JP2013242231A - Method of depositing protection film of gas sensor element, and gas sensor element including protection film formed thereby - Google Patents

Abstract

A method of forming a protective film for a gas sensor element capable of suppressing the occurrence of peeling, cracking and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element.
A protective film material is applied in advance to a portion (thinnest portion) where the thickness of the protective film is minimum in a predetermined range on the outer peripheral surface of a gas sensor element to which a protective film is to be disposed. By applying the protective film material over the entire surface, the thickness of the protective film at the site where the measured gas permeates the protective film (introduction site) when the measured gas is introduced into the gas sensor is more than necessary. Suppresses thickening.
[Selection] Figure 2

Description

  The present invention relates to a method for forming a protective film of a gas sensor element. More specifically, the present invention relates to a method for forming a protective film of a gas sensor element that can suppress the occurrence of peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. Furthermore, this invention relates also to the gas sensor element provided with the protective film formed by the said method.

  Conventionally, a gas sensor for detecting the concentration of a specific gas component (for example, oxygen, nitrogen oxide, ammonia, hydrogen, etc.) contained in exhaust gas discharged from an internal combustion engine such as a vehicle engine has been arranged in the exhaust gas flow path. It is known to control the combustion of the internal combustion engine and the exhaust gas purification device using the detection result of the gas sensor. As a gas sensor element used in such a gas sensor, for example, in the case of an oxygen sensor or an A / F (air-fuel ratio) sensor, a solid electrolyte layer having a flat plate-like oxygen ion conductivity and one of the solid electrolyte layers A measurement gas side electrode formed on the surface and in contact with the measurement gas; a porous diffusion resistance layer formed on the measurement gas side electrode and permeable to the measurement gas; and the other surface of the solid electrolyte layer A reference gas side electrode formed and in contact with the reference gas, a reference gas chamber forming layer having a reference gas chamber formed on the reference gas side electrode and introducing a reference gas, and an insulating base body having a heating element therein And a laminated gas sensor element.

  By the way, the exhaust gas as the gas to be measured includes, for example, components derived from engine oil such as phosphorus (P), calcium (Ca), zinc (Zn), silicon (Si), potassium (K), sodium ( It contains poisonous substances composed of components derived from gasoline additives such as Na) and lead (Pb). If the gas side electrode to be measured and the porous diffusion resistance layer provided in the multilayer gas sensor element as described above are contaminated by these poisonous substances, there is a risk of problems such as a decrease in response of the gas sensor and abnormal output. . Further, the exhaust gas also contains water vapor, which may condense into water droplets and adhere to the stacked gas sensor element.

  In addition, the laminated gas sensor element as described above is activated by heating to a high temperature (for example, a temperature of 700 ° C. or higher) by a heating means such as a heating element incorporated in the gas sensor element, and the solid electrolyte layer To exhibit ion conductivity with respect to specific ions. Therefore, when water droplets generated by condensation of water vapor contained in the gas to be measured adhere to (be wet with) the stacked gas sensor element as described above, a large thermal shock is applied to the stacked gas sensor element, so There is also a risk of cracking.

  Therefore, in this technical field, by forming a porous protective film having a predetermined thickness on the outer peripheral surface of the gas sensor element, the poisoning substance is captured in the porous protective film to prevent malfunction of the gas sensor element. In other words, it is known that the adhered water droplets are dispersed in the porous protective film to reduce thermal shock and prevent the entire device from cracking (for example, Patent Documents 1 and 2). See).

  The porous protective film as described above is, for example, a slurry in which a heat-resistant ceramic powder such as alumina having a predetermined particle size distribution is dispersed in a dispersion medium such as water or an organic solvent together with an inorganic binder and a dispersant. A portion of the gas sensor element that is exposed to the gas to be measured is immersed in the slurry (dipping) to form a film of the slurry (or paste) on the outer peripheral surface of the gas sensor element, and this is dried. It can be obtained by sintering (see, for example, Patent Document 1).

  By the way, in many cases, the cross-sectional shape of the gas sensor element including the laminated gas sensor element as described above is substantially rectangular. Even in such a gas sensor element having a substantially rectangular cross section, as described above, a slurry or paste in which a protective film forming material such as alumina, a dispersant, an inorganic binder, and the like are dispersed in a dispersion medium such as water or an organic solvent, For example, a porous protective film is formed by applying by wet methods such as dipping, brushing, spraying, and printing.

  However, in the case of a gas sensor element having such a substantially rectangular cross section, for example, due to the influence of surface tension or the like, in each plane constituting the outer peripheral surface of the gas sensor element (corresponding to each side constituting the substantially rectangular cross section), It is known that the thickness of the porous protective film is the thickest at the center portion of each plane, and is thin in the vicinity of a ridge line (corresponding to a corner in a substantially rectangular cross section) where each plane intersects.

  When the film thickness distribution as described above occurs in the porous protective film, when the protective film contracts due to drying and sintering of the porous protective film, the shrinkage amount increases as the film thickness increases (thicker). . As a result, a portion with a small (thin) film thickness (for example, a ridge line portion) is subjected to tensile stress accompanying shrinkage of the thick portion. When the strength of the tensile stress exceeds the tensile strength of the thin portion, the protective film is divided at the portion. When the porous protective film is divided in this way, as a result, the adhesion strength between the gas sensor element and the porous protective film is lowered, and the porous protective film is easily peeled off.

  Therefore, in this technical field, a multiple ridge surface portion including at least two ridge surfaces is provided along the longitudinal edge of the gas sensor terminal (in other words, the cross-sectional shape of the gas sensor element is changed from a substantially rectangular shape to, for example, By reducing the thickness fluctuation range in the film thickness distribution generated in the porous protective film, the peeling of the porous protective film covering the surface of the gas sensor element is suppressed. It has been proposed to provide a gas sensor element with excellent durability (see, for example, Patent Document 3).

  Further, on the outer peripheral surface of the gas sensor element, the location where the thickness of the porous protective film is maximized, the range of the thickness of the porous protective film at the location, and the location where the thickness of the porous protective film is minimized and the porosity at the location. It has also been proposed to sufficiently protect the porous protective film and the gas sensor element from cracks, cracks, etc. due to moisture, while prescribing the range of the thickness of the quality protective film and suppressing the increase in manufacturing cost. (For example, see Patent Document 4).

