The diffusivity of hydrogen through Pd-based solid solutions containing rhodium and platinum

An electrochemical technique has been used to measure the diffusivity of hydrogen in Pd-based solid solutions containing up to 60 at.% Pt and 33 at.% Rh in the temperature range 280–360 K. The results show that the addition of either substitutional solute to the Pd-lattice decreases the diffusivity of hydrogen in the f.c.c. matrix. The diffusion data have been discussed in terms of the cell model for solid solutions. In the case of the Pd-Pt-H system, the kinetic data are in accord with the predictions of the cell model and interaction parameters derived from independent thermodynamic studies.     

In Situ measurement of effective gas diffusivity through hematite pellets during stepwise reductions

The Wicke-Kallenbach (W-K) method for effective gas diffusivity measurements could only be used in the vicinity of ambient temperatures. However, in the present study, this technique has been extended to temperatures of about 1273 K through use of a high-temperature cement. This newly developed high-temperature W-K method was applied toin situ measurements of the diffusive and viscous fluxes through hematite pellets during stepwise reductions. When the sam-ple (acid and basic pellets) is reduced, it swells significantly. However, a gas-tight seal between the holder and specimen was successfully maintained by use of a high-temperature cement. This cement, composed mainly of Na₂O (20 mass pct) and SiO₂, separates into a solid and liquid phase at elevated temperatures. Thus, it can move in the same direction as the expansion of sample and thereby maintain the gas-tight seal. With this new technique, the structural param-eters of gases based on the “dusty gas model” for D’Arcy’s flow, Knudsen diffusion, and mo-lecular diffusion were obtained. These parameters were compared with those estimated through pore structure models.     

Modelling of droplet dynamic and thermal histories during spray forming—II. Effect of process parameters

A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomised droplets as a function of distance during spray forming. The model has been used to investigate the effects of the dynamic and thermal behaviour of individual gas atomised droplets and the cooling and solidification behaviour of the overall spray. The most influential parameters for a given alloy system, in order of importance, are: (i) droplet diameter and, therefore, the droplet size distribution within the spray; (ii) initial axial gas velocity at the point of atomisation and the subsequent gas velocity decay profile; (iii) melt mass flow rate; (iv) melt superheat at the point of atomisation; and (v) alloy composition. Experimental measurements of gas velocities and droplet size distributions during spray forming allow the spray solid fraction at deposition to be calculated and used in a subsequent computer model of billet heat flow to predict the billet top surface temperatures and solid fractions.     

Adsorption of gold on a resin-impregnated membrane in a fixed bed contractor

One of the main problems in operating a resin-in-pulp process is the separation of the ion-exchange beads from the slurry after loading. This problem can be overcome by impregnating an inert woven cloth with an ion exchange resin. Sheets of this membrane could be mounted in parallel within a vertical column to allow a clear solution or a slurry to pass between them.The use of a resin-impregnated cloth was tested in a fixed bed contractor by measuring the kinetics of adsorption of gold from a cyanide solution. The equilibrium isotherm was also determined and the resulting data gave a good fit to the homogeneous solid diffusion model. The internal diffusion coefficient was shown to be independent of solution flow rate and numerically in agreement with previous work. The external film transfer coefficient was fitted successfully by a modified Dawson correlation. The membrane can be easily eluted and regenerated under ambient conditions.     

Theory of Seepage into an Auger Hole in a Confined Aquifer Overlying a Gravel Substratum

