JP4744043B2 - Air-fuel ratio sensor element - Google Patents

Description

【００５４】
表１より、本発明であるガス拡散孔の内径が拡散律速を担う多孔質筒体の内径より小さい試料Ｎｏ．４、５、７、９、１０、１２、１３、１５、１７では良品率が８０％を超えることがわかる。これに対して、試料Ｎｏ．１、２、３、６、８、１１、１４、１６ではガス拡散孔の内径が多孔質筒体の内径と同じであるために、溶射時に溶射粉が多孔質筒体の内壁に付着し、被測定ガスの導入を妨げ、ポンピング電流が低下し、良品率が低下した。
【００５５】
以上詳述した通り、本発明の空燃比センサ素子によれば、安定したポンピング電流が得られ、良好な空燃比センサ素子を歩留まりよく提供できる。
【図面の簡単な説明】
【図１】本発明の空燃比センサ素子の一例を説明するための概略斜視図である。
【図２】図１の空燃比センサ素子におけるＸ−Ｘ縦断面図である。
【図３】本発明の空燃比センサ素子の製造方法を説明するための図であって、センサ素体を作製する工程を示す図である。
【符号の説明】
１は空燃比センサ素子、２は円筒管、３は基準電極、４は測定電極、５は空間部６は発熱体、７はセラミック絶縁層、８は固体電解質層、９は内側電極、１０は外側電極、１１はガス拡散孔、１２はリード電極、１３は端子電極、１６は多孔質筒体、３４はセラミック溶射層をそれぞれ示す。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio sensor element for controlling the ratio of air and fuel in an internal combustion engine such as an automobile. Specifically, the heating element and the sensor unit are integrated to generate heat with a high manufacturing yield. The present invention relates to an air-fuel ratio sensor element integrated with a body.
[0002]
[Prior art]
Currently, in an internal combustion engine such as an automobile, by detecting the oxygen concentration in the exhaust gas and controlling the amount of air and fuel supplied to the internal combustion engine based on the detected value, harmful substances from the internal combustion engine, For example, a method of reducing CO, HC, and NOx is adopted.
[0003]
This detection element is made of a solid electrolyte mainly composed of zirconia having oxygen ion conductivity, and a solid electrolyte type in which a pair of electrode layers are respectively formed on the outer surface and the inner surface of a cylindrical base body sealed at one end. An oxygen sensor element is used.
[0004]
In such an oxygen sensor, in general, 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 formed on the surface of the outer measurement electrode. An air-fuel ratio of the engine intake system is controlled by detecting a difference in oxygen concentration generated on both sides of the cylindrical base body at a predetermined temperature.
[0005]
On the other hand, a so-called wide-range air-fuel ratio sensor element (A / F sensor) used to control a wide range of air-fuel ratio is a ceramic porous material that becomes a gas diffusion-controlling layer having fine pores on the surface of a measurement electrode. A layer is provided, an applied voltage is applied to a substrate made of a solid electrolyte through a pair of electrodes, and a limit current value obtained at that time is measured to directly control the air-fuel ratio.
[0006]
The wide-area air-fuel ratio sensor element has a gas diffusion hole, and the detection gas is guided through the gas diffusion hole to the measurement electrode via the diffusion-controlling layer. Further, around the gas diffusion hole on the outer surface of the element, a protective layer made of spinel and having a thickness of 30 to 150 μm is provided for the protection of the electrode provided in the vicinity thereof and the relaxation of the generated thermal stress.
[0007]
[Problems to be solved by the invention]
However, when the protective layer made of the spinel or the like is formed by thermal spraying or the like, and the gas diffusion hole is provided in a direction perpendicular to the major axis direction of the sensor element, the thermal spray material is contained in the gas diffusion hole. Intruded and adhered to the surface of the diffusion-controlling layer, and the introduction of the detection gas was hindered, and the pumping current was lowered and varied.
