JP2002164153A - Ceramic heater and oxygen sensor using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a ceramic heater with an excellent property against heat shock like an abrupt temperature increase, and to obtain an oxygen sensor integrally composed of the ceramic heater and an oxygen concentration detection part or the like, with an excellent property against heat shock like an abrupt temperature increase, and with long life. SOLUTION: For the oxygen sensor comprising a ceramic heater embedding platinum heater inside a ceramic insulation layer, a detection part having a pair of porous platinum electrodes 3, 4 located at opposed positions of inner surface and outer surface of zirconia solid electrolyte base body 2 respectively, and a heating part embedding a platinum heater 7 inside the ceramic insulation layer 5 integrated with the solid electrolyte base body 2, the content of Na in the ceramic insulation layer 5 and the platinum heater 7 are made less than 50 ppm respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is for controlling the ratio of air to fuel in a ceramic heater and an internal combustion engine of an automobile or the like. Related to a long oxygen sensor.

[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 this detecting element, 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. 5, 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. A reference electrode 32 is formed, and a measurement electrode 33 is formed on the outer surface of the cylindrical tube 31 to be in contact with a gas to be measured such as exhaust gas. Various ceramic porous layers 34 are formed on the surface of the measurement electrode 33.

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 at a predetermined temperature, and to control an air-fuel ratio of an 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 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.

In both the above stoichiometric air-fuel ratio sensor and the wide area air-fuel ratio sensor, it is necessary to heat the detecting portion to an operating temperature of about 700 ° C. Therefore, inside the cylindrical tube, the detecting portion is heated to the operating temperature. Therefore, the bar-shaped heater 35 is inserted.

However, in recent years, the exhaust gas regulations have been strengthened, and CO, HC, NO
It has become necessary to detect x. In response to such a request, 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 detection 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 above problem, a reference electrode and a measurement electrode are provided on the inner and outer surfaces of a cylindrical tube made of a solid electrolyte, and a gas-permeable porous insulating layer is provided on the surface of the measurement electrode. A cylindrical oxygen sensor provided with a platinum heater in a gas non-permeable layer having a low gas permeability therein is also described in JP-A-10-206380.

On the other hand, the applicant of the present invention has disclosed a detecting section in which a reference electrode and a measuring electrode are formed on the inner surface and the outer surface of a cylindrical tube which is made of a ceramic solid electrolyte and one end of which is sealed. On the outer surface of the cylindrical tube, a ceramic insulating layer provided with an opening in the measurement electrode forming portion is laminated and formed so that the measurement electrode is exposed from the opening, and at least the ceramic insulation around the exposed measurement electrode is formed. An oxygen sensor was proposed, in which a heating section in which a platinum heater was embedded in the layer was integrally formed.

[0010]

This oxygen sensor has the advantage that it can be rapidly heated because it is a direct heating system, unlike the conventional indirect heating system. However, the life of the oxygen sensor is a major factor in shortening the life of the oxygen sensor in which the heating unit is integrated as described above, in addition to the deterioration of the detection unit.

[0011] The cause of the deterioration of the heating section is that the resistance of the Pt heater gradually increases due to repeated heating,
Eventually, the heating by the Pt heater becomes impossible, and there is a problem that the Pt heater is finally disconnected.

As a method for solving this problem, Japanese Patent Laid-Open Publication No.
No. 00-22158 proposes that the life of the Pt heater is improved by controlling the content of Na in the ceramic insulator in which the platinum heater is embedded to 50 ppm or less. However, as the life of the oxygen sensor is required to be further extended, it is not sufficient to extend the life of the Pt heater simply by reducing the content of Na in the ceramic insulator, and further improvement is required. It has been demanded.

Accordingly, the present invention provides a long-life ceramic heater excellent in thermal shock resistance such as rapid temperature rise and the like and integrally formed with a detecting portion for detecting oxygen concentration and the like, so It is an object of the present invention to provide an oxygen sensor having excellent impact resistance and a long heater life.

[0014]

The present inventor has studied the above problems and found that the use of a ceramic composition having a reduced amount of Na as the ceramic composition for forming the ceramic insulating layer can extend the life of the Pt heater. As can be seen, the Pt raw material forming the Pt heater also inevitably contains Na, and it has been found that the life is dramatically improved by reducing the amount of Na, leading to the present invention.

