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

Abstract

Provided are a method for efficiently producing metal particles, the 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. The present invention is a method for producing the metal particles including irradiating a reaction solution with microwave. The method includes: (i) a step of preparing a reaction solution containing a metal particle precursor, an organic substance, and a solvent; and (ii) a step of irradiating the reaction solution with microwaves while the reaction solution is flowing, a relationship between an absorbed power E of the microwaves (unit: W / mL) and a pressure P (unit: MPa) of the reaction solution satisfying Formula 1 of E × P ≥ 20 (Formula 1).

Description

Technical Field

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

[0002] Metal nanoparticles, which may have properties different from those of bulk materials, are used and studied in various applications, for example, as catalysts, ink materials, and electronic component materials.

[0003] For example, Patent Literature 1 discloses a method for producing chromium nanoparticles having an average particle diameter of 5.6 nm by irradiating ultrasonic waves at a frequency of 50 kHz and a power of 150 W at 40°C to 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 to thermally decompose the chromium nanoparticle raw material.

[0004] Patent Literature 2 discloses a method for producing metal fine particles, comprising allowing a reaction solution containing a metal precursor to flow through a flow tube, and irradiating the interior of the flow tube with electromagnetic waves uniformly and intensively along the longitudinal direction of the flow tube, thereby uniformly heating an electromagnetic-wave irradiation zone within the flow tube along the flow direction to produce metal fine particles.Citation ListPatent Literature

[0005] Patent Literature 1: JP 2015-129327 A Patent Literature 2: JP 2011-137226 A Summary of InventionTechnical Problem

[0006] In order to use metal nanoparticles in the field of electronics packaging, it is desirable to reduce the particle diameter of metal nanoparticles.

[0007] However, in order to produce metal nanoparticles with a nanoscale average particle diameter by a microwave irradiation method, high microwave intensity and high reaction temperature are required to suppress variation in formation of nuclei (nucleation) and particle growth.

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

[0009] As a result of various studies of means to solve the above-described problems, the present inventors have found that, in a method for producing metal particles by irradiating a reaction solution with microwaves, increasing the absorbed microwave power with respect to the reaction solution and the pressure applied to the reaction solution to a certain level or more shortens the reaction time. The present inventors further found that metal particles can be produced with a small particle diameter and a low amount of organic substance as a dispersant that can become impurities, and that the obtained metal particles have a low volumetric shrinkage ratio, thereby completing the present invention.

[0010] That is, the gist of the present invention is as follows. (1) A method for producing metal particles includes irradiating a reaction solution with microwave. The method comprises: (i) a step of preparing a reaction solution containing a metal particle precursor, an organic substance as a dispersant, and a solvent; and (ii) a step of irradiating the reaction solution with microwaves while the reaction solution is flowing, a relationship between an absorbed power E of the microwave (unit: W / mL) and a pressure P (unit: MPa) with respect to the reaction solution satisfying Formula 1 below, E × P ≥ 20 (2) The method according to (1), wherein in the step (i), an amount of the organic substance in the reaction solution is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particle precursor as metal. (3) The method according to (1) or (2), wherein in the step (ii), the relationship between the absorbed power E of the microwave (unit: W / mL) and the pressure P (unit: MPa) with respect to the reaction solution satisfies Formula 2 below, E × P ≥ 30 (4) Metal particles comprising an organic substance, wherein an amount of the organic substance is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particles, and wherein a median diameter (D50) by TEM is 20 nm or less. (5) A metal particle dispersion comprises: metal particles; an organic substance as a dispersant for the metal particles; and a solvent, wherein a content of the metal particles is from 1 weight% to 95 weight% with respect to a total weight of the metal particle dispersion, and wherein an amount of the organic substance is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particles, wherein a median diameter (D50) of the metal particles by TEM is 20 nm or less. (6) An apparatus for producing metal nanoparticles by irradiating a reaction solution with microwaves, the apparatus comprising: a pump that pumps the reaction solution; an irradiation device that irradiates the reaction solution pumped from the pump and flowing in a reaction tube together with the reaction tube with the microwaves; a pressure regulating device that regulates a pressure of the reaction solution in the reaction tube, downstream of the reaction tube; and a stirring device that stirs the reaction solution pumped from the pump between the pump and the reaction tube. (7) The apparatus for producing the metal nanoparticles according to (6), wherein the stirring device is positioned between the pump and the reaction tube where a stirring flow of the reaction solution having passed through the stirring device is sustained in the reaction tube. (8) The apparatus for producing the metal nanoparticles according to (6) or (7), wherein the stirring device includes a linear pipe through which the reaction solution flows and a twisted blade fixed within the pipe and twisted around an axis of the pipe. (9) The apparatus for producing the metal nanoparticles according to (8), wherein the twisted blade includes twisted blade elements having different twist directions around the axis, and the twisted blade elements are alternately arranged along an axial direction of the pipe. (10) The apparatus for producing the metal nanoparticles according to (8) or (9), wherein the twisted blade is made of a non-conductive material. Advantageous Effects of Invention

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

[0012] Fig. 1 is a schematic diagram of one embodiment of a production apparatus of the present invention. Fig. 2 is a schematic diagram illustrating one embodiment of a stirring device in the production apparatus of the present invention. Fig. 3 shows TEM images of silver particles in Comparative Example 1, Example 1, and Example 4. Fig. 4 is a graph showing a particle size distribution of silver particles in Example 1. Fig. 5 is a graph showing a relationship between D50 and a value obtained by multiplying an absorbed microwave power E (W / mL) with respect to a reaction solution by a pressure P (MPa) applied to the reaction solution (E × P). Fig. 6 is a graph showing a relationship between an organic substance amount and D50 in Examples and Comparative Examples. Fig. 7 is a graph showing a relationship between an organic substance amount and a volumetric shrinkage ratio at 120°C for 2 hours. Fig. 8 schematically illustrates a process from formation of nuclei of metal particles (silver particles as an example) to a growth of the particles in the case of a conventional technique or where E×P is less than 20. Fig. 9 schematically illustrates a process from formation of nuclei of metal particles (silver particles as an example) to a growth of the particles when E×P is 20 or more in the present invention. Description of Embodiments

[0013] The following describes preferable embodiments of the present invention in detail.

[0014] In the description, features of the present invention will be described with reference to the drawings as necessary. In the drawings, dimensions and shapes of respective components are exaggerated for clarification, and actual dimensions and shapes are not accurately illustrated. Accordingly, the technical scope of the present invention is not limited to the dimensions or the shapes of the respective components illustrated in the drawings. Note that metal particles, a method for producing the metal particles, an apparatus for producing the metal particles, and a dispersion containing the metal particles of the present invention are not limited to the embodiments below, and can be performed in various configurations where changes, improvements, and the like that a person skilled in the art can make are given without departing from the gist of the present invention. While the embodiments described in the description are each independent, two or more of the embodiments may be combined to constitute one embodiment of the present invention.

[0015] The term "to" described in the description is used to mean a range having values described before and after the term "to" as the lower limit value and the upper limit value, respectively. In numerical ranges described in stages in the description, an upper limit value or a lower limit value described in one numerical range may be replaced with an upper limit value or a lower limit value in another numerical range described in stages. The upper limit values or the lower limit values in the numerical ranges described in the description may be replaced with values indicated in the examples.

[0016] The present invention relates to a method for producing metal particles that includes the steps of (i) preparing a reaction solution containing a metal particle precursor, an organic substance, and a solvent, and (ii) irradiating the reaction solution with microwaves while causing the reaction solution to flow. (i) a step of preparing a reaction solution containing a metal particle precursor, an organic substance, and a solvent,

[0017] In the step (i), a reaction solution containing a metal particle precursor, an organic substance, and a solvent is prepared.

[0018] In the method of the present invention, the solvent used for the reaction solution is not limited insofar as the solvent is a polar solvent or an ionic liquid that can dissolve the materials, such as a metal particle precursor, an organic substance as a dispersant, and a reductant, and further can absorb microwaves. Examples of the solvent used for the reaction solution include a low boiling point solvent having a boiling point of 300°C or lower. While the low boiling point solvent is not limited, examples of the low boiling point solvent may include low boiling point polar solvents such as water; alcohols such as methanol and ethanol; polyhydric alcohol solvents such as ethylene glycol; ketone solvents such as acetone; dimethyl sulfoxide (DMSO); N,N-dimethylformamide (DMF); other organic solvents; or mixtures of two or more thereof. When the mixture is used as the solvent, the ratio between the respective components contained in the mixture is not limited and may be a ratio at which the components are miscible under experimental conditions.

[0019] The use of a low boiling point polar solvent as the solvent allows improvement of solvent handling and reduction of the burden on the environment.

[0020] The metal particle precursor is not limited insofar as it is a material that is dissolved in the solvent and generates metal ions, for example, ions of noble metals, base metals, and alloys, for example, metal ions of gold, silver, platinum, copper, nickel, iron, cobalt, or two or more thereof. Examples of the metal particle precursor may include: inorganic metal salts such as metal halides (for example, 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 in a base such as aqueous ammonia. As the metal particle precursor, it is preferable to use inexpensive metal nitrates, such as nickel nitrate and silver nitrate. In addition, a formate having reducing properties, for example, nickel formate, may be used as the metal particle precursor. Therefore, in one embodiment, when a salt composed of a metal ion and an organic anion having reducing properties as its counter anion, for example, a formate ion, is used as the metal particle precursor, it is not necessary to use a reducing agent.

[0021] A homogeneous reaction solution can be prepared by dissolving a metal particle precursor in a solvent.

[0022] The concentration of metal ions in the reaction solution is not limited as long as it is at or below the saturation concentration, but is usually 1 mmol / L (mM) or more, in one embodiment 10 mM or more, and usually 600 mM or less, 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.

[0023] 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 greatly increased, and the time, labor, and cost for the metal particle production can be reduced. In addition, the variation in the obtained metal particles will be reduced; in other words, the particle size distribution of the obtained metal particles will be narrower.

[0024] The reaction solution contains an organic substance as a dispersant. Dispersants include, but are not limited to, one or more selected from polyvinylpyrrolidone (PVP), dodecylamine (DDA), thiol-based polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylate salts, 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 not limited, but, for example, as a weight average molecular weight (Mw), is usually 1000 or more, in one embodiment 8000 or more, in one embodiment 10000 or more, and is usually 50000 or less, in one embodiment 40000 or less: for example, 1000 to 50000, in one embodiment 8000 to 50000, and in one embodiment 10000 to 40000. The amount of dispersant adsorbed on the metal particles is not limited; however, with respect to the total weight of the metal particles, it is usually 2 weight% or less, and in one embodiment 1 weight% or less. The lower limit for the amount of the dispersant is not particularly limited, since a smaller amount is preferred, but, with respect to the total weight of the metal particles, it is usually 0.1 weight% or more, in one embodiment 0.2 weight% or more, in one embodiment 0.3 weight%, and in one embodiment 0.4 weight% or more.

[0025] Conventionally, in the production of metal particles, the amount of organic substance as a dispersant required for particle diameter control during production had to be greater than the amount of organic substance required for dispersion. Accordingly, as the organic substance could not be removed, it remained in the final metal nanoparticles. According to the production method of the present invention, the amount of organic substance required for particle diameter control during production can be reduced, making it possible to produce metal particles with a reduced organic substance amount that contain only the amount of organic substance required for dispersion.

[0026] The reaction solution may further contain a reducing agent. A reducing agent is a material that can reduce metal ions to a metal having an oxidation number of 0 through an oxidation-reduction reaction.

[0027] The reducing agent is not limited. Reducing agents include, for example, citric acid or citrate, such as trisodium citrate, disodium citrate, monosodium citrate, oxalic acid or oxalate, such as sodium oxalate, ascorbic acid or ascorbate, such as sodium ascorbate, formic acid or formate, such as sodium formate, DMF, and mixtures of two or more of these. In one embodiment, the reducing agent for metal ions, especially silver ions, is DMF. Therefore, when DMF is used as the solvent for the reaction solution, a reducing agent other than DMF need not be used, since DMF can also act as a reducing agent.

[0028] The amount of the reducing agent is not limited as long as the metal ions can be reduced by an oxidation-reduction reaction to a metal having an oxidation number of 0, but with respect to the metal ions, it is usually 1.0 equivalent or more, in one embodiment 4.0 equivalents or more, and is usually 20 equivalents or less, in one embodiment 15 equivalents or less: for example, 1.0 equivalent to 20 equivalents, and in one embodiment 4.0 equivalents to 15 equivalents. The reducing agent for metal ions can also act as a dispersant if it contains 1 or more functional groups that can interact with metals, such as carboxyl, hydroxyl, and ether groups. If the reducing agent also acts as a dispersant, the reaction solution need not contain the dispersant described above, and the amount of reducing agent for the metal ions may be greater than the amount required to reduce the metal ions to metal having an oxidation number of 0 by an oxidation-reduction reaction.

[0029] While the reaction solution may be composed of the metal particle precursor, the solvent, the dispersant, and optionally the reductant described above, the reaction solution may further contain, in addition to these materials, an additive that is normally used in a conventional method for producing metal particles by microwave irradiation, such as chelating agents, for example ethylenediaminetetraacetic acid (EDTA) and / or salts thereof. The amount of additive is, although not limited, generally 10 weight% or less, and 3 weight% or less in one embodiment with respect to the total weight of the reaction solution. Since the additive is not necessarily added, the lower limit value of the additive is not limited.

[0030] The pH of the reaction solution is not particularly limited but is usually from pH 3 to pH 12.

