Scaly silver powder, method for producing same, and silver paste using same

Abstract

The present invention provides a method for producing fine scaly silver powder, the method being capable of suppressing an adverse effect on a resin by wet pulverization using water as a solvent.　This method for producing scaly silver powder is characterized in that a slurry containing granular silver powder, using water as a solvent, and having added thereto a nonionic surfactant having an HLB value of 3.9 to 13 and an addition amount of 2 to 10 wt% is subjected to wet pulverization to flake the granular silver powder, thereby obtaining scaly silver powder.　The method for producing scaly silver powder is further characterized in that D50% of the granular silver powder is 0.1 to 3 μm, and D50% of the scaly silver powder is 3 μm or less.

Description

Scale-like silver powder, method for producing the same, and silver paste using the same

[0001] This invention relates to flake-shaped silver powder, a method for producing the same, and a silver paste using the same.

[0002] Silver powder is increasingly being used as a filler in conductive pastes for forming electrodes and circuits. For such conductive pastes, flake-shaped silver powder is used to achieve high conductivity through surface contact. A known method for producing highly conductive flake-shaped silver powder involves wet grinding of granular silver powder. (Patent Document 1)

[0003] Furthermore, in recent years, as electrodes and circuits have become increasingly finer, there is a demand for flaky silver powder with a fine and sharp particle size distribution. In addition, environmental considerations such as VOC regulations are becoming more important, and there is a demand for the production of silver powder without using organic solvents. However, it is generally known that it is difficult to produce fine flaky silver powder using wet grinding without organic solvents.

[0004] Furthermore, Patent Document 2 points out that when fatty acids are added to the treatment agent for wet grinding, the fatty acids remaining in the silver powder may react with the resin, causing curing inhibition. To solve this problem, a method is disclosed in which spherical silver powder is wet-ground to obtain flake-shaped silver powder by using only nonionic surfactants as the treatment agent, adding a predetermined amount of a relatively hydrophilic nonionic surfactant when water is used as the solvent, and a relatively lipophilic nonionic surfactant when an organic solvent is used as the solvent. Accordingly, a method for producing silver powder without using organic solvents is disclosed by employing wet grinding using water and a nonionic surfactant as the solvent.

[0005] However, Patent Document 2 does not disclose a method for producing fine flake-shaped silver powder, and it is difficult to actually meet the demand for providing fine flake-shaped silver powder associated with the finening of circuits and the like. Regarding the finening of flake-shaped silver powder, Patent Document 3 points out that even if the silver particles of the raw material powder are finened in the expectation of finening the flake-shaped silver powder, the resulting flake-shaped silver powder will aggregate and contain coarse particles. Patent Document 3 discloses a method for producing fine flake-shaped silver powder of 3 to 5 μm size by adding fatty acids and nonionic surfactants to a post-reaction slurry containing silver particles obtained by a wet reduction reaction, or to an aqueous dispersion slurry immediately after washing the post-reaction slurry, and wet grinding the silver particles without drying.

[0006] Patent No. 3874634 Patent No. 4263799 Patent No. 5330724

[0007] According to the manufacturing method disclosed in Patent Document 3, it is possible to produce fine flaky silver powder with a size of 3 to 5 μm. However, because fatty acids are used, it is difficult to achieve both fine flaky silver powder and suppress adverse effects on thermosetting resins. Furthermore, the size of the obtained flaky silver powder is limited to 3 to 5 μm, and there is a demand for even finer flaky silver powder to meet the needs of increasingly fine circuits and other applications.

[0008] In view of the above issues, the present invention aims to provide a method for producing fine, flaky silver powder that can suppress adverse effects on resin by wet grinding using water as a solvent.

[0009] The present invention relates to a method for producing flaky silver powder, characterized in that a slurry containing granular silver powder, with water as the solvent and a nonionic surfactant having an HLB value of 3.9 to 13 and an addition amount of 2 to 10% by weight, is subjected to wet grinding to flak the granular silver powder and obtain flaky silver powder.

