Measuring and testing – Gas analysis – With compensation detail
Patent
1995-11-02
1997-05-13
Williams, Hezron E.
Measuring and testing
Gas analysis
With compensation detail
73 2331, 73 3105, 73 3106, 422 88, 422 94, 422 98, 334 34, G01N 2712
Patent
active
056294743
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a sensor for gases and vapours. For example, it may be used to detect carbon monoxide or water vapour. For convenience the invention will be described below with particular reference to the detection of carbon monoxide but it will be appreciated that its scope is not to be restricted thereto.
Detection of carbon monoxide is necessary in a number of different circumstances. For example, carbon monoxide is emitted when a fire starts to smoulder and can cause death before the fire begins to take hold. Most smoke detectors commercially available are based on one of two principles: diode;
In both cases a considerable quantity of smoke is needed before the alarm is triggered and in neither case is there detection of the early emission of carbon monoxide.
BACKGROUND OF THE INVENTION
Sensors for carbon monoxide have been previously proposed and one well known type is based on the; use of a metallic oxide semiconductor, usually tin oxide SnO.sub.2. The basic principle of their operation is the fact that the resistance of a layer of metallic oxide semiconductor changes in the presence of reactants such as organic vapours, carbon monoxide and even water vapour.
U.S. Pat. No. 4,453,151 discloses a sensor for detecting H.sub.2 S which is made from a mixture comprising metal oxides, activators, dopants and binders and which includes a molecular sieve material, e.g. a zeolite, to provide porosity in the product on a molecular scale.
However, previously proposed tin oxide sensors have not been entirely successful in that they have suffered from problems such as lack of sensitivity to very small amounts of the specific gases to be detected, over-sensitivity to surrounding conditions, e.g. humidity, and lack of specificity in operation. They are too readily poisoned by other extraneous gases and have a slow recovery time after an initial detection. The present invention, therefore, aims to provide an improved sensor of increased selectivity that can be used particularly for the detection of carbon monoxide emissions and that, unlike most previous sensors, can also be operated at room temperature.
Accordingly, the invention provides a sensor to detect emissions of gas or vapour, which comprises a substrate having a layer of a composition comprising a semiconductor metallic oxide, a catalyst and a rheological agent to induce porosity into the surface of the layer, the semiconductor metallic oxide being indium oxide or tannic oxide and the catalyst is present in the composition in an amount of from 3 to 30% by weight.
The invention also provides a method of making a sensor.
The substrate, which should be a good electrical insulator, may be, for example, a sheet of glass or ceramic material. A film of the sensor composition from 100 nanometres 1 mm thick is preferably formed on the substrate by applying a paste of the oxide, catalyst and rheological agent in water to the substrate and annealing at a temperature of e.g. from 500.degree. C. to 1000.degree. C. to form a hardened layer.
The proportions of the ingredients of the covering layer composition may be, for example, 3 to 30% catalyst by weight, 5 to 20% by weight theological agent, if desired additives (well known per se) to change the electrical conductivity of the layer, e.g. in an amount of from 0.5-5% by weight; and the balance stannic oxide or indium oxide.
The catalyst is chosen to give specificity of the sensitivity of the sensor to the gas, e.g. carbon monoxide, to be detected. It is preferably platinum, particularly in the form of platinum black, i.e. finely divided platinum, but other catalysts, e.g. palladium, rhodimn, ruthenium, osmium and irridium may be used.
The rheological aid may be chosen from, for example, kieselguhr and sepiolite. It is preferable to use Kieselguhr, (or diatomaceous earth), which is a mass of hydrated silica. It provides an improved open-pore structure surface for the sensing area of the: sensor and increases the available surface area for reaction with the gas to be detected.
To prep
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Keele University
Larkin Daniel S.
Williams Hezron E.
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