Abstract
During oxidation in air of finely grained manganese-substituted magnetites structure, the availability for oxidation of Mn 2+ ions in tetrahedral sites (A sites) is much less than that of Fe 2+ and Mn 3+ ions in octahedral sites (B sites). The oxidation kinetic of Mn 2+ ions is well interpreted by diffusion under variable working conditions with an activation energy of about 140 kJ mol −1. The defect phases γ obtained at 400°C undergo stoichiometry changes due to manganese ions in the temperature range 450–550°C as a function of oxygen pressure. Above 600°C, Mn 2+ ions that are not oxidized at lower temperatures are transformed into Mn 3+ with a phase change from a spinel to a corundum structure. The kinetic curves for this transformation are sigmoidal with an activation energy depending on the amount of Mn 3+ ions.
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