Effect of B2O3 Concentration and Sintering Temperature on Microstructure and Electrical Properties in the ZnO-Bi2O3-Based Varistors

The ZnO-Bi 2 O 3 -MnO 2 -Co 2 O 3 -based (ZBMCO) varistors were prepared via the sol–gel method. The effects of B 2 O 3 additive on the phase composition, microstructure, sintering temperature and electrical properties of the ZBMCO ceramics were studied. A single hexagonal ZnO phase was detected in all ZBMCO- x B 2 O 3 varistors sintered at high temperature (1100°C). Secondary phases Mn 0.31 Bi 1.69 O 2.85 , Zn 3 B 2 O 6 , Bi 24 B 2 O 39 and Bi 2 O 3 were detected in ZBMCO-1.0 wt.%B 2 O 3 varistors at a lower sintering temperature (900°C, 1000°C). The average grain size increased remarkably in the range of 15.27–26.12 μ m with an increase of the B 2 O 3 content. The ZBMCO-1.0 wt.%B 2 O 3 varistor showed the maximum relative density of 97.2% with high nonlinear coefficient (56.5) and low leakage current (0.07 μ A/cm 2 ) at 1100°C. The E 1mA decreased noticeably from 865 V/mm to 64 V/mm with an increase of the sintering temperature. When the sintering temperature is lower at 1000°C, the varistor exhibits relatively good electrical properties ( α = 48.7, I L = 0.68 μ A/cm 2 ). These results demonstrate that low melting point B 2 O 3 plays multiple roles in grain growth and microstructure.