Carbon nanotubes: The next industrial revolution?

Carbon nanotubes (CNTs) are hollow tubes made entirely of carbon. Tube diameters can be as low as one nanometer (10^-9m), while length can be hundreds of microns, giving aspect ratios above 130,000,000: 1, greater than any other substance. CNTs are unique because the bonding between the atoms makes them very strong and because they can be extremely good conductors of electricity and heat. Over the past 20 years, there has been unrelenting growth in R&D into CNTs, in both academia and industry.

There are many different types of carbon nanotubes, but they are normally categorised as either single-walled (SWNTs) or multi-walled nanotubes (MWNTs). A SWNT is a simple, hollow tube with only one layer, or wall. Individual nanotubes naturally align themselves into "ropes", held together by Van der Waals forces. MWNTs are a collection of nested tubes of continuously increasing diameters. They can range from one outer and one inner tube (a double-walled nanotube) to as many as 100 tubes or more. Many versions of MWNTs are now available, including films, yarns and surface coated tubes. Additionally, tubes can be structured to show different degrees of chirality, which is a combination of the tube’s diameter and twist. 

Until the mid-1980s, pure carbon was thought to exist in only two physical forms or allotropes; diamond and graphite. CNTs were discovered in 1991, after fullerenes were first recognised in 1985. In 2004, graphene was isolated, for the first time, as extremely thin, two-dimensional sheets with a thickness equivalent to a single atom of carbon. 

Source: dreamstime

CNT properties

CNTs and graphenes have extraordinary properties, providing a multitude of commercial applications. This report will focus on CNTs.

Electrical conductivity

Some CNTs are extremely good electrical conductors, behaving like metals, but able to conduct electricity much more effectively. When electrons travel through metal, there is resistance to movement as they collide with metal atoms. When electrons travel through CNTs, they behave like a wave travelling down a smooth channel, with no atoms to collide with. This is known as "ballistic transport" and most MWNTs behave in this way. Approximately two thirds of SWNTs are actually semi-conductors and only a third are conductors, depending on the diameter and twist of the tube.

Thermal conductivity

CNTs are very good thermal conductors along the axis of the tube, exhibiting a property known as "ballistic conduction", but are good insulators laterally to the tube axis. Ultra-small SWNTs have even been shown to exhibit superconductivity below 20 degrees K.

Density

CNTs are lightweight with a density of 1.3-1.4 g/cm³, just 25% that of steel. 

Strength

No other element within the periodic table has a lattice structure that bonds to itself with as much strength as carbon. Strength is provided by the interlocking carbon-carbon covalent bonds and the fact that each carbon nanotube is one large molecule, with no weak spots, such as the boundaries between the crystalline grains that form steel. The tensile strength of carbon nanotubes is approximately 200 times greater than that of steel of the same diameter. 

Elasticity

Nanotubes are strong but also elastic. Young’s modulus for carbon nanotubes, a measurement of how much force it takes to bend a material, is about five times higher than for steel. They are flexible to 20% without damage.

Surface area

Surface area is high, due to the small diameter of the tubes. SWNTs provide the highest surface area, with a theoretical value up to 1300m²/g.

SEM of CNTs with graphene foliates with TEM inset, deposited via microwave plasma enhanced CVD at Duke University, 2011.  B R Stoner, via Wikimedia Commons

Applications – a snapshot

Energy

Batteries

Some CNTs can conduct 1,000 times the density of electrical current, compared to copper. This has prompted a great deal of research into improving the performance and lifespan of batteries.

Graphenated CNTs, characterised by graphitic growths along the length of the tubes, have a very high surface area, low mass and high volumetric density and can behave like a supercapacitor. They have a much lower bulk than storage batteries and are capable of being charged and discharged for more than 10,000 cycles (less than 1,000 is the norm for rechargeable batteries), which promises great opportunities in the electric vehicle industry. Due to its flexibility, it can be densely packed into irregular spaces, freeing designers from the problem of finding large, flat, areas to house batteries.

