Method for producing silver particles

Abstract

To produce silver nanoparticles having a sharp particle size distribution with small variations in particle size by a method for using a water-based solvent to reduce and deposit silver particles.SOLUTION: A method for producing silver fine particles is used in the synthesis of silver fine particles in which a solution B having a silver compound dissolved therein is added to a solution A having an aliphatic amine and a reductant mixed in a water-based solvent, to reduce and deposit silver into the liquid. The average addition rate of silver by the solution B is 0.4 mmol / s or less relative to 1 kg of solution A just before the start of addition of the solution B.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to a method for producing fine silver particles useful as a material for silver paste for bonding, conductive paste, etc. [Background technology]

[0002] Silver fine particles with a particle diameter of about 100 nm or less (hereinafter sometimes referred to as "silver nanoparticles") can function as a filler that fills the gaps between the micron-order silver particles by mixing them with silver particles with a particle diameter on the order of microns in a silver paste. In a silver paste for bonding containing this type of mixed silver powder, a metal bonding layer can be formed by a bonding process at a relatively low temperature by utilizing the low melting temperature property of silver nanoparticles. In addition, since the gaps between the silver particles in the paste are small, it is also advantageous in obtaining a metal bonding layer with few voids. For example, Patent Document 1 discloses a silver paste in which silver nanoparticles with an average particle diameter of 10 to 40 nm and silver particles with a particle diameter on the order of microns coexist.

[0003] Patent Document 2 discloses a synthesis method in which a silver compound and a reducing agent are mixed in one go when reducing and precipitating silver fine particles in an aqueous solvent as a method for synthesizing silver fine particles in a short time at low cost. Specifically, the method shows an embodiment in which a silver compound is added in one go to an aqueous solvent containing a reducing agent and an organic protective agent, and then stirred for a predetermined time (Examples 1 to 3, 5 to 10), and an embodiment in which a reducing agent is added in one go to an aqueous solvent containing a silver compound and an organic protective agent, and then stirred for a predetermined time (Examples 4 and 11).

[0004] Patent Document 3 discloses a method for synthesizing silver particles by dropping a solution containing a silver compound and a coating agent (organic protective agent) into a solution containing a reducing agent while mixing and stirring the solution. In this method, the entire amount of the coating agent is contained in the silver compound liquid before the reduction reaction.

[0005] On the other hand, Patent Document 4 discloses a method of reducing and precipitating fine silver particles in an alcohol solvent in which a silver compound and an organic protective agent are dissolved, by utilizing the reducing power of the alcohol. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2020-164926 A [Patent Document 2] JP 2014-70264 A [Patent Document 3] JP 2012-31478 A [Patent Document 4] JP 2006-213955 A Summary of the Invention [Problem to be solved by the invention]

[0007] In order to develop a high-performance silver paste that can more stably suppress the formation of voids in metal bonding layers and conductors that utilize the sintering of silver particles, it is important to more precisely control the size of the voids that form between the silver particles dispersed in the paste. It has been found that the use of silver nanoparticles with as uniform a particle size as possible to fill the gaps between larger silver particles is extremely effective in controlling the void size. In other words, it is desirable to use silver nanoparticles with little variation in particle size and a sharp particle size distribution.

[0008] However, with the technology of Patent Document 2, it is difficult to obtain silver nanoparticles having a sharp particle size distribution in which the variation in particle size is sufficiently reduced. Patent Document 1 also uses the technology of Patent Document 2 to synthesize silver nanoparticles (paragraph 0042), and does not use silver nanoparticles having a sharp particle size distribution. With the technology of Patent Document 3, silver and a coating agent component may form a complex, and as a result, the particle size distribution of the obtained silver microparticles becomes broad.

[0009] According to the alcohol solvent reduction method of Patent Document 4, silver nanoparticles with little variation in particle size can be synthesized. However, the reduction reaction takes a long time, for example, 5 hours or more. In addition, since an alcohol solvent is used, the production cost is higher than reduction in a water solvent. In other words, the alcohol solvent reduction method has the disadvantages of low productivity and high cost, and is not suitable as a method for industrial mass production of silver nanoparticles.

[0010] The present invention aims to provide a technology that can efficiently produce silver nanoparticles having a narrow particle size distribution with little variation in particle size by a method of reducing and precipitating silver particles using an aqueous solvent. [Means for solving the problem]

[0011] In the reduction precipitation method of silver particles using an aqueous solvent, as typified by the above-mentioned Patent Document 2, the entire amount of the silver compound is mixed with the reducing agent all at once in order to complete the reduction reaction in a short time. The inventors have found that it is possible to significantly reduce the particle size variation of the obtained silver microparticles by controlling the mixing speed of the silver compound and the reducing agent, rather than mixing them all at once. However, it is necessary to mix the organic protective agent and the reducing agent in one liquid before starting the reduction reaction. The present invention is based on such findings.

