JP7772682B2 - Sensor element, gas sensor, and method of manufacturing sensor element - Google Patents

Description

The present invention relates to a sensor element, gas sensor, and method for manufacturing a sensor element that are suitable for use in detecting the concentration of a specific gas contained in combustion gas or exhaust gas from, for example, a combustor or internal combustion engine.

Gas sensors have traditionally been used to detect the concentration of specific components (such as oxygen) in the exhaust gas of internal combustion engines. These gas sensors contain a sensor element, which has a detection element portion consisting of a solid electrolyte and a pair of electrodes. Because the sensor element may be exposed to poisonous substances such as silicon and phosphorus contained in the exhaust gas, or to water droplets in the exhaust gas, the outer surface of the sensor element is coated with a porous protective layer to capture the poisonous substances and prevent water droplets from coming into direct contact with the sensor element. In other words, the entire circumference of the tip portion of the laminate, which is exposed to the gas to be measured (exhaust gas), is coated with a porous protective layer (see Patent Document 1).

A measurement chamber is formed inside the sensor element, facing one of the pair of electrodes, and the measurement target gas is introduced into the measurement chamber from the outside. A diffusion resistance section is interposed between the measurement chamber and the outside, adjusting the diffusion rate of the measurement target gas introduced into the measurement chamber. Therefore, the porous protective layer is in direct contact with the diffusion resistance section.

JP 2013-096792 A

8, the porous protective layer is produced by applying a coating liquid 400x, which is a mixture of ceramic particles 2 and a pore-forming material 410 such as burnable carbon, to the outer surface of the sensor element in the vicinity of the diffusion resistance portion 115, followed by drying and firing. The pore-forming material 410 is burned away during firing to form pores, while the ceramic particles 2 are bonded together to form the skeleton of the network structure of the porous protective layer.
Here, when the coating liquid 400x is applied, the moisture is absorbed into the liquid-permeable diffusion resistance portion 115, and the ceramic particles 2 and pore-forming material 410 in the coating liquid 21x also tend to aggregate toward the diffusion resistance portion 115, as shown by the arrows in Figure 8.

However, if the average particle size of the pore-forming material 410 becomes too large relative to the average particle size of the ceramic particles 2, a large amount of small-diameter ceramic particles 2 will aggregate in the large gaps G1 formed around the large-diameter pore-forming material 410. Meanwhile, few ceramic particles 2 will aggregate in the other small gaps G2, resulting in unevenness in the thickness of the skeleton formed after firing and the distribution of pores between the skeletons. This can lead to localized variations in the flow of exhaust gas that passes through the porous protective layer and enters the measurement chamber, potentially reducing detection accuracy.

The present invention therefore aims to provide a sensor element, a gas sensor, and a method for manufacturing a sensor element that suppresses the reduction in detection accuracy caused by the porous protective layer.

In order to achieve the above object, the present invention provides a sensor element comprising: a detection element portion provided with at least one cell having a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body; a measurement chamber to which one of the pair of electrodes faces; and a diffusion resistance portion for introducing a gas to be measured from the outside into the measurement chamber, the sensor element further comprising a porous protective layer in direct contact with the diffusion resistance portion and covering at least the diffusion resistance portion, the porous protective layer having ceramic particles forming a skeleton and pores formed in the gaps between the ceramic particles, a diameter ratio R expressed as (average diameter D1 (nm) of the pores/particle diameter D2 (nm) at which the cumulative number of the ceramic particles is 50%) being 100 or less , and a maximum diameter M1 (μm) of the pores being less than twice the average diameter D1 (μm) of the pores .

In this sensor element, the diameter ratio R is 100 or less, so that the average diameter (corresponding to D1) of the pore-forming material that is contained in the coating liquid for forming the porous protective layer and disappears to become pores does not become too large relative to the particle diameter D2 of the ceramic particles, and the sizes of the multiple gaps formed around adjacent pore-forming materials become uniform. As a result, the ceramic particles aggregate (disperse) evenly in each gap, and the size of the space (each gap) in which the ceramic particles can exist is limited, making it difficult for the ceramic particle aggregates to become large.
As a result, the thickness of the skeleton formed after baking the coating liquid and the distribution of pores between the skeletons are uniform, which prevents local variations in the flow of the target gas that passes through the porous protective layer and enters the measurement chamber, thereby preventing a decrease in detection accuracy.
Furthermore, with this sensor element, the particle size distribution of the pore-forming material becomes sharper, and the size of the above-mentioned gaps becomes more uniform, so that the distribution of pores also becomes more uniform.

