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Fig. 37. 3-D rendering of a high-resolution AFM image of a catalytic MWNT, grown at 720° C, and suspended on an alumina ultra-filtration membrane (the 0, 254, 508 A refer to the z axis). Rippling is observed on the inner side of the natural curvature of the MWNT, with a periodicity of about 16 nm (shown by the white arrows), inclined at 30° to the tube axis [7]

tube can be stripped off under axial stress. This has been observed in the experiments of Ruoff and coworkers [165] by fixing the ends of a MWNT to two AFM tips and measuring the elongation of the nanotube, while pulling until the tube broke. The tensile strength was found to be at least an order of magnitude lower than expected, assuming a homogeneous stress distribution over all carbon shells of the multi-wall tube. But because the support only made contact to the outside of the MWNT, it is the outermost shell which is mainly stressed. Ruoff and coworkers [165] found that above the tensile limit, the outermost tubes rupture first, followed by a sudden and large elongation. From TEM images, these researchers conclude that the ruptured outer shell then slides over the inner tubes, which then in turn bear the load [16,37].

To conclude this section on the measurement of mechanical properties, it was demonstrated that the Young's modulus is very high for individual, and well graphitized carbon nanotubes. Their high strength makes them promising candidates in reinforcement applications. There are many problems remaining to be overcome before composite materials can be fabricated, which reflect the exceptionally good mechanical properties of the individual nano-tubes. As well as optimizing the material properties of the individual tubes, the tubes must be bonded to a surrounding matrix in an efficient way to enable load transfer from the matrix to the tubes. To improve their mechanical properties, the nanotube surfaces should be functionalized, and cross-linking should be generated between the SWNTs within and between the ropes.

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