Boron nitride nanotube

Boron nitride nanotubes (BNNTs) are a polymorph of boron nitride. They were predicted in 1994 and experimentally discovered in 1995. Structurally they are similar to the carbon nanotube, which are cylinders with sub-micrometer diameters and micrometer length, except that carbon atoms are alternately substituted by nitrogen and boron atoms. However, the properties of BN nanotubes are very different: whereas carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius, a BN nanotube is an electrical insulator with a bandgap of ~5.5 eV, basically independent of tube chirality and morphology. In addition, a layered BN structure is much more thermally and chemically stable than a graphitic carbon structure.

All well-established techniques of carbon nanotube growth, such as arc-discharge, laser ablation and chemical vapor deposition, are used for mass-production of BN nanotubes at a tens of grams scale. BN nanotubes can also be produced by ball milling of amorphous boron, mixed with a catalyst (iron powder), under NH₃ atmosphere. Subsequent annealing at ~1100 °C in nitrogen flow transforms most of the product into BN. A high-temperature high-pressure method is also suitable for BN nanotube synthesis.

Electrical and field emission properties of BN nanotubes can be tuned by doping with gold atoms via sputtering of gold on the nanotubes. Doping rare-earth atoms of europium turns a BN nanotube into a phosphor material emitting visible light under electron excitation.Quantum dots formed from 3 nm gold particles spaced across the nanotubes exhibit the properties of field-effect transistors at room temperature.

Like BN fibers, boron nitride nanotubes show promise for aerospace applications where integration of boron and in particular the light isotope of boron (¹⁰B) into structural materials improves both their strength and their radiation-shielding properties; the improvement is due to strong neutron absorption by ¹⁰B. Such ¹⁰BN materials are of particular theoretical value as composite structural materials in future manned interplanetary spacecraft, where absorption-shielding from cosmic ray spallation neutrons is expected to be a particular asset in light construction materials. BN nanotubes have also shown potential in certain cancer treatments.

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