JP2005105376A - Silver fine particle and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide silver fine particles which have a small particle diameter and a shape of a high aspect ratio, can be fused and sintered at low temperatures and can form an electrically conductive thin film having excellent optical transparency and electrical conductivity, and to provide a method of relatively easily producing the silver fine particles. <P>SOLUTION: Each of the silver fine particles is almost a planar particle having two principal planes, and has a thickness of ≤50 nm and a major axis of ≤5,000 nm. In the method of producing the silver fine particles, a solution obtained by dissolving at least a polymer compound, a reducing agent and a silver salt is stirred at 25 to 60°C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to silver fine particles and a method for producing the same.

Conventionally, metal powders such as gold, silver, copper, and aluminum have high conductivity and metallic luster, and are used as conductive fillers. For example, it is mixed with an organic binder and used as a functional thin film forming paint such as a conductive paint, a conductive paste, a reflective film forming paint, and a metallic gloss film forming paint.
In particular, although approximately spherical silver particles have been widely used as the conductive filler powder, the contact area is small because the particles are in point contact. For this reason, when it is used as a conductive paste and printed on a base material in a predetermined pattern and then heat-treated to form a patterned film, the contact area between particles is small due to the small contact area between the particles. In order to obtain the required conductivity, there is a problem that the conductive filler powder must be added at a high concentration in order to obtain the necessary conductivity.

Therefore, silver particles having a high aspect ratio (major axis / thickness) such as a plate shape and a wire shape and allowing the particles to come into surface contact have been put to practical use. However, since the particle shape is deformed by mechanically compressing silver powder or the like, the produced silver particles have a large major axis of 10 μm or more and 15 μm or less, and are thick and flaky particles. End up.
For this reason, in order to obtain high electroconductivity, it was necessary to heat-process at the temperature of 600 degreeC or more, and to fuse | melt and sinter particles. Accordingly, there is a problem that sufficient conductivity cannot be obtained when it is desired to develop conductivity by heat treatment at a low temperature of 200 ° C. or lower. In addition, since the silver particles have a large major axis and a large thickness, it is difficult to form a thin conductive film when using the silver particles.

In addition, when forming a transparent electrode using a conductive filler, if almost spherical silver particles are used as the conductive filler, it is necessary to fill the silver particles at a high concentration in order to obtain high conductivity. It will decline. In addition, when the flaky silver particles are used as the conductive filler, since the particle shape is plate-like, high conductivity can be obtained at a low concentration compared to the spherical shape, but the particle thickness is large, and thus the permeability is high. There is a problem inferior to
As described above, when a transparent electrode is formed using substantially spherical or flaky silver particles as the conductive filler, it is currently impossible to achieve both light transmittance and conductivity.

In order to solve the above-mentioned problems, fine plate-like silver particles have been proposed (see Non-Patent Document 1). However, since silver particles are produced by a photochemical reduction method using ultraviolet rays, it is difficult to control the progress of the reaction, which is not practical. For this reason, the proposed silver particles are actually irregular in most particle shapes and partially include spherical particles, and a sufficient effect is not obtained.
As a method for synthesizing a conductive filler powder composed of fine plate-like particles, a method for producing plate-like crystals using Ag—Pd colloid as a crystal nucleus has been proposed (see Patent Document 1). However, the production method is complicated, and the particle size of the obtained plate-like particles is as large as 5 μm or more and 10 μm or less, and a sufficient effect is not obtained.
Science, 2001, 294, p. 1901-1903 JP-A-11-106806

  The object of the present invention has been made in view of the above-mentioned circumstances, has a shape with a small particle size and a large aspect ratio, can fuse and sinter particles at a low temperature, and has light transmission and conductivity. An object of the present invention is to provide a silver fine particle capable of forming an excellent conductive thin film and a method for producing the silver fine particle relatively easily.

The silver fine particles according to the present invention are substantially plate-like particles having two main surfaces, wherein the thickness of the particles is 50 nm or less and the major axis is 5000 nm or less.
Thereby, the reactivity of the particle surface can be increased, and since it is plate-like, most of the highly reactive particle surface is in contact with the adjacent particle surface. For this reason, even at a relatively low temperature of 200 ° C. or less, the particles can be fused and sintered.