[0010] By using a manufacturing method for flake-shaped silver powder with this configuration, it is possible to obtain fine flake-shaped silver powder with minimal adverse effects on the resin through water-based wet grinding, which has a low environmental impact.

[0011] Furthermore, in the above configuration of the method for producing flake-shaped silver powder according to the present invention, the D50% of the granular silver powder may be 0.1 to 3 μm.

[0012] Furthermore, in the above configuration of the method for producing flake-shaped silver powder according to the present invention, the D50% of the flake-shaped silver powder may be 3 μm or less.

[0013] By using a manufacturing method for flake-shaped silver powder with this configuration, it is possible to meet the increasingly demand for finer lines in circuits and other components.

[0014] Furthermore, in the method for producing flake-shaped silver powder according to the present invention, the nonionic surfactant may be an ester-based nonionic surfactant.

[0015] By using a manufacturing method for flake-shaped silver powder with this configuration, it is possible to effectively suppress coarse powdering due to aggregation.

[0016] The present invention relates to a method for producing silver paste, characterized by producing flake-shaped silver powder by the above-described manufacturing method and filling the flake-shaped silver powder into a resin.

[0017] By using a silver paste manufacturing method with this configuration, it is possible to produce a silver paste that has suppressed adverse effects on the resin.

[0018] The flaky silver powder according to the present invention is characterized in that the D50% is 3 μm or less and does not have organic matter with electrically biased functional groups on its surface.

[0019] By using flaky silver powder with this configuration, the increase in viscosity of the paste during paste preparation is suppressed, contributing to improved flexibility in silver paste production.

[0020] The silver paste according to the present invention is characterized by being filled with the aforementioned flaky silver powder.

[0021] By creating a silver paste with this configuration, we can provide a silver paste that can meet the demands associated with the fine-line design of circuits and other components.

[0022] According to the present invention, a method for producing fine, flaky silver powder can be provided that minimizes adverse effects on the resin by wet grinding using water as a solvent.

[0023] The table in Figure 1 shows the manufacturing conditions for the flake-shaped silver powder, the results of the characterization of the flake-shaped silver powder, and the results of the evaluation of its effect on the resin. Figure 2 is a graph showing the dependence of the HLB value and the amount of nonionic surfactant added on the manufactured flake-shaped silver powder size (D50%).

[0024] Embodiments of the present invention will be described below with reference to the drawings. However, none of the following embodiments are intended to provide a restrictive interpretation in determining the gist of the present invention. Furthermore, the same or similar reference numerals may be used for identical or similar components, and their descriptions may be omitted.

[0025] Furthermore, terms used in this specification to specify shapes, geometric conditions, and their degrees, such as "parallel," "orthogonal," and "identical," as well as values ​​of length and angle, shall not be strictly interpreted, but shall be interpreted to include a range that can be expected to perform similar functions.

[0026] (Embodiment 1) The following describes an embodiment of a method for producing flake-shaped silver powder. A water-based slurry containing granular silver powder as a raw material is prepared, and flake-shaped silver powder can be obtained by wet grinding, while suppressing coarse powdering due to aggregation. Furthermore, dried flake-shaped silver powder can be obtained by drying the water-based slurry containing the flake-shaped silver powder. For example, a water-based slurry can be prepared by charging silver powder as a raw material, water as a solvent, and an ester-based nonionic surfactant as an additive into a container, and then performing flake-forming treatment using wet grinding such as a ball mill. Note that a water-based slurry means a slurry that does not contain organic solvents and uses, for example, water as a solvent. Wet grinding using a water-based slurry is sometimes called water-based wet grinding.

[0027] Generally, it is difficult to grind coarse powder that has aggregated from flaky silver powder produced by wet grinding of granular silver powder. Therefore, the agglomeration must be broken up during wet grinding to prevent the generation of coarse powder. In this embodiment, a small crusher (cutter mill) is used to powderize the dried, block-like state, but it should be noted that this process is solely for powdering and is clearly distinct from breaking up the agglomeration during wet grinding.

