JP7175218B2 - Silver powder and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silver powder and a method for producing the same, and more particularly to a silver powder suitable for use in conductive pastes forming electrodes and circuits of electronic components such as solar cells and touch panel substrates, and a method for producing the same.

Conventionally, as a method of forming electrodes and circuits of electronic parts, after forming a firing-type conductive paste produced by adding silver powder together with glass frit into an organic vehicle and kneading them into a predetermined pattern on a substrate, , a method in which organic components are removed by heating at a temperature of 500° C. or higher and silver particles are sintered together to form a conductive film is widely used.

The silver powder for conductive paste used in such a method can be used to increase the density of conductor patterns and fine lines due to the miniaturization of electronic components, and to increase power generation efficiency by increasing the light collecting area of solar cells. In order to improve the performance, it is required that the grain size is moderately small and the grain size is uniform so as to correspond to the fine line of the finger electrode. In addition, there is a demand for a silver powder suitable for use in a conductive paste that can form conductive patterns and electrodes that allow electricity to flow efficiently even if the cross-sectional areas of the conductive patterns and electrodes are reduced due to finer lines. Therefore, a silver powder that can be heated at a lower temperature to sinter the silver particles is desired.

As a method for producing such silver powder for conductive paste, a wet reduction method is known in which silver powder is reduced and precipitated by adding a reducing agent to an aqueous reaction system containing silver ions (see, for example, Patent Document 1). ).

JP-A-8-176620 (paragraph number 0008-0013)

However, when silver powder having a particle size similar to that of silver powder produced by a conventional wet reduction method is used in a fired conductive paste, it may not be possible to form a conductive film with a low volume resistivity.
Therefore, in view of such conventional problems, the present invention has a particle size similar to that of silver powder produced by a conventional wet reduction method, and has a volume resistivity when used in a fired conductive paste. An object of the present invention is to provide a silver powder capable of forming a low conductive film and a method for producing the same.

As a result of intensive research to solve the above problems, the present inventors added a reducing agent to a silver complex aqueous solution containing benzoic acid to reduce and precipitate silver particles, thereby producing by a conventional wet reduction method. The inventors have found that it is possible to produce a silver powder that has a particle size similar to that of silver powder and that can form a conductive film with low volume resistivity when used in a sintered conductive paste, and has completed the present invention. rice field.

That is, the method for producing silver powder according to the present invention is characterized by adding a reducing agent to a silver complex aqueous solution containing benzoic acid to reduce and deposit silver particles. In this method for producing silver powder, the aqueous silver complex solution is preferably an aqueous silver ammine complex solution. In this case, the amount of benzoic acid in the silver ammine complex aqueous solution is preferably 0.05 to 6% by mass based on silver. Moreover, it is preferable to add a surface treating agent after reducing and depositing silver particles.

Further, the silver powder according to the present invention is characterized by containing benzoic acid inside the particles. The amount of benzoic acid contained inside the particles of this silver powder is preferably 0.001 to 1% by mass relative to silver. Further, the silver powder preferably has an average particle diameter _D50 of 0.3 to 5 μm as measured by a laser diffraction method, and preferably has a BET specific surface area of 0.1 to 3 m ² /g.

Also, a conductive paste according to the present invention is characterized by containing the above silver powder and an organic vehicle.

In the present specification, "average particle diameter _D50 by laser diffraction method" refers to volume-based cumulative 50% particle diameter ( _D50 ) measured by a laser diffraction particle size distribution device.

According to the present invention, silver powder that has a particle size similar to that of silver powder produced by a conventional wet reduction method and that can form a conductive film with low volume resistivity when used in a fired conductive paste is produced. can do.

In an embodiment of the method for producing silver powder according to the present invention, a reducing agent is added to an aqueous silver complex solution containing benzoic acid to reduce and deposit silver particles.

