JPS6034063B2 - Oxygen sensor for stoichiometric air-fuel ratio detection - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oxygen sensor for detecting a stoichiometric air-fuel ratio using an oxygen ion conductive solid electrolyte.

It goes without saying that in automobile engines and various combustion equipment, it is desirable to control the air-fuel ratio (ratio of air to fuel) so as to obtain optimal combustion conditions.

In particular, in automobile engines, the stoichiometric air-fuel ratio (mixture weight ratio of air and fuel (gasoline) is approximately 14.7
) It is desirable to activate the function of the three-way catalyst for purifying exhaust gas components and reduce harmful gas components such as C○, N○×, and HC, and at the same time reduce fuel consumption as much as possible by burning it near the . Therefore, a system has been implemented that performs closed-loop control while detecting the oxygen concentration in the exhaust gas so that the exhaust gas composition is always at the stoichiometric air-fuel ratio. Conventional oxygen sensors for detecting the oxygen concentration in exhaust gas from automobile engines, etc. have a structure in which electrode layers are formed on the inner and outer surfaces of a one-end closed U-shaped tube made of an oxygen ion-conducting solid electrolyte. occupies the mainstream.

However, such a horizontally built oxygen sensor has a problem in that although it has a high stoichiometric air-fuel ratio detection ability immediately after manufacture, it tends to generate an output that looks like the stoichiometric electromotive force has shifted over time. On the other hand, the present inventors provided a measurement side electron conductive layer 2 and a reference side electron conductive layer 3 on both the front and back surfaces of an oxygen ion conductive solid electrolyte 1 as shown in FIG. The oxygen ion conductive solid electrolyte or substrate 4 is sequentially laminated.
is made gas permeable, and a voltage measuring device 6 for detecting electromotive force is connected to both electron conductive layers 2 and 3 via lead wires 5, and in order to cause movement of oxygen ions within the solid electrolyte 1. Various experimental studies have also been conducted regarding an oxygen sensor having a structure in which a DC power supply device 7 is connected to the oxygen sensor.

In such an oxygen sensor, for example, when the positive electrode of the DC power supply 7 is connected to the measuring electrode 2 and the negative electrode is connected to the reference electrode 3, oxygen ions are transferred from the reference electrode 3 to the measuring electrode 2. By causing the movement, the oxygen partial pressure at the interface between the solid electrolyte 1 and the reference side electrode 3 is lowered, and oxygen is sucked into this lowered portion through the gas permeable solid electrolyte 1 or the substrate 4. By stabilizing the oxygen partial pressure in a low state, the stoichiometric air-fuel ratio and the lean side (air excess side) air-fuel ratio are detected. The air-fuel ratio and the rich side (fuel excess side) air-fuel ratio are detected. However, even the oxygen sensor shown in FIG. 1 has a problem in that the stoichiometric air-fuel ratio electromotive force characteristic produces an output that appears to have shifted after durability.

As described above, when the stoichiometric air-fuel ratio electromotive force characteristic shifts, the emission level of the exhaust gas deteriorates, and there is a considerable possibility that the exhaust gas composition may deviate from the regulation value.

The purpose of the present invention is to solve the above-mentioned drawbacks of the prior art, and to provide an oxygen sensor for stoichiometric air-fuel ratio detection that can detect the stoichiometric air-fuel ratio with high accuracy even after durability, has a simple structure, and is very inexpensive. The purpose is to provide sensors.

The oxygen sensor for detecting a stoichiometric air-fuel ratio of the present invention includes a first electron conductive layer and a zeroth electron conductive layer provided on the surface of a gas-permeable oxygen ion conductive solid electrolyte, and which is integrated with the first electron conductive layer. The interface FI with the electrolyte is made contactable with the gas to be measured, and the part other than the solid electrolyte at the interface FO' between the D electron conductive layer and the solid electrolyte is surrounded by a dense material layer. It is said that

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 shows an embodiment of the present invention, in which a first electron conductive layer 12 and a second electron conductive layer 13 are provided on the surface of a gas permeable oxygen ion conductive collective electrolyte 11.
It has a structure in which a voltage measuring device 16 for detecting electromotive force is connected to the electron conductive layer 12 and the zeroth electron conductive layer 13 via lead wires 15.

