JPS59109855A - Manufacture of wide-range air fuel ratio sensor - Google Patents

Abstract

PURPOSE:To obtain a sensor superior in sensibility and responsiveness by painting the paste obtained by mixing the powder of a metallic oxide oxidizing HC to CO and the powder of a porous electrode forming material having semicatalytic ability and a binder on the gas side to be measured of a solid electrolyte and sintering said paste. CONSTITUTION:The paste-state material obtained by mixing and kneading the powder consisting of 10-40wt% SnO2, In2O3, etc. capable of oxidizing HC to CO and the balance a porous electrode material such as powder of Mg, Pt and the binder of the quantity equivalent to 0.5-2 times volume ratio per the total quantity of powder is painted on the measuring gas side 6 of a solid electrolyte cylinder 1, and paste made solely of electrode material is painted on the inner surface of the cylinder. Next the cylinder is calcined at the m.p. or less of the electrode material and the metallic oxide and at a temperature capable of sintering the electrode material to obtain the sensor. Thus the metallic oxide 4 is allowed to oxist in the vicinity of a 3-phase point of the porous electrode 3, the electrolyte 1 and a gas to be measured, and the sensor of low variation in sensibility is obtained.

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, the technical concept of indirectly detecting the air-fuel ratio (A/F) by detecting the oxygen concentration in exhaust gas in engines installed in automobiles, for example, has become public knowledge. A so-called λ sensor, in which the electromotive force changes stepwise with the oxygen concentration corresponding to the stoichiometric air-fuel ratio as the boundary, is said to be used as a detection element for detecting the oxygen concentration in exhaust gas. According to the method, it can be determined whether the air-fuel ratio is larger or smaller than the stoichiometric air-fuel ratio.

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

For example, JP-A-57-76450 and JP-A-51-72
As shown in Japanese Patent Application No. 491 and No. 53-3407.7, an oxygen concentration detecting element that is capable of detecting oxygen concentrations other than stoichiometric air combustion has been proposed. However, in these conventional λ sensors, which are oxygen concentration detection elements formed by forming porous electrodes on both sides of a solid electrolyte, a protective layer is provided on the outside of the porous electrode on the side of the gas to be measured. The latter is a so-called amberometric sensor that controls the rate of gas diffusion to the electrode, while the latter poisons the porous electrode to dull its sensitivity, gradually decreasing the overall electromotive force characteristic and making it linear. Although it is designed to obtain electromotive force characteristics, linear electromotive force characteristics cannot be obtained in the lean region as with 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 the detection elements in question have insufficient detection sensitivity and are 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. We have now provided a wide-range air-fuel ratio sensor in which a metal oxide that oxidizes HC to generate CO is present near a three-phase point consisting of the electrode, solid electrolyte, and gas to be measured. Ta.

Here, the above-mentioned "semi-catalytic performance" will be explained. Figure 1 is rPhys by William J. Fleming.
icalPrinciplesGoverning
Non1deal Behavior of the Z
irconia Oxygen 5ensor J
(J.

Electrochemjcal 5ociety,
Vol, 124. No, 1゜January 1
977, pp, 2i 28 ) of the three-phase point due to electrocatalytic activity. It is a graph showing Co partial pressure change. The above-mentioned "semi-catalytic performance" refers to the following in Fig. 1:
It refers to the performance showing an activity of about "2" or less, which is said to have an activity of r Poor J (the effect will be described later). Such semi-catalytic performance is, for example, Ag
.

Au, etc., but Pt, which is generally considered to be highly active and is used in conventional λ sensors that exhibit step-like characteristics,
Such properties can be imparted depending on the material and firing conditions.

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 the conventional λ sensor.
Porous electrodes 2 and 3 are formed on both surfaces of the atmosphere 5 side and the gas to be measured 6 side, respectively. A layer of metal oxide 4 is further formed on the surface on the side of gas to be measured 6. Fig. 3 is an enlarged view of Fig. 2, and Fig. 4 is an enlarged view of Fig. 3. As shown in FIG. 4, the metal oxide 4 is layered so as to exist near the three-phase points mentioned above. The mechanism by which linear electromotive force characteristics are obtained will be explained in detail. As is generally known, the theoretical electromotive force V of the sensor is given by the Nernst formula. Here, R is the gas constant, T is the feed temperature, F
is Faraday's constant + Po2 (air) is the partial pressure of oxygen in the atmosphere + Po2 (exh) is the partial pressure of oxygen in the gas to be measured. By plotting the values derived from this equation, an electromotive force characteristic curve as shown in FIG. 5A can be obtained. However, many actual sensors exhibit electromotive force characteristics that cannot be explained by the Nernst equation, and it is difficult to explain them using the above-mentioned Wi L i am J, Fl emin g.
proposed Fleming's equivalent circuit, Mother Chill.

