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Dissertations / Theses on the topic 'Phase change materiaks'

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Published: 5 June 2025

Last updated: 16 July 2025

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Jost, Peter Christian Georg [Verfasser]. "Charge transport in phase-change materials / Peter Christian Georg Jost." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2013. http://d-nb.info/1043523359/34.

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Luckas, Jennifer. "Electronic transport in amorphous phase-change materials." Phd thesis, Université Paris Sud - Paris XI, 2012. http://tel.archives-ouvertes.fr/tel-00743474.

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Les matériaux à changement de phase montrent la combinaison exceptionnelle d'un contraste énorme dans leurs propriétés physiques entre la phase amorphe et cristalline allié à une cinétique de changement de phase extrêmement rapide. La grande différence en résistivité permet leur application dans les mémoires numériques. De plus, cette classe de matériaux montre dans leur état vitreux des phénomènes de transport électronique caractéristiques. Le seuil de commutation dénote la chute de la résistivité dans l'état amorphe au delà d'un champ électrique critique. Le phénomène de seuil de commutation permet la transition de phase en appliquant des tensions relativement faibles. Au-dessous de cette valeur critique l'état désordonné montre une conductivité d'obscurité activée en température ainsi qu'une résistance - dans les cellules mémoires et les couches minces également - qui augmente avec le temps. Cette évolution de la résistivité amorphe entrave le stockage à plusieurs niveaux, qui offrirait la possibilité d'accroître la capacité ou la densité de stockage considérablement. Comprendre les origines physiques de ces deux phénomènes est crucial pour développer de meilleures mémoires à changement de phase. Bien que ces deux phénomènes soient généralement attribués aux défauts localisés, la connaissance de la distribution de défauts dans les matériaux amorphes à changement de phase est assez limitée. Cette thèse se concentre sur la densité des défauts mesurée dans différents verres chalcogénures présentant l'effet de seuil de commutation. Sur la base d'expériences de photo courant modulé (MPC) et de spectroscopie par déviation photothermique, un modèle sophistiqué des défauts a été développé pour GeTe amorphe (a-GeTe) mettant en évidence les états de la bande de valence et plusieurs défauts. Cette étude sur a-GeTe montre que l'analyse des données MPC peut être grandement améliorée en prenant en compte la variation de la bande de l'énergie interdite avec la température. Afin de mieux appréhender l'évolution de la résistivité amorphe, la présente étude porte sur l'évolution avec les recuits et le vieillissement de la résistivité, de l'énergie d'activation du courant d'obscurité, de la densité des défauts, du stress mécanique, de l'environnement atomique et de l'énergie de la bande interdite mesurée par des méthodes optiques sur les couches minces de a-GeTe. Le recuit d'un échantillon entraîne un élargissement de la bande interdite et de l'énergie d'activation du courant d'obscurité. De plus, la technique MPC a révélé une diminution des défauts profonds dans les couches minces de a-GeTe vieillies. Ces résultats illustrent l'impact de l'annihilation des défauts et de l'élargissement de la bande interdite sur l'évolution de la résistivité des matériaux à changement de phase amorphe. Cette thèse présente également une étude sur les alliages à changement de phase GeSnTe. En augmentant la concentration d'étain, on observe une décroissance systématique de la résistivité amorphe, de l'énergie d'activation du courant d'obscurité, de la largeur de bande interdite et de la densité des défauts, qui conduisent à une résistivité amorphe plus stables dans les compositions riches en étain comme a-Ge2Sn2Te4. L'étude sur les alliages GeSnTe montre que les matériaux à changement de phase ayant une résistivité amorphe plus stable présentent une faible énergie d'activation du courant d'obscurité. À l'exemple du Ge2Sn2Te4 et GeTe la présente étude montre un lien étroit entre l'évolution de la résistivité et la relaxation du stress mécanique. L'étude sur les verres chalcogénures montrent que les matériaux ayant un grand champ d'électrique de seuil, bien connu d'après la littérature, présentent aussi une grande densité de défauts. Ce résultat implique que l'origine du phénomène de seuil de commutation se trouve dans un mécanisme de génération à travers la bande interdite et de recombinaison dans les défauts profonds comme proposé par D. Adler.

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Bugaje, Idris M. "Thermal energy storage in phase change materials." Thesis, University of Newcastle Upon Tyne, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335920.

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Huang, Bolong. "Theoretical study on phase change memory materials." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609986.

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Oliver, David Elliot. "Phase-change materials for thermal energy storage." Thesis, University of Edinburgh, 2015. http://hdl.handle.net/1842/17910.

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There is a current requirement for technologies that store heat for both domestic and industrial applications. Phase-change materials (PCMs) represent an important class of materials that offer potential for heat storage. Heat-storage systems are required to undergo multiple melt/freeze cycles without any change in melting-crystallisation point and heat output. Salt hydrates are attractive candidates on account of their high energy densities, but there are issues associated with potential crystallisation of lower-hydrates, long-term stability, and reliable nucleation. An extensive review of the PCMs in the literature, combined with an evaluation of commercially available PCMs led to the conclusion that many of the reported PCMs, lack at least one of the key requirements required for use as a heat-storage medium. The focus of this research was therefore to identify and characterise new PCM compositions with tailored properties. New PCM compositions based of sodium acetate trihydrate were developed, which showed improved properties through the use of selective polymers that retard the nucleation of undesirable anhydrous sodium acetate. Furthermore, the mechanism of nucleation of sodium acetate trihydrate by heterogeneous additives has been investigated using variable-temperature powder X-ray diffraction. This study showed that when anhydrous Na2HPO4 was introduced to molten sodium acetate trihydrate at 58°C the hydrogenphosphate salt is present as the dihydrate. On heating to temperatures in the range 75-90°C the dihydrate was observed to dehydrate to form anhydrous Na₂HPO4. This result explains the prior observation that the nucleator is deactivated on heating. The depression of melting point of sodium acetate trihydrate caused by the addition of lithium acetate dihydrate has also been investigated using differential scanning calorimetry and powder X-ray diffraction. It has been possible to tune the melting point of sodium acetate trihydrate thereby modifying its thermal properties. Studies of the nucleation of sodium thiosulfate pentahydrate, a potential PCM, led to the structural characterisation of six new hydrates using single crystal Xray diffraction. All of these hydrates can exist in samples with the pentahydrate composition at temperatures ranging from 20°C to 45°C. These hydrates are: α-Na₂S₂O₃·2H₂O, which formed during the melting of α-Na₂S₂O₃·5H₂O; two new pentahydrates, β-Na₂S₂O₃·5H₂O and γ-Na₂S₂O₃·5H₂O; Na₂S₂O₃·1.33 H₂O, β-Na₂S₂O₃·2H₂O and Na₂S₂O₃·3.67 H₂O, which formed during the melting of β- Na₂S₂O₃·5H₂O. Furthermore, new PCMs in the 75-90°C range were identified. The commercial impact and route to market of several of the PCMs are discussed in the final chapter.

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Kasali, Suraju Olawale. "Thermal diodes based on phase-change materials." Thesis, Poitiers, 2021. http://www.theses.fr/2021POIT2254.

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Nous étudions dans cette thèse la rectification thermique de diodes thermiques radiatives ou conductive constituées de matériaux à changement de phase.Cette thèse est divisée en trois parties. Dans les premières parties, nous modélisons comparativement les performances d’une diode thermique conductive sphérique et cylindrique constitués de VO2 présentant un transition de phase et des matériaux n’en présentant pas. Des expressions analytiques aux bornes des diodes sont dérivées. Des flux thermiques, des facteurs de rectifications ainsi que les profils de température à l’intérieur de la diode sont obtenus. Nos résul-tats montrent que les différentes géométries de diodes ont un impact significatif sur les profils de température et les flux thermiques, mais moins un sur les facteurs de rectification. Dans ce travail, nous avons obtenu des facteurs de rectification maximaux allant jusqu’à 20.8% et 20.7%, qui sont supérieurs à celui prédit pour une diode plane constituée de VO2. Nous montrons également que des facteurs de rectification similaires à ceux obtenus avec le VO2 dans les géométries sphériques et cylindriques peuvent être atteints avec des matériaux à changement de phase dont le contraste de conductivité est plus important que dans le cas du VO2. Dans la deuxième partie, nous étudions la rectification de diodes thermiques constituées de deux matériaux à changement de phase. Avec, l’idée de générer un facteur de redressement plus élevé que dans le cas d’une diode thermique conductive ne comprenant qu’un matériau à changement de phase unique. Là encore, le travail a conduit à l’établissement d’expressions explicites pour les profils de température, les flux thermiques et le facteur de rectification. Nous avons obtenu un facteur de rectification optimal de 60% avec une variation de température de 250 K couvrant les transitions métal-isolant des deux matériaux. Dans la troisième partie de notre travail, nous avons modélisé et optimisé la rectification thermique de diodes thermiques planes, cylindriques et sphériques radiatives à base de deux matériaux à changement de phase. Nous savons calculer et analyser les facteurs de rectification de ces trois diodes et obtenu les facteurs de rectification optimaux respectifs pour les trois géométries 82%, 86% et 90.5%. Nos résultats montrent que la géométrie sphérique est la meilleure pour optimiser la rectification des courants thermiques radiatifs. De plus, des facteurs de rectification potentiellement supérieurs à ceux prédits ici peuvent être réalisés en utilisant deux matériaux à changement de phase avec des contrastes d’émissivités plus élevés que ceux proposés ici. Ces résultats analytiques et graphiques fournissent un guide utile pour optimiser les facteurs de rectification des diodes thermiques conductives et radiatifs basées sur des matériaux à changement de phase de géométries différentes The thermal rectification of conductive and radiative thermal diodes based on phase-change materials, whose thermal conductivities and effective emissivities significant change within a narrow range of temperatures, is theoretically studied and optimized in different geometries. This thesis is divided into three parts. In the first part, we comparatively model the performance of a spherical and cylindrical conductive thermal diodes operating with vanadium dioxide (VO2) and non-phase-change materials, and derive analytical expressions for the heat flows, temperature profiles and optimal rectification factors for both diodes. Our results show that different diode geometries have a significant impact on the temperature profiles and heat flows, but less one on the rectification factors. We obtain maximum rectification factors of up to 20.8% and 20.7%, which are higher than the one predicted for a plane diode based on VO2. In addition, it is shown that higher rectification factors could be generated by using materials whose thermal conductivity contrast is higher than that of VO2. In the second part, on the other hand, we theoretically study the thermal rectification of a conductive thermal diode based on the combined effect of two phase-change materials. Herein, the idea is to generate rectification factors higher than that of a conductive thermal diode operating with a single phase-change material. This is achieved by deriving explicit expressions for the temperature profiles, heat fluxes and rectification factor. We obtain an optimal rectification factor of 60% with a temperature variation of 250 K spanning the metal-insulator transitions of VO2 and polyethylene. This enhancement of the rectification factor leads us to the third part of our work, where we model and optimize the thermal rectification of a plane, cylindrical and spherical radiative thermal diodes based on the utilization of two phase-change materials. We analyze the rectification factors of these three diodes and obtain the following optimal rectification factors of 82%, 86% and 90.5%, respectively. The spherical geometry is thus the best shape to optimize the rectification of radiative heat currents. In addition, potential rectification factors greater than the one predicted here can be realized by utilizing two phase-change materials with higher emissivities contrasts than the one proposed here. Our analytical and graphical results provide a useful guide for optimizing the rectification factors of conductive and radiative thermal diodes based on phase-change materials with different geometries

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Aboujaoude, Andrea E. "Nanopatterned Phase-Change Materials for High-Speed, Continuous Phase Modulation." University of Dayton / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1538243834791942.