[0008]
In order to avoid this, methods such as filling the gas diffusion holes before spraying and removing foreign materials after spraying have been considered, but it takes time to increase the work process, and when removing the spray material There was a problem that the diffusion control layer was damaged and the pumping current varied.
[0009]
Therefore, the present invention prevents the ceramic material forming a protective layer such as spinel from adhering to the diffusion rate-determining layer through the gas diffusion holes, and can provide a stable pumping current without variations and the like, with a high production yield. An object of the present invention is to provide an air-fuel ratio sensor element.
[0010]
[Means for Solving the Problems]
As a result of studying the above problems, the present inventor has found that a sensing part is formed by forming an electrode pair consisting of a reference electrode and a measurement electrode on the inner surface and the outer surface of the solid electrolyte substrate facing each other, and an outer surface of the measurement electrode. A solid electrolyte layer disposed through the space; a gas diffusion hole formed in the solid electrolyte layer; a porous cylinder formed around the diffusion hole in the space; and the solid electrolyte layer An air-fuel ratio sensor element comprising a ceramic protective layer formed around the gas diffusion hole on the outer surface of the gas diffusion hole, wherein the inner diameter of the gas diffusion hole on the outer surface side of the solid electrolyte layer is 0.2 mm or more. It said porous rather smaller than the inner diameter of the cylindrical body, and by the difference between the inner diameter of the porous cylindrical body is 0.2mm or more, the above problem is solved, yields the element of interest It has led to heading the present invention to be able to Ku production.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an example of the basic structure of the air-fuel ratio sensor element of the present invention will be described based on the schematic perspective view of FIG. 1 and the XX sectional view of the air-fuel ratio sensor element of FIG. 1 shown in FIG.
[0013]
The air-fuel ratio sensor element 1 of FIGS. 1 and 2 includes a solid electrolyte cylindrical base 2 (hereinafter referred to as a cylindrical tube 2) made of ceramics having oxygen ion conductivity, and the inner and outer surfaces thereof. The first electrode pair for constituting the sensor element is formed at a position facing each other. Specifically, a reference electrode 3 that is in contact with a reference gas such as air is formed on the inner surface of the cylindrical tube 2, and a measured gas such as exhaust gas is formed on the outer surface of the cylindrical tube 2 that faces the reference electrode 3. Measuring electrode 4 is formed in contact with each other, forming a sensing part.
[0014]
A solid electrolyte layer 8 having oxygen ion conductivity is formed on the upper surface of the measurement electrode 4 via the space 5, and the inner surface of the solid electrolyte layer 8 on the space 5 side is opposed to it. A second electrode pair consisting of an inner electrode 9 and an outer electrode 10 is formed as a pumping electrode on the outer surface of the space portion 5 of the solid electrolyte layer 8 to form a pumping portion.
[0015]
The solid electrolyte layer 8 including the second electrode pair 9 and 10 forming the pumping portion is formed with a small gas diffusion hole 11 for taking in the exhaust gas to be measured gas.
[0016]
Further, a porous cylinder 16 is provided in the space portion 5 in order to control the diffusion of the gas to be measured introduced into the space portion 5 via the gas diffusion holes 11.
[0017]
And on the outer surface of the sensor element, the three electrodes 4, 9, 10 that are in contact with the gas to be measured are prevented from being poisoned, the heat resistance of the sensor element itself is increased, and the sensor element is kept warm. Therefore, a ceramic sprayed layer 34 formed by thermal spraying or the like is formed.
[0018]
According to the present invention, it is important that the inner diameter x on the outer surface side of the solid electrolyte layer 8 in the gas diffusion hole 11 is smaller than the inner diameter y of the porous cylinder 16, thereby forming the ceramic sprayed layer 34. In addition, as a result of preventing the sprayed material from adhering to the inner surface of the porous cylinder 16, it is possible to obtain a stable pumping current without impairing the diffusion rate limiting property by the porous cylinder 16.
