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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 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Large-signal model of the bilayer graphene field-effect transistor targeting radio-frequency applications: Theory versus experiment

Bilayer graphene is a promising material for radio-frequency transistors because its energy gap might result in a better current saturation than the monolayer graphene. Because the great deal of interest in this technology, especially for flexible radio-frequency applications, gaining control of it requires the formulation of appropriate models for the drain current, charge, and capacitance. In this work, we have developed them for a dual-gated bilayer graphene field-effect transistor. A drift-diffusion mechanism for the carrier transport has been considered coupled with an appropriate field-effect model taking into account the electronic properties of the bilayer graphene. Extrinsic resistances have been included considering the formation of a Schottky barrier at the metal-bilayer graphene interface. The proposed model has been benchmarked against experimental prototype transistors, discussing the main figures of merit targeting radio-frequency applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Scale-Dependent Fracture-Matrix Interactions and Their Impact on Radionuclide Transport: Development of efficient particle-tracking methods

Matrix Diffusion and Adsorption within a rock matrix are important mechanisms for retarding transport of radionuclides in fractured rock. Due to computational limitations and difficulties in characterizing complex subsurface systems, diffusive exchange between a fracture network and surrounding rock matrix is often modeled using simplified conceptual representations. There is significant uncertainty in “effective” parameters used in these models, such as the “effective matrix diffusivity”. Often, these parameters are estimated by fitting sparse breakthrough data, and estimated values fall outside meaningful ranges, because simplified interpretive models do not consider complex three-dimensional flow. There is limited understanding of the relationship between the effective parameters and rock mass characteristics including network structure and matrix properties. There is also evidence for an apparent scale-dependence in “effective matrix diffusion” coefficients. These observations raise questions on whether fracture-matrix interaction parameters estimated from small-scale tracer tests can be used for predicting radionuclide fate and transport at the scale of DOE field sites. High-resolution three-dimensional Discrete-Fracture-Network-Matrix (DFNM) models based on well-defined local scale transport equations can help to address some of these questions. Due to tremendous advances in computational technology over the last 10 years, DFNM modeling in relatively large domains is now feasible. The overarching objective of our research is to use DFNM modeling to improve fundamental understanding of how effective parameters in conceptual models are related to fracture network structure and matrix properties. An advanced three-dimensional DFNM model is being developed, which combines upscaled particle-tracking algorithms for fracture-matrix interaction and a parallel fracture-network flow simulator. The particle-tracking algorithms allow complexity in flow fields at different scales, and track transport across fracture-matrix interfaces based on rigorous local approximations to the transport equations. This modeling approach can incorporate aperture variability, multi-scale preferential flow and matrix heterogeneity. We developed efficient particle-tracking methods for handling matrix diffusion and adsorption on fracture walls and demonstrated their efficiency for use within the context of large-scale complex fracture network models with variability in apertures across a network of fractures and within individual fractures.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2022

On heat conduction in an irregular magnetic field. Part 1

Anisotropic heat conduction in a plasma embedded in a magnetic field with irregular, possibly chaotic, field lines is discussed. If the collisional mean free path exceeds the electron gyroradius, the heat conductivity is much larger along the field lines than across them, and this enhances the transport across a domain where good flux surfaces do not exist. Recognising that anisotropic heat conduction may be cast in a variational form, and by constructing increasingly sophisticated trial functions that are based on invariant and almost-invariant structures under the magnetic field-line flow, bounds are derived on this enhancement and on the temperature variation along the magnetic field. In this way, remarkably accurate approximations for the temperature can be rapidly constructed without solving the diffusion equation, even in the small perpendicular-diffusion limit when the solution for the temperature is dominated by the fractal structure the magnetic field lines.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Shock-driven kinetic and diffusive mix in high-Z pusher ICF designs

Revolver and Double Shell Inertial Confinement Fusion capsule designs hope to achieve a robust volumetric thermonuclear burn via the use of a high-Z pusher shell filled with a cryogenic D–T fuel. Unfortunately, mix of the pusher material into the fuel (gas) may adversely impact the burn performance. Hydrodynamic instability of the metal/gas interface as the mix source is an obvious concern, but 1D effects may also be detrimental. Such effects include plasma diffusion at material interfaces, which has been the subject of numerous theoretical, computational, and experimental investigations. However, other 1D mix mechanisms may exist, which have yet to be thoroughly explored. In particular, plasma kinetic effects may drive the mix when a shock breaks out of the metal/gas interface. Using the state-of-the-art, hybrid (kinetic-ion/fluid electron), multi-ion Vlasov–Fokker–Planck code, iFP, we show herein that shock-driven kinetic effects can reconfigure the interface and the interfacial width subsequently grows diffusively. Finally, we consider any implications for high-Z pusher designs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas

