《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_fibrous monoliths-24-CARBON NANOTUBE

COMPOSITES SCIENCE AND TECHNOLOGY ELSEⅤIER Composites Science and Technology 61(2001)1899-1912 www.elsevier.com/locate/compscitech Advances in the science and technology of carbon nanotubes and their composites: a review Erik t. Thostensona, Zhifeng Ren, Tsu-Wei Chou* Department of Mechanical Engineering and Center for Composite Materials, University of Delaware, Newark, DE 19716, US.A Department of Physics, Boston College, Chestnut Hill, M. 02167, USA Received I May 2001; received in revised form 19 June 2001; accepted 21 June 2001 Abstract Since their first observation nearly a decade ago by lijima (lijima S. Helical microtubules of graphitic carbon Nature. 1991 354: 56-8), carbon nanotubes have been the focus of considerable research. Numerous investigators have since reported remarkable physical and mechanical properties for this new form of carbon. From unique electronic properties and a thermal conductivity higher than diamond to mechanical properties where the stiffness, strength and resilience exceeds any current material, carbon nanotubes offer tremendous opportunities for the development of fundamentally new material systems. In particular, the excep- tional mechanical properties of carbon nanotubes, combined with their low density, offer scope for the development of nanotube reinforced composite materials. The potential for nanocomposites reinforced with carbon tubes having extraordinary specific stiff- ness and strength represent tremendous opportunity for application in the 21st century. This paper provides a concise review of recent advances in carbon nanotubes and their composites. We examine the research work reported in the literature on the structure d processing of carbon nanotubes, as well as characterization and property modeling of carbon nanotubes and their composites C 2001 Elsevier Science Ltd. All rights reserved 1. Introduction consequence of their symmetric structure. Many researchers have reported mechanical properties of car- In the mid 1980s, Smalley and co-workers at Rice bon nanotubes that exceed those of any previously niversity developed the chemistry of fullerenes [2]. existing materials. Although there are varying reports in Fullerenes are geometric cage-like structures of carbon the literature on the exact properties of carbon nano- atoms that are composed of hexagonal and pentagonal tubes, theoretical and experimental results have shown faces. The first closed, convex structure formed was extremely high elastic modulus, greater than I TPa(the the C6o molecule. Named after the architect known for elastic modulus of diamond is 1.2 TPa)and reported designing geodesic domes, R. Buckminster Fuller, strengths 10-100 times higher than the strongest steel buckminsterfullerene closed cage of 60 carbon at a fraction of the weight. Indeed, if the reported atoms where each side of a pentagon is the adjacent side mechanical properties are accurate, carbon nanotubes of a hexagon similar to a soccer ball( the Coo molecule is may result in an entire new class of advanced materials often referred to as a bucky ball)[]. A few years later, To unlock the potential of carbon nanotubes for appli- their discovery led to the synthesis of carbon nanotubes. cation in polymer nanocomposites, one must fully Nanotubes are long, slender fullerenes where the walls understand the elastic and fracture properties of carbon of the tubes are hexagonal carbon (graphite structure) nanotubes as well as the interactions at the nanotube and often capped at each end matrix interface. Although this requirement is no dif- These cage-like forms of carbon have been shown ferent from that for conventional fiber-reinforced com- to exhibit exceptional material properties that are a posites [3], the scale of the reinforcement phase diameter has changed from micrometers(e.g. glass and carbon 4 Corresponding author. Tel: +1-302-831-2421; fax: +1-302-831 fibers)to nanometers In addition to the exceptional mechanical properties E-mail address: chou(@ me. udeledu(T.w. Chou) associated with carbon nanotubes, they also posses 0266-3538/01/ S.see front matter C 2001 Elsevier Science Ltd. All rights reserved. PII:S0266-3538(01)00094-X

