JPS59142457A - Production of wide range air-fuel ratio sensor - Google Patents

Abstract

PURPOSE:To allow a metallic oxide to exist uniformly at a high probability near the three-phase point and to improve bond strength by forming a porous electrode on a solid electrolyte and allowing the metallic oxide which is formed by a specific method and has prescribed properties to exist near the three-phase point. CONSTITUTION:Platinum paste is brushed on the inside and outside furface of a tubular solid electrolyte 1 consisting of ZrO2-6mol% Y2O3 and is calcined for 1hr at 1,050 deg.C to form the layers of porous electrodes 2, 3 on the atm. side and the measuring side, thereby forming a blank material 10 for the sensor. On the other hand, a liquid mixture 20 composed of 35wt% aq. SnCl4 soln. and 40% aluminum primary phosphate soln. at 2:1 by weight is prepd. preliminarily and the material 10 is immersed thereon to impregnate the liquid 20 in the voids of the electrode 3. The material 10 is then taken from the liquid 20 and after drying for 30min at 200 deg.C, the material is calcine for 1hr at 500 deg.C. If such operation is repeated three times, the metallic oxide (SnO2)4 having HC oxidizing power is deposited uniformly and substantially at high bond strength near the three-phase point.

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, is well-known. Detection element that detects oxygen concentration in exhaust gas,
For example, Japanese Patent Publication No. 52-46889, Japanese Patent Publication No. 52-46889,
As shown in Publication No. 7-10382, a so-called λ sensor is known in which the electromotive force changes stepwise with the oxygen concentration corresponding to the stoichiometric air-fuel ratio as the boundary. It can be determined 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 degree of richness, that is, the fuel is richer than the stoichiometric nitrous-fuel ratio, and the air-fuel ratio is set to be richer than the stoichiometric nitrate-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 and JP-A-53-34
As shown in Japanese Patent No. 077, 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 ones are limited to the aforementioned λ sensor,
In other words, in an oxygen concentration detection element 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 gas-to-be-measured side to control the rate of gas diffusion to the electrode. The latter is a link sensor whose sensitivity is dulled by poisoning the porous electrode, and the overall electromotive force characteristic gradually decreases to obtain a linear electromotive force characteristic.In the lean region, the above-mentioned Like the λ sensor, linear electromotive force characteristics cannot be obtained.

Moreover, the width of this linear electromotive force characteristic is, for example, 1.00~
The voltage is as small as about 200 mV, and therefore these detection elements 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 from William J. Fleming, 1-P.
physical principles
ng Non1deal Behaviorofthe
Zirconia Oxygen 5ensor
J (J, Electro-chemical 5o
society, Vol, 124, /161
, January 1977, pp, 21-28
), the three-phase point 02 due to electrocatalytic activity,
It is a graph showing C0 partial pressure change. The above “semi-catalytic performance”
In FIG. 1, it refers to the performance showing an activity of about "2", which is defined as rPoorJ, or less (the effect will be described later).

Such semi-catalytic performance is exhibited by, for example, Ag, Au, etc.
Even Pt, which is generally considered to be highly active and is used in conventional λ sensors exhibiting step-like characteristics, can be given such characteristics 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 a conventional λ sensor.
Porous electrodes 2 are placed on both surfaces of the atmosphere 5 side and the gas to be measured 6 side.
, 3 are formed, 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 be present in the vicinity of the three-phase points mentioned above.

Below, the air-fuel ratio is determined by the above configuration.

The mechanism by which linear electromotive force characteristics are obtained with respect to (A/F) will be explained in detail. As is generally known, the theoretical electromotive force (■) of a sensor is given by the Nernst formula (%): 1. Here R is the gas constant, T is the absolute temperature, F
is the Faraday constant, Po2 (air) is the partial pressure of oxygen in the atmosphere, and Po2 (exh) is the partial pressure of oxygen in the gas to be measured. If the values derived from this equation are plotted, 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 in order to explain these, Willian J. and Fleming mentioned above proposed Fleming's isochoric circuit model. The behavior of the wide range air/fuel ratio sensor of the present invention is explained by this Pleming equinodal circuit model with many parameters.

Here, a brief explanation of FleminH's isochoric 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 in the three-phase points, and according to this, the electromotive force V is V=fco ・Vco + (1-fco) VO2 It is expressed as Here, fco is the rate at which CO is adsorbed at the three-phase point, fCO=I<CO°Pco/(1+K.O”PCO→4
Ku. 2”PO2).

