GB1578769A

GB1578769A - Method of preparing gas sensor elements and sensor elements and sensor elements produced thereby

Info

Publication number: GB1578769A
Application number: GB4247377A
Authority: GB
Original assignee: Cerberus AG
Current assignee: Cerberus AG
Priority date: 1976-10-12
Filing date: 1977-10-12
Publication date: 1980-11-12
Also published as: DE2745697A1; JPS5348797A; FR2368035A1; CH595629A5; FR2368035B3

Description

(54) IMPROVED METHOD OF PREPARING GAS SENSOR ELEMENTS AND SENSOR ELEMENTS PRODUCED THEREBY (71) We, CERBERUS AG., of alte Landstrasse 411, CH-8708 Maennedorf Switzerland, a Swiss company., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method for the manufacture of a metal-oxide containing gas sensor, of which the electrical resistance alters in response to the effect of a gas to be detected, to a gas sensor made by this method and to the use of such a sensor.

It is known that certain heavy-metal oxides change in resistance under the influence of reducing gases. Known gas sensors employ this characteristic and contain as a gas-sensitive element a solid body in the form of a sintered bead, which contains one or more suitable heavy-metal oxides and which contains a heater device by means of which the element may be brought to a suitable temperature, as well as electrodes for determining a change in resistance. Since gases present in the atmosphere penetrate the solid body only with great difficulty and very slowly, such gas sensor elements react only slowly and only to large concentrations of a gas to be detected. For many applications they are therefore too. insensitive and do not react quickly enough.

Other known gas sensors avoid these disadvantages by disposing a thin metal oxide layer on a support material, e.g., ceramic, glass or quartz, and observing its resistance. This arrangement suffers from the disadvantage of poor stability and working life of such layers over long periods of time, and that their sensitivity to damage and their relatively high resistance make necessary complicated, faultliable and unstable evaluation circuits.

A further disadvantage of previously known metal oxide; gas sensors is their deficient selectivity as regards particular gases to be sensed. The usual sensors respond to almost all reducing gases, without the resistance alteration being pronounced for particular gases. In practice however it is often required to obtain only information as to particular gas components, for example, carbon monoxide, which results, for example, during a process of combustion. In general, known gas sensors do not possess sufficiently good selectivity for carbon monoxide, but possess a considerable, disturbing cross-sensitivity for other reducing gases such as hydrogen, methane, alcohol-vapour, etc. They are also very moisture-sensitive.

The object of the invention is to provide a method for the manufacture of a metaloxide containing gas sensor element with good sensitivity, rapid response, good longterm stability and good selectivity for a particular gas, e.g. for carbon monoxide, and correspondingly low cross-sensitivity for other reducing gases.

According to the present invention there is provided a method for the production of a gas sensor element, comprising providing a support member, applying an organic metal compound in the form of anisotropic crystals or acicular form to said support member and subjecting the crystals to a high temperature in an oxygen-containing atmosphere, so as to form from the organic metal compound a metal oxide layer of which the electrical resistance changes in response to a gas to be detected, and providing spaced-apart electrical contacts in contact with said layer.

The invention further provides a gas sensor element produced by the above method and including on said support member a gas-sensitive layer containing said metal oxide and having a porous structure.

in the method according to the invention an organic metal compound which crystallizes anisotropically in acicular form is employed as the starting material. By the method of the invention is formed a porous mat of needlelike crystals. Such a structure may be particularly readily obtained with heavy-metal phthalocyanines, specifically with copper, cobalt, platinum or iron phthalocyantine, as well as with mixtures that contain a plurality of the heavy metals mentioned above.

It is suitable to purify the metal phthalocyanines before further processing, e.g. by sublimation in vacuum or in an inert atmosphere, e.g. nitrogen.

"The purified metal phthalocyanine is now applied to a heat-resistant support, e.g. of ceramic, aluminium oxide, quartz or glass.

The support may have the form of a flat plate or of a tube, to the surface of which the sensor material is applied, and, in the case of a tube, space within the tube may contain a heater winding for heating the sensor to the necessary operating temperature may be placed. Electrical contacts are preferably applied to the support material before the application of the metal phthalocyanine, these contacts may be bright platinum contacts, that may be burnt on to the support material.

