Carbon Fibers

Carbon fibers represent an important class of graphite-related materials which are closely connected to carbon nanotubes, with regard to structure and properties. Despite the many precursors that can be used to synthesize carbon fibers, each having different cross-sectional morphologies (Fig. 3), the preferred orientation of the graphene planes is parallel to the fiber axis for all carbon fibers, thereby accounting for the high mechanical strength of carbon fibers [1]. Referring to the various morphologies in Fig. 3, the as-prepared vapor-grown fibers have an "onion skin" or "tree ring" morphology

GROWTH HOLLOW REGION CORE

Fig. 3. Sketch illustrating the morphology of Vapor-Grown Carbon Fibers (VGCF): (a) as-deposited at 1100° C [1], (b) after heat treatment to 3000° C [1]. The morphologies for commercial mesophase-pitch fibers are shown in (c) for a "PAC-man" cross section with a radial arrangement of the straight graphene ribbons and a missing wedge and (d) for a PAN-AM cross-sectional arrangement of graphene planes. In (e) a PAN fiber is shown, with a circumferential arrangement of ribbons in the sheath region and a random structure in the core

Fig. 3. Sketch illustrating the morphology of Vapor-Grown Carbon Fibers (VGCF): (a) as-deposited at 1100° C [1], (b) after heat treatment to 3000° C [1]. The morphologies for commercial mesophase-pitch fibers are shown in (c) for a "PAC-man" cross section with a radial arrangement of the straight graphene ribbons and a missing wedge and (d) for a PAN-AM cross-sectional arrangement of graphene planes. In (e) a PAN fiber is shown, with a circumferential arrangement of ribbons in the sheath region and a random structure in the core and after heat treatment to about 2500°C bear a close resemblance to carbon nanotubes (Fig. 3a). After further heat treatment to about 3000°C, the outer regions of the vapor grown carbon fibers form facets (Fig. 3b), and become more like graphite because of the strong interplanar correlations resulting from the facets [1,25]. At the hollow core of a vapor grown carbon fiber is a multiwall (and also a single wall) carbon nanotube (MWNT), as shown in Fig. 4, where the MWNT is observed upon fracturing a vapor grown carbon fiber [1]. Of all carbon fibers, the faceted vapor grown carbon fibers (Fig. 3b) are closest to crystalline graphite in both crystal structure and properties.

The commercially available mesophase pitch-based fibers, are exploited for their extremely high bulk modulus and high thermal conductivity, while the commercial PAN (polyacrylonitrile) fibers are widely used for their high tensile strength [1]. The high modulus of the mesophase pitch fibers is related to the high degree of c-axis orientation of adjacent graphene layers, while the high strength of the PAN fibers is related to defects in the structure. These structural defects inhibit the slippage of adjacent graphene planes relative to each other, and inhibit the sword-in-sheath failure mode (Fig. 5) that dominates the rupture of a vapor grown carbon fiber [1]. Typical diameters for individual commercial carbon fibers are ^ and they can be very long.

These fibers are woven into bundles called tows and are then wound up as a continuous yarn on a spool. The remarkable high strength and modulus of carbon fibers (Fig. 6) are responsible for most of the commercial interest

Fig. 4. (a) Carbon nanotube exposed on the breakage edge of a vapor grown carbon fiber as grown (a) and heat-treated at 3000° C (b). The sample is fractured by pulverization and the core diameter is ~5nm. (b) These photos suggest a structural discontinuity between the nanotube core of the fiber and the outer carbon layers deposited by chemical vapor deposition techniques. The photos show the strong mechanical properties of the nanotube core which maintain its form after breakage of the periphery [26]

Fig. 5. The sword-in-sheath failure mode of heat treated vapor grown carbon fibers. Such failure modes are also observed in multiwall carbon nanotubes [1]

in these fibers and these superior mechanical properties (modulus and tensile strength) should be compared to steel, for which typical strengths and bulk modulus values are 1.4 and 207 GPa, respectively [1]. The excellent mechanical properties of carbon nanotubes are closely related to the excellent mechanical properties of carbon fibers, though notable differences in behavior are also found, such as the flexibility of single wall carbon nanotubes and the good mechanical properties of multiwall nanotubes (with only a few walls) under compression, in contrast with carbon fibers which fracture easily under compressive stress along the fiber axis.

