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Carbon
nanotubes offer a range of attractive properties and could enhance many coatings
applications. However, the current barriers to mass-market use of these
materials include the availability of high-quality materials in commercial
quantities. Now a partnership between industry and academia has resulted in a
commercial manufacturing process for carbon nanotubes.
Industry Applications for Carbon
Nanotubes
Hardly a
day goes by without someone suggesting a new application for carbon nanotubes.
Since their discovery, academics and industrialists have researched or
considered their use in brake discs; fuel cells; advanced aerospace composites;
co-axial cable; conductive fuel lines; EMI (Electromagnetic Interference)
shielding in electronic
devices; conductive tyres; conductive inks; and, of course, the hotly disputed
space elevator – to name but a few. While some of these applications are clearly
closer to reality than others, possibly the most immediate products will be
developed in the coatings industry. The launch in April 2004 of the UK’s first
commercial manufacturing process for high-purity single-wall carbon nanotubes at
Thomas Swan & Co. Ltd., has cleared the way for the development of a wide
range of advanced coatings applications.
Composition and Classification of Carbon Nanotubes - Single-Wall
(SWNT) and Multi-Wall (MWNT)
For those
who have not come across them before, carbon nanotubes consist of molecular
cylinders of pure, hexagonally-arranged carbon atoms that resemble rolled-up
sheets of chicken wire with a diameter measured in a few nanometres (1 nanometre
is 1 billionth of a metre) and a length of many microns. They occur in two main
types, the single-wall carbon nanotube (SWNT) composed of a single cylinder of
carbon (Figure 1), and the multi-wall version (MWNT) (Figure 2) consisting of
concentric tubes or cylinders of carbon (effectively straws within straws). The
ends of the tubes are usually closed off by a carbon end-cap. Other variations
on this theme include the double-wall tube, ‘herringbone’ and ‘bamboo’
structures.
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Figure 1. Graphic depicting a single-wall carbon
nanotube. |
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Figure 2. Graphic depicting a multi-wall carbon
nanotube. |
Strength and Properties of Carbon
Nanotubes
Since their
discovery, and Sumio Iijima’s watershed paper in 1991, there has been
unprecedented academic and industrial interest in carbon nanotubes and their
potential use in a wide range of commercial applications. There is no denying
that the basic properties of both single and multi-wall carbon nanotubes are
truly remarkable. Recent studies have suggested that single-wall carbon
nanotubes have a tensile strength of 50-100 GPa and a modulus of 1-2 TPa. This
places them well ahead of steel in both strength and modulus and all for one
sixth the weight. Combine this with their high thermal and electrical
conductivity and you begin to understand why so many scientists and commercial
organisations have sat up and taken notice.
Properties of the Individual Carbon
Nanotube
The basic
properties of the individual carbon nanotube (both single and multi-wall) have
been considered in some fascinating potential applications. In essence carbon
nanotubes are advanced fillers - highly conductive (thermally and electrically),
extremely strong and light-weight. Add to this their apparent ability to absorb
electro-magnetic interference and the fascinating photo-acoustic effect (whereby
they spontaneously ignite when exposed to an intense light source) and you are
left with some intriguing possibilities for advanced coatings.
Electrical Conductivity of Carbon
Nanotubes
The
addition of low concentrations of carbon nanotubes to plastic it makes it
electrically conductive. This allows electrostatic painting to be used when
coating automotive parts, removing the need for costly primers.
Benefits of Modest Loadings of
Single-Wall Carbon Nanotubes
The high
aspect ratio of carbon nanotubes (length divided by diameter) means that they
form a percolating matrix across the non-conductive filler at much lower
loadings than traditional fillers. Carbon black typically requires 30 or 40% by
volume loading to confer conductivity, at which point the mechanical properties
of the composite are often severely degraded. In comparison
Cambridge University recently achieved a conductive
epoxy using only 0.005 wt% loading of aligned multi-wall carbon nanotubes. It is
also likely that modest loadings of single-wall carbon nanotubes will actually
enhance the structural properties of the composite.
