Improving the Microstructure of ZnO-Based Metal Oxide Varistors Using Cold Sintering

Metal oxide varistors (MOVs) are well established as surge arrestors to protect AC power transmission and distribution apparatus. Unlike the rare lightning-induced surges in AC application, MOVs are required to absorb energy repeatedly and frequently in DC circuit breakers (DCCBs). Therefore, further advances in lifetime and reliability are needed to develop MOVs for high-efficiency DCCBs. Microstructure in MOVs has been found to dictate device nonlinearity, voltage breakdown, and lifetime. In ZnO-based MOVs, smaller grains and increased grain homogeneity has been shown to extend the lifetime of MOVs, but the ability to control grain growth has previously been limited by the high temperatures needed to densify ZnO. Here, cold sintering is proposed as a route to minimize grain growth during the sintering process of ZnO-based MOVs. In cold sintering, increased pressure and liquid phase transport promote densification at lower temperatures. The microstructure of cold sintered ZnO MOVs processed with varied pressure and liquid phase compositions are compared to high temperature sintered samples to assess the potential for microstructure control. Density, the critical metric for fully sintered MOVs, was measured using a gas pycnometer, the microstructure was assessed using scanning electron microscopy (SEM), while impedance analysis was done to observe the electrical response. Sintering at 1100 °C for two hours resulted in grain sizes averaging 4.1 μm, but cold sintering at 400 MPa at 150 °C for 30 min reduced grain growth, resulting in grain sizes of 0.2-0.3 μm. Cold sintering was shown to produce homogenous microstructures with improved electrical properties while reducing energy consumption due to sintering at lower temperatures and times.