Diffusion in the system K2O-SrO-SiO2. IV - Mobility model, electrostatic effects, and multicomponent diffusion.
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Varshneya, A. K.↗
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Results for “effective diffusivity”
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Isentropic phase changes in dissociating gases and the method of sound dispersion for the investigation of homogeneous gas reactions with very high speed : Conclusion
The analytical results of Part I are also applied to sound dispersion by friction and heat conduction, An irreversible change of momentum, energy, and type of particle corresponding to friction, heat conduction, and diffusion effects can appear both in the direction of the sound field and traverse to it. Longitudinal damping, the coupling of longitudinal damping and that due to chemical and physical changes, and coupling of diffusion and compositional changes are treated for a plane sound wave of infinite extent. The same principles are also applied to sound effects in cylindrical tubes. The limitations of the method are discussed in some detail.
SOUND VELOCITY - GASES↗
Flow effects in a vertical CVD reactor
A model is presented to simulate the non-Boussinesq flow in a vertical, two-dimensional, chemical vapor deposition reactor under atmospheric pressure. Temperature-dependent conductivity, mass diffusivity, viscosity models, and reactive species mass transfer to the substrate are incorporated. In the limits of small Mach number and small aspect ratio, asymptotic expressions for the flow, temperature, and species fields are developed. Soret diffusion effects are also investigated. Analytical solutions predict an inverse relationship between temperature field and concentration field due to Soret effects. This finding is consistent with numerical simulations, assisting in the understanding of the complex interactions amongst the flow, thermal, and species fields in a chemically reacting system.
Young, G. W.↗
Hydrogen effects on thermal diffusivity and electrical resistivity of zircaloy cladding
Thermal diffusivity measurements on Zirconium-based cladding materials have historically been a challenge due to the difficulty to measure on specimens with curved geometries, including nuclear grade Zircaloy cladding materials. Here, in this work, we first used laser flash analysis method and four-probe configuration method to measure the thermal diffusivity and electrical resistivity of Zircaloy tubes respectively, which show good agreement with Zircaloy plates in this work, as well as previously published data. The consistent results proved the applicability of the laser flash analysis setup and four-probe configuration method for investigating thermal diffusivity and electrical resistivity of Zircaloy tubes. We further investigated the hydrogen effect on thermal diffusivity and electrical resistivity of Zircaloy. Hydrogen plays significant roles in the thermal diffusivity of Zircaloy, which depends on the hydrogen concentration. For higher hydrogen contents (1130 and 1820 wppm in this work), where phase transformation (α-Zr + δ-hydride → α-Zr + β-Zr) occurs at 567 °C, thermal diffusivity decreases as a function of temperature at the α-Zr + δ-hydride phase regime, while increase at higher temperature at the α-Zr + β-Zr phase regime. For low hydrogen concentration, where hydride dissolved into α-Zr matrix phase at higher temperature, the thermal diffusivity is lower than non-hydrided Zircaloy-4 with similar temperature-dependent trends. Such observation demonstrates the hydrogen effects on reducing thermal diffusivity of α-Zr phase. Hydrogen increases the electrical resistivity of Zircaloy. Similar to thermal diffusivity results, the phase transformation causes a reversal of temperature-dependent trends in electrical resistivity results in Zircaloy-4 with higher hydrogen contents.
