Intrinsic Diffusion Coefficients Research Articles

Phase boundary migration, Kirkendall marker shift and atomic mobilities in fcc Au–Pt alloys

The CALPHAD technique has been utilized to study the diffusion characteristics and atomic mobilities in fcc Au–Pt alloys, where the thermodynamics and kinetics descriptions are concurrently taken into consideration. For the investigated atomic mobilities, the end-members and interaction parameters are explored with the impurity diffusion coefficients, intrinsic diffusion coefficients and interdiffusion coefficients. Concentration curves in diffusion couples are computationally studied, the results of which are compared with the experimental values. In addition, the shift and distribution of Kirkendall markers are brought into discussion, based on the intrinsic diffusivities described in the lattice-fixed frame of reference.

Interdiffusion and the vacancy wind effect in Ni–Pt and Co–Pt systems

Abstract

Dehydration of acetic acid by pervaporation using filled IPN membranes

Polyvinyl alcohol (PVOH) was chemically modified by crosslink copolymerization of acrylic acid (AA) and acrylamide (AM) in aqueous solution of PVOH and finally crosslinking the copolymer of AA and AM designated as PAAAM with N, N′-methylenebisacrylamide (NMBA) and PVOH with glutaraldehyde to produce a full interpenetrating network (FIPN) membrane. Accordingly, a membrane containing PVOH:PAAAM of 1:0.5 designated as FIPN500 was synthesized. Filled FIPN membranes were synthesized by in situ incorporation of highly hydrophilic aluminosilicate filler during copolymerization of the monomers in PVOH matrix to produce three filled membranes designated as FIPN502, FIPN505, and FIPN510 containing 2, 5 and 10 mass%, respectively (of total polymer) of the filler. PVOH membrane cosslinked with 2 mass% glutaraldehyde, PAAAM copolymer modified PVOH membrane i.e. FIPN500 and the three filled FIPN membranes were used for sorption and pervaporative dehydration of acetic acid. The filled IPN membranes were found to show higher flux and water selectivity than the unfilled membranes. Among the three filled membranes, FIPN510 was found to show the highest flux (6.612 kg m 2 h −1 μm) and water selectivity (325.53) at 0.953 mass% water in feed. Interaction parameters, partial permeability, intrinsic membrane selectivity and concentration average diffusion coefficients for all the membranes were also evaluated.

Behavior of oxygen diffusion in buffer layers for coated conductors

We have evaluated the effect of annealing in oxygen atmosphere on the structure, texture and phase transformation of LZO films deposited on YSZ (yttria-stabilized zirconia) (0 0 l) single crystal substrates and textured NiW substrates by metal-organic deposition (MOD) method. The results show that the structure stability of the LZO films is heavily dependent on the oxygen partial pressure in annealing process. Then we have in details studied the behavior of oxygen diffusion in three kinds of buffer layer architectures on NiW substrates by varying the temperature, oxygen partial pressure and dwelling time in the annealing process. The oxygen diffusion within buffer layers leads to the oxidation of substrate, and even the texture and structure of buffer layers are destroyed with the increase of the thickness of the oxides layer related to NiW substrate. It reveals that the relative volume of oxides related to NiW substrate increases exponentially with the annealing temperature, and increases linearly with the annealing time at logarithmic scale. The relative intensity of texture peaks of buffer layers decreases and even disappears with the increase of the oxygen partial pressure in annealing process because of the acceleration of the oxidation reaction of substrate. The influence of annealing temperature, oxygen partial pressure and dwelling time on the oxygen diffusion is related to the intrinsic oxygen diffusion coefficient of buffer layers materials. Compared with the increase of oxygen partial pressure, the elongation of dwelling time shows a less effect on the oxidation rate of NiW substrate and a weak destruction of the texture of buffer layers. Except choosing the oxide materials with small oxygen diffusion coefficient as buffer layers in coated conductors, the degree of oxidation about NiW substrate could be greatly controlled and it would result in the less destruction of texture and structure of buffer layers by adjusting the annealing temperature, oxygen partial pressure and dwelling time in the process of YBCO deposition.

