Limiting current oxygen sensors

Yttrium stabilized zirconia (Zr02-Y203) as an electrolyte for reduction of molecular oxygen at elevated temperatures (400-800°C) has been already discussed in Section 6.23.4. In fact, both the reduction of oxygen and the oxidation of the oxide ion at the Pt/zirconia interface is reversible and the transport of both species in zirconia is so rapid it is possible to construct an electrochemical oxygen pump, which is the heart of the limiting current oxygen sensor described in this chapter (Saji, 1987). The overall electrochemical reaction that takes place at the porous Pt electrode is... 

Fig. 7.20 Response of limiting current oxygen sensor at 700°C to various partial pressures of oxygen (adapted from Pham and Glass, 1997)...

Liaw BY, Weppner W (1990) Low temperature limiting-current oxygen sensors using tetragonal zirconia as solid electrolytes. Solid State Ion 40(41 ) 428-432... 

Fig. 10. Structure of a limiting-current oxygen sensor with pinhole (Takeuchi and Igarashi 1988). (Reprinted by permission of the publisher, Kodansha Ltd.)...

Several limiting-current oxygen sensors of advanced design have been proposed. One example uses two ZrOa cells. In this configuration, one cell is used for pumping and the other cell is used to measure the oxygen concentration. A design which uses two sheets of zirconia for the two cells is reviewed by Sheppard (1992). 

In the device of Fig.6a (sensor with distributed cavity), depending on the dimensions of the pores of the diffusion barrier, the oxygen diffusion can be bulk diffusion or Knudsen diffusion(ll). In the former case, the sensor output (limiting current Ig) is given by Eq. (9). Knudsen diffusion occurs when the average pore diameter is much smaller than the mean free path of the gas molecules, in which case collisions between molecules and pore walls are the dominant events. In this case(121. Dq -k T /2,and... 

It is apparent that the double-cell sensor can be operated in the limiting current mode (which corresponds to large values of Vg, of the order of 600 to 800 mV). The advantage of this mode of operation is that errors associated with the measurement of small values of Vg (e.g, less than 100 mV) are avoided. However, limiting current operation is generally undesirable since the problems associated with complete depletion of oxygen at the cathode of the pump cell can seriously limit the performance of the device. It is... 

Yagi, H. and Hideaki, K. (1995) High-temperature humidity sensor using a limiting-current-type plane multi-oxygen sensor for direct firing system. Sens. Actuators B, B25 (1—3), 701—4. 

The current US emission limits for light duty vehicles are achieved for gasoline fueled cars by engines equipped with a controlled threeway catalyst system including an oxygen sensor and fuel injection. For the older engine types dual bed reduction/oxidation catalyst systems are often applied (Ref. 3). 

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