ZnO-based varistor ceramics are pivotal for overvoltage protection in electronic devices and power systems because of their highly nonlinear current-voltage characteristics. This behavior arises mainly from double Schottky barriers (DSBs) at grain boundaries. Formed during high-temperature sintering, these barriers keep varistors insulating at normal operating voltages but allow them to become conductive under surges, thereby protecting equipment across voltage ranges from several volts to megavolts. Yet dynamic formation and evolution during sintering are still not fully understood. Consequently, tracking the formation of the DSB throughout the entire sintering process is of great importance for developing advanced varistor ceramics.
Traditional techniques for characterizing DSBs, including I-V deconvolution, capacitance-voltage (C-V) testing, and conventional dielectric spectroscopy, are generally performed below 300°C. However, ZnO varistor ceramics however, are typically sintered above 1000°C, preventing their direct application during sintering. Consequently, existing studies have relied largely on control-variable approaches and rapid quenching to reconstruct DSB evolution at discrete stages. These ex-situ methods cannot capture continuous, real-time changes, making it difficult to establish the relationships among electrical response, grain-boundary barrier formation, microstructural evolution, and phase transformations.
Recently, a team of researchers led by Kangning Wu and Zhuolin Cheng from Xi'an Jiaotong University, China, reported an in-situ dielectric spectroscopy approach for tracking DSB formation throughout the sintering of ZnO varistor ceramics. The apparent activation energy (Ea) of DC conductance was identified as a descriptor for following DSB evolution. The electrical response was further correlated with densification, grain-boundary elemental redistribution, and Bi-rich phase evolution using dilatometry, SEM-EDS, and in-situ high-temperature XRD.
The team published their work in Journal of Advanced Ceramics on July 31, 2026.
"In this study, we employed in-situ dielectric spectroscopy to continuously monitor capacitance and conductance throughout the entire sintering process. The apparent activation energy of DC conductance was identified as a robust descriptor for tracking DSB evolution. By correlating the electrical responses with microstructural and phase evolution, we resolved the formation and evolution of DSBs during sintering," said Kangning Wu, an associate professor at the School of Electrical Engineering, Xi'an Jiaotong University (China), whose research focuses on ZnO varistor ceramics and dielectric physics.
"Dilatometry and SEM analyses showed that the major microstructural evolution occurred during heating and soaking, whereas cooling contributed little to dimensional change. The relative linear shrinkage increased from 0.30% at 700°C to 22.43% after soaking. SEM-EDS line scans further showed obvious enrichment of Bi and Sb at Zn-depleted grain-boundary regions after soaking, accompanied by local redistribution of Mn and Ni," said Wu.
In-situ XRD further revealed the continuous evolution of the Bi-rich phases during cooling. At 1080 °C, the Bi-rich phase was predominantly liquid and lacked long-range crystalline order. During cooling, the liquid progressively crystallized following the sequence liquid → β + δ → β → α + β → α. Below approximately 650°C, α-Bi2O3 increased as β-Bi2O3 decreased, with the β-to-α transition becoming particularly evident between 600 and 500°C.
"The in-situ dielectric spectra revealed a pronounced negative-permittivity response above approximately 700°C. Its frequency dependence was consistent with the Drude model, indicating a response dominated by thermally activated carriers released through the ionization of trap states in the ZnO grains and grain-boundary regions. During cooling, the Drude-like contribution was progressively suppressed, while positive dielectric relaxation associated with grains and grain boundaries re-emerged." said Wu.
Equivalent-circuit fitting showed that the dielectric response at 800°C was still dominated by the Drude-like branch. Below 800°C, a finite grain-boundary resistance (Rgb) emerged and increased continuously with decreasing temperature. Between 750 and 600°C, the free-carrier response and developing barrier-related relaxation coexisted. At 500°C, the spectra were mainly described by the grain and grain-boundary branches, demonstrating a transition to barrier-controlled carrier transport.
"During cooling from 800 to 650°C, Ea increased markedly from nearly zero to approximately 1.4 eV, accompanied by the emergence and continuous increase of Rgb, suggesting the progressive formation of DSBs. The evolution of Rgb and Ea reveals a clear separation between structural densification and electrical functionalization: densification and grain growth occur mainly during heating and soaking, whereas grain-boundary barriers form progressively during cooling." said Wu.
Upon further cooling, Ea exhibited a distinct peak in the 600-500°C range, coinciding with the progressive transformation from β-Bi2O3 to α- Bi2O3. Together with the grain-boundary elemental redistribution revealed by SEM-EDS, this correspondence suggests that the formation and subsequent modification of DSBs are accompanied by continuous changes in the grain-boundary chemical environment.
"These findings provide a mechanistic basis for distinguishing microstructural development from electrical functionalization and for optimizing the sintering process of grain-boundary-barrier-controlled functional ceramics," said Wu. "Although the exact temperature range of DSB formation may vary with material composition and cooling conditions, in-situ dielectric spectroscopy provides an effective approach for identifying the key stage of grain-boundary barrier development."
Other contributors include Jiale Wang, Jiatong Liu, Zhuang Tang, Yuguang Ge, Xuetong Zhao, and Jianying Li. The research involved Xi'an Jiaotong University, the State Key Laboratory of Disaster Prevention and Reduction for Power Grid Transmission and Distribution Equipment, Xi'an Tian Gong Electric Co., and Chongqing University.
About Author
Kangning Wu is an associate professor at the School of Electrical Engineering, Xi'an Jiaotong University, China. His research interests include dielectric physics, advanced electrical materials, and high-performance ZnO varistor ceramics. He serves as Deputy Secretary-General of the Engineering Dielectrics Committee of the China Electrotechnical Society and as a registered expert of IEC TC37. He has published more than 70 papers and received several scientific and technological awards.
Zhuolin Cheng is an assistant professor at the School of Electrical Engineering, Xi'an Jiaotong University, China. He received his B.Eng. and Ph.D. degrees from Xi'an Jiaotong University in 2019 and 2025, respectively. His research focuses on dielectric physics, high-performance ZnO varistor ceramics, and inorganic dielectrics for electrostatic chucks. His doctoral dissertation was selected for the 2026 Doctoral Dissertation Incentive Program of the China Electrotechnical Society.
Funding
This work was supported by the National Natural Science Foundation of China (No. 52577029), the State Key Laboratory of Electrical Insulation and Power Equipment (EIPE26306), and the Fundamental Research Funds for the Central Universities (No. xzy012026089).
DOI LINK: 10.26599/JAC.2026.9221356
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 2025 IF is 14, ranking in Top 1 (1/34, Q1) among all journals in "Materials Science, Ceramics" category, and its 2025 CiteScore is 24.6 (6/133) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508
