JP7629827B2 - Gas sensor element manufacturing method, gas sensor element, and gas sensor - Google Patents

Description

The present invention relates to a method for manufacturing a gas sensor element, a gas sensor element, and a gas sensor.

Gas sensors for detecting the concentration of specific components (oxygen, etc.) contained in exhaust gas from an internal combustion engine are known. This type of gas sensor has an elongated, plate-like gas sensor element inside. A detection section for detecting the specific component is provided at the tip of the gas sensor element. A porous protective layer is formed at the tip of the gas sensor element to cover the detection section (see, for example, Patent Document 1).

The porous protective layer is provided for the purpose of mitigating the thermal shock applied to the gas sensor element when water droplets originating from the moisture contained in the exhaust gas come into contact with the gas sensor element. When starting an internal combustion engine, the moisture in the exhaust gas condenses into water droplets, which may fly toward the gas sensor element along with the exhaust gas. Since the gas sensor element is used in a heated state, if water droplets adhere to the gas sensor element in such a state, the water droplets instantly evaporate, and a thermal shock is applied to the gas sensor element. The porous protective layer also has the function of preventing poisonous substances (silicon, phosphorus, etc.) contained in the exhaust gas from adhering to the detection section.

For example, as shown in Patent Document 1, a gas sensor element having a multi-layered porous protective layer is known. This porous protective layer has an inner porous protective layer and an outer porous protective layer that have different porosities and densities. The inner porous protective layer is formed so as to cover the tip of the gas sensor element, and the outer porous protective layer is formed so as to be superimposed on the inner porous protective layer. The outer porous protective layer has a smaller porosity and a higher density than the inner porous protective layer. Such an outer porous protective layer and inner porous protective layer are formed on the tip of the gas sensor element, for example, by dipping (dip method).

JP 2012-93330 A

When manufacturing a gas sensor element having a multi-layered porous protective layer, the outer porous protective layer may be formed in a multi-layer structure by multiple dipping steps. For example, when the outer porous protective layer is made of a first protective layer and a second protective layer overlying the first protective layer, a two-layered outer porous protective layer is formed by at least two dipping steps.

In this case, first, in order to form the first protective layer, the tip of the gas sensor element is immersed in a predetermined slurry, and a first coating film is formed on the tip of the gas sensor element. After that, the thickness of the first coating film is appropriately adjusted, and the first coating film is dried to obtain a first dry film. Next, the tip of the gas sensor element on which the first dry film is formed is immersed in the slurry, and a second coating film is formed on the first dry film. After that, the thickness of the second coating film is appropriately adjusted, and the second coating film is dried to obtain a second dry film. After the first dry film and the second dry film are formed in this way, they are fired to obtain a porous protective layer (outer porous protective layer).

When the porous protective layer is formed through multiple dipping processes, a coating film made of slurry is formed to cover the previously formed dry film. As a result, air bubbles present in the dry film move toward the coating film, and the air bubbles that move may break, resulting in defects such as concave (crater-like) depressions on the surface of the coating film. This is presumably because some of the moisture contained in the coating film moves to soak into the dry film, and the air bubbles that were previously in the dry film are pushed toward the coating film by the soaked moisture. If such defects are formed, the thickness of the finally obtained porous protective layer (the thickness of the part where the defects are formed) may be insufficient, and the water resistance of the gas sensor element may be reduced.

The object of the present invention is to provide a manufacturing method for a gas sensor element having a porous protective layer with excellent water resistance and in which defects caused by air bubbles are suppressed, a gas sensor element, and a gas sensor.

The means for solving the above problems are as follows.
<1> A method for manufacturing a gas sensor element including a rod-shaped main body portion having a rod shape extending from front to back and including a detection portion at a tip end thereof for detecting a specific gas component, and a porous protective layer covering the tip end, the method including a first immersion step of immersing the tip end in a first slurry to form a first coating film made of the first slurry on the tip end so as to encapsulate the detection portion, a first drying step of drying the first coating film to form a first dry film on the tip end, and a second drying step of drying the tip end on which the first dry film has been formed, the second drying step being performed by drying the tip end with a solvent having the same main component as the first slurry or the same main component as the first slurry. a second dipping step of dipping the first dry film in a second slurry containing a type of dispersoid to form a second coating film made of the second slurry on the first dry film so that a rear end of the first dry film is exposed and the detection portion is enclosed therein; a second drying step of drying the second coating film to form a second dry film on the first dry film so that the end is exposed; and a firing step of firing the first dry film and the second dry film to form a sintered body of the first dry film and the second dry film that constitute the porous protective layer at the tip portion.

<2> The method for manufacturing a gas sensor element described in <1>, in which, in the first immersion step, the first coating is formed so as to cover the area between the tip surface of the rod-shaped main body and a first reference position that is a predetermined first length away from the detection unit toward the rear end side of the rod-shaped main body, and in the second immersion step, the second coating is formed so as to cover the area between the tip surface and a second reference position that is a second length away from the detection unit toward the rear end side of the rod-shaped main body, the second length being shorter than the first length.

<3> The method for manufacturing a gas sensor element according to <1> or <2>, in which, in the second immersion step, the second coating film is formed so as to cover a portion of the tip portion that will be hotter than the portion where the end portion is formed when the gas sensor element is in use.

<4> The method for manufacturing a gas sensor element according to any one of <1> to <3>, wherein the porous protective layer has an outer porous protective layer made of the sintered body of the first dry film and the second dry film, and an inner porous protective layer interposed between the outer porous protective layer and the tip portion and formed on the tip portion so as to encapsulate the detection portion, and an inner dry film for forming the inner porous protective layer is formed in advance on the tip portion immersed in the first slurry in the first immersion step so as to encapsulate the detection portion, and the first coating film made of the first slurry is formed so as to cover the inner dry film on the tip portion.

<5> The method for manufacturing a gas sensor element described in any one of <1> to <4>, further comprising a first thickness adjustment step of removing a portion of the first coating film to adjust the thickness of the first coating film before the first drying step.

<6> The method for manufacturing a gas sensor element described in any one of <1> to <5>, further comprising a second thickness adjustment step of removing a portion of the second coating film to adjust the thickness of the second coating film before the second drying step.

<7> A gas sensor element having a rod-shaped body part extending from front to back and including a detection part at its tip for detecting a specific gas component, and a porous protective layer covering the tip, the porous protective layer having a first protective layer formed over the entire circumference at least in the axial direction in the area where the detection part is provided, and a second protective layer having the same main component as the first protective layer and provided outside the first protective layer so that the rear end of the first protective layer is exposed together with the area where the detection part is provided at least in the axial direction, and the porosity of the first protective layer and the porosity of the second protective layer are approximately the same.

<8> The gas sensor element described in <7>, in which the first protective layer is formed to cover the area between the tip surface of the rod-shaped main body and a first reference position that is a predetermined first length away from the detection unit toward the rear end side of the rod-shaped main body, and the second protective layer is formed to cover the area between the tip surface and a second reference position that is a second length away from the detection unit toward the rear end side of the rod-shaped main body, the second length being shorter than the first length.

<9> The gas sensor element according to <7> or <8>, in which the second protective layer covers a portion of the tip portion that becomes hotter than the portion where the end portion is formed during use.

<10> The gas sensor element described in any one of <7> to <9>, wherein the porous protective layer includes an outer porous protective layer including the first protective layer and the second protective layer, and an inner porous protective layer interposed between the outer porous protective layer and the tip portion and formed on the tip portion so as to enclose the detection portion.