[0031] In the present invention, the order of addition of each material, the addition temperature, the mixing method, and the mixing time in the preparation of the reaction solution are not limited, and mixing is performed such that a uniform reaction solution is prepared. In the present invention, the reaction is initiated after a homogeneous reaction solution has been prepared.

[0032] (ii) Irradiating the reaction solution with microwaves while the reaction solution is flowing. In the step (ii), the reaction solution prepared in the step (i) is irradiated with microwaves while it is flowing.

[0033] In the step (ii), the absorbed microwave power E (unit: W / mL) with respect to the reaction solution and the pressure P (unit: MPa) are adjusted to satisfy the relationship of the following Formula 1. E × P ≥ 20

[0034] In one embodiment, the absorbed microwave power E (unit: W / mL) with respect to the reaction solution and the pressure P (unit: MPa) are adjusted to satisfy the relationship of the following Formula 2. E × P ≥ 30

[0035] In one embodiment, the absorbed microwave power E (units: W / mL) with respect to the reaction solution and the pressure P (units: MPa) are adjusted to satisfy the relationship of the following Formula 3. E × P ≥ 35

[0036] Here, the absorbed microwave power E (absorbed power E of microwave) with respect to the reaction solution is calculated by dividing the intensity (W) of the microwave absorbed by the reaction solution, that is, the value expressed as (output - reflected power), namely the output of the microwave irradiation source minus the reflected power produced upon irradiation of the reaction solution-by the volume (mL) of the reaction solution irradiated at that output. The 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 by a power monitor of a microwave irradiation device.

[0037] In the present invention, by adjusting, with respect to the reaction solution, the absorbed microwave power E (unit: W / mL) and the pressure P (unit: MPa) so as to satisfy Formulae 1 to 3, the formation of nuclei of metal particles occurs simultaneously and uniformly, and growth of the formed nuclei also occurs simultaneously and uniformly, as a result of which metal particles having a small particle diameter can be obtained.

[0038] The method for flowing the reaction solution is not limited, provided that the reaction solution flows in any direction. The reaction solution can flow at a constant rate through the reaction tube, for example, a straight tube or a spiral tube.

[0039] In one embodiment, in the case where the reaction tube is a straight tube, the dimensions and shape are not limited as long as the configuration is such that the microwave is irradiated the entirety of the reaction tube uniformly. In one embodiment, the inner diameter of the tube is typically 1 mm or greater; in one embodiment, 2 mm or greater; in one embodiment, 4 mm or greater; and is 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 prism having a length of 100 mm, there is used the reaction tube having a tube inner diameter of 1 mm to 6 mm, a tube outer diameter of 3 mm to 8 mm (wall thickness: 1 mm), and a tube length of 100 mm.

[0040] The pressure P with respect to the reaction solution is not limited as long as it satisfies the above Formulae 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.

[0041] The flow velocity of the reaction solution is not limited as long as the pressure P with respect to the reaction solution satisfies Formulae 1 to 3 above, but is usually 1 m / min or higher and usually 100 m / min or lower, for example, 1 m / min to 100 m / min.

[0042] By setting the flow velocity of the reaction solution within the above range, the reaction solution can be flowed while applying a predetermined pressure to the reaction solution, and small metal particles can be formed even with little organic substance as a dispersant.

[0043] The absorbed microwave power (E) with respect to the reaction solution is not limited as long as the above Formulae 1 to 3 are satisfied, but, with respect to the volume of the reaction solution, is usually 20 W / mL or greater and usually 500 W / mL or less, for example, 20 W / mL to 500 W / mL.

[0044] When the output of the microwave irradiation source is adjusted to the above-described range, small metal particles with low organic substance amount can be formed owing to sufficient reducibility achieved by the microwave.

[0045] During microwave irradiation of the reaction solution, the reaction solution being microwaved is preferably stirred.

[0046] The stirring of the reaction solution during the reaction maintains the uniformity of the reaction solution against the non-uniformity of the reaction solution based on local changes in the concentration and viscosity of the reaction solution that can be caused by the reaction, suppresses segregation of metal particles, and also allows redissolving of the segregated metal particles.

[0047] Other microwave conditions in the method of the present invention are not particularly limited. In the method of the present invention, as explained above, the reaction is carried out by microwave irradiation using a microwave synthesis device so as to satisfy Formulae 1 to 3 while the reaction solution is flowing. When the reaction solution is irradiated with microwaves, the polar solvents contained in the reaction solution absorb the microwaves and convert them into thermal energy, thereby generating heat. Accordingly, in the reaction solution irradiated with microwaves, a uniform and rapid temperature rise occurs in the irradiated portion, and, in accordance with the temperature rise, a uniform and rapid reaction occurs.

[0048] It is preferable that microwave irradiation be uniformly applied to the target of the reaction, that is, the portion of the reaction solution in which the reaction occurs.

[0049] In microwave synthesis devices, in a vessel that contains the reaction solution, the material of the portion to which microwaves are applied is not limited as long as the reaction solution can be uniformly irradiated by microwaves. In a vessel that contains the reaction solution, the material of the portion to which microwaves are applied may, for example, when the microwaves are applied to the reaction solution from outside the reactor through the reactor, be a material that transmits the microwaves, that is, a material that does not absorb the microwaves, such as ceramic materials, glass, quartz, Teflon (registered trademark; PTFE), or silicone, namely a non-conductive material with small relative permittivity (ε) and dielectric loss angle tangent (tan δ). In the vessel that contains the reaction solution, the material of the portion that is not irradiated by the microwaves may, in addition to the above-described material, be a metal such as aluminum or stainless steel. With respect to the enclosure that houses the microwave irradiation source and the reaction tube irradiated with the microwaves, the material is not particularly limited as long as it does not permit leakage or absorption of the microwaves. Examples include a non-magnetic metal plate, such as an aluminum plate.

[0050] Microwaves are generated by a microwave irradiation source (a microwave oscillator (a magnetron)), and the microwave irradiation source can be used in either a single-mode system or a multimode system.

[0051] The frequency of microwaves generated by the microwave irradiation source may be varied as appropriate and is not particularly limited. The microwave frequency is usually 1 GHz or more, in one embodiment 2 GHz or more, and usually 10 GHz or less, for example, from 1 GHz to 10 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 supply.

[0052] The microwave is preferably uniform during irradiation, and the microwave irradiation conditions are preferably constant during microwave irradiation.

[0053] In the present invention, the temperature of the reaction solution elevated by microwave irradiation is the reaction temperature, and the reaction temperature may be appropriately varied according to the conditions of the reaction (type of solvent, pressure during the reaction, etc.) and is not particularly limited; typically, it is 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 below the boiling point of the solvent. For example, in the case where the solvent is water, the reaction temperature at atmospheric pressure is usually in the range of 25°C or higher and less than 100°C, and, in one embodiment, is 80°C to 90°C. In the present invention, since pressure is applied to the reaction solution, the reaction temperature of the present invention can be set higher than the boiling point of the solvent at atmospheric pressure. The boiling point of the solvent under an applied pressure may vary depending on the pressure. Thus, in one embodiment, the reaction temperature is at or above the boiling point of the solvent under atmospheric pressure and less than the boiling point of the solvent under the applied pressure.

[0054] By setting the reaction temperature to 25°C or higher, the reduction reaction from metal ions to metal particles occurs. By setting the reaction temperature below the boiling point of the solvent, the broadening of the particle size distribution (that is, irregularities in the particle diameter of the resulting metal particles) associated with a non-uniform reaction field that can arise due to boiling of the reaction solution is prevented. This enables the preparation of the metal particles having small and uniform particle diameters.

[0055] The microwave irradiation time for the reaction solution is the time required for the temperature of the reaction solution to reach the reaction temperature, and it may be appropriately varied depending on the reaction conditions (microwave conditions, metal type, solvent type, pressure during the reaction, reaction solution volume, reaction temperature, etc.) and is not particularly limited.

[0056] By irradiating the reaction solution with microwaves under the above-described conditions and bringing the temperature of the reaction solution to the reaction temperature, nuclei of metal particles are generated in the reaction solution.

[0057] In the method of the present invention, the reaction solution is flowed, and irradiation of the reaction solution with microwaves is usually continued until completion of the reaction.

[0058] In the method of the present invention, the reaction can be carried out at a high temperature by applying pressure to the reaction solution. Accordingly, the total reaction time can be significantly reduced as compared with the case in which no pressure is 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 application of pressure (reaction time under applied pressure / reaction time under atmospheric pressure) is usually 1.1 or greater, in one embodiment 1.5 or greater, in one embodiment 2.0 or greater, and is 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, and, for example, 5.0. Accordingly, the production method of the present invention enables significant time savings in the production of metal particles.

[0059] The completion of the reaction can be determined by observing that the absorbance derived from the metal particle precursor or the metal particles in the reaction solution no longer changes. For example, when the material of silver particle is used as the metal particle precursor, the change in absorbance of the reaction solution in the range of 280 nm to 780 nm is observed, and a time point at which the absorbance no longer changes is regarded as the completion point of the reaction.

[0060] In the method of the present invention, after the reaction is complete, the microwave irradiation of the reaction solution may be stopped, and the reaction solution may be subjected to heat retention by a heat retention device, such as a heater or a cooler.

[0061] The heat retention temperature of the reaction solution using the heat retention device is not particularly limited, but is usually at or below the reaction temperature. The lower limit of the heat retention temperature is not limited, but is usually 25°C or higher, and in one embodiment, 80°C or higher. The heat retention temperature, for example, when the solvent is water, at atmospheric pressure, is usually in a range of 25°C to less than 100°C, and in one embodiment is from 80°C to 90°C.

[0062] Switching the reaction solution from microwave irradiation to a heat retention device allows the heat retention device to adjust the reaction solution to the appropriate heat retention temperature, even if the temperature of the reaction solution become below the desired heat retention temperature at the time of the switchover.

[0063] The heat retention device used for heat retention of the reaction solution is not particularly limited provided that the temperature of the reaction solution can be maintained at the heat retention temperature, and a conventional heat retention device may be used. Heat retention devices include a heater, a heating mantle, an immersion heater, a water bath, an oil bath, a cooler, and the like.

[0064] The heat retention time for the reaction solution using the heat retention device is not particularly limited, but is usually 1 minute or more and is usually 15 minutes or less.

[0065] By keeping the reaction solution at a certain temperature with a heat retention device, the growth of the nuclei of metal particles in the resulting reaction solution can be promoted, and the metal particles can be further homogenized (aged).

[0066] The dispersion containing metal particles obtained according to the present invention may, as necessary, be subjected to separation and purification (for example, by salting out or centrifugation) by methods known in the art to obtain the desired metal particles or a dispersion containing metal particles.

[0067] The present invention also relates to a device for producing the metal particles, configured to efficiently carry out the method of the present invention.

[0068] A production apparatus 1 for producing metal particles is described below using Figs. 1 and 2. As shown in Fig. 1, the production apparatus 1 is a production apparatus for producing the metal particles by irradiating the reaction solution L with the microwave M.

[0069] The production apparatus 1 includes a containment tank 10 that contains the reaction solution L and a pump 20 that draws the reaction solution L from the containment tank 10 and delivers it under pressure. The reaction solution L flows through a path 5 from the containment tank 10 to the reaction apparatus 40. The slurry-like suspension containing metal particles produced by passage of reaction solution L through the reaction apparatus 40 flows continuously from the reaction apparatus 40 to a pressure regulating device 60, and the slurry also flows through the path 5. In this embodiment, for convenience, all of the fluid flowing through the path 5 is referred to as reaction solution L (before and after the reaction).

[0070] The production apparatus 1 includes a stirring device 30 and a reaction apparatus 40, as will be described below. The reaction apparatus 40, via the stirring device 30, includes a housing 41 in which a reaction tube 43 through which the reaction solution L flows is accommodated, and an irradiation device 42 that irradiates the reaction tube 43 within the housing 41 with the microwaves M.

[0071] Accordingly, using the irradiation device 42, both the reaction solution L, which is pumped by the pump 20 and flows in the reaction tube 43, and the reaction tube 43 can be irradiated with the microwave M. The material of the reaction tube 43 may be the same as the material of the vessel that contains the reaction solution described in the method of the present invention, and may be made of ceramic materials composed of silicon oxide such as glass or quartz, or of resin material such as PTFE. A pipe other than the path 5 forming the reaction tube 43 may also be constituted of these pipes; alternatively, because it is not irradiated by the microwave M, the pipe other than the path 5 forming the reaction tube 43 may be made of metal, such as stainless steel or aluminum. The reaction tube 43 may be a straight tube, but for example, a spiral pipe may be used. This allows the irradiation efficiency of the microwave M to be increased compared with a straight tube.

[0072] In one embodiment, in the case where the reaction tube 43 is a straight tube, the dimensions and shape are not limited, provided that the configuration allows the microwave M to uniformly irradiate the entirety of the reaction tube 43. For example, the inner diameter of the tube is usually 1 mm or more, in one embodiment 2 mm or more, in one embodiment 4 mm or more, and usually 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. For example, when the cavity is a rectangular prism having a length of 100 mm, the reaction tube 43 is preferably a reaction tube having a tube inner diameter of 1 mm to 6 mm, a tube outer diameter of 3 mm to 8 mm (wall thickness: 1 mm), and a tube length of 100 mm. The dimensions and shape of the reaction apparatus 40 are also not limited.