[0019]
The inner diameter x on the outer surface side of the solid electrolyte layer 8 in the gas diffusion hole 11 is preferably 0.2 mm or more, particularly 0.3 mm or more, and more preferably 0.5 mm or more. This is because if the inner diameter of the gas diffusion hole 11 is smaller than 0.2 mm, the gas diffusion hole 11 is easily clogged with the sprayed material.
[0020]
In particular, it is effective that the difference xy between the inner diameter x on the outer surface side of the solid electrolyte layer 8 in the gas diffusion hole 11 and the inner diameter y of the porous cylindrical body 16 is 0.2 mm or more, particularly 0.3 mm or more. It is.
[0021]
As the material of the porous cylinder 16, the same material as the above solid electrolyte material is used, and alumina, spinel and the like are used. The porous cylinder 16 also has a function of protecting the exhaust gas that has passed through the gas diffusion hole 11 from directly touching the measurement electrode 4 and the inner electrode 9.
[0022]
The ceramic solid electrolyte used in the present invention is made of a ceramic containing ZrO ₂ , and Y ₂ O ₃ and Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O ₃ , Dy are used as stabilizers. 1 to 30 mol% of rare earth oxides such as ₂ O ₃ in terms of oxide, preferably the partially stabilized ZrO ₂ or stabilized ZrO ₂ containing 3 to 15 mol% are used.
[0023]
Furthermore, for the purpose of improving the sinterability, Al ₂ O ₃ and SiO ₂ can be added to the ZrO ₂ , but if it is contained in a large amount, the creep properties at high temperatures are deteriorated. The total amount of Al ₂ O ₃ and SiO ₂ is preferably 5% by weight or less, particularly 2% by weight or less.
[0024]
On the other hand, a ceramic material such as alumina, spinel, forsterite, zirconia, or glass is preferably used as the ceramic insulating layer 7 in which the heating element 6 is embedded. Furthermore, glass can be used for the glass insulating layer as the ceramic insulating layer 7, but in this case, from the viewpoint of heat resistance, at least one of BaO, PbO, SrO, CaO, and CdO is contained by 5% by weight or more. It is desirable that the glass be crystallized glass.
[0025]
The ceramic insulating layer 7 is preferably composed of a dense ceramic having a relative density of 80% or more and an open porosity of 5% or less. This is because the mechanical strength of the oxygen sensor element itself can be increased as a result of the strength of the insulating layer being increased because the ceramic insulating layer 7 is dense.
[0026]
The heating element 6 embedded in the ceramic insulating layer 7 is preferably made of one kind of metal selected from the group consisting of platinum, rhodium, palladium and ruthenium, or two or more kinds of alloys. From the viewpoint of simultaneous sintering with the ceramic insulating layer 7, it is desirable to select a metal or alloy having a melting point higher than the firing temperature of the ceramic insulating layer 7.
[0027]
In addition to the above metals, the heating element 6 includes alumina, spinel, an alumina / silica compound, forsterite, zirconia that can be the above electrolyte, etc. from the viewpoint of preventing sintering and enhancing the adhesive strength with the ceramic insulating layer 7. Is desirably mixed in a volume ratio of 10 to 80%, particularly 30 to 50%.
[0028]
The solid electrolyte layer 8 that closes the space portion 5 is formed on the surface of the ceramic insulating layer 7 so as to cover the space portion 5. The solid electrolyte layer 8 is made of the same material as the solid electrolyte constituting the cylindrical tube 2 having oxygen ion conductivity, like the cylindrical tube 2. In particular, the solid electrolyte layer 8 is preferably made of the same material as that of the solid electrolyte constituting the cylindrical tube 2.
[0029]
In addition, the solid electrolyte layer 8 formed on the outer surface of the ceramic insulating layer 7 prevents heat from being emitted from the heating element 6. Further, the same solid electrolyte is formed on the upper and lower surfaces of the ceramic insulating layer 7. As a result, the stress caused by the difference in thermal expansion and firing shrinkage between the cylindrical tube 2 made of the solid electrolyte and the ceramic insulating layer 7 is applied. It also acts to relax and reduce the thermal stress as much as possible.
