JP2004325196A - Oxygen sensor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lengthy flat-shaped oxygen sensor element having a flat shape, excellent durability, heat resistance and stability, and free from breakage of a sensor element against a long-time operation. <P>SOLUTION: This oxygen sensor element is equipped with a sensor part 1 having a pair of electrodes 4, 5 on the inner wall surface of an air introduction hole 3a of a lengthy flat-shaped zirconia solid electrolyte substrate 3 having the air introduction hole and on the outer surface facing thereto, and a heater part 2 wherein a heating element 8 is buried in a ceramic insulating layer 7. In the sensor element, a part or the whole periphery of the heating element 8 is covered with a porous ceramic insulating layer 4a and a dense ceramic insulating layer 7b, and the heater part 2 is buried and integrated inside the zirconia solid electrolyte substrate 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００８５】
表１より、発熱体を取り巻く絶縁層が、多孔質セラミック絶縁層のみ、または緻密質セラミック絶縁層のみからなる場合、破損率が８０％以上と高いことがわかる。
【００８６】
これに対して、発熱体の周囲に多孔質セラミック絶縁層および緻密質セラミック絶縁層を設けた本発明の素子においては破損率が６０％以下まで低下することがわかる。また、表１から分かるように、本発明品は多孔質セラミック絶縁層を有するが、その周囲を緻密質セラミック絶縁層およびジルコニア固体電解質基体内に埋設されているため素子強度が３５ＭＰａ以上と優れる。このことは製造工程において、不注意による素子の破損を防止し。歩留まりを向上させるなど大きな製造上の利点でもある。
【００８７】
特に、多孔質セラミック絶縁層の気孔率が３〜２０％、厚みが５〜２００μｍ、さらには緻密質セラミック絶縁層の気孔率が１％以下、厚みが５〜１００μｍの素子は、破損率が５０％以下、素子強度が４０ＭＰａ以上の優れたものであった。
【００８８】
以上の結果から、多孔質セラミック絶縁層および緻密質セラミック絶縁層を設けた本発明品は、急激な熱サイクルを受けても優れた耐熱性、および耐久性を有するセンサ素子であることが容易に理解できる。
【００８９】
【発明の効果】
以上詳述した通り、本発明によれば、ヒータ部における発熱体を多孔質セラミック絶縁層で、さらに緻密質セラミック絶縁層によって覆うことによって、長時間運転に対してもクラックの発生や破壊することのない優れた安定性を有する平板状の酸素センサ素子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素子の一例を説明するための概略断面図である。
【図２】本発明における酸素センサ素子の概略平面図である。
【図３】図３（ｃ）の酸素センサ素子の応用例を説明するための概略斜視図である。
【図４】本発明の酸素センサ素子のさらに他の例を説明するために概略断面図である。
【図５】図１の酸素センサ素子の製造方法を説明するための分解斜視図である。
【図６】従来のヒータ一体型酸素センサ素子の構造を説明するための概略断面図である。
【図７】従来の他のヒータ一体型酸素センサ素子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 センサ部
２ ヒータ部
３ ジルコニア固体電解質基体
４ 基準電極
５ 測定電極
６ セラミック多孔質層
７ セラミック絶縁層
７ａ 多孔質セラミック絶縁層
７ｂ 緻密質セラミック絶縁層
８ 発熱体
９ ジルコニア固体電解質層[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an oxygen sensor element for controlling the ratio of air to fuel in an internal combustion engine such as an automobile.
[0002]
[Prior art]
Currently, in internal combustion engines such as automobiles, 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 has been adopted.
[0003]
As the detection element, a solid electrolyte type oxygen 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. Sensors are used. As a representative example of this oxygen sensor, as shown in a schematic sectional view of FIG. ₂ A reference electrode 42 made of platinum and in contact with a reference gas such as air is provided as a sensor portion on the inner surface of the cylindrical tube 41 made of a solid electrolyte and having a sealed end, and the outer surface of the cylindrical tube 41 is provided with an exhaust gas or the like. A measurement electrode 43 that is in contact with the gas to be measured is formed.
[0004]
In such an oxygen sensor, a so-called stoichiometric air-fuel ratio sensor (λ sensor), which is generally used for controlling the ratio of air to fuel near 1, is a ceramic porous layer as a protective layer on the surface of the measurement electrode 43. A ceramic insulating layer 44 is provided, and a difference in oxygen concentration generated on both sides of the cylindrical tube 41 at a predetermined temperature is detected to control the air-fuel ratio of the engine intake system. At this time, it is necessary to heat the stoichiometric air-fuel ratio sensor to an operating temperature of about 700 ° C. Therefore, a rod-shaped heater 45 is inserted inside the cylindrical tube 41 to heat the sensor unit to the operating temperature. I have.
