Silver nanoparticles and methods for producing the same

Abstract

To provide a silver nanoparticle which shows low volume resistivity even at a low sintering temperature, and a method for producing the same.SOLUTION: Provided is a core-shell type silver nanoparticle which comprises: a core part having a crystal structure; and a shell part having an amorphous structure for covering the circumference of the core part, and also provided is a method for producing the core-shell type silver nanoparticle by irradiating a reaction liquid with a microwave, which is being a method for adjusting the absorption power of the microwave.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to silver nanoparticles and a method for producing the same.

Background Art

[0002] In recent years, metal nanoparticles having properties different from those of bulk materials have been used and studied in various applications, such as catalysts, ink materials, and electronic component members.

[0003] In the field of electronics mounting, metal nanoparticles, such as silver nanoparticles, are being studied as lead-free bonding materials that can be bonded at low temperatures. Although lead-free solder is difficult to bond at 250°C or lower, lead-free solder containing metal nanoparticles utilizes the characteristics of metal nanoparticles, that is, having a lower melting point compared to bulk materials, while having the melting point of bulk materials when sintered for use in bonding, enabling bonding at 250°C or lower.

[0004] For example, Patent Document 1 discloses silver fine powder composed of silver particles having an average particle diameter D TEM : 3 nm to 20 nm, coated with an organic protective material composed of a primary amine B having 6 to 12 carbon atoms, and a method for producing the same.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] As silver nanoparticles that can be used for high heat-resistant bonding materials, particularly those having various excellent physical and chemical characteristics on the functional surface, those showing a low volume resistivity at a low sintering temperature are preferable for application to a wide range of base materials such as resins.

[0007] Therefore, the object of the present invention is to provide silver nanoparticles that exhibit low volume resistivity even at low sintering temperatures, and a method for producing the same. [Means for solving the problem]

[0008] In silver nanoparticles, reducing the particle size is one way to lower the sintering temperature. However, small particle sizes, such as silver nanoparticles of about 10 nm, require a large amount of protective agent (organic substance) to disperse stably in a dispersion (including paste). This protective agent can adversely affect low-temperature sintering properties.

[0009] Therefore, the present inventors investigated various means to solve the above problem and found that core-shell type silver nanoparticles produced by irradiating a reaction solution containing silver particle raw materials with microwaves having an absorbed power of 1 W / mL to less than 50 W / mL (1 W / mL or more and less than 50 W / mL) relative to the volume of the reaction solution to form and grow silver nanoparticles, and then amorphousizing the surface of the silver nanoparticles by irradiating them with microwaves having an absorbed power of 50 W / mL or more relative to the volume of the reaction solution, exhibit low volume resistivity even at low sintering temperatures, thus completing the present invention.

[0010] In other words, the gist of this invention is as follows: (1) Core-shell type silver nanoparticles comprising a core portion having a crystalline structure and a shell portion having an amorphous structure surrounding the core portion, wherein the average thickness of the shell portion is 1 nm or more as an average of 200 or more particles measured by TEM, and the average particle size of the core-shell type silver nanoparticles is 10 nm to 100 nm as an average of 200 or more particles when the particle size is the average of the major and minor axes of the particles in the TEM photograph. (2) A dispersion containing core-shell type silver nanoparticles as described in (1), wherein the content of core-shell type silver nanoparticles is 10% to 90% by volume relative to the total volume of the dispersion. (3) A method for producing core-shell type silver nanoparticles as described in (1) by irradiating a reaction solution with microwaves, comprising: (i) a first microwave irradiation step of irradiating the reaction solution with microwaves at an absorbed power of 1 W / mL to less than 50 W / mL based on the total volume of the reaction solution to form and grow the core portion of silver nanoparticles; and (ii) a second microwave irradiation step of irradiating the reaction solution in which the core portion of silver nanoparticles has been formed and grown in step (i) with microwaves at an absorbed power of 50 W / mL or more based on the total volume of the reaction solution to form the shell portion of silver nanoparticles. (4) The method according to (3), wherein the absorbed power of the microwaves in the first microwave irradiation step is 5 W / mL to 30 W / mL based on the total volume of the reaction solution. [Effects of the Invention]

