JPH10104194A - Stabilization of oxygen sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize the initial characteristics and capacity during use of an oxygen sensor used in the control of the air fuel ratio of an internal combustion engine. SOLUTION: A detection electrode 2 is provided to one surface of an oxygen ion conductive solid electrolyte and a reference electrode 3 is provided to the other surface thereof to constitute the detection element 1 of an oxygen sensor and at least the detection electrode 2 of the detection element 1 is heat-treated at 11501250 deg.C in the atmosphere for about 1hr and subsequnetly heat-treated at 700-800 deg.C under an atmosphere containing 30-70vol.% of hydrogen for about 1hr and subsequently heat-treated at 500-600 deg.C under an atmosphere not substnatlally containing oxygen and containing 1vol.% or less hydrogen for about 30min and, thereafter, this detection electrode is heat-treated at 200-300 deg.C under a non-oxidative atmosphere containing 10-30vol.% of moisture for about 1hr. Further, heat treatment in the atmosphere and heat treatment under a non-oxidative atmosphere containing moisture may be combined and heat treatment under a hydrogen-containing atmosphere may be further combined therewith.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for stabilizing an oxygen sensor in which a sensing element of an oxygen sensor is subjected to a heat treatment in a specific atmosphere to stabilize the performance after the initial use and after the endurance. The oxygen sensor stabilized by the method of the present invention is stable in a short time at the start of use, has a fast response, operates at a low temperature, and has little or no change in performance from the initial use to endurance.

[0002]

2. Description of the Related Art An oxygen sensor for measuring an oxygen concentration based on the principle of an oxygen concentration cell has been put into practical use by using a solid electrolyte having oxygen ion conductivity such as zirconia as a partition, providing a detection electrode made of a noble metal such as platinum, and a reference electrode. Have been. This oxygen sensor is used for purposes such as detecting the concentration of oxygen contained in exhaust gas from an internal combustion engine such as an automobile or a gas combustion device, grasping the combustion state of the internal combustion engine and controlling the air-fuel ratio. ing.

However, in the above-described oxygen sensor, the state of adsorption of oxygen on the surface of the detection electrode and the like are apt to change, and when this oxygen sensor is mounted, the accuracy of detecting oxygen in the early stage of use is deteriorated. There is a problem that is greatly changed. To suppress this, the oxygen sensor is usually exposed to high-temperature air, an inert gas, an exhaust gas, or the like before use, and its initial characteristics are stabilized.
Further, a method of stabilizing initial characteristics by alternately exposing an oxygen sensor before use to a lean atmosphere and a rich atmosphere has been proposed.

Further, Japanese Patent Publication No. 1-55406 discloses that
A method is disclosed in which a platinum electrode is heated while contacting a substance containing lead or sulfur to form a compound, and then heated again to decompose the compound, thereby activating the electrode. However, in these conventional methods, the characteristics of the oxygen sensor are not sufficiently stabilized, and a method of compensating for the change by shifting an initial engine control point or the like is adopted.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and when mounted, is stable in a short time, has a fast response, operates at a low temperature, and accurately detects the oxygen concentration. It is another object of the present invention to provide a stabilizing method for obtaining an oxygen sensor having little or no change in performance from the beginning of use to after endurance.

[0006]

A method for stabilizing an oxygen sensor according to a first aspect of the present invention is a method for stabilizing characteristics of an oxygen sensor, wherein a detection electrode is provided on one surface of a solid electrolyte having oxygen ion conductivity. Heat treatment at least the detection electrode of the detection element of the oxygen sensor provided with a reference electrode on the other surface at a temperature of 1000 to 1400 ° C. in an air atmosphere;
Then, under a non-oxidizing atmosphere containing 2 to 60% by volume of water,
The heat treatment is performed at a temperature of 150 to 400 ° C.

A second aspect of the present invention provides a method for stabilizing the characteristics of an oxygen sensor, in which a detection electrode is provided on one surface of a solid electrolyte having oxygen ion conductivity, and the other surface is provided. At least the above-mentioned detection electrode of the detection element of the oxygen sensor provided with the reference electrode is heat-treated at a temperature of 1000 to 1400 ° C. in an air atmosphere.
The heat treatment is performed at a temperature of 400 to 900 ° C. in an atmosphere containing hydrogen and then at a temperature of 150 to 400 ° C. in a non-oxidizing atmosphere containing 2 to 60% by volume of moisture.

