Rozprawy doktorskie na temat „NANO PHASE CHANGE”

This thesis studies the thermophysical properties and the phase change behaviour of EG-water and Salt-water based PCMs for cold storage applications, and investigates the role of adding MCNT on the thermophysical properties and the phase change processes. First, the structure of MCNT clusters is linked to the rheological behaviour of the nanofluids by fitting the experimental viscosity data to the modified K-D model. Second, the MCNT cluster information is used to predict thermal conductivity. The effective thermal conductivity of nanofluids not only relies on the particle concentration, but also depends on the particle cluster structure. The specific heat of MCNT nanofluids is decreasing proportionally with the concentration of MCNT. The supercooling degree of EG-water and salt-water based samples can be reduced by adding MCNT particles. The crystallization process of salt-water basefluid and nanofluid was observed and recorded under an optical microscope with cooling stage. Adding MCNT can promote the crystal growth rate.

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

Chalcogenide based Phase Change Materials are currently of great technological interest in the growing field of optoelectronics. Ge₂Sb₂Te₅ (GST) is the most widely studied phase change material, and it has been commercially used in both optical and electronic data storage applications, due to its ability to switch between two different atomic configurations, at high speed and with low power consumption, as well as its high optical and electrical contrast between amorphous and crystalline states. Despite its well-known optical and electrical properties, the operation in combination of optical and electrical domains has not yet been fully investigated. This work studies the operation of GST nano-devices exposed to a combination of optical and electrical stimuli or mixed mode by asking, is it possible to electrically measure an optically induced phase change, or vice versa? If so, how do the optical and electrical responses relate to each other, and is it possible to operate GST with a combination of optical and electrical signals? What are the technical constraints that need to be considered in order to fabricate GST devices that could be operated either optically or electrically? In order to answer these questions, experiments that characterized the optical and electrical responses of GST based nano-devices were performed. It was found that different crystallization mechanisms may have influence in the response, and that the thermal and optical design characteristics of the device play a key role in its operation. Finally a proof of principle, of an opto-electonic memory device that can be read electrically, reset optically and write electrically, is presented. This opens up possibilities for the development of new opto-eloectronic applications such as non-volatile interfaces between future photonics and electronics, high speed optical communication detectors, high speed cameras, artificial retinas and many more.

Latent heat storage is one of the most efficient ways of storing thermal energy. Organic phase change materials are latent heat storage materials and they have been widely used as suitable materials for thermal energy storage applications due to their high latent heat and small temperature difference between storing and releasing heat. In this thesis, micro- and nano- scaled phase change materials were fabricated for thermal energy storage. A novel microencapsulated phase change material slurry (MPCS) was introduced by dispersing microencapsulated phase change materials (MEPCMs) into water with an amount of surfactants and its thermal and rheological properties were also investigated. The results showed that MPCS fabricated in the current research are suitable for potential application as heat transfer media in the thermal energy storage. A new methodology was proposed to investigate the heat transfer characteristics of MPCSs. Experiments were carried out in laminar, transition and turbulent flow for MPCSs in a circular tube under constant heat flux, respectively. The experimental results demonstrated that in comparison to water as a heat transfer fluid at the same flow rate, the heat transfer of 10 wt. % MPCS could be enhanced by 10 % in transition flow condition while the PCM particles were in solid/liquid state, and the heat transfer of 5 wt. % MPCS could be enhanced by 21.9 % and 19.2 % in turbulent flow condition while the PCMs are in solid and solid/liquid states, respectively. Nevertheless, the heat transfer enhancement depends on the combination factors, including concentration of the slurry and flow rate of the slurry. A novel heat transfer fluid containing microencapsulated phase change material and multi-walled carbon nanotubes was prepared. The results showed that addition of MWCNTs to microencapsulated phase change material slurry can effectively improve the thermal conductivity of suspensions and it is also found that a blend of 10 wt. % MEPCM and 1 wt. % MWCNTs suspension can achieve the best thermal performance and stability among other blends in the experiment. A novel nanocapsule containing n-octadecane with an average 50 nm thick shell of poly (ethyl methacrylate) (PEMA), and with a core/shell weight ratio of 80/20 was synthesized by direct miniemulsion method. The results showed that PEMA/octadecane nanocapsule had good thermo-physical properties and had much higher encapsulation ratio (89.5%) and encapsulation efficiency (88.9%). For the first time, a novel PCM nanoparticle suspension (nano-PCS) was synthesized by direct miniemulsion method for thermal energy system application. It was found that the nano-PCSs had good thermo-physical properties and durability. All nano-PCSs presented narrow size distribution and stable particles. In comparison to the convectional PCM emulsion and MPCS, the nano-PCS tends to be more stable and is much easier and cheaper to fabricate in terms of the method and materials used, however, the heat transfer characteristics of the nano-PCS require further experimental investigation

