GB2267968A - Making resistance gas sensors. - Google Patents

Abstract

A resistance gas sensor which produces a significant resistance change on detecting low connotations of a gas such as carbon monoxide in air at ambient temperatures has a porous tin oxide zone containing a dispersion of precious metal on the surface of its pores. The sensor is made by impregnating porous tin oxide with a complex of precious metal salts, precipitating or co- precipitating the salts and heating the body to a temperature between the lowest decomposition temperature of the salt complex and at least 600 DEG C for a time sufficient to decompose the complex and deposit the precious metal. A heat treatment step, which may be the above step is performed at a temperature, and for a time which is an inverse function of that temperature so that the transducer resistance increases and, in the presence of a specific gas, in air at room temperature undergoes a specific reduction of resistance. A sensor for carbon monoxide showed a 50% resistance reduction to 95k OMEGA in one minute in response to 200ppm carbon monoxide in air at room temperature. <IMAGE>

Description

GAS SENSING TRANSDUCER ELEMENTS This invention relates to gas sensing transducer elements, of the resistance type in which the electrical resistance of the element changes in the presence of a particular gas or gases. The invention also relates to gas sensors that comprise such a transducer element, and to methods of making the transducer element.

Our United Kingdom patent specification GB 2 248 306A describes a method of making a gas sensor in the form of a porous tin dioxide transducer element in which the surface of the pores have been impregnated with platinum or other phases, the sensor having a body or thick film of porous tin oxide which, after being treated, has a metallic phase dispersed on the surface of the pores. The said patent specification teaches methods by which the response characteristics of the sensor over particular gas concentration ranges can be predetermined by the use of more than one metallic element in the dispersed phase on the surface of the pores.

As described in GB 2 248 306A, the additional phase or phases modifying the surface of the pores of the sensing element comprise fine particles produced from mixed metal salts, which are dispersed in the porous tin oxide body and then co-precipitated and decomposed in a manner which is otherwise generally similar to established methods of production. The final step in the dispersion of the metallic phase can be achieved by a heat treatment such as to obtain thermal decomposition of the co-precipitated salts. This is carried out at temperatures low enough, and over periods of time short enough, to avoid solid solution reactions between the tin oxide and metallic phases such as to give rise to the formation of unwanted further phases damaging to the gas response.

The above procedure can be used in order to deposit a single phase, such as platinum, on the surface of the pores of the sensing element. In this case the salt is precipitated, rather than co-precipitated, prior to heat treatment. Examples of a suitable time and temperature regime in the case where the metallic phase consists only of platinum are 350"C for one hour, or 600"C for 15 minutes.

Sensors made from 50% porous sintered tin oxide bodies, with platinum as the dispersed metallic phase, exhibit the conventional response (that is to say not the improved response disclosed in GB 2 248 306A).

However, for some purposes where a particular target gas is to be detected, for example in the detection of carbon monoxide as a toxic gas which may be accidentally present in the atmosphere, or present as a result of the early stages of a fire which it is desired to detect before it becomes a conflagration, sensitivity is required at very low concentrations of the target gas.

We have found that the response of sensors of the kind described above, when they are operating as resistors sensitive to carbon monoxide without being heated (i.e.

working at ambient temperature, typically in the range 3 - 35or) can be inhibited by relatively high concentrations of organic vapours that may be present in the surrounding atmosphere. Examples of such vapours are acetic acid, methylated spirit and siliconcontaining organic molecules. Additionally, after exposure to these vapours, these sensors exhibit long response times, typically many minutes, and their sensitivity is poor when expressed as a fractional change in resistance for a given change in gas concentration.

In air at ambient (room) temperatures, a typical porous tin oxide gas sensor made by a method such as is described in GB 2 248 306A has a normal resistance of the order of 10 kilohms or less. Its response to the presence of the gas to be detected (the "target gas"), e.g. carbon monoxide, in air at these temperatures, is rather slow, and can be too slow to be satisfactory in certain applications such as fire detection. In addition, because the normal resistance is relatively low, any reduction in response to the target gas, even if it represents a substantial percentage reduction in the resistance, will be only relatively small in absolute terms.

It is therefore desirable to improve the response to the target gas, not only by decreasing the response time of the sensor but also by enabling the resistance of the latter to fall by an amount (in the presence of the target gas at quite low concentrations at ambient temperatures) which is large enough to avoid being masked by the presence of inhibiting vapours as described above.

