They are an alternate form of high-density, lightweight, reversible energy storage based on the elastic deformations of CNTs.
Many previous studies on the mechanical properties of CNTs have revealed that they possess high stiffness, strength and flexibility.
The Young's modulus of CNTs is 1 TPa and they have the ability to sustain reversible tensile strains of 6%[1] and the mechanical springs based on these structures are likely to surpass the current energy storage capabilities of existing steel springs and provide a viable alternative to electrochemical batteries.
The process of elastic energy storage in a CNT involves deforming it under an applied load.
On removal of the applied load the energy released from the CNT can be used to perform mechanical work.
A CNT has the ability to deform reversibly and a spring made from it can undergo repeated charge-discharge cycles without fatigue.
Owing to this limited load transfer between the different layers of MWCNTs, single walled nanotubes (SWCNTs) are more useful structural materials for springs.
Springs for energy storage can be made of SWCNTs or MWCNTs arranged in dense bundles of long, aligned tubes called "forests" of CNTs [2] that are grown by chemical vapor deposition (CVD).
[3] A deformed CNT requires a support structure to carry the load of the spring prior to discharge.
For CNTs arranged in groups/bundles called "forests" as described earlier, efficient packing and good alignment in necessary between the tubes to achieve a high energy density.
A support structure is required to hold the CNT spring in the fully loaded configuration prior to its release.
The design of the support structure will depend on the scale of the spring, the deformation mode the CNT is being subjected to and the architecture of the rest of the system.
The material selected for the structure should have high strength, because the added mass and volume of the support contribute to reducing the energy density of the entire system.
A hollow cylindrical structure of CNT of length L, diameter d and mean radius r is considered.
So, a spring in axial tension should consist of either SWCNTs with small diameters or uniformly loaded MWCNTs with densely packed shells to maximize
The strain energy density must be reduced by a fill factor k to account for the spacing between the individual CNTs.
In reality, there may not be ideal packing within a bundle, as the actual fraction k may be lower than the value calculated.
The CNT is assumed to be a hollow cylindrical beam of length L, Young's modulus E, and thickness n.h, where n is the number of layers and h=0.34 nm is the thickness of one shell (taken equal to the separation between graphene sheets in graphite).
The continuous tube has a mean radius r and diameter d. The cylinder has inner and outer radii of
Therefore, in order for CNT springs to achieve a high energy density either SWCNTs with small diameters or MWCNTs with densely packed shells should be used.
A CNT spring made of bundles of densely packed 1 nm diameter SWCNTs stretched to a 10% strain is predicted to have an energy density of 3.4×106 kJ/m3.
[4] Whereas the current maximum energy density of a carbon-steel watch spring is reported to be between 1080 kJ/m3[5] and 3000 kJ/m3.
[6] Calculations show that when a support structure made of single crystal silicon carbide is used the energy density of CNT springs reduces to 1×106 kJ/m3.
A large number of CNTs are needed to store a significant amount of energy that can be used for macroscopic processes.
In order to achieve such a large amount of energy storage the CNT springs must maintain high stiffness and elasticity.
It is in practice quite difficult to have such high stiffness and elastic strains in yarns or fibers made up of assemblies of CNTs as they seldom maintain mechanical properties of an individual SWCNT.
This behavior occurs due to atomic defects and imperfect organisation.
The unequal amount of slack within each CNT due to the presence of atomic defects and tangling causes different CNTs to fracture at different strains.
Tensile tests of MWCNTs attached to atomic force microscope (AFM) tips at both ends show that fracture occurs at the outer shell in a way such that majority loading occurs at the outer shell and little load transfer occurs to the inner shells.
This causes the stiffness and strength of MWCNTs to be lower than they would be if the shells were loaded equally.