Enhanced electrical properties of ZnO varistor ceramics by spark plasma sintering: Role of annealing

Abstract ZnO varistor ceramics are synthesized by spark plasma sintering (SPS), and then annealed in a conventional furnace in air atmosphere. Dense microstructure with more than 98% of theoretical density is achieved, and homogenous ZnO grains with typical second phases such as Zn7Sb2O12 spinel, Zn2Bi3Sb3O14 pyrochlore are observed from X-ray diffraction spectra (XRD) and scanning electron microscopy (SEM). In addition, a metallic Bi phase is also observed, which is resulted from the reduction of Bi2O3 because of inherent low oxygen partial pressure of SPS, and it can be reoxidized after a post-annealing process. The breakdown field of the optimum ZnO varistor ceramic is 725 V/mm with a nonlinear coefficient of 53.5, which stems from its high Schottky barrier height of 0.60 eV. In analysis of dielectric spectra from 173 K to 293 K, two relaxation peaks with activation energy of 0.22 eV and 0.48–0.54 eV can be observed, which are attributed to the intrinsic zinc interstitial and defect of grain boundary or intergranular phase, respectively. The grain boundary resistances are discussed according to Cole-Cole plot, from which the activation energy controlling resistance at the grain boundary is determined to be 0.87 eV.