Anti-Static Properties of Carbon
Nanotubes May Be Used in Packaging for Electronics
The
anti-static properties of carbon nanotube coatings may also have potential
applications in packaging. Anti-static coatings are used in the electronics to
prevent damage to sensitive electronic components during shipping and storage.
Thermal Management Potential of
Carbon Nanotubes
Carbon
nanotubes (in their pure and undamaged form) are possibly the best thermally
conductive material ever discovered. As such they are being researched for their
heat management potential in a number of applications. As the electronics world
strives to fit more and more functionality into less and less space, overheating
is becoming a major problem. This is forcing greater pressure on materials and
coatings involved in electronics manufacturing to be thermally conductive in an
attempt to remove the heat from the device as quickly as possible.
Low Loadings of Carbon Nanotubes
Should Speed Up Curing Times
Another
aspect of thermal management is the demand for rapid curing times of polymeric
materials such as ink resins and elastomers. The use of low loadings of carbon
nanotubes should increase the speed of thermal dissipation from a coating or
resin allowing for a much faster curing time. This would clearly be of interest
in automated processes where the speed of curing limits the rate of production.
Electrostatic Dissipation Properties
of Carbon Nanotube Coatings
Electro-magnetic interference (EMI) shielding in laptops and mobile
phones is becoming increasingly important to preventing interference with and
from other portable electronic devices. As an extension to the electrostatic
dissipation properties of carbon nanotube coatings, a number of companies are
developing coatings that are designed to absorb EMI.
Using Carbon Nanotubes in Conductive
Inks for Spray-On Circuits and Coatings
A
significant amount of research is being directed into the use of carbon
nanotubes in conductive inks. The ability to spray on conductive ink opens up
some intriguing possibilities for ‘spray-on’ circuits and coatings. A leader in
this field is US-based Eikos which was awarded USD 860,000 in May this year from
the US Air Force Research Laboratory to develop transparent conductive polymers
specifically for military aircraft canopies.
While such
contracts indicate that the carbon nanotube market is already up and running in
some niche applications, there are still some major barriers to
overcome.
The Three Main Restrictions in the
Market-Place for Carbon Nanotubes
While there
are already some products in the market-place and a whole range of applications
in development, the carbon nanotube market has been constrained by three main
issues:
- A lack of
commercially-available material of consistently high
quality.
- Cost -
until recently single-wall carbon nanotubes were selling for EUR 400/g which is
clearly some way from commercial viability as a bulk
material.
- Successfully expressing the basic properties of the raw material in
the end application – the incredibly small scale of the material poses some
interesting challenges for advanced material and coatings
science.
Such
teething problems are not new. Carbon fibres took many years to be widely
accepted in the materials world from both a cost and performance aspect and have
only just made a regular appearance in popular sporting markets such as golf
equipment.
Early Applications of Carbon
Nanotubes now on the Market
Despite
this, some early applications of carbon nanotubes in products such as conductive
fuel lines in the automotive industry have already made a successful appearance
on the market. In addition, recent advances in the manufacture of carbon
nanotubes on an industrial scale will allow vital commercial research and
evaluation to be conducted and this will open the way to a wide range of fully
industrialised applications.
The
UK’s First Production Plant for
Single-Wall Carbon Nanotubes (SWNTs)
The
UK’s first commercial manufacturing
process for high-purity single-wall carbon nanotubes was announced by Thomas
Swan & Co in April 2004. The plant is the result of four years of
collaboration between Thomas Swan and Cambridge University’s Department of Material Science
and Department of Chemistry. The unique collaboration brought the academic
expertise of Cambridge University together with over 77 years of
chemical manufacturing experience from Thomas Swan.
Problems Encountered When Setting Up
this Plant for Carbon Nanotubes
The goal of
the project was to supply high-purity single and multi-wall carbon nanotubes in
commercial quantities and to back up this initial offering with the ability to
scale up the process to keep up with demand as the industrial market expanded.
The problem was that at the start of the programme no-one knew which production
method would supply both single and multi-wall tubes and be fully scalable. It
was this problem that was presented to Cambridge University.
