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At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Coating developments towards enabling aluminum as a bipolar plate material for PEM fuel cells

Bipolar plates represent a significant portion of the cost and weight of a proton exchange membrane fuel cell stack. As a result, there has been significant interest in using low weight and cost materials, such as aluminum. Aluminum has good mechanical and physical properties; however, it performs poorly in the corrosive environment within the fuel cell stack. To overcome the corrosion performance issue, there have been efforts towards developing coating processes to enable aluminum to attain corrosion targets. In an effort to enable aluminum as a viable bipolar plate material, a solid phase processing (diffusion bonding) approach for bonding a titanium foil to aluminum is investigated in this study. The microstructure, corrosion performance, mechanical properties and electrical resistance are investigated. To avoid the known reduction in contact resistance performance of titanium after exposure to bipolar plate media, the effect of addition of highly conductive gold particles after diffusion bonding, on contact resistance and corrosion behavior, is also studied and compared with diffusion bonded Ti to Al. In conclusion, the results indicate that diffusion bonded Ti to Al provides a viable alternative material combination for bipolar plate that avoids the micro pores and crevasses that are associated with vapor deposition or electroplating.

25 ENERGY STORAGE↗
DOE OSTI2023

Quantifying the Impact of Electric Fields on the Local Structure and Migration of Potassium Ions at the Orthoclase (001) Surface

Developing an understanding of the response of mineral/water interfaces to applied electric fields is central to detecting and interpreting signatures of interfacial processes in the subsurface. Here we focus density functional theory calculations on understanding K + cation binding and diffusion across the (001) surface of orthoclase feldspar under various applied electric fields with and without surface hydration. The calculations reveal how water ligands labilize surface K + cations for diffusion while also increasing their sensitivity to electric field effects on the binding energy at different surface sites. The calculations also show how the direction and strength of the electric field systematically affect surface cation mobility, sorption, and hydration behavior. Specifically, electric fields directed toward the surface increase the potassium binding energies, facilitate K + diffusion across multiple binding sites, and increase the propensity of surface K + ions to lose their hydration shells. Furthermore, the findings help fill a major knowledge gap into the impact of electric fields on mineral/water interface structure and dynamics more generally, featuring in this case a commonly found type of feldspar involved in a multitude of atmospheric and geochemical processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Theory of activated penetrant diffusion in viscous fluids and colloidal suspensions

We heuristically formulate a microscopic, force level, self-consistent nonlinear Langevin equation theory for activated barrier hopping and non-hydrodynamic diffusion of a hard sphere penetrant in very dense hard sphere fluid matrices. Penetrant dynamics is controlled by a rich competition between force relaxation due to penetrant self-motion and collective matrix structural (alpha) relaxation. In the absence of penetrant-matrix attraction, three activated dynamical regimes are predicted as a function of penetrant-matrix size ratio which are physically distinguished by penetrant jump distance and the nature of matrix motion required to facilitate its hopping. The penetrant diffusion constant decreases the fastest with size ratio for relatively small penetrants where the matrix effectively acts as a vibrating amorphous solid. Increasing penetrant-matrix attraction strength reduces penetrant diffusivity due to physical bonding. For size ratios approaching unity, a distinct dynamical regime emerges associated with strong slaving of penetrant hopping to matrix structural relaxation. A crossover regime at intermediate penetrant-matrix size ratio connects the two limiting behaviors for hard penetrants, but essentially disappears if there are strong attractions with the matrix. Activated penetrant diffusivity decreases strongly with matrix volume fraction in a manner that intensifies as the size ratio increases. We propose and implement a quasi-universal approach for activated diffusion of a rigid atomic/molecular penetrant in a supercooled liquid based on a mapping between the hard sphere system and thermal liquids. Calculations for specific systems agree reasonably well with experiments over a wide range of temperature, covering more than 10 orders of magnitude of variation of the penetrant diffusion constant.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Doping in Efficient Polycrystalline CdSeTe Solar Cells via AsCl 3 Vapor Annealing

