Effect of Bi_2O_3 on the ZnVMnCoTiO based varistor ceramic sintered at 800 °C

摘要

The effect of Bi_2O_3 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% V_2O_5-0.5 mol% MnCO_3-0.5 mol% Co_2O_3-0.5 mol% TiO_2-xmol% Bi_2O_3 (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 Bi_2O_3 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. Bi_2O_3 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 Bi_2O_3 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: Zn_3(VO_4)_2 and Zn_2TiO_4. Most of Bi_2O_3 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% Bi_2O_3 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/cm~2 leakage current density.