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

Abstract

[Problem] To provide a sensor element in which a decrease in the catalytic ability of a catalyst supported on a porous carrier is suppressed. [Solution] A sensor element comprising an oxygen ion-conductive solid electrolyte body, a working electrode that is provided on one surface of the solid electrolyte body and that is in contact with a gas to be measured, and a reference electrode that is provided on the other surface of the solid electrolyte body and that is in contact with a reference gas, the sensor element is further provided with a catalyst layer that is provided with: a porous carrier that covers the working electrode and is formed from ceramic particles; and one or more catalyst particles supported on the carrier and selected from the group consisting of Pt, Pd, Rh and Au, the carrier being obtained by bonding oxide particles formed from zirconium oxide, aluminum oxide or lanthanum oxide to a part of the surface of ceramic particles. The composition of the oxide particles is different from that of the ceramic particles, the diameter of the oxide particles is smaller than that of the ceramic particles when observed as the circle-equivalent diameter of a cross-sectional image, and the catalyst particles are supported on at least one of the surface of the oxide particles and the surface of the ceramic particles.

Description

Technical Field

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

[0002] As a gas sensor for detecting the oxygen concentration in exhaust gas of an automobile or the like, a gas sensor having a sensor element in which a detection electrode and a reference electrode are provided on the surface of a cylindrical or plate-like solid electrolyte is known. In addition, a porous electrode protection layer for preventing poisoning of the working electrode is formed on the surface of the working electrode.

[0003] Furthermore, the following technique has been developed: by loading catalyst particles of a noble metal such as Pt on the electrode protection layer and reacting a specific component in the exhaust gas that has passed through the porous protection layer with the catalyst particles, the detection accuracy, responsiveness of the gas, or the sensor output is stabilized (Patent Documents 1 and 2).

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-71632

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2019-117135 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] However, with the use of the gas sensor, due to the atmosphere and heat in the exhaust gas, there is a problem that the catalyst particles in the porous protection layer aggregate and coarsen, and the surface area of the catalyst and thus the catalytic ability decrease.

[0010] 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 suppress a decrease in the catalytic ability of a catalyst supported on a porous carrier.

[0011] Means for Solving the Problems

[0012] In order to solve the above technical problems, the sensor element of the present invention is characterized in that it includes a solid electrolyte body having oxygen ion conductivity, a working electrode provided on one surface of the solid electrolyte body and in contact with the gas to be measured, and a reference electrode provided on the other surface of the solid electrolyte body and in contact with a reference gas. The sensor element further includes a catalyst layer, and the catalyst layer includes: a porous carrier formed of ceramic particles covering the working electrode; and one or more catalyst particles selected from the group consisting of Pt, Pd, Rh, and Au and loaded on the carrier. The carrier is formed by the bonding of oxide particles formed of zirconia, alumina, or lanthanum oxide on a part of the surface of the ceramic particles. The oxide particles have a different composition from the ceramic particles and have a diameter smaller than that of the ceramic particles when observed in terms of the equivalent circle diameter of the cross-sectional image. The catalyst particles are loaded on at least one of the surface of the oxide particles and the surface of the ceramic particles.

[0013] According to this sensor element, since the carrier of the catalyst layer has a structure in which small-diameter oxide particles are bonded to a part of the surface of the ceramic particles, it is possible to suppress the coarsening of the catalyst particles due to aggregation caused by the atmosphere and heat in the exhaust gas during the use of the gas sensor. As a result, it is possible to suppress the reduction of the surface area of the catalyst particles and thus the catalytic ability.

[0014] Although the reason is uncertain, it is speculated that the oxide particles formed of zirconia, alumina, or lanthanum oxide are bonded to the ceramic particles, so that the surface states (such as potential) of the ceramic particles and the oxide particles change, and the bonding between the ceramic particles, the oxide particles, and the catalyst particles becomes stronger.

[0015] The gas sensor of the present invention is a gas sensor including a sensor element and a metal fitting main body for holding the sensor element, and is characterized in that the sensor element uses the sensor element described in claim 1.