The application of the metal phthalocyanine may be effected in a particularly advantageous manner by sublimation in an inert atmosphere, e,g. nitrogen or a noble gas, or in vacuum. There thus results a particularly well felted mat of acicular crystals with adequate porosity. However, the application may also be effected by de- position from a solution, provided that the cyanines employed are soluble. When copper phthálocyanine is employed, which has proved to be particularly suitable, sub- limation from the gaseous phase is however preferred. Even spreading of the material in the form of acicular crystals on the carrier is possible, though care must be taken to obtain sufficient porosity. The layer thickness may also be relatively great, for example, several ,am.

The sensor with the applied phthalocyanine body is finally heated in an oxygencontaining atmosphere, for example, in air, to a suitable oxidation temperature. In the case of copper phthalocyanine a temperature' of about 40(aBC is sufficient, and a time of a few minutes, in order to decompose and to oxidise the phthaiocyanine, so that there remains a sensor element that contains as its essential element copper oxide, or another heavy, metal if a different phthalocyanine has been employed.In contrast tb prior art sensor elements, however, the resulting metal oxide sensor element exhibits a porous structure, because of its production from a mat of acicular phthalocyanine crystals, so that the gas to be detected can readily enter into the sensor element and a larger and more rapid change in resistance results -than in the prior art sensor elements.

The operation may be advantageously further reinforced if during the decomposition there remains a residue of carbon.

For example, after the oxidation of a copper phthalocyanine sensor at 4000C for a period of, 5 minutes, a carbon residue of 7 weight percent was found. It is even possible that this carbon is present in a very active form and acts as a catalyst, so that the sensitivity of the sensor is enhanced. For higher oxidation temperatures, however, the carbon is lost through oxidation, but the porous skeleton of metal oxide remains.

It has proved to be advantageous, to follow the oxidation process, by a tempering step performed at a somewhat lower temperature and for a longer time, in order to stabilise the sensor. For example, when copper phthalocyanine was used, with de- composition in air at 400"C for a period of 5 minutes, a subsequent tempering at 375"C for 24 hours yielded an extraordinarily stable element, which did not change further in characteristics over a period of several weeks.

After this operation the sensor is ready for use. The appropriate operating temperature then lies below the temperature employed for the tempering step, for copper oxide, between 1500 and 350"C, for example.

As already mentioned, the use of copper phthalocyanine as the starting material yields particularly good results. In particular the selectivity of the copper oxide sensors thus produced for carbon monoxide in comparison to other reducing gases, such as hydrogen, methane and ammonia was extraordinarily high. For example, in an atmosphere of air having a carbon monoxide content of 50 parts per millnon (ppm) the resistance of a sensor layer 10 zm thick altered by 30% in a very short time. The cross-sensitivity for equal concentrations of other gases, on the contrary, always lay below 10% and as a rule, below 5%. The effect of water vapour was also relatively small. Thus the resistance altered by only 7% for an alteration in the relative humidity from 35% to 80%. Likewise the temperature dependence wa,s relatively small, at about 1% per 0C, 'so that complicated circuit arrangements for maintaining -a constant temperature could be dispensed with.

It has been remarked that other heavy metals may be used instead of copper, however, the selectivity for particular gases is then different The use of another heavy metal is therefore particularly recommended when a reducing gas other than carbon monoxide is to be detected.

In addition, there may be used instead of phthalocyanines, other metal-organic com- pounds which likewise yield acicular crystals which form a porous layer upon sublimation or crystallisation, e.g. heavy metal porphyrins or oxalates or other metalorganic compounds with great anisotropy upon crystallisation and strong polarisation.

Because of its high sensitivity and rapid change in resistance that result from a process of combustion a gas sensor produced in accordance with the method described above, particularly a copper oxide sensor, is especially suitable for the detection of slight traces of carbon monoxide. When connected to a circuit in which a predetermined change in resistance yields an alarm, it may be employed as a fire alarm device.

