JPH01232252A - Oxygen sensor element - Google Patents

Abstract

PURPOSE:To improve durability and to stably maintain exact air-fuel ratio control over a long period of time by constituting the sensor in such a manner that the metal oxide powder forming the 1st protective layer of a measuring electrode has 5-100mum average grain size and the average grain size of the metal oxide powder forming the 2nd protective layer is smaller than the average grain size of the 1st protective layer. CONSTITUTION:The oxygen sensor element is provided with a reference electrode 2 on one face side of a solid electrolyte 3 and the measuring electrode 4 on the other face side and brings the electrode 4 into contact with a gas to be measured. The electrode 4 is coated with the 1st protective layer 9 and the 2nd protective layer 10 formed by calcining the metal oxide powder which is chemically stable to the gas to be measured. This 1st protective layer 9 is formed by calcining the metal oxide powder having 5-100mum average powder grain size. The 2nd protective layer 10 provided on the outside of the 1st protective layer 9 is formed by calcining the metal oxide which is smaller in the average powder grain size than the material of the 1st protective layer. The stresses generated in the electrodes are dispersed first by the 1st protective layer 9 and further by the 2nd protective layer 10, by which the exfoliation of the electrodes is prevented as far as possible.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an oxygen sensor element for detecting oxygen concentration in various combustion equipment, particularly for air-fuel ratio control used to purify exhaust gas from an internal combustion engine. The present invention relates to an oxygen sensor element and a manufacturing method thereof.

[Prior art and problems] An oxygen sensor element for air-fuel ratio control consists of an oxygen ion-conducting solid electrolyte body and a pair of electrodes (reference electrode, measurement electrode) provided on the inner and outer surfaces of the solid electrolyte body. gas(
The most common type is one that is brought into contact with (exhaust gas).

However, when using this type of sensor element, the measurement electrode is exposed to high temperature exhaust gas and separates, so various studies and proposals have been made to prevent this separation and improve durability. being done.

For example, an oxygen sensor element in which the thickness of the measuring electrode itself is partially increased has been proposed (Japanese Patent Laid-Open No. 54-970).
89). However, in this type of oxygen sensor element,
The measurement electrode is formed by plating or vapor deposition, with a thickness of 2 to 3 μm.
Although it is effective when the thickness is below, it is no longer possible to prevent the electrode from peeling off when the film becomes thicker than 5 μm as in the case of paste printing.

In addition, an oxygen sensor element in which a porous membrane or a porous layer is interposed between a solid electrolyte body and a measuring electrode has been proposed (Japanese Patent Application Laid-open No. 53-12392 and No. 53-29187).
). However, in this type of oxygen sensor element, the mechanical strength of the solid electrolyte body may be deteriorated. or,
A portion of the measurement electrode exists within the pores of the porous membrane (layer), but during use, cracks may occur in the porous membrane due to electrode deterioration, i.e., volumetric expansion of the electrode due to adsorption or reaction of unburned components. Easy to do.

The present invention aims to solve this problem, that is, even in a high temperature and reducing atmosphere of unburned components (CO, etc.), the /111 constant electrode can be sufficiently protected from peeling.

The purpose of this project is to develop an oxygen sensor element that has excellent durability and can maintain accurate air-fuel ratio control stably for a long period of time.

[Means for solving the problem] As a result of intensive research in view of these points of view, the present inventor has found that
When covered with a single protective layer, the electrode itself is protected by volumetric expansion and miniaturization during the process of deterioration and wear and tear due to exposure to the gas to be measured (exhaust gas). It was discovered that the layer is pushed up and cracks occur, and if this is left untreated, electrode deterioration progresses at an accelerated rate.

Therefore.

As a result of historical research, we found extremely excellent results when we protected the electrode with multiple layers of different particle sizes, which led us to complete the present invention.The present invention solves the above-mentioned problems by the following means. .

(1) Based on one side of the solid electrolyte body? In an oxygen sensor element comprising a $ electrode and a measuring electrode on the other side, the measuring electrode is brought into contact with a gas to be measured.

The measurement electrode is coated with a plurality of protective layers formed by baking metal oxide powder that is chemically stable against the gas to be measured, and the number of protective layers is 71111 from the constant electrode side to the first protective layer, and from the second protective layer to the constant electrode side. Become.

