Defect and Diffusion Forum Vols. 258-260

Abstract: Droplets transport in homogeneous porous media has been found to be an attractive problem applicable in a lot of industrial and scientific sectors such as enhanced oil recovery, food production, plastics etc. As applications become wider, a predictive method for the process is warranted. To this end, it has been widely accepted that the collection of γ-order moments, Sγ, can describe the time evolution of any spatially averaged quantity like the mean diameter of spherical droplets, while it has been also found that Sγ satisfies the transport equations [1]. Here, the so-called “Sγ concept” is applied in a CFD module for the modeling of the transport processes occurring in a mixture of a continuous aqueous phase which includes a discontinuous one in the form of droplets. This mixture flows within a homogeneous porous medium under creeping or laminar flow conditions. The momentums of the particle size distribution are evaluated using the local flow conditions as obtained from CFD simulations for the processes considered. To solve the transport equations, the microstructure droplets formation/destruction has been also taken into account by using already known analytical expressions for the source terms representing the break up and coalescence of the droplets [2-4]. The proposed constitutive model adequately simulates the effect of porous geometry on the droplets size distribution and could be helpful in understanding the phenomena that take place in microscopic scale.

Abstract: Interdiffusion at the interface between a Co-36.5Ni-17.5Cr-8Al-0.5Y, MCrAlY coating and the underlying IN738 superalloy was studied in a large matrix of specimens isothermally heat treated for up to 12,000 hours at temperatures 875°C, 925°C or 950°C. Modelled results using the finite difference software DICTRA was compared with experimental average composition profiles measured across the interface using a new experimental approach.

Abstract: The Darken method is used to evaluate and compare different time scales in an ideal crystal lattice. The crystalline tungsten exemplifies presented approach. It is shown that in this metal and at 1273 K the time scales differ by nine orders of magnitude. Particular emphasis is given to the problem of deformation, the temperature and the mass diffusion time scales in tungsten quasicontinuum. The transport process in the nonuniform temperature and stress field is characterized by representative velocities. These velocities allow to quantify the time and length scales and can be used for the proper non-dimensialization and effective solution of the particular transport problems.

Abstract: This work presents experimental values of molecular diffusion coefficient, obtained with samples of building materials, using a non-stationary process of moisture absorption. The experiments covered four different types of building materials (with two selected ranges of relative humidity) and were carried out in the range of temperatures between 20°C and 40°C. The results show that the increase in temperature resulted in an increase in molecular diffusion coefficient and the relative humidity also influenced the molecular diffusion in a uniform manner. Finally, a good agreement with literature values was found.

Abstract: The limit of perfection of a blast-furnace smelting, including in its concept a minimal theoretically possible coke consumption, is determined by a thermodynamic equilibrium of oxidation-reduction reactions in separate zones of a furnace. It is possible to determine minimum coke consumption, for example, with the use of the mathematical model suggested by us. The following prerequisites underlie the mathematical model. Heat exchange in a blast furnace was completed, i.e. there is a small thermal gradient between gas and charge on a certain height. In a certain zone of the furnace at a reduction stage of magnetite the reaction of wustite reduction tends to a thermodynamic equilibrium. In this zone so-called zonal reduction realizes when the processes of reduction of various iron oxides combine in time. The significant role belongs here to diffusion of gas in pores of iron ore material. The solution of equations set for the bottom zone presented by a condition of a thermodynamic equilibrium, namely, the equilibrium constant of wustite reduction by carbon oxide, material and thermal balances, supplemented by the equation of the heat balance for the top zone, allows calculating the main indices of blast furnace smelting. Indices thus calculated are extremely accessible (minimum) at the given parameters of charge and blast. In the paper quantitative values of the coke consumption were presented at stage and zonal reduction of iron oxides.

