ELECTROPHORETIC DEPOSITION OF NANOPARTICLES FOR PHOTO-THERMAL SOLAR RECEPTORS

The production of hot water by using efficient photothermal solar collectors is growing in importance to limit the use of fossil fuels. Such surfaces have to display a high solar absorptance (α\u3e0.9), in the UV-VIS and near-IR regions (0.5-2µm) with a low thermal emittance (Ɛ\u3c0.1), in the mid-far infrared region (2-20µm) [1]. Black copper (CuO) has proved to be one of the viable solar-selective coatings owing to its nearly intrinsic properties with high solar absorptance (α\u3e0.9) and comparable emittance (Ɛ\u3c0.4). This study investigates the electrophoretic deposition (EPD) of dispersed nanoparticles (CuO) for the formation of submicron coating as a selective solar absorber [2]. To enhance the native CuO optical properties, a tandem system composed of CuO thin film deposited onto a highly IR reflecting metallic substrate (i.e. gold silicon wafer) is formed by electrophoretic deposition (EPD). Pure CuO thin films are obtained from EPD with a thickness control which is the key to drive the final optical properties. Prior to the electrophoretic deposition, the colloidal dispersion is analysed by laser doppler velocimetry coupled with small angle X-ray (SAXS) and dynamic light scatterings (DLS). The colloidal stability results mainly from Van der Waals and electro-steric interactions, which is a crucial factor in order to obtain some homogeneous and uniform films. Indeed, it not only affects the particle migration, but also influences the green coating density. Besides, some stabilizer additives as MgNO3 are also embedded during the EPD in order to modulate the density of the coating by acting as a binder. The deposition yield is studied relative to the different applied voltage range, deposition time and nanoparticle concentrations. The composition and the thickness of the coatings are analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy [3]. Finally, their optical selectivity thresholds are determined by reflectance spectroscopy from UV-vis-NIR spectra and Fourier transform (FTIR) spectra. The associated optimal film thickness range is finally discussed according to the EPD experimental conditions. Please click Additional Files below to see the full abstract

Selective absorbers based on carbon nanotube coating from Electrophoretic Deposition

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Couplages originaux entre Surfactants et Cristaux Liquides Thermotropes : <br />Microémulsions inverses et émulsions directes

This PhD thesis deals with two different experimental studies based on original couplings between liquid crystals and surfactants. The aim of the first study was to understand clearly the lyotropic organisation of mixture of didodecyldimethylammonium bromide (DDAB) and water in the isotropic 4-n-pentyl-4'-cyanobiphenyl liquid crystal (5CB). The formed microemulsion gets unstable when approaching the isotropic-to nematic 5CB or when slightly changing the composition by adding a co-surfactant. Our aim was to determine what the driving interactions were.The structure of this water-in-oil micro-emulsion has been investigated by small-angle neutron and X-rays scatterings far above the nematic-isotropic (N-I) phase transition of the liquid crystal. From quantitative analyses, we obtain the shape and size of the aggregates. Next, in order to obtain new mesophases (for example a lamellar phase) in a liquid crystal, we have explored the effect of adding a co-surfactant. The study of the phase diagrams reveals a small extension of the micro-emulsion domain. Our experimental results are explained by a large attractive Van der Waals interaction between inverse micelles. Under cooling, we have also shown by neutron scattering experiments the presence of “paranematic fluctuations” interactions occuring close to the isotropic-nematic phase transition. We discuss these interactions in the light of the scattering data.The goal of the second study was to formulate a direct emulsion of liquid crystal in water with new amphotropic surfactants that we have synthesized and characterized by NMR. These molecules were designed to be localized specifically to the water-5CB interface. A remarkable effect has been obtained with gemini molecules, which favour the formation of transient elongated cylindrical droplets composed by nematic 5CB. We have explored the static and dynamic properties of this new colloidal objects. The mechanisms controlling their formation have been discussed.Cette thèse décrit deux études expérimentales distinctes sur des couplages originaux entre un cristal liquide et des surfactants.La première étude porte sur l'organisation de systèmes lyotropes (surfactants et eau) dans un solvant thermotrope anisotrope (cristal liquide). L'existence d'une phase baptisée « nématique transparente » dans un tel système mixte thermotrope-lyotrope de type microémulsion a été en effet récemment débattue. Des expériences de diffusion dynamique de lumière, au voisinage de la transition isotrope-nématique, laissait supposer l'apparition d'une phase intermédiaire, résultant de la compétition entre l'apparition d'un ordre nématique dans le solvant et des effets d'ancrages sur les agrégats.Nous avons caractérisé, dans un premier temps, par diffusion de rayonnement X et neutrons, la morphologie et les interactions d'agrégats, composés de bromure de didodecyldimethylammonium (DDAB) et d'eau, dans le cristal liquide 4-n-pentyl-4'-cyanobiphenyl (5CB) en phase isotrope. L'étude de l'évolution des diagrammes de phases couplée à des expériences de diffusion (X, Lumière et neutrons) a permis de montrer que l'ajout de cosurfactants permettait de faire varier continûment la taille des nanoagrégats. L'ensemble de nos données expérimentales s'explique bien par la présence d'interactions fortes de Van der Waals entre micelles, ce qui justifie en particulier l'absence de transition microémulsion/ lamellaire gonflée dans ce système. L'analyse des facteurs de structure a permis par ailleurs de mettre en évidence une seconde interaction attractive entre micelle, causée par les fluctuations paranématiques du cristal liquide, intervenant essentiellement à l'approche de la transition isotrope-nématique du cristal liquide. Nous discutons de cette nouvelle interaction à la lumière des résultats de nos expériences de diffusion.La deuxième étude porte sur les émulsions directes eau-cristal liquide en présence de surfactants amphotropes que nous avons synthétisés et caractérisés par RMN. La formulation de ces surfactants visait à renforcer leur localisation exclusive à l'interface eau-cristal liquide. Les émulsions obtenues montrent la formation spectaculaire de gouttes allongées cylindriques de type filaments. Les propriétés statiques et dynamiques de ces objets ont été explorées et l'origine de cette instabilité est explicitée. La longueur des microtubes est modifiable par des gradients de concentration ou des variations de température ce qui nous a permis de discuter du mécanisme régissant leur morphologie