According to the invention in a first aspect, a method of making a gas sensing transducer element includes the steps of: (i) providing a body comprising a porous tin oxide zone, at least as a surface layer; and (ii) impregnating the porous tin oxide with a solution of at least one complex of precious metal salts, precipitating or co precipitating the said salts, heating the body to a temperature between the lowest decomposition temperature of a said salt and at least 600"C, and maintaining the said temperature for a time such as to decompose the complex, thereby depositing metallic particles in the pores of the tin oxide, wherein the method includes performing at least one heat treatment step at a treatment temperature, and for a time which is an inverse function of the treatment temperature, such as to increase the electrical resistance of the resulting transducer element to an extent such that, when the element is exposed to a specific gas in air at room temperature, its resistance undergoes a significant reduction.

The said time is an inverse function of the treatment temperature in the sense that the larger one of these parameters is, the smaller will be the other. The term "significant" is used here in the sense that it is large enough to constitute a decisive, easily measurable change that reveals the presence of relatively small quantities of the target gas in air at ambient temperature regardless of the presence of any inhibiting factors such as vapours in the air.

In this regard, the absolute value of the resistance drop in a sensor according to the invention can be at least an order of magnitude higher than in the prior art sensors discussed above, i.e. it is measurable in tens of kilohms rather than in an amount less than 10 kilohms. In this connection, the method of the invention can be made to give the transducer element a normal value of resistance in air at room temperature which is, in general terms, at least an order of magnitude greater than in the above mentioned prior art sensors, i.e. instead of being in a range of (say) 5 - 20 kilohms, it can be made to lie in a range of perhaps 50 - 200 kilohms.

Thus for example, in various gas sensors comprising transducer elements made according to the invention, the resistance in air at room temperature has been observed to fall by 100 or 200 kilohms, a reduction of about two-thirds, in the presence of 200 parts per million (ppm) of carbon monoxide. Such a reduction is significant.

It should be noted that the target (or "specified") gas need not be carbon monoxide. The transducer element of the invention can be adapted, using known technology, for sensing a wide variety of target gases, or mixtures of target gases, while remaining within the ambit of the invention.

According to the invention in a second aspect, a gas sensor comprises a resistive transducer element having a body with a porous tin oxide zone which contains deposited precious metal in metallic form and which has undergone heat treatment such that its electrical resistance has been enhanced by the metallic deposit and heat treatment, whereby its resistance undergoes a significant reduction in the presence of 200 parts per million of carbon monoxide in the air at room temperature.

The concentration value of 200 ppm is here chosen simply as an arbitrary figure, and in no way implies that the response is not significant at other concentrations of the target gas. Similarly, any sensor having a transducer element made by the method of the invention in its first aspect is of course within the scope of the invention.

The body of the transducer element may take any suitable form. In two preferred but non-limiting examples, it comprises either a porous tin oxide pellet, or an alumina tile carrying a plurality of precious metal electrodes spaced apart on the tile. In the former case, the porous tin oxide zone consists of the pellet itself. In the case of a transducer element comprising a substrate (which may be an alumina tile as mentioned above, or in any other suitable form and of any other suitable material), having electrodes arranged on it, the porous tin oxide layer is applied in the form of at least one surface layer bridging the electrodes.

In some versions of the method of the invention, step (i) includes the further step of treating the body with an aqueous tin oxide sol, and then heating it for a predetermined time and at a predetermined temperature so as to form a surface layer of tin oxide on the body.

This sol treatment (which may be repeated to a total of up to 30 times, to build up a desired layer thickness) is a preferred feature of the method when the body consists of a substrate with electrodes as mentioned above. It is also possible to employ the sol treatment when a tin oxide pellet is used, though in this case it is not thought to be necessary.

The sol treatment may be applied directly on the substrate; alternatively, a sub-layer of porous tin oxide may be applied directly on the substrate, bridging the electrodes, prior to treatment with the sol. This sub-layer may for example be printed (e.g.

by silk-screen printing), or it may be applied from a suitable tape, typically a plastics-bound tape. The sub-layer is of course fired by heating for a suitable time and at a suitable temperature before the sol treatment is commenced. Its thickness is preferably in the inclusive range 10 to 400 microns.

The effect of the sol treatment is in fact to reduce the resistance, but this is then increased by subsequent heat treatment in step (ii). For example, a typical value of resistance at the end of step (i), when the sol treatment has been applied 30 times, is 30 kilohms, this rising typically to 60 kilohms by the end of step (ii).

