May 25, 2006
Nanotubes squeeze out their contents
An international team has used multiwalled carbon nanotubes as nanoextruders that squeeze out their contents under the influence of an electron beam. The researchers, from Mainz University, Germany, the University of Helsinki and Helsinki University of Technology, Finland, Instituto Potosino de Investigaciòn Cientifica y Tecnològica, Mexico, and Rensselaer Polytechnic Institute, US, filled the tubes with hard materials such as iron, iron carbide and cobalt.
Irradiation with electrons removes carbon atoms from the walls of the nanotube, creating vacancies. The nanotube's crystal structure rearranges to remove these vacancies, which shrinks the tube's diameter and exerts pressure on the contents at its core.
"Vacancies in the graphite lattice have an unexpectedly high mobility at the temperature of this experiment (600 °C) and coalesce to form divacancies," explained the researchers. "Divacancies are unstable, however, and collapse by closing open bonds so that a coherent graphite layer is restored. Such a layer does not just consist of hexagonal rings only as the perfect graphite lattice, but also of pentagonal or heptagonal rings."
According to the researchers, the contraction caused by the nanotube restructuring is so strong that the crystals inside are massively deformed and squeezed out through the end of the tubes. Calculations showed that pressures could reach more than 40 GPa in the radial direction.
The technique enables study of the deformation of individual nano-sized metal crystals. "Carbon nanotubes serve as 'nanolaboratories' for the deformation of crystalline materials," said the researchers. "The high resolution of the electron microscope where the experiment is carried out permits the observation of the deformation process with atomic resolution."
The team reckons this is particularly useful as the mechanical properties of nanocrystalline materials are poorly understood – it's not possible to observe the deformation behaviour of single crystalline grains directly.
"Besides applying nanotubes as compression cells on the nanoscale, the new experiments should contribute to the understanding of the deformation mechanisms in nanocrystals," the researchers said.
The team reported their work in Science.
About the author
Liz Kalaugher is editor of nanotechweb.org.