JP3400511B2 - Sensor for measuring components of air-fuel mixture and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for measuring the concentration or change in concentration of a predetermined component of an air-fuel mixture, and a method for manufacturing the same.

[0002]

2. Description of the Prior Art It is known to measure the concentration or change in concentration of a given component of an air-fuel mixture by means of a sensor having a resistance, which takes advantage of the temperature dependence of the electrical conductivity of a given ceramic material. This applies in all cases to mixtures with components which react or decompose with one another to generate or consume heat. Assigning to this type of resistance a catalyst that promotes the reaction between these components, the temperature and thus the conductivity of the material forming the resistance,
It changes due to the heat generated or consumed by the reaction. Comparing this changed conductivity with the conductivity of the resistance, which indicates the temperature of the air-fuel mixture because the catalyst is not assigned, or the conductivity of the resistance, which indicates the temperature generated from this as long as the resistance is heated, the difference between the two From the value of, the concentration of the lacking reactive component in the air-fuel mixture can be determined.

For example, the concentration of ethylene in ethane can be measured as follows. That is, a mixture with a stoichiometric excess of hydrogen is guided through a hydrogenation catalyst, which is assigned to the resistance of the appropriate sensor, and the temperature of this resistance due to the reaction enthalpy and hence the conductivity of the resistance It can be measured by changing. If a sensor of this kind is calibrated with a reference gas with the other conditions being equal, a unique relationship between the change in conductivity and the ethylene concentration can be established.

In the case of an air-fuel mixture containing a plurality of reactive or degradable components, the above relationship becomes unclear and is not unique. This applies, for example, to the exhaust gases of internal combustion engines. The exhaust gas contains carbon monoxide and unburned hydrocarbons as oxidizable components, excess air as oxidant, and catalytically decomposable nitrogen oxides. These harmful substances must be substantially removed for environmental protection reasons. This is done by guiding the combustion gases over a suitable catalyst at temperatures above about 300 ° C., converting the harmful substances mentioned above into carbon dioxide, water and nitrogen.

The life of the above catalysts is limited. Its acting power decreases with time, so that more and more of the harmful substances are released into the atmosphere. For this reason, it is desirable to monitor the functional capabilities of the catalyst. The monitoring device indicates to the driver acoustically or optically that the catalyst has disappeared.

The function of the catalyst can be monitored by measuring the concentration of at least one of the abovementioned harmful substances at the inlet and outlet sides of the catalyst. However, this method is extremely expensive and prone to failure. Federal Republic of Germany Patent No. A1-4
According to the method disclosed in Japanese Unexamined Patent Publication No. 020383, a temperature-sensitive sensor is inserted into the exhaust gas or a part thereof at the outlet side of the catalyst, and the electrical conductivity generated based on the exothermic reaction due to the temperature rise of the resistance. The change is used to detect the deterioration of the function of the exhaust gas cleaning catalyst. In this case, the temperature-sensitive sensor has at least two NTC or PTC resistances, and further has a coating of the sensor on at least one of these resistances of the catalytically activated material, This material initiates the same reactions that proceed for the catalyst used for exhaust gas cleaning.

[0007]

The sensors as claimed in claims 1 to 8 are characterized by an improved sensitivity and a shorter response time than those of the prior art. The diaphragm as a resistor has a small mass and therefore a small heat capacity. In addition, the sensor is spatially separated from the sensor's high mass element by a region that blocks heat exchange, so the reaction enthalpy (ie, thermal tone) of the reaction proceeding at the catalyst.
Is larger and more than a large mass resistor with a correspondingly large heat capacity, or a large mass resistor with a small mass connected to a sensor element allowing rapid temperature adjustment. Causes a temperature change to appear quickly. The measuring sensitivity is also increased by the fact that only a thin diaphragm layer acts as the measuring resistance. In the case of a sensor configured as a complete substrate, the inner region contributes to the conductivity, but the temperature rise and thus the resistance change is mainly at the surface. A unique manufacturing advantage of the sensor according to the invention is that the sensor is characterized by claims 9-13.
It can be produced by the well-developed screen printing (or thick film) and ceramic film technology. This sensor is characterized by significantly less manufacturing irregularities during mass production. This is because the individual layers and elements are fixedly bonded to one another by means of cobalt sintering.

