The tubular conical helix of graphitic boron nitride

We employed a novel synthetic route for the generation of a boron nitride (BN) filamentary nanostructure. A pre-formed filamentary microstructure was used as a template instead of a conventional metal catalyst. BN nanotubes exhibiting the geometry of an Archimedes spiral were obtained via a post-heating process at temperatures above 1750°C on the intermediate BN microstructures pre-formed under heating to 1700°C. An enthalpy was found to primarily determine the structures of the resultant helical-conical nanotubes (HCNTs). Therefore, the structural parameters, particularly the apex angles of the cones, can be easily varied via simple annealing. This unique structural property is favourable to interlayer sliding in the HCNT structure, as was documented during electron beam induced deformation. HCNTs can be bent by an angle as high as 180° at room temperature and, then, fully recover the starting morphology after the release of an external stress, exhibiting a spring-like behaviour. The striking elasticity and flexibility of these nanotubes stem from both the high stiffness and the extraordinary flexibility of the BN filaments, and the ease of interlayer sliding in a graphitic structure. The novel BN tubular geometry broadens the range of known helical cone structures. Structural models were developed taking into account the disclination angles or apex angles, wrapping modes, coincidence site lattices and packing patterns of the BN HCNTs.