(Kco y KO2 is the scattering constant of valley C0,02) V
co is the electromotive force generated at the three-phase point where CO is adsorbed, and is Vco = Vco + (RT/2F)nn (Po21/
2(air)-Pcm(anode) / Pco2(
anode) ] Vo2 is the electromotive force generated where the three-phase point 02 is attracted, and VO2 = V0o2 + (RT/4F) An (Po2 (a
ir)/Po2(anode):]. Naori co and V'o2 are standard cell potentials in each electrochemical cell. PCO (anod
e), Pco2(ariode), Po2(a
node) are the CO2CO2 and 02 partial pressures at the three-phase points of the electrode on the gas to be measured side, respectively. The above formula is determined by the following two reactions at three-phase points.

02 + 4e-d 202- CO+ 0” d CO2+ 2e- The discrepancy 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, that is, the three-phase The electromotive force characteristics change sharply due to the difference between the 02 and 00 partial pressures at the point.As shown in Fig. 1 above,
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. That is, the electromotive force in the lean region is mainly dominated by CO. Flem
If the 00 partial pressure is increased in the 1-in region according to the formula of ing, the electromotive force will increase.

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) and is itself reduced to generate CO. For example, if this metal oxide is SnO2, asno2+ bl-IC(g)-+ csnO+ d
cqg)+ecO2(g) +fH2qg)+...
The following reaction occurs, and the further reduced SnO returns to SnO2 due to 02 in the gas to be measured. In other words, the so-called Re
Due to the action of dox, 5nOz constantly produces CO,
02 absorption.

The 02 partial pressure decreases due to the action of HC 2 and above, and the CO generated from l-1c increases the 00 partial pressure near the 3-phase point, so the electromotive force in the lean region increases as shown in Figure 5B. As a result, linear electromotive force characteristics can be obtained in the linear 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 is obtained from the above-mentioned one-in region to this rich region.

However, the HC concentration in the lean region is only about 1,000 to several hundred ppm. 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, this C
Even if the O concentration is, for example, 0.001%, it will not be oxidized (if it reaches the three-phase point, the 00 partial pressure change will be about ``2'' from ``4'' shown in Figure 1 above. 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 I-IC to generate CO, a metal oxide having a low ability to oxidize, filter, or oxidize HC is unsuitable. HC oxidation ability (CO generation ability) of various metal oxides
For example, by Tetsube Kiyoyama [Metal oxides and their catalytic action J]
(1979, Kodansha), Table 4.10 "Propylene oxidation reaction on various metal oxides" (p185) can be used as a guideline. paste, etc.), SnO2, Inz03. NiO, CO3O4 and CuO exhibit sufficient HC oxidation ability. Therefore, in this case, one or more of these metal oxides may be used. Note that the tendency for HC to be oxidized to produce CO is determined by the overall balance between the catalytic activity of the porous electrode and the IC oxidation ability of the metal oxide, so the porous electrode is When forming from a material with low catalytic activity, for example, 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, a porous electrode is
When formed from g paste, linear electromotive force characteristics can be obtained in lean head loading even if ZnO or MnO2 is used.

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 abundance ratio of the elements (that generates the 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. Calibration work becomes 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, the metal oxide that is fixed to the solid electrolyte or electrode and supported near the three-phase point must have sufficient adhesion strength. is practically essential. That is, as mentioned above, the electromotive force characteristics are linearized by supporting this metal oxide near the three-phase point, so if the metal oxide easily peels off from the vicinity of the three-phase point, Naturally, the linear electromotive force characteristics are lost.

Traditionally, in lambda sensors, etc., in order to increase the durability of the porous electrode on the measurement side, metal oxides (simply for the purpose of improving durability) have been coated on the porous electrode on the measurement side. In some cases, the metal oxide is sintered at a high temperature of about 2000'C to reduce its density.
It has been common practice to increase the strength of the text. However, as mentioned above, when the high-temperature sintering process described above is applied to a wide range air-fuel ratio sensor whose electromotive force characteristics are made linear by the presence of a metal oxide with HC oxidation ability near the three-phase point, the metal oxide The l-IC oxidation ability of the oxide is lost, and the linear electromotive force characteristics are impaired.

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 sufficiently improve the adhesion strength of metal oxides without losing their electromotive force properties.

The method for manufacturing a wide range air-fuel ratio sensor of the present invention includes a sensor material formed by forming the porous electrode on a solid electrolyte as described above, and a heat-resistant binder in a solution of a metal compound that generates the metal oxide. After immersing the material in the mixed liquid to sufficiently impregnate the deep part of the pores of the electrode with the mixed liquid, the material is taken out from the mixed liquid and heated to the thermal decomposition temperature of the metal compound. The method is characterized in that the metal oxide is produced.

