IMPROVEMENTS IN OR RELATING TO GAS SENSORS

Abstract: A sensor and a sensing method for use in a gas or gaseous mixture is provided. The sensor (9) includes a gas sensitive material (4), MO3-x, in which M is predominantely or exclusively Mo; and MO3-x is a substoichiometric molybdenum trioxide which exhibits a response in the form of an increase or a decrease in an electrical property of the material in the presence of a gas. This electrical property is measured by an electrical measuring means (6). The gas sensitive material (4) is in direct communication with two electrodes (2, 3) mounted to the gas sensitive material (4) and arranged for contact with a gas or gaseous mixture.

Description: IMPROVEMENTS IN OR RELATING TO GAS SENSORS BACKGROUND OF THE INVENTION A large number of semiconductor gas sensors are presently in use in many parts of the world largely to provide early warning of the development of an explosion hazard (e.g. escaping flammable gas) or the presence of toxic gases or vapors in ambient air. A sensing element normally comprising a semiconducting material and presenting a high surface-to-bulk ratio is deployed on a heated substrate between two metallic electrodes. The presence of gas posing a hazard is detected by a sensible change in the resistance of the semiconducting element by means of the electrodes that are incorporated in a suitable electric circuit. The device is thus a gas- sensitive resistor. The most commonly used material in gas sensitive resistors used to measure impure gases in air is tin dioxide. Tin dioxide sensors, though often useful in particular alarm functions, have generally been found to suffer from a lack of selectivity. The reactions that allow the detection of target gases normally involve the oxidation of the target gas at the semiconductor (oxide) surface and a concomitant change in the charge carrier density of the material. Unfortunately, changes in relative humidity also give rise to a sensible change in the conductivity of tin dioxide even though, in this case, no oxidation process is possible. In other words, changes in relative humidity amount to an interference with the detection of gases by tin dioxide even though the mechanisms involved in the two responses are different. Since the reactions that generate the resistance response take place at the oxide surface a very small amount of second phase additive may modify the behavior substantially. SUMMARY OF THE INVENTION The present invention relates to sensors and more particularly to sensors suitable for use in gases and gaseous mixtures . In a preferred embodiment, a sensor is provided that is suitable for use in a gas or gaseous mixture. The sensor includes a gas sensitive material (as hereinafter defined) that is capable of exhibiting a response in the form of an increase or a decrease in an electrical property of the material in the presence of a gas and that exhibits a small response to changes in the moisture content of the atmosphere. In another preferred embodiment, the gas sensitive material is provided with two or more electrodes in communication with the gas sensitive material and the 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 or 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" preferably embraces a gas as such and any material that may be present in a gaseous phase, one example of which is a vapor. The gas sensitive material is a material which responds to a target gas rather than to changes in relative humidity. Also, it will be appreciated that in this specification the term "gas sensitive material" means a preferred material which is gas (including vapor) 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 dependent, the sensor also preferably includes a temperature sensing means. The sensor may also 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 may 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 optimized and its response to interferents, such as changes in relative humidity may be minimized. The resistance and/or conductance and/or impedance 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 which have shown responses using a sensor in accordance with the present invention are H 2 , C 2 H A , NH 3 , C 3 H 8 , H 2 S, CH 4 and CO. In one preferred embodiment of the present invention, the gas sensitive material (as herein defined) , has two or more electrodes in communication with said gas sensitive material, and the gas sensitive material and the electrodes are in contact with the same gas. Preferably the gas sensitive material has porosity to give a satisfactory surface area for contact with the gas or gaseous mixture when in use. The gas sensitive material, for example, may be prepared from an oxide or from an appropriate precursor. The oxide or precursor may optionally be prepared by a gel process such as a sol-gel process or a gel precipitation process. The powder may be dried and calcined (e.g. for approximately 16 hours) at a temperature in the range of about 700 - 1000 °C depending upon 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 suitable powder. Subsequently, the fine powder may be pressed (e. g. with the optional addition of a binder, such as a solution of starch or polyvinyl alcohol) into any suitable shape (e. g. a