Couplages originaux entre surfactants et cristaux liquides thermotropes (microémulsions inverses et émulsions directes)

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Etude d'un système mixte Thermotrope-Lyotrope

International audienceNous souhaitons réaliser des systèmes lyotropes-thermotropes dans lesquels une organisationlyotrope coexiste avec un solvant cristal liquide thermotrope.Dans un premier temps, nous nous sommes intéressés à un système mixte de typemicroémulsion/ nématique, composé d’un mélange de molécules amphiphiles (bromure dedidodecyldimethylammonium DDAB), d’eau et de 5CB. Ce mélange avait été étudié dansl’équipe de Tanaka[1] qui avait discuté de l’existence d’une phase intermédiaire baptisée« nématique transparent » résultant de la compétition entre l’apparition d’un ordre nématiquedans le solvant et les effets d’ancrages sur les micelles. Diverses études que nous avonsmenées (dont une étude SAXS) infirment cependant ce point.Une étude soignée des diagrammes de phases couplée à des expériences de diffusions delumière nous a permis de montrer :- que l’ajout d’un cosurfactant (bromure de cetyltrimethyammonium CTAB) permetde faire varier continûment la taille des micelles, sans toutefois conduire àl’existence d’une phase lamellaire mixte.- que le 5CB possède des propriétés de cosurfactant et modifie fortement la courburespontanée des micelles.- qu’une interaction attractive entre micelles est présente et est vraisemblablementresponsable de l’absence de transition microémulsion/ lamellaire dans ce système.Cette première étude physico-chimique devrait nous conduire, par la suite, à l’étudequantitative des interactions entre micelles dans un solvant nématique.[1] J. Yamamoto, H. Tanaka, Nature, 409, 321 (2001)

Monolithic zirconia foams synthesis from emulsion stabilized by colloidal clusters

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Coupling between SAXS and Raman spectroscopy applied to the gelation of colloidal zirconium oxy-hydroxide systems

International audienceThe colloidal sol–gel transition based on zirconyl nitrate solution systems is investigated in this work. The different steps occurring in the transition have been identified by coupling small angle X-ray scattering with Raman spectroscopy and rheology measurements. The effect of the experimental conditions, such as the zirconium precursor concentration and pH, on the transition is studied. The precise mechanisms involved during the transition are based on a detailed understanding of the nanostructure of these systems. In particular, the dissolution of the zirconium salt leads to the formation of cyclic tetramers that self-organize into a cylindrical shape. We clearly demonstrate that increasing the pH induces a strong attractive interaction between the cylinders, giving rise to a mass fractal dimension. For each system, two characteristic pH values have been determined via rheological measurements analysis, where gelation is notably slow below the first pH value and precipitation occurs above the second one. The complete description of the quaternary system (zirconyl nitrate + acetylacetone + ammonia + water) is an efficient formulation guide for the further combination with a templating route leading to structured Zr-based materials

High-internal phase emulsions stabilized by colloidal Zr-based solid clusters

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Confinement-Induced Electronic Excitation Limitation of Anthracene: The Restriction of Intramolecular Vibrations

International audienceThe spectroscopic behavior of anthracene encapsulated in porous zeolitic zinc imidazolate framework ZIF-8 was studied. Nanoparticles of ZIF-8 with high crystallinity and large specific surface areas were prepared in one-pot synthesis. Monitoring single anthracene encapsulation in a ZIF-8 cell was done by careful control of the synthesis of the hybrid. The molecular electronic spectra of anthracene adsorbed on the external surface of ZIF-8 were compared with those of encapsulated anthracene. It was demonstrated that the encapsulation of anthracene induced unexpected confinement effects including the restriction of intramolecular vibrations in its absorption spectrum. It was shown that vibronic components in the high-energy part of the S0 → S1 absorption band totally collapse upon encapsulation. Our study highlights the mechanism of confinement-induced electronic excitation limitation related to the restriction of intramolecular vibrations, which evidences the role of host–guest interaction in infrared absorption and fluorescence emission properties

Linker-assisted structuration of tunable uranium-based hybrid lamellar nanomaterials

International audienceObtaining bulk materials ordered at the nanoscale using nanoparticles as building blocks is a new challenge for scientists and is of primary interest in some research areas. In this study, we developed a simple one-pot route towards nanostructured hybrid materials based on uranium oxide through a non-hydrolytic condensation process under mild solvothermal conditions (T = 160 °C) using ditopic organic linkers as structuring agents. The obtained materials present a sand rose morphology composed of agglomerated nanometric sheets with a lamellar substructure. Nanometric sheets are composed of inorganic fringes consisting of a studtite phase [(UO2)O2(H2O)2]·2H2O that alternates with organic fringes formed by the structuring organic linker. A multiparametric study based on SAXS experiments and TEM analyses has highlighted the control of structural parameters by varying some synthesis conditions. The interlamellar distances are tunable from 1.60 nm to 1.94 nm through the length of the structuring linker and the inter-sheet distance from 1.85 nm to 2.10 nm through synthesis temperature. This efficient one-pot route appears to be very promising for the production of tunable multiscale nanostructured hybrid materials and can be implemented in many research areas, paving the way for various applications