Cold Sintering Process for Fabrication of High Voltage Gradient ZnO Varistors

ZnO-based varistor ceramics exhibit exceptional nonlinear current-voltage (I-V) characteristics, rendering them suitable for application as surge suppressors in electronic circuits and overvoltage protectors in power systems. However, the conventional high-temperature sintering techniques face challenges in achieving fine-dense ZnO varistor ceramics with micro-nanometer grain sizes. In this study, we propose a novel strategy for producing high-density ZnO-based ceramics with controlled grain growth and customizable grain boundary structures. This is achieved through a low-temperature cold sintering process (CSP) utilizing a mixture of soluble metal salts (Bi, Co, Mn, etc.) and acetic acid as a transient liquid phase. Subsequently, the densification, microstructure, and electrical performance of the cold-sintered ZnO-based varistor ceramics were further enhanced by low-temperature annealing treatment at approximately 850°C for 3 h. The results demonstrate that following the annealing treatment, the relative densification of the ZnO-based varistor ceramics reaches up to ~99%. Additionally, the annealed varistor ceramics exhibit a high voltage gradient of ~2127.13 V/mm, with a high nonlinear coefficient of ~88.35, and a low leakage current density of below 0.03 μA/cm². The impressive I-$V$ properties can be attributed to the small grain size and the presence of Bi-rich high-resistance grain boundaries between ZnO grains induced by soluble metal salts during CSP, thus outperforming commercially available varistor ceramics. This work presents a promising approach for producing high voltage gradient ZnO varistors and offers valuable insights into designing ZnO varistors with tailored characteristics by manipulating microstructure via the CSP.