Metal particles, methods for manufacturing metal particles, and dispersions containing metal particles

Abstract

This invention provides a metal particle with low volume resistivity at a low firing temperature, a method for manufacturing the same, and a dispersion containing the metal particle. The invention relates to a metal particle having a main portion and auxiliary portions extending from the main portion, characterized in that the main portion is spherical and the auxiliary portions are plate-shaped or linear and thinner than the main portion.

Description

Metal particles, method for producing metal particles, and dispersion containing metal particles The present invention relates to metal particles, a method for producing metal particles, and a dispersion containing metal particles. By setting to a nano size, metal nanoparticles that sometimes have properties different from those of bulk materials are being used and studied in various applications such as catalysts, biomaterials, materials for inks, and components of electronic parts. For example, Patent Document 1 discloses a composite metal microparticle material, which is characterized in that spherical silver (Ag) nanoparticles synthesized using a silver (Ag) compound, a solvent, a reducing agent, and a dispersant are mixed with a conductive filler composed of non-spherical metal microparticles at a ratio within the range of 99:1 to 80:20. The aforementioned dispersant is a compound having at least any one of a mercapto group (-SH) and an amino group (-NH 2 ) Among them, and the addition amount of the aforementioned dispersant is 0.5 mol amount or more and 3.0 mol amount or less relative to the addition amount of the aforementioned silver (Ag) compound. Patent Document 2 discloses a conductive wiring material, which includes a plurality of first metal nanoparticles and a plurality of second metal nanoparticles having a particle diameter smaller than that of the plurality of first metal nanoparticles. By low-temperature firing, the second metal nanoparticles are melted to fill the spaces between the first metal nanoparticles. Patent Document 3 discloses silver fine powder, which is composed of silver particles having an average particle diameter D TEM : 3 to 20 nm covered with an organic protective material composed of a primary amine B having 6 to 12 carbon atoms. Patent Document 4 discloses a method for producing metal microparticles, in which a reaction solution containing a metal precursor is caused to flow into a flow tube, and electromagnetic waves are uniformly and intensively irradiated toward the inside of the flow tube over the length direction of the flow tube, and the electromagnetic wave irradiation space inside the flow tube is uniformly heated over the flow direction to generate metal microparticles. [Prior Art Documents] [Patent Documents] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-038141 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-279038 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-138242 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-137226 [Problems to be Solved by the Invention] In the field of electronic product mounting, in the composite metal fine particle material described in Patent Document 1 and the metal nanoparticles of the conductive wiring material described in Patent Document 2, by using a mixture of two types of fine particles having different particle diameters or shapes, the porosity of the sintered body formed by sintering the mixture is reduced. As a result, a low volume resistivity can be obtained. In such a mixture of two types of metal nanoparticles having different particle diameters and shapes, it is difficult to control the ratio of the two types of different metal nanoparticles to a desired ratio by a single synthesis. Accordingly, as usually described in Patent Document 1, two types of metal nanoparticles having different particle diameters and shapes are synthesized separately, classified as necessary, and further, it is necessary to uniformly mix them. However, in the manufacturing method of this mixture, since the number of steps increases, there are problems of low productivity and high cost. Also, for a mixture of two types of metal nanoparticles having different particle diameters and shapes, the firing temperature required to obtain a low volume resistivity is 180 ° C even if it is lowered, and it cannot be used for printing and firing on polyethylene terephthalate (PET) and polycarbonate (PC). On the other hand, as described in Patent Document 3, by using nanoparticles in the sintered body, the firing temperature can be lowered. However, in order to obtain nanoparticles with a small particle diameter, a large amount of protective agent is required. The organic protective agent that can remain in the particles hinders sintering and limits the effect of reducing the firing temperature. Accordingly, for such nanoparticles, the firing temperature is 120 ° C even if it is lowered, and firing at a temperature of 100 ° C or lower is difficult. Also, as described in Patent Document 4, by irradiating electromagnetic waves in a flow system, metal nanoparticles having a uniform particle size distribution of 100 nm or less can be synthesized. However, by generating spherical nanoparticles with a small particle diameter, the same problem as in Patent Document 3 occurs. Therefore, an object of the present invention is to provide metal particles having a low volume resistivity at a low firing temperature, a method for manufacturing the same, and a dispersion containing the metal particles. [Means for Solving the Problem] As a result of the inventors of the present invention studying various means for solving the above problems, it was found that in a method for manufacturing metal particles using a flow-through microwave synthesis apparatus, by setting the absorption power of microwaves for the reaction solution to 15 W / mL or more and setting the flow rate of the reaction solution to 1.0 m / minute or more, fine particles having two different shapes (structures) in one particle can be synthesized with a uniform particle size distribution, and the obtained fine particles have a volume resistivity of 30 Ω·cm or less even at a firing temperature of 80 ° C, and the present invention was completed. That is, the gist of the present invention is as follows. (1) A metal particle having a main part and an auxiliary part extending from the main part, the main part being spherical, the auxiliary part being plate-shaped or linear, and the thickness of the auxiliary part being thinner than the Heywood diameter of the main part. (2) A metal particle having a main part and an auxiliary part extending from the main part, the main part being spherical, the auxiliary part being spherical, and the Heywood diameter of the auxiliary part being smaller than the Heywood diameter of the main part. (3) A metal particle having a main part and an auxiliary part extending from the main part, the heat capacity of the auxiliary part being smaller than the heat capacity of the main part. (4) A metal particle having a main part and an auxiliary part extending from the main part, in the TEM image of the main part, the value of T / λ at the center is 1.2 times or more the value of T / λ at the end, and in the TEM image of the auxiliary part, the value of T / λ at the center is less than 2.0 times the value of T / λ at the end. The center means the centroid of the projected area of the main part or the auxiliary part, and the end means the edge area of the projected area of the main part or the auxiliary part. T represents the transmittance of the electron beam, and λ represents the wavelength of the electron beam. (5) A metal particle having a main part and an auxiliary part extending from the main part, the main part being spherical, the Heywood diameter of the main part being 2 nm to 100 nm, the auxiliary part being plate-shaped, the thickness of the auxiliary part being 2 nm to 40 nm, the length of the auxiliary part being 2 nm to 60 nm, the width of the auxiliary part being 2 nm to 60 nm, and the Heywood diameter of the main part being 1.1 times to 8.0 times the thickness of the auxiliary part. (6) The metal particle according to any one of (1) to (5), wherein the metal is silver. (7) A dispersion containing the metal particle according to any one of (1) to (6) and a solvent. (8) A powder containing the metal particle according to any one of (1) to (6), characterized in that the aforementioned metal particle is 10% or more based on the total number of metal particles contained in the powder. (9) A method for producing a metal particle including the step of irradiating a reaction solution with microwaves, which includes (i) a step of preparing a reaction solution containing a metal particle precursor and a solvent, and (ii) a step of irradiating microwaves while flowing the aforementioned reaction solution. The flow rate of the reaction solution is 1.0 m / minute or more, and the absorption power of microwaves for the reaction solution is 15 W / mL or more in the microwave irradiation part. (10) The method according to (9), wherein in the step (ii), the flow rate of the aforementioned reaction solution is 4 m / minute to 20 m / minute. (11) The method according to (9) or (10), wherein in the step (ii), the inner diameter of the reaction tube through which the aforementioned reaction solution flows is 1 mm to 20 mm.