The annealing effect on the performance of flash-sintered ZnO–Bi2O3–Sb2O3-based varistors

Annealing at a specific temperature enhances the performance of ZnO–Bi 2 O 3 –Sb 2 O 3 -based varistors prepared by the flash sintering method. The effect of the annealing temperature (800 and 950 °C) on the phase formation, microstructure, electrical properties, and stability of the ZnO varistors is investigated in this paper. Also, the properties of the varistors prepared with the proposed method are compared with the samples produced by a conventional sintering method used in industry. According to the X-ray analysis, the type and morphology of intergranular layers depend on the annealing temperature. The γ-Bi 2 O 3 phase forms during cooling from 800 °C and deteriorates the electrical properties of the varistor, while the β and δ-Bi 2 O 3 phases form in samples annealed at 950 °C with a positive effect on device performance. The estimated values of the donor density, the density of surface states, and potential barrier height by the thermionic emission model indicate the improvement in electrical properties due to the annealing at 950 °C, which was consistent with the grain boundary resistance enhancement and grain resistances reduction measured by impedance method. Therefore, the combination of flash sintering and annealing improves the nonlinear coefficient, leakage current, and voltage values measured as 48.8, 21 μA/mm 2 , and 421 V/mm, respectively. Moreover, the current creep test results show that the stability of flash-sintered varistors increases upon annealing. Eventually, the properties of flash-sintered ceramic varistors annealed at 950 °C were close to the characteristics of the conventionally sintered sample.