High Temperature Limiting Current 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 

For the hole of the diameter larger than the mean free path of O2 diffusion, the flux of oxygen, Jo, is given as 

Equation (7.35) yields the expression for steady-state current  

When the oxygen concentration inside the cavity reaches zero (i.e., Cq L) = 0) the current becomes mass transport limited that is, it reaches its limiting current value and (7.36) becomes 

for small values of partial pressure of oxygen in the ambient gas, the limiting current is directly proportional to the oxygen concentration. This approximation 

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Fig. 7.19 High temperature limiting current sensor (a) pinhole version and (b) porous diffii-sional barrier version...

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. 

There are several types of gas sensors in current use. Semiconductor-type sensors are sensitive to either a single gas or group of gases that modify electrical properties of the solids. A classical example is ZnO whose n-type conductivity changes proportionally to /- o 4, although this effect only becomes measurable at high temperatures due to diffusion limitations. 

Although there are many protonic conductors which are stable at low temperatures (< 200 °C), they are not suitable for high electrolytic current applications because of their large electrode polarization. Therefore, their usage is probably limited to sensors in which only a voltage signal is valuable. 

In this chapter we have attempted to provide an overview of the state of the art TDL-based sensors for temperature and species concentrations in industrial applications. To paint an accurate picture of current capabilities, we have discussed the limitations as well as the strengths of this class of sensors. As the work outlined here shows, TDL sensors offer an unprecedented capability for rapid in situ measurements with high accuracy. However, recent TDL temperature sensor installations in combustion environments have revealed difficulties... 

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