JP2000338078A - Heater one-piece type oxygen sensor and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an economical heater one-piece type oxygen sensor that relieves thermal stress caused by the difference of a thermal expansion coefficient, prevents an insulation layer from being easily released even when a sudden temperature cycle is added, and also can be manufactured at extremely low costs. SOLUTION: On the external surface of an oxygen sensor 1 where reference and measurement electrodes 3 and 4 are formed at opposing positions on the internal and external surfaces of a cylinder pipe 2 that is made of zirconia solid electrolyte and whose one end is sealed, a nonionic conductive porous insulation layer 5 such as spinel and a porous zirconia layer 6 are successively laminated, an electrical heating element 7 made of platinum is buried in the porous insulation layer 5 or between the insulation layer 5 and the zirconia layer 6, and at the same time the thickness of the porous insulation layer 5 from the electrical heating element 7 to the measurement electrode 4 is set to 1 to 100 μm.

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 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.

[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. 7A, an inner surface of a cylindrical tube 31 made of a ZrO ₂ solid electrolyte and having a sealed end is provided with:
Reference electrode 3 made of platinum and in contact with a reference gas such as air
2 and a measurement electrode 33 formed on the outer surface of the cylindrical tube 31 to be in contact with a gas to be measured such as exhaust gas.

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

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 method in which 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 and 33, and a limit current value obtained at that time is measured to control an air-fuel ratio in a lean to rich region. It is.

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. Therefore, the bar-shaped heater 35 is inserted.

However, in recent years, exhaust gas regulations have been strengthened, and CO, HC, NO
It has become necessary to detect x. 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 for avoiding the problem, FIG.
As shown in (b), an oxygen sensor 39 having a measurement electrode 37 and a reference electrode 38 formed on the surface of a flat solid electrolyte 36.
On the other hand, a laminated oxygen sensor of a flat plate type in which a heater 42 in which an internal heating element 41 of a ceramic insulating substrate 40 of a flat plate type is embedded is laminated and integrated is proposed in Japanese Utility Model Application Laid-Open No. 61-199666.

Recently, as shown in FIG. 8, a platinum reference electrode 44 and a measurement electrode 45 are provided on the inner and outer surfaces of a cylindrical tube 43 made of a zirconia solid electrolyte. A cylindrical heater-integrated oxygen sensor in which an insulating layer 46 made of porous spinel is provided and a heating element 48 made of platinum whose periphery is protected by a protective layer 47 such as alumina is embedded in the insulating layer 46 is also available. It is described in JP-A-10-206380.

The oxygen sensor integrated with these heaters has a feature that, unlike the conventional indirect heating method, the temperature can be rapidly raised because of the direct heating method, and the activation time of the sensing section is short. .

[0011]

However, since the above-described flat oxygen sensor has a flat plate shape,
There was a problem that the durability and heat resistance were poor, and as a result, the sensor was broken during operation.

Further, according to the heater-integrated oxygen sensor described in Japanese Patent Application Laid-Open No. 10-206380, the insulating layer 46 and the protective layer 47 made of spinel, alumina or the like are made of the zirconia solid electrolyte 43 and platinum. Since the thermal expansion coefficient is greatly different from that of the electrodes 44 and 45 and the heating element 48 made of platinum, thermal stress is easily generated in the sensor element. Therefore, when a rapid temperature cycle is applied, the insulating layer is easily peeled off. Had.

[0013] Regarding the production method, a zirconia powder is molded and fired to produce a solid electrolyte cylindrical body, and then platinum electrodes are formed on the outer and inner surfaces thereof. afterwards,
The process is complicated because an insulating layer made of spinel is formed on the electrolyte surface by plasma spraying, a paste-like platinum heater is applied to the surface, and the insulating layer is formed again by plasma spraying. There are problems such as high manufacturing costs. In addition, since the platinum electrodes 44 and 45 and the insulating layer 46 are also formed by the plasma spraying method, there is a problem that the bonding strength with the solid electrolyte 43 is weak.