36 MATERIALS SCIENCE↗
Interaction of Iron with Alumina Refractory Under Flash Ironmaking Conditions
A novel flash ironmaking technology (FIT) based on the direct reduction of iron ore concentrate by a reducing gas such as hydrogen, natural gas, or coal gas has been developed at the University of Utah. A proper choice of refractories is expected to play an important role in the development of FIT and its scale-up. In this work, the interaction of iron with alumina refractory under flash ironmaking conditions has been investigated. A thermodynamic basis for the interaction phenomena has been developed by considering the Fe-Al-O system, and a kinetic model based on solid-state diffusion is proposed to describe the growth of the hercynite (FeAl{sub 2}O{sub 4}) spinel formed as a result of the interaction between Fe, O (from H{sub 2}O and CO{sub 2}), and Al{sub 2}O{sub 3}. Experiments were conducted with Fe powders and pure Al{sub 2}O{sub 3} in the temperature range 1200 to 1500 °C under gas atmospheres relevant to flash ironmaking, and the reacted samples were analyzed using XRD, SEM-EDX, and EPMA techniques. The analyses of reacted samples confirmed the formation of the hercynite (FeAl{sub 2}O{sub 4}) spinel phase and the thickness of the FeAl{sub 2}O{sub 4} spinel as measured using elemental line scans in SEM-EDX and EPMA varied in the range of 0.5 to 1.3 mm. Results showed that the proposed model suitably describes the growth of the hercynite spinel layer, which obeyed the parabolic rate law at all temperatures. The parabolic rate constants were obtained and the diffusion-controlled growth of the FeAl{sub 2}O{sub 4} layer strongly depended on temperature. Furthermore, using the kinetic model, an expression for the effective diffusivity ( (D{sub eff})-bar ) was obtained and its values at the experimental temperatures were determined. The solid-state diffusion was an activated process, as expected, with an activation energy value of 231 kJ/mol.
36 MATERIALS SCIENCE↗
Simulating Dopant Diffusion in a Detalied Porous Structure
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) uses highpowered lasers to explore cutting edge fusion technology. Specialized optics, known as fiber lasers, generate and amplify the NIF’s high-powered lasers that facilitate fusion experiments. Fiber lasers are glass fibers that are doped with heavy elements to stimulate light emission. The fiber laser doping processing uses a predeposited layer of porous silica nanoparticles on the surface of the glass to absorb a dopant rich salt solution. Our project simulated the diffusion of dopants into porous silica with prescribed colloidal stacking geometries using COMSOL Multiphysics. With these simulations we identified the porosity and tortuosity of the porous nanoparticle structures as the primary parameters that will dictate how the dopants will diffusive into the porous surface of the glass. Porosity and Tortuosity are geometric parameters that can easily be input into more detailed simulations to estimate an effective diffusivity within the porous layer. Future work should account for surface reactions to accurately simulate the full-scale diffusion of the solution doping process.
36 MATERIALS SCIENCE↗
Performance of high-area-ratio annular dump diffuser using suction-stabilized-vortex flow control
A short annular dump diffuser having a geometry conductive to formation of suction stabilized toroidal vortices in the region of abrupt area change was tested. The overall diffuser area ratio was 4.0 and the length to inlet height ratio was 2.0. Performance data were obtained at near ambient temperature and pressure for inlet Mach numbers of 0.18 and 0.30 with suction rates ranging from 0 to 18 percent of total inlet mass flowrate. Results show that the exit velocity profile could be readily biased toward either wall by adjustment of inner and outer wall suction rates. Symmetric exit velocity profiles were inherently unstable with a tendency to revert to a hub or tip bias. Diffuser effectiveness was increased from about 38 percent without suction to over 85 percent at a total suction rate of 10 to 12 percent. At the same time diffuser total pressure loss was reduced from 3.1 percent to 1.1 percent at an inlet Mach number of 0.3.
Juhasz, A. J.↗
Adsorptive Transport of Noble Gas Tracers in Porous Media
The transport of noble gas radionuclides in porous media is relevant to the detection of underground nuclear detonations as well as the sequestration of reprocessing off-gases. However, in field tests releasing radioxenon underground, the quantity of radioxenon observed at the surface has fallen well below expectations1. This study examined the diffusivity of noble gases (Kr and Xe) and the inert molecular gas sulfur hexafluoride (SF6) in porous media to observe any unexpected behavior. To replicate the transport of radiogenic noble gases in underground media, a two-bulb gaseous diffusion apparatus was constructed. The two bulbs are connected with a column of 10-30 Ottawa sand and ordinary atmosphere filled both the bulbs and pore spaces. The tracer gases were diluted in an isolated bulb to approximately 1000 ppm. Once released, the gases were allowed to diffuse through the column. Aliquots were withdrawn at regular time intervals from both bulbs and concentrations were quantified using a Shimadzu QP2010 SE gas chromatograph-mass spectrometer. The effective diffusivity was then calculated using a maximum likelihood estimate on the quasi-steady state model.
