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

Abstract

Provided is a metal particle including a low volume resistivity at a low firing temperature, a method for producing the metal particles, and a dispersion containing the metal particles. The present invention relates to a metal particle including a main portion and an auxiliary portion extending from the main portion. The main portion has a spherical shape. The auxiliary portion has a plate shape or linear shape thinner than the main portion.

Description

Technical Field

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

[0002] Metal nanoparticles that may exhibit properties different from those of bulk materials when reduced to the nanoscale have been used and examined in various usages, for example, catalysts, biomaterials, ink materials, and electronic component materials.

[0003] For example, Patent Literature 1 discloses a composite metal fine particle material in which spherical silver (Ag) nanoparticles synthesized from a silver (Ag) compound, a solvent, a reducing agent, and a dispersant, and conductive fillers composed of non-spherical metal fine particles, are mixed with a ratio in a range of 99:1 to 80:20. The dispersant is a compound having at least any one of thiol group (-SH) and amine group (-NH 2 ). An additive amount of the dispersant to an additive amount of the silver (Ag) compound is 0.5 mol or more and 3.0 mol or less.

[0004] Patent Literature 2 discloses a conductive wiring material containing a plurality of first metal nanoparticles and a plurality of second metal nanoparticles having a particle size smaller than that of the plurality of first metal nanoparticles, in which the second metal nanoparticles are melted by low-temperature firing to be allowed to fill spaces between the first metal nanoparticles.

[0005] Patent Literature 3 discloses a silver fine powder that contains silver particles having an average particle diameter D TEM of 3 nm to 20 nm and are coated with an organic protective material composed of a primary amine (B) having 6 to 12 carbon atoms.

[0006] Patent Literature 4 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

[0007] Patent Literature 1: JP 2011-038141 A Patent Literature 2: JP 2006-279038 A Patent Literature 3: JP 2009-138242 A Patent Literature 4: JP 2011-137226 A Summary of InventionTechnical Problem

[0008] In the electronics packaging field, in the metal nanoparticles of the composite metal fine particle material disclosed in Patent Literature 1 and the conductive wiring material disclosed in Patent Literature 2, the use of the mixture of two types of fine particles different in particle size and shape lowers the porosity of the sintered body formed by sintering the mixture, thereby allowing providing the low volume resistivity.

[0009] In the mixture of two types of metal nanoparticles different in particle size and shape, it is difficult to produce the two types of different metal nanoparticles in a single synthesis while controlling the proportion of the respective metal nanoparticles to a desired proportion. Accordingly, generally, as disclosed in Patent Literature 1, it is necessary to separately synthesize two types of metal nanoparticles different in particle size and shape, optionally subject them to classification, and then uniformly mix the resulting nanoparticles.

[0010] However, the method for producing the mixture has a problem of low productivity and high cost due to the increased number of processes. Further, in the mixture of two types of metal nanoparticles different in particle size and shape, since the firing temperature necessary for obtaining the low volume resistivity is at least 180°C, such a mixture cannot be used by printing on polyethylene terephthalate (PET) or polycarbonate (PC) and by firing.

[0011] Meanwhile, as disclosed in Patent Literature 3, the use of nanoparticles for the sintered body enables to lower the firing temperature.

[0012] However, in order to obtain nanoparticles having small particle sizes, a large amount of protective agent is required. An organic protective agent that may remain in the particles may inhibit the sintering to limit the effect of lowering the firing temperature. Therefore, in such nanoparticles, the firing temperature is at least 120°C, and it is difficult to perform the sintering at the temperature of 100°C or lower.

[0013] As disclosed in Patent Literature 4, by the irradiation of electromagnetic waves into the flow system, metal nanoparticles having a uniform particle size distribution of 100 nm or less can be synthesized.

[0014] However, generation of spherical nanoparticles having small particle sizes causes a problem similar to that in Patent Literature 3.

[0015] Accordingly, it is an object of the present invention to provide a metal particle having a low volume resistivity at a low firing temperature, a method for producing the metal particles, and a dispersion containing the metal particles.Solution to Problem

[0016] The inventors examined various means to solve the above-described problem and consequently found that, in a method for producing metal particles using a flow-type microwave synthesis apparatus, by setting an absorbed microwave power with respect to a reaction solution to 15 W / mL or more and a flow velocity of the reaction solution to 1.0 m / min or higher, fine particles having two different shapes (structures) within a single particle can be synthesized with a uniform particle size distribution, and the obtained microparticles have a volume resistivity of 30 Ω•cm or less even at a firing temperature of 80°C, thus completing the present invention.

[0017] That is, the gist of the present invention is as follows. (1) A metal particle comprising a main portion and an auxiliary portion extending from the main portion. The main portion has a spherical shape, the auxiliary portion has a plate shape or linear shape, and the auxiliary portion has a thickness smaller than a Heywood diameter of the main portion. (2) A metal particle comprising a main portion and an auxiliary portion extending from the main portion. The main portion has a spherical shape, the auxiliary portion has a spherical shape, and the auxiliary portion has a Heywood diameter smaller than a Heywood diameter of the main portion. (3) A metal particle comprising a main portion and an auxiliary portion extending from the main portion. The auxiliary portion has a heat capacity smaller than a heat capacity of the main portion. (4) A metal particle comprising a main portion and an auxiliary portion extending from the main portion. A value of T / λ at a center part is 1.2 times or more of a value of T / λ at an end part in a TEM image of the main portion, a value of T / λ at a center part is less than 2.0 times of a value of T / λ at an end part in a TEM image of the auxiliary portion, and the center part means a center of gravity of a projected area of the main portion or the auxiliary portion, the end part means an edge part of the projected area of the main portion or the auxiliary portion, T indicates an electron beam transmittance, and λ indicates an electron beam wavelength. (5) A metal particle comprising a main portion and an auxiliary portion extending from the main portion. The main portion has a spherical shape, and the main portion has a Heywood diameter of 2 nm to 100 nm, the auxiliary portion has a plate shape, the auxiliary portion has a thickness of 2 nm to 40 nm, the auxiliary portion has a length of 2 nm to 60 nm, and the auxiliary portion has a width of 2 nm to 60 nm, and the Heywood diameter of the main portion is 1.1 times to 8.0 times of the thickness of the auxiliary portion. (6) In the metal particle according to any one of (1) to (5), the metal is silver. (7) A dispersion comprising the metal particles according to any one of (1) to (6), and a solvent. (8) In a powder comprising the metal particles according to any one of (1) to (6), the metal particles is 10% or more with respect to a total number of metal particles contained in the powder. (9) A method for producing metal particles including a step of irradiating a reaction solution with microwaves. The method comprises: (i) a step of preparing the reaction solution containing a metal particle precursor and a solvent; and (ii) a step of irradiating the reaction solution with microwaves while the reaction solution flows. A flow velocity of the reaction solution is 1.0 m / min or more, and an absorbed microwave power with respect to the reaction solution is 15 W / mL or more in a microwave irradiation zone. (10) In the method according to (9), the flow velocity of the reaction solution is 4 m / min to 20 m / min in the step (ii). (11) In the method according to (9) or (10), a reaction tube through which the reaction solution flows has an inner diameter of 1 mm to 20 mm in the step (ii). (12) In the method according to any one of (9) to (11), the metal particle precursor is a silver particle precursor in the step (i). Advantageous Effects of Invention

[0018] The present invention provides the metal particle having the low volume resistivity at the low firing temperature, the method for producing the metal particles, and the dispersion containing the metal particles.Brief Description of Drawings

[0019] Fig. 1 is a drawing schematically illustrating a positional relationship between a main portion and an auxiliary portion in one embodiment of a metal particle of the present invention. Fig. 2 is a drawing schematically illustrating one embodiment of the metal particle of the present invention. Fig. 3 is a drawing schematically illustrating one embodiment of the metal particle of the present invention. Fig. 4 is a drawing schematically illustrating one embodiment of the metal particle of the present invention. Fig. 5 is a drawing schematically illustrating a state where the metal particles according to one embodiment of the present invention are accumulated. Fig. 6 is a drawing schematically illustrating a state where the metal particles according to one embodiment of the present invention are formed. Fig. 7 is a TEM image of composite particles of Example 1. Fig. 8 is a graph illustrating a relationship between a flow velocity and an absorbed power in a production of silver particles of examples and comparative examples. Fig. 9 is a photograph illustrating portions indicated by white lines and crystal lattice planes thereof in the TEM image of the composite particles of Example 1. Description of Embodiments

[0020] The following describes preferable embodiments of the present invention in detail.

