JP4582899B2 - Ceramic heater and oxygen sensor using the same - Google Patents

Description

【００６１】
表１より、セラミック絶縁層がアルミナおよびＳｉＯ₂含有のアルミナの場合、セラミック絶縁層または白金ヒータ中の含有量が５０ｐｐｍを越える試料Ｎｏ．７、８、９、１４および１５ではヒータ断線に至る時間が３０００時間よりも短かった。また、セラミック絶縁層がスピネルの場合も、同様にセラミック絶縁層または白金ヒータ中のＮａが５０ｐｐｍを越える試料Ｎｏ．１８，１９および２０でもヒータ断線までの時間が短かった。また、いずれの試料ともマイナス極側のヒータが破損し，ＥＰＭＡ分析によると、破損からＮａの編析が見られた。
【００６２】
それに対して、セラミック絶縁層中のＳｉの含有量が酸化物換算で５重量％以下で、かつセラミック絶縁層および白金ヒータ中のＮａの含有量が５０ｐｐｍ以下の試料は全てヒータ断線に至る時間が４０５０時間以上と耐久性に優れたものであった。特に、セラミック絶縁層または白金ヒータ中のＮａの含有量が３０ｐｐｍ以下、セラミック絶縁層中のＳｉＯ_２の含有量が５重量％以下の試料はヒータの耐久時間が５０００時間以上と極めて優れた性能を示した。
【００６３】
【発明の効果】
以上詳述した通り、本発明によれば、セラミック絶縁層中に白金ヒータを内蔵するセラミックヒータにおいて、セラミック絶縁層中および白金ヒータ中のＮａ量を所定の範囲に制御することにより、白金ヒータの耐久性を飛躍的に向上させることができる結果、急速昇温などの熱衝撃性に優れた長寿命のセラミックヒータ、およびこれを酸素濃度などの検知部と一体的に形成してなり、急速昇温などの熱衝撃性に優れると同時に、ヒータ寿命の長い酸素センサを提供することができる。
【図面の簡単な説明】
【図１】本発明の酸素センサの一例の概略斜視図（ａ）およびＸ₁−Ｘ₁断面図（ｂ）である。
【図２】本発明の酸素センサの他の構造における要部断面図である。
【図３】（ａ）〜（ｃ）は図１の酸素センサを製造するための工程図である。
【図４】本発明の酸素センサの他の例を説明するための概略斜視図（ａ）およびＸ−Ｘ断面図（ｂ）である。
【図５】従来の酸素センサの構造を説明するための概略断面図である。
【符号の説明】
１ 酸素センサ
２ 円筒管
３ 基準電極
４ 測定電極
５ セラミック絶縁層
６ 開口部
７ 白金ヒータ
８ リード電極
９ 端子電極[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ceramic heater and an oxygen sensor for controlling a ratio of air and fuel in an internal combustion engine such as an automobile and having a long life of a heating unit in which a heating unit and a detection unit are integrated.
[0002]
[Prior art]
Currently, in an internal combustion engine such as an automobile, by detecting the oxygen concentration in the exhaust gas and controlling the amount of air and fuel supplied to the internal combustion engine based on the detected value, harmful substances from the internal combustion engine, For example, a method of reducing CO, HC, and NOx is adopted.
[0003]
The detection element is a solid electrolyte type oxygen mainly composed of a solid electrolyte mainly composed of zirconia having oxygen ion conductivity, and a pair of electrode layers formed on the outer surface and the inner surface of a cylindrical tube sealed at one end. A sensor is used. As a typical example of this oxygen sensor, as shown in FIG.₂A reference electrode 32 made of platinum and in contact with a reference gas such as air is formed on the inner surface of the cylindrical tube 31 made of a solid electrolyte and sealed at the tip, and a gas to be measured such as an exhaust gas is formed on the outer surface of the cylindrical tube 31. A measurement electrode 33 is formed in contact with. Various ceramic porous layers 34 are formed on the surface of the measurement electrode 33.
[0004]
In such an oxygen sensor, as a so-called theoretical air-fuel ratio sensor (λ sensor), which is generally used for controlling the ratio of air to fuel near 1, a ceramic porous layer serving as a protective layer is formed on the surface of the measurement electrode 33. A mass layer 34 is provided, and an oxygen concentration difference generated on both sides of the cylindrical tube at a predetermined temperature is detected to control the air-fuel ratio of the engine intake system.