[0011] The present invention provides silver nanoparticles that exhibit low volume resistivity even at low sintering temperatures, and a method for producing the same. [Brief explanation of the drawing]

[0012] [Figure 1] This is a transmission electron microscope (TEM) image of the core-shell type silver nanoparticles from Example 2. [Figure 2] Figure 2A is a graph showing the relationship between the microwave irradiation time at an absorbed power of 50 W / mL in the second microwave irradiation step in Comparative Example 1 and Examples 1-3 and the proportion of amorphous silver nanoparticles (amorphous ratio). Figure 2B is a graph showing the relationship between the amorphous ratio and volume resistivity in Comparative Example 1 and Examples 1-3. [Figure 3] Figure 3A is a graph showing the relationship between the microwave absorption power and the amorphous ratio in the second microwave irradiation step in Comparative Examples 2-3 and Examples 5-6. Figure 3B is a graph showing the relationship between the amorphous ratio and the volume resistivity in Comparative Examples 2-3 and Examples 5-6. [Figure 4] This graph shows the relationship between sintering temperature and volume resistivity for silver nanoparticles in Example 4, Comparative Examples 5 and 6.

Best Mode for Carrying Out the Invention

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail. In this specification, the features of the present invention will be described with reference to the drawings as appropriate. Note that the silver nanoparticles of the present invention and the method for producing the same are not limited to the following embodiments, and can be implemented in various forms with modifications and improvements that can be made by those skilled in the art without departing from the gist of the present invention.

[0014] In the core-shell type silver nanoparticles of the present invention, the core portion has a crystal structure. Here, "the core portion has a crystal structure" means that when the particles are observed by TEM (20,000 times magnification), a diffraction contrast of the crystal lattice is observed in the central portion of the particles.

[0015] The particle size of the core portion can be calculated by subtracting the shell portion (the average thickness of the shell portion × 2) from the average particle size of the core-shell type silver nanoparticles.

[0016] In the core-shell type silver nanoparticles of the present invention, the shell portion has an amorphous structure. Here, "the shell portion has an amorphous structure" means that when the particles are observed by TEM (20,000 times magnification), no diffraction contrast of the crystal lattice is observed on the particle surface (here, the surface means a portion having a thickness of at least 1 nm from the outermost surface of the particle toward the inside).

[0017] The average thickness of the shell portion is 1 nm or more, preferably 1 nm to 10 nm, as the average of 200 or more (for example, 200) particles measured by TEM.

[0018] Here, in TEM, the crystal state of the particles can be observed at a magnification of 20,000 times or more.

[0019] In the core-shell type silver nanoparticles of the present invention, since the shell portion has an amorphous structure within the above range, the particles come into contact with each other through the shell portion on the particle surface, that is, in an amorphous state. Compared with crystals, the atoms on the particle surface of amorphous are unstable and diffusion between particles is likely to occur, so sintering occurs at a low temperature. Therefore, when the core-shell type silver nanoparticles of the present invention are sintered, the shell portion which is amorphous is sintered at a sintering temperature lower than the temperature at which the original particles are sintered, and the shell portion binds the particles to each other. As a result, the core-shell type silver nanoparticles of the present invention can bind to each other even at a low sintering temperature and exhibit a low volume resistivity, that is, can form a conductive path.

[0020] The average particle diameter of the core-shell type silver nanoparticles is 10 nm to 100 nm, preferably 30 nm to 80 nm, as the average of 200 or more particles when the particle diameter is the average value of the major axis and minor axis of the particles in the TEM photograph.

[0021] When the average particle diameter of the core-shell type silver nanoparticles is within the above range, the amount of the protective agent required to disperse the particles can be suppressed when the particles are present in a dispersion or a paste.

[0022] In the core-shell type silver nanoparticles of the present invention, the ratio of the particles having an amorphous structure on the particle surface is usually 10 count% or more, preferably 15 count% or more when the particles are imaged 20,000 times by TEM and the particles with an amorphous surface (1 nm or more) among 200 or more particles are counted. The upper limit value of the ratio is not limited, and the larger the better.

