Role of Al2O3 Antireflective Layer on the Exceptional Durability of Mo-Si-N-Based Spectrally Selective Coatings in Air at High Temperature

Enhancing the stability of spectrally selective coatings (SSC) at high temperatures in air is necessary to push the concentrated solar power (CSP) technology to the next level. To avoid failures that might not be discovered for years, advanced knowledge of the chemical properties related to the degradation mechanisms with temperature is required. For this purpose, the Mo local environment is investigated here for Mo-Si3N4 and MoSi2-Si3N4 nanocomposites SSCs. The atomic short-range order around Mo proves that MoSi2 is the stable form that appears in high temperature vacuum annealing and that MoO3 formation is associated with optical degradation in air annealing. Deposition of Al2O3 capping layers with different techniques is found of paramount relevance in the long term performance of MoSi2-Si3N4 based SSCs. MoSi2-Si3N4 hybrid composite with Al2O3 capping layer shows exceptional functional stability even after 2900 h at 600 degrees C in air. Antireflective Al2O3 layer synthesized by atomic layer deposition (ALD) shows an extraordinarily efficient protection against oxidation with unprecedented MoSi2 stability in MoSi2-Si3N4 hybrid composite, and optimum long-term optical performance. On the other hand, Al2O3 layer deposited by sputtering blocks the formation of detrimental MoO3 at 600 degrees C but allows the formation of MoO2 in MoSi2-Si3N4 for long annealing times which, however, also provides excellent and stable optical functionality related to MoO2 high stability.