Effect of Zr-Addition on Electric Degradation Characteristics of ZnO Varistors

ZnO varistors of the excellent tolerance characteristics for electrical degradation were made by adding Bi2O3-MnO2-Co3O4-Cr2O3-SiO2-Sb2O3-NiO in ZnO. The tolerance characteristics for electrical degradation were evaluated by changing amount of ZrO2-additive. The evaluation methods are voltage-current characteristics, X-ray diffraction, scanning electron microscope, and energy dispersion X-ray spectroscopy. Monoclinic and tetragonal ZrO2 and the compounds originated in Zr were observed at both grain boundaries and triple points. Moreover, the compounds originated in both Zr and Sb improved the tolerance characteristics for electrical degradation. On the other hand, especially monoclinic ZrO2 deteriorated the tolerance characteristics for electrical degradation. It is one key factor of the improvements of the tolerance characteristics for electrical degradation that the mobility of oxide ions or interstitial Zn2+ ions was hindered by forming the compounds contained Zr, Sb, Si, and, Bi atoms.