Dissertations / Theses on the topic 'Non-Equilibrium conditions'

Notre travail porte sur la fusion dite solutale survenant lorsque l'on met deux métaux en contact à une température comprise entre leurs températures de fusion respectives. L'interface solide/liquide se retrouve initialement fortement hors-équilibre et la cinétique propre à son retour à l'équilibre semble mettre en défaut les modèles généralement employés pour décrire la solidification et la fusion. Pour progresser dans la compréhension du processus, nous avons abordé le problème sous trois angles complémentaires. Dans un premier temps, nous avons réalisé des expériences in-situ de fusion solutale du système Au-Ag en utilisant la tomographie par rayons X. L'analyse critique des résultats semble montrer que l'interface solide-liquide reste hors équilibre pendant la fusion solutale, avec la persistance inattendue de gradients de concentration à la fin des expériences. Dans un deuxième temps, dans le but de mieux comprendre les expériences, nous avons mis en œuvre un modèle reposant sur la thermodynamique des processus irréversibles appliquée aux échanges d'espèces chimiques à travers une interface solide/liquide abrupte. Une paramétrisation des coefficients de transfert interfaciaux permet au modèle de reproduire qualitativement les comportements observés dans les expériences. Enfin, nous avons cherché à justifier les paramètres cinétiques du modèle thermodynamique en utilisant la dynamique moléculaire (DM) dans le système Cu-Ni. Nous avons ainsi démontré que les coefficients interfaciaux dépendent des concentrations à l'interface, en accord avec la paramétrisation du modèle thermodynamique
Our work focuses on solutal melting, which occurs when two metals are brought into contact at a temperature between their respective melting temperatures. The solid/liquid interface is initially far from equilibrium, and the kinetics governing its return to equilibrium appear to challenge the models commonly used to describe solidification and melting. To advance our understanding of the process, we approached the problem from three complementary angles. First, we conducted in-situ experiments on the solutal melting of the Au-Ag system using X-ray tomography. Critical analysis of the results appears to indicate that the solid-liquid interface remains out of equilibrium during solutal melting, with the unexpected persistence of concentration gradients at the end of the experiments. Second, in an effort to better understand the experiments, we developed a model based on the thermodynamics of irreversible processes applied to the exchange of chemical species across a sharp solid/liquid interface. Parametrization of interfacial transfer coefficients enables the model to qualitatively reproduce the behaviors observed in the experiments. Finally, we sought to justify the kinetic parameters of the thermodynamic model using molecular dynamics (MD) in the Cu-Ni system. We thus demonstrated that the interfacial coefficients depend on the concentrations at the interface, consistent with the parametrization of the thermodynamic model

Cooling down a liquid below its melting temperature can lead to an amorphous frozen system commonly known as a glass. Despite glasses are common in everyday life, the nature of the glass transition still remains one of the most intriguing unsolved problems of condensed matter physics. In this Thesis the atomic length-scale rearrangements of glassy systems close to the glass transition temperature are explored under equilibrium and non-equilibrium conditions with X-Ray Photon Correlation Spectroscopy (XPCS). In particular, we explore the role of directional stresses on the dynamics in a colloidal glass of silica nanoparticles dispersed in a binary solvent. Our results show that the macroscopic stress stored in this glass relaxes via the cooperative ballistic motion of groups of particles with a characteristic size of the order of ten particle diameters. The role of stresses is further investigated in borate-based glasses, where the dynamics well below the glass transition are dictated by the recently discovered X-ray beam-induced dynamics. We show that these dynamics are related to the topology of the network, with peculiar similarities with the stress-phenomenology observed in colloids and metallic glasses.

Cooling down a liquid below its melting temperature can lead to an amorphous frozen system commonly known as a glass. Despite glasses are common in everyday life, the nature of the glass transition still remains one of the most intriguing unsolved problems of condensed matter physics. In this Thesis the atomic length-scale rearrangements of glassy systems close to the glass transition temperature are explored under equilibrium and non-equilibrium conditions with X-Ray Photon Correlation Spectroscopy (XPCS). In particular, we explore the role of directional stresses on the dynamics in a colloidal glass of silica nanoparticles dispersed in a binary solvent. Our results show that the macroscopic stress stored in this glass relaxes via the cooperative ballistic motion of groups of particles with a characteristic size of the order of ten particle diameters. The role of stresses is further investigated in borate-based glasses, where the dynamics well below the glass transition are dictated by the recently discovered X-ray beam-induced dynamics. We show that these dynamics are related to the topology of the network, with peculiar similarities with the stress-phenomenology observed in colloids and metallic glasses.

