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

The paper reports the preparation, structure and electrical properties of oxide ceramic semiconductors based on the series SrxLa1−x TiIVx+yCoIIyCoIII1−x−2yO3 with perovskite type stucture: 0<x<1, 0<y<(1−x)/2. The study starts from LaCoO3 which is highly conductive yielding metallic condutivity above 330°C. The upset trigonal distortion of LaCoO3 is reduced when SrII/TiIV is substituted for LaIII/CoIII corresponding to increasing values of x and also when 2 CoIII are introduced for TiIV/CoII 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 CoIII atoms of LaCoO3 is increasingly interrupted providing increasing values of the the resistivity value ρ25°C and of the B25/100°C value deduced from measurements at 25 and 100°C according to ρ(T )=ρ25°C eB/T. The range of variation of x an y makes possible to prepare ceramics with desired electrical properties within the limits of ρ25°C=1.1 Ωcm, B=1910 K and ρ25°C=1 bis 8×106 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.