Varistor properties and microstructure of ZnO-BaO ceramics

Ceramics in the system ZnO-BaO have been investigated for possible use as varistors. Specimens were prepared by the mixed oxide route, and were sintered at temperatures in the range 1000-1400 degrees C. The electrical properties were determined using d.c. A and impulse techniques. The ZnO-BaO ceramics have a two-phase microstructure comprising a ZnO phase and a Ba-rich grain boundary phase. Due to liquid phase sintering, the average grain sizes for the ZnO-BaO ceramics are large (typically 35-55 mu m for samples sintered at 1300 degrees C). This results in low breakdown fields, (less than or equal to 1000 Vcm(-1)). The maximum non-linearity exponent obtained for ZnO-BaO ceramics (similar to 14) is higher than that for binary ZnO-Bi2O3 ceramics. However, the high water solubility of the Ba-rich phase may restrict the use of ZnO-BaO ceramics.