An analytical solution is presented to the problem of steady groundwater flow seeping into a pumped cylindrical hole partially penetrating a homogeneous and anisotropic confined aquifer overlying a gravel substratum. Solutions are obtained for two general cases of the problem: (1) when the level of the pumped hole is below the confining layer; (2) when it is above it. The validity of the proposed theory is tested by comparing analytical predictions obtained for a few flow situations with corresponding results obtained by numerical means. The theory presented here can be utilized to convert the rate of rise of water in a pumped auger hole into directional conductivities of soil, in areas where water is found to be in a confined state overlying a gravel substratum. The study shows that the conductivity values calculated by neglecting the confining pressure of an artesian aquifer with a gravel base [i.e., by applying the existing (Boast and Kirkham in 1971), auger hole seepage theory for a phreatic aquifer with a gravel base to confined situations] may result in serious error; hence, the artesian head of an aquifer must be accounted for while computing the conductivity values. Further, it is observed that the area contributing flow to a pumped auger hole/well with a gravel base is mostly restricted to a short radial distance from the center of the hole, particularly for situations where the gravel substratum is located close to the bottom of the hole. This is in contrast to auger hole flow situations overlying an impervious substratum, where the domain contributing flow is mostly spread out to a considerable distance from the center of the hole.     

Cytophotometric determination of the relationship between the chief isoenzyme forms of lactate dehydrogenase in the neuron--neuroglia system in different fuctional states of the nervous system

The radioactive microsphere method was used to estimate simultaneoulsy the cardiac output and its distribution in the same rat by the use of an arterial reference sample obtained during microsphere distribution. Sufficient microspheres were injected so that all counted samples contained more than 400 spheres. No haemodynamic changes occurred during microsphere injection. The results for cardiac output of 253 +/- 11 ml/min per kg body weight agree with published estimates utilizing other techniques. The distribution of cardiac output also agrees with most published reports. This method should allow the rat to be conveniently used for certain haemodynamic studies when cardiac output and organ blood flow are necessary.     

Thermodynamic Framework for Analysis of Waste Containment Barrier Materials

In order to analyze the fate and estimate the transport rates of contaminants through a barrier system, textural parameters such as the specific surface, density, permeability, diffusion coefficient, and flow path tortuosity are usually measured or estimated. The magnitudes of transport parameters of barrier systems are expected to change in response to physicochemical reactions and other environmental stresses, the intensities of which may grow or wane over time. In essence, when discrete catastrophic events (for example, earthquakes) are discounted, the flaws that develop are macroscopic manifestations of microlevel processes. Processes such as crystallization and precipitation add solid material to pore spaces in barriers and can improve barrier performance. Conversely, processes that cause changes in state from solid to liquid (for example, material dissolution) degrade barriers through the creation of larger flow channels. An appreciation of the thermodynamics of contaminant/barrier interactions under various environmental (temperature, pressure, and moisture) conditions is a prerequisite for establishing the bounds for textural changes and estimating contaminant release rates from containment systems. Then, process kinetics can be used to estimate the rate at which such texture-controlling processes may occur. The alternative approach is to conduct numerous “test-and-see” factorial experiments of limited utility, in which one parameter is changed at a time. The latter approach consumes resources excessively, relative to an approach that involves the use of thermodynamics to minimize the number of tests. In this paper, long-term deterioration mechanisms are analyzed, and a framework for their assessment within the context of barrier system performance modeling is presented.     

Experimental Investigation of the Ce-Mg-Mn Isothermal Section at 723 K (450 °C) via Diffusion Couples Technique

The isothermal section of the Ce-Mg-Mn phase diagram at 723 K (450 °C) was established experimentally by means of diffusion couples and key alloys. The phase relationships in the complete composition range were determined based on six solid–solid diffusion couples and twelve annealed key alloys. No ternary compounds were found in the Ce-Mg-Mn system at 723 K (450 °C). X-ray diffraction and energy-dispersive X-ray spectroscopy spot analyses were used for phase identification. EDS line-scans, across the diffusion layers, were performed to determine the binary and ternary homogeneity ranges. Mn was observed in the diffusion couples and key alloys microstructures as either a solute element in the Ce-Mg compounds or as a pure element, because it has no tendency to form intermetallic compounds with either Ce or Mg. The fast at. interdiffusion of Ce and Mg produces several binary compounds (Ce _x Mg _y ) during the diffusion process. Thus, the diffusion layers formed in the ternary diffusion couples were similar to those in the Ce-Mg binary diffusion couples, except that the ternary diffusion couples contain layers of Ce-Mg compounds that dissolve certain amount of Mn. Also, the ternary diffusion couples showed layers containing islands of pure Mn distributed in most diffusion zones. As a result, the phase boundary lines were pointing toward Mn-rich corner, which supports the tendency of Mn to be in equilibrium with all the phases in the system.     

