Les mémoires à changement de phase (PCRAM) sont l'un des candidats les plus prometteurs pour la prochaine génération de mémoires non-volatiles du fait de leurs excellentes vitesses de fonctionnement et endurance. Cependant, deux inconvénients majeurs nécessitent une amélioration afin de permettre leur percée sur le marché des mémoires, à savoir un temps de rétention court à hautes températures et une consommation électrique trop importante. Cette thèse s'intéresse au développement de nouveaux matériaux à changement de phase afin de remplacer le matériau standard Ge2Sb2Te5, inadapté aux applications mémoires embarquées fonctionnant à hautes températures. Le comportement des matériaux binaires GeTe et GeSb a ainsi été évalué et comparé au matériau référence lors de la cristallisation de l'amorphe 'tel que déposé' mais aussi de l'amorphe 'fondu trempé'. En effet, il est important d'étudier le matériau dans son état amorphe 'fondu trempé' pour être au plus près de l'état du matériau cyclé dans les dispositifs. Ainsi, le mécanisme de cristallisation du GeTe déterminé par l'étude de la cristallisation de l'amorphe 'fondu trempé' par recuit laser est en accord avec l'observation MET in situ (recuit thermique) de la cristallisation. L'incorporation d'éléments 'dopants' dans ces matériaux binaires a également été évaluée afin d'augmenter à nouveau la stabilité thermique des matériaux non dopés. Certains éléments 'dopants' permettent une diminution du courant de reset, ou un retard à la formation de 'voids' au cours des cycles. / Phase Change Memories are suitable for the next generation of non volatiles memories due to high programmation speed and endurance. However, two major improvements need to be made in order to enter memories market, the short retention time at high temperature, and the important electric consumption. This thesis focuses on the development of new phase change materials to replace the reference material, Ge2Sb2Te5, insuitable for embedded memories applications working at high temperatures. The behavior of binary compounds GeTe and GeSb has been investigated and compared to the reference material during both the crystallization of the « as deposited » amorphous and the « melt quenched » amorphous materials. Indeeed it is important to study the « melt quenched » amorphous state of the material to be as close as possible to the cycled material in the devices. So, the crystallization mechanism of GeTe checked by the crystallization study of the amorphous « melt quenched » by laser annealing is in agreement with the in situ TEM observation (thermal annealing) of the crystallization. The addition of “doping” elements in the binary compounds has also been performed to improve the thermal stability of amorphous undoped materials. These “doping” elements allow a current reset decrease, or a later formation of « voids » during cycling.

abstract: Buildings consume a large portion of the world's energy, but with the integration of phase change materials (PCMs) in building elements this energy cost can be greatly reduced. The addition of PCMs into building elements, however, becomes a challenge to model and analyze how the material actually affects the energy flow and temperatures in the system. This research work presents a comprehensive computer program used to model and analyze PCM embedded wall systems. The use of the finite element method (FEM) provides the tool to analyze the energy flow of these systems. Finite element analysis (FEA) can model the transient analysis of a typical climate cycle along with nonlinear problems, which the addition of PCM causes. The use of phase change materials is also a costly material expense. The initial expense of using PCMs can be compensated by the reduction in energy costs it can provide. Optimization is the tool used to determine the optimal point between adding PCM into a wall and the amount of energy savings that layer will provide. The integration of these two tools into a computer program allows for models to be efficiently created, analyzed and optimized. The program was then used to understand the benefits between two different wall models, a wall with a single layer of PCM or a wall with two different PCM layers. The effect of the PCMs on the inside wall temperature along with the energy flow across the wall are computed. The numerical results show that a multi-layer PCM wall was more energy efficient and cost effective than the single PCM layer wall. A structural analysis was then performed on the optimized designs using ABAQUS v. 6.10 to ensure the structural integrity of the wall was not affected by adding PCM layer(s). / Dissertation/Thesis / M.S. Civil and Environmental Engineering 2013

