Synthesis of Lead-Free Air-Sintered BaTiO3-(Bi1/2Na1/2)TiO3-Based Positive-Temperature-Coefficient of Resistance Ceramics by Ca Addition and the Defect Chemistry of Semiconductivity

Lead-free n-type [Ba-0.9(Ba1/2Na1/2)(0.1)]TiO3-based positive-temperature-coefficient of resistance (PTC) ceramics with T-C > 158 degrees C were successfully synthesized by addition of 2-4 mol% CaO through a mixed oxide fabrication route and adopting an atmospheric sintering condition. All ceramics exhibited single phase BaTiO3 structure with 64.21(1) angstrom(3) cell volume. A drop in room-temperature resistivity (rho(RT)) by 7-8 orders magnitude in presence of the CaO was attributed to residual Ba2+ site Bi3+ acting as donor defect owing to filling up of vacancies introduced by the preferential evaporation of Na+ over Bi3+ by Ca2+. Increases in rho(RT) and the maximum PTC resistivity (rho(max)) from 0.316 to 2.11 k Omega.cm and from 0.085 to 2.94 M Omega.cm, respectively, with 2 to 4 mol% CaO addition were attributed to the gradual incorporation of Ca2+ into both Ba2+ and Ti4+ sites, which oxidized the electron hopping centers (Ti3+) through the generation of holes. The optimum CaO content (3.5 mol %), sintering time (2 h) and temperature (1160 degrees C) were established to achieve [Ba-0.9(Ba1/2Na1/2)(0.1)]TiO3-based semiconducting PTC ceramics by adopting an atmospheric sintering environment. (C) 2013 The Japan Society of Applied Physics