Composite filler and method for producing same

Abstract

Provided is a composite filler having heat dissipation (thermal conductivity) and higher electrical insulation than conventional electrical insulation. The composite filler forms a core-shell structure comprising a core material containing carbon, metal, and zinc oxide or zirconium oxide, and a shell material attached to the surface of the core material. The shell material comprises fumed oxide particles that have attached to a part or all of the surface of the core material as a result of mixing the fumed oxide particles and the core material by means of a dry ball mill. The fumed oxide particles are particles obtained as the result of transformation of bulky aggregated particles into agglomerated particles, the aggregated particles being formed by bead-shaped aggregation and fusion of spherical primary particles. In the composite filler, the core material constitutes 30-85 vol%, the shell material constitutes 15-70 vol%, the thermal conductivity is 0.075 W / m·K or more, the volume resistivity is 1.0 × 105 Ω·cm or more, and the dielectric breakdown voltage is 1 kV / mm or more.

Description

Composite filler and its manufacturing method

[0001] The present invention relates to a composite filler having a heat-dissipating core and an electrically insulating shell attached to the surface of the core, and a method for producing the same.

[0002] In recent years, electronic devices used in electric vehicles and other devices have become increasingly higher in voltage and current. This has led to an increase in the amount of heat generated by these devices, and one of the major challenges is how to dissipate this heat. Components used in these electronic devices are required to have high heat dissipation performance, i.e., thermal conductivity, as well as high electrical insulation for reliability reasons.

[0003] Conventionally, as a material having thermal conductivity and electrical insulation, thermally conductive particles including a composite core and an insulating material coating at least a portion of the composite core, and methods for producing the same have been disclosed (see, for example, Patent Document 1 (Claim 1, Claim 13, Paragraphs

[0021] ,

[0028] ,

[0037] ,

[0038] ,

[0049] ,

[0053] ,

[0058] ,

[0097] , and

[0098] )). The thermally conductive particles and a resin are mixed to produce a resin composition, which is then molded, and the molded product is used as a component of the electronic device.

[0004] The composite core includes a plurality of thermally conductive core particles and an organic binder binding the core particles together, the core particles being selected from the group consisting of metal particles, ceramic particles, carbon-based particles, and mixtures thereof, and the insulating material being selected from the group consisting of aluminum oxide (alumina), zinc oxide, talc, magnesium oxide, silicon dioxide, boehmite, boron nitride, mica, aluminum nitride, silicon nitride, zinc sulfide, sericite, and mixtures thereof.

[0005] The method for producing the thermally conductive particles involves mixing a plurality of core particles and an organic binder by compressive shear mixing to bond and mix the core particles with the organic binder to form a composite core, and then mixing the composite core and an insulating material by compressive shear mixing to at least partially coat the composite core with the insulating material.

[0006] The metal particles of the core particle include copper, silver, nickel, aluminum, or alloys thereof, and the carbon-based particles of the core particle include graphite, carbon nanotubes, fullerenes, graphene, carbon black, glassy carbon, carbon fiber, amorphous carbon, boron carbide, or mixtures thereof.

[0007] The thermally conductive particles thus configured have a thermal conductivity of at least 1×10 when measured at an applied voltage of 500 V on a cylinder of the thermally conductive particles having a diameter of 10 mm and a height of 3.0 mm. ４ Ω cm to 1 x 10 １０ It is characterized by exhibiting a volume resistivity in the Ω·cm range.

[0008] Special table 2017-504177 publication

[0009] The thermally conductive particles disclosed in Patent Document 1 have an insulation index of 1×10 when measured at 500 V. ４ Ω cm to 1 x 10 １０ However, the above-mentioned thermally conductive particles are insufficient in terms of electrical insulation as materials and components for electronic devices that are becoming increasingly high-voltage and high-current, and materials with even higher electrical insulation are desired.

[0010] An object of the present invention is to provide a composite filler having heat dissipation properties (thermal conductivity) and higher electrical insulation properties than conventional ones, and a method for producing the same.

[0011] A first aspect of the present invention is a composite filler that forms a core-shell structure comprising a core material made of carbon, metal, zinc oxide or zirconium oxide, and a shell material attached to the surface of the core material, wherein the shell material is formed by mixing fumed oxide particles and the core material in a dry ball mill, with the fumed oxide particles adhering to a part or all of the surface of the core material, and the fumed oxide particles are particles formed by agglomerating and fusing spherical primary particles in a beaded shape to form bulky aggregated particles, which are then transformed into agglomerated particles, and the core material accounts for 30 to 85% by volume when the composite filler is taken as 100% by volume. The shell material is in the range of 15% to 70% by volume, the thermal conductivity measured under the maximum compressive load condition (2400.0 gF) using a thermal conductivity measuring device TRIDENT manufactured by C-Therm and a compression test accessory (CTA) is 0.075 W / m K or more, and the volume resistivity measured using a high resistance / resistivity meter "Hiresta-UX" (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-HT800") or a low resistance / resistivity meter "Loresta-GX" (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-T700") and a powder resistivity measurement system (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-PD-51") is 1.0 x 10 ５ The dielectric strength is Ω·cm or more, and the breakdown voltage measured using an AC WITHSTAND VOLTAGE TESTER 7473 manufactured by Keisoku Gijutsu Kenkyusho Co., Ltd. is 1 kV / mm or more.

[0012] A second aspect of the present invention is the invention based on the first aspect, wherein the carbon serving as the core material is graphite, ketjen black, graphite or carbon black, and the metal serving as the core material is copper, zinc, tin, nickel, aluminum or stainless steel.

[0013] A third aspect of the present invention is an invention based on the first aspect, wherein the particle size of the composite filler is a particle size measured by a laser diffraction / scattering particle size distribution measuring device (manufactured by Horiba, Ltd., model "LA960") and is in the range of 1 μm to 300 μm.

[0014] A fourth aspect of the present invention is the invention based on the first aspect, wherein the fumed oxide particles are fumed silica particles or fumed alumina particles.

[0015] A fifth aspect of the present invention is a method for producing a composite filler by mixing fumed oxide particles, in which spherical primary particles have been agglomerated and fused together in a beaded shape to form bulky aggregated particles, and these agglomerated particles have been changed into agglomerated particles, with a core material made of carbon, metal, zinc oxide, or zirconium oxide in a volume ratio of fumed oxide particles:core material = 30 to 85:70 to 15 in a dry ball mill, thereby forming a core-shell structure in which the fumed oxide particles adhere as a shell material to part or all of the surface of the core material, wherein the fumed oxide particles are particles in which spherical primary particles have been agglomerated and fused together in a beaded shape to form bulky aggregated particles, and these agglomerated particles have been changed into agglomerated particles.

[0016] The composite filler according to the first aspect of the present invention is obtained by mixing and adhering a shell material, which is composed of particles in which spherical primary particles are aggregated and fused together in a beaded shape to form bulky aggregated particles, and which are transformed from these aggregated particles into agglomerated particles, to the surface of a core material made of carbon, metal, zinc oxide or zirconium oxide in a dry ball mill, and further by mixing and adhering the shell material to the surface of the core material made of carbon, metal, zinc oxide or zirconium oxide in a dry ball mill, and by adhering the shell material to the surface of the core material in a range of 30% to 85% by volume and 15% to 70% by volume, when the composite filler is taken as 100% by volume, the thermal conductivity is 0.075 W / m K or more and the volume resistivity is 1.0 × 10 ５ It is characterized by a dielectric strength of Ω·cm or more and a breakdown voltage of 1 kV / mm or more.

