CA1176721A - Microminiature palladium oxide gas detector and method of making same - Google Patents
Microminiature palladium oxide gas detector and method of making sameInfo
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- CA1176721A CA1176721A CA000425644A CA425644A CA1176721A CA 1176721 A CA1176721 A CA 1176721A CA 000425644 A CA000425644 A CA 000425644A CA 425644 A CA425644 A CA 425644A CA 1176721 A CA1176721 A CA 1176721A
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Abstract
Abstract of the Disclosure A microminiature palladium oxide gas detector and its method of manufacture. The detector comprises an extremely small coil of extremely fine wire such as platinum which is retained, sealed and insulated with an amorphous ceramic binder. The detector further comprises a catalyst applied to its exterior surface. The detector is manu-factured by winding the wire about a mandrel which is desirably molybdenum. The coil is then coated with the binder composition which preferably comprises reduced chromic and phosphoric acids. The binder is then cured to retain the coil and the mandrel is removed by etching or oxidation. The catalyst is then applied which is preferably palladium nitrate in a weakly acidic hydrolyzed solution which has been adjusted to a pH of about 3 with tertiary octyl amine. The catalyst is then dehydrated and calcined desirably using an automatic electronic pulse controlled machine to precisely adjust and control processing temperatures and times. When hydrolyzed palladium oxide is used as the catalyst, the processing temperature never reaches the point where the oxide becomes reduced to the metal.
Description
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This i5 a divisional application of copending application serial no. 317,423, filed December 5, 1978 BACKGROUND OF THE INVENTION
-a) Field of the Invention This invention relates to detectors or sensors for determining the presence of a combustible gas in air and more particularly relates to detec-tors which detect a change in temperature when a combustible gas is oxidized at the surface of the detector.
b) Histoxy of the Prior Art . _ In the prior art, gas detectors are known which comprise a metal wire which is connected as a resistance into a wheatstone bridge circuit and is electrically heated to a temperature of about 900C. At that temperature a combustible gas will oxidize at the surface of the wire, further increasing the temperature of the wire~ which in turn alters the electrical resistance of the wire which is detected by the wheatstone bridge circuit. Such detectors are unsatisfactory for many reasons. The wire must be manu-factured from a high melting metal such as palladium orplatinum which has catalytic properties. Even when such exotic metals are used, palladium or platinum metal vaporizes from the surface of the wire at the hi~h operating temperatures, thus causing early failure of the metal wire detector.
Additionally, even slight vaporization results in a change in the resistance of the wire, thus necessitating frequent zero adjustments to the bridge circuit.
Such aetectors additionally are insufficiently sensitive for many a plications since they are usually unable to 3~ detect the presence of a combust ble gas at ms/ f~J
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concentrations lower than abou-t 300 ppm. Furthermore, the large size and high operating temperatures of such detectors u~ilize an undesirably high amount of electrical energy.
These detectors also had undesirably long response times and were frequently position sensitiveO
A few of the disadvantages of the wire type com-bustible gas detector were overcome when it was recognized that the wire could be wound in a coil and coated with a ceramic material containing a compound such as palladium la chloride or chloroplatinic acid (H2PtC16 6H2O) which would convert to a ca~alytic metal upon application of sufficient heat. Alternatively, a catalytic metal could be condensed upon the ceramic coating by heating a catalytic metal in close proximity to the coatingO
Such detectors are disclosed in U.S. patents 3~092,799 issued June 4, 1963 and 3,200,011 issued August 10, 196~ to A. R. Baker, 3,564,474 issued February 16, 1971 to Firth et al and 2,816,863 issued December 17, 1957 to Page.
While such detectors were an improvement over the wire type detectors by preventing vaporiza-tion of the wire by means of the ceramic or refractory coating material, such detectors continued to have the disadvantage of vaporization of the catalytic metal at the surface of the detector, thus substantially reducing sensitivityO Additionally such detector sometimes continued to have varying responses depending upon the orientation of the detector. In order to maintain at least some sensitivity, such detectors were dependent upon diffusion of additional catalytic metals through the ceramic material to the surface. Such detectors therefore continued tv have characteristics which varied~over their useful life and continued to have poor sensitivity. The prior art
This i5 a divisional application of copending application serial no. 317,423, filed December 5, 1978 BACKGROUND OF THE INVENTION
-a) Field of the Invention This invention relates to detectors or sensors for determining the presence of a combustible gas in air and more particularly relates to detec-tors which detect a change in temperature when a combustible gas is oxidized at the surface of the detector.
b) Histoxy of the Prior Art . _ In the prior art, gas detectors are known which comprise a metal wire which is connected as a resistance into a wheatstone bridge circuit and is electrically heated to a temperature of about 900C. At that temperature a combustible gas will oxidize at the surface of the wire, further increasing the temperature of the wire~ which in turn alters the electrical resistance of the wire which is detected by the wheatstone bridge circuit. Such detectors are unsatisfactory for many reasons. The wire must be manu-factured from a high melting metal such as palladium orplatinum which has catalytic properties. Even when such exotic metals are used, palladium or platinum metal vaporizes from the surface of the wire at the hi~h operating temperatures, thus causing early failure of the metal wire detector.
Additionally, even slight vaporization results in a change in the resistance of the wire, thus necessitating frequent zero adjustments to the bridge circuit.
Such aetectors additionally are insufficiently sensitive for many a plications since they are usually unable to 3~ detect the presence of a combust ble gas at ms/ f~J
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concentrations lower than abou-t 300 ppm. Furthermore, the large size and high operating temperatures of such detectors u~ilize an undesirably high amount of electrical energy.
These detectors also had undesirably long response times and were frequently position sensitiveO
A few of the disadvantages of the wire type com-bustible gas detector were overcome when it was recognized that the wire could be wound in a coil and coated with a ceramic material containing a compound such as palladium la chloride or chloroplatinic acid (H2PtC16 6H2O) which would convert to a ca~alytic metal upon application of sufficient heat. Alternatively, a catalytic metal could be condensed upon the ceramic coating by heating a catalytic metal in close proximity to the coatingO
Such detectors are disclosed in U.S. patents 3~092,799 issued June 4, 1963 and 3,200,011 issued August 10, 196~ to A. R. Baker, 3,564,474 issued February 16, 1971 to Firth et al and 2,816,863 issued December 17, 1957 to Page.
While such detectors were an improvement over the wire type detectors by preventing vaporiza-tion of the wire by means of the ceramic or refractory coating material, such detectors continued to have the disadvantage of vaporization of the catalytic metal at the surface of the detector, thus substantially reducing sensitivityO Additionally such detector sometimes continued to have varying responses depending upon the orientation of the detector. In order to maintain at least some sensitivity, such detectors were dependent upon diffusion of additional catalytic metals through the ceramic material to the surface. Such detectors therefore continued tv have characteristics which varied~over their useful life and continued to have poor sensitivity. The prior art
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detectors were relatively large and in order to maintain their catalytic surfaces at usuable tem-perature, much energy was dissipated through con-duction, convection, and radiation. Additionally, the operating temperatures, although sli~htly lower than the operating temperatures frequently utilized with a wire detector, continued to be undesirably high, thus substantially reducing the useful life of the detector and utilizing undesir~bly high quantities of electrical energy.
Such energy re~uirements were not only wasteful but also required large batteries in portable instru-ments.
Of particular importance is the fact that prior art sensors lack zero stability in the absence of combustible gas and failed to stay in calibration when subjected to prolonged exposure to the low levels of combustible gas frequently present in industrial en vironments. These effects are due to the ~act that al~hough some prior art sensors used palladium oxide, the oxide was only a thin veneer on palladium metal.
When viewed under a microscope, prior art sensors have a blue-to-violet iridescent ~lack appearance characteristic of thin oxide films on palladium metal~ Some patches on the sensor re-quently show gray metal. By contrast, the sensor of the present inventlon varies in appearance from prior art sensors since iridescence and gray metal are not visible.
Furthermore, such sensors could not be made sufficiently small since the most suitable metals could not be wound in sufficiently small coils.
It is known that the la~ industry uses proprietary machines to wind very small coils o tungsten wire on mandrels; however, these machines are unable to wind such coils from metals such as platinum or palladium since the coils do not retain their shape.
BRIEF DESCRIPTION OF THE INVENTION
There is provided in accordance with this in-vention a detector for combustible gases in air which has high sensitivity, i.e., is able to detect com-bustible gases in concentrations as low as 1 to 10 ppm. The detector in accordance with the present in-vention i5 substantially smaller than prior art de-tectors and requires a comparatively low operating temperature which in turn reduces the amount of electrical energy utilized by the detector and increases the useful life of the detector. Furthermore, the detector in accordance with this invention, after an initial break-in period, has a more ùniform, i.e., less variable, sensitivity over its useful life.
Rather than using a catalytic metal, a detector of the present invention utilizes palladium oxide or palladium oxide hydrate (PdO-nH20) as the catalyst.
The new hydrate catalyst increases the sensitivity of the detector while at the same time permits lower operating temperatures. Additionally, the detector in accordance with the present invention utilizes a wire having a smaller diameter than the wires utilized in prior art combustible gas detectors. The use o~
the smaller wire accomplishes several purposes. One purpose is to increase the sensitivity of the detector since it requires less heat to alter the resistance of the thinner wire. Another purpose is to reduce the quantity of electrical energy needed to heat the wire to its opera~ing temperature. The thin wire ~ '7~ ~
further permits the use of a smaller quantity of protective ceramic material which reduces the amount of heat required to raise th~ temperature of the ceramic material surrounding the wire~ Such re-duction in the quantity of heat required to raise the temperature of the ceramic material decreases energy requirements of the sensor while at the same time reduces the amount of combustible gas which must be oxidized in order to increase the tem-perature of the sensor to produce a usable signal.
The detector in accordance with the invention is an element which, at a temperature of below about 600C, undergoes a change in electrical resistance $
when exposed to a combustible gas in air. The element comprises a wire~coated with a palladium ni-trate or ammonium chloropalladite solution which is heated to from about lOO~C to about 600C to form a palladium oxide composition. The prefe~red solution is a palladium nitrate solution which is most pre-ferably hydrolyzed in a weakly acid solution and heat-ed to from about 100C to about 600C, preferably 500~C
to 600C, to precipitate and to calcine palladium oxide hydrate. The wire comprises a metal or metal alloy having a melting temperature above 1500C and desirably has a circular cross section having a diameter of from about 0.0001 cm to about 0.0025 cm.
Desirably, a coating of ceramic materials is present between the wire and the palladium oxide hydrate.