Intrinsic and Interdiffusion in Cu-Sn System

Solid-solid diffusion couples assembled with disks of copper, tin and intermetallics (Cu3Sn and Cu6Sn5) were employed to investigate the Kirkendall effect in the copper-tin system at the temperature of 200 °C. In the Cu(99.9%)/Sn diffusion couple, inert alumina particles used as markers were identified in the Cu6Sn5 phase, while microvoids were observed at the Cu/Cu3Sn interface. The Cu(99.9%)/Sn and Cu(99.9%)/Cu6Sn5 diffusion couples annealed at 200 °C for 10 days were analyzed for intrinsic diffusion coefficients of Cu and Sn in the Cu6Sn5 and Cu3Sn phases, respectively with due consideration of changes in molar volume. Interdiffusion, integrated and effective interdiffusion coefficients were also calculated for the intermetallic phases. Diffusion couples annealed at 125-400 °C for various times were analyzed for the kinetic parameters such as growth rate constants and activation energies for the formation of Cu3Sn and Cu6Sn5 phases. Uncertainties in the calculated intrinsic diffusivities of Cu and Sn arise mainly from the non-planar morphologies of the interfaces and the non-planar distribution of the markers. Intrinsic diffusion coefficients based on average locations of the marker plane indicate that Cu is the faster diffusing component than Sn in both the Cu3Sn and Cu6Sn5 phases.

Diffusion and Growth of the μ Phase (Ni6Nb7) in the Ni-Nb System

Incremental diffusion couple experiments are conducted to determine the average interdiffusion coefficient and the intrinsic diffusion coefficients of the species in the Ni6Nb7 (μ phase) in the Ni-Nb system. Further, the tracer diffusion coefficients are calculated from the knowledge of thermodynamic parameters. The diffusion rate of Ni is found to be higher than that of Nb, which indicates higher defect concentration in the Ni sublattice.

The influence of the blast furnace slag replacement on chloride penetration in concrete

Corrosion of steel reinforcement due to chloride penetration is the greatest cause of durability problems in concrete; intense international research has been carried out to understand and avoid this. This paper summarises the results of a theoretical and experimental research programme investigating the influence of blast furnace slag on chloride-related transport properties. The relationship between the apparent chloride diffusion coefficient, electrical resistivity and compressive strength was measured. Chloride, hydroxide, sodium and potassium's intrinsic diffusion coefficients were obtained by using a computational model and an electrical migration test. The initial hydroxide composition of the pore solution, porosity and chloride binding capacity were also determined from the model. The results showed that blast furnace slag improved chloride penetration resistance, resulting in concrete which was less vulnerable to corrosion.

Diffusion in Co-Ni System Studied by Multifoil Technique

Diffusion couple experiments were performed in the Co-Ni binary system for determining inter-, impurity- and intrinsic-diffusion coefficients in the temperature range of 1050 - 1250°C. The activation energy and pre-exponential factor estimated for interdiffusion do not vary significantly with composition. The activation energy calculated for impurity diffusion experiments shows is higher than . Intrinsic diffusion coefficients estimated from the multifoil experiment show that Ni is the fastest diffusing species in this system.

Study of diffusion and atomic mobilities for fcc Ag–Cd and Ag–Sn solder alloys

Diffusion in Ag-based solders is a practical topic that has attracted world-wide attention. With the available experimental data in the literature, the atomic mobilities of Ag, Cd and Sn in fcc Ag–Cd and Ag–Sn alloys are critically explored in this work. The proposed atomic mobility parameters can reproduce a great majority of experimental data, including impurity diffusion coefficients, tracer diffusion coefficients, intrinsic diffusion coefficients and interdiffusion coefficients. The evolutions of composition profiles and Kirkendall marker shifts in Ag–Cd binary diffusion couples are also computationally studied. The atomic mobilities obtained in this work are essential to guide the design of candidate Ag-based alloys.

Stress Generation during Grain Boundary Interdiffusion

A grain boundary interdiffusion in a semi-infinite bicrystal under the conditions of negligible bulk diffusion is considered. We show that the inequality of intrinsic grain boundary diffusion coefficients of the two components leads to plating out of additional material at the grain boundary in the form of extra material wedge, which generates an elastic stress field in the vicinity of the grain boundary. We solved a coupled diffusion/elasticity problem and determined the time-dependent stress field and concentration distribution in the vicinity of the grain boundary.

