GeTe and Ge2Sb2Te5 alloys are potential candidates for non-volatile phase change random access memories (PCRAM). For electrical data storage applications the materials should have stable amorphous and crystalline phases, fast crystallization time, low power to switch, and high crystallization activation energy (to be stable at normal operating temperatures). Phase change memories can be tuned through compositional variations to achieve sufficient phase change contrast and thermal stability for data retention. Selenium is one of the attractive choices to use as an additive material owing to its flexible amorphous structure and a variety of possible applications in optoelectronics and solar cells. GeSb2Te3Se alloy, in which 25 at.% of Se substituted for Te, show a higher room temperature resistance with respect to parent GeSb2Te4 alloy, but the transition temperature is lowered which will affect the thermal stability. The RESET current observed for Sb65Se35 alloys were reduced and the crystallization speed increased 25 % faster with respect to Ge2Sb2Te5. Alloys of Ga-Sb-Se possess advantages such as higher crystallization temperatures, better data retention, higher switching speed, lower thermal conductivity and lower melting point than the GST, but the resistance ratio is limited to about two orders of magnitude. This affects the resistance contrast and data readability. It is with this background a study has been carried out in GeTe and GeSbTe system with Se doping. Studies on structural, thermal and optical properties of these materials all through the phase transition temperatures would be helpful to explore the feasibility of phase change memory uses. Thin films along with their bulk counterparts such as (GeTe)1-x Sex ( 0 < x ≤ 0.50) and (GST)1-xSex (0 < x ≤ 0.50), including GeTe and GST alloys, have been prepared. The results are presented in four chapters apart from the Introduction and Experimental techniques chapters. The final chapter summarizes the results. Chapter 1 provides an introduction to chalcogenide glasses, phase change memory materials and their applications. The fundamental properties of amorphous solids, basic phase change properties of Ge2Sb2Te5 and GeTe alloys and their applications are presented in detail. Various doping studies on GeTe and Ge2Sb2Te5 reported in literatures are reviewed. The limitations, challenges, future and scope of the present work are presented. In chapter 2, the experimental techniques used for thin film preparation, electrical characterizations, optical characterization and surface characterizations etc. are explained. Chapter 3 deals entirely on Ge2Sb2Te5 films studied throughout the phase transition, by annealing at different temperatures. Changes in sheet resistance, optical transmission, morphology and surface bonding characteristics are analyzed. The crystallization leads to an increase of roughness and the resistance changes to three orders of magnitude at 125 oC. Optical studies show distinct changes in transmittance during phase transitions and the optical parameters are calculated. Band gap contrast and disorder variation with annealing temperatures are explained. The surface bonding characteristics studied by XPS show Ge-Te, Sb-Te bonds are present in both amorphous and crystalline phases. The temperature dependent modifications of the band structure of amorphous GST films at low temperatures have been little explored. The band gap increment of around 0.2 eV is observed at low temperature (4.2 K) compared to room temperature 300 K. Other optical parameters like Urbach energy and B1/2 are studied at different temperatures and are evaluated. The observed changes in optical band gap (Eopt) are fitted to Fan’s one phonon approximation, from which a phonon energy (ћω) corresponding to a frequency of 3.59 THz resulted. The frequency of 3.66 THz optical phonons has already been reported by coherent phonon spectroscopy experiment in amorphous GST. This opens up an indirect method of calculating the phonon frequency of the amorphous phase change materials. Chapter 4 constitutes comparison of optical, electrical and structural investigation of GST and (GST)1-xSex films. It is well known that GST alloys have vacancy in their structure, which leads to the possibility of switching between the amorphous and crystalline states with minimum damage. Added Se may occupy the vacancy or change the bonding characteristics which intern may manifest in the possibility of change in optical and electrical parameters. The structural studies show a direct amorphous to hexagonal transition in (GST)1-xSex, where x ≥ 0.10 at.%. Raman spectra of the as deposited and annealed (GST)1-xSex films show structural modifications. The infrared transmission spectra indicate a shift in absorption edges from low to high photon energy when Se concentration increases in GST. Band gap values calculated from Tauc plot show the band gap increment with Se doping. It is noted that a small amount of Se doping increases the resistance of the amorphous and crystalline phases and maintains the same orders of resistance contrast. This will be beneficial as it improves the thermal stability and reduces the write current in a device. Switching studies show an increasing threshold voltage as the Se doping concentration increases. Chapter 5 comprises compositional dependent investigations of the bulk GeTe chalcogenides alloys added with different selenium concentrations. The XRD investigations on bulk (GeTe)1-xSex (x = 0.0, 0.02, 0.10, 0.20 and 0.50 at.%) alloys show that the crystalline structure of GeTe alloys does not affect ≤ 0.20 at.% of Se concentration. With increasing amount of Se concentration the alloys gets modified in to a homogeneous amorphous structure. This result has been verified from the XRD, Raman, XPS, SEM and DSC measurements. The possibility that Se occupying the Ge vacancy sites in GeTe structure is explained. Since Se is an easy glass former, the amorphousness increases in the alloys due to new amorphous phases formed by the Se with other elements. It is shown from Raman and XPS analysis that the Ge-Te bonds exists up to Se 0.20 at.% alloys. Ge-Se and GeTe2 bonds are increasing with increasing Se at.%. Melting temperature has found decreases and the reduction in melting point may reduces the RESET current. Further studies on switching behavior may bring out its usefulness. Chapter 6 deals with studies on (GeTe)1-xSex films for phase change memory applications based on the insight received from their bulk study. Even at low at.% addition of Se makes the as prepared (GeTe)1-xSex film amorphous. At 200 oC, GeTe crystalline structure is evolved and the intensity of the peaks reduces in the alloys with increase of Se content. At 300 oC, more evolved GeTe crystalline structure is seen compared to 200 oC annealed films whereas 0.20 at.% Se alloy remain amorphous. Resistance and thermal studies shows increase in crystallization temperature. It is expected that Se sits in the vacancies of the GeTe crystalline structural formation. This may also account for the increased threshold voltages with increasing Se doping. The band gap increase with increase of Se at.% signifying the possibility of band gap tuning in the material. Possible explanation for the increased order in GeTe due to Se doping is presented. The modifications in the alloy with Se addition can be explained with the help of chemical bond energy approach. Those bonds having higher energy leads to increased average bond energy of the system and hence the band gap. The XPS core level spectra and Raman spectra investigation clearly shows the GeTe bonds are replaced by Ge-Se bonds and GeTe2 bonds. The 0.10 at.% Se alloy is found to have a higher thermal stability in the amorphous state and maintains a gigantic resistance contrast compared to other Se concentration alloys. This alloy can be considered as an ideal candidate for multilevel PCM applications. Chapter 7 summarizes the major findings from this work and the scope for future work.

