JPH11211688A - Gas sensor with compound catalytic structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor device with a multi-layer catalyst support structure suitable for detecting gases present in air or gases present in the harsh environment of an automobile exhaust system, specifically hydrocarbons and nitrogen oxides. SOLUTION: A gas sensor 10 comprises a ceramic substrate 20 provided with a thermally sensitive resistor on one surface. A mixture of ceramic particles and glass powder is applied over the substrate 20 and the resistor and is sintered so that the glass may be fluidized to glue the ceramic particles to the substrate 20. A catalyst layer containing platinum or rhodium is adhered to a catalyst support 24, and a thermally sensitive resistor element 22 detects reactions between hydrocarbons or nitrogen oxides and the corresponding catalyst. The sensor 10 is suitable for detecting gases in the harsh environment of an automobile exhaust system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION This invention relates to sensors for detecting the presence of gas in air, and more particularly to a mixture of ceramic particles and glass adhered to both a base and a resistor element. The compound catalyst structure (ie, the composite catalyst structure, or in other words, the compound catalyst structure)
(cure)).

[0002]

BACKGROUND OF THE INVENTION The use of catalysts to increase the rate of chemical reactions is well known. A catalyst can be any substance that affects the rate of a chemical reaction without itself being consumed or undergoing a chemical change in the process. The catalyst may be inorganic, organic, or a complex formulation containing organic groups and metal halides.

[0003] The present invention relates to the use of catalysts in gas sensors. Gas sensors using catalysts work by causing a chemical reaction when the gas to be detected comes into contact with the catalyst. In many cases, chemical reactions result in temperature changes, which can be used to affect the electrical resistance of juxtaposed conductors. A typical sensing element is a conductor whose conductivity changes with temperature. Gas detection devices typically include both a catalyst sensor and a reference sensor without a catalyst. The difference in sensor resistance is the result of a catalytic reaction due to ambient gas concentration. In other words, the gas concentration can be ascertained by measuring the difference between the voltage before and after the conductor with the catalyst coating and the voltage before and after the conductor without the catalyst coating.

Examples of patents related to the present invention are listed below. That is, U.S. Pat. No. 4,045,177 is an apparatus for detecting combustible gas.

[0005] US Pat. No. 4,322,383 is a gas component detection device having two sensors made of metal oxide.

US Pat. No. 4,447,397 is a catalytic gas sensor having a filament coated with titanium dioxide (TiO ₂ ).

US Pat. No. 4,957,705 is an oxygen gas concentration detecting device.

[0008] US Patent No. 5,445,796 is an oxygen concentration sensor with a thermal resistance coating.

[0009] US Reissue Patent No. Re33,980 is a thick film gas sensing element.

[0010] By reference to these patents, the disclosure of these patents is incorporated herein by reference.

The above patents reflect the present state of the art as recognized by the applicant and disclose information that may have a direct bearing on the practice of this application. Duty of good faith that Applicants are aware of
of thought). However, these patents, alone or in combination, do not teach or clarify Applicants' invention.

[0012]

A problem with current gas sensors is that they are compact, inexpensive, and suitable for functioning in the harsh environment of automotive exhaust systems.
And no robust hydrocarbon or nitrogen oxide sensors. Sensors are needed to measure these gases to ensure that the vehicle complies with emissions requirements. Another problem is that current sensors use oxygen sensors that measure these gases indirectly. Oxygen sensors are not as accurate as more direct measurement methods.

Another problem is that, when subjected to very large temperature changes, subjected to vibrations and exposed to contaminants, it will operate properly without significantly degrading its performance over its expected life. It was difficult to design a possible sensor structure.

It should be noted that the above and other problems are solved by the present invention which will become apparent to those skilled in the art from the detailed description of the invention.

[0015]

SUMMARY OF THE INVENTION A feature of the present invention is to provide a gas sensor having a multilayer catalyst structure for directly detecting the presence of gas in air. In particular, the sensors detect hydrocarbons and nitrogen oxides directly by using a thermal resistor that responds to the exothermic reaction of the gas on the catalyst.

Another feature of the present invention is to provide an apparatus suitable for detecting gas in an harsh environment of an automotive exhaust system in an economical manner. The disclosed gas sensor provides these features by using a multilayer catalyst structure that is securely adhered to the surface of the sensor. The multi-layer catalyst structure can be deposited using conventional thick film techniques.

Yet another feature of the present invention is to provide an apparatus having a structure including a ceramic base and a resistor element (resistor element) disposed on the base. The electrical resistance of the resistor element changes according to the temperature change. A high surface area catalyst support structure comprising a mixture of glass and alumina particles is disposed on the base and over the resistor element. The catalyst material is deposited on the catalyst support structure to generate an exothermic reaction with the gas to be detected.

