Abstract
The emf of commercial automotive zirconia oxygen sensors exposed to O 2 /CO/CO 2 /N 2 or O 2 /H 2 /H 2 O/N 2 gas mixtures was measured as a function of R ′=(2P O 2 /P CO ) or (2P O 2 /P H 2 ) at several temperatures. A step-like transition from high to low emf occurs at the switch-point R s0 ′ that is almost independent of temperature. The high emf (at say R ′=0.5) first increases and then decreases with temperature for O 2 /CO/CO 2 /N 2 gas mixture, whereas it only decreases for O 2 /H 2 /H 2 O/N 2 . These results may be analyzed using the steady state model of gas sensors [A.D. Brailsford, M. Yussouff, E.M. Logothetis, M. Shane, Steady-state model of a zirconia oxygen sensor in a simple gas mixture, Sensors and Actuators, B 24/25 (1995) 362–365]. Two lumped parameters, R s ′ and ξ , are needed in the model to compute the emf. R s ′ represents the theoretical switch-point and ξ determines the theoretical high emf. To obtain good fit with experiment, we choose R s ′ close to the observed R s0 ′ and then vary ξ . The quantity ξ depends upon the fundamental processes taking place on the sensor electrode; the temperature dependence of its fitted values (and hence the high emf) may be understood in terms of rate constants for adsorption, desorption and reaction.
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