The invention further comprises a method for the manufacture of the element which comprises winding the wire around a mandrel having a diameter of from about 0.01 to about 0.1 cm to form a wire coil, the mandrel being more susceptible to oxidation than the ~ 7 ~d ~_ wire. After the coil is formed, it is coated with a ceramic precursor solution.
The ceramic precursor solution preferably contains phosphoric acid and chromic acid anhydride reduced by aluminum trihydrate, aluminum hydroxide or hydroxides of other metals. To the solution is added 5 to 10%
tabular a~umina, or other ull sintered high tem-perature oxide, having a particle ~ize of less than 0.5 micron. The proportions of the ingredients of the 1~ precursor solution should, on a dry basis, desirably havè
the molar ratios of about A1203:0.8 Cr203:3P2O5. Cerama-sind*~ manufactured by Aremco Products Inc., is a com-mercially available mixture containing aluminum phosphate and phosphoric acid which has been found suitable for blending with a water insoluble metal oxide such as A1203. The coil is then heated slowly to above about 255G to remove water oE crystalization and to dehydrate the coating. The coating i5 cured by raising the temperature to between 500~C to 600C
to form a cured ceramic binder coating having a thick-ness of from about 0.001 cm to about 0.025 cm. The mandrel is then removed by chemical or thermal oxidation.
The coil is then coated with a uni~orm solution containing ~rom about 1 to about 50, preferably from about 2 to about 10 weight percent of palladium nitrate, Pd(N03)2 4H20, or ammonium chloropalladite and the coil is heated to a temperature of from about 250C to about 750C for from about l to about 60 seconds to convert the palladium solution to a pal-ladium oxide composition.
In accordance with the method of the invention, prior to coating the coil with palladium nitrate solution, the coil is desirably coated with a solution of from about 5 to about 60 weight percent, preferably * trade mark 35 to 55% of aluminum nitrate, A1(NO3)3 9H20, and heated to convert the aluminum nitrate to aluminum oxide. The solutions can be as dilute as desired at the expense of applying additional coats. Permitting the solutions to stand at least 4, and preerably 24, hours before use is desirable. Solutions should be discarded after about one month.
The small size of the sensor, the high surface area o the catalyst support and good catalyst dispersion result in a sensor having improved speed of response and sensitivity and which is not position sensitive.
Independently of the manufacture of wire coils for sensor.s, miniature wire coils can be made in accordance with the invention for any purpose.
According to the method of the invention for making wire coils, a wire having a diameter o~ from about 0 0001 cm to about 0.005 cm may be wound about a mandrel which can be oxidized without oxidizing the wire. The wire, wound about the mandrel, is then coated with a ceramic precursor, the precursor is cured to form a ceramic binder and the mandrel is removed by oxidization.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a side elevational view of a detector connected to electrodes at about ten times actual de-tector size.
Figure 2 is an enlarged cross sectional side view of a sensor manufactured in accordance with the invention~
Figure 3 is an enlarged cross sectional view of a wire coil wound upon a mandrel.
Figure 4 is an enlarged sectional view as seen in the direction of the arrow shown in Figure 2.
DETAILED DESCRIPTION OF THE INVENTION
"Comb-ustible gas", as used herein means a gas, a vapor, or an atmospheric dispersion of fine particles which will react chemically at or near the surface of the detector and thereby change the detector temperature, when it is operated below abou-t 600C.
Examples oE such gases or vapors are carbon monoxide;
methanol; ethanol; isopropanol; formaldehyde; ethylene oxide; terpenes; aliphatic hydrocarbons such as methane~
ethane, propane, isobutylene, and octane; and cyclic hydrocarbons such as benzene, toluene, xylene, and nitrobenzene.
"Sensitivity" as used herein means the degree of ability to provide a meaningful signal which is distinguishable from background noise.
Comparative sensitivity measurements between detectors are made either by comparing the ratios of the changes in resistance from the air resistance, or by comparing millivolt bridge signal output for a specified input such as 2.5 volume percent CH~ in air.
'ICure'' or "curing" as used herein means hea~ing to a sufficient temperature to render a substance water insoluble and self-adheren~ at elevated te~peratures such as above about 80QCo The curing temperature in the case of phosphate and chromate containing ceramic precursor solutions is usually between about 440C and 600C.
In accordance with the invention, there is pro-vided an element; i.e., a detection device, for de-tecting the presence of a combustible gas in air. The combustible gas is usually the gas or vapor of a carbon containing compound but may also be other combustible gases such as hydrogen.
The element in accordance with the invention may detect particular combustible gases at temperatures well below 600C. For example, methane is detected at a temperature of between about 365C and 600C, hydrogen is detected at a temperature as low as about 50C, usually below 250C, and carbon monoxide is detected at a temperature of about 280C.
The detector element 10 in accordance with the invention, as seen in Figure 1 and Figure 2, is a wire 12 coated with a palladium compound. Microminiature sensors of this invention can be made using -8a-~ 7,~ ~
chloride compounds such as PdC12, H2PdC14, (NH4)2PdC14, etc. but require special aging or other treatments to remove the chlorine, which is a known catalyst poison.
At least two methods may be used. One consists of prolonged heating, i.e., (days~ at elevated tem-peratures, e.g., from about 200C to about 65QC which are well below those at which the oxide is destroyed.
The other method is to fuse the chloride compound with an alkaline metal carbonate to precipitate the chlorine so that later the chloride salt can be leached out by boiling water.
Preerably the detector is coated with hy-drolyzed palladium nitrate solution. The coated element for any of the c~mpounds is then heated to from about 100C to about 350C to convert the palladium compound such as palladium nitrate to a palladium oxide composition w~ich is desirably palladium oxide hydrate. Most o the impurities such as chlorine which can affect catalyst dispersion are re~oved at this temperature. Next, the detector tem-perature is raised preferably to from 500C to 600C
for further curing and stabilizing. If chlorine or hydrogen is present and the low temperature step is omitted, the catalyst particle size is increased by agglomeration and the detector sensitivity is reduced.
Palladium nitrate is preferably hydrolyzed in a weak-ly acidic solution prior to application so that the desired palladium oxide hydrate is obtained upon heating. Palladium nitrate complexes such as tetramine palladous nitrat~ [(NH3)4Pd}(NO3)2 may also be used.
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The wire 12 is preferably a metal or metal alloy, having a melting temperature above 1500C and having a high temperature coeEficient of resistanc~.
Examples of usuable metals for the wire are pla-tinum, palladium, nickel, rhodiuml ruthinium, iridium, vanadium, zirconium and their alloys.
- Particularly desirable metals for use in the wire of the element are platinum and palladium due to their high resistance to oxidation. A particularly desirable wire is manufactured from platlnum or a platinum alloy such as an alloy of about 90 weight percent platinum and about 10 weight percent rhodiurn.
Platinum is particularly desirable because of its high temperature coefficient of resistance.
Alloys of platinum and rhodium are desirable due to high strength.
The wire 12 of the element 10 ;s of a small diameter, preferably between about 0.0001 and about 0.005 cm and most preferably ~etween about 0.0008 and 0.002 cm. The small wire diameter permits more rapid heating of the wire when heat of combustion of the gas is detected at the palladium oxi~e surface of the element. The use of a small wire permits a very small detector, i.e., from about 0.005 to about 0.2 cm in diameter, to be manufactured.
The wire 12 may be turned or bent into any de-sirable shape and is preferably in the form of a coil 14, as seen in Figures 2 and 3, to permit the use of a long wire in a given volume. The coil di-ameter is between about 0.002 and about 0.1 cm. For ease of manufacture, the coil diameter is pr~ferably at least about 0.01 cm. The wire 12 may have any de-sirable cross section such as triangular, h~xagonal, ~ 7 round, rectangular or square.
When the wire 12 is a coil 14 in accordance with the preferred embodiment, it is usually formed by winding the wire around a mandrel 17 preferably having a diameter of from about 0.01 to about 0.1 cm to form the wire coil 14. In order to form such a small coil, precision winding is necessary.
It is desirable to have the turns of the coil close to the surface of the detector to optimize sen-sitivity and to maximize the length of the wire within the volume of the detector within the limits of wire strength. A large ratio of coil diameter to wire diameter is therefore desirable to maximize the wire length. Prior art methods used in winding lamp coils or mandrels to retain coil shape cannot be used since lamp coils use a small ratio of coil diameter to wire diameter which permits the shape of such coils to be retained by the stiffness of the wire. The mandrel 17 is manufactured from a material such as molybdenum whi~h is more susceptible to oxidation than the wire.
After the coil 14 is wound, it is desirably coated with a ceramic binder precursor solution.
Any suitable ceramic binder may be used. "Ceramic binder" as used herein includes high temperature in-organic contiguous compositions whether of crystalline or amorphous character. "Ceramic precursor" is a so-lution or dispersion which will form a ceramic binder upon heating to above about 250C. The resulting binder should be waterproof, be corrosion resistant, be electrically insulating, be able to fill the spaces between the turns of the coil prior to curing, and have sufficient strength to retain the shape of the coil and the spacing between turns. The ceramic pre-cursor should cure to the desired binder at a temperature well below 900C to minimize changes in wire xesistance due to vaporization of a portion of the metal wire.
The binder should have good surface area and other characteristics known to those skilled in the art, if the binder is used as the catalyst support. Otherwise, if a catalyst support is applied over the binder, the binder should provide a good ceramic adhesive surface with high thermal conductivity. 0f primary imp~rtance is the thermal stability of the binder, as well as free-dom from sintering following preliminary heat treatment and aging. The size, thermal conductivity, and radiation characteristics preferably are constan~ or at least changing only very slowly (weeks), to permit stable detector performance and freedom from long-term noise which would otherwise require frequent recalibration of the instrument.
Although numerous silicon-based binders and high temperature glasses can be used, for long-term sta-bility it is preferable not to use binders involving a silicon compound in combination with the very small diameter support wires of this .invention. Many re-searchers have found that in a reducing atmosphere in the presence of trace quantities of carbon, iron, or sulfur, minute quantities of silicon metal migrate to wires such as platinum causing lowering of the melting point, embrittlement, and changes in resistance These adverse results become increasingly important as the wire size is reduced to make very small detectors. See Thermocouple Temperature Measurement by P. A. Kinzie, pages 27, 34, 64, 73, and 174, published by John Wiley & Sons, 1973 and the references contained therein.
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It is f~rther desirable that the binder adhere to the wire of the coil to prevent shorting between turns and to provide good thermal conductivity. The binders should be free of l~mps or agglomerates which cannot penetrate and remain between the desirably very closely spaced turns of the coil.