[0017] The composite filler according to the second aspect of the present invention has high thermal conductivity because the carbon serving as the core material is graphite, ketjen black, graphite or carbon black, and the metal serving as the core material is copper, zinc, tin, nickel, aluminum or stainless steel.

[0018] The composite filler according to the third aspect of the present invention has a particle size in the range of 1 μm to 300 μm, and therefore has the characteristic that when this composite filler is mixed with a resin to produce a resin composition, the resin composition can be easily molded.

[0019] In the composite filler according to the fourth aspect of the present invention, the fumed oxide particles are fumed silica particles or fumed alumina particles, and therefore when attached to the surface of the core material as a shell material, the electrical insulation properties of the composite filler are further improved.

[0020] In a composite filler manufacturing method according to a fifth aspect of the present invention, fumed oxide particles are produced in which spherical primary particles aggregate and fuse together to form bulky aggregated particles, and these aggregated particles are transformed into agglomerated particles. These agglomerated particles are then mixed in a dry ball mill with a core material made of carbon, metal, zinc oxide, or zirconium oxide in a volume ratio of fumed oxide particles:core material of 30 to 85:70 to 15, thereby producing a composite filler. This allows the fumed oxide particles to adhere to the surface of the core material as a shell material in a relatively simple manner. The composite filler manufactured in this manner has the advantages of having heat dissipation properties (thermal conductivity) and higher electrical insulation than conventional materials.

[0021] 1 is a cross-sectional schematic diagram of a composite filler according to an embodiment of the present invention, having fumed oxide particles attached to the surface of a core material, the bulky agglomerates formed by aggregating and fusing spherical primary particles in a beaded shape, and the agglomerates transformed into agglomerate particles.

[0023] FIG. 1 is a diagram showing a state in which the agglomerate particles shown in FIG. 1 are formed by aggregating and fusing spherical primary particles in a beaded shape, and the bulky agglomerates transformed into agglomerate particles.

[0024] FIG. 1 is a scanning electron microscope (SEM) photograph showing an agglomerate particle, which is the smallest particle form formed by sintering silica primary particles.

[0025] FIG. 2 is a scanning electron microscope (SEM) photograph showing an agglomerate particle formed by gathering the agglomerate particles shown in FIG. 3.

[0026] FIG. 3 is a diagram superimposing the particle size distribution curve of the Sn powder of Example 1 and the particle size distribution curve of a composite filler obtained by mixing Sn powder and fumed alumina particles in a dry ball mill.

[0027] FIG. 4 is a scanning electron microscope (SEM) photograph of the Sn powder of Example 1.

[0028] FIG. 5 is a scanning electron microscope (SEM) photograph of the composite filler of Example 1. FIG. 1 is a diagram in which the particle size distribution curve of the Zn powder in Example 2 is superimposed on the particle size distribution curve of the composite filler obtained by mixing the Zn powder and fumed alumina particles in a dry ball mill. FIG. 2 is a scanning electron microscope (SEM) photograph of the Zn powder in Example 2. FIG. 3 is a scanning electron microscope (SEM) photograph of the composite filler in Example 2. FIG. 4 is a diagram in which the particle size distribution curve of the Ni powder in Example 3 is superimposed on the particle size distribution curve of the composite filler obtained by mixing the Ni powder and fumed alumina particles in a dry ball mill. FIG. 5 is a scanning electron microscope (SEM) photograph of the Ni powder in Example 3. FIG. 6 is a scanning electron microscope (SEM) photograph of the composite filler in Example 3. FIG. 7 is a diagram in which the particle size distribution curve of the SUS powder in Example 4 is superimposed on the particle size distribution curve of the composite filler obtained by mixing the SUS powder and fumed alumina particles in a dry ball mill. FIG. 8 is a scanning electron microscope (SEM) photograph of the SUS powder in Example 4. Fig. 1 is a scanning electron microscope (SEM) photograph of the composite filler of Example 4. Fig. 2 is a diagram in which the particle size distribution curve of the ZnO powder of Example 5 and the particle size distribution curve of the composite filler obtained by mixing ZnO powder and fumed alumina particles in a dry ball mill are superimposed. Fig. 3 is a scanning electron microscope (SEM) photograph of the ZnO powder of Example 5. Fig. 4 is a scanning electron microscope (SEM) photograph of the composite filler of Example 5.FIG. 1 is a diagram in which a particle size distribution curve of the Cu powder of Example 6 is superimposed on a particle size distribution curve of a composite filler obtained by mixing Cu powder and fumed alumina particles in a dry ball mill. FIG. 2 is a scanning electron microscope (SEM) photograph of the Cu powder of Example 6. FIG. 3 is a scanning electron microscope (SEM) photograph of the composite filler of Example 6. FIG. 4 is a diagram in which a particle size distribution curve of the Cu powder of Example 7 is superimposed on a particle size distribution curve of a composite filler obtained by mixing Cu powder and fumed alumina particles in a dry ball mill. FIG. 5 is a scanning electron microscope (SEM) photograph of the Cu powder of Example 7. FIG. 6 is a scanning electron microscope (SEM) photograph of the composite filler of Example 7. FIG. 7 is a diagram in which a particle size distribution curve of the Cu powder of Example 8 is superimposed on a particle size distribution curve of a composite filler obtained by mixing Cu powder and fumed alumina particles in a dry ball mill. FIG. 8 is a scanning electron microscope (SEM) photograph of the Cu powder of Example 8. FIG. 9 is a scanning electron microscope (SEM) photograph of the composite filler of Example 8. FIG. 1 is a diagram in which the particle size distribution curve of the Cu powder of Example 9 and the particle size distribution curve of the composite filler obtained by mixing the Cu powder and fumed alumina particles in a dry ball mill are superimposed. FIG. 2 is a scanning electron microscope (SEM) photograph of the Cu powder of Example 9. FIG. 3 is a scanning electron microscope (SEM) photograph of the composite filler of Example 9. FIG. 4 is a diagram in which the particle size distribution curve of the Cu powder of Example 10 and the particle size distribution curve of the composite filler obtained by mixing the Cu powder and fumed alumina particles in a dry ball mill and then surface-treating the mixture are superimposed. FIG. 5 is a scanning electron microscope (SEM) photograph of the Cu powder of Example 10. FIG. 6 is a scanning electron microscope (SEM) photograph of the composite filler of Example 10. FIG. 7 is a diagram in which the particle size distribution curve of the Al powder of Example 11 and the particle size distribution curve of the composite filler obtained by mixing the Al powder and fumed alumina particles in a dry ball mill are superimposed. FIG. 8 is a scanning electron microscope (SEM) photograph of the Al powder of Example 11. Fig. 1 is a scanning electron microscope (SEM) photograph of the composite filler of Example 11. Fig. 2 is a diagram in which the particle size distribution curve of the ZrO powder of Example 12 and the particle size distribution curve of the composite filler obtained by mixing the ZrO powder and fumed alumina particles in a dry ball mill are superimposed. Fig. 3 is a scanning electron microscope (SEM) photograph of the ZrO powder of Example 12. Fig. 4 is a scanning electron microscope (SEM) photograph of the composite filler of Example 12.FIG. 1 is a diagram in which the particle size distribution curve of the graphite powder of Example 13 is superimposed on the particle size distribution curve of a composite filler obtained by mixing graphite powder and fumed silica particles in a dry ball mill. FIG. 2 is a scanning electron microscope (SEM) photograph of the graphite powder of Example 13. FIG. 3 is a scanning electron microscope (SEM) photograph of the composite filler of Example 13. FIG. 4 is a diagram in which the particle size distribution curve of the Cu powder of Comparative Example 1 is superimposed on the particle size distribution curve of a composite filler obtained by mixing Cu powder and fumed silica particles by dry hand mixing. FIG. 5 is a scanning electron microscope (SEM) photograph of the Cu powder of Comparative Example 1. FIG. 6 is a scanning electron microscope (SEM) photograph of the composite filler of Comparative Example 1. FIG. 7 is a diagram in which the particle size distribution curve of the Cu powder of Comparative Example 2 is superimposed on the particle size distribution curve of a composite filler obtained by mixing Cu powder and fumed silica particles in a dry rotation-revolution mixer. FIG. 8 is a scanning electron microscope (SEM) photograph of the Cu powder of Comparative Example 2. FIG. 1 is a scanning electron microscope (SEM) photograph of the composite filler of Comparative Example 2. FIG. 2 is a diagram in which the particle size distribution curve of the Cu powder of Comparative Example 3 and the particle size distribution curve of the composite filler obtained by mixing Cu powder and fumed alumina particles in a dry ball mill are superimposed. FIG. 3 is a scanning electron microscope (SEM) photograph of the Cu powder of Comparative Example 3. FIG. 4 is a scanning electron microscope (SEM) photograph of the composite filler of Comparative Example 3. FIG. 5 is a diagram in which the particle size distribution curve of the Cu powder of Comparative Example 4 and the particle size distribution curve of the composite filler obtained by mixing Cu powder and fumed alumina particles in a dry ball mill are superimposed. FIG. 6 is a scanning electron microscope (SEM) photograph of the Cu powder of Comparative Example 4. FIG. 7 is a scanning electron microscope (SEM) photograph of the composite filler of Comparative Example 4.

