Improved chemical stability of ZnO–BaO based varistors

Non-linear current–voltage response of ZnO–BaO based ceramic materials makes them valuable for varistors manufacturing. Although this behaviour is quantitatively similar to that observed in the binary ZnO–Bi2O3 system, their commercial use is limited because the BaO rich secondary phase is severely damaged by moisture due to its high solubility in water. In the present work, doping with P2O5 to form BaZn2(PO4)2 and Zn3(PO4)2 has been studied in order to improve the resistance of ZnO–BaO materials to degradation by moisture. The water solubility of sintered samples has been evaluated by lixiviation experiments. Sintering behaviour and microstructure development have been followed by dilatometry and microstructure was analysed by scanning electron microscopy. The electrical properties of these materials evidence varistor behaviour with non-linearity values similar to those observed for binary materials ZnO–BaO and ZnO–Bi2O3.