Spectrally selective CrMoN/CrON tandem absorber for mid-temperature solar thermal applications

CrMoN(H)/CrMoN(L)/CrON tandem absorber was deposited on stainless steel (SS) and aluminum (Al) substrates by the sputtering method. The nitrogen flow rate was varied in order to obtain the CrMoN(H) (high metal content) and CrMoN(L) (low metal content) layers. The structural properties of the coatings were studied using the X-ray diffraction technique. The optimized tandem absorber on SS substrate exhibited high absorptance in the range of 0.90–0.92 and emittance in the range of 0.13–0.15. The optical constants of CrMoN(H), CrMoN(L) and CrON layers were obtained from spectroscopic ellipsometric measurements. The optical constants data revealed that the CrMoN(H) layer is the main absorber layer in the tandem absorber. The tandem absorber deposited on SS substrates was thermally stable up to 425 and 500 °C in air and vacuum for 7 h, respectively. The changes in the chemical composition of the tandem absorber were studied by x-ray photoelectron spectroscopy and micro-Raman spectroscopy techniques. The tandem absorber deposited on Al substrates qualified the performance criterion evaluation test and the service life time of the coating is more than 25 years.