<11> A gas sensor comprising the gas sensor element according to any one of <7> to <10>.

The present invention provides a method for manufacturing a gas sensor element having a porous protective layer with excellent water resistance and in which defects caused by air bubbles are suppressed, a gas sensor element, and a gas sensor.

FIG. 1 is a cross-sectional view of a gas sensor according to a first embodiment taken along an axial direction. FIG. 2 is an exploded perspective view showing a schematic diagram of a detection element and a heater portion that constitute the gas sensor element; FIG. 2 is an enlarged schematic view of the tip side of the gas sensor element shown in FIG. 1 . An explanatory diagram of the gas sensor element as viewed from the tip side along the axial direction. AA line cross-sectional view of FIG. 4B-B line cross-sectional view 1 is a flow chart showing a method for manufacturing the gas sensor element according to the embodiment; FIG. 13 is an explanatory diagram showing a state in which the gas sensor element body is waiting above the dipping tank in the first immersion step. FIG. 10 is an explanatory view showing a state in which the tip portion of the gas sensor element body, which has been lowered from the standby position, is immersed in the first slurry in the dipping tank in the first immersion step. FIG. 11 is an explanatory diagram showing a state in which a part (excess coating film) of the first peripheral surface coating film formed on the peripheral surface of the tip portion of the gas sensor element body is removed by using a scraping die in the first thickness adjustment process. FIG. 11 is an explanatory diagram showing a state in which a part (excess coating film) of the first tip coating film formed on the tip surface of the gas sensor element body is removed by using a scraping blade in the first thickness adjustment process. FIG. 13 is an explanatory diagram showing a schematic view of the tip side of the gas sensor element body after the first drying process. FIG. 1 is a schematic diagram illustrating the second immersion step. FIG. 11 is an explanatory diagram showing a state in which a part (excess coating film) of the second peripheral surface coating film formed on the peripheral surface of the tip portion of the gas sensor element body is removed by using a scraping die in the second thickness adjustment process. FIG. 11 is an enlarged vertical cross-sectional view of the gas sensor element body in the vicinity of its tip portion after a second thickness adjustment process; FIG. 13 is an explanatory diagram showing a schematic view of the tip side of the gas sensor element body after the second drying process. FIG. 17 is a vertical cross-sectional view of the gas sensor element body after the second drying step shown in FIG. 16 .

<Embodiment 1>
A first embodiment of the present invention will be described below with reference to Figs. 1 to 17. First, a configuration of a gas sensor (oxygen sensor) 1 including a gas sensor element 100 manufactured by a manufacturing method for a gas sensor element according to this embodiment will be described. Fig. 1 is a cross-sectional view of the gas sensor 1 according to the first embodiment cut along the axis L direction, and Fig. 2 is an exploded perspective view showing a schematic diagram of a detection element portion 300 and a heater portion 200 constituting the gas sensor element 100. In this specification, the lower side of the gas sensor 1 shown in Fig. 1 is referred to as the "front end side", and the opposite side (the upper side in Fig. 1) is referred to as the "rear end side".

As shown in FIG. 1, the gas sensor 1 includes a gas sensor element 100 consisting of a laminate of a detection element portion 300 and a heater portion 200, a metal shell 30 that holds the gas sensor element 100 and other components in a manner that accommodates them inside, and a protector 24 that is attached to the tip of the metal shell 30. The gas sensor element 100 is generally in the shape of an elongated plate (rod), and is disposed so that its longitudinal direction is aligned with the direction of the axis L. As will be described later, a porous protective layer 60 is formed on the tip side of the gas sensor element 100.

2, the heater section 200 is generally in the shape of a long, thin plate, and includes a first base 101 and a second base 103, which are mainly made of alumina, and a heating element 102, which is sandwiched between the first base 101 and the second base 103 and is mainly made of platinum. The heating element 102 has a heating portion 102a located at the tip side, and a pair of heater lead portions 102b extending from the heating portion 102a along the longitudinal direction (axis L direction) of the first base 101. The ends of the heater lead portions 102b are electrically connected to the heater side pads 120 via conductors formed in the heater side through holes 101a provided in the first base 101.

The detection element section 300, like the heater section 200, is generally in the shape of a long and thin plate, and includes an oxygen concentration detection cell 130 and an oxygen pump cell 140. The oxygen concentration detection cell 130 is composed of a first solid electrolyte body 105, and a first electrode 104 and a second electrode 106 formed on both sides of the first solid electrolyte body 105. The first electrode 104 is composed of a first electrode portion 104a and a first lead portion 104b extending from the first electrode portion 104a along the longitudinal direction (axis L direction) of the first solid electrolyte body 105. The second electrode 106 is composed of a second electrode portion 106a and a first lead portion 104b extending from the second electrode portion 106a along the longitudinal direction (axis L direction) of the first solid electrolyte body 105. The second electrode 106 is composed of a second electrode portion 106a and a second lead portion 106b that extends from the second electrode portion 106a along the longitudinal direction (axis L direction) of the first solid electrolyte body 105.

The end of the first lead portion 104b is electrically connected to the detection element side pad 121 via conductors formed in the first through hole 105a provided in the first solid electrolyte body 105, the second through hole 107a provided in the insulating layer 107 described later, the fourth through hole 109a provided in the second solid electrolyte body 109, and the sixth through hole 111a provided in the protective layer 111. The end of the second lead portion 106b is electrically connected to the detection element side pad 121 via conductors formed in the third through hole 107b provided in the insulating layer 107 described later, the fifth through hole 1-9b provided in the second solid electrolyte body 109, and the seventh through hole 111b provided in the protective layer 111.

The oxygen pump cell 140 is composed of a second solid electrolyte body 109, and a third electrode 108 and a fourth electrode 110 formed on both sides of the second solid electrolyte body 109. The third electrode 108 is composed of a third electrode portion 108a and a third lead portion 108b extending from the third electrode portion 108a along the longitudinal direction (axis L direction) of the second solid electrolyte body 109. The fourth electrode 110 is composed of a fourth electrode portion 110a and a fourth lead portion 110b extending from the fourth electrode portion 110a along the longitudinal direction (axis L direction) of the second solid electrolyte body 109.

The end of the third lead portion 108b is electrically connected to the detection element side pad 121 via a conductor formed in each of the fifth through hole 109b provided in the second solid electrolyte body 109 and the seventh through hole 111b provided in the protective layer 111. The end of the fourth lead portion 110b is electrically connected to the detection element side pad 121 via a conductor formed in the eighth through hole 111c provided in the protective layer 111. The second lead portion 106b and the third lead portion 108b are at the same potential.

The first solid electrolyte body 105 and the second solid electrolyte body 109 are made of a partially stabilized zirconia sintered body obtained by adding yttria (Y ₂ O ₃ ) or calcia (CaO) as a stabilizer to zirconia (ZrO ₂ ).

The heating element 102, the first electrode 104, the second electrode 106, the third electrode 108, the fourth electrode 110, the heater side pad 120, and the detection element side pad 121 can be formed of a platinum group element. Suitable platinum group elements for forming these include Pt, Ph, Pd, etc. These platinum group elements may be used alone or in combination of two or more kinds.

From the viewpoint of heat resistance and oxidation resistance, it is preferable that the heating element 102, etc. is mainly made of Pt. It is also preferable that the heating element 102, etc. contains a ceramic component in addition to the platinum group element that is the main component. From the viewpoint of adhesion, it is preferable that this ceramic component is the same component as the main material on the laminated side.

