JP6423139B2 - Flake silver powder, method for producing the same, and conductive paste - Google Patents

Description

  The present invention relates to a flaky silver powder, a method for producing the same, and a conductive paste.

Conventionally, a conductive paste in which silver powder is dispersed in an organic component has been used to form electrodes and circuits of electronic components and the like. Among such conductive pastes, resin-cured conductive pastes are brought into conduction by contact with silver powder due to resin volume shrinkage (see Patent Document 1). As silver powder blended in the resin curable conductive paste, flaky silver powder having a large contact area is used (see Patent Document 2).
In addition, it has been proposed to improve the characteristics of the resin-curable conductive paste by attaching a polyvalent carboxylic acid to the surface of silver powder (see Patent Document 3).
However, in the technique described in Patent Document 3, flaky silver powder and spherical silver powder are used in combination, and the polyvalent carboxylic acid to be attached to the silver powder is not optimized, so that sufficiently satisfactory performance is obtained. However, further improvements and improvements are desired.

JP 2002-150837 A Japanese Patent No. 3874634 Japanese Patent Application Laid-Open No. 2011-100573

  An object of the present invention is to solve the above-described problems and achieve the following objects. That is, an object of the present invention is to provide a flaky silver powder excellent in conductivity and suitably used for a resin curable conductive paste, a method for producing the same, and a conductive paste.

  The flaky silver powder of the present invention as a means for solving the above problems has ricinoleic acid on at least the surface.

  According to the present invention, there are provided a flaky silver powder, a method for producing the same, and a conductive paste that can solve the above-described problems, have excellent conductivity, and are suitably used for a resin-cured conductive paste. Can do.

FIG. 1 is a scanning electron micrograph of the flaky silver powder produced in Example 1. FIG. 2 is a scanning electron micrograph of the flaky silver powder prepared in Example 2. FIG. 3 is a scanning electron micrograph of the flaky silver powder produced in Comparative Example 1. FIG. 4 is a scanning electron micrograph of the silver powder produced in Comparative Example 2. FIG. 5 is a scanning electron micrograph of the silver powder produced in Comparative Example 3. 6 is a graph showing the volume resistivity of the conductive films of Examples 1 and 2 and Comparative Example 1. FIG.

(Flake silver powder)
The flaky silver powder of the present invention has ricinoleic acid on at least the surface.
Since the flaky silver powder has ricinoleic acid on the surface, the compatibility with the vehicle containing the resin is improved, and since it can be highly filled, the contact between the silver powders is increased and the conductivity is greatly improved. The inventor presumes. This effect is obtained specifically when ricinoleic acid is used as shown in the examples described later.

  Here, “having at least ricinoleic acid on the surface” means that the surface of flaky silver powder includes a state in which ricinoleic acid is attached to the surface of flaky silver powder by some method such as adsorption or coating. It is sufficient that ricinoleic acid is included at least in part, and the entire surface of the flaky silver powder may have ricinoleic acid, or a part of the surface of the flaky silver powder may have ricinoleic acid. . In addition, you may have ricinoleic acid inside the flaky silver powder.

The flake shape includes a flat plate, a thin rectangular parallelepiped, a flake shape, or a scale shape, and an aspect ratio (average major axis / average thickness) indicates a shape larger than 1.5. The aspect ratio is preferably 2 to 30, and more preferably 3 to 10. When the aspect ratio is 1.5 or less, the contact area between the flaky silver powders is not sufficient, and the conductivity of the conductive film formed using the conductive paste is sufficiently high when blended with the conductive paste. If it exceeds 30, it may be difficult to produce flaky silver powder.
The aspect ratio can be obtained by (average major axis L / average thickness T). Here, the “average major axis L” and the “average thickness T” indicate the average major axis and average thickness of 100 flaky silver powders measured with a scanning electron microscope.

As an average particle diameter which is the accumulation 50 mass% particle diameter (D50) by the laser diffraction scattering type particle size distribution measuring method of the said flaky silver powder, 1 micrometer-8 micrometers are preferable.
When the average particle size is less than 1 μm, the contact area between the flaky silver powders is not sufficient, and the conductivity of the conductive film formed using the conductive paste is sufficiently increased by blending with the conductive paste. When the thickness exceeds 8 μm, the unevenness on the outer periphery of the conductive film formed using the conductive paste is increased, and the viscosity of the conductive paste is reduced. Sometimes.

