Metal particles, method for producing metal particles, apparatus for producing metal particles, and dispersion containing metal particles

Abstract

The present invention provides: a method for efficiently producing metal particles; metal particles which are obtained by the production method; a production apparatus for executing the production method; and a dispersion which contains metal particles that are obtained by the production method. The present invention relates to a method for producing metal particles, the method including a step for irradiating a reaction solution with microwaves, and the method comprising: (i) a step for preparing a reaction solution that contains a metal particle precursor, an organic substance, and a solvent; and (ii) a step for irradiating the reaction solution with microwaves while flowing the reaction solution, wherein the absorption power E (unit: W / mL) of the microwaves to be applied to the reaction solution and the pressure P (unit: MPa) satisfy the relational expression E × P ≥ 20 (formula 1).

Description

Metal particles, metal particle manufacturing method, metal particle manufacturing apparatus, and dispersion containing metal particles

[0001] The present invention relates to metal particles, a method for producing metal particles, an apparatus for producing metal particles, and a dispersion containing metal particles.

[0002] Metal nanoparticles, which may have properties different from those of bulk materials, are being used and investigated for a variety of applications, such as catalysts, ink materials, and electronic component parts.

[0003] For example, Patent Document 1 discloses a method for producing chromium nanoparticles having an average particle size of 5.6 nm by irradiating a raw material solution containing 50 mL of 1-decanol as a solvent and 5 mmol of chromium carbonyl powder as a chromium nanoparticle raw material with ultrasonic waves at a frequency of 50 kHz and 150 W at 40°C to thermally decompose the chromium nanoparticle raw material.

[0004] Patent Document 2 discloses a method for producing metal microparticles, in which a reaction liquid containing a metal precursor is circulated through a flow pipe, and electromagnetic waves are uniformly and intensively irradiated into the flow pipe along the length of the flow pipe, thereby uniformly heating the electromagnetic wave-irradiated space within the flow pipe along the flow direction, thereby generating metal microparticles.

[0005] JP 2015-129327 A JP 2011-137226 A

[0006] In order to use metal nanoparticles in electronic packaging applications, it is desirable to reduce the particle size of the metal nanoparticles.

[0007] However, in order to produce metal nanoparticles with a nanoscale average particle size by microwave irradiation, high microwave intensity and high synthesis temperature are required to suppress variations in nucleation and particle growth.

[0008] Therefore, an object of the present invention is to provide a method for efficiently producing metal particles, metal particles obtained by the production method, a production apparatus for carrying out the production method, and a dispersion containing metal particles obtained by the production method.

[0009] As a result of examining various means for solving the above-mentioned problems, the inventors discovered that in a method for producing metal particles by irradiating a reaction liquid with microwaves, the reaction time can be shortened by setting the microwave absorption power of the reaction liquid and the pressure applied to the reaction liquid to a certain level or higher, and further, metal particles having a small particle size and a low content of organic matter as a dispersant that can become an impurity can be produced, and the resulting metal particles have a low volume shrinkage rate, thereby completing the present invention.

[0010] That is, the gist of the present invention is as follows: (1) A method for producing metal particles, comprising the step of irradiating a reaction solution with microwaves, the method comprising: (i) preparing a reaction solution containing a metal particle precursor, an organic substance as a dispersant, and a solvent, and (ii) irradiating the reaction solution with microwaves while flowing, wherein the relationship between the absorbed power E (unit: W / mL) of the microwaves and the pressure P (unit: MPa) of the reaction solution satisfies the following formula 1: ExP≧20 (Formula 1) (2) The method according to (1), wherein in step (i), the content of the organic substance in the reaction solution is 0.1 wt % to 2 wt % with respect to the total weight of the metal of the metal particle precursor. (3) The method according to (1) or (2), wherein in step (ii), the relationship between the absorbed power E (unit: W / mL) of the microwaves and the pressure P (unit: MPa) for the reaction solution satisfies the following formula 2: ExP≧30 (Formula 2). (4) Metal particles containing an organic substance, wherein the content of the organic substance is 0.1 wt % to 2 wt % with respect to the total weight of the metal particles, and the median diameter (D50) of the metal particles measured by TEM is 20 nm or less. (5) A metal particle dispersion comprising metal particles, an organic substance as a dispersant for the metal particles, and a solvent, wherein the content of the metal particles is 1 wt % to 95 wt % with respect to the total weight of the metal particle dispersion, the content of the organic substance is 0.1 wt % to 2 wt % with respect to the total weight of the metal particles, and the median diameter (D50) of the metal particles measured by TEM is 20 nm or less. (6) A manufacturing apparatus for producing metal particles by irradiating a reaction solution with microwaves, the manufacturing apparatus comprising: a pump for pressure-feeding the reaction solution; an irradiation device for irradiating the reaction solution, which is pressure-fed from the pump and flows through a reaction tube, with the microwaves together with the reaction tube; a pressure adjustment device for adjusting the pressure of the reaction solution in the reaction tube downstream of the reaction tube; and a stirring device for stirring the reaction solution pressure-fed from the pump between the pump and the reaction tube.(7) The apparatus for producing metal nanoparticles according to (6), wherein the agitator is disposed between the pump and the reaction tube at a position where an agitated flow of the reaction liquid that has passed through the agitator is maintained within the reaction tube. (8) The apparatus for producing metal nanoparticles according to (6) or (7), wherein the agitator has a straight pipe through which the reaction liquid flows and a twisted blade fixed within the pipe and twisted around the axis of the pipe. (9) The apparatus for producing metal nanoparticles according to (8), wherein the twisted blade is a blade in which twisted blade elements having different twist directions around the axis are arranged alternately along the axial direction of the pipe. (10) The apparatus for producing metal nanoparticles according to (8) or (9), wherein the twisted blade is made of a non-conductive material.

[0011] The present invention provides a method for efficiently producing metal particles, metal particles obtained by the production method, a production apparatus for carrying out the production method, and a dispersion containing metal particles obtained by the production method.

[0012] 1 is a diagram showing an outline of an embodiment of the manufacturing apparatus of the present invention. FIG. 2 is a diagram schematically showing an embodiment of a stirring device in the manufacturing apparatus of the present invention. FIG. 3 is a TEM image of silver particles in Comparative Example 1 and Examples 1 and 4. FIG. 4 is a graph showing the particle size distribution of silver particles in Example 1. FIG. 5 is a graph showing the relationship between D50 and the value (E×P) obtained by multiplying the absorbed power E (W / mL) of microwaves with respect to a reaction solution by the pressure P (MPa) applied to the reaction solution. FIG. 6 is a graph showing the relationship between the amount of organic matter and D50 in Examples and Comparative Examples. FIG. 7 is a graph showing the relationship between the amount of organic matter and the volumetric shrinkage rate at 120°C for 2 hours. FIG. 8 is a diagram schematically showing the process from nucleus formation to particle growth of metal particles (for example, silver particles) in the prior art or when E×P is less than 20. FIG. 9 is a diagram schematically showing the process from nucleus formation to particle growth of metal particles (for example, silver particles) in the present invention when E×P is 20 or more.