  However, as described above, when the thickness of the protective film on the outer peripheral surface of the gas sensor element is minimized (hereinafter, may be referred to as the “thinnest portion”), the protective film thickness is within a predetermined range. The thickness of the protective film in a portion other than the thinnest portion may be unnecessarily thick, which may adversely affect the performance as a gas sensor element. For example, when a gas to be measured is brought into contact with a detection site (for example, a gas to be measured side electrode) that detects a specific gas component to be detected by the gas sensor element by introducing the gas to be measured into the gas sensor element. If the thickness of the protective film at the introduction site corresponding to the portion where the measurement gas permeates the protective film becomes larger than necessary, the detection sensitivity and response speed of the gas sensor element may be reduced.

  As described above, in this technical field, formation of a protective film for a gas sensor element that can suppress the occurrence of peeling, cracking, and cracking of the protective film for the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. There is a need for a method.

JP 2006-126077 A JP 2007-121323 A JP 2010-107409 A JP 2009-080110 A

  As described above, in this technical field, the formation of a protective film for a gas sensor element capable of suppressing the occurrence of peeling, cracking, and cracking of the protective film for the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. There is a need for a method. The present invention has been made to meet such a demand. That is, the present invention provides a method for forming a protective film of a gas sensor element that can suppress the occurrence of peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. Is one purpose.

The above one purpose is
A method of forming a protective film, which forms a protective film in a predetermined range of the outer peripheral surface of the gas sensor element,
A first application step of applying a first protective film to a specific part included in the predetermined range; and after the first application step, applying the first protective film to the entire predetermined range A second coating step,
Including
The thickness of the first protective film is the smallest when the specific portion is applied with the first protective film over the predetermined range only by the second coating process without the first coating process. A part,
This is achieved by a method for forming a protective film.

  According to the present invention, there is provided a method for forming a protective film of a gas sensor element that can suppress the occurrence of peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. Can do.

It is a schematic diagram explaining the film-forming method of the protective film which concerns on one embodiment of this invention. It is a schematic diagram which compares the structure of the gas sensor element which concerns on one embodiment of this invention, and the structure of the gas sensor element which concerns on a prior art. It is a graph which compares the thickness of the protective film in the introduction site | part of a gas sensor element, and the specific site | part with the gas sensor element which concerns on one embodiment of this invention, and the gas sensor element which concerns on a prior art. It is a graph which compares the sensitivity of a gas sensor element with the gas sensor element which concerns on one embodiment of this invention, and the gas sensor element which concerns on a prior art.

  As described above, one object of the present invention is to provide a protective film for a gas sensor element that can suppress the occurrence of peeling, cracking, and cracking of the protective film for the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. It is to provide a film forming method. As a result of diligent research to achieve the above object, the present inventor protects a portion (thinnest portion) where the thickness of the protective film is minimum within a predetermined range of the outer peripheral surface of the gas sensor element on which the protective film is to be disposed. By applying the material of the film in advance and then applying the material of the protective film again over the entire predetermined range, the measured gas passes through the protective film when the measured gas is introduced into the gas sensor ( The present inventors have found that the thickness of the protective film at the introduction site) can be suppressed from becoming unnecessarily thick, and have come up with the present invention.

That is, the first embodiment of the present invention is:
A method of forming a protective film, which forms a protective film in a predetermined range of the outer peripheral surface of the gas sensor element,
A first application step of applying a first protective film to a specific part included in the predetermined range; and after the first application step, applying the first protective film to the entire predetermined range A second coating step,
Including
The thickness of the first protective film is the smallest when the specific portion is applied with the first protective film over the predetermined range only by the second coating process without the first coating process. A part,
This is a method for forming a protective film.

  As described above, the method for forming a protective film according to this embodiment is a method for forming a protective film in which a protective film is formed in a predetermined range on the outer peripheral surface of the gas sensor element. A first application step of applying a first protective film to a specific part included in the range, and a second application step of applying the first protective film to the entire predetermined range after the first application step ,including. The gas sensor element to which the protective film formed by the protective film forming method according to the present embodiment is applied is not particularly limited. For example, the specific gas sensor element included in the exhaust gas discharged from an internal combustion engine such as a vehicle engine is used. A gas sensor element that detects the concentration of the gas component can be used.

  Examples of the gas sensor element include an oxygen sensor and an A / F (air / fuel ratio) sensor as described above. Further, specific examples of these gas sensor elements include a solid electrolyte layer having oxygen ion conductivity, a measured gas side electrode formed on one surface of the solid electrolyte layer and in contact with the measured gas, and the measured A porous diffusion resistance layer formed on the gas side electrode and permeable to the gas to be measured; a reference gas side electrode formed on the other surface of the solid electrolyte layer and in contact with the reference gas; and on the reference gas side electrode Examples include a laminated gas sensor element formed by laminating a reference gas chamber forming layer having a reference gas chamber that is formed and introducing a reference gas, and an insulating substrate having a heating element therein.

  By the way, when the gas to be measured is exhaust gas, as described above, the gas to be measured is derived from engine oil such as phosphorus (P), calcium (Ca), zinc (Zn), and silicon (Si). It contains poisonous substances composed of components and components derived from gasoline additives such as potassium (K), sodium (Na), and lead (Pb). If the gas side electrode to be measured and the porous diffusion resistance layer provided in the multilayer gas sensor element as described above are contaminated by these poisonous substances, there is a possibility that it may lead to problems such as a decrease in responsiveness of the gas sensor and abnormal output. is there. Further, the exhaust gas also contains water vapor, which may condense into water droplets and adhere to the stacked gas sensor element.

  In addition, as described above, the stacked gas sensor element as described above is activated by heating to a high temperature (for example, a temperature of 700 ° C. or higher) by a heating means such as a heating element incorporated in the gas sensor element. Then, the solid electrolyte layer is made to exhibit ion conductivity with respect to specific ions. Therefore, when water droplets generated by condensation of water vapor contained in the gas to be measured adhere to (be wet with) the stacked gas sensor element as described above, a large thermal shock is applied to the stacked gas sensor element, so There is also a risk of cracking.

