Polymer nanocomposites (PNCs) are hybrid materials incorporating organic or inorganic nanoparticles (NPs) with at least one dimension in the submicron scale. Over the last two decades, these materials have drawn a remarkable attention due to their central role in industrial formulations and technological applications, extending from food packaging to smart coatings. Incorporating nanoparticles (NPs) to a polymer matrix can significantly alter the conformation and the mobility of the polymer chains in their proximity. Moreover, understanding the delicate balance between the enthalpic and entropic interactions is crucial to control and predict the ability of NPs to diffuse and disperse in the polymer matrix. The impact of these interactions on the structure and the dynamics of polymer chains and NPs is fully revealed in how a number of macroscopic properties changes, justifying the high interest on these materials for industrial applications. In this thesis, the impact on the structure, dynamics, viscosity and thermal conductivity of a number of microscopic properties is investigated by performing Molecular Dynamics (MD) simulations. Specifically, the PNC is represented by a coarse-grained model of a melt of linear homopolymer chains containing spherical NPs. Throughout this work, a number of parameters are modified in order to unveil possible patterns in the PNC’s performance. To this end, this work focuses on the consequences of modifying the NP size dispersity, NP-polymer chain relative size, and chains’ degree of stiffness. Four theoretical models describing the diffusivity of NPs, three of which include nano-scale corrections, have been averaged to study the dependence of dilute NPs’ diffusivity on the NP polydispersity index. By comparing these models to the simulation results at different degrees of polydispersity, it is possible to obtain a more complete picture of their validity as compared to the monodisperse case. Regarding the diffusion of polymer chains, simulation results were in good agreement with the experimental results previously obtained by Composto and coworkers (Soft Matter 2012, 8, 6512), which relate the chains’ diffusivity to the average interparticle distance. As far as the transport properties are concerned, they show a weaker dependence on the polydispersity index. By contrast, results on viscosity and thermal conducitivity show that they are conditioned by the polymer-NP specific interfacial area and the inverse average mass, respectively. These results are in good agreement with previous experimental results. A deeper examination of this intriguing deviation from viscosity predictions in traditional composites, reveals a non-trivial combination of thickening and thinning effects contributing to the final viscosity of the PNC. This thesis also address the influence of the chains’ stiffness on the dynamical and viscous behaviour. An isotropic-to-nematic phase transition is observed, regardless of the NP-monomer interactions, below which a monotonic increase of both properties is observed, whereas orientationally ordered systems dramatically modify them, resulting into a steep increase or a smooth decrease depending on the direction in which they are measured.

We have performed a first principles density functional theory (DFT) calculations to study the thermal conductivity in ZnO nanotubes, ZnO nanowires, and Si/Ge shell-core nanowires. We found the equilibrium configuration and the electric band structure of each nanostructure using DFT, the interatomic force constants and the phonon dispersion relations were calculated using DFPT as implemented in Quantum Espresso. In order to fundamentally understand the effect of atomic arrangements, we calculated the phonon conductance in a ballistic approach using a Green's function method. All ZnO nanostructures studied exhibit semiconducting behavior, with direct bandgap at the Gamma point. The calculated values for the bandgaps were larger than the value of the bandgap of the bulk ZnO. We were able to identify phonon modes in which the motion of Zn atoms is significant when it is compared with the motion of oxygen atoms. The thermal conductivity depends on the diameter of the nanowires and nanotubes and it is dramatically affected when the nanowire or nanotube is doped with Ga. For Si/Ge nanowires, the slope and the curvature of acoustic modes in the phonon dispersion relation increases when the diameter increases. For nanowires with the same number of atoms, the slope and curvature of acoustic modes depends on the concentration of Si atoms. We were able to identify phonon modes in which the motion of core atoms is significant when it is compared with motion of atoms on the nanowire's shell. The thermal conductivity in these nanostructures depends on the nanowire's diameter and on the Si atoms concentration.

Orientador: Prof. Dr. Sérgio Ricardo Lourenço / Tese ( doutorado)- Universidade Federal do ABC. Programa de Pós-Graduação em Energia, 2016.

