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At least 865 records · Page 48

All sources NASA NTRS DOE OSTI

DOE OSTI2020

Graphite Lithiation under Fast Charging Conditions: Atomistic Modeling Insights

The charging of lithium ion batteries in a fast and safe manner is critical for promoting the mass adoption of electric vehicles. Li intercalation in graphite electrodes is known to be one of the bottlenecks during the fast charging process. The mechanism of Li diffusion in highly polarized graphite anode at high current rates remains, however, not well understood. In this work, Density Functional Theory (DFT) calculations are used to gain insights into the Li diffusion process in graphite when it is far from equilibrium under fast charging conditions. The effect of uncompensated charges on Li mobility is determined in the highly polarized regions of the anode close to the interfaces. The extra charge was found to increase the interlayer spacing in the diffusion layer and adjacent channels, increasing the diffusivity and promoting the formation of Li clusters. A concerted diffusion mechanism at the edge of high-concentration Li domains is proposed to enhance the diffusion of Li.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRSDate not supplied

Hard Fault Protection for a Silicon Carbide-Based Aerospace Motor Drive

Due to increasingly high DC link voltages and further advancements in the current density of silicon carbide (SiC) MOSFETs, it has become evident that conventional IGBT protection methods are not sufficient to protect these devices from overcurrent during low-inductance fault events. The use of an air core Rogowski coil topology was explored to see if it could mitigate these hard fault events. The design of this circuit resulted in safe shutdown of a low impedance phase-tophase fault, tested up to DC link voltages of 1 kV.

High Voltage↗

NASA NTRS2016

Response of the Equatorial Ionosphere to the Geomagnetic DP 2 Current System

The response of equatorial ionosphere to the magnetospheric origin DP 2 current system fluctuations is examined using ground-based multiinstrument observations. The interaction between the solar wind and fluctuations of the interplanetary magnetic field (IMF) Bz, penetrates nearly instantaneously to the dayside equatorial region at all longitudes and modulates the electrodynamics that governs the equatorial density distributions. In this paper, using magnetometers at high and equatorial latitudes, we demonstrate that the quasiperiodic DP 2 current system penetrates to the equator and causes the dayside equatorial electrojet (EEJ) and the independently measured ionospheric drift velocity to fluctuate coherently with the high-latitude DP 2 current as well as with the IMF Bz component. At the same time, radar observations show that the ionospheric density layers move up and down, causing the density to fluctuate up and down coherently with the EEJ and IMF Bz.

Yizengaw, E.↗

NASA NTRS1995

Texturing of high T(sub c) superconducting polycrystalline fibers/wires by laser-driven directional solidification in an thermal gradient

This paper summarizes the technique of laser-driven directional solidification in a controlled thermal gradient of yttria stabilized zirconia core coated Y-Ba-Cu-O materials to produce textured high T(sub c) superconducting polycrystalline fibers/wires with improved critical current densities in the extended range of magnetic fields at temperatures greater than 77 K. The approach involves laser heating to minimize phase segregation by heating very rapidly through the two-phase incongruent melt region to the single phase melt region and directionally solidifying in a controlled thermal gradient to achieve highly textured grains in the fiber axis direction. The technique offers a higher grain growth rate and a lower thermal budget compared with a conventional thermal gradient and is amenable as a continuous process for improving the J(sub c) of high T(sub c) superconducting polycrystalline fibers/wires. The technique has the advantage of suppressing weak-link behavior by orientation of crystals, formation of dense structures with enhanced connectivity, formation of fewer and cleaner grain boundaries, and minimization of phase segregation in the incongruent melt region.

Varshney, Usha↗

NASA NTRS1990

Stabilization of high T(sub c) phase in bismuth cuprate superconductor by lead doping

It has widely been ascertained that doping of lead in Bi:Sr:Ca:Cu:O systems promotes the growth of high T(sub c) (110 K) phase, improves critical current density, and lowers processing temperature. A systematic investigation is undertaken to determine optimum lead content and processing conditions to achieve these. A large number of samples with cationic compositions of Bi(2-x)Pb(x)Sr2Ca2Cu3 (x = 0.2 to 2.0) were prepared by conventional solid state reaction technique. Samples of all compositions were annealed together at a temperature and characterized through resistance-temperature (R-T) measurements and x ray diffraction (XRD) to determine the zero resistance temperature, T(sub c)(0) and to identify presence of phases, respectively. The annealing temperature was varied between 790 C to optimize processing parameters. Results are given. In brief, an optimum process is reported along with composition of leaded bismuth cuprate superconductor which yields nearly a high T(sub c) single phase with highly stable superconducting properties.

