JPH1183789A - Oxygen sensor element and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a stable output in a prolonged use by covering at least one of a pair of electrodes with a protective layer having a close layer formed thereon to mostly eliminate the segregation of carbon at the electrodes. SOLUTION: This oxygen sensor element 1 comprises a cup-shaped solid electrolyte 10 which has the tip part 191 thereof closed and a base part 192 opened and an inner chamber 19 provided therein. A measuring electrode 11 is provided on the external surface 101 of the solid electrolyte 10 and a reference electrode 14 on the internal surface 109 facing the inner chamber 19. The measuring electrode 11 is covered with a protective layer 12 to protect it. The protective layer 12 is a porous layer which is formed by a plasma spray comprising a MgO.Al2 O3 spinel. A close layer 13 formed on the surface of the protective layer 12 is formed to close a tiny hole existing in the surface of the porous protective layer 12 and contains aluminum phosphate or the like. This hinders a gas to be measured from reaching a measuring electrode 11 thereby preventing the segregation of carbon in the measuring electrode 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor element mainly used for controlling combustion in an automobile internal combustion engine or the like, and a method for manufacturing the same.

[0002]

2. Description of the Related Art In order to detect the oxygen concentration in the exhaust gas of an automobile internal combustion engine, a gas sensor installed in the exhaust system of the internal combustion engine uses a ZrO ₂ solid electrolyte, for example, an oxygen concentration electromotive force type oxygen sensor. Those equipped with a sensor element are well known and have been put to practical use. Such a gas sensor includes, for example, a housing and an oxygen sensor element inserted and arranged in the housing, as shown in FIG. 4 described later.

[0003] The oxygen sensor element comprises a cup-shaped solid electrolyte body and an inner chamber provided therein. A measuring electrode is provided on the outer surface of the solid electrolyte body, and an inner surface facing the inner chamber is provided on the inner surface. A reference electrode is provided. The measurement electrode is provided with a protective layer for protecting the same. A heater is inserted into the inner chamber. The protective layer is formed from a ceramic coating layer.
Alternatively, for example, γ-
It is formed from a structure provided with an Al ₂ O ₃ layer.

In the oxygen sensor element, the exhaust gas passes through the protective layer (ceramic coating layer, γ-A
The sensor output can be obtained by reaching the measurement electrode after passing through the (l ₂ O ₃ layer). In addition, the protective layer is
It protects the catalytic action of the measurement electrode from being degraded by poisons such as P and Pb in the exhaust gas, and has a function of preventing the measurement electrode from peeling off from the solid electrolyte body.

[0005] However, when the operating condition of the internal combustion engine is fuel rich, that is, under conditions where the exhaust gas is oxygen-lean, the carbon monoxide (C
Carbon atoms in carbon compounds such as O) and hydrocarbons (HC) may be carbonized by the catalytic action of the measurement electrode and adhere to the interface between the measurement electrode and the protective layer.

For this reason, when the oxygen sensor element is used for a long period of time, the amount of carbon attached increases, and a phenomenon such as cracking of the protective layer or peeling of the protective layer occurs. There has been a problem that the characteristics of the oxygen sensor element are significantly reduced.

Therefore, it has been conventionally proposed to provide a dense glassy coating that does not allow exhaust gas to pass through the measurement electrode, particularly at a portion where carbon is likely to adhere (Japanese Patent Application Laid-Open No. 54-97490). In this technique, exhaust gas containing a carbon compound can be blocked by the vitreous coating. For this reason, the precipitation of carbon at the interface between the measurement electrode and the protective layer can be prevented. Therefore, it is possible to obtain an oxygen sensor element that can maintain good sensor characteristics for a long period of time.

[0008]

However, the above-mentioned prior art has the following problems. Here, the installation environment of the oxygen sensor element used in the combustion control of an automobile internal combustion engine or the like is an environment under severe conditions that are frequently exposed to a high-temperature atmosphere and a cooling / heating cycle. For this reason, it is usually required that each material constituting the oxygen sensor element has a high melting point and further has an appropriate coefficient of thermal expansion.

Here, considering the thermal expansion coefficient of the vitreous coating according to the above-mentioned prior art, this value is 40 to 7
It is disclosed as 0 × 10 ⁻⁷ / ° C. However, in this range, the difference in the coefficient of thermal expansion between the solid electrolyte body (the coefficient of thermal expansion is 75 to 85 × 10 ⁻⁷ / ° C.) that constitutes the oxygen sensor element is large, so that the glassy coating is formed by thermal stress. May crack or peel off. In a vitreous film in which cracks or peeling have occurred, a sufficient exhaust gas blocking effect cannot be exhibited.

Further, the surface of the vitreous coating is smooth. Therefore, the adhesive force between the vitreous film and the protective layer is weak, and the protective layer may be cracked or peeled off during use of the oxygen sensor element. Furthermore, the cracks and peeling may reach the protective layer where the vitreous coating is not provided. In the protective layer in which cracks and peeling have occurred, the protective effect on the measurement electrode cannot be sufficiently exhibited.

Further, in the above prior art, since the thickness of the vitreous coating is as large as 20 to 100 μm, a step is formed between the end of the vitreous coating and the surface of the protective layer. Stress may be concentrated, and as a result, cracking or peeling may occur in the protective layer.

In order to solve such a problem, the thickness of the protective layer is particularly increased in a portion where carbon is likely to adhere.
It has been proposed to prevent the passage of carbon compounds and the like (Japanese Utility Model Publication No. 57-30604). Further, a method of capturing the carbon compound with the protective layer (Japanese Patent Laid-Open No. 63-19549)
And a method of forming a trap layer for capturing a carbon compound on the protective layer (JP-A-63-306279).

