GB2066963A - Gas sensor elements and methods of manufacturing them - Google Patents
Gas sensor elements and methods of manufacturing them Download PDFInfo
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- GB2066963A GB2066963A GB8040618A GB8040618A GB2066963A GB 2066963 A GB2066963 A GB 2066963A GB 8040618 A GB8040618 A GB 8040618A GB 8040618 A GB8040618 A GB 8040618A GB 2066963 A GB2066963 A GB 2066963A
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- catalyst
- sensor element
- slurry
- bead
- alumina
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
- G01N27/14—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of an electrically-heated body in dependence upon change of temperature
- G01N27/16—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of an electrically-heated body in dependence upon change of temperature caused by burning or catalytic oxidation of surrounding material to be tested, e.g. of gas
Abstract
A gas sensor element for detecting the presence of a flammable gas comprises an electrical resistance filament 1 surrounded by a bead 2 which is formed by an array of alumina particles interspersed between and bound together only by particles of a catalyst for inducing catalytic oxidation of flammable gases to form an open porous gas diffusive skeletal matrix having a mean particle size of less than 20 nm. When the bead includes such a matrix, the number of active sites at which catalytic oxidation of flammable gases can take place is greatly increased with the result that the sensor element is very resistant to poisoning by atmospheric contaminants such as traces of silicone and sulphur compounds. The sensor thus has a very much longer life and does not require such frequent recalibration. <IMAGE>
Description
SPECIFICATION
Gas sensor elements and methods of manufacturing them ,This invention relates to a gas sensing element for sensing the presence of a flammable gas and
its method of manufacture and it is particularly concerned with the type of gas sensing element
known as a pellistor.
Such gas sensing elements have been widely used and their basic construction is described in
British Patent Specification No. 892,530. The elements are formed by a helically coiled filament
embedded within a bead of refractory material such as alumina and the outer surface of the
bead is coated with a layer of a catalyst. In use, an electrical heating current is passed through
the coil of such a sensor and then if the sensor is exposed to air including some flammable gas,
catalytic oxidation of the flammable gas takes place adjacent the surface of the bead which
results in an increase in its temperature and results in the electrical resistance of the filament
increasing. This change in electrical resistance is monitored by a resistance bridge circuit to give
an electrical indication of the presence and concentration of a flammable gas.
Such a sensor is usually made by supporting the filament vertically and dipping it into an
aqueous solution of a precursor of alumina, such as aluminium nitrate or hydroxide, and then
this compound is converted into alumina by the passage of an electrical heating current through
the filament. The dipping process is usually repeated to build up a bead of the required size
around the filament. A solution or dispersion of the catalyst is then applied to the outer surface
of the bead.
Whilst such sensors have been widely used, they have a poor resistance to poisoning and
since the environment in which a flammable gas sensor is used frequently contains materials
which poison the catalyst this means that the calibration of the sensor has to be checked
frequently as a gradual increase in the level of poisoning of the catalyst leads to a gradual
decrease in its sensitivity, and means that the sensor has to be replaced frequently.
British Patent Specification No. 1,387,412 in the name of English Electric Valve Company
Limited describes and claims a gas sensor element formed by a helical coil consisting of a
homogeneous mixture of an oxidation catalyst material and a substantially non-catalytic carrier
material. This specification attributes the tendency of the gas sensor element to change in its
electrical characteristics over a prolonged period to the diffusion of the catalyst into the carrier
material. It suggests that this problem is overcome by having a homogeneous mixture of cataylst
and carrier material. The specification only describes the bead as being made by a thermal
decomposition process from a mixture of catalyst precursors and aluminium salts.It particularly
discusses the bead as being formed from a solution of palladium chloride, platinum chloride,
concentrated hydrochloric acid, distilled water and aluminium nitrate solution.
Another, more recent, patent specification, No. 1 556,339, in the name of English Electric
Valve Company Limited acknowledges that changes in the electrical characteristics of gas
sensing elements take place as a result of poisoning of the catalyst and suggests the
incorporation of a zeolite into the bead to act as a molecular filter and absorb the catalyst
poisons and thereby prevent them poisoning the catalyst.
