JPH09113480A - Oxygen sensor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxygen sensor element in which stabilized performance can be sustained for a long term while preventing the poisoning of lead surely when the gas to be measured contains a lead component. SOLUTION: The oxygen sensor element comprises an inner electrode 2 of platinum applied to the inner surface of a ceramic tube 1 principally comprising zirconia, an outer electrode 3 of platinum applied to the outer surface of the ceramic tube 1, and a protective structure 4 provided on the surface of the outer electrode 3. The protective structure 4 comprises an inner protective layer 5, a lead trap layer 6, and an outer protective layer 7 laminated sequentially on the surface of the outer electrode 3. The inner protective layer 5 is a porous ceramic layer, the lead trap layer 6 is a porous ceramic layer carrying platinum as a catalyst metal, and the outer protective layer 7 is a porous ceramic layer having porosity in the range of 0.02-0.3cc/g and thickness in the range of 20-1000μm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor element used for an oxygen sensor for detecting the oxygen concentration in the exhaust gas of an internal combustion engine or various combustion equipment.

[0002]

2. Description of the Related Art Conventionally, for example, in an oxygen sensor used as a gas sensor for automobiles, a heat-resistant catalyst electrode (inner electrode, inner electrode, A device including an oxygen sensor element provided with an outer electrode) is known.

A noble metal electrode such as platinum or rhodium is used as the catalyst electrode of this oxygen sensor element.
Accurate equivalence ratio (air-fuel ratio) of air and fuel by equilibrating heat resistance that can withstand high temperatures of 0 ° C or higher and exhaust gas that is in a non-equilibrium state under normal conditions by the catalytic ability of the outer electrode.
This is so that the can be detected.

By the way, since the outer electrode of the oxygen sensor element is directly exposed to high-temperature and high-speed exhaust gas, the heat-resistant electrode is gradually deteriorated, and its catalytic ability is rapidly lost. Therefore, in order to protect against such problems, a protective coating of porous heat-resistant ceramics such as spinel is formed on the outer electrode by a method such as thermal spraying to increase the life of the oxygen sensor. It is trying to make it.

[0005]

However, since the unleaded gasoline is used as the fuel in the ordinary gasoline engine, the above-mentioned protective coat is sufficient, but in an area where the unleaded gasoline supply system is not yet sufficient, Since leaded gasoline is used, there is a problem that lead gas is mixed in the exhaust gas and the performance of the oxygen sensor is deteriorated.

That is, since the lead gas in the exhaust gas reaches the outer electrode through the protective coat and acts as a catalyst poison, the catalytic action of the outer electrode is hindered and the accurate air-fuel ratio of the exhaust gas is reduced. There is a problem that it cannot be detected. As a measure against this problem, for example, in Japanese Patent Laid-Open No. 62-187245, a lead trap layer made of alumina is laminated on a ceramic protective layer on an outer electrode to capture lead before it reaches the outer electrode. The technology is disclosed. Further, the same publication discloses a technique for optimizing lead capture performance within a range that does not affect the responsiveness of an oxygen sensor by defining the pore volume of alumina used for the lead trap layer and the thickness of the lead trap layer. Is disclosed. Furthermore, Japanese Patent Publication 5-765
In Japanese Patent Laid-Open No. 75-75, a catalyst metal (platinum) is included in a ceramic protective layer to form a lead trap layer, and the catalyst metal captures lead before reaching the outer electrode to prevent deterioration of sensor performance. Techniques for doing so are disclosed.

However, in the above-mentioned conventional lead trap layer, there is a problem that the lead trapping effect in the lead trap layer and its durability are not always sufficient. That is, in the above-mentioned technique, although a certain effect can be obtained by defining the pore volume of alumina used for the lead trap layer and the thickness of the layer, the lead trapping layer is sufficiently captured while ensuring the necessary gas permeability. There was a problem that I could not do it. Further, in the above-mentioned technique, since the catalyst metal is supported on the lead trap layer, the lead trapping ability is high, but the lead trapped in the lead trap layer is alloyed with the catalyst metal and trapped, so that the lead trapping of the catalyst metal is eventually taken. However, there is a problem that the lead capturing ability gradually decreases, and finally lead reaches the outer electrode.