Bifurcation physics of the magnetic island was investigated using the heat pulse propagation technique produced by the modulation of electron cyclotron heating. There are two types of bifurcation phenomena observed in LHD and DIII-D. One is a bifurcation of the magnetic topology between nested and stochastic fields. The nested state is characterized by the bi-directional (inward and outward) propagation of the heat pulse with slow propagation speed. The stochastic state is characterized by the fast propagation of the heat pulse with electron temperature flattening. The other bifurcation is between magnetic island with larger thermal diffusivity and that with smaller thermal diffusivity. The damping of toroidal flow is observed at the O-point of the magnetic island both in helical plasmas and in tokamak plasmas during a mode locking phase with strong flow shears at the boundary of the magnetic island. Associated with the stochastization of the magnetic field, the abrupt damping of toroidal flow is observed in LHD. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. Lastly, this observation suggests that this flow damping is due to the change in the non-diffusive term of momentum transport.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

A Physics-Based Analytical Model for Perovskite Solar Cells

Perovskites are promising next-generation absorber materials for low-cost and high-efficiency solar cells. Although perovskite cells are configured similar to the classical solar cells, their operation is unique and requires development of a new physical model for characterization, optimization of the cells, and prediction of the panel performance. Here, we develop such a physicsbased analytical model to describe the operation of different types of perovskite solar cells, explicitly accounting for nonuniform generation, carrier selective transport layers, and voltage-dependent carrier collection. Lastly, the model would allow experimentalists to characterize key parameters of existing cells, understand performance bottlenecks, and predict performance of perovskite-based solar panel—the obvious next step to the evolution of perovskite solar cell technology.

14 SOLAR ENERGY↗
DOE OSTI2025

Porous transport electrodes for oxygen evolution reaction in proton exchange membrane water electrolysis -cells: Materials, designs, and diagnoses

H 2 production using proton exchange membrane (PEM) water electrolysis (PEMWE) cells has received considerable attention because of the high efficiencies of these cells and no harmful emissions from the related process. In PEMWE cells, porous transport electrodes (PTEs) composed of a catalyst layer (CL) comprising O 2 evolution reaction (OER) catalysts, porous transport layer (PTL), and PEM play key roles in the stack performance and lifetime. Herein, Ir-based and non-precious-metal OER catalysts that are highly active and stable at low pH values and high anodic potentials are reviewed to understand their OER mechanisms. Various strategies are proposed for engineering CLs and PTLs to improve the interfacial properties and mass transfers of reactants and products to and from the active sites. Additionally, diagnoses of PTEs is significantly crucial for interpreting electrochemical processes and addressing their current challenges. Therefore, half-cell analyses, including diffusion electrode (DE), floating electrode (FE), and modified rotating disk electrode (MRDE) techniques, are explored, and membrane electrode assembly (MEA)-based analyses, such as the polarization technique, electrochemical impedance spectroscopy, and magnetic field analysis, are established. In conclusion, this study aims to provide an overview of recent technologies used for the engineering and diagnostic tools of PEMWE cells and insights into the advanced components and systems to be developed in this field.

Diagnosis of PEMWE Cells↗
DOE OSTI2019

Fluoroethylene Carbonate Breakdown Mechanisms and Energetics on Two Lithium Silicide Surfaces

Lithium-ion batteries are a leading energy storage technology. One challenge with lithium-ion batteries is the reductive decomposition of electrolyte on the surface, forming a passivating, solid–electrolyte interphase (SEI). The SEI prevents further electrolyte breakdown and consumption, but if not formed properly, may also consume lithium ions and prevent lithium-ion diffusion to the anode. Fluoroethylene carbonate (FEC) is currently one of the best electrolyte additives used to form a more robust SEI on silicon anodes. In this study, we use density functional theory (DFT) to investigate the spontaneous breakdown mechanisms, energies, and charge transfers of FEC on the surface of a silicon anode in a lesser lithiated LiSi and a more lithiated Li 15 Si 4 state. The reductive decomposition of FEC on LiSi and Li 15 Si 4 to F, CO 2 , and CH 2 CHO is energetically most favorable on both surfaces, but F, CO, and OCH 2 CHO can also be formed. The breakdown of FEC via either of the breakdown mechanisms is about 2 times more favorable on the Li 15 Si 4 surface than on the LiSi surface. Lastly, the Bader charge transferred from the anode to the FEC breakdown products is larger when forming F, CO 2 , and CH 2 CHO than when forming F, CO, and OCH 2 CHO and is also larger on the Li 15 Si 4 surface than on the LiSi surface.

