Microwave sintering of PTCR ceramics and their electrical properties

Sintering behavior and electrical properties of semiconducting oxides, including (Sr_0.2Ba_0.8)TiO₃ (SBT), (Sr_0.4Pb_0.6)TiO₃ (SPT), (Ba_0.7Pb_0.3)TiO₃ (BPT) materials, processed by a microwave sintering technique were examined. Sintering temperature needed in this process is generally ~150°C lower than and soaking time required is generally 80% shorter than those commonly used in the conventional sintering process. For SBT materials, post-annealing after sintering insignificantly modified the samples´ microstructure, but substantially alter their resistivity-temperature (ρ-T) characteristics. The re-oxidation which occurred at the grain boundary region is proposed as the mechanism of modification. Unique single high-T_c behavior and double-T_c characteristics in ρ-T curve were obtained for as-microwave sintered SPT and BPT materials, respectively. It is attributed to the formation of the “core-shell” microstructure, which, in turn, presumably resulted from the preferential reaction of liquid phase sintering aids, SiO₂, with the Pb-species. Post-annealing restored the single low-T_c ρ-T property, that is induced by the homogenization of the microstructure