[0021] 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 a metal particle, a method 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.

[0022] 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.

[0023] The present invention relates to a metal particle comprising a main portion and an auxiliary portion extending from the main portion.

[0024] Here, the "main portion" means a main part of the metal particle.

[0025] The "auxiliary portion" means a part that extends from the main portion and is smaller than the main portion in any physical property.

[0026] In one embodiment, the main portion has a thickness thicker than a thickness of the auxiliary portion. Here, the "thickness" means the shortest length passing through the center of gravity of the main portion or the auxiliary portion, and for example, the "thickness" means a diameter of a sphere in a case of a spherical shape and means a length in a perpendicular direction with respect to a surface expansion in a case of a plate shape. For example, the thickness of the main portion is generally 1.2 times or more, and 1.5 times or more in one embodiment, and generally 20 times or less, and ten times or less in one embodiment, of the thickness of the auxiliary portion. The thickness of the main portion is, for example, 1.2 times to 8.0 times, and 1.5 times to 5.0 times in one embodiment, of the thickness of the auxiliary portion.

[0027] In one embodiment, the main portion has a volume greater than a volume of the auxiliary portion. For example, the volume of the main portion is generally 1.2 times or more, and 1.5 times or more in one embodiment, of the volume of the auxiliary portion. Since the smaller auxiliary portion promotes sintering, the upper limit of a ratio of the main portion volume to the auxiliary portion volume (the main portion volume / the auxiliary portion volume) is, although not limited, for example, 50 or less, and 30 or less in one embodiment.

[0028] In one embodiment, the main portion has a weight greater than a weight of the auxiliary portion. For example, the weight of the main portion is generally 1.2 times or more, and 1.5 times or more in one embodiment, of the weight of the auxiliary portion. Since the smaller auxiliary portion promotes sintering, similarly to the volume, the upper limit of a ratio of the main portion weight to the auxiliary portion weight (the main portion weight / the auxiliary portion weight) is, although not limited, for example, 50 or less, and 30 or less in one embodiment.

[0029] In one embodiment, the main portion has a heat capacity greater than a heat capacity of the auxiliary portion. Here, the "heat capacity" is a state variable that represents the amount of heat required to raise the temperature of an object by 1°C when heat is transferred into or out of the object. The terms that "the main portion has a heat capacity greater than a heat capacity of the auxiliary portion" mean that the temperature change in the main portion is smaller than the temperature change in the auxiliary portion when heat is applied to the metal particle. For example, the heat capacity of the main portion is generally 1.2 times or more, and 1.5 times or more in one embodiment, of the heat capacity of the auxiliary portion. The upper limit of a ratio of the main portion heat capacity to the auxiliary portion heat capacity (the main portion heat capacity / the auxiliary portion heat capacity) is, although not limited, for example, 50 or less, and 30 or less in one embodiment.

[0030] With the main portion having the heat capacity greater than the heat capacity of the auxiliary portion, when the same amount of heat is applied to the main portion and the auxiliary portion in the metal particle comprising the main portion and the auxiliary portion of the present invention, the temperature rises to reach the melting point quickly in the auxiliary portion compared with the main portion. Therefore, the melted auxiliary portion can function as a joint and adhesive layer for sintering and joining the metal particle comprising the auxiliary portion and one or more other metal particles adjacent to the metal particle. As a result, the metal particle of the present invention, and a powder and a dispersion containing the metal particles of the present invention described below have the low volume resistivity even at a low firing temperature. Here, since the single metal particle of the present invention can be sintered and joined to one or more other adjacent metal particles, the metal particles of the present invention contained in the powder and the dispersion of the present invention described below are capable of providing the effect by comprising them with a certain proportion among a whole of the contained metal particles. Note that in the metal particle of the present invention, also in a case where any physical property, for example, one or more physical properties selected from the group consisting of the thickness, the weight, and the volume, of the main portion are greater than these physical properties of the auxiliary portion, it can be described, similarly to the above-described case, that the similar effect is provided.

[0031] In one embodiment, in a transmission electron microscope (TEM) image of the main portion, a value of T / λ at the center part is 1.5 times or more, and double or more in one embodiment, of a value of T / λ at an end part. Here, the center part means a center of gravity of a projected area of the main portion, the end part means an edge portion of the projected area of the main portion, T is a transmittance of an electron beam, and λ is a wavelength of the electron beam. Any one or more points may be selected as the end part. In one embodiment, when the main portion has a spherical shape, T / λ decreases as the distance from the center increases.

[0032] In one embodiment, in a TEM image of the auxiliary portion, a value of T / λ at the center part is less than double, and 1.5 times or less in one embodiment, of a value of T / λ at an end part. Here, the center part means a center of gravity of a projected area of the auxiliary portion, the end part means an edge portion of the projected area of the auxiliary portion, T is a transmittance of an electron beam, and λ is a wavelength of the electron beam. Any one or more points may be selected as the end part. In one embodiment, when the auxiliary portion has a plate shape or linear shape, the value of T / λ at the center part is approximately the same as the value of T / λ at the end part in the TEM image of the auxiliary portion.

[0033] In one embodiment, the metal particle of the present invention includes the main portion in a spherical shape and the auxiliary portion in a plate or linear shape. In one embodiment, the auxiliary portion forms the plate or linear shape from a plurality of, for example, two or more spherical bodies. In one embodiment, the main portion has a particle size greater than the thickness of the auxiliary portion. Here, the particle size means Heywood diameter, and the "Heywood diameter" means a projected area circle equivalent diameter of a sphere when an object is a sphere in a photograph, such as a TEM image. For example, the Heywood diameter of the main portion is generally 1.1 times or more, 1.2 times or more in one embodiment, and 1.5 times or more in one embodiment, and generally 20 times or less, and 10 times or less in one embodiment, of the thickness of the auxiliary portion. For example, the Heywood diameter of the main portion is 1.1 times to 8.0 times, 1.2 times to 8.0 times in one embodiment, and 1.5 times to 5.0 times in one embodiment, of the thickness of the auxiliary portion.

[0034] The metal particle of the present invention has the spherical main portion and the plate-shaped or linear auxiliary portion having the thickness smaller than the Heywood diameter of the main portion. Therefore, as illustrated in Fig. 5, when the metal particles are mutually overlapped, the auxiliary portions enter voids between the metal particles and act as joint and adhesive layers that sinter and join the main portions to each other, thereby allowing the sintering to be promoted. Details of the effect are similar to the effect provided when the heat capacity is different between the main portion and the auxiliary portion.

[0035] In one embodiment, the metal particle of the present invention has the main portion in a spherical shape and the auxiliary portion in a spherical shape, for example, an oval shape or spindle shape. In one embodiment, the Heywood diameter of the main portion is greater than the Heywood diameter of the auxiliary portion. For example, the Heywood diameter of the main portion is generally 1.1 times or more, 1.2 times or more in one embodiment, and 1.5 times or more in one embodiment, and generally 20 times or less, and 10 times or less in one embodiment, of the Heywood diameter of the auxiliary portion. For example, the Heywood diameter of the main portion is 1.1 times to 8.0 times, 1.2 times to 8.0 times in one embodiment, and 1.5 times to 5.0 times in one embodiment, of the Heywood diameter of the auxiliary portion.

[0036] The metal particle of the present invention has the spherical main portion and the spherical auxiliary portion having the Heywood diameter smaller than the Heywood diameter of the main portion. Therefore, when the metal particles are mutually overlapped, the auxiliary portions enter voids between the metal particles and act as joint and adhesive layers that sinter and join the main portions to each other, thereby allowing the sintering to be promoted. Details of the effect are similar to the effect provided when the heat capacity is different between the main portion and the auxiliary portion.

[0037] In the metal particle of the present invention, a positional relationship between the main portion and the auxiliary portion is not limited insofar as the auxiliary portion extends from the main portion.

[0038] In one embodiment, the main portion and an auxiliary object are joined to each other at a joint portion having a certain area. Here, because of the different thicknesses, the main portion and the auxiliary object can be distinguished, for example, by the difference in contrast in the TEM image. In an observation of the metal particles of the present invention with TEM, the metal particles of the present invention in the TEM image is observed as an image corresponding to a plan view having an image viewed in a direction allowing the thickness of the auxiliary portion to be seen as a front view due to the structure of the metal particle of the present invention.

