Effect of trace (Bi0.5Na0.5)TiO3 doping on PTCR properties of BaTiO3-based thermosensitive ceramics

Abstract

BaTiO₃-Bi_0.5Na_0.5TiO₃ (BT-BNT) material system has been extensively studied as a lead-free positive temperature coefficient resistance (PTCR) ceramic with a high Curie temperature. Although BNT can increase Curie temperature, it is at the expense of room temperature resistivity. In this study, the effects of trace BNT doping on the crystalline structure, microstructure and PTCR properties of La, Nb and Mn Co-doped BaTiO₃ PTCR ceramics were investigated. The results show that with the increase of BNT content, the lattice parameters, tetragonality and grain size decrease, while the Curie temperature increases monotonously. However, the room temperature resistivity decreases at first and then turn over to increase as the BNT content increases, reaching its minimum value at the sample doped with 0.25 mol % BNT. TEM study reveals that Na ⁺ ions and Bi³⁺ ones can be separated at grain boundaries. Bi³⁺ ions enriched in the grain interior affect the resistivity as donors, while Na⁺ ions enriched in the grain boundary affect the grain boundary barrier as acceptors. The impedance spectrum analysis proves that the resistivity jump is mainly determined by the grain boundary resistance. The ceramics doped with 0.25⁓0.50 mol% BNT sintered at 1330 °C for 30 min in air have good comprehensive PTCR properties, with room-temperature resistivities of 18.00⁓95.52 Ω cm, resistivity jumps of 1.45 × 10³⁓7.45 × 10³, and Curie temperatures of 133⁓138 °C. This study provides a new approach for the research of BT-BNT lead-free thermosensitive ceramics.