Structure and optical properties of Ag-Al2O3 nanocermet solar selective coatings prepared using unbalanced magnetron sputtering

Ag-Al₂O₃ nanocermet spectrally selective solar absorber coatings were prepared at different Ag contents on copper, silicon and glass substrates using unbalanced magnetron sputtering technique. Asymmetric bipolar pulsed direct current power supply and radio frequency power supply were used to sputter Ag and Al₂O₃ targets, respectively. The optimized coating exhibited high absorptance (α=0.93) in the visible region and low emittance (ε=0.04-0.05 at 82 °C) in the infrared region of the solar spectrum. Presence of the strong absorption band in the absorber coating is due to the surface plasmon resonance, i.e., collective oscillation of the conduction band electrons under the influence of the optical excitation. Atomic force microscopy, field emission scanning electron microscopy (FESEM), X-ray diffraction, micro-Raman spectroscopy, spectroscopic ellipsometry and spectrophotometer were used to characterize the nanostructure, composition and optical properties of these coatings. The face centered cubic crystalline structure of Ag nanoparticles inclusion in the amorphous alumina dielectric matrix was confirmed using X-ray diffraction. The size distribution and concentration of Ag nanoparticles embedded in Al₂O₃ dielectric matrix was studied using FESEM image analysis. The variations of refractive index and extinction coefficient with wavelength were obtained using phase modulation spectroscopic ellipsometry. The variation of absorption with wavelength in the UV-vis region was characterized using spectrophotometer. In order to study the thermal stability of the absorber coatings, they were annealed in vacuum at different temperatures (i.e., 200-400 °C) for 2 h. For the vacuum annealed coatings (heated up to 400 °C), chemical/micro-structural changes were studied using micro-Raman spectroscopy and FESEM. No shift in the Raman peaks for the Al₂O₃ was observed, confirming its structural stability in the absorber coatings with annealing in vacuum up to 400 °C. However, FESEM image analysis confirmed that the degradation in the vacuum annealed coatings was due to defragmentation of the Ag nanoparticles.