These range from super high-capacity batteries to materials so strong and light they could be used to build a space elevator. As these new forms of carbon start to become more affordable, they are appearing as structural reinforcement in consumer products. They are being added to bicycle frames, tennis rackets and even sports shoes. Despite this we haven’t seen the revolution in super-strong and light materials that has sometimes been suggested.
It has been suggested that graphene and carbon nanotubes (CNTs) could produce much stronger and lighter structures, more efficient electrical energy systems, nano-bots, cheap flexible solar cells, abundant freshwater and much more. In isolation, sheets of graphene and individual carbon nanotubes are extremely strong and stiff, as well as having low density.
Induced cross-linking
Carbon nanotubes have demonstrated 20 times the strength-to-weight ratio of the strongest carbon fibres. However, to be used as a structural reinforcement in composite materials, millions of nanotubes must be combined to form a fibre. Within these fibres the nanotubes slide past each other, resulting in relatively weak fibres. This is a bit like the soft graphite in a pencil, which is made of layers of graphene that slide past each other.
Just because a material has incredible strength at the molecular level, we can’t assume these properties will transfer to the material’s bulk strength. There is some encouraging research showing that irradiation of nanotube bundles can induce cross-linking. However, so far this only works with a few bundles containing a few nanotubes. Replacing carbon fibre would require fibres containing millions of crosslinked nanotubes. It is not clear whether this will be possible.
Carbon nanotubes are classified as either single-wall carbon nanotubes (SWCNTs) or multiwall carbon nanotubes (MWCNTs). MWCNTs are effectively a number of CNTs of different diameters located inside each other. The structural properties of CNTs are largely determined by diameter and defects. Raman spectroscopy is typically used to characterise SWCNTs, with a number of excitation modes used to identify tube diameter, purity, chiral angle and level of structural defects.
There are two main vibration modes identified by Raman spectroscopy of graphene: G (1,580cm-1) and D (1,350cm-1). In pure graphene the D mode cannot be observed directly owing to crystal symmetries but can be detected as an overture D2 peak. The ratio between the intensity of the G and D spikes in the spectrum is an indication of the purity of graphene, with bulk graphite producing a distinctly different ratio.
The ratio of G/D modes is an indication of defects and is the standard measure of structural quality, with ratios over 100 regarded as high quality. MWCNTs usually have more defects and are difficult to characterise optically. Chiral angle is the orientation of the hexagonal arrangements of carbon atoms within the walls of the tubes. It can be thought of as the direction that a flat sheet of graphene would be rolled to produce the CNT. Chiral angle primarily affects electrical properties, causing CNTs to be metallic or semiconducting.
Delivering strength
The National Graphene Institute and the Aerospace Technology Institute reported recently that graphene and CNTs are not yet ready to replace carbon fibre as the primary reinforcement in composites, but they do have potential as unstructured additives to resins and plastics. Graphene can be added to the resin in thermoset carbon-fibre composites. The carbon fibre provides stiffness while the graphene improves interlaminar shear strength and damage tolerance. This can enable reduced ply thickness. Graphene may also be the primary reinforcement in small high-performance polymer parts, providing stiffness and enabling these materials to replace metal.
For a number of years after being isolated, the cost of graphene and CNTs was thousands of dollars per gram, making use as a structural material uneconomic. Industrial production processes are now coming online and CNTs are available for a few dollars a gram. As a result, a range of consumer products is now being marketed as containing graphene, although they typically only contain very small quantities.
Sporting chance
The first company to supply carbon nanotube-enhanced materials commercially was Zyvex Technologies. In 2005 it worked with Easton Sports to produce a baseball bat featuring carbon nanotubes. Zyvex now supplies a range of prepreg, resins and adhesives enhanced with carbon nanotubes and graphene. Compared to carbon fibre alone, it is claimed that this combination increases tensile strength by 26%, compressive modulus by 12%, flexural modulus by 35%, interlaminar shear by 20%, and fracture toughness is nearly doubled.
The first company to produce a graphene product was Head with a graphene tennis racket in 2013. Head claimed the graphene allowed it to reduce weight by 20% while increasing strength by 30%.
Graphene and CNTs are now used in many products. For example, Dassi produces a bicycle that it claims is 30% lighter and twice as strong owing to the addition of just 1% graphene.
According to Dassi, the graphene is first added to the resin and a prepreg is then created. “We mix graphene with an epoxy resin we developed ourselves that is then electronically functionalised to disperse the graphene evenly within the resin. The carbon weave is then introduced into the resin mix, which in turn forms the graphene carbon material in a prepreg that can be used for laying-up components,” says the company.
It may seem strange that significant performance improvements are possible when experimental lab experiments show the bulk strength for larger samples of graphene is still lower than for carbon fibre. However, the carbon fibre is still providing the strength and stiffness in these products. The addition of graphene improves interlaminar shear strength, fracture toughness, and carbon-to-resin adhesion. It can also retard crack propagation.
Reinforcing printed parts
Another interesting application for graphene and CNTs is as a reinforcement within 3D-printed polymer parts. Print filaments containing graphene are available commercially from suppliers such as Directa Plus, Graphene 3D Lab and Haydale. As well as improved strength and stiffness, the carbon material also increases thermal conductivity, which can facilitate higher deposition rates.
It seems that composites can currently be improved by adding graphene and CNTs. Improvements in interlaminar shear strength and damage tolerance are possible and may under certain circumstances reduce mass by up to 50%. However, the order of magnitude improvements in strength to weight that it has been suggested graphene and CNTs could one day offer are not currently feasible. Significant developments would be required to enable cross-linking of graphene sheets or CNTs so that bulk material properties could approach the strength of isolated nano-structures.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.