Molecular Dynamics Simulations Of Heat Transfer In Carbon Nanotube And Its Interface With Semiconducting Mateirals

Carbon nanotubes are seamlessly rolled up from graphene sheets into cylinders. Before its preparation, its perfect qusi-one-dimensional structure theoratically predicts its extraordinary mechnical, electrical, optical and thermal properties. The realization of preparation makes it step out from theories to applications. Heat dissipation in electronics is one of the most serious energy problem badly needed to be solved; and that carbon nanotube has been confirmed to be with a remarkable axial thermal conductivity suggests its great potential in heat management in micro/nano devices. Therefore, how to control and manipulate the superior thermal conductivity of carbon nanotube in applications would need wide researches on its capability of heat transfer and internal or external influential factors, such as its length, defects and diameter, as well as environmental temperature and pressure.Using non-equilibrium molecular dynamics, the thesis investigated the relationship of thermal conductivity of single-walled carbon nanotube (SWNT) and its length, diameter, especially its chirality. Results showed some important conclusions.(1) with increasing the length of SWNT, its thermal conductivity increased while the extent of increase would decline; based on their relationship, thermal conductivity of infinite SWNT was calculated through extrapolation method, which was390.62±21.12W/m-K for (5,5) armchair SWNT.(2) when the tube length was only20nm, there was no obvious dependence of thermal conductivity on the diameter of SWNT.(3) for relative shorter SWNT, its thermal conductivity depended on its chirality and that the thermal conductivity of chrial SWNT with larger chiral angle was higher than that of chiral SWNT with smaller chiral angle; the thermal conductivity of armchair SWNT was higher than that of zigzag SWNT; however, with the increase of tube length, the influence of chirality was fading while the play of diameter became significant and the thermal conductivity enhanced with diameter; through extrapolation method, the relationship of thermal conductivity of infinite SWNT with its diameter and chirality turned out to be so insignificant that they affected thermal conductivity no more.(4) with LJ potential between tubes, thermal conductivity of SWNT hardly changed with the distance between tubes, which suggested that Van der Walls interactions between tubes could be ignored when thermal conductivity of isolated SWNT was studied.With extremely high axial thermal conductivity and elastic modulus, carbon nanotubes have been directly and vertically synthesized on substrate, as a good thermal interface material (TIM), to improve heat dissipation of electronics. However, the poor interfacial thermal conductance, rather than the high intrinsic thermal conductivity of CNT itself, dominates thermal transport through CNT to substrate, which results in exceptionally low capability of heat dissipation. To find out the mechanism of interfacial thermal conductance between CNT and substrate is one of the most important procedures to advance the application of CNT as TIM. Consequently, more and more researches, theories or experiments, have focused on this small interface of thermal boundary conductance (TBC). This thesis studied the Si-SWNT system with SWNT vertically aligned on Si substrate, to investigate how interfacial thermal conductance of Si-SWNT and internal thermal conductivity of this very SWNT would behave with the change of the length and diameter of SWNT, the size of substrate, temperature and substrate. As a result, interfacial thermal conductance and internal thermal conductivity of SWNT increased with diameter because of the increase of contact area at interface. In addition, contacting with Si substrate altered the pure ballistic transfer in SWNT, which leaded to the influential length of SWNT became shorter. With increase of length of SWNT, phonon modes with long wave length could exist, which benefited for internal thermal conductivity of SWNT and interfacial thermal conductance. However, when the phonon modes reached to saturation, both would come to a steady value. When the size of the substrate was8x6x6, interfacial thermal conductance was the highest while internal thermal conductivity of SWNT showed no obvious dependence on substrate size whoes change hardly affected the heat transfer in SWNT. The high temperature was a promotive factor to interfacial thermal conductance but it hindered the internal thermal conductivity of SWNT because of phonon-phonon scattering. Substituting Si substrate with SiC substrate, the interfacial thermal conductance significantly improved, owing to the wider phonon couplings of their vibrational density of states (VDOS).As TIM, carbon nanotubes can scarcely keep straight between two solid surfaces, so they are easy to bend or intersect with others under complex conditions. In this thesis, junctional thermal conductance between two SWNTs was studied. The results showed that junctional thermal conductance declined sharply when the cross angle of the two SWNTs changed from0°to15°, that was because the contact area was rapidly decreased.