The specific surface area of the flaky silver powder is not particularly limited and may be appropriately selected depending on the ^purpose, 0.4m ² / ^g~4m 2 / g is ^preferable, 0.5m ² / ^g~2m 2 / G is more preferable. When the specific surface area is less than 0.4 m ² / g, the viscosity of the conductive paste is lowered, and “sagging” is strongly generated in the outer peripheral portion of the conductive film formed using the paste, thereby forming a fine line. In the case of exceeding 4 m ² / g, when the conductive paste is formed by screen printing, the clogging of the screen and uneven printing may occur when the viscosity exceeds 4 m ² / g. .

<Ricinolic acid>
The ricinoleic acid is an unsaturated fatty acid represented by the following structural formula, and is naturally present in castor bean seeds. About 90% of the constituent fatty acids of castor oil are triglycerides of ricinoleic acid.

0.01 mass%-2 mass% are preferable with respect to the said flaky silver powder mass, and, as for the adhesion amount of the said ricinoleic acid, 0.05 mass%-1.0 mass% are more preferable.
Here, the amount of the surface treatment agent (including ricinoleic acid, fatty acids other than ricinoleic acid contained as impurities in commercially available ricinoleic acid, a dispersant, etc.) adhering to the silver powder can be measured as follows. In this method, only ricinoleic acid cannot be distinguished and measured, but the amount of ricinoleic acid attached can be predicted.

-Measurement of adhesion amount of surface treatment agent-
A predetermined amount (for example, 2 g) of the flaky silver powder is prepared and weighed accurately. An acid is added to the accurately weighed silver powder to obtain a silver solution. As the acid to be added, nitric acid having a concentration of 6M to 12M can be preferably used. The acid is added in an excess amount compared to the reaction equivalent of the precisely weighed silver powder. For example, if adding nitric acid with a concentration of 6M to 10M, add 20 mL. At the time of dissolution, care is taken to keep the temperature of the mixture at 50 ° C. or less, and the pH value of the silver solution is preferably 3 or less.
When the liquid temperature of the silver solution reaches room temperature (25 ° C.), an organic solvent that can dissolve ricinoleic acid, is liquid at 25 ° C., has a boiling point of 50 ° C. or less, and is insoluble in water (for example, And dichloromethane are preferred.) Is added in the same amount as the silver solution, and stirred sufficiently. With this configuration, ricinoleic acid is stably transferred to the organic phase. Therefore, the mixture of the silver solution and the organic solvent is separated into an aqueous phase and an organic phase by a centrifugal separation method or the like. When the separation is completed, the aqueous phase and the organic phase are separated, and a predetermined amount (for example, 2 mL) of a sample is taken from the organic phase.
After impregnating the preparative sample into a porous board, the porous board is heated to evaporate and dry the organic solvent. The carbon content of the dried porous board is measured using, for example, a carbon analyzer (manufactured by Horiba, Ltd., EMIA-510). The carbon analyzer is previously calibrated with a standard sample containing a known amount of a surface treatment agent, and a calibration curve is created. From the measured carbon amount, the surface treatment agent attached to the flaky silver powder can be quantified. This analysis can also determine whether or not a surface treatment agent is present.

-Quality of ricinoleic acid-
A predetermined amount (for example, 2 g) of the flaky silver powder is prepared and weighed accurately. Toluene is added to the precisely weighed silver powder and washed. The washing liquid is separated into solid and liquid, and the toluene washing filtrate is recovered. The recovered toluene wash filtrate is evaporated to obtain a dry residue. By analyzing this dry residue using, for example, an FT-IR apparatus (manufactured by ThermoELECTRON, Nicolet 4700), qualitative determination of ricinoleic acid can be made.

(Method for producing flaky silver powder)
The method for producing flaky silver powder of the present invention is a method for producing the flaky silver powder of the present invention, which includes a silver powder preparation step and a flaking step, and further includes other steps as necessary. .

<Silver powder production process>
The silver powder production step is a step of producing silver powder by a wet reduction method. The details of the wet reduction method are described, for example, in JP-A-7-76710.
In the silver powder preparation step, an alkali or complexing agent is added to the silver salt-containing aqueous solution to produce a silver oxide-containing slurry or a silver complex salt-containing aqueous solution, and ricinoleic acid is added as a reducing agent and a dispersing agent to reduce and precipitate silver particles. It is preferable to make it. Thereby, mixing of different types of fatty acids is reduced, and the above-mentioned effects specific to ricinoleic acid are easily exhibited.