Advances in the science and technology of carbon nanotubes and their composites: a review Erik T. Thostensona , Zhifeng Renb, Tsu-Wei Choua,* a Department of Mechanical Engineering and Center for Composite Materials, University of Delaware, Newark, DE 19716, USA bDepartment of Physics, Boston College, Chestnut Hill, MA 02167, USA Received 1 May 2001; received in revised form 19 June 2001; accepted 21 June 2001 Abstract Since their first observation nearly a decade ago by Iijima (Iijima S. Helical microtubules of graphitic carbon Nature. 1991; 354:56–8), carbon nanotubes have been the focus of considerable research. Numerous investigators have since reported remarkable physical and mechanical properties for this new form of carbon. From unique electronic properties and a thermal conductivity higher than diamond to mechanical properties where the stiffness, strength and resilience exceeds any current material, carbon nanotubes offer tremendous opportunities for the development of fundamentally new material systems. In particular, the excep￾tional mechanical properties of carbon nanotubes, combined with their low density, offer scope for the development of nanotube￾reinforced composite materials. The potential for nanocomposites reinforced with carbon tubes having extraordinary specific stiff- ness and strength represent tremendous opportunity for application in the 21st century. This paper provides a concise review of recent advances in carbon nanotubes and their composites. We examine the research work reported in the literature on the structure and processing of carbon nanotubes, as well as characterization and property modeling of carbon nanotubes and their composites. # 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction In the mid 1980s, Smalley and co-workers at Rice University developed the chemistry of fullerenes [2]. Fullerenes are geometric cage-like structures of carbon atoms that are composed of hexagonal and pentagonal faces. The first closed, convex structure formed was the C60 molecule. Named after the architect known for designing geodesic domes, R. Buckminster Fuller, buckminsterfullerene is a closed cage of 60 carbon atoms where each side of a pentagon is the adjacent side of a hexagon similar to a soccer ball (the C60 molecule is often referred to as a bucky ball) [2]. A few years later, their discovery led to the synthesis of carbon nanotubes. Nanotubes are long, slender fullerenes where the walls of the tubes are hexagonal carbon (graphite structure) and often capped at each end. These cage-like forms of carbon have been shown to exhibit exceptional material properties that are a consequence of their symmetric structure. Many researchers have reported mechanical properties of car￾bon nanotubes that exceed those of any previously existing materials. Although there are varying reports in the literature on the exact properties of carbon nano￾tubes, theoretical and experimental results have shown extremely high elastic modulus, greater than 1 TPa (the elastic modulus of diamond is 1.2 TPa) and reported strengths 10–100 times higher than the strongest steel at a fraction of the weight. Indeed, if the reported mechanical properties are accurate, carbon nanotubes may result in an entire new class of advanced materials. To unlock the potential of carbon nanotubes for appli￾cation in polymer nanocomposites, one must fully understand the elastic and fracture properties of carbon nanotubes as well as the interactions at the nanotube/ matrix interface. Although this requirement is no dif￾ferent from that for conventional fiber-reinforced com￾posites [3], the scale of the reinforcement phase diameter has changed from micrometers (e.g. glass and carbon fibers) to nanometers. In addition to the exceptional mechanical properties associated with carbon nanotubes, they also posses 0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(01)00094-X Composites Science and Technology 61 (2001) 1899–1912 www.elsevier.com/locate/compscitech * Corresponding author. Tel.: +1-302-831-2421; fax: +1-302-831- 3619. E-mail address: chou@me.udel.edu (T.-W. Chou)

