Optical, thermal and structural characteristics of carbon-in- silica selective solar absorbers coatings synthesized by Sol-Gel technique

Abstract

Carbon-in-silica composite films have been fabricated using sol-gel synthesis of silica and carbon precursor materials, followed by carbonization in an inert atmosphere. Four categories of samples were fabricated and investigated. These were the tetraethyl orthosilicate-only (TEOS-only), methyl-triethoxysilane (MTES), acetic acid anhydride (Ac2O) and soot (SOOT) samples. The carbon-in-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 scanning electron microscopy (SEM). The fine structure of the samples was studied by cross-section high-resolution transmission electron microscopy (X-HRTEM). Electron energy-loss spectroscopy (EELS) mapping was used to determine the carbon distribution. The highest solar absorptance attained with these coatings is 0.95 and the least emittance obtained is 0.1. An optimum performance of 0.88 for solar absorptance and 0.41 for thermal emittance has been achieved in a single sample. The sol-gel technique produced films with very flat surfaces and uniform thicknesses of the order of 1 μm. 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 level. 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, although Ac2O seemed to be a better choice of the two. The addition of soot in place of the carbon precursor did not yield a net advantage. This technique of production of selective solar surfaces is promising to compete favourably with other techniques, especially for its low cost of running and of initial investment.