JPH05307017A - Oxygen sensor - Google Patents

Abstract

PURPOSE:To enhance the low temp. activeness of an oxygen sensor of zirconia type. CONSTITUTION:Electrodes are formed on the inner and outer surfaces of a zirconia tube 11, and an electromotive force generated in compliance with the oxygen partial pressure ratio on the two surfaces is taken out from electrodes 12, 13. The electrodes 12, 13 are made of a material as a mixture of platinum and a complex oxide of perovskyte type (for example, La0.6Sr0.4CoO3).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor, and more particularly to a technique for improving low-temperature activity of an oxygen sensor that 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. 6 (Japanese Patent Laid-Open No. 58-20436).
No. 5, Japanese Utility Model Publication No. 59-31054, etc.). In FIG. 6, 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 extracting 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 on the platinum catalyst layer 4. 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 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.

[0004]

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 carried out, 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, 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. 7, 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 be normally operated. 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,
Moreover, 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 it is an object of the present invention to improve the low temperature activity of electrodes in an oxygen sensor so that the oxygen sensor can be operated at a low temperature without using heater heating. To aim.

[0007]

Therefore, in 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 the electrode on one surface contacted with a reference gas and the gas to be detected are contacted with each other. In the oxygen sensor for generating an electromotive force according to the oxygen partial pressure ratio between the electrode on the other surface and the electrode on the other surface, the electrode was formed of a mixed material of a noble metal and a perovskite complex oxide.

[0008]

With this structure, since the perovskite complex oxide is an electrode material having excellent low-temperature activity, the oxygen sensor can be operated at low temperature. In addition, by using a mixture of the perovskite-based composite oxide and a noble metal, even when used in an environment where the atmosphere changes drastically, the perovskite-based composite oxide present at the electrode interface of the solid electrolyte is It becomes possible to reach a gas that has been equilibrated by the catalytic action, and it is possible to prevent the perovskite-based composite oxide from decomposing due to a change in atmosphere.

[0009]

EXAMPLES Examples of the present invention will be described below. In FIG. 1 showing the first embodiment, electrodes 12 and 13 are provided respectively on respective inner and outer surfaces of a zirconia tube 11 (a base body of an oxygen ion conductive solid electrolyte) whose tip is closed,
A platinum catalyst layer 14 and a spinel protective layer 15 are provided outside the outer electrode 12.
Are stacked to form the element part of the 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 provided as follows. First, platinum Pt (noble metal) powder and perovskite-based composite oxide (for example, La _0.6 Sr _0.4 CoO).
₃ ) Mix with the organic solvent to form a paste. The paste is then calcined into a zirconia tube 11
Applied to the inner and outer surfaces of the outer electrode 12 and inner electrode
Provide 13. Then, a platinum catalyst layer 14 is formed on the outer electrode 12 film.
The platinum Pt paste that forms the film is printed and dried in this state.

When the drying is completed, the 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. Incidentally, the electrode
Platinum Pt and perovskite-based composite oxide constituting 12 and 13 were mixed at a ratio of 1: 1 (perovskite-based composite oxide addition ratio to platinum Pt: 50%).

The second embodiment shown in FIG. 2 shows a sensor structure in which the platinum catalyst layer 14 is omitted in the oxygen sensor shown in FIG. 1, and platinum platinum Pt powder and perovskite complex oxide (for example, La) are used. _0.6 Sr _0.4 CoO ₃ ) is mixed with an organic solvent to form a paste, which is printed on a calcined zirconia tube 11 to form electrodes 12 and 13. Then, set the zirconia tube 11, electrodes 12, 13 to 1400
After co-firing at a temperature of ~ 1500 ° C, Al ₂ O ₃ -MgO
A protective layer 15 made of spinel is formed.

Here, the platinum Pt and the perovskite complex oxide forming the electrodes 12 and 13 are mixed at a mixing ratio of 4: 1 (the addition ratio of the perovskite complex oxide to platinum Pt is 20%). Further, in the oxygen sensor structure shown in FIG. 3, an alumina protective layer 16 supporting platinum Pt is further laminated on the outer side of the spinel protective layer 15 with respect to the oxygen sensor structure shown in FIG.

By the way, in the oxygen sensor structures shown in FIGS. 1 to 3, the perovskite complex oxide mixed with platinum Pt as the material for the electrodes 12 and 13 is a complex oxide generally represented as ABO _3. As shown in FIG. 4, as compared with the case where platinum Pt is used as an electrode material, when a perovskite-based complex oxide having excellent low-temperature activity is used as an electrode material, theoretical electromotive force can be generated up to a relatively low temperature. You can

FIG. 4 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 is used as a perovskite-based composite oxide.
Sr _0.4 CoO ₃ , La _0.6 Sr _0.4 Co _0.98 Ni _0.02
Three _{O 3, La 0.6 Sr 0.4 Co} 0.98 Fe 0.02 O 3 and is cited as an example.