Gupta, Ram. P.↗

NASA NTRS1991

Stabilization of high Tc phase in bismuth cuprate superconductor by lead doping

It has been widely ascertained that doping of lead in Bi-Sr-Ca-Cu-O systems promotes the growth of high T sub c (110 K) phase, improves critical current density, and lowers processing temperature. A systematic study was undertaken to determine optimum lead content and processing conditions to achieve these properties. A large number of samples with cationic compositions of Bi(2-x)Pb(x)Sr2Ca2Cu3 (x = 0.2 to 2.0) were prepared by conventional solid state reaction technique. Samples of all compositions were annealed together at a temperature and characterized through resistance temperature (R-T) measurements and x ray diffraction to determine the zero resistance temperature, T sub c(0) and to identify presence of phases, respectively. The annealing temperature was varied between 790 and 880 C to optimize processing parameters. Results are given. In brief, an optimum process is reported along with composition of leaded bismuth cuprate superconductor which yields nearly a high T sub c single phase with highly stable superconducting properties.

Gupta, Ram. P.↗

DOE OSTI2022

High energy density redox flow device

Redox flow devices are described including a positive electrode current collector, a negative electrode current collector, and an ion-permeable membrane separating said positive and negative current collectors, positioned and arranged to define a positive electroactive zone and a negative electroactive zone; wherein at least one of said positive and negative electroactive zone comprises a flowable semi-solid composition comprising ion storage compound particles capable of taking up or releasing said ions during operation of the cell, and wherein the ion storage compound particles have a polydisperse size distribution in which the finest particles present in at least 5 vol % of the total volume, is at least a factor of 5 smaller than the largest particles present in at least 5 vol % of the total volume.

Chiang, Yet-Ming↗

DOE OSTI2024

High energy density redox flow device

Chiang, Yet-Ming↗

DOE OSTI2023

Understanding the Influence of Li 7 La 3 Zr 2 O 12 Nanofibers on Critical Current Density and Coulombic Efficiency in Composite Polymer Electrolytes

Composite polymer electrolytes (CPEs)are attractive materials for solid-state lithium metal batteries, owing to their high ionic conductivity from ceramic ionic conductors and flexibility from polymer components. As with all lithium metal batteries, however, CPEs face the challenge of dendrite formation and propagation. Not only does this lower the critical current density (CCD) before cell shorting, but the uncontrolled growth of lithium deposits may limit Coulombic efficiency (CE) by creating dead lithium. Here, we present a fundamental study on how the ceramic components of CPEs influence these characteristics. CPE membranes based on poly-(ethylene oxide) and lithium bis-(trifluoro methane sulfonyl)-imide(PEO-LiTFSI) with Li 7 La 3 Zr 2 O 12 (LLZO) nanofibers were fabricated with industrially relevant roll-to-roll manufacturing techniques. Galvanostatic cycling with lithium symmetric cells shows that the CCD can be tripled by including 50 wt % LLZO, but half-cell cycling reveals that this comes at the cost of CE. Varying the LLZO loading shows that even a small amount of LLZO drastically lowers the CE, from 88% at 0 wt % LLZO to 77% at just 2 wt % LLZO. Mesoscale modeling reveals that the increase in CCD cannot be explained by an increase in the macroscopic or microscopic stiffness of the electrolyte; only the microstructure of the LLZO nanofibers in the PEO-LiTFSI matrix slows dendrite growth by presenting physical barriers that the dendrites must push or grow around. This tortuous lithium growth mechanism around the LLZO is corroborated with mass spectrometry imaging. In conclusion, this work highlights important elements to consider in the design of CPEs for high-efficiency lithium metal batteries.

