JP7729241B2 - Method for producing metal nanoparticles - Google Patents

Description

The present invention relates to a method for producing metal nanoparticles.

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

A variety of devices and methods have also been devised for producing metal nanoparticles, and among these, devices and methods for producing metal nanoparticles using microwaves are attracting attention as they can synthesize large quantities of metal nanoparticles in a short period of time.

As a method for preparing such metal nanoparticles, for example, Patent Document 1 discloses a product production device that circulates a fluid through a reaction channel via a flow path and produces a predetermined product from the fluid within the reaction channel, and that is characterized by having a microwave generator for irradiating the reaction channel with microwaves.

Patent Document 2 discloses a metal nanoparticle manufacturing device that includes a chamber that contains a reaction liquid, multiple 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 that stirs the reaction liquid installed in the chamber.

Patent Document 3 discloses a microwave processing apparatus for processing an object to be processed using a single-mode standing wave, the single-mode standing wave being a standing wave of 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, the microwave processing apparatus having a cavity resonator that forms the single-mode standing wave, a microwave supply means that supplies microwaves into the cavity resonator that match the resonant 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 resonant frequency of the cavity resonator, and the object to be processed is placed along a position where the magnetic field strength is maximized.

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 in the microwave irradiation space toward the flow tube, heating the reaction liquid in the flow tube with this microwave irradiation and synthesizing metal microparticles. The method uses a microwave generator that is assigned a frequency band of 2.4 to 2.5 GHz or that meets the product's standard frequency band, and the average inner diameter of the portion of the flow tube placed in the microwave irradiation field is narrower than 2.84 mm.

Patent Document 5 discloses a microwave processing device that includes a cylindrical cavity rotatably supported on a fixed base and having an internal space for placing an object to be irradiated with microwaves, a rotary drive unit that rotates the cavity around the axis of the cylindrical shape, and a microwave generator that generates microwaves, the microwaves generated by the microwave generator being introduced into the internal space from the circumferential side of the cavity.

Japanese Patent Application Laid-Open No. 2006-188666 Japanese Patent Application Laid-Open No. 2019-077923 Japanese Patent Application Laid-Open No. 2020-080298 JP 2011-137226 A Japanese Patent Application Laid-Open No. 2021-125431

In the field of electronics packaging, metal nanoparticles are also being considered as high-heat-resistant bonding materials, for example, lead-free bonding materials that can be used for bonding 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.

In order to use metal nanoparticles as a highly heat-resistant bonding material, it is necessary to maintain a constant melting point for the metal nanoparticles. To maintain a constant melting point for metal nanoparticles, it is desirable to reduce the particle size of the metal nanoparticles and narrow the particle size distribution.

While progress is being made in the development of metal nanoparticles that can be used in 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.

The present invention aims 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.

In conventional technologies, for example, in flow-type metal nanoparticle manufacturing methods, mass production of metal nanoparticles has been achieved by making the reaction channel in the metal nanoparticle manufacturing equipment spiral, specifying the inner diameter of the reaction channel, using single-mode standing microwave waves, or rotating the microwave irradiation source; and, for example, in batch-type metal nanoparticle manufacturing methods, by using multiple microwave irradiation sources to irradiate the chamber containing the reaction solution.

However, in metal nanoparticle manufacturing methods, 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.

Furthermore, with the flow-through method, there is a possibility that the yield of metal nanoparticles may decrease due to, for example, the synthesis reaction not being completed as the reaction flow path increases.

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

The inventors of the present invention 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, the unique density characteristics of microwaves result in the existence of 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-unirradiated parts").

The inventors therefore investigated various means for solving the above-mentioned problems, and as a result, they discovered that in a method for producing metal nanoparticles by irradiating a reaction solution with microwaves, they reduced costs by using 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 reaction solution that was not irradiated with microwaves during microwave irradiation to less than 1%. This resulted in the production of metal nanoparticles that were small and uniform in particle size and could be produced inexpensively and efficiently, leading to the completion of the present invention.

That is, the gist of the present invention is as follows.
(1) A method for producing metal nanoparticles, comprising a step of irradiating a reaction solution with microwaves,
There is one microwave radiation source,
The steps are carried out in batch mode,
The method, wherein the volume fraction of the portion (non-microwave irradiated 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%.
(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.

The present invention provides a method for producing metal nanoparticles by irradiating a reaction solution with microwaves, which allows for the inexpensive and efficient preparation of metal nanoparticles with small and uniform particle sizes.