25 ENERGY STORAGE↗
DOE OSTI2017

High Efficiency Heat Exchanger for High Temperature and High Pressure Applications

CompRex, LLC (CompRex) specializes in the design and manufacture of compact heat exchangers and heat exchange reactors for high temperature and high pressure applications. CompRex’s proprietary compact technology not only increases heat exchange efficiency by at least 25 % but also reduces footprint by at least a factor of ten compared to traditional shell-and-tube solutions of the same capacity and by 15 to 20 % compared to other currently available Printed Circuit Heat Exchanger (PCHE) solutions. As a result, CompRex’s solution is especially suitable for Brayton cycle supercritical carbon dioxide (sCO2) systems given its high efficiency and significantly lower capital and operating expenses. CompRex has already successfully demonstrated its technology and ability to deliver with a pilot-scale compact heat exchanger that was under contract by the Naval Nuclear Laboratory for sCO2 power cycle development. The performance tested unit met or exceeded the thermal and hydraulic specifications with measured heat transfer between 95 to 98 % of maximum heat transfer and temperature and pressure drop values all consistent with the modeled values. CompRex’s vision is to commercialize its compact technology and become the leading provider for compact heat exchangers and heat exchange reactors for various applications including Brayton cycle sCO2 systems. One of the limitations of the sCO2 Brayton power cycle is the design and manufacturing of efficient heat exchangers at extreme operating conditions. Current diffusion-bonded heat exchangers have limitations on the channel size through which the fluid travels, resulting in excessive solid material per heat exchanger volume. CompRex’s design allows for more open area and shorter fluid proximity for increased heat transfer efficiency while sustaining the structural integrity needed for the application. CompRex is developing a novel improvement to its current heat exchanger design where fluids are directed to alternating channels so that each fluid is fully surrounded by the opposing fluid. As compared to similar existing compact heat exchangers, the new design converts most secondary surface area to primary surface area, eliminating fin inefficiencies. CompRex requests that all technical information about the heat exchanger designs be protected as proprietary information. To honor that request, only non-proprietay summaries are included in this report.

47 OTHER INSTRUMENTATION↗
DOE OSTI2013

Environmental Cleanup of the East Tennessee Technology Park Year One - Execution with Certainty SM - 13120

On August 1, 2011, URS - CH2M Oak Ridge LLC (UCOR) began its five-year, $1.4 billion cleanup of the East Tennessee Technology Park (ETTP), located on the U.S. Department of Energy's (DOE) Oak Ridge Reservation in Tennessee. UCOR will close out cleanup operations that began in 1998 under a previous contract. When the Contract Base scope of work [1] is completed in 2016, the K-25 gaseous diffusion building will have been demolished and all waste dispositioned, demolition will have started on the K-27 gaseous diffusion building, all contact-handled and remote-handled transuranic waste in inventory (approximately 500 cubic meters) will have been transferred to the Transuranic Waste Processing Center, previously designated 'No-Path-To-Disposition Waste' will have been dispositioned to the extent possible, and UCOR will have managed DOE Office of Environmental Management (EM)- owned facilities at ETTP, Oak Ridge National Laboratory (ORNL), and the Y-12 National Security Complex in a safe and cost-effective manner. Since assuming its responsibilities as the ETTP cleanup contractor, UCOR has completed its life-cycle Performance Measurement Baseline; received its Earned Value Management System (EVMS) certification; advanced the deactivation and demolition (D and D) of the K-25 gaseous diffusion building; recovered and completed the Tank W-1A and K-1070-B Burial Ground remediation projects; characterized, packaged, and shipped contact-handled transuranic waste to the Transuranic Waste Processing Center; disposed of more than 90,000 cubic yards of cleanup waste while managing the Environmental Management Waste Management Facility (EMWMF); and provided operations, surveillance, and maintenance activities at DOE EM facilities at ETTP, ORNL, and the Y-12 National Security Complex. Project performance as of December 31, 2012 has been excellent: - Cost Performance Index - 1.06; - Schedule Performance Index - 1.02. At the same time, since safety is the foundation of all cleanup work, UCOR's safety record goes hand in hand with its excellent project performance. Through calendar year 2012, UCOR's recordable injury rate was 0.33, and the company has worked close to 4 million hours without a lost work day injury. UCOR's safety record is one of the best in the DOE EM Complex. This performance was due, in large part, to the people and processes URS and CH2M HILL, the parent companies of UCOR, brought to the project. Key approaches included: - Selected and deployed experienced staff in key leadership positions throughout the organization; - Approached 'Transition' as the 'true' beginning of the cleanup project - kicking off a number of project initiatives such as Partnering, PMB development, D and D Plan execution, etc. - Established a project baseline for performance measurement and obtained EVMS certification in record time; - Determined material differences and changed conditions that warranted contract change - then quickly addressed these changes with the DOE client; - Aligned the project and the contract within one year - also done in record time; - Implemented Safety Trained Supervisor and Safety Conscious Work Environment Programs, and kicked off the pursuit of certification under DOE's Voluntary Protection Program. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2017