The invention is not in any one of these features, but rather in a particular combination of all features disclosed and claimed herein. The present invention is distinguished from the prior art for a particular function in this particular combination of all of its structures.

In order that the detailed description of the invention that follows may be more readily understood, and in order that its contribution to the art may be better understood, the more important features of the invention will be relatively broadly understood. Explained. Of course, additional features of the invention will be described hereinafter which form the subject of the dependent claims. One skilled in the art will appreciate that the concepts upon which this disclosure is based can be readily used as a basis for designing other structures, methods, and systems for implementing some of the objects of the invention. Let's do it. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

[0020]

BRIEF DESCRIPTION OF THE DRAWINGS The drawings of the present invention are not to scale. The accompanying drawings are only schematic and are not intended to show particular parameters of the invention. The accompanying drawings are only intended to illustrate exemplary embodiments of the present invention and therefore should not be considered as limiting the scope of the present invention. In the accompanying drawings, like reference numerals refer to like elements between the drawings.

The present invention provides a gas sensor having a multilayer catalyst structure (or a multilayer catalyst structure). This structure
Ideally suited for the detection of hydrocarbons and nitrogen oxides in automotive exhaust systems. Referring to FIG. 1, a plan view of the gas sensor 10 of the present invention showing a part of a substrate (base) 20 is shown. Substrate 20 is preferably made of a ceramic material, but other suitable dielectrics may be used. FIG. 1 shows only the portion of the substrate 20 that includes the catalyst support structure 24, and does not show the non-catalyzed sections of the support structure.

The remaining portion of substrate 20 can take any desired configuration that provides the necessary structural and thermal properties for the sensor. For example, the structure must be strong enough to withstand the shock and vibrations associated with automotive exhaust systems. In addition, the thermal properties must be such that any catalytic reaction that occurs on the catalyst support structure 24 can be detected by the thermal resistor element 22 disposed on the substrate 20 (ie, the substrate is It must not remove enough heat from the catalytic reaction to prevent the body element 22 from rising in temperature).

Conductors 25 and 26 are provided on substrate 20 and are electrically connected to resistor element 22. The conductors 25 and 26 are connected to a circuit (not shown) that can detect a change in resistance along the length of the resistor element 22 and a resulting voltage drop.

FIG. 2 shows a cross section along the resistor element 22. Resistor element 22 can be deposited on substrate 20 using any conventional thick film or thin film technology. Adhesion techniques are adherent enough to withstand the environment of the vehicle's exhaust system, and that the resistor is sufficiently hot to respond to temperature changes due to catalytic reactions on its upper support structure. Any technique may be used as long as the resistivity is high. The material used to form resistor element 22 can be selected using these same criteria. In the preferred embodiment, platinum has been found to be a satisfactory material for resistor element 22, and screen printing is a suitable method of deposition.

[0025] The conductors 25 and 26 can likewise be deposited using any conventional thick or thin film technique. Gold is selected as the conductor material in the preferred embodiment.

A very thin glass layer 23 covering the resistor element 22 is provided. The main purpose of this glass layer is to use chloroplatinic acid (eg, hexachloroplatinum (IV)
Those skilled in the art will readily understand that the addition of an acid) prevents conductive platinum left on top from causing a short circuit with the resistor element. Another obvious purpose of providing a glass layer is
The purpose is to prevent the resistor element 22 from causing a short circuit due to carbon dioxide that may adhere to the sensor.

As shown in FIGS. 2 and 3, the catalyst support structure 24 includes a mixture of alumina particles 34 and powdered glass 32. In a preferred embodiment, the mixture contains 20% LaRoche V700 alumina and 80% GA-4 glass from Nippon Electric Glass. The alumina is heated to about 600 ° C. before being added to the mixture.
And calcined for 1 hour. This results in alumina having a high surface area for catalyst coating.
Sufficient screening agent is added to the mixture so that the mixture has a paste-like consistency. The screening agent used in the preferred embodiment is
Contains organic solvents, solids to change rheological properties, and wetting agents.

The mixture is deposited on resistor element 22. Although screen printing is one suitable method for applying the mixture, it can also be applied using a doctor blade, brushing or the like. After applying the catalyst support structure 24, the entire assembly is heated to a temperature at which the used glass reflows. A temperature of 700 ° C. for one hour is sufficient to reflow the GA-4 glass 32 and firmly adhere it to both the alumina particles 34 and the substrate 20 as shown in FIG. The important thing is that the glass bonds very tightly with both the substrate and the catalyst support structure. this is,
This is because the sensor no longer functions when the alumina particles fall off.