[0005]
On the other hand, a so-called wide-area air-fuel ratio sensor (A / F sensor) used for controlling a wide range of air-fuel ratio is a ceramic porous material that becomes a gas diffusion-controlling layer having fine pores on the surface of the measurement electrode 33. A layer 34 is provided, and 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 combustion region.
[0006]
In both the theoretical air-fuel ratio sensor and the wide-range air-fuel ratio sensor, it is necessary to heat the detection unit to an operating temperature of about 700 ° C. Therefore, a rod heater is provided inside the cylindrical tube to heat the detection unit to the operating temperature. 35 is inserted.
[0007]
However, in recent years, 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 request, in the indirect heating type cylindrical oxygen sensor in which the heater 35 is inserted into the cylindrical tube 31 as described above, the time required for the detection unit to reach the activation temperature (hereinafter, There is a problem that exhaust gas regulations cannot be fully met because the activation time is slow.
[0008]
As a method for avoiding the 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, and further, Japanese Patent Application Laid-Open No. 10-206380 also discloses a cylindrical oxygen sensor in which a platinum heater is provided in a gas non-permeable layer having low gas permeability.
[0009]
On the other hand, the applicant of the present invention has previously described that the cylindrical portion is formed such that the reference electrode and the measurement electrode are formed on the inner surface and the outer surface of the cylindrical tube made of a ceramic solid electrolyte and sealed at one end, and the measurement electrode is exposed. A ceramic insulating layer having an opening in the measurement electrode forming portion is laminated on the outer surface of the tube so that the measurement electrode is exposed from the opening, and at least in the exposed ceramic insulating layer around the measurement electrode An oxygen sensor was proposed in which a heating unit with an embedded platinum heater was formed integrally.
[0010]
[Problems to be solved by the invention]
Unlike the conventional indirect heating method, this oxygen sensor has an advantage that rapid heating is possible because of the direct heating method. However, the lifetime of the oxygen sensor is not only the deterioration of the detection unit, but also the deterioration of the heating unit is a major factor in shortening the lifetime in the oxygen sensor in which the heating unit is integrated as described above.
[0011]
As a cause of the deterioration of the heating portion, there is a problem that the resistance of the Pt heater gradually increases due to repeated heating, and finally the heating by the Pt heater cannot be performed, and finally the Pt heater is disconnected.
[0012]
As a method for solving this problem, Japanese Patent Application Laid-Open No. 2000-221158 proposes that the life extension of the Pt heater is improved by controlling the Na content in the ceramic insulator in which the platinum heater is embedded to 50 ppm or less. Has been. However, in the demand for longer life of the oxygen sensor, it is not sufficient to extend the life of the Pt heater simply by reducing the Na content in the ceramic insulator in this way. It has been demanded.
[0013]
Therefore, the present invention is formed integrally with a long-life ceramic heater excellent in thermal shock resistance such as rapid temperature rise, and a detection unit such as oxygen concentration, so that thermal shock resistance such as rapid temperature rise is achieved. An object of the present invention is to provide an oxygen sensor which is excellent and has a long heater life.
[0014]
[Means for Solving the Problems]
As a result of studying the above problems, the present inventor can extend the life of a Pt heater by using a ceramic composition with a reduced amount of Na as a ceramic composition for forming a ceramic insulating layer. The Pt raw material to be inevitably contains Na, and it has been found that reducing the amount of Na dramatically improves the service life.
[0015]
  That is, the ceramic heater of the present invention is a ceramic heater in which a platinum heater is embedded in a ceramic insulating layer,The content of Si in the ceramic insulating layer is 5% by weight or less in terms of oxide,The ceramic insulating layer andSaidEach of the Na contents in the platinum heater is 50 ppm or less.
[0016]
  Further, as an oxygen sensor, a pair of oxygen sensors are provided at positions facing at least the inner and outer surfaces of the zirconia solid electrolyte substrate.WhiteAn oxygen sensor comprising a detection unit having a gold electrode, and a heating unit in which a platinum heater is embedded in a ceramic insulating layer integrated with the solid electrolyte substrate,The content of Si in the ceramic insulating layer is 5% by weight or less in terms of oxide,The content of Na in the ceramic insulating layer and the platinum heater is 50 ppm or less, respectively.