[0005]
However, in recent years, the exhaust gas regulations have been strengthened, and it has become necessary to detect CO, HC, and NOx immediately after starting the engine. In response to such a demand, as described above, in the indirect heating type cylindrical oxygen sensor in which the rod-shaped heater 45 is inserted into the cylindrical tube 41, the time required for the sensor section to reach the activation temperature (hereinafter, referred to as the time required). , Activation time) is slow, so that it is not possible to sufficiently cope with exhaust gas regulations.
[0006]
In recent years, as a method of avoiding this problem, as shown in a schematic sectional view of FIG. 7, a reference electrode 48 and a measurement electrode 47 are provided on the outer and inner surfaces of a flat substrate 46 made of a zirconia solid electrolyte, respectively, A heater-integrated oxygen sensor element in which a heater 50 made of platinum or tungsten is embedded in a ceramic insulating layer 49 made of ceramic has been proposed (see Patent Documents 1, 2, and 3).
[0007]
[Patent Document 1]
JP-A-2002-540399
[0008]
[Patent Document 2]
JP 2002-236104 A
[0009]
[Patent Document 3]
JP-A-2002-71628
[0010]
[Problems to be solved by the invention]
However, the flat heater integrated oxygen sensor as shown in FIG. 7 is a direct heating system, unlike the indirect heating system of FIG. 6, so that the heater 50 can rapidly raise the temperature of the sensor unit. Since the shape is a flat plate shape, the coefficient of thermal expansion between the zirconia solid electrolyte substrate 46 and the alumina ceramic insulating layer 49 is different, and the coefficient of thermal expansion between the heating element and the alumina ceramic insulating layer 49 is different. Due to the repetition of the temperature rise, cracks occur in the interface between the zirconia solid electrolyte substrate 46 and the alumina ceramic insulating layer 49 and in the alumina ceramic insulating layer 49, and the progress of the cracks may eventually lead to breakage. There was a problem.
[0011]
In order to solve such a problem, Patent Document 3 discloses that the ceramic insulating layer 49 is formed of a porous ceramic insulating layer so as to reduce stress caused by a difference in thermal expansion between the alumina ceramic insulating layer 49 and the heating element 50. Has been proposed.
[0012]
However, in Patent Document 3, although the thermal characteristics are improved as compared with the conventional oxygen sensor, the absolute strength of the ceramic insulating layer 49 is newly reduced by making the ceramic insulating layer 49 porous, A phenomenon that the bonding strength between the ceramic insulating layer 49 and the zirconia solid electrolyte substrate 46 was reduced was observed.
[0013]
Therefore, when a thermal cycle such as rapid temperature rise is applied, cracks occur in the ceramic insulating layer 49, and there is a problem that the element is broken.
[0014]
In addition, such a problem tends to decrease the absolute strength of the zirconia solid electrolyte substrate 46 and the alumina ceramic insulating layer 49 due to the downsizing of the element, and the occurrence of cracks and breakage is becoming very remarkable. This is a major factor that hinders miniaturization of the element.
[0015]
Further, in Patent Document 3, since the strength of the ceramic insulating layer 49 is low, there has been a problem that the element is easily damaged in the operation of incorporating the element into a metal housing.
[0016]
Accordingly, the present invention provides a flat oxygen sensor element having a flat shape, having excellent durability and heat resistance, and having excellent stability without cracking or breakage even during long-time operation. It is intended to do so.
[0017]
[Means for Solving the Problems]
As a result of studying the above problem, the present inventor has found that a sensor portion formed by forming a pair of electrodes via a zirconia solid electrolyte layer on a part of a long flat zirconia solid electrolyte substrate, In a oxygen sensor element having a heater section in which a heating element is embedded, a platinum heating element in the heater section is covered with a porous ceramic insulating layer, and a part or the entire periphery of the porous ceramic insulating layer is dense. Since the heater portion is formed so as to be covered with the ceramic insulating layer, and the heater portion is embedded and integrated inside the zirconia solid electrolyte substrate, a difference in thermal expansion coefficient between the heating element and the ceramic insulating layer is caused. In addition to relieving thermal stress, the absolute strength of the ceramic insulating layer and the bonding strength between the ceramic insulating layer and the solid electrolyte are increased, resulting in a flat sensor. Thereby preventing breakage of the element in the rapid Atsushi Nobori is a problem of the child, the heat resistance was found to be able to provide an excellent device durability.
[0018]
Further, in the present invention, since the ceramic insulating layer has a high strength, the frequency of element breakage in the sensor manufacturing process is extremely low, and as a result, the manufacturing yield is improved.
[0019]
It is effective that the porosity of the porous ceramic insulating layer covering the heating element is 5 to 20%, and the thickness of the porous ceramic insulating layer covering the heating element is 5 to 200 μm.