(12) The method according to any one of (9) to (11), wherein in the step (i), the metal particle precursor is a silver particle precursor. [Inventive Effect]. The present invention provides metal particles having a low volume resistivity at a low firing temperature, a method for producing the same, and a dispersion containing the metal particles. Hereinafter, preferred embodiments of the present invention will be described in detail. In this specification, the features of the present invention will be described with appropriate reference to the drawings. In the drawings, for clarity, the dimensions and shapes of each part are exaggerated and the actual dimensions and shapes are not correctly depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. Further, the metal particles, the method for producing metal particles, and the dispersion containing metal particles of the present invention are not limited to the following embodiments, and various forms such as changes and improvements that can be implemented by those having ordinary knowledge in the technical field of the present invention without departing from the gist of the present invention can be implemented. Also, although each embodiment described in this specification is independent, by combining two or more embodiments, it can also be defined as an embodiment of the present invention. The "~" described in this specification is used in the sense of a range having the numerical values described before and after it as the lower limit value and the upper limit value. In the numerical value ranges described step by step in this specification, the upper limit value or the lower limit value of a numerical value range described can be replaced with the upper limit value or the lower limit value described in other steps. The upper limit value or the lower limit value of the numerical value range described in this specification can be replaced with the value shown in the examples. The present invention relates to a metal particle having a main part and an auxiliary part extending from the main part. Here, the so-called "main part" means the main part of the metal particle. The so-called "auxiliary part" means a part extending from the main part and having any physical property smaller than that of the main part. In one embodiment, the thickness of the main part is thicker than the thickness of the auxiliary part. Here, the so-called "thickness" means the shortest length passing through the center of gravity of the main part or the auxiliary part. For example, if it is spherical, it means the diameter of the sphere, and if it is plate-shaped, it means the length in the direction perpendicular to the broad surface. For example, the thickness of the main part is usually 1.2 times or more, in one embodiment, 1.5 times or more, and usually 20 times or less, in one embodiment, 10 times or less, for example, 1.2 times to 8.0 times, in one embodiment, 1.5 times to 5.0 times the thickness of the auxiliary part. In one embodiment, the volume of the main part is larger than the volume of the auxiliary part. For example, the volume of the main part is usually 1.2 times or more, and in one embodiment, 1.5 times or more the volume of the auxiliary part. Also, since the smaller the auxiliary part, the more sintering is promoted, the upper limit of the ratio of the volume of the main part to the volume of the auxiliary part (volume of the main part / volume of the auxiliary part) is not limited, for example, 50 or less, and in one embodiment, 30 or less. In one embodiment, the weight of the main part is greater than the weight of the auxiliary part. For example, the weight of the main part is usually 1.2 times or more, and in one embodiment, 1.5 times or more the weight of the auxiliary part. Also, since the smaller the auxiliary part, the more sintering is promoted, when the volumes are the same, the upper limit of the ratio of the weight of the main part to the weight of the auxiliary part (weight of the main part / weight of the auxiliary part) is not limited, for example, 50 or less, and in one embodiment, 30 or less. In one embodiment, the heat capacity of the main part is greater than the heat capacity of the auxiliary part. Here, the so-called "heat capacity" is a state quantity that represents the amount of heat required to raise the temperature of an object by 1°C when heat enters or exits the object. The so-called "the heat capacity of the main part is greater than the heat capacity of the auxiliary part" means that when heat is applied to the metal particles, the temperature change of the main part is smaller than the temperature change of the auxiliary part. For example, the heat capacity of the main part is usually 1.2 times or more, and in one embodiment, 1.5 times or more the heat capacity of the auxiliary part. Also, the upper limit of the ratio of the heat capacity of the main part to the heat capacity of the auxiliary part (heat capacity of the main part / heat capacity of the auxiliary part) is not limited, for example, 50 or less, and in one embodiment, 30 or less. By having the heat capacity of the main part be larger than that of the auxiliary part, in the metal particles of the present invention having a main part and an auxiliary part, when the same amount of heat is applied to the main part and the auxiliary part, compared with the main part, since the auxiliary part raises the temperature more rapidly and reaches the melting point, the melted auxiliary part can be used as a bonding and joining layer for sintering and joining the metal particles having the auxiliary part and one or more other metal particles close to the metal particles. As a result, the metal particles of the present invention, the powder and the dispersion containing the metal particles of the present invention described below, even at a low firing temperature, have a low volume resistivity. Here, since one metal particle of the present invention can be sintered and joined with one or more other metal particles close to it, the metal particles of the present invention contained in the powder and the dispersion of the present invention described below can exhibit this effect by including a certain proportion in all of the metal particles they contain. Further, in the metal particles of the present invention, even when any physical property of the main part, for example, one or more physical properties selected from the group consisting of thickness, weight, and volume, is larger than that of the auxiliary part, the same effect as described above can be said to be exhibited. In one embodiment, in the transmission electron microscope (TEM) image of the main part, the value of T / λ at the center is 1.5 times or more, and in one embodiment, 2 times or more, of the value of T / λ at the end. Here, the center means the center of gravity of the projected area of the main part, the end means the edge area of the projected area of the main part, T is the transmittance of the electron beam, and λ represents the wavelength of the electron beam. Further, any one or more points can be selected for the end. In one embodiment, when the main part is spherical, T / λ decreases as it moves away from the center. In one embodiment, in the TEM image of the auxiliary part, the value of T / λ at the center is less than 2 times, and in one embodiment, 1.5 times or less, of the value of T / λ at the end. Here, the center means the center of gravity of the projected area of the auxiliary part, the end means the edge area of the projected area of the auxiliary part, T is the transmittance of the electron beam, and λ represents the wavelength of the electron beam. Further, any one or more points can be selected for the end. In one embodiment, when the auxiliary part is plate-shaped or linear, in the TEM image of the auxiliary part, the value of T / λ at the center and the value of T / λ at the end become about the same. In one embodiment, the metal particles of the present invention have a spherical main portion and a plate-like or linear auxiliary portion. In one embodiment, the auxiliary portion is formed into a plate-like or linear shape from a plurality of spheres, for example, two or more spheres. In one embodiment, the particle diameter of the main portion is larger than the thickness of the auxiliary portion. Here, the particle diameter means the Heywood diameter. The so-called "Heywood diameter" means the equivalent diameter of the projected area circle of the sphere when the object is regarded as a sphere in a photograph such as a TEM image. For example, the Heywood diameter of the main portion is usually 1.1 times or more, in one embodiment 1.2 times or more, in one embodiment 1.5 times or more, and usually 20 times or less, in one embodiment 10 times or less, for example 1.1 times to 8.0 times, in one embodiment 1.2 times to 8.0 times, in one embodiment 1.5 times to 5.0 times the thickness of the auxiliary portion. The metal particles of the present invention have a spherical main portion and a plate-like or linear auxiliary portion having a thickness smaller than the Heywood diameter of the main portion. As shown in FIG. 5, when the metal particles overlap each other, the auxiliary portion enters the gaps between the metal particles and is used as a bonding / adhesion layer for sintering the main portions to each other, thereby promoting sintering. The details of this effect are the same as those produced when the heat capacities of the main portion and the auxiliary portion are different. In one embodiment, the metal particles of the present invention have a spherical main portion and a spherical, for example, ellipsoidal or spindle-shaped auxiliary portion. In one embodiment, the Heywood diameter of the main portion is larger than the Heywood diameter of the auxiliary portion. For example, the Heywood diameter of the main portion is usually 1.1 times or more, in one embodiment 1.2 times or more, in one embodiment 1.5 times or more, and usually 20 times or less, in one embodiment 10 times or less, for example 1.1 times to 8.0 times, in one embodiment 1.2 times to 8.0 times, in one embodiment 1.5 times to 5.0 times the Heywood diameter of the auxiliary portion. The metal particles of the present invention have a spherical main portion and a spherical auxiliary portion having a Heywood diameter smaller than the Heywood diameter of the main portion. When the metal particles overlap each other, the auxiliary portion enters the gaps between the metal particles and is used as a bonding / adhesion layer for sintering the main portions to each other, thereby promoting sintering. The details of this effect are the same as those produced when the heat capacities of the main portion and the auxiliary portion are different. Regarding the positional relationship between the main portion and the auxiliary portion in the metal particles of the present invention, it is not limited as long as the auxiliary portion extends from the main portion. In one embodiment, the main part and the auxiliary part are joined to each other by a joint part having a certain area. Here, since the main part and the auxiliary part have different thicknesses, they can be discriminated by, for example, the difference in contrast in the TEM image. Further, when the metal particles of the present invention are observed by TEM, the metal particles of the present invention in the TEM image are observed as a plan view image when the image observed in the direction corresponding to the thickness of the auxiliary part that can understand the structure of the metal particles of the present invention is taken as a front view. In one embodiment, the angle formed by the contact plane in contact with the main part in the joint part of the main part and the auxiliary part and the auxiliary part is usually 0° (that is, the auxiliary part extends in the direction of the contact plane) to 90° (that is, the auxiliary part extends in the manner farthest from the main part or the contact plane). In other words, the auxiliary part can extend in any direction from the main part. FIG. 1 schematically shows the positional relationship between the main part and the auxiliary part in one embodiment of the metal particles of the present invention. Further, FIG. 1 is a view (hereinafter, also referred to as a "front view") observed in the direction corresponding to the thickness of the auxiliary part so that the metal particles of the present invention can be understood. In one embodiment, as shown in (1) of FIG. 1, the angle formed by the contact plane in contact with the main