Novel synthesis of monodisperse ZnO-based core/shell ceramic powders and applications in low-voltage varistors

Abstract In this article, monodisperse ZnO microspheres were successfully prepared by a facile ultrasound irradiation method. Then, by using the microspheres as the templates, the novel uniform ZnO/Co2O3-Bi2O3 core/shell composites were successfully synthesized via a situ precipitation–coating route due to the electrostatic interaction of [BiEDTA]− and [Co(NH3)6]3 +. Microstructural studies of the as-obtained powders were carried out using the techniques of the XRD, FTIR, SEM, TEM, TGA and EDS. The results show that the white ZnO powders as the core were spherical structure with the size of approximate 500 nm and the thickness of the additive metal oxide shell was about 80 nm, respectively. On the basis of the results, the spherical ZnO microspheres fully coated with additional transition metal oxides were identified and Bi12Co0.60O18.80 phase was observed after the ZnO composites were calcinated at 420 °C for 4 h. The formation mechanism of monodisperse ZnO microspheres and the ZnO-based core/shell structured composites were also brief introduced. The varistors ceramics sintered in air at 1250 °C for 2 h possesses a density of 5.64 g/cm3 corresponding to 97.7% of the theoretical density, with breakdown voltage of 78.6 V/mm and nonlinear coefficient of ~ 7.1.