Method for manufacturing electrodes for gas sensors

Abstract

To provide a method for manufacturing an electrode for a gas sensor, in which the reactivity of the electrode is improved by causing the electrode to include Pt and Au, and a decrease in productivity is suppressed.SOLUTION: A method for manufacturing a gas sensor electrode containing Pt particles and Au, includes: an alloying step of causing Au to be carried on a surface of first Pt particles with an average particle diameter of D1 (μm) as measured by a laser diffraction method, and alloying through heat-treatment to prepare alloy particles; a mixing step of mixing the alloy particles and second Pt particles with an average particle diameter of D2 (μm) smaller than D1 (μm) as measured by laser diffraction method; an electrode paste preparation step of preparing an electrode paste containing mixed particles obtained in the mixing step; an electrode forming step of forming the electrodes by coating the electrode paste onto predetermined positions on a sensor element.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing an electrode for a gas sensor that is suitably used in a gas sensor 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] A known gas sensor capable of measuring the concentration of a specific gas component in a measurement gas such as exhaust gas from an internal combustion engine of an automobile or the like has a configuration including one or more cells each having a pair of electrodes formed on the surface of an oxygen ion conductive solid electrolyte layer such as zirconia (see Patent Document 1). Here, each electrode is formed by screen printing a paste containing particles of a precious metal such as Pt, followed by firing. Furthermore, in gas sensors that detect NOx and ammonia, the electrodes must contain Pt and Au, and a technology is known for producing platinum-gold powder by precipitating and alloying Au particles on the surface of Pt particles (see Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-96888 [Patent Document 2] Patent No. 6795841 Summary of the Invention [Problem to be solved by the invention]

[0004] The smaller the particle size of the Pt particles contained in the electrode, the more reactivity the electrode has with the gas components being measured. However, fine Pt particles are difficult to handle, and depositing Au particles on their surface is difficult from a productivity standpoint. Furthermore, if Au particles are deposited on fine Pt particles and then alloyed, multiple Pt particles may sinter and bond, resulting in coarsening. Furthermore, if the Pt particles become coarse, the effect of improving the reactivity of the electrode cannot be achieved.

[0005] Therefore, an object of the present invention is to provide a method for manufacturing an electrode for a gas sensor that improves the reactivity of the electrode by incorporating Pt and Au into the electrode and that suppresses a decrease in productivity. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the present invention provides a method for manufacturing an electrode for a gas sensor comprising Pt particles and Au, and is characterized by comprising an alloying step of supporting Au on the surface of first Pt particles having an average particle size D1 (μm) measured by a laser diffraction method, followed by heat treatment to alloy the particles to prepare alloy particles; a mixing step of mixing the alloy particles with second Pt particles having an average particle size D2 (μm) measured by a laser diffraction method that is smaller than D1 (μm); an electrode paste preparation step of preparing an electrode paste containing the mixed particles obtained in the mixing step; and an electrode formation step of applying the electrode paste to a predetermined position of a sensor element to form an electrode.

[0007] According to this manufacturing method of an electrode for a gas sensor, the electrode can contain Au and stably contain two types of Pt particles with particle sizes D1 and D2, thereby improving the reactivity of the electrode and suppressing a decrease in productivity.

[0008] In the method for manufacturing the gas sensor electrode of the present invention, in the mixing step, the second Pt particles may be mixed in a ratio of 5 to 60 mass % with respect to the total amount of the first Pt particles and the second Pt particles. This method for manufacturing an electrode for a gas sensor can achieve the effect of improving the reactivity of the electrode by the fine second Pt particles, while further suppressing the decrease in productivity caused by the difficulty in handling the fine second Pt particles. [Effects of the Invention]

[0009] According to the present invention, the electrode for a gas sensor can be manufactured by incorporating Pt and Au into the electrode, thereby improving the reactivity of the electrode and suppressing a decrease in productivity. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view taken along the longitudinal direction of a gas sensor (NOx sensor) as an example to which the present invention is applied. [Figure 2] 2 is a cross-sectional view taken along the axis of a sensor element in the gas sensor of FIG. 1. FIG. [Figure 3] FIG. 2 is a diagram showing the composition (Au / Pt ratio) of the Ip2-electrode manufactured by the manufacturing method according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described. FIG. 1 is a longitudinal cross-sectional view (a cross-sectional view cut longitudinally along an axis AX) of a gas sensor (NOx sensor) 1 as an example to which the present invention is applied, and FIG. 2 is a cross-sectional view along the axis AX of a sensor element 10 in the gas sensor of FIG. 1.

