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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

X-Ray Microanalysis and Electron Energy Loss Spectrometry in the Analytical Electron Microscope: Review and Future Directions

This paper reviews and discusses future directions in analytical electron microscopy for microchemical analysis using X-ray and Electron Energy Loss Spectroscopy (EELS). The technique of X-ray microanalysis, using the ratio method and k(sub AB) factors, is outlined. The X-ray absorption correction is the major barrier to the objective of obtaining I% accuracy and precision in analysis. Spatial resolution and Minimum Detectability Limits (MDL) are considered with present limitations of spatial resolution in the 2 to 3 microns range and of MDL in the 0.1 to 0.2 wt. % range when a Field Emission Gun (FEG) system is used. Future directions of X-ray analysis include improvement in X-ray spatial resolution to the I to 2 microns range and MDL as low as 0.01 wt. %. With these improvements the detection of single atoms in the analysis volume will be possible. Other future improvements include the use of clean room techniques for thin specimen preparation, quantification available at the I% accuracy and precision level with light element analysis quantification available at better than the 10% accuracy and precision level, the incorporation of a compact wavelength dispersive spectrometer to improve X-ray spectral resolution, light element analysis and MDL, and instrument improvements including source stability, on-line probe current measurements, stage stability, and computerized stage control. The paper reviews the EELS technique, recognizing that it has been slow to develop and still remains firmly in research laboratories rather than in applications laboratories. Consideration of microanalysis with core-loss edges is given along with a discussion of the limitations such as specimen thickness. Spatial resolution and MDL are considered, recognizing that single atom detection is already possible. Plasmon loss analysis is discussed as well as fine structure analysis. New techniques for energy-loss imaging are also summarized. Future directions in the EELS technique will be the development of new spectrometers and improvements in thin specimen preparation. The microanalysis technique needs to be simplified and software developed so that the EELS technique approaches the relative simplicity of the X-ray technique. Finally, one can expect major improvements in EELS imaging as data storage and processing improvements occur.

Goldstein, J. I.↗
NASA NTRS2001

Imaging Electron Spectrometer (IES) Electron Preprocessor (EPP) Design

The Aerospace Corporation developed the Electron PreProcessor (EPP) to support the Imaging Electron Spectrometer (IES) that is part of the RAPID experiment on the ESA/NASA CLUSTER mission. The purpose of the EPP is to collect raw data from the IES and perform processing and data compression on it before transferring it to the RAPID microprocessor system for formatting and transmission to the CLUSTER satellite data system. The report provides a short history of the RAPID and CLUSTER programs and describes the EPP design. Four EPP units were fabricated, tested, and delivered for the original CLUSTER program. These were destroyed during a launch failure. Four more EPP units were delivered for the CLUSTER II program. These were successfully launched and are operating nominally on orbit.

Fennell, J. F.↗
NASA NTRS2001

Nano-Electronics and Bio-Electronics

Viewgraph presentation on Nano-Electronics and Bio-Electronics is discussed. Topics discussed include: NASA Ames nanotechnology program, Potential Carbon Nanotube (CNT) application, CNT synthesis,Computational Nanotechnology, and protein nanotubes.

Srivastava, Deepak↗
NASA NTRS2006

Electron-He(+) P-wave Elastic Scattering and Photoabsorption in Two-electron Systems

In a previous paper [Bhatia, Phys. Rev. A 69,032714 (2004)], electron-hydrogen P-wave scattering phase shifts were calculated using the optical potential approach based on the Feshbach projection operator formalism. This method is now extended to the singlet and triplet electron-He(+) P-wave scattering in the elastic region. Phase shifts are calculated using Hylleraas-type correlation functions with up to 220 terms. Results are rigorous lower bounds to the exact phase shifts and they are compared to phase shifts obtained from the method of polarized orbitals and close-coupling calculations. The continuum functions calculated here are used to calculate photoabsorption cross sections. Photoionization cross sections of He and photodetachment cross sections of H(-) are calculated in the elastic region, i.e. leaving He(+) and H in their respective ground states, and compared with previous calculations. Radiative attachment rates are also calculated.