[0013] Preferred embodiments of the present invention are described in detail below. In this specification, the features of the present invention are described with reference to the drawings as appropriate. In the drawings, the dimensions and shapes of each part are exaggerated for clarity and 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 metal particles, metal particle manufacturing method, metal particle manufacturing apparatus, and metal particle-containing dispersion of the present invention are not limited to the following embodiments, and can be embodied in various forms incorporating modifications and improvements that can be made by those skilled in the art without departing from the spirit of the present invention. Furthermore, although each embodiment described in this specification is independent, two or more of each embodiment can also be combined to form one embodiment of the present invention.

[0014] The symbol "to" used herein means a range having the numerical values ​​before and after it as the upper and lower limits. In numerical ranges described in stages in this specification, the upper or lower limit described in one numerical range may be replaced with the upper or lower limit described in another stage. The upper or lower limit of a numerical range described herein may be replaced with a value shown in the examples.

[0015] The present invention relates to a method for producing metal particles, which includes a step of irradiating a reaction liquid with microwaves, and the method includes a step of (i) preparing a reaction liquid containing a metal particle precursor, an organic substance, and a solvent, and a step of irradiating the reaction liquid with microwaves while flowing the reaction liquid.

[0016] (i) Step of preparing a reaction liquid containing a metal particle precursor, an organic substance, and a solvent In step (i), a reaction liquid containing a metal particle precursor, an organic substance, and a solvent is prepared.

[0017] In the method of the present invention, 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 a metal particle precursor, an organic dispersant, and a 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. When a mixture is used as a solvent, the ratio of each component contained in the mixture is not limited, as long as the components are miscible under the experimental conditions.

[0018] By using a low boiling point polar solvent as the solvent, the handling of the solvent can be improved and the burden on the environment can be reduced.

[0019] The metal particle precursor may be any material that dissolves in a solvent to produce metal ions, such as ions of precious metals, base metals, and alloys, such as gold, silver, platinum, copper, nickel, iron, and cobalt, or ions of two or more of these metals. Examples of metal particle precursors include inorganic metal salts such as metal halides, e.g., fluorides, chlorides, bromides, and iodides; metal sulfates, metal nitrates, metal phosphates, and metal cyanides; organic metal salts such as metal carboxylates and metal sulfonates; and metal complexes, including metal complex salts. The metal particle precursor 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 metal nitrates, such as nickel nitrate and silver nitrate, as the metal particle precursor. Alternatively, a formate salt having reducing properties, such as nickel formate, may be used as the metal particle precursor. Therefore, in one embodiment, when a salt composed of a metal ion and a reducing organic anion, such as a formate ion, as its counter anion is used as the metal particle precursor, a reducing agent may not be used.

[0020] A homogeneous reaction solution can be prepared by dissolving the metal particle precursor in a solvent.

[0021] The concentration of metal ions in the reaction solution is not limited as long as it is not more than the saturated concentration, but is usually 1 mmol / L (mM) or more, and in one embodiment 10 mM or more, and usually 600 mM or less, and in one embodiment 200 mM or less, for example, 1 mM to 600 mM, in one embodiment 10 mM to 200 mM, in one embodiment 20 mM to 180 mM, and in one embodiment 50 mM to 150 mM.

[0022] By setting the concentration of metal ions in the reaction solution within the above range, metal particles can be efficiently produced at a high concentration, the amount of metal particles that can be produced and recovered at one time can be significantly increased, and the time, labor, and cost required for producing metal particles can be reduced. In addition, the variation in the obtained metal particles can be reduced, in other words, the particle size distribution of the obtained metal particles can be narrowed.

[0023] The reaction liquid contains an organic substance as a dispersant. Examples of dispersants include, but are not limited to, one or more dispersants selected from polyvinylpyrrolidone (PVP), dodecylamine (DDA), thiol-based polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylates, cyclodextrin, aminopectin, methylcellulose, polyethyleneimine cellulose, aliphatic amines, aliphatic carboxylic acids, and tannic acid. When the dispersant is a polymer, the molecular weight of the dispersant is, but is not limited to, for example, a weight-average molecular weight (Mw) of typically 1,000 or more, in one embodiment 8,000 or more, and in one embodiment 10,000 or more, and typically 50,000 or less, in one embodiment 40,000 or less, for example 1,000 to 50,000, in one embodiment 8,000 to 50,000, and in one embodiment 10,000 to 40,000. The amount of dispersant adsorbed to the metal particles is not limited, but is usually 2 wt % or less, and in one embodiment, 1 wt % or less, relative to the total weight of the metal particles. Since a smaller amount of dispersant is preferable, the lower limit is not limited, but is usually 0.1 wt % or more, and in one embodiment, 0.2 wt % or more, and in one embodiment, 0.3 wt % or more, relative to the total weight of the metal particles.

[0024] Conventionally, when producing metal particles, the amount of organic substance used as a dispersant to control particle size during production was greater than the amount of organic substance required for dispersion. Furthermore, because the organic substance could not be removed, it remained in the final metal nanoparticles. According to the method of the present invention, the amount of organic substance required for particle size control during production can be reduced, making it possible to produce metal particles with a reduced amount of organic substance, which contain only the amount of organic substance required for dispersion.

[0025] The reaction solution may further contain 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.

[0026] 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; formic acid or formate salts such as sodium formate; DMF; and mixtures of two or more thereof. In one embodiment, the reducing agent for metal ions, particularly silver ions, is DMF. Therefore, when DMF is used as a solvent for the reaction solution, DMF can also function as a reducing agent, so that a reducing agent other than DMF does not need to be used.

[0027] 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 equivalent or more, in one embodiment 4.0 equivalents or more, and typically 20 equivalents or less, in one embodiment 15 equivalents or less, for example 1.0 to 20 equivalents, and in one embodiment 4.0 to 15 equivalents, relative to the metal ions. Note that when a 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 may also function as a dispersant. When the reducing agent also functions as a dispersant, the reaction solution need not contain the dispersant described above. Furthermore, the amount of reducing agent for metal ions may be an amount exceeding the amount necessary to reduce metal ions to a metal whose oxidation number is 0 through a redox reaction.

[0028] The reaction solution may be composed of the metal particle precursor, solvent, dispersant, and optionally reducing agent described above, but may also contain, in addition to these materials, additives that are commonly used in conventional methods for producing metal particles by irradiating microwaves, such as chelating agents, for example, ethylenediaminetetraacetic acid (EDTA) and / or salts thereof. The amount of additive is not limited, but is typically 10 wt % or less, and in one embodiment, 3 wt % or less, based on the total weight of the reaction solution. Since additives do not necessarily need to be added, there is no lower limit on the amount of additive.