The insulating layer 107 is formed between the oxygen pump cell 140 and the oxygen concentration detection cell 130. The insulating layer 107 is composed of an insulating portion 114 and a diffusion resistance portion 115. A hollow measurement chamber 107c is formed in the insulating portion 114 of the insulating layer 107 at a position corresponding to the second electrode portion 106a and the third electrode portion 108a. The measurement chamber 107c is connected to the outside in the width direction of the insulating layer 107, and a diffusion resistance portion 115 that realizes gas diffusion between the outside and the measurement chamber 107c under a predetermined rate-limiting condition is disposed in the connected portion.

The insulating section 114 is not limited to any particular material as long as it is a ceramic sintered body having insulating properties, and may be made of, for example, an oxide-based ceramic such as alumina or mullite.

The diffusion resistance portion 115 is a porous body made of alumina, and the speed at which the detection gas flows into the measurement chamber 107c is adjusted by the diffusion resistance portion 115 made of this porous body.

A protective layer 111 is formed on the surface of the second solid electrolyte body 109 so as to sandwich the fourth electrode 110. This protective layer 111 is composed of a porous electrode protective portion 113a for protecting the fourth electrode portion 110a from poisoning by sandwiching the fourth electrode portion 110a, and a reinforcing portion 112 for protecting the second solid electrolyte body 109 by sandwiching the fourth lead portion 110b. In addition, the gas sensor element 100 of this embodiment is an oxygen sensor element in which the direction and magnitude of the current flowing between the electrodes of the oxygen pump cell 140 are adjusted so that the voltage (electromotive force) generated between the electrodes of the oxygen concentration detection cell 130 becomes a predetermined value (for example, 450 mV), and the oxygen concentration in the measured gas is linearly detected according to the current flowing through the oxygen pump cell 140.

Returning to FIG. 1, the metal shell 30 is made of SUS430 and has a male threaded portion 31 for attaching the gas sensor 1 to the exhaust pipe and a hexagonal portion 32 to which an attachment tool is applied during attachment. The metal shell 30 is also provided with a metal side step 33 that protrudes radially inward, and the metal side step 33 supports a metal holder 34 for holding the gas sensor element 100. Inside the metal holder 34, a ceramic holder 35 and talc 36 are arranged in this order from the tip side. The talc 36 consists of a first talc 37 arranged in the metal holder 34 and a second talc 38 arranged at the rear end of the metal holder 34. The first talc 37 is compressed and filled in the metal holder 34, thereby fixing the gas sensor element 100 to the metal holder 34. The second talc 38 is compressed and filled in the metal shell 30, thereby ensuring sealing between the outer surface of the gas sensor element 100 and the inner surface of the metal shell 30. An alumina sleeve 39 is disposed on the rear end side of the second talc 38. The sleeve 39 is formed in a multi-stage cylindrical shape, and an axial hole 39a is provided along the axis L. The gas sensor element 100 is inserted into the sleeve 39 including the axial hole 39a. The crimped portion 30a on the rear end side of the metal shell 30 is bent inward, and the sleeve 39 is pressed against the front end side of the metal shell 30 via the stainless steel ring member 40 by the crimped portion 30a.

A metal protector 24 is attached by welding to the outer periphery of the tip side of the metal shell 30. The protector 24 has a double structure, with a cylindrical outer protector 41 with a uniform outer diameter on the outside and a cylindrical inner protector 42 with a rear end 42a with a larger outer diameter than the tip 42b on the inside. This protector 24 covers the tip of the gas sensor element 100 protruding from the tip of the metal shell 30 and has multiple gas intake holes 24a.

The tip side of the outer tube 25 made of SUS430 is inserted into the rear end side of the metal shell 30. The outer tube 25 has a tip portion 25a with an expanded diameter at the tip side, and the tip portion 25a is fixed to the metal shell 30 by laser welding or the like. A separator 50 is disposed inside the rear end side of the outer tube 25, and a holding member 51 is interposed in the gap formed between the separator 50 and the outer tube 25. The holding member 51 is fixed between the crimped outer tube 25 and the separator 50 while engaging with the protruding portion 50a that protrudes outward from the peripheral surface of the separator 50.

In addition, the separator 50 is provided with a through hole 50b penetrating from the front end to the rear end for inserting various lead wires 11, 12, 13 for the detection element section 300 and the heater section 200. For convenience of explanation, only three lead wires 11, 12, 13 are shown in FIG. 1, and the other lead wires are omitted. The through hole 50b accommodates a connection terminal 16 that connects the above-mentioned lead wires 11, etc. to the detection element side pad 121 of the detection element section 300 and the heater side pad 120 of the heater section 200. Each lead wire 11, etc. is configured to be connectable to a connector (not shown) on the outside, and electrical signals are input and output between an external device such as an ECU and each lead wire 11, etc. via such a connector.

Furthermore, a generally cylindrical rubber cap 52 is disposed on the rear end side of the separator 50 to close the opening 25b on the rear end side of the outer tube 25. This rubber cap 52 is secured to the outer tube 25 by crimping the outer tube 25 radially inward while housed within the rear end of the outer tube 25. The rubber cap 52 also has through holes 52a that run from the front end side to the rear end side for inserting the lead wires 11, etc.

Figure 3 is an enlarged view showing a schematic diagram of the tip side of the gas sensor element 100 shown in Figure 1. As shown in Figure 3, a porous protective layer 60 is formed on the tip side of the gas sensor element 100. In this specification, the gas sensor element 100 in a state in which the porous protective layer 60 is not formed is particularly referred to as the "gas sensor element main body (rod-shaped main body portion) 100a." Figure 4 is an explanatory diagram showing the gas sensor element 100 viewed from the tip side along the axis L direction.

The gas sensor element body 100a is rod-shaped and has a detection section 150 at its tip for detecting specific gas components (oxygen, etc.) contained in the exhaust gas. The detection section 150 is mainly composed of each electrode section (first electrode section 104a, second electrode section 106a, third electrode section 108a, and fourth electrode section 110a) of the detection element section 300, a measurement chamber 107c, and an electrode protection section 113a. FIG. 3 shows a porous electrode protection section 113a, which is a part of the detection section 150 and is formed so as to overlap with the fourth electrode section 110a in the thickness direction of the gas sensor element body 100a. A porous protection layer 60 is formed at the tip section 100b of the gas sensor element body 100a so as to cover the periphery of the detection section 150. The porous protection layer 60 has pores of a three-dimensional mesh structure to allow gas permeation.

As shown in FIG. 3, the porous protective layer 60 is formed on the tip portion 100b of the gas sensor element body 100a so as to cover the tip surface 100c and the peripheral surface 100d at the tip portion 100b of the gas sensor element body 100a.

The porous protective layer 60 can be formed by, for example, binding one or more ceramic particles selected from the group consisting of titania, alumina, spinel, zirconia, mullite, zircon, and cordierite by firing or the like. Pores can be formed between the ceramic particles by sintering a slurry containing these particles. Note that a burnable pore-forming material made of carbon, resin beads, organic or inorganic binders, etc. may be added to the slurry containing the above particles.

The porous protective layer 60 of this embodiment is a multi-layer type, and includes an inner porous protective layer 61 and an outer porous protective layer 62. FIG. 5 is a cross-sectional view taken along line A-A in FIG. 3, and FIG. 6 is a cross-sectional view taken along line B-B in FIG. 4. The inner porous protective layer 61 and the outer porous protective layer 62 differ in density, porosity (%), main components, etc. Specifically, the inner porous protective layer 61 has a structure with a lower density and a higher porosity (%) than the outer porous protective layer 62. The porosity of the inner porous protective layer 61 is, for example, 20 to 50%, and the porosity of the outer porous protective layer 62 is 10 to 20%.