Bhatia, A. K.↗
NASA NTRS1993

Total Electron Scattering and Electronic State Excitations Cross Sections for O_2, CO, and CH_4

Available electron collision cross section data concerning total and elastic scattering, vibrationalexcitation, and ionization for O_2, CO, and CH_4 have been critically reviewed, and a set of crosssections for modeling of planetary atmospheric behavior is recommended. Utilizing theserecommended cross sections, we derived total electronic state excitation cross sections and upperlimits for dissociation cross sections, which in the case of CH_4 should very closely equal the actualdissociation cross section.

Kanik, I.↗
NASA NTRS2014

D-Wave Electron-H, -He+, and -Li2+ Elastic Scattering and Photoabsorption in P States of Two-Electron Systems

In previous papers [A. K. Bhatia, Phys. Rev. A 85, 052708 (2012); 86, 032709 (2012); 87, 042705 (2013)] electron-H, -He+, and -Li2+ P-wave scattering phase shifts were calculated using the variational polarized orbital theory. This method is now extended to the singlet and triplet D-wave scattering in the elastic region. The long-range correlations are included in the Schrodinger equation by using the method of polarized orbitals variationally. Phase shifts are compared to those obtained by other methods. The present calculation provides results which are rigorous lower bonds to the exact phase shifts. Using the presently calculated D-wave and previously calculated S-wave continuum functions, photoionization of singlet and triplet P states of He and Li+ are also calculated, along with the radiative recombination rate coefficients at various electron temperatures.

D-wave↗
NASA NTRS2016

Hall2De Simulations with a First-Principles Electron Transport Model Based on the Electron Cyclotron Drift Instability

Several years of work have combined plasma measurements in a laboratory Hall thruster and r-z numerical simulations with Hall2De to isolate the spatial variation of the anomalous collision frequency needed in Ohm’s law to produce the observed thruster behavior. This numerical solution is used here to test the validity of a first-principles model of the anomalous transport in these devices before such model is implemented self-consistently in r-z fluid codes like Hall2De. The first-principles model employs quasi-linear theory and is based on the hypothesis that the Electron Cyclotron Drift Instability (ECDI) excites ion acoustic turbulence that, in turn, enhances the effective collision frequency in these devices. We find that a model of the ECDI that assumes Maxwellian velocity distributions for electrons and singly-charged, main-beam, cold ions (Ti=0.07 eV) is insufficient to explain the expected variation of the anomalous collision frequency both in the interior and exterior of the acceleration channel. When warm ions (~0.5-3 eV) are accounted for, the ECDI model in the channel interior appears more promising but fails by orders of magnitude in the near plume region due to the much higher Landau damping of the ion acoustic waves there. This implies that either (a) some process allows the ECDI instability to remain uninhibited by classical Landau damping or, (b) that a different instability (or instabilities) altogether, also insusceptible to Landau damping, is/are active in this region. A previous hypothesis, that convection of wave energy generated by the ECDI in the channel plays a significant role in the near plume, is not supported by the results of the simulations.

Mikellides, Ioannis G.↗
NASA NTRS2017

A First-Principles Model Based on Saturation of the Electron Cyclotron Drift Instability for Electron Transport in Hydrodynamics Simulations of Hall Thruster Plasmas

We present a computational model based on the hypothesis that the higher-than-expected electron mobility across magnetic field lines observed in Hall thrusters is due to the growth and eventual saturation of the electron cyclotron drift instability. The key improvement of this model with respect to previous work is that it decouples the saturation of the instability from the correlation that exists between mobility and magnitude of the wave perturbations in linear theory, thereby enabling first-principles simulations. The model has been incorporated in the 2-D (r-z) multi-fluids code Hall2De and simulations of the H6 thruster at 300 V, 20 A are numerically stable and achieve a steady-state solution at a computational cost that is not significantly higher than previous Hall2De simulations. The computed location of the acceleration region is found to be within 10% of the length of the acceleration channel compared to that inferred by experiments. The simulations also capture well the plasma gradients along the channel centerline of this thruster. To further establish the validity of this model, we plan to conduct simulations of other thrusters and/or operating conditions.

