| The energy problems and environmental issues derived from the sharply increasing consumption of fossil fuels have become the focus around the world. Therefore, it becomes an extremely urgent problem to develop and explore the clean and renewable energy. As a clean and renewable energy, solar energy has attracted great attention. Two main approaches are made use of solar energy, solar-thermal conversion and photoelectric conversion technology, etc. The solar-thermal conversion technology is a simple, direct energy conversion method with long utilization history. The solar selective absorbing coating is the main part of solar-thermal conversion device, its performance plays a crucial role in solar-thermal conversion efficiency and further application. In this report, high-performance solar selective absorbing coatings were designed by optical software and prepared by magnetron sputtering method based on the metal/dielectric multilayer structure. Furthermore, the thermal stability in air or in vacuum of the coatings was studied. The details were summarized as follows.Firstly, solar selective absorbing coatings of AIN/Ti/AIN/Ti/AIN were designed by optical software and fabricated by magnetron sputtering on quartz substrates deposited Cu films based on the metal/dielectric multilayer structure, and the thermal stability of the solar selective absorbing coatings were further studied. The optimized solar absorptance was about 0.959 designed by optical software, while the solar absorptance and thermal emissivity of the as-deposited coatings were about 0.95 and 0.07, respectively. The coatings were annealed in air or in vacuum to investigate its thermal stability, which was evaluated by the crystal structure, morphology and optical property. The variation of the solar absorptance, thermal emissivity and the PC value were less than 0.011,0.02 and 0.05, respectively, after annealing below 400℃ in air or 600℃ in vacuum. These results indicate that the solar absorbing coating with AlN/Ti/AlN/Ti/AIN multilayer structure have a good thermal stability at 400℃ in air or 600℃ in vacuum.Secondly, the colored solar-thermal collectors of Ti/SiO2/Ti/TiO2/SiO2 including purple, blue, blue green, yellow green and orange yellow were designed by optical multilayer software and then fabricated by magnetron sputtering. The multilayer solar selective absorbing coatings exhibited solar absorptance higher than 0.95 and thermal emissivity lower than 0.09, indicating they contain good optical properties. In addition, we also studied the thermal stability of the blue solar-thermal collectors by annealing at different temperatures in vacuum and in air. It was found that the solar absorptance of the coatings decreases and the thermal emissivity, the variation of the thermal emissivity and color change of the coatings increased when the temperature increases in vacuum in the annealing process. The optical property of the coatings was degenerated when the annealed temperature was no less than 500℃ in vacuum. The chemical element of the coating was oxidizated and the optical property of the coatings was degenerated when the annealed temperature was no less than 400℃ in air comparing with annealing in vacuum.Finally, the solar selective absorbing coatings of the Mo/Si3N4/Mo/Si3N4/SiO2 multilayer structure were designed and the thermal stability was further studied. The multilayer coating Mo/Si3N4/Mo/Si3N4/SiO2 exhibited high solar absorptance and low thermal emittance, and its thermal stability was investigated by annealing in air or in vacuum. The the solar selective absorbing coatings were thermally stable up to 625℃ in air and degraded at 650℃ in air from the characterization of the absorptance and emittance due to the oxidation of molybdenum. The solar selective absorbing coatings were much more stable in vacuum even annealed up to 800℃ comparing with the thermal stability in air. Therefore, the Mo/Si3N4/Mo/Si3N4/SiO2 coatings are suitable to be applied at high temperature as shown above. |
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