The inner porous protective layer 61 is formed on the tip portion 100b of the gas sensor element body 100a so as to enclose the detection portion 150.

The outer porous protective layer 62 is formed on top of the inner porous protective layer 61 and is formed to cover the tip portion 100b of the gas sensor element body 100a. Such an outer porous protective layer 62 is formed to enclose the inner porous protective layer 61. In particular, the outer porous protective layer 62 of this embodiment is multi-layered and is formed through multiple dipping processes as described below.

The outer porous protective layer 62 comprises a first protective layer 63 formed around the entire circumference in at least the region R in the axial direction where the detection unit 150 is provided, and a second protective layer 64 whose main component is the same as that of the first protective layer 63 and which is provided on the outside of the first protective layer 63 so that the rear end 63a of the first protective layer 63 is exposed together with the region R in which the detection unit 150 is provided in at least the axial direction L.

The "region R in the direction of axis L in which the detection section 150 is provided" is the minimum area in the direction of axis L that includes each part constituting the detection section 150 (first electrode section 104a, second electrode section 106a, third electrode section 108a, fourth electrode section 110a, measurement chamber 107c, and electrode protection section 113a) when the gas sensor element body 100a is viewed in the thickness direction as shown in FIG. 3.

In addition, between the first protective layer 63 and the second protective layer 64, "the main components are the same" means that the ceramic components constituting the first protective layer 63 are the same or at least 90% the same as the ceramic components constituting the second protective layer 64.

The porosity of the first protective layer 63 and the porosity of the second protective layer 64 are approximately the same. The porosity of the first protective layer 63 and the porosity of the second protective layer 64 are lower than the porosity of the inner porous protective layer 61.

The rear end 63a of the first protective layer 63 is not covered with the second protective layer 64 and is exposed, so the rear portion of the outer porous protective layer 62 is stepped by the end 63a of the first protective layer 63 and the end 64a of the second protective layer 64. Therefore, although the first protective layer 63 and the second protective layer 64 are integrated in a sintered body state, the end 63a of the first protective layer 63 and the end 64a of the second protective layer 64 can be clearly distinguished. Therefore, even in the portion where the first protective layer 63 and the second protective layer 64 overlap each other, the boundary between the first protective layer 63 formed by the first coating film (first dry film) described later and the second protective layer 64 formed by the second coating film (second dry film) can be inferred from the thickness of the end 63a of the first protective layer 63 and the thickness of the end 64a of the second protective layer 64.

In addition, in each cross-sectional view of FIG. 5 and FIG. 6, the boundary between the first protective layer 63 and the second protective layer 64, which is inferred from the end 63a of the first protective layer 63 and the end 64a of the second protective layer 64, is virtually shown.

The gas sensor element 100 manufactured by the manufacturing method of this embodiment is used in the gas sensor 1 described above.

Next, a method for manufacturing the gas sensor element according to the first embodiment will be described. FIG. 7 is a flow diagram showing a method for manufacturing the gas sensor element according to this embodiment. As shown in FIG. 7, the method for manufacturing the gas sensor element according to this embodiment includes a first immersion process S101, a first thickness adjustment process S102, a first drying process S103, a second immersion process S104, a second thickness adjustment process S105, a second drying process S106, and a firing process S107.

The first immersion step S101 is an immersion step for forming the first protective layer 63 of the outer porous protective layer 62. Specifically, the first immersion step S101 is a step for immersing the tip portion 100b of the gas sensor element body 100a (gas sensor element 100) in a first slurry S1 to form a first coating film made of the first slurry on the tip portion 100b so as to encapsulate the detection portion 150.

The first slurry S1 is a slurry for forming the first protective layer 63, and as the first slurry S1, for example, a slurry for forming an outer porous protective layer in a conventional gas sensor element is used. The first slurry S1 is obtained by mixing, for example, titania powder, spinel powder, alumina sol, organic binder (for example, acrylic copolymer), dispersant (for example, anionic polymer dispersant), defoamer (for example, amide wax-based defoamer), and water in a predetermined ratio and mixing them for a predetermined time.

Figure 8 is an explanatory diagram showing the state in which the gas sensor element body 100a is waiting above the dipping tank 400 in the first immersion step S101. In Figure 8, the container-shaped dipping tank 400 that contains the first slurry S1 and opens upward is shown, and the gas sensor element body 100a that is waiting in a stationary state above the dipping tank 400 is shown. The gas sensor element body 100a is fixed using a specified jig (not shown) with its longitudinal direction (axis L direction) aligned vertically and its tip portion 100b facing downward. The tip portion 100b of the gas sensor element body 100a is in a state facing the first slurry S1 contained in the dipping tank 400. In Figure 8, the surface of the gas sensor element body 100a on the side where the electrode protection portion 113a, which is part of the detection portion 150, is formed is shown facing the front side of the paper.

As shown in FIG. 8, the tip 100b of the gas sensor element body 100a to be immersed in the first slurry S1 has an inner dry film 610 for forming the inner porous protective layer 61 formed in advance so as to enclose the detection portion 150. The inner dry film 610 is formed by drying an inner coating film made of the slurry formed by immersing the tip 100b of the gas sensor element body 100a in the slurry for forming the inner porous protective layer 61. In the first immersion step S101, a first coating film 630 made of the first slurry S1 is formed so as to cover the inner dry film 610 of the tip 100b. As shown in FIG. 8, the position where the gas sensor element body 100a is stationary above the dipping tank 400 is called the "standby position".

The gas sensor element body 100a is configured to be movable up and down (vertically) between a standby position and a position inside the dipping tank 400 below (a dipping position, described below) while being fixed to a specified jig. There are no particular limitations on the mechanism for moving the gas sensor element body 100a back and forth, as long as it does not impair the object of the present invention, and a known reciprocating mechanism using a servo motor or the like can be used.

8, between the gas sensor element body 100a in the standby position and the dipping tank 400, the scraping die 500 and scraping blade 600 used in the first thickness adjustment step (first scraping step) performed after the first immersion step are arranged. The scraping die 500 includes a plate-shaped scraping body 501 arranged along the horizontal direction, a through hole 502 provided inside the scraping body 501 in a form penetrating in the thickness direction, and an opening edge 503 surrounding the through hole 502. The size of the through hole 502 and the opening edge 503 is set so that the gas sensor element body 100a on which the inner dry film 610 is formed can pass through the through hole 502 in the opening edge 503. In the standby position, the tip 100b of the gas sensor element body 100a on which the inner dry film 610 is formed is arranged so that it fits inside the opening edge 503 when viewed in a plan view.

As shown in FIG. 8, the scraping blade 600 is retracted to a position outside the through hole 502 so as not to overlap with the through hole 502 in the vertical direction, and this position is referred to as the "retracted position" of the scraping blade 600. In this embodiment, the scraping blade 600 is disposed on the upper surface 500a side of the scraping die 500. The scraping blade 600 is disposed at a height position such that a gap is formed between the scraping blade 600 and the upper surface 500a of the scraping die 500.

