GB2250823A

GB2250823A - Method for the preparation of a gas sensor

Info

Publication number: GB2250823A
Application number: GB9125334A
Authority: GB
Inventors: Per Brandt Rasmussen
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1990-11-30
Filing date: 1991-11-28
Publication date: 1992-06-17
Also published as: JPH04269649A; DE4038140C2; SE9103017D0; DK190091A; DE4038140A1; FR2670012A1; DK190091D0; NL9101879A; GB9125334D0; SE9103017L; CA2053049A1

Abstract

Two electrodes 3 and a gas-sensitive layer 4 connecting the two electrodes are applied to an electrically insulating carrier substrate. A catalyst in the form of a metal compound or base is added, in solution to the gas-sensitive layer and the sensor is heated to a temperature below the boiling point of the solution or below the temperature at which the compound decomposes to the metal. The solvent may be water or alcohol or a mixture thereof and the salt may be chloroplatinic acid (for an ammonia sensor) or the base borium hydroxide for a chlorine or hydrogen chloride sensor. The gas-sensing material is preferably tin oxide. Cuts 6 may be made in the gas-sensing layer 4 before the catalyst is applied to control the resistance of the sensor. <IMAGE>

Description

2 25J323 Method for the--preparation of a gas sensor.

The invention relates to a method for the preparation of a gas sensor, in which two electrodes and a gas-sensitive layer connecting the two electrodes are applied to an electrically insulating carrier substrate and a catalyst is added.

A method of this kind is known from DE 34 22 823 A. In that method, using a thin-film technique three electrodes are applied to the carrier substrate, two of the electrodes serving as measuring electrodes and one serving to heat the sensor. The gas-sensitive layer, which consists of tin oxide or tin oxide doped with aluminium, is arranged between the electrodes. A film that is used as catalyst is deposited by vapourdeposition on the gas-sensitive layer. This film consists of platinum or of other active metals. This gas sensor reacts in particular to hydrogen sulphide, its electrical conductivity rising as the concentration of gas increases. The gas sensor is typicaily operated at 280C.

US 4 197 089 discloses a gas sensor in which three electrodes are applied to a ceramic carrier substrate, one electrode serving as the measuring electrode, the second electrode serving as the heating electrode and the third electrode serving as a common earth electrode. The gas-sensitive layer between the electrodes consists in this case of a tungsten trioxide film. In this 2 - connection, a tungsten trioxide solution is prepared, and 'dripped between the electrodes. A fifteen-minute heating at a temperature of 600'C follows. In order to render the gas sensor sensitive to ammonia, a drop of platinic acid is then dripped between the electrodes, so that metallic platinum forms. The tungsten trioxide layer is then applied. This sensor requires an operating temperature of 150 to 3000C.

EP 141 033 A describes a method for the preparation of materials for a gas sensor, in which a metallic oxide is mixed with a metallic salt acting as catalyst, for example platinic acid, to form a solution. The solution is then exposed to ultra-violet radiation. The materiail thus treated is slowly heated to about 300C. After cooling and trimming to shape, the electrodes are joined on. Depending on the metallic oxide selected and the metallic salt selected, gas sensors with specific sensitivities to selected types of gas are obtained. These gas sensors are also able to operate at room temperature. Their preparation, however, is relatively costly.

The invention is based on the problem of providing an inexpensive method for preparing a gas sensor which is able to operate at room temperature and preferably reacts to a predetermined gas.

The present invention provides a method for the preparation of a gas sensor, in which two electrodes and a gas-sensitive layer connecting the two electrodes are applied to an electrically insulating carrier substrate and a catalyst is added, wherein a metallic salt dissolved in water or in an alcohol is added as catalyst to the gas-sensitive layer and the sensor is heated to a temperature below the boiling point of the metallic salt.

The above-mentioned problem is solved in a method of the kind mentioned in the introduction in that a metallic salt dissolved in water or in an alcohol is added as catalyst to the gas-sensitive layer and the sensor is heated to a temperature below the boiling point of the metallic salt.

The "metallic salt" mentioned in the two preceding paragraphs may, for example, be a salt in the narrower sense commonly used in the United Kingdom of the product of a reaction between an acid and a base, or, for example, it may be a base.