Numerical study of the influence of surface reaction probabilities on reactive species in an rf atmospheric pressure plasma containing humidity

The quantification and control of reactive species (RS) in atmospheric pressure plasmas (APPs) is of great interest for their technological applications, in particular in biomedicine. Of key importance in simulating the densities of these species are fundamental data on their production and destruction. In particular, data concerning particle-surface reaction probabilities in APPs are scarce, with most of these probabilities measured in low-pressure systems. In this work, the role of surface reaction probabilities, γ, of reactive neutral species (H, O and OH) on neutral particle densities in a He–H 2 O radio-frequency micro APP jet (COST-$\mu $APPJ) are investigated using a global model. It is found that the choice of γ, particularly for low-mass species having large diffusivities, such as H, can change computed species densities significantly. Furthermore the importance of γ even at elevated pressures offers potential for tailoring the RS composition of atmospheric pressure microplasmas by choosing different wall materials or plasma geometries.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Theoretical investigation of thermophysical properties in two-temperature argon-helium thermal plasma

The thermophysical properties of argon-helium thermal plasma have been studied in the temperature range from 5000 to 40 000 K at atmospheric pressure in local thermodynamic equilibrium and non-local thermodynamic equilibrium conditions. Two cases of thermal plasma considered are (i) ground state plasma in which all the atoms and ions are assumed to be in the ground state and (ii) excited state plasma in which atoms and ions are distributed over various possible excited states. The influence of electronic excitation and non-equilibrium parameter {theta} = T{sub e}/T{sub h} on thermodynamic properties (composition, degree of ionization, Debye length, enthalpy, and total specific heat) and transport properties (electrical conductivity, electron thermal conductivity, and thermal diffusion ratio) have been studied. Within the framework of Chapman-Enskog method, the higher-order contributions to transport coefficient and their convergence are studied. The influence of different molar compositions of argon-helium plasma mixture on convergence of higher-orders is investigated. Furthermore, the effect of different definitions of Debye length has also been examined for electrical conductivity and it is observed that electrical conductivity with the definition of Debye length (in which only electrons participate in screening) is less than that of the another definition (in which both the electrons and ions participate in screening) and this deviation increases with electron temperature. Finally, the effect of lowering of ionization energy is examined on electron number density, Debye length, and higher-order contribution to electrical conductivity. It is observed that the lowering of the ionization energy affects the electron transport-properties and consequently their higher-order contributions depending upon the value of the non-equilibrium parameter {theta}.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Final Report: The Influence of Novel Behavioral Strategies in Promoting the Diffusion of Solar Energy

This is the final report for a systematic, evidence-based project using an unprecedented series of large-scale field experiments to examine the effectiveness and cost-effectiveness of novel approaches to reduce the soft costs of solar residential photovoltaics. The approaches were based around grassroots marketing campaigns called ‘Solarize’ campaigns, that were designed to lower costs and increase adoption of solar technology. This study quantified the effectiveness and cost-effectiveness of the Solarize programs and tested new approaches to further improve the model.