At this time, the first electron conductive layer 12 and the solid electrolyte 11
The interface FI can be directly contacted with the gas to be measured by forming the first electron conductive layer 12, for example, from a thin film layer and making it permeable to oxygen molecules, and using the zero electron conductive layer 13 as a structural base. It is the same as surrounding the part other than the solid electrolyte 11 at the interface Fm between the second electronically conductive layer 13 and the solid electrolyte 11 with a dense material layer. It has a structure like this. The following describes changes in electromotive force when oxygen sensors having such a structure are sequentially held in the following gas atmospheres.

Note that the positive electrode side of the voltage measuring device 16 is connected to the first electron conductive layer 1.
2 will be explained. ■ Maintain in a rich atmosphere (gas composition with excess fuel due to stoichiometric air-fuel ratio).

@ Change the gas composition from a lean atmosphere to a lean atmosphere (a gas composition in which air is in excess of the stoichiometric air-fuel ratio).

■ Maintain in a stable atmosphere.

2. Change the gas composition from the lean atmosphere mentioned above to the Ritte atmosphere.

■ Maintain in the rich atmosphere mentioned above.

In the above gas atmosphere change from ■ to @, first,
In the state (2), the oxygen partial pressure P in the gas to be measured becomes the same value as the oxygen partial pressure PFO at the interface F between the 0th electron conductive layer 13 and the solid electrolyte 11, and is in an equilibrium state.

In addition, in a combustion reaction of gasoline or the like, when the first electron conductive layer 12 is made of a material with a catalytic effect, for example, the oxygen partial pressure P in the gas to be measured at a gas temperature of 600 qo
is a very small value of about 10 - phantom atm. Therefore, the electromotive force E, given by Nernst's equation, is E,=band n.

Band Tin = o.

Next, in the @ state, the gas to be measured changes from the rich atmosphere to the IJ
In this case, the oxygen partial pressure P in the gas to be measured changes from 10-atm in a rich atmosphere to about 10-3 atm in an IJ-n atmosphere.

Therefore, at this moment, since the oxygen partial pressure PF at the interface F is about 10-several atm, the electromotive force E2 at a gas temperature of 60,000 is E2=band nP
Point=〇. 〇496 (600 + 273) log false = 74 dishes v.

Next, in the state (2), when the rich atmosphere changes to the lean atmosphere, the oxygen partial pressure P in the gas to be measured changes to the oxygen partial pressure PF at the interface Fm between the 0th electron conductive layer 13 and the solid electrolyte 11. Although higher than B, the above solid electrolyte 11
Oxygen ions move from the measured gas through the
Due to the next reaction, the oxygen partial pressure PFO' at the interface FO gradually increases.

201-1020 Then, the oxygen partial pressure P=PF becomes equal to m on 10-3a and reaches an equilibrium state, and the electromotive force E3 is -E3: band n point band ln unit=.

becomes.

Next, in the eye condition, the gas to be measured changes from a lean atmosphere to a rich atmosphere, and at this time, the oxygen partial pressure P in the gas to be measured also changes from 10-3 atm in a lean atmosphere to about 10-dra in a Litte atmosphere. Change.

Therefore, at this moment, the oxygen partial pressure PFD at the interface m is about 10-3 atm, so the electromotive force E4 at a gas temperature of 60,000 is E4:band n life=
〇． 〇496 (600 plus 273) 10 special = -74 dishes v
becomes.

Next, in the @ state, the oxygen partial pressure P at the interface FO' is changed from the lean atmosphere to the rich atmosphere.
Since FO is higher than the oxygen partial pressure P in the gas to be measured,
Oxygen at the interface Fn' passes through the gas-permeable solid electrolyte 11 and the first electron conductive layer 12 and moves toward the gas to be measured, and the oxygen partial pressure PFn' at the interface FO gradually decreases.