[0029] The pH of the reaction solution is not limited, but is usually pH 3 to pH 12.

[0030] 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.

[0031] (ii) Step of irradiating with microwaves while flowing the reaction solution In step (ii), microwaves are irradiated while flowing the reaction solution prepared in step (i).

[0032] In step (ii), the absorbed power E (unit: W / mL) of the microwave to the reaction solution and the pressure P (unit: MPa) are adjusted to satisfy the relationship of the following formula 1: ExP≧20 (Formula 1).

[0033] In one embodiment, the absorbed power E (unit: W / mL) of the microwave to the reaction solution and the pressure P (unit: MPa) are adjusted to satisfy the relationship of the following formula 2: E×P≧30 (Formula 2).

[0034] In one embodiment, the absorbed power E (unit: W / mL) of the microwave to the reaction solution and the pressure P (unit: MPa) are adjusted to satisfy the relationship of the following formula 3: E×P≧35 (Formula 3).

[0035] Here, the absorbed power E of the microwaves in the reaction solution is calculated by dividing the intensity (W) of the microwaves absorbed by the reaction solution, i.e., the value obtained by subtracting the reflected power irradiated and reflected from the reaction solution from the output of the microwave irradiation source (output - reflected power), by the volume (mL) of the reaction solution irradiated with that output. Pressure P is the pressure (MPa) applied to the reaction solution, and is the pressure measured at the outlet of the reaction solution after microwave irradiation. The reflected power can be measured using a power monitor in the microwave irradiation device.

[0036] In the present invention, by adjusting the microwave absorption power E (unit: W / mL) and pressure P (unit: MPa) of the reaction solution so as to satisfy equations 1 to 3, the formation of metal particle nuclei occurs simultaneously and uniformly, and the growth of the formed nuclei also occurs simultaneously and uniformly, resulting in the production of metal particles with small particle sizes.

[0037] The method of flowing the reaction solution is not limited as long as the reaction solution flows in any direction. For example, the reaction solution can be flowed at a constant speed in a reaction tube, such as a straight tube or a spiral tube.

[0038] In one embodiment, when the reaction tube is a straight tube, the size and shape are not limited as long as the microwaves are uniformly irradiated throughout the reaction tube. In one embodiment, the inner diameter of the tube is typically 1 mm or more, in one embodiment, 2 mm or more, in one embodiment, 4 mm or more, and typically 20 mm or less, in one embodiment, 10 mm or less, for example, 1 mm to 20 mm, in one embodiment, 2 mm to 10 mm, and in one embodiment, 4 mm to 10 mm. In one embodiment, when the cavity is a rectangular parallelepiped with a length of 100 mm, the reaction tube used has an inner diameter of 1 mm to 6 mm, an outer diameter of 3 mm to 8 mm (wall thickness: 1 mm), and a length of 100 mm.

[0039] The pressure P applied to the reaction liquid is not limited as long as it satisfies the above formulas 1 to 3, but is usually 0.10 MPa or more and usually 1.0 MPa or less, for example, 0.10 MPa to 1.0 MPa.

[0040] The flow rate of the reaction liquid is not limited as long as the pressure P on the reaction liquid satisfies the above formulas 1 to 3, but is usually 1 m / min or more and usually 100 m / min or less, for example, 1 m / min to 100 m / min.

[0041] By setting the flow rate of the reaction liquid within the above range, the reaction liquid can be circulated while applying a predetermined pressure to the reaction liquid, and small metal particles can be formed even with a small amount of organic matter as a dispersant.

[0042] The microwave absorption power (E) of the reaction solution is not limited as long as it satisfies the above formulas 1 to 3, but is usually 20 W / mL or more and usually 500 W / mL or less, for example, 20 W / mL to 500 W / mL, relative to the volume of the reaction solution.

[0043] By adjusting the output of the microwave irradiation source within the above range, it is possible to form small metal particles with a low organic content due to sufficient reduction by microwaves.

[0044] While the reaction solution is being irradiated with microwaves, the reaction solution being irradiated with microwaves is preferably stirred.

[0045] By stirring the reaction solution during the reaction, the uniformity of the reaction solution can be maintained in response to non-uniformity of the reaction solution due to local changes in the concentration and viscosity of the reaction solution that may occur during the reaction, segregation of metal particles can be suppressed, and segregated metal particles can be re-dissolved.

[0046] Other microwave conditions in the method of the present invention are not limited. In the method of the present invention, as explained above, the reaction solution is irradiated with microwaves using a microwave synthesis apparatus while flowing so as to satisfy the above formulas 1 to 3, and the reaction proceeds. When the reaction solution is irradiated with microwaves, the polar solvent contained in the reaction solution absorbs the microwaves and converts them into thermal energy, thereby generating heat. Therefore, in the reaction solution irradiated with microwaves, a uniform and rapid temperature rise occurs in the irradiated portion, and a uniform and rapid reaction occurs in accordance with this temperature rise.

[0047] It is preferable that the microwaves be uniformly irradiated onto the target of reaction, that is, the part of the reaction solution where the reaction occurs.

[0048] In a microwave synthesis apparatus, the material of the portion of the container containing the reaction solution that is irradiated with microwaves 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 via the reactor, the material of the portion of the container containing the reaction solution that is irradiated with microwaves can be a material that transmits microwaves, i.e., does not absorb microwaves, such as ceramics, glass, quartz, Teflon (registered trademark, PTFE, etc.), silicone, or other non-conductive materials with a small relative dielectric constant ε or dielectric loss angle tan δ. In addition to the above-mentioned materials, metals such as aluminum and stainless steel can also be used for the portion of the container containing the reaction solution that is not irradiated with microwaves. Furthermore, the housing that houses the microwave irradiation source and the reaction tube that irradiates microwaves is not limited as long as it is a material that does not leak or absorb microwaves, and examples thereof include non-magnetic metal plates, such as aluminum plates.

[0049] The microwaves are generated by a microwave radiation source (microwave generator (magnetron)), which can be either a single-mode system or a multi-mode system.

[0050] The frequency of the microwaves generated from the microwave irradiation source can be changed as appropriate and is not limited. The microwave frequency is usually 1 GHz or higher, and in one embodiment, 2 GHz or higher, and usually 10 GHz or lower, for example, 1 GHz to 10 GHz. In the present invention, it is preferable to use 2.45 GHz, which is the frequency of an industrial microwave power source, as the microwave frequency.

[0051] The microwaves are preferably uniform during irradiation, and the microwave irradiation conditions are preferably constant during microwave irradiation.

