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La thèse porte sur la modélisation de la transformation de l'austénite en ferrite dans les aciers en mettant l'accent sur les conditions thermodynamiques et cinétiques aux interfaces alpha/gamma en cours de croissance de la ferrite. Dans une première partie, la thèse se concentre sur la description des équilibres thermodynamiques entre alpha et gamma à l'aide de la méthode CalPhad. Nous avons développé un nouvel algorithme hybride combinant la construction d'une enveloppe convexe avec la méthode classique de Newton-Raphson. Nous montrons ses possibilités pour des aciers ternaire Fe-C-Cr et quaternaire Fe-C-Cr-Mo dans des cas particulièrement difficiles. Dans un second chapitre, un modèle à interface épaisse a été développé. Il permet de prédire l'ensemble du spectre des conditions à l'interface alpha/gamma au cours de la croissance de la ferrite, de l'équilibre complet au paraéquilibre avec des cas intermédiaires des plus intéressants. Nous montrons que de nombreux régimes cinétiques particuliers dans les systèmes Fe-C-X peuvent être prévus avec un minimum de paramètres d'ajustement, principalement le rapport entre les diffusivités de l'élément substitutionnel dans l'interface épaisse et dans le volume d'austénite. Le troisième chapitre porte sur l'étude d'un modèle de champ de phase. Une analyse approfondie des conditions à l'interface données par le modèle est réalisée en utilisant la technique des développements asymptotiques. En utilisant les connaissances fournies par cette analyse, le rôle de la mobilité intrinsèque d'interface sur la cinétique et les régimes de croissance est étudié, à la fois dans le cas simple d'alliages binaires Fe-C et dans le cas plus complexe d'alliages Fe-C-Mn / Transformation in steels focusing on the thermodynamic and kinetics conditions at the alpha/gamma interfaces during the ferrite growth. The first chapter deals with the determination of thermodynamic equilibria between alpha and gamma with CalPhad thermodynamic description. We have developed a new hybrid algorithm combining the construction of a convex hull to the more classical Newton-Raphson method to compute two phase equilibria in multicomponent alloys with two sublattices. Its capabilities are demonstrated on ternary Fe-C-Cr and quaternary Fe-C-Cr-Mo steels. In the second chapter, we present a thick interface model aiming to predict the whole spectrum of conditions at an alpha/gamma interface during ferrite growth, from full equilibrium to paraequilibrium with intermediate cases as the most interesting feature. The model, despite its numerous simplifying assumptions to facilitate its numerical implementation, allows to predict some peculiar kinetics in Fe-C-X systems with a minimum of fitting parameters, mainly the ratio between the diffusivities of the substitutional element inside the thick interface and in bulk austenite. The third chapter deals with the phase field model of austenite to ferrite transformation in steels. A thorough analysis on the conditions at the interface has been performed using the technique of matched asymptotic expansions. Special attention is given to clarify the role of the interface mobility on the growth regimes both in simple Fe-C alloys and in more complex Fe-C-Mn alloys

Thesis advisor: Ying Ran / In this thesis, I investigated and implemented various numerical and simulation methods, including mean field theory, functional renormalization group method (fRG), density matrix renormalization group (DMRG) method etc., to find different quantum phases and quantum phase diagrams on models of correlated electronic systems. I found different phase diagrams with phases such as magnetism, superconductivity. By summarizing the strength and limitations of these methods, I investigated the projected entangled paired states (PEPS) with symmetry quantum number to sharply distinguish phases into crude classes and applied a variation of fast full update (FFU) prototype[58] to simulate different phases numerically. This method provides a promising, powerful and efficient way to simulate unconventional quantum phases and quantum phase diagrams in correlated electronic systems. / Thesis (PhD) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

In this dissertation we study the phases and phase transition properties of quantum ferromagnets and related magnetic materials. We first investigate the effects of an external magnetic field on the Goldstone mode of a helical magnet, such as MnSi. The field introduces a qualitatively new term into the dispersion relation of the Goldstone mode, which in turn changes the temperature dependences of the contributions of the Goldstone mode to thermodynamic and transport properties. We then study how the phase transition properties of quantum ferromagnets evolve with increasing quenched disorder. We find that there are three distinct regimes for different amounts of disorder. When the disorder is small enough, the quantum ferromagnetic phase transitions is generically of first order. If the disorder is in an intermediate region, the ferromagnetic phase transition is of second order and effectively characterized by mean-field critical exponents. If the disorder is strong enough the ferromagnetic phase transitions are continuous and are characterized by non-mean-field critical exponents.

