Improvement of the anisotropic thermal conductivity of h-BN filled epoxy composites by changing the filler shape to spherical

Abstract

Hexagonal boron nitride (h-BN) is extensively used as a thermally conductive filler owing to its high thermal conductivity and excellent electrical insulating properties. However, commercially available h-BN powder, which exhibits platelet shape morphology, results in composites with anisotropic thermal conductivity. In contrast to two-dimensional or one-dimensional h-BN fillers, research on polymer composites reinforced with spherical h-BN particles remains limited, likely because of the inherent challenges associated with their synthesis. Consequently, it is essential to develop a procedure for synthesizing larger-grain-sized h-BN particles with spherical morphologies, which may enable higher filler loadings in standard thermoset polymers. Furthermore, a spherical structure is effective in improving the thermal conductivity anisotropy, and larger-grain-sized spherical h-BN particles can also decrease the significant interfacial resistance at the filler–matrix interface, thereby improving the thermal conductivity of epoxy/h-BN composites. In this study, spherical h-BN fillers were successfully synthesized using iron borides (Fe–B_x) as the boron sources. This study aimed to improve the thermal conductivity anisotropy of epoxy/h-BN composites by synthesizing spherical Fe–B_x through carbothermal reduction of iron oxide–boric oxide mixtures (Fe₂O₃–B₂O₃), resulting in the fabrication of spherical h-BN. The thermal conductivity observed in the through-plane direction of the epoxy/h-BN composites was significantly enhanced, reaching 11.1 W m⁻¹ K⁻¹, with the incorporations of 50 vol% of spherical h-BN fillers of approximately 35.4 µm in particle size. This achievement signifies a threefold increase in the thermal conductivity compared to those of conventional platelet h-BN fillers.