Microwave Irradiation Device and Method for Producing Metal Nanoparticles

Abstract

To provide a microwave irradiator for producing metal nanoparticles allowed to prepare metal nanoparticles small in particle size and uniform, and a method of producing metal nanoparticles.SOLUTION: The present invention pertains to a microwave irradiator comprising a reaction container for receiving a reaction liquid containing a raw material of metal nanoparticles, a cooling mechanism for cooling the reaction container from external, and a microwave irradiation source for irradiating the reaction liquid with a microwave, in which a solid substance having a dielectric constant lower than a dielectric constant of the reaction liquid is provided within the reaction container, and a method of producing metal nanoparticles using the microwave irradiator.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a microwave irradiation device and a method for manufacturing metal nanoparticles.

Background Art

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

[0003] In addition, various devices and methods for manufacturing metal nanoparticles have been devised. Among them, a manufacturing device and a manufacturing method for metal nanoparticles using microwaves have attracted attention as those capable of synthesizing metal nanoparticles in a short time.

[0004] As such a manufacturing device for metal nanoparticles, for example, Patent Document 1 discloses a method for manufacturing metal fine particles in which a reaction solution containing a metal precursor is circulated in a flow tube, and electromagnetic waves are uniformly and intensively irradiated into the flow tube in the longitudinal direction of the flow tube, and the electromagnetic wave irradiation space in the flow tube is uniformly heated in the flow direction to generate metal fine particles.

[0005] Patent Document 2 discloses a manufacturing device for metal nanoparticles including a chamber for accommodating a reaction solution, a plurality of microwave irradiation sources installed in the chamber, the microwave irradiation ports of which are dome-shaped and are to be covered with the reaction solution, and a stirring mechanism for stirring the reaction solution installed in the chamber.

[0006] Non-Patent Document 1 discloses a temperature control reactor for a flow-through type microwave chemical reaction device capable of controlling the temperature of a reaction solution to be constant while irradiating microwaves.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

[0008] [Non-Patent Document 1] Mitsuhiro Matsuzawa et al., "Development of a Temperature-Controlled Reactor for a Flow-Processing-Type Microwave Chemical Reactor", The 75th Annual Meeting of the Chemical Engineering Society of Japan, 2010, p. 381 [Summary of the Invention] [Problems to be Solved by the Invention]

[0009] In the field of electronics packaging, metal nanoparticles are being studied as high-temperature-resistant bonding materials. Examples of high-temperature-resistant bonding materials include lead-free bonding materials (e.g., lead-free solder) that can be bonded at low temperatures. Usually, it is difficult to bond lead-free solder at 250°C or lower. However, lead-free solder containing metal nanoparticles has a lower melting point as a characteristic of the metal nanoparticles compared to bulk materials. On the other hand, lead-free solder containing metal nanoparticles has the melting point of a bulk material when it is used for bonding and sintered. Therefore, with lead-free solder containing metal nanoparticles, bonding at 250°C or lower becomes possible.

[0010] When using metal nanoparticles as a high-temperature-resistant bonding material, it is necessary to make the melting point of the metal nanoparticles constant. In order to make the melting point of the metal nanoparticles constant, it is desirable to reduce the particle size and further narrow the particle size distribution of the metal nanoparticles.

[0011] Although the development of metal nanoparticles that can be applied to such various uses is progressing, in the method of producing metal nanoparticles by irradiating a reaction solution with microwaves, it is difficult to prepare the metal nanoparticles while keeping the particle size small and uniform. Here, the particle size being uniform means that the particle size distribution is narrow.

[0012] Therefore, an object of the present invention is to provide a microwave irradiation device (a metal nanoparticle manufacturing device) for manufacturing metal nanoparticles capable of preparing metal nanoparticles having a small and uniform particle size, and a method for manufacturing metal nanoparticles.

Means for Solving the Problems

[0013] In a metal nanoparticle manufacturing apparatus and a manufacturing method for manufacturing metal nanoparticles by irradiating a reaction solution with microwaves, when the reaction solution is continuously irradiated with microwaves, the temperature of the reaction solution continuously rises. Therefore, in the prior art, the temperature of the reaction solution is controlled by shortening the irradiation time of microwaves to the reaction solution or flowing a refrigerant having a low dielectric constant around the outer periphery of the reaction tube containing the reaction solution.

