Bi–Er–O pre-synthesized phase (Bi1-xErx)2O3 doped ZnO varistors with high nonlinear coefficient and low leakage current

Abstract

The content of free oxygen Oad at the ZnO grain boundaries has an important impact on the double Schottky barrier and electrical properties of ZnO varistors. In this work, Bi–Er–O pre-synthesized (BE) phase (Bi1-xErx)2O3 (0.25 < x < 0.455) with high conductivity of Oad was prepared using Bi2O3 and Er2O3 as raw materials, and the effects of the BE phase doping on the phase, grain boundary properties, and electrical properties of ZnO–Bi2O3–Sb2O3–Co2O3–MnO2–Cr2O3–SiO2 varistors were studied. XRD analysis shows that the lattice constant of the BE phase (Bi1-xErx)2O3 is between (Bi0.75Er0.25)2O3 and BiErO3. With increasing doping content of the BE phase from 0 to 0.75 wt%, part of Er3+ still exists in the form of the BE phase (Bi1-xErx)2O3 at the ZnO grain boundaries, and its content increases with increasing doping content of the BE phase. The conductivity of Oad in (Bi1-xErx)2O3 is much higher than that in Bi2O3, resulting in the generation of a large number of negative O′ and O″ at the ZnO grain boundaries, and decreasing concentration of the intrinsic point defect zinc interstice Zni. So the donor concentration Nd decreases, the height of the grain boundary barrier increases, the depletion layer widens, and the electrical properties are improved. By analyzing the low-temperature permittivity spectra, it was calculated that with increasing doping content of the BE phase from 0 to 0.75 wt%, the concentration of oxygen vacancy VO• decreases inappreciably, but the concentration of zinc interstice Zni•• decreases by about 23%. When the doping amount of the BE phase is 0.75 wt%, the nonlinear coefficient α of the obtained ZnO varistors is 73.9 ± 0.6, and the leakage current density JL is only 0.3 ± 0.1 μA/cm2. This study provides an important reference for improving the electrical properties of ZnO varistors by manipulating the Bi-rich phase structure to improve the double Schottky barrier characteristics.