Effect of Co3O4-doping on the microstructure and electrical properties of novel ZnO–Cr2O3-based varistor ceramics

Novel ZnO–Cr2O3-based varistor ceramics have superior current-voltage (I–V) characteristics and, do not contain volatile, rare or toxic dopants. The results showed that doping with Co3O4 greatly affected the nonlinearity of the ZnO–Cr2O3-based ceramics, with the highest coefficient of nonlinearity α equal to 73, while the leakage current was lower than 0.2 μA/cm2. Such a varistor performance enhancement was explained by the substitution of Co2+ into the Zn sites leading to the formation of Zn interstitials and free charge carriers of ZnO grains as well as an increased concentration of electron-trapping interface states like adsorbed oxygen and consequently substantially increased electrostatic barriers at the grain boundaries. The optimal amount of added Co3O4 for high-performance ZnO–Cr2O3-based varistor ceramics was about 0.5 mol.%.