JP2001013101A - Heater integrated oxygen sensor element and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxygen sensor element and its manufacturing method in which a heater is integrated with a cylindrical oxygen sensor, and whose activation time is short and whose durability and heat resistance is excellent. SOLUTION: An oxygen sensor is equipped with a cylindrical tube 2 which is made of a ceramic solid electrolyte having oxygen ion conductivity and whose one end is closed, and a standard electrode 3 and a measuring electrode 4 which are respectively formed at face-to-face positions on an inner surface and an outer surface of the cylindrical tube 2. A ceramic insulating layer 5 is layered so that a portion or the whole of the measuring electrode 4 is exposed on the outer surface of the cylindrical tube 2. A heating body 7 is set at least close to the measuring electrode 4 of the ceramic insulating layer 5. The cylindrical tube 2 and the ceramic insulating layer 5 in which the heating body 7 is set, are simultaneously formed by baking, and a ceramic porous layer 11 is depositely formed on the surface of the measuring electrode 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heater-integrated oxygen sensor element for controlling the ratio of air to fuel in an internal combustion engine of an automobile or the like, and a method of manufacturing the same. To improve the time required to detect the oxygen concentration.

[0002]

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

As the detection element, a solid electrolyte mainly composed of a solid electrolyte mainly composed of zirconia having oxygen ion conductivity and having a pair of electrode layers formed on an outer surface and an inner surface of a cylindrical tube having one end sealed. Type oxygen sensors are used. As a typical example of this oxygen sensor, as shown in FIG. 11, the inner surface of a cylindrical tube 31 made of a ZrO ₂ solid electrolyte and having a sealed end is made of platinum and comes into contact with a reference gas such as air. Reference electrode 32
However, on the outer surface of the cylindrical tube 31, a measuring electrode 33 which is in contact with a gas to be measured such as exhaust gas is formed. Also,
Various ceramic porous layers 34 are provided on the surface of the measurement electrode 33.
Are formed.

In such an oxygen sensor, generally,
As a so-called stoichiometric air-fuel ratio sensor (λ sensor) used for controlling the ratio of air to fuel near 1, a porous layer 34 serving as a protective layer is provided on the surface of a measurement electrode 33. The oxygen concentration difference generated on both sides of the cylindrical tube 31 at the temperature is detected, and the air-fuel ratio of the engine intake system is controlled.

On the other hand, a so-called wide-range air-fuel ratio sensor (A / F sensor) used to control a wide range of air-fuel ratio is a gas diffusion-controlling layer having fine pores on the surface of a measurement electrode 33. A ceramic porous layer 34 is provided, an applied voltage is applied to a cylindrical tube 31 made of a solid electrolyte through a pair of electrodes 32, 33, and a limit current value obtained at that time is measured to control an air-fuel ratio in a lean burn region. is there.

[0006] In both the stoichiometric air-fuel ratio sensor and the wide area air-fuel ratio sensor, it is necessary to heat the sensing section to an operating temperature of about 700 ° C. Therefore, inside the cylindrical tube 31, the sensing section is heated to the operating temperature. For this purpose, a rod-shaped heater 35 is inserted.

[0007]

However, in recent years, the exhaust gas regulations have been strengthened, and it has become necessary to detect CO, HC and NOx immediately after the engine is started. In response to such a demand, as described above, in the cylindrical oxygen sensor of the indirect heating system in which the heater 35 is inserted into the cylindrical tube 31, the time required for the sensing unit to reach the activation temperature (hereinafter, referred to as “the activation temperature”). There is a problem that the exhaust gas control cannot be sufficiently performed because the activation time is long.

As a method of avoiding this problem, as shown in FIG. 12, a flat ceramic insulating substrate is provided for an oxygen sensor element 39 having a measurement electrode 37 and a reference electrode 38 formed on the surface of a flat solid electrolyte 36. 40 internal heating elements 41
Is proposed in Japanese Utility Model Laid-Open Publication No. 61-199666 and the like.

Recently, a reference electrode and a measurement electrode are provided on the inner and outer surfaces of a cylindrical tube made of a solid electrolyte.
A cylindrical heater-integrated oxygen sensor element in which a gas-permeable porous insulating layer is provided on the surface of the measurement electrode, and a platinum heating element is provided in a gas-impermeable layer having a low gas permeability therein. It is described in JP-A-10-206380.

[0010] Unlike the conventional indirect heating method, these heaters and the integrated oxygen sensor are of the direct heating method, so that the temperature can be rapidly raised and the activation time of the sensing section is short.

[0011] However, the above-mentioned flat-plate type oxygen sensor element has poor durability and heat resistance because of its flat shape, and as a result, there is a problem that the sensor is broken during operation.

Further, as described in JP-A-10-206380, the solid electrolyte portion is fired, the electrode is plated or sputtered, the electrode protective layer is plasma-sprayed, and the insulator layer is plasma-sprayed. The cylindrical heater-integrated oxygen sensor element manufactured by forming sequentially has a complicated manufacturing method and a large number of steps.
There is a problem that the yield is low and the manufacturing cost is high. In addition, a porous insulating layer is formed on the entire surface of the measuring electrode, and the heating element is buried in the insulating layer. There was a problem of chipping.

