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.