JPH06118054A - Manufacture of oxygen sensor - Google Patents

Abstract

PURPOSE:To improve the low temperature activity of a zirconia type oxygen sensor. CONSTITUTION:An electrode material paste in which platinum powder, an organic solvent, and an organic compound of each element (La, Sr, Co) forming a perovskite composite oxide (for example, La0.6Sr0.4CoO3) are mixed is formed, and each element forming the perovskite composite oxide is highly dispersed in the paste. The electrode material paste is applied to the inner and outer surfaces of a zirconia tube 11 laid in the temporarily sintered state, and the electrode material paste and the zirconia tube 11 are simultaneously sintered to form electrodes 12, 14. By such a sintering, the electrodes 12, 13 in which the perovskite composite oxide excellent in low temperature activity is uniformly adhered to the platinum surface can be provided.

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 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 portion structure as shown in FIG. 3 (Japanese Patent Laid-Open No. 58-20436).
No. 5, Japanese Utility Model Publication No. 59-31054, etc.). In FIG. 3, after applying platinum Pt paste to each part of the inner surface and the outer surface of the ceramic tube 1 having a closed tip formed of an oxygen ion conductive solid electrolyte containing zirconium oxide ZrO ₂ as a main component, By firing the ceramic tube 1, platinum electrodes 2 and 3 for taking out electromotive force are formed.

In order to protect the platinum catalyst layer 4 by further depositing platinum Pt on the outer surface of the ceramic tube 1 to form a platinum catalyst layer 4 and spraying a metal oxide such as magnesium spinel thereon. The protective layer 6 is formed.
In such a configuration, while the reference gas (for example, the atmosphere) is introduced into the inner cavity of the ceramic tube 1, the outside of the ceramic tube 1 is brought into contact with the gas to be detected (for example, the exhaust gas of the internal combustion engine), and the reference gas that comes into contact with the inner surface is The oxygen partial pressure (oxygen concentration) of the gas to be detected is generated by generating an electromotive force according to the ratio of the oxygen partial pressure and the oxygen partial pressure of the gas to be detected in contact with the outer surface between the electrodes 2 and 3. Is to detect.

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 ceramic tube in a calcined state is applied and then sintered is preferable as an electrode forming method.

[0005]

By the way, on the electrode, an electrochemical reaction that plays an important role in the oxygen sensor such as movement of electrons, adsorption and dissociation of oxygen molecules, and generation of oxygen ions is performed, and the oxygen sensor of the oxygen sensor is It is known that low temperature activation depends largely on the electrode performance. That is, the electromotive force E of the oxygen sensor is R is a gas constant, T is an absolute temperature,
It is known that when F is the Faraday constant and Ps and Pg are oxygen partial pressures at the reference electrode and the measurement electrode, respectively, E = (RT / 4F) · In (Ps / Pg) When is in a low temperature at a low temperature, the actual electromotive force is lower than the theoretical electromotive force obtained by the above equation, and it may be impossible to operate the oxygen sensor at a low temperature.

For example, in the case of a platinum electrode which has been generally used conventionally, as shown in FIG. 4, the element temperature is about 600 ° C.
In the following cases, the actual electromotive force is lower than the theoretical electromotive force derived from the equation of the electromotive force E, and the oxygen sensor cannot operate normally. Therefore, conventionally, by providing a heater for heating the sensor element when the temperature of the gas to be detected is low, low temperature operation can be performed. However, providing a heater causes an increase in the cost of the oxygen sensor,
Further, there is a problem that the power consumption is increased by the heater.

The present invention has been made in view of the above problems, and provides a method for manufacturing an oxygen sensor capable of improving the low-temperature activity of electrodes in the oxygen sensor, thereby improving the low-temperature operation of the oxygen sensor. The purpose is to

[0008]

Therefore, in the method for manufacturing an oxygen sensor 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 is brought into contact with a reference gas. And an oxygen sensor that generates an electromotive force according to the oxygen partial pressure ratio between the electrode on the other surface that is brought into contact with the gas to be detected, a noble metal powder, an organic solvent, and each element that constitutes the perovskite complex oxide. The electrode material paste mixed with the organic compound of 1 was applied to the substrate in the calcined state, and then the electrode material paste and the substrate were simultaneously sintered to form an electrode.

