Microwave irradiation device and method for producing metal nanoparticles

Abstract

To provide a microwave irradiation device for producing metal nanoparticles capable of efficiently preparing the metal nanoparticles having small and uniform particle size, and a method for producing the metal nanoparticles.SOLUTION: The present invention relates to a microwave irradiation device comprising two microwave irradiation sources, two waveguides, and one reaction section. The two microwave irradiation sources are arranged with their respective microwave emitting sections facing each other. The two microwave irradiation sources, the two waveguides, and the one reaction section are arranged so that the microwaves emitted from the two microwave irradiation sources pass through the two waveguides and impinge on an entire surface of the one reaction section. Furthermore, the reaction section is adjusted to a specific size.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

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

[0003] Furthermore, various devices and methods for producing metal nanoparticles have been devised. Among these, devices and methods for producing metal nanoparticles using microwaves have attracted attention as they can synthesize metal nanoparticles in a short time.

[0004] As an example of such a manufacturing apparatus for metal nanoparticles, Patent Document 1 discloses a chemical reaction apparatus including a reactor, a microwave generator that generates microwaves, and a waveguide that transmits the microwaves generated by the microwave generator to an unfilled space in the reactor. The reactor is a horizontal flow type. In the reactor, liquid contents flow horizontally with an unfilled space above. The reactor is divided into multiple chambers by partition plates. One type of partition plate is an overflow type partition plate through which the contents pass upward. Another type of partition plate is an underflow type partition plate through which the contents pass downward.

[0005] Patent Document 2 discloses a microwave processing apparatus having a cavity resonator, a microwave supplying means, and a control unit. The microwave processing apparatus processes an object to be processed using a single-mode standing wave. The single-mode standing wave is a TM mn0 (m and n are integers of 1 or more) or TE m0p(m and p are integers of 1 or more) mode standing waves. The cavity resonator forms the single-mode standing wave. The microwave supply means supplies microwaves into the cavity resonator that match the resonance frequency of the single-mode standing wave. The control unit controls the frequency of the microwaves supplied by the microwave supply means based on the resonance frequency of the cavity resonator. In the microwave processing apparatus, the object to be processed is placed along a position where the magnetic field strength is maximized. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-007566 [Patent Document 2] Japanese Patent Application Publication No. 2020-080298 Summary of the Invention [Problem to be solved by the invention]

[0007] Metal nanoparticles are also being considered as high-heat-resistant bonding materials in the field of electronics packaging. Examples of high-heat-resistant bonding materials include lead-free bonding materials (e.g., lead-free solder) that can be bonded at low temperatures. Normally, lead-free solder is difficult to bond at temperatures below 250°C. However, lead-free solder containing metal nanoparticles has a lower melting point than bulk materials due to the characteristics of the metal nanoparticles. On the other hand, lead-free solder containing metal nanoparticles retains the melting point of bulk materials when used for bonding and sintered. Therefore, lead-free solder containing metal nanoparticles enables bonding at temperatures below 250°C.

[0008] To use metal nanoparticles as a highly heat-resistant bonding material, it is necessary to keep the melting point of the metal nanoparticles constant. To keep the melting point of the metal nanoparticles constant, it is desirable to make the particle size of the metal nanoparticles small and narrow the particle size distribution.

[0009] Although progress has been made in the development of metal nanoparticles that can be used in such a variety of applications, it is difficult to prepare metal nanoparticles with small and uniform particle sizes in methods for producing metal nanoparticles by irradiating a reaction solution with microwaves. Here, uniform particle size means a narrow particle size distribution.

[0010] Therefore, an object of the present invention is to provide a microwave irradiation device (metal nanoparticle manufacturing device) for producing metal nanoparticles that can efficiently prepare metal nanoparticles with small and uniform particle sizes, and a method for producing metal nanoparticles. [Means for solving the problem]

[0011] In the prior art, mass production of metal nanoparticles has been attempted using a flow-type metal nanoparticle manufacturing apparatus and manufacturing method as follows. (i) Identify the inner diameter of the reaction channel in the metal nanoparticle production device. (ii) Uses a single-mode standing microwave wave. (iii) Stirring is performed using a partition plate.

[0012] However, in the metal nanoparticle manufacturing apparatus and manufacturing method, when the volume irradiated with microwaves is increased to improve productivity, the particle size of the synthesized metal nanoparticles may vary, resulting in a decrease in quality.

[0013] Furthermore, in the flow-through method, there is a possibility that the yield of metal nanoparticles may decrease due to incomplete synthesis reactions caused by an increase in the reaction flow path, for example.

[0014] The present inventors have investigated various reasons why the particle size of synthesized metal nanoparticles varies and the quality deteriorates when the volume of microwave irradiation is increased to improve productivity in a method for producing metal nanoparticles. As a result, the present inventors have discovered that in a method for producing metal nanoparticles by irradiating a reaction solution with microwaves, there are parts of the reaction solution that are irradiated with microwaves and parts that are not irradiated with microwaves. This can occur due to the unique density characteristics of microwaves. The parts that are irradiated with microwaves are parts of the reaction solution that absorb the microwaves. Furthermore, the parts that are not irradiated with microwaves are parts of the reaction solution that do not absorb the microwaves.