[0008] Further, a method for stabilizing an oxygen sensor according to a third invention is a method for stabilizing the characteristics of an oxygen sensor, wherein a detection electrode is provided on one surface of a solid electrolyte having oxygen ion conductivity, and the other surface is provided. At least the above-mentioned detection electrode of the detection element of the oxygen sensor provided with the reference electrode is heat-treated at a temperature of 1000 to 1400 ° C. in an air atmosphere.
Heat treatment at a temperature of 400 to 900 ° C. in an atmosphere containing hydrogen, and then in an atmosphere containing substantially no oxygen and containing 1% by volume or less of hydrogen or containing no hydrogen.
Heat treatment at a temperature of 00 to 800 ° C.
The heat treatment is performed at a temperature of 150 to 400 ° C. in a non-oxidizing atmosphere containing 60% by volume.

[0009] In the oxygen sensor, "stabilization" of the performance from the initial use to the end of use is important. If this stabilization is insufficient, the response will be slow and the oxygen sensor will have a large performance change from the beginning of use to the end of its use. Further, in general, the engine is controlled in a so-called lean region where the amount of supplied air is large, so that the exhaust gas contains many harmful substances. In the present invention, the crystal particles of platinum or the like, which are the main components of the detection electrode, are grown by the heat treatment in the air atmosphere according to the first to third inventions. In addition, the heat treatment in a non-oxidizing atmosphere containing water improves the catalytic activity of platinum or the like and promotes stabilization. Further, by combining the reduction of the detection electrode in a hydrogen atmosphere according to the second and third aspects of the present invention with the removal of hydrogen in the detection electrode by heat treatment in an atmosphere substantially free of oxygen according to the third aspect of the invention, the stabilizing effect is obtained. Can be further enhanced.

As the above-mentioned "solid electrolyte body having oxygen ion conductivity" (hereinafter referred to as "solid electrolyte body"), various ceramics, particularly ceramics containing zirconia as a main component, are suitable. This solid electrolyte body is prepared by mixing a raw material powder such as zirconium oxide and a powder of a sintering aid such as yttrium oxide, silicon oxide and magnesium oxide, granulating the mixture, shaping it into a predetermined shape, By calcination and then baking, the detection element body can be obtained.

The shape of the detection element body is usually a cylindrical shape or a plate shape with a bottom, and a pressure forming method such as a rubber press method, a thick film method, etc. Formed by a method such as a lamination method. Thereafter, if necessary, the detection element body is formed by calcining at a temperature lower than the firing temperature and then firing by a usual method. Prior to or after the firing, a detection electrode exposed to a test gas such as exhaust gas and a reference electrode exposed to a reference gas having a predetermined oxygen concentration such as the atmosphere are provided at predetermined positions. Is formed.

The above "detection electrode" and the above "reference electrode"
Is formed of a noble metal element which is an element having the above-mentioned catalytic action, for example, platinum, ruthenium, osmium, iridium, rhodium, palladium or the like, or formed as a thin film electrode made of a conductive material mainly containing these noble metal elements. Is done. In particular, the detection electrode is formed only of platinum having an excellent catalytic action, or a conductive material containing platinum as a main component and about 1 to 30% by weight of rhodium, palladium or the like. The formation of these electrodes can be performed by a conventional method such as a plating method, a sputtering method, and a method of thermally decomposing a salt of an electrode metal.

In the first to third aspects of the present invention, the above-mentioned "under air atmosphere, 1
Heat treatment at a temperature of 000 to 1400 ° C.
The metal crystal particles constituting the detection electrode have a high density and a large particle size. If the heat treatment temperature is lower than 1000 ° C., the densification and enlargement of the metal crystal particles do not proceed sufficiently. If the heat treatment temperature exceeds 1400 ° C., the detection electrodes may be thermally degraded.