pellet) . The pressing may be followed by a firing (e. g. at the same temperature as the calcination step(s) described above, or at a somewhat higher temperature, for approximately 16 hours) . In addition to assisting in the binding of the powder into desired shapes, the binder also burns out during the firing stage giving rise to porosity. As an alternative a powder for subsequent calcination may be prepared, for example, by spray drying a solution (e.g. an aqueous solution) of appropriate starting material (e.g. a metal oxalate, metal acetate or metal nitrate) . Electrodes may be applied to the prepared gas sensitive material 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 colloidal dispersion and firing at a temperature in the range of about 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 may 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 cove-red 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 about 450 - 950 °C to promote adhesion and mechanical integrity. The gas sensitive material disclosed by the present invention is comprised of a metal oxide of general formula M0 3.χ , in which formula M is predominantly or exclusively molybdenum. The oxide is thus derived from molybdenum trioxide, Mo0 3 , either by reduction so that in the formula M0 3.χ the value of x is invested with a finite value up to around 0.3, by a thermal treatment, or by substituting a small fraction of the molybdenum by a metal with a principal valence of less than 6 in order to stabilize the structure of the substoichiometric phase, M0 3 _ χ . In one example, the substoichiometric phase, M0 3.χ , may be stabilized by the incorporation of 7% of tantalum, which results in an overall stoichiometry Of M0 28 . Any one of a number of transition metal ions with stable valence of less than 6 could stabilize the required structure in accordance with the spirit of the present invention, provided that the substituent transition metal ion has a radius of a suitable size to match the structure. These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic representation of one form of a sensor in accordance with the present invention. Figures 2 and 3 represent diagrammatically a parallel plate sensor in accordance with the present invention and a partially completed parallel plate sensor respectively. Figure 4 is a diagrammatic representation of a coplanar sensor in accordance with the present invention. Figure 5 is the response, in terms of sensitivity ( (G- G 0 )/G 0 ), where G is conductance in gas and G 0 is conductance in air, and time, of a sensor of M0 3 _ χ . The M0 3.χ here was manufactured by heating Mo0 3 above its melting point, to 1000 °C, in an alumina crucible for 16 hours and regrinding the dark blue/purple material thus obtained. The sensor took the form of a cylindrical porous pellet, approximately 2 mm thick and 1 cm in diameter, with gold electrodes and was heated by an external tube furnace arranged coaxially with the pellet and with the gas concentrations indicated in a background atmosphere of air at about 500 °C. Figure 6 is a graph of the response in terms of sensitivity of a M0 3.χ sensor in air to 50 ppm hydrogen sulphide. Figure 7 is a graph of the response in terms of sensitivity of a M0 3 . χ sensor in air to 500 ppm ammonia. Figure 8 is a graph of the response in terms of sensitivity of a M0 3.χ sensor in dry air to a 10 minute pulse of wet air. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figure 1 of the drawings, there is shown a sensor 9 comprising a gas sensitive material 4 and, in contact with the gas sensitive material, gold electrodes 2 and 3. The gas sensitive material may be carried by a substrate (e.g. of alumina) (not shown). Conductors 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 4. In operation a gas or gaseous mixture is contacted with the gas sensitive material 4. The resistance and/or conductance 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 conductance and/or capacitance and/or impedance are observed as changes in the resistance and/or conductance and/or capacitance and/or impedance recorded by the measuring means 6. Referring now to Figure 2, 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 4 comprising a gas sensitive material in accordance with the present invention and a second electrode 3 (e.g. of gold) . A parallel plate sensor 9, as shown in Figure 2, may be fabricated by applying the first electrode 2 (e.g. of gold) to the insulating substrate 1 (e.g. by screen printing or sputtering) , forming a gas sensitive material layer 4 by deposition, for example by screen printing or 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 3 (e.g. of gold) on the gas sensitive material layer 4, (e.g. by screen printing or sputtering) . In order to facilitate understanding of the construction of the sensor of Figure 2 reference may be made to Figure 3, which shows a parallel plate sensor 9 of the type shown in the Figure 2 partially completed inasmuch as the second electrode 3 has not been formed. Figure 3 thus shows the insulating substrate 1, the first electrode 2 and the gas sensitive material layer 4 and it is seen that the portion of the first electrode 2 covered by the gas sensitive material layer 4 may preferably extend in area