Heat management in high thermal-density systems such as CPU chips, nuclear reactors and compact heat exchangers is confronting rising challenges due to ever more miniaturized and intensified processes. While searching for heat transfer enhancement, micro-channel flow boiling and the usage of high thermal potential fluids such as nanofluids are found to be efficient heat removal approaches. However, the limited understanding of micro-scale multiphase flows impedes wider applications of these techniques. In this thesis work, liquid-vapour phase change and multiphase flow heat transfer in micro-channels were experimentally investigated. Included are studies on the single phase friction, vapour dynamics, liquid meniscus evaporation, two-phase flow instabilities and heat transfer. An experimental system was built. Rectangular microchannels with different hydraulic diameters (571 μm, 762 μm and 1454 μm) and crosssectional aspect ratios were selected. Transparent heating was utilised by coating the micro-channels with a layer of tantalum on the outer surfaces. FC-72, n-pentane, ethanol, and ethanol-based Al2O3 nanofluids were used as working fluids. Pressures and temperatures at micro-channel inlet and outlet were acquired. Simultaneous visualisation and thermographic profiles were monitored. Single phase friction of pure liquids and nanofluids mostly showed good agreement with the conventional theory. The discrepancies were associated with hydrodynamic developing flow and the early transition to turbulent flow, but nanoparticle concentration showed minor impact. After boiling incipient, the single vapour bubble growth and flow regimes were investigated, exploring the influences of flow and thermal conditions as well as the micro-channel geometry on vapour dynamics. In addition, liquid meniscus evaporation as the main heat transfer approach at thin liquid films in micro-channels was studied particularly. Nanoparticles largely enhanced meniscus stability. Besides, flow instabilities were analyzed based on the pressure drop and channel surface temperature fluctuations as well as the synchronous visualization results. Moreover, study on flow boiling heat transfer was undertaken, the corresponding heat transfer characteristics were presented and the heat transfer mechanisms were elucidated. Furthermore, ten existing heat transfer correlations were assessed. A modified heat transfer correlation for high aspect ratio micro-channel flow boiling was proposed. The crucial role of liquid property and microchannel aspect-ratio on flow boiling heat transfer was highlighted.

L'information est devenue le bien le plus précieux au monde. Ce mouvement vers la nouvelle ère de l'information a été propulsé par la capacité à transmettre l'information plus rapidement, à la vitesse de la lumière. Il est donc apparu nécessaire de mener des recherches plus poussées pour contrôler plus efficacement les supports d'information. Avec les progrès réalisés dans ce secteur, la plupart des technologies actuelles de contrôle de la lumière se heurtent à certains obstacles tels que la taille et la consommation d'énergie et sont conçues pour être passives ou sont limitées technologiquement pour être moins actives (technologie Si-back). Même si rien ne voyage plus vite que la lumière, la vitesse réelle à laquelle les informations peuvent être transportées par la lumière est la vitesse à laquelle nous pouvons la moduler ou la contrôler. Ma tâche dans cette thèse visait à étudier le potentiel du VO2, un matériau à changement de phase, pour la nano-photonique, avec un accent particulier sur la façon de contourner les inconvénients du matériau et de concevoir et démontrer des dispositifs intégrés efficaces pour une manipulation efficace de la lumière à la fois dans les télécommunications et le spectre visible. En outre, nous démontrons expérimentalement que les résonances multipolaires supportées par les nanocristaux de VO2 (NC) peuvent être réglées et commutées dynamiquement en exploitant la propriété de changement de phase du VO2. Et ainsi atteindre l'objectif d'adaptation de la propriété intrinsèque basée sur le formalisme de Mie en réduisant les dimensions des structures de VO2 comparables à la longueur d'onde de fonctionnement, créant un champ d'application pour un métamatériau accordable défini par l'utilisateur
Information has become the most valuable commodity in the world. This drive to the new information age has been propelled by the ability to transmit information faster, at the speed of light. This erupted the need for finer researches on controlling the information carriers more efficiently. With the advancement in this sector, majority of the current technology for controlling the light, face certain roadblocks like size, power consumption and are built to be passive or are restrained technologically to be less active (Si- backed technology). Even though nothing travels faster than light, the real speed at which information can be carried by light is the speed at which we can modulate or control it. My task in this thesis aimed at investigating the potential of VO2, a phase change material, for nano-photonics, with a specific emphasis on how to circumvent the drawbacks of the material and to design and demonstrate efficient integrated devices for efficient manipulation of light both in telecommunication and visible spectrum. In addition to that we experimentally demonstrate the multipolar resonances supported by VO2 nanocrystals (NCs) can be dynamically tuned and switched leveraging phase change property of VO2. And thus achieving the target tailoring of intrinsic property based on Mie formalism by reducing the dimensions of VO2 structures comparable to the wavelength of operation, creating a scope for user defined tunable metamaterial

