Carbon nanotubes’ applications straddle many fields – what can this promising material do?

Nanotechnology is perhaps the single most versatile sphere of study today, which holds a tremendous potential for the future. An important part of the research focuses on carbon nanotubes’ application in different spheres. The following pages will deal with their properties, as well as potential uses, ranging from commercial to military, industrial, and personal purposes.

Carbon nanotubes’ applications are extremely diverse thanks to is remarkable properties

Carbon nanotubes, whether single or multiwalled, feature a wide range of properties which make them a truly universal tool, applicable in many spheres.

These properties include:

• High electrical conductivity
• High thermal conductivity
• Optimal aspect ratio
• Extremely high tensile strength
• High elasticity and flexibility
• A good ability to emit field electrons
• Low thermal expansion coefficient

We’ll take a further look at each of these properties in the following sections.

Nanotubes’ uses as field emitters make them a perfect material for electronics

The so called field emission occurs when electrons are emitted from a metal tip into a vacuum (or even another type of material) due to the application of an electrostatic field. And the small diameter and high aspect ratio (1000:1) of carbon nanotubes make them very apt for field emission. Even a small amount of voltages can create a strong electric field, potent enough to facilitate the emission of electrons. Nanotube-based field emitters have been recognised as more efficient electron sources than the traditional ones quite early, but their commercial use is still waiting to kick off properly. They could be put to work in many different fields, with perhaps the most viable option being lightweight flat-panel displays.

Other carbon nanotubes uses and applications are enabled by their thermal conductivity

According to the recent research, carbon nanotubes might be perhaps the best heat conductor around by a wide margin. The testing focused on super small straight-wall CNT’s and found out that they show signs of superconductivity at temperatures below 20°K (-235,15 °C). So in addition to being able to take on the role of conductors or semiconductors, CNTs may one day be used as tiny heat conduits as well!

Thanks to their tight structure, CNTs are highly resistant to axial strains, and show a high flexibility against non-axial strains. Their low levels of thermal expansion could make them useful as reinforcing materials in functional composite materials, as sensors or actuators.

Potential applications of carbon nanotubes as conductive additives is thanks to their high aspect ratio

CNTs can act as a conductive additive for all kinds of plastics, Thanks to its high aspect ratio, less CNT material will have to be used to achieve the same electrical conductivity in comparison to other conductive additives. Thanks to this, the polymer resin can maintain more of its toughness (in particular at low temperatures) and keep its other important performance properties. This makes carbon nanotubes a perfect additive, granting electric conductivity to plastics. CNTs high aspect ratio makes it therefore a superior additive material compared to the more traditional ones such as chopped carbon fibre, stainless steel fibre or carbon black.

Due to their potential to transfer electric current, applications of carbon nanotubes as conductors is an obvious choice

Depending on certain structural parameters, nanotubes can act as excellent conductors, so much so that they can be considered metallic in this regard. The degree of conductivity of CNTs is determined by their chiral angle (the orientation of the lattice towards the tube axis) and by their diameter. Depending on that, they can function in a metallic fashion or as semi-conductors.

The type of nanotube used is also an important factor in terms of conductivity. Experiments have shown that the multiwall CNTs featuring the “armchair” structure seem to be more efficient conductors than other types of CNTs. Furthermore, in multiwall tubes, the interwall reactions appear to distribute current in an uneven manner. Electric current flows without change through metallic single-walled tubes.

Semi-conducting SWTs behave in yet another manner – the transport current suddenly changes at different positions on the tube.

At this point, research has shown that single-walled nanotubes are the most conductive of all carbon fibres yet known. In theory, they could sustain an even greater current density, perhaps up to 1013 A/cm2.

While researching the conductivity of SWNTs, it has been found that some of them may contain defects. Luckily, even these defects could be put to favourable use, since they could enable these SWNTS to function as transistors.

And finally, recent findings suggest that SWNTs are able to route electrical signals at high speeds when they are used as interconnects on semi-conducting devices.

Further applications of nanotubes are enabled by their great strength and elasticity

The atoms forming the graphene sheet are arranged in a honeycomb lattice structure, where every atom is strongly connected to the three adjacent atoms. Thanks to this, the elastic modulus of graphite is one of the highest of all the known materials in the world.

This potentially makes carbon nanotubes peak high-strength fibres, outperforming, for example, steel. CNTs are extremely resistant to the application of outside force. Their tip will bend while pressed, but not take any damage. Once the pressure subsides, the nanotube’s tip will return back to its previous state. It is therefore highly suitable to serve, for example, as a probe tip to be used in scanning probe microscopy.

The measuring of the exact amount of pressure needed to push the unanchored end of a freestanding nanotube out of its equilibrium position is difficult, but estimates range from 1 TeraPascal up to 1,8 TeraPascal, with some findings suggesting an even higher value than that.

