A síntese de vidros pelo método sol-gel tornou-se recentemente de grande interesse, tanto do ponto de vista técnico como científico. Descrevemos neste trabalho o processo utilizado para a obtenção de aerogel de sílica (formação do gel &#8594 secagem hipercrítica &#8594 sinterização) e algumas características físicas e químicas, em diversas temperaturas de sinterização (Ts) dos aerogéis produzidos. Aerogeis de sílica foram preparados pela hidrólise e condensação de sóis de sistema TMOS metanol- H2O com razão molar TMOS: H2O = 4: 1 e razão em volume de TMOS na solução TMOS metanol = 0,2; 0,3; 0,4; 0,5 e 0,6. As suas propriedades estruturais tais como; a densidade aparente e da matriz, a área superficial BET, volume total dos poros, distribuição de tamanho dos poros, constante e perda dielétricas foram sistematicamente investigadas em função do tratamento térmico de sinterização. As propriedades de relaxação da água adsorvida na superfície altamente reativa foram estudadas através das técnicas dielétricas e de ressonância magnética nuclear em função do conteúdo de H2O, 0 &#60 &#952H2O &#60 6, e temperatura. Também foi feito um estudo da cinética de adsorção da água em aerogéis com varias temperaturas de sinterização. / The synthesis of glasses using the sol-gel method appeared recently of great interest either from the technological or from the scientific point of view. We describe the process used to obtain pure vitreous silica (gel formation &#8594 hypercritical drying &#8594 densification) as well as some physical and chemical characteristics of the various products fabricated. Silica aerogels have been prepared by hydrolysis and condensation of sols of composition TMOS methanol - H2O with molar ratio TMOS: H2O = 4.1 and volume ratio of TMOS in solution of TMOS-methanol = 0,2; 0,3; 0,4; 0,5 and 0,6. Their structural properties such as true and apparent densities, BET surface area, total pore volume, pore size distribution, dielectric constant and loss have been systematically investigated as a function of the densification heat treatment. The relaxation properties of water adsorbed on the highly reactive surface have been studied by dielectric and nuclear magnetic resonance techniques as a function of the H2O coverage (0 &#60 &#952H2O &#60 6) and temperature. The adsorption kinetics of the water was studied too in the aerogels with several heat temperature of sinterization.

Les aérogels de silice se classent parmi les matériaux dits superisolants grâce à leurs propriétés thermiques exceptionnelles. Leur très faible conductivité thermique (< 15 mW.m-1.K-1) représente un pouvoir d’isolation convoité par de nombreux secteurs d’activité comme l’isolation thermique du bâtiment, l’aérospatial, le transport, l’emballage, etc.Ces matériaux amorphes nanostructurés atteignent des taux de porosité supérieurs à 90% et sont architecturés sur plusieurs ordres de grandeur. Leur structure se compose d’un réseau poreux tridimensionnel de silice à l’échelle nanométrique. Celui-ci forme un agrégat de plusieurs centaines de nanomètres qui est lui-même la brique élémentaire d’un second réseau poreux à l’échelle mésoscopique. L'architecture 3D nanométrique imbriquée pores / particules, dont la taille est proche du libre parcours moyen des phonons, est à l’origine des propriétés thermiques exceptionnelles des aérogels de silice, cependant, elle est aussi la source de leurs faibles propriétés mécaniques.Cette thèse se focalise sur l’étude à l’échelle nanométrique du comportement mécanique des aérogels de silice par dynamique moléculaire. La première partie porte sur le potentiel interatomique utilisé comme paramètre d’entrée des simulations et plus particulièrement sur sa capacité à reproduire les surfaces de silice amorphe. Ce potentiel, développé pour modéliser les propriétés structurales et énergétiques de la silice amorphe dense, démontre une bonne transférabilité quant aux propriétés de surface. Il permet de simuler avec fiabilité des matériaux de grandes surfaces spécifiques avec des temps de calcul acceptables. La seconde partie s'intéresse successivement à la génération de la nanostructure des aérogels, à la caractérisation des textures formées et à l'étude du comportement mécanique. Les résultats montrent l’influence de la vitesse de déformation et des effets de taille sur l’étude du comportement mécanique en traction et en compression. Les propriétés élastiques sont correctement caractérisées et les mécanismes de déformation identifiés. Enfin des agrégats sphériques de l’ordre de la centaine de nanomètres sont générés afin d’étudier leur comportement mécanique sous compression. Les lois de comportement de ces agrégats, comparables en taille à ceux observés expérimentalement, pourront ensuite servir de paramètres d’entrée et nourrir les simulations aux échelles supérieures / Thanks to their exceptional thermal properties, silica aerogels are considered as superinsulating materials. Their very weak thermal conductivity (< 15 mW.m-1.K-1) stands for an important insulating power which has many industrial applications such as building insulation, aerospacial, transport, packaging, etc.Those amorphous architectured materials can reach more than 90% of porosity and present a porous architecture at different length scales. Their structure is composed by a three dimensional porous and tortuous network at nanometric scale. This network forms an aggregate of a hundred nanometers length which is also the particle of a second porous network at mesoscopic scale. The 3D nanometric architecture of pores / particles, is at the origin of the exceptional thermal properties but is also responsible of the weak mechanical properties.This thesis focuses on the study at nanoscale of the mechanical behavior of silica aerogels using Molecular Dynamics simulations. The first part of the thesis concerns the interatomic potential used as parameter of the simulations and his capacity of reproducing amorphous silica surface properties. This potential has been developped for dense amorphous silica. It also shows a good transferability to model amorphous silica surface properties. It can be used to simulate large volumes of high specific area materials with optimized computational time. The purpose of the second part of this thesis is to generate the porous nanostructure of silica aerogels, to characterize its structure and to study its mechanical properties. The results show the influences of the strain rate and of the simulation box size on the mechanical behavior during tensile and compression tests. Elastic properties are correctly computed and the deformation mechanisms are identified. Finally, hundred nanometers aggregates are generated and their behavior under compression is studied. The behavior laws of these aggregates, comparable in length with the experimental ones, provide precious information for a multiscale model