Accordingly, the present invention provides a heater-integrated oxygen sensor element in which a heater is integrated with a cylindrical oxygen sensor, the activation time is short, and durability and heat resistance are excellent. It is an object of the present invention to provide a production method for easily producing a.

[0014]

A heater-integrated oxygen sensor according to the present invention comprises a cylindrical tube made of a ceramic solid electrolyte having oxygen ion conductivity, one end of which is sealed, and an inner surface and an outer surface of the cylindrical tube facing each other. For an oxygen sensor including a reference electrode and a measurement electrode formed at respective positions, a ceramic insulating layer is laminated on the outer surface of the cylindrical tube so that a part or all of the measurement electrode is exposed, and the ceramic A heating element is buried at least in the vicinity of the measurement electrode of the insulating layer, and such an oxygen sensor simultaneously fires the cylindrical tube and the ceramic insulating layer in which the heating element is buried. That
A ceramic porous layer is formed on the surface of the measurement electrode, and is used as a stoichiometric air-fuel ratio sensor or a wide area air-fuel ratio sensor.

The method for manufacturing the heater-integrated oxygen sensor element includes the steps of manufacturing a cylindrical tube made of a ceramic solid electrolyte and having one end sealed, forming an opening at a predetermined location, and forming the opening. Producing an insulating ceramic green sheet having a heating element embedded in the vicinity,
A step of winding the insulating ceramic green sheet around the outer surface of the cylindrical tube to produce a cylindrical laminate, a step of firing the cylindrical laminate, and a measurement electrode on the surface inside the opening of the cylindrical tube. And a step of forming a reference electrode on the inner surface of the cylindrical tube facing the same, respectively.

Further, as another manufacturing method, a step of producing a cylindrical tube made of a ceramic solid electrolyte and having one end sealed, a step of producing a green sheet composed of the ceramic solid electrolyte, a step of producing a green sheet composed of the ceramic solid electrolyte, A step of laminating an insulating ceramic layer in which an opening is formed at a predetermined location on one surface side of the sheet and a heating element is buried in the vicinity of the opening; and forming the ceramic solid electrolyte on the outer surface of the cylindrical tube. A step of winding the other side of the green sheet to form a cylindrical laminate, a step of firing the cylindrical laminate, a measurement electrode on the surface of the ceramic solid electrolyte in the opening, and an inner surface of the cylindrical tube opposed thereto; And forming a reference electrode respectively.

It is preferable that the above-mentioned manufacturing method includes a step of forming a ceramic porous layer on the surface of the measurement electrode.

[0018]

According to the heater-integrated oxygen sensor element of the present invention, the outer surface of the cylindrical solid electrolyte is coated with the ceramic insulating layer having the built-in heating element, and the heating element is arranged near the measuring electrode. As a result, the heating efficiency of the sensing unit by the heating element can be increased, and the temperature can be rapidly increased. As a result, the sensor activation time can be shortened. Furthermore, since the heating element can be formed in the vicinity of the sensing part, the sensor activation time can be shortened, and excellent sensor responsiveness can be obtained as compared with the conventional flat plate type.

Moreover, since the oxygen sensor element of the present invention has a cylindrical shape in which a heater is integrated, it has higher strength against stresses from all directions as compared with a flat plate type oxygen sensor element. In addition, since the concentration of stress is small, the heat resistance and durability are excellent.

Further, according to the method of manufacturing a heater-integrated oxygen sensor element of the present invention, a cylindrical tube made of a solid electrolyte,
In order to manufacture by simultaneously firing a ceramic insulating layer containing a heating element, an oxygen sensor and a heater are separately manufactured as in the related art, and then the oxygen sensor is used by fitting the heater into the oxygen sensor. The manufacturing cost is extremely low as compared with the element, and it is excellent from the viewpoint of economy.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an example of an oxygen sensor element of the present invention schematic perspective view of FIG. 1 (a) and X ₁ -X ₁ cross-sectional view of (b) based. The oxygen sensor element 1 of FIG.
The inner surface of a cylindrical tube 2 made of a ceramic solid electrolyte having oxygen ion conductivity and having a sealed end, that is, a cylindrical tube 2 having a U-shaped cross section is brought into contact with a reference gas such as air as a first electrode. A reference electrode 3 is formed on the outer surface of the cylindrical tube 2 facing the reference electrode 3, and a measurement electrode 4 is formed as a second electrode in contact with a gas to be measured such as exhaust gas. ing.

Further, according to the present invention, the ceramic insulating layer 5 is formed on the surface of the measuring electrode 4 formed on the outer surface of the cylindrical tube 2 whose end is sealed or in the vicinity thereof.
A predetermined opening 6 is formed in the ceramic insulating layer 5 so that a part or the whole of the measurement electrode 4 is exposed, and a heating element 7 is provided in the ceramic insulating layer 5 near the opening 6. Is buried. The heating element 7 is connected to a terminal electrode 9 via a lead electrode 8, and by applying a current to the heating element 7 through these, the heating element 7 is heated, and the measurement electrode 4 and the cylindrical tube 2 are heated. And reference electrode 3
The sensing section made of is configured to be rapidly heated to a predetermined temperature.