The silver complex aqueous solution is prepared by adding ammonia water or an ammonium salt (ammonium ion NH ₄ ⁺ and anions such as chloride ion Cl ⁻ , nitrate ion NO ₃ ⁻ , sulfate ion SO ₄ ²⁻ to silver nitrate aqueous solution or silver chloride suspension). salt). In order to make the silver powder have an appropriate particle size (average particle size _D50 measured by laser diffraction method is 0.3 to 5 μm) in these silver complex aqueous solutions, ammonia water is added to the silver nitrate aqueous solution. It is preferred to use the resulting silver ammine complex aqueous solution (an aqueous solution of a silver ammine complex represented by the chemical formula [Ag(NH ₃ ) ₂ ] ⁺ ). When an aqueous silver ammine complex solution is used as the aqueous silver complex solution, if the silver concentration in the aqueous silver ammine complex solution is less than 0.1% by mass, the amount of silver powder that can be produced in one reaction is too small. If it exceeds 10% by mass, the viscosity of the reaction solution after precipitation of the silver particles may increase and the reaction solution may not be stirred uniformly. .1 to 10% by mass. Since the coordination number of ammonia in the silver ammine complex is 2, 2 mol or more (preferably 3 mol or more) of ammonia is added per 1 mol of silver. On the other hand, if the amount of ammonia added is too large, the complex becomes too stable and the reduction becomes difficult to proceed. Incidentally, if adjustments such as increasing the amount of the reducing agent added are performed, even if the amount of ammonia added exceeds 8 mol, the appropriate particle size (average particle size _D50 by laser diffraction method is 0.3 to 5 μm) can be obtained. It is possible to obtain silver dust.

The silver complex aqueous solution containing benzoic acid may be any silver complex aqueous solution containing benzoate ions (C ₆ H ₅ COO ⁻ ), and the silver complex aqueous solution and benzoic acid (C ₆ H ₅ COOH) or benzoate (for example, It can be obtained by mixing sodium benzoate (C ₆ H ₅ COONa) or ammonium benzoate (C ₆ H ₅ COONH ₄ ) (or aqueous solution of benzoic acid or benzoate).

The amount of benzoic acid in the silver ammine complex aqueous solution containing benzoic acid is preferably 0.05 to 6% by mass, more preferably 0.1 to 4% by mass, more preferably 0.2%, based on silver. Most preferred is ~2% by weight. When the amount of benzoic acid in the silver ammine complex aqueous solution containing benzoic acid is less than 0.05% by mass, the obtained conductive paste containing silver powder is used to form a conductive film with sufficiently low volume resistivity. If it exceeds 6% by mass, the viscosity of the obtained conductive paste containing silver powder becomes too high, so it is necessary to dilute the conductive paste, and the silver concentration of the conductive paste becomes low. A conductive film formed as a wiring may break.

Further, the aqueous silver ammine complex solution containing benzoic acid is preferably alkaline (pH is preferably higher than 7, more preferably 8 or higher), and an alkali such as sodium hydroxide is added as a pH adjuster. It is preferable to adjust the alkalinity by

The reducing agent may be any reducing agent that causes silver particles to be reduced and precipitated, and examples thereof include ascorbic acid, hydrogen peroxide solution, formic acid, tartaric acid, hydroquinone, pyrogallol, glucose, gallic acid, formaldehyde, hydrazine, hydrazine compounds, and alkanolamine. can be used, and it is preferred to use formaldehyde, hydrazine or a hydrazine compound. By using such a reducing agent, it is possible to obtain silver powder having an appropriate particle size (average particle size _D50 measured by laser diffraction method is 0.3 to 5 μm). The amount of the reducing agent added is preferably 1 equivalent or more relative to silver in order to increase the yield of silver. For example, it may be 10-20 equivalents.

As for the method of adding the reducing agent, it is preferable to add the reducing agent at a speed of 1 equivalent/minute or more in order to prevent aggregation of the silver powder. Moreover, during the reduction, it is preferable to stir the reaction solution so that the reaction can be completed in a shorter time. The temperature during the reduction reaction is preferably 5 to 80°C, more preferably 5 to 40°C.