[0012] The gas sensor 1 is a NOx sensor that includes a sensor element 10 capable of detecting the concentration of a specific gas (NOx) in exhaust gas, which is a measurement target gas, and is attached to an exhaust pipe (not shown) of an internal combustion engine for use. The gas sensor 1 includes a cylindrical metal shell 20 that has a threaded portion 21 formed at a predetermined position on its outer surface for fixing to the exhaust pipe. The sensor element 10 is in the form of an elongated plate extending in the direction of the axis AX, and is held inside the metal shell 20. More specifically, the gas sensor 1 includes a holding member 60 having an insertion hole 62 into which the rear end portion 10k (the upper end portion in FIG. 1) of the sensor element 10 is inserted, and six terminal members held inside the holding member 60. Note that FIG. 1 shows only two of the six terminal members (specifically, terminal members 75 and 76).

[0013] A total of six electrode terminals 13 to 18 (only electrode terminals 14 and 17 are shown in FIG. 1 ) that are rectangular in plan view are formed at the rear end 10k of the sensor element 10. The aforementioned terminal members are elastically abutted against and electrically connected to the electrode terminals 13 to 18, respectively. For example, the element abutting portion 75b of the terminal member 75 elastically abuts against and electrically connected to the electrode terminal 14. Furthermore, the element abutting portion 76b of the terminal member 76 elastically abuts against and electrically connected to the electrode terminal 17. Furthermore, six terminal members (terminal members 75, 76, etc.) are electrically connected to different lead wires 71, respectively. For example, as shown in Fig. 1, the core wire of lead wire 71 is crimped and held by lead wire holding portion 77 of terminal member 75. Also, the core wire of another lead wire 71 is crimped and held by lead wire holding portion 78 of terminal member 76.

[0014] In addition, an air inlet 10h opens on one of the main surfaces of the rear end 10k of the sensor element 10, further forward than the electrode terminal portions 13 to 15 and further rearward than the ceramic sleeve 45 described later (see Figure 2), and the air inlet 10h is arranged within the insertion hole 62 of the holding member 60. As a result, the reference atmosphere enclosed inside the outer cylinder 51 (described later) is introduced into the inside of the sensor element 10 through the atmosphere inlet 10h.

[0015] The metal shell 20 is a cylindrical member having a through hole 23 that penetrates in the direction of the axis AX. The metal shell 20 has a shelf portion 25 that protrudes radially inward and constitutes part of the through hole 23. The metal shell 20 holds the sensor element 10 in the through hole 23 with the front end portion 10s of the sensor element 10 protruding outward from its own front end side (downward in FIG. 1) and the rear end portion 10k of the sensor element 10 protruding outward from its own rear end side (upward in FIG. 1). Furthermore, an annular ceramic holder 42, two talc rings 43 and 44 formed by filling talc powder in an annular shape, and a ceramic sleeve 45 are arranged inside the through hole 23 of the metallic shell 20. In detail, the ceramic holder 42, the talc rings 43 and 44, and the ceramic sleeve 45 are arranged in this order, overlapping from the axial leading end side (the lower end side in FIG. 1) of the metallic shell 20 to the axial rear end side (the upper end side in FIG. 1), so as to surround the radial periphery of the sensor element 10.

[0016] A metal cup 41 is disposed between the ceramic holder 42 and the shelf portion 25 of the metallic shell 20. A crimping ring 46 is disposed between the ceramic sleeve 45 and the crimped portion 22 of the metallic shell 20. The crimped portion 22 of the metallic shell 20 is crimped so as to press the ceramic sleeve 45 toward the tip end side via the crimping ring 46. An outer protector 31 and an inner protector 32 made of metal (specifically, stainless steel) and having a plurality of holes are attached by welding to the front end portion 20b of the metallic shell 20 so as to cover the front end portion 10s of the sensor element 10. On the other hand, an outer cylinder 51 is attached by welding to the rear end portion of the metallic shell 20. The outer cylinder 51 has a cylindrical shape extending in the direction of the axis AX and surrounds the sensor element 10.