[0020]
The migration of Na, K, etc. is prevented by the content of Si in the porous ceramic insulating layer covering the heating element being 0.7% by weight or less, particularly 0.4% by weight or less in terms of oxide. can do.
[0021]
Further, the oxygen sensor element of the present invention is suitable when the sensor section and the heater section are formed by simultaneous firing.
[0022]
Further, in the oxygen sensor element of the present invention, the area of the electrode in contact with the gas to be measured is 8 to 18 mm among the pair of electrodes in the sensor section. ² In addition, the width in the direction orthogonal to the longitudinal direction of the element is preferably at least 5 mm or more from the tip of the element, which is suitable for downsizing to 2.0 to 3.5 mm. In addition, the durability can be improved.
[0023]
In order to reduce the size, a pair of electrodes of the sensor section is formed near the front end of the element, and an electrode pad for connecting a terminal is provided near the rear end of the element. It is desirable that the width in the direction orthogonal to the longitudinal direction is larger than the width of the element tip.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, examples of the basic structure of the oxygen sensor element of the present invention will be described below with reference to the drawings.
[0025]
FIG. 1 is a schematic sectional view for explaining one example of the oxygen sensor element of the present invention. These are generally called theoretical air twist ratio sensor elements, and each of the examples in FIG. 1 includes a sensor unit 1 and a heater unit 2.
[0026]
In the oxygen sensor element of FIG. 1, a zirconia solid electrolyte substrate 3 having oxygen ion conductivity made of a zirconia solid electrolyte and having an air introduction hole 3a having a sealed end is formed therein. A reference electrode 4 in contact with air and a measurement electrode 5 in contact with exhaust gas are formed on the opposing surface via the zirconia solid electrolyte layer 3b constituting the inner wall, forming a sensor unit 1 having a function of detecting oxygen concentration. are doing.
[0027]
Further, from the viewpoint of preventing the electrode from being poisoned by exhaust gas, a ceramic porous layer 6 may be formed on the surface of the measurement electrode 5 as an electrode protection layer.
[0028]
On the other hand, the heater section 2 has a heating element 8 made of platinum embedded in a ceramic insulating layer 7. In the oxygen sensor element of FIG. 1, the heater section 2 is embedded in a zirconia solid electrolyte base 3. Have been.
[0029]
According to the present invention, in the above-described structure of the heater section, the ceramic insulating layer 7 is formed of at least a two-layer structure, and the heating element 8 is covered with the porous ceramic insulating layer 7a. A major feature is that a part or the entire periphery of the ceramic insulating layer 7a is covered with the dense ceramic insulating layer 7b. The heater section 2 is embedded in the zirconia solid electrolyte substrate 3 by being covered with the zirconia solid electrolyte layer 9.
[0030]
By thus forming the ceramic insulating layer 7 in the two-layer structure as described above, the absolute strength of the ceramic insulating layer 7 is improved, and at the same time, the bonding strength between the zirconia solid electrolyte layer 9 and the ceramic insulating layer 7 is improved. As a result, destruction of the element can be prevented. Further, by embedding the ceramic insulating layer 7 in the zirconia solid electrolyte substrate 3, even if a crack enters the ceramic insulating layer 7, the surroundings are covered with the zirconia solid electrolyte layer 9 forming a part of the substrate 3. By doing so, destruction of the element is prevented.
[0031]
When the porosity of the porous ceramic insulating layer 7a is 5 to 20%, particularly 10 to 15%, the effect of alleviating stress can be sufficiently exerted, and at the same time, generation and breakage of cracks can be prevented. In addition, a decrease in the strength of the porous ceramic insulating layer 7a itself can be prevented, and a decrease in the strength of the oxygen sensor element itself can be suppressed. The porosity is defined as the area ratio of pores occupying a vertical section in the porous ceramic insulating layer 7a.
[0032]
The thickness of the porous ceramic insulating layer 7a, that is, the thickness t1 from the heating element 8 to the dense ceramic insulating layer 7b is in the range of 5 to 200 μm, particularly 10 to 50 μm, so that the zirconia solid electrolyte substrate 3 It is possible to reduce the thermal stress caused by the difference in the coefficient of thermal expansion from the ceramic insulating layer 7. The average size of the pores is effective when the diameter is 1 to 10 μm, particularly 3 to 5 μm.
[0033]
The porous ceramic insulating layer 7a is applied with a slurry in which a pore-forming agent such as resin beads is mixed at a ratio of 3 to 30% by volume with respect to the ceramic insulating layer composition, or the slurry is applied by a doctor blade method or the like. Into a sheet, arranged so as to sandwich the heating element 8, and then fired. The porosity and the pore diameter can be easily controlled by the particle size and the amount of the pore forming agent.
