Carbon fibers are manufactured by treating organic fibers precursors with heat and tension, leading

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Carbon fibers
Carbon fibers are manufactured by treating
organic fibers (precursors) with heat and
tension, leading to a highly ordered carbon
structure. The most commonly used precursors
include rayon-base fibers, polyacrylonitrile
(PAN), and pitch.
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PAN and Rayon (regenerated cellulose) made from
the 1970s Pitch-based process gives better
properties and lower cost Graphite is an
abusive term it means heat-treatment at T
where the crystalline order is similar to
graphite (gt 2000C) The structure of graphite is
anisotropic (unlike glass)
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(Note on the length of the C-C bond)

• Single bond
• Paraffinic 1.541 Å
• Diamond 1.544 Å
• Partial double bond
• Shortening of single bond in the presence of CC
  double bond (example aromatic ring) 1.53 Å
• Graphite 1.421 Å
• Double bond 1.337 Å
• Triple bond (C2H2) 1.204 Å

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Within each graphene plane, each C atom shares 3
strong covalent bonds with its neighbors.
Between each graphene plane, weaker VDW
bonding This is the source of anisotropy in
properties Objective of fabrication processes to
create a preferential orientation of graphitic
layers parallel to the fiber axis
Conversion from a precursor to high-modulus C
fiber through (generally) 3 main steps (1)
stabilization of precursor to avoid extensive
volatilisation or partial melting, (2)
longitudinal orientation of graphite-like
structure, (3) development of crystalline
ordering (graphitization)
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PAN (stable up to 180C)
Rigid ladder (stable up to 345C)
Step 1 Zipper reaction (cyclization or
transformation into a rigid ladder) PAN is a
stable linear polymer that is stable up to 180C
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2 HCN
Step 2 - dehydrogenation Tgt300C
Step 3 - denitrogenation 600-1300C Leads to 2D
carbon sheets
2 N2
Thermal treatment of PAN below 400C
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Two types of fibers are normally prepared from
PAN Type I high stiffness, low strength Type
II high strength Even in Type I, only 40 of
the modulus of graphite crystals is achieved
because of misalignment, imperfections, defects
etc.
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reactive peripheral atoms
reactive peripheral atoms
he0alable faults
Defects and chemical reaction possibilities on a
graphite lattice
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Lamellar model of carbon in cross-section
Fibrillar model of carbon fibre according to
Ruland
Model of skin-core organization in type I carbon
fibres
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Anisotropic Mesophase Pitch Based Process
Petroleum or coal tar pitch is converted into a
highly anisotropic mesophase or liquid crystal
phase through heating. Melt spinning is
performed, generating high shear stresses and
molecular orientation is generated parallel to
the fiber axis. This is followed by stabilization
in air and carbonization/graphitization
Main advantages (1) cheaper (2) no tension is
required during stabilization or graphitization
(unlike PAN-based process)
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Typical pitch molecule
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Pitch Based Process
graphite-like structure
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Alignment of mesophase pitch into a pitch filament
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Pitch-based
PAN-based
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High-modulus high-strength organic fibers

• Theoretical estimates for covalently bonded
  organics show strength of 20-50 GPa (or more) and
  modulus of 200 300 GPa
• Serious processing problems
• New fibers developed since the early 1970s high
  axial molecular orientation, highly planar,
  highly aromatic molecules
• Major fibers Kevlar (polyaramid) Spectra (PE)
  polybenzoxazole (PBO) and polybenzothiazole
  (PBT).

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Nylon 6,6
Poly(m-phenylene isophthalamide) (Nomex)
Poly(p-phenylene terephtalamide) PPT (Kevlar)
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• Aramid Fibers
• Aramid (aromatic polyamide) fibers
  poly(paraphenylene terephthalamide)
• Kevlar behaves as a nematic liquid crystal in
  the melt which can be spun
• Prepared by solution polycondensation of
  p-phenylene diamine and terephthaloyl chloride at
  low temperatures. The fiber is spun by extrusion
  of a solution of the polymer in a suitable
  solvent (for example, sulphuric acid) followed by
  stretching and thermal annealing treatment

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Producing Kevlar fibers
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Phase diagram of the anisotropic solution of PPT
in 100 H2SO4
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• Various grades of Kevlar fibers Kevlar-29, 49,
  and 149 (Kevlar-49 is the more commonly used in
  composite structures)
• X-ray diffraction the structure of Kevlar-49
  consists of rigid linear molecular chains that
  are highly oriented in the fiber axis direction,
  with the chains held together in the transverse
  direction by hydrogen bonds. Thus, the polymer
  molecules form rigid planar sheets.
• Strong covalent bonds in the fiber axis direction
  - high longitudinal strength
• Weak hydrogen bonds in the transverse direction -
  low transverse strength.
• Aramid fibers exhibit skin and core structures
  Core layers stacked perpendicular to the fiber
  axis, composed of rod-shaped crystallites with an
  average diameter of 50 nm. These crystallites are
  closely packed and held together with hydrogen
  bonds nearly in the radial direction of the
  fiber.

