High Barrier Effects of (0001)|(0001) Zinc Oxide Bicrystals: Implication for Varistor Ceramics with Inversion Boundaries

Inversion boundaries (IBs) of a head-to-head or (0001)|(0001) (C⁺|C⁺) configuration bisect virtually every grain in typical commercial ZnO varistor ceramics. They are most often considered electrically inactive, and the effect on the grain growth behavior has been recently addressed. In this work, various configurations of ZnO bicrystals were prepared using different source crystals and strong barrier effects were observed in some (0001)|(0001) (C⁻|C⁻) bicrystals using crystals with higher impurity contents. The crystallographic polarity and impurity effects were systematically examined by doping C⁺|C⁺ and C⁻|C⁻ bicrystals with single and double additives of Mn, Co, Ni, and Bi. Varying degrees of barrier effects including varistor-like behaviors were observed in C⁻|C⁻ bicrystals depending on dopants, while C⁺|C⁺ bicrystals consistently exhibited negligible effects. Because the IBs in ZnO varistor ceramics preferentially expose C⁻ surfaces in the grain boundaries, the superior property of commercial ZnO varistor ceramics is suggested to be assisted by the presence of IBs.