Binder-Free Boron Nitride Spheres Enhanced Quasi-Isotropic Thermal Conductivity of Polymer Composites

Heat dissipation has become increasingly crucial in modern highly integrated and miniaturised electronic devices to improve their reliability and performance. Thanks to the high thermal conductivity and electrical insulation, boron nitride nanosheets (BNNSs) are usually used as fillers to construct thermally conducive polymer composites for heat dissipation. However, limited to the low dispersibility, high aspect ratio, and anisotropic thermal conductivity of BNNSs, the resulted thermal conductive composites based on polymer matrix showed an unsatisfied thermal conductivity, especially in the out-of-plane direction, due to the "lie-down" structures of the BNNSs fillers. In this work, micro-sized, binder-free boron nitride spheres (BNSs) have been successfully synthesized using a two-step process of thermal spray drying and high-temperature sintering; and a facile and efficient method to measure the thermal conductivity of BNSs, based on the factors impacting the BNSs’ thermal conductivity, including precursor, polymer binder and sintering temperature, was developed. With optimised conditions, BNSs have a high, isotropic thermal conductivity of 37.2 W/mK. Based on the high thermal conductive and binder-free BNSs, a poly(vinyl alcohol) (PVA)/BNS composite film was successfully fabricated, and the out-of-plane thermal conductivity of this composite film is significantly enhanced to 8.1 W/mK, while the in-plane thermal conductivity, up to 10.6 W/mK, is not sacrificed, indicating the quasi-isotropy in thermal conductivity. The significant thermal conductivity enhancement (~3700%) of PVA is attributed to the formation of isotropic thermally conductive networks within the polymer matrix and strong interactions between BNNSs inside BNSs. This study provides a practical route to fabricate BN-enhanced composite films with isotropic thermal conductivity and promising materials that are valuable for heat dissipation in new-era advanced electronics and related applications.