E.T. Thostenson et al. Composites Science and Technology 61(2001)1899-1912 1901 instability of carbon nanotubes beyond linear response Their simulations show that carbon nanotubes are remarkably resilient, sustaining extreme strain with no signs of brittleness or plasticity. Although the chirality has a relatively small influence on the elastic stiffness, they concluded that the Stone-Wales transformation, a reversible diatomic interchange where the resulting structure is two pentagons and two heptagons in pairs snm plays a key role in the nanotube plastic deformation under tension The Stone- Wales transformation shown in Fig 3, occurs when an armchair nanotube is stressed Fig 4. TEM micrograph showing the layered structure of a multi- in the axial direction. Nardelli et al. 12] theorized that walled carbon nanotube the Stone- Wales transformation results in ductile frac- ture for armchair nanotubes are held together by secondary, van der Waals bonding Single-walled nanotubes are most desired for fundamental 2. 2. Nanotube tions of the carbon nanotubes. since the intra-tube interactions fur As mentioned before, fullerenes are closed, convex ther complicate the properties of carbon nanotubes cages that are composed of pentagons and exagons Indeed. both single and multi-walled nanotubes show The Stone-Wales transformation introduces a new unique properties that can be exploited for use in com defect in the nanotube structure, the heptagon. Hepta gons allow for concave areas within the nanotube. Thus, the heptagonal defects in nanotubes can result in many possible equilibrium shapes. Indeed, most nanotubes are 3. Pr ocess ssing of carbon nanotubes for composite not straight cylinders with hemispherical caps materials In addition to different tube morphologies resulting from defects, carbon nanotubes can be single walled or Since carbon nanotubes were discovered nearly a dec multi-walled structures. Fig. 4 shows a transmission ade ago, there have been a variety of techniques devel electron microscope (TEM) image showing the nano- oped for producing them. lijima [1] first observed multi structure of a multi-walled carbon nanotube where sev- walled nanotubes, and lijima et al. 13] and Bethune et al eral layers of graphitic carbon and a hollow core are evi- [14] independently reported the synthesis of single-walled dent. Multi-walled carbon nanotubes are essentially nanotubes a few years later. Primary synthesis method concentric single walled tubes, where each individual tube for single and multi-walled carbon nanotubes include can have different chirality. These concentric nanotubes arc-discharge [1, 15, laser ablation [16), gas-phase cata lytic growth from carbon monoxide [17], and chemical 食 vapor deposition(CVD)from hydrocarbons [18-201 methods. For application of carbon nanotubes in com posites, large quantities of nanotubes are required, and the scale-up limitations of the arc discharge and laser ablation techniques would make the cost of nanotube- 5 based composites prohibitive. During nanotube synthesis impurities in the form of catalyst particles, amorphous carbon, and non-tubular fullerenes are also produced T rate the tubes. The gas-phase processes tend to produce nanotubes with fewer impurities and are more amenable to large-scale processing. It is the authors' belief that gas 5 phase techniques, such as CVD, for nanotube growth offer the greatest potential for the scaling-up of nano- ube production for the processing of composites. In his section, we briefly review the primary techniques for producing carbon nanotubes and some of the benefits and draw backs of each technique lijima [1] first observed nanotubes synthesized from Fig. 3. The Stone- Wales transformation occurring in an armcha the electric-arc discharge technique. Shown schemati nanotu be under axial tension cally in Fig. 5, the arc discharge technique generallyinstability of carbon nanotubes beyond linear response. Their simulations show that carbon nanotubes are remarkably resilient, sustaining extreme strain with no signs of brittleness or plasticity. Although the chirality has a relatively small influence on the elastic stiffness, they concluded that the Stone-Wales transformation, a reversible diatomic interchange where the resulting structure is two pentagons and two heptagons in pairs, plays a key role in the nanotube plastic deformation under tension. The Stone-Wales transformation, shown in Fig. 3, occurs when an armchair nanotube is stressed in the axial direction. Nardelli et al. [12] theorized that the Stone-Wales transformation results in ductile frac￾ture for armchair nanotubes. 2.2. Nanotube morphology As mentioned before, fullerenes are closed, convex cages that are composed of pentagons and hexagons. The Stone-Wales transformation introduces a new defect in the nanotube structure, the heptagon. Hepta￾gons allow for concave areas within the nanotube. Thus, the heptagonal defects in nanotubes can result in many possible equilibrium shapes. Indeed, most nanotubes are not straight cylinders with hemispherical caps. In addition to different tube morphologies resulting from defects, carbon nanotubes can be single walled or multi-walled structures. Fig. 4 shows a transmission electron microscope (TEM) image showing the nano￾structure of a multi-walled carbon nanotube where sev￾eral layers of graphitic carbon and a hollow core are evi￾dent. Multi-walled carbon nanotubes are essentially concentric single walled tubes, where each individual tube can have different chirality. These concentric nanotubes are held together by secondary, van der Waals bonding. Single-walled nanotubes are most desired for fundamental investigations of the structure/property relationships in carbon nanotubes, since the intra-tube interactions fur￾ther complicate the properties of carbon nanotubes. Indeed, both single and multi-walled nanotubes show unique properties that can be exploited for use in com￾posite materials. 3. Processing of carbon nanotubes for composite materials Since carbon nanotubes were discovered nearly a dec￾ade ago, there have been a variety of techniques devel￾oped for producing them. Iijima [1] first observed multi￾walled nanotubes, and Iijima et al. [13] and Bethune et al. [14] independently reported the synthesis of single-walled nanotubes a few years later. Primary synthesis methods for single and multi-walled carbon nanotubes include arc-discharge [1,15], laser ablation [16], gas-phase cata￾lytic growth from carbon monoxide [17], and chemical vapor deposition (CVD) from hydrocarbons [18–20] methods. For application of carbon nanotubes in com￾posites, large quantities of nanotubes are required, and the scale-up limitations of the arc discharge and laser ablation techniques would make the cost of nanotube￾based composites prohibitive. During nanotube synthesis, impurities in the form of catalyst particles, amorphous carbon, and non-tubular fullerenes are also produced. Thus, subsequent purification steps are required to sepa￾rate the tubes. The gas-phase processes tend to produce nanotubes with fewer impurities and are more amenable to large-scale processing. It is the authors’ belief that gas￾phase techniques, such as CVD, for nanotube growth offer the greatest potential for the scaling-up of nano￾tube production for the processing of composites. In this section, we briefly review the primary techniques for producing carbon nanotubes and some of the benefits and drawbacks of each technique. Iijima [1] first observed nanotubes synthesized from the electric-arc discharge technique. Shown schemati￾cally in Fig. 5, the arc discharge technique generally Fig. 3. The Stone-Wales transformation occurring in an armchair nanotube under axial tension. Fig. 4. TEM micrograph showing the layered structure of a multi￾walled carbon nanotube. E.T. Thostenson et al. / Composites Science and Technology 61 (2001) 1899–1912 1901