With that being said, let’s jump to the individual applications nanotubes are suitable for

Perfect molecular symmetry and all the aforementioned properties single out carbon nanotubes as one of the most versatile materials on Earth. There is no other element capable of forming such strong bonds with itself such as carbon. It superb conductivity is caused by the possibility of the delocalised pi-electron donated by each of the atoms to freely move through the whole structure and not be tied to the donor atom. This makes it the first molecule to feature conductivity otherwise seen only in metals. What’s more, carbon nanotubes have a thermal conductivity which exceeds even than that of a diamond.

What makes CNTs superior to other materials is the low defect rate. Other materials might lose some of their properties or degrade easily due to the hardly avoidable presence of natural defects. However, due to their perfect, symmetrical molecular structure, the nanotubes are able to consistently perform and achieve close to their theoretical limits.

Obviously, the range of uses this material can be put to is almost miraculous. Its structure and properties will, in the near future, make it suitable for mass application in all of these following fields:

• Thermal materials
• Structural materials
• Electrical conductivity
• Energy storage
• Molecular electronics
• Fabrics and fibres
• Biomedicine
• Catalyst supports
• Water and air filtration
• Conductive plastics
• Ceramics
• Conductive adhesives

Uses of nanotubes and fullerenes in solar cells

Though they have yet to undergo a proper optimization to suit this purpose, SWNTs are capable of increasing solar cells efficiency even in their current state. They exhibit remarkable ability to absorb UV/Vis/NIR, which makes them perfect for the task. In the solar cell, carbon nanotubes form a complex together with fullerenes. These fullerenes can trap electrons, but are not able to make them flow – that task is enabled by nanotubes. The introduction of CNTs can help in reducing energy loss and also in preventing excessive photooxidation. Nanomaterials might be precisely the way forward in photovoltaics.

CNTS as thermal materials

It is the remarkable anisotropic thermal conductivity of carbon nanotubes that makes them suitable to fill in many roles involving heat transfer. For example, they could be very useful in electronics, namely advanced computing, where the temperature of some of the chips often goes over 100°C.

There is also work in progress with the aim of creating sophisticated heat conduits consisting of complex carbon nanotube structures and ribbons, which could truly revolutionise the world of heat conductivity in terms of sheer efficiency. Besides that, even composites containing relatively small amounts of CTNs appear to have an improved thermal conductivity.

CNTs as structural materials

Among the many properties of carbon nanotubes is also their high stiffness and resistance. This obviously makes them suitable in and of themselves, but also in composites where these qualities are in demand. Commercial composites of aluminium and multi-walled nanotubes manage to reach strengths similar to that of stainless steel, while having just a third of its density!

They might, for example, prove to be useful in:

• Aerospace industry
• Automotive industry
• Ship industry
• Anticorrosive coatings
• Military uses

Their electrical conductivity is also not to be underestimated

Since the discovery and widespread use of plastics, they have, in many ways, managed to replace metal. As far as structural applications are concerned, plastics have indeed managed to push metals aside in numerous ways. Today, without a doubt, they remain the most widespread and widely used material on earth. This age will, without a doubt, enter history as the age of plastic. Yet there is an area in which metals have retained their competitive edge, and that is electrical conductivity. Plastics exhibit remarkable insulator properties, but as conductors, they simply don’t cut it. The introduction of carbon nanotubes may very well change this fact and the edge that metals did have up to this point, could be significantly narrowed.

So far, the conductive deficiency of plastics used to be solved by loading them up with conductive fillers – for example carbon black and some larger graphite fibres. However, there is an offset to this solution, since the necessary loading tends to be quite high, causing the parts to become too heavy and the plastic involved to lose much of its structural properties.

However, carbon nanotubes feature a much higher aspect ratio (in fact the highest aspect ratio of all carbon fibres), and, as a result of that, a much lower loading is required to achieve the desired conductive effect. What’s more, since they’re naturally prone to form ropes, they are capable of creating especially long conductive pathways, even at extremely low loadings.

This behaviour makes CNTs applicable in many ways.

They are to be used for example in:

• EMI/RFI shielding composites
• Enclosure coatings
• Gaskets
• Electrostatic dissipation (ESD)
• Anti-static materials
• Conductive coatings
• Radar-absorbing materials (for example military stealth materials)

Besides all that, they can also function as conductive adhesives, for example in electronics packaging and interconnection applications as adhesives, coaxial cables and similar connectors.

Use in ceramic materials

A recent research found out that a ceramic material reinforced with carbon nanotubes proved to be far more tough and damage-resistant than standard ceramics, while also being more electrically and thermally conductive. Or, conversely, depending on the nanotube orientation, they might serve the function of a thermal barrier.