[0016] The manufacturing method of the sensor element according to the first aspect of the present invention is characterized in that it is the manufacturing method of the sensor element described in claim 1, and a slurry containing the ceramic particles and ions of zirconia, alumina, or lanthanum oxide that become the oxide particles is coated in a manner covering the working electrode, and then fired to manufacture the carrier.

[0017] The manufacturing method of the sensor element according to the second aspect of the present invention is characterized in that it is the manufacturing method of the sensor element described in claim 1, a slurry containing the ceramic particles is coated in a manner covering the working electrode, and then fired to manufacture a porous body that becomes the carrier, and then the porous body is impregnated with a solution containing ions of zirconia, alumina, or lanthanum oxide that become the oxide particles and fired.

[0018] Effect of the Invention

[0019] According to the present invention, a sensor element can be obtained in which a decrease in the catalytic ability of a catalyst supported on a porous carrier is suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A sectional view along the longitudinal direction of a gas sensor (oxygen sensor) according to an embodiment of the present invention.

[0021] Figure 2 A schematic exploded perspective view of the sensor element.

[0022] Figure 3 A partially enlarged sectional view of the front end side of the sensor element.

[0023] Figure 4 A schematic sectional view of the sensor element orthogonal to the axial direction.

[0024] Figure 5 A schematic sectional view of the catalyst layer.

[0025] Figure 6 A schematic diagram showing a method for measuring the particle size of ceramic particles.

[0026] Figure 7 A view showing a cross-sectional SEM image of the catalyst layer.

[0027] Figure 8 An enlarged view of a cross-sectional SEM image of the catalyst layer.

[0028] Figure 9 A view showing the evaluation results of gas sensing characteristics.

[0029] Figure 10 A view showing a cross-sectional SEM image of catalyst particles (Pt particles) after particle growth in the catalyst layer of Examples and Comparative Examples.

[0030] Figure 11 A view showing a cross-sectional SEM image of catalyst particles (Pt particles) after particle growth in the catalyst layer of Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, embodiments of the present invention will be described.

[0032] Figure 1 A sectional view along the longitudinal direction (axis L direction) of a gas sensor (oxygen sensor) 1 according to an embodiment of the present invention, Figure 2 A schematic exploded perspective view of the sensor element 100, Figure 3 A partially enlarged sectional view of the front end side of the sensor element 100,Figure 4 It is a schematic cross-sectional view orthogonal to the direction of axis L of the sensor element 100.

[0033] As Figure 1 shown, the gas sensor 1 has: a sensor element 100, a metal fitting body (main body metal fitting) 30 that holds the sensor element 100 and the like inside, and a protective member 24 and the like mounted on the front end portion of the metal fitting body 30. The sensor element 100 is arranged so as to extend along the direction of axis L.

[0034] In addition, a catalyst layer 20 is provided on the front end side of the sensor element 100 so as to cover the working electrode (see Figure 2 ).

[0035] As Figure 2 shown, the sensor element 100 includes an oxygen concentration detection unit 130, and the oxygen concentration detection unit 130 is composed of a solid electrolyte body 105 and a reference electrode 104 and a working electrode 106 formed on both surfaces of the solid electrolyte 105. The reference electrode 104 is formed by a reference electrode portion 104a and a reference lead portion 104L that extends along the length direction of the solid electrolyte body 105 from the reference electrode portion 104a. The working electrode 106 is formed by a working electrode portion 106a and a detection lead portion 106L that extends along the length direction of the solid electrolyte body 105 from the working electrode portion 106a.

[0036] It should be noted that the illustration of the catalyst layer 20 is omitted in Figure 2 .

[0037] The protective layer 111 is composed of a porous electrode protection portion 113a and a reinforcing portion 112. The porous electrode protection portion 113a prevents the working electrode portion 106a from being poisoned by sandwiching the working electrode portion 106a between the porous electrode protection portion 113a and the solid electrolyte body 105. The reinforcing portion 112 protects the solid electrolyte body 105 by sandwiching the detection lead portion 106L. It should be noted that the sensor element 100 of the present embodiment constitutes a so-called oxygen concentration difference electromotive force type gas sensor (λ sensor) that can detect the oxygen concentration using the value of the voltage (electromotive force) generated between the electrodes of the oxygen concentration detection unit 130.