Doping in cadmium telluride (CdTe) thin-film solar cells is a critical step in producing highly efficient CdTe solar modules. To date, copper (Cu) ex-situ diffusion doping and group V in situ doping (such as arsenic, As) have been effectively used in manufacturing CdTe solar modules. However, Cu doping is prone to rapid degradation, whereas the low activation ratio of the dopants constrains group V in situ doping. Recently, ex-situ group V doping has been developed, showing an improved doping activation ratio through a solution process. Here, in this study, we developed a vapor-based AsCl 3 doping method for diffusion doping of polycrystalline CdSeTe devices. AsCl 3 vapor annealing can promote the diffusion of As into the bulk CdSeTe through a surface chemical reaction between CdTe and AsCl 3 . This approach has led to a long carrier lifetime of over 72 ns, V oc of 850 mV, and power conversion efficiency of ~18% with Au metal electrodes. The vapor-based ex situ group V doping approach offers an effective means to perform group V diffusion doping into the CdSeTe device.

14 SOLAR ENERGY↗
DOE OSTI2020

A phase field study of the thermal migration of gas bubbles in UO 2 nuclear fuel under temperature gradient

Phase field models are developed to study the gas bubble migration in uranium dioxide nuclear fuel in which a large temperature gradient exists during the operation. In this work, thermal diffusion mechanism for nanosized gas bubbles and vapor transport process for micron-sized gas bubbles are considered, respectively. In both cases, gas bubbles migrate to the high-temperature area. Due to the velocity difference between leading and trailing edges of the gas bubbles, nanosized gas bubbles are elongated along the temperature gradient direction when thermal diffusion is dominated. Micron-sized gas bubbles are either compressed along temperature gradient direction to form lenticular shape bubbles or elongated along temperature gradient direction, depending on the location of the gas bubbles within the fuel pellet. Initial gas bubble radius has no significant effect on the gas bubble migration velocity for both thermal diffusion and vapor transport mechanisms. We notice that the shape change of the gas bubble due to vapor transport mechanism has no significant effect on the migration velocity. Furthermore, the center cavity formation is also captured by our model which is due to the migration and accumulation of lenticular gas bubbles at the center of the fuel pellet. The modeling results compare well with experimental observations and theoretical analysis in the literature.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2014

Origins of effective resistivity in collisionless magnetic reconnection

The mechanisms that provide effective resistivity for supporting collisonless magnetic reconnection have remained unsettled despite numerous studies. Some of these studies demonstrated that the electron pressure nongyrotropy generates the resistivity (η{sub npg}) in the electron diffusion region (EDR). We derive an analytical relation for the effective resistivity (η{sub kin}) by momentum balance in a control volume in the EDR. Both η{sub npg} and η{sub kin} mutually compare well and they also compare well with the resistivity required to support reconnection electric field E{sub rec} in multi-dimensional particle-in-cell simulations as well as in satellite observations when reconnection occurs in an EDR. But they are about an order of magnitude or so smaller than that required when the reconnection occurred in a much wider reconnecting current sheet (RCS) of half width (w) of the order of the ion skin depth (d{sub i}), observed in the Earth magnetosphere. The chaos-induced resistivity reported in the literature is found to be even more deficient. We find that for reconnection in RCS with w ∼ d{sub i}, anomalous diffusion, such as the universal Bhom diffusion and/or that arising from kinetic Alfven waves, could fairly well account for the required resistivity.

58 GEOSCIENCES↗
DOE OSTI2016

Band structure effects on resonant tunneling in III-V quantum wells versus two-dimensional vertical heterostructures

Since the invention of the Esaki diode, resonant tunneling devices have been of interest for applications including multi-valued logic and communication systems. These devices are characterized by the presence of negative differential resistance in the current-voltage characteristic, resulting from lateral momentum conservation during the tunneling process. While a large amount of research has focused on III-V material systems, such as the GaAs/AlGaAs system, for resonant tunneling devices, poor device performance and device-to-device variability have limited widespread adoption. Recently, the symmetric field-effect transistor (symFET) was proposed as a resonant tunneling device incorporating symmetric 2-D materials, such as transition metal dichalcogenides (TMDs), separated by an interlayer barrier, such as hexagonal boron-nitride. The achievable peak-to-valley ratio for TMD symFETs has been predicted to be higher than has been observed for III-V resonant tunneling devices. This work examines the effect that band structure differences between III-V devices and TMDs has on device performance. It is shown that tunneling between the quantized subbands in III-V devices increases the valley current and decreases device performance, while the interlayer barrier height has a negligible impact on performance for barrier heights greater than approximately 0.5 eV.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The role of surface passivation in controlling Ge nanowire faceting