[0015] Please refer to Figure 1. Figure 1 shows the microwave absorption intensity when microwaves are irradiated onto a reaction solution. In Figure 1A, the volume of the reaction solution (also referred to as the "reaction zone" in this specification) is 1 ml, and there is almost no unevenness in microwave intensity in the reaction zone. On the other hand, in Figure 1B, the reaction zone is 10 ml, and there is unevenness in microwave intensity in the reaction zone.

[0016] Figure 2 shows a schematic diagram of the degree of microwave penetration into the surface and interior of a reaction solution when microwaves are irradiated onto the reaction solution. As can be seen from Figure 2, microwaves penetrate up to a depth of several mm from the surface of the irradiated reaction solution. In other words, microwaves do not penetrate into a portion of the irradiated reaction solution that is more than a few mm below the surface. In the portions where microwaves do not penetrate, the reaction solution cannot absorb the microwaves.

[0017] The present inventors have therefore investigated various means for solving the above problems, and as a result, have found that in a method for producing metal nanoparticles by irradiating a reaction solution with microwaves, metal nanoparticles with small and uniform particle sizes can be efficiently prepared by adjusting the microwave irradiation source, reaction section, and reaction mode as follows, and have completed the present invention. (i) Two microwave irradiation sources are arranged so that their microwave emitting parts face each other. (ii) The reaction section to be irradiated with microwaves is adjusted so that the area (orthogonal projection area) of the plane perpendicular to the direction of microwave propagation (z-axis direction) (the plane (xy plane) of the reaction section extending along the x-axis and y-axis when viewed from the z-axis direction) is larger than the areas (orthogonal projection areas) of the planes parallel to the direction of microwave propagation (the plane (xz plane) of the reaction section extending along the x-axis and z-axis when viewed from the y-axis direction, and the plane (yz plane) of the reaction section extending along the y-axis and z-axis when viewed from the x-axis direction). (iii) The step of irradiating the reaction solution with microwaves is carried out batchwise.

[0018] That is, the gist of the present invention is as follows. (1) A microwave irradiation device comprising two microwave irradiation sources, two waveguides, and one reaction section, Two microwave irradiation sources are arranged with their microwave emitting portions facing each other, two microwave irradiation sources, two waveguides, and one reaction region are arranged so that microwaves emitted from the two microwave irradiation sources pass through the two waveguides and impinge on the entire surface of one reaction region; When the direction of microwave propagation is the z-axis direction and the directions perpendicular to the microwave propagation direction are the x-axis and y-axis directions, the orthogonal projection area of ​​the plane extending along the x-axis and y-axis (xy plane) in the reaction zone as viewed from the z-axis direction is larger than the orthogonal projection areas of the plane extending along the x-axis and z-axis (xz plane) in the reaction zone as viewed from the y-axis direction and the plane extending along the y-axis and z-axis (yz plane) in the reaction zone as viewed from the x-axis direction. Microwave irradiation device. (2) The microwave irradiation device according to (1), wherein the z-axis length of the xz plane and the yz plane in the reaction section is not more than twice the penetration depth of one microwave into the reaction section. (3) The orthogonal projection area of ​​the xy plane in the reaction section is 100 mm 2 ~3850mm 2 The microwave irradiation device according to (1) or (2), (4) A method for producing metal nanoparticles, comprising the step of irradiating a reaction solution with microwaves, (i) preparing two microwave irradiation sources, and arranging microwave emitting portions of the two microwave irradiation sources facing each other; (ii) placing the reaction mixture between two microwave irradiation sources; and (iii) irradiating the entire surface of the reaction solution with two microwaves from two microwave irradiation sources, It is carried out in batch mode, When the direction of microwave propagation is the z-axis direction and the directions perpendicular to the microwave propagation direction are the x-axis and y-axis directions, the orthogonal projection area of ​​the plane extending on the x-axis and y-axis (xy plane) in the reaction zone as viewed from the z-axis direction is adjusted to be larger than the orthogonal projection areas of the plane extending on the x-axis and z-axis (xz plane) in the reaction zone as viewed from the y-axis direction and the plane extending on the y-axis and z-axis (yz plane) in the reaction zone as viewed from the x-axis direction. Steps The method comprising: (5) The method according to (4), wherein in step (iii), the reaction mixture is not physically stirred. (6) The method according to (4) or (5), wherein in step (iii), the z-axis lengths of the xz plane and the yz plane in the reaction solution are not more than twice the penetration depth of one microwave into the reaction solution. (7) In step (iii), the orthogonal projection area of ​​the reaction solution in the xy plane is 100 mm 2 ~3850mm 2 The method according to any one of (4) to (6), wherein [Effects of the Invention]

