JPH0425754A - Oxygen concentration sensor - Google Patents

Abstract

PURPOSE:To obtain the sensor which is free from deterioration of the initial characteristics and durability and has stable characteristics by constituting the protective layer of an electrode of a multilayer structure which becomes a layer of small particle size as it is provided toward the inside. CONSTITUTION:The protective layers 24, 25, 26 having the same thickness are provided in order on the outside electrode 23 consisting of a platinum catalytic layer in a zirconia tube 22. Particle size is made smaller as the layer is provided in the inside. Therefore a thermally-sprayed low material is comparted into three classes having prescribed particle size by a classifier. The outside electrode is coated with the protective layers in order from the first layer 24 having small particle size by plasma coating. Since the third layer 26 has large particle size, gas and impurities are passed through it. Gas is properly squeezed by the first layer 24 having small particle size and the impurities are not trapped. Thereby the initial characteristics and durability are stabilized.

Description

[Detailed Description of the Invention] [Purpose of the Invention (Industrial Application Field) This invention is directed to the use of oxygen (
02) Regarding concentration sensor.

(Prior art) Oxygen concentration sensors are used to purify the exhaust gas of automobile engines. They generate an electromotive force based on the difference in oxygen concentration between the atmosphere and the exhaust gas, and use this signal to adjust the air-fuel ratio to the proper level to clean the exhaust gas. It is well known that - boat fishing is effective for purification.

The structure of this oxygen concentration sensor will be explained with reference to FIG.
A platinum thin film is coated on the inside of the tube body 1 of a zirconium (ZrO2) ceramic crucible by vacuum deposition or electroless plating to form an electrode 2 which also serves as a catalyst layer, and an electrode 3 is formed on the outside as well. These constitute the tube 4. Of these, it is common practice to provide a protective layer on the outer electrode (catalyst layer) 3 on the exhaust side to cover it. The tube 4 and housing 5 are combined to form an oxygen sensor 6.

A protector 8 having a slit 7 surrounding the chew 4 is integrally attached to the housing 5.

This protector 8 is located inside the exhaust pipe 9, that is, in the exhaust area. 10 is a metal plate 11 is a connecting pipe, 12 is an output terminal, 1
3 is an insulating plate 14 which is a distance piece, and 15 is a cover. Air with high oxygen concentration enters the inside of tube 4 as shown by arrow A from the outside of exhaust pipe 9 (right side in the figure), and exhaust gas with low oxygen concentration enters outside of tube 4 as shown by arrow E. When oxygen passes through the tube wall of the tube body through the platinum electrodes 2 and 3, which have an appropriate catalytic effect (semi-catalytic effect), based on the inner and outer oxygen concentration (partial pressure difference), the inner and outer electrodes 2 and 3 (electrode 3 is grounded via the housing 5), and a voltage signal is output from the output terminal 12.

The air-fuel ratio of the engine is controlled to an appropriate value by this signal, so that the exhaust purification device functions optimally.

(Problem to be Solved by the Invention) By the way, the protective layer covering the platinum catalyst layer on the surface of the zirconia tube has the function of protecting the catalyst layer from exhaust heat, and also protects against Co, HC, H2°, NOx, etc. in the exhaust gas. filter function that restricts gases other than oxygen, and impurity Pb in exhaust gas.
It has a trapping function to trap substances such as , Si or Carbon.

However, conventional products have the problem that the amount of gas passing through them varies in the initial stage of use, making the so-called initial characteristics unstable.Furthermore, after long-term use, the performance deteriorates due to the accumulation of coarse impurities, so-called There was a problem that the durability characteristics deteriorated. This is because conventionally, when forming a protective layer by coating a platinum catalyst with a thermal spraying raw material such as porous alumina by plasma spraying, the thermal spraying raw material had a wide particle size distribution. This is thought to be due to variations in the size or number of bores (micropores) in the protective layer depending on the product, which causes variations in the amount of gas permeation due to clogging caused by coarse impurities.

Therefore, the present invention aims to achieve
It is an object of the present invention to provide an oxygen concentration sensor having stable characteristics without deterioration of initial characteristics and durability performance, thereby solving the above-mentioned problems.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention coats the inner and outer surfaces of a tube body made of ceramics such as zirconia with a metal thin film such as platinum, and uses the metal thin film as an electrode. In the oxygen concentration sensor, the tube and the housing are combined so that one of the electrodes is covered with a protective layer and the electrode generates an electromotive force based on the oxygen concentration inside and outside the tube. It is characterized by having a multi-layered structure in which the particles become smaller toward the inside.

(Function) Since the outer side of the protective layer has a large particle size, coarse impurities will not clog the outer side and cause clogging. Therefore, gas smoothly permeates from the outside and is squeezed by the small particle size layer inside. As a result, the initial characteristics and durability characteristics do not deteriorate and stable functions are achieved.

(Example) An example of the present invention will be described below with reference to FIGS. 1 to 6. First, the configuration will be explained with reference to FIG. 1, which shows an exaggerated cross-section of the tube tip, and FIG. There is a protective layer (first layer) 24 and a protective layer (second layer) 25 on top of this in order.
and a protective layer (third layer) 26. These protective layers 24, 25 and 26 have the same thickness of 100 microns.

As can be seen from FIG. 2, these protective layers are formed so that the particle size is smaller toward the inner side and larger toward the outer side.

