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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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A core–shell cathode substrate for developing high-loading, high-performance lithium–sulfur batteries
Lithium–sulfur batteries with a high theoretical energy density would be a promising next-generation energy-storage system if their cell-fabrication parameters (e.g., sulfur loading/content and the electrolyte/sulfur ratio) are improved to a practically necessary level. In this article, we report the design of a three-dimensional core–shell carbon substrate, integrating a porous internal core with a conductive external carbon nanofiber shell. Such a carbon substrate encapsulates a high amount of sulfur as the active material core to form a high-loading core–shell cathode, attaining an ultra-high sulfur loading and content of, respectively, 23 mg cm -2 and 75 wt%. With distinguishable internal and external regions, the carbon substrate facilitates the redox reactions and hinders the polysulfide diffusion. Therefore, the core–shell cathodes exhibit a high areal capacity and energy density of, respectively, 14 mA h cm -2 and 27 mW h cm -2 during cycling. During resting, they achieve a long shelf-life of one month with a low capacity-fade rate of 0.25% per day.
Developing high safety Li-metal anodes for future high-energy Li-metal batteries: strategies and perspectives
Developing high-safety Li-metal anodes (LMAs) is extremely important for the application of high-energy Li-metal batteries (LMBs), especially Li–S and Li–O 2 battery systems. However, the notorious Li-dendrite growth problem results in serious safety concerns for any energy storage application. Through a recent combination of interface-based science, nanotechnology-based solutions and characterization methods, the LMA is now primed for a technological boom. In this review, the recent emerging strategies and perspectives on LMAs are summarized, following which the current huge evolution in interfacial chemistry regulation, optimizing electrolyte components, designing a rational ‘host’ for lithium metal, optimizing “solid-state electrolytes” and other emerging strategies for developing high-safety LMAs is highlighted. Furthermore, several state-of-the-art in situ/operando synchrotron-based X-ray techniques for high safety LMB research are introduced. With the further development of LMAs in the future, subsequent application in high energy LMBs is to be expected.
Localized High Concentration Electrolyte Behavior near a Lithium-Metal Anode Surface
Wide-scale practical application of rechargeable lithium-metal batteries remains a significant challenge due to dendrite growth. To overcome this challenge, electrolytes must be designed to allow for the formation of protective solid electrolyte interphase (SEI) layers on the highly reactive lithium-metal anode (LMA) surfaces. Recently, novel localized high-concentration electrolytes (LHCEs) were introduced as a potential solution to enable dendrite-free cycling of LMAs, by using an inert solvent to “dilute” the high concentration electrolytes. Ideally, the diluent itself does not dissolve the salt but is miscible with the solvent to form a localized high concentrated salt/solvent cluster surrounded by the diluent. However, detailed structure and potential surface reactions that may take place in LHCE environment are not yet clear. In this work, we investigated the reactivity of a 1M lithium bis(fluorosulfonyl)imide (LiFSI) in a mixture of dimethoxyethane (DME)/Tris(2,2,2-trifluoroethyl)orthoformate (TFEO) (1:3 by mol) electrolyte near a Li metal surface based on density functional theory and ab-initio molecular dynamics (MD) simulations. Selected liquid interfacial configurations were obtained from classical MD simulasults indicate that when salt and TFEO molecules are close to each other and to the surface, fluorine anions resulting from the fast salt anion decomposition can trigger a cascade of reactions that lead to the decomposition of TFEO. However, if the Li cation is initially solvated by DME and the anion forming a complex, the stability of the anion increases significantly. The Li solvated structure is implied in the LHCE concept; however statistically the larger amount of TFEO molecules suggest also the first scenario leading to TFEO decomposition. Therefore, the broader implication of our simulations is that the defluorination of TFEO may contribute, together with the anion decomposition, to the observed rapid formation of a stable SEI on the surface of the lithium metal; consequently, favorably affecting the stability of LMAs during battery operation.
Mechanism of high luminous efficacy in plasma display panel with high secondary electron emission coefficient cathode material analyzed through three-dimensional fluid model simulation
The mechanism to realize high luminous efficacy in a plasma display panel fabricated with a cathode material possessing a high secondary electron emission coefficient ({gamma}) for Ne and Xe ions was studied via three-dimensional numerical simulation. When a high {gamma} cathode material is used, the increased electron heating efficacy is responsible for increasing vacuum ultraviolet (VUV) efficacy with 10% Xe content gas. However, the continued availability of sufficient secondary electrons during the dynamic moving phase of the cathode sheath in which the electric field remains weakened causes increasing VUV efficacy with 30% Xe content gas. It was found that the improvement of the luminous efficacy of the plasma display panel with a high {gamma} cathode material is maximized under the condition of high Xe content gas because of the simultaneous increase of the electron heating efficacy and Xe excitation efficacy.
