The fine structure of the Kolmogoroff–Avrami theorem
The fundamental processes underlying phase changes at electrode surfaces are often described in terms of the Kolmogoroff–Avrami theorem.Some aspects of the 'fine structure' of this theorem are considered. Particular attention is paid to the case of two-dimensional nucleation and growth under the influence of an applied electrode potential (equivalent to supersaturation). A probabilistic analysis is presented for circularly-symmetric nuclei which obviates the need for concepts such as 'overlap' between emergent nuclei. The concept of a 'collision front' is introduced which is defined as the line of contact along which neighbouring nuclei collide. It is shown that for instantaneous nucleation (potentiostatic double step method) the extended set of collision fronts constitutes a net composed of irregular convex polygons. The properties of this net are investigated in some detail. In particular, the extent of applicability of the Kolmogoroff–Avrami equation is delineated, the distribution of collisional polygons is calculated, and an approximate treatment for the variance of the surface coverage function is presented. The general approach exposes clearly the relationship between the single nucleus and the multinuclear situations.