Influence of Spray Pyrolysis Deposition Parameters on the Optical Properties of Porous Alumina Films

Cermets, ceramic-metallic composites, have been extensively investigated for application as solar selective coatings due to their excellent optical properties. They consist of fine metal particles embedded in a ceramic matrix. The ceramic matrix main role is to increase the absorption coefficient (αsol>0.9) while the metallic particles decrease the thermal emittance (ɛT<0.1). Due to its good optical properties, and thermal and chemical stability, alumina is a promising candidate for a cermet matrix. The matrix morphology has an important role in the design of the cermet structures with optimum spectral selectivity. There are several methods to produce solar-thermal absorbers, and one low cost and upscale alternative is spray pyrolysis deposition (SPD). In this study it is shown that SPD parameters can be an important tool in tailoring pours alumina as matrix that can be infiltrated with metal and/or metal oxide particles for preparing solar selective coating. The results show that the matrix porosity can be significantly influenced by the thermal treatment, the addition of morphology controlling agents into the precursor solution and less by the number of spraying sequences.