Phonons are primary heat carriers in carbon nanotubes (CNTs) and graphene; a fundamental understanding of phonon transport in these nano-structures is required for the energy efficient design of their devices such as integrated circuit, flexible displays, and transparent electrodes. In this work, atomistic simulations have been performed to investigate thermal transport at interfaces of CNT and graphene that are typically encountered in their applications, e.g., CNT-CNT junctions on silicon oxide substrate, interfaces between shells of double-wall CNTs (DWNTs), and graphene-metal interfaces. Firstly, heat dissipation at CNT junctions supported on the silicon dioxide substrate is investigated using molecular dynamics (MD) simulations and methods for phonon spectrum analysis. The results show the inefficient heat removal from CNTs not making direct contact with the oxide substrate is responsible for the breakdown of CNT network. At interfaces between shells of DWNTs, the radial vibration modes are identified as phonons that are strongly coupled and can efficiently exchange energy between shells of DWNTs. Secondly, the thermal conductivity of suspended single layer graphene (SLG) and SLG supported on Cu is determined using equilibrium MD simulations following Green-Kubo method and relaxation time approximation approach at room temperature. It is demonstrated that the interaction with Cu substrate can significantly reduce the thermal conductivity of SLG, and that the reduction of thermal conductivity from three acoustic phonons is the major reason. Lastly, using atomistic Green’s function method and density function theory calculations, the thermal boundary conductance at interfaces across graphene layers sandwiched by different metals including Cu, Au, and Ti is predicted. The work shows how the bonding strength changes the graphene/metal and graphene/graphene phonon coupling, and demonstrated the transition of thermal transport mechanism from metal/graphene dominated resistance to graphene/graphene dominated resistance as the metal/graphene bonding strength increases in metal/MLG/metal structure.

This work presents the development and application of analytical, numerical, and experimental methods for the study of thermal and electrical transport in nanoscale systems, with special emphasis on those materials and phenomena which can be important in thermoelectric and semiconductor device applications. Analytical solutions to the Boltzmann transport equation (BTE) using the relaxation time approximation (RTA) are presented and used to study the thermal and electrical transport properties of indium antimonide (InSb), indium arsenide (InAs), bismuth telluride (Bi₂Te₃), and chromium disilicide (CrSi₂) nanowires. Experimental results for the thermal conductivity of single layer graphene supported by SiO₂ were analyzed using an RTA-based model and compared to a full quantum mechanical numerical BTE solution which does not rely on the RTA. The ability of these models to explain the measurement results as well as differences between the two approaches are discussed. Alternatively, numerical solutions to the BTE may be obtained statistically through Monte Carlo simulation for complex geometries which may prove intractable for analytical methods. Following this approach, phonon transport in silicon (Si) sawtooth nanowires was studied, revealing that thermal conductivity suppression below the diffuse surface limit is possible. The experimental investigation of energy transport in nanostructures typically involved the use of microfabricated devices or non-contact optical methods. In this work, two such approaches were analyzed to ascertain their thermal behavior and overall accuracy as well as areas for possible improvement. A Raman spectroscopy-based measurement design for investigating the thermal properties of suspended and supported graphene was examined analytically. The resulting analysis provided a means of determining from measurement results the thermal interface conductance, thermal contact resistance, and thermal conductivity of the suspended and supported graphene regions. Previously, microfabricated devices of several different designs have been used to experimentally measure the thermal transport characteristics of nanostructures such as carbon nanotubes, nanowires, and thin films. To ascertain the accuracy and limitations of various microdevice designs and their associated conduction analyses, finite element models were constructed using ANSYS and measurements of samples of known thermal conductance were simulated. It was found that designs with the sample suspended were generally more accurate than those for which the sample is supported on a bridge whose conductance is measured separately. The effects of radiation loss to the environment of certain device designs were also studied, demonstrating the need for radiation shielding to be at temperatures close to that of the device substrate in order to accurately calibrate the resistance thermometers. Using a suspended microdevice like those analyzed using finite element analysis, the thermal conductivities of individual bismuth (Bi) nanowires were measured. The results were correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk Bi in the same crystal direction, the thermal conductivity of a single-crystal Bi nanowires of 232 