abstract: Manufacture of building materials requires significant energy, and as demand for these materials continues to increase, the energy requirement will as well. Offsetting this energy use will require increased focus on sustainable building materials. Further, the energy used in building, particularly in heating and air conditioning, accounts for 40 percent of a buildings energy use. Increasing the efficiency of building materials will reduce energy usage over the life time of the building. Current methods for maintaining the interior environment can be highly inefficient depending on the building materials selected. Materials such as concrete have low thermal efficiency and have a low heat capacity meaning it provides little insulation. Use of phase change materials (PCM) provides the opportunity to increase environmental efficiency of buildings by using the inherent latent heat storage as well as the increased heat capacity. Incorporating PCM into concrete via lightweight aggregates (LWA) by direct addition is seen as a viable option for increasing the thermal storage capabilities of concrete, thereby increasing building energy efficiency. As PCM change phase from solid to liquid, heat is absorbed from the surroundings, decreasing the demand on the air conditioning systems on a hot day or vice versa on a cold day. Further these materials provide an additional insulating capacity above the value of plain concrete. When the temperature drops outside the PCM turns back into a solid and releases the energy stored from the day. PCM is a hydrophobic material and causes reductions in compressive strength when incorporated directly into concrete, as shown in previous studies. A proposed method for mitigating this detrimental effect, while still incorporating PCM into concrete is to encapsulate the PCM in aggregate. This technique would, in theory, allow for the use of phase change materials directly in concrete, increasing the thermal efficiency of buildings, while negating the negative effect on compressive strength of the material. / Dissertation/Thesis / M.S. Civil Engineering 2013

Estudamos o modelo de Ising com diluição de sítios numa rede de Bethe. a estrutura hierárquica da rede de Bethe leva de forma natural às relações de recorrência satisfeitas pelas distribuições de probabilidade dos campos efetivos. As quantidades termodinâmicas na rede de Bethe são então expressas explicitamente em termos das distribuições limite dos campos efetivos. As distribuições dos campos efetivos em T=0 são obtidas de forma numericamente exata (isto é, se desprezarmos os erros de arrendodamento) e também analiticamente em alguns casos selecionados. Encontramos no caso de interações ferromagnéticas um número sempre finito de campos efetivos possíveis, mas no caso de interações antiferromagnéticas esse número pode divergir para valores irracionais do campo aplicado. Esses resultados fornecem o diagrama de fases campo aplicado versus concentração, numericamente exato, para antiferromagnetismo diluído em T=0. As distribuições dos campos efetivos são determinadas aproximadamente para T>0 e utilizadas para o cálculo de diferentes grandezas termodinâmicas. Apresentamos as curvas de magnetização, energia livre, energia interna e entropia. Esses cálculos fornecem o diagrama de fases aproximado no espaço tridimensional de campo aplicado, temperatura e concentração. / The site diluted Ising model is studied on a Beth lattice. The hierarchical structure of the Bethe lattice leads naturally to recursion relations obeyed by the probability distributions of the effective fields. The thermodynamic quantities on the Bethe lattice are then explicitly written in terms of the limiting distributions of the effective fields. Numerically exact results (i.e. if we neglect roundoff errors) for the distributions of the effective fields for T = 0 are presented, together with analytic results for select cases. It is found that the number of effective fields is always finite in the case of ferromagnetic interactions , but it might diverge for irrational values of the applied field in the case of antiferromagnetic interactions. These results yeld a numerically exact applied field versus concentration phase diagram for diluted antiferromagnet at T = 0. The distributions of the effective fields are computed aproximately for T > 0 and used to evaluete various thermodynamic quantities. Curves for the magnetization, free energy, internal energy and entropy are displayed. These calculations give an approximate three-dimensional phase diagram in the space of applied field, temperature and concentration.