[0019] The present invention provides a microwave irradiation device for producing metal nanoparticles and a method for producing metal nanoparticles that can efficiently prepare metal nanoparticles with small and uniform particle sizes. [Brief explanation of the drawings]

[0020] [Figure 1]Figure 1A shows the microwave absorption intensity in a reaction solution when a 1 ml volume of the reaction solution is irradiated with microwaves, and Figure 1B shows the microwave absorption intensity in a 10 ml volume of the reaction solution when a 1 ml volume of the reaction solution is irradiated with microwaves. [Figure 2] 1 is a schematic diagram showing the degree of microwave penetration into the surface and inside of a reaction solution when the reaction solution is irradiated with microwaves. [Figure 3] 1 is a diagram schematically illustrating an embodiment of a microwave irradiation device of the present invention. [Figure 4] 1 is a graph showing the particle size distribution of silver nanoparticles obtained in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0021] Several embodiments of the present invention will be described in detail below. 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 drawings do not accurately depict the actual dimensions and shapes. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. The microwave irradiation device and method for producing metal nanoparticles of the present invention are not limited to the following embodiments. The microwave irradiation device and method for producing metal nanoparticles of the present invention can be implemented in various forms, including modifications and improvements that can be made by those skilled in the art, without departing from the spirit and scope of the present invention.

[0022] The present invention relates to a method for producing metal nanoparticles, which includes a step of irradiating a reaction solution with microwaves. In the production method of the present invention, in step (i), two microwave irradiation sources are prepared. Here, the microwave emitters of the two microwave irradiation sources are arranged facing each other. In step (ii), the reaction solution is placed between the two microwave irradiation sources. In step (iii), two microwaves from the two microwave irradiation sources are irradiated onto the entire surface of the reaction solution. Here, "the entire surface of the reaction solution" 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 and y-axis directions, the total orthogonal projection area of ​​the xy plane formed by the two microwaves from the two microwave irradiation sources as viewed from the z-axis direction is larger than the orthogonal projection area of ​​the xy plane of the reaction solution. In the following description, the relationship between the x-axis, y-axis, and z-axis is the same as above, and description of this relationship may be omitted. As will be explained below, the orthogonal projection area of ​​each microwave in the xy plane depends on the orthogonal projection area of ​​the waveguide through which the microwave passes. Furthermore, step (iii) is performed batchwise. Furthermore, the orthogonal projection area of ​​the plane extending along the x and y axes (xy plane) in the reaction zone as viewed from the z axis direction is adjusted to be larger than the orthogonal projection areas of the plane extending along the x and z axes (xz plane) in the reaction zone as viewed from the y axis direction and the plane extending along the y and z axes (yz plane) in the reaction zone as viewed from the x axis direction. Here, "the orthogonal projection area of ​​the xy plane is larger than the orthogonal projection areas of the xz plane and yz plane" means, in other words, that the area of ​​the xy plane at any position on the z axis in the reaction solution is larger than the area of ​​the xz plane at any position on the y axis and the area of ​​the yz plane at any position on the x axis.

[0023] The reaction solution used in the present invention is not limited. The reaction solution may have the same composition as that used in conventional methods for producing metal nanoparticles by irradiating microwaves. For example, examples of materials contained in the reaction solution are listed below.

[0024] 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, and cobalt nanoparticles, as well as 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 dissolve in a solvent and generate metal ions. Examples of 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 including 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, an inexpensive nitrate, such as silver nitrate, is used as the raw material for metal nanoparticles.

[0025] The concentration of metal ions in the reaction solution is not limited and is usually 0.1 mmol / L (mM) to 300 mM, and in one embodiment, 0.1 mM to 100 mM.

[0026] By setting the concentration of metal ions in the reaction solution within the above range, the variation in the obtained metal nanoparticles is reduced, in other words, the particle size distribution of the obtained metal nanoparticles is narrowed.

[0027] The reaction solution further contains a solvent. The solvent used in the reaction solution can dissolve and disperse materials such as raw materials for metal nanoparticles, a protective agent, and a reducing agent. 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. Examples of solvents used in the reaction solution include low-boiling-point solvents with a boiling point of 300°C or less. The low-boiling-point solvent is not limited. Examples of low-boiling-point solvents 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, and mixtures of two or more of these. In one embodiment, the solvent used in the reaction solution is water.

[0028] By using a low boiling point solvent as the solvent used in the reaction solution, the handling of the solvent can be improved and the burden on the environment can be reduced.

[0029] The reaction solution further 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 metal nanoparticles produced in the reaction solution and suppresses 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), and tannic acid. In one embodiment, the protective agent is PVP.

[0030] The amount of the protective agent is not limited and can be changed depending on 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.

[0031] By using a protective agent, it is possible to suppress aggregation of the generated metal nanoparticles.

