GB2149122A - Improvements in or relating to sensors - Google Patents

Abstract

The present invention relates to sensors and more particularly to sensors suitable for use in gases and gaseous mixtures. According to one aspect of the present invention there is provided a sensor suitable for use in a gas or gaseous mixture which sensor includes a gas sensitive material having an electrical characteristic variable in response to the presence of a gas and containing or comprising a complex oxide of niobium being a material having a general formula: A1-x Bx Nb2 O6 where A, B = Pb, Ba or Sr and 1>x>/=O or a general formula: A6 Bx Nb10-x O30. where: A is divalent (e.g. Sr, Ba or Pb) or trivalent (e.g. Bi) or tetravalent (e.g. Zr and Ce) and A may be partially substituted by a monovalent element such as K, Na, Li or Co and the value of x depends upon the valencies of 8 and A and is in the range 4>x>/=O. B may be chosen from divalent transition metals (e.g. Co, Ni and Mn) or trivalent elements (e.g. Fe) or tetravalent elements (e.g. Ti). B can be an element capable of exhibiting a variable valency in the solid. <IMAGE>

Description

SPECIFICATION Improvements in or relating to sensors The present invention relates to sensors and more particularly to sensors suitable for use in gases and gaseous mixtures.

According to one aspect of the present invention there is provided a sensor suitable for use in a gas or gaseous mixture which sensor includes a gas sensitive material (as hereinafter defined) containing or comprising a complex oxide of niobium (as hereinafter defined).

In this Specification the term "a complex oxide of niobium" means a material of the general formula: At X Bx Nb2 Of3 where A, B = Pb, Ba or Sr and 1 > x > O or a general formula: A6 Bx Nub10 x 030.

where A is divalent (e.g. Sr, Ba or Pb) or trivalent (e.g. Bi) or tetravalent (e.g. Zr and Ce) and A may be partially substituted by a monvalent element such as K, Na, Li or Cu and the value of x depends upon the valencies of B and A and is in the range 4 > xaO. B may be chosen from divalent transition metals (e.g. Co, Ni and Mn) or trivalent elements (e.g. Fe) or tetravalent elements (e.g. Ti). B can be an element capable of exhibiting a variable valency in the solid.

(It is to be understood that some materials falling within the general formulae hereinbefore given may have a tungsten bronze structure).

In one embodiment of the present invention a sensor comprises a gas sensitive material, (as hereinafter defined) containing or comprising a complex oxide of niobium (as hereinbefore defined), and two or more electrodes in communication with the gas sensitive material, and said gas sensitive material is arranged so as to be capable of being contacted with a gas or gaseous mixture.

A sensor in accordance with the present invention may be used as a gas sensor in quantitative and/or qualitative determinations with gases and gaseous mixtures.

The electrodes may be in direct communication with the gas sensitive material by being in contact therewith.

In this Specification the term "gas" embraces a gas as such and any material which may be present in a gaseous phase, one example of which is a vapour.

Also in this Specification the term "gas sensitive material" means a material which is gas (including vapour) sensitive in respect of an electrical property of the material.

It will be appreciated that the resistance and/or capacitance and/or impedance of the gas sensitive material depends upon the gas or gaseous mixture contacting the gas sensitive material. Thus, by measuring the resistance and/or capacitance and/or impedance of the gas sensitive material the composition of a gas or gaseous mixture can be sensed.

Since the resistance and/or capacitance and/or impedance of the gas sensitive material tends also to be temperature dependant, the sensor also preferably includes a temperature sensing means.

The sensor may also, optionally, include a heating means to enable operating temperature to be adjusted and/or contaminants to be burnt off if required.

It is to be understood that the sensitivity of a gas sensitive material within the general formulae hereinbefore given to a particular gas will depend upon the composition of the gas sensitive material. Thus, by selection of the composition of the gas sensitive material its response to a particular gas may be chosen.

