Researchers in France and the US have demonstrated that although multiwalled carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) are very similar in structure and properties, there is one critical property in which they differ significantly - friction.
Lydéric Bocquet, a visiting professor at MIT, and a team led by Alessandro Siria at Université de Lyon in France, have found that while CNTs are so slippery that they have a characteristic known as superlubricity, which makes them essentially frictionless materials, BNNTs are the exact opposite, they display a very high level of friction.
The team tested how easy it was to pull each type of nanotubes apart, like a "Christmas cracker" held between two supports - a quartz-tuning-fork-based atomic force microscope and a nanomanipulator so that the miniscule forces involved could be measured with precision. The apparatus allowed them to "telescope" the CNTs very easily until they break under the strain. By contrast, it was much harder to pull an inner tube out of an outer BNNT. [Nigues et al., Nature Mater; DOI: 10.1038/nmat3985]
BNNTs have not been studied with quite the intensity of CNTs, but side-by-side comparisons have always suggested that they are fundamentally the same physically, except in terms of their electrical properties, CNTs are conductors or semiconductors, BNNTs are insulators. As such, the team was shocked to discover such a fundamental difference between the physical properties of the two types of nanotube. "There's a hidden difference that we still do not fully understand," Bocquet says.
The researchers suggest that the high-friction tubes might be able to function as a kind of shock-absorbing material. "A large membrane of that material could dissipate a lot of energy," Bocquet says. Ironically, the material has long been produced as an industrial lubricant: Apparently its bulk lubricating properties are very different from the interlayer friction seen in the lab experiments.
The differences between how materials behave in bulk and at nanoscale "is typical of the kind of questions that are challenging now," Bocquet adds, but could ultimately allow the development of nanoelectromechanical systems and devices. "You could think of devising a kind of nanosyringe," for example, he says. "In some sense, the limit is just imagination." Fundamentally, structure and geometry are not the only things that matter in determining the behavior of different types of nanotubes.
"Behind this work is the question of dissipation at the nanoscale, beyond the well known macroscopic 'Coulomb-Amontons' laws of solid friction, Bocquet told Materials Today."Moreover, boron-nitride is widely used in industry, particularly for its thermal properties in the [electronics, polymer processing, metalworking and cosmetics] industry. So it is always interesting to highlight a new behavior, which was previously unknown and unexpected."
David Bradley blogs at http://www.sciencebase.com and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".