Transcript Slide 1

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.
Carbon Material
High modulus PAN-based
High strength PAN-based
High modulus Pitch-based
Fiber
Diameter
(mm)
Tensile
modulus
(GPa)
Tensile
strength
(GPa)
Density
(g/cm3)
Thermal
Expansion
Coeff.
(10-6 K-1)
6 - 8
290 590
2.5 –
3.9
1.70 –
1.94
-1.0 to –
1.2
5 - 8
230 295
3.5 –
7.1
1.76 –
1.82
-0.4 to –
1.0
10
520 830
2.1 –
2.2
2.03 –
2.18
-1.4
1
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 (> 2000°C)
The structure of graphite is anisotropic (unlike glass)
2
(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 C=C double bond
(example: aromatic ring) – 1.53 Å
Graphite – 1.421 Å
Double bond – 1.337 Å
Triple bond (C2H2) – 1.204 Å
3
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 volatilization or partial melting, (2) longitudinal
orientation of graphite-like structure, (3) development of
crystalline ordering (graphitization)
4
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 180°C
5
+ 2 HCN
Step 2 - dehydrogenation
T>300°C
Thermal treatment of
PAN below 400°C
Step 3 - denitrogenation
600-1300°C
+ 2 N2
Leads to 2D carbon sheets
6
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.
7
reactive peripheral
atoms
he0alable faults
reactive peripheral
atoms
Fibre
LM
Nominal
Modulus
(GPa)
200
MM
380
HM
750
Defects and chemical reaction possibilities
on a graphite lattice
8
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
9
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)
10
Typical pitch molecule
11
Pitch Based Process
pitch purification
mesophase formation
spinning
pitch precursor fiber
stabilization
carbonization
stabilization
surface treatment
sizing
carbon fiber
graphite-like structure
12
Alignment of
mesophase pitch
into a pitch
filament
13
PAN-based
Pitch-based
14
15
16
17
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).
18
Nylon 6,6
N
H
O
C
N H CH2 6
N
H
C
O
C
H
N
NH
O
O
C
CH2 4 C
Poly(m-phenylene isophthalamide)
(Nomex)
O
N
H
Poly(p-phenylene terephtalamide)
PPT (Kevlar)
N
H
O
O
O
C
C
C
N
H
N
H
19
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
20
Schematic representation of structure formation
during spinning, contrasting PPT and PET behavior
Liquid crystal
Conventional (PET)
Solution
Nematic structure
Low entropy
Extrusion
Random coil
High entropy
Solid
state
Extended chain structure
High chain continuity
High mechanical properties
Folded chain structure
Low chain continuity
Low mechanical properties
21
Producing Kevlar fibers
22
Phase diagram of the anisotropic solution of PPT
in 100% H2SO4
23
•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.
24
•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.
25
Kevlar fibers
0.51 nm
Schematic diagram of Kevlar® 49 fiber
showing the radially arranged
pleated sheets
Microstructure of aramid fiber
26
27
Kevlar - High flexibility but poor
compressive performance
Also low shear performance,
moisture-sensitive, UVsensitive
28
The Aramid fiber family
Twaron
(Akzo)
Twaron
HM
Kevlar
29
Kevlar
49
Kevlar
149
HM-50
(Teijin)
1.44
1.45
1.44
1.44
1.47
1.39
Tensile strength, GPa
2.8
2.8
2.8
2.8
2.8
3.0
Tensile modulus, GPa
80
125
62
124
186
74
Tensile strain, %
3.3
2.0
3.5
2.5
1.9
4.2
…
…
-2.0
…
--
…
…
…
59
…
--
…
Density, g/cm3
Coefficient of thermal
expansion,
10-6oC
Longitudinal:
0 to 100 oC
Radial:
0 to 100 oC
29
‫ב‬
Kevlar/epoxy
‫א‬
Note the fibrillar structure of the fiber
30
Kevlar fiber
•Little creep only
•Excellent temperature resistance (does not melt,
decomposes at ~500°C)
•Linear stress-strain curve until failure
•Low density : 1.44
•Negative CTE (why? mechanism?)
•Fiber diameter = 11.9 micron
•Fiber strength variability
31
Polyethylene fibers
Normal PE
Orientation low
Crystallinity < 60%
Dyneema or Spectra
Orientation > 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.
32
Extended chain polyethylene
minimum chain folding
UHMPE fibre structure: (a) macrofibril consists of array of microfibrils;
33
(b) microfibril; (c) orthorhombic unit cell; (d) view along chain axis
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 120°C
•Creep problems; weak interfaces
•Applications – ballistic impact-resistant structures
(however: very large strains are undesirable in
ballistic applications)
34
UHMWPE (Spectra) – high
flexibility and toughness, poor
interfacial bonding
35
Poly(p-phenylene benzobisthiazole)
PBT or PBZT
36
SiC
Dimethylchlorosilane
reaction
polysilane
polymerization
polycarbosilane
spinning
CH3
Na
n Cl
Si
Na
Cl
CH3
CH3 CH3 CH3
Si
Si
CH3 CH3
D
Si
Si
CH3 CH3 CH3
D
Si
CH3 CH3
CH3
CH3
Si
Si
CH3
CH3
CH3
CH2
Si
CH3
D
+
CH3
H
Si
CH3
CH3
CH2
Si
CH3
unfusing treatment
carbonization
SiC fiber
37
n
38
39
40
41
Flexibility, compressibility, and
limit performance of fibers
Kevlar
Spectra
42
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/m2][m4]/[m] = [N*m]
M = bending moment
I = second moment of area of cross-section
I   y dA
2
R = radius of curvature to the neutral surface of cross-section
43
E = Young’s modulus
EI = flexural rigidity (≈ resistance of beam to bending)
k= curvature = 1/R
Intuitively: the flexibility of a fiber is the highest when:
o
The radius of curvature is as small as possible (or the
curvature is as large as possible)
o
The bending moment necessary to reach a given curvature
is as small as possible
o The appropriate parameter to focus on is j = k/M = 1/EI,
which must be maximized for highest flexibility.
44
Moment of inertia
b
3
bh
I
12
I
d
M
M
h
4
64
M
d
M
45
Flexibility is thus defined as (for a circular fiber)
j  64 Ed
4
where E and d are the fiber bending modulus and
diameter, respectively
As seen, the effect of size (diameter) on flexibility
is dominant, and thus nanoscale reinforcement
promotes high flexibility. Low modulus also
promotes high flexibility.
Units of flexibility are [1/Nm2]
46
Performing a ‘gedanken’ experiment:
ASSUMING A CONSTANT DIAMETER:
material
d (m)
E (Pa)
j [N-1m-2]
E-glass
1.00E-05
72.0 E+9
28 E+9
max
HM carbon
1.00E-05
750 E+9
2.7 E+9
min
HS carbon
1.00E-05
250 E+9
8.2 E+9
Kevlar 49
1.00E-05
130 E+9
16 E+9
Nicalon
1.00E-05
190 E+9
11 E+9
(Steel)
1.00E-05
210 E+9
9.7 E+9
47
Using real diameters and moduli:
USING REAL DIAMETERS AND MODULI:
material
d (m)
E (GPa)
j [N-1m-2]
E-glass
1.10E-05
72.0 E+9
19 E+9
HM carbon
8.00E-06
750 E+9
6.6 E+9
HS carbon
8.00E-06
250 E+9
20 E+9
Kevlar 49
1.20E-05
130 E+9
7.6 E+9
Nicalon
1.50E-05
190 E+9
2.1 E+9
SWNT
1.10E-09
1200 E+9
1.16E+25 !
max
min
Glass fibers are thus much more tolerant to bending than HM
carbon fibers or even kevlar 49.
48
Flexibility of carbon
nanotubes
49
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. Displacements are
relatively small.
• When displacements grow very large: theory of the elastica
• NOTE: All the following methods can be used to measure the
(bending) Young’s modulus of a fiber (or a long & thin beam)
50
Definitions:
• ℓ – length of the bar
• EI – flexural rigidity of the bar
• P – load
• A – area of cross section of the
bar
REDRAW THIS !!
1. Small deflections - Euler
buckling of slender bar
P