In the sensor element of the present invention, the average pore diameter D1 may be 15 μm or less, and/or the particle diameter D2 may be 150 nm or more.
If the particle diameter D1 exceeds 15 μm, the pores of the resulting porous protective layer will be too large, making it difficult to sufficiently suppress water exposure or poisoning from the outside. If the particle diameter D2 is less than 150 nm, the particle diameter will be too small, making it difficult to handle the powder or to prepare a coating liquid.
Therefore, this sensor element can suppress such problems.

The gas sensor of the present invention comprises a sensor element that detects the concentration of a specific gas component in a gas to be measured and a housing that holds the sensor element, and is characterized in that the sensor element is a gas sensor element described in any one of claims 1 to 3.

The method for manufacturing a sensor element of the present invention includes a detection element portion provided with at least one cell having a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body, a measurement chamber to which one of the pair of electrodes faces, and a diffusion resistance portion for introducing a gas to be measured from the outside into the measurement chamber, the method comprising the steps of: preparing a coating liquid by mixing ceramic particles and a pore-forming material; and applying the coating liquid to the outer surface of the sensor element so as to cover the detection element portion with the coating liquid while being in direct contact with the diffusion resistance portion. and a porous protective layer forming step of drying and baking the applied coating liquid to eliminate the pore-forming material and form a porous protective layer having the ceramic particles that form a skeleton and pores that are formed in the burned-out portions of the pore-forming material, wherein the coating liquid has a diameter ratio R expressed as (average diameter D3 (nm) of the pore-forming material/particle diameter D2 (nm) at which the cumulative number of the ceramic particles is 50%) of 100 or less, and the maximum diameter (μm) of the pore-forming material is less than twice the average diameter D3 (μm) of the pore-forming material .

This invention provides a sensor element that suppresses the reduction in detection accuracy caused by the porous protective layer.

1 is a cross-sectional view taken along the longitudinal direction of a gas sensor (oxygen sensor) according to an embodiment of the present invention. FIG. 2 is a schematic exploded perspective view of a detection element and a heater. FIG. 2 is a partially enlarged cross-sectional view of the tip side of the detection element of FIG. 1 . FIG. 2 is a schematic cross-sectional view perpendicular to the axial direction of the sensor element. 3 is a schematic diagram showing the aggregation state of ceramic particles and a pore-forming material during the production of a porous protective layer by a method for producing a sensor according to an embodiment of the present invention. FIG. FIG. 10 is a diagram showing a method for measuring D1. FIG. 10 is a graph showing the relationship between the cost R of the porous protective layer and the maximum value ΔIp (%) of the pump current deviation, which is the variation in the sensor output. FIG. 1 is a schematic diagram showing the agglomeration state of ceramic particles and a pore-forming material during the production of a porous protective layer by a conventional sensor production method.

Hereinafter, an embodiment of the present invention will be described.
FIG. 1 is a cross-sectional view along the longitudinal direction (axis L direction) of a gas sensor (oxygen sensor) 1 according to an embodiment of the present invention, FIG. 2 is a schematic exploded perspective view of a detection element portion 300 and a heater portion 200, and FIG. 3 is a cross-sectional view perpendicular to the axis L direction of the detection element portion 300.

As shown in FIG. 1, the gas sensor 1 includes a sensor element 100 composed of a detection element portion 300 and a heater portion 200 laminated on the detection element portion 300, a metal shell (corresponding to the "housing" in the claims) 30 that holds the sensor element 100 and other components inside, and a protector 24 attached to the tip of the metal shell 30. The sensor element 100 is arranged to extend in the direction of the axis L.

As shown in FIG. 2, the heater section 200 comprises a first base 101 and a second base 103, each made primarily of alumina, and a heating element 102, made primarily of platinum, sandwiched between the first base 101 and the second base 103. The heating element 102 comprises a heating portion 102a located at the tip end and a pair of heater leads 102b extending from the heating portion 102a along the longitudinal direction of the first base 101. The terminals of the heater leads 102b are electrically connected to the heater-side pads 120 via conductors formed in heater-side through holes 101a provided in the first base 101. The laminate of the first base 101 and the second base 102 constitutes an insulating ceramic body.