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

[0015]

Means for Solving the Problems The present inventors have repeatedly studied the above-mentioned problems relating to the oxygen sensor in which the cylindrical heater is integrated, and as a result, a cylindrical tube made of a zirconia solid electrolyte and having one end sealed. A non-ion conductive porous insulating layer and a porous zirconia layer are sequentially laminated on the outer surface of an oxygen sensor in which a reference electrode and a measurement electrode are formed at opposing positions of an inner surface and an outer surface, respectively. A heating element made of platinum is embedded in the porous insulating layer or between the insulating layer and the zirconia layer, and the thickness of the porous insulating layer from the measurement electrode to the heating element is 1 to 100 μm. Or an oxygen sensor in which a reference electrode and a measurement electrode are formed at opposing positions on the inner surface and the outer surface of a cylindrical tube made of a zirconia solid electrolyte and sealed at one end. A non-ion conductive porous insulating layer having a thickness of 1 μm or more and a porous zirconia layer are sequentially laminated on the outer surface of the substrate, and a heating element made of platinum is embedded in the porous zirconia layer, and the measurement is performed. It has been found that the above object can be achieved by setting the total thickness of the porous insulating layer and the porous zirconia layer from the electrode to the heating element to 100 μm or less.

Further, as a manufacturing method, a reference electrode and a measurement electrode are formed at opposing positions of an inner surface and an outer surface of a cylindrical tube having one end sealed and formed using zirconia solid electrolyte powder. Make an elementary body,
A laminate of a non-ion conductive porous insulating layer and a porous zirconia layer, and a heating element made of platinum in the porous insulating layer or between the porous insulating layer and the porous zirconia layer. The thickness of the porous insulating layer from the measurement electrode to the heating element after burying and firing is 1 to 100.
μm heater element or 1 μm thick after firing
The above non-ion conductive porous insulating layer and the porous zirconia layer are laminated, and a heating element made of platinum is embedded in the porous zirconia layer, and the heating element from the measurement electrode to the heating element after firing. After producing a heater element in which the total thickness of the porous insulating layer and the porous zirconia layer is 100 μm or less, the heater element is placed on the outer surface of the sensor element, By forming a cylindrical laminated body by winding as described above and then firing the laminated body at the same time, the process can be simplified and the manufacturing cost can be reduced.

[0017]

According to the heater-integrated oxygen sensor of the present invention, the porous zirconia layer is provided on the surface of the measurement electrode formed on the surface of the cylindrical tube made of the zirconia solid electrolyte via the non-ion conductive porous insulating layer. Was formed, and a heating element made of platinum was formed in the porous insulating layer or the porous zirconia layer, or between the porous insulating layer and the porous zirconia layer. The sensor element can be efficiently heated, and the activation time can be shortened.

Even when the zirconia solid electrolyte and the non-ion conductive insulating layer have different thermal expansion characteristics, the thickness of the porous insulating layer can be reduced, and the surface of the porous insulating layer can be made porous. Since the zirconia layer is formed, the porous insulating layer is sandwiched between the two zirconia layers, so that the stress caused by the difference in thermal expansion is balanced, and the insulating layer and the heating element are prevented from peeling off. Can be.

In addition, in the present invention, in addition to the above-described sensor structure, since the oxygen sensor section and the heater section having the heating element are fired at once, the manufacturing is made in comparison with the conventional heater-integrated oxygen sensor. The cost is extremely low, and it is excellent from the economical viewpoint. In addition, the insulating layer, the heating element, and the zirconia layer can be firmly fixed to the sensor body including the solid electrolyte and the electrode for co-sintering. it can.

As a result, according to the heater-body oxygen sensor of the present invention, it is possible to provide a heater-integrated oxygen sensor having a short activation time, excellent durability and heat resistance, and a method of manufacturing the same.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The first embodiment of the oxygen sensor of the present invention will be described below.
Explaining the example schematic perspective view of FIG. 1 (a) and X ₁ -X ₁ cross-sectional view of (b) based. The oxygen sensor 1 of FIG. 1 is made of a zirconia solid electrolyte having oxygen ion conductivity,
A reference electrode 3 that is in contact with a reference gas such as air is formed as a first electrode on the inner surface of a cylindrical tube 2 having a U-shaped cross section with a sealed tip. On the outer surface facing the electrode 3, a measurement electrode 4 that is in contact with a gas to be measured such as exhaust gas is formed as a second electrode.

Further, according to the present invention, the non-ion conductive porous insulating layer 5 and the porous zirconia are formed on or around the surface of the measuring electrode 4 formed on the outer surface of the cylindrical tube 2 whose end is sealed. The layers 6 are sequentially provided, and a heating element 7 made of platinum is embedded in the porous insulating layer 5.