Ferroelectric materials have received lots of attention thanks to their intriguing properties such as the piezoelectric and pyroelectric effects, as well as the large dielectric constants and the spontaneous polarization which can potentially be used for information storage. In particular, perovskite crystal has a very simple unit cell structure yet a very rich phase transition diagram, which makes it one of the most intensively studied ferroelectric materials. In this dissertation, we use effective Hamiltonian, a first-principles-based computational technique to study the finite-temperature properties of ferroelectric perovskites. We studied temperature-graded (BaxSr1-x )TiO3 (BST) bulk alloys as well as the dynamics of nanodomain walls (nanowalls) in Pb(ZrxTi1-x )O3 (PZT) ultra-thin films under the driving force of an AC field. Our computations suggest that, for the temperature-graded BST, the polarization responds to the temperature gradient (TG), with the "up" and "down" offset observed in polarization components along the direction of TG, in agreement with the findings from experiments. For the nanowalls in PZT, the dynamics can be described by the damped-harmonic-oscillator model, and we observed a size-driven transition from resonance to relaxational dynamics at a critical thickness of 7.2 nm. The transition originates from the change in the effective mass of a nanowall as a film thickness increases. Some of the findings may find potential applications in various devices, such as thermal sensors, energy converters, or novel memory units.

Nitrogen constitutes a natural part of air and is a non-global warming gas. There have been recent attempts to use it as a working medium in high voltage circuit breakers to replace SF6, which is an excellent insulating and arc quenching gas but also a strong greenhouse gas with a Global Warming Potential (GWP) of 23,900 and a life time of 3,200 years. To employ the full potential of nitrogen for use in high voltage circuit breakers, the properties and radiation characteristics of its mixture with Polytetrafluoroethylene (PTFE) vapour, an electrical and thermal insulation material, need to be known, forming the objectives of the present work. This research has investigated the capability of nitrogen and SF6 on insulating and quenching an electric arc in a high voltage circuit breaker. Depending on the current of the electric arc range from 100 A to 330 A, the arc can be extinguished in 25-50 ms with nitrogen gas flow, while it takes 20-45 ms with SF6. The peak voltage of the electric arc with SF6 is 10% higher than it is with nitrogen. According to the preliminary data obtained in the experimental investigation, it seems possible that SF6 can be replaced by nitrogen as the operation gas in high voltage circuit breakers. Research on the properties and radiation characteristics of a gas mixture of nitrogen and PTFE is required to allow modelling of a nitrogen filled circuit breaker. Net Emission Coefficient (NEC) describes the radiation characteristics of the hottest area in an electric arc. The NEC of a nitrogen and PTFE mixture is computed by using the most advanced method considering both continuum and line radiation. The result shows that the influence of PTFE on the NEC cannot be neglected. The influence of uncertainty in atomic data on the calculated NEC is studied. A 50% adjustment in the uncertainty of the atomic data on high energy levels results in a 0.001% variation of the NEC because of the low population density of the particles with high energy levels involved. Due to the lack of experimental results of the mixture, the validity of the model is confirmed by a good agreement between the results calculated and those obtained by experiments or those predicted by the other research groups for pure nitrogen gas and pure PTFE vapour. Results show that influence of PTFE vapour on the NEC of the mixture is substantial. It has been determined that the NEC of the mixture cannot be estimated by NEC of the pure gases multiplied by their proportions in the mixture. Re-absorption has to be considered when the temperature is lower than 25,000 K. To solve the Radiation Transfer Equation (RTE), the P1 and Discrete Ordinate Method (DOM) have been used to calculate the radiation with re-absorption. The P1 approximation is computationally cheaper but it has poor accuracy. The accuracy of DOM is determined by the number of calculation bands. It is obvious that the more bands used, the higher the accuracy. A comparison of the DOM 8-5 and 5-3 methods indicates a balanced compromise between accuracy and cost of calculation with DOM 8-5. A two-temperature model was applied to solve the properties of the plasma in non-LTE condition when the temperature gradient is sufficiently high. A number of existing calculation methods are discussed. A modification of Godin's method has been proposed to satisfy the two-temperature model. Results of a gas mixture composition, obtained by different methods, are analysed. The modified method leads to results that agree very well with those from the existing methods but with better convergence that when the Newton-Raphson algorithm is used to obtain the solution. Results with different molar percentages of PTFE vapour have been presented and discussed. The results for pure nitrogen and pure PTFE vapour have been compared with existing data to verify the validity of the method.

This thesis treats the inversion of remote sensing measurements (infrared spectra) of middle and upper planetary atmospheres, through bayesian inversion strategy. The first part of the work regards the study of the middle and upper atmosphere of Titan, through inversion of VIMS measurements, a spectrometer aboard the Cassini mission. VIMS measured emission from CO, CH4, HCN and C2H2 in the 3-5 μm region, in conditions of non-Local Thermodynamic Equilibrium (non-LTE). The CO concentration in the upper stratosphere and mesosphere of Titan has been determined from the inversion of a set of day-side VIMS limb measurements around 4.7 μm. This represents the first measurement of the CO VMR (Volume Mixing Ratio) profile between 200 and 500 km on Titan. The latitudinal and seasonal variations in the distribution of CH4, HCN and C2H2 in the upper atmosphere of Titan between 500 and 1000 km were studied from VIMS measurements in the 2.8-3.5 μm region. The significance of the preliminary results is discussed with regard to the small coverage at the polar latitudes and the observed correlation between the retrieved CH4 VMR and SZA at high altitudes. A bias observed in the results of the inversion of CH4 brought to the development of SpectRobot (SR), a new code for radiative transfer modeling and inversion of atmospheric quantities, that considers the full 3D geometry of the problem. The second part of this work regards the study of the first measurements acquired by JIRAM, an infrared imager/spectrometer aboard the Juno mission to Jupiter. In the 3-4 μm region JIRAM measures the emission from H3+, which is a great tracer for the auroral phenomena, since it is produced by the precipitation of energetic electrons. Maps of the average retrieved H3+ column density, emission temperature and CH4 column density have been produced for the two auroral regions.

