Silver powder, method for producing the same, and electrically conductive resin composition

Abstract

The silver powder contains dendritic silver particles. The dendritic silver particles are dendritic, and have one main axis and a plurality of branches branching from the main axis. The thickness of the main axis in the dendritic silver particles is 10 nm or more and 280 nm or less. The number of the branches with respect to the axis length of the main axis is 6 branches / μm or more and 30 branches / μm or less. The proportion of the dendritic silver particles in all the silver particles is 50% by number or more. The silver powder is produced by electrolytic reduction of silver ions in an electrolyte containing silver ions and hydantoin or a derivative thereof.

Description

Technical Field

[0001] This invention relates to silver powder and a method for manufacturing the same. Additionally, this invention relates to conductive resin compositions containing silver powder. Background Technology

[0002] In recent years, attempts have been made to mix silver powder and resin to produce conductive resin compositions. For example, Patent Document 1 describes a silver powder in which, when the cumulative volumetric particle size measured under ultrasonic treatment is set as D50D and the cumulative volumetric particle size measured without ultrasonic treatment is set as D50N, the value of D50N / D50D is 1.0 to 10.0, and D50D is 1.0 to 15.0 μm. This document describes how mixing this silver powder with resin can impart sufficient conductivity to the resin.

[0003] Additionally, for example, Patent Document 2 describes a dendritic silver particle in which silver particles grow between the dendrites and have a flat, plate-like shape, with an average particle size (D50) of 0.5 μm to 50 μm and a BET specific surface area of ​​0.2 m². 2 / g~4.5m 2 / g. The document describes how mixing the silver powder with the resin can impart sufficient conductivity to the resin.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: U.S. Patent Application Publication No. 2018 / 326478

[0007] Patent Document 2: Japanese Patent Application Publication No. 2017-71819 Summary of the Invention

[0008] Regarding the silver powder described in Patent Document 1, since the dendritic silver particles constituting the silver powder are uniformly dispersed in the resin, the resin containing the silver powder has the advantage that its conductivity is difficult to change even when it is formed into a film and stretched.

[0009] Furthermore, the silver powder described in Patent Document 2 has the property that the bulk density increases accordingly with the amount of the dendrites in the dendritic silver particles constituting the silver powder bonded together.

[0010] On the other hand, there is a problem with silver powder added to resins to impart conductivity, resulting in high conductivity with a relatively small amount. Patent Documents 1 and 2 do not address this issue.

[0011] Therefore, the objective of this invention is to provide a silver powder that can impart high conductivity to resin with a small amount of additive.

[0012] This invention provides a silver powder comprising dendritic silver particles, wherein the dendritic silver particles are dendritic in shape, and the dendrites have a main axis and a plurality of branches branching from the main axis.

[0013] The spindle has a thickness of 10 nm or more and 280 nm or less.

[0014] The number of branches relative to the axial length of the main axis is more than 6 branches / μm and less than 30 branches / μm.

[0015] The dendritic silver particles account for more than 50% of all silver particles.

[0016] In addition, the present invention provides a conductive resin composition comprising a resin and the aforementioned silver powder.

[0017] Furthermore, the present invention provides a method for manufacturing silver powder, which includes a step of reducing silver ions by electrolyzing an electrolyte, wherein the electrolyte contains silver ions and hydantoin or its derivatives. Attached Figure Description

[0018] Figure 1 This is a scanning electron microscope image of the silver powder obtained in Example 1.

[0019] Figure 2 This is a scanning electron microscope image of the silver powder obtained in Example 2.

[0020] Figure 3 This is a scanning electron microscope image of the silver powder obtained in Example 3.

[0021] Figure 4 Scanning electron microscope images of the silver powder obtained in Comparative Example 1.

[0022] Figure 5 A graph is provided to illustrate the results of the thermomechanical analysis obtained in Example 1 and Comparative Example 1. Detailed Implementation

[0023] Hereinafter, based on preferred embodiments, with reference to the appendix. Figure 1 The present invention will now be described. The silver powder of the present invention is composed of an aggregate of silver particles. The silver powder of the present invention is composed of silver element and unavoidable impurities. One of the characteristics of the silver powder of the present invention lies in the morphology of the silver particles constituting it. Specifically, the silver particles are dendritic in shape. Dendritic refers to a shape having a main axis and multiple branches branching from that main axis.