36 MATERIALS SCIENCE↗

DOE OSTI2021

The Impact of Anomalous Resistivity in Vacuum Contaminant Plasmas on the Electrothermal Instability

This manuscript presents an assessment of the electrothermal instability (ETI) in the presence of anomalous resistivity (AR) in vacuum contaminant plasmas (VCP) when applied to a magnetized liner inertial fusion (MagLIF)-like load.Pulsed-power driven dielectrically coated metallic liners, like in MagLIF, experience the current-driven electrothermal instability which occurs when a material’s resistivity changes with temperature and is subject to ohmic heating. Large scale pulsed-power facilities that use magnetically insulated transmission lines (MITL) have been shown to generate low-density plasma which enters the target chamber and coalesces around the load. The low-density high-temperature vacuum contaminant plasmas (VCP) can parasitically divert current from the load through causing a short in the anode-cathode gap inside the target chamber. Resistive magnetohydrodynamic (MHD) simulations of these VCP experience unphysical runaway ohmic heating due to under predicting the resistivity by using a purely collisional resistivity model.AR provides a physics-based way to address this runaway heating through increasing the resistivity in a proportional way with the drift speed. In this work, 1D simulations probe the effect that AR in VCP has on the magnetic diffusion rate, and 2D simulations show how this effect manifests in the nonlinear striation form of the ETI for a MagLIF-like load. Beryllium and aluminum dielectrically coated liners are used for the 1D and 2D simulations in this work. The1D simulations show that a VCP causes a delay in the current delivery to the load by upwards of 8 ns at 60 ns into atypical current pulse for an 1 × 10 7 A scale pulsed-power accelerator. The 2D simulations show the delay observed in the 1D simulations is reduced substantially (4 ns) by 2D turbulence that disrupts the VCP layer early in time. Here, the 2Dsimulated ETI growth varies across AR models, more so for the beryllium liner than the aluminum liner because the beryllium liner shows an enhanced rate of penetration for the magnetic diffusion wave in comparison to the aluminum liner. The 2D simulations show the bulk dielectric thickness varies across AR models with the Davidson AR model beingthe largest and the Buneman AR model being the smallest, and in connection with the thickness the Rayleigh-Taylor bubble-spike distances varies correspondingly.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2020

Experimental study of high-k turbulence during an energy confinement degradation phase in EAST ohmic plasmas

In this paper, we present experimental studies of both high-k(r) ( cm(-1), and , respectively; ) and high- ( cm(-1), and , respectively; ) turbulence behavior during an ohmic energy confinement degradation phase in experimental advanced superconducting tokamak (EAST). High-k(r) turbulence from density fluctuation at and high- turbulence from density fluctuation at -0.97 were measured by tangential and poloidal CO2 laser collective scattering diagnostics, respectively. Note that k(r), , and are radial wavenumber, poloidal wavenumber, perpendicular wavenumber and ion gyroradius at electron temperature, respectively. Both high-k(r)/ turbulence power and energy confinement time are found to be temporally correlated to line-averaged electron density n(e) in the plasma current flat-top phase (I-p = 0.4 MA): when the n(e) shows continuous increase/decrease, the high-k(r)/ turbulence power increases/decreases correspondingly and the shows corresponding decrease/increase; the stable n(e) is related to stable both and high-k(r)/ turbulence power. Statistical results of high-k(r)/ turbulence power versus further imply that high-k(r)/ turbulence shows a strong correlation with plasma energy confinement degradation in high line-averaged n(e) condition, but high-k(r) and high- turbulence have relatively weak and no dependence on the transition between linear range in low-n(e) condition and energy confinement degradation in the high-n(e) condition, respectively. Moreover, profiles of electron temperature T-e and n(e) as well as their normalized gradients in a part of high- density fluctuation measurement region also have been given to qualitatively explain the enhancement of turbulence power with the increase of line-averaged n(e).