In situ transmission electron microscopy observations of nanowire morphologies indicate that during Au-catalyzed Ge nanowire growth, Ge facets can rapidly form along the nanowire sidewalls when the source gas (here, digermane) flux is decreased or the temperature is increased. This sidewall faceting is accompanied by continuous catalyst loss as Au diffuses from the droplet to the wire surface. We suggest that high digermane flux and low temperatures promote effective surface passivation of Ge nanowires with H or other digermane fragments inhibiting diffusion and attachment of Au and Ge on the sidewalls. Furthermore, these results illustrate the essential roles of the precursor gas and substrate temperature in maintaining nanowire sidewall passivation, necessary to ensure the growth of straight, untapered, <111>-oriented nanowires.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Simulation of the Plasma Afterglow in the Discharge Gap of a Subnanosecond Switch Based on an Open Discharge in Helium

The phenomenon of subnanosecond electrical breakdown in a strong electric field observed in an open discharge in helium at pressures of 6–20 Torr can be used to create ultrafast plasma switches triggering into a conducting state for a time shorter than 1 ns. To evaluate the possible repetition rate of such a subnanosecond switch, it is interesting to study the decay dynamics of the plasma remaining in the discharge gap after ultrafast breakdown. In this paper, a kinetic model based on the particle-in-cell Monte Carlo collision method is used to study the dynamics of the plasma afterglow in the discharge gap of a subnanosecond switch operating with helium at a pressure of 6 Torr. The simulation results show that the radiative, collisional-radiative, and three-body collision recombination mechanisms significantly contribute to the afterglow decay only while the plasma density remains higher than 10{sup 12} cm{sup −3}; the main mechanism of the further plasma decay is diffusion of plasma particles onto the wall. Therefore, the effect of recombination in the plasma bulk is observed only during the first 10–20 μs of the afterglow. Over nearly the same time, plasma electrons become thermalized. The afterglow time can be substantially reduced by applying a positive voltage U{sub c} to the cathode. Since diffusive losses are limited by the ion mobility, the additional ion drift toward the wall significantly accelerates plasma decay. As U{sub c} increases from 0 to +500 V, the characteristic time of plasma decay is reduced from 35 to 10 μs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Stable Metallic Enrichment in Conductive Filaments in TaO x –Based Resistive Switches Arising from Competing Diffusive Fluxes

Oxide–based resistive–switching devices hold promise for solid–state memory technology. Information encoding is accomplished by electrically switching the device between two nonvolatile states with low and high resistance states (LRS/HRS). It is generally accepted that the change between these states is due to the motion of oxygen vacancies forming a continuous (LRS) or gapped (HRS) filament between the electrodes. Direct assessments of filaments are rare due to their small size and the difficulty of locating the filament. Electron microscopy experiments reveal the filament structure and chemistry in TaO 2.0 ± 0.2 –based 150 × 150 nm 2 devices with cross–sectional geometry after forming with power dissipation lower than 1 mW. The filaments appear to be roughly hourglass–shaped with a diameter of less than 10 nm and are composed of Ta–rich and O–poor mostly amorphous material with local compositions as Ta–rich as TaO 0.4 . The as–formed HRS has a gap up to 10 nm wide located next to the anode and composed of nearly stoichiometric TaO 2.5 . The tantalum and oxygen distribution is consistent with filaments formed by the motion of both Ta and O driven by temperature gradients (Soret effect) and an electric field. As a result, this interpretation points towards a new compact model of resistive–switching devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Direct Nano‐Synthesis Methods Notably Benefit Mg‐Battery Cathode Performance

