JP7811892B2 - Method for manufacturing electrodes for gas sensors - Google Patents

Description

The present invention relates to a method for manufacturing a gas sensor electrode suitable for use in a gas sensor that detects the concentration of a specific gas contained in combustion gas or exhaust gas from a combustor, internal combustion engine, etc.

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, has a configuration including one or more cells, each cell having a pair of electrodes formed on the surface of an oxygen ion conductive solid electrolyte layer made of zirconia or the like (see Patent Document 1). Here, each electrode is formed by screen printing a paste containing precious metal particles such as Pt, followed by firing.
Furthermore, in gas sensors for detecting NOx and ammonia, it is necessary to include Pt and Au in the electrodes, 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).

JP 2013-96888 A Patent No. 6795841

However, 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 precipitating Au particles on their surface is difficult from a productivity standpoint. Furthermore, if Au particles are precipitated on fine Pt particles and then alloyed, there is a risk that multiple Pt particles will sinter and bond, causing them to become coarse. If the Pt particles become coarse, the effect of improving the reactivity of the electrode will not be achieved.

The present invention aims to provide a method for manufacturing electrodes for gas sensors that incorporate Pt and Au into the electrodes to improve their reactivity while suppressing declines in productivity.

To solve the above problem, the present invention provides a method for manufacturing a gas sensor electrode containing Pt particles and Au, and is characterized by comprising the following steps: an alloying step in which Au is supported on the surfaces of first Pt particles having an average particle size D1 (μm) measured by laser diffraction, and then heat-treated to alloy the particles to prepare alloy particles; a mixing step in which the alloy particles are mixed with second Pt particles having an average particle size D2 (μm) measured by laser diffraction that is smaller than D1 (μm); an electrode paste preparation step in which an electrode paste containing the mixed particles obtained in the mixing step is prepared; and an electrode formation step in which the electrode paste is applied to a predetermined position on a sensor element to form an electrode.

This method for manufacturing gas sensor electrodes allows the electrode to contain Au and stably contain two types of Pt particles with particle sizes D1 and D2, thereby improving the reactivity of the electrode and preventing a decline in productivity.

In the method for manufacturing a 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 improve the reactivity of the electrode by using fine second Pt particles, while further suppressing the decrease in productivity caused by the difficulty in handling the fine second Pt particles.

This invention makes it possible to manufacture gas sensor electrodes that contain Pt and Au, thereby improving the reactivity of the electrode and suppressing declines in productivity.

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. 2 is a cross-sectional view taken along the axis of a sensor element in the gas sensor of FIG. 1. FIG. FIG. 10 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.

Hereinafter, an embodiment of the present invention will be described.
FIG. 1 is a longitudinal cross-sectional view (a cross-sectional view cut in the longitudinal direction 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 taken along the axis AX of a sensor element 10 in the gas sensor of FIG. 1.

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. The gas sensor 1 includes a cylindrical metal shell 20 having a threaded portion 21 formed at a predetermined position on its outer surface for fastening 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 illustrates only two of the six terminal members (specifically, terminal members 75 and 76).

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, each of the six terminal members (terminal members 75, 76, etc.) is electrically connected to a different lead wire 71. For example, as shown in Fig. 1, the core wire of the lead wire 71 is crimped and held by the lead wire holding portion 77 of terminal member 75. Also, the core wire of another lead wire 71 is crimped and held by the lead wire holding portion 78 of terminal member 76.

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 confined inside the outer cylinder 51 (described later) is introduced into the inside of the sensor element 10 through the atmosphere inlet 10h.

The metallic shell 20 is a cylindrical member having a through hole 23 that penetrates in the direction of the axis AX. The metallic shell 20 has a shelf portion 25 that protrudes radially inward and constitutes part of the through hole 23. The metallic 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, 44 formed by filling talc powder in an annular shape, and a ceramic sleeve 45 are disposed inside the through hole 23 of the metallic shell 20. In detail, the ceramic holder 42, the talc rings 43, 44, and the ceramic sleeve 45 are disposed in this order, overlapping one another, 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.

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 against 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.

