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

Abstract

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 havebeen 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 electronmicroscopy, transmission electron microscopy, thermogravimetric analysis, differential scanning calorimetry, Fourier transforminfrared spectrum and the specific surface area of the samples was determined by means of the Brunauer?Emmett?Tellertechnique. The optical properties of pigments were assessed by means of a spectrophotometer. In all cases, powders exhibited thecrystalline 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 selectivesurfaces 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 washigher than 90%, as a consequence of the large width of the films.