The diversity of network applications over the Internet has propelled researchers to rethink the strategies in the transport layer protocols. Current applications either use UDP without end-to-end congestion control mechanisms or, more commonly, use TCP. TCP continuously probes for bandwidth even at network steady state and thereby causes variation in the transmission rate and losses. This thesis proposes TCP Carson, a modification of the window-scaling approach of TCP Reno to suit long-lived flows using loss-events as indicators of congestion. We analyzed and evaluated TCP Carson using NS-2 over a wide range of test conditions. We show that TCP Carson reduces loss, improves throughput and reduces window-size variance. We believe that this adaptive approach will improve both network and application performance.

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Dans ce travail de thèse, des méthodes de simulation numérique ont été utilisées afin de modéliser, à l’échelle moléculaire,l’interaction entre des aérosols carbonés et diverses molécules de l’atmosphère.L’objectif était de caractériser les propriétés physico-chimiques de ces aérosols, qui jouent un rôle majeur dans le climat, en particulier via leur capacité à former des noyaux de condensation pour les molécules d’eau. En premier lieu, les techniques de la dynamique moléculaire ont été appliquées à la détermination de l’organisation d’agrégats d’acide carboxylique sur lesquels sont adsorbées des molécules d’eau. Les résultats montrent une influence à la fois de la température, du taux d’humidité et du type d’acide considéré sur le comportement global des agrégats. Par contre, il a été montré qu’un mélange binaire d’acides n’a que peu d’influence sur ce comportement. En second lieu, des techniques de simulation moléculaires quantiques, à la fois statiques et dynamiques, ont été appliquées à l’étude de la chloration de surfaces de suie, modélisées par de grands HAP, ainsi qu’à la caractérisation de leur affinité pour les molécules d’eau. Les résultats ont montré une forte propension de Cl, Cl2 et HCl à former des HAP chlorés à la faveur de la présence de défauts structurels. Ces structures chlorées présentent une forte hydrophilicité, ce qui pourrait expliquer la nature fortement hygroscopique de suies émises lors d’incendies industriels. / In this PhD work, molecular simulation methods have been used in order to model, at the molecular scale, the interaction between carbonaceous aerosols andvarious atmospheric species. The aim wasto characterize the physico-chemical properties of these aerosols, which play a major role in climate forcing, in particular through their ability to act as cloud condensation nuclei.First, molecular dynamics techniques have been applied to determine the structure of carboxylic acid aggregates on which water molecules are adsorbed. The results of our simulations show an influence of the temperature, of the humidity and of the type of carboxylic acid considered on the global behavior of the aggregates. By contrast,a mixture of different types of acid molecules has no influence on this behavior. Secondly, quantum molecular simulation techniques have been applied to study the chlorination of soot surfaces, modeled by large PAHs, and their behavior with respect to water molecules adsorption. The results show a strong propensity of Cl, Cl2and HCl species to form chlorinated PAHsthanks to the presence of structural defects.These chlorinated structures show as trong hydrophilicity, which may explain the strong hygroscopic nature of soots emitted by industrial fires.

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Les verres de chalcogénure sont ont une grande importance dans des applications optoélectroniques, mais aussi pour leur réseau covalent qui leur procure une "variabilité structurale" accrue. Ce projet étudie des alliages Ge-Se et utilise l'association d'analyses dynamique moléculaire (AIMD), de simulations et de diffusion des rayons X afin de déterminer la structure et la dynamique des phases liquide et vitreuse sous pression. Les alliages de Ge-Se permettent la "formation de verre" sur une large gamme de conditions, ce qui permet de contrôler la rigidité du réseau en augmentant le nombre moyen de coordination en fonction de la teneur en Ge ce qui affectera les propriétés macroscopiques du matériau, comme la résistance au vieillissement, la dureté, la conductivité et la fragilité. Dans cette optique, nous étudions 10 compositions générées/calculées par AIMD, et 5 autres compositions déterminées expérimentalement (i.e par trempe ) couvrant les domaines transitions élastiques souple-rigide suivant les critères de stabilité isostatique de Maxwell. Dans le cas de l'état liquide, la comparaison au préalable des données de la expérimentale permet de valider les modèles structuraux, pour ensuite examiner la dynamique des liquides de Ge-Se à pression ambiante. En partant du postulat que la mobilité atomique du système décroit en devenant plus rigide, l'étude des coefficients de diffusion et de la viscosité à 1050 K a montré des anomalies flagrantes... / Among network forming glasses, chalcogenide glasses are of great importance not only for their optoelectronic applications, but also for the network structure that displays enhanced structural variability due to the covalent bonding network. In this project, we study the Ge-Se binary alloy as the target system in order to investigate the structure and dynamics of the liquid and glassy phases under pressure by using a combination of ab initio molecular dynamics (AIMD) simulations and X-ray scattering experiments...

