Des analyses structurales et pétrophysiques ont été menées dans le Massif de Melechov afin d’étudier les structures contrôlant la porosité, la perméabilité et la conductivité thermique de la roche. La structure du massif a été déterminée sur la base d’un jeu étendu de données incluant des mesures d’ASM et des mesures de terrain des structures ductiles et cassantes. Le système de fractures du massif a été décrit par quatre ensembles de fractures. Les données pétrophysiques mesurées ont servi pour caractériser l’effet de la fracturation et de l’altération sur la géométrie de l’espace poreux et en conséquence sur la perméabilité, la conductivité thermique et les propriétés élastiques du granite. Des propriétés pétrophysiques distinctes ont été identifiées pour du granite intact, du granite sain fracturé ainsi que pour du granite fracturé et altéré contenant des oxydes de fer, de la chlorite et des minéraux argileux. Une étude microstructurale détaillée, combinée à des mesures multi-directionnelles de vitesse des ondes P (VP) à pressions de confinement croissantes a été menée sur un échantillon du granite Lipnice aux schlieren. Les résultats indiquent que l’anisotropie des VP à basses pressions de confinement est contrôlée par des fissures inter-granulaires reliant les clivages sous-parallèles aux schlieren des micas et des feldspaths ainsi que par des fissures intra- ou trans-granulaires dans du quartz sous-parallèles aux fractures d’exfoliation. Une importante fermeture de la porosité de fissures à partir d’une profondeur de 500 m a été interprétée en termes d’élasticité des fissures manifestée par une augmentation rapide des VP avec la pression de confinement croissante. / Structural and petrophysical analysis have been conducted within the Melechov massif with focus on structures controlling the porosity, permeability and thermal conductivity of the rock. The structure of the massif has been constrained based on extensive dataset including AMS and field structural measurements of ductile and brittle structures. The fracture system of the massif has been described by four sets of fractures. The measured petrophysical data have been used to characterize the effect of fracturing and alteration on pore space geometry and in turn on permeability, thermal conductivity and elastic properties of the studied granite. Distinct petrophysical properties have been identified for pristine granite, for fractured fresh granite as well as for fractured granite altered by Fe-oxide, chlorite and clay minerals. A detailed microstructural study combined with multidirectional P-wave velocity measurements at high confining pressure and with AMS analysis has been conducted on a schlieren bearing sample of Lipnice granite. The granite VP anisotropy at low confining pressure was controlled by intergranular cracks interconnecting schlieren-subparallel cleavage cracks in micas and feldspars and by exfoliation fracture-subparallel intra- or trans-granular cracks in cleavage-free quartz. Major closing of the crack porosity linked to the schlieren granite below depth of 500 m has been interpreted in terms of crack compliance reflected by rapid increase in VP with confining pressure.

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.

Effective application of thermal interface materials (TIM) sandwiched between silicon and a heat spreader in a microelectronic package for improved heat dissipation is studied through thermal and mechanical characterization of high thermally conductive carbon nanotubes (CNTs) integrated into eutectic gallium indium liquid metal (LM) wetting matrix. Thermal conductivity data from Infrared microscopy tool reveals the dependence of experimental factors such as matrix types, TIM contacting interfaces, orientation of CNTs and wetting of CNTs in the matrix on the thermal behavior of TIM composite. Observed generalized trend on LM-CNT TIM shows progressive decrease in effective thermal conductivity with increasing CNT volume fractions. Further thermal characterizations LM-CNT TIM however show over 2x increase in effective thermal conductivity over conventional polymer TIMs (i.e. TIM from silicone oil matrix) but fails to meet 10x improvement expected. Poor wetting of CNT with LM matrix is hypothesized to hinder thermal improvement of LM-CNT TIM composite. Thus, wetting enhancement technique through electro-wetting and liquid crystal (LC) based matrix proposed to enhance CNT-CNT contact in LM-CNT TIM results in thermal conductivity improvement of 40 to 50% with introduction of voltage gradient of 2 to 24 volts on LM-CNT TIM sample with 0.1 to 1 percent CNT volume fractions over non voltage LM-CNT TIM test samples. Key findings through this study show that voltage tests on LC- CNT TIM can cause increased CNT-CNT networks resulting in 5x increase in thermal conductivity over non voltage LC-CNT TIM and over 2x improvement over silicone-CNT TIMs. Validation of LM wetting of CNT hypothesis further shows that wetting and interface adhesion mechanisms are not the only factors required to improve thermal performance of LM-CNT TIM. Anisotropic characteristic of thermal conductivity of randomly dispersed CNTs is a major factor causing lower thermal performance of LM-CNTs TIM composite. Other factors resulting in LM-CNT TIM decreasing thermal conductivity with increasing CNT loading are (i) Lack of CNT-CNT network due to large difference in surface tension and mass density between CNTs and LM in TIM composite (ii) Structural stability of MWCNT and small MFP of phonons in ~5um MWCNTs compared to the system resulted in phonon scattering with reduced heat flow (iii) CNT percolation threshold limit not reached owing to thermal shielding due to CNT tube interfacial thermal resistance. While mixture analytical models employed are able to predict thermal behaviors consistent with CNT-CNT network and CNT- polymer matrix contact phenomenon, these models are not equipped to predict thermo-chemical attributes of CNTs in LM-CNT TIM. Extent of LM-CNT wetting and LM-solid surface interfacial contact impacts on interfacial thermal resistance are investigated through LM contact angle, XPS/AES and SEM-EDX analyses on Au/Ni and Ni coated copper surfaces. Contact angle measurements in the range of 120o at both 55oC and 125oC show non wetting of LM on CNT, Au and Ni surfaces. Interface reactive wetting elemental composition of 21 days aged LM on Au/Ni and Ni surfaces reveals Ga dissolution in Au and Ni diffusion of ~0.32um in Au which are not present for similar analysis of 1 day LM on Au/Ni surface. Formation of Au-Ni-Ga IMC and IMC-oxide iono-covalency occurrence at the interface causes reduction in surface tension and reduction in interfacial contact resistance.