[0034]
The dense ceramic insulating layer 7b has a relative density of 80% or more, particularly 90% or more, more preferably 95% or more, a porosity of 3% or less, particularly 1% or less, and an average pore diameter of 3 μm or less. By using a high quality ceramic sintered body, the strength of the ceramic insulating layer 7 and the bonding strength between the ceramic insulating layer 7 and the zirconia solid electrolyte substrate 3 are increased. Further, the strength of the heater section 2 via the ceramic insulating layer 7 can be increased, and the strength of the entire element can be increased.
[0035]
The thickness t2 of the dense ceramic insulating layer 7b is desirably 5 to 100 μm, and particularly desirably 10 to 50 μm. Thereby, the absolute strength of the ceramic insulating layer and the bonding strength between the ceramic insulating layer and the zirconia solid electrolyte can be increased.
[0036]
In the present invention, the ceramic insulating layers 7a and 7b forming the heater section 2 are a) a sintered body mainly composed of alumina, b) a sintered body mainly composed of a composite oxide containing at least Al and Mg. , C) It is desirable to be constituted by at least one kind of sintered body selected from the group of sintered bodies mainly composed of a composite oxide of Al and at least one kind selected from the group consisting of Y and rare earth elements.
[0037]
a) The sintered body mainly composed of alumina contains alumina in an amount of 90% by mass or more. Further, in order to improve sinterability, alkaline earth elements such as Mg and Ca are used as components other than the main components. Metal oxide, SiO ₂ At least one selected from the group of 1 to 10% by mass in total.
[0038]
Also, b) a sintered body containing a composite oxide of Al and Mg as a main component is 20 to 90 mol% of Al in terms of oxide and Mg in terms of two components based on oxide of Al and Mg. It is desirable to contain it at a ratio of 10 to 80 mol% in terms of oxide.
[0039]
In this case, although depending on the composition to be used and the firing temperature, the crystal phase in the obtained ceramic insulating layer 4 is Al ₂ O ₃ Phase, MgO · Al ₂ O ₃ It is composed of two or three crystals of the (spinel) phase and the MgO phase. Of the above composition ranges, the Al content is particularly preferably 50 to 80 mol% in terms of oxide, and the Mg content is particularly preferably in the range of 20 to 50 mol% in terms of oxide.
[0040]
Further, c) as a composite oxide of Al and at least one selected from the group consisting of Y and a rare earth element, Al is an oxide based on two components based on the amount of Y and the rare earth element and the conversion of Al to the oxide. It is desirable that the amount of Y and the rare earth element be in the range of 10 to 80 mol% in conversion. Within the above range, it is desirable that the Al content be 50 to 80 mol%, and that at least one selected from the group consisting of Y and rare earth elements be 20 to 50 mol%. As the rare earth element, specifically, La, Yb, Nd, Dy, Sc, Sm, and Sc are suitably used.
[0041]
The crystal phase at this time is Al ₂ O ₃ And a composite oxide of Al and Y or a rare earth element, or two or three of Y or a rare earth element oxide. For example, as the composite oxide, for example, Al ₂ O ₃ And Y ₂ O ₃ When using 3Y ₂ O ₃ ・ 5Al ₂ O ₃ , 2Y ₂ O ₃ ・ Al ₂ O ₃ , And Y ₂ O ₃ And the like.
[0042]
Further, in the porous ceramic insulating layer 7a and the dense ceramic insulating layer 7b, alkali metals such as Na and K migrate to deteriorate electrical insulation of the heater portion 2 so that the total amount thereof is 50 ppm in terms of oxide. It is desirable to control below
The zirconia solid electrolyte in the zirconia solid electrolyte substrate 3, the zirconia solid electrolyte layers 9, 9 ', and the zirconia solid electrolyte layer 10 containing alumina in the oxygen sensor element of the present invention is ZrO. ₂ Consisting of a ceramic containing ₂ O ₃ And Yb ₂ O ₃ , Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O ₃ , Dy ₂ O ₃ Partially stabilized ZrO containing 1 to 30 mol%, preferably 3 to 15 mol% of at least one selected from the group of ₂ Or stabilized ZrO ₂ Is used. Further, in order to improve the sinterability, the above ZrO ₂ Against SiO ₂ Can be added and contained, but if contained in a large amount, the creep characteristics at high temperatures are deteriorated. ₂ Is preferably 5% by mass or less, particularly preferably 2% by mass or less.
[0043]
Furthermore, in the present invention, it is desirable that the minimum thickness S of the zirconia solid electrolyte layer 9 covering the heater section 2 is 20 μm or more, particularly 50 μm or more, and further preferably 100 μm or more. In some cases, pores are present inside the zirconia solid electrolyte layer 9, and are formed for the purpose of preventing the entry of water vapor through the pores and at the same time strengthening the bonding strength between the solid electrolytes.