The importance of studying the physico-chemical properties of nano-dimensional materials has gained significant attention in the fields of semiconductors, pharmaceuticals, materials science, and atmospheric chemistry owing to the differences in physical properties between macro- and nano-dimensional solids. Nonetheless, direct studies of physical properties of materials at nanoscale is limited in part due to their inherent size constraints and experimental limitations. However, development of atomic force microscopy (AFM) led to the implementation of methods to characterize a wide range of physical properties, including – but not limited to – mechanical properties, electrical properties, viscoelastic properties, and surface tension. Herein, the dissertation focuses on AFM-based method development for characterization of atmospheric particles as well as understanding the relationship between structure and physical properties of organic solids at both macro- and nano-dimensions. In the atmospheric chemistry realm, the combined aerosol effect on the climate and environment has significant uncertainty in part due to lack of direct characterization of their physico-chemical properties. The difficulty in assessing the physical and chemical properties arises due to the presence of diversified aerosol sources, which in turn influences the size, morphology, phase states and chemical compositions. Sea spray aerosols (SSAs) are the second-largest source of aerosols in the atmosphere. Studying SSAs – especially in submicrometer-dimensions – requires high-resolution microscopy techniques such as AFM. AFM can be used for imaging of individual aerosols, quantifying organic volume fraction for core-shell morphologies, measuring water uptake, quantifying surface tension of individual droplets, and measuring mechanical and viscoelastic properties of materials. Herein, we employed AFM-based morphology and force spectroscopy studies to correlate the 3D morphology, phase state, and viscoelastic properties of selected single-component chemical systems found in sea spray aerosol (SSA). We established a quantitative framework toward differentiation of the solid, semisolid and liquid phase states of individual particles by utilizing both relative indentation depth (RID) and viscoelastic response distance (VRD) data obtained from the force−distance plots. Moreover, we established a semi-quantitative and quick phase assessment by measuring the aspect ratio (AR) that refers the extent of particle spreading as a result of impaction. Overall, the established AFM-based quantitative and semi-quantitative phase identification method can be utilized to assess the phases of aerosols irrespective of chemical identity. Next, we investigated the factors that may control the electrical and mechanical properties of pharmaceutical and organic semiconducting materials in nano- and macro-dimensions. Understanding the structure-property relationship of materials, especially in the nano-dimension, is necessary for proper drug design and development of organic semiconducting materials. In this context, cocrystals provide a means to modulate the physico-chemical properties of organic solids. For example, the modulation of the mechanical properties is important in the pharmaceutical industry for improving the tabletability. The mechanical properties may be affected by packing arrangement, interaction strength and type, and atomic and chemical composition. Herein, we report the influence of alkane and alkene functional groups on the mechanical properties of organic solids based on salicylic acid (SA). The approach affords both isostructural and polymorphic solids. The isostructural alkane functional solid exhibits a two-fold larger Young’s modulus (YM) compared to the cocrystal with the alkene, where the YM refers to the stiffness of the material. Here, the higher YM values are attributed to the presence of a bifurcated weak C-H···O interactions involving the alkane and neighboring SA molecules. On the other hand, in the case of alkene polymorphisms, molecular packing with column arrangement shows higher YM values compared to the herringbone arrangements. Thus, functional groups and crystal arrangements influence the stiffness of the solid organic cocrystals. Moreover, we report the modulation of mechanical properties of salicylic acid (SA) through cocrystallization by variation of propane and butane functionality with bipyridine coformers. We show that the variation of propane and butane functionality in bipyridine coformer with salicylic acid leads to synthesis of cocrystal and salt-cocrystal, respectively. The AFM nanoindentation study revealed that the Young’s modulus values follow the order salicylic acid < cocrystal << salt-cocrystal. The highest Young’s modulus values of the salt-cocrystal, among the studied systems, are attributed to the presence of strong N+–H···O– and O–H···O– interactions. On the other hand, higher Young’s modulus values of the propane-based cocrystal compared to the macro-dimensional salicylic acid are attributed to the stronger O–H ···N hydrogen bonding. Thus, homologous alkane functional groups can influence the mechanical properties of the organic solid crystals. Additionally, in situ solid-solid polymorphic phase transformation and nucleation of a metastable and elusive polymorph of SA cocrystals in combination with 4,4’-bipyridine were studied. Understanding the solid-solid phase transformations and nucleation mechanisms are important for proper control over the parameters associated with the synthesis of targeted crystalline solids with desired crystal structure. Using in situ powder X-ray diffraction (PXRD) and terahertz time domain spectroscopy (THz-TDS) data we showed that the Form II polymorph transforms to Form I over time. AFM imaging and nanoindentation techniques were utilized to follow and quantify in real-time the solid-solid polymorphic transformation of the metastable Form II to the thermodynamically stable Form I on a single crystal basis. AFM in situ single crystal data revealed that the metastable Form II has a rod-shaped morphology and relatively high elasticity (Young’s modulus), which transforms to prism-shaped nanocrystals of much smaller sizes with significantly reduced elasticity. The AFM imaging reveals that the single crystals on the order of 80-150 nm to undergo catastrophic changes in morphology that are consistent with cracking and popping owing to a release of mechanical stress during the transformation. The nucleation mechanism for the polymorphic transformation is not spatially localized and occurs over the entire crystal surface. The higher mechanical properties of the metastable Form II is due to the presence of the additional interlayer C-H···O interactions. Furthermore, we have studied the electrical properties of boron-based cocrystals. More specifically, cocrystallization of a nonconductive 2,4-difluorophenylboronic ester catechol adduct of a 4,4’-bipyridine (BEA) host with two aromatic semiconducting guests (pyrene and tetrathiafulvalene) generated conductive cocrystals with variable charge carrier mobilities. Charge carrier mobilities of the cocrystals with either pyrene or tetrathiafulvalene were measured using conducting probe AFM (CP-AFM). The incorporation of π-rich aromatic guests through face-to-face and edge-to-face π-contacts results in electrically conductive cocrystals. The cocrystal with tetrathiafulvalene as a guest shows approximately 7 times higher charge carrier mobility than the cocrystal with pyrene. Overall, the current dissertation demonstrates the AFM-based method development and applications towards materials characterization to measure the morphological, electrical, mechanical, and phase-states at both nano- and macro-dimensions. The high spatial precision of the methods developed enables us to better understand the controlling factors for materials design and processing across nano- and macro-dimensions.

