Synthesis and growth of boron nitride nanotubes by a ball milling-annealing process

The synthesis and growth of boron nitride nanotubes (BNNTs) based on ball milling of crystalline boron powder followed by heat treatment were investigated. Fe-based stainless steel (STS) balls and milling vessels were used for milling, and the Fe impurity produced during milling acts as a catalyst for the generation of BNNTs during annealing under a nitrogen environment. Structural deformation of crystalline boron was observed for milled boron powder based on X-ray diffraction spectra and electron microscopy images. No chemical reactions of boron with nitrogen occurred during milling, and BNNTs were only synthesized during annealing. The BNNTs produced are basically multi-walled cylindrical- or bamboo-types mixed into nanotube clusters. The diameters of BNNTs are in the range of 50-150 nm, and numbers of the walls are 30-100 with a ∼0.3 nm gap on average. It was observed that BN was synthesized from amorphous boron coated onto the surface of the Fe particles. In addition, the types of grown nanotubes could be determined by the initial shapes of BN clusters on a Fe catalyst particle, which are nanoshells or opened nanocylinders. Yields of BNNTs were strongly dependent on the amorphous structure of the boron particles rather than on the residual crystalline boron particles in the milled samples.