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The Processing, Properties, and Structure of Carbon Fibers

Posted on: Thursday, 24 February 2005, 03:00 CST

This paper reviews the processing, properties, and structure of carbon fibers. Carbon fibers are derived from several precursors, with polyacrylonitrile being the predominant precursor used today. Carbon fibers have high strength (3-7 GPa), high modulus (200-500 GPa), compressive strength (1-3 GPa), shear modulus (10-15 GPa), and low density (1.75-2.00 g/cm^sup 3^). Carbon fibers made from pitch can have modulus, thermal, and electrical conductivities as high as 900 GPa, 1,000 W/mK, and 10^sup 6^ S/m, respectively. These fibers have become a dominant material in the aerospace industry and their use in the automotive and other industries is growing as their cost continues to come down.

INTRODUCTION

Carbon fibers contain at least 90% carbon by weight obtained by pyrolysis of an appropriate precursor fiber.1 Graphite is one form of carbon. In graphite, the sp2 hybridized carbon atoms are arranged in two-dimensional hexagonal planes.1 These graphitic planes are highly anisotropic due to the difference between in-plane and out- of-plane bonding of carbon atoms. The elastic modulus is much higher in the plane than it is perpendicular to the plane. The bonding between graphitic planes is van der Waals bonding so the planes can slide with respect to each another. Alignment of the graphitic planes parallel to the fiber axis leads to high tensile modulus and electrical and thermal conductivity parallel to the fiber axis.2

Polymeric materials, which leave a carbon residue and do not melt upon pyrolysis in an inert atmosphere, are generally considered candidates for carbon-fiber production.3 The historical development of carbon fiber has been traced extensively.4 The first carbon fibers were produced by T. Edison in the United States and J.W. Swan in England from a cellulose precursor for light-bulb filaments.5 Modern carbon fibers were developed in the late 1950s and early 1960s by W. Watt in England,6 A. Shindo in Japan,7 and R. Bacon in the United States.8 Though cellulose was the early precursor used for carbon fibers, today polyacrylonitrile (PAN) is the predominant carbon-fiber precursor, followed by petroleum pitch. Carbon fibers are also produced by decomposing gaseous hydrocarbons at high temperatures. The first account of vapor-grown carbon fiber (VGCF) production was in 1890.9

Figure 1. A PAN-based carbon-fiber processing flow chart.

PROCESSING OF CARBON FIBERS

PAN-Based Carbon Fibers

Polyacrylonitrile is favored as a carbon-fiber precursor due to its combination of tensile and compressive properties as well as the carbon yield.10 DuPont first developed PAN fibers in the 1940s for use as textile fiber. Its thermal stability was recognized soon thereafter, which led to further research on PAN fiber heat treatment. In the early 1960s, PAN fibers were first carbonized and graphitized by A. Shindo7 at the Government Industrial Research Institute, Osaka, Japan, and these fibers exhibited tensile strength and modulus of 0.75 GPa and 112 GPa, respectively. During the 196Os W. Watt and W. Johnson at Royal Aircraft Establishment in England," and R. Bacon at Union Carbide in the United States12 also developed a method for producing carbon fibers from PAN.

Producing carbon fibers from PAN involves polymerization of PAN, spinning of fibers, thermal stabilization, carbonization, and graphitization (Figure 1). The PAN homopolymer contains highly polar nitrile groups, hindering the alignment of the molecular chains during spinning. Therefore, a copolymer of PAN is used. The comonomer content generally ranges from 2% to 15%; typical comonomers are acrylic acid, methacrylic acid, and methacrylate. The use of comonomers partially disrupts the nitrile-nitrile interactions, allowing for better chain alignment. Typical carbon yield from PAN-based precursors is 50-60%.

Figure 2, The gases released during pyrolysis of PAN.16

Figure 3. (a) The Young's modulus and (b) tensile strength as a function of heat treatment temperature for both RAN and mesophase pitch-based carbon fibers.

The PAN fibers can be spun by wet, melt, dry, gel, and dry-jet wet spinning. In wet spinning, which is commonly used,13 the polymer is directly extruded in the coagulation bath and the fiber is subsequently drawn at ~ 10O0C. Wet-spun PAN precursor fibers typically have a circular or dog-bone-shaped cross section depending upon fiber-coagulation conditions. Using specially shaped spinnerets, other cross-sectional shapes, such as "C,""T," star, and trilobal shapes have also been processed to influence microstructure and properties of the resulting carbon fibers. The cross-sectional shape of the ultimate carbon fiber resembles the shape of the precursor fiber.