1900 E.T. Thostenson ef al/ Composites Science and Technology 61(2001) 1899-1912 uperior thermal and electric properties: thermally stable up to 2800C in vacuum, thermal conductivity about twice as high as diamond, electric-current-carrying capacity 1000 times higher than copper wires [4. These exceptional properties of carbon nanotubes have been investigated for devices such as field-emission display [5]. scanning probe microscopy tips [6], and micro- electronic devices [7, 8]. In this paper we provide an overview of the recent advances in processing, character- ization, and modeling of carbon nanotubes and their 0, Chiral Angle composites. This review is not intended to be compre hensive, as our focus is on exploiting the exceptional mechanical properties of carbon nanotubes toward the development of f macroscopic structural materials Fig. I. Schematic diagram showing how a hexagonal sheet of grap Indeed, the exceptional physical properties of carbon is rolled to form a carbon nanotube tifunctionalea a present the opportunity to develop mul- nanotubes al notube composites with tailored physical two limiting cases exist where the chiral angle is at 0o nd mechanical properties and 30. These limiting cases are referred to as ziz-zag (0)and armchair(30%) based on the geometry of the carbon bonds around the circumference of the nanotube 2. Atomic structure and morphology of carbon nanotubes The difference in armchair and zig-zag nanotube struc- tures is shown in Fig. 2. In terms of the roll-up vector, the Carbon nanotubes can be visualized as a sheet of ziz-zag nanotube is (n, 0)and the armchair nanotube graphite that has been rolled into a tube. Unlike dia-(n, n). The roll-up vector of the nanotube also defines the mond, where a 3-D diamond cubic crystal structure is nanotube diameter since the inter-atomic spacing of the formed with each carbon atom having four nearest carbon atoms is known neighbors arranged in a tetrahedron, graphite is formed The chirality of the carbon nanotube has significant as a 2-D sheet of carbon atoms arranged in a hexagonal implications on the material properties. In particular, array. In this case, each carbon atom has three nearest tube chirality is known to have a strong impact on the neighbors. 'Rolling sheets of graphite into cylinders electronic properties of carbon nanotubes. Graphite is forms carbon nanotubes. The properties of nanotubes considered to be a semi-metal, but it has been shown depend on atomic arrangement(how the sheets of gra- that nanotubes can be either metallic or semiconduct phite are 'rolled), the diameter and length of the tubes, ing, depending on tube chirality [9] and the morphology, or nano structure. Nanotubes Investigations on the influence of chirality on the exist as either single-walled or multi-walled structures, mechanical properties have also been reported. The ind multi-walled carbon nanotubes (MWCNTs) are analytical work of Yakobson et al. [10, 11] examined the simply composed of concentric single-walled carbon nanotubes(SWCNTs 2. Nanotube structure oO The atomic structure of nanotubes is described in terms of the tube chirality, or helicity, which is defined ooo ao op- ocp by the chiral vector, Ch, and the chiral angle, 6. In Fig. I we can visualize cutting the graphite sheet along the dotted lines and rolling the tube so that the tip of the chiral vector touches its tail. the chiral vector often known as the roll-up vector, can be described by the oooO following equation eo9φ8°。。8 here the integers (n, m) are the number of steps along the ziz-zag carbon bonds of the hexagonal lattice and al and a are unit vectors, shown in Fig. 1. The chiral strations of the atomic structure of (a) an armd angle determines the amount of 'twist in the tube. the a ziz-zag nanotube

superior thermal and electric properties: thermally stable up to 2800
C in vacuum, thermal conductivity about twice as high as diamond, electric-current-carrying capacity 1000 times higher than copper wires [4]. These exceptional properties of carbon nanotubes have been investigated for devices such as field-emission displays [5], scanning probe microscopy tips [6], and micro￾electronic devices [7,8]. In this paper we provide an overview of the recent advances in processing, character￾ization, and modeling of carbon nanotubes and their composites. This review is not intended to be compre￾hensive, as our focus is on exploiting the exceptional mechanical properties of carbon nanotubes toward the development of macroscopic structural materials. Indeed, the exceptional physical properties of carbon nanotubes also present the opportunity to develop mul￾tifunctional nanotube composites with tailored physical and mechanical properties. 