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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi

High-entropy alloys, near-equiatomic solid solutions of five or more elements, represent a new strategy for the design of materials with properties superior to those of conventional alloys. However, their phase space remains constrained, with transition metal high-entropy alloys exhibiting only face- or body-centered cubic structures. Here, we report the high-pressure synthesis of a hexagonal close-packed phase of the prototypical high-entropy alloy CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed to suppression of the local magnetic moments, destabilizing the initial fcc structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the transformation is sluggish, occurring over a range of >40 GPa. However, the behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This demonstrates a means of tuning the structures and properties of high-entropy alloys in a manner not achievable by conventional processing techniques.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Crossing the great divide between single-crystal reactivity and actual catalyst selectivity with pressure transients

The quantitative prediction of catalyst selectivity is essential to the design of efficient catalytic processes and requires a detailed knowledge of the reaction mechanism and rate constants. Here we present a study that accurately predicts, using the kinetics and a mechanism derived from fundamental studies on single-crystal gold, the product distribution resulting from the complex reaction network that governs the oxidative coupling of methanol, catalysed by nanoporous gold between 360 and 425 K and for a vast range of pressures. Analysis of the transient product responses to micropulses of methanol over nanoporous gold yields a precise understanding of the marked dependence of selectivity on pressure, surface oxygen coverage and temperature. The key to a high selectivity for methyl formate is the surface lifetime and abundance of the methoxy. In conclusion, this successful microkinetic modelling of catalytic reactions across a wide set of reaction conditions is broadly applicable to predicting catalytic selectivity and provides a pathway to designing more efficient catalytic processes.

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

High-pressure polymorphism in L -threonine between ambient pressure and 22 GPa

The crystal structure of L -threonine has been studied to a maximum pressure of 22.3 GPa using single-crystal X-ray and neutron powder diffraction. The data have been interpreted in the light of previous Raman spectroscopic data by Holanda et al. (J. Mol. Struct. (2015), 1092, 160–165) in which it is suggested that three phase transitions occur at ca. 2 GPa, between 8.2 and 9.2 GPa and between 14.0 and 15.5 GPa. In the first two of these transitions the crystal retains its P2 1 2 1 2 1 symmetry, in the third, although the unit cell dimensions are similar either side of the transition, the space group symmetry drops to P2 1 . The ambient pressure form is labelled phase I, with the successive high-pressure forms designated I', II and III, respectively. Phases I and I' are very similar, the transition being manifested by a slight rotation of the carboxylate group. Phase II, which was found to form between 8.5 and 9.2 GPa, follows the gradual transformation of a long-range electrostatic contact becoming a hydrogen bond between 2.0 and 8.5 GPa, so that the transformation reflects a change in the way the structure accommodates compression rather than a gross change of structure. Phase III, which was found to form above 18.2 GPa in this work, is characterised by the bifurcation of a hydroxyl group in half of the molecules in the unit cell. Density functional theory (DFT) geometry optimisations were used to validate high-pressure structural models and PIXEL crystal lattice and intermolecular interaction energies are used to explain phase stabilities in terms of the intermolecular interactions.

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

Enhanced Néel temperature in EuSnP under pressure

The high-pressure single crystal X-ray diffraction results for EuSnP are reported with no structural phase transition below ∼6.2 GPa. Temperature-dependent resistivity measurements up to 2.15 GPa indicate that the antiferromagnetic transition temperature ( T N ) is significantly enhanced under pressure.

Gui, Xin↗
DOE OSTI2015

Measurement of vacuum pressure with a magneto-optical trap: A pressure-rise method

The lifetime of an atom trap is often limited by the presence of residual background gases in the vacuum chamber. This leads to the lifetime being inversely proportional to the pressure. Here, we use this dependence to estimate the pressure and to obtain pressure rate-of-rise curves, which are commonly used in vacuum science to evaluate the performance of a system. We observe different rates of pressure increase in response to different levels of outgassing in our system. Therefore, we suggest that this is a sensitive method which will find useful applications in cold atom systems, in particular, where the inclusion of a standard vacuum gauge is impractical.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Design and construction of a cascading pressure reactor prototype for solar-thermochemical hydrogen production

Recent work regarding the efficiency maximization for solar thermochemical fuel production in two step cycles has led to the design of a new type of reactor—the cascading pressure reactor—in which the thermal reduction step of the cycle is completed in multiple stages, at successively lower pressures. This approach enables lower thermal reduction pressures than in single-staged reactors, and decreases required pump work, leading to increased solar to fuel efficiencies. In this work we report on the design and construction of a prototype cascading pressure reactor and testing of some of the key components. We specifically focus on the technical challenges particular to the design, and their solutions.

14 SOLAR ENERGY↗
DOE OSTI2016

The effect of sheared toroidal rotation on pressure driven magnetic islands in toroidal plasmas

The impact of sheared toroidal rotation on the evolution of pressure driven magnetic islands in tokamak plasmas is investigated using a resistive magnetohydrodynamics model augmented by a neoclassical Ohm’s law. Particular attention is paid to the asymptotic matching data as the Mercier indices are altered in the presence of sheared flow. Analysis of the nonlinear island Grad- Shafranov equation shows that sheared flows tend to amplify the stabilizing pressure/curvature contribution to pressure driven islands in toroidal tokamaks relative to the island bootstrap current contribution. Finally, as such, sheared toroidal rotation tends to reduce saturated magnetic island widths.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Modification of structural disorder by hydrostatic pressure in the superconducting cuprate YBa 2 Cu 3 O 6.73

Here, compelling efforts to improve the critical temperature (T c ) of superconductors have been made through high-pressure application. Understanding the underlying mechanism behind such improvements is critically important; however, much remains unclear. Here we studied ortho-III YBa 2 Cu 3 O 6.73 (YBCO) using x-ray scattering under hydrostatic pressure (HP) up to ~6.0GPa. We found the reinforced oxygen order of YBCO under HP, revealing an oxygen rearrangement in the Cu-O layer, which evidently shows the charge-transfer phenomenon between the CuO 2 plane and Cu-O layer. Concurrently, we also observed no disorder-pinned charge-density-wave signature in CuO 2 plane under HP. This indicates that the oxygen rearrangement modifies the quenched disorder state in the CuO 2 plane. Using these results, we appropriately explain why pressure condition can achieve higher T c compared with the optimal T c under ambient pressure in YBa 2 Cu 3 O 6+x . As an implication of these results, finally we have discussed that the change in disorder could make it easier for YBa 2 Cu 3 O 6+x to undergo a transition to the nematic order under an external magnetic field.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Pressure-Resistant Intermediate Valence in the Kondo Insulator SmB 6

Resonant x-ray emission spectroscopy was used to determine the pressure dependence of the f-electron occupancy in the Kondo insulator SmB 6 . Applied pressure reduces the f occupancy, but surprisingly, the material maintains a significant divalent character up to a pressure of at least 35 GPa. Thus, the closure of the resistive activation energy gap and onset of magnetic order are not driven by stabilization of an integer valent state. In conclusion, over the entire pressure range, the material maintains a remarkably stable intermediate valence that can in principle support a nontrivial band structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