Nous avons étudié la dynamique quantique d'un circuit supraconducteur constitué d'un SQUID dc couplé à un transistor à paires de Cooper fortement asymétrique (ACPT). Le SQUID dc est un qubit de phase contrôlé par un courant de polarisation et un champ magnétique. L'ACPT est un qubit de charge contrôlé par un courant de polarisation, un champ magnétique et une tension de la grille.

Nous avons mesuré par spectroscopie micro-onde les premiers niveaux d'énergie du circuit couplé en fonction des paramètres de contrôle. Les mesures des états quantiques des qubits de charge et de phase sont réalisées par une mesure d'échappement du SQUID dc avec une impulsion de flux nanoseconde appliquée dans celui-ci. La mesure de l'ACPT utilise un nouveau processus quantique : l'état excité de l'ACPT est transféré adiabatiquement vers l'état excité du SQUID durant l'impulsion de flux.

Notre circuit permet de manipuler indépendamment chaque qubit tout comme il permet d'intriquer les états quantiques des deux circuits. Nous avons observé des anti-croisements des niveaux d'énergie des deux qubits lorsqu'ils sont mis en résonance. Le couplage a été mesuré sur une large gamme de fréquence, pouvant varier de 60 MHz à 1.1 GHz. Nous avons réussi à obtenir un couplage variable entre le qubit de charge et le qubit de phase. Nous avons analysé théoriquement la dynamique quantique de notre circuit. Cette analyse a permis de bien expliquer le couplage variable mesuré par une combinaison entre un couplage Josephson et un couplage capacitif entre les deux qubits.

La microscopie thermique à sonde locale (SThM) est une technique qui permet de caractériser les propriétés thermiques de nanomatériaux et de mieux comprendre les transferts thermiques existants aux échelles sub-micrométriques. Pour correctement interpréter les mesures, les paramètres influençant le transfert thermique entre la pointe-sonde SThM et l’échantillon sont étudiés. Trois sondes résistives de SThM, se différenciant notamment par leur rayon de courbure micro ou nanométrique, sont tout d’abord caractérisées, et une méthodologie systématique de mesure en régime continu est proposée. Il est observé que la zone de sensibilité à la conductivité thermique des matériaux massifs plans est limitée à quelques W.m-1.K-1 pour toutes les pointes. Pour les matériaux les plus conducteurs, la mesure SThM est dominée par la résistance thermique de contact. Le transfert thermique par le (les) nanocontact(s) solide-solide entre la pointe et l’échantillon est dû à un transport conductif à la fois diffusif et balistique dans l’échantillon. Il est mis en évidence que la rugosité de surface impacte fortement la mesure SThM, diminuant le transfert thermique par le contact de plus de 50 % dans certains cas. Ces travaux sont mis à profit pour des caractérisations de nanomatériaux. La détermination de la conductivité thermique de couches minces de SiO2 sur substrat de silicium indique que les épaisseurs de quelques nanomètres jusqu’à 1 µm sont détectées par certaines pointes. La mesure de températures de changement de phase par microscopie SThM est également étudiée à l’aide d’un étalonnage sur des polymères massifs. L’application de cet étalonnage pour la caractérisation de couches minces de polymère confirme l’influence du substrat et de l’épaisseur de la couche sur la température déterminée par la pointe SThM. Ces travaux démontrent que la microscopie thermique permet d’obtenir des mesures quantitatives
Scanning thermal microscopy (SThM) is a technique that allows characterizing the thermal properties of nanomaterials and helps understanding heat transfer at submicron scales. To interpret the measurements, parameters influencing heat transfer between the probe and the sample are studied. Firstly, three resistive SThM probes, differing in particular by their micro and nanometric radii of curvature, are analyzed and a systematic methodology for the measurements is proposed. It is put forward that the sensitive zone to thermal conductivity of bulk planar materials is limited to few W.m-1.K-1 for the three probes. For the more conductive materials, SThM measurements are dominated by interfacial thermal resistance. Heat transfer at the solid-solid nanocontact between the probe and the sample can be both ballistic and diffusive. It is further demonstrated that surface roughness strongly impacts SThM measurements, decreasing heat transfer at the contact by more than 50 % in some cases. This work is used for characterizations of nanomaterials. The determination of the thermal conductivity of SiO2 thin film on silicon substrate indicates that thicknesses of a few nanometers up to 1 µm are detected by certain probes. Phase transition temperature measurement by SThM is also studied, using a calibration with bulk polymers. The application of this calibration for the characterization of polymer thin films demonstrates the influence of the substrate and the thin film thickness on the temperature determined by SThM. These results demonstrate that scanning thermal microscopy allows obtaining quantitative measurements