[0039] In one embodiment, an angle formed between a tangent plane that contacts the main portion at the joint portion of the main portion and the auxiliary portion and the auxiliary portion is generally from 0° (that is, the auxiliary portion extends in the tangent plane direction) to 90° (that is, the auxiliary portion extends furthest from the main portion or the tangent plane). In other words, the auxiliary portion may extend from the main portion in any direction.

[0040] Fig. 1 schematically illustrates the positional relationship between the main portion and the auxiliary portion in one embodiment of the metal particle of the present invention. Fig. 1 is a drawing illustrating the metal particle of the present invention when viewed in the direction allowing the thickness of the auxiliary portion to be seen (hereinafter also referred to as a "front view"). In one embodiment, as illustrated in Fig. 1(1), the angle formed between the tangent plane that contacts the main portion at the joint portion of the main portion and the auxiliary portion and the auxiliary portion is 90°. In one embodiment, as illustrated in Fig. 1(2), the angle formed between the tangent plane that contacts the main portion at the joint portion of the main portion and the auxiliary portion and the auxiliary portion is 45°. In one embodiment, as illustrated in Fig. 1(3), the angle formed between the tangent plane that contacts the main portion at the joint portion of the main portion and the auxiliary portion and the auxiliary portion is 0°.

[0041] In the metal particle of the present invention, crystal lattices of the main portion and the auxiliary portion are not limited.

[0042] The number of auxiliary portions may be one or more, for example, one, two, three, or four or more, for each main portion. In one embodiment, the metal particle includes one auxiliary portion for each main portion.

[0043] The Heywood diameters, volumes, weights, and heat capacities of the main portion and the auxiliary portion of the metal particle can be calculated based on the image observed with TEM and the types and properties of metals in the respective portions of the metal particle.

[0044] In one embodiment, the metal particle is a metal nanoparticle. Here, the "metal nanoparticle" means a metal particle having a particle size of generally 1 nm to 100 nm. Therefore, when the metal particle of the present invention is a metal nanoparticle, the metal particle of the present invention is a metal particle in which a Heywood diameter of the whole metal particle including the main portion and the auxiliary portion is 1 nm to 100 nm.

[0045] In one embodiment, the main portion has a spherical shape. Here, the "spherical shape" includes not only a perfectly spherical shape, but also an approximately spherical, ellipsoidal, and a polygonal shape with substantially equal sides when the main portion is observed with a transmission electron microscope (TEM).

[0046] In one embodiment, the particle size (Heywood diameter) of the main portion is, although not limited, generally 2 nm or more, and 5 nm or more in one embodiment, and generally 100 mm or less, 70 nm or less in one embodiment, and 60 nm or less in one embodiment. The particle size (Heywood diameter) of the main portion is, for example, 2 nm to 100 nm, 5 nm to 70 nm in one embodiment, 5 nm to 60 nm in one embodiment, and 6 nm to 60 nm in one embodiment.

[0047] Here, the particle size (Heywood diameter) of the main portion can be obtained, for example, by selecting any 100 or more metal particles having the main portion and the auxiliary portion in the TEM image, measuring values of diameter of the respective selected metal particles when the projected surface area of the metal particle is converted into the circle area, and averaging the measurement values.

[0048] In one embodiment, the auxiliary portion has a plate or linear shape thinner than the main portion. Here, the terms "thinner than the main portion" means that the auxiliary portion has the thickness smaller than the Heywood diameter of the main portion. The "plate shape" is a flat shape having a thickness smaller than the length and the width, the shape based on two directions of the length and the width is not limited, and examples of the shape include circular, approximately circular, oval, and polygonal, such as triangular, quadrangular (including square, rectangular, and trapezoidal), pentagonal, and hexagonal shapes. The "length" of the plate shape is a distance in a direction getting away from the main portion of the auxiliary portion, and the "width" of the plate shape is a distance in a direction perpendicular to the length of the auxiliary portion. In one embodiment, the auxiliary portion may have a plate shape in which the width and the thickness are small and have substantially equal size. The "linear shape" means a shape in which a cross section perpendicular to a longitudinal direction of the auxiliary portion has a circular, approximately circular, oval, or a polygonal shape with substantially equal sides. As described above, the auxiliary portion may form the plate or linear shape from a plurality of, for example, two or more spherical bodies.

[0049] Fig. 2 schematically illustrates one embodiment of the metal particle of the present invention. Fig. 2 is a front view of the metal particle of the present invention. In one embodiment, the thickness of the auxiliary portion as illustrated in Fig. 2 is, although not limited, generally 1 nm or more, 2 nm or more in one embodiment, and 5 nm or more in one embodiment, and generally 60 mm or less, and 40 nm or less in one embodiment. The thickness of the auxiliary portion is, for example, 1 nm to 40 nm, 2 nm to 40 nm in one embodiment, 5 nm to 40 nm in one embodiment, and 5 nm to 20 nm in one embodiment.

[0050] In one embodiment, the length of the auxiliary portion as illustrated in Fig. 2 is, although not limited, generally 2 nm or more, 3 nm or more in one embodiment, and 5 nm or more in one embodiment, and generally 80 mm or less, and 60 nm or less in one embodiment. The length of the auxiliary portion is, for example, 2 nm to 60 nm, 2 nm to 40 nm in one embodiment, and 5 nm to 30 nm in one embodiment.

[0051] Fig. 3 schematically illustrates one embodiment of the metal particle of the present invention. Fig. 3(1) is a front view of the metal particle of the present invention, and Fig. 3(2) is a plan view of the metal particle of the present invention. In one embodiment, when the plate shape of the auxiliary portion is quadrangular, the thickness of the auxiliary portion as illustrated in Fig. 3(1) is, although not limited, generally 1 nm or more, 2 nm or more in one embodiment, and 5 nm or more in one embodiment, and generally 60 mm or less, and 40 nm or less in one embodiment. The thickness of the auxiliary portion is, for example, 1 nm to 40 nm, 2 nm to 40 nm in one embodiment, 5 nm to 40 nm in one embodiment, and 5 nm to 20 nm in one embodiment. The length of the auxiliary portion as illustrated in Fig. 3(1) is, although not limited, generally 2 nm or more, 3 nm or more in one embodiment, and 5 nm or more in one embodiment, and generally 80 nm or less, and 60 nm or less in one embodiment. The length of the auxiliary portion is, for example, 2 nm to 60 nm, 2 nm to 40 nm in one embodiment, and 5 nm to 30 nm in one embodiment. Furthermore, the width of the auxiliary portion as illustrated in Fig. 3(2) is, although not limited, generally 2 nm or more, and 5 nm or more in one embodiment, and generally 100 nm or less, 80 nm or less in one embodiment, and 70 nm or less in one embodiment. The width of the auxiliary portion is, for example, 2 nm to 70 nm, 2 nm to 60 nm in one embodiment, and 5 nm to 50 nm in one embodiment. The ratio of the length to the width (the length / the width) of the auxiliary portion is, although not limited, generally 0.4 or more, and 0.5 times or more in one embodiment, and generally 10 times or less, and 5 times or less in one embodiment. The ratio of the length to the width of the auxiliary portion is, for example, 0.5 times to 5 times, and 1 times to 5 times in one embodiment. The ratio of the auxiliary portion length to the main portion Heywood diameter (the auxiliary portion length / the main portion Heywood diameter) is, although not limited, generally 0.2 or more, and 0.5 or more in one embodiment, and generally 4 or less, and 3 or less in one embodiment. The ratio of the auxiliary portion length to the main portion Heywood diameter is, for example, 0.2 to 4, and 0.5 to 3 in one embodiment.

[0052] Fig. 4 schematically illustrates one embodiment of the metal particle of the present invention. Fig. 4(1) is a front view of the metal particle of the present invention, and Fig. 4(2) is a plan view of the metal particle of the present invention. In one embodiment, when the plate shape of the auxiliary portion is circular, the circle equivalent diameter of the auxiliary portion as illustrated in Figs. 4(1) and 4(2) is, although not limited, generally 2 nm or more, and 5 nm or more in one embodiment, and generally 100 nm or less, and 80 nm or less in one embodiment. The circle equivalent diameter of the auxiliary portion is, for example, 2 nm to 60 nm, and 5 nm to 30 nm in one embodiment.