[0023] The core-shell type silver nanoparticles of the present invention may be in a powder state or in a state of a dispersion dispersed in a solvent such as water.

[0024] When the core-shell type silver nanoparticles of the present invention are in the state of a dispersion, the dispersion may contain, in addition to the core-shell type silver nanoparticles of the present invention and the solvent, a protective agent for dispersing the particles in the solvent. Examples of the (particle) protective agent are, but are not limited to, one or more protective agents selected from polyvinylpyrrolidone (PVP), thiol polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylate salts, cyclodextrin, aminopectin, methylcellulose, polyethyleneiminecellulose, aliphatic amines, and aliphatic carboxylic acids. The amount of the protective agent may be typically 0.1% to 10% by weight relative to the total weight of the dispersion. When the core-shell type silver nanoparticles of the present invention are in the state of a dispersion, the content of the core-shell type silver nanoparticles in the dispersion is typically 1% by volume or more relative to the total volume of the dispersion.

[0025] Furthermore, the present invention also relates to a dispersion containing core-shell type silver nanoparticles of the present invention, wherein the content of core-shell type silver nanoparticles is 10% to 90% by volume, preferably 10% to 80% by volume, relative to the total volume of the dispersion. The dispersion of the present invention may also be a paste.

[0026] As described above, the core-shell type silver nanoparticles of the present invention have a relatively large average particle size, so they can be introduced into a solvent constituting a dispersion, such as water, in the aforementioned content range.

[0027] The core-shell type silver nanoparticles and dispersions containing these particles of the present invention have a volume resistivity of typically 18 μΩ·cm or less, preferably 15 μΩ·cm or less, and more preferably 10 μΩ·cm or less at a sintering temperature of 120°C. Therefore, in addition to conventional catalysts, electronic component materials, and ink materials, they can be used as high-temperature-resistant lead-free bonding materials in the field of electronics packaging.

[0028] The core-shell type silver nanoparticles of the present invention can be produced by a method for producing core-shell type silver nanoparticles, which includes irradiating a reaction solution with microwaves, comprising: (i) a first microwave irradiation step of irradiating the reaction solution with microwaves at a relatively small absorbed power; and (ii) a second microwave irradiation step of irradiating the reaction solution in which the core portion of the silver nanoparticles has been formed and grown in step (i) with microwaves at an absorbed power greater than that of step (i).

[0029] The following describes each of the steps (i) and (ii).

[0030] (i) Step: First microwave irradiation step In step (i), the reaction solution is irradiated with microwaves to form and grow the core portion of the silver nanoparticles.

[0031] Herein, the reaction solution used in the present invention can be any, and is not limited to, a reaction solution that can be used in a conventional method for producing silver nanoparticles by microwave irradiation. For example, examples of materials contained in the reaction solution are listed below.

[0032] First, the reaction solution contains the raw materials for silver nanoparticles. The raw materials for silver nanoparticles are not limited as long as they can dissolve in a solvent and generate silver ions, but examples include inorganic salts of silver such as hydrochloride, sulfate, nitrate, and phosphate, and organic salts such as carboxylates and sulfonates. The raw materials for silver nanoparticles may also be prepared, for example, by dissolving a material containing metallic silver or silver salts with an acid such as nitric acid or ammonia water. It is preferable to use inexpensive silver nitrate as the raw material for silver nanoparticles.

[0033] The concentration of silver ions in the reaction solution is not limited, but is usually 0.1 mmol / L (mM) to 300 mM, preferably 0.1 mM to 100 mM.

[0034] By setting the concentration of silver ions in the reaction solution within the aforementioned range, the variability of the resulting silver nanoparticles is reduced, or in other words, the particle size distribution of the resulting silver nanoparticles becomes narrower.

[0035] Furthermore, the reaction solution contains a solvent. The solvent used in the reaction solution is not limited as long as it can dissolve materials such as the raw materials for silver nanoparticles, protective agents, and reducing agents, and can also absorb microwaves. Examples of solvents used in the reaction solution include low-boiling point solvents with a boiling point of 300°C or lower. Examples of low-boiling point solvents are not limited, but include water, alcohols such as methanol and ethanol, polyhydric alcohol solvents such as ethylene glycol, ketone solvents such as acetone, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), other organic solvents, or mixtures of two or more of these low-boiling point polar solvents. Water is preferred as the solvent used in the reaction solution.

