JP2022161370A - Silver micro-particle and method of producing the same - Google Patents

Abstract

To provide silver micro-particles that are less varied in particle size, have a sharp particle-size distribution and are capable of preferably maintaining the dispersibility in a lipophilic solvent.SOLUTION: Silver micro-particles comprise aliphatic amine, e.g. octyl amine, having 6 or more C on a particle surface, with an average particle size in terms of equivalent circle diameter of 70 nm or greater and 150 nm or smaller and a variation coefficient in particle size of 20% or smaller. The silver micro-particles are to be synthesized by a reduction process including the steps of: first reduction to add silver at an addition rate of 0.4 mmol/s per 1 kg of solution or lower into a solution having a reducer concentration of 0.8 molar equivalent/kg or lower and second reduction to add silver at an addition rate of 0.4 mmol/s per 1 kg of solution or lower after a reducer is newly added to a reaction liquid thereof.SELECTED DRAWING: Figure 6

Description

TECHNICAL FIELD The present invention relates to fine silver particles that are useful as a raw material for joining silver pastes, conductive pastes, and the like, and a method for producing the same.

In this specification, silver particles (silver particle(s)) having an average particle diameter of about 200 nm or less in equivalent circle diameter are particularly referred to as silver fine particle(s).

In order to form fine electrodes and circuits of electronic parts, there have been conventional conductive inks in which silver fine particles are dispersed in a dispersion medium, and conductive pastes in which silver fine particles are mixed with a binder resin and a solvent to form a paste. is coated on a substrate and then heated and baked at a low temperature of about 100 to 200° C. to sinter silver fine particles to form a silver conductive film.
The fine silver particles used in such conductive inks and conductive pastes have very high activity, are easily sintered even at low temperatures, and are unstable as particles as they are. Therefore, in order to prevent sintering and agglomeration of silver fine particles and to ensure the independence and storage stability of silver fine particles, the surface of silver fine particles is coated with an organic protective agent composed of an organic compound and dispersed in a solvent. There is known a method of storing as a fine silver particle dispersion liquid.
When a conductive ink or paste is applied to a substrate, a semiconductor element is placed on it, or a circuit pattern is printed, and then fired, the element is adhered to the substrate by a fusion bonding phenomenon. A circuit is formed by sintering the metal particles or contacting the metal particles due to curing shrinkage of the binder.
When screen printing is used to form fine electrodes, circuits, etc. of electronic parts, it is common to use a high-boiling lipophilic solvent for the silver paste.

In Patent Document 1, as a method for producing silver fine particles used in such conductive inks and conductive pastes, an aliphatic amine having 6 or more carbon atoms is added as an organic protective agent to water as a solvent, and a reducing agent and A method for producing fine silver particles by adding a silver compound (claim 1), and a method for producing fine silver particles by adding a silver compound to an aqueous solution in which an aliphatic amine as an organic protective agent and a reducing agent are mixed (claim 7) ) is disclosed. It is said that silver particles having an average primary particle diameter of 10 to 500 nm can be obtained by such a production method (claim 6). Further, in Examples, a manufacturing method is disclosed in which an aqueous solution of silver nitrate as a silver compound is added at once to an aqueous solution in which octylamine as an organic protective agent and hydrazine hydrate as a reducing agent are dissolved (paragraph 0022). .

In Patent Document 2, by using water-soluble carboxymethyl cellulose as a dispersant having a degree of etherification in a predetermined range, the average particle size is 10 to 100 nm, and the particle size distribution of the average particle size is A production method for obtaining silver nanoparticles in which the number of x(1±0.1) particles is 70% or more is disclosed.

JP 2014-070264 A JP 2016-020532 A

When producing a bonding silver paste using silver fine particles as disclosed in Patent Documents 1 and 2, since the silver fine particles are sintered at an assumed temperature, the melting temperature of the silver fine particles is a constant temperature without variation. is desirable. Here, in order to keep the melting temperature of the fine silver particles constant, it is preferable to keep the particle size of the fine silver particles constant and narrow the particle size distribution as much as possible. However, there is a problem that the fine silver particles obtained by the production method of Patent Document 1 have a wide particle size distribution.

Regarding this problem, since the fine silver particles obtained by the production method using water-soluble carboxymethyl cellulose as a dispersant of Patent Document 2 have a narrow particle size distribution, it is considered that there is an effect of improving the dispersion of the melting temperature of the fine silver particles. However, since water-soluble carboxymethyl cellulose is used as a dispersing agent for the fine silver particles obtained, acetate solvents such as methoxyethyl acetate and ether solvents such as ethylene glycol dimethyl ether, which are used as solvents for silver paste for screen printing, cannot be used. There is a problem that the dispersibility in a lipophilic solvent is poor and the fine silver particles aggregate. Here, it is thought that the dispersibility in a lipophilic solvent can be improved by using a lipophilic compound as an organic protective agent present on the surface of the fine silver particles. With the technique of Patent Document 2, it is difficult to apply a lipophilic organic protective agent.

An object of the present invention is to provide fine silver particles which have a sharp particle size distribution with little variation in particle size and can maintain good dispersibility in a lipophilic solvent.

In the method of reductively depositing silver particles using an aqueous solvent, as typified by the above-mentioned Patent Document 1, an operation is performed in which the entire amount of the silver compound is mixed with a reducing agent at once in order to complete the reduction reaction in a short period of time. rice field. The inventors controlled the mixing speed of the silver compound and the reducing agent instead of mixing them all at once, and reduced the concentration of the reducing agent that was present in the liquid during the reduction process involving the formation of silver crystal nuclei. It has been found that by suppressing it, it is possible to remarkably reduce the variation in the particle size of the fine silver particles obtained, and it becomes easier to control the average particle size of the fine silver particles to be obtained. The present invention is based on such findings.

The following inventions are disclosed in this specification as means for achieving the above object.
[1] Having an aliphatic amine having 6 or more carbon atoms on the particle surface, having an average particle diameter of 70 nm or more and 150 nm or less in circle equivalent diameter measured by SEM (scanning electron microscope) observation, and variation in particle diameter Silver fine particles having a modulus of 20% or less.
[2] The fine silver particles according to [1] above, wherein the aliphatic amine is octylamine.
[3] The fine silver particles according to [1] or [2] above, wherein the coefficient of variation of the particle size is 15% or less.
[4] A silver compound is added to a solution A(1) in which an aliphatic amine having 6 or more carbon atoms and a reducing agent are mixed in an aqueous solvent, and the concentration of the reducing agent is 0.8 molar equivalent/kg or less. By adding the dissolved solution B(1) under the conditions shown in X(1) below, silver is reduced and precipitated in the solution to form silver fine particles while accompanying the formation of silver crystal nuclei. process and
Silver fine particles obtained in the first reduction step are suspended in an aqueous solvent, an aliphatic amine having 6 or more carbon atoms is mixed, and a reducing agent newly mixed after the first reduction step is added. Silver is reduced and precipitated on the surface of the fine silver particles in the liquid by adding the solution B (2) in which the silver compound is dissolved to the solution A (2) containing silver under the conditions shown in X (2) below. a second reduction step of growing fine silver particles;
A method for producing fine silver particles containing
Here, N molar equivalents (N is a numerical value) means the stoichiometric number of moles of reducing agent required for the reaction to reduce N moles of monovalent silver ions to metallic silver.
X(1): The average rate of addition of silver by solution B(1) into solution A(1) was 0.4 mmol/s per 1 kg of solution A(1) immediately before the addition of solution B(1). Below.
X(2): The average rate of addition of silver by solution B(2) into solution A(2) was 0.4 mmol/s per 1 kg of solution A(2) immediately before the addition of solution B(2). Below.
[5] The amount of silver added by solution B(1) in the first reduction step, relative to the total amount of silver added by solution B(1) and solution B(2) in the first reduction step and the second reduction step The method for producing fine silver particles according to the above [4], wherein the content is 2% or more and 50% or less.
[6] The method for producing fine silver particles according to [4] or [5] above, wherein the aliphatic amine having 6 or more carbon atoms is octylamine.
[7] The method for producing fine silver particles according to any one of [4] to [6] above, wherein the reducing agent used in the first reduction step and the second reduction step is both hydrazine.
[8] As the solution A (1), in addition to an aliphatic amine having 6 or more carbon atoms and a reducing agent, an alkali metal hydroxide is further mixed, above [4] to [7] A method for producing silver fine particles according to any one of the above.
[9] As the solution A (2), in addition to an aliphatic amine having 6 or more carbon atoms and a reducing agent newly mixed after the completion of the first reduction step, a base newly mixed after the completion of the first reduction step The method for producing fine silver particles according to any one of the above [4] to [8], which uses a material containing a chemical substance.
[10] The method for producing fine silver particles according to [9] above, wherein the basic substance is one or more selected from ammonia and amines.