Next, embodiments of the sensor according to the present invention will be described with reference to the following description and drawings.

[0009]

The sensor according to the invention is suitable for measuring the concentration or the change in concentration of a given component of an air-fuel mixture which reacts in the presence of a catalyst. In the reaction, reduction-ie hydrogenation reactions, oxidation reactions or other reactions are targeted, for example synthesis- or decomposition reactions. The catalyst needs to be selected according to each reaction. For the reduction-or hydrogenation reaction, for example, plastina-, palladium- or chromium-copper is chosen. Suitable catalysts for the oxidation reaction in the absence of hydrogen are, for example, vanadium (V)-or cerium compounds. Platinum, rhodium and palladium can advantageously be used as catalysts in a known manner for the catalytic removal of carbon monoxide, hydrocarbons and / or nitrogen oxides from the exhaust gas.

The sensor according to the invention is suitable for measuring the concentration of a given component of a mixture when only two components which react with each other are present in the mixture. For example, after adding hydrogen to a trace amount of olefin as necessary, a correspondingly gaseous saturated hydrocarbon, or a trace amount of carbon monoxide in addition to carbon dioxide and hydrogen, in the reaction gas mixture of the hydrogen gas reaction. Measured at. The measurement of a given component is always easily and reliably reached at the next time. That is, it is reached when a unambiguous relationship is formed between the concentration of the component in question and the change in the electrical conductivity of the resistance, which is caused by the change in temperature due to the reaction enthalpy of the reaction proceeding in the catalyst.

However, the concentration of the predetermined component in the air-fuel mixture is not targeted for the absolute value, but its change is targeted in many cases. This is the case, for example, in the case of the complex occurrence of catalytic cleaning of the exhaust gases of internal combustion engines.
The sensor and method according to the invention are largely unsuitable for the measurement of individual components, since the reaction enthalpy (ie thermal tone) at the catalyst cannot be matched to a given reaction. However, the action of the exhaust gas cleaning catalyst can be measured reliably and easily as follows. That is, it can be measured by arranging a sensor having a catalyst layer according to the purpose at a position where the exhaust gas should already be cleaned. When the conductivity changes, this is a solid indicator that carbon monoxide, unburned hydrocarbons and / or nitrogen have passed through the exhaust gas cleaning catalyst and have been extinguished. The sensor can be located at the outlet of the exhaust gas cleaning catalyst, in the main fuel stream or in the auxiliary fuel stream. However, the sensor can also be provided at the height of the exhaust gas cleaning catalyst which is passed through. This is costly, but allows a prior warning, because the sensor will erase the catalytic area located on its inlet side at the point when the area present on its outlet side is still active. Because I will report. The substrate carrying the actual measuring element is made of an oxide (or ceramic) material, for example aluminum oxide, or preferably zirconium oxide (I) as in another element of the sensor.
V). When the sensor is to be used at significantly higher temperatures, for example in the case of exhaust gas monitoring of internal combustion engines, it is advantageous if the substrate has a heating device.

The sensor according to the invention has at least two resistors in the form of diaphragms. The material forming the diaphragm becomes conductive at extremely high temperatures and its conductivity changes with temperature. When a high measurement sensitivity is required, it is preferable to use a resistor made of an oxide material having a high temperature coefficient of conductivity. Suitable materials are titanium (IV) oxide, hafnium (IV) oxide, cerium oxide and, for example, readily available zirconium (IV) oxide. The oxide can be doped with activators, promoters or stabilizers. Zirconium (IV) oxide is used as a stabilizer, for example, a small amount of yttrium oxide (II
It can be doped with I) or with calcium oxide.