  As described above, the gas to be measured is not limited to the exhaust gas exhausted from the internal combustion engine, and poisonous substances and water vapor contained in the gas to be measured may cause problems such as a decrease in responsiveness of the gas sensor element and abnormal output. There is a risk of inviting or problems such as destruction of the gas sensor element due to thermal shock due to water. Therefore, in this technical field, as described above, by forming a porous protective film on the outer peripheral surface of the gas sensor element, the poisoning substance is captured in the porous protective film to prevent malfunction of the gas sensor element. In addition, it is known to disperse the adhered water droplets in the porous protective film to reduce thermal shock and prevent the gas sensor element from cracking.

  However, as described above, for example, when a slurry or paste obtained by dispersing ceramic particles constituting the porous protective film in a dispersion medium is applied to the outer peripheral surface of the gas sensor element, for example, due to the influence of surface tension or the like, the gas sensor In each plane constituting the outer peripheral surface of the element, the thickness of the porous protective film is the thickest at the central portion of each plane, and is thin in the vicinity of the ridgeline where each plane intersects. When the film thickness distribution occurs in the porous protective film in this way, when the protective film contracts as the porous protective film dries and sinters, the film becomes relatively thicker due to contraction of the thick (thick) part. The large tensile stress exceeds the tensile strength of the thin (thin) part where the film thickness is small (for example, the ridge line part), and the protective film is divided at the part. As a result, the adhesion strength between the gas sensor element and the porous protective film is increased. The porous protective film may be easily peeled off.

  Therefore, in this technical field, as described above, an attempt is made to keep the thickness of the protective film within a predetermined range at the position (the thinnest portion) where the thickness of the protective film is the smallest on the outer peripheral surface of the gas sensor element. ing. However, in the method for forming a protective film according to the prior art, when the thickness of the thinnest part falls within a predetermined range, a part other than the thinnest part (for example, an introduction part for introducing the gas to be measured into the gas sensor element) ) Is unnecessarily thick, and for example, the detection sensitivity and response speed of the gas sensor element may be reduced.

  However, as described above, the method for forming the protective film according to this embodiment includes the first application step of applying the first protective film to the specific portion included in the predetermined range, and the first application. After the step, a second application step of applying a first protective film to the entire predetermined range is included. Here, the specific portion has a thickness of the first protective film when the first protective film is applied to the entire predetermined range only by the second coating process without the first coating process. This is the thinnest part.

  In other words, in the method for forming a protective film according to the present embodiment, the protective film is protected at a portion (thinnest portion) where the thickness of the protective film is minimum within a predetermined range of the outer peripheral surface of the gas sensor element on which the protective film is to be disposed. After the film material is applied in advance, the protective film material is applied again over the entire predetermined range. Thereby, when the gas to be measured is introduced into the gas sensor, it is possible to prevent the protective film from becoming thicker than necessary at a site (introduction site) where the gas to be measured passes through the protective film. As a result, according to the method for forming a protective film according to this embodiment, it is possible to suppress the peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing the decrease in detection sensitivity of the gas sensor element.

  By the way, as described above, the protective film to be formed by the method for forming a protective film according to this embodiment is, for example, a slurry or paste obtained by dispersing ceramic particles constituting the protective film in a dispersion medium. It can form into a film by apply | coating to the outer peripheral surface of a gas sensor element. As a specific example, as described above, a slurry or paste in which a protective film forming material such as alumina, a dispersant, an inorganic binder, and the like are dispersed in a dispersion medium such as water or an organic solvent, for example, dipping, brushing, A (porous) protective film can be formed by applying to a predetermined range of the outer peripheral surface of the gas sensor element by a wet method such as spraying or printing. More specifically, the protective film as described above, for example, disperses heat-resistant ceramic powder such as alumina having a predetermined particle size distribution in a dispersion medium such as water or an organic solvent together with an inorganic binder and a dispersant. A slurry or paste is prepared, and a predetermined range of the outer peripheral surface of the gas sensor element, such as a portion exposed to the gas to be measured of the gas sensor element, is immersed in the slurry (dipping), for example, on the outer peripheral surface of the gas sensor element. The slurry or paste film can be formed, dried and sintered.

  Moreover, in the film-forming method of the protective film according to the present embodiment, the first protective film in the first application step of applying the first protective film to a specific part included in a predetermined range of the outer peripheral surface of the gas sensor element. The method of applying is not limited to a specific method, and for example, according to various factors such as the property of the material of the first protective film and the shape of the gas sensor element, various methods well known in the art are known. The method can be appropriately selected. Specifically, the method for applying the first protective film may be a wet method such as dipping, brushing, spraying, printing, or the like.

  In the first coating step included in the method for forming a protective film according to this embodiment, as described above, the first protective film is formed at a specific portion included in a predetermined range of the outer peripheral surface of the gas sensor element. Apply. Therefore, in the first application step, for example, a protective film may be applied only to a specific portion included in a predetermined range on the outer peripheral surface of the gas sensor element by using a technique such as masking.

  Furthermore, in the method for forming a protective film according to this embodiment, as described above, after the first application step, the second application step of applying the first protective film to the entire predetermined range. Is executed. The method of applying the first protective film in the second application step is not limited to a specific method, and may be caused by various factors such as the properties of the material of the first protective film and the shape of the gas sensor element. Accordingly, various methods known in the art can be selected as appropriate. Specifically, the method for applying the first protective film may be a wet method such as dipping, brushing, spraying, printing, or the like.

  In the second coating step included in the method for forming a protective film according to this embodiment, as described above, the first protective film is applied to the entire predetermined range of the outer peripheral surface of the gas sensor element. . Therefore, in the second application step, when the predetermined range corresponds to all of the outer peripheral surface of the gas sensor element, no special contrivance is required to limit the range where the protective film is applied. On the other hand, in the second application step, when the predetermined range corresponds to a part of the outer peripheral surface of the gas sensor element, a predetermined range of the outer peripheral surface of the gas sensor element is used in combination with a technique such as masking. You may make it apply | coat a protective film only to.

