Microstructural and electrical characterisation of melt grown high temperature protonic conductors

High temperature protonic conductors of SrCe1-xYxO3-delta and Sr3Ca1+xNb2-xO9-delta 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-delta 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 degrees 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. (c) 2006 Elsevier B.V. All rights reserved.