  There is no restriction | limiting in particular as silver powder obtained by the said silver powder preparation process, Although it can select suitably according to the objective, Spherical or indefinite shape silver powder is preferable. Here, when the silver powder is observed with a scanning electron microscope (SEM), the spherical shape means that the particle shape is spherical or substantially spherical, and the sphericity of 100 particles (sphericity: when the particles are observed with an SEM photograph) , (The diameter of the longest diameter portion) / (the diameter of the shortest diameter portion)) is 1.5 or less, and the indefinite shape is a particle shape other than the spherical shape when observed by SEM. Yes, it refers to silver powder that does not have the characteristics of a specific particle shape such as a columnar shape or a prismatic shape.

<Flakeing process>
The flaking step is a step of flaking ricinoleic acid as a silver powder and a lubricant using a ball.
In the flaking step, ricinoleic acid can be attached to the surface of the flaked silver powder by using ricinoleic acid as a lubricant.

  There is no restriction | limiting in particular as an apparatus which performs the said flaking process, According to the objective, it can select suitably, For example, mills, such as a ball mill, an attritor, etc. are mentioned.

  The amount of ricinoleic acid added during the flaking treatment is not particularly limited and may be appropriately selected depending on the intended purpose, but is 0.01% by mass to 2% by mass with respect to the silver powder to be flaked. preferable. When the addition amount is less than 0.01% by mass, the effect of adding ricinoleic acid may be insufficient, and when it exceeds 2% by mass, a sufficient aspect ratio may not be obtained.

The ball (media) is preferably a ball (media) having a diameter of 0.5 mm to 3 mm and a spherical shape. When the diameter of the ball (media) is less than 0.5 mm, when separating the flaky silver powder and the media after flaking, the efficiency of the separation decreases due to clogging of the media, etc. The average particle size of the obtained flaky silver powder may become excessive.
The material of the ball (media) is not particularly limited and may be appropriately selected depending on the purpose. Examples thereof include metals, ceramics such as alumina and zirconia, and among others, contamination to products. Considering the nation, stainless steel is preferable.
There is no restriction | limiting in particular as addition amount at the time of flaking process of the said ball | bowl (media), Although it can select suitably according to the objective, 1-50 times by mass is preferable with respect to the silver powder to flake-treat. . When the addition amount is less than 1 time, a sufficient aspect ratio may not be obtained, and when it exceeds 50 times, the amount of silver powder that can be flaked at one time decreases, and the processing cost increases. is there.

There is no restriction | limiting in particular in the processing time of the said flaking process, Although it can select suitably according to the objective, 10 minutes-50 hours are preferable. If the treatment time is less than 10 minutes, it may be difficult to obtain a flaky silver powder having a sufficient aspect ratio, and if it exceeds 50 hours, there is no effect and it is uneconomical. In addition, in the flaking process, it is not necessary for all the silver powder that has been added to be flaked, and silver powder that has not been flaked after the flaking process may be mixed .

<Other processes>
As said other process, a washing | cleaning process, a drying process, etc. are mentioned, for example.

(Conductive paste)
The conductive paste of the present invention is a conductive paste produced using the flaky silver powder of the present invention, and examples thereof include a resin curable conductive paste.
There is no restriction | limiting in particular as a viscosity of the said electrically conductive paste, Although it can select suitably according to the objective, 3 Pa * s-100 Pa * s are preferable at 25 degreeC, and 3 Pa * s-15 Pa * s are more preferable. .
When the viscosity of the conductive paste is less than 3 Pa · s, “bleeding” may occur during printing, and when it exceeds 100 Pa · s, uneven printing may occur.

There is no restriction | limiting in particular as a preparation method of the said electrically conductive paste, It can select suitably according to the objective from conventionally well-known methods, for example, it produces by mixing the said flaky silver powder with resin. be able to.
There is no restriction | limiting in particular as said resin, According to the objective, it can select suitably, For example, an epoxy resin, an acrylic resin, a polyester resin, a polyimide resin, a polyurethane resin, a phenoxy resin, a silicone resin, or these mixtures, etc. Is mentioned.
There is no restriction | limiting in particular also as content of the said flaky silver powder in the said electrically conductive paste, According to the objective, it can select suitably. In addition, you may mix the said flaky silver powder and other silver powder of this invention.