9 is an explanatory diagram showing the state in which the tip 100b of the gas sensor element body 100a, which has descended from the standby position, is immersed in the first slurry S1 in the dipping tank 400 in the first immersion step S101. When the gas sensor element body 100a (see FIG. 8), which has been waiting at the standby position, moves downward, the tip 100b on which the inner dry film 610 is formed passes through the through hole 502 in the opening edge portion 503 of the scraping mold 500 from the top to the bottom. At that time, the tip 100b on which the inner dry film 610 is formed passes through the through hole 502 without coming into contact with the opening edge portion 503. After that, the gas sensor element body 100a moves further downward, and the tip 100b is immersed in the first slurry S1 contained in the dipping tank 400. The height position of the tip portion 100b immersed in the first slurry S1 is set so that the tip portion 100b does not come into contact with the bottom of the dipping tank 400 and the first slurry S1 covers the inner dry film 610 formed on the tip portion 100b. More specifically, the height position of the tip portion 100b immersed in the first slurry S1 is set so that the first slurry S1 covers the area R1 between the tip surface 100c of the gas sensor element body (rod-shaped body portion) 100a and the first reference position M1, which is a position from the rear end of the detection portion 150 toward the rear end side of the gas sensor element body 100a (i.e., the upper side of FIG. 9) by a predetermined first length L1 in the axial direction L. The position of the gas sensor element body 100a set in this manner is referred to as the "first immersion position". The gas sensor element body 100a may be stationary at the first immersion position for a predetermined time as necessary, or may be immediately pulled up after reaching the first immersion position. In FIG. 9, the position corresponding to the tip surface 100c in the direction of the axis L is indicated by the symbol M0.

By this first immersion process S101, the tip portion 100b of the gas sensor element body 100a is immersed in the first slurry S1, so that a first coating film 630 made of the first slurry S1 is formed so as to encapsulate the detection portion 150 and the inner dry film 610.

Such a first coating 630 is formed so as to cover the area R1 between the tip surface 100c of the gas sensor element body (rod-shaped body portion) 100a and a first reference position M1, which is a position a predetermined first length L1 away from the detection portion 150 toward the rear end side of the gas sensor element body 100a, in the direction of the axis L. The "first length L1" is determined based on the range in the longitudinal direction (direction of the axis L) of the gas sensor element body 100a from the rear end of the detection portion 150 toward the rear end side of the gas sensor element body 100a, in which at least the porous protective layer 60 needs to be formed for the purpose of ensuring water resistance, etc.

The tip portion 100b of the gas sensor element body 100a that has been immersed in the first slurry S1 is then pulled up and subjected to the next first thickness adjustment process S102.

The first thickness adjustment step S102 is a step of adjusting the thickness of the first coating 630 by scraping off the first coating 630 made of the first slurry S1 adhering to the tip 100b of the gas sensor element body 100a and removing a part of the first coating 630. As described above, after the first immersion step S101, when the tip 100b of the gas sensor element body 100a is pulled up from the first slurry S1, the first coating 630 made of the first slurry S1 adhering to the tip 100b is formed. Then, after the first immersion step S101, if the thickness of the first coating 630 formed on the tip 100b exceeds the thickness required for the first protective layer 63 (outer porous protective layer 62), the excess coating is removed by the first thickness adjustment step S102.

In this specification, the coating film formed on the peripheral surface 100d of the tip portion 100b of the first coating film 630 that has adhered to the tip portion 100b after the first immersion step S101 is referred to as the "first peripheral surface coating film 631," and the coating film formed on the tip surface 100c of the tip portion 100b is referred to as the "first tip coating film 632." In this specification, the coating film that is located further forward than the tip surface 100c of the tip portion 100b in the direction of the axis L is referred to as the first tip coating film 632.

The first thickness adjustment process S102 of this embodiment includes a process of removing a part (excess coating) 631a of the first peripheral coating 631 formed on the peripheral surface 100d of the tip 100b of the first coating 630 attached to the tip 100b using the scraping die 500 (peripheral scraping process), and a process of removing a part (excess coating) 632a of the first tip coating 632 formed on the tip surface 100c of the tip 100b using the scraping blade 600 (tip surface scraping process). In this embodiment, the peripheral scraping process is performed first, and then the tip surface scraping process is performed.

10 is an explanatory diagram showing how a part (excess coating) 631a of the first peripheral surface coating 631 formed on the peripheral surface 100d of the tip 100b of the gas sensor element body 100a is removed using the scraping mold 500 in the first thickness adjustment process S102. FIG. 10 shows the content of the peripheral surface scraping process described above. After the first immersion process S101, if the first peripheral surface coating 631 that exceeds the required thickness is attached to the tip 100b of the gas sensor element body 100a that is pulled up, the excess coating (excess coating) 631a is scraped off by the opening edge portion 503 when the tip portion 100b passes through the opening edge portion 503 (through hole 502) of the scraping mold 500 from the bottom to the top. In this way, when the tip 100b passes through the opening edge 503, the excess coating 631a is separated from the first peripheral coating 631 by the required thickness.

A part of the separated excess coating film 631a falls due to its own weight and is returned to the dipping tank 400. The scraping mold 500 is, for example, a metal plate processed into a predetermined shape. As described above, the size of the opening edge portion 503 of the scraping mold 500 (the size of the through hole 502) is set so that the tip portion 100b of the gas sensor element body 100a on which the inner dry film 610 is formed can pass through, and the first coating film 630 (first peripheral coating film 631) of a thickness necessary for forming the first protective layer 63 of the outer porous protective layer 62 is formed around the inner dry film 610 of the tip portion 100b after the peripheral scraping process. In this embodiment, the opening edge portion 503 has a rectangular shape in a plan view and is fixed using a fixing device not shown. The longitudinal direction (long side direction) of the opening edge 503 is arranged along the lateral direction (width direction) of the gas sensor element body 100a, and the lateral direction (short side direction) of the opening edge 503 is arranged along the thickness direction of the gas sensor element body 100a.

Figure 11 is an explanatory diagram showing how a part (excess coating) 632a of the first tip coating 632 formed on the tip surface 100c of the gas sensor element body 100a is removed using the scraping blade 600 in the first thickness adjustment process S102. Figure 11 shows the contents of the tip surface scraping process described above. After the peripheral surface scraping process, the gas sensor element body 100a that has passed from the bottom to the top inside the opening edge portion 503 of the scraping mold 500 moves upward so that the position of the tip surface 100c of the tip portion 100b is at a predetermined height position for the tip surface scraping process. The predetermined height position of the tip surface 100c for the tip surface scraping process is set so that the thickness of the first tip coating 632 formed on the tip surface 100c becomes the thickness required as the first protective layer 63 of the outer porous protective layer 62 after the tip surface scraping process. In this embodiment, the height position of the tip surface 100c when the tip surface scraping process is performed is set to be the same as the height position of the tip surface 100c of the gas sensor element body 100a stopped at the standby position described above. In other embodiments, the height position of the tip surface 100c when the tip surface scraping process is performed may be different from the height position of the tip surface 100c of the gas sensor element body 100a stopped at the standby position.

Figure 11 shows the gas sensor element body 100a stationary in the standby position. In this state, the tip 100b of the gas sensor element body 100a is formed with a first coating 630 including a first peripheral surface coating 631 whose thickness has been adjusted by removing excess coating 631a. If the first tip coating 632 formed on the tip surface 100c of this first coating 630 exceeds the required thickness, the excess coating (excess coating 632a) is scraped off by the scraping blade 600.