14 SOLAR ENERGY↗
DOE OSTI2019

Exceeding 20% Efficiency with in situ Group V Doping in Polycrystalline CdTe Solar Cells

CdTe-based solar technology has achieved one of the lowest levelized costs of electricity among all energy sources as well as state-of-the-art field stability. Yet, there is still ample headroom to improve. For decades, mainstream technology has combined fast CdTe deposition with a CdCl2 anneal and Cu doping. The resulting defect chemistry is strongly compensated and limits the useful hole density to ~10^14 cm-3, creating a ceiling for fill factor, photovoltage and efficiency. In addition, Cu easily changes energy states and diffuses spatially, creating a risk of instabilities that must be managed with care. Here, we demonstrate a significant shift by doping polycrystalline CdSexTe1-x and CdTe films with As while removing Cu entirely from the solar cell. The absorber majority-carrier density is increased by orders of magnitude to 10^16-10^17 cm-3 without compromising the lifetime, and is coupled with a high photocurrent greater than 30 mA cm-2. We demonstrate pathways for fast dopant incorporation in polycrystalline thin films, improved stability and 20.8% solar cell efficiency.

14 SOLAR ENERGY↗
DOE OSTI2011

Oscillation modes of direct current microdischarges with parallel-plate geometry

Two different oscillation modes in microdischarge with parallel-plate geometry have been observed: relaxation oscillations with frequency range between 1.23 and 2.1 kHz and free-running oscillations with 7 kHz frequency. The oscillation modes are induced by increasing power supply voltage or discharge current. For a given power supply voltage, there is a spontaneous transition from one to other oscillation mode and vice versa. Before the transition from relaxation to free-running oscillations, the spontaneous increase of oscillation frequency of relaxation oscillations form 1.3 kHz to 2.1 kHz is measured. Fourier transform spectra of relaxation oscillations reveal chaotic behavior of microdischarges. Volt-ampere (V-A) characteristics associated with relaxation oscillations describes periodical transition between low current, diffuse discharge, and normal glow. However, free-running oscillations appear in subnormal glow only.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Experimental validation of thermodynamic mixture rules at extreme pressures and densities

Accurate modeling of a mixed material Equation of State (EOS) at high pressures (~1 to 100 Mbar) is critical for simulating inertial confinement fusion and high energy density systems. Here, this paper presents a comparison of two mixing rule models to the experiment to assess their applicability in this regime. The shock velocities of polystyrene, aluminum, and nickel aluminide (NiAl) were measured at a shock pressure of ~3 TPa (~30 Mbar) on the Omega EP laser facility (Laboratory for Laser Energetics, University of Rochester, New York). The resultant shock velocities were compared to those derived from the RAGE (Eulerian) hydrodynamics code to validate various mixing rules used to construct an EOS for NiAl. The simulated shock transit time through the sample (Al or NiAl) matched the measurements to within the ±45ps measurement uncertainty. The law of partial volume (Amagat) and the law of partial pressure (Dalton) mixture rules provided equally good matches to the NiAl shock data. Other studies showed that the Amagat mixing rule is superior, and we recommend it since our results also show a satisfactory match. In conclusion, the comparable quality of the simulation to data for the Al and NiAl samples implies that a mixture rule can supply an EOS for plasma mixtures with adequate fidelity for simulations where mixing takes place, such as advective mix in an Eulerian code or when two materials are mixed together via diffusion, turbulence, or other physical processes.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Design strategies for ion-sieving charge mosaic membranes toward sustainable lithium extraction

Membrane-based technologies are essential for realizing sustainable ion-sieving processes such as lithium extraction. Much effort was devoted to designing new membrane materials, whereas the effect of charge distribution has been largely overlooked. Here, we filled this knowledge gap by providing a quantitative transport model for selective ion diffusion through a charge mosaic membrane (CMM). Composed of alternating regions of Li-selective ceramic and anion-selective polymeric materials, CMMs offered the unique advantage of promoting the transport of both Li + and anions while blocking other cations. As a result, the membrane achieved a high Li/Mg selectivity of 62 and a high permeation rate of 59 mmol·m −2 ·h −1 at a low feeding concentration of 30 mM, without any external driving force. Systematical experiments revealed the influence of brine Mg/Li ratio and membrane ceramic/polymer ratio on the overall extraction rate, which was consistent with the prediction of our transport model. In conclusion, the model developed in this work not only presented the design strategies of CMMs for Li extraction but also provided guidance for the development of other ion-sieving processes in general.

25 ENERGY STORAGE↗