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°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 μm for samples sintered at 1300°C). This results in low breakdown fields, (≤1000 V cm-1). The maximum non-linearity exponent obtained for ZnO–BaO ceramics (∼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.