The tubular conical helix of graphitic boron nitride

Abstract

We employed a novel synthetic route for the generation of a boron nitride (BN) filamentarynanostructure. A pre-formed filamentary microstructure was used as a template insteadof a conventional metal catalyst. BN nanotubes exhibiting the geometry of anArchimedes spiral were obtained via a post-heating process at temperatures above1750 °C on the intermediate BN microstructures pre-formed under heating to1700 °C.An enthalpy was found to primarily determine the structures of the resultanthelical–conical nanotubes (HCNTs). Therefore, the structural parameters, particularly theapex angles of the cones, can be easily varied via simple annealing. This unique structuralproperty is favourable to interlayer sliding in the HCNT structure, as was documentedduring electron beam induced deformation. HCNTs can be bent by an angle as high as180° at room temperature and, then, fully recover the starting morphology after the release ofan external stress, exhibiting a spring-like behaviour. The striking elasticity and flexibilityof these nanotubes stem from both the high stiffness and the extraordinary flexibility ofthe BN filaments, and the ease of interlayer sliding in a graphitic structure. Thenovel BN tubular geometry broadens the range of known helical cone structures.Structural models were developed taking into account the disclination angles or apexangles, wrapping modes, coincidence site lattices and packing patterns of the BNHCNTs.