The binder should be unaffected by corrosion which might otherwise result from atmospheres fre-quently found in detector applications. In addition, the binder must be free of any catalyst poisons. The viscosity of the binder precursor should preferably be between 20 and 80 centipoise to permit appli-cation to the coil wound on the mandrel, as, for example, by a ine camel ,hair brush.
Many prior art binder compositions are un-suitable for use in accordance with this invention.
A binder which does, however, meet these and other desirable criteria for detector use is an inorganic polymer consisting of complex phosphate compounds of metals and metal oxides such as aluminum, chromium, zirconiwm, magnesium~ zinc, calcium, barium, tin, ti-tanium, thorium, and o~her metals which form acid-proof oxides.
A particularly preferred binder composition is prepared by heating 85% H3PO4, AlOOH, 60% H2CrO4, and H2O together at 100C to 130C for approximately one hour with slow mixing and constant stirring. Any lumps and particles are removed by straining. The re-sulting mixture contains aluminum and chromic acid phosphates in water solution. The mixture is allowed to stand overnight and 5 to 10 weight percent of a metal oxide powder such as alpha A12O3 powder is added.
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The alumina is preferably fully sintered at a high temperature, e.g., tabular alumina, and ground to a particle size of less than 0.5 micron, Water is then added to bring the binder to a viscosity of preferably about 50 centipoise for con~enience of application.
The amounts of the ingredients used should approximate in the dry state the formula molar ratios o~
Al2O3 : 3P2O5 to Al2O3 : O.8Cr2O3 : 3P2O5 the molar ratio of Cr2O3 to A12O3 varies from 0 to 0.8.
When ph~sphoric acid (P205) and aluminum oxide (A12O3) are used alone, various ceramic precursor for-mulation in molar ratios and the characteri~stics of the resulting binders are given below:
P25/A123 = 2.3 Precipitates solids, hardens on standing, makes poor bonds P2s/A123 = 3 Metastable P25/A123 = 3 to 3.5 Maximum bond strength and usable solution stability P25/A123 = 1.4 Usable solution but weak bonds because of ~xcess The best molar ratio is seen to be A12O3:3P2O5.
The appropriate molar ration for other ceramic pre-cursor systems can similarly be readily determined by one skilled in the art.
The binder containing chromic acid (Cr2O3) is superior in binding strengt~ and thermal stability and is usable for for several months before clouding begins to appear, at which time it should be discarded.
The starting binder is best made using water dispersible aluminum trihydrate, Al(OH)3 or aluminum monohydrate, AlOOH, such as Dispal* (Philadelphia Quartz Company) or sa~mal* (DuPont), and slowly add-in~ this mixture to heated 85% phosphoric acid. The * trade mark ~'7~
phosphoric acid, H2CrO4, is made by adding chromic oxide, CrO3, to water. The CrO3 may be mixed direct-ly or as a 50 to 60 weight percent water solution. The chromium is hexavalent and should be partially re-duced to the trivalent form for best adhesion and long term binder characteristics. Reduction o~ ~he chromium to trivalent form can be accomplished by reducing with hydroxides of the metals chosen for the binder, or~ if metals or oxides of metals are used, by reducing with inorganic hydroxides, or hydrogen peroxide. Preferably, the chromium is reduced by by using a low molecular weight organic amine, such as 3-amino-1-propanol, trithenolamine, or other alkanolamine or'an amine in combination with a carboxylic acid sequestering agent to pre-serve the solution, such as oxalic acid, citric acid, gluconic acid, sulfosalicylic acid~ tartaric acid. Other sequestering agents such as acetyl acetone may also be used. The latter combination enables the ceramic preeursor to stand for months without clouding or deteriorating. Sequestering agents to preserve the solution, such as carboxylic acids, may be used alone without an amine at a 1:1 ratio with the 85% H3PO4 in the ceramic precursor.
When the binder alone is used as the catalyst support, it has been found useful to make the binder from ammonium phosphate, NH4H2PO4 and metal nitrates, ( 3)3 9~20 and Cr(NO3)3 9H2O, in a molar ratio of NH4H2PO4 to metal nitrate of from 1.5:1 to 1.75-1. An amine is desirably used to adjust the pH.
The resulting binder when dried and heated to 500C
to 600C has a very large surface area and is par-ticularly suitable as a catalyst support.
After the coil 14 is coated with the ceramic precursor solution, it is heated slowly up to a temperature of above about 250C, then rapidly up to about 500C to about 600C to dehydrate and to cure the coating. The temperature preferably should not exceed 600C. The resulting binder coating 18 is desirably from about 0.0002 to about 0.004 cm thick. The resulting ceramic coating retains the shape of the coil after the mandrel is removed.
Other suitable ceramic precursor solutivns such as colloidal silica may be used to treat the coil to form a ceramic coil-retaining coating. Such coatings should be compatible with the wire9 thermally stable, and should contain no catalyst poisons.
After the aluminum phosphate coating is de~
hydrated and cured, the mandrel 17 is rPmoved by chemical or thermal oxidation.
A suitable metal for use as a mandrel 17 is molybdenum since it can be removed by dissolving it in an aqueous solution containing from about 15 to about 40 parts by weight sulfuric acid, from about 15 to about 40 parts by weight nitric acid, and from about 30 to about 70 parts by weight water. Al-ternatively, a molybdenum mandrel can be removed by heating the mandrel in air at a temperature of be-tween about 750C and about 1000C. Many other metals can be used as the material for the mandrel 17. Ex-amples of other metals which can be removed by chem-ical etching are alumînum, zinc, and iron. Mo-lybdenum is particularly desirable since essentially no residue remains upon the coil 14 after the mandrel ~ 7~ ~
17 is oxidized. In the preferred embodiment of the invention, the mandrel 17 comprises a molybdenum cylinder.
After the mandrel is removed, the coil 14 re-tains its shape due to the aluminum phosphate coat-ing 18 which becomes water insoluble, even to boil-ing water, and essentially gas imper~ious after de-hydration and cure. The preferred binder is amor-phous over a wide temperature range and does not under-go changes which would permit the passage of gases, vapors or liquids through the binder.
After the mandrel is removed by chemical or ther-mal oxidation, the coil 14 is preferably coated with an aqueous solution containing from about 2 to about 10 and most preferably from about 4 to about 7 weight percent of hydrolyzed palladium nitratè or with an aqueous solution containing from about 2 to about 10 weight percent of ammonium chloropalladite. And then the coil 14 is heated to a temperature of from about 250C to about 500C for from about 1 to about 60 ~econds to form a surface comprising palladium oxide hydrate or palladium oxide. The solution is preferably a palladium nitrate solution and the sur-face preferably comprises palladium oxide hydrate.
Whenever coating with a palladium nitrate solution is subsequently mentioned herein, it is to be under-stood that a coating with an ammonium chloropalladite or alkali chloropalladite solution can be sub-stituted with somewhat reduced detector sensitivity.
The palladium nitrate is desirably hydrolyzed in a weakly acid solution, i.e., pH 2 to 4.
Desirably, prior to coating the coils 14 with a solution of palladium nitrate, the coil .
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14 is coated with a solution containing from about 5 to about 60, preerably from about 35 to about 55, weight percent of al~minum nitrate and heated to a temperature of between about 250C
and about 750C to convert the aluminum nitrate to aluminum oxide. The resulting alum;num oxide coating 20 is preferably from about 0.001 to about 0.0~5 cm thick.
The resulting aluminum oxide coating 20 fur-ther protects the wire from vaporization and oxidation. If desired, more than one coating o aluminum nitrate can be applied over the coil for additional protection and strength.
Desirably and preferably, subsequent to coat-ing the wire with aluminum nitrate solution and prior to coating with palladium nitrate solution, the coil is coated with a solution containing from 5 to about 70, preferably from about 40 to about 65, weight percent of thorium nitrate and heated to be-tween 250 and 750C to convert the thorium nitrate to thorium oxide. The resulting thoriu~ oxide coat-ing 22 provides a ceramic surface having a large surface area upon which the palladium nitrate solution can be applied. Ihe resulting high surface area increases the sensitivity of the element lO.
The thorium oxide coating 22 is preferably from about 0.001 to about 0.025 cm thick.
Repeated applications and conversions of thor-ium solution can be made after application and con-version of aluminum nitrate and before application of palladium nitrate. Most desirably at least two coats of thorium nitrate solution are applied to the element 10 and up to about lO repeated appli~
cations may be desirable for certain applications.
Similarly, as previously discussed, repeated appli-cations and conversions of aluminum nitrate solution can be made prior to application of the thorium ni-trate or palladium nitrate solutions. Preferably at least two to four coats of aluminum nitrate so-lution are made prior to application of thorium nitrate and up to about 10 repeated applications and conversions of aluminum nitrate solution are desirable.
Multiple applications of palladium nitrate so-lution are desirably made upon the thorlum oxide surface. The solution is most preferably a 0.27 molar hydrolyzed palladi~m nitrate solution. De-sirably at least 15 coats of palladium nitrate solution are made and up to about 20 such appli-cations continue to improve the sensitivity o~
the element, After the palladium nitrate in the solution is converted to palladium oxide hydrate, th~ surface of the detector is not believed to have a contiguous palladium oxide hydrate coat;ng.
The surface of the detector seems to comprise palladium oxide hydrate concentrations in dis-crete zones or areas which are so small that the extreme magnification of an electron microscope is required to see them.
After the palladium nitrate is converted to palladium oxide hydrate, the resulting detector is desirably sensitized by heating the detector in an atmosphere containing from about 3 to about 9 volume percent o~ a combustible gas in air until the sensitivity is substantially increased, eg., by 50%. Desirably at least 8 volume percent of combustible gas for ammoni~ chloropalladite is used and less than about 6 volume percent of combustible gas for palladium nitrate i9 used. Most preferably, from about 8 to about 8.8 volume percent of a com-bustib~e gas for ammonium choropalladite solutions and 4.5 to 5.5 volume percent for palladium ni-trate solutions are used. The heating temperature is from about 650 to about 825C, preferably from about 725C to about 775C, for from about 1 to about 30 minutes. The combustible gas is preferably me-thane. Heating at elevated temperatures, i~e~, in excess of 775C, in an air-gas mixture is to be avoided since the detector can readily burn out ~mder those conditions. Furthermore, if the de-tector is heated much above 840C, the oxide is los~ and palladium metal remains. The oxide layer that builds back is thinner, tends to reduce the catalytic activity of the metal, and is itself less desirable catalytically than the oxide which was lost.