The behavior of the oxygen concentration detection element of the present invention is also explained by this Fleming equivalent circuit model having many parameters.

Here, a brief explanation of Fleming's equivalent circuit model will be given. The equivalent circuit model is based on the fact that a unique electromotive force is generated at each attraction point at the three-phase point, and according to this, the electromotive force V is v-foo-voo+(1-foo)Vo2. expressed.

Here f. 0 is the rate at which CO is adsorbed at the three-phase point, foo=KcoPco/(1+Kco'Pco+Ko2
'Po2) (I(oo + KO2 are respectively Co and 0
2 adsorption constant) voo is the electromotive force generated at the three-phase point OCO and is 1/2.

vco-Vco-1-(RT/2F)An[P02(a
zr)P(20(anode)/Pco2(anode)
):) +■o2 is the electromotive force generated where the three-phase point 02 is attracted, and ■o2-voo2+(RT/4F)tnCPo2(a1
rVPo2(anOde)]. Note that V'co, V
;2 is the standard potential in each electrochemical cell, Pco (anode).

pc02 (anode) + Po2' (anode) are Co and CO at the three-phase point of the electrode on the gas side to be measured, respectively.
2.02 partial pressure. The above formula is determined by the following two reactions at three-phase points.

02 + 4e g 2()2- Co +Od CO2+ 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. tI) 02. at the 3-phase point. The electromotive force characteristics vary greatly depending on the difference in Co partial pressure. As shown in FIG. 1, in the lean region, the 02 partial pressure is almost constant regardless of the catalyst activity, and it is the 00 partial pressure that changes significantly. In other words, the electromotive force in the lean region is dominated by CO. According to Ferning's equation, if the 00 partial pressure is increased in the lean region, the electromotive force will increase.

Based on the above, the mechanism by which linear electromotive force characteristics can be obtained will be explained. 1. The metal oxide is coated with 611]
It acts as an oxidation catalyst that oxidizes HC in constant gas (exhaust gas) and generates CO (which is itself reduced). For example, if this metal oxide is 51102, aS no 2 +bHC(g)→c SnO+ dc
o(g)+ eco 2 (g)+ fH20(g)+
The following reaction occurs, and the further reduced SnO returns to 5nOz due to 02 in the gas to be measured. Due to the so-called Redox effect, 5nOz constantly generates CO and 0
2. Perform absorption.

Due to the above action, the 902 partial pressure decreases, and dO generated by HC increases the 00 partial pressure near the three-phase point.
The electromotive force in the lean region increases as shown in FIG. 5B, and linear electromotive force characteristics are obtained in the lean region.

Since the porous electrode used has semi-catalytic performance, the electromotive force in the rich region is 5B.
As shown in the figure, the electromotive force decreases, and a linear electromotive force characteristic can be obtained from the above-mentioned 1-in region to this rich region.

By the way, the HC concentration in the lean region is only about 1,000 to several hundred ppm at most, and therefore the amount of COO 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 partial pressure change will be about 60% from "4" to "2" as shown in Figure 1 above. . Therefore, in order to fully demonstrate the effects of metal oxides,
It is necessary that the metal oxide be present near the three-phase point.

Since the above-mentioned metal oxide must act to oxidize HC to produce CO, a metal oxide having a low oxidizing ability to oxidize IHC is not suitable. The HC oxidation ability (CO generation ability) of various metal oxides can be found, for example, in “Metal oxides and their catalytic action” (1) by Tetsube Kiyoyama.
This can be determined using Table 4.101 Propylene oxidation reaction J on various metal oxides (P2S5) of 1979, Kodansha), but for example, porous electrodes containing PT as a main component (PT paste, etc.) can be used as a guide. ), when using
5nOz, I]1203 + NiO, CO3O4
and CuO exhibits sufficient HC oxidation ability. Therefore, in this case, one or a similar metal oxide may be used.゛It should be noted that HC is oxidized to C
The tendency for O to be produced is determined by the overall balance between the catalytic activity of the porous electrode and the HC oxidation ability of the metal oxide. In the case of forming the material from a material containing Ag, Au, etc. as a main component, a material having lower HC oxidation ability than the metal oxides mentioned above can also be used. For example, when a porous electrode is formed from Ag paste, Zllo,
By using MnO2, linear electromotive force characteristics can be obtained in the lean region.

