JP2013217733A - Gas sensor element manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor element manufacturing method of forming a porous protective layer constituting a gas sensor element by applying a molding method, the porous protective layer having a finely controlled film thickness and free from cracks.SOLUTION: A gas sensor element 100 includes a detection part 10, which is formed by laminating a solid electrolyte 3 with a pair of electrodes 4 arranged on both sides and a heat generator 7 including a heating source 6, and a porous protective layer 20 formed around the detection part 10. The manufacturing method of the gas sensor element 100 includes: injecting slurry S for forming the porous protective layer into a mold K to house the detection part 10; and curing the slurry S to form the porous protective layer 20 around the detection part 10 with the cured slurry S. The slurry S is prepared so that solid content formed of ceramic particles is contained in slurry solution, at 77 mass% or more, and pH of 3.4 or less.

Description

  The present invention relates to a method of manufacturing a gas sensor element that constitutes a gas sensor that is mounted on a vehicle and detects oxygen concentration in exhaust gas, and more particularly, to a gas sensor characterized by a method for forming a porous protective layer that constitutes the gas sensor element. The present invention relates to a method for manufacturing an element.

  Various industries are making various efforts to reduce environmental impact on a global scale. Among them, in the automobile industry, not only gasoline engine cars with excellent fuel efficiency, but also hybrid cars and electric cars. The development of the so-called eco-cars such as the above and the further improvement of its performance is being promoted every day.

  The measurement of the fuel consumption performance of a vehicle is performed by detecting the oxygen concentration in a gas to be measured such as exhaust gas with a gas sensor and obtaining the difference from this oxygen concentration using oxygen in the atmosphere as a reference gas.

  Since this gas sensor (exhaust gas sensor) detects the oxygen concentration in the exhaust gas at a high temperature atmosphere of 700 ° C or higher, if a water droplet in the exhaust gas collides with the gas sensor element constituting the gas sensor, thermal shock due to partial quenching occurs. This causes a problem that an output abnormality caused by a temperature drop at the time of flooding occurs in the element due to a volume change accompanying a temperature change.

  For this problem, for example, in the gas sensor element disclosed in Patent Document 1, the periphery of the element is surrounded by a hydrophobic porous protective layer to suppress the collision of water droplets (water resistance).

  Although this hydrophobic porous protective layer has a double structure, the formation method of each layer is to disperse hydrophobic slurry by dispersing hydrophobic heat-resistant particles, a dispersant, an inorganic binder, etc. in a dispersion medium such as water or an organic solvent. A gas sensor element in which a hydrophobic porous protective layer is formed in a predetermined range of the gas sensor element is obtained by preparing, immersing (dip) the gas sensor element in this, and drying.

  Patent Document 2 also discloses a method of forming a porous layer by applying a dip method around a solid electrolyte body constituting a gas sensor element. As a method of forming this porous layer, a dip tank and The solid electrolyte body is immersed in the slurry in the dip tank while the slurry is forcedly circulated between the viscosity adjusting tanks using a circulation pump, and a slurry film is provided at a predetermined position of the solid electrolyte body, and the porous layer is formed by firing. It is what.

  Thus, when the porous protective layer is formed by applying the dip method around the so-called detection part that constitutes the gas sensor element, the surface tension acting on the slurry during the drying process after the dip causes each part of the element. In addition, there is a high possibility that a film thickness distribution is generated in the porous protective layer. And when film thickness distribution arises in a porous protective layer in this way, possibility that the site | part with insufficient water resistance will generate | occur | produce will become high, and it will lead to the fall of a manufacturing yield.

  This problem is due to the fact that the control of the film thickness often depends on the process in the case of the dip method, but in order to solve the problem of the dip method, a mold having a cavity of a desired shape. A so-called molding method that can easily control the film thickness of the porous protective layer by using a glass has been widely used.

  The application of the molding method makes it very easy to control the thickness of the porous protective layer. However, in this molding method, the slurry is injected into the mold so that the film formability tends to deteriorate. Due to this poor film forming property, the porous protective layer is easily cracked, which leads to the problem that the water resistance after impact durability cannot be obtained sufficiently.

  Therefore, the present inventors have made use of the advantage of applying the molding method, and the method for forming a porous protective layer that can eliminate the above-described conventional problems when this molding method is applied. It has come to the idea of a method for manufacturing a gas sensor element.

JP 2010-169655 A JP 2002-323471 A

  The present invention has been made in view of the above-described problems, and relates to a method for manufacturing a gas sensor element that forms a porous protective layer constituting a gas sensor element by applying a molding method. An object of the present invention is to provide a method for manufacturing a gas sensor element that can form a porous protective layer that is less likely to cause the occurrence of the above.

