Method for producing metal nanoparticles

Abstract

To provide a metal nanoparticle production method for producing metal nanoparticles by irradiating a reaction liquid with microwaves, in which metal nanoparticles having a small and uniform particle diameter can be inexpensively and efficiently prepared.SOLUTION: This invention relates to a metal nanoparticle production method having a step of irradiating a reaction liquid with microwaves. Only one source of microwave irradiation is necessary. The step is executed in a batch type. The volume fraction of a microwave unirradiated part, where the power density of the microwaves absorbed into the reaction liquid upon the microwave irradiation is below 10 W / mL based on the volume of the reaction liquid, falls below 1%.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to 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, and among these, devices and methods for producing metal nanoparticles using microwaves have attracted attention as they can synthesize large amounts of metal nanoparticles in a short period of time.

[0004] As a method for preparing such metal nanoparticles, for example, Patent Document 1 discloses a product manufacturing apparatus for circulating a fluid through a flow path into a reaction channel and producing a predetermined product from the fluid in the reaction channel, the product manufacturing apparatus being characterized by comprising a microwave generator for irradiating microwaves into the reaction channel.

[0005] Patent Document 2 discloses an apparatus for producing metal nanoparticles, which includes a chamber for containing a reaction liquid, a plurality of microwave irradiation sources installed in the chamber, each of which has a dome-shaped microwave irradiation port that is covered by the reaction liquid, and a stirring mechanism for stirring the reaction liquid installed in the chamber.

[0006] Patent Document 3 discloses a microwave processing apparatus that processes an object to be processed by utilizing a single-mode standing wave, and the single-mode standing wave is a TM mn0 (m and n are integers of 1 or more) or TE m0pThe microwave processing apparatus disclosed includes a cavity resonator that forms a single-mode standing wave (m and p are integers of 1 or more), a microwave supply means that supplies microwaves into the cavity resonator that have a resonance frequency that matches the resonance frequency of the single-mode standing wave, and a control unit that controls the frequency of the microwaves supplied by the microwave supply means based on the resonance frequency of the cavity resonator, and the object to be processed is placed along a position where the magnetic field strength is maximized.

[0007] Patent Document 4 discloses a method for producing metal microparticles, in which a reaction liquid containing a metal precursor is circulated through a flow tube placed in a microwave irradiation space, and microwaves are irradiated from outside the flow tube toward the flow tube in the microwave irradiation space, heating the reaction liquid in the flow tube by this microwave irradiation and synthesizing metal microparticles.The method is characterized in that the microwave generator used is an apparatus to which a frequency band of 2.4 to 2.5 GHz is assigned or which is the standard frequency band for the product, and the average inner diameter of the part of the flow tube placed in the microwave irradiation field is narrower than 2.84 mm.

[0008] Patent Document 5 discloses a microwave processing device that includes a cylindrical cavity that is rotatably supported on a fixed base and has a space inside where an object to be irradiated with microwaves can be placed, a rotary drive unit that rotates the cavity around the axis of the cylindrical shape, and a microwave generator that generates microwaves, and the microwaves generated by the microwave generator are introduced into the internal space from the circumferential side of the cavity. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-188666 [Patent Document 2] Japanese Patent Application Publication No. 2019-077923 [Patent Document 3] Japanese Patent Application Publication No. 2020-080298 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-137226 [Patent Document 5] Patent Publication No. 2021-125431 Summary of the Invention [Problem to be solved by the invention]

[0010] In the field of electronics packaging, metal nanoparticles are being considered as high-heat-resistant bonding materials, for example, lead-free bonding materials that can be bonded at low temperatures. Lead-free solder is difficult to bond at temperatures below 250°C, but lead-free solder containing metal nanoparticles can make bonding at temperatures below 250°C possible by taking advantage of the properties of metal nanoparticles, namely, that they have a lower melting point than bulk materials, but when used for bonding and sintered, they retain the melting point of bulk materials.

[0011] 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.

[0012] Although progress is being made in the development of metal nanoparticles that can be used in such a variety of applications, it is difficult to inexpensively prepare metal nanoparticles that are small and uniform in particle size (here, uniform particle size means a narrow particle size distribution) using methods for producing metal nanoparticles by irradiating a reaction solution with microwaves.