nm diameter was found to be 3 - 6 times smaller than bulk between 100 K and 300 K. For polycrystalline Bi nanowires of 74 nm to 255 nm diameter the thermal conductivity was reduced by a factor of 18 - 78 over the same temperature range. Comparable thermal conductivity values were measured for polycrystalline nanowires of varying diameters, suggesting a grain boundary scattering mean free path for all heat carriers in the range of 15 - 40 nm which is smaller than the nanowire diameters. An RTA-based transport model for both charge carriers and phonons was developed which explains the thermal conductivity suppression in the single-crystal nanowire by considering diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 10¹⁹ cm⁻³. Using a similar experimental setup, the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of higher manganese silicide (HMS) nanostructures were investigated. Bulk HMS is a passable high temperature thermoelectric material which possesses a complex crystal structure that could lead to very interesting and useful nanoscale transport properties. The thermal conductivities of HMS nanowires and nanoribbons were found to be reduced by 50 - 60 % compared to bulk values in the same crystal direction for both nanoribbons and nanowires. The measured Seebeck coefficient data was comparable or below that of bulk, suggesting unintentional doping of the samples either during growth or sample preparation. Difficulty in determining the amorphous oxide layer thickness for nanoribbons samples necessitated using the total, oxide-included cross section in the thermal and electrical conductivity calculation. This in turn led to the determined electrical conductivity values representing the lower bound on the actual electrical conductivity of the HMS core. From this approach, the measured electrical conductivity values were comparable or slightly below the lower end of bulk electrical conductivity values. This oxide thickness issue affects the determination of the HMS nanostructure thermoelectric figure of merit ZT as well, though the lower bound values obtained here were found to still be comparable to or slightly smaller than the expected bulk values in the same crystal direction. Analytical modeling also indicates higher doping than in bulk. Overall, HMS nanostructures appear to have the potential to demonstrate measurable size-induced ZT enhancement, especially if optimal doping and control over the crystallographic growth direction can be achieved. However, experimental methods to achieve reliable electrical contact to quality four-probe samples needs to be improved in order to fully investigate the thermoelectric potential of HMS nanostructures. / text

<div>The rapid advance in modern electronics and photonics is pushing device design to the micro- and nano-scale, and the resulting high power density imposes immense challenges to thermal management. Promising materials like carbon nanotubes (CNTs) and graphene offer high thermal conductivity in the axial (or in-plane) directions, but their thermal transport in the radial (or cross-plane) directions are poor, limiting their applications. Hierarchical structures like pillared graphene, which is composed of many CNT-graphene junctions, have been proposed. However, thermal</div><div>interfacial resistance is a critical issue for thermal management of these systems. In this work, we have systematically explored thermal transport across interfaces,</div><div>particularly in pillared graphene and silicon/heavy-silicon.</div><div><br></div><div><div>First, by recognizing that thermal resistance of the 3D pillared graphene architecture primarily comes from CNT-graphene junctions, a simple network model of thermal transport in pillared graphene structure is developed. Using non-equilibrium molecular dynamics (NEMD), the resistance across an individual CNT-graphene junction with sp2 covalent bonds is found to be around 6 × 10−11 m2K/W, which is significantly lower than typical values reported for planar interfaces between dissimilar materials. Interestingly, when the CNT pillar length is small, the interfacial resistance</div><div>of the sp2 covalent junction is found to decrease as the CNT pillar length decreases, suggesting the presence of coherence effects. The junction resistance Rj is eventually</div><div>used in the network model to estimate the effective thermal conductivity, and the results agree well with direct MD simulation data, demonstrating the effectiveness of our model.</div></div><div><br></div><div><div>Then we identify three different mechanisms which can lead to thermal resistances across the pillared graphene junction: the material mismatch (phonon propagates from CNT to graphene), the non-planar junction (the phonon propagation direction must change), and defects (there are six heptagons at each junction). The NEMD results show that three mechanisms lead to similar resistance at the CNT-graphene junction, each at around 2.5 × 10−11 m2K/W.