Study of the oxidation kinetics of finely-divided magnetites III — Influence of chromium and aluminum surstitution

The oxidation kinetics of aluminum and chromium-substituted magnetites (Fe²⁺Cr³⁺_xAl³⁺_2?x)O^2?₄ (0< x < 2) into the γ defect phase of type $\text{γ(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Al}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext><mtext>?</mtext><mtext>y</mtext>}}\text{Cr}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>y</mtext>}}\text{)}{}_2\text{O}{}^{\mathrm{2?}}{}_3$ is found to be governed by the diffusion under variable working conditions for samples prepared at 700°C and whose size is less than 400 Å. When particle size is caused to increase by annealing the kinetic curves are sigmoidal but only for specimens with high chromium substitution. For compounds rich in chromium observations of morphology show facetting during oxidation but no fracturing was observed.