  Since the conductive paste of the present invention contains the flaky silver powder of the present invention, the conductive paste is excellent in conductivity, and is a collector electrode of a solar battery cell, an external electrode of a chip-type electronic component, an RFID, an electromagnetic wave shield, a vibrator It is suitably used for electrodes such as adhesion, membrane switch, electroluminescence, or electrical wiring.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

Example 1
<Preparation of flaky silver powder>
-Silver powder production process-
19 kg of 25 mass% ammonia aqueous solution was added to 375 kg of 2.7 mass% silver nitrate aqueous solution as silver ion aqueous solution, and silver ammine complex aqueous solution was produced | generated. To the resulting silver ammine complex aqueous solution, 25 kg of a 37% by mass formalin aqueous solution was added as a reducing agent. Immediately after adding the reducing agent, 10 g of stearic acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added as a dispersant to produce a slurry containing silver powder. The obtained slurry was filtered, washed with water, and then dried and heat-treated to obtain 10 kg of silver powder.

-Flaking process-
Add ricinoleic acid (manufactured by Wako Pure Chemical Industries, Ltd.) as a lubricant to the obtained silver powder in an amount of 0.2% by mass with respect to the silver powder, and mix well. Roll with SUS balls (diameter 1.6 mm). It put into the ball mill, flaking process was implemented on conditions with a rotation speed of 71 rpm, and processing time 15 hours, and flaky silver powder was obtained. In addition, after silver powder and SUS ball | bowl separation, in order to remove a coarse grain, the sieve of 40 micrometers of openings was applied. A scanning electron micrograph of the flaky silver powder obtained in Example 1 is shown in FIG.

(Example 2)
<Preparation of flaky silver powder>
-Silver powder production process-
19 kg of 25 mass% ammonia aqueous solution was added to 375 kg of 2.7 mass% silver nitrate aqueous solution as silver ion aqueous solution, and silver ammine complex aqueous solution was produced | generated. To the resulting silver ammine complex aqueous solution, 25 kg of a 37% by mass formalin aqueous solution was added as a reducing agent. Immediately after adding the reducing agent, 10 g of ricinoleic acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added as a dispersant to produce a slurry containing silver powder. The obtained slurry was filtered, washed with water, and then dried and heat-treated to obtain 10 kg of silver powder.

-Flaking process-
Add ricinoleic acid (manufactured by Wako Pure Chemical Industries, Ltd.) as a lubricant to the obtained silver powder in an amount of 0.2% by mass with respect to the silver powder, and mix well. Roll with SUS balls (diameter 1.6 mm). It put into the ball mill, flaking process was implemented on conditions with a rotation speed of 71 rpm, and processing time 15 hours, and flaky silver powder was obtained. In addition, after silver powder and SUS ball | bowl separation, in order to remove a coarse grain, the sieve of 40 micrometers of openings was applied. A scanning electron micrograph of the flaky silver powder obtained in Example 2 is shown in FIG.

(Comparative Example 1)
<Preparation of flaky silver powder>
-Silver powder production process-
19 kg of 25 mass% ammonia aqueous solution was added to 375 kg of 2.7 mass% silver nitrate aqueous solution as silver ion aqueous solution, and silver ammine complex aqueous solution was produced | generated. To the resulting silver ammine complex aqueous solution, 25 kg of a 37% by mass formalin aqueous solution was added as a reducing agent. Immediately after adding the reducing agent, 10 g of stearic acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added as a dispersant to produce a slurry containing silver powder. The obtained slurry was filtered, washed with water, and then dried and heat-treated to obtain 10 kg of silver powder.

-Flaking process-
To the obtained silver powder, stearic acid (manufactured by Wako Pure Chemical Industries, Ltd.) as a lubricant is added in an amount of 0.2% by mass with respect to the silver powder and mixed well, and rolled together with a SUS ball (diameter 1.6 mm). It put into the ball mill, flaking process was implemented on conditions with a rotation speed of 71 rpm, and processing time 15 hours, and flaky silver powder was obtained. In addition, after silver powder and SUS ball | bowl separation, in order to remove a coarse grain, the sieve of 40 micrometers of openings was applied. A scanning electron micrograph of the flaky silver powder obtained in Comparative Example 1 is shown in FIG.