[0013] Therefore, an object of the present invention is to provide a method for producing metal nanoparticles by irradiating a reaction solution with microwaves, which can inexpensively and efficiently prepare metal nanoparticles with small and uniform particle sizes. [Means for solving the problem]

[0014] In conventional technologies, for example, in a flow-type metal nanoparticle manufacturing method, mass production of metal nanoparticles has been attempted by making the reaction path in the metal nanoparticle manufacturing apparatus spiral, specifying the inner diameter of the reaction path, using a single-mode standing microwave wave, or rotating the microwave irradiation source; and, for example, in a batch-type metal nanoparticle manufacturing method, by using multiple microwave irradiation sources to irradiate the chamber containing the reaction solution.

[0015] However, in a method for producing metal nanoparticles, if 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.

[0016] 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.

[0017] Furthermore, the batch method requires the preparation of, for example, multiple microwave irradiation sources.

[0018] The inventors have investigated various reasons why the particle size of the synthesized metal nanoparticles varies and the quality deteriorates when the volume of microwave irradiation is increased to improve productivity in a method of manufacturing metal nanoparticles.As a result, they have discovered that in a method of manufacturing metal nanoparticles by irradiating a reaction solution with microwaves, due to the unique density characteristics of microwaves, there are parts of the reaction solution that are irradiated with microwaves (also parts where the reaction solution absorbs microwaves, referred to as "microwave irradiated parts") and parts that are not irradiated with microwaves (also parts where the reaction solution does not absorb microwaves, referred to as "microwave non-irradiated parts").

[0019] Therefore, the inventors have investigated various means for solving the above-mentioned problems, and as a result, in a method for producing metal nanoparticles by irradiating a reaction solution with microwaves, in order to reduce costs, they have used a single microwave irradiation source, performed the microwave irradiation step on the reaction solution in a batchwise manner, and further adjusted the volume fraction of the non-microwave-irradiated part of the reaction solution during microwave irradiation to be less than 1%.They have found that this method produces metal nanoparticles that are small and uniform in particle size at low cost and efficiently, and have completed the present invention.

[0020] That is, the gist of the present invention is as follows. (1) A method for producing metal nanoparticles, comprising the step of irradiating a reaction solution with microwaves, There is one microwave radiation source, The steps are carried out in batch mode, The volume fraction of the part where the microwave power density absorbed in the reaction solution during microwave irradiation is less than 10 W / mL based on the volume of the reaction solution (the part not irradiated with microwaves) is less than 1%. The method. (2) The method according to (1), wherein the reaction mixture is not physically stirred. (3) The method according to (1) or (2), wherein the metal nanoparticles are silver nanoparticles. (4) The method according to any one of (1) to (3), wherein the vessel containing the reaction solution is spiral-shaped. [Effects of the Invention]

[0021] The present invention provides a method for producing metal nanoparticles by irradiating a reaction solution with microwaves, which method enables the inexpensive and efficient preparation of metal nanoparticles with small and uniform particle sizes. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram schematically illustrating an example of an apparatus for carrying out the present invention. [Figure 2]1 is a diagram showing the microwave irradiation distribution (microwave absorption distribution) of the reaction solution during the production of silver nanoparticles obtained in Example 1. FIG. [Figure 3] FIG. 1 is a diagram schematically illustrating a silver nanoparticle production apparatus used in Comparative Example 1. [Figure 4] FIG. 2 is a diagram showing the microwave irradiation distribution of the reaction solution during the production of silver nanoparticles obtained in Comparative Example 1. [Figure 5] 1 shows TEM images of silver nanoparticles obtained in Example 1 and Comparative Example 1. [Figure 6] 1 is a graph showing the microwave power density absorbed by each mesh when 1 W of microwaves is irradiated onto the entire structure, and the volume fraction of that power density within the structure. [Figure 7] FIG. 10 shows a means for increasing the total volume of the reaction solution for parameters with high importance based on the results of machine learning. [Figure 8] FIG. 10 is a diagram showing a response surface for identifying optimal values ​​of parameters that affect the volume of a portion not irradiated with microwaves. DETAILED DESCRIPTION OF THE INVENTION

[0023] Preferred embodiments of the present invention will now 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, and the actual dimensions and shapes are not accurately depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. The method for producing metal nanoparticles of the present invention is not limited to the following embodiments, and can be implemented in various forms with modifications and improvements that can be made by those skilled in the art, without departing from the spirit of the present invention.

