Preparation And Properties Of Ultra-high Temperature Ceramic-based Solar Spectral Selective Absorber Coating

Based on the excellent properties of ultra-high temperature ceramics,such as high hardness,high strength,high temperature thermal stability and potential spectral selectivity,the controllable preparation and structure-activity relationship of ultra-high temperature ceramics based solar spectral selective absorption coating are studied in this paper.By magnetron sputtering technology,a series of ultra-high temperature ceramic materials,such as TiC,TiN,ZrC,TiB₂,HfC and HfB₂,are selected as the targets to prepare ultra-high temperature ceramic based solar selective absorber coating.Based on the optical simulation,controllable preparation,microstructure characterization and electromagnetic simulation study of optical absorption and failure mechanism,it provides experimental and theoretical support for the subsequent development of solar selective absorber coatings with better optical properties and thermal stability.The specific research results are as follows:（1）First,TiC-TiN-based solar selective absorber coatings were prepared on Si（100）and stainless steel（SS）substrates by magnetron sputtering system.The preparation technology and spectral selective absorption characteristics of TiC-TiN ultra-high temperature ceramic absorption layer were studied.In addition,Al₂O₃ layer is used as the anti-reflection layer for further optimization design.The absorptance（α）and emittance（ε）of SS/TiC-TiN/Al₂O₃ ultra-high temperature ceramic-based solar selective absorber coating is simulated based on commercial CODE and Matlab software combined with phase modulated spectral ellipsopolarimeter are annealed at 500℃ and 600℃,respectively.The chemical composition,structure and surface morphology of the coating are analyzed by XRD,XPS and SEM.After vacuum treatment at 500℃×100 h,the microstructure and optical properties of the coating have no significant changes,indicating that the coating can be used at 500℃ high temperature vacuum condition.The simulation results show that the photothermal conversion efficiency of the coating can reach 91.3%under the conditions of solar concentration ratio C=100 and operating temperature T=500℃.（2）The optical properties of SS/ZrC-TiB₂/SiO₂ solar selective absorber coating are studied by using ZrC-TiB₂ultra-high temperature ceramic material as the absorption layer and SiO₂ as the antireflection layer.The absorptivity and emissivity of the coating can reach 0.91 and 0.08（82℃）after optimized process.In order to test the thermal stability of the coating,the prepared samples are subjected to vacuum annealing treatment at different temperatures.The effect of annealing is characterized by SEM,AFM,XRD and Raman.The experimental results show that the optical properties of the coating failure significant at 400℃,and the failure mechanism may be caused by the graphitization of carbon components in the ZrC-TiB₂ ultra-high temperature ceramic absorbing layer.（3）The optical properties and thermal stability of SS/HfC-HfB₂/Al₂O₃ solar selective absorber coating were prepared and studied by using HfC-HfB₂ ultra-high temperature ceramic material as the absorption layer and Al₂O₃ as the anti-reflection layer.The results show that the absorption rate of the coating is 0.92 and the emissivity is 0.09（82℃）,respectively.Thermal stability test results show that the coating can maintain good thermal stability at 600℃ under vacuum condition and 400℃ under air condition.The simulation results show that the solar-thermal conversion efficiency can reach 84.3%under the conditions of concentration ratio C=100 and working temperature T=600℃.Based on TEM and AFM analysis,it is found that the annealing process causes significant changes in the surface and microstructure.The finite difference time domain（FDTD）method is used to further study and analyze the physical mechanism of absorption and failure.The final SS/HfC-HfB₂/Al₂O₃ solar selective absorber coating with simple structure,low cost and high performance can be used as a very promising candidate material for high temperature solar solarthermal conversion.