[0030]
The heating element 6 needs to be embedded in the ceramic insulating layer 7 without being in direct contact with the solid electrolyte layer 8 and the second electrode pair 9, 10 above the cylindrical tube 2 or the space 5. The thickness of the ceramic insulating layer 7 between the heating element 6 and the cylindrical tube 2 and the solid electrolyte layer 8 is desirably at least 2 μm, preferably 5 μm.
[0031]
The shape of the space portion 5 is not particularly limited, but when formed on the outer surface of the cylindrical tube 2, it is preferable that the length in the longitudinal direction of the cylindrical tube 2 is long or elliptical. In the case where the space portion 5 is rectangular, the corner portion is formed by a gently curved surface, thereby reducing the concentration of thermal stress on the corner portion of the space portion 5. Further, the height of the space portion is preferably 10 to 50 μm, particularly 20 to 30 μm.
[0032]
Each of the electrodes forming the first electrode pair 3, 4 and the second electrode pair 9, 10 is one selected from the group consisting of platinum, rhodium, palladium, ruthenium and gold, or two or more kinds. An alloy, particularly platinum, is preferably used. Further, for the purpose of preventing the grain growth of the metal in the electrode during the sensor operation and for the purpose of increasing the contact of the so-called three-phase interface between the metal particles related to the responsiveness, the solid electrolyte, and the gas, the above-described cylindrical tube 2 is used. Or it is desirable to mix the solid electrolyte component which comprises the solid electrolyte layer 8 in the said electrode in the ratio of 1-50 volume%, especially 10-30 volume%.
[0033]
Of the first electrodes 3 and 4, the reference electrode 3 formed on the inner surface of the cylindrical tube 2 may be formed on the inner surface portion of the measurement electrode 4 that faces the portion exposed to the space portion 5. The area of the measurement electrode 4 may be larger than the area of the exposed portion, for example, the entire inner surface of the cylindrical tube 2.
[0034]
The ceramic sprayed layer 34 formed on the outer surface of the sensor element is made of a porous material having an open porosity of 10 to 40% made of at least one selected from the group of zirconia, alumina, magnesia and spinel, and the thickness thereof is It is suitable that it is 10-200 micrometers, especially 50-150 micrometers.
[0035]
Furthermore, the space 5 is filled with a porous body having a larger porosity than the porous cylinder 16 in order to protect the electrodes 4, 9, and 10 and to increase the strength of the space 5. It is also possible to do.
[0036]
1 and FIG. 2, the A / F sensor has been described. However, the same structure can be used except that the electrodes 9 and 10 are not formed on the inner and outer surfaces of the solid electrolyte layer 8. It is.
[0037]
Next, a method for manufacturing the air-fuel ratio sensor element of the present invention will be described with reference to FIG. 3, taking the method for manufacturing the air-fuel ratio sensor element of FIGS.
(1) First, as shown in FIG. 3, in order to form the cylindrical tube 2, a hollow cylindrical tube body 20 with one end sealed is prepared. The cylindrical tube body 20 is formed by appropriately adding a molding organic binder to a ceramic solid electrolyte powder having oxygen ion conductivity such as zirconia, extrusion molding, isostatic pressing (rubber press), press formation, or the like. It is produced by the well-known method.
(2) And, as shown in FIG. 3, the electrode pattern 21 to be the reference electrode 3 is formed on the inner surface of the cylindrical tube body 20 made of the solid electrolyte, for example, a conductive paste containing platinum is placed inside the cylindrical tube body 20. Fill and discharge to form coating on the entire inner surface. In this way, the sensor element A is manufactured.
