Researchers have long been able to make nanotubes out of carbon, but they have struggled to craft them from boron nitride. The two have about the same strength, but boron nitride nanotubes (BNNTs) can survive temperatures that are twice as high as those carbon nanotubes can survive–800°C and higher. Scientists have only been able to create high-quality tubes a micron long; larger versions have been riddled with defects in the crystalline structure.
A new method for producing long, small-diameter, single- and few-walled, boron nitride nanotubes (BNNTs) in macroscopic quantities is reported. The pressurized vapor/condenser (PVC) method produces, without catalysts, highly crystalline, very long, small-diameter, BNNTs. Palm-sized, cotton-like masses of BNNT raw material were grown by this technique and spun directly into centimeters-long yarn. Nanotube lengths were observed to be 100 times that of those grown by the most closely related method. Self-assembly and growth models for these long BNNTs are discussed.
The team of materials scientists describe the first creation of high-quality, uniformly crystalline BNNTs in large quantities: Each piece of fiber is long enough that it can be spun into user-friendly yarn. To do this, the researchers aimed a laser at a cake of boron inside a chamber filled with nitrogen. (Originally an infrared laser was used, but the technique has been modified to use a conventional welding laser.) This forms a plume of boron gas that shoots upward. A cooled metal wire is then inserted into the gas, causing the gas to cool and form liquid droplets. The droplets combine with the nitrogen to self-assemble into BNNTs. “It’s like fuel-air-spark in an engine,” says team member and NASA aerospace scientist Michael Smith. “The reaction advances violently, creating the superlong tubes in just milliseconds.”
That explosive reaction quickly produces masses of high-quality BNNTs that look like mounds of cotton candy–more high-quality BNNTs than anyone has ever been able to make at once. The fibers show all of the important properties–strength, piezoelectric activity, conductivity, and stability at high temperatures–that have made BNNTs so sought after. And all with a method that can be done with commercially available materials and tools.
Success at building large amounts of inexpensive nanotubes opens the door for lighter, faster car frames; affordable space vehicles; and ultralightweight armor. Or on a smaller level, BNNTs could be used with pinpoint precision to attack cancer cells by sticking to tumors, absorbing neutrons from a targeted beam, and generating localized alpha radiation to kill the cancer.
“This is the start of a revolution in materials,” says Dennis Bushnell, a NASA engineer who has watched the work closely in the hopes of using BNNTs for space vehicles. Typical wisdom has been that high-quality carbon nanotubes were much easier to create than high-quality BNNTs, but this new, easy process may change that thinking and get nanotubes into a host of applications much faster, he says. “Just about everything can be made lighter, and hopefully, cheaper. You’re talking about energy savings all over the place.”