[0014] However, it has been found that when the irradiation time of microwaves to the reaction solution is shortened, the reaction rate of the metal nanoparticles becomes low. Furthermore, in order to increase the reaction rate of the metal nanoparticles, when the irradiation time of microwaves to the reaction solution is lengthened while flowing a refrigerant around the outer periphery of the reaction tube, the particle size of the synthesized metal nanoparticles may become non-uniform and the quality may deteriorate.

[0015] The inventors of the present invention have variously examined the cause of the variation in the particle size of the obtained metal nanoparticles when the reaction solution being heated by microwaves is cooled by a refrigerant from the outside of the reaction tube in a metal nanoparticle manufacturing apparatus and a manufacturing method for manufacturing metal nanoparticles by irradiating a reaction solution with microwaves. As a result, the inventors of the present invention have found that the temperature of the reaction solution in contact with the inner wall surface of the reaction tube in contact with the refrigerant on the outer wall surface of the reaction tube in the reaction solution becomes low, and the temperature of the reaction solution becomes high as it moves away from the portion (as it approaches the center of the reaction tube), that is, the temperature distribution of the reaction solution accommodated in the reaction tube is not uniform.

[0016] Therefore, the present inventors have variously studied means for solving the above problems. As a result, the present inventors have found that in a method for producing metal nanoparticles by irradiating a reaction solution with microwaves, by cooling the periphery of the reaction vessel with a refrigerant and introducing a solid substance that is difficult to absorb microwaves into the reaction vessel, the temperature distribution of the reaction solution in the reaction vessel becomes uniform, and metal nanoparticles having a small and uniform particle size can be prepared, and thus the present invention has been completed.

[0017] That is, the gist of the present invention is as follows. (1) A microwave irradiation device comprising a reaction vessel for containing a reaction solution containing a raw material for metal nanoparticles, a cooling mechanism for cooling the reaction vessel from the outside, and a microwave irradiation source for irradiating the reaction solution with microwaves, wherein a solid substance having a dielectric constant lower than that of the reaction solution is provided inside the reaction vessel. (2) The microwave irradiation device according to (1), wherein the relative dielectric constant of the solid substance is 10 or less. (3) The microwave irradiation device according to (1) or (2), wherein the solid substance is provided at the central portion of the reaction vessel. (4) The microwave irradiation device according to any one of (1) to (3), wherein the solid substance occupies 2% to 10% by volume based on the total volume of the reaction vessel. (5) A method for producing metal nanoparticles including a step of irradiating a reaction solution with microwaves, the method comprising: (i) a step of containing the reaction solution and a solid substance having a dielectric constant lower than that of the reaction solution in a reaction vessel; and (ii) a step of irradiating the reaction solution with microwaves while cooling the reaction vessel from the outside. (6) The method according to (5), wherein the relative dielectric constant of the solid substance is 10 or less. (7) The method according to (5) or (6), wherein the solid substance is disposed at the central portion of the reaction vessel. (8) The method according to any one of (5) to (7), wherein the volume of the solid substance is adjusted to occupy 2% to 10% by volume of the total volume of the reaction vessel.

Advantages of the Invention

[0018] The present invention provides a microwave irradiation apparatus for producing metal nanoparticles capable of preparing metal nanoparticles having a small and uniform particle size, and a method for producing metal nanoparticles.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0020] Hereinafter, some 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. In the drawings, the dimensions and shapes of each part are exaggerated for clarity. Therefore, the actual dimensions and shapes are not accurately depicted in the drawings. Accordingly, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. Note that the microwave irradiation device and the method for manufacturing metal nanoparticles of the present invention are not limited to the following embodiments. The microwave irradiation device and the method for manufacturing metal nanoparticles of the present invention 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.

[0021] The present invention relates to a method for manufacturing metal nanoparticles including a step of irradiating a reaction solution with microwaves, the method including: (i) a step of accommodating a reaction solution and a solid substance having a dielectric constant lower than that of the reaction solution in a reaction vessel; and (ii) a step of irradiating the reaction solution with microwaves while cooling the reaction vessel from the outside.

[0022] In the step (i) of the present invention, a reaction solution and a solid substance (simply referred to as "solid substance") having a dielectric constant lower than that of the reaction solution are accommodated in a reaction vessel.

[0023] In the present invention, the reaction vessel is a vessel for accommodating a reaction solution. As the material of the reaction vessel, a material known in the art can be used. As the material of the vessel for accommodating the reaction solution, a material that transmits microwaves, such as ceramics, glass (quartz), etc., is used for the portion that irradiates the reaction solution with microwaves through the vessel.