Wall-to-bed heat transfer in a circulating fluidized bed for the reduction of iron ore particles

A model is presented that describes the wall-to-bed heat transfer in a circulating fluidized bed (CFB) used for the prereduction of iron ore in the smelting-reduction iron-making process. The model incorporates the core-annulus type flow structure and the wall emulsion layer growing downward along the surface. Model predictions showed good agreements with measured data taken from the literature. The hydrodynamic behavior near the wall surface was able to be properly described by the core-annulus flow structure. A higher heat-transfer coefficient with higher solid circulation flux was obtained in the upper part of the bed because of the heat input caused by the lateral diffusion of particles from the core. The predicted and measured data also showed the minima in the heat-transfer coefficients in the lower part of the bed. Model predictions indicated that in the CFB for the reduction of iron ore particles, it is important to properly control the inlet temperature of the reducing gas, rather than that of the solid particles. The implications of the behavior of heat transfer in the CFB are discussed for the reduction of iron oxides.     

Particle Penetration through Inclined and L-Shaped Cracks

This paper presents a particle penetration model predicting particle penetration coefficient (Pp) through a narrow crack of arbitrary incline angles (θ). The objective was to simulate Pp for outdoor-to-indoor particle penetration for residential infiltration conditions. This model assumes laminar infiltration flow and considers particle deposition from both gravitational sedimentation and Brownian diffusion. For micron-sized particles, modeling results indicate that gravitational sedimentation is the major deposition mechanism. Pp increases monotonically with ∣θ∣ because effective particle sedimentation velocity (vs?cos?θ) decreases monotonically with ∣θ∣. For submicron-sized particles (0.1?μm), Brownian diffusion is the major particle deposition mechanism. Because Brownian diffusion is a nondirectional deposition mechanism, crack inclination did not affect Pp. This study applied this model to estimate Pp for L-shaped cracks, and validated modeling results with experiments. Experimental results indicated that inertial impaction and crack entrance cutoff effects were not significant particle deposition mechanisms for the test micron-sized particles. Gravitational sedimentation was the major deposition mechanism. An L-shaped crack can be simulated as the combination of horizontal and vertical sections. This model agreed reasonably with experimental results.     

A computational model of pulmonary gas transport incorporating effective diffusion

A computational model of gas transport in the lung is described which remedies many of the deficiencies of previous models, as listed by Chang and Farhi (1973), in that it allows for fluctuating lung dimensions, gas exchange, simultaneous convection and diffusion, and the enhanced effective diffusion that occurs when convective flow is also present. The results of calculations using the model are presented, showing the maximum effect of Taylor diffusion. The actual magnitude of Taylor diffusion, suitably modified to allow for the disturbed conditions within the lung, is considered in the light of recent experiments.     

Theory of Plastic Sand Flow with Fluid Pressure Effect

Fundamental principles of elastic–plastic mechanics of soils and rocks are given on the base of the original publications. The solid friction and dilatancy effects are included in the nonstandard form of nonassociative rule of plastic flow. The resulting hyperbolic system of equations is represented for a plane case. The slip surfaces are assumed to be jump tangential discontinuities of a velocity field. The possibility of limit equilibrium at slip surfaces is accounted for. The attempts to account for grain rotations, permitting study of slip surface structure, are discussed. The Biot–Frenkel model of interpenetrating continua is developed for plastic flow of porous saturated matrix. In this case the solid matrix state is determined by the effective stresses and pore pressure diffusion happens in plastically flowing matrix. To illustrate the theory possibilities, solutions for failure and mass sand flow, driven by the pore pressure gradient, are selected. They are important especially for oil/gas reservoirs with a weak matrix, typical for offshore geology.     

