Composite filler and manufacturing method of the same

Abstract

To provide a composite filler having an electric insulation property that is higher than the conventional electric insulation property while having a heat dissipation (a thermal conductivity).SOLUTION: A composite filler forms a core shell structure formed by a core material that is made of carbon, metal, zinc oxide, or zirconium oxide, and a shell material adhered to a front surface of the core material. The shell material is formed by adhering a fumed oxide particle to one part or a whole part of the front surface of the core material by mixing the fumed oxide particle with the core material through a dry ball mill. The fumed oxide particle is a particle that the number of primary particles in a real spherical type is aggregated and fused, and thus a bulky aggregation particle is formed, and that is changed from the aggregated particle to an agglomeration particle. In a composite filler, the core material is 30 to 85 volume%, the shell material is 15 to 70 volume%, a heat transmission ratio is 0.075 W / m K or more, a volume resistance is 1.0×105 Ω cm or more, and an insulation damage voltage is 1kV / mm or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a composite filler including a core having heat dissipation properties and a shell having electrical insulation properties attached to the surface of the core, and a method for manufacturing the same.

Background Art

[0002] In recent years, electronic devices used in electric vehicles and the like have been increasing in voltage and current. Along with this, the amount of heat generated from such electronic devices has also increased, and how to dissipate this heat has become one of the major issues. Parts used in these electronic devices are required to have high heat dissipation performance, that is, high thermal conductivity, and high electrical insulation from the viewpoint of reliability.

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

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

[0004] The above composite core includes a plurality of core particles having thermal conductivity and an organic binder that binds the core particles together, and the core particles are selected from the group consisting of metal particles, ceramic particles, carbon-based particles, and mixtures thereof, and the insulating material is 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 manufacturing the above-mentioned thermally conductive particles is a method in which a plurality of core particles and an organic binder are mixed by a compression-shear mixing method to bond and mix the core particles with the organic binder to form a composite core, and then the composite core and an insulating material are mixed by a compression-shear mixing method to at least partially coat the composite core with the insulating material.

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

[0007] When measured at an applied voltage of 500 V on the cylinder of the above-mentioned thermally conductive particles having a diameter of 10 mm and a height of 3.0 mm, the above-mentioned thermally conductive particles configured as described above have a volume resistivity in the range of at least 1×10 4 Ω·cm to 1×10 10 Ω·cm.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] The thermally conductive particles shown in Patent Document 1 have a volume resistivity in the range of 1×10 4 Ω·cm to 1×10 10 Ω·cm when measured at 500 V as an index of insulation. However, the above-mentioned thermally conductive particles are insufficient from the viewpoint of electrical insulation as materials and components for electronic devices with increasing high voltage and high current, and there is a further need for materials with higher electrical insulation.

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

Means for Solving the Problems

[0011] A first aspect of the present invention is a composite filler that forms a core-shell structure composed of a core substance made of carbon, metal, zinc oxide, or zirconium oxide, and a shell substance attached to the surface of the core substance, wherein the shell substance is formed by mixing fumed oxide particles and the core substance in a dry ball mill, so that the fumed oxide particles are attached to a part or all of the surface of the core substance, and the fumed oxide particles are particles in which spherical primary particles are aggregated and fused in a bead-like manner to form bulky aggregated particles, which are changed into agglomerated particles. When the composite filler is 100% by volume, the core substance is in the range of 30% to 85% by volume, and the shell substance is in the range of 15% to 70% by volume. The thermal conductivity measured under the maximum compression load condition (2400.0 gF) using a thermal conductivity measuring device TRIDENT and a compression test accessory (CTA) manufactured by C-Therm 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 resistance measuring system (manufactured by Mitsubishi Chemical Analytech Co., Ltd.: model number "MCP-PD-51") is 1.0×10 5 Ω·cm or more, and the dielectric breakdown voltage measured using an AC WITHSTAND VOLTAGE TESTER 7473 manufactured by Measurement Technology Research Institute Co., Ltd. is 1 kV / mm or more.

[0012] A second aspect of the present invention is an invention based on the first aspect, wherein the carbon serving as the core substance is graphite, ketjen black, graphite, or carbon black, and the metal serving as the core substance 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 the particle size when the volume distribution of the composite filler is measured by a laser diffraction particle size distribution analyzer, and is in the range of 1 μm to 300 μm.

[0014] A fourth aspect of the present invention is an 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 in which bulky aggregated particles are formed by the aggregation and fusion of spherical primary particles in a bead shape, and the fumed oxide particles changed from the aggregated particles to agglomerated particles and a core material composed of carbon, metal, zinc oxide or zirconium oxide are mixed by a dry ball mill at a volume ratio of fumed oxide particles:core material = 30 to 85:70 to 15, so that the fumed oxide particles adhere to a part or all of the surface of the core material as a shell material to form a core-shell structure, wherein the fumed oxide particles are particles formed by the aggregation and fusion of spherical primary particles in a bead shape and the formation of bulky aggregated particles, and the change from the aggregated particles to agglomerated particles.

