Exclusive Growth of Low-Aspect Ratio, Polyhedral h‑BN Crystals in Molten Li₂CO₃ as the Reactive Flux

Hexagonal boron nitride (h-BN) exhibits a wide range of properties, including insulation and high thermal conduction. Despite these advantages, h-BN often forms with a plate-like morphology, with an aspect ratio much greater than 1, which gives ineffective thermal conduction at the particle interfaces. The problem arises because of the plate-like morphology that renders the crystal a high aspect ratio, much greater than 1, in its typical growth form. Herein, we used the flux method to grow and develop the morphology of h-BN crystals with a much smaller aspect ratio. After screening the flux and heating conditions, we found that crystals of h-BN grown in molten Li₂CO₃ had a new polyhedral structure that was over 3 μm in size. These crystals had low aspect ratio (around 1), which was unusual, and nonequilibrium morphology. Crystallographic evaluation revealed that the polyhedral h-BN crystals were not single crystals but were polycrystals with oriented tablet-type single crystals. This unique morphology was further investigated by in situ thermal analyses of the phase transition, chemical reaction, and morphological change during the growth of h-BN in molten Li₂CO₃. Based on these results, we discuss the mechanism of the growth of the crystal and the role of the flux. Our observations could be explained based on the reactivity of the flux and its probable absorbed manner on h-BN, which is dependent on its crystal facets in the flux.