The migration of boundaries between single crystal and polycrystalline ZnO was investigated using single crystals with well-defined crystallographic faces. The

migration rate of the basal (00•1) planes through polycrystalline ZnO depended on the crystallographic polarity of the basal plane. Grain boundary migration in the [00•1] direction was much faster than in the [00•1] direction. Migration rates of boundaries in non-polar directions were somewhat slower than that in the [00•1] direction. Slow growth in the [00•1] direction and rapid growth in non-polar directions found in the present study help to explain the anisotropic growth of the grains that contain head-to-head inversion boundaries in ZnO varistor ceramics.

Effects of Polarity on Grain-Boundary Migration in ZnO. Lee, J.S., Wiederhorn, S.M.: Journal of the American Ceramic Society, 2004, 87[7], 1319-23