In order to make the above air-fuel ratio sensor suitable for practical use, it is necessary to oxidize metal oxides (HC and CO
It is necessary to ensure that the existence rate of (that which generates) does not vary depending on each sensor. 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 each sensor, the electromotive force of each element will increase. The power characteristics vary, making the job of moving the plane extremely complicated.

The present invention has been made in view of the above points, and the above-mentioned new wide-range air-fuel ratio sensor can be formed such that the metal oxide is present near the three-phase point at a uniform abundance rate among each sensor. The purpose is to provide a manufacturing method.

The method for manufacturing a wide range air-fuel ratio sensor of the present invention includes a metal oxide powder of 10 to 40% by weight that oxidizes HC to produce CO;
After applying a paste-like material made by mixing a powder whose remainder is electrode material powder and a binder in a volume ratio of 0.5 to 2 times the total amount of this powder on the surface of the solid electrolyte. , the paste-like material is fired at a temperature below the melting points of the electrode material and metal oxide and at which the electrode material can be sintered (this method is clearly shown in FIG. 6). .

In the paste-like material, the electrode material powder and the metal oxide powder are each uniformly dispersed, and these two powders are mixed at a constant ratio in any part of the paste-like material.

Therefore, if this paste-like material is applied to a large number of solid electrolytes and fired to form a large number of wide-range air-fuel ratio sensors,
In all sensors, the mixed structure of the electrode material and the metal oxide is the same, and the abundance of the metal oxide at the three-phase points is also the same.

The reason for limiting each of the above numerical conditions will be explained below.

In order to make the electromotive force characteristics sufficiently linear, it is necessary to sufficiently increase the abundance of metal oxides at the three-phase points. In order to increase the abundance of this metal oxide, it is sufficient to increase the mixing ratio of the metal oxide powder in the powder forming the paste-like material. However, on the other hand, if the mixing ratio of this metal oxide powder is increased and the mixing ratio of electrode material powder is lowered,
This time, the conductivity of the fired structure (which becomes the porous electrode) decreases and it no longer functions as an electrode. That is, the lower limit of the mixing ratio of the metal oxide powder in the powder is regulated from the viewpoint of linearization of the electromotive force characteristics of the sensor, and the upper limit is regulated from the viewpoint of the electrical conductivity of the porous electrode. As a result of fabricating various wide-range air-fuel ratio sensors with different mixing ratios of metal oxide powder and testing their performance, we found that the mixing ratio of metal oxide powder must be adjusted by a factor of 10 to obtain linear electromotive force characteristics. (
It has been found that in order to sufficiently maintain the conductivity of the porous electrode, the weight ratio needs to be 40% or less (particularly preferably 15 to 30%).

Next, the ratio of powder to binder will be explained. The table below shows the specific gravity of the materials preferably used as electrode materials and the metal oxides that oxidize HC to produce CO in the present invention.
In this way, the electrode material has a much higher specific gravity than metal oxides. Therefore, when the powder is mixed with a large amount of binder, - after applying the coust-like material to the solid electrolyte, the electrode material powder with a high specific gravity will settle down (toward the solid electrolyte surface side) on the upper surface of the solid electrolyte, and the specific gravity will decrease. The homogeneity of the paste-like material is hindered by the difference in specific gravity, such as small metal oxide powder floating to the surface and making it difficult to support the metal oxide at the three-phase point.

In addition, the binder is burnt off during the firing of the paste wood, and the burnt out portion becomes pores, but if a large number of pores occur near the three-phase point, the density of the metal oxide at the three-phase point decreases, causing a linear occurrence. Power characteristics cannot be obtained.

The above two problems become noticeable to the extent that they impede the practical use of the sensor when the binder is used in a volume ratio of more than twice the total amount of powder. Therefore, from the above point of view, it is preferable that the amount of binder be as small as possible, but if the amount of binder is reduced too much, the applicability of the paste material will deteriorate, so in practical terms, the volume ratio to the total amount of powder should be 0. .5 times more binder must be added.

Next, the firing temperature of the paste material will be explained. In order to form a porous electrode by firing a paste-like material, it is naturally necessary to fire the paste-like material at a high temperature that allows sintering of the electrode material.