Due to climatic change, increasing thermal loads inbuildings and rising living standards, comfort cooling inbuildings is becoming increasingly important and the demand forcomfort cooling is expanding very quickly around the world. Theincreased cooling demand results in a peak in electrical powerdemand during the hottest summer hours. This peak presents newchallenges and uncertainties to electricity utilities and theircustomers. Cool thermal storage systems have not only the potential tobecome one of the primary solutions to the electrical powerimbalance between production and demand, but also shift coolingenergy use to off-peak periods and avoid peak demand charges.It increases the possibilities of utilizing renewable energysources and waste heat for cooling generation. In addition, acool storage can actually increase the efficiency of combinedheat and power (CHP) generation provided that heat drivencooling is coupled to CHP. Then, the cool storage may avoidpeaks in the heat demand for cooling generation, and this meansthat the CHP can operate at design conditions in most oftime. Phase Change Materials (PCMs) used for cool storage hasobtained considerable attention, since they can be designed tomelt and freeze at a selected temperature and have shown apromising ability to reduce the size of storage systemscompared with a sensible heat storage system because they usethe latent heat of the storage medium for thermal energystorage. The goal of this thesis is to define suitable PCM candidatesfor comfort cooling storage. The thesis work combines differentmethods to determine the thermophysical properties oftetradecane, hexadecane and their binary mixtures, anddemonstrates the potential of using these materials as PCM forcomfort cooling storage. The phase equilibrium of the binarysystem has been studied theoretically as well asexperimentally, resulting in the derivation of the phasediagram. With knowledge of the liquid-solid phase equilibriumcharacteristics and the phase diagram, an improvedunderstanding is provided for the interrelationships involvedin the phase change of the studied materials. It has beenindicated that except for the minimum-melting point mixture,all mixtures melt and freeze within a temperature range and notat a constant temperature, which is so far often assumed in PCMstorage design. In addition, the enthalpy change during thephase transition (heat of fusion) corresponds to the phasechange temperature range; thus, the storage density obtaineddepends on how large a part of the phase change temperaturerange is valid for a given application. Differential Scanning Calorimetery (DSC) is one frequentlyused method in the development of PCMs. In this thesis, it hasbeen found that varying results are obtained depending on theDSC settings throughout the measurements. When the DSC runs ata high heating/cooling rate it will lead to erroneousinformation. Also, the correct phase transition temperaturerange cannot be obtained simply from DSC measurement. Combiningphase equilibrium considerations with DSC measurements gives areliable design method that incorporates both the heat offusion and the phase change temperature range. The potential of PCM storage for peak shaving in differentcooling systems has been demonstrated. A Computer model hasbeen developed for rapid phase equilibrium calculation. The useof phase equilibrium data in the design of a cool storagesystem is presented as a general methodology. Keywords:Comfort cooling, peak shaving, PCM, coolthermal storage system, DSC, phase change temperature range,the heat of fusion, phase equilibrium, phase diagram. Language:English

