Analysis of the diffusion-induced stress effect on the oxidation in finelydivided vanadium ferrites

The isothermal oxidation of the iron(II), vanadium(II), and vanadium(III) c ations in finely divided vanadium spinel ferrites which were oxidized in ca tion-deficient spinels have been studied by thermogravimetric analysis. The low temperature kinetics of oxidation of each oxidizable cation has been e xplained on the basis of a diffusion-induced stress effect, these stresses being generated in the ferrite particles by the chemical gradient induced d uring the oxidation process where the cations at the surface are more oxidi zed than those in the bulk. This has been made by considering that the chem ical diffusion coefficient is given by the relation (D) over tilde = D-0 ex p [-(E'(a)+pV(a))/RT], where D-0 is a pre-exponential factor, E'(a) an acti vation energy, V-a an activation volume, and p the isostatic pressure induc ed by the oxidation. When the V-ferrites contain same Co-cations, it has be en also revealed that the stresses have a significant influence upon the co ercivity.