[0032] The reaction solution further contains a reducing agent, which is a material capable of reducing metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction.

[0033] The reducing agent is not limited. Examples of the reducing agent include citric acid or citrates such as trisodium citrate, disodium citrate, and monosodium citrate, oxalic acid or oxalate salts such as sodium oxalate, ascorbic acid or ascorbate salts such as sodium ascorbate, DMF, and mixtures of two or more thereof. In one embodiment, the reducing agent for metal ions, particularly silver ions, is DMF.

[0034] The amount of reducing agent is not limited as long as it can reduce metal ions to a metal whose oxidation number is 0 through a redox reaction. The amount of reducing agent is typically 1.0 to 20 equivalents, and in one embodiment, 4.0 to 15 equivalents, relative to the metal ions. Note that a reducing agent for metal ions may also function as a protecting agent if it contains one or more functional groups capable of interacting with metals, such as a carboxy group, a hydroxy group, or an ether group. When the reducing agent also functions as a protecting agent, the reaction solution may not contain the protecting agent described above. Furthermore, in such cases, the amount of reducing agent for metal ions may be greater than the amount necessary to reduce metal ions to a metal whose oxidation number is 0 through a redox reaction.

[0035] The reaction solution may be composed of the raw materials for metal nanoparticles, a solvent, a protective agent, and a reducing agent described above. In one embodiment, the reaction solution may contain additives in addition to these materials. The additives are additives that can be commonly used in reaction solutions that can be used in conventional methods for producing metal nanoparticles by irradiating microwaves.

[0036] For example, the reaction solution may further contain a chelating agent as an additive, such as ethylenediaminetetraacetic acid (EDTA) and / or a salt of ethylenediaminetetraacetate.

[0037] The pH of the reaction solution is not limited and is usually pH 3 to pH 12.

[0038] In the present invention, the order of addition of each material, the addition temperature, the mixing method, the mixing time, etc. in preparing the reaction solution are not limited. In the present invention, the reaction solution is mixed so as to prepare a homogeneous reaction solution. In the present invention, the reaction is started after the homogeneous reaction solution is prepared.

[0039] In the present invention, microwaves are emitted from two microwave irradiation sources to the reaction solution described above. The two microwave irradiation sources are arranged with their microwave emitting portions facing each other. Therefore, the reaction solution is placed between the two microwave irradiation sources, and the microwaves emitted from each microwave irradiation source are irradiated onto the entire surface of the reaction solution.

[0040] By using two microwave irradiation sources to irradiate the reaction solution and irradiating the reaction solution with microwaves from the front and back, the microwaves can be distributed evenly not only to the surface region of the reaction solution but also to the internal region.

[0041] The relative positions of the two microwave irradiation sources need only be such that the microwave emitting portions of the microwave irradiation sources face each other. The relative positions of the two microwave irradiation sources can range from a position where the waveguides through which the microwaves pass completely overlap in a straight line when viewed from the z-axis direction to a position where the planes of the waveguides extending along the x-axis and y-axis (xy planes) do not overlap. The relative positions of the two microwave irradiation sources are set, for example, so that the orthogonal projection area of ​​the overlapping xy planes of the two waveguides when viewed from the z-axis direction is typically 12% to 100%, and in one embodiment, 12% to 51%, of the total orthogonal projection area of ​​the xy planes formed by the two waveguides. The shape of the waveguides is not limited, and can be, for example, a columnar shape, such as a rectangular parallelepiped, a cylindrical body, or a polygonal prism. The microwave irradiation range is determined depending on the shape of the waveguide.

[0042] By setting the positions of the two microwave irradiation sources as described above, the volume of the reaction solution that can be irradiated with microwaves can be increased.

[0043] The reaction solution to be irradiated with microwaves is adjusted so that the volume of the reaction solution as viewed from the z-axis direction, i.e., the orthogonal projection area of ​​the plane extending along the x-axis and y-axis in the reaction section (xy plane) is larger than the orthogonal projection areas of the plane extending along the x-axis and z-axis in the reaction section as viewed from the y-axis direction (xz plane) and the plane extending along the y-axis and z-axis in the reaction section as viewed from the x-axis direction (yz plane), so that the two microwaves from the two microwave irradiation sources can be irradiated across the entire surface of the reaction solution.

[0044] Typically, in a reaction, stirring with a stirrer or stir bar is required to maintain the reaction solution uniform. For example, in a flow-through reaction, transport of the reaction solution (control of the flow rate of the reaction solution) is required. However, by specifying the reaction zone as described above, microwaves can penetrate the entire reaction zone. Therefore, the nuclei of metal nanoparticles formed in the area irradiated with microwaves are prevented from becoming coarse due to the raw material of metal nanoparticles present in the area not irradiated with microwaves because the nuclei of metal nanoparticles have not yet formed. Therefore, in the present invention, the particle size of the resulting metal nanoparticles can be maintained small and uniform without physically stirring the reaction solution during the reaction.