The above heat treatment temperature is preferably 1100 to 130.
The temperature is preferably set to 0 ° C, more preferably 1150 to 1250 ° C. Due to the heat treatment in the air atmosphere, an appropriate number of through holes having an appropriate size are formed in the detection electrode between the metal crystal particles so that oxygen contacts the solid electrolyte body. still,
The heat treatment time is not particularly limited, but is preferably from 30 minutes to 3 hours, particularly from 40 minutes to 2 hours, for example, about 1 hour, since the densification of the crystal particles proceeds sufficiently and there is no risk of electrode deterioration.

Further, the heat treatment at the temperature of 400 to 900 ° C. in the atmosphere containing hydrogen according to the second and third inventions allows the metal crystal particles such as platinum constituting the detection electrode to be formed in the heat treatment in the air atmosphere. Oxygen attached to the surface or occluded in the surface layer can be reduced and removed. This hydrogen is preferably contained at 2% by volume or more, 5% by volume or more, and more preferably 10% by volume or more.

The above hydrogen content ratio is preferably 20 to 80% by volume, more preferably 25 to 70% by volume. Further, the heat treatment temperature is particularly 600 to 900 ° C., and furthermore 700
The temperature is preferably set to 800 ° C. If the amount ratio of hydrogen and the heat treatment temperature are in the above ranges, oxygen can be efficiently reduced and removed. In addition, the heat treatment time is not particularly limited, but 10 minutes to 3 hours, particularly 30 minutes to 2 hours,
For example, about 1 hour is preferable because oxygen can be sufficiently reduced.

Further, the heat treatment at a temperature of 400 to 800 ° C. in an atmosphere substantially containing no oxygen and containing 1% by volume or less of hydrogen or containing no hydrogen according to the third invention, In the heat treatment in the hydrogen atmosphere described above, it is possible to remove the hydrogen attached to or occluded on metal crystal particles such as platinum constituting the detection electrode. If oxygen is contained in this atmosphere, it is not preferable because oxygen is again attached to or occluded on the detection electrode. On the other hand, if the amount of hydrogen exceeds 1% by volume, the heat treatment in the hydrogen atmosphere described above is not preferable because the hydrogen attached to or absorbed on the detection electrode cannot be sufficiently removed. The atmosphere for this heat treatment is preferably an inert gas atmosphere, for example, an atmosphere of 100% by volume such as nitrogen gas.

The above heat treatment temperature is preferably 400 to 700.
C, more preferably 500 to 600C. When the heat treatment temperature is in the above range, hydrogen can be efficiently removed. The heat treatment time is not particularly limited, but is preferably 5 minutes to 2 hours, particularly 10 minutes to 1 hour, for example, about 30 minutes, since hydrogen can be sufficiently removed.

Further, in the first to third inventions, the above-mentioned "water content of 2 to 2"
By performing a heat treatment at a temperature of 150 to 400 ° C. in a non-oxidizing atmosphere containing 60% by volume, the catalytic activity of metal crystal particles such as platinum constituting the detection electrode can be improved. If the water content is less than 2% by volume, the catalytic activity will not be sufficiently improved. On the other hand, it is not easy to make the atmosphere containing more than 60% by volume of water due to restrictions of the apparatus. This atmosphere is preferably an inert gas containing water, for example, a nitrogen gas atmosphere, and must not contain a component that oxidizes the detection electrode. However, when it is difficult to provide an atmosphere containing no oxidizing component, a method of preventing oxidation by slightly mixing hydrogen may be employed.
In this step, hydrogen is not attached to the detection electrode because the processing temperature is low.

The above water content ratio is preferably 5 to 35% by volume,
More preferably, the content is 10 to 30% by volume. Also,
The heat treatment temperature is particularly 150 to 350 ° C., more preferably 200 to 3
The temperature is preferably set to 00 ° C. When the amount ratio of water and the heat treatment temperature are in the above ranges, the activity of the catalyst is greatly improved. The heat treatment time is not particularly limited, but may be 30
Minutes to 3 hours, particularly 40 minutes to 2 hours, for example, about 1 hour are preferable because the catalyst activity can be sufficiently improved.

