Effects of transition metal additives on redox stability and high-temperature electrical conductivity of (Fe,Mg)3O4 spinels

Magnetite-based spinels are considered as promising oxide materials to meet the requirements for ceramic consumable anodes in molten oxide pyroelectrolysis process, a breakthrough low-CO2 steel technology aimed to overcome the environmental impact of classical extractive metallurgy. The present work focuses on the assessment of phase relationships, redox stability and electrical conductivity of Fe2.6Me0.2Mg0.2O4 (M = Ni, Cr, Al, Mn, Ti) spinel-type materials at 300–1773 K and p(O2) from 10−5 to 0.21 atm. The oxidation state of substituting transition metal cation, affecting the fraction of Fe2+ in spinel lattice, was found to be a key factor, which determines the electronic transport and tolerance against oxidative decomposition, while the impact of preferred coordination of additives on these properties was less pronounced. At T > 650 K thermal expansion of Fe2.6Me0.2Mg0.2O4 ceramics exhibited complex behaviour, and, in highly oxidizing conditions, resulted in significant volume changes, unfavourable for high-temperature electrochemical applications.