Carbon fibers are manufactured by treating organic 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/cm 3) 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 1970 s 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 (C 2 H 2) – 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 180 C) Rigid ladder (stable up to 345 C) 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 N 2 Leads to 2 D 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 he 0 alable faults reactive peripheral atoms 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
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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 1970 s: high axial molecular orientation, highly planar, highly aromatic molecules • Major fibers: Kevlar (polyaramid); Spectra (PE); polybenzoxazole (PBO) and polybenzothiazole (PBT). 18
Nylon 6, 6 Poly(m-phenylene isophthalamide) (Nomex) Poly(p-phenylene terephtalamide) PPT (Kevlar) 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% H 2 SO 4 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
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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. 47 1. 39 Tensile strength, GPa 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/cm 3 Coefficient of thermal expansion, 10 -6 o. C Longitudinal: 0 to 100 o. C Radial: 0 to 100 o. C 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
Si. C Dimethylchlorosilane Na Na n reaction D polysilane + polymerization polycarbosilane D D n spinning unfusing treatment carbonization Si. C fiber 37
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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/m 2][m 4]/[m] = [N*m] M = bending moment I = second moment of area of cross-section 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 M M M h d M 45
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 dominant, and thus nanoscale reinforcement promotes high flexibility. Low modulus also promotes high flexibility. Units of flexibility are [1/Nm 2] 46
Performing a ‘gedanken’ experiment: ASSUMING A CONSTANT DIAMETER: material d (m) E (Pa) j [N-1 m-2] E-glass 1. 00 E-05 72. 0 E+9 28 E+9 max HM carbon 1. 00 E-05 750 E+9 2. 7 E+9 min HS carbon 1. 00 E-05 250 E+9 8. 2 E+9 Kevlar 49 1. 00 E-05 130 E+9 16 E+9 Nicalon 1. 00 E-05 190 E+9 11 E+9 (Steel) 1. 00 E-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-1 m-2] E-glass 1. 10 E-05 72. 0 E+9 19 E+9 HM carbon 8. 00 E-06 750 E+9 6. 6 E+9 HS carbon 8. 00 E-06 250 E+9 20 E+9 Kevlar 49 1. 20 E-05 130 E+9 7. 6 E+9 Nicalon 1. 50 E-05 190 E+9 2. 1 E+9 SWNT 1. 10 E-09 1200 E+9 1. 16 E+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 δ 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: curvature Bending moment 52
After solving and applying specific boundary conditions the following result is obtained: 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: thus 53
EULER’s THEORY OF BUCKLING A long and slender column has a low critical stress, whereas a short and broad column will buckle at a high stress. 54
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2. Large deflections – The Elastica P s y θ x 56
• For small deflections, an approximate expression for the curvature of the bar was used (d 2 y/dx 2). 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: • After solving and applying boundary conditions the following result is obtained: 57
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: 58
Cantilever P x δ Y (small deflections) 59
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