JPH0425755A - Oxygen sensor - Google Patents

Abstract

PURPOSE:To obtain an oxygen sensor capable of stably detecting the concn. of high-temp. exhaust gas in a long time by mixing platinum black with metallic oxide which is stable at high temp. and forming the outermost layer coating the protective layer of a catalytic layer of this mixture. CONSTITUTION:The outside of a protective layer 10 is coated with a pasty solvent 23 consisting of the mixture of platinum black and titania. Thereafter the outermost layer 21 is formed by roasting the coated layer at 1200 - 1400 deg.C. The surface of the outermost layer 21 forms the uneven shape and the specific surface area is drastically enlarged and also durability at high temp. is enhanced. The impurities contained in exhaust gas are effectively prevented in a long period from being infiltrated into the vapor deposited layer 9 of platinum. Thereby the concn. of exhaust gas is stably detected for a long period.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an oxygen sensor that is attached to, for example, an exhaust pipe of an automobile or the like and is suitably used to detect the oxygen concentration in exhaust gas.

[Conventional technology]

Generally, in engines such as automobiles, an oxygen sensor (02 sensor) is attached to the exhaust pipe or the like to detect the oxygen concentration in the exhaust gas in order to optimally feedback control the air-fuel ratio.

Therefore, FIGS. 3 to 6 show tube-type oxygen sensors according to the prior art.

In the figure, reference numeral 1 indicates a stepped cylindrical sensor body, and the sensor body 1 includes a holder 2 having a threaded portion 2A formed on the outer periphery of one end and a cylindrical fitting portion 2B on the other end. Consisting of a cap 3, one end of which is fixed to the fitting part 2B of the holder 2 by caulking parts 3A, 3A, . It is made of metal material such as stainless steel. The sensor main body 1 has a zirconia tube 5, which will be described later, protruding into an exhaust pipe (not shown).The male threaded portion 2A of the holder 2 is screwed into the exhaust pipe. Sensor body 1
It shows an insulating cylinder disposed within the insulating cylinder 4, which is made of a ceramic material such as alumina.

5 indicates a zirconia tube as an oxygen concentration detecting element, which is attached to the proximal end or the shoulder 2C of the holder 2 via a ring-shaped washer 6, and whose distal end protrudes outside the holder 2;
The zirconia tube 5 is made of zirconia (Zr) in which the weight ratio of the tetragonal phase to the monoclinic phase is a predetermined ratio.
02, weight ratio 88, 76%) was added to the powder of Ittria (Y
2O2, weight ratio 8.84%) and a binder are mixed together and fired to form a U-shaped cross section (as shown in FIG. The zirconia tube 5 has a solid electrolyte layer with a phase content of approximately 75%, and is designed to enhance thermal shock resistance against water intrusion, etc.The proximal end of the zirconia tube 5 is a large diameter opening 5A, and the distal end is closed. The end is 5B.

Reference numeral 7.8 indicates an inner electrode and an outer electrode provided on the inner and outer surfaces of the zirconia tube 5, respectively, and the electrode 7.8 is a paste-like material made of platinum (90% by weight) and zirconia (10% by weight). The material of 1 out of 5 zirconia tubes
By applying it to the outer surface, it is formed into an elongated band shape. The inner electrode 7 extends to the end face of the opening 5A of the zirconia tube 5 as a lead-out portion 7A, and the lead-out portion 7A is connected to a contact plate 11, which will be described later. Further, the outer electrode 8 is connected to the shoulder portion 2C of the holder 2 via a washer 6, and is grounded.

Reference numeral 9 indicates a platinum vapor deposited layer as a catalyst layer coated over the entire circumference of the outer surface of the zirconia tube 5 from the upper side of the outer electrode 8. (It is provided all around the outer surface of the zirconia tube 5 so as to cover the zirconia tube 5 and the outer electrode 8.The platinum vapor deposited layer 9 is formed on the outer surface of the zirconia tube 5 due to the difference in oxygen concentration inside and outside the zirconia tube Electrode 8 and inner electrode 7
The electromotive force generated between the inner electrode 7 and the inner electrode 7 is amplified by a catalytic action, and this electromotive force is outputted from the inner electrode 7 to the outside via the contact plate 12 and the like as a detection signal. 1
0 indicates a protective layer made of a porous spinel layer or the like, and the protective layer 10 is provided around the entire outer surface of the platinum vapor-deposited layer 9 to protect the platinum vapor-deposited layer 9. 11
indicates the outermost layer, and the outermost layer 11 is provided all around the outer surface of the protective layer 10 so as to cover the protective layer 10, and the outermost layer 11 is composed of lead (Pb) in the exhaust gas.

As a measure against poisoning by impurities such as phosphorus (P) and silicon (Si), it is formed of platinum-supported γ-alumina, which is made by supporting platinum (Pt) on alumina (Aρ203).

The platinum vapor deposited layer 9 is formed of a thin film of about 0.5 μm, and the zirconia tube 5.2 has a wall thickness of about 2 mm.
Electrode 7.8 of about 0 μm and protective layer 1 of about 50 μm
Although it is omitted in FIG. 4 because it is formed very thinly compared to the protective layer 10, it is provided over the entire circumference of the zirconia tube 5 like the protective layer 10.