Recently published European Patent Application No. 0,004,184 also discussed the poisoning
of the catalyst in a gas sensor element and discusses the preparation of the bead of the gas
sensor element at least partly by deposition of a slurry formed of finely ground alumina having a
particle size of less than 100W together with an aqueous binder. This bead is subsequently
impregnated with a catalyst solution.The specification also discloses that the bead may include
an initial coating of aluminium nitrate which is subsequently decomposed by a pulsed electrical
current and the specification describes this decomposition by a pulsed electric current as
resulting in an increase in the volume of the bead and suggests that it is this treatment of the
bead which contributes greatly to the resistance to poisoning of the completed gas sensor 'element.
According to a first aspect of this invention, a gas sensor element comprises an electrical
resistance filament surrounded by a bead which includes an array of alumina particles
interspersed between and bound together only by particles of a catalyst for inducing catalytic
oxidation of flammable gases to form an open porous gas diffusive skeletal matrix having a
mean particle size of less than 20nm.
According to another aspect of this invention, a method of making a gas sensor element
comprises depositing on an electrical resistance filament a slurry formed by a mixture of alumina
and at least one catalyst precursor in a substantially non-aqueous organic liquid, the mean
particle size in the slurry being less than 20 nm, removing the liquid and decomposing the at
least one catalyst precursor so that the filament is surrounded by a bead which includes an array
of alumina interspersed between and bound together only by particles of a catalyst for inducing
catalytic oxidation of flammable gases to form an open porous gas diffusive skeletal matrix
having a mean particle size of less than 20nm.
Preferably, the mean particle size of the catalyst precursors in the slurry and the mean particle size of the catalyst in the matrix is as small as possible and it is preferred that the mean particle size is below 5 nm. It is particularly important that the catalyst is in as finely divided a state as possible in the completed gas sensor and a preferred way of achieving and ensuring this is to subject the slurry fqrmed by the mixture of alumina and at least one catalyst precursor in a substantially non-aqueous organic liquid to a wet grinding stage before depositing it on the electrical resistance filament. Alumina is a very abrasive material and when the slurry is subjected to a wet grinding stage the alumina grinds the at least one catalyst precursor and reduces its particle size.
It is important to obtain the alumina in as finely a divided state as possible but there is a limit imposed to the degree of fineness which is obtained when alumina is subjected to a simple grinding or milling operation as the individual particles of alumina agglomerate together to form larger units and these larger units of apparently greater particle size prevent the alumina being ground more finely. A particularly preferred way of obtaining at least some alumina of a sufficiently small particle size is to subject the alumina to a pre-treatment in which the alumina is mixed into a slurry with a substantially non-aqueous organic liquid and subjected to ultrasonic vibrations followed by a sedimentation step, with only the upper fraction so obtaind then being mixed with the at least one catalyst precursor.This treatment with ultrasonic vibrations appears to break up at least some of the agglomerations of fine alumina particles and the sedimentation stage provides an effective size grading.
Preferably, the substantially non-aqueous organic liquid is a volatile liquid such as an alcohol, an ester, a ketone, a chlorinated aliphatic hydrocarbon, or an aliphatic hydrocarbon, for example petroleum ether. It is preferred to have the substantially non-aqueous organic liquid formed by methanol or ethanol. The catalyst is typically one of the noble metals such as platinum, palladium, or their salts and preferably, the catalyst is formed by a mixture of palladium and thorium. It has been found that a particularly good sensing element is obtained when the slurry is formed from a mixture of equal parts by weight of alumina, ammonium chloropalladite, and thorium nitrate.
It has also been found that better results are obtained if the filament is not dipped but, instead is supported horizontaily and then has the slurry deposited onto it from, for example, a pipette. Shrinkage occurs during the manufacture which can lead to damage of the filament or the matrix when it is supported horizontslly and so it is preferred that lead wires leading to the filament include kinks and are annealed so that the kinked portion can straighten to accommodate shrinkage of the matrix during removal of the liquid and subsequent decomposition of the catalyst precursors. After the slurry has been- deposited on the filament an electric current is passed through the filament to drive off the liquid and decompose the catalyst precursors.
The bead may be built up by applying more than one layer of material to the filament. One type of standard pellistor has a bead substantially 2mm in diameter and when a sensor in accordance with the present invention is required to have characteristics similar to this, conventional pellistor it is preferred that the matrix is built up in more than one layer and, typically, built up in three separate layers until the bead has a diameter of between 1.75 and 2.0mm. Another type of conventional pellistor has a bead less than 1 mm in diameter and when a sensor element in accordance with this invention is required to have characteristics similar to this type of conventional pellistor the matrix can be built up with only a single application of slurry.