Next, a method for forming the protective layer will be explained with reference to FIGS. 3 to 6.

First, the particle size distribution of the thermal spray raw material is adjusted. Protective layer raw material (
As shown in Figure 4, the particle size (μ
), and the vertical axis shows a mountain-shaped distribution in terms of weight percent, with a maximum value of 30μ, so in order to divide this into the particle size groups shown in FIG. 5, the classifier shown in FIG. 3 is used. In FIG. 3, 31 is an upper spiral chamber, 32 is a guide cone, and 33 is a guide cone.
3 is the classification chamber, 34 is the body wall of the classification chamber, 35 is the classification cone, 3
6 is an adjustment ring, 37 is a main shaft, 38 is a secondary air guide wall, 39 is a secondary air skirt, 40 is 2
Secondary air supply port, 41 is a butterfly valve for secondary air adjustment, 42 is a support stand, 43 is a portex pin, 44 is a bracket, 45 is a classifier outlet, 46 is an upper exhaust chamber, 47
is the raw material supply port. When the raw material powder having the particle size distribution shown in FIG. , fine particles are separated into an upper exhaust chamber 46, and coarse particles are separated into a lower exhaust port 45. So, in the middle of the upper part, fine particles (
(including secondary air) is sucked into the fan 48, fine particles are captured by the four bag filters, and then the cyclone 5o
Intermediate particles are captured and the raw material is divided into three classes (first layer, second layer, and third layer) within a predetermined particle size range.

Then, coating is performed by plasma spraying as shown in FIG. 6 in order to sequentially form the second and third layers starting with the first layer having the smallest particle size.

In FIG. 6, 51 is a powder nozzle, 52 is a thermal spraying raw material, 53 is a plasma gun, 54 is a plasma flame, and 55 is a zirconia tube. The protective layer thus has a multilayer structure as shown in FIGS. 1 and 2.

Next, the operation of the above embodiment will be explained with reference to FIG. 2 showing this embodiment.
This will be explained by comparing with FIGS. 8 to 11 which show the conventional method.

First, in the case of this embodiment, as shown in FIG. 2, since the outer third layer 26 has large particles, both gas and impurities pass through this third layer 26, and gas and is moderately squeezed and impurities are captured. As a result, initial characteristics and durability characteristics become stable.

On the other hand, in the conventional case, as shown in FIG. 8, the protective layer 60 is formed of only one layer having bores (micropores) 61, and the particle size distribution is not adjusted. layer 6
If the grain size is coarse, the bore 0 becomes a large bore 61a to increase the amount of gas permeation, or if the grain size is minute, as shown in FIG. 10, it becomes a small bore 61b to reduce the amount of gas permeation. This causes variations in the amount of gas permeation. In addition, as shown in FIG. 11, if the bore 61b is small, coarse impurities such as Pb, Si, and C will clog the voids of the bore 61b, causing clogging to prevent the formation of Co, H2, and other substances that should be moderately restricted. This has the disadvantage that gas permeation is not possible.

Note that the present invention is not limited to the above-mentioned embodiments, and the number of layers may be two, four, or more, for example.

[Effects of the Invention] As explained above, according to the present invention, it is possible to obtain a protective layer having stable characteristics as a protective layer of an oxygen concentration sensor by adjusting the particle size distribution of the thermal spray raw material.

[Brief explanation of drawings]

Fig. 1 is a sectional view of the tip of the main part of an oxygen concentration sensor showing an embodiment of the present invention, Fig. 2 is an enlarged view of the n part of Fig. 1, Fig. 3 is a sectional view of the classifier, and Fig. 4 is Figure 5 is a particle size distribution diagram of thermal spray raw materials, Figure 5 is a particle size group classification diagram, Figure 6 is a diagram of the principle of thermal spraying, Figure 7 is a cross-sectional view of an oxygen concentration sensor, Figure 8 is a cross-sectional view of a conventional protective layer, Figure 9 is a cross-sectional view of the same large bore.
FIG. 10 is a cross-sectional view of the small-bore type, and FIG. 11 is a cross-sectional view of the small-bore type showing a clogging state. 1...Tube body 2...Electrode (inner coating layer) 3
... Electrode (outer coating layer) 4 ... Tube 5 ... Housing 6 ... Oxygen concentration sensor 7 ... Slit
8... Protector 9... Exhaust pipe 21... Inner electrode 22... Zirconia tube 23... Outer electrode (platinum catalyst layer) 24... Protective layer (first layer) 25... Protection Layer (2nd layer) 26... Protective layer (3rd layer) Agent Patent attorney Hidekazu Miyoshi 21 Inner electrode 22/zirconia tube 23 Platinum catalyst layer 1 Figure 2 Figure 4 Figure 5 Figure 6 Figure 8 Figure 10 Figure 9 Figure 1111

Claims (1)

[Claims] (1) The inner and outer surfaces of a tube body made of ceramics such as zirconia are coated with a metal thin film of platinum or the like, and the metal thin film is formed as an electrode, and either one of the electrodes is covered with a protective layer. An oxygen concentration sensor comprising a tube and a housing in combination to generate an electromotive force based on the oxygen concentration, characterized in that the protective layer has a multilayer structure in which the particles become smaller toward the inside. Concentration sensor.