Advantages of the Invention

[0016] The composite filler according to the first aspect of the present invention is composed of a shell material formed by particles in which spherical primary particles are aggregated and fused in a bead shape to form bulky aggregated particles and then changed from the aggregated particles to agglomerated particles. The shell material is mixed and adhered to the surface of a core material composed of carbon, metal, zinc oxide or zirconium oxide by a dry ball mill. Also, when the composite filler is 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. Therefore, the thermal conductivity is 0.075 W / m·K or more, the volume resistivity is 1.0×10 5 Ω·cm or more, and the breakdown voltage is 1 kV / mm or more.

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

[0018] In the composite filler according to the third aspect of the present invention, since its particle size is in the range of 1 μm to 300 μm, when this composite filler is mixed with a resin to produce a resin composition, the resin composition is easy to mold.

[0019] In the composite filler according to the fourth aspect of the present invention, since the fumed oxide particles are fumed silica particles or fumed alumina particles, when they adhere to the surface of the core substance as the shell substance, the electrical insulation of the composite filler is further enhanced.

[0020] In the method for producing the composite filler according to the fifth aspect of the present invention, spherical primary particles aggregate and fuse in a bead-like manner to form bulky aggregated particles, and fumed oxide particles changed from these aggregated particles to agglomerated particles are mixed with a core substance composed of carbon, metal, zinc oxide or zirconium oxide in a volume ratio of fumed oxide particles:core substance = 30 - 85:70 - 15 using a dry ball mill to produce a composite filler. Therefore, in a relatively simple method, the fumed oxide particles adhere to the surface of the core substance as the shell substance. The composite filler thus produced has the characteristics of having heat dissipation (thermal conductivity) and having higher electrical insulation than conventional electrical insulation.

Brief Description of the Drawings

[0021]

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Mode for Carrying Out the Invention

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

[0023] When the core material 10 is carbon, examples of the carbon include graphite, ketjen black, graphite or carbon black. When the core material is metal, examples of the metal include copper, zinc, tin, nickel, aluminum or stainless steel. Zinc oxide or zirconium oxide as the core material is selected because it has a thermal conductivity as high as that of a metal. The particle size and particle size distribution of the core material vary depending on the material of carbon, metal, zinc oxide or zirconium oxide and its manufacturing method, and thus cannot be generally determined.

[0024] The shell material 12 of the present embodiment is formed by mixing fumed oxide particles and a core material with a dry ball mill at room temperature, so that the fumed oxide particles adhere to the surface of the core material. Examples of the 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 is measured by an image analysis method using a transmission electron microscope (TEM) and is in the range of 7 nm to 90 nm. When the fumed oxide particles and the core material are mixed by a dry hand-mixing method or a mixing method using a dry rotation-revolution type mixer, which are mixing methods other than the dry ball mill, the adhesion of the shell material to the core material is not sufficient, and a composite filler having desired thermal conductivity and electrical insulation cannot be obtained. As shown in the enlarged view of FIG. 1, the shell material 12 is formed including agglomerated particles 12a and voids 12b between the agglomerated particles.

[0025] When fumed oxide particles and a core material are mixed by dry ball milling at room temperature, in addition to the agglomerated particles of the fumed oxide particles being coated and adhering to the core material, the core material itself may be reduced in size, or the core materials of reduced size may aggregate. Therefore, when examining the particle size of the composite filler using a particle size distribution curve, the peak diameter in the particle size distribution is not limited to one, and multiple peak diameters may appear. For this reason, the average particle size of the composite filler cannot be determined unconditionally. However, from the particle size distribution curves in Examples 1 to 13 described later, the particle size of the composite filler is in the range of 1 μm to 300 μm. Also, for the same reason that the average particle size of the composite filler cannot be determined unconditionally, the average particle size of the shell material in the composite filler cannot be determined unconditionally. 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). Considering the peak diameter of the core material before composite and the main peak diameter of the composite filler in Examples 1 to 13 described later, when calculating the thickness of the shell material layer that can be estimated from the particle size distribution curve, the thickness of the shell material layer is in the range of 3 μm to 20 μm. The thickness of this shell material layer is twice that of 'd' shown in FIG. 1.

[0026] As shown in FIG. 2, taking the case where the fumed oxide particles are fumed silica particles as an example for the agglomerated particles 12a, by injecting a mixed gas of SiCl4, H2, and O2 as a vaporization raw material from a reactor which is a burner, aggregated particles which are the smallest particle form in which the primary particles are sintered are formed. Next, the aggregated particles gather by weak interactions such as hydrogen bonds and van der Waals forces to form agglomerated particles. The shell material, in an aggregate in which a plurality of composite fillers are gathered, is densely filled between the core materials and forms a three-dimensional network structure that densely fills the voids between the core materials. As a result, without impairing the high thermal conductivity of the core material, the volume resistivity of the composite filler as an aggregate is 1.0×10 5It can be increased to be higher than Ω·cm, and the breakdown voltage can be increased to be higher than 1 kV / mm. Therefore, high electrical insulation can be realized while maintaining the high thermal conductivity originally possessed by the core material. Fig. 3 shows agglomerated particles which are the smallest particle form formed by sintering of primary particles, and subsequently, Fig. 4 shows agglomerated particles formed by aggregation of agglomerated particles through weak interactions such as hydrogen bonds and van der Waals forces.

