Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Diffusion Behavior of Oversized Fission Products in bcc Fe Cladding: A First-Principles Study

Fuel-Cladding Chemical Interaction (FCCI) poses significant challenges in nuclear reactors, where fission products from nuclear fuel interact with Fe-based cladding materials, potentially compromising their structural integrity. This study investigates the diffusion behavior of oversized fission products, Pr, Nd, Ce, and La, within bcc Fe cladding using density functional theory (DFT), nudged elastic band (NEB) method, and self-consistent mean field (SCMF) theory. Our results reveal significant long-range vacancy binding energies, particularly up to the 6th nearest neighbor, with La exhibiting the strongest binding affinity, followed by Nd, Ce, and Pr. The NEB calculations indicate significant high barriers for the dissociation of 1nn vacancy-solute pairs for all fission products. The tracer diffusion coefficients of these fission products was derived in Arrhenius form. The significant trapping effect of vacancies by a very dilute amount of fission products reduces vacancy mobility, leading to an oversaturation of point defects, void nucleation, and swelling. These are critical issues for irradiated cladding materials. The tracer diffusion coefficients indicate that Nd diffuses the fastest, followed by La, Ce, and Pr. This study provides essential insights for developing advanced cladding materials and design strategies to mitigate FCCI, ultimately enhancing nuclear reactor safety and performance.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Electrochemical Impedance Spectroscopy and Cyclic Voltammetry Methods for Monitoring SmCl 3 Concentration in Molten Eutectic LiCl-KCl

Molten salt solutions consisting of eutectic LiCl-KCl and concentrations of samarium chloride (0.5 to 3.0 wt%) at 500 C were analyzed using both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV technique gave the average diffusion coefficient for Sm 3+ over the concentration range. Equipped with Sm 3+ diffusion coefficient, the Randles-Sevcik equation predicted Sm 3+ concentration values that agree with the given experimental values. From CV measurements; the anodic, cathodic, and half-peak potentials were identified and subsequently used as a parameter to acquire EIS spectra. A six-element Voigt model was used to model the EIS data in terms of resistance-time constant pairs. The lowest resistances were observed at the half-peak potential with the associated resistance-time constant pairs characterizing the reversible reaction between Sm 3+ and Sm 2+ . By extrapolation, the Voigt model estimated the polarization resistance and established a polarization resistance-concentration relationship.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS1972

Thermal conductivity in vibrationally excited gases.

Comparisons between experimental and theoretical values of the thermal conductivity for a series of nonpolar polyatomic gases show large discrepancies at higher temperatures where the vibrational contribution becomes increasingly important. Examination of various theoretical approaches indicates that the discrepancies can be attributed to approximating the coefficient characterizing the diffusion of vibrational energy, D sub vib, by the self-diffusion coefficient. The present theories cannot accurately predict D sub vib, and its measurement is extremely difficult. Consequently, the approach adopted by the author is to obtain D sub vib by using experimental values of the thermal conductivity. An attempt is made to correlate these values of D sub vib with vibrational relaxation time, so that values of D sub vib can be obtained for all nonpolar polyatomic gases.

Ahtye, W. F.↗
NASA NTRS1983

Plasmapause diffusion

The Bohm diffusion coefficient and observed electrostatic wave scattering are used as the bases of estimates of the smoothing effect that diffusion may have on steep plasmapause density gradients. The estimate for diffusion resulting from scattering by observed electrostatic waves is found to be much lower than that of the perpendicular Bohm diffusion coefficient for characteristic plasma temperatures and magnetic fields. This diffusion rate estimate may be too small, however, if the wave amplitudes are significantly higher for steep plasmapauses. The effects are therefore negligible for most considerations of macroscopic plasmapause dynamics, but may be significant in limiting drift wave instabilities and similar phenomena driven by the steepness of the plasmapause density gradient.

Horwitz, J. L.↗
NASA NTRS1990

Influence of convection on microstructure

The mechanism responsible for the difference in microstructure caused by solidifying the MnBi-Bi eutectic in space is sought. The objectives for the three year period are as follows: (1) completion of the following theoretical analyses - determination of the influence of the Soret effect on the average solid composition versus distance of off-eutectic mixtures directionally solidified in the absence of convection, determination of the influence of convection on the microstructure of off-eutectic mixtures using a linear velocity profile in the adjacent melt, determination of the influence of volumetric changes during solidification on microconvection near the freezing interface and on microstructure, and determination of the influence of convection on microstructure when the MnBi fibers project out in front of the bismuth matrix; (2) search for patterns in the effect of microgravity on different eutectics (for example, eutectic composition, eutectic temperature, usual microstructure, densities of pure constituents, and density changes upon solidification); and (3) determination of the Soret coefficient and the diffusion coefficient for Mn-Bi melts near the eutectic composition, both through laboratory experiements to be performed here and from data from Shuttle experiments.

