Studying The Effect Of Vacancies On The Thermal Conductivity Of Metal Coated Carbon Nanotubes Using Molecular Dynamics Atomistic Simulations

Breakthroughs in nanotechnology have made it possible for the size of electronic gadgets to be scaled down to micro/nano level. What follows however, is the side effect of high power densities and overheating as the heat dissipation is restricted to very small surface area. Thus, the need of an effective thermal management system is central to these developments. One way to counter this problem is to use Thermal Interface Materials (TIMs) which promote heat transfer between heat source and sink.Owing to their high thermal conductivity (up to 6000 W/mK), Carbon Nanotubes (CNTs) are thought of as strong candidates for TIMs. Unfortunately, they do not behave as expected when employed as Vertically Aligned CNT arrays, called as turfs. Possible causes include but are not restricted to bent/deformed CNTs, presence of impurities, defects, tube-tube contact resistance, and radiative heat losses to surroundings. Since the reduction in thermal conductivity of CNTs due to vacancies can be attributed to scattering of phonons at the site of the vacancies, we hypothesize that coating the defective CNTs with metallic layers will alleviate this reduction by providing additional pathways for heat transfer. Additionally, the metallic coatings will help decrease the contact resistance between co-axial tubes, protect the CNTs from surroundings to reduce radiative heat losses, and provide an added stiffness to CNTs, restricting their deformation. The current study focuses on employing Molecular Dynamics (MD) simulations to study the effect of vacancy concentration(s) on the thermal conductivity of metal-coated tri-walled CNTs (3WCNT).It should be noted that the metal's major contribution to heat transfer is by the virtue of its electrons, which the classical MD simulations are unable to capture. We therefore employ Two-Temperature Model (TTM), which can take into consideration the effect of electron-phonon interactions. Due to its ability to form uniform coatings around CNTs, Nickel has been chosen as the prime candidate for metal in this study.

To improve the energy efficiency in many electronics and machinery applications, advanced Thermal Interface Materials (TIMs) with high heat dissipation ability and more pliability must be employed. Among a variety of promising choices to make the advanced TIMs, Vertically Aligned Carbon Nanotube (VACNT) turfs (arrays) outstand with their exceptional mechanical and thermal properties. Individual CNTs are quite flexible due to their quasi-one-dimensional structure and presence of strong sp2 bonds among the carbon atoms gives them great strength. Also, the dominance of ballistic phonon transport in the CNTs endows them superior thermal conductivity when compared to many metallic substrates. However, the defects in CNTs, misaligned axial contacts between CNTs in a CNT turf, and the CNTs/substrate resistance reduce the practical thermal conductivity of the material. It is hypothesized that the application of metal coatings on each CNT in a CNT turf would enhance the overall thermal conductivity of the material and improve the connectivity between the CNT turfs and the metallic substrate. As the diameter of the CNTs in a CNT turf is in the order of several nanometers, Molecular Dynamics (MD) atomistic simulations is selected as a tool which provide a deeper understanding in studying the thermal transport at the fundamental level. Thermal conduction in the metals is electron dominant whereas regular MD procedures are incapable of considering the energy exchange between these electrons and phonons. Therefore, a different mechanism called Two-temperature Model (TTM) coupled with Non-Equilibrium MD is used in this study and proved to be effective. MD code to procure the coefficient of thermal conductivity (kappa) was developed and the effects of the metal thickness, number of walls in the CNT and the role of diameter of CNT on kappa of the metal-coated CNTs was individually investigated. It was shown that the increase in the thickness of metal coating would impede the kappa of individual CNTs following an inverse power trend. Also, it was found that among the number of shells in the CNT and its diameter, the former parameter tends to contribute more towards the thermal transport than the latter. The results of this work are capable of predicting the optimal design structure for metal-coated VACNT composite for advanced thermal management applications.

