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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

State-to-state reaction dynamics of {sup 18}O+{sup 32}O{sub 2} studied by a time-dependent quantum wavepacket method

The title isotope exchange reaction was studied by converged time-dependent wave packet calculations, where an efficient 4th order split operator was applied to propagate the initial wave packet. State-to-state differential and integral cross sections up to the collision energy of 0.35 eV were obtained with {sup 32}O{sub 2} in the hypothetical j{sub 0} = 0 state. It is discovered that the differential cross sections are largely forward biased in the studied collision energy range, due to the fact that there is a considerable part of the reaction occurring with large impact parameter and short lifetime relative to the rotational period of the intermediate complex. The oscillations of the forward scattering amplitude as a function of collision energy, which result from coherent contribution of adjacent resonances, may be a sensitive probe for examining the quality of the underlying potential energy surface. A good agreement between the theoretical and recent experimental integral and differential cross sections at collision energy of 7.3 kcal/mol is obtained. However, the theoretical results predict slightly too much forward scattering and colder rotational distributions than the experimental observations at collision energy of 5.7 kcal/mol.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Selected-ion flow tube temperature-dependent measurements for the reactions of O{sub 2}{sup +} with N atoms and N{sub 2}{sup +} with O atoms

The temperature variation of rate constants has been measured for the gas phase reactions of the oxycation O{sub 2}{sup +} with N atoms and of N{sub 2}{sup +} with O atoms from 120 to 400 K using a variable temperature-selected ion flow tube. Measured room temperature rate constants, 0.75 × 10{sup −10} cm{sup 3} s{sup −1} (±30%) for O{sub 2}{sup +} with N and 1.4 × 10{sup −10} cm{sup 3} s{sup −1} (±30%) for N{sub 2}{sup +} with O, are in agreement with previously reported values. A temperature dependence of T{sup −0.7(±0.3)} is observed for the O{sub 2}{sup +} + N reaction; however, the N{sub 2}{sup +} + O reaction is found to be independent of temperature. Calculations at varying levels of theory were used in tandem with experiments to evaluate likely pathways in potential energy surfaces for the reactions of concern.

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

Time dependent Doppler shifts in high-order harmonic generation in intense laser interactions with solid density plasma and frequency chirped pulses

High order harmonic generation from solid targets is a compelling route to generating intense attosecond or even zeptosecond pulses. However, the effects of ion motion on the generation of harmonics have only recently started to be considered. Here, we study the effects of ion motion in harmonics production at ultrahigh laser intensities interacting with solid density plasma. Using particle-in-cell simulations, we find that there is an optimum density for harmonic production that depends on laser intensity, which scales linearly with a{sub 0} with no ion motion but with a reduced scaling if ion motion is included. We derive a scaling for this optimum density with ion motion and also find that the background ion motion induces Doppler red-shifts in the harmonic structures of the reflected pulse. The temporal structure of the Doppler shifts is correlated to the envelope of the incident laser pulse. We demonstrate that by introducing a frequency chirp in the incident pulse we are able to eliminate these Doppler shifts almost completely.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Sequential double excitations from linear-response time-dependent density functional theory

Traditional UV/vis and X-ray spectroscopies focus mainly on the study of excitations starting exclusively from electronic ground states. However there are many experiments where transitions from excited states, both absorption and emission, are probed. In this work we develop a formalism based on linear-response time-dependent density functional theory to investigate spectroscopic properties of excited states. We apply our model to study the excited-state absorption of a diplatinum(II) complex under X-rays, and transient vis/UV absorption of pyrene and azobenzene.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Perspective: Fundamental aspects of time-dependent density functional theory

In the thirty-two years since the birth of the foundational theorems, time-dependent density functional theory has had a tremendous impact on calculations of electronic spectra and dynamics in chemistry, biology, solid-state physics, and materials science. Alongside the wide-ranging applications, there has been much progress in understanding fundamental aspects of the functionals and the theory itself. This Perspective looks back to some of these developments, reports on some recent progress and current challenges for functionals, and speculates on future directions to improve the accuracy of approximations used in this relatively young theory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Simple concentration-dependent pair interaction model for large-scale simulations of Fe-Cr alloys

