Thermal Conductivity and Thermal Rectification in Carbon Nanotubes - Reverse Non-Equilibrium Molecular Dynamics Simulations.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2010)
PhD thesis of Mohammad Alaghemandi -
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|Item Type:||Ph.D. Thesis|
|Title:||Thermal Conductivity and Thermal Rectification in Carbon Nanotubes - Reverse Non-Equilibrium Molecular Dynamics Simulations|
The purpose of this research is an investigation of the thermal conductivity () and thermal rectification of carbon nanotubes as well as the different factors which have an influence on these quantities. As computational tool we have used reverse non-equilibrium molecular dynamics (RNEMD) simulations. In chapter 1 we have briefly discussed the importance of research in nanoscale science. Furthermore the motivation for this work has been explained. In chapter 2 we have investigated the thermal conductivity of single-walled and multi-walled carbon nanotubes by RNEMD as a function of the tube length (L), temperature and chiral index. We found that the thermal conductivity in the ballistic-diffusive regime follows a L law. The exponent is insensitive to the diameter of the carbon nanotube; at room temperature has been derived for short carbon nanotubes. The temperature dependence of the thermal conductivity shows a peak between 250 and 500 K. We have also defined and shortly discussed the phenomenon of thermal rectification in mass-graded and extra-mass-loaded nanotubes. In chapter 3 the thermal rectification in nanotubes with a mass gradient has been studied in more detail. We predict a preferred heat flow from light to heavy atoms which differs from the preferential direction in one-dimensional (1D) monoatomic systems. This behavior of nanotubes is explained by anharmonicities caused by transverse motions which are stronger at the low mass end. The present simulations show an enhanced rectification with increasing tube length, diameter and mass gradient. Implications of the present findings for applied topics are mentioned concisely. In chapter 4 we have extended our work on thermal rectification from mass-graded quasi-one-dimensional nanotubes to the other model systems. Mass-graded polyacetylene-like chains behave like single-file chains as long as the mass gradient is hold by the backbone atoms. The thermal rectification in nanotubes with a gradient in the bond force constant (kr) has been studied, too. They show a preferred heat transfer from the region with large kr to the domain with small kr. Thermal rectification has been studied also in planar (2D) and 3D mass-graded systems where the heat flow followed a preferred direction similar to that observed in nanotubes. Additionally, a more realistic system has been implemented. Here a different number of carbon nanotubes have been grafted on both sides of a graphene sheet. We have found that the transfer of the vibrational energy as well as the generation of low-energy modes at atoms with large masses is responsible for the sign of the thermal rectification. In chapter 5 the thermal conductivity of carbon nanotubes (CNTs) with chirality indices (5,0), (10,0), (5,5) and (10,10) has been studied by reverse non-equilibrium molecular dynamics simulations as a function of different bondlength alternation patterns (r). The r dependence of the bond force constant (krx) in the MD force field has been determined with the help of an electronic band structure approach. From these calculations it follows that the r dependence of krx in tubes with not too small diameter can be mapped by a simple linear bondlength–bondorder correlation. A bondlength alternation with an overall reduction in the length of the nanotube causes an enhancement of while an alternation scheme leading to an elongation of the tube is coupled to a reduction of the thermal conductivity. This effect is more pronounced in CNTs with larger diameters.
|Place of Publication:||Darmstadt|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 540 Chemie|
|Divisions:||07 Fachbereich Chemie > Physical Chemistry|
|Date Deposited:||06 Jul 2010 10:07|
|Last Modified:||07 Dec 2012 11:58|
|Referees:||Müller-Plathe, Prof. Florian and Van der Vegt, Prof. Nico|
|Refereed:||22 March 2010|