[0038] On the other hand, a lower surface layer 103 and an air introduction hole layer 107 are laminated on the lower surface of the reference electrode 104 so as to sandwich the reference electrode 104 between the lower surface layer 103 and the air introduction hole layer 107 and the solid electrolyte body 105. The air introduction hole layer 107 is formed in a substantially "U" shape with an open rear end side, and the internal space surrounded by the solid electrolyte body 105, the air introduction hole layer 107, and the lower surface layer 103 constitutes an air introduction hole 107h. And the reference electrode 104 is exposed to the air (reference gas) introduced into the air introduction hole 107h.

[0039] A laminate including a lower surface layer 103, an air introduction hole layer 107, a reference electrode 104, a solid electrolyte body 105, a working electrode 106, and a protective layer 111 forms an element main body 300. In the present embodiment, the element main body 300 is plate-shaped.

[0040] Moreover, the end of the reference lead portion 104L is electrically connected to the detection element side gasket 121 on the solid electrolyte body 105 through a conductor formed on a through hole 105a provided in the solid electrolyte body 105. On the other hand, the protective layer 111 is shorter than the end of the detection lead portion 106L in the axial direction of the axis L, and the end of the detection lead portion 106L is exposed on the upper surface from the rear end of the protective layer 111 and is connected to an external terminal (not shown) for connection to an external circuit.

[0041] It should be noted that the solid electrolyte body 105 has oxygen ion conductivity. For example, partially stabilized zirconia (YSZ) in which yttrium oxide as a stabilizer is dissolved can be used as the main component. Here, the main component means a component exceeding 50 mass% in the solid electrolyte body 3s.

[0042] The reference electrode 104 and the working electrode 106 are formed mainly of Pt, for example. Here, "mainly of Pt" means that the component exceeding 50 mass% in the electrode is Pt.

[0043] The lower surface layer 103, the protective layer 111, and the air introduction hole layer 107 can be insulators such as alumina. The electrode protection portion 113a can be a porous body mainly composed of zirconia. The porous body can be formed, for example, by bonding one or more ceramic particles selected from the group consisting of alumina, spinel, zirconia, mullite, zircon, and cordierite through firing or the like. By sintering a slurry containing these particles, pores are formed in the framework of the coating film when the gaps between the ceramic particles and the organic or inorganic binder in the slurry are burned out.

[0044] Return to Figure 1 The metal fitting main body 30 is made of SUS430 and has an external thread portion 31 for mounting the gas sensor to the exhaust pipe and a hexagonal portion 32 for engaging with a mounting tool during installation. In addition, a metal fitting side step portion 33 protruding radially inward is provided on the metal fitting main body 30, and the metal fitting side step portion 33 supports a metal holder 34 for holding the sensor element 100.

[0045] Moreover, a ceramic holder 35 and talc 36 are sequentially arranged inside the metal holder 34 from the front end side. The talc 36 is composed of a first talc 37 disposed inside the metal holder 34 and a second talc 38 disposed across the rear end of the metal holder 34.

[0046] The sensor element 100 is fixed relative to the metal holder 34 by compressing and filling the first talc 37 within the metal holder 34. Additionally, by compressing and filling the second talc 38 within the metal fitting body 30, the sealing performance between the outer surface of the sensor element 100 and the inner surface of the metal fitting body 30 is ensured.

[0047] Then, an alumina plug sleeve 39 is disposed on the rear end side of the second talc 38. The plug sleeve 39 is formed in a multi-stage cylindrical shape, an axial hole 39a is provided along the axis, and the sensor element 100 penetrates therethrough. Moreover, the reinforcing portion 30a on the rear end side of the metal fitting body 30 is bent inward, and the plug sleeve 39 is pressed toward the front end side of the metal fitting body 30 with a stainless steel ring member 40 interposed therebetween.

[0048] In addition, on the outer periphery of the front end side of the metal fitting body 30, a metal protection member 24 is mounted by welding. The metal protection member 24 covers the front end portion of the sensor element 100 protruding from the front end of the metal fitting body 30 and has a plurality of intake holes 24a. The protection member 24 is formed in a double structure, a bottomed cylindrical outer protection member 41 having a uniform outer diameter is disposed on the outside, and a bottomed cylindrical inner protection member 42 having an outer diameter of the rear end portion 42a formed larger than that of the front end portion 42b is disposed on the inside.

