JPS5924382B2 - Manufacturing method for a sensor that mainly measures oxygen concentration in exhaust gas from internal combustion engines - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an oxygen enrichment cell for an internal combustion engine having an oxygen ion conducting solid electrolyte body having an inner surface coated at least in part with an internal electrode and an outer surface in contact with the oxygen of the waste gas. This invention relates to a method for manufacturing a sensor that electrochemically measures oxygen concentration in waste gas.

The internal combustion engine of a motor vehicle produces in its exhaust gas inter alia carbon monoxide, nitrogen oxides and unburned or partially burned hydrocarbons, which contribute to the impurity of the air.
In order to reduce the air impurity caused by these substances to a minimum value, it is necessary to remove these substances as fully as possible from the exhaust gases of motor vehicle internal combustion engines. This means that carbon monoxide and hydrocarbons are converted as completely as possible to their highest oxidation stage, carbon dioxide and, in the case of monohydrocarbons, monohydric, or nitrogen oxides are converted to elemental nitrogen and oxygen. Indicates that something must be done. The conversion of the harmful components of such waste gases into harmless compounds into carbon dioxide, nitrogen and water can be carried out, for example, by subjecting the waste gases to after-combustion through a catalyst at temperatures above about 600°C.

However, a prerequisite for this outcome is that the waste gas is adjusted in its composition in such a way that a practically complete reaction to harmless compounds is obtained. That is, the air-to-fuel ratio must be nearly stoichiometric, which, as is well known, exhibits a λ value of approximately 1. Regarding the oxygen concentration of the waste gas, for these λ≦1, there is no ``excess amount'' of oxygen that exceeds the equilibrium amount of the various possible reactions, but when λ>1, there is no excess amount of oxygen. This indicates that oxygen is present in the mixture, that is, when λ=1, the waste gas changes from a reducing state to an oxidizing state.
In order to maintain a λ value of approximately 1, it is necessary to arrange a sensor in the waste gas path, which measures, for example, the oxygen concentration and carries out a precise adjustment of the composition of the waste gas by means of a regulating device. Such known sensors are based on the principle of an oxygen concentration cell with an ion-conducting solid electrolyte.

German Published Patent Application No. 2010793 describes such a sensor, which is fixedly mounted on the wall of the waste gas exhaust pipe and is in contact with the outside air as a standard system for concentration cells.
The solid electrolyte is partially coated with platinum on both sides.
However, this sensor does not produce an obvious jump in potential at λ=1, but the potential changes gradually over a large λ range. However, for the use of such a sensor in a regulating device, it is particularly advantageous if this potential jump is sufficiently clear at λ=1. The aim of the invention is to obtain an electrochemical sensor for measuring the oxygen concentration in waste gases which under operating conditions produces a distinct potential jump at λ-1 and has a short response time and a useful life as long as possible. It is.

Such a clear potential jump can only be obtained if the components of the waste gas exist in thermodynamic equilibrium.

According to the present invention, the object is to produce an unsintered oxygen ion-conducting solid electrolyte body; to apply a paste of diluent oil and a finely divided ceramic material and a finely divided catalyst material in contact with gas equilibrium to the outer surface of the solid electrolyte body; The main body with the paste is coated on at least one part and sintered to form an electrode of the catalyst layer having micropores on the solid electrolyte, and the entire surface of the catalyst layer and the catalyst layer are coated and covered with the catalyst layer. The problem is solved by providing an outer protective layer that covers all the surfaces of the solid electrolyte body that are not covered, and that at least a part of this outer protective layer that covers the catalyst layer is porous.

In principle, an oxygen concentration cell makes it possible to measure the residual oxygen concentration of the waste gas as well as the oxygen concentration present in thermodynamic equilibrium.

Residual carbon in the waste gas, i.e., unreacted oxygen contained in the waste gas due to incomplete oxidation, is removed when the solid electrolyte is in contact with a catalytically inert metal or an inert electron-conducting oxide. Indicated by a concentration cell.

