JP4689859B2 - Heater integrated oxygen sensor element - Google Patents

Description

表１より、Ｓｉ含有量がＳｉＯ₂換算で０．４重量％以下の領域が全く存在しないか、または存在しても厚さが５μｍよりも薄い試料Ｎｏ．１，２では、ヒータの寿命が短かったのに対して、本発明に基づく試料Ｎｏ．３〜１１では寿命を７０００時間以上とすることができた。
【００４９】
実施例２
実施例１のヒータ素体を形成する際のジルコニアグリーンシート中のＳｉ量および厚みを種々変化させ、ついでＳｉ量をＳｉＯ₂換算で０．１重量％含有する５モル％Ｙ₂Ｏ₃含有のジルコニア粉末にポリビニルアルコールを添加してスラリーを形成し、これを約８μｍの厚みで塗布した以外は、実施例１と同様にして酸素センサ素子を作製した。
作製した酸素センサ素子に対して、ＥＰＭＡによってＳｉ量が０．４重量％以下の厚みを測定するとともに、第２固体電解質層の厚みとＳｉ量を測定した。第２固体電解質層の厚みはＳｉ量が０．４重量％を超える部分の総厚みとした。また。第２固体電解質層中のＳｉ量は、上記の第２固体電解質層の中心部分におけるＳｉ量をＳｉ含有量とした。
作製した素子に対して、室温から大気中１０００℃まで２０秒で昇温後、室温まで空冷するという温度サイクルを１サイクルとして、これを２０万回行った際の素子が破壊する割合（破壊率）を求めた。その際、試料はそれぞれ２０個として、数値はその平均値とした。結果を表２に示す。
【表２】

表２から、第２固体電解質層のＳｉ含有量がＳｉＯ₂換算で０．４重量％より低い試料Ｎｏ．１２および第２固体電解質層の厚みが１０μmより薄い試料Ｎｏ．１５では熱サイクルによる素子の破損率が高いことがわかる。
これに対して、第２固体電解質層のＳｉ含有量がＳｉＯ₂換算で０．４〜２重量％で厚みが１０μmより厚い試料は全て素子の破損率が２５％以下と低かった。
【００５０】
【発明の効果】
本発明のヒータ一体化酸素センサ素子は、発熱体によるジルコニア固体電解質基体の加熱効率を高め、急速昇温を行うことができる結果、センサ活性化時間を短縮することができ、しかもセラミック絶縁層と接する固体電解質の界面にはＳｉ含有量がＳｉＯ₂換算で０．４重量％以下である層が少なくとも５μｍの厚さで存在するため、ヒータ寿命が長くなるという効果がある。
【図面の簡単な説明】
【図１】 (ａ)は本発明の第１の実施形態にかかるヒータ一体化酸素センサ素子を示す概略斜視図であり、（ｂ）はそのＸ1 −Ｘ1断面図である。
【図２】本発明の概念を示す説明図である。
【図３】第１の実施形態にかかるヒータ一体化酸素センサ素子の製造方法を示す説明図である。
【図４】本発明の第２の実施形態にかかるヒータ一体化酸素センサ素子を示す断面図である。
【図５】 (ａ)は本発明の第４の実施形態にかかるヒータ一体化酸素センサ素子を示す概略斜視図であり、（ｂ）はそのＸ２ −Ｘ２断面図である。
【図６】本発明の第5の実施形態にかかるヒータ一体化酸素センサ素子を示す断面図である。
【図７】従来のヒータ一体化酸素センサ素子を示す断面図である。
【符号の説明】
１…ヒータ一体化酸素センサ素子、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…固体電解質層、１１…多孔質層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a heater-integrated oxygen sensor element for controlling the ratio of air and fuel in an internal combustion engine such as an automobile. More specifically, the present invention relates to a heater-integrated oxygen sensor having a long heater life in which a heating element and a sensor unit are integrated. The present invention relates to a sensor element.
[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.
As this 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 the outer surface and the inner surface of a cylindrical tube sealed at one end. A sensor is used.
[0003]
As a typical example of this oxygen sensor, as shown in FIG. 7, the inner surface of a cylindrical tube 51 made of a ZrO 2 solid electrolyte and sealed at the tip is made of platinum and is in contact with a reference gas such as air. The electrode 52 is formed on the outer surface of the cylindrical tube 51, and a measurement electrode 53 that is in contact with a gas to be measured such as exhaust gas is formed.
In such an oxygen sensor, a so-called theoretical air-fuel ratio sensor (λ sensor) that is generally used for controlling the ratio of air to fuel near 1 is a porous layer serving as a protective layer on the surface of the measurement electrode 53. 54 is provided to detect a difference in oxygen concentration generated on both sides of the cylindrical tube at a predetermined temperature to control the air-fuel ratio of the engine intake system.
[0004]
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 ceramic porous material that becomes a gas diffusion-controlling layer having fine pores on the surface of the measurement electrode 53. A layer 54 is provided, and an applied voltage is applied to a cylindrical tube 51 made of a solid electrolyte through a pair of electrodes 52 and 53, and a limit current value obtained at that time is measured to control an air-fuel ratio in a lean combustion region.
Both the theoretical air-fuel ratio sensor and the wide-range air-fuel ratio sensor need to heat the sensor section to an operating temperature of about 700 ° C. Therefore, a rod-like shape is provided inside the cylindrical tube to heat the sensor section to the operating temperature. A heater 55 is inserted.
