JP2867851B2 - Oxygen concentration sensor and method for forming trap layer thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for detecting an oxygen concentration in exhaust gas for controlling an air-fuel ratio of an internal combustion engine, and a method for forming a trap layer on the oxygen concentration sensor.

[0002]

2. Description of the Related Art FIG.
In the conventional oxygen concentration sensor shown in (a), 1 is a solid electrolyte element made of oxygen ion conductive zirconia (ZrO ₂ ), 2 is an inner electrode film made of platinum (Pt), and 3 is an outer electrode film also made of platinum. 4 is spinel (MgAl
Electrode protection layer made of ₂ O _4), 5 alumina (Al ₂ O
The trap layer 6 on the exhaust gas side composed of ₃ ) is an inner space into which air is introduced.

FIG. 3B shows an example of the trap layer 5. In this trap layer 5, a thickness T ₅ of about 20
μm (micron), and the particle size of the alumina particles is about 4 μm.
m. Since the trap layer 5 is made of fine-grained alumina, the contact area with the exhaust gas component is increased, and the trapping of the exhaust gas component, for example, lead (Pb) in the leaded fuel is improved. In addition, since the trap layer 5 is bonded between the fine-grain alumina, the bonding strength is increased, and the trap layer 5 is not easily peeled.

However, if the particle size of alumina particles is small,
The surface roughness of the trap layer 5 is reduced, and the trap layer 5
The exhaust gas component deposit A is likely to be formed as a dense continuous layer on the surface of the substrate. In particular, as shown in FIG. 3D, when the trap layer 5 is quenched, the exhaust gas component deposits A are more densified and tend to become a continuous layer. When the exhaust gas component deposit A forms a dense continuous layer as described above, it becomes difficult for oxygen molecules in the exhaust gas to flow through the solid electrolyte element 1, impairing gas reactivity as an oxygen concentration sensor and causing deterioration of characteristics. become.

To improve this, in another example shown in FIG. 3C, a more porous trap layer 7 is formed on the trap layer 5 on the electrode protection layer 4. In this trap layer 7, the thickness T ₇ is about 50 [mu] m, the particle diameter of the alumina particles is about 15 [mu] m. Since the surface roughness of the trap layer 7 is larger than the surface roughness of the trap layer 5,
It is difficult for the exhaust gas component deposit A to be formed as a continuous layer on the surface of the trap layer 7. Therefore, the flowability of oxygen molecules to the solid electrolyte element 1 is improved.

However, since the particle size of the alumina particles is large, the bond strength of the trap layer 7 determined by the bond between the particles is weakened, and the particles are easily separated. In order to prevent this peeling, the thickness T ₇ of the trap layer 7 must be increased, so that the total thickness T ₅ + T ₇ of the trap layers 5 and ₇ is increased, and the double trap layers 5 and 7 are removed. Along with the formation, the alumina dip treatment has been troublesome.

Another oxygen concentration sensor having an improved trap layer is disclosed in Japanese Patent Application Laid-Open No. 3-293552. In this trap layer, the fine particle powder is fired to form an aggregated coarse particle powder, and then the coarse particle powder is applied in the form of a paste and fired. Therefore, the trapping rate of the exhaust gas component can be increased, and the flowability of oxygen molecules in the exhaust gas can be improved, thereby improving the responsiveness of the oxygen concentration sensor.

However, the bonding strength of the trap layer, which is determined by the bonding force between the coarse particles, is weakened, and a disadvantage remains that it is easily peeled off. In addition, since coarse particles in which fine particles are aggregated must be formed in advance, such a work step is added before the coating step of the coarse particles, and these work steps are troublesome.

First, according to the present invention, the trap layer is made up of small particles to enhance the trapping rate of exhaust gas components, prevent separation, and at the same time improve the flow of oxygen molecules in the exhaust gas. It is intended to be.

Second, it is an object of the present invention to provide a method for easily forming such a trap layer.

[0011]

Means for Solving the Problems The oxygen concentration according to the present invention
In sensors, solid electrolytes with oxygen ion conductivity
One of the two surfaces of the quality element that is located on the reference air side
And electrodes on the other side located on the exhaust side
Then, on the exhaust side surface, fine-grained alumina (Al ₂ O ₃ )
Forming a trap layer through an electrode protection layer
I have. In particular, the particles of the alumina particles are formed in the trap layer.
Bubbles larger than the diameter are formed. Such a trap layer is made of oxygen-ion-conductive
A voltage is applied to the solid electrolyte element having the oxygen pump action to feed oxygen into the alumina dip,
The alumina dip is formed by firing.

