Abstract
Lead-free n-type [Ba0.9(Ba1/2Na1/2)0.1]TiO3-based positive-temperature-coefficient of resistance (PTC) ceramics with TC > 158 °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) Å3 cell volume. A drop in room-temperature resistivity (ρRT) 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 ρRT and the maximum PTC resistivity (ρmax) from 0.316 to 2.11 kΩ·cm and from 0.085 to 2.94 MΩ·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 °C) were established to achieve [Ba0.9(Ba1/2Na1/2)0.1] TiO3-based semiconducting PTC ceramics by adopting an atmospheric sintering environment.
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