[0027] When the composite filler is 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. It is preferable that the core material is in the range of 30% to 70% by volume, and it is preferable that the shell material is in the range of 30% to 70% by volume. If the core material is less than 30% by volume and the shell material exceeds 70% by volume, a composite filler having the desired thermal conductivity cannot be obtained. Also, if the core material exceeds 85% by volume and the shell material is less than 15% by volume, a composite filler having the desired volume resistivity and breakdown voltage cannot be obtained. By the shell material formed including agglomerated particles 12a made of fumed oxide particles and voids 12 (see the enlarged view of Fig. 1) adhering to the surface of the core material at a predetermined volume percentage by mixing with a dry ball mill, the composite filler of the present embodiment comes to have high thermal conductivity while having a high volume resistivity and a high breakdown voltage.

[0028] The composite filler of the present 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 the maximum compression load condition (2400.0 gF) using a thermal conductivity measuring device TRIDENT and a compression test accessory (CTA) manufactured by C-Therm Co., Ltd. Also, the composite filler of the present 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'). 5Ω cm or more, preferably 2.0×10 5 Furthermore, the composite filler of the present embodiment is characterized in that its dielectric 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 dielectric breakdown voltage is measured when a resin mixture is prepared by mixing 63 volume % of silicone resin (manufactured by Momentive Corp.: model number "YE5822A"), 7 volume % of hardener (manufactured by Momentive Corp.: model number "YE5822B"), and 30 volume % of composite filler, and the resin mixture and composite filler are mixed at 2000 rpm for 5 minutes in a rotation-revolution mixer (manufactured by Thinky Corp.: model number "ARE-310") to prepare a resin composition, and then the resin composition is placed in a mold with a cavity of length x width x depth of 15 cm x 15 cm x 2 mm, and the resin is cured by applying a pressure of 10 MPa (100 kg / cm2) at a temperature of 130°C and holding the pressure for 10 minutes in a heat press (manufactured by Kodaira Seisakusho Co., Ltd.: model number "PY15-EA") to the resin molded body. EXAMPLES

[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 as commercially available: (1) Tin powder (Sn, Fujifilm Wako Pure Chemical Industries, Ltd.: product number "206-01505") (2) Hydrophilic fumed alumina particles (Al2O3, manufactured by Evonik Corporation: product number "AEROXIDE (registered trademark) Alu C" (average primary particle size 13 nm)) (3) Zinc powder (Zn, manufactured by Hayashi Pure Chemical Industries, Ltd.: product number "26000085") (4) Nickel powder (Ni, manufactured by Fujifilm Wako Pure Chemical Corporation: product number '145 - 00982') (5) Stainless steel powder (SUS, manufactured by Fujifilm Wako Pure Chemical Corporation: product number '900919') (6) Zinc oxide powder (ZnO, manufactured by Hayashi Pure Chemical Industries, Ltd.: product number '26000375') (7) Copper powder (Cu, manufactured by Hayashi Pure Chemical Industries, Ltd.: product number '03003695') (8) Hydrophilic fumed alumina particles (Al2O3, manufactured by Evonik Industries AG: product number 'VP Alu 30' (average primary particle size 90 nm)) (9) Hydrophobic fumed alumina particles (Al2O3, manufactured by Nippon Aerosil Co., Ltd.: product number 'VP Alu C RK' (average primary particle size 13 nm)) (10) Isobutyltrimethoxysilane (IBTMO, manufactured by Evonik Industries AG: product number 'Dynasylan® IBTMO') (11) Aluminum powder (Al, manufactured by Hayashi Pure Chemical Industries, Ltd.: product number '01001325') (12) Zirconium oxide powder (ZrO, manufactured by Fujifilm Wako Pure Chemical Corporation: product number '264 - 00485') (13) Graphite powder (C, manufactured by Ito Graphite Industry Co., Ltd.: product number 'SG - BL40') (14) Hydrophilic fumed silica particles (manufactured by Nippon Aerosil Co., Ltd.: product number 'AEROSIL® 380S' (average primary particle size 7 nm))

[0032] <Example 1> As the core material, as shown in Fig. 5a, Sn (tin) powder having one peak particle size of 17 μm in the particle size distribution was prepared. As shown in Table 1, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 9.6 μm, the particle size at 50% was 18.4 μm, and the particle size at 90% was 40.6 μm. Also, as the shell material, hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') were prepared. Next, 30% by volume of the core material (Sn powder) and 70% by volume of the shell material (fumed alumina particles) were dry-mixed by rotating in a ball mill turntable (manufactured by Masuda Rika Kogyo Co., Ltd.: model number 'UNIVERSAL BALL MILL MODEL UBM-S') at room temperature in the atmosphere for 5 hours at a speed of 110 rpm using a pot mill containing alumina balls with a diameter of 5 mm manufactured by Nikkato Corporation to obtain a composite filler. As shown in Fig. 5a, the peaks of the particle size distribution of this composite filler were two with particle sizes of 12 μm and 67 μm. As shown in Table 1, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 5.9 μm, the particle size at 50% was 15.7 μm, and the particle size at 90% was 89.3 μm. Since the particle size in the particle size distribution of the Sn powder was in a wide range from 4 μm to 150 μm, it was difficult to calculate the thickness of the shell material layer. Fig. 5b shows a scanning electron microscope (SEM) photograph of the Sn powder, and Fig. 5c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0033]

Table 1

[0034] Starting from Example 1, in the other examples and comparative examples described below, the particle size of the core material or composite filler was measured using a laser diffraction / scattering particle size distribution measuring device (manufactured by Horiba, Ltd., model 'LA960'), and the particle size of the obtained core material or composite filler was used as the horizontal axis, and the particle size in the particle size distribution curve with the volume-converted particle frequency as the vertical axis. Also, the average primary particle size of the shell material was the particle size measured by the image analysis method of a transmission electron microscope (TEM).

