Vidros comerciais de base silicato do tipo sodo-cálcico, apresentam coloração pela presença de óxidos de metais de transição, tais como ferro e cobalto, ou pela presença de elementos não-metálicos, tais como o selênio. Geralmente, a presença de selênio metálico (Se°) num vidro silicato confere-lhe a cor rosa (depende de seu estado de oxidação). Desse modo, industrialmente, adiciona-se selênio à composição de vidros quando se deseja produzir vidros róseos ou, mais freqüentemente, quando se deseja mascarar a cor esverdeada conferida pela presença de ferro, o principal contaminante das matérias-primas naturais. Entretanto, o selênio é encontrado em pequenas quantidades na natureza, daí seu alto custo. Além disso, da quantidade inicial de selênio colocada na composição de vidros industriais sodo-cálcicos, quase 80% são vaporizados durante a etapa de fusão do vidro. Este trabalho é dedicado ao estudo do efeito da adição de diferentes compostos portadores de selênio como matéria-prima em substituição ao selênio metálico, visando a redução da volatilização de selênio durante a fusão. Além do selênio metálico, foram utilizados selenitos de sódio, de bário, de zinco e de ferro. Os vidros foram obtidos por fusões em escala de laboratório, em cadinhos de alta alumina, a 1500°C, em forno elétrico. A seguir determinou-se a composição química de cada vidro obtido, por fluorescência de raios X e sua cor, através da determinação de suas coordenadas cromáticas no sistema CIEL*a*b*. A análise dos resultados teve como principal conclusão que a concentração final de selênio no vidro é função apenas de sua quantidade no banho, independentemente do composto químico que lhe forneceu. / Commercial silicate glasses of the soda-lime system attain color by the presence of transition metal oxides, such as iron and cobalt, or by the presence of non-metallic elements, like selenium. Usually, the presence of metallic selenium (Se°) in a silicate glass results in a light pink coloration (depends on its oxidation state). Therefore, industrially, selenium is added to a glass composition when the objective is to obtain a pink color glass, or, more often, when the objective is to mask the greenish coloration resultant from the presence of iron, the most usual contaminant in the natural raw-materials. However, selenium occurs in the nature only in small quantities. Besides, from the initial selenium added to industrial soda-lime batch compositions, almost 80% volatilize during melting. This work is dedicated to the study of the effect of adding selenium from different raw-materials, substituting for the metallic selenium, with the principal objective of reducing the loss of this element during glass melting. Along with metallic selenium, sodium, barium, zinc, and iron selenites were employed. The glasses were obtained in laboratory scale meltings in alumina crucibles, at 1500°C, in an electrical furnace. After that, the chemical composition of each glass was obtained by X-ray fluorescence, and its color was measured according to the chromatic coordinates in the CIEL*a*b* method. The main conclusion that can be drawn from the analysis of the results is that the selenium concentration in the final glass is a function only of the quantity of selenium present in the melt, independently of the chemical compound which supplied such selenium.