[0053] Here, the length, the thickness, the length, the width, or the circle equivalent diameter of the auxiliary portion can be obtained, similarly to the thickness (Heywood diameter) of the main portion, for example, by selecting any 100 or more metal particles having the main portion and the auxiliary portion in the TEM image, measuring the length, the thickness, the length, the width, or the circle equivalent diameter of the auxiliary portion of the metal particle for each of the selected metal particles, and averaging the measurement values. The thickness of the auxiliary portion can be calculated, for example, by comparing the ratio of the electron beam transmittance to the electron beam wavelength (T / λ) in TEM-EELS at the center of the main portion (that is, the ratio of the electron beam transmittance to the electron beam wavelength (T / λ) corresponding to the length of Heywood diameter) with the ratio of the electron beam transmittance to the electron beam wavelength (T / λ) in TEM-EELS at the center of the auxiliary portion.

[0054] The metal particle of the present invention has the spherical main portion having the dimensions as described above and the plate-shaped or linear auxiliary portion having the thickness smaller than the Heywood diameter of the main portion. Therefore, as illustrated in Fig. 5, when the metal particles are mutually overlapped, the auxiliary portions enter voids between the metal particles and act as joint and adhesive layers that sinter and join the main portions to each other, thereby allowing the sintering to be promoted. Details of the effect are similar to the effect provided when the heat capacity is different between the main portion and the auxiliary portion.

[0055] In one embodiment, the auxiliary portion has a spherical shape. Here, the "spherical shape" includes not only a perfectly spherical shape, but also an approximately spherical, ellipsoidal, and a polygonal shape with substantially equal sides when the main portion is observed with a transmission electron microscope (TEM).

[0056] In one embodiment, the particle size (Heywood diameter) of the auxiliary portion is, although not limited, generally 1 nm or more, 2 nm or more in one embodiment, and 5 nm or more in one embodiment, and generally 60 mm or less, and 40 nm or less in one embodiment. The particle size (Heywood diameter) of the auxiliary portion is, for example, 1 nm to 40 nm, 5 nm to 40 nm in one embodiment, and 5 nm to 20 nm in one embodiment.

[0057] Here, the particle size (Heywood diameter) of the auxiliary portion can be obtained, for example, by selecting any 100 metal particles having the main portion and the auxiliary portion in the TEM image, measuring values of diameter of the respective selected metal particles when the projected surface area of the metal particle is converted into the circle area, and averaging the measurement values.

[0058] The metal particle of the present invention has the spherical main portion having the dimensions as described above and the spherical auxiliary portion having the Heywood diameter smaller than the Heywood diameter of the main portion. Therefore, when the metal particles are mutually overlapped, the auxiliary portions enter voids between the metal particles and act as joint and adhesive layers that sinter and join the main portions to each other, thereby allowing the sintering to be promoted. Details of the effect are similar to the effect provided when the heat capacity is different between the main portion and the auxiliary portion.

[0059] The metal of the metal particle is not limited. Examples of the metal include noble metals, base metals, and alloys, and for example, gold, silver, platinum, copper, nickel, iron, cobalt, and alloys of two or more thereof. For the metal, silver or nickel is preferable, and silver is more preferable. In the metal particle, generally, the main portion and the auxiliary portion are made of the same metal.

[0060] Since the metal particle of the present invention has the main portion and the auxiliary portion as described above, as illustrated in Fig. 5, when the metal particles are mutually overlapped, the auxiliary portions enter voids between the metal particles and act as joint and adhesive layers that sinter and join the main portions to each other, thereby allowing the sintering to be promoted. Details of the effect are similar to the effect provided when the heat capacity is different between the main portion and the auxiliary portion.

[0061] The present invention also relates to a powder containing the above-described metal particles of the present invention.

[0062] The powder of the present invention contains the metal particles of the present invention in an amount of generally 2% or more, 10% or more in one embodiment, 18% or more in one embodiment, and 20% or more in one embodiment with respect to the total number of metal particles contained in the powder. The upper limit of the metal particles of the present invention in the powder is not limited because the larger proportion of the metal particles of the present invention is preferable.

[0063] Here, in the powder of the present invention, the proportion of the metal particles of the present invention in the powder can be obtained, for example, by selecting any 300 or more metal particles in the TEM image, and calculating the proportion of the metal particles having the main portion and the auxiliary portion in all the selected metal particles.

[0064] The component other than the metal particles of the present invention in the powder of the present invention is not limited. In one embodiment, the powder of the present invention contains metal particles that are made of the same metal as that of the metal particle of the present invention and have a shape different from that of the metal particle of the present invention, in addition to the metal particle of the present invention. While the metal particle having the shape different from that of the metal particle of the present invention is not limited, examples thereof include metal particles having a shape, such as a spherical shape, a plate shape, and a linear shape. The size of the metal particle having the shape different from that of the metal particle of the present invention is, although not limited, generally similar to the size of the metal particle of the present invention, especially the size of the main portion of the metal particle of the present invention. The metal particle having the shape different from that of the metal particle of the present invention is preferably a metal particle without the auxiliary portion in the metal particle of the present invention, that is, a particle having only the main portion of the metal particle of the present invention.

[0065] The powder of the present invention may contain an organic substance that functions as a dispersant (also referred to as a "protective agent") used during production or during dispersion in a solvent. Examples of the dispersant include dispersants described below. The content of the dispersant is not limited. The content of the dispersant depends on, for example, the amount of dispersant used in the method for producing the metal particles of the present invention described below.

[0066] The powder of the present invention contains the metal particles having the main portion and the auxiliary portion extending from the main portion. The powder of the present invention is excellent in low-temperature sinterability and, for example, has the excellent sinterability at 80°C or lower. Furthermore, the sintered body formed from the powder has a low volume resistivity, for example, the volume resistivity of 30 µΩ•cm or less, 20 µΩ•cm or less in one embodiment, and 15 µΩ•cm or less in one embodiment.

[0067] The present invention also relates to a dispersion containing the above-described metal particles or powder of the present invention and a solvent for dispersing the metal particles or powder.

[0068] The content of the metal particles in the dispersion is not limited. For example, the content of the metal particles is generally 1 weight% or more, 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, and 70 weight% or more in one embodiment, and generally 95 weight% or less, 90 weight% or less in one embodiment, 85 weight% or less in one embodiment, and 80 weight% or less in one embodiment, with respect to the total weight of the dispersion. The content of the metal particles is, for example, 1 weight% to 95 weight%, 5 weight% to 90 weight% in one embodiment, and 50 weight% to 80 weight% in one embodiment, with respect to the total weight of the dispersion.

[0069] By increasing the content of the metal particles in the dispersion of the present invention, it is possible to reduce the solvent cost, the time, labor, and cost required for solvent volatilization after application of the dispersion, and also to reduce storage and transportation costs, thereby lowering the environmental burden.

[0070] The dispersion of the present invention may contain an organic substance that functions as a dispersant used during production or during dispersion in a solvent of the metal particles or powder of the present invention. Examples of the dispersant include dispersants described below. The content of the dispersant is not limited. The content of the dispersant depends on, for example, the amount of dispersant used in the method for producing the metal particles of the present invention described below.

[0071] The solvent contained in the dispersion of the present invention is not limited, and a solvent known in the technical field can be used. The solvent contained in the dispersion of the present invention is, for example, a solvent that is a liquid at 20°C and can be selected from, for example, water, alcohols, aldehydes, carboxylic acids, ethers, esters, amines, monosaccharides, polysaccharides, linear hydrocarbons, fatty acids, and aromatic compounds. Two or more of the above solvents may be used in combination.

[0072] The boiling point of the solvent is, although not specifically limited, generally 100°C or higher, 130°C or higher in one embodiment, and 150°C or higher in one embodiment, and generally 300°C or lower, 250°C or lower in one embodiment, and 200°C or lower in one embodiment. The boiling point of the solvent is, for example, 100°C to 300°C, 130°C to 250°C in one embodiment, and 150°C to 200°C in one embodiment. When the boiling point of the solvent is 100°C or higher, for example, in a case where the dispersion is used as an ink paste, it is possible to suppress volatilization of the solvent at room temperature and, as a result, to ensure the viscosity stability and coatability of the ink paste. When the boiling point of the solvent is 300°C or lower, it is possible, in a joining process such as firing, particularly pressureless firing, to suppress the solvent from remaining in the metal sintered body without being evaporated at the temperature at which a semiconductor element is connected to a support member, and, as a result, the properties of the metal sintered body can be maintained to be more proper.

