Preparation and thermal stability of a novel mid-temperature air-stable solar selective coating

Achievements of higher efficiency, excellent reliability, and lower cost in concentrating solar power plants are impeded by the operating temperature and thermal stability of solar selective coatings (SSCs) on solar collectors. Improving the air-stability of SSCs in mid-temperature is of great significance. We developed a multilayer SSC composed of Si3N4, a-Si, Ag, and Cr2O3 by magnetron sputtering. The amorphous silicon (a-Si) is served as the solar absorbing material. Optical constants of the a-Si thin films deposited at different sputtering powers and working pressures were measured by a spectroscopic ellipsometer after confirming their amorphous nature by XRD. Theoretical solar absorptance of the SSC is 88.38% after optimizing the thickness of the a-Si layer. It was noted from cross-sectional morphologies characterized by FESEM that the SSC exhibited a total thickness of 2 μm, and no obvious interfacial diffusion was observed after annealing at 400 °C in air. In addition, the experimental reflectance spectra revealed an improved solar absorptance of 78.08% and a decreased thermal emittance of 1.68% after heat treatment in air at 400 °C for 10 h. All these results show that the proposed SSC exhibits good solar selectivity in air and is a promising candidate for intermediate solar thermal applications.