Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting
of fibers about 5–10 μm in diameter and composed mostly of carbon atoms.
To produce carbon fiber, the carbon atoms are bonded together in crystals that are more
or less aligned parallel to the long axis of the fiber as the crystal alignment gives
the fiber high strength-to-volume ratio. Several thousand carbon fibers are bundled together
to form a tow, which may be used by itself or woven into a fabric.
The properties of carbon fibers, such as high stiffness, high tensile strength, low weight,
high chemical resistance, high temperature tolerance and low thermal expansion, make
them very popular in aerospace, civil engineering, military, and motorsports, along with other
competition sports. However, they are relatively expensive when compared to similar fibers,
such as glass fibers or plastic fibers. Carbon fibers are usually combined with other
materials to form a composite. When combined with a plastic resin and wound or molded it
forms carbon fiber reinforced polymer which has a very high strength-to-weight ratio,
and is extremely rigid although somewhat brittle. However, carbon fibers are also composed with
other materials, such as with graphite to form carbon-carbon composites, which have
a very high heat tolerance.
History In 1879, Thomas Edison baked cotton threads
or bamboo slivers at high temperatures carbonizing them into an all-carbon fiber filament used
in the first incandescent light bulb to be heated by electricity. In 1958, Roger Bacon
created high-performance carbon fibers at the Union Carbide Parma Technical Center,
now GrafTech International Holdings, Inc., located outside of Cleveland, Ohio. Those
fibers were manufactured by heating strands of rayon until they carbonized. This process
proved to be inefficient, as the resulting fibers contained only about 20% carbon and
had low strength and stiffness properties. In the early 1960s, a process was developed
by Dr. Akio Shindo at Agency of Industrial Science and Technology of Japan, using polyacrylonitrile
as a raw material. This had produced a carbon fiber that contained about 55% carbon.
The high potential strength of carbon fiber was realized in 1963 in a process developed
by W. Watt, L. N. Phillips, and W. Johnson at the Royal Aircraft Establishment at Farnborough,
Hampshire. The process was patented by the UK Ministry of Defence, then licensed by the
NRDC to three British companies: Rolls-Royce already making carbon fiber, Morganite, and
Courtaulds. Within a few years, Rolls-Royce took advantage of the new material's properties
to break into the American market with its RB-211 aero-engine with carbon fiber's compressor
blades. Unfortunately, the blades proved vulnerable to damage from bird impact. That and other
Rolls-Royce's problems became so great that the company was nationalized in 1971 and the
carbon-fiber production plant was sold off to form "Bristol Composites".
In the late 1960s, the Japanese took the lead in manufacturing PAN-based carbon fibers.
The 1970 joint technology agreement allowed Union Carbide to manufacture the Japan’s
Toray Industries superior product and USA to dominate the market. Morganite decided
that carbon-fiber production was peripheral to its core business, leaving Courtaulds as
the only big UK manufacturer. Continuing collaboration with the staff at Farnborough proved helpful
in the quest for higher quality and improvements in the speed of production as Courtaulds developed
two main markets: aerospace and sports equipment. However Courtaulds's big advantage as manufacturer
of the "Courtelle" precursor now became a weakness. Courtelle's low cost and ready availability
were potential advantages, but the water-based inorganic process used to produce it made
the product susceptible to impurities that did not affect the organic process used by
other carbon-fiber manufacturers. Nevertheless, during the 1980s Courtaulds
continued to be a major supplier of carbon fiber for the sports-goods market, with Mitsubishi
its main customer until a move to expand, including building a production plant in California,
turned out badly. The investment did not generate the anticipated returns, leading to a decision
to pull out of the area and Courtaulds ceased carbon-fiber production in 1991. Ironically
the one surviving UK carbon-fiber manufacturer continued to thrive making fiber based on
Courtaulds's precursor. Inverness-based RK Carbon Fibres Ltd concentrated on producing
carbon fiber for industrial applications, removing the need to compete at the quality
levels reached by overseas manufacturers. During the 1960s, experimental work to find
alternative raw materials led to the introduction of carbon fibers made from a petroleum pitch
derived from oil processing. These fibers contained about 85% carbon and had excellent
flexural strength. Also, during this period, the Japanese Government heavily supported
