Sensor element and gas sensor

Abstract

[Problem] To provide a sensor element and a gas sensor which achieve suppression of disconnection, due to water intrusion, of a lead that is connected to an electrode and that faces a communication hole to be in communication with the outside. [Solution] A sensor element 100 comprises a pair of electrodes 104a, 106a, and a pair of leads 104b, 106b respectively connected to the pair of electrodes. One lead 104b of the pair of leads is an inner lead disposed on the inside of the sensor element, and a portion of the lead 104b faces communication holes 105a, 106a, 107a, 109a, 111a provided in the sensor element to be in communication with the outside. The inner lead is configured to include particles 104M of at least one noble metal selected from the group consisting of Pt, Pd, Rh, and Au, particles 104S of ceramics, and pores 104V. The porosity of the inner lead is more than 0% but not more than 11%. The ratio RA of the total area of large diameter pores having a cross section area of at least 5 μm2, to the total area of pores in the cross section of the inner lead, is 8% or more.

Description

[Technical Field]

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

[0002] Gas sensors for detecting the concentration of specific components (such as oxygen) contained in exhaust gas from internal combustion engines are known. This type of gas sensor includes an elongated, plate-shaped sensor element. A detection section for detecting the specific component is provided at the tip of the sensor element (see Patent Document 1). The detection unit is composed of a pair of electrodes and a solid electrolyte body, with leads extending from each electrode toward the rear end. The reference lead connected to the reference electrode has an internal hole that allows gas inside and outside the element to communicate through the reference lead, thereby adjusting the pressure of the reference gas around the reference electrode. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-032240 Summary of the Invention [Problem to be solved by the invention]

[0004] However, if water that has entered the gas sensor or water generated by condensation comes into contact with the sensor element, the lead will absorb the water that has entered through the communicating holes. When the sensor element is subsequently heated, the water inside the lead will evaporate and expand, potentially damaging or breaking the lead.

[0005] An object of the present invention is to provide a sensor element and a gas sensor that are capable of suppressing breakage of leads connected to electrodes and facing communication holes to communicate with the outside due to water intrusion. [Means for solving the problem]

[0006] In order to achieve the above object, a sensor element of the present invention is a sensor element comprising a pair of electrodes and a pair of leads connected to the pair of electrodes, one of the pair of leads being an internal lead disposed inside the sensor element, a part of which faces a communication hole provided in the sensor element and communicates with the outside, the internal lead comprising one or more kinds of precious metal particles selected from the group consisting of Pt, Pd, Rh and Au, ceramic particles and pores, the porosity of the internal lead being greater than 0% and not more than 11%, and the cross-sectional area of ​​the internal lead being less than 5 μm with respect to the total area of ​​the pores in a cross section of the internal lead. 2 The ratio RA of the total area of ​​the large pores is 8% or more.

[0007] According to this sensor element, the porosity of the internal lead is greater than 0% and less than 11%, so that it has adequate gas permeability and can adjust the pressure of the reference gas around the reference electrode, and it can prevent the precious metal particles from being separated by the pores and breaking due to excessive porosity. Furthermore, because the ratio RA of the total area of ​​the large pores is 8% or more, water absorbed into the internal lead can easily escape from the large pores toward the communicating holes. As a result, when the sensor element is heated after water has entered it, the water inside the internal lead can be prevented from vaporizing and expanding, which can damage or break the internal lead.

[0008] In the sensor element of the present invention, the ratio RA may be less than 60%. This sensor element can prevent the internal leads from peeling off.

[0009] A gas sensor according to the present invention comprises the sensor element according to claim 1 or 2, and a metallic shell that holds the sensor element. [Effects of the Invention]

[0010] According to the present invention, a sensor element and a gas sensor are obtained that are capable of suppressing breakage of leads connected to electrodes and facing communication holes to communicate with the outside due to water intrusion. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a cross-sectional view of the gas sensor taken along the axial direction. [Figure 2] FIG. 2 is an exploded perspective view schematically illustrating a detection element portion and a heater portion that constitute a sensor element. [Figure 3] FIG. 2 is a diagram showing a cross-sectional SEM image including a first lead portion of Example 1. [Figure 4] FIG. 10 is a diagram showing a cross-sectional SEM image including a first lead portion of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described. First, the configuration of a gas sensor (oxygen sensor) 1 including a sensor element 100 according to this embodiment will be described. Fig. 1 is a cross-sectional view of the gas sensor 1 taken along the axis L, and Fig. 2 is an exploded perspective view schematically showing a detection element section 300 and a heater section 200 that constitute the sensor element 100. In this specification, the lower side of the gas sensor 1 shown in Fig. 1 will be referred to as the "front end side," and the opposite side (the upper side in Fig. 1) will be referred to as the "rear end side."

