Thermally conductive filler, method for producing thermally conductive filler, and thermally conductive resin composition

Abstract

To provide a thermally conductive filler which contains an industrially easily available fused alumina particles, has excellent thermal conductivity, has high filling property to a resin and hardly increases hardness of a resin excessively even when added to a resin and a method for producing the same and to provide a thermal conductive resin composition having excellent thermal conductivity and low hardness.SOLUTION: There is provided a thermally conductive filler which contains coarse inorganic particles and small-diameter inorganic particles, wherein the coarse inorganic particles include large-diameter fused alumina particles having an average particle diameter in the range of 20 μm or more and 50 μm or less and medium-diameter inorganic particles having an average particle diameter in the range of 1.0 μm or more and 10 μm or less in a mass ratio of 60:40 to 100:0, the small-diameter inorganic particles have an average particle diameter in the range of 0.1 μm or more and less than 1.0 μm and the content ratio of the small-diameter inorganic particles is in the range of 15 mass% or more and 30 mass% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a thermally conductive filler, a method for producing the thermally conductive filler, and a thermally conductive resin composition. [Background technology]

[0002] In recent years, with the advancement of electric vehicles, fuel cell vehicles, and the like, electrical components have become increasingly heavy currents, and the amount of heat generated by these electrical components has also been increasing. For example, lithium ion batteries for automobiles generate a large amount of heat because they continuously output high-current power for long periods of time, and this large amount of generated heat must be efficiently dissipated to the outside. For this reason, thermally conductive resin compositions with excellent thermal conductivity are sometimes used as heat dissipation members in parts of electrical components that output large currents, such as lithium ion batteries, that require insulation.

[0003] Conventionally, thermally conductive resin compositions include those in which a thermally conductive filler with excellent thermal conductivity is dispersed in a resin material with excellent insulating properties and moldability. Inorganic materials such as fused alumina (Al2O3) and silicon carbide (SiC) are known as thermally conductive fillers (Patent Document 1). Patent Document 1 describes α-alumina (fused alumina) with an average particle size of more than 10 μm and not more than 60 μm, and silicon carbide with an average particle size of not more than 10 μm. Furthermore, the use of a mixture of thermally conductive fillers with two different average particle sizes, large and small, has been investigated (Patent Document 2). Patent Document 2 describes the use of a mixture of silicon carbide with an average particle size of 50 to 100 μm and silicon carbide with an average particle size of not more than 10 μm, in a weight ratio of 1:1 to 3:1, as a thermally conductive filler with two different average particle sizes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-304946 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-6981 Summary of the Invention [Problem to be solved by the invention]

[0005] The thermally conductive filler used in the thermally conductive resin composition is preferably a material that is easily available industrially, has excellent thermal conductivity, is easily filled into the resin, and does not excessively increase the hardness of the resin when added to the resin. The fused alumina described in Patent Document 1 is an industrially easily available material, but there is a limit to how much fused alumina alone can improve thermal conductivity. Furthermore, silicon carbide described in Patent Document 2 has higher thermal conductivity than fused alumina, but is difficult to fill highly into the resin.

[0006] The present invention has been made in consideration of the above circumstances, and aims to provide a thermally conductive filler that uses materials that are easily available industrially, has excellent thermal conductivity, is highly fillable into resins, and does not excessively increase the hardness of resins when added to the resin; a method for producing the same; and a thermally conductive resin composition that has excellent thermal conductivity and low hardness. [Means for solving the problem]

[0007] In order to solve the above problems, the thermally conductive filler of the present invention comprises coarse inorganic particles and small-diameter inorganic particles, and the coarse inorganic particles have an average particle size in the range of 20 μm to 50 μm. Cubic and irregular shapes The composition contains large-diameter fused alumina particles and medium-diameter inorganic particles having an average particle size in the range of 1.0 μm or more and 10 μm or less in a mass ratio of 60:40 to 100:0, the average particle size of the small-diameter inorganic particles is in the range of 0.1 μm or more and less than 1.0 μm, and the content of the small-diameter inorganic particles is in the range of 15 mass% or more and 30 mass% or less.