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

碩士
國立臺灣大學
物理研究所
96
In this thesis, we study the formation of recording marks on crystalline Ge2Sb2Te5 phase-change nano thin film. We use an atomic force microscopy (AFM) and optical pump-probe system to investigate the topographic change and optical-thermal dependence of marks formation. From the experimental results, the process of recording mark formation is well studied in both incident power and pulse duration aspects. Through the complete experiments, the arbitrary pattern of recording marks can be written on phase-change material precisely by changing layered structure and tuning incident power and pulse duration. The special thermal-optical effect of phase-change material can be applied to the nano photonics in future.

Phase change phenomena have been of interest mainly due to large latent heats associated with the phase transition and independency on external energy to drive the phase change process. When combined with micro/nano structures, phase change systems become a promising approach to address challenges in high heat flux thermal management. The objective of this thesis is to implement micro and nano structured surfaces for better understanding the underlying fundamentals of evaporation and boiling phase change heat transfer and enhancing the heat transfer performance. First, we study single bubble dynamics on superheated superhydrophobic (SHB) surfaces and the corresponding heat transfer mechanism of water pool boiling. Because of the large contact angle, such surfaces are ideal for correlating pool boiling with single bubble dynamics by accurately controlling the number of nucleation sites in a defined area. The fundamental parameters of single bubble dynamics are collected and put into the heat flux partitioning model. We find that latent heat transport and bulk liquid water convection contribute together to the heat removal on superhydrophobic surfaces. Next, we present a novel method to fabricate silicon nanowires by one-step metal assisted chemical etching (MACE) on micro-structured surfaces with desired morphologies. Patterned vertically aligned silicon nanowires are fabricated on dense cavity/pillar arrays due to trapped hydrogen bubbles serving as an etching mask. Uniformly grown silicon nanowires on structured surfaces can be fabricated if extra energy is introduced to remove the trapped bubbles. An enhanced pool boiling heat transfer performance on such structured surfaces is demonstrated. Finally, we study the ultimate limits of water evaporation in single 2D nanochannels and 1D nanopores. These ultimate transport limits are determined by the maximum evaporation fluxes that liquid/vapor interfaces can provide regardless of liquid supply or vapor removal rates. A hybrid nanochannel design is utilized to provide sufficient liquid supply to the evaporating meniscus and evaporated vapor is efficiently removed by air jet impingement or a vacuum pump. The effect of nanoscale confinement on evaporation flux has been investigated, with feature size ranging from 16 nm to 310 nm. An ultra-high heat flux of 8500 W/cm2 is demonstrated in a single 16-nm nanochannel at 40 °C.
2017-09-09T00:00:00Z

碩士
國立臺灣海洋大學
光電科學研究所
97
The purpose of this thesis, we use Atomic Force Microscopy to measure the surface topography of nano structure made by Dynamic Optical Disk Tester and Pump-Probe Laser System on phase-change material Ge2Sb2Te5 thin film, to investigate the topographic change and optical-thermal dependence of marks formation. In this experiment, the nano ring structures with different power of phase change thin film are made by thermal-lithography. We compare the CCD image and surface topography image to know about the etching properties of phase change thin film with an alkaline etching solution NaOH. Then we write recording marks or line by Dynamic Optical Disk Tester and it becomes a segmental structure though the methods of sample separated, and compare the surface topography of different power by Atomic Force Microscopy.

碩士
國立臺灣海洋大學
光電科學研究所
96
In this thesis, the different writing strategies (2T8T, 2T6T, 2T4T, 2T1T, 2T=100 nm; 2T2T, 2T=50 nm ) and writing power were used to write the recording marks on the commercial rewritable DVD disk(DVD+RW). The Carrier to Noise Ratio (CNR) measurements were carried out using the dynamic optical disk tester. In order to understand the relationship with different writing strategy, writing power and recording mark size, we used the conductive atomic force microscope(C-AFM) to investigate the surface current signal of recording marks. In this experiment, the minimal size of recording mark is 16 nm.