Hydrogen Storage

CNTs have a significant capacity to store H₂ gas by adsorption on both their outer and inner surfaces and SWNTs have a bigger capacity than MWNTs. Most of the adsorbed gas (around 80%) is released when the containing pressure is reduced and the process is thought to be reversible. This has significant promise for the use in fuel cells in road vehicles.

Electronics

Lighting

CNTs offer the possibility of making new low-power, low-cost and brighter lights, which are significantly superior to light emitting diodes (LEDs). 

Circuit boards

Integrated circuit boards can be fabricated using micro-transistors, built with CNTs that measure only a few nanometers. Desktop computers currently use up to 500m transistors made out of silicon. Chips made using CNTs can theoretically contain billions of transistors, allowing for smaller and quicker computers.

US multinational technology corporation, IBM, announced a $3bn R&D investment fund in July 2014 to improve logic microchips. This will focus on reducing silicon-based chips from 10nm to 7nm and on new materials such as CNTs. 

Nantero, a US-based technology business specialising in CNT electronics, has raised $31.5m in a fifth round of funding for the use CNTs in memory chips, based on super-fast and dense technology which can be used in markets such as mobile computing and consumer electronics. In the future, Nantero expects to be able to store terabits of data on a single memory chip.

Display panels

CNTs can be used to direct electrons to illuminate pixels, resulting in a lightweight, millimetre thick "nano-emissive" display panel. Televisions may eventually use CNTs to create high-resolution displays, using less power than liquid crystal or plasma models. Scientists at Purdue University, US, and the University of Illinois Urbana-Champaign, US, are focusing on the flexibility of CNTs to develop very thin display units, which might be used as electronic newspapers or similar hand held, roll up devices.

Inks

Germany’s Linde Electronics GmbH & Co.KG recently announced the development of CNT ink for use in displays, sensors and other electronic devices. Potential applications include smartphones with a roll up screen and see-through GPS devices embedded in the windshield of a car. 

Single DNA strand immobilized on SWCNT-COOH surface Diagram courtesy of Leszek Stobinski, Head of Graphene Laboratory, Faculty of Chemical and Process Engineering, Warsaw University of Technology

Health

Dental implants

CNTs can be added to PMMA (poly methyl methacrylate) and other materials used for dental implants to form a structural scaffold, which improves strength and reduces shrinkage during polymerisation. There is strong competition in this application field from titanium dioxide (TiO₂) nanotubes. Nanotubes can be loaded with antibacterial substances, such as silver nanoparticles, to fight infection. 

Bone Implants

CNTs can be added to a number of substances currently used as bone implants, including PMMA, polylactic acid and aluminium, to improve mechanical strength and flexibility. The CNT scaffolds within the implants promote the growth of new bone cells by attracting hydroxyapatites. This can be further improved by coating the CNTs with collagen. Loading the tubes with antibacterial compounds, which are slow-released, can minimise the risk of infection.

Drugs delivery

CNTs have the ability to pass through cell membranes. Once the surface of the CNT is modified by the addition of a polar group, it is possible to attach drugs, genes and other molecules. Their high aspect ratio allows for a high loading capacity along the length of the tubes, without affecting cell penetration capability. 

Cancer

Chemotherapy currently affects the entire body system, causing damage to healthy tissue.

One of the aims of cancer therapy is to prevent metastasis (the spread of cancer cells) and improve the lack of selectivity of anti-cancer drugs. CNTs can provide a direct-delivery method, maximising the effect of chemotherapy on tumours while minimising harm to healthy tissue. Lab tests of nanotubes bound to an antibody produced by chickens have been shown to be useful in destroying breast cancer tumours. The antibody-carrying nanotubes are attracted to proteins produced by one type of breast cancer cell. Once attached to these cells, the nanotubes absorb light from an infrared laser, incinerating the nanotubes and the attached tumour.