[0035] <Example 2> As the core material, as shown in Fig. 6a, Zn (zinc) powder having a single peak particle size of 9 μm in the particle size distribution was prepared. As shown in Table 2, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 5.2 μm, the particle size at 50% was 8.0 μm, and the particle size at 90% was 12.2 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') as in Example 1 were prepared. 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 as in Example 1 to obtain a composite filler. As shown in Fig. 6a, the peaks of the particle size distribution of this composite filler were two, with particle sizes of 12 μm and 67 μm. As shown in Table 2, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 5.1 μm, the particle size at 50% was 12.2 μm, and the particle size at 90% was 50.4 μm. Since the particle size in the particle size distribution of the Zn powder was in a relatively narrow range from 3 μm to 22 μm, the thickness of the shell material layer was estimated to be 3 μm, which is 12 μm - 9 μm. Fig. 6b shows a scanning electron microscope (SEM) photograph of the Zn powder, and Fig. 6c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0036]

Table 2

[0037] <Example 3> As the core material, as shown in Fig. 7a, Ni (nickel) powder having one peak particle size of 10 μm in the particle size distribution was prepared. As shown in Table 3, the particle size at which the cumulative volume calculated from the smaller diameter side is 10% is 5.6 μm, the particle size at which it is 50% is 9.6 μm, and the particle size at which it is 90% is 16.5 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') as in Example 1 were prepared. Next, 30% by volume of the core material (Ni 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 Fig. 7a, the peak of the particle size distribution of this composite filler is one with a particle size of 12 μm. As shown in Table 3, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side is 10% is 3.3 μm, the particle size at which it is 50% is 9.2 μm, and the particle size at which it is 90% is 38.4 μm. Since the particle size in 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, which is 12 μm - 10 μm. Fig. 7b shows a scanning electron microscope (SEM) photograph of the Ni powder, and Fig. 7c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0038]

Table 3

[0039] <Example 4> As the core material, as shown in Fig. 8a, SUS (stainless steel) powder having one peak particle size of 89 μm in the particle size distribution was prepared. As shown in Table 4, the particle size at which the cumulative volume calculated from the smaller diameter side is 10% is 26.0 μm, the particle size at which it is 50% is 68.9 μm, and the particle size at which it is 90% is 160.3 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE (registered trademark) Alu C') as in Example 1 were prepared. 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 Fig. 8a, the peaks of the particle size distribution of this composite filler are two with particle sizes of 12 μm and 67 μm. As shown in Table 4, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side is 10% is 5.8 μm, the particle size at which it is 50% is 13.4 μm, and the particle size at which it is 90% is 57.6 μm. Since the particle size in the particle size distribution of the SUS powder was in a wide range from 8 μm to 300 μm, it was difficult to calculate the thickness of the shell material layer. Fig. 8b shows a scanning electron microscope (SEM) photograph of the SUS powder, and Fig. 8c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0040]

Table 4

[0041] <Example 5> As the core material, as shown in Fig. 9a, ZnO (zinc oxide) powder having three peak particle sizes of 0.09 μm, 2 μm, and 77 μm in the particle size distribution was prepared. As shown in Table 5, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 0.6 μm, the particle size at 50% was 2.6 μm, and the particle size at 90% was 38.9 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') as in Example 1 were prepared. Next, 30% by volume of the core material (ZnO 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 Fig. 9a, the peaks in the particle size distribution of this composite filler were two with particle sizes of 13 μm and 45 μm. As shown in Table 5, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 1.7 μm, the particle size at 50% was 14.5 μm, and the particle size at 90% was 63.3 μm. Since the particle size in the particle size distribution of the ZnO powder was in a wide range from 0.06 μm to 300 μm, it was difficult to calculate the thickness of the shell material layer. Fig. 9b shows a scanning electron microscope (SEM) photograph of the ZnO powder, and Fig. 9c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0042]

Table 5

[0043] <Example 6> As the core material, as shown in Fig. 10a, Cu (copper) powder having one peak particle size of 34 μm in the particle size distribution was prepared. As shown in Table 6, the particle size at which the cumulative volume calculated from the smaller diameter side is 10% is 13.5 μm, the particle size at which it is 50% is 30.6 μm, and the particle size at which it is 90% is 62.0 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE (registered trademark) Alu C') as in Example 1 were prepared. 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 Fig. 10a, the peaks of the particle size distribution of this composite filler are two, with particle sizes of 13 μm and 51 μm. As shown in Table 6, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side is 10% is 6.8 μm, the particle size at which it is 50% is 16.4 μm, and the particle size at which it is 90% is 66.3 μm. Since the particle size in the particle size distribution of the Cu powder was in a wide range from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. For convenience, the thickness of the shell material layer was estimated to be 17 μm of 51 μm - 34 μm. Fig. 10b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 10c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0044]