The reference electrode 3, the measuring electrode 4, and the heating element 7 are electrically connected to a lead electrode (not shown) provided on the surface of the cylindrical tube 2, the surface of the insulating layer 5, the surface of the zirconia layer 6, or the inside thereof. And a predetermined voltage or current is applied.

(Solid Electrolyte) In the present invention, the zirconia solid electrolyte constituting the cylindrical tube 2 is specifically Y ₂ O
₃ and Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂
O _3, Dy ₂ O ₃ 1~30 mol% of at least one selected from the group consisting of rare earth oxide in terms of oxide, such as, preferably 3 to 15 mol% content to partially stabilized ZrO ₂ or stabilized ZrO ₂ Is used. Further, by using ZrO ₂ in which Zr in ZrO ₂ is substituted with 1 to 20 atomic% of Ce for Ce, there is an effect that electron conductivity is increased and responsiveness is further improved.

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 Material) Each of the reference electrode 3 and the measurement electrode 4 formed on the inner surface and the outer surface of the cylindrical tube 2 is one selected from the group consisting of platinum, rhodium, palladium, ruthenium and gold, or 2 More than one alloy is used.
The zirconia solid electrolyte described above is used for the purpose of preventing the grain growth of the metal in the electrode during the operation of the sensor and to increase the contact point at the so-called three-phase interface between the metal particles, the solid electrolyte, and the gas related to the response. 1 to 50% by volume, especially 10%
You may mix in the said electrode in the ratio of 30 volume%.

(Porous Insulating Layer) On the other hand, it is necessary that the porous insulating layer 5 formed on the surface of the measuring electrode 4 itself has non-ionic conductivity. This is because the cylindrical tube 2 made of the solid electrolyte and the heating element 7 made of platinum are completely electrically separated from each other to insulate them.

In this structure, the thickness L ₁ of the insulating layer 5 from the surface of the measuring electrode 4 to the heating element 7 is ₁ to 100.
It is important that it is in the order of μm, in particular 5 to 50 μm, more particularly 10 to 40 μm. This is because the thickness L ₁ is 1 μm
When thinner than the electrical insulation becomes insufficient sensor and the heating element 7 measuring electrodes 4 both not exhibit the normal output voltage, and when the thickness L ₁ of more than 100 [mu] m, the heating element 7
The heating efficiency of the sensor unit is reduced, the activation time is lengthened, and the cylindrical tube 2 made of solid electrolyte and the insulating layer 5
The insulation layer is easily peeled off due to the thermal stress generated between them, and the rate of increase in the resistance of the heater increases.

As the porous insulating layer 5, a ceramic material such as alumina, spinel, forsterite, or glass is preferably used. When glass is used as the porous insulating layer 5, Ba is used from the viewpoint of heat resistance.
A glass containing at least one of O, PbO, SrO, CaO, and CdO in an amount of 5% by weight or more, particularly a crystallized glass is desirable.

Further, the porous insulating layer 5 needs to have predetermined pores in order for the gas to be measured to reach the measurement electrode 4, and if the porosity is too large, P ( The electrodes may be poisoned by (phosphorus) or Pb (lead). From this viewpoint, the porosity of the porous insulating layer 5 is 7 to
Desirably, it is 35%.

(Zirconia Layer) The porous zirconia layer 6 is made of Y ₂ O ₃ and Yb
₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O ₃ , Dy ₂
Partially stabilized Z containing at least one oxide selected from the group of rare earth oxides such as O ₃ in an oxide equivalent of 1 to 30 mol%, particularly 3 to 15 mol% for enhancing strength.
rO ₂ or stabilized ZrO ₂ is preferably used. Further, ZrO ₂ in which Zr in ZrO ₂ is substituted with 1 to 20 atomic% of Ce can also be preferably used.
Since it is desirable to have the same thermal expansion characteristics as the solid electrolyte constituting the cylindrical tube 2, it is most preferable to have the same composition as the zirconia solid electrolyte of the cylindrical tube 2.

Further, like the porous insulating layer 5, the porous zirconia layer 6 is preferably made of a porous material having a porosity of 7 to 35% so that the gas to be measured can reach the measuring electrode 4. The total thickness is 50 to 500 μm, and 100 to 200 μm from the viewpoint of gas permeability and heat retention of the heater.
m is desirable.

