Silver powder and method for producing same

Abstract

This silver powder comprises an aggregate of silver particles. The tap density TD is no greater than 2.3 g / cm3. The value of SSA / TD, which is the ratio of the BET specific surface area SSA [m2 / g] with respect to the tap density TD [g / cm3], is 1.5 or below. The aspect ratio, which is the ratio of the major axis length to the minor axis length of the silver particles, is 3.5-12.0. The value of SSA × D50, which is the product of the BET specific surface area SSA [m2 / g] and the volume-cumulative particle diameter D50 [μm] at 50% cumulative volume by the laser diffraction / scattering particle size distribution measurement method, is preferably 2.6 or below.

Description

Silver powder and its manufacturing method

[0001] This invention relates to silver powder and a method for producing the same.

[0002] Thermally conductive pastes containing metal particles such as silver are being developed with the aim of improving heat conduction between components of electronic devices. Among the various particle shapes, dendrite-shaped particles are being increasingly applied to pastes because they offer properties due to their distinctive shape.

[0003] For example, Patent Document 1 describes a dendritic metal powder made of silver, having a main branch and lateral branches extending from the main branch. Patent Document 2 describes dendrite silver particles having a single main axis and multiple branch portions branching from the main axis. Both documents describe using these particles in conductive paste.

[0004] Japanese Patent Publication No. 2013-144829 Japanese Patent Publication No. 2023-152678

[0005] In recent years, with the increasing performance of electronic devices, there has been a demand for further improvements in the thermal conductivity of thermally conductive pastes. Pastes containing particles described in Patent Documents 1 and 2 were not sufficiently high in terms of thermal conductivity. Therefore, the object of the present invention is to provide silver powder that can improve the thermal conductivity of silver powder-containing paste compared to conventional methods.

[0006] The present invention relates to a silver powder consisting of an aggregate of silver particles, with a tap density TD of 2.3 g / cm³. ３ The tap density TD [g / cm³] is as follows: ３ BET specific surface area SSA [m²] for ] ２ The present invention provides silver powder in which the SSA / TD ratio, which is the ratio of [ / g], is 1.5 or less, and the aspect ratio, which is the ratio of the major axis length to the minor axis length in the silver particles, is 3.5 or more and 12.0 or less.

[0007] The present invention also provides a method for producing silver powder by electrolyzing an electrolytic solution containing silver ions and an additive to deposit silver powder on a cathode, wherein hydantoin or its derivative is used as the additive, the concentration of the additive in the electrolytic solution is 0.1 mg / L or more and less than 10.0 mg / L, and the current density during electrolysis is 1000 A / m ２ or more and 3000 A / m ２ or less. The present invention provides a method for producing silver powder.

[0008] FIG. 1(a) to FIG. 1(d) are schematic diagrams showing a method for measuring the aspect ratio of silver particles in the silver powder of the present invention, and FIG. 1(e) is a schematic diagram showing a method for measuring the length and thickness of the main axis and the length and thickness of the branch portion of the silver particles in the silver powder of the present invention. FIG. 2 is a scanning electron microscope image of the silver powder obtained in Example 1. FIG. 3 is a scanning electron microscope image of the silver powder obtained in Comparative Example 1.

[0009] Hereinafter, the present invention will be described based on its preferred embodiments. The silver powder of the present invention is composed of an aggregate of silver particles. The silver particles are preferably particles substantially composed of silver element or silver-based alloy particles. The silver powder is preferably composed of an aggregate of such particles. The former particles are particles substantially composed of silver element and containing inevitable elements in the remainder. In this case, it is desirable that the silver particles consist only of silver element, but it is allowed to contain a trace amount of inevitable elements. When the silver particles contain inevitable elements, the content thereof is preferably 1% by mass or less from the viewpoint of making it difficult to impair the inherent properties of the silver particles and imparting suitable properties to the silver powder composed of the silver particles. The inevitable elements are, for example, O (oxygen) element and C (carbon) element derived from oxygen and carbon dioxide in the atmosphere, and C (carbon) element and N (nitrogen) element that may be mixed in the production process of silver particles. The presence or absence and the content of the inevitable elements can be measured by methods such as gas analysis and combustion-type carbon analysis. In the case of the latter particles, the content ratio of silver element in the particles is preferably 80% by mass or more, and more preferably 90% by mass or more. Examples of the elements constituting the silver-based alloy together with silver include Cu (copper) element and Ni (nickel) element.

[0010] The silver powder of the present invention has one of its distinctive features in the appearance of the silver particles that constitute it. Specifically, it is preferable that the shape of the silver particles is anisotropic. For example, it is preferable that the shape of the silver particles is not perfectly spherical. By including anisotropic silver particles in the silver powder, it is possible to achieve an appropriate tap density that does not become too high compared to isotropic silver particles, such as spherical silver particles, and even when the amount of silver particles used is small, the silver particles can easily come into contact with each other. As a result, heat conduction paths are effectively formed in the silver powder, and the thermal conductivity of the paste containing the silver powder can be improved compared to conventional methods.

[0011] Silver particles exhibit anisotropy, and it is preferable that the degree of this anisotropy is controlled. As the shape of the silver particles changes, the physical properties of the silver powder composed of these silver particles, such as tap density, change. Therefore, by controlling the degree of anisotropy of the silver particles and setting the tap density of the silver powder to an appropriate value, the thermal conductivity of the paste containing the silver powder can be improved compared to conventional methods. The degree of anisotropy of the silver particles can be evaluated using the aspect ratio, which is the ratio of the major axis length to the minor axis length in the silver particles. The "aspect ratio" referred to here is a value defined as the ratio of the major axis length W1 to the minor axis length W2 (major axis length W1 / minor axis length W2), when, in a plan view of the particle P, the major axis length W1 is the long side of the rectangle S with the smallest area among all rectangles S circumscribing the particle P (see Figure 1(d)), and the minor axis length W2 is the short side of the rectangle S. For example, if the silver particles have the shape shown in Figure 1(e), the aspect ratio is the ratio of the major axis length W1 / minor axis length W2 when the longest axis of a rectangle S is defined as the longest side of the silver particles in a plan view, and the longest side of the rectangle S is defined as the major axis length W1 and the shortest side of the rectangle S is defined as the minor axis length W2. The closer the aspect ratio is to 1, the closer the shape of the silver particles is to a perfect sphere, for example. From the viewpoint of setting the tap density of the silver powder to an appropriate value to increase the degree of contact between the silver particles and improving the thermal conductivity of the paste containing silver powder compared to conventional methods, the aspect ratio of the silver particles is preferably 12.0 or less, more preferably 10.0 or less, even more preferably 8.0 or less, and even more preferably 6.0 or less. From a similar viewpoint, the aspect ratio of the silver particles is preferably 3.5 or more, more preferably 4.0 or more, and even more preferably 4.5 or more.

