Hexagonal boron nitride powder and method for producing the same, and composition and heat dissipation material using the same

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

• having an average primary particle diameter of 0.5 μm or more and less than 10.0 μm, a 50% volume cumulative particle diameter D50 of 10.0 to 150.0 μm, a BET specific surface area of 1.0 m2/g or more and less than 10.0 m2/g, and a bulk density of 0.50 to 2.00 g/cm3, and
• having a peak A in a particle diameter range of 1.0 μm or more and less than 20.0 μm and a peak B in a particle diameter range of 20.0 μm or more and less than 200.0 μm in a particle size distribution curve representing volume-based frequency distribution,
• having a maximum peak C within a particle diameter range of 45.0 to 150.0 μm in a particle size distribution curve representing volume-based frequency distribution of a powder obtained by classifying the hexagonal boron nitride powder within a particle diameter range of more than 45 μm and 106 μm or less using a vibrating sieve, where c1 is a height of the maximum peak C,
• a ratio of a height a1 of the peak A to a height b1 of the peak B, a1/b1, is in a range of 0.07 to 0.80,
• wherein, after subjecting the hexagonal boron nitride powder to an ultrasonic treatment under the following condition 1 for 3 minutes,
• [condition 1] 50 mL of an aqueous dispersion of 0.12% by mass of the hexagonal boron nitride powder at 20° C. is placed in a 50 mL glass beaker having a barrel inner diameter of 40 mm and a height of 60 mm, an end of a tip of a vibrator of an ultrasonic generator is set at a height of 1 cm from a bottom surface of a central portion of the beaker, and ultrasonic treatment is performed at an output of 150 W and an oscillation frequency of 19.5 kHz,
• the ultrasonically treated hexagonal boron nitride powder has a ratio of a height a2 of a peak A to a height b2 of a peak B, a2/b2, in a range of 0.40 to 2.00, and a height c2 of a maximum peak C, and
• wherein a percent decrease in the height of the maximum peak C, (c1-c2)/c1×100, is 5% or more and less than 30%.