Sensor element, gas sensor, and method of manufacturing sensor element

Abstract

To provide a sensor element and a gas sensor with decrease in detection accuracy due to a porous protective layer being suppressed, and a manufacturing method of the sensor element.SOLUTION: A sensor element 100 comprises: a detection element part 300 including at least one of cells 130, 140 including solid electrolytes 105, 109 and a pair of electrodes 104, 106, 108, 110 disposed on the solid electrolytes; a measurement chamber 107c which one of the pair of electrodes faces; and a diffusion resistance part 115 introducing measuring object gas from the outside into the measurement chamber. The sensor element 100 further comprises a porous protective layer 21 that directly contacts the diffusion resistance part and at least covers the diffusion resistance part. The porous protective layer has ceramic particles as skeleton, and pores formed among the ceramic particles, and a diameter ratio R represented by (mean diameter D1(nm) of the pore / particle diameter D2(nm) at which the cumulative number of the ceramic particles is 50%) is 100 or less.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a sensor element, a gas sensor, and a method for manufacturing a sensor element that are suitably used for detecting the concentration of a specific gas contained in combustion gas or exhaust gas of, for example, a combustor or an internal combustion engine. [Background technology]

[0002] Gas sensors have been used to detect the concentration of specific components (such as oxygen) in the exhaust gas of an internal combustion engine. 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 covered with a porous protective layer to capture the poisonous substances and prevent water droplets from directly contacting the sensor element. Specifically, the entire circumference of the tip of the laminate, which is exposed to the gas to be measured (exhaust gas), is covered with a porous protective layer (see Patent Document 1).

[0003] 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 resistor is interposed between the measurement chamber and the outside to adjust 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 resistor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-096792 Summary of the Invention [Problem to be solved by the invention]

[0005] 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 near 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.

[0006] 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. On the other hand, 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.

[0007] Therefore, an object of the present invention is to provide a sensor element, a gas sensor, and a method for manufacturing a sensor element that suppresses a decrease in detection accuracy caused by a porous protective layer. [Means for solving the problem]

[0008] In order to solve the above problems, the sensor element of the present invention comprises a detection element section provided with at least one cell having a solid electrolyte body and a pair of electrodes arranged on the solid electrolyte body, a measurement chamber to which one of the pair of electrodes faces, and a diffusion resistance section for introducing a gas to be measured from the outside into the measurement chamber, the sensor element further comprising a porous protective layer that is in direct contact with the diffusion resistance section and covers at least the diffusion resistance section, the porous protective layer having ceramic particles that form a skeleton and pores formed by gaps between the ceramic particles, and 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. The maximum diameter M1 (μm) of the pores is less than twice the average diameter D1 (μm) of the pores. It is characterized in that:

[0009] With this sensor element, the diameter ratio R is 100 or less, so 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 is not 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 are 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.

[0010] 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 adequately prevent water contamination 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.

[0012] The gas sensor of the present invention is characterized in that it comprises a sensor element for detecting the concentration of a specific gas component in a gas to be measured and a housing for holding the sensor element, and the sensor element is a gas sensor element according to any one of claims 1 to 3.

[0013] 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 arranged 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; applying the coating liquid so as to cover the detection element portion on the outer surface of the sensor element while being in direct contact with the diffusion resistance portion; and drying and firing the applied coating liquid to remove 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, wherein the coating liquid is a porous protective layer having 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 ceramic particles is 50%) of 100 or less. The maximum diameter (μm) of the pore-forming material is less than twice the average diameter D3 (μm) of the pore-forming material. The present invention is characterized in that: [Effects of the Invention]

[0014] According to the present invention, a sensor element can be obtained in which the deterioration of detection accuracy caused by the porous protective layer is suppressed. [Brief explanation of the drawings]

[0015] [Figure 1] 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. [Figure 2] FIG. 2 is a schematic exploded perspective view of a detection element and a heater. [Figure 3] FIG. 2 is a partially enlarged cross-sectional view of the tip side of the detection element of FIG. [Figure 4] FIG. 2 is a schematic cross-sectional view perpendicular to the axial direction of the sensor element. [Figure 5] 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. [Figure 6] FIG. 1 shows a measurement method for D1. [Figure 7] FIG. 10 is a diagram 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. [Figure 8] 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. DETAILED DESCRIPTION OF THE INVENTION

[0016] 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.

[0017] 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 a "housing" in the claims) 30 that holds the sensor element 100 and other components therein, and a protector 24 attached to the tip of the metal shell 30. The sensor element 100 is disposed so as to extend in the direction of the axis L.

[0018] 2, the heater section 200 includes 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 includes 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 ends of the heater leads 102b are electrically connected to 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 corresponds to an insulating ceramic body.

[0019] 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 "one electrode" in the claims.

