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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Mixed excitonic nature in water-oxidized BiVO 4 surfaces with defects

BiVO 4 is a promising photocatalyst for efficient water oxidation, with surface reactivity determined by the structure of active catalytic sites. Surface oxidation in the presence of oxygen vacancies induces electron localization, suggesting an atomistic route to improve the charge transfer efficiency within the catalytic cycle. In this paper, we study the effect of oxygen vacancies on the electronic and optical properties at BiVO 4 surfaces upon water oxidation. We use density functional theory and many-body perturbation theory to explore the change in the electronic and quasiparticle energy levels and to evaluate the electron-hole coupling as a function of the underlying structure. Further we show that while the presence of defects alters the atomic structure and largely modifies the wave-function nature, leading to defect-localized states at the quasiparticle gap region, the optical excitations remain largely unchanged due to the substantial hybridization of defect and nondefect electron-hole transitions. Our findings suggest that defect-induced surface oxidation supports improved electron transport, both through bound and tunable electronic states and via a mixed nature of the optical transitions, expected to reduce electron-hole defect trapping.

36 MATERIALS SCIENCE↗
NASA NTRS1985

Analysis of the electron component of EAS at observational level 700 g x cm(-2) with a scale breaking interaction model and gammaisation hypothesis

Scale breaking model and gammaisation processes for high energies give a correct description of the longitudinal development of extensive air showers (E.A.S.). From the analysis of phenomenological characteristics of E.A.S. at Tien-Shan experiment, it follows that for energies near 10 to the 6 GeV the secondary particle multiplicity increases with energy faster than is predicted by the accepted scale breaking model.

Procureur, J.↗
NASA NTRS1987

Feasibility of hydroxyl concentration measurements by laser-saturated fluorescence in high-pressure flames

The effect of pressure on the laser-saturated fluorescence method for measuring OH concentration in high-pressure flames is studied using calculations for the burned-gas region of a stoichiometric H2-O2 flame at 2000 K. A numerical model of the excitation dynamics of OH is developed to explore the validity of the balanced cross-rate model at higher pressures. It is shown that depopulation of the laser-coupled levels is sensitive to collisions which depopulate v-double-prime (VDP) = 0 and to rate coefficients for rotational transfer in the ground state which are smaller than those in the excited state. In particular, it is shown that the depopulation of VDP = 0, and hence the laser-coupled levels, depends on the probability of electronic quenching to vibrational levels for which VDP is greater than 0 and vibrational relaxation to VDP = 0.

Carter, Campbell D.↗
NASA NTRS2004

Open-systems Architecture of a Standardized Command Interface Chip-set for Switching and Control of a Spacecraft Power Bus

This viewgraph presentation reviews the architecture of the The CIA-AlA chip-set is a set of mixed-signal ASICs that provide a flexible high level interface between the spacecraft's command and data handling (C&DH) electronics and lower level functions in other spacecraft subsystems. Due to the open-systems architecture of the chip-set including an embedded micro-controller a variety of applications are possible. The chip-set was developed for the missions to the outer planets. The chips were developed to provide a single solution for both the switching and regulation of a spacecraft power bus. The Open-Systems Architecture allows for other powerful applications.

power switch slice (PSS)↗
NASA NTRS1985

Cusp altitudinal electron temperature gradient - Dynamics Explorer 2 implications for heating mechanisms

Curtis et al. (1982) have shown that the levels of wave turbulence observed by Dynamics Explorer 2 (DE 2) are too low by several orders of magnitude to explain the high temperatures of the polar cusp ionosphere in terms of local deposition of energy. The low altitude plasma instrument (LAPI) showed high levels of superthermal electron fluxes. The present investigation has the objective to examine the arising questions more quantitatively by using the DE 2 electron temperature and superthermal electron flux measurements. It is shown that on the basis of DE 2 observations in the polar cusp, a consistent picture can be drawn regarding the ionospheric electron heating process. It is pointed out that the heating involves the generation of plasma waves by field-aligned electron beams of magnetosheath origin.

