Phase transformations of stoichiometric mixtures of hematite and iron under FAST conditions

Abstract

Since the mechanism of the synthesis of magnetite from a stoichiometric mixture of hematite and iron is still under debate, systematic studies of the phase transformations in such powder mixture processed under field assisted sintering conditions, are presented. Phase contributions, grain sizes and stoichiometries of the sintered composites were determined using scanning electron microscopy, high energy X-ray diffraction and Mößbauer spectroscopy. It was shown that with an increasing sintering temperature an accelerated growth of magnetite can be observed, while the amount of hematite decreases. Additionally, intermediate wustite phase was observed with a maximum intensity where iron vanished from the samples. Therefore, it was concluded that the transition from hematite - iron mixture to magnetite actually takes place in two steps. In the first step, iron reduces hematite to magnetite and oxidizes itself to wustite. In the second step, wustite enables the nucleation of magnetite and with the help of hematite it transforms into nearly pure stoichiometric magnetite at higher sintering temperatures. In composites sintered from pure hematite under the same conditions only a minor transition to highly nonstoichiometric magnetite was observed emphasizing the above mentioned route of transformation.