A new carbon structure discovered by chance during a lab accident
In the secrecy of the labs, it happens that accidents of “manipulation” allow impromptu discoveries. This is the case for structures evoking a forest of snow-covered fir trees, which are actually a network of micropyramids of soot, seen under a scanning electron microscope. About 5 micrometers in height each, hexagonal at the base, they appeared on a carbon microtube following an air leak in an experiment supposed to be carried out in an argon atmosphere.
“We discovered this interesting structure by chance”, acknowledges Valeriy Luchnikov, researcher at the Institute of Materials Science in Mulhouse (CNRS/University of Haute-Alsace). These pyramids, made up of multi-layered carbon nanoparticles, similar to carbon nano-onions, can emit an electric current called “electric tunnel” when they are subjected to an electric field. This study, carried out in collaboration with Yukie Saito (University of Tokyo), was the subject of a publication in ACS NanoDecember 13.
The two scientists were indeed looking for ways to grow nanostructures, but of a completely different nature. For their work, they use microtubes a few hundred micrometers in diameter prepared by rolling up thin layers of chitosan, a biopolymer derived from chitin, the main component of the shell of insects and crustaceans. The tubes are then heat treated by slowly heating them in an argon atmosphere to 1000°C. This turns chitosan into electrically conductive carbon.
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This is where their manipulations begin. By passing electricity through these now conductive microtubes, they raise their temperature to 2,400°C. These micro-ovens thus make it possible to subject nanoparticles inserted inside to incredible temperatures and to examine their behavior. But, wham, their system was leaking, and a small air leak disrupted everything. The oxygen caused the carbon to burn, but in insufficient quantities it left soot as a combustion residue. This dust is structured in an original way, forming these pyramids on the outer surface of the nanotubes.
The experiment was reproduced under an atmosphere of argon containing traces of oxygen (about 300 ppm). The phenomenon occurs above 2,300°C. How to explain this pyramid shape? Mystery. “It is still a scientific enigma, one can think that it is due to the thermophoretic force, caused by the important temperature gradient between the surface of the tube and its surroundings”, says Valeriy Luchnikov. Still, these dust pyramids can be hardened by reheating them in an oxygen-free atmosphere.
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