A tape-based transducer element which is essentially open-circuit at room temperature, has a typical resistance value of 200 kilohms after sol treatment, but a higher value than this after step (ii).

It can be seen from the foregoing that there is at least one heating stage in the method of the invention.

Indeed, in some cases there may be only one, this then being the heating stage of step (ii). We have found, surprisingly, that the dramatic increase in the normal resistance of the transducer element at ambient temperatures, accompanied by the substantial reduction in resistance in response to the presence of target gas, as discussed above, is obtained by suitable choice of the time and temperature of at least one heat treatment step during the manufacturing process. Where there is only one heating stage, this stage will be that heat treatment step.

It follows that the heat treatment step or steps may be, or include, at least one step that has to be carried out at least partly for firing purposes. By contrast with the method described in GB 2 248 306A, it will be seen that, in the meth-od of this invention, the time and temperature of the single heating stage, and/or of at least one other heating stage, are chosen to be such as to give the resistance characteristics of the transducer element of the invention. In particular, over and above the variables in the method already established for dispersing the precipitated salt or salts within the pores of the sensing element, the precise nature of this heat treatment has a substantial bearing on the magnitude and speed of response of the sensor, particularly in relation to its response to the presence of carbon monoxide.

We have observed an approximate correlation between the electrical resistance of the sensor and its rate of response: typically (but not always) the sensors with higher resistance show a deeper and a faster rate of response.

Step (ii) may be repeated up to 9 times.

In addition to the above, we have also found, again surprisingly, that when a treatment step is carried out at a temperature no greater (and optionally lower) than the original decomposition temperature of the precipitated or co-precipitated salts, its electrical resistance increases still further. Its response to carbon monoxide is enhanced, and its speed of reaction to a step function change in carbon monoxide concentration is further increased.

Accordingly, the method of this invention preferably includes the further step of applying an additional heat treatment step subsequent to step (ii), at a temperature no greater than that employed for heating in step (ii).

In general terms, it has been found that, in the transducer element having a porous tin oxide zone in which the surface of the pores are dotted with multiple metallic dispersions (e.g. where it comprises a porous tin oxide body, the element being substantially as described in GB 2 248 306A), the nature of this additional heat treatment, as well as that of any other heat treatment step, substantially determines the extent and rate of the response of the sensor, particularly to carbon monoxide.

The additional heat treatment can also be beneficial to the recovery of response capability where this has become inhibited through coming into contact with molecules in the atmosphere, other than those to which the sensor is intended to respond. As the sensor response is inhibited, its resistance in air drifts (typically to lower values), while the change in its resistance when exposed to a step function change in concentration of the test gas reduces in magnitude. In extreme cases of response inhibition, this resistance change can become negligible.

Embodiments of the invention will now be described, by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross section of one form of transducer according to the invention; and Figure 2 is a flow diagram showing various alternative ways of carrying out the method of the invention.

In general terms, the method illustrated in Figure 2 comprises two main steps (i) and (ii), followed by an optional additional heat treatment, to give a transducer element having a high electrical resistance (for example 50 kilohms or more) in air at room temperature and sensitive to carbon monoxide in the air at room temperature.

In step (i), the starting point is either a porous SnO2 pellet (not shown in the drawings) or an alumina tile 10, Figure 1, having platinum or gold electrodes 12 spaced apart by a distance which may for example be 250 microns. If an SnO2 pellet is used, this is fired without densification, with or without doping, for example with antimony at up to 1000 ppm, and the pellet constitutes a porous tin oxide zone.

In Figure 1, the transducer element, 20, comprises the substrate 10, the electrodes 12, and a porous tin oxide zone. This zone consists of a sub-layer 14 bridging the electrodes, together with a porous surface layer 16 which is applied on the sub-layer 14 so as to overlie, and/or coat the surface of the pores of, the latter.

The sub-layer 14 may be omitted, the surface layer 16 then being applied directly on to the substrate 10.

The sub-layer 14 is either silk-screen printed on to the tile 10 and fired at a suitable temperature and for a suitable time, or applied from a plastics-bound tape and then fired.

The surface layer 16 is applied by treatment with an aqueous sol of tin oxide, which is left to dry and then fired in a heat treatment stage for a suitable time and at a temperature of up to about 600 C. This sol treatment and heat treatment may be repeated up to nine times, to give the desired thickness of the porous tin oxide zone, typically from 10 to 400 microns.