The detection element unit 300 includes an oxygen concentration detection cell 130 and an oxygen pump cell 140. The oxygen concentration detection cell 130 is formed from 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 formed from a first electrode portion 104a and a first lead portion 104b extending from the first electrode portion 104a along the longitudinal direction of the first solid electrolyte body 105. The second electrode 106 is formed from a second electrode portion 106a and a second lead portion 106b extending from the second electrode portion 106a along the longitudinal direction of the first solid electrolyte body 105.
The oxygen concentration detection cell 130 and the oxygen pump cell 140 each correspond to a "cell" in the claims. The second electrode 106 and a third electrode 108 (described later) each correspond to a "one electrode" in the claims.

The terminal 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 below), 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. On the other hand, the terminal 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 below), 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.

On the other hand, the oxygen pump cell 140 is formed from 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 formed from a third electrode portion 108a and a third lead portion 108b extending from the third electrode portion 108a along the longitudinal direction of the second solid electrolyte body 109. The fourth electrode 110 is formed from a fourth electrode portion 110a and a fourth lead portion 110b extending from the fourth electrode portion 110a along the longitudinal direction of the second solid electrolyte body 109.

The terminal of the third lead portion 108b is electrically connected to the detection element side pad 121 via conductors formed in the fifth through hole 109b in the second solid electrolyte body 109 and the seventh through hole 111b in the protective layer 111. Meanwhile, the terminal 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 in the protective layer 111 (described below). 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, first electrode 104, second electrode 106, third electrode 108, fourth electrode 110, heater side pad 120, and detection element side pad 121 can be formed from a platinum group element. Suitable platinum group elements for forming these include Pt, Rh, and Pd, and these can be used alone or in combination of two or more.

However, in consideration of heat resistance and oxidation resistance, it is even more preferable that the heating element 102, first electrode 104, second electrode 106, third electrode 108, fourth electrode 110, heater side pad 120, and detection element side pad 121 be formed primarily from Pt. Furthermore, it is preferable that the heating element 102, first electrode 104, second electrode 106, third electrode 108, fourth electrode 110, heater side pad 120, and detection element side pad 121 contain a ceramic component in addition to the platinum group element that constitutes the primary component. From the standpoint of adhesion, it is preferable that this ceramic component be the same component as the primary material of the laminated side (e.g., the primary component of the first solid electrolyte body 105 and the second solid electrolyte body 109).

An insulating layer 107 is formed between the oxygen pump cell 140 and the oxygen concentration detection cell 130. The insulating layer 107 consists 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. This measurement chamber 107c is connected to the outside in the width direction of the insulating layer 107, and a diffusion resistance portion 115 is disposed in this communication portion to realize gas diffusion between the outside and the measurement chamber 107c under predetermined rate-limiting conditions.

The insulating portion 114 is not particularly limited as long as it is a ceramic sintered body with insulating properties, and examples include oxide ceramics such as alumina and mullite.

The diffusion resistance portion 115 is a porous body made of alumina. This diffusion resistance portion 115 regulates the flow rate of the detection gas into the measurement chamber 107c.

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 that sandwiches the fourth electrode portion 110a to protect the fourth electrode portion 110a from poisoning, and a reinforcing portion 112 that sandwiches the fourth lead portion 110b to protect the second solid electrolyte body 109.
In addition, the sensor element 100 of this embodiment corresponds to 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 (e.g., 450 mV), and the oxygen concentration in the measured gas is linearly detected in accordance with the current flowing through the oxygen pump cell 140.

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

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

Meanwhile, the front end of an outer tube 25 made of SUS430 is inserted into the rear end of the metal shell 30. The enlarged front end portion 25a of this outer tube 25 is fixed to the metal shell 30 by laser welding or the like. A separator 50 is disposed inside the rear end of the outer tube 25, and a retaining member 51 is interposed in the gap between the separator 50 and the outer tube 25. This retaining member 51 engages with the protruding portion 50a of the separator 50 (described below), and by crimping the outer tube 25, the outer tube 25 and separator 50 are fixed together.