part in the joint part of the main part and the auxiliary part and the auxiliary part is 90°. In one embodiment, as shown in (2) of FIG. 1, the angle formed by the contact plane in contact with the main part in the joint part of the main part and the auxiliary part and the auxiliary part is 45°. In one embodiment, as shown in (3) of FIG. 1, the angle formed by the contact plane in contact with the main part in the joint part of the main part and the auxiliary part and the auxiliary part is 0°. In the metal particles of the present invention, the crystal lattices of the main part and the auxiliary part are not limited. The number of auxiliary parts relative to one main part can be one or more, for example, one, two, three, or four or more. In one embodiment, the metal particle is composed of one auxiliary part relative to one main part. Further, the Heywood diameter, volume, weight, and heat capacity of the main part and the auxiliary part of the metal particle can be calculated based on the image observed by TEM, the type of metal of each part of the metal particle, and its properties. In one embodiment, the metal particle is a metal nanoparticle. Here, the so-called "metal nanoparticle" means a metal particle having a particle diameter generally of 1 nm to 100 nm. Accordingly, when the metal particle of the present invention is a metal nanoparticle, the metal particle of the present invention is a metal particle having a Heywood diameter of 1 nm to 100 nm for the entire metal particle including the main part and the auxiliary part. In one embodiment, the main part is spherical. Here, "spherical" includes not only a true sphere but also a slightly spherical shape, an ellipsoidal shape, a polygonal shape with almost equal sides, etc. when the main part is observed by a transmission electron microscope (TEM). In one embodiment, the particle diameter (Heywood diameter) of the main part is not limited, usually 2 nm or more, in one embodiment, 5 nm or more, and usually 100 mm or less, in one embodiment, 70 nm or less, in one embodiment, 60 nm or less, for example, 2 nm to 100 nm, in one embodiment, 5 nm to 70 nm, in one embodiment, 5 nm to 60 nm, in one embodiment, 6 nm to 60 nm. Here, the particle diameter (Heywood diameter) of the main part can be obtained, for example, by selecting 100 or more metal particles having a main part and an auxiliary part in an arbitrary TEM image, individually measuring the value of the diameter when the projected surface area of the metal particle is converted into the area of a circle, and averaging these values. In one embodiment, the auxiliary part is a plate-like or linear shape that is thinner than the thickness of the main part. Here, "thinner than the thickness of the main part" means having a thickness thinner than the Heywood diameter of the main part. The so-called "plate-like" refers to a flat shape having a thickness thinner than the length and width, and the shape in the two directions of the length and width is not limited. For example, circular, slightly circular, elliptical, polygonal shapes such as triangular, quadrangular (including square, rectangular, and trapezoidal), pentagonal, hexagonal, etc. can be cited. Further, the so-called "length" of the plate-like shape is the distance in the direction away from the main part of the auxiliary part, and the so-called "width" of the plate-like shape is the distance in the direction perpendicular to the length of the auxiliary part. In one embodiment, when the auxiliary part is plate-like, it can be defined as a shape with a small width and thickness, respectively showing the same size. The so-called "linear" means that the cross-section perpendicular to the length direction of the auxiliary part shows a circular, slightly circular, elliptical, or polygonal shape with almost equal sides. As described above, the auxiliary part can be formed into a plate-like or linear shape from a plurality of, for example, two or more spheres. FIG. 2 schematically shows an embodiment of the metal particle of the present invention. Further, FIG. 2 is a front view of the metal particle of the present invention. In one embodiment, the thickness of the auxiliary part as shown in FIG. 2 is not limited, usually 1 nm or more, in one embodiment, 2 nm or more, in one embodiment, 5 nm or more, and usually 60 mm or less, in one embodiment, 40 nm or less, for example, 1 nm to 40 nm, in one embodiment, 2 nm to 40 nm, in one embodiment, 5 nm to 40 nm, in one embodiment, 5 nm to 20 nm. In one embodiment, the length of the auxiliary portion shown in FIG. 2 is not limited, and is generally 2 nm or more, in one embodiment, 3 nm or more, in one embodiment, 5 nm or more, and generally 80 nm or less, in one embodiment, 60 nm or less, for example, 2 nm to 60 nm, in one embodiment, 2 nm to 40 nm, in one embodiment, 5 nm to 30 nm. FIG. 3 schematically shows an embodiment of the metal particles of the present invention. Incidentally, (1) in FIG. 3 is a front view of the metal particles of the present invention, and (2) in FIG. 3 is a plan view of the metal particles of the present invention. In one embodiment, when the shape of the plate-like auxiliary portion is a quadrangle, the thickness of the auxiliary portion shown in FIG. 3(1) is not limited, and is generally 1 nm or more, in one embodiment, 2 nm or more, in one embodiment, 5 nm or more, and generally 60 nm or less, in one embodiment, 40 nm or less, for example, 1 nm to 40 nm, in one embodiment, 2 nm to 40 nm, in one embodiment, 5 nm to 40 nm, in one embodiment, 5 nm to 20 nm. Also, the length of the auxiliary portion shown in FIG. 3(1) is not limited, and is generally 2 nm or more, in one embodiment, 3 nm or more, in one embodiment, 5 nm or more, and generally 80 nm or less, in one embodiment, 60 nm or less, for example, 2 nm to 60 nm, in one embodiment, 2 nm to 40 nm, in one embodiment, 5 nm to 30 nm. Further, the width of the auxiliary portion shown in FIG. 3(2) is not limited, and is generally 2 nm or more, in one embodiment, 5 nm or more, and generally 100 nm or less, in one embodiment, 80 nm or less, in one embodiment, 70 nm or less, for example, 2 nm to 70 nm, in one embodiment, 2 nm to 60 nm, in one embodiment, 5 nm to 50 nm. Incidentally, the ratio of the length to the width (length / width) of the auxiliary portion is not limited, and is generally 0.4 or more, in one embodiment, 0.5 times or more, and generally 10 times or less, in one embodiment, 5 times or less, for example, 0.5 times to 5 times, in one embodiment, 1 times to 5 times. Also, the ratio of the length of the auxiliary portion to the Heywood diameter of the main portion (length of the auxiliary portion / Heywood diameter of the main portion) is not limited, and is generally 0.2 or more, in one embodiment, 0.5 or more, and generally 4 or less, in one embodiment, 3 or less, for example, 0.2 to 4, in one embodiment, 0.5 to 3. FIG. 4 schematically shows an embodiment of the metal particles of the present invention. Here, (1) of FIG. 4 is a front view of the metal particles of the present invention, and (2) of FIG. 4 is a plan view of the metal particles of the present invention. In one embodiment, when the shape of the plate-like auxiliary part is circular, the equivalent circle diameter of the auxiliary part shown in (1) and (2) of FIG. 4 is not limited, and is usually 2 nm or more, and in one embodiment, is 5 nm or more, and is usually 100 nm or less, and in one embodiment, is 80 nm or less, for example, is 2 nm to 60 nm, and in one embodiment, is 5 nm to 30 nm. Here, the length, thickness, length, width or equivalent circle diameter of the auxiliary part is the same as the thickness (Heywood diameter) of the main part. For example, 100 or more metal particles having a main part and an auxiliary part in any TEM image can be selected. For each of the selected metal particles, the length, thickness, length, width or equivalent circle diameter of the auxiliary part of the metal particle is measured, and these are averaged and calculated. The thickness of the auxiliary part can be calculated, for example, by comparing the electron beam transmittance to electron beam wavelength ratio (T / λ) of TEM-EELS at the center of the main part (i.e., the electron beam transmittance to electron beam wavelength ratio (T / λ) of a length corresponding to the Heywood diameter) with the electron beam transmittance to electron beam wavelength ratio (T / λ) of TEM-EELS at the center of the auxiliary part. The metal particles of the present invention have a spherical main part and a plate-like or linear auxiliary part. The spherical main part has the dimensions as described above, and the plate-like or linear auxiliary part has a thickness smaller than the Heywood diameter of the main part. As shown in FIG. 5, when the metal particles overlap each other, the auxiliary part enters the gaps between the metal particles and serves as a bonding and joining layer for sintering the main parts, which can promote sintering. The details of this effect are the same as those produced when the heat capacities of the main part and the auxiliary part are different. In one embodiment, the auxiliary part is spherical. Here, "spherical" when observing the main part through a transmission electron microscope (TEM) includes not only a true spherical shape, but also a slightly spherical shape, an elliptical spherical shape, a polygonal shape with almost the same sides, etc. In one embodiment, the particle diameter (Heywood diameter) of the auxiliary part is not limited, and is usually 1 nm or more, and in one embodiment, is 2 nm or more, and in one embodiment, is 5 nm or more, and is usually 60 nm or less, and in one embodiment, is 40 nm or less, for example, is 1 nm to 40 nm, and in one embodiment, is 5 nm to 40 nm, and in one embodiment, is 5 nm to 20 nm. Here, for the particle size (Heywood diameter) of the auxiliary part, for example, 100 metal particles having a main part and an auxiliary part arbitrarily in the TEM image can be selected. For each of the selected metal particles, the value of the diameter when the projected surface area of the metal particle is converted into the area of a circle is measured, and these are averaged to obtain the result. The metal particles of the present invention have a spherical main part and a plate-like or linear auxiliary part. The spherical main part has the dimensions as described above. The plate-like or linear auxiliary part has a Heywood diameter smaller than the Heywood diameter of the main part. When the metal particles overlap each other, the auxiliary part enters the gaps between the metal particles and serves as a bonding / adhesion layer for sintering the main parts to each other, thereby promoting sintering. The details of this effect are the same as those produced when the heat capacities of the main part and the auxiliary part are different. The metal of the metal particles is not limited. Examples of the metal include noble metals, base metals, and alloys, such as