Microstructural and electrical characterisation of melt grown high temperature protonic conductors

High temperature protonic conductors of SrCe 1− x Y x O 3− δ and Sr 3Ca 1+ x Nb 2− x O 9− δ were fabricated by directional solidification to produce model microstructures. Elongated cells exhibited 〈100〉 direction parallel to the growth axis; low degree of disorientation was observed between the cells. In the simple perovskite SrCe 1− x Y x O 3− δ aluminum contamination caused the formation of intergranular second phase. Nuclear microprobe revealed that the second phase was enriched with hydrogen. Impedance spectroscopy revealed that the protons at grain boundaries have a lower mobility than within the cells. The cation distribution was not uniform in the complex perovskite. Inverse gradients in Ca 2+ and Nb 5+ were observed from the core to the shell of the cells. The Nb 5+ substitution decreased from x = 0.12 at the core to 0.07 at the shell. Higher Nb 5+/Ca 2+ ratio at the shell decreased the protonic conductivity. Nanodomain were observed in both perovskite compositions; they differentiate by a 90° rotation of the direction of oxygen cage tilting. In the complex perovskite, stoichiometric domains with an ordered distribution of Nb 5+ and Ca 2+ were surrounded by nonstoichiometric domains with a random distribution of these cations. Further work and analyses are required to understand the mechanism of proton transfer within domains and across domain interfaces.