[0024] The present invention relates to a method for producing metal nanoparticles, which includes a step of irradiating a reaction solution with microwaves, wherein the method uses one microwave irradiation source, the step is carried out batchwise, and the volume fraction of the part of the reaction solution where the microwave power density absorbed in the reaction solution during microwave irradiation is less than 10 W / mL based on the volume of the reaction solution (non-microwave irradiated part) is less than 1%.

[0025] Here, the reaction solution used in the present invention is not limited to the composition of a reaction solution that can be used in a conventional method for producing metal nanoparticles by irradiating microwaves. For example, examples of materials contained in the reaction solution are listed below.

[0026] First, the reaction solution contains raw materials for metal nanoparticles. Examples of 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. Silver nanoparticles are preferred as the raw materials for these metal nanoparticles. These raw materials are not limited as long as they can dissolve in a solvent and generate metal ions. Examples include inorganic salts such as hydrochlorides, sulfates, nitrates, and phosphates of metals, organic salts such as carboxylates and sulfonates, and complexes containing 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. It is preferable to use inexpensive nitrates, such as silver nitrate, as the raw material for metal nanoparticles.

[0027] The concentration of metal ions in the reaction solution is not limited, but is usually 0.1 mmol / L (mM) to 300 mM, preferably 0.1 mM to 100 mM.

[0028] 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.

[0029] The reaction solution further contains a solvent. The solvent used in the reaction solution is not limited as long as it is a polar solvent or ionic liquid that can dissolve materials such as raw materials for metal nanoparticles, a protective agent, and a reducing agent and can also absorb microwaves. Examples of solvents used in the reaction solution include low-boiling point solvents with a boiling point of 300°C or less. Examples of low-boiling point solvents include, but are not limited to, 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 low-boiling point polar solvents. Water is preferred as the solvent used in the reaction solution.

[0030] 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.

[0031] The reaction solution further contains a protective agent. The protective agent used in the reaction solution is a compound that bonds to part or all of the surface of the metal nanoparticles produced in the reaction solution and suppresses aggregation of the metal nanoparticles. Examples of protective agents include, but are not limited to, polyvinylpyrrolidone (PVP), thiol-based polymers, polyvinyl alcohol (PVA), and tannic acid. Tannic acid is preferred as the protective agent.

[0032] 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, preferably 0.2 to 10 times the amount of the metal substance.

[0033] By using a protective agent, it is possible to suppress aggregation of the generated metal nanoparticles.

[0034] 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.

[0035] Examples of reducing agents include, but are not limited to, citric acid or a salt thereof, such as trisodium citrate, disodium citrate, or monosodium citrate, oxalic acid or a salt thereof, such as sodium oxalate, ascorbic acid or a salt thereof, such as sodium ascorbate, a mixture of two or more thereof, etc. Citric acid and / or a salt thereof is preferred as a reducing agent for metal ions, particularly silver ions.

[0036] 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, but is typically 1.0 to 20 equivalents, and preferably 4.0 to 15 equivalents, relative to the metal ions. When the reducing agent for metal ions contains one or more functional groups capable of interacting with metals, such as a carboxy group, a hydroxy group, or an ether group, it can also function as a protecting agent. When the reducing agent also functions as a protecting agent, the reaction solution need not contain the protecting agent described above. Furthermore, 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.

[0037] The reaction solution may be composed of the raw materials for metal nanoparticles, a solvent, a protective agent, and a reducing agent described above, but in addition to these materials, it may also contain additives that can be commonly used in reaction solutions that can be used in conventional methods for producing metal nanoparticles by irradiating microwaves.

[0038] For example, the reaction solution may further contain a chelating agent as an additive, such as ethylenediaminetetraacetic acid (EDTA) and / or a salt thereof.

[0039] The pH of the reaction solution is not limited, but is usually pH3 to pH12.

[0040] 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, and the materials are mixed so as to prepare a homogeneous reaction solution. In the present invention, the reaction is started after the homogeneous reaction solution is prepared.

[0041] In the present invention, the reaction solution described above is irradiated with microwaves using a microwave synthesis apparatus equipped with one microwave irradiation source, to allow the reaction to proceed.

