Single-Crystal Zirconia Sensors

FIGURE 4.5 Sideway view of zirconia single-crystal sensor elements and oxygen probe. 

FIGURE 4.6 S ample of a zirconia single crystal made by an inductive high-frequency melting technique. (From Zhuiykov, S., Zirconia single crystal analyser for low-temperature measurements, Prvc. Control and Quality 11 (1998) 23-37. With permission.) 

Concentration of Chemical Elements as Impurities in Zirconia Single Crystal 

Impurity Element Concentration (%) Impurity Element Concentration (%) 

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For the zirconia-based gas sensors, the low level of threshold temperature, when the zirconia electrolytes possess pure ionic conductivity, is approximately 500-550°C for polycrystalline stractures [44 6] and around 380-420 C for single crystals [47, 48]. The conductivity of the YSZ-based electrolyte below these temperatures is compatible with the conductivity of isolators. Moreover, any reduction in operating... 

The emf of the zirconia single-crystal sensor with the Bi-tBijOj RE as a function of measuring temperature at the different oxygen concentrations is shown in Figure... 

It is also essential to know the cross-sensitivity of the zirconia single-crystal sensors to other gases. Sensors with porous Pt electrodes are known to be sensitive to gases such as CO at low temperatures [41], and in fact, this cross-sensitivity has been proposed as a principle for carbon monoxide sensors at low temperatures by some researchers [42, 43]. This effect is attributed to the ability of CO to compete successfully with oxygen for adsorption sites on Pt at temperatures from 500°C to 650°C. It was observed that the zirconia single-crystal sensor with thin-film Pt-Zr02-Y2O3 electrodes is less sensitive to CO than similar polycrystalhne sensors with porous Pt electrodes, but small em/errors still occur at 300-360°C. 

The zirconia single-crystal sensor is used both for periodic variations via short-time dipping into the melts, and for continuous measurement. It is vibration impact proof, needs no precalibration, and can be used for certifying other oxygen impurity control instruments. Small cylinders of the zirconia single crystal, mounted into a ceramic insulating tube, can work at allowable thermal shocks of up to 10°C per second... 

The conclusions concerning the proper design and performance of zirconia single-crystal sensors can be summarized as follows [8] ... 

Complex FCC oxides of the fluorite type represent oxygen-conduction solid electrolytes (SOE s). They comprise a typical class of materials for the manufacture of sensors of oxygen activity in complex gas mixtures, oxygen pumps, electrolyzers and high-temperature fuel elements. These materials are based on doped oxides of cerium and thorium, zirconium and hafnium, and bismuth oxide. Materials based on zirconium oxide, for example, yttrium stabilized zirconia (YSZ) are the most known and studied among them. This fact is explained both by their processibility and a wide spectrum of practical applications and by the possibility to conduct studies on single crystals, which have the commercial name "fianites" and are used in jewelry. 

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