The metal oxide powder of the first protective layer has an average particle size of 5 to 100 μm.
and the average particle size of the metal oxide powder in the second protective layer is smaller than that in the first protective layer.

Oxygen sensor element.

(2) An oxygen sensor probe that includes a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side, and the measurement electrode is brought into contact with a gas to be measured.

The measurement electrode is coated with a plurality of protective layers formed by baking metal oxide powder that is chemically stable against the gas to be measured, and the plurality of protective layers are arranged from the measurement electrode side to the base protective layer and the first and second protective layers. Consists of layers.

The metal oxide powder of the first protective layer located between the base protective layer and the second protective layer has an average particle size of 5 to 100 μl, and the average of the metal oxide powders of the base protective layer and the second protective layer The particle size is smaller than that of the first protective layer.

Oxygen sensor element.

[Preferred embodiments and operations] The shape of the element or the solid electrolyte body may be various shapes such as a two-bag shape, a plate shape, or a tube shape, as long as the front end is closed and the rear end is open. Each element of the element, such as a solid electrolyte body, may be combined to form the same shape as described above. Examples of solid electrolyte materials include ZrO3
It is preferable to use a material to which YO, CaO, etc. are added as a stabilizer. The reference electrode and the measurement electrode (layered) are both porous and made of Pt or Pt containing about 2% or less Rh.
It is recommended to use precious metals such as

The measuring electrode must be covered with a plurality of protective layers made of fired metal oxide powder that is chemically stable with respect to the gas to be measured. This is to prevent the measurement electrode from being directly exposed to the gas to be measured, reliably prevent peeling of the electrode, and improve durability. 2 In the present invention, a two-layer structure (the above ((
The target is a protective layer with a structure (1)) or a three-layer structure (structure (2) above), but more layers may be added as appropriate, and other elements such as a heater may be interposed between each layer. This does not exclude that.

Regarding the plurality of protective layers, at least the first one from the measuring electrode side.
A second protective layer must be deposited.

The first protective layer must be formed by firing a material with an average powder particle size of 5 to 100 μm. This is because the uneven portion exhibits an uneven state after firing, and this uneven portion exhibits a wedge function and contributes to increasing the adhesion between the electrode and the protective layer (including the first and second protective layers). The thickness is preferably 20 to 70 μm.

Outside the first protective layer there must be a second protective layer fired from a material having a smaller average powder particle size than the first protective layer material. This is to bond the protective layer to the electrode with a greater adhesive force than the stress generated by volumetric expansion due to electrode deterioration during use, ie, the adhesion of unburned components. That is.

The stress generated in the electrode is first applied to the first protective layer, and then this second protective layer.
It is dispersed by the protective layer, and peeling of the electrode can be prevented as much as possible.

The average powder particle size is 5 to [1 Qu3. Preferably 5-3
It is best to set it to 0 μ■.

In the configuration (2) above, the base protective layer is present inside the first protective layer. The base protective layer must also have a smaller average powder particle size than the first protective layer material. This is to increase the bonding force between the electrode and the protective layer (1.2 protective layer) by making it denser than the second protective layer. 0.5
~5. The thickness is preferably 1.5 to 2.5 μm.

Each protective layer may be made of a fired metal oxide that is thermally stable with respect to the gas to be measured, and preferably a ceramic made of fired alumina, spinel, zirconia, or a mixture thereof. It is preferable that the material of each layer is basically different. This is because it is easy to control the particle size of each powder within a specific range as described above.

However, materials of the same quality may be used as long as the particle size can be controlled. For example, the first protective layer may be calcined alumina, the second protective layer may be spinel, and the basic protective layer may be alumina.

In order to effectively exert the function of each of the above-mentioned protective layers and to prevent deterioration of the permeability and response of the gas to be measured, the thickness of each protective layer should be 20 to 150 μm for the first protective layer and 20 to 150 μm for the second protective layer. It is preferable that the thickness of the layer is 40 to 150 μm and the base protective layer is 10 to 30 μm. Further, from the same viewpoint, the porosity of the first protective layer is preferably 20 to 60%, the second protective layer is 5 to 40%, and the base protective layer is 20 to 90% of the first protective layer. In addition, at least one type of each protective layer contains unburned components (C) in the gas to be measured.
A catalyst that promotes the oxidation or reduction reaction of (o, etc.) may be supported as appropriate.