Lopez Ortega, Alejandro↗
DOE OSTI2020

Enhanced electron amplifier structure and method of fabricating the enhanced electron amplifier structure

An enhanced electron amplifier structure includes a microporous substrate having a front surface and a rear surface, the microporous substrate including at least one channel extending substantially through the substrate between the front surface and the rear surface, an ion diffusion layer formed on a surface of the channel, the ion diffusion layer comprising a metal oxide, a resistive coating layer formed on the first ion diffusion layer, an emissive coating layer formed on the resistive coating layer, and an optional ion feedback layer formed on the front surface of the structure. The emissive coating produces a secondary electron emission responsive to an interaction with a particle received by the channel. The ion diffusion layer, the resistive coating layer, the emissive coating layer, and the ion feedback layer are independently deposited via chemical vapor deposition or atomic layer deposition.

Mane, Anil U.↗
DOE OSTI2021

Convergent close-coupling calculations of electrons scattering on electronically excited molecular hydrogen

We use the adiabatic-nuclei molecular convergent close-coupling method to perform calculations of 0.01–1000 eV electrons scattering on the c 3 Π u , a 3 $Σ$$^{+}_{g}$, B 1 $Σ$$^{+}_{u}$, C 1 Π u , and EF 1 $Σ$$^{+}_{g}$ states of H 2 in the v=0 vibrational level. Elastic, superelastic, ionization, and grand-total cross sections are presented, as well as cross sections for excitation of the n=2–3 singlet and triplet states of H 2 (where n is the atomic-limit principle quantum number). Comparison with available theoretical results is made. Good agreement is found with the recent R-matrix results [J. Phys. B 53, 245203 (2020)] for most of the exchange and dipole-forbidden transitions, but not for the dipole-allowed transitions. Here, the sources of disagreement were found to be an unconverged partial-wave expansion and the utilization of the fixed-nuclei approximation (as opposed to adiabatic-nuclei) in the R-matrix calculations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Electronic and optical properties of plutonium metal and oxides from Reflection Electron Energy Loss Spectroscopy

Reflection Electron Energy Loss Spectra (REELS) have been acquired on plutonium δ-Pu-Ga alloys, α-Pu 2 O 3 and PuO 2 . The position of the peaks attributable to the Pu 6p excitation can be used as ‘fingerprint’ for these species. X-ray Photoelectron Spectroscopy has been used to aid interpretation of the REELS spectra. The band gap of PuO 2 was determined from the onset of the energy loss in the REELS spectrum. The REELS data have been evaluated within the dielectric response theory by means of the QUEELS-ε(k, w)-REELS software. In this work, the Energy Loss Function, dielectric function, refractive index and extinction coefficient for these materials have been determined in the ultraviolet and extreme ultraviolet range and are compared to previously reported results.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Broad Applicability of Electrochemical Impedance Spectroscopy to the Measurement of Oxygen Nonstoichiometry in Mixed Ion and Electron Conductors

Oxygen non-stoichiometry is a fundamental feature of mixed ion and electron conductors (MIECs). In this work a general electrochemical method for determining non-stoichiometry in thin film MIECs, via measurement of the chemical capacitance, is demonstrated using ceria and ceria-zirconia (Ce 0.8 Zr 0.2 O 2-δ ) as representative materials. A.C. impedance data are collected from both materials at high temperature (750-900 °C) under reducing conditions with oxygen partial pressure (pO 2 ) in the range 10-13 to 10-20 atm. Additional measurements of ceria-zirconia films are made under relatively oxidizing conditions with pO 2 in the range 0.2 to 10 -4 atm and temperatures of 800-900 °C. Under reducing conditions, the impedance spectra are described by a simple circuit in which a resistor is in series with a resistor and capacitor in parallel, and thickness dependent measurements are used to resolve the capacitance into interfacial and chemical terms. Under more oxidizing conditions, the impedance spectra (of Ce 0.8 Zr 0.2 O 2-δ ) reveal an additional diffusional feature, which enables determination of the ionic resistance of the film in addition to the capacitance, and hence the transport properties. A generalized mathematical formalism is presented for recovering the non-stoichiometry from the chemical capacitance, without recourse to defect chemical models. The ceria non-stoichiometry values are in good agreement with literature values determined by thermogravimetric measurements but display considerably less scatter and are collected on considerably shorter timescales. Here, the thermodynamic analysis of Ce 0.8 Zr 0.2 O 2-δ corroborates earlier findings that introduction of Zr into ceria enhances its reducibility.