[0045] The shape of the reaction zone is not limited as long as the xy plane, xz plane, and yz plane are specified as described above. The shape of the reaction zone has a surface smaller than the surface of the irradiated microwaves (the surface when the microwaves hit the reaction solution and / or the xy plane of the waveguide as viewed from the z-axis direction), and can be, for example, a sphere, a columnar body, such as a rectangular parallelepiped, a cylindrical body (tubular body), or a polygonal columnar body.

[0046] The z-axis length of the xz plane viewed from the y-axis direction in the reaction section and the yz plane viewed from the x-axis direction is determined based on the penetration depth into the reaction solution of microwaves irradiated from one microwave irradiation source.

[0047] The penetration depth D of one microwave into a reaction solution can be expressed by the following formula:

number

[0048] where f is the microwave frequency [Hz] and ε r is the relative permittivity of the dielectric, and tan δ is the dielectric loss angle of the dielectric. Two microwaves are irradiated to the reaction zone. Therefore, the maximum value of the z-axis length is twice the penetration depth D of one microwave into the reaction solution. For example, when the solvent of the reaction solution is water (5°C) and the microwave frequency is 2.45 GHz, the maximum value of the z-axis length is 10 mm. For example, when the solvent of the reaction solution is water (5°C), the z-axis length is adjusted to usually 5 mm to 26 mm, and in one embodiment, 5 mm to 10 mm, depending on the microwave frequency.

[0049] The orthogonal projection area of ​​the xy plane in the reaction zone as viewed from the z-axis direction is usually 1% to 33%, and in one embodiment 22%, of the orthogonal projection area of ​​the microwaves irradiated from the two microwave irradiation sources onto the reaction zone (the total orthogonal projection area of ​​the xy plane formed by the two waveguides as viewed from the z-axis direction). For example, the orthogonal projection area of ​​the xy plane in the reaction zone as viewed from the z-axis direction is usually 100 mm 2 ~3850mm 2 , in one embodiment 2640 mm 2 is.

[0050] The reaction unit can be formed as a container that contains a reaction liquid. Therefore, the reaction unit is a container that contains a reaction liquid adjusted so that the surface area relationship of each side is as described above. The reaction unit may be placed in a reaction chamber that is placed between two microwave irradiation sources. The material of the container that contains the reaction liquid is not limited as long as it can uniformly irradiate the reaction liquid with microwaves. As for the material of the container that contains the reaction liquid, a material that transmits microwaves, such as ceramics or glass (quartz), is used for the part that irradiates the reaction liquid with microwaves through the container.

[0051] The step of irradiating the reaction solution with microwaves is carried out batchwise, i.e., the step is carried out in a state where the reaction solution is not being transported, i.e., the microwaves are irradiated to the reaction solution that is not being transported.

[0052] By performing the step of irradiating the reaction solution with microwaves in a batch system, the synthesis reaction itself can be completed, thereby improving yield and productivity. Furthermore, in a batch system reaction, the problem of clogging of metal nanoparticles, which can occur in a flow system when the raw material for metal nanoparticles is highly concentrated, is less likely to occur.

[0053] When a reaction solution is irradiated with microwaves, the solvent contained in the reaction solution absorbs the microwaves and converts them into thermal energy, generating heat. Therefore, in the reaction solution irradiated with microwaves, a uniform and rapid temperature rise occurs in the irradiated area. In the reaction solution, a uniform and rapid reaction occurs in accordance with this temperature rise.

[0054] The microwaves are generated from a microwave radiation source (microwave generator (magnetron)). The microwave radiation source can be either a single-mode system or a multi-mode system. In one embodiment, the microwave radiation source is a single-mode system as used in Sim.

[0055] The output power of each of the two microwave irradiation sources is not limited, and is usually 1 W to 6000 W.

[0056] By adjusting the output power of each of the two microwave irradiation sources within the above range, metal nanoparticles with small and uniform particle size can be prepared using microwaves with the existing output power.

[0057] The frequency of the microwaves generated from each of the two microwave irradiation sources can be changed as appropriate and is not limited. The microwave frequency is usually 0.9 GHz to 10 GHz, and in one embodiment, 2 GHz to 6 GHz. In one embodiment, the microwave frequency is 2.45 GHz, which is the frequency of an industrial microwave power supply.

[0058] In one embodiment, the microwave irradiation is uniform during irradiation. In one embodiment, the microwave irradiation conditions are constant during microwave irradiation.

[0059] In the present invention, the temperature of the reaction solution raised by microwave irradiation is the reaction temperature. The reaction temperature can be appropriately changed depending on the reaction conditions (such as the type of metal, the type of solvent, and the pressure during the reaction), and is not limited. The reaction temperature is usually 25°C or higher, and in one embodiment, 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 below the boiling point of the solvent. For example, when the solvent is water, the reaction temperature is usually in the range of 25°C or higher and lower than 100°C, and in one embodiment, 80°C to 90°C, at atmospheric pressure.

