Redox stability and electrical conductivity of Fe_2.3Mg_0.7O_4±δ spinel prepared by mechanochemical activation

This paper addresses the potential of mechanochemical activation of MgO and α-Fe₂O₃ precursor powders to obtain Fe_2.3Mg_0.7O₄ ceramics with enhanced redox stability and electrical conductivity. X-ray diffraction (XRD) and Mössbauer spectroscopy suggest the initial formation of the spinel phase after 5h of high-energy milling in inert gas, but after 10h of mechanoactivation, the precursor still comprised hematite as a major phase with minor amounts of magnesiowustite as by-product. The activated mixtures can be nearly completely converted to spinel solid solution by heating to 1173K, whereas single-phase, dense spinel ceramics can be prepared by sintering at 1773K in inert atmosphere. These ceramics demonstrated redox stability under mildly reducing conditions (p(O₂)∼10Pa), as confirmed by XRD, thermogravimetry and electrical measurements. The electrical conductivity of Fe_2.3Mg_0.7O₄ at this oxygen partial pressure is lower compared to magnetite, but it is still as high as 60S/cm at 1073K and 15S/cm at room temperature. Cooling below 1473K in air results in a drop of conductivity due to segregation of hematite phase at the grain boundaries. However, the phase separation is kinetically stagnated at 1073K, and, after slight initial degradation, the retained electrical conductivity is more than 3 orders of magnitude higher compared to hematite and MgFe₂O₄ spinel.