(Heating Element) According to the oxygen sensor shown in FIG. 1, although the heating element 7 is embedded in the porous insulating layer 5,
The heating element 7 needs to be made of platinum (Pt) which is stable in the air due to the problem of simultaneous firing. This makes it possible to form the heating element 7 and the porous insulating layer 5 by simultaneous firing.

(Other Structure) According to the oxygen sensor of FIG.
The heating element 7 is embedded in the porous insulating layer 5, but the location where the heating element 7 is embedded is not limited to this. For example, an enlarged sectional view of a main part based on the second example of FIG. As shown in (1), it is also possible to provide between the porous insulating layer 5 and the porous zirconia layer 6.

Further, as shown in an enlarged sectional view of a main part based on the third example of FIG. 3, the heating element 7 may be embedded in the porous zirconia layer 6. This is because, as described above, the electrical insulation between both the heating element 7 and the measurement electrode 4 is insufficient, and the sensor does not exhibit a normal output voltage. Also,
The total thickness L ₂ of the porous insulating layer 5 and the porous zirconia layer 6 from the surface of the measurement electrode 4 to the heating element 7 is 100 μm.
It is important that: This is because the thickness L ₂ is 10
If the thickness exceeds 0 μm, the heating efficiency of the sensor unit by the heating element 7 decreases, and the activation time becomes longer.

According to the oxygen sensor of the present invention, the stoichiometric air-fuel ratio sensor is controlled by controlling the pore diameter of the pores of the porous insulating layer 5 and the porous zirconia layer 6 formed on the surface of the measuring electrode 4. λ sensor) or a wide-range air-fuel ratio sensor (A / F sensor).
In the latter wide-area air-fuel ratio sensor, when a gas diffusion-controlling layer (not shown) of spinel, zirconia, magnesia, alumina, or the like is further provided on the surface of the porous zirconia layer 6 by using a plasma spraying method or the like, higher accuracy is achieved. The air-fuel ratio can be controlled. At this time, the stoichiometric air-fuel ratio sensor controls the open porosity of the porous zirconia layer and the porous insulating layer in the range of 15% to 30%, and the open porosity of the porous zirconia layer and the porous insulating layer becomes 7% to 30%.
A wide-range air-fuel ratio sensor can be manufactured by controlling to 12%.

(Manufacturing Method) Next, a method of manufacturing the oxygen sensor of the present invention will be described with reference to FIG. 4 taking the oxygen sensor of FIG. 1 as an example. (Production of Sensor Element) First, a cylindrical oxygen sensor element 10 as shown in FIG. 4A is produced. This sensor element 10
One end is sealed to a zirconia solid electrolyte powder having oxygen ion conductivity by a well-known method such as extrusion molding, isostatic pressing (rubber pressing) or press forming by appropriately adding an organic binder for molding. Diameter 1
A 10 mm cylindrical molded body 11 is produced.

At this time, the solid electrolyte powder used was zirconia powder and Y ₂ O ₃ as a stabilizer.
And Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O
₃ , a mixed powder in which at least one selected from the group of rare earth oxides such as Dy ₂ O ₃ is added at a ratio of 1 to 30 mol%, preferably 3 to 15 mol% in terms of oxide, or zirconia and the above-mentioned stable powder. Coprecipitated raw material powder with an agent 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 sinterability, it is possible to add Al ₂ O ₃ or SiO ₂ to the solid electrolyte powder at a ratio of 5% by weight or less, particularly 2% by weight or less.

The patterns 12 and 13 serving as reference electrodes and measurement electrodes are formed on the inner and outer surfaces of the cylindrical molded body 11 made of the solid electrolyte by, for example, a slurry dipping method using a conductive paste containing platinum or a screen. It is formed by printing, pad printing, and roll transfer. At this time, for printing on the inner surface of the cylindrical tube, the conductor paste may be filled and discharged, and may be applied and formed on the entire inner surface.

(Heater Element) Next, a heater element 14 as shown in FIG. 4B is formed. For the heater element 14, first, a green sheet 15 for a porous zirconia layer having a predetermined thickness is prepared by a doctor blade method, an extrusion molding method, a pressing method, or the like using the same zirconia powder as described above.