However, in any of the methods, when the oxygen sensor element is used for a long time, the ability to capture the carbon compound is reduced, so that it has been difficult to sufficiently prevent the precipitation of carbon. In addition, since gaseous carbon monoxide (CO), hydrocarbon (HC), and the like can easily pass through the protective layer and the trap layer to reach the measurement electrode, sufficient effects can be obtained by the conventional technique. It was difficult.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an oxygen sensor element capable of maintaining a stable output during a long-term use with little carbon deposition on an electrode and a method of manufacturing the same. is there.

[0015]

According to a first aspect of the present invention, there is provided an oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on a surface of the solid electrolyte body, wherein at least one of the electrodes is covered with a protective layer. The oxygen sensor element is characterized in that the protective layer forms a dense layer densified from the inside on at least a part of its surface.

As described later, the oxygen sensor element comprises a so-called cup-shaped solid electrolyte body and an inner chamber provided therein, and the outer surface of the solid electrolyte body comes into contact with the gas to be measured. An oxygen sensor element having a measurement electrode and having a reference electrode on the inner surface facing the inner chamber can be mentioned.

Alternatively, as the oxygen sensor element,
A stacked oxygen sensor element comprising a substrate made of a plate-like solid electrolyte body, and a pair of a measurement electrode and a reference electrode provided on the substrate, wherein the measurement electrode is configured to be in contact with a gas to be measured. Can be. In any of the configurations, the electrode that comes into contact with the gas to be measured is provided with a protective layer (which corresponds to the measurement electrode in the above-described configuration) that covers the electrode.

The present invention can be applied to oxygen sensor elements having other configurations. Other structures include electrodes with pumping action,
An electrode having an electrode other than the pair of electrodes described above can be used.

The protective layer and the dense layer do not need to cover all the electrodes, but may be provided only partially.
In particular, since the dense layer blocks the gas to be measured, it is preferable that the dense layer be provided so as to avoid a portion related to the detection of the oxygen concentration in the electrode.

The operation of the present invention will be described below. The surface portion of the protective layer of the oxygen sensor element according to the present invention forms a dense layer. Therefore, the gas to be measured can be prevented from reaching the electrode, and the deposition of carbon on the electrode can be prevented. Further, in the present invention, since the gas to be measured can be prevented from reaching the electrode, the gaseous carbon compound can also be prevented from reaching the electrode.

The dense layer according to the present invention is a part of the protective layer. For this reason, there is almost no difference in the coefficient of thermal expansion between the protective layer and the dense layer, and it is possible to prevent the dense layer from being peeled off or cracked due to a thermal cycle or exposure to a high-temperature atmosphere. Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output in long-term use. Further, since there is no other coating between the protective layer and the electrode, the protective layer can strongly adhere to the electrode. For this reason, cracks and peeling of the protective layer can be prevented.

As described above, according to the present invention, it is possible to provide an oxygen sensor element in which carbon is hardly deposited on an electrode and a stable output can be maintained during long-term use.

A trap layer can be provided so as to cover the protective layer and the dense layer (see FIG. 6 described later). Thereby, it is possible to prevent the poisonous substance in the gas to be measured from reaching the electrode. Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output over a long period of use.

Note that the trap layer is a porous layer formed by a large number of particles, and has a function of capturing poisons such as P, Ca, and Pb contained in the gas to be measured.
In addition, the trap layer is formed by adding a slurry prepared by adding water, an inorganic binder, and a dispersant to heat-resistant particles such as γ-Al ₂ O ₃ so as to cover the protective layer and the dense layer. It can be formed by dipping or spraying on a surface and baking at 500 to 900 ° C. The baking temperature of the trap layer is preferably lower than the heating temperature for forming a dense layer described later.

Next, as in the second aspect of the present invention, the porosity of the dense layer is preferably 4% or less. Thereby, the effect according to the present invention can be reliably obtained. If the porosity is greater than 4%, the gas to be measured is insufficiently shut off, and the gas to be measured may reach the electrode while diffusing through the protective layer through the pores in the dense layer. . Further, the porosity is more preferably 3% or less.

Next, it is preferable that the porosity of the dense layer is 4% or less and the thickness is 5 to 20 μm. Thereby, the effect according to the present invention can be reliably obtained. If the porosity is greater than 4%, the gas to be measured may not be shut off as described above. When the thickness of the dense layer is less than 5 μm, if the dense layer is too thin and the porosity is large, the gas may pass through and the effect of blocking the gas may not be exhibited. On the other hand, when the thickness is more than 20 μm, it is difficult to allow the densified substance to penetrate into the protective layer, which may make the production difficult.

Next, it is preferable that the dense layer is formed near the mounting base of the oxygen sensor element. Since this portion is the portion where carbon is most easily deposited as described later, the effect according to the present invention can be more reliably obtained.

Here, the mounting base will be described. Generally, the oxygen sensor element is used by being mounted in an oxygen sensor constituted by a housing, an atmosphere side cover and the like.
When attaching the oxygen sensor element to the oxygen sensor, for example, as shown in FIG. In this case, a portion where the oxygen sensor element and the housing are in contact is a mounting base portion.

The vicinity of the mounting base is clear from the figure, but the oxygen sensor element and the housing are close to each other, so that the flow of the gas to be measured tends to be stagnant. Therefore, in the vicinity of the mounting base portion, the time of exposure to the carbon compound is longer than in other portions. For this reason, the electrode in the vicinity of the mounting base is a portion to which carbon is likely to adhere.