In addition, in FIG. 3, the protective layer 10. The platinum vapor deposited layer 9 and the outermost layer 11 are omitted from the illustration.

Reference numeral 12 denotes a contact plate disposed within the insulating cylinder 4. The contact plate 12 is formed by bending a conductive metal plate, and a disk-shaped contact portion 12A is provided at one end of the contact plate 12. The other end side is provided with a connecting portion 12B connected to a lead wire 13 led to the outside.

Then, the contact portion 12 of the contact plate 12
A is held between the end surface of the opening 5A of the zirconia tube 5 and the insulating cylindrical body 4 by the spring load of the disc spring 14, and is connected to the lead-out portion 7A of the inner electrode 7. Furthermore, 15 is a seal member disposed within the reduced diameter portion 3C of the cap 3 and seals around the lead wire 13, 16 is a protector fixed to the holder 2 to protect the zirconia tube 5, The protector 16 is formed with elongated holes 16A, 16A, . . . for introducing exhaust gas.

In the conventional oxygen sensor configured as described above, the sensor main body 1 is screwed onto the exhaust pipe etc. via the male threaded portion 2A, so that the distal end side of the zirconia tube 5 is projected into the exhaust pipe together with the protector 16. , the oxygen concentration in exhaust gas is detected. That is, since the exhaust gas is waste gas from burning a mixture of air and fuel, the oxygen concentration in the exhaust gas is lower than that of the atmosphere inside the zirconia tube 5, and the oxygen concentration in the exhaust gas is lower than that of the atmosphere inside the zirconia tube 5. There is a large difference in oxygen concentration between the two.

Therefore, oxygen ions pass through the zirconia tube 5 made of a solid electrolyte from the inside to the outside, and an electromotive force is generated between the inner electrode 7 and the outer electrode 8.

Since this electromotive force increases or decreases depending on the oxygen concentration in the exhaust gas, as shown in FIG. (without showing) was given 8 power,
The control unit corrects the fuel injection amount, etc. based on this detection signal, and performs feedback control of the air-fuel ratio.

[Problems to be Solved by the Invention] By the way, the above-mentioned conventional oxygen sensor is generally installed in an exhaust pipe and placed in high-temperature exhaust gas.
Platinum-supported γ-alumina, which is the material of the outermost layer 11, is fired at about 800° C. during the production of the zirconia tube 5 to form the outermost layer 11. However, in actual use, the temperature of the exhaust gas is the firing temperature (800
℃), and if the outermost layer 11 is exposed to high temperatures for a long time, the platinum-supported γ-alumina undergoes a phase transition to α-alumina, causing cracks, etc., and the lead in the exhaust gas, Impurities such as phosphorus and silicon are removed from the outermost layer 11 to the protective layer 10.
There is a problem in that the oxygen penetrates into the platinum vapor deposited layer 9 through the oxygen sensor, deteriorates the platinum vapor deposition layer 9, and changes the control characteristics of the air-fuel ratio by the oxygen sensor.

That is, as shown in FIG. 7, as a result of carrying out a durability test on the oxygen sensor at an exhaust gas temperature of 800°C, the outermost layer 11 deteriorated over a period of 300 hours as shown by characteristic line 14, and When the time exceeds 300 hours, the platinum vapor deposited layer 9 deteriorates and the control characteristics change, and it can be seen that this also changes the control characteristics of the air-fuel ratio by the oxygen sensor. Therefore, a problem arises in that it is impossible to always accurately detect the oxygen concentration from the oxygen sensor, and it becomes impossible to accurately correct the fuel injection amount, etc., and to perform feedback control of the air-fuel ratio.

The present invention has been made in view of the problems of the prior art described above.The present invention provides an oxygen sensor that can stably detect oxygen concentration over a long period of time even in high-temperature exhaust gas and greatly improves durability. It provides a sensor.

[Means for Solving the Problems] The feature of the structure adopted by the present invention in order to solve the above-mentioned problems is that the outermost layer is formed by mixing platinum black with a metal oxide that is stable at high temperatures. .

[Function] With the above configuration, the outermost layer made of metal oxide containing platinum black is resistant to high temperatures, so it is possible to prevent deterioration of the outermost layer even in high-temperature exhaust gas, and prevent impurities in the exhaust gas from reaching the catalyst layer. Intrusion can be prevented for a long period of time, and deterioration of the catalyst layer can be prevented for a long period of time.

C Example J Hereinafter, an example of the present invention will be described based on FIGS. 1 and 2. In the embodiment, the same components as those in the prior art described above are given the same reference numerals, and their explanations will be omitted.

In the figure, 21 indicates the outermost layer provided on the outer peripheral surface side of the zirconia tube 5 so as to cover the protective layer 10, similar to the outermost layer 11 described in the related art, and the outermost layer 21 is made of a metal that is stable at high temperatures. It is formed from a mixture of an oxide such as titania (TiO2) and platinum black.