[0052] In the present invention, the temperature of the reaction solution heated by microwave irradiation is the reaction temperature. This reaction temperature can be appropriately changed depending on the reaction conditions (such as the type of solvent and the pressure during the reaction). It is not limited, but is usually 25°C or higher, and in one embodiment, 80°C or higher. The upper limit of the reaction temperature is not limited, but is usually lower than 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 but lower than 100°C, and in one embodiment, 80°C to 90°C, at atmospheric pressure. Since pressure is applied to the reaction solution in the present invention, the reaction temperature can be higher than the boiling point of the solvent under atmospheric pressure. The boiling point of the solvent under the applied pressure may vary depending on the pressure. Therefore, in one embodiment, the reaction temperature is higher than the boiling point of the solvent under atmospheric pressure and lower than the boiling point of the solvent under the applied pressure.

[0053] By setting the reaction temperature to 25°C or higher, a reduction reaction from metal ions to metal particles 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 particles from becoming uneven due to the non-uniformity of the reaction field that can occur when the reaction solution boils, i.e., the broadening of the particle size distribution, and to prepare metal particles with small and uniform particle sizes.

[0054] The irradiation time of microwaves to the reaction solution is the time required for the temperature of the reaction solution to reach the reaction temperature, and 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.).

[0055] The reaction solution is irradiated with microwaves under the above conditions to raise the temperature of the reaction solution to the reaction temperature, thereby generating metal particle nuclei in the reaction solution.

[0056] In the method of the present invention, the reaction solution is circulated, and microwave irradiation of the reaction solution is generally continued until the reaction is completed.

[0057] In the method of the present invention, the reaction can be carried out at a high temperature by applying pressure to the reaction solution. Therefore, the total reaction time required for the reaction can be significantly shortened compared to when pressure is not applied. In one embodiment, the ratio of the reaction time in the method of the present invention to the reaction time in a method without applying pressure (reaction time under applied pressure / reaction time under atmospheric pressure) is usually 1.1 or more, in one embodiment 1.5 or more, in one embodiment 2.0 or more, and usually 10 or less, in one embodiment 8.0 or less, in one embodiment 6.0 or less, for example 1.1 to 10, in one embodiment 1.5 to 8.0, in one embodiment 2.0 to 6.0, for example 5.0. Therefore, by producing metal particles using the method of the present invention, a significant time reduction is possible.

[0058] The completion of the reaction can be determined by observing that the absorbance, etc., derived from the metal particle precursor material or metal particles in the reaction solution no longer changes. For example, when silver particles are used as the metal particle precursor material, the change in the absorbance of the reaction solution at 280 nm to 780 nm is observed, and the time when the absorbance no longer changes is regarded as the time when the reaction is complete.

[0059] In the method of the present invention, after the reaction is completed, the irradiation of the reaction solution with microwaves may be stopped, and the reaction solution may be kept warm using a heat-retaining device such as a heater or a cooler.

[0060] The temperature at which the reaction solution is kept warm by the warming device is not limited, but is usually the reaction temperature or lower. The lower limit of the warming temperature is not limited, but is usually 25°C or higher, and in one embodiment, 80°C or higher. For example, when the solvent is water, the warming temperature is usually in the range of 25°C or higher to lower than 100°C, and in one embodiment, 80°C to 90°C, at atmospheric pressure.

[0061] Even if the temperature of the reaction liquid falls below the desired temperature upon switching from microwave irradiation to the heat-retaining device, the temperature can be adjusted to an appropriate temperature by the heat-retaining device.

[0062] The heat-retaining device used to keep the reaction solution warm is not limited as long as it can maintain the temperature of the reaction solution at the warming temperature, and any conventional heat-retaining device can be used. Examples of the heat-retaining device include heaters such as mantle heaters, immersion heaters, water baths, oil baths, and coolers.

[0063] The time for keeping the reaction solution warm in the warming device is not limited, but is usually 1 minute or more and usually 15 minutes or less.

[0064] By keeping the reaction liquid warm using a heat retaining device, the growth of nuclei of metal particles in the reaction liquid produced can be promoted, and the metal particles can be made more uniform (aged).

[0065] The dispersion containing metal particles 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 particles or a dispersion containing metal particles.

[0066] The present invention also relates to an apparatus for producing metal particles for efficiently carrying out the method of the present invention.

[0067] A manufacturing apparatus 1 for producing metal particles will be described below with reference to Figures 1 and 2. As shown in Figure 1, the manufacturing apparatus 1 is a manufacturing apparatus for producing metal particles by irradiating a reaction liquid L with microwaves M.

[0068] The production apparatus 1 includes a storage tank 10 that stores a reaction liquid L, and a pump 20 that sucks the reaction liquid L from the storage tank 10 and pumps it. The reaction liquid L flows through a path 5 from the storage tank 10 to a reaction device 40. A slurry-like suspension containing metal particles, which is generated when the reaction liquid L passes through the reaction device 40, flows continuously from the reaction device 40 to a pressure adjustment device 60, and this slurry also flows through the path 5. In this embodiment, for convenience, all fluids flowing through the path 5 will be referred to as the reaction liquid L (before and after the reaction).

[0069] The production apparatus 1 includes a stirrer 30 (described later) and a reaction device 40. The reaction device 40 includes a housing 41 that houses a reaction tube 43 through which a reaction liquid L flows via the stirrer 30, and an irradiation device 42 that irradiates the reaction tube 43 in the housing 41 with microwaves M.

[0070] As a result, the reaction solution L, which is pumped by the pump 20 and flows through the reaction tube 43, can be irradiated with microwaves M together with the reaction tube 43 using the irradiation device 42. The material of the reaction tube 43 may be the same as the material of the container containing the reaction solution described in the method of the present invention, and may be made of a ceramic material made of silicon oxide such as glass or quartz, or a resin material such as PTFE. The piping other than the path 5 forming the reaction tube 43 may also be composed of these piping. Alternatively, the piping other than the path 5 forming the reaction tube 43 may be composed of a metal, such as stainless steel or aluminum, since it is not irradiated with microwaves M. The reaction tube 43 may be a straight pipe, or may be a spiral pipe, for example. This allows for increased irradiation efficiency of microwaves M compared to a straight pipe.

[0071] In one embodiment, when the reaction tube 43 is a straight tube, the dimensions and shape are not limited as long as the microwaves M are uniformly irradiated throughout the reaction tube 43. For example, the inner diameter of the tube is typically 1 mm or more, 2 mm or more in one embodiment, 4 mm or more in one embodiment, and typically 20 mm or less, 10 mm or less in one embodiment, for example, 1 mm to 20 mm, and 2 mm to 10 mm in one embodiment. For example, when the cavity is a rectangular parallelepiped with a length of 100 mm, the reaction tube 43 is preferably a reaction tube with an inner diameter of 1 mm to 6 mm, an outer diameter of 3 mm to 8 mm (wall thickness: 1 mm), and a length of 100 mm. The dimensions and shape of the reaction apparatus 40 are also not limited.