Ceramic materials tend to be very hard and resistant to both heat and chemical damage. However, they are also very fragile. But there is a way around – and CNTs play an important role in it. The researchers mixed aluminium oxide with 5-10% carbon nanotubes and a 5% milled niobium. The resulting mixture has been treated with an electrical pulse in a procedure named spark-plasm sintering. This procedure enables the consolidation of ceramic powders at a much quicker rate and at lower temperatures than is normal.

And the results are spectacular – the material created in this way exhibits five times more fracture toughness than standard alumina. Its electrical conductivity, compared to the previous ceramics made with nanotubes, is more than sevenfold. Its thermal properties are also quite interesting. The material conducts heat along the alignment of the nanotubes, but also reflects it at the right angle to the nanotubes. This way, it can serve as a promising material to be used in thermal barrier coatings.

Air and water filtration could prove to be another prospective use of these materials

Among the countless uses of CNTs, one could find their application as air and water filtration devices. It’s not just the blocking of smaller particles that these are capable of, but they can also kill of most bacteria. In this particular field, CNTs have already been put to a relatively widespread private and commercial use.

Storage of energy

The characteristics of CNTs are the same as those of materials that are currently used as electrodes in batteries and capacitors. It is these technologies in particular that are in high demand today. And CNTs could become a transforming material in this area. Not only can they boast an incredibly high surface area and a good electrical conductivity, but also the optimal linear geometry to make their surface easily accessible to the electrolytes.

They have the highest reversible capacity of all carbon materials suitable to be applicable in lithium-ion batteries. Besides, they are also the perfect materials for use in supercapacitor electrodes – and the market and potential investors are already seizing this opportunity.

Among other applications of CNT in this regards is their possible use in many fuel cell components. The previously mentioned high surface area, together with a high thermal conductivity, makes them suitable to be used as electrode catalyst supports in PEM fuel cells. Furthermore, their high electrical conductivity can make them useful in gas diffusion layers or current collectors.

CNTs could also fill further role in composite components in fuel cells in transport applications. This is due to their superb strength and durability.

Carbon nanotubes are also outstanding electrical emitters

In fact, they are the best of all the field emitters know, surpassing all other materials. This comes as no big surprise – their electrical conductivity, together with the extreme sharpness of their tip, enables the formation of a highly concentrated electric field – and thus an increased field emission.

Due to their sharp tip, they also emit at quite a low voltage, which is crucial to building low-power electrical devices that make use of this feature. Besides, they are also capable of carrying very high current density – as was mentioned previously, this number can go as high as 1013 A/cm2. Plus, the current maintains a great degree of stability at all times.

This quality of CNTs has been the source of great interest, and has been utilised quite early on, namely in field emission flat-panel displays. While the traditional cathode ray tube display features only one electron gun, field emission displays have a unique electron gun (sometimes even multiple ones) for every pixel on the display.

Due to their high current density, low operating voltages and a predictable behaviour, CNTs are among the top of all potential fiend emitters for this purpose. But aside from that, they can also function as emitters in the context of low-voltage lighting sources or even in electrone microscope sources.

They also hold many promises in the field of medicine

While the proper research into carbon nanotubes and their uses has not yet kicked off properly yet, it already appears that the contribution of this material to the medical sphere will be huge. Due to a large share of human body being carbon-based, CNTs are assumed to be quite compatible with it.

Cells are capable of growing on CNTs, suggesting no toxic or harmful effect of this material. They also don’t adhere to them, which could make them applicable as prosthetics coating.

Among other biomedical application of CNTs is (or can potentially be) the following:

• Vascular stents
• Help with neuron growth and regeneration
• Insertion of DNA strands into a cell
• Biosensors
• Tissue engineering

Nanotubes and molecular electronics

The plan to build electronic circuits from molecules has been the source of renewed interest in the recent years - after all, it is one of the chief ideas of nanotechnology. The smaller the scale, the more important the proper interconnections between switches and other active devices become. In this regard, carbon nanotubes could be the perfect material, offering optimal structure, precision and electrical conductivity to serve as connections or switches.

Application as textiles and fabrics

Their properties could make carbon nanotubes a material of the future for the textile and fabrics industry. Spinning the CNTs into fibres has been already done – another option is also coating them on textile fabrics. A wide range of applications is possible, such as woven fabrics, textiles, transmission line cables, body armor, vehicle armor and more. Some experiments even managed to combine CNTs with spider silk, resulting in a silk much stronger and durable than for example Kevlar or knotted fibres. CNTs can also be used to make textiles stain resistant.

And that may be just the start…

While there are countless current applications of nanotubes - hopefully this article gave you a clearer overview of some of them - and many more hypothetical ones, we dare say the full spectrum of their potential has not been discovered yet. The following years might bring about another breakthrough we have yet to see… Will nanomaterials become as defining for the 21st century as plastics were for the 20th?

In which area do you think carbon nanotubes excel the most? How do you personally see their future? Could you think of another ways in which they could be utilised? Let us know in the comment section!