TW202448601A - Mixed silver powder, method for producing mixed silver powder, and conductive paste - Google Patents

Abstract

The present invention provides a mixed silver powder capable of reducing a specific resistance of a conductive film. The mixed silver powder according to the present invention is a mixed silver powder containing first silver particles, second silver particles, and third silver particles. By observing cross sections of more than 100 silver particles on a polished section of a resin block obtained by embedding the mixed silver powder in the resin block and measuring circumferences of the silver particles and a length of a long side and a length of a short side of a rectangle circumscribing an outer shape of the silver particle and having the smallest area, silver particles corresponding to the length of the long side being 3 [mu]m or more are used as the first silver particles, silver particles corresponding to the length of the long side being 0.5 [mu]m or more and 3 [mu]m or less are used as the second silver particles, and silver particles corresponding to the length of the long side being 0.5 [mu]m or less are used as the third silver particles. The average aspect ratio of the first silver particles is 2 or more. The average aspect ratio of the second silver particles is 1.5 or more and less than 2. The average aspect ratio of the third silver particles is less than 1.5. The number ratio of the first silver particles in the mixed silver powder is more than 0.5% and less than 5%. The number ratio of the second silver particles in the mixed silver powder is more than 10%. The number ratio of the third silver particles in the mixed silver powder is more than 15%. According to the following formula (1): a ratio [alpha]= a perimeter of one of the second silver particles/(length of the long side of the second silver particle * 2 + length of the short side of the second silver particle * 2)…(1), a calculated average value of the ratio [alpha] is 0.84 or more. Polycarboxylic acid is attached to the surface of at least one silver particle selected from the group consisting of the first silver particles, the second silver particles and the third silver particles.

Description

Mixed silver powder, method for producing mixed silver powder, and conductive paste

The present invention relates to a mixed silver powder, a method for manufacturing the mixed silver powder and a conductive paste.

The method of applying or printing a conductive paste containing conductive metal powder on a base material such as a film, substrate, or electronic component and drying and curing it by heating to form electrodes or electrical wiring has been widely used since ancient times. However, with the recent improvement in the performance of electronic devices, electrodes and electrical wiring formed using conductive pastes are required to have lower resistance, and this requirement is becoming more stringent year by year.

In response to the above requirements, for example, Patent Document 1 proposes the following: for the purpose of obtaining a conductive paste composition having high conductivity (low resistance), a silver powder containing a fragmented silver powder and a spherical silver powder and having a polycarboxylic acid attached to the surface of at least one of the fragmented silver powder and the spherical silver powder is used as a conductive metal powder. [Prior Art Document] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 2011-100573

[The problem that the invention wants to solve]

Here, regarding the silver powder (silver powder) that can be used in the conventional conductive paste, there is room for further improvement in terms of reducing the specific resistance of a conductive film formed using the conductive paste.

Therefore, the object of the present invention is to provide a mixed silver powder capable of reducing the specific resistance of a conductive film. In addition, the object of the present invention is to provide a method for producing a mixed silver powder capable of reducing the specific resistance of a conductive film. In addition, the object of the present invention is to provide a conductive paste capable of reducing the specific resistance of a conductive film. [Means for Solving the Problem]

In order to solve the above problems, the inventors have repeatedly conducted diligent research, and as a result, the inventors have completed the present invention described below.

That is, the main structure of the present invention for solving the above-mentioned problems is as follows.

[1] A mixed silver powder comprising a first silver particle, a second silver particle and a third silver particle, wherein when observing more than 100 silver particles in a polished cross section of a resin block formed by embedding the mixed silver powder in a resin, and measuring the perimeter of the silver particles, and the long side length and short side length of a rectangle that circumscribes the outer shape of the silver particles and has the smallest area, the silver particles having a long side length of 3 μm or more are set as the first silver particles, the silver particles having a long side length of 0.5 μm or more and less than 3 μm are set as the second silver particles, and the silver particles having a long side length of less than 0.5 μm are set as the third silver particles, The average value of the aspect ratio of the first silver particles is greater than 2, The average value of the aspect ratio of the second silver particles is greater than 1.5 and less than 2, The average value of the aspect ratio of the third silver particles is less than 1.5, The number ratio of the first silver particles in the mixed silver powder is greater than 0.5% and less than 5%, the number ratio of the second silver particles in the mixed silver powder is greater than 10%, and the number ratio of the third silver particles in the mixed silver powder is greater than 15%, By the following formula (1): Ratio α = circumference of the second silver particle / (long side length of the second silver particle × 2 + short side length of the second silver particle × 2) ... (1) The calculated average value of the ratio α is greater than 0.84, A polycarboxylic acid is attached to the surface of at least one silver particle selected from the group consisting of the first silver particle, the second silver particle, and the third silver particle.

[2] The mixed silver powder as described in [1], wherein a polycarboxylic acid is attached to the surfaces of the first silver particles and the second silver particles.

[3] The mixed silver powder according to [1] or [2], wherein the content ratio of the polycarboxylic acid relative to the total of the first silver particles, the second silver particles, and the third silver particles is 0.01 mass % or more and 0.15 mass % or less.

[4] The mixed silver powder as described in any one of [1] to [3], wherein the polycarboxylic acid is at least one polycarboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, apple acid, diethylglutaric acid, 3-methyladipic acid, butylmalonic acid, maleic acid, diglycolic acid and citric acid.

[5] A method for producing a mixed silver powder, producing the mixed silver powder as described in any one of [1] to [4], the method comprising: a mixing step, mixing silver powder A containing silver particles A1 and silver particles A2 with silver powder B containing silver particles B to obtain a mixture; and a pre-treatment step, before the mixing step, performing a surface treatment agent attachment treatment on at least one of the silver powders A and B using a polycarboxylic acid, or a post-treatment step, performing a surface treatment agent attachment treatment on the mixture obtained in the mixing step using a polycarboxylic acid, When observing more than 100 silver particles in a polished cross section of a resin block formed by embedding the silver powder A in resin and more than 100 silver particles in a polished cross section of a resin block formed by embedding the silver powder B in resin, respectively, and measuring the perimeter of the silver particles, and the length of the long side and the length of the short side of a rectangle circumscribing the outer shape of the silver particles and having the smallest area, the average value of the aspect ratio of the silver particles A1 is greater than 2, the average value of the aspect ratio of the silver particles A2 is less than 2, and by the following formula (2): ratio β = perimeter of silver particle A2/(length of the long side of silver particle A2 × 2 + length of the short side of silver particle A2 × 2)…(2) The calculated average value of the ratio β is greater than 0.84, and the average value of the aspect ratio of the silver particles B is less than 1.5.

[6] The method for producing mixed silver powder as described in [5] comprises the pretreatment step, In the pretreatment step, the silver powder A is subjected to a surface treatment agent attachment treatment using the polycarboxylic acid.

[7] The method for producing a mixed silver powder according to [5] or [6], wherein the usage ratio of the polycarboxylic acid relative to the total of the silver powder A and the silver powder B is 0.01 mass % or more and 0.15 mass % or less.

[8] A method for producing a mixed silver powder as described in any one of [5] to [7], wherein the polycarboxylic acid used in the surface treatment agent adhesion treatment is at least one polycarboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, apple acid, diethylglutaric acid, 3-methyladipic acid, butylmalonic acid, maleic acid, diglycolic acid and citric acid.

[9] A conductive paste comprising the mixed silver powder, a binder and a solvent as described in any one of [1] to [4]. [Effect of the invention]

The present invention can provide a mixed silver powder capable of reducing the specific resistance of a conductive film. In addition, the present invention can provide a method for producing a mixed silver powder capable of reducing the specific resistance of a conductive film. In addition, the present invention can provide a conductive paste capable of reducing the specific resistance of a conductive film.

The mixed silver powder of the present invention is suitable for use as a conductive filler for a conductive paste. The conductive paste using the mixed silver powder of the present invention can be used to form a conductive pattern on a substrate or to form an electrode. The conductive paste using the mixed silver powder of the present invention can be printed on a substrate using, for example, screen printing, lithography, photolithography, etc. to form a conductive film such as a conductive pattern or an electrode (hereinafter, sometimes referred to as a "conductive film"). Furthermore, the mixed silver powder of the present invention can be obtained, for example, by the method for manufacturing the mixed silver powder of the present invention.

(Terms and measurement methods) First, before explaining the implementation form, the terms and measurement methods used in this manual are explained.

<Observation of Silver Particles> In this specification, observation of silver particles can be performed as follows: First, a mixed silver powder or silver powder is placed in a resin and a hardener and cured, and polished using a cross-section polisher to expose the cross section of the silver particles, and each silver particle is observed at a predetermined magnification (e.g., 5,000 times or 3,000 times) using a scanning electron microscope (SEM). Here, based on the contrast between the insulating resin and the conductive silver particles, the silver particles whose outlines are clear and whose peripheries can be clearly seen are considered to be silver particles whose cross sections are exposed. Then, in the polished cross section of the resin block formed by embedding the mixed silver powder or the silver powder in the resin, 100 or more silver particles are randomly selected from the silver particles whose periphery can be clearly seen, and the periphery of the silver particles is traced using image analysis software, thereby measuring the length of the long side of the rectangle of each silver particle (hereinafter, sometimes referred to as "long side length"), the length of the short side of the rectangle (hereinafter, sometimes referred to as "short side length"), the perimeter, the area of the silver particle, and the maximum length of the silver particle. Furthermore, as the resin and hardener, for example, "EpoFix resin" and "EpoFix hardener" manufactured by Struers can be used. In addition, as the cross-sectional polisher, for example, "ArBlade 5000" manufactured by Japan High-technologies Corporation can be used. In addition, as the scanning electron microscope, for example, "JEOL JSM-IT300LV" manufactured by JEOL Ltd. can be used. In addition, as the image analysis software, for example, "Image Analysis Particle Size Distribution Measurement Software Mac-View" manufactured by MOUNTECH Co., Ltd. can be used.

Here, referring to FIG1, the method for determining the length of the long side and the short side of the rectangle with the smallest area that circumscribes the outer shape of the cross section of the silver particle is described. As shown in FIG1, first, select (more than 100) silver particles as the measurement object. Then, rotate the selected silver particles 360 degrees, and simultaneously make all rectangles circumscribing the outer shape of the silver particles. Then, take out the rectangle with the smallest area from these rectangles, and determine the length of the long side and the short side of the rectangle.