We explore the nature of driven stochastic lattice systems with non-periodic boundary conditions. The systems consist of particle and holes which move by exchanges of nearest neighbor particle-hole pairs. These exchanges are controlled by the energetics associated with an internal Hamiltonian, an external drive and a stochastic coupling to a heat reservoir. The effect of the drive is to bias particle-hole exchanges along the field in such a way that a particle current can be established. Hard-core volume constraints limit the occupation of only one particle (hole) per lattice site. For certain regimes of the overall particle density and temperature, a system displays a homogeneous disordered phase. We investigate cooperative behavior in this phase by using two-point spatial correlation functions and structure factors. By varying the particle density and the temperature, the system orders into a phase separated state, consisting of particle-rich and particle-poor regions. The temperature and density for the co-existence state depend on the boundary conditions. By using Monte Carlo simulations, we establish co-existence curves for systems with shifted periodic boundary conditions.
Ph. D.

Stabilization of proteins is of great interest for the biotechnological society, industrial as well as research areas. Proteins with high stability are more suitable as reagents, easier to handle, store, transport and use in industrial processes. One way to stabilize a protein is to introduce a disulfide bridge into the structure by protein engineering. In this report the formation of a disulfide bridge between engineered cysteines in non ideal conformations in human carbonic anhydrase has been investigated. The disulfide bridge is not formed when the protein is in its native state. It is shown that when the protein is exposed to mild concentrations of urea in the presence of DTTox the disulfide bridge is formed. Also upon refolding in vitro, in a non oxidative environment, disulfide bridges are formed. This observation is worth to notice, since the disulfide bridge does not form to any appreciable extent when the protein is expressed and folded in vivo in Escherichia coli.

La combustion assistée par plasma a reçu une attention croissante dans les deux communautés de plasma et de combustion. Les décharges Nanoseconde Répétitive Pulsée (NRP) sont des techniques prometteuse et efficaces pour initier et contrôler les processus de la combustion en particulier quand les systèmes d’allumage conventionnels sont inefficaces ou trop coûteux en énergie. Néanmoins, les phénomènes rencontrés dans la combustion assistée par plasma sont encore mal connus. Les études numériques présentées dans la littérature sont limitées à des simulations 1-D et 2-D dans des conditions au repos. La complexité du problème augmente dans les configurations pratiques où le phénomène d’allumage est contrôlé par le mouvement du fluide ainsi que le mélange autour de la zone de décharge. La simulation numérique directe (DNS) est un outil de recherche puissant pour la compréhension des interactions plasma/combustion/écoulement. Toutefois, le coût de calcul de la combustion turbulente avec un nombre de Reynolds élevé et la cinétique chimique détaillée couplée avec le plasma hors-équilibre est prohibitif. Cette thèse présente un nouveau modèle de couplageplasma-combustion pour introduire les effets des décharges de plasma hors-équilibre dans le système d’équations qui décrit le phénomène de la combustion. Le modèle est construit en analysant les chemins par lesquels l’énergie électrique est transférée au gaz. Ce modèle de décharges NRP permet des simulations multidimensionalesDNS de la combustion et l’allumage assistés par plasma. Les phénomènes physiques complexes de l’allumage assisté par décharges multiples de plasma dans des mélanges au repos et en régime d’écoulement turbulent sont analysés dans cette thèse
Plasma-assisted combustion has received increasing attention in both plasma and combustion communities. Nanosecond Repetitively Pulsed (NRP) discharges are a promising and efficient technique to initiate and control combustion processes particularly when conventional ignition systems are rather ineffective or too energy costly. Even though a promising technique, the phenomena occurring in NRP discharges-assisted combustion are still poorly understood. The numerical studies presented in the literature are limited to 1-D and 2-D simulations in quiescent conditions. The problem complexity increases in practical configurations as ignition phenomena are also controlled by the flow and mixing field characteristics in and around the discharge channel. Direct Numerical Simulations (DNS) is a powerful research tool to understand these plasma/combustion/flow interactions. However, the computational cost of fully coupled detailed non-equilibrium plasma and combustion chemistry, and high Reynolds number simulations is prohibitive. This thesis presents a model to describe the effects of non-equilibrium plasma discharges in the set of equations governing the combustion phenomena. Based on the results reported in the literature, the model is constructed by analyzing the channels through which the electric energy is deposited. The two main channels by which the electrons produced during the discharge impact the reactive mixture are considered: 1) the excitation and the subsequent relaxation of the electronic states of nitrogen molecules, which leads to an ultrafast increase of the gas temperature and dissociation of species; and 2) the excitation and relaxation of vibrational states of nitrogen molecules which causes a much slower gas heating. This high level model of NRP discharges allows DNS studies of plasma-assisted combustion / ignition in high turbulent Reynolds number. The complex physics underlying plasma-assisted ignition by multiple discharges in both quiescent and turbulent flow conditions are discussed in the present thesis