Then, an equilibrium state is reached at P=PFO=10-2 oatm, and the electromotive force E5 is E5=band n voice.

Obi ln crocodile = o.

Therefore, the change in electromotive force when the above-mentioned changes from ■ to @ are performed continuously is as shown in FIG.

Therefore, in the measurement method described above, when changing from a rich atmosphere to a lean atmosphere, the electromotive force becomes positive when passing through the stoichiometric air-fuel ratio, and conversely, when changing from a lean atmosphere to a rich atmosphere, the electromotive force becomes positive when passing through the stoichiometric air-fuel ratio. Since the electromotive force becomes negative when this happens, the stoichiometric air-fuel ratio can be detected with high accuracy.

FIG. 3 shows another embodiment of the present invention, in which an oxygen ion conductive solid electrolyte 11 is used as a structural base, a first electron conductive layer 12 and a zero electron conductive layer 13 are provided on both sides of the structure base, and The interface FI between the first electron conductive layer 12 and the solid electrolyte 11 can be brought into direct contact with the gas to be measured, and the part other than the solid electrolyte 11 in the interface FDI between the zero electron conductive layer 13 and the solid electrolyte 11 is It has a structure in which the interface FO is covered with a dense material layer 14 made of an electrical insulator such as a glassy layer, and the interface FO is surrounded by the dense material layer 14.

Even with such a structure, the stoichiometric air-fuel ratio can be detected accurately as in the case shown in FIG.

FIG. 4 shows still another embodiment of the present invention.
In the case of the figure, solid electrolyte 11 is used as a structural base and covered with a dense material layer 14, but in the case of FIG. The solid electrolyte 11 is made to maintain the required strength, and can be formed from, for example, a thin film layer, without requiring any strength as a structural base.

When manufacturing such a structure, for example, the 0th electron conductive layer 13, the solid electrolyte 11, and the first electron conductive layer 12 are placed on the plate-like dense material layer 14 as a structural base.
This can be done by a process of sequentially laminating the layers. FIG. 5 shows still another embodiment of the present invention, in which a heating conductor 17 is disposed within the dense material layer 14 made of the electrical insulator shown in FIG.
It has a structure in which it is buried. That is, solid electrolyte 1
Since the oxygen ion conductivity of No. 1 is very poor at low temperatures, air-fuel ratio control cannot be performed, for example, when starting an automobile engine. Therefore, a heating conductor 17 is provided to heat the oxygen sensor in advance so that the stoichiometric air-fuel ratio can be detected with high accuracy even in a low temperature state. FIG. 6 shows still another embodiment of the present invention, in which the first electron conductive layer 12 is exposed from at least the surface of the first electron conductive layer 12, in the illustrated example, the upper surface of the dense material layer 14.
It has a structure in which the surface thereof is covered with a porous protective layer 18.

By covering with the porous protective layer 18 in this manner, at least the first electron conductive layer 12 is prevented from being altered by the gas to be measured such as exhaust gas as much as possible. As described above, the oxygen sensor of the present invention has the characteristic that it provides an output signal only when the air-fuel ratio is stoichiometric, and this characteristic remains unchanged even after durability.

In the oxygen sensor having the structure shown in FIGS. 2 to 6 described above, the materials for the first and second electron conductive layers 12 and 13 include Au, Ag, and SIC without catalytic action, or Tj02, Coo, WCr03, etc. oxide semiconductor or catalytic R↓Pd, Rh, melon, lr,
A single platinum group element such as Rt, an alloy thereof, or an alloy of a platinum group element and a base metal element can be used.

