Perovskite forming ceramics of the system SrxLa1−x TiIVx+yCoIIyCoIII1−x−2yO3 for NTC thermistor applications

Abstract The paper reports the preparation, structure and electrical properties of oxide ceramic semiconductors based on the series Sr x La 1− x Ti IV x + y Co II y Co III 1− x −2 y O 3 with perovskite type stucture: 0 x y x )/2. The study starts from LaCoO 3 which is highly conductive yielding metallic condutivity above 330°C. The upset trigonal distortion of LaCoO 3 is reduced when Sr II /Ti IV is substituted for La III /Co III corresponding to increasing values of x and also when 2 Co III are introduced for Ti IV /Co II into the lattice corresponding to increasing values of y . At high values of x and y othorhombic distortion occurs. At the same time, the interaction between the Co III atoms of LaCoO 3 is increasingly interrupted providing increasing values of the the resistivity value ρ 25°C and of the B 25/100°C value deduced from measurements at 25 and 100°C according to ρ ( T  )= ρ 25°C e B / T . The range of variation of x an y makes possible to prepare ceramics with desired electrical properties within the limits of ρ 25°C =1.1 Ωcm, B =1910 K and ρ 25°C =1 bis 8×10 6 Ωcm at B -values up to 6500 K. Dependent on composition, NTC ceramics for thermistor or insurance applications are accessible. Thermistors do not show aging even at higher temperature, e.g. at 500°C, provided the single phase state is achieved as a result of mixed oxide preparation and sintering. Hence, high temperature thermistor applications are also made possible. The semiconductor behavior can be understood using the conventional polaron state hopping model.