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.

"Cell traction force is generated in the cytoskeleton by actomyosin activity and plays an important role in many cellular processes. In previous cell traction force experiments performed by our lab, unexpectedly large variations were measured. Because these experiments were utilizing a cell population of randomized phase, and there had been no documented investigation into whether cell phase affected cell traction force generation or propagation, it was hypothesized that there would be a significant difference in traction force between S phase and the other phases of interphase, as the physical and chemical changes happening within the nucleus at this time might elicit changes within the cytoskeleton. To test this hypothesis, we characterized the time-evolution of traction forces from a population of synchronized 3T3 fibroblasts. 3T3 fibroblasts were synchronized in G1-phase via serum starvation. The transition times between cellular phases during the first cell cycle after synchronization were identified by BrdU and Hoechst staining at different time points. After phase transition times were approximated, the traction forces of 9 cells were measured in 4-hour intervals for 24 hours. The differences between traction forces measured in G1, S, and G2 phases are not significant, demonstrating that cellular phase does not significantly affect traction force magnitude."

The wave and tidal energy sectors have received much interest in recent years, from policy-makers attentive to the prospect that ocean energy technologies could be capable of contributing towards meeting environmental targets; from utility companies that expressed interest in developing, constructing and operating array projects to export large quantities of clean energy from ocean based resources; and from Small to Medium Enterprises (SMEs) and large multi-national Original Equipment Manufacturers (OEMs) that were interested in undertaking technological development to commercialise wave and tidal energy converters that could successfully harness the energy contained within the ocean waves and tides. Within the existing research, development and innovation environment that has largely dominated the development of wave and tidal energy to date – rapid development of large MW-scale devices capable of utility scale power generation – technology developers have failed to reach the level of deployed capacity that was initially anticipated, despite the significant level of investment that has taken place. Indeed, the expected contribution of ocean energy in the wider energy mix, which has been written into policy documentation at both national and European level, has so far failed to materialise in the form of prolific multiple device array deployments. The research, development and innovation environment has not delivered on its intended objective of demonstrating commercial technology readiness, and the historic development trajectories for ocean energy technologies may not represent the most cost-effective route to product commercialisation. This research explores the wave and tidal energy research, development, and innovation environment through extensive stakeholder engagement within the ocean energy sector, and through application of suitable techniques from innovation theory. The purpose of this research was three-fold. Firstly, an objective analysis of the development of the wave and tidal energy sectors – building a comprehensive understanding of their development to date through extensive stakeholder engagement, and comparing wave and tidal energy technology development with that of historic energy technologies that have successfully entered into commercial operation – was necessary in order to identify whether the attempt by ocean energy technologies for rapid up-scaling of technology is consistent with the development pathway that was followed by energy technologies which have successfully transitioned from novel invention to full commercial operation. This work identified several dichotomies that are present in the nascent stages of technology development in the wave and tidal energy sectors. Secondly, the uncertainties surrounding existing capital and revenue costs, and the uncertainties within the potential future cost reductions associated with current technology trajectories, could lead to unsustainable investment requirements. Commercialisation of wave and tidal energy technology is predicated upon significant cost reduction – the current technology costs are not feasible for large scale roll out of technology. A research focus on the economic uncertainty through application of learning theory and a learning investment sensitivity analysis was anticipated to demonstrate the economic impact of minor perturbations from idealised reference assumptions. The results from this work suggest that even minor perturbations in input parameters have substantial negative impact on overall investment requirements to bring technology to a level of cost competitiveness. Thirdly, the policy landscape surrounding wave and tidal energy development has not been specifically compared and contrasted, using a number of performance metrics, to a technology which was subject to similar expectations in the form of income streams – wind energy technology. The causes and motivations for the rapid transition to large-scale technologies and ‘accelerated innovation’ within ocean energy technology were considered within this research, which suggested that a mismatch between policy support and technological readiness could misguide and misdirect the innovation pathway, harming the commercialisation prospects of ocean energy technology. In order for the successful emergence of economically sustainable wave and tidal energy technologies, a paradigm shift may be necessary, a change from the current approach that has to date dominated technological development within both the wave and tidal energy sectors. This research draws together industry consultation with academic insight to identify an optimised innovation pathway, culminating in a policy appraisal to guide and inform economically sustainable development of wave and tidal energy technologies.