[0028] <Raw Materials> To produce fine, flaky silver powder with suppressed coarseness, for example, with a D50% of 1 to 3 μm, granular silver powder with a D50% of 0.1 to 3 μm is used as the raw material. While there is no limitation on the particle size distribution of the granular silver powder used as the raw material, it is preferable to set the particle size distribution according to the desired size of the flaky silver powder. When grinding the granular silver powder using a ball mill, the lower limit of the granular silver powder size is determined by the media size, and in reality, it is considered that the lower limit is 0.1 μm to 0.5 μm. Furthermore, the manufacturing method of the granular silver powder is not limited to known methods such as atomization, electrolysis, or chemical reduction. If the particle size can be made even smaller, it may be 0.1 μm or less. The granular silver powder is preferably in the form of dried powder, but is not limited to this.

[0029] <Additives> Nonionic surfactants are used as additives (lubricants) in water, which is the solvent, and ester-based nonionic surfactants are particularly used. In the case of ether-based nonionic surfactants, although adverse effects on the silver powder-filled resin are suppressed, coarse powdering occurs due to aggregation, making them unsuitable as additives. Ester-based nonionic surfactants include, for example, at least one of polyoxyethylene fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and sorbitan fatty acid esters. The amount of nonionic surfactant added is 2 to 10% by weight, and the HLB value is 3.9 to 13, preferably in the range of 3.9 to 10. The HLB value represents the affinity of the surfactant for water and oil, and is also called the hydrophilic-lipophilic balance value. The HLB value takes values ​​from 0 to 20. The closer the HLB value is to 0, the higher the affinity for oil, and the closer it is to 20, the higher the affinity for water. To suppress the aggregation of silver powder obtained by aqueous wet grinding, a nonionic surfactant with an HLB value of 10 or less (3.9 to 10), which is considered to have relatively high lipophilicity, is preferably used. However, by adding 2% by weight or more, the range in which the aggregation of silver powder can be suppressed can be expanded to 13 or less (3.9 to 13).

[0030] <Wet Grinding> The prepared aqueous slurry is subjected to wet grinding using a known grinder such as a ball mill or bead mill. The conditions for wet grinding are not particularly limited, and known conditions (for example, those described in Patent Document 1, etc.) can be used. After wet grinding, the slurry can be dried by a known method such as heating to obtain flake-shaped silver powder.

[0031] The evaluation results of specific examples are described in detail below. The table in Figure 1 shows the manufacturing conditions, characteristic evaluation results, and the results of filling the resin with the flake-shaped silver powder for silver paste formation and evaluating its effect on the resin. These physical properties all represent the results of systematically and quantitatively evaluating the characteristics (especially particle size and aggregation status) of the flake-shaped silver powder obtained by aqueous wet grinding. In addition, in the table in Figure 1, the number of coarse particles, the average area per coarse particle ("aggregated particle area" in the table in Figure 1), and the product of the number of coarse particles and the average area ("total aggregated particle area" in the table in Figure 1, sometimes simply referred to as "aggregation amount") were used as indicators of the degree of silver powder aggregation. The aggregation amount was evaluated by image analysis using SEM photographs. Details of the evaluation method are as follows. The BET value is the specific surface area (m²). ２ The values ​​represent ( / g) and were measured using a BET specific surface area analyzer (Macsorb) (manufactured by Mountec Co., Ltd.) based on the BET flow method (single-point method) according to JIS standard Z8830. D50% was measured using a laser diffraction particle size distribution analyzer (SALD-2300) (Shimadzu Corporation). HL (reduction loss) represents the amount of surface treatment agent attached. In the experiment, a 2g sample was heated at a temperature of 800°C for 0.5 hours in a H atmosphere. ２ , H ２ The sample weight was measured after heating under a gas flow rate of 0.5 L / min, and the value was calculated using the following formula: HL (%) = {(Sample weight before heating (g) - Sample weight after heating (g)) / Sample weight before heating (g)} × 100. The number of coarse powder particles was determined using image analysis software (ImageJ) at a magnification of 2000x for the cross-sectional image of the powder obtained by scanning electron microscope (SEM, Hitachi, Ltd.), with a particle size of 10 μm. ２ The number of coarse particles was measured. Agglomerated powder area (μm²) ２ ) is extracted using the coarse powder count, resulting in 10 μm particles. ２ This represents the average value of the aggregated area of ​​the coarse powder. Total aggregated powder area (aggregation amount) (μm ２ ) is extracted using the coarse powder count, resulting in 10 μm particles. ２This represents the total surface area of ​​the coarse powder. Viscosity (Pa·s) was measured using a cone-plate viscometer DV(HB)-NEXT (Blookfield) under the following conditions: Spindle used: CP-52 Temperature: 25°C Rotation speed: 0.5 rpm Maximum stress (MPa) was obtained by preparing a paste by adding various silver powders and thermosetting resins in a weight% (wt%) ratio of 85:15. A coating film was made from this paste measuring 60 mm × 12 mm × 0.5 mm, and cured at 150°C × 60 min. The coating film was then gripped at 40 mm using a universal material testing machine 5965 (INSTRON), and a tensile test was performed to obtain the maximum stress. G gauge (μm) refers to the grind gauge, and a paste is made by adding various silver powders and thermosetting resins in a weight% (wt%) ratio of 85:15. The coarse powder particle size is measured using a grind gauge (0-50 μm) (ERICHSEN Model 232), and the average value is taken from the three largest readings.