While 3D stacked multi-processor technology offers the potential for significant computing advantages, these architectures also face the significant challenge of small, localized hotspots with very large heat fluxes due to the placement of asymmetric cores, heterogeneous devices and performance driven layouts. In this thesis, a new thermal management solution is introduced that seeks to maximize the performance of microprocessors with dynamically managed power profiles. To mitigate the non-uniformities in chip temperature profiles resulting from the dynamic power maps, solid-liquid phase change materials (PCMs) with an embedded heat spreader network are strategically positioned near localized hotspots, resulting in a large increase in the local thermal capacitance in these problematic areas. Theoretical analysis shows that the increase in local thermal capacitance results in an almost twenty-fold increase in the time that a thermally constrained core can operate before a power gating or core migration event is required. Coupled to the PCMs are solid state coolers (SSCs) that serve as a means for fast regeneration of the PCMs during the cool down periods associated with throttling events. Using this combined PCM/SSC approach allows for devices that operate with the desirable combination of low throttling frequency and large overall core duty cycles, thus maximizing computational throughput. The impact of the thermophysical properties of the PCM on the device operating characteristics has been investigated from first principles in order to better inform the PCM selection or design process. Complementary to the theoretical characterization of the proposed thermal solution, a prototype device called a "Composite Thermal Capacitor (CTC)" that monolithically integrates micro heaters, PCMs and a spreader matrix into a Si test chip was fabricated and tested to validate the efficacy of the concept. A prototype CTC was shown to increase allowable device operating times by over 7X and address heat fluxes of up to ~395 W/cm2. Various methods for regenerating the CTC have been investigated, including air, liquid, and solid state cooling, and operational duty cycles of over 60% have been demonstrated.

The principal goal of this thesis is to develop a systematic method for the design of composite heat sinks (CHSs) that serve as passive and transient cooling devices for microelectronics. This is accomplished by posing the CHS design problem as a topology optimization problem wherein a phase-change material and a high-conductivity material are to be optimally distributed. Two different types of formulations are proposed. The first one aims to maximize the time of operation before a tolerable temperature is reached at the interface between a heat source and the CHS. The second one aims to minimize the maximum temperature across the heating interface for a given time of operation. The two materials are interpolated in topology optimization using the usual mixture law with penalty. The phase-change is modeled using the apparent heat capacity method in which the specific heat is taken as a nonlinear function of the temperature so that the latent heat absorption is accounted for at the melting point. The ensuing new transient topology optimization problem involving an interpolated material property that depends on the state variable is solved using continuous optimization algorithm. The validity of the phase-change modeling is verified with a one dimensional model as well as experimentation. Analytical sensitivity analysis is derived and verified with the finite difference derivatives. Several examples are solved to illustrate the intricacies of the problem and the effectiveness and the limitations of the proposed design method. Prototypes of an intuitively conceived CHS and optimized one are made. An experimental setup is devised to test the two prototypes. Based on the insight gained from the experiments, an improved conduction model is studied to also incorporate convective heat transfer also into the model.