(Solid Electrolyte) The ceramic solid electrolyte used in the present invention is made of a ceramic containing ZrO ₂ , specifically, Y ₂ O ₃ and Yb
₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O ₃ , Dy ₂
1 to 30 mol% of a rare earth oxide such as O _{3 in} terms of oxide;
Partially stabilized ZrO ₂ containing preferably 3 to 15 mol%
Alternatively, stabilized ZrO ₂ is used.

Further, the content of Zr in ZrO ₂ is 1 to 20 atomic%.
The use of ZrO _{2 in} which is substituted by Ce has the effect of increasing the electron conductivity and further improving the responsiveness.

Further, in order to improve the sinterability, the above Z
Al ₂ O ₃ or SiO ₂ can be added to and contained in rO ₂ , but if it is contained in a large amount, the creep characteristics at high temperatures deteriorate, so that Al ₂ O ₃ and S ₂
It is desirable that the total amount of iO ₂ be 5% by weight or less, particularly 2% by weight or less.

(Electrode) The reference electrode 3 and the measuring electrode 4 formed on the surface of the cylindrical tube 2 are made of platinum, rhodium,
1 selected from the group consisting of palladium, ruthenium and gold
Seeds, or two or more alloys are used. The ceramic solid electrolyte described above is used for the purpose of preventing grain growth of the metal in the electrode during operation of the sensor and to increase the so-called three-phase interface contact between the metal particles, the solid electrolyte, and the gas related to the response. 1 to 50% by volume, especially 10 to 30% by volume
May be mixed in the above electrode.

(Ceramic Insulating Layer) On the other hand, as the ceramic insulating layer 5 in which the heating element 7 is embedded, a ceramic material such as alumina, spinel, forsterite, zirconia, or glass is preferably used. At this time, when zirconia is used as the ceramic insulating layer, the zirconia itself is a solid electrolyte, and the zirconia is connected to the cylindrical tube 2 so that the leakage current from the heating element 7 does not affect the oxygen concentration detection. It is desirable to form an intermediate layer of alumina, spinel, forsterite, or the like. Furthermore, glass can be used for the glass insulating layer as the ceramic insulating layer 5, but in this case, from the viewpoint of heat resistance, BaO, Pb
A glass containing at least one of O, SrO, CaO, and CdO in an amount of 5% by weight or more, particularly, a crystallized glass is desirable.

The ceramic insulating layer 5 is desirably made of 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 1 itself can be increased as a result of the strength of the insulating layer being increased when the ceramic insulating layer 5 is dense.

(Heating Element) The ceramic insulating layer 5
It is preferable that the heating element 7 embedded in the inside is made of one kind of metal selected from the group consisting of platinum, rhodium, palladium and ruthenium, or two or more kinds of alloys. In terms of sinterability, it is desirable to select a metal or alloy having a melting point higher than the firing temperature of the ceramic insulating layer 5.

The structure of the heater section in which the heating element 7 is embedded in the ceramic insulating layer 5 is, as shown in the sectional view of FIG. 1B, formed on the surface of the cylindrical tube 2 made of a solid electrolyte. In addition to the structure in which the ceramic insulating layer 5 having the heating element 7 embedded therein is laminated, the heating element 7 is embedded inside the outer surface of the cylindrical tube 2 as shown in FIG. A ceramic insulating layer 5 of alumina, spinel, forsterite, or the like is formed, and a zirconia layer 10 can be formed on the outer surface of the ceramic insulating layer 5. The zirconia layer 10 relieves stress caused by a difference in thermal expansion and a difference in firing shrinkage between the solid electrolyte and the ceramic insulating layer 5,
To minimize the thermal stress as much as possible,
This is for keeping the heat generated by the heating element 7 buried therein to prevent a sudden change in temperature of the entire oxygen sensor element.

In this configuration, the heating element 7 is
As shown in FIG. 2A, it can be buried inside the ceramic insulating layer 5; as shown in FIG. 2B, it can be buried in the zirconia layer 10, or as shown in FIG. It can be disposed between the layer 5 and the zirconia layer 10.

In any case, the heating element 7 is provided without being in direct contact with the cylindrical tube 2 or the electrode via the ceramic insulating layer 5 having no solid electrolyte performance such as alumina. Is required, and the thickness of the ceramic insulating layer 5 between the cylindrical tube 2 and the heating element 7 is at least 2 μm.
m or more.

In the present invention, the heating element 7
Is buried in the vicinity of an opening 6 provided in the ceramic insulating layer 5. The shape of the opening 6 is such that the measurement electrode 4 is exposed on the side surface of the cylindrical tube in FIG. 3 is provided with a vertically long opening 6, which is made of a solid electrolyte as shown in a perspective view (a) of another example in FIG. 3 and a cross-sectional view taken along line X ₂ -X _{2 in} FIG. The measurement electrode 4 may be formed on the surface of the sealing portion at the tip of the cylindrical tube 2, and the ceramic insulating layer 5 in which the heating element 7 is embedded may be provided on the side surface near the tip. In this case, the leading end becomes the opening 6. Note that the shape of the opening 6 may be any of a circular shape and a square shape.