The resistance and/or conductance and/or impendance may be measured directly. Alternatively, the measurement may be carried out indirectly by incorporating the sensor in a feedback circuit of an oscillator such that the oscillator frequency varies with composition of the gas or gaseous mixture. Gas composition may then be determined using an electronic counter. The signal thus produced may be used to modulate a radio signal and thereby be transmitted over a distance (e.g. by telemetry or as a pulse train along an optical fibre).

Examples of gases and gaseous mixtures which have been used with a sensor in accordance with the present invention are 02 and low concentrations in air of H2, C2H4, CO, CH4, NH3. C3H8 and other hydrocarbons, oxides of sulphur, oxides of nitrogen. Cl2 and H2S.

According to another aspect of the present invention there is provided a method for effecting determinations in a gas or gaseous mixture which comprises contacting a sensor with the gas or gaseous mixture and measuring the electrical response of the sensor, said sensor including a gas sensitive material containing or comprising a complex oxide of niobium (as hereinbefore defined).

In one embodiment of the immediately preceding aspect of the present invention the sensor comprises a gas sensitive material containing or comprising a complex oxide of niobium (as hereinbefore defined), and two or more electrodes in communication with the said gas sensitive material, said gas sensitive material and said electrodes being in contact with the same gas or gaseous mixture.

It is preferred that the gas sensitive material has porosity to give a satisfactory surface area for contact with a gas or gaseous mixture when in use.

The gas sensitive material may, for example, be prepared from a mixture of powders of appropriate starting materials.

It will be understood that "appropriate starting materials" in this Specification means materials which can be processed to give the required gas sensitive material (e.g. where the gas sensitive material is to contain certain elements such as Pb, Ba and Sr appropriate starting materials may be powdered compounds of Pb, Ba and Sr). Oxides and oxide precursors are examples of materials from which the gas sensitive material may be prepared. The oxides or oxide precursors may be, for example, of laboratory reagent grade. Examples of oxide precursors are carbonates, nitrates, oxalates and acetates that may be converted to the corresponding oxide.

Oxides and oxide precursors may optionally be prepared by a gel process such as a sol-gel process or a gel precipitation process.

In preparing the gas sensitive material, by way of example, finely ground powders of the appropriate starting materials in appropriate proportions (i.e. in proportions appropriate to the desired composition of the desired gas sensitive material) may be thoroughly mixed in suspension (e.g. in acetone) by usig a mill apparatus in which materials are ground, mixed and dispensed (e.g. by use of small alumina ceramic balls agitated in a steel pot by a steel blade).

Mixing time and speed may be minimised to avoid unnecessary contamination of the starting materials.

After mixing the resulting powder mixture may be dried and calcined (e.g. for 16 hours) at a temperature in the range 700-13004 (conveniently -800"C or -1200"C) depending upon the melting temperature of the starting materials or the particular composition of gas sensitive material being prepared.

The product resulting from calcination, which may be in the form of a cake, may be ground as required to give a fine powder. (If required, grinding and calcination may be repeated several times in order to obtain a more fully reacted product powder).

Subsequently the fine powder may be pressed (e.g. with the optional addition of a binder, such as a solution of starch or PVA) into any suitable shape (e.g. a pellet).

The pressing may be followed by firing (e.g. at the same temperature as the calcination step(s) described above, or at a somewhat higher temperature, for -16 hours.

In addition to assisting binding the powder into the desired shape the binder also burns out during the firing stage and may give rise to porosity.

As an alterative to mixing powders in suspension a powder mixture for subsequent calcination may be prepared, for example, by spray drying a solution (e.g. an aqueous solution) of appropriate starting materials (e.g. metal oxalates, metal acetates or metal nitrates) in appropriate proportions.

Electrodes may be applied to the gas sensitive material once prepared in any suitable manner.

For example, electrodes (e.g gold electrodes) may be applied by means of screen printing or sputtering.