[0024] The shape of the reaction vessel is not limited. Examples of the shape of the reaction vessel include a reaction tube shape (cylindrical shape), a polygonal prism shape, such as a rectangular parallelepiped, a plate shape, etc. The reaction tube shape is preferred as the shape of the reaction vessel.

[0025] In the shape of the reaction vessel, when the traveling direction of the microwave irradiated in the step (ii) described below is the z-axis direction, and the directions perpendicular to the traveling direction of the microwave are the x-axis direction and the y-axis direction, the z-axis lengths of the xz plane viewed from the y-axis direction and the yz plane viewed from the x-axis direction in the reaction vessel are preferably determined based on the penetration depth of the microwave irradiated from the microwave irradiation source into the reaction solution.

[0026] The penetration depth D of one microwave into the reaction solution can be expressed by the following formula.

Equation

[0027] In the formula, f is the frequency of the microwave [Hz], and ε r is the relative permittivity of the dielectric, and tanδ is the dielectric loss angle of the dielectric. Therefore, when there is one microwave irradiation source, the maximum value of the z-axis length is the penetration depth D of the microwave into the reaction solution. For example, when the solvent of the reaction solution is water (5°C) and the frequency of the microwave is 2.45 GHz, the maximum value of the z-axis length is 5 mm. For example, when the solvent of the reaction solution is water (5°C), the z-axis length is usually adjusted to 2.5 mm to 13 mm, and in one embodiment, 2.5 mm to 5 mm, depending on the frequency of the microwave.

[0028] The reaction solution used in the present invention is not limited. The reaction solution can use the composition of a reaction solution that can be used in a method for producing metal nanoparticles by irradiating microwaves in the technical field. For example, examples of the materials contained in the reaction solution are given below.

[0029] First, the reaction solution contains raw materials for metal nanoparticles. Examples of the metal nanoparticles include noble metal nanoparticles, base metal nanoparticles, and alloy nanoparticles, such as gold nanoparticles, silver nanoparticles, platinum nanoparticles, copper nanoparticles, nickel nanoparticles, iron nanoparticles, cobalt nanoparticles, and alloy nanoparticles of these metals. In one embodiment, the metal nanoparticles are silver nanoparticles. The raw materials for these metal nanoparticles are not limited as long as they can be dissolved in a solvent to generate metal ions. Examples of the raw materials for metal nanoparticles include inorganic salts of metals such as metal hydrochlorides, metal sulfates, metal nitrates, and metal phosphates, organic salts of metals such as metal carboxylates and metal sulfonates, and metal complexes containing metal complex salts. The raw materials for metal nanoparticles may be prepared, for example, by dissolving a material containing a metal or a metal salt in an acid such as nitric acid or a base such as aqueous ammonia. In one embodiment, nitrate salts, such as silver nitrate, which are inexpensive, are used as the raw materials for metal nanoparticles.

[0030] The concentration of metal ions in the reaction solution is not limited. The concentration of metal ions in the reaction solution is usually 0.1 mmol / L (mM) to 300 mM, and in one embodiment, it is 0.1 mM to 100 mM.

[0031] By setting the concentration of metal ions in the reaction solution within the above range, the variation in the obtained metal nanoparticles becomes smaller. In other words, the particle size distribution of the obtained metal nanoparticles becomes narrower.

[0032] Furthermore, the reaction solution contains a solvent. The solvent used in the reaction solution can dissolve and disperse materials such as raw materials of metal nanoparticles, protective agents, and reducing agents. Furthermore, the solvent used in the reaction solution can absorb microwaves. Therefore, in one embodiment, the solvent used in the reaction solution is a polar solvent or an ionic liquid. In addition, examples of the solvent used in the reaction solution include low-boiling solvents having a boiling point of 300 °C or lower. The low-boiling solvent is not limited. Examples of the low-boiling solvent 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 low-boiling polar solvents such as mixtures of two or more of these. In one embodiment, the solvents used in the reaction solution are ethylene glycol and DMF.

[0033] By using a low-boiling solvent as the solvent used in the reaction solution, the handleability of the solvent can be improved and the environmental load can be reduced.