[0042] A single microwave radiation source can reduce costs.

[0043] The step of irradiating the reaction solution with microwaves is carried out batchwise, i.e., in a state where the reaction solution is not being transported. In other words, microwaves are irradiated onto the reaction solution that is not being transported.

[0044] By carrying out the step of irradiating the reaction solution with microwaves in a batchwise manner, the synthesis reaction itself can be completed, improving yield and productivity, and reducing the problem of clogging of metal nanoparticles that can occur in a continuous process when the raw materials for metal nanoparticles are used at high concentrations.

[0045] 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 part, and a uniform and rapid reaction occurs in accordance with this temperature rise.

[0046] Furthermore, in the step of irradiating the reaction liquid with microwaves, the microwaves are irradiated so that the volume ratio of the portion of the reaction liquid that has not been irradiated with microwaves is less than 1%.

[0047] Here, the microwave-unirradiated portion of the reaction solution refers to the portion where the power density of the microwaves absorbed in the reaction solution during microwave irradiation is less than 10 W / mL based on the volume of the reaction solution, and the volume fraction of the microwave-unirradiated portion of the reaction solution is {(Volume of the non-microwave irradiated part of the reaction solution / Volume of the reaction solution) × 100} It can be calculated as follows.

[0048] The volume of the part of the reaction solution that has not been irradiated with microwaves can be determined by calculating the microwave irradiation distribution (microwave absorption distribution) according to (1) to (3) below. (1) Using Microwave Sim (ANSYS Electronics Desktop), calculate the distribution of microwaves absorbed by the reaction solution. (2) The calculation model is analyzed in conjunction with Fluent in Workbench. (3) Using Fluent, obtain the volume of each mesh and the microwave power density absorbed by each mesh.

[0049] By irradiating microwaves so that the volume fraction of the non-microwave-irradiated parts of the reaction liquid is less than a specific value, the nuclei of the metal nanoparticles formed in the microwave-irradiated parts are prevented from becoming coarse due to the raw materials of the metal nanoparticles present in the non-microwave-irradiated parts because the nuclei of the metal nanoparticles have not yet formed. This makes it possible to maintain a small and uniform particle size of the resulting metal nanoparticles without physically stirring the reaction liquid during the reaction (for example, transporting the reaction liquid in a flow system (controlling the flow rate of the reaction liquid), or stirring with a stirrer or stir bar).

[0050] In a microwave synthesis apparatus, the material of the container that contains the reaction solution is not limited as long as it can uniformly irradiate the reaction solution with microwaves. For example, when microwaves are irradiated onto the reaction solution from outside the reactor through the reactor, materials that transmit microwaves, such as ceramics and glass (quartz), can be used. When microwaves are irradiated directly onto the reaction solution from above the reaction solution, materials that reflect microwaves, such as metals such as aluminum and stainless steel, can be used.

[0051] The shape of the vessel for containing the reaction solution is preferably spiral when it is made of a microwave-transparent material such as quartz.

[0052] By making the container containing the reaction liquid spirally shaped, it is possible to secure a larger volume of the microwave-irradiated portion of the reaction liquid with a single microwave irradiation source than with a straight tube, and it is possible to increase the volume fraction of the microwave-irradiated portion of the reaction liquid during microwave irradiation, i.e., to reduce the volume fraction of the portion of the reaction liquid that is not irradiated with microwaves.

[0053] The microwaves are generated from a microwave irradiation source (microwave oscillator (magnetron)), and the microwave irradiation source can be either a single-mode system or a multi-mode system. The single-mode system used in Sim is preferred.

[0054] The output of the microwave irradiation source is not limited as long as the volume ratio of the non-microwave-irradiated portion in the reaction liquid is less than 1%, but is usually 1W to 6000W.

[0055] By adjusting the output of the microwave irradiation source within the above range, metal nanoparticles with small and uniform particle sizes can be prepared using microwaves with the existing output.

[0056] The frequency of the microwaves generated from the microwave irradiation source can be changed as appropriate and is not limited, but is usually 1 GHz to 10 GHz, preferably 2 GHz to 6 GHz. In the present invention, it is preferable to use 2.45 GHz as the microwave frequency, which is the frequency of an industrial microwave power source.