Different Methods Used for
Manufacturing Carbon Nanotubes
Carbon
nanotubes can be manufactured using a variety of methods:
- Laser ablation uses a high-power laser to
vaporise a graphite source loaded with a metal catalyst. The carbon in the
graphite reforms as predominantly single-wall nanotubes on the metal catalyst
particles.
- Arc
discharge involves an electrical discharge from a carbon-based electrode in a
suitable atmosphere to produce both single and multi-wall tubes of high quality
but in low quantities.
- Chemical
vapour deposition (CVD) is where a hydrocarbon feedstock is reacted with a
suitable metal-based catalyst in a hot furnace to ‘grow’ nanotubes which are
subsequently removed from the substrate and catalyst by a simple acid
wash.
Using a System Based on Chemical
Vapour Deposition (CVD) Growth
The team at
Cambridge developed a novel system based on
CVD growth, as it enabled the production of both single and multi-wall nanotubes
of reasonably high quality and consistency while offering the greatest potential
for scale up. This concept was then taken to design stage (and patented by
Cambridge University). Using a team of chemical
engineers and an independent consultant, the plant was built at Swan’s site in
North East England towards the end of 2003 (Figure 3). Over the following months
the plant was commissioned and then optimised until the material that was being
produced was of consistently high quality.
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Figure 3. Photo showing the Swan nanotube plant in Consett,
County Durham. |
Benefits of Having Access to a
Regular Supply of High-Purity and Scaleable Material
A
consistent supply of high-purity material that is also scaleable is a major
stepping stone for the developing carbon nanotubes market. It allows companies
to conduct meaningful industrial research using commercial quantities of raw
material. In addition, this research can now be done with the knowledge that,
should an application prove itself, there is a scalable source of the same raw
material available to supply any ongoing development. Combine this with lowering
prices (Thomas Swan predicts that within a few years the price of single-wall
carbon nanotubes with be measured in tens of GBP per kg) and the market should
start to take off.
Problems in Seeing What has been
Manufactured
A major
problem in achieving a saleable product that is on average 100,000 times smaller
than a human hair is that it is rather hard to ‘see’ what you have made. The
advent of nanomaterial manufacturing brings to light the necessity for advanced
industrial and cost-effective analytical techniques. While Scanning Electron
Microscopes (SEM) provide the ability to view the sample in remarkable detail,
it only shows a minute fraction of the product (Figure 4). The analogy often
used is like looking at a football pitch through a jam jar. The grass beneath
you is green and healthy but you have no idea what it is like on the other side
of the pitch.
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Figure 4. Scanning Electron Micrograph (SEM) of Elicarb SW high-purity
single-wall carbon nanotubes. |
Tools and Techniques Used to Help
Manufacturers See what they are Making
This will
not be an easy problem to solve and while RAMAN techniques (which involve
studying the spectrum profile of a reflected laser from your sample) offer some
hope, it will be some time before a standard industry protocol for purity
measurement is identified and accepted. Thomas Swan currently bases purity
measurement on a combination of SEM, (Transmission Electron Microscopy) TEM, RAMAN and Thermal Gravimetry Analysis
(TGA) techniques,
combined with an experienced technical eye and has so far received very
encouraging feedback from early customers. As the company moves to scale the
process up, ongoing customer feedback and collaboration will become increasingly
vital.
Prices of Carbon Nanotubes Will Fall
as Demand Increases
Carbon
nanotubes are still in the very early stages of industrial development. While
they are still expensive compared to a fully commoditised product, such as
carbon black, the price will fall as the demand increases. Despite the problems
associated with analysis and downstream processing (namely dispersion) the basic
properties of the raw material offer a wide range of exciting applications that
are beginning to be realised.
What the Next Ten Years Might Hold
for Producers of Carbon Nanotubes
As the
research into these applications moves out of the lab and into industry it is
important that the supply of carbon nanotubes is consistent, of high quality and
performed under the appropriate regulatory and health and safety protocols.
Thomas Swan has taken the first step towards this goal and material is now
available to allow meaningful industrial evaluation and development. The next
ten years of development should see the creation of a whole new area of material
and coatings science and associated commercial applications.
Note: A complete list of references can be
found by referring to the original text. |