</div></div><div><br></div><div><div>Further, we have predicted the transmission function of individual phonon mode using the wave packet method at CNT-graphene junction. Intriguing phonon polarization conversion behavior is observed for most incident phonon modes. It is found that the polarization conversion dominates the transmission and is more significant at larger phonon wavelength. We attribute such unique phonon polarization conversion behavior to the dimensional mismatch across CNT-graphene interface. It is found that the transmission functions at the junction cannot be predicted by the conventional acoustic mismatch models due to the existence of dimensional mismatch. Further analysis shows that, the dimensionally mismatched interface, on one hand tends to reduce the transmission and conductance due to defects and the change of phonon propagation direction at the interface, while on the other hand tends to enhance the transmission and conductance due to the new phonon transport channel introduced by polarization conversion.</div></div><div><br></div><div><div>Finally, we address that many recent experiments have shown that the measured thermal boundary conductances (TBCs) significantly exceed those calculated using the Landauer approach. We identify that a key assumption that an interface is a local equilibrium system (different modes of phonons on each side of the interfaces are at the emitted phonon temperature Te), is generally invalid and can contribute to the discrepancy. We show that the measurable temperature for each individual mode is the ”modal equivalent equilibrium temperature” T rather than Te. Also,</div><div>due to the vast range of transmission functions, different phonon modes are out of local thermal equilibrium. Hence, the total conductance cannot be simply calculated as a summation of individual modal conductance. We modify the Landauer approach to include these effects and name it the ”Nonequilibrium Landauer approach”. Our approach has been used on the carbon nanotube (CNT)/graphene and Si/heavy-Si interfaces which are matched interfaces, and it gives 310% increases in TBC as compared to the conventional Landauer approach at CNT-graphene junction and even higher increase for Si/heavy-Si with small mass ratios. A convenient chart is created to estimate the conductance correction based on our approach, and it yields quite accurate results. Our work indicate that the measured high TBCs in experiments can be due to this nonequilibrium effect rather than the other proposed mechanisms, like inelastic phonon transmission and cross-interface electron-phonon coupling.</div></div><div><br></div><div><div>The results obtained in this study will provide a deeper understanding of nanoscale thermal transport across interfaces. This research also provides new perspectives of</div><div>atomic- and nano-scale engineering of materials and structures to enhance performance of thermal management.</div></div>

This dissertation focuses on the effects of superhydrophobic (SHPo) surfaces on thermal transport. The work is divided into two main categories: thermal transport without phase change and thermal transport with phase change. Thermal transport without phase change is the topic of four stand-alone chapters. Three address jet impingement at SHPo surfaces and the fourth considers natural convection at a vertical, SHPo wall. Thermal transport with phase change is the topic of a single stand-alone chapter exploring pool boiling at SHPo surfaces. Two chapters examining jet impingement present analytical models for thermal transport; one considered an isothermal wall and the other considered an isoflux wall. The chapter considering the isothermal scenario has been archivally published. Conclusions are presented for both models. The models indicated that the Nusselt number decreased dramatically as the temperature jump length increased. Further, the influence of radial position, jet Reynolds number, Prandtl number and isoflux versus isothermal heating become negligible as temperature jump length increased. The final chapter concerning jet impingement reports an experimental exploration of jet impingement at post patterned SHPo surfaces with varying microfeature pitch and cavity fraction. The empirical results show a decrease in Nusselt number relative to smooth hydrophobic surfaces for small pitch and cavity fraction and the isoflux model agrees well with this data when the ratio of temperature jump length to slip length is 3.1. At larger pitch and cavity fractions, the empirical results have higher Nusselt numbers than the SHPo surfaces with small pitch and cavity fraction but remain smaller than the smooth hydrophobic surface. We attribute this to the influence of small wetting regions. The chapter addressing natural convection presents an analytical model for buoyant flow at a vertical SHPo surface. The Nusselt number decreased dramatically as temperature jump length increased, with greater decrease occurring near the lower edge and at higher Rayleigh number. Thermal transport with phase change is the topic of the final stand-alone chapter concerning pool boiling, which has been archivally published. Surface heat flux as a function of surface superheat was reported for SHPo surfaces with rib and post patterning at varying microfeature pitch, cavity fraction, and microfeature height. Nucleate boiling is more suppressed on post patterned surfaces than rib patterned surfaces. At rib patterned surfaces, transition superheat decreases as cavity fraction increases. Increasing microfeature height modestly increases the transition superheat. Once stable film boiling is achieved, changes in surface microstructure negligibly influence thermal transport.