[0022] Next, an embodiment of the present invention will be described with reference to the drawings. As shown in Fig. 1, a composite filler 10 of this embodiment forms a core-shell structure consisting of a core material 11 made of carbon, metal, zinc oxide, or zirconium oxide, and a shell material 12 attached to the surface of this core material 11. In Fig. 1, the shell material 12 is attached to the entire surface of the core material 11, but it may also be attached to a part of the surface of the core material 11. Also, Fig. 1 shows an example in which the shape of the core material 11 is spherical, but the core material is not limited to a spherical shape and may be needle-like, rod-like, or flat plate-like.

[0023] When the core material 10 is carbon, examples of the carbon include graphite, ketjen black, graphite, and carbon black. When the core material is metal, examples of the metal include copper, zinc, tin, nickel, aluminum, and stainless steel. Zinc oxide and zirconium oxide are selected as core materials because they have high thermal conductivity comparable to that of metals. The particle size and particle size distribution of the core material cannot be determined in general terms because they vary depending on the material, such as carbon, metal, or zinc oxide or zirconium oxide, and the manufacturing method thereof.

[0024] In this embodiment, the shell material 12 is formed by mixing fumed oxide particles and a core material in a dry ball mill at room temperature, resulting in the fumed oxide particles adhering to the surface of the core material. Examples of fumed oxide particles include fumed silica particles or fumed alumina particles. The fumed oxide particles include hydrophilic fumed oxide particles and hydrophobic fumed oxide particles. The average primary particle size of the fumed oxide particles, as measured by image analysis using a transmission electron microscope (TEM), is in the range of 7 nm to 90 nm. Mixing the fumed oxide particles and the core material using methods other than a dry ball mill, such as a dry hand-mixing method or a dry rotation-revolution mixer, does not provide sufficient adhesion of the shell material to the core material, making it impossible to obtain a composite filler with the desired thermal conductivity and electrical insulation properties. As shown in the enlarged view of FIG. 1 , the shell material 12 is formed to include agglomerated particles 12a and voids 12b between the agglomerated particles.

[0025] When fumed oxide particles and a core material are mixed in a dry ball mill at room temperature, not only do agglomerated particles of the fumed oxide particles coat and adhere to the core material, but the core material itself may also become smaller, or the smaller core materials may aggregate together. Therefore, when examining the particle size of a composite filler using a particle size distribution curve, the particle size distribution may show not just one peak diameter, but multiple peak diameters. Therefore, the average particle size of the composite filler cannot be determined in a general way. However, based on the particle size distribution curves in Examples 1 to 13 described below, the particle size of the composite filler is in the range of 1 μm to 300 μm. For the same reason that the average particle size of the composite filler cannot be determined in a general way, the average particle size of the shell material in the composite filler cannot be determined in a general way either. However, while the average particle size of the core material is on the order of microns (μm), the average particle size of the shell material is on the order of nanometers (nm). In Examples 1 to 13 described below, the thickness of the shell material layer can be estimated from the particle size distribution curve, taking into account the peak diameter of the core material before compounding and the main peak diameter of the composite filler, and the thickness of the shell material layer is calculated to be in the range of 3 μm to 20 μm. This thickness of the shell material layer is twice the thickness "d" shown in FIG. 1.

[0026] As shown in FIG. 2, the agglomerated particles 12a are formed by vaporizing SiCl 2 as the vaporized raw material, taking the case where the fumed oxide particles are fumed silica particles as an example. ４ and H ２ and O ２ By injecting a mixed gas of these from a reactor (burner), aggregated particles, which are the smallest particle form formed by sintering primary particles, are formed, and then the aggregated particles gather together through weak interactions such as hydrogen bonds and van der Waals forces to form agglomerated particles. In an aggregate of a plurality of composite fillers, the shell material is densely packed between the core materials, forming a three-dimensional network structure that densely fills the voids between the core materials. As a result, the volume resistivity of the composite filler as an aggregate can be increased to 1.0 x 10 without impairing the high thermal conductivity of the core material. ５The dielectric constant can be increased to Ω·cm or more, and the breakdown voltage can be increased to 1 kV / mm or more. This allows for high electrical insulation while maintaining the high thermal conductivity inherent in the core material. Figure 3 shows aggregate particles, which are the smallest particle form formed by sintering primary particles, and Figure 4 shows agglomerated particles formed by the aggregation of aggregate particles through weak interactions such as hydrogen bonding and van der Waals forces.

