} over the catalyst. It is proposed that ethanol adsorbs through an ethoxy-intermediate that results in atomic hydrogen on the surface. This hydrogen aids the release of NO{sub 2} from Ag to the gas-phase which, can be subsequently adsorbed at {gamma}-Al{sub 2}O{sub 3} sites away from Ag. The disappearance of these nitrates/nitrites at higher temperatures proceeds in parallel with the increase in NO{sub x} reduction reactivity between 300 and 350 C observed in the kinetic study. It is therefore proposed that the consumption of nitrates is involved in the rate determining step for this reaction.

08 HYDROGEN↗
DOE OSTI2017

Parallel Fokker–Planck-DSMC algorithm for rarefied gas flow simulation in complex domains at all Knudsen numbers

A major challenge for the conventional Direct Simulation Monte Carlo (DSMC) technique lies in the fact that its computational cost becomes prohibitive in the near continuum regime, where the Knudsen number (Kn)—characterizing the degree of rarefaction—becomes small. In contrast, the Fokker–Planck (FP) based particle Monte Carlo scheme allows for computationally efficient simulations of rarefied gas flows in the low and intermediate Kn regime. The Fokker–Planck collision operator—instead of performing binary collisions employed by the DSMC method—integrates continuous stochastic processes for the phase space evolution in time. This allows for time step and grid cell sizes larger than the respective collisional scales required by DSMC. Dynamically switching between the FP and the DSMC collision operators in each computational cell is the basis of the combined FP-DSMC method, which has been proven successful in simulating flows covering the whole Kn range. Until recently, this algorithm had only been applied to two-dimensional test cases. In this contribution, we present the first general purpose implementation of the combined FP-DSMC method. Utilizing both shared- and distributed-memory parallelization, this implementation provides the capability for simulations involving many particles and complex geometries by exploiting state of the art computer cluster technologies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Synthesis, Structure, and Electrochemical Performance of High Capacity Li 2 Cu 0.5 Ni 0.5 O 2 Cathodes

Orthorhombic Li 2 NiO 2 , Li 2 CuO 2 , and solid solutions thereof have been studied as potential cathode materials for lithium-ion batteries due to their high theoretical capacity and relatively low cost. While neither endmember shows good cycling stability, the intermediate composition, Li 2 Cu 0.5 Ni 0.5 O 2 , yields reasonably high reversible capacities. A new synthetic approach and detailed characterization of this phase and the parent Li 2 CuO 2 are presented. The cycle life of Li 2 Cu 0.5 Ni 0.5 O 2 is shown to depend critically on the voltage window. The formation of Cu 1+ at low voltage and oxygen evolution at high voltage limit the electrochemical reversibility. In situ X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and gas evolution measurements are used to follow the chemical and structural changes that occur as a function of cell voltage.

25 ENERGY STORAGE↗
DOE OSTI2022

Revealing an Intermediate Cu–O/OH Superstructure on Cu(110)

Identifying the atomic structure and formation mechanism of intermediates during chemical transformations is challenging because of their short-lived nature. With a combination of microscopic and spectroscopic measurements and first-principles calculations, herein we report the formation of a metastable intermediate Cu–O/OH superstructure during the reaction of hydrogen with oxygen-covered Cu(110). This superstructure resembles the parent c(6 × 2)-O phase and can be termed as c(6 × 2)-(4O+2OH) with OH groups occupying the missing Cu sites between isolated Cu atoms. Using atomistic calculations, we elucidate the reaction pathways leading to the c(6 × 2)-(4O+2OH) formation via both molecular and dissociative H 2 adsorption. In this work, the results demonstrate the complex surface dynamics resulting from the parallel reaction pathways and may open up the possibility of directing the reaction dynamics by deliberately manipulating transient surface structure and composition.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Production of bio-based lactones as monomers for a circular polymer economy

To create a circular plastics economy, new polymers are being developed that can be chemically recycled. Circular polyesters are of particular interest and to this end, lactones are ideal monomers. Here, this Review examines catalytic routes to convert diols, hydroxy acids, and dicarboxylic acids to lactones, focusing on the development of scalable, atom-economic, and energy-efficient conversions of bio-derived feedstocks. Free energy analysis is used to inform process choices, such as reactor type, reaction phase, and use of solvent. Catalyst design principles are summarized for both direct (bio-substrate to lactone) and indirect (bio-substrate to intermediate to lactone) routes. Finally, we summarize literature that shows that many lactone precursors are readily accessible from various metabolic and chemo-catalytic pathways. Transitioning to bio-based monomers offers an opportunity to reduce reliance on fossil carbon resources, but requires advanced catalytic processes informed by mechanistic insights.

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

Long-term relaxation of orientational disorder and structural modifications in molecular nitrogen at high pressure

Up to 17 GPa, the crystalline phases of N 2 are characterized by pronounced orientational disorder, whereas the higher-pressure phases of molecular N 2 are ordered. This raises the question about long-term relaxation of orientational disorder within the low- to intermediate-pressure regime. Here, in this study, this question is addressed by comparing synthetic with natural, chemically pure, solid N 2 that resides as inclusions in diamonds at 300 K for about 10 8 years at pressures up to 11 GPa. It is shown that disorder prevails at 8.7 GPa, 300 K, where both synthetic and natural N 2 assume the same structure. However, at 10.8 GPa, natural solid N 2 exhibits monoclinic distortion and partial orientational ordering of the molecules, both of which are not observed in synthetic material. This difference is interpreted as the result of long-term structural relaxation. The ordering mechanism is examined and placed into the context of the δ- to ε-N 2 transition. We present explanations for the absence of complete ordering of δ-N 2 .

Tschauner, Oliver↗
DOE OSTI2017

Demonstration of a quantum key distribution network in urban fibre-optic communication lines

We report the results of the implementation of a quantum key distribution (QKD) network using standard fibre communication lines in Moscow. The developed QKD network is based on the paradigm of trusted repeaters and allows a common secret key to be generated between users via an intermediate trusted node. The main feature of the network is the integration of the setups using two types of encoding, i.e. polarisation encoding and phase encoding. One of the possible applications of the developed QKD network is the continuous key renewal in existing symmetric encryption devices with a key refresh time of up to 14 s. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