Synthesis, doping and electric bulk response of (Bi1/2Na1/2)xBa1-xTiO3 + CaO – based ceramics with positive temperature coefficient of resistivity (PTCR)

(Bi1/2Na1/2)xBa1-xTiO3 samples were prepared using the mixed-oxide route; their tetragonal to cubic phase transition temperature increases from 125°C for x = 0 to 190 °C for x = 0.2. (Bi1/2Na1/2)0.10Ba0.90TiO3 (x = 0.10)... [ view full abstract ]

(Bi_1/2Na_1/2)_xBa_1-xTiO₃ samples were prepared using the mixed-oxide route; their tetragonal to cubic phase transition temperature increases from 125°C for x = 0 to 190 °C for x = 0.2. (Bi_1/2Na_1/2)_0.10Ba_0.90TiO₃ (x = 0.10) ceramics with CaO addition sintered in air exhibit a positive temperature coefficient of resistivity (PTCR) behavior. Dense PTCR thermistor ceramics with low room-temperature resistivity and resistivity change r_max/r_min of 3.5 orders of magnitude at the Curie temperature were prepared using 0.05 wt% SiO₂ addition as sintering aid and Mn+Fe acceptor doping. Sintering at 1235°C with two hours holding time were found as optimum synthesis conditions for achieving a good electrical thermistor performance. Impedance spectroscopy confirms that the PTCR effect is a grain boundary phenomenon, since the grain interior exhibits semiconducting behavior that is unaffected across the phase transition. Impedance spectra could be fitted with a standard equivalent circuit consisting of two series RC elements to extract grain boundary and bulk resistance.