Optical design, microstructural characterization and high-temperature in-air stability study of solar selective coatings based on aluminium- (titanium, chromium) oxynitride multilayers

The development of new solar selective coatings (SSCs) operating in air at high temperatures is an actual challenge for the development of Generation 3 concentrated solar power (CSP) plants. In particular, current central tower systems operate at maximum temperatures of 550 ºC mainly due to the severe degradation that the state of the art absorber paints (i.e. Pyromark®) suffer at higher temperatures. Aluminium metal oxynitrides Al_yMe_1-yO_xN_1-x (Me = Ti, Cr) prepared by physical vapour deposition (i.e. cathodic vaccuum arc and HiPIMS) were selected as candidate materials for SSCs on the basis of stability considerations of the tentatively formed nitrides and oxides. The optical properties of these films can be controlled in a wide range from a metallic to a dielectric character by varying the oxygen and nitrogen content.
In the last years we have performed an extensive research on the design, fabrication and high-T exposure of SSCs based on aluminium-titanium [1,2] and aluminium-chromium oxynitrides [3]. Once single thin films were fully characterized by ion beam analysis, scanning and transmission electron microscopy and X-ray diffraction, complete SSCs were designed with optical simulations, based on measured optical constants of each of the individual layers, providing excellent optical selective properties in terms of solar absorptance (α) and thermal emittance (εRT). The selected multilayers stacks were deposited, obtaining excellent agreement between simulated and experimental reflectance spectra. Finally, the thermal stability in air of the complete deposited SSCs was analyzed by isothermal and cyclic heating tests simulating operating conditions. Al_yTi_1-yO_xN_1-x SSCs showed no degradation after 750h of cycles in air at 600ºC and these results were compared with in-situ high temperature annealing performed in vacuum at the multi-chamber cluster tool situated at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) [4], confirming that these stacks withstand breakdown at 600ºC in air and 800ºC in vacuum. Al_yCr_1-yO_xN_1-x SSCs stacks presented a good solar selectivity with solar absorptance > 95% and thermal emittance < 15%, and fulfilled the performance criterion after 600 and 700 ºC short term heating treatments. At 800 ºC, they underwent a further structural transformation, provoked by the oxidation of the inner layers, and they consequently lost their solar selectivity.

In this invited talk, we will summarize these results and present current research strategies to further improve the performace of the developed materials.

1. I. Heras, E. Guillén, F. Lungwitz, G. Rincón-Llorente, F. Munnik, E. Schumann, I. Azkona, M. Krause, R. Escobar-Galindo. Sol. Energy Mater. Sol. Cells 176 (2018) 81-92.
2. R. Escobar-Galindo, E. Guillén, I. Heras, G. Rincón-Llorente, M. Alcón-Camas, F. Lungwitz, F. Munnik, E. Schumann, I. Azkona, M. Krause. Sol. Energy Mater. Sol. Cells 185 (2018) 183-191.
3. T.C. Rojas, A. Caro, R. Escobar-Galindo, J.C. Sánchez López. High-temperature solar-selective coatings based on Cr(Al)N. Part 2: Design, spectral properties and thermal stability of multilayer stacks. Sol. Energy Mater. Sol. Cells. 218 (2020) 110812
4. R. Wenisch, F. Lungwitz, D. Hanf, R. Heller, J. Zscharschuch, R. Hübner, J. von Borany, G. Abrasonis, S. Gemming, R. Escobar-Galindo, M. Krause. Anal. Chem. 90 (13) (2018) 7837-7842.