This work is motivated by the need for large-scale simulations to extract physical information on the iron-chromium system that is a binary model alloy for ferritic steels used or proposed in many nuclear applications. From first-principles calculations and the experimental critical temperature we build a new energetic rigid lattice model based on pair interactions with concentration and temperature dependence. Density functional theory calculations in both norm-conserving and projector augmented-wave approaches have been performed. A thorough comparison of these two different ab initio techniques leads to a robust parametrization of the Fe-Cr Hamiltonian. Mean-field approximations and Monte Carlo calculations are then used to account for temperature effects. The predictions of the model are in agreement with the most recent phase diagram at all temperatures and compositions. The solubility of Cr in Fe below 700 K remains in the range of about 6 to 12%. It reproduces the transition between the ordering and demixing tendency and the spinodal decomposition limits are also in agreement with the values given in the literature.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Calculation of the Effect of Random Superfluid Density on the Temperature Dependence of the Penetration Depth

Microscopic variations in composition or structure can lead to nanoscale inhomogeneity in superconducting properties such as the magnetic penetration depth, but measurements of these properties are usually made on longer length scales. We solve a generalized London equation with a non-uniform penetration depth {lambda}(r), obtaining an approximate solution for the disorder-averaged Meissner screening. We find that the effective penetration depth is different from the average penetration depth and is sensitive to the details of the disorder. These results indicate the need for caution when interpreting measurements of the penetration depth and its temperature dependence in systems which may be inhomogeneous.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Changing the Mechanism for CO 2 Hydrogenation Using Solvent-Dependent Thermodynamics

A critical scientific challenge for utilization of CO2 is the development of catalyst systems that do not depend upon expensive or environmentally unfriendly reagents, such as precious metals, strong organic bases, and organic solvents. We have used thermodynamic insights to predict and demonstrate that the HCoI(dmpe)2 catalyst system, previously described for use in organic solvents, can hydrogenate CO2 to formate in water with bicarbonate as the only added reagent. Replacing tetrahydrofuran as the solvent with water changes the mechanism for catalysis by altering the thermodynamics for hydride transfer to CO2 from a key dihydride intermediate. The need for a strong organic base was eliminated by performing catalysis in water due to the change in mechanism. These studies demonstrate that the solvent plays a pivotal role in determining the reaction thermodynamics and thereby catalytic mechanism and activity. The research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

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

Enhancing hydrogen‐dependent growth of and carbon dioxide fixation by Clostridium ljungdahlii through nitrate supplementation

Abstract Synthesis gas (syngas) fermentation via the Wood–Ljungdahl pathway is receiving growing attention as a possible platform for the fixation of and renewable production of fuels and chemicals. However, the pathway operates near the thermodynamic limit of life, resulting in minimal adenosine triphosphate (ATP) production and long doubling times. This calls into question the feasibility of producing high‐energy compounds at industrially relevant levels. In this study, we investigated the possibility of co‐utilizing nitrate as an inexpensive additional electron acceptor to enhance ATP production during ‐dependent growth of Clostridium ljungdahlii , Moorella thermoacetica , and Acetobacterium woodii . In contrast to other acetogens tested, growth rate and final biomass titer were improved for C. ljungdahlii growing on a mixture of and when supplemented with nitrate. Transcriptomic analysis, labeling, and an electron balance were used to understand how electron flux was partitioned between and nitrate. We further show that, with nitrate supplementation, the ATP/adenosine diphosphate (ADP) ratio and acetyl‐CoA pools were increased by fivefold and threefold, respectively, suggesting that this strategy could be useful for the production of ATP‐intensive heterologous products from acetyl‐CoA. Finally, we propose a pathway for enhanced ATP production from nitrate and use this as a basis to calculate theoretical yields for a variety of products. This study demonstrates a viable strategy for the decoupling of ATP production from carbon dioxide fixation, which will serve to significantly improve the fixation rate and the production metrics of other chemicals from and in this host.