[0027] When the composite filler is taken as 100% by volume, the core material is in the range of 30% to 85% by volume, and the shell material is in the range of 15% to 70% by volume. The core material is preferably in the range of 30% to 70% by volume, and the shell material is preferably in the range of 30% to 70% by volume. If the core material is less than 30% by volume and the shell material is more than 70% by volume, a composite filler with the desired thermal conductivity cannot be obtained. Furthermore, if the core material is more than 85% by volume and the shell material is less than 15% by volume, a composite filler with the desired volume resistivity and breakdown voltage cannot be obtained. The shell material, which is formed by mixing agglomerated particles 12a made of fumed oxide particles and voids 12 (see the enlarged view in Figure 1), adheres to the surface of the core material at a predetermined volume percentage by mixing using a dry ball mill. This allows the composite filler of this embodiment to maintain high thermal conductivity while also exhibiting high volume resistivity and high breakdown voltage.

[0028] The composite filler of this embodiment is characterized in that its thermal conductivity is 0.075 W / m·K or more, preferably 0.100 W / m·K or more, when measured under maximum compressive load conditions (2400.0 gF) using a thermal conductivity measuring device TRIDENT and a compression test accessory (CTA) manufactured by C-Therm. Furthermore, the composite filler of this embodiment has a volume resistivity of 1.0 × 10 when measured using a high resistance / resistivity meter "Hiresta-UX" (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-HT800") or a low resistance / resistivity meter "Loresta-GX" (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-T700") and a powder resistivity measurement system (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-PD-51"). ５ Ω cm or more, preferably 2.0 × 10５ Furthermore, the composite filler of this embodiment is characterized in that its breakdown voltage is 1 kV / mm or more, preferably 2 kV / mm or more, when measured using an AC WITHSTAND VOLTAGE TESTER 7473 manufactured by Keisoku Gijutsu Kenkyusho Co., Ltd.

[0029] The thermal conductivity and volume resistivity are values ​​measured when the powder composite filler is used. The breakdown voltage was measured by mixing 63% by volume of a silicone resin (manufactured by Momentive Corp., model number "YE5822A"), 7% by volume of a curing agent (manufactured by Momentive Corp., model number "YE5822B"), and 30% by volume of a composite filler to prepare a resin mixture, mixing this resin mixture and the composite filler in a rotation-revolution mixer (manufactured by Thinky Corporation, model number "ARE-310") at 2000 rpm for 5 minutes to prepare a resin composition. The resin composition was then placed in a mold having a cavity measuring 15 cm length x 15 cm width x 2 mm depth, and heated at 130°C under 10 MPa (100 kg / cm) using a heat press (manufactured by Kodaira Seisakusho Co., Ltd., model number "PY15-EA"). ２ ) and maintained for 10 minutes to harden the resin, and the measured value was a resin molding.

[0030] Next, examples of the present invention will be described in detail together with comparative examples.

[0031] The raw materials used in the examples and comparative examples of the present invention are as follows. The following raw materials were used in their commercially available state: (1) Tin powder (Sn, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: Product No. 206-01505) (2) Hydrophilic fumed alumina particles (Al ２ O ３, Evonik Corporation: Product No. “AEROXIDE® Alu C” (average primary particle size 13 nm)) (3) Zinc powder (Zn, Hayashi Pure Chemical Industries, Ltd.: Product No. “26000085”) (4) Nickel powder (Ni, Fujifilm Wako Pure Chemical Industries, Ltd.: Product No. “145-00982”) (5) Stainless steel powder (SUS, Fujifilm Wako Pure Chemical Industries, Ltd.: Product No. “900919”) (6) Zinc oxide powder (ZnO, Hayashi Pure Chemical Industries, Ltd.: Product No. “26000375”) (7) Copper powder (Cu, Hayashi Pure Chemical Industries, Ltd.: Product No. “03003695”) (8) Hydrophilic fumed alumina particles (Al ２ O ３ , Evonik Corporation: Product number "VP Alu 30" (average primary particle size 90 nm) (9) Hydrophobic fumed alumina particles (Al ２ O ３ , manufactured by Nippon Aerosil Co., Ltd.: Product No. “VP Alu C RK” (average primary particle size 13 nm)) (10) Isobutyltrimethoxysilane (IBTMO, manufactured by Evonik Corporation: Product No. “Dynasylan® IBTMO”) (11) Aluminum powder (Al, manufactured by Hayashi Pure Chemical Industries, Ltd.: Product No. “01001325”) (12) Zirconium oxide powder (ZrO, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: Product No. “264-00485”) (13) Graphite powder (C, manufactured by Ito Graphite Industries Co., Ltd.: Product No. “SG-BL40”) (14) Hydrophilic fumed silica particles (manufactured by Nippon Aerosil Co., Ltd.: Product No. “AEROSIL® 380S” (average primary particle size 7 nm))

[0032] Example 1 As the core material, Sn (tin) powder was prepared, which has a particle size distribution with a single peak particle size of 17 μm as shown in Figure 5a, and has a 10% cumulative volume of 9.6 μm, a 50% cumulative volume of 18.4 μm, and a 90% cumulative volume of 40.6 μm, calculated from the smallest diameter side, as shown in Table 1. Hydrophilic fumed alumina particles (product number "AEROXIDE (registered trademark) Alu C") were prepared as the shell material. Next, 30% by volume of the core material (Sn powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed on a ball mill rotating table (Masuda Rika Kogyo Co., Ltd.: Model No. "UNIVERSAL BALL MILL MODEL UBM-S") by rotating a pot mill containing 5 mm diameter alumina balls manufactured by Nikkato Corporation at 110 rpm for 5 hours at room temperature in the atmosphere to obtain a composite filler. The particle size distribution of this composite filler had two peaks, at particle sizes of 12 μm and 67 μm, as shown in Figure 5a. As shown in Table 1, the particle size distribution of this composite filler, calculated from the smallest diameter, was 5.9 μm at 10% cumulative volume, 15.7 μm at 50% cumulative volume, and 89.3 μm at 90% cumulative volume. The particle size distribution of the Sn powder ranged from 4 μm to 150 μm, making it difficult to calculate the thickness of the shell material layer. Figure 5b shows a scanning electron microscope (SEM) image of the Sn powder, and Figure 5c shows a scanning electron microscope (SEM) image of the composite filler.

[0033]

[0034] The particle size of the core material or composite filler in Example 1 and other examples and comparative examples described below is the particle size on a particle size distribution curve obtained by measuring the core material or composite filler using a laser diffraction / scattering particle size distribution analyzer (manufactured by Horiba, Ltd., model "LA960"), with the particle size of the obtained core material or composite filler on the horizontal axis and the volumetric particle frequency on the vertical axis. The average primary particle size of the shell material is the particle size measured by image analysis using a transmission electron microscope (TEM).

[0035] Example 2 As a core material, Zn (zinc) powder was prepared, which had a particle size distribution with a single peak particle size of 9 μm as shown in Figure 6a, and a cumulative volume calculated from the smallest diameter side of 5.2 μm at 10%, 8.0 μm at 50%, and 12.2 μm at 90% as shown in Table 2. The same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (Zn powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill to obtain a composite filler. As shown in Figure 6a, the particle size distribution of this composite filler had two peaks, at particle sizes of 12 μm and 67 μm. As shown in Table 2, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 5.1 μm, a 50% cumulative volume of 12.2 μm, and a 90% cumulative volume of 50.4 μm. Because the particle size distribution of the Zn powder ranged from 3 μm to 22 μm in a relatively narrow range, the thickness of the shell material layer was estimated to be 3 μm, ranging from 12 μm to 9 μm. Figure 6b shows a scanning electron microscope (SEM) image of the Zn powder, and Figure 6c shows a scanning electron microscope (SEM) image of the composite filler.

