A new family of oxide ion conductors based on tricalcium oxy-silicate

Abstract

Tricalcium oxy-silicates, Ca₃(SiO₄)O and Ca_2.93Mg_0.07(Si_0.98Al_0.02O₄)O_0.99□_0.01, have been prepared as crystalline single phases. Ca₃(SiO₄)O and Ca_2.93Mg_0.07(Si_0.98Al_0.02O₄)O_0.99□_0.01 have triclinic and monoclinic structures, respectively. The samples show oxide anion conductivity with a small p-type electronic contribution under oxidizing conditions. At 1023 K, the oxide transport numbers range between 0.97 and 0.85 from reducing (dry 5%-H₂–Ar/air gradient) to oxidizing (O₂/air gradient) conditions in the 1023–1173 K interval. The thermal analyses showed a large weight loss on heating due to the presence of water in the materials. The monoclinic compound has ionic conductivities higher than those of the triclinic stoichiometric oxy-silicate, as expected due to the introduction of oxide vacancies. Typical total conductivities for these un-optimised solids are 10⁻⁵–10⁻⁴ S cm⁻¹ at 1100 K. These compounds may contain a small amount of water, ∼0.05 H₂O moles per chemical formula, and they display an important proton contribution under a humidified atmosphere.