JPH046909B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a wide-range air-fuel ratio sensor that detects the air-fuel ratio by detecting the oxygen concentration in engine exhaust gas over a wide range before and after the stoichiometric air-fuel ratio.

As is well known, for example, in an engine installed in a car, a technical concept is known in which the air-fuel ratio (A/F) is indirectly detected by detecting the oxygen concentration in exhaust gas. For example, as a detection element for detecting the oxygen concentration in exhaust gas,
As shown in Japanese Patent Publication No. 52-46889 and Japanese Patent Publication No. 57-10382, so-called λ sensors are known in which the electromotive force changes stepwise with the oxygen concentration corresponding to the stoichiometric air-fuel ratio as the boundary. The oxygen concentration detection element can determine whether the air-fuel ratio is larger or smaller than the stoichiometric air-fuel ratio.

On the other hand, in the case of automobiles, etc., when high output is required, such as during acceleration or high-load operation, the air-fuel ratio is set to a certain level of fuel richer than the stoichiometric air-fuel ratio. In order to improve fuel efficiency, it is desirable to operate the engine with the air-fuel ratio set to a level where the fuel is thinner than the stoichiometric air-fuel ratio, so-called lean. Naturally, the detection element cannot accurately detect an air-fuel ratio that deviates from the stoichiometric air-fuel ratio, and therefore cannot be used for control such as this to set the air-fuel ratio to an arbitrary value.

For example, JP-A-57-76450, JP-A No. 53-34077
As shown in the publication, an oxygen concentration detection element that is capable of detecting oxygen concentrations other than the stoichiometric air-fuel ratio has been proposed. However, these conventional devices are not suitable for the above-mentioned λ sensor, that is, an oxygen concentration detection element formed by forming porous electrodes on both sides of a solid electrolyte.
This is a so-called amperometric sensor in which a protective layer is provided on the outside of a porous electrode on the side of the gas to be measured to control the rate of gas diffusion to the electrode. It is designed to obtain linear electromotive force characteristics by dulling the overall electromotive force characteristics, and in the lean region, linear electromotive force characteristics cannot be obtained like the above-mentioned λ sensor. Moreover, the width of this linear electromotive force characteristic is small, for example, about 100 to 200 mV, and therefore these detection elements have insufficient detection sensitivity, making them unsuitable for practical use.

The inventors of the present invention have conducted extensive research in order to solve the above-mentioned drawbacks and to obtain a wide-range air-fuel ratio sensor that can continuously measure the air-fuel ratio. and is composed of the electrode, the solid electrolyte, and the gas to be measured.
We have now provided a wide-range air-fuel ratio sensor in which a metal oxide that oxidizes hydrocarbons (hereinafter referred to as HC) to generate CO is present near the phase point.

Here, the above-mentioned "semi-catalytic performance" will be explained.
Figure 1 is “Physical” by William J.Fleming.
Principles Governing Nonideal Behavior of
the Zirconia Oxygen Sensor” (J.Electro−
chemical Society, Vol.124, No.1, January
1977, pp. 21-28) is a graph showing changes in O ₂ and CO partial pressure at the three-phase point due to electrocatalytic activity. The above-mentioned "semi-catalytic performance" refers to a performance showing an activity of about "2" or less, which is considered to be "Poor" activity in FIG. 1 (the effect will be described later). For example, Ag, Au, etc. exhibit this kind of semi-catalytic performance, but even Pt, which is generally considered to be highly active and is used in conventional lambda sensors that exhibit step-like characteristics, may exhibit such properties depending on the material and firing conditions. Can be granted.

The above-mentioned "three-phase point" refers to the point where the solid electrolyte 1, the porous electrode 3 formed on the surface of the solid electrolyte 1, and the gas to be measured 6 are adjacent to each other, as shown by the broken line circle in FIG. It is.

The structure described above will now be schematically explained using FIGS. 2 to 4. As shown in FIG. 2, the solid electrolyte 1 is formed into, for example, a tubular shape that isolates the atmosphere 5 and the gas to be measured 6, similar to a conventional λ sensor, etc. Porous electrodes 2 and 3 are formed on both surfaces on the gas 6 side, respectively. A layer of metal oxide 4 is further formed on the surface on the side of gas to be measured 6. Figure 3 is an enlarged view of Figure 2, and Figure 4 further shows this third view.
As shown in FIGS. 3 and 4, the metal oxide 4 is an enlarged version of the metal oxide 4 described above.
It is stratified so that it exists near the phase point.