[0024] Although the manufacturing methods and conditions vary depending on the silver powder, the branches in the dendritic silver particles of the present invention extend at approximately a certain angle relative to the main axis, for example, in a manner that lies within a plane containing the main axis. Specifically, with the main axis as the axis of symmetry, the branches extend in a generally linearly symmetrical manner within the plane containing the axis of symmetry. Alternatively, the dendritic silver particles may be shaped such that the main axis lies on the intersection line of two or more planes and the branches lie within these two or more planes. Alternatively, the dendritic silver particles may also be shaped such that the branches extend radially from any position around the main axis at a certain angle relative to the main axis. In either case, it is preferable that adjacent branches are separated, and there are substantially no points where the branches join together.

[0025] Silver powder composed of dendritic silver particles, such as that described in Patent Document 1, is known to date in this art. In contrast, the morphology of the dendritic silver particles in the silver powder of the present invention is quite different from the morphology of currently known silver particles, and is extremely novel in this respect.

[0026] In detail, the dendritic axis of the silver particles constituting the silver powder of the present invention is finer than that of currently known dendritic silver particles. Specifically, the thickness of the axis is preferably 280 nm or less, more preferably 250 nm or less, and even more preferably 200 nm or less. If the main axis of the dendritic silver particles is fine, the dendrites are relatively well-developed, and it is easy for the silver particles to come into contact with each other. As a result, for example, even when the silver powder of the present invention is added to the resin in a smaller amount than before, it can exhibit the same level of conductivity as before. From this point of view, the finer the main axis of the dendritic silver particles, the better, but if the main axis is too fine, the particles will find it difficult to maintain a dendritic morphology. From this point of view, the thickness of the main axis is preferably 10 nm or more, more preferably 30 nm or more, and even more preferably 50 nm or more.

[0027] Based on the above points, the thickness of the main axis of the dendritic silver particles is preferably 10 nm or more and 280 nm or less, more preferably 30 nm or more and 250 nm or less, and even more preferably 50 nm or more and 200 nm or less.

[0028] When the thickness of the main shaft of the dendritic silver particles is as described above, the length of the main shaft is preferably 0.5 μm or more and 10.0 μm or less, more preferably 0.7 μm or more and 8.0 μm or less, and even more preferably 1.0 μm or more and 5.0 μm or less. By having the main shaft of the dendritic silver particles have a length within this range, combined with setting the thickness of the main shaft within the above range, it is easier for the silver particles to come into contact with each other. The method for measuring the thickness and length of the main shaft is described in the examples.

[0029] A further characteristic of the silver particles constituting the silver powder of the present invention is the large number of branches branching from the main axis of the dendrite. Specifically, the number of branches relative to the axial length of the main axis is preferably 6 or more, more preferably 8 or more, and even more preferably 10 or more. That is, the dendritic silver particles constituting the silver powder of the present invention have a structure with a very large number of branches per unit length of the main axis. By increasing the number of branches per unit length of the main axis, it is easier for the dendritic silver particles to come into contact with each other. As a result, even when the silver powder of the present invention is added to, for example, a resin in a smaller amount than before, it can exhibit the same level of conductivity as before. From this point of view, the more branches, the more preferred, but if the number of branches is too large, the branches are too close to each other, and the advantages of the dendritic structure are weakened. From this point of view, the number of branches is preferably 30 or less per μm, more preferably 27 or less per μm, and even more preferably 24 or less per μm.

[0030] Based on the above points, the number of branches relative to the axial length of the main axis is preferably 6 or more and 30 or less, more preferably 8 or more and 27 or less, and even more preferably 10 or more and 24 or less. The method for determining the number of branches is described in the examples.

[0031] Of the branches extending from the main axis, the average length of the longest branch is preferably 0.2 μm or more and 5.0 μm or less, more preferably 0.3 μm or more and 4.0 μm or less, and even more preferably 0.4 μm or more and 3.0 μm or less. When the branches of the dendritic silver particles have this range of length, combined with setting the number of branches within the aforementioned range, it is easier for the silver particles to come into contact with each other.