High-pressure cell for neutron reflectometry of supercritical and subcritical fluids at solid interfaces
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High mass resolution time of flight mass spectrometer for measuring products in heterogeneous catalysis in highly sensitive microreactors
We demonstrate a combined microreactor and time of flight system for testing and characterization of heterogeneous catalysts with high resolution mass spectrometry and high sensitivity. Catalyst testing is performed in silicon-based microreactors which have high sensitivity and fast thermal response. Gas analysis is performed with a time of flight mass spectrometer with a modified nude Bayard-Alpert ionization gauge as gas ionization source. The mass resolution of the time of flight mass spectrometer using the ion gauge as ionization source is estimated to m/{Delta}m > 2500. The system design is superior to conventional batch and flow reactors with accompanying product detection by quadrupole mass spectrometry or gas chromatography not only due to the high sensitivity, fast temperature response, high mass resolution, and fast acquisition time of mass spectra but it also allows wide mass range (0-5000 amu in the current configuration). As a demonstration of the system performance we present data from ammonia oxidation on a Pt thin film showing resolved spectra of OH and NH{sub 3}.
Design and performance of AERHA, a high acceptance high resolution soft x-ray spectrometer
A soft x-ray spectrometer based on the use of an elliptical focusing mirror and a plane varied line spacing grating is described. It achieves both high resolution and high overall efficiency while remaining relatively compact. The instrument is dedicated to resonant inelastic x-ray scattering studies. We set out how this optical arrangement was judged best able to guarantee performance for the 50 − 1000 eV range within achievable fabrication targets. The AERHA (adjustable energy resolution high acceptance) spectrometer operates with an effective angular acceptance between 100 and 250 μsr (energy dependent) and a resolving power well in excess of 5000 according to the Rayleigh criterion. The high angular acceptance is obtained by means of a collecting pre-mirror. Three scattering geometries are available to enable momentum dependent measurements with 135°, 90°, and 50° scattering angles. The instrument operates on the Synchrotron SOLEIL SEXTANTS beamline which serves as a high photon flux 2 × 200 μm{sup 2} focal spot source with full polarization control.
High-intensity positron microprobe at the Thomas Jefferson National Accelerator Facility
We present a conceptual design for a novel continuous wave electron-linac based high-intensity high-brightness slow-positron production source with a projected intensity on the order of 10{sup 10 }e{sup +}/s. Reaching this intensity in our design relies on the transport of positrons (T{sub +} below 600 keV) from the electron-positron pair production converter target to a low-radiation and low-temperature area for moderation in a high-efficiency cryogenic rare gas moderator, solid Ne. This design progressed through Monte Carlo optimizations of: electron/positron beam energies and converter target thickness, transport of the e{sup +} beam from the converter to the moderator, extraction of the e{sup +} beam from the magnetic channel, a synchronized raster system, and moderator efficiency calculations. For the extraction of e{sup +} from the magnetic channel, a magnetic field terminator plug prototype has been built and experimental results on the effectiveness of the prototype are presented. The dissipation of the heat away from the converter target and radiation protection measures are also discussed.
The ReactorSTM: Atomically resolved scanning tunneling microscopy under high-pressure, high-temperature catalytic reaction conditions
To enable atomic-scale observations of model catalysts under conditions approaching those used by the chemical industry, we have developed a second generation, high-pressure, high-temperature scanning tunneling microscope (STM): the ReactorSTM. It consists of a compact STM scanner, of which the tip extends into a 0.5 ml reactor flow-cell, that is housed in a ultra-high vacuum (UHV) system. The STM can be operated from UHV to 6 bars and from room temperature up to 600 K. A gas mixing and analysis system optimized for fast response times allows us to directly correlate the surface structure observed by STM with reactivity measurements from a mass spectrometer. The in situ STM experiments can be combined with ex situ UHV sample preparation and analysis techniques, including ion bombardment, thin film deposition, low-energy electron diffraction and x-ray photoelectron spectroscopy. The performance of the instrument is demonstrated by atomically resolved images of Au(111) and atom-row resolution on Pt(110), both under high-pressure and high-temperature conditions.