Vidros comerciais de base silicato do tipo sodo-cálcico, apresentam coloração pela presença de óxidos de metais de transição, tais como ferro e cobalto, ou pela presença de elementos não-metálicos, tais como o selênio. Geralmente, a presença de selênio metálico (Se°) num vidro silicato confere-lhe a cor rosa (depende de seu estado de oxidação). Desse modo, industrialmente, adiciona-se selênio à composição de vidros quando se deseja produzir vidros róseos ou, mais freqüentemente, quando se deseja mascarar a cor esverdeada conferida pela presença de ferro, o principal contaminante das matérias-primas naturais. Entretanto, o selênio é encontrado em pequenas quantidades na natureza, daí seu alto custo. Além disso, da quantidade inicial de selênio colocada na composição de vidros industriais sodo-cálcicos, quase 80% são vaporizados durante a etapa de fusão do vidro. Este trabalho é dedicado ao estudo do efeito da adição de diferentes compostos portadores de selênio como matéria-prima em substituição ao selênio metálico, visando a redução da volatilização de selênio durante a fusão. Além do selênio metálico, foram utilizados selenitos de sódio, de bário, de zinco e de ferro. Os vidros foram obtidos por fusões em escala de laboratório, em cadinhos de alta alumina, a 1500°C, em forno elétrico. A seguir determinou-se a composição química de cada vidro obtido, por fluorescência de raios X e sua cor, através da determinação de suas coordenadas cromáticas no sistema CIEL*a*b*. A análise dos resultados teve como principal conclusão que a concentração final de selênio no vidro é função apenas de sua quantidade no banho, independentemente do composto químico que lhe forneceu. / Commercial silicate glasses of the soda-lime system attain color by the presence of transition metal oxides, such as iron and cobalt, or by the presence of non-metallic elements, like selenium. Usually, the presence of metallic selenium (Se°) in a silicate glass results in a light pink coloration (depends on its oxidation state). Therefore, industrially, selenium is added to a glass composition when the objective is to obtain a pink color glass, or, more often, when the objective is to mask the greenish coloration resultant from the presence of iron, the most usual contaminant in the natural raw-materials. However, selenium occurs in the nature only in small quantities. Besides, from the initial selenium added to industrial soda-lime batch compositions, almost 80% volatilize during melting. This work is dedicated to the study of the effect of adding selenium from different raw-materials, substituting for the metallic selenium, with the principal objective of reducing the loss of this element during glass melting. Along with metallic selenium, sodium, barium, zinc, and iron selenites were employed. The glasses were obtained in laboratory scale meltings in alumina crucibles, at 1500°C, in an electrical furnace. After that, the chemical composition of each glass was obtained by X-ray fluorescence, and its color was measured according to the chromatic coordinates in the CIEL*a*b* method. The main conclusion that can be drawn from the analysis of the results is that the selenium concentration in the final glass is a function only of the quantity of selenium present in the melt, independently of the chemical compound which supplied such selenium.

Cette thèse porte sur l'étude et le comportement mécanique et vibrationnel du verre de silice. Des méthodes de dynamique moléculaire classique sont appliquées pour modéliser le verre à l'aide d'un potentiel BKS tronqué. La validité du modèle est testée au travers de comparaisonsstructurales et dynamiques avec des expériences de diffusion de rayons X et de neutrons. L'échantillon numérique est sollicité mécaniquement, et sa réponse à la déformation (compression hydrostatique, cisaillement à volume ou à pression cnostante dans les régimes élastiques et au-delà de la limite d'élasticité) est étudiée dans le cadre de la théorie classique de l'élasticité. L'utilisation de la dynamique moléculaire nous a permis de nous orienter vers une approche microscopique via l'étude du déplacement non-affine, qui semble expliquer des comportements macroscopiques encore peu décrits dans la littérature. En particulier, l'origine de l'anomalie du module de compressibilité dans la silice a pu être reliée à un comortement micro-plastique, et la courbe de charge (limite du domaine élastique) a été obtenue. En complément aux chargements mécaniques, des études expérimentales de spectroscopie Raman sous cellule enclume diament ont été réalisées et comparées aux spectres Raman modélisés à partir de configuration de silice chargées mécaniquement. Enfin, nous avons ou discuter de la validité de la description théorique ainsi que ses limites. / This thesis discusses the mechanical and vibrationnal behaviour of silica glasses. Here, classical molecular dynamics methods are used to model a glass using asmoothed and truncated BKS potential. The model is validated through structural and dynamical comparisons with X-rays and neutrons scattering experiments. The numerical sample in mechanically loaded, and the response to a strain (hydrostatic compression, shear at constant volume or pressure in the elastic domain and beyond) is studied in the frame of the classical theroy of elasticity. The use of classical molecular dynamics gave us a microscopic approach via the non-affine displacement study, to explain some macroscopic behaviour, which is not yet developed much in literature. In particular, we illustrated the origins of the bulk modulus anomaly of silica glasses because of its micro-plastic behaviour. we also obtained the yield surface (boundary between elastic and plastic behaviour) of silica glasses. Moreover, Raman spectroscopy is performed under diamond anvil cell, and the results were compared with the Raman spectra of numerically loaded glasses. At last; we discussed the validity of theoritical description and the limitation.

