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(O-2) 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. (C) 2014 Elsevier Ltd. All rights reserved.