The final step is to attach the catalyst to the catalyst support structure 24. In a preferred embodiment for a hydrocarbon sensor, platinum (platinum) is used as the catalyst.
Platinum is applied as a solution of chloroplatinic acid (eg, hexachloroplatinic (IV) acid) using a dropper or other suitable technique. Thereafter, the entire structure is reheated at a temperature high enough to reduce the acid to platinum. In the preferred embodiment, a temperature of 500 ° C was used.

The alumina particles 34 have various sizes and shapes, and have holes 36 on the surface. When a chloroplatinic acid (e.g., hexachloroplatinic (IV) acid) is applied and dried as described above, the surface of the particles 34, including the surface of the pores 36, is covered with a very thin layer of platinum.
Of course, some of the platinum will also adhere to the surface of glass 32.

Operation of the sensor The key to the operation of the sensor is the catalytic reaction of the gas to be detected and the ability of the resistor element to respond to this reaction by a resulting change in resistance.
For example, when a hydrocarbon gas comes into contact with a platinum catalyst, a chemical reaction occurs, the hydrocarbon is burned, and heat is generated. The greater the amount of hydrocarbons, the more heat is generated, thus increasing the resistance of the resistor element 22 accordingly.

Next, the resistance of the resistor element 22 is
Compare with the resistance of a reference sensor (not shown) of the same design, except that it is not covered by a catalyst, located in the same environment. The difference in resistance between the resistor element 22 and a reference sensor (not shown) is due to heat generated by the catalytic reaction. The difference in resistance indicates the hydrocarbon concentration in the exhaust stream.

Nitrogen Oxide Sensors Nitrogen oxide sensors can be manufactured using the same methods described above for hydrocarbon sensors except that a rhodium catalyst is used instead of platinum. Rhodium is applied to the catalyst support structure 24 in the form of rhodium chloride. Rhodium chloride is applied in a similar manner to chloroplatinic acid (eg, hexachloroplatinic (IV) acid) and is similarly heated to reduce the solution to pure rhodium.

Variations of the Preferred Embodiment Although the illustrated embodiment discusses a horseshoe shaped resistor element 22, those skilled in the art will appreciate that the disclosed gas sensor will work with other resistor patterns. . The horseshoe shape only provides an efficient means for placing the electrical conductors 25 and 26 close to each other for size limitations and manufacturability reasons.

Further, it is not necessary to attach the catalyst support structure 24 in a rectangular shape as shown in FIG. The catalyst support structure can be applied in any desired shape. This is also true for the shape of the substrate 20, as discussed above.

Furthermore, different types of ceramic particles other than alumina can be used in the catalyst support structure, and glass materials other than GA-4 can also be used in the present invention. The glass content in the mixture can vary from 40% to 90%. The rest are ceramic particles. When the glass content falls below 40%, the mixture does not adhere properly to the base, and when the glass content exceeds 90%, there is not enough high surface area particles to hold the catalyst. Any glass that adheres well to both the ceramic particles and the substrate can be used. In addition, other screening agents or water can be used to make the mixture into a paste for attaching to the substrate.

Although the present disclosure has discussed the detection of both hydrocarbon gases and nitric oxide, those skilled in the art of gas sensor fabrication will appreciate that
This design will readily apply to the detection of any number of gases that can produce an exothermic reaction when exposed to a suitable catalyst material disposed on a catalyst support structure.

While the present invention has been taught with particular reference to these embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. You will understand. The embodiments described above are to be considered in all respects only as illustrative and not restrictive. Accordingly, the scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are included within the scope of the claims.

[Brief description of the drawings]

FIG. 1 is a plan view showing a sensor of the present invention.

FIG. 2 is a cross-sectional view taken along line aa of FIG. 1, showing a multilayer catalyst structure before calcination.

FIG. 3 is an enlarged view of the circled portion of FIG. 2 showing the catalyst support structure in detail.