  By the way, the protective film formed on the outer peripheral surface of the gas sensor element may have a multilayer structure including a plurality of different layers. For example, in the case of a gas sensor used in an oxygen sensor or an A / F (air / fuel ratio) sensor having the above-described configuration, for example, a low molecular weight component (for example, hydrogen) contained in a measurement gas is oxidized (combusted). The catalyst support layer that supports the catalyst is arranged as a protective film, and the measured value deviation due to the difference in the diffusion rate between the low molecular weight component and other components in the porous diffusion resistance layer provided in the gas sensor element occurs. Can be suppressed. In addition, in such a gas sensor element, the trap layer for capturing the poisonous substance as described above is disposed on the outer peripheral surface of the catalyst supporting layer, and the catalyst supporting layer and the gas sensor element are contaminated with the poisoning substance. Can be prevented.

That is, the second embodiment of the present invention is:
A method for forming a protective film according to the first embodiment of the present invention, comprising:
A third application step of applying a second protective film to the entire predetermined range after the second application step;
Further including
This is a method for forming a protective film.

  As described above, the method for forming a protective film according to this embodiment further includes a third coating step of coating the second protective film over the entire predetermined range after the second coating step. In other words, the protective film formed on the outer peripheral surface of the gas sensor by the protective film forming method according to the present embodiment has a multilayer structure including at least two different layers. Thereby, the gas sensor to which the method for forming a protective film according to this embodiment is applied is not limited to the catalyst support layer and the trap layer as described above, for example, and includes at least two protective films on the outer peripheral surface thereof. Can do. As a matter of course, the number of layers of the protective film included in the multilayer structure is not limited to two, and the number required depending on the use of the gas sensor element and the function to be exhibited by the protective film. It goes without saying that these layers can be provided.

  By the way, as described above, the protective film to be formed by the method for forming a protective film according to the present invention includes, for example, a protective film forming material such as alumina, a dispersant, an inorganic binder, and the like such as water or an organic solvent. The slurry or paste dispersed in the dispersion medium is applied to a predetermined range on the outer peripheral surface of the gas sensor element by a wet method such as dipping, brushing, spraying, or printing, for example, and the slurry or paste. This film can be formed, dried and sintered. In addition, even if the protective film to be formed by the method for forming a protective film according to the present invention is not a ceramic film that is generally formed through a sintering process, the gas sensor can be formed by a wet method as described above. It is desirable that the method for forming the protective film includes a drying step of drying the applied protective film.

Therefore, the third embodiment of the present invention
A method for forming a protective film according to any one of the first and second embodiments of the present invention,
A drying step of drying the applied protective film after at least one of the first coating step, the second coating step, and the third coating step;
Further including
This is a method for forming a protective film.

  As described above, the method for forming a protective film according to this embodiment is applied after at least one of the first application process, the second application process, and the third application process. A drying step of drying the protective film. Thus, in the method for forming a protective film according to this embodiment, when the protective film to be formed is applied to the outer peripheral surface of the gas sensor by the wet method as described above, the applied protective film is used. The film can be dried to quickly stabilize the shape of the protective film, or to facilitate the formation of a further protective film.

  By the way, as described above, the protective film to be formed by the protective film forming method according to the present embodiment is, for example, a slurry or paste obtained by dispersing ceramic particles constituting the protective film in a dispersion medium. It can form into a film by apply | coating to the outer peripheral surface of a gas sensor element.

That is, the fourth embodiment of the present invention is
A method of forming a protective film according to any one of the first to third embodiments of the present invention,
The protective film material is a slurry or paste comprising ceramic particles,
The protective film is a porous ceramic film;
This is a method for forming a protective film.

  As described above, in the method for forming a protective film according to this embodiment, the material of the protective film is a slurry or paste containing ceramic particles, and the protective film is a porous ceramic film. As a specific example of the material of the protective film in the method for forming the protective film according to this embodiment, as described above, the protective film forming material such as alumina, the dispersant, the inorganic binder, and the like are dispersed in water or an organic solvent. Examples thereof include a slurry or paste dispersed in a medium. The material of the protective film can be applied to a predetermined range on the outer peripheral surface of the gas sensor element by a wet method such as dipping, brushing, spraying, or printing. The protective film thus formed can be a porous ceramic film.

  By the way, as mentioned above, when the protective film to be formed by the method for forming a protective film according to the present invention is the porous ceramic film as described above, the protective film is formed through a sintering process. It is common to be done.

Accordingly, the fifth embodiment of the present invention provides:
A method for forming a protective film according to the fourth embodiment of the present invention, comprising:
A firing step of firing the dried protective film after at least one of the drying steps;
Further including
This is a method for forming a protective film.

  As described above, the method for forming a protective film according to this embodiment further includes a baking step of baking the dried protective film after at least one of the drying steps. Thereby, in the method for forming a protective film according to the present embodiment, the porous ceramic film is formed by firing a slurry or paste containing ceramic particles applied to the outer peripheral surface of the gas sensor element as a material of the protective film. It can be formed as a protective film. As described above, the porous ceramic film captures poisonous substances to prevent malfunction of the gas sensor element, or disperses the attached water droplets inside to relieve the thermal shock and cause cracks in the entire element. It is a protective film suitable for preventing the above.

  By the way, as described so far, the present invention provides a protective film for a gas sensor element that can suppress the occurrence of peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. Relates to the film forming method. In addition, as described at the beginning, the present invention also relates to a gas sensor element including a protective film formed by the film forming method.

  Therefore, several embodiments of the present invention as a gas sensor element having a protective film formed by the method for forming a protective film according to various embodiments of the present invention including the above-described embodiments are listed below. To explain. However, the matters already described in the description of the above-described embodiment of the present invention as the method for forming the protective film will not be repeated here.

