Carbon fiber and carbon nanotubes

Where Found

Produced in industrialized nations, carbon fiber and related substances are made from plastics and materials derived from fossil fuels, such as (in the form of petroleum pitch) and coal. Efforts have been made to reclaim carbon-rich waste material from ash ponds produced by industrial plants.

Primary Uses

First used commercially in aircraft engines manufactured in England during the 1960s, carbon fiber—which appears most frequently as a key, strength-providing component in composites with plastic—is used for vehicle parts, safety products, sports equipment, construction, and many other applications. Increasingly, smaller variants, including vapor-grown carbon fibers and carbon nanotubes, have become available, leading to further diverse applications such as microcircuitry and nano-engineering.

Technical Definition

Carbon fibers are composed primarily of bonded carbon atoms that form long crystals aligned along their lengths. The atoms are bonded in hexagonal patterns, and, in both carbon fibers and carbon nanotubes, the grids formed by these bonded atoms wrap around to form the walls of long tubes. In carbon fibers, these tubes are wound together to form extremely thin strands, less than 0.010 millimeter thick. For common applications, thousands of fibers are twisted together into yarn, which is then combined with other materials to make composites. Carbon nanotubes are found at the molecular level, and because they are less than 2 nanometers thick, are exponentially smaller than carbon fibers.

Description, Distribution, and Forms

While the variants of carbon fiber share general properties such as strength, resistance to static and corrosion, and heat and electrical conductivity, specific qualities are associated with the materials from which the fiber is made. Fiber made from coal pitch is generally good at conducting heat, but relatively brittle, while fiber made from polyacrylonitrile (PAN) can handle more tension without breaking. Fiber made from petroleum pitch is flexible but cannot stand as much pressure.

Carbon nanotubes are molecules that can be either multi-walled or single-walled, and both forms consist of sheets of bonded carbon atoms. They are closely related to buckyballs, which are the spherical forms of the fullerene molecule.

History

Inventor Thomas Edison, in conjunction with his work on the lightbulb in the late 1800s, carbonized cotton and bamboo to make filaments for the bulbs. During World War II, contractors for the U.S. military gained experience in the use of fiber-reinforced composites in the manufacture of light-but-strong and corrosion-resistant aircraft, boats, and other vehicles. Although fiberglass was the most common material at this time, production techniques were similar to those that would be used for carbon-fiber composites.

The basic technique of heating polymers to make carbon fibers was established in the late 1950s by Roger Bacon, working at Union Carbide’s Parma Technical Center near Cleveland, Ohio. While the earliest manufacture of carbon fibers was achieved by carbonizing rayon, the advantages of PAN as a precursor were soon discovered. In 1961, Akio Shindo of the Government Industrial Research Institute in Japan published findings that the use of PAN as a precursor yielded the strongest fibers. Although observations of the structures had also been made independently by others, Sumio Iijima of Japan, who published his findings in 1991, is credited with disseminating knowledge about carbon nanotubes to the global community.

Obtaining Carbon Fiber and Carbon Nanotubes

Carbon fibers are processed from their precursors (PAN, rayon, pitch, and petroleum) using intense heat, often with the aid of catalytic chemicals. Usually, the precursors are first spun or extruded into fibers, which are then treated with chemicals so that they can be heated (between 1,000° and 3,000° Celsius), thus carbonizing the material. Both the raw materials used and the processes of production influence the properties of the resultant fibers. PAN, which is also used to make acrylic clothing, sails, and other products, is the most popular precursor material. Originally produced in England, Japan, and the United States, the fibers and their composite materials are now produced in most industrialized countries. As the desired sizes become smaller, isolating the particles and controlling the processes become increasingly difficult. In order to be seen and studied, carbon nanotubes require electron microscopes. Vapor-grown carbon fibers also require high heat and a catalytic vapor. Nanotubes can be obtained through laser ablation and arc discharge as well as with vapor deposition. Cost-effective mass-production techniques remain in development for carbon nanotubes.

Uses of Carbon Fiber

In England during the 1960s, Rolls-Royce and two other companies utilized processing research done by the British government and established carbon fiber production, primarily focusing on making blades for jet engines. Although many useful techniques were learned, the pioneering enterprise was not successful economically. In 1971, the Toray company in Japan began making large volumes of PAN-derived carbon-fiber yarn, which was used in many products. When the Cold War ended, emphasis shifted from military to commercial uses. However, defense applications continued to evolve, eventually to include parts for remote-controlled and stealth aircraft. In addition to the aircraft industry—which welcomed the weight-reduction advantages of carbon fiber-reinforced materials, which came in response to rising oil prices in the 1970s—carbon fiber started to appear in sports equipment such as golf-club shafts and fishing rods. Union Carbide, sometimes working with Toray, continued to develop PAN-based products.

Because of the ability of carbon fiber epoxy composites to withstand extreme conditions, both government agencies and private companies used them extensively in space vehicles and apparatuses. Carbon fiber can also conduct electricity and has been used in the construction of electrodes and many kinds of batteries and fuel cells. Because oxidized PAN fiber is fire-resistant, it has been used in protective clothing for firefighters and industry workers, for insulating cables, and as a safety measure to insulate flammable seat cushions in airplanes and other vehicles. Activated carbon fibers are useful in the design of many kinds of air filters, with applications ranging from poison chemical absorption to odor control.

In customized high-performance vehicles, cost is not a major concern, and racing bikes, cars, motorbikes, and boats have frequently used carbon fiber-reinforced materials. Graham Hawkes Ocean Technologies has developed carbon fiber electric submersible vehicles capable of diving to depths as great as 122 meters. In the field of music, these materials have made innovative new designs possible for guitars, cellos, and other stringed instruments and more durable classical guitar strings. Carbon fiber composite materials are used in medical and veterinary prosthesis products, including artificial limbs, and are also used in X-ray tables and other equipment.

In constructing support frames for concrete, the corrosion-resistant properties and relatively light weight of carbon fiber-reinforced material make it an attractive replacement for steel and welded wire. It has been used for repairing bridges, especially in England. Over time, the replacement of metal in so many industries could have a long-range impact on reductions in global resource consumption, not only of metals but also of fossil fuels, as a result of significantly lighter vehicles.

Uses of Carbon Nanotubes

While less commercially established than carbon fiber, carbon nanotubes are even stronger and are unmatched by any other substance in terms of strength-to-weight. Like carbon fiber, they can conduct electricity and heat, and their strength, conductivity, and microscopic dimensions make them ideal candidates for applications in nanotechnology.

Carbon nanotubes are used to provide greater strength in composites with carbon fibers, as well as other materials, with applications in many of the same areas as carbon fibers. Nanotechnology research focuses on medical uses of carbon nanotubes, because the nanotubes have the potential to work on the cellular level, delivering medicine and targeting cancer cells with heat. Carbon nanotubes have also been examined as an alternative to silicon in microcircuitry for computers and other devices. International Business Machines Corporation (IBM) has constructed logic gates using the nanotubes. Nantero, Inc., has used nanotubes to develop memory chips. Other teams are working on nano-engineering, using nanotubes to construct tiny machines.

Bibliography

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