In the silver fine particle structure, the main surface has a shape selected from a substantially triangular shape, a substantially pentagonal shape, and a substantially hexagonal shape.
Silver can be easily produced because it easily grows into the above-described particle shape, and the particle shape can be stably maintained, and can be stored for a long time.

In the structure of such silver fine particles, the aspect ratio, which is the ratio of the major axis to the thickness of the particles, is 3 or more.
Thereby, the orientation of the particles is improved, and the particles can be easily filled by overlapping the main surfaces in the thickness direction, thereby further increasing the contact area and improving the conductivity. Can do.

The method for producing silver fine particles according to the present invention is characterized in that a solution obtained by dissolving at least a polymer compound, a reducing agent, and a silver salt is stirred at a temperature of 25 ° C. or more and 60 ° C. or less.
As a result, any of the compounds used is inexpensive and available in large quantities, is easy to handle, has a simple manufacturing process, and can easily control manufacturing conditions. For this reason, plate-like silver fine particles having a thickness of 50 nm or less and a major axis of 5000 nm or less can be easily produced at a relatively low cost.

In the structure of the method for producing such silver fine particles, as the polymer compound, starch, sodium carboxymethyl cellulose, hydroxyethyl cellulose, gelatin, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylamide, polyethyleneimine, polycarboxylic acid, polycarboxylic acid ammonium salt, It is characterized by using at least one selected from alkali metal salts of polycarboxylic acid, polyacrylic acid, ammonium polyacrylate, alkali metal polyacrylate, and pectin.
As a result, in the solution, the silver ions are complexed with the polymer compound or are fixed to the inside or the surface of the polymer compound and uniformly dispersed in the solution. For this reason, silver ions can be reduced in the solution at a substantially uniform reduction rate.

In the configuration of the silver fine particle production method, as the reducing agent, ascorbic acid, ascorbic acid alkali metal salt, gentisic acid, hydroquinone, aminophenols, dihydroxynaphthalene, amidol, metol, phenidone, gallic acid, protocatechuic acid, pyrogallol , Using at least one selected from aminonaphthalene.
Thereby, silver ions can be reduced at an appropriate reduction rate, and plate-like silver fine particles can be produced.

According to the silver fine particles of the present invention, the particles can be fused and sintered even at a relatively low temperature of 200 ° C. or less, and the conductive material has high conductivity and good adhesion to the substrate. Conductive thin film can be manufactured.
Moreover, when forming a transparent electrode using the silver fine particle of this invention, since it is fine particle, a film thickness can be made thin and high translucency is realizable. For this reason, the transparent electrode which has high electroconductivity and favorable adhesiveness with a board | substrate as mentioned above, and has high translucency can be formed.

  Moreover, according to the method for producing silver fine particles of the present invention, all of the compounds used are inexpensive and available in large quantities, and are easy to handle. In addition, the manufacturing process is simple and the manufacturing conditions can be easily controlled. For this reason, plate-like silver fine particles having a thickness of 50 nm or less and a major axis of 5000 nm or less can be easily produced at a relatively low cost.

The silver fine particles of the present invention are substantially plate-like particles having two main surfaces.
By making the particle shape substantially plate-like, the particles are in surface contact with each other, so that the contact area can be made larger than that of the spherical particles. When the silver fine particles are heated, the contact portions between the particles are fused and sintered to form a conduction path. Therefore, in the silver fine particles of the present invention, many conduction paths are formed and the conductivity can be improved. .
Moreover, it is preferable that the main surface of the substantially plate-like particle has a shape selected from a substantially triangular shape, a substantially pentagonal shape, and a substantially hexagonal shape. Silver can be easily produced because it easily grows into the above-described particle shape, and the particle shape can be stably maintained, and can be stored for a long time.
In addition, in the above-mentioned particle shape, you may contain a part of particle | grains of the indefinite shape from which the corner | angular part and the side were partly missing.

