5.3.1 Composition and Effect of Additives

The semiconducting properties of ZnO were very well known [33] well before Matsuoka et al. [34] investigated the non-linear voltage-current characteristic of ZnO doped with alkali earth metal oxides. It was found that with appropriate amounts (0.5 mol%) of additives having an ionic radius larger than that of Zn ⁺² (which would not dissolve in the ZnO lattice and would thus form a segregation layer at the ZnO grain boundary), the ceramic exhibits a high coefficient of non-linearity a on its V-I characteristic. Furthermore, it was shown that the new material was exceptionally superior compared with conventionally used SiC varistors [35]. The non-linearity coefficient, ?, of early ZnO varistors was in the range 25 50. In contrast, that of SiC varistors was between 2 and 7 [36].

In 1971, Matsuoka [37] disclosed a large number of compositions which produce useful ZnO varistors. A coefficient ? = 50 was obtained when 99.9 percent pure ZnO is doped with 1 mol% Sb ₂O ₃ and 0.5 mol% of Bi ₂O ₃, CoO, MnO and Cr ₂O ₃ [38 42]. Adding more dopants has allowed the fabrication of ZnO varistors with large grain size and the reduction of the breakdown voltage of a single grain boundary [43 47]. Paraseodymium oxide rich composition, which does not contain any bismuth, was used to manufacture ZnO varistors with a two-phase microstructure and improve its electrical...