The retaining 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 disposed within the insertion hole 62 (see FIG. 1 ). A flange 65 that protrudes radially outward is formed at the rear end of the retaining member 60. The retaining member 60 is held by the internal support member 53 with the flange 65 abutting against the internal support member 53. The internal support member 53 is held to the external cylinder 51 by a crimped portion 51g of the external cylinder 51 that is crimped radially inward.
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 of the terminal members described above (such as the lead wire gripping portions 77 and 78) are disposed in the through holes 91.

An elastic seal member 73 made of fluororubber is disposed radially inside the rear end opening 51c located at the axial rear end (upper end in Figure 1) of the outer cylinder 51. This elastic seal member 73 has a total of six cylindrical insertion holes 73c extending in the direction of the axis AX. Each insertion hole 73c is defined by the 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 seal member 73 is elastically compressed and deformed in the radial direction by crimping the rear end opening 51c of the outer tube 51 radially inward, thereby tightly contacting the insertion hole surface 73b with 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.

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, respectively. The solid electrolyte bodies 111e, 121e, and 131e are embedded in these openings.

The solid electrolyte bodies 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 body 111e. A porous Ip1- electrode 113 is provided on the back side of the solid electrolyte body 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.

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) is provided in the first dense layer 118 on the rear end side of the void 10G to establish electrical connection with the electrode terminals 13 to 15.

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, so that 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 gap 10G, forming an air inlet 10h, and the gap 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.

Furthermore, a third dense layer 118B is interposed between the composite layer including the solid electrolyte body 111e (the solid electrolyte body 111e and the 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 (the Ip1+ electrode 112 side) of the porous layer 114B, with the Ip1+ electrode 112 protruding 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 positive electrode 112 is surrounded by the solid electrolyte body 111e.

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

The solid electrolyte body 121e is disposed opposite the solid electrolyte body 111e in the stacking direction, with the insulator 140 sandwiched between them. A porous Vs- electrode 122 is provided on the front surface side (top surface side in Figure 2) of the solid electrolyte body 121e. A porous Vs+ electrode 123 is provided on the back surface side (bottom surface side in Figure 2) of the solid electrolyte body 121e.

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 of the first measuring chamber 150 (on the right side in Figure 2) as a partition between the first measuring chamber 150 and the second measuring chamber 160 described later, limiting the amount of exhaust gas flowing per unit time.

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

The solid electrolyte body 131e is arranged opposite the solid electrolyte body 121e in the stacking direction, with the insulator 145 sandwiched between them. A porous Ip2+ electrode 132 and a porous Ip2- electrode 133 are provided on the surface side (top side in Figure 2) of the solid electrolyte body 131e.

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 formed 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. This 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 the 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.

The solid electrolyte body 131e and electrodes 132, 133 constitute the Ip2 cell 130 (second pump cell) for detecting the NOx concentration. This 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 gas) introduced into the second measurement chamber 160 flows between the electrodes 132 and 133.

In addition, in this embodiment, an alumina insulating layer 119 is formed on the back surface of the solid electrolyte body 111e in an area other than the Ip1-electrode 113, and the Ip1-electrode 113 contacts the solid electrolyte body 111e through a through-hole 119b (see Figure 4) that penetrates the alumina insulating layer 119 in the stacking direction.

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

Furthermore, in this embodiment, an alumina insulating layer 138 is formed on the surface of the solid electrolyte body 131e in areas 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 areas 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.

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 flows 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 flow of current Icp between the electrodes 122 and 123 sends oxygen from the first measuring chamber 150 into the reference oxygen chamber 170.

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 electrode 112 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 electrode 113 becomes negative, and oxygen is pumped out of the first measuring chamber 150 to outside the sensor element 10.

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. When NOx in the exhaust gas comes into contact with the electrode 133 in the second measuring chamber 160, a voltage Vp2 is applied between the electrodes 132 and 133, causing it to be decomposed (reduced) into nitrogen and oxygen on the electrode 133. The decomposed oxygen becomes oxygen ions and flows through the solid electrolyte body 131e, migrating into the reference oxygen chamber 170. At this time, residual oxygen left behind in the first measuring chamber 150 is also moved 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) via 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.