<Average value of aspect ratio> In this specification, the so-called "aspect ratio" is the ratio of the long side length to the short side length (long side length/short side length), and the so-called "average value of aspect ratio" refers to the average value of each aspect ratio of the silver particles within the range divided by the value of the long side length.

<Average value of short side length> In this specification, the "average value of short side length" refers to the average value of the length of each short side of the silver particles within the range divided according to the value of the long side length.

<Average value of shape factor> In this specification, the "shape factor" refers to the shape factor calculated by "π×(maximum length of silver particle/2) ² /area of silver particle", and the "average value of shape factor" refers to the average value of each shape factor of silver particles in the range divided according to the value of the long side length. The unit of the maximum length of silver particles is "μm", and the unit of the area of silver particles is "μm ² ".

<Measurement of Particle Size Distribution> In this specification, the volume-based cumulative 10% particle size ( _D10 ), cumulative 50% particle size ( _D50 ), cumulative 90% particle size ( _D90 ), cumulative 95% particle size ( _D95 ), and cumulative 100% (i.e., largest particle) particle size ( _DMAX ) of the mixed silver powder are measured using a laser diffraction-scattering particle size distribution measuring device (Microtrac MT-3300 EXII, manufactured by MicrotracBEL Co., Ltd.).

<Method for determining the content ratio of polycarboxylic acid> In this specification, the "content ratio of polycarboxylic acid" can be determined by the following method: first, polycarboxylic acid is dissolved in hydrochloric acid from a mixed silver powder to which polycarboxylic acid is attached, then the polycarboxylic acid is esterified in the hydrochloric acid solution in which the polycarboxylic acid is dissolved, then the esterified polycarboxylic acid is extracted into an organic solvent, then the amount of adipic acid ester extracted is obtained based on the calibration curve of polycarboxylic acid ester, and the amount of polycarboxylic acid is calculated by conversion calculation. For example, when the polycarboxylic acid is adipic acid, the content ratio of adipic acid can be determined according to the quantitative method of adipic acid described in Japanese Patent Laid-Open No. 2012-122880. Furthermore, when the polycarboxylic acid is adipic acid, the content ratio of adipic acid can also be determined by the following method: using hydrogen chloride-methanol reagent to methylate the adipic acid on the surface of the sample silver powder, extracting the generated dimethyl adipate into an organic solvent, and determining it by gas chromatography-mass spectrometry (GC-MS).

<Brunauer-Emmett-Teller (BET) specific surface area> In this manual, "BET specific surface area" can be measured using Macsorb HM-model 1210 (manufactured by MOUNTECH) and the BET single-point method based on nitrogen adsorption. In the measurement of BET specific surface area, the degassing conditions before the measurement are 60°C and 10 minutes.

<Heat loss (Ig-Loss) value of mixed silver powder> In this manual, the so-called "heat loss (Ig-Loss) value of mixed silver powder" indicates the change in mass when heated from room temperature to 800°C. Specifically, it indicates the amount of components other than silver in the mixed silver powder, and is an indicator of the amount of residual components in the mixed silver powder, such as residual components such as treatment agents or additives used in the production process of the mixed silver powder. Moreover, in this manual, the "heat loss (Ig-Loss) value of the mixed silver powder" can be calculated by precisely weighing (weighing value: w1) the mixed silver powder sample and placing it in a magnetic crucible, heating it to 800°C, and then keeping it at 800°C for 30 minutes, which is sufficient time to achieve constant weight, and then cooling and weighing it again (weighing value: w2), and then according to "heat loss (Ig-Loss) value (mass %) = (w1-w2)/w1×100".

<Tap density of mixed silver powder> In this manual, the "tap density of mixed silver powder" can be calculated by, for example, using a tap density measuring device (produced by Shibayama Scientific Co., Ltd., volumetric specific gravity measuring device SS-DA-2), weighing 30 g of silver powder sample and placing it in a 20 mL test tube, tapping it 1,000 times with a drop of 20 mm, and then calculating the tap density = sample mass (g) / sample volume after tapping (mL)".

<Circle equivalent diameter (Heywood diameter)> In this specification, the "circle equivalent diameter (Heywood diameter)" can be calculated using the area of the silver particle measured by observing the cross section of the silver particle. Specifically, the circle equivalent diameter (Heywood diameter) can be obtained by making a circle having the same area as the area of the silver particle.

<Viscosity of conductive paste> In this manual, "viscosity of conductive paste" can be set to the following value: for example, using 5XHBDV-IIIUC manufactured by Brookfield as a rotational viscometer, using CPE-52 as a cone spindle, setting the measurement temperature to 25°C, setting the cone spindle speed to 1 rpm, and rotating the cone spindle for 5 minutes.

(Mixed silver powder) The mixed silver powder of the present invention is a mixed silver powder comprising a first silver particle, a second silver particle and a third silver particle. When observing more than 100 silver particles in a polished cross section of a resin block formed by embedding the mixed silver powder in a resin, and measuring the perimeter of the silver particles and the length of the long side and the length of the short side of a rectangle circumscribing the outer shape of the silver particles and having the smallest area, the silver particles having a long side length of 3 μm or more are set as the first silver particles, the silver particles having a long side length of 0.5 μm or more and less than 3 μm are set as the second silver particles, and the silver particles having a long side length of less than 0.5 μm are set as the third silver particles. When the silver particles with a diameter of 20 μm are used as the third silver particles, the average value of the aspect ratio of the first silver particles is greater than 2, the average value of the aspect ratio of the second silver particles is greater than 1.5 and less than 2, the average value of the aspect ratio of the third silver particles is less than 1.5, the number ratio of the first silver particles in the mixed silver powder is greater than 0.5% and less than 5%, the number ratio of the second silver particles in the mixed silver powder is greater than 10%, and the number ratio of the third silver particles in the mixed silver powder is greater than 15%, by the following formula (1): Ratio α = circumference of the second silver particle / (long side length of the second silver particle × 2 + short side length of the second silver particle × 2) ... (1) The calculated average value of the ratio α (also called rectangularity) is greater than 0.84, and polycarboxylic acid is attached to the surface of at least one silver particle selected from the group consisting of the first silver particle, the second silver particle, and the third silver particle. If it is a mixed silver powder as described above, the specific resistance of the conductive film can be reduced. The reason is inferred to be that in the conductive film, the relatively small third silver particles enter the gaps between the relatively large first silver particles that are beneficial for reducing the specific resistance, thereby reducing the space rate of the conductive film and making the silver powder dense. As a result, the space without silver particles in the conductive film, that is, the part that becomes high resistance, is reduced, and the specific resistance is reduced. In addition, the reason is inferred to be that: in the conductive film, there are second silver particles with the long side length between the first silver particles and the third silver particles, and because the shape of the second silver particles is close to a rectangle, the first silver particles and the second silver particles or the second silver particles may be connected in a plane or line, thereby further reducing the space ratio of the conductive film, further reducing the part that becomes high resistance, and reducing the specific resistance. In addition, it is inferred that the silver particles are excellent in filling and sintering ease due to the tendency of having a larger aspect ratio as the length diameter increases, and the conductivity of the conductive film obtained is improved as a result. Furthermore, the perimeter of each silver particle, the length of the long side and the short side of the rectangle, the average value of the aspect ratio, the number ratio, and the ratio α can be adjusted by the characteristics of the silver powder that can be used as a raw material, the timing of the surface treatment agent attachment treatment described later, and the type and usage rate of the polycarboxylic acid that can be used in the surface treatment agent attachment treatment.

<First silver particles> In the mixed silver powder of the present invention, the first silver particles are silver particles having a long side length of 3 μm or more.

In the mixed silver powder of the present invention, the average value of the aspect ratio of the first silver particles needs to be greater than 2, preferably greater than 2.3, more preferably greater than 2.5, and further preferably greater than 3. If the average value of the aspect ratio of the first silver particles is greater than 2, the specific resistance of the conductive film can be effectively reduced. On the other hand, the average value of the aspect ratio of the first silver particles is, for example, less than 5, less than 4, or less than 3.5.

The average value of the short side length of the first silver particles is, for example, 0.6 μm or more, 1.0 μm or more, or 1.3 μm or more, and for example, 2.5 μm or less, 2.0 μm or less, or 1.9 μm or less.

The average value of the shape factor of the first silver particles is, for example, greater than 2, may be greater than 2.5, or may be greater than 3, and is, for example, less than 6, may be less than 5, or may be less than 4.

In the mixed silver powder of the present invention, the number ratio of the first silver particles in the mixed silver powder needs to be 0.5% or more, preferably 1% or more, more preferably 1.5% or more, and needs to be 5% or less, preferably 4% or less, and more preferably 3% or less. If the number ratio of the first silver particles in the mixed silver powder is within the above range, the specific resistance of the conductive film can be effectively reduced.

<Second silver particles> In the mixed silver powder of the present invention, the second silver particles are silver particles having a long side length of 0.5 μm or more and less than 3 μm.

In the mixed silver powder of the present invention, the average value of the aspect ratio of the second silver particles needs to be greater than 1.5, preferably greater than 1.6, and more preferably greater than 1.7, and needs to be less than 2, preferably less than 1.95, and more preferably less than 1.92. If the average value of the aspect ratio of the second silver particles is within the above range, the specific resistance of the conductive film can be effectively reduced.

In the mixed silver powder of the present invention, by the following formula (1): Ratio α = circumference of the second silver particle / (long side length of the second silver particle × 2 + short side length of the second silver particle × 2) ... (1) The calculated average value of the ratio α needs to be greater than 0.84, preferably greater than 0.85. The ratio α represents the rectangularity of the particle cross section. The closer the value of the ratio α is to 1, the closer the particle shape is to a rectangular shape. If the average value of the ratio α is greater than 0.84, the specific resistance of the conductive film can be effectively reduced. On the other hand, the average value of the ratio α is less than 1, for example, it can also be less than 0.95.

The average value of the short side length of the second silver particles is, for example, 0.3 μm or more, 0.4 μm or more, or 0.5 μm or more, and is, for example, 1 μm or less, 0.8 μm or less, or 0.7 μm or less.

The average value of the shape factor of the second silver particles is, for example, 1.5 or more, 1.7 or more, or 2 or more, and is, for example, 3 or less, 2.5 or less, or 2.3 or less.

In the mixed silver powder of the present invention, the number ratio of the second silver particles in the mixed silver powder needs to be more than 10%, preferably more than 30%. If the number ratio of the second silver particles in the mixed silver powder is more than 10%, the specific resistance of the conductive film can be effectively reduced.