Then, a slurry is prepared by appropriately adding an organic binder for molding to ceramic powder such as alumina, spinel, forsterite, or glass, which can form a porous insulating layer having non-ion conductivity. After applying to the green sheet 15 and then printing a conductive paste containing platinum powder on the heating element pattern by a screen printing method, a pad printing method, a roll transfer method, or the like,
By further applying a slurry thereon and drying the slurry, the heater element 14 composed of a laminate of the porous insulating layer 17 in which the heating element 16 is embedded and the zirconia layer green sheet 15 can be manufactured.

As another method, a green sheet for a porous zirconia layer and a green sheet for a porous insulating layer are prepared, and a heating element of platinum is applied to one of the green sheets by printing and then laminated. Heater element 1 made of a laminated body
4 can be produced. Incidentally, in order to finally control the porosity of the porous insulating layer and the porous zirconia layer to a predetermined range, with respect to the ceramic powder composition,
For example, a commercially available polyethylene powder can be added as a pore-forming agent to control the porosity after firing.

The thickness of each green sheet 15 is preferably from 60 to 600 μm, particularly preferably from 100 to 300 μm. If the thickness of the sheet is less than 60 μm, it is difficult to handle the sheet, and if it exceeds 600 μm, it becomes difficult to wind the sheet around the surface of the cylindrical molded body.

(Coiling) Next, as shown in FIG. 4C, the heater element 14 is wound around the surface of the cylindrical sensor element 10 to form a cylindrical laminate 18. At this time, in order to wind the heater element 14 around the sensor element 10, an adhesive such as an acrylic resin or an organic solvent is interposed between the heater element 14 and the sensor element 10, or a roller or the like is used. Can be mechanically bonded while applying pressure. At this time, the seams of the wound heater element 14 may be overlapped with each other at the end of the sheet or bonded at a predetermined interval in consideration of shrinkage during firing.

When the heater element 14 is wound,
In addition to winding the laminate of the porous insulating layer green sheet and the porous zirconia layer green sheet, it is also possible to wind the porous zirconia layer green sheet after winding the porous insulating layer green sheet. is there.

(Simultaneous Firing) Then, the cylindrical laminate 18 is simultaneously fired with the solid electrolyte constituting the sensor element 10 and the green sheet for the porous insulating layer and the green sheet for the porous zirconia layer in the heater element 14. By firing at a possible temperature, an oxygen sensor in which the heater element 14 and the sensor element 10 are integrated is manufactured.
Depending on the firing conditions, it is desirable to fire at 1300 to 1700 ° C. for about 1 to 10 hours in an inert atmosphere such as argon gas or the atmosphere.

(Other Manufacturing Method) The heating element 7 as shown in FIG. 2 is composed of the porous insulating layer 5 and the porous zirconia layer 6.
When the oxygen sensor buried in between is manufactured, a platinum paste is applied to the surface of the porous zirconia green sheet when the heater element 14 is manufactured, and then a slurry for the porous insulating layer is formed. A heater element may be prepared by applying or laminating green sheets, and then wound and fired in the same manner as described above.

When an oxygen sensor in which the heating element 7 as shown in FIG. 3 is embedded in the porous zirconia layer 6 is manufactured, when the heater element 14 is manufactured, the green for the porous zirconia layer is used. After applying a platinum paste to the surface of the sheet, laminating a green sheet for a porous zirconia layer or applying a slurry, and then further laminating a green sheet for a porous insulating layer or applying a slurry Thus, after the heater element body is manufactured, it can be manufactured by winding and simultaneous firing in the same manner as described above.

In some cases, a powder of alumina, spinel, zirconia, or the like is printed and coated on the surface of the porous zirconia layer of the oxygen sensor fired as described above by a sol-gel method, a slurry dipping method, a printing method, or the like. It is also possible to manufacture a wide range air-fuel ratio oxygen sensor by baking or coating the above ceramics by sputtering or plasma spraying to form a gas diffusion controlling layer.

In the oxygen sensor 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 tip, and may have a sensor strength and a manufacturing efficiency. For the sake of simplicity, the cylindrical tube may have a structure tapering toward the tip.

The reference electrode and the measurement electrode formed on the inner surface and the outer surface of the zirconia solid electrolyte may be formed at any position as long as they are opposed to each other across the solid electrolyte. Furthermore, it goes without saying that the heating element or the position of the electrode and the heater may be included in the element of the present invention as long as the above-mentioned relation is satisfied, regardless of the relation.