Les matériaux thermoélectriques (TE) offrent la possibilité de convertir directement un flux de chaleur en courant électrique pour recycler la chaleur perdue, par exemple par nos automobiles. Les skutterudites AyFe4-xCoxSb12, (A = Ce, Yb, …, 0 ≤ y < 1; x < 4) sont déjà de bons matériaux thermoélectriques dans le domaine de température 400–800K. Pour améliorer le coefficient Seebeck, des nano-inclusions de InSb ou GaSb (~50 nm) ont été générées à l’étape de frittage flash dans Ce0,3Fe1,5Co2,5Sb12 de type p. Elles n’ont pas eu l’effet escompté de filtrage en énergie des trous mais ont conduit à l’insertion de ~ 0,1 mol d’indium ou de gallium dans Ce0,3Fe1,5Co2,5Sb12 et à un facteur de mérite TE amélioré ZTmax = 0,7 (+ 20%) dans les deux cas . Pour réduire la conductivité thermique et améliorer leur performances TE, nous avons entrepris d’élaborer pour Co0,91Ni0,09Sb3 et Yb0,25Co4Sb12 de type n des microstructures à grains ultrafins (~ 100 nm) par broyage à haute énergie et frittage flash (SPS). Pour inhiber la croissance des grains lors du frittage, nous avons utilisé des additifs nanométriques (10 – 20nm), soit ajoutés ex-situ (CeO2, SiO2), soit générés in-situ (Yb, Yb2O3). Des facteurs de mérite TE ZTmax = 0,8 (+ 30%) et ZTmax = 1,4 ( + 10%) ont été obtenus respectivement pour Co0,91Ni0,09Sb3 et Yb0,25Co4Sb12 / The thermoelectric materials (TE) offer the possibility to convert a heat flow into an electric current for recycling heat wasted for example, by our automobiles. AyFe4-xCoxSb12 skutterudites, (A = Ce, Yb, …, 0 ≤ y < 1; x < 4) are already good thermoelectric materials in the 400 – 800 K temperature range. To improve the Seebeck coefficient, nano-inclusions of InSb or GaSb (~ 50 nm) were introduced during the spark plasma sintering step in p type Ce0.3Fe1.5Co2.5Sb12. They did not led to expected charge carriers energy filtering and but led to the insertion of ~ 0.1 mol of indium or gallium in Ce0.3Fe1.5Co2.5Sb12 and to figure of merit improved by 20 % (ZTmax = 0.7) in both cases. To reduce the thermal conductivity and improve their TE performance, we have developed for n type Co0.91Ni0.09Sb3 et Yb0.25Co4Sb12 an ultrafine grained microstructure (~ 100 nm) by high energy milling and spark plasma sintering (SPS). To inhibit grain growth during sintering, we used nanoscale additives (10 – 20nm) either added ex-situ (CeO2, SiO2) or precipitated in-situ (Yb, Yb2O3). The figure of merit ZTmax = 0,8 (+ 30%) et ZTmax = 1,4 ( + 10%) were thus obtained respectively in Co0,91Ni0,09Sb3 and Yb0,25Co4Sb12