The manufacturing of the oxygen sensor element is as follows.

In the case of plate-shaped or cylindrical elements, laminated printing techniques may be used. What is laminated printing technology?

This is a technique in which each component of an oxygen sensor cord is laminated and printed on a predetermined green sheet, the printed green sheet is attached to a base material, and the printed green sheet is baked and integrated.

However, the outermost layer directly exposed to the gas to be measured may be formed separately by thermal spraying, especially plasma spraying. This is because the adhesion strength between the thermal spray powders is strong and the durability is excellent. In the case of a single bag-shaped element, it is preferable to perform the stepwise deposition of each element. In this case, the '7IS electrode may be formed by a conventional plating process such as electroplating or chemical plating, or by a conventional vapor deposition method such as sputtering, vapor deposition, or screen printing. Formation of the protective layer includes methods of applying a solution or powder of the material by brushing, dipping, spraying, etc., followed by baking, and thermal spraying. Firing is preferably carried out at a temperature of 1400 to 1550°C.

Any of normal pressure, pressure sintering, and atmospheric pressure may be used.

In addition, when supporting a catalyst on the protective layer, it is preferable to carry out a immersion treatment in a noble metal salt solution, followed by drying and baking. may also be used.

Further, the oxygen sensor element may be manufactured by a so-called laminated printing method, in addition to a method in which each component is deposited and formed in stages. What is laminated printing technology?

Each component of the oxygen sensor element is laminated and printed on a predetermined green sheet, and this printed green sheet is used as a base material.
This refers to a technique of integrating firing and firing (see, for example, Japanese Patent Application Laid-Open No. 82-222159).

〔Example〕

An embodiment of the present invention will be described below based on the drawings.

FIGS. 1 to 3 show an embodiment of the structure of the means (2), and show a cylindrical oxygen sensor cord A obtained by laminated printing technology. still.

This example is an example equipped with a heater for heating the element body.

In FIG. 1, from below, base material 1. Reference electrode layer 2. Solid electrolyte layer (green sheet) 3° measurement electrode layer 4. Base protective layer 5. Insulating layer 61 heating element layer 7, insulating layer 8. The first protective layer 9 and the second protective layer 10 are located, and the layers 2 to 10 laminated and printed on the base material 1 are wound and baked to be integrated. The base material 1 is made of ceramic and is a hollow cylindrical body with an outer diameter of 3.2 mm and an inner diameter of 1.5 mm with one end closed. The reference electrode layer 2 and the measurement electrode layer 4 are each made of PT and have a thickness of 10
It is μm. The solid electrolyte layer 3 is YO-containing ZrO2
It is a green sheet with a thickness of 0.2 to 0.5 mm. The base protective layer 5 is made of Pt/A (203,
The thickness is 20μI. The insulating layers 6 and 8 are both dense A
i 203 and has a thickness of 20 to 30 μm. The heating element layer 7 is made of PT and has a thickness of 10 μm. Furthermore, 1
1 is a terminal for connecting the reference electrode to the outside, la...
is a connection port for bringing the reference gas into contact with the reference electrode layer 2;
3a is a conduction port for connecting the reference electrode layer 2 and the reference electrode terminal 11 located outside the solid electrolyte body 3; 6a, 6;
a, 8a, and 8a indicate openings formed in the insulating layers 6, 8 corresponding to the positions of the base protective layer 5.

The first protective layer 9 is made of A(203) with a powder particle size of 50 μm, has a thickness of 80 μm, and has a porosity of 40%.
is made of spinel with a powder particle size of 60 μm and has a thickness of 100 μm.
μI, porosity 15%. The base protective layer 5 has a powder particle size of 3
um A, made of e203 (Pt supported), thickness 20
μI, porosity 15%.

After the lamination and integration, the openings 5a and 6a.

A base protective layer 5 is provided on the outside of the measurement electrode layer 4 (the upper side in FIG. 3, the side where the gas to be measured enters) for the portion corresponding to ga, 8a.
．． First protective layer9. A second protective layer 10 is laminated (FIG. 2). Therefore, the measurement electrode layer 4 is connected to each protective layer 5, 9.1.
0 and firmly adheres.

