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Like most new materials, carbon nanotubes are expensive (costs range from $50/g and up), occasionally difficult to obtain, and frequently require further processing by the purchaser. At first glance, nanotubes are often considered to be too expensive to use in nearly all applications, since their cost is several times greater than gold. For a number of nanotube applications, such as composites, friction materials, and batteries, the high cost of nanotubes precludes even most development work. However, the high cost of nanotubes is something of a red herring—like most new materials, production volumes are very low, which makes production of these goods very labor intensive. If applications were found that required significant volumes of nanotubes, price reductions of at least a factor of five or larger would be possible.
Types of NanotubesTwo major types of nanotubes have been produced: single-wall and multi-wall. Single-wall nanotubes consist of one layer of carbon atoms, similar in structure to a graphene sheet, which is formed into a cylinder. Multi-wall nanotubes consist of several layers of graphene sheets rolled into a cylinder.
Single-wall nanotubes have been more extensively characterized than the multi-wall type. Furthermore, the electronic structure of these tubes has a larger semi-conductor band gap and is more amenable to theoretical understanding. Both single- and multi-wall nanotubes have high tensile strength and modulus.
Manufacturing MethodsIf carbon nanotubes are going to be used in a wide variety of applications, then the manufacturing cost of these materials must decrease. Currently, three methods of manufacturing nanotubes exist: arc discharge, laser ablation and chemical vapor deposition. All of these methods require evacuated chambers with laminar flow.
Nanotubes can be produced by flowing a precursor gas through a plasma discharge at very high temperature. This arc discharge technology produces high-quality nanotubes, but they are often accompanied by a large volume (up to 50%) of contaminants.
Laser ablation produces nanotubes by directing a high-energy laser at a carbon target. This technology produces high quality nanotubes with fewer contaminants than arc discharge, but the production rate remains low. Furthermore, this production technology is capital-intensive and is likely limited to research quantities.
Chemical vapor deposition produces nanotubes by heating a precursor gas and flowing the gas over a reactive metal surface. This technology can produce nanotubes in good yield—occasionally over 90%—with few contaminants. However, the tubes produced have a large number of defects—places where atoms and bonds are missing. This can be an advantage or disadvantage, depending on the application, since defects can also be viewed as reactive sites on the molecule.
Currently, all nanotube manufacturing methodologies produce nanotubes with a high degree of variability in their physical and electronic properties. Even relatively gross physical properties of nanotubes, such as length and diameter, can be difficult to control.
Overall, nanotube formation is still not a well-understood process, although one key factor is laminar flow in the reactor. Consequently, most nanotube reactors are limited to 4 in. in diameter to maintain laminar flow. Increasing production volumes of nanotubes requires more reactors, rather than larger reactors.
The two production processes that create nanotubes with relatively few defects, laser ablation and arc discharge, also produce significant amounts of undesired products such as carbon black and amorphous carbon. Since the overall yield of purified nanotubes is very low from the purification process, there is a dramatic increase in price (about 1000x greater) for purified nanotubes.
Nanotube ProducersNanotube producers can be divided into two categories: captive producers and open producers.
Captive Producers. Captive manufacturers of nanotubes produce nanotubes for internal use. Some captive producers of nanotubes also buy nanotubes on the open market.
Captive producers of nanotubes include academic, government and corporate laboratories. A number of scientists have decided to try producing their own nanotubes for a research project for a variety of reasons. It has been estimated that there are approximately two dozen captive producers of nanotubes worldwide. These producers can include large multinational firms, such as Honeywell or Electrovac, to individual researchers in a laboratory. Often, nanotube researchers will form clusters and organize resources to both produce and characterize nanotubes.
Open Producers. In contrast to captive producers, open producers of nanotubes sell their goods in the marketplace. Five firms currently sell nanotubes: Carbolex, (Kentucky), NanoLab (Massachusetts), Carbon Nanotechnologies, Inc. (Texas), MER (Arizona) and Dynamic Labs (UK). Nanotubes produced using all production methodologies are available. Carbon Nanotechnologies uses laser ablation and HiPCO (chemical vapor deposition); Carbolex uses arc discharge; and NanoLab uses chemical vapor deposition. Both single- and multi-wall nanotubes are also available. Carbon Nanotechnologies and Carbolex produce single-wall nanotubes, while the other firms produce either multi-wall or both single- and multi-wall nanotubes.
The current largest market for nanotubes is as a research material, and several of these firms intend to concentrate on that market, notably Carbon Nanotechnologies and Dynamic Labs. Even firms that are working on expanding the commercial applications of nanotubes, including Carbolex, NanoLab and Carbon Nanotechnologies (working on both research and commercial applications), report that the bulk of their sales currently are for research purposes.
Expanding ApplicationsBoth short-term (within five years) and long-term (10 years plus) applications exist for nanotubes. Many of the long-term applications (such as the highly touted uses in electronic circuits) require the ability to manufacture nanotubes with a specific structure, which is currently not technologically feasible.
Since nanotubes are primarily used as research materials today, the market for these goods is rather small. It has been estimated that the total worldwide production of nanotubes is between 4 and 10 kg, with a value of less than $5 million. However, additional applications may be successful within five years. These applications include microscope probe tips (already commercially available), field emission devices (including displays) and some membrane applications. All of these applications are commercially possible even with the current pricing structure of nanotubes. Overall, the market for applications requiring nanotubes could exceed $500 million in five years. Should the price of nanotubes decrease further in the short term, other nanotube applications involving energy storage devices—i.e., fuel cells, batteries and capacitors—may become feasible.
Longer-term markets for nanotubes are very difficult to estimate at this time since they rely heavily on the pace of technological development and require some breakthroughs in nanotube production technology. If nanotubes can be successfully used to make electronic circuits, which would eventually displace silicon, then the potential market for these circuits is enormous. However, such developments are at least a decade away.