Controllable synthesis of hexagonal boron nitride microspheres with densely interconnected nanosheets structure

Boron nitride microsphere (BNMS) has emerged as a promising filler for advanced composite materials due to its distinctive isotropic geometry and exceptional powder flow characteristics. However, challenges such as difficulties in controlling spherical morphology and crystal phase transformation have limited the development of research into the bottom-up synthesis of high-crystallinity BNMS. This study proposes a novel controllable synthesis strategy for BNMS with densely interconnected nanosheets structure. With the mixture of melamine (M) and boric acid (B) as precursors, structurally robust BNMS microparticles with high crystallinity were successfully prepared via a three-step approach of pre-pyrolysis, spray granulation, and high-temperature sintering (1500 °C) in sequence. The key for this synthesis route lies in the formation of a homogeneous boron, carbon, nitrogen and oxygen (BCNO) complexes intermediate via pre-pyrolysis. The obtained BNMS microparticles are composed of high-crystallinity hexagonal BN (h-BN) nanosheets, which form a three-dimensionally interconnected structure through an amorphous B–O–N interface phase. Meanwhile, the effects of the molar ratio of M to B (1:1–1:3) in the precursors and pre-pyrolysis temperatures (400–600 °C) on the morphology and crystallinity of BNMS microparticles were systematically investigated. The results indicate that a molar ratio of 1:2 and a pre-pyrolysis temperature of 500 °C facilitate the formation of dense microspheres with excellent crystallinity and integrity. This work provides an effective approach for the bottom-up preparation of high-crystallinity BNMS, and is of great significance for advancing its applications in composite materials.