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    Tesis

    Síntesis de óxidos nanoestructurados basados en aluminio, silicio y cobalto, para su empleo en superficies selectivas de colectores solares

    2013



    Tesista María Celeste GARDEY MERINO
    Ingeniera en Materiales - Instituto Sabato UNSAM/CNEA - Argentina
    Doctora en Ciencia y Tecnología, Mención Materiales - Instituto Sabato UNSAM/CNEA - Argentina
    Directora Dra. Patricia Graciela VÁZQUEZ, CINDECA, UNLP, CONICET - Argentina
    CodirectorDr. Gustavo LASCALEA, LISAMEN, CONICET - Argentina
    Lugar de realizaciónLaboratorio de Investigaciones y Servicios Ambientales - Mendoza - Argentina
    Centro de Investigación y Desarrollo en Ciencias Aplicadas - La Plata - Argentina
    Fecha Defensa 13/03/2013
    Jurado Dra. Ana Gabriela LEYVA, CNEA, UNSAM - Argentina
    Dr. Pedro Juan MORANDO, CNEA, UNSAM, CONICET - Argentina
    Dra. Nancy Esther QUARANTA, UTN FR San Nicolás, CICPBA - Argentina
    Código IS/TD 68/13

    Título completo

    Síntesis de óxidos nanoestructurados basados en aluminio, silicio y cobalto, para su empleo en superficies selectivas de colectores solares

    Resumen

    Para mejorar la eficiencia de colectores planos de baja temperatura utilizados en calefones solares a partir de pinturas que incluyan óxidos nanoestructurados, se propuso la obtención de óxidos de Co3O4 mediante síntesis por gelificación-combustión (GC) y de óxidos mixtos de Co-Si mediante sol-gel. A modo de entrenamiento y prueba para las síntesis por combustión de Co3O4 se propusieron síntesis novedosas por GC de Al2O3 utilizando ácido aspártico (Asp) como combustible y dos tipos de rutas: estequiométrica y con exceso de combustible. Se obtuvieron Al2O3 amorfa, además, de las fases gamma-Al2O3 y alpha-Al2O3 nanoestructuradas, luego de realizar calcinaciones adecuadas. Las muestras con fase gamma-alúmina presentan los valores más altos de área específica y los menores tamaños promedio de cristalita.Se presentaron novedosas síntesis por GC de Co3O4 utilizando diferentes combustibles como Asp, lisina monoclorohidrato, ácido etilendiaminotetraacético y tri-hidroxi-metil-aminometano, donde se estudiaron dos tipos de rutas: estequiométrica y con exceso de combustible. Se obtuvieron partículas de Co3O4 nanoestructuradas, de forma octaédrica, aglomeradas y con una distribución amplia del tamaño de partícula. Se observó una relación entre la cantidad de combustible utilizado y el tamaño de cristalita del Co3O4 obtenido.En adición, se estudió la obtención de óxidos mixtos de Co-Si por síntesis sol-gel, utilizando como precursor ortosilicato de tetraetilo para la obtención de sílice y los polvos nanoestructurados de Co3O4. En los materiales obtenidos se detectó SiO2 amorfa y la espinela cúbica Co3O4, por otro lado, se observaron, a simple vista, zonas cerámico-vítreas de color violeta o rosado, asociadas a la presencia de cationes de cobalto dispersos en la matriz silícica.Finalmente, los polvos de Co3O4 resultaron tener una alta absorbancia solar y se decidió utilizarlos para la fabricación de pinturas absorbedoras con resinas polivinílicas y alquídicas, en ambos casos, se lograron valores altos de absorción solar, sin embargo las alquídicas resultaron tener una mayor adhesión y una terminación más uniforme. Los valores de absorción solar y emitancia térmica resultaron altos para las superficies selectivas obtenidas al aplicar las pinturas alquídicas sobre sustratos metálicos. Los altos valores de emitancia térmica se asocian con el espesor relativamente alto de las capas de pinturas. Con espesores de pinturas menores, o el agregado de partículas metálicas, podrían obtenerse valores de emitancia mucho más bajos.

    Complete Title

    Synthesis of nanostructured oxides based on aluminum, silicon and cobalt, for use in solar collectors selective surfaces

    Abstract

    As an attempt for the improvement of efficiency of low temperature, flat collectors employed in solar heaters, using paints with nanostructured oxides as pigments, the synthesis of nanostructured Co3O4 via gel-combustion (GC) routes and the production of Co-Si based, mixed oxides by sol-gel were proposed. As a preliminary practise, for a proper training in combustion syntheses for Co3O4, novel GC routes for Al2O3 were proposed. These methods employed aspartic acid (Asp) as unique fuel, and two approaches were proposed: stoichiometric processes and fuel-rich processes. Amorph Al2O3, and nanostructured gamma-Al2O3 and alpha-Al2O3 were obtained in turn by proper calcination. Gamma-alumina samples exhibited the highest values for specific surface area and the minor average crystallite size values, as well. The novel alumina syntheses frame was extended to GC processes for preparation of Co3O4. Employing in each, assayed route four different fuels as Asp, lysine monochlorohydrate, ethylene diamine tetra acetic acid and tri-hydroxi-methyl-aminomethane. In each case, again, the same two approaches employed for alumina (stoichiometric and fuel-rich routes) were performed. Agglomerated, nanostructured, octahedral shaped particles of Co3O4 were obtained, exhibiting a wide particle sizes distribution. It was observed, in addition, a relationship between the amount of fuel employed and the average crystallite size for the particles of Co3O4 obtained. The preparation of Co-Si-based mixed oxides by sol-gel was also studied, employing tetra-ethyl-orthosilicate as a precursor for SiO2 further supplemented with the gel-combustion Co3O4 powders previously obtained. In the mixed oxide material prepared were observed both amorphous SiO2 and the cubic spinel phase of Co3O4. It was clearly seen by direct observation some glassy/ceramic zones colored in violet/purple or pink which can be associated with Co cations dispersed into the SiO2 matrix. Finally, Co3O4 powders exhibited a high solar absorbance were preferred for a laboratory scale manufacture of absorbing paints, with the aid of polyvinyl and alkyled resins. In both cases, high values of solar absorption were achieved, but alkyd paints resulted more adhesive and exhibited a more uniform covering. High values of solar absorption and thermal emittance were observed from selective surfaces made with suitable, metallic substrates covered with our alkyd paints. The observed, high values of thermal emittance are associated with the relative high thickness of the film in the covered areas. Presumably, diminishing the thickness and/or adding fine grained, metallic particles, the subsequent thermal emittance values would be lower. So, we have some hints for future development: to argument the solar radiation absorption of selective surfaces with a comprensive employment of mixed oxides as pigments. To diminish thermal emittance by thinning the paint films and/or adding fine grained, metallic particles as well. The development of sol-gel strategies yielding the desired mixed oxides in “one step” processes is another suitable option for the manufacture of coatings which allow thinner films to be obtained. Nowadays, these studies are part of a project which involves the building and installation of a solar heater, which collector will have a selective surface coating made employing the materials developed during this thesis studies. It was establish, in addition, a recent contact with research centers, specialized in paint studies, in order to enhance the performance parameters yet mentioned.

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