[0032] The method for determining the length of the longest branch among the branches extending from the main axis is described in the examples.

[0033] In the silver powder of the present invention, if the silver particles accounting for more than 50% of all silver particles are silver particles with a main axis thickness of 10 nm or more and 280 nm or less, and the number of dendrites relative to the axial length of the main axis is 6 or more and 30 or less, then the desired effect of the present invention can be fully exerted. The proportion of dendritic silver particles within this size range is further preferably 60% or more, and even more preferably 70% or more.

[0034] In order to determine the proportion of the above-mentioned dendritic silver particles, it is preferable to measure 50 or more silver particles as the total number of silver particles.

[0035] The silver powder of the present invention is highly porous because it comprises dendritic silver particles with a main axis of a specified size and a sufficient number of branches. When the porosity of the silver powder is expressed as tap density, the tap density of the silver powder of the present invention is preferably 1.0 g / cm³. 3 The following is a further preferred value: 0.8 g / cm³ 3 The following is a further preferred value: 0.7 g / cm³ 3 The following points highlight the advantage of the low tap density of the silver powder of the present invention, which allows it to exhibit the same level of conductivity as in the past, even when added to, for example, a resin in a smaller amount than before. The lower limit of the tap density is 0.4 g / cm³. 3 The method for determining tap density is described in the examples.

[0036] Related to the aforementioned tap density, the apparent density of the silver powder of the present invention is also low. Specifically, the apparent density of the silver powder of the present invention is preferably 0.2 g / cm³. 3 Above and 0.7g / cm 3 The following is a further preferred value: 0.25 g / cm³ 3 Above and 0.65g / cm 3 The following is a further preferred value: 0.3 g / cm³ 3 Above and 0.6 g / cm 3 The following describes how, by ensuring the apparent density of the silver powder of the present invention falls within this range, the same level of conductivity can be achieved even when the silver powder is added to, for example, a resin in a smaller amount than previously possible. The method for determining the apparent density is described in the examples.

[0037] The cumulative volumetric particle size D of the silver powder of the present invention at 50% capacity was determined by a laser diffraction scattering particle size distribution method. 50 Preferably, the particle size is 0.5 μm or more and 3.0 μm or less, more preferably 0.6 μm or more and 2.5 μm or less, and even more preferably 0.7 μm or more and 2.0 μm or less. This is achieved by adjusting the particle size D of the silver powder. 50 Within this range, contact between silver particles is more likely to occur. Particle size D 50 The determination method is described in the examples.

[0038] Furthermore, the BET specific surface area of ​​the silver powder of the present invention is preferably 2.0 m². 2 / g or more and 5.0m 2 / g or less, more preferably 2.4m 2 / g or more and 4.5m 2 / g or less, and more preferably 2.8m 2 / g or more and 4.0m 2 / g or less. By ensuring the BET specific surface area of ​​the silver powder is within this range, it is easier for the silver particles to come into contact with each other. The method for determining the BET specific surface area is described in the examples.

[0039] The silver powder of the present invention has the following characteristics: compared with conventional silver powder of the same particle size, the crystallite size of the silver particles is smaller. Specifically, the crystallite size of the silver is preferably 50 nm or less, more preferably 46 nm or less, and even more preferably 42 nm or less. A smaller crystallite size means that the shrinkage initiation temperature is lower when sintering the silver powder of the present invention. In other words, when comparing at a constant sintering temperature, it can be said that the silver powder of the present invention has a larger shrinkage rate compared with conventional silver powder of the same particle size. From this viewpoint, a smaller crystallite size is preferred, but as long as the crystallite size is as small as about 10 nm, the shrinkage initiation temperature will be sufficiently reduced.

[0040] Based on the above points, the crystallite size of silver in the silver powder of the present invention is preferably 10 nm or more and 50 nm or less, more preferably 20 nm or more and 46 nm or less, and even more preferably 30 nm or more and 42 nm or less. The method for measuring the crystallite size of silver is described in the examples.

