Air-Stability Improvement of Solar Selective Absorbers Based on TiW–SiO₂ Cermet up to 800 °C

A high-temperature air-stable solar selective absorber (SSA) based on TiW–SiO2 cermet is prepared by the co-sputtering method. The obtained SSA shows remarkable stability in spectrum, structure, and chemistry after air-annealing at 700 °C, demonstrating its resistance against air erosion at high temperature. Comparing with W-SiO2-based SSA, the addition of the Ti element is proved to be effective in enhancing the thermal stability of SSA. Nevertheless, as the temperature increases to 750 °C, perfectly round cavities appear and induce the deterioration of the coating. A phase transformation from α-W to β-W is found at the interface of TiW/HMVF (high metal volume fraction layer) during deposition. Consequently, the inverse phase transformation from β-W to α-W at above 750 °C results in small vacancies at the interface, being the incentive of cavity generation. Afterward, the violent morphological changes of oxidized TiW accelerate the cavities expansion. To enhance its tolerance ability of service temperature, a Cr barrier layer is introduced to prevent the diffusion of oxygen into the TiW layer. Therefore, the optimal SSA performs stably at 800 °C and the failure temperature is elevated to 850 °C, revealing that the air-stable TiW–SiO2-based SSA has outstanding potential in high-temperature photothermal conversion.