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

Abstract

Provided are metal particles having 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 metal particles having a main part and an auxiliary part extending from the main part. The main part is spherical and the auxiliary part has a plate shape or linear shape that is thinner than the main part.

Description

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

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

[0002] Metal nanoparticles, which can have properties different from those of bulk materials due to their nano-sized size, are being used and investigated for a variety of applications, such as catalysts, biomaterials, ink materials, and electronic component materials.

[0003] For example, Patent Document 1 discloses a method for producing a conductive filler comprising spherical silver (Ag) nanoparticles synthesized using a silver (Ag) compound, a solvent, a reducing agent, and a dispersant, and a conductive filler consisting of non-spherical metal fine particles, mixed in a ratio of 99:1 to 80:20, and the dispersant contains a thiol group (-SH), an amine group (-NH ２ The present invention discloses a composite metal microparticle material characterized in that the compound is a compound having at least one of the following:

[0004] Patent Document 2 discloses a conductive wiring material that includes a plurality of first metal nanoparticles and a plurality of second metal nanoparticles having a particle size smaller than that of the first metal nanoparticles, and that can be fired at a low temperature to melt the second metal nanoparticles and fill the spaces between the first metal nanoparticles.

[0005] Patent Document 3 discloses a polymer having an average particle diameter D ＴＥＭ : Discloses a silver fine powder consisting of silver particles of 3 to 20 nm.

[0006] Patent document 4 discloses a method for producing metal microparticles, in which a reaction liquid containing a metal precursor is circulated through a flow pipe, and electromagnetic waves are uniformly and intensively irradiated into the flow pipe along the length of the flow pipe, thereby uniformly heating the electromagnetic wave-irradiated space within the flow pipe along the flow direction, thereby generating metal microparticles.

[0007] JP 2011-038141 A JP 2006-279038 A JP 2009-138242 A JP 2011-137226 A

[0008] In the field of electronics packaging, the composite metal microparticle material described in Patent Document 1 and the metal nanoparticles in the conductive wiring material described in Patent Document 2 use a mixture of two types of microparticles with different particle sizes and shapes, which reduces the porosity of the sintered body formed by sintering the mixture, and as a result, the sintered body can have a low volume resistivity.

[0009] In such a mixture of two types of metal nanoparticles with different particle sizes and shapes, it is difficult to produce the two different types of metal nanoparticles in a single synthesis while controlling the ratio of each metal nanoparticle to a desired ratio. Therefore, as described in Patent Document 1, it is usually necessary to separately synthesize two types of metal nanoparticles with different particle sizes and shapes, optionally classify them, and then uniformly mix them.

[0010] However, the manufacturing method of this mixture has the problem of low productivity and high cost due to the large number of steps. Furthermore, in order to obtain a low volume resistivity for such a mixture of two types of metal nanoparticles with different particle sizes and shapes, the baking temperature must be at least 180°C, and therefore it cannot be used by printing and baking on polyethylene terephthalate (PET) or polycarbonate (PC).

[0011] On the other hand, as described in Patent Document 3, the firing temperature can be reduced by using nanoparticles in the sintered body.

[0012] However, to obtain nanoparticles with small particle size, a large amount of protective agent is required. The organic protective agent that may remain in the particles inhibits sintering and may limit the effect of lowering the firing temperature. Therefore, for such nanoparticles, the firing temperature is at most 120°C, and firing at temperatures below 100°C is difficult.

[0013] Furthermore, as described in Patent Document 4, metal nanoparticles with a uniform particle size distribution of 100 nm or less can be synthesized by irradiating electromagnetic waves into a flow system.

[0014] However, the generation of spherical nanoparticles with small particle diameters causes the same problems as those in Patent Document 3.

[0015] 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 producing the same, and a dispersion containing the metal particles.

[0016] The present inventors have investigated various means for solving the above problems, and as a result have found that in a method for producing metal particles using a flow-type microwave synthesis apparatus, by setting the microwave absorption power for the reaction liquid to 15 W / mL or more and the flow rate of the reaction liquid to 1.0 m / min or more, it is possible to synthesize microparticles having two different shapes (structures) per particle with a uniform particle size distribution, and the obtained microparticles have a volume resistivity of 30 Ω cm or less even when fired at a temperature of 80°C, thereby completing the present invention.

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

[0018] The present invention provides metal particles having low volume resistivity at low firing temperatures, a method for producing the same, and a dispersion containing the metal particles.

[0019] FIG. 1 is a diagram schematically showing the positional relationship between a main portion and an auxiliary portion in an embodiment of a metal particle of the present invention. FIG. 2 is a diagram schematically showing an embodiment of a metal particle of the present invention. FIG. 3 is a diagram schematically showing an embodiment of a metal particle of the present invention. FIG. 4 is a diagram schematically showing an embodiment of a metal particle of the present invention. FIG. 5 is a diagram schematically showing a state in which an embodiment of a metal particle of the present invention is deposited. FIG. 6 is a diagram schematically showing a state in which an embodiment of a metal particle of the present invention is formed. FIG. 7 is a TEM image of a composite particle of Example 1. FIG. 8 is a graph showing the relationship between flow rate and absorbed power in the production of silver particles of Examples and Comparative Examples. FIG. 9 is a photograph showing the area indicated by the white line in the TEM image of the composite particle of Example 1 and its crystal lattice plane.

[0020] Preferred embodiments of the present invention are described in detail below. In this specification, the features of the present invention are described with reference to the drawings as appropriate. In the drawings, the dimensions and shapes of each part are exaggerated for clarity and do not accurately depict the actual dimensions and shapes. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. The metal particles, metal particle manufacturing method, and metal particle-containing dispersion of the present invention are not limited to the following embodiments, and can be embodied in various forms incorporating modifications and improvements that can be made by those skilled in the art without departing from the spirit of the present invention. Furthermore, although each embodiment described in this specification is independent, two or more of each embodiment can also be combined to form one embodiment of the present invention.

[0021] The symbol "to" used herein means a range having the numerical values ​​before and after it as the upper and lower limits. In numerical ranges described in stages in this specification, the upper or lower limit described in one numerical range may be replaced with the upper or lower limit described in another stage. The upper or lower limit of a numerical range described herein may be replaced with a value shown in the examples.

[0022] The present invention relates to metal particles having a main portion and a secondary portion extending from the main portion.

[0023] Here, the "main part" means the main part of the metal particle.

[0024] The term "auxiliary portion" refers to a portion that extends from the main portion and has a certain physical property smaller than that of the main portion.

[0025] In one embodiment, the thickness of the main portion is greater than the thickness of the auxiliary portion. Here, "thickness" refers to the shortest length passing through the center of gravity of the main portion or auxiliary portion, for example, the diameter of the sphere if the main portion is spherical, or the length perpendicular to the planar extent if the main portion is plate-shaped. For example, the thickness of the main portion is typically 1.2 times or more, and in one embodiment, 1.5 times or more, and typically 20 times or less, and in one embodiment, 10 times or less, for example, 1.2 to 8.0 times, and in one embodiment, 1.5 to 5.0 times, the thickness of the auxiliary portion.