Phase change in solidification and melting can be described with the aid of discontinuous functions. The aim of this project is to establish effective methodologies for the solution of discontinuous phase-change problems. The classic capacitance method, which distributes the effect of any discontinuity present over a finite region (typically an element), can suffer from inaccurate energy transport. Improvement is possible with the application of the classic non-physical enthalpy method. However, this approach is known to suffer with the imposition of material velocity, which gives rise to negative thermal capacitance providing a source of error and instability. In order to improve on the performance of the capacitance method and the classic non-physical enthalpy method, this research introduces a series of new non-physical variables. Firstly, a new non-physical enthalpy is defined via the weak form of the energy transport equation. The classical non-physical enthalpy was defined using a temporal integral term. In the new definition, the non-physical enthalpy involves both a temporal and an advection term, which is shown to avoid the generation of negative capacitance and improve the stability of advection heat transfer in numerical methods. Secondly, control volume analysis is performed on weighted and unweighted forms of the governing energy equation involving non-physical enthalpy. The analysis is shown to reveal non-physical source terms that facilitate the removal of phase-change discontinuities. Thirdly, it is demonstrated in the thesis how a non-physical heat source must be introduced into the governing non-physical transport equation to remove discontinuities arising from non-physical terms related to advection. To demonstrate the accuracy and stability of the new method, it is implemented in the finite element method for both one-dimensional linear rod elements and two dimensional triangular elements. Update techniques and root finding methods, such as the predictor-corrector method, the secant method and the homotopy method, are applied to solve the non-linear system of equations, which are constructed with the new theory. Results returned from the one-dimensional numerical experiments are compared with exact solutions, which show reasonable accuracy. Numerical experiments for isothermal solidification with advection-diffusion in both one and two dimensions demonstrate the feasibility of the new methodology.

The high energy density and stable temperature fields of latent heat thermal storage devices (LHTSD) make them promising in a range of applications, including solar energy storage, solar cooking, home heating and cooling, and thermal buffering. The chief engineering challenge in building an effective LHTSD is to find a way to complement the storage capabilities provided by the low-conductivity phase-change material with a suitable enhanced heat transfer mechanism. The principal aim of this project is to develop a tool to improve the design of a small-scale LHTSD, such as one that might be used in solar cooking for a family. An effective small-scale storage device would need to absorb solar energy quickly, release the energy at a high temperature, be affordable, and be manageable within a small household. An LHTSD using solar salts fulfills the latter two requirements: solar salts, a near-eutectic mixture of NaNO3 and KNO3 (60/40% by mass) commonly used in thermal storage applications, are inexpensive and widely available, and the use of latent heat storage means a relatively small chamber can hold enough energy to cook a family meal. The challenge, however, is to design a device that absorbs and releases energy from the solar salts, which have a very low thermal conductivity. The most practical tool to improve the spread of heat through the salts is a finned metal core within the LHTSD. This project uses numerical simulation to determine the most effective design of this finned core. A Cartesian grid solver is developed that is capable of simulating the convection-dominated melting processes within the storage device. The phase boundary is tracked using the enthalpy method, and conjugate heat transfer is calculated with a strongly coupled implicit scheme. A number of techniques are then used to with this solver in order to better understand the factors that affect the performance of a LHTSD and to improve the design of such devices. The thesis is organized as an introductory section followed by three case studies. In the first section, the project is introduced, and the governing equations and core numerical methods are described. In addition, a set of test simulations demonstrate that results using the developed numerical scheme match those of a range of experimental and numerical benchmarks. Each of the case studies aims to adapt the numerical scheme to a more specific problem concerning LHTSDs. In the first, the performance of four fin designs are compared over long-term (48 hour) simulations; the aim is to test the potential performance of the four LHTSDs given realistic solar conditions in New Delhi, India. In the second case study, a set of physical experiments are performed in an empty and a finned LHTSD, and matched 3-dimensional numerical simulations are used to explore the thermal, melt, and flow behavior of the solar salts with the chambers. The final study uses the computational scheme to optimize the design of the finned core of an LHTSD over a large parameter space. To optimize the best design, the key parameters are first prescreened to find which three parameters have the largest effect on the objective equation. A machine-learning optimization code using the dynamic Kriging method (DKG) is then used to build a response surface from which the optimized design can be determined. These three cases demonstrate the potential of the numerical scheme to explore the performance of finned LHTSD designs in a range of ways: the scheme can be used to predict behavior of devices in realistic conditions, to explore the behavior of solar salts during the melting and solidification process, and to determine an optimal design within a large parameter space. In doing so, they show the potential of this tool to help improve the performance and practicality of small-scale LHTSDs.

