JP4074637B2 - Method for producing fine silver powder - Google Patents

Description

  The present invention relates to a method for producing fine silver powder by an electrolytic method.

  Silver powder is used for forming electrodes and circuits of various electronic parts such as internal electrodes of multilayer capacitors, conductor patterns of circuit boards, and electrodes of substrates for plasma display panels. In recent years, with the miniaturization, high density, and high precision of electronic parts, silver powder is also required to be finely divided and homogenized.

  As a method for producing silver powder, in addition to an electrolytic method (see Patent Document 1) in which an electrolytic solution containing silver ions is electrolyzed to deposit silver particles on an electrode, as disclosed in Patent Document 2, a silver nitrate solution and ammonia are used. A method of obtaining a highly dispersible spherical silver powder by a wet reduction process in which an aqueous silver ammine complex solution is produced with water and an organic reducing agent is added thereto. Furthermore, as disclosed in Patent Document 3, for example, sulfuric acid A method using a chemical reduction method in which a silver aqueous solution is reacted with polyvinyl pyrrolidone as one of sodium phosphinate, formaldehyde and hydroquinone as a reducing agent is known.

JP-A-8-209375 JP 2001-107101 A JP-A-6-122905

Among the various silver powder production methods as described above, the method using a reducing agent tends to increase the cost, and depending on the type of the reducing agent, it may cause problems in terms of the working environment such as the generation of a strange odor. is there. In comparison, the electrolytic method is superior in production efficiency because it can produce silver powder at a relatively low cost and can be operated at a high current density.
However, according to the electrolytic method, the particle diameter of the silver powder tends to become coarse, and there is a problem that it is difficult to obtain a fine silver powder required for forming a fine circuit.

  Therefore, the present invention is to provide a new method for producing fine silver powder so that fine silver powder can be obtained by an electrolytic method.

  The present invention proposes a method for producing fine silver powder, characterized in that electrolysis is performed using an aqueous silver complex salt solution as an electrolytic solution.

Fine silver powder having an average particle size (D50) of 10 μm or less can be obtained by electrolysis using a silver complex salt aqueous solution such as a silver ammine complex salt aqueous solution as an electrolyte. Moreover, the silver powder obtained is a silver powder mainly composed of dendritic particles with few spherical particles, and is excellent in terms of homogeneity. In addition, dendritic particles have a specific surface area larger than that of spherical powder, are easily affected by heat, and have physical characteristics that easily exhibit low-temperature sinterability.
Furthermore, since the conventional general electrolysis method is generally performed in an acidic solution, even if fine particles are precipitated, it is re-dissolved. However, a silver complex salt aqueous solution such as a silver ammine complex salt aqueous solution is used. By adjusting the pH and using it in the electrolyte, the precipitated fine silver particles can exist stably without re-dissolving, so that a dendritic fine silver powder that is homogeneous in terms of shape and size can be obtained. .

  By adding a water-soluble organic polymer to the electrolytic solution, the particle size of the obtained silver particles can be further reduced.

Further, by wet-grinding the silver powder obtained by electrolysis as described above, the trunk portion and the branched portion in the dendrite-like silver particles can be separated, and even finer needle-like silver particles can be obtained. .
And by classifying after wet pulverization, for example, a trunk portion and a branched portion can be separated, and each can be used for applications according to each particle size and shape.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, although the embodiment of the present invention is described in detail, the scope of the present invention is not limited to the following embodiment.
In this specification, “X to Y” (X and Y are arbitrary numbers) means “X or more and Y or less” unless otherwise specified, “preferably larger than X, Y It includes the meaning of “smaller”.

In the present embodiment, a manufacturing method for obtaining fine silver powder by electrolyzing an aqueous silver complex salt solution as an electrolytic solution will be described.
In the present invention, “electrolysis” includes both electrolytic collection using a DSE electrode and electrolytic purification using a silver electrode.

As the silver complex salt aqueous solution, a silver ammine complex salt aqueous solution (silver ammine complex salt aqueous solution) which is a monodentate ligand complex salt is preferably used.
Silver ammine complex salts, which are monodentate ligands, have a weaker binding power to silver ions and less steric hindrance than bidentate ligands or higher polydentate ligands. Suitable for analysis.

