GROWTH KINETICS OF TWO COMPOUND LAYERS BETWEEN ELEMENTARY SUBSTANCES

GROWTH KINETICS OF TWO COMPOUND LAYERS BETWEEN ELEMENTARY SUBSTANCES 

Growth kinetics of two chemical compound layers in a binary heterogeneous system have been theoretically treated, from a diffusional viewpoint, by V.I. Arkharov,1 46 K.P. Gurov el al.,22 B. Schroder and V. Leute,52 A.T. Fromhold and N. Sato,53 G.-X. Li and G.W. Powell,55 and other investigators. Diffusional considerations predict that (z) both layers must occur simultaneously and (ii) the thickness of each of them as well as their total thickness should increase parabolically with passing time. 

However, available experimental data provide evidence that this is not always the case. In fact, in many binary systems the layer growth is neither simultaneous, nor parabolic. Instead, a variety of kinetic laws (linear, asymptotic, paralinear, parabolic, etc.) are observed. Only a certain portion of the layer thickness-time kinetic dependence in the region of long reaction times and hence large layer thicknesses is close to a parabola. 

An unambiguous interpretation of these well-known experimental facts in the framework of the diffusional theory is hardly possible. To overcome considerable difficulties arisen, use is usually made of different additional (not very convincing) assumptions and suggestions. In contrast, from a physicochemical viewpoint, the phenomena and dependences observed in practice seem to be quite natural and easily explainable.134 136 139 141 199 These can therefore readily be expected to hold in binary systems of whatever chemical nature. 

It should be noted that a theoretical approach close to, but not identical with, the physicochemical one was proposed by U. Gosele and K.N. Tu (see also Ref. 30). It is worth mentioning that both approaches were described in the papers published in the same year 1982 (see Refs 25 and 134). Analysis of various stages of the growth process of two compound layers can be found in works by F.M. d Heurle and P. Gas,27 W.C. Johnson and G. Martin,29 J. Philibert33 34 and other investigators. 

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