The role of microstructure and processing on the proton conducting properties of gadolinium-doped barium cerate

The influence of grain boundary conductivity and microstructure on the electrical properties of BaCe_0.85Gd_0.15O_3–δhave been examined. Grain sizes were varied by sintering at various temperatures. Impedance data were analyzed using the brick layer model, and some new consequences of this model are presented. The specific grain boundary conductivity exhibits an activation energy of ~0.7 eV, and for similar processing routes, is independent of grain size. An isotope effect was observed, indicating that protons (or deuterons) are the mobile species. TEM investigations showed the intergranular regions to be free of any glassy phase that could account for the differences in bulk and grain boundary properties. Single-crystal fibers, grown by a modified float zone process, were notably barium deficient, and exhibited a low conductivity, comparable to that of polycrystalline Ba_0.96Ce_0.85Gd_0.15O_3–δ.