That is, the ceramic heater according to the present invention is a ceramic heater in which a platinum heater is embedded in a ceramic insulating layer, and the content of Na in the ceramic insulating layer and the content of Na in the platinum heater are each 50 ppm or less. It is characterized by the following.

Further, as the oxygen sensor, a detecting portion having a pair of porous platinum electrodes at least at positions on the inner and outer surfaces of the zirconia solid electrolyte substrate facing each other, and a ceramic insulating layer integrated with the solid electrolyte substrate are provided. An oxygen sensor comprising a heating unit having a platinum heater embedded therein,
The content of Na in each of the ceramic insulating layer and the platinum heater is 50 ppm or less.

According to the present invention, Na in the ceramic insulating layer and the platinum heater is easily diffused in the ceramic insulating layer by an electric field generated around the heater by energization of the heater, and moves to the high temperature portion on the negative electrode side, where it is concentrated. As a result, not only does the electric resistance of the ceramic insulating layer decrease, but also the electric resistance of the platinum heater increases, and as a result, the life of the oxygen sensor itself is significantly reduced. Therefore, according to the present invention, not only the ceramic insulating layer but also the amount of Na in the Pt heater is reduced to 50 pp.
By controlling the resistance to m or less, the electric resistance of the ceramic insulating layer and the platinum heater can be stabilized even during long-term use. As a result, the life of the ceramic heater and the oxygen sensor can be extended.

In the above structure, by forming the ceramic insulating layer from an oxide containing alumina and / or magnesia, not only high electrical insulation can be maintained, but also heat resistance can be improved. .

It is desirable to add a Si component for the purpose of improving the sinterability of the ceramic insulating layer. However, the content of Si in the ceramic insulating layer is preferably 5% by weight or less in terms of oxide. S in ceramic insulating layer
When the i content exceeds 5% by weight, when the heater is energized,
Since the diffusion of Na to the negative electrode in the ceramic insulating layer is promoted and the life of the heater is easily reduced, the content of Si in the ceramic insulating layer is preferably 5% by weight or less in terms of oxide.

Further, according to the present invention, a reference electrode is formed on the inner surface of the cylindrical tube, a measurement electrode is formed on the outer surface of the cylindrical tube, and a ceramic insulating layer in which a platinum heater is embedded is formed on the surface of the cylindrical tube. In addition, by providing an opening in the ceramic insulating layer and exposing a part or all of the measurement electrode from the opening, the heating efficiency of the detection unit by the heating unit can be increased, and the sensor activity of the oxygen sensor can be improved. Time can be shortened.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an example of an oxygen sensor of the present invention schematic perspective view of FIG. 1 (a) and X ₁ -X ₁ cross-sectional view (b)
It is explained based on.

The oxygen sensor 1 shown in FIG. 1 is generally called a theoretical air-twist ratio sensor (λ sensor), is made of a ceramic solid electrolyte having oxygen ion conductivity, and has one end sealed, that is, a longitudinal section. On the inner surface of the U-shaped cylindrical tube 2, a reference electrode 3 that is in contact with a reference gas such as air is formed as a first electrode, and on the outer surface of the cylindrical tube 2 facing the reference electrode 3. Has a measurement electrode 4 that is in contact with a gas to be measured such as exhaust gas as a second electrode.

According to the present invention, the ceramic insulating layer 5 is formed around the measuring electrode 4 formed on the outer surface of the cylindrical tube 2 having one end sealed. An opening 6 is formed in the ceramic insulating layer 5 so that the measurement electrode 4 is exposed, and a platinum heater 7 is buried in the ceramic insulating layer 5 around the opening 6 so that heating is performed. Part is formed.

In the present invention, it is important that Na in the ceramic insulating layer 5 and the platinum heater 7 is 50 ppm or less, preferably 30 ppm or less, and more preferably 20 ppm or less. If the Na content in the ceramic insulating layer 5 and the platinum heater 7 exceeds 50 ppm, Na moves to the minus pole side during heating of the heater, increasing the resistance of the platinum heater and shortening the life of the heater. That's why.

Next, the platinum heater 7 is connected to a terminal electrode 9 via a lead electrode 8 and is heated by passing an electric current through the platinum heater 7 through the lead electrode 8 so that the cylindrical tube 2 and the reference electrode 3 are heated. , And a mechanism for heating the detection unit including the measurement electrode 4.

