JP2003344351A - Oxygen sensor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a fine crack from occurring at a corner section in an air introduction hole, and to provide a long-life oxygen sensor. <P>SOLUTION: In the oxygen sensor element provided with a flat substrate 1 made of an air introduction hole 2 having a ceramic solid electrolyte; a reference electrode 3 formed on the inner wall of the air introduction hole 2, and a measuring electrode 4 formed on the outer surface of the substrate 1 opposite to the reference electrode 3; a corner section 8 where the two wall surfaces of the air introduction hole 2 orthogonally crossing each other is formed by a curved surface having a radius of curvature of at least 0.03 mm. Additionally, the cross-section shape of the air introduction hole meets relationship where a/b becomes 0.2 to 5.0 when the maximum length in a direction in parallel with a surface where the reference electrode is formed is set to be (a) and the maximum length in a direction at right to a surface where the reference electrode 3 is formed is set to be (b). <P>COPYRIGHT: (C)2004,JPO

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 for controlling the ratio of air to fuel in an internal combustion engine such as an automobile.

[0002]

2. Description of the Related Art Currently, in an internal combustion engine of an automobile or the like, the oxygen concentration in the exhaust gas is detected, and the amount of air and fuel supplied to the internal combustion engine is controlled based on the detected value so that Harmful substances such as CO, H
A method of reducing C and NOx is adopted.

As the detecting element, a solid electrolyte mainly composed of zirconia having oxygen ion conductivity is used, and a pair of electrode layers are formed on the outer surface and the inner surface of a cylindrical substrate whose one end is sealed. An electrolyte type oxygen sensor element is used.

In such an oxygen sensor, generally,
In a so-called theoretical air-fuel ratio sensor element (λ sensor) used for controlling the ratio of air to fuel near 1, a porous layer serving as a protective layer is provided on the surface of the outer measuring electrode,
The air-fuel ratio of the engine intake system is controlled by detecting the oxygen concentration difference generated on both sides of the cylindrical substrate at a predetermined temperature.

On the other hand, a so-called wide range air-fuel ratio sensor element (A / F) used to control a wide range of air-fuel ratios.
Sensor) is made of a solid electrolyte, as shown in FIG.
The measurement electrode 33 and the reference electrode 34 are adhered to the plate-shaped base 32 having the air introduction hole 31 of which one end is sealed at the position where the inner wall surface and the outer surface of the base 32 face each other. . Then, on the surface of the measurement electrode 33, the ceramic porous layer 3 serving as a gas diffusion rate controlling layer having fine pores.
5, the applied voltage is applied between the measurement electrode 33 and the reference electrode 34, the limiting current value obtained at that time is measured, and the air-fuel ratio is directly controlled. Further, both of the oxygen sensor elements (λ sensor, A / F sensor) function as sensors by being heated to 500 ° C. or higher by being heated to a predetermined temperature by the heating element.

The flat oxygen sensor element as shown in FIG. 5 is usually produced by stacking sheets of solid electrolyte powder and firing them. For example, as shown in FIG. The base 32 includes 32a, 32b, 32c
It is composed of a three-layer structure. Therefore, the cross-sectional shape of the air introduction hole 31 in the oxygen sensor element is necessarily rectangular.

[0007]

However, when the air introduction hole 31 has a rectangular cross section, when the sheets are stacked and fired, a solid electrolyte such as ZrO ₂ forming a sensor portion and a platinum such as an electrode are formed. Further, stress is generated due to the difference in thermal expansion from the alumina insulating layer, and cracks are likely to occur at the corners. This crack was found in the visual inspection hole 3
Since it is inside 1, it is very difficult to find it, and it is also difficult to find it by ultrasonic flaw detection or X-ray method if the crack is small. As a result, a problem arises that cracks grow during use and the sensor breaks. Therefore, according to the present invention, it is an object of the present invention to provide a long-life oxygen sensor element by preventing the generation of fine cracks that occur at the corners of the air introduction hole.