After sensitization7 the deteetor is de-sirably stabilized by operating the sensor under controlled conditions so that the operating characteristics will be essentially constant in actual use. One such stabilization method is to operate the detector at 650C for one hour in air, for 18 hours in an atmosphere containing 2.5 volume percent methane, and again in air for 31 hours.
The stabilization procedure is highly desirable when ammonium chloropalladite is used but may be shortened or eliminated when palladium nitrate is used.
It has been found in all steps requiring heat-ing during the manufacture o the detector following curing of the binder, that a stepwise temperature rise -2~-is preEerred over a gradual temperature change.
The binder should be heated slowly to at least 255C to avoid foaming during dehydration.
In prior art detec~ors, the palladium catalyst is activated by reducing the oxide to the metal by high heat or by treatment with hydrazine~ Next, the catalyst is aged in air or oxygen at a high tem-perature to reform the oxide. This procedure has been found to be highly undesirable since it results in a detector o~ lower sensitivity, shorter life, and ~ower calibration stability. The catalyst of this in-vention is activated by removing part, but not all, of the bound water and the water molecules bel;eved to be trapped in the large palladium oxide hydrate molecule, which after partial removal is believed to increase the surface area of the catalyst particles.
See 0. Glemser and G. Peuschel, Z. inorg. Chem., 281, 44-53, 1955; or Chemical Abstracts, 50: 4612g, ~956.
At no time in the processing is the original pal-ladlum oxide reduced all the way to the metal.
A marked difference between the detector of this invention and prior art detectoræ can be ob-served when detecting combustible gases in an at-mosphere of pure nitrogen. The new detector main-tains calibration for a long time of exposure to combustible gas, while prior art detectors lose calibration rather quickly.
The resulting element is utilized in a com-bustible gas sensing device which may comprise the element connected into a wheatstone bridge circuit which detects resistance changes in the element re-sulting from a temperature change in the device -2~-caused by a reaction of the device with a combustible gas. The circuit activates a warning means when a combustibLe gas is detected. The warning means may be any desirable warning device such as a light, buz-æerl or other alarm, ~.g., a mechanical vibrator.
Usually the warning means is a warning light to be used in conjunction with an auditory signaling means.
In order to balance the wheatstone bridge cir-cuit when such a circuit is used, a non-catalytic reference element is needed which has the same op-erating temperature resistance as the active element manufactured in accordance with this invention. The reference element is manufactured in t~e same way as the active element, except that it has no coating of palladium oxide. Suc~ a standard reference ele-ment therefore permits the wheatstone bridge circuit to register resistance changes in the detecting ele-ment which are due solely to detection of a com-bustible gas by the element.
The reference element, which does not contain a palladium oxide coating, is instead coated with a compound which does not react with a combustible gas but which does react with changes in air temperature and changes in humidity of the surrounding air in a manner similar to the reaction of the active element with temperature and humidity in the surrounding air.
A suitable coating for the reference element is potassium hydroxide. Alternatively, a thin layer of gold may be used as reference standard element sur- -face coating.
Alternatively, an integrated circuit diferential amplifier may be used to detect a change in detector resistance in place of the wheatstone bridge.
~7'~'7~ ~
The element in accordance with the present invention is able to rapidly detect minute quantities of combustible gas in air. For example, such an element is capable of detecting carbon monoxide in concentrations of about 1 ppm and is able to detect hydrogen and methane in concentrations of less than 10 ppm..
Additionally, the operating temperature of the element in accordance with the invention can be as low as 250C and sometimes even as low as 50C when hydrogen is being detected, as low as 280~C when carbon monoxide is being detected, and as low as rom about 400C to about 600~ when methane is being detected. Such low temperature operation substantially increases the useful lifé of the detection element as compared with prior art elements for detecting a combustible gas in air. Furthermore, the coating of the wire in accordance with the present`invention substantially increases the durability of the element by reducing oxidation and vaporization of t~e wire.
Operating at high temperatures, i.e., above 8005C, is to be avoided in order to prevent the palladium oxide coating from converting to metallic palladium as occurred in the prior art. Furthermore, the sen-sitivity of the novel detector remains more constant than the high temperature detectors of the prior art.
The size of the element manufactured in accor-dance with the invention is substantially smaller than known prior art elements for detecting a com-bustible gas. The reduced element size permits very small detection units to be manufactured which can -be unobtrusively used. The low temperature op-eration and small size substantially reduces ~ '7~
energy consumption of the element in accordance with the invention as compared with energy con-sumption required by prior art elements ~or de-tecting combustible gases in air, which permits the manufacture of portable instruments having a very small size and long battery life. The detector size may be from about 0.005 to about 0.25 cm in diameter and is preferably 0.02 to O.l cm in diameter. Some idea of the very small size of the detector can be appreciated when it is realized that the dot or period on a typewritten page would be within the above dimensions.
As seen in Figure l, the detector is connect-ed to electrodes 26 by means of wire 12. The elec-trodes 26 in turn form a part of a detector assembly 28 which can be plugged into an electronic circuit.
It has been found that the small size of the detector has the added advantage of increasing the shock resistance of the assembly due to the smaller relative mass of the detectQr lO.
In addition, the detector in accordance with the invention is a very small sphere with greatly reduced associated convection energy, which can be placed in flame arrestor cavities having a much smaller inside radius about the detector than is possible with the much larger prior art detectors. The limit in size reduction is a radius or separation distance between the detector and the flame arrestor wall of approx-imately 0.14 cm. The practical result is a significantly reduced detector response time, which is particularly important in quickly detecting the heavier molecular weight toxic and combustible gases and vapors which diffuse more slowly.
~ 7 ~ ~
The palladium oxide hydrate catalyst is believed to be a colloidal dispersion or sol with chemically bound and included water. It loses water when heated roughly in proportion to the temperature. It is be-lieved that not all of the bound water is released even at temperatures as high as 800C. A very small amount of water is bound in the hydrate so tenaciously that the decomposition temperature of the oxide must be reached before it can be released. In accordance with this invention, the processing of the detector is carried out so that the decomposition temperature of the hydrate or oxide is never reached. When most of the water is removed, the structure is believed to &
have a substantially increased surface area similar to dehydrated or activated aluminum catalyst carriers.
See J. R. Anderson, Structure o~ Metallic Catalysts, page 168, Academic Press, 1975. Dehydrated palladium hydroxide, palladium oxide, and palladium metal from reduced palladium chloride are not believed to have a similarly increased surface area.
To get the large palladium oxide hydrate molecule described by Glemser, the starting solution should be weakly acid. To achieve optimum catalyst dispersion in a reasonable number of coats, a concentration of palla~ium nitrate is required which results in a very acid solution having a pH at about 0.57 due to hy- -drolysis. If one attempts to bring the solution to a suitably higher pH value by titrating with NH40H, insoluble palladium hydroxide is precipitated. Ni-trate ions are therefore preferably removed, to pro-duce a weakly acid solution, by extraction with a ~ 7~
water insoluble, hi~h molecular weight organic amine.
See Yu. G. Frolov et al, Theoretical Aspects of Amine Extraction, Atomic Energy Review, 7 (1), 71-138, 1969. For use in this invention, the amine should be a liquid insoluble in water, have a boiling point above that of water, contain no chlorine or other catalyst p~isons, have a density substantially dif-ferent than water to facilitate separation, and be characterized by low toxicity. Tertiary octyl amine has been found to be particularly desirable, In the preferred embodiment, a sufficient amount of tertiary octyl amine is shaken with the catalyst solution to produce a hydrolyzed palladium nitrate solution with a pH between 2.5 and 4.5~
In all cases herein, where a hydrolyzed nitrate ls converted to an oxide hydrate or oxide at a tem-perature of between about 100C and 750C, the con-version takes place in an oxygen containing atmosphere such as alr and the conversion time i5 rom about 10 seconds to about 15 minutes.
An instrument utilizing the detector disclosed herein is not position sensitive, that is, the in-strument reading, either in air or when detecting a combustible gas, does not ~o off calibration due to changes in the înstrument position. It is be-lieved that the position stability o the detector is due to its small size and spherical shape. Prior art detectors frequently had oval, cylindrieal or flat shapes which are believed to contribute to their position sensitivity. To obtain meaningful instrument readings, i~ h~s been found that reference elements should be of the same shape and size as the detector.
-2~-~7 EXAMPLE I
A precision winding machine is used to wind thirteen turns of resistance grade platinum wire having a diameter of 0.00127 cm about a clean and deoxidi~ed molybdenum wire tQ form a coil. The molybdenum wire has a diameter of 0.0434 cm and the spacing between turns is maintained at about 0,00127 cm.
Five percent aluminum oxide ~A1203), which is completely sintered alpha alumina ground to a par-ticle siæe o 0.5 micron or smaller, is then blended into a solution containing chromic and phosphoric acids to form a ceramic precursor having ~ dry molar 1' ration of about A1203 : 0.8Cr203 : 3P205. T
ing blend is then coated over the coil as a binding cement. The mandrel containing the coated c~il is then heated to about 250C by passing an electric current through the mandrel to dry the cement. The mandrel is then further heated to about 530C to cure the binder.
The mandrel is then removed by dissolving it in an aeid solution containing 30% of 65% nitric acid solution, 30% of 98% sulfuric acid solution and 40%
distilled water.
The coil is then washed in distilled water and dried. The ends of the coi.ls are then welded to nickel supports or posts which are mounted to a base on 0.381 cm centers.
Solutions are prepared for further treating the coil. Each of the solutions is thoroughly mixed at room temperature and allowed to stand overnight before use. High purity chemicals and distilled water are used.
'7;~ L
The prepared solutions are as follows:
Aluminum nitrate solution:
Al(NO3)3-9H2O 44 weight per cent H2O 56 weight percent Thorium nitrate solution:
Th~NO3)~-41l2V 62 weight percent H2O 38 weight percent Catalyst solution:
Pd(NO3)2 5.9 weight percent H2O 94.1 weight percent The catalyst solution is shaken with tertiary octyl amine for less than ten minutes in a small separator funnel and then allowed to s~and to bring the pH to about 3. The amine rises to the top, or it is separated in a centrifuge, and the amine and any particulate residue is discarded. Care must be taken that the palladium nitrate solution is kept near room temperature, since PdO'nH2O will pre-cipitate at higher water temperatures.
The coil is heated by passing an electric current through it, i.e., ohmically, to a temperature of 600C for 25 seconds, which is believed to remove any residual moisture or organic compounds, and such heating is repeated when the coil has been allowed to stand for over 15 minutes at any time during the pro-cessing.