Based on the reaction with unburned gas in the gas to be measured during use, the stress generated in the constant electrode layer 4 is determined based on the stress generated in the constant electrode layer 4. distributed through the second protective layer. Therefore, the measurement electrode layer 4 can stably maintain its original state for a long period of time.

FIG. 4 shows an example of an oxygen sensor element (without a base protective layer) according to the means (1) of the present invention as another embodiment. Therefore, the measurement electrode layer 4 is the first. It firmly adheres to the second protective layer and the generated stress is dispersed, thereby providing reliable protection. Also, Figure 5 shows a single protective layer (
This corresponds to the base protective layer, and is the first one. This figure shows a conventional example in which the measurement electrode layer 4 is coated with a layer (without a second protective layer). Since the other configurations are the same as those of the previous embodiment, the same elements are given the same reference numerals and their explanations will be omitted.

[Test Example] Based on the oxygen sensor element of the above example, the following tests were conducted to examine each evaluation item. Comparative examples were also examined in the same manner.

The oxygen sensor element was subjected to a durability test using a Bunsen burner.

That is, 119 parts (tips) of each oxygen sensor element were heated to 700 to 850°C under incomplete combustion with almost no air introduced, and were made to last for 50,011 rs.

Evaluation item A: An oxygen sensor equipped with the above-mentioned heated oxygen sensor element is attached to a combustion tube (inner diameter width 43), and a burner flame is blown from a position separated by 1 mm to evaluate sensor responsiveness.

Evaluation item B: Similarly, after heating, the oxygen sensor is installed at a predetermined position on the actual engine vehicle, and the sensor is controlled to determine the temperature of λ located further downstream.
Check the scan value (control A/F average value) and evaluate the λ characteristic.

Evaluation item C: Evaluate the condition of the element surface by visual inspection.

These results are shown in the table below.

As is clear from the table of non-comparative examples, the oxygen sensor element according to the example has a higher score of A in each evaluation item than that of the comparative example.

Excellent results are shown for B and C. especially.

In the example, even when the measurement electrode layer 4 was formed with a thickness of 5 μm or more by thick film printing using paste, no change was observed and the original response (λ characteristic) and conductivity were maintained. It has excellent durability.

[Effects] As described above, according to the present invention, the first effect can be achieved even under conditions of exposure to high temperatures. Due to the presence of the second protective layer (and the third protective layer), the measuring electrode can be stably protected for a long time, and therefore, we have succeeded in providing an oxygen sensor element with a significantly improved durability life. It is extremely useful.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing each element of an oxygen sensor cord (and heater) according to an embodiment of the present invention. FIG. 2 is a radial cross-sectional view of the oxygen sensor element of the above example (regarding the area where the electrode is present). FIG. 3 is an axial cross-sectional view of the oxygen sensor element of the above example (regarding the area where the electrode is present). FIG. 4 is an axial cross-sectional view of an oxygen sensor element of another example (
3) and FIG. 5 are axial cross-sectional views of the conventional oxygen sensor element (parts corresponding to
(corresponding part in the figure). are shown respectively. Figure 1 Figure 2 Figure 3

Claims (2)

[Claims] (1) In an oxygen sensor element that has a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side, and the measurement electrode is brought into contact with a gas to be measured, the measurement electrode is chemically connected to the gas to be measured. It is coated with a plurality of protective layers made by firing stable metal oxide powder, and the plurality of protective layers consist of a first and a second protective layer from the measuring electrode side, and the metal oxide powder of the first protective layer has an average particle size. Diameter 5-100μm
and the average particle size of the metal oxide powder in the second protective layer is smaller than that in the first protective layer. (2) In an oxygen sensor element that has a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side, and the measurement electrode is brought into contact with a gas to be measured, the measurement electrode is chemically connected to the gas to be measured. It is coated with a plurality of protective layers formed by firing stable metal oxide powder, and the plurality of protective layers are composed of a base protective layer and first and second protective layers from the measuring electrode side, and a base protective layer and a second protective layer. The metal oxide powder of the first protective layer located between the two has an average particle size of 5 to 100 μm, and the average particle size of the metal oxide powder of the base protective layer and the second protective layer is that of the first protective layer. The oxygen sensor element is smaller than the oxygen sensor element.