Patent ID: 7595358

Claim:
A corundum for filling in resin, comprising a corundum X having an average secondary particle size of 35 to 80 μm and a specific surface area of 0.01 to 1 m 2 /g as measured by the nitrogen adsorption method (BET method), a corundum Y having an average secondary particle size of 1 to 5 μm and a specific surface area of 0.5 to 3 m 2 /g, and a corundum Z having an average secondary particle size of 0.3 to 1.5 μm and a specific surface area of 3 to 15 μm /g, the composition ratio by mass (X:Y:Z) being in the range surrounded by four points of (60:40:0), (95:0:5), (95:5:0) and (60:0:40) not including the borderlines in the ternary compositional diagram with an assumption that the entire is 100 mass %, wherein the amount of each of corundum Y and corundum Z is 5 mass % or more, and wherein a composition obtained by filling 300 parts by mass of the corundum into 100 parts by mass of silicone oil has a viscosity of less than 2,000 poise as measured at 35° C. by a Brookfield-type viscometer and a linseed oil absorption of 10 ml/100 g or less.