Thermal stability of electron beam evaporated AlxOy/Pt/AlxOy multilayer solar absorber coatings

• AlxOy/Pt/AlxOy multilayer absorber was thermally stable in air up to 500 °C for 2 h.
• The multilayer absorber coating was thermally stable in air up to 450 °C for 24 h.
• The coatings can be used for high temperature photothermal conversion applications.

This paper reports the effect of thermal annealing on AlxOy/Pt/AlxOy multilayer solar absorber coatings deposited by an electron beam (e-beam) vacuum evaporator onto copper substrate at room temperature. The samples were annealed at different temperatures for different duration in air. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), Raman spectroscopy, UV–vis spectrophotometer and emissometer. The results showed changes in morphology, structure, composition, and optical properties depend on both temperature and duration of annealing. The XRD pattern showed the intensity of Pt decreased with increasing annealing temperature and therefore, disappeared at high temperature. With increasing annealing temperature, an increase in the size of Pt particles was observed from SEM. The AlxOy/Pt/AlxOy multilayer solar absorber coatings deposited onto Cu substrate were found to be thermally stable up to 500 °C in air for 2 h with good spectral selectivity (α/ε) of 0.951/0.09. At 600 °C and 700 °C, the spectral selectivity decreased to 0.92/0.10 and 0.846/0.11 respectively, which is attributed to the diffusion of Cu and formation of CuO and Cu2O phases. The formation of CuO and Cu2O phases were confirmed by XRD, Raman spectroscopy and optical modelling. Long term thermal stability study showed the coatings were thermally stable in air up to 450 °C for 24 h.