Thermal aging characteristics of CrNxOy solar selective absorber coating for flat plate solar thermal collector applications

• Cu/CrNxOy/SiO2 coatings were prepared for flat plate solar thermal collector.
• The coating exhibited an absorptivity of 0.947 and an emissivity of 0.05 at 80 °C.
• The coatings on Cu substrates were thermally stable at 278 °C in air for 300 h.
• The structural, chemical, and optical evolution of the coatings were investigated.
• The Achilles' heel for the coatings to sustain high thermal stability was revealed.

A solar selective absorber coating of CrNxOy/SiO2 was prepared on Cu (Si) substrate using DC reactive magnetron sputtering technique. The coating exhibits a high absorptivity (α) of 0.947 and a low emissivity (ε) of 0.05 at 80 °C. The spectral selectivity (α/ε) of the coating on Cu substrate is stable (0.930/0.073) even after heat-treatment at 278 °C in air for 300 h, but decreased (0.904/0.135) at 278 °C for 600 h. The determinants to govern the thermal stability were investigated by micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) measurements, which reveal that the element diffusion whether throughout all the stacked layers or near the interface region, the chemical interactions adjacent to the interface, and the interface width broadening are the Achilles' heel for the solar thermal coatings to sustain high thermal stability.