Combustion Synthesis of Ultrafine Powders of Co3O4 for Selective Surfaces of Solar Collectors

Solar selective paints, with the addition of Co3O4 as a pigment, are used to improve energetic efficiency in solar collectors. Although Co3O4has been obtained by different methods, references about combustion synthesis are scarce. Co3O4 powders have been synthesized by stoichiometric and non-stoichiometric routes using aspartic acid (Asp) or tri-hydroxi-methyl-aminomethane (Tris) as fuels. The samples were calcined in air at 500 °C. They were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectrum and the specific surface area of the samples was determined by means of the Brunauer–Emmett–Teller technique. The optical properties of pigments were assessed by means of a spectrophotometer. In all cases, powders exhibited the crystalline structure of Co3O4. A minimum crystallite average size of 29 nm was observed for powders obtained by the “stoichiometric/Asp” combustion route, while a maximum value of 41 nm was stated for powders obtained by the “non-stoichiometric/Asp” combustion process. The average particle size ranged between 50 and 100 nm. The powders obtained by the“stoichiometric/Asp” method were selected to study their optical properties; their solar absorption value was 86%. Solar selective surfaces composed by Co3O4 pigments and an alkyd resin were obtained and applied over copper or aluminum substrates. In both cases, solar absorptance was of 93% and comparable with similar solar selective surfaces, but the thermal emittance value was higher than 90%, as a consequence of the large width of the films

Síntesis y caracterización de pinturas selectivas de CoCuMnOx para pinturas absorbentes solares

En este trabajo se propone estudiar la obtención de óxidos mixtos tipo espinela como CoMnCuOx mediante síntesis por combustión estequiométrica originales de un solo paso utilizando ácido aspártico como combustible y la preparación de una pintura absorbente con estos óxidos. Los polvos obtenidos se calcinaron a 500°C y se caracterizaron mediante Difracción de Rayos X, Microscopía Electrónica de Barrido y de Transmisión, además las propiedades ópticas de la pintura se estudiaron mediante reflectometría. Como resultado se obtuvieron partículas aglomeradas y poligonales de CoCuMnO₄ de un tamaño aproximado entre 20 y 100nm. En superficies de aluminio pintadas con la pintura absorbente se logró una absorción solar del 96,5% entre 500 y 1000nm de longitud de onda y temperaturas promedios mayores en comparación con superficies pintadas con aerosol comercial, expuestas al sol. Entonces, con estos valores de altos valores de absorción, se sugiere el uso de estos pigmentos en pinturas absorbentes solares.This work is aimed to study the production of mixed oxides spinel type as CoMnCuOx by means of original one-step stoichiometric combustion methods using aspartic acid as fuel and elaboration of absorbent enamel with these oxides. Once obtained the ashes from the combustion processes, they were calcined at 500°C. The obtained powders were characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and optical properties of enamels were analyzed by reflectometry. As a results agglomerated, polygonal nano-particles of CoCuMnO₄ with a particle size between 20 and 100 nm were obtained. Aluminum surfaces painted with absorbent enamel resulted with a solar absorption of 96,5% between 500 and 1000 nm length wave and average temperatures higher than surfaces with painted with an commercial aerosol. Then, considering these high absorption values suggest the possibility of utilizing these oxides as active pigments, in absorbent solar enamels.Asociación Argentina de Energías Renovables y Medio Ambiente (ASADES

Combustion synthesis of ultrafine powders of Co₃O₄ for selective surfaces of solar collectors

Solar selective paints, with the addition of Co₃O₄ as a pigment, are used to improve energetic efficiency in solar collectors. Although Co3O4 has been obtained by different methods, references about combustion synthesis are scarce. Co₃O₄ powders have been synthesized by stoichiometric and non-stoichiometric routes using aspartic acid (Asp) or tri-hydroxi-methyl-aminomethane (Tris) as fuels. The samples were calcined in air at 500 °C. They were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectrum and the specific surface area of the samples was determined by means of the Brunauer–Emmett–Teller technique. The optical properties of pigments were assessed by means of a spectrophotometer. In all cases, powders exhibited the crystalline structure of Co₃O₄. A minimum crystallite average size of 29 nm was observed for powders obtained by the “stoichiometric/Asp” combustion route, while a maximum value of 41 nm was stated for powders obtained by the “nonstoichiometric/ Asp” combustion process. The average particle size ranged between 50 and 100 nm. The powders obtained by the “stoichiometric/Asp” method were selected to study their optical properties; their solar absorption value was 86%. Solar selective surfaces composed by Co₃O₄ pigments and an alkyd resin were obtained and applied over copper or aluminum substrates. In both cases, solar absorptance was of 93% and comparable with similar solar selective surfaces, but the thermal emittance value was higher than 90%, as a consequence of the large width of the films.Centro de Investigación y Desarrollo en Ciencias Aplicada

Structure of nanoporous zirconia-based powders synthesized by different gel-combustion routes

Zirconia-based ceramics that retain their metastable tetragonal phase at room temperature are widely studied due to their excellent mechanical and electrical properties. When these materials are prepared from precursor nanopowders with high specific surface areas, this phase is retained in dense ceramic bodies. In this work, we present a morphological study of nanocrystalline ZrO2-2.8 mol% Y2O3 powders synthesized by the gel-combustion method, using different organic fuels - alanine, glycine, lysine and citric acid - and calcined at temperatures ranging from 873 to 1173 K. The nanopore structures were investigated by small-angle X-ray scattering. The experimental results indicate that nanopores in samples prepared with alanine, glycine and lysine have an essentially single-mode volume distribution for calcination temperatures up to 1073 K, while those calcined at 1173 K exhibit a more complex and wider volume distribution. The volume-weighted average of the nanopore radii monotonically increases with increasing calcination temperature. The samples prepared with citric acid exhibit a size distribution much wider than the others. The Brunauer-Emmett-Teller technique was used to determine specific surface area and X-ray diffraction, environmental scanning electron microscopy and transmission electron microscopy were also employed for a complete characterization of the samples