Wilcox, William R.↗
NASA NTRS1990

Collision integrals and high temperature transport properties for N-N, O-O, and N-O

Accurate collision integrals for the interactions of N(4S0) + N(4S0), O(3P) + O(3P), and N(4S0) + O(3P) are reported. These are computed from a semiclassical formulation of the scattering using the best available representations of all of the potential energy curves needed to describe the collisions. Spectroscopic curves and other accurate measured data are used where available; the results of accurate ab initio electronic structure calculations are used to determine the remaining potential curves. The high-lying states are found to give the largest contributions to the collision cross sections. The nine collision integrals needed to determine transport properties to second order are tabulated for translational temperatures in the range 250-100,000 K. The viscosity, thermal conductivity, diffusion coefficient, and thermal diffusion factor for a gas composed of nitrogen and oxygen atoms in thermal equilibrium have been calculated. It is found that the second-order contribution to the transport properties is small. Graphs of these transport properties for various mixture ratios are presented for temperatures in the range 5000-15,000 K.

Levin, E.↗
NASA NTRS1991

Diffusion of a multi-species component and its role in oxygen and water transport in silicates

The diffusion of a multispecies component is complicated by the different diffusion coefficient of each species and the interconversion reactions among the species. A diffusion equation is derived that incorporates the diffusive fluxes of all species contributing to the component's concentration. The effect of speciation on diffusion is investigated experimentally by measuring concentration profiles of all species developed during diffusion experiments. Data on water diffusion in rhyolitic glasses indicate that H2O molecules predominate over OH groups as the diffusing species at very low to high water concentrations. A simple theoretical relationship is drawn between the effective total oxygen diffusion coefficient and the total water concentration of silicates at low water content.

Zhang, Youxue↗
NASA NTRS1991

Comparison of observed and calculated implanted ion distributions outside Comet Halley's bow shock

This paper compares calculated and measured energy spectra of implanted H(+) and O(+) ions on the assumption that the pickup geometry is quasi-parallel and about 1 percent of the waves generated by the cometary pickup process propagate backward (toward the comet). The model provides a good description of the implanted O(+) and H(+) energy distribution near the pickup energies. The thickness of the implanted ion velocity distribution shells was nearly constant between 2.5 and 1.2 million km (just outside the shock) along the inbound Giotto trajectory. The explanation is that the velocity diffusion coefficient and characteristic diffusion time vary approximately as 1/r and r, respectively, and therefore their product (which determines the velocity shell thickness) remains nearly constant.

Gombosi, T. I.↗
NASA NTRS1989

Collision integrals and high temperature transport properties for N-N, O-O, and N-O

Accurate collision integrals are reported for the interactions of N(4 S 0) + N(4 S 0), O(3 P), and N(4 S 0) + O(3 P). These are computed from a semiclassical formulation of the scattering using the best available representations of all of the potential energy curves needed to describe the collisions. Experimental RKR curves and other accurate measured data are used where available; the results of accurate ab initio electronic structure calculations are used to determine the remaining potential curves. The high-lying states are found to give the largest contributions to the collision cross sections. The nine collision integrals, needed to determine transport properties to second order, are tabulated for translational temperatures in the range 250 K to 100,000 K. These results are intended to reduce the uncertainty in future predictions of the transport properties of nonequilibrium air, particularly at high temperatures. The viscosity, thermal conductivity, diffusion coefficient, and thermal diffusion factor for a gas composed of nitrogen and oxygen atoms in thermal equilibrium are calculated. It was found that the second order contribution to the transport properties is small. Graphs of these transport properties for various mixture ratios are presented for temperatures in the range 5000 to 15000 K.

