JPS61243354A - Oxygen sensor - Google Patents

Abstract

PURPOSE:To enable a prolonged use without lowering the response, by building up a second electrode with a platinum alloy layer containing either or both of rhodium and palladium to inhibit coagulation of the second electrode by heat. CONSTITUTION:A solid electrolyte 1 is formed in a tube with the detection end thereof closed and first and second electrodes 21 and 22 are formed on both surfaces thereof. The second electrode 22 is built up with a paste mixture of 0.1-50wt% of either or both of rhodium and palladium and platinum applied and baked on the surface of the solid electrolyte 1 and the first electrode 21 with a layer containing platinum. The oxygen sensor thus obtained is held with a holder 4, a spring 6 and a mounting flange 7 and transmits the electrical signal detected outside through a lead 5 to detect the concentration of oxygen in an emission gas. With such an arrangement, either or both of rhodium and palladium can inhibit the coagulation by the heat of platinum thereby improving the durability.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an oxygen sensor for detecting the oxygen concentration in exhaust gas discharged from an internal combustion engine such as an automobile.

[Conventional technology]

Currently, in order to purify exhaust gas from automobile engines, etc.
The oxygen concentration in the exhaust gas is measured to maintain the composition of the exhaust gas within a certain range.

This system feeds back the oxygen concentration measurement results to the fuel injection system to control the fuel injection amount and create an exhaust gas composition with a high purification rate. An oxygen sensor is used to measure this oxygen concentration.

Conventionally, zirconia (Zr02) has been used as an oxygen sensor.
The base material is a solid electrolyte such as, electrodes made of platinum etc. are provided on both surfaces of the base material, and a ceramic porous material made of magnesia alumina (MgO・A40s) spinel etc. is provided on the surface of the electrode that comes into contact with oxygen gas. Some have a protective layer.

A problem has arisen in these conventional oxygen sensors that the detection function deteriorates during use. The reason for this is that when the electrode (referred to as the second electrode) used as the air reference electrode of the oxygen sensor is excessively heated, the electrode aggregates due to the heat and loses electrical conductivity.

This is because the second electrode is not exposed to exhaust gas, and the first electrode (the electrode facing the gas to be measured in the oxygen sensor)
It is made of white metal formed by plating, sputtering or pasting.

This is because heat resistance is not sufficient. In particular, when the temperature of the exhaust gas from the engine is significantly off when driving at high speeds, or when an air reference electrode side (C heater is inserted) is used to volatilize and remove poisonous substances adhering to the surface of the sensor. Since the second electrode is exposed to high temperatures, its detection function as a sensor deteriorates over time.

[Problems that the invention attempts to correct]

This invention! d, J: An oxygen sensor that solves the problems of the conventional methods, suppresses aggregation of the second electrode due to heat, and can be used for a long period of time without deteriorating functions such as responsiveness as a sensor. It provides:

[Means for solving problems]

The oxygen sensor of the present invention is an oxygen sensor in which a first electrode is provided on the side of a solid electrolyte body facing a gas to be measured, and a second electrode is provided on the opposite side of the first electrode. , consisting of a layer containing platinum.

The stone 2 electrode is characterized by being made of a platinum alloy layer containing one or both of rhodium and palladium.

Hereinafter, the present invention will be explained using one drawing. FIG. 1 is an overall schematic sectional view showing an example of an oxygen sensor according to the present invention, and FIG. 2 is an enlarged sectional view of portion A in FIG. 1. Note that the structure of the oxygen sensor of the present invention is not limited to this.

In the present invention, the solid electrolyte body 1 is made of materials commonly used as solid electrolyte bodies for oxygen sensors, such as zirconia, thoria, ceria, ceria-lanthania, ceria-thria-wontania, and bismuth oxide. Further, preferably, yttrium (Y) is added to zirconia (Zr02).
), calclum (Ca), magnesium (Mg), ytterbium (Yb), cerium (Ce), scandium (SC).

A material to which an oxide such as lanthanum (La) or strontium <sr> is added is used.

This solid electrolyte body 2 is formed into a tubular shape with a closed detection end, for example, as shown in FIG. 1, and electrodes 2 are formed on both surfaces of the solid electrolyte body 1. That is, the first electrode 21 is placed on the side of the solid electrolyte body 1 facing the gas to be measured. [A second electrode 22 is formed on the opposite side of the pole 21.]

Further, the second electrode 22 is made of a platinum alloy layer containing one or both of rhodium and palladium.

By including one or both of rhodium and palladium in this platinum alloy, the heat resistance of the second electrode is improved and agglomeration of the electrode can be prevented. The blending amount of one or both of rhodium and palladium in the platinum alloy of the second electrode is preferably within the range of 1.1 to 50% by weight based on the total platinum public metal.