In this case, the cell exhibits an approximately steady potential course when changing from λ>1 to λ<1, but this is not clearly defined. This is because the residual oxygen concentration in the waste gas is not a clearly defined function of temperature and air/fuel ratio. The residual oxygen concentration of the waste gas, which is measured by means of an oxygen concentration cell, depends, inter alia, on the nature of the internal combustion engine, the temperature, the velocity of the gas at the surface of the solid electrolyte and other influences. The potential curve with respect to λ can be reproduced when measuring the oxygen concentration of the waste gas existing in thermodynamic equilibrium.

In this case, the potential E. depends solely on the temperature of the waste gas and the equilibrium oxygen concentration, corresponding to the Nernst equation. Its characteristic feature is a sudden change in potential reaching several 100 mV that occurs when changing to a reducing atmosphere at λ-1. Although the state of the potential jump at λ-1 does not depend on temperature, its magnitude is about 300 to 400 mV depending on the temperature. The oxygen concentration that exists in thermodynamic equilibrium can be measured when the surface of a thermal solid electrolyte exposed to waste gas at temperatures above 450°C is provided with a fixed layer on the entire surface that is in contact with the gas equilibrium. can. This layer must not have large pores or other surfaces directly exposed to waste gas. This is because otherwise the molecules of residual oxygen could directly reach the surface of the solid electrolyte, whereby a mixed potential is measured, whereby the potential no longer changes abruptly and the residual oxygen can be measured directly. As mentioned above, some degree of constant variation is brought about. Short response time required for adjustment: approx. 1-10
At 0 msec, one contact layer has micropores or micro-ruptures depending on the surface temperature of the H-substance electrolyte, which allows gas molecules to reach the three-phase boundary formed in the electrolyte based on Knudsen diffusion. Guaranteed where possible.

The thermodynamic equilibrium of the gas is adjusted during the passage through this hole.
When the thickness of the contact layer is 100 to 300 Å or less, gas permeation can also be carried out by so-called bulk diffusion without using micropores. The catalytically active layer covering the solid electrolyte on the waste gas side can contain platinum or a platinum alloy with aluminum, cobalt, nickel, chromium or other platinum metals as alloying constituents or an oxide system, such as optionally barium oxide or oxide. It consists of copper monochromium oxide blended with nickel or lanthanum-cobalt oxide blended with strontium oxide if necessary.

The average thickness of the layers is 0.02-20 μm. The pore density must be so high that at least 0.01% of the surface of the contacting layer consists of micropores or micropores.
In order to save on the material of the catalyst, it is necessary, inter alia, that if this material consists of platinum or mainly platinum metal, only a part of the surface of the solid electrolyte exposed to the waste gas is coated with a catalytic layer. is preferred.