Phase Change Materials have been the subject of increased research in modern times. Phase Change Materials, abbreviated as PCMs, are being used in a variety of applications in the energy conservation world. In this study, the effect of PCMs on a residential building’s energy consumption was evaluated at different locations across the United States and compared to the standard building at the same locations. An average American residential building was designed and modeled in SketchUp software. The building was evaluated for energy consumption at different locations across the United States using weather data for each chosen location. After the baseline results were collected, the building was re-evaluated, under the same conditions, with a Heptadecane embedded in the exterior walls as the chosen PCM for this study. The results of this study show that Phase Change Materials have a wide-ranging effect on the energy consumption of the designed building. Addition of the PCM to the building walls decreased total energy usage, over the course of a year, by 3.02 – 6.72%, depending on the location.

There is a renewed interest to use material as wood to construct large multi-storey buildings in Sweden, but lightweight material tends to increase the indoor temperature fluctuations during days with large changes in outdoor temperature. The problem can be resolved by integrating phase change material (PCM) in the construction. This increases thermal inertia which mitigates the fluctuations.           The scope of the study is to develop a simulation model in COMSOL Multiphysics, to validate the model experimentally and to determine the optimal position and thickness of a PCM layer in a multi-layer wall. The model, representing a building with the shape of a box, consists of two versions. The first version, called the test box, is modeled with 5 sides of pure gypsum and 1 side of PCM-gypsum composite. The second version without PCM, called the reference box, is modeled with 6 sides of pure gypsum. Since the study is focused on reducing the cooling load, the PCM gypsum composite material should function effectively during summer conditions in northern Sweden. The experimental part includes two real-life boxes, the experimental test box and reference box, built of the same type of material that is chosen for the simulation model boxes. A climate chamber is utilized for the temperature control of the two boxes while performing measurements to validate the simulation model. The simulation model showed deviations from the experimental measurements. The temperatures inside the climate chamber, at all five points of measurement, were lower than the equivalent points in the simulation. It was possible to compensate by adjusting the overall ambient temperature down with 0.6 °C in the simulation, resulting in smaller errors. The PCM positioning resulted in recommendations to place the PCM closest to the interior space. The testing of different PCM thicknesses showed the best heat storage for the thickest PCM layers, but the PCM storage efficiency should have been considered as well.

The area of nanophotonics has been the focus of researchers recently due to its high potential to overcome the limitations of scaling in electronic devices. One of the most popular devices in this field is the metasurface. A metasurface consists of a periodic or aperiodic array of spaced units called ’meta-atoms’, where the interaction between these neighboring elements provide unprecedented properties that cannot be obtained using a a regular array of antennas. By tuning the shape and structure of the meta-atoms, electromagnetic wave interaction with the metasurface can be manipulated to achieve a plethora of response characteristics. For active applications that require tunability of the response, a passive metasurface cannot be used to adapt to the varying operating conditions. Tunability of metasurfaces can then be achieved by using phase-changing materials. This type of materials can attain different optical properties by applying external stimulus such as heat, electric current, or laser pulses. The change in the optical properties would be beneficial for applications requiring reconfigurability or adaptation. In this thesis, I demonstrate the employment of volatile (Vanadium Dioxide) and non-volatile (Germanium Antimony Telluride) examples of phase-change materials to design reconfigurable metasurfaces operating at different bands in the infrared regime. I show metallic and dielectric-based structures that employ volatile and non-volatile phase-change materials, as well as apply physics such as plasmonics and bound states in the continuum to design and optimize metasurface structures for energy and biosensing applications. / Thesis / Doctor of Philosophy (PhD) / This thesis proposes methods to design and optimize reconfigurable and adaptive metasurfaces for energy harvesting, radiative cooling, and biosensing applications in the infrared range. The concept of phase-change metasurfaces is highlighted where a phase-change material (PCM) is employed to provide the tunable response. The properties of the PCM can be modified using several excitation methods such as thermal, electric, and laser excitation. The details of material selection, geometry configuration, as well as optimization procedures are demonstrated. Target applications for the study is harvesting from Earth’s ambient radiation around 10.6µm, adaptive cooling of spacecraft in the mid-infrared band 2.5 − 25µm, and trace biomarkers detection in the amide-I and amide-II bands (5.5−6.9µm). Full-wave numerical analysis was conducted using COMSOL Multiphysics software. Optimization was carried out using global optimization techniques implemented using Matlab and Python. The results show innovative designs for switchable absorbers, new approach for modeling of phase-change metasurfaces using deep learning, and employment of the physics of bound states in the continuum for the first time to implement a robust biosensing device. The results of this thesis would help advance the field of reconfigurable nanophotonics and related integrated applications.