[0008]

As a result of studying the above problems, the present inventor has found that a plate-like substrate made of a ceramic solid electrolyte having an air introduction hole and a reference electrode formed on the inner wall of the air introduction hole. And a measurement electrode formed on the outer surface of the base body facing the reference electrode, the corner portion where two wall surfaces of the air introduction hole intersect each other has a radius of curvature of 0.03 mm or more. The present invention has been found out that by forming a curved surface, cracks do not occur at the corners, and the target sensor can be manufactured with high yield.

Further, in the present invention, the corner portion, and further, the cross-sectional shape of the atmosphere introducing hole is a maximum length in a direction parallel to the surface on which the reference electrode is formed, and the reference is It is particularly effective that a / b satisfies the relationship of 0.2 to 5.0, where b is the maximum length in the direction perpendicular to the surface on which the electrodes are formed.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION An example of the basic structure of an oxygen sensor (theoretical air-fuel ratio sensor) of the present invention will be described below with reference to the schematic perspective view of FIG. 1 and the cross section of the oxygen sensor element of FIG. It will be described with reference to the drawings.

According to the oxygen sensor element of FIGS. 1 and 2, the solid electrolyte substrate 1 having oxygen ion conductivity is provided, and the sensor element is formed on the inner surface and the outer surface thereof at positions facing each other. A first electrode pair of is formed. Specifically, the inner surface of the substrate 1 is provided with an air introduction hole 2 with one end sealed, and air is introduced from the open end. A reference electrode 3 that is brought into contact with a reference gas such as air is formed inside the air introduction hole 2, and an outer surface of the substrate 1 facing the reference electrode 3 comes into contact with a measured gas such as exhaust gas. The measurement electrode 4 is formed. The measurement electrode 4 is electrically connected to the measurement electrode pad 6 by the measurement electrode lead 5. Similarly, the reference electrode 3 is electrically connected to the reference electrode pad 7 via a reference electrode lead or a through hole (not shown).

In the present invention, in the cross-sectional shape of the air introducing hole 2 of the oxygen sensor element, it is important that the corner portion 8 where two wall surfaces of the air introducing hole 2 intersect with each other is formed by a curved surface. Such a corner part 8
By forming the curved surface with a curved surface, the stress generated by the difference in thermal expansion between different materials used for the oxygen sensor is dispersed, so that the generation of cracks in the corner portion 8 can be prevented.

This curved surface preferably has a radius of curvature of 0.03 mm or more, particularly 0.
It is important that the thickness is 05 mm or more, further 0.1 mm or more, and if this radius of curvature is smaller than 0.03 mm, a remarkable effect cannot be obtained.

Further, in the present invention, in order to suppress the generation of cracks in the corner portion 8, the cross-sectional shape of the air introduction hole 2 has the maximum value in the direction parallel to the surface on which the reference electrode 3 is formed. When the length is a and the maximum length in the direction perpendicular to the surface on which the reference electrode 3 is formed is b, a / b is 0.2 to 5.
It is desirable to satisfy the relationship of 0. That is, when a / b deviates from the above range, the cross-sectional shape of the atmosphere introducing hole 2 is deformed from a rectangular shape at the stage of forming the atmosphere introducing hole 2, the corner portion becomes an acute angle, and it is easy to promote the generation of cracks.

The ceramic solid electrolyte used in the present invention is composed of a ceramic containing ZrO _2, and Y ₂ O ₃ and Yb ₂ O ₃ , Sc as stabilizers are used.
Partially stabilized ZrO ₂ containing 1 to 30 mol%, preferably 3 to 15 mol% of a rare earth oxide such as ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O ₃ , and Dy ₂ O _{3 in} terms of oxide, or stable. ZrO ₂
Is used.

The electrodes forming the reference electrode 3, the reference electrode lead, the reference electrode pad 7, the measurement electrode 4, the measurement electrode lead 5 and the measurement electrode pad 6 are all made of platinum, rhodium, palladium, ruthenium and gold. One kind or two or more kinds of alloys selected from the group, particularly platinum is preferably used.

