Microstructures and electrical properties of Sr_0.6Bi_0.4Fe_0.6Sn_0.4O₃–BaCo^II _0.02Co^III _0.04Bi_0.94O₃ thick-film thermistors with low room-temperature resistivity

The thick film NTC thermistor of compositions Sr_0.6Bi_0.4Fe_0.6Sn_0.4O₃ + BaCo^II _0.02Co^III _0.04Bi_0.94O₃, synthesized by solid state reaction, were prepared by screen-printing on alumina substrate. The microstructures, composition dependent, impedance characteristics, self-heating behaviours and thermistor properties were investigated. The relation between logarithm of resistivity and reciprocal of absolute temperature for the thick film thermistor was almost linear for all the compositions studied. The room-temperature resistivity, thermistor constant and activation energy of the films decreased with increasing BaCo^II _0.02Co^III _0.04Bi_0.94O₃ content and were in the range of 18.2–945.7 Ω cm, 1,753–2,649 K and 0.151–0.228 eV, respectively. The thick films showed the nearest-neighbor hopping or variable-range hopping model depended on the compositions. Impedance analysis indicated that the resistivity value of the thick films was mainly ascribed to the contribution of grains. At higher BaCo^II _0.02Co^III _0.04Bi_0.94O₃ content, a good self-heating effect of one thermistor film was observed.