Le travail présenté dans cette thèse porte sur la synthèse et les caractérisations thermiques et rhéologiques d’un nouveau type de nanofluide : le système Cu2O/Glycérol. La caractérisation est faite en fonction de la taille des particules mises en suspension, de la température et de la fraction volumique solide. Ce travail a nécessité la synthèse des nanoparticules et des nanofluides par la méthode de décomposition thermique des précuseurs organométalliques, qui présente un bon rendement en quantité de nanoparticules (17%). Et le montage d’un dispositif de caractérisation thermique utilisant la méthode 3ω. Finalement, nous avons passé à la caractérisation rhéologique et thermique de ces échantillons. Les résultats obtenus avec ce nouveau système sont intéressants, car l’augmentation de la conductivité thermique atteint des valeurs importantes : 120% et 35% respectivement pour des fractions volumiques aussi faibles que 0,625% et 0,078% de nanoparticules de 7 nm de diamètre, sans influence notable sur la viscosité du fluide hôte, permettant une bonne amélioration du bilan énergétique total. Nous avons observé que la concentration et la taille (surface) des nanoparticules sont des paramètres clefs du comportement de la conductivité thermique effective du nanofluide Cu2O/Glycérol. Nos mesures nous ont permis de déduire la prédominance des modifications de la surface des nanoparticules (par fonctionnalisation ou par réaction chimique secondaire) sur le mouvement brownien dans les transferts thermiques nanoparticules/ fluide hôte. / The work presented in this thesis involves the synthesis and thermal and rheological characterization of a new type of nanofluid : the Cu2O/glycerol system. The characterization was carried out as a function of the size of the particles in suspension, the temperature and the volume fraction of nanoparticles. The nanoparticles and nanofluids were synthesised by the thermal decomposition method, providing a good yield of nanoparticles (17%). Apparatus for thermal measurements using the 3ω method was constructed, and rheological and thermal characterization was carried out. Significant increases in thermal conductivity were observed : 120% and 35% for volume fractions as low as 0.625% and 0.078%, respectively, of 7-nm-diameter nanoparticles, without noticeable effect on the viscosity of the host fluid, leading to a considerable improvement in the energy content.We found that the concentration and surface area of the nanoparticles are key parameters influencing the behaviour of the effective thermal conductivity of the nanofluid. Surface modification of the nanoparticles by functionalization or secondary chemical reactions has a profound effect on the Brownian motion in the heat transfer between nanoparticles and fluid host.

In recent times, high power density trends and temperature constraints in integrated circuits have led to conventional cooling techniques not being sufficient to meet the thermal requirements. The ever-increasing desire to overcome this problem has led to worldwide interest in micro heat sink design of electronic components. It has been found that geometric configurations of micro heat sinks play a vital role in heat transfer performance. Therefore, an effective means of optimally designing these heat sinks is required. Experimentation has extensively been used in the past to understand the behaviour of these heat extraction devices. Computational fluid dynamics (CFD) has more recently provided a more cost-effective and less time-consuming means of achieving the same objective. However, in order to achieve optimal designs of micro heat sinks using CFD, the designer has to be well experienced and carry out a number of trial-and-error simulations. Unfortunately, this will still not always guarantee an accurate optimal design. In this dissertation, a design methodology which combines CFD with a mathematical optimisation algorithm (a leapfrog optimisation program and DYNAMIC-Q algorithm) is proposed. This automated process is applied to three design cases. In the first design case, the peak wall temperature of a microchannel embedded in a highly conductive solid is minimised. The second case involves the optimisation of a double row micropin-fin heat sink. In this case, the objective is to maximise the total rate of heat transfer with the effect of the thermal conductivity also being investigated. The third case extends the micropin-fin optimisation to a heat sink with three rows. In all three cases, fixed volume constraint and manufacturing restraints are enforced to ensure industrial applicability. Lastly, the trends of the three cases are compared. It is concluded that optimal design can be achieved with a combination of CFD and mathematical optimisation. / Dissertation (MEng)--University of Pretoria, 2011. / Mechanical and Aeronautical Engineering / Unrestricted

