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Boron nitride nanotubes (BNNTs) are the divas of the nanoworld. In possession of alluring properties, they are also notoriously temperamental compared to their carbon-based cousins. On the plus side, they can withstand high temperatures (well over 1100øC), according to Yoke Khin Yap, an associate professor of physics at Michigan Technological University. "Carbon nanotubes would burn like charcoal in a barbecue at half of those temperatures," he says.
In addition, the electrical properties of BNNTs are remarkably uniform. Perfect insulators, BNNTs could be doped with other materials to form designer semiconductors that could be used in high-powered electronics. Unfortunately, making nanotubes from boron and nitrogen is easier said than done. "Making carbon nanotubes is simpler, like cooking," says Yap.
BNNTs, on the other hand, have always been fussy, requiring special instrumentation, dangerous chemistry or temperatures of over 1500°C to assemble. On top of that, the products are shot through with impurities. "We've been stuck for more than 10 years because nobody could grow them well on substrates," says Yap. "But now we can."
As it turns out, BNNTs just needed a little encouragement. Yap and his team have grown virtual Persian carpets of the tiny fibers on a substrate made from simple catalysts, magnesium oxide, iron or nickel. The process uses the same instrumentation for growing carbon nanotubes (at about 1100°C) and has achieved high-quality results. The boron nitride nanotubes can be made to assemble exclusively on these catalysts, so the researchers can control precisely where they grow (see Figure 1). "I hope this encourages more researchers to grow BNNTs using the new technique," says Yap.
These transparent nanotube sheets have another interesting property: they shed water like a duck's back, a quality known as the lotus effect. "Water just slides away," says Yap. "Anything coated with it would not only be stain resistant, it would be protected from anything water-soluble." This super-hydrophobicity holds at all pH levels, so anything coated with the sheets would be protected from even the strongest acids and alkalis.
The research was funded through a National Science Foundation Career Grant. A paper detailing Yap's discoveries, "Patterned Growth of Boron Nitride Nanotubes by Catalytic Chemical Vapor Deposition," has been published online by Chemistry of Materials. Yap is also the editor of B-C-N Nanotubes and Related Nanostructures, a book detailing nanostructures constructed from one or multiple elements using boron, carbon and nitrogen.
For more information, visit www.mtu.edu.