[0017]
  According to the present invention, Na in the ceramic insulating layer and the platinum heater is easily diffused in the ceramic insulating layer by the electric field generated around by the energization of the heater, moved to the high temperature portion on the negative electrode side, concentrated, Not only does it lower the electrical resistance of the ceramic insulation layer, but it also increases the electrical resistance of the platinum heater, resulting in a significant reduction in the lifetime of the oxygen sensor itself. Therefore, according to the present invention, not only the ceramic insulating layer but also the amount of Na in the Pt heater is controlled to 50 ppm or less, so that the electrical 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.Further, Si component may be added for the purpose of improving the sinterability of the ceramic insulating layer, but the Si content in the ceramic insulating layer is 5% by weight or less in terms of oxide. This is because if the Si content in the ceramic insulating layer exceeds 5% by weight, the diffusion of Na in the ceramic insulating layer to the negative electrode is promoted and the life of the heater is likely to be shortened when the heater is energized.
[0018]
  In the above structure, by forming the ceramic insulating layer with an oxide containing alumina and / or magnesia, not only high electrical insulation can be maintained, but also heat resistance can be improved.In the configuration of the oxygen sensor, the pair of platinum electrodes is preferably porous.
[0020]
  Furthermore, according to the present invention,As the pair of platinum electrodes,Cylindrical tubeofA reference electrode is formed on the inner surface, and a measurement electrode is formed on the outer surface of the cylindrical tube.ofOn the surfaceSaidEmbedded platinum heaterSaidCeramic insulation layerButCoatingHas beenIn 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 Conversion time can be shortened.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an example of the oxygen sensor of the present invention is shown in FIG.₁-X₁A description will be given based on the cross-sectional view (b).
[0022]
The oxygen sensor 1 of FIG. 1 is generally called a theoretical air-twist ratio sensor (λ sensor), and is made of a ceramic solid electrolyte having oxygen ion conductivity, and one end is sealed, that is, the longitudinal section is U-shaped. A reference electrode 3 that is in contact with a reference gas such as air is attached and formed on the inner surface of the cylindrical tube 2 as a first electrode, and the outer surface of the cylindrical tube 2 facing the reference electrode 3 is a first electrode. As the second electrode, a measurement electrode 4 that is in contact with a gas to be measured such as exhaust gas is formed.
[0023]
Further, according to the present invention, the ceramic insulating layer 5 is deposited around the measurement electrode 4 formed on the outer surface of the cylindrical tube 2 whose one end is 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 embedded in the ceramic insulating layer 5 around the opening 6 to heat the ceramic insulating layer 5. Forming part.
[0024]
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, desirably 30 ppm or less, and further 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 negative electrode side when the heater is heated, thereby increasing the resistance of the platinum heater and reducing the life of the heater. Because.
[0025]
Next, the platinum heater 7 is connected to the terminal electrode 9 via the lead electrode 8, and is heated by passing an electric current through the platinum heater 7 through the lead electrode 8. The cylindrical tube 2, the reference electrode 3, and the measurement electrode 4 is a mechanism for heating the detection unit consisting of four.
[0026]
In the above oxygen sensor, a second opening may be provided at a contrasting position on the opposite side of the opening 6, and another electrode to be measured is provided in this opening in order to improve sensing sensitivity. Can also be formed.
[0027]
At this time, as the size of the entire element, the outer diameter of the entire oxygen sensor is 3 to 6 mm, particularly 3 to 4 mm, so that power consumption can be reduced and the sensing performance can be improved.
(Solid electrolyte material)
In the present invention, the ceramic solid electrolyte used to form the cylindrical tube 2 is ZrO.₂Specifically, it is made of ceramics containing₂O_ThreeAnd Yb₂O_Three, Sc₂O_Three, Sm₂O_Three, Nd₂O_Three, Dy₂O_ThreePartially stabilized ZrO containing 1 to 30 mol%, preferably 3 to 15 mol% of a rare earth oxide such as₂Or stabilized ZrO₂Is used.