Here, as the particle diameter, an equivalent circle diameter based on an image observed with a SEM (scanning electron microscope) can be adopted. Hereinafter, this average particle size may be referred to as "SEM average particle size".
The coefficient of variation (%) of particle size is expressed by the following equation.
Variation coefficient (%) = 100 × [standard deviation σ of distribution of particle diameter (nm)] / [average particle diameter (nm)]
The "molar equivalent" in "molar equivalent/kg" representing the concentration of the reducing agent in the liquid is the stoichiometric number of moles of the reducing agent required to reduce 1 mole of monovalent silver ion to metallic silver. be. For hydrazine, for example, 1 molar equivalent corresponds to 0.25 moles of hydrazine.
The particle size distribution of fine silver particles can be measured as follows.

(Method for measuring particle size distribution of fine silver particles)
A sample of the silver fine particles is observed with a SEM (scanning electron microscope), and in the SEM image of a randomly selected field of view, all particles that can grasp the entire contour of the particles are defined as particles to be measured. The area of the region surrounded by the outline of the particle on the image is measured, and the diameter of a circle equal to the area is taken as the equivalent circle diameter of the particle. This equivalent circle diameter measurement is performed using SEM images of one or more randomly selected fields of view so that the total number of particles to be measured is 100 or more, and the equivalent circle diameter data of all the particles to be measured Determine the particle size distribution (histogram) for For the average particle size, the arithmetic mean value of the circle-equivalent diameters of all particles to be measured is adopted.

ADVANTAGE OF THE INVENTION According to this invention, it became possible to provide the silver fine particle which has sharp particle size distribution with little dispersion|variation in a particle size, and to which the organic protective agent which has lipophilicity adheres. Compared to the method of adding the silver compound to the reducing agent solution all at once in a short time, which is employed in the conventional technique represented by Patent Document 1, the production method disclosed in the present specification requires less time to add the silver compound. become longer. However, when the scale of the reaction vessel is the same, according to the method of the present invention, it is possible to greatly increase the amount of silver compound that can be put in one batch of treatment, resulting in improved productivity. can be made

1 is a SEM photograph of fine silver particles obtained in Example 1. FIG. 2 is a SEM photograph of fine silver particles obtained in Example 2. FIG. SEM photograph of fine silver particles obtained in Example 10. FIG. 4 is a SEM photograph of fine silver particles obtained in Comparative Example 2. FIG. SEM photograph of silver fine particles obtained in Comparative Example 3. FIG. A histogram of particle diameters of fine silver particles obtained in Example 2. FIG. A histogram of particle diameters of fine silver particles obtained in Comparative Example 3. FIG.

[Silver Fine Particles]
The silver fine particles of the present invention have an aliphatic amine having 6 or more carbon atoms as an organic protective agent on the surface of the metallic silver particles, and have an average particle diameter of 70 nm or more and 150 nm or less in equivalent circle diameter. coefficient of variation is as small as 20% or less.

[Aliphatic amine]
Aliphatic amines are suitable as an organic protective agent when synthesizing fine silver particles because they are easily adsorbed to silver particles due to the presence of amino groups. Also, the organic protective agent adheres to the surface of the synthesized silver fine particles and prevents aggregation in the suspension of the fine silver particles or in the base material of the silver paste to ensure dispersibility. Here, it is believed that the amino group in the aliphatic amine binds to the hydrophilic functional group presumed to exist on the surface of the fine silver particles and contributes to the dispersion stability of the fine silver particles.

Since aliphatic amines having 6 or more carbon atoms in one molecule have moderate lipophilicity, they are also effective for ensuring the dispersibility of silver particles in a silver paste using a lipophilic solvent. More specifically, even when a lipophilic organic solvent such as an acetate-based solvent such as methoxyethyl acetate and an ether-based solvent such as ethylene glycol dimethyl ether is used to produce a silver paste for screen printing, the dispersibility of silver fine particles is improved. becomes good, and the problem of aggregation of fine silver particles can be avoided. The upper limit of the number of carbon atoms in the aliphatic amine in the fine silver particles of the present invention is not particularly limited. It may be necessary to increase the heating temperature for bonding. Silver fine particles originally have the merit of exhibiting low-temperature sinterability. When emphasizing the merit, it is preferable that the number of carbon atoms of the aliphatic amine is 18 or less.

From the viewpoint of ensuring sufficient dispersibility in a lipophilic solvent, the aliphatic amine is preferably a primary amine and an aliphatic amine having one amino group. Octylamine, hexylamine and oleylamine It is more preferable to use one or more selected types. Particular preference is given to using octylamine ( _{CH3 ( CH2} ) _7NH2 ). Such low-molecular-weight aliphatic amines have a boiling point lower than that of water-soluble carboxymethylcellulose used as a dispersant in Patent Document 2, for example. An organic protective agent (dispersant) with a low boiling point has the advantage of being more suitable for low-temperature sintering because it is easily volatilized during the heating process for sintering silver fine particles.

[Average particle size]
The fine silver particles of the present invention have an SEM average particle diameter of 70 nm or more from the viewpoint of sufficiently ensuring dispersion stability in the silver paste, and an SEM average particle diameter of 150 nm or less from the viewpoint of sufficiently ensuring low-temperature sinterability. do.

[Coefficient of variation of particle size]
The fine silver particles of the present invention are characterized by having a coefficient of variation in particle size in terms of equivalent circle diameter measured by SEM observation (hereinafter sometimes simply referred to as "variation coefficient in particle size") of 20% or less. and By using silver fine particles having such a sharp particle size distribution in a silver paste for bonding, it is possible to more stably suppress the formation of voids in metal bonding layers and conductors utilizing sintering of silver particles. . Moreover, the dispersion of the melting temperature of the fine silver particles can be reduced. By reducing the variation in the melting temperature of the fine silver particles, the fine silver particles in the paste can be locally and uniformly sintered at a predetermined temperature, increasing the effect of suppressing uneven sintering. More preferably, the coefficient of variation of particle size is 15% or less. It is preferable that the coefficient of variation of the particle size is as small as possible, and although there is no need to limit the lower limit, it may be controlled within a range of, for example, 5% or more in consideration of the manufacturability of the fine silver particles.

[Method for producing fine silver particles]
As a method for producing fine silver particles with a small coefficient of variation in particle size, here, in the production process of reducing and depositing silver in an aqueous solvent, a first reduction step involving the formation of silver crystal nuclei, and and a second reduction step mainly for growing fine silver particles by reducing and depositing silver on the surface of the fine particles.

[First reduction step]
[Solution A (1)]
First, a mixed solution is prepared by mixing an aliphatic amine and a reducing agent in an aqueous solvent. By adding a solution B(1) described later to this liquid, the reduction deposition reaction of silver proceeds in the liquid. In this specification, the liquid in which the aliphatic amine and the reducing agent are mixed and which receives the addition of the solution B(1) in the first reduction step is referred to as "solution A(1)". Solution A(1) immediately before starting addition of solution B(1) is sometimes referred to as "initial solution A(1)". The solution A(1) after starting the addition of the solution B(1) refers to the reaction solution (the solution in which the reduction reaction proceeds).

An aqueous solvent is a liquid medium containing water as a main component. In the initial solution A(1), the aqueous solvent can be composed of only pure water, but water and, for example, alcohol or a mixed solvent with other liquid organic medium. In the case of a mixed solvent, it is preferable that the mass ratio of water in the liquid medium is 70% or more. The aqueous solvent of solution A(1) may contain an alkali metal hydroxide in addition to the aliphatic amine and the reducing agent.

In the present invention, an aliphatic amine having 6 or more carbon atoms in one molecule is used as an organic protective agent. The organic protective agent adheres to the periphery of the fine silver particles that have been reduced and deposited in the liquid, prevents aggregation of the fine silver particles in the aqueous solvent, and has a role of ensuring dispersibility. As described above, since aliphatic amines having 6 or more carbon atoms have moderate lipophilicity, they are also effective for ensuring the dispersibility of silver particles in a silver paste using a lipophilic base material. From the viewpoint of fully exhibiting the advantage of low-temperature sinterability peculiar to silver fine particles, the number of carbon atoms in the aliphatic amine is more preferably 18 or less. Specifically, it is preferable to use one or more selected from octylamine, hexylamine and oleylamine as described above. Particular preference is given to using octylamine ( _{CH3 ( CH2} ) _7NH2 ).