The coil is then wetted with the foregoing solutions in the order specified in Table I to the temperature and for the times speciied in Table I.
TABLE I
Number of Coats Solution ~ erature Duration Degrees C Sec.
1 Aluminum nitra~e 250 10 1 Aluminum nitrate 250 10 50~ 10 2 Aluminum nitrate 350 10 2 Thorium ni~rate 250 10 Palladium cata~yst 350 10 solution 500 10 After coating is completed, the resulting coated coil, i.e., detector, is activated to maximize and to stabilize the detector performance. The activati~n is carried out in a metal chamber equipped with auto-matic controls which swit~h from one supply gas to 2~ another. The chamber has an internal volume of about 1 cubic centimeter. The detector is initially exposed to air flow of 1 liter per minute at room tem-perature followed by a combustible gas-air mixture at about 0.1 liter per minute for 15 seconds. The detector is then electrically, i.e., ohmically, heat-ed to 750C for about fifteen minutes. The electric current is then turned off and the detector ~s allowed to cool for about 10 seconds. The combustible gas 10w is then turned off and the air flow is again turned on.
After 10 more seconds, the activa~ed detector is re-moved ~rom tne chamber. The air-gas mixture which is used comprises about 5.5 percent by volume methane, i.e., C~14, in air.
The detector is then aged by electrically heating tha detector to 650~ for l hour at 0.3 liters per minute of dry air, followed by 16 hours at 0.3 ~liter per minute of air containing about 2.5 percent methane, followed by 31 hours in air at 0.3 liters per minute. When heated to ~50C, the resulting detector shows a 9 to 12% change in re-sistance when exposed to 2.5 volume percent methane in air. In addition, the detector expends only l50 milliwatts of energy during operation and has a volume of only 0.00018 cubic centimeters.
The discussion here~n is primarily directedto extremely small detectors which respond to oxidation reactions of combustible gases and their method of manu~acture; however, the dis~losure here-in with respect to small coils, is applicable to any detector utilizing a coil and operating at an ele-2~ vated temperature such as semiconductor gas detectors and solid electrolyte gas detectors.
t~
detectors were relatively large and in order to maintain their catalytic surfaces at usuable tem-perature, much energy was dissipated through con-duction, convection, and radiation. Additionally, the operating temperatures, although sli~htly lower than the operating temperatures frequently utilized with a wire detector, continued to be undesirably high, thus substantially reducing the useful life of the detector and utilizing undesir~bly high quantities of electrical energy.
Such energy re~uirements were not only wasteful but also required large batteries in portable instru-ments.
Of particular importance is the fact that prior art sensors lack zero stability in the absence of combustible gas and failed to stay in calibration when subjected to prolonged exposure to the low levels of combustible gas frequently present in industrial en vironments. These effects are due to the ~act that al~hough some prior art sensors used palladium oxide, the oxide was only a thin veneer on palladium metal.
When viewed under a microscope, prior art sensors have a blue-to-violet iridescent ~lack appearance characteristic of thin oxide films on palladium metal~ Some patches on the sensor re-quently show gray metal. By contrast, the sensor of the present inventlon varies in appearance from prior art sensors since iridescence and gray metal are not visible.
Furthermore, such sensors could not be made sufficiently small since the most suitable metals could not be wound in sufficiently small coils.
It is known that the la~ industry uses proprietary machines to wind very small coils o tungsten wire on mandrels; however, these machines are unable to wind such coils from metals such as platinum or palladium since the coils do not retain their shape.
BRIEF DESCRIPTION OF THE INVENTION
There is provided in accordance with this in-vention a detector for combustible gases in air which has high sensitivity, i.e., is able to detect com-bustible gases in concentrations as low as 1 to 10 ppm. The detector in accordance with the present in-vention i5 substantially smaller than prior art de-tectors and requires a comparatively low operating temperature which in turn reduces the amount of electrical energy utilized by the detector and increases the useful life of the detector. Furthermore, the detector in accordance with this invention, after an initial break-in period, has a more ùniform, i.e., less variable, sensitivity over its useful life.
Rather than using a catalytic metal, a detector of the present invention utilizes palladium oxide or palladium oxide hydrate (PdO-nH20) as the catalyst.
The new hydrate catalyst increases the sensitivity of the detector while at the same time permits lower operating temperatures. Additionally, the detector in accordance with the present invention utilizes a wire having a smaller diameter than the wires utilized in prior art combustible gas detectors. The use o~
the smaller wire accomplishes several purposes. One purpose is to increase the sensitivity of the detector since it requires less heat to alter the resistance of the thinner wire. Another purpose is to reduce the quantity of electrical energy needed to heat the wire to its opera~ing temperature. The thin wire ~ '7~ ~
further permits the use of a smaller quantity of protective ceramic material which reduces the amount of heat required to raise th~ temperature of the ceramic material surrounding the wire~ Such re-duction in the quantity of heat required to raise the temperature of the ceramic material decreases energy requirements of the sensor while at the same time reduces the amount of combustible gas which must be oxidized in order to increase the tem-perature of the sensor to produce a usable signal.
The detector in accordance with the invention is an element which, at a temperature of below about 600C, undergoes a change in electrical resistance $
when exposed to a combustible gas in air. The element comprises a wire~coated with a palladium ni-trate or ammonium chloropalladite solution which is heated to from about lOO~C to about 600C to form a palladium oxide composition. The prefe~red solution is a palladium nitrate solution which is most pre-ferably hydrolyzed in a weakly acid solution and heat-ed to from about 100C to about 600C, preferably 500~C
to 600C, to precipitate and to calcine palladium oxide hydrate. The wire comprises a metal or metal alloy having a melting temperature above 1500C and desirably has a circular cross section having a diameter of from about 0.0001 cm to about 0.0025 cm.
Desirably, a coating of ceramic materials is present between the wire and the palladium oxide hydrate.
The invention further comprises a method for the manufacture of the element which comprises winding the wire around a mandrel having a diameter of from about 0.01 to about 0.1 cm to form a wire coil, the mandrel being more susceptible to oxidation than the ~ 7 ~d ~_ wire. After the coil is formed, it is coated with a ceramic precursor solution.
The ceramic precursor solution preferably contains phosphoric acid and chromic acid anhydride reduced by aluminum trihydrate, aluminum hydroxide or hydroxides of other metals. To the solution is added 5 to 10%
tabular a~umina, or other ull sintered high tem-perature oxide, having a particle ~ize of less than 0.5 micron. The proportions of the ingredients of the 1~ precursor solution should, on a dry basis, desirably havè
the molar ratios of about A1203:0.8 Cr203:3P2O5. Cerama-sind*~ manufactured by Aremco Products Inc., is a com-mercially available mixture containing aluminum phosphate and phosphoric acid which has been found suitable for blending with a water insoluble metal oxide such as A1203. The coil is then heated slowly to above about 255G to remove water oE crystalization and to dehydrate the coating. The coating i5 cured by raising the temperature to between 500~C to 600C
to form a cured ceramic binder coating having a thick-ness of from about 0.001 cm to about 0.025 cm. The mandrel is then removed by chemical or thermal oxidation.
The coil is then coated with a uni~orm solution containing ~rom about 1 to about 50, preferably from about 2 to about 10 weight percent of palladium nitrate, Pd(N03)2 4H20, or ammonium chloropalladite and the coil is heated to a temperature of from about 250C to about 750C for from about l to about 60 seconds to convert the palladium solution to a pal-ladium oxide composition.
In accordance with the method of the invention, prior to coating the coil with palladium nitrate solution, the coil is desirably coated with a solution of from about 5 to about 60 weight percent, preferably * trade mark 35 to 55% of aluminum nitrate, A1(NO3)3 9H20, and heated to convert the aluminum nitrate to aluminum oxide. The solutions can be as dilute as desired at the expense of applying additional coats. Permitting the solutions to stand at least 4, and preerably 24, hours before use is desirable. Solutions should be discarded after about one month.
The small size of the sensor, the high surface area o the catalyst support and good catalyst dispersion result in a sensor having improved speed of response and sensitivity and which is not position sensitive.
Independently of the manufacture of wire coils for sensor.s, miniature wire coils can be made in accordance with the invention for any purpose.
According to the method of the invention for making wire coils, a wire having a diameter o~ from about 0 0001 cm to about 0.005 cm may be wound about a mandrel which can be oxidized without oxidizing the wire. The wire, wound about the mandrel, is then coated with a ceramic precursor, the precursor is cured to form a ceramic binder and the mandrel is removed by oxidization.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a side elevational view of a detector connected to electrodes at about ten times actual de-tector size.
Figure 2 is an enlarged cross sectional side view of a sensor manufactured in accordance with the invention~
Figure 3 is an enlarged cross sectional view of a wire coil wound upon a mandrel.
Figure 4 is an enlarged sectional view as seen in the direction of the arrow shown in Figure 2.
DETAILED DESCRIPTION OF THE INVENTION
"Comb-ustible gas", as used herein means a gas, a vapor, or an atmospheric dispersion of fine particles which will react chemically at or near the surface of the detector and thereby change the detector temperature, when it is operated below abou-t 600C.
Examples oE such gases or vapors are carbon monoxide;
methanol; ethanol; isopropanol; formaldehyde; ethylene oxide; terpenes; aliphatic hydrocarbons such as methane~
ethane, propane, isobutylene, and octane; and cyclic hydrocarbons such as benzene, toluene, xylene, and nitrobenzene.
"Sensitivity" as used herein means the degree of ability to provide a meaningful signal which is distinguishable from background noise.
Comparative sensitivity measurements between detectors are made either by comparing the ratios of the changes in resistance from the air resistance, or by comparing millivolt bridge signal output for a specified input such as 2.5 volume percent CH~ in air.
'ICure'' or "curing" as used herein means hea~ing to a sufficient temperature to render a substance water insoluble and self-adheren~ at elevated te~peratures such as above about 80QCo The curing temperature in the case of phosphate and chromate containing ceramic precursor solutions is usually between about 440C and 600C.
In accordance with the invention, there is pro-vided an element; i.e., a detection device, for de-tecting the presence of a combustible gas in air. The combustible gas is usually the gas or vapor of a carbon containing compound but may also be other combustible gases such as hydrogen.
The element in accordance with the invention may detect particular combustible gases at temperatures well below 600C. For example, methane is detected at a temperature of between about 365C and 600C, hydrogen is detected at a temperature as low as about 50C, usually below 250C, and carbon monoxide is detected at a temperature of about 280C.
