Effects of Reoxidation Process on Positive Temperature Coefficient of Resistance Properties of Sm-doped Ba0.85Ca0.15TiO3

The positive temperature coefficient resistance (PTCR) effects of Sm-doped BaTiO3 have been studied in terms of annealing temperature, annealing time, and additive content. (Ba0.85-xCa0.15Smx)0.997TiO3 samples were prepared by a liquid mix method developed by Pechini. The specimen was sintered in a reducing atmosphere (PO2<10-9 atm) at 1350 °C, followed by annealing processes at 900–1200 °C and PO2=1 atm. Room-temperature resistances were dependent on the donor concentration. At 0.5 mol % Sm, the electrical compensation mode of the donor impurity switched from electrons to cation vacancies. The specimens doped with Sm≤0.5 mol % showed low resistivities and large grains, whereas high resistivities and small grains were observed with Sm>0.5 mol %. The oxidation degree of grain boundaries increased with annealing temperature and time, leading to increased room-temperature resistances. This implies that the acceptor-state density at grain boundaries increases the potential barrier height.