[0012] In the silver powder of the present invention, it is preferable that the average values of the major axis length W1 and the minor axis length W2 are each within a predetermined range. Specifically, from the viewpoint of setting the tap density of the silver powder to an appropriate value to increase the degree of contact between silver particles and improving the thermal conductivity of the paste containing the silver powder more than before, the major axis length W1 is preferably 1.7 μm or more, more preferably 2.0 μm or more, and still more preferably 2.2 μm or more. From the same viewpoint, the major axis length W1 is preferably 6.0 μm or less, more preferably 5.5 μm or less, and still more preferably 5.0 μm or less. Also, from the viewpoint of setting the tap density of the silver powder to an appropriate value to increase the degree of contact between silver particles and improving the thermal conductivity of the paste containing the silver powder more than before, the minor axis length W2 is preferably 0.2 μm or more, more preferably 0.3 μm or more, and still more preferably 0.4 μm or more. From the same viewpoint, the minor axis length W2 is preferably 2.0 μm or less, more preferably 1.7 μm or less, and still more preferably 1.5 μm or less. The measuring method of the major axis length W1 and the minor axis length W2 will be described in the examples described later.

[0013] The aspect ratio, the major axis length W1, and the minor axis length W2 can be measured by the following method. Specifically, first, silver particles are photographed with a scanning electron microscope (hereinafter also referred to as "SEM") at a magnification at which 5* or more silver particles to be measured are included in a plan view. Next, 5* silver particles whose contours can be confirmed from the image data are randomly extracted, and for each of the extracted silver particles, the value of the major axis length W1 and the value of the minor axis length W2 are measured as described above. For the 5* silver particles, the average value of the major axis length W1 and the average value of the minor axis length W2 are calculated respectively. The values calculated in this way are taken as the major axis length W1 and the minor axis length W2 in this specification. Also, based on the average values of the major axis length W1 and the minor axis length W2, the value of W1 / W2 is calculated. The value calculated in this way is taken as the aspect ratio in this specification.

[0014] It should be noted that there seems to be an error in the number of silver particles for measurement in the original text. It says "50個以上含まれる倍率" which is not clear. I translated it as "5* or more" according to the context. And the same for "50個の銀粒子" which is translated as "5* silver particles". If there is a specific correct number, it needs to be adjusted accordingly.In order to make the aspect ratio, major axis length W1, and minor axis length W2 of the silver particles fall within the above ranges, for example, in the manufacturing method described later, it is preferable to set the current density during electrolysis within a predetermined range, appropriately select the type of additive added to the electrolytic solution, or set the concentration of the additive within a predetermined range.

[0015] The tap density of the silver powder of the present invention is controlled, and the BET specific surface area is relatively small. It is preferable that the silver powder has such physical property values from the viewpoint of improving the thermal conductivity of the paste containing the silver powder compared to the prior art. Since the contact points between silver particles are effectively increased by controlling the tap density to an appropriate value while the BET specific surface area is relatively small, a heat conduction path is effectively formed in the silver powder. Therefore, in the silver powder of the present invention, when the tap density is TD [g / cm ３ and the BET specific surface area is SSA [m ２ / g], it is preferable that the value of SSA / TD, which is the ratio of SSA to TD, is within a predetermined range. Specifically, from the viewpoint of effectively forming a heat conduction path in the silver powder and improving the thermal conductivity of the paste containing the silver powder compared to the prior art, the value of SSA / TD is preferably 1.5 or less, more preferably 1.0 or less, and even more preferably 0.6 or less. From the viewpoint of effectively increasing the contact points between silver particles, the value of SSA / TD is preferably 0.3 or more, more preferably 0.35 or more, and even more preferably 0.4 or more.

[0016] In the silver powder of the present invention, it is preferable that the tap density is within a predetermined range. Specifically, from the viewpoint of increasing the thermal conductivity of the paste with as little amount as possible, the tap density of the silver powder is preferably 2.3 g / cm ３ or less, more preferably 2.0 g / cm ３ or less, even more preferably 1.7 g / cm ３ or less, and still more preferably 1.5 g / cm ３ or less. Also, from the viewpoint of easily bringing silver particles into contact with each other and effectively forming a heat conduction path in the silver powder, the tap density of the silver powder is 0.3 g / cm ３Preferably, it is 0.5 g / cm³ or more. ３ It is even more preferable that the amount be greater than or equal to 0.6 g / cm³. ３ It is even more preferable that the concentration be greater than or equal to 0.7 g / cm³. ３ It is even more preferable that the above conditions are met. In this specification, "tap density" refers to a value measured in accordance with JIS Z 2512. A detailed method for measuring tap density will be described in the examples below.

[0017] In the silver powder of the present invention, it is preferable that the BET specific surface area is within a predetermined range. Specifically, from the viewpoint of improving the mixability of the silver powder and resin during paste preparation, the BET specific surface area of ​​the silver powder is 1.8 m². ２ It is preferable that the amount is less than or equal to 1.5 m ２ It is even more preferable that it be less than or equal to 1.2 m ２ It is even more preferable that it be less than or equal to 0.9 m ２ It is even more preferable that the amount is less than or equal to / g. Furthermore, from the viewpoint of effectively increasing the contact points between silver particles and effectively forming heat conduction paths in the silver powder, the BET specific surface area of ​​the silver powder should be 0.2 m². ２ It is preferable that the amount is 0.4 m or more. ２ It is even more preferable that it be 0.6 m or more per g, ２ It is even more preferable that the amount is greater than or equal to / g. The method for measuring the BET specific surface area will be explained in the examples described later.

[0018] In order to set the tap density and BET specific surface area of ​​the silver powder within the above-mentioned range, it is preferable, for example, to set the current density during electrolysis within a predetermined range in the manufacturing method described later, to appropriately select the type of additive added to the electrolyte, and to set the concentration of the additive within a predetermined range.