As described above, the oxygen sensor stabilized by the method of the present invention is quickly stabilized at the start of use, and has little or no change in performance from the beginning of use to after endurance. When the heat-treated detection element is housed in a protective tube socket and fixed and manufactured, for example, attached to an exhaust pipe of a gasoline engine with a displacement of 2000 cc and exposed to exhaust gas, the oxygen sensor starts a test. It starts to operate within 35 seconds, in particular 30 seconds, and even within 25 seconds, and especially within 20 seconds. In addition, λ (a value obtained by dividing the actual air-fuel ratio by the stoichiometric air-fuel ratio, for example, about 1 to 5 minutes after the start of the measurement of the air-fuel ratio, which is often used when evaluating the air-fuel ratio, Is).
The central value is 1.0015 to 1.0020, the fluctuation range is 0.0025 or less, and especially the central value is 1.0005 to 0.0.
010, the fluctuation range is 0.0020 or less, furthermore, the central value is 1.0000 to 1.0005, and the fluctuation range is 0.0015 or less, which is very excellent performance.

Further, the oxygen sensor of the present invention is attached to the exhaust pipe of a gasoline engine having a displacement of 2000 cc as described above, the engine speed is appropriately set to about 3000 rpm, and the endurance after being exposed to exhaust gas for, for example, 500 hours. The later [lambda] is approximately 1.0000 when stabilized by any of the first to third inventions. As described above, the λ of the oxygen sensor stabilized by the method of the present invention is very stable with little or no change in performance from the initial use to the end of use.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described specifically with reference to examples. (1) Production of Detecting Element 5 mol% of 99% pure Y ₂ O ₃ was mixed with 99% or more pure ZrO ₂ , and the mixture was wet-mixed and calcined at a temperature of 1300 ° C. Water was added to the calcined product, pulverized by a ball mill, a water-soluble binder was added, and the mixture was granulated by a spray drying method.

Thereafter, a bottomed cylindrical detection element main body having a shelf on the outer periphery of the intermediate portion was formed by a rubber press method, and was ground by a grindstone to adjust its shape. Then
After firing the detecting element body at a temperature of 1500 ° C. for 3 hours,
A platinum electrode having a thickness of 1 to 2 μm, which is exposed to a test gas such as an exhaust gas, was provided outside the main body of the detection element by an electroless plating method, and used as a detection electrode. Thereafter, a platinum electrode having a thickness of 1 to 2 μm exposed to the atmosphere was provided inside the detection element main body by an electroless plating method, and used as a reference electrode. Next, heat treatment was performed at a temperature of 1200 ° C. for 1 hour in an air atmosphere to improve the denseness of the detection electrode. Further, spinel powder was sprayed on the outer surface of the detection electrode to form a protective layer having a thickness of about 100 μm.

(2) Configuration of the detecting element FIG. 2 shows the detecting element obtained as described above. FIG.
2A is a front view before forming the protective layer, FIG. 2B is a front view after forming the protective layer, and FIG. 2C is a longitudinal sectional view after forming the protective layer. In FIG. 2A, reference numeral 1 denotes a detection element main body having a bottomed cylindrical shape, and 2 denotes a detection electrode provided on the outer peripheral surface of the bottom. 2B and 2C, reference numeral 4 denotes a protective layer, and in FIG.
Is a reference electrode provided on the inner peripheral surface at the bottom.

In the present invention, the structure of the detecting element is not limited to that shown in FIG. 2. For example, the detecting electrode and the reference electrode need not necessarily cover the entire bottom peripheral surface of the detecting element body as shown in FIG. However, it may be a band shape or the like. Further, as shown in FIG. 3, this detection element is made of a metal or the like having a high strength, and is housed and fixed in a protective tube socket 5 for protecting the detection element and attaching it to a predetermined position to produce an oxygen sensor. At a predetermined position of the exhaust pipe of the above, and put to practical use.

(3) Examples 1 to 3 and Comparative Example 1 Example 1 (corresponding to the first invention) The detection element manufactured in the above (1) was prepared by using 70% by volume of nitrogen and 3%.
20% by volume of the mixed gas consisting of 0% by volume of hydrogen and 100% by volume of the mixed gas by a wetter device.
Was added, and heat treatment was performed at 250 ° C. for 1 hour in this atmosphere to activate the catalytic performance of the detection electrode.

