**2. Properties**

Carbon nanotubes are endowed with exceptionally high material properties, very close to their theoretical limits, such as electrical and thermal conductivity, strength, stiffness, tough‐ ness and low density.

### **2.1. Mechanical properties of CNTs**

The strength of C-C bond gives a large interest in mechanical properties of nanotubes. Theoreti‐ cally, these should be stiffer than any other known substance. Young's modulus of the single walled carbon nanotubes (SWCNTs) can be as high as 2.8-3.6 TPa and 1.7-2.4 TPa for multiwal‐ led carbon nanotubes (MWCNTs) [2]which is approximately 10 times higher than steel, the

strongest metallic alloy known. Experimental values of Young's modulus for SWCNTs are re‐ ported as high as to 1470 GPa and 950 GPa [3, 4] for MWCNTs, nearly 5 times of steel. There are no direct mechanical testing experiments that can be done on individual nanotubes (nanoscopic specimens) to determine directly their axial strength. However, the indirect experiments like AFM provide a brief view of the mechanical properties as well as scanning probe techniques that can manipulate individual nanotubes, have provide some basic answers to the mechanical be‐ havior of the nanotubes [5]. The analysis performed on several MWCNTs gave average Young's modulus values of 1.8 TPa, which is higher than the in – plane modulus for single crystal graph‐ ite. So the high stiffness and strength combined with low density implies that nanotubes could serve as ideal reinforcement in composite materials and provide them great potential in applica‐ tions such as aerospace and other military applications.

of about 3500 W m−1 K−1 and MWCNTs have a peak value of ~ 3000 W m−1 K−1 at 320 K; com‐ pare this to copper, a metal well-known for its good thermal conductivity, which transmits 385 W m−1 K−1 [8]. Although for bulk MWCNTs foils, thermal conductivity limits to 20 W m −1 K−1 suggesting that thermally opaque junctions between tubes severely limit the large scale diffusion of phonons. The thermal conductivity of CNTs across axis (in the radial di‐ rection) is about 1.52 W m−1 K−1, which is about as thermally conductive as soil. Both SWCNT and MWCNT materials and composites are being actively studied for thermal man‐ agement applications, either as "heat pipes" or as an alternative to metallic addition to low thermal conductive materials. In case of composites, the important limiting factors are quali‐

Carbon Nanotubes and Their Composites http://dx.doi.org/10.5772/52897 195

A variety of synthesis methods now exist to produce carbon nanotubes. The three main pro‐ duction methods used for synthesis of CNTs are d.c. arc discharge, laser ablation and chemi‐

The carbon arc discharge method, initially used for producing C60 fullerenes, is the most common and perhaps easiest way to produce carbon nanotubes as it is rather simple to un‐ dertake. In this method two carbon rods placed end to end, separated by approximately 1mm, in an enclosure that is usually filled with inert gas (helium, argon) at low pressure (be‐ tween 50 and 700 mbar) as shown in Figure 1. Recent investigations have shown that it is also possible to create nanotubes with the arc method in liquid nitrogen [9]. A direct current of 50 to 100 A driven by approximately 20 V creates a high temperature (~4000K) discharge between the two electrodes. The discharge vaporizes one of the carbon rods (anode) and forms a small rod shaped deposit on the other rod (cathode). Large-scale synthesis of MWCNTs by a variant of the standard arc-discharge technique was reported by Ebbesen and Ajayan [10]. A potential of 18 V dc was applied between two thin graphite rods in heli‐ um atmosphere. At helium pressure of ~500 Torr, the yield of nanotubes was maximal of 75% relative to the starting graphitic material. The TEM analysis revealed that the samples consisted of nanotubes of two or more concentric carbon shells. The nanotubes had diame‐ ters between 2 and 20 nm, and lengths of several micrometers. The tube tips were usually

If SWCNT are preferable, the anode has to be filled with metal catalyst, such as Fe, Co, Ni, Y or Mo. Experimental results show that the width and diameter distribution depends on the composition of the catalyst, the growth temperature and the various other growth condi‐ tions. If both electrodes are graphite, the main product will be MWCNTs. Typical sizes for MWCNTs are an inner diameter of 1-3nm and an outer diameter of approximately 10nm. Because no catalyst is involved in this process, there is no need for a heavy acidic purifica‐

tion step. This means MWCNT can be synthesized with a low amount of defects.

ty of dispersion and interphase thermal barriers.

**3. Synthesis of CNTs**

cal vapor deposition (CVD).

capped with pentagons.

**3.1. d.c. arc discharge technique**

#### **2.2. Electrical properties of CNTs**

The nanometer dimensions of CNTs, together with the unique electronic structure of a graphene sheet, make the electronic properties of these one-dimensional (1D) structures extraordinary. The one dimensional structure of CNTs helps them in making a good electric conductor. In a 3D conductor the possibility of scattering of electrons is large as these can scatter at any angle. Espe‐ cially notable is the fact that SWCNTs can be metallic or semiconducting depending on their structure and their band gap can vary from zero to about 2 eV, whereas MWCNTs are zero-gap metals. Thus, some nanotubes have conductivities higher than that of copper, while others be‐ have more like silicon. Theoretically, metallic nanotubes having electrical conductivity of 105 to 106 S/m can carry an electric current density of 4 × 109 A/cm2 which is more than 1000 times great‐ er than copper metal and hence can be used as fine electron gun for low weight displays. Due to the large diameter of MWCNTs, their transport properties approaches those of turbostatic graphite. Theoretical study also shows that in case of MWCNTs the overall behavior is deter‐ mined by the electronic properties of the external shell. Conductivities of individual MWCNTs have been reported to range between 20 and 2 × 107 S/m [6], depending on the helicities of the out‐ ermost shells or the presence of defects [7].The electronic properties of larger diameter MWCNTs approach those of graphite. Nanotubes have been shown to be superconducting at low temperatures. As probably CNTs are not perfect at ends and end defects like pentagons or heptagons are found to modify the electronic properties of these nanosystems drastically. There is great interest in the possibility of constructing nanoscale electronic devices from nanotubes and some progress is being made in this area. SWCNTs have been recently used to form conduct‐ ing and semiconducting layers (source, drain and gate electrodes) in thin films transistors. So the high electrical conductivity of CNTs makes them an excellent additive to impart electrical con‐ ductivity in otherwise insulating polymers. Their high aspect ratio means that a very low load‐ ing is needed to form a connecting network in a polymer compared to make them conducting.

#### **2.3. Thermal properties of CNTs**

CNTs are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube axis. Experiments on individual tubes are extremely difficult but meas‐ urements show that a SWCNT has a room-temperature thermal conductivity along its axis of about 3500 W m−1 K−1 and MWCNTs have a peak value of ~ 3000 W m−1 K−1 at 320 K; com‐ pare this to copper, a metal well-known for its good thermal conductivity, which transmits 385 W m−1 K−1 [8]. Although for bulk MWCNTs foils, thermal conductivity limits to 20 W m −1 K−1 suggesting that thermally opaque junctions between tubes severely limit the large scale diffusion of phonons. The thermal conductivity of CNTs across axis (in the radial di‐ rection) is about 1.52 W m−1 K−1, which is about as thermally conductive as soil. Both SWCNT and MWCNT materials and composites are being actively studied for thermal man‐ agement applications, either as "heat pipes" or as an alternative to metallic addition to low thermal conductive materials. In case of composites, the important limiting factors are quali‐ ty of dispersion and interphase thermal barriers.