[0005]
[Problems to be solved by the invention]
However, in recent years, the tendency to tighten exhaust gas regulations has increased, 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 55 is inserted into the cylindrical tube 51 as described above, the time required for the sensor unit to reach the activation temperature (hereinafter, There was a problem that exhaust gas regulations could not be fully met because the activation time was slow.
[0006]
As a method for avoiding this 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 measuring electrode. Japanese Patent Laid-Open No. 10-206380 discloses a cylindrical heater-integrated oxygen sensor element in which a platinum heater is provided in a gas non-permeable layer having a low gas permeability.
[0007]
On the other hand, the applicant of the present invention, first, the sensor part formed by forming the reference electrode and the measurement electrode 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 formed on the outer surface of the cylindrical tube, and the measurement electrode is exposed from the opening, and at least the exposed ceramic insulating layer around the measurement electrode We proposed a heater-integrated oxygen sensor element in which a platinum heater (heating resistor) was embedded.
However, unlike these conventional indirect heating systems, these heater-integrated oxygen sensors are capable of rapid temperature increase because of the direct heating system, but there is a problem that the life of the heater is short.
[0008]
Accordingly, an object of the present invention is to provide a heater-integrated oxygen sensor element in which a heater is integrated and excellent in thermal shock properties such as rapid temperature rise and has a long heater life.
[0009]
[Means for Solving the Problems]
The present inventors have intensively studied that the Si content of the solid electrolyte plays an important role in the problem that the heater has a short life, and as a result of conducting an energetic study, Na was added to the heater high temperature part on the negative electrode side by energization. Alkali metals such as Ca and Mg, and alkaline earth metals such as Ca and Mg diffuse and segregate (hereinafter this phenomenon is called migration), which lowers the electrical resistance of the ceramic insulation layer and consequently reduces the life of the heater I found out that
In other words, migration that is a problem is that when the Si content in the solid electrolyte substrate is large, unavoidable components such as Na, Ca, and Mg in the solid electrolyte pass through the grain boundary phase mainly composed of Si when energized. This is because it diffuses into the grain boundary phase of the ceramic insulating layer and moves to and deposits on the negative electrode side of the high temperature portion of the heater to reduce the electrical resistance of the ceramic insulating layer. As a result, the electrical resistance of the ceramic insulating layer is reduced, and the plus side and the minus side of the heater are short-circuited.
[0010]
  Based on such knowledge, the present inventors further studied the relationship between the Si content in the solid electrolyte substrate in contact with the ceramic insulating layer and the migration, and as a result, from the interface of the ceramic insulating layer toward the solid electrolyte substrate. Si content of SiO with a thickness of at least 5 μm₂When there is a layer of 0.4% by weight or less in terms of conversion, the diffusion of Na, Ca, Mg, etc. through the grain boundary phase in the solid electrolyte substrate and the ceramic insulating layer is suppressed, and the heater life is extended. As a result, the present invention has been completed. That is, a heater-integrated oxygen sensor element of the present invention includes a zirconia solid electrolyte substrate, a sensor unit including a measurement electrode and a reference electrode formed at positions facing at least the inner and outer surfaces of the solid electrolyte substrate; And a heater part comprising a ceramic insulating layer formed on the outer surface of the solid electrolyte substrate excluding the site where the reference electrode is formed, and a heating element embedded in the ceramic insulating layer, Si content at the interface between the solid electrolyte substrate and the ceramic insulating layer is SiO.₂A layer of 0.4% by weight or less in terms of thickness exists with a thickness of at least 5 μmAnd the Si content formed on the outer surface of the heater part is SiO. ₂ A first solid electrolyte layer having a thickness of at least 5 μm which is 0.4 wt% or less in terms of conversion, and an Si content formed on the outer surface of the first solid electrolyte layer is SiO ₂ A second solid electrolyte layer exceeding 0.4% by weight in terms of conversionIt is characterized by that.
[0011]
The reason why the diffusion of Na, Ca, Mg, etc. through the grain boundary phase in the solid electrolyte substrate is suppressed is not clear at present, but the Si content is reduced to SiO.₂By providing a layer of 0.4% by weight or less in terms of conversion between the ceramic insulating layer, the solid electrolyte substrate and the solid electrolyte layer described later, the grain boundary phase from the solid electrolyte to the insulating layer becomes discontinuous, and Na, Ca It is considered that the grain boundary diffusion rate of Mg, etc. was extremely lowered, and as a result, the migration of Na, Ca, Mg, etc. to the minus side high temperature part of the heater was suppressed.
In the oxygen sensor element of the present invention, the sensor unit may be a cylindrical body or a flat plate. In particular, in the case of a cylindrical body, it is desirable to form a solid electrolyte layer on the outer surface of the ceramic insulating layer forming the heater portion in order to improve heat retention and thermal shock resistance. In that case, the Si content is also reduced at the interface between the ceramic insulating layer and the solid electrolyte layer.₂It is necessary to provide a layer having a thickness of 0.4% by weight or less in terms of a thickness of at least 5 μm.
[0012]
On the other hand, since Si has a function of improving the thermal shock resistance of the solid electrolyte substrate and the solid electrolyte, the solid electrolyte substrate and the solid electrolyte layer have a Si content of SiO.₂It is preferable that a region exceeding 0.4% by weight exists.