The detector element 10 in accordance with the invention, as seen in Figure 1 and Figure 2, is a wire 12 coated with a palladium compound. Microminiature sensors of this invention can be made using -8a-~ 7,~ ~
chloride compounds such as PdC12, H2PdC14, (NH4)2PdC14, etc. but require special aging or other treatments to remove the chlorine, which is a known catalyst poison.
At least two methods may be used. One consists of prolonged heating, i.e., (days~ at elevated tem-peratures, e.g., from about 200C to about 65QC which are well below those at which the oxide is destroyed.
The other method is to fuse the chloride compound with an alkaline metal carbonate to precipitate the chlorine so that later the chloride salt can be leached out by boiling water.
Preerably the detector is coated with hy-drolyzed palladium nitrate solution. The coated element for any of the c~mpounds is then heated to from about 100C to about 350C to convert the palladium compound such as palladium nitrate to a palladium oxide composition w~ich is desirably palladium oxide hydrate. Most o the impurities such as chlorine which can affect catalyst dispersion are re~oved at this temperature. Next, the detector tem-perature is raised preferably to from 500C to 600C
for further curing and stabilizing. If chlorine or hydrogen is present and the low temperature step is omitted, the catalyst particle size is increased by agglomeration and the detector sensitivity is reduced.
Palladium nitrate is preferably hydrolyzed in a weak-ly acidic solution prior to application so that the desired palladium oxide hydrate is obtained upon heating. Palladium nitrate complexes such as tetramine palladous nitrat~ [(NH3)4Pd}(NO3)2 may also be used.
~ 7;~
The wire 12 is preferably a metal or metal alloy, having a melting temperature above 1500C and having a high temperature coeEficient of resistanc~.
Examples of usuable metals for the wire are pla-tinum, palladium, nickel, rhodiuml ruthinium, iridium, vanadium, zirconium and their alloys.
- Particularly desirable metals for use in the wire of the element are platinum and palladium due to their high resistance to oxidation. A particularly desirable wire is manufactured from platlnum or a platinum alloy such as an alloy of about 90 weight percent platinum and about 10 weight percent rhodiurn.
Platinum is particularly desirable because of its high temperature coefficient of resistance.
Alloys of platinum and rhodium are desirable due to high strength.
The wire 12 of the element 10 ;s of a small diameter, preferably between about 0.0001 and about 0.005 cm and most preferably ~etween about 0.0008 and 0.002 cm. The small wire diameter permits more rapid heating of the wire when heat of combustion of the gas is detected at the palladium oxi~e surface of the element. The use of a small wire permits a very small detector, i.e., from about 0.005 to about 0.2 cm in diameter, to be manufactured.
The wire 12 may be turned or bent into any de-sirable shape and is preferably in the form of a coil 14, as seen in Figures 2 and 3, to permit the use of a long wire in a given volume. The coil di-ameter is between about 0.002 and about 0.1 cm. For ease of manufacture, the coil diameter is pr~ferably at least about 0.01 cm. The wire 12 may have any de-sirable cross section such as triangular, h~xagonal, ~ 7 round, rectangular or square.
When the wire 12 is a coil 14 in accordance with the preferred embodiment, it is usually formed by winding the wire around a mandrel 17 preferably having a diameter of from about 0.01 to about 0.1 cm to form the wire coil 14. In order to form such a small coil, precision winding is necessary.
It is desirable to have the turns of the coil close to the surface of the detector to optimize sen-sitivity and to maximize the length of the wire within the volume of the detector within the limits of wire strength. A large ratio of coil diameter to wire diameter is therefore desirable to maximize the wire length. Prior art methods used in winding lamp coils or mandrels to retain coil shape cannot be used since lamp coils use a small ratio of coil diameter to wire diameter which permits the shape of such coils to be retained by the stiffness of the wire. The mandrel 17 is manufactured from a material such as molybdenum whi~h is more susceptible to oxidation than the wire.
After the coil 14 is wound, it is desirably coated with a ceramic binder precursor solution.
Any suitable ceramic binder may be used. "Ceramic binder" as used herein includes high temperature in-organic contiguous compositions whether of crystalline or amorphous character. "Ceramic precursor" is a so-lution or dispersion which will form a ceramic binder upon heating to above about 250C. The resulting binder should be waterproof, be corrosion resistant, be electrically insulating, be able to fill the spaces between the turns of the coil prior to curing, and have sufficient strength to retain the shape of the coil and the spacing between turns. The ceramic pre-cursor should cure to the desired binder at a temperature well below 900C to minimize changes in wire xesistance due to vaporization of a portion of the metal wire.
The binder should have good surface area and other characteristics known to those skilled in the art, if the binder is used as the catalyst support. Otherwise, if a catalyst support is applied over the binder, the binder should provide a good ceramic adhesive surface with high thermal conductivity. 0f primary imp~rtance is the thermal stability of the binder, as well as free-dom from sintering following preliminary heat treatment and aging. The size, thermal conductivity, and radiation characteristics preferably are constan~ or at least changing only very slowly (weeks), to permit stable detector performance and freedom from long-term noise which would otherwise require frequent recalibration of the instrument.
Although numerous silicon-based binders and high temperature glasses can be used, for long-term sta-bility it is preferable not to use binders involving a silicon compound in combination with the very small diameter support wires of this .invention. Many re-searchers have found that in a reducing atmosphere in the presence of trace quantities of carbon, iron, or sulfur, minute quantities of silicon metal migrate to wires such as platinum causing lowering of the melting point, embrittlement, and changes in resistance These adverse results become increasingly important as the wire size is reduced to make very small detectors. See Thermocouple Temperature Measurement by P. A. Kinzie, pages 27, 34, 64, 73, and 174, published by John Wiley & Sons, 1973 and the references contained therein.
~ ~t7~
It is f~rther desirable that the binder adhere to the wire of the coil to prevent shorting between turns and to provide good thermal conductivity. The binders should be free of l~mps or agglomerates which cannot penetrate and remain between the desirably very closely spaced turns of the coil.
The binder should be unaffected by corrosion which might otherwise result from atmospheres fre-quently found in detector applications. In addition, the binder must be free of any catalyst poisons. The viscosity of the binder precursor should preferably be between 20 and 80 centipoise to permit appli-cation to the coil wound on the mandrel, as, for example, by a ine camel ,hair brush.
Many prior art binder compositions are un-suitable for use in accordance with this invention.
A binder which does, however, meet these and other desirable criteria for detector use is an inorganic polymer consisting of complex phosphate compounds of metals and metal oxides such as aluminum, chromium, zirconiwm, magnesium~ zinc, calcium, barium, tin, ti-tanium, thorium, and o~her metals which form acid-proof oxides.
A particularly preferred binder composition is prepared by heating 85% H3PO4, AlOOH, 60% H2CrO4, and H2O together at 100C to 130C for approximately one hour with slow mixing and constant stirring. Any lumps and particles are removed by straining. The re-sulting mixture contains aluminum and chromic acid phosphates in water solution. The mixture is allowed to stand overnight and 5 to 10 weight percent of a metal oxide powder such as alpha A12O3 powder is added.
~.~Lt~
The alumina is preferably fully sintered at a high temperature, e.g., tabular alumina, and ground to a particle size of less than 0.5 micron, Water is then added to bring the binder to a viscosity of preferably about 50 centipoise for con~enience of application.
The amounts of the ingredients used should approximate in the dry state the formula molar ratios o~
Al2O3 : 3P2O5 to Al2O3 : O.8Cr2O3 : 3P2O5 the molar ratio of Cr2O3 to A12O3 varies from 0 to 0.8.
When ph~sphoric acid (P205) and aluminum oxide (A12O3) are used alone, various ceramic precursor for-mulation in molar ratios and the characteri~stics of the resulting binders are given below:
P25/A123 = 2.3 Precipitates solids, hardens on standing, makes poor bonds P2s/A123 = 3 Metastable P25/A123 = 3 to 3.5 Maximum bond strength and usable solution stability P25/A123 = 1.4 Usable solution but weak bonds because of ~xcess The best molar ratio is seen to be A12O3:3P2O5.
The appropriate molar ration for other ceramic pre-cursor systems can similarly be readily determined by one skilled in the art.
The binder containing chromic acid (Cr2O3) is superior in binding strengt~ and thermal stability and is usable for for several months before clouding begins to appear, at which time it should be discarded.
The starting binder is best made using water dispersible aluminum trihydrate, Al(OH)3 or aluminum monohydrate, AlOOH, such as Dispal* (Philadelphia Quartz Company) or sa~mal* (DuPont), and slowly add-in~ this mixture to heated 85% phosphoric acid. The * trade mark ~'7~
phosphoric acid, H2CrO4, is made by adding chromic oxide, CrO3, to water. The CrO3 may be mixed direct-ly or as a 50 to 60 weight percent water solution. The chromium is hexavalent and should be partially re-duced to the trivalent form for best adhesion and long term binder characteristics. Reduction o~ ~he chromium to trivalent form can be accomplished by reducing with hydroxides of the metals chosen for the binder, or~ if metals or oxides of metals are used, by reducing with inorganic hydroxides, or hydrogen peroxide. Preferably, the chromium is reduced by by using a low molecular weight organic amine, such as 3-amino-1-propanol, trithenolamine, or other alkanolamine or'an amine in combination with a carboxylic acid sequestering agent to pre-serve the solution, such as oxalic acid, citric acid, gluconic acid, sulfosalicylic acid~ tartaric acid. Other sequestering agents such as acetyl acetone may also be used. The latter combination enables the ceramic preeursor to stand for months without clouding or deteriorating. Sequestering agents to preserve the solution, such as carboxylic acids, may be used alone without an amine at a 1:1 ratio with the 85% H3PO4 in the ceramic precursor.
When the binder alone is used as the catalyst support, it has been found useful to make the binder from ammonium phosphate, NH4H2PO4 and metal nitrates, ( 3)3 9~20 and Cr(NO3)3 9H2O, in a molar ratio of NH4H2PO4 to metal nitrate of from 1.5:1 to 1.75-1. An amine is desirably used to adjust the pH.
The resulting binder when dried and heated to 500C
to 600C has a very large surface area and is par-ticularly suitable as a catalyst support.
After the coil 14 is coated with the ceramic precursor solution, it is heated slowly up to a temperature of above about 250C, then rapidly up to about 500C to about 600C to dehydrate and to cure the coating. The temperature preferably should not exceed 600C. The resulting binder coating 18 is desirably from about 0.0002 to about 0.004 cm thick. The resulting ceramic coating retains the shape of the coil after the mandrel is removed.