Further, since the lower end portion of the oxygen sensor element is exposed to the gas to be measured, even if carbon adheres,
The carbon is separated and removed by the flow of the gas to be measured.
However, the flow of the gas to be measured is likely to be stagnant in the vicinity of the mounting base portion of the oxygen sensor element, so that the above-described peeling and removal is hard to occur. From the above points, the vicinity of the mounting base portion is a portion where carbon easily adheres.

It is preferable that the upper end of the dense layer is the mounting base portion and the lower end of the dense layer is 10 mm above the tip of the oxygen sensor element (see FIG. 5). If the upper end is below the mounting base, the gas to be measured may enter through the protective layer where the dense layer does not exist, causing carbon deposition on the electrode.
If the lower end is lower than 10 mm above, the gas to be measured hardly reaches the part of the oxygen sensor element which has a dominant role in detecting the oxygen concentration, and the response of the oxygen sensor element is reduced. May decrease.

Next, it is preferable that the dense layer contains at least one of Al and Mg phosphates. As a result, a dense layer having heat resistance is formed, there is no thermal deterioration in a high-temperature atmosphere, and since the dense layer is chemically bonded to the MgO.Al ₂ O ₃ spinel forming the protective layer, heat exchange with the protective layer is prevented. A dense layer having a small difference in expansion coefficient is formed, and cracking and peeling of the dense layer due to a cooling and heating cycle during use can be prevented.

Next, as in the invention of claim 6, it is preferable that the dense layer is filled with ceramic fine particles having a particle size of 0.1 μm or less, and the fine particles are sintered. By using ceramic fine particles, heat resistance is ensured, and since the coefficient of thermal expansion with the protective layer is small, cracking and peeling of the dense layer due to cooling and heating cycles during use can be prevented. In addition,
More preferably, fine particles having a size of 0.05 μm or less are used.

When the particle size is larger than 0.1 μm, the number of particles filled in the pores of the protective layer is small, and the gap between the particles is large. Also, since the voids after sintering are large,
There is a possibility that the gas to be measured passes through due to insufficient blockage of the pores and the gas blocking effect cannot be exhibited.

Next, a seventh aspect of the present invention is directed to a cup-shaped solid electrolyte body having a closed front end, an open base end, and an inner chamber provided therein, and a front end and a base end of the solid electrolyte body. A measurement electrode formed on at least a part of the outer surface of the portion and exposed to the gas to be measured, and a measurement electrode provided on the inner surface facing the inner chamber and facing the measurement electrode via the solid electrolyte body. A reference electrode provided therein, and a heater inserted into the inner chamber and heating a distal end side of the solid electrolyte body. The measurement electrode is covered with a protective layer. An oxygen sensor element is characterized in that a dense layer having a porosity smaller than that of the protective layer is formed on at least a part of the surface portion on the side of the portion (see FIG. 1 described later).

The base layer of the protective layer according to the present invention is a dense layer, and the porosity of the dense layer is smaller than that of the protective layer. That is, it is denser than the protective layer. For this reason, it is possible to prevent the gas to be measured from reaching the measurement electrode in the portion where the dense layer exists, thereby preventing carbon deposition at the measurement electrode in this portion. Further, in the present invention, an effect particularly effective for gaseous carbon compounds can be obtained.

The dense layer according to the present invention is a part of the protective layer. For this reason, there is almost no difference in the coefficient of thermal expansion between the protective layer and the dense layer, and it is possible to prevent the dense layer from being peeled off or cracked due to a thermal cycle or exposure to a high-temperature atmosphere.
Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output in long-term use. Further, since there is no other coating or the like between the protective layer and the measurement electrode, the protective layer can strongly adhere to the measurement electrode. For this reason, cracks and peeling of the protective layer can be prevented.

Further, a heater is inserted into the oxygen sensor element, and the heater is configured to mainly heat the tip side of the oxygen sensor element. For this reason, even if carbon adheres to the tip end side of the oxygen sensor element, the adhered carbon is burned and removed by the heat of the heater. However, on the base end side of the oxygen sensor element, combustion removal by the heat of the heater hardly occurs, and carbon is easily attached. Therefore, the effect of the present invention can be further ensured by providing the dense layer on the base end side.

As described above, according to the present invention, it is possible to provide an oxygen sensor element in which carbon is hardly deposited on the electrode and a stable output can be maintained during long-term use.

As shown in FIG. 1 to be described later, the base end side indicates a side of the protective layer closer to the base end of the oxygen sensor element. Similarly, the tip portion side also shows a side closer to the tip portion of the oxygen sensor element in the protective layer.

Next, as in the invention of claim 8, the porosity of the portion formed on the base end side of the protective layer is preferably 4% or less. Thereby, the effect according to the present invention can be reliably obtained. If the porosity is greater than 4%, the gas to be measured may not be shut off as in the case of the second aspect.

Next, as in the ninth aspect of the present invention, the thickness of the portion of the protective layer formed on the base end side is 5 to 20 μm.
m is preferable. Thereby, the effect according to the present invention can be reliably obtained. If the thickness of the dense layer is less than 5 μm, the dense layer may be too thin to allow gas to pass therethrough, as in the case of claim 2, and the gas blocking effect may not be exhibited. On the other hand, when the thickness is larger than 20 μm as in the case of the second aspect, it is difficult to infiltrate the densification substance into the inside of the protective layer, which may make the production difficult.