[0012] The above object is achieved by a method for producing silver fine particles, in which solution B, in which a silver compound is dissolved, is added to solution A, in which an aliphatic amine and a reducing agent are mixed in an aqueous solvent, to synthesize silver fine particles by reducing and precipitating silver in the solution, and the average addition rate of silver by solution B is 0.4 mmol / s or less calculated per kg of solution A immediately before the start of addition of solution B. In addition to the reducing agent and aliphatic amine, an alkali hydroxide may be further mixed into the above solution A.

[0013] The aliphatic amine may be, for example, one having 6 or more carbon atoms in one molecule. Specific examples include octylamine (CH3(CH2)7NH2), hexylamine (CH3(CH2)5NH2), and oleylamine (CH3(CH2)7CH=CH(CH2)7COOH). It is preferable to use one or more selected from these. As the reducing agent, for example, hydrazine can be used.

[0014] The liquid temperature during the reduction and precipitation of silver may be set to, for example, a range of 30 to 80° C. The fine silver particles obtained by this reduction and precipitation reaction have an average particle size of, for example, 100 nm or less. Effect of the Invention

[0015] According to the present invention, it is possible to produce silver nanoparticles with a narrow particle size distribution and small variation in particle size by a reduction method using an aqueous solvent. Compared with the method of adding a silver compound to a reducing agent solution in a short time all at once, which is used in conventional techniques such as Patent Document 2, the production method of the present invention requires a longer time to add the silver compound. However, when the reaction vessel has the same capacity, the method of the present invention makes it possible to significantly increase the amount of silver compound that can be added in one batch of processing, which ultimately improves productivity. [Brief description of the drawings]

[0016] [Figure 1] 1 is an SEM photograph of the silver microparticles obtained in Example 1. [Diagram 2] 1 is a histogram of particle sizes for the silver microparticles obtained in Example 1. [Diagram 3] 1 is an SEM photograph of the silver microparticles obtained in Example 2. [Figure 4] 4 is a histogram of particle sizes of the silver microparticles obtained in Example 2. [Diagram 5] 1 is an SEM photograph of the silver microparticles obtained in Example 3. [Figure 6] 1 is a histogram of particle sizes for the silver microparticles obtained in Example 3. [Figure 7] 1 is an SEM photograph of the silver microparticles obtained in Example 4. [Figure 8] 1 is a histogram of particle sizes for the silver microparticles obtained in Example 4. [Figure 9] SEM photograph of the silver microparticles obtained in Example 5. [Figure 10] 11 is a histogram of particle sizes for the silver microparticles obtained in Example 5. [Figure 11] SEM photograph of the silver microparticles obtained in Example 6. [Figure 12] 13 is a particle size histogram for the silver microparticles obtained in Example 6. [Figure 13] 1 is an SEM photograph of the silver microparticles obtained in Comparative Example 1. [Figure 14] 1 is a histogram of particle sizes for the silver microparticles obtained in Comparative Example 1. [Figure 15] SEM photograph of the silver microparticles obtained in Comparative Example 2. [Figure 16] 13 is a histogram of particle sizes for the silver microparticles obtained in Comparative Example 2. [Figure 17] 1 is an SEM photograph of the silver microparticles obtained in Comparative Example 3. [Figure 18] 13 is a histogram of particle sizes for the silver microparticles obtained in Comparative Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In the present invention, solution B, in which a silver compound is dissolved, is added to solution A, in which an aliphatic amine and a reducing agent are mixed in an aqueous solvent, to cause reduction precipitation of silver particles. At this time, the addition rate of silver added from solution B is controlled to obtain silver nanoparticles with a narrow particle size distribution.

[0018] [Solution A] A mixed solution is prepared by mixing an aliphatic amine and a reducing agent in an aqueous solvent. Solution B, which will be described later, is added to this solution, causing a silver reduction precipitation reaction to proceed within the solution. In this specification, the solution in which the aliphatic amine and reducing agent are mixed, which receives the addition of solution B, is called "solution A." Solution A immediately before the start of the addition of solution B is sometimes called "initial solution A." Solution A after the start of the addition of solution B refers to the reaction solution (the solution in which the reduction reaction proceeds).