It has been found that, by restricting the temperature to a range below the boiling point of the metallic salt, an especially sensitive catalyst layer can be produced, so that the gas sensor is able to work even at room temperature. The preparation method is relatively simple, since it is possible to work at relatively low temperatures when applying the catalyst. A deposit of the metal itself does not result from the heating because the temperature is kept below the temperature at which thermal decomposition of the catalyst compound occurs. on the contrary, the metallic compound, rather than the metal itself, serves as the catalyst.

The invention also provides a method for the preparation of a gas sensor, in which two electrodes and a layer of gas-sensitive material connecting the two electrodes are applied to an electrically insulating carrier substrate and the gas-sensitive material is treated with a metal-based catalyst so as to improve the sensitivity of the sensor, wherein the catalyst is added to the gas-sensitive material in the form of a solution of a compound of the metal and the sensor is heated without allowing its temperature to rise to a temperature at which the compound decomposes to yield the catalytic metal.

The solution may be a solution of the compound in water, or a solution of the compound in an alcohol, or a solution of the compound in a mixture of water and an alcohol.

The compound may, for example, be a salt, or it may, for example, be a base.

The metal may, for example, be platinum. In that case, preferably, the compound is chloroplatinic acid and the sensor is heated to a temperature not exceeding 115' C.

The metal may, for example, be barium. In that case, preferably, the compound is barium hydroxide and the sensor is heated to a temperature not exceeding 78' C.

In a preferred embodiment, the gas sensor is dried at a temperature below the boiling point. Heating to this temperature continues until the solvent content of the sensor, in particular of the catalyst layer or the gas-sensitive layer, has dropped to a predetermined percentage.

It is also preferable for the catalyst to be applied to the surface of the gas-sensitive layer. In this connection the potential for reaction with the gas to be detected is greatest. Moreover, this embodiment has fewest problems in manufacture.

Advantageously, the gas-sensitive layer and/or the electrodes are provided with cut-outs. In this manner it is possible to trim the resistance value of the gas sensor to a specific value. The gas sensor can thus be adapted to the sensitivity of evaluating devices.

In this connection, it is preferable for the cutouts to be produced with the help of a laser beam. Very fine structures can be obtained with a laser beam so that the resistance value can be set with great accuracy.

In this connection, it is an advantage for the gassensitive layer to be given a meandering configuration by the cut-outs. In this manner, a relatively large electrical resistance value can be produced. The resistance changes caused by the gas to be detected are 6 correspondingly large and can be readily detected.

Preferably, the cut-outs are produced prior to applying the catalyst layer. It is thus possible to ensure that the catalyst layer is not heated to a temperature above the required temperature.

Preferably, the two electrodes are applied by means of thick-film or thinfilm technology after the carrier substrate has been cleaned. Extremely accurate structures can be produced using thick-film or thin-film technology. In addition, the spatial extent of the electrodes can be limited relatively accurately. This is of particular advantage when valuable materials, such as gold, are used for the electrodes.

It is also an advantage for the gas-sensitive layer to be vacuumdeposited. In a method of this kind the thickness of the gas-sensitive layer can be set with great accuracy. The gas sensors can be prepared consistently with great accuracy.

Advantageously, the gas-sensitive layer is formed by a layer of tin which is oxidized at a temperature in the range from 400 to 5000C, especially in the range from 440' to 460C. It is easy to apply tin as a layer. Through heating to a temperature in the range mentioned, for example 450C, an oxidation can be achieved. In this connection, a layer of tin oxide (SnOx) is obtained, which has proved to be extraordinarily useful for detecting gas through change in electrical conductivity.

In this connection, the use of chloroplatinic acid (H2PtCl6(6H20)) in aqueous solution as catalyst is preferred. The sensor is then especially sensitive to ammonia.

It is an advantage therein for the sensor to be dried at a temperature in the range from 70C to 150C, in particular in the range from 105C to 1150C, for several hours, in particular 20 to 28 hours. These temperatures allow a very careful drying. The catalyst is then able to develop its advantageous response to ammonia.