According to a tenth aspect of the present invention, there is provided an oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, wherein at least one of the electrodes is covered with a protective layer. And a method for manufacturing an oxygen sensor element in which the protective layer has a dense layer densified from the inside at least in a part of its surface, wherein the protective layer reacts with a protective layer material constituting the protective layer. A method for manufacturing an oxygen sensor element, comprising applying a substance to the protective layer, impregnating the protective layer, and then heating to form the dense layer.

The heating temperature is desirably 500 to 1000 ° C. If the heating temperature is lower than 500 ° C., the heat history when using the oxygen sensor element may exceed the heating temperature. May occur and cracks may occur in the protective layer.

On the other hand, when the heating temperature exceeds 1000 ° C., there is a possibility that the heating temperature at the time of manufacturing the electrode may be exceeded, and the electrode may be agglomerated. The performance of the oxygen sensor element may be reduced because the electrode in which agglomeration occurs has reduced catalytic performance.

In the production of the oxygen sensor element, after a solid electrolyte body is produced, an electrode is produced thereon, and a protective layer is produced so as to cover the electrode. Thereafter, a dense layer can be formed on the protective layer.

In the present invention, by applying and heating the above substance, the above substance reacts with the material of the protective layer to form a compound. At this time, the compound expands in volume, and can block pores near the surface of the protective layer (see FIG. 3).

Therefore, according to the manufacturing method of the present invention, since a part of the surface of the protective layer forms a dense layer, the thermal expansion between the protective layer and the dense layer can be achieved. It is possible to obtain an oxygen sensor element having almost no difference in the rate and having almost no peeling or cracking of the dense layer due to exposure to a cooling / heating cycle or a high-temperature atmosphere. Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output in long-term use.

Further, since there is no other coating between the protective layer and the electrode, the protective layer can strongly adhere to the electrode. For this reason, cracks and peeling of the protective layer can be prevented. Since the dense layer can prevent the gas to be measured from reaching the electrode,
An oxygen sensor element with almost no carbon deposition on the electrode can be obtained.

As described above, according to the present invention, it is possible to provide a method of manufacturing an oxygen sensor element in which carbon is hardly deposited on an electrode and a stable output can be maintained during long-term use.

The type of substance used to form the dense layer depends on the type of the protective layer material, but reacts with the constituent material of the protective layer to form a thermally and chemically stable compound after the reaction. Preferably, the protective layer is made of MgO.
In the case of Al ₂ O ₃ spinel, phosphoric acid is preferably used. Thereby, the M for forming the protective layer is formed.
g, Al and phosphoric acid react with each other to form a phosphate, and further expand the volume, thereby closing the pores of the protective layer and shutting off the gas to be measured.

In preparing the dense layer, the above-mentioned substance is made into a liquid or a two-fold diluted solution or the like, and this is applied to a desired portion of the protective layer by a method such as brush painting and dipping. It is preferred to apply and then heat it. In addition, it is preferable that masking is performed in advance on portions where coating is unnecessary, that is, portions where a dense layer is unnecessary.

Alternatively, a 5 to 10-fold diluted solution of the above substance is prepared, applied to the protective layer, and heated. Further, the application of the 5 to 10-fold diluted solution and heating are repeated. The dense layer can be produced by the above method.

According to an eleventh aspect of the present invention, there is provided an oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, wherein at least one of the electrodes is covered with a protective layer. And a method for manufacturing an oxygen sensor element in which the protective layer has a dense layer densified from the inside on at least a part of its surface, wherein pores in the protective layer have a particle size of 0.1 μm. A method of manufacturing an oxygen sensor element, characterized in that the following fine particles are filled with ceramic particles and the fine particles are sintered to form the dense layer. More preferably, the particle size of the fine particles is set to 0.05 μm or less.

It is desirable that the fine particles are made of a material having the same quality as the protective layer or a material having the same quality as the protective layer after sintering. Further, as the fine particles, for example, heat-resistant particles such as alumina and silica, or particles capable of obtaining heat resistance after sintering can be used.

The fine particles are in the form of a sol or the like, and an aqueous solution containing the sol is brush-painted, adhered to the surface of the protective layer by dipping or the like, and impregnated to form pores in the protective layer. It is preferable to fill with. Further, the attachment of the fine particles is preferably performed in a reduced pressure atmosphere. Thereby, the fine particles can be filled into the inside of the pores.

The fine particles have a particle diameter (diameter) of 0.1.
When the diameter is larger than μm, the number of fine particles filled in the pores of the protective layer decreases, and the pores become insufficiently closed.
There is a possibility that a dense layer that cannot sufficiently block the gas to be measured may be obtained.

The sintering is performed at a temperature of 500 to 1000 ° C.
It is desirable to carry out by heating. If the temperature is lower than 500 ° C., the heat history when using the oxygen sensor element may exceed the above-mentioned temperature, and sintering of the fine particles may occur during the use of the oxygen sensor element, and the protective layer may be cracked. May occur.

On the other hand, if the temperature exceeds 1000 ° C., the temperature may exceed the heating temperature at the time of manufacturing the electrode, and aggregation or the like may occur on the electrode. The performance of the oxygen sensor element may be reduced because the electrode in which agglomeration occurs has reduced catalytic performance.

In the present invention, the fine pores in the protective layer are filled with ceramic fine particles having a particle size of 0.1 μm or less, and these fine particles are sintered. Thereby, the fine particles can be bonded to each other and further to the protective layer material constituting the protective layer. Therefore, pores near the surface of the protective layer can be closed (see FIGS. 7 and 8). More preferably, the particle size of the fine particles is set to 0.05 μm or less.