[0017] The holding member 60 is made of an insulating material (specifically, alumina) and is a cylindrical member having an insertion hole 62 that penetrates in the direction of the axis AX. The six terminal members (terminal members 75, 76, etc.) described above are arranged inside the insertion hole 62 (see FIG. 1). A flange portion 65 that protrudes radially outward is formed at the rear end of the holding member 60. The holding member 60 is held by the internal support member 53 in such a manner that the flange portion 65 abuts against the internal support member 53. The internal support member 53 is held to the external cylinder 51 by a crimped portion 51g that is crimped radially inward of the external cylinder 51. An insulating member 90 is disposed on the rear end surface 61 of the holding member 60. The insulating member 90 is made of an electrically insulating material (specifically, alumina) and has a cylindrical shape. A total of six through holes 91 are formed in the insulating member 90, penetrating in the direction of the axis AX. The lead wire gripping portions (lead wire gripping portions 77, 78, etc.) of the terminal members described above are disposed in the through holes 91.

[0018] An elastic seal member 73 made of fluororubber is disposed radially inside a rear end opening 51c located at the axial rear end (upper end in FIG. 1) of the outer cylinder 51. A total of six cylindrical insertion holes 73c extending in the direction of the axis AX are formed in the elastic seal member 73. Each insertion hole 73c is defined by an insertion hole surface 73b (cylindrical inner wall surface) of the elastic seal member 73. One lead wire 71 is inserted into each insertion hole 73c. ​​Each lead wire 71 extends to the outside of the gas sensor 1 through the insertion hole 73c of the elastic seal member 73. The elastic sealing member 73 is elastically compressed and deformed radially by crimping the rear end opening 51c of the outer tube 51 radially inward, thereby tightly adhering the insertion hole surface 73b and the outer peripheral surface 71b of the lead wire 71 and creating a watertight seal between the insertion hole surface 73b and the outer peripheral surface 71b of the lead wire 71.

[0019] 2, the sensor element 10 has a structure in which solid electrolyte bodies 111e, 121e, and 131e are stacked in the stacking direction with insulators 140 and 145 disposed therebetween. Furthermore, the sensor element 10 has a heater 161 stacked on the back surface of the solid electrolyte body 131e. The heater 161 includes plate-shaped insulators 162 and 163 primarily made of alumina and a heater pattern 164 (primarily made of Pt) embedded therebetween. The solid electrolyte bodies 111e, 121e, and 131e are each substantially rectangular, and rectangular openings are provided at the leading ends of the insulating layers 111s, 121s, and 131s, with the solid electrolyte bodies 111e, 121e, and 131e respectively embedded in these openings.

[0020] The solid electrolyte substrates 111e, 121e, and 131e are made of zirconia, a solid electrolyte, and have oxygen ion conductivity. A porous Ip1+ electrode 112 is provided on the front side of the solid electrolyte substrate 111e. A porous Ip1- electrode 113 is provided on the back side of the solid electrolyte substrate 111e. Furthermore, the front side of the Ip1+ electrode 112 is covered with a porous layer 114B. An Ip1+ lead (not shown) is connected to the Ip1+ electrode 112. An Ip1- lead 117 (not shown) is connected to the Ip1- electrode 113.

[0021] In addition, a gas-impermeable first dense layer 118 made of alumina or the like and having a void 10G is laminated on the surfaces of the Ip1+ electrode 112 and the Ip1+ lead, with the porous layer 114 exposed from the void 10G and the Ip1+ lead covered by the outer periphery of the first dense layer 118. The void 10G extends straight from the vicinity of the porous layer 114 to the portion communicating with the air inlet 10h. A through-hole (not shown) for electrical connection with the electrode terminals 13-15 is provided in the first dense layer 118 on the rear end side of the void 10G.