[0036] By using a low-boiling point solvent in the reaction solution, the handling of the solvent can be improved, and the environmental impact can be reduced.

[0037] Furthermore, the reaction solution contains a protective agent. The protective agent used in the reaction solution is a compound that binds to part or all of the surface of the silver nanoparticles generated in the reaction solution, and is a compound that suppresses aggregation of the silver nanoparticles. Examples of protective agents, though not limited to these, include polyvinylpyrrolidone (PVP), thiol polymers, polyvinyl alcohol (PVA), tannic acid, polyacrylates, cyclodextrin, aminopectin, methylcellulose, polyethyleneiminecellulose, aliphatic amines, aliphatic carboxylic acids, or mixtures of two or more of these.

[0038] The amount of protective agent is not limited and can be changed depending on the desired particle size of the silver nanoparticles, but is usually 0.1 to 20 times, preferably 0.2 to 10 times, the amount of silver.

[0039] By using a protective agent, the aggregation of generated silver nanoparticles can be suppressed.

[0040] Furthermore, the reaction solution contains a reducing agent. The reducing agent is a material that can reduce silver ions to silver with an oxidation state of 0 through a redox reaction.

[0041] Examples of reducing agents, though not limited to them, include citric acid or its salts, such as trisodium citrate, disodium citrate, monosodium citrate, oxalic acid or its salts, such as sodium oxalate, ascorbic acid or its salts, such as sodium ascorbate, or mixtures of two or more of these.

[0042] The amount of reducing agent is not limited as long as it can reduce the silver ions to a metal with an oxidation state of 0 through a redox reaction, but is usually 1.0 to 20 times the equivalent amount of the silver ions, preferably 4.0 to 15 times the equivalent amount. Furthermore, if the reducing agent for silver ions contains one or more functional groups that can interact with the metal, such as carboxyl groups, hydroxyl groups, or ether groups, it may also act as a protective agent. When the reducing agent also acts as a protective agent, the reaction solution does not need to contain the protective agent described above, and the amount of the reducing agent for silver ions may exceed the amount necessary to reduce the silver ions to a metal with an oxidation state of 0 through a redox reaction.

[0043] The reaction solution may consist of the silver nanoparticle raw materials, solvent, protective agent, and reducing agent described above, but it may also further contain additives that are commonly used in conventional methods for producing silver nanoparticles by microwave irradiation, such as chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and / or its salts.

[0044] The pH of the reaction solution is not limited, but is usually between pH 3 and pH 12.

[0045] In this invention, the order of addition of each material, the addition temperature, the mixing method, the mixing time, etc., in the preparation of the reaction solution are not limited, and the mixture is prepared in such a way that a homogeneous reaction solution is obtained. In this invention, the reaction is started after a homogeneous reaction solution has been prepared.

[0046] In this invention, the reaction solution described above is irradiated with microwaves using a microwave synthesis device to allow the reaction to proceed. When microwaves are irradiated onto the reaction solution, the polar solvent contained in the reaction solution absorbs the microwaves and generates heat energy by converting it into thermal energy. Therefore, in the reaction solution irradiated with microwaves, a uniform and rapid temperature rise occurs in the irradiated area, and a uniform and rapid reaction occurs in accordance with this temperature rise.

[0047] In a microwave synthesis apparatus, the material of the container holding the reaction solution is not limited as long as the reaction solution can be uniformly irradiated with microwaves. For example, when microwaves are irradiated into the reaction solution from outside the reactor through the reactor, materials that transmit microwaves, such as ceramics or glass, can be used. When microwaves are irradiated directly into the reaction solution from above, materials that reflect microwaves, such as aluminum or stainless steel, can be used.

[0048] Here, microwaves are generated from a microwave source (microwave oscillator (magnetron)), and the microwave source can be either a single-mode system or a multi-mode system.