FIG. 1 is a diagram schematically illustrating an example of an apparatus for carrying out the present invention. FIG. 2 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. 1 is a diagram schematically illustrating a silver nanoparticle production apparatus used in Comparative Example 1. FIG. 1 is a diagram showing the microwave irradiation distribution of the reaction solution during the production of silver nanoparticles obtained in Comparative Example 1. 1 shows TEM images of silver nanoparticles obtained in Example 1 and Comparative Example 1. 10 is a graph showing the microwave absorption power density of each mesh when 1 W of microwaves is irradiated onto the entire structure, and the volume fraction of the power density present within the structure. 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. 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.

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.

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 portion 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 portion) is less than 1%.

The reaction solution used in the present invention can have the same composition as a reaction solution that can be used in conventional methods for producing metal nanoparticles by irradiating microwaves, and is not limited to this. For example, examples of materials that can be contained in the reaction solution are listed below.

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. There are no particular restrictions on the raw materials for these metal nanoparticles, 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 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.

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

By keeping the concentration of metal ions in the reaction solution within this range, the variation in the resulting metal nanoparticles will be reduced; in other words, the particle size distribution of the resulting metal nanoparticles will be narrow.

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 the raw materials for the metal nanoparticles, the protective agent, and the reducing agent, and that 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.

Using a low-boiling point solvent as the solvent used in the reaction solution improves the ease of handling of the solvent and reduces the burden on the environment.

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 inhibits 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.

The amount of protective agent is not limited and can be changed depending on the desired particle size of the metal nanoparticles. The amount of protective agent is typically 0.1 to 20 times, and preferably 0.2 to 10 times, the amount of metal substance.

By using a protective agent, it is possible to prevent the generated metal nanoparticles from agglomerating together.

The reaction solution further contains a reducing agent. A reducing agent is a material that can reduce metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction.

Reducing agents include, but are not limited to, citric acid or its salts, such as trisodium citrate, disodium citrate, and monosodium citrate; oxalic acid or its salts, such as sodium oxalate; ascorbic acid or its salts, such as sodium ascorbate; and mixtures of two or more of these. Citric acid and/or its salts are preferred as reducing agents for metal ions, particularly silver ions.

The amount of reducing agent is not limited as long as it can reduce metal ions to a metal with an oxidation number of 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. Note that if 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 does not need to 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 with an oxidation number of 0 through a redox reaction.

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

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

The pH of the reaction solution is not limited, but is typically between pH 3 and pH 12.

In the present invention, the order of addition of each material, addition temperature, mixing method, 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 initiated after a homogeneous reaction solution has been prepared.

In the present invention, the reaction solution described above is irradiated with microwaves using a microwave synthesis apparatus equipped with a single microwave irradiation source, causing the reaction to proceed.

By using a single microwave irradiation source, costs can be reduced.

Furthermore, the step of irradiating the reaction solution with microwaves is carried out batchwise, i.e., while the reaction solution is not being transported. In other words, microwaves are irradiated onto the reaction solution that is not being transported.

By performing the step of irradiating the reaction solution with microwaves in a batch process, the synthesis reaction itself can be completed, improving yield and productivity, and reducing the problem of metal nanoparticle blockage that can occur in a continuous process when the metal nanoparticle raw material is used at a high concentration.

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 a reaction solution irradiated with microwaves, a uniform and rapid temperature rise occurs in the irradiated area, and as this temperature rises, a uniform and rapid reaction occurs.

Furthermore, in the step of irradiating the reaction liquid with microwaves, the microwaves are irradiated so that the volume fraction of the reaction liquid that is not irradiated with microwaves is less than 1%.

Here, the microwave-unirradiated portion of the reaction solution refers to a 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 ratio of the microwave-unirradiated portion of the reaction solution is
{(volume of the part of the reaction solution not irradiated with microwaves/volume of the reaction solution)×100}
It can be calculated as follows.

The volume of the portion 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), the distribution of microwaves absorbed by the reaction solution is calculated.
(2) The calculation model is subjected to coupled analysis with Fluent using Workbench.
(3) Using Fluent, obtain the volume of each mesh and the microwave power density absorbed by each mesh.

By irradiating microwaves so that the volume fraction of the non-microwave-irradiated portions of the reaction solution is less than a specific value, the nuclei of metal nanoparticles formed in the microwave-irradiated portions are prevented from becoming coarse due to the metal nanoparticle raw materials present in the non-microwave-irradiated portions where no metal nanoparticle nuclei have yet formed. This makes it possible to maintain a small and uniform particle size for the resulting metal nanoparticles without physically stirring the reaction solution during the reaction (for example, by transporting the reaction solution in a flow system (controlling the flow rate of the reaction solution), or by stirring with a stirrer or stir bar).

