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The fractured surfaces of Bi-doped and Bi, Co-doped ZnO varistors were investigated by means of cathodoluminescence. Three peaks were observed for the Bi-doped ZnO varistor. The 378 and 389 nm peaks in intragrains were intense compared to those at grain boundaries. On the other hand, the 526 nm peak was strong at grain boundaries and ascribed to deep levels formed at grain boundaries. In the case of further Co doping, the luminescence was extinct.
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Tanaka et al. (1995) studied this question.
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