Table 6

[0045] <Example 7> As the core material, as shown in Fig. 11a, Cu (copper) powder identical to that of Example 6 was prepared, which had one peak particle size of 34 μm in the particle size distribution, and as shown in Table 7, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 13.5 μm, the particle size at 50% was 30.6 μm, and the particle size at 90% was 62.0 μm. Also, as the shell material, hydrophilic fumed alumina particles (product number 'VP Alu 30') were prepared. 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 Fig. 11a, the peaks of the particle size distribution of this composite filler were two with particle sizes of 13 μm and 51 μm. As shown in Table 7, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 6.8 μm, the particle size at 50% was 16.4 μm, and the particle size at 90% was 66.3 μm, showing no difference from Example 6. Since the particle size in the particle size distribution of the Cu powder was in a wide range from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. For the sake of convenience, the thickness of the shell material layer was estimated to be 17 μm, which was 51 μm - 34 μm. Fig. 11b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 11c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0046]

Table 7

[0047] <Example 8> As the core material, as shown in Fig. 12a, Cu (copper) powder identical to that of Example 6 was prepared, which had one peak particle size of 34 μm in the particle size distribution, and as shown in Table 8, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 13.5 μm, the particle size at 50% was 30.6 μm, and the particle size at 90% was 62.0 μm. Also, as the shell material, hydrophobic fumed alumina particles (product number 'VP Alu C RK') were prepared. 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 Fig. 12a, the peak of the particle size distribution of this composite filler was one with a particle size of 12 μm, and as shown in Table 8, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 2.6 μm, the particle size at 50% was 9.2 μm, and the particle size at 90% was 26.5 μm. Since the particle size in the particle size distribution of the Cu powder was in a wide range from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. Fig. 12b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 12c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0048]

Table 8

[0049] <Example 9> As the core material, as shown in Fig. 13a, Cu (copper) powder identical to that of Example 6, which has one peak particle size of 34 μm in the particle size distribution and, as shown in Table 9, has a particle size of 13.5 μm with a cumulative volume of 10% calculated from the smaller diameter side, a particle size of 30.6 μm with a cumulative volume of 50%, and a particle size of 62.0 μm with a cumulative volume of 90%, was prepared. Also, as the shell material, hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') were prepared. 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. The peaks of the particle size distribution of this composite filler are two, with particle sizes of 13 μm and 45 μm, as shown in Fig. 13a, and as shown in Table 9, in the particle size distribution of this composite filler, the particle size with a cumulative volume of 10% calculated from the smaller diameter side is 7.3 μm, the particle size with a cumulative volume of 50% is 17.2 μm, and the particle size with a cumulative volume of 90% is 55.7 μm. Since the particle size in the particle size distribution of the Cu powder was in a wide range from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer, but for convenience, the thickness of the shell material layer was estimated to be 11 μm, which is 45 μm - 34 μm. Fig. 13b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 13c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0050]

Table 9

[0051] <Example 10> As the core material, as shown in Fig. 14a, Cu (copper) powder identical to that of Example 6, which has one peak particle size of 34 μm in the particle size distribution, and as shown in Table 10, has a particle size of 13.5 μm with a cumulative volume of 10% calculated from the smaller diameter side, a particle size of 30.6 μm with a cumulative volume of 50%, and a particle size of 62.0 μm with a cumulative volume of 90%, was prepared. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') as in Example 1 were prepared. 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, and then the surface of this mixed powder was hydrophobized by wet treatment (hereinafter referred to as C4 treatment) with isobutyltrimethoxysilane, a silane coupling agent, to obtain a composite filler. This C4 treatment involves putting the above mixed powder into a beaker containing IBTMO and heat-treating it at a temperature of 120 °C in the air using a magnetic stirrer with a hot plate (manufactured by Tokyo Rika Kikai Co., Ltd.: model number 'RH-1000'), and then drying it at 150 °C for 1 hour in the air. The dried product was used as the composite filler as it was. As shown in Fig. 14a, the peaks of the particle size distribution of this composite filler are three with particle sizes of 15 μm, 51 μm, and 300 μm. As shown in Table 10, in the particle size distribution of this composite filler, the particle size with a cumulative volume of 10% calculated from the smaller diameter side was 8.2 μm, the particle size with a cumulative volume of 50% was 40.3 μm, and the particle size with a cumulative volume of 90% was 298.9 μm. Since the particle size in the particle size distribution of the Cu powder was in a wide range from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer, but for the sake of convenience, the thickness of the shell material layer was estimated to be 17 μm of 51 μm - 34 μm. Fig. 14b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 14c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0052]

