Preparation And Properties Of SS/Mo/Al₂O₃ And SS/W/Al₂O₃ Solar Spectral Selective Absorbing Coatings For High Temperature Application

In solar thermal applications, to get more energy, many investigators improve the spectral selective coatings with good spectral selectivity and characters. The microstructure and materials of the spectral selective coating are the key factors to their properties. For spectrally selective solar absorbers coating, metal Mo was found important application in traditional metal-dielectric composites coating, which worked as infrared reflector and filler material.In this work, surface-textured Mo was fabricated by magnetron sputtering through the adjustment of deposition parameters, which exhibited the high absorptance of 0.80 and low emittance of 0.06. The single layer Mo deposited on stainless steel?SS? was characterized by X-ray diffraction?XRD?, ultra-high resolution scanning electron microscope?SEM?, atomic force microscope?AFM? and optical measurement. Combining surface roughness with their absorptance and emittance, we can choose the optimal thickness of the SS/Mo layer. Based on the SS/Mo coating, a spectrally selective coating?SS/Mo/Al₂O₃? was designed and fabricated. The coating shows an amorphous structure and exhibits an absorptance of 0.90 and an emittance of 0.08.The thermal stability of the coatings decides their lifetime. Thus in this thesis, we investigate the thermal stability of the SS/Mo/Al₂O₃ coatings in air and vacuum. The samples are annealed in air and vacuum at different temperatures for 2 h. The surface morphology, microstructure, composition distribution, diffusion and optical properties before and after high temperature annealing are characterized by scanning electronic microscopy?SEM?, atom force microscopy?AFM?, X-ray diffraction?XRD?, micro-Raman spectroscopy, UV-vis spectrophotometer and emissometer. Experimental results indicate that the coatings are stable in air at 350 oC and in vacuum at 600 oC. The spectral absorptance and thermal emittance are 0.87⁰.90 and 0.06⁰.08, respectively. The optical properties, surface morphology and microstructure have not change significant after annealing.The melting point of metal W is the highest metal in previous investigation. Thus it can improve the thermal stability of the solar spectral selective caotings. In this paper, single layer surface-textured metal W thin film is fabricated by magnetron sputtering through the adjustment of deposition parameters. The surface-textured single layer W exhibits a high absorptance of 0.74 and a low emittance of 0.05 due to the the high aspect ratio subwavelength structures. Based on the surface-textured W coating, the SS/W/Al₂O₃ tandem spectrally selective solar absorber is designed and fabricated, which exhibits the amorphous structure. Solar absorptance of 0.90 can be obtained with thermal emittance of 0.06.