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Monday, 23 January 2012

Carbon Nanotubes Chart Their Own Course

R&D Magazine
Oct 18, 2011

Optical image of a completed graphene integrated circuit (IC) including contact pads. The probes for testing the circuit (P1-P4) are also shown. The scale bar is 100 µm. Credit: IBM

Not since in the development of carbon fiber in the 1960s has a material stimulated so much research interest as carbon nanotubes. Presaged by the designs of Buckminster Fuller—who lent his name to the C-60 atom, or fullerene—the first glimpse of naturally occurring carbon nanotubes in the early 1990s by James Tour's laboratory at Rice University quickly generated the excitement of materials scientists worldwide.

Unlike most materials inventions, which have been developed for specific applications within a given industry (a new alloy grade for metals processing, for example), the carbon nanotube was quickly hailed as a potential replacement or complement for a wide range of well-established commodity products, from the alloys used to strengthen steel to the copper used to carry electric circuit.

An allotrope of carbon, and closely related to graphite, carbon nanotubes feature simple carbon-carbon bonds of varying length, and as a consequence have a remarkably high modulus of tensile strength. The conductivity of this form of carbon makes the nanotube a nearly perfect conductor of electricity when produced in a pure form. More importantly, the variability of the form allows a band gap, an energy range where no electrons can exist.

Prompted by such extraordinary performance, eight U.S. federal agencies formed the National Nanotechnology Initiative, which has grown to more than $10 billion in research and business development funding.

Unlocking the true power of the nanotube
As with any nanostructured material, the challenge in producing carbon nanotubes and its related forms depends on understanding the manner in which nanoscale building blocks and processes integrate and assemble into larger systems and how these processes can be precisely controlled to achieve predictable products.

Engineers are now learning how to design nanomaterials that can seamlessly and functionally integrate with tissues of the body to perform biological functions. They are wrestling with the development of "top-down" and "bottom-up" engineering approaches to control properties. This is a critical step prior to the development of nanodevices. Finally, analytical instruments and techniques must be adapted to allow precise characterization.

In their purest form, nanotubes are of the single-walled variety. Early on, developers produced multi-level tubes, also known as double- or multi-walled nanotubes. Easier to produce, this type of nanotube was billed as a bulk additive and dominated production levels for early nanotube materials.

Multi-walled nanotubes (MWNTs) consist of multiple rolled layers (concentric tubes) of graphite.

Restrictions on the relative diameters of the individual tubes typically means that MWNTs are zero-gap materials. They are typically used for bulk or composite applications, and are measured by percent/weight. The amount dictates how much a given amount of nanotubes is worth. For MWNTs, of which an estimated 3,400 tons were produced in 2010, this is often just 10 cents or less per gram. This low cost has led to some experimentation in combined bulk MWNTs with other types materials, particularly lightweight metals in aircraft.

Single-walled carbon nanotubes (SWNTs) are much more difficult to produce. Their unique properties are the direct result of a distinctive structure composed of carbon-carbon bonds more closely related to those in graphite than to those in diamond. Diamond has four-coordinated carbon, featuring an sp3 hybridization. Graphite involves three-coordinated carbons, in which three electrons are in sp2 hybridization and one is delocalized.

Fullerenes and nanotubes also have carbon bonds with sp2 hybridization such as graphite, but unlike the graphite structure, which is made up of flat planar honeycomb, the structures of fullerenes and nanotubes involve a high degree of curvature.

Single-wall carbon nanotubes exhibit unique properties due to their unusual structure. They consist of a hollow cylinder of carbon approximately 1 nm in diameter, up to 1,000 times as long as it is wide. This structure has remarkable optical and electronic properties, tremendous strength and flexibility, and high thermal and chemical stability. As a result, carbon nanotubes are expected to have dramatic impact on several industries, including displays, electronics, health care, and composites.

In recent years, other forms of carbon nanotubes have been researched, some of them boasting strange or substantial increases in certain properties. The nanotorus, for example, is a carbon nanotube bent into a ring, causing a large increase in its potential magnetic moment. Fullerenes have been attached to carbon nanotubes to form bud-like structures that show strong field-emissive properties.

Graphene is perhaps the most significant new R&D target for nanostructured carbon research. This single-atom thick layer of carbon has been celebrated for properties that resemble those of carbon nanotubes.

Research in graphene is growing and attracting investment, but carbon nanotubes have had a head start in the marketplace.

Undoubtedly, graphene will show its clout in the marketplace in the coming years. For now it has severely limited practical applications.  
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