La cristallisation est beaucoup utilisée pour isoler des molécules d’intérêt pharmaceutique sous formes énantiopures, ce qui nécessite que les énantiomère s’auto-discriminent à l’état solide. Malheureusement, ce comportement est imprévisible et rare (5-10%), car les deux énantiomères cristallisent majoritairement sous forme de composé racémique, empêchant toute résolution chirale par cristallisation. Réalisant que des conditions hors équilibres peuvent surmonter ces statistiques défavorables, nous avons exploré les comportements à l’état solide et les propriétés thermodynamiques de différentes séries de molécules analogues dérivées de trois composés model chiraux : le praziquantel, la proxyphylline, le paclobutrazol. Nos investigations ont conduit à trois approches clés pour améliorer les méthodes de résolution chirale par cristallisation : (i) une stratégie d’évolution dirigée basée sur les différences d’énergie entre les phases cristallines racémique et énantiopure, (ii) l’identification de caractéristiques moléculaires spécifiques favorisant la cristallisation de cristaux énantiopures et (iii) la conception d’une structure cristalline avec des capacités de discrimination chirale efficaces reposant sur une stratégie d’association hôte-invité. Ces avancées, reposant sur l’étude de dérivés chimiquement apparentés, ouvrent de nouvelles possibilités, précédemment inconsidérées, pour l’obtention d’énantiomères purs qui sont essentiels dans notre vie quotidienne
Crystallization is widely used for isolating biorelevant enantiopure molecules, which requires enantiomers to self-sort into separate enantiopure crystals. Unfortunately, this behaviour is unpredictable and rare (5-10%), as both enantiomers predominantly crystallize together into racemic crystals, hindering any such chiral sorting. Recognizing that non-equilibrium conditions may overcome these unfavourable statistics, we explored the solid-state landscapes and thermodynamic properties of different series of numerous analogous molecules derived from three distinct chiral cores: Praziquantel, Proxyphylline and Paclobutrazol. Our investigations led to three key approaches for enhancing crystallization-based chiral resolution methods: (i) A directed evolution strategy based on the energy differences between racemic and enantiopure crystal phases, (ii) the identification of specific molecular features favouring the crystallization of enantiopure crystals and (iii) engineering a crystal structure with efficient chiral discrimination capabilities that relies on a host-guest association strategy. These insights relying on the study of chemically related derivatives open new, previously unconsidered possibilities for isolating pure enantiomers that are essential in our daily lives

Cette thèse, dont le cadre général est l'optimisation, traite de problèmes d'optimisation non-lisse et de problèmes de théorie des jeux. Elle est constituée de quatre parties. Dans la première, nous présentons le contexte et l'introduction. Dans la deuxième partie, nous discutons quelques règles de calcul sous-différentiel dans des espaces généraux, et présentons notamment certaines formules plus fortes que l'état de l'art, autant dans le cas convexe que dans le cas non convexe. L'accent est mis sur les règles de calcul et conditions d'optimalité approchées et "fuzzy", pour lesquelles aucune condition de qualification n'est requise. Dans la troisième partie, nous considérons des jeux bi-niveaux à plusieurs meneurs et plusieurs suiveurs. Après quelques résultats d'existence dans le cas d'un seul meneur optimiste et dans le cas de plusieurs meneurs, nous étendons des résultats existants concernant la relation entre le problème bi-niveau original et sa reformulation obtenue grâce au remplacement des problèmes des suiveurs par la concaténation de leurs conditions d'optimalité (KKT). Finalement, dans la quatrième partie, nous abordons quelques problèmes de quasi-équilibre, qui sont une généralisation des problèmes d'équilibre de Nash et des inégalités quasi-variationnelles. Nous prouvons ainsi de nouveaux résultats d'existence qui permettent de relâcher les hypothèses standard
This thesis is within the framework of optimization and deals with nonsmooth optimization and with some problems of game theory. It is divided into four parts. In the first introductory part, we give the context and some preliminary results. In the second part we discuss about subdifferential calculus rules in general spaces providing of some improved formulas in both the convex and the non-convex cases. Here the focus is on approximate or fuzzy calculus rules and optimality conditions, for which no qualification conditions are required. In the third part, we discuss about the so-called Multi-Leader-Follower Games. We give an existence result for the case of a single optimistic leader and multiple followers, and extend some results concerning the relation between the original problem with the reformulation obtained by replacing the followers' problem by the concatenation of their KKT conditions. Finally, in the fourth part we study quasi-equilibrium problems which are a general formulation for studying Nash equilibrium problems and quasi-variational inequalities. We provide some new existence results that relax some of the standard hypotheses

