Study On Ag-based Spectral Selective Coatings

Solar power generation is mainly consisted of thermal power generation and photovoltalic power generation.In the thermal field,the higher the temperature of the heat-transfer fluid in the parabolic trough collector,the higher the Carnot efficiency of power plants.With the transition of the heat-conducting medium from synthetic oil to more efficient molten salt,high-temperature solar selective absorbing coating with low thermal emittance has been vital to solar energy utilization at medium-high temperatures.In the photovoltalic field,the temperature of photovoltalic module will elevate after receiving solar radiation,which has a negative effect on the photoelectric conversion efficiency.The coating on the photovoltalic cover plate has the function of enhancing transmittance in the visible region and reflecting infrared radiation,which can intelligently control the temperature of photovoltalic module and then improve the performance.Therefore,there is an urgent request to develop a spectral selective transmittal coating with high visible and low infrared transmittance.On the basis of microstructures and spectral reflecting characteristics of Ag films,the studys of Ag-based spectral selective absorption/transmission coating were carried out respectively,including the preparation of Ag/TiAlONM/TiAlOND/AlON coating and its thermal stability and adhesion,and the optimization of SiN/Ag/SiN coating and its thermal stability.The main conclusions were obtained as follows:A series of Ag films were deposited on stainless steel by magnetron sputtering.The effects of different thicknesses,substrate roughnesses and deposition temperatures on both morphologies and optical properties are investigated.It shows that the spectrum reflectance tends to decrease with an increase of the substrate roughness and deposition temperature.With the thickness increasing,the reflectance increases at first and then decreases due to the growing and coarsening of grains.The minimum thermal emittance of 0.6%can be obtained with thickness of 120 nm.Ag/TiAlONM/TiAlOND/AlON coating was optimized by the optical fitting method of TFcale,and then it was deposited by magnetron sputtering.The actual thickness and optical properties of the coating are consistent with the theoretical designs.When the thickness of each layer is 120 nm,68 nm,27 nm and 35 nm,the absorptance of coating is 95.3%and the emittance at 400℃ is only 5.8%.Compared to the previous Cu-based spectral selective absorbing coating,Ag-based coating has lower emittance and high spectral selectivity after annealing at 600℃ for 96 h.In addition,100 nm TiN as the adhesive layer can effectively improve adhesion to 46.71 N between the Ag-based coating and stainless steel.SiN/Ag/SiN coating was optimized by the optical fitting method of TFcale,and then it was deposited by magnetron sputtering.When the thickness of each layer of SiN/Ag/SiN coating is 30 nm,15 nm and 35 nm,the transmittance is 73.1%in visible region and 17.2%in near infrared region respectively.Using single-variable method to optimize the thickness of each layer,it is found that as the thickness of inner and outer SiN layers increases,the visible and near-infrared transmittances increases and then decreases.After annealing at 200℃ for 16 h,the spectral selective transmission remains constant.However,when the temperature is higher than 300℃,the spectral selective transmission gradually recedes with annealing time increasing because of the interdiffusion between the Ag film and SiN layers.