gold, silver, platinum, copper, nickel, iron, cobalt, and alloys of two or more of these. As the metal, silver or nickel is preferred, and silver is more preferred. In the metal particles, the main part and the auxiliary part are usually made of the same metal. The metal particles of the present invention have the main part and the auxiliary part as described above. As shown in FIG. 5, when the metal particles overlap each other, the auxiliary part enters the gaps between the metal particles and serves as a bonding / adhesion layer for sintering the main parts to each other, thereby promoting sintering. The details of this effect are the same as those produced when the heat capacities of the main part and the auxiliary part are different. Furthermore, the present invention relates to a powder (powder) containing the metal particles of the present invention as described above. In the powder of the present invention, the metal particles of the present invention usually account for 2% or more, in one embodiment, 10% or more, in one embodiment, 18% or more, and in one embodiment, 20% or more with respect to the total number of metal particles contained in the powder. Also, the upper limit of the metal particles of the present invention in the powder is not limited because the higher the proportion of the metal particles of the present invention, the better. Here, in the powder of the present invention, the proportion of the metal particles of the present invention in the powder can be obtained by, for example, selecting 300 or more arbitrary metal particles in the TEM image and calculating the proportion of the metal particles having a main part and an auxiliary part among all the selected metal particles. The components other than the metal particles of the present invention in the powder of the present invention are not limited. In one embodiment, the powder of the present invention may contain the same metal as the metal particles of the present invention and metal particles having a shape different from that of the metal particles of the present invention, in addition to the metal particles of the present invention. As the metal particles having a shape different from that of the metal particles of the present invention, there is no limitation, and for example, metal particles having a spherical shape, a plate shape, a linear shape, etc. may be mentioned. The size of the metal particles having a shape different from that of the metal particles of the present invention is not limited, and is usually the same as the size of the metal particles of the present invention, particularly the size of the main part of the metal particles of the present invention. As the metal particles having a shape different from that of the metal particles of the present invention, it is preferably a metal particle that does not have an auxiliary part in the metal particle of the present invention, that is, a particle composed only of the main part of the metal particle of the present invention. The powder of the present invention may contain an organic substance as a dispersant (also referred to as a "protective agent") for dispersion during manufacturing or in a solvent. As the dispersant, those described below may be mentioned. The content of the dispersant is not limited. The content of the dispersant depends on, for example, the amount of the dispersant used in the method for manufacturing the metal particles of the present invention described below. The powder of the present invention contains metal particles having a main part and an auxiliary part extending from the main part, and is excellent in low-temperature sinterability. For example, it has excellent sinterability at 80°C or lower. Furthermore, the sintered body formed from the powder has a low volume resistivity, for example, 30 μΩ·cm or less, in one embodiment, 20 μΩ·cm or less, and in one embodiment, 15 μΩ·cm or less. The present invention also relates to a dispersion containing the metal particles or powder of the present invention described above and a solvent for dispersing the metal particles or powder. The content of the metal particles contained in the dispersion is not limited. For example, the content of the metal particles is usually 1% by weight or more, in one embodiment, 5% by weight or more, in one embodiment, 10% by weight or more, in one embodiment, 20% by weight or more, in one embodiment, 50% by weight or more, in one embodiment, 70% by weight or more, relative to the total weight of the dispersion, and is usually 95% by weight or less, in one embodiment, 90% by weight or less, in one embodiment, 85% by weight or less, in one embodiment, 80% by weight or less. For example, it is 1% by weight to 95% by weight, in one embodiment, 5% by weight to 90% by weight, and in one embodiment, 50% by weight to 80% by weight. By increasing the content of the metal particles in the dispersion of the present invention, the cost of the solvent, the time, labor, and cost consumed by the volatilization of the solvent after coating the dispersion, as well as the storage cost and transportation cost, etc. can be reduced, and the environmental load can be reduced. The dispersion of the present invention may contain an organic substance as a dispersant during the production of the metal particles or powder of the present invention or during the dispersion of the solvent. As the dispersant, those described below can be cited. The content of the dispersant is not limited. The content of the dispersant depends on the amount of the dispersant used in, for example, the method for producing the metal particles of the present invention described below. The solvent contained in the dispersion of the present invention can be a solvent known in the art and is not limited. It can be selected from, for example, solvents that are liquid at 20°C, such as water, alcohols, aldehydes, carboxylic acids, ethers, esters, amines, monosaccharides, polysaccharides, linear hydrocarbons, fatty acids, aromatic compounds, etc. Two or more of the above solvents can also be used in combination. The boiling point of the solvent is not particularly limited. Usually, it is 100°C or higher. In one embodiment, it is 130°C or higher. In one embodiment, it is 150°C or higher, and usually, it is 300°C or lower. In one embodiment, it is 250°C or lower. In one embodiment, it is 200°C or lower. For example, it is 100°C to 300°C. In one embodiment, it is 130°C to 250°C. In one embodiment, it is 150°C to 200°C. When the boiling point of the solvent is 100°C or higher, for example, when the dispersion is used as an ink paste, the evaporation of the solvent at room temperature can be suppressed. As a result, the viscosity stability, coatability, etc. of the ink paste can be ensured. Also, when the boiling point of the solvent is 300°C or lower, at the temperature for connecting the semiconductor element and the support member by a firing process, especially a non-pressure firing process, the solvent can be prevented from remaining in the metal sintered body without being evaporated. As a result, the characteristics of the metal sintered body can be better maintained. As the solvent, it is preferably selected from the above solvents as a solvent suitable for the dispersion of metal particles. Specifically, from the viewpoint of improving the thermal conductivity, electrical conductivity, and bonding strength of the metal sintered body, it is preferably a solvent having an alcohol structure, an ether structure, or an ester structure. As the solvent contained in the dispersion of the present invention, for example, butyl cellosolve, carbitol, butyl cellosolve acetate, carbitol acetate, ethylene glycol diethyl ether, dipropylene glycol methyl ether acetate, dipropylene glycol monon-butyl ether, dipropylene glycol monon-methyl ether, terpineol, ethylene glycol, isobornyl cyclohexanol, tributyrin, etc. can be cited. As the solvent contained in the dispersion of the present invention, ethylene glycol is preferred. When the dispersion of the present invention is used as an ink paste, the amount of the solvent contained in the ink paste varies depending on the content of the metal particles contained in the dispersion, etc. Usually, it is 5% by weight or more and usually 99% by weight or less relative to the total weight of the ink paste. In one embodiment, it is 90% by weight or less. When the dispersion of the present invention is used as an ink paste, by adjusting the amount of the solvent to the above range, the viscosity of the ink paste can be adjusted to an appropriate viscosity range described later. Furthermore, the volume shrinkage due to the evaporation of the solvent during the sintering of the ink paste can be suppressed, and the denseness of the formed silver sintered body can be improved. Within the scope that does not impede the effects of the present invention, the dispersion of the present invention may further contain components other than the metal particles and the solvent of the present invention. Examples of the components that can be added other than the metal particles and the solvent of the present invention include those well-known in the technical field, and are not limited. For example, carboxylic acids having a boiling point of 400 °C or less under one atmospheric pressure and being solid at 20 °C, such as stearic acid, lauric acid, docosanoic acid, sebacic acid, 1,16-octadecanedioic acid, etc., additives, metal particles other than the metal particles of the present invention, anti-settling agents for the metal particles in the dispersion, soldering fluxes for promoting the sintering of the metal particles, etc. The amount of the components that can be added other than the metal particles and the solvent of the present invention is usually 0% by weight or more and usually 10% by weight or less, relative to the total weight of the dispersion. In one embodiment, it is 1% by weight or less, for example, 0% by weight to 10% by weight, and in one embodiment, 0% by weight to 1% by weight. When the dispersion of the present invention is used as an ink paste, when the viscosity of the ink paste is measured with a cone-plate viscometer, it is usually 10 Pa･s or more and usually 1000 Pa･s or less. This viscosity can be appropriately adjusted by the type and amount of the metal particles, the type and amount of the polymer as the dispersant, the type and amount of the solvent, etc. By adjusting the viscosity of the ink paste to the above range, the coatability of the ink paste can be improved, and bleeding after the ink paste is coated can be prevented. Since the dispersion of the present invention contains the metal particles of the present invention, when the solvent in the dispersion volatilizes and the metal particles overlap with each other, by assisting the part to enter the gaps between the metal particles, it is used as a bonding･adhesive layer between the main sintering parts, and sintering can be promoted. The details of this effect are the same as those produced when the heat