Study of Carbon Nanotubes Under High Pressure

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interest are the mechanical properties of carbon nanotubes [2]. Due to the structural perfection, the Young's modulus approaches to the theoretical value of graphene and therefore carbon nanotubes should have the highest mechanical strength due to the strongest C-C sp 2 bond. It has already been observed experimentally that the Young's modulus of multiwalled carbon nanotubes approaches to the theoretical value of about 2 TPa [3]. On the other hand, single-walled carbon nanotubes (SWCNTs) [4] represent another extreme regime in the mechanical properties of carbon nanotubes: they are highly ductile and soft in radial directions, as has been well demonstrated by both experimental observations and theoretical simulations [5]. However, there has been no sufficient experimental data to quantify the softness of SWCNTs in radial directions. Another interesting feature of carbon nanotubes is their polygonization, either self-induced by the balance between the van der Waals forces and the strain energy induced by curvature when their diameters are large [6] or externally induced by applying pressure [7-8]. Diamond anvil cell is a powerful device to introduce high pressure and especially when it is combined with in situ synchrotron X-ray diffraction, a lot of insightful structural information on the mechanical deformation of carbon nanotubes can be obtained. It has been recently reported that SWCNTs undergo 179 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society

reversible volume reduction up to about 3 GPa as measured in a piston-cylinder apparatus [7], which provides non-hydrostatic pressure. We have studied the elastic deformation of single-walled carbon nanotubes utilizing a diamond anvil cell to exert hydrostatic pressure. Structural information was obtained from the changes of the Bragg reflection from the hexagonal closed packing nanotube lattice using in situ synchrotron X-ray diffraction. In the present paper, the elastic behavior of SWCNTs under hydrostatic pressure will be described and the compressibility of SWCNTs in the linear regime has been deduced. It was also found that the raft-like bundles of SWCNTs undergo reversible deformation under hydrostatic pressure up to about 4 GPa, after which plastic deformation occurred. The structural consequences of these transformations will also be discussed. EXPERIMENTAL Single-walled carbon nanotubes were produced by laser evaporation of graphite target mixed with Ni/Co catalysts. The pristine carbon nanotubes aggregate to form raft-like bundles [9] in which most of the individual SWCNTs of a narrow diameter distribution are stacked in hexagonal closed packing, though their helicities assume a rather uniform distribution [10]. Loosely tangled carbon nanotube material was put into a gasketed diamond anvil cell and hydrostatic pressure was applied to the SWCNTs via a pressure medium made of ethanolmethanol, in which a small number of ruby crystals were also embedded for the purpose of pressure measurement. In situ synchrotron X-ray diffraction data were

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