Phase change material (PCM)-based ocean thermal energy harvesting is a relatively new method, which extracts the thermal energy from the temperature gradient in the ocean thermocline. Its basic idea is to utilize the temperature variation along the ocean water depth to cyclically freeze and melt a specific kind of PCM. The volume expansion, which happens in the melting process, is used to do useful work (e.g., drive a turbine generator), thereby converting a fraction of the absorbed thermal energy into mechanical energy or electrical energy. Compared to other ocean energy technologies (e.g., wave energy converters, tidal current turbines, and ocean thermal energy conversion), the proposed PCM-based approach can be easily implemented at a small scale with a relatively simple structural system, which makes it a promising method to extend the range and service life of battery-powered devices, e.g, autonomous underwater vehicles (AUVs). This dissertation presents a combined theoretical and experimental study of the PCM-based ocean thermal energy harvesting approach, which aims at demonstrating the feasibility of the proposed approach and investigating possible methods to improve the overall performance of prototypical systems. First, a solid/liquid phase change thermodynamic model is developed, based on which a specific upperbound of the thermal efficiency is derived for the PCM-based approach. Next, a prototypical PCM-based ocean thermal energy harvesting system is designed, fabricated, and tested. To predict the performance of specific systems, a thermo-mechanical model, which couples the thermodynamic behaviors of the fluid materials and the elastic behavior of the structural system, is developed and validated based on the comparison with the experimental measurement. For the purpose of design optimization, the validated thermo-mechanical model is employed to conduct a parametric study. Based on the results of the parametric study, a new scalable and portable PCM-based ocean thermal energy harvesting system is developed and tested. In addition, the thermo-mechanical model is modified to account for the design changes. However, a combined analysis of the results from both the prototypical system and the model reveals that achieving a good performance requires maintaining a high internal pressure, which will complicate the structural design. To mitigate this issue, the idea of using a hydraulic accumulator to regulate the internal pressure is proposed, and experimentally and theoretically examined. Finally, a spatial-varying Robin transmission condition for fluid-structure coupled problems with strong added-mass effect is proposed and investigated using fluid structure interaction (FSI) model problems. This can be a potential method for the future research on the fluid-structure coupled numerical analysis of AUVs, which are integrated with and powered by the PCM-based thermal energy harvesting devices. / Doctor of Philosophy / The global ocean, which covers about 71% of the Earth’s surface, absorbs a great amount of heat from the sunshine everyday, making it a reliable and renewable source of thermal energy. Also, the temperature of the ocean water varies with depth, which provides a necessary condition (i.e, a temperature gradient) to extract the thermal energy. If harvested and converted into electrical energy using small scale portable devices, the ocean thermal energy can be a potential energy resource to provide power for autonomous underwater vehicles (AUVs), which are conventionally powered by on-board rechargeable batteries. To this end, this dissertation presents a study of using solid/liquid phase change materials (PCMs) to extract thermal energy from the temperature gradient in the ocean. The basic idea is to use the warm surface water and deep cold water to melt and freeze the PCM cyclically. In the meantime, the volume of PCM will expand and contract accordingly. Therefore, a turbine generator can be driven by the volume expansion in the melting process, thereby converting a fraction of the absorbed thermal energy into electrical energy. This study includes four key aspects. First, to evaluate the theoretical full potential of the PCM-based approach, a solid/liquid phase change thermodynamic model – which represents an idealized energy harvester – is developed. Based on the thermodynamic model, an upperbound of the thermal efficiency is derived. Secondly, two prototypical systems, as well as a thermo-mechanical model which can predict the performance of specific designs, are developed. Third, for the purposes of performance improvement and pressure regulation, the latter of which is associated with the structural safety, a hydraulic accumulator is added to the existing system and its effects are examined using both experimental and theoretical methods. Finally, a computational method is proposed and demonstrated, which can be a potential tool for the design of PCM-based ocean thermal energy harvesting systems when they are integrated with exiting AUVs.