Highly thermally conductive silicone composition and cured product thereof

A highly thermally conductive silicone composition comprising:

• (A) an organopolysiloxane;
• (B) a spherical magnesium oxide powder with an average sphericity of 0.8 or more, an average particle size of 80 to 150 m, and a purity of 98% by mass or more;
• (C) a thermally conductive filler comprising (C-I) and (C-II), and the following component (C-I) and component (C-II) being contained in a volume ratio (C-I):(C-II) of 2:8 to 8:2,
• (C-I) a spherical aluminum oxide powder with an average sphericity of 0.8 or more and an average particle size of 7 to 60 m, a proportion of coarse particles of 96 to 150 m being 0.1 to 30% by mass of an entirety of the component (C-I) in a laser diffraction type particle size distribution;
• and
• (C-II) a spherical or irregularly shaped aluminum oxide powder with an average particle size of 0.1 to 4 m;
• and
• (D) an ion-trapping agent comprising either or both of a cation exchange type ion-trapping agent and an amphoteric ion exchange type ion-trapping agent, one or more elements selected from Zr, Bi, Sb, Mg, and Al being supported on the ion-trapping agent,
• wherein the component (B) is 3,300 to 6,500 parts by mass based on 100 parts by mass of the component (A),
• the component (D) is 0.01 to 10 parts by mass based on 100 parts by mass of the component (A),
• the component (B) and the component (C) are contained in a volume ratio (B):(C) of 5:5 to 9.5:0.5, and a total amount of the component (B) and the component (C) is 80 to 90% by volume in the composition, and
• the composition has a thermal conductivity of 7.0 W/m·K or more in a Hot Disk method in accordance with ISO 22007-2, and the composition has a viscosity of 30 to 800 Pa·s at 25° C. as measured with a spiral viscometer at a rotational speed of 10 rpm.