Fabrication of Bismuth-based Zinc Oxide Varistors Having the High Varistor Voltage by Adding Silicon Oxides and the Improvement of Resistance of Electrical Degradation by Adding Boron Oxide

Zinc oxide (ZnO) varistors have excellent nonlinear V-J characteristics and are used as one element for protecting electronic equipment and electric power systems from abnormally high voltage such as surge voltage. The... [ view full abstract ]

Zinc oxide (ZnO) varistors have excellent nonlinear V-J characteristics and are used as one element for protecting electronic equipment and electric power systems from abnormally high voltage such as surge voltage. The protection function of a varistor is determined by varistor voltage and resistance to electrical degradation. Some commercial bismuth (Bi)-based ZnO varistors contain antimony (Sb) to increase the varistor voltage and improve the resistance to electrical degradation. However, Sb is toxic. To fabricate high voltage-varistors without Sb or rare metals, SiO₂ and B₂O₃ was added to Bi-Mn-Co added ZnO varistor. The effects of addition of SiO₂ and simultaneous addition of SiO₂ and B₂O₃ on the varistor voltage and the resistance to electrical degradation were investigated. The molar ratio of Bi₂O₃, MnO₂, and Co₃O₄ was the same in each sample. The sample consisting of basic additives, X (0–40) mol% SiO₂ and Y (0–1) mol% B₂O₃ is denoted ZBMC-SiXBY. Fig.1 shows the relation between both the varistor voltage per 1 mm thickness and the grain size of ZnO and the Si content. The varistor voltage increased proportional to Si content and reached to about 2300 V/mm for Si content of 40 mol%. It is found that the varistor voltage is determined by the number of the grain boundary between ZnO grains. However, the resistance to electrical degradation for the sample ZBMC-Si20B0 was deteriorated as shown in Fig.2 which shows the time dependence of the leakage current density. The resistance to electrical degradation was improved drastically by the addition of B₂O₃ (sample ZBMC-Si20B1) as shown in Fig.2. Therefore, it is speculated that the electrical degradation is caused by the motion of Zn²⁺ and oxide ions (O^2-) under the voltage application, as a result the deformation of double Schottky barriers, and the motion of the ions was prevented by B³⁺ ions.