Here, a method for forming the outermost layer 21 on the zirconia tube 5 will be described with reference to FIG. 2.

In the figure, 22 is a container, and 23 is a solvent contained in the container 22, which is made by mixing titania with platinum black and making it into a paste with an organic solvent. It is formed by adding it to powder and dissolving it into a paste with an organic solvent. Then, this solvent 23 is applied to the surface of the protective layer 10 of the zirconia tube 5 as shown by the two-dot chain line so as to cover the protective layer 10, and then heated to a temperature of 1000°C or more, preferably about 1200°C to 1400°C. By firing the outermost layer 21 together with the zirconia tube 5 at the bottom, the outermost layer 21 is formed in a stable state.

The oxygen sensor according to this embodiment has the above-described configuration, and its basic operation is not particularly different from that of the prior art.

However, in this embodiment, after coating the outside of the protective layer 10 with a paste-like solvent 23 made of a mixture of platinum black and titania, the outermost layer 21 is baked at a temperature of about 1200°C to 1400°C. forming the outermost layer 2
1 covers the protective layer 10, the outermost layer 2
The surface of 1 has an uneven shape due to platinum black particles, which can greatly expand its specific surface area, improve durability under high temperatures, and eliminate lead, phosphorus, silicon, etc. contained in exhaust gas. Impurities can be effectively prevented from entering the platinum deposited layer 9 over a long period of time.

That is, as a result of carrying out an endurance test of the oxygen sensor using high-temperature exhaust gas in the same manner as the prior art, it was found that its control characteristics remained unchanged even after an endurance period of 600 hours, as shown by the characteristic line 24 shown by the dotted line in FIG. There was almost no change, and no cracks or the like occurred in the outermost layer 21 due to the high-temperature exhaust gas, confirming that deterioration of the platinum vapor deposited layer 9 could be effectively prevented.

Therefore, according to this embodiment, the oxygen concentration can be detected stably over a long period of time, the life of the oxygen sensor can be extended, and the correction of the fuel injection amount, etc., and the feedback control of the air-fuel ratio can be accurately performed. This can be done over a long period of time.

In addition, in the above embodiment, the case where titania was used as the metal oxide of the outermost layer 21 was explained as an example, but instead of this, it is also possible to use a metal oxide that is stable at high temperatures such as zirconia (ZrO2). It is.

Further, in the above embodiment, a tube type oxygen concentration detection element using a zirconia tube 5 was explained as an example, but the present invention is not limited to this, and for example, a plate type oxygen concentration detection element formed in a flat plate shape from zirconia In this case, the outer electrode side, that is, the outermost layer that comes into direct contact with the exhaust gas, may be formed of platinum black and a metal oxide such as titania.

[Effects of the Invention] As detailed above, according to the present invention, the outermost layer covering the outside of the protective layer of the oxygen concentration detection element is formed of a mixture of platinum black and a metal oxide that is stable at high temperatures. This prevents the outermost layer from cracking under high temperatures, reliably prevents impurities contained in exhaust gas from entering the catalyst layer, and greatly improves high-temperature durability. can. Therefore, defective characteristics of the oxygen sensor can be prevented, stable oxygen concentration can be detected even during long-term use in high-temperature exhaust gas, and the life of the oxygen sensor can be extended.

[Brief explanation of drawings]

1 and 2 show an embodiment of the present invention, FIG. 1 is a detailed cross-sectional view showing an enlarged main part of the front end of the zirconia tube, FIG. 2 is an explanatory view showing the process of forming the outermost layer, 3 to 7 show the prior art, FIG. 3 is a vertical cross-sectional view showing the overall configuration of an oxygen sensor, FIG. 4 is a vertical cross-sectional view of a zirconia tube, and FIG. 5 is a zirconia tube in FIG. 4. 6 is a characteristic diagram of the detection signal, and FIG. 7 is a comparison of the durability test results of air-fuel ratio control using an oxygen sensor with the conventional technology and this example. FIG. DESCRIPTION OF SYMBOLS 1... Sensor main body, 5... Zirconia tube (oxygen concentration detection element), 7... Inner electrode, 8... Outer electrode, 9... Platinum vapor deposition layer (catalyst layer), 10... Protective layer, 21...outermost layer. Patent Applicant Japan Electronics Co., Ltd. Agent Patent Attorney Kazuhiko Hirose

Claims (1)

[Claims] It consists of a sensor body, and an oxygen concentration detection element made of zirconia, the base end side of which is attached to the sensor body, the distal end side of which protrudes outside the sensor body, and the oxygen concentration detection element has inner and outer electrodes on its inner and outer surfaces. Along with providing an outer electrode,
The outer surface of the oxygen concentration detection element includes a catalyst layer that covers the outer electrode from the outside, a protective layer that covers the catalyst layer from the outside, and a protective layer that covers the protective layer from the outside to prevent impurities in the exhaust gas from reaching the catalyst layer. 1. An oxygen sensor comprising an outermost layer for preventing intrusion into the oxygen sensor, wherein the outermost layer is formed by mixing platinum black with a metal oxide that is stable at high temperatures.