[0008] The thermally conductive filler of the present invention, configured as described above, has high thermal conductivity because fine small-sized inorganic particles with an average particle size of 0.1 μm or more but less than 1.0 μm are present in the gaps between the coarse inorganic particles. Furthermore, the thermally conductive filler of the present invention contains large-sized fused alumina particles, medium-sized inorganic particles, and small-sized inorganic particles in the above-described ranges, so that when added to a resin, the hardness of the resin is unlikely to increase excessively. Furthermore, the large-sized fused alumina particles used in the thermally conductive filler of the present invention are easily available industrially. Therefore, by using the thermally conductive filler of the present invention, a thermally conductive resin composition with excellent thermal conductivity and low hardness can be obtained at low cost.

[0009] Here, the thermally conductive filler of the present invention may be configured so that the thermal conductivity at 20°C is 30 W / mK or more. In this case, a thermally conductive resin composition having excellent thermal conductivity can be obtained more reliably.

[0010] The thermally conductive filler of the present invention may be configured so that when 1100 parts by mass of the filler is added to 100 parts by mass of silicone resin, the hardness measured by an Asker C hardness scale is 65 or less. In this case, since the hardness of the resin when added to the silicone resin is the above value, it can be added to various resins other than silicone resin without excessively increasing the hardness.

[0011] The method for producing the thermally conductive filler of the present invention is a method for producing the thermally conductive filler of the present invention, wherein the average particle size is in the range of 20 μm or more and 50 μm or less. Cubic and irregular shapes The composition is such that large-diameter fused alumina particles, medium-diameter inorganic particles having an average particle size in the range of 1.0 μm or more and 10 μm or less, and small-diameter inorganic particles having an average particle size in the range of 0.1 μm or more and less than 1.0 μm are mixed in a mass ratio of 60:40 to 100:0, and the content of the small-diameter inorganic particles is in the range of 15 mass% to 30 mass%. According to the method for producing the thermally conductive filler of the present invention configured as described above, the large-diameter fused alumina particles, the medium-diameter inorganic particles, and the small-diameter inorganic particles are mixed in the above-mentioned ratio, so that the thermally conductive filler of the present invention can be produced industrially advantageously.

[0012] The thermally conductive resin composition of the present invention contains a resin and the thermally conductive filler of the present invention. The thermally conductive resin composition of the present invention having the above-described configuration contains the thermally conductive filler of the present invention, and therefore has excellent thermal conductivity and low hardness.

[0013] Here, the thermally conductive resin composition of the present invention may be configured so that the thermal conductivity at 40°C is 3.25 W / mK or more. In this case, the thermally conductive resin composition has high thermal conductivity at 40° C., and therefore can be advantageously used as a heat dissipation member for electric parts that output large currents, such as lithium ion batteries. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a thermally conductive filler that contains industrially readily available electrofused alumina particles, has excellent thermal conductivity, is highly fillable into resins, and is unlikely to excessively increase the hardness of resins when added to the resin; a method for producing the same; and a thermally conductive resin composition that has excellent thermal conductivity and low hardness. [Brief explanation of the drawings]