Moreover, it is preferable to deposit the silver particles by reduction with a reducing agent, and then add the surface treatment agent so that the surface treatment agent adheres to the surface of the silver particles. Fatty acids, fatty acid salts, surfactants, organometallic compounds, chelating agents, polymer dispersants, and the like can be used as the surface treatment agent. Propionic acid, caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, acrylic acid, oleic acid, linoleic acid, arachidonic acid, ricinoleic acid and their salts and emulsions are used as fatty acids and fatty acid salts. be able to. As chelating agents, azoles such as benzotriazole or salts thereof, succinic acid, malonic acid, glutaric acid, adipic acid and the like can be used.

It is preferable to perform solid-liquid separation of the silver-containing slurry obtained by reducing and depositing the silver particles, and wash the resulting solid matter with pure water to remove impurities in the solid matter. The end point of this washing can be determined by the electric conductivity of the water after washing, and it is preferable to wash until the electric conductivity becomes 0.5 mS/m or less.

Since the lumpy cake obtained after this washing contains a lot of water, it is preferable to obtain dried silver powder with a dryer such as a vacuum dryer. The drying temperature is preferably 100° C. or less in order to prevent the silver powder from sintering during drying.

In addition, the obtained silver powder may be subjected to dry pulverization treatment or classification treatment. Instead of this crushing, the silver powder is put into a device that can mechanically fluidize the particles, and the particles of the silver powder are mechanically collided with each other to remove unevenness and angular portions on the surface of the silver powder. A smoothing surface smoothing treatment may be performed. Classification may be performed after pulverization or smoothing. Drying, pulverizing and classifying may be performed using an integrated device capable of drying, pulverizing and classifying.

An embodiment of the silver powder according to the present invention can be produced by the method for producing silver powder described above. Embodiments of silver powder according to the present invention contain benzoic acid inside the particles. The amount of benzoic acid contained inside the particles of this silver powder is preferably 0.001 to 1% by mass relative to silver. In addition, the average particle diameter (volume-based cumulative 50% particle diameter) _D50 of silver powder measured by the laser diffraction method is expected to increase the density and fineness of conductor patterns due to the miniaturization of electronic components when silver powder is used in conductive pastes. It is preferably 0.3 to 5 μm and 0.5 μm so as to correspond to line formation and to correspond to fine line formation of finger electrodes in order to increase the light collecting area of the solar cell and improve the power generation efficiency. More preferably ~3 μm. Also, the BET specific surface area of the silver powder is preferably 0.1 to 3 m ² /g, more preferably 0.2 to 2 m ² /g. If the BET specific surface area is less than 0.1 m ² /g, the particle size of the silver powder becomes large, and when wiring or the like is drawn using such a large silver powder in the conductive paste, it becomes difficult to draw fine wiring. , the viscosity of the conductive paste becomes too high if it is greater than 3 m ² /g, and thus the conductive paste must be diluted. sometimes.

Examples of the silver powder and the method for producing the same according to the present invention will now be described in detail.

[Example 1]
To 3500 g of an aqueous solution of silver nitrate containing 49 g of silver, 155 g of industrial ammonia water having a concentration of 28% by mass (5.6 molar equivalents of ammonia per 1 mol of silver) was added to obtain an aqueous silver ammine complex solution. To this silver ammine complex aqueous solution, 6 g of a sodium hydroxide aqueous solution having a concentration of 20% by mass and 5.31 g of a sodium benzoate aqueous solution containing 4.7% by mass of benzoic acid (0.34 g of 20% by mass of sodium hydroxide are added 0.25 g of benzoic acid was added to an alkaline aqueous solution dissolved in 4.72 g of water, and an aqueous solution of sodium benzoate (0.50% by mass of benzoic acid with respect to silver) was added and dissolved completely by ultrasonic waves. was adjusted to 20° C., 380 g of an aqueous formaldehyde solution having a concentration of 23 mass % (as a reducing agent) was added with stirring to obtain a slurry containing silver particles. To this slurry, 6.5 g of a 1.6% by weight stearic acid aqueous solution (as a surface treatment agent) was added, and after sufficient stirring, the stirring was stopped to allow the silver particles to settle. The liquid in which the silver particles precipitated was filtered, washed with water until the electrical conductivity of the liquid after passing water became 0.5 mS/m or less, dried, and pulverized to obtain silver powder. The pH of the filtrate after filtration was 8.5.