Furthermore, as shown in a perspective view (a) and X ₃ -X ₃ cross-sectional view of another example of FIG. 4, the ceramic insulating layer 5
It is also possible to provide a large number of openings 6 in such a manner that the measurement electrode 4 is exposed from each of the openings 6 and a heating element 7 is buried in the vicinity of the openings 6. Further, the measuring electrode 4 may be formed in a large area on the outer surface of the cylindrical tube 2, and a part of the electrode portion may be exposed through an opening 6 provided in the ceramic insulating layer 5 laminated on the surface. Alternatively, the measurement electrode 4 may be formed only in the portion of the ceramic insulating layer 5 located at the opening 6, and the entire measurement electrode 4 may be exposed from the opening 6.

On the other hand, the reference electrode 3 formed on the inner surface of the cylindrical tube 2 made of a solid electrolyte may be formed on the inner surface at a portion facing the portion of the measurement electrode 4 exposed from the opening 6. It may be formed over a large area including a portion facing the exposed portion of the measurement electrode 4, for example, over the entire inner surface.

(Porous Layer) In the oxygen sensor element of the present invention, as shown in an enlarged sectional view of a main part of FIG. 5, the oxygen sensor element is exposed through an opening 6 formed in the ceramic insulating layer 5 of the measuring electrode 4. The ceramic porous layer 11 can be formed in the portion.

The ceramic porous layer 11 is provided on the exposed surface of the measurement electrode 4 with zirconia, in order to prevent the measurement electrode 4 from being poisoned by the exhaust gas and lowering the output voltage.
It can be provided as a porous protective layer of alumina, magnesia or spinel. An oxygen sensor provided with such a protective layer is generally a stoichiometric air-fuel ratio sensor (λ
Sensor) element. In this case,
The open porosity of the ceramic protective layer 11 is 10 to 40%.
It is desirable to be composed of a porous body.

In another embodiment, at least one type of gas diffusion-controlling layer selected from the group consisting of zirconia, alumina, spinel, magnesia, and γ-alumina having fine pores on the surface of the exposed measurement electrode 4 is formed of ceramic porous material. The quality layer 11 is formed. As such a gas diffusion controlling layer, a porous body having an open porosity of 5 to 30% is desirable. In addition, from the viewpoint of further preventing poisoning of the exhaust gas, it is desirable to provide the above-mentioned ceramic protective layer made of alumina or spinel on the surface of the gas diffusion control layer. Such a heater-formed oxygen sensor element can be applied as a wide-range air-fuel ratio sensor element (A / F sensor).

(First Manufacturing Method) Next, a first manufacturing method of the oxygen sensor element of the present invention will be described with reference to FIG. 6 by taking the method of manufacturing the oxygen sensor element of FIG. 1 as an example. In order to manufacture the oxygen sensor element of FIG. 1, first, a cylindrical tube 12 as shown in FIG. This cylindrical tube 12
An organic binder for molding is appropriately added to a ceramic solid electrolyte powder having oxygen ion conductivity such as zirconia, and one end is sealed by a known method such as extrusion molding, isostatic pressing (rubber pressing) or press forming. It is produced by producing a stopped cylindrical molded body having a diameter of 1 to 10 mm.

At this time, the solid electrolyte powder used was Y ₂ O ₃ as a stabilizer with respect to zirconia powder.
And Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O
₃ , a rare earth oxide powder such as Dy ₂ O ₃
A mixed powder added at a ratio of ３０30 mol%, preferably 3-15 mol%, or a coprecipitated raw material powder of zirconia and the above-mentioned stabilizer is used. It is also possible to use ZrO ₂ powder or coprecipitated material, a 1-20 atomic% of Zr in ZrO ₂ was replaced by Ce.

Further, for the purpose of improving the sinterability, Al ₂ O ₃ or SiO ₂ may be added to the solid electrolyte powder at a ratio of 5% by weight or less, particularly 2% by weight or less.

Next, the cylindrical tube 12 made of the solid electrolyte is used.
On the inner and outer surfaces of the substrate, patterns 13 and 14 serving as reference electrodes and measurement electrodes are formed by a slurry dipping method using a conductive paste containing platinum or the like, or by screen printing, pad printing, or roll transfer. At this time, for printing on the inner surface of the cylindrical tube 12, the conductive paste may be filled and discharged, and may be applied and formed on the entire inner surface.

Next, as shown in FIG.
7 and an insulating ceramic green sheet 15 in which a heating element 16 is buried near an opening 17 is formed. The insulating ceramic green sheet 15 is prepared by first using a ceramic powder such as alumina, spinel, forsterite, zirconia, or glass for forming a ceramic insulating layer, and appropriately adding a forming organic binder to prepare a slurry. Using this slurry, a green sheet having a predetermined thickness is produced by a doctor blade method, an extrusion molding method, a pressing method, or the like. The thickness of the green sheet is from 50 to 500 μm, especially from 100 to 3 from the viewpoint of handling of the sheet.
A range of 00 μm is particularly preferred.