Thermal management is a key challenge to improving the performance of microelectronic devices. For many high performance applications, the thermal resistance between chip and heat sink may account for half of the total thermal budget. Chip-level heat dissipation is therefore a critical bottleneck to the development of advanced microelectronics with high junction temperatures. Recently aligned carbon nanotube arrays have been developed as possible next generation thermal interface materials to overcome this thermal limitation, however the thermal physics of these nanoscale interfaces remains unclear. In this thesis, the thermal interface resistance between a carbon nanotube and adjoining carbon, silicon, or copper substrate is investigated through non-equilibrium molecular dynamics simulation. Phonon transmission is calculated using a simplified form of the diffuse mismatch model with direct simulation of the phonon density of states. The results of theory and simulation are reported as a function of temperature in order to estimate the importance of anharmonicity and inelastic scattering. The results of this work provide a better understand of the mechanisms of thermal transport to assist future CNT TIM research and development.

To fulfill the potential of carbon nanotube (CNT) as thermal interface material (TIM), the packing density of CNT array needs improvement. In this work, two potential ways to increase the packing density of CNT array are tested. They are liquid precursor(LP)CVD and cycled catalyst deposition method. Although LP-CVD turned out to be no help for packing density increase, it is proved to enhance the CNT growth rate. The packing density of CNT array indeed increases with the cycle number. The thermal conductivity of the CNT array increases with the packing density. This work is believed to be a step closer to the real life application of CNT in electronic packaging industry.

abstract: The rapid progress of solution-phase synthesis has led colloidal nanocrystals one of the most versatile nanoscale materials, provided opportunities to tailor material's properties, and boosted related technological innovations. Colloidal nanocrystal-based materials have been demonstrated success in a variety of applications, such as LEDs, electronics, solar cells and thermoelectrics. In each of these applications, the thermal transport property plays a big role. An undesirable temperature rise due to inefficient heat dissipation could lead to deleterious effects on devices' performance and lifetime. Hence, the first project is focused on investigating the thermal transport in colloidal nanocrystal solids. This study answers the question that how the molecular structure of nanocrystals affect the thermal transport, and provides insights for future device designs. In particular, PbS nanocrystals is used as a monitoring system, and the core diameter, ligand length and ligand binding group are systematically varied to study the corresponding effect on thermal transport. Next, a fundamental study is presented on the phase stability and solid-liquid transformation of metallic (In, Sn and Bi) colloidal nanocrystals. Although the phase change of nanoparticles has been a long-standing research topic, the melting behavior of colloidal nanocrytstals is largely unexplored. In addition, this study is of practical importance to nanocrystal-based applications that operate at elevated temperatures. Embedding colloidal nanocrystals into thermally-stable polymer matrices allows preserving nanocrystal size throughout melt-freeze cycles, and therefore enabling observation of stable melting features. Size-dependent melting temperature, melting enthalpy and melting entropy have all been measured and discussed. In the next two chapters, focus has been switched to developing colloidal nanocrystal-based phase change composites for thermal energy storage applications. In Chapter 4, a polymer matrix phase change nanocomposite has been created. In this composite, the melting temperature and energy density could be independently controlled by tuning nanocrystal diameter and volume fractions. In Chapter 5, a solution-phase synthesis on metal matrix-metal nanocrytal composite is presented. This approach enables excellent morphological control over nanocrystals and demonstrated a phase change composite with a thermal conductivity 2 - 3 orders of magnitude greater than typical phase change materials, such as organics and molten salts. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2015