Les composés homologues de formule chimique (PbSe)5(Bi2Se3)3m avec m = 1, 2 et 3 se caractérisent par une structure lamellaire où alternent des couches de PbSe avec m couches de Bi2Se3. Ces composés, que l’on retrouve à l’état naturel, ont récemment suscité un intérêt pour la thermoélectricité en raison de leur remarquable aptitude à ne conduire que très faiblement la chaleur. L’objectif des travaux de cette thèse est d’étudier en détail le transport électrique et thermique de ces matériaux et de sonder leurs performances pour la génération d’électricité. Pour atteindre ces objectifs, des techniques de synthèse par métallurgie de poudres ont été mises en œuvre. Les matériaux résultants ont été ensuite caractérisés finement. Les caractérisations ont porté sur des analyses physico-chimiques mais aussi sur des mesures de propriétés électriques et thermiques aussi bien à basses températures (2 – 300 K) pour identifier les mécanismes microscopiques qui gouvernent le transport qu’à hautes températures (300 – 723 K) pour déterminer leur domaine d’application optimal. Une étude détaillée de leur structure cristalline a été menée en combinant des mesures de diffraction des rayons X sur monocristal et des analyses de microscopie électronique à transmission à haute résolution. Les mesures des propriétés physiques de ces composés ont confirmé leur potentiel pour des applications en génération d’électricité à températures moyennes. De nombreuses substitutions ont été entreprises afin de tenter d’optimiser davantage les performances de ces composés. Les éléments en substitution ont été choisi pour soit augmenter (m = 1) ou au contraire diminuer (m = 2 et 3) la concentration en électrons. Ces travaux ont permis de démontrer pour la première fois la possibilité de doper ces matériaux avec de nombreux éléments tels que l’iode, le sodium, l’argent ou le tellure. D’autre part, une étude détaillée des propriétés thermiques de ces matériaux a été réalisée par diffusion inélastique des neutrons sur poudre afin de dévoiler l’origine microscopique des très faibles valeurs de conductivité thermique de réseau mesurées / The homologous series of compounds of general chemical formula (PbSe)5(Bi2Se3)3m with m = 1, 2 et 3 is characterized by a lamellar crystal structure where PbSe layers alternate with m Bi2Se3 layers. These compounds, that can be found as minerals, have recently focused attention for thermoelectric applications owing to their remarkable ability to poorly conduct heat. In order to evaluate their thermoelectric performances, the present work dealt with their synthesis by powder metallurgy techniques followed by measurements of their transport properties not only at low temperatures (2 – 300 K) with the aim to identify the basic mechanisms governing the transport but also at high temperatures (300 – 723 K) to determine their optimum temperature range. A detailed study of their crystalline structure has been carried out by a combination of X-ray diffraction on high-quality single crystals and high-resolution transmission electron microscopy. Measurements of their transport properties have confirmed the potential of these materials for power generation applications at mid temperatures. Numerous substitutions have been studied to optimize further their thermoelectric performances. The elements in substitution have been chosen to either increase (m = 1) or decrease (m= 2 and 3) the electron concentration. This work has demonstrated for the first time the possibility to dope these materials with various elements such as iodine, sodium, silver or tellurium. Moreover, a detailed study of the thermal properties of these compounds has been performed by means of powder inelastic neutron scattering in order to unveil the microscopic origin of the very low lattice thermal conductivity values measured