[0009]

With this structure, since each element forming the perovskite complex oxide is mixed in the form of an organic compound in the electrode paste composed of the organic solvent and the noble metal powder,
Each element constituting the perovskite-based complex oxide is highly dispersed in the paste, so that the perovskite-based complex oxide generated by sintering adheres to the noble metal uniformly.

The perovskite complex oxide is an electrode material excellent in low-temperature activity, and since it adheres uniformly to the noble metal as described above, it can enable the oxygen sensor to operate at low temperature.

[0011]

EXAMPLES Examples of the present invention will be described below. In FIG. 1 showing an embodiment, electrodes 12 and 13 are respectively provided on a part of the inner and outer surfaces of a zirconia tube 11 (a substrate of an oxygen ion conductive solid electrolyte) whose tip is closed, while on the outside of the outer electrode 12. The platinum catalyst layer 14 and the spinel protective layer 15 are laminated to form the element portion of the oxygen concentration cell type oxygen sensor.

In the above oxygen sensor, a reference gas (for example, the atmosphere) is introduced into the inner cavity of the zirconia tube 11, while the outside of the zirconia tube 11 is brought into contact with a gas to be detected (for example, exhaust gas of an internal combustion engine),
An electromotive force corresponding to the ratio of the oxygen partial pressure of the reference gas that contacts the inner surface to the oxygen partial pressure of the gas to be detected that contacts the outer surface is generated between the electrodes 12 and 13, and is detected by the electromotive force. The oxygen partial pressure (oxygen concentration) of the gas can be detected.

Here, the electrodes 12 and 13 are as follows.
Is provided. First, platinum Pt (noble metal) powder and organic
Solvent and perovskite complex oxide (eg La_0.6
Sr _0.4CoO₃) Constituting each element (for example, La, S
r, Co, etc.) organic compounds (resinate or acetate)
And are mixed to produce an electrode material paste. And said
Inner surface of calcinated zirconia tube 11
Also, the outer electrode 12 and the inner electrode 13 are provided by coating on the outer surface.
It Next, a platinum catalyst layer 14 is formed on the outer electrode 12 film.
Platinum Pt paste is printed and dried in this state.
Be done.

After drying, zirconia tube 1
1, the electrodes 12, 13 and the platinum catalyst layer 14 are co-fired at a temperature of 1400-1500 ° C., and the protective layer 15 of Al ₂ O ₃ —MgO spinel is formed as the outermost layer after the firing, as shown in FIG. An oxygen sensor having such a structure was produced. In addition, platinum Pt
The addition rate of perovskite-based complex oxide is 20 to 50
It was set to about%.

When the organic compound of each element constituting the perovskite complex oxide is mixed in the electrode paste composed of platinum powder and the organic solvent as described above, each element constituting the perovskite complex oxide is pasted. It can be highly dispersed in the interior, and thus the perovskite-based composite oxide produced after sintering can be uniformly attached to platinum.

Here, in the oxygen sensor shown in FIG. 1, the platinum catalyst layer 14 may be omitted, or the platinum catalyst layer 14 may be omitted and platinum P is formed outside the spinel protective layer 15.
A t-supported alumina protective layer 16 may be laminated,
It does not limit the laminated structure. By the way, in the electrodes 12 and 13 after sintering, the perovskite-based composite oxide that is uniformly attached to the platinum surface by the above-described manufacturing method is a composite oxide generally represented as ABO ₃ .
As shown in FIG. 2, if a perovskite-based complex oxide, which is more excellent in low-temperature activity than platinum Pt, is used as an electrode material,
The theoretical electromotive force can be generated up to a relatively low temperature.

FIG. 2 shows the electromotive force of the oxygen sensor when the oxygen partial pressure of the reference gas is 0.21 and the oxygen partial pressure of the gas to be detected is 0.01 for each material used as the electrode material.
In addition to t, La _0.6 as a perovskite-based composite oxide
Sr _0.4 CoO ₃ , La _0.6 Sr _0.4 Co _0.98 Ni _0.02
Three types of O ₃ and La _0.6 Sr _0.4 Co _0.98 Fe _0.02 O ₃ are given as examples, and the characteristics when only platinum or only perovskite complex oxide is used as an electrode are shown.