[0073] For the solvent, it is preferable to select a solvent suitable for dispersing the metal particles from among the above-described solvents. Specifically, from the viewpoint of providing improved thermal conductivity, electrical conductivity, and bonding strength of the metal sintered body, it is preferable to select a solvent having an alcohol structure, an ether structure, or an ester structure. Examples of the solvent 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. For the solvent contained in the dispersion of the present invention, ethylene glycol is preferable.

[0074] When the dispersion of the present invention is used as an ink paste, the amount of the solvent contained in the ink paste may be changed depending on the content of metal particles contained in the dispersion. The amount of the solvent contained in the ink paste is generally 5 weight% or more, and generally 99 weight% or less, and 90 weight% or less in one embodiment, with respect to the total weight of the ink paste.

[0075] When the dispersion of the present invention is used as an ink paste, by adjusting the amount of the solvent to the above-described range, the viscosity of the ink paste can be adjusted to an appropriate viscosity range described later, and further, volume shrinkage caused by volatilization of the solvent during sintering of the ink paste can be suppressed, thereby allowing the improvement of the density of the resulting silver sintered body.

[0076] The dispersion of the present invention may further contain components other than the metal particle of the present invention and the solvent, insofar as they do not impair the effects of the present invention. The components that may be added other than the metal particle of the present invention and the solvent are not limited, and components known in the technical field may be added. Examples of the components include: additives, such as carboxylic acid that has a boiling point at atmospheric pressure of 400°C or lower and is a solid at 20°C, such as stearic acid, lauric acid, docosanoic acid, sebacic acid, and 1,16-octadecanedioic acid; metal particles other than the metal particle of the present invention; a sedimentation inhibitor for the metal particles in the dispersion; and a flux for promoting sintering of the metal particles. The amount of the components that may be added other than the metal particle of the present invention and the solvent is generally 0 weight% or more, and generally 10 weight% or less, and 1 weight% or less in one embodiment, with respect to the total weight of the dispersion. The amount of the components is, for example, 0 weight% to 10 weight%, and 0 weight% to 1 weight% in one embodiment, with respect to the total weight of the dispersion.

[0077] When the dispersion of the present invention is used as an ink paste, the viscosity of the ink paste is generally 10 Pa•s or more and generally 1000 Pa•s or less in the measurement using a cone-and-plate viscometer. The viscosity can be appropriately adjusted depending on the type and the amount of the metal particles, the type and the amount of a polymer as the dispersant, the type and the amount of the solvent, and the like.

[0078] By adjusting the viscosity of the ink paste to the above-described range, the coatability of the ink paste can be improved, and bleeding of the ink paste after application can be avoided.

[0079] Since the dispersion of the present invention contains the metal particles of the present invention, when the metal particles are mutually overlapped after volatilization of the solvent in the dispersion, the auxiliary portions enter voids between the metal particles and act as joint and adhesive layers that sinter and join the main portions to each other, thereby allowing the sintering to be promoted. Details of the effect are similar to the effect provided when the heat capacity is different between the main portion and the auxiliary portion. Accordingly, since the dispersion of the present invention contains the metal particle comprising the main portion and the auxiliary portion extending from the main portion, the dispersion of the present invention is excellent in low-temperature sinterability and, for example, has the excellent sinterability at 80°C or lower after volatilization of the solvent in the dispersion. Furthermore, the sintered body formed from the dispersion has a low volume resistivity, for example, the volume resistivity of 30 µΩ•cm or less, 20 µΩ•cm or less in one embodiment, and 15 µΩ•cm or less in one embodiment.

[0080] The present invention also relates to a method for producing the metal particles of the present invention. The metal particles of the present invention can be produced, in a method for producing the metal particles including a step of irradiating a reaction solution with microwaves, by irradiating the reaction solution with the microwaves while flowing the reaction solution, and controlling a flow velocity of the reaction solution and an absorbed microwave power during the irradiation.(i) Step of Preparing Reaction Solution Containing Metal Particle Precursor and Solvent

[0081] In Step (i), a reaction solution containing a metal particle precursor and a solvent is prepared.

[0082] In the method of the present invention, the solvent used for the reaction solution is not limited insofar the solvent is a polar solvent or an ionic liquid that can dissolve the materials, such as a metal particle precursor, 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 the 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 with which the components are miscible under experimental conditions.

[0083] By using the low boiling point polar solvent as the solvent, the handlability of the solvent can be improved and the environmental burden can be lowered.

[0084] 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 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.

[0085] By dissolving the metal particle precursor in the solvent, a homogeneous reaction solution can be prepared.

[0086] The concentration of metal ions in the reaction solution is, although not limited insofar as it is equal to or less than the saturation concentration, generally 1 mmol / L (mM) or more, and 1.5 mM or more in one embodiment, and generally 500 mM or less, and 300 mM or less in one embodiment. The concentration of metal ions in the reaction solution is, for example, 1 mM to 500 mM, and 1.5 mM to 300 mM in one embodiment.

[0087] By setting the concentration of metal ions in the reaction solution to the above-described range, the metal particles can be efficiently generated with a high concentration, thereby allowing the amount of metal particles that can be generated and collected once to be significantly increased and allowing the time, labor, and cost required for producing the metal particles to be reduced. The variation in the obtained metal particles is reduced, in other words, the particle size distribution of the obtained metal particles is narrowed.

[0088] The reaction solution may contain a dispersant. Although the dispersant is not limited, examples of the dispersant include one or more dispersants selected from polyvinylpyrrolidone (PVP), thiol-based polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylates, cyclodextrin, aminopectin, methyl cellulose, polyethyleneimine cellulose, aliphatic amines, aliphatic carboxylic acids, tannic acid, and chelating agents such as ethylenediaminetetraacetic acid (EDTA) and / or salts thereof. The molecular weight of dispersant is, although not limited, for example, generally 1000 or more, and 10000 or more in one embodiment, and generally 50000 or less, and 40000 or less in one embodiment, in terms of weight average molecular weight (Mw). The molecular weight of dispersant is, for example, 1000 to 50000, 8000 to 50000 in one embodiment, and 10000 to 40000 in one embodiment, in terms of weight average molecular weight (Mw). The amount of dispersant is generally 0.1 weight% or more, and 0.5 weight% or more in one embodiment, and generally 10 weight% or less, and 5 weight% or less in one embodiment, with respect to the total weight of the reaction solution. The amount of dispersant is, for example, 0.1 weight% to 10 weight%, and 0.5 weight% to 5 weight% in one embodiment, with respect to the total weight of the reaction solution.

[0089] Furthermore, the reaction solution may contain a reductant. The reductant is a material that can reduce the metal ions to a metal having the oxidation number of 0 through a redox reaction.

[0090] The reductant is not limited. Examples of the reductant may include: citric acid or a citrate such as trisodium citrate, disodium citrate, or monosodium citrate; oxalic acid or an oxalate such as sodium oxalate; ascorbic acid or an ascorbate such as sodium ascorbate; formic acid or a formate such as sodium formate; DMF; and mixtures of two or more thereof. In one embodiment, the reductant of the metal ion, especially silver ion, is DMF. Therefore, when DMF is used as the solvent of the reaction solution, since DMF can act also as the reductant, it is not necessary to use the reductant other than DMF.

[0091] Although the amount of reductant is not limited insofar as the metal ions are reduced to a metal having the oxidation number of 0 through a redox reaction, the amount of reductant is generally 1.0 equivalent or more, and 4.0 equivalent or more in one embodiment, and generally 20 equivalent or less, and 15 equivalent or less in one embodiment, with respect to the metal ions. The amount of reductant is, for example, 1.0 equivalent to 20 equivalent, and 4.0 equivalent to 15 equivalent in one embodiment, with respect to the metal ions. When the reductant for the metal ions contains one or more functional groups capable of interacting with metals, such as carboxyl groups, hydroxyl groups, or ether groups, it can also function as the dispersant. When the reductant functions also as the dispersant, the reaction solution does not need to contain the above-described dispersant, and the amount of the reductant for the metal ions may be an amount exceeding the amount necessary for reducing the metal ions to a metal having the oxidation number of 0 through a redox reaction.

[0092] While the reaction solution may be composed of the metal particle precursor, the solvent, the dispersant, and the reductant described above, the reaction solution may further contain, in addition to these materials, an additive and the like that may be generally used in conventional methods for producing metal particles by irradiation with microwaves. 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.

[0093] While the pH of the reaction solution is not limited, it is generally pH 3 to pH 12.

