Models for growth kinetics of A-15 compounds by solid state diffusion

Abstract

In the formation of A-15 superconducting compounds by solid state diffusion, the time exponent in the growth law under different experimental conditions varies widely from about 0·25 to 1·0. Specific models of growth for different operative rate-controlling conditions are proposed. When the diffusion of B atoms in the matrix is rate-controlling, the thickness of the reacted compound layer increases as t^1/2 or t^2/3 . When the diffusion of B atoms through the compound layer is rate controlling, a t^1/2 dependence both for bulk diffusion and grain-boundary diffusion is predicted. When substantial grain growth occurs in the reacted layer during the diffusion anneal, the time exponent observed could be as low as 1/4. Experimental data in support of the predictions of the proposed models are presented.