Hexagonal boron nitride powder, method for producing same, resin composition and resin sheet

1. A hexagonal boron nitride powder comprising an aggregate of primary particles of hexagonal boron nitride, wherein

the powder has a ratio of an average longer diameter (L 1 ) to an average thickness (d 1 ) of the primary particles, [L 1 /d 1 ], of 10 or more and 25 or less, a tap density of 0.80 g/cm 3 or more, and a BET specific surface area of less than 5.0 m 2 /g,

a particle size distribution curve showing a frequency distribution based on volume of the hexagonal boron nitride powder is a bimodal distribution curve having a first peak in a range of a particle diameter of 500 μm or less and a second peak existing on a smaller particle size side of the first peak, and

a peak height ratio of a height of the second peak (H B ) to a height of the first peak (H A ), [(H B )/(H A )], is 0.90 or less.

2. The hexagonal boron nitride powder according to claim 1 , wherein the powder has one maximum peak in a range of a particle size of 45 μm or more and 150 μm or less in a particle size distribution curve showing a frequency distribution based on volume of a hexagonal boron nitride powder classified to have a particle size of 45 μm or more and 95 μm or less, and has a decrease rate of the maximum peak of 35% or more and 70% or less when a dispersion liquid obtained by dispersing the classified hexagonal boron nitride powder in water is subjected to an ultrasonic treatment for 1 minute, the peak decrease rate being calculated by the following expression (1):

Peak decrease rate=[(maximum peak height before treatment ( a ))−(maximum peak height after treatment ( b ))]×100/(maximum peak height before treatment ( a ))  (1).

3. The hexagonal boron nitride powder according to claim 1 , having a crystallite size of 260 Å or more and 1000 Å or less.

4. The hexagonal boron nitride powder according to claim 1 , wherein the average longer diameter (L 1 ) of the primary particles is 0.5 μm or more and 20 μm or less.

5. A resin composition comprising 10% by volume or more and 90% by volume or less of the hexagonal boron nitride powder according to claim 1 .

6. A method for producing the hexagonal boron nitride powder according to claim 1 , the method comprising a step of mixing 100 parts by mass of a mixed powder comprising 50% by mass or more and 90% by mass or less of a boron nitride fine powder (A) and 10% by mass or more and 50% by mass or less of a boron compound (B) represented by a composition formula B 2 O (3+X) H 2X wherein X=0 to 3, 1.0 part by mass or more and 20 parts by mass or less in terms of carbon of a carbon source (C), and 0.01 parts by mass or more and 5.0 parts by mass or less of a calcium compound (D), molding a resultant mixture, and then firing a resultant under an atmosphere comprising a nitrogen gas, wherein

the boron nitride fine powder (A) has a ratio of an average longer diameter (L 2 ) to an average thickness (d 2 ) of primary particles thereof, [L 2 /d 2 ], of 2.0 or more and 15 or less, a 50% volume cumulative particle size D 50 of 0.20 μm or more and 5.0 μm or less, a BET specific surface area of 5.0 m 2 /g or more and 30 m 2 /g or less, and a crystallite size of 150 Å or more and 400 Å or less.

7. The method for producing the hexagonal boron nitride powder according to claim 6 , wherein the carbon source (C) is at least one selected from the group consisting of graphite and boron carbide.

8. The method for producing the hexagonal boron nitride powder according to claim 6 , wherein the carbon source (C) is boron carbide, and the boron carbide has a 50% volume cumulative particle size D 50 of 1.0 μm or more and 200 μm or less.