  The method for preparing the silver complex aqueous solution is not particularly limited. For example, when preparing an aqueous silver ammine complex salt solution, it can be prepared by adding ammonia or aqueous ammonia to an aqueous solution containing silver ions such as an aqueous silver nitrate solution, or by further adding an ammonium salt such as ammonium sulfate. You can also At this time, the ammonium salt serves as a supply source of ammonium ions and functions as a pH buffering agent. Therefore, the amount of ammonia or ammonia water added can be suppressed, and pH adjustment of the electrolyte can be facilitated.

  The pH of the silver complex salt aqueous solution is preferably adjusted to 3 to 11, particularly 4 to 11, and particularly preferably 5 to 10. When the pH is lower than 3, the precipitated particles are dissolved and the shape is not stable. On the other hand, when the pH exceeds 11, not only the ammonia gas volatilizes and causes bad odor, but also it is not economical because there is no difference in the generated particles. In particular, by adjusting the pH of the aqueous silver complex salt solution to a range of 5 to 10, the precipitated fine silver particles are present more stably without dissolving (re-dissolving), so the quality in terms of shape and size Can produce a more stable fine silver powder.

  The silver concentration in the electrolytic solution is preferably adjusted to 0.5 g / L to 50 g / L, particularly 1 g / L to 30 g / L. If it is less than 0.5 g / L, the silver deposition rate becomes slow, and silver powder cannot be obtained efficiently. Moreover, since it will become unstable for the shape of the silver particle to produce | generate when it exceeds 50 g / L, it is unpreferable.

NH ₃ / Ag ⁺ in the electrolytic solution is preferably adjusted to a molar ratio of 2 or more, particularly 2 to 20. If it is less than 2, complex formation is insufficient and silver is precipitated. Moreover, when it exceeds 20, it is uneconomical and there is a possibility that the working environment is deteriorated due to the bad smell of ammonia gas.
Specifically, for example, an aqueous silver nitrate solution and aqueous ammonia, or further an ammonium salt such as ammonium sulfate, is preferably mixed so that the molar ratio of silver and NH ₃ is within the predetermined range.

The electrolysis conditions, the current density is preferably 10~1000A / m ^2, more preferably 30~800A / m ^2, more preferably from 50~500A / m ^2. If it is less than 10 A / m ² , the silver deposition rate becomes slow and the grains become coarse. Moreover, when it becomes higher than 1000 A / m < ² >, the temperature in a solution will rise and the shape of silver powder will not be stabilized. In addition, ammonia is more easily volatilized and the running cost is high, which is uneconomical.

  The silver powder deposited on the electrode plate is scraped off at appropriate intervals, and the silver powder scraped off from the electrode plate is filtered, washed, and dried to obtain fine silver powder. At this time, the method of filtration, washing and drying is not particularly limited, and a general method may be adopted.

When silver powder is produced as described above, all particles have a dendritic shape, and a fine silver powder having an average particle diameter (D50) of 10 μm or less, preferably 5 μm to 10 μm can be obtained.
The dendrite shape is a shape formed by branching and growing from a trunk portion to a planar shape.

Furthermore, the dendritic fine silver powder can be further atomized by adding a water-soluble organic polymer to the electrolytic solution and performing electrolysis as described above.
Examples of the water-soluble organic polymer include gelatin, polyvinyl alcohol, water-soluble starch, glue, water-soluble carboxylate, etc. Among them, gelatin is preferable.
At this time, the water-soluble organic polymer is preferably added so as to be 0.05 g / L to 5 g / L with respect to the electrolytic solution. If it is less than 0.05 g / L, a sufficient effect cannot be obtained, and if it exceeds 5 g / L, the particle shape becomes unstable.

  In addition, by wet-grinding the silver powder obtained by electrolysis as described above, the trunk portion and the branched portion of the dendrite-like silver particles can be separated, thereby obtaining finer needle-like silver particles. Can do.

  As the wet pulverization means, since the silver particles are soft, it is preferable to employ a wet pulverization means that does not use a medium (a pulverization medium such as beads or balls) so that the shape can be maintained. For example, a wet jet mill is preferably used. Can be used.

Furthermore, by classifying following the above-mentioned wet pulverization, for example, the trunk portion and the branched portion can be separated, both of which are extremely fine particles, among which depending on the characteristics of the trunk portion and the branched portion. Can be used for various purposes.
In this case, as a classification method, in addition to centrifugal classification, any of a method of passing a mesh of a certain size such as a vibration sieve or an in-plane sieve, or a method of separation by airflow may be employed.

