Mechanism of the low temperature oxidation of synthetic magnetites

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Abstract

At about 200°C, synthetic magnetites are oxidised to maghemites without any significant changes in morphology and surface area. This oxidation is accompanied by a contraction of the cubic unit cell. During oxidation the excess Fe probably moves towards the surface and combines with oxygen to form a film of maghemite, the thickness of which increases with time. The data on changes in Fe(II) content during the oxidation can be fitted to an equation describing the diffusion in a sphere. At 190°C the diffusion coefficients of Fe are in the range 0.8 to 2.1 × 10−15 cm2/sec and decrease with the increasing isomorphous substitution of Co, Ni and Zn for Fe(II) in the magnetite.

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