Alternatively to preparing a sensor by forming a pellet and applying electrodes as disclosed above, a sensor in accordance with the present invention may be formed in any suitable manner. Thus, for example, a parallel plate configuration may be fabricated by applying a first electrode (e.g. of gold) to an insulating substrate (e.g. by screen printing or sputtering), forming a gas sensitive material layer covering at least a portion of the first electrode (e.g. by deposition, for example by screen printing or doctor-blading, from a suspension or a colloidai dispersion and firing at a temperature in the range 450-950"C to promote adhesion and mechanical integrity) and forming a second electrode (e.g. of gold) on the gas sensitive material layer (e.g. by screen printing or sputtering).

The second electrode is preferably permeable to facilitate access of gas or gaseous mixture in which the sensor is to be used to the gas sensitive material layer.

By way of further example, a coplanar configuration can be used in the preparation of a sensor in accordance with the present invention.

In such a coplanar configuration interdigitated electrodes (e.g. of gold) may be formed on an insulating substrate (e.g. by screen printing,or by sputtering, or by photolithography and etching). The interdigitated electrodes are subsequently covered with a gas sensitive material layer (e.g by means of deposition, for example by screen printing or doctor-blading, from a suspension or a colloidal dispersion) and firing at a temperature in the range of 450-950"C to promote adhesion and mechanical integrity.

Sensors in accordance with the present invention fabricated in a coplanar configuration may include another layer or layers interposed between the gas sensitive material layer and the electrodes. By way of example, an interposed layer may be a layer of a dielectric material, or a layer for promoting adhesion of the gas sensitive material (e.g. a layer of glss material or a layer fabricated from a powder prepared from a gel). By way of further example, a layer for promoting adhesion may be interposed between a dielectric layer and the gas sensitive material layer.

By way of further example, sensors in accordance with the present invention may be fabricated by depositing a gas sensitive material layer on electrodes of any suitable configuration for example those fabricated in the form of "wander tracks". By way of yet further example, a gas sensitive material layer may be deposited onto a semi-conductor device such as a field effect transistor, MOS capacitor or gate-controlled diode.

It is believed that certain gas sensitive materials in accordance with the present invention may be used to effect determinations in a gas or a' gaseous mixture at non-elevated temperature (e.g.

at ambient temperature such as room temperature).

The present invention will now be further described with reference to Table I which gives Examples of gas sensitive material in accordance with the present invention together with examples of gases to which they are sensitive.

In the case of the Examples listed in Table I the gas sensitive material in each case comprises a pellet (-2mm thick and 1 cm in diameter); sputtered gold electrodes were used on opposing faces of the pellet and the sensor constituted thereby was mounted between gold foils in a in a flowing gas stream (of chosen composition) while electrical measurements were made.

Also in the case of the Examples listed in Table I in each case the gas sensitive material was prepared by mixing appropriate finely ground starting materials in suspension in acetone in a mill, drying, calcining (at a temperature in the range 700-1300"C (a preferred temperature was 800'C) for -16 hours, grinding to a fine powder and pressing and firing for -16 hours (at a preferred temperature in the range 800"C to 1000"C) to give pellets.

Table I Example Gas Sensitive Gas Sensitivity No. Material 1 KCu3Nb7021 H2, CO, CH4 02 C2H4, NH3 2 RbFe2Nb7021 H20, 112, CO, 02 3 Sr0.75Ba0.25Nb2O6 H2O 4 Sr6FeNb9O30 H2, CO, C2H4, NH3, C3H8, SO2 5 Ba6FeNb9O30 H2O, H2, CO, O2, C2H4, NH3 6 Pb6FeNbgO30 H20, H2, 02' C2H4, NH3 7 Ba9NiNb14O45 H2O 8 Ce9Ni7Nb8O45 H2O 9 Bi6Fe4Nb6O30 H2O, H2, CO, O2, C2H4, NH3 10 Ba2NaNb5O15 H2O 11 Ba2KNb5O15 H2O, H2, C2H4, NH3 12 Sr2NaNb5015 H20 13 Ba6Ti2Nb8O30 H2O, CO, H2, C2H4,F NH3, C3H8, N02, S02, and H2S 14 Sr6Ti2Nb8O30 CO, CH4, H2, O2, C2H4, NH3 15 Ca6Ti2Nb8030 H20 16 Pb0.55Ba0.45Nb2O6 H2O, H2 C2H4, NH3 17 PbNb2O6 H20, H2, CO, C114, C3H8, Cl2, NO2, SO2 and H2S 18 Ba0.5Sr0.5Nb2O6 H2O, O2 CH4, CO, H2, C2H4, NH3 19 BaNb2O6 H2O, H2.CH4, C2114, NH3, CO.

