PtAl 2O 3 selective absorber coatings for photothermal conversion up to 600°C

Abstract

PtAl 2O 3 cermet films (Pt volume fraction 0.18–0.53) have been deposited on optically polished glass substrates by co-sputtering using composite targets. The optical transmittance and reflectance in the range 300–3000 nm and the electrical conductivity have been measured. The electrical percolation and the optical cross over occur around 0.43 Pt volume fraction. Cermet films with a composition close to this percolation threshold show a good solar spectral selectivity which can be improved when they are overcoated by an Al 2O 3 film. Solar absorptances greater than 0.95 and emittances of about 0.14 have been obtained. These selective absorbers deposited on stainless steel show a good stability up to 400°C in air. Between 400 and 600°C, the diffusion of certain elements of the substrate into the coating and their oxidization contribute to the degradation of the optical properties. Coatings deposited on iron, nickel, cobalt base superalloy substrates are stable over 600°C and appear to be attractive for collector applications operating at high temperatures and under a high concentration of solar flux.