[0019] An aqueous solvent is a liquid medium containing water as a main component. In the initial solution A, the aqueous solvent can be composed of pure water alone, but it can also be a mixed solvent of water and, for example, alcohol or other liquid organic medium, as long as it does not impede the effect of the present invention (sharpening the particle size distribution of the obtained silver particles). In the case of a mixed solvent, it is preferable that the mass ratio of water in the liquid medium is 70% or more. In addition to the aliphatic amine and the reducing agent, the aqueous solvent of solution A can further contain an alkali hydroxide.

[0020] In the present invention, an aliphatic amine is used as the organic protective agent. The organic protective agent adheres to the periphery of fine silver particles precipitated by reduction in the liquid, prevents the aggregation of silver particles in the aqueous solvent, and ensures dispersibility. Since the presence of amino groups makes the aliphatic amine easily adsorbed to silver particles, it is suitable as an organic protective agent when synthesizing silver nanoparticles. In addition, since an aliphatic amine having 6 or more carbon atoms in one molecule has moderate hydrophobicity, it is also effective for ensuring the dispersibility of silver particles in a silver paste using a hydrophobic base material. When it is important to have excellent dispersibility in a liquid or paste, it is desirable to use an aliphatic amine having 6 or more carbon atoms in one molecule. If the molecular weight is large, the boiling point becomes high, making it difficult to utilize the low-temperature sintering property of silver fine particles in the silver paste. It is more preferable to use an aliphatic amine having 18 or less carbon atoms in one molecule. One or more types of aliphatic amines can be used. In terms of molecular weight, for example, an aliphatic amine having a molecular weight of 100 to 400 is suitable. Specifically, hexylamine, heptylamine, octylamine, nonamine, decaamine, laurylamine, myristylamine, palmitylamine, stearylamine, octadecylamine, etc., as well as oleylamine, which is an unsaturated amine, can be mentioned. The aliphatic amine may be used alone or in combination of two or more. In particular, it is preferable to use one or more selected from the group consisting of octylamine, hexylamine, and oleylamine, and it is most preferable to use octylamine.

[0021] Aliphatic amines having 6 or more carbon atoms are generally poorly soluble in water. When using such an aliphatic amine, a part of the aliphatic amine molecules is dissolved in the aqueous solvent by thoroughly stirring the mixture during preparation of the initial solution A, and the majority of the aliphatic amine molecules that are not completely dissolved are uniformly suspended in the liquid. The molar ratio of the aliphatic amine mixed in the initial solution A is preferably 0.05 to 6 times the total molar number of silver added by solution B. The content of the aliphatic amine in the initial solution A may be adjusted, for example, within the range of 0.4 to 43 g of aliphatic amine per kg of solution A.

[0022] According to the research of the inventors, in order to generate silver nanoparticles with small variation in particle size, it is extremely effective to prepare a solution in which an organic protective agent and a reducing agent coexist in advance and wait for the silver compound to be slowly added. Therefore, in the present invention, a liquid in which an aliphatic amine, which is an organic protective agent, and a reducing agent are mixed in an aqueous solvent is prepared as the initial solution A.

[0023] As the reducing agent, various substances capable of reducing silver ions supplied by the silver compound to metallic silver are applicable, but it is more preferable to apply a basic reducing agent. For example, hydrazine (N2H4) and sodium borohydride (NaBH4) are suitable, and hydrazine is particularly suitable. It is desirable to completely dissolve the reducing agent in an aqueous solvent in the preparation of solution A. The total amount of reducing agent mixed in the initial solution A is preferably in the range of 1 to 4 equivalents relative to the total amount of silver added by solution B. Here, 1 equivalent means the amount of reducing agent required to reduce 1 mole of silver ions to metallic silver. For example, when the silver compound is silver nitrate (I) and the reducing agent is hydrazine, the amount of hydrazine required to reduce 1 mole of silver ions to metallic silver is 0.25 moles, so 1 equivalent of hydrazine to silver refers to 0.25 times the amount of hydrazine in terms of molar ratio to silver. The content of the reducing agent in the initial solution A may be adjusted, for example, in the range of 0.9 to 3.6 g per 1 kg of solution A.

[0024] It was also found that it is advantageous to include an alkali hydroxide in solution A in order to generate silver nanoparticles with a small variation in particle size. As the alkali hydroxide, it is preferable to use one or more selected from sodium hydroxide and potassium hydroxide in terms of cost and availability. As a result of various studies, it is effective to include an amount of alkali hydroxide necessary to make the initial pH of solution A 11.5 or higher. Although the mechanism is not yet clear, it is speculated that when the pH is increased to above the above level using an alkali hydroxide, the effect of suppressing the generation of silver crystal nuclei (described later) is more likely to be obtained. In particular, when the temperature of the reaction solution is 45°C or lower, the effect of including an alkali hydroxide in solution A and making the initial pH of solution A 11.5 or higher is large. The content of the alkali hydroxide may be adjusted, for example, so that the molar ratio to the content of the reducing agent in solution A is in the range of 0.10 to 1.0.