[0073] The production apparatus 1 includes a heat retention device 50 and the pressure regulating device 60, in that order, downstream of the reaction apparatus 40. The heat retention device 50 is an optional device and need not be installed. Since the post-reaction reaction solution L (specifically, a slurry-like suspension containing metal particles) flowing through the path 5 is heated in the reaction apparatus 40, the heat retention device 50 is a device that maintains this reaction solution L at the heat retention temperature in the method of the present invention described above, that is, from the reaction temperature down to a predetermined temperature lower than the reaction temperature. Specifically, the reaction solution L flowing in the path 5 is heated or cooled using a heat retention device, and the reaction solution L maintained at a predetermined temperature is discharged to the pressure regulating device 60. Besides the above-described heater, a cooler may also be used as the heat retention device 50.

[0074] The pressure regulating device 60, located downstream of the reaction apparatus 40 (the reaction tube 43), regulates the pressure of the reaction solution L in the reaction tube 43; in one embodiment, it regulates the pressure of the reaction solution L in the path 5 from the pump 20 to the pressure regulating device 60. Specifically, the pressure regulating device 60 adjusts the pressure of the reaction solution L in the path 5 by throttling the discharge rate of the reaction solution L passing through it. This allows, within a range that does not exceed the discharge pressure of the pump 20, the pressure of the reaction solution L in the reaction tube 43 to be increased to at or above atmospheric pressure. The reaction solution L pumped by the pressure regulating device 60 is collected in a collection tank 70.

[0075] In this way, the reaction solution L delivered by the pump 20 is pressure-fed into the reaction tube 43. The pressure of the pumped reaction solution L is regulated in a pressurized state by the pressure regulating device 60. By irradiating the reaction solution L under pressure with the microwave M using the irradiation device 42, the boiling point of the reaction solution L can be raised compared to atmospheric pressure conditions, thereby increasing the formation rate of metal particles.

[0076] Meanwhile, in the reaction solution L, when metal particles are generated, the concentration and viscosity of the reaction solution L may change locally, and the reaction solution L may pulsate slightly. This may impair the uniformity of microwave irradiation applied to the reaction solution L, which may lead to segregation of metal particles in the reaction solution L. In this regard, the production apparatus 1 includes, between the pump 20 and the reaction tube 43 of the reaction apparatus 40, the stirring device 30 that stirs the reaction solution L pumped from the pump 20.

[0077] If the segregation of metal particles in reaction solution L can be suppressed, the configuration of the stirring device 30, its structure, and the position at which it is disposed are not particularly limited. In this embodiment, the stirring device 30 is, as an example, equivalent in structure to the static mixer shown in Fig. 2.

[0078] The stirring device 30 includes a straight pipe 31 through which the reaction solution L flows, and twisted blades 32 that are fixed within the pipe 31 and twisted around the axis CL of the pipe 31. With the twisted blades 32 disposed in the pipe 31, a flow path 33 (the path 5) through which the reaction solution L flows is formed within the pipe 31.

[0079] The twisted blades 32 may be helically twisted in the same direction about the axis CL, but in this embodiment, they have the structure described below. Specifically, the twisted blades 32 are blades in which twisted blade elements 32a and 32b, having different twist directions around the axis, are alternately arranged along the axial direction of the pipe 31.

[0080] Specifically, the twisted blade element 32a and the twisted blade element 32b are each formed into a twisted shape from a flat plate. Relative to the twist direction of the twisted blade element 32a, the twisted blade element 32a is twisted in the opposite twist direction. The twisted blade element 32a and the twisted blade element 32b are alternately connected along the axial direction of the pipe 31.

[0081] Accordingly, for the reaction solution L that has passed through the twisted blades 32, a stable stirring flow is readily generated along the axial direction of the pipe 31, so segregation of metal particles in the reaction tube 43 is readily reduced, and furthermore the segregated metal particles readily undergo redissolving. In particular, it is preferable that the twisted blades be blades in which twisted blade elements 32a and 32b having different twist directions about the axis are alternately arranged along the axial direction of the pipe 31. Accordingly, flows of reaction solutions La and Lb stirred in different directions are formed, and a stirring flow around the axis is readily formed, such that segregation of metal particles in the reaction tube 43 is further reduced more readily, and the segregated metal particles are more readily redissolved.

[0082] The material of the twisted blades 32 is not particularly limited. The material of the twisted blades 32 is made of a non-conductive material that neither absorbs nor reflects microwaves when installed immediately before the reaction apparatus 40. For example, the material exemplified for the reaction tube 43 is preferred; it may be ceramic materials composed of silicon oxide such as glass, quartz, or a resin material such as PTFE. It is also preferable that the pipe 31 be made of a similar material. Thus, by forming the twisted blades 32 of a non-conductive material, it is possible to prevent the microwave M from reaching the twisted blades via the reaction solution L from the reaction tube 43. This allows efficient irradiation of the reaction solution in the reaction tube 43 with the microwave M.

[0083] Preferably, the stirring device 30 is disposed at a position closer to the reaction apparatus 40 in the path 5 between the pump 20 and the reaction apparatus 40. More specifically, the stirring device 30 is preferably disposed at a position at which the stirring flow of the reaction solution L that has passed through the stirring device 30 is sustained within the reaction tube 43. For example, it may be at a position near the inlet of the reaction apparatus 40 where the reaction solution L flows in, and it is not particularly limited to this position as long as the stirring flow is sustained.

[0084] In this way, the stirring flow of the reaction solution L through the stirring device 30 is sustained in the reaction tube 43, such that localized changes in the concentration and viscosity of the reaction solution L can be suppressed during the formation of metal particles. Thereby, within the reaction tube 43, the reaction solution L becoming non-uniform is suppressed, segregation of the metal particles in the reaction solution L is suppressed, and further, redissolving of the segregated metal particles is enabled.

[0085] Accordingly, the metal particle production apparatus 1 of the present invention can, without causing changes in physical properties such as the volume and viscosity of the reaction solution L before and after application of pressure and a decrease in the absorption rate (uniformity) of the applied microwave based on such changes in physical properties, further prevent segregation of the metal particles and redissolve segregated metal particles, thereby enabling the production method for metal particles of the present invention to be carried out efficiently.

[0086] The present invention also relates to the metal particles containing an organic substance that can be obtained by the method or production apparatus of the present invention, the metal particles having a small particle diameter and a low amount of organic substance. Here, "organic substance" is a compound that is attached to metal particles to uniformly disperse the metal particles in a solvent or a diluent; it is non-volatile and is distinguished from organic compounds used as a solvent or a diluent.

[0087] In one embodiment, the metal particles are metal nanoparticles. As used herein, "metal nanoparticles" denotes metal particles having a particle diameter typically from 1 nm to 100 nm. Accordingly, in the case where the metal particles of the present invention are metal nanoparticles, the metal particles of the present invention are metal particles in which the Heywood diameter of the entire metal particle, including a main portion and an auxiliary portion, is 1 nm to 100 nm.

[0088] In one embodiment, the metal particles have a spherical shape. Herein, "spherical" includes, when the metal particles are observed with a transmission electron microscope (TEM), not only a perfectly spherical shape but also a approximately spherical shape, an ellipsoidal shape, and a polygonal shape whose sides are substantially the same.

[0089] The median diameter (D50) of the metal particles of the present invention, as measured by TEM, is 20 nm or less, and in one embodiment is 10 nm or less. Since a smaller D50 is preferable, the lower limit thereof is not particularly limited, but is usually 1.0 nm or more.

[0090] Here, the measurement method for the D50 of metal particles by TEM is as follows.

[0091] First, a TEM image of metal particles is acquired. Next, arbitrarily select 500 metal particles in the TEM image. Next, for each of the selected metal particles, the diameter value is measured by converting the projected surface area of the metal particle to the area of a circle. They are then plotted as particle diameter, with particle diameter (nm) on the x-axis and cumulative number of pieces (%) on the y-axis. Finally, the particle diameter at which the cumulative number of pieces reaches 50% can be determined from the graph as D50.

[0092] The particle size distribution of the metal particles of the present invention has a narrow distribution.

[0093] The amount of organic substance, particularly a dispersant, contained in the metal particles (a powder formed from the metal particles) of the present invention is 2 weight% or less, in one embodiment 1 weight% or less, and in one embodiment 0.8 weight% or less, with respect to the total weight of the metal particles. The lower limit of the amount of the organic substance is not particularly limited, since less is preferable; however, it is usually 0.1 weight% or more, in one embodiment 0.2 weight% or more, in one embodiment 0.3 weight% or more, and in one embodiment 0.4 weight% or more, with respect to the total weight of the metal particles.

[0094] Herein, the measurement method for the amount of the organic substance contained in the metal particles is as follows.

[0095] First, purified water such as ion-exchanged water is added to metal particles or to a dispersion containing metal particles, and it is purified using a solid-liquid separator, such as by centrifugation, until the electrical conductivity of the slurry is several 10 µS / cm or less. The measurement of electrical conductivity may be performed using a hand-held type general-purpose type. This step is performed for the purpose of eliminating impurities, water-soluble by-products, and the like that may be present in the metal particles or in a dispersion containing metal particles. Among the organic substances added during production for use as a dispersant or for similar purposes, that which is adsorbed onto the metal particles is not removed in the relevant step. The purified metal particles are then dispersed in a low boiling point solvent (alcohol-based, for example, methanol or ethanol) to prepare a metal particle-containing slurry. The slurry is then applied to the glass surface to form a film. The film thus formed is thereafter dried in a furnace at 80°C for about 1 hour, peeled from the glass, and collected as an organic substance measurement sample. Finally, the collected organic substance measurement sample is powdered, and the C content (weight%) of the organic substance is measured using a CS analyzer (combustion method).

[0096] The metal particles of the present invention have a low organic substance amount and a small particle diameter and have excellent low-temperature sinterability. Furthermore, the sintered body formed from the metal particles has a low volume resistivity, and furthermore, the volumetric shrinkage ratio during sintering is small.

[0097] Specifically, the metal particles of the present invention have a low organic substance amount and a small median diameter. The metal particles of the present invention have a low volumetric shrinkage ratio during sintering, for example, a volumetric shrinkage ratio, based on the volume of the metal particles in a press-compacted state before sintering, which is usually 60% or less, in one embodiment 50% or less, and in one embodiment 15% or less. The lower limit of the volumetric shrinkage ratio due to sintering of metal particles in the present invention is not limited, since a smaller rate is preferable.

[0098] Furthermore, the metal particles of the present invention can be dispersed sufficiently in the solvent without the need for additional dispersants because they contain the minimum amount of organic substance as a dispersant necessary to disperse the metal particles in the solvent. This is because, in the metal particles of the present invention, the dispersant adheres to the metal particles immediately after formation of the metal particles, such that it is uniformly adhered into the metal particles, and because, even after reaching a semi-dry state or a dry condition, when added to a solvent, it can again exhibit its function as the dispersant. Thus, the metal particles of the present invention are redispersible metal particles. In the case where the metal particles contain no dispersant at all, once they reach a semi-dry state or a dry condition, redispersion is difficult, especially because the cohesive force of the nano-sized metal particles is large. In addition, when mixing metal particles containing no dispersant at all with a dispersant, once the metal particles are in a semi-dry state or a dry condition, it is difficult to mix them uniformly due to the large cohesive force of the metal particles.

[0099] The present invention also relates to a dispersion comprising the metal particles of the present invention described above, an organic substance serving as a dispersant for the metal particles, and a solvent for dispersing the metal particles and the organic substance.

[0100] In the dispersion according to 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 the metal particles to disperse in the solvent. For example, the content of metal particles, with respect to the total weight of the dispersion, is usually 1 weight% or more, in one embodiment 5 weight% or more, in one embodiment 10 weight% or more, in one embodiment 20 weight% or more, in one embodiment 50 weight% or more, in one embodiment 70 weight% or more, and is usually 95 weight% or less, in one embodiment 90 weight% or less, in one embodiment 85 weight% or less, in one embodiment 80 weight% or less, for example, from 1 weight% to 95 weight%, in one embodiment from 5 weight% to 90 weight%, and in one embodiment from 50 weight% to 80 weight%.

[0101] By increasing the content of metal particles in the dispersion of the present invention, it is possible to reduce solvent cost; the time, labor, and cost required for volatilization of the solvent after application of the dispersion; and storage and transportation costs, thereby reducing environmental impact.

[0102] In the dispersion of the present invention, the amount of an organic substance as a dispersant for the metal particles, with respect to the total weight of the metal particles, is 2 weight% or less, 1 weight% or less in one embodiment, and 0.8 weight% or less in one embodiment. The lower limit of the amount of the organic substance is not particularly limited, since less is preferable; however, with respect to the total weight of the metal particles, it is usually 0.1 weight% or more, in one embodiment 0.2 weight% or more, in one embodiment 0.3 weight% or more, and in one embodiment 0.4 weight% or more.

[0103] A small amount of dispersant in the dispersion of the present invention allows the sintered body formed from the dispersion to have low volume resistivity and a low volumetric shrinkage ratio.

[0104] The solvents contained in the dispersion of the present invention may be any known in the art and are not particularly limited; for example, they may be selected from solvents that are liquid at 20°C, water, alcohols, aldehydes, carboxylic acids, ethers, esters, amines, monosaccharides, polysaccharides, straight-chain hydrocarbons, fatty acids, and aromatic compounds, and the like. Two or more of the above solvents may be used in combination.