[0012]

A large number of bubbles are exposed on the surface of the trap layer, so that the surface of the trap layer becomes rough, and it becomes difficult for exhaust gas component deposits to be formed as a continuous layer on the surface of the trap layer.

On the other hand, since the particle size of alumina particles is small,
The contact area with the exhaust gas is increased, and the capture of the exhaust gas component is improved. Further, since the bonding is performed between the alumina particles having a small particle diameter, the bonding strength of the trap layer is increased, and the trap layer is hardly peeled.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An oxygen concentration sensor according to one embodiment of the present invention will be described below with reference to FIGS.

The solid electrolyte element 1 shown in FIG. 1 (a) has a cylindrical shape with a bottom. As shown in FIG. 2, a raw material comprising oxygen ion conductive zirconia (ZrO ₂ ) and yttria (Y ₂ O ₃ ) is used. It is formed after granulation.

The inner and outer peripheral surfaces (both surfaces) of the solid electrolyte element 1
Of which is platinum on one inner peripheral surface
The (Pt) electrode film 2 is chemically plated and exhausted.
(Pt) electrode film 3 on the other outer peripheral surface
Are chemically plated and subsequently hydrogen-aged. The electrode protection layer 4 is formed on the outer periphery of the outer electrode film 3,
Spinel (MgAl ₂ O ₄ ) is plasma sprayed. The thickness T ₄ of the electrode protection layer 4 is about 110 μm (micron).

The outer periphery of the electrode protection layer 4 is coated with alumina (A
1 ₂ O ₃ ). In the present embodiment, an improvement is added to the step of forming the trap layer 5.

The particle size of the alumina fabricated particles separated by the sieve is about 2 μm smaller than that of the conventional one in order to increase the viscosity between the particles and to smooth the slurry.
m. The binder of the alumina slurry is of a long-chain alumina sol type in order to increase viscosity and strength and to prevent shrinkage after firing. As shown in FIG. 1B, such an alumina slurry is first dipped. The thickness T ₅ is about 20 to 30
μm.

Next, as shown in FIG. 1B, a voltage is applied by using the inner electrode film 2 as a negative electrode and the outer electrode film 3 as a positive electrode,
Raise the temperature of the solid electrolyte element 1 to 350 ° C. or higher. In this state, oxygen ions move from the inside to the outside of the solid electrolyte element 1 by a so-called oxygen pump action and are supplied into the trap layer 5. Therefore, as shown in FIG.
Due to the oxygen ions, many bubbles 5a are formed in the trap layer 5.
Occurs. Thereafter, the trap layer 5 is further baked. The thickness T ₅ of the trap layer 5 is about 40 to 50 .mu.m. Most of the bubbles 5a are larger than the particle size of the alumina particles, and the volume of each bubble 5a is larger than the volume of the alumina particles.

In such an oxygen concentration sensor, the atmosphere (reference air) is introduced into the inner space 6, and the exhaust gas of the engine is introduced into the space facing the trap layer 5 outside the space. An electromotive force generated in the solid electrolyte element 1 is detected between the two electrode films 2 and 3 according to the concentration difference between the oxygen concentration and the oxygen concentration of the exhaust gas, so that the oxygen concentration in the exhaust gas can be measured. ing.