In a microwave synthesis apparatus, the material of the container that holds the reaction solution is not limited as long as it allows microwaves to be uniformly irradiated onto the reaction solution. 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, materials that reflect microwaves, such as metals such as aluminum and stainless steel, can be used.

When the container containing the reaction solution is made of a microwave-transparent material, such as quartz, it is preferable that it be spiral-shaped.

By making the container that holds the reaction solution spiral-shaped, it is possible to ensure a larger volume of the microwave-irradiated portion of the reaction solution with a single microwave irradiation source than with a straight tube, and this increases the volume fraction of the microwave-irradiated portion of the reaction solution during microwave irradiation, i.e., reduces the volume fraction of the non-microwave-irradiated portion of the reaction solution.

Microwaves are generated from a microwave irradiation source (microwave oscillator (magnetron)), which can be either a single-mode system or a multi-mode system. The single-mode system used in SIM is preferred.

The output of the microwave irradiation source is not limited as long as the volume fraction of the reaction solution not irradiated by microwaves is less than 1%, but is typically between 1W and 6000W.

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 at the existing output.

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

It is preferable that the microwaves are uniform during irradiation, and that the microwave irradiation conditions are constant during microwave irradiation.

In the present invention, the temperature of the reaction solution raised by microwave irradiation is the reaction temperature, which can be varied appropriately depending on the reaction conditions (such as the type of metal, the type of solvent, and the pressure during the reaction). While not limited, the reaction temperature is typically 25°C or higher, and preferably 80°C or higher. The upper limit of the reaction temperature is typically below the boiling point of the solvent, but is not limited thereto. For example, when the solvent is water, the reaction temperature is typically in the range of 25°C or higher but lower than 100°C, preferably 80°C to 90°C, at atmospheric pressure.

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 particle size distribution from becoming wider, making it possible to prepare metal nanoparticles with small, uniform particle sizes.

The microwave irradiation time for the reaction solution is the time it takes for the temperature of the reaction solution to reach the reaction temperature. It varies 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.), and is not limited; however, it is usually 0.1 to 300 seconds, preferably 10 to 60 seconds.

By irradiating the reaction solution with microwaves under the above conditions and raising the temperature of the reaction solution to the reaction temperature, metal nanoparticle nuclei and then metal nanoparticles are produced in the reaction solution.

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, if 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 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 time is observed, and the reaction is completed when the absorbance no longer changes.

As mentioned above, in the present invention, there is no need to stir the reaction solution using a stirring mechanism, such as a propeller stirrer, vibration stirrer, or magnetic stirrer.

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

In addition, although the present invention is carried out in a batch system, it can also be carried out using a flow-type synthesis apparatus because it does not require a stirring function. For example, when using a flow-type synthesis apparatus, the present invention can be carried out by filling the reaction solution 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 carrying out the reaction while the reaction solution is not flowing, i.e., while transport of the reaction solution is stopped, and then discharging (transporting) the reaction solution after the reaction is complete. By carrying out the present invention in this manner, although it is necessary to stop the transport of the reaction solution during the reaction, the reaction solution can be transported quickly after the reaction is complete, allowing for the continuous synthesis of metal nanoparticles. Furthermore, because the generation and transport of metal nanoparticles are independent of each other, the transport rate can be increased as a countermeasure against piping blockage caused by the generated metal nanoparticles.

The present invention can be implemented with a simple configuration consisting 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 single microwave irradiation source. Therefore, metal nanoparticles can be produced with low equipment costs.

For example, in an apparatus for implementing the present invention, 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%, the container containing the reaction solution is adjusted to a spiral shape, and further, for each parameter shown in Figure 1, the tube pitch 1 of the container is usually adjusted to (tube inner diameter + 3) mm, for example, 10.5 mm to 11.7 mm, for example, 11.7 mm, and the tube inner diameter 2 of the container is usually adjusted to 7.5 mm to 8.7 mm, for example, 8.7 mm, and the volume is adjusted to The bending radius 3 of the vessel is typically adjusted to (tube inner diameter x 2) mm, for example, 15.0 mm to 17.4 mm, for example, 17.4 mm; the reaction tube length 4 of the vessel (height of the chamber in which the vessel is installed) is typically adjusted to 100 mm or less, for example, 100 mm; the width 5 of the chamber in which the vessel is installed (chamber width) is typically adjusted to 39.2 mm to 160 mm, for example, 39.2 mm; and the thickness 6 of the chamber in which the vessel is installed (chamber thickness) is typically adjusted to 44.6 mm to 105.3 mm, for example, 44.6 mm. When a vessel containing such a reaction liquid is filled with the reaction liquid, the volume of the reaction liquid can typically be greater than 41 mL to 56 mL. Table 1 summarizes examples of each parameter.