In the present invention, it is more preferable that the sensor part and the heater part are simultaneously fired and formed. As a result, after manufacturing the oxygen sensor part and the heater part individually as in the prior art, the manufacturing cost is extremely low compared to the oxygen sensor element used by fitting the heater in the oxygen sensor, It is excellent from the viewpoint of economy.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
<First Embodiment>
An example of the oxygen sensor element of the present invention will be described with reference to the drawings. FIG. 1A is a schematic perspective view showing an example of the oxygen sensor element of the present invention, and FIG.
The oxygen sensor element 1 in FIG. 1 is generally called a theoretical air-twist ratio sensor (λ sensor), and is made of a ceramic solid electrolyte such as zirconia having oxygen ion conductivity, and has a cylindrical tube 2 sealed at the tip. A reference electrode 3 that is in contact with a reference gas such as air is deposited on the inner surface of the (solid electrolyte substrate) as a first electrode, and the outer surface of the cylindrical tube 2 facing the reference electrode 3 is As the second electrode, a measurement electrode 4 that is in contact with a measurement gas such as exhaust gas is deposited. A sensor part is formed by the reference electrode 3, the cylindrical tube 2 made of a zirconia solid electrolyte, and the measurement electrode 4.
[0014]
On the outer surface of the cylindrical tube 2 whose tip is sealed,₂O_ThreeA ceramic insulating layer 5 is deposited and formed, and an opening 6 is formed in the ceramic insulating layer 5 so that a part or all of the measurement electrode 4 is exposed.
Further, a heating resistor (hereinafter referred to as a platinum heater) made of platinum or the like for heating the sensor portion is embedded in the ceramic insulating layer 5 around the opening 6, and these constitute a heater portion. ing. A ceramic solid electrolyte layer 8 such as zirconia is laminated on the surface of the ceramic insulating layer 5 in order to increase the heating efficiency by the heating resistor 7.
[0015]
The platinum heater 7 is connected to the terminal electrode 10 via the lead portion 9, and the heater 7 is heated by passing a current through the platinum heater 7 through the lead electrode 9. It becomes the mechanism which heats the sensor part which becomes. In the present invention, a second opening may be provided in a contrasting position on the opposite side of the opening 6, and another electrode to be measured may be formed in this opening in order to improve the sensitivity of sensing. it can.
Hereinafter, each component will be described.
[0016]
(Cylindrical tube 2 and solid electrolyte layer 8)
The solid electrolyte constituting them is ZrO.₂ Made of ceramics, specifically Y₂ O_Three  And Yb₂ O_Three, Sc₂ O_Three, Sm₂ O_Three, Nd₂ O_Three, Dy₂ O_ThreePartially stabilized ZrO containing 1 to 30 mol%, preferably 3 to 15 mol% of a rare earth oxide such as₂ Or stabilized ZrO₂ Is used. ZrO₂ ZrO in which 1 to 20 atomic% of Zr was substituted with Ce₂ By using, there is an effect that the electron conductivity is increased and the responsiveness is further improved.
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.
[0017]
At that time, in the present invention, the Si content is particularly reduced at the interface of the cylindrical tube 2 and the solid electrolyte layer 8 in contact with the ceramic insulating layer 5.₂It is important that a layer having a thickness of not more than 0.4% by weight and having a thickness of 5 μm or more is formed. If the Si content of this layer exceeds 0.4% by weight or the thickness is less than 5 μm, Na, Ca, Mg, etc. migrate to the high temperature part on the minus side of the platinum heater, and the electrical insulation of the ceramic insulating layer Increases resistance, resulting in reduced heater life.
In particular, the Si content is SiO₂It is desirable that a layer of 0.4% by weight or less in terms of thickness exists in a thickness of 10 μm or more, particularly 20 μm or more.
[0018]
The other part of the solid electrolyte has a Si content of SiO.₂ZrO containing 0 to 2% by weight in terms of conversion₂ Specifically, it is made of ceramics.₂ O_ThreeAnd Yb₂ O_Three, Sc₂ O_Three , Sm₂ O_Three , Nd₂ O_Three , Dy₂ O_Three Partially stabilized ZrO containing 1 to 30 mol%, preferably 3 to 15 mol% of a rare earth oxide such as₂ Or stabilized ZrO₂ Is used. ZrO₂ ZrO in which 1 to 20 atomic% of Zr was substituted with Ce₂ By using, there is an effect that the electron conductivity is increased and the responsiveness is further improved. In the present invention, particularly in improving the thermal shock resistance of the device, the Si content in the solid electrolyte of the other part is SiO.₂The amount is more than 0.4% by weight, preferably 0.5% by weight or more, more preferably 1% by weight or more.
[0019]
FIG. 2 is a conceptual diagram showing the distribution of Si content in the cylindrical tube 2 and the solid electrolyte layer 8. In FIG. 2, 8a and 2a have a Si content of SiO.₂It is a layer having a thickness of not more than 0.4% by weight and having a thickness of 5 μm or more. In contrast, 8b and 2b each have a Si content of SiO.₂The amount of the layer is more than 0.4% by weight, preferably 0.5% by weight or more.
Further, the Si content of the cylindrical tube 2 and the solid electrolyte layer 8 itself is SiO.₂Although it may be 0.4% by weight or less in terms of conversion, Si is SiO in the solid electrolyte.₂When the content exceeds 0.4% by weight, particularly 0.5% by weight or more, the thermal shock resistance is improved. Accordingly, the cylindrical tube 2 and / or the solid electrolyte layer 8 has a Si content of SiO.₂It is desirable that there is a layer containing more than 0.4% by weight in terms of conversion.