With a conventional sensor the catalyst, or the catalyst precursor, may be activated by heating the bead in air, or in the presence of a hydrocarbon gas. With a sensor of conventional construction, this heating step merely activates the catalyst. Alternatively with conventional sensors the catalyst may be activated by other means but, with a sensing element in accordance with this invention, it is especially preferred that the sensor is treated by exposing it to a stoichiometric mixture of a hydrocarbon gas and air whilst a current is passed through the filament, the current passing through the filament being the typical operating current of the sensing element in use. Catalytic oxidation of the hydrocarbon gas takes place and the bead glows brightly.We have discovered that when a sensor element in accordance with the invention is treated in this way, the sensor is considerably more robust and the matrix has a greater mechanical strength.
Preferably, the completed gas sensor element in accordance with this invention is subjected to a constant "soak" for a period of time, typically 96 hours.
Sensor elements in accordance with this invention are very resistant to poisoning by atmospheric impurities and we believe that this results from the very finely divided and dispersed state of the catalyst, together with the open porous gas diffusive skeletal nature of the matrix. These two factors mean that a sensor in accordance with this invention has a very much greater number of available sites at which catalytic oxidation of flammable gases can take place than any of the other various different types of gas sensor element which have been described previously. When a catalyst is applied to a conventional gas sensor element, simply by coating
the outer surface of the bead or by impregnating the bead with a catalyst, there is a very much
greater concentration of catalyst on the surface of the bead than throughout the remainder of
the bead.This greater concentration at the surface is more accessible to any impurities in the
atmosphere and so more likely to be poisoned but, perhaps more importantly, this greater concentration of catalyst at the surface of the bead means that the catalyst is far more likely to
be sintered during its conditoning or during use and this sintering and joining together of
particles of catalyst reduces the number of active catalytic sites that are available.
Another factor affecting the state of division of the catalyst particles is their nature of
formation. When the catalyst is derived from a solution the particles of catalyst precursor that
grow during removal of the solvent vary considerably and uncontrollably in size and they are
typically very large in comparison with the particle sizes with which this invention is concerned.
Also, when both the catalyst precursors and the alumina precursors are obtained from solution
as in the case in the homogeneous matrix disclosed in U.K. Patent Specification No. 1,387,142
it is possible for at least some of the catalyst to be completely trapped inside crystals and
platelets of alumina and so not be available as a site of catalytic activity.
With the sensor in accordance with this invention, the very fine particle size of the catalyst
precursor ensures that, upon decomposition of the precursors, the catalyst is in a very finely
divided state. Also, the fine particle size of at least some of the alumina particles and their
intimate mixture with the finely divided particles of catalyst precursor and, catalyst ensure that
the alumina particles are interspersed between adjacent finely divided particles of catalyst to
prevent them becoming sintered together eight during manufacture or use of the gas sensor
element. When the slurry is subjected to a pre-treatment with ultrasonic vibrations not all of the
agglomerations of small alumina particles are broken up with the result that some of these large
agglomerations of alumina particles are distributed throughout the bead.This inhomogeneity in
the nature of the bead does not affect the sensor elements adversely and it is believed that it
may even contribute to the better results that are obtained by providing gas paths through
which the inflammable gases may enter the bead more easily.
The catalyst acts as the sole binder to bind together the array of alumina particles and this
results in a more open and porous matrix than with any of the previous sensors described above
and thus the active catalytic sites distributed throughout the beads are readily accessible by
flammable gas moiecules diffusing through the bead. Equally, the flammable gas molecules can
readily diffuse through any of the agglomerations of alumina particles since these also have an
open porous nature and are even free of particles of catalyst and so are likely to be even more
porous than the remainder of the matrix.
In the prior art devices, such as that described in European Patent Application 0,004,1 84, the alumina particles although small in size are firstly bound together by the aqueous binder
which fills some of the interstices between the adjacent alumina particles and then the
subsequent application of the solution of catalyst means that further of the interstices between
the alumina particles are filled. This reduces the porosity of the matrix still further and so
prevents ready diffusion of flammable gases into the bead.
