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Results for “Effective diffusion coefficient”

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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Temperature and moisture analysis in composites

Advanced fiber-reinforced polymeric matric composites have emerged as strong candidate materials for airframe applications. Favorable aspects include high strength, stiffness, and low density. Temperature and Moisture Analysis in Composites (TMAC) program was developed to study effect of variations in diffusion coefficients, surface properties, panel tilt, ground reflection, and geographical location on moisture-concentration profiles and average moisture contents of composite laminates.

Tenney, D. R.↗
NASA NTRS1984

Solar-flare-induced Forbush decreases - Dependence on shock wave geometry

It is argued that the principal mechanism for the association of Forbush decreases with the passage of a solar flare shock wave is prolonged containment of cosmic ray particles behind the flare compression region, which acts as a semipermeable obstacle to particle motion along the field lines, leading to additional adiabatic cooling of the particles. Liouville's theorem is used to calculate the instantaneous distribution function at 1 AU for each particle arriving at the earth. By averaging over a large number of individual estimates, a representative estimate of the omnidirectional phase space density and the corresponding particle intensity is obtained. The energy change of individual particles at the shocks is found to be small in comparison to the energy lost by adiabatic cooling of the cosmic rays between the shock wave and the sun. The effects of particle rigidity, diffusion coefficient, and flare longitude on the magnitude of the Forbush decrease are quantitatively investigated.

Thomas, B. T.↗
NASA NTRS1985

Heat Flow and Segregation in Directional Solidification

This research is composed of three major components: (1) development of interface morphology control for automated Bridgman growth of semiconductor systems; (2) comparative analysis of segregation during crystal growth in a reduced gravity environment and in the presence of magnetic fields; and (3) consequences of seeding by meltback in Bridgman growth under reduced gravity conditions. In attempts to optimize furnace design for crystal growth in a reduced gravity environment, an analytical approach to heat transfer was developed. It was thus found that charge confining crucibles diminish the ability to control the growth interface morphology through its position within the gradient zone. A heat pipe hot zone system for Bridgman growth, in reduced gravity environment, of crystal with diameters up to 16 mm was developed. For growth of Ga-doped germanium in the multipurpose (ASTP) furnace, it was found that the application of transverse magnetic fields (up to 36 kg) does not substantially increase the effective distribution coefficient; i.e., diffusion-controlled segregation observed in reduced gravity environment cannot be reached nor approached by magnetic field induced melt stabilization.

Witt, A. F.↗
NASA NTRS1987

Rate of energy gain and maximum energy in diffusive shock acceleration

The problem of diffusive shock acceleration of fast charged particles is reexamined with emphasis on the rate of energy gain, and the maximum energy which can be attained in a given circumstance. The direction of the average magnetic field at the shock is shown to have a large effect. If the perpendicular diffusion coefficient is much smaller than the parallel coefficient, particles can gain much more energy if the shock is quasi-perpendicular than if it is quasi-parallel. The maximum energy attainable can be substantially higher (by a factor of 100 or more) than previous discussions would predict, in cases where the shock is quasi-perpendicular. The energy gain increases as kappa-perpendicular decreases. The principal limitation comes from the requirement that diffusion be a valid approximation to the particle motion, and that the particle be able to diffuse fast enough to encounter the shock many times.

Jokipii, J. R.↗
NASA NTRS1975

External field effects on diffusion and solidification derived from the free-volume model

Expressions for the diffusion coefficient and the solidification rate from the free-volume model of liquids developed by Turnbull and Cohen have been used to estimate the effects which microgravity and magnetic fields will have on these quantities. The mathematical formalism describing changes of the diffusion coefficient and the solidification rate is the same for both the microgravity and magnetic field cases, but the difference between the magnitudes of the two effects is quite large. The change in the two parameters is found to be less than .0001% for the microgravity case and on the order of 0.1 to 1.1% for the magnetic field case for four representative materials. The diffusion coefficient and the solidification rate are found to increase under the influence of an applied magnetic field, and this is in agreement with experimental observations.