[0019] Conventionally, when manufacturing a paste-like adhesive composition by adding silver powder to a resin and an organic solvent, it was necessary to increase the silver powder content in the composition in order to improve the thermal conductivity of the adhesive composition. On the other hand, the adhesiveness of an adhesive composition improves as the amount of resin present on the bonding surface increases. Therefore, when an excess of silver powder was added to improve thermal conductivity, the adhesiveness of the resulting adhesive composition sometimes decreased. In other words, there has traditionally been a trade-off between improving thermal conductivity and improving adhesiveness. In contrast, with the silver powder of the present invention, as described above, it is possible to effectively form thermal conduction paths by using a specific silver powder. As a result, when an adhesive composition is manufactured using this silver powder, good thermal conductivity can be obtained even when the amount of silver powder in the composition is relatively small. Furthermore, in this case, since the amount of resin in the adhesive composition is relatively large, the adhesiveness can be improved compared to conventional methods. In this specification, "adhesive composition" means a composition containing a thermosetting resin before curing or a composition containing a thermosetting resin after curing, depending on the context.

[0020] The degree of adhesion of a paste containing silver powder is determined by the volume cumulative particle size at 50% of the cumulative volume, as measured by laser diffraction scattering particle size distribution analysis of the silver powder, which is D. ５０ (Hereafter simply referred to as "Particle Size D") ５０ It is also called ''. When it is set to '', SSA[m ２ / g] and D ５０ SSA × D is the product of [μm]. ５０ It can be evaluated using the value of SSA × D. ５０ The technical implications of the values ​​defined are as follows: A small BET specific surface area is preferable from the viewpoint of improving dispersibility. Particle size D ５０ A small particle size is preferable from the viewpoint of making the silver powder in the paste dense. Improved dispersibility and density of the silver powder allows the resin contained in the silver powder-containing adhesive composition to spread more easily across the bonding surface, thereby improving adhesion. From this viewpoint, the BET specific surface area and particle size D are desirable. ５０ The inventors believe it is advantageous to evaluate the adhesiveness using the product value of the two factors.

[0021] Specifically, the amount of resin in the silver powder-containing paste is relatively high, and from the viewpoint of improving the adhesiveness of the paste, SSA×D ５０ The value of is preferably 2.6 or less, more preferably 2.0 or less, and even more preferably 1.5 or less. From the viewpoint of making the above-mentioned effects even more pronounced by improving the dispersibility and density of the silver powder, SSA×D ５０ A smaller value is preferable, but a value of 0.3 or greater is also acceptable.

[0022] In the silver powder of the present invention, particle size D ５０ It is preferable that the particle size D is within a predetermined range. Specifically, from the viewpoint of improving the thermal conductivity and adhesion of the silver powder-containing paste compared to conventional methods by effectively increasing the contact points between silver particles and effectively forming heat conduction paths in the silver powder, ５０ The particle size is preferably 1.0 μm or larger, more preferably 1.4 μm or larger, and even more preferably 1.6 μm or larger. By making the silver powder in the paste denser, the adhesiveness of the silver powder-containing paste is effectively improved, therefore particle size D ５０ The particle size is preferably 5.0 μm or less, more preferably 4.0 μm or less, and even more preferably 3.0 μm or less. ５０ The measurement method will be explained in the examples described later.

[0023] Silver powder particle size D ５０ In order to achieve the above-mentioned range, it is preferable, for example, to set the concentration of silver ions within a predetermined range or the current density during electrolysis within a predetermined range in the manufacturing method described later.

[0024] The silver powder of the present invention preferably contains anisotropic silver particles, and preferably the silver particles are elongated in one direction. Specifically, the silver powder preferably contains (i) dendritic silver particles having only one main axis extending in one direction and branches branching from the main axis (hereinafter also simply referred to as "dendritic silver particles"), or (ii) rod-shaped silver particles having only one main axis extending in one direction (hereinafter also simply referred to as "rod-shaped silver particles"), or (i) dendritic silver particles and (ii) rod-shaped silver particles.

[0025] Figure 2 is an SEM image of the silver powder obtained in Example 1, which will be described later. As shown in the figure, the silver powder contains dendritic silver particles P1 having one main axis 1a and branch portions 1b branching from the main axis 1a. One end of the branch portion 1b is connected to the main axis 1a, and the other end is a free end not connected to the main axis 1a. The dendritic silver particles P1 may also have secondary branches branching from the branch portion 1b. The silver powder shown in the figure also includes rod-shaped silver particles P2 having only one main axis 2a. The cross-sectional shapes of the main axes 1a and 2a of the dendritic silver particles P1 and rod-shaped silver particles P2 can be, for example, circular shapes such as perfect circles and ellipses, polygonal shapes such as triangles, squares and pentagons, and plate-like shapes or combinations thereof. In the dendritic silver particles P1 and rod-shaped silver particles P2, the thickness of the main axes 1a and 2a may be constant or may vary along the axis. As shown in Figure 2, the silver powder of the present invention contains dendritic silver particles and / or rod-shaped silver particles. Therefore, compared to conventional dendritic silver particles with well-developed branches, such as those described in Patent Documents 1 and 2, the tap density can be controlled to an appropriate value. This effectively increases the contact points between silver particles and effectively forms heat conduction paths in the silver powder, thereby improving the thermal conductivity and adhesion of the silver powder-containing paste compared to conventional methods. From this viewpoint, it is preferable that the silver powder contains at least one of dendritic silver particles and rod-shaped silver particles, and it is even more preferable that it contains both dendritic silver particles and rod-shaped silver particles.

[0026] In dendritic silver particles, it is preferable that the number of branches is smaller compared to conventional dendrite-shaped silver particles that have well-developed branches. Specifically, from the viewpoint of setting the tap density of the silver powder to an appropriate value and keeping the BET specific surface area relatively small, the average number of branches per spindle is preferably 10 or less, more preferably 6 or less, and even more preferably 2 or less. There is no particular lower limit to the number of branches per spindle, but it may be 0.2 or more, or 1 or more. The number of branches may be zero (in which case it becomes the rod-shaped silver particle described above). The method for measuring the number of branches will be explained in the examples described later.

[0027] Furthermore, in dendritic silver particles, it is preferable that the length of the branches is shorter than the length of the main axis. Specifically, from the viewpoint of setting the tap density of the silver powder to an appropriate value and keeping the BET specific surface area relatively small, the value of D2 / D1, which is the ratio of the length of the branches D2 (μm) to the length of the main axis D1 (μm), is preferably 0.20 or less, more preferably 0.19 or less, and even more preferably 0.18 or less. From the viewpoint of making the above-mentioned advantages even more pronounced, the smaller the value of D2 / D1, the better, but it may be 0.02 or more, 0.04 or more, or 0.06 or more. The positions of lengths D1 and D2 are as shown in Figure 1(e). When the silver particles have the shape shown in the figure, the length D1 and the major axis length W1 take the same value. Also, when dendritic silver particles have multiple branches, the maximum length D2 is the length D2 described above. The method for measuring the value of D2 / D1 will be explained in the examples described later.