EAST tokamak↗

DOE OSTI2025

Modeling Structured Electrodes and Graded Porosity for Improving Discharge Rate Capability in Ultra-Thick Graphite|LiNi 0.6 Mn 0.2 Co 0.2 O 2 Batteries

Long-range electric vehicles (EVs) require high-energy-density batteries that also meet the power demands of high current charge and discharge. Ultra-thick (>100 μm) Lithium-ion battery electrodes are critical to enable this need, but slow ion transport in conventional uniform electrodes (UEs) reduces battery capacity at increasing charge/discharge rates. We present a 3D computational analysis on the impact of structured electrode (SE) and graded electrode (GE) geometries on the discharge rate capability of ultra-thick graphite|LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC-622) battery cells based on the footprint of a commercial EV pouch cell. SE cathodes with either a “grid” or “line” geometry and GEs with two layers of porosity were modeled. Based on the results of 230 models, we found that the electrolyte volume fraction is a key parameter that impacts capacity improvements in UEs, GEs, and SEs at 2 C–6 C discharge rates. SEs have the greatest discharge rate capability, outperforming GEs and UEs due to reduced Lithium-ion concentration gradients across the electrode thickness, which mitigates electrolyte depletion at high rates. The best SE model has a “grid” geometry with gravimetric and volumetric energy density improvements of 0.9%–4% at C/2–2 C and 18%–24% at 4 C–6 C relative to UEs.

25 ENERGY STORAGE↗

NASA NTRS1986

Requirements for high-efficiency solar cells

Minimum recombination and low injection level are essential for high efficiency. Twenty percent AM1 efficiency requires a dark recombination current density of 2 x 10 to the minus 13th power A/sq cm and a recombination center density of less than 10 to the 10th power /cu cm. Recombination mechanisms at thirteen locations in a conventional single crystalline silicon cell design are reviewed. Three additional recombination locations are described at grain boundaries in polycrystalline cells. Material perfection and fabrication process optimization requirements for high efficiency are outlined. Innovative device designs to reduce recombination in the bulk and interfaces of single crystalline cells and in the grain boundary of polycrystalline cells are reviewed.

Sah, C. T.↗

DOE OSTI2020

Understanding asymmetries using integrated simulations of capsule implosions in low gas-fill hohlraums at the National Ignition Facility

Current capsule implosions at the National Ignition Facility (NIF) using high-density-carbon ablators and laser energies close to 2 MJ have shown neutron yields in excess of 50 kJ. Improving on this performance requires understanding of the different degradation mechanisms. For many NIF implosions, nuclear diagnostic signatures have inferred residual hot-spot velocities that correlate with fuel areal density variations consistent with a low-order mode-1 asymmetry. A current working hypothesis attributes these asymmetries to a combination of beam to beam variations in the laser delivery and possibly coupling to target features, such as the diagnostic holes needed for x-ray imaging. Recently, a new source has been identified, thickness variation in the ablator shell. To gain better understanding and eventually mitigate the causes of <ρR> asymmetries, 3D integrated simulations using the actual delivered laser powers are needed. To capture the effect of the diagnostic holes (DHs) using direct numerical simulation would require significantly large computational resources. Instead, our 3D simulations make use of a subgrid model developed using highly-resolved 2D simulations that include several details of the DH engineering complexity. Simulations of NIF shots using hohlraums without DHs, to isolate the effect of beam-to-beam variations, reproduce fairly well the observed nuclear diagnostic signatures. Similarly, reasonable agreement between data and simulations is also obtained in the presence of diagnostic holes. To account for the remaining discrepancies a sensitivity study of ablator thickness variation showed that 1% thickness asymmetries are comparable in effect to 1% peak drive mode-1 asymmetries. Furthermore, this study identified sensitivity to variations in the imbedded doped layer (needed to shield the DT ice from the high energy x-rays generated in the hohlraum) thickness even when the inner and outer surface of the ablator are perfectly spherical.

radiation hydrodynamics↗

NASA NTRS1984

Plasma response to the injection of an electron beam

The results of Vlasov-Poisson-solver numerical simulations of the detailed temporal response of a Maxwellian plasma to the sudden injection of an electron beam are presented in graphs and maps and discussed. Phenomena characterized include ion bursts, electron shocks and holes, plasma heating and expulsion, density gradients; cavitons, deep-density-front and solitary-pulse propagation down the density gradient, and Bunemann-mode excitation leading to formation of a virtual cathode and double layers which are at first monotonic or have low-potential-side dips or high-potential-side bumps and become strong as the electron-current density decreases. The strength of the double layer is found to be roughly proportional to the beam energy.

