Effect of Bi2O3 on the ZnVMnCoTiO based varistor ceramic sintered at 800 °C

The effect of Bi2O3 on ZnO-based varistor ceramic sintered around 800 °C is still not fully understood, because most of the ceramic cannot reach full densification at this low-temperature. Therefore, (balanced) ZnO–0.5 mol% V2O5–0.5 mol% MnCO3–0.5 mol% Co2O3–0.5 mol% TiO2–xmol% Bi2O3 (x = 0, 0.01, 0.05, 0.1 and 0.2) ceramic was designed and prepared by the solid-state sintering at 800 °C. The effect of Bi2O3 variation on the microstructure and electrical nonlinearity of the sample material was studied by using methods of XRD SEM, STEM, EDS and E–J test. Bi2O3 can further activate the reaction at the interface between the V-rich liquid phase and ZnO grains during sintering. This enhanced interface reaction is the key reason to the extremely low-temperature sintering of the ceramic at 800 °C. The variation of Bi2O3 in the 0–0.2 mol% range barely influences the constituents of the ceramic, which consists of ZnO main phase and two other secondary phases: Zn3(VO4)2 and Zn2TiO4. Most of Bi2O3 tends to segregate at grain boundaries. As a result, a soaking time longer than 4 h is needed to allow Bi ions diffusing into enough grain boundaries and improving the nonlinearity of ZnO based varistor ceramic. The sample containing 0.01 mol% Bi2O3 and sintered at 800 °C for 8 h has the best properties of a 43.3 nonlinear coefficient, a 2300 V/mm breakdown voltage and a 0.13 mA/cm2 leakage current density.