[0015] [Figure 1] 1 shows the particle size distribution of large-diameter fused alumina particles used in the examples. [Figure 2] 1 shows the particle size distribution of medium-sized inorganic particles used in the examples. [Figure 3] 1 shows the particle size distribution of small-diameter inorganic particles used in the examples. [Figure 4] 1 shows an estimated value of particle size distribution of the mixed powder produced in Example 1 of the present invention. [Figure 5] FIG. 2 is a ternary diagram showing the compositions of mixed powders produced in Examples 1 to 6 of the present invention and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0016] A thermally conductive filler, a method for producing the thermally conductive filler, and a thermally conductive resin composition according to one embodiment of the present invention will be described below. Note that the following embodiment is specifically described to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[0017] (thermal conductive filler) The thermally conductive filler contains coarse inorganic particles and small inorganic particles. The coarse inorganic particles are composed of large fused alumina particles with an average particle size ranging from 20 μm to 50 μm, and medium-sized inorganic particles with an average particle size ranging from 1.0 μm to 10 μm, in a mass ratio of 60:40 to 100:0. The average particle size of the small inorganic particles is in the range of 0.1 μm to less than 1.0 μm. The average particle sizes of the large fused alumina particles, medium-sized inorganic particles, and small-sized inorganic particles were measured using a laser diffraction / scattering particle size distribution analyzer (MT3300EXII: Microtrack Bell Corporation). The content of the small inorganic particles is in the range of 15% to 30% by mass.

[0018] The shape of the large-diameter fused alumina particles is not particularly limited, and may be, for example, spherical, cubic, or amorphous. The large-diameter fused alumina particles may be crushed, which is easily available industrially. The average particle size of the large-diameter fused alumina particles is preferably in the range of 25 μm to 45 μm, and more preferably in the range of 30 μm to 40 μm.

[0019] The shape of the medium-sized inorganic particles is not particularly limited and may be, for example, spherical, cubic, or amorphous. The medium-sized inorganic particles are preferably made of a material with a thermal conductivity of 30 W / mK or higher at 20°C. Examples of materials that can be used for the medium-sized inorganic particles include silicon carbide, alumina, aluminum nitride, boron nitride, silicon nitride, magnesium oxide, and zinc oxide. The average particle size of the medium-sized inorganic particles is preferably in the range of 2 μm to 6 μm, and more preferably in the range of 3 μm to 5 μm.

[0020] The shape of the small-diameter inorganic particles is not particularly limited, and may be, for example, spherical, cubic, or amorphous. The small-diameter inorganic particles are preferably made of a material having a thermal conductivity of 30 W / mK or more at 20°C. Examples of materials that can be used for the small-diameter inorganic particles include silicon carbide, alumina, aluminum nitride, boron nitride, silicon nitride, magnesium oxide, and zinc oxide. The average particle size of the small-diameter inorganic particles is preferably in the range of 0.20 μm to 0.80 μm, and more preferably in the range of 0.30 μm to 0.70 μm.

[0021] In the thermally conductive filler of this embodiment, the ratio of the average particle size of the medium-sized inorganic particles to the average particle size of the large-sized fused alumina particles is preferably 8 / 10 or less, more preferably 5 / 10 or less, and particularly preferably 3 / 10 or less. Furthermore, the ratio of the average particle size of the medium-sized inorganic particles to the average particle size of the large-sized fused alumina particles is preferably 1 / 20 or more. Furthermore, the ratio of the average particle size of the small-sized inorganic particles to the average particle size of the large-sized fused alumina particles is preferably 9 / 100 or less, more preferably 6 / 100 or less, and particularly preferably 4 / 100 or less. Furthermore, the ratio of the average particle size of the small-sized inorganic particles to the average particle size of the large-sized fused alumina particles is preferably 1 / 200 or more.