In step (ii), the pellet or tile, previously prepared by step (i), is impregnated with a solution containing at least one compound of at least one precious metal, the salt or salts then being precipitated or coprecipitated in a known manner, after which a heat treatment stage is performed such as to decompose the salts and deposit a dispersion of the metal or metals on the surface of the tin oxide pores.Step (ii) is generally as described in GB 2 248 306A and therefore need not be described in any greater detail here, except that the time, and the temperature Tii for the heating stage, may be (and, if this is the only heating stage, will be) chosen to be such as to enhance the resistance of the final transducer element and its response (in terms of resistance drop and/or response time) to the presence of carbon monoxide in air at ambient temperature, in the manner already discussed above.

Step (ii) may optionally be repeated from one to nine times, after which, or instead of which, an additional heat treatment step may be performed at a temperature Tiii the same as, or less than, the temperature Tii in the heat treatment stage of step (ii), in order to enhance still further the resistance of the transducer element and its response to carbon monoxide in air at room temperature.

Several examples are given below by way of illustration.

Example 1 A 50% porous tin oxide sensor, on the surface of the pores of which platinum has been dispersed, was subjected to a heat treatment of 350"C for one hour. It had a resistance in air at 25"C of 7.5 kilohms. When exposed to 200 parts per million of carbon monoxide at the same temperature, the resistance fell to 4.1 kilohms. The response time of the sensor was of the order of five minutes when the change in carbon monoxide concentration was presented to the sensor as a step function. Such a response time is rather long for some of the applications, particularly fire detection, in which it might be employed.

Subsequently this same sensor was given an additional heat treatment in which it was heated at 250"C for 8 hours. Its normal resistance, i.e. its resistance in air at room temperature, rose to 150 kilohms, falling to approximately 50 kilohms when exposed to 200 parts per million of carbon monoxide at the same temperature, a drop of 100 kilohms (about 67%) as compared with only 3.4 kilohms without the additional heat treatment.

The response time was also substantially improved, being less than one minute compared with the five minutes previously observed. This more rapid response approaches that required for the more demanding applications contemplated for the sensor.

Example 2 A sensor with similar physical characteristics to the sensor described in Example 1 and again treated with platinum metal, was exposed to test gases and gases containing various molecules known to inhibit the response of catalytic gas detectors. As a result of this treatment, the resistance of the sensor declined to 2 kilohms, and it became substantially insensitive to carbon monoxide. However, when the sensor was heated at 275"C for 90 minutes, its resistance in air at room temperature increased to 290 kilohms and fell, in 200 ppm of carbon monoxide, by 195 kilohms (67.2%) to 95 kilohms in less than one minute.

Example 3 An alumina tile, bearing a pair of interdigitated gold electrodes 250 microns apart, is wetted with an aqueous sol containing 100 grams/litre of SnO2 so as to cover the electrode array. It is then left to dry, after which it is fired to 600"C for 15 minutes in air. This process is repeated a number of times (up to a total of 30). The tin oxide layer which is thus built up is then impregnated in the manner described in the document GB 2 248 306A, and allowed to dry. It is then heated once again, to 600"C for 15 minutes, so as to decompose the complex metallic platinum.

The resulting transducer element then has a resistance range of 1000 to 50,000 ohms measured between the electrodes, and is highly sensitive to the presence of carbon monoxide in air at room temperature.

Example 4 A sensor comprises a transducer element prepared in the same way as in Example 3, except that the treatment with the precious metal complex and decomposition to metallic platinum is repeated a number of times (between 2 and 10 times).

The sensor thus prepared has a resistance range of 1000 to 50,000 ohms and is sensitive to carbon monoxide in air at room temperature.

Example 5 A sensor comprises a transducer element prepared by firing a layer of -porous tin oxide having a thickness which is typically from 10 to 400 microns, and which may for example be formed by silk-screen printing or from a plastic bound tape, on to an alumina substrate with interdigitated platinum electrodes 250 microns apart. Precious metal is then applied as in Example 3 or Example 4.

The sensor thus prepared has a resistance range of 1000 ohms to 100,000 ohms and is sensitive to carbon monoxide in air at room temperature.

Example 6 A sensor comprises a transducer element prepared by firing a layer of porous tin oxide, e.g.

from a plastic bound tape, on to an alumina substrate, impregnating this with tin oxide from a sol as in Example 3, and adding precious metal as in Example 3 or Example 4. The sensor thus prepared has a resistance range of 1000 ohms to 100,000 ohms, and is sensitive to carbon monoxide in air at room temperature.