20 CaNb2O6 Cl2 21 Ca2Nb2O7 C3H8, Cl2, H2S The present invention will now be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of one form of sensor in accordance with the present invention: Figure 2 and Figure 2a represent diagrammatically a parallel plate sensor in accordance with the present invention and a partially completed parallel plate sensor respectively; Figure 3 is a diagrammatic representation of a coplanar sensor in accordance with the present invention; Figure 4 is a diagrammatic representation of a further form of sensor in accordance with the present invention;; Figure 5 is the response, in terms of resistance at 1 OKHz (indicated by the line R10KHZ) and time, of a sensor of the form used in the Examples given in Table I at temperatures indicated by the line T with the gases and gaseous mixtures indicated using Sr6FeNbgO30 as the gas sensitive material; Figure 6 is the response, in terms of resistance at 1 OKHz (indicated by the line R10KHz) and time, of a sensor of the form used in the Examples given in Table I at temperatures indicated by the line T with the gases and gaseous mixtures indicated using Ba6FeNb9O30 as the gas sensitive material;; Figure 7 is the response, in terms of resistance at 1 OKHz (indicated by the line R10KHz) and permittivity at 1OKHz (indicated by the line C10KHZ) of a sensor of the form used in the Examples in Table I at temperatures indicated by the line T with the gases and gaseous mixtures indicated using Bi6 Fe4Nb6030 as the gas sensitive material; Figure 8 is the response, in terms of resistance at 1 OKHz (indicated by the line R10KHZ) and permittivity at 1OKHz (indicated by the line C1 0KHz) of a sensor of the form used in the Examples in Table I at temperatures indicated by the line T with the gases and gaseous mixtures indicated using Ba6Ti2Nb8030 as the gas sensitive material;; Figure 9 is the response, in terms of resistance at 1 OKHz (indicated by the line RloKHZ) and permittivity at 1 OKHz (indicated by the line CloKHZ) of a sensor of the form used in the Examples in Table I at temperatures indicated by the line T with the gases and gaseous mixtures indicated using Pbo ssBaO ssNb206 as the gas sensitive material;; Figure 10 is the response, in terms of resistance at 10KHz (indicated by the line RlOKH,) and permittivity at 1 OKHz (indicated by the line C1 0KHZ) of a gas sensor of the form used in the Examples in Table I at temperatures indicated by the line T with the gases and gaseous mixtures indicating using BaNB2O6 as the gas sensitive material; Figure 11 is the response in terms of resistance at 1 OKHz (indicated by the line RloKHZ) of a sensor of the form used in the Examples in Table I at a temperature of 470"C using BaO sSrO sNb206 as the gas sensitive material;; Referring now to Fig. 1 of the drawings there is shown a sensor comprising a gas sensitive material 1 and, in contact with the gas sensitive material 1 gold electrodes 2 and 3. (The gas sensitive material 1 may be carried by a substrate (e.g. of alumina) (not shown)).

Conductors 4 and 5 are provided to connect the electrodes 2 and 3 respectively to electrical measuring means 6 for measuring the resistance and/or capacitance and/or impedance of the gas sensitive material 1.

In operation a gas or gaseous mixture is contacted with the gas sensitive material.