[0032] Generally, when seeking to improve the electrical and thermal conductivity of silver paste, a higher silver ratio is desirable, and when compounding resin with silver powder, silver powder with the lowest possible viscosity is required. Therefore, we evaluated not only the effect on the resin during curing, but also the viscosity of the resin filled with silver powder.

[0033] In the evaluation example shown in Figure 1, granular silver powder (dried powder) with a D50% content of 2.7 μm was used as the raw material, and polyoxyethylene fatty acid ester was used as the ester-based nonionic surfactant, polyoxyethylene alkylphenyl ether as the ether-based nonionic surfactant, alkylamine as the cationic surfactant, fatty acid salt as the anionic surfactant, and alkyldimethylaminoacetic acid betaine as the amphoteric surfactant. Specifically, as an example, the following were used: - Ester-based nonionic surfactant: Polyoxyethylene fatty acid ester (Example 1, Comparative Example 2: EMALEX TEG-di-O (PEG-3 dioleate manufactured by Nippon Emulsion Co., Ltd.)) - Ether-based nonionic surfactant: Polyoxyethylene alkylphenyl ether (Comparative Example 11: Polyoxyethylene (10) octylphenyl ether (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)) - Cationic surfactant: Alkylamine (Comparative Examples 13-14: Hexylamine (manufactured by Tokyo Chemical Industry Co., Ltd.)) - Anionic surfactant: Fatty acid salt (Comparative Example 15: Sodium oleate (manufactured by NOF Corporation)) - Amphoteric surfactant: Alkyldimethylaminoacetic acid betaine (Comparative Examples 16-17: Rikabion A-700 (stearyldimethylaminoacetic acid betaine manufactured by Shin Nippon Rika))

[0034] <Effects of Ionic Surfactants> Referring to Figure 1, the effects of ionic surfactants on resins will be explained. In Comparative Examples 13 and 14, where a cationic surfactant was used as an additive in the aqueous solvent, the resin coating was brittle, and its maximum stress could not be measured. The same was true for Comparative Examples 16 and 17, where an amphoteric surfactant was used. The reason for the brittleness of the coating is thought to be the reaction between the surfactant remaining in the silver powder and the resin (see Patent Document 2). In Comparative Example 15, where an anionic surfactant was used as the surfactant, although it was possible to measure the stress of the coating, the viscosity (0.5 rpm viscosity) of the resin with added silver powder was remarkably high. It is thought that the viscosity increased due to the electrical bias of the surfactant remaining inside and on the surface of the silver powder. On the other hand, no adverse effects on the resin were confirmed for nonionic surfactants. (Examples 1-11, Comparative Examples 1-12) Thus, adverse effects on the resin were confirmed for ionic surfactants, and it was confirmed that they are unsuitable as additives.