In this way, in the case of a solid electrolyte with an annular structure, it is sufficient if only the lower part is coated with a catalytically active layer. This is because the jump in potential is not dependent on the amount of gas to be measured, but simply on the difference in the oxygen partial pressure on both sides of the solid electrolyte. However, in this case, as mentioned above, in order to avoid direct contact between the waste gas and the solid electrolyte, the parts that are not covered with the catalytic layer are covered with an airtight layer that is resistant to the waste gas. must have been done. In order to be sufficiently reliable in this case, the contacting layer preferably partially overlaps the gas-tight layer. In order to electrically contact the contacting layer, it is sufficient to run an elongated conductor from this layer to the point where the potential is to be received. This strip conductor is preferably covered with a gas-tight layer. This hermetic layer may consist of a material that has at least a lower ionic conductivity than the solid electrolyte or an electronically conductive material. Among others, potassium aluminum silicate or barium aluminum silicate or barium calcium aluminum silicate or silicate glass or borate glass as substrates and TiO2, MnO
This is the case for semiconductor glazes with additions of 1 or Fe3O4. The solid electrolyte may consist of isotropically stabilized zirconium dioxide, thorium dioxide or mullite, and these materials may have additions of melting agents. Particular preference is given to containing 1 to 5% by weight of silicon dioxide and optionally aluminum oxide, preferably 1.5 to 5% by weight, in addition to the calcium oxide necessary to stabilize the isotropic phase by containing a coalescing agent. The use of zirconium dioxide containing a total amount of 3% by weight, the aluminum oxide component being less than or at most equal to the silicon dioxide component. In particular, the composition of the raw material of zirconium dioxide containing a melting agent is 85 mol% of monoclinic zirconium dioxide and 1 mol% of calcium oxide.
5 mol% and 1.5% of silicon dioxide and aluminum oxide relative to the sum of zirconium dioxide and calcium oxide.
It is advantageous to contain up to 3% by weight. The composition of such a solid electrolyte is such that, compared to the commonly used stabilized zirconium dioxide ceramic, parts manufactured from it have a 1800%
C. instead of about 1600.degree. C., which has the advantage of reducing the manufacturing costs of such parts.

Furthermore, relatively inexpensive raw materials can be used for the production (monoclinic zirconium dioxide, chalk, kaolin, talc), which has an advantageous effect on the production costs.
Another advantage when using said fusing agents is the good processability of the raw materials, which is caused, for example, by the elasticity of kaolin. Finally, compared to the commonly used zirconium dioxide ceramics, an increased mechanical strength as well as a greater resistance to temperature shocks can be demonstrated. When sintered, the additive forms a glass phase together with calcium oxide and a portion of zirconium dioxide, which results in tight sintering at relatively low temperatures. For example, when talc is used as a raw material for silicon dioxide, the sintering is performed using an additional magnesium oxide content of the sintering agent. ~ Slightly suppressed compared to fusing agents, but nevertheless still tight parts can be sintered at significantly lower temperatures than with zirconium dioxide without fusing agents. As raw materials for the coalescing agent, silicates, such as kaolin, wollastonite talc and similar minerals, as well as pure oxides or materials which are converted into oxides by heat treatment, are preferably used due to cost, such as kaolin, wollastonite talc and similar minerals, as well as mixtures thereof. . However, the most favorable results are obtained when using kaolin. As a raw material for zirconium dioxide, stabilized zirconium dioxide is not necessary, and it is preferable to use conventional monoclinic zirconium dioxide, which is supplemented with calcium raw materials necessary for stabilizing the isotropic phase and forming the glass, e.g. Mix chalk. Since the waste gas is corrosive at least at the operating temperature and the catalytically active layer is unstable to mechanical influences due to its small thickness, this should not be exposed to mechanical and chemical adverse influences. It is necessary to protect the sensor to ensure a sufficient service life. This protection must be gas permeable, so that waste gases can reach the catalytic layer, but the protection must be in such a way that it does not increase or significantly increase the response time of the sensor. According to a further development of the invention, this is achieved in that the contacting layer is bonded to a thin porous protective layer, which protective layer preferably has a thickness of 5 to 5001 tm. To create such a protective layer, all materials that are resistant to waste gases at the operating temperature of the sensor and capable of adhering well to the catalyst layer,
For example metals or metal alloys, such as copper or nickel/chromium alloys; also oxides, mixtures of several oxides, hard materials,
For example, the use of carbides, borides, nitrides, silicates of transition metals, such as high-melting sintered glass, kaolin or talc, optionally with the addition of fusing agents such as feldspar, nepheline syenite or wollastonite. I can do it. Various methods exist for providing this protective layer.

