Oxygen sensors structure

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

For a sensor with good electrodes (high Jg), the response time is determined by the characteristics of the diffusion barrier and the sensor structure. For the sensor of Fig.6b, for example, a simple rate equation analysis for Pv, the oxygen pressure inside v, predicts that for a cylindrical aperture... 

Non-Diffusion-Limited Sensor Structures. In this section, we will discuss a number of devices which do not require for their operation the existence of a barrier to the diffusion of oxygen. 

Figure 1.1 Multilayer sensor structure showing control of oxygen, glucose, and electroactive interferences.

Several fluorides form the cubic fluorite structure (including CaF2 for which it is named), and are excellent fluoride ion conductors [50] (see Chapter 2). With the introduction of an oxide phase, either during fabrication or in situ during operation, fluoride ion conductors can be used as electrolytes in oxygen sensors [51]. One ofthe advantages of using fluoride electrolytes is that they tend to remain pure ionic... 

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