[0026] In one embodiment, the volume of the main portion is larger than the volume of the auxiliary portion. For example, the volume of the main portion is typically 1.2 times or more, and in one embodiment, 1.5 times or more, the volume of the auxiliary portion. Note that since sintering is promoted as the auxiliary portion becomes smaller, there is no upper limit to the ratio of the volume of the main portion to the volume of the auxiliary portion (volume of the main portion / volume of the auxiliary portion), but it is, for example, 50 or less, and in one embodiment, 30 or less.

[0027] 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 typically 1.2 times or more, and in one embodiment, 1.5 times or more, the weight of the auxiliary part. Note that, since sintering is promoted more easily as the auxiliary part becomes smaller, there is no upper limit to 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), as with the volume, but it is, for example, 50 or less, and in one embodiment, 30 or less.

[0028] In one embodiment, the heat capacity of the main portion is greater than the heat capacity of the auxiliary portion. Here, "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 is transferred to or from the object. "The heat capacity of the main portion is greater than the heat capacity of the auxiliary portion" means that when heat is applied to the metal particles, the temperature change of the main portion is smaller than the temperature change of the auxiliary portion. For example, the heat capacity of the main portion is typically 1.2 times or more the heat capacity of the auxiliary portion, and in one embodiment, 1.5 times or more. The upper limit of the ratio of the heat capacity of the main portion to the heat capacity of the auxiliary portion (heat capacity of the main portion / heat capacity of the auxiliary portion) is not limited, but is, for example, 50 or less, and in one embodiment, 30 or less.

[0029] Because the heat capacity of the main portion is greater than that of the auxiliary portion, when the same amount of heat is applied to the main portion and the auxiliary portion, the auxiliary portion of the metal particle of the present invention, having both a main portion and an auxiliary portion, rises in temperature and reaches its melting point more quickly than the main portion. Therefore, the molten auxiliary portion can act as a bonding / adhesive layer for sintering and bonding the metal particle having the auxiliary portion to one or more other metal particles adjacent to the metal particle. As a result, the metal particle of the present invention and the powder and dispersion containing the metal particle of the present invention described below have low volume resistivity even at low firing temperatures. Here, since each metal particle of the present invention can be sintered and bonded to one or more other nearby metal particles, the metal particles of the present invention contained in the powder and dispersion described below can exhibit the above effect by being contained in a certain proportion of the total metal particles contained therein. It can also be explained that the same effect can be exhibited when any physical property of the main portion, such as one or more physical properties selected from the group consisting of thickness, weight, and volume, is larger than the corresponding physical property of the auxiliary portion.

[0030] In one embodiment, in a transmission electron microscope (TEM) image of the main portion, the T / λ value at the center is 1.5 times or more, and in one embodiment, 2 times or more, of the T / λ value at the edge. Here, the center refers to the center of gravity of the projected area of ​​the main portion, the edge refers to the edge of the projected area of ​​the main portion, T is the transmittance of the electron beam, and λ is the wavelength of the electron beam. Note that one or more arbitrary points can be selected as the edge. In one embodiment, when the main portion is spherical, T / λ decreases with increasing distance from the center.

[0031] In one embodiment, in a TEM image of the auxiliary part, the T / λ value at the center is less than twice the T / λ value at the end, and in one embodiment, is 1.5 times or less. Here, the center refers to the center of gravity of the projected area of ​​the auxiliary part, the end refers to the edge of the projected area of ​​the auxiliary part, T is the transmittance of the electron beam, and λ is the wavelength of the electron beam. Note that one or more arbitrary points can be selected as the end. In one embodiment, when the auxiliary part is plate-shaped or linear, in a TEM image of the auxiliary part, the T / λ value at the center and the T / λ value at the end are approximately the same.

[0032] In one embodiment, the metal particle of the present invention has a spherical main portion and a plate-like or linear auxiliary portion. In one embodiment, the auxiliary portion is plate-like or linear and is composed of a plurality of, for example, two or more, spheres. In one embodiment, the particle size of the main portion is larger than the thickness of the auxiliary portion. Here, particle size refers to the Heywood diameter, and "Heywood diameter" refers to the diameter of a circle equivalent to the projected area of ​​a sphere when the object is a sphere in a photograph such as a TEM image. For example, the Heywood diameter of the main portion is typically 1.1 times or more, in one embodiment 1.2 times or more, and in one embodiment 1.5 times or more, but typically 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, and in one embodiment 1.5 times to 5.0 times the thickness of the auxiliary portion.

[0033] The metal particles of the present invention have spherical main portions and plate-like or linear auxiliary portions having a thickness smaller than the Heywood diameter of the main portions, and when the metal particles overlap, the auxiliary portions enter the gaps between the metal particles and act as a bonding / adhesion layer that sinters the main portions together, thereby accelerating sintering, as shown in Figure 5. The details of this effect are similar to the effect that occurs when the heat capacities of the main portions and auxiliary portions are different.

[0034] In one embodiment, the metal particles of the present invention have a spherical main portion and a spherical, e.g., 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 typically 1.1 times or more, in one embodiment 1.2 times or more, and in one embodiment 1.5 times or more, and typically 20 times or less, in one embodiment 10 times or less, for example, 1.1 to 8.0 times, in one embodiment 1.2 to 8.0 times, and in one embodiment 1.5 to 5.0 times the Heywood diameter of the auxiliary portion.

[0035] The metal particles of the present invention have spherical main portions and spherical auxiliary portions having a Heywood diameter smaller than that of the main portions, and when the metal particles overlap, the auxiliary portions enter the gaps between the metal particles and act as a bonding / adhesion layer that sinters the main portions together, thereby promoting sintering. The details of this effect are similar to the effect that occurs when the heat capacities of the main portions and the auxiliary portions are different.

[0036] In the metal particles of the present invention, the positional relationship between the main portion and the auxiliary portion is not limited as long as the auxiliary portion extends from the main portion.

[0037] In one embodiment, the main part and the auxiliary part are joined to each other at a joint portion of a certain area. Here, the main part and the auxiliary part have different thicknesses, and therefore can be distinguished, for example, by the difference in contrast in a TEM image. 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 an image corresponding to a plan view when a view from a direction that allows the thickness of the auxiliary part to be seen is taken as a front view, due to the structure of the metal particles of the present invention.

[0038] In one embodiment, the angle formed by the auxiliary portion and a tangent plane tangent to the main portion at their junction is typically between 0° (i.e., the auxiliary portion extends in the direction of the tangent plane) and 90° (i.e., the auxiliary portion extends furthest from the main portion or the tangent plane). In other words, the auxiliary portion can extend in any direction from the main portion.

[0039] FIG. 1 schematically shows 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 view of the metal particle of the present invention viewed from a direction in which the thickness of the auxiliary portion can be seen (hereinafter also referred to as a "front view"). In one embodiment, as shown in FIG. 1 (1), the angle formed by the tangent plane tangent to the main portion at the joint between the main portion and the auxiliary portion and the auxiliary portion is 90°. In one embodiment, as shown in FIG. 1 (2), the angle formed by the tangent plane tangent to the main portion at the joint between the main portion and the auxiliary portion and the auxiliary portion is 45°. In one embodiment, as shown in FIG. 1 (3), the angle formed by the tangent plane tangent to the main portion at the joint between the main portion and the auxiliary portion and the auxiliary portion is 0°.

