Isotropic enhancement of the thermal conductivity of polymer composites by dispersion of equiaxed polyhedral boron nitride fillers

Recently, hexagonal boron nitride (h-BN) has attracted attention as a high-thermal-conductivity filler for insulating polymers used in electronic devices. However, since the crystal structure of h-BN is analogous to that of graphite, h-BN exhibits a platelet crystalline shape with the anisotropy of high thermal conductivity for the in-planar direction and an extremely low thermal conductivity for the trans-planar direction. Consequently, the composites dispersed by conventional h-BN platelets indicated considerably low thermal conductivity in the trans-planar direction owing to the orientation of h-BN platelets. In this study, the anisotropic thermal conductivity was improved by dispersion of equiaxed h-BN fillers. Equiaxed h-BN fillers with spherical polyhedron shape were successfully synthesized through liquid-phase crystal growth by heating an h-BN powder with a rare-earth oxide. The common low thermal conductivity observed for the trans-planar direction was improved in the epoxy composites including equiaxed h-BN fillers, which attained 21.4 Wm −1 K −1 corresponding to a six-times-higher thermal conductivity than platelet h-BN filler.