Structure–property relationships in thermally-annealed multi-walled carbon nanotubes

The mechanical properties of individual multi-walled carbon nanotubes (MWCNTs) synthesized by a catalytic chemical vapor deposition (CVD) method followed by a series of high temperature annealing steps at 1200, 1800, 2200 and 2600 °C are investigated by a manipulator tool operated inside a scanning electron microscope. To investigate the relationship between the MWCNT structure and mechanical properties, such MWCNTs with a significantly different nanostructure are separately tested in tension, and subsequently observed their nanostructure and fracture morphology by a transmission electron microscope. The results show that the thermal annealing is effective for improving both the strength and modulus of the catalytic CVD-grown MWCNTs. The MWCNTs annealed at 1800, 2200 and 2600 °C display enhancements to their strengths by factors of ∼5.4, ∼5.1 and ∼15.6, and moduli by factors of ∼5.9, ∼13.2 and ∼18.9, respectively, compared to the MWCNTs annealed at 1200 °C. This effect is associated with the degree of waviness of the graphitic planes along the nanotube axis as well as the degree of crystallinity of the MWCNTs: the strength and modulus of the MWCNTs increases with a higher degree of orientation of the 0 0 2 graphitic planes and with a lower degree of defect concentration in the MWCNT structure.