No abstract

No abstract

536.46+539.3+531Changes in the properties of a substance due to a chemical reaction (change in the concentration of components), thermal expansion upon heating, and inhomogeneities in the structure of the substance (both in the initial state and during heating and reaction) determine the volume changes dV/Vo = (V -Vo)/Vo of the substance. For many solids, these changes are small and have no effect on heat and mass transfer processes. For high heating rates or significant differences in the properties of the reactants and products, a volume change dV can lead to the occurrence of stresses in the reaction zone and even to damage (accumulation of defects and inhomogeneities in the structure compared with the initial state). We write these changes in the formVo k where aT = (1/3) (OV/OT)Vo 1 is the coefficient of linear thermal expansion; a} = (1/3)(OV/ON•)Vo-' is the coefficient of concentration expansion for each component; ctr = (1/3)(OV/c3vp)Vo 1 is the coefficient of structural expansion; Nk is the concentration of the kth component; vp is the volume of damage (discontinuities, pores, cracks, etc.). We can speak in general about a change in structure inhomogeneity of one or another type. For simplicity of analysis, we restrict the consideration to overall volume changes. In expansion (1), only first-order terms are taken into account, by virtue of the smallness of the coefficients aT, ak, and ctc in accordance with their meaning for a solid medium. Let us determine how small changes (1) manifest themselves in heat and diffusion equations. For this recall that with temperature and volume changes in the system, the free energy F is the main thermodynamic potet~tial [1]. Its total differential is written asHere dF ~ is the change in the free energy due to heating and chemical reaction:k where #k is the chemical potential of the kth component; S is the entropy of a unit volume; and dF' is the change in the free energy during deformation and damage:dF' = ~_, aodr j + ~_, X,,ndw,m,ij lmTomsk State University, Tomsk 634050.

A model for the saturation of the surface layer of a thin metal plate with an impurity from the environment under uniaxial mechanical loading is proposed and investigated. The effect of stresses and strains on the diffusion process is analyzed. It is shown that, first, due to the deformation of the crystal lattice of the base, stresses that occur in local volumes lead to a change in the diffusion activation energy; second, stresses influence impurity transfer (this effect is similar to mass transfer by pressure diffusion in liquids). The joint effect of the two types of influences of stresses and strains on the behavior of the system at various geometrical and physical sample parameters is numerically investigated.Introduction. As is known, stresses and strains occurring in the zone where diffusion takes place (diffusion zone) have an impact on diffusion processes. Many studies have been devoted to the investigation of this phenomenon (for instance, see [1][2][3]).The appearance of concentration stresses in the case of a binary system is due, first, to the difference between the atomic sizes of the diffusing substance and the base, and, second, to the inequality of the partial diffusion coefficients of the impurity and base, which leads to inequality of the opposite partial fluxes and the appearance of redundant vacancies causing stresses [4]. The stresses caused by the appearance of atoms of another substance in the lattice of the base are called concentration stresses, and the stresses due to the difference in diffusion mobility between the atoms of the base and the impurity atoms are called diffusion stresses. It is difficult to separate one type of stresses from the other. Stresses and strains in the diffusion zone eventually occur due to heterogeneity of concentration fields. From now on, concentration stresses and strains will be considered from this perspective.The dependence of diffusion processes on stresses has motivated the Development of methods for controlling impurity redistribution. One of these methods is the use of an additional external load. To investigate the role of external loading in the presence of internal stresses, it is necessary to solve the problem of mechanical equilibrium of the sample taking into account the possible feedback between diffusion and mechanical processes. In the case where inertia forces may be neglected (due to the low rate of diffusion processes) under quasistatic loading, the problem is divided into two parts: the problem of mechanical equilibrium and the nonlinear diffusion problem. The purpose of this work is to investigate the impact of various physical factors on the parameters of the diffusion zone under quasistatic uniaxial loading.Problem of Mechanical Equilibrium. Let us consider a plate of length L, width h, and depth δ (L h δ) which is subjected to uniaxial mechanical loading. The magnitude of the external load p is specified. Impurity from the environment or impurity from a previously applied layer containing an excess of the diffusing element can penetr...

We report a fast and cost-effective strategy towards the preparation of superhydrophobic composites where a double-sided adhesive tape is paved with charcoal particles. The composites are mechanically robust, and resistant to strong chemical agents.

No abstract

No abstract

A coupled model of coating formation on the surface of a part of a cylindrical shape during deposition from the plasma is proposed. This model takes into account the phenomena of thermal diffusion, diffusive thermal conductivity, and mass transfer under the action of the stress gradient, and the formation of chemical compounds. The coating growth rate is considered to be a given function of the particle velocity and particle concentration near the surface of the growing coating. The problem is solved numerically. It is shown that diffusion cross-fluxes, diffusive thermal conductivity, and thermal diffusion during the growth process reduce the width of the transition zone between the substrate and the coating. This effect becomes most essential if the substrate has a low thermal conductivity. Accounting for stresses arising in the coating-substrate system during the deposition process changes the effective transfer coefficients and significantly affects the result of modeling the distribution of chemical elements and their compounds in the coating.