[0036]

[0037] Example 3 As a core material, Ni (nickel) powder was prepared, which has a particle size distribution with a single peak particle size of 10 μm as shown in FIG. 7A, and has a 10% cumulative volume particle size of 5.6 μm, a 50% cumulative volume particle size of 9.6 μm, and a 90% cumulative volume particle size of 16.5 μm as shown in Table 3, calculated from the smallest diameter side. Furthermore, the same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (Ni powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed using the same dry ball mill as in Example 1, in the same manner as in Example 1, to obtain a composite filler. As shown in Figure 7a, the particle size distribution of this composite filler had a single peak at a particle size of 12 μm. As shown in Table 3, the particle size distribution of this composite filler, calculated from the smallest diameter, had a particle size of 3.3 μm at 10% of the cumulative volume, 9.2 μm at 50% of the cumulative volume, and 38.4 μm at 90% of the cumulative volume. Because the particle size distribution of the Ni powder was in a relatively narrow range from 3 μm to 30 μm, the thickness of the shell material layer was estimated to be 2 μm, between 12 μm and 10 μm. Figure 7b shows a scanning electron microscope (SEM) image of the Ni powder, and Figure 7c shows a scanning electron microscope (SEM) image of the composite filler.

[0038]

[0039] Example 4 As a core material, a stainless steel (SUS) powder was prepared. The powder had a particle size distribution with a peak particle size of 89 μm (as shown in FIG. 8A), and as shown in Table 4, the cumulative volume calculated from the smallest diameter side had a 10% particle size of 26.0 μm, a 50% particle size of 68.9 μm, and a 90% particle size of 160.3 μm. Furthermore, the same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (SUS powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill to obtain a composite filler. As shown in Figure 8a, the particle size distribution of this composite filler had two peaks, at particle sizes of 12 μm and 67 μm. As shown in Table 4, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 5.8 μm, a 50% cumulative volume of 13.4 μm, and a 90% cumulative volume of 57.6 μm. Because the particle size distribution of the SUS powder ranged widely from 8 μm to 300 μm, it was difficult to calculate the thickness of the shell material layer. Figure 8b shows a scanning electron microscope (SEM) image of the SUS powder, and Figure 8c shows a scanning electron microscope (SEM) image of the composite filler.

[0040]

[0041] Example 5 As a core material, ZnO (zinc oxide) powder was prepared, which had a particle size distribution with three peak particle sizes of 0.09 μm, 2 μm, and 77 μm as shown in FIG. 9A, and a cumulative volume calculated from the smallest diameter side with a 10% particle size of 0.6 μm, a 50% particle size of 2.6 μm, and a 90% particle size of 38.9 μm as shown in Table 5. The same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (ZnO powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed using the same dry ball mill as in Example 1, in the same manner as in Example 1, to obtain a composite filler. As shown in Figure 9a, the particle size distribution of this composite filler had two peaks, at particle sizes of 13 μm and 45 μm. As shown in Table 5, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 1.7 μm, a 50% cumulative volume of 14.5 μm, and a 90% cumulative volume of 63.3 μm. Because the particle size distribution of the ZnO powder ranged from 0.06 μm to 300 μm, it was difficult to calculate the thickness of the shell material layer. Figure 9b shows a scanning electron microscope (SEM) image of the ZnO powder, and Figure 9c shows a scanning electron microscope (SEM) image of the composite filler.

[0042]

[0043] Example 6 As a core material, Cu (copper) powder was prepared, which had a particle size distribution with a peak particle size of 34 μm as shown in FIG. 10A, and a cumulative volume calculated from the smallest diameter side of 10% of the total particle size of 13.5 μm, 50% of the total particle size of 30.6 μm, and 90% of the total particle size of 62.0 μm as shown in Table 6. Furthermore, the same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (Cu powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed using the same dry ball mill as in Example 1, in the same manner as in Example 1, to obtain a composite filler. As shown in Figure 10a, the particle size distribution of this composite filler had two peaks, at particle sizes of 13 μm and 51 μm. As shown in Table 6, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 6.8 μm, a 50% cumulative volume of 16.4 μm, and a 90% cumulative volume of 66.3 μm. Because the particle size distribution of the Cu powder ranged from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. However, for convenience, the thickness of the shell material layer was estimated to be 17 μm, i.e., 51 μm - 34 μm. Figure 10b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Figure 10c shows a scanning electron microscope (SEM) photograph of the composite filler.

[0044]

[0045] Example 7 The same Cu (copper) powder as in Example 6 was prepared as the core material. The powder had a particle size distribution with a peak particle size of 34 μm (as shown in FIG. 11a), and as shown in Table 7, the 10% cumulative volume particle size, 50% cumulative volume particle size, and 90% cumulative volume particle size were 13.5 μm, 30.6 μm, and 62.0 μm, respectively, calculated from the smallest diameter side. Hydrophilic fumed alumina particles (product number "VP Alu 30") were also prepared as the shell material. Next, 30% by volume of the core material (Cu powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill as in Example 1 to obtain a composite filler. As shown in Figure 11a, the particle size distribution of this composite filler had two peaks, at particle sizes of 13 μm and 51 μm. As shown in Table 7, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 6.8 μm, a 50% cumulative volume of 16.4 μm, and a 90% cumulative volume of 66.3 μm, similar to Example 6. Because the particle size distribution of the Cu powder ranged from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. However, for convenience, the thickness of the shell material layer was estimated to be 17 μm, i.e., 51 μm - 34 μm. Figure 11b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Figure 11c shows a scanning electron microscope (SEM) photograph of the composite filler.

[0046]

[0047] Example 8 As a core material, the same Cu (copper) powder as in Example 6 was prepared. The Cu powder had a particle size distribution with a single peak particle size of 34 μm as shown in FIG. 12a, and as shown in Table 8, the 10% cumulative volume particle size, 50% cumulative volume particle size, and 90% cumulative volume particle size were 13.5 μm, 30.6 μm, and 62.0 μm, respectively, calculated from the smallest diameter side. Hydrophobic fumed alumina particles (product number "VP Alu C RK") were also prepared as a shell material. Next, 30% by volume of the core material (Cu powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill to obtain a composite filler. As shown in Figure 12a, the particle size distribution of this composite filler had a single peak at a particle size of 12 μm. As shown in Table 8, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 2.6 μm, a 50% cumulative volume of 9.2 μm, and a 90% cumulative volume of 26.5 μm. Because the particle size range of the Cu powder in its particle size distribution was wide, from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. Figure 12b shows a scanning electron microscope (SEM) image of the Cu powder, and Figure 12c shows a scanning electron microscope (SEM) image of the composite filler.