[0105] The boiling point of the solvent is not particularly limited, but is usually 100°C or higher, in one embodiment 130°C or higher, in one embodiment 150°C or higher, and is usually 300°C or lower, in one embodiment 250°C or lower, in one embodiment 200°C or lower: for example, 100°C to 300°C, 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 volatilization of the solvent at room temperature can be suppressed, and as a result, the viscosity stability and coatability of the ink paste can be ensured. If the boiling point of the solvent is 300°C or lower, in a bonding process involving firing, particularly non-pressurized firing, at the temperature at which the semiconductor device is connected to the support member, retention of the solvent in the sintered metal without evaporation can be suppressed, and as a result, the properties of the sintered metal can be better maintained.

[0106] As the solvent, it is preferable to select a solvent suitable for dispersion of silver particles from among the solvents mentioned above. Specifically, it is preferable to select a solvent having an alcohol structure, an ether structure, or an ester structure because it results in favorable thermal conductivity, electrical conductivity, and adhesive strength of the sintered metal. 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, isobornyl cyclohexanol, and tributyrin. Ethylene glycol is a preferred solvent contained in the dispersion of the present invention.

[0107] When the dispersion of the present invention is used as an ink paste, the amount of solvent contained in the ink paste can vary depending on the content of the metal particles contained in the dispersion and the like; however, with respect to the total weight of the ink paste, the amount of solvent is usually 5 weight% or more and usually 99 weight% or less, and in one embodiment 90 weight% or less.

[0108] By adjusting the amount of solvent within the range set forth above, the viscosity of the ink paste can be adjusted to the appropriate viscosity range set forth below, and, furthermore, volume shrinkage caused by volatilization of the solvent during sintering of the ink paste can be suppressed, thereby improving the density of the resulting silver sintered body.

[0109] The dispersion of the present invention may further contain components other than the metal particles of the present invention and solvent, to the extent that they do not interfere with the effects of the present invention. Components that may be added other than to the metal particles of the present invention and the solvent include, without limitation, materials known in the art, for example, additives such as carboxylic acids that have a boiling point of 400°C or lower under atmospheric pressure and are solid at 20°C, for example, 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 the metal particles in the dispersion; and fluxing agents for promoting sintering of the metal particles. The amount of components that may be added other than the metal particles of the present invention and the solvent is, with respect to the total weight of the dispersion, usually 0 weight% or more and usually 10 weight% or less; in one embodiment, 1 weight% or less; for example, from 0 weight% to 10 weight%; and in one embodiment, from 0 weight% to 1 weight%.

[0110] When the dispersion of the present invention is used as an ink paste, the viscosity of the ink paste, when measured with a cone-plate viscometer, is usually 10 mPa·s or more and usually 10000 Pa·s or less. As described above, the viscosity can be appropriately adjusted based on the aspect ratio and amount of plate-like silver particles, the type and amount of a polymer as a dispersant, and the type and amount of a solvent.

[0111] By adjusting the viscosity of the ink paste to the above-described range, the coatability of the ink paste can be improved, and bleeding after application of the ink paste can be prevented.

[0112] Metal particles or dispersions produced by the present invention can be used as conductive wiring materials in the field of electronics packaging, in addition to catalysts, electronic component materials, materials for inks, etc., and, for example, can reduce the number of processes for preparing inks used for wiring boards.Examples

[0113] The following is a description of several examples of the present invention, but the present invention is not intended to be limited to those examples.1. Synthesis of metal particles

[0114] Silver particles or nickel particles were produced in accordance with the conditions shown in Table 3, 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. In Table 1, the weight average molecular weight of PVP is 40000. In Table 3, E is the absorbed power (W / mL) that the reaction solution absorbed from the microwave irradiated by the irradiation device 42, which irradiates the microwave M. P is the pressure (MPa) applied to the reaction solution, as measured at the pressure regulating device 60. In the irradiation device 42 which irradiates the microwave M of the production apparatus 1, a rectangular prism having a length of 100 mm was used as the cavity, and the following one was used as the reaction tube. Reaction tube configuration Tube inner diameter: 1 mm to 6 mm Tube outer diameter: 3 mm to 8 mm (wall thickness: 1 mm) Tube length: 100 mm [Table 1] Table 1 Reaction Solution of Silver ParticlesTypeAgNO 3 PVPDMFConcentration100 mM20 mM8900 mM Solvent: Water, DMF [Table 2] Table 2 Reaction Solution of Nickel ParticlesTypeNickel FormateDDAConcentration100 mM20 mM Solvent: EG [Table 3] Table 3 Synthesis ConditionsType of MetalNumberAbsorbed Microwave Power EPressure PE×PAgExample 190 W / ml0.4 MPa36Example 292 W / ml0.4 MPa37Example 3200 W / ml0.15 MPa30Example 4200 W / ml0.1 MPa20Comparative Example 175 W / ml0.15 MPa11.3Comparative Example 233 W / ml0.4 MPa13.2Comparative Example 327 W / ml0.4 MPa10.8Comparative Example 450 W / ml0.1 MPa5Comparative Example 5150 W / ml0.1 MPa15NiExample 5200 W / ml0.1 MPa20

[0115] In these experiments, the total reaction time in the Examples was about one-fifth that of the total reaction time when the reaction was performed without applying pressure. This is because the boiling point of the reaction solution to which pressure was applied became higher than the boiling point of the reaction solution under atmospheric pressure, allowing the temperature of the reaction solution during the reaction to be set higher.2. Assessment Results(Transmission electron microscopy (TEM) evaluation)

[0116] TEM images were taken of the dispersions obtained in Examples 1 to 5 and Comparative Examples 1 to 5. As an example, Fig. 3 shows TEM images of silver particles in Comparative Example 1 and Examples 1 and 4.

[0117] Additionally, 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 arbitrarily selected in the TEM image. Next, for each selected one of the silver particles or nickel particles, the diameter value was measured when the projected surface area of the silver particle or nickel particle was converted to the area of a circle. They were then plotted as particle diameter, with particle diameter (nm) on the x-axis and cumulative number of pieces (%) on the y-axis. As an example, Fig. 4 shows the particle size distribution of silver particles in Example 1. Finally, the particle diameter at which the cumulative number of pieces reaches 50% was determined from the graph as D50. Table 4 and Fig. 5 show the results. [Table 4]Table 4 Synthesis Conditions and ResultsType of MetalNumberAbsorbed Microwave Power EPressure PE×PD50AgExample 190 W / ml0.4 MPa365.6 nmExample 292 W / ml0.4 MPa375.1 nmExample 3200 W / ml0.15 MPa3011 nmExample 4200 W / ml0.1 MPa208.2 nmComparative Example 175 W / ml0.15 MPa11.347 nmComparative Example 233 W / ml0.4 MPa13.235 nmComparative Example 327 W / ml0.4 MPa10.859 nmComparative Example 450 W / ml0.1 MPa5114 nmComparative Example 5150 W / ml0.1 MPa1524 nmNiExample 5200 W / ml0.1 MPa2012 nm

[0118] The results in Table 4 and Fig. 5 show that when the value obtained by multiplying the absorbed microwave power E (W / mL) with respect to the reaction solution by the pressure P (MPa) applied to the reaction solution (E × P) is 20 or greater, the D50 of the resulting metal particles decreases, in particular, to 15 nm or less.

[0119] Subsequently, for Examples 1, 3, and 4 and Comparative Examples 3 and 5, the organic substance amount and its relationship to D50 were measured. The organic substance amount was measured as follows.

[0120] First, purified water such as ion-exchanged water was added to the dispersion containing the examples or comparative examples, and a solid-liquid separator, such as centrifugation, was used to purify the slurry until the electrical conductivity of the slurry was several tens µS / cm or less. A handheld-type general-purpose type was used for electrical conductivity measurement. This step was carried out for the purpose of eliminating impurities, water-soluble by-products, and the like from the dispersion. The purified particles were then dispersed in a low boiling point solvent (alcohol-based, for example, methanol or ethanol) to prepare a metal particle-containing slurry. Subsequently, the slurry was applied to glass, thereby forming a film. The formed film was then dried in a furnace at 80°C for about 1 hour, peeled off from the glass, and collected as an organic substance amount measurement sample. Finally, the collected organic substance amount measurement sample was pulverized to a powder, and the organic carbon (C) content (weight%) was measured using a CS analyzer (combustion method). Fig. 6 shows the results.

[0121] Fig. 6 shows that the metal particles produced by the production method of the present invention can maintain a small D50 even when the organic substance amount in the metal particles is reduced.

[0122] Next, the volumetric shrinkage ratio during sintering at 120°C for 2 hours was measured for Examples 1, 3, and 4 and Comparative Examples 3 and 5, for which the organic substance amount had been measured. The volumetric shrinkage ratio was measured as follows. (1) First, metal particles purified with purified water such as ion-exchanged water using a solid-liquid separator such as centrifugation were collected on filter paper, and ethanol was added and filtration under reduced pressure was performed to effect solvent replacement. (2) Subsequently, the cake layer remaining on the filter paper after solvent replacement was collected, solvents such as butyl carbitol, butyl acetate, and terpineol were added to adjust the concentration of metal particles to 80 weight%, deagglomeration and kneading were performed using a rotation / revolution mixer, and a metal particle-containing paste was prepared. (3) The resulting paste was applied onto a glass slide using a metal mask having a thickness of 100 µm, a width of 15 mm, and a length of 15 mm (100 µm x 15 mm x 15 mm). (4) Firing was performed at 120°C for 2 hours in a constant-temperature chamber. (5) After firing, the film thickness was measured at 5 predetermined points using a micrometer, and the average value was used as the film thickness. (6) From the dimensions and film thickness of the metal mask and the dimensions and film thickness after firing, calculated the volume of the paste before firing and the volume of the sintered body after firing, and obtained the volumetric shrinkage ratio using the following Formula.

[0123] Fig. 7 shows the results. Fig. 7 shows that, as the organic substance amount in the metal particles decreases, the volumetric shrinkage ratio also decreases.

[0124] Further, to confirm the redispersibility of the silver particles and nickel particles produced and purified in Examples of the present invention, the silver particles and nickel particles produced and purified in Examples in a semi-dry state or a dry condition were redispersed in various solvents, for example, aqueous solvents and non-polar solvents. As a result, confirmation was obtained that the metal particles in the example were capable of redispersion in various solvents.

[0125] Figs. 8 and 9 schematically illustrate the manner of formation of metal particles based on the results of the above-described comparative examples and examples.

[0126] Fig. 8 schematically shows, either by conventional techniques or when E×P is less than 20, the formation of the nuclei of metal particles (silver particles as an example) and the growth of the particles. As shown in Fig. 8, when the absorbed microwave power is low and no pressure is applied, the reaction temperature is low, the formation of the nuclei is also non-uniform, and the nuclei are of varying sizes. As a result, nuclei with different sizes are present in the reaction solution, and the smaller nuclei that are formed later adhere to the larger nuclei that have been formed earlier, resulting in larger particles.

[0127] Fig. 9 schematically illustrates the progression from formation of the nuclei of metal particles (silver particles as an example) to growth of the particles when E × P is 20 or greater in the present invention. As shown in Fig. 9, when the absorbed microwave power is high and the pressure is applied, the reaction temperature becomes higher, the formation of nuclei also occurs uniformly, and no variation in nucleus size occurs. As a result, in the presence of nuclei with uniform size, nuclei with similar size adhere to each other and grow, resulting in small particles. The dispersant required to disperse the metal particles present in the reaction solution can also be uniformly attached to the uniformly formed particles, and a the minimum amount of dispersant will be sufficient. Furthermore, by conducting the reaction in a high-temperature atmosphere, the reaction time can be significantly shortened, for example, to 1 / 5. The method for forming metal particles of the present invention is characterized in that the reaction temperature is increased by applying a certain level or more of absorbed microwave power and pressure with respect to the reaction solution, thereby causing the reduction of metal ions by microwave heating all at once. The types of metal particles produced by the method of the present invention are not limited as long as the metal ions are reduced by microwave heating, and can include not only the above-described metal particles, that is, silver particles and nickel particles, but also, for example, particles of gold, platinum, copper, iron, or cobalt.

[0128] All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entireties.

Examples

examples

[0113]The following is a description of several examples of the present invention, but the present invention is not intended to be limited to those examples.

1. Synthesis of metal particles

[0114]Silver particles or nickel particles were produced in accordance with the conditions shown in Table 3, 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. In Table 1, the weight average molecular weight of PVP is 40000. In Table 3, E is the absorbed power (W / mL) that the reaction solution absorbed from the microwave irradiated by the irradiation device 42, which irradiates the microwave M. P is the pressure (MPa) applied to the reaction solution, as measured at the pressure regulating device 60. In the irradiation device 42 which irradiates the microwave M of the production apparatus 1, a rectangular prism having a length of 100 mm was used as the cavity, and the following one was used as the reaction tube. Rea...

Technical Field

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

[0002] Metal nanoparticles, which may have properties different from those of bulk materials, are used and studied in various applications, for example, as catalysts, ink materials, and electronic component materials.

[0003] For example, Patent Literature 1 discloses a method for producing chromium nanoparticles having an average particle diameter of 5.6 nm by irradiating ultrasonic waves at a frequency of 50 kHz and a power of 150 W at 40°C to 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 to thermally decompose the chromium nanoparticle raw material.

[0004] Patent Literature 2 discloses a method for producing metal fine particles, comprising allowing a reaction solution containing a metal precursor to flow through a flow tube, and irradiating the interior of the flow tube with electromagnetic waves uniformly and intensively along the longitudinal direction of the flow tube, thereby uniformly heating an electromagnetic-wave irradiation zone within the flow tube along the flow direction to produce metal fine particles.Citation ListPatent Literature

[0005] Patent Literature 1: JP 2015-129327 A Patent Literature 2: JP 2011-137226 A Summary of InventionTechnical Problem

[0006] In order to use metal nanoparticles in the field of electronics packaging, it is desirable to reduce the particle diameter of metal nanoparticles.

