Study On Solar Selective Absorbers Based Metal/Dielectric Multilayers Thin Films

With the development of human society,people's demand for energy has increased rapidly.Traditional fossil resources become non-renewable,meanwhile the exploitation of fossil resources gets more and more difficult.Solar energy,as an inexhaustible source of green energy,is fully capable of meeting human energy needs.The useage of solar energy mainly includes photoelectric conversion and photothermal conversion.The most critical component is the collector in the solar-thermal utilization process,and the key technology for the collector to achieve high absorption is the solar selective absorption thin films.Therefore,in the photothermal conversion system,the solar selective absorption thin film plays a vital role.Among the typical six solar selective absorption thin film structures,metal/dielectric multilayer thin films have been widely considered and studied due to their advantages of great spectral selectivity absorption,good thermal stability,simple structure and low cost.In this thesis,the application prospects of solar thermal conversion,the basic concept of solar selective absorbers and the domestic and overseas's general development background were reviewed.Firstly,a metal/dielectric six-layer film structure of[SiO₂?57.3 nm?/Cr?2.9 nm?/SiO₂?72.9nm?/Cr?6.6 nm?/SiO₂?57.6 nm?/Cu?>100.0 nm?was designed and fabricated.The film fabricated by the magnetron sputtering device has a high absorption in the solar spectrum and a low thermal radiation at 600 K.In addition,the sample has excellent wide-angle selective absorption characteristics as demonstrated from the variable angle reflection spectrum under p light and s light.When the sample was annealed for 12 h under a vacuum pressure of 49 mTorr at 673 K,the sample showed a good thermal stability,indicating that the sample could work stably at 673 K for a long time.To improve the thermal stability,transition metal of tungsten?W?with higher metling point?3410±20??was chosen to replace chromium?Cr?in the six-layer film structure.The fabricated sample showed a six-layer film structure with the TEM?transmission electron microscopy?calibrated film parameters as[Cu?500.0 nm?/SiO₂?63.5 nm?/W?11.0 nm?/SiO₂?60.0 nm?/W?5.4 nm?/SiO₂?75.5 nm?].The sample has a high average absorption in the spectral range of250-2500 nm.In the experiment,the radiation characteristics of the sample were characterized in real time by using an infrared camera at a high temperature,rather than reduction from the optical spectra at room-temperature condition,which proved the sample having a low thermal radiation.Finally,the film structure was successfully designed and prepared by a combination of metal/dielectric multilayer film and surface texturing.The experimental results show that the solar absorptance of the surface roughened metal/dielectric multilayer film structure is increased compared with the untextured film structure.It demonstrated that changing the surface roughness is a very effective method to enhance the performance of solar selective absorbers.The textured sample was prepared by only increasing the deposition power of copper,which can be compatible comparing to the conventional fabrication process for mass production.When the temperature exceeds 600 K,the heat radiation rate is greatly increased.Therefore,the designed film samples are suitable for working temperature below 600 K.