Este trabalho apresenta uma revisão de técnicas para medir propriedades térmicas de lmes, seguida de enfoque na termo-re etância transiente (TTR). Dentre as tecnologias existentes para medir propriedades térmicas, métodos ópticos são preferidos devido à sua natureza não-destrutiva, potencial de alta resolução temporal e espacial e calibração independente de contato físico. A implementação experimental deste método é apresentada, assim como a teoria da linha de transmissão utilizada para tratamento por Transformada de Laplace da equação de Fourier unidimensional do calor. Para facilitar o cálculo de invers ão desta Transformada, uma aproximação numérica, empregando o método Stehfest, foi usada. Experimentalmente, a evolução temporal da temperatura normalizada é mostrada para um lme de Au sobre Si e para lmes de Cu sobre substratos de vidro e Si, assim como foram utilizadas técnicas complementares de caracterização dos lmes (per lometria, elipsometria, microscopia de força atômica, eletrônica de varredura e de transmissão). Para o filme de ouro com espessura de 4:6µm, a teoria apresenta boa concordância com os resultados experimentais, já que o valor encontrado para a condutividade térmica do ouro está entre 230W/m.K e 280W/m.K, próximo e abaixo do valor da condutividade térmica do Au em volume (318W/m.K), indicando a validade do método implementado. Para lmes de cobre, porém, os resultados iniciais não apresentam a mesma concordância, e possíveis causas são discutidas. Futuramente, a TTR implementada poderá ser utilizada para determinação da condutividade térmica de lmes nos dielétricos ou semicondutores, e possivelmente na caracterização da componente transversal em filmes anisotrópicos. / This work presents a review of techniques to measure thermal properties off films, followed by a focused attention to the transient termo-re ectance (TTR). Amongst the existing technologies to measure thermal properties, optical methods are preferred due their nondestructive nature, high potential of spacial and temporal resolution, and independence from physical contact. The experimental implementation of this method is presented, as well as the theory of the transmission line theory used in the Laplace Transform treatment of the Fourier one-dimensional heat conduction equation. To facilitate the calculation of the Transform inversion, a numerical method, using the Stehfest method, was used. Experimentally, evolution of the normalized temperature is shown for a lm of Au on Si and for films of Cu on glass and Si substrates, whereas complementary techniques were used for film characterization (pro lometry, ellipsometry, atomic force microscopy, scanning and transmission eletron microscopy). For the Au film 4:6µm thick, the theory presents good agreement with the experimental results, and the value found for the thermal conductivity of the gold film is between 230W/m.K and 280W/m.K, near and below the bulk Au thermal conductivity (318W/m.K), indicating the validity of the method implementation. For Cu films, however, the initial results do not present the same agreement, and possible causes are discussed. In the future, the implemented TTR could be used for determination of the thermal conductivity of dielectric or semicondutors thin films, and possibly in the characterization of the transversal component in anisotropic films.

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

abstract: Planetary surface studies across a range of spatial scales are key to interpreting modern and ancient operative processes and to meeting strategic mission objectives for robotic planetary science exploration. At the meter-scale and below, planetary regolith conducts heat at a rate that depends on the physical properties of the regolith particles, such as particle size, sorting, composition, and shape. Radiometric temperature measurements thus provide the means to determine regolith properties and rock abundance from afar. However, heat conduction through a matrix of irregular particles is a complicated physical system that is strongly influenced by temperature and atmospheric gas pressure. A series of new regolith thermal conductivity experiments were conducted under realistic planetary surface pressure and temperature conditions. A new model is put forth to describe the radiative, solid, and gaseous conduction terms of regolith on Earth, Mars, and airless bodies. These results will be used to infer particle size distribution from temperature measurements of the primitive asteroid Bennu to aid in OSIRIS-REx sampling site selection. Moving up in scale, fluvial processes are extremely influential in shaping Earth's surface and likely played an influential role on ancient Mars. Amphitheater-headed canyons are found on both planets, but conditions necessary for their development have been debated for many years. A spatial analysis of canyon form distribution with respect to local stratigraphy at the Escalante River and on Tarantula Mesa, Utah, indicates that canyon distribution is most closely related to variations in local rock strata, rather than groundwater spring intensity or climate variations. This implies that amphitheater-headed canyons are not simple markers of groundwater seepage erosion or megaflooding. Finally, at the largest scale, volcanism has significantly altered the surface characteristics of Earth and Mars. A field campaign was conducted in Hawaii to investigate the December 1974 Kilauea lava flow, where it was found that lava coils formed in an analogous manner to those found in Athabasca Valles, Mars. The location and size of the coils may be used as indicators of local effusion rate, viscosity, and crustal thickness. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2018

