| The thesis systematically studied the optical performance and composition structure of each functional layer of Al2O3:Cr-based mid-and high-temperature solar selective absorbing coatings.Combined with optical design software,the coating was successfully prepared.The coating exhibits a high absorptance,a low emittance and great thermal stability,which was suitable for photo-thermal conversion at mid-and high-temperature.First,tungsten thin films were deposited on three different substrates at different substrate temperature using PDC magnetron sputtering process.The coatings were measured and characterized to obtain their IR reflectance,film resistivity,crystal structure and their preferred orientation,and surface morphology.By adjusting the oxygen flow rate between 0.5-2 sccm,a series of Al2O3: Cr films were deposited using alloy Cr Al?70:30at%?and Cr Al?55:45at%?targets.Al2O3:Cr?high-metal-volume fraction,HMVF?and Al2O3:Cr?low-metal-volume fraction,LMVF?films were studied by elliptical polarization spectrometer,XPS and other characterization methods in order to study the effect of oxygen flow rate on the optical properties and composition of single-layer Al2O3: Cr films.According to the optical constant of each single-layer thin film,Al2O3: Cr midand high-temperature solar selective absorbing coatings with excellent spectral selective absorber characteristics was designed using TFCalc optical design software.Based on this,the Al2O3:Cr mid-and high-temperature solar selective absorbing coatings were deposited by magnetron sputtering.The coatings structure was W/Al2O3:Cr?HMVF?/ Al2O3:Cr?LMVF?/Si O2.Finally,the absorptance ? and emittance ? of the as-deposited coating were studied by spectrophotometer,fourier transform infrared spectroscopy?FTIR?.Thermal steabilities of the coatings in vacuum and air were also studied by SEM,XRD and spectroscopy.The reasons caused the degradation of the coatings were analyzed.The main results are shown as follows:1.The crystal structure of the tungsten films is the completely thermodynamically stable BBC ?-W with [110] preferred orientation when the film is deposited at 300 ? substrate temperature.The film exhibits a low emittance less than 0.07?500 ??.Through the method of substrate heating in film deposition process,the metastable phase ?-W is completely transformed into the stable phase BBC ?-W at a small film thickness of 150 nm,which could be potentially applied to large area coating technology for IR reflective layer of the mid-and high-temperature solar selective absorbing coatings.2.The Al2O3:Cr-based mid-and high-temperature solar spectrum selective absorbing coatings prepared by magnetron sputtering is dark green in appearance,without obvious peeling,cracks.The absorber layers were Al2O3:Cr?HMVF?and Al2O3:Cr?LMVF?whose refractive index decreases from bottom to top.The absorptance of the coatings was as high as 96.1% and the emittance is 8%?400 ??.The coating has excellent solar spectrum selective absorption performance.The surface of the coating is composed of irregular particles with a particle size between 200-400 nm.Due to the similar composition of the two absorber sublayers,there is no obvious interface between the absorber layers.The absorber layer and the anti-reflection layer are both amorphous.3.Annealing at 400 ? for 100 h in vacuum or in air did not cause obvious change in absorptance and thermal emittance.The photothermal conversion efficiency remains at above 90% at 100?.However,as the annealing temperature rose to 500 ? in air,the degradation of the coatings accelerated.The absorptance decreased to 87.5% after a 500? heatment for 100 h in air.The degradation of the coatings was found to be the oxidiation of Al2O3:Cr from the morphologies and structures. |
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