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Milisic, Edina. "Modelling of energy storage using phase-change materials (PCM materials)." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for energi- og prosessteknikk, 2013. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-23506.

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Unfortunately the global conventional fuels in reserves are running out while the world energy consumption is increasing very fast. All scientists agreed that Renewable energies is one of the best solutions for energy supply in many parts of the world. Renewable energies are solar energy, wind energy, bio energy, geothermal energy, tidal energy, and hydropower. Approximately all these forms of energy are hampered by their high costs. Moreover, solar energy, wind energy and tidal energy are characterized by their intermittent nature, as they are not available all the time. This intermittent problem can be solved by energy storage.Energy storage components improve the energy efficiency of systems by reducing the mismatch between supply and demand. For this purpose, phase-change materials are particularly attractive since they provide a high-energy storage density at a constant temperature which corresponds to the phase transition temperature of the material. The aim of this thesis is to Is to describe the state of the art progress in applying PCM materials for energy storage (essentially in tanks), and opportunities of their future applications, describe physical properties of typically used PCM materials, present a mathematical model of the energy balance during the energy storage (charge) and energy discharge from the PCM material. Mathematical model is based on one-dimensional (1D) analysis. The mathematical model consist of charging process and discharging process. During charging process the heat transfer fluid passes through the storage tank in order to transfer its thermal energy to the phase change material tube. During the discharging process, the cold water passes through the storage tank to acquire the thermal energy stored by the phase change material tube. Different solutions utilizing PCM was assessed. It was presented different Phase Change Materials for energy storage. This assessment indicated that salt hydrates are the most energy intensive of the PCM possibilities. When we use the Paraffin for energy storage we had less energy stored then with salt hydrates used like medium for energy storage. This assessment indicated that when we use PCM as a medium for energy storage we accumulate significantly more energy than in the case when we use water as a medium for energy storage.There are some weaknesses in the PCM model. It was assumed that the temperature in the tank was uniform. This will not apply for the real case where the heat transfer fluid temperature will increase while transferring through the tank. For a realistic case, the temperature of the first elements will decrease rapidly because of large temperature difference between the heat transfer fluid and the PCMs in the tank.

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Giovanardi, Fabio <1984&gt. "Analysis of charge-transport properties in GST materials for next generation phase-change memory devices." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2013. http://amsdottorato.unibo.it/5583/4/giovanardi_fabio_tesi.pdf.

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The quest for universal memory is driving the rapid development of memories with superior all-round capabilities in non-volatility, high speed, high endurance and low power. The memory subsystem accounts for a significant cost and power budget of a computer system. Current DRAM-based main memory systems are starting to hit the power and cost limit. To resolve this issue the industry is improving existing technologies such as Flash and exploring new ones. Among those new technologies is the Phase Change Memory (PCM), which overcomes some of the shortcomings of the Flash such as durability and scalability. This alternative non-volatile memory technology, which uses resistance contrast in phase-change materials, offers more density relative to DRAM, and can help to increase main memory capacity of future systems while remaining within the cost and power constraints. Chalcogenide materials can suitably be exploited for manufacturing phase-change memory devices. Charge transport in amorphous chalcogenide-GST used for memory devices is modeled using two contributions: hopping of trapped electrons and motion of band electrons in extended states. Crystalline GST exhibits an almost Ohmic I(V) curve. In contrast amorphous GST shows a high resistance at low biases while, above a threshold voltage, a transition takes place from a highly resistive to a conductive state, characterized by a negative differential-resistance behavior. A clear and complete understanding of the threshold behavior of the amorphous phase is fundamental for exploiting such materials in the fabrication of innovative nonvolatile memories. The type of feedback that produces the snapback phenomenon is described as a filamentation in energy that is controlled by electron–electron interactions between trapped electrons and band electrons. The model thus derived is implemented within a state-of-the-art simulator. An analytical version of the model is also derived and is useful for discussing the snapback behavior and the scaling properties of the device. Lo sviluppo dei sistemi di memoria di futura generazione è guidato principalmente dalla ricerca di una tecnologia in grado di superare quelle attuali in ogni loro specifica di funzionamento, dalla ritenzione di dato alla velocità di accesso, migliorandone la durata e riducendo il dispendio energetico. Il sottosistema delle memorie assorbe una parte significativa delle risorse del macro sistema costituito dal calcolatore, tanto da aver quasi raggiunto il limite tecnologico nel caso delle odierne memorie di tipo DRAM. La soluzione più promettente sembra essere quella delle memorie a cambiamento di fase (PCM), in grado di colmare anche i limiti mostrati dalla tecnologia Flash nell’ambito della durata e scalabilità. I materiali che consentono di realizzare dispostivi a cambiamento di fase pilotato elettricamente appartengono alla famiglia dei calcogenuri. Tra i diversi composti calcogenuri quello attualmente identificato come soluzione più promettente è il Ge2Sb2Te5 (GST). Il trasporto di carica all’interno di dispositivi di memoria realizzati con tali materiali è stato modellato considerando l’azione di due contributi differenti: hopping di cariche intrappolate e moto di elettroni liberi in stati estesi. Il GST mostra un comportamento elettrico pressoché Ohmico in fase cristallina mentre, in fase amorfa, risulta essere poco conduttivo per basse correnti fino al superamento di una tensione di soglia oltre la quale si assiste al passaggio da uno stato altamente resistivo ad uno altamente conduttivo, caratterizzato da un andamento a resistenza differenziale negativa (NDR). Il meccanismo retroattivo che induce il fenomeno di snapback viene descritto come filamentazione in energia controllata dalle interazioni tra elettroni liberi ed elettroni intrappolati. Il modello fisico ricavato è stato implementato all’interno di un simulatore di dispositivi di ultima generazione ed è stato in seguito riprodotto in una versione analitica semplificata in grado, però, di permettere una prima analisi del comportamento elettrico del dispositivo e delle sue proprietà di scaling.

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Ocampo, Carlos Andrés Ríos. "Phase-change materials for photonic memories and optoelectronic applications." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:1c2c3179-ef9f-4fbf-b91c-c4d2f7ee7ed5.

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The content of this thesis encompasses the fundamentals, modelling, chip design, nanofabrication process, measurement setup, and experimental results of devices exploiting the optical properties of phase-change chalcogenide materials. Special attention is paid to integrated Si3N4 nanophotonic circuits for optical switching and memory applications, as well as to multilayer stacks for colour modulation. Herein, the implementation of the first robust, non-volatile, phase-change photonic memory is presented. By utilising optical near-field effects for Read, Write and Erase operations, bit storage of up to eight transmission levels is demonstrated in a single device employing Ge2Sb2Te5 as the active material. These on-chip memory cells feature single-shot read-out of the transmission state and switching energies as low as 13.4pJ at speeds approaching 1GHz. The capability to readily switch between intermediate states is also demonstrated, a feature that requires complex iteration-based algorithms in electronic phase-change memories. This photonic memory is not only the first truly non-volatile memory---a long-term elusive goal in integrated photonics---but could also potentially represent the first multi-level memory, including electronic counterparts, that requires no computational post-processing or drift correction. These findings provide a pathway towards solving the throughput limitations of current computer architectures by eliminating the so-called von-Neumann bottleneck and portend a new paradigm in all-photonic memory, non-conventional computing, and tunable photonic devices. Finally, novel capabilities in electro-optic colour modulation using phase-change materials are demonstrated. In particular, this thesis offers the first implementation of Ag3In4Sb76Te17-based optical cavities for colour modulation on low-dimensional multilayer stacks. Moreover, "gray-scale" image writing is demonstrated by establishing intermediate levels of crystallisation via voltage modulation. This finding, in turn, corresponds to the first demonstration of nonvolatile colour-depth modulation in the emerging phase-change materials nanodisplay technology, featuring resolutions down to 50nm. Furthermore, a comprehensive comparison is carried out for two types of materials: growth- (Ag3In4Sb76Te17) and nucleation-dominated (Ge2Sb2Te5) alloys in terms of colour, energy efficiency, and resolution. These results provide new tools for the new generation of bistable and ultra-high-resolution displays and smart glasses while allowing for other potential applications in photonics and optoelectronics.

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Bruns, Gunnar [Verfasser]. "Electronic switching in phase-change materials / Gunnar Bruns." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2012. http://d-nb.info/1020843993/34.

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Hong, Yan. "Encapsulated nanostructured phase change materials for thermal management." Doctoral diss., University of Central Florida, 2011. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/4929.

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A major challenge of developing faster and smaller microelectronic devices is that high flux of heat needs to be removed efficiently to prevent overheating of devices. The conventional way of heat removal using liquid reaches a limit due to low thermal conductivity and limited heat capacity of fluids. Adding solid nanoparticles into fluids has been proposed as a way to enhance thermal conductivity of fluids, but recent results show inconclusive anomalous enhancements in thermal conductivity. A possible way to improve heat transfer is to increase the heat capacity of liquid by adding phase change nanoparticles with large latent heat of fusion into the liquid. Such nanoparticles absorb heat during solid to liquid phase change. However, the colloidal suspension of bare phase change nanoparticles has limited use due to aggregation of molten nanoparticles, irreversible sticking on fluid channels, and dielectric property loss. This dissertation describes a new method to enhance the heat transfer property of a liquid by adding encapsulated phase change nanoparticles (nano-PCMs), which will absorb thermal energy during solid-liquid phase change and release heat during freeze. Specifically, silica encapsulated indium nanoparticles, and polymer encapsulated paraffin (wax) nanoparticles have been prepared using colloidal method, and dispersed into poly-alpha]-olefin (PAO) and water for high temperature and low temperature applications, respectively. The shell, with a higher melting point than the core, can prevent leakage or agglomeration of molten cores, and preserve the dielectric properties of the base fluids. Compared to single phase fluids, heat transfer of nanoparticle-containing fluids have been significantly enhanced due to enhanced heat capacities. The structural integrity of encapsulation allows repeated uses of nanoparticles for many cycles.; By forming porous semi crystalline silica shells obtained from water glass, supercooling has been greatly reduced due to low energy barrier of heterogeneous nucleation. Encapsulated phase change nanoparticles have also been added into exothermic reaction systems such as catalytic and polymerization reactions to effectively quench local hot spots, prevent thermal runaway, and change product distribution. Specifically, silica-encapsulated indium nanoparticles, and silica encapsulated paraffin (wax) nanoparticles have been used to absorb heat released in catalytic reaction, and to mitigate the gel effect during polymerization, respectively. The reaction rates do not raise significantly owing to thermal buffering using phase change nanoparticles at initial stage of thermal runaway. The effect of thermal buffering depends on latent heats of fusion of nanoparticles, and heat releasing kinetics of catalytic reactions and polymerizations. Micro/nanoparticles of phase change materials will open a new dimension for thermal management of exothermic reactions. ID: 029809237; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Thesis (Ph.D.)--University of Central Florida, 2011.; Includes bibliographical references (p. 164-191). Ph.D. Doctorate Mechanical Materials and Aerospace Engineering Engineering and Computer Science

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Khoo, Chee Ying. "Evaluation of phase change materials for reconfigurable interconnects." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/62678.

