Effect of SiO₂ addition on the microstructure and electrical properties of ZnO-based varistors

The microstructure and electrical properties of ZnO-based varistors with the SiO₂ content in the range of 0–1.00mol% were prepared by a solid reaction route. The varistors were characterized by scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectrometry, inductively coupled plasma-atomic emission spectrometry, and X-ray photoelectron spectroscopy. The results indicate that the average grain size of ZnO decreases with the SiO₂ content increasing. A new second phase (Zn₂SiO₄) and a glass phase (Bi₂SiO₅) are found. Element Si mainly exists in the grain boundary and plays an important role in controlling the Bi₂O₃ vaporization. The electric measurement shows that the incorporation of SiO₂ can significantly improve the nonlinear properties of ZnO-based varistors, and the nonlinear coefficients of the varistors with SiO₂ are in the range of 36.8–69.5. The varistor voltage reaches the maximum value of 463 V/mm and the leakage current reaches the minimum value of 0.11 μA at the SiO₂ content of 0.75mol%.