Abstract Rechargeable magnesium batteries are promising candidates for next‐generation electrochemical energy storage, but their development is severely hindered by sluggish solid‐state diffusion and significant desolvation penalties of the divalent cation. Studies suggest that nano‐sized electrode materials alleviate these issues by shortening diffusion lengths and increasing electrode/electrolyte interaction. Here, the effect of particle size and synthetic methodology on the electrochemical performance of four sulfide cathode materials in Mg batteries is investigated: layered TiS 2 , CuS, spinel Ti 2 S 4 , and CuCo 2 S 4 . In these sulfide hosts, the direct preparation of nano‐dimensional crystallites is critical to activate or improve electrochemistry. Even promising cathode materials can appear electrochemically inert when micron‐sized particles are investigated (e.g., CuCo 2 S 4 ), and mechanical milling leads to surface degradation of active material which severely limits performance. However, nano‐sized CuCo 2 S 4 prepared directly reaches a capacity nearly double that of ball‐milled material and delivers 350 mAh g −1 at 60 °C. This work provides synthetic considerations which may be crucial in the discovery and design of novel Mg cathode materials, so that promising candidates are not overlooked. By extension, in oxide materials where Mg 2+ diffusion is expected to be much more sluggish, this factor is anticipated to be even more important when screening for new hosts.

Blanc, Lauren E.↗
DOE OSTI2018

Dynamical Theory of X-Ray Diffraction for Restricted Beams: III. Analysis of Experimental Data for a Porous Crystal

Using the dynamical theory of X-ray diffraction for restricted beams, we have performed quantitative analysis of the structure of an InP(001) porous crystal layer on the basis of experimental data obtained by high-resolution X-ray diffractometry. The parameters of the porous crystal have been determined by simulating coherent and diffuse scattering. The influence of the instrumental function and dynamical effects on the results of investigation has been demonstrated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Hetero-nanostructured materials for high-power lithium ion batteries

The development of high technology electrical devices increased the importance of higher power density or higher capacity at high current density. Especially, rapid charge/discharge issues remain problematic for electric vehicle commercialization. After extensive investigation, researchers introduced hetero-nanostructured materials to the field of lithium ion batteries (LIBs), aiming to enhance the power density or improve the capacity at high current density and life cycle capability. Hetero-nanostructured materials consist of current collectors and directly attached active nanomaterials. Carbon, carbon nanotube (CNT), graphene, Nickel (Ni), Copper (Cu) and Aluminum (Al) were used for current collector, aiming to improve the electron transfer and the cyclability, due to high electrical conductivity and superior buffering effects. Also, Hetero-nanostructure can produce a favorable lithium diffusion condition by creating a lithium diffusion pathway. This article presents an explanation of important factors for high power density or high capacity at high current density. It summarizes the capacity of electrode materials at high current density, including structural descriptions and material types.

Lee, Jaewon↗
DOE OSTI2014

Charge-trapping characteristics of fluorinated thin ZrO{sub 2} film for nonvolatile memory applications

The effects of fluorine treatment on the charge-trapping characteristics of thin ZrO{sub 2} film are investigated by physical and electrical characterization techniques. The formation of silicate interlayer at the ZrO{sub 2}/SiO{sub 2} interface is effectively suppressed by fluorine passivation. However, excessive fluorine diffusion into the Si substrate deteriorates the quality of the SiO{sub 2}/Si interface. Compared with the ZrO{sub 2}-based memory devices with no or excessive fluorine treatment, the one with suitable fluorine-treatment time shows higher operating speed and better retention due to less resistance of built-in electric field (formed by trapped electrons) against electron injection from the substrate and smaller trap-assisted tunneling leakage, resulting from improved ZrO{sub 2}/SiO{sub 2} and SiO{sub 2}/Si interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Self-diffusion of liquid deuterium hydride and liquid tritium

Here, we present a quasi-elastic neutron scattering study of liquid deuterium hydride carried out using the Disk Chopper Spectrometer at the National Institute of Standards and Technology. Under saturated vapor pressure, the self-diffusion constant of deuterium hydride obeys an Arrhenius law D = D 0 exp(-E A /k B T), where the prefactor D 0 is given by D 0 = 9.5 ± 1.2 Å 2 /ps and the activation energy is given by E A = 58 ± 2 K. We apply the quantum law of corresponding states to the known diffusion constants of the hydrogen isotopologues. From this application, we estimate that D 0 ≈ 9.1 Å 2 /ps and E A ≈ 75 K in liquid tritium. Young’s theory of quantum-mechanical effects in van der Waals fluids is shown to apply to the diffusion constants of the liquid hydrogens. Our results underscore the importance of nuclear quantum effects in shaping the properties and behavior of the hydrogen isotopologues.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Computational Exploration of the Li-Electrode|Electrolyte Interface in the Presence of a Nanometer Thick Solid-Electrolyte Interphase Layer [Computational exploration of the Li-electrode|electrolyte interface complicated by a nanometer thin solid-electrolyte interphase (SEI) layer]