abstract: Emergent environmental issues, ever-shrinking petroleum reserves, and rising fossil fuel costs continue to spur interest in the development of sustainable biofuels from renewable feed-stocks. Meanwhile, however, the development and viability of biofuel fermentations remain limited by numerous factors such as feedback inhibition and inefficient and generally energy intensive product recovery processes. To circumvent both feedback inhibition and recovery issues, researchers have turned their attention to incorporating energy efficient separation techniques such as adsorption in in situ product recovery (ISPR) approaches. This thesis focused on the characterization of two novel adsorbents for the recovery of alcohol biofuels from model aqueous solutions. First, a hydrophobic silica aerogel was evaluated as a biofuel adsorbent through characterization of equilibrium behavior for conventional second generation biofuels (e.g., ethanol and n-butanol). Longer chain and accordingly more hydrophobic alcohols (i.e., n-butanol and 2-pentanol) were more effectively adsorbed than shorter chain alcohols (i.e., ethanol and i-propanol), suggesting a mechanism of hydrophobic adsorption. Still, the adsorbed alcohol capacity at biologically relevant conditions were low relative to other `model' biofuel adsorbents as a result of poor interfacial contact between the aqueous and sorbent. However, sorbent wettability and adsorption is greatly enhanced at high concentrations of alcohol in the aqueous. Consequently, the sorbent exhibits Type IV adsorption isotherms for all biofuels studied, which results from significant multilayer adsorption at elevated alcohol concentrations in the aqueous. Additionally, sorbent wettability significantly affects the dynamic binding efficiency within a packed adsorption column. Second, mesoporous carbons were evaluated as biofuel adsorbents through characterization of equilibrium and kinetic behavior. Variations in synthetic conditions enabled tuning of specific surface area and pore morphology of adsorbents. The adsorbed alcohol capacity increased with elevated specific surface area of the adsorbents. While their adsorption capacity is comparable to polymeric adsorbents of similar surface area, pore morphology and structure of mesoporous carbons greatly influenced adsorption rates. Multiple cycles of adsorbent regeneration rendered no impact on adsorption equilibrium or kinetics. The high chemical and thermal stability of mesoporous carbons provide potential significant advantages over other commonly examined biofuel adsorbents. Correspondingly, mesoporous carbons should be further studied for biofuel ISPR applications. / Dissertation/Thesis / M.S. Chemical Engineering 2011

A síntese de vidros pelo método sol-gel tornou-se recentemente de grande interesse, tanto do ponto de vista técnico como científico. Descrevemos neste trabalho o processo utilizado para a obtenção de aerogel de sílica (formação do gel &#8594 secagem hipercrítica &#8594 sinterização) e algumas características físicas e químicas, em diversas temperaturas de sinterização (Ts) dos aerogéis produzidos. Aerogeis de sílica foram preparados pela hidrólise e condensação de sóis de sistema TMOS metanol- H2O com razão molar TMOS: H2O = 4: 1 e razão em volume de TMOS na solução TMOS metanol = 0,2; 0,3; 0,4; 0,5 e 0,6. As suas propriedades estruturais tais como; a densidade aparente e da matriz, a área superficial BET, volume total dos poros, distribuição de tamanho dos poros, constante e perda dielétricas foram sistematicamente investigadas em função do tratamento térmico de sinterização. As propriedades de relaxação da água adsorvida na superfície altamente reativa foram estudadas através das técnicas dielétricas e de ressonância magnética nuclear em função do conteúdo de H2O, 0 &#60 &#952H2O &#60 6, e temperatura. Também foi feito um estudo da cinética de adsorção da água em aerogéis com varias temperaturas de sinterização. / The synthesis of glasses using the sol-gel method appeared recently of great interest either from the technological or from the scientific point of view. We describe the process used to obtain pure vitreous silica (gel formation &#8594 hypercritical drying &#8594 densification) as well as some physical and chemical characteristics of the various products fabricated. Silica aerogels have been prepared by hydrolysis and condensation of sols of composition TMOS methanol - H2O with molar ratio TMOS: H2O = 4.1 and volume ratio of TMOS in solution of TMOS-methanol = 0,2; 0,3; 0,4; 0,5 and 0,6. Their structural properties such as true and apparent densities, BET surface area, total pore volume, pore size distribution, dielectric constant and loss have been systematically investigated as a function of the densification heat treatment. The relaxation properties of water adsorbed on the highly reactive surface have been studied by dielectric and nuclear magnetic resonance techniques as a function of the H2O coverage (0 &#60 &#952H2O &#60 6) and temperature. The adsorption kinetics of the water was studied too in the aerogels with several heat temperature of sinterization.