Highly efficient terahertz wave filter for high-power laser beam separation
In this study, we design and fabricate a two-layer device based on the Rayleigh scattering theory for effectively separating high-energy pump-laser-generated terahertz (THz) waves. The basic layer is comprised of cyclo olefin polymer and silicon nanoparticles, which can obstruct the propagation of the 800-nm, high-energy pump laser through scattering and absorption effects while permitting THz waves to pass through. In order to improve the laser damage threshold of the basic layer, an additional layer, which is composed of hollow silica nanoparticles, is used to diffuse the incident high-energy laser beam. Through this two-layer structure, a high 800-nm laser threshold and highly transparent THz region filter are fabricated.
O{sub 3}-sourced atomic layer deposition of high quality Al{sub 2}O{sub 3} gate dielectric for normally-off GaN metal-insulator-semiconductor high-electron-mobility transistors
High quality Al{sub 2}O{sub 3} film grown by atomic layer deposition (ALD), with ozone (O{sub 3}) as oxygen source, is demonstrated for fabrication of normally-off AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs). Significant suppression of Al–O–H and Al–Al bonds in ALD-Al{sub 2}O{sub 3} has been realized by substituting conventional H{sub 2}O source with O{sub 3}. A high dielectric breakdown E-field of 8.5 MV/cm and good TDDB behavior are achieved in a gate dielectric stack consisting of 13-nm O{sub 3}-Al{sub 2}O{sub 3} and 2-nm H{sub 2}O-Al{sub 2}O{sub 3} interfacial layer on recessed GaN. By using this 15-nm gate dielectric and a high-temperature gate-recess technique, the density of positive bulk/interface charges in normally-off AlGaN/GaN MIS-HEMTs is remarkably suppressed to as low as 0.9 × 10{sup 12 }cm{sup −2}, contributing to the realization of normally-off operation with a high threshold voltage of +1.6 V and a low specific ON-resistance R{sub ON,sp} of 0.49 mΩ cm{sup 2}.
Testing the high turbulence level breakdown of low-frequency gyrokinetics against high-frequency cyclokinetic simulations
This paper presents numerical simulations of the nonlinear cyclokinetic equations in the cyclotron harmonic representation [R. E. Waltz and Zhao Deng, Phys. Plasmas 20, 012507 (2013)]. Simulations are done with a local flux-tube geometry and with the parallel motion and variation suppressed using a newly developed rCYCLO code. Cyclokinetic simulations dynamically follow the high-frequency ion gyro-phase motion which is nonlinearly coupled into the low-frequency drift-waves possibly interrupting and suppressing gyro-averaging and increasing the transport over gyrokinetic levels. By comparing the more fundamental cyclokinetic simulations with the corresponding gyrokinetic simulations, the breakdown of gyrokinetics at high turbulence levels is quantitatively tested over a range of relative ion cyclotron frequency 10 < Ω*{sup }< 100 where Ω*{sup }= 1/ρ*, and ρ* is the relative ion gyroradius. The gyrokinetic linear mode rates closely match the cyclokinetic low-frequency rates for Ω*{sup }> 5. Gyrokinetic transport recovers cyclokinetic transport at high relative ion cyclotron frequency (Ω*{sup }≥ 50) and low turbulence level as required. Cyclokinetic transport is found to be lower than gyrokinetic transport at high turbulence levels and low-Ω* values with stable ion cyclotron (IC) modes. The gyrokinetic approximation is found to break down when the density perturbations exceed 20%. For cyclokinetic simulations with sufficiently unstable IC modes and sufficiently low Ω*{sup }∼ 10, the high-frequency component of cyclokinetic transport level can exceed the gyrokinetic transport level. However, the low-frequency component of the cyclokinetic transport and turbulence level does not exceed that of gyrokinetics. At higher and more physically relevant Ω*{sup }≥ 50 values and physically realistic IC driving rates, the low-frequency component of the cyclokinetic transport and turbulence level is still smaller than that of gyrokinetics. Thus, the cyclokinetic simulations do not account for the so-called “L-mode near edge short fall” seen in some low-frequency gyrokinetic transport and turbulence simulations.