[0035] <Condition Dependence of Nonionic Surfactant> Figure 2 is a graph showing the HLB value and additive amount dependence of nonionic surfactants for flake-shaped silver powder (D50%) produced using ester-based nonionic surfactants as additives, based on the table in Figure 1. In Figure 2, the white triangles, black squares, and black diamonds represent data for conditions where the surfactant additive amount is 1%, 3%, and 6% by weight, respectively.

[0036] When the amount of nonionic surfactant added is 1% by weight, it can be seen that the silver powder size D50% increases significantly when the HLB value is 2.8 (Comparative Example 1), 11.1 (Comparative Example 8), and 16.0 (Comparative Example 9). Furthermore, from the table in Figure 1, it can be confirmed that the amount of silver powder aggregation (product of the number of coarse particles and area) also increases under the conditions of HLB values ​​of 2.8, 11.1, and 16.0.

[0037] Thus, when the HLB value exceeds 10, the size of the silver powder produced (D50%) increases, even though the affinity for water is relatively higher. Patent Document 2 states that when the solvent is water, a highly hydrophilic HLB value is preferable, and that the use of nonionic surfactants with an HLB value lower than 6 is inappropriate. However, the graph in Figure 2 shows that, in order to produce fine, flaky silver powder while suppressing coarse powdering due to aggregation, the range of usable HLB values ​​shows a trend opposite to that considered appropriate.

[0038] Thus, the reason why the amount of aggregation increases in the high HLB value range is thought to be as follows: In the high HLB value range, although the affinity to water increases, the affinity between the nonionic surfactant and the silver powder decreases. Therefore, the protective power of the silver powder decreases, and the effect of preventing aggregation of fine silver powder decreases. Accordingly, in wet grinding using fine silver powder as the raw material, an HLB value range with relatively low affinity to water that can enhance the effect of preventing aggregation of silver powder is preferably used. However, it has also been confirmed that the amount of aggregation increases if the HLB value is too low. The reason for this is thought to be that although the affinity with silver powder increases, the water dispersibility decreases.

[0039] Furthermore, when the amount of nonionic surfactant added was increased to 3% by weight and 6% by weight, a tendency was observed for the resulting flake-like silver powder size D50% to be smaller compared to when the additive amount was 1% by weight (Examples 1-11). Also, from the table in Figure 1, it can be seen that flake-like silver powder with a D50% size of 1 μm to 3 μm can be obtained. As a result, even when the HLB value is 12.3, fine flake-like silver powder with a D50% size of 3 μm or less can be obtained, thus expanding the range of usable HLB values. In addition, from the table in Figure 1, it can be seen that aggregation is also reduced by increasing the amount of additive.

[0040] In addition, the amount of the nonionic surfactant was further increased to 10% by weight, and the same evaluation was carried out. As a result, it was confirmed that fine flaky silver powder could be obtained, similar to the case where the addition amount was 6% by weight. It is considered that the protective power of the silver powder increased and the applicable HLB value range expanded due to the increase in the addition amount.

[0041] The data with the addition amount of the nonionic surfactant being 1% by weight is also interpreted as data emphasizing the HLB value dependency. From the perspective of the HLB value dependency, it is interpreted that the margin can be further increased within the range of the HLB value of 3.9 to 10.

[0042] For silver paste formation, the obtained flaky silver powder was filled into a resin and the reactivity was confirmed. As a result, sufficient strength of the coating film was confirmed. Also, it was confirmed that the viscosity (0.5 rpm viscosity) of the resin filled with the flaky silver powder was 1000 Pa·s or less. On the other hand, for comparison, flaky silver powder obtained by conventional wet grinding using acetone as a solvent and an anionic surfactant as an additive was filled into a resin and the viscosity was investigated. As a result, the viscosity of the resin was 1500 Pa·s. From this, by using the flaky silver powder obtained by aqueous wet grinding using the nonionic surfactant under the above conditions, it is possible to provide a high-filled paste with low viscosity and high coating film strength.