The typical diameter of the PAN precursor fiber is about 15 m, which ultimately results in a carbon fiber of about 7 m diameter. When processed under comparable conditions, the tensile strength of the carbon increases with decreasing fiber diameter. Therefore, higher-tensile-strength carbon fibers can be produced by decreasing the diameter of the PAN precursor fiber, and indeed 5 m diameter carbon fibers with tensile strength of 7 GPa have been reported. However, the current fiber-production technology makes it difficult and more expensive to process a PAN fiber with a diameter significantly less than 15 m. On the other hand, much smaller diameter PAN-based carbon fibers (100 nm to about 2 m) can be processed by electrospinning. Carbonization of the electrospun PAN fibers has the potential to yield carbon fibers with tensile strength close to the theoretical value.

Polyacrylonitrile fiber is stabilized under stress between 200C and 300C in an oxidizing atmosphere.14 During oxidative stabilization, PAN goes through chemical changes that result in increased density.15 The stabilization process causes cyclization of PAN, leading to the formation of what is termed as the ladder polymer. During this step, some hydrogen evolution and oxygen pick- up also occurs. Since oxidative stabilization is a diffusion- controlled process, the stabilization of the 15 m diameter PAN fiber generally takes about 2 h, and stabilization time can vary with the copolymer composition. The stabilized fibers are carbonized in nitrogen in the 1,000C to 1,700C range. Various gases evolved during pyrolysis of PAN16 are shown in Figure 2. Stretching during stabilization minimizes the need for stress during carbonization and graphitization. During the carbonization process, carbon content increases to above 90% and a three-dimensional, near-amorphous carbon structure with microcrystals forms. These fibers can be further heat-treated between 2,000C and 3,000C17 in an inert environment for graphitization. Nitrogen cannot be used in the graphitization process as it will react with the carbon to form nitrides. Modulus monotonically increases with heat-treatment temperature, while maximum strength is obtained at about 1,500C to 1,600C (Figure 3).18

Figure 4. A model of carbonaceous mesophase: a lamellar liquid crystal.23

Pitch-Based Carbon Fibers

Both isotropic and mesophase pitches are used to produce carbon fibers with low (100 GPa) and high moduli (up to 900 GPa), respectively. Pitch is produced from petroleum or coal tar which is made up of fused aromatic rings. Pitch-based carbon fibers are theoretically capable of modulus equal to that of graphite single crystal (about 1,050 GPa). This is significantly higher than the highest modulus obtained from PAN-based carbon fibers of 650 GPa.10 Pitch-based carbon fibers also demonstrate better electrical and thermal properties than PAN-based fibers. Isotropic pitch-based carbon fibers were first commercialized in the 1960s but mesophase pitch was not commercialized until the 1980s.19 Mesophase pitch- based carbon-fiber production is an expensive process when compared to PAN-based carbon-fiber production. Production of pitch-based carbon fibers involves melt spinning of pitch precursor fibers, stabilization (oxidation), carbonization, and graphitization.

Isotropic pitch has a softening point between 40C and 120C.20 Mesophase pitch is an anisotropic liquid crystal state of pitch consisting of disc-like aromatic molecules also known as carbonaceous pitch with a softening point around 300C. This form of pitch is produced by pyrolysis of isotropic pitch between 300C and 500C.21,22 A schematic diagram of the mesophase pitch23 is shown in Figure 4. Before spinning, isotropic and mesophase pitches are purified using several methods. The molecular weight of pitch is typically in the 150 g/mole to 1,000 g/mole range, with the average molecular weight being about 450 g/mole. Pitch is melt spun into a continuous fiber that can be drawn. The spinning temperature for mesophase pitch is around 350C. The cross section of the spinneret hole not only controls the shape of the fiber but can also be used to control the microstructure of the final carbon fibers. The transverse microstructures of the pitch-based carbon fibers also changes with specific spinning conditions24 (Figure 5). The diameter of the pitch-based carbon fibers is typically about 10 m.