Other suitable ceramic precursor solutivns such as colloidal silica may be used to treat the coil to form a ceramic coil-retaining coating. Such coatings should be compatible with the wire9 thermally stable, and should contain no catalyst poisons.
After the aluminum phosphate coating is de~
hydrated and cured, the mandrel 17 is rPmoved by chemical or thermal oxidation.
A suitable metal for use as a mandrel 17 is molybdenum since it can be removed by dissolving it in an aqueous solution containing from about 15 to about 40 parts by weight sulfuric acid, from about 15 to about 40 parts by weight nitric acid, and from about 30 to about 70 parts by weight water. Al-ternatively, a molybdenum mandrel can be removed by heating the mandrel in air at a temperature of be-tween about 750C and about 1000C. Many other metals can be used as the material for the mandrel 17. Ex-amples of other metals which can be removed by chem-ical etching are alumînum, zinc, and iron. Mo-lybdenum is particularly desirable since essentially no residue remains upon the coil 14 after the mandrel ~ 7~ ~
17 is oxidized. In the preferred embodiment of the invention, the mandrel 17 comprises a molybdenum cylinder.
After the mandrel is removed, the coil 14 re-tains its shape due to the aluminum phosphate coat-ing 18 which becomes water insoluble, even to boil-ing water, and essentially gas imper~ious after de-hydration and cure. The preferred binder is amor-phous over a wide temperature range and does not under-go changes which would permit the passage of gases, vapors or liquids through the binder.
After the mandrel is removed by chemical or ther-mal oxidation, the coil 14 is preferably coated with an aqueous solution containing from about 2 to about 10 and most preferably from about 4 to about 7 weight percent of hydrolyzed palladium nitratè or with an aqueous solution containing from about 2 to about 10 weight percent of ammonium chloropalladite. And then the coil 14 is heated to a temperature of from about 250C to about 500C for from about 1 to about 60 ~econds to form a surface comprising palladium oxide hydrate or palladium oxide. The solution is preferably a palladium nitrate solution and the sur-face preferably comprises palladium oxide hydrate.
Whenever coating with a palladium nitrate solution is subsequently mentioned herein, it is to be under-stood that a coating with an ammonium chloropalladite or alkali chloropalladite solution can be sub-stituted with somewhat reduced detector sensitivity.
The palladium nitrate is desirably hydrolyzed in a weakly acid solution, i.e., pH 2 to 4.
Desirably, prior to coating the coils 14 with a solution of palladium nitrate, the coil .
~ 3~7,~
14 is coated with a solution containing from about 5 to about 60, preerably from about 35 to about 55, weight percent of al~minum nitrate and heated to a temperature of between about 250C
and about 750C to convert the aluminum nitrate to aluminum oxide. The resulting alum;num oxide coating 20 is preferably from about 0.001 to about 0.0~5 cm thick.
The resulting aluminum oxide coating 20 fur-ther protects the wire from vaporization and oxidation. If desired, more than one coating o aluminum nitrate can be applied over the coil for additional protection and strength.
Desirably and preferably, subsequent to coat-ing the wire with aluminum nitrate solution and prior to coating with palladium nitrate solution, the coil is coated with a solution containing from 5 to about 70, preferably from about 40 to about 65, weight percent of thorium nitrate and heated to be-tween 250 and 750C to convert the thorium nitrate to thorium oxide. The resulting thoriu~ oxide coat-ing 22 provides a ceramic surface having a large surface area upon which the palladium nitrate solution can be applied. Ihe resulting high surface area increases the sensitivity of the element lO.
The thorium oxide coating 22 is preferably from about 0.001 to about 0.025 cm thick.
Repeated applications and conversions of thor-ium solution can be made after application and con-version of aluminum nitrate and before application of palladium nitrate. Most desirably at least two coats of thorium nitrate solution are applied to the element 10 and up to about lO repeated appli~
cations may be desirable for certain applications.
Similarly, as previously discussed, repeated appli-cations and conversions of aluminum nitrate solution can be made prior to application of the thorium ni-trate or palladium nitrate solutions. Preferably at least two to four coats of aluminum nitrate so-lution are made prior to application of thorium nitrate and up to about 10 repeated applications and conversions of aluminum nitrate solution are desirable.
Multiple applications of palladium nitrate so-lution are desirably made upon the thorlum oxide surface. The solution is most preferably a 0.27 molar hydrolyzed palladi~m nitrate solution. De-sirably at least 15 coats of palladium nitrate solution are made and up to about 20 such appli-cations continue to improve the sensitivity o~
the element, After the palladium nitrate in the solution is converted to palladium oxide hydrate, th~ surface of the detector is not believed to have a contiguous palladium oxide hydrate coat;ng.
The surface of the detector seems to comprise palladium oxide hydrate concentrations in dis-crete zones or areas which are so small that the extreme magnification of an electron microscope is required to see them.
After the palladium nitrate is converted to palladium oxide hydrate, the resulting detector is desirably sensitized by heating the detector in an atmosphere containing from about 3 to about 9 volume percent o~ a combustible gas in air until the sensitivity is substantially increased, eg., by 50%. Desirably at least 8 volume percent of combustible gas for ammoni~ chloropalladite is used and less than about 6 volume percent of combustible gas for palladium nitrate i9 used. Most preferably, from about 8 to about 8.8 volume percent of a com-bustib~e gas for ammonium choropalladite solutions and 4.5 to 5.5 volume percent for palladium ni-trate solutions are used. The heating temperature is from about 650 to about 825C, preferably from about 725C to about 775C, for from about 1 to about 30 minutes. The combustible gas is preferably me-thane. Heating at elevated temperatures, i~e~, in excess of 775C, in an air-gas mixture is to be avoided since the detector can readily burn out ~mder those conditions. Furthermore, if the de-tector is heated much above 840C, the oxide is los~ and palladium metal remains. The oxide layer that builds back is thinner, tends to reduce the catalytic activity of the metal, and is itself less desirable catalytically than the oxide which was lost.
After sensitization7 the deteetor is de-sirably stabilized by operating the sensor under controlled conditions so that the operating characteristics will be essentially constant in actual use. One such stabilization method is to operate the detector at 650C for one hour in air, for 18 hours in an atmosphere containing 2.5 volume percent methane, and again in air for 31 hours.
The stabilization procedure is highly desirable when ammonium chloropalladite is used but may be shortened or eliminated when palladium nitrate is used.
It has been found in all steps requiring heat-ing during the manufacture o the detector following curing of the binder, that a stepwise temperature rise -2~-is preEerred over a gradual temperature change.
The binder should be heated slowly to at least 255C to avoid foaming during dehydration.
In prior art detec~ors, the palladium catalyst is activated by reducing the oxide to the metal by high heat or by treatment with hydrazine~ Next, the catalyst is aged in air or oxygen at a high tem-perature to reform the oxide. This procedure has been found to be highly undesirable since it results in a detector o~ lower sensitivity, shorter life, and ~ower calibration stability. The catalyst of this in-vention is activated by removing part, but not all, of the bound water and the water molecules bel;eved to be trapped in the large palladium oxide hydrate molecule, which after partial removal is believed to increase the surface area of the catalyst particles.
See 0. Glemser and G. Peuschel, Z. inorg. Chem., 281, 44-53, 1955; or Chemical Abstracts, 50: 4612g, ~956.
At no time in the processing is the original pal-ladlum oxide reduced all the way to the metal.
A marked difference between the detector of this invention and prior art detectoræ can be ob-served when detecting combustible gases in an at-mosphere of pure nitrogen. The new detector main-tains calibration for a long time of exposure to combustible gas, while prior art detectors lose calibration rather quickly.
The resulting element is utilized in a com-bustible gas sensing device which may comprise the element connected into a wheatstone bridge circuit which detects resistance changes in the element re-sulting from a temperature change in the device -2~-caused by a reaction of the device with a combustible gas. The circuit activates a warning means when a combustibLe gas is detected. The warning means may be any desirable warning device such as a light, buz-æerl or other alarm, ~.g., a mechanical vibrator.
Usually the warning means is a warning light to be used in conjunction with an auditory signaling means.
In order to balance the wheatstone bridge cir-cuit when such a circuit is used, a non-catalytic reference element is needed which has the same op-erating temperature resistance as the active element manufactured in accordance with this invention. The reference element is manufactured in t~e same way as the active element, except that it has no coating of palladium oxide. Suc~ a standard reference ele-ment therefore permits the wheatstone bridge circuit to register resistance changes in the detecting ele-ment which are due solely to detection of a com-bustible gas by the element.
The reference element, which does not contain a palladium oxide coating, is instead coated with a compound which does not react with a combustible gas but which does react with changes in air temperature and changes in humidity of the surrounding air in a manner similar to the reaction of the active element with temperature and humidity in the surrounding air.
A suitable coating for the reference element is potassium hydroxide. Alternatively, a thin layer of gold may be used as reference standard element sur- -face coating.
Alternatively, an integrated circuit diferential amplifier may be used to detect a change in detector resistance in place of the wheatstone bridge.
~7'~'7~ ~
The element in accordance with the present invention is able to rapidly detect minute quantities of combustible gas in air. For example, such an element is capable of detecting carbon monoxide in concentrations of about 1 ppm and is able to detect hydrogen and methane in concentrations of less than 10 ppm..
Additionally, the operating temperature of the element in accordance with the invention can be as low as 250C and sometimes even as low as 50C when hydrogen is being detected, as low as 280~C when carbon monoxide is being detected, and as low as rom about 400C to about 600~ when methane is being detected. Such low temperature operation substantially increases the useful lifé of the detection element as compared with prior art elements for detecting a combustible gas in air. Furthermore, the coating of the wire in accordance with the present`invention substantially increases the durability of the element by reducing oxidation and vaporization of t~e wire.
Operating at high temperatures, i.e., above 8005C, is to be avoided in order to prevent the palladium oxide coating from converting to metallic palladium as occurred in the prior art. Furthermore, the sen-sitivity of the novel detector remains more constant than the high temperature detectors of the prior art.
The size of the element manufactured in accor-dance with the invention is substantially smaller than known prior art elements for detecting a com-bustible gas. The reduced element size permits very small detection units to be manufactured which can -be unobtrusively used. The low temperature op-eration and small size substantially reduces ~ '7~
energy consumption of the element in accordance with the invention as compared with energy con-sumption required by prior art elements ~or de-tecting combustible gases in air, which permits the manufacture of portable instruments having a very small size and long battery life. The detector size may be from about 0.005 to about 0.25 cm in diameter and is preferably 0.02 to O.l cm in diameter. Some idea of the very small size of the detector can be appreciated when it is realized that the dot or period on a typewritten page would be within the above dimensions.