capacities of the main part and the auxiliary part are different. Accordingly, the dispersion of the present invention contains metal particles having a main part and an auxiliary part extending from the main part. After the solvent in the dispersion volatilizes, it has excellent low-temperature sinterability. For example, it has excellent sinterability at 80 °C or less. Furthermore, the sintered body formed from this dispersion has a low volume resistivity, for example, 30 μΩ･cm or less, and in one embodiment, 20 μΩ･cm or less, and in one embodiment, 15 μΩ･cm or less of volume resistivity. The present invention also relates to a method for manufacturing the metal particles of the present invention. The metal particles of the present invention can be manufactured by irradiating microwaves while circulating the reaction solution and controlling the flow rate of the reaction solution and the absorption power of the microwaves in a method for manufacturing metal particles that includes a step of irradiating microwaves to the reaction solution. (i) Step of preparing a reaction solution containing a metal particle precursor and a solvent In the step of (i), a reaction solution containing a metal particle precursor and a solvent is prepared. In the method of the present invention, the solvent used in the reaction solution is not limited as long as it can dissolve materials such as metal particle precursors, dispersants, and reducing agents, and can further absorb polar solvents or ionic liquids for microwaves. As the solvent used in the reaction solution, for example, low-boiling solvents with a boiling point of 300 °C or lower can be cited. The low-boiling solvent is not limited, and examples include water, alcohols such as methanol and ethanol, polyol 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 of these low-boiling polar solvents. In addition, when a mixture is used as the solvent, the ratio of each constituent substance contained in the mixture is not limited, and each constituent substance can be in a mixed ratio under experimental conditions. By using a low-boiling polar solvent as the solvent, the operability of the solvent can be improved, and the environmental load can be reduced. The metal particle precursor is not limited as long as it can dissolve in a solvent to generate metal ions, such as ions of precious metals, base metals, and alloys, such as gold, silver, platinum, copper, nickel, iron, cobalt, etc., or ions of two or more of these metals. As the metal particle precursor, for example, metal halides such as fluorides, chlorides, bromides, iodides, metal sulfates, metal nitrates, metal phosphates, metal cyanides, etc., metal inorganic salts, metal carboxylates, metal sulfonates, etc., metal organic salts, metal complexes containing metal complex salts, etc. The metal particle precursor can also be prepared by dissolving a material containing a metal or a metal salt in an acid such as nitric acid or a base such as ammonia water. As the metal particle precursor, it is preferably to use nitrates of inexpensive metals, such as nickel nitrate and silver nitrate. In addition, formates with reducibility can be used as the metal particle precursor. Accordingly, in one embodiment, when a salt composed of a metal ion and a reducing organic anion, such as formate ion, as its counter anion is used as the metal particle precursor, a reducing agent may not be used. By dissolving the metal particle precursor in the solvent, a uniform reaction solution can be prepared. The concentration of metal ions in the reaction solution is not limited as long as it is below the saturation concentration. Usually, it is 1 mmol / L (mM) or more, and in one embodiment, it is 1.5 mM or more, and usually 500 mM or less. In one embodiment, it is 300 mM or less, for example, 1 mM to 500 mM, and in one embodiment, it is 1.5 mM to 300 mM. By setting the concentration of metal ions in the reaction solution within the aforementioned range, metal particles can be efficiently generated at a high concentration, significantly increasing the amount of metal particles that can be generated and recovered in one batch, and reducing the time, labor, and cost required for manufacturing metal particles. Additionally, the non-uniformity of the obtained metal particles is reduced, in other words, the particle size distribution of the obtained metal particles becomes narrower. The reaction solution may contain a dispersant. The dispersant is not limited, and examples include one or more dispersants selected from polyvinylpyrrolidone (PVP), thiol-based polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylate, cyclodextrin, pectin amine, methyl cellulose, polyethyleneimine cellulose, aliphatic amines, aliphatic carboxylic acids, tannic acid, and chelating agents such as ethylenediaminetetraacetic acid (EDTA) and / or its salts. The molecular weight of the dispersant is not limited. For example, as the weight average molecular weight (Mw), it is usually 1000 or more, and in one embodiment, it is 10000 or more, and usually 50000 or less, and in one embodiment, it is 40000 or less. For example, it is 1000 to 50000, and in one embodiment, it is 8000 to 50000, and in one embodiment, it is 10000 to 40000. The amount of the dispersant relative to the total weight of the reaction solution is usually 0.1 wt% or more, and in one embodiment, it is 0.5 wt% or more, and usually 10 wt% or less, and in one embodiment, it is 5 wt% or less. For example, it is 0.1 wt% to 10 wt%, and in one embodiment, it is 0.5 wt% to 5 wt%. Furthermore, the reaction solution may contain a reducing agent. The reducing agent is a material that can reduce metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction. The reducing agent is not limited. Examples of the reducing agent include 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, mixtures of two or more of these, etc. In one embodiment, the reducing agent for metal ions, especially silver ions, is DMF. Accordingly, when using DMF as the solvent of the reaction solution, since DMF can also be used as a reducing agent, a reducing agent other than DMF does not need to be used. The amount of the reducing agent is not limited as long as it can reduce the metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction. Generally, it is 1.0 equivalent or more relative to the metal ions. In one embodiment, it is 4.0 equivalents or more, and generally 20 equivalents or less. In one embodiment, it is 15 equivalents or less. For example, it is 1.0 equivalent to 20 equivalents. In one embodiment, it is 4.0 equivalents to 15 equivalents. Further, when the reducing agent of the metal ions contains one or more functional groups such as carboxyl, hydroxyl, and ether groups that can interact with the metal, it can also be used as a dispersant. When the reducing agent can also be used as a dispersant, the reaction solution may not contain the dispersant described above. Also, the amount of the reducing agent of the metal ions can be an amount exceeding the amount required to reduce the metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction. Although the reaction solution can also be composed of the metal particle precursor, solvent, dispersant, and reducing agent described above, in addition to these materials, it can further contain additives and the like that are usually used in the conventional method of manufacturing metal particles by irradiating microwaves. The amount of the additive is not limited. Generally, it is 10% by weight or less relative to the total weight of the reaction solution. In one embodiment, it is 3% by weight or less. Since the additive may not be added, the lower limit value of the additive is not limited. The pH of the reaction solution is not limited and is generally pH 3 to pH 12. In the present invention, the addition order, addition temperature, mixing method, mixing time, etc. of each material in the preparation of the reaction solution are not limited, and they are mixed in such a way as to prepare a uniform reaction solution. In the present invention, the reaction starts after preparing a uniform reaction solution. (ii) Step of irradiating microwaves while flowing the reaction solution described above. In the step (ii), the reaction solution prepared in the step (i) is flowed while irradiating microwaves. If the reaction solution flows in any direction, the flow method of the reaction solution is not limited. The reaction solution can flow at a certain speed in, for example, a reaction tube, such as a straight tube or a spiral tube. In one embodiment, when the reaction tube is a straight tube, if the structure is such that the irradiated microwaves are uniformly irradiated on the entire reaction tube, the size and shape are not limited. In one embodiment, the inner diameter of the tube is generally 1 mm or more. In one embodiment, it is 2 mm or more. In one embodiment, it is 4 mm or more, and generally 20 mm or less. In one embodiment, it is 10 mm or less. For example, it is 1 mm to 20 mm. In one embodiment, it is 2 mm to 20 mm. In one embodiment, it is 4 mm to 10 mm. In one embodiment, when the cavity is a cuboid with a length of 100 mm, as the reaction tube, a reaction tube with an inner diameter of 1 mm to 6 mm, an outer diameter of 3 mm to 8 mm (wall thickness: 1 mm), and a length of 100 mm is used. The flow rate of the reaction solution is 1.0 m / minute or more, in one embodiment, it is 1.5 m / minute or more, in one embodiment, it is 4 m / minute or more, and usually 50 m / minute or less, in one embodiment, it is 20 m / minute or less, for example, 1.0 m / minute to 50 m / minute, in one embodiment, it is 1.5 m / minute to 50 m / minute, in one embodiment, it is 4 m / minute to 20 m / minute. By setting the flow rate of the reaction solution within the above range, as shown in FIG. 6, after forming the main part in a spherical shape in the reaction solution, the auxiliary part extends along the flow of the reaction solution from the main part, and metal particles having the main part of the present invention and the auxiliary part extending from the main part can be formed. Accordingly, the auxiliary part can extend along the flow of the reaction solution from any part of the hemispherical surface on the upstream side of the main part with respect to the flow of the reaction solution, forming the metal particles of A to C in FIG. 1. Accordingly, in the method for forming metal particles of the present invention by the method of the present invention, since it is reduced by microwave heating of metal ions at a flow rate of the reaction solution above a certain level, as the type of metal particles manufactured by the method of the present invention, the metal ions only need to be metals reducible by microwave heating, that is, the aforementioned metal particles are not limited to silver particles, and for example, particles of gold, platinum, copper, nickel, iron, and cobalt can also be cited. The absorption power of microwave for the reaction solution