[0041] Related to the crystallite size of silver, the shrinkage rate of the silver powder of the present invention at 150°C is preferably 0.3% or more, more preferably 0.5% or more, and even more preferably 0.7% or more. Considering that the melting point of bulk silver is 961.8°C, from the viewpoint of low-temperature sintering, this shrinkage rate of the silver powder of the present invention at 150°C is extremely advantageous. From this viewpoint, a higher shrinkage rate of the silver powder at 150°C is preferred, but as long as the shrinkage rate is as high as about 0.3%, the benefits of low-temperature sintering can be fully obtained. The shrinkage rate of the silver powder is determined by thermomechanical analysis. The measurement method is described in the examples.

[0042] As described above, in the dendritic silver particles constituting the silver powder of the present invention, each branch forms a certain angle relative to the main axis. The angle between the main axis and the branch is preferably an average angle of 30 degrees or more and 80 degrees or less on the acute angle side, more preferably 40 degrees or more and 75 degrees or less, and even more preferably 50 degrees or more and 70 degrees or less. By making the main axis and the branch within the above-mentioned angle range, it is easy for the dendritic silver particles to come into contact with each other.

[0043] The method for measuring the average angle is described in the embodiments.

[0044] In the silver powder of the present invention, the branches branching from the main axis in the dendritic silver particles constituting therein preferably have multiple secondary branches branching from the main axis. By having secondary branches in addition to the main branches, it is easy for the dendritic silver particles to come into contact with each other. As a result, even when the silver powder of the present invention is added to, for example, a resin in a smaller amount than before, it can exhibit the same level of conductivity as before.

[0045] Each secondary branch extends at approximately a certain angle relative to the branch, for example, within a plane containing the branch. Specifically, with the branch as an axis of symmetry, each secondary branch extends in a generally linearly symmetrical manner within a plane containing that axis of symmetry. Alternatively, the dendritic silver grain may be shaped such that the branch lies on the intersection line of two or more planes, and each secondary branch lies within those two or more planes. Alternatively, the dendritic silver grain may also be shaped such that each secondary branch extends radially from any position around the axis of the branch at an angle relative to the main axis.

[0046] Sub-branches are finer structures than branches, so sometimes adjacent sub-branches may combine with each other depending on the situation, but from the point of view of reducing the bulk density of silver powder, it is desirable for adjacent sub-branches to be separated.

[0047] Next, a suitable method for manufacturing the silver powder of the present invention will be described. The silver powder of the present invention is preferably manufactured by electrolyzing an electrolyte containing silver ions to reduce the silver ions to silver. In the process of manufacturing silver powder by electrolysis, the anode and cathode are immersed in an electrolyte containing silver ions, and a direct current voltage is applied between the two electrodes. The silver reduced by electrolysis is deposited at the cathode.

[0048] As the anode used in electrolysis, a well-known insoluble anode plate (DSE (manufactured by Permelec Electrode Ltd.)) can be cited as an example. For example, titanium electrodes coated with iridium oxide or titanium electrodes coated with ruthenium oxide are preferred as insoluble anode plates. On the other hand, there are no particular restrictions on the type of cathode, and materials that do not affect the reduction of silver ions can be appropriately selected. For example, stainless steel can be used.

[0049] As a condition for electrolysis, the preferred current density is 10–2000 A / m. 2 More preferably 30–1500 A / m 2 More preferably, it is 50–1000 A / m 2 By setting the current density to 10 A / m 2 The above methods can increase the silver precipitation rate and suppress the coarsening of dendritic silver particles. Additionally, by setting the current density to 2000 A / m... 2 The following methods can suppress the temperature rise of the electrolyte and stabilize the shape of the dendritic silver particles.

[0050] The electrolyte temperature is preferably set to below 80°C, more preferably below 60°C, and even more preferably below 40°C. By setting the electrolyte temperature to below 80°C, excessive convection of the electrolyte is suppressed, which avoids difficulties in controlling the shape of dendrites caused by an excessive increase in the silver ion supply rate.