[0022] In the thermally conductive filler of this embodiment, the small-sized inorganic particles are present in the voids between the coarse inorganic particles (between large-sized fused alumina particles, between large-sized fused alumina particles and medium-sized inorganic particles, and between medium-sized inorganic particles), thereby improving conductivity. To facilitate the presence of the small-sized inorganic particles in the voids between the coarse inorganic particles, in this embodiment, the content ratio of the large-sized fused alumina particles to the medium-sized inorganic particles is set to a mass ratio of 60:40 to 100:0. The content ratio (mass ratio) of the large-sized fused alumina particles to the medium-sized inorganic particles is preferably 80:20 to 100:0, and more preferably 80:20 to 90:10. Furthermore, if the content of small-diameter inorganic particles is too low, the number of small-diameter inorganic particles interposed between the coarse inorganic particles will decrease, resulting in a reduced effect of improving conductivity. On the other hand, if the content of small-diameter inorganic particles is too high, the small-diameter inorganic particles will disperse in the resin of the thermally conductive resin composition, potentially resulting in an excessively high hardness of the thermally conductive resin composition. For this reason, in this embodiment, the content of small-diameter inorganic particles relative to the entire thermally conductive filler is set to a range of 15% by mass to 30% by mass. The content of small-diameter inorganic particles is preferably in the range of 20% by mass to 30% by mass. In addition, in the thermally conductive filler of this embodiment, the particle size distribution of the entire thermally conductive filler preferably has two or more peaks. A bimodal distribution with two peaks is preferred, and a trimodal distribution with three peaks is more preferred. Having a bimodal or higher particle size distribution allows particles of different particle sizes to easily enter gaps, allowing for a greater filling amount.

[0023] The thermally conductive filler of this embodiment preferably has a thermal conductivity of 30 W / mK or more as a whole. It is more preferable that the large-diameter fused alumina particles, medium-diameter inorganic particles, and small-diameter inorganic particles constituting the thermally conductive filler each have a thermal conductivity of 30 W / mK or more.

[0024] The thermally conductive filler of this embodiment is preferably added in an amount of 600 to 1500 parts by mass, more preferably 800 to 1300 parts by mass, per 100 parts by mass of the resin. When the amount of the thermally conductive filler of this embodiment is within the above range, the thermal conductivity of the thermally conductive resin composition can be improved without excessively increasing its hardness. When 1100 parts by mass of the thermally conductive filler of this embodiment is added to 100 parts by mass of the silicone resin, the hardness measured with an Asker C hardness scale is preferably 65 or less.

[0025] The thermally conductive filler of this embodiment, configured as described above, has high thermal conductivity because fine small-sized inorganic particles with an average particle size of 0.1 μm or more but less than 1.0 μm are present in the gaps between the coarse inorganic particles. Furthermore, the thermally conductive filler of this embodiment contains large-sized fused alumina particles, medium-sized inorganic particles, and small-sized inorganic particles in the above-described ranges, so that when added to a resin, the hardness of the resulting resin is unlikely to be excessively high. Furthermore, the large-sized fused alumina particles used in the thermally conductive filler of this embodiment are easily available industrially. Therefore, by using the thermally conductive filler of this embodiment, a thermally conductive resin composition with excellent thermal conductivity and low hardness can be obtained at low cost.

[0026] When the thermally conductive filler of this embodiment has a thermal conductivity of 30 W / mK or more at 20° C., a thermally conductive resin composition with excellent thermal conductivity can be more reliably obtained. Furthermore, when the thermally conductive filler of this embodiment has a hardness of 65 or less as measured by an Asker C hardness scale when 1,100 parts by mass of the filler is added to 100 parts by mass of a silicone resin, the filler can be added to various resins other than silicone resins without excessively increasing the hardness.

[0027] (Method for manufacturing thermally conductive filler) In the method for producing a thermally conductive filler of this embodiment, large-sized fused alumina particles, medium-sized inorganic particles, and small-sized inorganic particles are mixed. There are no limitations on the order in which the large-sized fused alumina particles, medium-sized inorganic particles, and small-sized inorganic particles are mixed. For example, the large-sized fused alumina particles, medium-sized inorganic particles, and small-sized inorganic particles may be mixed simultaneously, or a mixture of large-sized fused alumina particles and medium-sized inorganic particles may be mixed with small-sized inorganic particles, or a mixture of large-sized fused alumina particles and small-sized inorganic particles may be mixed with medium-sized inorganic particles. Furthermore, mixing may be performed by either a dry method or a wet method.