Vapor-grown carbon fibers can be prepared over a wide range of diameters (from ~10nm to more than 100|j.m) and these fibers have central hollow cores. A distinction is made between vapor grown carbon fibers and fibers

Fig. 6. The breaking strength of various types of carbon fibers plotted as a function of Young's modulus. Lines of constant strain can be used to estimate the breaking strains [1,27,28]

with diameters in the range 10-100 nm, which are called nanofibers, and exhibit properties intermediate between those of typical vapor grown carbon fibers, on the one hand, and MWNTs, on the other [24]. The preparation of vapor grown carbon fibers is based on the growth of a thin hollow tube of about 100 nm diameter (a nanofiber) by a catalytic process based on ultra-fine particles (~10 nm diameter) which have been super-saturated with carbon from the pyrolysis of a hydrocarbon gas at ^1050°C [1,29]. The thickening of the vapor-grown carbon fiber occurs through an epitaxial growth process, whereby the hydrocarbon gas is dehydrogenated at the ^1050°C growth temperature, and the carbon deposit is adsorbed on the surface of the growing fiber. Subsequent heat treatment to ^2500°C anneals the disordered carbon deposit and results in vapor grown carbon fibers with a tree ring coaxial cylinder morphology [29]. Further heat treatment to 2900°C results in faceted fibers (Fig. 3b) which exhibit structural and electronic properties very close to those of single crystal graphite [1,29]. If the growth process is stopped before the thickening step starts, MWNTs are obtained [30].

These vapor grown carbon fibers show (h, k, l) X-ray diffraction lines indicative of the 3D graphite structure and a semimetallic band overlap and carrier density similar to 3D graphite. The infrared and Raman spectra are essentially the same as that of 3D graphite. Vapor-grown carbon fibers and nanofibers with micrometer and several tens of nanometer diameters, respectively, provide intermediate materials between conventional mesophase pitch-derived carbon fibers (see Fig. 3) and single wall carbon nanotubes. Since their smallest diameters (~10nm) are too large to observe significant quantum confinement effects, it would be difficult to observe band gaps in their electronic structure, or the radial breathing mode in their phonon spectra. Yet subtle differences are expected between nanofiber transport properties, electronic

structure and Raman spectra relative to the corresponding phenomena in either MWNTs of graphitic vapor grown carbon fibers. At present, little is known in detail about the structure and properties of nanofibers, except that their properties are intermediate between those of vapor grown carbon fibers and MWNTs.

4.1 History of Carbon Fibers in Relation to Carbon Nanotubes

We provide here a brief review of the history of carbon fibers, the macroscopic analog of carbon nanotubes, since carbon nanotubes have become the focus of recent developments in carbon fibers.

The early history of carbon fibers was stimulated by needs for materials with special properties, both in the 19th century and more recently after World War II. The first carbon fiber was prepared by Thomas A. Edison to provide a filament for an early model of an electric light bulb. Specially selected Japanese Kyoto bamboo filaments were used to wind a spiral coil that was then pyrolyzed to produce a coiled carbon resistor, which could be heated ohmically to provide a satisfactory filament for use in an early model of an incandescent light bulb [31]. Following this initial pioneering work by Edison, further research on carbon filaments proceeded more slowly, since carbon filaments were soon replaced by a more sturdy tungsten filament in the electric light bulb. Nevertheless research on carbon fibers and filaments proceeded steadily over a long time frame, through the work of Schutzenberger and Schutzenberger (1890) [32], Pelabon [33], and others. Their efforts were mostly directed toward the study of vapor grown carbon filaments, showing filament growth from the thermal decomposition of hydrocarbons.