[0043] <Nonionic surfactant type dependency> Comparative Examples 10 to 12 all show examples using ether-based nonionic surfactants. When the addition amount was 1% by weight, at the high hydrophilic HLB values of 13.6 (Comparative Example 11) and 18.0 (Comparative Example 12), the size D50% of the produced flaky silver powder was as large as 10 μm, and the amount of aggregation was also large. Also, even under the condition where the addition amount of the nonionic surfactant was increased and the HLB value was lowered to 8.3 (Comparative Example 10), although the size D50% of the produced flaky silver powder decreased to 4.1 μm, it was larger than the target 3 μm, and the amount of aggregation was a high value (2604 μm ２)(It can be understood that aggregation of the flaky silver powder has occurred. Thus, in the case of an ether-based nonionic surfactant, it can be understood that it is difficult to prevent aggregation of the obtained silver powder and produce fine silver powder. By using an ester-based nonionic surfactant as an additive, coarsening due to aggregation can be effectively suppressed.)

[0044] As described above, in an aqueous slurry using water as a solvent, an ester-based nonionic surfactant is added as an additive in an amount of 2 to 10% by weight, and wet grinding is performed in a range where the HLB value is 3.9 to 13, preferably 3.9 to 10, whereby fine (D50% is 1 to 3 μm) flaky silver powder with coarsening due to aggregation suppressed can be produced. By filling the thus obtained flaky silver powder into a curable resin, it is possible to provide a silver paste that suppresses adverse effects on the resin and meets the requirements associated with fine line formation of circuits and the like. The silver powder wet-ground with a fatty acid added as an additive has a fatty acid, which is an organic substance having a functionally polarized group on the surface, and the viscosity will increase. However, since water is used as a solvent and only a nonionic surfactant is used as an additive for processing by wet grinding, the flaky silver powder does not have an organic substance having a functionally polarized group inside and on the surface. Therefore, an increase in the viscosity of the paste during paste preparation is suppressed, and it can be said that this flaky silver powder is excellent in terms of handling (freedom in silver paste preparation). For example, it is also possible to produce a highly filled silver paste without using a solvent. Also, if the silver powder contains a large amount of coarsened powder due to aggregation, it may cause disconnection or short circuit, or aggregation breakdown may occur in the paste filled with the silver powder, resulting in a decrease in the strength of the coating film. However, in the case of flaky silver powder with a small D50% and less aggregation as described above, a decrease in the strength of the coating film can be suppressed.)

[0045] According to the present invention, it is possible to provide fine flaky silver powder with coarsening due to aggregation suppressed by aqueous wet grinding with less environmental load. By using the obtained flaky silver powder, for example, it is possible to provide a silver paste that can meet the requirements for fine line formation of electrodes, circuits, etc., and various applications are possible, and the industrial applicability is high.)

Claims

1. A method for producing flaky silver powder, characterized by subjecting a slurry containing granular silver powder, water as a solvent, and a nonionic surfactant having an HLB value of 3.9 to 13 and an addition amount of 2 to 10% by weight to wet grinding, thereby flaking the granular silver powder to obtain flaky silver powder.

2. The method for producing flake-like silver powder according to claim 1, characterized in that 50% of the D of the granular silver powder is 0.1 to 3 μm.

3. The method for producing flake-shaped silver powder according to claim 2, characterized in that 50% of the flake-shaped silver powder has a diameter of 3 μm or less.

4. The method for producing flake-shaped silver powder according to any one of claims 1 to 3, characterized in that the nonionic surfactant is an ester-based nonionic surfactant.

5. A method for producing silver paste, characterized by producing flake-shaped silver powder by the manufacturing method described in any one of claims 1 to 3, and filling a resin with the flake-shaped silver powder.

6. A flaky silver powder characterized in that D50% is 3 μm or less and does not have organic matter with electrically biased functional groups on its surface.

7. A silver paste characterized by being filled with the flaky silver powder described in claim 6.

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