Therefore, according to the manufacturing method of the present invention, since a part of the surface of the protective layer forms a dense layer, the thermal expansion coefficient between the protective layer and the dense layer is as described in claim 1. Thus, an oxygen sensor element can be obtained in which there is almost no difference, and there is almost no peeling or cracking of the dense layer due to exposure to a cooling / heating cycle or a high-temperature atmosphere. Therefore, it is possible to obtain an oxygen sensor element that can maintain a stable output in long-term use.

Further, since there is no other coating or the like between the protective layer and the electrode, the protective layer can strongly adhere to the electrode. For this reason, cracks and peeling of the protective layer can be prevented. Since the dense layer can prevent the gas to be measured from reaching the electrode,
An oxygen sensor element with almost no carbon deposition on the electrode can be obtained.

As described above, according to the present invention, it is possible to provide a method of manufacturing an oxygen sensor element in which carbon is hardly deposited on an electrode and a stable output can be maintained during long-term use.

[0064]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 An oxygen sensor element and a method of manufacturing the same according to an embodiment of the present invention will be described with reference to FIGS. As shown in FIGS. 1 and 2, the oxygen sensor element 1 according to the present embodiment includes a solid electrolyte body 10 and a pair of a measurement electrode 11 and a reference electrode 14 provided on the surface of the solid electrolyte body 10. The measurement electrode 11 is covered with a protective layer 12, and the protective layer 12 has a dense layer 13 densified from the inside at a part of its surface.

The structure of the oxygen sensor element 1 will be described in detail. As shown in FIG.
Is a cup-shaped solid electrolyte body 10 having a closed top end 191 and an open base end 192, and an inner chamber 1 provided therein.
9 and the outer surface 10 of the solid electrolyte body 10
Reference numeral 1 denotes a measurement electrode 11 and an inner surface 1 facing the inner chamber 19.
09 is provided with a reference electrode 14. In addition, the inner room 1
A heater (not shown) is inserted in 9 and the measurement electrode 1
1 is provided with a protective layer 12 for protecting it.

The protective layer 12 is made of MgO.Al ₂ O ₃ spinel. In addition, as shown in FIG.
2 is a porous layer formed by plasma spraying. Reference numeral 129 denotes MgO.Al ₂ O ₃ spinel particles. The dense layer 13 on the surface of the protective layer 12 is formed so as to close pores existing on the surface of the porous protective layer 12, and contains aluminum phosphate or the like.

A projection 105 is formed on the outer periphery of the solid electrolyte member 10, and the projection 105 has a tapered portion 108. The protective layer 12,
The dense layer 13 is formed. In the tapered portion 108, the oxygen sensor element 1 according to the present example is attached to a housing 30 of the oxygen sensor 3 described later (see FIG. 4). That is, the tapered portion 108 is
It will be the mounting base part.

Next, the oxygen sensor 3 equipped with the oxygen sensor element 1 will be described. As shown in FIG. 4, the oxygen sensor 3 includes a housing 30 and an oxygen sensor element 1 inserted and arranged in the housing 30. A measured gas chamber 33 is formed below the housing 30, and a double measured gas side cover 330 for protecting the tip of the oxygen sensor element 1 is provided. Further, an introduction hole 331 for introducing a measured gas is provided in the measured gas side cover.

A rod-shaped heater 34 is inserted into the inner chamber 19 of the oxygen sensor element 1. The heater 34 is inserted and arranged so as to secure a desired clearance between the heater 34 and the inner surface 109 of the inner chamber 19.

Above the housing 30, three atmospheric side covers 31, 32, and 33 are provided. Lead wires 391-39 are attached to the upper ends of the atmosphere side covers 32 and 33, respectively.
3 is provided with an elastic insulating member 39 into which 3 is inserted. The lead wires 391 and 392 take out a current generated in the solid electrolyte body 2 as a signal and send it to the outside.
The lead wire 393 is for supplying electricity to the heater 34 to generate heat.

At the lower ends of the leads 391 and 392, connection terminals 383 and 384 are provided.
3, 384 establishes continuity with the terminals 381, 382 fixed to the oxygen sensor element 1. Note that the terminals 381 and 382 are fixed to the ends of the measurement electrode and the reference electrode in the oxygen sensor element 1.

Next, a method for manufacturing the oxygen sensor element 1 will be described in detail. Weigh raw material powder such as ZrO ₂ ,
After mixing, a cup-shaped molded body as shown in FIG. 1 is produced. Next, the formed body is fired to form the solid electrolyte body 10. Next, the outer surface 10 of the solid electrolyte
The measurement electrode 11 and the reference electrode 14 are formed on the first electrode 1 and the inner surface 109 by using any method such as chemical plating, vapor deposition, paste application, and baking.

Next, the surface of the measurement electrode 11 was coated with MgO
· A Al ₂ O ₃ spinel and plasma spraying to form the protective layer 12. The protective layer 12 has a thickness of 50 to 300 μm, a porosity of 5 to 30%, and an average pore diameter of 500 to 5000 °. Next, phosphoric acid having a concentration of 100% is applied to the surface of the protective layer 12 with a brush. Thereafter, the solid electrolyte body 10 is heated to a temperature of 900 ° C.

By this heating, the phosphoric acid and the protective layer 12
Reacts with the MgO.Al ₂ O ₃ spinel in the above to form aluminum phosphate and the like. Further, the volume expansion of the aluminum phosphate or the like causes the protection layer 12
The pores on the surface portion are closed. As a result, the porosity of the surface of the protective layer 12 becomes about 3%, and the dense layer 13 which does not allow the gas to be measured to pass is formed. The thickness of this dense layer is 10 μm. Thus, the oxygen sensor element 1 according to the present example was obtained.

