Avaliação integrada do envelhecimento térmico e natural de superfícies solares seletivas multicamadas a base de cromo e sílica via análises óptica, morfológica e eletroquímica

Abstract

This research aimed to develop and conduct an integrated evaluation of the optical, microstructural, and electrochemical performance of Cr/Cr₂O₃-based solar selective surfaces produced by magnetron sputtering, targeting applications in solar collectors operating under severe thermal conditions. Three coating configurations were investigated: S3 (Cr/Cr₂O₃ + Cr₂O₃), S4 (Cr/Cr₂O₃ + SiO₂), and S8 (Cr/Cr₂O₃). The coatings were characterized immediately after deposition (2021) and after two years of natural aging (2023), as well as after accelerated thermal treatments at 200 °C and 350 °C. Solar absorptance, determined by UV–Vis–NIR spectroscopy, remained high for all systems, ranging from 94.93% to 97.61% in 2021 and from 91.86% to 96.45% in 2023, demonstrating excellent optical stability. FTIR analysis indicated low emissivity in the mid-infrared region, associated with high spectral reflectance in this range, thereby confirming the selective behavior of the coatings. Band gap estimation revealed values between 0.85 and 4.38 eV in 2021 and between 1.53 and 4.56 eV in 2023, indicating chemical evolution of the films and the predominance of Cr₂O₃- related phases after aging. Morphological analyses by SEM and profilometry revealed homogeneous films with thicknesses ranging from 64.3 to 270 nm and average surface roughness (Ra) below 35 nm, demonstrating that morphology exerted a limited influence on solar absorptance. Electrochemical tests by EIS showed that the film response is strongly governed by microstructure and interfacial properties, with the S4 system standing out due to its higher charge transfer resistance and superior interfacial stability at elevated temperatures. Overall, the results demonstrate that multilayer engineering—particularly through the incorporation of SiO₂ is a key factor in achieving durable and high-performance solar selective surfaces.