[0051] From the viewpoint of successfully obtaining the desired dendritic silver particles, it is advantageous to circulate the electrolyte during electrolysis. To circulate the electrolyte, an electrolysis apparatus can be used, for example, comprising a closed flow path, an electrolytic cell disposed within that flow path, and a pump disposed within that flow path. Driving the pump causes the electrolyte to flow in one direction within the electrolytic cell. The anode and cathode used in electrolysis simply need to be immersed in the electrolytic cell in a state where they are facing each other.

[0052] When electrolysis is performed while circulating the electrolyte, it is advantageous to adjust the electrolyte flow rate, i.e., the circulation rate, from the viewpoint of successfully obtaining the desired dendritic silver particles. Specifically, the electrolyte circulation rate is preferably set to 0.1 mL / (min·cm). 2 ) or more and 30.0 mL / (min·cm) 2 Below this, it is further preferred to set it to 0.2 mL / (min·cm). 2 ) or more and 20.0 mL / (min·cm) 2 The following is a further preferred setting: 0.3 mL / (min·cm). 2 ) or more and 10.0 mL / (min·cm) 2 The following is a calculation method: The circulation rate is calculated by dividing the electrolyte flow rate (mL / min) by the cathode current-carrying area (cm²). 2 And calculated.

[0053] The electrolyte supplied for electrolysis contains a silver compound that serves as a silver ion source. Water-soluble silver salts, such as silver nitrate, are preferably used as the silver compound. The concentration of silver ions in the electrolyte is preferably set to 0.1 g / L or more and 50 g / L or less, more preferably 0.5 g / L or more and 30 g / L or less, and even more preferably 1.0 g / L or more and 20 g / L or less. By setting the concentration of silver ions to 0.1 g / L or more, the silver deposition rate can be increased to a level that is industrially acceptable. Furthermore, by setting the silver deposition rate to 50 g / L or less, the desired dendritic silver particles can be readily obtained.

[0054] The electrolyte preferably contains a supporting electrolyte to improve its ionic conductivity. A water-soluble salt that does not affect electrolysis can be used as the supporting electrolyte. In particular, when ammonium sulfate is used as the supporting electrolyte, it is easier to set the pH of the electrolyte within a range where the deposited silver particles are difficult to dissolve, thus successfully obtaining the desired dendritic silver particles, which is therefore preferred. Specifically, the pH of the electrolyte is preferably set to 6 or higher and 10 or lower. The concentration of the supporting electrolyte in the electrolyte is preferably set such that the pH of the electrolyte is within this range, specifically preferably 10 g / L or higher and 100 g / L or lower, more preferably 20 g / L or higher and 80 g / L or lower, and even more preferably 30 g / L or higher and 60 g / L or lower.

[0055] The electrolyte preferably contains hydantoin or its derivatives (hereinafter collectively referred to as "hydantoin-type"). It is believed that hydantoin-types will form aggregates with silver ions in the electrolyte. By performing electrolysis in the state where these aggregates have formed, the desired dendritic silver particles can be successfully obtained.

[0056] Examples of hydantoin derivatives include hydantoin and its alkyl, hydroxyalkyl, phenyl, amino, carboxylalkyl, and halogen derivatives. 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-dihydroxymethyl-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 hydantoin derivatives can be used alone or in combination of two or more.

[0057] The concentration of hydantoin in the electrolyte is preferably set to 0.01 g / L or more and 10.0 g / L or less, more preferably 0.03 g / L or more and 5.0 g / L or less, and even more preferably 0.1 g / L or more and 3.0 g / L or less. By setting the concentration of hydantoin in the electrolyte within this range, the desired dendritic silver particles can be successfully obtained.

[0058] Electrolysis under the above conditions results in the deposition of dendritic silver particles at the cathode. The deposited dendritic silver particles are then scraped off the cathode and recovered to obtain the target silver powder. Because the silver particles constituting this powder are dendritic, a smaller amount of the powder can impart conductivity to the resin compared to silver powder composed of, for example, spherical silver particles.

[0059] Organic surface treatment agents can also be applied to silver powder obtained through electrolysis. Applying organic surface treatment agents to silver powder can suppress the aggregation of silver particles. Furthermore, by appropriately selecting the organic surface treatment agent, the affinity with other materials can be controlled. Examples of organic surface treatment agents include saturated fatty acids, unsaturated fatty acids, nitrogen-containing organic compounds, sulfur-containing organic compounds, and silane coupling agents.

