Advances in ZnO varistors in China during the past 30 years—fundamentals, processing, and applications

The intrinsic mission in the field of electrical insulation is to make the insulator or the power equipment withstand voltage as high as possible. On one hand, extensive research has focused on improving insulation materials or optimizing the design to endure higher voltage, whereas on the other hand, efforts have addressed the suppression of voltage surges because this indirectly improves the capability of the equipment to withstand higher voltages. ZnO varistors, exhibiting excellent nonlinear I-V characteristics, are specialized in discharging transient surges and limiting the voltage to a magnitude that is not harmful to power equipment. Because of their good price-to-performance ratio, ZnO varistors are the most common overvoltage protection component in AC power protectors. The discovery of the nonlinear I-V characteristics of ZnO ceramics in the 1960s [1] trigged interest in scientific research and industry [2]–[7]. The mysteries about the interfacial states at the grain boundaries and the point defect structures have attracted much attention from theoretical and experimental aspects. As a result, ZnO varistors developed rapidly to meet new challenges from the power system and electronic sectors. ZnO varistors developed rapidly during the past 30 years, not only in China but also in the world, and benefited from the contributions of the universities on the fundamentals and key processing techniques. The objective of this article is to give an overview of the research carried out over the last 30 years that has contributed to the development of ZnO varistors in a broad sense. We emphasize, in our view, the remarkable progress in the fundamental research, which has revealed the physical origin of the obtained properties, the key processing techniques, which have provided important guidance on how to obtain ZnO ceramics with the required performance, and the advanced products now available from industry.