[0034] Furthermore, the reaction solution contains a protective agent. The protective agent used in the reaction solution is a compound that binds to a part or the whole surface of the metal nanoparticles generated in the reaction solution and suppresses the aggregation of the metal nanoparticles. The protective agent is not limited. Examples of the protective agent include polyvinylpyrrolidone (PVP), thiol-based polymers, polyvinyl alcohol (PVA), tannic acid, and the like. In one embodiment, the protective agent is PVP.

[0035] The amount of the protective agent is not limited and can be changed according to the desired particle size of the metal nanoparticles. The amount of the protective agent is usually 0.1 to 20 times, and in one embodiment, 0.2 to 10 times the amount of the metal substance.

[0036] By using the protective agent, the aggregation of the generated metal nanoparticles can be suppressed.

[0037] Furthermore, the reaction solution contains a reducing agent. The reducing agent is a material that can reduce metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction.

[0038] The reducing agent is not limited. Examples of the reducing agent include citric acid or citrate, such as trisodium citrate, disodium citrate, monosodium citrate, oxalic acid or oxalate, such as sodium oxalate, ascorbic acid or ascorbate, such as sodium ascorbate, DMF, mixtures of two or more of these, and the like. In one embodiment, the reducing agent for metal ions, particularly silver ions, is DMF.

[0039] The amount of the reducing agent is not limited as long as it can reduce metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction. The amount of the reducing agent is usually 1.0 to 20 equivalents, and in one embodiment, 4.0 to 15 equivalents, relative to the metal ions. Incidentally, when the reducing agent for metal ions contains one or more functional groups capable of interacting with the metal, such as a carboxy group, a hydroxy group, and an ether group, it can also act as a protecting agent. When the reducing agent also acts as a protecting agent, the reaction solution may not contain the protecting agent described above. Also, in such a case, the amount of the reducing agent for metal ions may be an amount exceeding the amount necessary to reduce metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction.

[0040] The reaction solution may be composed of the raw material of the metal nanoparticles, the solvent, the protecting agent, and the reducing agent described above. In one embodiment, the reaction solution can also contain an additive in addition to these materials. The additive is an additive that can be usually used in a reaction solution used in a conventional method for producing metal nanoparticles by irradiating microwaves.

[0041] For example, the reaction solution may further contain a chelating agent, such as ethylenediaminetetraacetic acid (EDTA) and / or ethylenediaminetetraacetate, as an additive.

[0042] The pH of the reaction solution is not limited, and the pH of the reaction solution is usually pH 3 to pH 12.

[0043] In the present invention, the order of addition, addition temperature, mixing method, mixing time, etc. of each material in the preparation of the reaction solution are not limited. In the present invention, the reaction solution is mixed so that a uniform reaction solution is prepared. In the present invention, the reaction is started after a uniform reaction solution is prepared.

[0044] In the present invention, the solid substance having a dielectric constant lower than that of the reaction solution is a solid substance that hardly absorbs microwaves. Therefore, the relative dielectric constant of the solid substance is usually 10 or less, preferably 6 or less. The lower limit value of the relative dielectric constant of the solid substance is not limited because the more difficult it is for the solid substance to absorb microwaves, the better. The relative dielectric constant of the solid substance is usually 1 or more, preferably 2 or more. Examples of the solid substance include plastics such as Teflon (registered trademark), polystyrene, silicone, nylon, epoxy, melamine, and ceramics such as quartz, borosilicate glass, wood, and paper.

[0045] Since the solid substance has a dielectric constant lower than that of the reaction solution, in step (ii) described below, even when microwaves are irradiated, it hardly absorbs microwaves and therefore hardly generates heat.

[0046] The solid substance is disposed inside the reaction vessel. The solid substance is preferably disposed at a position equally distant from the wall surface of the reaction vessel (the inner wall surface of the outer wall surface that will come into contact with the refrigerant in the reaction vessel), that is, at the central portion of the reaction vessel inside the reaction vessel. For example, when the reaction vessel is a reaction tube, the solid substance preferably has the same central axis as the central axis of the reaction tube and is disposed in a cylindrical shape having a diameter smaller than the diameter of the reaction tube. The solid substance may be hollow, because the relative dielectric constant of air is about 1. Even when the solid substance is disposed in the reaction tube, the reaction solution is not divided into two or more, and the continuity of the reaction solution is maintained.