<Third silver particles> In the mixed silver powder of the present invention, the third silver particles are silver particles having a long side length of less than 0.5 μm.

In the mixed silver powder of the present invention, the average value of the aspect ratio of the third silver particles needs to be less than 1.5, preferably less than 1.4, and further preferably less than 1.35. If the average value of the aspect ratio of the third silver particles is less than 1.5, the specific resistance of the conductive film can be effectively reduced. On the other hand, the average value of the aspect ratio of the third silver particles is, for example, greater than 1.1, greater than 1.2, or greater than 1.25.

The average value of the short side length of the third silver particles is, for example, 0.1 μm or more, 0.2 μm or more, or 0.25 μm or more, and is, for example, 0.5 μm or less, or 0.4 μm or less.

The average value of the shape factor of the third silver particles is, for example, 1.2 or more, 1.4 or more, or 1.6 or more, and is, for example, 2.5 or less, 2 or less, or 1.75 or less.

In the mixed silver powder of the present invention, the number ratio of the third silver particles in the mixed silver powder needs to be 10% or more, preferably 13% or more, and more preferably 15% or more. If the number ratio of the third silver particles in the mixed silver powder is within the above range, the specific resistance of the conductive film can be effectively reduced.

<Polycarboxylic acid> The mixed silver powder of the present invention has a polycarboxylic acid attached to the surface of at least one silver particle selected from the group consisting of the first silver particle, the second silver particle, and the third silver particle. In addition, in terms of effectively reducing the specific resistance of the conductive film, the mixed silver powder preferably has a polycarboxylic acid attached to the surface of the first silver particle and the second silver particle.

Here, the polycarboxylic acid is not particularly limited, and existing known ones can be cited. In terms of being able to effectively reduce the specific resistance of the conductive film, the polycarboxylic acid is preferably an aliphatic polycarboxylic acid, and more preferably a saturated aliphatic polycarboxylic acid. More specifically, the polycarboxylic acid is preferably at least one polycarboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, apple acid, diethylglutaric acid, 3-methyladipic acid, butylmalonic acid, maleic acid, diglycolic acid and citric acid, and adipic acid is particularly preferred.

The content ratio of the polycarboxylic acid relative to the total of the first silver particles, the second silver particles, and the third silver particles is preferably 0.01 mass % or more, more preferably 0.02 mass % or more, preferably 0.15 mass % or less, more preferably 0.1 mass % or less, and further preferably 0.05 mass % or less. If the content ratio of the polycarboxylic acid is within the above range, the specific resistance of the conductive film can be effectively reduced.

<Characteristics of Mixed Silver Powder> The cumulative 50% particle size (D ₅₀ ) of the mixed silver powder on a volume basis obtained by laser diffraction scattering particle size distribution measurement is, for example, 1 μm or more and, for example, 3 μm or less.

The BET specific surface area of the mixed silver powder is, for example, 0.2 m ² /g or more, 0.4 m ² /g or more, or 0.5 m ² /g or more, and is, for example, 1 m ² /g or less, 0.8 m ² /g or less, or 0.7 m ² /g or less.

The intensive heat loss (Ig-Loss) value of the mixed silver powder is, for example, 0.05 mass % or more, 0.1 mass % or more, or 0.3 mass % or more, for example, 3 mass % or less, 1 mass % or less, or 0.5 mass % or less.

The tap density of the mixed silver powder is, for example, 3 g/mL or more, may be 4 g/mL or more, and may be 5 g/mL or more, and for example, 7 g/mL or less, and may be 6 g/mL or less.

(Method for producing mixed silver powder) The method for producing mixed silver powder of the present invention comprises: a mixing step, mixing silver powder A containing silver particles A1 and silver particles A2 with silver powder B containing silver particles B to obtain a mixture; and a pre-treatment step, before the mixing step, using a polycarboxylic acid to carry out a surface treatment agent attachment treatment on at least one of the silver powders A and B, or a post-treatment step, using a polycarboxylic acid to carry out a surface treatment agent attachment treatment on the mixture obtained in the mixing step, before burying the silver powder A in a tree. When observing more than 100 silver particles in the polished cross section of a resin block formed by embedding silver powder B in resin and measuring the perimeter of the silver particles and the length of the long side and the length of the short side of the rectangle circumscribing the outer shape of the silver particles and having the smallest area, the average value of the aspect ratio of the silver particles A1 is greater than 2, the average value of the aspect ratio of the silver particles A2 is less than 2, and the following formula (2): Ratio β = circumference of silver particle A2 / (long side length of silver particle A2 × 2 + short side length of silver particle A2 × 2) ... (2) The calculated average value of ratio β (also called rectangularity) is 0.84 or more, and the average value of aspect ratio of silver particle B is less than 1.5. If the above-mentioned mixed silver powder production method is used, a mixed silver powder capable of reducing the specific resistance of the conductive film can be obtained. In addition, the mixed silver powder of the present invention can be obtained by the mixed silver powder production method of the present invention. Furthermore, the observation of the cross section of the silver particles in the mixed silver powder production method of the present invention can be carried out by the same method as the observation of the cross section of the silver particles of the mixed silver powder of the present invention.

The method for producing the mixed silver powder of the present invention may further include, in addition to the pre-treatment step, the mixing step, and the post-treatment step, other steps such as a crushing step of crushing the silver particles A and/or the silver particles B before the pre-treatment step or the mixing step.

<Silver powder A> Silver powder A contains silver particles A1 having a large aspect ratio, and silver particles A2 having a smaller aspect ratio than silver particles A1 and a large rectangular shape.

[Silver particles A1] Silver particles A1 are silver particles having an aspect ratio of 2 or more. The number ratio of silver particles A1 in silver powder A is, for example, 5% or more and 50% or less.

The average value of the aspect ratio of the silver particles A1 is 2 or more, preferably 2.3 or more, more preferably 2.5 or more, and further preferably 3 or more. If the average value of the aspect ratio of the silver particles A1 is 2 or more, a mixed silver powder that can effectively reduce the specific resistance of the conductive film can be obtained. On the other hand, the average value of the aspect ratio of the silver particles A1 is, for example, 5 or less, 4 or less, or 3.5 or less.

[Silver particles A2] Silver particles A2 are silver particles having an aspect ratio smaller than that of silver particles A1 and a rectangularity of the particle shape (ratio β described below) of 0.84 or more. Silver particles A2 are silver particles having an aspect ratio of less than 2. Moreover, for silver particles having an aspect ratio of less than 2, the rectangularity is measured, and the average value thereof is 0.84 or more. The number ratio of silver particles A2 in silver powder A is preferably, for example, 50% to 95%.

The average value of the aspect ratio of the silver particles A2 is preferably greater than 1.4 and less than 2. If the average value of the aspect ratio of the silver particles A2 is within the above range, a mixed silver powder that can effectively reduce the specific resistance of the conductive film can be obtained.

In silver powder A, by the following formula (2): Ratio β = circumference of silver particle A2 / (long side length of silver particle A2 × 2 + short side length of silver particle A2 × 2) ... (2) The calculated average value of the ratio β needs to be greater than 0.84, preferably greater than 0.85. The ratio β represents the rectangularity of the particle cross section. The closer the value of the ratio β is to 1, the closer the particle shape is to a rectangular shape. If the average value of the ratio β is greater than 0.84, the specific resistance of the conductive film can be effectively reduced. On the other hand, the average value of the ratio β is less than 1, for example, less than 0.95.

[Preparation method of silver powder A] Regarding the preparation method of silver powder A, there is no particular limitation as long as the silver particles A and silver particles B contained in the silver powder A meet the prescribed conditions, and a conventionally known method can be used. Below, an example of the preparation method of silver powder A is described, but the silver powder A used in the method for producing the mixed silver powder of the present invention is not limited thereto.

Silver powder A can be prepared by a method comprising the following steps: a reduction step, in which a reducing agent is added to an aqueous silver ammine complex solution to obtain a first liquid; a surface treatment agent addition step, in which a surface treatment agent is added to the first liquid to obtain a second liquid; a separation step, in which the spherical silver powder having an aspect ratio of less than 1.5 is separated from the second liquid and dried; and a partial flattening step, in which the spherical silver powder, a lubricant and a medium are stirred in a container to partially flatten the spherical silver powder to such an extent that silver particles A1 having a large aspect ratio and silver particles A2 having an aspect ratio smaller than that of the silver particles A1 and a large rectangularity are formed.

-Reduction step- In the reduction step, a reducing agent is added to the silver-ammonia complex aqueous solution to obtain a first solution. Here, in the reduction step, by adding the reducing agent, silver ions are reduced and silver particles (hereinafter referred to as core particles) can be precipitated.

Examples of the reducing agent include hydrazine, hydrazine compounds, and formalin.

The silver-ammonia complex aqueous solution can be produced by adding ammonia water or ammonium salt to a raw material solution such as a silver nitrate aqueous solution or a silver oxide suspension. A pH adjuster can also be added to the raw material solution or the silver-ammonia complex aqueous solution. As the pH adjuster, a general acid or base can be used, for example, nitric acid, sodium hydroxide, etc.

-Surface treatment agent adding step- In the surface treatment agent adding step, the surface treatment agent is added to the first liquid obtained in the reduction step to obtain a second liquid. Here, in the surface treatment agent adding step, the surface treatment agent can be coated on the surface of the core particles by adsorption or the like. In addition, hereinafter, the core particles coated with the surface treatment agent are referred to as coated particles. The second liquid is a suspension (so-called slurry) or a dispersion in which the coated particles are dispersed.

Examples of the surface treatment agent include fatty acids such as stearic acid, palmitic acid, linoleic acid, linolenic acid, and oleic acid. Among these, unsaturated fatty acids such as linoleic acid, linolenic acid, and oleic acid are preferred.

The amount of the surface treatment agent added in the surface treatment agent adding step is usually 0.05 mass % or more and 0.15 mass % or less relative to the mass of silver contained in the silver ammine complex aqueous solution.

The amount of the surface treatment agent that is attached to the core particles by the surface treatment agent addition step and remains after the drying step is a value measured using the spherical silver powder after the drying step described later in a state where the type of the surface treatment agent is determined. In addition, the type of the surface treatment agent can be determined by qualitative analysis of the surface treatment agent volatilized by heating the spherical silver powder using gas chromatography.