  In order to achieve the above object, a method of manufacturing a gas sensor element according to the present invention includes a solid electrolyte body having a pair of electrodes on both sides and a heating element including a heat source to form a detection unit. A method for manufacturing a gas sensor element having a porous protective layer around a part, wherein the detection part is accommodated in a mold and a slurry for forming a porous protective layer is injected, or porous protection is provided in the mold A slurry for forming a layer is injected to accommodate the detection unit, and the slurry is cured to form a porous protective layer formed by curing the slurry around the detection unit. The solid content of ceramic particles is contained in the slurry solution, the solid content is 77 mass% or more, and the pH is adjusted to 3.4 or less.

  The gas sensor element manufacturing method of the present invention is a so-called molding method in which a porous protective layer is formed around a detection portion to manufacture a gas sensor element by injecting slurry as a material for the porous protective layer into a mold. In addition to being able to form the thickness of the porous protective layer uniformly around the entire periphery of the detector by applying this molding method, the film formability is improved by improving the slurry used Therefore, it is a manufacturing method that can form a porous protective layer that is difficult to crack.

  The slurry to be injected into the mold is a slurry in which a solid content of ceramic particles is contained in a slurry solution, and the solid content is 77% by mass or more and has a pH of 3.4 or less.

  As the ceramic particles, alumina particles, silicon carbide particles, aluminum nitride particles, or the like can be applied. For example, particles in which noble metal catalyst particles are supported on the alumina particles are included in the slurry solution. As the noble metal catalyst, palladium or rhodium alone or two or more kinds of alloys of palladium, rhodium and platinum can be applied.

  Also, as the slurry solution, a slurry solution is produced from silica sol or nitric acid whose pH is adjusted, and the slurry is prepared so that the solid content of ceramic particles is 77 mass% or more and the pH is 3.4 or less. .

  According to the verification by the present inventors, a porous protective layer formed by applying a molding method and using a slurry having a solid content of 77% by mass or more and a pH of 3.4 or less is resistant to impact after impact durability. It has been demonstrated that the porous protective layer is excellent in water repellency and hardly cracks.

  It is desirable that the solid content of the ceramic particles is prepared in the range of 79 to 90% by mass. This is because if the solid content exceeds 90% by mass when the molding method is applied, the fluidity of the slurry is deteriorated and casting becomes difficult.

  By this manufacturing method, the thickness of the porous protective layer can be formed, for example, with a thickness of about 80 μm or less around the entire detection portion without thickness distribution. Of course, it is also possible to form the film by changing the film thickness for each part of the detection part.For example, a part having a film thickness of 70 μm, a part of 10 μm, etc. A porous protective layer having a film thickness may be formed. Thus, the porous protective layer formed to have a desired film thickness distribution under control is completely different from the porous protective layer having the film thickness distribution formed by the dip method. It is.

  Environmental safety regulations have their own standards around the world. For example, in Europe, it is required to be below the sensor activation time that satisfies the EuroVI emission emission regulations that apply after 2014. ing. Increasing the thickness of the porous protective layer improves water resistance (water cracking prevention performance), but on the other hand, sensor activation time becomes longer. However, according to the gas sensor provided with the gas sensor element obtained by the manufacturing method of the present invention described above, the environmental standards of each country or each region where the standards are expected to become increasingly strict in the future including the above-mentioned EuroVI. It is possible to sufficiently satisfy the moisture resistance and the sensor activation time, which can reduce the environmental impact load and provide an environment-friendly vehicle that is desired to spread worldwide. It can contribute.

  As can be understood from the above description, according to the method for manufacturing a gas sensor element of the present invention, a molding method in which a slurry that is a material of a porous protective layer is injected into a molding die is applied, and further, the molding is injected into the molding die. The slurry is a slurry in which the solid content of ceramic particles is contained in the slurry solution, and the solid content is 77 mass% or more, and the pH is adjusted to 3.4 or less. In addition to being able to form a porous protective layer with a uniform thickness (thickness of the film is different for each part), the water resistance after impact durability is improved, and it is difficult to crack A gas sensor element having a protective layer can be manufactured.

It is the schematic diagram explaining the gas sensor element of this invention. It is the schematic diagram explaining the manufacturing method of the gas sensor element in order of (a), (b). It is a figure which shows the relationship between the solid content rate in the slurry for porous protective layer formation, and the frequency | count of dripping which leads to an element crack among the experimental results which measured the sensor water exposure life after impact durability. It is a figure which shows the relationship between the pH of the slurry for porous protective layer formation, and the frequency | count of dripping which leads to an element crack among the experimental results which measured the sensor water exposure lifetime after impact durability.

  Hereinafter, embodiments of a gas sensor element manufacturing method and a gas sensor element manufactured by the manufacturing method according to the present invention will be described with reference to the drawings.