[0013] 1, the gas sensor 1 includes a sensor element 100 formed by a laminate of a detection element section 300 and a heater section 200, a metal shell 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 generally in the shape of an elongated plate, and is disposed so that its longitudinal direction is along the direction of the axis L. A porous protective layer 20 is formed on the tip side of the sensor element 100.

[0014] 2, the heater section 200 has an elongated plate-like shape overall, and includes a first base 101 and a second base 103 made primarily of alumina, and a heating element 102 made primarily of platinum and sandwiched between the first base 101 and the second base 103. The heating element 102 has 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 (axis L direction) of the first base 101. The ends of the heater leads 102b are electrically connected to the heater pads 120 via conductors formed in heater-side through holes 101a provided in the first base 101.

[0015] The detection element section 300 , like the heater section 200 , is generally in the shape of an elongated 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 second lead portion 106b extending from the second electrode portion 106a along the longitudinal direction (axis L direction) of the first solid electrolyte body 105.

[0016] 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. 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.

[0017] The first electrode portion 104a and the second electrode portion 106a correspond to a "pair of electrodes" in the claims, and the first lead portion 104b corresponds to "one lead (internal lead)" in the claims. The first through-hole 105a, the second through-hole 107a, the fourth through-hole 109a, and the sixth through-hole 111a correspond to the "communicating holes" in the claims.

[0018] 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.

[0019] An end of the third lead portion 108b is electrically connected to the detection element side 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. An end of the fourth lead portion 110b is electrically connected to the detection element side pad 121 via a conductor formed in an 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.

[0020] The first solid electrolyte body 105 and the second solid electrolyte body 109 are made of a partially stabilized zirconia sintered body made by adding yttria (Y2O3) or calcia (CaO) as a stabilizer to zirconia (ZrO2). 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, Rh, and Pd. These platinum group elements may be used alone or in combination of two or more.

[0021] From the viewpoint of heat resistance and oxidation resistance, the heating element 102 and the like are preferably formed mainly of Pt. Furthermore, the heating element 102 and the like preferably contain a ceramic component in addition to the platinum group element that is the main component. From the viewpoint of adhesion, this ceramic component is preferably the same component as the main material on the laminated side.

[0022] 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 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 communicates with the outside in the width direction of the insulating layer 107, and a diffusion resistance portion 115 is disposed in the communicated portion to realize gas diffusion between the outside and the measurement chamber 107c under predetermined rate-controlling conditions. The insulating portion 114 is not limited as long as it is a ceramic sintered body having insulating properties, and may be made of, for example, oxide 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.

[0023] 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. Note that the 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 (e.g., 450 mV), thereby linearly detecting the oxygen concentration in the measurement gas in accordance with the current flowing through the oxygen pump cell 140.

[0024] Here, when looking at a cross section of the sensor element 100 perpendicular to the axis L direction, the outer edge consisting of the protective layer 111 and the first substrate 101 forms the long side, and the two sides along the stacking direction form the short sides, forming a rectangular cross section.

[0025] 1, the metal shell 30 is made of SUS430 and has a male thread portion 31 for attaching the gas sensor 1 to an exhaust pipe and a hexagonal portion 32 to which an attachment tool is applied during attachment. The metal shell 30 also has a metal-side step 33 that protrudes radially inward, and the 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 tip side. The talc 36 is made up of a first talc 37 disposed within the metal holder 34 and a second talc 38 disposed at 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.

[0026] 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 and has an axial hole 39a formed along the axis L, and the sensor element 100 is inserted into the sleeve 39 including the axial hole 39a. A crimped portion 30a on the rear end side of the metallic shell 30 is bent inward, and the sleeve 39 is pressed against the front end side of the metallic shell 30 via a stainless steel ring member 40 by the crimped portion 30a.

[0027] A metal protector 24 is attached by welding to the outer periphery of the front end side of the metallic shell 30. The protector 24 has a double structure, with a cylindrical outer protector 41 with a uniform outer diameter and a closed bottom disposed on the outside, and a cylindrical inner protector 42 with a closed bottom formed so that the outer diameter of a rear end 42a is larger than the outer diameter of a front end 42b disposed on the inside. The protector 24 covers the front end portion of the sensor element 100 protruding from the front end of the metallic shell 30, and has a plurality of gas intake holes 24a.

[0028] 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 a front end portion 25a that expands in diameter toward the front end, and the front end portion 25a is 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 a 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 a protrusion 50a that protrudes outward from the circumferential surface of the separator 50.