The amount of surface treatment agent attached to spherical silver powder can be determined by the quantitative analysis method of fatty acids described in Japanese Patent No. 5622543. Specifically, first, the silver powder is dissolved with acid, then mixed with an organic solvent, and the total amount of the surface treatment agent is extracted from the organic solvent phase. A predetermined amount of the organic solvent phase is then separated, and the carbon content of the solids remaining after evaporation and drying is measured using a carbon-sulfur analyzer, which can be calculated.

For example, when the surface treatment agent is determined to be stearic acid and the spherical silver powder does not contain a carbon source other than stearic acid, the method for determining stearic acid is as follows.

In standard solutions with different stearic acid contents (mg), the calibration curve is obtained by measuring each carbon amount (strength) using a carbon-sulfur analyzer, and its slope is set as A (strength/mg). In addition, regarding the stearic acid mass X (mg) and concentration Y (%) in the silver powder, a predetermined amount b (mL) is taken from the treatment agent obtained by extracting the treatment agent from the total organic solvent amount a (mL) by treating the silver powder, and the carbon amount obtained by measuring the residual solid matter is set as C (strength), and the amount of silver powder dissolved in the acid is set as M (g). In this case, the stearic acid mass X and concentration Y can be calculated by the following formula (A) and formula (B), respectively. X(mg)=(C/A×a/b)…(A) Y(%)=X/(M×1000)×100…(B)

When oleic acid is used as the treatment agent, the carbon content is measured in the same manner as described above. For oleic acid, the calibration curve of stearic acid is also used for calculation. The molecular weight of stearic acid is 284.48, and the carbon content is 216.19. The molecular weight of oleic acid is 282.46, and the carbon content is 216.19. Therefore, the oleic acid concentration Y' can be calculated by the following formula (C). Oleic acid concentration Y' (%) = Y × (216.19/284.48) × (282.46/216.19) ... (C)

The amount of the surface treatment agent attached is preferably 0.01 mass % or more and 0.11 mass % or less relative to the mass of the spherical silver powder.

-Separation step- In the separation step, the coated particles are separated from the second liquid. The aggregate of the coated particles separated and dried in the separation step is called spherical silver powder. In addition, in the separation step, a washing and recovery step of recovering the coated particles from the second liquid and washing them, and a drying step of drying the coated particles can be arbitrarily performed.

In the cleaning and recovery step, for example, the second liquid is dehydrated to make the aggregate of coated particles into a filter cake shape, and the filter cake of the aggregate of coated particles is cleaned. The cleaning in the cleaning and recovery step can be performed, for example, using pure water. The dehydration in the cleaning and recovery step can be performed, for example, by decanting or a filter press. The end point of the cleaning can be determined by the conductivity of the cleaning water. Specifically, the cleaning can be determined to be complete when the conductivity of the cleaning water becomes below a specified value. The cleaned coated particles can be provided to the drying step in a coagulated state such as a filter cake shape.

In the drying step, the aggregate of the coated particles containing water and in a coagulated state is dried. In the drying step, a vacuum drying or an air flow type dryer can be used. In the drying step, the following operation can also be performed: a high-pressure air flow is blown to the aggregate of the coated particles, or the filter cake or the silver powder in the drying process is put into a stirrer with a stirring rotor and stirred, thereby giving a dispersing force to the filter cake or the silver powder in the drying process to promote dispersion or drying.

In the drying step, the temperature of the silver powder is usually below 100° C. If the temperature of the silver powder is below 100° C., sintering of silver particles in the silver powder can be effectively suppressed.

The dried spherical silver powder may be in a lump form, so it may be crushed or classified during or after the drying step for the purpose of improving the handling properties of the spherical silver powder. Here, the so-called improvement of the handling properties of the spherical silver powder refers to the proper loosening of the spherical silver powder in order to ensure the fluidity to a degree that does not hinder the addition of the lubricant described later or the supply operation into the device in the step described later (partial flattening step), or to ensure good handling efficiency in the device.

The spherical silver powder obtained in the separation step has a specific surface area calculated from the specific surface area obtained by the BET method after mixing with the lubricant described later (BET specific surface area) of 1.3 μm or more and 2.0 μm or less. Here, the specific surface area can be calculated using the following formula (D) based on the specific surface area A (m ² /g) obtained by the BET method after mixing the spherical silver powder with the lubricant and the density ρ of silver. Specific surface area (μm) = 6/(A×ρ)… (D)

-Partial flattening step- In this step, spherical silver powder, lubricant and medium are stirred in a container to obtain silver powder A in which the spherical silver powder is partially flattened to such an extent that silver particles A1 with a large aspect ratio and silver particles A2 with a small aspect ratio and a large rectangular shape can be formed.

Here, the step of partially flattening the spherical silver powder includes a lubricant mixing step of mixing the spherical silver powder with the lubricant for the purpose of uniformly dispersing the lubricant on the surface of the spherical silver powder. In the lubricant mixing step, the lubricant may be added to the spherical silver powder that is appropriately loosened by crushing, and the mixture may be stirred and mixed using a crusher. For example, the silver powder may be put into a Henschel mixer (manufactured by Mitsui Mining Co., Ltd., FM mixer, model FM75, using SO type stirring blades) or a jet mill mixer, and the lubricant may be added and stirred to mix the spherical silver powder with the lubricant. After the lubricant mixing step, the spherical silver powder mixed with the lubricant may be in an agglomerated state to a certain extent. In this step, the coated particles in the spherical silver powder mixed with the lubricant are partially flattened by colliding with the medium in a small number of collisions in the container, and the silver particles are coated with the lubricant, that is, silver powder A. Among the coated particles in the silver powder, the particles with a relatively large number of collisions are silver particles A1 with a large aspect ratio, and the particles with a relatively small number of collisions are silver particles A2 with a small aspect ratio and a large rectangular degree. The number of collisions with the medium is controlled by adjusting the filling ratio of the medium and the silver powder in the stirring container and the amount of energy estimated by the rotation speed and time of the stirring container so that the silver particles A1 and the silver particles A2 coexist (not just the silver particles A1).

Examples of lubricants include fatty acids such as stearic acid, palmitic acid, linolenic acid, linolenic acid, and oleic acid. Among these, unsaturated fatty acids such as linolenic acid, linolenic acid, and oleic acid are preferred. The amount of lubricant added is, for example, 0.05 mass % or more and 0.3 mass % or less relative to the mass of the spherical silver powder.

For the partial flattening step, a so-called media mill such as a ball mill or a bead mill can be used, for example. The medium can be made of known materials such as stainless steel (SUS) or zirconia. Stirring in the media mill can be performed by rotating the container or the stirring paddle, or by vibrating the container.

In addition, the step of partially flattening preferably includes a step of further performing a disintegration treatment using a Henschel mixer or a jet mill mixer after the collision with the medium. It is also preferred that the disintegration treatment disintegrates the silver particles that have become too large due to the collision with the medium, so that the values of the cumulative 90% particle size (D ₉₀ ), the cumulative 95% particle size (D ₉₅ ), and the cumulative 100% (i.e., the largest particle) particle size (D _MAX ) are reduced. Furthermore, in the step of performing the disintegration treatment, it is preferred to use a jet mill mixer in terms of being able to effectively reduce the various particle sizes.

Here, when a jet mill mixer is used in the crushing step, the crushed silver powder can be treated with a cyclone or the like in the partial flattening step to remove small particles (chips, etc., for example, particles with a diameter less than 0.1 μm). Furthermore, the cyclone can also be used as a device that adds additional airflow to improve the collection efficiency.

<Silver powder B> Silver powder B contains silver particles B.

The average value of the aspect ratio of the silver particles B needs to be less than 1.5, preferably less than 1.4, and further preferably less than 1.35. If the average value of the aspect ratio of the silver particles B is less than 1.5, a mixed silver powder that can effectively reduce the specific resistance of the conductive film can be obtained. On the other hand, the average value of the aspect ratio of the particles B is, for example, greater than 1.1, greater than 1.2, or greater than 1.25.

The average value of the shape factor of the silver particles B is, for example, not less than 1, may be not less than 1.3, or may be not less than 1.5, and is, for example, not more than 1.7, or may be not more than 1.6.

The average value of the circle equivalent diameter (Heywood diameter) of the silver particles B is, for example, 0.1 μm or more, and may be 0.3 μm or more, and, for example, 1 μm or less, and may be 0.5 μm or less.

The volume-based cumulative 50% particle size (D ₅₀ ) of the silver powder B obtained by laser diffraction scattering particle size distribution measurement is, for example, 0.3 μm or more, or 0.5 μm or more, and, for example, 1.3 μm or less, or 1 μm or less.

Hereinafter, the pre-treatment step, the mixing step, and the post-treatment step in the method for producing the mixed silver powder of the present invention will be described.

<Pretreatment step> In the pretreatment step, before the mixing step, at least one of the silver powders A and B is subjected to a surface treatment agent adhesion treatment using a polycarboxylic acid. Here, in the pretreatment step, in order to obtain a mixed silver powder that can effectively reduce the specific resistance of the conductive film, it is preferred to use a polycarboxylic acid to carry out a surface treatment agent adhesion treatment on the silver powder A.

In the pre-treatment step, the surface treatment agent adhesion treatment of the silver powder using polycarboxylic acid is not particularly limited, and can be performed using, for example, a sample mill, a Henschel mixer, or other disintegrator. Furthermore, the surface treatment agent adhesion treatment of the silver powder using polycarboxylic acid is usually performed for a period of more than 1 minute and less than 30 minutes.

The polycarboxylic acid used in the surface treatment agent attachment treatment as the pretreatment step is not particularly limited, and existing known ones can be cited. In terms of obtaining a mixed silver powder that can effectively reduce the specific resistance of the conductive film, the polycarboxylic acid is preferably an aliphatic polycarboxylic acid, and more preferably a saturated aliphatic polycarboxylic acid. More specifically, the polycarboxylic acid is preferably at least one polycarboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, apple acid, diethyl glutaric acid, 3-methyl adipic acid, butylmalonic acid, maleic acid, diglycolic acid and citric acid, and adipic acid is particularly preferred.

Here, the polycarboxylic acid may be used in the form of a solution dissolved in a solvent. Examples of the solvent that can be used to dissolve the polycarboxylic acid include alcohols such as methanol, ethanol, and propanol. Among these, ethanol is preferred.