[0060] By raising the reaction temperature to 25°C or higher, a reduction reaction from metal ions to metal nanoparticles occurs. On the other hand, if the reaction solution boils, the reaction field becomes non-uniform. When the reaction field becomes non-uniform, the particle size of the resulting metal nanoparticles becomes irregular, which can result in a broadening of the particle size distribution. Therefore, by setting the reaction temperature below the boiling point of the solvent, it is possible to prevent the broadening of the particle size distribution. Therefore, by setting the reaction temperature within the above range, it is possible to prepare metal nanoparticles with small and uniform particle sizes.

[0061] The microwave irradiation time for the reaction solution is the time required for the temperature of the reaction solution to reach the reaction temperature. The microwave irradiation time is not limited and may be appropriately changed depending on the reaction conditions (microwave conditions, type of metal, type of solvent, pressure during the reaction, amount of reaction solution, reaction temperature, etc.). The microwave irradiation time is usually 0.1 to 300 seconds, and in one embodiment, 10 to 60 seconds.

[0062] By irradiating the reaction solution with microwaves under the above conditions and raising the temperature of the reaction solution to the reaction temperature, nuclei of metal nanoparticles and further metal nanoparticles are produced in the reaction solution.

[0063] The completion of the reaction can be determined by observing the absorbance of the raw material for the metal nanoparticles in the reaction solution or the absorbance derived from the metal nanoparticles. For example, when the metal nanoparticles are silver nanoparticles and an inorganic salt is used as the raw material for the silver nanoparticles, the change in the absorbance of the reaction solution at 280 nm to 780 nm over the incubation time is observed, and the reaction is completed when the absorbance no longer changes. Alternatively, the change in the absorbance of the reaction solution at 280 nm to 780 nm derived from the silver nanoparticles over the incubation time is observed, and the reaction is completed when the absorbance no longer changes.

[0064] In the present invention, as described above, it is not necessary to stir the reaction solution using a stirring mechanism such as a propeller stirrer, a vibration stirrer, or a magnetic stirrer.

[0065] Even without stirring the reaction solution, the reaction zone is shaped to a specific shape during microwave irradiation, allowing microwaves to be irradiated uniformly across the reaction zone. As a result, metal nanoparticles are generated uniformly in the reaction solution, and the reaction solution can be maintained uniformly.

[0066] The present invention is carried out in a batch system, but does not require a stirring function. Therefore, the present invention can also be carried out using a flow-type synthesis apparatus. When a flow-type synthesis apparatus is used, the present invention is carried out, for example, as follows. First, the reaction liquid is placed in a reaction section provided in the microwave irradiation apparatus of the present invention described above or below. Second, the reaction is carried out in a state where the reaction liquid is not flowing, i.e., with the transport of the reaction liquid stopped. Third, after the reaction is completed, the reaction liquid is pumped out (transported). By carrying out the present invention in this manner, although the transport of the reaction liquid needs to be stopped during the reaction, the reaction liquid can be transported quickly after the reaction is completed, allowing metal nanoparticles to be continuously synthesized. Furthermore, since the generation and transport of metal nanoparticles are independent of each other, the transport rate can be increased. Therefore, clogging of piping by the generated metal nanoparticles can be prevented.

[0067] The present invention also relates to a microwave irradiation device capable of efficiently carrying out the above-described method for producing metal nanoparticles. Accordingly, the microwave irradiation device of the present invention comprises two microwave irradiation sources, two waveguides, and one reaction section. The two microwave irradiation sources are arranged with their respective microwave emitting sections facing each other. The two waveguides are arranged so that the microwaves emitted from the two microwave irradiation sources pass through. The reaction section is located at the end of the two waveguides opposite the microwave irradiation sources. Therefore, the two microwave irradiation sources, two waveguides, and one reaction section are arranged so that the microwaves emitted from the two microwave irradiation sources pass through the two waveguides and impinge on the entire surface of the one reaction section. When the direction of microwave propagation is the z-axis direction and the directions perpendicular to the direction of microwave propagation are the x-axis and y-axis directions, one reaction section is configured so that the orthogonal projection area of ​​the plane extending along the x-axis and y-axis (xy plane) in the reaction section as viewed from the z-axis direction is larger than the orthogonal projection areas of the plane extending along the x-axis and z-axis in the reaction section as viewed from the y-axis direction (xz plane) and the plane extending along the y-axis and z-axis in the reaction section as viewed from the x-axis direction (yz plane).

[0068] The details of the two microwave irradiation sources, two waveguides, and one reaction section in the microwave irradiation device of the present invention are as described above.