Abstract: Reduction and sintering are complicated processes demanding use of mathematical modelling - consolidation of multicomponent materials. The most representative of them are products formed during consolidation during sintering of powders. They have common physical and chemical characteristics both from the viewpoint of initial (multicomponent) materials, and from the viewpoint of technological parameters. Such processes simultaneously involve both the reduction and the sintering with simultaneous consolidation of particles into materials. The sintering of powders of crystals is a flexible and effective way of production of polycrystalline composite materials with a given structure and properties. In this paper a mathematical model of the process of sintering suggested by us is considered.

Abstract: Porous silicon (PS) layers with porosity of 60% on n-type (111) Si substrates were prepared by anodic etching under the white illumination. Metal (Cu, Ag or Au)/PS/Si and metal (Cu, Ag or Au)/Si structures have been fabricated by evaporation of thin metal film onto the PS or Si surface, respectively. The diffusion annealing of structures was carried out in air at 100-250oC. Examination of Cu, Ag and Au concentration distribution in PS layer and monocrystalline Si substrate was performed by successive removal of thin layers from sample and measuring the energy dispersive X-ray fluorescence (EDXRF) intensity of CuKα1, AgKα1 and AuLβ1 peak. The effective diffusion coefficients for investigated metals along PS surfaces decrease in series Cu, Ag and Au and temperature dependences are described as D(Cu)=7.8 exp(-0.62eV/kT), D(Ag)=4.2x10 exp(-0.72eV/kT) and D(Au)=1.2x102 exp(-0.81eV/kT). Diffusion coefficients of Cu, Ag and Au along PS surfaces are larger (by a factor of 104-105) than those into monocrystalline Si. The diffusion mechanism of Cu, Ag and Au along PS surfaces is discussed and data on influence of diffusion of these metals on humidity-sensitive characteristics of metal(Cu, Ag or Au)/PS Schottky type gas sensors is also presented. Diffusion of metals of I group in PS is accompanied by increase of humidity-sensitivity of metal/PS structures by a factor of 1.2-1.4.

Abstract: The liquid phase of adhesives is most readily described by the theory of solutions. Adhesives act on phase boundaries. Yet right there, the fact that an adhesive is a solution does not fully describe its special performance. Some of the outstanding properties of adhesives are designed by the use non- soluble components, like Nanofillers, fibre reinforcements, color pigments, additives for the improvement of electrical and heat conductivity, etc. We would like to compare near phase boundary properties inside adhesives and outside near the substrates. We expect a relation between rheology and thermodynamical parameters of adhesion. Solutions are characterized by their chemical potential. We expect the above relationship to manifest itself by a change of the chemical potential near a phase boundary.

Abstract: A theoretical investigation of peculiarities in the formation of diffusion potential barrier for oxygen atoms in NiO1-δ crystal is performed. The approximation of pair interionic potentials set in analytical expressions is used for the evaluation of the potential barrier for the diffusing oxygen atom. It is shown that the potential energy of diffusing oxygen atom originates first of all from compensation between positive contribution of short-range interaction and negative contributions of the long-range Coulomb interaction. It is also influenced by effects of degenerate term splitting due to the low-symmetry crystal fields, created by the diffusing atom and vacancy, lattice relaxation and redistribution of excess charges in the vicinity of diffusing oxygen atom.

Abstract: The tracer diffusion coefficient D* and the mobility of protons in proton-conducting oxides of the АIIВIV 1-xRIII xO3-δ family have been calculated using the Monte Carlo method as functions of temperature and concentration x of the acceptor impurity RIII. The effect of protondopant interactions (proton trapping) and the effect of protonic sites blocking caused by protonproton and proton - oxygen vacancy interactions, are analyzed. It is shown that the proton diffusivity depends significantly on the dopant content and is considerably reduced already at small x. The D* value weakly depends on the concentration of oxygen vacancies at fixed, not too large values of x. The conductivity dependence on the doping concentration σ(x) can have a maximum due to the proton-defect and proton-proton interactions. The Haven ratio deviates slightly from unity at the expected intensity of interparticle correlations. The calculated values of both the diffusivity activation energy increase (with x), and the location of the σ(x) maxima agree with the experimental data for a number of proton-conducting oxides.