In FIG. 2, the solid line indicates the above theoretical formula E.
= (RT / 4F) · In (Ps / Pg), the theoretical electromotive force is shown. When an electrode containing only platinum Pt is used, the actual electromotive force against the theoretical electromotive force is shown in the temperature range below about 600 ° C. A decrease in electromotive force occurs. On the other hand, when the perovskite-based composite oxide is used as the electrode material, the lower limit temperature at which the theoretical electromotive force can be generated is lowered, and La _0.6 S
In the case of r _0.4 CoO ₃ , around 300 ° C., La _0.6 Sr _0.4 Co
_{With 0.98} Ni _0.02 O ₃ , near 200 ° C, La _0.6 Sr _0.4 C
In the case of _0.98 Fe _0.02 O ₃ , around 250 ° C. is the lower limit temperature at which the theoretical electromotive force can be obtained.

Therefore, when the perovskite complex oxide shown above is used as the electrode material, the low temperature activity is improved as compared with the oxygen sensor using the platinum electrode, and the low temperature operation can be achieved without using the heater. Become. However, when the oxygen sensor is used in an environment where the atmosphere changes drastically, for example, when measuring the oxygen concentration in the exhaust gas of an internal combustion engine, the perovskite complex oxide used as the electrode material is The electrode function may be lost due to the decomposition of unburned components contained in, and the perovskite complex oxide cannot be used alone as an electrode material.

In order to obtain a low temperature activation effect by using the perovskite complex oxide as an electrode material of an oxygen sensor, an electrode of the perovskite complex oxide exists at the electrode interface of the solid electrolyte (zirconia), and It is necessary to sufficiently equilibrate the gas that reaches the electrode of the perovskite-based composite oxide so as to avoid decomposition of the perovskite-based composite oxide.

Therefore, as described above, a mixed material of platinum Pt, which is a general noble metal as an electrode material, and a perovskite complex oxide is used as an electrode material, and it is equilibrated by the catalytic action of platinum Pt. The gas was made to reach the electrode of the perovskite complex oxide existing at the electrode interface of zirconia. However, since each element (La, Sr, Co, Ni, Fe, etc.) forming the perovskite-based complex oxide is mixed as an organic compound into the electrode paste at the time of forming the electrode, the perovskite-based complex oxide is formed. Each element is highly dispersed in the paste, so that the perovskite-based composite oxide produced by sintering can be evenly attached to platinum. The effect of activity can be maximized.

As the noble metal mixed with the perovskite complex oxide, ruthenium Ru, rhodium Rh and the like contained in the same platinum group may be used in addition to the above platinum Pt.

[0023]

As described above, according to the present invention, the electrode material paste obtained by mixing the noble metal powder, the organic solvent, and the organic compound of each element constituting the perovskite complex oxide is prepared in a calcined state. Since the electrode material paste and the substrate are simultaneously sintered to form the electrode after being applied to the substrate, the perovskite-based complex oxide excellent in low-temperature activity is uniformly attached to the surface of the noble metal. By mixing the perovskite-based composite oxide in the electrode, the expected low-temperature activation effect can be maximized.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the effect of low temperature activation when a perovskite complex oxide is used as an electrode material.

FIG. 3 is a cross-sectional view showing a structural example of a conventional oxygen sensor.

FIG. 4 is a diagrammatic view showing how an electromotive force is reduced at a low temperature in an oxygen sensor using a conventional platinum electrode.

[Explanation of symbols]

 11 Zirconia tube 12, 13 Electrode 14 Platinum catalyst layer 15 Protective layer

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] June 23, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

After drying, zirconia tube 1
1, electrodes 12 and 13, the platinum catalyst layer 14 is fired simultaneous, by forming the protective layer 15 by Al ₂ O ₃ -MgO spinel on the outermost layer after such firing, the oxygen sensor having the structure shown in FIG. 1 Was produced. The addition rate of the perovskite complex oxide to platinum Pt was set to about 20 to 50%.

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI Technical display location C04B 41/90 Z C23C 28/00 B H01B 1/06 A 7244-5G

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 on one surface is in contact with a reference gas and the electrode on the other surface is in contact with a gas to be detected. In an oxygen sensor that generates an electromotive force according to the oxygen partial pressure ratio, an electrode material paste prepared by mixing a noble metal powder, an organic solvent, and an organic compound of each element forming the perovskite-based complex oxide is calcinated. 2. The method for manufacturing an oxygen sensor, comprising: applying the electrode material paste and the substrate at the same time after applying to the substrate to form an electrode.