Study Of Medium-Frequency Magnetron Sputtering Of Ni-AlN Solar Selective Absorbing Films

Solar selective absorbing films, which are a key technology for solar energy thermal utilization, were restricted in application for their disadvantages such as poor high temperature stability, poor process stability and complex fabrication process. The industrial fabrication technology for solar selective absorbing films with excellent optical properties and high temperature stability at low cost can be developed through the combination of the Ni-AlN material system and medium frequency magnetron reactive sputtering.The influence of medium frequency magnetron reactive sputtering process on the structure and properties of TiN were studied, and it is shown that nitrogen flow rate and Ti target power are the key factors.The influence of medium frequency magnetron reactive sputtering process, such as nitrogen flow rate, argon flow rate, Al target current, and magnetic field on Al target surface, on the discharge characteristics of Al target and the microstructures of deposited AlN films were studied. It was found that the discharge characteristics of Al target was obviously affected by nitrogen flow rate, argon flow rate, Al target current, and magnetic field on Al target surface; Al target voltage was decreased with increasing nitrogen flow rate, and the decreasing rate of voltage with increasing nitrogen flow rate was obviously increased after the nitrogen flow rate exceed a critical value; the critical nitrogen flow rate corresponding with serious poisoning of Al target could be increased by decreasing argon flow rate or increasing Al target current. With increasing the nitrogen flow rate, the nitrogen content in AlN films was increased, the phase composition was changed from mainly pure aluminum to pure AlN, and the density of AlN films were improved also. The synthesis of pure AlN with an ideal stoichiometric proportion became more difficult and the crystalline structure of AlN films was changed from nanocrystalline to amorphous phase with increasing argon flow rate or decreasing Al target current; the density of AlN films could be improved with increasing argon flow rate and Al target current.The reactive cosputtering process and microstructure of Ni-AlN composite films were studied. It is shown that the critical nitrogen flow rate corresponding with serious poisoning of Al target was increased when Al target was ignited with Ni target. Ni-AlN films, having finer grains on their surface and a smaller surface roughness, was composed of amorphous AlN and nanocrystalline Ni; the atomic ratio of nitrogen to aluminum was lower than the ideal stoichiometric proportion of AlN and nickel content was obviously higher than aluminum content in Ni-AlN composite films.The influences of the structure of graded Ni-AlN solar selective absorbing films on their properties were studied and the graded Al-AlN solar selective absorbing films were fabricated. It it shown that the visible light reflectance of graded Ni-AlN solar selective absorbing films was reduced with increasing the layer number of Ni-AlN absorbing layer, and it could be further obviously decreased after depositing AlN antireflection layer. The visible light reflectance of graded Ni-AlN solar selective absorbing films was reduced with decreasing the composition gradual slope of graded Ni-AlN solar selective absorbing films; the visible light reflectance of graded Ni-AlN solar selective absorbing films was less than 15% and an obvious spectral selectivity was found when the deposition duration of every Ni-AlN absorbing layer was 10min.