[0094] In the present invention, the adding order, the addition temperature, the mixing method, the mixing time, and the like of the materials are not limited in the preparation of the reaction solution, and the materials are mixed to prepare a homogeneous reaction solution. In the present invention, the reaction is started after homogeneous reaction solution is prepared.(ii) Step of Irradiating Reaction Solution with Microwaves While Flowing Reaction Solution

[0095] In Step (ii), microwaves are irradiated while the reaction solution prepared in Step (i) flows.

[0096] The flowing method of the reaction solution is not limited insofar as the reaction solution flows in any direction. The reaction solution can be flowed at a constant rate through, for example, a reaction tube such as a straight tube or a spiral tube.

[0097] In one embodiment, when the reaction tube is a straight tube, the dimensions and shape of the reaction tube are not limited insofar as the microwaves to be irradiated are uniformly irradiated to the entire reaction tube. In one embodiment, the tube has an inner diameter of generally 1 mm or more, 2 mm or more in one embodiment, and 4 mm or more in one embodiment, and generally 20 mm or less, and 10 mm or less in one embodiment. The inner diameter of the tube is, for example, 1 mm to 20 mm, 2 mm to 20 mm in one embodiment, and 4 mm to 10 mm in one embodiment. In one embodiment, when a cavity is a rectangular parallelepiped with a length of 100 mm, 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 is used for the reaction tube.

[0098] The flow velocity of the reaction solution is 1.0 m / min or more, 1.5 m / min or more in one embodiment, and 4 m / min or more in one embodiment, and generally 50 m / min or less, and 20 m / min or less in one embodiment. The flow velocity of the reaction solution is, for example, 1.0 m / min to 50 m / min, 1.5 m / min to 50 m / min in one embodiment, and 4 m / min to 20 m / min in one embodiment.

[0099] By setting the flow velocity of the reaction solution to the above-described range, as illustrated in Fig. 6, in the reaction solution, after the spherical main portion is formed, the auxiliary portion extends from the main portion along the flow of the reaction solution, thereby allowing the formation of the metal particle comprising the main portion and the auxiliary portion extending from the main portion of the present invention. Therefore, the auxiliary portion can extend from any portion of the hemisphere surface at the upstream side of the flow of the reaction solution in the main portion along the flow of the reaction solution, and the metal particles of Figs. 1A to 1C are formed. Accordingly, since the method for forming the metal particles of the present invention according to the method of the present invention uses the reduction of the metal ions by microwave heating at a certain flow velocity or more of the reaction solution, the type of the metal particles produced by the method of the present invention may be a metal whose metal ions can be reduced by microwave heating. The metal particles produced by the method of the present invention may include not only the above-described metal particle, that is, silver particle, but also, for example, particles of gold, platinum, copper, nickel, iron, and cobalt.

[0100] The absorbed microwave power with respect to the reaction solution is 15 W / mL or more, 20 W / mL or more in one embodiment, and 50 W / mL or more in one embodiment, and generally 1000 W / mL or less, 800 W / mL or less in one embodiment, and 500 W / mL or less in one embodiment, in a microwave irradiation zone (corresponding to a volume of the reaction solution irradiated with the microwaves). The absorbed microwave power with respect to the reaction solution is, for example, 15 W / mL to 1000 W / mL, 20 W / mL to 800 W / mL in one embodiment, and 50 W / mL to 500 W / mL in one embodiment, in a microwave irradiation zone (corresponding to a volume of the reaction solution irradiated with the microwaves). Here, the "absorbed microwave power with respect to the reaction solution" means a value obtained by subtracting a reflected power irradiated and reflected on the reaction solution from the output of the microwave irradiation source, that is, a value represented by (output - reflected power). The reflected power can be measured using a detector in a microwave irradiation apparatus.

[0101] By adjusting the absorbed microwave power with respect to the reaction solution to the above-described range, the metal particle comprising the main portion and the auxiliary portion extending from the main portion of the present invention can be formed due to sufficient reducibility of the microwaves.

[0102] Other conditions of the microwaves in the method of the present invention are not limited. In the method of the present invention, the microwaves are irradiated by using a microwave synthesis apparatus while the above-described reaction solution flows, thereby promoting the reaction. When the reaction solution is irradiated with the microwaves, the polar solvent contained in the reaction solution absorbs the microwaves and converts them into a heat energy, thereby generating heat. Therefore, in the reaction solution irradiated with the microwaves, the temperature rises uniformly and rapidly at the irradiated portion, and a uniform and rapid reaction occurs in accordance with the temperature rise.

[0103] The microwaves are preferably uniformly irradiated to a target to be reacted, that is, a portion to be reacted in the reaction solution.

[0104] In the microwave synthesis apparatus, the material of the portion to be irradiated with the microwaves of the container that accommodates the reaction solution is not particularly limited insofar as it can be uniformly irradiated to the reaction solution. In the container that accommodates the reaction solution, for example, when microwaves are irradiated to the reaction solution from outside the reactor through the reactor, as the material of the portion to which the microwaves are irradiated, materials that allow the microwaves to pass through, that is, does not absorb the microwaves may be used. Examples of the materials include ceramics, glass, quartz, Teflon (registered trademark, PTFE and the like), and silicone, which are non-conductive materials having low dielectric constant (ε) and dielectric loss tangent (tan δ). In the container that accommodates the reaction solution, for the material of the portion where microwaves are not irradiated, in addition to the above-described materials, a metal such as aluminum or stainless steel, may be used. In the housing that accommodates the microwave irradiation source and the reaction tube to which microwaves are irradiated, the material is not particularly limited insofar as it does not allow leakage or absorption of microwaves, and examples of such materials include non-magnetic metal plates, such as aluminum plates.

[0105] The microwaves are generated from a microwave irradiation source (microwave oscillator (magnetron)), and the microwave irradiation source may be of either a single-mode system or a multi-mode system.

[0106] The frequency of the microwaves generated from the microwave irradiation source can be appropriately changed and is not particularly limited. The frequency of the microwaves is generally 1 GHz or more, and 2 GHz or more in one embodiment, and generally 10 GHz or less, and 6 GHz or less in one embodiment. The frequency of the microwaves is, for example, 1 GHz to 10 GHz, and 2 GHz to 6 GHz in one embodiment. In the present invention, as the frequency of the microwaves, it is preferred to use 2.45 GHz that is a frequency of industrial microwave power source.

[0107] The microwaves are preferably uniform during the irradiation, and the above-described irradiation conditions of the microwaves are also preferably constant during the microwave irradiation.

[0108] In the present invention, the temperature of the reaction solution increased by the microwave irradiation is a reaction temperature, and the reaction temperature may be changed as necessary based on the reaction conditions (solvent type, pressure during the reaction, and the like). The reaction temperature is, although not limited, generally 25°C or higher, and 80°C or higher in one embodiment. The upper limit of the reaction temperature is, although not limited, generally less than the boiling point of the solvent. For example, when the solvent is water, the reaction temperature is generally in a range of 25°C or higher and less than 100°C, and from 80°C to 95°C in one embodiment, at atmospheric pressure.

[0109] By setting the reaction temperature to 25°C or higher, a reduction reaction from metal ions to metal particles occurs, and by setting the reaction temperature to less than the boiling point of the solvent, it is possible to prevent variation in the particle size of the produced metal particles, that is, broadening of the particle size distribution, which may result from non-uniformity of the reaction field caused by boiling of the reaction solution. Accordingly, metal particles having a small and uniform particle size can be prepared.

[0110] The irradiation time of the microwaves to the reaction solution is the time required for the temperature of the reaction solution to reach the reaction temperature, and it can be appropriately changed depending on the reaction conditions (for example, the microwave conditions, the type of metal, the type of solvent, the pressure during the reaction, the amount of the reaction solution, and the reaction temperature). The irradiation time of the microwaves to the reaction solution is, although not limited, generally 0.1 seconds or more, and 0.5 seconds or more in one embodiment, and generally 600 seconds or less, and 30 seconds or less in one embodiment. The irradiation time of the microwaves to the reaction solution is, for example, 0.1 seconds to 600 seconds, and 0.5 seconds to 300 seconds in one embodiment. In the method of the present invention, since the reaction is carried out in a flow system, the irradiation time of the microwaves to the reaction solution can be adjusted by changing the length of the microwave irradiation zone or by circulating the reaction solution.