[0040] In the metal particles of the present invention, there are no limitations on the crystal lattices of the main portion and the auxiliary portion.

[0041] The number of auxiliary parts per main part may 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 per main part.

[0042] The Heywood diameter, volume, weight, and heat capacity of the main portion and auxiliary portion of the metal particle can be calculated based on the image observed by TEM, the type of metal in each portion of the metal particle, and its properties.

[0043] In one embodiment, the metal particles are metal nanoparticles. Here, "metal nanoparticles" refers to metal particles having a particle size typically between 1 nm and 100 nm. Therefore, when the metal particles of the present invention are metal nanoparticles, the Heywood diameter of the entire metal particle, including the main portion and the auxiliary portion, is between 1 nm and 100 nm.

[0044] In one embodiment, the main portion is spherical. Here, "spherical" includes not only a perfect sphere when the main portion is observed with a transmission electron microscope (TEM), but also an approximately spherical shape, an oval sphere, a polygonal shape with approximately the same sides, and the like.

[0045] In one embodiment, the particle size (Heywood diameter) of the main portion is, but is not limited to, typically 2 nm or more, in one embodiment 5 nm or more, and typically 100 mm or less, in one embodiment 70 nm or less, and 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, and in one embodiment 6 nm to 60 nm.

[0046] Here, the particle size of the main part (Heywood diameter) can be determined by, for example, selecting 100 or more metal particles having any main part and auxiliary part in a TEM image, measuring the diameter value for each selected metal particle when the projected surface area of ​​the metal particle is converted into the area of ​​a circle, and averaging these values.

[0047] In one embodiment, the auxiliary portion is plate-shaped or linear, having a thickness thinner than that of the main portion. Here, "thinner than the main portion" means having a thickness thinner than the Heywood diameter of the main portion. "Plate-shaped" refers to a flat shape having a thickness thinner than its length and width. The shape based on the two directions of length and width is not limited, and examples include circular, approximately circular, elliptical, polygonal, such as triangular, quadrilateral (including square, rectangular, and trapezoidal), pentagonal, and hexagonal. The "length" of a plate-shaped portion refers to the distance from the main portion in the auxiliary portion, and the "width" of a plate-shaped portion refers to the distance perpendicular to the length of the auxiliary portion. In one embodiment, the auxiliary portion can have a shape in which the width and thickness are small but are of equal size. "Linear" refers to a shape in which the cross section perpendicular to the longitudinal direction of the auxiliary portion is circular, approximately circular, elliptical, or polygonal with approximately identical sides. As described above, the auxiliary portion may be formed into a plate-shaped or linear shape from multiple, for example, two or more, spheres.

[0048] Fig. 2 schematically shows one embodiment of a 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 shown in Fig. 2 is not limited, but is usually 1 nm or more, in one embodiment 2 nm or more, in one embodiment 5 nm or more, and usually 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, and in one embodiment 5 nm to 20 nm.

[0049] In one embodiment, the length of the auxiliary portion as shown in FIG. 2 is not limited, but is typically 2 nm or more, in one embodiment 3 nm or more, in one embodiment 5 nm or more, and is typically 80 mm or less, in one embodiment 60 nm or less, for example, 2 nm to 60 nm, in one embodiment 2 nm to 40 nm, and in one embodiment 5 nm to 30 nm.

[0050] Figure 3 schematically shows an embodiment of the metal particles of the present invention. Note that (1) in Figure 3 is a front view of the metal particles of the present invention, and (2) in Figure 3 is a plan view of the metal particles of the present invention. In one embodiment, when the plate-like shape of the auxiliary part is a quadrilateral, the thickness of the auxiliary part as shown in (1) of Figure 3 is not limited, but is usually 1 nm or more, in one embodiment it is 2 nm or more, in one embodiment it is 5 nm or more, and is usually 60 nm or less, in one embodiment it is 40 nm or less, for example, it is 1 nm to 40 nm, in one embodiment it is 2 nm to 40 nm, in one embodiment it is 5 nm to 40 nm, and in one embodiment it is 5 nm to 20 nm. Also, the length of the auxiliary part as shown in (1) of Figure 3 is not limited, but is usually 2 nm or more, in one embodiment it is 3 nm or more, in one embodiment it is 5 nm or more, and is usually 80 nm or less, in one embodiment it is 60 nm or less, for example, it is 2 nm to 60 nm, in one embodiment it is 2 nm to 40 nm, and in one embodiment it is 5 nm to 30 nm. Further, the width of the supplementary part as shown in (2) of Figure 3 is not limited, but is usually 2 nm or more, in one embodiment it is 5 nm or more, and is usually 100 nm or less, in one embodiment it is 80 nm or less, in one embodiment it is 70 nm or less, for example, it is 2 nm to 70 nm, in one embodiment it is 2 nm to 60 nm, and in one embodiment it is 5 nm to 50 nm. Note that the ratio of the length to the width (length / width) of the supplementary part is not limited, but is usually 0.4 or more, in one embodiment it is 0.5 times or more, and is usually 10 times or less, in one embodiment it is 5 times or less, for example, it is 0.5 times to 5 times, and in one embodiment it is 1 times to 5 times. Also, the ratio of the length of the auxiliary part to the Heywood diameter of the main part (length of the auxiliary part / Heywood diameter of the main part) is not limited, but is usually 0.2 or more, in one embodiment it is 0.5 or more, and is usually 4 or less, in one embodiment it is 3 or less, for example, it is 0.2 to 4, and in one embodiment it is 0.5 to 3.

[0051] Figure 4 schematically shows one embodiment of a metal particle of the present invention. Figure 4 (1) is a front view of a metal particle of the present invention, and Figure 4 (2) is a plan view of a metal particle of the present invention. In one embodiment, when the plate-like shape of the auxiliary part is circular, the circle-equivalent diameter of the auxiliary part as shown in Figure 4 (1) and (2) is not limited, but is usually 2 nm or more, in one embodiment 5 nm or more and usually 100 nm or less, in one embodiment 80 nm or less, for example 2 nm to 60 nm, and in one embodiment 5 nm to 30 nm.

[0052] Here, the length, thickness, length, width, or equivalent circle diameter of the auxiliary portion can be determined, similarly to the thickness (Heywood diameter) of the main portion, by, for example, selecting 100 or more metal particles having any main portion and auxiliary portion in a TEM image, measuring the length, thickness, length, width, or equivalent circle diameter of the auxiliary portion of each selected metal particle, and averaging the measurements. Note that the thickness of the auxiliary portion can be calculated, for example, by comparing the ratio (T / λ) of the electron beam transmittance to the electron beam wavelength in TEM-EELS at the center of the main portion (i.e., the ratio (T / λ) of the electron beam transmittance to the electron beam wavelength at a length equivalent to the Heywood diameter) with the ratio (T / λ) of the electron beam transmittance to the electron beam wavelength in TEM-EELS at the center of the auxiliary portion.