carbon fiber development at home and several Japanese companies such as Toray, Nippon Carbon,
Toho Rayon and Mitsubishi started their own development and production. As they subsequently
advanced to become market leaders, companies in USA and Europe were encouraged to take
up these activities as well, either through their own developments or contractual acquisition
of carbon fiber knowledge. These companies included Hercules, BASF and Celanese USA and
Akzo in Europe. Since the late 1970s, further types of carbon
fiber yarn entered the global market, offering higher tensile strength and higher elastic
modulus. For example, T400 from Toray with a tensile strength of 4,000 MPa and M40, a
modulus of 400 GPa. Intermediate carbon fibers, such as IM 600 from Toho Reyon with up to
6,000 MPa were developed. Carbon fibers from Toray, Celanese and Akzo found their way to
aerospace application from secondary to primary parts first in military and later in civil
aircraft as in McDonnell Douglas, Boeing and Airbus planes. By 2000 the industrial applications
for highly sophisticated machine parts in middle Europe was becoming more important.
Structure and properties
Each carbon filament thread is a bundle of many thousand carbon filaments. A single such
filament is a thin tube with a diameter of 5–8 micrometers and consists almost exclusively
of carbon. The earliest generation had diameters of 16–22 micrometers. Later fibers have
diameters that are approximately 5 micrometers. The atomic structure of carbon fiber is similar
to that of graphite, consisting of sheets of carbon atoms arranged in a regular hexagonal
pattern, the difference being in the way these sheets interlock. Graphite is a crystalline
material in which the sheets are stacked parallel to one another in regular fashion. The intermolecular
forces between the sheets are relatively weak Van der Waals forces, giving graphite its
soft and brittle characteristics. Depending upon the precursor to make the fiber,
carbon fiber may be turbostratic or graphitic, or have a hybrid structure with both graphitic
and turbostratic parts present. In turbostratic carbon fiber the sheets of carbon atoms are
haphazardly folded, or crumpled, together. Carbon fibers derived from Polyacrylonitrile
are turbostratic, whereas carbon fibers derived from mesophase pitch are graphitic after heat
treatment at temperatures exceeding 2200 °C. Turbostratic carbon fibers tend to have high
tensile strength, whereas heat-treated mesophase-pitch-derived carbon fibers have high Young's modulus and
high thermal conductivity. Applications
The global demand on carbon fiber composites was valued at roughly US$10.8 billion in 2009,
which declined 8–10% from the previous year. It is expected to reach US$13.2 billion by
2012 and to increase to US$18.6 billion by 2015 with an annual growth rate of 7% or more.
Strongest demands come from aircraft & aerospace, wind energy, as well as from the automotive
industry with optimized resin systems. Composite materials
Carbon fiber is most notably used to reinforce composite materials, particularly the class
of materials known as carbon fiber or graphite reinforced polymers. Non-polymer materials
can also be used as the matrix for carbon fibers. Due to the formation of metal carbides
and corrosion considerations, carbon has seen limited success in metal matrix composite
applications. Reinforced carbon-carbon consists of carbon fiber-reinforced graphite, and is
used structurally in high-temperature applications. The fiber also finds use in filtration of
high-temperature gases, as an electrode with high surface area and impeccable corrosion
resistance, and as an anti-static component. Molding a thin layer of carbon fibers significantly
improves fire resistance of polymers or thermoset composites because a dense, compact layer
of carbon fibers efficiently reflects heat. The increasing use of carbon fiber composites
is displacing aluminum from aerospace applications in favor of other metals because of galvanic
corrosion issues. Textiles
Precursors for carbon fibers are polyacrylonitrile, rayon and pitch. Carbon fiber filament yarns
are used in several processing techniques: the direct uses are for prepregging, filament
winding, pultrusion, weaving, braiding, etc. Carbon fiber yarn is rated by the linear density
or by number of filaments per yarn count, in thousands. For example, 200 tex for 3,000 filaments
of carbon fiber is three times as strong as 1,000 carbon filament yarn, but is also three
times as heavy. This thread can then be used to weave a carbon fiber filament fabric or
cloth. The appearance of this fabric generally depends on the linear density of the yarn
and the weave chosen. Some commonly used types of weave are twill, satin and plain. Carbon
filament yarns can be also knitted or braided. Microelectrodes
Carbon fibers are used for fabrication of carbon-fiber microelectrodes. In this application
typically a single carbon fiber with diameter of 5–7 μm is sealed in a glass capillary.