According to the above method, the dissolved metal compound reaches deep into the electrode pores, so the metal oxide abundance rate near the three-phase point depends on the electrode pore size, electrode film thickness, sensor surface shape, etc. It is uniform among each element with a high abundance rate.

In order to increase the metal oxide abundance at the three-phase point, the above operation may be repeated several times. As the metal compound, compounds, sulfates, nitrates, etc. of metals (i.e., Sn, In, Ni, Co, Cu, Mn5Zn, etc.) that produce metal oxides as described above may be used, and as a solution thereof, Although an aqueous solution, an alkaline solution, an acid solution, an organic solvent solution, etc. can be used, an aqueous solution that does not contain many extra elements other than the above-mentioned metals is particularly suitable.

Since a heat-resistant binder is mixed into the metal compound solution in which the sensor material is immersed, the metal oxide obtained by thermally decomposing the metal compound is firmly fixed to the electrode or solid electrolyte by this heat-resistant binder. Ru. Of course, treatment with such a binder does not expose the metal oxide to high heat and does not impair the HC oxidation ability of the metal oxide. Furthermore, since a heat-resistant binder is used as the binder, the binder will not be thermally degraded even if the sensor is exposed to high-temperature exhaust gas, and the adhesion strength of the metal oxide will not deteriorate.

Next, embodiments of the present invention will be described with reference to the drawings.

This will be explained below with reference to FIG. As a tubular solid electrolyte 1 as shown in Fig. 2, Zr02-6 manufactured by Nippon Insulator
Using a solid electrolyte tube of mole Y2O3, Pt paste was applied to the inner and outer surfaces of the solid electrolyte tube by brushing.
After drying at 20° C., it was fired at 1050° C. for 1 hour in an electric furnace to form a sensor material 10 in which a porous electrode 2 on the atmosphere side with a film thickness of about 15 μm and a porous power buying vi3 on the measurement side were layered (No. 6
Diagram a).

Separately, a mixed solution 20 is prepared by mixing a 35% by weight 5nC14 aqueous solution and a 40% by weight primary aluminum phosphate solution at a weight ratio of 2:1.
The sensor material 10 is immersed in the liquid mixture 20 to impregnate the electrode gaps (pores) of the measurement side (external) porous electrode 3 (FIG. 6b). After that, add 10 pieces of sensor material to 20 pieces of mixed liquid.
The metal oxide 4, which is a metal oxide 4 having HC oxidation ability, was removed by taking it out, drying it at 200°C for 30 minutes, and then calcining it at 300°C for 1 hour in an oxygen atmosphere (Fig. 6C).
nOz (made by thermally decomposing the above 5nCJ 4),
Primary aluminum phosphate 30 as heat-resistant binder 30
Therefore, it is strongly reinforced and supported near the three-phase point. By repeating the process of immersing the sensor material 10 in the mixed solution 20 three more times, 5n02 is sufficiently supported near the three-phase point, and moreover, the SnO2 solid electrolyte 1 or the measurement-side porous electrode 3 A wide range air-fuel ratio sensor was obtained by the manufacturing method of one embodiment of the present invention, in which the adhesion strength to the air-fuel ratio sensor was significantly increased by monobasic aluminum phosphate.

The wide-range air-fuel ratio sensor of the above example was attached to the exhaust system of a reciprocating engine, and a pliers test was conducted. The air-fuel ratio (
When the electromotive force was measured while changing A/F) from 11 to 18, linear electromotive force characteristics as shown in FIG. 7 were obtained.

The mixing ratio of the heat-resistant binder will be described below. As will be described in detail below, when a heat-resistant binder is added in an amount of 20% by weight or more based on the total amount of the metal compound and binder, the adhesion strength of the metal oxide produced from the metal compound is improved. The adhesion strength of the metal oxide improves as the mixing ratio of the heat-resistant binder increases, but if too much heat-resistant binder is added, the abundance rate of the metal oxide near the three-phase point decreases, and the linearity of the sensor decreases. The mixing ratio at which linear electromotive force characteristics are lost varies somewhat depending on the type of metal compound and heat-resistant binder. When monobasic aluminum phosphate is used as a binder, as shown in Figure 8, the
When 0% by weight or more of primary aluminum phosphate is mixed,
Linear electromotive force characteristics are lost. For other combinations of metal compounds and heat-resistant binders, the mixing ratio at which the linear electromotive force characteristic of the sensor is lost varies slightly, but is approximately the same. Therefore, the mixing ratio of the heat-resistant binder to the total amount (metal compound dry binder) is preferably set to about 20 to 60% by weight, preferably about 30 to 50% by weight.

