Investigation of dielectric relaxation and degradation behavior of two-step sintered ZnO varistors

Abstract

In this study, ZnO varistors using sub-micron varistor powders by two-step sintering were fabricated in contrast to conventional sintering. The microstructures, electrical properties, and the degradation behavior under a severe dc electric field and a high temperature were systematically studied. The dielectric spectroscopy, including impedance and electric modulus, was also measured. The results showed that two-step sintering would effectively decrease the grain size, uniform grain size distribution and strengthen densification. Two-step sintered ZnO varistors (TSZV) acquired better electrical properties with the following sintering schedule: rising up to 1150 °C, immediately down to 850 °C, and soaking for 10 h. For this two-step sintered sample, the breakdown field was 402.26 V/mm, the leakage current was 0.98 μA/cm2, and the nonlinear coefficient was 60.88. With the help of electric modulus spectroscopy from −110 to 200 °C, three dielectric relaxation peaks with activation energy of 0.21–0.23 eV, 0.31–0.41 eV, and 0.71–0.78 eV originated from zinc interstitial, oxygen vacancy and interface states, respectively were detected and a peak with an activation energy of 0.60 eV originated from DC conductance was also detected for the TSZV samples and the conventional sintered sample. The differences in dielectric relaxation suggested different defect trap states at the grain boundary inside ZnO varistors. It was also found that TSZV were more likely to degrade when compared with the conventional sintered varistor, and some explanations were given for the worse aging stability of two-step sintering.