ZnO-based varistor ceramics have been widely used in overvoltage protection. However, conventional sintering technologies face significant challenges in enhancing the voltage gradient without compromising other performances. The high sintering temperature (typically ~1200°C) and long soaking time in conventional technologies not only cause energy loss but also lead to issues like grain overgrowth and the volatilization of the Bi-rich phase at grain boundaries, deteriorating the electric properties of ZnO-based varistor ceramics.
To tackle these challenges, scholars from Chongqing University, Xi'an Jiaotong University, Tsinghua University, and State Grid Jiangxi Electric Power Research Institute in China has developed a novel approach to prepared ultrahigh voltage-gradient ZnO-based varistor ceramics through a hybrid cold sintering/spark plasma sintering (CSP-SPS) and post-annealing process. This route aims to optimize the microstructure of the ceramics and inhibits the volatilization of the Bi-rich phase, thereby improving their electrical properties.
The team published their work in Journal of Advanced Ceramics on March 20, 2025.
"Novel hybrid cold sintering/spark plasma sintering technologies, combined with a post-annealing process, can effectively densify ZnO-based varistor ceramics at a low temperature. It not only reduces energy consumption but enables the formation of a fine and homogeneous microstructure. Moreover, the low annealing temperature helps to prevent the volatilization of the Bi-rich phase at grain boundaries, which is crucial for enhancing the voltage gradient and other electrical properties of the ZnO-based varistor ceramics." said by Xuetong Zhao, the corresponding author of the study.
The researchers prepared multi-doped ZnO-based varistor ceramics using the CSP-SPS method and post-annealing process. The samples were first densified at 300 °C and then annealed at the temperature ranging from 700 to 900 °C. XRD, TEM, and XPS, etc. were employed to analyze the crystal structure, microstructure, and electrical properties. The results showed that the ZnO-based varistor ceramics prepared using this method exhibited an ultrahigh voltage gradient of ~1832.71 V/mm, a high nonlinear coefficient of ~106.69, and a low leakage current density of less than 0.2 μA/cm².
In addition, the researchers explored the relationship between the microstructure, Schottky barrier at grain boundaries, and the ultrahigh voltage gradient of the ZnO-based varistor ceramics. It is found that the post-annealing process provided sufficient energy for phase transformation and the formation of high-resistance grain boundaries, which produces an increased Schottky barrier height and improved electrical performance.
This work was supported by the Joint Funds of the National Natural Science Foundation of China (Grant no. U23B20115) and the Fok Ying-Tong Education Foundation, China (No. 171050).
About the Authors
Dr. Xuedong Zhao is a Professor in the School of Electrical Engineering at Chongqing University. He received his Ph.D. in Electrical Engineering from Xi'an Jiaotong University in 2013 and worked as a post-doctor at Pennsylvania State University from 2016 to 2018. His research focuses on:
- Dielectric physics and its applications
- Novel functional ceramic materials
- Organic insulating/energy storage materials
Shenglin Kang is a Ph.D. candidate in the School of Electrical Engineering at Chongqing University. His research focuses on developing high-performance functional ceramic materials based on cold sintering process.
About Journal of Advanced Ceramics
Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen . JAC's 2023 IF is 18.6, ranking in Top 1 (1/31, Q1) among all journals in "Materials Science, Ceramics" category, and its 2023 CiteScore is 21.0 (top 5%) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508
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