Paul, Matthew R.↗
Hydrogen Diffusion Coefficient Measures on Thin Film Uranium Oxide
Thin films of uranium oxides, putatively UO 2 and U 3 O 8 , were deposited on palladium/silver (75/25) foil discs (10 μm thick, 10 mm diameter). Methodology for sealing these foils, applying a hydrogen pressure (~ 1 atm) to the oxide side, and measuring the pressure on the permeate side (opposite side of foil) is reported. The experimental apparatus is held at 100 °C to speed diffusion, aided by the pressure differential across the foil (~1 atm (750 torr) on oxide side, initially low vacuum on permeate side, ~1x 10 -4 torr). Early results indicate an effective diffusion coefficient of roughly 7.66 x 10 -17 +/- 2.22 x 10 -17 cm 2 /sec for UO 2 . These values are in line with expectations and prior measures relative to large lag times for system baseline (substrate only). Evaluation of the technique for U 3 O 8 thin films suggests further development will be needed to extend the technique to more oxidized films (U 3 O 8 , UO 3 ). Characterization of the foils (thickness and speciation) post diffusion experiments will be carried out by SIGMA (Eric Tegtmeier and Andy Richards) in FY24 which will sharpen the uncertainty quantification for UO 2 diffusion coefficients, aiding the nucleation model for DRACO.
36 MATERIALS SCIENCE↗
Processes of diffusion in a limited ensemble of charged particles in a static magnetic field
The dynamics of a limited ensemble of charged particles in a spatially uniform static magnetic field has been studied analytically and numerically. Calculations have been performed for particles with various masses and charges in a wide range of the parameters of the systems under study. It has been shown that the transverse diffusion coefficient in an ensemble of particles in the magnetic field for weakly nonideal systems is described by the Townsend formula. Analytical estimates have been proposed for the analysis of the effective diffusion radius of the ensemble of particles owing to their thermal motion. The proposed relations have been tested by the numerical simulation of the problem.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Directional Solidification and Characterization of Hg(0.89) Mn(0.11)Te
Two boules of Hg(0.89)Mn(0.11)Te(MMT) were solidified using the vertical Bridgman-Stockbarger method. Translation rates of 0.09 and 0. 18 microns/s were used. The influence of growth rate on axial compositional homogeneity in the MMT boules was evaluated experimentally by conducting precision density measurements on radial slices taken from each boule. In addition, Plane Front Solidification theory and segregation coefficient (k) data for the Hg(1-x)Mn(x)Te system were used to fit theoretical composition profiles to the measured MMT axial composition profiles. The strong correlation between the measured and calculated MMT axial composition profiles indicates diffusion dominated axial solute redistribution in the boules under the applied growth conditions. The analysis of the MMT axial composition profiles by Plane Front Solidification theory allowed the calculation of the effective diffusion coefficient (D(eff) = 3.5 x l0(exp -5) sq cm/s). The k-values for the Hg(1-x)Mn(x)Te system and the D(sub eff) - value were then used to verify that both boules were solidified under conditions which did not exceed the Constitutional Supercooling Criteria under ideal conditions. Finally, a preliminary examination of the radial compositional variation in each MMT was made using Fourier Transform Infra-Red Spectroscopy (FTIR). The radial homogeneity in the MMT boules was found to be comparable for both translation rates.
