GB2248306A

GB2248306A - Gas sensing resistance transducer element

Info

Publication number: GB2248306A
Application number: GB9116820A
Authority: GB
Inventors: Peter Mcgeehin
Original assignee: Capteur Sensors and Analysers Ltd
Current assignee: Capteur Sensors and Analysers Ltd
Priority date: 1990-08-06
Filing date: 1991-08-05
Publication date: 1992-04-01
Anticipated expiration: 2011-08-05
Also published as: GB9116820D0; GB2248306B; GB9017209D0

Abstract

The element comprises a porous SnO2 body containing a fine dispersion of a metal such that the resistance variation response characteristic of the transducer is partially suppressed at gas concentrations up to about 10 ppm, while being substantially unsuppressed at concentrations in the approximate range 10 - 300 ppm. The metal may be Pb, Ir, Au, Rh, Pb, Ni or Bi, traces of alkali or alkaline earth metals or a congener of any of these metals. The transducer may also include Pt, Pd, Ir or Rh, As, Sb, P or ammonium. The dispersion is formed by solution impregnation with a salt, preferably a nitrate, and precipitation of the metal, preferably with subsequent heat treatment to control particle size.

Description

TRANSDUCER ELEMENTS This invention relates to transducer elements suitable for use as sensors for the detection and quantitative assessment of various gases, wherein use is made of the change in electrical resistance of the sensor material in response to the presence of certain specific gas constituents in the atmosphere.

A preferred sensor device of this type comprises elements of porous, sintered tin oxide (SnO2) through which a finely divided second phase of pure platinum is dispersed. In the production of such devices, it is known to impregnate the SnO2 body with a platinum (Pt) compound in solution, with subsequent precipitation to produce the dispersion.

Electrodes are then added by, for example, vacuum deposition of gold or other metal.

Elements of this type afford ready means of quantitatively detecting the presence of a variety of gases, e.g. CO and H2. They are finding important applications for monitoring combustion processes in industry, as well as in the domestic sphere.

It is the higher concentrations of the gases to be monitored that represent the most important field of activity in terms of health and industrial hazards, for example from about 10 parts per million (ppm) of the atmosphere up to about 300 ppm. In this context, the sensors are often required to actuate alarm or shut-down means in industrial processes and other situations. At very low concentrations, e.g. up to 10 ppm, while the capability of detection is clearly desirable, such contents may in many cases be within the tolerated plant error or even be deemed innocuous.

A practical drawback of the types of sensors referred to is that, at these low gas concentrations, their sensitivity is extremely high, i.e. small changes in concentration produce large resistance variations; whereas in the presence of those higher concentrations that are of greatest interest in practice, the slope of the resistance/gas concentration curve (with resistance on the ordinate and gas concentration on the abscissa) is very much reduced. At sufficiently high values of gas concentration this amounts to a saturation effect, i.e. a value of the slope that is substantially zero.

In other words, the sensor is at its most sensitive under those conditions in which it is least useful, at the expense of its sensitivity under the conditions in which it is most likely to be needed.

We have realised that the controlled addition of various selected substances, in addition to or in place of the platinum, and preferably in conjunction with the solution impregnation procedure in which these substances are introduced onto the surface of the pores of the SnO2 body, leads to modification of the response characteristic of the element, such that the response of the transducer to the gas constituent, in terms of the resistance of its sensing element, is partially inhibited in the region of up to about 10 ppm, while remaining high in the concentration range upwards of 10 ppm.

This partial inhibition of the response at low gas concentrations is expressed as a reduction in the slope of the characteristic; on the other hand the range of gas concentrations for which the response remains unsaturated is substantially extended. In other words, a markedly larger proportion of the total resistance change (normally in the form of decreasing resistance) now occurs over those regions that are of the greatest importance in practice for atmosphere control purposes.

According to the invention in a first aspect, in a gas sensing resistance transducer element in the form of a porous tin oxide (sun02) body having a finely divided metal dispersion therein, the dispersion comprises at least one phase consisting of a metal such that the electrical resistance of the transducer varies in response to the concentration of a given gas applied to the transducer, with at least one said metal being selected from a group such as to cause the variation in resistance to be at least partially inhibited at concentrations of the said gas below about 10 parts per million.