[0049] On the other hand, the front end side of a SUS430 outer cylinder 25 is inserted into the rear end side of the metal fitting body 30. The front end portion 25a after the front end side of the outer cylinder 25 is expanded in diameter is fixed to the metal fitting body 30 by laser welding or the like. A partition member 50 is disposed inside the rear end side of the outer cylinder 25, and a holding member 51 is interposed in the gap between the partition member 50 and the outer cylinder 25. The holding member 51 engages with a protruding portion 50a of the partition member 50 described later and is fixed by the outer cylinder 25 and the partition member 50 by pressing the outer cylinder 25.

[0050] In addition, in the partition member 50, a through hole 50b for inserting the leads 11, 12 for the sensor element 100 (in Figure 1 which, the lead 12 overlaps with the lead 11 in the depth direction and thus is not shown) is provided so as to penetrate from the front end side to the rear end side. Connection terminals 16 for connecting the leads 11 to 12 and the detection element side gasket 121 of the sensor element 100 are accommodated in the through hole 50b. Each of the leads 11 to 12 is connected to a connector (not shown) outside. An external device such as an ECU and each of the leads 11 to 12 perform input and output of electrical signals through the connector. In addition, although each of the leads 11 to 12 is not shown in detail, it has a structure in which a wire is covered with an insulating coating formed of resin.

[0051] Moreover, a substantially cylindrical rubber cap 52 for closing the opening 25b on the rear end side of the outer cylinder 25 is disposed on the rear end side of the separator 50. In a state where the rubber cap 52 is installed inside the rear end of the outer cylinder 25, the rubber cap 52 is fixed to the outer cylinder 25 by pressing the outer periphery of the outer cylinder 25 toward the radially inner side. Through holes 52a for inserting the lead wires 11 to 15 are also provided in the rubber cap 52 so as to penetrate from the front end side to the rear end side.

[0052] Next, the catalyst layer 20 will be described. As Figure 3 , Figure 4 shown, the catalyst layer 20 is a porous layer provided to cover the entire circumference of the front end side of the sensor element 100 (element main body 300).

[0053] The catalyst layer 20 is formed so as to cover the front end face of the sensor element 100 (element main body 300) and extend toward the rear end side along the axis L direction, and is formed so as to completely surround the front and back faces and the two side faces of the sensor element 100 (element main body 300) as Figure 4 shown. Further, when viewed from the axis L direction, the catalyst layer 20 covers at least the region including the reference electrode portion 104a and the working electrode portion 106a of the sensor element 100 (element main body 300) (this region constitutes the detection portion), and further extends from this region to the rear end.

[0054] In the sensor element 100, it may be exposed to poisoning substances such as silicon and phosphorus contained in the exhaust gas or attached with water droplets in the exhaust gas. Therefore, by coating the catalyst layer 20 on the outer surface of the sensor element 100, poisoning substances can be captured or direct contact between the water droplets and the sensor element 100 can be suppressed.

[0055] In addition, as Figure 5 shown, the catalyst layer 20 includes a porous carrier 23 formed of ceramic particles and catalyst particles 60 of one or more noble metals selected from the group consisting of Pt, Pd, Rh, and Au supported on the carrier 23.

[0056] The catalyst particles 60 react with specific components in the exhaust gas passing through the catalyst layer 20 (burn unburned gas components), thereby improving the gas detection accuracy, responsiveness, or stabilizing the sensor output. For example, the responsiveness of the gas sensor in an environment with a high gas flow rate can be improved.

[0057] A simple explanation will be given here. When the gas flow rate becomes faster, the unburned gas does not burn sufficiently on the working electrode 106 and remains inside the catalyst layer 20. Moreover, as the electrode reaction approaches the equilibrium state, for example, CO gas, which is one of the unburned gases remaining inside the catalyst layer 20, reaches the working electrode 106 in sequence and reacts, and sometimes the actual gas concentration cannot be reflected.