Aliphatic amines having 6 or more carbon atoms are generally sparingly soluble in water. When such an aliphatic amine is used, a part of the aliphatic amine molecule is dissolved in the aqueous solvent by sufficiently stirring during the preparation of the initial solution A(1), and it is not completely dissolved. Most of the aliphatic amine molecules are evenly suspended in the liquid. The number of moles of the aliphatic amine mixed in the initial solution A (1) is 0.05 with respect to the total number of moles of silver added by solution B (1) and solution B (2) in the subsequent step. A molar ratio of up to 6 times is preferable, and a molar ratio of 0.05 to 2 times is more preferable. The content of the aliphatic amine in the initial solution A(1) may be adjusted, for example, in the range of 0.4 to 43 g of aliphatic amine per 1 kg of solution A(1).

According to the research of the inventors, in order to generate fine silver particles with a small variation in particle size, a solution in which an organic protective agent and a reducing agent coexist is prepared in advance, and the silver compound is slowly added. is extremely effective. Therefore, in the present invention, as the initial solution A(1), a liquid in which an aliphatic amine as an organic protective agent and a reducing agent are mixed in an aqueous solvent is prepared.

As the reducing agent, various substances capable of reducing silver ions supplied by a silver compound to metallic silver are applicable, but it is more preferable to use a basic reducing agent. For example, hydrazine (N ₂ H ₄ ) and sodium borohydride (NaBH ₄ ) are suitable, and hydrazine is particularly suitable. It is desirable that the reducing agent is completely dissolved in the aqueous solvent in the preparation of solution A(1).

In the first reduction step involving the formation of silver crystal nuclei, the concentration of the reducing agent mixed in the initial solution A(1) should be 0.8 molar equivalents or less per 1 kg of the initial solution A(1). is very effective. Here, 1 molar equivalent means the amount of reducing agent required to reduce 1 mol of silver ion to metallic silver. For example, when the silver compound is silver(I) nitrate and the reducing agent is hydrazine, the amount of hydrazine required to reduce 1 mol of silver ions to metallic silver is 0.25 mol. In contrast, 1 molar equivalent of hydrazine refers to 0.25 times the molar ratio of hydrazine to silver. Therefore, when hydrazine is used, the initial solution A ( 1) should have a hydrazine concentration of 0.2 mol/kg or less. By limiting the concentration of the reducing agent mixed in the initial solution A(1) to a low value as described above, the synergistic effect of controlling the silver addition rate to be slow, which will be described later, generates It is presumed that the effect of suppressing the number of crystal nuclei generated in the first reduction step can be obtained, and the variation in the particle size of the silver fine particles generated in the first reduction step can be reduced. As a result, variation in the particle size of the fine silver particles finally obtained in the second reduction step can be reduced. More preferably, the concentration of the reducing agent mixed in the initial solution A(1) is 0.2 molar equivalents or less per 1 kg of the initial solution A(1). In consideration of productivity, the concentration of the reducing agent mixed in the initial solution A(1) is preferably 0.001 molar equivalent or more per 1 kg of the initial solution A(1), 0.02 It is more preferable to use the molar equivalent or more. When hydrazine is used, the concentration of hydrazine mixed in the initial solution A(1) is preferably 0.004 mol or more per 1 kg of the initial solution A(1), preferably 0.08 mol or more. is more preferable.

The total amount of the reducing agent contained in the initial solution A(1) is preferably in the range of 1 to 4 times the total amount of silver added by the solution B(1). Here, the n-fold equivalent of the reducing agent (n is a numerical value) is expressed by the following formula: stoichiometric amount (mol)].

In order to generate fine silver particles having a small variation in particle size, it is advantageous to allow the solution A(1) to contain an alkali metal hydroxide. As the alkali metal hydroxide, it is preferable to employ one or more selected from sodium hydroxide and potassium hydroxide from the viewpoint of cost and availability. As a result of various investigations, it was found that it is effective to contain the necessary amount of alkali metal hydroxide to adjust the pH of the initial solution A(1) to 11.5 or higher. Although the mechanism has not been elucidated, when the pH is increased above the above using an alkali metal hydroxide, the effect of suppressing the generation of silver crystal nuclei is likely to be obtained. It is speculated that In particular, when the temperature of the reaction solution is 45° C. or lower, the effect of adding an alkali metal hydroxide to the solution A(1) so that the initial pH of the solution A is 11.5 or higher is significant. The content of the alkali metal hydroxide may be adjusted, for example, within a range in which the molar ratio to the content of the reducing agent in the solution A(1) is 0.10 or more and 1.0 or less. Along with the addition of the silver compound, the alkali metal hydroxide may be replenished in a timely manner. You can get great results just by putting it on. In addition, it is not preferable to use ammonia as a pH adjuster in the first reduction step. Since ammonia forms a complex with silver, it may inhibit silver crystal nucleation.

Here, the pH value described in this specification is measured using a glass electrode based on JIS Z8802, and is calibrated using an appropriate buffer solution according to the pH range to be measured as a pH standard solution. measured by a meter. In addition, the pH described in this specification is a value obtained by directly reading the measured value indicated by the pH meter compensated by the temperature compensating electrode under the condition of the liquid temperature of the liquid to be measured. In suspensions using poorly water-soluble amines, if the temperature of the liquid is, for example, 50°C or higher, the measured value may not be stable if the pH is measured at that temperature. The pH value of the solution A(1) may be evaluated from the pH value measured after cooling the sample liquid separated from the solution A(1) to 40° C. or lower.

[Solution B (1)]
As a liquid for introducing silver ions into the above solution A(1), an aqueous solution in which a silver compound is dissolved in an aqueous solvent is prepared. This liquid is referred to herein as "solution B(1)". The silver compound is not particularly limited as long as it dissolves in an aqueous solvent and functions as a source of silver ions, but silver nitrate (AgNO ₃ ) is preferable from the viewpoint of cost and ease of handling. The aqueous solvent of the solution B(1) may consist of only pure water, or as a mixed solvent of water and, for example, alcohol or other liquid organic medium, if necessary, as long as the effects of the present invention are not impaired. good too. The concentration of the silver compound in solution B(1) may be, for example, in the range of 0.3 to 3 mol/kg in terms of the number of moles of silver.

[Addition of silver to solution A(1)]
By slowly adding the above solution B(1) (silver compound solution) little by little to the above solution A(1) (mixed solution of organic protective agent and reducing agent) being stirred in a reaction vessel, A reduction deposition reaction of silver is allowed to proceed in the solution A(1). According to the research of the inventors, the average addition rate of silver by solution B (1) should be 0.4 mmol/s or less per 1 kg of solution A (1) immediately before the addition of solution B (1). is extremely effective for sharpening the particle size distribution of the fine silver particles obtained in the first reduction step. Here, "per 1 kg of solution A(1)" is converted based on the initial mass of solution A(1) (total mass of solvent and solute). More preferably, the above average addition rate is set in the range of 0.01 to 0.4 mmol/s in terms of 1 kg of solution A(1).

The method of adding solution B(1) into solution A(1) may be continuous or intermittent (for example, injection at regular intervals). Addition of the solution B(1) may be performed with a device such as a pump, or may be performed manually. The pump is not particularly limited as long as it can control the liquid feeding speed, and for example, various industrial pumps, tube pumps, diaphragm pumps, etc., which control the number of revolutions by an inverter can be used. In any addition method, for example, the average of the added amount measured at 1-minute intervals (average at that time in the time segment of less than 1 minute immediately before the end of addition) is the solution B (1) It is more preferable to keep the supply rate as constant as possible so that the amount of the solution A(1) immediately before the addition is 0.4 mmol/s or less per 1 kg of solution A(1).

In addition, the 1-second average addition rate of silver in the previous 1 second at all points from the start of addition of solution B (1) to the end of addition was calculated as It is more effective to maintain the conversion at 2.0 millimoles/second or less. Here, the "1 second average addition rate of silver in the previous 1 second" at a certain point is, for example, if 12.3 seconds have passed since the addition start, 1 second from 11.3 seconds to 12.3 seconds from the start of addition The amount of silver added (in millimoles) corresponds to the "1-second average addition rate of silver in the previous 1 second (in millimoles/second)". When the elapsed time from the start of addition is within 1 second, the amount of silver added (mmol) up to that point is regarded as "1 second average addition rate of silver in the previous 1 second (mmol/sec)". .

