JPH05307015A - Manufacture of oxygen sensor - Google Patents

Abstract

PURPOSE:To activate the catalyst function of an electrode and allow a sensor to be actuated at a low temp. by applying a paste as electrode material consisting of a noble metal and complex oxide to a base in a provisionally baked state, and sintering the paste and the base simultaneously. CONSTITUTION:A tubular base 1 with the tip blocked is fabricated from a green sheet of zirconia ZrO2 as a solid electrolyte having oxygen ion conductivity. After provisionally baking the base 1, paste as mixture of complex oxide (for example, ZrO2+Y2O3) and platinum Pt is applied to the inner and outer surfaces of the base 1, and the resultant is sintered at a high temp. between 1100-1300 deg.C to form electrodes 2, 3. At this time, gas is generated from the complex oxide to suppress grain growth of platinum Pt, which causes enlargement of the surface area of the platinum Pt, and thereby, the catalyst function is activated. Accordingly, starting is practicable even at a low temp. without using any heater. On the outer surface of the base 1, a platinum catalyst layer 4 and a protection layer 5 are further provided, and brought into contact with the gas to be inspected is generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an oxygen sensor, and more particularly to a manufacturing technique for improving low-temperature activity of an oxygen sensor which generates an electromotive force according to an oxygen partial pressure ratio.

[0002]

2. Description of the Related Art Conventionally, as an oxygen sensor using an oxygen ion conductive solid electrolyte, there is one having a sensor part structure as shown below (Japanese Patent Laid-Open No. 58-20436).
No. 5, Japanese Utility Model Publication No. 59-31054, etc.). That is, a platinum electrode is formed on each of the inner surface and the outer surface of a zirconia tube having a closed tip formed from an oxygen ion conductive solid electrolyte containing zirconium oxide ZrO ₂ as a main component, and further outside the outer platinum electrode. Then, a protective layer is formed with a metal oxide such as magnesium spinel.

In such a structure, while a reference gas (for example, atmosphere) is introduced into the inner cavity of the zirconia tube,
The outside of the zirconia tube is brought into contact with the gas to be detected (for example, exhaust gas from an internal combustion engine), and the oxygen partial pressure of the reference gas that contacts the inner surface and the oxygen partial pressure of the gas that contacts the outer surface are determined according to the ratio. By generating an electromotive force between the electrodes, the oxygen partial pressure (oxygen concentration) of the gas to be detected is detected.

As the method for forming the electrode, there are a vacuum vapor deposition method, an electroless plating method, a paste method and the like. However, since the electrode has high adhesion strength and excellent durability, the electrode material is formed into a paste. It is said that a paste method, in which a zirconia tube in a calcined state is applied and then sintered, is preferable as an electrode forming method.

[0005]

By the way, in the conventional oxygen sensor, the internal resistance becomes large at a low temperature,
In addition, the catalyst has a characteristic of generating a higher electromotive force than that at high temperature due to a large difference in oxygen partial pressure because the catalyst activity is deteriorated, and the problem that the oxygen sensor cannot normally operate from a low temperature state There is. In particular, when the electrode is formed by the paste method, grain growth of the platinum electrode occurs during sintering, which is one of the causes of deteriorating the catalytic activity.

For this reason, when it is desired to use the oxygen sensor in a low temperature state, a heater for heating the element is provided, and the heater is used to reduce internal resistance and catalytic activity from a low ambient temperature state. I was doing
However, the provision of the heater causes an increase in the cost of the oxygen sensor, and there is a new problem that the element may be destroyed when the heater voltage leaks.

The present invention has been made in view of the above problems, and an object thereof is to provide a manufacturing method capable of improving the catalytic activity of an electrode in an oxygen sensor, and to improve the low temperature operation of the oxygen sensor. To do.

[0008]

Therefore, according to the present invention, an electrode is formed on each of the inner and outer surfaces of a substrate made of an oxygen ion conductive solid electrolyte and brought into contact with a reference gas, and the electrode on one surface is brought into contact with a gas to be detected. A method of manufacturing an oxygen sensor that generates an electromotive force according to an oxygen partial pressure ratio between the other surface of the electrode and an electrode material paste composed of a noble metal and a complex oxide, on the base body in a calcined state. After coating, the electrode material paste and the substrate were simultaneously sintered to prepare an electrode.

[0009]

According to such a manufacturing method, gas is generated from the composite oxide during sintering, grain growth of the noble metal is suppressed, the grain size of the noble metal after sintering is reduced, and the catalytic activity can be achieved by increasing the surface area. become.

[0010]

EXAMPLES Examples of the present invention will be described below. Referring to FIG. 1 showing an oxygen sensor structure of an example, a zirconia tube 1 having a closed tip formed of an oxygen ion conductive solid electrolyte containing zirconium oxide ZrO ₂ as a main component.
Platinum electrodes 2 and 3 for extracting electromotive force are formed on the inner surface and the outer surface of the (base), respectively.

Further, a platinum catalyst layer 4 is further formed on the outer surface of the zirconia tube 1, and a protective layer 5 for protecting the platinum catalyst layer 4 with an oxide metal such as magnesium spinel is formed thereon. ing. In such a configuration, a reference gas (for example, the atmosphere) is introduced into the inner cavity of the zirconia tube 1 while the zirconia tube 1
Electromotive force according to the ratio of the oxygen partial pressure of the reference gas that contacts the inner surface and the oxygen partial pressure of the detected gas that contacts the outer surface Is generated between the electrodes 2 and 3 to detect the oxygen partial pressure (oxygen concentration) of the gas to be detected.