in the microwave irradiation part (corresponding to the volume of the reaction solution irradiated with microwave) is 15 W / mL or more, in one embodiment, it is 20 W / mL or more, in one embodiment, it is 50 W / mL or more, and usually 1000 W / mL or less, in one embodiment, it is 800 W / mL or less, in one embodiment, it is 500 W / mL or less, for example, 15 W / mL to 1000 W / mL, in one embodiment, it is 20 W / mL to 800 W / mL, in one embodiment, it is 50 W / mL to 500 W / mL. Herein, the so-called "absorption power of microwave for the reaction solution" is a value represented by the value obtained by subtracting the reflected power reflected on the reaction solution from the output of the microwave irradiation source (output - reflected power). Incidentally, the reflected power can be measured by a detector in the microwave irradiation device. By adjusting the absorption power of microwave for the reaction solution within the above range, due to the sufficient reducibility of microwave, metal particles having the main part and the auxiliary part extending from the main part of the present invention can be formed. Other conditions of the microwave in the method of the present invention are not limited. In the method of the present invention, while flowing the reaction solution described above, a microwave synthesis device is used to irradiate microwaves and cause the reaction to proceed. When irradiating the reaction solution with microwaves, the polar solvent contained in the reaction solution absorbs the microwaves and is converted into thermal energy to generate heat. Accordingly, in the reaction solution irradiated with microwaves, a uniform and rapid temperature rise occurs in the irradiated part, and a uniform and rapid reaction is generated according to this temperature rise. The microwave is preferably uniformly irradiated on the object causing the reaction, that is, the part causing the reaction in the reaction solution. In the microwave synthesis device, the material of the part irradiating the microwave in the container containing the reaction solution is not limited as long as the reaction solution can be uniformly irradiated with microwaves. When irradiating the reaction solution with microwaves from the outside of the reactor through the reactor in the container containing the reaction solution, a material that transmits microwaves, that is, does not absorb microwaves, such as ceramics, glass, quartz, Teflon (registered trademark, PTFE, etc.), polysiloxane, etc., a non-conductive material with a small relative permittivity ε or dielectric loss tangent tanδ can be used. In addition, for the material of the part in the container containing the reaction solution that is not irradiated with microwaves, in addition to the above materials, metals such as aluminum and stainless steel can be used. Also, for the housing of the reaction tube that houses the microwave irradiation source and microwaves, as long as it is a material that does not leak and absorb microwaves, it is not limited. For example, a non-magnetic metal plate such as an aluminum plate can be cited. The microwave is generated from a microwave irradiation source (microwave oscillator (magnetron)), and either a single-mode system or a multi-mode system can be used for the microwave irradiation source. The frequency of the microwave generated from the microwave irradiation source can be appropriately changed and is not limited. The frequency of the microwave is usually 1 GHz or more, and in one embodiment, it is 2 GHz or more, and usually 10 GHz or less. In one embodiment, it is 6 GHz or less. For example, it is 1 GHz to 10 GHz, and in one embodiment, it is 2 GHz to 6 GHz. In the present invention, as the frequency of the microwave, it is preferably to use the frequency of 2.45 GHz of an industrial microwave power supply. The microwave is preferably uniform during irradiation, and the above-mentioned irradiation conditions of the microwave are preferably constant during irradiation of the microwave. In the present invention, the temperature of the reaction solution heated by the irradiation of the microwave is the reaction temperature, and this reaction temperature can be appropriately changed by the reaction conditions (type of solvent, pressure during reaction, etc.) and is not limited. Usually, it is 25°C or more, and in one embodiment, it is 80°C or more. The upper limit of the reaction temperature is not limited and is usually less than the boiling point of the solvent. For example, when the solvent is water, at one atmosphere, it is usually in the range of 25°C or more and less than 100°C, and in one embodiment, it is 80°C to 95°C. By setting the reaction temperature at 25 °C or higher, a reduction reaction from metal ions to metal particles is induced. By setting the reaction temperature below the boiling point of the solvent, it is possible to prevent the disorder of the particle diameters of the metal particles generated due to the non-uniformity of the reaction field accompanying the boiling of the reaction solution, that is, to prevent the spread of the particle size distribution, and it is possible to prepare metal particles with a small and uniform particle diameter. The irradiation time of the microwave on the reaction solution is the time required for the temperature of the reaction solution to reach the reaction temperature. It is not limited by appropriately changing the reaction conditions (microwave conditions, type of metal, type of solvent, pressure during reaction, amount of reaction solution, reaction temperature, etc.). Usually, it is 0.1 second or more. In one embodiment, it is 0.5 second or more, and usually, it is 600 seconds or less. In one embodiment, it is 30 seconds or less. For example, it is 0.1 second to 600 seconds. In one embodiment, it is 0.5 second to 300 seconds. Also, in the method of the present invention, since the reaction is carried out in a flow-through manner, the irradiation time of the microwave on the reaction solution can be adjusted by adjusting the length of the microwave irradiation part or by circulating the reaction solution. By irradiating the reaction solution with microwave under the above-mentioned conditions, the temperature of the reaction solution is raised to the reaction temperature, and nuclei (main parts) of metal particles are generated in the reaction solution. The nuclei (main parts) generated in the reaction solution then flow along the flow of the reaction solution, as shown in FIG. 6, in the direction along the flow (from the main part, toward the upstream direction of the reaction solution), and auxiliary parts are formed. In the method of the present invention, the reaction solution is in circulation, and the irradiation of the microwave on the reaction solution continues until the normal reaction is completed. The completion of the reaction can be determined by observing that the absorbance etc. of the metal particle precursor material or metal particles in the reaction solution no longer change. For example, when using the above-mentioned material of silver particles as the metal particle precursor material, the change in the absorbance of the reaction solution in the range of 280 nm to 780 nm is observed, and the time point when the absorbance no longer changes is taken as the completion time point of the reaction. If necessary, the dispersion liquid containing the metal particles obtained by the present invention can be separated and purified (such as salting out or centrifugation) by methods well-known in the art, and metal particles as the object, powders containing metal particles, or dispersions containing metal particles can be obtained. In the powder or dispersion containing the metal particles obtained by the method of the present invention, the metal particles of the present invention usually account for 2% or more, in one embodiment, 10% or more, in one embodiment, 18% or more, and then, in one embodiment, 20% or more, relative to the total number of metal particles contained in the powder or dispersion. Also, the upper limit of the metal particles in the powder is not limited because the more the proportion of metal particles, the better. The metal particles obtained by the method of the present invention include metal particles having a main part and auxiliary parts extending from the main part. The metal particles, the powder and the dispersion containing the same have excellent low-temperature sinterability, for example, below 80°C, and have excellent sinterability. Furthermore, the sintered body formed by the metal particles has a low volume resistivity, for example, 30 μΩ·cm or less, and in one embodiment, 20 μΩ·cm or less, and in one embodiment, 15 μΩ·cm or less. The metal particles of the present invention, the powder and the dispersion containing the same, and the metal particles, the powder and the dispersion manufactured by the method of the present invention can be used as conductive wiring materials in the field of electronic product mounting, in addition to the fields such as catalysts, electronic component members, and materials for inks, etc., and can reduce the number of steps for manufacturing inks used in wiring boards, etc. [Examples] Hereinafter, although several embodiments of the present invention will be described, the present invention is not intended to be limited to those shown in the embodiments. 1. Particle synthesis Examples 1 to 13 and Comparative Example 1 Silver particles were manufactured according to the conditions shown in Table 1. First, solutions containing silver nitrate (AgNO 3 ) and solutions containing pure water, a reducing agent (DMF), and a dispersant (protective agent) (PVP or EDTA) (the molecular weights in the table represent weight average molecular weights) were respectively prepared. Next, using a flow-through microwave synthesis device, the above two solutions were mixed to prepare a reaction solution, and while flowing the reaction solution, microwave (MW) was irradiated. The composition shown in Table 1 is the composition of the reaction solution. In the flow-through microwave synthesis device, a rectangular parallelepiped with a length of 100 mm was used as the cavity system, and the following was used as the reaction tube. Structure of the reaction tube - Inner diameter of the tube: 1 mm to 6 mm - Outer diameter of the tube: 3 mm to 8 mm (wall thickness: 1 mm) - Length of the tube: 100 mm In individual experiments, the reaction temperature was 60°C to 140°C. Also, the amount of the dispersant was set to 0.1 wt% to 10 wt%. Comparative Example 2 Synthesis was carried out using a batch-type microwave synthesis device with the composition of the reaction solution shown in Table 1. 