This thesis presents a research into the solder interconnects made through the reactive bonding process based on the self-propagating reaction. A numerical study of soldering conditions in the heat affected zone (HAZ) during bonding was initially carried out in order to understand the self-propagating reactive bonding and the related influencing factors. This was subsequently followed by an extensive experimental work to evaluate the feasibility and reliability of the reactive bonding process to enable the optimisation of processing parameters, which had provided a detailed understanding in terms of interfacial characteristics and bonding strengths. In addition, by focusing on the microstructure of the bonds resulted from the self-propagating reactions, the interfacial reactions and microstructural evolution of the bonded structures and effects of high-temperature aging were studied in details and discussed accordingly. To study the soldering conditions, a 3D time-dependent model is established to describe the temperature and stress field induced during self-propagating reactions. The transient temperature and stress distribution at the critical locations are identified. This thus allows the prediction of the melting status of solder alloys and the stress concentration points (weak points) in the bond under certain soldering conditions, e.g. ambient temperature, pressure, dimension and type of solder materials. Experimentally, the characterisation of interconnects bonded using various materials under different technical conditions is carried out. This ultimately assists the understanding of the feasibility, reliability and failure modes of reactive bonding technique, as well as the criteria and optimisation to form robust joints. The formation of phases such as intermetallic compounds (IMCs) and mechanism of interfacial reactions during reactive bonding and subsequent aging are elaborated. The composition, dimension, distribution of phases have been examined through cross-sectional observations. The underlying temperature and stress profile determining the diffusion, crystallization and growth of phases are defined by numerical predictions. XXI Through the comparative analysis of the experimental and numerical results, the unique phases developed in the self-propagating joints are attributed to the solid-liquid-convective diffusion, directional solidification and non-equilibrium crystallization. The recrystallization and growth of phases during aging are revealed to be resulted from the solid-state diffusion and equilibration induced by the high-temperature heating. In conclusion, the interfacial reactions and microstructural evolution of interconnect developed through self-propagating reactive bonding are studied and correlated with the related influencing factors that has been obtained from these predictions and experiments. The results and findings enable the extensive uses of self-propagating reactive bonding technology for new design and assembly capable of various applications in electronic packaging. It also greatly contributes to the fundamentals of the crystallization and soldering mechanism of materials under the non-equilibrium conditions.

Le transport du courant électrique sur de longues distances nécessite l'emploi de la haute-tension pour minimiser les pertes en énergie. Toutefois, lorsqu'un défaut survient sur le réseau, il faut pouvoir couper le courant électrique. À cette fin, on utilise des appareils spécifiques comme les disjoncteurs haute-tension qui doivent être capables de couper l'arc électrique qui se forme lors de l'ouverture des contacts électriques. Afin de concevoir et d'améliorer les disjoncteurs, les industriels recourent de plus en plus aux modélisations numériques simulant le comportant de l'arc électrique et du plasma qui le sous-tend. Toutefois, les phénomènes ayant lieu dans le plasma sont complexes et nécessitent une expertise bien spécifique, disponible dans les laboratoires de recherche publique. Cette thèse s'inscrit dans un partenariat entre l'industriel Siemens et l'équipe AEPPT du LAPLACE dans le but de créer une banque de données, nécessaire aux modélisations numériques, afin d'étudier spécifiquement la phase d'extinction de l'arc électrique. Cette banque porte sur les propriétés thermodynamiques et radiatives du plasma. Les coefficients de transport, essentiels, ont fait l'objet d'une autre thèse effectuée en parallèle de celle-ci par G. Vanhulle. Le calcul de ces propriétés est relativement bien connu si le plasma est à l'équilibre thermodynamique. Néanmoins, lors de l'extinction de l'arc, les électrons libres et les particules lourdes (molécules, atomes et ions) ne sont plus nécessairement à l'équilibre thermique et sont décrits par des températures différentes (on parle alors de plasma 2T). L'originalité de cette thèse réside dans l'extension du calcul des propriétés d'un plasma SF_6-C_2 F_4 en conditions hors équilibre thermique. Nous commençons par discuter le calcul des températures internes des espèces lourdes (température d'excitation des niveaux électroniques, vibrationnels et rotationnels) puis le calcul des fonctions de partition, indispensables pour obtenir les autres propriétés du plasma. Pour obtenir la composition chimique 2T du plasma, nous avons développé une approche fondée sur la cinétique chimique qui est basée sur un ensemble d'équations de bilan de peuplement, permettant de limiter les approximations physiques simplificatrices. Toutefois, cette approche n'est pas adaptée pour calculer une grande banque de données. Nous avons donc opté pour une méthode de calcul de la composition selon deux lois d'action de masse multi-températures. À l'aide des compositions chimiques obtenues, nous calculons ensuite les propriétés thermodynamiques et radiatives. Les propriétés thermodynamiques (densité, enthalpie et capacité thermique) sont décrites et les résultats sont discutés. Les propriétés radiatives sont plus complexes à obtenir. Nous présentons en détails les processus radiatifs et les phénomènes d'élargissement des raies atomiques permettant d'obtenir les coefficients d'émission et d'absorption du plasma. Enfin, nous discutons le transfert du rayonnement à travers le plasma et nous présentons nos résultats en utilisant la méthode du coefficient d'émission nette permettant de déterminer la divergence du flux radiatif dans les modèles magnétohydrodynamiques
To transport the electrical current on large distances, we have to use high voltage to minimize the energy losses. However, if an electrical fault occurs in the distribution network, we should be able to turn off the electrical current. For this purpose, we use specific devices: the high voltage circuit breakers that should to shut off the electrical arc formed when the electrical contacts are separated. To design and to upgrade the high voltage circuit breakers, the manufacturers are increasingly using numerical modelisations that simulate the electrical arc and the plasma behaviors. Nevertheless, the physics phenomena in the plasma are complex. To describe them, a specific expertise, available in the public research laboratories, is necessary. This thesis is in a partnership between the Siemens company and the AEPPT group in LAPLACE to create a databank to study the extinction phase of electrical arc. This data bank includes thermodynamic and radiative properties of plasma. The transport coefficients have been studied in another thesis conducted in parallel by G. Vanhulle. The calculation of these properties is relatively well known if the plasma is in local thermodynamic equilibrium. But, during the extinction of arc, the free electrons and the heavy particles (molecules, atoms and ions) are not necessary in thermal equilibrium and are describe by different temperatures (the plasma is referred to as 2T plasma). The originality of this thesis is in the extension of the calculation of properties for a SF_6-C_2 F_4 plasma out of thermal equilibrium. First, we discuss the calculation of internal temperatures of heavy particles (excitation temperatures of electronic, vibrational and rotational levels). Then, we report the calculation of partition functions that are essential to get the others properties. To get the 2T chemical composition of the plasma, we developed an approach based on a set of balance equations. In this method, the physical approximations are limited but it is not adapted for the massive calculation of a databank. Therefore, we opted for a method based on two multi-temperature laws of mass action. With the chemical compositions, we calculated the thermodynamic and radiative properties. The thermodynamic properties (mass density, enthalpy and heat capacity) are described and the results are discussed. The radiative properties are complex. We describe in details the radiative processes and the broadening phenomena of atomic lines to get the emission and absorption coefficients. Finally, we discuss the transfer of radiation across the plasma. We show our results resulting of the net emission coefficient method to get the divergence of radiation flux used in magneto-hydrodynamic model

