Effect of Annealing Process on Intrinsic Point Defects and Electrical Properties of ZnO Varistor Ceramics

Based on the orthogonal test method, the effects of four parameters of heat treatment, namely the heating rate, holding temperature, holding time and cooling rate, on the phase composition, microstructure, intrinsic point defects and barrier structure of ZnO varistor ceramics were studied. The electrical properties of ceramics were comprehensively analyzed to obtain the optimal heat treatment process for ZnO varistor ceramics. The results show that the heat treatment under the conditions of cooling and heating rates of 1 ℃·min⁻¹, a holding temperature of 600 ℃, and a holding time of 2 h caused the bismuth-rich phase to transform from γ-Bi₂O₃ to α-Bi₂O₃, the average grain size slightly increased, the grain size distribution became more concentrated, the element distribution became more uniform, the microstructure of the ceramic became more uniform, and the relative density was greater; the defect equilibrium state of the grains, with the contents of Zni^•• and V_O^• remaining at a relatively low level and the barrier height being relatively high. The heat treatment process parameters, in order of their influence on the electrical properties of ZnO varistor ceramics from largest to smallest, were holding temperature, cooling rate, holding time, and heating rate. The optimal heat treatment process was as follows: heating rate of 6 ℃·min⁻¹, heat treatment temperature of 600 ℃, holding time of 2 h, and cooling rate of 1 ℃·min⁻¹. After heat treatment of ZnO ceramics under this condition, the barrier height increased from 0.58 eV before heat treatment to 0.70 eV, the voltage gradient increased from 355.0 V·mm⁻¹ to 370.1 V·mm⁻¹, the leakage current density decreased from 1.38 µA·cm⁻² to 0.41 µA·cm⁻², and the nonlinear coefficient remained at 72.