Miller, R. I.↗
DOE OSTI2020

Se diffusion in CdTe thin films for photovoltaics

Manipulating CdSeTe bandgrading to enhance photocurrent and carrier lifetime is an essential part of high-performance CdTe photovoltaics (PVs). Here, we examine Se diffusion kinetics in single-crystal and polycrystalline CdTe during deposition, thermal annealing, and CdCl 2 treatments. Se distributions are determined by dynamic secondary-ion-mass spectroscopy and Auger electron spectroscopy depth profiling and coupled with electron backscatter diffraction images of the crystalline structure. Effective bulk and grain boundary diffusion coefficients are determined by analytical models and discussed in the context of processing and film morphology. Se is found to diffuse in CdTe at much higher rates during CdCl 2 treatments than with thermal processing alone. GB diffusion also occurs at a significantly faster rate than bulk diffusion. As a result of these two effects, the near interface bulk and GB Se diffusion during CdCl 2 treatments dominates the bandgrading profiles in CdTe PVs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1993

Advances in Studies of Electrode Kinetics and Mass Transport in AMTEC Cells

Previous work reported from JPL has included characterization of electrode kinetics and alkali atom transport from electrodes including Mo, W, WRh x (Mn), in sodium AMTEC cells and vapor exposure cells; and Mo in potassium vapor exposure cells. These studies were generally performed in cells with small area electrodes (about 1 to 5 cm 2 ), and device geometry had little effect on transport. Alkali metal diffusion coefficients through these electrodes have been characterized, and approximate surface diffusion coefficients were derived in cases of activated transport. A basic model of electrode kinetics and transport at the alkali metal vapor/pourous metal electrode/alkali beta"-alumina solid electrolyte (BASE) three phase boundary has been proposed which accounts for electrochemical reaction rates with a collision frequency near the three phase boundary and tunneling from the porous electrode partially covered with adsorbed alkali metal atoms. The small electrode effect in AMTEC cells has been discussed in several papers, but quantitative investigations have described only the overall effect and the important contribution of electrolyte resistance. The quantitative characterization of transport losses in cells with large area electrodes has been limited to simulations of large area electrode effects, or characterization of transport losses from large area electrodes with significant longitudinal temperature gradients. This paper describes new investigations of electrochemical kinetics and transport, with four 14.4 cm 2 WPt 3.5 electrodes, including the influence of electrode size on the mass transport loss in the AMTEC cell. These electrodes exhibit very slow sintering, as well as excellent sodium transport properties, making them attractive candidates for AMTEC power conversion use. However, the facile sodium transport in WPt 3.5 electrodes makes characterization of the transport process difficult.

electrode↗
NASA NTRS1981

Effects of particle drift on the transport of cosmic rays. IV - More realistic diffusion coefficients

Results from numerical simulations of cosmic-ray modulations by the solar wind are presented which show that the scattering mean free path should be larger than the particle gyroradius in the average magnetic field. It is found that the difference between drift and no-drift solutions is not as great as in previous simulations, which violated the mean free path constraint stated. Profound effects are still noted for the drifts, which determine the origin of the bulk of the cosmic rays seen at any given time in the inner solar system. Accordingly, during the 1975 solar minimum, the positively charged cosmic rays seen in the inner solar system came primarily from the outer boundary near the heliospheric poles while negative particles came from the equatorial regions of the boundary.

Jokipii, J. R.↗
DOE OSTI2020

Leveraging Sequential Doping of Semiconducting Polymers to Enable Functionally Graded Materials for Organic Thermoelectrics

With the ability to modulate electronic properties through molecular doping coupled with ease in processability, semiconducting polymers are at the forefront in enabling organic thermoelectric devices for thermal energy management. In contrast to uniform thermoelectric material properties, an alternative route focuses on functionally graded materials (FGMs) where one spatially controls and optimizes transport properties across the length of a thermoelectric material. While primarily studied in the context of inorganic materials, the concept of FGMs for organic thermoelectrics has not been explored. Herein, we introduce how molecular doping of semiconducting polymers enables spatial compositional control of thin-film FGMs. Specifically, we utilize sequential vapor doping of poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno [3,2-b]thiophene] (PBTTT) with the small molecule acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) to fabricate the simplest form of FGMs – double- segmented thin films. The two thin-film segments are of equal length (7.5 mm), but each set to different doping levels. Our study focuses on understanding the thermoelectric properties (Seebeck coefficient, α, and electronic conductivity, σ) and structural properties (through X-ray scattering, UV-vis-NIR spectroscopy, and Raman spectroscopy) within and across the two segments. We observe the presence of a small diffuse interfacial region of 0.5 – 1 mm between the two segments where the doping level and transport properties vary continuously. Despite the diffuse interface, the measured effective Seebeck coefficient (αeff) across the two segments is simply the average of α within each segment. Importantly, this experimental result is consistent with reported mathematical models describing αeff in graded thermoelectric materials. Finally, our results demonstrate the facile fabrication and characterization of functionally graded organic thermoelectric materials, providing guidelines for further development on more complex FGMs.