At the tip the capillary is either sealed with epoxy and polished to make carbon-fiber
disk microelectrode or the fiber is cut to a length of 75–150 μm to make carbon-fiber
cylinder electrode. Carbon-fiber microelectrodes are used either in amperometry or fast-scan
cyclic voltammetry for detection of biochemical signaling.
Catalysis PAN-based nanofibers can efficiently catalyze
the first step in the making of synthetic gasoline and other energy-rich products out
of carbon dioxide. The process uses a “co-catalyst” system in three steps: EMIM–CO2 complex
formation; adsorption of EMIM–CO2 complex on reduced carbon atoms and Carbon monoxide
formation. The first step uses an ionic liquid, while
graphitic carbon structures doped with other reactive atoms replaced silver to produce
the final output. The carbon nanofibre catalyst exhibited negligible overpotential for carbon
dioxide reduction and more than an order of magnitude higher current density compared
with silver under similar experimental conditions. The reduction derived from the reduced carbons
rather than to electronegative nitrogen dopants. The performance came from the nanofibrillar
structure and high binding energy of key intermediates to the carbon nanofibre surfaces.
Synthesis Each carbon filament is produced from a precursor
polymer such as polyacrylonitrile, rayon, or petroleum pitch. For synthetic polymers
such as PAN or rayon, the precursor is first spun into filament yarns, using chemical and
mechanical processes to initially align the polymer atoms in a way to enhance the final
physical properties of the completed carbon fiber. Precursor compositions and mechanical
processes used during spinning filament yarns may vary among manufacturers. After drawing
or spinning, the polymer filament yarns are then heated to drive off non-carbon atoms,
producing the final carbon fiber. The carbon fibers filament yarns may be further treated
to improve handling qualities, then wound on to bobbins.
A common method of manufacture involves heating the spun PAN filaments to approximately 300 °C
in air, which breaks many of the hydrogen bonds and oxidizes the material. The oxidized
PAN is then placed into a furnace having an inert atmosphere of a gas such as argon, and
heated to approximately 2000 °C, which induces graphitization of the material, changing the
molecular bond structure. When heated in the correct conditions, these chains bond side-to-side,
forming narrow graphene sheets which eventually merge to form a single, columnar filament.
The result is usually 93–95% carbon. Lower-quality fiber can be manufactured using pitch or rayon
as the precursor instead of PAN. The carbon can become further enhanced, as high modulus,
or high strength carbon, by heat treatment processes. Carbon heated in the range of 1500–2000 °C
exhibits the highest tensile strength, while carbon fiber heated from 2500 to 3000 °C
exhibits a higher modulus of elasticity. Manufacturers
Major manufacturers of carbon fibers include Toho Tenax, Cytec Industries, Formosa Plastics,
Hexcel, Mitsubishi Rayon, SGL Carbon, Toray Industries and Zoltek. Manufacturers typically
make different grades of fibers for different applications. Higher modulus carbon fibers
are typically more expensive. See also
Carbon fiber reinforced polymer Carbon fiber reinforced ceramic material
Carbon nanotube ESD materials
References
External links Making Carbon Fiber
How carbon fiber is made Carbon Fibres – the First Five Years A 1971
Flight article on carbon fibre in the aviation field