Singh, N.↗

DOE OSTI2025

Machine learning for improved current-density reconstruction from two-dimensional vector magnetic images

The reconstruction of electrical current densities from magnetic field measurements is an important technique with applications in materials science, circuit design, quality control, plasma physics, and biology. Analytic reconstruction methods exist for planar currents, but break down in the presence of high-spatial-frequency noise or large standoff distance, restricting the types of systems that can be studied. Here, we demonstrate the use of a deep convolutional neural network for current density reconstruction from two-dimensional images of vector magnetic fields acquired by a quantum diamond microscope . Trained network performance significantly exceeds analytic reconstruction for data with high noise or large standoff distances. This machine learning technique can perform quality inversions on lower-signal-to-noise-ratio data, significantly reducing the data collection time and permitting reconstructions of weaker and three-dimensional current sources. Published by the American Physical Society 2025

Reed, Niko R. (ORCID:0009000305222403)↗

DOE OSTI2021

High-Performance All-Polymer Solar Cells and Photodetectors Enabled by a High-Mobility n-Type Polymer and Optimized Bulk-Heterojunction Morphology

All-polymer solar cells (all-PSCs) and all-polymer photodetectors (all-PPDs) are promising for application in flexible electronics, but their performance is greatly limited by the lack of polymer acceptors and the difficulties in morphology control. Herein, we report the use of a high-mobility n-type polymer semiconductor, PNDI-DTBT, for fabricating high-performance all-PSCs and all-PPDs. By pairing PNDI-DTBT with a matched polymer donor, a prominent power conversion efficiency of 8.5% was obtained in all-PSCs, demonstrating the great potential of high-mobility n-type polymers with a conventional donor–acceptor skeleton for photovoltaic application. Moreover, all-PPDs with a low dark current density of 1.32 × 10 –8 A cm –2 at –0.1 V bias and a high specific detectivity of 4.77 × 10 12 Jones were achieved, which belong to the best results of organic photodetectors. Morphology investigations revealed that the formation of a bicontinuous interpenetrating network with optimal phase separation scale, high domain purity, and preferential vertical composition distribution in the bulk-heterojunction active layer contribute to the remarkable device performance. Furthermore, these results suggest that combining high-mobility n-type polymers and morphology optimization is fruitful to achieving high-performance all-PSCs and all-PPDs.

36 MATERIALS SCIENCE↗

DOE OSTI2023

High-Sensitivity Low-Energy Ion Spectroscopy with Sub-Nanometer Depth Resolution Reveals Oxidation Resistance of MoS 2 Increases with Film Density and Shear-Induced Nanostructural Modifications of the Surface

For decades, density has been attributed as a critical aspect of the structure of sputter-deposited nanocrystalline molybdenum disulfide (MoS 2 ) coatings impacting oxidation resistance and wear resistance. Despite its importance, there are few examples in the literature that explicitly investigate the relationship between the density and oxidation behaviors of MoS 2 coatings. Aging and oxidation are primary considerations for the use of MoS 2 coatings in aerospace applications as they inevitably experience prolonged storage in water and oxygen-rich environments prior to use. Oxidation that is either limited to the first few nanometers or through the bulk of the coating can result in seizure due to high initial coefficients of friction or component failure from excessive wear. High-sensitivity low-energy ion spectroscopy (HS-LEIS) and Rutherford backscattering spectrometry (RBS) are both used to understand the extent of oxidation throughout the first ~10 nanometers of the surface of pure sputtered nanocrystalline MoS 2 coatings after high-temperature aging and how it is impacted by the density of coatings as measured by RBS. Results show that low-density coatings (ρ = 3.55 g/cm 3 ) exhibit a more columnar microstructure and voiding, which act as pathways for oxidative species to penetrate and interact with edge sites, causing severe surface and subsurface oxidation. Furthermore, HS-LEIS of surfaces sheared prior to oxidation reveals that the oxidation resistance of low-density MoS 2 coatings can be significantly improved by shear-induced reorientation of the surface microstructure to a basal orientation and elimination of pathways for oxygen into the bulk through compaction of surface and subsurface voids.

36 MATERIALS SCIENCE↗