JPH0221258A - Manufacture of oxygen concentration sensor - Google Patents

Abstract

PURPOSE:To achieve a higher response and a higher durability of the title device by a method wherein a plasma spraying of Ti and TiH is performed on an electrode on a substrate and between electrodes to form a sprayed layer which is oxidized in an oxidizing atmosphere by heating to form a TiO2 layer. CONSTITUTION:Platinum electrodes 2A and 2B are formed on a substrate 1 with a platinum heater 5 buried thereinto and a plasma spraying gun 6 is used to spray a Ti metal powder or TiH powder by an Ar plasma to form a sprayed layer 7 covering a part on the electrodes 2A and 2B and a part therebetween. Then, the sprayed layer 7 is oxidized in atmospheric air by heating to form a TiO2 layer 3. Thereafter, Al2O3 powder is sprayed covering the layer 3 to form a protective layer 4. This enables highly accurate formation of the TiO2 uniformly thereby permitting the production of a sensor excellent in durability with a better response to a change in the oxidation density.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a sensor for detecting the concentration of oxygen contained in various gases such as exhaust gas from an internal combustion engine. The present invention relates to a method for manufacturing an oxygen concentration sensor.

2. Description of the Related Art In modern automobiles, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is feedback-controlled in accordance with the oxygen concentration in the exhaust gas, so the oxygen concentration in the exhaust gas is detected. As such an oxygen concentration sensor, one using titania (TiO2) as a detection element has been developed. T i O2 has been attracting attention because its electrical resistance changes directly in response to oxygen concentration, which allows it to be easily configured into a sensor.

FIG. 3 shows a typical example of an oxygen concentration sensor using TiO2 as a detection element.

In FIG. 3, a pair of electrodes 2A and 2B made of a conductive material such as platinum (Pt) or silver (A9) are formed at intervals on a substrate 1 made of an electrically insulating material such as alumina (Al2O2). , its electrode 2A.

A TiO2 layer 3 as a detection element is formed so as to cover a portion above 2B and a portion between electrodes 2A and 2B, and a protective layer 4 such as Ai'203 is further formed on the TiO2 layer 3. Note that a platinum heater 5 for heating the element portion is embedded inside the substrate 1. In such an oxygen concentration sensor, since the resistance value between the electrode 2A and the two layers changes depending on the oxygen concentration, the detected oxygen concentration can be extracted as an electrical signal.

By the way, in the conventional oxygen concentration sensor using TiO2, methods for forming a TiO2 layer as a detection element include coating or dipping a slurry of TiO2 powder, as disclosed in Utility Model Application Publication No. 61-170060@. PVD method such as sputtering or ion blasting as shown in Japanese Patent Application Laid-Open No. 61-247950.
A method is known in which a TiO2 layer is formed by a method, and a method in which a TiO2 powder paste is printed by screen printing and then fired as shown in Japanese Patent Application Laid-Open No. 57-39341.

Problems to be Solved by the Invention As mentioned above, among the methods of forming a TiO2 layer in an oxygen concentration sensor using T r 02 as a detection element, the method of forming a TiO2 layer by coating or dipping does not
The thickness of the iO2 layer becomes extremely large, several hundreds of layers or more, and as a result, the responsiveness to changes in oxygen concentration must become low.Moreover, it is difficult to control the thickness, so when manufacturing a large number of sensors, it is difficult to It has been difficult to manufacture a sensor with uniform characteristics.

On the other hand, PVD such as sputtering and ion brating
When using the method, extremely thin Ti with a thickness of about 1 to 2 #
Since the O2 layer can be obtained with high precision, it is thought that it is possible to manufacture a sensor with excellent responsiveness to changes in oxygen concentration, but in reality, the T i O2 layer is too thin and has poor durability. Thermal deterioration of TiO2 and peeling between the substrate and electrodes and the TiO2 layer are likely to occur, and furthermore, deterioration due to impurities in the gas to be measured and reducing atmosphere is likely to occur.As a result, although the initial response is good, the response gradually decreases. In addition, there were other problems such as the high cost since a vacuum system was required for manufacturing, making it unsuitable for mass production.

Furthermore, in the case of using the screen printing method,
-39341 states that it is possible to form a TiO2 layer with a thickness of 50 to 100 layers, but if you want to obtain a thickness of 50 layers after firing, you must print with a thickness of 100 layers or more. If a thick layer is printed using the screen printing method, the thickness will be uneven and the thickness accuracy will be significantly reduced.
Therefore, there is a problem in that it is difficult to obtain a sensor with uniform characteristics.

This invention was made against the background of the above circumstances, and T
When manufacturing an oxygen concentration sensor using iO2 as a detection element, Ti
It is an object of the present invention to provide a method capable of manufacturing an oxygen concentration sensor having excellent responsiveness and durability by forming the 02 layer with high precision.

Means for Solving the Problems In the method of the present invention, when manufacturing an oxygen concentration sensor using TiO2 as a detection element, a pair of electrodes is formed in advance on a substrate made of an electrically insulating material, and a Form a sprayed layer by plasma spraying Ti or TiH so as to continuously cover the area and the area above the electrode, and then heat the sprayed layer at high temperature in an oxidizing atmosphere to oxidize the sprayed layer. This is characterized by producing TiO21m.

Function: In the method of the present invention, a pair of electrodes is formed in advance on a substrate made of an electrically insulating material, and Ti powder (metallic Ti
TiH powder) or TiH powder (titanium hydride powder) is plasma sprayed. During this thermal spraying, T
In the case of iH powder, decomposition and slight oxidation occur, and in the case of Ti powder, slight oxidation also occurs, so T
A layer consisting mainly of iO is obtained. If the sprayed layer is then heated at a high temperature in an oxidizing atmosphere, the oxidation of the sprayed layer will progress and the T
This becomes an iO2 layer. This TiO2 has a rutile crystal structure and generates oxygen defects depending on the oxygen concentration, so the electrical resistance changes depending on the oxygen concentration, and therefore it can be operated as a hydrogen concentration sensor.

