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 °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 μm) that the absorber exhibits. The as-deposited absorber has a solar absorptance, α, of 92 % and room temperature emissivity, εRT, 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 °C further improve the solar absorptance and emissivity of the absorber (α = 96 %, εRT = 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® 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 °C. Therefore, the developed coating shows the best thermal in-air long-term stability up to 700 °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.