A need was identified to test multi–layer vacuum super insulation (MLVSI) used in cryogenic applications for hydrogen storage. The study focuses on the application of commercially available MLVSI to a locally patented liquid hydrogen cryogenic storage system. This led to an investigation of different types of multi–layer vacuum insulation configurations, as well as further research on tank inlet coupling configurations. It includes the manufacturing of a liquid nitrogen testing cryostat to be able to test and evaluate the system performance. The first set of tests was based on the development of an inlet coupling configuration to limit heat transfer through the inner tank inlet, of a double cryogenic tank system in order to reduce gas boil–off. The couplings were manufactured in the form of a bellow to handle cryogenic vacuum levels, while ensuring low heat transfer rates between inner and outer tanks. It was found that various coupling designs can be considered to limit gas boil–off. The second set of tests was conducted on a specific MLVSI configuration to determine its effectiveness to insulate the spherical header surface of a typical hydrogen storage vessel. The installation procedure, to limit heat transfer and boil–off due to edge effects in this configuration was investigated. It was found that insulation–overlap–edge effects will always have an impact on insulation performance when a spherical header of a storage vessel is insulated, due to its specific geometry. A time efficient way to install MLVSI on such a spherical header is presented and evaluated. Further investigations were carried out by combining findings into one single system to determine the performance of an optimised insulated cryogenic system. It was found that copper plate discs installed between the vanes of a bellowed inlet/outlet nozzle is the most promising to limit heat transfer to the cryogenic fluid. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2012.

A need was identified to test multi–layer vacuum super insulation (MLVSI) used in cryogenic applications for hydrogen storage. The study focuses on the application of commercially available MLVSI to a locally patented liquid hydrogen cryogenic storage system. This led to an investigation of different types of multi–layer vacuum insulation configurations, as well as further research on tank inlet coupling configurations. It includes the manufacturing of a liquid nitrogen testing cryostat to be able to test and evaluate the system performance. The first set of tests was based on the development of an inlet coupling configuration to limit heat transfer through the inner tank inlet, of a double cryogenic tank system in order to reduce gas boil–off. The couplings were manufactured in the form of a bellow to handle cryogenic vacuum levels, while ensuring low heat transfer rates between inner and outer tanks. It was found that various coupling designs can be considered to limit gas boil–off. The second set of tests was conducted on a specific MLVSI configuration to determine its effectiveness to insulate the spherical header surface of a typical hydrogen storage vessel. The installation procedure, to limit heat transfer and boil–off due to edge effects in this configuration was investigated. It was found that insulation–overlap–edge effects will always have an impact on insulation performance when a spherical header of a storage vessel is insulated, due to its specific geometry. A time efficient way to install MLVSI on such a spherical header is presented and evaluated. Further investigations were carried out by combining findings into one single system to determine the performance of an optimised insulated cryogenic system. It was found that copper plate discs installed between the vanes of a bellowed inlet/outlet nozzle is the most promising to limit heat transfer to the cryogenic fluid. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2012.

Cette thèse a pour objectif de moduler la nanoporosité et la densité de gels Résorcinol-Formaldéhyde (RF) afin d’étudier leurs performances dans des applications industrielles telles que les supercondensateurs ou les matériaux d’isolation thermique. En effet, c’est leur texture, et plus particulièrement leur porosité, qui confère à ces matériaux de telles propriétés. Ce travail de thèse a consisté à étudier les paramètres de synthèse des systèmes RF en ajoutant un additif et à utiliser de nouveaux précurseurs. Notre choix s’est d’abord porté sur un polyélectrolyte cationique qui est susceptible de modifier les mécanismes de synthèse des gels ainsi que leur procédé de fabrication. Dans un second temps, l’emploi de nouveaux précurseurs a ouvert de nouvelles possibilités de synthèse de ces matériaux. Enfin, l’aspect sécuritaire du procédé a été abordé afin d’évaluer tous les risques liés à la synthèse de gels RF pour faciliter le transfert du procédé à plus grande échelle. / The objective of this thesis is to modulate the nanoporosity and the density of Resorcinol-Formaldehyde (RF) gels in order to study their performances in industrial applications such as supercapacitors or thermal insulation materials. Indeed, it is their texture and more particularly their porosity which give these materials such properties. This thesis work consisted in studying synthesis parameters of RF systems by adding an additive and using new precursors. We chose a cationic polyelectrolyte which is likely to modify the synthesis mechanisms of the gels as well as their elaboration process. In a second time, the use of new precursors offered new possibilities of synthesis of these materials. Finally, process security was discussed in order to evaluate all risks linked to RF gels synthesis in order to facilitate the scaling-up of the process.