Example 2 (corresponding to the second invention) The detection element prepared in the above (1) was replaced with 70% by volume of nitrogen and 3%.
Heat treatment was performed at 750 ° C. for 1 hour in an atmosphere consisting of 0% by volume of hydrogen to reduce and remove oxygen adhering to and occluded on the detection electrode. Then, 70% by volume of nitrogen and 30% by volume
To a gas mixture consisting of
20% by volume of water was added to 100% by volume of the mixed gas, and heat treatment was performed at 250 ° C. for 1 hour in this atmosphere to activate the catalytic performance of the detection electrode.

Example 3 (corresponding to the third aspect of the invention) The detection element manufactured in the above (1) was replaced with 70% by volume of nitrogen and 3%.
Heat treatment was performed at 750 ° C. for 1 hour in an atmosphere consisting of 0% by volume of hydrogen to reduce and remove oxygen adhering to and occluded on the detection electrode. Thereafter, heat treatment is performed at 550 ° C. for 30 minutes in an atmosphere of 100% by volume of nitrogen to adhere to the detection electrode.
The occluded hydrogen was removed. Next, to a mixed gas composed of 70% by volume of nitrogen and 30% by volume of hydrogen, 20% by volume of water is added to 100% by volume of the mixed gas by a wetter device, and heat treatment is performed at 250 ° C. for 1 hour in this atmosphere. This activated the catalytic performance of the detection electrode.

Comparative Example 1 The detection element prepared in the above (1) was prepared by mixing 70% by volume of nitrogen with 3%
Heat treatment was performed at 750 ° C. for 1 hour in an atmosphere consisting of 0% by volume of hydrogen to reduce and remove oxygen adhering to and occluded on the detection electrode.

(4) Performance Evaluation of Air-Fuel Ratio Control of Oxygen Sensor The detection elements of Examples 1 to 3 and Comparative Example 1 are housed and fixed in a protective tube socket to produce an oxygen sensor shown in FIG. This oxygen sensor was attached to an exhaust pipe of a 2000 cc gasoline engine, and λ was measured. That is, the air-fuel ratio control is executed by the attached oxygen sensor, and the standard λ measuring device (model “MEXA-110”, Horiba, Ltd.) is used for the exhaust gas discharged from the exhaust pipe at that time.
λ ”). About 30 engine speed
00 rpm and 2 to exhaust gas at a temperature of 800 to 900 ° C.
The λ at the beginning of use after exposure for up to 3 minutes and the λ after endurance after continuous exposure for 500 hours were measured. The results are shown in FIG.
In each of the examples and the comparative examples, the same heat treatment was applied to ten detection elements, and the same performance evaluation was performed.

In FIG. 1, E3, E2, E1, C
Reference numeral 1 denotes the evaluation results of λ at the initial stage of use of the oxygen sensor using the heat-treated detection elements of Example 3, Example 2, Example 1, and Comparative Example 1, respectively. FIG. 1 shows λ after durability.
(After the endurance, the results of the oxygen sensor using the detection element subjected to the heat treatment in any of Examples 1 to 3 and Comparative Example 1 are almost the same.) In FIG. 1, the vertical line indicating the result indicates the range of variation in the air-fuel ratio control results of the ten detection elements.

(5) Results of Performance Evaluation of λ Control The operability at a low temperature was compared between Example 3 and Comparative Example 1. As a result, the average value of each of the ten detection elements is:
In Example 3, the oxygen sensor started to operate 20 seconds after the start of the test, while in Comparative Example 1, the operation started 40 seconds later. As described above, it can be seen that the oxygen sensor of the present invention starts operating in a short time, and the function as the oxygen sensor operates from a low temperature.

According to the results shown in FIG. 1, in the third embodiment in which the heat treatment of the third invention is performed, λ in the initial stage of use is approximately 1.00.
It is controlled in a narrow range centered on 00, and is a value close to λ after durability. Further, Example 2 in which the heat treatment of the second invention was applied, Example 1 in which the heat treatment of the first invention was applied
In comparison with Comparative Example 1, λ in the initial stage of use gradually shifts to the lean side as compared with Example 3.
At the same time, the width of λ also increases, and the performance of λ decreases, making it difficult to use. As described above, in the present invention, the stabilization method of the third invention has the largest effect, and the stabilization effect of the second invention and the first invention is small. However, even with the stabilizing method of the first invention, it can be seen that λ in the initial stage of use is controlled to be closer to 1 and controlled to a narrower range as compared with the comparative example.