[0020] An end of the first lead portion 104b is electrically connected to the detection element pad 121 via conductors formed in a first through-hole 105a provided in the first solid electrolyte body 105, a second through-hole 107a provided in the insulating layer 107 (described later), a fourth through-hole 109a provided in the second solid electrolyte body 109, and a sixth through-hole 111a provided in the protective layer 111. On the other hand, an end of the second lead portion 106b is electrically connected to the detection element pad 121 via conductors formed in a third through-hole 107b provided in the insulating layer 107 (described later), a fifth through-hole 109b provided in the second solid electrolyte body 109, and a seventh through-hole 111b provided in the protective layer 111.

[0021] 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.

[0022] An end of the third lead portion 108b is electrically connected to the detection element pad 121 via conductors formed in a fifth through-hole 109b provided in the second solid electrolyte body 109 and a seventh through-hole 111b provided in the protective layer 111. On the other hand, an end of the fourth lead portion 110b is electrically connected to the detection element pad 121 via a conductor formed in an eighth through-hole 111c provided in the protective layer 111, which will be described later. The second lead portion 106b and the third lead portion 108b are at the same potential.

[0023] 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 (Y2O3) or calcia (CaO) as a stabilizer to zirconia (ZrO2).

[0024] 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 made of a platinum group element. Suitable platinum group elements for these include Pt, Rh, and Pd, and these can be used alone or in combination of two or more.

[0025] 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 are 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 viewpoint 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).

[0026] An 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 resistor 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 communicates with the outside in the width direction of the insulating layer 107, and a diffusion resistor portion 115 is disposed in the communication portion to realize gas diffusion between the outside and the measurement chamber 107c under predetermined rate-controlling conditions.

[0027] The insulating portion 114 is not particularly limited as long as it is a ceramic sintered body having insulating properties, and examples thereof include oxide ceramics such as alumina and mullite.

[0028] The diffusion resistance portion 115 is a porous body made of alumina, and serves to regulate the flow rate of the detection gas into the measurement chamber 107c.

[0029] Furthermore, 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 according to the current flowing through the oxygen pump cell 140.

[0030] Returning to FIG. 1 , the metal shell 30 is made of SUS430 and has a male thread 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, and 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 this 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 disposed on the rear end side of the second talc 38. This sleeve 39 is formed in a multi-stage cylindrical shape, has an axial hole 39a formed along the axis, and has the sensor element 100 inserted therein. 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 a stainless steel ring member 40.

[0031] A metal protector 24 having a plurality of gas intake holes 24a is attached by welding to the outer periphery of the front end side of the metallic shell 30. The metal protector 24 covers the front end portion of the sensor element 100 protruding from the front end of the metallic shell 30. The protector 24 has a double structure, with an outer protector 41 in the form of a closed-end cylinder having a uniform outer diameter on the outside and an inner protector 42 in the form of a closed-end cylinder with a rear end 42a whose outer diameter is larger than that of a front end 42b on the inside.

[0032] Meanwhile, the front end side of an outer tube 25 made of SUS430 is inserted into the rear end side of the metallic shell 30. The outer tube 25 has an expanded diameter front end portion 25a fixed to the metallic 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 between the separator 50 and the outer tube 25. This holding member 51 engages with a protruding portion 50a of the separator 50 (described later) and fixes the outer tube 25 and the separator 50 together by crimping the outer tube 25.

[0033] The separator 50 also has through-holes 50b extending from the front end to the rear end thereof for inserting lead wires 11-15 for the detection element section 300 and the heater section 200 (note that the lead wires 14, 15 are not shown). Connection terminals 16 are housed within the through-holes 50b, connecting the lead wires 11-15 to the detection element side pads 121 of the detection element section 300 and the heater side pads 120 of the heater section 200. The lead wires 11-15 are externally connected to connectors (not shown). Electrical signals are input and output between the lead wires 11-15 and external devices such as an ECU via these connectors. Although not shown in detail, the lead wires 11-15 have a structure in which the conductors are covered with an insulating film made of resin.

[0034] Furthermore, a substantially cylindrical rubber cap 52 is disposed on the rear end side of the separator 50 to close an opening 25b on the rear end side of the outer tube 25. This rubber cap 52 is fixed to the outer tube 25 by crimping the outer periphery of the outer tube 25 radially inward while attached to the rear end of the outer tube 25. Through holes 52a are also formed in the rubber cap 52 from the front end side to the rear end side, through which the lead wires 11 to 15 are inserted, respectively.

[0035] 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. That is, 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, covering the entire periphery of the tip side portion of the sensor element 100.

[0036] An outer porous layer 23 is formed covering 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 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).

[0037] 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 direction of the axis L, and is formed to completely surround all four surfaces of the sensor element 100 (laminated body), including 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 to form the skeleton by firing or other processes.