The resistance and/or capacitance and or impedance is measured by the electrical measuring means 6. Changes in the composition of the gas or gaseous mixture which result in a change of resistance and/or capacitance and/or impedance are observed as changes in the resistance and/or capacitance and/or impedance recorded by the measuring means 6.

Referring now to Fig. 2 of the drawings there is shown (in plan view) an insulating substrate 1 (e.g. an alumina ceramic tile) upon which is formed a first electrode 2 (e.g. of gold), a gas sensitive material layer 3 comprising a gas sensitive material in accordance with the present invention and a second electrode 4 (e.g. of gold).

A parallel plate sensor as shown in Fig. 2 may be fabricated by applying the first electrode 2 (e.g. of gold) to the insulating substrate (e.g. by screen printing or sputtering), forming a gas sensitive material layer 3 covering at least a portion of the first electrode 2 (e.g. by deposition, for example by screen printing doctor-blading, from a suspension or a colloidal dispersion and firing at a temperature in the range 450-950"C to promote adhesion and mechanical integrity) and forming a second electrode 4 (e.g. of gold) on the gas sensitive material layer 3 (e.g. by screen printing or sputtering).

To facilitate understanding of the construction of the sensor of Fig. 2 reference may be made to Fig. 2a which shows a parallel plate sensor of the type shown in the Fig. 2 partially completed inasmuch as the second electrode 4 has not been formed. Fig. 2a thus shows the insulating substrate 1, the first electrode 2 and the gas sensitive material layer 3 and it can be seen that the portion of the first electrode 2 covered by the gas sensitive material layer 3 may extend in area to substantially the same extent as the second electrode 4.

In operation the first electrode 2 and second electrode 4 are connected to an electrical measuring means (not shown) for measuring the resistance and/or capacitance and/or impedance of the gs sensitive material layer 3 and the sensor is contacted with a gas or gaseous mixture. The resistance and/or capacitance and/or impedance is measured by the electrical measuring means and changes in the composition of the gas or gaseous mixture which result in a change of resistance and/or capacitance and/or impedance are observed as changes in the resistance and/or capacitance and/or impedance recorded by the measuring means.

Referring now to Fig. 3 there is shown (plan view) an insulating substrate 1 (e.g. an alumina ceramic tile) upon which are formed electrodes 2 and 3 (e.g. both of gold), and a gas sensitive material layer 4 (comprising a gas sensitive material in accordance with the present invention) covering at least a portion of both electrodes 2 and 3. It will be seen from the lines shown in dotted form in Fig. 3 the portions of the first electrode 2 and second electrode 3 covered by the gas sensitive material layer 4 are interdigitated.

The first electrode 2 and the second electrode 3 may be provided on the insulating- substrate 1 by any suitable method. For example the methods disclosed for providing electrodes 2 and 4 in the parallel plate sensor described herein before with reference to Fig. 2 and Fig. 2a may be used.

The gas sensitive material layer 4 shown in Fig. 3 may be prepared by any suitable method.

For example the methods disclosed for preparing gas sensitive material layer 2 in Fig. 2 and Fig.

2a may be used.

Referring now to Fig. 4 of the drawings there is shown a diagrammatic representation in cross-section of a sensor having an insulating substrate 1, electrodes represented as 2, a dielectric layer 3 and a gas sensitive material layer 4.

The electrodes 2 and the layers 3 and 4 may be prepared by any suitable method. Thus, for example, screen printing or sputtering or photolithography and etching may be used as is appropriate.

Referring now to Figs. 5 and 6 of the drawings there is shown respectively the response of Sr6FeNbgO30, Ba6FeNb9O30, Bi6Fe4Nb6O30, Ba6Ti2Nb8O30, Pb0.55Ba0.55Nb2O6, BaNb2O6 and Ba0.5Sr0.5Nb2O6 as gas sensitive material in the gases and gaseous mixtures indicated.

