Thermally conductive silicone composition and thermally conductive silicone sheet

A thermally conductive silicone composition comprising:

• (A) 100 parts by mass of a linear organopolysiloxane having at least two alkenyl groups per each molecule and at least one of the at least two alkenyl groups is a vinyl group, and a kinetic viscosity of the linear organopolysiloxane at 25° C. is from 300 to 50,000 mm2/s;
• (B) an organohydrogenpolysiloxane having, per each molecule, at least two hydrogen atoms directly bonded to silicon atoms, the component (B) being in an amount where the number of the hydrogen atoms directly bonded to the silicon atoms is 0.2 to 2.0 times larger than the number of the alkenyl groups in the component (A), wherein the organohydrogenpolysiloxane (B) is represented by the following general formula (3):
• wherein R7 independently represents a hydrogen atom, a substituted or unsubstituted alkyl group having 1 to 3 carbon atoms, or a substituted or unsubstituted phenyl group, and n is an integer of from 1 to 200;
• (C) 2,600 to 4,680 parts by mass of a thermally conductive filler containing magnesium oxide, aluminum oxide and aluminum hydroxide;
• (D) 140 to 250 parts by mass of a surface treatment agent of the thermally conductive filler, which is a dimethylpolysiloxane with one end of the molecular chain thereof being blocked by a trialkoxy group, as represented by the following general formula (1):
• wherein R5 independently represents an alkyl group having 1 to 6 carbon atoms, and c is an integer of 5 to 100;
• (E) a platinum group metal-based curing catalyst in an amount of 0.1 to 1,000 ppm in terms of mass of platinum group metal elements with respect to the component (A);
• and
• (F) an effective amount of an addition reaction control agent,
• wherein the thermally conductive filler as the component (C) is such that the magnesium oxide is contained by an amount of 21 to 36% by mass per the total amount of the component (C), the aluminum oxide is contained by an amount of 40 to 56% by mass per the total amount of the component (C), and the aluminum hydroxide is contained by an amount of 12 to 28% by mass per the total amount of the component (C);
• and the magnesium oxide has an average particle size of 60 to 90 μm, the aluminum oxide has an average particle size of 1 to 70 μm, and the aluminum hydroxide has an average particle size of 8 to 50 μm.