(Comparative Example 2)
<Preparation of spherical silver powder>
3,887 g of a silver nitrate solution containing 43.16 g of Ag was prepared, 97.1 g of an aqueous ammonia solution having a concentration of 28% by mass was added thereto to prepare an aqueous reaction system containing silver ions, and the liquid temperature was 34.5 ° C. It was.
To the aqueous reaction system containing silver ions, 0.1 mass% (43.16 mg) of polyethyleneimine (PEI) having a molecular weight of 600 with respect to Ag mass is added, and 7.5 g of a hydrazine aqueous solution is further added as a reducing agent. And a slurry containing silver powder was obtained.
Furthermore, 0.23% by mass of stearic acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the resulting slurry containing silver powder, and after sufficient stirring, the mixture was aged. The aged slurry was filtered, washed with water, and crushed to obtain silver powder. A scanning electron micrograph of the silver powder obtained in Comparative Example 2 is shown in FIG.

(Comparative Example 3)
<Preparation of spherical silver powder>
3,887 g of a silver nitrate solution containing 43.16 g of Ag was prepared, and 97.1 g of 28 mass% ammonia aqueous solution was added thereto to prepare an aqueous reaction system containing silver ions, and the liquid temperature was 34.5 ° C. It was.
To an aqueous reaction system containing silver ions, polyethyleneimine (PEI) having a molecular weight of 600 was added in an amount of 0.1% by mass (43.16 mg) with respect to the Ag mass, and 7.5 g of a hydrazine aqueous solution was further added as a reducing agent. Stirring to obtain a slurry containing silver powder.
Further, 0.18% by mass of ricinoleic acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the resulting slurry containing silver powder, and after sufficient stirring, was aged. The aged slurry was filtered, washed with water, and crushed to obtain silver powder. A scanning electron micrograph of the silver powder obtained in Comparative Example 3 is shown in FIG.

  Next, about the silver powder of Examples 1-2 and Comparative Examples 1-3, the average particle diameter, the aspect-ratio, and the specific surface area were measured as follows. The results are shown in Table 1.

<Aspect ratio measurement method>
The aspect ratio of each produced silver powder was determined from (average major axis L / average thickness T). Here, “average major axis L” and “average thickness T” indicate the average major axis and average thickness of 100 particles measured with a scanning electron microscope.

<Method for measuring specific surface area>
The specific surface area (BET) was determined using a MONOSORB apparatus (manufactured by Yuasa Ionics Co., Ltd.) using 70% He, 30% N ₂ carrier gas, 3 g of silver powder in the cell and degassing at 60 ° C. for 10 minutes. The BET one-point method was used for measurement.

<Measurement of average particle size>
0.3 g of each prepared silver powder was placed in 30 mL of isopropyl alcohol, treated with a 45 W ultrasonic cleaner for 5 minutes, and then the particle size was measured using a Microtrac 9320-X100 (Honeywell-Nikkiso Co., Ltd.) for the treatment liquid. . The cumulative 50 mass% particle size (D50) at that time was defined as the average particle size of the silver powder, and the cumulative 10 mass% particle size (D10) and cumulative 90 mass% particle size (D90) values were also determined.

Next, each produced silver powder, polyester resin, and a solvent are mixed with the following composition, and a kneading process is performed by passing a roll gap from 100 μm to 20 μm using a three-roll (manufactured by Otto Herman, EXAKT80S). Thus, the conductive pastes of Examples 1-2 and Comparative Examples 1-3 were obtained. The obtained conductive paste was completely kneaded.
・ Each silver powder: 75 parts by mass ・ Polyester resin (byron 300, manufactured by Toyobo Co., Ltd.): 7.5 parts by mass ・ Solvent (ECA (diethylene glycol mono-n-ethyl ether), Wako Pure Chemical Industries Ltd.) 17.5 parts by mass)

  Next, the viscosity and thixo ratio of the obtained conductive paste were measured as follows. The results are shown in Table 1.

<Viscosity and thixo ratio of conductive paste>
The viscosity of the obtained conductive paste was measured using an E-type viscometer (manufactured by BROOKFIELD, DV-III +) under conditions of a cone spindle CP-52, a paste temperature of 25 ° C., and a rotation speed of 5 rpm.
Also, a five-minute 1 rpm (shear rate a time of 2 sec ^-1), 5 rpm by measuring the value of one minute (shear rate 10 sec ^-1), were determined thixotropic ratio (TI value) = 1rpm / 5rpm.