Radiotherapy is used in clinics to treat cancer with highly energetic ionizing particles. The radiation dose can be measured indirectly by means of radiation detectors or dosimeters. The dose deposited in a detector can be related to dose deposited in a point within the patient. In theory, however, this is only possible under charged particle equilibrium (CPE). The motivation behind the dissertation was driven by the difficult, yet crucial, dosimetry in non-CPE regions. Inaccurate dose assessment performed with standard dosimetry using ionization chambers may significantly impact the outcomes of radiotherapy treatments. Therefore, advanced dosimetry methods tailored specifically to suit non-CPE conditions must be used. This work aims to improve dosimetry in two types of non-CPE conditions that pose dosimetric challenges: regions near interfaces of tissues with low- and high- density media and in small photon fields. To achieve the main dissertation objectives, an enhanced film dosimetry protocol with a novel film calibration approach was implemented. This calibration method is based on the percent depth dose (PDD) tables and was shown to be efficient and accurate. As a result, the PDD calibration method was used for the film dosimetry process throughout the dissertation work. Monte Carlo (MC) calculations for the small field dosimetry were performed using phase-space files (PSFs) provided by Varian for TrueBeam linac. The MC statistical uncertainty in these types of calculations is limited by the number of particles (due to latent variance) in the used PSFs. This study investigated the behaviour of the latent variances (LV) with beam energy, depth in phantom, and calculation resolution (voxel size). LV was evaluated for standard 10x10 cm2 fields as well as small fields (down to 1.3 mm diameter). The results showed that in order to achieve sub-percent LV in open 10x10 cm2 field MC simulations a single PSF can be used, whereas for small SRS fields (1.3—10 mm) more PSFs (66—8 PSFs) would have to be summed. The first study in this dissertation compared the performance of several dosimetric methods in three multi-layer heterogeneous phantoms with water/air, water/lung, and water/steel interfaces irradiated with 6 and 18 MV photon beams. MC calculations were used, along with Acuros XB, anisotropic analytical algorithm (AAA), GafChromic EBT2 film, and MOSkin dosimeters. PDDs were calculated and measured in these heterogeneous phantoms. The result of this study showed that Acuros XB, AAA, and MC calculations were within 1% in the regions with CPE. At media interfaces and buildup regions, differences between Acuros XB and MC were in the range of +4.4% to -12.8%. MOSkin and EBT2 measurements agreed to MC calculations within ~ 2.5%-4.5%. AAA did not predict the backscatter dose from the high-density heterogeneity. For the third, multilayer lung phantom, 6 MV beam PDDs calculated by all treatment planning system (TPS) algorithms were within 2% of MC. 18 MV PDDs calculated by Acuros XB and AAA differed from MC by up to 3.2 and 6.8%, respectively. MOSkin and EBT2 each differed from MC by up to 3%. All dosimetric techniques, except AAA, agreed within 3% in the regions with particle equilibrium. Differences between the dosimetric techniques were larger for the 18 MV than the 6 MV beam. This study provided a comparative performance evaluation of several advanced dosimeters in heterogeneous phantoms. This combination of experimental and calculation dosimetry techniques was used for the first time to evaluate the dose near these interfaces. The second study in the dissertation aims to improve dose measurement accuracy in small radiotherapy fields. Field output factors of 6 MV beams from TrueBeam linear accelerator (linac) collimated with 1.27-40 mm diameter cones were calculated and measured using MC and EBT3 films. A set of detector specific correction factors for two widely used dosimeters (EFD-3G diode and PTW-60019 microDiamond detectors) were determined based on GafChromic EBT3 film measurements and calculated using MC methods. MC calculations were performed for microDiamond detector in parallel and perpendicular orientations relative to the beam axis. The result of this study showed that the measured OFs agreed within 2.4% for fields ≥10 mm. For the cones of 1.27, 2.46, and 3.77 mm diameter maximum differences were 17.9%, 1.8% and 9.0%, respectively. MC calculated OF in water agreed with those obtained using EBT3 film within 2.2% for all fields. MC calculated output correction factors for microDiamond detector in fields ≥10 mm ranged within 0.975-1.020 for perpendicular and parallel orientations. MicroDiamond detector correction factors calculated for the 1.27, 2.46 and 3.77 mm fields were 1.974, 1.139 and 0.982 with detector in parallel orientation, and these factors were 1.150, 0.925 and 0.914 in perpendicular orientation. EBT3 and MC obtained correction factors agreed within 3.7% for fields of ≥3.77 mm and within 5.9% for smaller cones. This work provided output correction factors for microDiamond and EFD-3G detectors in very small fields of 1.27 – 3.77 mm diameter and demonstrated over and under-response of these detectors in such fields. These correction factors allow improve the accuracy of dose measurements in small photon fields using these detectors.
Graduate
2019-08-30