[0058] Therefore, by loading the catalyst layer 20 with the catalyst particles 60, a part of the unburned gas reacts with the catalyst particles 60 in the catalyst layer 20 and burns, so that the responsiveness of the gas sensor in an environment with a high gas flow rate can be improved.

[0059] Of course, the effects brought about by loading the catalyst layer 20 with the catalyst particles 60 are not limited to this.

[0060] However, due to the atmosphere and heat in the exhaust gas generated during the use of the gas sensor 1, the catalyst particles 60 in the catalyst layer 20 aggregate and coarsen, reducing the surface area of the catalyst and thus the catalytic ability.

[0061] Therefore, the present invention inhibits the reduction of the catalytic ability of the catalyst particles 60 loaded on the carrier 23 by making the structure of the carrier 23 as follows.

[0062] That is, as Figure 5 shown, the carrier 23 has a structure in which oxide particles 22, which are different in composition from the ceramic particles 21 and have a diameter smaller than that of the ceramic particles 21, are bonded to a part of the surface of the ceramic particles 21. As a result, a part of the surface of the ceramic particles 21 is exposed, and the other parts of the surface are covered by the oxide particles 22.

[0063] Moreover, the catalyst particles 60 are formed by being granularly dispersed on at least one of the surfaces of the ceramic particles 21 and the oxide particles 22 constituting the carrier 23.

[0064] The ceramic particles 21 preferably contain, for example, at least one selected from alumina, alumina-magnesia spinel, zirconia, and titanium dioxide, and alumina-magnesia spinel can be exemplified.

[0065] The oxide particles 22 are composed of zirconia, alumina, or lanthanum oxide. As the composition of zirconia, for example, ZrO2 is exemplified, but it may also contain non-stoichiometric compounds of Zr and oxygen.

[0066] When the carrier 23 has a structure in which the small-diameter oxide particles 22 are bonded to a part of the surface of the ceramic particles 21, it is possible to suppress the aggregation and coarsening of the catalyst particles 60 caused by the atmosphere and heat in the exhaust gas generated during the use of the gas sensor. As a result, it is possible to suppress the reduction of the surface area of the catalyst particles 60 and thus the catalytic ability.

[0067] Although the reason is uncertain, it is speculated that the oxide particles 22 composed of zirconia, alumina, or lanthanum oxide are bonded to the ceramic particles 21, so that the surface states (such as potential) of the ceramic particles 21 and the oxide particles 22 change, and the bonding between the ceramic particles 21, the oxide particles 22, and the catalyst particles 60 becomes stronger.

[0068] Among them, the ceramic particles 21 and the oxide particles 22 can be identified by performing elemental analysis on the cross-sectional sample of the catalyst layer 20 using EPMA (Electron Probe Micro Analyzer) and EDS (Energy Dispersive X-ray Spectroscopy).

[0069] In addition, regarding the particle size of the ceramic particles 21 and the oxide particles 22, the equivalent circle diameter of each of the ceramic particles 21 and the oxide particles 22 identified by elemental analysis in the cross-sectional sample of the catalyst layer 20 (such as the above-mentioned EPMA image, EDS image, etc.) is obtained for determination.

[0070] It should be noted that in the cross-sectional sample, for three or more ceramic particles 21, the particle sizes of the ceramic particles 21 and the oxide particles 22 are compared with the oxide particles 22 bonded to the surface of the ceramic particles 21. In addition, as Figure 5 shown in E, the oxide particles 22 further bonded to the surface of the oxide particles 22 bonded to the surface of the ceramic particles 21 (without passing through the ceramic particles 21) are not within the scope of the object.

[0071] However, as Figure 5 shown, the respective ceramic particles 21 are integrated by sintering, and the boundary A - B sometimes becomes unclear.

[0072] Therefore, as Figure 6 shown, in the case where it is considered that the ceramic particle 21x and the adjacent ceramic particle 21y are sintered and bonded, the boundary is determined as follows.