[0057] The microwaves are preferably uniform during irradiation, and the microwave irradiation conditions are preferably constant during microwave irradiation.

[0058] In the present invention, the temperature of the reaction solution raised by microwave irradiation is the reaction temperature, which 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, but is usually 25°C or higher, preferably 80°C or higher. The upper limit of the reaction temperature is not limited, but is usually below the boiling point of the solvent. For example, when the solvent is water, the reaction temperature is usually in the range of 25°C or higher and lower than 100°C, preferably 80°C to 90°C, at atmospheric pressure.

[0059] By setting the reaction temperature at 25°C or higher, a reduction reaction from metal ions to metal nanoparticles occurs, and by setting the reaction temperature below the boiling point of the solvent, it is possible to prevent the particle size of the resulting metal nanoparticles from becoming uneven due to the non-uniformity of the reaction field that can occur when the reaction solution boils, i.e., to prevent the broadening of the particle size distribution, and it is possible to prepare metal nanoparticles with small and uniform particle sizes.

[0060] The microwave irradiation time for the reaction solution is the time required for the temperature of the reaction solution to reach the reaction temperature, and is not limited, but may be changed appropriately depending on the reaction conditions (microwave conditions, type of metal, type of solvent, pressure during the reaction, amount of reaction solution, reaction temperature, etc.). It is usually 0.1 to 300 seconds, and preferably 10 to 60 seconds.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] Even without stirring the reaction solution, the volume fraction of the non-microwave-irradiated portion of the reaction solution during microwave irradiation is set to a certain value or less, so that the microwaves are irradiated uniformly throughout the reaction solution, allowing metal nanoparticles to be generated uniformly in the reaction solution and maintaining the reaction solution uniform.

[0065] Furthermore, although the present invention is carried out in a batch system, since a stirring function is not required, it can also be carried out using a flow-type synthesis apparatus. For example, when using a flow-type synthesis apparatus, the reaction solution is filled to a portion of the reaction tube where the volume ratio of the non-microwave-irradiated portion of the reaction tube is less than 1% when microwaves are irradiated, and the reaction is carried out in a state where the reaction solution does not flow, i.e., the transport of the reaction solution is stopped, and the reaction solution can be pumped out (transported) after the reaction is completed. By carrying out the present invention in this manner, although the transport of the reaction solution needs to be stopped during the reaction, the reaction solution can be transported quickly after the reaction is completed, allowing for continuous synthesis of metal nanoparticles. Furthermore, since the generation and transport of metal nanoparticles are independent of each other, the transport rate can be increased as a countermeasure against the clogging of piping by the generated metal nanoparticles.

[0066] The present invention can be implemented with a simple configuration of one microwave irradiation source and one container in which the volume ratio of the non-microwave-irradiated portion is less than 1% when microwaves are irradiated by the one microwave irradiation source, and therefore metal nanoparticles can be produced with low equipment costs.

[0067] For example, in order to maximize the volume of the reaction solution while maintaining the volume ratio of the non-microwave irradiated portion of the reaction solution from one microwave irradiation source at less than 1% as an apparatus for carrying out the present invention, the vessel containing the reaction solution is adjusted to a spiral shape, and further, for each parameter shown in FIG. 1, the vessel tube pitch 1 is adjusted to usually (tube inner diameter + 3) mm, for example, 10.5 mm to 11.7 mm, for example, 11.7 mm, the vessel tube inner diameter 2 is adjusted to usually 7.5 mm to 8.7 mm, for example, 8.7 mm, and the vessel tube pitch 1 is adjusted to usually (tube inner diameter + 3) mm, for example, 10.5 mm to 11.7 mm, for example, 11.7 mm. The bending radius 3 of the vessel is typically (tube inner diameter x 2) mm, e.g., 15.0 mm to 17.4 mm, e.g., 17.4 mm. The reaction tube length 4 of the vessel (height of the chamber in which the vessel is installed) is typically 100 mm or less, e.g., 100 mm. The width 5 of the chamber in which the vessel is installed (chamber width) is typically 39.2 mm to 160 mm, e.g., 39.2 mm. The thickness 6 of the chamber in which the vessel is installed (chamber thickness) is typically 44.6 mm to 105.3 mm, e.g., 44.6 mm. When the vessel containing such a reaction solution is filled with the reaction solution, the volume of the reaction solution can typically be greater than 41 mL to 56 mL. Table 1 summarizes examples of each parameter.