A catalyst is assigned to at least one of the resistors. This catalyst acts on the reaction of the reactive components of the mixture. The catalyst and resistive material should be coordinated with each other as follows:
It must be adjusted so that the catalyst has sufficient conductivity (and a temperature coefficient of conductivity as high as possible) in the range where it begins to act. The sensor according to the invention should be used to monitor the action of a catalyst for cleaning the exhaust gases of internal combustion engines. The sensor preferably contains a catalyst which is the same as the catalyst used as exhaust gas cleaning catalyst. This is usually platinum, rhodium and / or palladium, titanium (IV) oxide
Alternatively, it works well with resistors made of zirconium (IV) oxide. These resistors work perfectly under the actual exhaust gas temperature of 150 ° C to 700 ° C. The sensor according to the invention responds significantly more sensitively and very quickly when the catalyst is placed directly on the resistor. In all cases, the catalyst should be placed in the space near the resistance.

The sensor according to the invention usually has two resistors. Sensors with more than two resistances can be used as a precaution, for example, when the measurement cannot be represented by only two resistances because the gas flow can be non-uniform. In either case, it is necessary to provide at least one resistance, the catalyst is provided in the space near the resistance, and at least one resistance is provided otherwise.

A significantly advantageous characteristic of the sensor according to the invention is the result of a combination of the two configurations indicated in the characterizing part of claim 1. At least the associated resistance of the catalyst is designed as a diaphragm. The diaphragm is separated from the substrate by a region that blocks heat exchange over most of its area. With this combination, (a) the reaction enthalpy of the reaction in the catalyst is converted into a highly resistive temperature change, and (b) this is done quickly.

The diaphragm is typically a thickness of. 10 to 200 [mu] m, advantageously in 10~100μm especially 40-70
The thickness is μm. Its expansion rate is in the mm range. Sensors for the exhaust gases of internal combustion engines typically have a coefficient of expansion of 2 × 10 mm, for example. The sensor is separated over most of its area, preferably 50-100%, from the substrate by a region that blocks heat exchange. In this case, a cavity is used. Sensors of the type in which this region is filled with a porous material having a correspondingly low thermal conductivity are mechanically more stable. It is advantageous to use the oxide material, for example zirconium (IV) oxide as described above. It is preferable that the hole portion has a value of 30 to 90% by volume. The porous layer contributes little to conductivity, even when it is formed from a resistive material, and thus does not substantially degrade the measurement sensitivity of the sensor. The distance between the diaphragm and the substrate-from which the diaphragm is separated by a cavity or a porous material-typically has a value of 10-100 μm.
Alternatively, however, a ceramic sheet provided with voids in the diaphragm can be used as spacers.

Basically, the resistance of the uncatalyzed measuring element does not have to be designed as a diaphragm. Manufacture of the sensor is, of course, easier when both resistors are diaphragms.

FIG. 1 shows a sensor according to the invention in cross section. It can be used to monitor the action of the exhaust gas catalyst. On a substrate 1 made of zirconium (IV) oxide with a heater 2, a frame 6 also made of zirconium (IV) oxide is provided. The heater is embedded in an insulating layer of aluminum oxide. On this frame are provided resistors 4 and 5 in the form of diaphragms. A region 7 is provided as a cavity between the substrate 1 and the diaphragms 4, 5. Instead of a cavity, a porous zirconium (IV) oxide layer can be provided between the substrate 1 and the resistors 4, 5. In this case, the frame 6 can be omitted and the porous layer 7 can extend to the edges of the resistors 4, 5. Electrodes 8, 9 and 10 are provided on the layer, both electrodes 8 being conductively connected to each other. This is shown in FIG. Between electrodes 8 and 9 and electrode 8
And 104, the regions of resistors 4 and 5 are provided.
The conductivity (or resistance value) of the resistance is measured in this region. Only a small part of the diaphragm acts the function of resistance. A cover layer 11 made of aluminum oxide is applied onto the measuring element consisting of the resistor 4 and the electrodes 8, 9 and a catalytic layer 12 made of platinum, rhodium and / or palladium is applied thereover. To be done. The cover layer 11 can be omitted if the layer 12 acting as a catalyst is not electrically conductive. The catalyst is surrounded by a porous protective layer 13. A second measuring element consisting of the resistor 5, the electrodes 8, 10, the cover layer 11 and the porous protection 14 is correspondingly formed; however, no catalytic layer is provided in this measuring element. The cover layer 11 is also omitted. The porous protective layers 13, 14 are preferably formed from zirconium (IV) oxide.