[0028] The mixing ratio of the large-diameter fused alumina particles to the medium-diameter inorganic particles is 60:40 to 100:0, preferably 80:20 to 100:0, and more preferably 80:20 to 90:10, by mass. The mixing ratio of the small-diameter inorganic particles is such that the content of the small-diameter inorganic particles relative to the total amount of the large-diameter fused alumina particles, the medium-diameter inorganic particles, and the small-diameter inorganic particles is 15% by mass or more and 30% by mass or less, and preferably 20% by mass or more and 30% by mass or less.

[0029] According to the method for producing the thermally conductive filler of the present embodiment configured as described above, the large-diameter fused alumina particles, the medium-diameter inorganic particles, and the small-diameter inorganic particles are mixed in the above-mentioned ratio, so that the thermally conductive filler of the present embodiment can be produced industrially advantageously.

[0030] (Thermal conductive resin composition) The thermally conductive resin composition of this embodiment includes a resin and the thermally conductive filler of this embodiment. The thermally conductive resin composition may contain, for example, 600 parts by mass or more and 1500 parts by mass or less of the thermally conductive filler per 100 parts by mass of the resin. The thermal conductivity at 40°C is preferably 3.25 W / mK or more, and more preferably 3.40 W / mK or more.

[0031] The resin contained in the thermally conductive resin composition is not particularly limited, and known resins can be used. The resin may be a thermoplastic resin or a thermosetting resin. Examples of the resin include hydrocarbon resins, unsaturated polyester resins, acrylic resins, vinyl ester resins, epoxy resins, xylene formaldehyde resins, guanamine resins, diallyl phthalate resins, phenolic resins, furan resins, polyimide resins, melamine resins, urea resins, and silicone resins. In this embodiment, the resin includes rubber.

[0032] The thermally conductive resin composition of this embodiment, configured as described above, contains the thermally conductive filler of this embodiment, and therefore has excellent thermal conductivity and low hardness. Therefore, it can be used as a heat dissipation material for various components. In particular, a thermally conductive resin composition having a thermal conductivity of 3.25 W / mK or more at 40°C can be advantageously used as a heat dissipation material for electrical components that output a large current, such as lithium-ion batteries.

[0033] Although one embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This embodiment can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. This embodiment and its modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Example]

[0034] The results of confirmation experiments conducted to confirm the effectiveness of the present invention will be described below. The large-diameter fused alumina particles, medium-diameter inorganic particles, and small-diameter inorganic particles used in this example are as follows: The particle size distribution was measured using a laser diffraction / scattering particle size distribution analyzer (MT3300EXII: Microtrack Bell Corporation). (1) Large-diameter fused alumina particles Alumina particles with an average particle size of 33.3 μm and a thermal conductivity of 30 W / mK or more at 20°C. Figure 1 shows the particle size distribution. (2) Medium-sized inorganic particles Low soda alumina particles with an average particle size of 3.9 μm and a thermal conductivity of 30 W / mK or more at 20°C. Figure 2 shows the particle size distribution. (3) Small diameter inorganic particles Low soda alumina particles with an average particle size of 0.49 μm and a thermal conductivity of 30 W / mK or more at 20°C. Figure 3 shows the particle size distribution.

[0035] [Experimental Example 1] The large-diameter fused alumina particles and the medium-diameter inorganic particles were weighed and placed in a mortar and mixed with a pestle to prepare a mixed powder so that the content ratio (mass ratio) of the large-diameter fused alumina particles to the medium-diameter inorganic particles and the content of the small-diameter inorganic particles were the values ​​shown in Table 1 below. 0.5 g of silicone rubber (KE-1051J A, manufactured by Shin-Etsu Chemical Co., Ltd.) was placed in a container, and the above mixed powder was added in 0.5 g increments. The mixture was kneaded in a centrifugal defoamer at 2000 rpm for 2 minutes. The amount of mixed powder added when the mixture was touched with a finger and became brittle and unable to be molded was taken as the limiting loading amount. The results are shown in Table 1 below.