碩士
國立臺北科技大學
化學工程研究所
98
The study is mainly focused on using melamine-formaldehyde to plate on phase change material（PCM）such as the wax material of octadecane、nonadecane and eicosane to get microencapsulated phase change materials (McPCMs). In order make McPCMs to have the function of antiseptic and modulate temperature, we added a nanometer silver film outside the McPCMs by using plating to increase the efficacy of antiseptic. Therefore, the main research is divided into two parts. In the first part, we adjusted varies of experience to improve the sizse of the microcapsule, the cover completeness, and the yield. In the second part, we used chemical plating on McPCMs, which was made from the first part. We also used Sn2+ and Ag(TEA)2+ to proceed redox reaction and produced accumulation of nanometer silver on surface of particle. When the McPCMs, which we got from the first part, passed through the high-speed emulsification machine to 11000rpm, the microcapsule size of McPCMs went down below 5um. The mass ratio of the shell and the cell was 25.59:74.41. However, in the second part, we found an ideal result that accumulated quantities of nanometer silver getting attached to the surface of McPCMs. By analyzing and calculating, we know the plating nanometer silver has about 11.56% in the total mass ratio of McPCMs.

碩士
國立臺灣師範大學
光電科技研究所
95
In this study, we first investigate the optical response on phase-change material Ge2Sb2Te5 with an optical pump-probe system (Static tester, Optica co.). From the CCD images and reflectance of recorded mark matrix, the process of recording mark formation and the reflectance change can be analyzed and categorized. An Auger electron spectrum (VG Scientific,Microlab 350.) is also applied to analyze the component and oxidization of different types of recording marks. For further understanding of the phase-change state of recording marks, we use a conductive atomic force microscope (C-AFM) with a heating stage (Asylum Research co.) to investigate the topographic change and the surface current distribution of recording marks on phase-change material at different temperature. From the experimental results, the process and the phase transition of recording marks on phase-change material Ge2Sb2Te5 can be obtained to have the further usage in ultra-high density recording.

碩士
國立中央大學
物理研究所
94
The purpose of the study is to utilize a pump-probe laser system, a static tester, to analyze the formation mechanism and the thermal properties of recorded marks on the thin film of the phase-change material Ge2Sb2Te5. The variation of reflectance on the phase-change recording layer can be acquired simultaneously as the recorded mark is being written. The information of optical reflectance on the interface of the phase-change recording layer can help observe marks written with either different powers or durations, respectively. The detailed change of reflectance provide us criterion to analyze both the formation mechanism of recorded marks and the state of recorded area on the recording layer. With the help of conductive-atomic force microscopy (C-AFM), not only the crystalline, as-deposited, and amorphous states can be characterized but also the area of recorded marks can be determined especially for the size of marks below diffraction limit. The area of recorded marks with respect to laser writing power or laser duration suggest that both writing power and laser duration have strong influence on the size of recorded marks. The effective specific heat can also be estimated by analyzing the relation between absorption power and area of recorded marks.

The increasing growth of industrialization and population has led to a global shortage of drinkable water, which has motivated researchers to find an alternate approach to supply this need. Solar stills (SSs) are solar-powered systems that can produce drinking water, although they have a low output problem. In present research work, two different types of SS- a conventional SS and an advanced SS have been studied. To enhance the yield (productivity) of advanced solar stills, an external condenser (EC), water heating coil, and Nano-phase change material (ZnO-PCM) have been used. Performance of conventional SS and advanced SS were compared in three different sets of investigation under the same climatological conditions. The maximum thermal efficiency and improved yield are obtained as 46% & 77%, 53% &119%, and 51% & 113% for ASS (heating coil), ASS-EC, and ASS-ZnO-PCM respectively. Thus, the productivity of ASS (heating coil) was enhanced by around 36% and 42% using ZnO-PCM and an EC, respectively. A study of economic analysis was conducted as well and observed that the desalinated freshwater's acquired prices were 0.030, 0.023, and 0.021 $/l for CSS, ASS-ZnO-PCM, and ASS-EC, correspondingly.

碩士
臺灣大學
物理研究所
98
Phase change materials has different optical and electrical properties in crystalline and amorphous state, it has been applied to versatile areas such as optical data storage, phase change memory, nanolithography. In this paper, we present a laser-induced forward transfer technique to fabricate the pattern with phase change material Ge2Sb2Te5. The as-deposited Ge2Sb2Te5 alloy films on a transparent substrate are transferred to the receiver substrate after a femto-second laser pulse irradiation (wavelength is 800 nm, and pulse duration is 140 femto-second). The dots patterns are fabricated with different volume and height-width ratio by changing the laser fluence and the thickness of the donor film. The topography of receiver substrate is studied by atomic force microscopy (AFM) and the optical measure system, the transfer properties are analyzed. According to the AFM measured information, we found that the dot diameter is function of Ge2Sb2Te5 donor film thickness and laser fluence. The dot size is around 14 nm (thickness) x 1500 nm (diameter). Fabrication of patterns composed of dots deposited on the receiver substrate was measured. This technique provides a simple way to form arbitrary pattern and has potential in future production of optical components, MEMS and phase-change memory.