Price, M. W.↗
Dendritic solidification. III - Some further refinements to the model for dendritic growth under an imposed thermal gradient
Some further refinements to a simple model for dendritic solidification in a binary alloy melt under an imposed positive thermal gradient are presented. Two new expressions for the dendrite tip undercooling have been obtained and shown to yield a limiting value of Delta T sub 0 and very small growth rates. Here Delta T sub 0 is the equilibrium solidification range of the alloy. At very large growth rates, all three tip undercooling expressions reach the same limiting value depending on the value of a dimensionless parameter lambda which is related to the effective diffusion distance ahead of the dendrite tip. The predicted tip undercoolings are, however, somewhat lower at intermediate growth rates. An improved calculation for the solute buildup at the dendrite tip due to curvature effects is also included.
Laxmanan, V.↗
HIGH-FIDELITY SIMULATION OF SOOT FORMATION AND THERMAL RADIATION IN A LABORATORY-SCALE RICH-QUENCH-LEAN BURNER
High-fidelity simulations of a swirl-stabilized turbulent spray flame in a laboratory-scale aero-combustor have been performed to evaluate the predictability of state-of-the-art models in capturing soot formation. The simulations employ a complex chemical mechanism developed for Jet-A with PAH chemistry, coupled with the Hybrid Method of Moments (HMOM) soot model, and a Lagrangian dilute spray model for the fuel injection. Two simulations are performed to compare the results when thermal radiation is neglected or included in the solution with a mean spectral model. Modeling closures for the soot differential diffusion effects in mixture fraction space, as well as turbulence-radiation interaction are also evaluated using the data generated by the simulations. Given the degree of complexity of the simulation, the results showed good agreement with experimental measurements of the spatial distribution of the soot volume fraction ensemble average. A closer agreement with the experiment is observed when thermal radiation is included in the solution. Thermal radiation is observed to reduce the flame temperature and increase the flame intermittency, denoted by the increase in the temperature standard deviation in mixture fraction space. The reduction in temperature also leads to a reduction in PAH production and soot volume fraction. Turbulence is observed to have different effects on radiative emission depending on the mixture fraction. Turbulent scalar fluctuations significantly enhance radiative emission in fuel lean mixtures and can also play a role for fuel rich conditions. The statistical description of the turbulence-radiation interaction, previously proposed in the literature, was observed to correctly reproduce the high-fidelity results. Model coefficients were provided for swirl-stabilized flames. The soot differential diffusion model, previously proposed in the literature, based on the residual between the exact term and its model approximation, was also evaluated. The residual correction term further improved the agreement with exact differential diffusion term evaluated with the high-fidelity simulation data in mixture fraction space. The results suggest that the effective turbulent Lewis number can be equal to unity in simulations of turbulent non-premixed recirculating flames.
Soriano, Bruno [Sandia National Laboratories (SNL)↗
Density functional theory calculations of self- and Xe diffusion in U 3 Si 2
Uranium silicide, U 3 Si 2 , has been proposed as an advanced nuclear fuel to be used in light water reactors (LWRs). Development of this alternative to the predominant current fuel, UO 2 , is motivated by enhanced accident tolerance as a result of higher thermal conductivity as well as improved fuel cycle economics through increased uranium density. In order to accurately model the fuel performance of U 3 Si 2 , the diffusion rate of point defects, which is related to self-diffusion, and of fission gas atoms must be determined. DFT calculations are used to predict the U and Si point defect concentrations, the corresponding self-diffusivities, the preferred Xe trap site and the Xe diffusivity. Effects of irradiation are not considered. A low defect formation energy and a high entropy for Si interstitials give rise to Si-rich non-stoichiometry at elevated temperatures. Both U and Si self-diffusion and Xe diffusion are anisotropic as a consequence of the tetragonal crystal structure of U 3 Si 2 . Si diffusion occurs by interstitial mechanisms in both the a-b plane and along the c axis, while the U c axis diffusion rate is controlled by a vacancy mechanism. Interstitial diffusion of U is very fast in the a-b plane of the U 3 Si 2 crystal structure. Xe atoms prefer to occupy U vacancy trap sites. The highest Xe diffusion rate occurs by a vacancy mechanism in both the a-b plane and along the c axis. The diffusion rate is similar in the a-b plane and along the c axis. U and Si self-diffusion and Xe diffusion are all faster in U3Si2 than intrinsic U and Xe diffusion in conventional UO 2 nuclear fuel.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