FIG. 2 shows the same sensor in plan view. As shown, the substrate 1 (reaching the edges of the measuring element), the resistors 4, 5 and the cavities below which are shown by dashed lines, both measuring elements with electrodes 8, 9, 10. The finger-like structure of FIG. 1, the catalyst layer 12 provided on only one side and the porous protective layers 13 and 14 on both sides are shown.

The known techniques for producing the sensor according to the invention are preferably used, preferably multi-layer techniques, mainly involving screen printing on ceramic sheets. The substrate 1 can be formed from two sheets of zirconium (IV) oxide. The heating meander body 2, for example made of platinum, is preferably applied by means of a screen printing technique. The same applies to the insulating layer 3.

A frame 6 is attached onto the substrate 1. The frame can be formed from sheets, but can also be manufactured by screen printing. The frame includes a space 7. However, this is a porous zirconium (IV) oxide
Can be filled with. In this case, first of all, a mixture of zirconium (IV) oxide and soot particles is contained, from which porous zirconium (IV) oxide is formed after copper sintering. Combustion or decomposition products can escape through the frame 6. The frame is also not densely sintered under the soot decomposition temperature. When the cavity is to be left, it is advantageous in manufacturing technology to first fill it with the following materials. That is, this material decomposes when heated to the high temperatures reached during subsequent copper sintering,
Leave cavities behind as they escape through the disassembly product frame 6. Soot particles are also suitable for this purpose. The area between the substrate 1 and the diaphragms 4, 5 is omitted by omitting the frame, and the porous zirconium (IV) oxide is formed by the screen printing technique.
It can also be filled with. This type of sensor is mechanically stable enough for most practical purposes.

On top of the frame 6 or the abovementioned mixture-from which the porous zirconium (IV) oxide is formed-zirconium (IV) oxide 4, 5 is printed. The platinum electrodes 8, 9, 10 and the cover 11 are printed on this. By depositing a platinum layer or a platinum cermet layer (a mixture of platinum and ceramic particles) on the measuring element having the diaphragm 4, the platinum catalyst 12
Be applied. Finally, the porous oxide cover layers 13 and 14 are formed from zirconium (IV) oxide as follows. That is, a layer is deposited from the aforementioned mixture of zirconium (IV) oxide and soot particles, and porous zirconium oxide (I
V) is formed. Cobalt sintering completes the manufacturing process to complete the sensor-the layer and the element being fixedly bonded to each other.

The sensor according to the invention is conventionally used for measuring the concentration or change in concentration of a given component in a mixture.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a sensor.

FIG. 2 is a plan view of a sensor.