Additive manufacturing (AM) is a manufacturing technique that adds material, such as polymers, ceramics, and metals, in patterned layers to build three-dimensional parts for applications related to medicine, aviation, and energy. AM processes for metals like selective laser melting (SLM) hold the unique advantage of fabricating metal parts with complex architectures that cannot be produced by conventional manufacturing techniques. Thermal transport can be a focal point of unique AM products and is likewise important to metal AM processes. This dissertation investigates AM metal meshes with spatially varied thermal conductivities that can be used to maximize the charge and discharge rates for thermal energy storage and thermal management by phase change materials (PCMs). Further, manufacturing these meshes demands excellent thermal control in the metal powder bed for SLM processes. Since the thermal conductivities of metal powders specific to AM were previously unknown, we made pioneering measurements of such powders as a function of gas infiltration. In the past, thermal transport was improved in phase change materials for energy storage by adding spatially homogeneous metal foams or particles into PCMs to create composites with uniformly-enhanced (UE) thermal conductivity. Spatial variation can now be realized due to the emergence of metal AM processes whereby graded AM meshes are inserted into PCMs to create PCM composites with spatially-enhanced (SE) thermal conductivity. As yet, there have been no studies on what kind of spatial variation in thermal conductivity can further improve charge and discharge rates of the PCM. Making such mesh structures, which exhibit unsupported overhangs that limit heat dissipation pathways during SLM processes, demands understanding of heat diffusion within the surrounding powder bed. This inevitably relies on the precise knowledge of the thermal conductivity of AM metal powders. Currently, no measurements of thermal conductivity of AM powders have been made for the SLM process. In chapter 2 and 3, we pioneer and optimize the spatial variation of metal meshes to maximize charge and discharge rates in PCMs. Chapter 2 defines and analytically determines an enhancement ratio of charge rates using spatially-linear thermal conductivities in Cartesian and cylindrical coordinates with a focus on thermal energy storage. Chapter 3 further generalizes thermal conductivity as a polynomial function in space and numerically optimizes the enhancement ratio in spherical coordinates with a focus on thermal management of electronics. Both of our studies find that higher thermal conductivities of SE composites near to the heat source outperform those of UE composites. For selected spherical systems, the enhancement ratio reaches more than 800% relative to existing uniform foams. In chapter 4, the thermal conductivities of five metal powders for the SLM process were measured using the transient hot wire method. These measurements were conducted with three infiltrating gases (He, N2, and Ar) within a temperature range of 295-470 K and a gas pressure range of 1.4-101 kPa. Our measurements indicate that the pressure and the composition of the gas have a significant influence on the effective thermal conductivity of the powder. We find that infiltration with He provides more than 300% enhancement in powder thermal conductivity, relative to conventional infiltrating gases N2 and Ar. We anticipate that this use of He will result in better thermal control of the powder bed and thus will improve surface quality in overhanging structures.

A typical electronic or photonic device may consist of several materials each one potentially meeting at an interface or terminating with a free-surface boundary. As modern device dimensions reach deeper into the nanoscale regime, interfaces and boundaries become increasingly influential to both electrical and thermal energy transport. While a large majority of the device community focuses on the former, we focus here on the latter issue of thermal transport which is of great importance in implementing nanoscale devices as well as developing solutions for on-chip heat removal and waste heat scavenging. In this document we will discuss how modern performance enhancing techniques (strain, nanostructuring, alloying, etc.) affect thermal transport at boundaries and across interfaces through the avenue of three case studies. We use first-principles Density Functional Perturbation Theory to obtain the phonon spectrum of the materials of interest and then use the dispersion data as input to a phonon Boltzmann Transport model. First, we investigate the combined effects of strain and boundary scattering on the in-plane and cross-plane thermal conductivity of thin-film silicon and germanium. Second, we review a recently developed model for cross-dimensional (2D-3D) phonon transport and apply it to 3D-2D-3D stacked interfaces involving graphene and molybdenum disulfide 2D-layers. Third, we combine relevant models from earlier Chapters to study extrinsic effects, such as line edge roughness and substrate effects, on in-plane and through-plane thermal transport in 1H-phase transition metal dichalcogenide (TMD) alloys. Through these investigations we show that: (1) biaxial strain in Si and Ge thin-films can modulate cross-plane conductivity due to strong boundary scattering, (2) the thermal boundary conductance between 2D-3D materials can be enhanced in the presence of an encapsulating layer, and (3) the thermal conductivity of 1H-phase TMDs can be reduced by an order of magnitude through the combination of nanostructuring, alloying, and substrate effects.