The application of commercial herbicide restricts weed growth and significantly improves control over crop vitality and yield. Despite their utility in the agriculture sector, herbicides have the potential to contaminate local water sources. To minimize environmental impacts, the development of efficient separation processes to clean-up contaminated water bodies is necessary. However, complex speciation and conformational flexibility in the condensed phase poses a significant challenge. In this work, we investigate structure and speciation of three common organic herbicides (glyphosate, atrazine, and metolachlor) in aqueous solution. We employ the PBE-D3 density functional to perform ab initio molecular dynamics (MD) simulations in the canonical and isothermal-isobaric ensembles. We analyze MD trajectories to understand hydrogen bonding dynamics and lifetime as well as diffusional and vibrational characteristics. To enhance configurational sampling, we conduct metadynamics simulations to obtain the free energies of dissociation and intramolecular proton transfer of glyphosate.

This work 1) draws insights on molten salt structure and properties directly from ab initio molecular dynamics (AIMD) simulations, 2) demonstrates the advantageous symbiosis of computational and experimental collaborations on molten salt research, and 3) simultaneously generates ab initio data sets for fitting an interatomic potential model for classical molecular dynamics (MD) simulations. This work discusses the motivations for AIMD simulations of molten salts, thermophysical properties and structural characteristics of interest, advanced methodologies for AIMD simulations, and several completed AIMD studies on molten salts. Of import are the methodological contributions of this work to AIMD simulations, primarily the radical increase in generalized gradient planewave energy cutoff used to more accurately model the electron distribution across a highly-polarizable molten salt. Cutoffs of up to 2500 Rydbergs are used in this work, but 2000 Rydbergs is found to be sufficient for most AIMD NpT modelling of molten fluorides. The equilibrium liquid density of eutectic FLiNaK as a function of temperature is found to agree with the experimental density reported by Chrenkova et al. to within 0.2%, and the equilibrium liquid density of eutectic FMgNaK is found to agree with experimental measurements reported herein to within 4%. Self-diffusion coefficients in FMgNaK are also considered, with applicability to other halide salts. Molybdenum, Cesium, iodide, nickel, hydrogen, oxide, and uranium complexation are examined. It is found that solvation strength can be qualitatively determine via AIMD simulations, and that poorly solvated solutes will minimize the surface area of interaction with the salt solution. Cesium in particular is shown to be volatile or retainable in FLiBe at 500, 650, and 800 ËšC based on complexation and validated experimentally. It is shown that the chemical potential of an anion varies between melts as influenced by the different cations present in each melt. Hence, attempts to use a common electrochemical reference reaction for different salt mixtures are at best an approximation.

This work demonstrates how ICP-MS can be employed to evaluate cesium volatilization from LiF-BeF2 (Flibe) salt with ab initio molecular dynamics studies used as corroborating data to better understand cesium behavior. Using mixtures of Flibe with 2 mol% CsF, it was found that cesium was stable within the salt melt at temperatures between 500-650°C over a time span of 8 hours. At 800°C, cesium vaporized from the salt at a rate of 0.83% / hr with a mass flux of 0.0023 g Cs / cm2hr. The atomistic modeling results show poor solvation of Cs at 500°C and 800°C, with stability preferred at 650°C. Specialized equipment and procedures were needed to enable this work, especially those required for safe handling of beryllium containing salts. The methods, custom equipment, and important considerations for working with high-temperature fluoride salts are detailed in this thesis.

A fundamental quantum mechanical modelling approach is used for calculating liquid diffusion parameters in cemented carbides. Up to now, no detailed description of diffusion for alloying elements in a liquid Co matrix is available. Neither are experimental measurements found in the literature for the self- or impurity diffusion in the liquid Co system. State of the art application is the description of gradient formation in cemented carbide systems using DICTRA. In this work it is assumed that diffusion during sintering of cemented carbides takes place mainly in the liquid Co binder phase. With this assumption one can calculate the diffusion coefficient for different alloying elements like W, Ti, N and C in a liquid Co matrix phase. The mean square displacement (MSD) of the diffusing atoms is used to obtain the diffusion coefficients which could be simulated by Ab initio Molecular Dynamics (AIMD). By fitting the computed temperature dependence with the Arrhenius relation one can determine the frequency factor and the activation energy which allows to give a quantitative description of the diffusion. Three methods will be used for validating the data from this work. Available estimated literature values based on calculations (scaling laws, a modified Sutherland equation and classical molecular dynamics) will be used to compare the results in a first instance. The general agreement for diffusion in liquid metals will be done by comparison with experimental data for the liquid Fe system. In a last step, the diffusion values obtained by this work will be used to create a kinetic database for DICTRA. The gradient simulations will be compared with experimentally measured gradients. The AIMD simulations are performed for binary diffusion systems to investigate the diffusion between the liquid Co matrix and one type of alloying element. In a second approach the diffusion for a multicomponent systems with Co, W, Ti and C has been performed. The results from the present AIMD simulations could be shown to be in good agreement with the literature. Only two DICTRA simulations could be performed within the timeframe of this work. Both are predicting a ~3 times bigger gradient zone whereas the initial choice of the labyrinth factor λ = f could be identified as a possible source of disagreement. A labyrinth factor of λ = f2 with the calculated mobility values from the AIMD calculations should give improved results. Although the results from those simulations are not available to this date. The two approaches of the diffusion simulations in the binary and multicomponent system are giving matching results. The non-metallic elements C and N are diffusing two times faster than the fastest metallic element Co. The diffusivity of Ti is slightly lower than Co and W could be identified as the element with the slowest diffusion within the liquid Co matrix. Further investigations of the liquid structure could indicate the tendency to form bonds between C and W and between C and Ti. This gives slowed down diffusion of C in the multicomponent system compared to the diffusion in the binary Co-C system.

