Microstructure, hydrogen distribution and electrical properties of melt grown high temperature protonic conductors

Abstract

A Sr 3 ( Ca 1 + x Nb 2 - x ) O 9 - δ High Temperature Protonic Conductor has been produced using the melt growth process with the aim of reducing the grain-boundary blocking effect in proton conduction. The microstructure is characteristic of a cellular growth with cell dimension of 10– 20 μ m in width and 100 μ m in length. The cells have distinct core and shell regions. A composition gradient in bivalent to pentavalent cation ratio is observed from the core to the shell. The cores act as channels for hydrogen diffusion. Nano-scaled domains have been revealed inside the cells identified as domains of 1:1 order of the cations on the B sites and orientation variants in the oxygen octahedra tilting. Ion Beam Analyses using a Nuclear Microprobe reveal only a slight hydrolysis of the surface after heat treatment of 10 h at 500 ∘ C in wet air. Protonic conductivity is discussed and improvements are suggested by optimizing the calcium to niobium ratio of the constituent elements and designing larger cells.