The reason why fine silver particles with little variation in particle size are produced by adding silver from solution B(1) slowly and little by little as described above is that the number of crystal nuclei generated per unit time is suppressed. It is presumed to be an effect caused by The unit of silver crystal nuclei when the average silver addition rate of solution B (1) is maintained at 0.4 mmol/s or less per 1 kg of solution A (1) immediately before the addition of solution B (1) It is considered that the effect of controlling the number of occurrences per time is effectively exhibited. It is more preferable that the average rate of addition of silver by solution B(1) be 0.2 mmol/s or less per 1 kg of solution A(1) immediately before the addition of solution B(1), and more preferably 0.1 mmol/s. It is more preferable to: In addition, it is desirable to secure a time of 1 minute or more from the start of the addition of silver by the solution B(1) to the end of the addition, and it is usually set within a range of 60 minutes or less.

When adding the solution B(1) to the solution A(1), the solution A(1) is stirred. It is preferable to continue stirring for, for example, 1 minute or longer even after the addition of solution B(1) is completed. The temperature of the solution A (1) (that is, the reaction solution) when proceeding with the reduction deposition reaction of silver must be set within the range of the melting point or more and the boiling point or less of the solution, but usually within the range of 30 to 80°C. Favorable conditions can be found. From the viewpoint of making the particle size of the resulting silver particles as uniform as possible, it is more effective to maintain a constant liquid temperature from the start to the end of the reaction.

The main role of this first reduction step is to sequentially form new silver crystal nuclei in the liquid. When a silver crystal nucleus is formed, silver ions present near the crystal nucleus are precipitated around the crystal nucleus to form fine solid silver particles. In the first reduction step, the amount of silver to be added is preferably set so as to synthesize fine silver particles having an average particle diameter of about 10 to 70 nm in terms of equivalent circle diameter. A reduction process for growing the fine silver particles to a desired size is the second reduction process. Therefore, the amount of silver to be reduced and deposited in the first reduction step may be a small proportion of the total amount of silver to be reduced and deposited by the end of the second reduction step. According to the study of the inventors, the amount of silver added by the solution B(1) in the first reduction step is equal to the amount of silver added by the solution B(1) and the solution B(2) in the first reduction step and the second reduction step. is preferably 2% or more and 50% or less with respect to the total amount of

[Second reduction step]
[Solution A (2)]
A solution is prepared in which silver fine particles obtained in the first reduction step are suspended in an aqueous solvent and an aliphatic amine having 6 or more carbon atoms is mixed. Usually, it is efficient to subject the reaction solution after the first reduction step to the second reduction step as it is in the reaction vessel. In the second reduction step, the solution B (2) in which the silver compound is dissolved is added to the solution A (2) containing the reducing agent, which is newly mixed after the completion of the first reduction step. advances the reduction deposition reaction of Here, the liquid containing the fine silver particles obtained in the first reduction step and the reducing agent newly mixed after the first reduction step on the side receiving the addition of the solution B(2) in the second reduction step is defined as " Solution A (2)”. Solution A(2) immediately before starting addition of solution B(2) is sometimes referred to as "initial solution A(2)". The solution A(2) after starting the addition of the solution B(2) refers to the reaction solution in the second reduction step (the solution in which the reduction reaction proceeds). “After completion of the first reduction step” means “a reaction in which silver ions introduced into solution A(1) by solution B(1) are reduced to metallic silver by a reducing agent in solution A(1)”. means that it is after the

When the reaction solution after the first reduction step is used in the second reduction step, the reducing agent is mixed with the reaction solution while stirring the solution to dissolve the reducing agent in the solution, A solution A (2) is obtained. When the second reduction step is performed after once collecting the silver fine particles obtained in the first reduction step, the silver fine particles obtained in the first reduction step are suspended in an aqueous solvent, and An aliphatic amine and a reducing agent are mixed to obtain solution A(2). In that case, the amount of the solution A(2) may be set so that the content of the silver fine particles to be suspended is in the range of, for example, 0.04 to 4.0 mass %. As the aqueous solvent, a mixed solvent of water and, for example, alcohol or other liquid organic medium can be used as long as the effect of the present invention (sharpening of the particle size distribution of the silver particles obtained) is not impaired. In the case of a mixed solvent, it is preferable that the mass ratio of water in the liquid medium is 70% or more. The aqueous solvent of solution A(2) may further contain a basic substance newly mixed after the completion of the first reduction step. A basic substance is a substance that exhibits basicity when dissolved in pure water (pH=7).

The initial solution A(2) contains an aliphatic amine having 6 or more carbon atoms in the same manner as the initial solution A(1). The aliphatic amine having 6 or more carbon atoms functions as an organic protective agent for ensuring the dispersibility of silver particles in the process of growth in liquid, and also functions as a buffering agent for suppressing pH fluctuations. When the reaction solution after the first reduction step is used as the initial solution A (2), since the aliphatic amine having 6 or more carbon atoms is already mixed in the reaction solution, the initial solution A ( It is not necessary to newly add an aliphatic amine having 6 or more carbon atoms at the stage of preparing 2). However, as will be described later, it is possible to select an aliphatic amine having 6 or more carbon atoms as a basic substance to be newly mixed for pH adjustment. The content of the aliphatic amine having 6 or more carbon atoms contained in the initial solution A(2) is preferably in the range of, for example, 0.03 to 1.0 mol per 1 kg of the initial solution A(2). When the reaction solution after the first reduction step is used as the initial solution A(2), the content in the above preferable range can usually be covered by the aliphatic amine having 6 or more carbon atoms present in the reaction solution. can. Here, the content of the aliphatic amine having 6 or more carbon atoms per 1 kg of the initial solution A (2) includes the content of 6 or more carbon atoms adhering to the surface of the silver fine particles suspended in the liquid. Also included are aliphatic amines.

As a reducing agent, the same substance as in the first reduction step or a different substance can be used. As in the first reduction step, it is more preferred to apply a basic reducing agent. For example, hydrazine (N ₂ H ₄ ) and sodium borohydride (NaBH ₄ ) are suitable, and hydrazine is particularly suitable. In the first reduction step, from the viewpoint of suppressing the formation of silver crystal nuclei, the concentration of the reducing agent in the liquid was limited to 0.8 molar equivalents or less per 1 kg of the initial solution A(1). However, with such a low concentration of reducing agent content, it is difficult to grow the fine silver particles to a size of 70 nm or more in SEM average particle size with good productivity. Therefore, in the second reduction step, a reduction reaction is allowed to proceed for the purpose of growing (increasing the size of) the fine silver particles in solution A(2) to which a reducing agent is newly added.

It is considered that unreacted reducing agent usually remains in the reaction solution after the first reduction step. The amount of residual reducing agent depends on the amount of reducing agent mixed in solution A(1) and the amount of silver ions introduced into solution A(1) by solution B(1). When the reaction solution after the first reduction step is used in the second reduction step, the remaining reducing agent can also be used for the reduction reaction in the second reduction step. In this case, the amount r(2) of the reducing agent newly added to the solution A(2) in the second reduction step may be set so as to satisfy the following formula [1].
s(1)+s(2)≦n(1)×r(1)+n(2)×r(2) …[1]
here,
s(1) is the number of moles of silver added into solution A(1) by solution B(1);
s(2) is the number of moles of silver added by solution B(2) into solution A(2);
r (1) is the number of moles of reducing agent contained in solution A (1);
r (2) is the number of moles of the reducing agent newly added in solution A (2);
n(1) is the number of moles of monovalent silver ions that can be stoichiometrically reduced to metallic silver by 1 mole of the reducing agent contained in solution A(1);
n(2) is the number of moles of monovalent silver ions that can be stoichiometrically reduced to metallic silver by 1 mole of the reducing agent newly added to solution A(2);
is.
When both the reducing agent contained in solution A(1) and the reducing agent newly added to solution A(2) are hydrazine, n(1)=n(2)=4.

In the second reduction step, the amount of the reducing agent additionally added after the first reduction step is If a part is mixed, as long as the initial mixed amount is sufficient to reduce the silver ions introduced into the solution A (2) at the beginning of the addition of the solution B (2), in the second reduction step During the progress of the reduction reaction, the remaining necessary amount of the reducing agent can be added intermittently or dividedly. However, considering the stability of the operation, the total amount of the reducing agent to be additionally added after the first reduction step, that is, the amount of the reducing agent corresponding to the above r(2) is preliminarily added to the initial solution A(2). It is preferable to mix and dissolve them sufficiently. When the total amount of the reducing agent to be additionally added after the first reduction step is premixed in the initial solution A (2), per 1 kg of the initial solution A (2) containing the mass of the suspended silver fine particles The content of the reducing agent is preferably 0.4 molar equivalents/kg or more, more preferably in the range of 0.6 to 2.0 molar equivalents/kg.

