Comments and recommendations on the use of the Avrami equation for physico‐chemical kinetics

Abstract

Abstract Over the last four decades, numerous reports have appeared on the physico‐chemical kinetics, especially crystallization kinetics, based on the Avrami equation, ϕ = exp [− Kt n ] where ϕ is the fraction of material unchanged at time t , K is an overall rate constant and “ n ” is the Avrami exponent indicative of process mechanism. The usage of the Avrami equation has been limited to the determination of “ n ” and its temperature dependence. It is shown that the evaluation of K and the activation energy (E) using this equation is erroneous since K and E are both influenced by “ n ” although such would be unexpected from the Avrami equation. On the other hand, if one uses a modified expression, ϕ = exp[− Kt ] n , then in addition to the value of “ n ”, correct values of K and E are obtained. This retains the original correspondence of the Avrami equation to nuclea‐tion and crystal growth processes but extends its applicability to (i) correctly evaluate K and E parameters, and (ii) correctly compare transformation rates when the systems differ in their n values. Experimental data are presented to support these conclusions.