Hereinafter, the air-fuel ratio (A/
The mechanism by which linear electromotive force characteristics are obtained for F) will be explained in detail. As is generally known, the theoretical electromotive force V of the sensor is given by the Nernst equation: V=(RT/4F)ln [Po ₂ (air) /Po ₂ (exh)]. Here, R is the gas constant, T is the absolute temperature, F is Faraday's constant, Po ₂ (air) is the partial pressure of oxygen in the atmosphere, and Po ₂ (exh) is the partial pressure of oxygen in the gas to be measured. If the electrode has a catalytic activity high enough to bring the gas into equilibrium, plotting the values derived from this equation will yield an electromotive force characteristic curve as shown in FIG. 5A. However, many actual sensors exhibit electromotive force characteristics that cannot be explained by the Nernst equation, and to explain these, the aforementioned William J. Fleming proposed Fleming's equivalent circuit model. The behavior of the wide-range air-fuel ratio sensor of the present invention is also determined by Fleming, which has many parameters.
This is explained by the equivalent circuit model of

Here we will briefly explain Fleming's equivalent circuit model. The equivalent circuit model is based on the fact that a unique electromotive force is generated at each attraction point in the three-phase points, and according to this, the electromotive force V is V=f _cp・Vco+(1−f _cp ) Represented by Vo ₂ .
Here _, f _cp is the rate at which CO is adsorbed at the _{three - phase} point _. The electromotive force generated where CO is adsorbed at the three-phase point is Vco = Vco + (RT/2F) ln [Po ₂ ^1/2 (air), Pco (anode) / Pco ₂ (anode)], Vo ₂ is the electromotive force generated at the three-phase point where O ₂ is adsorbed, and is Vo ₂ = Vo ₂ + (RT/4F) ln [Po ₂ (air) / Po ₂ (anode)]. Note that Vco and Vo ₂ are standard cell potentials for each electrochemical cell. Pco (anode),
Pco ₂ (anode) and Po ₂ (anode) are the partial pressures of CO, CO ₂ and O ₂ at the three-phase point of the electrode on the gas side to be measured, respectively. The above formula is determined by the following two reactions at three-phase points.

O ₂ +4e ^- 2O ^2- CO + O ^2- CO ₂ +2e ^- The difference between the electromotive force characteristics of the actual sensor and the electromotive force characteristics of the theoretical sensor is mainly due to the insufficient catalytic performance of the cathode. In other words, O ₂ and CO at the three-phase point
The electromotive force characteristics change greatly due to the difference in partial pressure. As shown in FIG. 1 above, in the lean region, the O ₂ partial pressure is almost constant regardless of the catalyst activity;
What changes greatly is the CO partial pressure. In other words, the electromotive force in the lean region is mainly dominated by CO.
According to Fleming's equation, increasing the CO partial pressure in the lean region will increase the electromotive force.

Based on the above, we will explain the mechanism by which linear electromotive force characteristics are obtained. First, the metal oxide acts as an oxidation catalyst that oxidizes HC in the gas to be measured (exhaust gas) (self is reduced) and generates CO. For example, if this metal oxide is SnO ₂ , the following reaction occurs: aSnO ₂ + bHC(g) → cSnO + dCO(g) + eCO ₂ (g) + fH ₂ O(g) +..., and the further reduced SnO is It returns to _SnO2 due to _O2 in the gas. In other words, SnO ₂ constantly generates CO and O ₂ due to the so-called Redox effect.
Perform absorption.

SnO ₂ HC ―→ ←― O ² SnO O ₂ partial pressure decreases due to the above action, and CO generated from HC increases the CO partial pressure near the three-phase point, so the electromotive force in the lean region becomes As shown in Fig. 5B, the electromotive force increases and a linear electromotive force characteristic is obtained in the lean region.

Since a porous electrode having semi-catalytic performance is used as the porous electrode, the electromotive force in the rich region decreases as shown in Figure 5B, and the electromotive force is linear from the lean region to this rich region. Power characteristics will be obtained.