[0028]
ZrO₂ZrO in which 1 to 20 atomic% of Zr was substituted with Ce₂By using, there is an effect that the electron conductivity is increased and the responsiveness is further improved.
[0029]
Furthermore, for the purpose of improving the sinterability, the above ZrO₂In contrast, Al₂O_ThreeAnd SiO₂However, if a large amount is added, the creep characteristics at high temperatures deteriorate, so Al₂O_ThreeAnd SiO₂The total amount of added is preferably 5% by weight or less, particularly 3% by weight or less.
[0030]
Further, the content of Na in the solid electrolyte is preferably 200 ppm or less, particularly 100 ppm, from the viewpoint of preventing diffusion penetration from the solid electrolyte to the ceramic insulating layer.
(Ceramic insulation 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 preferably used. At this time, for the purpose of improving the sinterability of the ceramic insulating layer, it is desirable to add a small amount of Si component, but as its content, the effect is seen at 0.1 wt% or more in terms of oxide, If the Si content exceeds 5% by weight, the diffusion and knitting of Na in the ceramic insulating layer is promoted and the life of the platinum heater is likely to be shortened, so the Si content is in the range of 0.1 to 5% by weight. Is desirable. As Si content, 0.5 to 3 weight% is desirable. In particular, 0.5 to 2% by weight is desirable from the viewpoint of preventing the diffusion of Na.
[0031]
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 mechanical strength of the oxygen sensor itself can be increased as a result of the strength of the insulating layer being increased because the ceramic insulating layer 5 is dense. Furthermore, the Na content in the ceramic insulating layer 5 is indispensable to extend the life of the heater to 50 ppm, particularly 30 ppm or less, for the reasons described above.
(Platinum heater)
In addition, platinum is used as the heater 7 embedded in the ceramic insulating layer 5, but in some cases, an alloy of platinum and at least one selected from the group of rhodium, palladium, and ruthenium is used. You can also. It is necessary to set the Na content to 50 ppm or less with respect to 100 wt% for both the platinum simple substance heater and the alloy heater. When the Na content exceeds 50 ppm, the life of the heater is deteriorated. The Na content is particularly preferably 30 ppm or less. Further, it is desirable to select a metal or alloy having a melting point higher than the firing temperature of the ceramic insulating layer 5 in terms of co-sinterability with the ceramic insulating layer 5.
[0032]
In addition to the above metals, the heater 7 is selected from the group of alumina, spinel, alumina / silica compound, forsterite, and the above-mentioned zirconia, which can be used as an electrolyte, from the viewpoint of preventing sintering and increasing the adhesion between the insulating layer and the metal. At least one kind of ceramics can be mixed in a volume ratio of 10 to 80%, particularly 30 to 50%. In this case, the content of Na is 50 ppm or less for the ceramics to be mixed. It is necessary that the Na amount of the heater as a whole satisfies the above range.
[0033]
As shown in the cross-sectional view of FIG. 2, the heat generating portion in which the platinum heater 7 is embedded inside the ceramic insulating layer 5 can form a zirconia layer 10 on the outer surface of the ceramic insulating layer 5. This zirconia layer 10 relieves the stress caused by the difference in thermal expansion and firing shrinkage between the solid electrolyte and the ceramic insulating layer 5 so as to reduce the thermal stress as much as possible, and also functions as a heat retaining layer and is heated by the heater 7. Efficiency can be increased.
[0034]
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)
The reference electrode 3 and the measurement electrode 4 deposited and formed on the inner surface and the outer surface of the cylindrical tube 2 are each made of one or more alloys selected from the group consisting of platinum, rhodium, palladium, ruthenium and gold. . In addition, for the purpose of preventing the grain growth of the metal in the electrode during the sensor operation and for the purpose of increasing the contact at the so-called three-phase interface between the metal particles, the solid electrolyte and the gas related to the responsiveness, The components may be mixed in the electrode in a proportion of 1 to 50% by volume, particularly 10 to 30% by volume.
[0035]
In the present invention, the shape of the measurement electrode 4 exposed in the first opening 6 is composed of a vertically long rectangular shape and an elliptical shape as shown in FIG. desirable.
[0036]
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 portion facing the portion exposed from the opening 6 of the measurement electrode 4. An area larger than the partial area, for example, the entire inner surface of the cylindrical tube 2 may be formed.
