A hexagonal boron nitride powder having an average longer diameter (L) of primary particles in the hexagonal boron nitride powder of more than 10.0 μm and 30.0 μm or less, an average thickness (D) of the primary particles in the hexagonal boron nitride powder of 1.0 μm or more, a ratio of the average longer diameter (L) to the average thickness (D), [L/D], of 3.0 or more and 5.0 or less, and a content of primary particles having a ratio of a longer diameter (1) to a thickness (d), [l/d], of 3.0 or more and 5.0 or less of 25% or more, a method for producing the hexagonal boron nitride powder, and a resin composition and a resin sheet each containing the hexagonal boron nitride powder.

The present invention provides a hexagonal boron nitride powder that has a highly reduced content of magnetic foreign bodies and is excellent in electrical insulation, and a production method capable of producing the hexagonal boron nitride powder at low cost.

The hexagonal boron nitride powder includes single particles and/or aggregated particles of hexagonal boron nitride and has a total content of Co, Cr, Cu, Fe, Mg, Mn, Ni, Ti, Zn, and Al of not more than 20 ppm. The method for producing the hexagonal boron nitride powder with a reduced content of magnetic foreign bodies includes a plurality of specific steps.

One aspect of the present disclosure provides a hexagonal boron nitride powder having a purity of 98 mass % or more and a specific surface area of less than 2.0 m.sup.2/g.

Provided is a boron nitride sintered body having a porous structure, the boron nitride sintered body including a lump particle formed by aggregation of primary particles of boron nitride and having a particle diameter of 15 μm or more. Provided is a method for manufacturing a boron nitride sintered body, the method including: a nitriding step of firing a raw material powder containing boron carbide in an atmosphere containing nitrogen to obtain a fired product including lump particles each having a core part with primary particles of boron carbonitride aggregated and a shell part surrounding the core part; and a firing step of molding and heating a blend containing the fired product including lump particles and a sintering aid to obtain the boron nitride sintered body having a porous structure and including lump particles of boron nitride.

Provided is a boron nitride powder including: an agglomerated particle obtained by aggregating flaky primary particles, in which in-plane directions of the primary particles are oriented in a direction parallel to a short-side direction of the agglomerated particle. In addition, provided is a method for producing a boron nitride powder including: a nitriding step of firing a boron carbide powder having an aspect ratio of 1.5 to 10 in a nitrogen pressurized atmosphere to obtain a fired product; and a crystallization step of heating a formulation that contains the fired product and a boron source to produce flaky boron nitride primary particles and obtaining a boron nitride powder containing an agglomerated particle obtained by aggregating the primary particles.

A surface-treated aggregated boron nitride powder is prepared by using the boron nitride powder as the raw material, adding an oxidizer to the boron nitride aggregated grains, wet-pulverizing or wet-crushing the grains for surface modification treatment of the particles and allowing reaction of the particles with a metal coupling agent. The surface-treated boron nitride aggregated grains are formed by aggregation of hexagonal h-BN primary particles; (B) 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 the surface of 10 nm; (C) have a crushing strength of 5 MPa or more; and (D) have an average particle diameter of 20 μm or more and 100 μm or less.

The present invention relates to aggregated boron nitride particles including hexagonal boron nitride primary particles aggregated, having a specific surface area measured by the BET method of 2 to 6 m.sup.2/g and a crushing strength of 5 MPa or more. The thermally conductive resin composition of the present invention includes the aggregated boron nitride particles of the present invention. The heat dissipation member of the present invention includes the thermally conductive resin composition of the present invention. According to the present invention, aggregated boron nitride particles that can suppress the formation of voids in a heat dissipation member and can improve the insulation breakdown characteristics and the thermal conductivity of a heat dissipation member, a thermally conductive resin composition including the aggregated boron nitride particles, and a heat dissipation member using the thermally conductive resin composition can be provided.

A surface-treated aggregated boron nitride powder is prepared by using the boron nitride powder as the raw material, adding an oxidizer to the boron nitride aggregated grains, wet-pulverizing or wet-crushing the grains for surface modification treatment of the particles and allowing reaction of the particles with a metal coupling agent. The surface-treated boron nitride aggregated grains are formed by aggregation of hexagonal h-BN primary particles; (B) 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 the surface of 10 nm; (C) have a crushing strength of 5 MPa or more; and (D) have an average particle diameter of 20 μm or more and 100 μm or less.

A boron nitride particle having a bent shape. A resin composition containing the boron nitride particle and a resin. A method for producing a resin composition including a step of preparing the boron nitride particle and a step of mixing the boron nitride particle with a resin.

To provide a hexagonal boron nitride powder which contains agglomerates, has a maximum torque calculated by measuring in accordance with JIS-K-6217-4 of 0.20 to 0.50 Nm, a DBP absorption rate of 50 to 100 ml/100 g, a tap bulk density of 0.66 to 0.95 g/cm.sup.3 and reduced anisotropy of heat conduction and can provide high heat conductivity and dielectric strength to a resin composition produced by filling a resin therewith and a process for producing the powder by carrying out a reduction nitriding reaction using boron carbide.