An organic surface treatment may be applied to the silver powder obtained as described above. Aggregation can be suppressed by applying an organic surface treatment to the silver particles. Moreover, the affinity with other materials can be controlled by appropriately selecting the organic surface treatment agent.
At this time, as the organic surface treatment, for example, a film made of a saturated fatty acid, an unsaturated fatty acid, a nitrogen-containing organic compound, a sulfur-containing organic compound, a silane coupling agent, or the like may be formed on the silver particle surface. Among these organic compounds, it is preferable to use oleic acid, capric acid or stearic acid. As a film forming method, a known method such as a dry method or a wet method may be employed.

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

<Particle size measurement>
Take a small amount of silver powder in a beaker, add a few drops of 3% Triton X solution (Kanto Chemical), blend in the powder, add 50 mL of 0.1% SN Dispersant 41 solution (San Nopco), Then, the dispersion | distribution process was performed for 2 minutes using the ultrasonic disperser TIP20 (Nippon Seiki Seisakusho make, OUTPUT: 8, TUNING: 5), and the sample for a measurement was prepared.
Powder characteristic values (Dmin, D10, D50, D90, Dmax) of this measurement sample were measured using a laser diffraction / scattering particle size distribution analyzer MT3300 (manufactured by Nikkiso).

<Measurement of specific surface area>
The specific surface area was measured by the BET 1-point method with a monosorb manufactured by Yuasa Ionics.

Example 1
12.6 g of silver nitrate was dissolved in 0.8 L of pure water, 24 mL of 25% ammonia water and 40 g of ammonium sulfate were added to prepare an aqueous silver ammine complex salt solution (silver concentration 10 g / L, NH ₃ / Ag ⁺ molar ratio). 12, 20 ° C., pH 9.4).
Using this silver ammine complex salt aqueous solution as an electrolytic solution, a DSE electrode plate is used for both the anode and the cathode, electrolysis is performed at a current density of 200 A / m ² and a solution temperature of 20 ° C., and silver particles electrodeposited by a scraper at appropriate intervals The electrode plate was scraped off and electrolyzed for 1 hour.
Thereafter, the silver powder obtained by scraping off was filtered with Nutsche, washed with pure water and further with alcohol, and dried in an air atmosphere at 70 ° C. for 12 hours to obtain a dendritic silver powder.

  A scanning electron microscope (SEM) observation image of the obtained silver powder is shown in FIG. Table 1 shows the powder characteristics obtained by measuring the obtained silver powder with a laser diffraction / scattering particle size distribution analyzer.

(Example 2)
1.26 g of silver nitrate was dissolved in 0.8 L of pure water, 2.4 mL of 25% aqueous ammonia and 4 g of ammonium sulfate were added to prepare an aqueous silver ammine complex salt solution (silver concentration: 1 g / L, NH ₃ / Ag ⁺ Molar ratio 12, 20 ° C., pH 9.4).
Using this silver ammine complex salt aqueous solution as an electrolytic solution, a DSE electrode plate is used for both the anode and the cathode, electrolysis is performed at a current density of 200 A / m ² and a solution temperature of 20 ° C., and silver particles electrodeposited by a scraper at appropriate intervals The electrode plate was scraped off and electrolyzed for 1 hour.
Thereafter, the silver powder obtained by scraping off was filtered with Nutsche, washed with pure water and further with alcohol, and dried in an air atmosphere at 70 ° C. for 12 hours to obtain a dendritic silver powder.

  Table 1 shows the powder characteristics obtained by measuring the obtained silver powder with a laser diffraction / scattering particle size distribution analyzer.

(Example 3)
12.6 g of silver nitrate was dissolved in 0.8 L of pure water, 24 mL of 25% ammonia water and 40 g of ammonium sulfate were added to prepare an aqueous silver ammine complex salt solution (silver concentration 10 g / L, NH ₃ / Ag ⁺ molar ratio). 12, 20 ° C., pH 9.4).
Using this silver ammine complex salt aqueous solution as an electrolytic solution, a DSE electrode plate is used for both the anode and the cathode, electrolysis is performed at a current density of 700 A / m ² and a solution temperature of 20 ° C., and silver particles electrodeposited by a scraper at appropriate intervals The electrode plate was scraped off and electrolyzed for 1 hour.
Thereafter, the silver powder obtained by scraping off was filtered with Nutsche, washed with pure water and further with alcohol, and dried in an air atmosphere at 70 ° C. for 12 hours to obtain a dendritic silver powder.