The present invention has been made in order to solve such a problem, and even if the gas to be measured contains a lead component, the poisoning of lead can be reliably prevented and stable for a long period of time. An object is to provide an oxygen sensor element that can maintain performance.

[0009]

In order to solve the above problems, the invention of claim 1 provides a reference gas side electrode on one surface of a solid electrolyte substrate having oxygen ion conductivity and a detection gas on the other surface. A side electrode is formed, and a porous inner protective layer made of ceramic is further provided on the surface side of the detection gas side electrode, and a porous lead trap layer containing a lead trap substance is provided on the surface side of the inner protective layer. In an oxygen sensor element in which a porous outer protective layer made of ceramic is formed on the surface side of a lead trap layer, the outer protective layer has a pore volume of 0.02 cc / g or more and 0.3 cc / g or less and a thickness thereof. The gist is an oxygen sensor element having a thickness of 20 μm or more and 1000 μm or less.

Here, when the pore volume is smaller than the lower limit value and is dense or when the thickness exceeds the upper limit value, clogging occurs and gas permeability is deteriorated, which is not preferable. Further, when the porosity volume ratio is higher than the upper limit value or when the thickness is lower than the lower limit value, the lead component easily reaches the lead trap layer, the life of the lead trap layer is shortened, and the sensor This is not preferable because the performance will deteriorate.

The detection gas side electrode, the inner protective layer, the lead trap layer, and the outer protective layer may be arranged in this order from the inner side to the outer side. For example, another layer may be provided between these layers. Layers and the like may be arranged. Specifically, two lead trap layers and two inner protective layers may be formed, or another type of protective layer may be formed.

The invention according to claim 2 is based on the oxygen sensor element according to claim 1, characterized in that the pore volume of the outer protective layer is 0.05 cc / g or more and 0.2 cc / g or less. . A third aspect of the invention provides the oxygen sensor element according to the first or second aspect, wherein the thickness of the outer protective layer is 50 μm or more and 200 μm or less.

The invention of claim 4 is characterized in that the inner protective layer, the lead trap layer, and the outer protective layer are sequentially laminated on the surface of the detection gas side electrode.
3. The oxygen sensor element according to any one of 3 above.

Examples of the solid electrolyte having oxygen ion conductivity include zirconia in which an oxide of calcium, magnesium or the like or an oxide of a rare earth element such as yttrium is in solid solution. Further, examples of the material of the reference gas side electrode and the detection gas side electrode include platinum, palladium, iridium, rhodium and the like and alloys thereof.

The material for the inner protective layer and the outer protective layer is
Examples thereof include magnesium aluminum spinel, alumina, magnesia, calcia, and the like, and mixtures thereof. Examples of the catalyst metal used for the lead trap layer include platinum, palladium, iridium, rhodium and the like. The catalyst metal can be used by mixing with a ceramic material such as magnesium aluminum spinel, alumina, magnesia, zirconia.

[0016]

With the structure of the first aspect of the present invention, for example, lead fine particles in exhaust gas are first captured by the outer protective layer and hardly reach the lead trap layer. Therefore,
Compared with a conventional lead trap layer using a catalytic metal, the trapping ability of the lead trap layer does not deteriorate, and the effect can be maintained for a long time. In addition, the lead once trapped by the outer protective layer gradually begins to diffuse into the exhaust gas due to the high temperature exhaust gas. In other words, even once the lead is once trapped, it eventually leaves the outer protective layer.
In the process, if the lead concentration in the outer protective layer gradually increases, the degree of lead emission in the outer protective layer will soon be in proportion to the degree of lead trapping, and the lead concentration in the outer protective layer will saturate, The above lead accumulation does not proceed.