Table 10

[0053] <Example 11> As the core material, as shown in Fig. 15a, Al (aluminum) powder having one peak particle size of 30 μm in the particle size distribution was prepared. As shown in Table 11, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 20.3 μm, the particle size at 50% was 30.2 μm, and the particle size at 90% was 47.9 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE (registered trademark) Alu C') as in Example 1 were prepared. 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 to obtain a composite filler. As shown in Fig. 15a, the peak of the particle size distribution of this composite filler was one with a particle size of 45 μm. As shown in Table 11, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 10.2 μm, the particle size at 50% was 35.3 μm, and the particle size at 90% was 60.1 μm. Since the particle size in the particle size distribution of the Al powder was in a relatively narrow range from 12 μm to 77 μm, the thickness of the shell material layer was estimated to be 15 μm of 45 μm - 30 μm. Fig. 15b shows a scanning electron microscope (SEM) photograph of the Al powder, and Fig. 15c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0054]

Table 11

[0055] <Example 12> As the core material, as shown in Fig. 16a, ZrO (zirconium oxide) powder having one peak particle size of 45 μm in the particle size distribution was prepared. As shown in Table 12, the particle size at which the cumulative volume calculated from the smaller diameter side is 10% is 2.0 μm, the particle size at which it is 50% is 35.9 μm, and the particle size at which it is 90% is 65.1 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE (registered trademark) Alu C') as in Example 1 were prepared. 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 to obtain a composite filler. As shown in Fig. 16a, the peaks of the particle size distribution of this composite filler are two, with particle sizes of 12 μm and 59 μm. As shown in Table 12, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side is 10% is 3.2 μm, the particle size at which it is 50% is 11.4 μm, and the particle size at which it is 90% is 52.7 μm. Since the particle size in the particle size distribution of the ZrO powder was in a relatively wide range from 0.2 μm to 150 μm, it was difficult to calculate the thickness of the shell material layer. For convenience, the thickness of the shell material layer was estimated to be 14 μm, which is 59 μm - 45 μm. Fig. 16b shows a scanning electron microscope (SEM) photograph of the ZrO powder, and Fig. 16c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0056]

Table 12

[0057] <Example 13> As the core material, as shown in Fig. 17a, graphite powder having one peak particle size of 39 μm in the particle size distribution was prepared. As shown in Table 13, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 15.8 μm, the particle size at 50% was 34.2 μm, and the particle size at 90% was 57.9 μm. As the shell material, hydrophilic fumed silica particles (product number 'AEROSIL® 380S') were prepared. 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 as in Example 1 to obtain a composite filler. As shown in Fig. 17a, the peaks of the particle size distribution of this composite filler were two with particle sizes of 12 μm and 59 μm. As shown in Table 13, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 5.3 μm, the particle size at 50% was 18.3 μm, and the particle size at 90% was 76.7 μm. Since the particle size in the particle size distribution of the graphite powder was in a relatively wide range from 4 μm to 101 μm, it was difficult to calculate the thickness of the shell material layer. For the sake of convenience, the thickness of the shell material layer was estimated to be 20 μm of 59 μm - 39 μm. Fig. 17b shows a scanning electron microscope (SEM) photograph of the graphite powder, and Fig. 17c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0058]

Table 13

[0059] <Comparative Example 1> As the core material, as shown in Fig. 18a, Cu (copper) powder identical to that of Example 6, which has one peak particle size of 34 μm in the particle size distribution, and as shown in Table 14, has a particle size of 13.5 μm at 10% cumulative volume calculated from the smaller diameter side, 30.6 μm at 50% cumulative volume, and 62.0 μm at 90% cumulative volume, was prepared. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') as in Example 1 were prepared. Next, 30% by volume of the core material (Cu powder) and 70% by volume of the shell material (fumed alumina particles) were put into a polyethylene bag in the air, and a composite filler was obtained by the hand-mixing method of mixing the polyethylene bag with gloved hands for 1 minute. As shown in Fig. 18a, the peak of the particle size distribution of this composite filler is one with a particle size of 12 μm. As shown in Table 14, in the particle size distribution of this composite filler, the particle size at 10% cumulative volume calculated from the smaller diameter side is 6.7 μm, 11.4 μm at 50% cumulative volume, and 20.4 μm at 90% cumulative volume. Since the particle size distribution of the composite filler after hand-mixing became one sharp peak and the Cu powder of the core material became weakly aggregated enough to be dispersed by hand-mixing, it was difficult to calculate the thickness of the shell material layer. Fig. 18b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 18c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0060]

Table 14

[0061] <Comparative Example 2> As the core material, as shown in Fig. 19a, Cu (copper) powder identical to that of Example 6, which has one peak particle size of 34 μm in the particle size distribution, and as shown in Table 15, has a particle size of 13.5 μm at 10% cumulative volume calculated from the smaller diameter side, 30.6 μm at 50% cumulative volume, and 62.0 μm at 90% cumulative volume, was prepared. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') as in Example 1 were prepared. Next, 30% by volume of the core material (Cu powder) and 70% by volume of the shell material (fumed alumina particles) were mixed in the atmosphere at 2000 rpm for 3 minutes using a rotation and revolution mixer (manufactured by Shinchi Co., Ltd., Awatori Renkitaro: model number 'ARE-310') to obtain a composite filler. The peaks of the particle size distribution of this composite filler are two, with particle sizes of 13 μm and 67 μm as shown in Fig. 19a, and as shown in Table 15, in the particle size distribution of this composite filler, the particle size at 10% cumulative volume calculated from the smaller diameter side is 11.2 μm, 50% cumulative volume is 51.2 μm, and 90% cumulative volume is 91.9 μm. Since the peak diameter of 67 μm was the main one in the particle size distribution of the composite filler and there was a significant difference from Example 6 which was compounded by a dry ball mill, the thickness of the shell material layer was estimated to be 33 μm of 67 μm - 34 μm. Fig. 19b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 19c shows a scanning electron microscope (SEM) photograph of the composite filler respectively.

