Papers by Keyword: Chemical Diffusion

Abstract: The new technique of atomic-scale X-ray Photon Correlation Spectroscopy (aXPCS) makesuse of a coherent X-ray beam to study the dynamics of various processes in condensed matter systems.Particularly atomistic migration mechanisms are still far from being understood in most of intermetallicalloys and in amorphous systems. Special emphasis must be given to the opportunity to measureatomistic diffusion at relatively low temperatures where such measurements were far out of reach withpreviously established methods. The importance of short-range order is demonstrated on the basis ofMonte Carlo simulations.

Abstract: Crystal structure,thermal expansion, oxygen non-stoichiometry, electrical conductivity and diffusion characteristics of two analogous LaFe_0.7Ni_0.3O_3‑d and PrFe_0.7Ni_0.3O_3‑d compositions were investigated depending on temperature (201000 °C) and oxygen partial pressure (0.6–21000 Pa). The found oxygen diffusion and oxygen exchange coefficients for the both compositions at similar conditions are near to each other and varied in the range of 110^‑7110^‑5 cm²s^‑1 and 510^‑6110^‑4 cms^‑1, respectively.

Abstract: In their paper, R. Merkle et al [R. Merkle, J. Maier, K.D. Becker and M. Kreye, Phys. Chem. Chem. Phys. 6, 3633 (2004)] conducted an experimental study on the chemical diffusion of oxygen in Fe-doped SrTiO3 single crystals driven by large changes in the oxygen ambient partial pressure. The stoichiometry dependence of the chemical diffusion coefficient was derived on the basis of the concept of conservative ensembles for two independent trapping reactions, which then served for calculating the evolution of vacancy profiles. The theoretical predictions were compared to the experimental results. In the framework of the same model, in the present communication, the chemical diffusion of oxygen was analyzed by the concept of a dynamic reaction front [M. Sinder, J. Pelleg, Phys. Rev. E 61, 4935 (2000); Z. Koza, Phys. Rev. E 66, 011103 (2002)]. We show, that by using a quasi-chemical reaction rate profile, it is possible to obtain information relating to the position and width of the zone where the reaction takes place. It is indicated, that the reaction rate distribution can be directly calculated from measured concentration profiles of the immobile reactant.

Abstract: The model of the heterogeneous reaction between the alloy and oxidant is shown. The alloy reacts with oxidant and forms oxide scale. The reaction rate is controlled by the interdiffusion in alloy and the chemical diffusion through the compact scale. In this work we extend the Wagner model by introducing i) the variable instantaneous rate constant, ii) the composition dependent diffusivities of the alloy components and iii) the finite geometry of the oxidized alloy. The model allows us to predict the life time of the alloy and the evolution of the components concentration. The comparison of Wagner’s results and our extended analysis is shown.

Abstract: Tracer diffusion experiments have historically furnished much of the information about fundamental diffusion processes as embodied in such quantities as tracer correlation factors and vacancy-atom exchange frequencies. As tracer diffusion experiments using radiotracers are rather less often performed nowadays, it is important to be able to process other diffusion data to provide similar fundamental information. New procedures that are primarily based around the random alloy model have been established recently for analyzing chemical diffusion data in binary and ternary alloy systems. These procedures are reviewed here. First, we review the random alloy model, the Sum-rule relating the phenomenological coefficients and three diffusion kinetics formalisms making use of the random alloy. Next, we show how atom-vacancy exchange frequency ratios and then component tracer correlation factors can be extracted from chemical diffusion data in alloy systems. Examples are taken from intrinsic diffusion and interdiffusion data in a number of binary and ternary alloys.

Abstract: In this paper, we present and discuss some of the theoretical procedures that have been established recently for binary and ternary alloy systems for the purposes of analyzing chemical diffusion data (interdiffusion and intrinsic diffusion) alone and chemical diffusion data in combination with tracer diffusion data. Emphasis is put on extracting information about diffusion mechanisms by way of tracer correlation factors/vacancy-wind factors. Examples are taken from the intrinsic diffusion, interdiffusion and tracer diffusion data in the Ag-Cd and Ag-Cd-Zn, Fe-Ni-Cr and Cu-Fe-Ni alloy systems.

Abstract: The six-jump-cycle (6JC) diffusion mechanism is used to analyze the behavior of vacancy-wind factors and collective correlation factors in partially ordered B2 intermetallic compounds at stoichiometric and near- stoichiometric compositions. Expressions for the vacancywind factors are obtained in the framework of the four-frequency model where the two sublattices exist a priori. The phenomenological coefficients on the two sublattices that remain hitherto independent in 6JC mechanism are connected through a microscopic detailed balance condition. The present results for collective correlation factors when compared with our earlier calculation based on taking the harmonic mean of the sublattice correlation factors show much better agreement with Monte Carlo simulation results. The collective correlation factors and tracer correlation factors are used to calculate the vacancy-wind factors. Our results for vacancy-wind factors agree qualitatively with the simulation data when the frequency ratio ( α ) of structural and antistructural atoms jumps decreases up to the order of unity.

Abstract: In this overview, we discuss the sum-rule relating phenomenological coefficients in randomly mixed systems and consider several applications to collective diffusion problems. These applications include intrinsic diffusion multicomponent alloys, chemical diffusion in strongly ionic mixed cation crystals and demixing of cations in randomly mixed quaternary transition metal oxides. In each case a substantial simplification is possible as a result of the sum-rule.