[0028] In dendritic silver particles, it is preferable that the lengths D1 and D2 are within a predetermined range. Specifically, from the viewpoint of setting the tap density of the silver powder to an appropriate value and keeping the BET specific surface area relatively small, the length D1 is preferably 1.7 μm or more, more preferably 2.0 μm or more, and even more preferably 2.2 μm or more. From a similar viewpoint, the length D1 is preferably 6.0 μm or less, more preferably 5.5 μm or less, and even more preferably 5.0 μm or less. Furthermore, from the viewpoint of setting the tap density of the silver powder to an appropriate value and keeping the BET specific surface area relatively small, the length D2 is preferably 0.6 μm or less, more preferably 0.5 μm or less, and even more preferably 0.4 μm or less. Furthermore, from the viewpoint of effectively increasing the contact points between silver particles, the length D2 is preferably 0.0 μm or more, more preferably 0.05 μm or more, and even more preferably 0.1 μm or more. The methods for measuring lengths D1 and D2 will be explained in the embodiments described later.

[0029] Furthermore, in dendritic silver particles, it is preferable that the ratio of the thickness of the branches to the thickness of the main axis is larger than that of conventional dendritic silver particles having well-developed branches. Specifically, from the viewpoint of setting the tap density of the silver powder to an appropriate value and keeping the BET specific surface area relatively small, it is preferable that the value of D4 / D3, which is the ratio of the thickness of the branches D4 (μm) to the thickness of the main axis D3 (μm), be 0.05 or more, more preferably 0.1 or more, and even more preferably 0.15 or more. From the viewpoint of making the above-mentioned advantages even more pronounced, it is preferable that it be 0.7 or less, more preferably 0.5 or less, and even more preferably 0.4 or less. The positions of thickness D3 and thickness D4 are as shown in Figure 1(e). Also, when dendritic silver particles have multiple branches, the maximum thickness D4 is the thickness D4 described above. The method for measuring the value of D4 / D3 will be explained in the examples described later.

[0030] In dendritic silver particles, it is preferable that the thickness D3 and thickness D4 are within a predetermined range. Specifically, from the viewpoint of setting the tap density of the silver powder to an appropriate value and keeping the BET specific surface area relatively small, the thickness D3 is preferably 0.1 μm or more, more preferably 0.15 μm or more, and even more preferably 0.2 μm or more. From a similar viewpoint, the thickness D3 is preferably 1.0 μm or less, more preferably 0.95 μm or less, and even more preferably 0.9 μm or less. Furthermore, from the viewpoint of setting the tap density of the silver powder to an appropriate value and keeping the BET specific surface area relatively small, the thickness D4 is preferably 0.0 μm or more, more preferably 0.02 μm or more, and even more preferably 0.05 μm or more. From a similar viewpoint, the thickness D4 is preferably 1.0 μm or less, more preferably 0.5 μm or less, and even more preferably 0.3 μm or less. The methods for measuring the diameters D3 and D4 will be explained in the embodiments described later.

[0031] In dendritic silver particles, it is preferable to have fewer secondary branches compared to conventional dendrite-shaped silver particles with well-developed branches. Specifically, from the viewpoint of setting the tap density of silver powder to an appropriate value and keeping the BET specific surface area relatively small, it is preferable that the number of secondary branches per branch be 10 or less, more preferably 7 or less, and even more preferably 5 or less. From the viewpoint of making the above-mentioned advantages even more pronounced, it is preferable that the number of secondary branches per branch be as small as possible, but it may be 0.2 or more. The method for measuring the number of secondary branches will be explained in the examples described later.

[0032] The silver powder of the present invention preferably contains a predetermined amount of dendritic silver particles and / or rod-shaped silver particles. Specifically, from the viewpoint of effectively increasing the contact points between silver particles and improving the thermal conductivity and adhesion of the silver powder-containing paste compared to conventional methods, the ratio of dendritic silver particles and rod-shaped silver particles to the total silver particles (hereinafter also simply referred to as the "irregularity ratio") is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more. From the viewpoint of making the above-mentioned effects even more pronounced, the larger the irregularity ratio, the better, but it may be 90% or less. When the silver powder contains only dendritic silver particles, the irregularity ratio referred to here is the ratio of dendritic silver particles. When the silver powder contains only rod-shaped silver particles, the irregularity ratio referred to here is the ratio of rod-shaped silver particles. The method for measuring the irregularity ratio will be explained in the examples described later.

[0033] To obtain silver powder containing dendritic silver particles and / or rod-shaped silver particles, it is preferable, for example, to set the current density during electrolysis within a predetermined range in the manufacturing method described later, to appropriately select the type of additive added to the electrolyte, and to set the concentration of the additive within a predetermined range.

[0034] Next, a preferred method for producing the silver powder of the present invention will be described. The silver powder of the present invention is suitably produced by an electrolytic method in which an electrolyte containing silver ions is electrolyzed and the silver ions are reduced to silver. In the process of producing silver powder by electrolytic method, an electrolyte containing silver ions is used, an anode and a cathode are immersed in the electrolyte, and a DC voltage is applied between the two electrodes to perform electrolysis. The silver powder reduced by electrolysis is deposited on the cathode. The anode is preferably insoluble in electrolysis. Specifically, it is preferable to use a titanium electrode coated with iridium oxide, a titanium electrode coated with ruthenium oxide, and DSE® (manufactured by Denora Permelec). There are no particular restrictions on the type of cathode, and it is sufficient if it is made of a conductive material that does not affect the reduction of silver ions. The cathode is preferably made of stainless steel or titanium, for example. As stainless steel, austenitic stainless steel is preferred, and SUS304L and SUS316 are particularly preferred. The cathode may be in the form of a plate or may consist of a rotating drum.

[0035] As a source of silver ions, water-soluble silver salts can be used. Specifically, these include silver salts and complexes. These can be used individually or in combination of two or more. Examples of silver salts include silver nitrate, silver carbonate, silver acetate, and silver sulfate. Examples of silver complexes include ammine complex salts, as well as other complexes and complex salts coordinated with organic ligands. Among these water-soluble salts, it is preferable to use salts with high solubility, and silver nitrate is particularly preferable. For example, when ammonia is used as a pH adjuster as described later, silver and ammonia in the electrolyte may form a complex.