[0047] The volume of the solid substance is arranged to occupy usually 2 to 10% by volume, preferably 3 to 5% by volume of the total volume of the reaction vessel. For example, when the reaction vessel is a reaction tube and the shape of the solid substance is a columnar shape having the same central axis as the central axis of the reaction tube and a diameter smaller than the diameter of the reaction tube, with respect to the inner diameter of the reaction tube, usually 8 mm to 14 mm, for example 11 mm, the diameter of the solid substance is usually 1 mm to 3 mm, for example 2 mm.

[0048] After the reaction vessel is filled with the reaction solution and the solid substance, subsequently, as step (ii), while cooling the reaction vessel from the outside, the reaction solution is irradiated with microwaves.

[0049] Cooling from the outside of the reaction vessel can be carried out by a method known in the art. For example, cooling from the outside of the reaction vessel can be carried out by a cooling mechanism for cooling the reaction vessel from the outside. For example, the cooling mechanism is a mechanism for continuously contacting the outer wall surface of the reaction vessel with a refrigerant cooled to usually 5°C to 100°C (depending on the holding temperature of the irradiated object and / or the irradiation intensity of the microwaves, etc.), that is, a low dielectric constant medium that is difficult to absorb microwaves (for example, Fluorinert (trademark), manufactured by 3M Japan Limited).

[0050] Irradiation of the reaction solution with microwaves is carried out such that the microwaves from the microwave irradiation source are irradiated onto the entire surface of the reaction vessel (reaction solution) facing the microwave irradiation source. Here, "the microwaves are irradiated onto the entire surface of the reaction vessel" means that when the microwave propagation direction is the z-axis direction and the directions perpendicular to the microwave propagation direction are the x-axis direction and the y-axis direction, the orthographic projection area of the xy plane formed by the microwaves from the microwave irradiation source as viewed from the z-axis direction is larger than the orthographic projection area of the xy plane of the reaction solution. Note that the orthographic projection area of the xy plane of the microwaves depends on the orthographic projection area of the xy plane of the waveguide through which the microwaves pass.

[0051] The microwaves to be irradiated may be one or two or more.

[0052] Microwaves are generated from a microwave irradiation source (a microwave oscillator (magnetron)). The microwave irradiation source can be used in either a single-mode system or a multi-mode system. In one embodiment, the microwave irradiation source is a single-mode system used in Sim.

[0053] The output of the microwave irradiation source is not limited. The output of the microwave irradiation source is usually from 1 W to 6000 W.

[0054] By adjusting the output of the microwave irradiation source within the above range, metal nanoparticles with small and uniform particle sizes can be prepared by microwaves with the existing output.

[0055] The frequency of the microwaves generated from the microwave irradiation source can be appropriately changed and is not limited. The frequency of the microwaves is usually from 0.9 GHz to 10 GHz, and in one embodiment, it is from 2 GHz to 6 GHz. In one embodiment, as the frequency of the microwaves, 2.45 GHz, which is the frequency of an industrial microwave power supply, is used.

[0056] In one embodiment, the microwaves are uniform during irradiation. In one embodiment, the above-mentioned microwave irradiation conditions are constant while the microwaves are being irradiated.

[0057] In the present invention, the temperature of the reaction solution heated by microwave irradiation is the reaction temperature. The reaction temperature can be appropriately changed according to the reaction conditions (type of metal, type of solvent, pressure during the reaction, etc.) and is not limited. The reaction temperature is usually 25°C or higher, and in one embodiment, it is 80°C or higher. The upper limit of the reaction temperature is not limited. In one embodiment, the upper limit of the reaction temperature is usually lower than the boiling point of the solvent. The reaction temperature is, for example, when the solvent is water, at atmospheric pressure, usually in the range of 25°C or higher and lower than 100°C, and in one embodiment, it is from 80°C to 90°C.

[0058] By setting the reaction temperature to 25°C or higher, a reduction reaction from metal ions to metal nanoparticles occurs. On the other hand, when the reaction solution boils, the non-uniformity of the reaction field occurs. When the non-uniformity of the reaction field occurs, the particle size of the generated metal nanoparticles is disturbed, and as a result, the particle size distribution may spread. Therefore, by setting the reaction temperature below the boiling point of the solvent, the spread of the particle size distribution can be prevented. Therefore, by setting the reaction temperature within the above range, metal nanoparticles with a small and uniform particle size can be prepared.