Numerical estimates of the ability of a green roof to reduce energy consumption in buildings are plagued by a lack of accuracy in thermal properties that are input to the model. An experimental study into the thermal conductivity at different temperatures and moisture contents was performed using four different commercially available substrates for green roofs. In the unfrozen state, as moisture content increased, thermal conductivity increased linearly. In the phase transition zone between +5 ºC and -10ºC, as temperature decreased, thermal conductivity increased sharply during the transition from water to ice. When the substrate was frozen, thermal conductivity varied exponentially with substrate moisture content prior to freezing. Power functions were found between thermal conductivity and temperature (when shifted up by +10.001ºC). Two equally sized, green roof test cells were constructed and tested to compare various roof configurations including a bare roof, varying media thickness for a green roof, and vegetation. The results show that compared with the bare roof, there is a 75% reduction in the interior temperature amplitude for the green roof with 150mm thick substrate. When a sedum mat was added, there’s a 20% reduction in the amplitude of the inner temperature as compared with the cell without sedum mat. / Graduate

Building waste and disposable packaging are a major component in today's landfills. Most of these are structural or thermally insulative polymer foams that do not degrade over a long period of time. Currently, there is a push to replace these foams with thermoplastic or biodegradable foams that can either be recycled or composted. We propose the use of compostable soy-based polyurethane foams (PU) with kenaf core modifiers that will offer the desired properties with the ability to choose responsible end-of-life decisions. The effect of fillers is a critical parameter in investigating the thermal and mechanical properties along with its effect on biodegradability. In this work, foams with 5%, 10%, and 15% kenaf core content were created. Two manufacturing approaches were used: the free foaming used by spray techniques and the constrained expansion complementary to a mold cavity. Structure-property relations were examined using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermal conductivity, compression values, scanning electron microscopy (SEM), x-ray micro-computed tomography (micro-CT), and automated multiunit composting system (AMCS). The results show that mechanical properties are reduced with the introduction of kenaf core reinforcement while thermal conductivity and biodegradability display a noticeable improvement. This shows that in application properties can be improved while establishing a responsible end-of-life choice.

Two-dimensional van der Waals materials have been a subject of intense research interest in recent years. High thermal conductivity of graphene can be utilized for many thermal management applications. In spite of possessing very high electron mobility, graphene can’t be used as transistors because of the absence of band gap; however transition metal dichalcogenides are another class of two-dimensional van der Waals materials with inherent band gap and show a great promise for future nanoelectronic applications. But in order to tailor these properties for commercial applications, we should develop a better understanding of the effect of extrinsic factors like size, rough edges, grain boundaries, mass-impurities, interaction with substrate etc. on thermal and electrical transport. Most materials exhibit a smooth ballistic-to-diffusive type of thermal transport in which when the sample size is small as compared to mean-free-path of phonons the transport is ballistic, whereas, when the sample size is large as compared to phonon mean-free-path, phonons undergo multiple scattering events and the thermal transport becomes diffusive in nature. However, graphene exhibits an atypical thermal transport behavior where thermal conductivity shows an increasing logarithmic trend even for samples far greater than the mean-free-path of phonons. We show that this anomalous behavior can be attributed to the significant contribution coming from momentum-conserving normal phonon-phonon scattering. Secondly, graphene grain boundaries have been found to significantly reduce thermal conductivity even in the presence of substrates. In spite of numerous studies on the effect of grain boundaries (GBs) on thermal conductivity in graphene, there lacks a complete correlation between GB resistance and misorientation angle across graphene GBs. We show a direct correlation between thermal GB resistance and mismatch angles with low angle mismatch can be captured only by GB roughness, whereas, large mismatch angles will lead to the formation of a disordered patch at the interface and it could significantly deteriorate the overall thermal conductivity even in the presence of substrates. GBs are found to affect electrical transport in two-dimensional systems as well. Owing to the excellent electronic properties and compactness of these two-dimensional materials, high quality 2D heterojunctions are the subject of intense research interest in recent years. Graphene-MoS2 heterojuctions are found to form ohmic contacts and show great potential for future nanoelectronic applications. We show that the interface resistance in Gr-MoS2 heterojuctions can affect the overall resistance of the device if the channel (MoS2) length is small at low carrier densities, whereas, at high carrier densities interface resistance do not play much role in determining the resistance of the entire device. However, if graphene and MoS2 grains are misorientated then interface resistance can play a crucial role in determining the overall resistance of the device. We also show a weak dependence of misorientation angles on GB resistance across MoS2 grain boundaries.

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