[0062]

Table 15

[0063] <Comparative Example 3> As the core material, as shown in Fig. 20a, copper (Cu) powder identical to that of Example 6 was prepared, which had one peak particle size of 34 μm in the particle size distribution, and as shown in Table 16, the particle size at which the cumulative volume % calculated from the smaller diameter side was 10% was 13.5 μm, the particle size at 50% was 30.6 μm, and the particle size at 90% was 62.0 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE (registered trademark) Alu C') as in Example 1 were prepared. 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 to obtain a composite filler. As shown in Fig. 20a, the peaks of the particle size distribution of this composite filler were two, with particle sizes of 13 μm and 59 μm. As shown in Table 16, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 4.1 μm, the particle size at 50% was 13.8 μm, and the particle size at 90% was 71.0 μm. Since the particle size in the particle size distribution of the Cu powder was in a wide range from 5 μm to 175 μm, it was difficult to calculate the thickness of the shell material layer. For convenience, the thickness of the shell material layer was estimated to be 25 μm, which was 59 μm - 34 μm. Fig. 20b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 20c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0064]

Table 16

[0065] <Comparative Example 4> As the core material, as shown in Fig. 21a, copper (Cu) powder identical to that of Example 6 was prepared, which had one peak particle size of 34 μm in the particle size distribution, and as shown in Table 17, the particle size at which the cumulative volume % calculated from the smaller diameter side was 10% was 13.5 μm, the particle size at 50% was 30.6 μm, and the particle size at 90% was 62.0 μm. Also, as the shell material, the same hydrophilic fumed alumina particles (product number 'AEROXIDE® Alu C') as in Example 1 were prepared. 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 to obtain a composite filler. As shown in Fig. 21a, the peak of the particle size distribution of this composite filler was one with a particle size of 30 μm. As shown in Table 17, in the particle size distribution of this composite filler, the particle size at which the cumulative volume calculated from the smaller diameter side was 10% was 14.0 μm, the particle size at 50% was 27.1 μm, and the particle size at 90% was 50.2 μm. The particle size distribution of the composite filler after mixing became a sharp single peak slightly shifted to the smaller particle size side, and it was difficult to calculate the thickness of the shell material. Fig. 21b shows a scanning electron microscope (SEM) photograph of the Cu powder, and Fig. 21c shows a scanning electron microscope (SEM) photograph of the composite filler, respectively.

[0066]

Table 17

[0067] <Comparative Example 5> A filler consisting only of the Cu (copper) powder of the core material, which was the same as that of Example 6, was prepared.

[0068] <Comparative Example 6> A filler consisting only of Al (aluminum) powder, which was the same as that of Example 11, was prepared.

[0069] <Comparative Example 7> A filler consisting only of Sn (tin) powder, which was the same as that of Example 1, was prepared.

[0070] Table 18 shows the details of the core materials, shell materials, and their mixing methods in Examples 1 to 13 and Comparative Examples 1 to 4 described above. Table 19 shows the composition of the composite filler and the physical properties of the composite filler. Note that the fillers in Comparative Examples 5 to 7 consist only of non-composite core materials, but are referred to as composite fillers for convenience.

[0071]

Table 18

[0072]

Table 19

[0073] <Comparative measurement of physical property values> The thermal conductivities of the composite fillers in Examples 1 to 13 and Comparative Examples 1 to 4 and the fillers in Comparative Examples 5 to 7 were measured using the apparatus described above. Also, regarding the volume resistivity of these composite fillers and fillers, when the volume resistivity is 10 6 Ω·cm or more, a high resistance / resistivity meter and a powder resistance measurement system were used, and when the volume resistivity is less than 10 6 Ω·cm, a low resistance / resistivity meter and a powder resistance measurement system were used for measurement, respectively. Furthermore, the dielectric breakdown voltages of the resin molded bodies prepared by the method described above using these composite fillers and fillers were measured using the apparatus described above. 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 was normalized 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 mixing rather than by the dry ball mill mixing method, the measurement of the volume resistivity was carried out in a state where pressure was applied to the powder and compressed, so 1.9×10 6Although a seemingly high value of Ω·cm is obtained, in reality, the mixing force is extremely insufficient, so the shell cannot be formed. Since the compatibility with the resin is very poor, it is difficult to produce a molded body and the breakdown voltage cannot be measured. Also, in the composite filler of Comparative Example 2 where the core material and the shell material were mixed using a rotating and revolving mixer that does not rely on the dry ball mill mixing method, compared with the composite filler of Example 6 mixed by dry ball mill, the shell material layer is thicker and there are many voids in the shell, and the volume resistivity was 2.9×10 2 Ω·cm, which was low.