In the oxygen sensor described above, a second opening may be provided at a symmetrical position opposite to the opening 6, and another opening may be provided at this opening to improve the sensitivity of sensing. An electrode to be measured can also be formed.

At this time, the size of the whole element is oxygen.
The outer diameter of the entire sensor is 3-6 mm, especially 3-4 mm
Power consumption and sensing
Performance can be enhanced. (Solid electrolyte material) In the present invention, the cylindrical tube 2 is formed.
The ceramic solid electrolyte used for_TwoTo
Containing ceramics, specifically, Y_TwoO_ThreeYou
And Yb_TwoO _Three, Sc_TwoO_Three, Sm_TwoO_Three, Nd_TwoO_Three, Dy_Two
O_Three1 to 30 mol% of a rare earth oxide such as
Partially stabilized ZrO preferably containing 3 to 15 mol%_Two
Or stabilized ZrO_TwoIs 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, for the purpose of improving 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 SiO ₂
Is preferably 5% by weight or less, particularly 3% by weight or less in total.

The content of Na in the solid electrolyte is 200 ppm or less, particularly 100 ppm, from the viewpoint of preventing diffusion from the solid electrolyte into the ceramic insulating layer.
Is desirable. (Ceramic insulating layer) On the other hand, as the ceramic insulating layer 5 in which the platinum heater 7 is embedded, an oxide containing alumina and / or magnesia, particularly, an alumina material, a spinel material, or a composite compound material of alumina and spinel is preferable. Used for At this time, for the purpose of improving the sinterability of the ceramic insulating layer, it is desirable to add a small amount of a Si component.
The effect can be seen at weight% or more, but the content of Si is
If it exceeds 5% by weight, diffusion and knitting of Na in the ceramic insulating layer are promoted, and the life of the platinum heater is easily reduced. Therefore, the Si content is desirably in the range of 0.1 to 5% by weight.
The Si content is desirably 0.5 to 3% by weight. In particular, 0.5 to 2% by weight is desirable from the viewpoint of preventing the diffusion of Na.

The ceramic insulating layer 5 is preferably made of a dense ceramic having a relative density of 80% or more and an open porosity of 5% or less. This is because the denseness of the ceramic insulating layer 5 increases the strength of the insulating layer, thereby increasing the mechanical strength of the oxygen sensor itself. Further, the content of Na in the ceramic insulating layer 5 is 50
It is indispensable to reduce the heater life to 30 ppm or less, especially 30 ppm or less. (Platinum heater) Platinum is used as the heater 7 embedded in the ceramic insulating layer 5. In some cases, platinum and at least one selected from the group consisting of rhodium, palladium and ruthenium are used. Alloys can also be used. It is necessary to make the content of Na 50 ppm or less with respect to 100% by weight for both the platinum simple heater and the alloy heater. When the content of Na exceeds 50 ppm, the life of the heater is deteriorated. It is particularly desirable that the content of Na be 30 ppm or less. Further, it is desirable to select a metal or an alloy having a melting point higher than the firing temperature of the ceramic insulating layer 5 from the viewpoint of simultaneous sintering with the ceramic insulating layer 5.

Further, in addition to the above metals, the heater 7 contains alumina, from the viewpoint of preventing sintering and increasing the adhesive strength with the insulating layer.
Spinel, alumina / silica compound, forsterite, and at least one ceramic selected from the group consisting of zirconia that can be the above-mentioned electrolyte, in a volume ratio of 10 to 8
0%, especially 30 to 50%, can be mixed. At this time, the content of Na should be 50 ppm or less with respect to the ceramics to be mixed, and the Na content of the entire heater should satisfy the above range. is necessary.

A platinum heater 7 is provided inside the ceramic insulating layer 5.
As shown in the cross-sectional view of FIG.
A zirconia layer 10 can be formed on the outer surface of the ceramic insulating layer 5. The zirconia layer 10 reduces stress caused by a difference in thermal expansion and a difference in firing shrinkage between the solid electrolyte and the ceramic insulating layer 5 so that the thermal stress is reduced as much as possible. Efficiency can be increased.

At this time, the thickness of the ceramic insulating layer 5 between the cylindrical tube 2 and the heater 7 and between the zirconia layer 10 and the heater 7 is preferably at least 2 μm or more. (Electrode) 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.
Alternatively, 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. The components may be mixed in the electrode at a ratio of 1 to 50% by volume, particularly 10 to 30% by volume.