[0037]
(Aperture)
The shape of the opening 6 is preferably rectangular or elliptical as described above. In the case where the opening 6 of the ceramic insulating layer 5 has a rectangular shape, the corner of the opening 6 should have a gentle curve or a structure having a c-plane to concentrate the thermal stress on the corner of the opening 6. From the viewpoint of relaxing
[0038]
(Porous layer)
In the oxygen sensor of the present invention, a ceramic porous layer 11 is formed on the surface of the measurement electrode 4 exposed in the opening 6 of the ceramic insulating layer 5 as shown in the enlarged sectional view of the main part of FIG. can do. The ceramic porous layer 11 is formed for the following two purposes.
[0039]
First, it is provided for the purpose of preventing the measurement electrode 4 from being poisoned by exhaust gas and lowering the output voltage. The exposed surface of the measurement electrode 4 is made of zirconia, alumina, magnesia, spinel or the like. It is formed as a porous protective layer. An oxygen sensor provided with such a protective layer can generally be used as a theoretical air-fuel ratio sensor (λ sensor) element. In this case, the ceramic porous body 11 is preferably made of a porous body having an open porosity of 10 to 40%.
[0040]
Second, it functions as at least one gas diffusion rate-determining layer selected from the group consisting of zirconia, alumina, spinel, magnesia or 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 control layer, a porous body having an open porosity of 5 to 30% is desirable.
[0041]
In addition, it is desirable to provide the ceramic protective layer made of alumina or spinel as described above from the viewpoint of preventing exhaust gas poisoning on the surface of the ceramic porous layer 11 serving as the gas diffusion controlling layer. Such a heater built-in oxygen sensor can be applied as a wide-range air-fuel ratio sensor element (A / F sensor) described later.
(Production method)
Next, a method for manufacturing the oxygen sensor of the present invention will be described using 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. This cylindrical tube 2 is well-known such as extrusion molding, hydrostatic pressure molding (rubber press) or press formation by appropriately adding a molding organic binder to a ceramic solid electrolyte powder having oxygen ion conductivity such as zirconia. It is produced by the method.
[0042]
At this time, the solid electrolyte powder used is Y as a stabilizer against zirconia powder.₂O_ThreeAnd Yb₂O_Three, Sc₂O_Three, Sm₂O_Three, Nd₂O_Three, Dy₂O_ThreeA mixed powder obtained by adding a rare earth oxide powder such as 1 to 30 mol%, preferably 3 to 15 mol% in terms of oxide, or a coprecipitation raw material powder of zirconia and the stabilizer is used. ZrO₂ZrO in which 1 to 20 atomic% of Zr was substituted with Ce₂A powder or a coprecipitation raw material can also be used. Furthermore, for the purpose of improving the sinterability, the solid electrolyte powder is mixed with Al.₂O_ThreeAnd SiO₂Can be added at a ratio of 5% by weight or less, particularly 3% by weight or less.
[0043]
(2) And, on the inner surface and the outer surface of the cylindrical tube 2 made of the solid electrolyte, the patterns 3 and 4 to be the reference electrode and the measurement electrode are, for example, a slurry dip method or screen printing using a conductive paste containing platinum. It is formed by pad printing and roll transfer. At this time, it is efficient to print the reference electrode on the inner surface of the cylindrical tube 2 by filling and discharging the conductive paste and coating the entire inner surface. In this way, the sensor element A is manufactured.
[0044]
(3) Next, a heater element B as shown in FIG. 3B is formed. The heater element B is prepared by first using zirconia powder to appropriately add a molding organic binder to prepare a slurry, and using this slurry, a ceramic having a predetermined thickness by a doctor blade method, an extrusion molding method, a pressing method, or the like. A green sheet 2 for forming an insulating layer is prepared. The thickness of one green sheet is particularly preferably in the range of 50 to 500 μm, particularly 100 to 300 μm from the viewpoint of sheet handling.
