Microstructure and Oxide Ion Conductivity in a Dense La9.33(SiO4)6O2 Oxy‐Apatite

Abstract

Oxy‐apatites are one of the most promising oxide ion conducting electrolytes for intermediate‐temperature solid oxide fuel cells, those operating close to 950 K. La9.33(SiO4)6O2 has the simplest stoichiometry among the oxy‐apatite family of compounds, and the range of oxide ion conductivity reported for dense ceramic pellets is very high, almost two orders of magnitude at a given temperature. The spread in conductivity values including porous pellets is obviously larger. Here, we show that dense pellets of La9.33(SiO4)6O2 ceramics prepared by three different methods, conventional, spark plasma, and reaction sintering (RS), have different bulk conductivities. Bulk activation energies are equal for all the samples studied. Furthermore, the grain boundary conductivity cannot be described by the simple brick‐layer model. These experimental findings suggest segregation at the grain boundary region, with slightly different compositions for different sintering conditions, likely leaving the grain interior with a compositional gradient. A two‐step RS led to dense pellets with the highest bulk and grain boundary oxide ion conductivities.