It may also be used for the measurement of carbon monoxide in exhaust gases of internal combustion engines.

WHAT WE CLAIM IS:- 1. A method for the production of a gas sensor element, comprising providing a support member applying an organic metal compound in the form of anisotropic crystals of acicular form to said support member, subjecting the crystals to a high temperature in an oxygen-containing atmosphere, so as to form from the organic metal compound a metal oxide layer of which the electrical resistance changes in response to a gas to be detected, and providing spaced-apart electrical contacts in contact with said layer.

2. A method in accordance with claim 1, wherein said organic metal compound is a heavy metal phthalocyanine.

3. A method in accordance with claim 2, wherein said metal compound is copper phthalocyanine.

4. A method in accordance with any one of claims 1 to 3, wherein the organic metal compound is purified by sublimation in vacuum or in an inert atmosphere before being applied to the support.

5. A method in accordance with any one of claims 1 to 4, wherein the organic metal compound is applied by sublimation in an inert atmosphere or in a vacuum to a said support member of heat-resistant material.

6. A method in accordance with any one of claims 1 to 5, wherein after the formation of a metal oxide at a raised temperature the sensor element is subjected to a tempering step at a temperature below that of oxidation.

7. A method in accordance with claim 3, wherein the copper phthalocyanine is oxidised at a temperature of about 400"C.

8. A gas sensor element prepared by the method of claim 1 and including on said support member a gas-sensitive layer containing said metal oxide and having a porus structure.

9. A gas sensor element in accordance with claim 8 and containing copper oxide.

10. A gas sensor element in accordance with claim 9 and containing also a residue of carbon.

11. A gas sensor element in accordance with any one of claims 8 to 10, wherein said support member is provided with said electrical contacts.

12. A gas sensor element in accordance with any one of claims 8 to 11, wherein said support member has the form of a flat plate.

13. A gas sensor element in accordance with any one of claims 8 to 11, wherein said support member has the form of a tube, upon the outer surface of which is applied the layer formed from the organic metal compound and the interior of which contains a heating device.

14. A gas sensor element in accordance with any one of claims 1 to 13 when used for the detection of carbon monoxide.

15. A gas sensor element in accordance with any one of claims 8 to 13 when used for the detection of carbon monoxide formed in a process of combustion.

16. A fire alarm device mcorporating a gas sensor element in accordance with any one of claims 8 to 13.

17. A fire alarm device in accordance with claim 16 and further including an evaluation circuit arranged to yield an alarm signal upon a predetermined change in the resistance of the gas sensor element.

18. A gas sensor element in accordance with any one of claims 8 to 13 when used to measure the carbon monoxide content of exhaust gases of an internal combustion engine.

19. A method for the production of a gas sensor element in accordance with claim 1 and substantially as herein described.

20. A gas sensor element produced by the method of any one of claims 1 to 7 or 19.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. monoxide is to be detected. In addition, there may be used instead of phthalocyanines, other metal-organic com- pounds which likewise yield acicular crystals which form a porous layer upon sublimation or crystallisation, e.g. heavy metal porphyrins or oxalates or other metalorganic compounds with great anisotropy upon crystallisation and strong polarisation. Because of its high sensitivity and rapid change in resistance that result from a process of combustion a gas sensor produced in accordance with the method described above, particularly a copper oxide sensor, is especially suitable for the detection of slight traces of carbon monoxide. When connected to a circuit in which a predetermined change in resistance yields an alarm, it may be employed as a fire alarm device. It may also be used for the measurement of carbon monoxide in exhaust gases of internal combustion engines. WHAT WE CLAIM IS:-

1. A method for the production of a gas sensor element, comprising providing a support member applying an organic metal compound in the form of anisotropic crystals of acicular form to said support member, subjecting the crystals to a high temperature in an oxygen-containing atmosphere, so as to form from the organic metal compound a metal oxide layer of which the electrical resistance changes in response to a gas to be detected, and providing spaced-apart electrical contacts in contact with said layer.

9. A gas sensor element in accordance with claim 8 and containing copper oxide.