In the present invention, the shape of the measurement electrode 4 exposed in the first opening 6 is as shown in FIG.
It is desirable to be composed of a vertically long rectangular or elliptical shape as shown in FIG.

On the other hand, the reference electrode 3 formed on the inner surface of the cylindrical tube 2 made of a solid electrolyte is connected to the opening 6 of the measuring electrode 4.
It may be formed on the inner surface portion facing the more exposed portion, and may be formed on an area larger than the exposed portion area of the measurement electrode 4, for example, on the entire inner surface of the cylindrical tube 2.

(Opening) The shape of the opening 6 is preferably a rectangle or an ellipse as described above. When the opening 6 of the ceramic insulating layer 5 has a rectangular shape, the corner may have a gentle curve or a c-shaped structure, so that thermal stress is concentrated on the corner of the opening 6. From the viewpoint of alleviating the above.

(Porous Layer) In the oxygen sensor of the present invention, as shown in the enlarged sectional view of the main part of FIG. 2, the surface of the measuring electrode 4 exposed in the opening 6 of the ceramic insulating layer 5 The ceramic porous layer 11 can be formed.
The ceramic porous layer 11 is formed for the following two purposes.

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

Second, it functions as at least one kind of gas diffusion-controlling layer selected from the group consisting of zirconia, alumina, spinel, magnesia and γ-alumina having fine pores on the exposed surface of the measurement electrode 4. As the ceramic insulating layer 11 serving as such a gas diffusion controlling layer, a porous body having an open porosity of 5 to 30% is desirable.

From the viewpoint of further preventing the exhaust gas from being poisoned, it is desirable to provide the ceramic protective layer made of alumina or spinel described above on the surface of the ceramic porous layer 11 serving as the gas diffusion-controlling layer. .
Such a heater-embedded oxygen sensor can be applied as a wide-range air-fuel ratio sensor element (A / F sensor) described later. (Manufacturing Method) Next, a method for manufacturing the oxygen sensor according to the present invention will be described with reference to the method for manufacturing the oxygen sensor in FIG. 1 as an example. (1) First, a hollow cylindrical tube 2 having one end sealed as shown in FIG. The cylindrical tube 2 is formed by adding an appropriate organic binder to a ceramic solid electrolyte powder having oxygen ion conductivity, such as zirconia, or the like, by extrusion molding, isostatic pressing (rubber pressing), or press forming. It is produced by the method described above.

At this time, the solid electrolyte powder used was Y ₂ O ₃ and Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O ₃ , Dy as stabilizers with respect to zirconia powder. _Two
A mixed powder obtained by adding a rare earth oxide powder such as O ₃ at a ratio of 1 to 30 mol%, preferably 3 to 15 mol% in terms of oxide, or a coprecipitated raw material powder of zirconia and the above 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, it is possible to add Al ₂ O ₃ or SiO ₂ to the solid electrolyte powder in an amount of 5% by weight or less, particularly 3% by weight or less.

(2) On the inner and outer surfaces of the cylindrical tube 2 made of the solid electrolyte, patterns 3 and 4 serving as a reference electrode and a measurement electrode are formed by a slurry dipping method using a conductive paste containing platinum, for example. Formed by screen printing, pad printing, and roll transfer. At this time,
For printing the reference electrode on the inner surface of the cylindrical tube 2, it is efficient to fill and discharge the conductive paste and apply and form the entire surface of the inner surface. Thus, the sensor body A is manufactured.

(3) Next, a heater element B as shown in FIG. 3B is formed. The heater element B is prepared by first preparing a slurry by using zirconia powder and appropriately adding an organic binder for molding, and using the slurry to form a ceramic having a predetermined thickness by a doctor blade method, an extrusion molding method, a pressing method, or the like. Green sheet 2 for forming insulating layer
Is prepared. The thickness of one green sheet is 50 to 500 μm, particularly 100 to 30 μm, from the viewpoint of sheet handling.
A range of 0 μm is particularly preferred.