[0053] The metal particles of the present invention have spherical main portions having the dimensions described above and plate-like or linear auxiliary portions having a thickness smaller than the Heywood diameter of the main portions, so that when the metal particles overlap, the auxiliary portions enter the gaps between the metal particles and act as a bonding / adhesion layer that sinters the main portions together, thereby accelerating sintering, as shown in Figure 5. The details of this effect are similar to the effect that occurs when the heat capacities of the main portions and auxiliary portions are different.

[0054] In one embodiment, the auxiliary portion is spherical. Here, "spherical" includes not only a perfect sphere when the main portion is observed with a transmission electron microscope (TEM), but also an approximately spherical shape, an oval sphere, a polygonal shape with approximately the same sides, and the like.

[0055] In one embodiment, the particle diameter (Heywood diameter) of the auxiliary portion is, but is not limited to, typically 1 nm or more, in one embodiment 2 nm or more, in one embodiment 5 nm or more, and typically 60 nm or less, in one embodiment 40 nm or less, for example, 1 nm to 40 nm, in one embodiment 5 nm to 40 nm, and in one embodiment 5 nm to 20 nm.

[0056] Here, the particle diameter of the auxiliary portion (Heywood diameter) can be determined by, for example, selecting 100 metal particles having any main portion and auxiliary portion in a TEM image, measuring the diameter value when the projected surface area of ​​each selected metal particle is converted into the area of ​​a circle, and averaging these values.

[0057] The metal particles of the present invention have spherical main portions having the dimensions described above and spherical auxiliary portions having a Heywood diameter smaller than that of the main portions, so that when the metal particles overlap, the auxiliary portions fill the gaps between the metal particles and act as a bonding / adhesion layer that sinters the main portions together, thereby promoting sintering. The details of this effect are similar to the effect that occurs when the heat capacities of the main portions and the auxiliary portions are different.

[0058] The metal in the metal particles is not limited. Examples of metals include noble metals, base metals, and alloys, such as gold, silver, platinum, copper, nickel, iron, cobalt, and alloys of two or more of these. The metal is preferably silver or nickel, and more preferably silver. In metal particles, the main part and the auxiliary part are usually composed of the same metal.

[0059] By having the main portion and auxiliary portion described above, the metal particles of the present invention can promote sintering by allowing the auxiliary portion to fill the gaps between the metal particles when the metal particles overlap, and act as a bonding / adhesion layer that sinters the main portions together, as shown in Figure 5. The details of this effect are similar to the effect that occurs when the heat capacities of the main portion and auxiliary portion are different.

[0060] The present invention also relates to a powder containing the metal particles of the present invention described above.

[0061] The powder of the present invention contains the metal particles of the present invention in an amount of typically 2% or more, in one embodiment 10% or more, in one embodiment 18% or more, and in one embodiment 20% or more of the total number of metal particles contained in the powder. The upper limit of the amount of the metal particles of the present invention in the powder is not limited, as a higher proportion of the metal particles of the present invention is preferable.

[0062] Here, in the powder of the present invention, the proportion of the metal particles of the present invention in the powder can be determined, for example, by selecting 300 or more arbitrary metal particles from a TEM image and calculating the proportion of metal particles having a main portion and an auxiliary portion among all the selected metal particles.

[0063] 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 contains, in addition to the metal particles of the present invention, metal particles that are the same metal as the metal particles of the present invention but have a different shape from the metal particles of the present invention. Examples of metal particles that have a different shape from the metal particles of the present invention include, but are not limited to, metal particles having a spherical, plate-like, linear, or other shape. The size of the metal particles that have a different shape from the metal particles of the present invention is not limited, but is usually the same as the size of the metal particles of the present invention, particularly the size of the main portion of the metal particles of the present invention. As the metal particles that have a different shape from the metal particles of the present invention, metal particles that do not have the auxiliary portion of the metal particles of the present invention, i.e., particles consisting only of the main portion of the metal particles of the present invention, are preferred.

[0064] The powder of the present invention may contain an organic substance as a dispersant (also referred to as a "protective agent") during production or dispersion in a solvent. Examples of dispersants include the dispersants described below. The content of the dispersant is not limited. The content of the dispersant depends, for example, on the amount of dispersant used in the method for producing metal particles of the present invention described below.

[0065] The powder of the present invention contains metal particles having a main portion and an auxiliary portion extending from the main portion, and has excellent low-temperature sinterability, for example, excellent sinterability at 80°C or less. Furthermore, a sintered body formed from the powder has a low volume resistivity, for example, a volume resistivity of 30 μΩ cm or less, in one embodiment 20 μΩ cm or less, and in one embodiment 15 μΩ cm or less.

[0066] 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.

[0067] The content of the metal particles in the dispersion is not limited. For example, the content of the metal particles is, relative to the total weight of the dispersion, 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, and in one embodiment 70% by weight or more, and is usually 95% by weight or less, in one embodiment 90% by weight or less, in one embodiment 85% by weight or less, and in one embodiment 80% by weight or less, for example, 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.

[0068] By increasing the metal particle content of the dispersion of the present invention, it is possible to reduce the cost of the solvent, the time, labor, and cost required to evaporate the solvent after applying the dispersion, as well as storage costs and transportation costs, thereby reducing the environmental impact.

[0069] The dispersion of the present invention may contain an organic substance as a dispersant for use in producing the metal particles or powder of the present invention or dispersing them in a solvent. Examples of dispersants include the dispersants described below. The content of the dispersant is not limited. The content of the dispersant depends, for example, on the amount of dispersant used in the method for producing metal particles of the present invention described below.

[0070] The solvent contained in the dispersion of the present invention may be any solvent known in the art, and may be selected from, but is not limited to, solvents that are liquid at 20°C, such as water, alcohols, aldehydes, carboxylic acids, ethers, esters, amines, monosaccharides, polysaccharides, straight-chain hydrocarbons, fatty acids, and aromatics, and two or more of the above-mentioned solvents may be used in combination.

[0071] The boiling point of the solvent is not particularly limited, but is typically 100°C or higher, and in one embodiment, 130°C or higher, and in one embodiment, 150°C or higher, and typically 300°C or lower, and in one embodiment, 250°C or lower, and in one embodiment, 200°C or lower, for example, 100°C to 300°C, and in one embodiment, 130°C to 250°C, and in one embodiment, 150°C to 200°C. If the boiling point of the solvent is 100°C or higher, for example, when the dispersion is used as an ink paste, the solvent can be prevented from volatilizing at room temperature, thereby ensuring the viscosity stability and coatability of the ink paste. Furthermore, if the boiling point of the solvent is 300°C or lower, the solvent can be prevented from remaining in the metal sintered body without evaporating at the temperature at which the semiconductor element is connected to the support member during firing, particularly during a bonding process using pressureless firing, and as a result, the properties of the metal sintered body can be better maintained.

[0072] As the solvent, it is preferable to select a solvent suitable for dispersing metal particles from the solvents described above. Specifically, it is preferable to select a solvent having an alcohol structure, an ether structure, or an ester structure in order to improve the thermal conductivity, electrical conductivity, and adhesive strength of the metal sintered body. Examples of solvents contained in the dispersion of the present invention include butyl cellosolve, carbitol, butyl cellosolve acetate, carbitol acetate, ethylene glycol diethyl ether, dipropylene glycol methyl ether acetate, dipropylene glycol mono-n-butyl ether, dipropylene glycol mono-n-methyl ether, terpineol, ethylene glycol, isobornylcyclohexanol, and tributyrin. As the solvent contained in the dispersion of the present invention, ethylene glycol is preferred.