Claims (22)

1. A sensor suitable for use in a gas or gaseous mixture which sensor includes a gas sensitive material (as hereinbefore defined) containing or comprising a complex oxide of niobium (as hereinbefore defined).

2. A sensor as claimed in Claim 1 wherein the complex oxide of niobium has the general formula: A x Bx Nb2 6 where A, B is Pb, Ba or Sr and 1 > x#0.

3. A sensor as claimed in Claim 1 wherein the complex oxide of niobium has a general formula: A6 Bx Nb10-x O30.

where A is divalent or trivalent or tetravalent and the value of x depends upon the valencies of B and A and is in the range 4 > x#0

4. A sensor as claimed in Claim 3 wherein A is Sr, Ba, Pb, Bi, Zr or Ce.

5. A sensor as claimed in Claim 3 or Claim 4 wherein A is partially substituted by a monovalent element.

6. A sensor as claimed in Claim 3 wherein B is Co, Ni, Mn, Fe or Ti.

7. A sensor as claimed in any one of Claims 1 to 6 wherein two or more electrodes are provided in communication with the said gas sensitive material and said gas sensitive material is arranged so as to be capable of being contacted with a gas or gaseous mixture.

8. A sensor as claimed in Claim 7 wherein the electrodes are in direct contact with the gas sensitive material by being in contact therewith.

9. A sensor as claimed in any one of the preceding claims wherein the sensor includes a temperature sensing means.

10. A sensor as claimed in any one of the preceding claims wherein the sensor includes a heating means.

11. A sensor as claimed in any one of the preceding claims wherein the gas sensitive material has porosity to give surface area for contact with a gas or gaseous mixture when in use.

12. A sensor as claimed in any one of the preceding claims wherein the gas sensitive material comprises KCu3Nb702,, RbFe2Nb7 021, Sr0.75Ba0.25Nb2O6, Sr6FeNb9O30, Ba6FeNbgO30, Pb6FeNbgO30, Ba9NiNb14O45, Ce9Ni7Nb8045, Bi6Fe4Nb6O30, Ba2NaNbsO,s, Ba2KNbsOlst Sr2NaNb5015, Ba6Ti2Nb8O30, Sr6Ti2Nb8 030, Ca6Ti2Nb8030, Pb0 55Ba0 45N b206, PbNb2O6, Ba0.5Sr0.5Nb2O6, BaNb2O6, CaNb2O6 or Ca2Nb2O7.

13. A method for effecting determinations in a gas or gaseous mixture which comprises contacting a sensor with the gas or gaseous mixture and measuring the electrical response of the sensor, said sensor including a gas sensitive material containing or comprising a complex oxide of niobium (as hereinbefore defined).

14. A method as claimed in Claim 1 3 wherein two or more electrodes are provided in communication with the gas sensitive material and said gas sensitive material and said electrodes are in contact with the same gas or gaseous mixture.

15. A method as claimed in Claim 13 or Claim 14 wherein the resistance of the sensor is measured.

16. A method as claimed in Claim 13 or Claim 14 wherein the conductance of the sensor is measured.

1 7. A method as claimed in Claim 1 3 or Claim 14 wherein the impedance of the sensor is measured.

1 8. A method as claimed in any one of Claims 1 3 to 1 7 wherein O2 is detected by the sensor.

19. A method as claimed in any one of Claims 13 to 17 wherein H2, C2H4, CO, CH4, NH3, an oxide of sulphur, an oxide of nitrogen, Cl2 or H2S is detected, in air, by the sensor.

20. A sensor substantially as hereinbefore described with reference to any one of the Figs.

1, 2, 2a, 3 or 4 of the accompanying drawings.

21. A method for preparing a sensor substantially as hereinbefore described with reference to any of the Figs. 2, 2a, 3 or 4 of the accompanying drawings.

22. A method for effecting determinations in a gas or gaseous mixture substantially as hereinbefore described with reference to any one of the Figs. 1, 2, 2a, 3 or 4.