Exceptional high-temperature in-air stable solar absorber coatings based on aluminium titanium oxynitride nanocomposites

The in-air stability of a novel solar absorber coating type based on aluminium titanium oxynitride nanocomposites deposited by Physical Vapour Deposition (PVD) is studied up to temperatures of 800 degrees C. The microstructural and morphological characterization by high resolution electron microscopy and glancing angle X-ray diffraction reveals the creation of a nanocomposite structure formed by crystalline AlTiN nanoparticles inserted in an oxide matrix. This nanocomposite structure (nc-AlTiON) is responsible for the high absorption within the whole solar wavelength range (0.3-2.5 mu m) that the absorber exhibits. The as-deposited absorber has a solar absorptance, alpha, of 92 % and room temperature emissivity, epsilon RT, of 70 %. The deposition of an antireflecting Al2O3 top layer lowers the reflectance of the sample in the UV-Vis-NIR region and, consequently the solar absorptance increases up to 93.5 %. Post-deposition thermal treatments of 2 h at 800 degrees C further improve the solar absorptance and emissivity of the absorber (alpha = 96 %, epsilon RT = 60 %). Thus, this nc-AlTiON/Al2O3 coating presents solar performances that match, within the experimental errors, the ones calculated for the commercial absorber paint Pyromark (R) at C = 1000 (typical of central tower receivers) or even surpass it for concentration factors typical of parabolic trough receivers (C = 100) under the same operating temperatures. Very importantly, the studied nc-AlTiON/Al2O3 solar absorber coating shows no degradation after 1000 h of thermal cycling in air between 300 and 700 degrees C. Therefore, the developed coating shows the best thermal in-air long-term stability up to 700 degrees C reported so far for PVD-based solar absorbers under cycling conditions, and it can be considered as a promising absorber candidate for Generation 3 (GEN3) CSP plants.