PTCR characteristics in porous (Ba,Sr)TiO3 ceramics produced by adding partially oxidized Ti powders

Porous and fine-grained (Ba,Sr)TiO3 ceramics were fabricated in vacuum by the addition of partially oxidized Ti powders. The positive temperature coefficient of resistivity (PTCR) characteristics in the (Ba,Sr)TiO3 ceramics appeared after paste-baking treatment at 580°C in air due to the adsorption of oxygen at grain boundaries, in the same behavior as ordinary air-sintered BaTiO3 ceramics. It was found that the phase transition temperature from the tetragonal to cubic structure of (Ba,Sr)TiO3 ceramics depends on the oxygen content and grain size. The electrical properties of the (Ba,Sr)TiO3 ceramics were discussed, based on the microstructure, resistivity of grain boundaries, donor concentration of grains, and the electrical potential barrier of grain boundaries. The (Ba,Sr)TiO3 ceramics containing the 5 vol.% partially oxidized Ti powders sintered at 1450°C showed a large PTCR effect greater than six orders of magnitude.