[0049] In step (i), the output (absorbed power) of the microwave irradiation source is 1 W / mL to less than 50 W / mL, preferably 5 W / mL to 30 W / mL, and more preferably 5 W / mL to 20 W / mL, based on the total volume of the reaction solution.

[0050] (i) By adjusting the absorbed power of the microwave irradiation source to the aforementioned range in step (i), the core portion of the core-shell type silver nanoparticles can be formed and grown.

[0051] The microwaves in step (i) can be generated under conditions known in the art, except that the absorbed power of the microwave irradiation source is within the aforementioned range.

[0052] For example, the frequency of microwaves generated from a microwave irradiation source can be changed as appropriate and is not limited, but is usually 1 GHz to 10 GHz, preferably 2 GHz to 6 GHz. In this invention, it is preferable to use 2.45 GHz as the frequency of the microwave, which is the frequency of an industrial microwave power supply.

[0053] It is preferable that microwaves are uniformly irradiated onto the object to be reacted, that is, the part of the reaction solution where the reaction takes place.

[0054] It is preferable that the microwaves are uniform during irradiation, and that the microwave irradiation conditions remain constant throughout the microwave irradiation.

[0055] In the present invention, the temperature of the reaction solution heated by microwave irradiation is the reaction temperature, which can be appropriately changed depending on the reaction conditions (type of solvent, pressure during reaction, etc.) and is not limited, but is usually 25°C or higher, preferably 80°C or higher. The upper limit of the reaction temperature is not limited, but is usually below the boiling point of the solvent. For example, when the solvent is water, the reaction temperature at atmospheric pressure is usually in the range of 25°C or higher and less than 100°C, preferably 80°C to 90°C.

[0056] By raising the reaction temperature to 25°C or higher, the reduction reaction from silver ions to silver nanoparticles occurs. By keeping the reaction temperature below the boiling point of the solvent, the disorder in the particle size distribution of the resulting silver nanoparticles, i.e., the broadening of the particle size distribution, which can occur due to the non-uniformity of the reaction field caused by the boiling of the reaction solution, is prevented, and silver nanoparticles with small and uniform particle size can be prepared.

[0057] The microwave irradiation time for the reaction solution is the time it takes for the reaction solution to reach the reaction temperature. This time is not limited to the microwave irradiation time, but is appropriately changed depending on the reaction conditions (microwave conditions, type of solvent, pressure during the reaction, amount of reaction solution, reaction temperature, etc.). It is usually between 0.1 seconds and 300 seconds, preferably between 0.1 seconds and 5 seconds.

[0058] (ii) Step: Second microwave irradiation step In step (ii), the reaction solution in which the core portion of the silver nanoparticles was formed and grown in step (i) is continuously irradiated with microwaves of a higher irradiation intensity than in step (i) to form the shell portion of the silver nanoparticles.

[0059] In step (ii), the output (absorbed power) of the microwave irradiation source is 50 W / mL or more, preferably 50 W / mL to 600 W / mL, and more preferably 100 W / mL to 400 W / mL, based on the total volume of the reaction solution.

[0060] (ii) By adjusting the absorbed power of the microwave irradiation source to the aforementioned range in step (ii), the shell portion of the core-shell type silver nanoparticles can be formed.

[0061] In step (ii), the microwave irradiation time of the reaction solution is appropriately changed depending on the reaction conditions (microwave conditions, type of solvent, pressure during reaction, amount of reaction solution, reaction temperature, etc.) and is not limited, but is usually 0.1 seconds to 30 minutes, preferably 1 second to 15 minutes, and more preferably 5 seconds to 10 minutes.

[0062] Furthermore, the microwaves used in process (ii) may be the same as those used in process (i), except for the output power (absorbed power) of the microwave irradiation source.

[0063] Step (ii) can be completed by stopping microwave irradiation, allowing it to cool at room temperature, and obtaining silver nanoparticles.

