Resistive gas sensing, especially for detection of ozone

Details Resistive gas sensing, especially for detection of ozone Resistive gas sensing, especially for detection of ozone

1997-08-25

1998-09-22

Williams, Hezron E.

Measuring and testing

Gas analysis

Detector detail

73 232, 73 3102, 422 90, 422 94, G01N 2704, B05D 140, H01C 700

Patent

active

058116621

DESCRIPTION:

BRIEF SUMMARY
This invention relates to resistive gas sensing, especially for the detection of ozone, and in particular to methods of sensing ozone, sensors and sensing apparatus suitable for use in the measurement of ozone concentrations, and methods of making such sensors.
Our co-pending United Kingdom patent application GB 95 08115.4, filed on 21 Apr. 1995 with a priority date of 21 Apr. 1994, and not published at the date of the present application, discloses the use of tungsten trioxide (WO.sub.3) as a material for gas sensors when incorporated in a fail-safe gas detection system in which the electrical resistance of the sensors increases with an increase in the concentration of a target gas consisting of chlorine, NO.sub.2 or ozone.
The document WO95/00836, in the name of the present Applicants, discloses the use of WO.sub.3 as the sensing material in gas sensors for the selective sensing of chlorine or NO.sub.2 at a concentration of the order of 0.5 ppm (parts per million) in air, in such a way as to distinguish over the presence of any SO.sub.2 or CO, the sensor having a heated thin-film sensing element which is preferably in the range of 10 to 200 micrometers thick.
It is well known to use sensors made from tungsten trioxide for the detection of hydrogen sulphide.
The document DE-A-3 213 286 (Hitachi) discloses a sensor in which the sensing material is WO.sub.3 doped with platinum, for use in the detection, at a temperature in the range 250.degree. to 400.degree. C., of hydrogen, NO.sub.2 or CO. The disclosure in the Hitachi document shows that this sensor gives a measurable signal only at concentrations of these gases in excess of about 1000 ppm, the doped WO.sub.3 sensor being one of an array of sensors, of different materials, which are scanned in a particular way so as to give signals from the individual sensors.
Ozone in air can be toxic at concentrations of less than 1 ppm or 1000 ppb ("ppb" means parts per billion, where 1 billion is 1000 million). In the United Kingdom, occupational limits for exposure to ozone lie in the range 100-300 ppb.
One object of this invention is to provide gas sensors which are sensitive to ozone at these rarefied concentrations, in order for example to provide continuous monitoring of an atmosphere, giving electrical output signals which can be used to trigger warning devices and/or means for remedying hazardous occupational exposure levels of this gas.
In these circumstances, it is of course desirable to distinguish the presence of ozone from that of other gases which may be present at similar concentrations but which are harmless at these levels.
It is therefore another object of the invention to provide a sensor capable of detecting ozone at concentrations of less than 1 ppm in air in the presence of other gases, for example H.sub.2 S, CO, hydrocarbons, ammonia, hydrogen, ethanol, chlorine, or NO.sub.2.
We have been surprised to find that these requirements can be satisfied by using WO.sub.3 as the sensitive material in a sensor in a form having a very open porous microstructure with an especially large surface area exposed to the target gas.
According to the invention in a first aspect, a resistive gas sensor comprising a sensing element containing tungsten trioxide as gas-sensitive material, is characterised in that the sensing element is a porous oxide layer in which the oxide is at least 99% pure, its porosity being in the inclusive range 30-60%, macroscopic flaws having a dimension larger than 5 times the mean pore size being absent, and the layer having a thickness of less than 50 micrometer and uniform to an accuracy better than .+-.20%.
The sensor preferably includes an electrically isolated heating element in thermal contact with the oxide layer (referred to below as a heated sensor).
According to the invention in a second aspect, gas sensing apparatus including a resistive gas sensor, and resistance measuring means connected with the sensor, for measuring values taken by the electrical resistance of the sensor in response to the concentration of a target gas to w

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