Study On The Thermal Stability Of AlON/TiAlON_D/TiAlON_M/Cu Solar Selective Coating

High temperature evacuated collector tube is an important component of parabolic trough solar thermal power system, while solar selective coating is the key material to realize the solar-thermal conversion in the evacuated collector tube. Usually, the coating works under high temperature and vacuum condition. During the long term and high temperature service condition, organic heat transfer medium in the stainless steel tube decomposes slowly and releases hydrogen gas. The hydrogen gas would permeate into the annular vacuum of the tube, resulting in a high temperature and hydrogen atmosphere working condition. Elements diffusion, oxidation and phase transformation may occur in vacuum condition and lead to optical properties degradation of solar selective coating. Hydrogen can change the materials’optical and mechanical properties and phase structure at high temperature. Therefore, study on the thermal stability of solar selective coating in vacuum and hydrogen condition has great signification for improvement of the collector tube’s working reliability.In this article, AlON/TiAlOND/TiAlONM/Cu solar selective coating was used to study the thermal stability in vacuum and hydrogen atmosphere. The as-deposited coating has an absorptance of 0.940 and emittance of 0.08 at 82 ℃. The UV-VIS-NIR spectrophotometer, emissometer, XRD, SEM, TEM, AES and ellipsometer were used to study the absorptance/emittance, phase structure, micro-morphology, profile chemical composition and optical constants of the coating, and the thermal degradation mechanisms of the coating were subsequently discussed. The main research results were reported as below.(1) After annealed at 500～600 ℃ in vacuum condition, the micro-morphology and multilayer structure of the coating keep stable. The absorptance of the coating decreases with annealing temperature increase and time extension, while the emittance of the coating keeps unchanged. The AlON and TiAlONM layer get crystallized and form hexagonal AlON phase and cubic TiAlON phase, respectively, after annealed at 550 ℃. In the process of annealing, the N atoms diffuse from TiAlON double-absorbing layer into AlON layer, whereas the O atoms diffuse from AlON layer into TiAlON double-absorbing layer. Crystallization of AlON and TiAlONM layer results in the change of refractive index and extinction coefficient and the elements diffusion also broadens the interfaces of the sub-layers, leading to the change of layer thickness. These two factors combine to destroy the optimized structure and decrease the absorptance of the coatings. However, after annealed at 600 ℃, the absorptance of the coating suffers a serious and sudden decrease. Except for the degradation discussed above, the crystallization of TiAlOND layer after annealed at 600℃ makes its optical properties transfer from semiconductor to dielectric, resulting in the further degradation of the absorptance.(2) The AlON/TiAlOND/TiAlONM/Cu solar selective coating has a little reduction effect on the hydrogen permeation process at 450～500 ℃. The absorptance of the coating decreases with annealing temperature increase and time extension, and the degradation extent is larger than the coating annealed in vacuum. After annealed below 550 ℃, the micro-morphology and multilayer structure of the coating keep stable. While after the coating is annealed above 600 ℃, cracks and bubbles appear on the surface and the multilayer structure gets destroyed. The AlON layer and TiAlONM layer get crystallized and form hexagonal AlON phase and cubic TiAlON phase, respectively. The analysis results of chemical composition show the hydrogen gas despoils O atoms from the layer, resulting in decreasing of O element concentration. The concentration of N, A1 and Ti element keep stable in each layer after annealing.(3) The destruction of optimized structure caused by layer crystallization, O content decrease in AlON layer and interface diffusion is the main reason for the degradation of the coating’s optical properties. When the annealing temperature rises above 600 ℃, the n and k values of TiAlOND and TiAlONM layer decrease obviously after annealing, resulting in the decrease of absorptance.(4) At high annealing temperature, the hydrogen gas reacts with AlON layer and makes the surface roughness increase, resulting in the further degradation of absorptance. When the annealing temperature rises upon 600 ℃, H atoms dissolved in Cu layer gather in interface region and form hyddrogen bubbles during the cooling process, resulting in the formation of cracks and bubbles. Thus the structure stability of the coating become worse.