[0020]
In particular, as shown in FIG. 2, the Si content on the outer surface of the ceramic insulating layer 5 is SiO.₂The first solid electrolyte layer 8a of 0.4% by weight or less in terms of conversion, and the Si content is SiO₂By providing the second solid electrolyte layer 8b exceeding 0.4% by weight in terms of conversion, it is possible to improve durability against a thermal shock from the outer surface where the heater-integrated oxygen sensor element contacts the exhaust gas.
In particular, the Si content of the second solid electrolyte layer is SiO₂It is 0.5% by weight or more in terms of conversion, and the thickness is desirably 10 μm or more, particularly 20 μm or more.
[0021]
Further, for the purpose of improving the sinterability, it is preferable that the cylindrical tube 2 and the solid electrolyte layer 8 contain 5% by weight or less, particularly 2% by weight or less of alumina.
The Na content in these solid electrolytes is preferably 200 ppm or less, particularly 100 ppm or less, from the viewpoint of reliably preventing diffusion penetration from the solid electrolyte into the ceramic insulating layer and extending the life of the heater.
[0022]
(Ceramic insulating layer 5)
As the ceramic insulating layer 5 in which the platinum heater 7 is embedded, alumina, spinel, and a composite compound material of alumina and spinel are preferably used. At this time, it is desirable to add a small amount of Si for the purpose of improving the sinterability of the ceramic insulating layer.₂It is necessary to control to 5% by weight or less in terms of conversion. If the Si content exceeds 5% by weight, the diffusion and segregation of Na in the ceramic insulating layer is promoted during energization, and the life of the platinum heater is reduced. The content of Si is particularly preferably 3% by weight or less. In particular, 2% by weight or less is desirable from the viewpoint of preventing the diffusion of Na, Ca and the like.
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 element itself can be increased as a result of the strength of the insulating layer being increased because the ceramic insulating layer 5 is dense.
[0023]
(Platinum heater 7)
As the heater 7 embedded in the ceramic insulating layer 5, platinum is mainly used. However, in some cases, an alloy of platinum and rhodium, palladium, or ruthenium can also be used. At this time, in order to form a long-life heating element, it is preferable to set the Na content to 50 ppm or less with respect to 100 wt% for both the platinum heater and the alloy heater. When the Na content exceeds 50 ppm, Na diffuses to the negative electrode side when energized and reacts with platinum to increase the resistance of the heater. 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.
[0024]
In addition to the above metals, the heater 7 contains alumina, spinel, an alumina / silica compound, forsterite, or a zirconia ceramic that can be used as the above electrolyte from the viewpoint of preventing sintering and enhancing the adhesion between the insulating layers. Although it can mix in the range of 10-80% by volume ratio, especially 30-50%, it is desirable that content of Na shall be 50 ppm or less also with respect to the ceramic mixed at this time.
[0025]
(Heater structure)
  As shown in the sectional view of FIG. 1 (b), the heater part structure in which the platinum heater 7 is embedded in the ceramic insulating layer 5 has the heater 7 embedded in the surface of the cylindrical tube 2 made of solid electrolyte. IsA ceramic insulating layer 5 is formed.Further, a zirconia solid electrolyte layer 8 is formed on the outer surface of the ceramic insulating layer 5. The solid electrolyte layer 8 is for reducing the stress caused by the thermal expansion difference and the firing shrinkage difference between the cylindrical tube 2 which is the solid electrolyte base and the ceramic insulating layer 5, and making the thermal stress as small as possible. . At this time, the thickness of the ceramic insulating layer 5 between the cylindrical tube 2 and the heater 7 and between the zirconia solid electrolyte layer 8 and the heater 7 is preferably 2 μm or more.
[0026]
(electrode)
The reference electrode 3 and the measurement electrode 4 deposited on the surface of the cylindrical tube 2 are each made of one or more alloys selected from the group of rhodium, palladium, ruthenium and gold in addition to platinum. It is formed. In the present invention, the area where the thickness of the measurement electrode 4 is 6 μm or less is preferably 50% or more of the total measurement electrode area.
Further, regarding the formation of the measurement electrode 4, the average crystal particle size is 0 when the average crystal particle size is 0.1 μm or less and the oxygen ion conductive powder, for example, the above-mentioned solid electrolyte powder is contained in 1-50 vol% crystal. It is preferable to use a platinum group metal of 5 to 4 μm as a starting material.
Further, the shape of the measurement electrode 4 exposed from the opening 6 is preferably composed of a vertically long rectangular shape, an elliptical shape, or the like as shown in FIG.
[0027]
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 at least on the inner surface portion facing the portion exposed from the opening 7 of the measurement electrode 4. The area may be larger than the exposed area, for example, the entire inner surface of the cylindrical tube 2.
[0028]
(Opening 6)
As the shape of the opening 6, for example, a rectangular shape or an elliptical shape is preferable. In the case where the shape of the opening 6 is a rectangular shape, the corner of the opening 6 should have a gentle curve or a c-plane structure from the viewpoint of alleviating the concentration of thermal stress on the corner of the opening 6. preferable.
[0029]
(Production method)
Next, a method for manufacturing the heater-integrated oxygen sensor element shown in FIG. 1 will be described with reference to FIG.
(1) First, a hollow cylindrical tube 12 having one end sealed as shown in FIG. This cylindrical tube 12 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.