As seen in Figure l, the detector is connect-ed to electrodes 26 by means of wire 12. The elec-trodes 26 in turn form a part of a detector assembly 28 which can be plugged into an electronic circuit.
It has been found that the small size of the detector has the added advantage of increasing the shock resistance of the assembly due to the smaller relative mass of the detectQr lO.
In addition, the detector in accordance with the invention is a very small sphere with greatly reduced associated convection energy, which can be placed in flame arrestor cavities having a much smaller inside radius about the detector than is possible with the much larger prior art detectors. The limit in size reduction is a radius or separation distance between the detector and the flame arrestor wall of approx-imately 0.14 cm. The practical result is a significantly reduced detector response time, which is particularly important in quickly detecting the heavier molecular weight toxic and combustible gases and vapors which diffuse more slowly.
~ 7 ~ ~
The palladium oxide hydrate catalyst is believed to be a colloidal dispersion or sol with chemically bound and included water. It loses water when heated roughly in proportion to the temperature. It is be-lieved that not all of the bound water is released even at temperatures as high as 800C. A very small amount of water is bound in the hydrate so tenaciously that the decomposition temperature of the oxide must be reached before it can be released. In accordance with this invention, the processing of the detector is carried out so that the decomposition temperature of the hydrate or oxide is never reached. When most of the water is removed, the structure is believed to &
have a substantially increased surface area similar to dehydrated or activated aluminum catalyst carriers.
See J. R. Anderson, Structure o~ Metallic Catalysts, page 168, Academic Press, 1975. Dehydrated palladium hydroxide, palladium oxide, and palladium metal from reduced palladium chloride are not believed to have a similarly increased surface area.
To get the large palladium oxide hydrate molecule described by Glemser, the starting solution should be weakly acid. To achieve optimum catalyst dispersion in a reasonable number of coats, a concentration of palla~ium nitrate is required which results in a very acid solution having a pH at about 0.57 due to hy- -drolysis. If one attempts to bring the solution to a suitably higher pH value by titrating with NH40H, insoluble palladium hydroxide is precipitated. Ni-trate ions are therefore preferably removed, to pro-duce a weakly acid solution, by extraction with a ~ 7~
water insoluble, hi~h molecular weight organic amine.
See Yu. G. Frolov et al, Theoretical Aspects of Amine Extraction, Atomic Energy Review, 7 (1), 71-138, 1969. For use in this invention, the amine should be a liquid insoluble in water, have a boiling point above that of water, contain no chlorine or other catalyst p~isons, have a density substantially dif-ferent than water to facilitate separation, and be characterized by low toxicity. Tertiary octyl amine has been found to be particularly desirable, In the preferred embodiment, a sufficient amount of tertiary octyl amine is shaken with the catalyst solution to produce a hydrolyzed palladium nitrate solution with a pH between 2.5 and 4.5~
In all cases herein, where a hydrolyzed nitrate ls converted to an oxide hydrate or oxide at a tem-perature of between about 100C and 750C, the con-version takes place in an oxygen containing atmosphere such as alr and the conversion time i5 rom about 10 seconds to about 15 minutes.
An instrument utilizing the detector disclosed herein is not position sensitive, that is, the in-strument reading, either in air or when detecting a combustible gas, does not ~o off calibration due to changes in the înstrument position. It is be-lieved that the position stability o the detector is due to its small size and spherical shape. Prior art detectors frequently had oval, cylindrieal or flat shapes which are believed to contribute to their position sensitivity. To obtain meaningful instrument readings, i~ h~s been found that reference elements should be of the same shape and size as the detector.
-2~-~7 EXAMPLE I
A precision winding machine is used to wind thirteen turns of resistance grade platinum wire having a diameter of 0.00127 cm about a clean and deoxidi~ed molybdenum wire tQ form a coil. The molybdenum wire has a diameter of 0.0434 cm and the spacing between turns is maintained at about 0,00127 cm.
Five percent aluminum oxide ~A1203), which is completely sintered alpha alumina ground to a par-ticle siæe o 0.5 micron or smaller, is then blended into a solution containing chromic and phosphoric acids to form a ceramic precursor having ~ dry molar 1' ration of about A1203 : 0.8Cr203 : 3P205. T
ing blend is then coated over the coil as a binding cement. The mandrel containing the coated c~il is then heated to about 250C by passing an electric current through the mandrel to dry the cement. The mandrel is then further heated to about 530C to cure the binder.
The mandrel is then removed by dissolving it in an aeid solution containing 30% of 65% nitric acid solution, 30% of 98% sulfuric acid solution and 40%
distilled water.
The coil is then washed in distilled water and dried. The ends of the coi.ls are then welded to nickel supports or posts which are mounted to a base on 0.381 cm centers.
Solutions are prepared for further treating the coil. Each of the solutions is thoroughly mixed at room temperature and allowed to stand overnight before use. High purity chemicals and distilled water are used.
'7;~ L
The prepared solutions are as follows:
Aluminum nitrate solution:
Al(NO3)3-9H2O 44 weight per cent H2O 56 weight percent Thorium nitrate solution:
Th~NO3)~-41l2V 62 weight percent H2O 38 weight percent Catalyst solution:
Pd(NO3)2 5.9 weight percent H2O 94.1 weight percent The catalyst solution is shaken with tertiary octyl amine for less than ten minutes in a small separator funnel and then allowed to s~and to bring the pH to about 3. The amine rises to the top, or it is separated in a centrifuge, and the amine and any particulate residue is discarded. Care must be taken that the palladium nitrate solution is kept near room temperature, since PdO'nH2O will pre-cipitate at higher water temperatures.
The coil is heated by passing an electric current through it, i.e., ohmically, to a temperature of 600C for 25 seconds, which is believed to remove any residual moisture or organic compounds, and such heating is repeated when the coil has been allowed to stand for over 15 minutes at any time during the pro-cessing.
The coil is then wetted with the foregoing solutions in the order specified in Table I to the temperature and for the times speciied in Table I.
TABLE I
Number of Coats Solution ~ erature Duration Degrees C Sec.
1 Aluminum nitra~e 250 10 1 Aluminum nitrate 250 10 50~ 10 2 Aluminum nitrate 350 10 2 Thorium ni~rate 250 10 Palladium cata~yst 350 10 solution 500 10 After coating is completed, the resulting coated coil, i.e., detector, is activated to maximize and to stabilize the detector performance. The activati~n is carried out in a metal chamber equipped with auto-matic controls which swit~h from one supply gas to 2~ another. The chamber has an internal volume of about 1 cubic centimeter. The detector is initially exposed to air flow of 1 liter per minute at room tem-perature followed by a combustible gas-air mixture at about 0.1 liter per minute for 15 seconds. The detector is then electrically, i.e., ohmically, heat-ed to 750C for about fifteen minutes. The electric current is then turned off and the detector ~s allowed to cool for about 10 seconds. The combustible gas 10w is then turned off and the air flow is again turned on.
After 10 more seconds, the activa~ed detector is re-moved ~rom tne chamber. The air-gas mixture which is used comprises about 5.5 percent by volume methane, i.e., C~14, in air.
The detector is then aged by electrically heating tha detector to 650~ for l hour at 0.3 liters per minute of dry air, followed by 16 hours at 0.3 ~liter per minute of air containing about 2.5 percent methane, followed by 31 hours in air at 0.3 liters per minute. When heated to ~50C, the resulting detector shows a 9 to 12% change in re-sistance when exposed to 2.5 volume percent methane in air. In addition, the detector expends only l50 milliwatts of energy during operation and has a volume of only 0.00018 cubic centimeters.
The discussion here~n is primarily directedto extremely small detectors which respond to oxidation reactions of combustible gases and their method of manu~acture; however, the dis~losure here-in with respect to small coils, is applicable to any detector utilizing a coil and operating at an ele-2~ vated temperature such as semiconductor gas detectors and solid electrolyte gas detectors.
Claims (2)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for making a miniature platinum or palladium wire coil, comprising:
(a) winding a wire having a diameter of from about 0.0001 cm to about 0.005 cm about a mandrel, said mandrel being made of a material which can be removed by oxidation without oxidizing said wire;
(b) coating the wire wound about said mandrel with a ceramic precursor;
(c) drying and curing the ceramic precursor to form a ceramic binder; and (d) removing said mandrel by oxidation;
said ceramic precursor comprising aluminum phosphate, phosphoric acid, chromium phosphate, submicron aluminum oxide and water.
(a) winding a wire having a diameter of from about 0.0001 cm to about 0.005 cm about a mandrel, said mandrel being made of a material which can be removed by oxidation without oxidizing said wire;
(b) coating the wire wound about said mandrel with a ceramic precursor;
(c) drying and curing the ceramic precursor to form a ceramic binder; and (d) removing said mandrel by oxidation;
said ceramic precursor comprising aluminum phosphate, phosphoric acid, chromium phosphate, submicron aluminum oxide and water.
2. A miniature platinum or palladium wire coil, comprising: wound wire having a diameter or from about 0.0001 cm to about 0.005 cm and held with a ceramic binder comprising a cured ceramic precursor comprising aluminum phosphate, phosphoric acid, chromium phosphate, submicron aluminum oxide and water.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000425644A CA1176721A (en) | 1977-12-21 | 1983-04-11 | Microminiature palladium oxide gas detector and method of making same |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US862,974 | 1977-12-21 | ||
| US05/862,974 US4193964A (en) | 1977-12-21 | 1977-12-21 | Microminiature palladium oxide gas detector and method of making same |
| CA000317423A CA1150971A (en) | 1977-12-21 | 1978-12-05 | Microminiature palladium oxide gas detector and method of making same |
| CA000425644A CA1176721A (en) | 1977-12-21 | 1983-04-11 | Microminiature palladium oxide gas detector and method of making same |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000317423A Division CA1150971A (en) | 1977-12-21 | 1978-12-05 | Microminiature palladium oxide gas detector and method of making same |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1176721A true CA1176721A (en) | 1984-10-23 |
Family
ID=27166002
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000425644A Expired CA1176721A (en) | 1977-12-21 | 1983-04-11 | Microminiature palladium oxide gas detector and method of making same |
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| Country | Link |
|---|---|
| CA (1) | CA1176721A (en) |
-
1983
- 1983-04-11 CA CA000425644A patent/CA1176721A/en not_active Expired
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