Comparison of structure and solar-selective absorbance properties of Al2O3 thin films with Al and Ni reflector interlayers

Abstract

This work aims at evaluating the selective solar absorbance (SSA) of Al2O3 thin films deposited by physical vapor deposition (PVD) with two different metallic reflector interlayers. Al2O3 thin films are deposited on the same substrate stainless steel (SS) 304L, with two different interlayers between the thin film and the substrate, namely, nickel (Ni) and aluminum (Al) are used as the reflector interlayer SS304L/Al/Al2O3 and SS304L/Ni/Al2O3. A scanning electron microscope (SEM) was utilized to characterize the chemical composition by energy dispersive X-ray analysis (EDX) and surface morphology of the deposited thin films. The phases of the thin films were analyzed and identified by X-ray diffraction (XRD) to detect the present phases. The surface topography and the thickness of the deposited thin films were investigated using an atomic force microscope (AFM). The optical properties of the substrate and the deposited thin films (absorbance & emittance) in two conditions were identified by Fourier transform infrared spectroscopy (FTIR) and spectrophotometer. The obtained results demonstrate that both SS304L/Al/Al2O3 and SS304L/Ni/Al2O3 show good performance, such as high solar absorbance and low thermal emittance. However, the Al/Al2O3 thin film provides high selectivity (absorbance/emittance (α/ε)) of 0.916/0.05, compared to 0.913/0.15 for the Ni/Al2O3 coating. The effect of different properties and microstructure on the efficiency of deposited thin films showed that the SS304L/Al/Al2O3 has higher absorbance (92%) in visible and ultraviolet (UV) regions; and lower emittance (5%) than the SS304L/Ni/Al2O3. This work shows that the intermediate IR layer has a more pronounced effect on the emittance properties rather than the absorbance properties of the Al/Al2O3 layer.