ZnO varistor ceramics have some advantages of high nonlinear coefficient, low leakage current and low production cost, which are widely used in the protection of power systems and electronic equipment. With the development of electronic devices in the direction of better performance and miniaturization, the electrical performance of ZnO varistor ceramics needs to be further improved. It is thus necessary to prepare ZnO varistor ceramics with a high breakdown voltage and a uniform fine grain size. In common ZnO-Bi₂O₃-Sb₂O₃ varistor ceramics, Bi₂O₃ and Sb₂O₃ are the two most important elements, and Bi₂O₃ and Sb₂O₃ doped in a certain proportion play an important role in the optimization of electrical properties and microstructure regulation of ZnO varistor ceramics. The melting point of Bi₂O₃ is 825 ℃ and the melting point of Sb₂O₃ is 655 ℃ in the high-temperature sintering process volatile, resulting in the loss of components and the increased porosity, structural uniformity, deterioration of electrical properties of ZnO varistor ceramics. Therefore, reducing the sintering temperature of ZnO varistor ceramics (i.e., below 1 000 ℃) is a necessity, which is expected to provide the experimental basis for the realization of pure silver internal electrode co-firing of multi-layer chip ZnO varistor elements.

ZnO-Bi₂O₃ based varistor ceramics with a Bi∶Sb molar ratio of 4∶1 were prepared by a traditional electronic ceramics process. ZnO powder was mixed with Bi₂O₃, Sb₂O₃ powder, agate ball and deionized water in a certain proportion and then the mixed slurry was ground in a planetary ball mill. The ground mixed slurry was dried at 110 ℃, then ground and passed through the 100 mesh screen. 5% polyvinyl alcohol (PVA) solution was used as a binder for granulation, and then pressed into green billets with a diameter of 13 mm and a thickness of 2 mm. The green billet was firstly discharged in a muffle furnace at a rate of 2 ℃/min to 500 ℃, and then sintered in a sintering furnace at different temperatures (i.e., 880, 900, 920, 950, and 980 ℃, respectively). The obtained sample was polished and silvered. The final sample size was φ10 mm×1 mm. The phase was analyzed by X-ray powder diffraction (XRD). The backscattering mode of field emission scanning electron microscopy (FESEM) was used to characterize the surface morphology of the samples, and the distribution of elements on the surface of the samples was analyzed by X-ray energy spectroscopy (EDS). The electrical properties of the samples were measured by a varistor tester, and the grain boundary characteristic parameters were measured by wideband dielectric impedance spectrometry.

In this experiment, 800 ℃ is selected as a turning point. When the temperature is higher than 800 ℃, the sample is heated at a lower rate and the sintering temperature is below 1 000 ℃, obtaining fully developed ZnO grains. The grain size of the two groups of samples is small and evenly distributed. The molar ratio of Bi₂O₃ and Sb₂O₃ of the two groups of samples is 4∶1. When the sintering temperature is 920 ℃ or above, the breakdown voltage of the two groups of samples is similar. At the Bi₂O₃∶Sb₂O₃ molar ratio of 4∶1, the breakdown voltage of the sample is more related to the Bi₂O₃∶Sb₂O₃ molar ratio rather than to the total doping amount of Bi₂O₃ and Sb₂O₃. This means that the breakdown voltage regulation can be achieved with less doping of Bi₂O₃ and Sb₂O₃. The high breakdown voltage ZnO varistor ceramics can be prepared by sintering at a low temperature, and the breakdown voltage of doped with the same proportion of Bi₂O₃ and Sb₂O₃ is similar in the less addition of Bi₂O₃ and Sb₂O₃, thus providing an effective way for the low-carbon preparation of ZnO varistor ceramics.

Low-temperature sintering could effectively inhibit the growth of ZnO grains. The breakdown voltage of the two groups of samples was high, ranging from 599-1 154 V/mm. ZnO grains of all the samples were fully developed, and the average size of ZnO grains of the samples was 2.7-3.6 μm. In addition, the grain size distribution and microstructure of the sample were more uniform. The optimum comprehensive electrical performance of the sample sintered at 900 ℃ was obtained (i.e., the breakdown voltage of 1 154 V/mm, the nonlinear coefficient of 13.7, and the leakage current of 39 μA).