[0072] The production apparatus 1 is equipped with a heat retention device 50 and a pressure adjustment device 60, in that order, downstream of the reaction device 40. The heat retention device 50 is an optional device and does not necessarily have to be installed. Since the post-reaction reaction liquid L (specifically, a slurry-like suspension containing metal particles) flowing through the path 5 is heated in the reaction device 40, the heat retention device 50 is a device that keeps this reaction liquid L at the heat retention temperature in the method of the present invention described above, i.e., at a predetermined temperature between the reaction temperature and the reaction temperature lower than the reaction temperature. Specifically, the reaction liquid L flowing through the path 5 is heated or cooled using the heat retention device, and the reaction liquid L maintained at the predetermined temperature is released to the pressure adjustment device 60. In addition to the heater described above, a cooler can also be used as the heat retention device 50.

[0073] The pressure regulator 60 is located downstream of the reaction apparatus (reaction tube 43) 40 and regulates the pressure of the reaction liquid L in the reaction tube 43, in one embodiment, the pressure of the reaction liquid L in the path 5 from the pump 20 to the pressure regulator 60. Specifically, the pressure regulator 60 regulates the pressure of the reaction liquid L in the path 5 by throttling down the discharge rate of the reaction liquid L passing through it. This makes it possible to increase the pressure of the reaction liquid L in the reaction tube 43 to atmospheric pressure or higher, within a range that does not exceed the discharge pressure of the pump 20. The reaction liquid L pressure-fed by the pressure regulator 60 is recovered in a recovery tank 70.

[0074] In this way, the reaction liquid L sent by the pump 20 is pumped into the reaction tube 43. The pressure of the pumped reaction liquid L is adjusted in a pressurized state by the pressure adjustment device 60. By irradiating the pressurized reaction liquid L with microwaves M using the irradiation device 42, the boiling point of the reaction liquid L can be increased compared to that in an atmospheric pressure state, and therefore the production rate of metal particles can be increased.

[0075] Incidentally, when metal particles are generated in the reaction liquid L, the concentration and viscosity of the reaction liquid L may change locally, causing the reaction liquid L to pulsate slightly. This may impair the uniformity of microwave irradiation of the reaction liquid L, and cause the metal particles in the reaction liquid L to segregate. For this reason, the production apparatus 1 is provided with an agitator 30 between the pump 20 and the reaction tube 43 of the reaction device 40, which agitates the reaction liquid L pumped from the pump 20.

[0076] There are no particular limitations on the device configuration, structure, or location of the agitator 30, as long as it can suppress segregation of metal particles in the reaction solution L. In this embodiment, the agitator 30 has, as an example, a structure equivalent to that of the static mixer shown in FIG.

[0077] The stirring device 30 has a straight pipe 31 through which the reaction liquid L flows, and a twisted blade 32 that is fixed within the pipe 31 and twisted around an axis CL of the pipe 31. With the twisted blade 32 disposed in the pipe 31, a flow path 33 (path 5) through which the reaction liquid L flows is formed within the pipe 31.

[0078] The twist blade 32 may be twisted spirally around the same axis CL, but in this embodiment, it has the following structure: Specifically, the twist blade 32 is a blade in which twist blade elements 32 a, 32 b having different twist directions around the axis are alternately arranged along the axial direction of the pipe 31.

[0079] Specifically, the twist blade element 32 a and the twist blade element 32 b are formed by twisting a flat plate material. The twist blade element 32 a is twisted in the opposite twist direction to the twist direction of the twist blade element 32 a. Such twist blade elements 32 a and twist blade elements 32 b are alternately connected along the axial direction of the pipe 31.

[0080] As a result, a stable agitated flow is easily generated in the reaction liquid L that has passed through the twisted impeller 32 along the axial direction of the pipe 31, which makes it easy to reduce segregation of metal particles in the reaction tube 43 and further makes it easy to re-dissolve the segregated metal particles. In particular, it is preferable that the twisted impeller be a blade in which twisted impeller elements 32a, 32b having different twist directions around the axis are alternately arranged along the axial direction of the pipe 31. As a result, flows of the reaction liquids La, Lb that are agitated in different directions are formed and a agitated flow is easily formed around the axis, which makes it easy to further reduce segregation of metal particles in the reaction tube 43 and further makes it easy to re-dissolve the segregated metal particles.

[0081] The material of the twist blade 32 is not limited. When the twist blade 32 is placed immediately before the reaction apparatus 40, it is made of a non-conductive material that does not absorb or reflect microwaves. For example, the materials exemplified for the reaction tube 43 are preferred, and examples of such materials include ceramic materials made of silicon oxide such as glass and quartz, and resin materials such as PTFE. It is also preferable that the piping 31 is made of a similar material. By using a non-conductive material for the twist blade 32 in this way, it is possible to prevent microwaves M from reaching the twist blade from the reaction tube 43 via the reaction liquid L. This allows microwaves M to be efficiently irradiated onto the reaction liquid in the reaction tube 43.

[0082] Incidentally, the agitator 30 is preferably disposed at a position closer to the reaction device 40 in the path 5 between the pump 20 and the reaction device 40. More specifically, the agitator 30 is preferably disposed at a position where the agitated flow of the reaction liquid L that has passed through the agitator 30 is maintained in the reaction tube 43. For example, the agitator 30 may be disposed near the inlet into which the reaction liquid L of the reaction device 40 flows, and is not particularly limited to this position as long as the agitated flow is maintained.

[0083] In this way, the agitated flow of the reaction liquid L that has passed through the agitator 30 is maintained within the reaction tube 43, thereby preventing local changes in the concentration and viscosity of the reaction liquid L during metal particle generation. This prevents the reaction liquid L from becoming non-uniform within the reaction tube 43, prevents segregation of metal particles in the reaction liquid L, and further allows the segregated metal particles to be re-dissolved.

[0084] Therefore, the metal particle manufacturing apparatus 1 of the present invention can efficiently carry out the metal particle manufacturing method of the present invention without causing changes in the physical properties such as the volume and viscosity of the reaction liquid L before and after the application of pressure, or a decrease in the absorption rate (uniformity) of the irradiated microwaves due to such changes in physical properties, and furthermore, can prevent segregation of metal particles and re-dissolve segregated metal particles.

[0085] The present invention also relates to metal particles containing an organic substance, which can be obtained by the method or production apparatus of the present invention, and which have a small particle size and a low organic substance content. Here, the "organic substance" refers to a compound that is attached to the metal particles to uniformly disperse the metal particles in a solvent or solvent, and is non-volatile and is distinguished from organic compounds used as a solvent or solvent.

[0086] In one embodiment, the metal particles are metal nanoparticles. Here, "metal nanoparticles" refers to metal particles having a particle size typically between 1 nm and 100 nm. Therefore, when the metal particles of the present invention are metal nanoparticles, the Heywood diameter of the entire metal particle, including the main portion and the auxiliary portion, is between 1 nm and 100 nm.

[0087] In one embodiment, the metal particles are spherical. Here, "spherical" includes not only a perfect sphere when the metal particles are observed under a transmission electron microscope (TEM), but also an approximately spherical shape, an oval sphere, a polygonal shape with approximately equal sides, and the like.

