Zirconia-based oxygen sensor

FIGURE 6.3 Front and back views of the planar zirconia-based oxygen sensor with the Pt-SE and Fe-FeO-RE. 

FIGURE 6.6 Contribution of different components into the total error of measurement for the zirconia-based oxygen sensor at 500°C (a) and 1000°C (b) at the temperature deviation of the RE ( 0.5°C). i component of error by instability of setting reference oxygen pressure by Me-MeO RE 2 component of error by the thermoelectric effect 3 component of error by inaccuracy of setting and measuring of the SEC temperature and 4 component of error by the secondary measuring device. 

Tobias, P., Macak, K., Helmersson, U., Lundstrbm, I. and Lloyd Spetz, A. (2000), Zirconia based oxygen sensor without the need of a reference electrode, in Proceedings of the 8th International Meeting on Chemical Sensors, Basel, Switzerland, 149. 

An excellent example of the advantages and limitations of IS is the use of this technique to examine the effect of various electrode materials on the properties of zirconia-based oxygen sensors at temperatures below 600°C (Matsui [1981], Badwal [1983], Mizusaki et al. [1983], Badwal et al. [1984]). The most common electrode material is platinum. However, the charge-transfer reaction (I) at the electrodeelectrolyte interface is restricted to regions at or near lines of three-phase (gas-electrode-electrolyte) contact ... 

The zirconia-based pump-sensor device can be used for controlling the oxygen partial pressure in closed systems typical applications include the oxygen permeation flux measurements, oxygen monitoring in molten metals, and coulometric titration. 

The uncertainty of commercial calibration gas mixtures is typically 1% of the amount fraction, and the contribution of this to the overall measurement uncertainty is much greater than the intrinsic error of the oxygen sensor for amount fractions of oxygen that are 0.1 ppm and above [2]. Consequently, test procedures that require calibrated gas mixtures will be limited by the uncertainty of those gas mixtures, and therefore carmot be used to test the accuracy of oxygen and other zirconia-based gas sensors, assuming the uncertainty of the sensor calibration is no worse than that of the test mixtures. At present, the uncertainty of commercial calibration gas mixtures is a limitation to verifying the accuracy of these gas sensors. 

Electrochemical Sensors for Aerospace Applications, Fig. 2 (a) Schematic diagram of the zirconia-based oxygen microsensor. The sensor design is aimed towards limiting the flow of oxygen to the electrodes, as well as minimal size, weight, and power consumptions, (b) Picture of packaged zirconia-based O2 sensor [4]... 

In general Zr02 oxygen sensors consist of a tube-like solid-state Zr02 electrolyte where the electronic conductivity is based on oxygen ion charge carrier transport. The inner and outer surface of the yttrium-doped and stabilized zirconia tube is covered by porous platinum electrodes. 

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). 

The most thoroughly developed sensor based on a solid electrolyte is the oxygen sensor using a stabilised zirconia electrolyte. This type of sensor is one of the most successful commercial sensors to date. They are widely used in industry, especially in the analysis of exhaust gases from combustion engines. The following configuration is used in the Oj sensors ... 

Several oxygen sensors based on oxygen pumping with stabilised zirconia have been reported (Hetrick, Fate and Vassell, 1981). This type of oxygen sensor is able to measure the oxygen partial pressure in the exhaust gas from the engine in lean burn. The operating principle of the... 

One of the successfully commercialized sensors in the last century was the zirconia-based potentiometric oxygen sensor (A,-sensor), which has greatly improved... 

If the SE and RE of the zirconia-based sensor are exposed to different oxygen partial pressures, P02 (gas) and 7 02 (reference), this induces different chemical potentials of oxygen ions in zirconia at the interfaces with gas phases. In order that the electrochemical potential remains constant, the electrical potential has to be different. Therefore, the output emf of the electrochemical cell (3.2), represented as a difference between potentials on the RE and SE, obeys the well-known Nemst s law ... 

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