Peculiarities of kinetic dependences in solid-gas systems

Kinetics of interaction of solids with gases have been considered in the books by V.I. Arkharov,1 O. Kubaschewski and B.E. Hopkins,4 K. Hauffe,6 U.R. Evans,7 W.Jost,81.N. Frantsevich et al.,11 P. Kofstad,13,14 N.A. Kolobov and M.M. Samokhvalov,17 A.T. Fromhold,18,19 N.Birks and G.H. Mei- 

Therefore, consideration will here be restricted to a few comments regarding the shape of the layer thickness-time dependence. First, the most probable dependence in the case of formation of non-volatile chemical compounds between solid A and gas B will be analysed, assuming that at a given temperature B is insoluble in A. Then, the influence of evaporation on the layer-growth kinetics will be illustrated. Also, soft oxidation of chemical compounds will briefly be considered. 

Although there are no essential differences in the processes of formation of the layers of chemical compounds at the interface of either two solid substances, or a solid and a liquid, or a solid and a gas, nevertheless in the latter case the experimentally observed layer thickness-time dependences are more diversified and complicated than in the first two cases since the growth kinetics of compound layers in the solid-gas systems are usually investigated by thermogravimetry over a comparatively long time range. 

Under such conditions, significant changes of the layer-growth mechanism may take place, and this must be reflected on the kinetic dependences observed. Therefore, the number of equations proposed for the mathematical description of those dependences is considerably greater than in the case of systems formed, for example, by two solids. Of these, different forms of linear, parabolic, paralinear, cubic and logarithmic laws are employed most frequently. 

According to U.R. Evans,7 logarithmic equations (direct, reverse and asymptotic) are low-temperature laws. The reverse logarithmic law is valid if the layer-growth process is controlled by the electric potential gradient. This is probably the case with thin films of ionic compounds.6,145 The 

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