Les matériaux amorphes sont omniprésents dans la vie quotidienne. Ils comprennent des verres "dures" et "mous". Le systèmes amorphes durs sont généralement considérés comme des matériaux de structure, dont les propriétés et l'utilisation sont comparable à celles des solides cristallins. Les verres mous sont généralement considérés comme des fluides complexes, décrits pour leurs propriétés rhéologiques et les applications correspondantes pratiques. Les matériaux amorphes peuvent soit présenter un comportement de type solide ou de flux en fonction de leur charge mécanique: tous sont des fluides à limite apparente d'élasticité. Leurs limites d'utilisation sont souvent définies par l'apparition de bandes de cisaillement, une forme extrême de localisation vu dans des verres moléculaires ainsi que dans les matériaux granulaires. Il ya maintenant des preuves considérables que ce sont les conséquences de l'existence d'une structure désordonnée au niveau des constituants élémentaires. Les études de la plasticité des solides amorphes, sont encore gênés par l'absence de tout défaut identifiable responsable de la réponse plastique. Il est maintenant reconnu que la plasticité est le résultat net des réarrangements locaux, ou "transformations de cisaillement", impliquant des petits groupes de particules. Ces réarrangements sont thermiquement - activées et sont ubiquitaires dans le processus de relaxation de déformation structurelle des verres à basse température. Malheureusement, ils se déroulent sur des échelles de temps long par rapport à ceux qui sont accessibles aux simulations de dynamique moléculaire. Certains nouveaux outils très prometteurs, cependant, ouvrent la voie vers des algorithmes accélérés pour la simulation de systèmes thermiques. Ils sont basés sur les méthodes numériques développées au cours de ces deux dernières décennies pour déterminer les transitions thermiquement activés dans les systèmes atomiques. Un intérêt particulier ici est la technique d'activation-relaxation (ART). Dans cette étude, nous allons montrer que, même si une recherche exhaustive des points de selle pour des solides désordonnés est impossible, ART peut identifier assez de points de selles pour construire des échantillons statistiquement pertinents, à partir desquelle des distributions stationnaires peuvent être calculées. Le but de cette thèse strictement numérique était de prédire les cinétiques thermiquement activées dans des verres telles que celles rencontrées expérimentalement. La nature de ces événements microscopique qui se produisent dans miscroscopic systèmes désordonnés a été étudiée à la fois sous des contraintes mécaniques et dans des conditions de vieillissement. Nous étudions deux grandeurs décrivant ces événements au sein de l'approximation harmonique de la théorie de l'état de transition, c'est à dire l'énergie d'activation et la fréquence d'attaque. Etant donné que dans la définition d'une fréquence d'attaque la courbure du minimum initial et le point de selle sont présents, nous voulions voir (pour les verres métalliques et verres de silice) s'il y avait une relation entre les fréquences de tentative et les énergies d'activation d'un événement donné. Cette corrélation a été précédemment observée pour un large éventail de phénomènes et est appelé la règle de compensation Meyer-Neldel. Nous aussi tentons de répondre si le simple potentiel BKS sans sommation d'Ewald est capable de reproduire le polyamorphisme observé dans les verres de silice soumis à une compression hydrostatique. Outre les processus activés thermiquement, les analyses structurelles de verres métalliques et de silice ont été réalisées. Les ordres de courte et moyenne portée ont été caractérisées par deux méthodes: pavages de Voronoı pour les verres métalliques, nous fournissant des informations sur les conformations voisines proches, et dans le cas de la silice, les statistiques de distributions de chaînes moléculaires. / Amorphous materials are ubiquitous in everyday life. They comprise "hard" and"soft" glasses. Hard amorphous systems are usually seen as structure materials, with properties and use comparable to those of crystalline solids. Soft glasses are usually seen as complex fluids, described in terms of their rheological properties with the corresponding practical applications (concrete, paints, drilling mud, cosmetic gels, creams or foams, etc). Amorphous materials can either present a solid-like behaviour or flow depending on their mechanical load: all are yield-stress fluids. Their usage limits are often defined by the