Therefore, at this point, if the performance such as sufficient output and responsiveness of the oxygen sensor can be maintained, the outer protective layer will not be clogged even after long-term use, and the performance of the oxygen sensor will be affected. Never. That is, in the present invention, by regulating the pore volume and thickness of the outer protective layer within the above-mentioned ranges, it is possible to make the gas permeability appropriate and to keep the lead capturing performance high. As a result, the long-term protection ability against lead can be maintained without impairing the sensor output and responsiveness.

Furthermore, in the present invention, by providing a lead trap layer containing a catalytic metal under the outer protective layer, the lead that has passed through the outer protective layer is completely captured, and the detection gas side electrode (outer electrode) is formed. It is possible to greatly reduce the influence of lead poisoning in the. In the structure in which the inner protective layer, the lead trap layer, and the outer protective layer are sequentially laminated, the total thickness thereof is 2
The thickness is preferably 0 μm or more and 1000 μm or less.

According to the second aspect of the present invention, the pore volume of the outer protective layer is set to 0.05 cc / g or more and 0.2 cc / g or less, so that the gas permeability and the lead capturing performance are excellent in balance, and the long-term lead lead The protection ability can be further enhanced. In the invention of claim 3, the thickness of the outer protective layer is 50 μm or more 2
Since it is set to be not more than 00 μm, the gas permeability and the lead capturing performance are well balanced, and the long-term protection ability against lead can be further enhanced.

The pore volume of the outer protective layer is 0.05 cc.
/ G or more and 0.2 cc / g or less and its thickness is 50
When the thickness is in the range from μm to 200 μm, the long-term protection capability against lead is further enhanced, which is preferable. Claim 4
In the invention described above, since the inner protective layer, the lead trap layer, and the outer protective layer are sequentially stacked on the surface of the detection gas side electrode, long-term lead The protection ability can be fully exerted.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a cross section of the basic structure of the oxygen sensor element of the present embodiment. As shown in FIG. 1, in the oxygen sensor element of the present embodiment, an inner electrode 2 made of platinum is provided on the inner surface of a bottomed ceramic tube 1 containing zirconia as a main component, and the outer surface of the ceramic tube 1 is provided. The outer electrode 3 made of platinum is provided, and the protective structure 4 is further provided on the surface of the outer electrode 3.

As shown in the enlarged view of the cross section of FIG. 2, the protective structure 4 has an inner protective layer 5, a lead trap layer 6, and an outer protective layer 7 sequentially laminated from the surface of the outer electrode 3. . Of these, the inner protective layer 5 has a thickness of 50 to 150 μm.
Of the lead trap layer 6
Is a porous layer made of a mixture of platinum and zirconia having a thickness of 10 to 60 μm, and the outer protective layer 7 has a pore volume of 0.02 to 0.3 cc / g and a thickness of 20 to 1
It is a porous layer made of spinel in the range of 000 μm.

Next, an example of a method of manufacturing the oxygen sensor element of this embodiment will be described. Platinum is plated on a part of the inner surface and the outer surface of the ceramic tube 1 whose main part is zirconia with the tip closed to form an inner electrode 2 and an outer electrode 3 for extracting electromotive force.

Next, a metal oxide such as magnesium aluminum spinel is sprayed on the surface of the outer electrode 3 to form a porous inner protective layer 5. Next, for example, a particle size of 0.5
~ 2.5 μm platinum powder and zirconia powder were mixed at a weight ratio of 9: 1 to form a paste, and the paste was
Applying a thickness of 0 to 70 μm on the inner protective layer 5,
The lead trap layer 6 is formed by firing at 00 to 800 ° C.

After the firing treatment, a metal oxide such as magnesium aluminum spinel is melt-squinted to form a porous outer protective layer 7. (Experimental Example) Next, an experimental example performed for confirming the effect of the oxygen sensor element of the present embodiment will be described.