[0053]

EXAMPLES The present invention will be described with reference to examples. (Example 1) The average particle size produced by the coprecipitation method is 0.
A polyvinyl alcohol solution is added to 8 μm of 5 mol% Y ₂ O ₃ containing zirconia powder to prepare a kneaded material, and a hollow cylindrical molded body having an outer diameter of 5 mm and an inner diameter of 3 mm sealed at one end is extruded. Produced. Then, a conductive paste in which platinum was dispersed was applied to the inner and outer surfaces of the cylindrical molded body at opposing positions via a solid electrolyte to form a reference electrode and a measurement electrode, thereby producing a sensor element body.

A commercially available 3 μm polyethylene powder was added as a pore-forming agent to the zirconia powder, and a polyvinyl alcohol solution was further added to prepare a slurry. The slurry was prepared by a doctor blade method for a porous zirconia layer of various thicknesses. A green sheet was produced.

Further, a slurry for forming a porous insulating layer was prepared by adding polyethylene powder to spinel powder having an average particle diameter of 0.5 μm. Then, using these, the following 3
Various kinds of oxygen sensors were produced and evaluated as follows.

(Oxygen sensor in FIG. 1) A slurry for forming a porous insulating layer of spinel is applied to the surface of the green sheet for zirconia layer in various thicknesses and dried, and then a conductive paste of platinum powder is screened on the surface. Printing was applied to the heating element pattern by printing so that the thickness after firing was about 10 μm. Thereafter, a slurry for forming a porous insulating layer of spinel was applied in various thicknesses on the surface of the heating element pattern to produce a heater element composed of a sheet-like laminate. Then, the heater element was wound around the sensor element to produce a cylindrical laminated body. Thereafter, the cylindrical laminate was fired in the air at 1500 ° C. for 2 hours to produce an oxygen sensor for controlling the stoichiometric air-fuel ratio having the heating element structure shown in FIG. The open porosity of the manufactured oxygen sensor was measured by the Archimedes method. As a result, the porous insulating layer was 18% and the porous zirconia layer was 19%. Table 1 shows the thickness of each layer in the manufactured oxygen sensor.

(Oxygen sensor of FIG. 2) A conductor paste of platinum was printed and applied on the surface of the green sheet for the zirconia layer by a screen printing method so that the thickness after firing was about 10 μm. Then, a zirconia slurry is applied to the area other than the heating element pattern portion, and a slurry for forming a porous insulating layer is applied thereon in various thicknesses so as to embed the heating element pattern thereon. The body was made. Thereafter, according to the above-described method, a cylindrical laminated body is formed by winding around the surface of the sensor element.
The operation was performed at 500 ° C. for 2 hours to produce an oxygen sensor having a stoichiometric air-fuel ratio control having a heating element structure in FIG. The open porosity of the fabricated oxygen sensor was measured by the Archimedes method. As a result, the porous insulating layer was 18%, and the porous zirconia layer was 20%.
%Met. Table 2 shows the thickness of each layer in the manufactured oxygen sensor.

(Oxygen sensor in FIG. 3) A slurry of zirconia is applied to the surface of the green sheet for the zirconia layer, and a conductor paste of platinum is printed and applied on the surface of the green sheet for the heating element pattern by a screen printing method. Then, a slurry for forming a porous insulating layer was applied in various thicknesses to prepare a sheet-like laminate. Thereafter, according to the above-described method, a cylindrical laminate was wound around the surface of the sensor element body, and then performed at 1500 ° C. for 2 hours to produce an oxygen sensor having a stoichiometric air-fuel ratio control. At this time, the open porosity of the porous insulating layer and the porous zirconia layer was about 18% and 19%, respectively.
%Met. Table 1 shows the thickness of each layer in the manufactured oxygen sensor.

(Evaluation method) Activation time The above oxygen sensor was measured at 700 ° C. in air.
, Ie, the activation time was measured, and the results are shown in Tables 1 and 2.

Durability (resistance change) For each manufactured oxygen sensor, in order to evaluate the durability of the device, the device was rapidly heated from room temperature to 700 ° C. in the air in 20 seconds and maintained at that temperature for 60 seconds. After that, the temperature cycle of rapid cooling to room temperature was defined as one cycle, and when this cycle was repeated 100,000 times, the rate of increase of the resistance value of the heating element from the initial value was measured. The results are shown in Tables 1 and 2.

Durability (Damage) Also, the presence or absence of damage of the element when the temperature cycle was repeated 200,000 times was examined.