[0022] Furthermore, a gas-impermeable second dense layer 115 made of alumina or the like is laminated on the surface of the first dense layer 118 to close the gap 10G. As a result, the Ip1+ electrode 112 covered with the porous layer 114 is disposed in the gap 10G surrounded by the dense layers 115 and 118, preventing contact with the gas to be measured. The second dense layer 115 has a rectangular opening at a position overlapping the rear end of the void 10G, forming an air inlet 10h, and the void 10G is connected to the air inlet 10h. The air inlet 10h opens further rearward than the first porous body 151 (described later) and can introduce air rather than exhaust gas. This allows the Ip1+ electrode 112 to be exposed to air introduced from the air inlet 10h through the porous layer 114.

[0023] Furthermore, a third dense layer 118B is interposed between the composite layer including the solid electrolyte body 111e (solid electrolyte body 111e and insulating layer 111s) and the first dense layer 118, and a rectangular opening 118Bh is provided on the tip side of the third dense layer 118B. The opening 118Bh is filled with the porous layer 114B, and the Ip1+ electrode 112 is formed on the lower surface (Ip1+ electrode 112 side) of the porous layer 114B, and the Ip1+ electrode 112 protrudes downward from the lower surface of the third dense layer 118B. The protruding portion of the Ip1+ electrode 112 is covered with the solid electrolyte body 111e. In this way, the side surface of the Ip1+ electrode 112 is surrounded by the solid electrolyte body 111e.

[0024] The solid electrolyte body 111e and the electrodes 112, 113 constitute an Ip1 cell (pump cell) 110. The Ip1 cell 110 pumps oxygen in and out (so-called oxygen pumping) between the atmosphere in contact with the electrode 112 (the atmosphere in the gap 10G, which is different from the measurement gas outside the sensor element 10) and the atmosphere in contact with the electrode 113 (the atmosphere in the first measurement chamber 150, described later, i.e., the measurement gas outside the sensor element 10) in response to a pump current Ip1 flowing between the electrodes 112, 113.

[0025] The solid electrolyte body 121e is disposed opposite the solid electrolyte body 111e in the stacking direction, with the insulator 140 sandwiched therebetween. A porous Vs-electrode 122 is provided on the front surface side (upper surface side in FIG. 2) of the solid electrolyte body 121e. A porous Vs+ electrode 123 is provided on the back surface side (lower surface side in FIG. 2) of the solid electrolyte body 121e.

[0026] A first measurement chamber 150 is formed between the solid electrolyte bodies 111e and 121e as an internal space of the sensor element. This first measurement chamber 150 is an internal space into which the measurement gas (exhaust gas) flowing through the exhaust passage is first introduced into the sensor element 10, and is in communication with the outside of the sensor element 10 through a first porous body (diffusion resistance portion) 151 that is gas-permeable and water-permeable. The first porous body 151 is provided on the side of the first measurement chamber 150 as a partition between the sensor element 10 and the outside, and limits the amount of exhaust gas flowing into the first measurement chamber 150 per unit time (diffusion rate). A second porous body 152 is provided at the rear end side (right side in Figure 2) of the first measuring chamber 150 as a partition between the first measuring chamber 150 and the second measuring chamber 160 described later, which limits the amount of exhaust gas flowing per unit time.

[0027] The solid electrolyte body 121e and the electrodes 122 and 123 constitute a Vs cell (detection cell) 120. This Vs cell 120 generates an electromotive force mainly in response to the oxygen partial pressure difference between the atmospheres separated by the solid electrolyte body 121e (the atmosphere in the first measurement chamber 150 in contact with the electrode 122 and the atmosphere in the reference oxygen chamber 170 in contact with the electrode 123).

[0028] The solid electrolyte body 131e is disposed opposite the solid electrolyte body 121e in the stacking direction, with the insulator 145 sandwiched therebetween. A porous Ip2+ electrode 132 and a porous Ip2- electrode 133 are provided on the front surface side (upper surface side in FIG. 2) of the solid electrolyte body 131e.