The gas sensor element in accordance with this invention thus has a much greater number of
active catalytic sites available at which catalytic oxidation of flammable gas can take place and
consequently there are naturally a greater number that have to be poisoned before the device
ceases to operate. More importantly with the greater availability of the catalytic sites at which
catalytic oxidation can take place, it is believed that the rate controlling step in the oxidation of
the flammable gas in the region of the bead is the rate of diffusion of a flammable gas through
a flammable gas depletion layer surrounding the bead instead of the rate of oxidation of the gas.
The rate of diffusion of the flammable gas through a flammable gas depletion layer surrounding
the bead is substantially slower since it is a bulk transport mechanism, unlike the rate of
oxidation and consequently, since there are so many sites available in the bead at which
catalytic oxidation can take place, all the while that there is an excess of these sites, it is the rate
of diffusion through the depletion layer around the bead which provides the mechanism limiting
the heat generated by the catalytic oxidation of the flammable gas. Thus, if some of the catalytic
sites are poisoned, provided that there is still an excess number of catalytic sites, there will be
no change in the rate of oxidation of the flammable gas and hence no change in the increase in
temperature of the bead for a particular concentration of flammable gas.
We have also found that sensors in accordance with this invention have a very much longer
life than those manufactured by the conventional processes and this is principally due to their
greater resistance to poisoning. However, sensors in accordance with this invention also have a
high mechanical strength and toughness and consequently are also very resistant to mechanical
damage.
One example of a sensor and a method of making it, in accordance with this invention, will
now be described and its performance contrasted with that of conventional sensors with
reference to the accompanying drawings; in which:
Figure 1 is a perspective view illustrating the initial pre-treatment of the filament;
Figure 2 is a partly cut-away perspective view of the complete sensor element;
Figures 3a and 3b are photo-micrographs to the same scale of part of the matrix of a sensor in accordance with this invention and of a conventional sensor, respectively;
Figure 4 is a graph of sensitivity against time contrasting the characteristics of a sensor in accordance with this invention with those of a conventional device;
Figure 5 is a graph of sensitivity against time contrasting the characteristics of a sensor in accordance with this invention with those of a conventional device;;
Figure 6 is a graph showing the average loss in sensitivity against time for a group of sensors in accordance with this invention against a group of conventional sensors in a field trial;
A filament formed by a coil 1 is prepared by winding eleven turns of 0.05 mm hard drawn thermopure platinum wire having a resistance of 47 ohms per metre around a mandrel having a diameter of 0.5mm against a tension of 1 5 grams. The coil 1 is then cleaned by immersion firstly in a solution of potassium permanganate in concentrated sulphuric acid, washed in tap water and then washed with a mixture of dilute nitric acid and hydrogen peroxide. The coil is then washed with de-ionised water and dried.One of the free ends of the wire leading to the coil is connected to a clip 2 and a pair of forceps 3 is used to hold the wire leading to the coil immediately adjacent the coil as shown in Fig. 1. The variable electric power supply is connected to the clip 2 and the forceps 3 and a current is passed along the lead wire between the clip 2 and the forceps 3 and the current is adjusted until the lead wire glows at a dull red heat. The wire leading to the coil is then bent into a kink 4 as shown in Fig. 1. The current to this portion of the lead wire is then reduced slowly to anneal this portion. A similar process is carried out on the opposite lead into the coil and then the lead wires of the coil are welded to a header including a pair of support posts 5 mounted in a standard can.
25 grams of alumina powder. gamma grade, having a mean particle size of less than or equal to 500A is mixed with 200 ml of methanol and agitated in an ultrasonic bath for 30 minutes.
The mixture is then allowed to settle for 7 minutes and the supernatant slurry is decanted off the top of the sediment. The supernatant slurry is then left for 48 hours to settle. The methanol is decanted off the settled alumina and the alumina dried and stored. This dried alumina powder has a typical particle size of 200nm or less. 1 gram of this dried alumina powder is mixed with one gram each of ammonium chlorpalladite and thorium nitrate. These three are placed in a grinding jar containing agate grinding elements and 7 ml of methanol. The compounds are mixed and ground together in a micronising mill for 1 5 minutes to provide a smooth slurry.The alumina particles or crystallites are very abrasive and, together with the agate grinding elements rapidly reduce the ammonium chloropalladite and thorium nitrate to crystallites having a typical particle size of 5 nm or less. The grinding step also ensures that complete and intimate mixing of the alumina, ammonium chloropalladite and thorium nitrate takes place.
