Sensor for detecting nitrogen oxide

Details Sensor for detecting nitrogen oxide Sensor for detecting nitrogen oxide

1996-04-12

1997-04-29

Snay, Jeffrey

Chemical apparatus and process disinfecting, deodorizing, preser

Analyzer, structured indicator, or manipulative laboratory...

Means for analyzing gas sample

422 98, 436116, 204432, 338 34, G01N 2712

Patent

active

056246404

DESCRIPTION:

BRIEF SUMMARY
PRIOR ART

The invention relates to a sensor for detecting nitrogen oxides (NO, NO.sub.2, N.sub.2 O.sub.4) as generically defined by the preamble to claim 1. For determining specific components of a test gas, the use of sensors has been disclosed whose sensitive element is a semiconductor material whose electrical resistance changes when in contact with the specific gas components. This kind of sensor is used in particular to determine the oxygen content in exhaust gasses, for example from internal combustion engines, but is also used to determine methane, carbon monoxide, or alcohol content. In particular, semiconducting metal oxides such as tin oxide (SnO.sub.2), zinc oxide (ZnO), titanium oxide (TiO.sub.2), or tungsten oxide (WO.sub.3) are used as the semiconducting materials, depending upon the purpose.
These known gas sensors are usually manufactured using thick-film or thin-film technology. Strip conductors, by means of which the resistance change is later measured, as well as the semiconducting oxide, are deposited on an insulating, preferably ceramic substrate, for example of aluminum oxide (Al.sub.2 O.sub.3). In order on the one hand to increase the sensitivity of the sensor--this depends on temperature--and on the other hand to assure the maintenance of the dynamic equilibrium of adsorption and desorption, it is customary to heat the substrate with the sensor device. Heating devices required for this can be disposed according to known proposed embodiments, for example on the underside of the substrate--if the sensor device is mounted on the top side--, or they can be integrated into the substrate, or be disposed between the substrate surface and the sensor device.
For example, EP-OS 313 390 discloses a sensor of this kind. In the sensor described there, the heating device and sensor device are disposed on one side of a substrate of aluminum oxide (Al.sub.2 O.sub.3). Tin oxide (SnO.sub.2) is proposed as the semiconducting material to detect methane gas, tungsten oxide (WO.sub.3) for detecting carbon monoxide, or lanthanum nickel oxide (LaNiO.sub.3) for detecting alcohol.
DE-OS 36 24 217 discloses a sensor in which the heating device is integrated into the substrate. Once again aluminum oxide (Al.sub.2 O.sub.3) is suggested as the material for the substrate, the gas-sensitive semiconductor layer in this case is comprised of porous titanium oxide (TiO.sub.2) which is enriched with another metal oxide. The described sensor is provided in particular for regulating the fuel/air ratio in an exhaust gas by measuring the oxygen content.
Although these known sensors based on semiconducting oxides have proven to be practical for detecting CO, H.sub.2, and hydrocarbons, they are unsuited for detecting nitrogen monoxide (NO) or nitrogen dioxide (NO.sub.2). However, their detection in particular is of considerable significance for the verification of diesel exhaust gasses. These contain primarily nitrogen monoxide (NO), which is converted in minutes almost completely into nitrogen dioxide (NO.sub.2) or dinitrogen tetroxide (N.sub.2 O.sub.4). Because of the different effects of nitrogen monoxide (NO) or nitrogen dioxide (NO.sub.2) on the semiconducting oxide, this reacts with a complex resistance behavior, which cannot be suitably evaluated. The use of phthalocyanine layers, which detect NO.sub.x molecules contained in the exhaust, is therefore known for the verification of diesel exhaust gasses. For example, the verification of diesel exhaust gasses can in turn be used to interrupt the supply of external air, in which diesel exhaust gasses have been detected, to rooms or vehicles by means of a control flap in ventilators or air conditioners. The precise knowledge of the percentage of nitrogen oxides (NO, NO.sub.2, . . . ) in exhaust gases is of particular interest for monitoring and regulating internal combustion engines or furnaces. This particularly applies if catalysts and other processes are used to optimize combustion and reduce undesirable exhaust gas components such as NO, CO, or hydrocarbons.
The objec

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Journal of Materials Science, vol. 25, No. 5, May 1990, London, Great Britain, pp. 2632-2636; Y. Sadaoka et al "Effect of NO2 in air on the electrical conductance of In203 films with and without added ZnO . . . Sputtering".

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