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 nanotubesuperior 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