The said group of metals preferably includes Pb; Ir; Au; Rh; Pd; Ni; Bi; traces of one or more alkali metals; traces of alkali earth metals; a congener of any of the foregoing.

The dispersion may have at least two said phases, one of which consists of an element selected from Pt; Pd; Ir; Rh, the other phase or phases being selected from the said group.

According to the invention in a second aspect, a gas sensing resistance transducer element comprises a tin oxide (no2) body containing a dispersion of finely divided Pt and at least one additive such that the slope of the response characteristic of the element is reduced in the presence of concentrations of a given gas up to about 10 parts per million, with the slope being numerically greater than zero at gas concentrations up to beyond 100 parts per million. Using such an element, the range of gas concentration over which there is a significant slope in the resistance/gas concentration characteristic is extended to above 300 parts per million.

According to the invention in a third aspect, a method of making a gas sensing transducer element includes making a porous, sintered tin oxide (SnO2), and subsequently performing at least one dispersion stage, each dispersion stage comprising impregnating the tin oxide body with a solution containing at least one compound of at least one metal, and precipitating the metal or metals from solution on the surface of the pores of the body to produce a finely divided dispersion, the said metal or metals being so chosen that the electrical resistance of the transducer element is variable in response to the concentration of a given gas applied to the transducer at least above a concentration of about 10 parts per million, but so that such variation is at least partly inhibited at concentrations below about 10 parts per million.

According to the invention in a fourth aspect, a method of making a transducer element includes impregnating a tin oxide (no2) body with a Pt compound to produce a dispersion of finely divided Pt, and also includes introducing at least one additive to the Pt or Pt compound, so as to modify the response characteristic of the element in such a way that variation of its electrical resistance, in response to the concentration of a given gas applied to the element, is at least partially inhibited at concentrations in the approximate range 0 - 10 parts per million, but substantially uninhibited in the approximate range 10 - 300 parts per million.

According to the invention in a fifth aspect, a method of making a transducer element includes making a porous, sintered tin oxide (no2) body and adding one or more chemical elements to the surface of the pores of the body by means of a solution impregnation process, the said chemical elements substantially or wholly excluding Pt, and at least one of the said chemical elements being such as to promote electronic equilibrium between a gas or gases, applied to the element for detection of the gas or gases, and the body of the transducer element. It will be noted that in this case, platinum is not present, at least to any significant extent, in the resulting transducer element.

The invention also embraces a gas sensing transducer array comprising a plurality of sensor elements having electrical connections for connection of the array to signal processing means, in which at least one of the elements is an element according to the invention or made by a method according to the invention.

A relatively large number of additive or platinumsubstitutive substances are found to give this effect of reducing the response at low concentrations.

The following are examples of such additives and of the manner of their incorporation, each being suitable for use either alone or in any suitable combination with one of more of the others: (1) The additional phase or phases modifying the surface of the pores comprise fine particles, produced from mixed metal salts which are dispersed in the porous Sino2 and which are then co-precipitated and decomposed, in a manner which is otherwise similar to the established production process. Examples of salts from which the additional phase or phases may be produced are those of Pb, Ir, Au, Rh, Pd, Ni and Bi.

The preferred salts are nitrates of any or all of these metals: the salts may be employed individually or mixed together, and/or used in conjunction with a suitable platinum salt.

(2) Traces of an alkali or one or more alkaline earth metals, preferably in the form of their nitrates or chlorides are introduced, with or without other metals as set out in (1).

(3) Traces of decomposed salts of semi-metals, such as As, Sb or P, may be introduced to the metals referred to in Examples 1 and 2. The ammonium salts of the semi-metals are preferred.

(4) In any of the examples in (1), (2) or (3), the conditions of a subsequent heat treatment may be used to control the particle size of the modified metal and the extent of interaction between the constituents.

The above materials are quoted by way of example, but benefit may similarly be derived by the use of many of their respective congeners in the atomic table.

Transducer (sensor) elements according to the invention, and/or made by one or more methods according to the invention, can readily be disposed in sensor arrays to give sensitive determinations over a wide range of gas concentrations. If the array includes at least one sensor element of the conventional kind, i.e. one in which any metal such as to cause any significant inhibition of the variation in resistance at gas concentrations below about 10 ppm is absent or substantially absent, then the array can be made sensitive to gas concentrations over the whole range, from zero up to a value at least as high as 300 ppm.

