Nonohmic Properties of Zinc Oxide Ceramics

Nonohmic properties of ZnO ceramics with five additives of Bi₂O₃, CoO, MnO, Cr₂O₃, and Sb₂O₃ are studied in relation to sintering temperature, additive content, and temperature dependence. The observation of electron photomicrographs and X-ray microanalysis proves a ceramic microstructure such that ZnO and these five oxides form, at the grain boundaries, segregation layers which are responsible for the nonohmic properties. The electrical resistivity and dielectric constant of segregation layers are estimated to be 10¹³ ohm-cm, and 170, respectively by using a simple model. The electric field strength corresponding to the steep rise in the current is also estimated to be 10⁴ V/cm by taking account of the concentration of applied voltage at the segregation layer. In view of these data and simple model, a possible explanation for nonohmic properties is discussed.