Zirconia electrolytes content

A unique proposed applieation for an yttria-stabilized zirconia is in carbon monoxide detection. A platinum electrode is attached on both sides of a zirconia electrolyte. One side is covered with a platinum eatalyst on a porous alxunina substrate and the Pt electrode is not in direct contact with the sample gas. Platinum on the substrate acts as a catalyst for CO oxidation. A cross-sectional view of the CO sensor is shown in fig. 15 (Okamoto et al. 1980) (the operating temperature is around 300°C). When carbon monoxide exists in the atmosphere, most will be eatalytically oxidized by the oxygen in air during difiusion through the porous substance. Therefore, the gas that reaches the Pt electrode is not CO but a CO2-O2 mixture. On the other hand, on the surface of the platinum electrode without the catalyst, carbon monoxide is oxidized to CO2 and causes an anomalous EMF. This potential shows a one-to-one correspondence to the CO concentration. The typical performance of the CO sensor in air at 300°C is shown in fig. 16 (Okamoto et al. 1980). The EMF output increases with the CO content, but the slope of the curve decreases gradually. This sensor can operate at temperatures between 260 and 350 C and no speeial O2 reference gas is necessary. 

J. Xue, and R. Dieckmann. Oxygen partial pressure dependence of the oxygen content of zirconia-based electrolytes in Ionic and Mixed Conducting Ceramics Second International Symposium 94-12, 191-208 (1994) ES Meeting San Francisco, California. 

Sensor. The control of the exhaust composition was essential to maintain the air-to-fuel ratio close to stoichiometric for simultaneous conversion of all three pollutants. This control came about with the invention of the 02 sensor.21,22 The sensor head of this device was installed in the exhaust immediately at the inlet to the catalyst and was able to measure the 02 content instantly and precisely. It generates a voltage consistent with the Nemst equation in which the partial pressure of 02 (P02)exhaust in the exhaust develops a voltage (E) relative to a reference. The exhaust electrode was Pt deposited on a solid oxygen ion conductor of yttrium-stabilized zirconia (Zr02). The reference electrode, also Pt, was deposited on the opposite side of the electrolyte but was physically mounted outside the exhaust and sensed the partial pressure (P02)ref in the atmosphere. E0 is the standard state or thermodynamic voltage. R is the universal gas constant, T the absolute temperature, n the number of electrons transferred in the process, and F the Faraday constant. 

Politova, T.I., Irvine, J.T.S. Investigation of scandia-yttria-zirconia system as an electrolyte material for intermediate temperature fuel cells—influence of yttria content in system (Y2O3)x(Sc2O i)0i x)(ZrO2)( 9)- Solid State Ionics 2004,168,153-65. 

Another type of electrical conductivity observed in ceramics is ionic conductivity, which often occurs appreciably at elevated temperature a widely used material exhibiting this behavior is zirconia doped with other oxides such as calcia (CaO) or yttria (Y2O3). For this material, atomic oxygen is the mobile ionic species. Doped zirconia finds widespread use as oxygen sensors, especially as part of automobile emission control systems, where the oxygen content of the exhaust gas is monitored to control the air-to-fuel ratio. Other applications of ionic conducting ceramics are as the electrolyte phases in solid-oxide fuel cells and in sodium-sulfur batteries. 

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