Characterization of NO x sensor using doped In 2 O 3
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To develop a highly sensitive gas sensor monitoring NOx, various kinds of n-type semiconductors made of In2O3 were prepared, and the relations between doped elements and gas sensitivities or response times were studied. Consequently, it was found that the samples doped with less than 1 at.% alkali-earth metal components have high sensitivities and responsiveness. The gas-absorbing phenomena were investigated using highly sensitive thermal analysis. From the result, it was indicated that alkali-earth component-doped In2O3 materials have higher adsorption ability of NOx than pure In2O3 has.
I. INTRODUCTION
To control the ventilation system in vehicles, a sensor system has been examined from several years ago, which switches the recirculating or air-input mode depending on the concentration of pollutants in air outside the vehicle. Although automobiles with only a combustible-gas sensor were launched, it was difficult for the system to be applied to diesel car-exhausted gas. Thereupon, it has been examined whether a NO2 sensor for diesel exhausts can be coupled with a combustible-gas sensor for gasoline exhaust. We have developed a high-sensitivity and practical-use gas sensor for simultaneous detection of combustible gas and NO2. This sensor is actually installed in automobiles at present and is working without any problems. However, the sensor has a weak point in that the sensitivity is offset when gasoline exhausts and Diesel exhausts contact the gas sensor simultaneously. If we can use a combustible-gas sensor and a NO2 sensor individually, this kind of problem would be solved. As to a NOx sensor, we have a demand that NOx is detected selectively in extremely low concentration range (at least 0.5 ppm) for practical use. Furthermore, the response speed should be fast and, if possible, the sensor resistance must be in the comparatively low range of 10 k to 100 k⍀ in air. However, there is not yet any such highly sensitive NOx sensor with high gas selectivity and high response speed. In2O3 has C-type sesquioxide structure and is very stable up to high temperatures even if a considerably large amount of oxygen vacancies exists in the lattice.1,2 Although the band gap of pure In2O3 is as high as 3.5 to
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Address all correspondence to this author. e-mail: esaka@chem. tottori-u.ac.jp J. Mater. Res., Vol. 16, No. 5, May 2001
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4.0 eV at room temperature, the electrical conductivity is very high due to existence of a considerably large amount of oxygen vacancies. Doped In2O3 materials are formed by introduction of the carrier dopants3 and show high electronic conductivity with high carrier concentration of 1.4 × 1021 cm−3 and carrier mobility of 1.03 × 102 cm2 V−1 s−1.4–6 With regard to semiconductor gas sensors, three gas sensing mechanism models are proposed;7–10 the grain boundary control, the neck control, and the grain control. Each crystallite of semiconductor oxide included in the sensor element has an electron-depleted surface layer (space-charge layer) of
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