In the present experimental example, the lead trap layer and the outer protective layer of the oxygen sensor element were formed by the method shown in Table 1 below, and samples No. 1 to 29 (Examples No. 1 to 23, Comparative example
No. 24-29) was created. Then, an oxygen sensor using this oxygen sensor element was attached to an exhaust pipe of an actual engine, and the air-fuel ratio was controlled based on the output of the oxygen sensor. Then, the state (cycle) of fluctuation of the air-fuel ratio was measured as the control frequency. This control frequency is shown in Table 2.

Next, a durability test of this sensor was conducted.
Specifically, after 100 or 200 hours, it was similarly attached to the exhaust pipe of an actual engine for control, and the control air-fuel ratio was measured. The difference between this air-fuel ratio and the initial air-fuel ratio is shown in Table 2 (change in control A / F).

It should be noted that the larger the control frequency, the better the response, and the smaller the change in the control air-fuel ratio, the better the durability.

[0029]

[Table 1]

[0030]

[Table 2]

As is apparent from this experimental example, when the porosity of the outer protective layer is 0.02 to 0.3 cc / g and the thickness thereof is within the range of the present invention of 20 to 1000 μm, (response Change in the control air-fuel ratio (due to changes in
It can be seen that excellent sensor performance can be exhibited over a long period of time. Particularly, the porosity of the outer protective layer is 0.05 to 0.2c.
In the case of c / g or the thickness of 50 to 200 μm,
It can be seen that the change in the control air-fuel ratio is even smaller and excellent sensor performance can be exhibited over a longer period of time.

On the other hand, those outside the scope of the present invention are:
Due to a large delay in response or insufficient output, the change in the control air-fuel ratio becomes so large that it cannot be measured, or the control cannot be performed, which is not preferable. It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes without departing from the gist of the present embodiment.

[0033]

According to the first aspect of the present invention, the porosity and thickness of the outer protective layer are defined in the above ranges, so that the gas permeability is made appropriate and the lead protective layer is reached without impairing the responsiveness. Since the lead component can be limited, stable performance of the oxygen sensor can be exhibited for a long period even when a fuel containing the lead component is used.

According to the second aspect of the present invention, since the porosity and thickness of the outer protective layer are further defined within the above ranges, stable oxygen sensor performance can be exhibited for a longer period of time.

[Brief description of the drawings]

FIG. 1 is a sectional view of a basic structure of an oxygen sensor.

FIG. 2 is an enlarged sectional view showing a part of the basic structure of the oxygen sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Ceramic tube 2 ... Inner electrode 3 ... Outer electrode 5 ... Inner protective layer 6 ... Lead trap layer 7 ... Outer protective layer

Claims (4)

[Claims] 1. A reference gas side electrode is formed on one surface of a solid electrolyte substrate having oxygen ion conductivity, and a detection gas side electrode is formed on the other surface, and a ceramic is formed on the surface side of the detection gas side electrode. To form a porous inner protective layer, a porous lead trap layer containing a lead trap material on the surface side of the inner protective layer, and a porous outer protective layer made of ceramic on the surface side of the lead trap layer. In the oxygen sensor element, the pore volume of the outer protective layer is 0.02 cc / g or more.
3 cc / g or less, and the thickness is 20 μm or more and 10
An oxygen sensor element characterized by having a thickness of 00 μm or less. 2. The outer protective layer has a pore volume of 0.05 c
The oxygen sensor element according to claim 1, wherein the oxygen sensor element has a c / g or more and 0.2 cc / g or less. 3. The thickness of the outer protective layer is 50 μm or more 2
The oxygen sensor element according to claim 1 or 2, wherein the oxygen sensor element has a thickness of not more than 00 µm. 4. The oxygen sensor according to claim 1, wherein the inner protective layer, the lead trap layer, and the outer protective layer are sequentially laminated on the surface of the detection gas side electrode. element.