Investigation of the solidus boundaries and microstructure in the ZnO-PrO1.5-CoO system

ZnO ceramics used as varistors are prepared with cobalt oxide as an essential additive to improve nonohmic properties. Because some of its effects during the liquid-phase sintering remain unexplained, we characterize the liquid-phase formation temperatures and phase reactions in the system ZnO-PrO1.5-CoO. Using differential thermal analysis (DTA) during sintering, we detect new thermal phenomena. An addition of cobalt oxide to ZnO-PrO1.5 mixtures (ZnO-5 mol% PrO1.5-10 mol% CoO) significantly decreases the liquid-phase formation temperature to 1272 +/- 5 degrees C, which is about 110 degrees C lower compared to those in the ZnO-PrO1.5 system. Characterization of ceramics quenched during sintering allows us to describe an isoplethal section with PrO1.5 contents of 5 mol% and solubility limit of Co in ZnO.