GB2061520A

GB2061520A - Hydrogen Sulphide Detector

Info

Publication number: GB2061520A
Application number: GB8032423A
Authority: GB
Original assignee: Draegerwerk AG and Co KGaA
Current assignee: Draegerwerk AG and Co KGaA
Priority date: 1979-10-20
Filing date: 1980-10-08
Publication date: 1981-05-13
Also published as: DE2942516A1; FR2468119B1; DE2942516C2; GB2061520B; FR2468119A1

Abstract

A device for detecting the presence of hydrogen sulphide consists of a ceramic substrate (1) having on one face a semiconductive layer (5) and having on the other face a layer (6) of ferro-electric material. The layer (5) has an electrical conductivity which varies when it comes into contact with hydrogen sulphide, which variation can be detected by a measuring device connected to wires (3), and is a layer of sintered metal oxide having gold and platinum atoms disposed upon the individual metal oxide particles forming the layer. The device is heated by passing a current through the layer (6) to raise the temperature of layer (5) to a value at which the layer (5) is sensitive to the presence of hydrogen sulphide. <IMAGE>

Description

SPECIFICATION Device for Detecting Hydrogen Sulphide This invention relates to a device for detecting hydrogen sulphide.

As hydrogen sulphide is highly toxic it is important to be able to measure the gas even in relatively low concentrations. It is true that low concentrations of the unpleasant-smelling gas can be sensed by smell, the smell threshold being approximately 0.1 ppm. However, the danger for persons resides in the fact that detection by smell is not possible at higher concentrations (i.e.

approximately 100 ppm). A concentration of 700 ppm is an immediate danger to life. The necessity for an exactly measuring and easy to use detecting device for hydrogen sulphide is therefore self-evident.

A known semiconductive device for detecting hydrogen sulphide in the atmosphere is described in U.S. Patent No. 3,901,067. This device consists of an inert and heat-resistant carrier body provided with a semiconductive layer made of tin oxide doped with zinc, cadmium, aluminium, gallium, indium, tellurium, arsenic, antimony, bismuth or palladium. The semiconductive layer is applied onto the carrier body as a tin salt solution, such as tin chloride in glycerol, and is then formed by heating the solution in an oxidising atmosphere. In one embodiment, the carrier body has an outer and an inner electrode for measuring the conductivity of the semiconductive layer, and a resistance heating element. The electrodes are arranged between the semiconductive layer and the carrier body. In a particular embodiment, the outer electrode may simultaneously act as a heater.The temperature of the heating element, and thus also of the semiconductive layer, is kept at the operating temperature by means of a thermistor in contact with the carrier body and connected into the heating circuit.

The disadvantage of this device is an unspecified base conductivity of the semiconductive layer. Experience has shown that reaction of the tin salt to form tin oxide takes place only in an incomplete manner. The surplus of tin thereby produced and/or residual anionic constituents cause an undesirably high electrical base conductivity which is not very suitable for detecting H2S because of its greatly reducing effect. The action of hydrogen sulphide on the semiconductive layer leads to irreversible damage, rendering calibration necessary after a short period of time. In the case of devices which are in constant use, this is a great disadvantage.

Another gas detecting device for measuring hydrogen or reducing gases in the atmosphere is described in U.S. Patent No. 3,479,257. This device has, as a gas-sensitive layer, a metal film which is vacuum coated onto a carrier and which is converted subsequently in an oxidising atmosphere into the corresponding metal oxide. A further non-continuous layer, formed on the metal oxide layer by vacuum evaporation or by blowing with a gas current, acts as a catalyst. This layer consists of, inter alia, platinum, gold or their mixtures.

The disadvantages described above in connection with the other known gas detecting device are also applicable to this gas detecting device. The conversion of the metal film into the metal oxide takes place only in an incomplete manner. The action of hydrogen sulphide then leads to irreversible damage which makes necessary frequent subsequent calibrations involving a great deal of time and work.

It is desirable to stabilize the gas-sensitive layer of gas detection devices for the detection of hydrogen sulphide in order thus to achieve an increased period of time in which the measured values can be reproduced.

According to the present invention, there is provided a device for detecting the presence of hydrogen sulphide, comprising a substrate made of electrically-insulating and heat-resisting material and provided with means for heating it, the substrate having on a surface thereof a semiconductive layer whose electrically conductivity varies when it comes into contact with hydrogen sulphide, the layer being a layer of sintered metal oxide having gold and platinum atoms disposed upon the individual metal oxide particles forming the layer.

Preferably, the layer is formed by applying a gold-containing and platinum-containing suspension of a metal oxide to the substrate, and by sintering the applied suspension. The platinum may be introduced into the suspension in the form of an aqueous solution of a salt or salts thereof.

Preferably, the suspension is agitated in a ball mill, preferably for approximately 1 5 minutes, so as to finely grind and thoroughly mix the constituents of the suspension. The suspension is preferably applied to the substrate by means of a piston stroke pipette, and the suspension is preferably sintered at approximately 5000C in a tube furnace.

The suspension medium of the suspension is preferably glycerol. The gold and platinum content of the suspension, in total, is preferably 0.01 to 1% by weight, and the suspension preferably contains gold and platinum in substantially equal quantities.

A preferred metal oxide is tin oxide, and a preferred substrate is one made of alumina.

The thickness of the semiconductive layer is preferably 10 to 20 microns.

Preferably, the means for heating the substrate is a ferro-electric ceramic material, the substrate preferably being a flat body having the semiconductive layer on one face and having a layer of the ferro-electric ceramic material on the other face.