The scraping blade 600 is made of, for example, a metal plate processed into a predetermined shape. The scraping blade 600 is configured using a known reciprocating mechanism or the like so that the tip 601 can reciprocate along the horizontal direction. In the first thickness adjustment process S102, the height position of the tip 601 of the scraping blade 600 and the height position of the tip 100b of the gas sensor element body 100a are set so that the first tip coating 632 of the required thickness is left on the tip surface 100c. In FIG. 11 and other figures, the gas sensor element body 100a is arranged so that its longitudinal direction is aligned along the vertical direction (up and down direction). The axis L direction of such a gas sensor element body 100a is aligned along the vertical direction. Therefore, it can be said that the scraping blade 600 moves along a direction perpendicular to the axis L direction of the gas sensor element body 100a.

During the first immersion step and the peripheral surface scraping step, the scraping blade 600 remains stationary at the retracted position, as shown in FIG. 8 and the like. When the gas sensor element body 100a is returned to the standby position after the peripheral surface scraping step and stops, the scraping blade 600 moves horizontally to scrape off the excess coating 632a of the first tip coating 632. The scraping blade 600 moves horizontally from the retracted position toward the gas sensor element body 100a, and further moves horizontally above the opening edge 503 to cross the opening edge 503 in the longitudinal direction. When the scraping blade 600 moves horizontally to cross the opening edge 503, the tip 601 scrapes off the excess coating 632a that exceeds the required thickness from the first tip coating 632 to the required thickness. In this way, if the first tip coating 632 formed on the tip surface 100c exceeds the required thickness, the excess coating (excess coating 632a) is scraped off by the scraping blade 600. After scraping off the excess coating of the first tip coating 632, the scraping blade 600 moves horizontally in the opposite direction to when scraping, so as to return to the retracted position.

By going through the first thickness adjustment process S102 as described above, the thickness of the first coating film 630 formed on the tip portion 100b of the gas sensor element body 100a is adjusted.

After the first thickness adjustment process S102, the first coating film 630 formed on the tip portion 100b of the gas sensor element body 100a is dried in the first drying process S103 to become the first dry film 633. The first drying process S103 is a process for drying the first coating film 630 to form the first dry film 633 on the tip portion 100b of the gas sensor element body 100a.

The first drying step S103 is performed, for example, by placing the gas sensor element body 100a in a heating tank and heating it at a temperature of 80°C to 120°C for 1 to 5 minutes. FIG. 12 is an explanatory diagram showing the tip side of the gas sensor element body 100a after the first drying step S103. As shown in FIG. 12, a first dry film 633 is formed on the tip portion 100b of the gas sensor element body 100a. In the axial direction L (longitudinal direction), the first dry film 633 is formed so as to cover the area R1 between the tip surface 100c of the gas sensor element body (rod-shaped main body portion) 100a and the first reference position M1, which is a position from the rear end of the detection portion 150 toward the rear end side of the gas sensor element body 100a (i.e., the upper side of FIG. 12) by a predetermined first length L1. In FIG. 12, the position corresponding to the tip surface 100c in the axial direction L is indicated by the symbol M0. FIG. 12 also shows the rear end 633a of the first dry film 633 at the first reference position M1.

As shown in FIG. 7, the first drying step S103 is followed by the second immersion step S104. FIG. 13 is an explanatory diagram showing the contents of the second immersion step S104. The second immersion step S104 is an immersion step for forming the second protective layer 64 of the outer porous protective layer 62. Specifically, the second immersion step S104 is a step of immersing the tip portion 100b of the gas sensor element body 100a (gas sensor element 100) on which the first dry film 633 is formed, into the second slurry S2, and forming a second coating film 640 made of the second slurry S2 on the first dry film 633 so that the rear end portion 633a of the first dry film 633 is exposed and the detection portion 150 is enclosed.

The second immersion step S104 of this embodiment is performed using another dipping tank 400A prepared separately from the dipping tank 400 used in the first immersion step S101. The basic configuration of the dipping tank 400A is the same as that of the dipping tank 400. The second slurry S2 contained in the dipping tank 400A is a slurry for forming the second protective layer 64, and the second slurry is composed of "the same slurry as the first slurry S1" or "a slurry containing the same type of dispersoid as the main component of the first slurry S1".

When the same slurry as the first slurry S1 is used as the second slurry S2, the first slurry S1 to be used as the second slurry S2 is prepared separately from the first slurry S1 used in the first soaking process S101.

In addition, in this specification, "a slurry containing the same type of dispersoid as the main component of the first slurry S1" means a slurry containing dispersoids that are the same or 90% or more the same as the substances (dispersoids) dispersed in the dispersion medium (water) among the components constituting the first slurry S1.

It is preferable that the dispersion medium used in the second slurry S2 is the same type as the dispersion medium used in the first slurry S2.

Figure 13 is an explanatory diagram showing the state in which the tip 100b of the gas sensor element body 100a, which has been lowered from the standby position, is immersed in the second slurry S2 in the dipping tank 400A in the second immersion step S104. Before the second immersion step S104 is started, the gas sensor element body 100a is waiting at a standby position above the dipping tank 400A, as in the first immersion step S101. At this time, the gas sensor element body 100a is fixed using a specified jig (not shown) along its longitudinal direction (axis L direction) with its tip 100b facing downward.

When the second immersion step S104 is started, as shown in Fig. 13, the gas sensor element body 100a moves downward and passes through the through hole 502A in the opening edge portion 503A of the scraping die 500A used in the second thickness adjustment step S105 described later from the top to the bottom. At that time, the tip portion 100b on which the first dry film 633 is formed passes through the through hole 502A without coming into contact with the opening edge portion 503A. After that, the gas sensor element body 100a moves further downward, and the tip portion 100b is immersed in the second slurry S2 contained in the dipping tank 400A. The height position of the tip portion 100b immersed in the second slurry S2 is set so that the tip portion 100b does not contact the bottom of the dipping tank 400A, the rear end portion 633a of the first dry film 633 is exposed, and the second coating film 640 made of the second slurry S2 is formed on the first dry film 633 so as to enclose the detection portion 150. More specifically, the height position of the tip portion 100b immersed in the second slurry S2 is set so that the second slurry S2 covers the area R2 between the tip surface 100c of the gas sensor element body (rod-shaped body portion) 100a and the second reference position M2, which is a position from the rear end of the detection portion 150 toward the rear end side of the gas sensor element body 100a (i.e., the upper side of FIG. 13) by a second length L2 shorter than the first length L1 described above, in the axial direction L. The position of the gas sensor element body 100a set in this manner is referred to as the "second immersion position". If necessary, the gas sensor element body 100a may remain stationary at the second immersion position for a predetermined time, or may be immediately pulled up after reaching the second immersion position. In FIG. 13, the position corresponding to the tip surface 100c in the direction of the axis L is indicated by the symbol M0.

In the second immersion step S104, as in the first immersion step S101, the gas sensor element body 100a is configured to be able to move back and forth between the standby position and a position (second immersion position) inside the dipping tank 400A located below it.

As shown in FIG. 13, a cutting die 500A and a cutting blade 600A are disposed at a predetermined location above the dipping tank 400A to be used in the second thickness adjustment process (second cutting process) that is performed after the second immersion process.

By the second immersion step S104 as described above, the tip portion 100b of the gas sensor element body 100a is immersed in the second slurry S2, so that the rear end portion 633a of the first dry film 633 is exposed, and a second coating film 640 made of the second slurry S2 is formed on the first dry film 633 so as to encapsulate the detection portion 150.