The thicknesses of the porous insulating layer and the porous zirconia layer and the thickness L ₂ from the measurement electrode to the heating element in the table were measured at 10 points using a scanning electron microscope photograph, and the average values were shown. It is a thing. The results are shown in Tables 1 and 2.

(Comparative Product) For comparison, a similar evaluation was performed on a commercially available oxygen sensor having a heating element inserted in a cylindrical tube as shown in FIG. 7A.

FIGS. 1 and 2 show another comparative example.
The oxygen sensor having no porous zirconia layer (sample Nos. 17 and 21) and the oxygen sensor of FIG. 3 having no insulating layer (sample No. 22) were prepared and evaluated in the same manner.

[0065]

[Table 1]

As shown in Table 1, in the oxygen sensor having the structure shown in FIGS. 1 and 2, the element was normal in the samples No. 2 and No. 3 in which the thickness of the porous insulating layer from the measurement electrode to the heating element was less than 1 μm. No output voltage was indicated. In Samples No. 15 and No. 16 in which the thickness of the porous insulating layer from the measurement electrode to the heating element exceeded 100 μm, the activation time was prolonged. On the other hand, the products of the present invention in which the thickness of the porous insulating layer from the measurement electrode to the heating element was 1 to 100 μm showed an activation time of 17 seconds or less, which was shorter than that of a commercially available product.

Regarding the durability of the element, the rate of increase in the resistance value of the element after 100,000 cycles was 6.1% for the conventional oxygen sensor (sample No. 1), and the thickness from the measurement electrode to the heating element was 6.1%. Samples No. 15 and No. 16 having a thickness of more than 100 μm were as large as 5.1% and 5.6%, respectively, but the samples having a thickness of 100 μm or less were as small as 2.1% or less. Sample No. 17 having no zirconia layer,
No. 21 failed in each of about 130 to 180,000 temperature cycles, but all of the devices of the present invention did not break in 200,000 cycles.

[0068]

[Table 2]

As shown in Table 2, in the oxygen sensor having the structure shown in FIG. 3, the sample No. 2 in which the thickness of the porous insulating layer is thinner than 1 μm.
In Nos. 2 and 23, the element did not show a normal output voltage.
In Sample No. 29, in which the thickness from the measurement electrode to the heating element exceeded 100 μm, the activation time was prolonged. On the other hand, in the case of the present invention in which the thickness of the insulating layer is 1 μm or more and the thickness from the measurement electrode to the heating element is 100 μm or less, the activation time is 18 seconds or less and the activation time is shorter than that of the commercially available product. Indicated.

Regarding the durability of the device, the devices did not operate in the samples No. 22 and No. 23 in which the thickness of the insulating layer was thinner than 1 μm. In sample No. 29, in which the thickness from the measurement electrode to the heating element was greater than 100 μm, the rate of increase in resistance was as high as 3.6%, but the thickness of the insulating layer was 1%.
μm or more, the thickness from the measurement electrode to the heating element is 100 μm
Each of the following present invention products was a small value of 1.8% or less.

From this, it can be understood that the oxygen sensor of the present invention shown in FIGS. 1 to 3 has a short activation time and is excellent in durability.

Next, the sensing characteristics of Sample No. 5 in Table 1 were evaluated. Air to the sensor's reference electrode,
HC, CO, H _2, and air were supplied to the measurement electrodes at a predetermined excess air ratio, and the relationship between the output voltage at 700 ° C. and the excess air ratio was shown in FIG. From FIG. 5, it can be seen that the output voltage changes abruptly near the predetermined excess air ratio, that is, the so-called stoichiometric air-fuel ratio.

(Embodiment 2) According to the method of Embodiment 1, FIG.
A wide-range air-fuel ratio sensor having a heating element structure was fabricated. At this time, firing of the cylindrical laminate was performed at 1550 ° C. in the atmosphere for 2 hours. The open porosity of the obtained insulating layer and zirconia layer was 8% and 10%, respectively. The thickness of the insulating layer was 27 μm. Thereafter, at 700 ° C., air was supplied to the reference electrode of the air-fuel ratio sensor, and HC, CO,
A voltage was applied while supplying H ₂ and air so as to have a predetermined air-fuel ratio, and a limit current value at that time was obtained. FIG. 7 shows the relationship between the limit current value and the air-fuel ratio. As shown in FIG. 6, the limit current value is shown by a single curve with respect to the air-fuel ratio, and it was found that the oxygen sensor of the present invention can be applied as a wide-range air-fuel ratio sensor.