  Table 1 shows the powder characteristics obtained by measuring the obtained silver powder with a laser diffraction / scattering particle size distribution analyzer.

Example 4
12.6 g of silver nitrate was dissolved in 0.8 L of pure water, 24 mL of 25% aqueous ammonia and 40 g of ammonium sulfate were added to prepare an aqueous silver ammine complex salt solution (silver concentration 10 g / L, NH ₃ / Ag ⁺ molar ratio). 12, 50 ° C., pH 9.4).
Using this silver ammine complex salt aqueous solution as an electrolytic solution, a DSE electrode plate is used for both the anode and the cathode, electrolysis is performed at a current density of 200 A / m ² and a solution temperature of 50 ° C., and silver particles electrodeposited by a scraper at appropriate intervals The electrode plate was scraped off and electrolyzed for 1 hour.
Thereafter, the silver powder obtained by scraping off was filtered with Nutsche, washed with pure water and further with alcohol, and dried in an air atmosphere at 70 ° C. for 12 hours to obtain a dendritic silver powder.

  Table 1 shows the powder characteristics obtained by measuring the obtained silver powder with a laser diffraction / scattering particle size distribution analyzer.

(Example 5)
12.6 g of silver nitrate was dissolved in 0.8 L of pure water, 24 mL of 25% aqueous ammonia and 40 g of ammonium sulfate were added, and gelatin was further added at a rate of 0.1 g / L to prepare a silver ammine complex salt aqueous solution. (Silver concentration 10 g / L, NH ₃ / Ag + molar ratio 12, 20 ° C., pH 9.4).
Using this silver ammine complex salt aqueous solution as an electrolytic solution, a DSE electrode plate is used for both the anode and the cathode, electrolysis is performed at a current density of 200 A / m ² and a solution temperature of 20 ° C., and silver particles electrodeposited by a scraper at appropriate intervals The electrode plate was scraped off and electrolyzed for 1 hour.
Thereafter, the silver powder obtained by scraping off was filtered with Nutsche, washed with pure water and further with alcohol, and dried in an air atmosphere at 70 ° C. for 12 hours to obtain a dendritic silver powder.

  Table 1 shows the powder characteristics obtained by measuring the obtained silver powder with a laser diffraction / scattering particle size distribution analyzer.

The silver powder obtained in Example 1 was a fine silver powder having an average particle size of 10 μm or less, and as seen in FIG. 1, all the particles had a uniform dendrite shape.
Although the silver powder obtained in Example 2 had a smaller precipitation amount than the silver powder obtained in Example 1, almost the same dendritic fine silver powder was obtained. However, when considering production efficiency, it is considered that the silver concentration is preferably 1 g / L or more.
Compared with the silver powder obtained in Example 1, the silver powder obtained in Example 3 had an increased precipitation amount, and almost the same dendritic fine silver powder was obtained. However, caution is required because the liquid temperature rises due to the increase in current density, and the shape tends to vary, and ammonia tends to volatilize.
As for the silver powder obtained in Example 4, dendritic fine silver powder almost similar to the silver powder obtained in Example 1 was obtained. However, when the electrolysis temperature is raised, the shape tends to vary, and ammonia tends to volatilize.
The silver powder obtained in Example 5 was significantly smaller in particle size than the silver powder obtained in Example 1.

2 is a scanning electron microscope (SEM, magnification 5000 times) observation image of the silver powder obtained in Example 1. FIG.

Claims (3)

A silver ammine complex salt aqueous solution prepared by adding an ammonium salt together with ammonia to a silver nitrate solution, wherein an electrolysis is performed using a silver ammine complex salt aqueous solution having an NH ₃ / Ag molar ratio adjusted to 2 to 20 as an electrolyte. And D50 exhibiting a dendritic shape: a method for producing fine silver powder of 5 μm to 10 μm.     2. The method for producing fine silver powder according to claim 1, wherein the silver concentration in the silver ammine complex salt aqueous solution is adjusted to be larger than 1 g / L and smaller than 30 g / L. The method for producing fine silver powder according to claim 1 or 2, wherein the pH of the aqueous silver ammine complex salt solution is adjusted to be higher than 5 and lower than 10.

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