[0068] [Table 1]

[0069] The dispersion liquid containing metal nanoparticles obtained by the present invention can be subjected to separation and purification (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.

[0070] The metal nanoparticles produced by the method for producing metal nanoparticles of the present invention are characterized by their small particle size and narrow variance in particle size distribution.

[0071] The average particle size of metal nanoparticles can be measured by TEM images or absorbance of a dispersion containing the metal nanoparticles. When the average particle size of 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, preferably 1 nm to 20 nm.

[0072] 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).

[0073] 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]

[0074] 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.

[0075] 1. Preparation of Silver Nanoparticles Example 1 43 mg of silver nitrate as a raw material for silver nanoparticles, 43 mg of tannic acid as a protective agent, and 1.471 g of trisodium citrate as a reducing agent and protective agent were added to water, a polar solvent, and stirred for 5 minutes to dissolve each material in water, preparing 10 mL of reaction solution.

[0076] The resulting reaction solution was poured into a test tube, and without stirring, microwaves were absorbed at a power density of 10 W / mL based on the total volume of the reaction solution until the temperature of the reaction solution reached 90°C, and the reaction was carried out for 5 minutes to obtain silver nanoparticles.

[0077] As shown in Figure 2, when the microwave irradiation distribution in the reaction solution during the reaction in Example 1 was calculated using the following (1) to (3), the volume fraction of the part where the power density of the microwaves absorbed in the reaction solution was less than 10 W / mL based on the volume of the reaction solution (the part of the reaction solution that was not irradiated with microwaves) was less than 1% (approximately 0.9%). Calculation of microwave irradiation distribution (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.

[0078] Example 2 43 mg of silver nitrate as a raw material for silver nanoparticles, 43 mg of tannic acid as a protective agent, and 1.47 g of trisodium citrate as a reducing agent and protective agent were added to water, a polar solvent, and stirred for 5 minutes to dissolve each material in water, preparing 10 mL of reaction solution.

[0079] The resulting reaction solution was poured into a test tube, and while stirring with a stirrer, microwaves were absorbed at a power density of 10 W / mL based on the total volume of the reaction solution until the temperature of the reaction solution reached 90°C, and the reaction was carried out for 5 minutes to obtain silver nanoparticles.

[0080] When the microwave irradiation distribution in the reaction solution during the reaction in Example 2 was calculated according to the following (1) to (3), the volume ratio of the part of the reaction solution that was not irradiated with microwaves was less than 1% (approximately 0.9%). Calculation of microwave irradiation distribution (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.

[0081] Comparative Example 1 To 1 L of water, a polar solvent, 0.56 g of silver nitrate as a raw material for silver nanoparticles, 66.7 g of PVP as a protective agent, and 1.03 g of trisodium citrate as a reducing agent were added, and the mixture was stirred for 5 minutes to dissolve each material in water, preparing 1 L of reaction solution.

[0082] The resulting reaction solution was placed in the reaction chamber of the apparatus shown schematically in Figure 3, and while stirring with a stirrer, microwaves were absorbed at a power density of 10 W / mL based on the total volume of the reaction solution until the temperature of the reaction solution reached the reaction temperature of 90°C, and the reaction was carried out for 40 minutes to obtain silver nanoparticles.

[0083] As shown in FIG. 4, when the microwave irradiation distribution in the reaction solution during the reaction in Comparative Example 1 was calculated according to the following (1) to (3), the volume fraction of the part of the reaction solution that was not irradiated with microwaves was more than 2%. Calculation of microwave irradiation distribution (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] 2. TEM images of silver nanoparticle dispersions The silver nanoparticles obtained in Example 1 and Comparative Example 1 were photographed using a transmission electron microscope (TEM), and the results are shown in FIG.

[0085] 5, it was found that the silver nanoparticles obtained in Example 1 had a small and uniform particle size, whereas the silver nanoparticles obtained in Comparative Example 1 had a large and non-uniform particle size. Therefore, it was found that in order to produce silver nanoparticles with a small and uniform particle size, the volume fraction of the portion of the reaction solution that was not irradiated with microwaves must be less than 1%.