2. Atomic structure and morphology of carbon nanotubes Carbon nanotubes can be visualized as a sheet of graphite that has been rolled into a tube. Unlike dia￾mond, where a 3-D diamond cubic crystal structure is formed with each carbon atom having four nearest neighbors arranged in a tetrahedron, graphite is formed as a 2-D sheet of carbon atoms arranged in a hexagonal array. In this case, each carbon atom has three nearest neighbors. ‘Rolling’ sheets of graphite into cylinders forms carbon nanotubes. The properties of nanotubes depend on atomic arrangement (how the sheets of gra￾phite are ‘rolled’), the diameter and length of the tubes, and the morphology, or nano structure. Nanotubes exist as either single-walled or multi-walled structures, and multi-walled carbon nanotubes (MWCNTs) are simply composed of concentric single-walled carbon nanotubes (SWCNTs). 2.1. Nanotube structure The atomic structure of nanotubes is described in terms of the tube chirality, or helicity, which is defined by the chiral vector, C~h, and the chiral angle, . In Fig. 1, we can visualize cutting the graphite sheet along the dotted lines and rolling the tube so that the tip of the chiral vector touches its tail. The chiral vector, often known as the roll-up vector, can be described by the following equation: C~h ¼ na~1 þ ma~2 ð1Þ where the integers (n, m) are the number of steps along the ziz-zag carbon bonds of the hexagonal lattice and a~1 and a~2 are unit vectors, shown in Fig. 1. The chiral angle determines the amount of ‘twist’ in the tube. The two limiting cases exist where the chiral angle is at 0
and 30
. These limiting cases are referred to as ziz-zag (0
) and armchair (30
) based on the geometry of the carbon bonds around the circumference of the nanotube. The difference in armchair and zig-zag nanotube struc￾tures is shown in Fig. 2. In terms of the roll-up vector, the ziz-zag nanotube is (n, 0) and the armchair nanotube is (n, n). The roll-up vector of the nanotube also defines the nanotube diameter since the inter-atomic spacing of the carbon atoms is known. The chirality of the carbon nanotube has significant implications on the material properties. In particular, tube chirality is known to have a strong impact on the electronic properties of carbon nanotubes. Graphite is considered to be a semi-metal, but it has been shown that nanotubes can be either metallic or semiconduct￾ing, depending on tube chirality [9]. Investigations on the influence of chirality on the mechanical properties have also been reported. The analytical work of Yakobson et al. [10,11] examined the Fig. 2. Illustrations of the atomic structure of (a) an armchair and (b) a ziz-zag nanotube. Fig. 1. Schematic diagram showing how a hexagonal sheet of graphite is ‘rolled’ to form a carbon nanotube. 1900 E.T. Thostenson et al. / Composites Science and Technology 61 (2001) 1899–1912

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 generally

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, 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

E.T. Thostenson et al. Composites Science and Technology 61(2001)1899-1912 growing nanotubes feedthrough connection connection Fig. 5. Schematic illustration of the arc-discharge technique (after Fig. 6. Schematic of the laser ablation process(after Ref (4D Ref.[22) involves the use of two high-purity graphite rods as the Smalley and his co-workers at Rice University have anode and cathode. The rods are brought together refined the process to produce large quantities of single- under a helium atmosphere and a voltage is applied walled carbon nanotubes with remarkable purity. The until a stable arc is achieved. The exact process variables So-called HiPco nanotubes(high-pressure conversion of depend on the size of the graphite rods. As the anode is carbon monoxide) have received considerable attention consumed, a constant gap between the anode and cath- as the technology has been commercialized by Carbon ode is maintained by adjusting the position of the Nanotechnologies Inc(Houston, TX) for large-scale anode. The material then deposits on the cathode to production of high-purity single-walled carbon nano- form a build-up consisting of an outside shell of fused tubes material and a softer fibrous core containing nanotubes Other gas-phase techniques utilize hydrocarbon gases and other carbon particles. To achieve si as the carbon source for production of both single and nanotubes, the electrodes are doped with a small multi-walled carbon nanotubes via CVD [25-28. Niko amount of metallic catalyst particles [13-15, 21, 22 laev and co-workers [17 point out that hydrocarbons first used for the initial synthes of fullerenes. Over the years, the technique has been As a consequence, nanotubes grown from hydrocarbons improved to allow the