The second applications-driven stimulus to carbon fiber research came in the 1950's from the needs of the space and aircraft industry for strong, stiff light-weight fibers that could be used for building lightweight composite materials with superior mechanical properties. This stimulation led to great advances in the preparation of continuous carbon fibers based on polymer precursors, including rayon, polyacrylonitrile (PAN) and later mesophase pitch. The late 1950's and 1960's was a period of intense activity at the Union Carbide Corporation, the Aerospace Corporation and many other laboratories worldwide. This stimulation also led to the growth of a carbon whisker [23] (see Sect. 3), which has become a benchmark for the discussion of the mechanical and elastic properties of carbon fibers. The growth of carbon whiskers was also inspired by the successful growth of single crystal whisker filaments at that time for many metals such as iron, non-metals such as Si, and oxides such as Al2O3, and by theoretical studies [34], showing superior mechanical properties for whisker structures [35]. Parallel efforts to develop new bulk synthetic carbon materials with properties approaching single crystal graphite led to the development of highly oriented pyrolytic graphite (HOPG) in 1962 by Ubbelohde and co-workers [36,37], and HOPG has since been used as one of the benchmarks for the characterization of carbon fibers.

While intense effort continued toward perfecting synthetic filamentary carbon materials, and great progress was indeed made in the early 1960's, it was soon realized that long term effort would be needed to reduce fiber defects and to enhance structures resistive to crack propagation. New research directions were introduced because of the difficulty in improving the structure and microstructure of polymer-based carbon fibers for high strength and high modulus applications, and in developing graphitizable carbons for ultra-high modulus fibers. Because of the desire to synthesize more crystalline filamentous carbons under more controlled conditions, synthesis of carbon fibers by a catalytic Chemical Vapor Deposition (CVD) process proceeded, laying the scientific basis for the mechanism and thermodynamics for the vapor phase growth of carbon fibers in the 1960's and early 1970's [1]. In parallel to these scientific studies, other research studies focused on control of the process for the synthesis of vapor grown carbon fiber [38,39,40,41], leading to the more recent commercialization of vapor grown carbon fibers in the 1990's for various applications. Concurrently, polymer-based carbon fiber research has continued worldwide, mostly in industry, with emphasis on greater control of processing steps to achieve carbon fibers with ever-increasing modulus and strength, and on fibers with special characteristics, such as very high thermal conductivity, while decreasing costs of the commercial products.

As research on vapor grown carbon fibers on the micrometer scale proceeded, the growth of very small diameter filaments less than 10 nm (Fig. 7), was occasionally observed and reported [42,43], but no detailed systematic studies of such thin filaments were carried out. An example of a very thin vapor grown nanofiber along with a multiwall nanotube is shown in the bright field TEM image of Fig. 7 [42,43,44,45].

Reports of thin filaments below 10 nm inspired Kubo [46] to ask whether there was a minimum dimension for such filaments. Early work [42,43] on vapor grown carbon fibers, obtained by thickening filaments such as the fiber denoted by VGCF (Vapor Grown Carbon Fiber) in Fig. 7, showed very sharp lattice fringe images for the inner-most cylinders. Whereas the outermost layers of the fiber have properties associated with vapor grown carbon fibers,

Fig. 7. High-resolution TEM micrograph showing carbon a vapor grown carbon nanofiber (VGCF) with an diameter less than 10 nm and a nanotube [42,43,44,45]

there may be a continuum of behavior of the tree rings as a function of diameter, with the innermost tree rings perhaps behaving like carbon nanotubes.

Direct stimulus to study carbon filaments of very small diameters more systematically [11] came from the discovery of fullerenes by Kroto and Smal-ley [10]. In December 1990 at a carbon-carbon composites workshop, papers were given on the status of fullerene research by Smalley [47], the discovery of a new synthesis method for the efficient production of fullerenes by Huffman [48], and a review of carbon fiber research by Dresselhaus [49]. Discussions at the workshop stimulated Smalley to speculate about the existence of carbon nanotubes of dimensions comparable to Ceo. These conjectures were later followed up in August 1991 by discussions at a fullerene workshop in Philadelphia [50] on the symmetry proposed for a hypothetical single-wall carbon nanotubes capped at either end by fullerene hemispheres, with suggestions on how zone folding could be used to examine the electron and phonon dispersion relations of such structures. However, the real breakthrough on carbon nanotube research came with Iijima's report of experimental observation of carbon nanotubes using transmission electron microscopy [11]. It was this work which bridged the gap between experimental observation and the theoretical framework of carbon nanotubes in relation to fullerenes and as theoretical examples of 1D systems. Since the pioneering work of Iijima [11], the study of carbon nanotubes has progressed rapidly.

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