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Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010. Cataloged from PDF version of thesis. Includes bibliographical references (p. 76-80). The possible use of programmable integrated circuit interconnect vias using an indirectly heated phase change material is evaluated. Process development and materials investigations are examined. Devices capable of multiple cycles between on/off states for reconfigurable applications have been successfully demonstrated in a standard CMOS-compatible technology. Building computer chips with these vias would create a new kind of field programmable gate array (FPGA), whereby the design can be reconfigured depending on its application. The phase change reprogrammable-via is nonvolatile, unlike SRAM-based technology. It also has a relatively low on-state resistance and occupies less real estate on the chip. As the "switches" are placed at the metallization level, it provides flexibility for the designer to place them. Programmable-via can operate at a relatively low voltage compared to FLASH-based technology. Similar to the case of antifuses, programmable-via interconnect structures are projected to be radiation hard. However, the most challenging part of implementation is the circuit design. Issues such as integration of materials and design with current tools need to be overcome. A lack of expert personnel in this area also makes the implementation of programmable-via FPGAs complicated. The market for FPGA is promising due to the attraction of the programmable logic market. An intellectual Property (IP) analysis indicates there exist a significant new space for exploration in this area. The best-suited business model is as a new start-up that demonstrates feasibility and develops intellectual property. The potential commercialization of such technology is also discussed. Although this concept is promising result, more research is needed to show the reliability and feasibility of such a technology in complex circuits. It will take some time before this approach can be considered for production. by Chee Ying Khoo. M.Eng.

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Kotze, Johannes Paulus. "Thermal energy storage in metallic phase change materials." Thesis, Stellenbosch : Stellenbosch University, 2014. http://hdl.handle.net/10019.1/96049.

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Thesis (PhD) -- Stellenbosch University, 2014. ENGLISH ABSTRACT: Currently the reduction of the levelised cost of electricity (LCOE) is the main goal of concentrating solar power (CSP) research. Central to a cost reduction strategy proposed by the American Department of Energy is the use of advanced power cycles like supercritical steam Rankine cycles to increase the efficiency of the CSP plant. A supercritical steam cycle requires source temperatures in excess of 620°C, which is above the maximum storage temperature of the current two-tank molten nitrate salt storage, which stores thermal energy at 565°C. Metallic phase change materials (PCM) can store thermal energy at higher temperatures, and do not have the drawbacks of salt based PCMs. A thermal energy storage (TES) concept is developed that uses both metallic PCMs and liquid metal heat transfer fluids (HTF). The concept was proposed in two iterations, one where steam is generated directly from the PCM – direct steam generation (DSG), and another where a separate liquid metal/water heat exchanger is used – indirect steam generation, (ISG). Eutectic aluminium-silicon alloy (AlSi12) was selected as the ideal metallic PCM for research, and eutectic sodium-potassium alloy (NaK) as the most suitable heat transfer fluid. Thermal energy storage in PCMs results in moving boundary heat transfer problems, which has design implications. The heat transfer analysis of the heat transfer surfaces is significantly simplified if quasi-steady state heat transfer analysis can be assumed, and this is true if the Stefan condition is met. To validate the simplifying assumptions and to prove the concept, a prototype heat storage unit was built. During testing, it was shown that the simplifying assumptions are valid, and that the prototype worked, validating the concept. Unfortunately unexpected corrosion issues limited the experimental work, but highlighted an important aspect of metallic PCM TES. Liquid aluminium based alloys are highly corrosive to most materials and this is a topic for future investigation. To demonstrate the practicality of the concept and to come to terms with the control strategy of both proposed concepts, a storage unit was designed for a 100 MW power plant with 15 hours of thermal storage. Only AlSi12 was used in the design, limiting the power cycle to a subcritical power block. This demonstrated some practicalities about the concept and shed some light on control issues regarding the DSG concept. A techno-economic evaluation of metallic PCM storage concluded that metallic PCMs can be used in conjunction with liquid metal heat transfer fluids to achieve high temperature storage and it should be economically viable if the corrosion issues of aluminium alloys can be resolved. The use of advanced power cycles, metallic PCM storage and liquid metal heat transfer is only merited if significant reduction in LCOE in the whole plant is achieved and only forms part of the solution. Cascading of multiple PCMs across a range of temperatures is required to minimize entropy generation. Two-tank molten salt storage can also be used in conjunction with cascaded metallic PCM storage to minimize cost, but this also needs further investigation. AFRIKAANSE OPSOMMING: Tans is die minimering van die gemiddelde leeftydkoste van elektrisiteit (GLVE) die hoofdoel van gekonsentreerde son-energie navorsing. In die kosteverminderingsplan wat voorgestel is deur die Amerikaanse Departement van Energie, word die gebruik van gevorderde kragsiklusse aanbeveel. 'n Superkritiese stoom-siklus vereis bron temperature hoër as 620 °C, wat bo die 565 °C maksimum stoor temperatuur van die huidige twee-tenk gesmelte nitraatsout termiese energiestoor (TES) is. Metaal fase veranderingsmateriale (FVMe) kan termiese energie stoor by hoër temperature, en het nie die nadele van soutgebaseerde FVMe nie. ŉ TES konsep word ontwikkel wat gebruik maak van metaal FVM en vloeibare metaal warmteoordrag vloeistof. Die konsep is voorgestel in twee iterasies; een waar stoom direk gegenereer word uit die FVM (direkte stoomopwekking (DSO)), en 'n ander waar 'n afsonderlike vloeibare metaal/water warmteruiler gebruik word (indirekte stoomopwekking (ISO)). Eutektiese aluminium-silikon allooi (AlSi12) is gekies as die mees geskikte metaal FVM vir navorsingsdoeleindes, en eutektiese natrium – kalium allooi (NaK) as die mees geskikte warmteoordrag vloeistof. Termiese energie stoor in FVMe lei tot bewegende grens warmteoordrag berekeninge, wat ontwerps-implikasies het. Die warmteoordrag ontleding van die warmteruilers word aansienlik vereenvoudig indien kwasi-bestendige toestand warmteoordrag ontledings gebruik kan word en dit is geldig indien daar aan die Stefan toestand voldoen word. Om vereenvoudigende aannames te bevestig en om die konsep te bewys is 'n prototipe warmte stoor eenheid gebou. Gedurende toetse is daar bewys dat die vereenvoudigende aannames geldig is, dat die prototipe werk en dien as ŉ bevestiging van die konsep. Ongelukkig het onverwagte korrosie die eksperimentele werk kortgeknip, maar dit het klem op 'n belangrike aspek van metaal FVM TES geplaas. Vloeibare aluminium allooie is hoogs korrosief en dit is 'n onderwerp vir toekomstige navorsing. Om die praktiese uitvoerbaarheid van die konsep te demonstreer en om die beheerstrategie van beide voorgestelde konsepte te bevestig is 'n stoor-eenheid ontwerp vir 'n 100 MW kragstasie met 15 uur van 'n TES. Slegs AlSi12 is gebruik in die ontwerp, wat die kragsiklus beperk het tot 'n subkritiese stoomsiklus. Dit het praktiese aspekte van die konsep onderteken, en beheerkwessies rakende die DSO konsep in die kollig geplaas. In 'n tegno-ekonomiese analise van metaal FVM TES word die gevolgtrekking gemaak dat metaal FVMe gebruik kan word in samewerking met 'n vloeibare metaal warmteoordrag vloeistof om hoë temperatuur stoor moontlik te maak en dat dit ekonomies lewensvatbaar is indien die korrosie kwessies van aluminium allooi opgelos kan word. Die gebruik van gevorderde kragsiklusse, metaal FVM stoor en vloeibare metaal warmteoordrag word net geregverdig indien beduidende vermindering in GLVE van die hele kragsentrale bereik is, en dit vorm slegs 'n deel van die oplossing. ŉ Kaskade van verskeie FVMe oor 'n reeks van temperature word vereis om entropie generasie te minimeer. Twee-tenk gesmelte soutstoor kan ook gebruik word in samewerking met kaskade metaal FVM stoor om koste te verminder, maar dit moet ook verder ondersoek word.

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Veelken, Henrik [Verfasser]. "Hotspot cooling using phase change materials / Henrik Veelken." München : Verlag Dr. Hut, 2021. http://d-nb.info/1232847038/34.

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Ihalawela, Chandrasiri A. "Sb-Te Phase-change Materials under Nanoscale Confinement." Ohio University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1449245846.

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Geri, Michela. "Dynamics and rheology of soft phase-change materials." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/121885.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019 Cataloged from PDF version of thesis. Includes bibliographical references (pages 325-353). Many industrial processes involve multicomponent or composite materials in which one component can undergo a phase transition leading to the appearance of a solid phase dispersed in a liquid-like continuous phase. Examples of soft phase-change materials can be found in a variety of applications from food products (e.g., organogels, casein gels and gelatin), pharmaceutical products (e.g., tissue mimicking phantoms and encapsulating agents), cosmetics (e.g., foundations and lipsticks), and in the oil and gas industry, where formation of paraffin waxes and clathrate hydrates represent major issues for upstream production and flow assurance. Historically, phase-changing materials have been exploited for their unique thermal properties in energy storage applications, however soft solids and complex fluids that undergo phase transformation have broader impact in industrial and biomedical applications because of the dramatic changes in mechanical properties that result from the conditions across the phase transition. Typically, these soft phase-change materials are part of the broader class of elasto-visco-plastic materials, showing both viscoelasticity at small deformations and plasticity at large deformations. However, their material properties are greatly influenced by the specific processing conditions during formation, such as temperature and applied deformation, leading to a thermo-rheological complexity that still poses major challenges for their experimental and theoretical characterization. In this Thesis, we develop novel experimental protocols and theoretical frameworks to characterize and describe the complex rheological behavior of soft phase-changing materials, under both linear and non-linear deformations. We focus mainly on two types of materials that are of major importance in the oil and gas industry: paraffin gels, as model waxy crude oils, and clathrate hydrate suspensions. In the limit of small deformations, we are usually interested in measuring the frequency response of the material as it evolves, or mutates, over time. Current state-of-the-art techniques have major limitations in providing both time- and frequency-resolution primarily due to the type of input signals used. To overcome this, we develop a robust excitation signal that allows us to perform time-resolved mechanical spectroscopy of fast mutating systems. Inspired by the biosonar signals of bats and dolphins, we introduce a joint frequency- and amplitude- modulated chirp signal. Combining experiments and numerical simulations, we show that there exists an optimized range of amplitude modulation that minimizes the estimation error while reducing the total acquisition time by almost two orders of magnitude. With this new technique, which we call the Optimally Windowed Chirp (or OWCh), we then explore the phase transition during gelation of a series of mutating, phase-changing materials, including casein gels, gelatin and paraffin gels. To address large, non-linear deformations, we start from a thorough investigation of the steady state and transient response of paraffin gels under shear. We develop a robust protocol that enables us to systematically extract the main rheological features including the thermokinematic memory (i.e. the effect of thermal and shear history on the rheological behavior of the gel) and thixotropy (i.e. the time-dependent behavior under constant applied deformation). We show that these features can be understood in terms of microstructural rearrangements of the underlying solid particle network, which can be quantified through differential scanning calorimetry, birefringence imaging and rheometry. Based on this understanding, we present a constitutive framework that captures all of the different features while respecting thermodynamic and objectivity constraints. We also investigate mechanical instabilities that may arise during rheological measurements. Combining ultrasonic image velocimetry and rheometry, we show that both shear banding and slip can take place during steady shear below a critical value of the shear rate. However, the thixotropic nature of these materials precludes the banding instability from growing in the sheared region of the gap, ensuring that the measured stress response corresponds to the real bulk behavior. Finally, we study the visco-plastic response of clathrate hydrate suspensions. To do so, we develop a novel method to robustly control their formation, which so far has been a major issue in experimental studies due to uncontrolled nucleation and growth of hydrate crystals. Our method, based on the use of "frozen emulsions", decreases the induction time by orders of magnitude while guaranteeing that all the water droplets initially frozen into ice particles are converted into hydrate particles. Rheological measurements for different water volume fractions and shear rates reveal that the macroscopic rheological response is again governed by rearrangements of the microstructure; however, due to the very strong interparticle forces (which are the result of a continuous sintering process) the microstructure evolves towards a fully connected network that behaves as a porous solid structure. Incorporating this limit into our theoretical model, we show that the framework developed for softer interparticle interaction can also capture the macroscopic plastic response of hydrate suspensions. The results from this Thesis have the potential to impact many industrial processes that involve soft phase-change materials, such as flow assurance and oil extraction, thermal energy storage, gas transport and storage, and other processes where the dynamics of gelation are used to control the rheological properties of the ultimate product. by Michela Geri. Ph. D. Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineering

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CAMPI, DAVIDE. "Atomistic simulations of thermal transport and vibrational properties in phase-change materials." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2016. http://hdl.handle.net/10281/101863.

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Phase change materials are a class of compounds employed for data storage applications such as rewritable optical disks (DVD-RW, Blue-Ray disks) and more recently for non-volatile electronic memories of new generation, named phase change memories (PCM)[1]. These applications rely on a fast (50 ns) and reversible transition between a crystalline and the amorphous phases upon heating. The strong optical and electronic contrast between the crystal and the amorphous allows discriminating between the two phases that correspond to the two states of the memory, i.e. the 0 and 1 bits. In this work I have studied, by means of first principle and classical calculations, the structural, vibrational and thermal properties of some of the most promising and widely used phase-change materials such as GeTe, Ge2Sb2Te5 (GST), GeTe-Sb2Te3 superlattices and InSbTe (IST) alloys. The first part of the thesis is focused on the calculation of bulk thermal conductivity and thermal boundary resistances(TBR) between the active media and the surrounding dielectrics and metallic electrodes. Since in PCMs the phase changes corresponding to the memory writing/erasing processes are induced by Joule heating, heat dissipation and transport are key factors that greatly affect the power consumption and the switching speed of the memory cell. Moreover these quantities also influence the thermal cross-talks among the different bits in a memory array which can rise serious reliability issues, especially in ultrascaled devices. Bulk thermal has been computed on the basis density functional calculations [2] for crystalline GeTe, Sb2Te3 and GST. These calculations allowed to identify the origin of the large variability in experimental measurement for GeTe and the origin of the glass-like thermal conductivity in GST. An estimate of IST thermal conductivity was also obtained based on the minimal thermal conductivity model and ab-initio phonons. Thermal boundary resistance at different interface of crystalline GST, IST and GeTe with dielectrics and metals have been estimated from ab-initio phonons and the Diffuse Mismatch Model. The calculations revealed that an important contribution to the TBR comes from the electron-phonon coupling within GST and GeTe. For the GeTe amorphous/crystalline interface, which is also present in the device, we used an interatomic potential generated with a Neural Network (NN) method [3] and non-equilibrium molecular dynamics simulations. In the second part of the thesis we calculated the vibrational Raman spectra GeTe multilayers and of different GeTe-Sb2Te3 superlattices and intermixed compounds which are proposed to be the basis of the so called interfacial phase-change memories, a new type of device with very low power consumption[4]. The comparison between theoretical and experimental spectra allowed the identification of the growth mechanism of GeTe thin films on silicon and could allow the identification of the structures in the superlattices. In the last part we studied the nanowires of Sb2Te3 and GeTe. In particular we studied the energetic of Sb2Te3 surfaces by mean of ab initio calculations in order to explain the observation of a new Sb2Te3 crystal structure in nanowires that turned out to be stabilized by the low dimensionality. Finally we extended the bulk NN potential for GeTe previously developed in our group, enabling the possibility to study the properties of GeTe surfaces and nanowires. [1] M. Wuttig and N. Yamada, Nature Materials 6, 824 (2007). [2] L. Paulatto et al., Phys. Rev. B 87, 214303 (2013). [3] G.C. Sosso et al., Phys. Rev. B 86, 104301 (2012). [4] R.E.Simpson et al., Nature Nanotechnology 6, 501 (2011).

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Navarrete, Argilés Nuria. "Development of nanofluidst based on nanoencapsulated phase change materials." Doctoral thesis, Universitat Jaume I, 2020. http://hdl.handle.net/10803/669273.

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The present thesis deals with the synthesis and characterisation of nanofluids including phase change materials as the nanoparticles. The main aim of the work is to develop new materials that suppose an improvement on the heat transfer properties and heat storage capacity with respect to the current available technologies. Several nanofluids based on thermal oils and molten salts have been synthetised and characterised in order to study the influence of the addition of nanoencapsulated phase change materials in their thermal properties such as thermal conductivity, heat thernal capacity, specific heat and heat storage capacity. Additionally, the use of Atomic Layer Deposition to synthetise material layers of a few nanometers as encapsulation and their influence on the nanofluids characteristics have been studied. a presente tesis pretende lidiar con la síntesis y caracterización de nanofluidos que incluyen materiales de cambio de fase como nanopartículas. El objetivo principal del trabajo es desarrollar nuevos materiales que supongan una mejora en las propiedades de transferencia de calor y capacidad de almacenamiento térmico respecto a las tecnologías disponibles en la actualidad. Varios nanofluidos basados en aceite térmico y en sales fundidas han sido sintetizados y caracterizados con el objetivo de estudiar la influencia de la introducción de materiales de cambio de fase nanoencapsulados en sus propiedades térmicas tales como conductividad térmica, capacidad de transferencia de calor, calor específico y capacidad de almacenamiento térmico. A ello se añade el uso de la deposición de capa atómica para sintetizar encapsulados con cortezas de muy pocos nanómetros, y el estudio de su influencia en las características de los nanofluidos.

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Woda, Michael [Verfasser]. "Electrical transport in crystalline phase change materials / Michael Woda." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2012. http://d-nb.info/1019306114/34.

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Susman, Gideon. "The application of phase change materials to cool buildings." Thesis, Brunel University, 2012. http://bura.brunel.ac.uk/handle/2438/7639.

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Five projects improve understanding of how to use PCM to reduce building cooling energy. Firstly, a post-installation energy-audit of an active cooling system with PCM tank revealed an energy cost of 10.6% of total cooling energy, as compared to an identical tankless system, because PCM under%cooling prevented heat rejection at night. Secondly, development of a new taxonomy for PCM cooling systems allowed reclassification of all systems and identified under-exploited types. Novel concept designs were generated that employ movable PCM units and insulation. Thirdly, aspects of the generated designs were tested in a passive PCM sail design, installed in an occupied office. Radiant heat transfer, external heat discharge and narrow phase transition zone all improved performance. Fourthly, passive PCM product tests were conducted in a 4.2 m3 thermal test cell in which two types of ceiling tile, with 50 and 70% microencapsulated PCM content, and paraffin/copolymer composite wallboards yielded peak temperature reductions of 3.8, 4.4 and 5.2 °C, respectively, and peak temperature reductions per unit PCM mass of 0.28, 0.34 and 0.14 °C/kg, respectively. Heat discharge of RACUS tiles was more effective due to their non-integration into the building fabric. Conclusions of preceding chapters informed the design of a new system composed of an array of finned aluminium tubes, containing paraffin (melt temperature 19.79 °C, latent heat 159.75 kJ/kg) located below the ceiling. Passive cooling and heat discharge is prioritised but a chilled water loop ensures temperature control on hotter days (water circulated at 13 °C) and heat discharge on hotter nights (water circulated at 10 °C). Test cell results showed similar passive performance to the ceiling tiles and wallboards, effective active temperature control (constant 24.6˚C air temperature) and successful passive and active heat discharge. A dynamic heat balance model with an IES% generated UK office’s annual cooling load and PCM temperature%enthalpy functions predicted annual energy savings of 34%.

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Pendyala, Swetha. "Macroencapsulation of Phase Change Materials for Thermal Energy Storage." Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4200.

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The use of a latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy. Latent heat storage enables high-energy storage density which reduces the footprint of the system and the cost. However, PCMs have very low thermal conductivities making them unsuitable for large-scale use without enhancing the effective thermal conductivity. In order to address, the low thermal conductivity of the PCMs, macroencapsulation of PCMs has been adopted as an effective technique. The macroencapsulation not only provides a self-supporting structure of PCM and separates the PCM from thermal fluids but also enhances the heat transfer rate. The current work involves study of various concepts of encapsulation of low cost inorganic PCMs. Sodium nitrate (NaNO3), a low cost PCM, was selected for thermal storage in a temperature range of 300 - 500˚C. Various techniques like electroless coatings, coatings using silicates, coatings with metal oxide (SiO2) and sand encapsulation are discussed. A novel technique of metal oxide coating was developed where firstly a high temperature polymer, such as, polymer (stable > 500˚C) was coated over PCM pellets, and cured, so that the pellet becomes insoluble in water as well as several organic solvents and later the metal oxide is coated over the pellet using self-assembly, hydrolysis, and simultaneous chemical oxidation at various temperatures. The coated PCM pellets were characterized.

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Kurklu, Ahmet. "Energy management in greenhouses using phase change materials (PCMs)." Thesis, University of Reading, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239466.

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Stringer, Karl Stephen. "Phase change storage materials and modelling a MIND environment." Thesis, University of Ulster, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.333915.

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Al-Shahrablee, Ammar Adel Hasah. "Reconfigurable three-terminal logic devices using phase-change materials." Thesis, University of Exeter, 2018. http://hdl.handle.net/10871/35293.

