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

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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 ($P_{\text{O$_{2}$}}<10^{-9}$ atm) at 1350 °C, followed by annealing processes at 900–1200 °C and $P_{\text{O$_{2}$}}=1$ 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 $\text{Sm}\leq 0.5$ mol % showed low resistivities and large grains, whereas high resistivities and small grains were observed with $\text{Sm}>0.5$ 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.
2007-03-15