GROWTH-KINETICS OF INTERMEDIATE COMPOUNDS AT A PLANAR SOLID-SOLID OR SOLID-LIQUID INTERFACE BY DIFFUSION MECHANISMS

A diffusional model of interface displacement kinetics is proposed for the growth of n intermediate compounds at an initially planar interfa ce between two semi-infinite phases. The model is based on the solutio n of Fick's equations with the restrictive assumptions of simultaneous growth of n intermediate phases, unidirectional diffusion flow, and l ocal equilibrium conditions. The velocity of each interface follows th e parabolic law and the (n+1) kinetic coefficients are expressed as a function of boundary concentrations and diffusion coefficients of all the phases via (n+1) nonlinear equations. A parametric study of the ki netic coefficients, corresponding to realistic situations of initial s olid-solid or solid-liquid interface, is developed for systems with on e or two intermediate layers. If two interacting initial phases alpha and beta are such that the chemical diffusion coefficient D-alpha (in alpha) is smaller than D-beta (in beta), it is found that the interfac e velocities are enhanced by: (a) increases in D-beta, (b) increases i n the solubility limit in beta, and (c) reduced miscibility gaps at th e interfaces. Moreover, the widths of the intermediate layers are incr eased by: (a) decreases in D-beta and (b) increases in the diffusion c oefficients and solubility limits in these intermediate phases. (C) 19 97 American Institute of Physics.