Effects of Tm₂O₃ and Bi₂O₃ Co-Doped with Equal Content of Nb₂O₅ on the Properties of TiO₂-Based Varistor Ceramics

In this work, equal amount (0.6 mol.%) of Nb₂O₅ and Tm₂O₃ was respectively designed as donor and acceptor dopants, 0.3 mol.% SiO₂ was added as sintering aid in order to improve the comprehensive properties of TiO₂-based ceramics and the sintering temperature of varistor ceramics was optimized. It was found that the varistor sintered at 1350 °C possessed the best integrated electrical properties with the breakdown voltage (E_b) of 3.04 V/mm, the nonlinear coefficient (α) of 4.75, the giant dielectric constant (ε_r) of 1.4 × 10⁵ and the dielectric loss (tanδ) of 0.29, which was derived from synthetic effect of grain size, the highest density and grain boundary characteristics. Partial Tm³⁺ dissolved in the TiO₂ lattice led to lattice distortion, while the others aggregated on the grain boundary to form Tm₂Ti₂O₇. Meanwhile, the effects of Bi₂O₃ co-doping on the properties and microstructure of TiO₂ varistors were discussed, indicating that the varistor co-doped with 0.5 mol.% of Bi₂O₃ and 0.1 mol.% of Tm₂O₃ exhibited a higher nonlinear coefficient (α = 6), but deteriorated the dielectric properties. The secondary phase changed from Tm₂Ti₂O₇ to Bi₂Ti₂O_7, and the liquid phase sintering promoted the mass transfer and the grain growth when the Bi₂O₃ doping amount exceeded 0.2 mol.%.