Sol–gel Al₂O₃ powders—matrix in solar thermal absorbers

Porous alumina powders for solar thermal applications, prepared by sol–gel were comparatively investigated, starting from an organo-metallic precursor (aluminum isopropoxide) and an inorganic precursor (aluminum chloride hexahydrate). As morphology controlling agents, HCl and poly (ethylene glycol) were added in the precursor solutions. X-Ray Diffraction, Atomic Force Microscopy, Differential Scanning Calorimetry, Fourier Transform Infrared Spectra and UV–VIS-NIR spectroscopy were used to characterize the materials. The chemical composition, the phase structure and morphological properties are influenced by the additive type and by the heat treatment temperature. The results shows that optimized conditions lead to high quality matrix for solar-thermal absorbers, with absorptance in the visible region varying from 0.13 to 0.47 and low thermal emittance (ε_T = 0.02–0.09) in the infrared region of the solar spectrum.