The header is mounted in a jig so that the coil 1 4s horizontal and with an electrical power supply connected between the support posts 5. A drop of slurry is placed on the platinum coil 1 using a small pipette and any excess slurry removed by lightly touching the base of the coil 1.
The slurry adheres to the coil 1. The slurry on the coil is allowed to dry for 3 minutes in air and then a current of 200 mA is passed through the coil for 1 minute. The current through the coil is then increased to a current of 275 mA for two minutes. This drives off the methanol. A further drop of slurry is then added to the coil and a current of 200 mA passed through the coil for one minute. A current of 300 mA is then passed through the coil for 2 minutes followed by a current of 360 mA for one minute. Further drops of slurry are placed on the coil with this three stage heating regime following each addition until a bead of material around the coil having a diameter of between 1.75 and 2.0 mm is formed.After completion, a current of 400 mA is passed through the coil for fifteen minutes and during this stage the ammonium chloropalladite and thorium nitrate at least partly decompose.
The sensor is then conditioned by applying a current of 400 mA, a typically operating current, through the coil and allowing it to run in air for five minutes. A flow of methane and air in stoichiometric proportions with 13% methane is then introduced over the surface of the bead for five minutes. During this conditioning process the catalyst precursors complete their decompose tion to provide a very finely divided catalyst formed by particles of palladium metal and thorium oxide which bind together the alumina particles or crystallites. The sensor glows bright red during this period and this is followed by a further five minute period with an air flow passing over the beads. This current is then continued at least overnight with the sensors in static air to provide a constant current "soak" for the sensors to enable their characteristics to stabilise. This constant current soak may last a number of days, typically for days.
Figs. 3a and 3b are photomicrographs of the bead produced using an electron microscope and it is clear from these Figures that there is a striking contrast between the bead of the device in accordance with this invention and that of a conventional device. The bead of the conventional device was formed from a solution of an alumina precursor and the photographs show the typical structure of such devices, which is that they are formed by large fractured platelets of alumina which typically have a size of 8000 nm, together with some sintered lumps of catalyst. The platelets themselves are substantially impermeable to inflammable gases.In contrast to this the example of the sensor in accordance with this invention includes a number of white round regions, which are formed by agglomerations of alumina particles, which have a typical bulk mean size of 1 50 nm, and which are permeable to the inflammable gases. The agglomerations of alumina particles are surrounded by a porous open gas diffusive skeletal matrix consisting of finely divided catalyst and alumina. It is this porous open gas diffusive skeletal matrix formed by alumina particles interspersed between and bound together only by particles of the catalyst which provide the large number of active catalytic sites that are then available at which catalytic oxidation of the flammable gases can take.place.
Fig. 4 illustrates the results of a laboratory test in which a sensor made in accordance with this invention and a standard pellistor made by a method in accordance with the description of
British Patent Specification 892,530 were exposed to a mixture of 50% L.E.L. (Lower Explosive
Limit) petrol vapour in air. Lead compounds from the petrol vapour tends to poison the catalysts of both pellistors and the graph shown in Fig. 5 shows that they both start with similar initial declines in their sensitivity but then, the reduction in sensitivity of the sensor in accordance with this invention levels off whereas the sensitivity of the standard pellistor continued to decline.
Fig. 5 illustrates the results of another laboratory test again using a sensor in accordance with this invention and a standard pellistor in accordance with British Specification 892,530. In this test, both pellistors were exposed to a concentraton of 10 parts per million of hexamethyldisiloxane (HMDS) in air. The graph shows a marked difference in the rate of poisoning of the conventional pellistor and that in accordance with this invention.
To confirm these laboratory test results, a field trial was carried out on an offshore platform in the Dunlin Alpha oilfield in the North Sea. The nature of the catalyst poison had not been defined but tests indicated that it was probably sulphur based. At each of ten locations around the platform both a sensor made in accordance with this invention and a standard pellistor made in accordance with British Patent Specification No. 892,530 were installed. The sensors were all calibrated initially with a 0.95% butane/air mixture and then recalibrated at monthly intervals with the same gas/air mixture. Table I shows the result of the tests for the first six months with the sensors in accordance with this invention being marked with an asterisk. The average of these results is shown in the graph illustrated in Fig. 6.These show that the sensor in accordance with this invention is at least ten times more resistant to poisoning by the atmospheric impurities present in this location than the conventional sensor.
