Impedance spectroscopy was used to distinguish between the grain interior and grain boundary conductivities of ceramic electrolytes with various grain-size distributions. A so-called brick-layer microstructural model was used to estimate the true grain-boundary conductivity and to relate the electrical properties of the ceramic to microstructural parameters. High-resolution electron microscopic images showed that insulating intergranular phases did not necessarily have to be present in order to cause a high grain-boundary resistance. A clear relationship between the grain-boundary resistance and the number of grain boundaries was observed in samples with a mean grain size of 3 or above. In the case of samples with sub-micron average grain sizes, the measured grain-boundary conductivities were inconsistent with the predictions of the model and were significantly higher than those predicted.

G.M.Christie, F.P.F.Van Berkel: Solid State Ionics, 1996, 83[1-2], 17-27