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 reduction reactions occurring at low temperatures in finely divided spinel oxides such as Mn-Zn, Mo, and V ferrites is presented. When the Fe-based spinel oxides contain more than one oxidizable cation (for instance Fe2+, Mo3+ and Mo4+ in Mo ferrites), it has been demonstrated that each cation oxidizes in a well defined range in temperature in close relation with the cation-oxygen distance of the cation considered. The model involves both thermodynamics and kinetics considerations : thermodynamics involves the minimization of the Gibbs free energy and the kinetics oxidation has been explained on the basis of a diffusion-induced stress effect. Finite element calculations have been developped and applied to compute the features of the reaction in the case of V, Mn-Zn and Mo ferrites. In these materials, the model enables the evolution with time of oxidation to be explained. In the second part of the paper, the influence of the oxidation and reduction reactions occurring during all the stages of sintering upon both physical and mechanical properties is evidenced