Triaxial Compression on Semi-solid Alloys

Multi-axial compression of the mushy zone occurs in various pressurized casting processes. Here, we present a drained triaxial compression apparatus for semi-solid alloys that allow liquid to be drawn into or expelled from the sample in response to isotropic or triaxial compression. The rig is used to measure the pressure-dependent flow stress and volumetric response during isothermal triaxial compression of globular semi-solid Al-15 wt pct Cu at 70 to 85 vol pct solid. Analysis of the stress paths and the stress–volume data show that the combination of the solid fraction and mean effective pressure determines whether the material undergoes shear-induced dilation or contraction. The results are compared with the critical state soil mechanics (CSSM) framework and the similarities and differences in behavior between equiaxed semi-solid alloys and soils are discussed.

     

Interval isotachophoresis for purification and isolation of ionogenic species

The stability of continuous flow electrophoresis (CFE) was tested by performing uninterrupted continuous flow cationic isotachophoresis for 15 h, using a mixture of two synthetic isoelectric point (pI) markers as the model sample. This sample as well as the other operational electrolytes were continuously pumped into the CFE separation chamber via 10 individual inlets. The operational electrolyte system and the arrangement of the electrolyte streams were selected in accord with previously published results, providing a stable electrophoretic regime. The chamber had 96 individual outlets and here 96 individual fractions were collected in microwell plates in regular time intervals and scanned photometrically. The scanned photometric patterns proved that CFE is sufficiently stable for an overnight operation.     

Microsegregation in cellular solidification

Microsegregation in a binary alloy solidified in the form of deep cells is predicted using a simplified finite difference model. The model accounts for solid state diffusion and for flow of liquid between cells driven by solidification shrinkage. Cell tip undercooling is predicted using the expression originally derived by Boweret al. Cells are assumed to be cylindrical, and solid state diffusion along the cell axis is ignored, simplifying considerably prediction of solid state diffusion and cell shape behind the tip, which are treated as a one-dimensional moving boundary problem. Experiments were conducted on binary Al-4.5 wt pct Cu, solidified in the cellular growth regime using a Bridgman furnace. Microsegregation in the samples was measured and is compared to predictions; good agreement is found, both for cell heights and microsegregation in the fully solidified material. It is found that intercellular fluid flow exerts a small, but discernable, influence on microsegregation and cell shape. Formerly Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Effect of Adsorbent Particle Layering on Performance of Conventional and Tapered Fixed-Bed Adsorbers

Experimental and modeling studies were conducted for the adsorption of phenol from aqueous solutions onto activated carbon in fixed beds with the adsorbent particles layered according to particle size. In the conventional stratified cylindrical adsorber (SCA), the particles were layered according to natural stratification, and increased in size with column depth. In the reverse stratified tapered adsorber (RSTA), the particle size decreased with column depth, and the fluid velocity decreased in the direction of flow. Experimental data indicate that for a uniform particle size distribution, the breakthrough time for the RSTA was about 60% higher than for the SCA under identical carbon loading and flow conditions. The homogeneous solid phase diffusion model with Linear-Freundlich isotherm was used to model the layered adsorbers. It provides excellent predictions for breakthrough curves at various column depths. Bed capacity utilization can be increased with the RSTA due to the sharpening of the solute front, and this will translate into lower capital and operating costs for the carbon adsorption system due to the smaller unit required, lower carbon inventory, and lower pumping costs.     

Alloy formation during the sintering of molybdenum-chromium powder mixtures

Conclusions A molybdenum-chromium alloy forms through preferential diffusion of chromium into pure molybdenum, with the formation of a stable molybdenum-base intermediate solid solution, followed, after the disappearance of pure molybdenum, by diffusion of chromium into the intermediate solid solution, resulting in a steady increase in the latter's concentration. The picture of alloy formation in the molybdenum-chromium system differs radically from that in a system exhibiting full intersolubility of components in the solid state, such as the copper-nickel system.Translated from Poroshkovaya Metallurgiya, No. 8(224), pp. 27–32, August, 1981.     