[0074]

As described above in detail, according to the present invention, the responsiveness and the detection accuracy are excellent, the thermal stress caused by the difference in the coefficient of thermal expansion is relaxed, and a rapid temperature cycle is applied. -Insulation layer is hardly peeled off and heater has excellent durability-
A body oxygen sensor can be provided. In addition, since such an oxygen sensor can be manufactured by simultaneously firing the sensor element and the heater element, the manufacturing cost is extremely low and the oxygen sensor is excellent in terms of economy.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view (a) for explaining an example of a heater-integrated oxygen sensor of the present invention, and an enlarged sectional view (b) of a main part thereof.

FIG. 2 is an enlarged sectional view of a main part for explaining another example of the heater-integrated oxygen sensor of the present invention.

FIG. 3 is an enlarged sectional view of a main part for explaining still another example of the heater-integrated oxygen sensor of the present invention.

FIG. 4 is a process chart for explaining a method of manufacturing the heater-integrated oxygen sensor of FIG. 1 of the present invention.

FIG. 5 shows the relationship between the output voltage of the oxygen sensor of Sample No. 5 and the excess air ratio according to the embodiment of the present invention.

FIG. 6 shows a relationship between a limiting current value and an air-fuel ratio in the oxygen sensor according to the second embodiment of the present invention.

FIG. 7 is a schematic sectional view of a conventional cylindrical oxygen sensor (a).
And a schematic cross-sectional view (b) of a heater-integrated flat plate type oxygen sensor.

FIG. 8 is an enlarged sectional view of a main part of a conventional cylindrical oxygen sensor integrated with a heater.

[Explanation of symbols]

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

Claims (6)

[Claims] 1. An oxygen sensor comprising a cylindrical tube made of a zirconia solid electrolyte and having one end sealed and having a reference electrode and a measurement electrode formed at opposing positions on an inner surface and an outer surface, respectively. Are sequentially laminated with a porous insulating layer and a porous zirconia layer, in the porous insulating layer, or between the porous insulating layer and the zirconia layer,
A heating element made of platinum is embedded, and the thickness of the porous insulating layer from the heating element to the measurement electrode is 1 to 10
A heater-integrated oxygen sensor having a thickness of 0 μm. 2. A heater-integrated oxygen sensor according to claim 1, wherein said non-ion conductive porous insulating layer is mainly composed of spinel. 3. An outer surface of an oxygen sensor having a reference electrode and a measurement electrode formed at opposing positions of an inner surface and an outer surface of a cylindrical tube made of a zirconia solid electrolyte and having one end sealed, and having a thickness of 1 μm or more. A non-ion conductive porous insulating layer and a porous zirconia layer are sequentially laminated, and a heating element made of platinum is embedded in the porous zirconia layer, and the porous layer from the heating element to the measurement electrode is formed. The total thickness of the porous insulating layer and the porous zirconia layer is 100 μm.
m or less. 4. The heater-integrated oxygen sensor according to claim 3, wherein said porous insulating layer having no oxygen ion conductivity is mainly composed of spinel. 5. A process for forming a sensor element by forming a reference electrode and a measurement electrode at opposing positions on an inner surface and an outer surface of a cylindrical tube having one end sealed and formed using zirconia solid electrolyte powder. a, a non-ion conductive porous insulating layer and a porous zirconia layer are laminated, and heat is generated from platinum in the porous insulating layer or between the porous insulating layer and the porous zirconia layer. The thickness of the porous insulating layer from the heating element after firing to the measurement electrode is 1 to 100 μm.
Or a non-ion conductive porous insulating layer having a thickness of 1 μm after firing and a porous zirconia layer, and a heating element made of platinum embedded in the porous zirconia layer. Forming a heater element in which the total thickness of the porous insulating layer and the porous zirconia layer from the heating element to the measurement electrode after firing is 100 μm or less; b. A step c of preparing a cylindrical laminate by winding the heater element on the outer surface of the heater so that the porous insulating layer is on the inside, and a step d of firing the cylindrical laminate. A method for manufacturing a heater-integrated oxygen sensor, comprising the steps of: 6. The method according to claim 5, wherein said non-ion conductive porous insulating layer is mainly composed of spinel.