As the solid electrolyte powder used at this time, Si is SiO.₂Y as a stabilizer against zirconia powder containing 0 to 2% by weight in terms of conversion₂ O_Three And Yb₂ O_Three , Sc₂ O_Three , Sm₂ O_Three , Nd₂ O_Three , Dy₂ O_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. ZrO₂ ZrO in which 1 to 20 atomic% of Zr was substituted with Ce₂ A 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.₂ O_Three It is also possible to add 5% by weight or less, particularly 2% by weight or less.
[0030]
(2) Slurry the patterns 13 and 14 to be the reference electrode and the measurement electrode on the inner surface and the outer surface of the cylindrical tube 12 made of the solid electrolyte, for example, using a conductive paste containing platinum.DickOr by screen printing, pad printing or roll transfer. At this time, it is efficient to print the reference electrode on the inner surface of the cylindrical tube 12 by filling the cylindrical tube 12 with the conductive paste and then discharging and coating the conductive paste on the entire inner surface. In this way, the sensor element A is manufactured. The conductive paste for electrode formation is prepared by mixing a solid electrolyte powder having a crystal particle size of 0.1 μm or less and platinum powder with a binder, terpineol, etc., and rotating and mixing with a three roll etc. under a pressure for 24 hours or more. The prepared, ultrafine solid electrolyte powder is almost completely impregnated in platinum. As the solid electrolyte powder to be impregnated, the same material as the above-mentioned solid electrolyte can be used, but among them, 8 to 12 mol% Y₂O_ThreeStabilized zirconia in which is dissolved is particularly preferred. Further, when the electrode is made more porous, the solid electrolyte powder can be added to the platinum powder impregnated with the solid electrolyte powder. Although it depends on the firing temperature, the total ceramic amount is preferably in the range of 10 to 60% by volume with respect to the metal.
[0031]
(3) Next, a heater element B as shown in FIG. 3B is formed. First, by using the above zirconia powder, a molding organic binder is appropriately added thereto to prepare a slurry, and this slurry is used to prepare a ceramic solid electrolyte having a predetermined thickness by a doctor blade method, an extrusion molding method, a pressing method, or the like. A green sheet 15 for forming a 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.
At this time, the zirconia powder has a Si content after firing of SiO.₂Since it is formed so as to contain 0.4% by weight or less in terms of conversion, the green sheet 15 has a Si content of SiO after firing.₂In terms of 0.4% by weight or less, the thickness becomes a layer of 5 μm or more after firing.
A ceramic insulating layer 16 is formed on the surface of the green sheet 15 by printing alumina, spinel or a composite oxide powder of alumina and spinel by screen printing, pad printing, roll transfer, or the like. Thereafter, a conductive paste containing platinum powder is printed by a screen printing method, a pad printing method, a roll transfer method or the like to apply a platinum heater pattern 17 including a lead portion, and then the ceramic insulating layer is further formed thereon. The formed powder is applied to form a ceramic insulating layer 18 to obtain a sheet-like laminate 19 in which the platinum heater pattern 17 is embedded. Thereafter, the opening 20 is formed by punching or the like.
[0032]
(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. In order to wind the heater element B around the sensor element A, a slurry is prepared by adding a binder to the zirconia powder adjusted so that the Si content after baking as an adhesive is 0.4% by weight or less, The slurry is interposed between the heater element B and the sensor element A so as to be bonded, or in some cases, mechanically bonded while applying pressure with a roller or the like. 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.
In any case, the solid electrolyte layer obtained by firing the slurry at the interface of the cylindrical tube in contact with the ceramic insulating layer has a Si content of SiO.₂It is necessary to control the thickness so that it is 0.4% by weight or less and the thickness is 5 μm or more.
[0033]
(5) The sensor element body A and the sensor element body B can be integrated by firing the cylindrical laminate obtained above at a temperature at which the respective constituent elements can be fired simultaneously. For example, the heater element body and the sensor element body are simultaneously baked by baking at 1300 to 1700 ° C. for about 1 to 10 hours in an inert atmosphere such as argon gas. Thereby, the element shown in FIG. 1 of the present invention can be manufactured.
[0034]
(Other manufacturing methods)
As another manufacturing method, the Si content is SiO.₂A zirconia sheet of 0.4% by weight or less in terms of conversion, and a Si content of SiO₂Two or more zirconia sheets of 0 to 2% by weight, preferably more than 0.4% by weight and less than or equal to 2% by weight are laminated, and the Si content in the zirconia sheet in contact with the ceramic insulating layer is SiO.₂The Si content in the zirconia sheet in contact with the gas to be measured is 0.4% by weight or less in terms of conversion, and the SiO content is SiO.₂After preparing to be more than 0.04 wt% and 2 wt% or less in terms of conversion, the Si content is SiO₂A surface obtained by printing a ceramic insulating layer, a platinum heater, or the like on the surface of 0.4% by weight or less of the converted zirconia sheet may be wound around and integrated with the zirconia cylindrical tube on which the electrode is formed according to the method described above.
[0035]
Further, after the heater body B formed in (3) above is wound around the surface of the cylindrical tube 12 having no electrode to produce a cylindrical laminate, electrode paste is screen printed on the cylindrical laminate, and the pad After being applied to the inner surface of the cylindrical tube 12 and the surface of the cylindrical tube in the opening 20 of the heater element B by printing, roll transfer method or dipping method, simultaneous firing can be performed as described in (5) above.
As another method, the heater element body B formed by the above (3) is wound around the surface of the cylindrical tube 12 having no electrode to produce a cylindrical laminate, and this is used for the inner surface of the cylindrical tube 12 and the heater. An electrode paste can be printed in the opening 20 in the element body B and baked.
