The Role of Co Ions on the Appearance of Non-linear I-V Characteristics in ZnO based Ceramics

The addition of transition metal elements such as Co is necessary to realize highly non-linear ZnO based varistors. The relation between the oxidation states of Co ions and the appearance and disappearance of the non-linear I-V characteristics was studied in ZnO-CoO based compositions. Pressed bodies were sintered at 1430°C in an oxygen atmosphere followed by annealing at various temperatures in air. The I-V characteristics of these specimens were then measured. The appearance and disappearance of the non-linear I-V characteristics was found to reversibly occur depending on the annealing temperatures and to be closely related to the oxidation states of the Co ions.