Curtis, S. A.↗
NASA NTRS1988

Space plasma contactor research, 1987

A simple model describing the process of electron collection from a low pressure ambient plasma in the absence of magnetic field and contactor velocity effects is presented. Experimental measurments of the plasma surrounding the contactor are used to demonstrate that a double-sheath generally develops and separates the ambient plasma from a higher density, anode plasma located adjacent to the contactor. Agreement between the predictions of the model and experimental measurements obtained at the electron collection current levels ranging to 1 A suggests the surface area at the ambient plasma boundary of the double-sheath is equal to the electron current being collected divided by the ambient plasma random electron current density; the surface area of the higher density anode plasma boundary of the double-sheath is equal to the ion current being emitted across this boundary divided by the ion current density required to sustain a stable sheath; and the voltage drop across the sheath is determined by the requirement that the ion and electron currents counterflowing across the boundaries be at space-charge limited levels. The efficiency of contactor operation is shown to improve when significant ionization and excitation is induced by electrons that stream from the ambient plasma through the double-sheath and collide with neutral atoms being supplied through the hollow cathode.

Wilbur, Paul J.↗
NASA NTRS1985

Textured carbon on copper: A novel surface with extremely low secondary electron emission characteristics

Experimentally determined values of true secondary electron emission and relative values of reflected primary electron yield for a range of primary electron beam energies and beam impingement angles are presented for a series of novel textured carbon surfaces on copper substrates. (All copper surfaces used in this study were oxygen-free, high-conductivity grade). The purpose of this investigation is to provide information necessary to develop high-efficiency multistage depressed collectors (MDC's) for microwave amplifier traveling-wave tubes (TWT's) for communications and aircraft applications. To attain the highest TWT signal quality and overall efficiency, the MDC electrode surface must have low secondary electron emission characteristics. While copper is the material most commonly used for MDC electrodes, it exhibits relatively high levels of secondary electron emission unless its surface is treated for emission control. The textured carbon surface on copper substrate described in this report is a particularly promising candidate for the MDC electrode application. Samples of textured carbon surfaces on copper substrates typical of three different levels of treatment are prepared and tested for this study. The materials are tested at primary electron beam energies of 200 to 2000 eV and at direct (0 deg) to near-grazing (85 deg) beam impingement angles. True secondary electron emission and relative reflected primary electron yield characteristics of the textured surfaces are compared with each other and with those of untreated copper. All the textured carbon surfaces on copper substrate tested exhibited sharply lower secondary electron emission characteristics than those of an untreated copper surface.

Curren, A. N.↗
NASA NTRS1986

Semiconductor superlattice photodetectors

Superlattice photodetectors were investigated. A few major physical processes in the quantum-well heterostructures related to the photon detection and electron conduction mechanisms, the field effect on the wave functions and the energy levels of the electrons, and the optical absorption with and without the photon assistance were studied.

Chuang, S. L.↗
NASA NTRS1994

Theoretical determination of chemical rate constants using novel time-dependent methods

The work completed within the grant period 10/1/91 through 12/31/93 falls primarily in the area of reaction dynamics using both quantum and classical mechanical methodologies. Essentially four projects have been completed and have been or are in preparation of being published. The majority of time was spent in the determination of reaction rate coefficients in the area of hydrocarbon fuel combustion reactions which are relevant to NASA's High Speed Research Program (HSRP). These reaction coefficients are important in the design of novel jet engines with low NOx emissions, which through a series of catalytic reactions contribute to the deterioration of the earth's ozone layer. A second area of research studied concerned the control of chemical reactivity using ultrashort (femtosecond) laser pulses. Recent advances in pulsed-laser technologies have opened up a vast new field to be investigated both experimentally and theoretically. The photodissociation of molecules adsorbed on surfaces using novel time-independent quantum mechanical methods was a third project. And finally, using state-of-the-art, high level ab initio electronic structure methods in conjunction with accurate quantum dynamical methods, the rovibrational energy levels of a triatomic molecule with two nonhydrogen atoms (HCN) were calculated to unprecedented levels of agreement between theory and experiment.

