The New, New Nano Stuff
Boron nitride nanostructures offer tougher alternatives to carbon counterparts.
There’s a new nanotube in town, and it’s not based on carbon. Laurence Marks, a materials science professor at Northwestern University, and graduate student Erman Bengü have developed a boron nitride nanomaterial by depositing ions of boron and nitrogen on a hot, electrically charged tungsten surface in a high vacuum.
As they discovered, the resulting yarn-like substance, or “wool,” contains a surprising variety of boron nitride nanostructures, including single-wall nanotubes, spherical fullerenes, cone-shaped “nano horns” and sphere-within-a-sphere “buckyonions.”
“Boron nitride nanostructures have been something of a neglected orphan to carbon,” Marks says. “They’re not better for all applications, but they might be in some instances.”
For starters, boron nitride nanomaterials should withstand higher temperatures than their carbon counterparts and won’t oxidize as readily. This could result in the development of new nanoparticles for coatings that won’t corrode and for machine parts that can better withstand high-heat conditions.
In addition, boron nitride nanotubes are likely to be semiconductors with predictable electronic properties, regardless of their diameter. In contrast, carbon nanotubes can be highly conductive, like metal, or semiconductive, like silicon, depending on how their edges align when they fold themselves into tubes of hexagonal molecules that resemble chicken wire. The problem is that a batch of carbon nanotubes will likely contain a mixture of both conducting and semiconducting tubes that are hard to separate.
Why were the scientists working with boron nitride in the first place?
“We were actually trying to make cubic boron nitride, so these results were something of an interesting accident,” says Marks. Cubic boron nitride, the second-hardest known substance after diamond, is used as a coating for high-temperature cutting tools.
Boron nitride nanomaterials have been an elusive research target ever since carbon buckyballs and tubes were discovered in the 1980s. While researchers have been able to produce boron nitride nanotubes, those nanostructures were always exposed to air and “contaminated” by stray air molecules in the attempt to get a clear image of them.
By growing and imaging the hair-like nanomaterial in an almost complete vacuum, Marks has achieved a significant first.
To explain why the boron nitride “wool” exhibits so many different shapes and structures, Marks theorizes that the hexagonal pattern of boron and nitrogen atoms is periodically interrupted by fourfold and eightfold rings-joints that may enable them to bend in different directions. In contrast, carbon nanotubes are mostly composed of hexagons with occasional five- or seven-molecule rings.
The even-numbered rings in boron nitride may make it a much more stable material, he says.
Marks continues to work with boron nitride in the lab but notes that the deposition technique he used to produce the boron nitride “wool” is very close to commercial processes in use today.
“It’s just a variant on the method used to produce the titanium nitride that makes the chrome-plated door handles in your hardware store strong enough to offer a lifetime guarantee,” he says.
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