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Conventional solid-state and mass storage memories (such as SRAM, DRAM and the hard disk drive HDD) are facing many technological challenges to meet the ever-increasing demand for fast, low power and cheap data storage solutions. This is compounded by the current conventional computer architectures (such as the von Neumann architecture) with separate processing and storage functionalities and hence data transfer bottlenecks and increased silicon footprint. Beyond the von Neumann computer architecture, the combination of arithmetic-logic processing and (collocally) storage circuits provide a new and promising alternative for computer systems that overcome the many limitations of current technology. However, there are many technical challenges that face the implementation of universal blocks of both logic and memory functions using conventional silicon technology (transistor-transistor logic - TTL, and complementary metal oxide semiconductors - CMOS). Phase-change materials, such as Ge2Sb2Te5 (GST), provide a potential complement or replacement to these technologies to provide both processing and, collocally, storage capability. Existing research in phase-change memory technologies focused on two-terminal non-volatile devices for different memory and logic applications due to their ability to achieve logic-resistive switching in nanosecond time scale, their scalability down to few nanometer-scale cells, and low power requirements. To perform logic functionality, current two-terminal phase-change logic devices need to be connected in series or parallel circuits, and require sequential inputs to perform the required logic function (such as NAND and NOR). In this research programme, three-terminal (3T) non-volatile phase-change memories are proposed and investigated as potential alternative logic cells with simultaneous inputs as reconfigurable, non-volatile logic devices. A vertical 3T logic device structure is proposed in this work based on existing phase-change based memory cell architecture and original concept work by Ovshinsky. A comprehensive, multi-physics finite-element model of the vertical 3T device was constructed in Comsol Multiphysics. This model solves Laplace's equation for the electric potential due to the application of voltage sources. The calculated electric potential and fields provide the Joule heating source in the device, which is used to compute the temperature distribution through solution of the heat diffusion equation, which is necessary to activate the thermally-driven phase transition process. The physically realistic and computationally efficient nucleation- growth model was numerically implemented to model the phase change and resistance change in the Ge2Sb2Te5 (GST) phase-change material in the device, which is combined with the finite- element model using the Matlab programming interface. The changes in electrical and thermal conductivities in the GST region are taken into account following the thermally activated phase transformations between the amorphous-crystalline states using effective medium theory. To determine the appropriate voltage and temperature conditions for the SET and RESET operations, and to optimise the materials and thicknesses of the thermal and heating layers in the device, comprehensive steady-state parametric simulations were carried out using the finite-element multi-physics model. Simulations of transient cycles of writing (SET) and erasing (RESET) processes using appropriate voltage pulses were then carried out on the designed vertical 3T device to study the phase transformations for practical reconfigurable logic operations. The simulations indicated excellent resistance contrast between the logic 1 and 0 states, and successfully demonstrated the feasibility of programming the logic functions of NAND and NOR gates using this 3T configuration.

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Ma, Bingyin. "Phase change materials for controllable stiffness of robotic joints." Thesis, University of Leeds, 2018. http://etheses.whiterose.ac.uk/21471/.

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Snake-like manipulators are well suited for operation in restricted and confined environments where the manipulator body can bend around obstacles to place an end effector at a difficult to access location. They require high stiffness when self-supporting weight against gravity and undertake precision manipulation task, but also require soft properties when operating in complex and delicate environments. A controllable stiffness manipulator has the potential to meet the application demands as it can switch between rigid and soft state. This thesis experimentally investigates the properties of four materials, (low melting point solder, hot-melt adhesive, low melting point alloy and granular material) as candidates for mechanically altering the stiffness of the joints/modules in snake-like manipulators. These materials were evaluated for bonding strength, repeatability, and activation time. Modules for a snake-like manipulator were fabricated using 3D printing and silicone casting techniques including, for the first time, variable stiffness joints that use hot-melt adhesive and low melting point alloy. These modules were evaluated for stiffness properties and low melting point solder based module was found to achieve a stiffness change 150X greater than the state of the art granular material approach. In addition, the proposed modules were able to support 25X of their own weight.

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GABARDI, SILVIA. "First principles simulations of phase change materials for data storage." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2015. http://hdl.handle.net/10281/76292.

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I materiali a cambiamento di fase sono calcogenuri a base di tellurio di notevole interesse tecnologico per la realizzazione di memorie ottiche (DVD) e di memorie elettroniche non volatili di nuova concezione, le memorie a cambiamento di fase o PCM. Questi dispositivi si basano su una veloce (50 ns) e reversibile transizione di fase amorfo-cristallo indotta per riscaldamento. Le due fasi corrispondono ai due stati di memoria che possono essere distinti grazie alla grande differenza tra le proprietà ottiche ed elettroniche dell'amorfo e quelle del cristallo. Nonostante il Ge2Sb2Te5 (GST) sia il materiale attualmente usato nelle PCM, si stanno studiando nuovi materiali con una temperatura di cristallizzazione più alta per aumentare la stabilità termica delle PCM. A questo proposito in questa tesi sono state studiate, attraverso simulazioni di dinamica molecolare ab-initio, diverse leghe ad alta temperatura di cristallizzazione con composizione In3Sb1Te2, In13Sb11Te3 e Ga4Sb6Te3. Queste leghe sono state studiate sperimentalmente e proposte come sostituti del GST, ma le proprietà strutturali e l'origine microscopica dell'elevata temperatura di cristallizzazione della fase amorfa di questi composti non è ancora del tutto chiara. Sono stati, quindi, generati modelli di qualche centinaio di atomi della fase amorfa raffreddando dal liquido in centinaia di ps allo scopo di trovare una relazione tra la struttura dell'amorfo e l'alta temperatura di cristallizzazione di queste leghe. La topologia di legame dei modelli amorfi risulta principalmente tetraedrica, molto diversa dalla geometria della fase cristallina che presenta invece intorni ottaedrici. La presenza di strutture tetraedriche nell'amorfo, assenti invece nella fase cristallina, può quindi costituire un ostacolo alla cristallizzazione con l'effetto di innalzare la temperatura di cristallizzazione rispetto al GST che presenta una geometria di legame prevalentemente ottaedrica sia nell'amorfo che nel cristallo. Nella seconda parte di questo lavoro è stato affrontato il problema del drift, che consiste in un aumento della resistenza elettrica della fase amorfa con il tempo. Questo fenomeno rappresenta un problema nelle celle PCM in quanto modifica le caratteristiche elettriche del dispositivo; tuttavia, manca ancora una spiegazione completa del meccanismo microscopico alla base di questo processo. Il drift sembra però legato al fenomeno del rilassamento strutturale che si verifica nei semiconduttori amorfi e che modifica nel tempo gli stati di difetto in prossimità degli edge delle bande di valenza e di conduzione, da cui dipende la conduzione nella fase amorfa. Per studiare il fenomeno del drift sono stati generati modelli di grandi dimensioni (circa duemila atomi) di GeTe amorfo raffreddando dal liquido in 100 ps attraverso simulazioni di dinamica molecolare classica con un potenziale Neural-Network. Una volta rilassati ab-initio, i modelli presentano diversi stati nel gap localizzati su catene di atomi di Ge. Dopo aver riscaldato i modelli a 500 K in modo da accelerare il processo di drift, si osserva una riduzione del numero di catene di Ge e di legami omopolari Ge-Ge con un conseguente allargamento del gap e riduzione dell'ampiezza delle code di Urbach che possono giustificare un aumento della resistenza. Si propone quindi che il drift sia dovuto al rilassamento strutturale della fase amorfa che porta alla riduzione delle catene di legami omopolari di Ge. Phase change materials based on chalcogenide alloys are of great technological importance because of their use in optical data storage devices (DVDs) and electronic non-volatile memories of new concept, the Phase Change Memory cell (PCM). These applications rely on a fast (50 ns) and reversible change between the crystalline and the amorphous phases upon heating. The two phases correspond to the two states of the memory that can be discriminated thanks to a large difference in their optical and electronic properties. Although Ge2Sb2Te5 (GST) is the compound presently used as active layer in PCMs, alternative materials with a higher crystallization temperature are under scrutiny in order to increase the thermal stability of the PCM devices. In this respect, we analysed, by means of ab-initio molecular dynamics simulations, different high crystallization temperature alloys with composition In3Sb1Te2, In13Sb11Te3 and Ga4Sb6Te3, which have been experimentally proposed as substitute of GST. However, the structural properties and the microscopical reason of the high thermal stability of the amorphous phases of these compounds is still unclear. We, thus, generated models of the amorphous phase of few hundreds of atoms by quenching from the melt in few hundreds of ps aiming at finding out a relation between the structural properties of the amorphous phase and the high crystallization temperature of these alloys. The topology of our amorphous models turned out to be mostly tetrahedral which differs from the octahedral-like geometry of the crystalline phases. The presence of tetrahedral structures in the amorphous which are absent in the crystalline phase, probably hinders the crystallization process resulting in a higher crystallization temperature with respect to GST which display a mostly octahedral-like structures in both amorphous and the crystalline phase. In the second part of this work we addressed the issue of the resistance drift phenomenon, which consists of an increase of the electrical resistance of the amorphous phase with time. This effect is detrimental in PCMs since it changes the electrical characteristics of the devices. This process is believed to be due to an aging of the amorphous phase which modifies during time the defect states in the proximity of the valence and conduction band edges which control the electrical conductivity. The microscopic origin of the structural relaxations leading to the drift is still unknown. To address this problem, we generated large models (about two thousand atoms) of amorphous GeTe by quenching from the melt in 100 ps with classical molecular dynamics simulations by using a neural-network potential. Once relaxed by first principles, the models showed the presence of several in-gap states localized on chains of Ge atoms. After an annealing at 500 K, performed to accelerate the drift process, Ge chains and homopolar Ge-Ge bonds reduce in number resulting in a band gap widening and a reduction of the Urbach tails at the band edges which can account for the increase of the resistance. We thus propose that the resistance drift originates from structural relaxations leading to the removal of Ge chains.

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Huang, Ruomeng. "Confined nanoscale chalcogenide phase change material and memory." Thesis, University of Southampton, 2015. https://eprints.soton.ac.uk/379321/.

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The miniaturization of memory devices has been one of the major driving forces in the exploration of ever faster, smaller and more efficient memory concepts. Among all the competitors for the next generation of non-volatile memory, phase change materials based random access memory has emerged as a leading candidate. A better understanding of nanoscale properties of phase change materials and the ability of selective depositing them into confined nanostructures are substantially important in the long march towards smaller more densely packed memory bits. A novel top-down spacer etch technique has been developed for fabricating sub hundred nanometre phase change Ge2Sb2Te5 nanowires. Taking advantage of this technique which allows precise control over nanowire position and geometries, the contact properties between phase change material and metallic electrode in nanoscale can be quantitatively investigated. The results reveal a specific contact resistance of 7.56 x 10-5.

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Carrizales, Juarez Ricardo. "Radiation pattern reconfigurable antennas based on Phase Change Materials (PCM) integration." Electronic Thesis or Diss., Limoges, 2024. http://www.theses.fr/2024LIMO0003.

