Development of component-integrated high-temperature NOx-sensors using doped metal-oxides as electrodes for emission-control

Abstract

Semiconducting metal oxides such as SnO2 and TiO2 can detect NO/NO2 if doped with trivalent elements (e.g. Cr3+, Al3+, etc.). It is reported that the selectivity of TiO2-sensors towards NO2 in oxygen containing environment increases by doping with aluminium at operation temperatures as high as 500°C (1). Moreover, the incorporation of Cr up to 10 at% into TiO2 yields p-type conductivity resulting in sensor stability and increased selectivity towards NO2 (2). Increased sensor selectivity is important for NOx-detection from exhaust and combustion gases, since most of which are oxidising and influence the conductivity of the sensor material. This work presents the results obtained from the sensor electrodes prepared by reactive sputtering and have thin Cr- and Al-doped TiO2 and Al-doped SnO2-layers. By changing the coating parameter during magnetron sputtering, the sensing ability of thin layered NO2-sensory can be improved significantly. This work displays the influence of surface structuring as well as chemical and crystallographic changes on the sensors response behaviour. Sensor characteristics towards NO and NO2 were tested at the temperature range of 300°-900°C, depending on the application and material. Doping of semiconducting metal oxides resulted in the alteration of microstructure and phase sequence, consequently in the sensing properties of the electrode material. Moreover, the function of the sensor in context of the NOx-reducing catalyst material was also evaluated. Sensing properties of the semi-conducting oxide electrodes can be adjusted by tailoring the doping to yield the specific sensing abilities.