[0007] However, in order to produce metal nanoparticles with a nanoscale average particle diameter by a microwave irradiation method, high microwave intensity and high reaction temperature are required to suppress variation in formation of nuclei (nucleation) and particle growth.

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

[0009] As a result of various studies of means to solve the above-described problems, the present inventors have found that, in a method for producing metal particles by irradiating a reaction solution with microwaves, increasing the absorbed microwave power with respect to the reaction solution and the pressure applied to the reaction solution to a certain level or more shortens the reaction time. The present inventors further found that metal particles can be produced with a small particle diameter and a low amount of organic substance as a dispersant that can become impurities, and that the obtained metal particles have a low volumetric shrinkage ratio, thereby completing the present invention.

[0010] That is, the gist of the present invention is as follows. (1) A method for producing metal particles includes irradiating a reaction solution with microwave. The method comprises: (i) a step of preparing a reaction solution containing a metal particle precursor, an organic substance as a dispersant, and a solvent; and (ii) a step of irradiating the reaction solution with microwaves while the reaction solution is flowing, a relationship between an absorbed power E of the microwave (unit: W / mL) and a pressure P (unit: MPa) with respect to the reaction solution satisfying Formula 1 below, E × P ≥ 20 (2) The method according to (1), wherein in the step (i), an amount of the organic substance in the reaction solution is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particle precursor as metal. (3) The method according to (1) or (2), wherein in the step (ii), the relationship between the absorbed power E of the microwave (unit: W / mL) and the pressure P (unit: MPa) with respect to the reaction solution satisfies Formula 2 below, E × P ≥ 30 (4) Metal particles comprising an organic substance, wherein an amount of the organic substance is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particles, and wherein a median diameter (D50) by TEM is 20 nm or less. (5) A metal particle dispersion comprises: metal particles; an organic substance as a dispersant for the metal particles; and a solvent, wherein a content of the metal particles is from 1 weight% to 95 weight% with respect to a total weight of the metal particle dispersion, and wherein an amount of the organic substance is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particles, wherein a median diameter (D50) of the metal particles by TEM is 20 nm or less. (6) An apparatus for producing metal nanoparticles by irradiating a reaction solution with microwaves, the apparatus comprising: a pump that pumps the reaction solution; an irradiation device that irradiates the reaction solution pumped from the pump and flowing in a reaction tube together with the reaction tube with the microwaves; a pressure regulating device that regulates a pressure of the reaction solution in the reaction tube, downstream of the reaction tube; and a stirring device that stirs the reaction solution pumped from the pump between the pump and the reaction tube. (7) The apparatus for producing the metal nanoparticles according to (6), wherein the stirring device is positioned between the pump and the reaction tube where a stirring flow of the reaction solution having passed through the stirring device is sustained in the reaction tube. (8) The apparatus for producing the metal nanoparticles according to (6) or (7), wherein the stirring device includes a linear pipe through which the reaction solution flows and a twisted blade fixed within the pipe and twisted around an axis of the pipe. (9) The apparatus for producing the metal nanoparticles according to (8), wherein the twisted blade includes twisted blade elements having different twist directions around the axis, and the twisted blade elements are alternately arranged along an axial direction of the pipe. (10) The apparatus for producing the metal nanoparticles according to (8) or (9), wherein the twisted blade is made of a non-conductive material. Advantageous Effects of Invention

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

[0012] Fig. 1 is a schematic diagram of one embodiment of a production apparatus of the present invention. Fig. 2 is a schematic diagram illustrating one embodiment of a stirring device in the production apparatus of the present invention. Fig. 3 shows TEM images of silver particles in Comparative Example 1, Example 1, and Example 4. Fig. 4 is a graph showing a particle size distribution of silver particles in Example 1. Fig. 5 is a graph showing a relationship between D50 and a value obtained by multiplying an absorbed microwave power E (W / mL) with respect to a reaction solution by a pressure P (MPa) applied to the reaction solution (E × P). Fig. 6 is a graph showing a relationship between an organic substance amount and D50 in Examples and Comparative Examples. Fig. 7 is a graph showing a relationship between an organic substance amount and a volumetric shrinkage ratio at 120°C for 2 hours. Fig. 8 schematically illustrates a process from formation of nuclei of metal particles (silver particles as an example) to a growth of the particles in the case of a conventional technique or where E×P is less than 20. Fig. 9 schematically illustrates a process from formation of nuclei of metal particles (silver particles as an example) to a growth of the particles when E×P is 20 or more in the present invention. Description of Embodiments

[0013] The following describes preferable embodiments of the present invention in detail.

[0014] In the description, features of the present invention will be described with reference to the drawings as necessary. In the drawings, dimensions and shapes of respective components are exaggerated for clarification, and actual dimensions and shapes are not accurately illustrated. Accordingly, the technical scope of the present invention is not limited to the dimensions or the shapes of the respective components illustrated in the drawings. Note that metal particles, a method for producing the metal particles, an apparatus for producing the metal particles, and a dispersion containing the metal particles of the present invention are not limited to the embodiments below, and can be performed in various configurations where changes, improvements, and the like that a person skilled in the art can make are given without departing from the gist of the present invention. While the embodiments described in the description are each independent, two or more of the embodiments may be combined to constitute one embodiment of the present invention.

[0015] The term "to" described in the description is used to mean a range having values described before and after the term "to" as the lower limit value and the upper limit value, respectively. In numerical ranges described in stages in the description, an upper limit value or a lower limit value described in one numerical range may be replaced with an upper limit value or a lower limit value in another numerical range described in stages. The upper limit values or the lower limit values in the numerical ranges described in the description may be replaced with values indicated in the examples.

[0016] The present invention relates to a method for producing metal particles that includes the steps of (i) preparing a reaction solution containing a metal particle precursor, an organic substance, and a solvent, and (ii) irradiating the reaction solution with microwaves while causing the reaction solution to flow. (i) a step of preparing a reaction solution containing a metal particle precursor, an organic substance, and a solvent,

[0017] In the step (i), a reaction solution containing a metal particle precursor, an organic substance, and a solvent is prepared.

[0018] In the method of the present invention, the solvent used for the reaction solution is not limited insofar as the solvent is a polar solvent or an ionic liquid that can dissolve the materials, such as a metal particle precursor, an organic substance as a dispersant, and a reductant, and further can absorb microwaves. Examples of the solvent used for the reaction solution include a low boiling point solvent having a boiling point of 300°C or lower. While the low boiling point solvent is not limited, examples of the low boiling point solvent may include low boiling point polar solvents such as water; alcohols such as methanol and ethanol; polyhydric alcohol solvents such as ethylene glycol; ketone solvents such as acetone; dimethyl sulfoxide (DMSO); N,N-dimethylformamide (DMF); other organic solvents; or mixtures of two or more thereof. When the mixture is used as the solvent, the ratio between the respective components contained in the mixture is not limited and may be a ratio at which the components are miscible under experimental conditions.

[0019] The use of a low boiling point polar solvent as the solvent allows improvement of solvent handling and reduction of the burden on the environment.

[0020] The metal particle precursor is not limited insofar as it is a material that is dissolved in the solvent and generates metal ions, for example, ions of noble metals, base metals, and alloys, for example, metal ions of gold, silver, platinum, copper, nickel, iron, cobalt, or two or more thereof. Examples of the metal particle precursor may include: inorganic metal salts such as metal halides (for example, 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 in a base such as aqueous ammonia. As the metal particle precursor, it is preferable to use inexpensive metal nitrates, such as nickel nitrate and silver nitrate. In addition, a formate having reducing properties, for example, nickel formate, may be used as the metal particle precursor. Therefore, in one embodiment, when a salt composed of a metal ion and an organic anion having reducing properties as its counter anion, for example, a formate ion, is used as the metal particle precursor, it is not necessary to use a reducing agent.

[0021] A homogeneous reaction solution can be prepared by dissolving a metal particle precursor in a solvent.

[0022] The concentration of metal ions in the reaction solution is not limited as long as it is at or below the saturation concentration, but is usually 1 mmol / L (mM) or more, in one embodiment 10 mM or more, and usually 600 mM or less, 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.

[0023] 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 greatly increased, and the time, labor, and cost for the metal particle production can be reduced. In addition, the variation in the obtained metal particles will be reduced; in other words, the particle size distribution of the obtained metal particles will be narrower.

[0024] The reaction solution contains an organic substance as a dispersant. Dispersants include, but are not limited to, one or more selected from polyvinylpyrrolidone (PVP), dodecylamine (DDA), thiol-based polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylate salts, 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 not limited, but, for example, as a weight average molecular weight (Mw), is usually 1000 or more, in one embodiment 8000 or more, in one embodiment 10000 or more, and is usually 50000 or less, in one embodiment 40000 or less: for example, 1000 to 50000, in one embodiment 8000 to 50000, and in one embodiment 10000 to 40000. The amount of dispersant adsorbed on the metal particles is not limited; however, with respect to the total weight of the metal particles, it is usually 2 weight% or less, and in one embodiment 1 weight% or less. The lower limit for the amount of the dispersant is not particularly limited, since a smaller amount is preferred, but, with respect to the total weight of the metal particles, it is usually 0.1 weight% or more, in one embodiment 0.2 weight% or more, in one embodiment 0.3 weight%, and in one embodiment 0.4 weight% or more.

[0025] Conventionally, in the production of metal particles, the amount of organic substance as a dispersant required for particle diameter control during production had to be greater than the amount of organic substance required for dispersion. Accordingly, as the organic substance could not be removed, it remained in the final metal nanoparticles. According to the production method of the present invention, the amount of organic substance required for particle diameter control during production can be reduced, making it possible to produce metal particles with a reduced organic substance amount that contain only the amount of organic substance required for dispersion.

[0026] The reaction solution may further contain a reducing agent. A reducing agent is a material that can reduce metal ions to a metal having an oxidation number of 0 through an oxidation-reduction reaction.

[0027] The reducing agent is not limited. Reducing agents include, for example, citric acid or citrate, such as trisodium citrate, disodium citrate, monosodium citrate, oxalic acid or oxalate, such as sodium oxalate, ascorbic acid or ascorbate, such as sodium ascorbate, formic acid or formate, such as sodium formate, DMF, and mixtures of two or more of these. In one embodiment, the reducing agent for metal ions, especially silver ions, is DMF. Therefore, when DMF is used as the solvent for the reaction solution, a reducing agent other than DMF need not be used, since DMF can also act as a reducing agent.

[0028] The amount of the reducing agent is not limited as long as the metal ions can be reduced by an oxidation-reduction reaction to a metal having an oxidation number of 0, but with respect to the metal ions, it is usually 1.0 equivalent or more, in one embodiment 4.0 equivalents or more, and is usually 20 equivalents or less, in one embodiment 15 equivalents or less: for example, 1.0 equivalent to 20 equivalents, and in one embodiment 4.0 equivalents to 15 equivalents. The reducing agent for metal ions can also act as a dispersant if it contains 1 or more functional groups that can interact with metals, such as carboxyl, hydroxyl, and ether groups. If the reducing agent also acts as a dispersant, the reaction solution need not contain the dispersant described above, and the amount of reducing agent for the metal ions may be greater than the amount required to reduce the metal ions to metal having an oxidation number of 0 by an oxidation-reduction reaction.

[0029] While the reaction solution may be composed of the metal particle precursor, the solvent, the dispersant, and optionally the reductant described above, the reaction solution may further contain, in addition to these materials, an additive that is normally used in a conventional method for producing metal particles by microwave irradiation, such as chelating agents, for example ethylenediaminetetraacetic acid (EDTA) and / or salts thereof. The amount of additive is, although not limited, generally 10 weight% or less, and 3 weight% or less in one embodiment with respect to the total weight of the reaction solution. Since the additive is not necessarily added, the lower limit value of the additive is not limited.

[0030] The pH of the reaction solution is not particularly limited but is usually from pH 3 to pH 12.

[0031] In the present invention, the order of addition of each material, the addition temperature, the mixing method, and the mixing time in the preparation of the reaction solution are not limited, and mixing is performed such that a uniform reaction solution is prepared. In the present invention, the reaction is initiated after a homogeneous reaction solution has been prepared.

[0032] (ii) Irradiating the reaction solution with microwaves while the reaction solution is flowing. In the step (ii), the reaction solution prepared in the step (i) is irradiated with microwaves while it is flowing.

[0033] In the step (ii), the absorbed microwave power E (unit: W / mL) with respect to the reaction solution and the pressure P (unit: MPa) are adjusted to satisfy the relationship of the following Formula 1. E × P ≥ 20

[0034] In one embodiment, the absorbed microwave power E (unit: W / mL) with respect to the reaction solution and the pressure P (unit: MPa) are adjusted to satisfy the relationship of the following Formula 2. E × P ≥ 30

[0035] In one embodiment, the absorbed microwave power E (units: W / mL) with respect to the reaction solution and the pressure P (units: MPa) are adjusted to satisfy the relationship of the following Formula 3. E × P ≥ 35

[0036] Here, the absorbed microwave power E (absorbed power E of microwave) with respect to the reaction solution is calculated by dividing the intensity (W) of the microwave absorbed by the reaction solution, that is, the value expressed as (output - reflected power), namely the output of the microwave irradiation source minus the reflected power produced upon irradiation of the reaction solution-by the volume (mL) of the reaction solution irradiated at that output. The 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 by a power monitor of a microwave irradiation device.