When the electron conductive layers 12 and 13 made of the above materials are laminated in the form of a thin film, a physical vapor deposition method such as sputtering or vacuum evaporation, an electrochemical method such as plating, or coating and baking using a paste is performed. As shown in FIG. 2, when one of the second electron conductive layers 13 is used as a structural base and also serves as a dense material layer, compaction molded compacts or cut-out materials can be used. etc. can be used.
In addition, the materials of the oxygen ion conductive solid electrolyte 11 include Ca○, Y203, SK], Mg○, Ti02, W0
3. Zr02 stabilized with Ta205, etc., or N
Bi203 stabilized with Q05,S, W03, Ta205, Y203, and even Tho2-Y203,C
Known materials such as a○-Y203 can be used.
When forming the solid electrolyte 11 of the above material into a thin film, a physical vapor deposition method such as sputtering or vacuum evaporation, an electrochemical method, or a coating and baking method using a paste may be employed. In the case where the solid electrolyte 11 is used as the structural base as shown in FIG. 3, a powder-molded combustion body or the like can be used.

Further, as the material for the dense material layer 14, forsterite, alumina, mullite, spinel, etc. can be used when it is an electrical insulator, and when it is an electrically conductive material, a single metal such as steel or platinum, or An alloy or the like can be used, and furthermore, a mixture of ceramics such as cermet and metal can be used.

When covering the dense material layer 14 as in the case of FIG. 3, it can be deposited by a bonding firing method, a printing firing method, a thermal spraying method using plasma or the like, and as shown in FIGS. When using the dense material layer 14 as a structural base as in the case shown in the figure, a compacted sintered body, a cut-out material, or the like can be used. Furthermore, Pt, W, or the like can be used as the material for the heating conductor 17, and when forming the conductor 17, a thin wire or a printed film made of paste can be used.

Further, as the porous protective layer 18 shown in FIG. 6, calcium zirconate (Ca0-Z2), alumina (A
I203), spinel, etc. can be used, and these can be attached by methods such as bonding and firing, or thermal spraying with plasma or the like.

Experimental Example 1 An oxygen sensor having the structure shown in FIG. 3 was manufactured and evaluated.

That is, on both sides of the solid electrolyte 11 made of a composite body (5 x 3.5 x 0.3 side) obtained by press-molding two powders of 8 wt% Y203-Z and then firing them, as shown by diagonal lines in FIG. (However, only on one side) Print platinum paste on a 4 x 3 area, hold a platinum wire with a diameter of 0.25 ribs as the lead wire 15 on each side, dry it as it is, and then heat it in the atmosphere at 1350 qo x Firing was performed for 1 hour to form the first and zeroth electronic conductive layers 12 and 13.

The platinum layer thickness at this time was 3±0.5 meters. next,
A glass paste was printed to cover the entire surface of the second electronically conductive layer 13, which is one of the platinum layers, and after drying,
Firing was performed for C×1 hour to form a dense material layer 14.

The thickness of the dense material layer 14 obtained here was 200 mm and 10 mm. Oxygen sensor body 2 manufactured in this way
0 was attached to the holder shown in FIG. As shown in FIG. 9, the oxygen sensor body 20 is made of stainless steel louvers 2.
1, the gas to be measured flows in and out through gas passage holes 21a formed in the louver 21. This louver 21 is welded and fixed to a stainless steel holder 23 provided on the outer periphery of the alumina protection tube 22 to protect it.
There is an insertion hole 2 for the platinum lead wire 15 in the alumina protection tube 22.
2a, and the platinum lead wire 15 passing through it.
penetrates through the rubber filler 24 and the alumina insulator 25. The outside of the alumina insulator 25 is protected by a stainless steel tube 26, which is welded to the stainless steel holder 23 and has a fixing nut 27 on its outer periphery. ing. Therefore, in order to investigate the basic characteristics of the oxygen sensor body 20 mounted on the above-mentioned holder, rich gas and lean gas were alternately poured into the oxygen sensor body 20 at intervals of 3, and changes in electromotive force were investigated.

The gas temperature at this time was 600° C., and the obtained characteristics are shown in FIG. As is clear from FIG. 10, almost the same characteristics as the theoretical electromotive force characteristics shown in FIG. 7 described above can be obtained. Note that the response speed when the electromotive force suddenly changes from 0 depends on the gas flow rate and the reaction rate of the catalyst, but the response speed when the electromotive force returns to 0 depends on the oxygen moving within the solid electrolyte 11. Since it is thought that it depends on the amount, it is possible to obtain a better response speed when the solid electrolyte 11 has appropriate porosity, that is, oxygen molecule permeability, than when it is dense. can.

