Multi-walled carbon nanotubes made by catalytic-pyrolysis of ethylene have curved bodies and exhibit entangled masses. Their average length is on the scale of microns, which is not suitable for carbon nanotubes to be used as a nanometer-scale material. Boiling carbon nantubes in concentrated nitric acid or ginding in a ball mill can break the nanotubes. The average length of the nanotubes will be reduced and they will be relatively straight. In addition, on boiled carbon nanotubes, we find -OH, >C-O and -COOH functional groups. In this state, the nanotubes can be dispersed evenly in a matrix when used to make a composite, which can result in an enhancement of the composite's properties, especially the mechanical properties.
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