A carbon nanotube is a tube made of carbon with diameters typically measured in nanometers. Single-walled carbon nanotubes are one of the allotropes of carbon, intermediate between fullerene cages and flat graphene, with diameters in the range of one nanometer. Although not made this way, single-walled carbon nanotubes can be idealized as cutouts from a two-dimensional hexagonal lattice of carbon atoms rolled up along one of the Bravais lattice vectors of the hexagonal lattice to form a hollow cylinder. In this construction, periodic boundary conditions are imposed over the length of this roll-up vector to yield a helical lattice of seamlessly bonded carbon atoms on the cylinder surface . Multi-walled carbon nanotubes consisting of nested single-walled carbon nanotubes weakly bound together by van der Waals interactions in a tree ring-like structure. If not identical, these nanotubes are very similar to Oberlin, Endo, and Koyama's long straight and parallel carbon layers cylindrically arranged around a hollow nanotube . Multi-walled carbon nanotubes are also sometimes used to refer to double-walled and triple-walled carbon nanotubes. Carbon nanotubes can exhibit remarkable electrical conductivity [3, 4], while others are semiconductors [5, 6]. They also have exceptional thermal conductivity [7, 8] and tensile strength [9, 10] because of their nanostructure and strength of the bonds between carbon atoms. In addition, they can be chemically modified [11, 12]. These properties are expected to be valuable in many areas of technology, such as electronics, optics, composite materials, nanotechnology, and other applications of materials science.
Within the last twenty years, as the properties of carbon nanotubes have been better understood, interests in carbon nanotubes have greatly increased within and outside of the research community [13, 14]. One key to making use of these properties is the synthesis of carbon nanotubes in sufficient quantities for them to be used industrially [15, 16]. For example, large quantities of carbon nanotubes may be needed if they are to be used as high strength components of carbon nanotubes in macroscale three-dimensional structures [17, 18]. Carbon nanotubes are known to have extraordinary tensile strength, including high strain to failure and relatively high tensile modulus [19, 20]. Carbon nanotubes may also be highly electrically and thermally conductive while being resistant to fatigue, radiation damage, and heat [21, 22]. For example, carbon nanotubes can be good thermal conductors along the nanotube, where each individual carbon nanotube can have thermal conductivities potentially in excess of 2000 W/(m·K) [23, 24]. However, this thermal conductivity is anisotropic, exhibiting properties with different values when measured in different directions and is dramatically reduced when a large ensemble of carbon nanotubes is used in a sheet or mat [25, 26]. Accordingly, it would be desirable to provide a material that can take advantage of the characteristics and properties of carbon nanotubes, so that efficient and light-weight devices, such as shielding and thermal insulators, can be manufactured in a cost-effective manner.
Because of a remarkable combination of their properties, carbon nanotubes are being considered as prime candidate materials for nano-scale device applications [27, 28]. Consequently, considerable effort has been invested in characterizing properties of carbon nanotubes, particularly their electronic and mechanical properties [29, 30]. Surprisingly, despite the importance of thermal management in nano-scale devices, there has been relatively little progress in characterizing thermal conductivity of carbon nanotubes. This is partly due to challenges associated with nano-scale experimental measurements, but it is also a result of technological difficulties of synthesizing high-quality, well-ordered carbon nanotubes [31, 32]. Consequently, theoretical computations of thermal conductivity of carbon nanotubes are presently very essential. Theoretical computations of thermal conductivity of materials can be classified as two main approaches: first principles based atomistic simulations and continuum computations based on transport theories. The atomistic approach is particularly useful for nano-scale devices where the experimental determination of the thermal conductivity is quite challenging [33, 34]. The main advantage of the continuum approach is that it enables an analysis of relatively large systems [35, 36]. However, the approach entails the knowledge of certain parameters such as phonon relaxation time and phonon density of states which must be determined using either experimental measurements or by theoretical computations. An addition shortcoming of the continuum approach is that solving the governing differential transport equation may be quite difficult in some cases. Because of the aforementioned limitations of the continuum approach, the first principles based atomistic simulations are increasingly getting more attention as a means of predicting thermal properties [37, 38]. Besides not requiring the prior knowledge of the model parameters, atomic-scale computations enable quantification of the effect of microstructure on thermal properties [39, 40]. Furthermore, atomistic simulations can be used to determine the parameters for the continuum models discussed above and, therefore, help bridge gap between atomistic-scale and continuum-level computations [41, 42]. While single-walled carbon nanotubes exhibit extremely high thermal conductivity, the effects of different factors on the heat conduction properties of the carbon-based nanostructured material are poorly understood.
This study relates to the heat conduction properties of single-walled carbon nanotubes. The effects of different factors on the heat conduction properties of single-walled carbon nanotubes were investigated by using the nonequilibrium molecular dynamics method. Computational simulations were performed using molecular dynamics to investigate the heat transport properties of single-walled carbon nanotubes. The intrinsic thermal conductivity of carbon nanotubes was determined to understand the characteristics of thermal transport in the nanostructured materials. The mechanism of phonon transport in carbon nanotubes was discussed. The physical factors limiting heat conduction in carbon nanotubes were provided. The objective is to gain insight into the fundamental characteristics of thermal transport in carbon nanotubes. Particular emphasis is placed on the dependence of different physical factors on the thermal conductivity of carbon nanotubes, with an attempt to improve the heat conduction properties for the carbon-based nanostructured material.