[0044]
The zirconia solid electrolyte layer 9 and the zirconia solid electrolyte layer 3b, which form part of the zirconia solid electrolyte substrate 3, are preferably made of the same material, and have a relative density of 90% or more, particularly 95% or more. Desirably, it consists of
[0045]
Each of the reference electrode 4 and the measurement electrode 5 formed on the surface of the base 3 is made of platinum or an alloy of platinum and one selected from the group consisting of rhodium, palladium, ruthenium and gold. The above-mentioned ceramic solid electrolyte is used for the purpose of preventing grain growth of metal in the electrode at the time of operation of the sensor and to increase the contact point of the so-called three-phase interface between platinum particles, solid electrolyte, and gas related to responsiveness. The components may be mixed in the electrode at a ratio of 1 to 50% by volume, particularly 10 to 30% by volume. The shape of the electrode may be square or elliptical. Further, the thickness of the electrode is preferably 3 to 20 μm, particularly preferably 5 to 10 μm.
[0046]
The heating element 8 and the leads (not shown) embedded in the ceramic insulating layer 7 in the heater section 2 are made of platinum alone or an alloy of platinum and one selected from the group consisting of rhodium, palladium and ruthenium. Alternatively, W alone, or one alloy selected from the group consisting of W, Mo, and Re can be used.
[0047]
When platinum is used as the heating element 8, the same ceramic powder forming the ceramic insulating layer is used in an amount of 10 to 40% by volume, particularly 20%, in addition to alumina, from the viewpoint of preventing the growth of platinum grains during firing. It is preferable to add 〜30% by volume. In this case, the resistance ratio between the heater 8 and the lead is preferably controlled in the range of 9: 1 to 7: 3 at room temperature in any case.
[0048]
The ceramic porous layer 6 formed on the surface of the measurement electrode 5 has a thickness of 10 to 800 μm, particularly 100 to 500 μm, and has a porosity of 10 to 50% from the group of zirconia, alumina, γ-alumina and spinel. Desirably, it is formed of at least one selected from the following.
[0049]
Further, the oxygen sensor element of the present invention is most suitably used for a small-sized oxygen sensor element. Specifically, in order to achieve excellent gas responsiveness as well as miniaturization of the element, the electrode area of the measurement electrode 5 is 8 to 18 mm. ² It is desirable that the width in the direction orthogonal to the longitudinal direction at a portion of 5 mm or more, particularly 10 mm or more from the tip of the element is 2.0 to 3.5 mm. According to the present invention, by controlling the area of the measurement electrode 5 and the width of the tip portion within the above ranges, the rapid temperature rise of the measurement electrode 5 by the heater can be enhanced, and the gas responsiveness by the sensor can be improved. .
[0050]
In the oxygen sensor element of the present invention, as shown in the plan view of FIG. 2, a pair of electrode pairs 5 of the sensor unit 1 is formed near the tip of the element, and a terminal is connected near the rear end of the element. Although the electrode pad 11 is provided, the width L1 in the direction perpendicular to the longitudinal direction of the rear end of the element is larger than the width L2 of the front end of the element when setting the electrode area and the width in the above ranges. Alternatively, it is desirable that the width of the sensor element increases continuously or discontinuously from the front end to the rear end of the element. In particular, it is appropriate that the maximum width L1 in the direction orthogonal to the longitudinal direction of the rear end portion where the electrode pad 11 is formed is 3.7 to 5 mm, particularly 4.0 to 4.5 mm.
[0051]
In this case, as a specific structure of the oxygen sensor element, specifically, as shown in FIG. 2A, in other words, the width continuously increases from the front end to the rear end of the element. FIG. 2C shows a case where the width of the element becomes wider, as shown in FIG. 2B, and a case where the width of the element becomes wider at the step portion v between the front end and the rear end. As shown, a tapered portion p is provided between the front end portion and the rear end portion, and the width thereof is partially continuous and widened.
[0052]
As described above, by increasing the width of the portion where the electrode pad 11 is provided and making the width L1 of the portion where the electrode pad 11 is formed larger than the width L2 of the tip end of the element, the sensor portion can be downsized. A connector or a metal pin can be easily and firmly attached to the electrode pad 11.
[0053]
Further, according to the present invention, using the oxygen sensor element shown in FIG. 2C, for example, as shown in FIG. 3, a mounting jig 12 for mounting the oxygen sensor element on a holder is provided as a part of the element. Can be attached.
[0054]
Further, from the viewpoint of element strength, the oxygen sensor element of the present invention has a thickness of the entire element of 0.8 to 3 mm, particularly 1 to 2 mm, and an element length of 45 to 55 mm, particularly 45 to 50 mm. This is preferable from the relationship between the rapid temperature rise and the degree of attachment of the element to the engine.
