In-situ tracking reveals key stages of grain-boundary barriers formation in ZnO varistor ceramics

What is it about?

ZnO varistor ceramics play a vital role in protecting electronic devices and power systems from voltage surges. Their unique electrical properties arise mainly from double Schottky barriers at grain boundaries. However, when and how these barriers form during high-temperature sintering have not been fully understood. In this study, we used in-situ dielectric spectroscopy to continuously monitor the electrical response of ZnO varistor ceramics throughout the entire sintering process. We identified the apparent activation energy (Ea) of DC conductance as a key descriptor for tracking grain-boundary barrier evolution. By correlating electrical responses with densification, microstructural changes, and phase evolution, we divided the sintering process into five distinct stages. We found that densification and grain growth occur mainly during heating and soaking, whereas grain-boundary barriers form progressively during cooling. In particular, 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 grain-boundary resistance, indicating progressive barrier formation. A subsequent Ea peak between 600 and 500 °C coincided with the transformation from β-Bi₂O₃ to α-Bi₂O₃ and the later modification of the barriers. These findings reveal the key stages of grain-boundary barrier formation and evolution during sintering.

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Photo by Kier in Sight Archives on Unsplash

Photo by Kier in Sight Archives on Unsplash

Why is it important?

Understanding the formation and evolution of grain-boundary barriers is essential for developing advanced ZnO varistor ceramics. However, conventional characterization methods generally rely on measurements after sintering or rapid quenching at selected temperatures. These approaches cannot capture continuous, real-time changes, making it difficult to identify the critical stages of barrier development during high-temperature sintering. The key contribution of our study is providing a new in-situ approach for monitoring grain-boundary barrier formation and evolution. By identifying the apparent activation energy (Ea) of DC conductance as a key descriptor, this approach enables the continuous tracking of barrier evolution throughout sintering. Importantly, we identified the critical temperature range of approximately 800–650 °C during cooling, when grain-boundary barriers progressively form. This provides an effective way to identify the key stage of barrier development. Our findings also reveal 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. These findings provide a mechanistic basis for understanding grain-boundary barrier development and optimizing the sintering process of ZnO varistor ceramics. The approach may also benefit other functional ceramics whose electrical properties are controlled by grain boundaries.

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