[0025] The alkali hydroxide may be replenished at appropriate times along with the addition of the silver compound, but based on the inventors' findings to date, a high effect can be obtained simply by adjusting the initial pH of solution A to within the above range.

[0026] Here, the pH value described in this specification is measured using a glass electrode based on JIS Z8802, and is measured by a pH meter calibrated with an appropriate buffer solution according to the pH range to be measured as a pH standard solution. The pH described in this specification is a value obtained by directly reading the measured value indicated by a pH meter compensated by a temperature compensation electrode under the condition of the liquid temperature of the liquid to be measured. In a suspension using a poorly water-soluble amine, if the liquid temperature is, for example, 50°C or higher, the measured value may not be stable when the pH is measured at that temperature. In that case, the pH of solution A may be evaluated by measuring the pH value of a sample liquid taken from solution A after cooling it to 40°C or lower.

[0027] [Solution B] As a liquid for introducing silver ions into the above-mentioned solution A, an aqueous solution in which a silver compound is dissolved in an aqueous solvent is prepared. This liquid is referred to as "solution B" in this specification. The silver compound is not particularly limited as long as it dissolves in an aqueous solvent and functions as a silver ion supply source, but silver nitrate (AgNO3) is preferable from the viewpoint of cost and ease of handling. As in the case of the above-mentioned solution A, the aqueous solvent of solution B may be composed of pure water alone, or may be a mixed solvent of water and, for example, alcohol or other liquid organic medium as necessary within a range that does not impair the effects of the present invention. The concentration of the silver compound in solution B may be, for example, in the range of 0.3 to 3 mol / kg in terms of the number of moles of silver.

[0028] [Synthesis of silver particles] The above solution B (silver compound solution) is slowly added little by little to the above solution A (mixed solution of organic protective agent and reducing agent) being stirred in a reaction vessel, thereby allowing the reduction and precipitation reaction of silver to proceed in solution A. According to the inventors' research, it is extremely effective to set the average addition rate of silver by solution B to 0.4 mmol / s or less per 1 kg of solution A just before the start of addition of solution B in order to sharpen the particle size distribution of the obtained silver nanoparticles. Here, the conversion of "per 1 kg of solution A" is performed based on the initial mass of solution A (total mass of solvent and solute). It is more preferable to set the above addition rate in the range of 0.01 to 0.4 mmol / s per 1 kg of solution A.

[0029] Solution B may be added to solution A continuously or intermittently (e.g., by injection at regular intervals). Solution B may be added using a device such as a pump or manually. In either case, it is more preferable to maintain a constant supply rate as much as possible so that the average of the amount added measured at one-minute intervals (the average for that time period in a time segment of less than one minute immediately before the end of addition) is 0.4 mmol / s or less per kg of solution A immediately before the start of addition of solution B.

[0030] It is more effective to maintain the average silver addition rate per second in the previous second at all points from the start to the end of the addition of solution B at 2.0 mmol / sec or less, calculated per kg of solution A immediately before the start of the addition of solution B. Here, the "average silver addition rate per second in the previous second" at a certain point in time is, for example, the amount of silver (mmol) added in the one second from 11.3 seconds to 12.3 seconds after the start of addition, which corresponds to the "average silver addition rate per second in the previous second (mmol / sec)". For points within 1 second of the time that has elapsed since the start of addition, the amount of silver (mmol) added up to that point in time is regarded as the "average silver addition rate per second in the previous second (mmol / sec)".

[0031] It is presumed that the reason why silver nanoparticles with small particle size variation can be obtained by slowly and gradually adding silver from solution B as described above is due to the effect of suppressing the number of crystal nuclei generated per unit time. It is considered that the effect of controlling the number of silver crystal nuclei generated per unit time is effectively exerted when the average addition rate of silver from solution B is maintained at 0.4 mmol / s or less converted to 1 kg of solution A immediately before the start of adding solution B. It is more preferable to set the average addition rate of silver from solution B to 0.2 mmol / s or less converted to 1 kg of solution A immediately before the start of adding solution B. In addition, it is desirable to ensure that the time from the start to the end of the addition of silver from solution B is 1 minute or more, and it is usually set to a range of 60 minutes or less.