[0036] [Table 1]

[0037] The results in Table 1 confirm that mixtures in which the blending ratio (mass ratio) of large-diameter fused alumina particles to medium-diameter inorganic particles is 60:40 to 100:0 and the content of small-diameter inorganic particles is in the range of 15% by mass or more and 30% by mass or less have a limit filling amount of 1,300 parts by mass or more, and that the amount filled into the silicone rubber is high.

[0038] [Example 1 of the present invention] A mixed powder was prepared by mixing 68% by mass of large-sized fused alumina particles, 17% by mass of medium-sized inorganic particles, and 15% by mass of small-sized inorganic particles using a pestle and mortar. The resulting mixed powder had a content ratio of large-sized fused alumina particles to medium-sized inorganic particles of 80:20. A few grams of the mixed powder was added to a rubber mixture containing 1 g of silicone rubber (KE-1051J A) and 1 g of silicone rubber (KE-1051J B), and the mixture was kneaded in a centrifugal defoamer at 2000 rpm for 2 minutes, repeatedly until a total of 22 g of the mixed powder was added to the rubber mixture. The rubber mixture with the added mixed powder was left to stand at room temperature (25°C) for 24 hours to harden, yielding a resin composition.

[0039] The particle size distribution of the mixed powder obtained in Invention Example 1 was estimated from the particle size distributions of the large-diameter fused alumina particles, the medium-diameter inorganic particles, and the small-diameter inorganic particles and their blending amounts. The results are shown in Figure 4. Figure 4 shows that the mixed powder obtained in Invention Example 1 has three peaks: 33.3 μm, 3.9 μm, and 0.49 μm.

[0040] [Invention Examples 2 to 6, Comparative Examples 1 to 3] A resin composition was obtained in the same manner as in Example 1 of the present invention, except that the ratios of the large-diameter fused alumina particles, medium-diameter inorganic particles, and small-diameter inorganic particles in the mixed powder were set to the ratios shown in Table 2 below.

[0041] Table 1 shows the content ratio (mass ratio) of large-diameter fused alumina particles to medium-diameter inorganic particles in the mixed powders produced in Examples 1 to 6 of the present invention and Comparative Examples 1 to 3. FIG. 5 shows a ternary diagram illustrating the compositions of the mixed powders produced in Examples 1 to 6 of the present invention and Comparative Examples 1 to 3.

[0042] [evaluation] The thermal conductivity and hardness of the resin compositions obtained in Inventive Examples 1 to 6 and Comparative Examples 1 to 3 were measured as follows. The results are shown in Table 1.

[0043] (thermal conductivity) The cured resin composition was molded into 28 mm × 28 mm × 3 mm and 28 mm × 28 mm × 5 mm pieces, and measured using a thermal conductivity measuring device (IE-1237, manufactured by Iwasaki Tsushinki Co., Ltd.). The measurement conditions were a high temperature side of the device at 80°C and a low temperature side at 15°C, and the measurement was performed at a sample temperature of approximately 40°C. The average of the thermal conductivity of the resin composition at a thickness of 3 mm and the thermal conductivity of the resin composition at a thickness of 5 mm was taken as the thermal conductivity of the resin composition at 40°C. (Hardness) Since the resin composition of the present invention remains deformable even after curing, the resin composition was molded into a 7 mm thick specimen using a hand press to prepare a specimen for hardness measurement. The hardness of the specimen was measured using an Asker C hardness tester.