[0073] First, if the contour P of the ceramic particle 21x narrows between points A - B to form a neck, the direction parallel to the straight line C1 connecting A - B is set as L. Second, regarding the distance between A - B, when the longest lengths parallel to the direction L in the contours of all the ceramic particles 21x, 21y connected to the ceramic particle 21x are set as Lx and Ly respectively, if the length of C1 is shorter than either of Lx and Ly, it is regarded that the two ceramic particles 21x, 21y are sintered and bonded between A - B, and the straight line C1 is set as the boundary between the two ceramic particles 21x, 21y.

[0074] In addition, when the ceramic particle 21x is interrupted in the above-mentioned field of view, the outer edge C2 of the field of view is regarded as a part of the contour P of the ceramic particle 21x.

[0075] Further, when depicting the outermost contour P of the ceramic particles 21x, in the case of overlapping with the oxide particles 22x, 22z, the contours P1, P2 of the boundaries between the oxide particles 22x, 22z and the ceramic particles 21x are regarded as part of the contour P of the ceramic particles 21x. On the other hand, the oxide particles 22y existing inside the contour P of the ceramic particles 21x are ignored.

[0076] Therefore, the straight lines C1, C2 are regarded as part of the contour P of the ceramic particles 21x, and the area surrounded by the entire contour P ( Figure 6 the shaded part) is set as the circle equivalent diameter of the ceramic particles 21x.

[0077] Next, a method for manufacturing the sensor element according to the embodiment of the present invention will be described. In this method for manufacturing the sensor element, with respect to the carrier 23 of the catalyst layer 20, a slurry containing the ceramic particles 21 and ions of zirconia, alumina, or lanthanum oxide that become the oxide particles 22 is coated on the surface of the front end side of the sensor element 100 so as to cover the working electrode 106 (working electrode portion 106a), and then fired.

[0078] The ions that become the oxide particles are contained, for example, in an aqueous solution of oxyacetatozirconium as a complex. Moreover, the aqueous solution, the ceramic particles 21, a binder, and a solvent such as water or PGA can be added to prepare a slurry.

[0079] When firing this slurry, the oxide particles 22 are precipitated from the ions that become the oxide particles and bind to a part of the surface of the ceramic particles 21 to obtain the carrier 23.

[0080] As another method, when, for example, a porous layer formed of the ceramic particles 21 is infiltrated with a solution containing Zr ions (such as zirconium nitrate solution) and heat-treated, the oxide particles 22 are precipitated on the ceramic particles 21 and bind to a part of the surface of the ceramic particles 21 to obtain the carrier 23.

[0081] In addition, when the fired carrier 23 is immersed in a solution containing catalyst ions (such as dinitrodiammineplatinum nitrate solution) and heat-treated, fine catalyst particles 60 are precipitated on the surface of the carrier.

[0082] The present invention is not limited to the above embodiments. The sensor element only needs to have a solid electrolyte body, a working electrode, and a reference electrode, and can be applied to the oxygen sensor (oxygen sensor element) of this embodiment, but is not limited to these uses, and of course also relates to various modifications and equivalents included in the idea and scope of the present invention.

[0083] For example, the present invention can also be applied to an all-region oxygen sensor having an oxygen pump unit, an NO sensor for detecting the NO concentration in the gas to be measured x in the gas to be measured, x an NO sensor (NO x sensor element), an HC sensor for detecting the HC concentration (HC sensor element), etc. In addition, the sensor element can be cylindrical, or can be a binary sensor or a linear sensor.

[0084] In addition, the gas sensor can also have a heater that generates heat by being energized.

[0085] Examples

[0086] <Evaluation of Gas Sensing Characteristics>

[0087] Manufacturing Figure 1 , Figure 2 the plate-shaped sensor element (oxygen sensor element) 100 shown.

[0088] A slurry containing alumina particles as ceramic particles 21, an aqueous solution of zirconium acetate as a complex containing ions of zirconia as oxide particles 22, and a binder and water is coated on the surface of the front end side of the sensor element 100 so as to cover the working electrode 106 (working electrode portion 106a), and fired to obtain a carrier 23. The content of the oxide particles 22 (zirconia) relative to the carrier 23 is set to 5% by mass.

[0089] In addition, the fired carrier 23 is immersed in a solution containing catalyst Pt ions (dinitrodiammine Pt nitrate solution) and heat-treated. This is taken as an example.