[0048]

[0049] Example 9 As a core material, the same Cu (copper) powder as in Example 6 was prepared. The powder had a particle size distribution with a peak particle size of 34 μm as shown in FIG. 13a, and a cumulative volume of 10% of the total volume calculated from the smallest diameter side, with a particle size of 13.5 μm, a 50% particle size of 30.6 μm, and a 90% particle size of 62.0 μm as shown in Table 9. Hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") were also prepared as a shell material. Next, 70% by volume of the core material (Cu powder) and 30% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill as in Example 1 to obtain a composite filler. As shown in Figure 13a, the particle size distribution of this composite filler had two peaks, at particle sizes of 13 μm and 45 μm. As shown in Table 9, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 7.3 μm, a 50% cumulative volume of 17.2 μm, and a 90% cumulative volume of 55.7 μm. Because the particle size distribution of the Cu powder ranged from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. However, for convenience, the thickness of the shell material layer was estimated to be 11 μm, i.e., 45 μm - 34 μm. Figure 13b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Figure 13c shows a scanning electron microscope (SEM) photograph of the composite filler.

[0050]

[0051] Example 10 As a core material, the same Cu (copper) powder as in Example 6 was prepared. The Cu powder had a particle size distribution with a peak particle size of 34 μm (see FIG. 14a), and as shown in Table 10, the 10% cumulative volume particle size calculated from the smallest diameter side was 13.5 μm, the 50% cumulative volume particle size was 30.6 μm, and the 90% cumulative volume particle size was 62.0 μm. Furthermore, the same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (Cu powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed using the same dry ball mill as in Example 1, followed by the same procedure as in Example 1. The surface of this mixed powder was then hydrophobized by wet treatment with isobutyltrimethoxysilane, a silane coupling agent (hereinafter referred to as C4 treatment), to obtain a composite filler. This C4 process involves adding the mixed powder to a beaker containing IBTMO, heating it in air at 120°C using a magnetic stirrer with a hot plate (Tokyo Rika Kikai Co., Ltd.: Model No. RH-1000), and then drying it in air at 150°C for 1 hour. The dried product was used as a composite filler. As shown in Figure 14a, the particle size distribution of this composite filler had three peaks: 15 μm, 51 μm, and 300 μm. As shown in Table 10, in the particle size distribution of this composite filler, the particle size at 10% of the cumulative volume calculated from the smallest diameter side was 8.2 μm, the particle size at 50% was 40.3 μm, and the particle size at 90% was 298.9 μm. Although it was difficult to calculate the thickness of the shell material layer because the particle size in the particle size distribution of the Cu powder was in a wide range from 5 μm to 175 μm, for convenience the thickness of the shell material layer was estimated to be 17 μm (51 μm-34 μm). Figure 14b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Figure 14c shows a scanning electron microscope (SEM) photograph of the composite filler.

[0052]

[0053] Example 11 As a core material, an aluminum (Al) powder was prepared, which had a particle size distribution with a peak particle size of 30 μm as shown in FIG. 15A, and a cumulative volume calculated from the smallest diameter side with a 10% particle size of 20.3 μm, a 50% particle size of 30.2 μm, and a 90% particle size of 47.9 μm as shown in Table 11. The same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 85% by volume of the core material (Al powder) and 15% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill as in Example 1 to obtain a composite filler. As shown in Figure 15a, the particle size distribution of this composite filler had a single peak at a particle size of 45 μm. As shown in Table 11, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 10.2 μm, a 50% cumulative volume of 35.3 μm, and a 90% cumulative volume of 60.1 μm. Because the particle size distribution of the Al powder ranged from 12 μm to 77 μm in a relatively narrow range, the thickness of the shell material layer was estimated to be 15 μm, ranging from 45 μm to 30 μm. Figure 15b shows a scanning electron microscope (SEM) image of the Al powder, and Figure 15c shows a scanning electron microscope (SEM) image of the composite filler.

[0054]

[0055] Example 12 ZrO (zirconium oxide) powder was prepared as a core material. The powder had a particle size distribution with a peak particle size of 45 μm (as shown in FIG. 16A), and as shown in Table 12, the 10% cumulative volume particle size, 50% cumulative volume particle size, and 90% cumulative volume particle size were 2.0 μm, 35.9 μm, and 65.1 μm, respectively, calculated from the smallest diameter side. The same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (ZrO powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill as in Example 1 to obtain a composite filler. As shown in Figure 16a, the particle size distribution of this composite filler had two peaks, at particle sizes of 12 μm and 59 μm. As shown in Table 12, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 3.2 μm, a 50% cumulative volume of 11.4 μm, and a 90% cumulative volume of 52.7 μm. Because the particle size distribution of the ZrO powder ranged from 0.2 μm to 150 μm, it was difficult to calculate the thickness of the shell material layer. However, for convenience, the thickness of the shell material layer was estimated to be 14 μm, i.e., 59 μm - 45 μm. Figure 16b shows a scanning electron microscope (SEM) photograph of the ZrO powder, and Figure 16c shows a scanning electron microscope (SEM) photograph of the composite filler.

[0056]

[0057] Example 13 As a core material, graphite powder was prepared, which had a particle size distribution with a single peak particle size of 39 μm as shown in FIG. 17A, and a cumulative volume calculated from the smallest diameter side with a 10% particle size of 15.8 μm, a 50% particle size of 34.2 μm, and a 90% particle size of 57.9 μm as shown in Table 13. Hydrophilic fumed silica particles (product number "AEROSIL® 380S") were also prepared as a shell material. Next, 30% by volume of the core material (graphite powder) and 70% by volume of the shell material (fumed silica particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill to obtain a composite filler. As shown in Figure 17a, the particle size distribution of this composite filler had two peaks, at particle sizes of 12 μm and 59 μm. As shown in Table 13, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 5.3 μm, a 50% cumulative volume of 18.3 μm, and a 90% cumulative volume of 76.7 μm. Because the particle size distribution of the graphite powder ranged from 4 μm to 101 μm, it was difficult to calculate the thickness of the shell material layer. However, for convenience, the thickness of the shell material layer was estimated to be 20 μm, i.e., 59 μm - 39 μm. Figure 17b shows a scanning electron microscope (SEM) photograph of the graphite powder, and Figure 17c shows a scanning electron microscope (SEM) photograph of the composite filler.

[0058]

[0059] Comparative Example 1 As a core material, the same Cu (copper) powder as in Example 6 was prepared. The Cu powder had a particle size distribution with a single peak particle size of 34 μm as shown in FIG. 18a, and had a 10% cumulative volume particle size of 13.5 μm, a 50% cumulative volume particle size of 30.6 μm, and a 90% cumulative volume particle size of 62.0 μm, as calculated from the smallest diameter side, as shown in Table 14. Furthermore, the same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (Cu powder) and 70% by volume of the shell material (fumed alumina particles) were placed in a polyethylene bag in the air, and the polyethylene bag was mixed with gloved hands for 1 minute to obtain a composite filler. As shown in Figure 18a, the particle size distribution of this composite filler had a single peak at a particle size of 12 μm. As shown in Table 14, the particle size distribution of this composite filler, calculated from the smallest diameter, had a particle size of 6.7 μm at 10% of the cumulative volume, 11.4 μm at 50% of the cumulative volume, and 20.4 μm at 90% of the cumulative volume. After manual mixing, the particle size distribution of the composite filler had a single sharp peak. This likely indicates that the core material, Cu powder, had formed agglomerates weak enough to be broken up by manual mixing, making it difficult to calculate the thickness of the shell material layer. Figure 18b shows a scanning electron microscope (SEM) image of the Cu powder, and Figure 18c shows a scanning electron microscope (SEM) image of the composite filler.