08 HYDROGEN↗
DOE OSTI2022

Control of electronic topology in a strongly correlated electron system

It is becoming increasingly clear that breakthrough in quantum applications necessitates materials innovation. In high demand are conductors with robust topological states that can be manipulated at will. This is what we demonstrate in the present work. We discover that the pronounced topological response of a strongly correlated “Weyl-Kondo” semimetal can be genuinely manipulated—and ultimately fully suppressed—by magnetic fields. We understand this behavior as a Zeeman-driven motion of Weyl nodes in momentum space, up to the point where the nodes meet and annihilate in a topological quantum phase transition. The topologically trivial but correlated background remains unaffected across this transition, as is shown by our investigations up to much larger fields. Our work lays the ground for systematic explorations of electronic topology, and boosts the prospect for topological quantum devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The role of electron correlations in the electronic structure of putative Chern magnet TbMn6Sn6

Abstract A member of the RMn 6 Sn 6 rare-earth family materials, TbMn 6 Sn 6 , recently showed experimental signatures of the realization of a quantum-limit Chern magnet. In this work, we use quantum Monte Carlo (QMC) and density functional theory with Hubbard U (DFT + U ) calculations to examine the electronic structure of TbMn 6 Sn 6 . To do so, we optimize accurate, correlation-consistent pseudopotentials for Tb and Sn using coupled-cluster and configuration–interaction (CI) methods. We find that DFT + U and single-reference QMC calculations suffer from the same overestimation of the magnetic moments as meta-GGA and hybrid density functional approximations. Our findings point to the need for improved orbitals/wavefunctions for this class of materials, such as natural orbitals from CI, or for the inclusion of multi-reference effects that capture the static correlations for an accurate prediction of magnetic properties. DFT + U with Mn magnetic moments adjusted to the experiment predict the Dirac crossing in bulk to be close to the Fermi level, within ~120 meV, in agreement with the experiments. Our non-stoichiometric slab calculations show that the Dirac crossing approaches even closer to the Fermi level, suggesting the possible realization of Chern magnetism in this limit.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Active control of electron cyclotron emission radiometer channel frequencies for improved electron temperature measurements

As advanced scenarios are developed for tokamak operations, the demand for flexibility of the electron cyclotron emission (ECE) channels’ locations has increased. The tunable feature of yttrium iron garnet (YIG) filters provides this spatial flexibility. Here, we present a method of performing ECE measurements on fixed flux surfaces instead of fixed frequencies. This is achieved by adjusting YIG filters utilized in the intermediate frequency section to frequencies associated with flux surfaces in regions of interest during the discharge. Here, the key components are the application of tunable YIG filters and a control program that calculates the filter settings using flux information from real-time reconstruction equilibria (EFIT). This fast procedure facilitates T e measurements in regions of interest to investigate plasma dynamic behaviors.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Probing the Electronic and Opto-Electronic Properties of Multilayer MoS 2 Field-Effect Transistors at Low Temperatures

Transition metal dichalcogenides (TMDs)-based field-effect transistors (FETs) are being investigated vigorously for their promising applications in optoelectronics. Despite the high optical response reported in the literature, most of them are studied at room temperature. To extend the application of these materials in a photodetector, particularly at a low temperature, detailed understanding of the photo response behavior of these materials at low temperatures is crucial. Here we present a systematic investigation of temperature-dependent electronic and optoelectronic properties of few-layers MoS 2 FETs, synthesized using the mechanical exfoliation of bulk MoS 2 crystal, on the Si/SiO 2 substrate. Our MoS 2 FET show a room-temperature field-effect mobility μ FE ~40 cm 2 ·V -1 ·s -1 , which increases with decreasing temperature, stabilizing at 80 cm 2 ·V -1 ·s -1 below 100 K. The temperature-dependent (50 K < T < 300 K) photoconductivity measurements were investigated using a continuous laser source λ = 658 nm (E = 1.88 eV) over a broad range of effective illuminating laser intensity, P eff (0.02 μW < P eff < 0.6 μW). Photoconductivity measurements indicate a fractional power dependence of the steady-state photocurrent. The room-temperature photoresponsivity (R) obtained in these samples was found to be ~2 AW -1 , and it increases as a function of decreasing temperature, reaching a maximum at T = 75 K. The optoelectronic properties of MoS 2 at a low temperature give an insight into photocurrent generation mechanisms, which will help in altering/improving the performance of TMD-based devices for various applications.

2D materials↗