As the reduction reaction of silver progresses, the pH of the liquid tends toward the acidic side. The reduction rate of silver depends on the pH of the liquid, and a high reduction rate is obtained in the basic region. In the second reduction step responsible for the growth of fine silver particles, the speed of the reduction reaction has a greater effect on productivity. It is desirable to add In the second reduction step, one or more selected from ammonia and amines are preferably used as the basic substance. Since these substances have a buffering action, the allowable range of the addition amount of the pH adjuster is widened, and can contribute to stable operation with a uniform pH distribution in the reaction tank. Aliphatic amines such as octylamine, hexylamine, oleylamine and methylamine are suitable as the amine used as the basic substance. The amount of the basic substance added to the solution A(2) is preferably such that the pH of the solution during the reduction reaction is maintained within the range of 8.0 to 12.0. Although it can be added at any time during the reduction reaction, from the viewpoint of obtaining fine silver particles with a small variation in particle size by promoting a stable reduction reaction, the entire amount of the basic substance is mixed in advance in the initial solution A (2). It is preferable to keep For example, when the reducing agent in the second reduction step is hydrazine, the amount of the basic substance is in the range of 1.5 to 8 mol in the case of ammonia per 1 mol of monovalent silver ions to be reduced. is preferred, and octylamine is preferably used in the range of 0.5 to 2 mol. As described above, the initial solution A(2) contains an aliphatic amine having 6 or more carbon atoms. A substance similar to the aliphatic amine having 6 or more carbon atoms may be used as the basic substance. However, the basic substance referred to here refers to a substance newly mixed into the solution A(2) after the completion of the first reduction step.

[Solution B (2)]
As a liquid for introducing silver ions into the above solution A(2), an aqueous solution in which a silver compound is dissolved in an aqueous solvent is prepared. This liquid is referred to herein as "solution B(2)". Descriptions of the silver compound, solvent, and silver concentration used in solution B(2) are the same as those for solution B(1) described above, and are therefore omitted. A silver compound solution stored in advance in a predetermined tank can be used as solution B(1) in the first reduction step and used as solution B(2) in the second reduction step.

[Addition of silver to solution A(2)]
The solution B (2) (silver compound solution) is added to the solution A (2) (suspension of silver fine particles containing a newly added reducing agent) that is being stirred in a reaction vessel. By slowly adding little by little, the reduction deposition reaction of silver is allowed to proceed in the solution A(2). As in the first reduction step, the average addition rate of silver from solution B(2) is 0.4 mmol/kg of solution A(2) immediately before the addition of solution B(2). s or less is extremely effective in sharpening the particle size distribution of the fine silver particles obtained in the second reduction step. Here, "per 1 kg of solution A(2)" is converted based on the initial mass of solution A(2) (solvent, solute, and total mass of silver fine particles suspended in the liquid). More preferably, the average addition rate is set in the range of 0.01 to 0.4 mmol/s in terms of 1 kg of solution A(2).

The method of adding solution B(2) into solution A(2) may be continuous or intermittent (for example, injection at regular intervals), as in the first reduction step. good too. Addition of the solution B(2) may be performed with a device such as a pump, or may be performed manually. In any addition method, for example, the average of the added amount measured at 1-minute intervals (average at that time in the time segment of less than 1 minute immediately before the end of addition) is the solution B (2) It is more preferable to keep the supply rate as constant as possible so that the amount is 0.4 mmol/s or less per 1 kg of solution A (2) just before the start of addition.

In addition, the 1-second average addition rate of silver in the previous 1 second at all points from the start of addition of solution B (2) to the end of addition was calculated as It is more effective to maintain the conversion at 2.0 millimoles/second or less. Since this point is also the same as the first reduction step, the explanation is omitted.

By slowly and little by little adding silver from the solution B(2) as described above, silver can be added on the surface of already formed silver fine particles with little variation in particle size without causing non-uniform nucleation. It is presumed that reduction precipitation occurs, and as a result, it becomes possible to grow silver fine particles while maintaining a state in which there is little variation in particle size. When the average silver addition rate of solution B(2) is maintained at 0.4 mmol/s or less per 1 kg of solution A(2) immediately before the addition of solution B(2), the above action is effectively exhibited. It is considered that More preferably, the average rate of silver addition by solution B(2) is 0.2 mmol/s or less per 1 kg of solution A(2) just before the start of addition of solution B(2). In addition, it is desirable to ensure a time of 1 minute or more from the start of the addition of silver by the solution B(2) to the end of the addition, and it is usually set within the range of 400 minutes or less.

When adding solution B(2) to solution A(2), solution A(2) is kept under stirring. It is preferable to continue stirring for, for example, one minute or longer even after the addition of solution B(2) is completed. As a result, the unreduced silver ions introduced into the solution A(2) can be almost completely reduced and precipitated, which is advantageous for improving the yield of silver. The temperature of the solution A(2) (that is, the reaction solution) when proceeding with the reduction deposition reaction of silver can usually be found to be in the range of 30 to 80° C., as in the first reduction step. From the viewpoint of making the particle size of the resulting silver particles as uniform as possible, it is more effective to maintain a constant liquid temperature from the start to the end of the reaction.

In this second reduction step, based on the silver fine particles having a small particle size (for example, an average particle size of 10 to 70 nm) synthesized in the first reduction step, the size of the silver fine particles is reduced to a desired average size within the range of 70 to 150 nm. This is a step of growing to a particle size. The average particle size of the fine silver particles obtained in the second reduction step can be controlled by the total amount of silver added in the second reduction step.

[Production volume per 1 kg of reaction solution]
In the method for producing fine silver particles in which silver is added all at once disclosed in Patent Document 1, the amount of fine silver particles obtained from 1 kg of the reaction solution is about 5 g. In contrast, in the method for producing fine silver particles according to the present invention, 30 g or more of fine silver particles can be obtained from 1 kg of the reaction solution at the end of the second reduction step. That is, according to the present invention, the production amount of fine silver particles per batch can be greatly increased.

[Example 1]
(First reduction step)
(Preparation of initial solution A (1))
After putting 1343.3 g of pure water as a solvent in a 5 L reaction tank and adjusting the temperature to 40 ° C., octylamine as an aliphatic amine having 6 or more carbon atoms (special grade manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., molecular weight 129 .24) 115.3 g, 2.4 g of an 80% by mass hydrazine monohydrate aqueous solution (manufactured by Otsuka Chemical Co., Ltd.) as a reducing agent, and 0.6 g of a 50% sodium hydroxide aqueous solution as an alkali metal hydroxide. was added, and while blowing nitrogen gas at a flow rate of 1 L/min, the liquid was stirred by rotating a stirring rod equipped with blades at 345 rpm by an external motor. By this stirring, the molecules of the organic protective agent are sufficiently and uniformly suspended in the solvent, and the reducing agent and the alkali metal hydroxide are sufficiently dissolved in the solvent, resulting in the initial solution A (1) (that is, the solution described later). An organic protective agent/reducing agent-containing solution at a stage before addition of B(1)) was obtained.

The hydrazine concentration in the initial solution A(1) was 0.026 mol/kg, which corresponds to a reducing agent concentration of 0.104 molar equivalents/kg. The molar ratio of octylamine contained in the initial solution A(1) to the total amount of silver added in the first reduction step (molar ratio to Ag) is 1.15. The concentration of alkali metal hydroxide added to the initial solution A(1) is 0.005 mol/kg. The pH of this initial solution A(1) (40° C.) was measured and found to be 11.6.

(Preparation of silver compound aqueous solution)
As a silver compound, 131.9 g of silver nitrate crystals (manufactured by Toyo Kagaku Kogyo Co., Ltd.) were dissolved in 287.1 g of pure water to obtain 419 g of an aqueous silver nitrate solution having a silver concentration of 1.85 mol/kg. A portion of this liquid is used as solution B(1) in the first reduction step, and the remainder is used as solution B(2) in the second reduction step.

