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.

This work investigates the thermal performance of buildings in Cyprus and application of a particular passive technology; Phase Change Materials (PCMs) for the ultimate aim of reducing indoor air temperatures and energy supplied for the cooling season.PCMs for passive building applications are emerging technology and have not been tested for the buildings of Cyprus neither by computer simulations nor by practical applications. In this work, particular PCM end product; wallboard, having phase change temperature of 26 oC is employed together with various construction materials and simulated for buildings of Cyprus. Description of the current state in Cyprus has been carried out in terms of low energy building studies, widely used building fabric and building statistics. There is a huge gap in Cyprus in the field of energy performance and thermal comfort of buildings, which creates big room for research. Climatic design of buildings has been abandoned resulting in poor thermal comfort and increased energy consumption. There is still no regulation in place regarding the thermal performance of buildings in North Cyprus.Recent weather data of different Cyprus locations has been investigated and compared with the simulation weather data files that are employed in this work. The author has demonstrated that Finkelstein-Schafer statistics between recent weather data of Cyprus and simulation weather data files are close enough to obtain accurate results.Dynamic thermal simulations has been carried out by using Energy Plus, which is a strong and validated thermal simulation program that can model PCMs. Simulations are done for two different building geometry; “simple building” and “typical building” by employing different construction materials. Simple building is a small size box shaped building and typical building is a real existing building and selected by investigation of the building statistics.Simulation results showed that with this particular PCM product, indoor air temperatures and cooling energies supplied to simple building is reduced up to 1.2 oC and 18.64 % when heavier construction materials are used and up to 1.6 oC and 44.12 % when lighter construction materials are used. These values for typical building are found to be 0.7 oC, 3.24 % when heavier construction materials are used and 1.2 oC, 3.64 % when lighter construction materials are used. It is also found that, if thinner walls and slabs are used in the buildings the effectiveness of the PCM lining increases in significant amount.

Dans le cadre de la réduction des consommations d'énergies primaires des bâtiments, de nouveaux matériaux de constructions sont amenés à être développés. Les réglementations thermiques poussent les nouvelles constructions à être économes en énergie. Elles doivent aussi être moins impactantes sur l'environnement tout en garantissant le confort des occupants.Dans ce travail est présenté le développement d'un nouveau matériau de construction composite intégrant un matériau à changement de phase (MCP).Les MCP sont capables d'échanger passivement de l'énergie thermique avec leur environnement. Il permettent ainsi une régulation passive de la température intérieure.Suite à un état de l'art, sur les MCP et le plâtre, est présenté la synthèse et la caractérisation physico-chimique d'un nouveau MCP à transition solide-solide.L'incorporation du MCP préalablement synthétisé à un matériau de construction de type plâtre est ensuite développée. Le matériau composite ainsi obtenu est caractérisé thermiquement et mécaniquement.Dans un dernier temps des évaluations environnementales du MCP et du matériau composite sont réalisées. / In a context of reduction of energy consumption in buildings, new buildings materials are developed. Thermal regulations require energy efficiency to buildings. They must be less impacting on the environment while ensuring occupant comfort.In this work is presented the development of a new composite building material incorporating a phase change material.PCM are able to exchange passively heat energy with their environment. It thus allow a passive control of the interior temperature of buildings.After a state of the art on PCM and plaster, a part is dedicated to synthesis and physicochemical characterisation of a new solid/solid PCM. In a third part the incorporation of the PCM previously synthesized in plaster is then developped. The composite material is mechanically and thermally characterized.In a last time environmental assessments of the PCM and the composite material are performed.

The introduction of phase change material fluid and nanofluid in micro-channel heat sink design can significantly increase the cooling capacity of the heat sink because of the unique features of these two kinds of fluids. To better assist the design of a high performance micro-channel heat sink using phase change fluid and nanofluid, the heat transfer enhancement mechanism behind the flow with such fluids must be completely understood. A detailed parametric study is conducted to further investigate the heat transfer enhancement of the phase change material particle suspension flow, by using the two-phase non-thermal-equilibrium model developed by Hao and Tao (2004). The parametric study is conducted under normal conditions with Reynolds numbers of Re=600-900 and phase change material particle concentrations ¡Ü0.25 , as well as extreme conditions of very low Reynolds numbers (Re < 50) and high phase change material particle concentration (0.5-0.7) slurry flow. By using the two newly-defined parameters, named effectiveness factor and performance index, respectively, it is found that there exists an optimal relation between the channel design parameters, particle volume fraction, Reynolds number, and the wall heat flux. The influence of the particle volume fraction, particle size, and the particle viscosity, to the phase change material suspension flow, are investigated and discussed. The model was validated by available experimental data. The conclusions will assist designers in making their decisions that relate to the design or selection of a micro-pump suitable for micro or mini scale heat transfer devices. To understand the heat transfer enhancement mechanism of the nanofluid flow from the particle level, the lattice Boltzmann method is used because of its mesoscopic feature and its many numerical advantages. By using a two-component lattice Boltzmann model, the heat transfer enhancement of the nanofluid is analyzed, through incorporating the different forces acting on the nanoparticles to the two-component lattice Boltzmann model. It is found that the nanofluid has better heat transfer enhancement at low Reynolds numbers, and the Brownian motion effect of the nanoparticles will be weakened by the increase of flow speed.