Researchers at the University of Connecticut have developed a sensor that uses nanotubes and gold nanoparticles to detect proteins that indicate the presence of oral cancer. Tests have shown this sensor to be accurate and it provides results in less than an hour.

Synthetic muscles

Scientists at the University of Texas at Dallas (UTD) are developing "nano-muscles", composed of thin aerogel sheets of CNTs. Applying a small positive charge onto the sheets, results in a repulsive force from existing positive charges, activating the muscles. High strength (30 times stronger than natural muscles) and flexibility means that the muscles can operate at extreme temperatures, making them especially attractive for space applications. This technology is receiving attention as a possible means for endowing soldiers with superhuman strength through the use of exoskeletons.

TEM of MWNTs, courtesy of the Nanoalaytical platform at Monash University, Malaysia Pooria Pasbakhsh

Environment

Oil Absorption

CNTs are being developed to clean up oil spills. Researchers have found that adding boron atoms during the growth of carbon nanotubes causes them to grow into a sponge-like material that can absorb many times its weight in oil. These nanotube sponges are made to be magnetic, simplifying recovery once filled with oil.

Desalination and water purification

CNTs can be used as pores in membranes to run reverse osmosis desalination plants. Water molecules pass through the smoother walls of CNTs more easily than through other types of nanopores, requiring less power. Other researchers are using CNTs to develop small, inexpensive water purification devices needed in developing countries. An inexpensive nanotube-based sensor can detect bacteria in drinking water. 

Food quality

Scientists at the Massachusetts Institute of Technology (MIT) have developed a sensor based on modified CNTs that can detect amines released by decaying meat. This may help to prevent food poisoning and reduce food wastage.

Vehicles

Adding CNTs to polymers makes them lighter and stronger, properties which, if imparted to vehicle components, allow for more efficient fuel consumption and improved safety.

Very minor additions of CNTs to a polymer matrix can make non-conductive polymers conductive, reducing problems with static electricity that may result in a fire.  They can replace carbon black in tyres to improve wear properties

Aircraft

In aircraft wings, the conductivity of CNTs can provide de-icing and lightning strike protection, combined with weight reduction. 

Sport

CNTs are already used in sport applications such as skis, tennis rackets, baseball bats, hockey sticks, fishing rods and golf shafts, where they provide lighter weight but higher strength and flexibility.

Cleanrooms

MWNTs have been used in polycarbonate and other polymers to make cleanrooms for the production of computer chips and hard drives, because they dissipate static electricity and, therefore, will not attract airborne contaminants.

Concrete

According to Eden Energy Ltd in Perth, Australia, CNT-enriched concrete should significantly reduce the quantity of concrete required for structures and reduce, or even perhaps eliminate, the need for reinforcing steel. This will provide cheaper, lighter and stronger structures with more flexible designs. 

Manufacture of CNTs

Arc discharge

A carbon rod (usually made of graphite) is subjected to a very high voltage, which is then discharged, rapidly heating the rod and vapourising some of the carbon. As the carbon vapour cools, CNTs are produced. This method generally involves the use of a vacuum chamber and an inert gas supply and gives a yield of up to 30% by weight, producing both SWNTs and MWNTs.

Laser ablation

A powerful laser is used to rapidly heat carbon (usually graphite) and vapourise it. This procedure benefits from the ability to precisely control temperature and pulse times, thus allowing for strict control over the parameters of the CNTs. 70% of the carbon can be turned into CNTs.

Chemical vapour deposition (CVD)

CVD is the most widely used method to produce CNTs. The procedure involves injecting hydrocarbon compounds (commonly methane or ethane) into a high temperature zone in a furnace. The hot zone contains a substrate with a nano-scale surface film of iron, nickel or cobalt, which catalyses CNT growth. The size of the metal particles determines the diameters of the nanotubes. 

Global production

There are currently more than 100 CNT manufacturing companies, with the majority of the market share held by a few large-scale manufacturers (see table).

