Thermally Driven and Ball-Milled Hematite to Magnetite Transformation

Abstract

In this work, a study on the dynamics of transformation from hematite (α-Fe₂O₃) to magnetite (Fe₃O₄) by following two solid-state reaction methods is carried out. One of the procedures consists of a thermal treatment under a 20% H₂ and 80% N₂ atmosphere at 375°C, whereas the second method involves a planetary ball mill to induce the transformation. The phases evolution as a function of the thermal treatment time ranging from 0 up to 25 min every 2.5 min, and from 0 up to 6 hours every hour in the case of the milling method, was followed by using room-temperature Mössbauer spectroscopy and X-ray diffraction analysis. Results evidence a well-behaved structural transformation for which highly stoichiometric Fe₃O₄ as a single phase was obtained for treatment times above 12.5 min in the case of the thermally treated samples. Differently from this a less stoichiometric magnetite characterized by a distribution of hyperfine fields for milling times above 3 hours in the case of the ball milled samples was obtained. For reaction times below 12.5 min, two interpretation models based on the presence of an anion-deficient magnetite Fe₃O_4−δ and the presence of maghemite accounting for the intermediate states during the thermal transformation are also presented and discussed.