[0088] The metal particles of the present invention have a median diameter (D50) of 20 nm or less, and in one embodiment, 10 nm or less, as measured by TEM. Since a smaller D50 is preferable, the lower limit is not limited, but is usually 1.0 nm or more.

[0089] Here, the method for measuring D50 of metal particles by TEM is as follows.

[0090] First, a TEM image of the metal particles is taken. Next, 500 random metal particles are selected from the TEM image. Next, for each of the selected metal particles, the diameter value is measured when the projected surface area of ​​the metal particle is converted into the area of ​​a circle. Next, these are used as particle diameters, and a graph is created with the particle diameter (nm) on the x-axis and the cumulative number (%) on the y-axis. Finally, the particle diameter at which the cumulative number becomes 50% can be determined from the graph as D50.

[0091] Furthermore, the particle size distribution of the metal particles of the present invention is narrow.

[0092] The content of organic matter, particularly dispersant, contained in the metal particles (powder (powder) formed from metal particles) of the present invention is 2 wt % or less, in one embodiment 1 wt % or less, and in one embodiment 0.8 wt % or less, relative to the total weight of the metal particles. Since a lower content of the organic matter is preferable, the lower limit is not limited, but is usually 0.1 wt % or more, in one embodiment 0.2 wt % or more, in one embodiment 0.3 wt % or more, and in one embodiment 0.4 wt % or more, relative to the total weight of the metal particles.

[0093] The method for measuring the content of organic matter contained in the metal particles is as follows.

[0094] First, purified water such as ion-exchanged water is added to metal particles or a dispersion containing metal particles, and the slurry is purified using a solid-liquid separator such as a centrifuge until the electrical conductivity of the slurry reaches several tens of μS / cm or less. A handy, general-purpose separator can be used to measure the electrical conductivity. This process is performed to remove impurities and water-soluble by-products that may be contained in the metal particles or the dispersion containing metal particles. Organic substances added during manufacturing as dispersants or other purposes that are adsorbed to the metal particles are not removed in this process. Next, the purified metal particles are dispersed in a low-boiling point solvent (alcohol-based, such as methanol or ethanol) to prepare a metal particle-containing slurry. The slurry is then applied to a glass surface to form a film. The formed film is then dried in an oven at 80°C for approximately one hour, peeled off from the glass, and collected as a sample for organic matter measurement. Finally, the collected organic matter measurement sample is powdered, and the amount of C (weight %), an organic substance, is measured using a CS meter (combustion method).

[0095] The metal particles of the present invention are metal particles with a low organic content and small particle size, have excellent low-temperature sintering properties, and further, sintered bodies formed from the metal particles have low volume resistivity and a small volume shrinkage rate during sintering.

[0096] Specifically, the metal particles of the present invention have a low organic content and a small median diameter. The metal particles of the present invention have a low volumetric shrinkage rate during sintering, for example, a volumetric shrinkage rate of typically 60% or less, in one embodiment 50% or less, and in one embodiment 15% or less, relative to the volume of the pressure-processed metal particles before sintering. Note that, since a smaller volumetric shrinkage rate due to sintering of the metal particles of the present invention is preferable, there is no lower limit.

[0097] Furthermore, since the metal particles of the present invention contain the minimum amount of organic matter required as a dispersant to disperse the metal particles in a solvent, they can be sufficiently dispersed in a solvent without the need for additional dispersants. This is because the dispersant adheres to the metal particles immediately after their formation, thereby uniformly adhering throughout the metal particles. Even if the metal particles reach a semi-dry or dry state, the dispersant function can be restored by adding the dispersant to a solvent. Therefore, the metal particles of the present invention are redispersible. Note that if the metal particles do not contain any dispersant, once they reach a semi-dry or dry state, they are difficult to redisperse due to the strong cohesion of nano-sized metal particles. Furthermore, when metal particles that do not contain any dispersant are mixed with a dispersant, once they reach a semi-dry or dry state, the strong cohesion of the metal particles makes it difficult to mix them uniformly.

[0098] The present invention also relates to a dispersion containing the metal particles of the present invention described above, an organic substance as a dispersant for the metal particles, and a solvent for dispersing the metal particles and the organic substance.

[0099] In the dispersion of the present invention, the content of metal particles is not limited. This is because the metal particles of the present invention contained in the dispersion of the present invention contain the minimum amount of dispersant necessary for dispersing the metal particles in the solvent. For example, the content of metal particles is typically 1 wt % or more, in one embodiment 5 wt % or more, in one embodiment 10 wt % or more, in one embodiment 20 wt % or more, in one embodiment 50 wt % or more, in one embodiment 70 wt % or more, and typically 95 wt % or less, in one embodiment 90 wt % or less, in one embodiment 85 wt % or less, in one embodiment 80 wt % or less, for example, 1 wt % to 95 wt %, in one embodiment 5 wt % to 90 wt %, and in one embodiment 50 wt % to 80 wt %, based on the total weight of the dispersion.

[0100] By increasing the metal particle content of the dispersion of the present invention, it is possible to reduce the cost of the solvent, the time, labor, and cost required to evaporate the solvent after applying the dispersion, as well as storage costs and transportation costs, thereby reducing the environmental impact.

[0101] In the dispersion of the present invention, the content of the organic substance as a dispersant for the metal particles is 2% by weight or less, in one embodiment 1% by weight or less, and in one embodiment 0.8% by weight or less, relative to the total weight of the metal particles. Since a lower content of the organic substance is preferable, the lower limit is not limited, but is usually 0.1% by weight or more, in one embodiment 0.2% by weight or more, in one embodiment 0.3% by weight or more, and in one embodiment 0.4% by weight or more, relative to the total weight of the metal particles.

[0102] In the dispersion of the present invention, the amount of dispersant is small, so that the sintered body formed from the dispersion can have a low volume resistivity and a low volume shrinkage rate.

[0103] The solvent contained in the dispersion of the present invention may be any solvent known in the art, and may be selected from, but is not limited to, solvents that are liquid at 20°C, water, alcohols, aldehydes, carboxylic acids, ethers, esters, amines, monosaccharides, polysaccharides, straight-chain hydrocarbons, fatty acids, aromatics, etc., and two or more of the above-mentioned solvents may also be used in combination.

[0104] The boiling point of the solvent is not particularly limited, but is typically 100°C or higher, and in one embodiment, 130°C or higher, and in one embodiment, 150°C or higher, and typically 300°C or lower, and in one embodiment, 250°C or lower, and in one embodiment, 200°C or lower, for example, 100°C to 300°C, and in one embodiment, 130°C to 250°C, and in one embodiment, 150°C to 200°C. If the boiling point of the solvent is 100°C or higher, for example, when the dispersion is used as an ink paste, the solvent can be prevented from volatilizing at room temperature, thereby ensuring the viscosity stability and coatability of the ink paste. Furthermore, if the boiling point of the solvent is 300°C or lower, the solvent can be prevented from remaining in the metal sintered body without evaporating at the temperature at which the semiconductor element is connected to the support member during firing, particularly during a bonding process using pressureless firing, and as a result, the properties of the metal sintered body can be better maintained.

