For more than 30 years, ion conducting metal oxide ceramics have been in use for exhaust gas monitoring systems in the automotive sector (e.g. Bosch’s lambda sensor). Recently, fast exhaust gas sensors based on semiconducting metal oxides have proven to be a promising alternative. These resistive-type oxygen sensors take advantage of the fact that the conductivity at high temperature is correlated with the oxygen partial pressure pO2 of the ambient atmosphere. There is thus no need for a reference partial pressure; furthermore, such sensors are composed of simple structures which can be miniaturized at low cost.
Strontium titanate in particular exhibits interesting features which may be significantly improved by addition of dopants. For instance, the temperature dependence of the characteristic curve (conductivity vs. pO2) of SrTiO3 can be completely suppressed by adding iron: The materials system Sr(Ti0.65Fe0.35)O3, a solid solution of two perovskites, shows a temperature independence and thus an unambiguous curve for a certain pO2 range of technical interest (Fig. 1). This is of great importance for exhaust gas sensing where temperature changes in the range of 100 K are likely to occur.
Fig. 1: Characteristic curve of Sr(Ti0.65Fe0.35)O3 for different temperatures
However, the response behaviour of such oxygen sensors currently is sufficiently fast (t90 < 30 ms) only at high temperatures (nearly 900 °C). For an application as a universal sensor basis, though, a reduction of the operation temperatures is of great importance due to the fact that all catalytic processes necessary for selectivity (NOx, CO2 ...) only take place at lower temperatures.
The sensor response is determined by the kinetics of oxygen exchange between sample and gas phase. This exchange consists of a surface transfer reaction through solid-gas interfaces and the subsequent bulk diffusion of oxygen vacancies. The strongly temperature dependent response behaviour is mainly the result of an inhibited surface transfer of oxygen. This transfer summarizes a rather intricate reaction scheme (adsorption of gas molecules, dissociation, ionization...). The rate of at least one of these elementary processes is decreased considerably at lower temperatures, thus becoming a bottleneck for the overall rate of oxygen incorporation.
The surface reaction may be significantly enhanced through various surface modifications, e.g. by catalytic coatings with thin metal or oxide films. Moreover, investigations performed on doped and undoped SrTiO3 single crystals and thin films clearly showed that the surface transfer kinetics can be strongly influenced by thin coatings of alkaline earth metal oxides (CaO, SrO, BaO). These investigations have been carried out by a frequency-domain analysis of the response signal obtained from a pO2 modulation in a fast kinetic measurement setup (Figs. 2, 3). By means of this system-analytical method, not only can short response times be determined at temperatures up to 1000 °C but also a distinction between kinetic behaviour either controlled by bulk diffusion or by surface transfer reaction is possible.
Publications for further reading
- C. Tragut, K. H. Härdtl: Kinetic Behaviour of Resistive Oxygen Sensors, Sensors and Actuators B 4 (1991), 425-429
- W. Menesklou, H.-J. Schreiner, K. H. Härdtl, E. Ivers-Tiffée: High temperature oxygen sensors based on doped SrTiO3, Sensors and Actuators B 59 (1999), 184-189
- W. Menesklou, H.-J. Schreiner, R. Moos, K. H. Härdtl, E. Ivers-Tiffée, Sr(Ti,Fe)O3: Material for a Temperature Independent Resistive Oxygen Sensor, in: M. Wun-Fogle et al. (eds.): Materials for Smart Systems III, Warrendale, PA, USA: Materials Research Society Proceedings 604 (2000), 305-310
- E. Ivers-Tiffée, K. H. Härdtl, W. Menesklou, J. Riegel: Principles of solid state oxygen sensors for lean combustion gas control, Electrochimica Acta 47 (2001), 807-814
- S. F. Wagner, W. Menesklou, Th. Schneider, E. Ivers-Tiffée: Kinetics of Oxygen Incorporation into SrTiO3 Investigated by Frequency-Domain Analysis, J. Electroceramics 13 (2004), 645-651
- Ch. Argirusis, S. Wagner, W. Menesklou, C. Warnke, T. Damjanovic, G. Borchardt, E. Ivers-Tiffée, Enhancement of Oxygen Surface Exchange Kinetics of SrTiO3 by Alkaline Earth Metal Oxides, Physical Chemistry Chemical Physics 7 (2005), 3523-3525
- T. Schneider, C. Peters, S. Wagner, W. Menesklou, E. Ivers-Tiffée, Sr(Ti,Fe)O3-δ Exhaust Gas Sensors, in: S. Seal et al. (eds.): Semiconductor Materials for Sensing, Warrendale, PA, USA: Materials Research Society Proceedings 828 (2005), 139-144
- S. F. Wagner, C. Warnke, W. Menesklou, C. Argirusis, T. Damjanovic, G. Borchardt, E. Ivers-Tiffée, Enhancement of Oxygen Surface Kinetics of SrTiO3 by Alkaline Earth Metal Oxides, Solid State Ionics 177 (2006), 1607-1612