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Kevlar fibers
Schematic diagram of Kevlar 49 fiber showing the
radially arranged pleated sheets
Microstructure of aramid fiber
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Kevlar - High flexibility but poor compressive
performance
Also low shear performance, moisture-sensitive,
UV-sensitive
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The Aramid fiber family
29
?
?
Kevlar/epoxy
Note the fibrillar structure of the fiber
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Kevlar fiber

• Little creep only
• Excellent temperature resistance (does not melt,
  decomposes at 500C)
• Linear stress-strain curve until failure
• Low density 1.44
• Negative CTE
• Fiber diameter 11.9 micron
• Fiber strength variability

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Polyethylene fibers
Normal PE Orientation low Crystallinity lt 60
Dyneema or Spectra Orientation gt
95 Crystallinity up to 85
Stretching
Entanglement network
Fibrillar crystal
The theoretical elastic modulus of the covalent
C-C bond in the fully extended PE molecule is 220
Gpa. Experimental value in PE fibres - 170 Gpa.
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Extended chain polyethylene
minimum chain folding
UHMPE fibre structure (a) macrofibril consists
of array of microfibrils (b) microfibril (c)
orthorhombic unit cell (d) view along chain axis
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UHMWPE

• UHMWPE (Spectra or Dyneema) are highly
  anisotropic fibers
• Even higher specific properties than Kevlar
  because of lower density (0.98 g/cc)
• Limited to use below 120C
• Creep problems weak interfaces
• Applications ballistic impact-resistant
  structures

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UHMWPE (Spectra) high flexibility and
toughness, poor interfacial bonding
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Poly(p-phenylene benzobisthiazole) PBT or PBZT
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SiC
Dimethylchlorosilane
Na
Na
n
reaction
polysilane
polymerization
polycarbosilane
spinning
unfusing treatment
carbonization
SiC fiber
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Flexibility, compressibility, and limit
performance of fibers
Kevlar
Spectra
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FLEXIBILITY Intense bending strains and stresses
applied to fibers during manufacturing operations
(weaving, knitting, filament winding,
etc) Definition of flexibility Bending of an
elastic beam M (EI)/R k(EI)
Units N/m2m4/m
Nm M bending moment I second moment of
area of cross-section R radius of curvature to
the neutral surface of cross-section
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• E Youngs modulus
• EI flexural rigidity ( resistance of beam to
  bending)
• curvature 1/R
• Intuitively the flexibility of a fiber is the
  highest when
• The radius of curvature is as small as
  possible (or the curvature is as large as
  possible)
• The bending moment necessary to reach a given
  curvature is as small as possible
• The appropriate parameter to focus on is j
  k/M, which must be maximized for highest
  flexibility.

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Moment of inertia
b
M
M
h
M
M
d
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Flexibility is thus defined as (for a circular
fiber) where E and d are the fiber bending
modulus and diameter, respectively As seen, the
effect of size (diameter) on flexibility is by
far the strongest, and thus nanoscale
reinforcement promotes high flexibility. Low
modulus also promotes high flexibility. Units of
flexibility are 1/Nm2
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Performing a gedanken experiment
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Using real diameters and moduli
Glass fibers are thus much more tolerant to
bending than HM carbon fibers or even kevlar 49.
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Flexibility of carbon nanotubes
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Compressibility

• The compressive strength of single fibers is very
  difficult to measure and is usually inferred from
  the behavior of composites including the fibers.
• Euler buckling is one possible mode of
  compressive failure it occurs when a fiber under
  compression becomes unstable against lateral
  movement of its central region.

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What is this?
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EULERs WORK ON BUCKLING
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Hollow cylindrical tube of length L, diameter d,
and wall thickness t
(for a tube with fixed end)
NOTES - The critical buckling load is not a
function of the strength of the material (yield
and ultimate strength do not appear), but only
of the elastic modulus (E) and geometry via the
moment of inertia (I)
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BUCKLING OF MWNTs
?crit ENT(m?r/L)2
(2K/?)(L/m?r)2 Euler Matrix term
m number of 1/2? at initial buckling L, r
tube fragment length, outer radius K
foundation modulus from tube/matrix
interaction, strong or weak
Lourie et. al. PRL 81, p1638 (1998)
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COMPRESSIVE STRENGTH OF MWNTS 150 GPa
EXPERIMENT BY LOURIE, COX AND WAGNER (PRL
1998) Later calculated by Srivastava et al (PRL
1999)
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• A long and slender column has a low critical
  stress, whereas a short and broad column will
  buckle at a high stress.
• A high critical stress is obtained when (EI) is
  maximized, or when L is reduced, or when
  additional lateral support is provided (a very
  short column has almost infinite critical stress,
  which is not physical).
• .
• To maximize the flexural rigidity EI (and thus
  the critical stress)
• Use a material with higher Young's modulus, or
• Distribute the material in such a way as to
  increase the moment of inertia I of the cross
  section.
• To increase the moment of inertia Distribute
  the material farther from the centroid of the
  cross-section. Therefore, a hollow tubular member
  is generally more economical for use as a column
  than a solid member having the same
  cross-sectional area (or same weight!).
• Reducing the wall thickness of a tubular member
  and increasing its lateral dimensions while
  keeping the cross sectional area constant (or
  weight constant) also increases the critical load
  because I is increased. This has of course a
  limit, because eventually the wall becomes too
  thin and instable ("localized buckling)

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LIMIT OF FIBER PERFORMANCE

• Question What is the expected optimum
  theoretical performance of a fiber (per unit
  weight)?
• Based on the Orowan-Polanyi model,
• Normalizing the strength and modulus

(where r is density)
(units are m)
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Example
For the C-C bond in the basal plane of graphite,
we have
g 4.2 J/m2 a0 1.5 Å r 1.8 g/cc 1800
kg/m3 17652 N/m3
Therefore
Plotted as follows
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