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

Abstract

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 °C, which is about 110 °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.