(Addition of solution B (1) to solution A (1))
While the liquid temperature of solution A(1) in the reactor was maintained at 40° C. and stirring was continued, 41.9 g of solution B(1) was added to solution A(1) over 10 minutes. The solution B(1) was introduced into the solution A(1) by continuously injecting the solution B(1) into the solution A(1) with a constant flow rate controlled by a tube pump. The average addition rate of silver added into solution A(1) by solution B(1) is 0.089 mmol/s per kg of initial solution A(1). In this case, the 1-minute average of the addition amount measured at 1-minute intervals (hereinafter referred to as "1-minute average addition rate") is the same as the above average addition rate, and even if the pulsation of the tube pump is taken into account, , The maximum value of the 1-second average addition rate of silver in the previous 1 second at all points from the start of addition of solution B (1) to the end of addition is 2.0 mmol / s or less per 1 kg of initial solution A (1) (The same applies to the first reduction step of Examples 2 to 13 and all the reduction steps of Comparative Example 3 below.). The initial amount of reducing agent in solution A(1) was 1.98 times the total amount of silver added in the first reduction step.
After the addition of solution B(1) was completed, stirring was continued for an additional 2 minutes while the liquid temperature of the reaction liquid was maintained at the above temperature to complete the first reduction step. Thus, a reaction liquid in which fine silver particles were suspended was obtained. Table 1 shows the initial solution A(1), solution B(1), and reaction conditions in the first reduction step (the same applies to the following examples).

(Second reduction step)
(Preparation of initial solution A (2))
While stirring the reaction solution obtained in the first reduction step in which the silver fine particles are suspended, 80 mass % hydrazine monohydrate aqueous solution (manufactured by Otsuka Chemical Co., Ltd.) 21 is added as an additional reducing agent into the reaction solution. 0.9 g and 202.2 g of 26.1 mass % aqueous ammonia solution as a basic compound were added and mixed to obtain an initial solution A (2). The amount of the newly added reducing agent in the initial solution A (2) occupied by 1 kg of the initial solution A (2) (not including the reducing agent added to the initial solution A (1). Hereinafter referred to as "additional reducing agent concentration") corresponds to 0.81 molar equivalents/kg. The concentration of the basic substance newly mixed in the initial solution A(2) is 1.80 mol/kg. In addition, in Table 2, the basic substance newly mixed in the initial solution A(2) is described as "additional basic substance". The pH of this initial solution A(2) (40° C.) was measured to be 11.2.

(Addition of solution B (2) to solution A (2))
While maintaining the liquid temperature of solution A(2) in the reaction tank at 40° C. and continuing to stir, 377.1 g of the above silver compound aqueous solution was added as solution B(2) to solution A(2). was added over 90 minutes. The solution B(2) was introduced into the solution A(2) by continuously injecting the solution B(2) into the solution A(2) with a constant flow rate controlled by a tube pump. The average addition rate of silver added into solution A(2) by solution B(2) is 0.075 mmol/s/kg of initial solution A(2). In this case, the 1-minute average addition rate is the same as the above average addition rate, and even if the pulsation of the tube pump is taken into account, the previous addition rate at all points from the start of addition of solution B (2) to the end of addition is The maximum value of the 1-second average addition rate of silver in 1 second is 2.0 mmol/s or less per 1 kg of the initial solution A(2) (the same applies to the second reduction step of Examples 2 to 13 below). The initial amount of reducing agent in solution A(2) was 2.00 equivalents relative to the total amount of silver added in the second reduction step.
After the addition of solution B(2) was completed, stirring was continued for an additional 2 minutes while maintaining the liquid temperature of the reaction liquid at the above temperature. Thus, a reaction liquid in which fine silver particles were suspended was obtained. Table 2 shows the initial solution A (2), solution B (2), and reaction conditions in the second reduction step (the same applies to the following examples).

(Evaluation of obtained fine silver particles)
(Particle size distribution of fine silver particles)
A sample for SEM observation was prepared by applying the reaction liquid obtained in the second reduction step onto a carbon tape. The sample was observed with a SEM (scanning electron microscope, SM-7200, manufactured by JEOL Ltd.) at a magnification of 80,000 times, and the particle size distribution (particle size distribution). Image analysis software (manufactured by Asahi Kasei Engineering Co., Ltd., Azo-kun (registered trademark)) was used to measure the particle size (the same applies to each example described later). As a result, the silver fine particles obtained in this example had an average particle diameter of 97.1 nm and a coefficient of variation of 12.5%. Here, the total number of particles to be measured in the particle diameter measurement was 161. FIG. 1 illustrates an SEM photograph of the fine silver particles obtained in this example. The length of the white scale bar shown slightly rightward near the center of the lower part of the photograph corresponds to 100 nm.

(Production amount per 1 kg of reaction liquid)
The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 39.8 g. Here, since the reduction reaction of the added silver ions has been completed, the amount of production was calculated assuming that all the added silver ions were fine silver particles (the same applies to the following examples).
These results are shown in Table 3 (same for each example below).

[Example 2]
In the preparation of the silver compound aqueous solution, 237.5 g of silver nitrate crystals were dissolved in 1272.2 g of pure water to prepare an aqueous silver nitrate solution having a silver concentration of 0.93 mol/kg. ), adding 83.9 g of solution B(1) to solution A(1) in the first reduction step, and adding 1425.8 g of solution B(2) to solution A(2) in the second reduction step. was added over 170 minutes.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.089 mmol/s per kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.073 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle size of 124.0 nm and a coefficient of variation of 12.6%. Here, the total number of particles to be measured in the particle diameter measurement was 245. FIG. 2 shows an SEM photograph of the fine silver particles obtained in this example. The length of the white scale bar shown slightly rightward near the center of the lower part of the photograph corresponds to 100 nm. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 47.2 g.

[Example 3]
In the second reduction step, an experiment was conducted under the same conditions as in Example 2, except that the amount of the 80% by mass hydrazine monohydrate aqueous solution contained in the initial solution A(2) was 41.3 g. .

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.089 mmol/s per kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.073 mmol/s/kg of initial solution A(2).

The silver fine particles obtained in this example had an average particle diameter of 128.4 nm and a coefficient of variation of 13.9%. Here, the total number of particles to be measured in the particle diameter measurement was 234. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 46.9 g.

[Example 4]
In the first reduction step, the amount of 50% sodium hydroxide aqueous solution contained in the initial solution A (1) was 1.9 g, and in the second reduction step, the initial solution A (2) was contained. An experiment was conducted under the same conditions as in Example 2, except that the amount of the 80% by mass hydrazine monohydrate aqueous solution was 41.3 g.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.089 mmol/s per kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.073 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle diameter of 90.3 nm and a coefficient of variation of 11.4%. Here, the total number of particles to be measured in the particle diameter measurement was 385. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 46.9 g.

[Example 5]
(First reduction step)
(Preparation of initial solution A (1))
After putting 2149.2 g of pure water as a solvent in a 5 L reaction tank and adjusting the temperature to 40 ° C., octylamine as an aliphatic amine having 6 or more carbon atoms (special grade manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., molecular weight 129 .24) 40.1 g, 3.9 g of an 80% by mass hydrazine monohydrate aqueous solution (manufactured by Otsuka Chemical Co., Ltd.) as a reducing agent, and 1.0 g of a 50% sodium hydroxide aqueous solution as an alkali metal hydroxide. was added, and while blowing nitrogen gas at a flow rate of 1 L/min, the liquid was stirred by rotating a stirring rod equipped with blades at 345 rpm by an external motor. By this stirring, the molecules of the organic protective agent were sufficiently and uniformly suspended in the solvent, and the reducing agent and the alkali metal hydroxide were sufficiently dissolved in the solvent to obtain an initial solution A(1).

The hydrazine concentration in the initial solution A(1) was 0.028 mol/kg, which corresponds to a reducing agent concentration of 0.112 molar equivalents/kg. The molar ratio of octylamine contained in the initial solution A(1) to the total amount of silver added in the first reduction step (molar ratio to Ag) is 1.15. The concentration of alkali metal hydroxide added to the initial solution A(1) is 0.006 mol/kg.

(Preparation of silver compound aqueous solution)
As a silver compound, 211.1 g of silver nitrate crystals (manufactured by Toyo Kagaku Kogyo Co., Ltd.) was dissolved in 459.2 g of pure water to obtain 670.3 g of an aqueous silver nitrate solution having a silver concentration of 1.85 mol/kg. A portion of this liquid is used as solution B(1) in the first reduction step, and the remainder is used as solution B(2) in the second reduction step.

(Addition of solution B (1) to solution A (1))
While the liquid temperature of solution A(1) in the reactor was maintained at 40° C. and stirring was continued, 67.0 g of solution B(1) was added to solution A(1) over 10 minutes. The solution B(1) was introduced into the solution A(1) by continuously injecting the solution B(1) into the solution A(1) with a constant flow rate controlled by a tube pump. The average addition rate of silver added into solution A(1) by solution B(1) is 0.094 mmol/s per kg of initial solution A(1). The initial amount of reducing agent in solution A(1) was 2.01 times the total amount of silver added in the first reduction step.
Thus, a reaction liquid in which fine silver particles were suspended was obtained.