In this way, an aqueous dispersion of the material forming the protective layer is applied onto the contact-active layer by brushing, dipping or spraying, or by sprinkling the material as a powder, and subsequently heated to a temperature above the operating temperature of the sensor. Can be baked at any temperature.
To obtain a high porosity, materials can be added that burn or sublimate during sintering. Furthermore, there is a method of providing a protective layer in porous form by plasma irradiation. Finally, a protective layer of chromium oxide can be provided by reactive vapor deposition or chromium electroplating followed by oxidation. It is particularly advantageous that the protective layer is capable of regulating the gas equilibrium by itself, so that in the event of damage to the catalytically active layer provided on the solid electrolyte, a noticeable jump in the potential is obtained. This is the case when the situation is such that

This is achieved in that the protective layer itself consists of a ceramic material that contacts the gas equilibrium adjustment. in this case,
Copper oxide especially with barium oxide or nickel oxide
Chromium is preferred. Furthermore, the already provided contact-inactive porous protective layer can be impregnated with one or more noble metal solutions and subsequently activated by pyrolysis of the noble metal compound to the noble metal. Another method is to provide a contact layer on the solid electrolyte.
One method is to apply a paste made of fine ceramic material and fine catalyst material mixed with diluent oil onto the solid electrolyte before sintering, and then sinter the solid electrolyte tube with the applied layer.

In this way it is achieved that the catalytically active layer is combined with the surface of the solid electrolyte, whereby a high adhesion strength of the layer is obtained. Ceramic materials include materials having at least approximately the same coefficient of thermal expansion as that of the material of the solid electrolyte, such as isotropically stabilized zirconium dioxide, optionally with addition of smelting agents, such as feldspar, nepheline syenite or wollastonite; Magnesium monospermite or magnesia olivine falls under this category. The volume ratio of catalytic material to ceramic material in the paste should be about 1:5 or higher, thereby ensuring that the contacting material forms a bonding layer. In this case, the bonds occur at the boundaries of the solid electrolyte crystallite particles. The invention will now be described with reference to the accompanying drawings. FIG. 1 shows a sensor whose outer surface is entirely coated with white metal.

The sensor consists of a solid electrolyte in the form of a tube 10 closed on one side and equipped with a flange 11 at its open end. Inside the tube there is an internal electrode 12, which has the form of a conductor strip and consists of a noble metal or a material that conducts electrons at the operating temperature, for example a single or mixed oxide. The outer surface of the solid electrolyte tube 10 is entirely covered with a platinum layer 13,
This layer reaches up to the flange 11. The platinum layer itself is fully covered with a porous protective layer 14, and only a portion of the platinum layer is optionally present at the flange 11 to extract the potential. FIG. 2 shows an enlarged sectional view of the flange 11 at the point where the platinum layer 13 and the porous protective layer 14 terminate.

In this case, a portion of the platinum layer 13 is not coated with a porous protective layer 14 in order to provide electrical contact to the platinum layer 13. FIG. 3 shows an embodiment in which the lower part of the solid electrolyte tube is simply coated with a platinum layer 13.

Since the other parts of the outer surface are entirely covered with the airtight layer 15, the waste gas cannot directly reach the solid electrolyte 10. Furthermore, the platinum layer is coated with a porous protective layer 14.
To bring the platinum layer into electrical contact (FIG. 4), an elongated conductor strip 16 is passed from the platinum layer 13 to the flange 11. The internal electrodes 12 in FIG. 3 are provided in close contact as in FIG. 1. In FIG. 5, the platinum layer 13 and the gas-tight layer 15 overlap on the solid electrolyte tube, and the porous protective layer 14 is slightly stretched over the gas-tight layer 15 to ensure sufficient protection of the platinum layer 13. There is. To produce the sensor according to the invention, 84.8% by weight of monoclinic zirconium dioxide, 12.1% by weight of chalk and 3.1% by weight of kaolin as a coalescing agent are dry-ground for 4 hours. , mix.