[0069] FIG. 3 shows a schematic diagram of one embodiment of the microwave irradiation device of the present invention. The microwave irradiation device shown in FIG. 3 is configured in accordance with the microwave irradiation device of the present invention described above. Therefore, the microwave irradiation device of the present invention includes two microwave irradiation sources 1 and 1', two waveguides 2 and 2', and one reaction unit 3. One reaction unit 3 is installed in a reaction chamber 4 installed between the two microwave irradiation sources 1 and 1' (between the two waveguides 2 and 2'). The two waveguides 2 and 2' each have a rectangular parallelepiped shape. The x- and y-planes of the two waveguides 2 and 2' partially overlap when viewed from the z-axis direction. One reaction unit 3 has a cylindrical shape.

[0070] The dispersion liquid containing metal nanoparticles obtained by the metal nanoparticle manufacturing apparatus or manufacturing method of the present invention can be separated and purified (e.g., salting out or centrifugation) by methods known in the art, as needed, to obtain the desired metal nanoparticles and / or a dispersion liquid containing metal nanoparticles.

[0071] Metal nanoparticles produced by the metal nanoparticle production apparatus or production method of the present invention are characterized by small particle size and narrow variance in particle size distribution.

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

[0073] The particle size distribution of metal nanoparticles can be determined from a TEM image of a dispersion containing the metal nanoparticles or the half-width of the absorbance peak. When measuring the particle size distribution of metal nanoparticles based on the half-width of the absorbance peak, the smaller the half-width, the narrower the particle size distribution, i.e., the smaller the particle size variation. The half-width of the absorbance peak of a dispersion containing metal nanoparticles indicates the distance (width) between the wavelengths at two absorbance points that are half the maximum value of the absorbance peak. For example, when silver nanoparticles are used as the metal nanoparticles, the half-width of the absorbance peak of a dispersion containing silver nanoparticles produced by the production method of the present invention is smaller than the half-width of the absorbance peak of a dispersion containing silver nanoparticles produced by conventional microwave synthesis. In other words, the silver nanoparticles produced by the production method of the present invention are uniform silver nanoparticles with a narrow particle size distribution (small particle size variation and uniform particle size).

[0074] The metal nanoparticles produced by the method for producing metal nanoparticles of the present invention can be used not only as conventional catalysts and electronic component materials, but also as highly heat-resistant bonding materials for electronic components and wiring materials due to their ability to be sintered at low temperatures. [Example]

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

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

[0077] The resulting reaction solution was placed in a reaction vessel in an apparatus configured as shown in Figure 3. Without stirring, the reaction solution was allowed to absorb microwaves from each of two microwave irradiation sources at a power density of 10 W / mL based on the total volume of the reaction solution. The temperature of the reaction solution was allowed to reach 90°C, and the reaction was carried out for 10 minutes, yielding silver nanoparticles.

[0078] As described above, the reaction vessel used in the reaction of Example 1 satisfied the requirements of the reaction section of the apparatus of Fig. 3. Therefore, when the microwave propagation direction is the z-axis direction and the directions perpendicular to the microwave propagation direction are the x-axis and y-axis directions, the reaction vessel was adjusted so that the orthogonal projection area of ​​a plane extending along the x-axis and y-axis (xy plane) in the reaction section viewed from the z-axis direction would be larger than the orthogonal projection areas of a plane extending along the x-axis and z-axis (xz plane) in the reaction section viewed from the y-axis direction and a plane extending along the y-axis and z-axis (yz plane) in the reaction section viewed from the x-axis direction.

[0079] Comparative Example 1 In Example 1, the microwave irradiation to the reaction solution was changed to one-sided irradiation, i.e., microwave irradiation from one irradiation source, the waveguide through which the microwaves passed was installed so that the waveguide and the reaction solution overlapped in a straight line when viewed from the z-axis direction, and microwaves were absorbed at a power density of 10 W / mL based on the total volume of the reaction solution. The same procedures as in Example 1 were carried out.

[0080] 2. Particle size distribution of silver nanoparticle dispersion The silver nanoparticles obtained in Example 1 and Comparative Example 1 were subjected to UV-vis absorption measurement, and the results are shown in FIG.

[0081] 4, it was found that the silver nanoparticles obtained in Example 1 were small and uniform in particle size, whereas the silver nanoparticles obtained in Comparative Example 1 were large and non-uniform in particle size (i.e., an absorbance peak was present near 600 nm). Therefore, it was found that in order to produce silver nanoparticles with small and uniform particle size, it was necessary to configure the reaction section so that, when the microwave propagation direction is the z-axis direction and the directions perpendicular to the microwave propagation direction are the x-axis and y-axis directions, the orthogonal projection area of ​​the plane extending along the x-axis and y-axis in the reaction section as viewed from the z-axis direction (xy plane) is larger than the orthogonal projection areas of the plane extending along the x-axis and z-axis in the reaction section as viewed from the y-axis direction (xz plane) and the plane extending along the y-axis and z-axis in the reaction section as viewed from the x-axis direction (yz plane).