Further, the thickness of the silver fine particles is 50 nm or less, and the major axis is 5000 nm or less. The thickness of the silver fine particles is preferably 1 nm or more and 50 nm or less, more preferably 1 nm or more and 30 nm or less. The major axis is preferably 5 nm or more and 1000 nm or less, more preferably 10 nm or more and 200 nm or less.
Here, in this specification, as shown in FIG. 1, the particle diameter in the direction perpendicular to the main surface of the particle is defined as the particle thickness. In addition, when the outline of the main surface of the particle is sandwiched between two parallel lines, the minimum interval between the two parallel lines is defined as the minor axis, and between the two intersections of the line segment and the outline perpendicular to the minor axis. The maximum distance is defined as the major axis.
Examples of the method for measuring the thickness or major axis of the particles include a method of observing the particle shape with a transmission electron microscope (TEM) and measuring the particle size.
It should be noted that the thickness and major axis of almost all of the silver particles only need to be within the above-described range, and even if a very small amount of silver particles having a thickness or major axis outside the above-described range is contained, the effects of the present invention are hindered. There is nothing.

In general, it is known that the surface activity (surface reactivity) increases exponentially as the particle size of the metal particles decreases. When the silver fine particles have a thickness of 50 nm or less and a major axis of 5000 nm or less, the reactivity of the particle surface can be enhanced. In particular, since the silver fine particles are plate-like, most of the highly reactive particle surfaces are in contact with the adjacent particle surfaces.
For this reason, even at a relatively low temperature of 200 ° C. or less, the particles can be fused and sintered.

As described above, for example, when silver fine particles are applied as a conductive paste on a substrate such as glass, and then sintered by heat treatment to form a conductive thin film such as a wiring pattern, at a relatively low temperature of 200 ° C. or less. Even in such a case, the particles can be fused and sintered, and high conductivity can be obtained. Moreover, since the particle surface has high reactivity, the silver fine particles can be firmly adhered to the substrate by heat treatment. Therefore, high electrical conductivity and good adhesion with the substrate can be achieved.
In the case of an approximately spherical silver ultrafine particle (silver colloid) of a size of several tens of nanometers used as a conventional conductive filler, even if the particle diameter is small and the surface reactivity of the particle is high, the particles are in point contact, and the contact area Since it was difficult to increase the particle size, the particles could not be fused and sintered at a relatively low temperature of 200 ° C. or lower. On the other hand, according to the silver fine particles of the present invention, as described above, the particles can be fused and sintered even at a low temperature, and the silver colloid used as a conventional conductive filler can be heated at a low temperature. High conductivity that was not obtained and adhesion to the substrate can be realized.

  In addition, since the silver fine particles are as thin as 50 nm or less, when the transparent electrode is formed as the conductive thin film, the film thickness can be reduced and high translucency can be realized. For this reason, the transparent electrode which has high electroconductivity and favorable adhesiveness with a board | substrate as mentioned above, and has high translucency can be formed.

  Furthermore, by setting the thickness and major axis of the silver fine particles to the above-described ranges, excellent particle packing properties can be realized. For this reason, for example, when silver fine particles are kneaded with resin and applied to a substrate such as glass to form a functional thin film such as a reflective film or a conductive thin film, a thin film filled with silver fine particles is formed. In addition, a functional thin film having a smooth film surface and excellent reflection characteristics and conductivity can be realized.

  The aspect ratio (major axis / thickness) of the silver fine particles is preferably 3 or more, more preferably 3 or more and 20 or less. Thereby, the orientation of the particles is improved, and the particles are easily filled by overlapping the main surfaces in the thickness direction. For this reason, a contact area can be enlarged further and electroconductivity can be improved.

When the silver fine particle thickness is thicker than 50 nm, the reactivity of the particle surface is low. For example, when forming a conductive thin film by sintering, as in the case of using conventional flaky silver particles, A heating temperature of about 200 ° C. is not preferable because the particles cannot be fused and sintered, and thus high conductivity cannot be achieved.
In addition, when the major axis of the silver fine particles is longer than 5000 nm, the packing property of the particles is low. For example, when forming a functional thin film by kneading with a resin, the packing property of the particles in the thin film is low, and the surface of the film is smooth. This is not preferable because the properties, reflection characteristics, and conductivity are deteriorated.

