Effect of Ho2O3 doping on the microstructure and electrical properties of ZnO–Bi2O3–Sb2O3–Cr2O3–Co2O3–MnO2-based varistors

Ho2O3 doped ZnO–Bi2O3–Sb2O3–Cr2O3–Co2O3–MnO2-based varistors (ZBSCCM based varistors) was fabricated by solid-state reaction and subsequent sintering process. Microstructure characterization by X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicated the Ho2O3 phase with the doping content no more than 0.8 mol% was distributed along the grain boundaries of ZnO grains and could restrain the growth of ZnO grain. However, excessive Ho2O3 doping (>1.0%) delayed the full densification of the green pellets, leading to the decrease in relative density of the sample. The electrical properties were investigated by means of direct current measurement. It was found that proper amount of Ho2O3 doped in the ZBSCCM based varistors was able to improve the electrical properties. Particularly, the varistor doped with 0.8 mol% of Ho2O3 possessed the highest breakdown field (1578.5 V/mm), an excellent nonlinear coefficient α (86.3) and a small leakage current density (1.06 μA/cm2).