The array will have suitable electrical connections, in the usual way, for its connection to appropriate signal processing means for analysing, displaying, recording or measuring the signals from the various elements in the array.

Claims

1. A gas sensing resistance transducer element in the form of a porous tin oxide (no2) body having a finely divided metal dispersion therein, wherein the dispersion comprises at least one phase consisting of a metal such that the electrical resistance of the transducer varies in response to the concentration of a given gas applied to the transducer, with at least one said metal being selected from a group such as to cause the variation in resistance to be at least partially inhibited at concentrations of the said gas below about 10 parts per million.

2. A transducer element according to Claim 1, wherein the said group of metals includes Pb; Ir; Au; Rh; Pd; Ni; Bi; traces of one or more alkali metals; traces of alkali earth metals; a congener of any of the foregoing.

3. A transducer element according to Claim 1 or Claim 2, having at least two said phases, one of which consists of an element selected from Pt; Pd; Ir; Rh, the other phase or phases being selected from the said group.

4. A transducer element according to any one of Claims 1 to 3, including traces of at least one semi-metal added to at least one metal selected from the said group.

5. A gas sensing resistance transducer element comprising a tin oxide (SnO2) body containing a dispersion of finely divided Pt and at least one additive such that the slope of the response characteristic of the element is reduced in the presence of concentrations of a given gas up to about 10 parts per million, with the slope being numerically greater than zero at gas concentrations up to beyond 100 parts per million.

6. A method of making a gas sensing transducer element, including making a porous, sintered tin oxide (no2), and subsequently performing at least one dispersion stage, each dispersion stage comprising impregnating the tin oxide body with a solution containing at least one compound of at least one metal, and precipitating the metal or metals from solution on the surface of the pores of the body to produce a finely divided dispersion, the said metal or metals being so chosen that the electrical resistance of the transducer element is variable in response to the concentration of a given gas applied to the transducer at least above a concentration of about 10 parts per million, but so that such variation is at least partly inhibited at concentrations below about 10 parts per million.

7. A method according to Claim 6, wherein the or each compound is selected from: compounds of Pb, Ir, Au, Rh, Pd, Ni, Bi, or congeners thereof; traces of compounds of alkali metals or congeners thereof; traces of compounds of alkali earth metals or congeners thereof.

8. A method according to Claim 6 or Claim 7, wherein the body is impregnated with at least two compounds, one of which is a salt of Pt.

9. A method according to any one of Claims 6 to 8, wherein at least one said compound is a salt.

10. A method according to Claim 9, wherein at least one said compound is a nitrate.

11. A method according to Claim 7, or any one of Claims 8 to 10 when dependent on Claim 7, in which traces of a chloride of at least one alkali or alkali earth metal or a congener thereof are introduced.

12. A method according to any one of Claims 6 to 11, including adding traces of at least one salt containing a semi-metal.

13. A method according to Claim 12, wherein the salt containing a semi-metal is an ammonium salt.

14. A method according to any one of Claims 6 to 13, including heat treatment subsequent to the dispersion stage or stages to control particle size and the extent of interaction between the constituents.

15. A method of making a transducer element, including impregnating a tin oxide (SnO2) body with a Pt compound to produce a dispersion of finely divided Pt, and also including introducing at least one additive to the Pt or Pt compound, so as to modify the response characteristic of the element in such a way that variation of its electrical resistance, in response to the concentration of a given gas applied to the element, is at least partially inhibited at concentrations in the approximate range 0 - 10 parts per million, but substantially uninhibited in the approximate range 10 - 300 parts per million.

16. A method of making a transducer element, including making a porous, sintered tin oxide (no2) body and adding one or more chemical elements to the surface of the pores of the body by means of a solution impregnation process, the said chemical elements substantially or wholly excluding Pt, and at least one of the said chemical elements being such as to promote electronic equilibrium between a gas or gases, applied to the element for detection of the gas or gases, and the body of the transducer element.

17. A gas sensing transducer array comprising a plurality of sensor elements having electrical connections for connection of the array to signal processing means, the elements including at least one element according to any of Claims 1 to 5 and/or at least one element made by a method according to any of Claims 6 to 16.

18. A transducer array according to Claim 17, further including at least one element in which any metal such as to cause significant inhibition of variation in electrical resistance of the element at gas concentrations below about 10 parts per million, is absent.