[0059] The irradiation time of the microwave to the reaction solution is the time for the temperature of the reaction solution to reach the reaction temperature and maintain that temperature. The irradiation time of the microwave is appropriately changed according to the reaction conditions (microwave conditions, type of metal, type of solvent, pressure during reaction, amount of reaction solution, reaction temperature, etc.) and is not limited. The irradiation time of the microwave is usually 0.1 second to 300 seconds, and in one embodiment, it is 10 seconds to 60 seconds.

[0060] By irradiating the reaction solution with microwave under the above conditions to reach the reaction temperature of the reaction solution, nuclei of metal nanoparticles and further metal nanoparticles are generated in the reaction solution. Furthermore, in the present invention, since a cooling mechanism is arranged in the reaction vessel, the microwave can be irradiated for a long time while maintaining the temperature of the reaction solution at the reaction temperature, and as a result, the yield of the metal nanoparticles can be improved.

[0061] The completion of the reaction can be determined by observing the absorbance etc. of the raw material of the metal nanoparticles or the metal nanoparticles in the reaction solution. For example, when the metal nanoparticles are silver nanoparticles and an inorganic salt is used as the raw material of the silver nanoparticles, the change in the absorbance of the reaction solution at 280 nm to 780 nm with the heat preservation time is observed, and the time point when the absorbance no longer changes is taken as the completion time point of the reaction. Alternatively, the change in the absorbance of 280 nm to 780 nm derived from the silver nanoparticles in the reaction solution with the heat preservation time is observed, and the time point when the absorbance no longer changes is taken as the completion time point of the reaction.

[0062] In the present invention, the reaction solution may be stirred by a stirring mechanism, such as a propeller stirrer, a vibrating stirrer, a magnetic stirrer, or the like.

[0063] By stirring the reaction solution, the metal nanoparticles generated in the reaction solution can be uniformly dispersed, and the reaction solution can be kept uniform.

[0064] The step of irradiating the reaction solution with microwaves may be a batch type or a flow type (continuous flow type).

[0065] In the present invention, while cooling the reaction vessel from the outside, by irradiating the reaction solution with microwaves, the increase in the temperature of the reaction solution due to the microwaves can be suppressed by the cooling mechanism, and the irradiation of the reaction solution with microwaves can be carried out for a long time. At this time, if there is no solid substance having a dielectric constant lower than that of the reaction solution inside the reaction vessel, the temperature of the reaction solution in contact with the inner wall surface of the reaction vessel in contact with the refrigerant on the outer wall surface becomes low, and the temperature of the reaction solution at a location away from the reaction vessel wall surface, for example, the central portion of the reaction vessel, becomes high. That is, the temperature distribution of the reaction solution becomes non-uniform. When the temperature distribution of the reaction solution becomes non-uniform, the resulting metal nanoparticles may also become non-uniform. In the present invention, by disposing a solid substance having a dielectric constant lower than that of the reaction solution inside the reaction vessel, since the solid substance does not absorb microwaves and does not generate heat, the temperature rise of the reaction solution in contact with the solid substance is relatively suppressed compared to the reaction solution not in contact with the solid substance. As a result, the difference between the temperature of the reaction solution in contact with the inner wall surface of the reaction vessel in contact with the refrigerant on the outer wall surface and the temperature of the reaction solution in contact with the solid substance becomes small. Therefore, the variation in the temperature distribution of the reaction solution becomes small, and the temperature distribution of the reaction solution becomes uniform. When the temperature distribution of the reaction solution becomes uniform, the resulting metal nanoparticles also become uniform.

[0066] The present invention also relates to a microwave irradiation device capable of efficiently implementing the method for producing metal nanoparticles described above. Therefore, the microwave irradiation device of the present invention includes a reaction vessel for accommodating a reaction solution containing a raw material of metal nanoparticles, a cooling mechanism for cooling the reaction vessel from the outside, and a microwave irradiation source for irradiating the reaction solution with microwaves, and a solid substance having a dielectric constant lower than that of the reaction solution is provided inside the reaction vessel.

[0067] Details of the reaction vessel, the cooling mechanism, the microwave irradiation source, and the solid substance having a dielectric constant lower than that of the reaction solution in the microwave irradiation device of the present invention are as described above.

[0068] FIG. 1A schematically shows an embodiment in which a reaction solution is accommodated in the microwave irradiation device of the present invention. The microwave irradiation device shown in FIG. 1A is configured according to the microwave irradiation device of the present invention described above. Therefore, the microwave irradiation device of the present invention includes a reaction vessel for accommodating a reaction solution 1 containing a raw material of metal nanoparticles, a cooling mechanism (refrigerant flow path) 2 for cooling the reaction vessel from the outside, and a microwave irradiation source 3 for irradiating the reaction solution 1 with microwaves. A solid substance 4 having a dielectric constant lower than that of the reaction solution 1 is provided inside the reaction vessel.