production of single-walled can have substantial amorphous carbon deposits on the nanotubes [16, 23, 24]. In this technique, a laser is used to surface of the tubes and will require further purification vaporize a graphite target held in a controlled atme steps. Although the disassociation of hydrocarbons at sphere oven at temperatures near 1200C. The general low temperatures affects the purity of the as-processed set-up for laser ablation is shown in Fig. 6. To produce nanotubes, the lower processing temperature enables single-walled nanotubes, the graphite target was doped the growth of carbon nanotubes on a wide variety of with cobalt and nickel catalyst [16]. The condensed substrates, including glass material is then collected on a water-cooled targel in- synthesize aligned arrays of carbon nanotubes with One unique aspect of CVD techniques is its ability to Both the arc-discharge and the laser-ablation ques are limited in the volume of sample they can pro- controlled diameter and length. The synthesis of well- duce in relation to the size of the carbon source(the aligned straight carbon nanotubes on a variety of sub- anode in arc-discharge and the target in laser ablation). strates has been accomplished by the use of plasma In addition, subsequent purification steps are necessary enhanced chemical vapor deposition(PeCvd)where the to separate the tubes from undesirable by-products. plasma is excited by a DC source[18-20]or a microwave These limitations have motivated the development of source [29-33]. Fig 7a and b shows the ability to grow gas-phase techniques, such as chemical vapor deposition straight carbon nanotubes over a large area with (CVD), where nanotubes are formed by the decomposi- excellent uniformity in diameter, length, straightness, tion of a carbon-containing gas. The gas-phase techni- and site density. Adjusting the thickness of the catalyst ques are amenable to continuous processes since the layer controls the diameter of the tubes, shown in carbon source is continually replaced by flowing gas In Fig. &a and b addition, the final purity of the as-produced nanotubes In CVd growth of straight carbon nanotube arrays, can be quite high, minimizing subsequent purification described by Ren et al. [19]a substrate is first coated a layer of nickel catalyst. High-purity ammonia is then Nikolaev et al. [17 describe the gas-phase growth of used as the catalytic gas and acetylene as the carbon single-walled carbon nanotubes with carbon monoxide source. a direct-current power generates the required as the carbon source. They reported the highest yields of plasma, and a deeply carbonized tungsten filament single walled nanotubes occurred at the highest acces- assists the dissociation of the reactive gases and supplies ble temperature and pressure(1200C, 10 atm). heat to the substrate. Control over the nanotube length

involves the use of two high-purity graphite rods as the anode and cathode. The rods are brought together under a helium atmosphere and a voltage is applied until a stable arc is achieved. The exact process variables depend on the size of the graphite rods. As the anode is consumed, a constant gap between the anode and cath￾ode is maintained by adjusting the position of the anode. The material then deposits on the cathode to form a build-up consisting of an outside shell of fused material and a softer fibrous core containing nanotubes and other carbon particles. To achieve single walled nanotubes, the electrodes are doped with a small amount of metallic catalyst particles [13–15,21,22]. Laser ablation was first used for the initial synthesis of fullerenes. Over the years, the technique has been improved to allow the production of single-walled nanotubes [16,23,24]. In this technique, a laser is used to vaporize a graphite target held in a controlled atmo￾sphere oven at temperatures near 1200
