Anomalous low-high transition of ceria doped SnO2 sensors exposed to synthetic automobile exhaust gas

The low-high transition behavior of SnO2-based sensors and the effect of ceria as a major additive are studied. The response of the sensors is studied in the presence of synthetic automobile exhaust gases comprising air and CO and/or C2H6 reducing gases (fuel), with different weight ratios of the air and fuel, defined as air-to-fuel ratio. A sharp S-shape transition is observed in the resistance of the sensor, when the air-to-fuel ratio of the synthetic gas varies. The resistance reads low values if the air-to-fuel ratio is less than a critical value, while passing through the transition point, the sensor resistance shows a dramatic increase. Using a catalytic burning chamber (CBC) just before the sensor, results in a sharp S-shape transition at the stoichiometirc air-to-fuel ratio. For pure SnO2 sensors and without a CBC, this transition point occurs at values other than the expected stoichiometric ratio, whereas for ceria doped SnO2 samples this transition point approaches the stoichiometric value. Using a composition of 10 wt.% ceria in SnO2 leads to a sharp S-shape behavior right at the stoichiometric value. The electrical as well as physical characteristics of ceria-SnO2 layers have been investigated. The proposed structure can be used as a reference-less lambda-ratio sensor for automotive applications.