First, the sixth embodiment of the present invention is:
A gas sensor element comprising a protective film formed in a predetermined range of the outer peripheral surface,
The protective film is
A first application step of applying a first protective film to a specific part included in the predetermined range; and after the first application step, applying the first protective film to the entire predetermined range A second coating step,
A method for forming a protective film containing
The thickness of the first protective film is the smallest when the specific portion is applied with the first protective film over the predetermined range only by the second coating process without the first coating process. A part,
Formed by the method of forming the protective film,
It is a gas sensor element.

In addition, the seventh embodiment of the present invention provides
A gas sensor element according to the sixth embodiment of the present invention,
The method for forming the protective film comprises:
A third application step of applying a second protective film to the entire predetermined range after the second application step;
Further including
It is a gas sensor element.

Furthermore, the eighth embodiment of the present invention provides:
A gas sensor element according to any one of the sixth and seventh embodiments of the present invention,
The method for forming the protective film comprises:
A drying step of drying the applied protective film after at least one of the first coating step, the second coating step, and the third coating step;
Further including
It is a gas sensor element.

The ninth embodiment of the present invention provides
A gas sensor element according to any one of the sixth to eighth embodiments of the present invention,
The protective film material is a slurry or paste comprising ceramic particles,
The protective film is a porous ceramic film;
It is a gas sensor element.

Furthermore, the tenth embodiment of the present invention provides:
A gas sensor element according to the ninth embodiment of the present invention,
The method for forming the protective film comprises:
A firing step of firing the dried protective film after at least one of the drying steps;
Further including
It is a gas sensor element.

  By the way, as described above, in the method for forming a protective film according to the present invention, the portion (the thinnest) in which the thickness of the protective film is the smallest in a predetermined range on the outer peripheral surface of the gas sensor element on which the protective film is to be disposed. The protective film material is applied in advance to the entire predetermined range after the protective film material is previously applied in the vicinity (specific part) of the portion. Thus, when the gas to be measured is introduced into the gas sensor when the thickness of the protective film at the specific site is sufficiently secured, the thickness of the protective film at the site (introduction site) through which the gas to be measured passes through the protective film is reduced. It can suppress becoming thicker than necessary. As a result, according to the method for forming a protective film according to the present invention, it is possible to suppress the occurrence of peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element.

  Therefore, in the gas sensor element including the protective film formed by the method for forming the protective film according to various embodiments of the present invention, the protective film material is preliminarily applied to the specific portion, and then the entire predetermined range is protected. By re-applying the material of the film, it is suppressed that the thickness of the protective film at the introduction site is unnecessarily thick, so that a decrease in the detection sensitivity of the gas sensor element is suppressed and the protective film is peeled, cracked, or cracked Occurrence is suppressed.

  In the above, it is desirable that the thickness of the protective film at the specific portion is equal to or greater than the minimum thickness required for suppressing the peeling, cracking and cracking of the protective film. On the other hand, the thickness of the protective film at the introduction site is preferably equal to or greater than the minimum thickness required to suppress the occurrence of peeling, cracking, and cracking of the protective film, similarly to the thickness of the protective film at the specific site. In addition, the thickness is desirably less than the minimum thickness that causes a decrease in detection sensitivity of the gas sensor element (desirably thin enough not to cause a decrease in detection sensitivity of the gas sensor element). Thus, as a result of intensive studies by the present inventors, a desirable range of the thickness of the protective film at the specific site and the introduction site has been found.

That is, the eleventh embodiment of the present invention is
A gas sensor element according to any one of the sixth or tenth embodiments of the present invention,
The thickness of the protective film at the introduction site where the gas to be measured passes through the protective film when the gas to be measured whose concentration is to be detected by the gas sensor element is introduced into the gas sensor. Is at least 1 and less than 2 times the thickness of the protective film at the specific site,
It is a gas sensor element.

  As described above, in the gas sensor element according to the present embodiment, the gas to be measured is protected when the gas to be measured, whose concentration is to be detected by the gas sensor element, is introduced into the gas sensor. The thickness of the protective film at the introduction site, which is a part that permeates the membrane, is one or more times and less than twice the thickness of the protective film at the specific site. Here, as described above, when the first protective film is applied to the entire predetermined range only by the second application process without the first application process, the specific part is the first part. This is the region where the thickness of the protective film is the thinnest.

  In other words, in the gas sensor element according to the present embodiment, the thickness of the protective film at the introduction site is equal to or less than twice the thickness at the specific site while sufficiently maintaining the thickness of the protection film at the specific site. limit. Thereby, in the gas sensor element which concerns on this embodiment, peeling of a protective film of a gas sensor element, generation | occurrence | production of a crack, and a crack can be suppressed, suppressing the fall of the detection sensitivity of a gas sensor element.

  In addition, from the viewpoint of maintaining the detection sensitivity of the gas sensor element, the thickness of the protective film at the specific portion is desirably the minimum thickness required to suppress the occurrence of peeling, cracking, and cracking of the protective film. . Therefore, when the thickness of the protective film at the introduction site in the gas sensor element according to the present embodiment is less than the thickness of the protective film at the specific site, there is a possibility that the protective film may be peeled off, cracked or cracked at the introduction site. is there. From this point of view, it is desirable that the thickness of the protective film at the introduction site in the gas sensor element according to the present embodiment is at least one time (that is, equal to or greater) than the thickness of the protective film at the specific site. On the other hand, when the thickness of the protective layer at the introduction site is twice or more than the thickness of the protective film at the specific site, there is a risk of problems such as a decrease in detection sensitivity of the gas sensor element. Therefore, the thickness of the protective film at the introduction site in the gas sensor element according to this embodiment is preferably less than twice the thickness of the protective film at the specific site.

  As described above, specific examples of the gas sensor element according to the present invention include an oxygen sensor and an A / F (air / fuel ratio) sensor. As a specific configuration of such a gas sensor element, a solid electrolyte layer having oxygen ion conductivity, a measured gas side electrode formed on one surface of the solid electrolyte layer and in contact with the measured gas, and the measured gas A diffusion resistance layer that is formed on the side electrode and transmits the gas to be measured; a reference gas side electrode that is formed on the other surface of the solid electrolyte layer and that is in contact with the reference gas; and is formed on the reference gas side electrode. Examples include a configuration in which a reference gas chamber forming layer having a reference gas chamber into which a reference gas is introduced is laminated.