[0055]
Further, the oxygen sensor element of the present invention is also applied to a wide-range sensor element as shown in FIG. FIG. 4 is a schematic cross-sectional view for explaining a typical structure. According to the oxygen sensor element of FIG. 4, an electrode pair of the reference electrode 4 and the measurement electrode 5 is formed on the opposite surface of the base 3, and the space 13 is formed by the substrate 13 above the measurement electrode 5. The substrate 13 is provided with a diffusion hole 15 having a size of 0.1 to 0.5 mm for taking in exhaust gas.
[0056]
In such an oxygen sensor, a pumping cell is formed by a pair of electrodes 4 and 5 sandwiching the zirconia solid electrolyte layer 3b, and the exhaust gas is controlled by controlling the current flowing between the electrode pairs in accordance with the oxygen concentration in the exhaust gas. Controls the air-fuel ratio in the gas.
[0057]
The space 14 may be filled with a porous ceramic to increase the strength of the element. Further, the diffusion hole 15 can be formed not only on the upper surface of the element but also on the side surface or the tip. Further, the diffusion holes 15 function as holes for taking in a certain amount of exhaust gas into the space. Therefore, the diffusion hole 15 may be formed by a large number of holes, or may be formed by a ceramic porous ceramic insulating layer.
[0058]
The reference electrode 4 formed on the lower surface of the base 3 is formed on the inner wall of the air introduction hole 3a. Immediately below the air introduction hole 3a, an alumina ceramic insulating layer 7 in which a heating element 8 made of W or Pt is embedded is covered with a zirconia solid electrolyte layer 9. By heating the heating element 8, the sensor unit 1 is heated. In the oxygen sensor of the present invention, as another example, pumping electrodes can be formed on both surfaces of the substrate 13.
[0059]
Also in such an oxygen sensor element, by forming the ceramic insulating layer 7 by the porous ceramic insulating layer 7a and the dense ceramic insulating layer 7b, the same effect as described above can be exerted.
[0060]
Next, the manufacturing method of the oxygen sensor element of the present invention will be described with reference to the manufacturing method of the oxygen sensor element of FIG. 1 using Pt as the heating element and alumina as the ceramic insulating layer. Description will be made based on an exploded perspective view.
[0061]
First, a green sheet 21 of a solid electrolyte is prepared. The green sheet 21 is formed, for example, by adding an organic binder for molding to a ceramic solid electrolyte powder having oxygen ion conductivity of zirconia as appropriate, for example, by a doctor blade method, extrusion molding, or hydrostatic molding (rubber press). Alternatively, it is manufactured by a known method such as press forming.
[0062]
Next, on both surfaces of the green sheet 21, a pattern 22, a lead pattern 23, a through hole (not shown), and the like which become the measurement electrode 5 and the reference electrode 4, respectively, are formed by using a conductive paste containing platinum, for example. After printing by slurry dipping, screen printing, pad printing, or roll transfer, the green sheet 25 and the green sheet 26 in which the air introduction holes 24 are formed may be interposed with an adhesive such as an acrylic resin or an organic solvent, or a roller or the like. A laminate A for the sensor part is produced by mechanically bonding while applying pressure by using
Further, as the conductive paste containing platinum used at this time, a paste containing an organic resin component such as ethyl cellulose in platinum particles containing 1 to 50% by volume, particularly 10 to 30% by volume of zirconia is used. By using this, the sensitivity of the electrode can be increased. At this time, a porous slurry for forming the porous ceramic insulating layer 6 may be printed and formed on the surface of the pattern serving as the measurement electrode 5.
[0063]
Next, the surface of the zirconia green sheet 27 is coated with Al. ₂ O ₃ An insulating paste in which an organic resin and a solvent are added to the powder and mixed is printed by a slurry dipping method, screen printing, pad printing, or roll transfer to form a dense ceramic insulating layer 28a.
[0064]
Next, a slurry dip method or screen printing is performed by using an insulating paste obtained by adding 3 to 30% by volume of a pore forming agent such as resin beads or carbon powder having an average particle diameter of 1 to 10 μm to the ceramic powder composition. The porous ceramic insulating layer 29a is formed by printing using pad printing or roll transfer.
[0065]
Next, as shown in FIG. 2, a heater pattern 30 and a lead pattern 31 are printed on the surface of the porous ceramic insulating layer 29a. Then, a porous ceramic insulating layer 29b is formed on the heater pattern 30 and the lead pattern 31 in the same manner as described above. Thereafter, the dense ceramic insulating layer 28b is formed by printing again in the same manner as described above.
[0066]
In order to embed the porous ceramic insulating layers 29a and 29b and the dense ceramic insulating layers 28a and 28b in the zirconia solid electrolyte substrate, they were used to form a green sheet 21 around these insulating layers. The zirconia solid electrolyte paste is printed and applied to form the zirconia solid electrolyte layer 32.
[0067]
Then, the zirconia green sheets 33 are laminated again to produce a laminate B of the heater section 2.