In the pretreatment step, the usage rate of the polycarboxylic acid relative to the total of the silver powder A and the silver powder B is preferably 0.01 mass % or more, more preferably 0.02 mass % or more, preferably 0.15 mass % or less, more preferably 0.1 mass % or less, and further preferably 0.05 mass % or less. If the content ratio of the polycarboxylic acid is within the above range, a mixed silver powder that can effectively reduce the specific resistance of the conductive film can be obtained.

<Mixing step> In the mixing step, silver powder A containing silver particles A1 and silver particles A2 is mixed with silver powder B containing silver particles B to obtain a mixture. Furthermore, when a pretreatment step is performed before the mixing step, the mixture obtained in the mixing step can be equivalent to the target mixed silver powder.

The mixing ratio of silver powder B to silver powder A (silver powder B/silver powder A) is, for example, 10/90 or more, 20/80 or more, or 30/70 or more, and for example, 90/10 or less, 60/40 or less, or 40/60 or less, on a mass basis. If the mixing ratio of silver powder B to silver powder A is within the above range, a mixed silver powder that can effectively reduce the specific resistance of the conductive film can be obtained.

<Post-treatment step> In the post-treatment step, the mixture obtained in the mixing step is subjected to a surface treatment agent attachment treatment using a polycarboxylic acid. Furthermore, the mixture subjected to the surface treatment agent attachment treatment in the post-treatment step can be equivalent to the target mixed silver powder.

In the post-treatment step, the surface treatment agent adhesion treatment of the mixture using the polycarboxylic acid is not particularly limited, and can be performed using, for example, a sample mill, a Henschel mixer, or other disintegrator. Furthermore, the surface treatment agent adhesion treatment of the mixture using the polycarboxylic acid is usually performed for more than 1 minute and less than 30 minutes.

The polycarboxylic acid used in the surface treatment agent attachment treatment as the post-treatment step is not particularly limited, and existing known ones can be cited. In terms of obtaining a mixed silver powder that can effectively reduce the specific resistance of the conductive film, the polycarboxylic acid is preferably an aliphatic polycarboxylic acid, and more preferably a saturated aliphatic polycarboxylic acid. More specifically, the polycarboxylic acid is preferably at least one polycarboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, apple acid, diethylglutaric acid, 3-methyl adipic acid, butylmalonic acid, maleic acid, diglycolic acid and citric acid, and adipic acid is particularly preferred.

Here, the polycarboxylic acid may be used in the form of a solution dissolved in a solvent. Examples of the solvent that can be used to dissolve the polycarboxylic acid include alcohols such as methanol, ethanol, and propanol.

In the post-treatment step, the usage rate of the polycarboxylic acid relative to the total of the silver powder A and the silver powder B is preferably 0.01 mass % or more, more preferably 0.02 mass % or more, preferably 0.15 mass % or less, more preferably 0.1 mass % or less, and further preferably 0.05 mass % or less. If the content ratio of the polycarboxylic acid is within the above range, a mixed silver powder that can effectively reduce the specific resistance of the conductive film can be obtained.

(Conductive paste) The conductive paste of the present invention includes the mixed silver powder, binder and solvent of the present invention, and may optionally include components other than the mixed silver powder, binder and solvent (hereinafter, sometimes referred to as "other components"). Since the conductive paste of the present invention includes the mixed silver powder of the present invention, the specific resistance of the conductive film can be reduced.

The content of the mixed silver powder in the conductive paste is preferably 50% by mass or more, more preferably 80% by mass or more, preferably 98% by mass or less, and more preferably 95% by mass or less. If the content of the mixed silver powder in the conductive paste is within the above range, the specific resistance of the conductive film can be effectively reduced.

Examples of the binder for the conductive paste include epoxy resin, acrylic resin, polyester resin, polyimide resin, polyurethane resin, phenoxy resin, silicone resin, ethyl cellulose, etc. These may be used alone or in combination of two or more.

Examples of the solvent (i.e., dispersion medium) of the conductive paste include terpineol, butyl carbitol, butyl carbitol acetate, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (texanol), etc. These may be used alone or in combination of two or more.

Other components that may be included in the conductive paste include, for example, glass frit, dispersant, surfactant, viscosity adjuster, etc. These may be used alone or in combination of two or more.

In the production, i.e., dispersion or mixing of the conductive paste, ultrasonic dispersion, disperser, three-roll mill, ball mill, bead mill, twin-shaft kneader, rotation-revolution mixer, etc. can be used.

The viscosity of the conductive paste is not particularly limited and can be appropriately selected according to the purpose. Under the conditions of a paste temperature of 25°C and a rotation speed of 1 rpm, it is preferably 150 Pa･s or more, more preferably 200 Pa･s or more, preferably 800 Pa･s or less, and more preferably 750 Pa･s or less. If the viscosity of the conductive paste is 150 Pa･s or more, the occurrence of "bleeding" during printing can be effectively suppressed. On the other hand, if the viscosity of the conductive paste is 800 Pa･s or less, the occurrence of uneven printing can be effectively suppressed.

The conductive paste of the present invention is suitable for forming a conductive film, that is, forming a conductive pattern on a substrate, or forming an electrode. For example, it can be suitably used to form a conductive film by coating or printing directly on various substrates such as silicon wafers for solar cells, films for touch panels, and glass for electroluminescence (EL) elements, or on a transparent conductive film that is further provided on the substrate as needed. The conductive film obtained using the conductive paste of the present invention can be suitably used, for example, as a collector electrode of a solar cell unit, an external electrode of a chip-type electronic component, radio-frequency identification (RFID), electromagnetic wave shielding, vibrator bonding, diaphragm switches, electrodes for electroluminescence, or electrical wiring, etc.

The conductive paste can be printed on the substrate by screen printing, lithography, photolithography, etc. to form a conductive film of a desired shape. [Example]

Hereinafter, the present invention will be described in more detail using examples, but the present invention is not limited to the following examples.

(Example 1) <Preparation of silver powder A> [Reduction step] First, 167.1 kg of 26 mass % ammonia aqueous solution was added to 2922 kg of silver nitrate aqueous solution containing 68.8 kg of silver as a silver ion aqueous solution to generate a silver-ammonia complex aqueous solution. Furthermore, 266 kg of 6 mass % hydrazine aqueous solution was added to the silver-ammonia complex aqueous solution as a reducing agent to obtain a first liquid. The rate of addition of the reducing agent was set to 80 L/min.

[Surface treatment agent addition step] After 5 minutes from the end of the reducing agent addition, 68.8 g (0.1 mass % relative to the mass of silver contained in the silver-ammonia complex aqueous solution (calculated by oleic acid 68.8 g/silver 68800 g×100)) of oleic acid as a surface treatment agent was added. After adding the surface treatment agent, the mixture was stirred for 5 minutes to obtain a second liquid. The second liquid was in the form of a slurry containing coated particles.

[Separation step] The second liquid is filtered, washed with water, and then dried to obtain spherical silver powder.

[Lubricant mixing step] 16.25 kg of the spherical silver powder was added to a Henschel mixer (Mitsui Mining Co., Ltd., FM mixer, model FM75, using SO type stirring blades), stirred at 900 rpm for 1 minute, and then 37.4 g (0.23 mass % relative to the mass of the spherical silver powder (calculated by oleic acid 37.4 g/spherical silver powder 16250 g × 100)) of oleic acid as a lubricant was added, and the mixture was mixed and stirred at 2200 rpm for 20 minutes. This step was performed in multiple batches to obtain a lubricant-mixed silver powder in which the lubricant was dispersed on the surface of the silver particles.

[Partial flattening step] 32 kg of the lubricant-mixed silver powder and 256 kg of SUS balls (diameter 1.6 mm) were placed in a vibration mill (FVR-20 model, manufactured by Central Chemical Industry Co., Ltd.) and processed at a vibration speed of 780 vpm for 135 minutes to partially flatten the lubricant-mixed silver powder.

The partially flattened silver powder (hereinafter also referred to as the second silver powder) was separated from the SUS ball and then crushed by stirring at 2600 rpm for 25 minutes using the Henschel mixer. Furthermore, in order to remove coarse particles from the crushed second silver powder, it was screened using a dry screen device (manufactured by Freund-Turbo Co., Ltd., TS125×200 type/mesh 27 μm screen) to obtain silver powder A.

[Cross-sectional observation of silver powder A] The silver particles in silver powder A were observed, and the average value of the aspect ratio of silver particle A1 was calculated by the following method, and the average value of the aspect ratio of silver particle A2 was calculated by the following formula (2): Ratio β = circumference of silver particle A2 / (long side length of silver particle A2 × 2 + short side length of silver particle A2 × 2) ... (2) The average value of the calculated ratio β. Specifically, first, silver powder A was placed in a resin (EpoFix resin manufactured by Struers) and a hardener (EpoFix hardener manufactured by Struers) and cured, and then polished using a cross-section polisher (ArBlade 5000 manufactured by High-technologies Corporation of Japan) to expose the cross section of the silver particles. Each silver particle was observed at 5,000 times magnification using a scanning electron microscope (JEOL JSM-IT300LV manufactured by JEOL Ltd.). Here, based on the contrast between the insulating resin and the conductive silver particles, the silver particles whose outlines are clear and whose peripheries can be clearly seen are regarded as silver particles exposed in the cross section. Then, in the polished cross section of the resin block in which the silver powder A is embedded in the resin, 100 or more silver particles are randomly selected from the silver particles whose peripheries can be clearly seen, and the peripheries of the silver particles are traced using image analysis software (Mac-View, an image analysis type particle size distribution measurement software manufactured by MOUNTECH Co., Ltd.), thereby measuring the length of the long side, the length of the short side, and the perimeter of each silver particle. Furthermore, the length of the long side and the length of the short side are automatically calculated to be the values when the area of the rectangle circumscribing the outer shape of each silver particle is the smallest. Then, the silver particles with an aspect ratio of 2 or more are set as silver particles A1, and the silver particles with an aspect ratio of less than 2 are set as silver particles A2. The average value of the aspect ratio is calculated for silver particles A1, and the average value of the aspect ratio and the average value of the ratio β are calculated for silver particles A2. The results are shown in Table 1.

