Microstructural and electrical characterisation of melt grown high temperature protonic conductors

High temperature protonic conductors of SrCe1−xYxO3−δ and Sr3Ca1+xNb2−xO9−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 SrCe1−xYxO3−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 Ca2+ and Nb5+ were observed from the core to the shell of the cells. The Nb5+ substitution decreased from x = 0.12 at the core to 0.07 at the shell. Higher Nb5+/Ca2+ 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 Nb5+ and Ca2+ 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.