TABLE I
I- I Sensor Initial RECALIBRATION FIGURE / CUMULATIVE % CHANGE Number Calibration D A T E (0,tane) 16.4.80 | 20.5.80 9.7.80 ' 6.8.80 1 11.9.80 AS/10* 60 67 52 / 60 / 60 59 +12 / -13 / -15 / -1 -15 12 3 5 60 58 / 30 / 12 ~ w - 3 / - 52 / -40 NO RESULT AS 1 * 5 58 5 58 60 1 -11 -.15 /-17 /-20 1 /-20 ilsJ8 + 63 20 o 70 -u;;0 ~ -77 ~ -q5 521256 -60 50 Lt5 No RESULT AS/LSY -60 -17 - 38 -80 -93 RESULT 521272 60 50 12 iULT Ja/16 60 LL J2/272 60 50 5 12 NO RE SULT 9 60 9 O -85 -100 N O R E S U-L T AS 13* 90 90 90 ~"t- 78 90 85 O ~-'~ 0 -13 0 -6 12/362 55 ~''~ c v 60 62 72 -8 -8 -8 ,-'~ - , +13 AS 18* , 0 75 ~, ' 50 57 5 5 +25 +4 ~-v~ -1 -7 O AS 11* 0 0 8 7 Water No . O ,-~ +13 ,-'~ -11 Damaged Result 12 3 3 60 25 15 - N 0 R E S U L T RESULT 80 73 90 80 70 82 - +12 O -13 +2 J-2 252 60 55 ~- 63 30 20 o No ~ -8 -4 -52 ,-'~ -84 Result AS/9* 60 e: 58 / No 30 -,S't 60 ;; > ' 7 - -3 Result - ,-' < -3 ,-- < -3 +22 127367 0 30 20 ~,- < N O R E S U L T -50 -50 -85 J2 2 9 60 No 30 / 25 20 25 Result ,-'~ -50 -80 - - ,,f'- 8 AS 1 * -O 5 50 70 5,8 -7 -22 -10 o -22 ,-' < -10 -/ < Average DcP * +1 -2 -7 -7 -2 Loss of Standard sensitivity Pellistor -30 -53 -76 > -82 > -82 Duration of 15.3.80 32 66 116 144 180 Appraisal (Days) .
Claims (12)
1. A gas sensor element comprising an electrical resistance filament surrounded by a bead which includes an array of alumina particles interspersed between and bound together only by particles of a catalyst for inducing catalytic oxidation of flammable gases to form an open porous
gas diffusive skeletal matrix having a mean particle size of less than 20nm.
2. A gas sensor element according to claim 1, in which the particles of the catalyst have a
mean particle size of less than 5 nm.
3. A method of making a gas sensor element comprising depositing on an electrical
resistance filament a slurry formed by a mixture of alumina and at least one catalyst precursor in
a substantially non-aqueous organic liquid, the mean particle size in the slurry being less than 20nm, removing the liquid and decomposing the at least one catalyst precursor so that the
filament is surrounded by a bead which includes an array of alumina particles interspersed
between and bound together only by particles of a catalyst for inducing catalytic oxidation of
flammable gases to form an open porous gas diffusive skeletal matrix having a mean particle
size of less than 20nm.
4. A method according to claim 3, in which the slurry is subjected to a wet grinding stage
before being deposited on the electrical resistance filament.
5. A method according to claim 4, in which the mean particle size of the at least one
catalyst precursor is reduced to 5 nm.
6. A method accoding to claim 3, 4 or 5, in which the alumina is subjected to a pre
treatment in which it is mixed into a slurry with a substantially non-aqueous organic liquid and
subjected to ultrasonic vibrations followed by a sedimentation step, only the upper fraction so
obtained then being mixed with the at least one catalyst precursor.
7. A method according to any one of claims 3 to 6, in which the slurry is formed from a
mixture of equal parts by weight of alumina, ammonium chloropalladite, and thorium nitrate.
8. A method according to any one of claims 3 to 7, in which the non-aqueous organic liquid
is ethanol or methanol.
9. A method according to any one of claims 3 to 8, in which the electrical resistance
filament is supported horizontally whilst the slurry is deposited onto it.