(Gas sensor element)
The gas sensor element 100 shown in FIG. 1 protects the surroundings of the detection unit 10 that detects the oxygen concentration in the exhaust gas from moisture in the exhaust gas, and the moisture reaches the detection unit 10. It is mainly composed of a porous protective layer 20 that suppresses the occurrence of output abnormality due to the temperature drop of the detection unit 10 and traps passing hydrogen gas, carbon monoxide gas, and the like.

  The detection unit 10 surrounds the measured gas side electrode 41 through the measured gas space 8 and the solid electrolyte layer 3 having a pair of electrodes 4 including a measured gas side electrode 41 and a reference gas side electrode 42 on both sides. A porous diffusion resistance layer 2 that forms a gas layer 8 to be measured together with the porous diffusion resistance layer 2, and a reference gas space protection layer that surrounds the reference gas side electrode 42 through the reference gas space 9. 5, a heat source 6 and a heat source substrate 7.

  The heat source 6 includes a heater serving as a heat generator inside, and is heated and controlled so as to form a heating region of the gas sensor element 100 and reach its activation temperature.

  In the cross-sectional shape shown in the figure, the detection unit 10 has a corner cut out in a tapered shape, and this cutout ensures the thickness of the porous protective layer 20 at that portion of the detection unit 10.

  The solid electrolyte layer 3 is made of zirconia, and both the measured gas side electrode 41 and the reference gas side electrode 42 are made of platinum. The shielding layer 1 and the reference gas space protection layer 5 both have a gas-impermeable internal structure, and are both made of alumina.

  A voltage having a linear correlation between the oxygen concentration difference and the current is applied to the pair of electrodes 4, the measurement gas is brought into contact with the measurement gas side electrode 41, and a reference gas such as the atmosphere is applied to the reference gas side electrode 42. The current value generated between the electrodes is measured according to the difference in oxygen concentration between the two, and the air-fuel ratio of the vehicle engine can be specified based on the measured current.

  The porous diffusion resistance layer 2 is provided at a position that defines the measurement gas space 8 around the measurement gas side electrode 41 in order to suppress the introduction amount of the measurement gas to the measurement gas side electrode 41. Hydrogen gas, carbon monoxide gas, oxygen gas, or the like constituting the exhaust gas introduced through the porous protective layer 20 around the detection unit 10 further enters the measured gas space 8 through the porous diffusion resistance layer 2. It has been introduced.

  The porous protective layer 20 is a porous layer made of alumina particles on the surface of which noble metal catalyst particles (not shown) are supported. The distribution mode of the noble metal catalyst particles in the porous protective layer 20 is the same as that of the porous protective layer 20. It may be a region, or may be only a side region corresponding to the porous diffusion resistance layer 2 adjacent to the measured gas side electrode 41. Further, even if the amount of noble metal catalyst particles supported in the porous protective layer 20 is distributed, for example, a relatively large amount of noble metal catalyst particles may be supported in a region corresponding to the porous diffusion resistance layer 2. Good. Here, as the noble metal catalyst particles, palladium or rhodium is preferably used alone or as an alloy of two or more of palladium, rhodium and platinum.

The alumina particles constituting the porous protective layer 20 are prepared with a small diameter (for example, an average particle diameter of about 10 μm or less) and a low specific surface area (for example, about 10 m ² / g or less) as much as possible. In addition, by forming the porous protective layer 20 from alumina particles having a low specific surface area, it is possible to form the gas sensor element 10 having a short sensor activation time and good water resistance.

  The thickness of the porous protective layer 20 shown in the figure is a uniform thickness (for example, about 70 μm) around the entire circumference of the detection unit. It may be a layer. For example, the film thickness of the part corresponding to the notch on the side of the heat source substrate 7 may be about 10 μm, and the film thickness of other parts may be about 70 μm.

  The gas sensor element 100 shown in the figure is fixed in the housing through an insulator made of, for example, an insulating material, and an element cover is provided at the tip of the housing to form a gas sensor.

(Gas sensor element manufacturing method)
Next, a method for manufacturing the gas sensor element will be described with reference to FIG. 2a and 2b are flowcharts of the method for manufacturing the gas sensor element in that order. In the manufacturing method shown in the figure, a porous protective layer forming slurry S contained in a container Y is injected into a cavity C of a mold K to form a porous protective layer around a detection portion, and a gas sensor element is formed. This is done by applying the molding method to be manufactured.

  By applying the molding method, there is a high possibility that a film thickness distribution generated in the porous protective layer, which is a problem when applying the conventional general dip method, and a portion having insufficient water resistance due to this will occur. Thus, a porous protective layer having a uniform film thickness can be formed on the entire circumference of the detection unit. It is easy to control the film thickness by changing the film thickness for each part of the detection part, and the porous protective layer formed in this way has a film thickness distribution due to low predictability in the case of the dip method. Is completely different.

