Effects of Bi1/2Na1/2TiO3 on the Curie temperature and the PTC effects of BaTiO3-based positive temperature coefficient ceramics

In order to prepare lead-free BaTiO3-based PTC with middle Curie point, the incorporation of Bi1/2Na1/2TiO3 (abbreviated to NBT) into BaTiO3-based ceramics was investigated on samples containing 0.1, 0.5, 1, 1.5, 1.6, 1.8, 2 mol% of NBT. It was found that for all samples containing up to 1.5 mol%, the Curie temperature was obviously improved with the increasing of NBT content, and the TC raised to 150 °C from 97 °C just only 1.5 mol% NBT added. Because AO bonds were weakened, which induced that TiO bonds were strengthened correspondingly, and the actions between Ti4+ (a departure of center) and its near O2− were so strong that Ti4+ could not resume its seat except that it was under the higher temperature, hence the Curie temperature was improved. At the same time, the PTC effects were also meliorated which attributed to vast electron capture centers formed by Bi2O3 acting with MnO. However, when the amount of NBT exceeded 1.5 mol%, the samples were dielectric material and had no PTC effects, but it was worth noting that the ρ–T curve of the samples sintered in reducing atmosphere was linear.