Previous work has suggested spontaneous ordering of DNA-BNNT hybrid systems, but aligned structures were only observed in dried films of DNA-wrapped BNNTs after solvent removal by filtration 25 or evaporation 26. Thus far, the formation of BNNT liquid crystals has not been attained, possibly because of sample impurities (e.g., hexagonal boron nitride (h-BN) and elemental boron 24), which hinder the individualization and alignment of BNNTs. This less effective charge stabilization (and charge localization on the nitrogen atoms) is likely to lead to some residual long-range attraction of BNNTs in CSA (which is absent for CNTs in CSA 23), consistent with the higher-than-predicted isotropic-nematic phase transitions observed in this work. However, the bond structure of BNNTs would localize protons on the nitrogen atoms-unlike in CNTs, where shared π electrons favor delocalization and hence further stabilization of positive charges when CNTs are dissolved in acids 22.
hypothesized that protonation of nitrogen atoms of the outer BNNT wall confers a net positive charge to their surface positively charged BNNTs repel each other leading to their individualization, as is the case for CNTs 21. Dispersions of individualized BNNTs can be achieved using chlorosulfonic acid (CSA) 21. Yet, wet spinning requires a solvent for the nanotubes and preferably the formation of a nanorod liquid crystal. Of these routes, wet spinning appears most easily adapted to processing BNNTs because it is independent of synthesis method. Thus far, none of these methods has been effectively employed to produce ordered BNNT materials. For example, closely packed and highly aligned CNT fibers yield high performance (e.g., tensile strength 13 above 4 GPa and electrical conductivity 13 above 10 MS/m) via multiple routes, including direct spinning 14, 15, wet spinning 16, 17, 18, and carpet spinning 19, 20. The inherent properties of nanoscale building blocks can be translated to the macroscopic scale by controlling long-range ordering, as has been achieved with carbon nanotubes (CNTs) 11, structural analogs of BNNTs that share many of their desirable properties, apart from superior thermal stability 12. Future improvements in material quality and processing techniques will enable high-performance neat BNNT articles with extraordinary properties for use in extreme environments. However, the utility of BNNTs is not yet fully realized, because their remarkable properties have only been observed at the microscopic level 10. These properties are desirable for many applications, including aerospace, electronics, and energy-efficient materials. In addition to being mechanically strong 3, 4, BNNTs are thermally conductive 5, electrically insulating 2, neutron-shielding 6, piezoelectric 7, and thermally stable up to 900 ☌ in air 8, 9.
Chemically, BNNTs are composed of alternating boron and nitrogen atoms in a hexagonally-bonded sheet, scrolled to form a seamless cylindrical structure that gives rise to several unique properties 2. Boron nitride nanotubes (BNNTs) are high aspect ratio rod-like nanostructures a few nanometers in diameter and microns long 1.
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