[0029] The separator 50 is also provided with insertion holes 50b that extend from the front end to the rear end, through which various lead wires 11, 12, and 13 for the detection element section 300 and the heater section 200 are inserted. For ease of explanation, only three lead wires 11, 12, and 13 are shown in FIG. 1 , and other lead wires are omitted from the illustration. Connection terminals 16 that connect the lead wires 11, etc. to the detection element pads 121 of the detection element section 300 and the heater pads 120 of the heater section 200 are housed within the insertion holes 50b. Each lead wire 11, etc. is configured to be connectable to an external connector (not shown), and electrical signals are input and output between each lead wire 11, etc. and an external device such as an ECU via such a connector.

[0030] 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 tube 25 radially inward while housed within the rear end of the outer tube 25. The rubber cap 52 also has insertion holes 52a that penetrate from the front end side to the rear end side, for inserting the lead wires 11 and the like.

[0031] Next, the first lead portion 104b, which is a characteristic feature of the present invention, will be described. The first lead portion 104b is made of a gas-permeable porous material including pores, and communicates with the outside by facing the communication holes (first through-hole 105a, second through-hole 107a, fourth through-hole 109a, and sixth through-hole 111a) provided in the sensor element 100. The detection element side pad 121 electrically connected to the sixth through-hole 111a is also made of a gas-permeable porous material. The reference gas generated in the first electrode portion is discharged to the outside via the first lead portion 104b and the communication hole.

[0032] In this case, if water enters the sensor element 100, the first lead portion 104b will absorb the water that has entered through the communicating hole, and when the sensor element 100 is subsequently heated, the water will evaporate and expand inside the first lead portion 104b, which may cause damage or breakage of the first lead portion 104b.

[0033] Therefore, in the present invention, the first lead portion 104b is configured to contain one or more kinds of precious metal particles selected from the group consisting of Pt, Pd, Rh, and Au, ceramic particles, and pores, and has a porosity of more than 0% and 11% or less, and a cross-sectional area of ​​5 μm 2 with respect to the total area of ​​the pores in the cross section. 2 The ratio RA of the total area of ​​the large pores above is 8% or more.

[0034] The reason why the porosity of the first lead portion 104b is greater than 0% is that if the first lead portion 104b does not have pores, it becomes gas impermeable and the pressure of the reference gas in the first electrode portion 104a cannot be adjusted. On the other hand, the higher the porosity of the first lead portion 104b, the easier it is for gas to permeate, but if the porosity exceeds 11%, the noble metal particles, which are conductive materials in the first lead portion 104b, are separated by the pores and are prone to breakage. The porosity is preferably 5% or more, and more preferably 8% or more.

[0035] Furthermore, in the first lead portion 104b, the cross-sectional area is 5 μm 2 When the ratio RA of the total area of ​​the large pores is 8% or more, water absorbed into the first lead portion 104b can easily escape from the large pores toward the communicating holes. As a result, when the sensor element 100 is heated after water has entered it, the water inside the first lead portion 104b is prevented from vaporizing and expanding, which would otherwise damage or break the first lead portion 104b. The ratio RA is preferably less than 60%. If the ratio RA is 60% or more, there is a risk that the first lead portion 104b may peel off.

[0036] RA is more preferably 40% or less, even more preferably 30% or less, and most preferably 21.6% or less. Furthermore, RA is preferably 10% or more, more preferably 15% or more, and most preferably 20% or more. The range of RA is, for example, preferably 10 to 40%, more preferably 15 to 30%, and most preferably 20 to 21.6%.

[0037] The porosity of the first lead portion 104b is measured based on a cross-sectional SEM image including the first lead portion 104b as shown in Fig. 3. Fig. 3 is a cross-sectional SEM image of an example described later. First, the cross-sectional SEM image is binarized to extract the pores 104V, which are voids (binarization software: ImageJ), and the area of ​​each pore and the number of pores per area are calculated using the Analyze Particle command. The pores 104V are the darkest parts in the cross-sectional SEM image, and the noble metal particles 104M that make up the first lead portion 104b are the brightest parts. The ceramic particles 104S that make up the first lead portion 104b are of intermediate brightness.

[0038] Next, parallel lines BL are drawn from the ends of the pores 104V that are connected to the precious metal particles 104M and are located outermost in the thickness direction of the first lead portion 104b (the vertical direction in FIG. 3) to the surface direction of the first lead portion 104b (the horizontal direction in FIG. 3). The parallel lines BL are drawn on both the upper and lower sides of the first lead portion 104b. Then, the area of ​​the cross-sectional SEM image surrounded by the two parallel lines BL is regarded as the cross-sectional area of ​​the first lead portion 104b, and the porosity is calculated from the total area of ​​the pores 104V.

