The Investigation On The Fabrication, Thermal Stability And Optical Properties Of High-temperature-resistant AgAl-Al₂O₃ Nanocermet Films

Simultaneously possessing high absorption in the visible and near-infrared region and low emissivity in the infrared region, cermet is often used as the high-temperature solar selective absorption coating material. As Ag shows high antioxidant capacity and excellent absorption in the visible region due to the surface plasmon resonance (SPR) effect, Ag nanoparticles are considered as the promising candidate to be used in the cermet. Alumina (Al2O3) is one of the commonly used ceramics in solar absorbers, owing to its low refractive index, good stability at high temperatures and ease of fabrication. Therefore, silver nanoparticles embedded Al2O3 ceramic matrix are proposed to construct nanocermet absorbing layers for high temperature solar selective absorbption coatings. Unfortunately, however, Ag nanoparticles are easily subjected to agglomeration and outward diffusion when working at elevated temperatures, which would limit its high-temperature applications. In this paper we will focus on the AgAl alloyed nanoparticle embedded Al2O3 nanocerment films, with the purpose of solvoing the stability issues of the cermet used at high temperatures.AgAl-Al2O3 nanocermet films were successfully prepared by cosputtering of silver, aluminum and alumina targets. The addition of Al would allow the easier nucleation (heterogeneous nucleation mode) and growth of nanoparticles in the cermet. As a result, the nanoparticle size decreased and particle density increased, and the shape of the particles turned out to be near-spherical as the doping content of Al increased. When the Al target and Ag target power were respectively set to be 8 W and 9 W, AgAl-Al2O3 nanocermet films exihibited excellent thermal tolerance even at 500℃ for 260 h under nitrogen ambient. The preferential outward diffusion and oxidation of Al atoms from the AgAl bimetallic alloy nanoparticles during the annealing process were crucial to suppress the migration of Ag atoms. Namely, there was a fresh alumina layer capped on the AgAl nanoparticle surface, which could boost the diffusion barrier and prevent Ag element from long-range diffusion. Moreover, appropriate volume fraction of AgAl benefited the thermal stability of AgAl-Al2O3 films.The optical property simulation on the Ag-Al2O3 absorption layer could provide the theoretical basis on the optimization design of AgAl-Al2O3 film solar selective absorbing coatings. By adjusting the power of Ag target and substrate bias, the Ag-Al2O3 cermets with continuous metal volume fraction were obtained. Then the optical constants of these cermets could be availabe by using spectroscopic ellipsometry. Based on the optical constants of Ag, Al2O3 and Ag-Al2O3, we have successfully built a physical model on the solar absorption coating, which is with the structure of the Al2O3/low-metal-volume fraction Ag-Al2O3/high-metal-volume fraction Ag-Al2O3/Al2O3/Ag/silica. According to the simulating results, the thicknese of each layer, in the order of the above-mentioned multilayer structure, is 30,20,60, 40, and 90 nm, respectively. The designed solar selective absorber showed high absorption (93.5%), and the infrared emissivity was quite low as expected.