[0045]
Thereafter, alumina, spinel or composite oxide powder of alumina and spinel with reduced Na content is printed on the surface of the formed green sheet 2 by a screen printing method, a pad printing method, a roll transfer method or the like to form a ceramic insulating layer 5a. To do. After this, after applying the platinum heater pattern 7 including the lead portion by similarly printing the conductive paste containing platinum powder with reduced Na amount in the present invention by the screen printing method, the pad printing method, the roll transfer method or the like. Then, the above-mentioned ceramic insulating layer forming powder is applied again thereon to obtain a sheet-like laminate in which the ceramic insulating layer 5b and the platinum heater pattern 7 are embedded. Thereafter, the opening 6 is formed by punching or the like.
[0046]
(4) Next, as shown in FIG. 3 (c), the heater element B is wound around the surface of the cylindrical sensor element A to produce a cylindrical laminate. At this time, in order to wrap the heater element B around the sensor element A, an adhesive such as an acrylic resin or an organic solvent is interposed between the heater element B and the sensor element A, or a roller or the like. Can be bonded mechanically while applying pressure. At this time, the seam of the wound heater element B may be overlapped with each other at a predetermined interval in consideration of shrinkage during firing.
[0047]
(5) The sensor element body A and the heater element body B can be integrated by firing the cylindrical laminate at a temperature at which the respective constituent elements can be fired simultaneously. For example, the heater element B and the sensor element A can be simultaneously fired by firing at 1300 to 1700 ° C. for about 1 to 10 hours in an inert atmosphere such as argon gas.
(Other manufacturing methods)
In addition, as another manufacturing method, after winding the heater element body B formed by the above (3) around the surface of the cylindrical tube 2 having no electrode to produce a cylindrical laminated body, 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 in the heater element B by screen printing, pad printing, roll transfer method or dipping method, and then co-fired as in (5) above. You can also
[0048]
As another method, after the heater element body 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, and this is used for the inner surface of the cylindrical tube 2 and the heater. An electrode paste can be printed in the opening 6 in the element body B and baked, or can be formed by sputtering or plating.
(Method for forming porous layer)
In the case of producing the above-described theoretical air-fuel ratio sensor or a wide-range air-fuel ratio oxygen sensor described later, after firing the cylindrical laminate, a powder of alumina, spinel, zirconia or the like is applied to the surface of the measurement electrode by the sol-gel method, A ceramic porous layer or a gas diffusion-controlling layer is formed by applying and baking by a slurry dip method, a printing method, or the like, or coating the ceramic by a sputtering method or a plasma spraying method. 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 when firing the cylindrical laminated body, and fire it simultaneously with the cylindrical laminated body.
(Other sensor structures)
The present invention can also be applied to an air-fuel ratio sensor element. As another application example of the present invention, FIG. 4A shows a schematic perspective view of an air-fuel ratio sensor element, and FIG. 4B shows an X1-X1 cross-sectional view of FIG.
[0049]
According to the air-fuel ratio sensor element 20 of FIG. 4, the sensor element is configured in the cylindrical tube 2 made of a ceramic solid electrolyte having oxygen ion conductivity and sealed at one end, in other words, the U-shaped longitudinal section. A first electrode pair is formed. Specifically, a reference electrode 3 that is in contact with a reference gas such as air is formed on the inner surface of the cylindrical tube 2, and a gas to be measured such as an exhaust gas is formed on the outer surface facing the reference electrode 3 of the cylindrical tube 2. A measuring electrode 4 is formed in contact therewith.
[0050]
A space 6 is formed on the outer surface of the cylindrical tube 2 so that a part or all of the measurement electrode 4 is exposed, and the platinum heater 7 of the present invention is embedded around the space 6. A ceramic insulating layer 5 is provided.
[0051]
A solid electrolyte layer 21 having oxygen ion conductivity is formed on the upper surface of the space portion 6 so as to close the space portion 6, and the inner surface of the solid electrolyte layer 21 on the space portion 6 side A second electrode pair consisting of an inner electrode 22 and an outer electrode 23 is formed on the outer surface of the solid electrolyte layer 21 opposite to it. The solid electrolyte layer 21 and the second electrode pair 22 and 23 function as a pump cell for controlling the oxygen concentration in the space 6 to a predetermined concentration. On the surface of the outer electrode 23, a ceramic porous layer 26 serving as a protective layer having a thickness of 10 to 200 μm and an open porosity of 10 to 40% made of at least one selected from the group consisting of zirconia, alumina, magnesia and spinel. Is formed. Furthermore, the space 6 can be filled with a porous ceramic or a filler of ceramic powder having large particles.
