Materials & Applications

How Carbon Nanotubes are Redefining Strength of Materials

Carbon Nanotubes

What is 100 times stronger than steel, yet lighter than aluminium? Carbon nanotubes are among the strongest materials ever discovered, yet engineers are only just starting to unlock their full potential.

Carbon nanotubes are an allotrope of carbon, meaning that they are one of several possible arrangements of atoms that carbon can take. In nature, pure carbon is found either in the form of graphite, a soft flaky solid, or diamond, which is transparent and the hardest naturally-occurring material.

Carbon nanotubes are more closely related to graphite than diamond. Though graphite is very soft, it is constructed of layers of carbon atoms arranged in sheets one atom thick. These individual sheets are called graphene, and carbon nanotubes are what you would get from rolling a graphene sheet into the shape of a tube.

Carbon nanotubes

Graphene is a remarkable material in its own right. Like carbon nanotubes, it is incredibly strong. The 2010 Nobel Prize in Physics was awarded to the team that first isolated a sheet of graphene, and graphene is so strong that, in theory, a sheet of graphene weighing about one milligram could hold a cat.

This hypothetical “cat hammock” would be only one atom thick and completely invisible to the human eye. Carbon nanotubes are formed from one or more layers of graphene arranged into a tube and are also extremely strong.

Graphene cat hammock
According to the Nobel Academy document, if a one square meter hammock made of graphene were "tied between two trees you could place a weight of approximately 4 kilograms before it would break. It should thus be possible to make an almost invisible hammock out of graphene that could hold a cat without breaking.

At present, carbon nanotubes are primarily used to improve the properties of polymer composites. Carbon nanotubes can offer a range of beneficial properties:

  • Very high strength
  • High strength-to-weight ratio
  • High electrical conductivity
  • High thermal conductivity
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Carbon nanotube properties

When compared to other reinforcement materials, carbon nanotubes are considerably stronger than other fibres used in fibre-reinforced polymer composites. Recent research has also been focused on the development of functionally graded polymers, where carbon nanotubes are distributed strategically within a polymer structure to grant it custom mechanical properties.

MaterialStrength (data sources in links)
Multiwalled carbon nanotubes11 – 63 GPa
Carbon fibre3.5 – 5.5 GPa
Glass fibre3.5 – 4.6 GPa
Kevlar3.0 GPa
Steel0.23 – 0.73 GPa

Carbon nanotubes also offer good electrical and thermal conductivity, which makes them useful in electronics packaging applications or as additives to polymers and adhesives to make them conductive. Traditionally, metals have been the primary material used as electrical and thermal conductors in electronics because polymers and ceramics offered poor electrical and thermal conductivity by comparison.

However, by adding carbon nanotubes, several polymers can be made conductive, which opens up new possibilities for faster, less expensive electronics manufacturing.

Material Electrical Conductivity
(data sources in links)
Thermal Conductivity
(data sources in links)
Single-walled carbon nanotubes102 – 106 S/cm6000 W/mK
Multiwalled carbon nanotubes103 – 105 S/cm2000 W/mK
Diamond10-2 – 10-15 S/cm900 – 2320 W/mK
Graphite3.3 – 4000 S/cm2.2-298 W/mK
Copper4.3•109– 5.9•109 S/cm305 – 385 W/mK

Carbon nanotube-reinforced composites

If graphene and carbon nanotubes are so incredibly strong, then why aren’t we using them in everything? A part with strength comparable to graphene or carbon nanotubes would be practically indestructible compared to any other material.

In order to understand the challenge of taking advantage of the incredible strength of carbon nanotubes and graphene, we can look at the reason graphite is soft. Individual sheets of graphene are extremely strong, but graphite is soft because the bonds between the sheets of graphene are weak.

Individual carbon nanotubes are one of the strongest materials ever discovered, but they must be connected together in order for their strength to be useful.

Carbon nanotubes graphene
Carbon nanotubes are what you would get from rolling a graphene sheet into the shape of a tube.