Next, the performance of the oxygen sensor element according to this embodiment will be described with reference to FIG. With respect to the oxygen sensor element according to the present example, Samples 1 to 13 were prepared by changing the position where the dense layer was provided and the thickness T of the dense layer variously as shown in Table 1. The durability and initial response of these samples were measured as shown below.

First, the position where the dense layer is provided will be described. F in the table is a distance between a contact position between the oxygen sensor element and the housing and a densification start position (upper end of the dense layer) as shown in FIG. The contact position between the oxygen sensor element and the housing is the mounting base in this example.

G in the table is the distance between the densification end position (the lower end of the dense layer) and the tip of the oxygen sensor element as shown in FIG. And F is 5-5 mm,
An oxygen sensor element in which G was changed in the range of 0 to 10 mm was produced. In addition, "-" in F indicates that the dense layer was provided beyond the contact position between the oxygen sensor element and the housing to a position facing the inside of the housing.
Further, an oxygen sensor element in which the thickness T of the dense layer was changed in the range of 2 to 20 μm was manufactured. The thickness T of the dense layer is illustrated in FIG.

Next, Table 1 shows the performance of the oxygen sensor element.
As shown in Table 2, durability and initial response were shown. The durability was determined based on the adhesion and appearance of the protective layer after the accelerated durability test. In the accelerated endurance test, the oxygen sensor shown in FIG. 4 in which the oxygen sensor elements serving as each sample were attached to the exhaust system of a 2000 cc in-line four-cylinder engine equipped with a fuel injection device was arranged. Further, the exhaust gas temperature was set to 850 ° C., and the excess air ratio in the exhaust gas was set to 0.90 (reducing atmosphere).
The durability time was 100 hours.

After completion of the accelerated durability test, the protective layer of the oxygen sensor element on each sample was evaluated for taping. In this evaluation, the case where the protective layer was peeled was determined as x, and the case where the protective layer was not peeled was evaluated as ○. In addition, the appearance of the protective layer on the oxygen sensor element on each sample was visually observed to check for cracks or peeling. The case where there was a crack or peeling in the protective layer was evaluated as x, and the case where there was no crack or peeling was evaluated as ○.

The measurement of the initial response was performed as follows. The oxygen sensor according to FIG. 4 in which an oxygen sensor element serving as each sample was attached to the exhaust system of a 2000 cc in-line six-cylinder engine with a fuel injection device was arranged.

Then, by using unleaded gasoline,
100r. p. m. And the temperature of the tip of the oxygen sensor element was maintained at 690 to 710 ° C. Then, when switching from λ = 0.9 to λ = 1.1, the output of the oxygen sensor element is changed from 0.6V to 0.3V.
Was measured for the time to change. This time is 150m
The case of less than s was judged as ○, the case of 150 ms or more and less than 200 ms was judged as Δ, and the case of 200 ms or more was judged as ×. The above measurement was performed before and after the durability test.

In the same table, Samples 3 to 9, Sample 12,
According to 13, an oxygen sensor having high initial response and high durability is provided by providing at least a dense layer between a portion where the protective layer and the housing come into contact with each other and a portion 10 mm above the tip of the oxygen sensor element. It was found that the device was obtained.

Next, the operation and effect of this embodiment will be described. The surface portion of the protective layer 12 of the oxygen sensor element 1 according to the present embodiment forms the dense layer 13, thereby preventing the gas to be measured from reaching the measurement electrode 11. Therefore, the deposition of carbon on the measurement electrode 11 can be prevented.

The dense layer 13 according to the present embodiment is a part of the protective layer 12. Therefore, the protective layer 12 and the dense layer 13
There is almost no difference in the coefficient of thermal expansion between them, and thermal stress is generated between them by exposure to a cooling / heating cycle or a high-temperature atmosphere, so that separation of the dense layer 13 and generation of cracks can be prevented. Therefore, it is possible to obtain the oxygen sensor element 1 that can maintain a stable output in long-term use.

The protective layer 12 and the measuring electrode 11
Since no other film or the like exists between the protective layer 1 and the
2 can strongly adhere to the measurement electrode 11. For this reason, cracking, peeling, and the like of the protective layer 12 can also be prevented.

As described above, according to the present embodiment, it is possible to provide an oxygen sensor element capable of maintaining a stable output over a long period of use with almost no carbon deposition on the electrode and a method of manufacturing the same. Can be. Further, according to the manufacturing method of this example, an excellent oxygen sensor element as described above can be manufactured.

In the present embodiment, the dense layer 13 is formed on a tapered portion 108 serving as a mounting base of the oxygen sensor element 1. Since this portion is the portion where carbon is most likely to be deposited as described later, the effect according to this example can be obtained more reliably.

That is, the oxygen sensor element 1 has a heater 3
The heater 34 is configured to mainly heat the vicinity of the tip 101 of the oxygen sensor element 1. For this reason, even if carbon adheres in the vicinity of the tip end portion 101 of the oxygen sensor element 1, the adhered carbon is burned and removed by the heat of the heater 34. However, at the mounting base,
The combustion removal by the heat of the heater 34 hardly occurs.

Further, as shown in FIG. 4, the tip 101 of the oxygen sensor element 1 faces the introduction hole 331 for introducing the gas to be measured into the gas chamber 33 to be measured. The carbon is separated and removed by the flow of the gas to be measured. However, the flow of the gas to be measured is likely to be stagnant at the mounting base portion of the oxygen sensor element 1, so that the above-mentioned peeling and removal is unlikely to occur.

