Thermoelectric-Based Gas Sensors

Thermoelectric gas sensors are other field of possible application of metal oxides. The thermoelectric effect is the direct conversion of temperature differences to electric voltage. A thermoelectric device 

It was found that metal oxides in thermoelectric gas sensors may be applied as catalysts instead of Pt for specific gases such as hydrocarbons (SnO ) (Rettig and Moos 2007a) and CO (TiO iAu) (Nishibori et al. 2006). 

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Table 5.3 Thin-film materials used in thermoelectric-based gas sensors...

Rettig, F. and Moos, R. (2007b), Direct thermoelectric hydrocarbon gas sensors based on Sn02, IEEE Sensors Journal, 1,1490-6. 

Rettig F, Moos R (2007b) Direct thermoelectric hydrocarbon gas sensors based on SnO. IEEE Sens J 7 1490-1496 Rettig F, Moos R (2008) Morphology dependence of thermopower and resistance in semiconducting oxides with space charge regions. Solid State Ionics 179 2299-2307... 

It was shown in this section that it is possible to manufacture accurate, rapid and sensitive DTEGs.The design of the direct thermoelectric gas sensors can be developed knowledge-based. Since intrinsic materials show the best sensitivity, the internal resistance of the gas sensitive layers has to be considered, and the insulation and equipotential layers have to be applied. An appropriate temperature modulation frequency needs to be selected to achieve good results. 

The direct thermoelectric gas sensors introduced in the preceding sections were based on semiconducting oxide materials. However, other materials besides electronic conductors can be employed as materials for DTEGs. Several years ago, it was shown that the thermopower p of an electrochemical cell with Ft electrodes separated by an oxygen ion conductor follows Equation [7.23] (e.g. Ahlgren and Poulsen, 1995) ... 

Thermoelectric-, pyroelectric-, and thermoconductivity-based devices are other representatives of thermometric gas sensors (Korotcenkov 2011). In particular the thermal conductivity technique for detecting gas is suitable for the measurement of high (vol. %) concentrations of binary gas mixes. The heated sensing element is exposed to the sample and the reference element is enclosed in a sealed compartment (see Fig. 1.14). If the thermal conductivity of the sample gas is higher than that of the reference, then the temperature of the sensing element decreases. The higher their thermal conductivity, the lower the concentration which can be measured (Table 1.14). Power loss of a single filament thermistor by heat conduction via the ambient gas can be expressed as... 

Ahlgren EO, Poulsen FW (1995) Thermoelectric power of stabilized zirconia. Sohd State Ionics 82 193—201 Ahmad A, Walsh 1, Wheat TA (2003) Effect of processing on the properties of tin oxide-based thick-film gas sensors. 

We showed above that wide-bandgap semiconductors are promising for the development of high temperature gas sensors such as FET, Schottky, and MOS-based devices aimed at operation in corrosive environments. However, there are fields of gas sensing where semiconductors with much smaller band gap and lower chemical stability can be applied. Thermoelectric gas sensors are applicable in this... 

Early bolometers used, as thermometers, thermopiles, based on the thermoelectric effect (see Section 9.4) or Golay cells in which the heat absorbed in a thin metal film is transferred to a small volume of gas the resulting pressure increase moves a mirror in an optical amplifier. A historical review of the development of radiation detectors until 1994 can be found in ref. [59,60], The modern history of infrared bolometers starts with the introduction of the carbon resistor, as both bolometer sensor and absorber, by Boyle and Rogers [12], The device had a number of advantages over the Golay cell such as low cost, simplicity and relatively low heat capacity at low temperatures. 

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