[0068]
Thereafter, the laminate A of the sensor unit 1 and the laminate B of the heater unit 2 are mechanically adhered to each other by interposing an adhesive such as an acrylic resin or an organic solvent or applying pressure with a roller or the like. After bonding and integration, they are fired. The firing is performed in the air or in an inert gas atmosphere at a temperature in the range of 1300 ° C. to 1700 ° C. for 1 to 10 hours.
[0069]
Thereafter, if necessary, at least one type of ceramic selected from the group consisting of alumina, zirconia, and spinel is formed on the surface of the fired measurement electrode by a plasma spraying method or the like, so that the heater unit is integrated. A sensor element can be formed.
[0070]
In the example of FIG. 5, the heating element pattern 30 is a meander (waveform) pattern having a turn in a direction orthogonal to the longitudinal direction of the element, but the heating element pattern is not limited to this. Instead, a meandering pattern having a turn in the longitudinal direction of the element may be used.
[0071]
In the example of FIG. 1, the dense ceramic insulating layer 7b is formed only above and below the porous ceramic insulating layer 7a. However, the dense ceramic insulating layer 7b is, of course, a side surface of the porous ceramic insulating layer 7a. The same effect can be obtained by forming the entire circumference including the above.
[0072]
【Example】
The stoichiometric air-fuel ratio sensor element shown in FIG. 1 was manufactured as follows according to FIG.
[0073]
First, 5 mol% Y containing 0.1 wt% each of alumina and silica ₂ O ₃ A slurry was prepared by adding a polyvinyl alcohol solution to the contained zirconia powder, and a zirconia green sheet 21 having a thickness of 0.4 mm after sintering was prepared by extrusion molding.
[0074]
Thereafter, on both surfaces of the zirconia green sheet 21, a conductive paste containing platinum powder containing 30% by volume of zirconia made of yttria having an average particle diameter of 0.1 μm and 8 mol% in a crystal was screen-printed and measured. After printing and forming the pattern 22 of the electrode and the reference electrode and the lead pattern 23, the zirconia green sheet 25 and the zirconia green sheet 26 in which the air introduction holes 24 are formed are laminated with an acrylic resin adhesive to form the sensor unit laminate A. Obtained.
[0075]
Next, on the surface of the zirconia green sheet 27 produced in the same manner as described above, a dense ceramic insulating layer is formed by printing and applying an insulating paste made of alumina powder having an average particle diameter of 0.3 μm and having a thickness of 3 after firing. The ceramic insulating layer 28a was formed to have a thickness of about 140 μm. Thereafter, the porous ceramic insulating layer was formed such that a paste in which a pore forming agent (flow beads; average particle diameter: about 4 μm) was added to the above-mentioned insulating paste in an amount of 5 to 40% by volume was fired to have a thickness of 3 to 320 μm. Then, a heater pattern 30 and a lead pattern 31 were screen-printed on the surface thereof using a paste of platinum powder containing 10% by volume of alumina.
[0076]
Thereafter, a porous ceramic insulating layer 29b and a dense ceramic insulating layer 29a were formed on these patterns by using the respective insulating pastes as described above.
[0077]
Then, around this laminate, 5 mol% Y ₂ O ₃ The zirconia solid electrolyte layer 32 was formed by applying a paste made of the contained zirconia powder by screen printing so as to have the same height as the laminate. Then, a zirconia green sheet 33 was further laminated to produce a heater section laminate in which the heater pattern 30 was embedded between the porous ceramic insulating layers 29a and 29b.
[0078]
Thereafter, the sensor section laminate and the heater section laminate were laminated and baked at 1500 ° C. for 1 hour to produce a sensor element with an integrated heater.
[0079]
In addition, based on FIG. 2 (b), the manufactured oxygen sensor elements all had a measurement electrode area of 12 mm. ² The length of the element was 50 mm, the width of the portion from the tip of the element to 20 mm was 3 mm (L2 = 3 mm), the maximum width L2 of the electrode pad formation portion was 4.5 mm, and the length was 10 mm. In addition, the thickness of the element was 1.5 mm.
[0080]
The thickness S of the zirconia solid electrolyte layer covering the heater was 200 μm.
[0081]
According to the above-described method, 20 sensor elements each having a different porosity of the porous body and a different thickness are produced, and the temperature is raised from room temperature to 1000 ° C. in about 20 seconds, and then rapidly cooled to room temperature by a fan. The cycle was defined as one cycle, and the damage rate after about 200,000 times was obtained. At this time, the mirror-polished surfaces of the porous ceramic insulating layer and the dense ceramic insulating layer in the heater portion were measured at 10 points of a 2000 × scanning electron microscope photograph, and the area ratio of pores in the cross-sectional area was measured. The average value was defined as the porosity. In addition, the thickness of the porous solid electrolyte layer was similarly measured by a scanning electron microscope photograph. Table 1 shows the results.