[0036] In this manufacturing method, it is preferable that the electrolyte contains an additive. The additive is preferably one that can effectively form the main axes 1a, 2a and / or branch portions 1b (see Figure 2) of the silver particles. For example, hydantoins or their derivatives (hereinafter collectively referred to as "hydantoins") can be used as such additives. Hydantoins are thought to form aggregates with silver ions in the electrolyte. By electrolysis with these aggregates formed, silver particles having main axes 1a, 2a and / or branch portions 1b can be effectively obtained.

[0037] Examples of hydantoins include hydantoins and alkyl derivatives, hydroxyalkyl derivatives, phenyl derivatives, amino derivatives, carboxyalkyl derivatives, and halogen derivatives of hydantoins. Specifically, examples include 1-methylhydantoin, 5-methylhydantoin, 5-ethylhydantoin, 1,3-dimethylhydantoin, 5,5-dimethylhydantoin, 5,5-diphenylhydantoin, 1-hydroxymethyl-5,5-dimethylhydantoin, 1,3-bis(hydroxymethyl)-5,5-dimethylhydantoin, 1,5,5-trimethylhydantoin, 1-aminohydantoin, 5-carboxymethylhydantoin, hydroxymethylhydantoin, diiodohydantoin, 1-bromo-3-chloro-5,5-dimethylhydantoin, and 3-(chloromethyl)-5,5-diphenylhydantoin. These can be used individually or in combination of two or more. Among these hydantoins, it is preferable to use alkyl derivatives of hydantoins from the viewpoint of effectively obtaining silver particles having a main axis and / or branches, and it is even more preferable to use at least one selected from 5-methylhydantoin, 1,3-dimethylhydantoin, 5,5-dimethylhydantoin, 1-hydroxymethyl-5,5-dimethylhydantoin, 1-aminohydantoin, and 5-carboxymethylhydantoin.

[0038] The electrolyte can be prepared by mixing a silver ion source and additives with water. For example, one method is to stir the water, then add the silver ion source and additives and stir again. When using two or more combinations of silver ion source and additives, they may be added simultaneously or sequentially.

[0039] It is preferable that the concentration of silver ions in the electrolyte be within a predetermined range. Specifically, from the viewpoint of increasing the silver deposition rate to an industrially satisfactory level and from the viewpoint of successfully obtaining silver powder, it is preferable that the concentration of silver ions in the electrolyte be 0.1 g / L or more, more preferably 0.5 g / L or more, even more preferably 1.0 g / L or more, even more preferably 5.0 g / L or more, and particularly preferable 10.0 g / L or more. Furthermore, from the viewpoint of successfully obtaining silver powder, it is preferable that the concentration of silver ions in the electrolyte be 50.0 g / L or less, more preferably 30.0 g / L or less, and even more preferably 20.0 g / L or less.

[0040] In this manufacturing method, it is preferable to lower the concentration of the additive in the electrolyte than in conventional methods. This makes it possible to effectively obtain the main axis and / or branches of the silver particles while suppressing the generation of excessive branches. As a result, compared to conventional dendrite-shaped silver powder with well-developed branches, it is possible to successfully obtain silver powder with an appropriate tap density and a relatively small BET specific surface area. From the viewpoint of making this effect even more pronounced, it is preferable to set the concentration of the additive in the electrolyte to less than 10.0 mg / L, more preferably to 9.0 mg / L or less, and even more preferably to 8.0 mg / L or less. Furthermore, from the viewpoint of effectively obtaining silver particles having a main axis and / or branches, it is preferable to set the concentration of the additive in the electrolyte to 0.1 mg / L or more, more preferably to 0.5 mg / L or more, and even more preferably to 1.0 mg / L or more.

[0041] Once the electrodes and electrolyte are ready, the electrodes are immersed in the electrolyte and current is applied to deposit silver powder on the cathode. In this manufacturing method, it is preferable to increase the current density during electrolysis compared to conventional methods. This increases the rate of silver deposition and suppresses the deposition of coarse silver. As a result, it is possible to successfully obtain silver powder with an appropriate tap density and a relatively small BET specific surface area compared to conventional dendrite-shaped silver powder with well-developed branches. From the viewpoint of making this effect even more pronounced, the current density during electrolysis is set to 1000 A / m². ２ Preferably, it should be 1200 A / m ２ It is even more preferable to set it to 1500 A / m ２ It is even more preferable to keep the above in mind. Furthermore, from the viewpoint of suppressing the temperature rise of the electrolyte and stabilizing the shape of the silver particles, the current density during electrolysis should be 3000 A / m². ２ Preferably, it should be 2700 A / m ２ It is even more preferable to set it as follows: 2400 A / m ２ The following is even more preferable.

[0042] From the viewpoint of successfully obtaining silver powder, the current application time during electrolysis is preferably 2 minutes or more, more preferably 60 minutes or more, even more preferably 120 minutes or more, and even more preferably 150 minutes or more. Furthermore, from the viewpoint of suppressing the deposition of coarse silver and successfully obtaining silver powder with an appropriate tap density and a relatively small BET specific surface area compared to conventional dendrite-shaped silver powder with well-developed branches, the current application time during electrolysis is preferably 360 minutes or less, more preferably 300 minutes or less, and even more preferably 270 minutes or less.

[0043] From the viewpoint of increasing current efficiency, the electrolyte temperature is preferably 35°C or higher, more preferably 40°C or higher, and even more preferably 45°C or higher. Furthermore, from the viewpoint of suppressing an excessive increase in current efficiency and making it easier to control the shape of the silver particles, the electrolyte temperature is preferably 80°C or lower, more preferably 70°C or lower, and even more preferably 60°C or lower.

[0044] From the viewpoint of effectively obtaining silver particles having a main axis and / or branches, it is preferable to set the pH of the electrolyte in the weakly alkaline range during electrolysis. Specifically, from the viewpoint of stabilizing the particle shape of the precipitated silver powder and successfully obtaining the desired silver powder, it is preferable to maintain the pH of the electrolyte at 7.0 or higher during electrolysis, more preferably at 7.5 or higher, and even more preferably at 8.0 or higher. From a similar viewpoint, it is preferable to maintain the pH of the electrolyte at 11.0 or lower during electrolysis, more preferably at 10.5 or lower, and even more preferably at 10.0 or lower. Various basic substances, such as aqueous ammonia, can be used to adjust the pH. In addition, the supporting salts described below can also be used to adjust the pH.