[0060] The silver powder obtained in this way readily allows the silver particles constituting it to come into contact with each other, thus easily imparting high conductivity to the resin by adding the silver powder to it. For example, the silver powder of the present invention can be suitably used in the form of a conductive resin composition comprising the silver powder and the resin. For example, the silver powder of the present invention can be mixed with resin, organic solvent, and glass frit to prepare a conductive paste. Alternatively, the silver powder of the present invention can be mixed with organic solvent to prepare a conductive ink. By applying the conductive paste or conductive ink thus obtained to the surface of the object to which the application is intended, a conductive film having a desired pattern can be obtained.

[0061] Example

[0062] The present invention will now be described in more detail through embodiments. However, the scope of the present invention is not limited to these embodiments. Unless otherwise specified, "%" refers to "mass %".

[0063] [Example 1]

[0064] (1) Preparation of electrolyte

[0065] Prepare an electrolyte with the following composition.

[0066] • Pure water: 30L

[0067] Silver nitrate: 10 g / L (converted to silver ion concentration)

[0068] Ammonium sulfate: 100g / L

[0069] Alkyl derivatives of hydantoin: 0.6 g / L

[0070] (2) Electrolysis

[0071] Electrolysis of the electrolyte is carried out under the following conditions.

[0072] • Anode: DSE (Permelec Electrode Ltd.) electrode

[0073] Cathode: SUS316 plate

[0074] • Electrode distance: 5cm

[0075] Current density: 500A / m 2

[0076] • Electrolyte circulation rate: 4.0 mL / (min·cm) 2 )

[0077] • Electrolyte temperature: 25℃

[0078] After electrolysis, the silver powder was washed with 5L of pure water using a suction filter. Next, 1L of a 0.3% stearic acid solution in acetone was sprinkled onto the silver powder for surface treatment. Then, the silver powder was dried in a dryer.

[0079] [Example 2]

[0080] Silver powder was obtained in the same manner as in Example 1, except that the amount of alkyl derivative of hydantoin added was changed to 0.03 g / L.

[0081] [Example 3]

[0082] In addition to changing the electrolyte circulation rate to 2.0 mL / (min·cm) 2 In addition to the above, silver powder was obtained in the same manner as in Example 1.

[0083] [Comparative Example 1]

[0084] This comparative example is equivalent to the embodiment of Patent Document 1.

[0085] (1) Preparation of electrolyte

[0086] Prepare an electrolyte with the following composition.

[0087] • Pure water: 30L

[0088] Silver nitrate: 20 g / L (converted to silver ion concentration)

[0089] Citric acid: 0.1g / L

[0090] Nitric acid: 10g / L

[0091] (2) Electrolysis

[0092] Electrolysis of the electrolyte is carried out under the following conditions.

[0093] • Anode: DSE electrode

[0094] Cathode: SUS316 plate

[0095] • Electrode distance: 5cm

[0096] • Current density: 750A / m 2

[0097] • Electrolyte circulation rate: 4.0 mL / (min·cm) 2 )

[0098] • Electrolyte temperature: 25℃

[0099] The subsequent operations were performed in the same manner as in Example 1 to obtain silver powder.

[0100] [Evaluation 1]

[0101] The silver powders obtained in Examples 1 to 3 were observed using scanning electron microscopy (SEM). The results are shown below. Figure 1 (Example 1) Figure 2 (Example 2) Figure 3 (Example 3) and Figure 4 (Comparative Example 1). If aggregates are present, the particles overlap, making it difficult to determine the dendrite shape. Therefore, classification is performed beforehand to remove aggregates. In addition, after silver powder is sprinkled on the sample stage, air blowing is used to suppress the overlap of particles.

