Thermal_&_Electrical_Conductivity_of_Carbon_Nanotube_(Cnt)

THERMAL & ELECTRICAL CONDUCTIVITY OF CARBON NANOTUBE (CNT) As a nano-graphitic structure, carbon nanotubes (CNTs) are very attractive thermal properties. The thermal properties of carbon nanotubes are dominated by phonons. The low-temperature specific heat and thermal conductivity show direct evidence of 1-D quantization of the phonon band structure. Measurements show single-wall carbon nanotubes (SWNTs) room-temperature thermal conductivity is about 3500 W/mK, over 200W/mK for bulk carbon nanotubes (SWNTs), and over 3000 W/mK for individual multiwalled carbon nanotubes (MWNTs).The addition of nanotubes to epoxy resin can double the thermal conductivity for a loading of only 1%, showing that nanotube composite materials may be useful for thermal management applications. In general, the thermal conductivity K is a tensor quality, but for this discussion, it is only important to consider the diagonal elements: where C is the specific heat, and vz and τ are the group velocity and relaxation time of a given phonon state. At low temperatures, the relaxation time is determined by scattering of fixed impurities, defects, sample boundaries, etc. and is roughly constant. Therefore, in ordinary materials, the low-temperature thermal conductivity has the same temperature dependence as the specific heat. However, in anisotropic materials, this relationship does not strictly hold. CNTs conductivity has been shown to be a function of their chirality, the degree of twist as well as their diameter. CNTs can be either metallic or semi-conducting in their electrical behavior.  Conductivity in MWNTs is quite complex. Some types of "armchair" structured CNTs appear to conduct better than other metallic CNTs. Furthermore, interwall reactions within multi walled nanotubes have been found to redistribute the current over individual tubes non-uniformly. However, there is no change in current across different parts of metallic single-walled nanotubes. The behavior of the ropes of semi-conducting single walled nanotubes is different, in that the transport current changes abruptly at various positions on the CNTs. The conductivity and resistivity of ropes of single walled nanotubes has been measured by placing electrodes at different parts of the CNTs. The resistivity of the single walled nanotubes ropes was of the order of 10–4 ohm.cm at 27°C. This means that single walled nanotube ropes are the most conductive carbon fibers known. The current density that was possible to achieve was 10-7 A/cm2, however in theory the single walled nanotube ropes should be able to sustain much higher stable current densities, as high as 10-13 A/cm2. It has been reported that individual single walled nanotubes may contain defects. These defects also allow the single walled nanotubes to act as transistors. Likewise, joining CNTs together may form transistor-like devices.