(Second reduction step)
(Preparation of initial solution A (2))
While stirring the reaction solution obtained in the first reduction step in which the silver fine particles are suspended, 80 mass % hydrazine monohydrate aqueous solution (manufactured by Otsuka Chemical Co., Ltd.) 35 is added as an additional reducing agent into the reaction solution. 0 g and 323.5 g of a 26.1 mass % ammonia aqueous solution as a basic compound were added and mixed to obtain an initial solution A (2). The additional reducing agent concentration per kg of initial solution A(2) corresponds to 0.85 molar equivalents/kg. The concentration of the basic substance newly mixed in the initial solution A(2) is 1.90 mol/kg.

(Addition of solution B (2) to solution A (2))
While maintaining the liquid temperature of solution A(2) in the reaction tank at 40° C. and continuing stirring, 603.3 g of the remaining silver compound aqueous solution was added as solution B(2) to solution A(2). was added over 90 minutes. The solution B(2) was introduced into the solution A(2) by continuously injecting the solution B(2) into the solution A(2) with a constant flow rate controlled by a tube pump. The average addition rate of silver added into solution A(2) by solution B(2) is 0.079 mmol/s/kg of initial solution A(2). The initial amount of reducing agent in solution A(2) was 2.00 equivalents relative to the total amount of silver added in the second reduction step.
After the addition of solution B(2) was completed, stirring was continued for an additional 2 minutes while maintaining the liquid temperature of the reaction liquid at the above temperature. Thus, a reaction liquid in which fine silver particles were suspended was obtained.

(Evaluation of fine silver particles)
The particle size distribution of the fine silver particles obtained in the second reduction step was examined by the method described in Example 1. As a result, the silver fine particles obtained in this example had an average particle diameter of 104.5 nm and a coefficient of variation of 12.9%. Here, the total number of particles to be measured in the particle diameter measurement was 354. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 41.6 g.

[Example 6]
In the first reduction step, the amount of octylamine contained in the initial solution A(1) was 184.5 g, and in the second reduction step, the amount of octylamine contained in the initial solution A(2) was 26.1 g. An experiment was conducted under the same conditions as in Example 5, except that the amount of the mass % aqueous ammonia solution was 161.8 g.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.088 mmol/s/kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.079 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle diameter of 87.6 nm and a coefficient of variation of 19.4%. Here, the total number of particles to be measured in the particle diameter measurement was 358. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 40.8 g.

[Example 7]
In the first reduction step, the amount of octylamine contained in the initial solution A(1) was 184.5 g, and in the second reduction step, the amount of octylamine contained in the initial solution A(2) was 26.1 g. An experiment was conducted under the same conditions as in Example 5, except that the amount of the mass % aqueous ammonia solution was 242.6 g.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.088 mmol/s/kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.077 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle size of 98.2 nm and a coefficient of variation of 13.6%. Here, the total number of particles to be measured in the particle diameter measurement was 349. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 41.8 g.

[Example 8]
In the preparation of the silver compound aqueous solution, 131.9 g of silver nitrate crystals were dissolved in 706.8 g of pure water to prepare an aqueous silver nitrate solution having a silver concentration of 0.93 mol/kg. ), in the first reduction step, the amount of octylamine contained in the initial solution A (1) was 25.6 g, and the amount of solution B (1) added was 83.9 g; In the second reduction step, 90.2 g of octylamine (special grade manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) was newly added to the initial solution A (2) in addition to hydrazine monohydrate and aqueous ammonia solution. The experiment was conducted under the same conditions as in Example 1, except that the amount of solution B(2) added was 754.8 g.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.095 mmol/s per kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.073 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle size of 101.1 nm and a coefficient of variation of 11.0%. Here, the total number of particles to be measured in the particle diameter measurement was 161. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 33.2 g.

[Example 9]
In the preparation of the aqueous silver compound solution, 105.5 g of silver nitrate crystals were dissolved in 565.4 g of pure water to prepare an aqueous silver nitrate solution having a silver concentration of 0.93 mol/kg. ), in the first reduction step, the amount of octylamine contained in the initial solution A (1) was 25.6 g, and the amount of solution B (1) added was 83.9 g; In the second reduction step, the initial solution A (2) does not contain an aqueous ammonia solution, and 70.2 g of octylamine (special grade manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) is newly added to the solution B (2). ) was carried out under the same conditions as in Example 1, except that the amount of added was 587.0 g.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.095 mmol/s per kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.085 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle size of 78.0 nm and a coefficient of variation of 17.0%. Here, the total number of particles to be measured in the particle size measurement was 263. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 31.6 g.

[Example 10]
An experiment was conducted under the same conditions as in Example 5, except that the reaction temperature in the first reduction step and the second reduction step was 50°C.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.094 mmol/s/kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.079 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle diameter of 95.3 nm and a coefficient of variation of 9.3%. Here, the total number of particles to be measured in the particle diameter measurement was 438. FIG. 3 shows an SEM photograph of the fine silver particles obtained in this example. The length of the white scale bar shown slightly rightward near the center of the lower part of the photograph corresponds to 100 nm. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 41.6 g.

[Example 11]
Example 5 except that in the first reduction step, the initial solution A (1) did not contain an alkali hydroxide, and the reaction temperature in the first reduction step and the second reduction step was 50 ° C. Experiments were conducted under the same conditions as

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.094 mmol/s/kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.079 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle size of 131.4 nm and a coefficient of variation of 13.2%. Here, the total number of particles to be measured in the particle diameter measurement was 228. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 41.6 g.

[Example 12]
An experiment was conducted under the same conditions as in Example 1, except that the amount of octylamine contained in the initial solution A(1) in the first reduction step was 12.5 g.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.095 mmol/s per kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.080 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle diameter of 102.3 nm and a coefficient of variation of 11.0%. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 41.8 g.

[Example 13]
An experiment was conducted under the same conditions as in Example 1, except that solution B(2) was added to solution A(2) over 45 minutes in the second reduction step.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.089 mmol/s per kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.150 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle size of 92.5 nm and a coefficient of variation of 13.7%. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 39.8 g.

[Comparative Example 1]
This is an example of synthesizing fine silver particles by a conventional method of adding an aqueous silver compound solution at once without adopting a two-stage reduction process. For the sake of convenience, the reduction step is hereinafter referred to as the first reduction step.
(First reduction step)
(Preparation of initial solution A (1))
After putting 3192.5 g of pure water as a solvent in a 5 L reaction tank and adjusting the temperature to 40 ° C., octylamine (special grade manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., molecular weight 129.24) 60.8 g and 80 mass % hydrazine monohydrate aqueous solution (manufactured by Otsuka Chemical Co., Ltd.) 5.8 g and 50% sodium hydroxide aqueous solution 1.5 g are added, and stirred with a blade while blowing nitrogen gas at a flow rate of 1 L / min. The bar was rotated at 345 rpm by an external motor to agitate the liquid. By this stirring, the molecules of the organic protective agent were sufficiently and uniformly suspended in the solvent, and the reducing agent and the alkali metal hydroxide were sufficiently dissolved in the solvent to obtain an initial solution A(1).

The hydrazine concentration in the initial solution A(1) was 0.028 mol/kg, which corresponds to a reducing agent concentration of 0.112 molar equivalents/kg. The molar ratio of octylamine contained in the initial solution A(1) to the total amount of silver added in the reduction step (molar ratio to Ag) is 2.55. The concentration of alkali metal hydroxide added to the initial solution A(1) is 0.006 mol/kg.

(Preparation of silver compound aqueous solution)
As a silver compound, 31.4 g of silver nitrate crystals (manufactured by Toyo Kagaku Kogyo Co., Ltd.) were dissolved in 167.9 g of pure water to obtain 199.3 g of an aqueous silver nitrate solution having a silver concentration of 0.93 mol/kg. The entire amount of this liquid is used as solution B(1).

(Addition of solution B (1) to solution A (1))
While the temperature of solution A(1) in the reaction tank was maintained at 40° C. and stirring was continued, 199.3 g of solution B(1) was added to solution A(1) over 3 seconds. After that, stirring was continued for an additional 2 minutes while maintaining the liquid temperature of the reaction liquid at the above temperature. Thus, a reaction liquid in which fine silver particles were suspended was obtained. In this reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 18.95 mmol/s per kg of initial solution A(1).