Thereafter, a conductive paste containing platinum powder is printed as a heating element 16 on the surface of the formed green sheet on the heating element pattern by a screen printing method, a pad printing method, a roll transfer method, or the like. The heating element 16 is embedded by stacking the above-mentioned green sheets or applying a slurry of ceramic powder by a printing method or a transfer method. The opening 17 is formed by punching or the like after or before the formation of the heating element 16.

Next, as shown in FIG. 6C, the insulating ceramic green sheet 15 is wound around the surface of the cylindrical tube 12 to form a cylindrical laminate 18. At this time, in order to wind the insulating ceramic green sheet 15 around the surface of the cylindrical tube 12, an adhesive such as an acrylic resin or an organic solvent is interposed between the insulating ceramic green sheet 15 and the cylindrical tube 12. Glue it,
Alternatively, they can be mechanically bonded while applying pressure with a roller or the like.

At this time, the seams of the wound insulating ceramic green sheet 15 may be overlapped with each other at the ends of the sheet or bonded at a predetermined interval in consideration of shrinkage during firing.

Then, the cylindrical laminate 18 is transferred to the cylindrical tube 1.
The solid electrolyte and the insulating ceramic green sheet 15 constituting the two can be integrated by firing simultaneously. A sintered body in which the heater element 15 and the sensor element 12 are integrated is manufactured.

More specifically, when zirconia is used as the solid electrolyte, firing may be performed at 1300 to 1700 ° C. for about 1 to 10 hours in an inert atmosphere such as argon gas or the atmosphere.

In the above-mentioned manufacturing method, the measurement electrode and the reference electrode are formed on the cylindrical tube 12 before the insulating ceramic green sheet 15 is wound. The electrode paste is not limited to the cylindrical tube 12 but is applied to the cylindrical laminate 18 by winding the insulating ceramic green sheet 15 around the surface of the cylindrical tube 12 by a dipping method, screen printing, pad printing, or roll transfer. Opening 1 in inner surface and insulating ceramic green sheet 15
After printing on the surface of the cylindrical tube 12 in 7, it may be fired under the above conditions.

Further, without forming the measurement electrode and the reference electrode, a cylindrical laminate 18 is formed according to the above-described method, and after firing, the opening 18 is formed on the inner surface of the cylindrical tube 12 and the ceramic insulating layer. The electrode paste may be printed and baked therein, or may be formed by a thin film method such as a sputtering method or a plating method.

(Second Manufacturing Method) Next, a second manufacturing method of the oxygen sensor element of the present invention will be described with reference to FIG. 7 by taking the manufacturing method of the oxygen sensor element of FIG. 1 as an example. First, a cylindrical tube 20 as shown in FIG. 7A is manufactured in the same manner as in the first manufacturing method. A reference electrode (not shown) may be formed on the inner surface of the cylindrical tube 20 by a slurry dipping method using a conductive paste containing platinum or the like, or by screen printing, pad printing, or roll transfer.
At this time, the reference electrode may be printed on the inner surface of the cylindrical tube 20 by filling the above-mentioned conductive paste into the cylindrical tube 20, discharging the paste, and applying the conductive paste to the entire inner surface.

Next, a solid electrolyte green sheet 21 as shown in FIG. 7B is prepared using a solid electrolyte having the same composition as that of the cylindrical tube 20. The green sheet can be prepared by using a solid electrolyte powder, appropriately adding a forming organic binder to prepare a slurry, and using this slurry by a doctor blade method, an extrusion forming method, a pressing method, or the like. The thickness of the solid electrolyte green sheet is 50 to 500 μm from the viewpoint of handling of the sheet.
m, particularly preferably in the range of 100 to 300 μm.

As shown in FIG. 7B, the measurement electrode 22 may be formed by printing and applying a conductive paste such as platinum on a predetermined portion of the surface of the solid electrolyte green sheet 21.

As shown in FIG. 7C, an opening 23 is formed at a predetermined location on one surface side of the solid electrolyte green sheet 21 and a heating element 2 is provided near the opening 23.
4 are laminated.

For example, the insulating ceramic green sheet 25a is manufactured by the same method as the insulating ceramic green sheet 15 in the first manufacturing method. The thickness of the insulating ceramic green sheet 25a is 50 to 500 μm, particularly 10 μm, from the viewpoint of sheet handling.
The range of 0 to 300 μm is particularly preferred. Then, an opening 23 is formed at a predetermined position of the green sheet 25a by punching or the like.

Thereafter, the opening 2 of the green sheet 25a is formed.
After printing a conductive paste containing platinum powder as a heating element 24 in the vicinity of 3 on the heating element pattern by a screen printing method, a pad printing method, a roll transfer method or the like, another green sheet 25b is further placed thereon. The heating element 24 is embedded in the insulating ceramic layer 25 by stacking or applying a slurry of ceramic powder by a printing method or a transfer method. The opening 23 may be formed after the heating element 24 is embedded.

Then, the insulating ceramic layer 25 in which the heating element 24 is embedded is replaced with the solid electrolyte green sheet 21.
By laminating the electrodes so that the measurement electrodes 22 formed on the surfaces thereof are exposed from the openings 23, a laminate as shown in FIG. 7C can be manufactured.

