Transition metal oxide gas sensor

A sensor and a sensing method for use in a gas or gaseous mixture is provided. The sensor includes a gas sensitive material, MO.sub.3-x, in which M is predominantly or exclusively MO and MO.sub.3-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. The gas sensitive material is in communication with two or more electrodes and is arranged for being contacted with a gas or gaseous mixture. The electrodes are in direct communication with the gas sensitive material by being in contact therewith. The sensor incorporates a temperature sensor. The sensor includes a heating element.

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.sub.2, C.sub.2 H.sub.4, NH.sub.3, C.sub.3
 H.sub.8, H.sub.2 S, CH.sub.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 sixteen hours)
 at a temperature in the range of about 700-1000.degree. 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 sixteen 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 range of about 450-950.degree.
 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 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 about 450-950.degree. 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 MO.sub.3-x, in which formula M is
 predominantly or exclusively molybdenum. The oxide is thus derived from
 molybdenum trioxide, MoO.sub.3, either by reduction so that in the formula
 MO.sub.3-x 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 six in order
 to stabilize the structure of the substoichiometric phase, MO.sub.3-x.
 In one example, the substoichiometric phase, MO.sub.3-x, may be stabilized
 by the incorporation of 7% of tantalum, which results in an overall
 stoichiometry Of MO.sub.2.8. 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring now to FIG. 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 operations 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. Sensor 9 may include temperature
 sensing means 17 for sensing temperature and heating means 15 for heating
 the sensor.
 Referring now to FIG. 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 FIG. 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.degree. 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
 FIG. 2, reference may be made to FIG. 3, which shows a parallel plate
 sensor 9 of the type shown in FIG. 2 partially completed inasmuch as the
 second electrode 3 has not been formed. FIG. 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 FIG. 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 FIG. 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 FIG. 2 and FIG. 3 may be used.
 The gas sensitive material layer 4 shown in FIG. 4 may be prepared by any
 suitable method. For example, the methods disclosed for preparing gas
 sensitive material layer 4 in FIG. 2 and FIG. 3 may be used.
 FIG. 5 is the response, in terms of sensitivity and time, of a sensor of
 MO.sub.3-x. The MO.sub.3-x here was manufactured by heating MoO.sub.3
 above its melting point, to 1000.degree. C., in an alumina crucible for
 sixteen hours and regrinding the dark blue/purple material thus obtained.
 The sensor took the form of a cylindrical porous pellet, approximately two
 mm thick and one 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.degree.
 C.
 Gases that the sensor may detect include, but are not limited to, hydrogen,
 ethene, ammonia, ozone, propane, methane, carbon monoxide, chlorine,
 nitrogen dioxide, sulphur dioxide, or hydrogen sulphide.
 FIG. 5 shows the gas response of a MO.sub.3-x 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. The y-axis shows the sensitivity of the sensor,
 which is a function of the conductance in clean air, Go, and the
 conductance in (air plus the gas to be detected), G, as follows:
EQU S=(G-Go)/Go
 The value of the sensitivity changes as the composition of the atmosphere
 is altered at times indicated on the x axis.
 The graph shows that at the start (time zero) the sensor is in air so that
 the sensitivity is zero. As soon as the first gas (carbon dioxide) is
 introduced (at a concentration of 1% in air), right after time zero, the
 sensitivity rises to reach a peak at a value near 7.0. As soon as the
 atmosphere is returned to pure air (at around ten minutes on the x-axis),
 the sensitivity begins to drop to reach zero once more (at fourteen
 minutes). The second gas, methane, is then introduced, also at a
 concentration of 1%, so that the sensitivity rises once more to a new peak
 at around 2.2.
 FIGS. 6, 7, and 8 are the responses of a thick film sensor of MoO.sub.0.93
 Ta.sub.0.07 O.sub.2.8 to H.sub.2 S, to NH.sub.3, and to moisture,
 respectively. The MO.sub.3-x was a thick film of Mo.sub.0.93 Ta.sub.0.07
 O.sub.2.8 manufactured by firing together the constituent oxides at
 800.degree. C. The response is given in terms of sensitivity, which is
 defined as (G-G.sub.0)/G.sub.0, where G is the conductance in gas and
 G.sub.0 is the conductance in air.
 FIG. 6 is a graph of the gas response in terms of sensitivity of a
 MO.sub.3-x sensor at 250.degree. C. in air to a five minute pulse of fifty
 parts per million of hydrogen sulfide. The graph shows that when hydrogen
 sulfide is introduced (at three minutes) the sensitivity starts rising and
 reaches a peak. When the atmosphere is returned to air the sensitivity
 drops (eight minutes).
 FIG. 7 is a graph of the gas response in terms of sensitivity of a
 MO.sub.3-x sensor at 250.degree. C. in air to a five minute pulse of 500
 parts per million of ammonia. The graph shows that when ammonia is
 introduced (at one minute) the sensitivity starts rising and reaches a
 peak. When the atmosphere is returned to air the sensitivity drops (six
 minutes).
 FIG. 8 is a graph of the gas response in terms of sensitivity of a
 MO.sub.3-x sensor at 250.degree. C. in dry air to a ten minute pulse of
 wet air (passed through a bubbler of water at room temperature). The graph
 shows the sensitivity of the sensor to the saturation of an atmosphere of
 air (two minutes) and ending with water vapor (twelve minutes).
 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.