Enhanced solar reflectance of sputtered dielectric-coated aluminum for solar thermal systems

Aluminum (Al)-based solar reflectors (SRs) are essential components in concentrated solar power (CSP) systems, as they reflect sunlight across a broad wavelength range in high-temperature solar thermal applications, thereby impacting overall system performance and optimal energy generation in industrial processes. However, environmental degradation often reduces their optical performance and durability under extreme operating environments. To address these challenges, we have designed and fabricated bilayer SiO₂-TiO₂ thin films on Al substrates using radio-frequency (RF) magnetron sputtering. These films significantly improve solar reflectance (R) through constructive interference, while also enhancing nano-mechanical adhesion strength and resistance to environmental degradation. The resulting bilayer films exhibited dense, crack-free morphologies with a low surface roughness of 1.72 nm, minimizing scattering losses. The bilayer films achieved a solar-weighted reflectance (SWR) of 94.82% with a peak R of 99.93% in the wavelength range of 300–2500 nm, exhibiting angular-insensitive behaviour. Moreover, a high R of 97% was maintained over the wavelength range of 2.5-15 μm, indicating suppressed thermal radiation losses. Environmental durability testing under salt spray exposure and in an accelerated weathering chamber revealed excellent stability, with only 0.87% and 2.23% decreases in SWR after 480 h and 144 h, respectively. The high-temperature stability of the optical performance shows minimal SWR loss of 0.44% after 144 h at 250 °C. Additionally, the nano-mechanical adhesion strength of the bilayer films increases from 49.8 mN to 52.3 mN, due to the synergy of SiO₂'s strain accommodation and TiO₂'s hardness, ensuring uniform stress distribution and robust interfacial integrity. These results highlight the potential of bilayer films as a high-performance, durable solution for Al-based SR for advanced solar thermal applications.