<p>Due to climatic change, increasing thermal loads inbuildings and rising living standards, comfort cooling inbuildings is becoming increasingly important and the demand forcomfort cooling is expanding very quickly around the world. Theincreased cooling demand results in a peak in electrical powerdemand during the hottest summer hours. This peak presents newchallenges and uncertainties to electricity utilities and theircustomers.</p><p>Cool thermal storage systems have not only the potential tobecome one of the primary solutions to the electrical powerimbalance between production and demand, but also shift coolingenergy use to off-peak periods and avoid peak demand charges.It increases the possibilities of utilizing renewable energysources and waste heat for cooling generation. In addition, acool storage can actually increase the efficiency of combinedheat and power (CHP) generation provided that heat drivencooling is coupled to CHP. Then, the cool storage may avoidpeaks in the heat demand for cooling generation, and this meansthat the CHP can operate at design conditions in most oftime.</p><p>Phase Change Materials (PCMs) used for cool storage hasobtained considerable attention, since they can be designed tomelt and freeze at a selected temperature and have shown apromising ability to reduce the size of storage systemscompared with a sensible heat storage system because they usethe latent heat of the storage medium for thermal energystorage.</p><p>The goal of this thesis is to define suitable PCM candidatesfor comfort cooling storage. The thesis work combines differentmethods to determine the thermophysical properties oftetradecane, hexadecane and their binary mixtures, anddemonstrates the potential of using these materials as PCM forcomfort cooling storage. The phase equilibrium of the binarysystem has been studied theoretically as well asexperimentally, resulting in the derivation of the phasediagram. With knowledge of the liquid-solid phase equilibriumcharacteristics and the phase diagram, an improvedunderstanding is provided for the interrelationships involvedin the phase change of the studied materials. It has beenindicated that except for the minimum-melting point mixture,all mixtures melt and freeze within a temperature range and notat a constant temperature, which is so far often assumed in PCMstorage design. In addition, the enthalpy change during thephase transition (heat of fusion) corresponds to the phasechange temperature range; thus, the storage density obtaineddepends on how large a part of the phase change temperaturerange is valid for a given application.</p><p>Differential Scanning Calorimetery (DSC) is one frequentlyused method in the development of PCMs. In this thesis, it hasbeen found that varying results are obtained depending on theDSC settings throughout the measurements. When the DSC runs ata high heating/cooling rate it will lead to erroneousinformation. Also, the correct phase transition temperaturerange cannot be obtained simply from DSC measurement. Combiningphase equilibrium considerations with DSC measurements gives areliable design method that incorporates both the heat offusion and the phase change temperature range.</p><p>The potential of PCM storage for peak shaving in differentcooling systems has been demonstrated. A Computer model hasbeen developed for rapid phase equilibrium calculation. The useof phase equilibrium data in the design of a cool storagesystem is presented as a general methodology.</p><p><b>Keywords:</b>Comfort cooling, peak shaving, PCM, coolthermal storage system, DSC, phase change temperature range,the heat of fusion, phase equilibrium, phase diagram. Language:English</p>

The theoretical part of the diploma thesis deals with introduction to the issue of refrigeration systems with thermal energy storage. Possibilities of thermal energy storage with phase change are introduced. The thermodynamic principle of functioning of the particular refrigeration system type is explained here as well as its coefficient of performance and fundamental components. In addition, several examples of ice-making systems are discussed. In the practical part the design of a particular refrigeration system is calculated. The main part of the design is the cold evaporator with direct evaporation and the possibility of ice production and its usage for thermal energy storage. The calculation of the overall two-phase heat transfer is carried out.

