Brussels, 13 Jan 2004
Nanotubes and other nanostructured forms of carbon exhibit remarkable properties but practical applications remain difficult. CARBEN and CARDECOM have made valuable progress, resulting in specialist spin-offs.
Carbon is a remarkably versatile material that can occur in numerous forms – from mechanically hard crystalline diamond or the semi-metallic, layered crystal graphite, to amorphous powders with very large specific surface areas. As a result, it already finds a huge range of uses, including: as a decorative gemstone, industrial abrasive, lubricant, electrode material, energy storage medium, absorbent/filter and fuel source.
New forms offer remarkable properties
In the 1990s, discoveries of new forms of the element – fullerenes and nanotubes – generated great interest among the global scientific community, which envisaged still more breakthrough applications derived from their unique nanostructural make-up and properties.
Nanotubes, which are graphite sheets formed into seamless hollow tubes with diameters ranging from 1 nm upwards, exhibit the highest tensile strengths of any solid. Their electronic characteristics vary from metallic to semiconducting, according to the diameter and structural characteristics of the tube. To date, however, commercial exploitation has been limited by practical difficulties in producing the tubes consistently, cost-effectively and in sufficient quantities for industrial processing.
An interesting alternative to nanotubes is a nanostructured carbon made by assembling carbon clusters produced by supersonic beams. Its performance is comparable to that of nanotubes for many applications, while synthesis is easier and cheaper.
In the three-year CARBEN project, the partners therefore opted to focus mainly on this route for research into larger-scale manufacturing of materials, and to explore end-uses that could capitalise on its electrochemical and energy storage qualities. All were targeted at recognised societal needs and, through improved efficiencies, would also bring environmental benefits. However, given the current state of nanotechnological knowledge, all carried a relatively high degree of risk.
Leading the consortium, which completed its work in January 2003, was the University of Cambridge (UK). Other academic partners were the universities of Fribourg (CH) and Milan (IT). Representing industry were: Microcoat, an Italian SME manufacturing vacuum coating systems; Swiss power capacitors specialist Montena Components; and Regenesys (formerly National Power, and then a sub-division of UK electricity utility Innogy).
A key objective was for Microcoat to scale up the University of Milan's patented cluster-beam deposition technology, in order to produce nano-carbon with a maximised surface area for applications in electrochemistry, energy storage and electron emission.
Electrical and electronics applications
Although inexpensive electrodes made from porous carbon are already in widespread use, over 90% of their surface area is inactive because the pores are dead-ended or have diameters less than the 1 nm needed to permit electrochemical interactions. Nanotubes, on the other hand, have more controllable porosity – and, theoretically, could give a fully active surface of up to 3000 m2/g.
It was envisaged that these could be employed in super-capacitors with energy densities of at least 7 Wh/kg and power densities of 10 kW/kg, making them suitable for use in electric road vehicles and train motors.
By developing electrodes suitable for bulk storage of electricity in oxidation-reduction-reaction (redox) fuel cells, it would also be possible to bridge demand peaks and troughs without resorting to the intermittent use of less efficient generating plants – or of hydroelectric pumped storage, which is only feasible in certain geographical areas. Other proposed options – such as batteries, compressed air or superconductors – cannot provide sufficient capacity.
The third application, electronic field emission, is of potential interest for flat panel displays, microwave devices and high power switching. Most such materials require applied electrical fields of over 500 MV/m to initiate emission, but many forms of carbon emit in fields of just 10 to 20 MV/m. Moreover, the sharp geometries of nanostructures give rise to much higher current densities.
Valuable progress in several areas
In the event, CARBEN made valuable progress in several areas, although cost factors and parallel advances in competitive technologies mean the prospects for commercial exploitation are generally longer-term.
"We and Fribourg both successfully utilised chemical vapour deposition techniques to grow mats of aligned nanotubes on surfaces," says CARBEN co-ordinator Dr John Robertson, of Cambridge University. "This has provided valuable insights into the growth mechanisms, and now forms the basis for further studies."
By mid-term, Microcoat had completed a medium-sized cluster-beam deposition source, which was used to provide test samples for the other partners. Although early results obtained with thin capacitor films were disappointing, the Italian SME subsequently finalised a larger-scale source capable of delivering thick films. From this, Montena was able to produce a prototype super-capacitor that approached the power density goal, so proving the technology. Cost nevertheless remains a serious barrier.