[0038] 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 near 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 21x 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 21x also tend to aggregate toward the diffusion resistance portion 115.

[0039] 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.

[0040] As shown in Figure 6, D1 is measured by drawing an imaginary line V with 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 particles passing through this imaginary line V as the pore diameter. Then, the average value of all the gap distances (pore diameters) S1, S2, ... passing through the imaginary line V is 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.

[0041] Similarly, 100 ceramic particles were randomly selected from a 40,000x magnification SEM (scanning electron microscope) photograph of the 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 reaches 50% was defined as D2, with the finest particles being zero.

[0042] 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.

[0043] If the particle size D2 is less than 150 nm, the particle size becomes too small, which may make it difficult to handle the powder or to prepare a coating liquid. The lower limit of the diameter ratio R is not particularly limited, but when D1=15 μm (15000 nm) and D2=150 nm, the diameter ratio R=100.

[0044] The maximum pore diameter M1 (μm) may be less than twice the average pore 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.

[0045] The porous protective layer 21 can be formed by binding, for example, one or more ceramic particles selected from the group consisting of alumina, spinel, zirconia, mullite, zircon, and cordierite by firing or the like. 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-out portions of the pore-forming material become pores, which is preferable because it allows the porous protective layer 21 to have a high porosity, as described below. Examples of pore-forming materials that can be used include carbon, resin beads, and particles of organic or inorganic binders.

[0046] 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 an SEM (scanning electron microscope) photograph of the cross section of the porous protective layer 21 at a magnification of 1000 times 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.

[0047] The outer porous layer 23 can be formed by binding 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.

[0048] The diffusion resistance section 115 can also be formed by bonding one or more ceramic particles selected from the group consisting of alumina and zirconia through 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 section 115 is formed by laminating each layer simultaneously and firing them together before firing the sensor element 100 (detection element section 200), as in a known manufacturing method. The thickness of the diffusion resistance portion 115 is preferably 10 to 50 μm.

[0049] The outer porous layer 23 may not be provided, 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.

[0050] The method for manufacturing 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, and a step of applying the coating liquid 21x to the diffusion resistance portion of the sensor element 100. 115 and a porous protective layer forming process in which the applied coating liquid 21x is dried and fired 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 burnt-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.

[0051] Here, "applying a coating liquid (slurry)" refers to any of dipping, thermal spraying, printing and spraying methods using a coating liquid, and there are no limitations on the application method. Furthermore, the pore-forming material 250 is not limited to being burned away when the coating liquid 21x is baked, but also includes cases where the pore-forming material 250 made of resin is dissolved (disappeared) in a solvent before baking.

[0052] 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 sintered. In this case, after the slurry (coating liquid) to become the porous protective layer 21 is applied and sintered, 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 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.

[0053] The present invention is not limited to the above-described embodiment and can be applied to any gas sensor (gas sensor element) having a detection element portion having a solid electrolyte body and a pair of electrodes, and can be applied to the oxygen sensor (oxygen sensor element) of the present embodiment. However, the present invention is not limited to these applications and covers 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. [Example]

[0054] The following coating liquid A, which will become the porous protective layer 21, was adjusted to an appropriate viscosity and applied to the surface (front, back, and both side surfaces) of the tip side 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 air 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. 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.

[0055] 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 air 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 compounding) were weighed, and an organic solvent was added and stirred to prepare the slurry.

[0056] The diffusion resistance portion 115C was prepared by preparing a slurry in which 100% by mass of alumina powder and a plasticizer were dispersed by wet mixing. The plasticizer consisted of butyral resin and DBP. Using this slurry, the layers were simultaneously laminated and fired together before firing the sensor element 100, as in a known manufacturing method. The obtained sensor elements 100 were assembled to manufacture the gas sensor 1.

[0057] 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.

[0058] 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-section SEM of the protective layer for one of the sensors, and this was used as D1 for the entire Example Group A. The same was true for 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 feed 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 for Examples A to C and the Comparative Example group was also considered to be the same.

[0059] [Table 1]

[0060] 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 the sensor output was reduced, and the deterioration of the detection accuracy was suppressed. This is thought to be because the flow of the 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%, the sensor output varied, and the detection accuracy decreased. [Explanation of symbols]

[0061] 1 Gas sensor 2. Ceramic particles 21 Porous protective layer 30 Housing 104, 106, 108, 110 Pair of electrodes 106, 108 One electrode 107c Measurement room 105, 109 Solid electrolyte body 100 sensor elements 115 Diffusion resistance section 130, 140 cells 250 Pore forming material 300 Detector element L axis

Claims

1. 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.

2. 2. The sensor element according to claim 1, wherein the average pore diameter D1 is 15 μm or less and / or the particle diameter D2 is 150 nm or more.

3. 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.

4. 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.

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