FIG. 4 is an enlarged view showing the structure of FIG. 3 after firing and reflowing the glass.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Gas sensor 20 Substrate 22 Heat sensitive resistor element 24 Catalyst support structure 25, 26 Conductor

Claims (30)

[Claims] 1. A gas sensor for detecting a concentration of a gas, comprising: a) a base; and b) a resistor element disposed on the base, the resistor element having an electric resistance that changes according to a temperature change. C) a multilayer catalyst support structure, comprising: ceramic particles;
A multilayer catalyst support structure comprising a mixture of the ceramic particles with each other and a molten glass bonding the support structure to the base and the resistor element; and d) promoting an exothermic reaction of the gas. A gas sensor, comprising: a catalyst disposed on both the molten glass and the ceramic particles of the catalyst support structure for increasing a temperature of a resistor element. 2. The gas sensor according to claim 1, wherein the base includes a ceramic. 3. The gas sensor according to claim 2, wherein the ceramic includes alumina. 4. The gas sensor according to claim 1, wherein the catalyst includes platinum which causes a catalytic reaction when exposed to a hydrocarbon. 5. The gas sensor according to claim 1, wherein the catalyst contains rhodium which causes a catalytic reaction when exposed to nitrogen oxides. 6. The gas sensor according to claim 1, wherein the ceramic particles are high surface area alumina particles. 7. The gas sensor according to claim 6, wherein the high surface area alumina particles have pores, and the pores have a catalyst layer covering a portion of the surface of the pores. 8. The gas sensor according to claim 1, further comprising a conductor disposed on the base and electrically connected to the resistor element. 9. The gas sensor according to claim 8, wherein the conductor includes gold. 10. The multi-layer catalyst structure according to claim 1, wherein said alumina catalyst is composed of 20% alumina particles and 80% powdered glass and 60% alumina particles.
2. The gas sensor according to claim 1, wherein the gas sensor is in the range of up to 40% powder glass. 11. A method of manufacturing a gas sensor, comprising: a) providing a base with a thermal resistor element; and b) applying a mixture of ceramic particles and powdered glass to the base for use as a catalyst support structure. C) heating the base and the catalyst support structure to a temperature that reflows the glass, thereby bonding the ceramic particles together and bonding the catalyst support structure to the base; d) after step c) applying a catalyst in the form of a solution on an exposed surface of both the ceramic particles and the reflowed glass of the catalyst support structure; e) after the step d) the base And reheating the catalyst support structure to reduce the catalyst in solution to a pure catalyst. 12. The method of claim 11, wherein said base comprises a ceramic. 13. The method of claim 12, wherein said ceramic comprises alumina. 14. The method of claim 11, wherein said ceramic particles are high surface area alumina. 15. The method according to claim 14, wherein the alumina particles are calcined before mixing with the powdered glass. 16. The mixture comprises 20% ceramic particles.
12. The method of claim 11, wherein the method ranges from 80% and powdered glass to 60% ceramic particles and 40% powdered glass. 17. The method of claim 16, wherein said ceramic particles are alumina. 18. The method of claim 11, wherein said catalyst is platinum for initiating a reaction with a hydrocarbon. 19. The method of claim 11, wherein said catalyst is rhodium for initiating a reaction with nitrogen oxides. 20. The ceramic particles and powdered glass are mixed with a screening agent including an organic solvent, a solid to change the rheological properties, and a wetting agent to form a paste prior to application to a base. 12. The method according to 11. 21. The method according to claim 11, wherein the heating temperature for reflowing the glass is 700 ° C. 22. Providing the base with a conductor electrically connected to a thermal resistor element,
The method according to claim 11. 23. The method of claim 22, wherein said conductor comprises gold. 24. The method of claim 11, wherein the powdered glass comprises a GA-4 type glass. 25. The method according to claim 11, wherein the mixture is applied by screen printing. 26. A method for manufacturing a gas sensor, comprising: a) providing a base with a gold conductor on the upper side electrically connected to a thermal resistor element; and b) maintaining a high surface area. For the purpose, 600 alumina particles
Calcining at 1 ° C. for 1 hour; c) making a mixture containing 20% alumina particles and 80% powdered glass; d) organic solvents, solids for changing the rheological properties, and Adding a screening agent including a wetting agent to the mixture to form a paste; e) screen printing the mixture on the base over the resistor element for use as a catalyst support structure; f) heating the assembly to a temperature of 700 ° C., reflowing the glass, bonding the alumina particles together, and adhering the mixture to the base; g) after step f), the catalyst support structure Applying a catalyst solution on the exposed surfaces of both the alumina particles and the reflowed glass of the body; h) after step g), reheating the assembly to 500 ° C , Method characterized by a step of reducing the catalyst solution is pure catalyst. 27. The catalyst solution is platinum acid.
The method of claim 26. 28. The method of claim 26, wherein said catalyst solution is rhodium chloride. 29. The method of claim 26, wherein the pure catalyst is platinum for initiating a reaction with a hydrocarbon. 30. The method of claim 26, wherein said pure catalyst is rhodium for initiating a reaction with nitrogen oxides.