x
Y
51
Assumptions:
a)
The stress is below proportional limit
b)
The deflection δ is (relatively) small
Objective: to find the minimum load at which the
beam bends sidewise.
Equation to solve:
d2y
EI 2  P(  y )
dx
curvature
Bending
moment
52
After solving and applying specific boundary conditions
the following result is obtained:
Pcr   1 4 
 2 EI

2
Dividing by the cross-sectional area and substituting r (radius of gyration,
which for a hollow cylinder, taken as an example, is the radius of the cylinder),
the critical stress is:
r
thus
I
A
2
Pcr

E
1
 cr 
  4
2
A
 / r 
53
EULER’s
THEORY OF
BUCKLING
Pcr  k
2

2
EI
A long and slender column has a low critical stress,
whereas a short and broad column will buckle at a high stress.
54
55
2. Large deflections – The Elastica
P
s
y


x
56
• For small deflections, an approximate expression for
the curvature of the bar was used (d2y/dx2). For
large deflections, it is necessary to introduce the
exact expression for the curvature of the bar: dθ/ds
(s is the distance along the bar, θ is the deflection
angle at each point).
• Equation to be solved:
d
EI
  Py
ds
• After solving and applying boundary conditions the
following result is obtained:
57

K ( p) EI
4
2
Pcr
l
2
where p = sin(/2) ( is the deflection at the upper end of the
bar) and K(p) is the complete elliptic integral of the first kind
(values are tabulated numerically in handbooks).
Note: In the previous (Euler, small deformations) solution the
bar could have any value of deflection, provided the deflection
remained small. Now we have a connection between the load
and the deflection. At small values of  (small deflections) the
solution approaches the critical load given by Euler solution.
The stress is:
 cr
Pcr K ( p)2 E


2
A
l / r 
58
Cantilever
P
x
3EI
P  3


Y
(small deflections)
59