By the way, the HC concentration in the lean region is only about 1,000 to several hundred ppm at most. Therefore, the amount of CO produced by the action of the metal oxide is also extremely small. However, if this CO is generated near the three-phase point, it will reach the three-phase point without being oxidized by the porous electrode. For example, even if the concentration of CO is 0.001%, if it reaches the three-phase point without being oxidized, the CO partial pressure will change from "4" shown in FIG. 1 to about "2". Therefore, in order to fully exhibit the effect of the metal oxide, it is necessary to make the metal oxide exist near the three-phase point.

Since the above-mentioned metal oxide must act to oxidize HC and generate CO, a metal oxide having a small oxidizing ability to oxidize HC is unsuitable. The HC oxidation ability (CO generation ability) of various metal oxides can be found, for example, in the table in "Metal Oxides and Their Catalytic Actions" by Tetsuro Kiyoyama (1979, Kodansha).
This can be determined using 4.10 "Propylene oxidation reaction on various metal oxides" (p185) as a guide, but for example, when using a porous electrode containing Pt as a main component (such as Pt paste), _SnO2 ,
In ₂ O ₃ , NiO, Co ₃ O ₄ and CuO exhibit sufficient HC oxidation ability. Therefore, in this case, one or more of these metal oxides may be used. The tendency for HC to be oxidized to generate CO is determined by the overall balance between the catalytic activity of the porous electrode and the HC oxidation ability of the metal oxide. When forming the oxide layer from a material having low catalytic activity, for example, a material containing Ag, Au, etc. as a main component, a material having a lower HC oxidation ability than the above-mentioned metal oxides can also be used. For example, when the porous electrode is formed from Ag paste, linear electromotive force characteristics can be obtained in the lean region even if ZnO or MnO ₂ is used.

In order to make the above air-fuel ratio sensor suitable for practical use, it is necessary to
It is necessary to ensure that the abundance of carbon dioxide (which generates CO by oxidizing carbon dioxide) does not vary depending on each element, and that the abundance of these substances is uniform even at a high abundance. In other words, the presence of metal oxides near this three-phase point increases the electromotive force in the lean region, so if the presence rate of metal oxides varies between elements, the electromotive force characteristics of each element will change. Variations and calibration work are extremely complicated. Moreover, if the metal oxide abundance rate is low, naturally a sufficient increase in electromotive force in the lean region cannot be obtained, and the linearization of the electromotive force characteristics is slowed down.

On the other hand, in the wide range air-fuel ratio sensor used for the above-mentioned applications, it is practically essential that the porous electrode on the surface thereof has sufficient durability. Conventionally, in order to improve the durability of this porous electrode, a protective coating has been formed by plasma spraying a heat-resistant metal oxide on the electrode.
As mentioned above, when such plasma spraying is performed on an element in which a metal oxide that generates CO is present near the three-phase point, the metal oxide near the three-phase point melts and the oxidation ability of the oxide itself is reduced. In addition, the thin oxide layer is absorbed into the spinel during melting, and its amount decreases, making it impossible to exhibit linear electromotive force characteristics.

The present invention has been made in view of the above points, and provides the above-mentioned new wide-range air-fuel ratio sensor in which the metal oxide is present near the three-phase point at a uniform and high abundance rate between each element, and is linear. The object of the present invention is to provide a manufacturing method that can be formed to have sufficient durability without impairing the electromotive force characteristics.

The method for manufacturing the wide-range air-fuel ratio sensor of the present invention involves forming a porous electrode on the surface of the solid electrolyte as described above, and then plasma-spraying a refractory metal oxide onto the electrode on the side that comes into contact with the gas to be measured. A sensor material formed with a porous protective coating is immersed in a solution of a metal compound that generates a metal oxide that generates CO, and the solution is passed through the pores of the protective coating and deep into the pores of the electrode. By impregnating the material, and then taking the material out of the solution and heating it to the thermal decomposition temperature of the metal compound, the material is impregnated near the three-phase point.
It is designed to generate metal oxides that generate CO.

According to the above method, the dissolved metal compound reaches the deep part of the electrode pores, so the metal oxide abundance rate near the three-phase point depends on the size of the electrode pores, the electrode film thickness, the element surface shape, etc. Even if the abundance rate is high, it will be uniform among each element.

Moreover, since the protective film is formed by plasma spraying before the formation of the metal oxide that generates CO, the metal oxide that is involved in linearizing the electromotive force characteristics is not destroyed by plasma spraying. There is no need to put it away.