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Ce travail porte sur l'étude, la caractérisation et activation optique de couches minces de matériaux à changement de phase (PCM) pour leur intégration dans des dispositifs reconfigurables haute fréquence, notamment des systèmes antennaires. Le principe de reconfiguration est obtenu en exploitant le contraste élevé de résistivité des PCMs entre son état amorphe (forte résistivité) et son état cristallin (faible résistivité). Ce changement d'état est non volatil et réversible et est obtenu par une courte impulsion laser. Les propriétés électromagnétiques du PCM ont également été caractérisées dans la bande des fréquences d'ondes millimétriques (environ 30 GHz) et utilisées pour la conception et la simulation précise d'un réseau d'antennes multi-reconfigurables (en état de polarisation et en fréquence de fonctionnement) fonctionnant à des fréquences autour de 30 GHz This work focuses on the study, characterization and optical activation of thin films of phase change materials (PCM) for their integration in high-frequency reconfigurable devices, in particular antenna systems. The principle of reconfiguration is achieved by exploiting the high contrast in resistivity of PCMs between its amorphous state (high resistivity) and crystalline state (low resistivity). This change of state is non-volatile and reversible and is achieved by a short laser pulse. The electromagnetic properties of PCMs were also characterized in the millimeter-wave band frequency (around 30 GHz) and used for the design and accurate simulation of a multi-reconfigurable (in polarization state and operational frequency) array of antennas working at frequencies around 30 GHz

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SELMO, SIMONE. "Functional analysis of In-based nanowires for low power phase change memory applications." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2017. http://hdl.handle.net/10281/153247.

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Phase change memories (PCMs), based on chalcogenide alloys (mainly Ge2Sb2Te5), are the most promising candidate for the realization of “Storage Class Memories”, which would fill the gap between ‘‘operation’’ and ‘‘storage’’ memories. PCMs are also one of the few currently available technologies for the implementation of nanoeletronic synapses in high density neuromorphic systems. The main improvements needed in order to exploit the full potential of PCMs in these innovative applications are the reduction of the programming currents and power consumption, and further cell downscaling. Thanks to their nano-sized active volume to be programmed and self-heating behavior, phase change nanowires (NWs) are expected to exhibit improved memory performances with respected to commonly used thin-film/heater-based structures. The Ph. D. Thesis of the candidate reports the study of the phase change properties of ultra-thin In-based NWs for low power consuming PCMs, exploring the more promising features of this class of materials with respect to the commonly considered Ge-Sb-Te alloys. In particular, the self-assembly of In-Sb-Te, In-doped Sb and In-Ge-Te NWs was successfully achieved by Metal Organic Chemical Vapour Deposition (MOCVD), coupled to vapour-liquid-solid mechanism, catalysed by catalyst nanoparticles. The parameters influencing the NW self-assembly were studied and the compositional, morphological and structural analysis of the grown structures was performed. In all cases, NWs of several μm in length and with diameters as small as 15 nm were obtained. The experimental contribution of the Ph. D. candidate to the NWs growth study was mainly related to the substrates preparation, catalyst deposition and, morphological and elemental analysis of the grown samples. Moreover, the Ph. D. candidate has performed the functional analysis of In3Sb1Te2 and In-doped Sb NW-based PCM devices. To conduct that analysis, a suitable fabrication procedure of the devices and an appropriate electrical measuring set-up have been identified. Reversible and well reproducible phase change memory switching was demonstrated for In3Sb1Te2 and In-doped Sb NW devices, showing low working parameters, such as “RESET” voltage, current and power. The obtained results support the conclusion that In-based ultra-thin NWs are potential building blocks for the realization of ultra-scaled, high performance PCM devices. Phase change memories (PCMs), based on chalcogenide alloys (mainly Ge2Sb2Te5), are the most promising candidate for the realization of “Storage Class Memories”, which would fill the gap between ‘‘operation’’ and ‘‘storage’’ memories. PCMs are also one of the few currently available technologies for the implementation of nanoeletronic synapses in high density neuromorphic systems. The main improvements needed in order to exploit the full potential of PCMs in these innovative applications are the reduction of the programming currents and power consumption, and further cell downscaling. Thanks to their nano-sized active volume to be programmed and self-heating behavior, phase change nanowires (NWs) are expected to exhibit improved memory performances with respected to commonly used thin-film/heater-based structures. The Ph. D. Thesis of the candidate reports the study of the phase change properties of ultra-thin In-based NWs for low power consuming PCMs, exploring the more promising features of this class of materials with respect to the commonly considered Ge-Sb-Te alloys. In particular, the self-assembly of In-Sb-Te, In-doped Sb and In-Ge-Te NWs was successfully achieved by Metal Organic Chemical Vapour Deposition (MOCVD), coupled to vapour-liquid-solid mechanism, catalysed by catalyst nanoparticles. The parameters influencing the NW self-assembly were studied and the compositional, morphological and structural analysis of the grown structures was performed. In all cases, NWs of several μm in length and with diameters as small as 15 nm were obtained. The experimental contribution of the Ph. D. candidate to the NWs growth study was mainly related to the substrates preparation, catalyst deposition and, morphological and elemental analysis of the grown samples. Moreover, the Ph. D. candidate has performed the functional analysis of In3Sb1Te2 and In-doped Sb NW-based PCM devices. To conduct that analysis, a suitable fabrication procedure of the devices and an appropriate electrical measuring set-up have been identified. Reversible and well reproducible phase change memory switching was demonstrated for In3Sb1Te2 and In-doped Sb NW devices, showing low working parameters, such as “RESET” voltage, current and power. The obtained results support the conclusion that In-based ultra-thin NWs are potential building blocks for the realization of ultra-scaled, high performance PCM devices.

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Bonnebaigt, Rachael. "Modelling the effects on building temperatures of phase change materials." Thesis, University of Cambridge, 2016. https://www.repository.cam.ac.uk/handle/1810/261553.

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In this thesis, three problems motivated by the possibility of use of phase change materials in buildings are presented. First, a model for the response of phase change material (PCM) to a varying room temperature is extended, and applied to realistic situations in a building. For a spatially varying distribution of PCM through a wall, an equation estimating the appropriate depth for PCM inclusion is found. For cooling at the exterior surface of the wall, a wall thickness and a PCM mass fraction that give a low maximum daily interior surface temperature, without redundant PCM, are identified. The hot upper layer for a displacement ventilated space is cooler but deeper when a PCM ceiling interacting with the exhaust from an air heat pump is used. During hot weather, the maximum daily interior surface temperatures are reduced when using two PCMs with different, appropriate, melting ranges. In each extension, PCM reduces temperature fluctuations substantially when used in appropriate quantities and locations. Second, experimental results are presented, demonstrating that, for a buoyancy source vertically distributed over a full wall, detrainment qualitatively changes the shape of the ambient buoyancy profile in a sealed space. Theoretical models with one-way entrainment predict stratifications that are qualitatively different from the stratifications measured in experiments. A peeling plume model, where density and vertical velocity vary linearly across the plume, so that plume fluid “peels” off into the ambient at intermediate heights, more accurately captures the shape of the ambient buoyancy profiles measured in experiments than a one-way-entrainment model does. For a half wall source, however, detrainment is not observed in the experiments, and a one-way-entrainment model is appropriate. Third, experimental results are presented for a time-varying, vertically distributed buoy- ancy source. For a source whose buoyancy flux decreases linearly with time, the peeling plume model is appropriate, as it was for a constant flux source. For a source providing a buoyancy flux that increases linearly with time, however, the one-way-entrainment model is the most appropriate model for predicting the stratification that develops in a sealed space. A PCM wall solidifying provides a buoyancy source which decreases linearly with time, then is constant, then again decreases linearly with time. Experiments show that, in a space with such a buoyancy source, the first front moves quickly through the space, so the whole space x is stratified for most of the experiment. The shape of the stratification changes only slightly after this point, with heat being added uniformly throughout the space, so that the ambient buoyancy profile simply translates, with little change in its shape.

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Volker, Hanno [Verfasser]. "Disorder and electrical transport in phase-change materials / Hanno Volker." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2013. http://d-nb.info/1044570776/34.

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Luckas, Jennifer Maria [Verfasser]. "Electronic transport in amorphous phase-change materials / Jennifer Maria Luckas." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2013. http://d-nb.info/1033024333/34.

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Wang, Fuqiao. "The passive use of phase change materials in refrigeration systems." Thesis, London South Bank University, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.433624.

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Graham, M. J. "Encapsulated salt hydrate phase change materials for thermal energy storage." Thesis, University of Liverpool, 2017. http://livrepository.liverpool.ac.uk/3012709/.

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Mey, Hennie. "Carbon black : enhancing phase change materials for direct solar application." Diss., University of Pretoria, 2016. http://hdl.handle.net/2263/61312.

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A solar simulator was used to test whether a carbon black additive could increase the solar absorption of a low temperature organic PCM (consisting of a eutectic mixture of palmitic acid and stearic acid). Various PCM and carbon black composites (0.01 % to 6 %) were tested, with the 0.06 % carbon black composites showing the fastest temperature increase, reaching 75 °C much quicker (350 % faster) than the pure PCM. All of the tested PCM composites reached 75 °C in less than half the time it took the pure PCM. It can therefore be seen that carbon black is very effective at increasing the solar absorption of the PCM. The carbon black did not have a negative impact on the melting/solidifying onset temperature or the latent heat of the PCM. This proves that at these low concentrations carbon black can help reduce the shortcomings of the PCM without adversely affecting its energy storage properties. The optimal carbon black concentration changes with the size of the PCM: a shallow PCM layer (2 cm) showed the fastest temperature increase at higher concentrations (between 0.06 % and 0.5 % carbon black), while the deep PCM layer (9 cm) showed the fastest temperature increase at lower concentrations (between 0.01 % and 0.08 % carbon black). The poor optical properties of the PCM were vastly improved by the carbon black, making the composite an effective direct solar absorber. The carbon black, however, does not provide meaningful thermal conductivity enhancements. Therefore additional heat transfer enhancements (like graphite) are needed if this novel PCM composite is to be used in a combined system (direct solar absorber, heat transfer fluid and energy storage system). Dissertation (MEng)--University of Pretoria, 2016. Chemical Engineering MEng Unrestricted

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Guo, Pengfei. "Design and fabrication of photonic devices using phase change materials." University of Dayton / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=dayton154448809502474.

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Al-Maghalseh, Maher. "Compact solar thermal energy storage systems using phase change materials." Thesis, Northumbria University, 2014. http://nrl.northumbria.ac.uk/23579/.

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The present research explores numerically and experimentally the process of melting and solidification of Phase Change Materials (PCM) in a latent heat thermal energy storage system (LHTESS). Further, the study will investigate various methods of intensification of heat transfer in such materials by means of metallic fins, filling particles or nanoparticles and by choosing the optimal system geometry for a rapid development of free convection flows during the melting process. The study includes three main parts. First, 3D CFD modelling was performed for the melting performance of a shell-and-tube thermal storage system with n-Octadecane as a PCM. The predicted model was in very good agreement with experimental data published in open literature. A series of numerical calculations were then undertaken to investigate the effect of nanoparticles on the heat transfer process. Dimensionless heat transfer correlations were derived for the system with Pure PCM and PCM mixed with nano-particles. In the second part of this study the experimental studies were carried out in order to investigate the performance of the laboratory thermal storage system with paraffin as the PCM. The thermal storage system was connected to evacuated tube solar collectors and its performance was evaluated in various conditions. 3D CFD model of the system was developed and numerical simulations were run for constant heat source conditions. Computational results were compared with experimental data obtained on the test rig at Northumbria University. Comparison revealed that the developed CFD model is capable to describe process of heat transfer in the system with high accuracy and therefore can be used with high confidence for modelling further cases. Finally, 3D CFD model was developed to predict the transient behaviour of a latent heat thermal energy storage system (LHTESS) in the form of a rectangular container with a central horizontal pipe surrounded by paraffin as PCM (melting temperature is 60 oC). Water was used as a heat transfer fluid (HTF). The enhancement of heat transfer in specific geometries by using external longitudinal fins on the tube and metallic porous matrix were numerically investigated. The influence of the number of fins and porosity of the matrix on the temperature distribution, melting process, melting time and natural convection phenomena were studied. Dimensionless heat transfer correlations were derived for calculation of the Nusselt number as function of Fourier, Stefan and Rayleigh numbers. These correlations to be used in the further designing process of similar thermal storage units at Northumbria University.