Dateo, Christopher E.↗
NASA NTRS2004

Inner Radiation Belt Representation of the Energetic Electron Environment: Model and Data Synthesis Using the Salammbo Radiation Belt Transport Code and Los Alamos Geosynchronous and GPS Energetic Particle Data

The highly energetic electron environment in the inner magnetosphere (GEO inward) has received a lot of research attention in resent years, as the dynamics of relativistic electron acceleration and transport are not yet fully understood. These electrons can cause deep dielectric charging in any space hardware in the MEO to GEO region. We use a new and novel approach to obtain a global representation of the inner magnetospheric energetic electron environment, which can reproduce the absolute environment (flux) for any spacecraft orbit in that region to within a factor of 2 for the energy range of 100 KeV to 5 MeV electrons, for any levels of magnetospheric activity. We combine the extensive set of inner magnetospheric energetic electron observations available at Los Alamos with the physics based Salammbo transport code, using the data assimilation technique of "nudging". This in effect input in-situ data into the code and allows the diffusion mechanisms in the code to interpolate the data into regions and times of no data availability. We present here details of the methods used, both in the data assimilation process and in the necessary inter-calibration of the input data used. We will present sample runs of the model/data code and compare the results to test spacecraft data not used in the data assimilation process.

Friedel, R. H. W.↗
NASA NTRS1993

Sampling and position effects in the Electronically Steered Thinned Array Radiometer (ESTAR)

A simple engineering level model of the Electronically Steered Thinned Array Radiometer (ESTAR) is developed that allows an identification of the major effects of the sampling process involved with this technique. It is shown that the ESTAR approach is sensitive to aliasing and has a highly non-uniform sensitivity profile. It is further shown that the ESTAR approach is strongly sensitive to position displacements of the low-density sampling antenna elements.

Katzberg, Stephen J.↗
NASA NTRS1984

Secondary electron emission characteristics of ion-textured copper and high-purity isotropic graphite surfaces

Experimentally determined values of true secondary electron emission and relative values of reflected primary electron yield for untreated and ion textured oxygen free high conductivity copper and untreated and ion textured high purity isotropic graphite surfaces are presented for a range of primary electron beam energies and beam impingement angles. This investigation was conducted to provide information that would improve the efficiency of multistage depressed collectors (MDC's) for microwave amplifier traveling wave tubes in space communications and aircraft applications. For high efficiency, MDC electrode surfaces must have low secondary electron emission characteristics. Although copper is a commonly used material for MDC electrodes, it exhibits relatively high levels of secondary electron emission if its surface is not treated for emission control. Recent studies demonstrated that high purity isotropic graphite is a promising material for MDC electrodes, particularly with ion textured surfaces. The materials were tested at primary electron beam energies of 200 to 2000 eV and at direct (0 deg) to near grazing (85 deg) beam impingement angles. True secondary electron emission and relative reflected primary electron yield characteristics of the ion textured surfaces were compared with each other and with those of untreated surfaces of the same materials. Both the untreated and ion textured graphite surfaces and the ion treated copper surface exhibited sharply reduced secondary electron emission characteristics relative to those of untreated copper. The ion treated graphite surface yielded the lowest emission levels.

Curren, A. N.↗
DOE OSTI2021

Significant disparity of non-basal dislocation activities in hot-rolled highly-textured Mg and Mg-3Al-1Zn alloy under tension

It is well-established that Mg-3Al-1Zn (AZ31) alloy exhibits much better tensile ductility than pure Mg. However, the underlying mechanisms for such difference still remain relatively unexplored at the dislocation level. In this work, we deformed hot-rolled highly-textured pure Mg and AZ31 samples with similar initial microstructures (i.e., grain size and texture) under tension along the rolling direction. Apparent shear banding was only observed in the pure Mg samples from the early stage of the deformation. Shear bands could act as easy paths for crack propagation, leading to low ductility in pure Mg. Apparent shear banding was not noted in deformed AZ31 samples. We then investigated the deformation mechanisms at the dislocation level using transmission electron microscopy. Systematic tilting experiments and statistical analyses of multiple grains at different strain levels revealed a significant disparity of non-basal dislocation activities between pure Mg and AZ31. For pure Mg, dislocations were activated since the early stage of plastic deformation. For AZ31, dislocations were mostly absent at all strain levels, even in the strain-to-failure samples. Non-basal dislocations, including prismatic and pyramidal dislocations, were observed. The promotion of the non-basal dislocation activities and the suppression of dislocations in AZ31 are expected to offer more sustainable hardening, which could elucidate the absence of apparent shear banding and much-improved ductility in AZ31 compared to pure Mg.

