A boron nitride powder includes boron nitride aggregated grains that are formed by aggregation of scaly hexagonal boron nitride primary particles, the boron nitride powder having the following characteristic properties (A) to (C): (A) the primary particles of the scaly hexagonal boron nitride 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) the boron nitride powder has an average particle diameter of 20 μm or more and 100 μm or less. Also provided are a method for producing the same and a thermally conductive resin composition including the same.

In an embodiment, a method for preparing expanded hexagonal boron nitride comprises mixing a boron compound and a carbon template in an organic solvent; removing the organic solvent to provide a dried mixture of the boron compound and the carbon template; exposing the dried mixture to a nitrogen-containing gas under conditions effective to provide a crude product comprising hexagonal boron nitride; removing the carbon template from the crude product to provide the expanded hexagonal boron nitride.

A hexagonal boron powder having a purity of 98% by mass or more and a specific surface area of less than 2.0 m.sup.2/g.

A hexagonal boron nitride powder whose maximum absorption peak within the range of 3,100 to 3,800 cm.sup.1 of the diffuse reflectance fourier transform infrared spectrum is existent at 3,530 to 3,590 cm.sup.1 and which is able to provide high heat conductivity, dielectric strength and copper foil peel strength to a resin composition obtained by filling the powder into a resin, and a process for producing the above boron nitride powder by mixing together an oxygen-containing boron compound, a carbon source having a sulfur concentration of 1,000 to 10,000 ppm and an oxygen-containing calcium compound in a specific ratio and reduction nitriding the mixture.

A hexagonal boron nitride (hBN) powder containing an aggregate of primary particles of hBN, in which the powder has a primary particle size of less than 10 m, a ratio of an average longer diameter (L.sub.1) to an average thickness (d.sub.1) of the primary particles, [L.sub.1/d.sub.1], of 5.0 or more and 20 or less, and a BET specific surface area of less than 10 m.sup.2/g. The powder has one maximum peak in a range of a particle size of 45 m or more and 150 m or less in a particle size distribution curve of a hexagonal boron nitride powder classified to have a particle size of 45 m or more and 106 m or less, and has a decrease rate of the maximum peak of 10% or more and less than 40%.

To provide a boron nitride powder having excellent heat conductivity and high particle strength. Provided is a boron nitride powder which comprises bulky boron nitride formed such that scaly primary particles of hexagonal boron nitride are aggregated to form bulky particles, and which has the following characteristics (A) to (C): (A) a particle strength of the bulky particles at a cumulative breakdown rate of 63.2% is 5.0 MPa or more; (B) an average particle size of the boron nitride powder is 2 m or more and 20 m or less; and (C) an orientation index of the boron nitride powder as determined from X-ray diffraction is 20 or less.

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 m and 30.0 m or less, an average thickness (D) of the primary particles in the hexagonal boron nitride powder of 1.0 m 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.

A boron nitride powder includes boron nitride aggregated grains that are formed by aggregation of scaly hexagonal boron nitride primary particles, the boron nitride powder having the following characteristic properties (A) to (C): (A) the primary particles of the scaly hexagonal boron nitride have an average long side length of 1.5 m or more and 3.5 m or less and a standard deviation of 1.2 m 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) the boron nitride powder has an average particle diameter of 20 m or more and 100 m or less. Also provided are a method for producing the same and a thermally conductive resin composition including the same.

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.

In an aspect, a method for preparing hexagonal boron nitride comprises mixing a boron compound and a carbon template in an organic solvent; removing the organic solvent to provide a dried mixture of the boron compound and the carbon template; exposing the dried mixture to a nitrogen-containing gas under conditions effective to provide a crude product comprising hexagonal boron nitride; removing the carbon template from the crude product to provide the hexagonal boron nitride.