Chemical sensors yttria

Micromachined and microfabricated electrochemical sensors have been used either per se, or as part of a sensor system, in many practical applications. This includes various biosensors and chemical sensors reported in research literature. An example of a practical electrochemical sensor is the yttria-stabilized zirconium dioxide potentiometric oxygen sensor used for fuel-air control in the automotive industry. Thick-film metallization is used in the manufacture of this sensor. Even though the sensor is not microsize, this solid electrolyte oxygen sensor has proven to be reliable in a relatively hostile environment. It is reasonable to anticipate that a smaller sensor based on the same potentiometric or the voltammetric principle can be developed using advanced microfabrication and micromachining techniques. 

CaO has been used to some degree as a stabilizer and is attractive due to its low cost. Its ionic conductivity, however, is approximately an order of magnitude less than an equivalent yttria stabilized body. There has also been some question about the chemical stability of a CaO stabilized body, although this may be more of a factor with a partially stabilized body than a fully stabilized body. Calcia fully stabilized ZrO has been and may still be used in commercial production of oxygen sensors. 

A similar situation occurs with sensors based on several types of solid electrolytes (Fergus 2008). For example, carbonate and sulfate electrolytes could be used with CO and SO sensors. However, those electrolytes generally do not provide adequate stability (see Chap. 6 (Vol. 1)), and therefore the most promising sensors use common electrolytes, such as Nasicon, P-alumina, and yttria-stabilized zirconia (YSZ). These electrolytes require auxiliary electrodes to provide the desired response, but they provide good stability and long operating lives. Therefore, while optimizing the reactions responsible for a gas sensor s sensitivity, one should also aim to maximize the chemical, structural and time stability of the device. 

A typical electrochemical NOx sensor design involves the use of two electrodes on an oxygen-ion conducting ceramic, such as yttria-stabilized zirconia (YSZ), as shown in Fig. la. Both chemical and electrochemical reactivity at each electrode is critical to sensor performance [8, 18-20]. We have obtained optimal results with a Pt electrode covered with Pt-containing zeolite Y (PtY) as the reference electrode and WO3 as the sensing electrode [17, 21, 22]. These electrodes were identified by temperature programmed desorption of NO from NOx/02-exposed PtY and WO3, and the ability of PtY and WO3 to equilibrate a mixture of NO and O2. Significant reactivity differences were found between the PtY and WO3, with the latter... 

Fig. 9.13 Typical configurations of potentiometric oxygen sensors with yttria-stabilized zirconia solid electrolyte, where the oxygen chemical potential over porous metallic reference electrode is fixed by supplying a gas mixture of known composition (a) and where a solid-state RE is used (b)...

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