[0069] As shown in FIG. 1A, by providing the solid substance 4 inside the reaction vessel, in the microwave irradiation distribution (FIG. 1B) in the reaction solution 1, the reaction solution 1 in contact with the solid substance 4 is away from the refrigerant and is thus hardly affected by the cooling by the refrigerant, but is less likely to absorb microwaves (less likely to generate heat by microwaves) compared to the reaction solution 1 not in contact with the solid substance 4. On the other hand, the reaction solution 1 in contact with the inner wall surface of the reaction vessel in contact with the cooling mechanism 2 on the outer wall surface is close to the refrigerant and is thus easily cooled by the refrigerant, but since it is not in contact with the solid substance 4, it is relatively more likely to absorb microwaves compared to the reaction solution 1 in contact with the solid substance 4. As a result, the temperature distribution (FIG. 1C) of the reaction solution 1 becomes uniform.

[0070] The dispersion liquid containing the metal nanoparticles obtained by the production apparatus or production method of the metal nanoparticles of the present invention can be separated and purified (for example, salting out or centrifugation) by a method known in the art as necessary, to obtain the target metal nanoparticles and / or the dispersion liquid containing the metal nanoparticles.

[0071] The metal nanoparticles produced by the production apparatus or production method of the metal nanoparticles of the present invention are characterized by having a small particle size and a small dispersion of particle size distribution.

[0072] The average particle size of the metal nanoparticles can be measured by a TEM image or absorbance of the dispersion liquid containing the metal nanoparticles. When measuring the average particle size of the metal nanoparticles by absorbance, the smaller the maximum value of the absorbance peak, the smaller the average particle size. For example, when using silver nanoparticles as the metal nanoparticles, the average particle size of the silver nanoparticles is usually 30 nm or less, and in one embodiment, it is 1 nm to 20 nm.

[0073] The particle size distribution of the metal nanoparticles can be grasped by a TEM image or the half-value width of the absorbance peak of the dispersion liquid containing the metal nanoparticles. When measuring the particle size distribution of the metal nanoparticles by the half-value width of the absorbance peak, the smaller the half-value width, the narrower the particle size distribution, that is, the smaller the variation in particle size. The half-value width of the absorbance peak of the dispersion liquid containing the metal nanoparticles indicates the distance (width) between the wavelengths at two points of absorbance that are half of the maximum value of the absorbance peak. For example, when using silver nanoparticles as the metal nanoparticles, the half-value width of the absorbance peak of the dispersion liquid containing the silver nanoparticles produced by the production method of the present invention is smaller than the half-value width of the absorbance peak of the dispersion liquid containing the silver nanoparticles produced by the conventional synthesis using microwaves. That is, the silver nanoparticles produced by the production method of the present invention are uniform silver nanoparticles with a narrow particle size distribution (small variation in particle size and uniform particle size).

[0074] The metal nanoparticles produced by the method for producing metal nanoparticles of the present invention can be used as a high heat-resistant bonding material for electronic components and as a wiring material due to their property of being sinterable at low temperatures, in addition to conventional catalysts, electronic component members, etc.

Examples

[0075] Hereinafter, several examples of the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.

[0076] 1. Preparation of silver nanoparticles Example 1 To 25 ml of DMF, which is a polar solvent and a reducing agent, 0.85 g of silver nitrate as a raw material for silver nanoparticles and 3.33 g of PVP as a protective agent were added and dissolved to prepare a reaction solution.

[0077] The obtained reaction solution was put into a reaction vessel in the apparatus having the configuration shown in FIG. 1. Here, as the reaction vessel, a reaction tube with a diameter of 11 mm was used, as the solid substance, a columnar solid substance made of Teflon (registered trademark) with a diameter of 2 mm was used, and as the refrigerant, Florinate was used. Without stirring the reaction solution, microwave irradiation was carried out from a microwave irradiation source so that the reaction solution absorbed microwaves with a power density of 10 W / mL based on the total volume of the reaction solution, the temperature of the reaction solution was raised to 120 °C, and while maintaining the temperature, microwave irradiation was finally carried out for 30 seconds to carry out the reaction and obtain silver nanoparticles.