Examples of methods in accordance with the invention for the preparation of a gas sensor and a sensor in accordance with the invention made by the method will now be described, by way of example only, with reference to the accompanying drawing, in which:

Fig. I shows a gas sensor and Fig. 2 shows a cross-section through the gas sensor on an enlarged scale.

Referring to the accompanying drawing, a gas sensor 1 comprises a carrier substrate 2, which can be in the form of a ceramic substrate (for example, A1203) or a Sisubstrate with an insulator (nitride or oxide). On the substrate there are two electrodes 3. A gas-sensitive layer 4 is vacuumdeposited between the electrodes 3. It consists of tin oxide (Snox). On the gas-sensitive layer 4 there is a catalyst 5. The gas-sensitive layer 4 is provided with cut-out portions 6, which isolate adjacent portions of the gas-sensitive layer 4 electrically from one another-, so that the gassensitive layer 4 follows a meandering course. similarly, the electrodes 3 can also be provided with cut-out portions (not illustrated).

To prepare the gas sensor, the substrate 2 is first of all cleaned. Then two electrodes 3 consisting of an inert metal, for example platinum or gold, are applied by means of thick-film or thin-film technology, that is to say, they are printed on and thereafter sintered at about 850'C. After renewed cleaning and drying, the substrate 2 equipped with the electrodes 3 is placed in a vacuum system. There, the gas-sensitive layer 4 is vacuumdeposited, for example by means of thin-film technology (reactive vacuum-deposition).. The gas-sensitive layer has an electrical connection with the two electrodes. The gas-sensitive layer here consists of tin (Sn) with a thickness of about 100 nm. After the application of the tin layer, the entire assembly is heated to about 4500C, whereby the Sn-layer is converted into a layer of tin oxide (Sno.). With the help of a laser beam the cut-out portions 6 are then cut into the gas-sensitive layer.

A solution of a catalyst or a catalyst mixture, which later forms the layer 5, is applied to the gassensitive layer. By this means the sensitivity to a selected type of gas is enhanced. The gas-sensitive material is treated with a metal-based catalyst and the 9 catalyst is applied to the gas-sensitive material in the form of a compound of the metal in solution and the sensor is dried by heating without allowing its temperature to rise to a temperature at which the compound decomposes to yield the catalytic metal. The solution may be a solution of the compound in water, a solution of the compound in an alcohol, or a solution of the compound in a mixture of water and an alcohol. The compound may, for example, be a salt or it may, for example, be a base. Examples of suitable metals are platinum and barium. When the metal is platinum, the compound is preferably a platinum salt, preferably a chloride. When the metal is barium, the compound is preferably the base, barium hydroxide.

The following EXAMPLES illustrate treatment with metal-based catalysts in accordance with the invention: EXAMPLE 1 A gas sensor as described above with reference to the accompanying drawing is made by applying chloroplatinic acid (H2PtC16(6H20)) in aqueous solution to the gas-sensitive layer 4. The gas sensor is then dried for about 24 hours at 1150C. This temperature lies below the decomposition point of the chloroplatinic acid. A sensor of this kind is especially selective towards ammonia. As the concentration of ammonia increases the electrical resistance between the two electrodes 3 changes.

EXAMPLE 2

A gas sensor as described above with reference to the accompanying drawing is made by applying barium hydroxide (Ba(OH)2(8H20)) in aqueous solution to the gassensitive layer 4. The gas sensor is then dried for about 24 hours at 780C. This temperature lies below the decomposition point of the barium hydroxide. A sensor of this kind is especially selective towards chlorine and hydrogen chloride. As the concentration of chlorine or hydrogen chloride increases the electrical resistance between the two electrodes 3 changes.

Other metal-based catalysts may be used in the invention to provide sensitivity towards other gases and they are applied as solutions of compounds of the metal followed by heating over an extended period, preferably more than five hours, more preferably more than ten hours, and yet more preferably between twenty and twentyeight hours, without allowing the temperature of the compound to rise to a temperature at which the compound decomposes to yield the catalytic metal itself.

C L A 1 M S.

