Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Means for analyzing gas sample
Patent
1996-10-28
1998-09-29
Pyon, Harold Y.
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Means for analyzing gas sample
422 90, 422 98, 422 83, 73 3106, 73 232, G01N 3064
Patent
active
058142819
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/GB95/00986 filed on May 1, 1995.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the technical field of resistive gas sensing, that is to say methods of detecting or measuring a target gas in an atmosphere, and apparatus for use in performing such methods, and more particularly resistive gas sensors per se and methods of making such sensors.
2. Discussion of the Prior Art
A resistive gas sensor, or gas sensing resistor, generally comprises a body of porous semiconducting oxide carrying particles of a precious metal, such that the electrical resistance of the sensor varies on exposure to small concentrations of certain reactive gases in air.
It is known from prior art that a semiconducting oxide surface decorated with small amounts of metals such as platinum, palladium and silver show a variation of conductivity at ambient temperature (less than 100.degree. C., and typically in the range 20 to 50.degree. C.) on exposure to the above mentioned small concentrations of reactive gases in air. Examples of gases showing this effect are hydrogen, ammonia, ethene and carbon monoxide. Conjectures have been made that the effect depends on the presence of a sub-monolayer coverage of sufficiently small particles of the metal decorating the surface of the oxide, which is prepared in the form of a porous body. However, these conjectures have never been reduced to practice. The literature is silent on what an optimum composition might be, or how it might be achieved, and the previous work has not resulted in useful devices.
SUMMARY OF THE INVENTION
Five steps are involved in the preparation of such sensors: made. This includes, for example: the precipitation of the oxide or hydroxide from solution; control of the solution conditions; precipitation temperature; the rate at which reagent is added in order to control agglomerate size; and control of the concentration of solution additives, the purpose of which is to act as grain-growth inhibitors or promoters on the oxide (step 2 below), or to influence the nucleation and growth of the precious metal particles grown on the surface in steps 4 and 5 below. artefact, by controlled calcination to obtain a desired specific surface area, porosity, and balance between macro-porosity and micro-porosity. comprising a metal salt or complexion or organo-metallic complex colloidal dispersion. In particular, the impregnation time and temperature, the concentration of metal in the impregnating solution, and the nature and concentration of counter-ions and other cations present, require to be controlled. change the composition of the liquid within the pores, this being followed by drying, or drying. for example by heating, with control of the heat-treatment time and temperature, and the gaseous atmosphere.
An object of the invention is to achieve reliable manufacture of ambient temperature gas sensors based on composites of a semiconducting oxide and one or more metals, particularly precious metals (e.g. Pt, Pd, Ir, Rh, Ag). The present document discloses particular characterisations which can be carried out to confirm that the desired microstructure has been obtained. If these characteristics are obtained, regardless of the details of the method of preparation, then a sensor having the required attributes of a reliable, ambient-temperature indication will be achieved. The invention is particularly directed at the detection of carbon monoxide at low concentrations (10 to 1000 ppm) in air at temperatures at or below 100.degree. C., but this is without limitation as to either the identity of the target gas or the range of concentrations.
We have found that, in pursuit of the above object, the requirements for the choice of metal and semiconducting oxide can be framed very simply, in terms of: the energy of adsorption on the metal of the gas to be measured (the target gas); the electronic work function of the metal, both with adsorption from the air of oxygen only, and with adsorption of the gas to be detected; the work fu
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Ridley Ruth
Sizeland Eliot
Williams David Edward
Capteur Sensor & Analysers, Ltd.
Middleton James B.
Pyon Harold Y.
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