to substantially the same extent as the second electrode 3. In operation, the first electrode 2 and second electrode 3 are connected to an electrical measuring means (not shown) for measuring the resistance and/or capacitance and/or impedance of the gas sensitive material layer 4 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 electrical measuring means. Referring now to Figure 4, 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. It is seen from the lines shown in dotted form in Figure 4 that 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 3 in the parallel plate sensor described hereinbefore with reference to Figure 2 and Figure 3 may be used. The gas sensitive material layer 4 shown in Figure 4 may be prepared by any suitable method. For example, the methods disclosed for preparing gas sensitive material layer 4 in Figure 2 and Figure 3 may be used. Figure 5 is the response, in terms of sensitivity and time, of a sensor of M0 3 . χ . The M0 3.χ here was manufactured by heating Mo0 3 above its melting point, to 1000 °C, in an alumina crucible for 16 hours and regrinding the dark blue/purple material thus obtained. The sensor took the form of a cylindrical porous pellet, approximately 2 mm thick and 1 cm in diameter, with gold electrodes and was heated by an external tube furnace arranged coaxially with the pellet and with the gas concentrations indicated in a background atmosphere of air at 500 °C. Gases that the sensor may detect include, but are not limited to, hydrogen, ethene, ammonia, ozone, propane, methane, carbon monoxide, chorine, nitrogen dioxide, sulphur dioxide, or hydrogen sulphide. Figure 5 shows the gas response of a M0 3.χ sensor in air. The first peak 11 is the response to 1% of carbon monoxide. The second peak 13 is the response to 1% of methane. Figures 6, 7 and 8 are the responses of a thick film sensor of Mo Q 93 Ta 007 O 2 δ to H 2 S , to NH 3 and to moisture, respectively. The M0 3.χ was a thick film of Mo Q 93 Ta Q 07 O 28 manufactured by firing together the constituent oxides at 800 °C. The response is given in terms of sensitivity, which is defined as (G-G 0 )/G 0 , where G is the conductance in gas and G Q is the conductance in air. Figure 6 is a graph of the gas response in terms of sensitivity of a M0 3.χ sensor at 250 °C in air to a 5 minute pulse of 50 parts per million of hydrogen sulfide. Figure 7 is a graph of the gas response in terms of sensitivity of a M0 3.χ sensor at 250 °C in air to a 5 minute pulse of 500 parts per million of ammonia. Figure 8 is a graph of the gas response in terms of sensitivity of a M0 3.χ sensor at 250 °C in dry air to a 10 minute pulse of wet air (passed through a bubbler of water at room temperature) . While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.

Claims: I claim: 1. A sensor for use in a gas or gaseous mixture which sensor includes a gas sensitive material, 0 3 _ χ , (as hereinbefore defined in which M is predominantly or exclusively Mo and M0 3.χ is a substoichiometric Molybdenum trioxide) which exhibits a response in the form of an increase or a decrease in an electrical property of the material in the presence of a gas. 2. The sensor as claimed in claim 1, wherein the gas sensitive material is provided with two or more electrodes 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. 3. The sensor as claimed in claim 2 , wherein the electrodes are in direct communication with the gas sensitive material by being in contact therewith. 4. The sensor as claimed in claim 1, wherein the sensor incorporates a temperature sensing means. 5. The sensor as claimed in claim 1, wherein the sensor includes a heating means. 6. A method for effecting determination in a gas or gaseous mixture which comprises providing a gas sensor with a gas sensitive material having a formula M0 3.χ in which M is predominantly or exclusively Mo and 0 3 _ χ is a substoichiometric Molybdenum trioxide contacting the sensor with gas or gaseous mixture and measuring the electrical response of the sensor, the gas sensitive material as herein before defined exhibiting a response in the form of an increase or a decrease in an electrical property of the material in presence of the gas or gaseous mixture. 7. The method as claimed in claim 6, further comprising two or more electrodes in communication with the gas sensitive material, and contacting the gas sensitive material and the electrodes with the same gas or gaseous mixture. 8. The method as claimed in claim 6, further comprising providing the gas sensitive material with a porosity and increasing surface area for contact with the gas or gaseous mixture. 9. The method as claimed in claim 6, wherein the measuring further comprises measuring the resistance of the sensor. 10. The method as claimed in claim 6, wherein the measuring further comprises measuring the capacitance of the sensor. 11. The method as claimed claim 6, wherein the measuring further comprises measuring the impedance of the sensor. 12. The method as claimed in claim 6, further comprising detecting hydrogen, ethene, ammonia, propane, methane, carbon monoxide, or hydrogen sulphide with the sensor.

Applicants: MOSELEY PATRICK T, MOSELEY, PATRICK, T.
Application year: 1999
Main classification: G01N27
IPC codes: G01N27