[0078] Example 2 The same procedure as in Example 1 was carried out, except that a columnar solid substance made of melamine with a diameter of 2 mm was used as the solid substance in Example 1.

[0079] Example 3 The same procedure as in Example 1 was carried out, except that a columnar solid substance made of epoxy with a diameter of 2 mm was used as the solid substance in Example 1.

[0080] Comparative Example 1 The same procedure as in Example 1 was carried out, except that no solid substance was accommodated in the reaction vessel in Example 1.

[0081] Comparative Example 2 In Comparative Example 1, the procedure of Comparative Example 1 was repeated, except that a cooling mechanism was not used during the reaction, and microwave irradiation was terminated when the temperature of the reaction solution reached 120°C.

[0082] Figure 2 shows the microwave irradiation profiles of Comparative Examples 1 and 2. Figure 3 shows the relationship between the microwave irradiation time and the yield in Example 1. Figure 4A schematically shows the reaction solution temperature T A (°C) near the pole of the reaction solution 1 in the broken line part in the reaction vessel and the reaction solution temperature T B (°C) near the refrigerant. Figure 4B shows the reaction solution temperature T A near the pole of the reaction solution 1 in the reaction vessel and the reaction solution temperature T B near the refrigerant in Comparative Example 1 and Examples 1 to 3, respectively. The temperature difference (T A -T B ) is shown. For (T A -T B ) in Comparative Example 1 and Examples 1 to 3 in Figure 4B, Comparative Example 1 is (122 - 107 = 15), Example 1 is (123 - 110 = 13), Example 2 is (127 - 117 = 10), and Example 3 is (122 - 114 = 8).

[0083] From Figure 2, it was found that by using a cooling mechanism during the reaction, even if microwave irradiation was continued after the temperature of the reaction solution reached the reaction temperature by microwave irradiation, the temperature of the reaction solution could be maintained at the reaction temperature. Also, from Figure 3, it was found that in an environment where the microwave irradiation time could be extended by using a cooling mechanism during the reaction, the yield increased as the microwave irradiation time increased. Furthermore, from Figure 4, it was found that by arranging a pole with a low relative permittivity in the reaction vessel, the temperature difference between the two ends and the center of the reaction tube could be reduced.

[0084] 2. Particle Size Distribution of Silver Nanoparticle Dispersion The silver nanoparticles obtained in Example 1 and Comparative Example 1 were measured by UV-vis absorption. The results are shown in Figure 5.

[0085] As shown in Fig. 5, the silver nanoparticles obtained in Example 1 have a small and uniform particle size (i.e., there is a large absorbance peak only around 400 nm), while the silver nanoparticles obtained in Comparative Example 1 have a large and non-uniform particle size (i.e., there are absorbance peaks other than around 400 nm). Therefore, it was found that in order to produce silver nanoparticles with a small and uniform particle size, it is necessary to contain a solid substance having a dielectric constant lower than that of the reaction solution in the reaction vessel.

Explanation of Symbols

[0086] 1: Reaction solution, 2: Cooling mechanism, 3: Microwave irradiation source, 4: Solid substance

Claims

1. A reaction vessel for containing a reaction solution containing a raw material of metal nanoparticles, a cooling mechanism for cooling the reaction vessel from the outside, and a microwave irradiation source for irradiating the reaction solution with microwaves A microwave irradiation device comprising: inside the reaction vessel, a solid substance having a dielectric constant lower than that of the reaction solution is provided. Microwave irradiation device.

2. The microwave irradiation device according to claim 1, wherein the relative dielectric constant of the solid substance is 10 or less, and the solid substance is provided at the central portion of the reaction vessel.

3. The microwave irradiation device according to claim 2, wherein the solid substance occupies 2% to 10% by volume of the total volume of the reaction vessel.

4. A method for producing metal nanoparticles, comprising the step of irradiating a reaction solution with microwaves, (i) a step of containing a reaction solution and a solid substance having a dielectric constant lower than that of the reaction solution in a reaction vessel, and (ii) a step of irradiating the reaction solution with microwaves while cooling the reaction vessel from the outside Including the above method.

5. The method according to claim 4, wherein the relative dielectric constant of the solid substance is 10 or less, and the solid substance is disposed at the central portion of the reaction vessel.

6. The method according to claim 5, wherein the volume of the solid substance is adjusted to occupy 2% to 10% by volume of the total volume of the reaction vessel.

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