Probing passive diffusion of flagellated and deflagellated Escherichia coli

Using particle-tracking techniques, the translational and rotational diffusion of paralyzed E. coli with and without flagella are studied experimentally. The position and orientation of the bacteria are tracked in the lab frame and their corresponding mean-square displacements are analyzed in the lab frame and in the body frame to extract the intrinsic anisotropic translational diffusion coefficients as well as the rotational diffusion coefficient for both strains. The deflagellated strain is found to show an anisotropic translational diffusion, with diffusion coefficients that are compatible with theoretical estimates based on its measured geometrical features. The corresponding translational diffusion coefficients of the flagellated strain have been found to be reduced as compared to those of the deflagellated counterpart. Similar results have also been found for the rotational diffusion coefficients of the two strains. Our results suggest that the presence of flagella --even as a passive component-- has a significant role in the dynamics of E. coli, and should be taken into account in theoretical studies of its motion.

Grain boundary interdiffusion and stresses in thin polycrystalline films

We consider the kinetics of tensile stress relaxation in thin metal film attached to inert substrate, controlled by chemical interdiffusion along the grain boundaries. We assume that the source of diffusing atoms is located at the surface of the film. We show that the kinetics of stress relaxation in the film can be either accelerated or slowed down if compared with the same kinetics in a single-component film, depending on the difference of intrinsic GB diffusion coefficients of the two components. In the case of faster matrix atoms, the tensile stress in the film significantly increases beyond its initial value at the beginning of interdiffusion process, while in the case of faster diffuser atoms, the compressive stresses develop in the film at the intermediate stages of stress evolution.

Theory of the Kirkendall effect during grain boundary interdiffusion

A grain boundary interdiffusion in a semi-infinite bicrystal under the conditions of negligible bulk diffusion is considered. We show that the inequality of the intrinsic grain boundary diffusion coefficients of the two components leads to plating out of additional material at the grain boundary in the form of a wedge of extra material, which generates an elastic stress field in the vicinity of the grain boundary. We solved a coupled diffusion/elasticity problem and determined the time-dependent stress field and concentration distribution in the vicinity of the grain boundary. We show that diffusion of embrittling impurities along the grain boundary generates tensile stresses at the boundary which are high enough to cause intergranular fracture.

The effect of large vacancy concentration on intrinsic and interdiffusion coefficients: A first-principle study of B2-NiAl

Most treatments of interdiffusion in metallic alloys implicitly or explicitly rely on the assumption of a dilute vacancy concentration. Important intermetallic compounds such as B2-NiAl, however, are characterized by large vacancy concentrations. Here we investigate how large vacancy concentrations affect intrinsic and interdiffusion coefficients. We use B2-NiAl as an example and calculate all parameters that enter into a phenomenological description of interdiffusion from first principles, including thermodynamic factors, the effect of a concentration dependent volume, intrinsic diffusion and interdiffusion coefficients. We find that in Al-rich alloys, characterized by high vacancy concentrations, the thermodynamic factor of Al is larger than that of Ni by several orders of magnitude, while in Ni-rich alloys, where the vacancy concentration is very low, they are identical. Similarly, the effects of changing cell volume on intrinsic diffusion coefficients are very different for Al and Ni for the alloy compositions in which the vacancy concentration is high. Comparison of calculated interdiffusion coefficients with experimentally measured values show acceptable agreement in Ni-rich alloys but poor agreement in Al-rich alloys.

ChemInform Abstract: Anomalous Behavior of Cumene Cracking in Small-Pore Catalysts.

Abstract The reaction sequence in small-pore catalysts has been postulated to be as follows: adsorption of the reactant from the gas phase to the outside surface of the catalyst to form a condensed-type phase; diffusion of the condensed molecules along the pore walls analogous to surface diffusion, then reaction on active sites, followed by diffusion of the products to the external surface of the catalyst; finally, desorption of the products. This is in contrast to the conventional diffusion, adsorption, and reaction model which is suitable for larger-pore catalysts. Kinetic data from the cumene cracking reaction over REY zeolites are used in this work. The data were obtained at a wide range of temperatures, from 225 to 575°C, sufficient to include the regions of kinetic control and strong pore diffusion. Under these conditions and using the above model, the kinetic and diffusion parameters for cumene cracking are obtained. The intrinsic reaction rate constant and diffusion coefficient over REY zeolites are 2.5 × 106 cm3/g/s and 1.9 × 10−9 cm2/s at 500°C, with activation energies of 33 and 15 kcal/mol, respectively.