The overall aim of this study was to obtain a facile method of synthesizing graphite nanoplatelets from commercial expandable graphite and use these as functional fillers in rotational moulding applications and phase change materials for energy storage. Two commercial expandable graphites were evaluated as precursors for the synthesis of graphite nanoplatelets. Microwave radiation treatment was shown to be more efficient in exfoliating expandable graphite than furnace heating. The expandable graphite with better exfoliating characteristics was selected. XRD results of this graphite showed that it was a high stage graphite intercalation compound. Graphite nanoplatelets with an average particle size of 13 μm and an estimated thickness of about 76 nm were prepared by microwave exfoliation and ultrasonication-assisted liquid phase exfoliation in isopropanol from the selected expandable graphite. Prior to the selection of isopropanol as the ultrasonication media, various exfoliation media that encompassed different solvents and water with various surfactants had been evaluated, on the basis of their acoustic cavitation characteristics. The graphite nanoplatelets were used as a functional additive to fabricate linear low density polyethylene (LLDPE) and poly(ethylene-co-vinyl acetate) (EVA) based nanocomposites using the rotational moulding (rotomoulding) process. The dry blending approach yielded surface resistivities within the static dissipation range (antistatic) at filler loadings as low as 0.25 wt.% (0.1 vol.%). However, even at this low graphite content, impact properties were significantly reduced compared to the neat polymers. Bilayer mouldings via the double dumping method proved to be a feasible approach to achieve both acceptable mechanical properties and antistatic properties. This was achieved by rotomoulding nanocomposites with a 1 mm outer layer containing the filler and a 2 mm inner layer of neat LLDPE. Excellent fire resistance, in terms of cone calorimeter testing, was achieved when the outer layer also contained 10 wt.% expandable graphite. Pseudo binary mixtures of stearyl alcohol/commercial triple pressed stearic acid where prepared and characterized as a new phase change material (PCM) for energy storage. A facile method of preparing highly thermally conductive stearyl alcohol/stearic acid phase change material/graphite nanoplatelets (GNPs) nanocomposites was developed. Inclusion of the GNPs in the PCM matrix reduced the enthalpy of melting and crystallization marginally. However, the PCM/nanocomposite exhibited negligible super cooling. At 10 wt.% loading, the graphite nanoplatelets enhanced the thermal conductivity of the PCM by close to 600 % and 1200 % in the solid and molten states, respectively. Thermal conductivity modelling showed that the substantial thermal conductivity enhancement was as a result of relatively low interfacial thermal resistance between the PCM matrix and GNPs. The PCM/GNPs nanocomposites also showed excellent thermal reliability after being subjected to accelerated thermal cycling tests of 100 melting and freezing cycles. Settling tests showed the PCM/GNPs nanocomposite with 10 wt.% GNPs was stable after 60 days, with no apparent separation between the PCM matrix and the graphite nanoplatelets.
Thesis (PhD)--University of Pretoria, 2016.
National Research Foundation (NRF)
Department of Science and Technology
Chemical Engineering
PhD
unrestricted

Open celled micro and nano foams fabricated from polymers and metals have attracted tremendous attention in the recent past because of their applications in numerous areas such as catalyst carriers, filtration media, ion exchange membranes and tissue engineering scaffolds. In this study open celled polymer micro- and nano foams with controllable pore size and porosity were fabricated via solid state foaming of immiscible blends. The polymer foams were used as templates for fabricating nickel foams using an ethanol based electroless plating process. Thermal conductivity of micro- and nano foams was studied as a function of pore size and porosity using finite element and molecular dynamics based models. The effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management was investigated via numerical models. Open celled micro foams were fabricated via solid state foaming of ethylene acrylic acid (EAA) and polystyrene (PS) co-continuous blends. Blending temperature was the main parameters affecting the formation of co-continuous structure. Gas saturation and foaming studies were performed to determine ideal processing conditions for the blend. The results indicated that saturation pressure and foaming temperature were major process parameters determining the porosity of the foamed samples. Open celled polymer templates were obtained by selective extraction of PS phase using dichloromethane (DCM). Foaming resulted in faster extraction of PS and also in a higher porosity. Open celled nano foams were fabricated via solid state foaming of polyetherimide (PEI) and polyethersulfone (PES). The effect of process parameters namely saturation pressure and temperature, desorption time, and foaming temperature and time on porosity and pore size was studied. A high gas concentration and foaming temperature were required to obtain nano pore-sized foams. Throughout the cross section there existed regions with varying pore size and porosity and solid skins at the surface regions of the foam. A solvent surface dissolution process using dimethylformamide (DMF) was employed to access the internal porous structure. Micro- and nano cellular nickel foams were fabricated from EAA and PES templates via electroless plating. The structure of the nickel foams was an inverse of the polymer templates. Ethanol based electroless plating solutions were used to ensure infiltration into the porous structure because of the small pore sizes. Finite element and molecular dynamics based models were developed to predict thermal conductivity of polymer foams as a function of pore size and porosity. Pore sizes ranging from 1 nm to 1 mm were studied. Models were partially validated using experimental data. The results showed that pore size has significant effect on thermal conductivity even for microcellular and conventional foams. When the pore size is reduced to the nanometer scale, the thermal conductivity of the nano foam dramatically reduces and the value could be lower than that of air for certain porosity levels. The extremely low thermal conductivity of polymer nanofoams is possibly due to increased phonon-phonon scattering in the solid phases of the polymer matrix in addition to low thermal conductivity of gas trapped in nano sized pores. Finite element based models were also developed to study the effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management applications. The results showed that foams with smaller pore sizes can delay the temperature rise of the heat source for an extended period of time by rapidly dissipating heat in the phase change material. The lower temperatures resulting from the use of a smaller pore size metal foam could significantly increase the lifetime of IC chips.
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碩士
國立臺灣師範大學
光電科技研究所
96
In this study, we use Atomic Force Microscopy to measure the surface topography of nano structure made by Thermal-lithography and dynamic optical disk tester on phase-change material Ge2Sb2Te5 thin film. First, we use Thermal-lithography to make a single line structure on different thickness of phase change thin film. We compare the CCD images and surface topography images to know more about the physical properties of laser interaction on phase change thin film. Then we write over recording marks by Dynamic Optical Disk Tester and it becomes a segmental structure. We can control this repetitive segmental structure by changing writing strategy. As the same, we get the surface topography by atomic force microscopy and count the periods of these structures with different laser writing power. Finally, we measure the smallest line width of single line width is 144nm and the periods of the segmental structure are 141nm.