Separator 50 also has through-holes 50b extending from the front to the rear end for inserting lead wires 11-15 for detection element section 300 and heater section 200 (lead wires 14, 15 are not shown). Connection terminals 16 are housed within through-holes 50b, connecting lead wires 11-15 to detection element pads 121 of detection element section 300 and heater pads 120 of heater section 200. Each lead wire 11-15 is externally connected to a connector (not shown). Electrical signals are input and output between each lead wire 11-15 and external devices such as an ECU via this connector. Although not shown in detail, each lead wire 11-15 has a structure in which the conductor is covered with an insulating coating made of resin.

Furthermore, a roughly cylindrical rubber cap 52 is disposed at the rear end of the separator 50 to close the opening 25b at the rear end of the outer tube 25. This rubber cap 52 is attached to the rear end of the outer tube 25 by crimping the outer periphery of the outer tube 25 radially inward while it is fitted inside the rear end of the outer tube 25. Through holes 52a are also formed in the rubber cap 52 from the front end to the rear end, through which the lead wires 11-15 are inserted, respectively.

Next, the porous protective layer 21, which is a characteristic feature of the present invention, will be described.
3 is a partially enlarged cross-sectional view of the tip end side of the sensor element 100 in Fig. 1, and shows that a porous protective layer 21 is provided directly on the surface (the outer surface of the tip end side of the sensor element 100) of the laminate of the detection element section 300 and the heater section 200. In other words, the porous protective layer 21 is in direct contact with the diffusion resistance section 115 and covers at least the diffusion resistance section.
In this example, the porous protective layer 21 is provided on the outer surface of the sensor element 100 including the diffusion resistance portion 115 so as to cover the entire periphery of the tip end portion of the sensor element 100 .

An outer porous layer 23 is formed to cover the outer surface of the porous protective layer 21 , and these two layers are collectively referred to as the “tip protective layer” 20 .
The "tip end portion of the sensor element 100" refers to the portion in the axial direction L, as shown in Figure 3, from the very tip of the sensor element 100 to at least the rear end of the measuring chamber 107c (including the second measuring chamber if there is a second measuring chamber connected to the measuring chamber, such as in the case of a NOx sensor element).

The tip protective layer 20 including the porous protective layer 21 includes the tip surface of the sensor element 100, is formed to extend toward the rear end along the axis L direction, and is formed to completely surround the four surfaces of the sensor element 100 (laminated body), namely the front, back, and both side surfaces (see Figure 4).
The diffusion resistance portion 115, the porous protective layer 21, and the outer porous layer 23 each have ceramic particles that form a skeleton and pores formed between the ceramic particles, and these pores form a three-dimensional network structure that allows gas to pass through. Furthermore, multiple ceramic particles are bonded together by firing or the like to form the skeleton.

In the porous protective layer 21, the diameter ratio R expressed as (average pore diameter D1 (nm)/particle diameter D2 (nm) at which the cumulative number of ceramic particles becomes 50%) is 100 or less.
5, the porous protective layer 21 is produced by applying a coating liquid 21x, which is a mixture of ceramic particles 2 and a pore-forming material 250 such as burnable carbon, to the outer surface of the sensor element in the vicinity of the diffusion resistance portion 115, followed by drying and firing. The pore-forming material 250 is burned away during firing to form pores, while the ceramic particles 2 are bonded together to form the skeleton of the network structure of the porous protective layer.
When the coating liquid 21 x is applied, the moisture is absorbed into the liquid-permeable diffusion resistance portion 115 , and the ceramic particles 2 and pore-forming material 250 in the coating liquid 21 x also tend to aggregate toward the diffusion resistance portion 115 .

Therefore, when the above-mentioned diameter ratio R is 100 or less, the average diameter D3 (nm) of the pore-forming material 250 (corresponding to the average diameter D1 (nm) of the pores after the pore-forming material 250 has disappeared) does not become too large relative to the particle diameter D2 of the ceramic particles 2, and the sizes of the multiple gaps G3, G4 formed around adjacent pore-forming materials 250 become uniform. As a result, the ceramic particles 2 aggregate (disperse) uniformly in each of the gaps G3, G4, and the size of the space in which the ceramic particles 2 can exist (each of the gaps G3, G4) is limited, making it difficult for the aggregates of the ceramic particles 2 to become large.
As a result, the thickness of the skeleton formed after firing and the distribution of pores between the skeletons are uniform, which prevents local variations in the flow of the measurement target gas (exhaust gas, etc.) that passes through the porous protective layer 21 and enters the measurement chamber 107c, thereby preventing a decrease in detection accuracy.