Emerson, David F.↗
DOE OSTI2019

Modeling Magneto‐Photoabsorption Using Time‐Dependent Complex Generalized Hartree‐Fock

Abstract Magneto‐optical spectroscopy provides a glimpse into the hidden electronic structure of molecules and materials that normally cannot be resolved with regular optical absorption spectroscopy. The magneto‐optical signal is difficult to simulate computationally due to the necessity of modeling perturbations from both uniform magnetic and oscillating electric fields. In this work, we introduce a time‐dependent complex generalized Hartree‐Fock framework for calculating electronic magneto‐optical spectra of atomic and molecular systems. Complex‐valued gauge including atomic orbitals are used in a non‐perturbative treatment of the uniform magnetic field. The magneto‐photoabsorption spectra of atomic alkalis and aromatic systems are investigated with a focus on understanding how spectroscopic signals change in the presence of a uniform magnetic field.

Stetina, Torin F.↗
DOE OSTI2018

pH-Dependent Interactions between Keggin Heteropolyanions in Dilute Solutions: pH-Dependent Interactions between Keggin Heteropolyanions in Dilute Solutions

Previously, Keggin phosphotungstate heteropolyanions (HPA), PW 12 O 40 3– , are known to interact via short-range attraction and long-range repulsion (SALR) at moderate and high solution concentrations and low-pH (pH = 1). These interactions were identified through the observation of structure factor peaks in small-angle X-ray scattering (SAXS) data reported previously (J. Phys. Chem. C 2016, 120, 1317). Here we describe the interactions between the same heteropolyanions with extremely low concentrations at low (pH = 0) and high pH (pH = 4.7) conditions. The solution chemistry of Keggin heteropolyanions at these two pH values offers a unique way to change the charge of the cluster anions without changing the structure (from the perspective of SAXS) and, at the same time, changing the hydrogen bonding interactions. The structure factors obtained from concentration dependent SAXS data reveal that the change in pH changes the nature of SALR interactions, consistent with the presence of the plenary PW 12 O 40 3– anion at pH = 0 and the monovacant lacunary PW 11 O 39 7– anion at pH = 4.7. At the low pH condition, the attractive interactions dominate to provide a peak near Q = 0 Å –1 momentum transfer in the structure factors, whereas at pH = 4.7 the repulsive interactions dominate, thereby eliminating the peak near Q = 0 Å –1 . The experimental findings presented here are in agreement with the recent liquid state theory and computer simulation predictions on SALR interactions.

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

Regime dependence of ice cloud heterogeneity – a convective life‐cycle effect?

Cloud condensate varies on scales smaller than those typically resolved by global weather and climate models. In order to accurately predict the radiative and microphysical process rates representative of the entire model grid box, the effect of the subgrid‐scale heterogeneity of cloud must be taken into account. In this study, observed ice water content retrieved from A‐Train satellite observations is used to explore how spatial ice condensate variability, characterized by the fractional standard deviation (FSD, the standard deviation divided by the mean), varies with cloud regime. FSD is generally lower for overcast cloud scenes, but an additional predictor based on convective activity is needed to capture the high FSD associated with more turbulent clouds and reproduce the observed latitudinal and height variations of FSD. Convective clouds that extend only a few kilometres above the freezing level are likely to be smaller at an earlier stage in their life cycle, and actively growing with a higher FSD. In contrast, more mature clouds reaching the tropopause are larger and generally have a lower variability and smaller FSD. To capture this life‐cycle effect, a new parametrization is tested which uses the ratio of a model's convectively detrained condensate to the existing cloud condensate mass as a proxy for the cloud's convective life stage to highlight areas with enhanced condensate variability. The parametrization is scale‐adaptive, situation‐dependent and captures seasonally varying global patterns and the zonal mean vertical structure of observed ice condensate variability well. Ground‐based observations obtained from five Atmospheric Radiation Measurement sites provide independent confirmation that the parametrization satisfactorily captures condensate variability in high‐altitude ice clouds.