The BET specific surface area of the silver powder thus obtained was measured using a BET specific surface area measuring device (Macsorb HM-model 1210 manufactured by Mountec Co., Ltd.) in the measuring device at 60 ° C. After deaeration by flowing a Ne—N ₂ mixed gas containing nitrogen, the BET specific surface area was 0.76 m ² /g when measured by the BET one-point method.

In addition, the particle size distribution of the obtained silver powder was measured by a laser diffraction particle size distribution analyzer (Microtrac MT3300EXII manufactured by Nikkiso Co., Ltd.) using isopropanol (IPA) as a dispersion medium. The % particle size ( _D10 ) was 1.1 μm, the cumulative 50% particle size ( _D50 ) was 1.6 μm, the cumulative 90% particle size ( _D90 ) was 2.5 μm, and the _D90 / _D50 ratio was 1. .6.

In addition, 8 mL of ultrapure water and 4 mL of nitric acid (precise analysis nitric acid (concentration 60 to 61% by mass) manufactured by Kanto Chemical Co., Ltd.) were added to 1.0 g of the obtained silver powder, and thermally decomposed at 70 ° C. in a water bath. Then, add 10 mL of ultrapure water and 1.5 mL of hydrochloric acid (hydrochloric acid for precision analysis manufactured by Kanto Chemical Co., Ltd. (concentration 35 to 37% by mass)) and stir, and when the liquid becomes transparent, allow to cool for 20 minutes. After that, it was filtered, the volume was adjusted to 100 mL with ultrapure water, and the volume was further diluted 100 times with ultrapure water. When analyzed by heavy pole LC/MS), it was confirmed that 0.34% by mass of benzoic acid in total was present on the surface and inside of the silver powder.

Further, to 1 g of the obtained silver powder, 20 mL of hydrochloric acid aqueous solution obtained by mixing hydrochloric acid (hydrochloric acid for precision analysis (concentration 35 to 37% by mass) manufactured by Kanto Kagaku Co., Ltd.) and ultrapure water at a volume ratio of 1:1 was added. Extract by heating at 70 ° C. in a water bath, allow to cool for 20 minutes, separate solid and liquid by filtration, and add the above 1:1 hydrochloric acid aqueous solution to a constant volume of 50 mL. When diluted 1000 times with pure water and analyzed by the above liquid chromatograph mass spectrometer (LC/MC), silver does not dissolve in hydrochloric acid, so 0.33% by mass of benzoic acid is present on the surface of the silver powder. confirmed to exist.

In addition, 8 mL of ultrapure water and 4 mL of nitric acid (nitric acid for precision analysis manufactured by Kanto Chemical Co., Ltd. (concentration 60 to 61% by mass)) were added to the residue obtained by the above solid-liquid separation, and the mixture was heated to 70°C in a water bath. Decompose by heating, add 10 mL of ultrapure water and 1.5 mL of hydrochloric acid to the resulting solution, stir, heat at 70°C in a water bath until the solution becomes transparent, allow to cool for 20 minutes, and then filter. , A constant volume of 100 mL with ultrapure water, further diluted 100 times with ultrapure water, and analyzed by the above liquid chromatograph mass spectrometer (LC / MC), found that 0.004% by mass inside the silver powder was confirmed to contain benzoic acid.