2. Evaluation Results (Evaluation by Transmission Electron Microscope (TEM)) For the dispersions of Examples 1 to 13 and Comparative Examples 1 to 2 obtained, TEM images were taken. The observation method and particle size distribution measurement method of TEM are as follows. First, the silver nanoparticles synthesized under each condition were dropped onto a TEM grid and dried to prepare a sample. Then, TEM observation was performed, and for 100 to 300 or more randomly selected silver particles, the projected area circle equivalent diameter (Heywood diameter) of the main part and the auxiliary part, etc. were determined to evaluate the particle size and size distribution. TEM Measurement Conditions - Apparatus: JEM-ARM300F - TEM Grid: Formvar, Cu grid 150P - Acceleration Voltage: 200 kV Figure 7 shows a TEM image of the silver particles (composite particles) of Example 1 including the main part and the auxiliary part as an example. In Figure 7, the thickly colored part is a spherical particle with a plurality of crystal planes appearing, that is, the main part, and the thinly colored part is a particle with a thinner thickness compared to those with higher contrast, that is, the auxiliary part. From the obtained TEM image, the proportion (number proportion) of the composite particles in all 300 randomly selected particles, the particle diameter (Heywood diameter) of the main part in 100 composite particles, the thickness, width, and length of the auxiliary part (when the auxiliary part is a circular plate, the width and length are the same) were calculated as the average values. Also, for the thickness of the auxiliary part, the T value analysis of the electron beam transmittance during TEM-EELS measurement was performed. Specifically, in the TEM image, the thickness of the auxiliary part was analyzed by the difference in the T values of particles with different contrasts. The main part is composed of a plurality of crystal planes, and since the particle scale observed in the TEM image is consistent with the thickness in the depth direction, the value of the T value is determined. A specific example will be described using Figure 9. In Figure 9, the average value of the Heywood diameter of the main part is 21.9 nm. When calculating T / λ from the T value and the λ value, T / λ of the main part is 0.40, and T / λ of the auxiliary part is 0.25. Accordingly, the thickness of the auxiliary part can be calculated as 0.25×21.9÷0.4 = 13.7 nm. The results are shown in Table 2. From the results in Table 2 and Fig. 7, it is understood that by comparing Examples 1 to 13, Comparative Example 1, and Comparative Example 2, silver particles (composite particles) of the present invention having a main part and an auxiliary part extending from the main part can be produced by setting the flow rate of the reaction solution at 1.0 m / min or more. In this experiment, a flow rate of 1.0 m / min corresponds to a flow rate of about 13 mL / min. Furthermore, it is understood that by comparing Examples 1 to 13 with Comparative Example 1, in order to increase the proportion of composite particles, together with an increase in the flow rate, it is sufficient to set the absorption power of microwaves for the reaction solution in the microwave irradiation section at 15 W / mL or more. Fig. 8 shows the relationship between the flow rate and the absorption power in the production of silver particles in Examples and Comparative Examples. In addition, in the TEM image shown in Fig. 9, the lattice plane of the portion indicated by the white line of the composite particles is measured. From the results of Fig. 9, in the composite particles, a plane where crystallization peculiar to the main part or the auxiliary part easily progresses was not observed. Next, in order to more precisely grasp the shapes of the main part and the auxiliary part of the composite particles of the present invention, the ratio (T / λ) of the transmission rate of the electron beam to the wavelength was measured at the center (center of gravity) and the end (edge region of any one point) of the main part of the composite particles, and at the center (center of gravity) and the end (edge region of any one point) of the auxiliary part by TEM images. The results are shown in Table 3. From Table 3, it is understood that in the main part, T / λ is very different between the center and the end, and this difference increases as the equivalent diameter of the projected area circle increases. Thereby, it is understood that the main part is spherical, and accordingly, the larger the particle diameter, the thicker the thickness of the center part, and the larger {(T / λ of the center part) / (T / λ of the end part)} becomes. On the other hand, it is understood that in the auxiliary part, T / λ is almost the same between the center and the end. Thereby, it is understood that the auxiliary part is plate-shaped, and the thickness is almost the same between the center and the end. (Volume Resistivity Evaluation) The volume resistivity was measured for the powders of the dispersions of Examples 1 to 13 and Comparative Examples 1 to 2 obtained by drying. The volume resistivity was measured as follows. (1) The silver particles in the silver particle dispersion synthesized in the comparative example or the example were centrifuged to precipitate them at 20,000 rpm, and the supernatant was removed. (2) The operation in (1) was performed 6 times using pure water as the washing liquid for filtration. (3) The dispersion obtained in (2) was concentrated by an evaporator until the solid content became 15% by weight to prepare an ink. Incidentally, the solid content of the obtained dispersion was measured as follows. First, a small amount of the dispersion was sampled and heated at 150 °C for 2 hours to volatilize the solvent, and the residue in the dispersion was measured. Then, a part of the residue was heated to 500 °C by a thermogravimetric (TG) analyzer to burn out the organic components, and the solid content of the residue was measured. (4) The ink obtained in (3) was dropped into a slide chamber (slide & chamber, 8 well, manufactured by Watson). (5) The silver particles dropped in (4) were sintered at 80 °C in a Yamato DN63 Constant Temperature Oven air-blowing constant temperature thermostat, and the volume resistivity of the sintered product was measured. The volume resistivity was measured with a resistivity meter (Loresta GX MCP-T700, manufactured by Nitto Seiko Analysis Technology Co., Ltd.). (6) The film thickness of the sintered product whose resistance was measured in (5) was measured by observing the cut surface of the sintered product exposed by cutting with SEM. The results are shown in Table 2. In Table 2, it was found that by comparing Examples 1 to 13 with Comparative Example 1, by increasing the proportion of the composite particles of the present invention relative to the total silver particles in the powder or the dispersion, the volume resistivity can be increased. Specifically, it was found that the powders containing composite particles in Examples 1 to 13, compared with the powders in Comparative Examples 1 to 2 that completely did not contain or hardly contained composite particles, had a lower volume resistivity when the powder was sintered at a low temperature of 80 °C, especially less than 30 Ω·cm. That is, it was found that the composite particles of the present invention are metal particles showing excellent sinterability, and in order to fully exhibit their performance, it is preferably to increase the proportion of the composite particles of the present invention relative to the total metal particles in the powder or the dispersion. Furthermore, in order to confirm the usefulness of the composite particles of the present invention, measurements of volume resistivity were also carried out on spherical particles only (average particle diameter: 13 nm) (Comparative Example 3), plate-like particles only (average of width and length: 40 nm, average of thickness: 5 nm) (Comparative Example 4), a mixed powder of spherical particles (average particle diameter: 42 nm) and plate-like particles (average of width and length: 50 nm, average of thickness: 10 nm) at a weight ratio of 1:1 (Comparative Example 5), and a mixed powder of spherical particles and plate-like particles (8 nm and 15 nm, manufactured by HARIMA Kasei Co., Ltd.) (Comparative Example 6). As a result, the volume resistivity of the sintered bodies of the individual powders at 80°C was 32.8 μΩ·cm for Comparative Example 3, 51 μΩ·cm for Comparative Example 4, 78 μΩ·cm for Comparative Example 5, and 1173 μΩ·cm for Comparative Example 6. The volume resistivity ratios of Comparative Examples 3 to 6 were higher than those of the powder containing the composite particles of the present invention. It was found that in the composite particles of the present invention, when the same amount of heat is applied to the main part and the auxiliary part, compared with the main part, the auxiliary part rapidly raises the temperature to the melting point, so the melted auxiliary part serves to sinter and bond the metal particles having the auxiliary part and one or more other metal particles close to the metal particles, and the bonding and subsequent layer. As a result, the powder or dispersion containing the composite particles can have a low volume resistivity even at a low firing temperature, that is, 80°C, and particularly has a volume resistivity of 30 μΩ·cm or less. Also, it was found that since one silver particle of the present invention can sinter and bond with one or more other silver particles, even if the proportion of the composite particles relative to all the silver particles in the powder or dispersion is not 100% in terms of the number ratio, by having a certain proportion, for example, 2% or more, and in one embodiment, 10% or more, the effect of reducing the volume resistivity of the sintered body formed from the powder or dispersion at low temperature can be exerted. All publications, patents, and patent applications cited in this specification are hereby incorporated by reference in their entirety and made a part of this specification. [Fig. 1] A diagram schematically showing the positional relationship between the main part and the auxiliary part of an embodiment of the metal particles of the present invention. [Fig. 2] A diagram schematically showing an embodiment of the metal particles of the present invention. [Fig. 3] A diagram schematically showing an embodiment of the metal particles of the present invention. [Fig. 4] A diagram schematically showing an embodiment of the metal particles of the present invention. [Fig. 5] A diagram schematically showing the state of an embodiment of depositing the metal particles of the present invention. [Fig. 6] A diagram schematically showing the state of forming an embodiment of the metal particles of the present invention. [Fig. 7] A TEM image of the composite particles of Example 1. [Fig. 8] A graph showing the relationship between the flow rate and the absorption power in the production of silver particles in the Examples and Comparative Examples. [Fig. 9] A photograph of the portion indicated by the white line in the TEM image of the composite particles of Example 1 and its crystal lattice plane.

