Reinterpretation of the Reaction Kinetics of Nickel Ferrite

Abstract

The kinetics of formation of nickel ferrite from the oxide components has been studied using saturation magnetization to follow the course of the reaction. The data has been interpreted in terms of an empirical expression developed by Tammann. This expression, which gives quite satisfactory agreement with the data, is C = A logt+B, where C is the percent of reaction product, t is the time, and A and B are constants. The sharp breaks that occur in the percent reaction vs log time curves indicate that three separate and distinct mechanisms occur. These are interpreted as (1) an initial surface diffusion of significance at temeratures in the 600° to 800°C range, (2) bulk diffusion in the temperature range from 700° to 950°C, and(3) crystallization into a relatively defect-free crystal lattice above temperatures of 950°C. Activation energies for the surface diffusion and bulk diffusion are 30 and 18 kcal, respectively. Both the low activation energy for the bulk diffusion in comparison to the surface diffusion, and the high initial reaction rate at higher temperatures, suggest that the reaction proceeds through iron diffusing into an active metastable NiO phase.