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A novel WAlN/WAlON/Al 2 O 3 coating was successfully deposited on stainless steel (SS) substrate using reactive DC and RF magnetron sputtering. Excellent spectrally selective property with a high absorptance of 0.958 in the solar spectrum region and low emittance of 0.08 in the infrared region were achieved by tailoring the target power, deposition time and the reactive flow rates of N 2 and O 2 . In the present solar selective coating, W layer acts as a back reflector and diffusion barrier, WAlN as the main absorber layer, WAlON as the semi-absorber layer, whereas the topmost Al 2 O 3 layer works as an anti-reflecting layer. In reference to the thermal stability, the absorber deposited on SS substrates exhibited high solar selectivity (α/ε) of 0.920/0.11, when heat treated in air up to 500°C for 2 hours. Taken together, the present study demonstrated that the WAlN/WAlON/Al 2 O 3 -based selective absorbing coating with excellent thermal stability could be a promising material for photo-thermal conversion at temperatures of up to 500°C.

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Spectral emissivity is considered as one of the most critical thermophysical properties influencing photothermal conversion efficiency of solar selective absorbers. In addition, long-term stability at high temperature and thermal shock resistance are the performance-limiting properties of spectrally selective absorbers. In this context, this study reports the variation of emissivity with a change in emergence angles and operational temperatures for the newly developed W/WAlN/WAlON/ Al 2 O 3 absorber. An analysis of the experimental results demonstrates that hemispherical emissivity values at elevated temperature are comparable while calculated using both room temperature and high temperature reflectance data. Hence, the applicability of the room temperature measurement method is validated to evaluate high temperature emissivity. The analysis of angular measurements indicates an insignificant difference between hemispherical and near-normal emissivity values for W/WAlN/WAlON/Al 2 O 3 . The study suggests that hemispherical emissivity can be well approximated from near-normal emissivity values by avoiding complex angular measurement procedure. Importantly, one can achieve a combination of high solar absorptance (α = 0.90), low thermal emittance (ε = 0.15), and appreciable heliothermal efficiency (η = 87% at a concentration factor of 100) at 500 °C for the W/WAlN/WAlON/Al 2 O 3 absorber. Thermal stability of this absorber was established by observing an insignificant change in the reflectance spectra while annealed at 80, 200, 300, and 400 °C. In addition, thermal cycling test for 30 times between room temperature and 450 °C in a high flux (40−60 kW/m 2 ) solar simulator confirmed the efficacy of W/WAlN/WAlON/Al 2 O 3 as a promising multilayer solar absorber for high temperature applications.

In this paper, we report the solar absorptance property of a new tandem absorber consisting of layers W/WAlN/WAlON/Al 2 O 3 that holds promise for solar thermal energy harvesting. The coating was prepared by DC/RF magnetron sputtering on stainless steel substrate. The performance of the film was investigated by measuring the reflectance spectra in the wavelength range of 250-2500 nm and most importantly by varying the incident angle from 18° to 68°. The effect of thermal annealing on the optical properties, microstructure and morphology of the solar selective absorber coating was also explored. The thermal annealing of the coating at 350°C till 550 hrs in air did not result in any significant change in the spectral properties of the absorber coating. The excellent thermal stability and wide range of angular absorptance of this multilayer coating indicate the potential for application as selective coating in mid temperature photothermal conversion systems.