[0037] In the present invention, by adjusting, with respect to the reaction solution, the absorbed microwave power E (unit: W / mL) and the pressure P (unit: MPa) so as to satisfy Formulae 1 to 3, the formation of nuclei of metal particles occurs simultaneously and uniformly, and growth of the formed nuclei also occurs simultaneously and uniformly, as a result of which metal particles having a small particle diameter can be obtained.

[0038] The method for flowing the reaction solution is not limited, provided that the reaction solution flows in any direction. The reaction solution can flow at a constant rate through the reaction tube, for example, a straight tube or a spiral tube.

[0039] In one embodiment, in the case where the reaction tube is a straight tube, the dimensions and shape are not limited as long as the configuration is such that the microwave is irradiated the entirety of the reaction tube uniformly. In one embodiment, the inner diameter of the tube is typically 1 mm or greater; in one embodiment, 2 mm or greater; in one embodiment, 4 mm or greater; and is 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 prism having a length of 100 mm, there is used the reaction tube having a tube inner diameter of 1 mm to 6 mm, a tube outer diameter of 3 mm to 8 mm (wall thickness: 1 mm), and a tube length of 100 mm.

[0040] The pressure P with respect to the reaction solution is not limited as long as it satisfies the above Formulae 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.

[0041] The flow velocity of the reaction solution is not limited as long as the pressure P with respect to the reaction solution satisfies Formulae 1 to 3 above, but is usually 1 m / min or higher and usually 100 m / min or lower, for example, 1 m / min to 100 m / min.

[0042] By setting the flow velocity of the reaction solution within the above range, the reaction solution can be flowed while applying a predetermined pressure to the reaction solution, and small metal particles can be formed even with little organic substance as a dispersant.

[0043] The absorbed microwave power (E) with respect to the reaction solution is not limited as long as the above Formulae 1 to 3 are satisfied, but, with respect to the volume of the reaction solution, is usually 20 W / mL or greater and usually 500 W / mL or less, for example, 20 W / mL to 500 W / mL.

[0044] When the output of the microwave irradiation source is adjusted to the above-described range, small metal particles with low organic substance amount can be formed owing to sufficient reducibility achieved by the microwave.

[0045] During microwave irradiation of the reaction solution, the reaction solution being microwaved is preferably stirred.

[0046] The stirring of the reaction solution during the reaction maintains the uniformity of the reaction solution against the non-uniformity of the reaction solution based on local changes in the concentration and viscosity of the reaction solution that can be caused by the reaction, suppresses segregation of metal particles, and also allows redissolving of the segregated metal particles.

[0047] Other microwave conditions in the method of the present invention are not particularly limited. In the method of the present invention, as explained above, the reaction is carried out by microwave irradiation using a microwave synthesis device so as to satisfy Formulae 1 to 3 while the reaction solution is flowing. When the reaction solution is irradiated with microwaves, the polar solvents contained in the reaction solution absorb the microwaves and convert them into thermal energy, thereby generating heat. Accordingly, in the reaction solution irradiated with microwaves, a uniform and rapid temperature rise occurs in the irradiated portion, and, in accordance with the temperature rise, a uniform and rapid reaction occurs.

[0048] It is preferable that microwave irradiation be uniformly applied to the target of the reaction, that is, the portion of the reaction solution in which the reaction occurs.

[0049] In microwave synthesis devices, in a vessel that contains the reaction solution, the material of the portion to which microwaves are applied is not limited as long as the reaction solution can be uniformly irradiated by microwaves. In a vessel that contains the reaction solution, the material of the portion to which microwaves are applied may, for example, when the microwaves are applied to the reaction solution from outside the reactor through the reactor, be a material that transmits the microwaves, that is, a material that does not absorb the microwaves, such as ceramic materials, glass, quartz, Teflon (registered trademark; PTFE), or silicone, namely a non-conductive material with small relative permittivity (ε) and dielectric loss angle tangent (tan δ). In the vessel that contains the reaction solution, the material of the portion that is not irradiated by the microwaves may, in addition to the above-described material, be a metal such as aluminum or stainless steel. With respect to the enclosure that houses the microwave irradiation source and the reaction tube irradiated with the microwaves, the material is not particularly limited as long as it does not permit leakage or absorption of the microwaves. Examples include a non-magnetic metal plate, such as an aluminum plate.

[0050] Microwaves are generated by a microwave irradiation source (a microwave oscillator (a magnetron)), and the microwave irradiation source can be used in either a single-mode system or a multimode system.

[0051] The frequency of microwaves generated by the microwave irradiation source may be varied as appropriate and is not particularly limited. The microwave frequency is usually 1 GHz or more, in one embodiment 2 GHz or more, and usually 10 GHz or less, for example, from 1 GHz to 10 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 supply.

[0052] The microwave is preferably uniform during irradiation, and the microwave irradiation conditions are preferably constant during microwave irradiation.

[0053] In the present invention, the temperature of the reaction solution elevated by microwave irradiation is the reaction temperature, and the reaction temperature may be appropriately varied according to the conditions of the reaction (type of solvent, pressure during the reaction, etc.) and is not particularly limited; typically, it is 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 below the boiling point of the solvent. For example, in the case where the solvent is water, the reaction temperature at atmospheric pressure is usually in the range of 25°C or higher and less than 100°C, and, in one embodiment, is 80°C to 90°C. In the present invention, since pressure is applied to the reaction solution, the reaction temperature of the present invention can be set higher than the boiling point of the solvent at atmospheric pressure. The boiling point of the solvent under an applied pressure may vary depending on the pressure. Thus, in one embodiment, the reaction temperature is at or above the boiling point of the solvent under atmospheric pressure and less than the boiling point of the solvent under the applied pressure.

[0054] By setting the reaction temperature to 25°C or higher, the reduction reaction from metal ions to metal particles occurs. By setting the reaction temperature below the boiling point of the solvent, the broadening of the particle size distribution (that is, irregularities in the particle diameter of the resulting metal particles) associated with a non-uniform reaction field that can arise due to boiling of the reaction solution is prevented. This enables the preparation of the metal particles having small and uniform particle diameters.

[0055] The microwave irradiation time for the reaction solution is the time required for the temperature of the reaction solution to reach the reaction temperature, and it may be appropriately varied depending on the reaction conditions (microwave conditions, metal type, solvent type, pressure during the reaction, reaction solution volume, reaction temperature, etc.) and is not particularly limited.

[0056] By irradiating the reaction solution with microwaves under the above-described conditions and bringing the temperature of the reaction solution to the reaction temperature, nuclei of metal particles are generated in the reaction solution.

[0057] In the method of the present invention, the reaction solution is flowed, and irradiation of the reaction solution with microwaves is usually continued until completion of the reaction.

[0058] In the method of the present invention, the reaction can be carried out at a high temperature by applying pressure to the reaction solution. Accordingly, the total reaction time can be significantly reduced as compared with the case in which no pressure is 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 application of pressure (reaction time under applied pressure / reaction time under atmospheric pressure) is usually 1.1 or greater, in one embodiment 1.5 or greater, in one embodiment 2.0 or greater, and is 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, and, for example, 5.0. Accordingly, the production method of the present invention enables significant time savings in the production of metal particles.

[0059] The completion of the reaction can be determined by observing that the absorbance derived from the metal particle precursor or the metal particles in the reaction solution no longer changes. For example, when the material of silver particle is used as the metal particle precursor, the change in absorbance of the reaction solution in the range of 280 nm to 780 nm is observed, and a time point at which the absorbance no longer changes is regarded as the completion point of the reaction.

[0060] In the method of the present invention, after the reaction is complete, the microwave irradiation of the reaction solution may be stopped, and the reaction solution may be subjected to heat retention by a heat retention device, such as a heater or a cooler.

[0061] The heat retention temperature of the reaction solution using the heat retention device is not particularly limited, but is usually at or below the reaction temperature. The lower limit of the heat retention temperature is not limited, but is usually 25°C or higher, and in one embodiment, 80°C or higher. The heat retention temperature, for example, when the solvent is water, at atmospheric pressure, is usually in a range of 25°C to less than 100°C, and in one embodiment is from 80°C to 90°C.

[0062] Switching the reaction solution from microwave irradiation to a heat retention device allows the heat retention device to adjust the reaction solution to the appropriate heat retention temperature, even if the temperature of the reaction solution become below the desired heat retention temperature at the time of the switchover.

[0063] The heat retention device used for heat retention of the reaction solution is not particularly limited provided that the temperature of the reaction solution can be maintained at the heat retention temperature, and a conventional heat retention device may be used. Heat retention devices include a heater, a heating mantle, an immersion heater, a water bath, an oil bath, a cooler, and the like.

[0064] The heat retention time for the reaction solution using the heat retention device is not particularly limited, but is usually 1 minute or more and is usually 15 minutes or less.

[0065] By keeping the reaction solution at a certain temperature with a heat retention device, the growth of the nuclei of metal particles in the resulting reaction solution can be promoted, and the metal particles can be further homogenized (aged).

[0066] The dispersion containing metal particles obtained according to the present invention may, as necessary, be subjected to separation and purification (for example, by salting out or centrifugation) by methods known in the art to obtain the desired metal particles or a dispersion containing metal particles.

[0067] The present invention also relates to a device for producing the metal particles, configured to efficiently carry out the method of the present invention.

[0068] A production apparatus 1 for producing metal particles is described below using Figs. 1 and 2. As shown in Fig. 1, the production apparatus 1 is a production apparatus for producing the metal particles by irradiating the reaction solution L with the microwave M.

[0069] The production apparatus 1 includes a containment tank 10 that contains the reaction solution L and a pump 20 that draws the reaction solution L from the containment tank 10 and delivers it under pressure. The reaction solution L flows through a path 5 from the containment tank 10 to the reaction apparatus 40. The slurry-like suspension containing metal particles produced by passage of reaction solution L through the reaction apparatus 40 flows continuously from the reaction apparatus 40 to a pressure regulating device 60, and the slurry also flows through the path 5. In this embodiment, for convenience, all of the fluid flowing through the path 5 is referred to as reaction solution L (before and after the reaction).

[0070] The production apparatus 1 includes a stirring device 30 and a reaction apparatus 40, as will be described below. The reaction apparatus 40, via the stirring device 30, includes a housing 41 in which a reaction tube 43 through which the reaction solution L flows is accommodated, and an irradiation device 42 that irradiates the reaction tube 43 within the housing 41 with the microwaves M.

[0071] Accordingly, using the irradiation device 42, both the reaction solution L, which is pumped by the pump 20 and flows in the reaction tube 43, and the reaction tube 43 can be irradiated with the microwave M. The material of the reaction tube 43 may be the same as the material of the vessel that contains the reaction solution described in the method of the present invention, and may be made of ceramic materials composed of silicon oxide such as glass or quartz, or of resin material such as PTFE. A pipe other than the path 5 forming the reaction tube 43 may also be constituted of these pipes; alternatively, because it is not irradiated by the microwave M, the pipe other than the path 5 forming the reaction tube 43 may be made of metal, such as stainless steel or aluminum. The reaction tube 43 may be a straight tube, but for example, a spiral pipe may be used. This allows the irradiation efficiency of the microwave M to be increased compared with a straight tube.

[0072] In one embodiment, in the case where the reaction tube 43 is a straight tube, the dimensions and shape are not limited, provided that the configuration allows the microwave M to uniformly irradiate the entirety of the reaction tube 43. For example, the inner diameter of the tube is usually 1 mm or more, in one embodiment 2 mm or more, in one embodiment 4 mm or more, and usually 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. For example, when the cavity is a rectangular prism having a length of 100 mm, the reaction tube 43 is preferably a reaction tube having a tube inner diameter of 1 mm to 6 mm, a tube outer diameter of 3 mm to 8 mm (wall thickness: 1 mm), and a tube length of 100 mm. The dimensions and shape of the reaction apparatus 40 are also not limited.

[0073] The production apparatus 1 includes a heat retention device 50 and the pressure regulating device 60, in that order, downstream of the reaction apparatus 40. The heat retention device 50 is an optional device and need not be installed. Since the post-reaction reaction solution L (specifically, a slurry-like suspension containing metal particles) flowing through the path 5 is heated in the reaction apparatus 40, the heat retention device 50 is a device that maintains this reaction solution L at the heat retention temperature in the method of the present invention described above, that is, from the reaction temperature down to a predetermined temperature lower than the reaction temperature. Specifically, the reaction solution L flowing in the path 5 is heated or cooled using a heat retention device, and the reaction solution L maintained at a predetermined temperature is discharged to the pressure regulating device 60. Besides the above-described heater, a cooler may also be used as the heat retention device 50.

[0074] The pressure regulating device 60, located downstream of the reaction apparatus 40 (the reaction tube 43), regulates the pressure of the reaction solution L in the reaction tube 43; in one embodiment, it regulates the pressure of the reaction solution L in the path 5 from the pump 20 to the pressure regulating device 60. Specifically, the pressure regulating device 60 adjusts the pressure of the reaction solution L in the path 5 by throttling the discharge rate of the reaction solution L passing through it. This allows, within a range that does not exceed the discharge pressure of the pump 20, the pressure of the reaction solution L in the reaction tube 43 to be increased to at or above atmospheric pressure. The reaction solution L pumped by the pressure regulating device 60 is collected in a collection tank 70.

