Mechanical Properties of Carbon Nanotubes in the Radial Direction

Mechanical Properties of Carbon Nanotubes in the Radial (transverse) Direction

Carbon nanotube is the one of the strongest materials in nature. Carbon nanotubes (CNTs) are basically long hollow cylinders of graphite sheets. Although a graphite sheet has a 2D symmetry, carbon nanotubes by geometry have different properties in axial and radial directions. It has been shown that CNTs are very strong in the axial direction [M.-F. Yu et al., Science 287,637 - 640 (2000)] . Young's modulus on the order of 270-950 GPa and tensile strength of 11-63 GPa were obtained.

On the other hand, there were evidences that in the radial direction they are rather soft. The first TEM observation of radial elasticity suggested that even the van der Waals forces can deform two adjacent nanotubes [R. S. Ruoff, et al., Nature 364, 514 - 516 (1993)] . Later, nanoindentations with atomic force microscope were performed by several groups to quantitatively measure radial elasticity of multiwalled carbon nonotubes [I. Palaci, et al., PRL 94, 175502 (2005) ] , [M.-F. Yu, et al.,PRL 85, 1456-1459 (2000) ] . Young's modulus of on the order of several GPa showed that CNTs are in fact very soft in the radial direction.

Radial direction elasticity of CNTs is important especially for carbon nanotube composites where the embedded tubes are subjected to large deformation in the transverse direction under the applied load on the composite structure.

One of the main problems in characterizing the radial elasticity of CNTs is the knowledge about the internal radius of the CNT; carbon nanotubes with identical outer diameter may have different internal diameter (or the number of walls). Recently a new method using atomic force microscope was introduced to find the exact number of layers and hence the internal diameter of the CNT. In this way, mechanical characterization is more accurate [M. Minary-Jolandan, M.-F. Yu, Journal of Applied Physics 103, 073516 (2008)] .

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