Next, the method for producing silver fine particles of the present invention will be described below.
First, a polymer compound and a silver salt are dissolved in a solvent.
The polymer compound can be used without any particular limitation as long as it can be a ligand and can stably form a complex with silver ions, or can fix silver ions inside or on the surface, such as starch, Sodium carboxymethyl cellulose, hydroxyethyl cellulose, gelatin, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylamide, polyethyleneimine, polycarboxylic acid, polycarboxylic acid ammonium salt, polycarboxylic acid alkali metal salt, polyacrylic acid, polyacrylic acid ammonium salt, polyacrylic Examples thereof include at least one selected from acid alkali metal salts and pectin.
The silver salt is not particularly limited as long as it is soluble in the solvent to be used. For example, when the solvent is water, silver nitrate and silver perchlorate can be preferably used.
The solvent is not particularly limited as long as the polymer compound, silver salt, and reducing agent (described later) to be used are uniformly dissolved. For example, polar organic solvents such as ethyl alcohol, toluene and the like can be used. Nonpolar organic solvents, water and the like can be mentioned.

  The compounding ratio of the polymer compound and the silver salt is 3 parts by weight or more and 50 parts by weight or less, preferably 5 parts by weight or more and 40 parts by weight or less, with respect to 1 part by weight of silver. Preferably they are 10 to 30 weight part. Thus, the silver ions can be complexed with the polymer compound or fixed inside or on the surface of the polymer compound and uniformly dispersed in the solvent.

Next, a reducing agent is added and dissolved in the solution in which the polymer compound and the silver salt are dissolved.
As the reducing agent, weak organic reducing agents can be applied, for example, ascorbic acid, alkali metal ascorbate, gentisic acid, hydroquinone, aminophenols, dihydroxynaphthalene, amidol, methol, phenidone, gallic acid, protocatechuic acid , Pyrogallol, and at least one selected from aminonaphthalene.
Here, the weak organic reducing agent is capable of reducing noble metals such as Au, Ag, and Pd that have a low ionization tendency, but has a reducing power that cannot reduce base metals such as Cu, Ni, and Fe. It is a reducing agent consisting of organic compounds. By using such a reducing agent, silver ions can be reduced at an appropriate reduction rate, and plate-like silver fine particles can be produced.
The compounding ratio of the reducing agent and the silver salt is preferably 1 part by weight or more of the reducing agent with respect to 1 part by weight of silver. Thereby, silver ions can be sufficiently reduced.

As described above, a solution in which the polymer compound, the reducing agent, and the silver salt are dissolved is prepared, and the solution is stirred at 25 ° C. or more and 60 ° C. or less. Thereby, the silver ion in a solution is reduce | restored with a reducing agent, and silver fine particles are produced | generated.
By making the said solution into 25 degreeC or more and 60 degrees C or less, the reductive reaction of silver ion can fully be performed.
When silver ions are reduced, silver fine particles are produced as a colloidal dispersion. Although it can be applied to conductive materials as a dispersion, the produced colloidal dispersion is subjected to a normal washing method such as electrodialysis to remove by-products, and then a dispersion of high-purity silver fine particles Application to conductive materials is also recommended.

In the solution, the silver ions are complexed with the polymer compound, or are fixed inside or on the surface of the polymer compound and uniformly dispersed in the solution. When silver ions are reduced by the reducing agent in a state of being uniformly dispersed in the solution, the silver ions are reduced at a substantially uniform reduction rate in the solution. For this reason, there is no local rapid reduction reaction in the solution, and silver is generated by the reduction to grow crystals to form plate-like silver fine particles.
According to the production method of the present invention, all of the compounds used are inexpensive and available in large quantities, are easy to handle, the production process is simple, and the production conditions can be easily controlled. For this reason, plate-like silver fine particles having a thickness of 50 nm or less and a major axis of 5000 nm or less can be easily produced at a relatively low cost.