"The electrolytic characteristics of nano-carbons proved basically unsuitable for mass power storage, so effort was switched into the development of other electrode materials, for example by incorporating metallic catalysts," Dr Robertson adds.
"Field emission tests on nanostructured films did show satisfactory performance, and we identified new ways of depositing catalyst dots on which the nanotubes are grown. Present costs are too high for their use in consumer equipment such as high-resolution computer screens, but there could be opportunities in niche markets where performance is the prime consideration."
Nanotube focus in follow up
Know-how gained during CARBEN has been shared with various Information Society Technologies (IST) projects covering related technology areas – and a follow-up initiative, CARDECOM, was launched in May 2002.
Although co-ordinated once again by the University of Cambridge, CARDECOM assembles a new consortium – including a Chinese SME – and has only a partial overlap in terms of the applications addressed.
"In this project, we are oriented more specifically towards nanotubes," Dr Robertson remarks. "The intended outputs include the technology to use the tubes in electron guns for cathode ray tube displays and scanning electron microscopes, methods for low temperature and large area growth of nanotubes, improved catalysts for fuel cell electrodes, and nanotube/polymer composites with improved mechanical and conductive properties.
"Joining us are the Max Planck Institute for Solid State Physics, Stuttgart; France's Montpellier University II; and the Centre de Recherches Scientifiques et Techniques de l'Industrie des Fabrications Métalliques(CRIF/WTCM) from Belgium. On the European industrial side, Philips in the Netherlands is building on our earlier work in field emission. Nanoledge, an SME spin-off from Montpellier University, will act as the main supplier of nanotubes to the other participants – while another SME, CCR, Germany, is developing and building a large area plasma deposition system to make nanotube arrays."
Chinese SME Yangtze Nanomaterials in Shanghai is also working with the CARDECOM project on the use of nanotubes in nanocomposites for electromagnetic shielding. Yangtze was able to participate in the FP5 GROWTH project as a result of the EU-China scientific co-operation agreement signed in the 1998 and in the framework of the implementation agreement on materials signed in 2001.
Low temperature breakthrough
At present, nanotubes are grown mainly on silicon and glass substrates, by processes involving temperatures in the region of 550ºC. One aim of CARDECOM is to lower the required temperature dramatically, to a level at which polymer substrates can be used. This would allow the manufacture of novel moulded components, such as electromagnetic screens for use in aircraft and mobile telephones. Because nanotubes have a very high length-to-diameter ratio, they form networks that make such composites conductive with minimal carbon loadings. And their exceptional physical strength lends itself to the production of tough, lightweight structural composites.
"After our first year, we succeeded in growing nanotubes of moderate quality on silicon at just 120ºC by plasma-enhanced chemical vapour deposition, which is a good start," says Dr Robertson. This is helping Nanoledge to build a strong position in what will be a vital industry for the future.
"We could also demonstrate reasonable conductivity in polymer composites with carbon loadings as low as 0.01%. And recently, we have developed a colloidal process that could lead to effective conductive coatings for environment-friendly electrostatic painting of automobiles. Fortunately, we had an existing stock of nanotubes, as production cannot keep pace with demand for experimentation. Nanoledge, for example, has a current capacity for a few grams a week of purified product."
Knowledge for the future
In the face of current competition from liquid crystal display (LCD) screens, electronics manufacturers' interest in field emission displays has waned. Philips is nevertheless continuing its studies, with an emphasis on the development of high performance sources for electron microscopes, where cost is less critical. Brightness levels 20 times higher than those of today could be reached within two years.
Philips is also pursuing the use of field emission from nanotubes in miniature x-ray generators – an application where there is no alternative competition. These generators could be used for local medical imaging, and also for detecting errors in printed circuit boards.
"The leading edge nature of this research means that it would be unrealistic to expect early commercialisation of our results in all the sectors," Dr Robertson concludes. "But with the eyes of the world on nanotechnologies as the foundation for the next industrial revolution, it is crucial for Europe to remain at the forefront of knowledge acquisition in this challenging field."