The extraordinary potential of nanoscale materials and nano-constituent composite materials coupled with rapid progress in the capability to synthesize these materials has created a demand for their property characterization. To develop a better understanding of thermal transport in nanoscale wires and nanotubes, this thesis utilizes both theoretical and experimental techniques to determine how nanostructure geometry, surface properties, and heat treatment affect their ability to conduct thermal energy. As an example, the effect of heat treatment on the thermal conductivity of commercially available, chemical vapor deposition-grown, multiwalled carbon nanotubes (MWCNTs) is presented. The measurement device is implemented inside a scanning electron microscope equipped with nanomanipulators and a gas injected electron beam deposition system for repeatable in-situ sample characterization. MWCNT samples treated to 3000 C show a 5-fold increase in average thermal conductivity as compared to the as-synthesized samples. By including an estimation of thermal contact resistance due to phonon impedance mismatch, the average thermal conductivity of the heat-treated MWCNT specimens is estimated to be 228 W/m-K. The results suggest that heat treatment is a viable method to improve thermal conductivity and highlight the importance of MWCNTs quality in thermal management applications. Classical MD simulations are used to investigate the sensitivity of thermal conductivity to side-wall defects in SWCNTs. Vacancy repair is evident with heat treatment. 3000 C heat treatment of SWCNTs having varying degrees of defect concentrations is found to generally increase thermal conductivity by at least 10 %. The results suggest that phonon mean free path in (6,6) SWCNTs is nearly equally impeded by side-wall functionalization as compared to atomic vacancies. Classical MD simulations estimate that 2 atom % of hydrogenation and 1.5 - 2 % vacancy concentrations reduce thermal conductivity to the same degree. As compared to non-functionalized SWCNTs, the results suggest that the use of chemically functionalized SWCNTs as a second phase material in multifunctional CNT-polymer composites may reduce thermal transport. Nanoscale effects on thermal properties are further evaluated by an investigation into the dependency of specific heat and ballistic thermal conductance on cross-sectional geometry in free-standing isotropic non-metallic crystalline nanostructures. Analysis of phonon confinement is performed using dispersion relations found by numerically solving the Pochhammer-Chree frequency equation for a tube. These dispersion relations are used to evaluate the specific heat and ballistic thermal conductance in the nanostructures as a function of the nanostructure geometry, size, and surface stiffness. 1D, 2D, and 3D geometric phonon confinement regimes are recognized and found to depend on both the nanostructure's wall thickness and outer radius. Compared to nanowires, the frequency reduction of acoustic phonon modes in thinner walled nanotubes is shown to elevate the ballistic thermal conductance of the thin-walled nanotube between 0.2 K and 150 K. At 20 K, the ballistic thermal conductance of the thin-walled nanotube is found to be 300% greater than that of a solid nanowire. Surface modification of nanotubes and nanowires is performed using a multilayer elastic model to increase the average Young's modulus of the first three atomic surface layers. The acoustic stiffening of the interior and exterior lateral walls of nanotubes is found to have a contrasting effect on specific heat and thermal conductivity as compared to the outer surface modification of the nanowire. A 10% reduction in specific heat and a 2% reduction in lattice thermal conductivity at 50 K occur in a crystalline nanotube having a 10 nm outer and 5 nm inner diameter. In contrast, at the same temperature, an approximate 30% increase in thermal conductivity and specific heat occurs when the acoustically stiffened surface is applied to the outer diameter of a nanowire with a solid cross-sectional area. The simplified model has the potential to investigate the acoustic engineering of nanowires and nanotubes by inducing surface stiffening or softening via appropriate surface chemical functionalization protocols or coatings.

This thesis studies the effective thermal conductivity of randomly oriented, percolated carbon nanotubes. To that end, a multiscale analysis approach was adopted. At the nanoscale, molecular dynamics simulation was performed to determine the thermal conductivity coefficient of a single carbon nanotube. Then, thermal conductivity of two carbon nanotubes positioned at different angles were studied after determining the equilibrium positions of the two nanotubes at various relative positions. Finally, using the data obtained in the previous analyses, the effective thermal conductivity of randomly oriented carbon nanotubes was calculated using the finite element model where each nanotube was modeled as a continuous rod.

Le présent travail vise à apporter certaines pistes de solution concernant certaines controverses sur l'estimation de la conductivité thermique des nanotubes de carbone par simulation de dynamique moléculaire à l'équilibre avec conditions aux limites périodiques et la formule de Green-Kubo. Entre autre, différents auteurs obtiennent des résultats pouvant parfois varier de plusieurs ordres de grandeur pour un même type de nanotube. H n'y a toutefois que très peu d'études jusqu'à ce jour tentant d'expliquer ces contradictions. Dans la première partie du projet, on détermine les paramètres numériques pouvant influencer la conductivité thermique calculée avec une méthode de dynamique moléculaire à l'équilibre. On effectue ensuite une analyse de sensibilité pour plusieurs de ces paramètres afin de déterminer de quelle manière ils influencent la conductivité thermique calculée (chapitres 3 et 4). Finalement, on présente une étude sur le phénomène de fréquence de coupure lors du calcul de la conductivité thermique (chapitre 5).