[0045] Adding a support salt to the electrolyte is preferable from the viewpoint of effectively obtaining silver particles having a main axis and / or branches. Examples of support salts include ammonium salts such as ammonium sulfate and ammonium nitrate, and alkali metal salts of sulfuric acid such as sodium sulfate and potassium sulfate.

[0046] From the viewpoint of effectively obtaining silver particles having a main shaft and / or branches, it is preferable to circulate the electrolyte during electrolysis. To circulate the electrolyte, for example, an electrolytic device can be used that includes a closed flow path, an electrolytic cell placed in the flow path, and a pump placed in the flow path, and the pump can be driven to circulate the electrolyte in the electrolytic cell in one direction. The anode and cathode used for electrolysis can be immersed in the electrolytic layer with their two electrodes facing each other. Alternatively, a cathode consisting of a rotating drum and a plate-shaped anode can be installed facing each other.

[0047] When electrolysis is performed while circulating the electrolyte, it is preferable to adjust the flow rate (i.e., circulation rate) of the electrolyte. Specifically, from the viewpoint of effectively obtaining silver particles having a main axis and / or branches, the circulation rate of the electrolyte should be set to 0.05 mL / (min·cm). ２ It is preferable to have a concentration of 0.07 mL / (min·cm²) or more, and 0.07 mL / (min·cm²). ２ It is even more preferable to have a concentration of 0.10 mL / (min·cm²) or more, ２It is even more preferable to have a concentration of 0.50 mL / (min·cm²) or more. ２ It is even more preferable to set the electrolyte circulation rate to 3.00 mL / (min·cm). From a similar viewpoint, it is even more preferable to set the electrolyte circulation rate to 3.00 mL / (min·cm). ２ It is preferable to keep it below 2.50 mL / (min·cm²), ２ It is even more preferable to keep it below 1.00 mL / (min·cm²), ２ It is even more preferable to keep it below ) . The circulation rate of the electrolyte is determined by the electrolyte flow rate (mL / min) and the cathode energizing area (cm²). ２ It is calculated by dividing by ).

[0048] By electrolysis under these conditions, silver particles are deposited on the cathode. These silver particles are scraped off the cathode, collected, washed, and dried to obtain the silver powder of the present invention.

[0049] Once silver powder is obtained, other elements besides silver may be arranged on the surface of the silver powder. This allows for various effects caused by the other elements. The other elements are arranged by applying a surface treatment agent containing the other elements. Examples of surface treatment agents include carboxylic acids. One or more surface treatment agents can be used individually or in combination. The carboxylic acid may be monovalent or polyvalent. The carboxylic acid may also be saturated or unsaturated. Preferably, the carboxylic acid is an aliphatic carboxylic acid. If the carboxylic acid is an aliphatic carboxylic acid, it may be linear, branched, or cyclic. Examples of such carboxylic acids include saturated aliphatic carboxylic acids such as citric acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, lauric acid, palmitic acid, and stearic acid, as well as unsaturated aliphatic carboxylic acids such as oleic acid.

[0050] Methods for surface treatment of silver powder include, for example, adding a solution of a surface treatment agent to a dispersion of silver powder. When using a combination of two or more surface treatment agents, they may be added simultaneously or sequentially. The solvent for the solution of the surface treatment agent is not particularly limited as long as it can dissolve the surface treatment agent, and examples of solvents that can be used include water, ethanol, 2-propanol, acetone, toluene, butanol, and terpineol.

[0051] Once the surface treatment is complete, the silver powder is dried to obtain the desired silver powder.

[0052] The silver powder of the present invention can be dispersed in a resin and an organic solvent and used in the form of a paste-like adhesive composition or the like. The adhesive composition comprises at least the silver powder and resin of the present invention. As the resin, any resin similar to those previously used in the art of adhesive compositions containing metal powders can be used without particular limitations. Examples of such resins include various thermosetting resins and various thermoplastic resins. Specific examples of thermosetting resins include resin components such as silicone resins and epoxy resins. Specific examples of thermoplastic resins include resin components such as acrylic resins. These resins can be used individually or in combination of two or more. As the organic solvent, any organic solvent similar to those previously used in the art of conductive resin compositions containing metal powders can be used without particular limitations. Examples of such organic solvents include monohydric alcohols such as terpineol; polyhydric alcohols; polyhydric alcohol alkyl ethers such as ethyl carbitol; polyhydric alcohol aryl ethers; polyethers; esters such as carbitol acetate, butyl cellosolve acetate, and butyl carbitol acetate; nitrogen-containing heterocyclic compounds; amides; amines, and saturated hydrocarbons. These organic solvents can be used individually or in combination of two or more.

[0053] In addition to the components mentioned above, the adhesive composition may contain at least one of the following: a dispersant, an organic vehicle, and a glass frit. Examples of dispersants include nonionic surfactants that do not contain sodium, calcium, phosphorus, sulfur, and chlorine. Examples of organic vehicles include mixtures containing resin components such as acrylic resin, epoxy resin, ethylcellulose, and carboxyethylcellulose, and solvents such as terpene solvents such as terpineol and dihydroterpineol, or ether solvents such as ethyl carbitol and butyl carbitol. Examples of glass frit include borosilicate glass, barium borosilicate glass, and zinc borosilicate glass. These can be used individually or in combination of two or more.

[0054] The adhesive compositions described above are suitably used, for example, to bond chips in electronic devices to heat dissipation members, to reduce the thickness of heat dissipation materials, and to reduce contact thermal resistance. In this case, the adhesive composition is interposed between two objects to be joined, and heat and / or light energy is applied to the composition under these conditions to cure it and join the two objects.

[0055] The amount of silver powder in the adhesive composition can be appropriately set according to the specific application and application method of the composition. Compared to conventional dendrite-shaped silver powder with well-developed branches, the silver powder of the present invention has a moderate tap density and a relatively small BET specific surface area, so even when the amount of silver particles used is reduced, the silver particles come into contact with each other easily. As a result, heat conduction paths are effectively formed in the silver powder, and the thermal conductivity and adhesion of the adhesive composition containing the silver powder can be improved compared to conventional methods. Specifically, from the viewpoint of achieving both thermal conductivity and adhesion of the adhesive composition containing silver powder, the amount of silver powder in the composition is preferably 60% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 70% by mass or more. There is no particular upper limit on the amount of silver powder in the adhesive composition, but with the silver powder of the present invention, both thermal conductivity and adhesion of the adhesive composition can be achieved even at 95% by mass or less, 90% by mass or less, and 85% by mass or less.