Les Ruddlesden Popper RExA2-xMnO4 (RE : La, Nd et A : Sr, Ca) ont été étudiées en tant matériaux d'électrode pour sSOFC. Les LaxSr2-xMnO4±δ (x=0.25, 0.4, 0.5, 0.6), NdxSr2-xMnO4±δ (x=0.4, 0.5) et NdxCa2-xMnO4±δ (x=0.25, 0.4, 0.5) ont été synthétisées. Les matériaux RExSr2-xMnO4±δ (RE : La, Nd) sont stables sous atmosphère réductrice avec des TECs compatibles avec ceux des électrolytes. L'étude In situ par HT-NPD du composé La0.5Sr1.5MnO4±δ (L5S15M), sous hydrogène, révèle la formation de lacunes d'oxygène sur les sites équatoriaux de la couche pérovskite. Des valeurs élevées de conductivité électrique ont été obtenues (35.6 S cm−1 sous air et 1.9 S cm−1 sous H2/Ar). Les propriétés électrochimiques de L5S15M ont été examinées par EIS. L'influence sur les performances électrochimiques à la fois de la température de frittage et de la composition de l'électrode a été étudiée dans les deux atmosphères, cathodiques et anodiques. Du côté de la réduction de l'oxygène, le transfert d'électrons entre l'électrode et l'oxygène, et l'incorporation des ions oxygène dans l'électrode sont les principales étapes limitantes. L'augmentation de la température de frittage à 1250 °C conduit à l'accumulation de Sr à l'interface GDC/YSZ. Pour l'oxydation de l'hydrogène, le transfert de charge à l'interface électrode/électrolyte, l'adsorption dissociative de l'hydrogène et la diffusion de surface ont été les étapes limitantes. Finalement, les résultats extraits des mesures par EIS ont permis de comprendre la nature des processus mis en place au sein de l'électrode, révélant que le comportement électrochimique de L5S15M pouvait être amélioré par la modification de la surface de l'électrode. / The Ruddlesden Popper RExA2-xMnO4 (RE: La, Nd and A: Sr, Ca) have been studied as electrode materials for symmetrical Solid Oxide Fuel Cells. The LaxSr2-xMnO4±δ (x=0.25, 0.4, 0.5, 0.6), NdxSr2-xMnO4±δ (x=0.4, 0.5) and NdxCa2-xMnO4±δ (x=0.25, 0.4, 0.5) were successfully synthesized. The RExSr2-xMnO4±δ materials (RE: La, Nd) are stable in reducing atmosphere with electrolyte-compatible TECs. In situ HT-NPD of La0.5Sr1.5MnO4±δ (L5S15M), under flowing hydrogen, reveals formation of oxide-ion vacancies on the equatorial sites of the perovskite layer. High electrical conductivities were obtained (35.6 S cm−1 in air and 1.9 S cm−1 in H2/Ar). Electrochemical properties of L5S15M electrode were investigated by EIS. The influence of both sintering temperature and electrode composition on the electrochemical performance was studied in both cathode and anode atmosphere. For oxygen reduction, the electron transfer between the electrode and oxygen, and the incorporation of oxygen ions into the electrode are the main rate-limiting steps. Increasing the sintering temperature to 1250°C leads to Sr accumulation at the GDC/YSZ interface. For hydrogen oxidation, charge transfer at the electrode/electrolyte interface, dissociative adsorption and surface diffusion were the limiting steps. The best compromise between sintering temperature and composition is reached for pure L5S15M electrode sintered at 1150 °C. Finally, the results extracted from EIS measurements allowed understanding the nature of processes taking place within the electrode, proposing that the electrochemical behavior of L5S15M could be improved with modification of the electrode’s surface.

