Diffusion in polycrystalline magnesiowüstite determined by reduction kinetics

Abstract

Polycrystalline magnesiowüstite samples (5 wt% Fe 2O 3 doped) were reduced (or oxidized) at various temperatures in CO 2CO gas mixtures and N 2O 2 gas mixtures. The reaction was followed continuously with a Cahn microbalance. The kinetics in the high oxygen partial pressure region (0.18−9.6 × 10 −6 atm) were controlled by cation vacancy diffusion. The chemical diffusion coefficient of iron in magnesiowüstite was found to increase linearly with the concentration of cation vacancies. The activation energy for diffusion was found to be 45 kcal/mol. The pseudochemical diffusion coefficient of iron in magnesiowüstite evaluated from the kinetics of reduction and oxidation in low oxygen partial pressure atmospheres (10 −7–10 −10 atm) was found to decrease as the concentration of cation vacancies increased. The reduction kinetics could be interpreted best by assuming an initial desorption of oxygen on the surface of the solid solution, and the migration of oxygen away from the grain boundaries of the sample followed by an associated solid state diffusion process.