Next, a method of manufacturing the oxygen sensor element of the present invention will be described with reference to FIGS. First, as shown in FIG. 3, a molding organic binder is appropriately added to a ceramic solid electrolyte powder having oxygen ion conductivity such as zirconia to prepare sheets 10, 11 and 12, and isostatic pressing is performed. By a known method such as (rubber press) or press forming, these three sheets are integrated to form a laminated body A. On the surface of the sheet 10, the measurement electrode 4, the measurement electrode lead 5, the measurement electrode pad 6, the reference electrode pad 7
Is formed in advance by printing a conductive paste containing platinum, for example. The reference electrode 3 and the reference electrode lead are printed on the back side of the sheet 12 in the same manner. Further, through holes are provided below the reference electrode pads 7 of the sheets 10, 11 and 12, and the conductive paste is injected so that the reference electrode pads 7 and the reference electrode leads are electrically connected. In this way, next, in order to form the air introduction hole 2 in the same manner, tapes 13, 14, and 15 whose insides are cut out in a U-shape are produced, and are laminated and integrated by the same method as described above to form a laminated body. Form B.

Then, the sheets 16, 17 and 18 for forming the bottom of the air introduction hole 2 are prepared, and the laminated body C is formed by laminating and integrating in the same manner as above.

Then, the three laminated bodies A, B and C produced by the above method are integrated and fired.

According to the present invention, the corner portion 8 of the air introduction hole 2 is coated with a slurry containing a solid electrolyte powder composition which is substantially the same as the component forming the substrate 1 to form the corner portion 8. 8 is formed by a curved surface. To apply such a slurry, for example, by pouring the slurry into the air introduction hole 2 formed in the laminated molded body before firing, and then discharging the slurry,
The viscosity of the slurry allows the slurry to adhere to the corner portion 8.

However, if the size of the atmosphere introducing hole 2 becomes smaller as the oxygen sensor element becomes smaller, it becomes difficult to fill and discharge the slurry as described above, and the slurry filling the atmosphere introducing hole 2 becomes clogged. There are cases.

In such a case, the following method is used. First, before the three laminated bodies A, B, and C manufactured by the above method are integrated, a carbon core 19 shown in FIG. 4A is arranged on the surface of the reference electrode pattern 3 of the laminated body A. To do. Further, as shown in FIG. 4A, a slurry obtained by diluting the solid electrolyte powder with a solvent such as alcohol is applied to the surface of the core 19 to form a solid electrolyte layer 20.

Then, the laminate A having the core 19 and the solid electrolyte layer 20, and the laminates B and C are laminated and pressure-bonded to be integrated. At this time, since the core 19 is inserted into the atmosphere introducing hole 2, the atmosphere introducing hole 2 is not largely deformed even if pressure is applied. Further, at this time, the solid electrolyte layer 20 formed on the surface of the core 19 is accumulated in the corner portion 8 of the air introduction hole 2 to form the curved corner portion 8.

Then, the laminate is fired in a nitrogen atmosphere. The carbon core 19 installed in the air introduction hole 2 is removed by heat, and a curved surface having a predetermined radius of curvature is formed at the corner portion 8 of the air introduction hole 2.
The oxygen sensor element of the present invention can be formed.

[0025]

EXAMPLE An oxygen sensor element was manufactured based on the example of the manufacturing method shown in FIGS.

First, an acrylic binder and a toluene solution are added to zirconia powder containing 5 mol% Y ₂ O ₃ to prepare a slurry, and a slurry is prepared by a doctor blade method to a thickness of about 20.
A 0 μm zirconia green sheet was produced and cut with a cutter so that the dimensions after firing were 5 mm in width and 50 mm in length. (A) Prepare three green sheets 10, 11 and 12 and print platinum paste on the surface of one sheet 10,
The measurement electrode 4, the measurement electrode lead 5, the measurement electrode pad 6, and the reference electrode pad 7 were formed. Also, another sheet 1
The reference electrode 3 and the reference electrode lead 5 were printed on 2. Then, through holes were formed in the three sheets of the above-mentioned two sheets 10 and 12 and the one sheet 11 which was not printed, at positions below the reference electrode pads, and the platinum paste was filled therein. After that, the above three sheets 10, 11, and 12 were integrated by press molding by press molding to produce a laminate A. (B) Three green sheets 13, 14 and 15 were prepared and cut into a U-shape so that the cross-sectional shape of the air introduction hole was a × b in Table 1. Then, the U-shaped sheets 13, 14, and 15 were integrated by press molding to produce a laminated body B. (C) Three green sheets 16, 17, and 18 were prepared and integrated by press molding to produce a laminate C. (D) Next, on the surface of the laminated body A on which the reference electrode 3 is formed,
Core 19 made of carbon with a width of 0.8 mm and a length of 40 mm
Was placed on the reference electrode 3. Further, a slurry in which zirconia powder containing 5 mol% Y ₂ O ₃ and toluene is mixed is applied to the surface of the core 19 by atomization to form a solid electrolyte layer 20.
Was formed. At this time, the curvature of the corner portion of the air introduction hole was finally controlled by adjusting the coating amount of the slurry.