Thus, the twelfth embodiment of the present invention is
A gas sensor element according to any one of the sixth to eleventh embodiments of the present invention,
A solid electrolyte layer having oxygen ion conductivity;
A measured gas side electrode and a reference gas side electrode respectively disposed on one main surface and the other main surface of the solid electrolyte layer;
A diffusion resistance layer disposed on a main surface opposite to the solid electrolyte layer of the measured gas side electrode and capable of transmitting the measured gas;
A reference gas chamber forming layer that is disposed on a surface of the reference gas side electrode opposite to the solid electrolyte layer and forms a reference gas chamber that is brought into contact with the reference gas side electrode by introducing a reference gas;
Comprising
It is a gas sensor element.

  As described above, the gas sensor element according to this embodiment includes a solid electrolyte layer having oxygen ion conductivity, and a gas side electrode to be measured disposed on one main surface and the other main surface of the solid electrolyte layer, respectively. And a reference gas side electrode, a diffusion resistance layer disposed on the main surface of the measurement gas side electrode opposite to the solid electrolyte layer and capable of transmitting the measurement gas, and the reference gas side electrode A reference gas chamber forming layer disposed on a surface opposite to the solid electrolyte layer and forming a reference gas chamber for introducing a reference gas and bringing it into contact with the reference gas side electrode. The gas sensor element according to the present embodiment having such a configuration has a specific component (a measurement target gas and a reference gas) to be detected in the gas to which the measurement gas side electrode and the reference gas side electrode are in contact (respectively, Specific component contained in the gas to be measured (for example, exhaust gas discharged from the internal combustion engine) based on the electromotive force generated between the gas side electrode to be measured and the reference gas side electrode in accordance with the difference in the concentration of oxygen) The concentration of (oxygen) can be detected.

  By the way, the identification contained in the gas to be measured based on the electromotive force generated between the pair of electrodes arranged with the solid electrolyte layer sandwiched according to the difference in the concentration of the specific component (oxygen) to be detected as described above. As described above, the gas sensor element that detects the concentration of the component is activated by heating the solid electrolyte layer to a high temperature (for example, a temperature of 700 ° C. or higher) by heating means such as a heating element built in the gas sensor element. In general, the solid electrolyte layer is made to exhibit ion conductivity with respect to specific ions (oxygen ions). Therefore, it is desirable that the gas sensor element according to this embodiment having the above-described configuration further includes heating means for heating the gas sensor element, particularly the solid electrolyte layer.

Accordingly, the thirteenth embodiment of the present invention provides:
A gas sensor element according to the twelfth embodiment of the present invention,
Heating means for heating the gas sensor element;
Further comprising
It is a gas sensor element.

  As described above, the gas sensor element according to this embodiment further includes a heating unit that heats the gas sensor element in addition to the components included in the gas sensor element according to the twelfth embodiment of the present invention. Thereby, in the gas sensor element according to the present embodiment, the solid electrolyte layer is activated by heating to a high temperature (for example, a temperature of 700 ° C. or more), and the ion conductivity of the solid electrolyte layer with respect to specific ions (oxygen ions). Can be expressed. The configuration of the heating means may be any as long as the gas sensor element, particularly the solid electrolyte layer can be heated. For example, the heating means may be configured as an insulating substrate having a heating element therein, and may be laminated on the reference gas chamber forming layer.

  By the way, as described above, a solid electrolyte layer having oxygen ion conductivity, a measured gas side electrode formed on one surface of the solid electrolyte layer and in contact with the measured gas, and the measured gas side electrode A diffusion resistance layer that is formed and permeates the gas to be measured; a reference gas side electrode that is formed on the other surface of the solid electrolyte layer and is in contact with the reference gas; and a reference gas that is formed on the reference gas side electrode is introduced In an oxygen sensor, an A / F (air / fuel ratio) sensor, or the like formed by laminating a reference gas chamber forming layer having a reference gas chamber to be measured, the measured gas passes through the diffusion resistance layer and the measured gas side electrode Led to. Therefore, in such a gas sensor element, the introduction portion, which is a portion that passes through the protective film when the gas to be measured is introduced into the gas sensor, corresponds to a portion where the diffusion resistance layer and the protective film are in contact with each other. Become.

That is, the fourteenth embodiment of the present invention is
A gas sensor element according to any one of the twelfth and thirteenth embodiments of the present invention,
The introduction portion corresponds to a portion where the diffusion resistance layer and the protective film are in contact with each other;
It is a gas sensor element.

  As described above, in the gas sensor element according to this embodiment, the introduction site corresponds to a portion where the diffusion resistance layer and the protective film are in contact with each other. That is, in the gas sensor element according to the present embodiment, the gas to be measured around the gas sensor element permeates the protective film and diffuses through the introduction portion corresponding to the portion where the protective film and the diffusion resistance layer are in contact with each other. It enters the resistance layer and is led to the measurement gas side electrode through the diffusion resistance layer.

  In an oxygen sensor, an A / F (air-fuel ratio) sensor, or the like having the above-described configuration, notched surfaces that form an obtuse angle with the main surface of the diffusion resistance layer are formed at both ends of the diffusion resistance layer. Generally, a contact surface between the cross section of the diffusion resistance layer constituting the cutout surface and the protective film corresponds to the introduction portion. However, the above description regarding the diffusion resistance layer, the protective film, the introduction site, and the like is merely an example, and the configuration of the gas sensor element according to the present embodiment should not be construed as being limited to the above description.

  As described above, in general oxygen sensors, A / F (air-fuel ratio) sensors, and the like, notched surfaces that form an obtuse angle with the main surface of the diffusion resistance layer are formed at both ends of the diffusion resistance layer. In addition, the diffusion resistance layer is often formed as a porous ceramic film, like the protective film. As a result, in the vicinity of both ends of the diffusion resistance layer, such a notch surface is not formed, and the reference gas chamber forming layer (and, if present, is often not formed of the same material as the protective film) There is a tendency that a thicker protective film is formed as compared with the corner (ridge line portion) of the gas sensor element on the side where the means is disposed. In other words, in a general oxygen sensor, A / F (air-fuel ratio) sensor, etc., the thickness of the protective film is the largest at the peripheral portion of the main surface of the reference gas chamber forming layer opposite to the reference gas side electrode. It tends to be thin.