As shown in FIG. 6, the protective layer 12
In addition, the trap layer 15 can be provided outside the dense layer 13. Thereby, it is possible to prevent the poisonous substance in the gas to be measured from reaching the measurement electrode 11. Therefore, it is possible to obtain the oxygen sensor element 1 that can maintain a stable output in a long-term use.

[0091]

[Table 1]

Embodiment 2 This embodiment is directed to an oxygen sensor element having a dense layer formed by filling pores in a protective layer with ceramic fine particles having a particle size of 0.1 μm or less and sintering these fine particles. A method for manufacturing a dense layer according to an example will be described with reference to FIGS.

The oxygen sensor element according to this embodiment has the same structure as that of the first embodiment, and includes a cup-shaped solid electrolyte body 10 and a pair of measurement electrodes 11 provided on the surface of the solid electrolyte body 10. Consists of a reference electrode. The measuring electrode 11 is covered with a protective layer 12, and the protective layer 12
Is a dense layer 13 densified from the inside on a part of the surface portion.
Is formed. The solid electrolyte body 10 has an inner chamber 19 into which a heater (not shown) is inserted.

The protective layer 12 and the dense layer 13 according to this example will be described. The protective layer 12 according to the present embodiment is also made of MgO.Al ₂ O ₃ spinel as in the first embodiment. Further, as shown in FIG. 7, the protective layer 12 is a porous layer formed by plasma spraying. Reference numeral 129 denotes MgO.
Al ₂ O ₃ spinel particles. Then, the protective layer 1
The pores on the surface of No. 2 are closed by a large number of fine particles 139, and the dense layer 13 is formed here. The fine particles 139 are made of Al ₂ O ₃ particles, and the particle size is 0.1 μm or less. The porosity of the dense layer 13 is 3%. Others are the same as the oxygen sensor element of the first embodiment.

A method for manufacturing a dense layer according to this example will be described below. The solid electrolyte member 10, the measurement electrode 11, the protective layer 12, and the like are manufactured in the same manner as in the first embodiment. Next, an alumina sol solution was applied to the protective layer 12 by brush painting. Thereby, the fine particles 139 of alumina present in the alumina sol were filled in the pores on the surface of the protective layer 12.

Next, the alumina sol solution applied together with the solid electrolyte is heated. Thereby, the fine particles 1
39 are mutually sintered or fine particles 139 and the protective layer 1
The particles 129 constituting the particles 2 were sintered to form the dense layer 13.

Next, the performance of the oxygen sensor element according to this embodiment will be described with reference to FIG. In the oxygen sensor element according to this example, the position of the dense layer 13 was changed variously as shown in Table 2, and the sample 21 was manufactured.
~ 29 were prepared. The durability and initial response of these samples were measured in the same manner as in Example 1. F and G in the table have the same meanings as in the first embodiment, Table 1, and FIG.

Samples 23 to 25 and Table 2 in Table 2
8 and 29, by providing at least a dense layer between the portion where the protective layer contacts the housing related to the oxygen sensor and the portion 10 mm above the tip of the oxygen sensor element as in the first embodiment. High initial response,
It was found that a highly durable oxygen sensor element was obtained. In addition, it was found that also in the manufacturing method according to the present embodiment, an excellent oxygen sensor element can be manufactured as in the first embodiment.

[0099]

[Table 2]

Embodiment 3 This embodiment describes an oxygen sensor element having a dense layer made of glass as shown in FIG. The oxygen sensor element according to the present embodiment has the same structure as that of the first embodiment, and includes a cup-shaped solid electrolyte body 10 and a pair of a measurement electrode 11 and a reference electrode provided on the surface of the solid electrolyte body 10. Consisting of The measurement electrode 11 is covered with a protective layer 12, and the protective layer 12 has a dense layer 13 densified from the inside at a part of its surface. The solid electrolyte body 10 has an inner chamber, into which a heater (not shown) is inserted.

The dense layer will be described. The dense layer 13 according to the present embodiment is made of a vitreous heat-resistant coating. More specifically, a solution in which a powder of MgO and Al ₂ O ₃ and a low melting point glass such as borosilicate glass are dissolved in a resin such as ethyl cellulose is formed into a slip shape, and this is coated with a brush to obtain the desired protective layer 12. At a temperature of 800 to 10
It consists of a film formed by baking at 00 ° C.

If the temperature is lower than 800 ° C., the glass in the material does not melt sufficiently, the adhesion to the protective layer becomes insufficient, and the dense layer may peel off during use of the oxygen sensor element. is there. On the other hand, if the temperature is higher than 1000 ° C., the electrodes may aggregate.

The thermal expansion coefficient of the dense layer 13 according to the present example has an expansion coefficient substantially equal to that of the protective layer. That is, the coefficient of thermal expansion of the protective layer 12 is 75 to 85 × 10 ⁻⁷ / ° C., and the coefficient of expansion of the dense layer 13 is 75 to 85 × 10 ⁻⁷ / ° C.

The above-mentioned film may be made of any material having a coefficient of thermal expansion equal to or close to that of the protective layer 12 or a material which does not soften at a temperature of about 500 to 700 ° C. which is the operating temperature of the oxygen sensor element. It may be composed of a substance other than the material.

Next, the performance of the oxygen sensor element according to this embodiment will be described with reference to FIG. That is, an oxygen sensor element (sample 31) provided with a dense layer in which F and G according to FIG. 5 have the values shown in Table 3 was prepared, and the durability and initial response were measured as in the first embodiment. did.
As a result, as can be seen from the table, it was found that the oxygen sensor element having the dense layer according to this example had excellent durability and initial response.