[0111] By irradiating the reaction solution with the microwaves under the above-described conditions to cause the temperature of the reaction solution to reach the reaction temperature, nuclei (main portions) of the metal particles are generated in the reaction solution. The nuclei (main portions) generated in the reaction solution then flow along the flow of the reaction solution, and as illustrated in Fig. 6, auxiliary portions are formed in the direction along the flow (from the main portions toward the upstream side of the reaction solution).

[0112] In the method of the present invention, the reaction solution is flowing, and the irradiation of the microwaves to the reaction solution is generally continued until the reaction is completed.

[0113] 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.

[0114] The dispersion containing the metal particles obtained by the method of the present invention can be subjected to separation and purification (for example, salting-out or centrifugation) by methods known in the technical field as necessary, to obtain the desired metal particle, powder containing the metal particle, or dispersion containing the metal particles.

[0115] The powder or dispersion containing the metal particles obtained by the method of the present invention contains the metal particles of the present invention in an amount of generally 2% or more, 10% or more in one embodiment, 18% or more in one embodiment, and 20% or more in one embodiment, with respect to the total number of metal particles contained in the powder or dispersion. The upper limit of the metal particles in the powder is not limited because the larger proportion of the metal particles is preferable.

[0116] The metal particles obtained by the method of the present invention contains the metal particle comprising the main portion and the auxiliary portion extending from the main portion. The metal particles and the powder and dispersion containing the metal particles are excellent in low-temperature sinterability and, for example, has the excellent sinterability at 80°C or lower. Furthermore, the sintered body formed from the metal particles has a low volume resistivity, for example, the volume resistivity of 30 µΩ•cm or less, 20 µΩ•cm or less in one embodiment, and 15 µΩ•cm or less in one embodiment.

[0117] The metal particles of the present invention, the powder and dispersion containing it, the metal particles produced by the method of the present invention, and the powder and dispersion containing it can be used as a conductive wiring material in the electronics packaging field, in addition to fields of the catalyst, the electronic component material, the ink material, and the like, and for example, can reduce the number of processes in the production of ink used for wiring board and the like.[Examples]

[0118] The following describes some examples of the present invention but is not intended to limit the present invention to those described in the examples.1. Particle SynthesisExamples 1 to 13 and Comparative Example 1

[0119] Silver particles were produced under conditions illustrated in Table 1.

[0120] First, a solution containing silver nitrate (AgNO 3 ) and a solution containing pure water, a reductant (DMF), and a dispersant (protective agent) (PVP or EDTA) (molecular weight in the table represents the weight average molecular weight) were each prepared. Subsequently, using a flow-type microwave synthesis apparatus, the above two solutions were mixed to prepare a reaction solution, and while the reaction solution was being flowed, microwaves (MW) were irradiated thereto. The compositions illustrated in Table 1 are compositions of the reaction solution. In the flow-type microwave synthesis apparatus, a rectangular parallelepiped with a length of 100 mm was used as a cavity, and the following was used as the reaction tube.Configuration of reaction tube

[0121] Tube inner diameter: 1 mm to 6 mm Tube outer diameter: 3 mm to 8 mm (wall thickness: 1 mm) Tube length: 100 mm

[0122] In each experiment, the reaction temperature was 60°C to 140°C. The amount of the dispersant was 0.1 weight% to 10 weight%.Comparative Example 2

[0123] The synthesis was carried out using a batch-type microwave synthesis apparatus with the composition of the reaction solution illustrated in Table 1. [Table 1]Synthesis ConditionsAg Concentration AgNO 3 [mM]ReductantProtective AgentAbsorbed MW Power [W / mL]Flow Velocity [m / min]Example 1100DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)2008Example 2100DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)30015Example 3100DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)6008Example 4100DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)101.5Example 5100DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)508Example 610DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)2008Example 7500DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)4008Example 8500DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)2008Example 9500DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)2008Example 10500DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)40015Example 11500DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)4004Example 12100DMF+H 2 O 4000 mMPVP (Molecular Weight 10000)2008Example 13100Citric Acid 600 mMEDTA2008Comparative Example 1100DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)38Comparative Example 2100DMF+H 2 O 4000 mMPVP (Molecular Weight 40000)2000 2. Evaluation Result(Transmission Electron Microscope (TEM) Evaluation)

[0124] TEM images were taken for the obtained dispersions of Examples 1 to 13 and Comparative Examples 1 and 2. The observation method by TEM and the particle size distribution measurement method are as follows. First, silver nanoparticles synthesized under each condition were dropped onto a TEM grid and dried to prepare samples. Thereafter, TEM observation was performed, and for 100 to 300 or more silver particles randomly selected, the projected area circle equivalent diameters (Heywood diameters) and the like of the main portion and the auxiliary portion were determined to evaluate the particle size and the size distribution.TEM measurement conditions

[0125] Apparatus: JEM-ARM300F TEM grid: Formvar, Cu Mesh 150P Accelerating voltage: 200 kV

[0126] Fig. 7 illustrates a TEM image of silver particles (composite particles) including the main portion and the auxiliary portion in Example 1 as an example. In Fig. 7, the dark-colored portion is a spherical particle in which multiple crystal lattice planes appear, that is, the main portion, and the light-colored portion is a particle having a smaller thickness compared with the portion of higher contrast, that is, the auxiliary portion. From the obtained TEM image, the ratio (number ratio) of composite particles among a total of randomly selected 300 particles, and the average values of the particle diameter (Heywood diameter) of the main portions, and the thickness, width, and length of the auxiliary portions (in the case where the auxiliary portion has a circular plate shape, the width and length are the same) in 100 composite particles were calculated. The thickness of the auxiliary portion was analyzed by comparing T values, which represent the transmittances of the electron beam during TEM-EELS measurement. Specifically, the thickness of the auxiliary portion was analyzed based on the difference in T values between particles having different contrasts in the TEM image. The main portion is composed of multiple crystal planes, and since the particle scale observed in the TEM image corresponds to the thickness in the depth direction, the T value thereof is determined. A specific example is explained with reference to Fig. 9. In Fig. 9, the average value of the Heywood diameter of the main portion was 21.9 nm. By the calculation of T / λ from the T value and the λ value, T / λ of the main portion was 0.40 and T / λ of the auxiliary portion was 0.25. Therefore, the thickness of the auxiliary portion was calculated as 0.25 × 21.9 / 0.4 = 13.7 nm. Table 2 illustrates the result. [Table 2]Metal Particles (Composite Particles)Volume ResistivityFormation of Main and Auxiliary PortionsComposite Particle Proportion [%]Main Portion Particle Size [nm]Auxiliary Portion Length / Main Portion Particle Size [nm]Auxiliary Portion Thickness [nm]Auxiliary Portion Width [nm]Firing at 80°C [µΩ·cm]Example 1Formed18141.25711.5Example 2Formed16122.1112614.2Example 3Formed16141.11298.9Example 4Formed19230.7151510.6Example 5Formed22191.2112010.9Example 6Formed21111.631011.2Example 7Formed25281.231812.3Example 8Formed28501.464813.5Example 9Formed34680.886528.9Example 10Formed26123.461624.7Example 11Formed21381.2273427.3Example 12Formed25131.291311.5Example 13Formed29131.181218.3Comparative Example 1Formed1180.916553Comparative Example 2Not Formed0240.00062

[0127] By comparing Examples 1 to 13 and Comparative Example 1 with Comparative Example 2 in the results illustrated in Table 2 and Fig. 7, it was found that the silver particle having the main portion and the auxiliary portion extending from the main portion of the present invention (composite particle) can be produced by setting the flow velocity of the reaction solution to 1.0 m / min or more. In this experiment, the flow velocity of 1.0 m / min corresponds to a flow rate of approximately 13 mL / min. Furthermore, by comparing Examples 1 to 13 with Comparative Example 1, it was found that, in order to increase the proportion of composite particles, the absorbed microwave power with respect to the reaction solution in the microwave irradiation zone may be set to 15 W / mL or more in addition to the increase of the flow velocity. Fig. 8 illustrates a relationship between the flow velocity and the absorbed power in the production of silver particles of Examples and Comparative Examples. In the TEM image illustrated in Fig. 9, crystal lattice planes at portions indicated by white lines of composite particles were measured. As a result of Fig. 9, no planes where crystallization characteristic of the main portion or the auxiliary portion readily proceeds were observed in the composite particles.