<p dir="ltr">A major goal of applied electrocatalysis research has been the development of electrode materials that are active, selective, stable, and cost effective in producing electricity or desired products. In recent years, developments in <i>ab initio</i> methods for the simulation of catalyst surfaces, and electrochemical reactions occurring on them, have enabled the development of a fundamental understanding of the processes occurring at the solid-liquid interface at an atomistic scale. In combination with experiments, these calculations are helpful in elucidating design principles that can then inform electrocatalyst design. In this work, we describe the application of density functional theory, <i>ab initio</i> molecular dynamics, and high throughput materials informatics approaches to understand oxygen and carbon based electrochemistries, with relevance to electricity conversion and environmental protection. We also introduce an approach, based on a Born-Haber cycle analysis, to quantify adsorbate stabilization from solvent molecules that are ubiquitous for any electrochemical reaction occurring at solid-liquid interfaces.</p><p dir="ltr">The oxygen reduction reaction (ORR) occurs at the cathode in hydrogen fuel cells and, in conjunction with the hydrogen oxidation reaction (HOR) at the anode, produces electricity and water. While platinum group metals are the current state-of-the-art catalysts for the ORR, their high cost has necessitated an extensive search for alternatives. To this end, we investigated iron-nitrogen-carbon (Fe-N-C) catalysts, which are platinum group metal-free and have been shown experimentally to have reasonable activity compared to platinum. Despite their potential as cost effective materials, however, these catalysts are not durable over long-term operation of fuel cells, impeding their commercial adoption. The mechanisms of deactivation of the iron-nitrogen-carbon catalysts under aqueous acidic electrochemical reaction conditions remain debated, and deciphering them is complicated due to the complex structure of the catalyst. We attempt to address these challenges by first examining the structural aspects of the catalyst, sampling numerous potential active site configurations, determining their in-situ structure, and linking them to intrinsic activity and intrinsic stability descriptors. Our findings reveal that an activity-stability tradeoff exists, with the most active sites being most prone to stability issues. Additionally, we explored the role of hydrogen peroxide, a side product of ORR, in degrading Fe-N-C catalysts. This analysis demonstrated that hydrogen peroxide strongly oxidizes the catalyst surface, resulting in an activity loss in the catalyst. Based on these insights, we propose design principles to enhance the activity and stability of Fe-N-C catalysts.</p><p dir="ltr">In additional work, we compared the predictions for the Fe-N-C catalysts with ORR analysis on platinum catalysts, and we further analyzed the oxygen evolution reaction (OER) on iridium oxides and the carbon dioxide reduction reaction (CO₂R) on copper catalysts in water electrolyzers. For ORR on platinum, we identified the formation of hydroxyl and water adsorbate rings on stepped surfaces, akin to hexagonal rings found on terraces but largely absent on Fe-N-C catalysts. The ORR follows an associative mechanism involving proton coupled electron transfer to these ring structures. Furthermore, we provided activity descriptors that aligned with experimental observations, showing a higher activity on stepped surfaces compared to terraces. For OER on iridium oxides, we examined transformations of IrO₂ (110) surfaces, and we pinpointed oxidation of bridge and coordinatively unsaturated top sites as key charge transfer steps that correlate with peaks in cyclic voltammograms. Finally, for CO₂R on copper, we investigated the role of water as a proton source under neutral or alkaline conditions, providing insights into the effect of coverages of surface species on the kinetics of water dissociation that, in turn, can provide protons for CO₂ reduction and the competing hydrogen evolution reaction.</p><p dir="ltr">Through this work, we have gained a deeper understanding of the properties of various catalytic materials under conditions specific to each type of electrochemistry. We elucidated the relationships between the in-situ structure, activity, and stability for the electrocatalysts, and identified key factors influencing catalyst performance. Integrating such insights from a computational perspective with experimental approaches holds great potential in making significant advancements in developing sustainable energy technologies and ultimately contributing to a greener and more energy-efficient future.</p>