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 800℃ and then pressed in to pellets which were sintered at 950℃ 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 μm down to 5.3 μm, 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 α of 66.7 and the leakage current density J_L of 1.1 µA/cm².