[0060]

[0061] Comparative Example 2 As a core material, the same Cu (copper) powder as in Example 6 was prepared. The Cu powder had a particle size distribution with a single peak particle size of 34 μm as shown in FIG. 19a, and had a 10% cumulative volume particle size of 13.5 μm, a 50% cumulative volume particle size of 30.6 μm, and a 90% cumulative volume particle size of 62.0 μm, as shown in Table 15, calculated from the smallest diameter side. Furthermore, the same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as a shell material. Next, 30% by volume of the core material (Cu powder) and 70% by volume of the shell material (fumed alumina particles) were mixed in air at 2000 rpm for 3 minutes in a rotation-revolution mixer (Thinki Corporation, Awatori Rentaro: model number "ARE-310") to obtain a composite filler. As shown in Figure 19a, the particle size distribution of this composite filler had two peaks, at 13 μm and 67 μm. As shown in Table 15, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 11.2 μm, a 50% cumulative volume of 51.2 μm, and a 90% cumulative volume of 91.9 μm. Because the particle size distribution of the composite filler had a main peak diameter of 67 μm and was significantly different from that of Example 6, which was compounded using a dry ball mill, the thickness of the shell material layer was estimated to be 33 μm (67 μm - 34 μm). Figure 19b shows a scanning electron microscope (SEM) image of the Cu powder, and Figure 19c shows a scanning electron microscope (SEM) image of the composite filler.

[0062]

[0063] Comparative Example 3 The same copper (Cu) powder as in Example 6 was prepared as the core material. The powder had a particle size distribution with a peak particle size of 34 μm as shown in FIG. 20a, and a cumulative 10% particle size of 13.5 μm, a 50% particle size of 30.6 μm, and a 90% particle size of 62.0 μm, calculated from the smallest diameter side, as shown in Table 16. The same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as the shell material. Next, 25% by volume of the core material (Cu powder) and 75% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill as in Example 1 to obtain a composite filler. As shown in Figure 20a, the particle size distribution of this composite filler had two peaks, at particle sizes of 13 μm and 59 μm. As shown in Table 16, the particle size distribution of this composite filler, calculated from the smallest diameter, had a 10% cumulative volume of 4.1 μm, a 50% cumulative volume of 13.8 μm, and a 90% cumulative volume of 71.0 μm. Because the particle size distribution of the Cu powder ranged from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. However, for convenience, the thickness of the shell material layer was estimated to be 25 μm, i.e., 59 μm - 34 μm. Figure 20b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Figure 20c shows a scanning electron microscope (SEM) photograph of the composite filler.

[0064]

[0065] Comparative Example 4 The same copper (Cu) powder as in Example 6 was prepared as the core material. The powder had a particle size distribution with a peak particle size of 34 μm as shown in FIG. 21a, and a cumulative 10% particle size, 50% particle size, and 90% particle size, calculated from the smallest diameter side, of 13.5 μm, 30.6 μm, and 62.0 μm, as shown in Table 17. The same hydrophilic fumed alumina particles (product number "AEROXIDE® Alu C") as in Example 1 were prepared as the shell material. Next, 90% by volume of the core material (Cu powder) and 10% by volume of the shell material (fumed alumina particles) were dry-mixed in the same manner as in Example 1 using the same dry ball mill as in Example 1 to obtain a composite filler. As shown in Figure 21a, the particle size distribution of this composite filler had a single peak at a particle size of 30 μm. As shown in Table 17, the particle size distribution of this composite filler, calculated from the smallest diameter, had a particle size of 14.0 μm at 10% of the cumulative volume, 27.1 μm at 50% of the cumulative volume, and 50.2 μm at 90% of the cumulative volume. After mixing, the particle size distribution of the composite filler had a single sharp peak slightly shifted toward the smallest diameter, making it difficult to calculate the thickness of the shell material. Figure 21b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Figure 21c shows a scanning electron microscope (SEM) photograph of the composite filler.

[0066]

[0067] Comparative Example 5 A filler consisting only of Cu (copper) powder as the core material was prepared in the same manner as in Example 6.

[0068] Comparative Example 6 A filler consisting of only Al (aluminum) powder was prepared in the same manner as in Example 11.

[0069] Comparative Example 7 As in Example 1, a filler consisting of only Sn (tin) powder was produced.

[0070] The contents of the core materials and shell materials in Examples 1 to 13 and Comparative Examples 1 to 4, as well as the methods for mixing these materials, are shown in Table 18. The compositions and physical properties of the composite fillers are shown in Table 19. Note that the fillers in Comparative Examples 5 to 7 consist of only uncomposite core materials, but are referred to as composite fillers for convenience.

[0071]

[0072]

[0073] <Comparative Measurement of Physical Properties> The thermal conductivity of each of the composite fillers of Examples 1 to 13 and Comparative Examples 1 to 4, and the fillers of Comparative Examples 5 to 7 was measured using the above-mentioned device. ６ If the volume resistivity is 10 Ω cm or more, a high resistance resistivity meter and a powder resistance measurement system are used. ６ When the dielectric strength was less than Ω cm, measurements were made using a low-resistance resistivity meter and a powder resistance measurement system, respectively. Furthermore, the breakdown voltages of these composite fillers and resin molded articles produced using the fillers by the above-described method were measured using the above-described equipment. These results are shown in Table 19. Table 19 also shows the thermal conductivity ratios of Examples 1 to 13 and Comparative Examples 1 to 4, 6, and 7, when the thermal conductivity of the filler made of Cu powder in Comparative Example 5 is standardized to 100.

[0074] <Evaluation> As is clear from Tables 18 and 19, in the composite filler of Comparative Example 1 in which the core material and the shell material were mixed by hand without using the dry ball mill mixing method, the volume resistivity was measured in a compressed state by applying pressure to the powder, and therefore the volume resistivity was measured at 1.9 × 10 ６ Although a seemingly high value of Ω cm was obtained, in reality, the mixing force was significantly insufficient, so that a shell was not formed, and the compatibility with the resin was very poor, making it difficult to produce a molded body, and the breakdown voltage could not be measured. Furthermore, the composite filler of Comparative Example 2, in which the core material and shell material were mixed using a rotation / revolution type mixer rather than the dry ball mill mixing method, had a thicker shell material layer and many voids in the shell compared to the composite filler of Example 6, which was mixed using a dry ball mill, and the volume resistivity was 2.9 × 10 ２ It was low at Ω·cm.