C. The general set-up for laser ablation is shown in Fig. 6. To produce single-walled nanotubes, the graphite target was doped with cobalt and nickel catalyst [16]. The condensed material is then collected on a water-cooled target. Both the arc-discharge and the laser-ablation techni￾ques are limited in the volume of sample they can pro￾duce in relation to the size of the carbon source (the anode in arc-discharge and the target in laser ablation). In addition, subsequent purification steps are necessary to separate the tubes from undesirable by-products. These limitations have motivated the development of gas-phase techniques, such as chemical vapor deposition (CVD), where nanotubes are formed by the decomposi￾tion of a carbon-containing gas. The gas-phase techni￾ques are amenable to continuous processes since the carbon source is continually replaced by flowing gas. In addition, the final purity of the as-produced nanotubes can be quite high, minimizing subsequent purification steps. Nikolaev et al. [17] describe the gas-phase growth of single-walled carbon nanotubes with carbon monoxide as the carbon source. They reported the highest yields of single walled nanotubes occurred at the highest acces￾sible temperature and pressure (1200
C, 10 atm). Smalley and his co-workers at Rice University have refined the process to produce large quantities of single￾walled carbon nanotubes with remarkable purity. The so-called HiPco nanotubes (high-pressure conversion of carbon monoxide) have received considerable attention as the technology has been commercialized by Carbon Nanotechnologies Inc (Houston, TX) for large-scale production of high-purity single-walled carbon nano￾tubes. Other gas-phase techniques utilize hydrocarbon gases as the carbon source for production of both single and multi-walled carbon nanotubes via CVD [25–28]. Niko￾laev and co-workers [17] point out that hydrocarbons pyrolize readily on surfaces heated above 600–700
C. As a consequence, nanotubes grown from hydrocarbons can have substantial amorphous carbon deposits on the surface of the tubes and will require further purification steps. Although the disassociation of hydrocarbons at low temperatures affects the purity of the as-processed nanotubes, the lower processing temperature enables the growth of carbon nanotubes on a wide variety of substrates, including glass. One unique aspect of CVD techniques is its ability to synthesize aligned arrays of carbon nanotubes with controlled diameter and length. The synthesis of well￾aligned, straight carbon nanotubes on a variety of sub￾strates has been accomplished by the use of plasma￾enhanced chemical vapor deposition (PECVD) where the plasma is excited by a DC source [18–20] or a microwave source [29–33]. Fig. 7a and b shows the ability to grow straight carbon nanotubes over a large area with excellent uniformity in diameter, length, straightness, and site density. Adjusting the thickness of the catalyst layer controls the diameter of the tubes, shown in Fig. 8a and b. In CVD growth of straight carbon nanotube arrays, described by Ren et al. [19] a substrate is first coated with a layer of nickel catalyst. High-purity ammonia is then used as the catalytic gas and acetylene as the carbon source. A direct-current power generates the required plasma, and a deeply carbonized tungsten filament assists the dissociation of the reactive gases and supplies heat to the substrate. Control over the nanotube length Fig. 5. Schematic illustration of the arc-discharge technique (after Ref. [22]). Fig. 6. Schematic of the laser ablation process (after Ref. [4]). 1902 E.T. Thostenson et al. / Composites Science and Technology 61 (2001) 1899–1912

E T. Thostenson et al/ Composites Science and Technology 61(2001)1899-1912 Fig. 7. Micrographs showing the straightness of MWCNTs grown via PECVD [191 Fig 8. Micrographs showing control over the nanotube diameter:(a)40-50 nm and(b)200-300 nm aligned carbon nanotubes [19]- and graphitization is accomplished by changing the tube furnace. Fig. 10 is a SEM micrograph of the fur growth time and temperature, respectively, and applica- nace -grown carbon nanotubes showing the same ran- tion of the dC plasma results in tube growth in the dom, curled structure associated with thermal CVD direction of the plasma. The use of an alternating micro-(Shown in Fig 9). The outer diameters of these tubes wave frequency source to excite the plasma results in the range from 10-50 nm. These tangled, spaghetti-like growth of carbon nanotubes that occur directly normal to nanotubes can be produced at a larger quantity and lower the surface of the substrate. Bower et al. [29] showed that cost than PECVD tubes but there is less control over in microwave plasma-enhanced CVD (MPECV length, diameter, and structure alignment of the carbon nanotubes results from the self- bias that is imposed on the surface of the substrate from the microwave plasma. Fig 9a shows the alignment of 4. Characterization of carbon nanotubes carbon nanotubes grown normal to the surface of an optical glass fiber. To gain further