[0032] When adding solution B to solution A, solution A is kept in a stirring state. It is preferable to continue stirring for, for example, 1 minute or more after the addition of solution B is completed. This allows the unreduced silver ions introduced into solution A to be reduced and precipitated almost completely, which is also advantageous for improving the silver yield. The temperature of solution A (i.e., the reaction liquid) during the silver reduction and precipitation reaction must be set in the range of the melting point or higher and the boiling point or lower of the liquid, and suitable conditions can usually be found in the range of 30 to 80°C. From the viewpoint of making the particle size of the obtained 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.

[0033] [Silver fine particles] The surface of the silver particles produced by the above method has molecules of aliphatic amine, which is an organic protective agent, adsorbed thereon, and each silver particle is dispersed in the reaction liquid, preventing aggregation. That is, a slurry in which silver particles are dispersed in the liquid is obtained. The silver particles dispersed in this slurry are nanoparticles with an average particle diameter of, for example, about 100 nm or less. These silver particles have a sharp particle size distribution (particle size histogram) with little variation in particle size. In particular, silver particles with an average particle diameter of less than 70 nm, which has a sharp particle size distribution with a coefficient of variation of particle size of 20% or less, can be produced by a rational method using an aqueous solvent. By using silver particles with such a sharp particle size distribution in a silver paste for bonding, it is expected that the formation of voids in a metal bonding layer or a conductor using sintering of silver particles can be more stably suppressed. In addition, the melting temperature of the silver particles is more uniform, and the effect of suppressing uneven sintering in the metal bonding layer can be expected. Here, the particle diameter can be a circle equivalent diameter based on an image observed by a SEM (scanning electron microscope). The coefficient of variation (%) is expressed by the following formula: Coefficient of variation (%) = 100 x [standard deviation of particle size (nm) distribution σ] / [average particle size (nm)] The particle size distribution of the silver particles can be measured as follows.

[0034] (Method for measuring particle size distribution of silver particles) A sample of silver microparticles is observed by SEM (scanning electron microscope), and all particles whose outlines can be grasped in the SEM image of a randomly selected visual field are regarded as measurement target particles. The area of ​​the area surrounded by the outline of the particle on the image of each measurement target particle is measured, and the diameter of the circle equal to the area is regarded as the equivalent circle diameter of the particle. This measurement of the equivalent circle diameter is performed on SEM images of one or more randomly selected visual fields so that the total number of measurement target particles is 100 or more, and a particle size distribution (histogram) is obtained for the data of the equivalent circle diameters of all the measurement target particles. The average particle diameter is the arithmetic mean value of the equivalent circle diameters of all the measurement target particles. Note that when the average particle diameter is small, for example, less than 20 nm, observation by TEM (transmission electron microscope) may be performed instead of the above observation by SEM. EXAMPLES

[0035] [Example 1] (Preparation of initial solution A) A 5L reaction vessel was charged with 3192.5g of pure water as a solvent and adjusted to 40℃, and then 60.8g of octylamine (special grade, molecular weight 129.24, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as an aliphatic amine of the organic protective agent, 5.8g of 80% by mass hydrazine monohydrate aqueous solution (Otsuka Chemical Co., Ltd.) as a reducing agent, and 1.48g of 50% aqueous sodium hydroxide solution as an alkali hydroxide were added, and while blowing in nitrogen gas at a flow rate of 1L / min, the liquid was stirred by rotating a stirring rod equipped with blades at 345 rpm by an external motor. This stirring allowed the molecules of the organic protective agent to be sufficiently and uniformly suspended in the solvent, while the reducing agent and alkali hydroxide were sufficiently dissolved in the solvent, and an initial solution A (i.e., a solution containing an organic protective agent and a reducing agent at a stage before the addition of solution B described below) was obtained. The pH of this initial solution A (40℃) was measured and found to be 11.6.

[0036] The amount of organic protective agent contained in the initial solution A was 2.5 in terms of molar ratio to the total amount of silver added (Ag molar ratio). The amount of reducing agent contained in the initial solution A was 2.0 in terms of equivalent to the total amount of silver added (Ag equivalent). The molar ratio of alkali hydroxide (NaOH) contained in the initial solution A to the reducing agent was 0.20.

[0037] (Preparation of solution B) Solution B was obtained by dissolving 31.4 g of silver nitrate crystals (manufactured by Toyo Kagaku Kogyo Co., Ltd.) as a silver compound in 167.9 g of pure water.