In addition, 27.0 g of the obtained silver powder and 3.3 g of Texanol (CS-12 manufactured by JNC Co., Ltd.) were kneaded with a propeller-less rotary-revolution stirring and degassing device (AR250 manufactured by Thinky Co., Ltd.), followed by three rolls. A conductive paste was obtained by kneading with (80S manufactured by EXAKT). This conductive paste was printed on the surface of a silicon substrate in a line shape of width 250 μm×length 55 mm using a screen printer (MT-320T manufactured by Microtec Co., Ltd.), and dried at 200° C. for 10 minutes using a hot air dryer. After heating for 10 minutes, it was fired at a peak temperature of 770° C. with an in-out time of 22.9 seconds in a high-speed IR furnace (high-speed firing test 4-chamber furnace manufactured by NGK INSULATORS, LTD.). For the conductive film thus obtained, the average thickness (from one end to the other end in the length direction) was measured with a surface roughness/contour shape measuring machine (Surfcom 480B-12 manufactured by Tokyo Seimitsu Co., Ltd.). The average thickness was 16.1 μm, and the resistance value was 0.289Ω when measured with a digital multimeter (R6551 manufactured by Advantest Co., Ltd.). Further, the volume resistivity of the conductive film was calculated (from this resistance value and the volume obtained from the film thickness, line width and length) to be 2.11 μΩ·cm.

[Comparative Example 1]
Instead of the aqueous sodium benzoate solution, 5.12 g of an aqueous benzotriazole sodium solution containing 4.8% by mass of benzotriazole (0.61 g of 40% by mass of benzotriazole sodium was added to 4.51 g of pure water, and the mixture was completely ultrasonicated. Silver powder was obtained in the same manner as in Example 1, except that an aqueous benzotriazole sodium solution (0.5% by mass of benzotriazole relative to silver) was used.

When the BET specific surface area and particle size distribution of the silver powder thus obtained were measured in the same manner as in Example 1, the BET specific surface area was 0.56 m ² /g, and the volume-based cumulative 10% particle diameter ( D ₁₀ ) was 1.2 µm, the cumulative 50% particle size (D ₅₀ ) was 2.0 µm, the cumulative 90% particle size (D ₉₀ ) was 3.3 µm, and the D ₉₀ /D ₅₀ ratio was 1.7. rice field.

Also, using the obtained silver powder, a conductive film was produced by the same method as in Example 1, and the average thickness and resistance value thereof were measured, and the volume resistivity was calculated. 5 μm, the resistance value was 0.307Ω, and the volume resistivity was 2.44 μΩ·cm.

[Comparative Example 2]
Instead of the sodium benzoate aqueous solution, 5.88 g of an L-lysine aqueous solution containing 5.3% by mass of L-lysine (0.31 g of L-lysine was added to 5.57 g of pure water and completely dissolved with ultrasonic waves. A silver powder was obtained in the same manner as in Example 1, except that an aqueous solution of L-lysine (0.6% by mass of L-lysine relative to silver) was used.

When the BET specific surface area and particle size distribution of the silver powder thus obtained were measured in the same manner as in Example 1, the BET specific surface area was 0.53 m ² /g, and the volume-based cumulative 10% particle diameter ( D ₁₀ ) was 1.2 μm, the cumulative 50% particle size (D ₅₀ ) was 2.1 μm, the cumulative 90% particle size (D ₉₀ ) was 3.6 μm, and the D ₉₀ /D ₅₀ ratio was 1.7. rice field.

Also, using the obtained silver powder, a conductive film was produced in the same manner as in Example 1, the average thickness and resistance value thereof were measured, and the volume resistivity was calculated. It had a thickness of 9 μm, a resistance value of 0.321Ω, and a volume resistivity of 2.32 μΩ·cm.

Table 1 shows the properties of the silver powders obtained in these examples and comparative examples.

The silver powder according to the present invention can be used for producing a conductive paste as a silver powder that can be fired at a lower temperature. It can be used for electrodes and circuits of electronic parts such as chip parts and touch panels, as well as electromagnetic wave shielding materials.

Claims (4)

A method for producing silver powder, comprising adding a reducing agent to an aqueous silver complex solution containing benzoic acid to reduce and deposit silver particles. 2. The method for producing silver powder according to claim 1, wherein the aqueous silver complex solution is an aqueous silver ammine complex solution. 3. The method for producing silver powder according to claim 2, wherein the amount of benzoic acid in the silver ammine complex aqueous solution is 0.05 to 6% by mass relative to silver. 4. The method for producing silver powder according to any one of claims 1 to 3, wherein a surface treatment agent is added after the silver particles are reduced and precipitated.