Effects Of Composite Additives Preparation On The Properties Of ZnO Varistor Ceramics

Zn O varistor ceramics are widely used in electrical power systems and electronic circuits due to their excellent nonlinear voltage-current characteristic and ability of absorbing surge energy. And they have become the most widely used varistor ceramics nowadays. In this paper, Zn O varistor ceramic additives were calcined under different calcination temperature schedules to prepare composite additives powder and then J, K series Zn O varistor ceramics were prepared via solid mechanical mixing method. Besides, on the basis of composite additives preparation, R series Zn O varistor ceramics doped with different amount of Ce O2 and Y2O3 were prepared. The effects of composite additives preparation with different calcination temperature schedules and different doping amount of Ce O2 and Y2O3 on the properties of Zn O varistor ceramics were studied.1. The additives were calcined under different temperature schedules so that J, K series Zn O varistor ceramics composite additives powder were prepared. The test results showed that the additives reacted with each other and homogeneous composite oxides, mostly Bi1.81((Mn Sb)O6)O0.72, were generated in J, K series additives after calcined at 600℃~875℃ and 825℃~925℃ respectively. This would be helpful to improve the microstructure uniformity and the electrical properties of Zn O varistor ceramics.2. The composite additives powder were mixed with Zn O by solid mechanical mixing method and J, K series Zn O varistor ceramics were prepared. The test results showed that J1~J6 Zn O varistor ceramics with calcined additives contained more small intergranular grains than J0 Zn O varistor ceramics whose additives were not calcined. Owing to the pinning effect of these grains, the growth of the Zn O grains was inhibited and the number of the grain boundaries per unit thickness was increased so that the breakdown electrical field was enhanced. The uniformity of the grain distribution of K1~K5 Zn O varistor ceramics with calcined additives was better than that of K0 Zn O varistor ceramics whose additives were not calcined. With appropriate additives calcination, the microstructure uniformity of the Zn O varistor ceramics could be improved, the breakdown electrical field could be increased, the voltage ratio could be lowered, the 2ms rectangular impulse current capacity as well as the AC aging property could be enhanced. J6 Zn O varistor ceramics with additives calcined at 850℃ for 90 min exhibited excellent comprehensive electrical properties. Its breakdown electrical field was 243V/mm, voltage ratio reached 1.68, the passing rate of 300 A 2ms rectangular impulse current test was 100% and aging coefficient was 0.85. K5 Zn O varistor ceramics with additives calcined at 925℃ for 90 min also exhibited excellent comprehensive electrical properties. Its breakdown electrical field was 264V/mm, voltage ratio reached 1.84, the passing rate of 300 A 2ms rectangular impulse current test was 100% and aging coefficient was 0.70.3. On the basis of composite additives preparation, different amount of Ce O2 and Y2O3 were doped and R series Zn O varistor ceramics were prepared via solid mechanical mixing method. The test results showed that the Ce O2 doped mainly existed in the form of Ce O2 oxide in the grain boundary layer and caused pinning effect to the grain boundary along with the spinel phase so that the breakdown electrical field was increased. And the Y2O3 doped not only existed in the form of Y2O3 oxide in the grain boundary layer, but also solubilized with Bi2O3 and existed in the form of Y-containing Bi-rich phase, which would inhibit Bi2O3 from promoting the growth of Zn O grains and also led to the increase of the breakdown electrical field. With the increasing of Ce O2 doping amount, the breakdown electrical field of R1~R3 Zn O varistor ceramics was raised largely and almost linearly. The doping of Y2O3 also caused a large increase of the breakdown electrical field. With appropriate doping of Ce O2 and Y2O3, the breakdown electrical field of R series Zn O varistor ceramics could be largely increased. R3 Zn O varistor ceramics doped with 1.5wt% Ce O2 exhibited excellent comprehensive electrical properties. Its breakdown electrical field reached 303V/mm, voltage ratio was 1.89, the passing rate of 300 A 2ms rectangular impulse current test was 80% and aging coefficient was 0.85.