[0086] Although the results for the silver nanoparticles obtained in Example 2 are not shown, the silver nanoparticles obtained in Example 2 were equivalent to the silver nanoparticles obtained in Example 1. This indicates that stirring is not essential when irradiating the reaction solution with microwaves.

[0087] 3. Simulation Experiment The apparatus shown in FIG. 1 was used as an apparatus for carrying out the present invention, and a simulation was carried out to maximize the volume of the reaction liquid while maintaining the volume ratio of the non-microwave-irradiated portion of the reaction liquid at less than 1%.

[0088] The simulation method is shown below. 1. First, the microwave irradiation distribution was calculated using Sim (ANSYS Electronics Desktop) using the following (1) to (3), and the volume of the area not irradiated with microwaves was quantified. Calculation of microwave irradiation distribution (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.

[0089] Figure 6 shows the microwave power density absorbed by each mesh and the volume fraction of the structure where this power density exists when the entire structure is irradiated with microwaves at 1 W. Here, the non-microwave-irradiated areas are those where the microwave power density absorbed is 0 to less than 10 W / mL based on the volume of the reaction solution when the microwave irradiation source output is 6000 W.

[0090] In addition, in Figure 6, when the output of the microwave irradiation source is set to 6000 W, that is, when the x-axis of Figure 6 is multiplied by 6000, the power density becomes 10 W / mL, which is 1667 W / m 3 When it is desired to change the output of the microwave irradiation source, in this simulation experiment, the power density that becomes 10 W / mL when multiplied by the output of the microwave irradiation source used is calculated on the x-axis of Figure 6, and the microwave-irradiated area can be distinguished from the non-microwave-irradiated area using this value as the boundary.

[0091] 2. Next, the parameters that contribute to the change in the volume of the non-microwave-irradiated area were identified using machine learning (random forest).

[0092] Here, we fixed the tube length at 100 mm, and created models by varying the tube inner diameter, tube pitch, bending radius, chamber width, and chamber thickness, calculated the volume fraction of the area not irradiated with microwaves, and used the obtained data as learning data to calculate the importance (degree of influence) of the objective variable. Table 2 shows an example of a created model.

[0093] [Table 2]

[0094] Table 3 shows the importance (degree of influence) of each parameter (explanatory variable), and Figure 7 shows the means for increasing the total volume of the reaction solution for parameters with high importance based on the results of machine learning.

[0095] [Table 3]

[0096] 3. Furthermore, the optimal values ​​of the parameters that reduce the volume of the non-microwave-irradiated portion were identified using a response surface. Figure 8 shows the response surface for identifying the optimal values ​​of the parameters that affect the volume of the non-microwave-irradiated portion, and Table 4 shows the maximum tube inner diameter, chamber width, and chamber thickness when selected under conditions that result in a volume ratio of the non-microwave-irradiated portion of less than 1% in order to improve the mass production efficiency of metal nanoparticles.

[0097] [Table 4]

[0098] Tables 2 to 4 and Figures 6 to 8 show that in order to reduce the volume fraction of the unexposed area, it is preferable to increase the tube inner diameter, decrease the chamber width, and increase the chamber thickness, with the tube inner diameter, chamber width, and chamber thickness being the most influential parameters. Table 5 summarizes the optimal values ​​for each parameter. When the device is fabricated using the parameters in Table 5, the total volume of the reaction solution can be made 56 mL.

[0099] [Table 5] [Explanation of symbols]

[0100] 1. Tube pitch, 2. Tube inner diameter, 3. Bending radius, 4. Reaction tube length, 5. Chamber width, 6. Chamber thickness

Claims

1. A method for producing metal nanoparticles, comprising a step of irradiating a reaction solution with microwaves, There is one microwave radiation source, the microwave radiation source is a single-mode system; The vessel containing the reaction solution is spiral-shaped, The steps are carried out in batch mode, The batch method means that the reaction solution is not being transported. The volume fraction of the portion where the microwave power density absorbed in the reaction solution during microwave irradiation is less than 10 W / mL based on the volume of the reaction solution is less than 1%. The method.

2. The method of claim 1 , wherein the reaction mixture is not physically stirred.

3. The method of claim 1 or 2, wherein the metal nanoparticles are silver nanoparticles.

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