[0073] When the dispersion of the present invention is used as an ink paste, the amount of solvent contained in the ink paste may vary depending on the content of metal particles contained in the dispersion, etc., but is usually 5% by weight or more and usually 99% by weight or less, and in one embodiment 90% by weight or less, of the total weight of the ink paste.

[0074] When the dispersion of the present invention is used as an ink paste, by adjusting the amount of solvent within the above-mentioned range, the viscosity of the ink paste can be adjusted to an appropriate viscosity range described below, and further, volume shrinkage associated with the evaporation of the solvent when the ink paste is sintered can be suppressed, thereby improving the density of the silver sintered body that is formed.

[0075] The dispersion of the present invention may further contain components other than the metal particles and solvent of the present invention, provided that the effects of the present invention are not impaired. Examples of components that may be added in addition to the metal particles and solvent of the present invention include those known in the art, and are not limited thereto. Examples include additives such as carboxylic acids that have a boiling point of 400°C or less at atmospheric pressure and are solid at 20°C, such as stearic acid, lauric acid, docosanoic acid, sebacic acid, and 1,16-octadecanedioic acid; metal particles other than the metal particles of the present invention; anti-settling agents for metal particles in the dispersion; and fluxing agents for promoting sintering of metal particles. The amount of components that may be added in addition to the metal particles and solvent of the present invention is typically 0% by weight or more and typically 10% by weight or less, and in one embodiment, 1% by weight or less, for example, 0% by weight to 10% by weight, and in one embodiment, 0% by weight to 1% by weight, based on the total weight of the dispersion.

[0076] When the dispersion of the present invention is used as an ink paste, the viscosity of the ink paste, as measured with a cone-plate viscometer, is usually 10 Pa s or more and usually 1,000 Pa s or less. The viscosity can be appropriately adjusted by the type and amount of metal particles, the type and amount of polymer as a dispersant, the type and amount of solvent, etc.

[0077] By adjusting the viscosity of the ink paste to fall within the above range, the applicability of the ink paste can be improved and bleeding after application of the ink paste can be prevented.

[0078] By including the metal particles of the present invention in the dispersion, when the metal particles overlap after the solvent in the dispersion has evaporated, the auxiliary portions penetrate into the gaps between the metal particles and act as a bonding / adhesive layer that sinters the main portions together, thereby promoting sintering. The details of this effect are similar to the effect that occurs when the main portion and the auxiliary portion have different heat capacities. Therefore, the dispersion of the present invention includes metal particles having a main portion and an auxiliary portion extending from the main portion. After the solvent in the dispersion has evaporated, the dispersion has excellent low-temperature sinterability, for example, excellent sinterability at 80°C or less. Furthermore, the sintered body formed from the dispersion has a low volume resistivity, for example, a volume resistivity of 30 μΩ·cm or less, in one embodiment, 20 μΩ·cm or less, and in one embodiment, 15 μΩ·cm or less.

[0079] 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 by a method for producing metal particles comprising a step of irradiating a reaction solution with microwaves, in which the reaction solution is irradiated with microwaves while flowing, and by controlling the flow rate of the reaction solution and the absorbed power of the microwaves.

[0080] (i) Step of preparing a reaction liquid containing a metal particle precursor and a solvent In step (i), a reaction liquid containing a metal particle precursor and a solvent is prepared.

[0081] In the method of the present invention, the solvent used in the reaction solution is not limited as long as it is a polar solvent or ionic liquid that can dissolve materials such as a metal particle precursor, a dispersant, and a reducing agent and can absorb microwaves. Examples of solvents used in the reaction solution include low-boiling-point solvents with a boiling point of 300°C or less. Examples of low-boiling-point solvents include, but are not limited to, water, alcohols such as methanol and ethanol, polyhydric alcohol solvents such as ethylene glycol, ketone solvents such as acetone, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), other organic solvents, and mixtures of two or more of these low-boiling-point polar solvents. When a mixture is used as a solvent, the ratio of each component contained in the mixture is not limited, as long as the components are miscible under the experimental conditions.

[0082] By using a low boiling point polar solvent as the solvent, the handling of the solvent can be improved and the burden on the environment can be reduced.

[0083] The metal particle precursor may be any material that dissolves in a solvent to produce metal ions, such as ions of precious metals, base metals, and alloys, such as gold, silver, platinum, copper, nickel, iron, and cobalt, or ions of two or more of these metals. Examples of metal particle precursors include inorganic metal salts such as metal halides, fluorides, chlorides, bromides, and iodides; metal sulfates, metal nitrates, metal phosphates, and metal cyanides; organic metal salts such as metal carboxylates and metal sulfonates; and metal complexes, including metal complex salts. The metal particle precursor may be prepared, for example, by dissolving a material containing a metal or a metal salt in an acid such as nitric acid or a base such as aqueous ammonia. It is preferable to use inexpensive metal nitrates, such as nickel nitrate and silver nitrate, as the metal particle precursor. Alternatively, formates, which have reducing properties, may be used as the metal particle precursor. Therefore, in one embodiment, when a salt composed of a metal ion and a reducing organic anion, such as a formate ion, as its counter anion is used as the metal particle precursor, a reducing agent may not be used.

[0084] A homogeneous reaction solution can be prepared by dissolving the metal particle precursor in a solvent.

[0085] The concentration of the metal ions in the reaction solution is not limited as long as it is not more than the saturated concentration, but is usually 1 mmol / L (mM) or more, and in one embodiment 1.5 mM or more, and usually 500 mM or less, and in one embodiment 300 mM or less, for example, 1 mM to 500 mM, and in one embodiment 1.5 mM to 300 mM.

[0086] By setting the concentration of metal ions in the reaction solution within the above range, metal particles can be efficiently produced at a high concentration, the amount of metal particles that can be produced and recovered at one time can be significantly increased, and the time, labor, and cost required for producing metal particles can be reduced. In addition, the variation in the obtained metal particles can be reduced, in other words, the particle size distribution of the obtained metal particles can be narrowed.

[0087] The reaction solution may contain a dispersant. Examples of dispersants include, but are not limited to, polyvinylpyrrolidone (PVP), thiol-based polymers, polyvinyl alcohol (PVA), polyacrylic acid, polyacrylates, cyclodextrin, aminopectin, methylcellulose, 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 the dispersant is not limited, but is, for example, a weight average molecular weight (Mw) of typically 1,000 or more, in one embodiment 10,000 or more, and typically 50,000 or less, in one embodiment 40,000 or less, for example 1,000 to 50,000, in one embodiment 8,000 to 50,000, and in one embodiment 10,000 to 40,000. The amount of the dispersant is generally 0.1% by weight or more, and in one embodiment 0.5% by weight or more, and generally 10% by weight or less, and in one embodiment 5% by weight or less, for example, 0.1 to 10% by weight, and in one embodiment 0.5 to 5% by weight, based on the total weight of the reaction liquid.