Carbon nanotubes (CNTs) have drawn great interest and shown great promise in recent years in the areas of composite materials, sensors, and small electronic devices owing in a large part to their extraordinary mechanical properties. Yet an enormous scatter is observed in available laboratory results on the stiffness and strength of CNTs. Surface defects including vacancies, pentagon and heptagons have been commonly observed in CNT samples, and are found to have significant influence on the mechanics of CNTs. However, any link between the randomness in CNTs mechanical properties and CNT defects has not been investigated systematically before. Moreover, the fracture of CNTs due to mechanical loading is an important issue likely to affect the durability and reliability of CNT-based materials and devices; yet, based on the author's knowledge, the fracture resistance of CNTs has not been quantified before. This dissertation, trying to build up these missing links, studies the effects of randomly distributed vacancies and Stone-Wales (SW or 5-7-7-5) defects on the mechanical properties of single-walled nanotubes (SWNTs) using the technique of atomistic simulation (AS), and quantifies the fracture resistance of zigzag SWNTs with fracture mechanics concepts. Basic principles and key issues of atomistic simulation and multiscale modeling are reviewed. A series of displacement controlled tensile tests of CNTs are modeled with atomistic simulation. Armchair and zigzag SWNTs, with and without defects are studied. A modified Morse potential is adopted to model the interatomic forces. Time histories of energies, displacements and forces are generated from the simulations, and three mechanical properties & mdash;stiffness, ultimate strength and ultimate strain & mdash;are further calculated. Effects of loading speed and geometry are discussed. Further details of CNT structure changes, especially the evolution of defects during the loading process are monitored. In studying the fracture resistance of CNTs, the strain energy release rate, G, is computed through a series of simulated mechanical loading of zigzag SWNTs with preexisting cracks of various lengths. A significant dependence of the critical strain energy release rate, Gc, on crack length, a, is observed: Gc increases with a initially, and tends to reach a constant value as a becomes large. The temperature dependence of Gc is also investigated up to 500K: Gc drops substantially as temperature increases for all tube diameters.

Carbon nanotubes are formed by folding a graphene sheet. They have gained a lot of attention during the last decade due to their extra ordinary mechanical, thermal and electrical properties. Molecular dynamics simulations have been used extensively for studying the mechanical properties of carbon nanotubes. In this thesis, a quantum mechanics and molecular dynamics level multi-scale modeling and analysis of single walled carbon nanotubes is presented. This dissertation reports many findings based on these simulations such as some parameters that affect the correctness of the results obtained by molecular dynamics simulation like the boundary conditions and the displacement increment. The effects of the strain rate and the length of the nanotube on the mechanical properties of carbon nanotubes under uniaxial tension are also reported.^A simplification for calculating the virial stresses with multibody potential is derived to use for calculating the stresses in carbon nanotubes and compared with the stresses calculated using continuum mechanics engineering stresses. Simulation of unraveling of carbon nanotubes during field emission is simulated using Molecular dynamics simulations. The force required to start the unraveling in carbon nanotubes with different chiralities is reported as well as the maximum force that can be sustained by the atomic chain. Due to the nonlinearity in the current-voltage relation of carbon nanotubes, the traditional Joule's law for calculating joule heating in carbon nanotubes can not be used. In this thesis, the joule heating and the electron-induced wind forces per unit length of carbon nanotubes are calculated using a quantum mechanical formulation based on the energy and momentum exchange between the electrons and the phonons.^Two approaches were used in the calculations; the first one is based on formulating an integral form that makes use of the relaxation time approximation into the modified Fermi-Dirac distribution for the electron occupation probability. The other approach uses the Ensemble Monte Carlo simulations and tracks the energy and the phonon exchange during the simulation time. The results are used to calculate the effective charge number in carbon nanotubes at different temperatures. The methods proposed in this thesis for calculating the joule heating and the effective charge number can be used for any nanoscale material, and can be extended to include effects like phonon-phonon interaction and hot phonon effects.

Carbon nanotubes are one of the newest materials to be discovered, being barely 20 years old. They are also the most promising one, with one particular sample of multi-walled nanotube attaining a tensile strength of 63GPa, and with carbon nanotubes in general having a specific strength of up to 48000kNm/kg: effectively a direct exploitation of the covalent sp2 bonding between carbon atoms. Plastic deformation begins at about 5% strain. The nanotubes can be produced in lengths of up to 550mm, and thicknesses as small as 4.3Å; making them perfect reinforcement fibres for composites. They also have many other properties which may be useful in electronics, gas storage , etc. The present compilation focuses on the various characteristic types of defect which are found in carbon nanotubes, plus the relatively limited number of diffusion studies which have been performed. The 418 entries cover the period from 1994 to 2014.