Novel Niobium-Doped Titania Varistor with Added Barium and Bismuth

Characteristics of the sintered compacts and the microstructures and electrical properties of Nb-doped TiO₂ varistors with added Ba and Bi were studied at various sintering temperatures ranging from 1250° to 1400°C. Adding both Ba and Bi to Nb-doped TiO₂ ceramics resulted in a maximum intergranular phase and a minimum weight loss at 1350°C. In contrast, adding either Ba or Bi alone produced no such maxima and minima. The intergranular phases included mainly a Bi₂Ti₂O₇ crystal phase, apt to occur at a triple junction, and a Ba-rich amorphous phase that surrounded the grains, but discontinuously. The intergranular phases varied consistently with variation in electrical properties. The optimum conditions for the most efficient boundary barrier layer, with the lowest weight loss and the highest resistivity at low frequencies, were 1350°C with both Ba and Bi addition. The highest values for α (∼9.5), V_gb10 (∼0.8 V), and E_B (∼0.42 e V) support that finding. The effective relative dielectric constant, K_eff∼ 20 000, also was obtained under optimum conditions. The single addition of either Ba or Bi, however, produced nearly the opposite results, as discussed in this paper.