Tracer diffusion through F-actin: effect of filament length and cross-linking

We have determined diffusion coefficients for small (50- to 70-nm diameter) fluorescein-thiocarbamoyl-labeled Ficoll tracers through F-actin as a function of filament length and cross-linking. fx45 was used to regulate filament length and avidin/biotinylated actin or ABP-280 was used to prepare cross-linked actin gels. We found that tracer diffusion was generally independent of filament length in agreement with theoretical predictions for diffusion through solutions of rods. However, in some experiments diffusion was slower through short (< or = 1.0 micron) filaments, although this result was not consistently reproducible. Measured diffusion coefficients through unregulated F-actin and filaments of lengths > 1.0 micron were more rapid than predicted by theory for tracer diffusion through rigid, random networks, which was consistent with some degree of actin bundling. Avidin-induced cross-linking of biotinylated F-actin did not affect diffusion through unregulated F-actin, but in cases where diffusion was slower through short filaments this cross-linking method resulted in enhanced tracer diffusion rates indistinguishable from unregulated F-actin. This finding, in conjunction with increased turbidity of 1.0-micron filaments upon avidin cross-linking, indicated that this cross-linking method induces F-actin bundling. By contrast, ABP-280 cross-linking retarded diffusion through unregulated F-actin and decreased turbidity. Tracer diffusion under these conditions was well approximated by the diffusion theory. Both cross-linking procedures resulted in gel formation as determined by falling ball viscometry. These results demonstrate that network microscopic geometry is dependent on the cross-linking method, although both methods markedly increase F-actin macroscopic viscosity.     

Modeling of microsegregation in macrosegregation computations

A general framework for the calculation of micro-macrosegregation during solidification of metallic alloys is presented. In particular, the problems of back diffusion in the primary solid phase, of eutectic precipitation at the end of solidification, and of remelting are being addressed for an open system,i.e., for a small-volume element whose overall solute content is not necessarily constant. Assuming that the variations of enthalpy and of solute content are known from the solution of the macroscopic continuity equations, a model is derived which allows for the calculation of the local solidification path (i.e., cooling curve, volume fraction of solid, and concentrations in the liquid and solid phases). This general framework encompasses four microsegregation models for the diffusion in the solid phase: (1) an approximate solution based upon an internal variable approach; (2) a modification of this based upon a power-law approximation of the solute profile; (3) an approach which approximates the solute profile in the primary phase by a cubic function; and (4) a numerical solution of the diffusion equation based upon a coordinate transformation. These methods are described and compared for several situations, including solidification/remelting of a closed/open volume element whose enthalpy and solute content histories are known functions of time. It is shown that the solidification path calculated with method 2 is more accurate than using method 1, and that 2 is a very good approximation in macrosegregation calculations. Furthermore, it is shown that method 3 is almost identical to that obtained with a numerical solution of the diffusion equation (method 4). Although the presented results pertain to a simple binary alloy, the framework is general and can be extended to multicomponent systems.     

Ternary Diffusion. Solutions with Diffusion Coefficients Linearly Dependent on Concentrations

A numerical solution was devised for the set of differential equations characterizing interdiffusion in a ternary system in which the partial diffusion coefficients are linear functions of the concentrations. A one-step fourth order Runge-Kutta method was used in the computer program. The concentration-penetration curves obtained accurately reflect the behavior of an interdiffusing ternary ideal solid solution provided that the tracer diffusion coefficients are constant and the ratios of the latter are not very different from unity. A theorem was derived which specifies the conditions for which the points of inflection of the three concentration-penetration curves occur at the same position in the diffusion couple. Also, it was shown that in an ideal interdiffusing ternary solid solution, zero-flux plane positions cannot correspond exactly to the points of intersection of the diffusion path and iso-activity lines drawn through the terminal alloys of the diffusion couple, but approach the latter in the limiting case. Finally, a method was developed to calculate the partial interdiffusion coefficients at a composition point on the diffusion path of an ideal ternary solid solution diffusion couple through the use of information obtained only from that one diffusion couple.