In the present invention, the overall element size is 3 to 6 mm, particularly 3 to 4 mm, so that the power consumption can be reduced and the sensing performance can be enhanced.
[0036]
<Second Embodiment>
FIG. 4 is a longitudinal sectional view showing another embodiment of the heater-integrated oxygen sensor element according to the present invention. In FIG. 4, the same components as those shown in FIG.
That is, in this oxygen sensor element, as shown in FIG. 4, 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. The ceramic porous layer 11 is formed for the following two purposes.
[0037]
The first is provided for the purpose of preventing the measurement electrode 4 from being poisoned by the 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 porous layer can generally be used as a theoretical air-fuel ratio sensor (λ sensor) element. In this case, the ceramic porous layer 11 (protective layer) is preferably made of a porous body having an open porosity of 10 to 40%. The ceramic porous layer 11 can be formed on the electrode surface as shown in FIG. 5, but may be formed all around the tip of the element.
[0038]
Second, it is formed as at least one gas diffusion rate-determining layer selected from zirconia, alumina, spinel, magnesia and γ-alumina having fine pores on the exposed surface of the measurement electrode 4. As such a gas diffusion control layer, a porous body having an open porosity of 5 to 30% is desirable. Further, it is desirable to provide the ceramic protective layer made of alumina or spinel on the surface of the porous layer 11 (gas diffusion rate controlling layer) from the viewpoint of further preventing the exhaust gas from being poisoned. In this case as well, the ceramic porous layer can be formed all around the tip of the element in addition to the electrode surface. Such a heater-integrated oxygen sensor element can be applied as a wide-range air-fuel ratio sensor element (A / F sensor) described later.
[0039]
Next, a method for forming the ceramic porous layer 11 will be described. When producing a theoretical air-fuel ratio sensor or a wide-range air-fuel ratio oxygen sensor, after firing the cylindrical laminate, powders of alumina, spinel, zirconia, etc. are sol-gel method, slurry dip method, printing method, etc. A ceramic protective layer or a gas diffusion rate controlling layer, which is the ceramic porous layer 11, is formed by applying and printing by baking, or coating the ceramics by sputtering or plasma spraying. As another method, it is also possible to form the ceramic protective layer or the gas diffusion-controlling layer on the surface of the measurement electrode in advance when the cylindrical laminated body is manufactured, and fire it at the same time as the cylindrical laminated body. .
Others are the same as the first embodiment.
[0040]
<Third Embodiment>
  FIG. 5 is a longitudinal sectional view showing an embodiment of an air-fuel ratio sensor element according to the present invention. FIG. 5 (a) is a schematic perspective view of the air-fuel ratio sensor element 25, and FIG.SoIt is a principal part enlarged view of X2-X2 cross section. In FIG. 5, the same components as those shown in FIG. That is, the air-fuel ratio sensor element 25 is made of a ceramic solid electrolyte having oxygen ion conductivity and is sealed at one end. In other words, the air-fuel ratio sensor element 25 is a first for constituting the sensor element in the cylindrical tube 2 having a U-shaped longitudinal section. Electrode pairs are 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. The measuring electrode 4 of the present invention that contacts is formed.
[0041]
In addition, a space 26 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 a platinum heater 7 is embedded around the space 26. Layer 5 is provided.
A solid electrolyte layer 28 having oxygen ion conductivity is formed on the upper surface of the space portion 26 so as to close the space portion 26, and the inner surface of the solid electrolyte layer 28 on the space portion 26 side; A second electrode pair made up of an inner electrode 29 and an outer electrode 30 is formed on the outer surface of the solid electrolyte layer 28 facing it. The solid electrolyte layer 28 and the second electrode pair 29 and 30 function as a pump cell for controlling the oxygen concentration in the space 26 to a predetermined concentration.
[0042]
In addition, a small diffusion hole 31 is formed in the solid electrolyte layer 28 including the second electrode pair 29 and 30 for taking in an exhaust gas serving as a gas to be measured. The space 26 is provided with a porous body 27 for controlling the diffusion.
The platinum heater 7 disposed in the ceramic insulating layer 5 is connected to the terminal electrode 33 via the lead portion 32, and the heater 7 is heated by passing a current through the heater 7 through these, and the reference electrode 3. In addition, the sensor is composed of a cylindrical tube 2 made of a solid electrolyte provided with the measurement electrode 4 and a sensor unit made up of the solid electrolyte layer 28 provided with the second electrode pairs 29 and 30 described above.
[0043]
Also in the element having such a structure, the Si content is similar to that described above at the interface between the cylindrical tube 2 made of a solid electrolyte 2 and the ceramic insulating layer 5 and at the interface with the ceramic insulating layer 5 of the solid electrolyte layer 28.₂The solid electrolyte layers 34 and 35 of 0.4% by weight or less in terms of thickness are present at a thickness of at least 5 μm.
[0044]
The solid electrolyte layer 28 itself has a Si content of SiO.₂You may form in 0.4 weight% or less in conversion. In this case, in order to improve thermal shock resistance, it is preferable to provide a second solid electrolyte layer on the outer surface. The solid electrolyte layer 28 has a Si content of SiO.₂Even if the layer exceeds 0.4% by weight in terms of conversion, the Si content on the inner surface of the solid electrolyte layer 28 in contact with the ceramic insulating layer 5 is SiO.₂It is only necessary that a layer having a conversion of 0.4% by weight or less exists in a thickness of at least 5 μm.