Furthermore, the main surface of the particles is substantially triangular by adjusting the polymer compound used, the type of reducing agent, the silver salt, the reducing agent, the compounding ratio of the polymer compound, the temperature of the solution during reduction, etc. Silver fine particles having a shape selected from a substantially pentagonal shape and a substantially hexagonal shape and having an aspect ratio of 3 or more can also be obtained.
In the present invention, the order in which the polymer compound, the reducing agent, and the silver salt are dissolved in the solvent is not particularly limited. First, the polymer compound and the silver salt are dissolved in the solvent, and the silver ions are converted into the polymer compound. It is preferable that the reducing agent is added after being complexed or fixed to the inside or surface of the polymer compound and uniformly dispersed in the solvent. As a result, silver ions can be reduced in a state where silver ions are uniformly dispersed, and silver ions can be reduced at a substantially uniform reduction rate in a solution, whereby silver fine particles having a uniform particle shape and particle shape can be produced.

EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited by these Examples.
[Example 1]
After dissolving 0.3 parts by weight of gelatin as a polymer compound in 100 parts by weight of water, 0.024 parts by weight of silver nitrate was added to and dissolved in the resulting aqueous solution, and the liquid temperature was maintained at 60 ° C. Ascorbic acid was added in an amount of 0.03 part by weight, and the reaction was completed while stirring for 2 hours.

[Example 2]
Silver fine particles were produced in the same manner as in Example 1 except that hydroxyethyl cellulose was used as the polymer compound.
[Example 3]
Silver fine particles were produced in the same manner as in Example 1 except that hydroquinone was used as the reducing agent.
[Example 4]
Silver fine particles were produced in the same manner as in Example 1 except that silver perchlorate was used as the silver salt.

[Example 5]
Silver fine particles were produced in the same manner as in Example 1 except that ethyl alcohol was used as a solvent.
[Example 6]
Silver fine particles were produced in the same manner as in Example 1 except that the solution in which the polymer compound, the reducing agent and the silver salt were dissolved was stirred at 25 ° C.

[Comparative Example 1]
Silver fine particles were produced in the same manner as in Example 1 except that agar was used as the polymer compound.
[Comparative Example 2]
Silver fine particles were produced in the same manner as in Example 1 except that sodium borohydride was used as the reducing agent.

About the manufactured silver fine particle, the crystal structure and the shape of particle | grains were observed with the transmission electron microscope (TEM), and also the magnitude | size of particle | grains was measured.
Further, with respect to the silver fine particles produced in Example 1 and Comparative Example 1, 95 parts by weight of silver fine particles and 5 parts by weight of phenol resin as an organic binder were mixed to obtain a conductive paste. This conductive paste was applied on a glass substrate and then heated at 200 ° C. to form a transparent electrode film having a thickness of 50 nm. And the total light transmittance and surface resistance were measured about the obtained transparent electrode film.
Table 1 shows the evaluation results of the thickness and major axis of the silver particles and the formed transparent electrode.

It was found that the silver fine particles produced in Examples 1 to 6 were dispersed in the solvent without agglomeration and had high dispersibility in the solvent.
As a result of confirming the crystal structure with a transmission electron microscope (TEM), it was confirmed to be a silver crystal.
FIG. 2 is a TEM photograph of the silver fine particles produced in Example 1. In the silver fine particles produced in Examples 1 to 6, as shown in FIG. 2, the particles were substantially plate-shaped, and the shapes of the main surfaces thereof were triangular, pentagonal, and hexagonal.
As shown in Table 1, the plate-like silver fine particles produced in Examples 1 to 6 had a thickness of 50 nm or less and a major axis of 5000 nm or less. The aspect ratio was 3 or more. Such silver fine particles can be fused and sintered even at a low temperature of 200 ° C. as in Example 1, and a transparent electrode can be formed. It was found that the formed transparent electrode had a thin film thickness of 50 nm, high optical transparency, a smooth surface, low surface resistance, and excellent electrical conductivity.