[0038] (Synthesis of silver particles) The temperature of solution A in the reaction vessel was maintained at 40°C, and the entire amount of solution B was added to this solution A over 10 minutes while stirring was continued. Solution B was introduced into solution A by continuously injecting solution B, which was controlled to a constant flow rate by a tube pump, into solution A. The average addition rate of silver added to solution A by solution B and the average of the amount added measured at 1-minute intervals (hereinafter referred to as the "average addition rate per minute") were both 0.094 mmol / s per kg of initial solution A. In this case, even when the pulsation of the tube pump is taken into consideration, the maximum value of the average addition rate per second of silver in the last 1 second at all points from the start to the end of the addition of solution B is 2.0 mmol / s or less per kg of initial solution A (similar to the following Examples 2 to 13 and Comparative Example 3). After the addition of solution B was completed, stirring was continued for another 2 minutes while maintaining the temperature of the reaction solution at the above temperature. In this way, a slurry in which the generated silver particles were dispersed in the liquid was obtained. The reaction time (i.e., the time from the start of addition of solution B to the end of stirring) is 12 minutes.

[0039] (Particle size distribution of silver particles) The obtained slurry was applied onto a carbon tape to prepare a sample for SEM observation. The sample was observed at a magnification of 80,000 times using an SEM (scanning electron microscope, manufactured by JEOL Ltd., SM-7200), and the particle size distribution (particle size distribution) was obtained according to the above-mentioned "Method for measuring particle size distribution of silver microparticles". Image analysis software (manufactured by Asahi Kasei Engineering Co., Ltd., A-zo-kun (registered trademark)) was used to measure the particle size (similarly in the examples and comparative examples 1 to 3 described later). As a result, the average particle size of the silver microparticles obtained in this example was 49.2 nm, and the coefficient of variation was 13.0%. Here, the total number of particles measured in the particle size measurement was 566. Figure 1 shows an example of an SEM photograph of the silver microparticles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. Figure 2 shows a particle size histogram for the silver microparticles obtained in this example. The initial solution A, solution B, synthesis conditions, average particle size of the obtained silver particles, and coefficient of variation are summarized in Tables 1 and 2 along with the following examples.

[0040] [Example 2] In the synthesis of silver microparticles, the experiment was carried out under the same conditions as in Example 1, except that the entire amount of solution B was added to solution A over 5 minutes. In this case, the average addition rate and the average addition rate per minute of silver added to solution A by solution B were both 0.189 mmol / s per 1 kg of initial solution A. The average particle size of the silver microparticles obtained in this example was 50.2 nm, the coefficient of variation was 16.7%, and the total number of particles measured in the measurement of particle size was 528. Figure 3 shows an example of an SEM photograph of the silver microparticles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. Figure 4 shows a histogram of particle sizes for the silver microparticles obtained in this example.

[0041] [Example 3] In the synthesis of silver microparticles, the experiment was carried out under the same conditions as in Example 1, except that the entire amount of solution B was added to solution A over 30 minutes. In this case, the average addition rate and the average addition rate per minute of silver added to solution A by solution B were both 0.031 mmol / s per 1 kg of initial solution A. The average particle size of the silver microparticles obtained in this example was 49.9 nm, the coefficient of variation was 11.0%, and the total number of particles measured in the measurement of particle size was 505. Figure 5 shows an example of an SEM photograph of the silver microparticles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. Figure 6 shows a histogram of particle sizes for the silver microparticles obtained in this example.

[0042] [Example 4] The experiment was carried out under the same conditions as in Example 1, except that in the preparation of the initial solution A, the amount of 50% aqueous sodium hydroxide solution added was changed from 1.48 g to 2.46 g. The pH of the initial solution A (40°C) was measured and found to be 11.8. The average particle size of the silver microparticles obtained in this example was 44.1 nm, the coefficient of variation was 10.7%, and the total number of particles measured in the particle size measurement was 526. Figure 7 shows an example of an SEM photograph of the silver microparticles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. Figure 8 shows a histogram of particle sizes for the silver microparticles obtained in this example.

[0043] [Example 5] The experiment was carried out under the same conditions as in Example 1, except that in the preparation of the initial solution A, the amount of 50% aqueous sodium hydroxide solution added was changed from 1.48 g to 4.92 g. The pH of the initial solution A (40°C) was measured and found to be 12.0. The average particle size of the silver microparticles obtained in this example was 32.2 nm, the coefficient of variation was 12.4%, and the total number of particles measured in the particle size measurement was 425. Figure 9 shows an example of an SEM photograph of the silver microparticles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. Figure 10 shows a histogram of particle sizes for the silver microparticles obtained in this example.