If the blending amount is less than 11% by weight, there is a risk that the agglomeration suppressing effect of the electrode will be reduced; on the other hand, if it exceeds 50% by weight, the effect commensurate with the blending amount may not be obtained, and the cost may increase. It gets expensive.

Note that the thickness of the platinum alloy layer 22 of the second electrode is α1 to 2μ.
It is desirable that it be within the range of In. When llL is less than 1pm.

There is a risk that the heat resistance of the electrode will decrease, and if it is larger than 2 μm, the heat resistance will not be improved to the extent that the thickness of one layer is increased, and the cost will increase.

The second electrode can be formed by applying a paste of a mixture of rhodium or palladium, or both, and platinum to the surface of the solid electrolyte body 1 opposite to the first electrode, and baking it; There is a method of forming one or both of these and platinum by sputtering, vapor deposition, plating, etc.

The first electrode 21 is made of a layer containing platinum, such as a platinum layer, or a platinum alloy layer containing one or both of rhodium and palladium similar to the second electrode.

In the case of a platinum layer, the formation method is the same as in the conventional method, and in the case of a platinum-plated gold layer similar to the second electrode, it is formed in the same manner as the second electrode. Also.

The thickness of the layer containing platinum of the first electrode is preferably within the range of α1 to 2 μrn. If it is less than 0.1 μtn, the heat resistance of the electrode may decrease, and if it is greater than 2 μtn, the responsiveness as a single sensor will decrease.

In addition, when the first electrode is a platinum layer, in order to improve the poisoning resistance, there is a patent application filed earlier by the present applicant in 1982-153.
Like the invention described in No. 775.

A layer made of one or both of rhodium and palladium may be formed on the surface of the first electrode 21.

Furthermore, a protective layer 5 may be formed on the surface of the first electrode 21 to prevent the attachment of combustion products in the exhaust gas. The material of the protective layer is magnesia alumina (Mg0
-kl 10s) spinel, alumina (h4 Ug
) etc.

As described above, the oxygen sensor according to the present invention □ is manufactured, and as shown in FIG. 1, it is held by the holder 4,
It is used in the exhaust gas discharged from automatic gear 1 etc. The pneumatic signal detected by the oxygen sensor is connected to lead 5
This signal is sent to the outside to detect the oxygen concentration in the exhaust gas. Also in Figure 1.

6 indicates a spring, and 7 indicates an attachment 7 lunge.

〔Effect of the invention〕

According to the present invention, it is possible to prevent agglomeration of the air reference electrode (second electrode) due to heat and provide an oxygen sensor with excellent durability.

This effect is achieved because the second electrode of the oxygen sensor according to the present invention is made of a platinum alloy layer containing one or both of rhodium and palladium, and one layer of rhodium or palladium is coated with heat of platinum. This is thought to be due to the suppression of a-nucleus. Also, the oxygen sensor of the present invention is as follows.

The responsiveness is stable, the oxygen concentration detection function does not deteriorate even after long-term use, and it has excellent durability.

〔Example〕

Experimental examples and examples of the F1 invention will be explained below.

Experimental Example 1 A second polarizing metal layer (sample) with a thickness of 4000A made of platinum (Pt)-10% by weight (hereinafter referred to as %) rhodium (ah) alloy was deposited on a zirconia substrate as a solid electrolyte by sputtering. Point 1) was formed.

For comparison, a metal layer for the second pin electrode (sample point C1) made of only Pt and having a thickness of 4000 Å was formed by sputtering on a zirconia substrate in the same manner as above.

At the same time, a heat resistance test was conducted on the above two types of metal layers to examine their cohesiveness due to heat. That is, the metal layer of the disaccharide was heated in air at a temperature of 800'C for 1 hour and 10 hours.

After the heating test, the surfaces of the two types of metal layers were observed using an 8EM (scanning electron microscope). 8EM photographs of the metallographic structure of the surface of the metal layer are shown in FIGS. 5 and 6 (all magnifications are 2000x).

The samples with a heating time of 1 hour are shown in Figure 6 (sample point 1) and @Figure 4 (sample AC1), and the samples with a heating time of 10 hours are shown in Figure 5 (sample A I) and Figure 6 ( Sample point 01
).

As can be seen from FIGS. 4 and 6, in sample C1, agglomeration of Pt progressed and large holes were formed between the electrodes, and there were also places where the electrodes were interrupted. On the other hand, as shown in Figures 3 and 5, in sample &1, there were no large holes or breaks in the electrode, maintaining good conductivity and fully functioning as an electrode. 1. It can be seen that the oxygen sensor according to the present invention has excellent agglomeration resistance.

In addition, the metal layer of the above two oaks was heated at 800'C in air.

A heat resistance test was conducted by heating at temperatures of 900°C and 1000°C for 1 hour, and the cohesiveness due to heat was investigated.