Others are the same as the first embodiment.
[0045]
<Fourth Embodiment>
  FIG. 6A is a schematic perspective view showing an example of a heater-integrated flat plate type oxygen sensor element according to the present invention, and FIG. 6B is an X3-X3 sectional view thereof. The flat oxygen sensor element 39 shown in FIG. 6 has an air introduction hole 41 provided in the solid electrolyte substrate 40, a measurement electrode 42 provided outside the solid electrolyte substrate 40, and a solid electrolyte plate 40a of the solid electrolyte substrate 40. A reference electrode 43 is formed on the inner surface of the air introduction hole 41 facing each other to form a sensor part. On the surface of the measurement electrode 42, the above-mentionedofA porous ceramic layer 47 similar to the above is formed. In addition, a heater portion in which a platinum heater 45 is embedded in a ceramic insulating layer 44 is integrally formed on one surface of the solid electrolyte substrate 40. According to the present invention, even in such a flat plate type oxygen sensor element, the Si content at the interface between the ceramic insulating layer 44 and the solid electrolyte substrate 40 is SiO.₂By forming 0.4% by weight or less of the solid electrolyte layer 46 with a thickness of 5 μm or more, diffusion from the solid electrolyte base 40 to the heater portion such as Na can be prevented. Such a flat plate oxygen sensor element is formed by laminating sheets of solid electrolytes and applying a platinum heater.TIt can be manufactured by laminating and laminating sheets of a ceramic insulating layer and co-firing. Also in that case, the solid electrolyte layer 46 with a small amount of Si can be formed by sandwiching a sheet or by interposing it between the ceramic insulating layer sheet and the solid electrolyte sheet as an adhesive.
[0046]
Although an example of the present invention has been described above, the present invention is not limited to these structures, and a sensor unit having a pair of porous platinum electrodes at opposite positions on both sides of a solid electrolyte, and ceramic insulation And a heater part in which a platinum heater is embedded in the layer, preferably a heater-integrated oxygen sensor element in which an electrode and a heater are produced by simultaneous firing, in contact with a ceramic insulating layer and a solid electrolyte substrate and a solid electrolyte layer Needless to say, it can be applied to all of the above.
[0047]
【Example】
EXAMPLES Hereinafter, although an Example is given and the heater integrated oxygen sensor element of this invention is demonstrated in detail, this invention is not limited to a following example.
[0048]
Example 1
The embodiment of the present invention will be described by taking the oxygen sensor element shown in the figure as an example.
Commercial alumina powder and SiO₂Multiple types of 5 mol% Y in which the amount of Si was changed by changing the amount₂ O_Three Containing zirconia powder, platinum powder containing 10 wt% alumina, 8 mol% Y₂O_ThreeA mixed powder of platinum containing 30% by volume of zirconia powder containing selenium was prepared.
First, Si is changed to SiO₂5 mol% Y containing 1.0% by weight in terms of conversion₂ O_Three  A cylindrical sensor element having one end sealed so that an outer diameter after sintering is about 3.5 mm and an inner diameter is 2 mm by extruding and adding a polyvinyl alcohol solution to the contained zirconia powder. The body was molded. On the surface of this molded body, the above 8 mol% Y is used as a measurement electrode.₂O_ThreeA platinum powder containing zirconia containing sinter is printed and applied so that the firing is about 10 μm, and the paste made of the above platinum powder is also applied to the inside of the molded body so as to be 10 μm to form a reference electrode. A sensor element was prepared.
On the other hand, the amount of Si is reduced to SiO.₂5 mol% Y containing 1.0% by weight in terms of conversion₂ O_Three  A 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. A rectangular opening corresponding to the shape of the measurement electrode was opened in the green sheet by punching.
Then, Si amount is changed to SiO₂5 mol% Y each containing 0 to 3.0 wt% in terms of conversion₂ O_Three  Polyvinyl alcohol was added to the contained zirconia powder to form a slurry, which was printed on a portion other than the opening. Next, after alumina powder is applied to the surface to a thickness of about 10 μm, the above-mentioned conductor paste made of platinum powder containing alumina is screen-printed around the opening so that the thickness of the heating element pattern is about 10 μm. Further, a heater element having a structure as shown in FIG. 3B was prepared, in which alumina powder was applied to a thickness of about 10 μm and a platinum heater was embedded.
Next, on the surface of the cylindrical sensor element body, the Si amount is changed to SiO.₂5 mol% Y each containing 0 to 3.0 wt% in terms of conversion₂ O_Three An acrylic resin was added as an adhesive to the zirconia powder contained, and the heater element was wound around the sensor element to produce a cylindrical laminate. Then, this cylindrical laminated body was baked at 1500 ° C. for 2 hours in the air and baked and integrated.
Thereafter, a ceramic protective layer made of spinel having a porosity of about 30% was formed on the surface of the measurement electrode in the opening with a thickness of 100 μm to produce a theoretical air-fuel ratio sensor as shown in FIG.
The produced element was heated at 800 ° C. in the atmosphere and the time until the heater was disconnected was measured. At that time, the number of samples was 10 and the arithmetic average value of the time was obtained.
In this experiment, the amount of Si at the interface between the ceramic insulating layer, the solid electrolyte substrate and the solid electrolyte layer was analyzed using EPDM (electron probe microanalyzer), and the amount of Si was 0.4 weight at the interface. Table 1 shows the thickness of the region below%.
