High-temperature conductivity, stability and redox properties of Fe3−xAlxO4 spinel-type materials

Iron-based oxides are considered as promising consumable anode materials for high temperature pyroelectrolysis. Phase relationships, redox stability and electrical conductivity of Fe3−xAlxO4 spinels were studied at 300–1773 K and p(O2) from 10−5 to 0.21 atm. Thermogravimetry/XRD analysis revealed metastability of the sintered ceramics at 300–1300 K. Low tolerance against oxidation leads to dimensional changes of ceramics upon thermal cycling. Activation energies of the total conductivity corresponded to the range of 16–26 kJ/mol at 1450–1773 K in Ar atmosphere. At 1573–1773 K and p(O2) ranging from 10−5 to 0.03 atm, the total conductivity of Fe3−xAlxO4 is nearly independent of the oxygen partial pressure. The conductivity values of Fe3−xAlxO4 (0.1 ≤ x ≤ 0.4) at 1773 K and p(O2) ∼10−5 to 10−4 atm were found to be only 1.1–1.5 times lower than for Fe3O4, showing high potential of moderate aluminium additions as a strategy for improvement of refractoriness for magnetite without significant deterioration of electronic transport.