6, D1 is measured by drawing an imaginary line V having a length equivalent to 50 μm on an SEM (scanning electron microscope) photograph of the cross section of the porous protective layer 21 directly above the diffusion resistance portion 115, and measuring the distances S1, S2, ... between the gaps between particles passing through the imaginary line V as the pore diameter. The average value of all the gap distances (pore diameters) S1, S2, ... passing through the imaginary line V is then defined as D1. Note that gaps of 500 nm or less are not considered to be the pore diameter.
FIG. 6 is a cross-sectional SEM image of Example Group B.

Similarly, 100 ceramic particles were randomly selected from a 40,000x magnification SEM (scanning electron microscope) photograph of the above cross section (in the SEM photograph, the ceramic particles appear as white images against the background pores), and the particle diameter (circular equivalent diameter of the particle area) of each particle was measured using image analysis software. Then, for the 100 particles, the particle diameter at which the cumulative number of particles is 50% was determined to be D2, with the finest particles being zero.

The average diameter D1 may be 15 μm or less and/or the particle diameter D2 may be 150 nm or more.
If the average diameter D1 exceeds 15 μm, the porous protective layer 21 may become vulnerable to external stress.
Furthermore, when the average diameter D1 is 15 μm or less, the specific surface area of each pore-forming material increases, making it easier for the pore-forming materials to bond together, and as a result, the resulting pores tend to be interconnected, resulting in good breathability.

If the particle diameter D2 is less than 150 nm, the particle diameter becomes too small, which may make it difficult to handle the powder or to prepare a coating liquid.
Although there is no particular lower limit to the diameter ratio R, when D1=15 μm (15,000 nm) and D2=150 nm, the diameter ratio R=100.

The maximum diameter M1 (μm) of the pores may be less than twice the average diameter D1 (μm).
In this way, when M1<(D1×2), the particle size distribution of the pore-forming material 250 becomes sharper, and the sizes of the above-mentioned gaps G3 and G4 become more uniform, so the distribution of the pores also becomes more uniform.
The maximum diameter M1 is the maximum gap distance included in the imaginary line among the measurements of D1 described above.

The porous protective layer 21 can be formed by bonding, for example, one or more ceramic particles selected from the group consisting of alumina, spinel, zirconia, mullite, zircon, and cordierite, by firing or other methods. Pores can be formed in the skeleton of the coating by sintering a slurry (coating liquid) containing these particles. However, if a dissipative (burnable) pore-forming material is added to a slurry containing the above particles and then sintered, the burned-off pore-forming material becomes pores. This allows the porous protective layer 21 to have a high porosity, as described below, and is therefore preferable. Pore-forming materials that can be used include, for example, carbon, resin beads, and organic or inorganic binder particles.

The thickness of the porous protective layer 21 is preferably 20 to 800 μm.
The porosity (void ratio) of the porous protective layer 21 is preferably 40 to 85%. The porosity is measured by binarizing a 1000x magnification SEM (scanning electron microscope) photograph of the cross section of the porous protective layer 21 using image analysis software into a dark image of the pores in the background and a brighter image, and then measuring the area occupied by the pores.

The outer porous layer 23 can be formed by bonding one or more ceramic particles selected from the group consisting of alumina, spinel, zirconia, mullite, zircon, and cordierite by firing, etc. By sintering a slurry containing these particles, pores are formed in the gaps between the ceramic particles and in the skeleton of the coating when the organic or inorganic binder in the slurry is burned away.
The thickness of the outer porous layer 23 is preferably 100 to 800 μm.

The diffusion resistance portion 115 can also be formed by bonding one or more ceramic particles selected from the group consisting of alumina and zirconia by firing or the like. By sintering a slurry containing these particles, gaps between the ceramic particles are formed, and pores are formed in the skeleton of the coating when the organic or inorganic binder in the slurry is burned away. Note that the diffusion resistance portion 115 is formed by laminating each layer simultaneously and firing them together before firing the sensor element 100 (detection element portion 200), as in a known manufacturing method.
The thickness of the diffusion resistance portion 115 is preferably 10 to 50 μm.