[0029] A reference oxygen chamber 170 is formed as an isolated small space between the Ip2+ electrode 132 and the Vs+ electrode 123. This reference oxygen chamber 170 is configured by an opening 145b formed in the insulator 145. A porous ceramic body is disposed inside the reference oxygen chamber 170 on the Ip2+ electrode 132 side. A second measurement chamber 160 serving as an internal space of the sensor element is formed at a position facing the Ip2-electrode 133 in the stacking direction. The second measurement chamber 160 is composed of an opening 145c penetrating the insulator 145 in the stacking direction, an opening 125 penetrating the solid electrolyte body 121e in the stacking direction, and an opening 141 penetrating the insulator 140 in the stacking direction. The first measurement chamber 150 and the second measurement chamber 160 communicate with each other through a gas-permeable and water-permeable second porous body 152. Therefore, the second measurement chamber 160 communicates with the outside of the sensor element 10 through the first porous body 151, the first measurement chamber 150, and the second porous body 152.

[0030] The solid electrolyte body 131e and the electrodes 132, 133 constitute an Ip2 cell 130 (second pump cell) for detecting the NOx concentration. The Ip2 cell 130 transfers oxygen (oxygen ions) derived from NOx decomposed in the second measurement chamber 160 to the reference oxygen chamber 170 through the solid electrolyte body 131e. At this time, a current corresponding to the concentration of NOx contained in the exhaust gas (measurement target gas) introduced into the second measurement chamber 160 flows between the electrodes 132 and 133.

[0031] In this embodiment, an alumina insulating layer 119 is formed on the rear surface of the solid electrolyte body 111e in a region other than the Ip1-electrode 113, and the Ip1-electrode 113 is in contact with the solid electrolyte body 111e through a through-hole 119b (see FIG. 4) that penetrates the alumina insulating layer 119 in the stacking direction.

[0032] Furthermore, in this embodiment, an alumina insulating layer 128 is formed on the surface of the solid electrolyte body 121e in a region other than the Vs-electrode 122, and the Vs-electrode 122 contacts the solid electrolyte body 121e through a through-hole (not shown) that penetrates the alumina insulating layer 128 in the stacking direction. Furthermore, an alumina insulating layer 129 is formed on the rear surface of the solid electrolyte body 121e in an area other than the Vs+ electrode 123, and the Vs+ electrode 123 is in contact with the solid electrolyte body 121e through a through-hole (not shown) that penetrates the alumina insulating layer 129 in the stacking direction.

[0033] Furthermore, in this embodiment, an alumina insulating layer 138 is formed on the surface of the solid electrolyte body 131e in a region other than the Ip2+ electrode 132, and the Ip2+ electrode 132 contacts the solid electrolyte body 131e through a through-hole (not shown) that penetrates the alumina insulating layer 138 in the stacking direction. Furthermore, an alumina insulating layer 138 is formed on the surface of the solid electrolyte body 131e in a region other than the Ip2- electrode 133, and the electrode 133 contacts the solid electrolyte body 131e through a through-hole (not shown) that penetrates the alumina insulating layer 138 in the stacking direction.

[0034] Here, the detection of NOx concentration by the gas sensor 1 of this embodiment will be briefly described. The solid electrolyte bodies 111e, 121e, and 131e of the sensor element 10 are heated and activated as the temperature of the heater pattern 164 rises, thereby causing the Ip1 cell 110, the Vs cell 120, and the Ip2 cell 130 to operate. Exhaust gas flowing through the exhaust passage (not shown) is introduced into the first measuring chamber 150 while the flow rate is restricted by the first porous body 151. At this time, a weak current Icp is flowing through the Vs cell 120 from the electrode 123 side to the electrode 122 side. As a result, oxygen in the exhaust gas can receive electrons from the electrode 122 in the first measuring chamber 150, which is the negative electrode, and become oxygen ions, which flow through the solid electrolyte body 121e and migrate into the reference oxygen chamber 170. In other words, the current Icp flowing between the electrodes 122 and 123 sends oxygen from the first measuring chamber 150 into the reference oxygen chamber 170.