(Winding) Next, as shown in FIG. 7D, the other side of the solid electrolyte green sheet 21 is wound around the surface of the cylindrical tube 20 to form a cylindrical laminate 26. At this time, in order to wind the solid electrolyte green sheet 21 around the surface of the cylindrical tube 20, an adhesive such as an acrylic resin or an organic solvent is interposed between the solid electrolyte green sheet 21 and the cylindrical tube 20 to be bonded. Or by mechanical bonding while applying pressure with a roller or the like.

At this time, the seams of the wound solid electrolyte green sheet 21 may be overlapped with each other at the end of the sheet or bonded at a predetermined interval in consideration of shrinkage during firing.

(Simultaneous firing) Then, the cylindrical laminate 2
6 can be integrated by simultaneously firing the solid electrolyte constituting the cylindrical tube 20 and the solid electrolyte green sheet 21 and the insulating ceramic layer 25.

Specifically, when zirconia is used as the solid electrolyte, firing may be performed at 1300 to 1700 ° C. for about 1 to 10 hours in an inert atmosphere such as argon gas or the atmosphere.

(Other Method of Forming Electrodes) In the above-described second manufacturing method, the formation of the measurement electrode and the reference electrode is performed in the following manner.
The method is not limited to the above method, and the cylindrical laminate 2 in which the solid electrolyte green sheet 21 is wound around the surface of the cylindrical tube 20 is used.
An electrode paste such as platinum is applied to the inner surface of the cylindrical tube 20 and the surface of the solid electrolyte green sheet 21 in the opening 23 in the insulating ceramic layer 25 by a dipping method, screen printing, pad printing, or roll transfer. After that, baking may be performed under the above conditions.

Further, without forming the measuring electrode and the reference electrode, a cylindrical laminated body 26 is formed according to the above-described method, and after firing, the cylindrical laminated body 26 is opened, and the opening 23 in the inner surface of the cylindrical tube 21 and the ceramic insulating layer is formed. The electrode paste may be printed and baked therein, or may be formed by a thin film method such as a sputtering method or a plating method.

(Method of Forming Porous Layer) When the above-described stoichiometric air-fuel ratio sensor or wide-range air-fuel ratio oxygen sensor is manufactured, in the first and second methods, alumina, spinel, , Zirconia and other ceramic powders are printed and coated by sol-gel method, slurry dipping method, printing method, etc. and baked, or the above ceramics are coated by sputtering method or plasma spraying method to form ceramic protective layer and gas diffusion controlling layer I do. Also,
As another method, it is also possible to form a ceramic protective layer or a gas diffusion-controlling layer on the surface of the measurement electrode in advance when producing the cylindrical laminate, and to form the same by firing simultaneously with the cylindrical laminate.

In the first and second manufacturing methods, before the firing, a slurry of zirconia powder is applied to the surface of the insulating ceramic layer in which the heating element is embedded,
The zirconia layer 10 as shown in FIG. 2 can be formed by further laminating green sheets produced using zirconia powder and then firing.

(Other Structure) In the oxygen sensor element of the present invention, in the shape of a cylindrical tube made of a solid electrolyte, the sealed one end may have a spherical tip or a cylindrical shape. In addition, the cylindrical tube may have a structure in which the cylindrical tube becomes thinner in a tapered shape toward the tip from the viewpoint of sensor strength and ease of manufacture.

[0067]

EXAMPLES Example 1 Commercially available spinel powder, alumina powder, zirconia powder containing 5 mol% Y ₂ O ₃ ,
Each platinum powder was prepared. First, 5 mol% Y ₂ O ₃
A polyvinyl alcohol solution was added to the containing zirconia powder to prepare a kneaded material, and the outer diameter was about 5 mm by extrusion molding.
A cylindrical molded body having an inner diameter of 3 mm and sealed at one end was prepared.

A slurry was prepared by adding a polyvinyl alcohol solution to the spinel powder, and a green sheet of about 200 μm was prepared. After forming an opening in a green sheet made of this spinel powder by punching,
A conductor paste containing platinum powder was screen-printed in the vicinity of the opening in the form of a heating element pattern, and then a spinel powder was further applied thereon to produce a heater element in which the heating element was embedded.

Next, the heater element was wound around the surface of the cylindrical sensor element using an acrylic resin as an adhesive to form a cylindrical laminated body. Thereafter, the cylindrical laminate is fired in the air at 1500 ° C. for 2 hours,
Fired and integrated.

Thereafter, a porous reference electrode and a measuring electrode made of platinum were formed by electroless plating to a thickness of 2 μm on the surface inside the opening of the cylindrical tube and the entire inner surface of the cylindrical tube.

Then, a ceramic protective layer made of spinel and having a porosity of 30% and having a porosity of 200 μm was formed on the surface of the measurement electrode in the opening by plasma spraying to produce a stoichiometric air-fuel ratio sensor as shown in FIG. .