Potential for graphite companies

The preferred manufacturing route for CNTs is CVD, which does not require natural graphite as a raw material. High quality graphene can be made by exfoliating natural graphite, the higher the purity the better, but even graphene will not have a major impact on global graphite production. For most commercial applications, small quantities of CNTs or graphene go a long way, due to their high surface area and high performance. Only a small fraction of the graphite produced in any mine is likely to be used to make CNTs or graphene.

Some companies have, however, invested in the use of graphite for CNTs and graphene, including Australia’s Western Mining Network Ltd, which last year signed a memorandum of understanding to acquire up to 51% of South Korean company, Carbon Nano-Material Technology Co. Ltd, which holds patents for manufacturing CNTs and graphene. In North America, American Graphite Technologies, which is investigating graphite deposits in Quebec, Canada, has partnered with CheapTubes for the same purpose.

Global market

The CNT market is currently limited, due to inconsistent quality and high prices. These issues will be resolved through research and improvements to manufacturing during the next few years. As production volumes increase, prices will come down. Governments in China, Japan, the US and India have increased funding for nanotechnology, which is expected to significantly benefit CNTs. 

Graphene is unlikely to become a large volume end market for natural graphite, although it could offer a significant revenue stream for some companies.  Source: dreamstime

Prices for CNTs vary considerably. SWNTs are more expensive than MWNTs at typically $50/g and $1/g, respectively, while grades which have been modified by the addition of other molecules (i.e., functionalised) or encapsulated, may cost more than $2,000/g. Ten years ago, MWNTs cost closer to $40/g, giving an indication of the substantial progress already made in production techniques. 

The global CNT market is expected to reach $3.42bn by 2022, according to a recent study by Grand View Research Inc., while projections by Future Markets put the figure at $2.3bn by 2020. Global CNT market demand stood at 5,064.1 tonnes in 2014 and is anticipated to exceed 20,000 tonnes by 2022, growing at a compound annual growth rate of over 18% between 2015 and 2020.

According to technology portal, Nanowerk, plastics and composites, with sales of $472.9m in 2010, represented 69% of the CNT market, with electronics accounting for 10% and energy 8%. 

The electronics and data storage market is likely to grow significantly from 2016, particularly in China, South Korea, Singapore, Taiwan and Japan. The energy sector will also witness rapid growth, owing to enhanced performance requirements for batteries, wind turbine blades and photovoltaic cells.

Health and ecological concerns 

In the UK, the occupational use of nanomaterials is regulated under the Control of Substances Hazardous to Health Regulations (COSHH) 2002. The Health and Safety Executive has published a document, entitled "Risk management of carbon nanotubes", which warns that inhaling CNTs can cause lung inflammation and fibrosis. 

In the US, the National Institute for Occupational Safety and Health (NIOSH) published Bulletin 65 in 2013 warning about the potential hazards CNTs might pose, including respiratory problems.

In Europe, the CNT industry is going through the process of Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH). Some MWNTs are already registered.

Conclusions

There seems little doubt that many features of our lives could be changed by the use of CNTs in the near future. However, there are a number of technical problems that need to be solved, including precise chirality (twist) of the tubes, which is crucial in the production of semi-conducting versus conducting SWNTs.

In many cases, the price of CNTs is simply too high and work must be done to enable cheaper mass production. 

Further, some potential end users are waiting for verification that CNTs are safe to use through registration to REACH or other regulatory bodies.

Acknowledgements

Nanowerk. Understanding Nano.com.  PR newswire: "Global Markets and Technologies for Carbon Nanotubes"

Tom Grace: "An introduction to carbon nanotubes". Nano Science Instruments Inc. Tibi Puiu, ZME Science

Journal of Nanomedicine & Nanotechnology.  Dr. Bojan Boskovic, Cambridge Nanomaterials Technology Ltd.

Michael De Volder, Phys.org. "Carbon Nanotubes" Chris Scoville, Robin Cole, Jason Hogg, Omar Farooque & Archie Russell 

*Frank Hart is technical director at First Test Minerals Ltd