[0105] As the solvent, it is preferable to select a solvent suitable for dispersing silver particles from the solvents described above. Specifically, it is preferable to select a solvent having an alcohol structure, an ether structure, or an ester structure in order to improve the thermal conductivity, electrical conductivity, and adhesive strength of the metal sintered body. Examples of solvents contained in the dispersion of the present invention include butyl cellosolve, carbitol, butyl cellosolve acetate, carbitol acetate, ethylene glycol diethyl ether, dipropylene glycol methyl ether acetate, dipropylene glycol mono-n-butyl ether, dipropylene glycol mono-n-methyl ether, terpineol, ethylene glycol, isobornylcyclohexanol, and tributyrin. As the solvent contained in the dispersion of the present invention, ethylene glycol is preferred.

[0106] When the dispersion of the present invention is used as an ink paste, the amount of solvent contained in the ink paste may vary depending on the content of metal particles contained in the dispersion, etc., but is usually 5% by weight or more and usually 99% by weight or less, and in one embodiment 90% by weight or less, of the total weight of the ink paste.

[0107] By adjusting the amount of solvent within the above-mentioned range, the viscosity of the ink paste can be adjusted to an appropriate viscosity range described below, and further, volumetric shrinkage due to evaporation of the solvent when the ink paste is sintered can be suppressed, thereby improving the density of the silver sintered body that is formed.

[0108] The dispersion of the present invention may further contain components other than the metal particles and solvent of the present invention, provided that the effects of the present invention are not impaired. Examples of components that may be added in addition to the metal particles and solvent of the present invention include those known in the art, and are not limited thereto. Examples include additives such as carboxylic acids that have a boiling point of 400°C or less at atmospheric pressure and are solid at 20°C, such as stearic acid, lauric acid, docosanoic acid, sebacic acid, and 1,16-octadecanedioic acid; metal particles other than the metal particles of the present invention; anti-settling agents for metal particles in the dispersion; and fluxing agents for promoting sintering of metal particles. The amount of components that may be added in addition to the metal particles and solvent of the present invention is typically 0% by weight or more and typically 10% by weight or less, and in one embodiment, 1% by weight or less, for example, 0% by weight to 10% by weight, and in one embodiment, 0% by weight to 1% by weight, based on the total weight of the dispersion.

[0109] When the dispersion of the present invention is used as an ink paste, the viscosity of the ink paste, as measured with a cone-plate viscometer, is usually 10 mPa s or more and usually 10,000 Pa s or less. As described above, the viscosity can be appropriately adjusted by the aspect ratio and amount of the plate-like silver particles, the type and amount of the polymer used as a dispersant, the type and amount of the solvent, etc.

[0110] By adjusting the viscosity of the ink paste to fall within the above range, the applicability of the ink paste can be improved and bleeding after application of the ink paste can be prevented.

[0111] The metal particles or dispersion produced by the present invention can be used in fields such as catalysts, electronic component materials, and ink materials, as well as conductive wiring materials in the electronics packaging field, and can reduce the number of steps in producing inks used, for example, on wiring boards.

[0112] 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.

[0113] 1. Synthesis of Metal Particles Silver particles or nickel particles were produced using the production apparatus 1 for producing metal particles shown in FIGS. 1 and 2 and the reaction solution shown in Table 1 or 2 under the conditions shown in Table 3. In Table 1, the weight average molecular weight of PVP is 40,000. In Table 3, E is the absorbed power (W / mL) of microwaves irradiated from the irradiation device 42 that irradiates microwaves M and absorbed by the reaction solution, and P is the pressure (MPa) applied to the reaction solution measured by the pressure adjustment device 60. Note that a rectangular parallelepiped with a length of 100 mm was used as the cavity in the irradiation device 42 that irradiates microwaves M of the production apparatus 1, and the following reaction tube was used. Reaction tube configuration: Inner diameter of tube: 1 mm to 6 mm; Outer diameter of tube: 3 mm to 8 mm (wall thickness: 1 mm); Length of tube: 100 mm

[0114]

[0115]

[0116]

[0117] In this experiment, the total reaction time required in the example was about one-fifth of the total reaction time required when the reaction was carried out without applying pressure. This is because the boiling point of the reaction solution under pressure was higher than that under atmospheric pressure, allowing the temperature of the reaction solution to be raised during the reaction.

[0118] 2. Evaluation Results (Transmission Electron Microscope (TEM) Evaluation) TEM images were taken of the obtained dispersions of Examples 1 to 5 and Comparative Examples 1 to 5. Figure 3 shows TEM images of silver particles in Comparative Example 1 and Examples 1 and 4 as examples.

[0119] Furthermore, the particle size distribution was measured from the obtained TEM image. The particle size distribution was measured as follows. First, 500 silver particles or nickel particles were randomly selected from the TEM image. Next, for each of the selected silver particles or nickel particles, the diameter value was measured when the projected surface area of ​​the silver particles or nickel particles was converted into the area of ​​a circle. Next, these were used as particle diameters, and a graph was created with the particle diameter (nm) on the x-axis and the cumulative number (%) on the y-axis. FIG. 4 shows the particle size distribution of the silver particles in Example 1 as an example. Finally, the particle diameter at which the cumulative number becomes 50% was determined from the graph as D50. The results are shown in Table 4 and FIG. 5.

[0120]

[0121] From the results in Table 4 and Figure 5, it was found that when the value (E x P) obtained by multiplying the microwave absorption power E (W / mL) of the reaction solution by the pressure P (MPa) applied to the reaction solution is 20 or more, the D50 of the resulting metal particles becomes small, particularly 15 nm or less.

[0122] Subsequently, the amount of organic matter was measured and the relationship with D50 was measured for Examples 1, 3, and 4 and Comparative Examples 3 and 5. The amount of organic matter was measured as follows.

[0123] First, purified water such as ion-exchanged water was added to the dispersion containing the example or comparative example, and the slurry was purified using a solid-liquid separator such as a centrifuge until the electrical conductivity of the slurry reached several tens of μS / cm. A handy, general-purpose separator was used to measure the electrical conductivity. This process was carried out to remove impurities and water-soluble by-products contained in the dispersion. The purified particles were then dispersed in a low-boiling point solvent (alcohol-based, e.g., methanol or ethanol) to prepare a metal particle-containing slurry. The slurry was then applied to a glass surface to form a film. The formed film was then dried in an oven at 80°C for approximately one hour, peeled off from the glass, and collected as a sample for measuring the amount of organic matter. Finally, the collected sample for measuring the amount of organic matter was powdered, and the amount of C (wt%), an organic substance, was measured using a CS meter (combustion method). The results are shown in Figure 6.

