OXIDATION AND REDUCTION REACTIONS IN SPINEL OXIDES - INFLUENCE UPON SOME PHYSICAL-PROPERTIES

In the first part of the paper, a complete model for oxidation and red uction reactions occurring at low temperatures in finely divided spine l oxides such as Mn-Zn, Mo, and V ferrites is presented. When the Fe-b ased spinel oxides contain more than one oxidizable cation (for instan ce Fe2+, Mo3+ and Mo4+ in Mo ferrites), it has been demonstrated that each cation oxidizes in a well defined range in temperature in close r elation with the cation-oxygen distance of the cation considered. The model involves both thermodynamics and kinetics considerations: thermo dynamics involves the minimization of the Gibbs free energy and the ki netics oxidation has been explained on the basis of a diffusion-induce d stress effect. Finite element calculations have been developped and applied to compute the features of the reaction in the case of V, Mn-Z n and Mo ferrites. In these materials, the model enables the evolution with time of oxidation to be explained. In the second part of the pap er, the influence of the oxidation and reduction reactions occurring d uring all the stages of sintering upon both physical and mechanical pr operties is evidenced.