[0075] In this way, the reaction solution L delivered by the pump 20 is pressure-fed into the reaction tube 43. The pressure of the pumped reaction solution L is regulated in a pressurized state by the pressure regulating device 60. By irradiating the reaction solution L under pressure with the microwave M using the irradiation device 42, the boiling point of the reaction solution L can be raised compared to atmospheric pressure conditions, thereby increasing the formation rate of metal particles.

[0076] Meanwhile, in the reaction solution L, when metal particles are generated, the concentration and viscosity of the reaction solution L may change locally, and the reaction solution L may pulsate slightly. This may impair the uniformity of microwave irradiation applied to the reaction solution L, which may lead to segregation of metal particles in the reaction solution L. In this regard, the production apparatus 1 includes, between the pump 20 and the reaction tube 43 of the reaction apparatus 40, the stirring device 30 that stirs the reaction solution L pumped from the pump 20.

[0077] If the segregation of metal particles in reaction solution L can be suppressed, the configuration of the stirring device 30, its structure, and the position at which it is disposed are not particularly limited. In this embodiment, the stirring device 30 is, as an example, equivalent in structure to the static mixer shown in Fig. 2.

[0078] The stirring device 30 includes a straight pipe 31 through which the reaction solution L flows, and twisted blades 32 that are fixed within the pipe 31 and twisted around the axis CL of the pipe 31. With the twisted blades 32 disposed in the pipe 31, a flow path 33 (the path 5) through which the reaction solution L flows is formed within the pipe 31.

[0079] The twisted blades 32 may be helically twisted in the same direction about the axis CL, but in this embodiment, they have the structure described below. Specifically, the twisted blades 32 are blades in which twisted blade elements 32a and 32b, having different twist directions around the axis, are alternately arranged along the axial direction of the pipe 31.

[0080] Specifically, the twisted blade element 32a and the twisted blade element 32b are each formed into a twisted shape from a flat plate. Relative to the twist direction of the twisted blade element 32a, the twisted blade element 32a is twisted in the opposite twist direction. The twisted blade element 32a and the twisted blade element 32b are alternately connected along the axial direction of the pipe 31.

[0081] Accordingly, for the reaction solution L that has passed through the twisted blades 32, a stable stirring flow is readily generated along the axial direction of the pipe 31, so segregation of metal particles in the reaction tube 43 is readily reduced, and furthermore the segregated metal particles readily undergo redissolving. In particular, it is preferable that the twisted blades be blades in which twisted blade elements 32a and 32b having different twist directions about the axis are alternately arranged along the axial direction of the pipe 31. Accordingly, flows of reaction solutions La and Lb stirred in different directions are formed, and a stirring flow around the axis is readily formed, such that segregation of metal particles in the reaction tube 43 is further reduced more readily, and the segregated metal particles are more readily redissolved.

[0082] The material of the twisted blades 32 is not particularly limited. The material of the twisted blades 32 is made of a non-conductive material that neither absorbs nor reflects microwaves when installed immediately before the reaction apparatus 40. For example, the material exemplified for the reaction tube 43 is preferred; it may be ceramic materials composed of silicon oxide such as glass, quartz, or a resin material such as PTFE. It is also preferable that the pipe 31 be made of a similar material. Thus, by forming the twisted blades 32 of a non-conductive material, it is possible to prevent the microwave M from reaching the twisted blades via the reaction solution L from the reaction tube 43. This allows efficient irradiation of the reaction solution in the reaction tube 43 with the microwave M.

[0083] Preferably, the stirring device 30 is disposed at a position closer to the reaction apparatus 40 in the path 5 between the pump 20 and the reaction apparatus 40. More specifically, the stirring device 30 is preferably disposed at a position at which the stirring flow of the reaction solution L that has passed through the stirring device 30 is sustained within the reaction tube 43. For example, it may be at a position near the inlet of the reaction apparatus 40 where the reaction solution L flows in, and it is not particularly limited to this position as long as the stirring flow is sustained.

[0084] In this way, the stirring flow of the reaction solution L through the stirring device 30 is sustained in the reaction tube 43, such that localized changes in the concentration and viscosity of the reaction solution L can be suppressed during the formation of metal particles. Thereby, within the reaction tube 43, the reaction solution L becoming non-uniform is suppressed, segregation of the metal particles in the reaction solution L is suppressed, and further, redissolving of the segregated metal particles is enabled.

[0085] Accordingly, the metal particle production apparatus 1 of the present invention can, without causing changes in physical properties such as the volume and viscosity of the reaction solution L before and after application of pressure and a decrease in the absorption rate (uniformity) of the applied microwave based on such changes in physical properties, further prevent segregation of the metal particles and redissolve segregated metal particles, thereby enabling the production method for metal particles of the present invention to be carried out efficiently.

[0086] The present invention also relates to the metal particles containing an organic substance that can be obtained by the method or production apparatus of the present invention, the metal particles having a small particle diameter and a low amount of organic substance. Here, "organic substance" is a compound that is attached to metal particles to uniformly disperse the metal particles in a solvent or a diluent; it is non-volatile and is distinguished from organic compounds used as a solvent or a diluent.

[0087] In one embodiment, the metal particles are metal nanoparticles. As used herein, "metal nanoparticles" denotes metal particles having a particle diameter typically from 1 nm to 100 nm. Accordingly, in the case where the metal particles of the present invention are metal nanoparticles, the metal particles of the present invention are metal particles in which the Heywood diameter of the entire metal particle, including a main portion and an auxiliary portion, is 1 nm to 100 nm.

[0088] In one embodiment, the metal particles have a spherical shape. Herein, "spherical" includes, when the metal particles are observed with a transmission electron microscope (TEM), not only a perfectly spherical shape but also a approximately spherical shape, an ellipsoidal shape, and a polygonal shape whose sides are substantially the same.

[0089] The median diameter (D50) of the metal particles of the present invention, as measured by TEM, is 20 nm or less, and in one embodiment is 10 nm or less. Since a smaller D50 is preferable, the lower limit thereof is not particularly limited, but is usually 1.0 nm or more.

[0090] Here, the measurement method for the D50 of metal particles by TEM is as follows.

[0091] First, a TEM image of metal particles is acquired. Next, arbitrarily select 500 metal particles in the TEM image. Next, for each of the selected metal particles, the diameter value is measured by converting the projected surface area of the metal particle to the area of a circle. They are then plotted as particle diameter, with particle diameter (nm) on the x-axis and cumulative number of pieces (%) on the y-axis. Finally, the particle diameter at which the cumulative number of pieces reaches 50% can be determined from the graph as D50.

[0092] The particle size distribution of the metal particles of the present invention has a narrow distribution.

[0093] The amount of organic substance, particularly a dispersant, contained in the metal particles (a powder formed from the metal particles) of the present invention is 2 weight% or less, in one embodiment 1 weight% or less, and in one embodiment 0.8 weight% or less, with respect to the total weight of the metal particles. The lower limit of the amount of the organic substance is not particularly limited, since less is preferable; however, it is usually 0.1 weight% or more, in one embodiment 0.2 weight% or more, in one embodiment 0.3 weight% or more, and in one embodiment 0.4 weight% or more, with respect to the total weight of the metal particles.

[0094] Herein, the measurement method for the amount of the organic substance contained in the metal particles is as follows.

[0095] First, purified water such as ion-exchanged water is added to metal particles or to a dispersion containing metal particles, and it is purified using a solid-liquid separator, such as by centrifugation, until the electrical conductivity of the slurry is several 10 µS / cm or less. The measurement of electrical conductivity may be performed using a hand-held type general-purpose type. This step is performed for the purpose of eliminating impurities, water-soluble by-products, and the like that may be present in the metal particles or in a dispersion containing metal particles. Among the organic substances added during production for use as a dispersant or for similar purposes, that which is adsorbed onto the metal particles is not removed in the relevant step. The purified metal particles are then dispersed in a low boiling point solvent (alcohol-based, for example, methanol or ethanol) to prepare a metal particle-containing slurry. The slurry is then applied to the glass surface to form a film. The film thus formed is thereafter dried in a furnace at 80°C for about 1 hour, peeled from the glass, and collected as an organic substance measurement sample. Finally, the collected organic substance measurement sample is powdered, and the C content (weight%) of the organic substance is measured using a CS analyzer (combustion method).

[0096] The metal particles of the present invention have a low organic substance amount and a small particle diameter and have excellent low-temperature sinterability. Furthermore, the sintered body formed from the metal particles has a low volume resistivity, and furthermore, the volumetric shrinkage ratio during sintering is small.

[0097] Specifically, the metal particles of the present invention have a low organic substance amount and a small median diameter. The metal particles of the present invention have a low volumetric shrinkage ratio during sintering, for example, a volumetric shrinkage ratio, based on the volume of the metal particles in a press-compacted state before sintering, which is usually 60% or less, in one embodiment 50% or less, and in one embodiment 15% or less. The lower limit of the volumetric shrinkage ratio due to sintering of metal particles in the present invention is not limited, since a smaller rate is preferable.

[0098] Furthermore, the metal particles of the present invention can be dispersed sufficiently in the solvent without the need for additional dispersants because they contain the minimum amount of organic substance as a dispersant necessary to disperse the metal particles in the solvent. This is because, in the metal particles of the present invention, the dispersant adheres to the metal particles immediately after formation of the metal particles, such that it is uniformly adhered into the metal particles, and because, even after reaching a semi-dry state or a dry condition, when added to a solvent, it can again exhibit its function as the dispersant. Thus, the metal particles of the present invention are redispersible metal particles. In the case where the metal particles contain no dispersant at all, once they reach a semi-dry state or a dry condition, redispersion is difficult, especially because the cohesive force of the nano-sized metal particles is large. In addition, when mixing metal particles containing no dispersant at all with a dispersant, once the metal particles are in a semi-dry state or a dry condition, it is difficult to mix them uniformly due to the large cohesive force of the metal particles.

[0099] The present invention also relates to a dispersion comprising the metal particles of the present invention described above, an organic substance serving as a dispersant for the metal particles, and a solvent for dispersing the metal particles and the organic substance.

[0100] In the dispersion according to 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 the metal particles to disperse in the solvent. For example, the content of metal particles, with respect to the total weight of the dispersion, is usually 1 weight% or more, in one embodiment 5 weight% or more, in one embodiment 10 weight% or more, in one embodiment 20 weight% or more, in one embodiment 50 weight% or more, in one embodiment 70 weight% or more, and is usually 95 weight% or less, in one embodiment 90 weight% or less, in one embodiment 85 weight% or less, in one embodiment 80 weight% or less, for example, from 1 weight% to 95 weight%, in one embodiment from 5 weight% to 90 weight%, and in one embodiment from 50 weight% to 80 weight%.

[0101] By increasing the content of metal particles in the dispersion of the present invention, it is possible to reduce solvent cost; the time, labor, and cost required for volatilization of the solvent after application of the dispersion; and storage and transportation costs, thereby reducing environmental impact.

[0102] In the dispersion of the present invention, the amount of an organic substance as a dispersant for the metal particles, with respect to the total weight of the metal particles, is 2 weight% or less, 1 weight% or less in one embodiment, and 0.8 weight% or less in one embodiment. The lower limit of the amount of the organic substance is not particularly limited, since less is preferable; however, with respect to the total weight of the metal particles, it is usually 0.1 weight% or more, in one embodiment 0.2 weight% or more, in one embodiment 0.3 weight% or more, and in one embodiment 0.4 weight% or more.

[0103] A small amount of dispersant in the dispersion of the present invention allows the sintered body formed from the dispersion to have low volume resistivity and a low volumetric shrinkage ratio.

[0104] The solvents contained in the dispersion of the present invention may be any known in the art and are not particularly limited; for example, they may be selected from solvents that are liquid at 20°C, water, alcohols, aldehydes, carboxylic acids, ethers, esters, amines, monosaccharides, polysaccharides, straight-chain hydrocarbons, fatty acids, and aromatic compounds, and the like. Two or more of the above solvents may be used in combination.

[0105] The boiling point of the solvent is not particularly limited, but is usually 100°C or higher, in one embodiment 130°C or higher, in one embodiment 150°C or higher, and is usually 300°C or lower, in one embodiment 250°C or lower, in one embodiment 200°C or lower: for example, 100°C to 300°C, 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 volatilization of the solvent at room temperature can be suppressed, and as a result, the viscosity stability and coatability of the ink paste can be ensured. If the boiling point of the solvent is 300°C or lower, in a bonding process involving firing, particularly non-pressurized firing, at the temperature at which the semiconductor device is connected to the support member, retention of the solvent in the sintered metal without evaporation can be suppressed, and as a result, the properties of the sintered metal can be better maintained.

[0106] As the solvent, it is preferable to select a solvent suitable for dispersion of silver particles from among the solvents mentioned above. Specifically, it is preferable to select a solvent having an alcohol structure, an ether structure, or an ester structure because it results in favorable thermal conductivity, electrical conductivity, and adhesive strength of the sintered metal. 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, isobornyl cyclohexanol, and tributyrin. Ethylene glycol is a preferred solvent contained in the dispersion of the present invention.

[0107] When the dispersion of the present invention is used as an ink paste, the amount of solvent contained in the ink paste can vary depending on the content of the metal particles contained in the dispersion and the like; however, with respect to the total weight of the ink paste, the amount of solvent is usually 5 weight% or more and usually 99 weight% or less, and in one embodiment 90 weight% or less.