[0035] If the oxygen concentration of the exhaust gas introduced into the first measuring chamber 150 is lower than a predetermined value, current Ip1 is passed through the Ip1 cell 110 so that the electrode 112 side becomes negative, and oxygen is pumped into the first measuring chamber 150 from outside the sensor element 10. On the other hand, if the oxygen concentration of the exhaust gas introduced into the first measuring chamber 150 is higher than a predetermined value, current Ip1 is passed through the Ip1 cell 110 so that the electrode 113 side becomes negative, and oxygen is pumped out of the first measuring chamber 150 to outside the sensor element 10.

[0036] In this way, the exhaust gas whose oxygen concentration has been adjusted in the first measuring chamber 150 is introduced into the second measuring chamber 160 through the second porous body 152. NOx in the exhaust gas that has come into contact with the electrode 133 in the second measuring chamber 160 is decomposed (reduced) into nitrogen and oxygen on the electrode 133 by applying a voltage Vp2 between the electrodes 132 and 133, and the decomposed oxygen becomes oxygen ions that flow through the solid electrolyte body 131e and move into the reference oxygen chamber 170. At this time, the residual oxygen that was left behind in the first measuring chamber 150 also moves into the reference oxygen chamber 170 by the Ip2 cell 130. As a result, a current derived from NOx and a current derived from the residual oxygen flow through the Ip2 cell 130. The oxygen that moves into the reference oxygen chamber 170 is released to the outside (atmosphere) through the Vs+ electrode 123 and Vs lead, and the Ip2+ electrode 132 and Ip2+ lead, which are in contact with the inside of the reference oxygen chamber 170. For this reason, the Vs+ lead and Ip2+ lead are porous.

[0037] Here, because the concentration of residual oxygen left unpumped in the first measuring chamber 150 is adjusted to a predetermined value as described above, the current derived from that residual oxygen can be considered to be approximately constant and is little affected by fluctuations in the current derived from NOx, so that the current flowing through the Ip2 cell 130 is proportional to the NOx concentration. Therefore, the current Ip2 flowing through the Ip2 cell 130 can be detected, and the NOx concentration in the exhaust gas can be detected based on the current value.

[0038] In this embodiment, the Ip1+ electrode 112 is disposed in a gap 10G surrounded by the dense layers 115 and 118 to prevent contact with the gas to be measured, and is exposed to the atmosphere introduced through the atmosphere inlet 10h. As a result, the Ip1+ electrode 112 always uses the air as the reference atmosphere, so that the atmosphere of the Ip1+ electrode 112 is kept constant even if the oxygen atmosphere in the gas to be measured changes.

[0039] Next, a method for manufacturing a gas sensor electrode according to an embodiment of the present invention will be described. As an example, it is assumed that the Vs-electrode 122 constituting the Vs cell (detection cell) 120 is manufactured by the method of this embodiment.

[0040] First, Au is supported on the surface of first Pt particles having an average particle size D1 (μm) measured by laser diffraction, and then the particles are alloyed by heat treatment to prepare alloy particles (alloying step). As a method for supporting Au on the surfaces of the first Pt particles, for example, the method described in Japanese Patent No. 6795841 can be adopted. In this method, the platinum powder and gold compound solution are stirred under reduced pressure (for example, at 70°C or below) while the solvent is evaporated to dryness, resulting in a highly dispersed gold compound supported on the surface of the platinum powder particles. This results in a coating of the gold compound on the platinum particle surface. The gold compound can be, for example, an aqueous solution containing chloroauric acid (III) or sodium gold (I) sulfite. However, the method for supporting Au is not limited to the above method, and other known methods may be used.

[0041] Next, according to the method described above, the first Pt particles carrying the Au compound are heat-treated at 560 to 650°C. Heat treatment at 560 to 650°C exceeds the decomposition temperature of the gold compound (approximately 360°C), so the gold compound is decomposed into metallic gold, and at the same time, the bond between the platinum particle surface and the gold is strengthened (alloyed), resulting in highly dispersed gold being carried on the platinum particle surface, and alloy particles can be obtained.

