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,

• comprising boron as an impurity element at a content of 1.00 to 30.00% by mass and oxygen as an impurity element at a content of 0 to 1.00% by mass, and
• having a peak in a particle diameter range of 10.0 μm or more and less than 100.0 μm and a 50% volume cumulative particle diameter D50 (d1) of 30.0 to 200.0 μm in a particle size distribution curve representing volume-based frequency distribution,
• wherein the hexagonal nitride powder has a structure such that, when the hexagonal boron nitride powder is ultrasonically treated under the following condition 1 for 1 minute, a ratio of a height (a1) of the peak before the ultrasonic treatment to a height (a2) of the peak after treatment, a2/a1, being 0.80 to 1.00 and a ratio of a 50% volume cumulative particle diameter D50 (d1) before ultrasonic treatment to a 50% volume cumulative particle diameter D50 (d2) after the ultrasonic treatment, d2/d1, being 0.80 to 1.00,
• [condition 1] a powder sample dispersion obtained by mixing 0.06 g of the hexagonal boron nitride powder, 0.005 g of a detergent (“Mama Lemon (registered trademark)”, manufactured by Lion Corporation) and 50 g of water (20° C.) is placed in a 50 mL glass beaker having a barrel inner diameter of 40 mm and a height of 60 mm, and 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, and
• wherein the boron as an impurity element is a boron atom not bound to a nitrogen atom and which does not form a hexagonal boron nitride molecule.