[0108] By adjusting the amount of solvent within the range set forth above, the viscosity of the ink paste can be adjusted to the appropriate viscosity range set forth below, and, furthermore, volume shrinkage caused by volatilization of the solvent during sintering of the ink paste can be suppressed, thereby improving the density of the resulting silver sintered body.

[0109] The dispersion of the present invention may further contain components other than the metal particles of the present invention and solvent, to the extent that they do not interfere with the effects of the present invention. Components that may be added other than to the metal particles of the present invention and the solvent include, without limitation, materials known in the art, for example, additives such as carboxylic acids that have a boiling point of 400°C or lower under atmospheric pressure and are solid at 20°C, for example, 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 the metal particles in the dispersion; and fluxing agents for promoting sintering of the metal particles. The amount of components that may be added other than the metal particles of the present invention and the solvent is, with respect to the total weight of the dispersion, usually 0 weight% or more and usually 10 weight% or less; in one embodiment, 1 weight% or less; for example, from 0 weight% to 10 weight%; and in one embodiment, from 0 weight% to 1 weight%.

[0110] When the dispersion of the present invention is used as an ink paste, the viscosity of the ink paste, when measured with a cone-plate viscometer, is usually 10 mPa·s or more and usually 10000 Pa·s or less. As described above, the viscosity can be appropriately adjusted based on the aspect ratio and amount of plate-like silver particles, the type and amount of a polymer as a dispersant, and the type and amount of a solvent.

[0111] By adjusting the viscosity of the ink paste to the above-described range, the coatability of the ink paste can be improved, and bleeding after application of the ink paste can be prevented.

[0112] Metal particles or dispersions produced by the present invention can be used as conductive wiring materials in the field of electronics packaging, in addition to catalysts, electronic component materials, materials for inks, etc., and, for example, can reduce the number of processes for preparing inks used for wiring boards.Examples

[0113] The following is a description of several examples of the present invention, but the present invention is not intended to be limited to those examples.1. Synthesis of metal particles

[0114] Silver particles or nickel particles were produced in accordance with the conditions shown in Table 3, 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. In Table 1, the weight average molecular weight of PVP is 40000. In Table 3, E is the absorbed power (W / mL) that the reaction solution absorbed from the microwave irradiated by the irradiation device 42, which irradiates the microwave M. P is the pressure (MPa) applied to the reaction solution, as measured at the pressure regulating device 60. In the irradiation device 42 which irradiates the microwave M of the production apparatus 1, a rectangular prism having a length of 100 mm was used as the cavity, and the following one was used as the reaction tube. Reaction tube configuration Tube inner diameter: 1 mm to 6 mm Tube outer diameter: 3 mm to 8 mm (wall thickness: 1 mm) Tube length: 100 mm [Table 1] Table 1 Reaction Solution of Silver ParticlesTypeAgNO 3 PVPDMFConcentration100 mM20 mM8900 mM Solvent: Water, DMF [Table 2] Table 2 Reaction Solution of Nickel ParticlesTypeNickel FormateDDAConcentration100 mM20 mM Solvent: EG [Table 3] Table 3 Synthesis ConditionsType of MetalNumberAbsorbed Microwave Power EPressure PE×PAgExample 190 W / ml0.4 MPa36Example 292 W / ml0.4 MPa37Example 3200 W / ml0.15 MPa30Example 4200 W / ml0.1 MPa20Comparative Example 175 W / ml0.15 MPa11.3Comparative Example 233 W / ml0.4 MPa13.2Comparative Example 327 W / ml0.4 MPa10.8Comparative Example 450 W / ml0.1 MPa5Comparative Example 5150 W / ml0.1 MPa15NiExample 5200 W / ml0.1 MPa20

[0115] In these experiments, the total reaction time in the Examples was about one-fifth that of the total reaction time when the reaction was performed without applying pressure. This is because the boiling point of the reaction solution to which pressure was applied became higher than the boiling point of the reaction solution under atmospheric pressure, allowing the temperature of the reaction solution during the reaction to be set higher.2. Assessment Results(Transmission electron microscopy (TEM) evaluation)

[0116] TEM images were taken of the dispersions obtained in Examples 1 to 5 and Comparative Examples 1 to 5. As an example, Fig. 3 shows TEM images of silver particles in Comparative Example 1 and Examples 1 and 4.

[0117] Additionally, 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 arbitrarily selected in the TEM image. Next, for each selected one of the silver particles or nickel particles, the diameter value was measured when the projected surface area of the silver particle or nickel particle was converted to the area of a circle. They were then plotted as particle diameter, with particle diameter (nm) on the x-axis and cumulative number of pieces (%) on the y-axis. As an example, Fig. 4 shows the particle size distribution of silver particles in Example 1. Finally, the particle diameter at which the cumulative number of pieces reaches 50% was determined from the graph as D50. Table 4 and Fig. 5 show the results. [Table 4]Table 4 Synthesis Conditions and ResultsType of MetalNumberAbsorbed Microwave Power EPressure PE×PD50AgExample 190 W / ml0.4 MPa365.6 nmExample 292 W / ml0.4 MPa375.1 nmExample 3200 W / ml0.15 MPa3011 nmExample 4200 W / ml0.1 MPa208.2 nmComparative Example 175 W / ml0.15 MPa11.347 nmComparative Example 233 W / ml0.4 MPa13.235 nmComparative Example 327 W / ml0.4 MPa10.859 nmComparative Example 450 W / ml0.1 MPa5114 nmComparative Example 5150 W / ml0.1 MPa1524 nmNiExample 5200 W / ml0.1 MPa2012 nm

[0118] The results in Table 4 and Fig. 5 show that when the value obtained by multiplying the absorbed microwave power E (W / mL) with respect to the reaction solution by the pressure P (MPa) applied to the reaction solution (E × P) is 20 or greater, the D50 of the resulting metal particles decreases, in particular, to 15 nm or less.

[0119] Subsequently, for Examples 1, 3, and 4 and Comparative Examples 3 and 5, the organic substance amount and its relationship to D50 were measured. The organic substance amount was measured as follows.

[0120] First, purified water such as ion-exchanged water was added to the dispersion containing the examples or comparative examples, and a solid-liquid separator, such as centrifugation, was used to purify the slurry until the electrical conductivity of the slurry was several tens µS / cm or less. A handheld-type general-purpose type was used for electrical conductivity measurement. This step was carried out for the purpose of eliminating impurities, water-soluble by-products, and the like from the dispersion. The purified particles were then dispersed in a low boiling point solvent (alcohol-based, for example, methanol or ethanol) to prepare a metal particle-containing slurry. Subsequently, the slurry was applied to glass, thereby forming a film. The formed film was then dried in a furnace at 80°C for about 1 hour, peeled off from the glass, and collected as an organic substance amount measurement sample. Finally, the collected organic substance amount measurement sample was pulverized to a powder, and the organic carbon (C) content (weight%) was measured using a CS analyzer (combustion method). Fig. 6 shows the results.

[0121] Fig. 6 shows that the metal particles produced by the production method of the present invention can maintain a small D50 even when the organic substance amount in the metal particles is reduced.

[0122] Next, the volumetric shrinkage ratio during sintering at 120°C for 2 hours was measured for Examples 1, 3, and 4 and Comparative Examples 3 and 5, for which the organic substance amount had been measured. The volumetric shrinkage ratio was measured as follows. (1) First, metal particles purified with purified water such as ion-exchanged water using a solid-liquid separator such as centrifugation were collected on filter paper, and ethanol was added and filtration under reduced pressure was performed to effect solvent replacement. (2) Subsequently, the cake layer remaining on the filter paper after solvent replacement was collected, solvents such as butyl carbitol, butyl acetate, and terpineol were added to adjust the concentration of metal particles to 80 weight%, deagglomeration and kneading were performed using a rotation / revolution mixer, and a metal particle-containing paste was prepared. (3) The resulting paste was applied onto a glass slide using a metal mask having a thickness of 100 µm, a width of 15 mm, and a length of 15 mm (100 µm x 15 mm x 15 mm). (4) Firing was performed at 120°C for 2 hours in a constant-temperature chamber. (5) After firing, the film thickness was measured at 5 predetermined points using a micrometer, and the average value was used as the film thickness. (6) From the dimensions and film thickness of the metal mask and the dimensions and film thickness after firing, calculated the volume of the paste before firing and the volume of the sintered body after firing, and obtained the volumetric shrinkage ratio using the following Formula.

[0123] Fig. 7 shows the results. Fig. 7 shows that, as the organic substance amount in the metal particles decreases, the volumetric shrinkage ratio also decreases.

[0124] Further, to confirm the redispersibility of the silver particles and nickel particles produced and purified in Examples of the present invention, the silver particles and nickel particles produced and purified in Examples in a semi-dry state or a dry condition were redispersed in various solvents, for example, aqueous solvents and non-polar solvents. As a result, confirmation was obtained that the metal particles in the example were capable of redispersion in various solvents.

[0125] Figs. 8 and 9 schematically illustrate the manner of formation of metal particles based on the results of the above-described comparative examples and examples.

[0126] Fig. 8 schematically shows, either by conventional techniques or when E×P is less than 20, the formation of the nuclei of metal particles (silver particles as an example) and the growth of the particles. As shown in Fig. 8, when the absorbed microwave power is low and no pressure is applied, the reaction temperature is low, the formation of the nuclei is also non-uniform, and the nuclei are of varying sizes. As a result, nuclei with different sizes are present in the reaction solution, and the smaller nuclei that are formed later adhere to the larger nuclei that have been formed earlier, resulting in larger particles.

[0127] Fig. 9 schematically illustrates the progression from formation of the nuclei of metal particles (silver particles as an example) to growth of the particles when E × P is 20 or greater in the present invention. As shown in Fig. 9, when the absorbed microwave power is high and the pressure is applied, the reaction temperature becomes higher, the formation of nuclei also occurs uniformly, and no variation in nucleus size occurs. As a result, in the presence of nuclei with uniform size, nuclei with similar size adhere to each other and grow, resulting in small particles. The dispersant required to disperse the metal particles present in the reaction solution can also be uniformly attached to the uniformly formed particles, and a the minimum amount of dispersant will be sufficient. Furthermore, by conducting the reaction in a high-temperature atmosphere, the reaction time can be significantly shortened, for example, to 1 / 5. The method for forming metal particles of the present invention is characterized in that the reaction temperature is increased by applying a certain level or more of absorbed microwave power and pressure with respect to the reaction solution, thereby causing the reduction of metal ions by microwave heating all at once. The types of metal particles produced by the method of the present invention are not limited as long as the metal ions are reduced by microwave heating, and can include not only the above-described metal particles, that is, silver particles and nickel particles, but also, for example, particles of gold, platinum, copper, iron, or cobalt.

[0128] All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entireties.

Examples

examples

[0113]The following is a description of several examples of the present invention, but the present invention is not intended to be limited to those examples.

1. Synthesis of metal particles

[0114]Silver particles or nickel particles were produced in accordance with the conditions shown in Table 3, 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. In Table 1, the weight average molecular weight of PVP is 40000. In Table 3, E is the absorbed power (W / mL) that the reaction solution absorbed from the microwave irradiated by the irradiation device 42, which irradiates the microwave M. P is the pressure (MPa) applied to the reaction solution, as measured at the pressure regulating device 60. In the irradiation device 42 which irradiates the microwave M of the production apparatus 1, a rectangular prism having a length of 100 mm was used as the cavity, and the following one was used as the reaction tube. Rea...

Claims

1. A method for producing metal particles including irradiating a reaction solution with microwave, the method comprising: (i) a step of preparing a reaction solution comprising a metal particle precursor, an organic substance as a dispersant, and a solvent; and (ii) a step of irradiating the reaction solution with microwaves while the reaction solution is flowing, a relationship between an absorbed power E of the microwaves (unit: W / mL) and a pressure P (unit: MPa) with respect to the reaction solution satisfying Formula 1 below, E × P ≥ 202. The method according to claim 1, wherein in the step (i), an amount of the organic substance in the reaction solution is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particle precursor as metal.

3. The method according to claim 1 or 2, wherein in the step (ii), the relationship between the absorbed power E of the microwave (unit: W / mL) and the pressure P (unit: MPa) with respect to the reaction solution satisfies Formula 2 below, E × P ≥ 304. Metal particles comprising an organic substance, wherein an amount of the organic substance is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particles, and wherein a median diameter (D50) 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 a content of the metal particles is from 1 weight% to 95 weight% with respect to a total weight of the metal particle dispersion, and wherein an amount of the organic substance is from 0.1 weight% to 2 weight% with respect to a total weight of the metal particles, wherein a median diameter (D50) of the metal particles by TEM is 20 nm or less.

6. An apparatus for producing metal nanoparticles by irradiating a reaction solution with microwaves, the apparatus comprising: a pump that pumps the reaction solution; an irradiation device that irradiates the reaction solution pumped from the pump and flowing in a reaction tube together with the reaction tube with the microwaves; a pressure regulating device that regulates a pressure of the reaction solution in the reaction tube, downstream of the reaction tube; and a stirring device that stirs the reaction solution pumped from the pump between the pump and the reaction tube.

7. The apparatus for producing the metal nanoparticles according to claim 6, wherein the stirring device is positioned between the pump and the reaction tube where a stirring flow of the reaction solution having passed through the stirring device is sustained in the reaction tube.

8. The apparatus for producing the metal nanoparticles according to claim 6 or 7, wherein the stirring device includes a linear pipe through which the reaction solution flows and a twisted blade fixed within the pipe and twisted around an axis of the pipe.

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

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

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