Inorganic Compounds

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 carbon nanotubes, which are cylinders with sub-micrometer diameters and micrometer lengths, except that carbon atoms are alternately substituted by nitrogen and boron atoms.

The properties of BNNTs are very different: whereas carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius, a BNNT 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.

Owing to the unique atomic structure, BNNT has numerous excellent intrinsic properties such as superior mechanical strength , high thermal conductivity, electrically insulating behavior, piezoelectric property, neutron shielding capability, and oxidation resistance.

High stiffness and excellent chemical stability makes BNNTs ideal material for reinforcement in polymers, ceramics and metals. For instance, buckypaper-based BNNT/epoxy composites and polyurethane-modified buckypaper composites have been successfully developed.  These composite materials exhibit Young’s moduli over twice the value for neat epoxy and 20 times the value for unimpregnated buckypaper. BNNTs are also one of the most promising classes of material for reinforcing aluminum-based structures.

BNNTs also exhibit good thermal conductivity. This renders them useful for applications in nanoelectronics where heat dissipation is critical. This also makes BNNTs multifunctional as it not only improves the stiffness of composites, but also yields high thermal conductivity along with high transparency. The combination of high stiffness and high transparency is already exploited in the development of BNNT-reinforced glass composites.18 Other intrinsic properties of BNNTs such as good radiation shielding ability, high electrical resistance and excellent piezoelectric properties are likely to promote interest for integrating them in new applications.