Although graphene sheets have 2D symmetry, carbon nanotubes by geometry have different properties in axial and radial directions.
[1] Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively.
(For illustration, this translates into the ability to endure tension of a weight equivalent to 6,422 kilograms-force (62,980 N; 14,160 lbf) on a cable with cross-section of 1 square millimetre (0.0016 sq in).
Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to ≈100 gigapascals (15,000,000 psi), which is in agreement with quantum/atomistic models.
Since carbon nanotubes have a low density for a solid of 1.3 to 1.4 g/cm3, its specific strength of up to 48,000 kN·m·kg−1 is the best of known materials, compared to high-carbon steel's 154 kN·m·kg−1.
[citation needed] Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multi-walled carbon nanotubes and carbon nanotube bundles down to only a few GPa.
This limitation has been recently addressed by applying high-energy electron irradiation, which crosslinks inner shells and tubes, and effectively increases the strength of these materials to ≈60 GPa for multi-walled carbon nanotubes and ≈17 GPa for double-walled carbon nanotube bundles.
Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional, or bending stress.
The first transmission electron microscope observation of radial elasticity suggested that even the van der Waals forces can deform two adjacent nanotubes.
[2] Later, nanoindentations with atomic force microscope were performed by several groups to quantitatively measure radial elasticity of multiwalled carbon nanotubes[3][4] and tapping/contact mode atomic force microscopy was also performed on single-walled carbon nanotubes.
[5] Young's modulus of on the order of several GPa showed that CNTs are in fact very soft in the radial direction.
A complete phase diagram giving the transition to the radially collapsed geometry as function of diameter, pressure and number of tube-walls has been produced from semiempirical grounds.
In 2008, a method using an atomic force microscope was introduced to determine the exact number of layers and hence the internal diameter of the CNT.
By applying a voltage as low as 1.3 V, the extreme water repellant surface can be switched to a superhydrophilic one.
Because CNTs are themselves 1D materials, the well-known generation and multiplication mechanisms (such as a Frank-Read source) for 1D dislocations do not apply.
[10][11] This activation energy barrier partially explains the low ductility of CNTs (~6–15%) at room temperature.
The defect structure reduces strain because the heptagon geometry is able to stretch more than the original hexagonal rings, while the C-C bond remains about the same length.
Defect motion occurs when the work done by an applied stress overcomes it, such that the required bending curvature is inversely proportional to the diameter of the CNT:[13] Similarly, thermal vibrations can provide the energy required for defect nucleation and motion.
In fact, a combination of stress and high temperature is required to induce observable plastic deformation in CNTs.
Climb of kinks is evidenced by the fact that they do not always move along the close-packed planes in CNTs, but rather along the length of a tube.
It is proposed that elevated temperatures allow for the diffusion of vacancies, so that defects climb through a process similar to that observed in 3D crystalline materials.