That is, the fifteenth embodiment of the present invention is
A gas sensor element according to any one of the twelfth to fourteenth embodiments of the present invention,
The specific portion corresponds to a peripheral portion of a main surface of the reference gas chamber forming layer opposite to the reference gas side electrode;
It is a gas sensor element.

  As described above, in the gas sensor element according to this embodiment, the specific portion corresponds to the peripheral portion of the main surface of the reference gas chamber forming layer opposite to the reference gas side electrode. However, as described above, in the gas sensor element according to the present invention, after the protective film material is applied in advance to a specific portion, the protective film material is applied again over the predetermined range to form the protective film. The Thereby, since the thickness of the protective film in a specific site | part is fully ensured, peeling of a protective film, a crack, and generation | occurrence | production of a crack are suppressed.

  Several embodiments of the present invention will be described below with reference to the accompanying drawings. However, the following description is for illustrative purposes only, and the scope of the present invention should not be construed as being limited to the following description.

1. Method for Forming Protective Film According to the Present Invention Here, a method for forming a protective film according to one embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram for explaining a method of forming a protective film according to one embodiment of the present invention as described above.

  First, as shown in FIG. 1 (a), as a gas sensor element for forming a protective film by the method for forming a protective film according to this example, a solid electrolyte layer having oxygen ion conductivity, one of the solid electrolyte layers A measuring gas side electrode formed on the surface (upper side in FIG. 1) and in contact with the gas to be measured, and a reference gas side electrode formed on the other surface of the solid electrolyte layer and in contact with the reference gas (Shaded portion), a porous diffusion resistance layer that is formed on the measured gas side electrode (upper side in FIG. 1) and transmits the measured gas, and on the reference gas side electrode (upward in FIG. 1) A reference gas chamber forming layer having a reference gas chamber for introducing a reference gas formed on the lower side, and a heating element formed on the reference gas chamber forming layer (lower side in FIG. 1). Insulating substrate , Was adopted a formed by laminating multilayered gas sensing element.

  As shown in FIG. 1A, notched surfaces are formed at both ends of the porous diffusion resistance layer, and the cross section of the porous diffusion resistance layer constituting the notch surface corresponds to the introduction site (FIG. 1). (A part and B part in (a)). On the other hand, multiple ridge surface portions each formed of a plurality of cutout surfaces are formed at the peripheral edge portion (both end portions) of the main surface opposite to the reference gas side electrode of the reference gas chamber forming layer (FIG. 1A). C part and D part). However, when the protective film is formed on the gas sensor element by the protective film forming method according to the prior art, the protective film in the C part and the D part has the smallest thickness, and the protective film having a sufficient thickness in these parts When the is formed, the thickness of the protective film in the A part and the B part as the introduction site becomes excessive. That is, the C part and the D part correspond to the specific part described above.

  However, the method for forming the protective film according to the present embodiment is the first coating process (the white area in FIG. 1) in which the first protective film is applied to a specific portion (C part and D part in FIG. 1A). (Shown by an arrow (1)), a second coating step (shown by a hollow arrow (2) in FIG. 1) for applying a first protective film to the entire outer peripheral surface of the gas sensor element after the first coating step, and After the second application step, a third application step (indicated by a white arrow (3) in FIG. 1) for applying the second protective film to the entire outer peripheral surface of the gas sensor element is included.

  As described above, according to the method for forming the protective film according to the present embodiment, the protective film in the specific portions C and D is protected without excessively increasing the thickness of the protective film in the portions A and B that are the introduction portions. A sufficient film thickness can be secured. As a result, as represented by the dashed arrows in FIG. 1D, the introduction of the gas to be measured into the gas sensor element in the A part and the B part which are the introduction parts is not hindered. That is, according to the method for forming a protective film according to the present embodiment, the gas sensor element capable of suppressing the occurrence of peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. Can be provided.

2. Structure of Gas Sensor Element According to the Present Invention Next, the structure of the gas sensor element according to one embodiment of the present invention will be described with reference to FIG. FIG. 2 is a schematic diagram comparing the structure of the gas sensor element according to one embodiment of the present invention and the structure of the gas sensor element according to the prior art as described above. FIG. 2A shows a gas sensor element according to the present embodiment including a protective film formed by the film forming method described with reference to FIG. 1, for example, and FIG. Such a gas sensor element is represented.

  As shown in FIG. 2 (a), in the gas sensor element according to the present embodiment, the thicknesses of the protective films in the A part and the B part that are the introduction parts are not excessive, and the C part and the D part that are the specific parts. The thickness of the protective film is sufficiently secured. As a result, in the gas sensor element according to the present embodiment shown in FIG. 2A, it is possible to suppress the occurrence of peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. it can.

  On the other hand, as shown in FIG. 2B, in the gas sensor element according to the related art, when the thickness of the protective film in the C part and the D part which are specific parts is sufficiently secured, as a result, A which is the introduction part. The thickness of the protective film in the part and the part B becomes excessive. As a result, in the gas sensor element according to the prior art shown in FIG. 2 (b), the protective film of the gas sensor element can be prevented from peeling, cracking, and cracking, but the detection sensitivity of the gas sensor element is prevented from being lowered. It is not possible.

3. Next, the thickness of the protective film provided in the gas sensor element according to one embodiment of the present invention will be described with reference to FIG. FIG. 3 is a graph comparing the thickness of the protective film at the introduction part and the specific part of the gas sensor element between the gas sensor element according to one embodiment of the present invention and the gas sensor element according to the prior art as described above. More specifically, FIG. 3 shows the respective introduction sites (gray) of the gas sensor element (shaded line) according to the present embodiment shown in FIG. 2A and the gas sensor element (gray) according to the prior art shown in FIG. It is a graph showing the thickness of the protective film in A part and B part) and a specific site | part (C part and D part).