  Here, the slurry S injected into the cavity C is a slurry solution produced from silica sol, nitric acid or the like, and contains a solid content of ceramic particles of 77 mass% or more, and is adjusted to a pH of 3.4 or less.

  Generally, when the solid content ratio in the slurry increases, the viscosity of the slurry increases, and the film formability in the mold tends to be poor, but the viscosity of the slurry in the illustrated example is adjusted to pH 3.4 or less. The film forming property is reduced and the film formability is improved. Moreover, when the solid content rate in the slurry is as high as 77% by mass or more, the shrinkage rate of the slurry is lowered, and this increases the cracking suppression effect. Thus, by defining the pH and solid content ratio of the slurry, it is possible to form a porous protective layer that has good film formability and is less prone to cracking.

  After injecting the slurry S into the cavity C as shown in FIG. 2a, a gas sensor element precursor 100 'having a columnar shape and having a detector is inserted into the cavity C as shown in FIG.

  In the insertion posture of the gas sensor element precursor 100 ′, a separation between the wall surface of the cavity C and the precursor 100 ′ is defined, and thereby a porous protective layer whose thickness is precisely controlled is provided around the detection unit. Can be formed. Of course, regarding the method of manufacturing the gas sensor element, it is possible to use a method opposite to the illustrated example, that is, a method of injecting slurry after the gas sensor element is accommodated in the cavity.

[Experiments and results of sensor wet life after impact durability]
The inventors create slurries with various blends shown in Table 1 below, and apply various molding methods around the members corresponding to the detectors to form a porous protective layer and produce various gas sensor element test specimens. did.

  A pendulum test was applied to each gas sensor element test body as an impact resistance test, and an impact with an acceleration of 7400 G was applied.

  The gas sensor element test body after the impact resistance test was heated to 700 ° C., 0.3 μL of water droplets were dropped a maximum of 130,000 times, and the number of drops until the element cracked was measured. Hereinafter, Table 2 shows the solid content ratio and pH of the slurry in each gas sensor element test piece, and the number of times until water cracking after impact durability. Moreover, FIG. 3 is a diagram showing the relationship between the solid content in the slurry for forming the porous protective layer and the number of times of dripping leading to element cracking, among the experimental results of measuring the sensor wet life after impact durability, FIG. 4 is a graph showing the relationship between the pH of the slurry for forming the porous protective layer and the number of times of dripping leading to element cracking, among the experimental results of measuring the sensor wet life after impact durability.

  In Table 2, since all of the test specimens of the examples had no element cracking at 130,000 times, this evaluation was evaluated as “good”, while all of the test specimens of the comparative examples could confirm element cracking at 1000 times or less. This evaluation is set as x. From the table and FIGS. 3 and 4, the porous protective layer formed from the slurry having a solid content of 77 mass% or more (preferably 79 mass% or more) and a pH of 3.4 or less is obtained. Compared to porous protective layers made of slurries prepared in other numerical ranges, the number of times of water cracking after impact durability has improved dramatically from 1000 times to 130,000 times and has excellent crack resistance. It has been demonstrated to be a porous protective layer. In addition, the solid content contained in the slurry solution is 79 to 90% by mass in consideration of the reason that the casting of the slurry becomes extremely difficult when the solid content rate exceeds 90% by mass when the molding method is applied. It is desirable to stipulate in the range.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Shielding layer, 2 ... Porous diffusion resistance layer, 3 ... Solid electrolyte layer, 4 ... A pair of electrodes, 41 ... Gas to be measured side electrode, 42 ... Reference gas side electrode, 5 ... Reference gas space protective layer, 6 ... Heat source (heater), 7 ... Heat source substrate, 8 ... Gas space to be measured, 9 ... Reference gas space, 10 ... Detector, 20 ... Porous protective layer, 100 ... Gas sensor element, 100 '... Gas sensor element precursor, S ... Slurry, K ... Mold

Claims (1)

A method of manufacturing a gas sensor element in which a solid electrolyte body having a pair of electrodes on both sides and a heating element including a heat source are stacked to form a detection unit, and a porous protective layer is provided around the detection unit. And
The detection part is accommodated in the mold and the slurry for forming the porous protective layer is injected, or the slurry for forming the porous protective layer is injected into the mold and the detection part is accommodated, and the slurry is A porous protective layer is formed by curing the slurry around the detection unit by curing,
The slurry is a method for producing a gas sensor element in which a solid content of ceramic particles is contained in a slurry solution, the solid content is 77 mass% or more, and the pH is adjusted to 3.4 or less.

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