碩士
國立臺灣海洋大學
光電科學研究所
93
The purpose of this thesis is to study the nonlinear optical properties of near-field optical disk structure by using Z-scan measurements with a Q-switched Nd:YAG semiconductor pulse laser (wavelength of 532nm, pulse width of 0.71ns, and 15.79kHz repetition rate). We investigate the nonlinear optical properties of AgOx layer and phase-change recording layer in near-field optical disk. The interactions between the AgOx layer and phase-change recording layer of AgOx near-field optical disk structure are studied by Z-scan. Measurements of open-aperture transmittance, close-aperture transmittance, open-aperture and close-aperture reflectance of Z-scan with various input laser powers are carried out on AgOx near-field optical disk structure. Experimental results show reverse saturable absorption (RSA) of AgOx structure sandwiched by ZnS-SiO2. Because of the decomposition of AgOx nano thin film, the high transmittance of AgOx structure happened in the Z-scan. The nonlinear optical properties of the interactions between AgOx thin film and phase-change recording layer are clearly observed by Z-scan technique and can be very useful for the developments of the near-field optical data storage and nano-photonic devices in future.

abstract: Non-volatile memories (NVM) are widely used in modern electronic devices due to their non-volatility, low static power consumption and high storage density. While Flash memories are the dominant NVM technology, resistive memories such as phase change access memory (PRAM) and spin torque transfer random access memory (STT-MRAM) are gaining ground. All these technologies suffer from reliability degradation due to process variations, structural limits and material property shift. To address the reliability concerns of these NVM technologies, multi-level low cost solutions are proposed for each of them. My approach consists of first building a comprehensive error model. Next the error characteristics are exploited to develop low cost multi-level strategies to compensate for the errors. For instance, for NAND Flash memory, I first characterize errors due to threshold voltage variations as a function of the number of program/erase cycles. Next a flexible product code is designed to migrate to a stronger ECC scheme as program/erase cycles increases. An adaptive data refresh scheme is also proposed to improve memory reliability with low energy cost for applications with different data update frequencies. For PRAM, soft errors and hard errors models are built based on shifts in the resistance distributions. Next I developed a multi-level error control approach involving bit interleaving and subblock flipping at the architecture level, threshold resistance tuning at the circuit level and programming current profile tuning at the device level. This approach helped reduce the error rate significantly so that it was now sufficient to use a low cost ECC scheme to satisfy the memory reliability constraint. I also studied the reliability of a PRAM+DRAM hybrid memory system and analyzed the tradeoffs between memory performance, programming energy and lifetime. For STT-MRAM, I first developed an error model based on process variations. I developed a multi-level approach to reduce the error rates that consisted of increasing the W/L ratio of the access transistor, increasing the voltage difference across the memory cell and adjusting the current profile during write operation. This approach enabled use of a low cost BCH based ECC scheme to achieve very low block failure rates.
Dissertation/Thesis
Ph.D. Electrical Engineering 2014

碩士
元智大學
企業管理學系
96
Taiwan semiconductor industry plays an important role in the world. Wafer foundry and memory take high percentage in the output value of semiconductor industry. Nonvolatile memory’s importance increase and grow rapidly after portable devices become popular let memory industry pay more attentions on new nonvolatile memory. Phase Change Memory was considered the first one to be commercialized. Patents analysis can represent the degree of technology and improvement, and companies can take it for reference. Technology life cycle and product life cycle are commonly identified by industry and companies consult them to decide resources distribution and develop strategy. This research use the patents analyze technology life cycle, and use the output values to analyze product life cycle, from the relationship of them to forecast the development and trend of Phase Change Memory.