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The effect of Al addition in ZnO varistors revealed to be the suppression of grain growth, not the lowering of the electric resistance of ZnO. This Al effect can explain the change in the E-J characteristics by the following three points. (i) The grain boundary becomes thinner and the proportion of boundary layers with a thickness of 10 nm or less increases, which enlarges the effective cross-sectional area of the tunnel current generation. (ii) The delay in densification during the sintering makes it easier for pores to remain at the grain boundary, which increases discontinuities of boundary layers between ZnO grains and increases leakage current, resulting in reduced non-ohmic properties in the low current range. (iii) Abnormal grain growth of ZnO is suppressed and the particle size becomes more uniform, which improves non-ohmic characteristics. This paper proposes a new barrier model for the conduction mechanism of ZnO varistors, based on the tunneling effect and taking into account the influences caused by microstructural inhomogeneities specific to polycrystalline ceramics, such as discontinuities in grain boundary layers and variations in ZnO grain size.
Okinaka et al. (2023) studied this question.