The effects of MnO2 on the microstructure and electrical properties based on ZnO-Bi2O3-Sb2O3-Cr2O3-Co2O3 varistors

In this work, nano ZnO powders, Bi2O3, Sb2O3, Cr2O3, Co2O3 and a various content of MnO2 were blended thoroughly and pre-calcined at 800degree celsius and then pressed in to pellets which were sintered at 950degree celsius to form varistor ceramics. Subsequently, the effects of MnO2 on the microstructure and electrical properties of the ZnO-based varistor were investigated. It was found that the amount of spinel phase (Zn7Sb2O12) and Bi2O3 phase increased with the MnO2 increasing, while the content of pyrochlore (Zn2Bi3Sb3O14) phase decreased. As a result, the growth of ZnO grain was reduced with the average grain size from 9.5 mu m down to 5.3 mu m, leading to the increase of breakdown field of ZnO-based varistor. Particularly, the ZnO-based varistor with 1.2 mol% MnO2 exhibited the best comprehensive electrical performance with the breakdown field E-b of 901.4 V/mm, the nonlinear coefficient alpha of 66.7 and the leakage current density J(L) of 1.1 mu A/cm(2).