This is why carbon nanotubes are often used as an additive in other materials, usually polymers, to improve their properties. The carbon nanotubes add strength and the “matrix” material they are dispersed in holds everything together. But this leaves us with the question: Why are those tiny carbon nanotubes so much stronger than bulk materials? They are strong because they are small.

Ultra-strong nanomaterials

The key to carbon nanotubes’ strength lies in the fact that they get close to reaching the theoretical strength of carbon due to their small size. The theoretical strength of a material is the stress which would be required to break a perfect crystal free from any defects.

For example, the theoretical strength of pure iron is 31.8 GPa, while bulk steels have strengths in the range 270-740 MPa, less than 2.5% of the theoretical strength. This is because tiny defects known as dislocations make the bulk steels susceptible to plastic deformation and failure at lower stresses compared to a hypothetical defect-free crystal.

SEM image of aligned carbon nanotubes
SEM image of aligned carbon nanotubes. © Fraunhofer IKTS

Bulk materials never get close to their theoretical strengths because, even with extremely careful processing, larger-scale materials inevitably end up with microstructural defects that reduce their strength. This is also the reason that, unfortunately, humans are unlikely to ever fabricate a large and perfect sheet of graphene like the one-atom-thick graphene cat hammock described in the 2010 Nobel Prize award ceremony.

Such a large sheet would be certain to contain defects that would reduce its strength, and the cat would tear through the hammock. Thus, the processing of most bulk materials is focused on limiting the effect of their microstructural defects rather than eliminating defects entirely. Creating large amounts of completely defect-free material is practically impossible with current technology.

An electron microscope image of electrochemically grown TiO2 nanotubes. 10,000 times smaller than the width of a human hair, the tubes are filled with organic polymer in a new technique for "growing" solar cells with the potential to be cheaper than current solar cells.

Synthesizing a defect-free material is much more feasible if the material volume is very, very small. Put simply, a very small amount of material is statistically less likely to contain a defect than a large one, and small volumes of material are easier to create by chemical growth processes that introduce few defects.

Thus, it is possible to create large quantities of defect-free nanotubes, but not possible to create a large monolithic piece of defect-free material. Carbon nanotubes are ultra-strong because they are ultra-small which makes it possible for them to be defect-free.

Carbon nanotubes being spun to form a yarn.

It is also worth keeping in mind that the strength of materials is measured by dividing the force required to break a specimen by that specimen’s cross-sectional area, resulting in units like the megapascal (MPa) which is equivalent to one newton per square millimetre (N/mm2). Thus, strength measurements automatically compensate for the amount of material in a specimen, and consequently, we can compare the strength of a 1 cm diameter steel rod with that of a 1 µm diameter carbon nanotube.

The tiny nanotube is much more likely to be completely defect-free, and therefore extremely strong. But to bundle up enough nanotubes to create a part of equal size to the steel rod, we would have to create a fibre-reinforced composite.

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Their small size, and resulting lack of defects are what make carbon nanotubes an ultra-strong nanomaterial, with individual multiwall carbon nanotubes having reported strengths of 11 – 63 GPa, which is close to carbon’s theoretical strength of 156.0 GPa. Other materials can also be made into ultra-small, defect-free “whiskers”, including iron, which has a reported strength of 13 GPa in nano-whisker form, a much closer value to iron’s theoretical strength than bulk steels.

Many other materials have been synthesized in nanotube, nanowire, or whisker form under laboratory conditions, but carbon nanotubes are one of the few ultra-strong nanomaterials available in commercial quantities from suppliers like Goodfellow.

Conclusions

Carbon nanotubes are one of the strongest materials ever discovered because their extremely small size makes it possible for them to be defect-free and come close to achieving the theoretical strength of carbon. This is why carbon nanotubes are most commonly used to enhance the properties of other materials, such as when they are added to polymer matrices to improve their strength, electrical conductivity, and thermal conductivity.

The strength of carbon nanotubes is orders of magnitude higher than other fibres commonly used in fibre-reinforced composites. Carbon nanotubes’ good electrical and thermal conductivity also makes it possible to create conductive polymers for electronic applications where metals are traditionally used.

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