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Electrophysical characteristics of the near-surface layer of semiconductor ceramics for ZnO varistors

Electrophysical properties of near-surface layer of ZnO varistor ceramics forming during high-temperature annealing in oxidizing gaseous atmosphere are studied. Microstructure of near-surface layer and its electrical properties have features which are determined by an increased concentration of oxygen at surface of pressed ceramic billets in comparison with bulk during annealing. From the data of electron raster microscopy of cross-chipping of varistors, it is defined that the average value of thickness of near-surface layer is 40 μm. It is found that the specific resistivity value of near-surface layer is 5.3·1012 Ω·cm. The electrical capacitance of near-surface layer is calculated from capacitance measurement data for varistors  containing such layer under two electrodes and without layers. It is shown that the electrical capacitance of near-surface layer exceeds that of varistor by more than an order. Conclusion is made that the near-surface layer gives a weak contribution to dielectric properties of ZnO varistor ceramics. However contribution of this layer to electrical conductivity of ceramics in pre-breakdown region is significant.