Boron and sodium co-doped ZnO varistor with high stability of pulse current surge

Abstract The effect of boron and sodium co-doping on the stability of ZnO varistors subjected to large pulse-current surges was investigated. The relationship among the microstructure, phase composition, electric properties and pulse-current aging behavior were systematically discussed. No additional phase is generated with the co-doping of boron and sodium ions. The energy dispersive spectroscopy (EDS) analysis confirms that sodium ions can permeate into the grains and occupy the interstitial sites, while the boron ions stay at the grain boundary to form glass phase and decrease the concentration of the interstitial defect. Moreover, the co-doping behavior leads to a trend that the grain size increases firstly and then decrease, which is the main reason for the change in the breakdown field ( E 1mA/cm 2 ). The result reveals that with 0.24 mol% B 3+ , E 1mA/cm 2 increases to a maximum value of 4187 V/cm and then decreases drastically with increasing content of Na + dopant from 0.12 to 0.48 mol%. Meanwhile, the nonlinear coefficient of the samples decreases and the leakage current increases with the increase of Na + content. The varistor doped with 0.24 mol% Na + and 0.24 mol% B 3+ exhibits the optimum I-V stability. After subjecting to a surge current of 6000 A, the variation rate of the breakdown field (%Δ E 1mA/cm 2 ) is −2.6%, which is almost five times lower than that of the sample without boron and sodium. In addition, the ZnO varistor with the B/Na ratio of 0.24/0.24 shows much better stability compared to the samples added with B 3+ or Na + singly. The finding of this study provides an effective method for improving the protective effect of surge protection equipment and the stability of power systems.