Long-time low-impact ball milling of multi-wall carbon nanotubes

A systematic study on the morphological changes experienced by multi-wall carbon nanotubes during long-time ball milling in a vibrating mill was performed. Samples were collected in the time interval 0-200 h and analyzed with TEM and N2 adsorption. The change of the mean nanotube length with time could be approximated with a second-order exponential decay function. The length distribution function changed from the original log-normal to normal after 140 h of milling. Specific surface area, surface fractal dimension, the dimension of the capillary condensation and the pore size distribution (PSD) curve were calculated from the N2 adsorption isotherms. These morphological descriptors all experienced characteristic changes after ∼40 h of milling: the specific surface area decreased, the fractal dimension increased and the maximum of the PSD curve shifted downwards by ∼0.5 nm. Results indicate that ball milling of carbon nanotubes is a rather complex process and that it can cause several morphological changes to samples besides the well-known cutting effect.