[0088] The reaction solution may further contain a reducing agent, which is a material capable of reducing metal ions to a metal with an oxidation number of 0 through an oxidation-reduction reaction.

[0089] The reducing agent is not limited. Examples of the reducing agent include citric acid or citrates such as trisodium citrate, disodium citrate, and monosodium citrate; oxalic acid or oxalate salts such as sodium oxalate; ascorbic acid or ascorbate salts such as sodium ascorbate; formic acid or formate salts such as sodium formate; DMF; and mixtures of two or more thereof. In one embodiment, the reducing agent for metal ions, particularly silver ions, is DMF. Therefore, when DMF is used as a solvent for the reaction solution, DMF can also function as a reducing agent, so that a reducing agent other than DMF does not need to be used.

[0090] The amount of reducing agent is not limited as long as it can reduce metal ions to a metal whose oxidation number is 0 through a redox reaction, but is typically 1.0 equivalent or more, in one embodiment 4.0 equivalents or more, and typically 20 equivalents or less, in one embodiment 15 equivalents or less, for example 1.0 to 20 equivalents, and in one embodiment 4.0 to 15 equivalents, relative to the metal ions. Note that when a reducing agent for metal ions contains one or more functional groups capable of interacting with metals, such as a carboxy group, a hydroxy group, or an ether group, it may also function as a dispersant. When the reducing agent also functions as a dispersant, the reaction solution need not contain the dispersant described above. Furthermore, the amount of reducing agent for metal ions may be an amount exceeding the amount necessary to reduce metal ions to a metal whose oxidation number is 0 through a redox reaction.

[0091] The reaction solution may be composed of the metal particle precursor, solvent, dispersant, and reducing agent described above, but may also contain additives that are commonly used in conventional methods for producing metal particles by irradiating microwaves. The amount of additive is not limited, but is usually 10 wt % or less, and in one embodiment, 3 wt % or less, based on the total weight of the reaction solution. Since the additive does not need to be added, there is no lower limit for the additive.

[0092] The pH of the reaction solution is not limited, but is usually pH 3 to pH 12.

[0093] In the present invention, the order of addition of each material, the addition temperature, the mixing method, the mixing time, etc. in preparing the reaction solution are not limited, and the materials are mixed so as to prepare a homogeneous reaction solution. In the present invention, the reaction is started after the homogeneous reaction solution is prepared.

[0094] (ii) Step of irradiating with microwaves while flowing the reaction solution In step (ii), microwaves are irradiated while flowing the reaction solution prepared in step (i).

[0095] The method of flowing the reaction solution is not limited as long as the reaction solution flows in any direction. For example, the reaction solution can be flowed at a constant speed in a reaction tube, such as a straight tube or a spiral tube.

[0096] In one embodiment, when the reaction tube is a straight tube, the size and shape are not limited as long as the microwaves are uniformly irradiated throughout the reaction tube. In one embodiment, the inner diameter of the tube is typically 1 mm or more, in one embodiment, 2 mm or more, in one embodiment, 4 mm or more, and typically 20 mm or less, in one embodiment, 10 mm or less, for example, 1 mm to 20 mm, in one embodiment, 2 mm to 20 mm, or in one embodiment, 4 mm to 10 mm. In one embodiment, when the cavity is a rectangular parallelepiped with a length of 100 mm, the reaction tube used has an inner diameter of 1 mm to 6 mm, an outer diameter of 3 mm to 8 mm (wall thickness: 1 mm), and a length of 100 mm.

[0097] The flow velocity of the reaction liquid is 1.0 m / min or more, in one embodiment 1.5 m / min or more, and in one embodiment 4 m / min or more, and is usually 50 m / min or less, and in one embodiment 20 m / min or less, for example, 1.0 m / min to 50 m / min, in one embodiment 1.5 m / min to 50 m / min, and in one embodiment 4 m / min to 20 m / min.

[0098] By setting the flow rate of the reaction solution within the above range, as shown in Figure 6, a spherical main portion is formed in the reaction solution, and then an auxiliary portion extends from the main portion along the flow of the reaction solution, thereby forming metal particles of the present invention having a main portion and an auxiliary portion extending from the main portion. Therefore, the auxiliary portion can extend along the flow of the reaction solution from any part of the hemispherical surface of the main portion that is upstream of the flow of the reaction solution, resulting in the formation of metal particles such as those shown in Figures 1A to 1C. Therefore, since the method for forming metal particles of the present invention involves the reduction of metal ions by microwave heating at a flow rate of the reaction solution equal to or greater than a certain level, the type of metal particles produced by the method of the present invention may be any metal whose metal ions can be reduced by microwave heating. Examples of the metal particles produced by the method of the present invention include not only the above-mentioned metal particles, i.e., silver particles, but also, for example, gold, platinum, copper, nickel, iron, and cobalt particles.

[0099] The microwave absorption power of the reaction solution in the microwave irradiation section (corresponding to the volume of the reaction solution irradiated with microwaves) is 15 W / mL or more, in one embodiment 20 W / mL or more, in one embodiment 50 W / mL or more, and typically 1000 W / mL or less, in one embodiment 800 W / mL or less, in one embodiment 500 W / mL or less, for example, 15 W / mL to 1000 W / mL, in one embodiment 20 W / mL to 800 W / mL, and in one embodiment 50 W / mL to 500 W / mL. Here, "microwave absorption power of the reaction solution" refers to the value obtained by subtracting the reflected power irradiated and reflected by the reaction solution from the output of the microwave irradiation source, i.e., a value expressed as (output - reflected power). The reflected power can be measured using a detector in the microwave irradiation device.

[0100] By adjusting the microwave absorption power for the reaction solution within the above range, the metal particles of the present invention having a main portion and an auxiliary portion extending from the main portion can be formed due to sufficient reduction by the microwave.

[0101] Other microwave conditions in the method of the present invention are not limited. In the method of the present invention, the reaction solution described above is irradiated with microwaves using a microwave synthesis device while flowing, and the reaction proceeds. When the reaction solution is irradiated with microwaves, the polar solvent contained in the reaction solution absorbs the microwaves and converts them into thermal energy, thereby generating heat. Therefore, in the reaction solution irradiated with microwaves, a uniform and rapid temperature rise occurs in the irradiated part, and a uniform and rapid reaction occurs in accordance with this temperature rise.

[0102] It is preferable that the microwaves be uniformly irradiated onto the target of reaction, that is, the part of the reaction solution where the reaction occurs.

[0103] In a microwave synthesis apparatus, the material of the portion of the container containing the reaction solution that is irradiated with microwaves is not limited as long as it can uniformly irradiate the reaction solution with microwaves. For example, when microwaves are irradiated onto the reaction solution from outside the reactor via the reactor, the material of the portion of the container containing the reaction solution that is irradiated with microwaves can be a material that transmits microwaves, i.e., does not absorb microwaves, such as ceramics, glass, quartz, Teflon (registered trademark, PTFE, etc.), silicone, or other non-conductive materials with a small relative dielectric constant ε or dielectric loss angle tan δ. In addition to the above-mentioned materials, metals such as aluminum and stainless steel can also be used for the portion of the container containing the reaction solution that is not irradiated with microwaves. Furthermore, the housing that houses the microwave irradiation source and the reaction tube that irradiates microwaves is not limited as long as it is a material that does not leak or absorb microwaves, and examples thereof include non-magnetic metal plates, such as aluminum plates.

