Doping effect of Ta⁵⁺ ions on microstructure and electrical properties of BaTiO₃–(Bi_0.5Na_0.5)TiO₃ ceramics with positive temperature coefficient of resistivity

Lead-free 0.912Ba_0.97TiO₃–0.088(Bi_0.5Na_0.5)TiO₃–xTa₂O₅ (0 ≤ x ≤ 0.005) ceramics with positive temperature coefficient of resistivity (PTCR) were prepared by solid reaction sintering of high-purity metal oxides and carbonates reagents. The doping effect of Ta⁵⁺ ions on the microstructure and electrical properties of the samples was investigated. X-ray diffraction analysis indicated that all the samples were of a single tetragonal perovskite structure with the calculated c/a value first increased and then decreased with increasing x. The Raman shift at 305 cm⁻¹ became strong firstly and then declined, and the broad band at 722 cm⁻¹ narrowed initially and then broadened with the increase in Ta⁵⁺ content. The Curie temperature raised first and then decreased, presenting the highest value up to 170 °C for the samples with x = 0.001. Moreover, after the incorporation of Ta⁵⁺ ions, the PTCR (defined by the resistivity jump with the ratio of maximum to minimum ones) decreased first and then increased, presenting the best PTCR performance for the samples with x = 0.005. And with increasing x, the room temperature resistivity decreased first and then increased, displaying the lowest room temperature resistivity for the samples with x = 0.003.