The effect of secondary phases on electrical properties of ZnO-based ceramic films prepared by a sol–gel method

Abstract

The ZnO-based ceramic films doped with Bi2O3, Sb2O3, MnO, Co2O3 and Cr2O3 were prepared for use as film varistors by a sol–gel method. The formation and the changes of the phases in the films doped with different dopants and annealed at different temperatures were investigated via X-ray diffraction analysis. Three secondary phases, i.e., Bi2O3, Zn7Sb2O12 spinel and ZnCr2O4, were detected in the films when the annealing temperature was above 550 °C. The lattice constants of ZnO and Zn7Sb2O12 spinel phase changed with dopants and the annealing temperature, indicating that the diffusion of the ions into the crystals of ZnO and spinel phase had taken placed. The redistribution of the ions changed the constituents of the intergranular phases and the relevant defect species in ZnO grains, and affected intensively the electrical properties of the films, which were used as film varistors. The highest nonlinear coefficient (α) with the lowest leakage current was achieved when the film, which was doped with Bi2O3, Sb2O3, MnO and Cr2O3, was annealed at 750 °C.