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 implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. This embodiment and its modifications are included in the scope of the invention and its equivalents described in the claims, as well as in the scope and gist of the invention.

EXAMPLES

The results of confirmation experiments conducted to confirm the effectiveness of the present invention will be described below.

The large fused alumina particles, the medium inorganic particles, and the small inorganic particles used in this example are as follows: The particle size distribution was measured using a laser diffraction scattering type particle size distribution measuring device (MT3300EXII: Microtrack Bell Corporation).

(1) Large 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. FIG. 1 shows the particle size distribution.

(2) Medium 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. FIG. 2 shows the particle size distribution.

(3) Small 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. FIG. 3 shows the particle size distribution.

Experimental Example 1

Each particle was weighed and placed in a mortar and mixed using a pestle so that the content ratio (mass ratio) of the large fused alumina particles to the medium inorganic particles and the content rate of the small inorganic particles were the values shown in Table 1 below to prepare a mixed powder.

0.5 g of silicone rubber (KE-1051JA, Shin-Etsu Chemical Co., Ltd.) was placed in a container, and the above mixed powder was added in 0.5 g portions and kneaded for 2 minutes at 2000 rpm in a centrifugal defoamer. The amount of mixed powder added when the resulting mixture was touched with a finger and became brittle and unable to be molded was taken as the limit filling amount. The results are shown in Table 1 below.

TABLE 1

Filling limit (Mass parts/100 mass parts of silicone rubber)

Large fused alumina particles:Medium inorganic particles

40:60
50:50
60:40
70:30
80:20
85:15
90:10
100:0

Content
40
—
—
1100
1100
1100
900
—
—

of Small
35
—
—
1200
1200
1200
1200
1300
1300

inorganic
30
—
1000
1300
1300
1300
1400
1500
1400

particles
25
900
1300
1400
1400
1400
1400
1400
1400

(mass %)
20
1000
1400
1300
1400
1500
1400
1500
1300

15
1100
1300
1400
1400
1500
1300
1400
1300

10
—
1300
1300
1300
1300
1200
1100
1000

5
—
1200
1400
1300
1200
1100
—
—

0
—
—
1200
1100
1000
1100
—
—

From the results in Table 1, it was confirmed that a mixture in which the blending amounts (mass ratio) of the large fused alumina particles and the medium inorganic particles are in a ratio of 60:40 to 100:0 and the content of small inorganic particles is in the range of 15% by mass or more and 30% by mass or less has a limit filling amount of 1,300 parts by mass or more, and thus has a high filling amount into the silicone rubber.

Example (Ex.) 1 of the Present Invention

A mixed powder was prepared by mixing 68% by mass of the large-diameter fused alumina particles, 17% by mass of the medium-diameter inorganic particles, and 15% by mass of the small-diameter inorganic particles using a pestle and mortar. The resulting mixed powder had a content ratio of large-diameter fused alumina particles to medium-diameter inorganic particles of 80:20.