Effect of reoxidation on positive temperature coefficient of resistance behavior for BaTiO₃-K_0.5Bi_0.5TiO₃ ceramics

As a positive temperature coefficient of resistivity (PTCR) material, (1-x)BaTiO₃-xK_0.5Bi_0.5TiO₃ (BT-KBT, 0.05≦ x ≦0.15) ceramics without any donor doping were prepared by a conventional oxide mixing method. All samples were sintered in an Ar atmosphere at 1280 ~ 1350°C, subsequently, reoxidized at 800 ~ 1100°C in a gas mixture (99 %Ar–1 %O₂). The PTCR behavior of BT-KBT ceramics were investigated in terms of KBT content, reoxidation temperature and time. The results showed that the BT-KBT ceramics exhibited an abrupt increase in their resistivity near the Curie temperature (Tc) after annealing in gas mixture, Tc of 0.9BT-0.1KBT ceramic was shifted to a higher temperature (~150°C). Furthermore, the room-temperature resistivity (ρ_RT) of ceramic samples sintered in Ar and reoxidized in a gas mixture decreased to 10² Ω·cm. The jump in resistivity (maximum resistivity [ρ_max]/minimum resistivity [ρ_min]) was enhanced by three orders of magnitude through a suitable reduction–reoxidation method without sacrificing the ρ_RT.