Materials for temperature independent resistive oxygen sensors for combustion exhaust gas control

Acceptor and donor doped SrTi1−xFexO3−δ materials for novel temperature independent resistive oxygen sensors for lean-burn engine exhaust gases were prepared and characterized by X-ray diffraction. Their electrical resistance, R, was investigated in the oxygen partial pressure range from 10−4 to 1 bar between 700°C and 1000°C. Doped and undoped samples with x=0.3 obey an R∝pO2−1/5 power law. Undoped samples show negligible temperature dependence in a small oxygen partial pressure (pO2) range between 10−2 and 3×10−2 bar. Acceptor (Ga) doping shifts the pO2 range of negligible temperature dependence to lower pO2, whereas donor (La) doping results in a right-hand shift to higher pO2. Combined acceptor and donor doping leads to an extended pO2 range of negligible temperature dependence. At pO2=10−1 bar the temperature coefficient of resistance (TCR) of this material composition is always below 250 ppm/K in the complete investigated temperature range. At about 725°C and 975°C TCR=0 can be found.