Next, a film was formed from the produced conductive paste by screen printing on a polyethylene terephthalate (PET) film. Screen printing conditions are as follows.
-Printing device: MS-300 manufactured by Murakami Co., Ltd.
-Printing conditions: A squeegee pressure of 0.3 MPa, a film having a width of 500 μm and a length of 37.5 mm was formed.
The obtained film was heat-treated at 130 ° C. for 30 minutes using an air circulation dryer to form a conductive film.
About the obtained electrically conductive film, average thickness, volume resistivity, and adhesive strength were evaluated as follows. The results are shown in Table 1.

<Average thickness of conductive film>
Using the surface roughness meter (SE-30D, manufactured by Kosaka Laboratory Ltd.), measure the level difference between the part where the film is not printed on the PET film and the part of the conductive film. Thus, the average thickness of the conductive film was measured.

<Volume resistivity of conductive film>
The resistance value at the position of the length (interval) of the conductive film was measured using a digital multimeter (manufactured by ADVANTEST, R6551). The volume of the conductive film was determined from the size (film thickness, width, length) of the conductive film, and the volume resistivity was determined from this volume and the measured resistance value. In addition, the value of the volume resistivity of Examples 1, 2 and Comparative Example 1 is shown in FIG.

<Adhesion strength>
Using a cutter knife and a crosscut guide on a printed 8.5 mm x 10 mm solid pattern, 1 mm width x 11 pieces, vertical and horizontal cuts were made, and a grid of 10 squares x 10 squares (100 squares) was produced. . A cellophane tape (registered trademark) (Oriente tape No. 830, manufactured by Sekisui Chemical Co., Ltd.) is stretched on the cut and rubbed well with a nail, and then the cellotape is peeled off rapidly (0.5 sec to 1 sec). The number of cells in which the film remained completely without peeling off was counted.

* Volume resistivity: “OR” means outside the measurable range.

Examples of the aspect of the present invention include the following.
<1> A flaky silver powder having ricinoleic acid at least on the surface.
<2> The flaky silver powder according to <1>, wherein the aspect ratio (average major axis / average thickness) is greater than 1.5.
<3> The method for producing flaky silver powder according to any one of <1> to <2>,
A silver powder production process for producing silver powder by a wet reduction method;
A flaky silver powder production method comprising a flaking process in which ricinoleic acid as a silver powder and a lubricant is flaked using a ball.
<4> In the silver powder production process, an alkali or complexing agent is added to the silver salt-containing aqueous solution to produce a silver oxide-containing slurry or a silver complex salt-containing aqueous solution, and ricinoleic acid is added as a reducing agent and a dispersing agent to reduce silver particles. It is a manufacturing method of the flaky silver powder as described in said <3> made to precipitate.
<5> A conductive paste comprising the flaky silver powder according to any one of <1> to <2>.

Claims (8)

Possess the ricinoleic acid at least on the surface,
A flaky silver powder characterized by being used as a silver powder blended in a resin-curable conductive paste .   The flaky silver powder according to claim 1, wherein the aspect ratio (average major axis / average thickness) is larger than 1.5.   The flaky silver powder according to any one of claims 1 to 2, wherein a cumulative 50 mass% particle size (D50) by a laser diffraction / scattering particle size distribution measurement method is 1 µm to 8 µm. A method for producing the flaky silver powder according to any one of claims 1 to 3,
A silver powder production process for producing silver powder by a wet reduction method;
A method for producing a flaky silver powder, comprising a flaking step of lysinol acid as a silver powder and a lubricant using a ball. In the silver powder production process, an alkali or complexing agent is added to a silver salt-containing aqueous solution to produce a silver oxide-containing slurry or a silver complex salt-containing aqueous solution, and ricinoleic acid is added as a reducing agent and a dispersing agent to reduce and precipitate silver particles. Item 5. A method for producing flaky silver powder according to Item 4. A conductive paste comprising the flaky silver powder according to claim 1 and a curable resin. The conductive paste according to claim 6, wherein the curable resin is an epoxy resin, an acrylic resin, a polyester resin, a polyimide resin, a polyurethane resin, a phenoxy resin, a silicone resin, or a mixture thereof. The conductive paste according to claim 6, wherein the curable resin is a polyester resin.