[0044] [Example 6] The experiment was carried out under the same conditions as in Example 1, except that in the preparation of the initial solution A, the amount of 50% sodium hydroxide aqueous solution added was changed from 1.48 g to 0.74 g. The pH of the initial solution A (40°C) was measured and found to be 11.5. The average particle size of the silver microparticles obtained in this example was 54.1 nm, the coefficient of variation was 19.4%, and the total number of particles measured in the particle size measurement was 328. Figure 11 shows an example of an SEM photograph of the silver microparticles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. Figure 12 shows a particle size histogram for the silver microparticles obtained in this example.

[0045] [Example 7] The experiment was carried out under the same conditions as in Example 1, except that the temperature of solution A was maintained at 45° C. The average particle size of the silver microparticles obtained in this example was 46.9 nm, the coefficient of variation was 10.9%, and the total number of particles measured in the particle size measurement was 397.

[0046] [Example 8] Except for keeping the temperature of solution A at 50° C. and not adding alkali hydroxide, the experiment was carried out under the same conditions as in Example 1. The average particle size of the silver particles obtained in this example was 49.7 nm, the coefficient of variation was 12.5%, and the total number of particles measured in the particle size measurement was 504.

[0047] [Example 9] The experiment was carried out under the same conditions as in Example 1, except that the temperature of solution A was maintained at 50° C. The average particle size of the silver microparticles obtained in this example was 44.6 nm, the coefficient of variation was 13.2%, and the total number of particles measured in the particle size measurement was 496.

[0048] [Example 10] Except for keeping the temperature of solution A at 60° C. and not adding alkali hydroxide, the experiment was carried out under the same conditions as in Example 1. The average particle size of the silver particles obtained in this example was 44.6 nm, the coefficient of variation was 12.9%, and the total number of particles measured in the particle size measurement was 574.

[0049] [Example 11] The experiment was carried out under the same conditions as in Example 1, except that the temperature of solution A was maintained at 60° C. The average particle size of the silver microparticles obtained in this example was 42.8 nm, the coefficient of variation was 14.9%, and the total number of particles measured in the particle size measurement was 346.

[0050] [Example 12] The experiment was carried out under the same conditions as in Example 1, except that the temperature of solution A was maintained at 35° C. The average particle size of the silver microparticles obtained in this example was 48.7 nm, the coefficient of variation was 17.4%, and the total number of particles measured in the particle size measurement was 574.

[0051] [Example 13] The experiment was carried out under the same conditions as in Example 1, except that the temperature of solution A was maintained at 30° C. The average particle size of the silver microparticles obtained in this example was 49.1 nm, the coefficient of variation was 19.3%, and the total number of particles measured in the particle size measurement was 366.

[0052] [Comparative Example 1] The experiment was carried out under the same conditions as in Example 1, except that no alkali hydroxide was added to solution A, and the entire amount of solution B was added to solution A over 0.05 minutes (3 seconds) in the synthesis of silver fine particles. In this case, the average addition rate of silver added by solution B to solution A is 18.879 mmol / s per kg of initial solution A. The maximum value of the average addition rate of silver per second in the last second at all points from the start to the end of the addition of solution B can be considered to be 18.879 mmol / s, which is almost equal to the above average addition rate per kg of initial solution A. The average particle size of the silver fine particles obtained in this example was 44.4 nm, the coefficient of variation was 22.8%, and the total number of particles measured in the measurement of particle size was 606. Figure 13 shows an example of an SEM photograph of the silver fine particles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. Figure 14 shows a histogram of particle sizes for the silver fine particles obtained in this example.

[0053] [Comparative Example 2] The experiment was carried out under the same conditions as in Example 1, except that no alkali hydroxide was added to solution A, and the entire amount of solution B was added to solution A over 0.25 minutes (15 seconds) in the synthesis of silver microparticles. In this case, the average addition rate of silver added by solution B to solution A is 3.716 mmol / s per kg of initial solution A. The maximum value of the average addition rate of silver per second in the last second at all points from the start to the end of the addition of solution B can be considered to be 3.716 mmol / s, which is almost equal to the above average addition rate per kg of initial solution A. The average particle size of the silver microparticles obtained in this example was 57.5 nm, the coefficient of variation was 24.6%, and the total number of particles measured in the measurement of particle size was 578. Figure 15 shows an example of an SEM photograph of the silver microparticles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. Figure 16 shows a histogram of particle sizes for the silver microparticles obtained in this example.