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Ozdenefe, Murat. "Phase change materials and thermal performance of buildings in Cyprus." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/phase-change-materials-and-thermal-performance-of-buildings-in-cyprus(a7b37f53-22de-47d4-ad19-2596ee75a558).html.

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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.

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White, Jason Franklin Suppes Galen J. "Flammability characterization of fat and oil derived phase change materials." Diss., Columbia, Mo. : University of Missouri--Columbia, 2005. http://hdl.handle.net/10355/6252.

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The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Title from PDF of title page (University of Missouri--Columbia, viewed on February 12, 2010). Thesis advisor: Dr. Galen J. Suppes. Includes bibliographical references.

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Goff, Michael J. "Fat and oil derivatives for use as phase change materials /." free to MU campus, to others for purchase, 2004. http://wwwlib.umi.com/cr/mo/fullcit?p3144418.

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Dyball, Dianne L. "An innovative wall-lining for buildings incorporating phase change materials." Thesis, University of Brighton, 2013. https://research.brighton.ac.uk/en/studentTheses/efdce4e8-dbb0-407b-9d57-ac45820e6743.

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The use of thermal energy storage in buildings is well understood but contemporary buildings with highly insulated, lightweight construction have low thermal inertia. This causes them to respond rapidly to external temperature changes, which results in significant internal temperature fluctuations and overheating. This research addresses this problem by developing a thermal interactive wall-lining through the inclusion of phase change materials. Phase change materials (PCMs) not only absorb sensible heat but also absorb and release latent heat during phase transition. This research set out to determine if it is possible to locate phase change materials on the surface of a room and if they can effectively improve the thermal performance of a room. Using the scientific method the suitability of different types of PCMs was investigated for inclusion within a vinyl matrix. Experiments evaluated the maximum quantity of PCM loading and thermal analysis identified the largest potential heat storage capacity for the phase change wall-lining. Following successful laboratory experiments a pilot scale prototype phase change wall-lining was manufactured and tested. The test involved an experiment comprising two thermally matched chambers to evaluate the thermal performance of the phase change wall-lining in a controlled environment. The results demonstrated the phase change wall-lining can reduce internal temperatures by more than 2°C and delay the time taken to reach extreme temperatures. The effect of different air flow rates on the ability to charge and discharge the phase change wall-lining have been evaluated to identify the required operating criteria for use in buildings. This research has developed an innovative phase change wall-lining that reduces internal peak temperatures, minimises diurnal temperature fluctuations by storing excess heat and improves the thermal comfort. The outcomes provide a greater understanding of the interaction between air and PCMs when located on the surface of a room, and indicate that such materials have the potential to improve thermal performance of new and existing buildings.

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Perumal, Karthick. "Epitaxial growth of Ge-Sb-Te based phase change materials." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2013. http://dx.doi.org/10.18452/16815.

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Ge-Sb-Te basierte Phasenwechselmaterialen sind vielersprechende Kandidaten für die Anwendung in optischen und elektrischen nicht-flüchtigen Speicheranwendungen. Diese Materialien können mit Hilfe von elektrischen oder optischen Pulsen reversibel zwischen der kristallinen und amorphen Struktur geschaltet werden. Diese stukturellen Phasen zeigen einen großen Unterschied in ihren elektronischen Eigenschaften, der sich in einer starken Änderung der optischen Reflektivität und des elektrischen Widerstands zeigt.Diese Studie befasst sich mit epitaktischem Wachstum und Analyse der epitaktischen Schichten. Der erste Teil der Arbiet befasst sich mir dem epitaktischen Wachstum von GeTe. Dünne GeTe Schichten wurden auf Si(111) und Si(001) Substraten mit einer Gitterfehlanpassung von 10.8% präpariert. Auf beiden Substraten bildet sich in der GeTe Schicht die [111] Oberflächenfacette parallel zur Si(001) und Si(111) Oberfläche aus. Während des inertialen Wachstums findet eine Phasentransformation von amorph zu kristallin statt. Diese Phasentransformation wurde mittels azimuthaler in-situ Beugung hochenergetischer Elektronen sowie in-situ Röntgenbeugung unter streifendem Einfall untersucht. Der zweite Teil der Arbeit wird die Epitaxie sowie die strukturelle Charakterisierung dünner Sb2Te3 Schichten dargestellt. Der dritte Teil umfasst die Epitaxie terniärer Ge-Sb-Te Schichten . Zum Wachstum wurden sowohl die Substrattemperatur als auch die Ge, Sb und Te Flüsse variiert. Es wird gezeigt, dass die Komposition der Schicht stark von der Wachtumstemperatur abhängt und nur entlang der pseudibinären Verbindungslinie von GeTe-Sb2Te3 variiert. Zur Kontrolle des Wachstums wurde dabei die in-situ Quadrupol Massenspektroskopie verwendet. Es zeigen sich diverse inkommensurate Beugungsmaxima entlang der [111] Oberflächennormalen der Schichten, anhand derer die Ausbildung einer Lehrstellen Ordnung in Form einer Überstruktur diskutiert wird. Ge-Sb-Te based phase change materials are considered as a prime candidate for optical and electrical data storage applications. With the application of an optical or electrical pulse, they can be reversibly switched between amorphous and crystalline state, thereby exhibiting large optical and electrical contrast between the two phases, which are then stored as information in the form of binary digits. Single crystalline growth is interesting from both the academic and industrial perspective, as ordered Ge-Sb-Te based metamaterials are known to exhibit switching at reduced energies. The present study deals with the epitaxial growth and analysis of Ge-Sb-Te based thin films. The first part of the thesis deals with the epitaxial growth of GeTe. Thin films of GeTe were grown on highly mismatched Si(111) and (001) substrates. On both the substrate orientations the film grows along [111] direction with an amorphous-to-crystalline transition observed during the initial stages of growth. The amorphous-to-crystalline transition was studied in-vivo using azimuthal reflection high-energy electron diffraction scans and grazing incidence x-ray diffraction. In the second part of the thesis epitaxy and characterization of Sb2Te3 thin films are presented. The third part of the thesis deals with the epitaxy of ternary Ge-Sb-Te alloys. The composition of the films are shown to be highly dependent on growth temperatures and vary along the pseudobinary line from Sb2Te3 to GeTe with increase in growth temperatures. A line-of-sight quadrupole mass spectrometer was used to reliably control the GeSbTe growth temperature. Growth was performed at different Ge, Sb, Te fluxes to study the compositional variation of the films. Incommensurate peaks are observed along the [111] direction by x-ray diffraction. The possibility of superstructural vacancy ordering along the [111] direction is discussed.

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CASCONE, YLENIA. "Optimisation of opaque building envelope components with Phase Change Materials." Doctoral thesis, Politecnico di Torino, 2017. http://hdl.handle.net/11583/2687833.

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The objective of the present thesis is to provide a methodological approach for the design of responsive building envelope components through the application of optimisation analyses. In detail, this approach was applied to opaque building envelope components with Phase Change Materials (PCMs). Since multi-objective optimisation problems generally result in a series of trade-off solutions called Pareto-front, the main focus was to investigate which values assumed by the optimisation variables led to the optimal set of solutions. In this way, the optimisation analysis was used as a tool to gain knowledge on specific problems. After an overview on PCMs and on the application of optimisation analyses to the building envelope for improving the energy efficiency of buildings, three levels of analysis were explored; material level, component level and building level. At the material level, the optimisation approach was applied to estimate the temperature-dependent specific heat curve of PCMs through best-fit of experimental data. Given the measured surface temperatures of a sample as boundary conditions and the known thermo-physical properties of the materials to a numerical model, the curve which minimised the difference between measured and simulated heat fluxes on both faces of the sample was found. At the component level, “equivalent” parameters for the dynamic thermal characterisation of opaque building envelope components with PCM were proposed. Starting from the definition of the traditional dynamic thermal properties according to ISO 13786:2007, a monthly equivalent periodic thermal transmittance and the corresponding time shift were defined by imposing steady-periodic conditions with monthly average external air temperature and solar irradiance profiles while keeping a constant air temperature on the internal side. Then, the monthly equivalent values were synthesised in a unique yearly value by means of a simple average. A parametric model was subsequently developed to describe PCM-enhanced multi-layer walls with simultaneous use of at most two PCMs, and an optimisation analysis was carried out for three locations (Palermo, Torino and Oslo) to find wall layout and PCMs' thermo-physical properties (melting temperature, melting temperature range, latent heat of fusion and thermal conductivity) which minimise yearly equivalent periodic thermal transmittance, overall PCM thickness and thickness of the wall. At the building level, the investigations focused on the application of optimisation analyses for the energy retrofit of office buildings. Three retrofit options on the opaque envelope components were considered in the aforementioned locations; intervention either on the external side of the wall, on the internal side of the wall, or on both sides of the wall. Moreover, either the same retrofit solution for all the walls or a different wall solution for each orientation were considered. In both cases, a maximum of two PCM materials could be selected by the optimisation algorithm. With regard to the objective functions, the problem was faced under two points of view. On one side, optimisations were run with three objectives to minimise the building energy need for heating, cooling and the investment cost. On the other side, the optimisations were performed with two objectives to minimise primary energy consumption and global cost. Only for the climate of Oslo, where heating is mostly electric and no cooling system was adopted, the minimisation objectives were primary energy consumption, global cost and thermal discomfort. Even though a proper optimisation of the thermo-physical properties of PCMs was found to be especially advisable when the operation of the HVAC system implies a non-trivial solution, the results of these analyses allowed to propose a few design guidelines for PCM selection and application. However, for the analysed case studies, PCM prices need to be reduced in order to become a cost-effective retrofit option.

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Gunasekara, Saman Nimali. "Phase Equilibrium-aided Design of Phase Change Materials from Blends : For Thermal Energy Storage." Doctoral thesis, KTH, Kraft- och värmeteknologi, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-212440.