The outer porous layer 23 does not necessarily have to be provided, and another porous layer may be provided between the porous protective layer 21 and the outer porous layer 23, or another porous layer may be provided outside the outer porous layer 23.

A manufacturing method of a sensor element according to an embodiment of the present invention includes a coating liquid preparation step of preparing a coating liquid 21x by mixing ceramic particles 2 and a pore-forming material 250; an application step of applying the coating liquid 21x so as to be in direct contact with and cover at least the diffusion resistance portion 115 of the sensor element 100 described above; and a porous protective layer formation step of drying and firing the applied coating liquid 21x to eliminate the pore-forming material 250 and form a porous protective layer 21 having ceramic particles 2 as a skeleton and pores formed in the burned-out portions of the pore-forming material 250, wherein the coating liquid 21x has a diameter ratio R expressed as (average diameter D3 (nm)/particle diameter D2 (nm)) of 100 or less.

Here, "applying a coating liquid (slurry)" refers to any of dipping, thermal spraying, printing and spraying methods using a coating liquid, and the application method is not limited.
Furthermore, the pore-forming material 250 is not limited to being burned away when the coating liquid 21x is baked, but may also be made of a resin and dissolved (disappeared) in a solvent before baking.

As a method for manufacturing the porous protective layer 21 and the outer porous layer 23, slurries to become the porous protective layer 21 and the outer porous layer 23 may be applied in order by a dipping method or the like and then sintered. In this case, the slurry (coating liquid) to become the porous protective layer 21 may be applied and sintered, and then the slurry to become the outer porous layer 23 may be applied and sintered. Alternatively, the slurries to become the porous protective layer 21 and the outer porous layer 23 may be applied in order and then sintered at the same time.
Alternatively, the porous protective layer 21 and the outer porous layer 23 may be manufactured by a thermal spraying method, a printing method, or a spraying method. Furthermore, the porous protective layer 21 and the outer porous layer 23 may be formed by different methods selected from the dipping method, the thermal spraying method, the printing method, and the spraying method.

The present invention is not limited to the above embodiment and can be applied to any gas sensor (gas sensor element) having a detection element portion with a solid electrolyte body and a pair of electrodes, and can be applied to the oxygen sensor (oxygen sensor element) of this embodiment. However, it goes without saying that the present invention is not limited to these uses and extends to various modifications and equivalents that fall within the spirit and scope of the present invention. For example, the present invention may be applied to a NOx sensor (NOx sensor element), an HC sensor (HC sensor element) that detects HC concentration, etc.

The following coating liquid A, which will become the porous protective layer 21, was adjusted to an appropriate viscosity and applied to the front surface (front, back, and both side surfaces) of the plate-shaped sensor element 100 shown in Figures 1 and 2 by a dipping method to a thickness of 200 µm. Thereafter, in order to volatilize excess organic solvent in the coating liquid A, the coating liquid A was dried for several hours in a dryer set at 200°C, and the porous protective layer 21 was baked in the atmosphere at 1100°C for 3 hours.
Coating solution A: 40 vol% alumina powder (charge D50 = 150 nm), 60 vol% carbon powder (charge D50 = changed between 1.2 and 13.0 μm), and 10 wt% alumina sol (external blend) were weighed, and an organic solvent was added and stirred to prepare the coating solution A. The D50 diameter was determined by laser diffraction scattering. The D50 diameter of the alumina powder dispersed in coating solution A is approximately equivalent to D2 (measured by the above-mentioned method) measured from a cross-sectional SEM of the porous protective layer 21 obtained by baking coating solution A. M1 was also determined from a cross-sectional SEM of the porous protective layer 21.

Next, the following slurry B, which would become the outer porous layer 23, was adjusted to an appropriate viscosity and applied to the surface of the porous protective layer 21 by a dipping method to a thickness of 150 μm or more. Thereafter, in order to volatilize excess organic solvent in the slurry B, the slurry was dried for several hours in a dryer set at 200° C., and the outer porous layer 23 was fired in the atmosphere at 1100° C. for 3 hours.
Slurry B: 20 vol % alumina powder (average particle size 0.1 μm), 80 vol % spinel powder (average particle size 40.0 μm), and 10 wt % alumina sol (external blend) were weighed, and an organic solvent was added and stirred to prepare the slurry.