It will be understood that the metallic dispersion in the pores may be of platinum alone, platinum with other metallic elements, or other suitable combinations of elements, and that it may be oxidised at its surface.

Since the heat treatment in step (ii) follows the decomposition of the precipitate in the pores, it can be accomplished during a single heat treatment cycle comprising that heat treatment stage and any or all subsequent heat treatment stages, in succession.

Alternatively the additional heat treatment step, if carried out, may constitute a separate cycle. This can be performed over a shorter or a longer time depending on the combination of properties required, for example resistance, response speed, response magnitude.

The depth and speed of response can be measured with the sensor working in the temperature range 0 to 1500C, but more particularly when it is operating at ambient temperature. This can be up to 20"C above actual room temperature.

It will be seen from the foregoing that: the invention provides heat treatment, and single or repeated treatments with tin oxide sol and/or precious metals derived from a complex, such as to determine the depth of response and rate of response to CO (in addition to variables not described relating to the production process); and the additional heat treatment step, and again single or repeated treatments as mentioned above, leads to recovery of the response of sensors exhibiting an inhibited response; and there is an approximate correlation between the speed and magnitude of the sensor response, and sensor resistance after heat treatment.

Claims (20)

1. A method of making a gas sensing resistance transducer element, including the steps of: (i) providing a body comprising a porous tin oxide zone, at least as a surface layer; and (ii) impregnating the porous tin oxide with a solution of at least one complex of precious metal salts, precipitating or co precipitating the said salts, heating the body to a temperature between the lowest decomposition temperature of a said salt and at least 600 C, and maintaining the said temperature for a time such as to decompose the complex, thereby depositing metallic particles in the pores of the tin oxide, wherein the method includes performing at least one heat treatment step at a treatment temperature, and for a time which is an inverse function of the treatment temperature, such as to increase the electrical resistance of the resulting transducer element to an extent such that, when the element is exposed to a specific gas in air at room temperature, its resistance undergoes a significant reduction.

2. A method according to Claim 1, wherein the body is a porous tin oxide pellet, fired without densification.

3. A method according to Claim 2, wherein the tin oxide is doped with antimony at up to 1000 parts per million.

4. A method according to any one of the preceding Claims, wherein step (i) includes the further step of treating the body with an aqueous tin oxide sol, and then heating it for a predetermined time and at a predetermined temperature so as to form a surface layer of porous tin oxide on the body.

5. A method according to Claim 4, wherein the body treated with the tin oxide sol is heated in air for 15 minutes at 600"C.

6. A method according to Claim 4 or Claim 5, wherein the step of treating the body with tin oxide sol and then heating it is performed up to 30 times.

7. A method according to any one of Claims 4 to 6, wherein step (i) comprises providing a substrate having a plurality of precious metal electrodes spaced apart thereon, and applying the said porous tin oxide zone on the substrate in the form of a layer bridging the electrodes.

8. A method according to Claim 7, wherein the substrate is in the form of a tile.

9. A method according to Claim 7 or Claim 8, wherein the substrate is of alumina.

10. A method according to any one of Claims 7 to 9, wherein the electrodes are spaced apart by 250 microns.

11. A method according to any one of Claims 7 to 10, wherein the step of treating the body with sol is performed directly on the substrate.

12. A method according to any one of Claims 7 to 10, wherein step (i) further includes applying directly to the substrate a sub-layer of porous tin oxide bridging the electrodes prior to treatment with the sol.

13. A method according to Claim 12, wherein the sublayer is applied by printing and firing.

14. A method according to Claim 12, wherein the sublayer is applied to the substrate from a tape and then fired.

15. A method according to any one of Claims 12 to 14, wherein the sub-layer is applied to a thickness in the range 10 to 400 microns.

16. A method according to any one of the preceding Claims, wherein step (ii) is repeated up to 9 times.

17. A method according to any one of the preceding Claims, including the further step of applying an additional heat treatment step subsequent to step (ii), at a temperature no greater than that employed for heating in step (ii).

18. A gas sensor comprising a resistive transducer element having a body with a porous tin oxide zone which contains deposited precious metal in metallic form and which has undergone heat treatment such that its electrical resistance has been enhanced by the metallic deposit and heat treatment, whereby its resistance undergoes a significant reduction in the presence of 200 parts per million of carbon monoxide in the air at room temperature.

19. A gas sensor according to Claim 18, wherein the said reduction is in excess of 50%.

20. A gas sensor made by the method of, and having the characteristics described in, any one of the Examples set forth in the foregoing description.