After the endurance, λ of any oxygen sensor including the comparative example is controlled to a narrow range centered at 1.0000, but the oxygen sensor stabilized by the method of the present invention, especially (3) In the oxygen sensor stabilized by the method of the present invention, λ is approximately 1.0 from the initial use as described above.
It can be seen that the oxygen sensor is controlled in a narrow range around 000, and has very stable performance. on the other hand,
In the case of the comparative example, the performance changes greatly from the initial stage of use to after the endurance, indicating that the oxygen sensor is difficult to use.

It should be noted that the present invention is not limited to the above specific embodiments, but can be variously modified within the scope of the present invention according to the purpose and application. For example, following the step of heat treatment at a temperature of 1000 to 1400 ° C. in an air atmosphere, Pb, Zn, Sn and S
And the like can be attached and contained in a small amount. Thereby, the detection electrode is further stabilized, and the performance of the air-fuel ratio control of the oxygen sensor is further improved.

[0037]

According to the oxygen sensor stabilizing methods of the first to third aspects of the present invention, a specific multi-step heat treatment is performed, whereby the oxygen sensor is quickly stabilized at the start of use, and at the time of mounting and from the beginning of use. It is possible to obtain an oxygen sensor with little or no change in performance after durability. Therefore, the air-fuel ratio of an automobile engine or the like can be controlled accurately and stably from a relatively low temperature.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of λ measured at the beginning of use and after endurance measured by air-fuel ratio detecting elements using the oxygen sensors of Examples 1 to 3 and Comparative Example 1.

FIG. 2A is a front view of a detection element before a protective layer is formed, and FIG. 2B is a front view of the detection element after a protective layer is formed.
(C) is a longitudinal sectional view of the detection element after the formation of the protective layer.

FIG. 3 is a front view of an oxygen sensor obtained by housing and fixing a detection element in a metal protection tube socket.

[Explanation of symbols]

1; detection element body; 2; detection electrode; 3; reference electrode, 4;
Protective layer, 5; metal protective tube socket.

Claims (3)

[Claims] 1. A method for stabilizing the characteristics of an oxygen sensor, comprising: a detection electrode provided on one surface of a solid electrolyte having oxygen ion conductivity, and a reference electrode provided on another surface. Heat-treating at least the detection electrode of the element at a temperature of 1000 to 1400 ° C. in an air atmosphere;
Then, under a non-oxidizing atmosphere containing 2 to 60% by volume of water,
A method for stabilizing an oxygen sensor, comprising performing heat treatment at a temperature of 150 to 400C. 2. A method for stabilizing the characteristics of an oxygen sensor, comprising: a detection electrode provided on one surface of a solid electrolyte having oxygen ion conductivity, and a reference electrode provided on the other surface. Heat-treating at least the detection electrode of the element at a temperature of 1000 to 1400 ° C. in an air atmosphere;
Thereafter, heat treatment is performed at a temperature of 400 to 900 ° C. in an atmosphere containing hydrogen, and then heat treatment at a temperature of 150 to 400 ° C. in a non-oxidizing atmosphere containing 2 to 60% by volume of water. Stabilization method. 3. A method for stabilizing the characteristics of an oxygen sensor, comprising the steps of: detecting an oxygen sensor in which a detection electrode is provided on one surface of a solid electrolyte having oxygen ion conductivity and a reference electrode is provided on the other surface. Heat-treating at least the detection electrode of the element at a temperature of 1000 to 1400 ° C. in an air atmosphere;
Thereafter, heat treatment is performed at a temperature of 400 to 900 ° C. in an atmosphere containing hydrogen, and then 400 to 800 ° C. in an atmosphere containing substantially no oxygen and containing 1% by volume or less of hydrogen or containing no hydrogen. Heat treatment at a temperature of 150 to 4% in a non-oxidizing atmosphere containing 2 to 60% by volume of water.
A method for stabilizing an oxygen sensor, comprising performing heat treatment at a temperature of 00 ° C.