Formation of a new phase from chemical interactions at an interface. Nucleation control

The phenomena considered constitute a particular case of the general question of the formation of a new solid phase from a chemical interaction between two adjoining phases, one of which acting as a substrate should be solid also, while the second one can indifferently assume any physical state, solid, liquid or gaseous (as in oxidation). The conditions under which nucleation becomes the rate-determining process in such reactions are analyzed in terms of the classical theory of nucleation. Quite often, the growth of the new phase is proportional to the square-root of time, indicating diffusion control; germination comes to play the preponderant role when the change in free energy ΔG is sufficiently small. When germination becomes very difficult, it may take place only at such high temperatures that diffusion ceases to play any significant role and the reaction assumes a quasi explosive character. While the presence of equilibrium points, eutectoid or oxide decomposition, provides the possibility of making ΔG arbitrarily as small as desired, nucleation-control does not depend on such equilibrium points; the one necessary condition is that ΔG be small. Some practical aspects are considered.