[0104] The microwaves are generated by a microwave radiation source (microwave generator (magnetron)), which can be either a single-mode system or a multi-mode system.

[0105] The frequency of the microwaves generated from the microwave irradiation source can be changed appropriately and is not limited. The microwave frequency is usually 1 GHz or higher, and in one embodiment, 2 GHz or higher, and usually 10 GHz or lower, and in one embodiment, 6 GHz or lower, for example, 1 GHz to 10 GHz, and in one embodiment, 2 GHz to 6 GHz. In the present invention, it is preferable to use 2.45 GHz, which is the frequency of an industrial microwave power source, as the microwave frequency.

[0106] The microwaves are preferably uniform during irradiation, and the microwave irradiation conditions are preferably constant during microwave irradiation.

[0107] In the present invention, the temperature of the reaction solution raised by microwave irradiation is the reaction temperature, which can be appropriately changed depending on the reaction conditions (such as the type of solvent and the pressure during the reaction) and is not limited, but is usually 25°C or higher, and in one embodiment, 80°C or higher. The upper limit of the reaction temperature is not limited, but is usually lower than the boiling point of the solvent. For example, when the solvent is water, the reaction temperature is usually in the range of 25°C or higher and lower than 100°C, and in one embodiment, 80°C to 95°C, at atmospheric pressure.

[0108] By setting the reaction temperature to 25°C or higher, a reduction reaction from metal ions to metal particles occurs, and by setting the reaction temperature below the boiling point of the solvent, it is possible to prevent the particle size of the resulting metal particles from becoming uneven due to the non-uniformity of the reaction field that can occur when the reaction solution boils, i.e., the broadening of the particle size distribution, and to prepare metal particles with small and uniform particle sizes.

[0109] The microwave irradiation time for the reaction solution is the time required for the temperature of the reaction solution to reach the reaction temperature, and is appropriately changed depending on the reaction conditions (microwave conditions, type of metal, type of solvent, pressure during the reaction, amount of reaction solution, reaction temperature, etc.), but is not limited to, and is usually 0.1 seconds or more, and in one embodiment 0.5 seconds or more, and usually 600 seconds or less, and in one embodiment 30 seconds or less, for example, 0.1 seconds to 600 seconds, and in one embodiment 0.5 seconds to 300 seconds. In the method of the present invention, the reaction is carried out in a flow system, and therefore the microwave irradiation time for the reaction solution can be adjusted by adjusting the length of the microwave irradiation section or circulating the reaction solution.

[0110] By irradiating the reaction solution with microwaves under the above conditions and raising the temperature of the reaction solution to the reaction temperature, 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, thereby forming auxiliary parts in the direction of the flow (from the main parts toward the upstream direction of the reaction solution) as shown in Figure 6.

[0111] In the method of the present invention, the reaction solution is circulated, and microwave irradiation of the reaction solution is generally continued until the reaction is completed.

[0112] The completion of the reaction can be determined by observing that the absorbance, etc., derived from the metal particle precursor material or metal particles in the reaction solution no longer changes. For example, when silver particles are used as the metal particle precursor material, the change in the absorbance of the reaction solution at 280 nm to 780 nm is observed, and the time when the absorbance no longer changes is regarded as the time when the reaction is complete.

[0113] The dispersion containing metal particles obtained by the method of the present invention can be subjected to separation and purification (e.g., salting out or centrifugation) by methods known in the art, as needed, to obtain the desired metal particles, powder containing metal particles, or dispersion containing metal particles.

[0114] The powder or dispersion containing metal particles obtained by the method of the present invention contains the metal particles of the present invention in an amount of typically 2% or more, in one embodiment 10% or more, in one embodiment 18% or more, and in one embodiment 20% or more, based on the total number of metal particles contained in the powder or dispersion. Note that the upper limit of the metal particles in the powder is not limited, as a higher proportion of metal particles is preferable.

[0115] The metal particles obtained by the method of the present invention include metal particles having a main portion and an auxiliary portion extending from the main portion, and the metal particles and powders and dispersions containing the metal particles have excellent low-temperature sinterability, for example, excellent sinterability at 80°C or less, and further, a sintered body formed from the metal particles has a low volume resistivity, for example, a volume resistivity of 30 μΩ cm or less, in one embodiment 20 μΩ cm or less, and in one embodiment 15 μΩ cm or less.

[0116] The metal particles of the present invention, powders and dispersions containing the metal particles, and metal particles, powders and dispersions containing the metal particles produced by the method of the present invention can be used in fields such as catalysts, electronic component materials, and ink materials, as well as conductive wiring materials in the electronics packaging field, and can reduce the number of steps in producing inks used, for example, on wiring boards.

[0117] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to those shown in these examples.

[0118] 1. Particle Synthesis Examples 1 to 13 and Comparative Example 1 Silver particles were produced according to the conditions shown in Table 1. First, silver nitrate (AgNO ３A solution containing 1,000 ppm toluene (DMF) and 1,000 ppm toluene (PVP) was added to the reaction mixture. Another solution containing pure water, a reducing agent (DMF), and a dispersant (protecting agent) (PVP or EDTA) (the molecular weights in the table indicate weight-average molecular weights) was prepared. Subsequently, the two solutions were mixed using a flow-type microwave synthesizer to prepare a reaction solution. While the reaction solution was flowing, microwaves (MW) were irradiated. The composition shown in Table 1 is the composition of the reaction solution. In the flow-type microwave synthesizer, a rectangular parallelepiped with a length of 100 mm was used as the cavity, and the following reaction tube was used. Reaction tube configuration: Tube inner diameter: 1 mm to 6 mm Tube outer diameter: 3 mm to 8 mm (wall thickness: 1 mm) Tube length: 100 mm In each experiment, the reaction temperature was 60°C to 140°C. The amount of dispersant was 0.1 wt% to 10 wt%.

[0119] Comparative Example 2 Synthesis was carried out using the reaction solution composition shown in Table 1, using a batch microwave synthesis apparatus.

[0120]

[0121] 2. Evaluation Results (Transmission Electron Microscope (TEM) Evaluation) TEM images were taken of the dispersions obtained in Examples 1 to 13 and Comparative Examples 1 and 2. The TEM observation method and 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 a sample. TEM observation was then performed, and the projected area circle equivalent diameter (Heywood diameter) of the main and auxiliary parts of 100 to 300 or more randomly selected silver particles was determined, and the particle size and size distribution were evaluated. TEM Measurement Conditions: Apparatus: JEM-ARM300F; TEM Grid: Formvar, Cu Mesh 150P; Acceleration Voltage: 200 kV

[0122] FIG. 7 shows an example TEM image of silver particles (composite particles) comprising a main portion and an auxiliary portion in Example 1. In FIG. 7, the darker colored areas are spherical particles in which multiple crystal lattice planes appear, i.e., the main portion, and the lighter colored areas are particles with a thinner thickness compared to particles with a higher contrast, i.e., the auxiliary portion. From the obtained TEM image, the proportion (number proportion) of composite particles out of 300 randomly selected particles, the particle diameter (Heywood diameter) of the main portion out of 100 composite particles, and the average thickness, width, and length of the auxiliary portion (if the auxiliary portion is circular and plate-shaped, the width and length are the same) were calculated. The thickness of the auxiliary portion was analyzed by comparing the T value, which is the transmittance of the electron beam during TEM-EELS measurement. Specifically, the thickness of the auxiliary portion was analyzed based on the difference in the T value between particles with different contrast in the TEM image. The main portion is composed of multiple crystal planes, and the particle scale when viewed in the TEM image corresponds to the thickness in the depth direction, so the T value can be determined. A specific example will be described using FIG. 9. In FIG. 9, the average Heywood diameter of the main portion was 21.9 nm. When T / λ was calculated 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 to be 0.25 × 21.9 ÷ 0.4 = 13.7 nm. The results are shown in Table 2.

