Low cost selective solar absorber coatings : Characteristics of carbon-in-silica synthesized with sol-gel technique

Carbon-silica composite films have been fabricated using sol-gel synthesis of silica-carbon precursor composites, followed by carbonization in an inert atmosphere. Four categories of samples were studied. These were the tetraethyl orthosilicate-only (TEOS-only), methyl trimethoxysilane (MTES), acetic acid anhydride (Ac2O) and soot (SOOT) samples. The carbon-silica films, spin-coated on aluminium and steel substrates, have been investigated for selective solar absorber functionality. Optical measurements were performed on these samples to determine solar absorptance and thermal emittance of each. The morphology of the films was studied by electron microscopy. Electron energy-loss spectroscopy (EELS) mapping was used to determine the carbon distribution. An optimum performance of 0.88 for solar absorptance and 0.41 for thermal emittance has been achieved. The sol-gel technique produced films with very flat surfaces and uniform thicknesses in the 1 μm range. The fine structure showed homogeneous mixing of the carbon and silica in the TEOS-only samples while the separate additions of MTES and Ac2O resulted in segregation of composition of the silica and carbon at nano-scale. The addition of 20 wt% MTES or 15 wt% Ac2O to the TEOS-only sols also helped to reduce the cracks in the TEOS-only samples. The addition of soot in place of the carbon precursor did not yield a net advantage.