10. A method according to any one of claims 3 to 9, in which leads of the electrical
resistance filament are bent into a kink and annealed before the slurry is deposited onto the
filament.
11. A method according to any one of claims 3 to 10, in which the sensor element is
conditioned by heating the bead in an atmosphere formed by a stoichiometric mixture of a
hydrocarbon gas and air.
1 2. A method according to any one of claims 3 to 11, in which the completed gas sensor
element is subjected to a constant current "soak" for a number of days.
1 3. A method of making a gas sensor element according to claim 3, substantially as
described with reference to the accompanying drawings.
1 4. A gas sensor element when made by a method in accordance with any one of claims 3
to
12.
1 5. A gas sensor element according to claim 1 constructed substantially as described with
reference to the accompanying drawings.
Priority Applications (1)
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GB8040618A GB2066963B (en) | 1980-01-02 | 1980-12-18 | Gas sensor elements and methods of manufacturing them |
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GB8000040 | 1980-01-02 | ||
GB8040618A GB2066963B (en) | 1980-01-02 | 1980-12-18 | Gas sensor elements and methods of manufacturing them |
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GB2066963B GB2066963B (en) | 1983-07-27 |
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Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2489961A1 (en) * | 1980-09-05 | 1982-03-12 | Nat Res Dev | ELEMENT ASSOCIATING A CATALYTIC MATERIAL AND AN ELECTRIC RESISTANCE FOR THE DETECTION OF COMBUSTIBLE GASES |
GB2121180A (en) * | 1982-05-01 | 1983-12-14 | English Electric Valve Co Ltd | Catalytic combustible-gas detectors |
GB2125554A (en) * | 1982-08-16 | 1984-03-07 | Sieger J & S Ltd | Catalytic gas detector |
US5314828A (en) * | 1990-06-12 | 1994-05-24 | Catalytica, Inc. | NOx sensor and process for detecting NOx |
US5486336A (en) * | 1990-06-12 | 1996-01-23 | Catalytica, Inc. | NOX sensor assembly |
WO1998022387A1 (en) * | 1996-11-18 | 1998-05-28 | The University Of Connecticut | Nanostructured oxides and hydroxides and methods of synthesis therefor |
EP1632771A1 (en) * | 2003-06-12 | 2006-03-08 | Riken Keiki Co., Ltd. | Catalytic combustion type gas sensor and method for manufacture thereof |
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1980
- 1980-12-18 GB GB8040618A patent/GB2066963B/en not_active Expired
Cited By (11)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2489961A1 (en) * | 1980-09-05 | 1982-03-12 | Nat Res Dev | ELEMENT ASSOCIATING A CATALYTIC MATERIAL AND AN ELECTRIC RESISTANCE FOR THE DETECTION OF COMBUSTIBLE GASES |
US4457954A (en) * | 1980-09-05 | 1984-07-03 | National Research Development Corporation | Catalytic gas-sensitive elements |
GB2121180A (en) * | 1982-05-01 | 1983-12-14 | English Electric Valve Co Ltd | Catalytic combustible-gas detectors |
GB2125554A (en) * | 1982-08-16 | 1984-03-07 | Sieger J & S Ltd | Catalytic gas detector |
US5314828A (en) * | 1990-06-12 | 1994-05-24 | Catalytica, Inc. | NOx sensor and process for detecting NOx |
US5486336A (en) * | 1990-06-12 | 1996-01-23 | Catalytica, Inc. | NOX sensor assembly |
WO1998022387A1 (en) * | 1996-11-18 | 1998-05-28 | The University Of Connecticut | Nanostructured oxides and hydroxides and methods of synthesis therefor |
US6162530A (en) * | 1996-11-18 | 2000-12-19 | University Of Connecticut | Nanostructured oxides and hydroxides and methods of synthesis therefor |
US6517802B1 (en) | 1996-11-18 | 2003-02-11 | The University Of Connecticut | Methods of synthesis for nanostructured oxides and hydroxides |
EP1632771A1 (en) * | 2003-06-12 | 2006-03-08 | Riken Keiki Co., Ltd. | Catalytic combustion type gas sensor and method for manufacture thereof |
EP1632771A4 (en) * | 2003-06-12 | 2010-09-15 | Riken Keiki Kk | Catalytic combustion type gas sensor and method for manufacture thereof |
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GB2066963B (en) | 1983-07-27 |
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