To evaluate the manufactured oxygen sensor, air was used as the reference electrode, and HC, CO, H ₂ and air (O ₂₎ were used as the measurement electrodes.
₂ ) was supplied so as to have a predetermined excess air ratio. 700
FIG. 8 shows the result of measuring the activation time of the sensor element when the above-mentioned mixed gas was supplied such that the excess air ratio was 0.95 at ° C. At this time, the activation time of a cup-shaped oxygen sensor in which a heater was inserted inside a commercially available cylindrical tube was also measured for comparison.

From the results shown in FIG. 8, the activation of the sensor element of the commercially available cup-shaped oxygen sensor element requires about 50 seconds, whereas the oxygen sensor element with the heater of the present invention operates in 15 seconds. You can see that

FIG. 9 shows the relationship between the output voltage at 700 ° C. and the excess air ratio of the heater-integrated oxygen sensor element of the present invention. This shows that the output voltage changes rapidly when the excess air ratio is around 1. It is apparent from this that the stoichiometric air-fuel ratio sensor of the present invention has a function sufficient to control the mixture ratio of fuel and air near the stoichiometric air-fuel ratio.

Further, the heater-integrated oxygen sensor element manufactured here, an oxygen sensor made of a flat zirconia solid electrolyte as shown in FIG. 12, and a heating element made of platinum are embedded with alumina ceramics as an insulating substrate. The reliability when a temperature cycle was applied to a flat-plate-type heater-integrated oxygen sensor element in which stacked heaters were integrated was compared. At this time, the number of samples was 20 each. The reliability was evaluated from room temperature to 700 ° C. in 20 seconds, and then rapidly cooled to room temperature. This was defined as one cycle, and the rate of increase in the resistance value of the heater embedded in the sensor after the test was performed 100,000 times with respect to the initial value was examined.

As a result, the average value of the rate of increase in the resistance value of the heater of the flat plate-shaped heater-integrated oxygen sensor was 3.5%. In comparison, the average value of the rate of increase in resistance of the product of the present invention was 0.4%, and it was confirmed that the product had excellent heat cycle resistance.

(Example 2) In the sensor element of Example 1, instead of spraying spinel on the surface of the measurement electrode, zirconia powder was applied to the surface of the measurement electrode by a slurry dipping method and baked at 1000 ° C for 1 hour. Thus, a gas diffusion controlling layer having a porosity of about 15% was formed, and a wide-range air-fuel ratio sensor element was manufactured.

FIG. 10 shows the relationship between the limit current value at 700 ° C. and the air-fuel ratio of the wide-range air-fuel ratio sensor element. This figure 1
From 0, it can be seen that the limit current value and the air-fuel ratio are represented by a single curve. This indicates that the oxygen sensor of the present invention has a sufficient function of detecting the ratio of air to fuel even in the lean burn region. The rise time of the oxygen sensor of the present invention provided with the gas diffusion-controlling layer up to 700 ° C. was similarly 15 seconds.

Example 3 5 mol% Y ₂ O _{3 of} Example 1
Polyvinyl alcohol was added to the zirconia powder obtained by the coprecipitation method, and a green sheet having a thickness of about 300 μm was produced by an extrusion molding method. An insulating layer made of alumina was applied to the surface of the green sheet by a screen printing method. After that, an opening was formed by punching, and a conductive paste containing platinum powder was screen-printed on the heating element pattern near the opening. Then, a paste containing alumina powder as an insulating layer was applied to the surface of the platinum heating element pattern by screen printing and dried to prepare a heater element.

Next, the above-mentioned heater element was wound around the surface of the cylindrical sensor element produced in Example 1 to produce a cylindrical laminated body.
It was baked at 0 ° C. for 2 hours.

Thereafter, in the same manner as in Example 1, a reference electrode and a measurement electrode made of platinum were formed on the inner and outer surfaces of the sealed distal end of the cylindrical tube made of the solid electrolyte by plating, and then measured by plasma spraying. A 200 μm-thick ceramic protective layer made of spinel and having a porosity of 28% was formed on the surface of the electrode to produce a theoretical air-fuel ratio sensor.

Further, with respect to the cylindrical laminate after firing,
After forming a reference electrode and a measurement electrode made of platinum on the inner surface of the cylindrical tube made of the solid electrolyte and the outer surface inside the opening by plating in the same manner as in Example 1, a slurry containing zirconia powder was formed on the surface of the measurement electrode. After the application, the coating was baked at 1100 ° C. to form a gas diffusion-controlling layer having a porosity of about 15% with a thickness of 50 μm. Thus, a wide-area air-fuel ratio sensor element having the same shape as that of FIG.

Thereafter, the sensor characteristics were evaluated in accordance with Example 1. As a result, the activation time of the oxygen sensor element with heater of the present invention up to 700 ° C. was 17 times with the stoichiometric air-fuel ratio sensor.
It was 16 seconds with the wide area air-fuel ratio sensor.

The wide-range air-fuel ratio sensor 700 of the present invention
For the relationship between the output voltage and the excess air ratio in ° C,
The same result as in FIG. 9 was obtained. The relationship between the limiting current value at 700 ° C. and the air-fuel ratio of the wide-range air-fuel ratio sensor provided with the gas diffusion-controlling layer was similar to that shown in FIG.