Claims

1. A metal particle having a main part and an auxiliary part extending from the main part, the main part being spherical and the auxiliary part being plate-shaped, the thickness of the auxiliary part being thinner than the Heywood diameter of the main part.

2. A metal particle having a main part and an auxiliary part extending from the main part, the auxiliary part being plate-shaped, the auxiliary part having a smaller heat capacity than the main part.

3. A metal particle having a main part and an auxiliary part extending from the main part, the auxiliary part being plate-shaped, wherein in the TEM image of the main part, the T / λ value of the central part is more than 1.2 times the T / λ value of the end part, and in the TEM image of the auxiliary part, the T / λ value of the central part is 2.0 times the T / λ value of the end part, wherein the central part refers to the centroid of the projected area of ​​the main part or the auxiliary part, and the end part refers to the edge region of the projected area of ​​the main part or the auxiliary part, T represents the transmittance of the electron beam, and λ represents the wavelength of the electron beam.

4. A metal particle having a main part and an auxiliary part extending from the main part, the main part being spherical with a Heywood diameter of 2 nm to 100 nm, the auxiliary part being plate-shaped with a thickness of 2 nm to 40 nm, a length of 2 nm to 60 nm, a width of 2 nm to 60 nm, and a Heywood diameter of the main part being 1.1 to 8.0 times the thickness of the auxiliary part.

5. Metal particles as described in any of claims 1 to 4, wherein, The metal is silver.

6. A dispersion comprising metal particles as claimed in any one of claims 1 to 5 and a solvent.

7. A powder comprising metal particles as claimed in any one of claims 1 to 5, wherein the metal particles comprise 10% or more of the total number of metal particles contained in the powder.

8. A method for manufacturing metal particles according to any one of claims 1 to 5, comprising the step of irradiating a reaction solution with microwaves, comprising the steps of (i) and (ii): (i) preparing a reaction solution containing a metal particle precursor and a solvent, and (ii) irradiating the reaction solution with microwaves while the reaction solution is flowing through it, wherein the flow rate of the reaction solution is 1.0 m / min or more, and the microwave absorption power of the reaction solution in the microwave irradiation section is 15 W / mL or more.

9. As in request item 8, wherein, In step (ii), the flow rate of the aforementioned reaction solution is 4 m / min to 20 m / min.

10. As in request item 8 or 9, wherein, In step (ii), the inner diameter of the reaction tube through which the aforementioned reaction liquid flows is 1 mm to 20 mm.

11. As in request item 8 or 9, wherein, In step (i), the metal particle precursor is the silver particle precursor.

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