Levin, E.↗
NASA NTRS2013

Conductance of Ion Channels - Theory vs. Experiment

Transmembrane ion channels mediate a number of essential physiological processes in a cell ranging from regulating osmotic pressure to transmission of neural signals. Kinetics and selectivity of ion transport is of critical importance to a cell and, not surprisingly, it is a subject of numerous experimental and theoretical studies. In this presentation we will analyze in detail computer simulations of two simple channels from fungi - antiamoebin and trichotoxin. Each of these channels is made of an alpha-helical bundle of small, nongenomically synthesized peptides containing a number of rare amino acids and exhibits strong antimicrobial activity. We will focus on calculating ionic conductance defined as the ratio of ionic current through the channel to applied voltage. From molecular dynamics simulations, conductance can be calculated in at least two ways, each involving different approximations. Specifically, the current, given as the number of charges transferred through the channel per unit of time, can be obtained from the number of events in which ions cross the channel during the simulation. This method works well for large currents (high conductance values and/or applied voltages). If the number of crossing events is small, reliable estimates of current are difficult to achieve. Alternatively, conductance can be estimated assuming that ion transport can be well approximated as diffusion in the external potential given by the free energy profile. Then, the current can be calculated by solving the one-dimensional diffusion equation in this external potential and applied voltage (the generalized Nernst-Planck equation). To do so three ingredients are needed: the free energy profile, the position-dependent diffusion coefficient and the diffusive flux of ions into the channel. All these quantities can be obtained from molecular dynamics simulations. An important advantage of this method is that it can be used equally well to estimating large and small currents. In addition, once the free energy profile becomes available the full current-voltage dependence can be readily obtained. For both channels we carried out calculations using both approaches. We also tested the main assumptions underlying the diffusive model, such as uncorrelated nature of individual crossing events and Fickian diffusion. The accuracy and consistency of different methods will be discussed. Finally we will discuss how comparisons between calculated and measured ionic conductance and selectivity of transport can be used for determining structural models of the channels.

Pohorille, Andrew↗
DOE OSTI2025

Water Dynamics of Superacid Aromatic Proton Exchange Membranes for Fuel Cell Applications

Proton exchange membranes (PEMs) with high conductivity are of critical importance for the development of fuel cells, electrolyzers, and other electrochemical technologies. In this research, poly(1,1,2,2-tetrafluoro-2-phenoxyethane-1-sulfonic acid) (PTPS) with an aromatic polymer main chain and a perfluorinated superacidic polymer side chain was synthesized. The water dynamics of PTPS were characterized across various length scales using a combination of Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) and compared with Nafion, a standard perfluorinated PEM, and sulfonated poly(ether sulfone) (SPES 40), an aromatic PEM without perfluorinated superacid side chains. The T 1 and T 2 relaxation times of water in the samples probed by NMR increase from SPES 40 to PTPS to Nafion, indicating that the local motion of the water molecules becomes faster. This trend corresponds well with the relative fraction of bulk-like water determined using FTIR. At larger length scales, the diffusion coefficient of water was characterized using pulsed-field gradient NMR (PFG-NMR). At a longer diffusion time (Δ = 100 ms), PTPS has a smaller diffusion coefficient compared with both Nafion and SPES 40, due to restricted diffusion, and this effect is also evident in the proton conductivity of the hydrated membranes. From this comparison, it is apparent that the aromatic backbone and side chain type greatly influence the water dynamics in PEMs at various length scales and the water dynamics significantly impact the bulk proton conductivity. These insights will lead to new designs for aromatic PEMs and help to identify bottlenecks in current materials.

25 ENERGY STORAGE↗
DOE OSTI2025

Ultrafast and Highly Mobile Photocarriers in Monolayer WSe 2 Doped by α-RuCl 3

We report an experimental investigation of photocarrier dynamics in a heterostructure formed by a WSe 2 monolayer stacked on a multilayer α-RuCl 3 flake. The samples were fabricated using mechanical exfoliation and dry transfer techniques. Photoluminescence measurements showed that the excitonic photoluminescence of WSe 2 is quenched by more than 3 orders of magnitude upon contact with α-RuCl 3 . Transient absorption measurements revealed an ultrashort photocarrier lifetime of 0.5 ps in the heterostructure. Spatially resolved transient absorption microscopy measurements were employed to probe the transport properties of these photocarriers, resulting in a room-temperature diffusion coefficient of 370 cm 2 s –1 , which is higher than the exciton diffusion coefficients of most previously studied 2D semiconductors. Furthermore, these results suggest that α-RuCl 3 -doped WSe 2 could be utilized in high-speed optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Diffusion behavior of lanthanide fission products in bcc Fe cladding: A first-principles study