[0044] [Table 2]

[0045] The results in Table 2 show that the resin compositions obtained in Examples 1 to 6 of the present invention, which used mixed powders containing large-sized fused alumina particles, medium-sized inorganic particles, and small-sized inorganic particles within the ranges of the present invention, had high thermal conductivities of 3.25 W / mK or more and low hardnesses of 65 or less as measured by an Asker C hardness scale. The reason why the resin compositions obtained in Examples 1 to 6 of the present invention exhibit high thermal conductivities is that the small-sized inorganic particles are interposed between the coarse inorganic particles (large-sized fused alumina particles, medium-sized inorganic particles), thereby increasing the thermal conductivity of the resin compositions. The reason why the resin compositions obtained in Examples 1 to 6 of the present invention exhibit low hardness is that the small-sized inorganic particles are interposed between the coarse inorganic particles, resulting in a low amount of small-sized inorganic particles dispersed in the resin.

[0046] In contrast, the resin compositions obtained in Comparative Examples 1 to 3 used the same large-diameter fused alumina particles, medium-diameter inorganic particles, and small-diameter inorganic particles as those used in Inventive Examples 1 to 6, yet the thermal conductivity of the resin compositions was lower than 3.25 W / mK and the hardness measured by an Asker C hardness scale exceeded 65. In particular, the hardness of the resin composition in Comparative Example 1 was significantly high, at 80. The resin composition of Comparative Example 1 contained a higher proportion of small-diameter inorganic particles than the range of the present invention, resulting in a relatively reduced amount of large-diameter fused alumina particles and medium-diameter inorganic particles, presumably resulting in a low thermal conductivity. The resin composition of Comparative Example 1 also contained a higher amount of small-diameter inorganic particles dispersed in the resin, presumably resulting in a high hardness. The resin composition of Comparative Example 2 contained a lower proportion of small-diameter inorganic particles than the range of the present invention, resulting in a relatively increased amount of large-diameter fused alumina particles and medium-diameter inorganic particles, resulting in a large number of voids not interposed by small-diameter inorganic particles, presumably resulting in a low thermal conductivity. The resin composition of Comparative Example 3 had a lower thermal conductivity because the ratio of large-sized fused alumina particles to medium-sized inorganic particles was lower than the range of the present invention. The resin composition of Comparative Example 3 also had a higher ratio of medium-sized inorganic particles and small-sized inorganic particles to large-sized fused alumina particles, which increased the specific surface area of ​​the entire thermally conductive filler, which is thought to have resulted in a high hardness of 75.

Claims

1. Contains coarse inorganic particles and small inorganic particles, The coarse inorganic particles comprise cubic and amorphous large-diameter fused alumina particles having an average particle size in the range of 20 μm to 50 μm, and medium-diameter inorganic particles having an average particle size in the range of 1.0 μm to 10 μm, in a mass ratio of 60:40 to 100:0, The small inorganic particles have an average particle size in the range of 0.1 μm or more and less than 1.0 μm, The thermally conductive filler has a content of the small-diameter inorganic particles in the range of 15% by mass or more and 30% by mass or less.

2. 2. The thermally conductive filler according to claim 1, which has a thermal conductivity of 30 W / mK or more at 20°C.

3. 3. The thermally conductive filler according to claim 1, wherein the hardness measured by an Asker C hardness scale when 1100 parts by mass of the filler is added to 100 parts by mass of a silicone resin is 65 or less.

4. 4. A method for producing a thermally conductive filler according to claim 1, comprising mixing large-diameter fused alumina particles having a cubic or amorphous shape and an average particle size of 20 μm or more and 50 μm or less, medium-diameter inorganic particles having an average particle size of 1.0 μm or more and 10 μm or less, and small-diameter inorganic particles having an average particle size of 0.1 μm or more and less than 1.0 μm in a mass ratio of 60:40 to 100:0, and in an amount such that the content of the small-diameter inorganic particles is 15% by mass or more and 30% by mass or less.

5. A thermally conductive resin composition comprising a resin and the thermally conductive filler according to claim 1 .

6. 6. The thermally conductive resin composition according to claim 5, which has a thermal conductivity at 40°C of 3.25 W / mK or more.

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