  In particular, as in Example 1, silver fine particles having a thickness of 1 nm or more and 30 nm or less, a major axis of 10 nm or more and 200 nm or less, and an aspect ratio of 5 or more and 20 or less, have a highly reactive particle surface. Since most of them are in contact with the adjacent particle surfaces, the particles can be more easily fused and sintered, and higher conductivity can be realized.

On the other hand, the silver fine particles produced in Comparative Example 1 are minute spherical particles having a particle diameter of about 10 nm as shown in FIG. 3 as a result of observing the particle shape by TEM. I understood.
In Comparative Example 1, agar was used as the polymer compound, but silver ions are not fixed inside or on the surface of the agar, and it is considered difficult to achieve a state of being uniformly dispersed in the solvent.
For this reason, when a reducing agent is added to a solution to reduce silver ions, the silver ions are non-uniform, and therefore the silver ion reduction reaction is locally concentrated in the solution. For this reason, when an abrupt reduction reaction occurs locally, and this abrupt reduction reaction produces a large amount of silver as a fine seed crystal, and then the grains cannot be grown sufficiently and become substantially spherical silver particles. Conceivable.

Further, the silver fine particles produced in Comparative Example 2 were also substantially spherical particles as in FIG. 3, and the particle diameter was found to be about 5 nm.
In Comparative Example 2, sodium borohydride was used as the reducing agent. However, since this sodium borohydride is highly reducible, a rapid reduction reaction occurred, and this reduction reaction caused a large amount of silver as a fine seed crystal. It is considered that the particles are generated, and the grains cannot be sufficiently grown, resulting in fine substantially spherical particles.

  In the silver fine particles produced in the above-described Comparative Examples 1 and 2, even if the particle diameter is small and the reactivity of the particle surface is high, the particles are in point contact and the contact area is small. For this reason, as shown in Table 1, the surface resistance has increased.

  The silver fine particles of the present invention can be used for conductive paints, conductive pastes, reflective film forming paints, metallic glossy film forming paints, and other various functional thin film forming paints or pastes. The functional thin film formed by these can be fused and sintered by heat treatment at a low temperature of 200 ° C., and high conductivity can be realized. Printed circuit boards, plasma display panels (PDP) and liquid crystal displays (LCD) It can be used for wiring of various flat panel displays. It can also be used as an alternative solder.

It is a schematic diagram which shows the major axis and thickness of particle | grains. 2 is a TEM photograph of silver fine particles produced in Example 1. FIG. 4 is a TEM photograph of silver fine particles produced in Comparative Example 1.

Claims (6)

A silver fine particle, which is a substantially plate-like particle having two main surfaces, wherein the particle has a thickness of 50 nm or less and a major axis of 5000 nm or less. The silver fine particles according to claim 1, wherein the main surface has a shape selected from a substantially triangular shape, a substantially pentagonal shape, and a substantially hexagonal shape. The silver fine particles according to claim 1, wherein an aspect ratio which is a ratio of a major axis to a thickness of the particles is 3 or more. A method for producing silver fine particles, wherein a solution obtained by dissolving at least a polymer compound, a reducing agent, and a silver salt is stirred at a temperature of 25 ° C. or higher and 60 ° C. or lower. Examples of the polymer compound include starch, sodium carboxymethylcellulose, hydroxyethylcellulose, gelatin, polyvinyl alcohol, polyvinylpyrrolidone, polyacrylamide, polyethyleneimine, polycarboxylic acid, polycarboxylic acid ammonium salt, polycarboxylic acid alkali metal salt, and polyacrylic acid. 5. The method for producing silver fine particles according to claim 4, wherein at least one selected from ammonium acrylate, polyacrylic acid ammonium salt, polyacrylic acid alkali metal salt, and pectin is used. As the reducing agent, at least one or more selected from ascorbic acid, alkali metal ascorbate, gentisic acid, hydroquinone, aminophenols, dihydroxynaphthalene, amidol, methol, phenidone, gallic acid, protocatechuic acid, pyrogallol, aminonaphthalene The method for producing silver fine particles according to claim 4, wherein:

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