After that, a laminated body A having the core 19 arranged therein, a laminated body B having the air introduction hole 2 formed therein, and a laminated body C are stacked in this order and then integrated by press molding to produce a sensor laminated molded body. did. Then, this sensor laminated compact was fired in a nitrogen atmosphere at 1500 ° C. for 2 hours. In addition, as a comparative example, an oxygen sensor element (Sample No. 1) was similarly prepared by a conventional method in which the core 19 and the solid electrolyte layer 20 were not formed at all in the step (d).

In this way, several kinds of oxygen sensor elements having different radiuses of curvature of the corner portion 8 of the air introduction hole 2 and different shapes of the air introduction hole 2 were produced. For each oxygen sensor element,
Fifty pieces were manufactured and subjected to a temperature cycle test at a high temperature of 800 ° C. and a low temperature of 30 ° C. for 5000 cycles, and then cut to check whether or not a crack was generated in the corner portion 8 of the air introduction hole 2,
The number of cracked devices was investigated.

[0029]

[Table 1]

From Table 1, in the conventional oxygen sensor element in which the corner portion of the air introduction hole is angular and the curvature radius is smaller than 0.03 mm, cracks were observed in 80% of the oxygen sensor elements in the thermal cycle test.

On the other hand, according to the present invention, the incidence rate of cracks was reduced to 30% or less by forming the corner portion of the air introduction hole by the curved surface. In particular, the radius of curvature is 0.05 mm or more, and the shape a / b ratio of the air introduction hole is 0.
In the case of 2 to 5.0, the crack occurrence rate could be reduced to 10% or less.

[0032]

As described in detail above, according to the oxygen sensor element of the present invention, the concentration of stress can be suppressed by forming the corner portion of the atmosphere introducing hole with a predetermined curved surface, and the atmosphere introducing hole can be suppressed. It is possible to provide an oxygen sensor element having excellent stability without cracks at the corners.

[Brief description of drawings]

FIG. 1 is a schematic perspective view for explaining an example of an oxygen sensor element of the present invention.

FIG. 2 is a vertical cross-sectional view taken along the line XX of the oxygen sensor of FIG.

FIG. 3 shows an exploded view for explaining a method for manufacturing the oxygen sensor element of the present invention.

FIG. 4A is a schematic perspective view and FIG. 4B is a cross-sectional view taken along line YY for explaining a method for forming a curved surface at a corner portion of an air introduction hole in the oxygen sensor element of the present invention.

FIG. 5 is a schematic sectional view of a conventional oxygen sensor element.

[Explanation of symbols]

1 base 2 atmosphere introduction hole, 3 is a reference electrode 4 measuring electrodes, 5 measuring electrode lead, 6 measuring electrode pad, 7 Reference electrode pad, 8 corners,

Claims (2)

[Claims] 1. A plate-shaped substrate made of a ceramic solid electrolyte having an air introduction hole, a reference electrode formed on the inner wall of the air introduction hole, and a measurement electrode formed on the outer surface of the substrate facing the reference electrode. In the oxygen sensor element having and,
An oxygen sensor element, characterized in that a corner portion where two wall surfaces of the air introduction hole intersect each other at right angles is formed by a curved surface having a curvature radius of 0.03 mm or more. 2. A cross-sectional shape of the air introduction hole has a maximum length in a direction parallel to a surface on which the reference electrode is formed, and a maximum length in a direction perpendicular to the surface on which the reference electrode is formed. The oxygen sensor element according to claim 1, wherein a / b satisfies a relation of 0.2 to 5.0, where b is b.