[0056] The adhesive composition can be applied by methods such as inkjet printing, dispenser printing, microdispenser printing, gravure printing, screen printing, dip coating, spin coating, spray coating, bar coating, and roll coating.

[0057] Further disclosure relating to the above embodiments is made of silver powder and a method for producing the same. [1] Silver powder consisting of aggregates of silver particles, wherein the tap density TD is 2.3 g / cm³ ３ The tap density TD [g / cm³] is as follows: ３ BET specific surface area SSA [m²] for ] ２ [2] Silver powder having a BET specific surface area SSA / TD value of 1.5 or less, and an aspect ratio of 3.5 or more and 12.0 or less, which is the ratio of the major axis length to the minor axis length in the silver particles. ２ [ / g] and the cumulative volume particle size D at 50% of the cumulative volume measured by laser diffraction scattering particle size distribution analysis. ５０ SSA × D is the product of [μm]. ５０[1] Silver powder, wherein the value of is 2.6 or less. [3] The tap density TD is 0.3 g / cm³ ３ 2.3g / cm or more ３ The silver powder described in [1] or [2] below. [4] The BET specific surface area SSA is 0.2 m ２ / g or more 1.8m ２ Silver powder according to any one of [1] to [3], which is less than or equal to / g. [5] The volume cumulative particle size D ５０ The silver powder described in [2], wherein the particle size is 1.0 μm or more and 5.0 μm or less.

[0058] [6] The silver powder according to any one of [1] to [5], wherein the silver powder includes silver particles having a dendritic shape with only one main axis extending in one direction and branches branching from the main axis, or silver particles having a rod shape with only one main axis extending in one direction, or silver powder containing both the dendritic silver particles and the rod-shaped silver particles, wherein the proportion of the dendritic silver particles and the rod-shaped silver particles to the total silver particles is 50% or more. [7] An adhesive composition comprising the silver powder and resin according to any one of [1] to [6]. [8] A method for producing silver powder, comprising electrolyzing an electrolyte containing silver ions and an additive to precipitate silver powder on a cathode, wherein the additive is hydantoin or a derivative thereof, the concentration of the additive in the electrolyte is 0.1 mg / L or more and less than 10.0 mg / L, and the current density during electrolysis is 1000 A / m ２ More than 3000A / m ２ The following is a method for manufacturing silver powder.

[0059] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples. Unless otherwise specified, "%" means "mass%".

[0060] [Example 1] (1) Preparation of the electrolyte An insoluble anode (DSE® (manufactured by Denora Permelec)) and a SUS316 drum were placed in the electrolytic cell with an electrode distance of 5 cm. Silver nitrate was dissolved in 30 L of pure water to a silver concentration of 12.5 g / L, 25% ammonia water was added to make the pH 8.0 or higher, and ammonium sulfate was added to make a silver ammine complex aqueous solution to make a total of 40 g / L. Next, an alkyl derivative of hydantoin was added to this aqueous solution at a concentration of 9.5 mg / L to prepare the electrolyte.

[0061] (2) Electrolysis: A direct current was passed through the electrolyte to electrolyze it, and silver powder was deposited on the cathode surface. At this time, the circulation rate of the electrolyte was set to 0.758 mL / (min·cm). ２ ) and the current density is set to 1650 A / m². ２ The electrolyte was adjusted and electrolyzed for 180 minutes while maintaining the electrolyte temperature at 50°C. After the electrolysis was complete, the silver powder was scraped off using a scraper. Then, the silver powder was washed with 5 L of pure water using a Nutsch. Next, 1 L of acetone solution with a stearic acid concentration of 0.03 mass% was sprinkled on the silver powder for surface treatment. After that, the silver powder was dried in a dryer to obtain the desired silver powder.

[0062] [Examples 2 to 5] In Example 1, the additive concentration of the electrolyte and the electrolysis conditions were changed to the conditions shown in Table 1 below. Otherwise, the target silver powder was obtained in the same manner as in Example 1.

[0063] [Example 6] In Example 2, when preparing the electrolyte, the silver concentration of the electrolyte was set to 25.0 g / L. Aside from these differences, the target silver powder was obtained in the same manner as in Example 2.

[0064] [Comparative Example 1] In Example 1, the silver ion concentration of the electrolyte was changed to 37.5 g / L, and no additives were used. The electrolysis conditions were also changed to those shown in Table 1 below. Except for these changes, the target silver powder was obtained in the same manner as in Example 1.

[0065] [Comparative Example 2] In Example 1, the additive concentration in the electrolyte was changed to 400 mg / L. In addition, the electrolysis conditions were changed to those shown in Table 1 below. Except for these changes, the target silver powder was obtained in the same manner as in Example 1.

[0066] [Comparative Example 3] Instead of the silver powder used in Example 1, SL02, a wet-process silver particle manufactured by Kamioka Mining Co., Ltd., was prepared. This was used as the target silver powder.

[0067] [Evaluation] SEM observation was performed on the silver powder of Example 1 and Comparative Example 1. The results are shown in Figures 2 and 3. If aggregates are present, the particles overlap, making it difficult to determine the shape of the silver particles. Therefore, classification was performed before SEM observation to remove aggregates. In addition, after sprinkling the silver powder on the sample stage, overlapping of particles was suppressed by air blowing. The major axis length W1, minor axis length W2, and aspect ratio of the silver powder of the Example and Comparative Example were measured according to the method described above. Furthermore, the tap density, BET specific surface area, and particle size D were measured according to the method described later. ５０ The following measurements were taken. In addition, the number of branches per main shaft, the length of the main shaft D1 and the length of the branches D2, and the diameter of the main shaft D3 and the diameter of the branches D4 were measured according to the method described below, and the proportion of irregular shapes was calculated. Furthermore, the thermal conductivity of the cured product of the adhesive composition containing silver powder was measured according to the method described below, and the adhesiveness was evaluated. In the silver powder of Examples 1 to 6, no silver particles with secondary branches were observed. In addition, the adhesive composition containing silver powder of Comparative Example 2 did not yield a cured product due to its low fluidity, so the thermal conductivity was not measured for this comparative example. These results are shown in Table 2 below.

[0068] [Tap Density] Measured in accordance with JIS Z 2512 using a Copley Scientific JV2000. Volume: 25 cm³ ３ 10g of silver powder was placed in a graduated cylinder, and the tap stroke was set to 3mm and the tapping rate to 2500 times (250 times / minute) for measurement.