[0054] [Comparative Example 3] In the synthesis of silver microparticles, the experiment was carried out under the same conditions as in Example 1, except that the entire amount of solution B was added to solution A over 1 minute. In this case, the average addition rate of silver added to solution A by solution B was 0.944 mmol / s per 1 kg of initial solution A. The average particle size of the silver microparticles obtained in this example was 43.8 nm, the coefficient of variation was 25.5%, and the total number of particles measured in the particle size measurement was 402. FIG. 17 shows an example of an SEM photograph of the silver microparticles obtained in this example. The length of the white scale bar shown slightly to the right near the center of the bottom of the photograph corresponds to 100 nm. FIG. 18 shows a histogram of particle sizes for the silver microparticles obtained in this example.

[0055] [Comparative Example 4] In this example, silver nanoparticles were synthesized by the alcohol solvent reduction method as follows. 107.9g of oleylamine and 13.7g of silver nitrate crystals were added to 161.2g of isobutanol, and dissolved at room temperature by stirring with a magnetic stirrer. The solution was transferred to a vessel equipped with a reflux condenser and placed on an oil bath. While blowing nitrogen gas into the vessel at a flow rate of 400mL / min, the solution was heated at a temperature increase rate of 2°C / min while stirring with a magnetic stirrer at a rotation speed of 200 rpm, and refluxed at a temperature of 100°C for 5 hours to complete the reaction.

[0056] After the reaction was completed, the slurry obtained was subjected to solid-liquid separation and washing in the following steps 1 to 4 to obtain a precipitate. 1. After the reaction is completed, the resulting slurry is centrifuged at 5,000 rpm for 60 minutes using a Hitachi Koki centrifuge CF7D2, and the supernatant is discarded. 2. Add ethanol to the precipitate and disperse it using an ultrasonic disperser. 3. Repeat steps 1. to 2. three times. 4. Repeat step 1 above, discard the supernatant and obtain the precipitate. Next, kerosene was added to the obtained precipitate to disperse the precipitate, and the obtained liquid was centrifuged to remove the precipitate, thereby obtaining a dispersion liquid. The silver fine particles in the obtained dispersion liquid were observed at a magnification of 300,000 times using a TEM (transmission electron microscope, manufactured by JEOL Ltd., JEM-1011). On the TEM image, spherical particles were observed to be well dispersed at a predetermined interval. Based on the TEM image, the particle size distribution (particle size distribution) was obtained according to the above-mentioned "Method for measuring the particle size distribution of silver fine particles". Image analysis software (manufactured by Asahi Kasei Engineering Co., Ltd., A-zo-kun (registered trademark)) was used to measure the particle size. As a result, the average particle size of the silver fine particles obtained in this example was 6.6 nm, and the coefficient of variation was 10.5%.

[0057] [Table 1]

[0058] [Table 2]

[0059] In each Example where silver was added slowly and gradually within the range according to the present invention, silver microparticles with a smaller coefficient of variation could be synthesized compared to Comparative Examples 1 to 3 where silver was added at a higher rate. On the other hand, in the alcohol solvent reduction method of Comparative Example 4, silver microparticles with a small average particle size and coefficient of variation could be synthesized, but the treatment time was very long.

Claims

1. In the synthesis of silver fine particles, a solution B in which a silver compound is dissolved is added to a solution A in which octylamine and hydrazine are mixed in an aqueous solvent, and silver is reduced and precipitated in the solution. A method for producing fine silver particles, comprising the steps of: preparing solution A immediately before the start of addition of solution B, in which 0.05 to 6 times the molar amount of octylamine relative to the total molar amount of silver to be added by solution B and 1 to 4 equivalents of hydrazine relative to the total amount of silver to be added by solution B are mixed; and the average addition rate of silver by solution B is 0.4 mmol / s or less calculated per 1 kg of solution A immediately before the start of addition of solution B.

2. A method for producing silver microparticles as described in claim 1, wherein the concentration of the silver compound in solution B is 0.3 to 3 mol / kg in terms of the number of moles of silver.

3. A method for producing silver microparticles as described in claim 2, wherein the silver compound is silver nitrate.

4. A method for producing silver microparticles described in any one of claims 1 to 3, wherein the average addition rate of silver by solution B is 0.2 mmol / s or less per 1 kg of solution A immediately before the start of addition of solution B.

5. The method for producing fine silver particles according to any one of claims 1 to 4, wherein solution A contains an alkali hydroxide in addition to hydrazine and octylamine.

6. The method for producing fine silver particles according to any one of claims 1 to 5, wherein the reduction and precipitation of silver is carried out at a solution temperature of 30 to 80°C.

7. The method for producing fine silver particles according to any one of claims 1 to 6, wherein fine silver particles having an average equivalent circle diameter of 100 nm or less are synthesized.

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