insight into the Significant challenges exist in both the micromechanical mechanism for tube alignment, the tubes were grown for characterization of nanotubes and the modeling of the two minutes under the microwave- induced plasma fol- elastic and fracture behavior at the nano-scale Challenges lowed by 70 min with the plasma off. Fig 9b shows the in characterization of nanotubes and their composites results of this experiment. The upper portion of the include (a) complete lack of micromechanical character nanotubes are straight, indicating alignment in the ization techniques for direct property measurement, (b) plasma, and the base shows a random, curled structure tremendous limitations on specimen size, (c)uncertainty associated with thermal CVD. In addition, the growth in data obtained from indirect measurements, and (d) rate under the plasma enhancement was 40 times faster inadequacy in test specimen preparation techniques and than the thermal CVD lack of control in nanotube alignment and distribution In addition to highly aligned arrays of carbon nano- In order better to understand the mechanical proper tubes, large quantities of carbon nanotubes can be pro- ties of carbon nanotubes, a number of investigators have essed by conventional CVd techniques. Unlike attempted to characterize carbon nanotubes directly PECVD, which requires the use of specialized plasma Treacy et al. [34] first investigated the elastic modulus of equipment, tangled carbon nanotubes are grown in nanotu bes by measuring, in the

and graphitization is accomplished by changing the growth time and temperature, respectively, and applica￾tion of the DC plasma results in tube growth in the direction of the plasma. The use of an alternating micro￾wave frequency source to excite the plasma results in the growth of carbon nanotubes that occur directly normal to the surface of the substrate. Bower et al. [29] showed that in microwave plasma-enhanced CVD (MPECVD) alignment of the carbon nanotubes results from the self￾bias that is imposed on the surface of the substrate from the microwave plasma. Fig. 9a shows the alignment of carbon nanotubes grown normal to the surface of an optical glass fiber. To gain further insight into the mechanism for tube alignment, the tubes were grown for two minutes under the microwave-induced plasma fol￾lowed by 70 min with the plasma off. Fig. 9b shows the results of this experiment. The upper portion of the nanotubes are straight, indicating alignment in the plasma, and the base shows a random, curled structure associated with thermal CVD. In addition, the growth rate under the plasma enhancement was 40 times faster than the thermal CVD. In addition to highly aligned arrays of carbon nano￾tubes, large quantities of carbon nanotubes can be pro￾cessed by conventional CVD techniques. Unlike PECVD, which requires the use of specialized plasma equipment, tangled carbon nanotubes are grown in a tube furnace. Fig. 10 is a SEM micrograph of the fur￾nace-grown carbon nanotubes showing the same ran￾dom, curled structure associated with thermal CVD (shown in Fig. 9). The outer diameters of these tubes range from 10–50 nm. These tangled, spaghetti-like nanotubes can be produced at a larger quantity and lower cost than PECVD tubes, but there is less control over length, diameter, and structure. 4. Characterization of carbon nanotubes Significant challenges exist in both the micromechanical characterization of nanotubes and the modeling of the elastic and fracture behavior at the nano-scale. Challenges in characterization of nanotubes and their composites include (a) complete lack of micromechanical character￾ization techniques for direct property measurement, (b) tremendous limitations on specimen size, (c) uncertainty in data obtained from indirect measurements, and (d) inadequacy in test specimen preparation techniques and lack of control in nanotube alignment and distribution. In order better to understand the mechanical proper￾ties of carbon nanotubes, a number of investigators have attempted to characterize carbon nanotubes directly. Treacy et al. [34] first investigated the elastic modulus of isolated multi-walled nanotubes by measuring, in the Fig. 7. Micrographs showing the straightness of MWCNTs grown via PECVD [19]. Fig. 8. Micrographs showing control over the nanotube diameter: (a) 40–50 nm and (b) 200–300 nm aligned carbon nanotubes [19]. E.T. Thostenson et al. / Composites Science and Technology 61 (2001) 1899–1912 1903