Preparation, Structure And Electrical Properties Of TiO₂ Based Varistor Ceramics

TiO₂ based multifunction ceramics are a kind of novel functional ceramics with dual functions of low-voltage varistor and gain-boundary layer capacitor. The low breakdown voltage, high non-linear coefficient and large dielectric constant make the material useful in miniaturized electronic devices as transient high-voltage protector and surge arrestor. In this research, a technique for producing TiO₂ seed grains was developed by using NaCl as an accelerator for grain growth at high temperature. The influence of sintering temperature, various oxide additives and pre-prepared seed grains on the structure and multifunction of TiO₂ based ceramics has been examined. The relation between the multifunction and structure feature has been tentatively explained.TiO₂ seed grains were prepared at high temperature using NaCl as a grain growth accelerator. The role of NaCl and growth condition on the phase purity and morphology of the seed grains has been investigated. It was found that adding a moderate amount of NaCl greatly promoted the growth of TiO₂ grains, and grain growth condition has a remarkable impact on the morphology of the resulting seed grains. It has been ascertained that the seed grains with a length of about 15μm can be obtained at a growth temperature of 1200℃when adding 30.0% NaCl.The influence of sintering temperature and various oxide additives on the structure and electrical properties of TiO₂ based ceramics has been investigated. The results indicate that increasing sintering temperature obviously promoted the average grain size, but increased the loss of bismuth during high temperature sintering and consequently lowered the height of grain-boundary potential barriers. Adding moderate amounts of BaO, Bi₂O₃ or SiO₂, to different extents, serves to modify the electrical properties of TiO₂ based ceramics. By contrast, no positive effect was found when adding MnO₂ additive. At a sintering temperature of 1400℃, adding 0.50% BaO, 0.50% Bi₂O₃ and 0.90% SiO₂ led to modified electrical properties, including a dielectric constant of 1.8×10⁴, a breakdown voltage of 49V/mm and a non-linear coefficient of 4.2.Based on this composition, the influence of pre-prepared TiO₂ seed grains on the structure and multifunction of TiO₂ based ceramics has been investigated. The results demonstrated that adding pre-prepared TiO₂ seed grains acts as an effect route in modifying the microstructure and in turn improving the electrical properties. Adding a proper amount of seed grains resulted in a decline of breakdown voltage and dissipation factor together with an enhancement of non-linear coefficient and dielectric constant. At a sintering temperature of 1400℃, adding 25.0% seed grains yielded a breakdown voltage of 19.5V/mm, a non-linear coefficient of 4.8, a dielectric constant of 4.4×10⁴ and a dissipation factor of 5%.