occurrence of shear-banding, an extreme form of localization seen in molecular glasses as well as in granular materials. There is now considerable evidence that they are consequences of the existence of a disordered structure at the level of the elementary constituents (atoms, particles,...). Studies of plasticity in amorphous solids, are still hampered by the lack of any identifiable defect responsible for the plastic response. It is now acknowledged that plasticity is the net result of local rearrangements, or "shear transformations", involving small clusters of (say a few tens of) particles. These rearrangements are thermally--activated and are ubiquitous processes in the structural relaxation and deformation of glasses at low temperatures. Unfortunately, they take place over timescales long compared to those accessible to direct Molecular Dynamics simulations. Some extremely promising new tools, however, are opening the route towards accelerated algorithms for the simulation of thermal systems. They are based on numerical methods developed over these last two decades to determine thermally activated transitions in atomic systems. Of particular interest here is the Activation-Relaxation Technique (ART), an eigenvector-following method that allows the identification of activated states and paths in the potential energy landscape of atomic systems. In this study, we will show that although an exhaustive search for saddle points in case of disordered solids is unfeasible (because of the exponential number of activated states), ART can identify enough saddles to build statistically relevant samples, from which stationary distributions can be computed. The purpose of this strictly numerical thesis was the prediction of thermally activated kinetics in glasses such as those encountered experimentally. The nature of such miscroscopic events occuring in disordered systems was studied both under mechanical stress and in ageing conditions. We investigate two quantities that describe thermally-activated events within the harmonic approximation of the transition state theory, i.e. activation energy and attempt frequency.Since in the definition of an attempt frequency the curvature of the initial minimum and the saddle point are present, we wanted to see if there was a relation between attempt frequencies and activation energies of a given event in two types of systems: metallic glasses and silica glasses. Such correlation had been observed before for a wide range of phenomena and is referred to as the Meyer-Neldel compensation rule. We also attempt to answer if the simple BKS potential without Ewald summation is able to reproduce polyamorphism observed in silica glasses subject to hydrostatic compression and characterized mainly in terms of coordination numbers. Apart from thermally activated processes, the structural analyses of metallic and silica glasses were performed. The short and medium range orders were characterized using two methods: Voronoi tesselations for metallic glasses, providing us information about near neighbor conformations, and in case of silica, statistics of ring distributions.

Estudos sistemáticos utilizando a técnica de SAXS foram realizados no síncrotom do LURE (Orsay) com feixe intenso de geometria pontua:, para descrever a cinética e as estruturas obtidas durante as etapas de transformação sol -> gel úmido -> gel seco -> vidros de sílica . As análises foram feitas em termos de uma lei de potência I = q-? cujo expoente está relacionado a dimensão fractal de massa ou de superfície das estruturas. Devido a polidispersividade das unidades espalhadoras as dimensões fractais verificadas podem ser resultados de dimensões reais encobertas por uma distribuição de tamanhos. As curvas obtidas para a cinética de gelificação e envelhecimento dos géis úmidos mostram expoentes que indicam fractalidade de massa e, posteriormente, de superfície. Uma microestrutura composta, que pode resultar de uma agregação limitada por difusão (DLA) seguido de agregação de agregados (C-C) foi observado nas soluções básicas. Por outro lado, os agregados ácidos parecem surgir do crescimento contínuo de cadeias até atingir a interligação com outros agregados com uma cinética que pode ser descrita pelo modelo DLA. No estado de aerogeis outras técnicas como picnometria de mercúrio e hélio, microscopia eletrônica de transmissão e adsorção de gás de nitrogênio, foram também aproveitadas. Nestes materiais encontramos em uma faixa estreita de escala, um expoente indicando fractalidade de massa, embora com superfície