[Table 1]

From Table 1, Si content is SiO₂Sample No. 4 having no thickness of 0.4% by weight or less in terms of conversion or having a thickness of less than 5 μm even though it exists. In Nos. 1 and 2, the life of the heater was short, whereas the sample No. 1 according to the present invention was used. In 3-11, the lifetime could be 7000 hours or more.
[0049]
Example 2
The Si amount and thickness in the zirconia green sheet when forming the heater element of Example 1 were variously changed, and then the Si amount was changed to SiO.₂5 mol% Y containing 0.1% by weight in terms of conversion₂O_ThreeAn oxygen sensor element was produced in the same manner as in Example 1 except that polyvinyl alcohol was added to the contained zirconia powder to form a slurry, and this was applied to a thickness of about 8 μm.
The thickness of the second solid electrolyte layer and the amount of Si were measured with respect to the produced oxygen sensor element while measuring the thickness of the Si amount of 0.4 wt% or less by EPMA. The thickness of the second solid electrolyte layer was the total thickness of the portion where the Si amount exceeded 0.4% by weight. Also. The amount of Si in the second solid electrolyte layer was defined as the Si content in the central portion of the second solid electrolyte layer.
A ratio of destruction of the device when the device was subjected to 200,000 times with a temperature cycle of raising the temperature from room temperature to 1000 ° C. in the air in 20 seconds and then air-cooling to room temperature. ) At that time, the number of samples was 20, and the numerical value was the average value. The results are shown in Table 2.
[Table 2]

From Table 2, the Si content of the second solid electrolyte layer is SiO.₂Sample No. lower than 0.4% by weight in terms of conversion. Sample No. 12 and the thickness of the second solid electrolyte layer are thinner than 10 μm. 15 indicates that the damage rate of the element due to thermal cycling is high.
On the other hand, the Si content of the second solid electrolyte layer is SiO.₂All samples having a thickness of 0.4 to 2% by weight and a thickness of more than 10 μm had a low element breakage rate of 25% or less.
[0050]
【The invention's effect】
The heater-integrated oxygen sensor element of the present invention can increase the heating efficiency of the zirconia solid electrolyte substrate by the heating element and can rapidly increase the temperature. As a result, the sensor activation time can be shortened, and the ceramic insulating layer and At the interface of the solid electrolyte in contact, the Si content is SiO₂Since the layer of 0.4% by weight or less in terms of thickness exists at a thickness of at least 5 μm, there is an effect that the heater life is extended.
[Brief description of the drawings]
1A is a schematic perspective view showing a heater integrated oxygen sensor element according to a first embodiment of the present invention, and FIG. 1B is an X1-X1 cross-sectional view thereof.
FIG. 2 is an explanatory diagram showing the concept of the present invention.
FIG. 3 is an explanatory diagram showing a method for manufacturing the heater-integrated oxygen sensor element according to the first embodiment.
FIG. 4 is a cross-sectional view showing a heater integrated oxygen sensor element according to a second embodiment of the present invention.
5A is a schematic perspective view showing a heater integrated oxygen sensor element according to a fourth embodiment of the present invention, and FIG. 5B is an X2-X2 cross-sectional view thereof.
FIG. 6 is a cross-sectional view showing a heater integrated oxygen sensor element according to a fifth embodiment of the present invention.
FIG. 7 is a cross-sectional view showing a conventional heater-integrated oxygen sensor element.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Heater integrated oxygen sensor element, 2 ... Cylindrical tube, 3 ... Reference electrode, 4 ... Measurement electrode, 5 ... Ceramic insulating layer, 6 ... Opening part, 7 ... Platinum heater, 8 ... Solid electrolyte layer, 11 ... Porous layer

Claims (6)

A sensor unit comprising a zirconia solid electrolyte substrate, and a measurement electrode and a reference electrode formed at positions facing at least the inner and outer surfaces of the solid electrolyte substrate;
A heater comprising: a ceramic insulating layer formed on the outer surface of the solid electrolyte substrate excluding the measurement electrode and reference electrode forming portions; and a heater unit including a heating element embedded in the ceramic insulating layer. A body oxygen sensor element,
A layer having a Si content of 0.4 wt% or less in terms of SiO ₂ is present at a thickness of at least 5 μm at the interface between the solid electrolyte substrate and the ceramic insulating layer ;
A first solid electrolyte layer having a thickness of at least 5 μm formed on the outer surface of the heater portion and having a Si content of 0.4% by weight or less in terms of SiO ₂ , and the outer surface of the first solid electrolyte layer And a second solid electrolyte layer having a Si content exceeding 0.4 wt% in terms of SiO ₂ .   The heater-integrated oxygen sensor element according to claim 1, wherein the sensor portion is formed of a cylindrical body, and the heater portion is formed on an outer surface of the cylindrical body.   The heater-integrated oxygen sensor element according to claim 1, wherein the sensor portion is formed of a flat plate, and the heater portion is laminated on the flat plate. _2. The heater-integrated oxygen sensor element according to claim 1, wherein the layer having a thickness of at least 5 μm and having a Si content of 0.4% by weight or less in terms of SiO ₂ is a solid electrolyte layer. The heater-integrated oxygen sensor element according to claim 1, wherein the second solid electrolyte layer is a layer having a thickness of 10 µm or more. Heater-integrated oxygen sensor element according to any one of claims 1 to 3 obtained by forming by baking said sensor unit and the heater unit at the same time.

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