Experimental Example 2 An oxygen sensor having basically the same structure as shown in FIG. 4 was manufactured and an evaluation experiment was conducted.

That is, as shown in FIG. 11a, an alumina fired body (5×3.
On one surface of the delicate material layer 14 consisting of
Print platinum paste as shown by diagonal lines in Figure 11b,
Drying was carried out in the atmosphere for 10,000 x 2 minutes and baking was performed in the air at 1,300 o 0 x 1 hour to form the second electronically conductive layer 13.

The thickness of the platinum layer obtained here was 3 to 4 mm. Thereafter, as indicated by diagonal lines in FIG. 11c, 8% by weight Y203-Z
A solid electrolyte paste was printed by mixing and kneading two powders (average particle size 0.5 m) and lacquer at a weight ratio of 1:1, dried in the atmosphere for 30 minutes at 100°C, and then dried in the air. The oxygen ion conductive solid electrolyte 11 was laminated by firing for 138,000 x 3 hours inside. The film thickness of the solid electrolyte 11 obtained here is 30±2 mm, and extremely fine porous material is formed that can appropriately transmit oxygen molecules. Next, the same platinum paste as above was printed on the solid electrolyte 11 as shown by diagonal lines in FIG. Firing was performed to form the first electronically conductive layer 12. The thickness of the platinum layer obtained here is 3
~ It was of 4 Buddhas. Thereafter, a porous protective layer was formed by depositing calcium zirconate powder over the entire surface of the device by plasma spraying.

Therefore, an evaluation experiment was conducted by attaching the oxygen sensor main body 20 to the holder shown in FIG. 9.

That is, here, the temperature of the gas to be measured was gradually raised from a low temperature state, and during this period, the stoichiometric air-fuel ratio of the gas to be measured was detected. The results are shown in FIG. 12 with overlapping lines. As shown in FIG. 12, it is clear that in the case of this experimental example, electromotive force starts to be generated from around 370 oo. Experimental example
3 An oxygen sensor having the structure shown in Fig. 6 was manufactured and an evaluation experiment was conducted.

That is, after printing platinum paste on one of two alumina green sheets, the other alumina green sheet is pressed and dried and fired to maintain the strength as a structural base and generate heat generated by the platinum inside. A dense material layer 14 in which a conductor 17 was embedded was formed.
Next, the oxygen sensor main body 20 was manufactured in exactly the same manner as in Experimental Example 2 shown in FIG. 11, and was attached to the holder shown in FIG. 9 to conduct an evaluation experiment. Note that the resistance of the heating conductor 17 at room temperature was about 30. Therefore, the heating conductor 17 receives a constant current of 0. With pine flowing, the temperature of the gas to be measured was gradually raised from a low temperature, and the stoichiometric air-fuel ratio was detected during this period. This result is shown in FIG. 12 as a solid line. As shown in FIG. 12, the heating conductor 1
It is clear that when 7 is provided, an electromotive force is generated from a low temperature of about 150°C compared to the case of Experimental Example 2 where this is not provided, and the heating conductor 17 is It can be seen that it can be applied more effectively in lower temperature conditions by providing this. Comparative Example A durability test was conducted on an oxygen sensor equipped with the constant current DC power supply 7 shown in FIG. 1 and the oxygen sensor manufactured in Experimental Example 2.

That is, the oxygen sensor shown in FIG. 1 was tested with three constant currents A flowing between the measurement side electrode 2 and the reference side electrode 3.

In this durability test, after 50,000 runs on a vehicle bench, changes in electromotive force were investigated by changes in air-fuel ratio at a measured gas temperature of 60,000. The results are shown in FIG.