lisa. Os resultados estão de acordo com um modelo de estrutura que tem uma matriz de SiO2 densa. A estrutura fractal desaparece durante o tratamento térmico de densificação devido ao rearranjo estrutural. / Systematic SAXS studies have been performed at the LURE synchrotron (Orsay) using an intense beam of point-like cross-section to describe the kinetic and the structure obtained during the sol-> humid gel -> dry gel ->silica glass. The analysis were done in terms of a power law, I = q-? , whose exponent is related to mass and surface fractal dimensions of the structures. Due to polidispersity of the scattering units the found fractal dimensions can be \"smeared\" dimensions that result from a size distribution. All the kinetic and aging humids curves showed evidenced of fractal structures. A composed microstructure that can be originated by clustering of clusters (C-C) of primary units grown by diffusion limited-aggregation (DLA), was observed for basic gels. By other hand, the acidic cluster seems to grow continuously reaching gelation through crosslinking of the clusters. The DLA model seems to describe this process. In the aerogels study others techniques like density measurements using Hg and He, TEM and adsorption of N2 gas were done along with SAXS measurements. Here we could find a narrow range in which the system is a mass fractal, although its surface has been already smoothened. The results agree with a dense SiO2 matrix model. During the densification no fractal structure was verified due to structural rearrangements at high temperature.

Estudos sistemáticos utilizando a técnica de SAXS foram realizados no síncrotom do LURE (Orsay) com feixe intenso de geometria pontua:, para descrever a cinética e as estruturas obtidas durante as etapas de transformação sol -> gel úmido -> gel seco -> vidros de sílica . As análises foram feitas em termos de uma lei de potência I = q-? cujo expoente está relacionado a dimensão fractal de massa ou de superfície das estruturas. Devido a polidispersividade das unidades espalhadoras as dimensões fractais verificadas podem ser resultados de dimensões reais encobertas por uma distribuição de tamanhos. As curvas obtidas para a cinética de gelificação e envelhecimento dos géis úmidos mostram expoentes que indicam fractalidade de massa e, posteriormente, de superfície. Uma microestrutura composta, que pode resultar de uma agregação limitada por difusão (DLA) seguido de agregação de agregados (C-C) foi observado nas soluções básicas. Por outro lado, os agregados ácidos parecem surgir do crescimento contínuo de cadeias até atingir a interligação com outros agregados com uma cinética que pode ser descrita pelo modelo DLA. No estado de aerogeis outras técnicas como picnometria de mercúrio e hélio, microscopia eletrônica de transmissão e adsorção de gás de nitrogênio, foram também aproveitadas. Nestes materiais encontramos em uma faixa estreita de escala, um expoente indicando fractalidade de massa, embora com superfície lisa. Os resultados estão de acordo com um modelo de estrutura que tem uma matriz de SiO2 densa. A estrutura fractal desaparece durante o tratamento térmico de densificação devido ao rearranjo estrutural. / Systematic SAXS studies have been performed at the LURE synchrotron (Orsay) using an intense beam of point-like cross-section to describe the kinetic and the structure obtained during the sol-> humid gel -> dry gel ->silica glass. The analysis were done in terms of a power law, I = q-? , whose exponent is related to mass and surface fractal dimensions of the structures. Due to polidispersity of the scattering units the found fractal dimensions can be \"smeared\" dimensions that result from a size distribution. All the kinetic and aging humids curves showed evidenced of fractal structures. A composed microstructure that can be originated by clustering of clusters (C-C) of primary units grown by diffusion limited-aggregation (DLA), was observed for basic gels. By other hand, the acidic cluster seems to grow continuously reaching gelation through crosslinking of the clusters. The DLA model seems to describe this process. In the aerogels study others techniques like density measurements using Hg and He, TEM and adsorption of N2 gas were done along with SAXS measurements. Here we could find a narrow range in which the system is a mass fractal, although its surface has been already smoothened. The results agree with a dense SiO2 matrix model. During the densification no fractal structure was verified due to structural rearrangements at high temperature.