The diffusion resistor 115C was prepared by wet-mixing 100% alumina powder and a plasticizer. The plasticizer consisted of butyral resin and DBP. This slurry was used to form the sensor element 100. The layers were laminated together and fired together before firing, similar to a known manufacturing method.
The obtained sensor element 100 was assembled to manufacture the gas sensor 1.

The obtained gas sensor 1 was tested using a model gas testing machine. A stoichiometric (λ=1) model gas was flowed through the piping, and the element temperature was controlled to 720°C. The current Ip flowing through the oxygen pump cell 140, which indicates the detection output, was measured for a predetermined time t. The average IpIp-AVE of the current Ip during time t was calculated, and compared with the measured values of each current Ip during time t to determine the maximum deviation ΔIp (%) from IpAVE. ΔIp = {|IpAVE - each measured value of Ip|/IpAVE} × 100.
The results are shown in Table 1 and FIG.

In "Example Group A," a large number of sensors with porous protective layers were manufactured under the same conditions, and D1 was measured from a cross-sectional SEM of the protective layer for one of the sensors, and this was used as D1 for the entire Example Group A. The same applies to Example Groups B and C and the Comparative Example Group.
Furthermore, with regard to D2, since the alumina powders used in Examples A to C and the Comparative Example group all had the same charge D50 value, for convenience, D2 was measured from a cross-sectional SEM of the protective layer for one sensor in Example Group A, and for convenience, D2 was also considered to be the same for Examples A to C and the Comparative Example group.

7, in the example groups A to C in which the diameter ratio R was 100 or less, ΔIp was reduced to 1% or less, the variation in sensor output was reduced, and the deterioration of detection accuracy was suppressed. This is thought to be because the flow of exhaust gas passing through the porous protective layer became uniform.
On the other hand, in the comparative example group in which D1 was increased so that the diameter ratio R exceeded 100, ΔIp exceeded 1%, and the sensor output varied, resulting in a decrease in detection accuracy.

REFERENCE SIGNS LIST 1 gas sensor 2 ceramic particles 21 porous protective layer 30 housing 104, 106, 108, 110 pair of electrodes 106, 108 one electrode 107c measuring chamber 105, 109 solid electrolyte body 100 sensor element 115 diffusion resistance portion 130, 140 cell 250 pore-forming material 300 detection element portion L axis

Claims (4)

A sensor element including: a detection element portion provided with at least one cell having a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body; a measurement chamber facing one of the pair of electrodes; and a diffusion resistance portion for introducing a measurement target gas from the outside into the measurement chamber,
further comprising a porous protective layer that is in direct contact with the diffusion resistance portion and covers at least the diffusion resistance portion,
the porous protective layer has ceramic particles as a skeleton and pores formed between the ceramic particles;
a diameter ratio R expressed as (the average diameter D1 (nm) of the pores/the particle diameter D2 (nm) at which the cumulative number of the ceramic particles becomes 50%) is 100 or less,
A sensor element characterized in that the maximum diameter M1 (μm) of the pores is less than twice the average diameter D1 (μm) of the pores . The sensor element described in claim 1, wherein the average pore diameter D1 is 15 μm or less and/or the particle diameter D2 is 150 nm or more. A gas sensor comprising a sensor element for detecting the concentration of a specific gas component in a measurement gas, and a housing for holding the sensor element,
3. A gas sensor comprising the gas sensor element according to claim 1 or 2 . A method for manufacturing a sensor element including: a detection element portion provided with at least one cell having a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body; a measurement chamber facing one of the pair of electrodes; and a diffusion resistance portion for introducing a measurement target gas from the outside into the measurement chamber,
a coating solution preparation step of preparing a coating solution by mixing ceramic particles and a pore-forming material;
a coating step of coating the coating liquid on the outer surface of the sensor element so as to cover the detection element portion while being in direct contact with the diffusion resistance portion;
a porous protective layer forming step of drying and baking the applied coating liquid to eliminate the pore-forming material, thereby forming a porous protective layer having the ceramic particles as a skeleton and pores formed in the burnt-out portions of the pore-forming material,
a diameter ratio R of the coating liquid, expressed as (average diameter D3 (nm) of the pore-forming material/particle diameter D2 (nm) at which the cumulative number of the ceramic particles becomes 50%) , is set to 100 or less, and the maximum diameter (μm) of the pore-forming material is set to less than twice the average diameter D3 (μm) of the pore-forming material .