[0039] Among the pores 104V, those that are not connected to the precious metal particles 104M (that is, those that are farther from the precious metal particles 104M than the parallel line BL) are considered to be nests (voids) inside other layers adjacent to the first lead portion 104b and are therefore excluded. Furthermore, when another noble metal particle 104M is located outermost in the thickness direction than a pore 104V connected to the noble metal particle 104M, the position of that noble metal particle 104M is defined as the parallel line BL.

[0040] The cross-sectional area of ​​the first lead portion 104b is 5 μm 2 The total area of ​​the large pores is calculated by creating a histogram from the number of individual pores 104V per area obtained by image analysis as described above. Then, the ratio RA of the total area of ​​the large pores is calculated from the total area of ​​the pores obtained by image analysis as described above.

[0041] It goes without saying that the present invention is not limited to the above-described embodiments, but covers various modifications and equivalents that fall within the spirit and scope of the present invention. The gas sensor is not limited to an oxygen sensor, but may be, for example, a NOx sensor. [Example]

[0042] Using Pt as the noble metal particles of the first lead portion 104 and alumina as the ceramic particles, a plurality of sensor elements 100 shown in FIGS. 1 and 2 were fabricated with the porosity of the first lead portion 104 varied. Each of the obtained sensor elements 100 was immersed in water to allow the water to penetrate into the first lead portion 104, and then dried. Next, the heater portion 200 of the sensor element 100 was driven to heat it to a predetermined temperature, and a characteristics test was performed to measure the puncture (disconnection) rate of the first lead portion 104. The puncture (disconnection) rate indicates the rate at which the lead was damaged or disconnected.

[0043] Furthermore, a cross-sectional SEM image including the first lead portion 104 was taken for a sensor element 100 (for which the puncture rate had not been measured) from the same lot as the sensor element 100 for which the puncture (disconnection) rate had been measured. Based on this cross-sectional SEM image, the porosity of the first lead portion 104 and the ratio RA of the total area of ​​the large-diameter pores were calculated by the image analysis described above. The results obtained are shown in Table 1, Figures 3 and 4.

[0044] [Table 1]

[0045] As shown in Table 1, in Example 1, where the porosity was 11% or less and the ratio RA of the total area of ​​large pores was 8% or more, the puncture (breakage) rate was 0%, and damage and breakage of the leads was suppressed. On the other hand, in Comparative Examples 1 and 2, in which the porosity was 11% or less but the ratio RA of the total area of ​​large pores was less than 8%, the puncture (disconnection) rate was 100%, and the leads were damaged and disconnected. Furthermore, in the case of Comparative Example 3, in which the ratio RA of the total area of ​​large pores was 8% or more but the porosity exceeded 11%, the puncture (disconnection) rate was also high, and the leads were damaged and disconnected.

[0046] 3 and 4 are cross-sectional SEM images including the first lead portion 104 of Example 1 and Comparative Example 1, respectively, and it can be seen that the cross-sectional area of ​​each pore 104V is smaller in Comparative Example 1, making it more difficult for absorbed water to escape. [Explanation of symbols]

[0047] 1 Gas sensor 30 Metal body 100 sensor element 104a first electrode portion (a pair of electrodes) 104b First lead portion (one lead (internal lead)) 104M precious metal particles 104S ceramic particles 104V Pore 105a First through hole (communicating hole) 106a second electrode portion (a pair of electrodes) 106b Second lead section (pair of leads) 107a Second through hole (communicating hole) 109a 4th through hole (communicating hole) 111a 6th through hole (communicating hole)

Claims

1. a pair of electrodes; a pair of leads connected to the pair of electrodes, respectively; one of the pair of leads is an internal lead disposed inside the sensor element, and a part of the internal lead faces a communication hole provided in the sensor element to communicate with the outside, the internal lead is configured to contain one or more kinds of precious metal particles selected from the group consisting of Pt, Pd, Rh, and Au, ceramic particles, and pores; The porosity of the internal lead is greater than 0% and less than or equal to 11%; The cross-sectional area of ​​the pores in the cross section of the internal lead is 5 μm ２ A sensor element characterized in that the ratio RA of the total area of ​​the large diameter pores is 8% or more.

2. 2. The sensor element according to claim 1, wherein the ratio RA is less than 60%.

3. The sensor element according to claim 1 or 2; a metal shell that holds the sensor element; A gas sensor comprising:

Images