[0052]
The solid electrolyte layer 21 having the second electrode pair 22 and 23 is formed with a small diffusion hole 25 for taking in exhaust gas as a gas to be measured, and in the space 6 of the diffusion hole 25. A diffusion rate-determining body 27 is formed around.
[0053]
The platinum heater 7 disposed in the ceramic insulating layer 5 is connected to the terminal electrode 9 through the lead electrode 8 in the same manner as the oxygen sensor of FIG. As a result, the heater 7 is heated to heat the cylindrical tube 2 made of a solid electrolyte having the reference electrode 3 and the measurement electrode 4 and the detection portion made of the solid electrolyte layer 21 having the second electrode pair 22 and 23 described above. It is a mechanism.
[0054]
As described above, the oxygen sensor is specifically described as an example using the ceramic heater of the present invention, but the present invention can be applied to ceramic heaters of any shape in which a platinum heater is embedded in a ceramic insulating layer, Further, the oxygen sensor as an application example is not limited to the above structure, and at least a pair of porous platinum electrodes opposed to the inside and outside of the solid electrolyte, such as a bag-shaped or flat-plate type oxygen sensor. Needless to say, the present invention can be applied to all of the oxygen sensors including the detection unit having the platinum heater embedded in the ceramic insulating layer and the element having the platinum heater built in the other gas sensor.
[0055]
【Example】
Example 1
The embodiment of the present invention will be described by taking the oxygen sensor shown in the figure as an example.
Commercially pure 99.8% Na with about 10-200 ppm, SiO₂Of 0 to 7% by weight of alumina powder and spinel powder (MgAlO_Four) And 5 mol% Y₂O_ThreeA contained zirconia powder and a platinum powder containing about 8 to 200 ppm of Na were prepared. First, 5 mol% Y₂O_ThreeA polyvinyl alcohol solution is added to the contained zirconia powder to prepare a clay, and after extrusion, a cylindrical molded body having one end sealed so that the outer diameter is about 3 to 5 mm and the inner diameter is 1 mm is prepared. A rectangular measurement electrode pattern and a lead pattern made of platinum paste as shown in Table 1 were printed and applied to the surface, and the reference electrode was formed by applying platinum paste to the entire interior surface of the molded body. . The thicknesses of the measurement electrode and the reference electrode were adjusted to be about 5 μm after firing.
[0056]
5 mol% Y₂O_ThreeA polyvinyl alcohol solution was added to the contained zirconia powder to prepare a slurry, and a green sheet having a thickness of about 200 μm was prepared. On the green sheet, second openings having the same size and the same shape were respectively opened by punching so as to be located on the opposite side of the first openings having various rectangular shapes corresponding to the shape of the measurement electrode. After that, the alumina powder or spinel powder is applied to a portion other than the opening to a thickness of about 20 μm, and then a conductive paste containing platinum powder is screen printed so that the heating element pattern has a thickness of about 20 μm around the opening. Further, a heater element having a structure as shown in FIG. 3 was prepared by applying alumina powder or spinel powder to a thickness of about 20 μm thereon and embedding a heating element.
[0057]
Next, the heater element body was wound around the surface of the cylindrical sensor element body using an acrylic resin as an adhesive to produce a cylindrical laminate. Thereafter, this cylindrical laminate is formed in the atmosphere with an insulating layer made of SiO.₂For samples not containing SiO 2 at 1500 ° C. for 2 hours,₂The sample containing was fired at 1350 ° C. for 2 hours.
[0058]
Thereafter, a ceramic porous layer made of spinel having a porosity of about 30% was formed with a thickness of 100 μm on the surface of the measurement electrode in the opening to produce a theoretical air-fuel ratio sensor as shown in FIG.
[0059]
The Na content in the ceramic insulating layer and platinum heater of the produced oxygen sensor was determined by EPMA analysis using a Na calibration curve. In the evaluation test, the element was heated to 900 ° C. in the atmosphere, and the time until the heater was disconnected was measured. The results are shown in Table 1.
