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.
CNTs and graphenes have extraordinary properties, providing
a multitude of commercial applications. This report will focus
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.
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.
CNTs are lightweight with a density of 1.3-1.4
g/cm3, just 25% that of steel.
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.
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 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
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
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
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
CNTs offer the possibility of making new low-power, low-cost
and brighter lights, which are significantly superior to light
emitting diodes (LEDs).
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
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.
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
Single DNA strand immobilized on SWCNT-COOH
Diagram courtesy of Leszek Stobinski, Head of Graphene
of Chemical and Process Engineering, Warsaw University
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
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.
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
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.
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
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.
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.
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
In aircraft wings, the conductivity of CNTs can provide
de-icing and lightning strike protection, combined with weight
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.
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.
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
Manufacture of CNTs
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
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
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.
Leading CNT manufacturers
Showa Denko KK
Hyperion Catalysis Int.
New patented process based on plasma
New patented process based on methane
1CVD = Chemical vapour deposition CCVD
= Catalytic chemical vapour
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
unlikely to become a large volume end market for
although it could offer a significant revenue stream
for some companies.
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
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
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
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
In Europe, the CNT industry is going through the process of
Registration, Evaluation, Authorisation and Restriction of
Chemicals (REACH). Some MWNTs are already registered.
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
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.
Nanowerk. Understanding Nano.com.
PR newswire: "Global Markets and Technologies for
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 &
*Frank Hart is technical director at First Test Minerals