Exploring the effects of silica and zirconia additives on electrical and redox properties of ferrospinels

The interplay of unique cation arrangement and redox coupling in ferrospinels offers a wide range of magnetic and catalytic properties, evaluated mostly for low and intermediate temperature applications. This work focuses on high-temperature properties of magnesium-substituted magnetite, for prospective high temperature applications such as electrodes for pyroelectrolysis, energy conversion, catalysis, etc. The effects of silica and zirconia additions to (Fe,Mg)3O4 are studied, with emphasis on structural, electronic transport and redox properties. Up to at least 2% Zr4+ can be dissolved in the spinel lattice by sintering in inert atmosphere at 1773 K, resulting in a moderate conductivity decrease and lower tolerance against oxidative decomposition. Silica additions are accommodated by magnesium exsolution and formation of (Fe,Mg)2SiO4 and (Mg,Fe)O phase impurities rather than by the substitution in spinel lattice, as confirmed by combined structural, microstructural and electrical conductivity studies. Minor amounts of silica in ferrospinels do not result in the conductivity decrease and apparently provide more stable thermo-chemical expansion behaviour, being favourable for prospective high-temperature applications, including anode materials in silicate-based melts.