[0123]

[0124] Comparing the results in Table 2 and Figure 7 between Examples 1 to 13 and Comparative Example 1 and Comparative Example 2 reveals that by increasing the flow rate of the reaction solution to 1.0 m / min or more, it is possible to produce silver particles (composite particles) having a main portion and an auxiliary portion extending from the main portion according to the present invention. In this experiment, a flow rate of 1.0 m / min corresponds to a flow rate of approximately 13 mL / min. Furthermore, comparing Examples 1 to 13 with Comparative Example 1 reveals that, in order to increase the proportion of composite particles, it is necessary to increase the flow rate and increase the microwave absorption power of the reaction solution to 15 W / mL or more in the microwave irradiation section. Figure 8 shows the relationship between flow rate and absorbed power in the production of silver particles in the Examples and Comparative Examples. The crystal lattice planes of the composite particles were measured at the locations indicated by the white lines in the TEM images shown in Figure 9 . The results in Figure 9 show that no surfaces specific to the main portion or auxiliary portion that are prone to crystallization were observed in the composite particles.

[0125] Next, in order to understand the shapes of the main part and auxiliary part in more detail in the composite particle of the present invention, the transmittance and wavelength ratio (T / λ) of the electron beam at the center (center of gravity) and edge (edge ​​at any one point) of the main part in the composite particle, and at the center (center of gravity) and edge (edge ​​at any one point) of the auxiliary part were measured using TEM images. The results are shown in Table 3.

[0126]

[0127] From Table 3, it was found that in the main portion, T / λ differs significantly between the center and the end, and that this difference increases as the diameter equivalent to the projected area circle increases. This shows that the main portion is spherical, and therefore, as the particle size increases, the thickness of the center increases, and {(T / λ at the center) / (T / λ at the end)} increases. On the other hand, it was found that in the auxiliary portion, T / λ is almost the same at the center and the end. This shows that the auxiliary portion is plate-shaped, and the thickness is almost the same at the center and the end.

[0128] (Evaluation of Volume Resistivity) The volume resistivity was measured for the powder obtained by drying the dispersions obtained in Examples 1 to 13 and Comparative Examples 1 and 2. The volume resistivity was measured as follows.

[0129] (1) The silver particles in the silver particle dispersion synthesized in the comparative example or example were centrifuged at 20,000 rpm to settle, and the supernatant was removed. (2) The procedure in (1) was repeated six times, using pure water as a filtration wash. (3) The dispersion obtained in (2) was concentrated using an evaporator until the solids content reached 15% by weight, thereby preparing an ink. The solids content of the resulting 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 residual content of the dispersion was measured. Then, a portion of the residual content was heated to 500°C in a thermogravimetric (TG) measurement device to burn off the organic components, and the solids content of the residual content 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, a constant temperature incubator with a blower, and the volume resistivity of the sintered product was measured using a resistivity meter (Loresta GX MCP-T700, manufactured by Nitto Seiko Analytech Co., Ltd.). (6) The film thickness of the sintered product whose resistance was measured in (5) was measured by cutting the sintered product and observing the exposed cut surface with a SEM.

[0130] The results are shown in Table 2. Comparing Examples 1 to 13 with Comparative Example 1 in Table 2 reveals that the volume resistivity can be improved by increasing the ratio of the composite particles of the present invention to the total silver particles in the powder or dispersion. Specifically, it was found that the powders containing the composite particles of Examples 1 to 13 have lower volume resistivities, particularly 30 Ω cm or less, when sintered at a low temperature of 80°C, compared to the powders of Comparative Examples 1 and 2 that contain no or almost no composite particles. In other words, the composite particles of the present invention are metal particles that exhibit excellent sinterability, and it was found that in order to fully exhibit their performance, it is preferable to increase the ratio of the composite particles of the present invention to the total metal particles in the powder or dispersion.

[0131] In addition, to confirm the usefulness of the composite particles of the present invention, the volume resistivity was also measured for spherical particles only (average particle size: 13 nm) (Comparative Example 3), plate-like particles only (average width and length: 40 nm, average thickness: 5 nm) (Comparative Example 4), a mixed powder of spherical particles (average particle size: 42 nm) and plate-like particles (average width and length: 50 nm, average thickness: 10 nm) in 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 Chemical Co., Ltd.) (Comparative Example 6). As a result, the volume resistivity of the sintered body of each powder 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 resistivities of Comparative Examples 3 to 6 were higher than that of the powder containing the composite particles of the present invention.

[0132] In the composite particles of the present invention, when the same amount of heat is applied to the main portion and the auxiliary portion, the auxiliary portion rises in temperature and reaches its melting point more quickly than the main portion, and the molten auxiliary portion acts as a bonding / adhesive layer for sintering and bonding a metal particle having the auxiliary portion to one or more other metal particles adjacent to the metal particle. As a result, a powder or dispersion containing the composite particles can have a low volume resistivity, particularly a volume resistivity of 30 μΩ·cm or less, even at a low firing temperature, i.e., 80°C. Furthermore, because one silver particle of the present invention can be sintered and bonded to one or more other silver particles, it has been found that the proportion of composite particles relative to the total silver particles in the powder or dispersion does not have to be 100% by number, but a certain proportion, for example, 2% or more, in one embodiment, 10% or more, can exhibit the effect of reducing the volume resistivity of a sintered body formed from the powder or dispersion at low temperatures.

[0133] All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety.

Claims

1. A metal particle having a main portion and an auxiliary portion extending from the main portion, wherein the main portion is spherical, the auxiliary portion is plate-like or linear, and the thickness of the auxiliary portion is thinner than the Heywood diameter of the main portion.

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

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

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

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

6. Metal particles 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 containing the metal particles according to any one of claims 1 to 6, wherein the metal particles account for 10% or more of the total number of metal particles contained in the powder.

9. A method for producing metal particles, comprising the step of irradiating a reaction liquid with microwaves, the method comprising the steps of: (i) preparing a reaction liquid containing a metal particle precursor and a solvent; and (ii) irradiating the reaction liquid with microwaves while flowing, wherein the flow rate of the reaction liquid is 1.0 m / min or more, and the absorbed power of the microwaves for the reaction liquid is 15 W / mL or more in the microwave irradiation section.

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

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

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

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