  As shown in FIG. 3, in the gas sensor element (gray) according to the prior art, the thickness of the protective film in the introduction part (A part and B part) is the same as the thickness of the protective film in the specific part (C part and D part). It is remarkably thick compared. On the other hand, in the gas sensor element (shaded line) according to the present embodiment, the thickness of the protective film at the introduction site (A portion and B portion) is slightly larger than the thickness of the protective film at the specific site (C portion and D portion). Although it is thick, it is almost the same.

  As described above, in the gas sensor element according to the present embodiment, the thickness of the protective film at the specific portion can be sufficiently ensured without the protective film at the introduction portion being excessively thick. Next, the detection sensitivity of the gas sensor element according to the present embodiment provided with the protective film having an appropriate thickness thus obtained will be described below.

4). Detection Sensitivity of Gas Sensor Element According to the Present Invention Next, the detection sensitivity of the gas sensor element according to one embodiment of the present invention will be described with reference to FIG. FIG. 4 is a graph comparing the sensitivity of the gas sensor element between the gas sensor element according to one embodiment of the present invention and the gas sensor element according to the prior art as described above. In the present embodiment, the maximum inclination of the detection signal that fluctuates corresponding to the oxygen concentration in the state where imbalance occurs in the multi-cylinder engine is adopted as an index of detection sensitivity.

  As shown in FIG. 4, the gas sensor element according to one embodiment of the present invention showed significantly higher detection sensitivity than the gas sensor element according to the prior art. This is because, as described above, in the gas sensor element according to the present example, the thickness of the protective film at the specific site can be sufficiently secured without the protective film thickness at the introduction site being excessive. It is thought to be caused. That is, according to the present invention, there is provided a method for forming a protective film of a gas sensor element that can suppress the occurrence of peeling, cracking, and cracking of the protective film of the gas sensor element while suppressing a decrease in detection sensitivity of the gas sensor element. This example clearly suggested that this can be done.

  As described above, according to the present invention, in the gas sensor element, it is possible to effectively suppress degradation of detection sensitivity and damage to the gas sensor element due to contamination by poisonous substances contained in the gas to be measured and water exposure due to condensed water of water vapor. can do. For the purpose of illustrating the present invention, several embodiments having specific configurations have been described. However, the scope of the present invention is not limited to these exemplary embodiments, and patents Needless to say, modifications can be made as appropriate within the scope of the claims and the description of the specification.

Claims (15)

A method of forming a protective film, which forms a protective film in a predetermined range of the outer peripheral surface of the gas sensor element,
A first application step of applying a first protective film to a specific part included in the predetermined range; and after the first application step, applying the first protective film to the entire predetermined range A second coating step,
Including
The thickness of the first protective film is the smallest when the specific portion is applied with the first protective film over the predetermined range only by the second coating process without the first coating process. A part,
A method for forming a protective film. It is the film-forming method of the protective film of Claim 1, Comprising:
A third application step of applying a second protective film to the entire predetermined range after the second application step;
Further including
A method for forming a protective film. A method for forming a protective film according to any one of claims 1 and 2,
A drying step of drying the applied protective film after at least one of the first coating step, the second coating step, and the third coating step;
Further including
A method for forming a protective film. A method for forming a protective film according to any one of claims 1 to 3,
The protective film material is a slurry or paste comprising ceramic particles,
The protective film is a porous ceramic film;
A method for forming a protective film. It is the film-forming method of the protective film of Claim 4, Comprising:
A firing step of firing the dried protective film after at least one of the drying steps;
Further including
A method for forming a protective film. A gas sensor element comprising a protective film formed in a predetermined range of the outer peripheral surface,
The protective film is
A first application step of applying a first protective film to a specific part included in the predetermined range; and after the first application step, applying the first protective film to the entire predetermined range A second coating step,
A method for forming a protective film containing
The thickness of the first protective film is the smallest when the specific portion is applied with the first protective film over the predetermined range only by the second coating process without the first coating process. A part,
Formed by the method of forming the protective film,
Gas sensor element. The gas sensor element according to claim 6,
The method for forming the protective film comprises:
A third application step of applying a second protective film to the entire predetermined range after the second application step;
Further including
Gas sensor element. The gas sensor element according to any one of claims 6 and 7,
The method for forming the protective film comprises:
A drying step of drying the applied protective film after at least one of the first coating step, the second coating step, and the third coating step;
Further including
Gas sensor element. A gas sensor element according to any one of claims 6 to 8,
The protective film material is a slurry or paste comprising ceramic particles,
The protective film is a porous ceramic film;
Gas sensor element. The gas sensor element according to claim 9, wherein
The method for forming the protective film comprises:
A firing step of firing the dried protective film after at least one of the drying steps;
Further including
Gas sensor element. The gas sensor element according to any one of claims 6 to 10,
The thickness of the protective film at the introduction site, which is a portion through which the gas to be measured permeates when the gas to be measured for detecting the concentration of the specific component by the gas sensor element is introduced into the gas sensor, 1 or more times and less than 2 times the thickness of the protective film at the site,
Gas sensor element. The gas sensor element according to any one of claims 6 to 11,
A solid electrolyte layer having oxygen ion conductivity;
A measured gas side electrode and a reference gas side electrode respectively disposed on one main surface and the other main surface of the solid electrolyte layer;
A diffusion resistance layer disposed on a main surface opposite to the solid electrolyte layer of the measured gas side electrode and capable of transmitting the measured gas;
A reference gas chamber forming layer that is disposed on a surface of the reference gas side electrode opposite to the solid electrolyte layer and forms a reference gas chamber that is brought into contact with the reference gas side electrode by introducing a reference gas;
Comprising
Gas sensor element. The gas sensor element according to claim 12,
Heating means for heating the gas sensor element;
Further comprising
Gas sensor element. The gas sensor element according to any one of claims 12 and 13,
The introduction portion corresponds to a portion where the diffusion resistance layer and the protective film are in contact with each other;
Gas sensor element. The gas sensor element according to any one of claims 12 to 14,
The specific portion corresponds to a peripheral portion of a main surface of the reference gas chamber forming layer opposite to the reference gas side electrode;
Gas sensor element.

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