Catalytic gas sensor

Details Catalytic gas sensor Catalytic gas sensor

1996-06-05

1999-05-11

Achutamurthy, Ponnathapura

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Waste gas purifier

422 8212, 422 83, 422 94, 422 95, 422 96, 422 97, 422 98, F01N3/10

Patent

active

059025565

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to a catalytic gas detector, and to a method of manufacturing such a detector.
Catalytic gas detectors are used in the continuous monitoring of atmospheres to detect the presence of a low molecular weight flammable gases, such as methane. The principle of operation relies on the fact that small concentrations of hydrocarbon gases are cationically oxidized in air at elevated temperatures over a suitable catalyst such as palladium rhodium, platinum or iridium. The resultant exotherm reaction produces a temperature rise which is sensed by a sensor. There is a direct relationship between the degree of temperature rise and the proportion of combustible gas in the atmosphere being measured.
Sensors of the type described, which are commonly known as pellistors, comprise a coil of a noble metal, such as platinum, covered with a bead of porous catalytic material. The change in temperature referred to above leads to a change in resistance of the coil, which is detected in a Wheatstone bridge-type configuration of which the coil forms a part.
In the case of portable sensing instruments which are reliant on battery power, it is important that the power consumption of the pellistor be as low as possible.
There are two ways of achieving this end: surroundings;
In order to satisfy the first requirement, published UK patent application GB2238617A describes a thin wire in the form of a conductive track deposited on a non-conductive and thermally isolating layer, such as silicon nitride or silicon oxde, which is deposited upon the silicon substrate. Photolithographic processes are commonly used to define the conductive track or heater element. The bead is usually formed by the deposition of a material such as porous alumina on top of the heater element.
In order to satisfy the second requirement, the substrate beneath the isolating layer is generally etched away so that a suspended membrane is formed upon which the heater element is located, thereby increasing the thermal isolation of the heater element, which results in a significant decrease in power consumption. This is important for portable sensing instruments which are reliant on battery power.


SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a catalytic detector for a flammable gas comprising a substrate and a sensing structure suspended from the substrate,the sensing structure including: electrically conductive track terminating in at least two electrically conductive bridging leads; structure being etched away so as to isolate thermally the sensing structure, and the temperature sensing structure being supported by the bridging leads.
Preferably the leads extend from the conductive track to terminal pads supported on the substrate, the leads having a thickness and cross-sectional profile which allows them to provide complete support for the sensing structure.
Conveniently, the electrically conductive track is sandwiched between a substrate adhesion layer for facilitating adhesion of the conductive track and a superstrate diffusion barrier layer.
Typically the adhesion layer and the diffusion barrier layer are arranged to prevent buckling or twisting of the conductive track when subjected to thermal stress.
The catalytic bead advantageously comprises a catalyst precursor distributed within a refractory carrier.
The refractory carrier typically comprises alumina and the catalyst precursor comprises palladium (II) chloride or palladium (II) nitrate.
Preferably the heating and temperature sensing elements are combined in the form of a single integrated layer-deposited meandering track of platinum.
Typically, the substrate is formed from a planar wafer of single crystal silicon having a base face and a top face, the base face of the wafer being coated with an inert film and the top face being coated with a dielectric film, with the inert film being a silicon dioxide film and the dielectric film being an alumina film.
The leads may have a thickness matching that of the track and

REFERENCES:
patent: 4574264 (1986-03-01), Takahashi et al.
patent: 4935289 (1990-06-01), Kikuchi et al.
patent: 5783154 (1998-07-01), Althianz et al.

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