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ZnO Varistors: From Grain Boundaries to Power Applications
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Book Chapter
ZnO Varistors: From Grain Boundaries to Power Applications
2021
Overview
ZnO‐based varistors are today the established technology for overvoltage surge protection on all voltage levels, ranging from a few volts in electronics up to >1 MV in ultra‐high voltage power systems. All these applications are based on the highly nonlinear, fast, and reversible electrical switching characteristics of a single grain boundary with a switching voltage of ≈3.2–3.6 V in the semiconducting polycrystalline ZnO ceramics. For the many intriguing steady‐state and time‐dependent transport properties, the model of the double Schottky barrier at each grain boundary provides a solid frame for the microscopic understanding, provided the knowledge gained on the electronic bulk and interface defects states and the hot electron effects at breakdown are consistently integrated into the model. The commonalities of the many different varistor formulations in use, and the parameters to be optimized to achieve a specific target for the electrical performance, then become evident from the ‘double Schottky barrier defect’ model. Interfacial and bulk defects, characteristic for the ZnO crystal lattice, control the grain boundary potential barriers and hence the I‐V characteristics. Their densities vary with the material formulation and processing parameters, whereas their energy positions are at fixed positions within the band gap of ZnO, independent of the manufacturing parameters used.
The paper describes the present state of understanding of the varistor phenomena on the micro‐, meso‐, and macroscopic length scales, which are relevant for the basic and applied research on varistor materials and their applications. Besides the single junction properties, the main technically and scientifically challenging topics needed to be understood are the filamentary current flow in real microstructures, the inhomogeneities present on all length scales and the long‐term stability. Knowing the limits in energy absorption and high current withstand stresses are key inputs for the development engineer for the design of surge arresters consisting of a few or many hundred individual varistor elements. The scientific backbone summarized in the present overview is a good basis to tackle the few remaining open questions in future varistor research.
Publisher
John Wiley & Sons, Ltd
Subject
ISBN
9781119529477, 1119529476