Next, a method of manufacturing the oxygen sensor having the above structure will be described below with reference to the flow chart of FIG. First, a tubular substrate (zirconia tube 1) having a closed tip is produced from a green sheet of zirconia ZrO ₂ as an oxygen ion conductive solid electrolyte (S1).

Next, this substrate (zirconia tube 1) is calcined to prepare for application of the electrode material paste (S
2). On the other hand, a plurality of oxides to be mixed with the platinum paste is prepared (S3). This composite oxide is, for example, ZrO ₂ + Y
₂ O ₃ or ZrO ₂ + CeO ₂ or the like, which is produced by the coprecipitation method.

When the composite oxide (ZrO ₂ + Y ₂ O ₃ or ZrO ₂ + CeO ₂ ) is prepared, it and platinum P
A paste mixed with t (noble metal) is prepared as an electrode material (S4). The composite oxide (ZrO ₂ +
The mixing ratio of Y ₂ O ₃ or ZrO ₂ + CeO ₂ ) and platinum Pt is about 60 to 80% platinum Pt and about 20 to 40% complex oxide.

Then, the electrode material paste is applied to each part (electrode forming portion) of the inner and outer surfaces of the base body (zirconia tube 1) in the calcined state (S5). When the electrode material paste is applied, the substrate (zirconia tube 1) and the applied electrode material paste are simultaneously sintered at a high temperature of about 1100 to 1300 ° C (S6). As a result, the electrodes 2 and 3 are formed on the inner and outer surfaces of the base (ceramic tube 1), respectively.

Here, as shown in FIG. 3, during the sintering
And ZrO contained in the electrode material paste ₂+ Y₂O₃Some
Is ZrO₂+ CeO₂O from complex oxides such as₂Or H₂
O gas is generated, and by the generation of such gas, platinum Pt
Grain growth is suppressed and the particle size of the platinum Pt electrode after sintering is small.
You can Specifically, the electrode material binder that does not contain complex oxides
If the electrodes are formed in the same way as above by
Causes the particle size of the platinum electrode to become 10-20 μm by sintering.
However, as described above,₂+ Y₂O₃Or ZrO₂+
CeO₂Mixing complex oxides such as
With, the particle size of the platinum electrode can be reduced to about 1 to 10 μm.
It

As the particle size of the platinum electrode becomes smaller, the surface area becomes larger and the catalytic performance is activated. Therefore, good catalytic activity is obtained even at low temperatures, and the oxygen sensor operates at low temperature without the need for a heater. Can be made possible. Therefore, it is possible to provide an oxygen sensor that can be operated at a low temperature at low cost, and since a heater is not required, there is no fear of element destruction due to leakage of the heater voltage.

As described above, if the catalytic activity is improved and the low temperature operation is promoted, if the oxygen concentration is provided to detect the oxygen concentration in the exhaust gas of the internal combustion engine, the exhaust gas in the exhaust gas is discharged immediately after the engine is started. The oxygen concentration can be measured, and the air-fuel ratio feedback control based on the information of the oxygen concentration can be performed early to improve the exhaust property at the engine start (when the engine is cold).

If the electrode material is applied in paste form to the substrate (zirconia tube) that has been calcined as described above and the substrate and the electrode are simultaneously sintered, the adhesion strength of the electrode to the substrate is high. Further, the effect that the heat change of platinum is small is obtained as in the case of forming the electrode by the electrode paste containing no complex oxide.

That is, the paste-like electrode material is
Manufacture of applying to Luconia tube) and then sintering at the same time
In the method, high adhesion strength is obtained, and platinum
It has the effect of less heat change, but platinum grain growth occurs.
It has the drawback that it causes poor catalytic activity
However, as shown in this example, the platinum paste
Complex oxide (ZrO₂+ Y₂O₃Some
Iha ZrO₂+ CeO ₂) Is mixed to form platinum particles.
The length can be suppressed, and the paste method called grain growth
The drawbacks could be resolved.

Further, platinum oxide is mixed with complex oxide (ZrO
Mixing ₂ + Y ₂ O ₃ or ZrO ₂ + CeO ₂ ) also increases the affinity for the substrate and obtains high adhesion strength. In this embodiment, platinum Pt is used as the noble metal.
However, other noble metals having heat resistance and a catalytic function may be used.

[0022]

As described above, according to the method of manufacturing an oxygen sensor of the present invention, the paste of the electrode material is applied to the substrate in the calcined state, and the paste and the substrate are simultaneously sintered to form the electrode. In the method described above, gas can be generated from the composite oxide mixed in the electrode material paste at the time of sintering to suppress grain growth of the electrode, so that catalytic activity in the electrode can be achieved and low temperature operation of the oxygen sensor can be achieved. The effect is to be able to achieve.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of an oxygen sensor structure.

FIG. 2 is a flowchart showing steps of a manufacturing method according to the present invention.

FIG. 3 is a state diagram showing how gas is generated during sintering.

[Explanation of symbols]

 1 Ceramic tube 2 and 3 electrodes

Claims (1)

[Claims] 1. An electrode is formed on each of the inner and outer surfaces of a substrate made of an oxygen ion conductive solid electrolyte, and the electrode is on one surface in contact with a reference gas and the electrode on the other surface is in contact with a gas to be detected. A method for manufacturing an oxygen sensor that generates an electromotive force according to an oxygen partial pressure ratio, wherein an electrode material paste comprising a noble metal and a complex oxide is applied to the base body in a calcined state, and then the electrode material paste and A method of manufacturing an oxygen sensor, which comprises simultaneously sintering a substrate and an electrode.