Stabilization is a necessary step for pitch precursor fibers and takes pla\ce at temperatures between 200C and 300C. Pitch fibers will soften and melt at higher temperatures because they behave like a thermoplastic. During stabilization, the thermoplastic must be converted to a thermoset so it can undergo hightemperature carbonization. The degree of stabilization is carefully controlled; otherwise, during carbonization the fiber will melt if there is not sufficient stabilization. On the other hand, prolonged stabilization leads to a decrease in the final carbon-fiber mechanical properties. The stabilization time for Isotropic pitch is typically greater than that for the mesophase pitch. Pitch precursor fibers undergo carbonization and graphitization. The carbon yield for pitch-based fibers-about 70-80%-is highest of all the precursors.

Figure 5. Typical transverse microstructures for pitchbased carbonfibersshowing (a) radial with wedge, (b) radial, and (c) concentric microstructure.24

Table I. Structural Parameters of Various Carbon Fibers*

Gas-Phase-Grown Carbon Fibers

Gas-phase-grown carbon fibers, also known as VGCF, are made by decomposing gaseous hydrocarbons at temperatures between 300C and 2,500C in the presence of a metal catalyst such as iron or nickel that is either fixed to a substrate or fluidized in space.25 Typical substrates are carbon, silicon, and quartz while hydrocarbons can be benzene, acetylene, or natural gas. O.P. Bahl et al. have traced the historical development of VGCF.10 There are a number of reports on the development of VGCF between 1890 and the 1980s.9,26-31 However, the development of VGCF in general and vapor-grown carbon nano fibers in particular picked up steam during 1990s, at least partly due to the recognition of and ensuing development of carbon nanotubes. The Pittsburgh Coke and Chemical Company attempted to commercialize VGCF in the 1950s.28 The properties of the carbon fibers are affected by the residence time of thermal decomposition and the temperature of the furnace. Growth mechanisms for VGCF have been proposed by R.T.K. Baker et al.,32 T. Baired et al.,33 and A. Oberlin.34 The diameter of VGCF ranges from 0.1 m to 100 m with circular, helical, and twisted cross sections.10,35 Vapor-grown carbon nano fibers can now be obtained from Applied Sciences36 and Showa Denko.37

Carbon Nanotubes

Carbon nanotubes (CNT) were first reported by S. Iijima38 in 1991 and since then have been the subject of intensive research due to their remarkable mechanical,39 electrical,40 and thermal41 properties. Carbon nanotubes can be classified into single-wall nanotubes (SWNTs, typical diameter 0.7 nm to 1.5 nm), double-wall nanotubes (typical diameter, 2 nm to 5 nm), and multi-wall nanotubes (MWNT, typical diameter 5 nm to 50 nm). Single-wall nanotubes are composed of a single graphene layer rolled into a seamless cylinder. They can be semiconducting or metallic, depending on the diameter and chiral angle42 (0 to 30) of the tube. Double-wall nanotubes consist of two concentric tubes while MWNT are made of more than two concentric tubes. Nanotubes are synthesized by several methods including arc discharge,43 catalytic chemicalvapor deposition, and the high-pressure carbon monoxide process.44

Figure 6. A schematic of basic structural units arranged in a carbon fiber.51

Single-wall carbon nanotubes can be thought of as the ultimate carbon fiber because of their perfect graphitic structure, low density, and alignment with respect to each layer giving them exceptional engineering properties and light weight. The elastic modulus parallel to the nanotube axis is estimated to be ~640 GPa45 and the tensile strength, ~37 GPa.46 The electrical and thermal conductivity of SWNTs at 300 K are 106 S/m47 and -3,000 W/mK,48 respectively. The combination of density (1.3 g/cm^sup 3^) and mechanical, thermal, and electrical properties of SWNTs is unmatched, as there are no other materials with this combination of properties. The translation of these properties into macroscopic structures is the current challenge for the material scientists and engineers.

Table II. Properties of Various High-Performance Fibers*

STRUCTURE, PROPERTIES, AND MORPHOLOGY OF CARBON FIBERS

Structure and Morphology

The fine structure of carbon fibers consists of basic structural units of turbostratic carbon planes.49 The distance between turbostratic planes is generally >0.34 nm while the distance between perfect graphite planes is 0.3345 nm.49 Carbon fibers typically exhibit a skincore texture that has been confirmed using optical microscopy.50The skin can result from higher preferred orientation and a higher density of material at the fiber surface.l4 The formation of the skin is also associated with the coagulation conditions during PAN precursor fiber spinning. Figure 6 is a schematic of the basic structural units for carbon fibers based on various characterizations.51