Forming the electrodes of air-fuel ratio sensors by firing a paste-like material has been done for a long time, as shown in Japanese Patent Application Laid-Open No. 52-46889. For this purpose, firing was performed at a high temperature that would melt the electrode material. However, when calcining is performed at such a high temperature, the catalytic activity of the electrode and the metal oxide, especially the HC oxidation ability of the metal oxide, is significantly reduced. This is because the specific surface area of the ultrafine metal oxide material decreases due to melting or shishitaring, and active sites are lost. As already mentioned, the wide range air-fuel ratio sensor manufactured by the method of the present invention obtains linearization of the electromotive force characteristics by the HC oxidation ability of the metal oxide, and the HC oxidation ability is determined by the HC oxidation ability of the electrode. Since it is determined by the overall balance between the catalytic activity and the catalytic activity of the metal oxide, the catalytic activity of these electrodes and metal oxides must never be changed. Therefore, in the method of the present invention, unlike conventional methods, the firing temperature must not be set higher than the melting point of the electrode material and the metal oxide.

Specifically, when PT (melting point: 1773°C) is used as the electrode material, the temperature is 900 to 1100°C.

550 to 80 when using Ag (melting point 960°C)
It is preferable to set the firing temperature to about 0°C.

Note that preparing the metal oxide powder so that the particle size is smaller than that of the electrode material powder is advantageous in supporting the metal oxide at the three-phase point.

(If we assume in FIG. 3 that 4 is an electrode and 3 is a metal oxide, it will be clear that metal oxides with large particle diameters are difficult to be supported at the three-phase points of each electrode.) Further, by setting the particle size of the metal oxide powder to be small, the specific surface area of the metal oxide increases and the HC oxidation ability is improved.

Examples of the present invention will be described below.

[First Example] Ag paste made by kneading Ag powder as an electrode material and a binder and SnO2 powder, which is a metal oxide having HC oxidation ability, were mixed at a weight ratio of 7:3,
Thoroughly knead and apply the paste material to a thickness of about 20 μm on the outer surface (measurement side surface) of the solid electrolyte tube. The above Ag paste is applied as is to the inner surface (atmospheric side surface) of the solid electrolyte tube. Next, this solid electrolyte tube was fired at a temperature of 650°C for 1 hour, and an Ag electrode carrying SnO2 at the three-phase point was sintered on the outer surface, and an Ag electrode was sintered on the inner surface. Wide range air-fuel ratio sensor No. according to the manufacturing method of the example
, got 1.

This No. 1 sensor was attached to the exhaust system of a reciprocating engine and tested. When we measured the electromotive force by changing the air-fuel ratio A/F from 11 to 18 while maintaining the exhaust gas temperature near the sensor at 550℃, we obtained linear electromotive force characteristics as shown in Figure 7. .

[Second Example] A PT paste made by kneading PT powder as an electrode material and a binder, and In20a powder, which is a metal oxide having HC oxidation ability, were mixed at a weight ratio of 4:1, and the mixture was sufficiently mixed. The paste material is applied to the outer surface (measurement side surface) of the solid electrolyte tube to a thickness of about 15 μm. The above PT paste is applied as is to the inner surface (atmosphere side surface) of the solid electrolyte tube. Next, this solid electrolyte tube was fired at a temperature of 800°C for 1 hour, and a PT electrode carrying n203 at the three-phase point was sintered on the outer surface, and a PT electrode was sintered on the inner surface.
Wide-range air-fuel ratio sensor No. 2 was obtained by the manufacturing method of the second embodiment of the present invention.

When the electromotive force characteristics of this No. 2 sensor were measured in the same test as in the first embodiment, the results shown in FIG. 8 were obtained.

[Comparison with other manufacturing methods]

In order to compare the inter-individual performance differences of wide range air-fuel ratio sensors made by the manufacturing method of the present invention with those of sensors made by other manufacturing methods, 50 of the above-mentioned No. 1 sensors were formed; According to the method, an electrode was formed by coating a solid electrolyte with Ag paste in advance and then firing it, and this was coated with 5nO
Fifty sensors with a ratio of Ag:SnO2 of approximately 7:3 were formed by immersing them in a bath solution consisting of Z particles, a binder, and a viscosity adjusting liquid and then firing them, and the electromotive force of each sensor was measured by the test described above. .

The variation in electromotive force characteristics among individuals in each sensor group is
The results shown in FIG. 9 of the electromotive force at each air-fuel ratio A/F were obtained. As clearly shown in FIG. 9, in the wide range air-fuel ratio sensor manufactured by the manufacturing method of the present invention, the difference in electromotive force performance between individual sensors is suppressed to a low level. That is, it has been found that according to the manufacturing method of the present invention, sensors with constant performance can be manufactured in large quantities.

According to the present invention as described in detail above, a wide range air-fuel ratio sensor can be obtained which has sufficiently linear electromotive force characteristics and has little difference in performance between individual sensors and is suitable for practical use.

[Brief explanation of drawings]

Figure 1 shows the air-fuel ratio at the air-fuel ratio sensor and the three-phase point 02.
．． A graph showing the relationship with Co partial pressure for each electrode catalyst activity. Figure 2 is a schematic cross-sectional view showing the structure of a wide range air-fuel ratio sensor manufactured by the method of the present invention. Figure 3 is an enlarged view of Figure 2. Figure 4 is an enlarged view of Figure 3, Figure 5A is a graph showing the relationship between the air-fuel ratio and the electromotive force in the theoretical sensor for each temperature of the gas to be measured, and Figure 5B is a graph showing the relationship between the air-fuel ratio and the electromotive force in the theoretical sensor. FIG. 6 is an explanatory diagram schematically showing the manufacturing method of the wide-range air-fuel ratio sensor of the present invention, and FIG. FIG. 8 is a graph showing the electromotive force characteristics with respect to the air-fuel ratio of the wide-range air-fuel ratio sensor manufactured according to the first embodiment of the present invention. FIG. 9 is a graph showing the electromotive force characteristics for the wide range air-fuel ratio sensor manufactured by the manufacturing method of the present invention,
It is a graph showing a comparison between individual performance differences with wide-range air-fuel ratio sensors manufactured by other manufacturing methods. 1... Solid electrolyte 2... Atmospheric side porous electrode 3...
・Measurement side porous electrode 4...Metal oxide 6...
Gas to be measured Diagram 1
) Fig. 2, Fig. 3 Fig. 4 Fig. 5B Fig. 1.0□ Fig. 7 A/F Fig. 8 A/F Fig. 9 A/F FIB Japanese February 1958 248 Commissioner of the Patent Office Mr. 2, Name of the invention Wide area air fuel 1 and sensor manufacturing method 3. 11iRelationship with the person making the amendment (!l- Patent applicant 4, agent 5-2-1 Roppongi, Minato-ku, Tokyo None 6. Number of inventions increased by amendment None 7. Amendment Subject Akira nJ4 [Detailed description of the invention: (1ηB, Contents of amendment 1) Meigoj4M page 3rd to 4th lines of procedural emphasis (method) % formula % 1, case indication patent application Sho No. 57-220510 2, Name of the invention: Method for manufacturing a wide range air-fuel ratio sensor 3, Person making the amendment Relationship to the case Patent applicant 4, Agent, 5-2 Roppongi, Minato-ku, Tokyo, January 5, Date of amendment order March 9, 1981 (Shipping date: March 29, 1988)
(1) Take the 1b shell of the Meiken circle and remove it from the 1st wa. The mixing ratio of the oxide powder needs to be at least a factor of 10 (M amount ratio) to obtain linear electromotive force characteristics, and less than a factor of 40 (M amount ratio) to maintain sufficient conductivity of the porous electrode. (particularly preferably 15 to 30%). Next, the ratio of powder to binder will be explained. The table below shows the specific gravity of materials preferably used as electrode materials in the present invention and materials preferably used as metal oxides for converting HC into In to produce CO.

Claims (1)

[Claims] A porous electrode having semi-catalytic performance is formed on the surface of the solid electrolyte, and a metal oxide that oxidizes HC to generate CO is located near the three-phase point consisting of the electrode, the solid electrolyte, and the gas to be measured. A method for producing a wide range air-fuel ratio sensor comprising: a powder consisting of 10 to 40 parts by weight of the metal oxide powder, the remainder being electrode material powder, and a volume ratio relative to the total amount of the powder; After applying a paste material mixed with 0.5 to 2 times the amount of binder to the surface of the solid electrolyte, the paste material is heated to a temperature below the melting point of the electrode material and metal oxide and of the electrode material. A method for manufacturing a wide range air-fuel ratio sensor characterized by firing at a temperature that allows sintering.