Measuring and testing – Gas analysis – Detector detail
Patent
1997-06-20
1999-08-24
Chilcot, Richard
Measuring and testing
Gas analysis
Detector detail
73 2331, 422 90, 422 94, G01N 2704, G01N 2712, B32B 900
Patent
active
059426764
DESCRIPTION:
BRIEF SUMMARY
PRIOR ART
The invention concerns a sensor for the detection of combustible gases in a test gas, having a sensitive layer on the basis of a semiconducting metal oxide that is deposited on an insulating, ceramic substrate, and for which the electrical resistance provides a statement of the concentration of combustible gases in a test gas. The invention further relates to a process for the manufacture of a sensitive layer of type for gas sensors, which determine the combustible gas component in a test gas by evaluating the electrical resistance of the sensitive layer. The use of sensors for detecting specific components of a test gas is known, which sensors have a sensitive element composed of a semiconductor material, the electrical resistance of which changes when coming in contact with the specific gas component. Sensors of this type are used, in particular, for determining the oxygen content in exhaust gases, e.g. from piston-type internal combustion engines, but also for determining methane, carbon-monoxide or alcohol. The semiconductor materials used are in particular semiconducting metal oxides such as stannic oxide (SnO.sub.2), zinc oxide (ZnO), titanium oxide (TiO.sub.2) or tungsten oxide (WO.sub.3), depending on the purpose. These known sensors normally are produced with the thick or thin-layer technique. Conductor tracks, by means of which the change in resistance is determined later on, are applied onto an insulating, preferably a ceramic substrate, e.g. made of aluminum oxide (Al.sub.2 O.sub.3), as well as the semiconducting metal oxide. In order to increase on the one hand the sensor sensitivity--which depends on the temperature--and to ensure on the other hand that the thermodynamic balance of absorption and desorption is maintained, it is standard practice to heat the substrate with the sensor arrangement. In accordance with known suggestions, the heating devices needed for this are arranged, for example, on the underside of the substrate--wherein the sensor arrangement is located on the top--or they can be integrated into the substrate or arranged between substrate surface and sensor arrangement. A sensor of this type is known, for example, from the EP-OS 313 390. The sensor following from this reference has a substrate composed of aluminum oxide (Al.sub.2 O.sub.3) with a heating device as well as a sensor arrangement placed onto one side. As semiconducting material, stannic oxide (SnO.sub.2) is suggested for detecting methane, tungsten oxide (WO.sub.3) for detecting carbon monoxide or lanthanum nickel oxide (LaNiO.sub.3) for detecting alcohol.
A similar sensor, for which the heating device is integrated into a substrate composed of aluminum oxide (Al.sub.2 O.sub.3), is known from the DE-OS 36 24 217. The gas-sensitive semiconductor layer for this sensor is composed of a porous titanium dioxide (TiO.sub.2) enriched with a second metal oxide. The described sensor is provided in particular for regulating the air/fuel ratio in an exhaust gas by measuring the oxygen content.
These known sensors on a semiconductor basis have proven themselves in practical operations for detecting combustible gases, such as carbon monoxide (CO), hydrogen (H.sub.2) and hydrocarbons. However, all known sensors have a tendency to a transverse sensitivity relative to nitric oxides, such as occur, for example, in the gaseous atmosphere during combustion processes or in automotive exhaust gases. If used in nitric-oxide containing test gases, the known sensors therefore provide inaccurate results or require special correction devices.
It is the object of the invention to provide a sensor as well as a process for manufacturing a sensitive layer, which permits a sufficiently good detection of the share of combustible gases even in test gases with a nitric-oxide component.
The above object is generally achieved according to a first aspect of the invention by a sensor for detecting combustible gases in a test gas, including a sensitive layer on the basis of a semiconducting metal oxide that is deposited on an insulating, cer
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Potthast Heidrun
Schumann Bernd
Chilcot Richard
Kunitz Norman N.
Robert & Bosch GmbH
Spencer George H.
Wiggins J. David
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