[0042] D1 (μm) can be, for example, 2.5 to 13.5 μm. If D1 is less than 2.5 μm, the electrode becomes too dense, which may result in reduced pump performance. If D1 exceeds 13.5 μm, the electrode film becomes too uneven, which may result in disconnection.

[0043] The measurement of particle size distribution by laser diffraction utilizes the fact that when particles are irradiated with light, the amount and pattern of scattered light vary depending on the particle diameter (Mie scattering theory). However, since the method for calculating particle size distribution from scattered light differs depending on the particle size measuring instrument, in the present invention, measurement is performed using a laser diffraction / scattering particle size distribution measuring instrument (Model LA-750, manufactured by Horiba, Ltd., light source: He-Ne laser (632.8 nm), 1 mW). The particle size distribution is measured using a sample prepared by adding and mixing the sample dropwise into pure water to form a uniform dispersion. In addition, electrodes are generally manufactured by applying an electrode paste to a predetermined position by screen printing or the like, and since the electrode paste generally contains ceramic particles in addition to the electrode metal material, the particle size of the electrode metal material in the electrode paste is measured according to the following procedure. Note that the particle size of the metal material before it is made into a paste may also be measured. 1) Heat the electrode paste (for example, hold at 500°C for 1 hour) to remove the binder and solvent components. 2) Measure the particle size distribution of the remaining solids 3) The solid content is immersed in heated aqua regia to dissolve the precious metal components (electrode metal material). 4) Measure the particle size distribution of the remaining solids 5) Calculate the average particle size of the electrode metal material and ceramic particles from the difference in particle size distribution between 2) and 4).

[0044] Next, the Pt—Au alloy particles obtained in the alloying step are mixed with second Pt particles having an average particle size D2 (μm) smaller than D1 (μm) as measured by laser diffraction (mixing step). For mixing, it is preferable to use a mixer such as a hybrid mixer (rotating / revolving mixer) or a roll mill (tri-roll). Compared to methods that grind particles using a grinder, a mixer is less likely to crush the soft Au and can suppress large fluctuations in particle size. If the Au becomes crushed and becomes large enough to resemble a foil, when it is prepared into an electrode paste (described later) and screen-printed to form an electrode, the Au may get caught on the screen and be captured, which could result in variations in the amount of Au contained in the electrode. D2 (μm) can be, for example, 0.5 to 5.0 μm. If D2 is less than 0.5 μm, excessive sintering may occur during firing, resulting in disconnection. If D2 exceeds 5.0 μm, the electrode film may become uneven, resulting in disconnection.

[0045] In the mixing step, the second Pt particles may be mixed in an amount of 10 to 60 mass % with respect to the total amount of the first Pt particles and the second Pt particles. If the mixing ratio of the second Pt particles is less than 10 mass %, the effect of improving the reactivity of the electrode due to the fine second Pt particles may not be obtained. If the mixing ratio of the second Pt particles exceeds 60 mass %, the productivity may decrease due to the difficulty in handling the fine second Pt particles.

[0046] Next, an electrode paste containing the mixed particles obtained in the mixing step is prepared (electrode paste preparation step). The composition of the electrode paste includes, in addition to the mixed particles, for example, ceramic particles, a binder, and a solvent. The ceramic particles are used as a co-base to enhance the adhesion of the electrode to the solid electrolyte. The ceramic particles are preferably made of alumina and / or zirconia. An example of zirconia is YSZ (yttria-partially stabilized zirconia).

[0047] Examples of binders include ethyl cellulose, polyvinyl butyral, cellulose acetate, alkyd, phenol, acrylic, epoxy, polyurethane, etc. The binder refers to a material that dissolves in the solvent contained in the electrode paste. Examples of the solvent include butyl carbitol, butyl carbitol acetate, terpineol, terpineol acetate, ethyl carbitol, ethyl carbitol acetate, dihydroterpineol, and propylene glycol diacetate. The solid components of the electrode paste are the mixed particles and ceramic particles, and the binder dissolves in the solvent to give the paste a predetermined viscosity.