[0075] In the composite filler of Comparative Example 3 where the volume percentage of the core material during mixing is 25% by volume and the volume percentage of the shell material is 75% by volume, which is not in an appropriate range, since the shell material is excessive with respect to the core material, the breakdown voltage is as high as 5.0 kV / mm, but the thermal conductivity is as low as 0.070 W / m·K. Also, in the composite filler of Comparative Example 4 where the volume percentage of the core material during mixing is 90% by volume and the volume percentage of the shell material is 10% by volume, which is not in an appropriate range, the thermal conductivity was as high as 0.112 W / m·K and was excellent, but since the amount of the shell material was insufficient and proper insulation could not be achieved, the volume resistivity was 2.6×10 -3 Ω·cm, which was low, and the breakdown voltage was extremely low at 0 kV / mm.

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

[0077] In contrast, the fumed oxide particles of the shell material are spherical primary particles that aggregate and fuse in a bead-like manner to form bulky aggregated particles, which are particles that have changed from these aggregated particles to agglomerated particles. When the shell material and the core material are mixed in a dry ball mill and the composite filler is 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. In the composite fillers of Examples 1 to 13, the thermal conductivity is 0.075 W / m·K or more, the volume resistivity is 1.0×10 5 Ω·cm or more, and the breakdown voltage is 1 kV / mm or more.

[0078] In particular, when comparing the composite filler of Example 6 (Cu: fumed alumina particles = 30% by volume: 70% by volume) and the composite filler of Example 9 (Cu: fumed alumina particles = 70% by volume: 30% by volume) with respect to thermal conductivity, while the materials of the core material (Cu powder) and the shell material (fumed alumina particles) are the same respectively, the thermal conductivity of Example 9 with a rich core material was as high as 0.128 W / m·K, whereas the thermal conductivity of Example 6 with a poor core material was as low as 0.086 W / m·K.

[0079] Regarding the volume resistivity and the breakdown voltage, the volume resistivity and the breakdown voltage of Example 6 with a rich shell material were 5.7×10 5 Ω·cm and 4.6 kV / mm respectively, whereas the volume resistivity and the breakdown voltage of Example 9 with a poor shell material were 7.6×10 6 Ω·cm and 1.2 kV / mm respectively. In terms of the volume resistivity, Example 9 was higher than Example 6, but in terms of the breakdown voltage, Example 6 was higher than Example 9. However, this is a frequently occurring result. The volume resistance value is measured for a sample compressed under high pressure rather than the powder itself, and the applied voltage is at most about 0.1 to 0.5 kV. Therefore, for the discussion of insulation properties, it is more reliable to use the breakdown voltage.

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

[0081] Furthermore, in the composite filler of Example 8 where the fumed oxide particles are hydrophobic fumed alumina particles, the volume resistivity was 3.6×10 10 Ω·cm, which was higher than the volume resistivity of the composite fillers of other examples where the fumed oxide particles are hydrophilic. This is presumably due to the fact that hydrophobic fumed alumina particles have a higher volume resistivity than hydrophilic fumed alumina particles.

Industrial Applicability

[0082] The composite filler of the present invention can be used as a cooling member for heat-generating bodies such as semiconductor chips, transistors, lithium-ion secondary batteries, or LED light sources in communication devices and in-vehicle electronic devices, a cooling member for stators built into motor housings, a cooling member for power conversion devices built into inverter cases, a heat-radiating member for heat generated in the sliding or rotating parts of actuators, and the like.

Claims

1. A composite filler that forms a core-shell structure composed 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 is formed by mixing fumed oxide particles and the core material in a dry ball mill, so that the fumed oxide particles adhere to part or all of the surface of the core material. The fumed oxide particles are particles in which spherical primary particles are aggregated and fused in a bead-like manner to form bulky aggregated particles, which are then changed into agglomerated particles. When the composite filler is 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 thermal conductivity measured under the maximum compression load condition (2400.0 gF) using a thermal conductivity measuring device TRIDENT and a compression test accessory (CTA) manufactured by C-Therm is 0.075 W / m·K or more. The volume resistivity measured using a high resistance / resistivity meter 'Hiresta-UPX' (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 1.0×10 5 Ω·cm or more, and A composite filler characterized in that the dielectric breakdown voltage measured using an AC WITHSTAND VOLTAGE TESTER 7473 manufactured by Measurement Technology Research Institute Co., Ltd. is 1 kV / mm or more.

2. The composite filler according to claim 1, 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.

3. The composite filler according to claim 1, wherein the particle size of the composite filler is the particle size when the volume distribution of the composite filler is measured using 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.

4. The composite filler according to claim 1, wherein the fumed oxide particles are fumed silica particles or fumed alumina particles.

5. A method for producing a composite filler, comprising forming a core-shell structure in which fumed oxide particles are attached as a shell material to a part or all of the surface of a core material by mixing fumed oxide particles, which are formed by aggregation and fusion of spherical primary particles into bulky aggregated particles and then changing from the aggregated particles to agglomerated particles, and a core material composed of carbon, metal, zinc oxide or zirconium oxide in a volume ratio of fumed oxide particles:core material = 30 to 85:70 to 15 using a dry ball mill, wherein the fumed oxide particles are particles formed by aggregation and fusion of spherical primary particles into bulky aggregated particles and then changing from the aggregated particles to agglomerated particles.

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