Ahlgrimm, Maike↗
DOE OSTI2018

Temperature Dependence of Uranium and Vanadium Adsorption on Amidoxime-Based Adsorbents in Natural Seawater

Recent advances in the development of amidoxime-based adsorbents have made it highly promising for seawater uranium extraction. However, there is a great need to understand the influence of temperature on the uranium sequestration performance of the adsorbents in natural seawater. Here in this work, the apparent enthalpy and entropy of the sorption of uranium (VI) and vanadium (V) with amidoxime-based adsorbents were determined in natural seawater tests at 8, 20, and 31 °C that cover a broad range of ambient seawater temperature. The sorption of U was highly endothermic, producing apparent enthalpies of 57 ± 6.0 and 59 ± 11 kJ mol -1 and apparent entropies of 314 ± 21 and 320 ± 36 J K-1 mol -1 , respectively, for two adsorbent formulations. In contrast, the sorption of V showed a much smaller temperature sensitivity, producing apparent enthalpies of 6.1 ± 5.9 and -11 ± 5.7 kJ mol -1 and apparent entropies of 164 ± 20 and 103 ± 19 J K -1 mol -1 , respectively. This new thermodynamic information suggests that amidoxime-based adsorbents will deliver significantly increased U adsorption capacities and improved selectivity in warmer waters. A separate field study of seawater uranium adsorption conducted in a warm seawater site (Miami, FL, USA) confirm the observed strong temperature effect on seawater uranium mining. Lastly, this strong temperature dependence demonstrates that the warmer the seawater where the amidoxime-based adsorbents are deployed the greater the yield for seawater uranium extraction.

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

Morphology-dependent phase transformation of y-Al2O3

The phase transformations of platelet- and rod-shaped ?-Al2O3 were investigated and compared to that of a commercial sample by XRD, BET surface area measurements, Transmission Electron Microscopy (TEM), solid state 27Al-NMR and ethanol temperature programmed desorption (TPD) after sequential annealing in air up to 1100 oC. After annealing at 1100 oC, commercial ?-Al2O3 mostly transformed into a-Al2O3 with drastic surface area reduction (from 200 m2/g to 25 m2/g). Interestingly, platelet- and rod-shaped ?-Al2O3 which showed exactly the same XRD patterns transformed into different phases upon the high temperature calcinations. Platelet-shaped ?-Al2O3 transformed into ?-phase while the rod-shaped ?-Al2O3 transformed into the d-phase and not to the a-polymorph. Both platelet- and rod-shaped aluminas retained significantly higher surface area (~60 m2/g) than the commercial one after the same treatment at 1100 oC. These results suggest that the phase transformation in ?-Al2O3 is strongly affected by not only the crystal structure of the starting material, but its morphology as well. Ethanol TPD from platelet- and rod-shaped alumina after 1100 oC annealing, showed significantly different desorption profiles which suggest different surface characteristics even though they had almost the same surface areas. These different phase transformations were also supported by solid state 27Al-NMR. After 1100oC annealing commercial alumina showed the presence of mostly octahedral Al3+ ions, but the other two samples displayed even higher number of tetrahedral Al3+ ions than the initial ?-Al2O3. Morphological changes were also confirmed by TEM. All these results consistently suggest the morphology-dependent phase transformations of ?-Al2O3 and the improved thermal stability of platelet- and rod- shaped ?-Al2O3 in comparison to a commercial ?-Al2O3. J.Sz. acknowledges the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences & Biosciences for the support of this work.

Lee, Jaekyoung↗
DOE OSTI2011

The Min Oscillator Uses MinD-Dependent Conformational Changes in MinE to Spatially Regulate Cytokinesis.

In E. coli, MinD recruits MinE to the membrane, leading to a coupled oscillation required for spatial regulation of the cytokinetic Z ring. How these proteins interact, however, is not clear because the MinD-binding regions of MinE are sequestered within a six-stranded {beta} sheet and masked by N-terminal helices. minE mutations that restore interaction between some MinD and MinE mutants were isolated. These mutations alter the MinE structure leading to release of the MinD-binding regions and the N-terminal helices that bind the membrane. Crystallization of MinD-MinE complexes revealed a four-stranded {beta} sheet MinE dimer with the released {beta} strands (MinD-binding regions) converted to {alpha} helices bound to MinD dimers. These results identify the MinD-dependent conformational changes in MinE that convert it from a latent to an active form and lead to a model of how MinE persists at the MinD-membrane surface.

59 BASIC BIOLOGICAL SCIENCES↗