[0075] In the composite filler of Comparative Example 3, in which the volume percent of the core material when mixed was 25 volume percent and the volume percent of the shell material was 75 volume percent, which was outside the appropriate range, the amount of shell material was in excess relative to the core material, so the breakdown voltage was high at 5.0 kV / mm, but the thermal conductivity was low at 0.070 W / m K. In addition, in the composite filler of Comparative Example 4, in which the volume percent of the core material when mixed was 90 volume percent and the volume percent of the shell material was 10 volume percent, which was outside the appropriate range, the thermal conductivity was high at 0.112 W / m K, but the volume resistivity was low at 2.6 x 10 because the amount of shell material was insufficient to provide proper insulation. －３ The dielectric strength was as low as Ω·cm, and the breakdown voltage was remarkably low at 0 kV / mm.

[0076] The filler of Comparative Example 5, which is made only of a core material of Cu powder, had a high and excellent thermal conductivity of 0.126 W / m·K, but the volume resistivity was 1.8×10 －４ The filler of Comparative Example 6, which consisted only of the core material of Al powder, had a high and excellent thermal conductivity of 0.191 W / m K, but a low volume resistivity of 1.4 × 10 －４ The filler of Comparative Example 7, which was made only of a core material of Sn powder, had a high and excellent thermal conductivity of 0.181 W / m K, but a low volume resistivity of 1.0 × 10 －４ The dielectric strength was as low as Ω·cm, and the breakdown voltage was remarkably low at 0 kV / mm.

[0077] In contrast, the fumed oxide particles of the shell material are particles in which spherical primary particles are aggregated and fused together in a beaded shape to form bulky aggregated particles, and these aggregated particles are transformed into agglomerated particles. The shell material and core material are mixed in a dry ball mill, and when the composite filler is taken as 100% by volume, the core material is in the range of 30% by volume to 85% by volume and the shell material is in the range of 15% by volume to 70% by volume. In the composite fillers of Examples 1 to 13, the thermal conductivity is 0.075 W / m K or more and the volume resistivity is 1.0 x 10 ５ The dielectric strength was Ω·cm or more, and the breakdown voltage was 1 kV / mm or more.

[0078] In particular, when comparing the thermal conductivity of the composite filler of Example 6 (Cu:fumed alumina particles = 30 vol%:70 vol%) and the composite filler of Example 9 (Cu:fumed alumina particles = 70 vol%:30 vol%), in which the core material (Cu powder) and the shell material (fumed alumina particles) were the same but the volume percentages were different, Example 9, which had a rich core material, had a high thermal conductivity of 0.128 W / m K, while Example 6, which had a poor core material, had a low thermal conductivity of 0.086 W / m K.

[0079] Regarding the volume resistivity and the dielectric breakdown voltage, the volume resistivity and the dielectric breakdown voltage of Example 6, which is rich in the shell material, were 5.7×10 ５ The volume resistivity and breakdown voltage of Example 9, which had a poor shell material, were 7.6×10 ６ The values ​​were Ω cm and 1.2 kV / mm. The volume resistivity of Example 9 was higher than that of Example 6, but the breakdown voltage of Example 6 was higher than that of Example 9, which is a result that often occurs. Since the volume resistivity is measured not on the powder itself but on a sample compressed under high pressure, and the applied voltage is at most about 0.1 to 0.5 kV, it is more reliable to use the breakdown voltage when discussing insulation properties.

[0080] Furthermore, when considering Example 10, in which a composite filler made of the same material and volume percentage as the core material and shell material of Example 6 was subjected to C4 treatment, the thermal conductivity of Example 6 was 0.086 W / m K, while the thermal conductivity of Example 10 was 0.105 W / m K, which was slightly higher. Furthermore, when comparing the volume resistivities, the volume resistivity of Example 6 was 5.7 × 10 ５ The volume resistivity of Example 10 was 1.9 × 10 ７ Ω cm, which was significantly higher. Furthermore, comparing the breakdown voltage, the breakdown voltage of Example 6 was 4.6 kV / mm, while the breakdown voltage of Example 10 was lower at 1.3 kV / mm. This is thought to be because the C4 treatment of the composite filler replaced the air at the core-shell interface and within the shell material layer.

[0081] Furthermore, the composite filler of Example 8, in which the fumed oxide particles were hydrophobic fumed alumina particles, had a volume resistivity of 3.6 × 10 １０ The volume resistivity of the composite fillers in other examples, in which the fumed oxide particles were hydrophilic, was higher at Ω cm, which is believed to be due to the hydrophobic fumed alumina particles having a higher volume resistivity than the hydrophilic fumed alumina particles.

[0082] The composite filler of the present invention can be used as a cooling member for a heat-generating body such as a semiconductor chip, transistor, lithium ion secondary battery, or LED light source in a communication device or an in-vehicle electronic device; a cooling member for a stator built into a motor housing; a cooling member for a power conversion device built into an inverter case; a heat dissipation member for heat generated in a sliding part or a rotating part of an actuator, etc.

Claims

1. A composite filler that forms a core-shell structure consisting of a core material made of carbon, metal, zinc oxide or zirconium oxide and a shell material attached to the surface of the core material, the shell material being formed by mixing fumed oxide particles with the core material in a dry ball mill, the fumed oxide particles being attached to a part or all of the surface of the core material, the fumed oxide particles being particles in which spherical primary particles are aggregated and fused together in a beaded shape to form bulky aggregate particles, and the aggregate particles are transformed into agglomerated particles, the core material being in the range of 30% to 85% by volume and the shell material being in the range of 15% to 70% by volume when the composite filler is taken as 100% by volume, the thermal conductivity measured under maximum compressive load conditions (2400.0 gF) using a thermal conductivity measuring device TRIDENT manufactured by C-Therm and a compression test accessory (CTA) is 0.075 W / m・K or more, The volume resistivity measured using a high resistance / resistivity meter "Hiresta-UX" (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-HT800") or a low resistance / resistivity meter "Loresta-GX" (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-T700") and a powder resistivity measurement system (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-PD-51") was 1.0 x 10 ５ Ω·cm or more, and a dielectric breakdown voltage measured using an AC WITHSTAND VOLTAGE TESTER 7473 manufactured by Keisoku Gijutsu Kenkyusho Co., Ltd. is 1 kV / mm or more.

2. The composite filler according to claim 1, wherein the carbon core material is graphite, ketjen black, graphite or carbon black, and the metal core material is copper, zinc, tin, nickel, aluminum or stainless steel.

3. The composite filler according to claim 1, wherein the particle size of the composite filler is in the range of 1 μm to 300 μm when the volume distribution of the composite filler is measured using a laser diffraction / scattering type particle size distribution measuring device (manufactured by Horiba, Ltd., model "LA960") 4. The composite filler according to claim 1, wherein said fumed oxide particles are fumed silica particles or fumed alumina particles.

5. A method for producing a composite filler, comprising mixing fumed oxide particles in which spherical primary particles are agglomerated and fused together to form bulky agglomerates, and these agglomerates are changed into agglomerate particles with a core material made of carbon, metal, zinc oxide or zirconium oxide in a volume ratio of fumed oxide particles:core material = 30-85:70-15 in a dry ball mill, thereby forming a core-shell structure in which the fumed oxide particles are attached as a shell material to part or all of the surface of the core material, wherein the fumed oxide particles are particles in which spherical primary particles are agglomerated and fused together to form bulky agglomerates, and these agglomerates are changed into agglomerate particles.

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