<Preparation of Silver Powder B>"AG-2-1C" manufactured by DOWA High-Tech Co., Ltd. was prepared as silver powder B. The volume-based cumulative 50% particle size (D ₅₀ ) of the silver powder B measured by a laser diffraction scattering particle size distribution measuring device was 0.80 μm, and the average shape factor obtained by measuring the external shapes of more than 100 particles using image analysis particle size distribution measuring software Mac-View manufactured by MOUNTECH Co., Ltd. was 1.53, the average aspect ratio was 1.3, and the average Heywood diameter was 0.34 μm.

<Production of mixed silver powder> [Pretreatment step] First, prepare a solution in which 0.056 g of adipic acid (manufactured by Junsei Chemical Co., Ltd.) is dissolved in 0.504 g of an ethanol solvent. Then, add the solution to 120 g of the obtained silver powder A, mix for 4.5 minutes using a sample mill (manufactured by Kyoritsu Riko Co., Ltd., SK-M10), and treat the surface of the silver powder A with adipic acid. Oleic acid is used as a surface treatment agent or lubricant in the production process of silver powder A, but this treatment allows adipic acid, which is a further surface treatment agent, to adhere to the surface of the silver powder A.

[Mixing step] The silver powder A and silver powder B obtained in the pre-treatment step, which have been treated with the surface treatment agent, are mixed in a mass ratio of 60:40 (silver powder B/silver powder A=40/60) to produce a mixture (i.e., mixed silver powder). The obtained mixed silver powder is used to perform the various measurements shown below. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in FIG2.

[Measurement of BET specific surface area] The BET specific surface area of the obtained mixed silver powder was measured using a BET specific surface area measuring device (Macsorb HM-model 1210, manufactured by MOUNTECH) and a BET single-point method based on nitrogen adsorption. In the measurement of the BET specific surface area, the degassing conditions before the measurement were set to 60°C and 10 minutes. The results are shown in Table 2.

[Measurement of particle size distribution] The volume-based cumulative 10% particle size ( _D10 ), cumulative 50% particle size (D50), cumulative 90% particle size ( _D90 ), cumulative 95% particle size ( _D95 ), and cumulative 100% (i.e., largest particle) particle size ( _DMAX ) of the obtained mixed _{silver powder were measured by the following method. In addition, the ratio of the difference between the cumulative 90% particle size (D90 )} and the cumulative 10% particle size ( _D10 ) to the cumulative 50% particle size ( _D50 ) was calculated [( _D90 - _D10 )/ _D50 ]. The results are shown in Table 2. 0.1 g of the mixed silver powder was added to 40 mL of isopropyl alcohol (IPA), dispersed for 2 minutes using an ultrasonic homogenizer (device name: US-150T, manufactured by Nippon Seiki Co., Ltd.; 19.5 kHz, chip tip diameter 18 mm), and then measured using a laser diffraction-scattering particle size distribution measuring device (Microtrac BEL Co., Ltd., Microtrac MT-3300 EXII).

[Determination of Ig-Loss value] The Ig-Loss value of the obtained mixed silver powder is obtained by accurately weighing (weighing value: w1) 3 g of the mixed silver powder sample and placing it in a magnetic crucible, heating it to 800°C, and then keeping it at 800°C for 30 minutes, which is sufficient time for achieving constant weight, and then cooling and weighing it again (weighing value: w2), and calculating it according to "Ig-Loss value (mass %) = (w1-w2)/w1×100". The results are shown in Table 2.

[Measurement of tap density] The tap density of the obtained mixed silver powder was measured using a tap density measuring device (produced by Shibayama Scientific Co., Ltd., volume specific gravity measuring device SS-DA-2). 30 g of silver powder was weighed and placed in a 20 mL test tube, and tapped 1,000 times with a drop of 20 mm. The tap density was calculated according to "tap density = sample mass (g) / sample volume after tapping (mL)". The results are shown in Table 2.

[Cross-sectional observation of mixed silver powder] The silver particles in the mixed silver powder were observed and the following calculations were performed by the following method: (A) The overall average of the shape factor, the overall average of the long side length, the overall average of the short side length, and the overall average of the aspect ratio of the mixed silver powder (silver particles as a whole) were calculated (B) The number ratio, the average value of the shape factor, the average value of the short side length, and the average value of the aspect ratio of the first silver particles were calculated (C) The number ratio, the average value of the shape factor, the average value of the short side length, and the average value of the aspect ratio of the second silver particles were calculated, and the following formula (1) was used: Ratio α = circumference of the second silver particle / (long side length of the second silver particle × 2 + short side length of the second silver particle × 2) ... (1) Calculated average value of ratio α (D) The number ratio, average value of shape factor, average value of short side length and average value of aspect ratio were calculated for the third silver particles. Specifically, first, the mixed silver powder was placed in a resin (EpoFix resin manufactured by Struers) and a hardener (EpoFix hardener manufactured by Struers) and cured, and then polished using a cross-section polisher (ArBlade 5000 manufactured by High-technologies Corporation of Japan) to expose the cross section of the silver particles. Each silver particle was observed using a scanning electron microscope (JEOL JSM-IT300LV manufactured by JEOL Ltd.) at 5,000 times (3,000 times in the case of Comparative Example 3 described later). Here, based on the contrast between the insulating resin and the conductive silver particles, the silver particles whose outlines are clear and whose peripheries can be clearly seen are considered as the silver particles exposed in the cross section. Then, in the polished cross section of the resin block in which the mixed silver powder is embedded in the resin, 100 or more silver particles are randomly selected from the silver particles whose peripheries can be clearly seen, and the peripheries of the silver particles are traced using image analysis software (Mac-View, an image analysis type particle size distribution measurement software manufactured by MOUNTECH Co., Ltd.), thereby measuring the length of the long side, the length of the short side, the perimeter, the area of the silver particles, and the maximum length of the silver particles. Furthermore, the long side length and short side length are automatically calculated to be the values when the area of the rectangle circumscribing the outer shape of each silver particle is the smallest. Using these measured values, (A) the overall average of the shape factor, the overall average of the long side length, the overall average of the short side length, and the overall average of the aspect ratio are calculated for the mixed silver powder (silver particles as a whole). The results are shown in Table 2. Next, silver particles with a long side length of 3 μm or more are set as the first silver particles, silver particles with a long side length of 0.5 μm or more and less than 3 μm are set as the second silver particles, and silver particles with a long side length of less than 0.5 μm are set as the third silver particles, and the following calculations are performed: (B) The number ratio, the average value of the shape factor, the average value of the short side length, and the average value of the aspect ratio are calculated for the first silver particles (C) The number ratio, the average value of the shape factor, the average value of the short side length, the average value of the aspect ratio, and the average value of the ratio α are calculated for the second silver particles (D) The number ratio, the average value of the shape factor, the average value of the short side length, and the average value of the aspect ratio are calculated for the third silver particles. The results are shown in Table 2.

<Preparation of conductive paste> The conductive paste was prepared by mixing the silver powder obtained above with an Ag content of 91%, epoxy resin A (EP4901E manufactured by ADEKA) with an epoxy resin content of 3.8%, epoxy resin B (jER1009 manufactured by Mitsubishi Chemical) with an epoxy resin content of 1.0%, a hardener (boron trifluoride monoethylamine complex manufactured by Wako Junyaku Co., Ltd.) with an epoxy resin content of 0.2%, and a solvent (BCA (Butyl Carbitol Acetate) with an epoxy resin content of 4.0%) at 1200 °C. rpm for 30 seconds, and then three rollers (80S manufactured by EXAKT) were used to pass through the roller gap at 100 μm to 20 μm and knead to obtain a conductive paste. The viscosity of the conductive paste obtained was measured and adjusted to 300 Pa･s using BCA. The viscosity of the conductive paste was measured using a viscometer (DV-III, CP-52 cone manufactured by Brookfield) at 25°C and 1 rpm for 5 minutes.

<Formation of conductive film> The conductive paste obtained above was printed with a line pattern of 500 μm in width and 37.5 mm in length using a screen printer (MT-320T manufactured by Micro-tec) at a squeegee pressure of 0.18 MPa to form a conductive paste film. Then, the obtained film was heated and hardened at 200°C for 30 minutes using an atmospheric circulation dryer to form a conductive film.

[Specific resistance value] For the obtained conductive film, the step difference between the unprinted film portion and the conductive film portion on the alumina substrate was measured using a surface roughness meter (Surfcom 480B-12 manufactured by Tokyo Seimitsu Co., Ltd.), thereby measuring the average thickness of the conductive film. Then, the resistance value of the conductive film was measured using a digital multimeter (R6551 manufactured by ADVANTEST). Then, the volume of the conductive film was calculated based on the size of the conductive film (average thickness, width, length), and the specific resistance value was calculated based on the volume and the measured resistance value. The results are shown in Table 2.

(Example 2) Except that the mixed silver powder was produced by the following method, various operations and measurements were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in Figure 3.

<Production of mixed silver powder> [Pretreatment step] "AG-2-1C" was prepared as silver powder B, and then adipic acid was used to perform a surface treatment agent adhesion treatment. The volume-based cumulative 50% particle size ( _D50 ) of the silver powder B measured using a laser diffraction scattering particle size distribution measuring device was 0.80 μm, and the average shape factor obtained by measuring the shape of more than 100 particles using image analysis particle size distribution measuring software Mac-View manufactured by MOUNTECH Co., Ltd. was 1.53, the average aspect ratio was 1.3, and the average Heywood diameter was 0.34 μm. For the surface treatment agent adhesion treatment using adipic acid, first, a solution was prepared by dissolving 0.084 g of adipic acid (manufactured by Junsei Chemical Co., Ltd.) in 0.756 g of an ethanol solvent. Then, the solution was added to 120 g of "AG-2-1C" and mixed for 4.5 minutes using a sample mill (manufactured by Kyoritsu Riko Co., Ltd., SK-M10), and the surface of the silver powder B was treated with adipic acid.

[Pulverization step] First, 120 g of silver powder A obtained by the same method as the preparation method of silver powder A in Example 1 was pulverized using a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) for 4.5 minutes.

[Mixing step] The silver powder A obtained in the crushing step and the silver powder B treated with a surface treatment agent are mixed in a mass ratio of 60:40 (silver powder B/silver powder A=40/60) to produce a mixture (i.e., mixed silver powder).

(Example 3) Except that the mixed silver powder was produced by the following method, various operations and measurements were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in Figure 4.

<Production of mixed silver powder> [Mixing step] First, 72 g of silver powder A obtained by the same method as the preparation method of silver powder A in Example 1 and 48 g of silver powder B are mixed to obtain a mixture.