The dispersion liquid containing metal nanoparticles obtained by 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 dispersion liquid containing metal nanoparticles.

Metal nanoparticles produced by the metal nanoparticle production method of the present invention are characterized by their small particle size and narrow particle size distribution.

The average particle size of metal nanoparticles can be measured using TEM images or absorbance of a dispersion containing the metal nanoparticles. When measuring the average particle size of metal nanoparticles using 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 typically 30 nm or less, preferably 1 nm to 20 nm.

The particle size distribution of metal nanoparticles can be determined from TEM images 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 points of absorbance 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 manufacturing 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 manufacturing method of the present invention are uniform silver nanoparticles with a narrow particle size distribution (small particle size variation and uniform particle size).

Metal nanoparticles produced by the metal nanoparticle production method 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.

The following describes several examples of the present invention, but it is not intended that the present invention be limited to those examples.

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, thereby preparing 10 mL of a reaction solution.

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. The reaction was carried out for 5 minutes, yielding silver nanoparticles.

As shown in FIG. 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 calculation model was subjected to coupled analysis with Fluent using Workbench.
(3) Using Fluent, the volume of each mesh and the microwave power density absorbed by each mesh were obtained.

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, thereby preparing 10 mL of a reaction solution.

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. The reaction was carried out for 5 minutes, yielding silver nanoparticles.

When the microwave irradiation distribution in the reaction solution during the reaction in Example 2 was calculated using the following (1) to (3), the volume fraction of the non-microwave-irradiated portion in the reaction solution 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 calculation model was subjected to coupled analysis with Fluent using Workbench.
(3) Using Fluent, the volume of each mesh and the microwave power density absorbed by each mesh were obtained.

Comparative Example 1
To 1 L of water as 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, thereby preparing 1 L of a reaction solution.

The resulting reaction solution was placed in the reaction chamber of the device 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. The reaction was carried out for 40 minutes, yielding silver nanoparticles.

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 exceeded 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 calculation model was subjected to coupled analysis with Fluent using Workbench.
(3) Using Fluent, the volume of each mesh and the microwave power density absorbed by each mesh were obtained.

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). The results are shown in FIG.

Figure 5 shows 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. Therefore, in order to produce silver nanoparticles with small and uniform particle size, it was found that the volume fraction of the non-microwave-irradiated portion of the reaction solution must be less than 1%.

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. Therefore, it was found that stirring is not essential when irradiating the reaction solution with microwaves.

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

The simulation method is shown below.
1. First, the microwave irradiation distribution was calculated using Sim (ANSYS Electronics Desktop) according to 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 calculation model was subjected to coupled analysis with Fluent using Workbench.
(3) Using Fluent, the volume of each mesh and the microwave power density absorbed by each mesh were obtained.

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

In Fig. 6, when the output of the microwave irradiation source is set to 6000 W, that is, when the x-axis of Fig. 6 is multiplied by 6000, the microwave non-irradiated portion and the microwave irradiated portion are distinguished at the boundary of 1667 W/ ^m3 , which is the power density that becomes 10 W/mL. 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 according to the output of the microwave irradiation source used on the x-axis of Fig. 6 can be calculated, and the microwave non-irradiated portion and the microwave irradiated portion can be distinguished at that value.

2. Next, machine learning (random forest) was used to identify the parameters that contribute to changes in the volume of the non-microwave-irradiated areas.

Here, we created a model in which the tube length was fixed at 100 mm and the tube inner diameter, tube pitch, bending radius, chamber width, and chamber thickness were variously changed, and calculated the volume fraction of the area not irradiated with microwaves. The obtained data was used as training data to calculate the importance (degree of influence) of the objective variable. Table 2 shows an example of the model we created.

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.

3. Furthermore, the optimal values of the parameters that reduce the volume of the microwave-unirradiated 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 microwave-unirradiated 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 microwave-unirradiated portion of less than 1% in order to improve the mass production efficiency of metal nanoparticles.

Tables 2-4 and Figures 6-8 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 an apparatus is fabricated using the parameters in Table 5, the total volume of the reaction solution can be made 56 mL.

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

Claims (3)

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 method as described above, wherein the volume fraction of the portion 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 is less than 1%. The method of claim 1, wherein the reaction mixture is not physically stirred. The method of claim 1 or 2, wherein the metal nanoparticles are silver nanoparticles.