[Explanation of symbols]

1 substrate 2 heater 3 insulating layers 4,5 resistance 6 frames 7 Porous layer 8, 9, 10 electrodes 11 cover layer 12 Active layer 13 Protective layer 14 Porous protective layer

Front Page Continuation (72) Inventor Johann Riegel, Federal Republic of Germany Beitheimheim-Bissingen Eichenweg 27 (56) References JP 62-49251 (JP, A) JP 60-243549 (JP, JP, A) JP 57-178149 (JP, A) JP 62-220850 (JP, A) JP 1-203957 (JP, A) JP 63-172948 (JP, A) JP 56 -18381 (JP, A) JP-A-2-179459 (JP, A) JP-A-61-201148 (JP, A) Actually developed 2-37360 (JP, U) (58) Fields investigated (Int.Cl) . ^7, DB name) G01N 27/00 - 27/24

Claims (15)

(57) [Claims] 1. A sensor for measuring the concentration or change in concentration of a predetermined component of an air-fuel mixture, the air-fuel mixture reacting in the presence of a catalyst under the generation or consumption of heat, the sensor comprising: ), At least two resistors (4, 5) made of a material that conducts electrically under significantly higher temperatures, the conductivity of the material varying with temperature, A catalyst (12) is assigned to at least one of the resistors (4), which acts on the reaction of the components to change the temperature of the resistance, while at least one of the plurality of resistors is In the above-mentioned measuring sensor, in which the catalyst is not assigned to one resistor (5), so that the temperature of the resistor does not change, at least one resistor (4) to which the catalyst is assigned is configured as a diaphragm, The diaphragm is A sensor for measuring the components of an air-fuel mixture, characterized in that it is separated from the substrate (1) by a region (7) that blocks heat exchange over most of its area. 2. Sensor according to claim 1, characterized in that both resistors (4, 5) are constructed as a diaphragm and are separated from the substrate (1) by an area (7) which blocks heat exchange. 3. The thickness of the diaphragm layer is from 10 to 100.
The sensor according to claim 1 or 2, wherein the sensor has a size of μm. 4. A diaphragm made of zirconium oxide (I
Sensor according to any one of claims 1 to 3, formed from V). 5. The sensor according to claim 1, wherein the catalyst (12) promotes the oxidation of carbon monoxide and / or the oxidation of hydrocarbons with oxygen. 6. The sensor according to claim 1, wherein the distance between the diaphragm and the substrate in the region (7) is 10 to 100 μm. 7. The sensor according to claim 6, wherein the spacing is adjusted by a frame of one sheet or by using a printed layer of a mixture of oxide and soot particles. 8. A sensor according to claim 1, wherein the region (7) is a cavity or is filled with a porous material. 9. In a method of manufacturing a sensor, wherein two resistors (4, 5) are configured as a diaphragm and are separated from the substrate (1) by an area (7) that blocks heat exchange, in a method of manufacturing an oxide material. on the substrate (1) comprising, placed frame (6) made of an oxide material, it is stretched a Daiyafura arm (4, 5) functioning as a resistor to the frame, the electrode onto the diaphragm ( 8, 9, 10) , the electrode is covered with an insulating oxide cover layer (11), and the catalyst (12) is coated over the insulating oxide cover above the diaphragm (4). A catalyst, as well as an insulating oxide cover layer (11) and an oxide porous protective layer (13, 1).
Cover with 4), characterized in that this way to each other fixedly bond the layers structure formed a layer and the element of the sintered under a temperature 1300 to 1600 ° C., the sensor Manufacturing method. 10. A diaphragm made of zirconium oxide (I
The method of claim 9 formed from V). 11. The frame (6) is filled with a material that forms a porous oxide layer when heated to a high temperature.
Alternatively, the method according to Item 10. 12. Substrate (1) and diaphragm (4,5)
11. The method according to claim 9 or 10, wherein the space between and under the omission of the frame (6) is filled with a material that forms a porous oxide layer when heated to high temperatures. 13. Method according to claim 9 or 10, characterized in that the frame (6) is filled with a material that decomposes when heated to high temperatures to form cavities. 14. Use for measuring the composition or composition change of exhaust gas of an internal combustion engine ,
A method of using the sensor according to claim 1. 15. A layer (12) acting as said catalyst.
Is a non-conductive material, the insulating oxide cover layer described above is used.
The method according to claim 9, wherein (11) is omitted.