36 MATERIALS SCIENCE↗
NASA NTRS1982

Effect of vacuum processing on outgassing within an orbiting molecular shield

The limiting hydrogen number density in an orbiting molecular shield is highly dependent on the outgassing rates from the materials of construction for the shield, experimental apparatus, and other hardware contained within the shield. Ordinary degassing temperatures used for ultrahigh vacuum studies (less than 450 C) are not sufficient to process metals so that the contribution to the number density within the shield due to outgassing is less than the theoretically attainable level (approximately 200 per cu. cm). Pure aluminum and type 347 stainless steel were studied as candidate shield materials. Measurements of their hydrogen concentration and diffusion coefficients were made, and the effects of high temperature vacuum processing (greater than 600 C) on their resulting outgassing rates was determined. The densities in a molecular shield due to the outgassing from either metal were substantially less ( 0.003) than the density due to the ambient atomic hydrogen flux at an orbital altitude of 500 km.

Outlaw, R. A.↗
NASA NTRS1984

Effect of growth parameters on compositional variations in directionally solidified HgCdTe alloys

A series of Hg(1-x)Cd(x)Te alloy crystals was grown by directional solidification with compositions ranging from x = 0.2 to x = 0.4. The measured axial compositional profiles were interpreted in terms of an exact numerical solution of the appropriate diffusion equation that takes into account both the variations of the segregation coefficient and solidification rate with composition. The solutions for all growth rates agree generally well with the experimental data, and confirm earlier observations that employed approximate analytical solutions. The effective mass diffusion coefficients showed no significant composition or melt temperature dependence. Slightly higher diffusion coefficients were obtained, however, for the highest growth rates.

Szofran, F. R.↗
DOE OSTI2022

Atomistic thermodynamics and kinetics of dicalcium silicate dissolution

Low-temperature cement manufacturing has garnered academic and industrial attention for its low environmental footprints. However, the sluggish hydration kinetics of the resultant cement affects their early-age strength development. This motivates fundamental studies to unravel the mechanistic picture of the dissolution process and discover science-informed pathways to accelerate hydration. Standard atomistic simulations seldomly exceed a microsecond making them impractical to study slow dissolution processes. We are using rare event sampling techniques, we provide the mechanistic picture of Ca 2+ ion dissolution from a kink site on the dicalcium silicate surface. The Ca 2+ ion dissolution is comprised of two sequential stages: breaking restraints from the kink sites to form a ledge adatom and detaching from the ledge/terrace adatom sites into the solution. The first and second stages feature free energy barriers of ~63 kJ/mol and ~ 29 kJ/mol respectively, making the first stage the rate-limiting step of the entire Ca 2+ dissolution kinetics. Using the reactive flux method, the rate and equilibrium constants for each reaction step are calculated, which yield the Ca 2+ ion activity of ~1.03 × 10 -5 . The diffusion calculations indicate that the surface effects lower the self-diffusion coefficient of Ca 2+ ions at the solid-water interface.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Dynamics of deuterium retention and desorption from plasma-facing materials in fusion reactor-relevant conditions

Hydrogen isotopes retention and desorption during and after discharges in fusion devices are still not well understood due to the complex device conditions and limitations of in-situ diagnostics and measurements. Here, we simulated well-diagnosed recent experiments at the DIII-D facility to benchmark our ITMC-DYN integrated package of modeling deuterium diffusion, retention, and desorption during and after D discharge irradiation. Modeling results were compared with detail experimental data of D desorption fluxes for various irradiation conditions. We predicted the temporal evolution of free and trapped D distribution in tungsten (W) plasma-facing material (PFM). Effects of key parameters namely diffusion coefficient, recombination rate, trapping energies against different defect types, were examined in these simulations. Existing experimental data of these parameters in literature varies significantly which makes it harder to identify key mechanisms and physics responsible for hydrogen isotope retention and desorption. The purpose of this work is to accurately simulate recent well-diagnosed reactor experiments given the uncertainties in such parameters and identify mechanisms responsible for the retention and desorption. We implemented the best identified diffusion, recombination, and trapping parameters in ITMC-DYN package that integrate both various collisional and thermal processes. We predicted, for example, that sample cooling between discharges in DIII-D operations can significantly affect the spatial distribution of trapped D in W under reactor irradiation conditions. Correct prediction of desorption spectra from samples irradiated during 10 DIII-D discharges showed that up to 35% of D can be retained in high binding energy defects such as vacancy clusters or voids.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Substrate-Dependent Exciton Diffusion and Annihilation in Chemically Treated MoS 2 and WS 2