Thereafter, alumina, spinel or a composite oxide powder of alumina and spinel with a reduced Na content is screen-printed on the surface of the green sheet 2 by molding,
The ceramic insulating layer 5a is formed by printing using a pad printing method, a roll transfer method, or the like. Thereafter, a conductive paste containing platinum powder with a reduced amount of Na according to the present invention is similarly printed by a screen printing method, a pad printing method, a roll transfer method, etc., and a platinum heater pattern 7 including a lead portion is applied. The above-mentioned ceramic insulating layer forming powder is further applied thereon to obtain a sheet-like laminate in which the ceramic insulating layer 5b and the platinum heater pattern 7 are embedded. After that, the opening 6 is manufactured by forming by punching or the like.

(4) Next, as shown in FIG. 3 (c), the heater element B is wound around the surface of the cylindrical sensor element A to form a cylindrical laminate. At this time, the heater element B
Is wound around the sensor body A by bonding an adhesive such as an acrylic resin or an organic solvent between the heater body B and the sensor body A, or by mechanically applying pressure with a roller or the like. Can be adhered to. At this time, the seams of the wound heater element B may be overlapped with each other at the sheet ends or bonded at a predetermined interval in consideration of shrinkage during firing.

(5) The sensor body A and the heater body B can be integrated by firing the cylindrical laminate at a temperature at which the respective components can be fired simultaneously. The firing is performed, for example, at 1300 to 1700 ° C. in an inert atmosphere such as an argon gas or in the air.
By firing for about 10 hours, the heater element B and the sensor element A can be simultaneously fired. (Other manufacturing method) As another manufacturing method, after the heater element B formed by the above (3) is wound around the surface of the cylindrical tube 2 having no electrode, a cylindrical laminated body is manufactured.
The electrode paste is applied to the inner surface of the cylindrical tube 2 and the surface of the cylindrical tube in the opening 6 of the heater element B by screen printing, pad printing, a roll transfer method or an immersion method on the cylindrical laminated body. Co-firing can also be performed as in 5).

As another method, the heater element B formed by the above (3) is wound around the surface of the cylindrical tube 2 having no electrode to form a cylindrical laminated body. The electrode paste may be printed and baked in the inner surface and the opening 6 in the heater element B, or may be formed by a sputtering method or a plating method. (Method for Forming Porous Layer) In the case of manufacturing the above-described theoretical air-fuel ratio sensor and the wide-range air-fuel ratio oxygen sensor described later,
After firing the cylindrical laminate, alumina,
A powder such as spinel or zirconia is printed and applied by a sol-gel method, a slurry dipping method, a printing method, etc., and baked, or the ceramic is coated by a sputtering method or a plasma spraying method to form a ceramic porous layer or a gas diffusion-limiting layer. Form. As another method, it is also possible to form a ceramic porous layer or a gas diffusion-controlling layer on the surface of the measurement electrode in advance and manufacture the cylindrical laminated body by firing simultaneously with the cylindrical laminated body. (Other Sensor Structures) The present invention can be applied to an air-fuel ratio sensor element. FIG. 4 shows another application example of the present invention.
FIG. 4A is a schematic perspective view of an air-fuel ratio sensor element, and FIG.
(B) shows an X1-X1 cross-sectional view of (a).

According to the air-fuel ratio sensor element 20 shown in FIG. 4, the sensor element is formed in a cylindrical tube 2 made of a ceramic solid electrolyte having oxygen ion conductivity and having one end sealed, in other words, a U-shaped vertical section. Is formed. Specifically, a reference electrode 3 that is in contact with a reference gas such as air is formed on an inner surface of the cylindrical tube 2, and a measurement target gas such as exhaust gas is formed on an outer surface of the cylindrical tube 2 that faces the reference electrode 3. A measuring electrode 4 to be in contact is formed.

A space 6 is formed on the outer surface of the cylindrical tube 2 so that part or all of the measurement electrode 4 is exposed, and the platinum heater 7 of the present invention is provided around the space 6. Are embedded in the ceramic insulating layer 5.

On the upper surface of the space 6, a solid electrolyte layer 21 having oxygen ion conductivity is formed so as to close the space 6.
The inner surface on the side of the first space portion 6 and the outer surface of the solid electrolyte layer 21 opposed thereto have a second
Are formed. The solid electrolyte layer 21 and the second pair of electrodes 22 and 23 function as a pump cell for controlling the oxygen concentration in the space 6 to a predetermined concentration. Further, on the surface of the outer electrode 23, a ceramic porous layer 26 serving as a protective layer having an open porosity of 10 to 40% and a thickness of 10 to 200 μm and made of at least one selected from the group consisting of zirconia, alumina, magnesia and spinel. Are formed. Further, the space 6 can be filled with a porous ceramic or a filler of ceramic powder having a large particle size.