The solid line a in FIG. 13 is the electromotive force characteristic of the oxygen sensor in FIG. 1 before durability, and shows a sudden change in electromotive force at the stoichiometric air-fuel ratio. However, the electromotive force characteristics of the oxygen sensor shown in Figure 1 after the endurance test show that the change in the electromotive force at the stoichiometric air-fuel ratio is somewhat gradual compared to before the endurance cycle, as shown by the broken line b in Figure 13. , the characteristics have changed as if the stoichiometric air-fuel ratio had shifted to the rich side. Therefore, the oxygen sensor shown in FIG. 1 cannot accurately detect the stoichiometric air-fuel ratio after durability. On the other hand, the oxygen sensor of the present invention has a characteristic of generating an electromotive force only when the air-fuel ratio is set to the stoichiometric air-fuel ratio. It can be seen that there is no change at all in the electromotive force characteristics afterwards.

This is because, as described above, the oxygen sensor of the present invention has the characteristic of generating an electromotive force only at the stoichiometric air-fuel ratio, and this characteristic does not change even after durability. As is clear from the detailed description above, the oxygen sensor for detecting the stoichiometric air-fuel ratio of the present invention can detect the stoichiometric air-fuel ratio with high accuracy even after durability, and the stoichiometric air-fuel ratio can be detected with high accuracy even after durability. Since there is no electromotive force signal that indicates a shift in the fuel ratio electromotive force characteristics, it is possible to control the air-fuel ratio in automobile engines with high accuracy over a long period of time, and the durability of the oxygen sensor and peripheral parts is improved. It has an extremely excellent effect in that it can significantly improve the performance and reduce the cost.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional explanatory diagram of a conventional oxygen sensor for detecting an air-fuel ratio, FIGS. 2 to 6 are schematic cross-sectional explanatory diagrams of an oxygen sensor for detecting a theoretical air-fuel ratio in each embodiment of the present invention,
The figure is a graph showing the theoretical electromotive force characteristics of the oxygen sensor according to the present invention, Figure 8 is an explanatory diagram of the external appearance of the oxygen sensor manufactured in Experimental Example 1, Figure 9 is a cross-sectional view of the holder for mounting the oxygen sensor, and Figure 10 is a graph showing the theoretical electromotive force characteristics of the oxygen sensor according to the present invention. The figure is a graph showing the electromotive force characteristics of the oxygen sensor detected in Experimental Example 1, and Figures 11a to 11d are Experimental Example 2.
Fig. 12 is a graph showing the relationship between the measured gas temperature and electromotive force measured in Experimental Examples 2 and 3, and Fig. 13 is a graph showing the relationship between the air-fuel ratio and the electromotive force measured in Comparative Example. It is a graph showing the relationship with electromotive force. 11...Oxygen ion conductive solid electrolyte, 12.
...First electron conductive layer, 13...0th electron conductive layer, 14...Dense material layer. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 8 Figure 6 Figure 7 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13

Claims (1)

[Claims] 1. A first electron conductive layer and a second electron conductive layer are provided on the surface of a gas permeable oxygen ion conductive solid electrolyte, and an interface FI between the first electron conductive layer and the solid electrolyte is provided. oxygen for stoichiometric air-fuel ratio detection, characterized in that the interface FII between the second electron conductive layer and the solid electrolyte, other than the solid electrolyte, is surrounded by a dense material layer. sensor. 2. The oxygen sensor for detecting a stoichiometric air-fuel ratio according to claim 1, wherein the dense material layer is made of a good electrical conductor. 3. The oxygen sensor for detecting a stoichiometric air-fuel ratio according to claim 2, wherein the dense material layer is a second electron conductive layer. 4. The oxygen sensor for detecting a stoichiometric air-fuel ratio according to claim 1, wherein the dense material layer is made of an electrical insulator and a solid electrolyte is used as a structural base. 5. The oxygen sensor for detecting a stoichiometric air-fuel ratio according to claim 1, wherein the dense material layer is made of an electrical insulator, and the electrical insulator is used as a structural base. 6. The oxygen sensor for detecting a stoichiometric air-fuel ratio according to claim 5, wherein a heat generating conductor is provided on the structural base. 7. The oxygen sensor for stoichiometric air-fuel ratio detection according to any one of claims 1 to 6, wherein at least the surface of the first electron conductive layer is covered with a porous protective layer.