The reliability was evaluated in the same manner as in Example 1 except that the heat treatment was started up from room temperature to 700 ° C. in 20 seconds and then rapidly cooled down to room temperature in one cycle, and this was performed 200,000 times. The increase rate of the value with respect to the initial value was examined for each of 100 theoretical air-fuel ratio sensor elements and the wide area air-fuel ratio sensor. As a result, the rate of increase in resistivity was as small as 0.5% for the stoichiometric air-fuel ratio sensor and 0.4% for the wide-range air-fuel ratio sensor.

[0086]

As described in detail above, according to the oxygen sensor element of the present invention, since it has a cylindrical shape and has a structure in which the oxygen sensor and the heater are simultaneously fired and integrated, a flat plate type heater is provided. Because it has higher strength than the integrated oxygen sensor, it is superior in heat resistance and durability, and a heater is provided adjacent to the electrode compared to the conventional oxygen sensor with a built-in heater inside the cylindrical tube. Therefore, the rate of temperature rise is increased, and the sensor response is superior to or higher than that of a flat-plate stacked sensor element. In addition, since the manufacturing process can be simplified, the manufacturing cost is reduced, and the manufacturing cost is excellent.

[Brief description of the drawings]

1 shows a (a) a schematic perspective view and (b) X ₁ -X ₁ cross-sectional view for explaining an example of an oxygen sensor element of the present invention.

FIG. 2 is an enlarged sectional view of a main part for explaining various structures of a heater unit.

FIG. 3 shows (a) a schematic perspective view and (b) a sectional view taken along line X ₂ -X ₂ for explaining another example of the oxygen sensor element of the present invention.

FIG. 4 shows (a) a schematic perspective view and (b) a sectional view taken along line X ₃ -X ₃ for explaining another example of the oxygen sensor element of the present invention.

FIG. 5 is a schematic cross-sectional view of a case where a porous layer is formed on a measurement electrode surface of the oxygen sensor element of FIG.

FIG. 6 shows a process chart of a method of manufacturing the sensor element of FIG. 1 as an example of a method of manufacturing the oxygen sensor element of the present invention.

FIG. 7 shows a process chart of a method for manufacturing the sensor element of FIG. 1 as another example of the method for manufacturing the oxygen sensor element of the present invention.

FIG. 8 shows the result of measuring the activation time of the sensor element when the mixed gas was supplied at 700 ° C. so that the excess air ratio was 0.95 in Example 1.

FIG. 9 shows the relationship between the output voltage at 700 ° C. and the excess air ratio of the heater-integrated oxygen concentration sensor element of Example 1.

FIG. 10 shows a relationship between a limit current value at 700 ° C. and an air-fuel ratio of a wide area air-fuel ratio sensor.

FIG. 11 is a schematic sectional view of a conventional cylindrical oxygen sensor.

FIG. 12 is a schematic perspective view of a conventional heater-integrated flat plate type oxygen sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Oxygen sensor element 2 Cylindrical tube 3 Reference electrode 4 Measurement electrode 5 Ceramic insulating layer 6 Opening 7 Heating element 11 Porous layer

Claims (6)

[Claims] 1. A cylindrical tube made of a ceramic solid electrolyte having oxygen ion conductivity and sealed at one end, and a reference electrode and a measurement electrode formed at opposing positions on an inner surface and an outer surface of the cylindrical tube. A ceramic insulating layer is laminated on the outer surface of the cylindrical tube so that part or all of the measuring electrode is exposed, and a heating element is embedded at least in the vicinity of the measuring electrode in the ceramic insulating layer. A heater-integrated oxygen sensor element comprising: 2. The heater-integrated oxygen sensor element according to claim 1, wherein the cylindrical tube and the ceramic insulating layer in which the heating element is embedded are simultaneously fired. 3. The heater-integrated oxygen sensor element according to claim 1, wherein a ceramic porous layer is formed on the surface of the measurement electrode. 4. A process for producing a cylindrical tube made of a ceramic solid electrolyte and having one end sealed, an insulating ceramic green having an opening formed in a predetermined location and a heating element embedded near the opening. A step of producing a sheet; a step of winding the insulating ceramic green sheet around the outer surface of the cylindrical tube to produce a cylindrical laminate; a step of firing the cylindrical laminate; and the opening of the cylindrical tube Forming a measurement electrode on the inner surface of the portion and a reference electrode on the inner surface of the cylindrical tube opposed to the measurement electrode, respectively. 5. A step of forming a cylindrical tube having one end made of a ceramic solid electrolyte and sealing the cylindrical tube, a step of forming a green sheet made of the ceramic solid electrolyte, and one surface side of the ceramic solid electrolyte green sheet. A step of laminating an insulating ceramic layer in which an opening is formed at a predetermined location and a heating element is buried in the vicinity of the opening; and Forming a cylindrical laminate by winding; baking the cylindrical laminate; forming a measurement electrode on the surface of the ceramic solid electrolyte in the opening, and forming a reference electrode on the inner surface of the cylindrical tube opposed thereto. And a method for manufacturing a heater-integrated oxygen sensor element. 6. The method for manufacturing a heater-integrated oxygen sensor element according to claim 4, further comprising the step of forming a ceramic porous layer on the surface of said measurement electrode.