[0064] Therefore, in the method for producing core-shell type silver nanoparticles of the present invention, the core portion of the core-shell type silver nanoparticle is formed and grown by irradiating with microwaves with a relatively small absorption power in step (i), and the core portion of the core-shell type silver nanoparticle is transformed into a shell portion by irradiating with microwaves with a larger absorption power than in step (i) in step (ii), thereby producing the core-shell type silver nanoparticles of the present invention. In step (ii), the microwaves are reflected from the particle surface, and the addition of high energy causes a specific reaction to proceed at the particle surface, making it possible to synthesize amorphous particles with an irregular atomic arrangement. In amorphous particles, the atoms on the particle surface are unstable, diffusion between particles occurs easily, and conductive paths can be formed even at low temperatures. [Examples]

[0065] The following describes some embodiments of the present invention, but the present invention is not intended to be limited to those shown in these embodiments.

[0066] 1. Investigation of microwave irradiation time in the second microwave irradiation process Comparative Example 1 (1) Silver nitrate, DMF, and PVP were dissolved in water to prepare aqueous solutions of silver nitrate, DMF, and PVP, respectively, so that their concentrations in the reaction solution were 100 mM silver nitrate, 2000 mM DMF, and 600 mM PVP. (2) The aqueous solutions prepared in (1) were mixed together to prepare the reaction solution. (3) The reaction solution prepared in (2) was irradiated with microwaves while stirring the reaction solution at an absorbed power of 10 W / mL based on the total volume of the reaction solution until the temperature of the reaction solution reached 90°C, thereby synthesizing silver nanoparticles.

[0067] Example 1 (1) Silver nitrate, DMF, and PVP were dissolved in water to prepare aqueous solutions of silver nitrate, DMF, and PVP, respectively, so that their concentrations in the reaction solution were 100 mM silver nitrate, 2000 mM DMF, and 600 mM PVP. (2) The aqueous solutions prepared in (1) were mixed together to prepare the reaction solution. (3) The reaction solution prepared in (2) was irradiated with microwaves at an absorbed power of 10 W / mL based on the total volume of the reaction solution, while stirring the reaction solution, until the temperature of the reaction solution reached 90°C (first microwave irradiation step). (4) The reaction solution heated to 90°C in (3) was irradiated with microwaves for 1 second at an absorbance power of 50 W / mL based on the total volume of the reaction solution, while stirring the reaction solution, to synthesize silver nanoparticles (second microwave irradiation step).

[0068] Example 2 Silver nanoparticles were synthesized in the same manner as in Example 1, except that the microwave irradiation time in the second microwave irradiation step of Example 1 (4) was set to 5 seconds.

[0069] Example 3 Silver nanoparticles were synthesized in the same manner as in Example 1, except that the microwave irradiation time in the second microwave irradiation step of Example 1 (4) was set to 10 seconds.

[0070] Example 4 Silver nanoparticles were synthesized in the same manner as in Example 1, except that the second microwave irradiation step in (4) of Example 1 was modified as follows. (4) The reaction solution heated to 90°C in (3) was intermittently irradiated with microwaves at an absorbed power of 50 W / mL based on the total volume of the reaction solution, while stirring the reaction solution, and the temperature of the reaction solution was maintained at 90°C for 15 minutes (900 seconds) to synthesize silver nanoparticles.

[0071] Table 1 summarizes the proportion of core-shell type silver nanoparticles (amorphous proportion) where the particle surface (1 nm or larger) is amorphous, the average particle size, and the volume resistivity for the silver nanoparticles of Comparative Example 1 and Examples 1-4. Figure 1 shows a TEM image of Example 2. Figure 2A shows the relationship between the irradiation time and the amorphous proportion of microwaves with an absorbed power of 50 W / mL in the second microwave irradiation step in Comparative Example 1 and Examples 1-3. Figure 2B shows the relationship between the amorphous proportion and the volume resistivity in Comparative Example 1 and Examples 1-3.

[0072] Here, the amorphous ratio was calculated by counting the number of particles with a amorphous surface (1 nm or larger) among 200 or more particles when the particles were imaged at 20,000x magnification using TEM.