Typical structural parameters for the selected pitch- and PAN- based carbon fibers are given in Table I. The crystallite size in the high-modulus pitch-based fibers is as high as 25 nm along the c axis, 64 nm along the a axis parallel to the fiber axis, and 88 nm along a-axis perpendicular to the fiber axis. Crystallite dimensions in fibers such as K-1100 are expected to be even larger. The crystallite size in the PAN-based carbon fibers (T300 and IM-8) is in the 1.5 nm to 5 nm range. High-modulus pitch-based carbon fibers exhibit high orientation, while the orientation of the pan-based carbon fibers is relatively low. High-modulus pitch-based carbon fibers (P-100 and P-120) also exhibit graphitic sheet-like morphology (Figure 7a and b), as well clear evidence of the three- dimensional order from x-ray diffraction .52 Due to the formation of microdomains that can bend and twist, carbon fibers contain defects, vacancies, dislocations, grain boundaries, and impurities.19 Low interlayer spacing, large crystallite size, high degree of orientation parallel to the fiber axis, low density of defects, and high degree of crystallinity are characteristics of the high tensile modulus and high thermal and high electrical conductivity fibers. Porosity in carbon fibers is measured using small-angle x-ray scattering.53This data can be used to estimate the size, shape, and orientation of the pores. Pore size, pore size distribution, and pore orientation changes as the fiber undergoes increasing heat treatment and tension.

Table III. Elastic Constants for Single-Crystal Graphite57

Properties

Figure 7. A scanning-electron micrograph of pitch-based P-100 fiber at (a) low and (b) high magnification.52

The axial compressive strength of PAN-based carbon fibers is higher than those of the pitch-based fibers (Figure 9), and it decreases with increasing modulus in both cases. It is understood that higher orientation, higher graphitic order, and larger crystal size all contribute negatively to the compressive strength. The PAN- based carbon fibers typically fail in the buckling mode, while pitchbased fibers fail by shearing mechanisms58 (Figure 10). This suggests that the compressive strength of intermediate modulus PAN- based carbon fibers may be higher than what is being realized in the composites.

Changes in the fiber geometry, effective fiber aspect ratio, fiber/matrix interfacial strength, and matrix stiffness can result in fiber compressive strength increase, until the failure mode changes from buckling to shear. Highcompressive-strength fibers also exhibit high shear modulus (Figure 11). The Compressive strength dependence of pitch- and PAN-based carbon fibers on various structural parameters has been studied52 and the compressive strength of high-performance fibers as well as compression test methods have been reviewed.59

The electrical and thermal conductivities increase with increasing fiber tensile modulus and carbonization temperature (Figure 12).60 Electrical conductivity of PAN-based carbon fibers is in the range of 10^sup 4^ S/m to 10^sup 5^ S/m, while that of the pitchbased carbon fibers is 10^sup 5^S/mto 10^sup 6^S/m. Electrical conductivity increases with temperature because as the temperature is raised the density and carrier (electrons and holes) mobility increases. Defects are known to cause carrier scattering. An increase in modulus is due to increased orientation of the carbon planes; this decreases the concentration of defects and subsequently decreases carrier scattering. Thermal conductivity of pitchbased carbon fibers is in the range of 20 W/mK to 1,000 W/mK. Carbon- fiber resistance to oxidation increases with the degree of graphitization. For carbon fibers, thermal gravimetric analysis in air shows the initial weight loss above 400C, sharp weight loss in the 500C to 600C range, and total weight loss by 850C. Axial coefficient of thermal expansion of the 200 GPa to 300 GPa modulus carbon fibers is in the range of -0.4 10^sup -6^/C to -0.8 10^sup - 5^/C and that for the high modulus (700 GPa to 900 GPa) carbon fibers it is about -1.6 10^sup -6^C.

Figure 8. The graphite tensile modulus versus misorientation angle.

Figure 9.The compressive strength versus tensile modulus for FAN and Pitch-based carbon fibers.