[0048] Next, the electrode paste is applied to predetermined positions of the sensor element 10 to form electrodes (electrode formation process). In this embodiment, the electrode paste is applied to the surfaces of the solid electrolyte bodies 111e and 121e to form the Ip1-electrode 113 and the Vs-electrode 122. Although the method for applying the electrode paste is not limited, screen printing is preferred. In screen printing, the electrode paste is first printed onto the target object from above a predetermined screen mask (such as a metal mask). The printed electrode pattern is then dried and fired at a predetermined temperature (e.g., 1000°C or higher) to form the electrode. It should be noted that "screen printing" includes printing in which the screen mask and the target are spaced apart, as well as contact printing in which the screen mask comes into contact with the target.

[0049] The present invention is not limited to the above-described embodiment, and various modifications and equivalents are included within the spirit and scope of the present invention. For example, the present invention can be applied to oxygen sensors such as a full-range air-fuel ratio sensor in addition to a NOx sensor, but is not limited to these applications.

[0050] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples. [Example]

[0051] The first Pt particles and the chloroauric (III) acid solution were stirred under reduced pressure to evaporate the solvent and dry up, resulting in gold compounds being supported on the surfaces of the first Pt particles. The first Pt particles with the Au compounds supported were then heat-treated. This decomposed the gold compounds into metallic gold, and simultaneously alloyed the first Pt particles with gold. Next, the Pt—Au alloy particles and second Pt particles having a particle size smaller than that of the first Pt particles were mixed using a tri-roll.

[0052] Next, an electrode paste containing the above mixed particles, zirconia particles, a binder, and a solvent was prepared, and was applied to the surface of the solid electrolyte body 131e of the sensor element 10 shown in Fig. 2. After application, the sensor element 10 was fired at 1000°C or higher to form the Vs-electrode 122. For comparison, first Pt particles, second Pt particles, and Au particles (average particle size measured by laser diffraction method: 0.6 μm) were simply mixed and used as the electrode material for the electrode paste, and a Vs-electrode 122 was similarly formed.

[0053] The composition (Au / Pt ratio: mass ratio) of the Vs-electrode 122 in the obtained sensor element 10 was determined by TOF-SIMS analysis. The obtained results are shown in Fig. 3. Fig. 3 shows the relative values ​​of the Au / Pt ratio in the sensor element 10 of each example and comparative example, when the target value of the Au / Pt ratio (dashed line in Fig. 3) is set to 1.

[0054] 3, in Examples 1 to 3, the composition (Au / Pt ratio) of the Vs-electrode 122 falls within the upper and lower limits of the target value, and the electrode stably contains Au and Pt particles of two different particle sizes. This improves the reactivity of the electrode and prevents a decrease in productivity. On the other hand, in the comparative example, the composition (Au / Pt mass ratio) of the Vs-electrode 122 was outside the target upper and lower limits, resulting in a decrease in the proportion of Au contained in the electrode, which may result in a decrease in the reactivity of the electrode. In the comparative example, when the Au particles were mixed with the Pt particles, the soft Au particles were crushed and expanded into foil-like particles. When the electrodes were formed by screen printing, the foil-like Au particles were caught on the screen and did not reach the electrodes. [Explanation of symbols]

[0055] 1 Gas sensor 10 Sensor element 122 electrode (Vs-electrode)

Claims

1. A method for manufacturing an electrode for a gas sensor including Pt particles and Au, comprising the steps of: an alloying step of supporting Au on the surface of first Pt particles having an average particle size D1 (μm) measured by a laser diffraction method, and then alloying the first Pt particles by heat treatment to prepare alloy particles; a mixing step of mixing the alloy particles with second Pt particles having an average particle size D2 (μm) measured by a laser diffraction method that is smaller than D1 (μm); an electrode paste preparation step of preparing an electrode paste containing the mixed particles obtained in the mixing step; an electrode forming step of applying the electrode paste to predetermined positions of a sensor element to form electrodes; 1. A method for manufacturing an electrode for a gas sensor, comprising:

2. 2. The method for manufacturing an electrode for a gas sensor according to claim 1, wherein in the mixing step, the second Pt particles are mixed in a ratio of 5 to 60 mass % with respect to the total amount of the first Pt particles and the second Pt particles.

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