[0090] As a comparative example, except that the aqueous solution containing no ions of zirconia is used as the catalyst layer 20, the carrier 23 is produced in the same manner as above, and the fired carrier 23 is immersed in a solution containing catalyst Pt ions (dinitrodiammine Pt nitrate solution) and heat-treated.

[0091] Next, the above sensor element 100 is assembled into the gas sensor 1, and the gas sensing characteristics are evaluated by observing the output difference of the sensor output under two different specified gas compositions (a gas rich in H2 and a gas rich in CO). The gas sensing characteristics refer to the degree of influence of the composition of the gas to be measured on the sensor output of the component to be measured, and the lower the value of the gas sensing characteristics, the better.

[0092] The obtained results are shown in Figures 7 - 11 .

[0093] Figure 7 , Figure 8Shows a cross-sectional SEM image of the catalyst layer 20.

[0094] Figure 9 Shows the evaluation results of the gas sensing characteristics. Figure 10 、 Figure 11 Respectively show cross-sectional SEM images of the catalyst particles 60 (Pt particles) after particle growth in the catalyst layer 20 of the example and the comparative example.

[0095] As Figures 7 - 8 shown, it can be seen that small-diameter particles of zirconia that become oxide particles 22 are precipitated on a part of the surface of the alumina particles that are the ceramic particles 21. In addition, in this example, it can be seen that fine Pt particles that become catalyst particles 60 are precipitated on the surfaces of both the ceramic particles 21 and the oxide particles 22.

[0096] As Figure 9 shown, in the case of the example using the carrier 23 in which small-diameter particles of zirconia that become oxide particles 22 are precipitated on a part of the surface of the alumina particles that are the ceramic particles 21, the gas sensing characteristics are maintained well for a long time.

[0097] On the other hand, in the case of the example using only the alumina particles that are the ceramic particles 21 as the carrier 23, the gas sensing characteristics deteriorate over time.

[0098] Moreover, as Figure 10 、 Figure 11 shown, it has been found that in the example, the particle size of the Pt particles after particle growth is at most about 20 nm, while in the comparative example, it coarsens to about 50 nm.

[0099] Description of reference numerals

[0100] 1: Gas sensor; 20: Catalyst layer; 21: Ceramic particles; 22: Oxide particles; 23: Carrier; 30: Metal fitting body; 60: Catalyst particles; 100: Sensor element; 104: Reference electrode; 106: Working electrode; 105: Solid electrolyte body.

Claims

1. A sensor element, characterized in that, It has: a solid electrolyte body with oxygen ion conductivity, a working electrode provided on one surface of the solid electrolyte body and in contact with the gas to be measured, and a reference electrode provided on the other surface of the solid electrolyte body and in contact with a reference gas. The sensor element further includes a catalyst layer, and the catalyst layer includes: a porous carrier formed of ceramic particles covering the working electrode; and one or more catalyst particles selected from the group consisting of Pt, Pd, Rh, and Au supported on the carrier. The carrier is formed by the bonding of oxide particles made of zirconia, alumina, or lanthanum oxide on a part of the surface of the ceramic particles. The oxide particles have a different composition from the ceramic particles and have a diameter smaller than that of the ceramic particles when observed by the equivalent circle diameter of the cross-sectional image. The catalyst particles are supported on at least one of the surface of the oxide particles and the surface of the ceramic particles.

2. A gas sensor comprising a sensor element and a metal fitting main body for holding the sensor element, characterized in that the sensor element uses the sensor element according to claim 1.

3. A method for manufacturing the sensor element according to claim 1, characterized in that a slurry containing the ceramic particles and ions of zirconia, alumina, or lanthanum oxide that become the oxide particles is coated in a manner covering the working electrode, and fired to manufacture the carrier.

4. A method for manufacturing the sensor element according to claim 1, characterized in that a slurry containing the ceramic particles is coated in a manner covering the working electrode, fired to manufacture a porous body that becomes the carrier, and then the porous body is impregnated with a solution containing ions of zirconia, alumina, or lanthanum oxide that become the oxide particles and fired.

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