Such a second coating 640 is formed in the direction of axis L so as to cover a region R2 between the tip surface 100c of the gas sensor element body (rod-shaped body portion) 100a and a second reference position M2, which is a position away from the rear end of the detection portion 150 toward the rear end side of the gas sensor element body 100a by a second length L2 that is shorter than the first length L1 described above. The "second length L2" is determined based on the range from the rear end of the detection portion 150 toward the rear end side of the gas sensor element body 100a in the longitudinal direction (direction of axis L) of the gas sensor element body 100a, where there is a particularly high demand for ensuring water resistance.

The second coating 640 is formed so as to cover the portion of the tip 100b that becomes hotter than the portion where the end 633a of the first dry film 633 is formed when the gas sensor element 100 is in use.

In the second immersion step S104, the tip portion 100b of the gas sensor element body 100a immersed in the second slurry S2 is then pulled up and subjected to the next second thickness adjustment step S105.

The second thickness adjustment process S105 is a process for adjusting the thickness of the second coating film 640 by scraping off the second coating film 640 made of the second slurry S2 adhering to the tip portion 100b of the gas sensor element body 100a and removing a portion of the second coating film 640. After the second immersion process S104, if the thickness of the second coating film 640 formed on the tip portion 100b exceeds the thickness required for the second protective layer 64 (outer porous protective layer 62), the excess portion of the coating film is removed in the second thickness adjustment process S105.

In this specification, the coating film formed on the peripheral surface 100d of the tip portion 100b of the second coating film 640 that has adhered to the tip portion 100b after the second immersion step S104 is referred to as the "second peripheral surface coating film 641," and the coating film formed on the tip surface 100c of the tip portion 100b is referred to as the "second tip coating film 642." In this specification, the coating film that is located further forward than the tip surface 100c of the tip portion 100b in the direction of the axis L is referred to as the second tip coating film 642.

The second thickness adjustment process S105 of this embodiment, like the first thickness adjustment process S102 described above, includes a process of removing a part (excess coating) 631a of the second peripheral surface coating 641 formed on the peripheral surface 100d of the tip portion 100b of the second coating film 640 attached to the tip portion 100b using the scraping die 500A (peripheral surface scraping process), and a process of removing a part (excess coating) of the second tip coating 642 formed on the tip surface 100c of the tip portion 100b using the scraping blade 600A (tip surface scraping process).

The basic configuration of the leveling die 500A is the same as that of the leveling die 500 in the first thickness adjustment process described above, and includes a plate-shaped leveling body 501A arranged along the horizontal direction, a through hole 502A provided inside the leveling body 501A in a manner penetrating the thickness direction, and an opening edge 503A surrounding the through hole 502A.

The basic configuration of the scraping blade 600A is the same as that of the scraping blade 600 in the first thickness adjustment process described above. As shown in FIG. 13, the scraping blade 600A is retracted to a position outside the through hole 502A so as not to overlap with the through hole 502A in the vertical direction. The scraping blade 600A is also arranged on the upper surface 500Aa side of the scraping mold 500A.

14 is an explanatory diagram showing how a part (excess coating) 641a of the second peripheral coating 641 formed on the peripheral surface 100d of the tip 100b of the gas sensor element body 100a is removed using the scraping mold 500A in the second thickness adjustment process S105. FIG. 14 shows the content of the peripheral scraping process in the second thickness adjustment process S105. After the second immersion process S104, if the second peripheral coating 641 that exceeds the required thickness is attached to the tip 100b of the gas sensor element body 100a that is pulled up, the excess coating (excess coating) 641a is scraped off by the opening edge 503A when the tip 100b passes through the opening edge 503A (through hole 502A) of the scraping mold 500A from the bottom to the top. In this way, when the tip 100b passes through the opening edge 503A, the excess coating 641a is separated from the second peripheral coating 641 by the required thickness.

The size of the opening edge 503A of the scraping mold 500A (the size of the through hole 502A) is set so that the tip 100b of the gas sensor element body 100a can pass through when it descends from the standby position in the second immersion step S104, and so that when the gas sensor element body 100a is pulled up from the second slurry S2, a second coating film 640 (second peripheral coating film 641) of a thickness required to form the second protective layer 64 is formed around the first dry film 633 of the tip 100b.

Then, the gas sensor element body 100a is pulled further upward, and a portion of the second tip coating 642 formed on the tip surface 100c is removed using the scraping blade 600A. The method of adjusting the thickness of the second tip coating 642 using the scraping blade 600A is basically the same as the method of adjusting the thickness of the first tip coating 632 using the scraping blade 600 in the first thickness adjustment step. The scraping blade 600A is configured using a known reciprocating mechanism or the like so that its tip 601A can reciprocate along the horizontal direction.

Figure 15 is an enlarged vertical cross-sectional view of the configuration of the gas sensor element body 100a near the tip portion 100b after the second thickness adjustment process. As shown in Figure 15, a first dry film 633 is formed to cover the inner dry film 613 formed on the tip portion 100b. Then, a second coating film 640 is formed to overlap the first dry film 633. As described above, the rear end 633a of the first dry film 633 is not covered by the second coating film 640, and protrudes further toward the rear end side of the gas sensor element body 100a than the end 640a of the second coating film 640.

In this way, when the second coating film 640 is formed on the first dry film 633, the air bubbles present in the first dry film 633 move toward the second coating film 640, and the bubbles are prevented from bursting so as to break through the second coating film 640. This is because even if some of the moisture contained in the second coating film 640 moves so as to soak into the first dry film 633 (see the arrow indicated by the symbol 70), the air bubbles present in the first dry film 633 move toward the rear end 633a of the first dry film 633 and are released to the outside from the end 633a. Since the rear end 633a of the first dry film 633 is not covered with the second coating film 640, the end 633a of the first dry film 633 can function as an "exit" for the air bubbles generated in the first dry film 633.

After the second thickness adjustment process, a second drying process S106 is performed as shown in FIG. 7. The second drying process is a process of drying the second coating film 640 to form a second dry film 643 on the first dry film 633 so that the end portion 633a is exposed. Through this second drying process S106, the second coating film 640 is dried to become the second dry film 643.

The second drying step S106 is performed, for example, by placing the gas sensor element body 100a in a heating tank and heating it at a temperature of 80 to 120°C for 1 to 5 minutes, as in the first drying step S103 described above. FIG. 16 is an explanatory diagram showing the tip side of the gas sensor element body 100a after the second drying step S106. As shown in FIG. 16, a second dry film 643 is formed on the tip portion 100b of the gas sensor element body 100a. In the axial direction L (longitudinal direction), the second dry film 643 is formed so as to cover the area R2 between the tip surface 100c of the gas sensor element body (rod-shaped body portion) 100a and the second reference position M2, which is a position from the rear end of the detection portion 150 toward the rear end side of the gas sensor element body 100a by a second length L2 shorter than the first length l1 described above. In FIG. 16, the position corresponding to the tip surface 100c in the axial direction L is indicated by the symbol M0. FIG. 16 also shows the rear end 643a of the second dry film 643 at the second reference position M2, and the rear end 633a of the first dry film 633 at the first reference position M1.

Figure 17 is a longitudinal cross-sectional view of the gas sensor element body 100a after the second drying step S106 shown in Figure 16. Figure 17 shows a schematic cross-sectional configuration of the gas sensor element body 100a cut along the axis L direction. In the longitudinal direction (axis L direction), the range Ha from the tip surface 100c (position M0) of the gas sensor element body 100a to the second reference position M2 becomes higher in temperature due to heat generation by the heater section 200 when the gas sensor 1 is used, compared with the range Hb from the second reference position M2 to the first reference position M1. A second coating film 640 (second dry film 643) for forming the second protective layer 64 of the gas sensor 1 is formed so as to cover such range Ha.