This material is subsequently shaped by the compaction-grinding method customary in the manufacture of spark plug insulators: the material is brought to a pressure of about 50
It is compressed radially (quasi-non-equilibrium) at 0 kgw/Cd and brought into the corresponding shape by circular polishing of the reduced body. The parts are then sintered in an electric furnace on a zirconium dioxide underlay. The sintering temperature is 1600° C. with a heating time of approximately 10 hours, and the residence time at this sintering temperature is 1 hour. A zirconium dioxide ceramic is thus obtained, which has a flexural strength of 2800 kgw/Cd, whereas a flexural strength of about 1900 kgw/Cd is mentioned for commercially available stabilized zirconium dioxide ceramics. In order to ensure uniform dosing when manufacturing multiple products, the material is preferably granulated in a conventional manner using known binders or compression aids prior to compression. Inside this tube, electrodes are provided in the form of conductor strips by applying a platinum suspension with a brush.

A conductor strip 16 (FIG. 4) is then applied in the same manner to the outer surface of the solid electrolyte tube 10, the conductor strip 16 starting from the subsequently provided platinum layer 13 and passing as far as the flange 11.

Both conductors are then baked together at a temperature of 1000-1300°C. The next step is to provide an airtight layer on the surface of the tube that is not coated with platinum in order to prevent the waste gas from coming into direct contact with the solid electrolyte.

In order to create this airtight layer, powder components such as quartz, kaolin, feldspar, and chalk are used in a mixture of SiO2 and 72% by weight Al2.
6% by weight of O3l, 8.5% by weight of K2O and CaOl. 5
Mix in proportions consisting of % by weight. This mixture is stirred with water to form a thick liquid suspension and is applied with a brush to the surface of the solid electrolyte tube which is not coated with platinum, in which case according to FIG. Care should be taken so that it slightly overlaps the platinum layer 13. The solid electrolyte tube with the coating suspension is heat treated at a temperature of 1350° C. for 8 hours, and the holding time is 1 hour at said temperature. As a final step, the platinum layer is further provided with a porous protective layer of aluminum oxide with a thickness of approximately 100 μm using plasma irradiation techniques.

Sensors exist which exhibit a clear potential jump at λ=1 and can therefore be used in a superior manner in regulating devices for regulating the optimal waste gas composition, in which case a protective platinum layer is The useful life of this sensor is therefore greater than that of conventional known sensors of this kind, and the response time is no greater than in a sensor with an unprotected platinum layer.

It should be noted that in the sensor according to the invention the materials as well as the individual steps can be varied in the manner described above.

[Brief explanation of the drawing]

1 is a longitudinal section through one embodiment of the sensor according to the invention, FIG. 2 is an enlarged longitudinal section through the wall of the sensor of FIG. 1, and FIG. 3 is a longitudinal section through another embodiment of the sensor. FIG. 4 is a top view of the sensor in FIG. 3, and FIG.
The figure is an enlarged longitudinal sectional view of the wall of the sensor of FIGS. 3 and 4. 10... Solid electrolyte tube, 11... Flange, 12... Internal electrode, 13... Platinum layer, 14... Porous protection Layer, 15...
・Airtight layer, 16... Strip conductor.

Claims (1)

[Claims] 1 Oxygen concentration battery, which has an oxygen ion-conducting solid electrolyte body having an inner surface covered at least in part with an internal electrode and an outer surface in contact with the oxygen of the waste gas, electrochemically detects the oxygen concentration in the waste gas of an internal combustion engine. In the method of manufacturing a sensor for measuring the temperature of the solid electrolyte, an unsintered oxygen ion-conducting solid electrolyte body is prepared, and a paste of a fine-grained ceramic material and a fine-grained catalyst material and diluting oil in contact with the gas equilibrium is added to the solid electrolyte body. the main body with the paste is sintered to form an electrode of the catalyst layer with micropores on the solid electrolyte; A method for manufacturing a sensor, characterized in that an outer protective layer is provided that covers the entire surface of a solid electrolyte body that is not coated with a catalyst layer, and at least a portion of the outer protective layer that covers the catalyst layer is porous.