Cette thèse s'intéresse à la préparation, par la technique de la lyophilisation, des aérogelsà base de celluloses et de charges minérales destinés à une utilisation potentielle dans le domainede l'isolation thermique. Le premier objectif de la thèse a été la caractérisation de différentescelluloses (cellulose (PBPD), nanocristaux (NCC) et nanofibrilles oxydées (NFCs)), les chargesminérales (principalement la zéolithe) et les différents aérogels résultants de différentescombinaisons des matériaux de départ utilisés. Nous avons utilisé pour la caractérisation desmatériaux de départ et des aérogels des techniques d'analyse telles que la diffraction des rayonsX (DRX), la BET, le MEB et le potentiel zêta. Nous avons également caractérisé les propriétésmécaniques des aérogels par des essais de compression et leurs propriétés de conductionthermique dans le régime non stationnaire par la technique du fil chaud. Il s’est avéré qu’unestructuration multi-échelles de ces différentes celluloses favorise la création de méso etnanoporosités au détriment de la macroporosité. Ceci favorise le confinement de l’air dans le bioaérogelpar effet de Knüdsen et améliore ses propriétés d’isolation thermique. D'autre part lesnanoparticules (organiques et inorganiques) permettent d'avoir des aérogels de très bonnespropriétés mécaniques. Le troisième objectif était d'essayer d'autres charges minérales (autres quela zéolithe) dans les différentes celluloses et d’explorer les propriétés morphologiques,structurales, thermiques et mécaniques. Cette étude a permis de montrer l'importance descaractéristiques morphologiques et géométriques des charges minérales dans le contrôle despropriétés physiques et mécaniques des aérogels bio-hybrides. / This thesis focuses on the preparation, using freeze drying technique, of aerogels madefrom cellulose and mineral fillers intended for potential use in the field of thermal insulation. Thefirst goal of this thesis was the characterization of different cellulose (cellulose (PBPD)nanocrystals (NCC) and oxidized nanofibrils (NFCs)), the inorganic filler (mainly zeolite) and theresulting aerogels prepared by various combinations. We used for the characterization of thestarting materials and the aerogels analytical techniques such as x-ray diffraction (XRD), BET,SEM and the zeta potential. We also characterized the mechanical properties of the aerogels bycompression tests and their thermal conduction properties in the non-steady state by the hot wiretechnique. It has been found that multi-scale structure of these celluloses promotes the creation ofmeso and nanoporosities to the detriment of macroporosity. This promotes the confinement ofthe air in the bio-aerogel by Knudsen effect and improves their thermal insulation properties. Onthe other hand, the nanoparticles (organic and inorganic) allow the aerogels to have very goodmechanical properties. The third objective was to try other mineral fillers (other than the zeolite)in combination with the different cellulose and explore the morphological, structural, thermaland mechanical of the corresponding aerogels. This study has allowed showing the importance ofmorphological and geometrical characteristics of the mineral fillers in controlling physical andmechanical properties of the bio-hybrid aerogels.

Le but de ce travail de thèse est de développer des aérogels biosourcés, mécaniquement résistants et thermiquement très isolants (voire super-isolants). Les aérogels à base de cellulose, souvent appelés « aérocelluloses », sont connus pour être très poreux et extrêmement légers. Ils présentent en revanche une grande dispersion de tailles de pores, donnant de propriétés thermiques relativement modestes. Nous avons étudié plusieurs approches pour améliorer la morphologie des aérocelluloses: la modification du solvant, la réticulation chimique de la cellulose et la formation d'hybrides avec d'autres polymères. La réticulation de la cellulose a réellement permis d'affiner la structure poreuse de l'aérocellulose vers une nano-structuration ce qui a amélioré la conductivité thermique, s'approchant du domaine de la super-isolation (0.026 W.m-1.K-1). Un autre polysaccharide, la pectine, a été utilisé pour préparer un aérogel également poreux et très léger, « l'aéropectine ». L'aéropectine et l'aérocellulose présentent de fortes similitudes dans leur morphologie. Cependant, l'aéropectine possède de meilleures propriétés thermiques, super-isolantes (0.020 W.m-1.K-1), grâce à la nano-structuration du réseau poreux. Ces aérogels sont 100% biosourcés avec un faible impact environnemental, et sont très prometteurs non seulement pour l'isolation thermique mais également pour de nombreuses autres applications, telle que la libération contrôlée de médicaments ou la catalyse. La formation d'aérogel de silice à l'intérieur de la structure poreuse d'aéropectine a augmenté la surface spécifique jusqu'à 700 m2/g et a permis de diminuer la conductivité thermique (0.017 W.m-1.K-1). / The work aims at developing a new generation of bio-based aerogels, mechanically robust and thermally very insulating (super-insulating). Cellulose aerogels, called “aerocelluloses”, are known to be very porous and ultra-lightweight materials but present a wide range of pores and therefore moderate thermal insulating properties. We studied several approaches for tuning aerocellulose towards a finer and nanostructured morphology: modification of solvent, cellulose crosslinking and formation of cellulose-based hybrids. It was cellulose cross-linking that greatly improved aerocellulose structure towards a nano-structuration, reflected by the increase of specific surface area and the decrease of thermal conductivity, close to super-insulation (0.026 W.m-1.K-1). Another polysaccharide, pectin, was used for preparing a highly porous and very lightweight aerogel called “aeropectin”. Aeropectin and aerocellulose were compared; they present many similarities in their morphology. However, aeropectin has better thermal properties, reaching super-insulation (0.020 W.m-1.K-1). These bio-aerogels are 100% bio-based, environmentally friendly and present a high potential not only for thermal insulation but also for a broad range of other applications such as controlled drug release and catalysis. The formation of silica aerogel directly inside the porous structure of aeropectin increased the specific surface area up to 700 m²/g and decreased thermal conductivity (0.017 W.m-1.K-1).