[0102] [Evaluation 2]

[0103] For the silver powder obtained in the examples, the thickness and length of the main axis of the dendritic silver particles, the number of branches, and the length of the longest branch extending from the main axis were determined using the following methods. Additionally, the average angle between the main axis and the branches was measured. Furthermore, the percentage of dendritic silver particles with a main axis thickness of 10 nm or more and 280 nm or less, and a branch count relative to the axial length of the main axis of 6 branches / μm or more and 30 branches / μm or less was determined. These results are shown in Table 1.

[0104] [Thickness and length of the main axis, number of roots in the branches, length of the longest branch]

[0105] Using a scanning electron microscope at a magnification of 10,000x (in this example) that allows for the discernible shape of the entire particle, 50 particles were observed across 15 fields of view. The thickness and length of the main axis, the number of branches, and the length of the longest branch extending from the main axis were measured for each particle, and their average values ​​were calculated.

[0106] [Number of dendritic silver particles %]

[0107] The number of dendritic silver particles with a main shaft diameter of 10 nm or more and 280 nm or less and a number of branches of 6 or more and 30 or less per μm, obtained by measuring each particle using the aforementioned method, was measured. This number was divided by 50 and then multiplied by 100 to calculate the total number.

[0108] [The average angle between the main axis and the branches]

[0109] Fifty particles were observed using a scanning electron microscope at 10,000x magnification across 15 fields of view. The acute angle between the principal axis and the branch of each particle was measured, and the average value was calculated for each particle.

[0110] [Evaluation 3]

[0111] For the silver powder obtained in the examples, the tap density, apparent density, BET specific surface area, and particle size D were determined using the following methods. 50 The results are shown in Table 1.

[0112] [Tap density]

[0113] According to JIS Z 2512, measurements were performed using a Copley Scientific JV2000. The volume was 25 cm³. 3 10g of silver powder was placed in a graduated cylinder, and the vibration stroke was set to 3mm and the vibration frequency was set to 2500 times (250 times / minute) for measurement.

[0114] Apparent density

[0115] According to JIS Z 2504, the bulk density was measured using a bulk density measuring instrument (for metal powder, model: JIS-Z-2504, funnel aperture 5.0 mm) manufactured by the Kōchi Scientific Apparatus.

[0116] [BET specific surface area]

[0117] Monosorb was prepared using Mounttech Co., Ltd., and the determination was performed using the BET single-point method.

[0118] Particle size D 50 ]

[0119] 0.2 g of silver powder was placed in a beaker, and 0.07 g of Triton X-100 (manufactured by Kanto Chemical) was added to mix with the silver powder. Next, the silver powder was added to 40 mL of water with a dispersant (dispersant: 0.3% SN-PW-43 solution (manufactured by SAN NOPCO)). Then, the mixture was dispersed using an ultrasonic disperser US-300AT (manufactured by Nippon Seiki Co., Ltd.) at 300 watts for 3 minutes to prepare a sample for measurement. Using this sample, the volumetric cumulative particle size D was measured using a laser diffraction scattering particle size distribution measuring device MT3300II (manufactured by Nikkiso). 50 .

[0120] [Evaluation 4]

[0121] For the silver powder obtained in the examples, the crystallite size, shrinkage rate at 150°C, and shrinkage initiation temperature of the silver were determined using the following methods. The results are shown in Table 1.

[0122] [Silver crystallite size]

[0123] X-ray diffraction was performed using a RINT2000 X-ray diffractometer manufactured by Rigaku Electric Co., Ltd. The crystallite size was calculated using the obtained diffraction peaks via the Scherer method. The X-ray diffraction conditions were set as follows: 2θ / θ = 5–80 degrees, step size = 0.01 degrees, scan rate = 0.2 degrees / minute, characteristic X-rays = Cu-Kα1 rays, and a 1D detector. The crystallite size was calculated based on the half-width at half-maximum (WHM) of the Ag(200) peak, using 0.94 as the Scherer constant.

[0124] [Shrinkage rate and shrinkage onset temperature at 150℃]

[0125] A TMA / SS6300 thermomechanical analysis apparatus manufactured by Hitachi High-Tech Science Corporation was used. 0.5 g of silver powder was placed in a dedicated mold with an inner diameter of 3.8 mm to obtain a sample subjected to a load of 1.0 kN. This sample was mounted in the thermomechanical analysis apparatus and heated from 25 °C to 800 °C at a heating rate of 10 °C / min under a mixed atmosphere of 49 mN load, 99 vol% nitrogen, and 1 vol% hydrogen. The rate of thermal expansion (%) was monitored over time starting from 25 °C, and the absolute value of the negative expansion rate at 150 °C was defined as the rate of contraction at that temperature.