A nanometer thick passivation layer will spontaneously form on Li-metal in battery applications due to electrolyte reduction reactions. This passivation layer in rechargeable batteries must have “selective” transport properties: blocking electrons from attacking the electrolytes, while allowing Li + ion to pass through so the electrochemical reactions can continue. The classical description of the electrochemical reaction, Li + + e → Li 0 , occurring at the Li-metal|electrolyte interface is now complicated by the passivation layer and will reply on the coupling of electronic and ionic degrees of freedom in the layer. We consider the passivation layer, called “solid electrolyte interphase (SEI)”, as “the most important but the least understood in rechargeable Li-ion batteries,” partly due to the lack of understanding of its structure–property relationship. In predictive modeling, starting from the ab initio level, we find that it is an important tool to understand the nanoscale processes and materials properties governing the interfacial charge transfer reaction at the Li-metal|SEI|electrolyte interface. Here, we demonstrate pristine Li-metal surfaces indeed dissolve in organic carbonate electrolytes without the SEI layer. Based on joint modeling and experimental results, we point out that the well-known two-layer structure of SEI also exhibits two different Li + ion transport mechanisms. The SEI has a porous (organic) outer layer permeable to both Li + and anions (dissolved in electrolyte), and a dense (inorganic) inner layer facilitate only Li + transport. This two-layer/two-mechanism diffusion model suggests only the dense inorganic layer is effective at protecting Li-metal in electrolytes. This model suggests a strategy to deconvolute the structure–property relationships of the SEI by analyzing an idealized SEI composed of major components, such as Li 2 CO 3 , LiF, Li 2 O, and their mixtures. After sorting out the Li+ ion diffusion carriers and their diffusion pathways, we design methods to accelerate the Li + ion conductivity by doping and by using heterogonous structure designs. We will predict the electron tunneling barriers and connect them with measurable first cycle irreversible capacity loss. We note that the SEI not only affects Li + and e – transport, but it can also impose a potential drop near the Li-metal|SEI interface. Our challenge is to fully describe the electrochemical reactions at the Li - metal|SEI|electrolyte interface. This will be the subject of ongoing efforts.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2021

Temperature and salt controlled tuning of protein clusters

The formation of molecular assemblies in protein solutions is of strong interest both from a fundamental viewpoint and for biomedical applications. While ordered and desired protein assemblies are indispensable for some biological functions, undesired protein condensation can induce serious diseases. As a common cofactor, the presence of salt ions is essential for some biological processes involving proteins, and in aqueous suspensions of proteins can also give rise to complex phase diagrams including homogeneous solutions, large aggregates, and dissolution regimes. Here, we systematically study the cluster formation approaching the phase separation in aqueous solutions of the globular protein BSA as a function of temperature (T), the protein concentration (c p ) and the concentrations of the trivalent salts YCl 3 and LaCl 3 (c s ). As an important complement to structural, i.e. time-averaged, techniques we employ a dynamical technique that can detect clusters even when they are transient on the order of a few nanoseconds. By employing incoherent neutron spectroscopy, we unambiguously determine the short-time self-diffusion of the protein clusters depending on c p , c s and T. We determine the cluster size in terms of effective hydrodynamic radii as manifested by the cluster center-of-mass diffusion coefficients D. For both salts, we find a simple functional form D(c p , c s , T) in the parameter range explored. The calculated inter-particle attraction strength, determined from the microscopic and short-time diffusive properties of the samples, increases with salt concentration and temperature in the regime investigated and can be linked to the macroscopic behavior of the samples.

59 BASIC BIOLOGICAL SCIENCES↗