| The development of solar spectrum selective absorbing coatings has been a very active research subject in the field of solar thermal technology since the 1950s. Many scholars have made a lot of work on the selection of coating material, structural design and preparation method. And a series of coating systems have been developed. However, these coatings are difficult to achieve the practical application in industrial under medium-high temperature conditions because of some limitations. Therefore, various factors should be taken into consideration. And we need to find a new preparation method and adequate material system to develop a new spectral selective absorbing coating which could be stable under medium-high temperature and able to be cheaply prepared on an industrial scale, in order to promote the development of China's solar thermal technology.In this paper, we developed Ni-NiO-Al2O3 selective absorption coatings of metal ceramic by plasma spraying (APS) technology, and we developed Ni-Mo metal selective absorption coatings by HVOF technology. Then we used the sol-gel thin film technology as a secondary method, SnO2/Ni-NiO-Al2O3 and Al2O3/Ni-Mo composite coatings were prepared. The component, structure, topography and performance of coatings and composite powder were characterized and analyzed by XRD, SEM, EDS, spectrophotometer and infrared spectrometer, et al. The influence of the surface topography, component proportion, superficial treatment and heat treatment of coatings on their spectral selective absorption properties was mainly studied.The results showed that Ni-NiO-Al2O3 coatings occurring in porous and layered structure stacked by deformed particles, their surface were rough surface of diffuse reflective type. Thickness of coatings had a small impact on their optical properties. When the content of Ni of composite powder increased,αandεof coatings all gradually decreased, quality factorα/εgradually increased. When the content of Ni of composite powder was 60 wt.%, the corresponding coating had a best performance of selective absorption, reached a solar absorptance of 0.85 and a thermal emittance of 0.68, and the quality factor a/s reached 1.25. Superficial treatment had a big impact on Ni-NiO-Al2O3 coatings'optical performance, after polishing treatment,αandεof coatings all decreased, but a/s improved. After coating treatment we obtained SnO2/Ni-NiO-Al2O3 composite coatings, the optical properties of coatings were significantly improved than uncoated before, a increased to 0.86, s sharply fell to 0.50. However, due to optical trap formed by large holes of the surface of coatings, it was difficult to further reduceε. The results of heat resistance test showed that Ni-NiO-Al2O3 coatings and composite coatings all had high thermal stability below 300℃, and after treatment at 500℃, composite coatings had a better heat resistance.Ni-Mo metallic coatings were composed mostly of three phases which were Ni, Mo and NiO. The surfaces of coatings were micro-rough and diffuse surface which consisted of particles and millipore, this surface had a good spectral selective absorption role. Under technologic conditions involved in experiment, when the content of Mo of coating increased,αshowed a regressive trend andεfirst decreased and then increased, correspondingly the colors of coatings gradually faded from the deep. When the grain diameter r of composite powder were less than or equal to 45μm, and MNi:MMo of powder was 7:3, coatings had better spectral selectivity, a was 0.85,εwas 0.24, andα/εreached 3.54. The results of superficial treatment of coatings showed that surface roughness Ra of coatings gradually decreased with the deepening of grinding,αandεwere all gradually decreased. When the surface roughness Ra of coating was 2.156μm as a result of grinding treatment, the coating had a preferred selective absorption performance,αwas 0.84,εwas 0.16, andα/εreached 5.25. After adding an anti-reflection layer of Al2O3 on top of the absorbing layer,αof the coating reached 0.92, having a substantial enhances the utilization of sunlight, simultaneously coatings had good thermal stability and thermal shock resistance.
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