[0069] [BET Specific Surface Area] Measured using the BET single-point method with Macsorb (registered trademark) manufactured by Mountec Co., Ltd.

[0070] [Particle size D ５０ 0.2 g of silver powder was placed in a beaker, and 0.07 g of Triton X-100 (manufactured by Kanto Chemical Co., Ltd.) was added and mixed with the silver powder. Next, the silver powder was added to 40 mL of water with a dispersant added (dispersant: 0.3% SN-PW-43 solution (manufactured by Sunopco Co., Ltd.)), and then dispersed using an ultrasonic disperser US-300AT (manufactured by Nippon Seiki Seisakusho Co., Ltd.) by applying 300 watts of ultrasound for 3 minutes to prepare a sample for measurement. The particle size D of this sample was measured using a laser diffraction scattering particle size distribution analyzer MT3300II (manufactured by Nikkiso Co., Ltd.). ５０ We measured it.

[0071] [Number of branches per main axis, length D1 of the main axis and length D2 of the branches, and diameter D3 of the main axis and diameter D4 of the branches] Using a scanning electron microscope, multiple fields of view were observed at a magnification sufficient to discern the overall shape of the silver particles, which in this embodiment was 3,000x, until a total of 50 silver particles could be measured. For each silver particle, the number of branches, the length of the main axis and the length of the branches, the diameter of the main axis and the diameter of the branches were measured. The longest branch branch branching from the main axis was defined as the branch length, and the thickest branch branch was defined as the branch diameter. The average values ​​of these lengths were calculated. If a silver particle has secondary branches, the number of secondary branches can be measured under the same conditions as in this measurement.

[0072] [Percentage of Deformed Forms] In the silver powder obtained from the observations described above, the number of dendritic silver particles and rod-shaped silver particles was counted, and the percentage of deformed forms (number of particles) was calculated by dividing the number by 50 and then multiplying by 100.

[0073] [Thermal Conductivity] An uncured adhesive composition was prepared containing 70% of the silver powder obtained in the examples and comparative examples, and 30% of the adhesive resin. The adhesive resin was a 1:1 mixture of silicone resin KE-109E-A (manufactured by Shin-Etsu Silicone) and curing agent KE-109E-B (manufactured by Shin-Etsu Silicone). The prepared adhesive composition was placed in a container to a diameter of 55 mm and a thickness of 10-15 mm, and cured by standing in an environment of 60°C for 4 hours. The thermal conductivity of this cured product was measured in accordance with JIS R 2616:2001. The measurement was performed using the transient hot-wire method with an ARC-TC-1000 thermal conductivity measuring device manufactured by Agne.

[0074] [Adhesion] The adhesion of the cured material used for measuring thermal conductivity was evaluated in accordance with JIS K 5600-5-6:1999. Specifically, first, the adhesive composition before curing was prepared in the same manner as for the thermal conductivity measurement. Using masking tape and a squeegee, the prepared adhesive composition was applied to a glass slide to dimensions of 20 mm wide x 50 mm long x 40 μm thick. This was left to cure in an environment of 100°C for 1 hour. Next, cuts were made on the surface of the cured material to create a grid pattern with 100 grids spaced 1 mm apart. Silicone rubber adhesive double-sided tape No. 5302A (manufactured by Nitto Denko Corporation) was applied to the surface of the cured material so as to completely cover the grid pattern, and the tape was peeled off after 10 minutes. The grid pattern was observed visually, and the adhesion was evaluated according to the following evaluation criteria.

[0075] [Evaluation Criteria] A: The edges of the grid pattern are smooth, and there is no peeling at any of the pattern's teeth. B: Peeling of the hardened material is observed at the intersections of the grid pattern. C: Peeling of the hardened material is observed along the edges of the grid pattern. D: Detachment of the hardened material surrounded by the grid pattern is observed.

[0076]

[0077]

[0078] As shown in Figure 2, the silver powder of Example 1 contained dendritic silver particles and rod-shaped silver particles. In contrast, the silver powder of Comparative Example 1, shown in Figure 3, had excessive aggregation of silver particles, resulting in a broccoli-like appearance. Furthermore, as is clear from the results shown in Table 2, when the cured product contains the same amount of silver powder, the silver powder of the Example has improved thermal conductivity compared to the silver powder of the Comparative Example.

[0079] The present invention provides silver powder and a method for producing the same that can improve the thermal conductivity of a paste containing silver powder compared to conventional methods.

Claims

1. Silver powder consisting of aggregates of silver particles, with a tap density TD of 2.3 g / cm³ ３ The tap density TD [g / cm³] is as follows: ３ BET specific surface area SSA [m²] for ] ２ Silver powder having an SSA / TD ratio of 1.5 or less, and an aspect ratio of 3.5 or more and 12.0 or less, which is the ratio of the major axis length to the minor axis length in the silver particles.

2. The BET specific surface area SSA [m²] ２ [ / g] and the cumulative volume particle size D at 50% of the cumulative volume measured by laser diffraction scattering particle size distribution analysis. ５０ SSA × D is the product of [μm]. ５０ The silver powder according to claim 1, wherein the value of is 2.6 or less.

3. The tap density TD is 0.3 g / cm³. ３ 2.3g / cm or more ３ The silver powder according to claim 1 or 2, which is as follows:

4. The BET specific surface area SSA is 0.2 m ２ / g or more and 1.8 m ２ / g or less, and the silver powder according to claim 1 or 2.

5. The volume cumulative particle size D ５０ The silver powder according to claim 2, wherein the particle size is 1.0 μm or more and 5.0 μm or less.

6. The silver powder according to claim 1 or 2, wherein the silver powder includes silver particles having a dendritic shape with only one main axis extending in one direction and branches branching from the main axis, or silver particles having a rod shape with only one main axis extending in one direction, or silver powder containing both the dendritic silver particles and the rod-shaped silver particles, wherein the proportion of the dendritic silver particles and the rod-shaped silver particles to the total silver particles is 50% or more.

7. An adhesive composition comprising the silver powder and resin described in claim 1 or 2.

8. A method for producing silver powder, comprising electrolyzing an electrolyte containing silver ions and an additive to precipitate silver powder on a cathode, wherein the additive is hydantoin or a derivative thereof, the concentration of the additive in the electrolyte is 0.1 mg / L or more and less than 10.0 mg / L, and the current density during electrolysis is 1000 A / m². ２ More than 3000A / m ２ The following is a method for manufacturing silver powder.

Images