The optimal dimensions of hexagonal-boron nitride nanosheets as thermally conductive fillers: The thinner the better?

Layered solid particles such as hexagonal boron nitride (h-BN) are widely used as thermally conductive fillers in polymer composites. Exfoliated sheets of the layered particles (nanosheets) have been considered a significant asset to enhance thermal conductivity of the composites. Theoretical and experimental studies have reported that maximally exfoliated h-BN nanosheets (BNNS) would possess superior thermal conductivity. Accordingly, considerable efforts have been devoted to development of the single- or few-layered BNNS as thermally conductive fillers. As for thermal conductivity, however, the nanosheet fillers cannot be free from several drawbacks. Taking h-BN as an example, we discuss if the thinner nanosheets are always superior solid fillers. Based on significant preceding papers in the related disciplines, positive and negative factors of the thermally conductive nanosheets are examined in the short review. Contrary to common belief, 10 layers BNNS or slightly thicker ones were found to be the most valuable as thermally conductive fillers. Since the methodology presented here avails for other layered solid particles, it would advance the technological field of the functional composite materials. More broadly, in the present paper, we attempted to bridge the huge gap between knowledge about nano-sized materials and functional advancement of practically utilized materials.