Fuel-cladding chemical interaction poses significant challenges in nuclear reactors, where fission products generated from nuclear fuel interact with Fe-based cladding materials, potentially compromising their structural integrity. This study investigates the diffusion behavior of lanthanide fission products, Lanthanum (La), Cerium (Ce), Praseodymium (Pr), and Neodymium (Nd), within body-centered cubic (bcc) Fe cladding using the density functional theory, nudged elastic band method, and self-consistent mean field theory. Our results reveal significant vacancy binding energies, particularly with the 1st and 2nd nearest neighbors, which diminish beyond the 5th nearest neighbor, with La exhibiting the strongest binding affinity, followed by Nd, Ce, and Pr. The nudged elastic band calculations indicate significant high barriers for the dissociation of 1st nearest neighbor vacancy-solute pairs for all fission products. The tracer diffusion coefficients of these fission products were derived in an Arrhenius form, with a magnetic correction that accounts for the high-temperature paramagnetic state. The significant trapping effect of vacancies caused by a very dilute concentration of fission products reduces vacancy mobility, potentially leading to modifications in point defect supersaturation, void nucleation, and swelling under irradiation. These represent critical challenges for irradiated cladding materials. The tracer diffusion coefficients indicate that Nd diffuses the fastest, followed by La, Ce, and Pr. Furthermore, this study provides essential insights for understanding fission product transport in cladding materials and informs future design strategies to mitigate fuel-cladding chemical interaction, ultimately enhancing nuclear reactor safety and performance.

Diffusion↗
DOE OSTI2020

The Effect of Acetylation on Iron Uptake and Diffusion in Water Saturated Wood Cell Walls and Implications for Decay

Acetylation is widely used as a wood modification process that protects wood from fungal decay. The mechanisms by which acetylation protects wood are not fully understood. With these experiments, we expand upon the literature and test whether previously observed differences in iron uptake by wood were a result of decreased iron binding capacity or slower diffusion. We measured the concentration of iron in 2 mm thick wood sections at 0, 10, and 20% acetylation as a function of time after exposure to iron solutions. The iron was introduced either strongly chelated with oxalate or weakly chelated with acetate. The concentrations of iron and oxalate in solution were chosen to be similar to those found during brown rot decay, while the concentration of iron and acetate matched previous work. The iron content of oxalate-exposed wood increased only slightly and was complete within an hour, suggesting little absorption and fast diffusion, or only slight surface adsorption. The increase in iron concentration from acetate solutions with time was consistent with Fickian diffusion, with a diffusion coefficient on the order of 10 -16 m 2 s -1 . The rather slow diffusion rate was likely due to significant binding of iron within the wood cell wall. The diffusion coefficient did not depend on the acetylation level; however, the capacity for iron absorption from acetate solution was greatly reduced in the acetylated wood, likely due to the loss of OH groups. We explored several hypotheses that might explain why the diffusion rate appears to be independent of the acetylation level and found none of them convincing. Implications for brown rot decay mechanisms and future research are discussed.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

The Impact of Parameterized Lateral Mixing on the Antarctic Circumpolar Current in a Coupled Climate Model

This study examines the impact of changing the lateral diffusion coefficient ARedi on the transport of the Antarctic Circumpolar Current (ACC). The lateral diffusion coefficient ARedi is poorly constrained, with values ranging across an order of magnitude in climate models. The ACC is difficult to accurately simulate, and there is a large spread in eastward transport in the Southern Ocean (SO) in these models. This paper examines how much of that spread can be attributed to different eddy parameterization coefficients. A coarse-resolution, fully coupled model suite was run with A Redi = 400, 800, 1200, and 2400 m 2 s -1 . Additionally, two simulations were run with two-dimensional representations of the mixing coefficient based on satellite altimetry. Relative to the 400 m 2 s -1 case, the 2400 m 2 s -1 case exhibits 1) an 11% decrease in average wind stress from 50° to 65°S, 2) a 20% decrease in zonally averaged eastward transport in the SO, and 3) a 14% weaker transport through the Drake Passage. The decrease in transport is well explained by changes in the thermal current shear, largely due to increases in ocean density occurring on the northern side of the ACC. In intermediate waters these increases are associated with changes in the formation of intermediate waters in the North Pacific. We hypothesize that the deep increases are associated with changes in the wind stress curl allowing Antarctic Bottom Water to escape and flow northward.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1994

