Effect of Zn doping on stability of ZnO varistors under high pulse-current stress

The electrical properties, especially the degradation behavior against pulse-current stress, of the ZnO-based varistors with different doping contents of Zn have been investigated. The results show that the breakdown field (E1mA/cm2) and the nonlinear coefficient (α) decreases with the increasing content of Zn due to the increase in the amount of the non-effective grain boundaries and the reduced barrier height. The doped Zn ions, acting as donors, enhance the electron concentration in grains and result in the decrease of the clamping voltage ratio (K) for the pulse-current. The varistors with 3.0 mol% doped Zn possess a K in the range from 2.11 to 2.41, and after applying a surge-current of 6000 A, the variation rate of the breakdown field (% ΔE1mA/cm2) is only −9.8%, almost three times smaller than that of the samples without Zn doping.