Pressure-induced emergence of unusually high-frequency transverse excitations in a liquid alkali metal: Evidence of two types of collective excitations contributing to the transverse dynamics at high pressures
Unlike phonons in crystals, the collective excitations in liquids cannot be treated as propagation of harmonic displacements of atoms around stable local energy minima. The viscoelasticity of liquids, reflected in transition from the adiabatic to elastic high-frequency speed of sound and in absence of the long-wavelength transverse excitations, results in dispersions of longitudinal (L) and transverse (T) collective excitations essentially different from the typical phonon ones. Practically, nothing is known about the effect of high pressure on the dispersion of collective excitations in liquids, which causes strong changes in liquid structure. Here dispersions of L and T collective excitations in liquid Li in the range of pressures up to 186 GPa were studied by ab initio simulations. Two methodologies for dispersion calculations were used: direct estimation from the peak positions of the L/T current spectral functions and simulation-based calculations of wavenumber-dependent collective eigenmodes. It is found that at ambient pressure, the longitudinal and transverse dynamics are well separated, while at high pressures, the transverse current spectral functions, density of vibrational states, and dispersions of collective excitations yield evidence of two types of propagating modes that contribute strongly to transverse dynamics. Emergence of the unusually high-frequency transverse modes gives evidence of the breakdown of a regular viscoelastic theory of transverse dynamics, which is based on coupling of a single transverse propagating mode with shear relaxation. The explanation of the observed high-frequency shift above the viscoelastic value is given by the presence of another branch of collective excitations. With the pressure increasing, coupling between the two types of collective excitations is rationalized within a proposed extended viscoelastic model of transverse dynamics.
A reactor for high-throughput high-pressure nuclear magnetic resonance spectroscopy
The design of a reactor for operando nuclear magnetic resonance (NMR) monitoring of high-pressure gas-liquid reactions is described. The Wisconsin High Pressure NMR Reactor (WiHP-NMRR) design comprises four modules: a sapphire NMR tube with titanium tube holder rated for pressures as high as 1000 psig (68 atm) and temperatures ranging from −90 to 90 °C, a gas circulation system that maintains equilibrium concentrations of dissolved gases during gas-consuming or gas-releasing reactions, a liquid injection apparatus that is capable of adding measured amounts of solutions to the reactor under high pressure conditions, and a rapid wash system that enables the reactor to be cleaned without removal from the NMR instrument. The WiHP-NMRR is compatible with commercial 10 mm NMR probes. Reactions performed in the WiHP-NMRR yield high quality, information-rich, and multinuclear NMR data over the entire reaction time course with rapid experimental turnaround.
High-entropy alloys as high-temperature thermoelectric materials
Thermoelectric (TE) generators that efficiently recycle a large portion of waste heat will be an important complementary energy technology in the future. While many efficient TE materials exist in the lower temperature region, few are efficient at high temperatures. Here, we present the high temperature properties of high-entropy alloys (HEAs), as a potential new class of high temperature TE materials. We show that their TE properties can be controlled significantly by changing the valence electron concentration (VEC) of the system with appropriate substitutional elements. Both the electrical and thermal transport properties in this system were found to decrease with a lower VEC number. Overall, the large microstructural complexity and lower average VEC in these types of alloys can potentially be used to lower both the total and the lattice thermal conductivity. These findings highlight the possibility to exploit HEAs as a new class of future high temperature TE materials.
High Voltage Test-Stand Research Done on ICRF Antenna Elements of the High-Harmonic Fast-Wave System of NSTX
The twelve-strap high-harmonic fast-wave (HHFW) antenna on NSTX has exhibited a high-voltage standoff around 25 kV during previous experimental campaigns; this standoff needs to be improved for increased power coupling. During the recent NSTX-U upgrade period, a test-stand was set up with two antenna straps along with Faraday screens for testing purposes. Using a diagnostic suite consisting of a fast camera, a residual gas analyzer, a pressure gage, high-voltage probes, and an infrared camera, several interesting discoveries were made, leading to possible improvements of the antenna RF voltage operation level. First, arcing was observed outside the Faraday shields towards the low-voltage ("grounded") end of the straps (faraday shield box ends); this arcing was successfully eliminated by installing an additional grounding point between the Faraday shield box and the vessel wall. Second, considerable outgassing was observed during the RF pulse and the amount of outgassing was found to decrease with increasing RF power, possibly indicative of multipacting. Finally, infrared camera measurements of heating on the Faraday shield assembly suggest that the return currents on the Faraday shield box are highly localized at the box sides and possibly account for the pressure increase observed. Computations of these RF currents using Microwave Studio show qualitative agreement with the heated regions. New grounding points between the antenna box and the vessel have been implemented in NSTX-U, where future tests will be done to determine if the high-voltage standoff has improved. Further antenna improvements will be sought through future experiments on the test stand.