Radial diffusion models of energetic electrons and Jupiter's synchrotron radiation. 2: Time variability

We used a radial diffusion code for energetic electrons in Jupiter's magnetosphere to investigate variations in Jupiter's radio emission due to changes in the electron phase space density at L shells between 6 and 50, and due to changes in the radial diffusion parameters. We suggest that the observed variations in Jupiter's radio emission are likely caused by changes in the electron phase space density at some boundary L(sub 1) is greater than 6, if the primary mode of transport of energetic electrons is radial diffusion driven by fluctuating electric and/or magnetic fields induced by upper atmospheric turbulence. We noticed an excellent empirical correlation, both in phase and relative amplitude, between changes in the solar wind ram pressure and Jupiter's synchrotron radiation if the electron phase space density at the boundary L(sub 1) (L(sub 1) is approximately equal to 20-50) varies linearly with the square root of the solar wind ram pressure, f is approximately (N(sub s)nu(exp 2 sub s))(exp 1/2). The calculations were carried out with a diffusion coefficient D(sub LL) = D(sub n)L(exp n) with n = 3. The diffusion coefficient which best fit the observed variations in Jupiter's synchrotron radiation D(sub 3) = 1.3 +/- 0.2 x 10(exp -9)/s is approximately 0.041/yr, which corresponds to a lagtime of approximately 2 years. We further show that the observed short term (days-weeks) variations in Jupiter's radio emission cannot be explained adequately when radial diffusion is taken into account.

De Pater, Imke↗
DOE OSTI2021

Radiation driven diffusion in γU-Mo

A monolithic fuel design based on a U-Mo alloy has been selected as the fuel type for conversion of the United States High-Performance Research Reactors (HPRRs). A critical phenomenon of interest with U-Mo monolithic fuel is the large amount of swelling that takes place during operation, particularly at high fission densities. The accurate prediction of fuel evolution under irradiation requires implementation of correct thermodynamic and kinetic properties into mesoscale and continuum level fuel performance modeling codes. One such property where there exists incomplete data is the diffusion of relevant species under irradiation. Fuel performance swelling predictions rely on an accurate representation of diffusion in order to determine the rate of fission gas swelling and the local microstructural evolution. In this work, we present molecular dynamics simulations of the radiation driven diffusion of U, Mo and Xe in U-Mo nuclear fuels. Diffusion coefficients for each species are determined over a range of temperatures and compositions. In this work, updated diffusion coefficients are presented that are applicable under irradiation that incorporate both intrinsic and radiation driven diffusion.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2023

Ab Initio Simulations of Tritium Diffusion in Al 2 O and Intermetallic Al 12 (TM) 2.34 Aluminide Coating Phases

Density functional theory simulations have been carried out to investigate the diffusion of interstitial tritium in Al 12 (TM) 2.34 and Al 2 O bulk phases. In Al 12 (TM) 2.34 the transition metal (TM) sites are occupied on average by 58.93 at.% Fe, 18.52 at.% Cr, and 22.54 at.% Ni. While the insertion of interstitial tritium in Al 2 O lead to a strong disordering of the structure, we only investigated interstitial tritium in Al 12 (TM) 2.34 and its iron end-member (i.e., Al 12 Fe 2.34 ). Nine diffusion pathways have been investigated along the a-, b-, and c -axis of Al 12 (TM) 2.34 . The direction of fastest diffusion for interstitial tritium is found to be along the b-axis, with a calculated diffusion coefficient of ≈10 -10 m 2 .s -1 at 600 K, which is at least one order of magnitude faster than those previously calculated for interstitial tritium in other Al-rich phases such as Fe2Alx, Fe4Al13, and FeNiAl5 for which D T =10 -11 m 2 .s -1 , ≈10 -12 m 2 .s -1 , and D T ≈10 -13 m 2 .s -1 respectively. The comparison of the energy landscape between Al 12 (TM) 2.34 and Al 12 Fe 2.34 for the fastest diffusion pathways in each axis direction, found that some pathways are not sensitive to transition metal mixing, while other are more affected, leading to higher energy barrier in Al 12 Fe 2.34 in part due to a more energetically favorable formation of Fe—T bond compared to Ni—T. However, we found that both materials have similar diffusion coefficients as the fastest diffusion pathways occurs along pathways that are not very sensitive to transition metal mixing.

36 MATERIALS SCIENCE↗