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Climate change is no longer imminent but eminent. To combat climate change, effective, efficient and smart energy use is imperative. Thermal energy storage (TES) with phase change materials (PCMs) is one attractive choice to realize this. Besides suitable phase change temperatures and enthalpies, the PCMs should also be robust, non-toxic, environmental-friendly and cost-effective. Cost-effective PCMs can be realized in bulk blends. Blends however do not have robust phase change unless chosen articulately. This thesis links bulk blends and robust, cost-effective PCMs via the systematic design of blends as PCMs involving phase equilibrium evaluations. The key fundamental phase equilibrium knowledge vital to accurately select robust PCMs within blends is established here. A congruent melting composition is the most PCM-ideal among blends. Eutectics are nearly ideal if supercooling is absent. Any incongruent melting composition, including peritectics, are unsuitable as PCMs. A comprehensive state-of-the-art evaluation of the phase equilibrium-based PCM design exposed the underinvestigated categories: congruent melting compositions, metal alloys, polyols and fats. Here the methods and conditions essential for a comprehensive and transparent phase equilibrium assessment for designing PCMs in blends are specified. The phase diagrams of the systems erythritol-xylitol and dodecane-tridecane with PCM potential are comprehensively evaluated. The erythritol-xylitol system contains a eutectic in a partially isomorphous system unlike in a non-isomorphous system as previous literature proposed. The dodecane-tridecane system forms a probable congruent minimum-melting solid solution, but not a maximum-melting liquidus or a eutectic as was previously proposed. The sustainability aspects of a PCM-based TES system are also investigated. Erythritol becomes cost-effective if produced using glycerol from bio-diesel production. Olive oil is cost-effective and has potential PCM compositions for cold storage. A critical need exists in the standardization of methods and transparent results reporting of the phase equilibrium investigations in the PCM-context. This can be achieved e.g. through international TES collaboration platforms. Energi är en integrerad del av samhället men energiprocesser leder till miljöbelastning, och klimatförändringar. Därför är effektiv energianvändning, ökad energieffektivitet och smart energihantering nödvändigt. Värmeenergilagring (TES) är ett attraktivt val för att bemöta detta behov, där ett lagringsalternativ med hög densitet är s.k. fasomvandlingsmaterial (PCM). Ett exempel på ett billigt, vanligt förekommande PCM är systemet vatten-is, vilket har använts av människor i tusentals år. För att tillgodose de många värme- och kylbehov som idag uppstår inom ett brett temperaturintervall, är det viktigt med innovativ design av PCM. Förutom lämplig fasförändringstemperaturer, entalpi och andra termofysikaliska egenskaper, bör PCM också ha robust fasändring, vara miljövänlig och kostnadseffektiv. För att förverkliga storskaliga TES system med PCM, är måste kostnadseffektivitet och robust funktion under många cykler bland de viktigaste utmaningarna. Kostnadseffektiva PCM kan bäst erhållas från naturliga eller industriella material i bulkskala, vilket i huvudsak leder till materialblandningar, snarare än rena ämnen. Blandningar uppvisar dock komplexa fasförändringsförlopp, underkylning och/eller inkongruent smältprocess som leder till fasseparation. Denna doktorsavhandling ger ny kunskap som möjliggör att bulkblandningar kan bli kostnadseffektiva och robusta PCM-material, med hjälp av den systematiskutvärdering av fasjämvikt och fasdiagram. Arbetet visar att detta kräver förståelse av relevanta grundläggande fasjämviktsteorier, omfattande termiska och fysikalisk-kemiska karakteriseringar, och allmänt tillämpliga teoretiska utvärderingar. Denna avhandling specificerar befintlig fasjämviktsteori för PCM-sammanhang, men sikte på att kunna välja robusta PCM blandningar med specifika egenskaper, beroende på tillämpning. Analysen visar att blandningar med en sammansättning som leder till kongruent smältande, där faser i jämvikt har samma sammansättning, är ideala bland PCM-blandningar. Kongruent smältande fasta faser som utgör föreningar eller fasta lösningar av ingående komponenter är därför ideala. Eutektiska blandningar är nästan lika bra som PCM, så länge underkylning inte förekommer. Därmed finns en stor potential för att finna och karakterisera PCM-ideala blandningar som bildar kongruent smältande föreningar eller fasta lösningar. Därigenom kan blandningar med en skarp, reversibel fasändring och utan fasseparation erhållas – egenskaper som liknar rena materialens fasändringsprocess. Vidare kan man, via fasdiagram, påvisa de blandningar som är inkongruent smältande, inklusive peritektiska blandningar, som är direkt olämpliga som PCM. Denna avhandling ger grundläggande kunskap som är en förutsättning för att designa PCM i blandningar. Genom en omfattande state-of-the-art utvärdering av fas-jämviktsbaserad PCM-design lyfter arbetet de PCM-idealiska blandningarna som hittills inte fått någon uppmärksamhet, såsom kongruenta smältande blandningar, och materialkategorierna metallegeringar, polyoler och fetter. Resultatet av arbetet visar dessutom att vissa PCM-material som ibland föreslås är direkt olämpliga då fasdiagram undersöks, bl a pga underkylning och även peritektiska system med fasseparation och degradering av kapaciteten (t ex Glauber-salt och natriumacetat-trihydrat). Denna avhandling specificerar och upprättar grundläggande teori samt tekniker, tillvägagångssätt och förhållanden som är nödvändiga för en omfattande och genomsynlig fasjämviktsbedömning, för utformning av PCM från blandningar för energilagering. Med detta som bas har följande fasdiagramtagits fram fullständigt: för erytritol-xylitol och för dodekan-tridekan, med PCM-potential för låg temperaturuppvärmning (60-120 °C) respektive frysning (-10 °C till -20 °C) utvärderas fullständigt. Erytritol-xylitol systemet har funnits vara eutektiskt i ett delvis isomorft system, snarare än ett icke-isomorft system vilket har föreslagits tidigare litteratur. Dodekan-tridekan systemet bildar ett system med kongruent smältande fast lösning (idealisk som en PCM) vid en minimumtemperatur, till skillnad från tidigare litteratur som föreslagt en maximumtemperatur, eller ett eutektiskt system. Teoretisk modellering av fasjämvikt har också genomförts för att komplettera det experimentella fasdiagrammet för systemet erytritol-xylitol. Efter granskning av de metoder som använts tidigare i PCM-litteraturen har här valts ett generiskt tillvägagångssätt (CALPHAD-metoden). Denna generiska metod kan bedöma vilken typ av material och fasändring som helst, till skillnad från en tidigare använda metoder som är specifika för materialtyper eller kemiska egenskaper. Denna teoretiska studie bekräftar termodynamiskt solvus, solidus, eutektisk punkt och erytritol-xylitol fasdiagrammet i sin helhet. Vad gäller hållbarhetsaspekter med PCM-baserad TES, lyfter denna avhandling fokus på förnybara och kostnadseffektiva material (t.ex. polyoler och fetter) som PCM. Som exempel har här undersökts erytritol och olivolja, med förnybart ursprung. Erytritol skulle kunna bli ett kostnadseffektivt PCM (163 USD/kWh), om det produceras av glycerol vilket är en biprodukt från biodiesel/bioetanolframställning. Olivolja är ännu ett kostnadseffektivt material (144 USD/kWh), och som här har påvisats innehålla potentiella PCM sammansättningar med lämpliga fasändringsegenskaper för kylatillämpningar. En övergripande slutsats från denna avhandling är att det finns ett behov av att standardisera tekniker, metoder och transparent resultatrapportering när det gäller undersökningar av fasjämvikt och fasdiagram i PCM-sammanhang. Internationella samarbetsplattformar för TES är en väg att koordinera arbetet. QC 20170830

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Khillarkar, Dipendra B. "Melting of a phase change material in horizontal annuli." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape8/PQDD_0025/MQ50629.pdf.

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Khillarkar, Dipendra B. "Melting of a phase change material in horizontal annuli." Thesis, McGill University, 1998. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=21307.

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Numerical experiments were carried out to determine tube geometries for more efficient thermal storage. A finite element simulation code developed earlier, which solves the two dimensional governing conservation equations was employed to examine the thermal performance of horizontal annuli of the following configurations: (a) Square external tube with a circular tube inside---Annulus Type A; (b) Circular external tube with a square tube inside---Annulus Type B. Effects of the Rayleigh number as well as heating of the inside, outside or both walls at a temperature above the melting point of the material were studied. Flow and temperature patterns within the melt, local heat flux distributions at the heating surface and the cumulative energy charged as a function of time are presented and discussed. To enhance the heat transfer rate during melting in horizontal annular containers various innovative passive methods were examined. Eccentric annular configurations are identified as superior to concentric tubular geometries due to the vertically upward orientation of the buoyancy force in the melt phase at higher Rayleigh numbers. In addition to this the effect of flipping the container at pre-selected times after initiation of melting as a measure to increase the heat transfer rate during the last stage of the melting process is also examined and discussed.

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Mahdavi, Nejad Alireza. "Numerical Study of Heat and Mass Transfer Using Phase Change Materials." Digital WPI, 2018. https://digitalcommons.wpi.edu/etd-dissertations/500.

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Phase Change Materials (PCM) absorb and release heat at preset temperatures. Due to their relatively high values of latent heat, they are capable of storing and releasing large amounts of energy during phase change. When a PCM is in its solid phase, it will absorb heat as the external temperature rises. The temperature of the PCM will mirror the external temperature until the melting point of PCM is reached. At this stage, the PCM will begin to melt with almost no change in its temperature. PCM plays an opposite role when the external temperature drops. It releases the stored energy back while going through phase change from liquid phase to solid phase. The present work is a numerical study towards fundamental understanding of the impact of using PCM on enhancement of heat and mass transfer in several scenarios. A numerical analysis has been carried out to determine the impact of presence of PCM on the insulating characteristics of paper board packaging. Two different cases of a layered PCM and uniformly dispersed PCM within the packaging wall are considered. The numerical results illustrate significant reduction in exchange of heat between the exterior and the interior of the packaging. Specifically, the unique concept of utilizing PCM in drying of paper is proposed and a numerical investigation is performed to determine the corresponding transport characteristics. The results indicate that the PCM acts as a heat source and a heat sink alternatingly throughout the conventional paper drying process, enhancing the drying energy efficiency. This study also included presence of gas-fired infrared emitters in the drying process as well for which the spectral absorption coefficient of PCM was measured and incorporated into the theoretical model. Finally, the impact of the presence of PCM in convective air-drying of moist paper is numerically investigated. The hot air ow is generated by an in-line jet nozzle. The air impinges on the exposed surface of the moist paper while the other side is considered to be perfectly insulated. The results provide the corresponding air flow field as well as air temperature distribution in between the nozzle exit and the surface of the moist paper. The results also reveal the enhancement of drying rates with PCM, fundamentally confirming the role of PCM on enhancing the energy efficiency of convective drying of moist paper.

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Guo, Qiang M. Eng Massachusetts Institute of Technology. "Evaluation on the thin-film phase change material-based technologies." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/37684.

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Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. Includes bibliographical references (leaves 68-69). Two potential applications of thin film phase-change materials are considered, non-volatile electronic memories and MEMS (Micro-Electro-Mechanical Systems) actuators. The markets for those two applications are fast growing and rapidly changing, so new materials technologies with superior performance are of great interest. Devices made with thin film phase change materials are shown to offer significant performance improvements for memory applications and new triggering mechanisms for MEMS actuation. IP (Intellectual Property) analyses for both markets indicate significant new IP space in each of them. Rough estimations for cost and revenue are provided. by Qiang Guo. M.Eng.

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