Dynamics of Semiflexible Dendrimers in Dilute Solutions

The dilute solution dynamic properties of dendrimers are theoretically studied in the framework of an optimized Rouse−Zimm approach. Semiflexibility is implemented by modeling topological restrictions on the directions and orientations of the respective bond vectors. This accounts for the neglect of excluded volume interactions in the limit of short spacers. The transport properties, particularly, the behavior of intrinsic viscosity and translational diffusion coefficient are analyzed as functions of flexibility and dendrimer generational growth. The intrinsic viscosity reveals a characteristic maxima, as is experimentally observed in some cases. The value and position of the maxima varies with the topology of the molecule and is mainly dictated by the longest relaxation times. The diffusion coefficient shows a steady decrease for all topologically different conformations up to the fourth generation. The inclusion of hydrodynamic interactions accelerates the dynamics of all semiflexible dendrimer models by decreasing the respective range of intrinsic viscosity and diffusion coefficient values. The structural differences between the different conformations are manifest in the subsequent generations. These topologically dependent flexibility parameters provide suitable approximations to explore the intramolecular dynamics of real dendrimers.

Study of atomic mobilities and diffusion characteristics in bcc Ti–Ta and Ta–W alloys

In order to assist the design of bcc Ti–Ta and Ta–W alloys, the atomic mobilities of Ti, Ta and W are studied critically with the DICTRA software (DIffusion Controlled TRAnsformation), based on the published thermodynamic description, self-diffusion coefficients, impurity diffusion coefficients, intrinsic diffusion coefficients and interdiffusion coefficients. Good agreement is obtained from the comprehensive comparisons between the calculated and measured diffusion coefficients. To further verify the obtained atomic mobilities, the concentration profiles and Kirkendall marker shift in bcc Ti–Ta and Ta–W diffusion couples are explored computationally, the results of which can reproduce satisfactorily the experimental measurements.

Oxygen Diffusion and Electrical Conductivity Measurements in Uranium Dioxide

In this paper, we describe two experimental set-ups which enable the measurement of electrical properties and intrinsic diffusion coefficients in UO2. Electrical conductivity measurements are insured by a standard four point Kelvin-Bridge method. In parallel, the gas-solid isotopic exchange method is used to load the samples with 18O tracer atoms, the concentration profile of which are subsequently characterized using SIMS and chromatic confocal microscopy. An application of both types of measurements on a UO2 single crystal is given. The diffusion study was carried out at 750°C, and the electrical conductivity study was performed between 1000°C and 1300°C at oxygen potentials at which the material exhibits extrinsic behaviour. We show how a careful use of both measurements in conjunction can be an indication of the operative migration mechanism.

Numerical Determination of Intrinsic Diffusion in Fe-Cr-Al Systems

The intrinsic diffusion coefficients in diffusion aluminide coatings based on Fe-30Cr were determined at 1000oC. The diffusion fluxes were given by the Nernst Planck formulae and the Darken method for multicomponent systems was applied. This paper summarizes some numerical results to determine the composition dependent diffusivities in Fe-Cr-Al systems. The method presented in this study to obtain average intrinsic diffusion coefficients is as an alternative to the Dayananda method. Our method based on empirical parameters allowed us to predict the concentration profile during the interdiffusion process.

Assessing Concentration Dependence of FCC Metal Alloy Diffusion Coefficients Using Kinetic Monte Carlo

Intrinsic diffusion coefficients have been calculated for a solid solution binary fcc metal alloy with vacancies using grand canonical and kinetic Monte Carlo (MC) methods for a variety of model Hamiltonians. Model Hamiltonians include a kinetically and thermodynamically ideal case, solute-vacancy attraction and repulsion, and solute-solute attraction and repulsion. These model Hamiltonians are chosen to have constant average activation energies in order to focus on contributions from other thermodynamic and kinetic factors. The thermodynamic factor calculated using MC is compared to a mean-field regular solution model. It is shown that the mean-field model accurately predicts the thermodynamic factors for each model alloy Hamiltonian except for the alloys with a solute-solute interaction and concentration that are in the spinodal region (as predicted by the regular solution model). The MC determined concentration-dependent intrinsic diffusion coefficients are compared to values determined from the dilute five-frequency model and Darken and Manning analytical approximations. The results indicate that for a solid solution with known average barriers and vacancy concentration, Darken and Manning approximation-based analytic expressions and mean-field theory can be used to predict concentration-dependent diffusion coefficients within a factor of approximately three, provided the system is outside of the spinodal region. The good accuracy of this approximate approach follows from the fact that the thermodynamic factor is the main contribution to the concentration dependence of the diffusion constants, and that this thermodynamic factor is well described by mean-field theory.