As shown in FIG. 7, the second drying step S106 is followed by the firing step S107. The firing step S107 is a step of firing the first dry film 633 and the second dry film 643 to form a sintered body of the first dry film 633 and the second dry film 643 that constitute the porous protective layer 60 at the tip portion 100b. There are no particular limitations on the firing temperature, firing time, and other conditions in the firing step, as long as the first dry film 633, the second dry film 643, and the like are sintered to obtain the porous protective layer 60, and the firing step is appropriately set according to the purpose. The firing step is performed, for example, at 1080°C for 3 hours.

By this firing step S107, the first dry film 633 becomes the first protective layer 63 after firing, and the second dry film 643 becomes the second protective layer 64 after firing. In other words, when the first dry film 633 and the second dry film 643 are fired, they become the outer porous protective layer 62 consisting of the first protective layer 63 and the second protective layer 64. Also, by the firing step S107, the inner dry film 613 becomes the inner porous protective layer 61 after firing (see Figures 3 to 6).

As described above, in the manufacturing method of the gas sensor element of this embodiment, the gas sensor element 100 having the porous protective layer 60 formed on the tip portion 100b of the gas sensor element body 100a is obtained through the first immersion process S101, the first thickness adjustment process S102, the first drying process S103, the second immersion process S104, the second thickness adjustment process S105, the second drying process S106, and the firing process S107.

In particular, in the manufacturing method of the gas sensor element of this embodiment, when forming the second coating film 640 in the second immersion step S104, the second coating film 640 is formed on the first dry film 633 so that the rear end portion 633a of the first dry film 633 is exposed, so that a gas sensor element 100 is obtained that has a porous protective layer 60 with excellent water resistance and in which defects caused by air bubbles are suppressed.

<Other embodiments>
The present invention is not limited to the embodiments described above and illustrated in the drawings, and the following embodiments, for example, are also included within the technical scope of the present invention.

(1) In the above embodiment 1, the porous protective layer formed on the tip of the gas sensor element body includes an inner porous protective layer. However, as long as the object of the present invention is not impaired, for example, in other embodiments, the first coating film may be formed directly on the tip of the gas sensor element body.

(2) In the above-mentioned embodiment 1, in the first immersion step, the first coating film is formed on the tip portion of the gas sensor element body by a single dipping (immersion step). However, as long as the object of the present invention is not impaired, in other embodiments, for example, in the first immersion step, the tip portion of the gas sensor element body may be immersed multiple times in the first slurry for the purpose of ensuring the thickness of the first coating film, etc.

(3) In the above embodiment 1, in the second immersion step, the second coating film is formed on the tip portion of the gas sensor element body by a single dipping (immersion step). However, as long as the object of the present invention is not impaired, in other embodiments, for example, the tip portion of the gas sensor element body may be immersed multiple times in the second slurry as the second immersion step for the purpose of ensuring the thickness of the second coating film, etc.

1...gas sensor, 60...porous protective layer, 61...inner porous protective layer, 62...outer porous protective layer, 63...first protective layer, 63a...end, 64...second protective layer, 100...gas sensor element, 100a...gas sensor element body (rod-shaped body), 100b...tip, 150...detection part, 630...first coating film, 633...first dry film, 633a...end, 640...second coating film, 643...second dry film, S1...first slurry, S2...second slurry, S101...first immersion step, S103...first drying step, S104...second immersion step, S106...second drying step, S107...firing step

Claims (11)

A method for manufacturing a gas sensor element comprising a rod-shaped body portion extending from front to back and including a detection portion at a tip end thereof for detecting a specific gas component, and a porous protective layer covering the tip end, comprising:
a first dipping step of dipping the tip portion into a first slurry to form a first coating film made of the first slurry on the tip portion so as to encapsulate the detection portion;
a first drying step of drying the first coating film to form a first dry film on the tip portion;
a second immersion step of immersing the tip portion on which the first dry film is formed in a second slurry that is the same as the first slurry, to form a second coating film made of the second slurry on the first dry film so that a rear end of the first dry film is exposed and the detection unit is enclosed therein;
a second drying step of drying the second coating film to form a second dry film on the first dry film so that the end portion is exposed;
and a firing step of firing the first dry film and the second dry film to form a sintered body of the first dry film and the second dry film that constitute the porous protective layer at the tip portion. In the first immersion step, the first coating film is formed so as to cover an area between a tip surface of the rod-shaped main body and a first reference position that is a predetermined first length away from the detection unit toward a rear end side of the rod-shaped main body,
2. The method for manufacturing a gas sensor element according to claim 1, wherein in the second immersion step, the second coating film is formed so as to cover an area between the tip surface and a second reference position that is spaced from the detection portion by a second length that is shorter than the first length toward a rear end side of the rod-shaped main body. 3. The method for manufacturing a gas sensor element according to claim 1, wherein in the second immersion step, the second coating film is formed so as to cover a portion of the tip portion that will have a higher temperature than a portion of the end portion when the gas sensor element is used. the porous protective layer has an outer porous protective layer made of the sintered body of the first dry film and the second dry film, and an inner porous protective layer interposed between the outer porous protective layer and the tip portion and formed on the tip portion so as to enclose the detection portion;
4. The method for manufacturing a gas sensor element according to claim 1, wherein in the first immersion step, an inner dry film for forming the inner porous protective layer is pre-formed on the tip portion immersed in the first slurry so as to enclose the detection portion, and the first coating film made of the first slurry is formed so as to cover the inner dry film of the tip portion. The method for manufacturing a gas sensor element according to any one of claims 1 to 4, further comprising a first thickness adjustment step of removing a portion of the first coating film to adjust the thickness of the first coating film before the first drying step. The method for manufacturing a gas sensor element according to any one of claims 1 to 5, further comprising a second thickness adjustment step of removing a portion of the second coating film to adjust the thickness of the second coating film before the second drying step. A gas sensor element comprising a rod-shaped body portion extending from front to back and including a detection portion at a tip end thereof for detecting a specific gas component, and a porous protective layer covering the tip end,
the porous protective layer includes a first protective layer formed around the entire circumference at least in the axial direction in a region where the detection section is provided, and a second protective layer having the same main component as the first protective layer, provided at least in the axial direction in a region where the detection section is provided, and provided outside the first protective layer so that a rear end of the first protective layer is exposed;
The gas sensor element, wherein the porosity of the first protective layer and the porosity of the second protective layer are approximately the same. the first protective layer is formed so as to cover an area between a tip surface of the rod-shaped main body and a first reference position that is a predetermined first length away from the detection unit toward a rear end side of the rod-shaped main body,
8. The gas sensor element according to claim 7, wherein the second protective layer is formed so as to cover an area between the tip surface and a second reference position that is spaced from the detection portion by a second length that is shorter than the first length toward a rear end side of the rod-shaped main body. The gas sensor element according to claim 7 or 8, wherein the second protective layer covers a portion of the tip portion that becomes hotter than the portion where the end portion is formed during use. The gas sensor element according to any one of claims 7 to 9, wherein the porous protective layer has an outer porous protective layer including the first protective layer and the second protective layer, and an inner porous protective layer interposed between the outer porous protective layer and the tip portion and formed on the tip portion so as to enclose the detection portion. A gas sensor comprising the gas sensor element according to any one of claims 7 to 10.

Images