1. A method for the preparation of a gas sensor, in which two electrodes and a layer of gas-sensitive material connecting the two electrodes are applied to an electrically insulating carrier substrate and the gassensitive material is treated with a metal-based catalyst so as to improve the sensitivity of the sensor, wherein the catalyst is added to the gas-sensitive material in the form of a solution of a compound of the metal and the sensor is heated without allowing its temperature to rise to a temperature at which the compound decomposes to yield the catalytic metal. A method as claimed in claim 1, wherein the is a solution of the compound in water. A method as claimed in claim 1, wherein the is a solution of the compound in an alcohol. A method as claimed in claim 1, wherein the solution is a solution of the compound in a mixture of water and an alcohol.

5. A method as claimed in any preceding claim, wherein the compound is a salt.

6. A method as claimed in any one of claims 1 to 4, wherein 7.

solution 3. solution 4.

the compound is a base.

A method as claimed in any preceding claim, wherein the metal is platinum.

8. A method as claimed in claim 7, wherein the compound is chloroplatinic acid and the sensor is heated to a temperature not exceeding 115' C.

9. A method as claimed in any one of claims 1 to 6, wherein the metal is barium.

10. A method as claimed in claim 9, wherein the compound is barium hydroxide and the sensor is heated to a temperature not exceeding 78 C.

11. A method for the preparation of a gas sensor, in which two electrodes and a gas-sensitive layer connecting the two electrodes are applied to an electrically insulating carrier substrate and a catalyst is added, wherein a metallic salt dissolved in water or in an alcohol is added as catalyst to the gas-sensitive layer and the sensor is heated to a temperature below the boiling point of the metallic salt.

12. A method as claimed in claim 11, wherein the heating of the gas sensor consists of drying at a temperature below the boiling point.

13. A method as claimed in any preceding claim, wherein the catalyst is applied to the surface of the gas-sensitive layer.

14. A method as claimed in any preceding claim, wherein the gas-sensitive layer and/or the electrodes are provided with cut-out portions.

15. A method as claimed in claim 14, wherein the cut-out portions are produced by means of a laser beam.

16. A method as claimed in claim 14 or 15, wherein the gas-sensitive layer is given a meandering configuration by the cut-out portions.

17. A method as claimed in one of claims 14 to 16, wherein the cut-out portions are made before the catalyst layer is applied.

18. A method as claimed in any preceding claim, wherein the two electrodes are applied by thick-film technology after the carrier substrate has been cleaned.

19. A method as claimed in any preceding claim, wherein the two electrodes are applied by thin-film technology after the carrier substrate has been cleaned.

20. A method as claimed in any preceding claim, wherein the gas-sensitive layer is vacuum-deposited.

21. A method as claimed in any preceding claim, wherein the gas-sensitive layer is formed by a layer of tin which is oxidized at a temperature in the range from 4000 to 500'C.

22. A method as claimed in claim 21, wherein the tin is oxidized at a temperature in the range from 440' to 460'C.

23. A method as claimed in any preceding claim, wherein chloroplatinic. acid in aqueous solution is used as catalyst.

24. A method as claimed in claim 23, wherein the sensor is dried at a temperature in the range from 70 to 115'C.

25. A method as claimed in claim 24, wherein the sensor is dried at a temperature in the range from 105' to 115, C.

26. A method as claimed in any preceding claim, wherein batium hydroxide in aqueous solution is used as catalyst.

27. A method as claimed in claim 26, wherein the sensor is dried at a temperature in the range from 700 to 78'C.

28. A method as claimed in any preceding claim, wherein the sensor is heated for more than five hours.

29. A method as claimed in claim 28, wherein the sensor is heated for more than ten hours.

30. A method as claimed in claim 29, wherein the sensor is heated for between 20 and 28 hours.

31. A method for the preparation of a gas sensor, the method being carried out-substantially as herein described.

32. A method as claimed in claim 31 carried out substantially as herein described in EXAMPLE 1.

33. A method as claimed in claim 31 carried out substantially as herein described in EXAMPLE 2.

34. A gas sensor made by a method as claimed in any preceding claim.

35. A gas sensor substantially as herein described with reference to, and as illustrated by, the accompanying drawing.