In FIG. 4, the solid line is the theoretical formula E described above.
= (RT / 4F) · In (Ps / Pg), the theoretical electromotive force is shown. When a platinum Pt electrode is used, the actual electromotive force is about the theoretical electromotive force in the temperature range below 600 ° C. 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, resulting in La _0.6 Sr _0.4.
For CoO ₃ , near 300 ° C, La _0.6 Sr _0.4 Co _0.98 N
i _0.02 O ₃ around 200 ° C., La _0.6 Sr _0.4 Co _0.98
For Fe _0.02 O ₃ , the temperature around 250 ° C. is the lower limit temperature at which the theoretical electromotive force can be obtained.

Therefore, when the above-described perovskite complex oxide 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, such as 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 decomposition due to the arrival 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-based composite oxide as an electrode material for an oxygen sensor, an electrode of the perovskite-based composite oxide exists at the electrode interface of the solid electrolyte (zirconia), and It is necessary to sufficiently equilibrate the gas reaching 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 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. Here, FIG. 5 shows the relationship between the mixing ratio with respect to platinum Pt and the lower limit temperature at which theoretical electromotive force can be generated when La _0.6 Sr _0.4 CoO ₃ is used as the perovskite complex oxide. As shown in this figure, as the mixing ratio of the perovskite-based composite oxide to platinum Pt is increased from 0%, the lower limit temperature at which theoretical electromotive force can be generated decreases, and the electrode containing only platinum is used. It is possible to operate the oxygen sensor from a lower temperature than when it was used.

Therefore, in the oxygen sensor structure as shown in FIGS. 1 to 3, if a mixed material of platinum and perovskite complex oxide is used as the electrode material, the temperature is lower than that of the oxygen sensor using the platinum electrode. It can be operated from time to time, and the heater for activating the sensor element can be eliminated. By the way, when the mixing ratio of the perovskite-based composite oxide to platinum Pt exceeds 50%, the lowering rate of the lower limit temperature becomes slow, and it is assumed that the limit temperature is around 350 ° C.

Therefore, even if the perovskite-based composite oxide is mixed in a ratio of more than 50% with respect to platinum Pt, no remarkable effect is obtained in terms of low temperature activity.
Rather, for the purpose of avoiding the decomposition of the perovskite-based composite oxide due to a change in atmosphere, it is assumed that the mixing ratio of the perovskite-based composite oxide to platinum Pt is preferably suppressed to 50% or less.

A conventional platinum electrode as shown in FIG. 1 or 2 was used as an oxygen sensor structure having an electrode formed of a mixed material of platinum and perovskite complex oxide as described above. In addition to being able to apply the sensor structure as it is, by providing a protective layer 16 supporting platinum Pt on the outermost layer as shown in FIG. 3, a gas equilibrated with respect to the perovskite complex oxide forming the electrode can be obtained. The effect of reaching can be obtained more stably.

Further, 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.

[0025]

As described above, according to the present invention, in the oxygen sensor for generating an electromotive force according to the oxygen partial pressure ratio by the electrodes formed on the inner and outer surfaces of the substrate made of the oxygen ion conductive solid electrolyte, By forming the electrode with a mixed material of a noble metal and a perovskite-based composite oxide, low-temperature activity is improved, yet decomposition of the perovskite-based composite oxide can be avoided, and stable low-temperature operation can be achieved. The effect is that you will be able to.

[Brief description of drawings]

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

FIG. 2 is a sectional view showing a second embodiment of the oxygen sensor structure.

FIG. 3 is a sectional view showing a third embodiment of the oxygen sensor structure.

FIG. 4 is a diagram showing the effect of low temperature activation when a perovskite complex oxide is used as an electrode material.

FIG. 5 is a diagram showing a correlation between a mixing ratio of a perovskite complex oxide to platinum and a low temperature activation effect.

FIG. 6 is a sectional view showing a structural example of a conventional oxygen sensor.

FIG. 7 is a diagram 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

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[Procedure amendment]

[Submission date] June 23, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

When the drying is completed, the 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 platinum Pt and the perovskite complex oxide forming the electrodes 12 and 13 were mixed at a mixing ratio of 1: 1 (the addition ratio of the perovskite complex oxide to platinum Pt was 50%).

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

A second embodiment shown in FIG. 2 is shown in FIG.
In the oxygen sensor, the platinum catalyst layer 14 is omitted.
It shows a structure of platinum, Pt powder and perovskite
Based complex oxides (eg La_0.6Sr_0.4CoO₃)
It is mixed with machine solvent to form a paste, which is calcined
Printed on zirconia tube 11 to form electrodes 12 and 13
It Then, the zirconia tube 11, the electrodes 12, 13The sameTime
After firing, Al₂O ₃-MgO spinel protective layer
Forming fifteen.

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. In the oxygen sensor that generates an electromotive force according to the oxygen partial pressure ratio, the electrode is formed of a mixed material of a noble metal and a perovskite complex oxide.