Metastable b-Ti alloys are titanium alloys with sufficient b stabilizer alloying additions such that it's possible to retain single b phase at room temperature. These alloys are of great advantage compared to a/b alloys since they are easily cold rolled, strip produced and can attain excellent mechanical properties upon age hardening. Beta 21S, a relatively new b titanium alloy in addition to these general advantages is known to possess excellent oxidation and corrosion resistance at elevated temperatures. A homogeneous distribution of fine sized a precipitates in the parent b matrix is known to provide good combination of strength, ductility and fracture toughness. The current work focuses on a study of different mechanisms to engineer homogeneously distributed fine sized a precipitates in the b matrix. The precipitation of metastable phases upon low temperature aging and their influence on a precipitation is studied in detail. The precipitation sequence on direct aging above the w solvus temperature is also assessed. The structural and compositional evolution of precipitate phase is determined using multiple characterization tools. The possibility of occurrence of other non-classical precipitation mechanisms that do not require heterogeneous nucleation sites are also analyzed. Lastly, the influence of interstitial element, oxygen on a precipitation during the oxidation of Beta 21S has been determined. The ingress of oxygen and its influence on microstructure have also been correlated to measured mechanical properties.

In Chapter 1, we briefly review the thermodynamics of polymer solutions, including the ideal solution based on the Flory-Huggins lattice chain model. The entropy change in the mixing of macromolecules with small solvent molecules is much smaller than that in the mixing of two kinds of small molecules. Therefore, the effect of the solution temperature is also smaller. / In Chapter 2, we describe the basic difference between the complicate polymeric and simple Newtonian fluid and list some dimensionless numbers relevant to various physical phenomena, including the Reynolds number (Re) and the capillary number (Ca), respectively related to the inertial effect and the interfacial tension. As a fluid goes down to the micro scale, the inertial effect is usually negligible so that the flow becomes laminar. However, the interfacial tension starts to play a significant role, which leads to the development of some droplet-based (or digital) microfluidic systems. Using small droplets leads to the following advantages: (1) the reagents are limited within a small boundary of each droplet; and (2) no complex microfluidic device is required. / In Chapter 3, we use PNIPAM in water as a model system to detail how to construct a polymer phase diagram by using a microfluidic device, including the choice of the carrier fluid, the principle and experimental procedure of forming concentration-controllable PNIPAM droplets, the determination of PNIPAM concentration in each droplet by using a fluorescein probe, the effect of fluorescein on the phase transition, and the detection of the phase transition by dark field viewing. For comparison, we also did the normal LLS measurement of the phase transition of PNIPAM in water. / In Chapter 4, PS in cyclohexane is used as a model system to illustrate how to handle organic solvents because cyclohexane swells the PDMS channels. The swelling is much eliminated by directly loading the PS solution into the junction via glass capillaries. Since the addition of a fluorescence concentration probe dramatically influences the PS phase transition, we have to use a so-called parallel experimental method to produce concentration-controllable PS droplets. In this method, several PDMS chips from the same batch are used in the formation of small PS droplets. When the numbers of small PS droplets produced in the same procedure are similar to each other, the PS concentrations in different corresponding droplets are comparable. Therefore we are able to index the PS concentration in each droplet by comparing it with the droplets prepared by the same procedure, but with some added fluorescence probes. / In Chapter 5, on the basis of numerous experiments, we find inorganic salts play a significant role on the droplet forming. Thus we propose that droplet formation is a kinetics governed process when two immiscible liquids meet each other in microchannels. / In this thesis we have proposed and established a new method of constructing polymer phase diagrams. By employing the droplets-based microfluidic system, we are able to form an array of droplets of polymer solutions with several nanoliters in size. Each droplet has a controllable composition. The array of polymer droplets can be transferred and stored in a glass capillary; there the turbidity of each droplet due to the difference of scatted light immediately after the phase transition can be monitored under a microscope via dark field viewing, when the solution temperature changes. Therefore, we are able to construct a polymer phase diagram by simply combing each phase transition temperature with its corresponding compositions of polymer solution droplets. / This method has two distinguished advantages; namely, minimal sample consumption and much reduced experimental time required for the phase transition to reach its equilibrium at each given temperature. This is because the greatly increased surface-to-volume ratio allows rapid diffusion and fast heat transfer. To demonstrate the principle, we have chosen PS in cyclohexane with an upper critical solution temperature (UCST) and Poly(N-isopropylacrylamide) (PNIPAM) in water with a lower critical solution temperature (LCST) as two model systems. Primarily established phase diagrams of these two polymer solutions have demonstrated the feasibility of using droplets-based microfluidic system to construct polymer phase diagrams. / Shi, Feng. / Advisers: Chi Wu; Bo Zheng. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3532. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.