Masmoudi, Yasmine (2006) Study of the supercritical co2 drying for the elaboration of nanostructured materials: application to monolithic silica aerogels. PhD thesis Energétique, ENSMP - CEP Centre Energétique et Procédés, ENSMP.

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Abstract

Aerogel-like nanostructured materials present a wide set of potential application fields. Among these materials, silica aerogels are known in particular for their thermal super-insulation capability and their transparency in the visible range. Their integration in double-glazing should offer an energetic gain in the building sector, especially through the reduction of heat consumption.
For such an application, silica gels synthesised through a sol-gel process, should be dried supercritically in order to obtain large dimensions transparent and monolithic aerogel sheets.
This thesis work aims to contribute to the amelioration of the supercritical CO2 drying process efficacity while focusing particularly on the supercritical CO2 washing phase.
The phenomena occurring during this phase were studied by coupling experimental and theoretical approaches.
The experimental approach is based on the instrumentation of a drying system and notably the implementation of an analysis loop. This metrological tool makes it possible to monitor on line the degree of advancement of the washing phase.
The theoretical approach is based on an analytical model coupling diffusion phenomena through the nanoporosity of the gels and mass transfer phenomena in the autoclave.
This double approach has allowed firstly to quantify the diffusion phenomena in reference experimental conditions. The effective diffusion coefficient of a model gel nanostructure was so determined. A first estimation of the washing phase duration was also obtained.
Secondly, the influence of the variation of silica aerogel nanostructure on the diffusion phenomena was studied. The obtained results have led to a first correlation between the materials permeability and the effective diffusion coefficient.
This study has also underlined the interest of an aging treatment by dissolution-reprecipitation phenomena prior to drying in order to shorten the supercritical washing phase duration.

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Table of content

Introduction
I. Elaboration d'aérogels de silice
1 - Présentation générale
2 - Synthèse sol-gel
2.1 - Sol
2.1.1 - Les précurseurs
2.1.2 - Les mécanismes réactionnels: hydrolyse et polycondensation
2.2 - Transition sol-gel
2.2.1 - Principe
2.2.2 - Paramètres réactionnels
2.3 - Vieillissement
2.3.1 - Principe
2.3.2 - Paramètres réactionnels
3 - Séchage
3.1 - Problématique générale
3.2 - Séchage évaporatif
3.2.1 - Comportement des gels au cours du séchage évaporatif
3.2.2 - Améliorations apportées au séchage évaporatif
3.3 - Séchage par cryodessication
3.4 - Séchage supercritique
3.4.1 - Séchage dans les conditions supercritiques du solvant
3.4.2 - Séchage dans les conditions supercritiques du CO2
4 - Procédé de séchage au CO2 supercritique
4.1 - Phase de Lavage
4.2 - Phase de dépressurisation
5 - Structure et propriétés des aérogels de silice
II. Banc de séchage: Analyse en ligne de la phase de lavage au CO2 supercritique.
1 - Introduction
2 - Présentation générale du banc de séchage
2.1.1 - Ligne d'alimentation en CO2
2.1.2 - Autoclave
2.1.3 - Ligne d'évacuation
3 - Mise en place de la boucle d'analyse
3.1 - Description générale
3.1.1 - Fonctionnement du m-CPG
3.1.2 - Fonctionnement du ROLSI
3.2 - Procédure d'analyse
3.2.1 - Prélèvement des échantillons
3.2.2 - Chauffage de la ligne
3.2.3 - Analyses discontinues
3.2.4 - Analyses continues
4 - Méthode d'analyse chromatographique
4.1 - Définition de la méthode d'analyse
4.2 - Etalonnage du micro-chromatographe en phase gaz
4.3 - Tests de validation
5 - Conclusions
III. Etude du lavage au CO2 supercritique pour une nanostructure modèle
1 - Introduction
2 - Conditions expérimentales de référence
2.1 - Matériau modèle
2.1.1 - Mode de synthèse
2.1.2 - Propriétés structurales
2.2 - Conditions de pression et de température
2.3 - Procédure expérimentale de lavage
2.3.1 - Description générale
2.3.2 - Début de lavage
3 - Identification analytique du coefficient de diffusion effectif
3.1 - Suivi en ligne de la phase de lavage supercritique
3.1.1 - Analyse en ligne
3.1.2 - Quantification
3.2 - Identification du coefficient de diffusion effectif par un modèle analytique
3.2.1 - Modèle analytique de transfert de matière
3.2.2 - Méthode d'ajustement
3.3 - Résultats
3.3.1 - Evolution expérimentale de la phase de lavage supercritique
3.3.2 - Identification du coefficient de diffusion effectif
3.3.3 - Durée de lavage supercritique
4 - Conclusions
IV. Influence de la nanostructure sur l'évolution de la phase de lavage au CO2
supercritique
1 - Introduction
2 - Influence de la concentration du sol en précurseur
2.1 - Composition du sol
2.2 - Influence sur les matériaux
2.2.1 - Propriétés structurales
2.2.2 - Propriétés physiques
2.3 - Influence sur l'évolution de la phase de lavage
2.3.1 - Suivi en ligne de la phase de lavage
2.3.2 - Influence sur les phénomènes de diffusion
2.4 - Résumé
3 - Influence du mode de vieillissement
3.1 - Mode de vieillissement
3.2 - Influence sur les matériaux
3.2.1 - Propriétés structurales
3.2.2 - Propriétés physiques
3.3 - Influence sur l'évolution de la phase de lavage
3.3.1 - Suivi en ligne de la phase de lavage
3.3.2 - Influence sur les phénomènes de diffusion.
3.4 - Résumé
4 - Corrélation entre les phénomènes de diffusion et la perméabilité des gels
5 - Conclusions
V. Conclusions et perspectives
1 - Conclusions générales
2 - Perspectives.
Annexes
Références bibliographiques

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