Defect and Diffusion Forum Vols. 273-276

Abstract: Osmotic dehydration of eggplant slices in sucrose and sodium chloride solutions at different solution concentrations, stirring levels and time of process is studied. Water loss and solids gain kinetics were experimentally determined and modelled using a diffusional model and equilibrium values were predicted by means of Azuara model. Modelling was satisfactory and involved effective coefficients of diffusion useful to quantify the different mass transfer fluxes.

Abstract: It is known that two main interdiffusion coefficients, ık Dii and ık Djj , as well as two cross interdiffusion coefficients, ık Dij and ık Dji , are necessary for understanding the atomic diffusion for ternary system. Here, k is the host element of ternary system, and i and j are solute elements. These four interdiffusion coefficients are obtained from a series of experiments using two kinds of ternary diffusion couples. In general, it is believed that ık Dij and ık Dji indicate the same sign to each other, but there are a lot of experimental data showing that ık Dij and ık Dji indicate opposite sign [1]. In such a case, the physical meaning of the cross interdiffusion coefficient has not always been understood thoroughly. The purposes of this study are to measure the interdiffusion coefficients by a series of experiments and to elucidate the physical meaning of the two cross interdiffusion coefficients on the basis of the consideration about the relationship between the thermodynamic functions and the cross interdiffusion coefficients. It is concluded that ık Dij exhibits the opposite sign to ık Dji without contradicting the Onsarger’s reciprocity theorem when the ( 2 2 ) c j ∂ G ∂c shows the opposite sign to ( 2 2 ) c i ∂ G ∂c . Here, c G is Gibbs free energy of the ternary system.

Abstract: We model the grain boundary tracer diffusion problem by constructing a 3D structure consisting of cubic grains each of equal volume. We build the structure in such a way that no four cubes have a common edge. It is shown that the transition point between Harrison Type-A and Type-B kinetics regimes occurs at a diffusant diffusion length roughly an order of magnitude smaller than for the extensively studied case of parallel grain boundary slabs. For two dimensional squares the transition point occurs at a diffusion length roughly a factor of five smaller than for parallel grain boundary slabs. Thus we can draw the conclusion that dimensionality and geometric shape are both important factors in the parametric analysis of the grain boundary diffusion problem.

Abstract: Net fluxes of vacancies commonly occur during chemical interdiffusion in alloys, ionic conductivity and the annealing out of radiation damage. When atoms with different jump rates diffuse in a net flux of vacancies the phenomenon of the vacancy-wind effect will occur. This effect, first discovered by the late Dr John Manning, is a subtle phenomenon arising from a disturbed distribution of vacancies with respect to a given moving atom or species of atom. In this paper, the vacancy-wind effect is discussed and its visualization, performed for the first time by computer simulation, is demonstrated.

Abstract: In this paper, we discuss different possible cation diffusion mechanisms in YSZ and LSGM. Monte Carlo simulations are also reported of tracer diffusivities in LSGM for a postulated cluster mechanism. These simulations extend earlier simulations on un-doped material. The limits of the ratio of the diffusivities are consistent with experimental tracer diffusion findings over a wide range of cation-vacancy exchange frequencies. We also develop relationships between the phenomenological coefficients and use these relationships to predict possible demixing and interdiffusion experimental outcomes.

Abstract: We report on the computer simulation (using molecular dynamics and lattice relaxation) to explore tracer and chemical diffusion (carbon) kinetics in austenite at low carbon contents. It was found by molecular dynamics simulations that the detailed balance relations are not valid for the second nearest neighbours in the movements of the carbon interstitial atoms. The effect of a possible split energy level at the second nearest neighbour positions is analysed from a theoretical point of view.

Abstract: The presence of atomic oxygen at internal metal-ceramic oxide interfaces signifi- cantly affects the physical properties of the interfaces which in turn affects the bulk properties of the material. This problem is addressed for the model system Ag-MgO from a phenomenolog- ical point of view using the finite element method. The performed parametric studies investigate the influence of different kinetic parameters of the diffusion-segregation system.

Abstract: The interfacial reactions between a solidified Fe shell and Ti were investigated within the framework of steel alloying. Ti cylinders were immersed into liquid Fe for various durations and subsequently water-quenched. An Fe shell solidifies around the Ti. At the interface between Fe and Ti a reaction zone is formed. Initially it consists of a liquid eutectic layer, though in later stages all stable phases at elevated temperature can be found in the quenched microstructure. The larger part of this reaction zone is fluid at high temperature and both Ti and Fe dissolve into it. Moreover, intermetallic compound formation and mixing of Fe and Ti generate extra heat, shortening the time required for shell melt-back. The reaction zone reaches thicknesses up to 40 % of the initial sample’s radius and when the shell has completely remolten, a discontinuity in the thickness-time profile is expected. The exact morphology of the reaction zone at high temperature remains to be determined: presence of a solid layer of Fe2Ti may impede mixing in the initial stages.

Abstract: The TEXAS-V code was tuned for TROI-13 and used for analyzing the parametric findings of the TROI experiments. The calculations for the melt composition are relatively similar to the TROI experimental results, in which the melt composition has quite an effect on the steam explosion occurrence probabilities and a steam explosion itself. The void fraction difference due to a particle size difference appears to cause this composition dependency of a steam explosion. The water depth effect in the TEXAS-V code seems to be consistent with the TROI experiments to some degree. The water area effect of the TEXAS-V calculations isn’t exactly harmonious with that of the TROI experiments. This indicates that TEXAS-V code as a 1-dimensional code or as a numerical steam explosion has a limitation in estimating an area effect. The experimental information on the particle size and the void fraction during mixing is very helpful for a verification of this relation.

Abstract: Visible photoluminescence was generated in standard soda-lime-silica glass powder, mechanically milled in a high-energy attrition mill. The broad emission band maximum shows a linear dependence on the exciting wavelength, suggesting the possibility to tune the PL emission. The photoluminescence was attributed to defect generation related to unsatisfied chemical bonds due to the high surface area. Raman scattering and ultraviolet-visible optical reflectance measurements corroborate this assertion. Transmission electron microscopy measurements indicate that the powder is composed by nanocrystallites with about 10-20 nanometers immersed in an amorphous media.