The oxygen concentration sensor according to the present embodiment has the following features. (A) Many bubbles 5a are exposed on the surface of the trap layer 5,
Despite the small particle size of the alumina particles in the trap layer 5, the surface roughness of the trap layer 5 becomes coarse. Therefore,
Exhaust gas component deposits A are less likely to be formed as a continuous layer on the surface of the trap layer 5, and the flow of oxygen molecules to the solid electrolyte element 1 is improved. (B) a porous by trapping layer 5 are a number of bubbles 5a, the thickness T ₅ of the trap layer 5 the number and size are the same alumina particles increases. Therefore, the contact area of the alumina particles with the exhaust gas component is also increased, and the trapping performance can be improved. (C) When the trap layer 5 is made porous by a large number of bubbles 5a, the thermal conductivity decreases. Therefore, even when the oxygen concentration sensor is exposed to the high-temperature gas, the oxygen concentration sensor is hardly affected by the heat, and the function of protecting the oxygen concentration sensor from heat is further improved. This function is also promoted by the increase in the thickness T ₅ of the trap layer 5 as shown in (b). (D) When the trap layer 5 is porous due to the large number of bubbles 5a, the heat capacity of the trap layer 5 increases. Therefore, even if the temperature of the exhaust gas suddenly drops, rapid cooling of the surface of the trap layer 5 can be prevented, and as shown in FIG. Let cool. As a result, densification of the deposit A is prevented, and crystallization of the deposit A is promoted. Therefore, the flowability of oxygen molecules to the solid electrolyte element 1 is improved. (E) As described above, even if the surface roughness of the trap layer 5 is made rough or the trap layer 5 is made porous, the contact area with the exhaust gas increases because the particle size of the alumina particles is small, and the exhaust gas Good trapping of components such as lead (Pb) in leaded fuel is achieved. Further, since the bonding is performed between the alumina particles having a small particle diameter, the bonding strength of the trap layer 5 is increased, and the trap layer 5 is hardly peeled. In order to provide such a function, the particle diameter is preferably 6 μm or less. (F) When forming the trap layer 5 having a great effect as shown in the above (a) to (e), a new process P as shown in FIG. And it can be performed at low cost.

In the above-described embodiment, a so-called test tube type having a bottomed cylindrical shape is exemplified in an oxygen concentration sensor in which both electrode films 2 and 3 are joined to each other with an oxygen ion conductive solid electrolyte element 1 therebetween. However, it can be applied to various types of oxygen concentration sensors having other shapes and functions.

[0023]

According to the oxygen concentration sensor according to the present invention,
By the action of the bubbles in the trap layer, the flowability of oxygen molecules in the exhaust gas is improved, and the responsiveness can be improved. In addition, since the trap layer has a small particle size, the trapping rate of the exhaust gas component can be increased and peeling can be prevented.

According to the method for forming a trap layer of an oxygen concentration sensor according to the present invention, a trap layer having such bubbles can be easily formed.

[Brief description of the drawings]

FIG. 1A is a longitudinal sectional view showing an oxygen concentration sensor according to the present embodiment, and FIG. 1B shows a state in which a voltage is applied between both electrode films after dipping alumina on an electrode protection layer. It is a partially enlarged sectional view, (c) is a partially enlarged sectional view showing a state in which a large number of bubbles are formed in this alumina trap layer, and (d) is an exhaust gas component deposit on the trap layer which is crystallized by cooling. FIG. 4 is a partially enlarged cross-sectional view showing a state in which

FIG. 2 is a manufacturing process diagram of the oxygen concentration sensor according to the present embodiment.

3A is a longitudinal sectional view showing a conventional oxygen concentration sensor, FIG. 3B is a partially enlarged sectional view showing one example of a trap layer, and FIG. 3C is a part showing another example of a trap layer. It is an expanded sectional view, and (d) is a partial expanded sectional view showing the state where exhaust gas components adhering on the trap layer were quenched.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Solid electrolyte element, 2 ... Electrode film, 3 ... Electrode film, 4 ... Electrode protection layer, 5 ... Trap layer, 5a ... Bubble, A ... Exhaust gas component deposit.

Claims (2)

(57) [Claims] 1. A solid electrolyte element having oxygen ion conductivity.
One of the two sides of the
Electrodes are formed on the other side disposed on the air side, respectively.
In an oxygen concentration sensor in which a trap layer formed of fine-grain alumina (Al ₂ O ₃ ) is formed on the exhaust side surface via an electrode protection layer , the trap layer has a size larger than the particle size of the alumina particles.
An oxygen concentration sensor characterized by forming a bubble . 2. A solid electrolyte element having oxygen ion conductivity.
A step of dipping fine-grained alumina (Al ₂ O ₃ ) on the element ; a step of applying a voltage to the solid electrolyte element to feed oxygen into the alumina dip by an oxygen pump action; and a step of firing the alumina dip A method for forming a trap layer of an oxygen concentration sensor, comprising: