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This is Part I of a two-part discussionof carbon fibers and
carbon nanotubes.This article covers carbon fibers, andPart II will
cover carbon nanotubes inthe October issue.
M. Rashid KhanKing Abdullah University of Science &
TechnologySaudi Arabia
Andrew R. BarronRichard E. Smalley Institute for Nanoscale
Scienceand Technology, Rice University
Houston, Texas
Carbon fiber has been described as a fibercontaining at least
90% carbon preparedby the controlled pyrolysis and thermaltreatment
of selected feedstocks and fibers.
The ability to fabricate nanomaterials (often in theform of
nanoparticles) with strictly controlled size,shape, and structure,
has inspired the applicationof nanochemistry to numerous fields
including catal-
ysis, optics, and electronics.The synthesis of well-defined
nanoparticles hasresulted in several prominent milestones in
theprogress of nanoscience, including the discovery offullerenes
and carbon nanotubes. The latter ma-terial represents a link
between well-establishedcarbon materials and the new field of
nanotech-nology. The applications of these carbon materialsin areas
as diverse as construction, transportation,medicine, electronics,
and energy suggests that thefirst technology age of the 21st
century will be thecarbon age.
This article discusses carbon fibers, describingmanufacturing
methods, properties, and several
current and potential applications.
Making carbon fibersCarbon fibers are manufactured by the
controlled
thermal treatment of organic precursors (polyacry-lonitrile,
pitch, or rayon) in fibrous form. Mostcarbon fiber is made from
polyacrylonitrile (PAN).
When PAN is thermally treated in an inert atmos-phere at 400 to
600C, the heat causes the cyano re-peat units to form rings. At
still higher temperaturesof 600 to 2000C, hydrogen atoms boil off,
the carbonbecomes more concentrated, and the compoundforms a series
of fused pyridine rings. During fur-ther heat treatment, nitrogen
is expelled and the
rings join together to form ribbons with even greaterpercentages
of carbon. As heat treatment continues,the ribbons become wider and
more nitrogen is ex-pelled, leaving ribbons that are almost pure
carbonin the graphite form. Bundles of these ribbons canbe packed
together to form fibers, hence the namecarbonfiber.
When combined with a matrix material such asepoxy, carbon fibers
form an advanced compositematerial with high strength and elastic
modulus and
relatively low density. It is this unique combinationof
properties that makes these materials highly ad-vantageous in many
advanced applications.
PAN fibersPAN fibers are white with a density of 1.17 g/cm3
and a molecular structure comprised of oriented,long-chain
molecules. Stabilization involvesstretching and heating the PAN
fibers to approxi-mately 200 to 300C in an oxygen-containing
atmos-phere to further orient and then crosslink the mole-cules,
such that they can survive higher-temperatureheat treatment without
decomposing.
After spinning, stretching of fibers during stabi-
lization and heat treatment helps orient the mole-cules to
develop high tensile modulus and tensilestrength. Aseries of
heat-treatments at carboniza-tion temperatures ranging from 1000 to
1500C inan inert atmosphere, result in a material with about95%
carbon content.
An additional high heat treatment step is includedafter
carbonization for some very high-modulusfibers. During
carbonization, the fibers shrink in di-ameter and lose
approximately 50% in weight. Mostmanufacturers use an electrolytic
oxidation processthat creates carboxyl, carbonyl, and hydroxyl
groupson the surface for enhanced bonding. Asizing orfinish is then
applied to minimize handling damage
ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2008 47
CARBON FIBERS:OPPORTUNITIES AND
CHALLENGES
Fig. 1 The Boeing 787 Dreamliner has a carbon fiber composite
fuselage forreduced weight and improved fatigue properties.
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during spooling, and to enhance bonding with ma-trix resins. The
fiber is then spooled.
Today, some manufacturers use a modified tex-tile-type PAN
precursor, and others use an aero-space-type precursor.
The textile-type precursor is made on a verylarge scale in
modified acrylic textile fiber plants in
tows consisting of 200,000 filaments. The tows arethen split
into smaller bundles (approximately48,000 filaments) after
carbonization for spooling.
Aerospace precursor is made in smaller specialtyplants and
processed in 3000 to 12,000 filament towsthat can be assembled into
24,000 or larger tows aftercarbonization. Because it is processed
in smaller towsizes, the aerospace-type precursor is less fuzzy,
andis favored by the aerospace industry for which it wasoriginally
developed.
Physical properties can be similar for both types.Manufacturing
cost is lower for the textile-type pre-cursor (targeted for
industrial applications), due tohigher line throughputs, larger
economies of scale,
and less handling of smaller tow bundles. Greatercarbon contents
and densities are achieved throughhigher-temperature heat
treatments that remove ni-trogen and provide greater crystalline
perfection.Electrical and thermal conductivity also increasewith
increasing crystalline perfection and purity.
Pitch-based fibersPetroleum pitch is processed in a
complicated
way. Asimplified figure is shown (Fig. 2). Pitch is acomplex
mixture of aromatic hydrocarbons, andcan be made not only from
petroleum, but also fromcoal tar, asphalt, or PVC. Raw material
selection isimportant to the final fiber properties.
Pitches must be processed through a pre-treat-ment step to
develop the required viscosity and mo-lecular weight in preparation
for making high-per-formance carbon fibers. The pre-processed
pitchcontains a mesophase, a term for a disk-like liquid
crystal phase that develops in regions of long-termordered
molecules favorable to manufacture of high-performance fibers.
Without this step, the result isan isotropic carbon fiber with low
strength and lowmodulus of less than 50 GPa.
Process details of the final composition andmethod of spinning
mesophase pitches are gener-ally held secret by the manufacturers.
Once spun,the stabilization, carbonization, surface
treatment,application of sizing, and spooling of pitch-based
fibers follow a sequence similar to the manufactureof PAN-based
fibers. Actual process parameters,such as temperatures, ramp rates,
and time at tem-perature for stretch and stabilization, are
differentfor pitch than for PAN.
Carbon fiber compositesComposites made from carbon fiber are
five times
stronger than steel for structural parts, yet are stillfive
times lighter. In comparison to aluminum,carbon fiber composites
are seven times strongerand two times stiffer, yet 1.5 times
lighter. Carbonfiber composites have fatigue properties
superior
to all known metals, and, when coupled with theproper resins,
carbon fiber composites are one ofthe most corrosion resistant
materials available.
Certain mesophase-pitch-based carbon fibers pos-sess thermal
conductivity three times greater thancopper. The electrical
conductivity of PAN and pitch-based carbon fibers is used to
dissipate static elec-tricity in a wide variety of computer related
prod-ucts. They do not melt or soften with heat, enablingsuch high
temperature applications as rocket noz-zles and aircraft brakes. In
fact, their strength actu-ally increases with temperature in
non-oxidizing at-mospheres. These unique properties are the
resultof the fiber microstructure, in both the axial and
transverse directions.On a finer scale, each ribbon-like
crystallite is com-prised of multiple layers. Each layer is made
ofcarbon atoms arranged like chicken wire in a hexag-onal structure
characteristic of graphite, called agraphene plane. Strong covalent
C-C bonds withinthe layer plane give the potential for high
strengthand stiffness. Weak van der Waals bonding betweenthe layer
planes gives rise to poor shear resistance,but also allows thermal
and electrical conductivity.Loose electrons and thermal energy in
the form ofphonons take advantage of the weak bonding be-tween
layer planes and use the inter-plane space asa corridor to
travel.
The width of the ribbons, the number of graphenelayers
comprising their thickness, and the length ofthe ribbons help
determine the electrical and thermalcharacteristics of the carbon
fiber, as well as con-tribute to fiber modulus. Larger and more
oriented
48 ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2008
Fig. 2 Schematic diagram of processes for PAN and pitch-based
carbon fibers.
PANPROCESS
PAN fibers Stretch Oxidation Pre-carbonization
CarbonizationPITCHPROCESS
Petroleum Meltspin Oxidation Pre-carbonization
CarbonizationpitchTake up
CARBON FIBER Sizing Surface treatment
Fatigue strength of welded steel pipe Vs. carbon fiber strand
with potted terminationComponent Cyclic stress, MPa Life,
cycles
250 metric tons (560,00 lb), 54 mm (2 inch) diameter carbon
fiber strand with Max: 861, Min: 296, 2,000,000 +potted termination
(extrapolation: 127 mm rope has a strength Range: 565 (No
failure)of 1636 metric tons)
X60 steel pipe, 609 mm (24 inch) OD and 20 mm (0.8 inch) wall
thickness Max: 220, Min: 21, 300,000(1636 metric ton yield load)
Range: 199 (Failure)
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graphene planes result in higher thermal and electrical
con-ductivity. Many transverse textures are possible, including
acommon one described as onion skin. In this structure,the graphene
layer planes at the fiber surface align like thelayers of an onion.
In the center core region of the fiber, thelayers are randomly
oriented. Most of the micro structuralpores and flaws are found in
either the transition from theskin to the random core region, or in
the core region; flaws re-sulting from damage induced during
precursor or carbonfiber processing are observed on the surface.
Flaw size and
flaw density reduce the strength of a carbon fiber.Strength is
an average effect because the fiber is bundled
with thousands or millions of other fibers in a composite.Fiber
manufacturers control the strength of the overall fiberbundle
through rigorous process control.
Composite applicationsCarbon fiber composites have often lowered
total system
costs through reduced maintenance, faster processingspeeds, and
improved reliability. They are ideally suited toapplications where
strength, stiffness, lower weight, andoutstanding fatigue
characteristics are critical requirements.They are also ideal for
applications in which high temper-ature operation and chemical
inertness are important.
Aerospace: Early growth of the carbon fiber industry wasdriven
almost exclusively by the need for higher performanceaircraft made
possible with carbon fiber composites.
Satellites: Space-based satellites incorporate very highmodulus
pitch-based carbon fibers, partly for the high stiff-ness-to-weight
ratios, and partly for their negative axial co-efficient of thermal
expansion.
Automotive: Carbon/carbon brakes, liquefied naturalgas tankage,
specialty auto and truck panels, and drive shaftsall are made of
carbon fiber-reinforced thermoplastics andthermosets. Applications
will expand significantly whencarbon fiber prices are reduced below
$20 /kg.
Sporting goods: Lighter weight and higher stiffnessgolf shafts
allow manufacturers to place more weight in theclub head, which
increases club head speed for improveddistance.
Carbon fiber tethers are the leading contender for oilplatforms
in water depths beyond 5000 ft; this shift is basedon its
lightweight and high stiffness, which minimize thenatural frequency
of the platform due to wave motion.
Carbon composite rebar is being developed to combatthe high
costs of corrosion induced structural damage.
Carbon fiber fabrics saturated with resin are appliedto concrete
bridge columns in Japan for seismic protection.The high stiffness
of the carbon minimizes movement of theconcrete, and the inertness
to corrosion insures long termprotection.
Semi permeable membranes with defined mass trans-port properties
make carbon fibers the material of choice asthe electrode in
polymer electrolyte fuel cells.
Carbon composite rollers spin faster and have lessdeflection
than steel rollers for papermaking and similarindustries.
For more information: M. Rashid Khan is with Intellectual
Assets& Technology Management, Saudi Aramco, Saudi
Arabia;[email protected]; [email protected]. Current
af-filiation is with King Abdullah University of Science &
Technology(KAUST), Saudi Arabia.Andrew R. Barron is Associate Dean
of Industry Interactions andTechnology Transfer at the Wiess School
of Natural Sciences, RiceUniversity, Houston, TX 77005;
arb@rice.
ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2008 49
Jaret FrafjordPhysical Testing Engineer
Y-12 National Security Complex
I went to my first MS&T last yearand it was so good that
Imattending again this year.
You meet a lot of ASM members from other fields, and
youre definitely going to meet someone who sharesyour technical
interests. Im interested in failure analy-
sis and quality assurance, and I get answers at MS&T.
If you just started your career, Id recommend
attending the Emerging Professionals program
its presented by and geared toward young members.
Through technical sessions and panels, you can
find out what others are doing and how theyre
succeeding. Learn great advice that you can apply
to advance your own career path.
Not only is MS&T a great conference, its a great
way to meet people who can help you becomesuccessful in your
career.
Visit asminternational.org/mst to learn more
about technical programs and social events
designed specifically for you.
Materials Science and TechnologyPITTSBURGH, OCTOBER 5-9
A GREAT
WAY TO
ADVANCE
YOUR
CAREER.
Member
since 2000
AM&P ad Frafjord.indd 1 8/1/08 3:39:36