[0082] 3. Simulation Experiment Based on the apparatus shown in Figure 3, in an apparatus for carrying out the present invention, when the microwave propagation direction is the z-axis direction and the directions perpendicular to the microwave propagation direction are the x-axis and y-axis directions, a simulation was carried out to make the orthogonal projection area of ​​the plane extending on the x-axis and y-axis (xy plane) in the reaction section viewed from the z-axis direction larger than the orthogonal projection areas of the plane extending on the x-axis and z-axis in the reaction section viewed from the y-axis direction (xz plane) and the plane extending on the y-axis and z-axis in the reaction section viewed from the x-axis direction (yz plane).

[0083] The simulation method is shown below. (1) The distribution of microwaves absorbed by the reaction solution was calculated using Microwave Sim (ANSYS Electronics Desktop). (2) The above calculation model was subjected to coupled analysis with Fluent in Workbench. (3) Using Fluent, the volume of each mesh and the microwave power density absorbed by each mesh were obtained.

[0084] As a result of the simulation, it was found that the two waveguides can be arranged in a range from a position where the two waveguides completely overlap in a straight line (a structure in which, when viewed from the z-axis direction, the orthogonal projection area where the x-y planes of the two waveguides overlap is 100% of the total orthogonal projection area of ​​the x-y planes formed by the two waveguides) to a position where the planes extending along the x-axis and y-axis (x-y planes) of the two waveguides just do not overlap. Furthermore, it was found that the arrangement of the two waveguides is preferably such that the x-y planes of the two waveguides partially overlap (a structure in which, when viewed from the z-axis direction, the orthogonal projection area where the x-y planes of the two waveguides overlap is 12% to 51% of the total orthogonal projection area of ​​the x-y planes formed by the two waveguides). It was also found that the orthogonal projection area of ​​the xy plane in the reaction zone as viewed from the z-axis direction is preferably 1% to 33%, particularly 22%, of the orthogonal projection area of ​​the microwaves irradiated from the two microwave irradiation sources onto the reaction zone (the total orthogonal projection area of ​​the xy plane formed by the two waveguides as viewed from the z-axis direction). The orthogonal projection area of ​​the xy plane in the reaction zone as viewed from the z-axis direction is preferably 100 mm 2 ~3850mm 2 , especially 2640mm 2 It was.

[0085] It was also found that the shape of the reaction zone can be either a cylinder or a rectangular parallelepiped, and that a cylinder is the preferred shape of the reaction zone. [Explanation of symbols]

[0086] 1, 1': microwave irradiation source, 2, 2': waveguide, 3: reaction section, 4: reaction chamber

Claims

1. A microwave irradiation device comprising two microwave irradiation sources, two waveguides, and one reaction section, Two microwave irradiation sources are arranged with their microwave emitting portions facing each other, two microwave irradiation sources, two waveguides, and one reaction region are arranged so that microwaves emitted from the two microwave irradiation sources pass through the two waveguides and impinge on the entire surface of one reaction region; 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 orthogonal projection area of ​​the plane extending along the x-axis and y-axis (xy plane) in the reaction section as viewed from the z-axis direction is larger than the orthogonal projection areas of the plane extending along the x-axis and z-axis (xz plane) in the reaction section as viewed from the y-axis direction and the plane extending along the y-axis and z-axis (yz plane) in the reaction section as viewed from the x-axis direction. Microwave irradiation device.

2. 2. The microwave irradiation device according to claim 1, wherein the z-axis lengths of the xz plane and the yz plane in the reaction zone are equal to or less than twice the penetration depth of one microwave into the reaction zone.

3. The orthogonal projection area of ​​the xy plane in the reaction section is 100 mm ２ ~3850mm ２ The microwave irradiation device according to claim 1 or 2,

4. A method for producing metal nanoparticles, comprising a step of irradiating a reaction solution with microwaves, (i) preparing two microwave irradiation sources, and arranging the microwave emitting portions of the two microwave irradiation sources facing each other; (ii) placing the reaction mixture between two microwave irradiation sources; and (iii) irradiating the entire surface of the reaction solution with two microwaves from two microwave irradiation sources, It is carried out in batch mode, 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 orthogonal projection area of ​​the plane extending along the x-axis and y-axis (xy plane) in the reaction section as viewed from the z-axis direction is adjusted to be larger than the orthogonal projection areas of the plane extending along the x-axis and z-axis (xz plane) in the reaction section as viewed from the y-axis direction and the plane extending along the y-axis and z-axis (yz plane) in the reaction section as viewed from the x-axis direction. Steps The method comprising:

5. The method according to claim 4, wherein in step (iii), the reaction mixture is not physically stirred.

6. The method according to claim 4 or 5, wherein in step (iii), the z-axis lengths of the xz plane and the yz plane in the reaction solution are not more than twice the penetration depth of one microwave into the reaction solution.

7. In step (iii), the orthogonal projection area of ​​the xy plane of the reaction solution is 100 mm ２ ~3850mm ２ The method of claim 6, wherein

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