Atomically thin semiconductors such as monolayer MoS 2 and WS 2 exhibit nonlinear exciton–exciton annihilation at notably low excitation densities (below ~10 excitons/μm 2 in exfoliated MoS 2 ). In this work, we show that the density threshold at which annihilation occurs can be tuned by changing the underlying substrate. When the supporting substrate is changed from SiO 2 to Al 2 O 3 or SrTiO 3 , the rate constant for second-order exciton–exciton annihilation, k XX [cm 2 /s], is reduced by 1 or 2 orders of magnitude, respectively. Using transient photoluminescence microscopy, we measure the effective room-temperature exciton diffusion coefficient in bis(trifluoromethane)sulfonimide-treated MoS 2 to be in the range D = 0.03–0.06 cm 2 /s, corresponding to a diffusion length of L D = 350 nm for an exciton lifetime of τ = 18 ns, which does not depend strongly on the substrate. We discuss possible mechanisms for the observed behavior, including substrate permittivity, long-range exciton–exciton or exciton–charge interactions, defect-mediated Auger recombination, and spatially inhomogeneous exciton populations arising from substrate-induced disorder. Exciton annihilation limits the overall efficiency of 2D semiconductor devices operating at high exciton densities; the ability to tune these interactions via the underlying substrate is an important step toward more efficient optoelectronic technologies featuring atomically thin materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

A Pore‐Scale Investigation of Mineral Precipitation Driven Diffusivity Change at the Column‐Scale

Abstract Mineral precipitation affects the pore structure and thus transport properties of porous media. In this study, we investigated the pore‐scale dynamics of precipitation in diffusion controlled systems and the resulting impacts on the effective diffusivity, using a micro‐continuum reactive transport model. Forty two‐dimensional pore structures representing both idealized and realistic geometries were simulated with consideration of different precipitation scenarios and rates. A homogeneous nucleation scenario reproduced patterns observed in previous experimental study showing mixing‐induced precipitation, and a surface growth scenario captured the pattern for mineral precipitation on a substrate with the same or similar mineral structures. In all cases, local precipitation resulted in the reduction in the average porosity of the domain ( Φ ) until the cessation of diffusive transport and the termination of precipitation. The minimum porosity reached was referred to as the critical porosity ( Φ c ). The effective diffusivity ( D eff ) decreased with Φ and dropped sharply to effectively zero, that is, the critical effective diffusivity ( D eff c ), as Φ c was reached. These pore‐scale dynamics can be captured by a revised relationship that explicitly considers the critical porosity and the corresponding effective diffusivity, and the pre‐exponential coefficient and the exponent of the relationship varied with initial pore structure and the precipitation kinetics. Overall, the homogeneous nucleation scenario results in systematically larger Φ c and coefficients that give rise to a sharper decrease in diffusivity as Φ c is approached, compared to the surface growth scenario. The revised relationship was also implemented at continuum scale and used to examine column scale diffusivity change and reactions.

58 GEOSCIENCES↗
NASA NTRS1962

Thermodynamic and Transport Property Correlation Formulas for Equilibrium Air from 1,000 Degrees Kelvin to 15,000 Degrees Kelvin

The thermodynamic properties, density and temperature, as well as transport property parameters involving viscosity, Prandtl number (including diffusion effects), and gaseous radiation absorption coefficients have been correlated as a function of enthalpy at four pressure levels (10 (sup -1), 10 (sup 0), 10, and 10 (sup 2) atmospheres). The correlation formulas are written in a generalized form for which coefficients for a particular property and pressure level are tabulated. The correlation formulas are useful in digital computer programs for non-adiabatic viscous flow problems.

AIR↗
NASA NTRS1994

Ion transport and loss in the Earth's quiet ring current. 2: Diffusion and magnetosphere-ionosphere coupling

We have studied the transport and loss of H(+), He(+), and He(++) ions in the Earth's quiet time ring current (1 to 300 keV/e, 3 to 7 R(sub E), Kp less than 2+, absolute value of Dst less than 11, 70 to 110 degs pitchangles, all LT) comparing the standard radial diffusion model developed for the higher-energy radiation belt particles with measurements of the lower energy ring current ions in a previous paper. Large deviations of that model, which fit only 50% of the data to within a factor of 10, suggested that another transport mechanism is operating in the ring current. Here we derive a modified diffusion coefficient corrected for electric field effects on ring current energy ions that fit nearly 80% of the data to within a factor of 2. Thus we infer that electric field fluctuations from the low-latitude to midlatitude ionosphere (ionospheric dynamo) dominated the ring current transport, rather than high-latitude or solar wind fluctuations. Much of the remaining deviation may arise from convective electric field transport of the E less than 30 keV particles. Since convection effects cannot be correctly treated with this azimuthally symmetric model, we defer treatment of the lowest-energy ions to a another paper. We give chi(exp 2) contours for the best fit, showing the dependence of the fit upon the internal/external spectral power of the predicted electric and magnetic field fluctuations.

Sheldon, R. B.↗