Research Doctorate - Doctor of Philosophy (PhD)
This thesis encompasses an investigation of the efficacy of a dielectric barrier discharge for treatment of chlorinated hydrocarbons in a non-oxidative environment by converting them to environmentally benign and potentially valuable products, most notably polymers. The approach for this research is rather unconventional in the sense that unlike the majority of research in this field, we have not singularly targeted the destruction of the chemical, but have rather focussed on its conversion to the aforementioned value added products. The four chlorinated hydrocarbons investigated in this research are: 1,2-dichloroethane (C₂H₄Cl₂), chloroform (CHCl₃), dichloromethane (CH₂Cl₂) and carbon tetrachloride (CCl₄). The thesis also investigates the effect of methane on the reaction of these chlorinated hydrocarbons as well as on the structure of the resultant polymer. By employing a non-oxidative atmosphere we have been able to successfully obviate the formation of a number of unwanted and hazardous by-products such as COCl₂ and CO, a notorious hindrance widely noted in the literature for treatment of waste chlorinated hydrocarbons. Qualitative and quantitative analysis of gas phase products was afforded by utilising analytical equipment such as a (micro) GC, GC-MS, FT-IR and Ion Chromatograph (IC). For each chapter in this thesis, a comprehensive mass balance accounting for both gaseous and solid products is provided at a representative applied voltage. There exists a significant knowledge gap in understanding the structure of polymers or solids that can be obtained from non-equilibrium plasma treatment of the target hazardous chlorinated hydrocarbons; and hence a substantial part of this thesis is directed towards characterisation of these polymers. Analytical techniques such as NMR (1-D and 2-D), GPC and FT-IR were utilised for this purpose. The research pertaining to C₂H₄Cl₂ and CHCl₃ is predominantly experimental in nature. However in case of CH₂Cl₂ and CCl₄, the research also has a computational chemistry aspect to it, used to gain insight into the reaction mechanism. Quantum chemical calculations performed on the Gaussian09 software suite were utilised in conjunction with the experimental data to determine the most probable routes of decomposition and also to explain the formation of some of the major gas phase products. In general, the conversion levels of the investigated chlorinated hydrocarbons and methane were found to increase with increasing applied voltage. The maximum conversion levels attained for 1,2-dichloroethane, dichloromethane ,chloroform and carbon tetrachloride were 91.5 % (18.5 kV), 80.8 % (16 kV), 66.8 % (16 kV), and 37 % (18 kV) respectively. In case of C₂H₄Cl₂, CHCl₃ and CH₂Cl₂, it was found that addition of methane had a net inhibiting effect on the conversion levels of the chlorinated hydrocarbons. For example the EDC conversion level at 16 kV in absence of methane is 87.9 %, while decreasing to 78.6 % upon methane addition. However, in case of carbon tetrachloride a very interesting phenomenon was observed; the addition of methane promoted the conversion of CCl₄ to gaseous and polymeric products. An additional study was conducted to examine the effect of hydrogen on the reaction of CHCl₃ in the non-equilibrium plasma. The results indicated that hydrogen, like methane, had an inhibiting effect on CHCl₃ conversion and had no significant effect on the structure of the polymer. An important attribute of the polymers obtained from individual studies of the chlorinated hydrocarbons in this thesis is that they are predominantly non-crosslinked in nature and are comprised of –(CH₂-CHC1)- as a major component of their polymeric chain structure, which is also the repeating main chain group in poly (vinyl chloride). NMR analyses of the polymers obtained from the reaction of methane and the chlorinated hydrocarbons revealed that bulk of the polymer is similar to the polymers obtained from the reaction of the respective chlorinated hydrocarbons in absence of methane. Nonetheless, the most significant outcome of methane addition with respect to the resultant polymer was evidenced in case of C₂H₄Cl₂ where the addition of methane resulted in the virtual elimination of unsaturation in the polymer, which is basically a structural defect. The peaks which indicate presence of unsaturation in the polymer are evidenced between 5.6-5.9 ppm chemical shift in the ¹H NMR for the polymer obtained from reaction of EDC in absence of methane. Conversely, these peaks are absent in the ¹H NMR of the polymer obtained from C₂H₄Cl₂ + CH₄ reaction. This effect however, was not observed in case of any other chlorinated hydrocarbons investigated in this study. The computational study in case of CH₂Cl₂ was performed by employing the B3LYP/6-31 G(2df,p) level of theory for initial optimisation of geometries and ZPVE calculations. However, for increased accuracy of energy calculations we subjected the optimised structures to a G4MP2 level of theory. This computational study was mainly directed towards explaining the formation of two major species formed during CH₂Cl₂ decomposition, viz. CHCl₃ and C₂HCl₃. Transition state structures for this study were determined using the Synchronous Transit-Guided Quasi-Newton (STQN) method, more specifically the QST3 function. IRC calculations were also employed in this study.

