Highly c-axis oriented ZnO varistors are able to be successfully fabricated by industrial tape casting and lamination process, which use fine templated ZnO. They have Lotgering factor of 0.637 which is an index of degree of orientation, and are composed of isotropic fine grains of 2.2–3.0 µm. Significant variations are obtained in electrical properties of grain boundaries by crystalline orientation. Break down voltage per a grain boundary (Vgb) shows a decrease in perpendicular directions to c-axis alignment from those in parallel (−27 % in parallel to casting direction, −14 % in transverse), simultaneously with an increase in εr of relative permittivity. The orientational control is more likely to contribute enhancing the protection performance of ZnO based multilayer ceramic varistors.
Higashi et al. (Sat,) studied this question.