Influence of sintering time and donor concentration on the PTCR effect of La-doped BaTiO₃-Na_0.5Bi_0.5TiO₃ ceramics

Lead-free positive temperature coefficient of resistivity (PTCR) materials with higher Curie point are important for their high temperature applications. In the present study, 0.99BaTiO₃ −0.01Na_0.5Bi_0.5TiO₃-xLa₂O₃ (99BT-1NBT-100xLa, x = 0.001, 0.002, 0.003, 0.004) ceramics were prepared by a conventional solid-state reaction method. The phase structure, microstructure, and PTCR effect of these ceramics were investigated. X-ray diffraction analysis indicates that all the samples are of a single tetragonal perovskite structure. The SEM images of all samples show the grain shape is irregular. And the grain size is around 3–30 µm. The PTCR effects occur at a temperature of about 150 °C. The room-temperature resistivity (ρ₂₅) drops with the increase of donor concentration and the decrease of sintering time. The resistivity ratio (ρ_max/ρ_min) first increased and then decreased with increasing sintering time and donor concentration. The optimum composition for a large PTCR effect is 99BT-1NBT-0.3La, having a high jump of resistivity of 2.63 orders of magnitude sintered at 1400 °C for 30 min in air. A possible relation between the PTCR effect with the dopant behavior are discussed. This work may help on the study of lead-free PTCR ceramics from the view of control of doping elements and sintering procedure.