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The properties are summarized and compared with other carbon allotropes in Table 1. Other excellent physical properties of CNTs have also attracted much attention. Although there is no consensus on the exact mechanical properties of CNTs, theoretical and experimental results have shown exceptional mechanical properties of CNTs with YoungРІР‚в„ўs modulus of 1.2 TPa and tensile strength of 50РІР‚вЂњ200 GPa ( Coleman et al., 2006). It is well known that the mechanical properties of CNTs exceed those of any existing materials. This sp 2structure provides CNTs with higher mechanical properties compared to any materials, even diamonds. The basic structure of CNTs is comprised of sp 2carbons. The graphite plane of nanotube surface coordinates ( Jin & Yuan, 2003). Each MWCNT contains a multi-layer of graphene, and each layer can have different chiralities, so the prediction of its physical properties is more complicated than that of SWCNT ( Jin & Yuan, 2003). For a given ( n, m) nanotube, if ( 2n + m) is a multiple of 3, then the nanotube is metallic, otherwise the nanotube is a semiconductor. The chirality of nanotubes has significant impact on their transport properties, particularly the electronic properties. If m= 0, the nanotubes are called РІР‚ВРІР‚ВzigzagРІР‚Сњ. If n = m, the nanotubes are called РІР‚ВРІР‚ВarmchairРІР‚Сњ. Using this ( n, m) scheme, the three types of nanotubes are characterized. To describe this structure, a chiral vector is defined as OA = na 1 + ma 2, where a 1and a 2are unit vectors for the hexagonal lattice of the graphene sheet, nand mare integers, along with a chiral angle РћС‘, which is the angle of the chiral vector with respect to the xdirection.

1 shows a segment of a single graphene plane that can be transformed into a carbon nanotube by rolling up into a cylinder. The exact magnitudes of these properties depend on the diameter and chirality of the nanotubes and whether their structure is single- or multi-walled. CNTs exhibit excellent mechanical, electrical, thermal and magnetic properties. CNTs are synthesized in a variety of ways, such as arc discharge, laser ablation, high pressure carbon monoxide (HiPCO), and chemical vapor deposition (CVD) ( Dresselhaus, 1997). In contrast, MWCNT diameters range from 5 to a few hundred nanometers because their structure consists of many concentric cylinders held together by van der Waals forces. SWCNT have diameters between 1 and 10 nm and normally capped ends. CNT lengths can be as short as a few hundred nanometers or as long as several micrometers. Two structural forms of CNTs exist: single-walled (SWCNTs) and multi-walled (MWCNTs) nanotubes. They can be described as a graphite sheet rolled up into a nanoscale tube. These properties make nanotubes ideal, not only for a wide range of applications but also as a test-bed for fundamental scientific studies ( Baughman et al., 2002). Since the first discovery of carbon nanotubes (CNTs) in 1991, CNTs have generated enormous research activities in many areas of science and engineering due to their combined exceptional mechanical, thermal and electronic properties.

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