[Post-treatment step] Prepare a solution in which 0.0336 g of adipic acid (manufactured by Junsei Chemical Co., Ltd.) is dissolved in 0.3024 g of an ethanol solvent. Then, add the solution to 120 g of the mixture obtained in the mixing step, mix for 7.5 minutes using a sample mill (manufactured by Kyoritsu Riko Co., Ltd., SK-M10), and treat the surface of the mixture with adipic acid to produce a mixed silver powder.

(Comparative Example 1) Without mixing silver powder B, 120 g of silver powder A was taken, and a solution prepared by dissolving adipic acid (0.0336 g of adipic acid manufactured by Junsei Chemical Co., Ltd.) in 0.3024 g of ethanol solvent was added, and mixed for 4.5 minutes using a sample mill to allow adipic acid as a further surface treatment agent to adhere to the surface of the silver powder. Other operations and measurements were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in Figure 5.

(Comparative Example 2) Except that the mixed silver powder was produced by the following method, various operations and measurements were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in Figure 6.

<Production of mixed silver powder> [Pulverization step] First, 120 g of silver powder A obtained by the same method as the preparation method of silver powder A in Example 1 was pulverized using a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) for 4.5 minutes.

[Mixing step] The silver powder A obtained in the crushing step is mixed with the silver powder B in a mass ratio of 60:40 (silver powder B/silver powder A=40/60) to produce a mixture (i.e., mixed silver powder).

(Comparative Example 3) In the surface treatment agent addition step in Example 1, no surface treatment agent was added, the second liquid was filtered, washed with water, and dried, and then heat-treated at 150°C for 10 hours to promote the aggregation of particles. In the lubricant mixing step, the lubricant added was set to 48.8 g (0.30 mass % relative to the mass of the silver powder provided for the lubricant mixing step), and a partial flattening step was performed as follows. The mixed silver powder was produced by the following method. Except for this, various operations and measurements were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in Figure 7. Furthermore, the cross-sectional observation of the mixed silver powder in Comparative Example 3 was performed using a scanning electron microscope at 3,000 times magnification to observe each silver particle.

[Partial flattening step] 32 kg of lubricant-mixed silver powder and 256 kg of SUS balls (diameter 1.6 mm) were placed in a vibration mill (manufactured by Central Chemical Industry Co., Ltd., FVR-20 model) and subjected to partial flattening treatment at a vibration number of 1200 vpm for 120 minutes to partially flatten the lubricant-mixed silver powder and obtain a second silver powder. Since the vibration number is greater than that of Example 1, the flattening is further advanced compared to Example 1, and the number of silver particles with a large aspect ratio increases.

After the second silver powder was separated from the SUS ball, it was stirred and crushed at 2600 rpm for 25 minutes using the Henschel mixer. In addition, in order to remove coarse particles from the crushed second silver powder, it was screened using a dry screen device (manufactured by Freund-Turbo Co., Ltd., TS125×200 type/mesh 27 μm screen) to obtain silver powder A.

<Production of mixed silver powder> [Pretreatment step] "AG-2-1C" was prepared as silver powder B, and then a surface treatment agent adhesion treatment was performed using adipic acid. For the surface treatment agent adhesion treatment using adipic acid, first, a solution was prepared by dissolving 0.084 g of adipic acid (manufactured by Junsei Chemical Co., Ltd.) in 0.756 g of ethanol solvent. Then, the solution was added to 120 g of "AG-2-1C", mixed for 4.5 minutes using a sample mill (manufactured by Kyoritsu Riko Co., Ltd., SK-M10), and the surface of silver powder B was treated with adipic acid.

[Mixing step] The obtained silver powder A and the silver powder B which has been subjected to the surface treatment agent adhesion treatment are mixed in a mass ratio of 60:40 (silver powder B/silver powder A=40/60) to produce a mixture (i.e., mixed silver powder).

[Table 1] Embodiment Comparison Example 1 2 3 1 2 3 raw material Silver powder A The average value of the aspect ratio of silver particles A1 3.31 3.31 3.31 3.31 3.31 3.84 Average value of aspect ratio of silver particles A2 1.47 1.47 1.47 1.47 1.47 1.65 The average value of the ratio β of silver particles A2 0.86 0.86 0.86 0.86 0.86 0.86 Usage [Quantity] 60 60 60 100 60 60 Silver powder B The average value of the aspect ratio of silver particles B 1.3 1.3 1.3 - 1.3 1.3 Usage [Quantity] 40 40 40 0 40 40 Preparation method Pre-processing step or post-processing step Processing Timing Pre-treatment Pre-treatment Post-processing - - Pre-treatment Types of polycarboxylic acids Adipic acid Adipic acid Adipic acid Adipic acid - Adipic acid Usage rate of polycarboxylic acid [mass %] 0.028 0.028 0.028 0.028 - 0.028 Processing objectSilver powder Silver powder A Silver powder B Silver powder A, Silver powder B Silver powder A - Silver powder B

[Table 2] Embodiment Comparison Example 1 2 3 1 2 3 Mixed silver powder Types of polycarboxylic acids Adipic acid Adipic acid Adipic acid Adipic acid - Adipic acid BET specific surface area [m ² /g] 0.67 0.65 0.63 0.41 0.63 0.61 Particle size distribution D ₁₀ [µm] 0.6 0.6 0.6 1.3 0.6 0.9 D ₅₀ [µm] 1.9 1.8 1.8 2.9 1.9 6.0 D ₉₀ [µm] 5.6 5.5 5.4 6.4 5.6 15.6 D ₉₅ [µm] 7.4 7.3 6.9 8.0 7.4 19.6 D _MAX [µm] 18.5 18.5 15.6 18.5 18.5 44.0 (D ₉₀ -D ₁₀ )/D ₅₀ 2.6 2.7 2.7 1.8 2.7 2.5 Heat loss value [mass %] 0.36 0.37 0.38 0.38 0.34 0.37 Tight density [g/mL] 5.9 5.7 5.6 5.5 5.0 4.8 Cross-section observation Overall Overall average shape factor 2.13 2.10 2.03 2.67 2.17 2.45 Overall average of long side length [μm] 1.01 1.03 0.87 2.11 1.15 1.19 Overall average of short side length [μm] 0.59 0.59 0.54 1.10 0.66 0.59 Overall average aspect ratio 1.76 1.73 1.69 2.26 1.82 1.97 First Silver Particles Number ratio [%] 2.4 3.4 1.4 15.9 3.0 6.3 The average shape factor 3.99 3.01 2.62 2.73 3.11 6.05 Average value of short side length [μm] 1.64 1.67 1.70 2.04 1.84 1.46 Average aspect ratio 3.35 2.58 2.30 2.24 2.58 5.27 Second Silver Particles Number ratio [%] 80.8 76.6 78.5 84.1 86.2 79.4 The average shape factor 2.18 2.17 2.12 2.66 2.20 2.28 Average value of short side length [μm] 0.61 0.62 0.57 0.92 0.66 0.56 Average aspect ratio 1.81 1.80 1.79 2.26 1.86 1.85 The average value of α 0.86 0.85 0.85 0.85 0.85 0.86 Third Silver Particles Number ratio [%] 16.8 20.0 20.1 0.0 10.8 14.3 The average shape factor 1.62 1.67 1.63 - 1.61 1.76 Average value of short side length [μm] 0.33 0.32 0.34 - 0.33 0.35 Average aspect ratio 1.27 1.30 1.23 - 1.28 1.23 Specific resistance [µΩ･cm] 6.8 7.3 7.7 9.2 28.1 9.2

As is also clear from Table 2, it is known that the mixed silver powder of Examples 1 to 3 can reduce the specific resistance of the conductive film compared with the mixed silver powder of Comparative Examples 1 to 3.

In addition, the content ratio of adipic acid in the mixed silver powder was measured by the following method. 0.1 g to 0.2 g of the sample silver powder was weighed, and 1 mL of Hydrogen Chloride-Methanol Reagent (5%-10%) [for esterification] (manufactured by Tokyo Chemical Industry Co., Ltd.) as a methylation reagent was added using a graduated pipette, and ultrasonicated for 1 minute and heated in a 50°C hot water bath for 30 minutes. Then, ultrasonicated for 1 minute, the adipic acid dissolved by these operations was methylated to generate dimethyl adipate. Next, 5 mL of hexane and 4 mL of distilled water were added to the methylated sample solution, and the solution was shaken for 1 minute four times to extract dimethyl adipate into the organic solvent (hexane). Then, using arachidic acid methyl ester as an internal standard substance, hexane was added to the organic solvent in which dimethyl adipate was extracted to a constant volume of 25 mL. The obtained solution was measured using a GC-MS (GC: Agilent Technologies 7980B, MS: Agilent Technologies 5977B) as a gas chromatography quantitative analysis device based on the MS ionization (EI) method, and the amount of adipic acid was calculated by conversion calculation based on the amount of dimethyl adipate obtained. By comparing the calculated amount of adipic acid with the amount of sample silver powder, the amount of adipic acid (mass %) contained in the sample silver powder is quantified.

The amount of adipic acid contained in the mixed silver powder was investigated by the method for measuring the content ratio of adipic acid. The results showed that the amount of adipic acid contained in Example 1 was 0.021 mass%, in Example 2 was 0.023 mass%, in Example 3 was 0.024 mass%, in Comparative Example 1 was 0.025 mass%, and in Comparative Example 3 was 0.022 mass%. The amount of adipic acid contained in Comparative Example 2, to which no adipic acid was added, was less than 0.001 mass%.

(Example 4) Except that the step of partially flattening the silver powder A and the production of the mixed silver powder were performed by the following method, various operations and measurements were performed in the same manner as in Example 1. The results are shown in Tables 3 and 4. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in Figure 8.

<Preparation of Silver Powder A> [Partial Flattening Step] 32 kg of the above-mentioned silver powder mixed with lubricant and 256 kg of SUS balls (diameter 1.6 mm) were put into a vibration mill (FVR-20 model, manufactured by Central Chemical Industry Co., Ltd.) and processed at a vibration speed of 780 vpm for 135 minutes to partially flatten the silver powder mixed with lubricant.

After the partially flattened silver powder is separated from the SUS ball, it is crushed using a jet mill mixer (jet mill CJ-25 manufactured by Nisshin Engineering) under the condition that the supply amount of compressed air (0.7 MPa) is set to 11.6 ^m3 per 1 kg of silver powder. Then, the crushed silver powder is processed using a cyclone under the condition that the air volume used in the air transport per 1 kg of silver powder is 46 ^m3 , and small fine particles (chips, such as particles less than 0.1 μm) are removed from the crushed silver powder. The cyclone can also be used with a device that adds additional air flow to improve the collection efficiency.