[0106]

[Table 3]

Fourth Embodiment As shown in FIG. 10, this embodiment is a limiting current type oxygen sensor element 4. The oxygen sensor element 4 according to the present embodiment includes a cup-shaped solid electrolyte body 10 and an inner chamber 19 provided therein, and a measuring electrode 421 and insulating layers 411 and 412 are provided on the outer surface of the solid electrolyte body 10. The reference electrode 14 is provided on the inner surface facing the inner chamber 19. A heater (not shown) is inserted into the inner chamber 19, and protective layers 43, 4 for protecting the heater are inserted into the measuring electrode 421.
Further, a poisoning substance trap layer 423 is provided on the outside thereof.

The protection layer 43 is made of MgO.Al ₂
A porous layer made of O ₃ spinel and formed by plasma spraying. The dense layer 13 on the surface of the protective layer 43 is formed so as to close the pores on the surface of the porous protective layer 43, and contains aluminum phosphate. In addition, the dense layer 13 is provided in the first embodiment.
As shown in (1), phosphoric acid is applied to the surface of the protective layer 43 by brush painting, and is heated to heat. Others are the same as the first embodiment. Further, the oxygen sensor element 4 of the present example has the same effect as the first embodiment.

[Brief description of the drawings]

FIG. 1 is an explanatory longitudinal sectional view of an oxygen sensor element according to a first embodiment.

FIG. 2 is a diagram showing an oxygen sensor element according to the first embodiment;
FIG.

FIG. 3 is an enlarged explanatory view of a main part of the oxygen sensor element according to the first embodiment.

FIG. 4 is an explanatory cross-sectional view of the oxygen sensor according to the first embodiment and provided with the oxygen sensor element according to the present embodiment.

FIG. 5 is an explanatory diagram showing F and G in Table 1 according to the first embodiment.

FIG. 6 is an explanatory longitudinal sectional view of the oxygen sensor element provided with the trap layer according to the first embodiment.

FIG. 7 is an enlarged explanatory diagram of a main part of an oxygen sensor element according to a second embodiment.

FIG. 8 is an explanatory diagram of a protective layer and a dense layer according to the second embodiment.

FIG. 9 is an enlarged explanatory view of a main part of an oxygen sensor element according to a third embodiment.

FIG. 10 is an explanatory longitudinal sectional view of an oxygen sensor element according to a fourth embodiment.

[Explanation of symbols]

 1,4. . . Oxygen sensor element, 10. . . Solid electrolyte body, 101. . . Outer surface, 109. . . Inner surface, 11. . . Measuring electrode, 12, 43. . . 12. protective layer; . . 13. dense layer, . . Reference electrode, 191. . . Tip, 192. . . Base end,

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiromi Sano 1-1-1, Showa-cho, Kariya-shi, Aichi Pref.

Claims (11)

[Claims] 1. An oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on a surface of the solid electrolyte body, wherein at least one of the electrodes is covered with a protective layer, and the protective layer is An oxygen sensor element, wherein a dense layer densified from the inside is formed on at least a part of a surface portion. 2. The oxygen sensor element according to claim 1, wherein the porosity of the dense layer is 4% or less. 3. The oxygen sensor element according to claim 1, wherein the porosity of the dense layer is 4% or less and the thickness is 5 to 20 μm. 4. The method according to claim 1, wherein:
The oxygen sensor element according to claim 1, wherein the dense layer is formed near a mounting base of the oxygen sensor element. 5. The method according to claim 1, wherein:
An oxygen sensor element, wherein the dense layer contains at least one of Al and Mg phosphates. 6. The method according to claim 1, wherein:
An oxygen sensor element characterized in that the dense layer is filled with ceramic fine particles having a particle size of 0.02 μm or less, and the fine particles are sintered. 7. The distal end is closed, the proximal end is opened,
A cup-shaped solid electrolyte body having an inner chamber therein, a measurement electrode formed on at least a part of an outer surface of a distal end and a base end of the solid electrolyte body, and exposed to a gas to be measured; A reference electrode provided on the inner surface facing the inner chamber and opposed to the measurement electrode via the solid electrolyte body; and a reference electrode inserted into the inner chamber for heating the tip side of the solid electrolyte body In addition to the heater, the measurement electrode is covered with a protective layer, and a dense layer having a smaller porosity than the protective layer is formed on at least a part of the surface of the protective layer on the base end side. An oxygen sensor element characterized in that: 8. The oxygen sensor element according to claim 7, wherein the porosity of the dense layer is 4% or less. 9. The oxygen sensor element according to claim 7, wherein the dense layer has a thickness of 5 to 20 μm. 10. An oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on the surface of the solid electrolyte body, wherein at least one of the electrodes is covered with a protective layer, and the protective layer is A method for manufacturing an oxygen sensor element in which a dense layer densified from the inside is formed on at least a part of a surface portion, wherein a substance that reacts with a material for a protective layer constituting the protective layer is applied to the protective layer. And a method for producing an oxygen sensor element, wherein the dense layer is formed by impregnating and then heating. 11. An oxygen sensor element comprising a solid electrolyte body and a pair of electrodes provided on a surface of the solid electrolyte body, wherein at least one of the electrodes is covered with a protective layer, and the protective layer is A method for manufacturing an oxygen sensor element in which a dense layer densified from the inside is formed on at least a part of a surface portion, wherein pores in the protective layer are filled with ceramic fine particles having a particle size of 0.1 μm or less. And manufacturing the oxygen sensor element by sintering the fine particles to form the dense layer.