Temperature-independent resistive oxygen exhaust gas sensor for lean-burn engines in thick-film technology

Strontium titanate based materials (SrTi 0.65Fe 0.35O 3− δ ; STF or La 0.05Sr 0.95Ti 0.65Fe 0.35O 3− δ ; LSTF) are suggested in the literature as resistive oxygen gas sensors due to their temperature-independent but oxygen concentration-dependent resistance characteristic. This contribution reports on the difficulties that had to be overcome by trying to transfer the properties of the pure material to a real exhaust gas compatible thick-film sensor device. Two main problems are discussed: the transfer to thick-film technique and the sensor behavior in real exhaust gas. In order to maintain the properties of the pure material, an additional diffusion barrier layer between substrate and sensor film turned out to be essential. A thick-film spinel layer (SrAl 2O 4) was shown to give the best performance. By using this additional layer, all the properties of the pure material were successfully transferred to a thick-film gas sensor device. During the real exhaust gas experiments a strong deterioration of the sensor characteristic due to sulfur dioxide in the exhaust gas was observed. The poor stability against sulfur compounds in the exhaust seems to be a problem of the earth alkaline constituent of the titanate sensor material and cannot be improved easily. However, by applying an additional newly developed porous sulfur adsorber film made from earth alkaline carbonates as sulfur adsorbing components, long-term stable exhaust gas sensors can be obtained.