[0073] The volume resistivity was measured as follows: (1) The silver nanoparticle dispersions synthesized in the comparative example or example were allowed to settle by centrifugation, and the supernatant was removed. (2) The procedure in (1) was repeated until the conductivity of the dispersion reached 10 μS. (3) The dispersion obtained in (2) was concentrated using an evaporator until the concentration of silver nanoparticles reached 15% by weight to prepare an ink. (4) The ink obtained in (3) was dropped onto a glass plate and dried at room temperature for more than 12 hours. (5) The silver nanoparticles dried in (4) were sintered at 120°C, and the resistance of the sintered material was measured using the four-terminal method. (6) The film thickness of the sintered material whose resistance was measured in (5) was determined by observing the exposed cross-section of the sintered material using a scanning electron microscope (SEM).

[0074] [Table 1]

[0075] Table 1 and Figure 2A show that irradiating with microwaves with an absorbed power of 50 W / mL for more than 1 second results in an amorphous content of 10 count% or more. Furthermore, Table 1 and Figure 2B show that as the amorphous content increases, the volume resistivity decreases. In addition, the results from Example 4 show that even if the microwave irradiation time in step (4) of Example 1 is 15 minutes, it is possible to produce silver nanoparticles with an amorphous structure.

[0076] 2. Investigation of microwave absorption power in the second microwave irradiation process Comparative Example 2 (1) Silver nitrate, DMF, and PVP were dissolved in water to prepare aqueous solutions of silver nitrate, DMF, and PVP, respectively, so that their concentrations in the reaction solution were 100 mM silver nitrate, 2000 mM DMF, and 600 mM PVP. (2) The aqueous solutions prepared in (1) were mixed together to prepare the reaction solution. (3) The reaction solution prepared in (2) was irradiated with microwaves at an absorbed power of 10 W / mL based on the total volume of the reaction solution, while stirring the reaction solution, until the temperature of the reaction solution reached 90°C (first microwave irradiation step). (4) The reaction solution heated to 90°C in (3) was irradiated with microwaves for 5 seconds at an absorbed power of 10 W / mL based on the total volume of the reaction solution, while stirring the reaction solution, to synthesize silver nanoparticles (second microwave irradiation step).

[0077] Comparative Example 3 Silver nanoparticles were synthesized in the same manner as in Comparative Example 2, except that in the second microwave irradiation step of (4) of Comparative Example 2, the absorbed power of the microwaves was set to 30 W / mL based on the total volume of the reaction solution.

[0078] Example 5 Silver nanoparticles were synthesized in the same manner as in Comparative Example 2, except that in the second microwave irradiation step of (4) of Comparative Example 2, the absorbed power of the microwaves was set to 50 W / mL based on the total volume of the reaction solution.

[0079] Example 6 Silver nanoparticles were synthesized in the same manner as in Comparative Example 2, except that in the second microwave irradiation step of (4) of Comparative Example 2, the absorbed power of the microwaves was set to 100 W / mL based on the total volume of the reaction solution.

[0080] Table 2 summarizes the amorphous ratio, average particle size, and volume resistivity for Comparative Examples 2-3 and Examples 5-6. Figure 3A shows the relationship between the absorbed power of the microwaves in the second microwave irradiation step (4) and the amorphous ratio in Comparative Examples 2-3 and Examples 5-6. Figure 3B shows the relationship between the amorphous ratio and volume resistivity in Comparative Examples 2-3 and Examples 5-6.

[0081] [Table 2]

[0082] Table 2 and Figure 3A show that after heating the reaction solution to 90°C in the first microwave irradiation step, irradiating it with microwaves with an absorbed power of 50 W / mL or more in the second microwave irradiation step resulted in an amorphous content of 10 count% or more. Furthermore, Table 2 and Figure 3B show that as the amorphous content increases, the volume resistivity decreases.

[0083] 3. Examination of the first and second microwave irradiation processes Comparative Example 4 (1) Silver nitrate, DMF, and PVP were dissolved in water to prepare aqueous solutions of silver nitrate, DMF, and PVP, respectively, so that their concentrations in the reaction solution were 100 mM silver nitrate, 2000 mM DMF, and 600 mM PVP. (2) The aqueous solutions prepared in (1) were mixed together to prepare the reaction solution. (3) The reaction solution prepared in (2) was irradiated with microwaves at an absorbed power of 50 W / mL based on the total volume of the reaction solution, while stirring the reaction solution, until the temperature of the reaction solution reached 90°C (first microwave irradiation step). (4) The reaction solution heated to 90°C in (3) was intermittently irradiated with microwaves at an absorbed power of 50 W / mL based on the total volume of the reaction solution, while stirring the reaction solution, and the temperature of the reaction solution was maintained at 90°C for 15 minutes to synthesize silver nanoparticles (second microwave irradiation step).