SURFACE TREATMENT

Surface treatment67 and surface properties of carbon fibers have been reviewed because carbon fibers used in composites are often coated or surface treated to improve interaction between the fiber surface and the matrix. Surface treatment usually results in the development of specific polar groups and/or roughness on the surface for enhanced interaction with the matrix. Surface treatment can be oxidative (e.g., in oxygen, nitric acid, or other oxidizing media) or non-oxidative. Non-oxidative treatment includes grafting of polymers or vapor-phase deposition of pyrolytic carbon on the carbon- fiber surface. Carbon fibers can be treated with plasma, and they can be sized by applying a thin coating of epoxy resin or other polymers to make them compatible with a particula\r matrix. Interlaminar shear strength of surfacetreated carbon fibers is reported to be in the range of 30 MPa to 90 MPa, while the BET surface area for these surfacetreated carbon fibers is typically 25 m^sup 2^/g to 60 m^sup 2^/g. Since carbon fibers degrade in the presence of oxygen above 400C and are stable in inert environment up to above 2,000C, they can be protected from oxidative degradation by the application of a coating such as SiC, Si^sub 3^N^sub 4,^ BN, and Al^sub 2^O^sub 3^.

APPLICATIONS OF CARBON FIBERS

Worldwide production of carbon fibers has increased from 15 million kg in 1997 to 20 million kg in 2002, and it is projected to reach 27 million kg by 2007.68 Carbon fiber costs have come down significantly since the 1980s, and the PAN- and pitch-based fiber production technology appears to have matured. While intermediate- modulus carbon fibers can now be purchased for about $20/kg, high- modulus, highly conducting fibers can cost as much as $3,000/kg. Carbon fibers are used in aerospace, aircraft, nuclear, sporting goods, biomedical, and high-end automotives. The high strength, high modulus, and low density of carbon fibers make them suitable for aerospace and sporting-goods applications. Carbon fibers are also used for chemical protective clothing, electromagnetic shielding, and as non-woven fire retardant.

Carbon-fiber-reinforced composites are made with polymer, metal, ceramic, and carbon matrices. Although the composite materials do not yield the same mechanical properties as the fibers alone, the matrix adds other properties to the composite for specific applications and holds the fiber together. As compared to most matrices, carbon fiber's coefficient of thermal expansion is typically two orders of magnitude lower, therefore it can improve the dimensional stability of the composite. While the early development of carbon fibers was prompted by defense and NASA applications, carbon fiber use in the civilian aerospace industry is rapidly increasing. For example, in a Boeing 767, carbon-fiber composites made up 3% of the total materials.69 This increased to 7% for the latest Boeing 777 model, while the Boeing 7E7, to roll off the assembly line in 2008, will have about 50 wt.% composite materials and will use 20% less fuel than airliners of the same size.

FUTURE PROSPECTS

A carbon-fiber modulus (~900 GPa) close to the theoretical value ( 1,060 GPa) has been achieved, while the experimental tensile strength achieved to date is only 10% to 20% of the theoretical estimates (>30 GPa). One hundred years ago, the only available fibers were natural cellulosic fibers such as cotton, and protein fibers such as wool and silk, as well as the man-made liber, rayon. The tensile strength of these fibers was less than 0.5 GPa. Today, a number of fibers are available with tensile strength ten times this value. Although carbon-fiber processing from PAN and pitch appears to have matured, PAN/CNT composite fibers have been processed,70 exhibiting improved tensile modulus and strength and reduced thermal shrinkage when compared to the control PAN fibers. These PAN/CNT composite fibers are good candidates for the development of next- generation carbon fibers with improved tensile strength and modulus while retaining compressive strength.71 Carbon-fiber tensile strength increases with decreasing diameter. Therefore, it is expected that the small-diameter electrospun PAN fibers (~100 nm) may exhibit significantly higher tensilestrength values than achieved so far.

Figure 10. (a) Kink bands in PAN-based carbon fiber after recoil compression under high deformation; (b) shear bands in high modulus mesophase pitch carbon fibers after moderate deformation.58

Figure 11. The compressive strength versus shear modulus of various carbon fibers. Adapted from Reference 52.

Figure 12. (a) The electrical conductivity dependence on tensile modulus for PANand pitch-based carbon fibers. (b)Thermal conductivity dependence on tensile modulus for pitch- and PAN-based carbon fibers. Adapted from References 61-66.

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Marilyn Minus and Satish Kumar are with the School of Polymer, Textile and Fiber Engineering at Georgia Institute of Technology in Atlanta, Georgia.

For more information, contact Satish Kumar, Georgia Institute of Technology, School of Polymer, Textile and Fiber Engineering,801 Ferst Drive, NW MRDC-1, Atlanta, Georgia 30332-0295; (404) 894- 7550; fax (404) 894-8780; e-mail satish.kumar@ptfe.gatech.edu.

Copyright Minerals, Metals & Materials Society Feb 2005

Source: JOM

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