Mechanical properties of carbon nanotubes
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The mechanical properties of carbon nanotubes reveal them as one of the strongest materials in nature. Carbon nanotubes (CNTs) are long hollow cylinders of
Strength
Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of
Under excessive tensile strain, the tubes will undergo
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.
CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional, or bending stress.
Material | Young's modulus (TPa) | Tensile strength (GPa)
|
Elongation at break (%) |
---|---|---|---|
Single-Walled Carbon Nanotubes (SWNT)E | ≈1 (from 1 to 5) | 13–53 | 16 |
Armchair SWNTT | 0.94 | 126.2 | 23.1 |
Zigzag SWNTT | 0.94 | 94.5 | 15.6–17.5 |
Chiral SWNT | 0.92 | ||
MWNTE | 0.2–0.8–0.95 | 11–63–150 | |
Stainless steelE | 0.186–0.214 | 0.38–1.55 | 15–50 |
Kevlar–29&149E | 0.06–0.18 | 3.6–3.8 | ≈2 |
EExperimental observation; TTheoretical prediction
Radial elasticity
On the other hand, there was evidence that in the radial direction they are rather soft. The first
Radial direction elasticity of CNTs is important especially for carbon nanotube composites where the embedded tubes are subjected to large deformation in the transverse direction under the applied load on the composite structure.
One of the main problems in characterizing the radial elasticity of CNTs is the knowledge about the internal radius of the CNT; carbon nanotubes with identical outer diameter may have different internal diameter (or the number of walls). In 2008, a method using an
Hardness
Standards single-walled carbon nanotubes can withstand a pressure up to 25 GPa without [plastic/permanent] deformation. They then undergo a transformation to superhard phase nanotubes. Maximum pressures measured using current experimental techniques are around 55 GPa. However, these new superhard phase nanotubes collapse at an even higher, albeit unknown, pressure.[citation needed]
The bulk modulus of superhard phase nanotubes is 462 to 546 GPa, even higher than that of diamond (420 GPa for single diamond crystal).
Wettability
The surface wettability of CNT is of importance for its applications in various settings. Although the intrinsic contact angle of graphite is around 90°, the contact angles of most as-synthesized CNT arrays are over 160°, exhibiting a superhydrophobic property. By applying a voltage as low as 1.3V, the extreme water repellant surface can be switched to a superhydrophilic one.[citation needed]
Kinetic properties
Multi-walled nanotubes are multiple concentric nanotubes precisely nested within one another. These exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing. This is one of the first true examples of molecular nanotechnology, the precise positioning of atoms to create useful machines. Already, this property has been utilized to create the world's smallest rotational motor. Future applications such as a gigahertz mechanical oscillator are also envisioned.
Defects
As with any material, the existence of a
Plastic deformation
A typical 3D material undergoes
Instead, CNTs undergo plastic deformation through the formation and movement of defects, primarily topological defects such as the
Applied stresses can overcome the energy barrier needed to move 5-7 defect pairs. Another way of understanding this is that when strained, a CNT releases strain by forming these defects spontaneously. For example, in (5,5) tubes, a critical tensile strain of ~5% results in defect generation. 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.[14] Bending the tubes beyond a critical curvature has the same effect. This behavior can be approximated by a simple, semi-quantitative analysis. Applying a stress over a tube of length and diameter does work approximately equal to on the tube, where is the Burgers vectors for the defect, is the bending curvature, and relates the Young's modulus of the CNT to that of graphene. The energy increase resulting from the defect creation and the separation of the 5-7 pairs is approximately given by . Here, is the dislocation core energy and gives the interaction energy between the defect pairs. 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. This has been achieved in the literature via the application of a current, which causes resistive heating in the material.[15] For CNTs subjected to temperatures above 1500K, elongations up to 280% have been reported. This kind of behavior is called superplasticity.[16] At these high temperatures, kinks may form and move by climb as well as glide. 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. When kinks do glide along close-packed planes in CNTs, they follow a helical path. 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.[17]
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