Further, a small diffusion hole 25 for taking in an exhaust gas to be measured is formed in the solid electrolyte layer 21 having the second pair of electrodes 22 and 23, and the space of the diffusion hole 25 is formed. A diffusion controlling body 27 is formed around the inside of the portion 6.

Further, the platinum heater 7 disposed in the ceramic insulating layer 5 is connected to the terminal electrode 9 via the lead electrode 8 as in the oxygen sensor of FIG. The heater 7 is heated by passing an electric current, and the cylindrical tube 2 made of a solid electrolyte including the reference electrode 3 and the measurement electrode 4 and the above-described second electrode pair 22, 2
This is a mechanism for heating the detection section composed of the solid electrolyte layer 21 having the structure 3.

Although the oxygen sensor has been specifically described as an example using the ceramic heater of the present invention, the present invention can be applied to any shape ceramic heater in which a platinum heater is embedded in a ceramic insulating layer. In addition, the oxygen sensor as an application example is not limited to the above-described structure, and the element structure is a bag-like tube, or a flat plate-type oxygen sensor. A detection unit having a platinum electrode;
Further, it goes without saying that the present invention can be applied to all oxygen sensors having a platinum heater embedded in a ceramic insulating layer and to other gas sensors having a platinum heater incorporated therein.

[0055]

(Embodiment 1) An example of an oxygen sensor shown in FIG.
An embodiment of the present invention will be described. Na with commercial purity of 99.8%
About 10 to 200 ppm, SiO_Two0 to 7% by weight
Alumina powder and spinel powder (MgAlO_Four)
And 5 mol% Y_TwoO _ThreeContaining zirconia powder and Na
Prepare platinum powder containing 8 to 200 ppm
Was. First, 5 mol% Y_TwoO_ThreeZirconia powder containing poly
A clay is prepared by adding a vinyl alcohol solution and extruded.
After sintering, the outer diameter becomes about 3-5mm and inner diameter becomes 1mm depending on the shape.
To make a cylindrical molded body with one end sealed
And a rectangular shape as shown in Table 1 made of platinum paste
Print the measurement electrode pattern and lead pattern
Cloth and also apply platinum paste to the entire inside of the molded body.
It was applied to form a reference electrode. The measurement electrode and the base
Adjust the thickness of the quasi-electrode so that it becomes about 5 μm after firing.
Was.

A slurry was prepared by adding a polyvinyl alcohol solution to zirconia powder containing 5 mol% of Y ₂ O _{3, and} a green sheet having a thickness of about 200 μm was prepared.
In the green sheet, second openings having the same size and the same shape were opened by punching so as to be located on opposite sides of the first openings having various rectangular shapes corresponding to the shape of the measurement electrode. Thereafter, the above-mentioned alumina powder or spinel powder was applied to a portion other than the opening by about 20 μm.
m, a conductor paste containing platinum powder is screen-printed around the opening to form a heating element pattern having a thickness of about 20 μm, and an alumina powder or a spinel powder is further formed thereon to about 20 μm. A heater element having the structure shown in FIG.

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 laminate. Thereafter, the cylindrical laminate was fired in air at 1500 ° C. for 2 hours for a sample containing no SiO ₂ in the insulating layer, and 2 hours at 1350 ° C. for a sample containing SiO ₂ in the insulating layer.

Thereafter, a ceramic porous layer having a porosity of about 30% and having a porosity of about 30% was formed by plasma spraying on the surface of the measurement electrode in the opening to a thickness of 100 μm.
The stoichiometric air-fuel ratio sensor shown in FIG.

The Na content in the ceramic insulating layer and the platinum heater of the manufactured oxygen sensor was determined by EPMA analysis using a calibration curve of Na. In the evaluation test, the element was heated to 900 ° C. in the atmosphere, and the time until the heater was disconnected was measured. Table 1 shows the results. In Table 1, the same evaluation was performed for a commercially available heater-integrated flat plate type oxygen sensor for comparison.