Master of Science / Department of Mechanical and Nuclear Engineering / Amy R. Betz / The purpose of this research was to test if biphilic surfaces mitigate frost and ice formation. Frost, which forms when humid air comes into contact with a surface that is below the dew point and freezing temperature of water, hinders engineering systems such as aeronautics, refrigeration systems, and wind turbines. Most previous research has investigated increasingly superhydrophobic materials to delay frost formation; however, these materials are dependent on fluctuating operating conditions and surface roughness. Therefore, the hypothesis for this research was that a biphilic surface would slow the frost formation process and create a less dense frost layer, and water vapor would preferentially condense on hydrophilic areas, thus controlling where nucleation initially occurs. Preferential nucleation can control the size, shape, and location of frost nucleation. To fabricate biphilic surfaces, a hydrophobic material was coated on a silicon wafer, and a pattern of hydrophobic material was removed using photolithography to reveal hydrophilic silicon-oxide. Circles were patterned at various pitches and diameters. The heat sink was comprised of two parts: a solid bottom half and a finned upper half. Half of the heat sink was placed inside a polyethylene base for insulation. Tests were conducted in quiescent air at room temperature, 22 °C, and two relative humidities, 30% and 60%. Substrate temperatures were held constant throughout all tests. All tests showed a trend that biphilic surfaces suppress freezing temperature more effectively than plain hydrophilic or hydrophobic surfaces; however, no difference between pattern orientation or size was noticed for maximum freezing temperature. However, the biphilic patterns did affect other aspects such as time to freezing and volume of water on the surface. These effects are from the patterns altering the nucleation and coalescence behavior of condensation.

Master of Science / Department of Mechanical and Nuclear Engineering / Bruce R. Babin / A significant body of literature exists for experiments in spray cooling applications that utilize one-dimensional heat transfer through a metal ingot to determine the average surface heat flux. Due to inherent non-uniformities in spray distributions, measurements that account for the two-dimensional effects are required. In this study, an infrared (IR) camera was used to capture the two-dimensional temperature distribution formed when spraying an electrically heated NiChrome surface with three different fluids. IR thermography captured the thermal response of the un-sprayed side of a 0.005-inch (0.125mm) think strip of NiChrome exposed to spray from a 90° full-cone nozzle at low mass fluxes (0.025 – 0.045 lb/ft[superscript]2-s / 0.122 – 0.220 kg/m[superscript]2-s) from a distance of approximately 5 to 11 inches (13 to 28cm). Results were measured for surface average temperatures ranging from 150 to 600°F (65 – 315°C).

The increasing demand for energy in combination with a greater awareness for our environmental impact have encouraged the development of sustainable energy sources, including materials for energy storage. Latent heat thermal energy storage by the use of phase change material (PCM) have become an area of great interest. It is a reliable and efficient way to reduce energy consumption. PCMs store and release latent heat, which means that the material can absorb the excess of heat energy, save it and release it when needed. By introducing soy wax as a biobased PCM and apply it on textile, one can achieve a thermoregulation material to be used in buildings and smart textiles. By replacing the present most used PCM, paraffin, with soy wax one cannot only decrease the use of fossil fuel, but also achieve a less flammable material. The performance of soy wax PCM applied on a textile fabric have not yet been investigated but can be a step towards a more sustainable energy consumption. The soy wax may also broaden the application for PCM due to its low flammability. The aim is to develop an environmental friendly latent heat thermal energy storage material to be used within numerous application fields.

Phase-change contrast agents (PCCAs) are an innovative form of imaging agent with practical applications in both the research and clinical settings. PCCAs are derived from gaseous microbubbles, which are able to act as targeted-contrast agents through conjugation of a ligand that is selective for an overexpressed receptor or biomarker in a given disease. Gaseous microbubbles can be condensed to liquid phase nanodroplets, which should be sufficiently small to extravasate into cells and/or tissues given their size and stability. Once liquid nanodroplets have internalized within a given tissue, they can be "activated" back into gaseous microbubbles with ultrasound at clinically used frequencies and energy outputs. This is purposeful as microbubbles provide much greater ultrasound reflectivity than nanodroplets. In this study, PCCAs and/or microbubbles act as a targeting agent in multiple scenarios. The projects in this study include- examination of binding and internalization of targeted PCCAs with different gaseous cores within MDA-MB-231 breast cancer cells, vaporization of liquid phase nanodroplets through application of acoustic energy via focused ultrasound (FUS), and targeting vulnerable plaque in the heart with different types of targeted microbubbles under varying shear-stresses.