Zhao, Dexin↗
NASA NTRS1996

Electronic Structure of La(sub 1-x)Ca(sub x)CoO(sub 3) (x = 0, 0.5)

X-ray photoemission measurements of the core levels and valence electronic structure of LaCoO(sub 3) and La(sub 0.5)Ca0.5CoO(sub 3) high quality epitaxial films are presented. Shifts of the core levels and main valence band features are consistent with a doping-induced change in the chemical potential.

epitaxial films Fermi level↗
DOE OSTI2022

Electron-only reconnection and associated electron heating and acceleration in PHASMA

In this work, using incoherent Thomson scattering, electron heating and acceleration at the electron velocity distribution function (EVDF) level are investigated during electron-only reconnection in the PHAse Space MApping (PHASMA) facility. Reconnection arises during the merger of two kink-free flux ropes. Both push and pull type reconnection occur in a single discharge. Electron heating is localized around the separatrix, and the electron temperature increases continuously along the separatrix with distance from the X-line. The local measured gain in enthalpy flux is up to 70% of the incoming Poynting flux. Notably, non-Maxwellian EVDFs comprised of a warm bulk population and a cold beam are directly measured during the electron-only reconnection. The electron beam velocity is comparable to, and scales with, electron Alfvén speed, revealing the signature of electron acceleration caused by electron-only reconnection. The observation of oppositely directed electron beams on either side of the X-point provides “smoking-gun” evidence of the occurrence of electron-only reconnection in PHASMA. 2D particle-in-cell simulations agree well with the laboratory measurements. The measured conversion of Poynting flux into electron enthalpy is consistent with recent observations of electron-only reconnection in the magnetosheath [Phan et al., Nature 557, 202 (2018)] at similar dimensionless parameters as in the experiments. The laboratory measurements go beyond the magnetosheath observations by directly resolving the electron temperature gain.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Networked HIL Simulation System for Modeling Large-scale Power Systems

This paper presents a network hardware-in-the-loop (HIL) simulation system for modeling large-scale power systems. Researchers have developed many HIL test systems for power systems in recent years. Those test systems can model both microsecond-level dynamic responses of power electronic systems and millisecond-level transients of transmission and distribution grids. By integrating individual HIL test systems into a network of HIL test systems, we can create large-scale power grid digital twins with flexible structures at required modeling resolution that fits for a wide range of system operating conditions. This will not only significantly reduce the need for field tests when developing new technologies but also greatly shorten the model development cycle. In this paper, we present a networked OPAL-RT based HIL test system for developing transmission-distribution coordinative Volt-VAR regulation technologies as an example to illustrate system setups, communication requirements among different HIL test systems, and connection mechanisms. Impacts of communication delays, information exchange cycles, and computing delays are illustrated. Simulation results show that the performance of a networked HIL test system is satisfactory.

digital twin, distribution system, co-simulation, ↗
DOE OSTI2023

A unique van Hove singularity in kagome superconductor CsV 3-x Ta x Sb 5 with enhanced superconductivity

Van Hove singularity (VHS) has been considered as a driving source for unconventional superconductivity. A VHS in two-dimensional (2D) materials consists of a saddle point connecting electron-like and hole-like bands. In a rare case, when a VHS appears at Fermi level, both electron-like and hole-like conduction can coexist, giving rise to an enhanced density of states as well as an attractive component of Coulomb interaction for unconventional electronic pairing. However, this van Hove scenario is often destroyed by an incorrect chemical potential or competing instabilities. Here, by using angle-resolved photoemission measurements, we report the observation of a VHS perfectly aligned with the Fermi level in a kagome superconductor CsV 3-x Ta x Sb 5 (x~ 0.4), in which a record-high superconducting transition temperature is achieved among all the current variants of AV 3 Sb 5 (A = Cs, Rb, K) at ambient pressure. Doping dependent measurements reveal the important role of van Hove scenario in boosting superconductivity, and spectroscopic-imaging scanning tunneling microscopy measurements indicate a distinct superconducting state in this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