GB2234074A

GB2234074A - Gas sensor

Info

Publication number: GB2234074A
Application number: GB9016147A
Authority: GB
Inventors: Patrick Timothy Moseley; Bruce Cedric Tofield
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1989-07-22
Filing date: 1990-07-23
Publication date: 1991-01-23
Also published as: GB9016147D0

Abstract

A gas sensor, suitable for use in the sensing of carbon monoxide, C2H4, hydrogen or ammonia, includes a heater (1) made of a positive temperature coefficient of resistance material which is in thermal communication with a gas sensitive material (2). The positive temperature coefficient of resistance material (1) is used as a self-regulating heater and may be a donor-doped polycrystalline barium titanate. The gas sensitive material may be Na0.1, Nb0.1, W0.9O3 which is either deposited directly on the heater or on one side of a substrate on the other side of which is deposited the heater. The semiconductor gas sensitive material may be coated with a noble metal e.g. palladium. <IMAGE>

Description

Sensor The present invention relates to sensors and finds one application in the sensing of carbon monoxide.

According to the present invention there is provided a gas sensor comprising a heater made of a material having a positive coefficient of resistance sufficient to enable the said material to act as a self-limiting heater, a gas sensitive material in combination with the heater and means for supplying electrical power to the heater.

The positive temperature coefficient of resistance material may be supplied, in operation, with electrical power thereby to act as a self-regulating heater for the gas sensitive material. Thus, it is possible to avoid the need for separate heating and temperature control means as is required in known gas sensors. The positive temperature coefficient of resistance material can be any suitable substance, for example, a donor-doped polycrystalline barium titanate.

The gas sensitive material may be any gas sensitive material which gives an acceptable response to the gas to be detected at the temperature of operation of the positive temperature coefficient of resistance material.

Thus, for example, where the positive temperature coefficient of resistance material is such that it can operate as a self-regulating heater at a temperature of about 1000C the gas sensitive material may be chosen to be a material which gives an acceptable response to the selected gas to be detected at about 1000C.

For example, the gas sensitive material nay he a semiconducting oxide having platinum upon its surface.

electrically the positive temperature coefficient of resistance material from the gas sensitive material a resistive ceramic material may be interposed (e.g. as a layer) between the positive temperature coefficient of resistance material and the gas senstive material. The resistive ceramic material may be, for example, silica.

By way of example, a gas sensor having a gas sensitive material comprising Na0.1NbO.lW0.903 carrying a surface coating of platinum has been observed to respond strongly to 1% carbon monoxide in air at about 1000C; the response was noted as a reduction of resistance of from about 1 M Q in air to about 1 K SL in air containing 1% carbon monoxide.

Examples of gases which may be sensed by a gas sensor in accordance with the present invention are carbon monoxide, C2H4, hydrogen and ammonia.

The present invention will now be further described, by way of example only, with reference to the single Figure of the accompanying Drawing and with reference to the Example.

The Figure shows, in diagrammatic form, a gas sensor in accordance with the present invention.

Referring to the Figure there is shown a gas sensor having a substrate 1, consisting of a positive temperature coefficient material of resistance, carrying a gas sensitive material 2 consisting of a semi-conducting oxide 2a having a surface coating of platinum 2h.

Conductors 3 and 4 are provided for supplying electrical energy to the substrate 1.

Conductors 5 and 6 are provided for allowing the resistance of the gas sensitive material 2 to be measured.

In operation electrical energy is supplied to the substrate 1 by means of conductors 3 and 4, thereby to raise the temperature of the positive coefficient of resistance material to a value at which the material acts as a self regulating heater for the gas sensitive material, and the gas sensor is contacted with a gas to be tested.

The substrate 1 maintains the temperature of the gas sensitive material of resistance 2 at the operating temperature of the gas sensitive materials and electrical measurements by means of conductors 5 and 6 allow the resistance of the gas sensitive material 2 to be measured.

(It will be appreciated that the resistance may be measured as such or in any other convenient way (e.g. by determination of conductance)).

The sensor may be calibrated in terms of resistance against a number of concentrations of selected gas to be detected (e.g. carbon monoxide) and thereafter used to detect concentrations of the gas.

Where the gas sensitive material comprises a semiconducting oxide in combination with platinum it is believed that platinum abstracts charge carriers from the semiconducting oxide thereby to give the semiconducting oxide an artificially high resistance.

Sorption of gas (e.g. CO) onto the platinum is believed to allow charge carriers to return to the semiconducting oxide thereby to effect a reduction in resistance thereof.

Example A powder comprising na0.1Nb0.1W0.903 is prepared by solid state reaction of stoichiometric quantities of constituent oxides by mixing the oxides and firing to induce reaction.

The powder is applied to a substrate comprising a donor-doped polycrystalline barium titanate by screen printing and firing at 10000C in air for 8 hours.

Platinum is introduced to the available surface area of the Na0,1Nb0,10,903 by means of an ammoniacal platinum hydroxide solution. Electrodes are provided on the Na0.1Nb0.1W0.903 and on the donor-doped polycrystalline barium titanate by sputtering and conductors are attached thereto.

Claims

1. A gas sensor comprising a heater made of a material having a positive coefficient of resistance sufficient to enable the said material to act as a self-limiting heater1 a gas sensitive material in combination with the heater and means for supplying electrical power to the heater.

2. A gas sensor according to claim 1 wherein the gas sensitive material is deposited directly on the heater.

3. A gas sensor according to claim 1 wherein the heater is deposited on one sid of a substrate and the gas sensitive material is deposited upon the other side of the substrate.

4. A gas sensor according to claim 1, 2 or 3 wherein the gas sensitive material comprises a semi-conducting oxide material having a surface coating of a noble metal.

5. A gas sensor according to claim 4 wherein the semi-conducting oxide material comprises a mixture of the oxides of sodium, niobium and tungsten and the noble metal is palladium.

6. A gas sensor according to claim 6 wherein the semi-conducting oxide material has the formulation Na01Nb0 1W0.903.

7. A gas sensor according to any preceding claim wherein the said material having a positive temperature coefficient of resistance sufficient to enable it to act as a heater comprises polycrystalline barium titanate doped with an electron donor material.

8. A gas sensor substantially as hereinbefore described and with reference to the accompanying drawing.