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 the current flowing through the Ip2 cell 130 is proportional to the NOx concentration. Therefore, by detecting the current Ip2 flowing through the Ip2 cell 130, the NOx concentration in the exhaust gas can be detected based on that current value.

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

Next, we will explain a method for manufacturing a gas sensor electrode according to an embodiment of the present invention. As an example, we will assume that the Vs-electrode 122 that constitutes the Vs cell (detection cell) 120 is manufactured using this embodiment.

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 first Pt particles are heat-treated to be alloyed, thereby preparing 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 the above method, the platinum powder and gold compound solution are stirred under reduced pressure (e.g., below 70°C) 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, sodium gold(I) sulfite, or the like.
However, the method for supporting Au is not limited to the above method, and other known methods may be used.

Next, according to the method described above, the first Pt particles carrying the Au compound are heat-treated at 560-650°C. Heat treatment at 560-650°C exceeds the decomposition temperature of the gold compound (approximately 360°C), which decomposes the gold compound into metallic gold while simultaneously strengthening the bond between the platinum particle surface and the gold (alloying), resulting in a highly dispersed gold support on the platinum particle surface and the production of alloy particles.

D1 (μm) can be, for example, 2.5 to 13.5 μm. If D1 is less than 2.5 μm, the electrode may become too dense, resulting in reduced pump performance. If D1 exceeds 13.5 μm, the electrode film may become too uneven, potentially resulting in breakage.

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). Furthermore, the particle size distribution is measured using a sample prepared by dropwise mixing the sample in 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) Immerse the solids 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 distributions between 2) and 4).

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), a roll mill (tri-roll), etc. Compared to methods that grind particles using a crusher, a mixer is less likely to crush the soft Au and can suppress large fluctuations in particle size.
If the Au is crushed and enlarged into a foil-like shape, when the Au is prepared into an electrode paste described below and screen-printed to form an electrode, the Au may get caught and captured on the screen, which may cause 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.

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.

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 body. The ceramic particles are preferably made of alumina and/or zirconia. Examples of zirconia include YSZ (yttria-partially stabilized zirconia).

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.

Next, the electrode paste is applied to predetermined positions of the sensor element 10 to form electrodes (electrode formation step). 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.

The present invention is not limited to the above-described embodiments, and it goes without saying that various modifications and equivalents fall within the spirit and scope of the present invention. For example, the present invention can be applied to oxygen sensors such as full-range air-fuel ratio sensors in addition to NOx sensors, but is not limited to these applications.

The present invention will be explained in detail below using examples, but the present invention is not limited to these examples.

The first Pt particles and the chloroauric (III) acid solution were stirred under reduced pressure to evaporate the solvent and dry up, thereby supporting the gold compound on the surface of the first Pt particles. Next, the first Pt particles with the Au compound supported thereon were heat-treated. This decomposed the gold compound into metallic gold, and simultaneously alloyed the first Pt particles with gold.
Next, the Pt--Au alloy particles were mixed with second Pt particles having a particle size smaller than that of the first Pt particles using a tri-roll.

Next, an electrode paste containing zirconia particles, a binder, and a solvent in addition to the mixed particles 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 of the electrode paste, and a Vs-electrode 122 was similarly formed.

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.

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 two types of Pt particles with 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 ratio: mass ratio) of the Vs-electrode 122 was outside the upper and lower limits of the target value, and the proportion of Au contained in the electrode decreased. This raises concerns about a decrease in the reactivity of the electrode.
In the comparative example, when the Au particles were mixed with each Pt particle, the soft Au particles were crushed and expanded to form a foil, and when the electrodes were formed by screen printing, the foil-like Au got caught on the screen and did not reach the electrodes.

1 Gas sensor 10 Sensor element 122 Electrode (Vs-electrode)

Claims (2)

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: The method for manufacturing a gas sensor electrode according to claim 1, characterized in that in the mixing step, the second Pt particles are mixed in a ratio of 5 to 60 mass% relative to the total amount of the first Pt particles and the second Pt particles.