The stabilizer additive, namely the gold and platinum, in the metal oxide present in powder form results in a gas detecting device which is stable for a long time, i.e. which operates in a reproducible manner. This is not achieved without the addition of stabilizers nor even by the addition alone of any one precious metal. The uniform distribution of the gold and platinum atoms on the microscopic surfaces of the individual metal oxide crystallites forming the sintered layer also gives rise to a long term stability.

Metal oxides other than tin oxide, for example zinc oxide and ferric oxide, are also suitable for the detection of hydrogen sulphide. Any chemical compound containing gold and platinum may be used as the precious metal salt. Instead of glycerol, liquids such as ethylene glycol which have a sufficiently high viscosity and vaporise or are decomposed below the sintering temperature can be used as the suspension medium.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing in which: Figure 1 is a plan view of a gas detection device of the invention; and Figure 2 is a sectional view along line Il-Il of Figure 1.

Referring to the Figures, there is shown a ceramic substrate made of alumina and having dimensions of, for example, 2.2 mix1.4 mmx0.5 mm. The upper surface of the substrate is provided with gold electrodes 2 which are constructed in a comb-like manner which can be applied by a screen printing method. Two gold lead-in wires 3 are fixed, by means of thermocompression, to the electrodes 2 via contacts 4. They lead to an operating voltage source and to a measuring device for measuring the electrical conductivity of a gas-sensitive semiconductive layer 5 provided on the upper surface of the substrate 1. This semiconductive layer 5, shown as a shaded region in Figure 1, fills the space between the two electrodes 2 and partly covers the electrodes 2.The lower surface of the substrate 1 is provided with a heater 6 in the form of a cold conductor made of a ferroelectric ceramic material, such as titanate ceramic, with a self-regulating critical temperature. The heater is supplied with current via gold wires 7, contacts 8 and electrodes 9. The operating temperature of the gas detecting device is approximately 1 500 C, since, at this temperature, the sensitivity of the semiconductive layer 5 for hydrogen sulphide is good, while its sensitivity for other reducing gases is so small as to be negligible.

The heater and the electrodes 9 have any appropriate shape. For example, the electrodes 9 may have a rectilinear shape.

The semiconductive layer 5 is produced according to the following method. Powdered tin oxide, SnO2, with an approximately stoichiometric composition, together with hexachloroplatinic acid, H2[PtCl.6H20, and tetrachloroauric acid, H[AuC14].3H20, as stabilizer additive, are used.

The SnO2 powder is suspended in glycerol, and to the suspension an aqueous solution of the acids, in the form of salts, is added. The gold and platinum are present in equal amounts in this solution. In order to achieve a homogenous distribution of the precious metals in the suspension, it is agitated for approximately 1 5 minutes in a ball mill so that the constituents are finely ground and thoroughly mixed. The finegrained suspension thereby produced is then applied by dropping onto the upper surface of the substrate 1 by means of a piston stroke pipette and is then heated in a tube furnace at 5000C.

During the course of this heating, the solvent and the suspension medium gradually vaporise, and the precious metal salts decompose and leave behind platinum and gold as non-volatile constituents distributed in an approximately homogenous manner. Sintering then takes place in an oxidising atmosphere. A finely pored sintered layer is produced, with a good structural and adhesive strength. The layer thickness is approximately 10-20 microns.

Claims

1. A device for detecting the presence of hydrogen sulphide, comprising a substrate made of electrically-insulating and heat-resisting material and provided with means for heating it, the substrate having on a surface thereof a semiconductive layer whose electrically conductivity varies when it comes into contact with hydrogen sulphide, the layer being a layer of sintered metal oxide having gold and platinum atoms disposed upon the individual metal oxide particles forming the layer.

2. A device as claimed in claim 1, the layer having been formed by applying a gold-containing and platinum-containing suspension of a metal oxide to the substrate, and by sintering the applied suspension.

3. A device as claimed in claim 2, the gold and platinum having been introduced into the suspension in the form of an aqueous solution of a salt or salts thereof.

4. A device as claimed in claim 2 or 3, the suspension having been agitated in a ball mill so as to finely grind and thoroughly mix the constituents of the suspension.

5. A device as claimed in claim 4, the suspension having been agitated for approximately 15 minutes.

6. A device as claimed in any of claims 2 to 5, the suspension having been applied to the substrate by means of a piston stroke pipette.

7. A device as claimed in any of claims 2 to 6, the suspension having been sintered at approximately 5000C. in a tube furnace.

8 A device as claimed in any of claims 2 to 7, the suspension medium of the suspension being glycerol.

9. A device as claimed in any of claims 2 to 8, the gold and platinum content of the suspension, in total, being 0.01 to 1% by weight.

10. A device as claimed in any of claims 2 to 9, the suspension containing gold and platinum in substantially equal quantities.

11. A device as claimed in any of claims 1 to 10, wherein the metal oxide is tin oxide.

12. A device as claimed in any of claims 1 to 11, wherein the substrate is made of alumina.

13. A device as claimed in any of claims 1 to 12, wherein the thickness of the semiconductive layer is 10 to 20 microns.

14. A device as claimed in any of claims 1 to 13, wherein the means for heating the substrate is a ferro-electric ceramic material.

1 5. A device as claimed in claim 14, wherein the substrate is a flat body having the semiconductive layer on one face and having a layer of the ferro-electric ceramic material on the other face.

16. A device as claimed in claim 1, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.