We demonstrate a versatile phase-change sub-micron contrast agent providing three modes of contrast enhancement: 1) photoacoustic imaging contrast, 2) ultrasound contrast with optical activation, and 3) ultrasound contrast with acoustic activation. This agent, which we name 'Cy-droplet', has the following novel features. It comprises a highly volatile perfluorocarbon for easy versatile activation, and a near-infrared optically absorbing dye chosen to absorb light at a wavelength with good tissue penetration. It is manufactured via a 'microbubble condensation' method. The phase-transition of Cy-droplets can be optically triggered by pulsed-laser illumination, inducing photoacoustic signal and forming stable gas bubbles that are visible with echo-ultrasound in situ. Alternatively, Cy-droplets can be converted to microbubble contrast agents upon acoustic activation with clinical ultrasound. Potentially all modes offer extravascular contrast enhancement because of the sub-micron initial size. Such versatility of acoustic and optical 'triggerability' can potentially improve multi-modality imaging, molecularly targeted imaging and controlled drug release. (C) 2017 The Authors. Published by Elsevier GmbH.

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Energy storage with the help of different materials is something that has been around for a long time. Structures such as concrete or brick use their high thermal mass to store energy in the form of temperature increase, sensible heat, of the respective material. Energy storage in the form of sensible heat is relatively small in capacity, which means that large masses of building material are required to give a significant effect. Concrete, for example, requires 4 kJ/kg to increase in temperature by 1 °C. Now society has begun to look more closely at other materials that can be used for energy storage and temperature stabilization in buildings. Phase change materials are unique materials that use the change in phases between different aggregation states to store energy in the form of latent heat. It is mainly phase change between solid and liquid that is used since gaseous form would involve large volume changes. An everyday example of a material that undergoes a phase change is water. Water requires 334 kJ/kg to go from 0 ˚C ice (solid) to 0 ˚C water (liquid).  Water is a very powerful phase change material but cannot be used in buildings as it melts at 0 ˚C. Therefore, other materials have been developed to meet the requirement that the melting take place at the desired temperature, usually room temperature. The different phase change materials can be divided into three different groups: organic, inorganic and eutetic materials. All groups come with their own advantages and disadvantages. Organic materials are stable materials in the sense that they can phase change in repeated cycles which makes them the popular choice when it comes to buildings. One disadvantage that most organic materials possess is that they are flammable. The choice of phase change material will affect the quality of the building and it is therefore important that the knowledge about these materials is adopted before they become more widespread.

The thesis focuses on the polymer hollow fibres heat exchangers. The main object of the research is the formation and process of condensation on the outer surface of fibres and the effect of phase change on the heat transfer. The study deals with the influence of the geometry of the heat exchnager on the heat transfer and the condensation process. Fatigue tests of polymeric hollow fibres exchangers are also part of the work. The work provides an overview of the possible use of such heat exchangers in industrial applications associated with condensation.

This thesis is devoted to the fabrication, optical characterization and switching behaviour of the prototypical chalcogenide-based phase-change material Ge2Sb2Te5, which is employed in non-volatile optical and electrical data storage devices. While common studies on conventional memory applications of Ge2Sb2Te5 are based on reversible amorphous to polycrystalline phase transitions, this thesis particularly focuses on the use of Ge2Sb2Te5 thin films of epitaxial, single-crystalline like nature aiming to gain deeper insights into structure-property correlations and novel switching pathways. The first part of this thesis deals with the growth of epitaxial Ge2Sb2Te5 thin films on Si(111) substrates by pulsed laser deposition with strong emphasis on controlling the degree of structural order in the thin films resulting from the distribution of intrinsic vacancies in the crystalline state of the material. As a result, highly vacancy-ordered epitaxial Ge2Sb2Te5 thin films in the thermodynamically stable as well as in the metastable crystalline phase are obtained, possessing a pronounced nanostructuring due to periodically spaced Van-der-Waals gaps or vacancy layers. Besides that, epitaxial Ge2Sb2Te5 thin films with complete disordered vacancy distributions are realized. Based on the achieved single-phase quality of the epitaxial thin films, a classification of the optical property contrast of different crystalline Ge2Sb2Te5 phases with respect to their vacancy ordering is presented. Beyond that, the impact of vicinal substrate surfaces on the phase, structure as well as on surface pattern formation in epitaxial Ge2Sb2Te5 thin films is investigated. The second part of this thesis employs epitaxial Ge2Sb2Te5 thin films as a model system to follow ns-laser induced structural modifications ranging from reversible crystalline to amorphous phase transitions to interface assisted epitaxial recrystallization processes. In particular, by applying single ns-laser pulses to the thin films, the transition from the vacancy ordered stable to the vacancy disordered metastable crystalline structure of Ge2Sb2Te5 via a transient molten phase is realized while the epitaxial nature of the thin films is preserved. This transition mechanism provides access to ultrafast crystal growth dynamics in an epitaxial phase-change material thin film model system offering the advantage of high crystalline quality and application-relevant sizing. By introducing a method that combines time-resolved reflectivity measurements with high resolution scanning transmission electron microscopy, crystal growth velocities upon fast cooling after single ns-laser pulse irradiation are determined. As a result, an increase in crystal growth velocity from 0.4 to 1.7 m/s with increasing laser fluence is observed with the maximum rate of 1.7 m/s as the upper detectable limit of the studied material.

>Magister Scientiae - MSc / Phase change material (PCM) through latent heat of molten salt, is a convincing way for thermal energy storage in CSP applications due to its high volume density. Molten salt, with (60% NaNO3 and 40% KNO3) has been used extensively for energy storage however; the low thermal conductivity and specific heat have limited its large implementation in solar applications. For that, molten salt with the additive of silver nanowires (AgNWs) was synthesized and characterized. This research project aims to investigate the thermophysical properties enhancement of nanosalt (Mixture of molten salt and silver nanowires). The results obtained showed that by simply adjusting the temperature, Silver nanowires with high aspect ratio have been synthesized through the enhanced PVP polyol process method. SEM results revealed a network of silver nanowires and TEM results confirmed the presence of silver nanowires with an average diameter of 129 nm and 16 μm in length.