The results are shown in Table 1.

Table 1 Experimental Example 1 A 21st sample of 2000A thick made of platinum (Pt)-10% palladium (Pd) alloy by sputtering on a zirconia substrate as a solid electrolyte body! Electrode metal layer (sample/fE
L2) was formed.

This electrode metal layer was heated in air at 800° C. for 10 hours to conduct a heat resistance test. After the test, the surface of the metal layer was observed using 8EM. A photograph of the metallographic structure of the surface of the metal layer taken by 8EM is shown in FIG. 7 (magnification: 2000 times).

As is clear from FIG. 7, no agglomeration of the metal layer was observed in the electrode metal layer according to the present invention, as in sample AO1 shown in FIG. 6, indicating that the metal layer has excellent aggregation resistance.

Example 1゜ This embodiment will be explained based on FIGS. 1 and 2.

Zirconia as a solid electrolyte body has a diameter of approximately 7 ff.

The first plating method consists of forming a hollow tube 1 with a length of 30 mm and a thickness of 11 rIl, and forming a 1 μm Pt layer by plating on the flllll (1 ml outside the hollow tube) of the zirconia tube 1 facing the gas to be measured. A second electrode 22 made of platinum/rhodium alloy (10% rhodium) and having a thickness of 4000 Å was formed.

First, the surface of the first electrode 21 is coated with MgO.Al.

A protective layer 6 made of 0.03 spinel and having a thickness of 100 .mu.III was formed by plasma spraying to produce two oxygen sensors according to the present invention (sample 1 and 3).

Further, for comparison, a comparative oxygen sensor (sample 502) was manufactured in the same manner as above except that an electrode having a thickness of 4000 Å made only of platinum was formed as the second electrode 22.

The above two types of oxygen sensors are held by a holder 4,
It was connected to lead wire 5 and tested for durability as a sensor.

The durability test was conducted using unleaded gasoline with a displacement of 200
Attach the above oxygen sensor to the exhaust pipe of an engine running at 0cc, 6 cylinders, 9 revolutions at 4000 rpm, and set the exhaust gas temperature to 900°CK. ) was measured. To measure the response time (rich-lean), take out the oxygen sensor from the exhaust pipe and set the temperature at the tip of the sensor to 4.
Set at 50'C, excess air ratio α9 using model gas
This was done by measuring the response time (rich-lean) of the output voltage change when switching from (reducing atmosphere) to 1.1 (oxidizing atmosphere).

The results are shown in Table 2. In addition, it is the response time (rich-lean) that can actually be put into practical use as an oxygen sensor.
It is necessary that the value of is 260 m9ec or less.

Table 2 As is clear from Table 2, the oxygen sensor according to the present invention exhibits better response than the comparative example, has less variation in response time, and has excellent durability. You can see that

Example 2 The second electrode 22 was made of Pt with a thickness of 1 μm by plating.
An oxygen sensor (sample 4) according to the present invention was manufactured in the same manner as in Example 1 except that a -5% Rh alloy layer was formed.

For comparison, a layer made of only Pt with a thickness of 1 μm was formed as the second electrode in the same manner as above.

A comparison oxygen sensor (sample AO3) 1c was manufactured.

A durability test was conducted using the above two types of oxygen sensors. In this durability test, the oxygen sensor was placed in an electric furnace, heated in air at 900°C for an SOO period, and then
Example 1: Oxygen sensor response time (rich-lean)
Measurements were made in the same manner.

The results are shown in Table 3. As is clear from Table 6, 2
It can be seen that the oxygen sensor of the present invention has excellent one-time durability compared to the comparative example.

Table 5

[Brief explanation of drawings]

Figure 1 is a schematic cross-sectional view showing an embodiment of the oxygen sensor of the present invention, Figure 2 is an enlarged cross-sectional view of part A in Figure 1, and Figures 5, 5, and 7 are experiments of the present invention. FIGS. 4 and 6 are 8EM photographs showing the metal structure of the electrode surface after the heat resistance test in Examples 1 and 2. It is an 8EM photograph showing the metal structure of the electrode surface after a heat resistance test in a comparative example. DESCRIPTION OF SYMBOLS 1... Solid electrolyte body, 2... Electrode 21... First electrode, 22... Second electrode 6... Protective layer FIG.

Claims (2)

[Claims] (1) In an oxygen sensor in which a first electrode is provided on the side of a solid electrolyte body facing the gas to be measured, and a second electrode is provided on the opposite side of the first electrode, the first electrode is made of a layer containing platinum. An oxygen sensor characterized in that the second electrode is made of a platinum alloy layer containing one or both of rhodium and palladium. (2) The oxygen sensor according to claim (1), wherein the platinum alloy metal containing one or both of rhodium and palladium has a thickness of 0.1 to 2 μm.