碩士
國立臺灣大學
土木工程學研究所
106
Molecular dynamics (MD) simulation has been widely applied in studying materials behavior in the past decades. However, owning to its great demands on the modeling fidelity, MD is often limited by the length scale it can handle. Although it is much desired to develop multiscale modeling to overcome this limitation, the development of dynamic multiscale modeling is limited by the spurious wave-reflection problems on the interface between atoms and continuum. This study has developed a multiscale semi-analytical formulation for arbitrary lattices in MD. This method decomposes MD system into a coupled-domain of real and virtual domains, dissembles the contribution from the virtual domain in frequency, and derives the time-history kernel function (THKF) accounting for the interaction of virtual domain implicitly. The virtual atoms on the interface can further be controlled by this numerical method after pairing of degree-of-freedom of THKF and atoms on the interface. By doing so, the dynamics simulation without considering virtual atoms will be equivalent to the original full MD simulation. In a broad sense, multiscale interface (MI) from this study is a two-way implicit interface across real domains, which is not limited to the number of real domains and the way of coupling. However, in the special case of single real domain, multiscale interface reduces to the classical absorbing boundary condition (ABC). There is some key difference between these two applications. ABC totally ignores the information of the virtual domain, thus the time-cutoff of convolution can reduce the computational cost and maintain the bound of the non-reflecting system. On the other hand, due to the transition of the information between different real domains, the simulation of MI will result in spurious wave reflection if THKF are interfered by the time-cutoff. In addition, THKF analyzed from the MI has strong physical meanings of real space that contains the size effect, which can be further developed to resolve length scale limitation in MD. In this thesis, we also explore the possibility of applying machine learning (ML) in ABC based on the function form in THKF. The preliminary verification has been completed and ML for ABC can reduce the computational efforts substantially. This poses a good direction to develop ML for MI in the future.

Due to failure of the continuum hypothesis for higher Knudsen numbers, rarefied gases and microflows of gases are particularly difficult to model. Macroscopic transport equations compete with particle methods, such as the direct simulation Monte Carlo method (DSMC) to find accurate solutions in the rarefied gas regime. Due to growing interest in micro flow applications, such as micro fuel cells, it is important to model and understand evaporation in this flow regime. To gain a better understanding of evaporation physics, a non-steady simulation for slow evaporation in a microscopic system, based on the Navier-Stokes-Fourier equations, is conducted. The one-dimensional problem consists of a liquid and vapor layer (both pure water) with respective heights of 0.1mm and a corresponding Knudsen number of Kn=0.01, where vapor is pumped out. The simulation allows for calculation of the evaporation rate within both the transient process and in steady state. The main contribution of this work is the derivation of new evaporation boundary conditions for the R13 equations, which are macroscopic transport equations with proven applicability in the transition regime. The approach for deriving the boundary conditions is based on an entropy balance, which is integrated around the liquid-vapor interface. The new equations utilize Onsager relations, linear relations between thermodynamic fluxes and forces, with constant coefficients that need to be determined. For this, the boundary conditions are fitted to DSMC data and compared to other R13 boundary conditions from kinetic theory and Navier-Stokes-Fourier solutions for two steady-state, one-dimensional problems. Overall, the suggested fittings of the new phenomenological boundary conditions show better agreement to DSMC than the alternative kinetic theory evaporation boundary conditions for R13. Furthermore, the new evaporation boundary conditions for R13 are implemented in a code for the numerical solution of complex, two-dimensional geometries and compared to Navier-Stokes-Fourier (NSF) solutions. Different flow patterns between R13 and NSF for higher Knudsen numbers are observed which suggest continuation of this work.
Graduate