[0060]

[Table 1]

As shown in Table 1, when the ceramic insulating layer was made of alumina and SiO ₂ -containing alumina, the content of Sample No. 50 in the ceramic insulating layer or the platinum heater exceeded 50 ppm. In 7, 8, 9, 14, and 15, the time required for heater disconnection was shorter than 3000 hours. Similarly, when the ceramic insulating layer is spinel, the sample N in which the Na in the ceramic insulating layer or the platinum heater exceeds 50 ppm is also used.
o. Also in 18, 19 and 20, the time until heater disconnection was short. In addition, the heater on the minus pole side was broken in all samples, and according to EPMA analysis, Na was segregated due to the breakage.

On the other hand, all of the samples in which the content of Na in the ceramic insulating layer or the platinum heater was 50 ppm or less were excellent in durability, with the time to heater disconnection being 3500 hours or more. In particular, a sample in which the content of Na in the ceramic insulating layer or the platinum heater is 30 ppm or less and the content of SiO ₂ in the ceramic insulating layer is 5% by weight or less has extremely excellent heater durability of 5000 hours or more. Indicated.

[0063]

As described above in detail, according to the present invention, in a ceramic heater having a platinum heater built in the ceramic insulating layer, the amount of Na in the ceramic insulating layer and the platinum heater is controlled to a predetermined range. As a result, the durability of the platinum heater can be dramatically improved, and as a result, a long-life ceramic heater with excellent thermal shock resistance, such as rapid temperature rise, and this can be formed integrally with the detection unit for oxygen concentration, etc. And has excellent thermal shock properties such as rapid temperature rise.
An oxygen sensor having a long heater life can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view of an example of an oxygen sensor of the present invention (a).
And X ₁ -X ₁ sectional view (b).

FIG. 2 is a sectional view of a main part of another structure of the oxygen sensor of the present invention.

FIGS. 3A to 3C are process diagrams for manufacturing the oxygen sensor of FIG.

FIGS. 4A and 4B are a schematic perspective view and a sectional view taken along line XX for explaining another example of the oxygen sensor of the present invention.

FIG. 5 is a schematic sectional view for explaining the structure of a conventional oxygen sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Oxygen sensor 2 Cylindrical tube 3 Reference electrode 4 Measurement electrode 5 Ceramic insulating layer 6 Opening 7 Platinum heater 8 Lead electrode 9 Terminal electrode

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H05B 3/03 H05B 3/44 3/12 C04B 35/04 A 3/18 35/10 D 3/44 G01N 27/58 BF term (for reference) 2G004 BB01 BC02 BD05 BE13 BE22 BF03 BF09 BJ02 BK02 BL08 3K092 PP16 PP20 QA02 QB02 QB26 QB45 QB76 QC02 QC20 QC26 QC31 QC38 QC49 RB03 RB04 RB17 RB23 RB17 RB23 RB17 RB23 RB17 RD23 4G030 AA07 AA36 AA37 BA12

Claims (8)

[Claims] 1. A ceramic heater having a platinum heater embedded in a ceramic insulating layer, wherein each of the ceramic insulating layer and the platinum heater has a Na content of 50 p.
pm or less. 2. The ceramic heater according to claim 1, wherein said ceramic insulating layer is made of an oxide containing alumina and / or magnesia. 3. The ceramic heater according to claim 1, wherein the content of Si in the ceramic insulating layer is 5% by weight or less in terms of oxide. 4. A detection section having a pair of porous platinum electrodes at least at positions facing the inner and outer surfaces of a zirconia solid electrolyte substrate, and a platinum heater embedded in a ceramic insulating layer integrated with the solid electrolyte substrate. An oxygen sensor comprising: a heating unit comprising: a ceramic insulating layer; and a platinum heater, wherein each of the contents of Na in the platinum heater is 50 ppm or less. 5. The oxygen sensor according to claim 4, wherein said ceramic insulating layer is made of an oxide containing alumina and / or magnesia. 6. The oxygen sensor according to claim 4, wherein the content of Si in the ceramic insulating layer is 5% by weight or less in terms of oxide. 7. The oxygen sensor according to claim 4, wherein said zirconia solid electrolyte substrate is formed of a cylindrical tube having one end sealed. 8. A cylindrical electrode having a reference electrode formed on an inner surface thereof, a measurement electrode formed on an outer surface of the cylindrical tube, and a ceramic insulating layer having a platinum heater embedded therein formed on the surface of the cylindrical tube. Providing an opening in the insulating layer,
8. The oxygen sensor according to claim 4, wherein a part or the whole of the measurement electrode is exposed from the opening.