(3) Next, as shown in FIG. 3, a green sheet 22 is formed using a solid electrolyte slurry that forms the cylindrical tube 20, and alumina or the like is formed in a region of the green sheet 22 where the measurement electrode 4 is not formed. After the insulating slurry is applied to form the insulating layer 23a, the heating element pattern 25 including the lead pattern is printed and formed again, and the insulating slurry 23 is applied again to form the insulating layer 23b. It is embedded in the insulating layers 23a and 23b. Then, the solid electrolyte slurry in which the green sheet 22 is formed is applied to the surface of the insulating layer 23 b including the opening 26 where the insulating layers 23 a and 23 b are not formed, and the solid electrolyte body 27 is filled in the opening 26.
[0038]
Then, the measurement electrode pattern 28 is formed on the surface of the solid electrolyte body 27, and the porous cylinder 29 is formed by applying a ceramic slurry, arranging a previously formed cylinder, or the like. The inner diameter of the cylindrical body at this time is designed so that the inner diameter after firing is larger than a gas diffusion hole described later. In this way, the heater element B is formed.
[0039]
Further, after the gas introduction hole 32 is formed in the solid electrolyte green sheet 33, the pumping element body C is formed by printing and applying the electrode patterns 30 and 31 for pumping on both surfaces thereof. At this time, the radius of the gas introduction hole 32 formed in the grease sheet 33 is made smaller than the inner diameter of the cylindrical body 29 after firing and 0.2 mm or more.
[0040]
The heater element B and the pumping element C are wound around the surface of the sensor element A, and the green sheet 22 side of the laminate produced as described above is wound around the surface of the sensor element A, and then fired simultaneously. Can be formed.
[0041]
For example, when zirconia is used as the solid electrolyte and alumina is used as the ceramic insulating layer, the baking may be performed at 1300 to 1700 ° C. for about 1 to 10 hours in an inert atmosphere such as argon gas or in the air.
[0042]
In the above manufacturing method, the ceramic insulating layers 23a and 23b in which the heating element pattern 25 is embedded by slurry application are formed on the surface of the solid electrolyte green sheet 22. However, the ceramic insulating layer is not used without using the solid electrolyte green sheet 22. It is also possible to form 23a and 23b by a laminate of green sheets.
[0043]
In the above manufacturing method, the surface of the ceramic insulating layer 23b is covered when the solid electrolyte body 27 is filled and formed in the opening 26. However, there is no problem even with the opening 26 alone.
[0044]
By coating the outer surface of the sensor element A body produced as described above with a ceramic material made of at least one selected from the group of zirconia, alumina, magnesia and spinel with a thickness of 10 to 200 μm by a thermal spraying method. The air-fuel ratio sensor element of the present invention can be formed.
[0045]
【Example】
First, a slurry was prepared by adding a polyvinyl alcohol solution to zirconia powder containing 5 mol% Y ₂ O ₃ , sintered by extrusion molding, and then sealed at one end so that the outer diameter was about 4 mm and the inner diameter was 1 mm. A cylindrical tube body was manufactured, and a platinum paste was applied to the entire inner surface of the molded body to form a reference electrode, thereby preparing a sensor body A.
[0046]
A slurry was prepared by adding an acrylic binder and a toluene solution to zirconia powder containing 5 mol% Y ₂ O _3, and a zirconia green sheet having a thickness of about 200 μm was prepared by a doctor blade method.
[0047]
Thereafter, a slurry containing alumina powder is applied to the surface of the region where the measurement electrode of the zirconia green sheet is not formed so as to have a thickness of about 10 μm after firing by screen printing, and then the thickness after firing using platinum paste is 10 μm. A heating element pattern was formed by printing, and the alumina slurry was applied again so that the heating element pattern was embedded, thereby forming an alumina insulating layer in which the heating element was embedded on the surface of the zirconia green sheet.
[0048]
Then, after applying and filling a slurry of zirconia powder containing 5 mol% Y ₂ O _{3 on the} electrode forming region of the zirconia green sheet where the alumina insulating layer is not formed, the electrode paste as a measurement electrode is applied to the surface. Coating was formed.
[0049]
The thicknesses of the measurement electrode and the reference electrode were adjusted to be about 10 μm after firing.