In order to increase the metal oxide abundance at the three-phase point, the above operation may be repeated several times. The above-mentioned metal compounds include metals that produce metal oxides such as those mentioned above (i.e., Sn, In, Ni, Co,
Cu, Mn, Zn, etc.) chlorides, sulfates, nitrates, etc. of metals other than the above-mentioned metals can be used. It is preferable to use one that does not contain many extra elements.

Examples of the present invention will be described below.

[First Embodiment] Hereinafter, a description will be given with reference to FIG. 6. Manufactured by Nippon Insulator Co., Ltd. as a tubular solid electrolyte 1 as shown in Figure 2.
Using ZrO ₂ −6 mol Y ₂ O ₃ , coat both sides of the solid electrolyte 1 with Pt paste by brushing,
It was baked at 1050°C for 1 hour to form a porous electrode 2 on the atmosphere side and a porous electrode 3 on the measurement side with a film thickness of about 20 μm (6th
Diagram a). Next, on this electrode is a refractory metal oxide.
Plasma sprayed MgAl ₂ O ₄ spinel, film thickness 50μ,
A protective coating 10 having a porosity of about 15% was formed (FIG. 6b).

The outside of this sensor material 20 (measurement side porous electrode 3 side) is immersed in a tin chloride () (SnCl ₄ ) aqueous solution prepared at 35% by weight (Fig. 6c), and then exposed to an ammonia gas atmosphere. insoluble in water
Precipitate Sn(OH) ₄ to prevent the tin solution from flowing down,
Dry at 200℃ for 1 hour, then at a temperature of 800℃ for 30 minutes.
By heating for minutes (FIG. 6d), SnO ₂ was formed.

The above treatment from SnCl ₄ aqueous solution immersion is continued for 3 more times.
By repeating this process several times, an air-fuel ratio sensor was obtained in which SnO ₂ was sufficiently supported at the three-phase points of the porous electrode 3 (FIG. 6e).

This air-fuel ratio sensor was installed in the exhaust system of a reciprocating engine and tested. While maintaining the exhaust gas temperature near the sensor at 550℃, the air-fuel ratio is set to A/
When the electromotive force was measured while changing F from 11 to 18, linear electromotive force characteristics as shown in FIG. 7 were obtained.

[Comparison with other manufacturing methods]

(1) The solid line in Figure 8 indicates the electromotive force of a wide range air-fuel ratio sensor manufactured by a manufacturing method that is essentially the same as the method of the above example except that the step of forming a protective film by plasma spraying described above is omitted. characteristics (exhaust gas temperature is 600℃).

Next, in order to improve the durability of this sensor with linear electromotive force characteristics, MgAl ₂ O ₄ spinel was sprayed onto the surface of the porous electrode on the measurement side of the sensor to form a protective coating. When the electromotive force characteristics of the sensor after adding this protective film were measured, results as shown by the broken line in FIG. 8 were obtained. In other words, the linear electromotive force characteristic with respect to the air-fuel ratio is completely lost. This loss of linear electromotive force characteristics is due to the metal oxide (SnO ₂ in this example; melting point 1127°C) that was present in the electrode pores and three-phase point melting due to the high heat of the molten spinel.
Loss of HC oxidation ability. Metal oxide (SnO ₂ ) is a fine particle with low adhesion strength and is supported thinly, so it is absorbed by the molten spinel and the density at the three-phase point decreases. It is considered that

(2) 3 metal oxides that oxidize HC to generate CO
In addition to the method of immersing the sensor material in the metal compound solution described above, the air-fuel ratio sensor supported near the phase point can be prepared by mixing and dispersing metal oxide particles in a binder and immersing the sensor material in this. can also be formed. Hereinafter, a case will be described in which a protective film is formed on a sensor manufactured by this method.

In the case of such a manufacturing method, the metal oxide is supported in the porous electrode pores and three-phase points, and
A layer is also formed on the surface of the porous electrode. For example, using SnO ₂ as a metal oxide, the SnO ₂ and
When the sensor material is immersed in a mixture of ethyl silicate condensate as a binder and ethyl alcohol as a viscosity adjusting liquid at a weight ratio of 3:1:1, the SnO ₂ has a thickness of approximately 30 to 40μ. (See the cross-sectional micrograph in Figure 9 [magnification: 500x]).