(Evaluation of fine silver particles)
The particle size distribution of the fine silver particles obtained in the reduction step was examined by the method described in Example 1. As a result, the silver fine particles obtained in this example had an average particle diameter of 44.4 nm and a coefficient of variation of 22.8%. Here, the total number of particles to be measured in the particle size measurement was 606. The production amount of fine silver particles per 1 kg of the obtained reaction solution was calculated to be 5.8 g.

[Comparative Example 2]
This is an example of synthesizing fine silver particles by a conventional method of adding an aqueous solution of a silver compound all at once without adopting a two-step reduction process, without adding an alkali hydroxide. For the sake of convenience, the reduction step is hereinafter referred to as the first reduction step.
(First reduction step)
(Preparation of initial solution A (1))
After putting 3422.0 g of pure water as a solvent in a 5 L reaction tank and adjusting the temperature to 40 ° C., octylamine (special grade manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., molecular weight 129.24) 51.1 g and 80 mass % hydrazine monohydrate aqueous solution (manufactured by Otsuka Chemical Co., Ltd.) 15.0 g and 50% sodium hydroxide aqueous solution 1.5 g are added, and stirred with a blade while blowing nitrogen gas at a flow rate of 1 L / min. The bar was rotated at 345 rpm by an external motor to agitate the liquid. By this stirring, the molecules of the organic protective agent were sufficiently and uniformly suspended in the solvent, and the reducing agent and the alkali metal hydroxide were sufficiently dissolved in the solvent to obtain an initial solution A(1).

The hydrazine concentration in the initial solution A(1) was 0.069 mol/kg, which corresponds to a reducing agent concentration of 0.276 molar equivalents/kg. The molar ratio of octylamine contained in the initial solution A(1) to the total amount of silver added in the reduction step (molar ratio to Ag) is 2.0.

(Preparation of silver compound aqueous solution)
As a silver compound, 33.6 g of silver nitrate crystals (manufactured by Toyo Kagaku Kogyo Co., Ltd.) were dissolved in 180.0 g of pure water to obtain 213.6 g of an aqueous silver nitrate solution having a silver concentration of 0.93 mol/kg. The entire amount of this liquid is used as solution B(1).

(Addition of solution B (1) to solution A (1))
While the liquid temperature of solution A(1) in the reactor was maintained at 40° C. and stirring was continued, 213.6 g of solution B(1) was added to solution A(1) over 3 seconds. After that, stirring was continued for an additional 2 minutes while maintaining the liquid temperature of the reaction liquid at the above temperature. Thus, a reaction liquid in which fine silver particles were suspended was obtained. In this reduction step, the average addition rate of silver added into solution A(1) by solution B(1) is 18.98 mmol/s per kg of initial solution A(1).

(Evaluation of fine silver particles)
The particle size distribution of the fine silver particles obtained in the reduction step was examined by the method described in Example 1. As a result, the fine silver particles obtained in this example had an average particle diameter of 76.6 nm and a coefficient of variation of 34.5%. FIG. 4 illustrates an SEM photograph of the fine silver particles obtained in this example. One graduation on the scale shown in the lower right of the photograph corresponds to 100 nm. Here, the total number of particles to be measured in the particle diameter measurement was 195. The production amount of fine silver particles per 1 kg of the obtained reaction solution was calculated to be 5.8 g.

[Comparative Example 3]
In this example, the initial solution A (1) of the first reduction step contains a reducing agent in an amount sufficient for the reduction reaction in all steps, and the initial solution A (2) of the second reduction step contains an additional This is an experimental example in which no reducing agent was contained.

In the first reduction step, the amount of octylamine contained in the initial solution A (1) was 25.6 g, and the amount of 80% by mass hydrazine monohydrate aqueous solution was 24.3 g; The experiment was carried out under the same conditions as in Example 1, except that the initial solution A (2) did not contain additional hydrazine in the process. Here, the hydrazine concentration in the initial solution A(1) was 0.279 mol/kg, which corresponds to a reducing agent concentration of 1.116 molar equivalents/kg.

In the first reduction step, the average addition rate of silver added by solution B(1) into solution A(1) is 0.093 mmol/s/kg of initial solution A(1).
In the second reduction step, the average addition rate of silver added by solution B(2) into solution A(2) is 0.079 mmol/s/kg of initial solution A(2).

The fine silver particles obtained in this example had an average particle diameter of 118.6 nm and a coefficient of variation of 23.2%. Here, the total number of particles to be measured in the particle diameter measurement was 288. FIG. 5 shows an SEM photograph of the fine silver particles obtained in this example. The length of the white scale bar shown slightly rightward near the center of the lower part of the photograph corresponds to 100 nm. The production amount of fine silver particles per 1 kg of the reaction solution obtained in the second reduction step was calculated to be 41.6 g.

Fine silver particles having an average particle size of 70 nm or more are prepared by a method in which the reduction reaction proceeds in the two-step process defined in the present invention, and silver is slowly added little by little under the conditions defined in the present invention in each reduction process. In each example in which the above was synthesized, fine silver particles having a small variation coefficient of particle size, for example, 20% or less, and little variation in particle size could be stably obtained. It can be seen that it is sufficiently possible to synthesize fine silver particles having an average particle size of 70 nm or more and having a variation coefficient of particle size of 15% or less. In addition, according to the production method of the present invention, it is possible to greatly increase the production amount of fine silver particles per batch compared to the conventional method of adding silver all at once (e.g., Comparative Examples 1 and 2). confirmed.

FIG. 6 exemplifies a particle size histogram of the fine silver particles obtained in Example 2. As shown in FIG.
FIG. 7 illustrates a histogram of particle diameters of the fine silver particles obtained in Comparative Example 3. As shown in FIG.

Claims (10)

Silver fine particles having an aliphatic amine having 6 or more carbon atoms on the particle surface, an average particle diameter of 70 nm or more and 150 nm or less in equivalent circle diameter, and a variation coefficient of the particle diameter of 20% or less. 2. The fine silver particles according to claim 1, wherein said aliphatic amine is octylamine. 3. The fine silver particles according to claim 1, wherein the coefficient of variation of the particle size is 15% or less. A silver compound is dissolved in a solution A (1) in which an aliphatic amine having 6 or more carbon atoms and a reducing agent are mixed in an aqueous solvent, and the concentration of the reducing agent is 0.8 molar equivalent/kg or less. a first reduction step of reducing and precipitating silver in the solution by adding solution B(1) under the conditions shown in X(1) below to generate fine silver particles while accompanying the formation of silver crystal nuclei;
Silver fine particles obtained in the first reduction step are suspended in an aqueous solvent, an aliphatic amine having 6 or more carbon atoms is mixed, and a reducing agent newly mixed after the first reduction step is added. Silver is reduced and precipitated on the surface of the fine silver particles in the liquid by adding the solution B (2) in which the silver compound is dissolved to the solution A (2) containing silver under the conditions shown in X (2) below. a second reduction step of growing fine silver particles;
A method for producing fine silver particles containing
Here, N molar equivalents (N is a numerical value) means the stoichiometric number of moles of reducing agent required for the reaction to reduce N moles of monovalent silver ions to metallic silver.
X(1): The average rate of addition of silver by solution B(1) into solution A(1) was 0.4 mmol/s per 1 kg of solution A(1) immediately before the addition of solution B(1). Below.
X(2): The average rate of addition of silver by solution B(2) into solution A(2) was 0.4 mmol/s per 1 kg of solution A(2) immediately before the addition of solution B(2). Below. The amount of silver added by solution B(1) in the first reduction step is 2% with respect to the total amount of silver added by solution B(1) and solution B(2) in the first reduction step and the second reduction step. 5. The method for producing fine silver particles according to claim 4, wherein the ratio is 50% or more. 6. The method for producing fine silver particles according to claim 4, wherein the aliphatic amine having 6 or more carbon atoms is octylamine. 7. The method for producing fine silver particles according to any one of claims 4 to 6, wherein the reducing agent used in the first reduction step and the second reduction step is both hydrazine. The solution A (1) according to any one of claims 4 to 7, wherein in addition to an aliphatic amine having 6 or more carbon atoms and a reducing agent, a mixture of an alkali metal hydroxide is used. A method for producing the described silver fine particles. As the solution A (2), in addition to the aliphatic amine having 6 or more carbon atoms and the reducing agent newly mixed after the completion of the first reduction step, the basic substance newly mixed after the completion of the first reduction step is added. 9. The method for producing fine silver particles according to any one of claims 4 to 8, wherein the silver particles are used. 10. The method for producing fine silver particles according to claim 9, wherein the basic substance is one or more selected from ammonia and amines.

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