Mesoscopic analyses of the impact of morphology and operating conditions on the transport resistances in a proton-exchange-membrane fuel-cell catalyst layer
Exploring the origins of local transport resistance and characterizing the oxygen transport resistances in the catalyst layer (RCL) are critical for cost reduction. In this paper, a comprehensive mesoscopic model for simulating coupled transport processes of oxygen and water vapor for different structural parameters under different operating conditions in reconstructed microstructures is proposed. The local transport resistance is calculated after achieving the limiting current density and the effective diffusivity of oxygen. The results demonstrate that RCL increases greatly with decreasing platinum loading (LPt) and the transport resistances in other components of the cell dominate for high-loadings. Both the reduced oxygen permeation coefficient in the ionomer thin-film and the adsorption resistance account for the origins of local transport resistance. The local transport resistance increases with the bare carbon ratio for a constant LPt and Pt/C ratio due to the decreased effective ionomer surface, and increases with the I/C ratio due to the increased ionomer thickness and decreased Knudsen diffusivity. Due to the presence of liquid water, a slight decrease followed by an increase of the local transport resistance versus relative humidity is obtained. The contribution of ionomer thin-films to RCL is more sensitive to liquid saturation compared with that of pores. This journal is
Mu, YT↗
Tidal waves within the thermosphere
The eigenfunctions of the atmosphere (the Hough functions within the lower atmosphere below about 100 km) change their structure and their propagation characteristics within the thermosphere due to dissipation effects such as heat conduction, viscosity, and ion drag. Wave dissipation can be parameterized to a first-order approximation by a complex frequency, the imaginary term of which simulates an effective ion drag force. It is shown how the equivalent depth, the attenuation, and the vertical wavelength of the predominant symmetric diurnal tidal modes change with height as functions of effective ion drag. The boundary conditions of tidal waves are discussed, and asymptotic solutions for the wave parameters like pressure, density, temperature, and wind generated by a heat input proportional to the mean pressure are given. Finally, diffusion effects upon the minor constituents within the thermosphere are described.
Volland, H.↗
AN INVESTIGATION OF THE EFFECTS OF GASEOUS DIFFUSION ON THE RATE OF OXIDATION OF A METAL FORMING A VOLATILE OXIDE
Effects of gaseous diffusion on rate of oxidation of metal forming volatile oxide
OXIDATION↗
Diffusion Blurring of GaAs Quantum Wells Grown at Low Temperature
The processes of the diffusion blurring of a periodic system of GaAs quantum wells separated by AlGaAs barriers are studied by photoluminescence spectroscopy. The system is grown by molecular-beam epitaxy at a low temperature (200°C) and additionally doped with Sb and P isovalent impurities. Postgrowth annealing at the temperature 750°C for 30 min induces an increase in the energy corresponding to the photoluminescence peak of the e1–hh1 exciton state in quantum wells because of blurring of the epitaxial GaAs/AlGaAs interfaces due to enhanced Al–Ga interdiffusion in the cation sublattice. For the Al concentration profile defined by linear diffusion into quantum wells, the Schrödinger equation for electrons and holes is solved. It is found that the experimentally observed energy position of the photoluminescence peak corresponds to the Al–Ga interdiffusion length 3.4 nm and to the effective diffusion coefficient 6.3 × 10{sup –17} cm{sup 2} s{sup –1} at the temperature 750°C. This value is found to be close to the corresponding value for GaAs quantum wells grown at low temperatures without additional doping with Sb and P impurities. From the results obtained in the study, it is possible to conclude that enhanced As–Sb and As–P interdiffusion in the anion sublattice only slightly influences the processes of Al–Ga interdiffusion in the cation sublattice.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