Le transport thermique dans les nanostructures semi-conductrices est un sujet de recherche très actuel, couvrant de larges domaines applicatifs dont l’auto-échauffement des composants nanoélectroniques et la conversion d’énergie par effet thermoélectrique. La modélisation du transport thermique à l’échelle nanométrique est complexe car la longueur des dispositifs devient du même ordre de grandeur que le libre parcours moyen des porteurs de chaleurs (phonons). L’hypothèse de pseudo-équilibre local n’est plus pertinente, de plus des effets de confinements peuvent aussi apparaitre. Il faut donc développer des outils de modélisation spécifiques.Pour prendre en compte les effets de confinement, j'ai calculé les relations de dispersions des phonons dans les nanostructures. Pour cela, j’ai mis en œuvre une méthode atomistique semi-empirique nommée ABCM (« Adiabadic Bond Charge Model »). J’ai pu ainsi calculer, dans l'ensemble de la zone de Brillouin (« Full Band »), la dispersion des phonons dans du silicium et du germanium en phase Zinc-Blende et aussi en phase Wurtzite.En outre, afin d’évaluer la résistance thermique d’interface, une extension originale du modèle « Acoustic Mismatch Model », entièrement « full-band », a été développée. Grâce à l’approche « Full-Band » la dépendance à l’orientation relative des cristaux de chaque côté de l’interface a été étudiée. Les effets d’orientations sur la transmission ont aussi été étudiés dans des nanofils polyphasés nouvellement synthétisés dans le laboratoire.En parallèle, pour étudier le transport des phonons, j'ai développé un simulateur Monte Carlo particulaire qui utilise les dispersions « Full-Band » calculées en ABCM. Ce type de simulateur est très polyvalent et permet de décrire l’ensemble des régimes de transports (du balistique au diffusif). De plus, comme il utilise une dispersion « Full-Band » les effets de confinement peuvent aussi être inclus. Ce simulateur m’a permis d’étudier les effets d’un changement d’orientation des plans cristallographiques du cristal sur la conductivité thermiques dans des nanofils de silicium et de germanium. J’ai ainsi évalué l’anisotropie du flux thermique dans ces nanostructures. / The heat transfer in semiconducting nanostructures is a current research topic, covering a wide range of applications including self-heating in nanoelectronic devices and energy conversion via thermoelectric effect. The modeling of heat transport at the nanometer scale is complex as the device length is in the same order of magnitude than the mean free path of heat carriers (phonons). The local pseudo-equilibrium assumption is no longer relevant, moreover confinement effects can also appear. Therefore development of specific modeling tools is highly desirable.To take into account the confinement effects, I have calculated the phonon dispersion relations in nanostructures. For this, I have implemented an atomistic semi-empirical method called ABCM (Adiabadic Bond Charge Model). I have calculated, in the entire Brillouin zone (Full Band approach), the dispersion relationship of phonons in both Silicon and Germanium for both Zinc-Blende and Wurtzite phases.In addition, to evaluate the thermal interface resistance, an original extension of the Acoustic Mismatch Model, completely full band, was developed. Within this approach, the dependence on the relative orientation of crystals has been studied in polytype nanowires that were recently synthesized in the laboratory.In parallel, to study the transport of phonons, I developed a particle Monte Carlo simulator that uses Full-Band dispersions calculated via ABCM. This kind of simulator is very versatile and can describe all transport regimes (from ballistic to diffusive one). Moreover, as it uses a "Full-Band" dispersion confinement effects can also be included. This simulator allowed me to study the effects of a change in orientation of the crystallographic planes on the thermal conductivity in both silicon and germanium nanowires. I have thus evaluated the anisotropy of the heat fluxes in these nanostructures.