<Production of mixed silver powder> [Mixing step] 84 g of the obtained silver powder A and 36 g of silver powder B are mixed to obtain a mixture.

[Post-treatment step] Prepare a solution in which 0.0336 g of adipic acid (manufactured by Junsei Chemical Co., Ltd.) is dissolved in 0.3024 g of an ethanol solvent. Then, add the solution to 120 g of the mixture obtained in the mixing step, mix for 4.5 minutes using a sample mill (manufactured by Kyoritsu Riko Co., Ltd., SK-M10), and treat the surface of the mixture with adipic acid to produce a mixed silver powder.

(Comparative Example 4) Without mixing silver powder B, 120 g of silver powder A was taken, and a solution prepared by dissolving adipic acid (0.0336 g of adipic acid manufactured by Junsei Chemical Co., Ltd.) in 0.3024 g of ethanol solvent was added, and mixed for 4.5 minutes using a sample mill to allow adipic acid as a further surface treatment agent to adhere to the surface of the silver powder. Other operations and measurements were performed in the same manner as in Example 4. The results are shown in Tables 3 and 4. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in Figure 9.

(Comparative Example 5) Except that the mixed silver powder was produced by the following method, various operations and measurements were performed in the same manner as in Example 4. The results are shown in Tables 3 and 4. In addition, a scanning electron microscope photograph of the obtained mixed silver powder is shown in FIG10.

<Production of mixed silver powder> [Pulverization step] First, 120 g of silver powder A obtained by the same method as the preparation method of silver powder A in Example 4 was pulverized using a sample mill (SK-M10 manufactured by Kyoritsu Riko Co., Ltd.) for 4.5 minutes.

[Mixing step] The silver powder A obtained in the crushing step is mixed with the silver powder B in a mass ratio of 70:30 (silver powder B/silver powder A=30/70) to produce a mixture (i.e., mixed silver powder).

[Table 3] Embodiment Comparison Example 4 4 5 raw material Silver powder A The average value of the aspect ratio of silver particles A1 2.98 2.98 2.98 Average value of aspect ratio of silver particles A2 1.45 1.45 1.45 The average value of the ratio β of silver particles A2 0.85 0.85 0.85 Usage [Quantity] 70 100 70 Silver powder B The average value of the aspect ratio of silver particles B 1.3 - 1.3 Usage [Quantity] 30 0 30 Preparation method Pre-processing step or post-processing step Processing Timing Post-processing - - Types of polycarboxylic acids Adipic acid Adipic acid - Usage rate of polycarboxylic acid [mass %] 0.028 0.028 - Processing objectSilver powder Silver powder A, Silver powder B Silver powder A -

[Table 4] Embodiment Comparison Example 4 4 5 Mixed silver powder Types of polycarboxylic acids Adipic acid Adipic acid - BET specific surface area [m ² /g] 0.61 0.47 0.61 Particle size distribution D ₁₀ [µm] 0.6 1.0 0.6 D ₅₀ [µm] 1.8 2.3 1.9 D ₉₀ [µm] 4.1 4.3 4.3 D ₉₅ [µm] 5.1 5.3 5.4 D _MAX [µm] 11.0 11.0 13.1 (D ₉₀ -D ₁₀ )/D ₅₀ 2.0 1.5 1.9 Heat loss value [mass %] 0.38 0.37 0.34 Tight density [g/mL] 5.4 5.6 5.0 Cross-section observation Overall Overall average shape factor 2.15 2.36 2.07 Overall average of long side length [μm] 0.95 1.78 0.97 Overall average of short side length [μm] 0.55 1.01 0.58 Overall average aspect ratio 1.83 2.00 1.76 First Silver Particles Number ratio [%] 1.8 5.1 3.2 The average shape factor 3.01 2.75 2.13 Average value of short side length [μm] 1.86 1.73 2.25 Average aspect ratio 2.35 2.26 1.70 Second Silver Particles Number ratio [%] 81.1 93.6 83.8 The average shape factor 2.23 2.35 2.15 Average value of short side length [μm] 0.57 0.98 0.56 Average aspect ratio 1.92 2.00 1.84 The average value of α 0.85 0.85 0.85 Third Silver Particles Number ratio [%] 17.1 1.3 13.0 The average shape factor 1.72 1.72 1.55 Average value of short side length [μm] 0.32 0.43 0.34 Average aspect ratio 1.32 1.11 1.22 Specific resistance [µΩ･cm] 5.6 7.6 18.0

As is also clear from Table 4, it is known that the mixed silver powder of Example 4 can reduce the specific resistance of the conductive film compared with the mixed silver powder of Comparative Examples 4 and 5.

In addition, the amount of adipic acid contained in the mixed silver powder was investigated by the above-mentioned method for measuring the content ratio of adipic acid. The result was 0.026% by mass in Example 4 and 0.026% by mass in Comparative Example 4. The amount of adipic acid not added in Comparative Example 5 was less than 0.001% by mass. [Industrial Applicability]

The present invention can provide a mixed silver powder capable of reducing the specific resistance of a conductive film. In addition, the present invention can provide a method for producing a mixed silver powder capable of reducing the specific resistance of a conductive film. In addition, the present invention can provide a conductive paste capable of reducing the specific resistance of a conductive film.

without

FIG1 is a schematic diagram for explaining a method for determining the length of the long side and the short side of a rectangle that circumscribes the outer shape of a silver particle cross section and has the smallest area. FIG2 is a scanning electron microscope photograph of the mixed silver powder of Example 1. FIG3 is a scanning electron microscope photograph of the mixed silver powder of Example 2. FIG4 is a scanning electron microscope photograph of the mixed silver powder of Example 3. FIG5 is a scanning electron microscope photograph of the mixed silver powder of Comparative Example 1. FIG6 is a scanning electron microscope photograph of the mixed silver powder of Comparative Example 2. FIG7 is a scanning electron microscope photograph of the mixed silver powder of Comparative Example 3. FIG8 is a scanning electron microscope photograph of the mixed silver powder of Example 4. FIG9 is a scanning electron microscope photograph of the mixed silver powder of Comparative Example 4. FIG10 is a scanning electron microscope photograph of the mixed silver powder of Comparative Example 5.

Claims (9)

A mixed silver powder, comprising a first silver particle, a second silver particle and a third silver particle, wherein when observing more than 100 silver particles in a polished cross section of a resin block formed by embedding the mixed silver powder in a resin, and measuring the perimeter of the silver particles, and the long side length and short side length of a rectangle that circumscribes the outer shape of the silver particles and has the smallest area, the silver particles having a long side length of 3 μm or more are set as the first silver particles, the silver particles having a long side length of 0.5 μm or more and less than 3 μm are set as the second silver particles, and the silver particles having a long side length of less than 0.5 μm are set as the third silver particles, The average value of the aspect ratio of the first silver particles is greater than 2, The average value of the aspect ratio of the second silver particles is greater than 1.5 and less than 2, The average value of the aspect ratio of the third silver particles is less than 1.5, The number ratio of the first silver particles in the mixed silver powder is greater than 0.5% and less than 5%, the number ratio of the second silver particles in the mixed silver powder is greater than 10%, and the number ratio of the third silver particles in the mixed silver powder is greater than 15%, By the following formula (1): Ratio α = circumference of the second silver particle / (long side length of the second silver particle × 2 + short side length of the second silver particle × 2) ... (1) The calculated average value of the ratio α is greater than 0.84, A polycarboxylic acid is attached to the surface of at least one silver particle selected from the group consisting of the first silver particle, the second silver particle, and the third silver particle. The mixed silver powder as described in claim 1, wherein polycarboxylic acid is attached to the surfaces of the first silver particles and the second silver particles. The mixed silver powder according to claim 1 or 2, wherein the content ratio of the polycarboxylic acid to the total of the first silver particles, the second silver particles, and the third silver particles is 0.01 mass % or more and 0.15 mass % or less. The mixed silver powder as described in claim 1 or 2, wherein the polycarboxylic acid is at least one polycarboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, apple acid, diethylglutaric acid, 3-methyladipic acid, butylmalonic acid, maleic acid, diglycolic acid and citric acid. A method for producing mixed silver powder, producing the mixed silver powder as described in claim 1, the method comprising: a mixing step, mixing silver powder A containing silver particles A1 and silver particles A2 with silver powder B containing silver particles B to obtain a mixture; and a pre-treatment step, before the mixing step, using a polycarboxylic acid to perform a surface treatment agent attachment treatment on at least one of the silver powders A and B, or a post-treatment step, using a polycarboxylic acid to perform a surface treatment agent attachment treatment on the mixture obtained in the mixing step, When observing more than 100 silver particles in a polished cross section of a resin block formed by embedding the silver powder A in resin and more than 100 silver particles in a polished cross section of a resin block formed by embedding the silver powder B in resin, respectively, and measuring the perimeter of the silver particles, and the length of the long side and the length of the short side of a rectangle circumscribing the outer shape of the silver particles and having the smallest area, the average value of the aspect ratio of the silver particles A1 is greater than 2, the average value of the aspect ratio of the silver particles A2 is less than 2, and by the following formula (2): ratio β = perimeter of silver particle A2/(length of the long side of silver particle A2 × 2 + length of the short side of silver particle A2 × 2)…(2) The calculated average value of the ratio β is greater than 0.84, and the average value of the aspect ratio of the silver particles B is less than 1.5. The method for producing mixed silver powder as described in claim 5 includes the pre-treatment step, In the pre-treatment step, the silver powder A is subjected to surface treatment agent attachment treatment using the polycarboxylic acid. The method for producing a mixed silver powder as described in claim 5 or 6, wherein the usage rate of the polycarboxylic acid relative to the total of the silver powder A and the silver powder B is 0.01 mass % or more and 0.15 mass % or less. A method for producing a mixed silver powder as described in claim 5 or 6, wherein the polycarboxylic acid used in the surface treatment agent attachment treatment is at least one polycarboxylic acid selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, apple acid, diethylglutaric acid, 3-methyladipic acid, butylmalonic acid, maleic acid, diglycolic acid and citric acid. A conductive paste comprises the mixed silver powder, a binder and a solvent as described in claim 1 or 2.