| The main requirements for the solar selective coatings which are a key technology for photo-thermal utilization, are high absorptance (a) in the solar spectral range (0.3-2.5μm) and low emittance (ε) in the infrared region (2.5-50μm). Many kinds of solar selective coatings including semiconductor coating, optical interference coating, multi-graded coating, metal-ceramic coating and porous coating have been developed. Especially, M-Al2O3 (M=Pt, Ni, Mo, Cu) and M-AlN (M=W, Mo, stainless steel) cermets have been succeed in commercial application. However, these coatings were restricted in low and middle temperature application for their poor high temperature stability. It is important for parabolic troughs current application to research the solar selective coatings which behave good thermal stability, excellent optical performance (α≥0.94 andε≤0.05) and preparation process.Ti-Al-N coatings are promising candidate for solar selective coating due to their high hardness, excellent anti-oxidation, super wear-resistance, low resistivity and low infrared radiation characteristic. In this paper, the preparation process and optical properties of Ti-Al-N and AlN coatings were studied. Moreover, the four-layer structure solar selective coatings with Ti-Al-N and AlN film as absorb layer and anti-reflection layer have been designed and prepared. The microstructure and thermal stability of the solar selective coatings were investigated.Ti-Al-N coatings have been prepared by magnetron sputtering. The influence of magnetron reactive sputtering process on the microstructure and phase structure of Ti1-xAlxN was studied. It is shown that Al content is the key factor. The coatings with high Al content (x≥0.67) consist of hexagonal-AlN and cubic-(Ti, Al) N phases, while the coatings with low Al content (x<0.67) has major cubic-(Ti, Al) N phase. As Al content increased, the roughness of Ti1-xAlxN coating's surface decreases and columnar texture becomes ambiguous. The micro-structure investigation shows that the particles of cubic Ti3AlN distributes into the amorphous matrix. The optical properties are determined by the Ti-N bonds in coatings. The increase of both refractive index and extinction coefficient with wavelength indicates that Ti0.5Al0.5N coating exhibits metallic characteristic. However, extinction coefficient reaches to the maximum value at the wavelength of～1000nm, and then decreases monotonically indicates that the Ti0.25Al0.75N coating has intermediate characteristic.The effect of N2/Ar flow ratio on phase structure and microstructure of AlNX films was studied. With increasing N2/Ar flow ratio, the coatings consist of single Al phase and then cubic-AlN phase, and the surface becomes much smoother and denser, the deposition rate decreases and the optical transmission increases from 0 to 83%. The transmittance of AlNx coatings in 300-2500nm wavelength reaches to 90%, and is zero over 2500nm. The refractive index and extinction coefficient are 1.9 and zero, respectively.Four-layer structure solar selective coatings with Ti-Al-N and AlN film as absorb layer and anti-reflection layer were designed. The properties of deposited coating which behave high absorptance of 0.945 and low emittance of 0.04 (82℃) are in good agreement with the theoretical result. The optimized thickness of Ti0.5Al0.5N, Tio.25Al0.75N and AlN layer are 50 nm,18 nm and 22nm, respectively.The high absorptance is achieved by reasons listed below:Firstly, Ti0.5Al0.5N is main absorber layer. Single layer Ti0.5Al0.5N gets the absorptance increased to 78.44% from 26.29%. Secondly, the micro-rough interface between AlN and Ti0.25Al0.75N layers. Thirdly, Tio.5Al0.5N and Ti0.25Al0.75N layers could not crystallize completely and form island structure, i.e. nanocrystalline distributed in amorphous matrix. Solar radiation may be reflected many times and absorbed by crystal grains.The thermal stability of Al/Tio.sAl0.5N/Ti0.25Al0.75N/AlN has been studied. Heat treatment of the selective coating in air for 2h showed that the coating was stable up to 475℃with a solar selectivity of 0.92/0.06. At 500℃, the solar selectivity decreased significantly (α/ε=0.7877/0.32), which could be attributed to diffusion of O element into absorber layer, and form Ti-Al-O-N layer. The interface between two absorber and AlN anti-reflection layer disappears after annealed at 500℃. Besides, the curved interface between Al layer and absorber layer is the main reason of increase of emittance. |
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