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Cette thèse se concentre sur l'étude de l'impact de divers défauts de réseau, c'est-à-dire de dislocations, de parois entre phases inversées, de décalages de mailles et de gaps, sur la conductivité thermique de nano-fils par simulation de dynamique moléculaire et les calculs de fonctions de Green atomiques. Tout d'abord, nous calculons la conductivité thermique de nano-fils de silicium orientés <110> incluant une dislocation spirale par la dynamique moléculaire de non-équilibre. Nous constatons qu'avec l'inclusion d'une dislocation, le taux de diffusion phonon-phonon est amélioré de façon significative en raison de l'existence du champ de déformation induit. Ce processus de diffusion anharmonique augmente avec le vecteur de Burger. Par conséquent, la conductivité thermique de nano-fils disloqués est largement réduite et le pourcentage de réduction est proportionnel à la grandeur du vecteur de Burger. Deuxièmement, le concept de nano-fils de super-réseau anti-phase est proposé et leur conductivité thermique est étudiée avec la dynamique moléculaire d'équilibre. On constate que la frontière anti-phase peut diffuser fortement les phonons et réduire la vitesse de groupe des phonons. Le jeu entre le transport cohérent de phonons et la diffusion de surface conduit à une conductivité thermique minimale à une période de longueur spécifique. La combinaison de la diffusion des phonons à l'interface et la diffusion de surface des nanofils réduit la conductivité thermique de SiC de deux ordres de grandeur, ce qui est d'un grand intérêt pour les applications en thermoélectricité. Troisièmement, nous démontrons que le transport des phonons peut être entravé en grande partie dans un nano-fil de Si avec une structure en zig-zag périodique. Une conductivité thermique plus faible est observée du fait d'un pur effet géométrique, qui produit une disparition complète des directions principales de polarisation de phonon à une période de longueur spécifique. La conductivité thermique minimale et la longueur de période correspondante sont dépendantes du diamètre. L'avantage de cette structure est qu'elle supprime en grande partie le transport thermique sans détériorer le transport d'électrons. Enfin, la transition entre la conduction de la chaleur et le rayonnement de champ proche dans un système de chaîne de clusters de SiO2 est étudiée avec la méthode des fonctions de Green. Trois régions de variation de la conductance dans ce domaine de largeur de gap sont identifiées, plus particulièrement, la région liée à la conduction où les électrons des deux corps sont mis en commun au milieu du gap, la région de champ proche prédominée par des interactions de charges de surface, et la région de champ proche prédominée par des interactions dipôle-dipôle de volume. Cette étude fournit finalement une description de la transition entre le rayonnement et la conduction de la chaleur dans les gaps de dimensions inférieures à quelques nanomètres. / This thesis is focused on the investigation of the impact of various lattice defects, i.e., screw dislocations, anti-phase boundaries, twinning boundaries, and vacuum gaps, on the thermal conductivity of nanowires by molecular dynamic simulations and Green's function calculations. We firstly calculated the thermal conductivity of <110> Si nanowires with a screw dislocation in the center through non-equilibrium molecular dynamics.We find that with the inclusion of a dislocation, the phonon-phonon scattering rate is enhanced dramatically due to the dislocation-induced strain field. This anharmonic scattering process increases with the Burger's vector. As a result, the thermal conductivity of dislocated nanowires is largely reduced and the reduction percentage is proportional to the magnitude of Burger's vector. Secondly, the concept of anti-phase superlattice nanowire is proposed and its thermal conductivity is investigated with equilibrium molecular dynamics. It is found that the anti-phase boundary can strongly scatter phonons and reduce the phonon group velocity. The interplay between phonon coherent transport and boundary scattering results in a minimum thermal conductivity at a specific period length. The combination of anti-phase boundary scattering and nanowire surface scattering reduces the thermal conductivity of SiC by two orders of magnitude, which is of great interest for potential thermoelectric applications. Thirdly, we demonstrate that phonon transport can be hindered to a large extent in a Si nanowire with periodically distributed twinning boundaries. A minimum thermal conductivity is observed due to a pure geometrical effect, which produces a thorough disappearance of favored phonon polarization directions at a specific period length. The minimum thermal conductivity and the corresponding period length are diameter dependent. The advantage of this structure is that it largely suppresses the thermal transport without deteriorating the electron transport. Finally, the transition from heat conduction to near field radiation in a SiO2 cluster chain system is investigated with the phonon Green's function. Three conductance variation regions within the studied distances are identified, more specifically, the heat conduction region with shared electrons in the middle of a gap, the near field region predominated by surface charge interactions, and the near field region predominated by volume dipole-dipole interactions. This study finally provides a description of the transition between radiation and heat conduction in gaps smaller than a few nanometers.