Surface-treated aggregated boron nitride powder, aggregated boron nitride powder, and thermally conductive resin composition

A surface-treated aggregated boron nitride powder comprising surface-treated boron nitride aggregated grains, wherein the surface-treated boron nitride aggregated grains are prepared by

• adding an oxidizer to boron nitride aggregated grains,
• wet-pulverizing or wet-crushing the boron nitride aggregated grains for surface modification treatment and allowing reaction of the boron nitride aggregated grains with a metal coupling agent to form surface-treated boron nitride aggregated grains,
• wherein the boron nitride aggregated grains are formed by aggregation of scaly hexagonal boron nitride primary particles, and have the following characteristics (a) to (c):
• (a) the scaly hexagonal boron nitride primary particles have an average long side length of 1.5 μm or more and 3.5 μm or less and a standard deviation of 1.2 μm or less;
• (b) the boron nitride aggregated grains have a grain strength of 8.0 MPa or more at a cumulative breakdown rate of 63.2% and a grain strength of 4.5 MPa or more at a cumulative breakdown rate of 20.0%;
• and
• (c) boron nitride powder of the surface-treated aggregated boron nitride powder has an average particle diameter of 20 μm or more and 100 μm or less,
• and wherein the surface-treated aggregated boron nitride aggregated grains have the following characteristics (A) to (C):
• (A) the surface-treated boron nitride aggregated grains are formed by aggregation of hexagonal h-BN primary particles;
• (B) the surface-treated boron nitride aggregated grains have any one or more of Si, Ti, Zr, Ce, Al, Mg, Ge, Ga, and V in an amount of 0.1 atm % or more and 3.0 atm % or less in its composition on a surface of 10 nm;
• and
• (C) the surface-treated boron nitride aggregated grains have a crushing strength of 5 MPa or more.