Here, the thickness of the TiO2 layer that is finally generated and the sprayed layer before it are substantially the same, and the thickness of the solvent layer formed by plasma spraying is controlled with high precision to about several tens of square meters. and therefore the final generated T
The thickness of the iO2 layer can also be controlled with high precision to about several tens of degrees. The thinner the TiO2 layer is, the better the responsiveness to oxygen concentration is, but if it is too thin, the durability will be reduced.

If the thickness of the TiO2 layer is on the order of several tens of square meters, excellent responsiveness can be obtained without impairing durability.

The thickness of the TiO2 layer is specifically preferably about 10 to 100 mm, more preferably about 20 to 70 mm.

Furthermore, instead of using Ti or TiH as the thermal spraying material as in the present invention, TiO2 can be used to achieve T
It is also possible to form an i O2 layer, but when TiO2 is irradiated with Fg in this way, according to experiments by the present inventors, a non-stoichiometric compound of Ti(h-x) is produced, and if left as is, the oxygen concentration will decrease. It cannot be used as a sensor and requires oxidation treatment after thermal spraying. However, setting the conditions for oxidation treatment when non-stoichiometric compounds are generated is extremely difficult. In the case of the method of the invention, there is no particular difficulty in setting the conditions for oxidation.According to the experiments of the present inventors, when TiO2 is directly sprayed onto a substrate made of 1203 or the like, the adhesion to the substrate is low. It was found that one layer was easily peeled off and could not be put to practical use. On the other hand, when Ti or TiH is thermally sprayed and then oxidized as in the present invention, sufficient adhesion to a substrate made of A11203 or the like can be obtained, and peeling resistance is also good.

EXAMPLE An electrically insulating substrate 1 made of A&203 as shown in FIG. 1(A) was prepared. Note that the substrate 1 has a structure in which a platinum heater 5 is embedded in advance. On this substrate 1, a pair of platinum electrodes 2A and 2B are placed as shown in FIG. 1(B).
was formed by screen printing method. Specifically, platinum paste is screen printed in the shape of an electrode on the substrate 1,
Electrode 2A was heated at about 900°C for 1 hour in the atmosphere.
2B was formed. Note that the electrodes 2A and 2B can also be formed by screen printing a platinum paste on the substrate 1 in its unfired state, that is, in the state of an alumina green sheet, and then degreasing and firing.

Next, as shown in FIG. 1 <C>, the plasma spray gun 6
Ti metal powder or TiH
The powder is sprayed onto the parts on electrodes 2A and 2B and on electrode 2A.
A sprayed layer 7 as shown in FIG. 1 (D>) was formed to cover the area between , 2B.

The thickness of the solvent layer at this time was on the order of several tens of diagonals.

Next, the sprayed layer was oxidized by heating at 1100° C. for 2 hours in the atmosphere to form a TiO2 layer 3 as shown in FIG. 1(E). Then T! Al22 to cover 02 layer 3
O3 powder was thermally sprayed to form a protective layer 4 as shown in FIG. 1(F). Note that this protective layer 4 is for preventing deposits such as carbon and oil mist.

The oxygen concentration sensor obtained by the method of the present invention obtained as described above was installed in the exhaust gas line of an automobile engine, and its durability performance was investigated.

For comparison, TiO2 layer 3 was formed by sputtering 1
The durability performance of oxygen concentration sensors formed to a thickness of about 2 tJJn was similarly investigated. Note that the sensors of this comparative example were the same as those described above except for the TiO2 layer 3.

As a result, the oxygen concentration sensor according to the method of this invention is
It was found that the sensor had significantly superior durability compared to the oxygen concentration sensor of the comparative example using the sputtering method, and that there was little deterioration in response characteristics when exposed to exhaust gas for a long time. Specifically, the self-feedback output waveform of the sensor of the comparative example after being exposed to exhaust gas for a long time was as shown by the broken line B in FIG. The output waveform is shown by the solid line A in Figure 2. From this figure, it can be seen that the sensor according to the method of this invention has a short response cycle and little deterioration in response characteristics even after being exposed to exhaust gas for a long time. , it can be seen that the durability is good.

Effects of the Invention According to the method of the present invention, a TiO2 layer as a detection element can be formed uniformly and with high precision to a thickness of about several tens of square meters, and therefore has good responsiveness to changes in oxygen concentration. An oxygen concentration sensor with excellent durability can be obtained, and since a plasma spraying method is applied, mass production is possible, and the cost can be significantly reduced compared to when using a PVD method such as sputtering.

[Brief explanation of the drawing]

Figures 1 (A) to (F) are schematic vertical cross-sectional views showing the embodiments of the present invention, and Figure 2 is the self-feedback response characteristics of the oxygen concentration sensors of the embodiments of the present invention and comparative examples. FIG. 3 is a schematic longitudinal sectional view of a conventional general oxygen concentration sensor using TiO2. 1...Substrate, 2A, 2B...electrode, 3...T
lO2 layer, 7... solvent layer. Figure 1 5 c-ta

Claims (1)

[Claims] In manufacturing an oxygen concentration sensor using TiO_2 as a detection element, a pair of electrodes is formed in advance on a substrate made of an electrically insulating material, and the part between the electrodes and the part above the electrodes are continuously covered. An oxygen concentration sensor characterized by forming a sprayed layer by plasma spraying Ti or TiH, and then heating the sprayed layer at high temperature in an oxidizing atmosphere to oxidize the sprayed layer, thereby generating a TiO_2 layer. manufacturing method.