As shown in FIG. 8, changing the mixing ratio changes the abundance of metal oxides in the vicinity of the three-phase point, so changing the mixing ratio changes the air-fuel ratio of the sensor.

The electromotive force gradient can be changed freely.

Next, we will discuss the effect of improving the adhesion strength of metal oxides by adding a heat-resistant binder. The wide-range air-fuel ratio sensor according to the first embodiment and the wide-range air-fuel ratio sensor formed by the same manufacturing method as that of the first embodiment except for adding a heat-resistant binder were prepared, and instructions were given for each. The table below shows the results of a touch test, an ultrasonic vibration test, and an actual bench exhaust gas test. For the finger touch test (2), the state after finger touch is shown, and for the ultrasonic vibration test (2), the sensor is immersed in a cleaning solution consisting of 50°C Etol-(45K Ilz). The amount of SnO2 peeled off after applying ultrasonic waves for 10 minutes is shown as a percentage of the initial amount supported.
2. in the actual exhaust gas of sec. ．． 10. The percentage of the amount of 51102 peeled off after exposing the sensor for 30 hours to the amount originally supported is shown.

In the above example, the resistance is for the entire amount, but as a result of conducting the adhesion strength test as described above with various mixing ratios, it was found that the mixing ratio of monobasic aluminum phosphate to the total amount was about 20% by weight. It has been found that a practically significant adhesion strength improvement effect of 5 nOz can be obtained.

Furthermore, changes in the electromotive force characteristics of each sensor after 30 hours of durability in the bench actual exhaust gas test are shown in FIG. 9 (without binder) and FIG. 10 (with binder).

As shown in FIG. 9, the electromotive force of the sensor without binder was significantly reduced in the single region, confirming the peeling of the metal oxide.

On the other hand, the electromotive force of the binder-added sensor according to the above example shows almost no change, which proves that peeling of metal oxides is extremely small.

As explained in detail above, according to the present invention, the metal oxide abundance near the three-phase point is high, the solid state opening is small,
Furthermore, a wide-range air-fuel ratio sensor having high adhesion strength of the metal oxide and sufficiently durable for practical use can be obtained.

[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, Fig. 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, 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 air-fuel ratio and electromotive force in a theoretical sensor for each ambient temperature, and Fig. 5B is a wide-range air-fuel ratio sensor manufactured by the method of the present invention. FIG. 6 is an explanatory diagram schematically showing a manufacturing method of a wide range air-fuel ratio sensor according to an embodiment of the present invention, and FIG. Graph showing the electromotive force characteristics with respect to air-fuel ratio of the sensor manufactured by the method shown in FIG. , Fig. 9 is a graph comparing the electromotive force characteristics of a wide range air-fuel ratio sensor without a heat-resistant binder before and after an actual exhaust gas durability test. It is a graph showing a comparison of electromotive force characteristics of a fuel ratio sensor before and after an actual exhaust gas durability test. 1... Solid heavy solute 2... Atmospheric side porous electrode 3.
・・Measurement side porous real electrode 4・Metal oxide 6・Measurement gas 10・Sensor material 20・Mixture
Liquid 30...Heat-resistant binder Figure 1 Sora Shirenhi (Difference with Rikyo Lantern Seven 147 J Figure 2 Figure 3 Figure 5A Figure A/F Shoes 14.7 *Sword-I-Giant Sword I Cod lI Beam 7 14.7 and 4) Figure 4 Figure 58 (G)
(b) (C)
(d) Figure 7 A/F Figure 8 A/F Figure 9 A/F Figure 10 A/F

Claims (1)

[Claims] A porous electrode having semi-catalytic performance is formed on the surface of the solid electrolyte, and a metal oxidation film that oxidizes IC to generate CO is placed near the three-phase point consisting of the electrode, solid electrolyte, and gas to be measured. A method for manufacturing a wide range air-fuel ratio sensor in which a sensor material is formed by forming a porous electrode on a solid electrolyte in a heat-resistant solution of a metal compound that generates a metal oxide. After immersing the material in a mixed solution containing a binder to sufficiently impregnate the deep part of the pores of the electrode with the mixed solution, the material is taken out from the mixed solution and heated to the thermal decomposition temperature of the metal compound. 1. A method for manufacturing a wide range air-fuel ratio sensor, comprising producing the metal oxide reinforced and supported with a binder in the vicinity of a point.