[0126] Furthermore, the temperature at which the absolute value of the negative expansion rate reaches 0.3% is defined as the contraction initiation temperature. The results of the thermomechanical analysis are shown below. Figure 5 .

[0127] [Evaluation 5]

[0128] The silver powder obtained in the examples was mixed with resin to prepare a conductive resin composition, and the resistivity of the conductive film obtained from the conductive resin composition was measured. The results are shown in Table 1.

[0129] [Resistivity of conductive film]

[0130] Conductive resin compositions were prepared using the silver powder obtained in the examples and comparative examples.

[0131] A conductive resin composition formed by mixing silver powder, epoxy resin, and 2-methylimidazole was prepared. Three conductive resin compositions were prepared with silver powder content of 30%, 50%, and 60%, respectively. The mass ratio of epoxy resin to 2-methylimidazole in each conductive resin composition was set to 97:3.

[0132] Next, the paste was coated onto a glass plate. A 200mm wide rod coater was used for coating. The gap was set to 100μm. The resulting coating was dried in an atmospheric hot air drying oven at 110°C for 60 minutes to obtain a conductive film with a thickness of 80μm. The resistance of the conductive film was measured using a resistivity meter (Mitsubishi Chemical MCP-T600) using the four-probe method.

[0133] [Table 1]

[0134]

[0135] ※1 The number of dendritic silver particles with a main shaft thickness of 10 nm or more and 280 nm or less, and a number of branches of 6 or more and 30 or less per μm were counted.

[0136] As clearly shown in Table 1, the silver powder of the examples has finer main axes and more branches compared to the silver powder of the comparative examples. Furthermore, in Figures 1 to 3 In the image shown, secondary branches branching off from the main branch are observed.

[0137] Furthermore, the results shown in Table 1 clearly demonstrate that the conductive film containing the silver powder of the embodiments has a lower resistivity compared to the conductive film containing the silver powder of the comparative examples. It is worth noting that the film containing the silver powder of the embodiments exhibits conductivity even at an extremely low silver powder content of 30%.

[0138] Industrial availability

[0139] The silver powder according to the present invention can impart high conductivity to resin with a small amount added. Furthermore, such silver powder can be easily manufactured according to the manufacturing method of the present invention.

Claims

1. A type of silver powder, comprising dendritic silver particles, The dendritic silver particles are dendritic in shape, and the dendrites have only one main axis and multiple branches branching from the main axis. The spindle has a thickness of 10 nm or more and 280 nm or less. The number of branches relative to the axial length of the main axis is more than 6 branches / μm and less than 30 branches / μm, wherein, The dendritic silver particles account for more than 50% of all silver particles. The shrinkage rate of the silver powder at 150°C, as determined by thermomechanical analysis, is greater than 0.3%.

2. The silver powder according to claim 1, wherein its tap density is 1.0 g / cm³. 3 the following.

3. The silver powder according to claim 1, wherein the crystallite size is 10 nm or more and 50 nm or less.

4. The silver powder according to any one of claims 1 to 3, wherein, The average angle between the main shaft and each branch is greater than 45 degrees and less than 80 degrees.

5. The silver powder according to any one of claims 1 to 3, further comprising a plurality of secondary branches branching from the branch.

6. A conductive resin composition comprising a resin and the silver powder according to any one of claims 1 to 5.

7. A method for manufacturing silver powder according to any one of claims 1 to 5, comprising the step of electrolyzing an electrolyte to reduce silver ions. The electrolyte contains silver ions and hydantoin derivatives. The hydantoin derivative is an alkyl derivative of hydantoin, a hydroxyalkyl derivative of hydantoin, a phenyl derivative of hydantoin, an amino derivative of hydantoin, a carboxylalkyl derivative of hydantoin, or a halogen derivative of hydantoin.

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