[0128] Subsequently, to obtain the shapes of the main portion and the auxiliary portion of the composite particles of the present invention in more detail, the ratio of the transmittance to the wavelength (T / λ) of the electron beam was measured from the TEM image at the center part (center of gravity) and the end part (edge portion of any one point) of the main portion and the center part (center of gravity) and the end part (edge portion of any one point) of the auxiliary portion of the composite particles. Table 3 illustrates the result. [Table 3]Main PortionAuxiliary PortionProjected Area Circle Equivalent DiameterT / λThicknessT / λCenterEndCenter / EndCenterEndCenter / EndComposite Particle 121.900.400.0220.0013.700.250.251.00Composite Particle 212.100.210.0211.107.200.130.140.93Composite Particle 35.700.100.024.8018.200.330.321.03

[0129] From Table 3, it was found that, in the main portion, T / λ is significantly different between the center part and the end part, and the difference increases as the projected area circle equivalent diameter increases. Thus, it was found that, the main portion has a spherical shape, and therefore, the thickness of the center part increases and a value of {(T / λ at center part) / (T / λ at end part)} increases as the particle size increases. Meanwhile, it was found that T / λ is approximately the same at the center part and the end part in the auxiliary portion. Thus, it was found that the auxiliary portion has a plate shape, and the thickness is approximately the same at the center part and the end part.(Volume Resistivity Evaluation)

[0130] For the powders obtained by drying the obtained dispersions of Examples 1 to 13 and Comparative Examples 1 and 2, the volume resistivity was measured. The volume resistivity was measured as follows.

[0131] (1) The silver particles in the silver particle dispersion liquid synthesized in the comparative examples or the examples were precipitated by centrifugation at 20000 rpm, and the supernatant was removed. (2) The operation of (1) was performed six times using pure water as a washing liquid for filtration. (3) The dispersion liquid obtained in (2) was concentrated by an evaporator until the solid content reached 15 weight%, thereby producing an ink. The solid content of the obtained dispersion liquid was measured as follows.

[0132] First, a small amount of the dispersion liquid was sampled and heated at 150°C for two hours to evaporate the solvent, thus measuring the residue in the dispersion liquid. Subsequently, a portion of the residue was heated up to 500°C using a thermogravimetric analyzer (TG analyzer) to burn off the organic components, thereby measuring the solid content of the residue. (4) The ink obtained in (3) was dropped onto a slide chamber (Slide & Chamber, 8-well, manufactured by Watson). (5) The silver particles dropped in (4) were sintered at 80°C in a forced-circulation constant-temperature oven (Yamato DN63 Constant Temperature Oven), and the volume resistivity of the sintered body was measured using a resistivity meter (Loresta-GX MCP-T700, manufactured by Nittoseiko Analytech Co., Ltd.). (6) The film thickness of the sintered body whose resistance was measured in (5) was determined by observing the cross section exposed by cutting the sintered body with SEM.

[0133] Table 2 illustrates the result. In Table 2, by comparing Examples 1 to 13 with Comparative Example 1, it was found that by increasing the proportion of the composite particles of the present invention with respect to the total silver particles in the powder or dispersion, the volume resistivity can be improved. Specifically, it was found that the powders containing the composite particles of Examples 1 to 13 exhibited the low volume resistivity, particularly the volume resistivity of 30 Ω•cm or less, when the powder was sintered at a low temperature of 80°C compared with the powders containing little or no composite particles of Comparative Examples 1 and 2. That is, it was found that the composite particle of the present invention is metal particles exhibiting the excellent sinterability, and in order to fully provide the performance, it is preferable to increase the proportion of the composite particles of the present invention with respect to the total metal particles in the powder or dispersion.

[0134] In order to confirm the usefulness of the composite particle of the present invention, the volume resistivity was measured also for the powder containing only spherical particles (average particle diameter: 13 nm) (Comparative Example 3), the powder containing only plate-shaped particles (average width and length: 40 nm, average thickness: 5 nm) (Comparative Example 4), a mixed powder having a weight ratio of 1:1 between spherical particles (average particle diameter: 42 nm) and plate-shaped particles (average width and length: 50 nm, average thickness: 10 nm) (Comparative Example 5), and a mixed powder of spherical particles and plate-shaped particles (8 nm and 15 nm, manufactured by Harima Chemicals, Inc.) (Comparative Example 6). As a result, the volume resistivity of the sintered body at 80°C of each powder was 32.8 µΩ•cm in Comparative Example 3, 51 µΩ•cm in Comparative Example 4, 78 µΩ•cm in Comparative Example 5, and 1173 µΩ•cm in Comparative Example 6. The volume resistivity in Comparative Examples 3 to 6 was greater than that of the powder containing the composite particle of the present invention.

[0135] It was found that, in the composite particle of the present invention, when the same amount of heat is applied to the main portion and the auxiliary portion, the temperature of the auxiliary portion rises more rapidly and reaches its melting point earlier compared with the main portion. Accordingly, the molten auxiliary portion functions as a joint and adhesive layer that sinters and joins the metal particle comprising the auxiliary portion to one or more other metal particles adjacent to the metal particle. As a result, the powder or dispersion containing the composite particles was able to exhibit the low volume resistivity, particularly the volume resistivity of 30 µΩ•cm or less even at the low firing temperature, that is, 80°C. Further, it was found that, since one silver particle of the present invention can be sintered and joined to one or more other silver particles, when the proportion of the composite particles with respect to the total silver particle in the powder or dispersion is a certain proportion, for example, 2% or more, and 10% or more in one embodiment in number ratio, even when it is not 100%, the effect of reducing the volume resistivity at low temperature of the sintered body formed from the powder or dispersion can be provided.

[0136] All publications, patents, and patent applications cited in this specification are incorporated herein by reference in their entireties.

Claims

1. A metal particle comprising a main portion and an auxiliary portion extending from the main portion, wherein the main portion has a spherical shape, wherein the auxiliary portion has a plate shape or linear shape, and wherein the auxiliary portion has a thickness smaller than a Heywood diameter of the main portion.

2. A metal particle comprising a main portion and an auxiliary portion extending from the main portion, wherein the main portion has a spherical shape, wherein the auxiliary portion has a spherical shape, and wherein the auxiliary portion has a Heywood diameter smaller than a Heywood diameter of the main portion.

3. A metal particle comprising a main portion and an auxiliary portion extending from the main portion, wherein the auxiliary portion has a heat capacity smaller than a heat capacity of the main portion.

4. A metal particle comprising a main portion and an auxiliary portion extending from the main portion, wherein a value of T / λ at a center part is 1.2 times or more of a value of T / λ at an end part in a TEM image of the main portion, wherein a value of T / λ at a center part is less than 2.0 times of a value of T / λ at an end part in a TEM image of the auxiliary portion, and wherein the center part means a center of gravity of a projected area of the main portion or the auxiliary portion, the end part means an edge part of the projected area of the main portion or the auxiliary portion, T indicates an electron beam transmittance, and λ indicates an electron beam wavelength.

5. A metal particle comprising a main portion and an auxiliary portion extending from the main portion, wherein the main portion has a spherical shape, and the main portion has a Heywood diameter of 2 nm to 100 nm, wherein the auxiliary portion has a plate shape, the auxiliary portion has a thickness of 2 nm to 40 nm, the auxiliary portion has a length of 2 nm to 60 nm, and the auxiliary portion has a width of 2 nm to 60 nm, and wherein the Heywood diameter of the main portion is 1.1 times to 8.0 times of the thickness of the auxiliary portion.

6. The metal particle according to any one of claims 1 to 5, wherein the metal is silver.

7. A dispersion comprising the metal particles according to any one of claims 1 to 6, and a solvent.

8. A powder comprising the metal particles according to any one of claims 1 to 6, wherein the metal particles is 10% or more with respect to a total number of the metal particles contained in the powder.

9. A method for producing metal particles including a step of irradiating a reaction solution with microwaves, the method comprising: (i) a step of preparing the reaction solution containing a metal particle precursor and a solvent; and (ii) a step of irradiating the reaction solution with microwaves while the reaction solution flows, wherein a flow velocity of the reaction solution is 1.0 m / min or more, and wherein an absorbed microwave power with respect to the reaction solution is 15 W / mL or more in a microwave irradiation zone.

10. The method according to claim 9, wherein the flow velocity of the reaction solution is 4 m / min to 20 m / min in the step (ii).

11. The method according to claim 9 or 10, wherein a reaction tube through which the reaction solution flows has an inner diameter of 1 mm to 20 mm in the step (ii).

12. The method according to any one of claims 9 to 11, wherein the metal particle precursor is a silver particle precursor in the step (i).

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