[0082]
With the electrode surface as the lower surface (tensile surface), a three-point bending test was performed on 10 elements at a span of 10 mm using the tip portions, and the average value of the element strength was determined.
[0083]
For comparison, an oxygen sensor including only a porous ceramic insulating layer and only a dense ceramic insulating layer as a ceramic insulating layer of the heater portion was manufactured in the same manner as above, and the above characteristics were measured.
[0084]
[Table 1]

[0085]
Table 1 shows that when the insulating layer surrounding the heating element is composed of only the porous ceramic insulating layer or only the dense ceramic insulating layer, the breakage rate is as high as 80% or more.
[0086]
On the other hand, in the device of the present invention in which the porous ceramic insulating layer and the dense ceramic insulating layer are provided around the heating element, the breakage rate is reduced to 60% or less. Further, as can be seen from Table 1, the product of the present invention has a porous ceramic insulating layer, but since the periphery thereof is embedded in the dense ceramic insulating layer and the zirconia solid electrolyte substrate, the element strength is excellent at 35 MPa or more. This prevents inadvertent damage to the device during the manufacturing process. This is also a major manufacturing advantage such as improving the yield.
[0087]
In particular, an element having a porous ceramic insulating layer having a porosity of 3 to 20% and a thickness of 5 to 200 μm, and a dense ceramic insulating layer having a porosity of 1% or less and a thickness of 5 to 100 μm, has a failure rate of 50%. % Or less, and the element strength was excellent at 40 MPa or more.
[0088]
From the above results, the product of the present invention provided with the porous ceramic insulating layer and the dense ceramic insulating layer can easily be a sensor element having excellent heat resistance and durability even when subjected to a rapid thermal cycle. It can be understood.
[0089]
【The invention's effect】
As described in detail above, according to the present invention, the heating element in the heater section is covered with the porous ceramic insulating layer and further covered with the dense ceramic insulating layer, so that cracks are generated or broken even for long-time operation. It is possible to provide a flat oxygen sensor element having excellent stability without any problem.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view for explaining an example of an oxygen sensor element of the present invention.
FIG. 2 is a schematic plan view of an oxygen sensor element according to the present invention.
FIG. 3 is a schematic perspective view for explaining an application example of the oxygen sensor element of FIG. 3 (c).
FIG. 4 is a schematic sectional view for explaining still another example of the oxygen sensor element of the present invention.
FIG. 5 is an exploded perspective view for explaining a method of manufacturing the oxygen sensor element of FIG.
FIG. 6 is a schematic sectional view for explaining the structure of a conventional heater-integrated oxygen sensor element.
FIG. 7 is a schematic cross-sectional view for explaining the structure of another conventional heater-integrated oxygen sensor element.
[Explanation of symbols]
1 Sensor section
2 Heater section
3 Zirconia solid electrolyte substrate
4 Reference electrode
5 measuring electrode
6. Ceramic porous layer
7 Ceramic insulation layer
7a Porous ceramic insulating layer
7b Dense ceramic insulating layer
8 Heating element
9 Zirconia solid electrolyte layer

Claims (7)

An oxygen sensor having a sensor section in which a pair of electrodes are formed on a part of a long flat zirconia solid electrolyte substrate via a zirconia solid electrolyte layer, and a heater section in which a heating element is embedded in a ceramic insulating layer. In the element, the heating element in the heater portion is covered with a porous ceramic insulating layer, and a part or the entire periphery of the porous ceramic insulating layer is covered with a dense ceramic insulating layer. An oxygen sensor element, wherein a part is embedded and integrated in the zirconia solid electrolyte substrate. The oxygen sensor element according to claim 1, wherein the porosity of the porous ceramic insulating layer covering the heating element is 5 to 20%. 3. The oxygen sensor element according to claim 1, wherein the thickness of the porous ceramic insulating layer covering the heating element is 5 to 200 [mu] m. The oxygen sensor according to any one of claims 1 to 3, wherein the content of Si in the porous ceramic insulating layer covering the heating element is 0.7% by weight or less in terms of oxide. element. The oxygen sensor element according to any one of claims 1 to 4, wherein the sensor section and the heater section are formed by simultaneous firing together with the zirconia solid electrolyte substrate. Of the pair of electrodes in the sensor section, the area of the electrode in contact with the gas to be measured is 8 to 18 mm ² , and the width in the direction perpendicular to the longitudinal direction of the element is at least 5 mm or more from the tip of the element. The oxygen sensor element according to any one of claims 1 to 4, wherein the diameter is 2.0 to 3.5 mm. A pair of electrodes of the sensor section is formed near the front end of the element, and an electrode pad for connecting a terminal is provided near the rear end of the element. The oxygen sensor element according to claim 1, wherein a width of the oxygen sensor element is larger than a width of the element tip.

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