[0084] Comparative Example 5 Silver nanoparticles were synthesized in the same manner as in Comparative Example 4, except that in the first and second microwave irradiation steps of Comparative Example 4 (3) and (4), the absorbed power of the microwaves was set to 10 W / mL based on the total volume of the reaction solution.

[0085] Comparative Example 6 (1) Silver nitrate, DMF, and PVP were dissolved in water to prepare aqueous solutions of silver nitrate, DMF, and PVP, respectively, so that their concentrations in the reaction solution were 100 mM silver nitrate, 2000 mM DMF, and 600 mM PVP. (2) The aqueous solutions prepared in (1) were mixed together to prepare the reaction solution. (3) The reaction solution prepared in (2) was heated in an oil bath to 90°C while stirring the reaction solution. (4) The reaction solution heated to 90°C in (3) was kept at 90°C in an oil bath for 15 minutes while stirring to synthesize silver nanoparticles.

[0086] Table 3 summarizes the amorphous content, average particle size, and volume resistivity of Comparative Examples 4-6.

[0087] [Table 3]

[0088] Table 3 shows that even if the first microwave irradiation step is omitted, or the second microwave irradiation step is omitted, or if the reaction is carried out in an oil bath, it is not possible to create an amorphous structure of silver nanoparticles.

[0089] Table 4 and Figure 4 show the relationship between sintering temperature and volume resistivity for silver nanoparticles in Example 4, Comparative Examples 5 and 6. Note that volume resistivity was measured by varying the sintering temperature in the volume resistivity measurement method described above.

[0090] [Table 4]

[0091] Table 4 and Figure 4 show that amorphous structures of silver nanoparticles can be created by performing the first and second microwave irradiation steps, and these core-shell type silver nanoparticles exhibit low volume resistivity even at low sintering temperatures.

Claims

1. A core portion having a crystalline structure, A shell portion having an amorphous structure that surrounds the core portion. Silver nanoparticles comprising core-shell type silver nanoparticles, The proportion of core-shell type silver nanoparticles in silver nanoparticles is between 10 count% and 38 count% when silver nanoparticles are imaged at 20,000x magnification using TEM and core-shell type silver nanoparticles are counted among 200 or more silver nanoparticles. The average thickness of the shell portion of the core-shell type silver nanoparticles is 1 nm or more, as measured by TEM for 200 or more silver nanoparticles. The average particle size of silver nanoparticles, when the particle size is defined as the average of the major and minor axes of the particles in a TEM image, is between 10 nm and 100 nm on average for 200 or more silver nanoparticles. Silver nanoparticles containing core-shell type silver nanoparticles.

2. A dispersion containing silver nanoparticles, including core-shell type silver nanoparticles, as described in claim 1, wherein the content of silver nanoparticles including core-shell type silver nanoparticles is 10% by volume to 90% by volume relative to the total volume of the dispersion.

3. A method for producing silver nanoparticles containing core-shell type silver nanoparticles as described in claim 1 by irradiating a reaction solution with microwaves, (i) A first microwave irradiation step in which microwaves are irradiated onto the reaction solution with an absorbed power of 1 W / mL to less than 50 W / mL based on the total volume of the reaction solution to form and grow the core portion of silver nanoparticles, wherein the reaction solution includes a solvent, a raw material for silver nanoparticles that dissolves in the solvent to generate silver ions, a protective agent and a reducing agent, and (ii) A second microwave irradiation step in which the reaction solution in which the core portion of silver nanoparticles was formed and grown in step (i) is irradiated with microwaves at an absorbed power of 50 W / mL or more based on the total volume of the reaction solution to form the shell portion of the silver nanoparticles. A method that includes this.

4. The method according to claim 3, wherein the absorbed power of microwaves in the first microwave irradiation step is 5 W / mL to 30 W / mL based on the total volume of the reaction solution.

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