After spraying MgAl ₂ O ₄ spinel onto the porous electrode of the sensor thus formed to form a protective coating, the electromotive force characteristics were measured.
In this case, linear electromotive force characteristics were not lost. This is because the SnO ₂ layer on the electrode acts as a protective layer for the spinel, and the
This is thought to be because the thermal spraying heat of spinel does not affect SnO ₂ .

However, as shown in the photograph of FIG. 10 (magnification: 500 times), which is an enlarged photograph of the cross section of the element after spinel spraying, cracks were generated between the spinel protective coating and the porous electrode. this is
This is thought to occur when the SnO ₂ layer is melted and absorbed by the heat of the spinel. Because of this cracking, the spinel protective coating has extremely weak adhesion to the porous electrode, easily peels off from the electrode, and no longer functions as a protective coating.

As an example, the above protective film is peeled off at 780°C.
In a temperature shock test where one cycle was heating and holding at 200℃ for 10 minutes and then holding at 200℃ for 5 minutes, 50 samples were tested at test cycles of 200, 500, and 1000 cycles, and if even a slight amount of peeling was observed, the samples were peeled off. Assuming that, number of peeled pieces / 50 pieces ×
As a result of calculating the peeling rate as a percentage of 100, it was found that approximately 30, 60, and 90% of peeling occurred in each cycle. On the other hand, similar tests were conducted on the wide-range air-fuel ratio sensor according to the embodiment of the present invention described above.
No peeling of the protective coating occurred during all cycles.

As explained in detail above, according to the manufacturing method of the present invention, the metal oxide abundance near the three-phase point is high, the electromotive force characteristics are uniform and sufficiently linear, there is little difference in performance between individual products, and the material is strong. The protective coating increases durability and provides a wide-range air-fuel ratio sensor that is sufficiently durable for practical use.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the air-fuel ratio and O ₂ and CO partial pressure at the three-phase point in the air-fuel ratio sensor for each electrode catalyst activity, and Figure 2 is a graph of the wide-range air-fuel ratio sensor manufactured by the method of the present invention. A schematic cross-sectional diagram showing the structure,
Fig. 3 is an enlarged view of Fig. 2, Fig. 4 is an enlarged view of Fig. 3, Fig. 5A is a graph showing the relationship between the air-fuel ratio and the electromotive force in the theoretical sensor at each atmospheric temperature, and Fig.
Figure B is an explanatory diagram showing the electromotive force characteristics of a wide range air-fuel ratio sensor manufactured by the method of the present invention in comparison with the characteristics of a λ sensor, and Figure 6 is a method of manufacturing a wide range air-fuel ratio sensor according to an embodiment of the present invention. FIG. 7 is a graph showing the electromotive force characteristics with respect to the air-fuel ratio of the sensor manufactured by the method shown in FIG. 6, and FIG. FIG. 9 is a graph showing the electromotive force characteristics of a sensor with a protective film manufactured by a method other than the method of the present invention; FIG. 10
The figure is a micrograph showing an enlarged cross-sectional structure of the surface of the sensor shown in FIG. 9 with a spinel sprayed protective coating formed thereon. 1... Solid electrolyte, 2... Atmospheric side porous electrode,
3... Porous electrode for measurement, 4... Metal oxide, 6
... Gas to be measured, 10... Protective coating, 20... Sensor material.

Claims (1)

[Claims] 1 A porous electrode with semi-catalytic performance is formed on the surface of a solid electrolyte, and a metal that oxidizes hydrocarbons to generate CO is placed near a three-phase point consisting of the electrode, solid electrolyte, and gas to be measured. This is a method for manufacturing a wide range air-fuel ratio sensor in which an oxide is present, in which a porous electrode is formed on the surface of a solid electrolyte, and then a refractory metal oxide is placed in a plasma chamber on the electrode on the side that contacts the gas to be measured. The sensor material is immersed in a solution of a metal compound that generates a metal oxide that generates CO, and the solution is passed through the pores of the protective coating to the electrode. The material is impregnated deep into the pores of the material, and then the material is taken out of the solution and heated to the thermal decomposition temperature of the metal compound, thereby producing a metal oxide that produces the CO in the vicinity of the three-phase point. A method for manufacturing a wide range air-fuel ratio sensor.