[0050]
Next, after forming the inner electrode, the pumping electrode pair of the outer electrode and the lead wire on both surfaces of another zirconia green sheet produced in the same manner as described above, a porous cylinder that performs diffusion rate control on one electrode surface and A zirconia powder containing 8 mol% Y ₂ O ₃ was formed to a thickness of 30 μm. The inner diameter of the porous cylinder was the size shown in Table 1. The inner electrode and the outer electrode have substantially the same area. After that, the gas diffusion hole was opened with a drill so as to coincide with the center of the porous cylinder. The inner diameter of the gas diffusion hole is shown in Table 1.
[0051]
Then, the heater element B and the pumping element C were sequentially wound around the surface of the sensor element A to prepare a cylindrical laminate. Note that an acrylic binder was used as an adhesive for bonding between the element bodies during winding. Then, this cylindrical laminated body was baked at 1500 ° C. in the air for 2 hours to complete 50 sensor elements each.
[0052]
The evaluation of the manufactured air-fuel ratio sensor element was performed by applying 0.5 V between the electrodes of the pumping unit at 800 ° C. in the atmosphere to pump oxygen in the space into the atmosphere, and making the element showing a pumping current of 4 mA to 5 mA as a non-defective product. The ratio (non-defective product rate) was examined.
[0053]
[Table 1]

[0054]
From Table 1, the sample No. 1 in which the inner diameter of the gas diffusion hole according to the present invention is smaller than the inner diameter of the porous cylindrical body responsible for diffusion rate control. 4, 5, 7, 9, 10, 12, 13, 15, and 17 show that the non-defective rate exceeds 80%. In contrast, sample no. In 1, 2, 3, 6, 8, 11, 14, and 16, since the inner diameter of the gas diffusion hole is the same as the inner diameter of the porous cylinder, the sprayed powder adheres to the inner wall of the porous cylinder during spraying, The introduction of the gas to be measured was hindered, the pumping current decreased, and the yield rate decreased.
[0055]
As described above, according to the air-fuel ratio sensor element of the present invention with a stable pumping current is obtained, it can be provided with high yield good air-fuel ratio sensor element.
[Brief description of the drawings]
FIG. 1 is a schematic perspective view for explaining an example of an air-fuel ratio sensor element of the present invention.
2 is an XX longitudinal sectional view of the air-fuel ratio sensor element of FIG. 1. FIG.
FIG. 3 is a view for explaining the method for manufacturing the air-fuel ratio sensor element of the present invention, and is a view showing a process of manufacturing a sensor element body.
[Explanation of symbols]
1 is an air-fuel ratio sensor element, 2 is a cylindrical tube, 3 is a reference electrode, 4 is a measurement electrode, 5 is a space 6 is a heating element, 7 is a ceramic insulating layer, 8 is a solid electrolyte layer, 9 is an inner electrode, 10 is The outer electrode, 11 is a gas diffusion hole, 12 is a lead electrode, 13 is a terminal electrode, 16 is a porous cylinder, and 34 is a ceramic sprayed layer.

Claims (1)

A sensing part formed by forming an electrode pair consisting of a reference electrode and a measurement electrode on the inner surface and the outer surface of the solid electrolyte substrate facing each other; and a solid electrolyte layer disposed on the outer surface of the measurement electrode via a space part; A gas diffusion hole formed in the solid electrolyte layer; a porous cylinder formed around the gas diffusion hole in the space; and a gas diffusion hole formed around the outer surface of the solid electrolyte layer. and the air-fuel ratio sensor element formed by and a ceramic thermal sprayed layer, wherein along with the outer surface of the inner diameter of the solid electrolyte layer is 0.2mm or more in the gas diffusion holes, rather smaller than the inner diameter of the porous cylindrical body And the difference with the internal diameter of the said porous cylinder is 0.2 mm or more, The air fuel ratio sensor element characterized by the above-mentioned .

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