In Table 1, the same evaluation was performed for a commercially available heater-integrated flat plate oxygen sensor for comparison.
[0060]
[Table 1]

[0061]
From Table 1, the ceramic insulating layer is alumina and SiO.₂In the case of the alumina containing, the sample No. in which the content in the ceramic insulating layer or the platinum heater exceeds 50 ppm. In 7, 8, 9, 14, and 15, the time to reach the heater break was shorter than 3000 hours. Similarly, in the case where the ceramic insulating layer is spinel, the sample No. in which the Na in the ceramic insulating layer or the platinum heater exceeds 50 ppm is similarly applied. In 18, 19, and 20, the time until the heater was disconnected was short. In all samples, the heater on the negative electrode side was damaged, and according to EPMA analysis, Na was knitted from the damage.
[0062]
  On the other hand,The content of Si in the ceramic insulating layer is 5% by weight or less in terms of oxide, andCeramic insulation layerandAll samples with a Na content of 50 ppm or less in the platinum heater take time to break the heater.4050It was excellent in durability over time. In particular, the Na content in the ceramic insulating layer or platinum heater is 30 ppm or less, and the SiO in the ceramic insulating layer.₂Samples with a content of 5% by weight or less showed extremely excellent performance with a heater durability time of 5000 hours or more.
[0063]
【The invention's effect】
As described above in detail, according to the present invention, in the ceramic heater in which the platinum heater is incorporated in the ceramic insulating layer, the amount of Na in the ceramic insulating layer and the platinum heater is controlled within a predetermined range. As a result of the dramatic improvement in durability, a long-life ceramic heater with excellent thermal shock resistance, such as rapid temperature rise, and an oxygen concentration detection unit are integrally formed. It is possible to provide an oxygen sensor that is excellent in thermal shock properties such as temperature and has a long heater life.
[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 X₁-X₁It is sectional drawing (b).
FIG. 2 is a cross-sectional view of a main part in another structure of the oxygen sensor of the present invention.
FIGS. 3A to 3C are process diagrams for manufacturing the oxygen sensor of FIG. 1;
FIG. 4 is a schematic perspective view (a) and an XX sectional view (b) for explaining another example of the oxygen sensor of the present invention.
FIG. 5 is a schematic cross-sectional view for explaining the structure of a conventional oxygen sensor.
[Explanation of symbols]
1 Oxygen sensor
2 Cylindrical tube
3 Reference electrode
4 Measuring electrodes
5 Ceramic insulation layer
6 opening
7 Platinum heater
8 Lead electrode
9 Terminal electrode

Claims (7)

A ceramic heater obtained by embedding a platinum heater in the ceramic insulating layer, wherein along with the content of Si in the ceramic insulating layer is 5 wt% or less in terms of oxide, the ceramic insulating layer and in the platinum heater A ceramic heater, wherein the content of Na in each is 50 ppm or less. The ceramic heater according to claim 1, wherein the ceramic insulating layer is made of an oxide containing alumina and / or magnesia. And at least a detection section having a pair of platinum electrodes at opposite positions of the inner and outer surfaces, formed by embedding a platinum heater in the ceramic insulating layer which is integrated with the solid electrolyte base heating portion of the zirconia solid electrolyte substrate an oxygen sensor for the content of Si of the ceramic insulating layer together with more than 5% by weight in terms of oxide, the content of Na of the ceramic insulating layer and in said platinum heater is 50ppm or less, respectively An oxygen sensor characterized by that. The oxygen sensor according to claim 3, wherein the pair of platinum electrodes is porous . 5. The oxygen sensor according to claim 3 , wherein the ceramic insulating layer is made of an oxide containing alumina and / or magnesia. Said zirconia solid electrolyte substrate, an oxygen sensor according to any one of claims 3 to 5, characterized in that it consists of a cylindrical tube one end of which is sealed. As the pair of platinum electrodes, a reference electrode formed on the inner surface of the cylindrical tube, becomes the measuring electrode is formed on an outer surface of said cylindrical tube, said ceramic insulating layer which is embedded the platinum heater on the surface of the cylindrical tube coated have Rutotomoni, an opening provided in the ceramic insulating layer, the oxygen sensor according to claim 6, characterized in that allowed expose part or all of the measuring electrode through the opening.

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