[0124] From FIG. 6, it was found that the metal particles produced by the method of the present invention can maintain a small D50 even if the amount of organic matter in the metal particles is reduced.

[0125] Next, the volume shrinkage ratios of Examples 1, 3, and 4, and Comparative Examples 3 and 5, in which the amount of organic matter was measured, were measured when sintered at 120° C. for 2 hours. The volume shrinkage ratios were measured as follows.

[0126] (1) First, metal particles purified with purified water such as ion-exchanged water using a solid-liquid separator such as a centrifuge were collected on filter paper, and solvent substitution was performed by adding ethanol and filtering under reduced pressure.

[0127] (2) Next, the cake layer remaining on the filter paper after the solvent replacement was collected, and solvents such as butyl carbitol, butyl acetate, and terpineol were added so that the concentration of metal particles was 80% by weight. The mixture was then crushed and kneaded using a planetary centrifugal mixer to prepare a paste containing metal particles.

[0128] (3) The obtained paste was applied onto a slide glass using a metal mask having a thickness of 100 μm, a width of 15 mm, and a length of 15 mm (100 μm×15 mm×15 mm).

[0129] (4) The substrate was baked in a thermostatic oven at 120° C. for 2 hours.

[0130] (5) After firing, the film thickness was measured at five predetermined points using a micrometer, and the average value was taken as the film thickness.

[0131] (6) From the dimensions and film thickness of the metal mask and the dimensions and film thickness after firing, the volume of the paste before firing and the volume of the sintered body after firing were calculated, and the volume shrinkage rate was calculated using the formula {(volume of paste before firing−volume of sintered body after firing) / (volume of paste before firing)}×100.

[0132] The results are shown in Figure 7. From Figure 7, it was found that as the amount of organic matter in the metal particles decreased, the volume shrinkage rate also decreased.

[0133] Furthermore, in order to confirm the redispersibility of the silver particles and nickel particles produced and purified in the examples of the present invention, the semi-dry or dry silver particles and nickel particles produced and purified in the examples were redispersed in various solvents, such as aqueous solvents, non-polar solvents, etc. As a result, it was confirmed that the metal particles of the examples can be redispersed in various solvents.

[0134] 8 and 9 are schematic diagrams showing the state of metal particle generation based on the results of the comparative example and the example.

[0135] Figure 8 shows a schematic diagram of the process from nucleation to growth of metal particles (e.g., silver particles) in the prior art or when Ex × P is less than 20. As shown in Figure 8, when the microwave absorption power is weak and no pressure is applied, the reaction temperature is low, nuclei are generated unevenly, and the size of the nuclei varies. As a result, nuclei of different sizes exist in the reaction solution, and the smaller nuclei formed later adhere to the larger nuclei formed earlier, resulting in larger particles.

[0136] Figure 9 shows a schematic diagram of the process from nucleation to particle growth of metal particles (e.g., silver particles) when Ex × P is 20 or greater in the present invention. As shown in Figure 9, when microwave absorption power is strong and pressure is applied, the reaction temperature increases, nuclei are generated uniformly, and there is no variation in nuclei size. As a result, when nuclei of uniform size are present, nuclei of similar size adhere to each other and grow, resulting in small particles. Furthermore, the dispersant required to disperse the metal particles present in the reaction solution can be uniformly attached to the uniformly formed particles, so only a minimal amount of dispersant is required. Furthermore, by carrying out the reaction under a high-temperature atmosphere, the reaction time can be significantly reduced, for example, to one-fifth of the original time. Furthermore, the method for forming metal particles of the present invention by the method of the present invention is characterized by applying a certain level of microwave absorption power and pressure to the reaction solution, thereby raising the reaction temperature and instantly causing the reduction of metal ions by microwave heating.Therefore, the type of metal particles produced by the method of the present invention may be any metal whose metal ions can be reduced by microwave heating, and may include not only the above-mentioned metal particles, i.e., silver particles and nickel particles, but also particles of gold, platinum, copper, iron, cobalt, etc.

[0137] All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety.

Claims

1. A method for producing metal particles comprising the steps of irradiating a reaction liquid with microwaves, the method comprising the steps of: (i) preparing a reaction liquid containing a metal particle precursor, an organic substance as a dispersant, and a solvent; and (ii) irradiating the reaction liquid with microwaves while flowing, wherein the relationship between the absorbed power E (unit: W / mL) of the microwaves and the pressure P (unit: MPa) for the reaction liquid satisfies the following formula 1: E × P ≧ 20 (Formula 1).

2. The method according to claim 1, wherein in step (i), the content of the organic matter in the reaction solution is 0.1% by weight to 2% by weight based on the total weight of the metal particle precursor in terms of metal.

3. The method according to claim 1 or 2, wherein in step (ii), the relationship between the absorbed power E (unit: W / mL) of the microwaves and the pressure P (unit: MPa) for the reaction liquid satisfies the following formula 2: E×P≧30 (Formula 2).

4. Metal particles containing an organic substance, wherein the content of the organic substance is 0.1% by weight to 2% by weight based on the total weight of the metal particles, and the median diameter (D50) measured by TEM is 20 nm or less.

5. A metal particle dispersion comprising metal particles, an organic substance serving as a dispersant for the metal particles, and a solvent, wherein the content of the metal particles is 1 wt % to 95 wt % relative to the total weight of the metal particle dispersion, the content of the organic substance is 0.1 wt % to 2 wt % relative to the total weight of the metal particles, and the median diameter (D50) of the metal particles measured by TEM is 20 nm or less.

6. A manufacturing apparatus for producing metal particles by irradiating a reaction liquid with microwaves, the manufacturing apparatus comprising: a pump for pressure-feeding the reaction liquid; an irradiation device for irradiating the reaction liquid, which is pressure-feeded from the pump and flows inside the reaction tube, with the microwaves together with the reaction tube; a pressure adjustment device downstream of the reaction tube for adjusting the pressure of the reaction liquid in the reaction tube; and a stirring device between the pump and the reaction tube for stirring the reaction liquid pressure-feeded from the pump.

7. The apparatus for producing metal nanoparticles described in claim 6, wherein the stirring device is disposed between the pump and the reaction tube at a position where the stirred flow of the reaction liquid that has passed through the stirring device is maintained within the reaction tube.

8. The apparatus for producing metal nanoparticles described in claim 6 or 7, wherein the stirring device comprises a straight pipe through which the reaction liquid flows, and a twisted blade fixed within the pipe and twisted around the axis of the pipe.

9. The apparatus for producing metal nanoparticles according to claim 8, wherein the twisted blade is a blade in which twisted blade elements having different twist directions around the axis are arranged alternately along the axial direction of the pipe.

10. The apparatus for producing metal nanoparticles according to claim 8 or 9, wherein the twisted blade is made of a non-conductive material.

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