Probing the spectrally selective property of NbB2-based tandem absorber coating for concentrated solar power application

Solar selective coating with good thermal stability is the primary requirement for a concentrated solar power (CSP) plant to function with better photothermal efficiency. In recent years, ultra-high-temperature ceramic-based coatings have been explored as potential materials for solar selective coatings. In this context, NbB2/Nb(BN)/Al2O3 tandem absorber coating was designed to be fabricated on a stainless-steel substrate by the radio frequency magnetron sputtering of spark plasma sintered ceramic target. In the bulk form, the NbB2 ceramic exhibits high solar absorptance (alpha = 0.756) and thermal emissivity (epsilon = 0.43), whereas the amorphous single NbB2 coating exhibits alpha/epsilon = 0.716/0.13. Reactive sputtering of NbB2 in nitrogen produced a semi-transparent coating with an optical bandgap of similar to 2.80 eV and was used as the secondary absorber layer. Raman and X-ray photoelectron spectroscopy analyses reveal mild oxygen incorporation in the absorber layers. The developed SS/NbB2/Nb(BNO)/Al2O3 tandem absorber exhibits a good solar absorptance of 0.950 and a moderate thermal emissivity of 0.15 at room temperature. The coatings exhibited good thermal stability when heated in vacuum for 5 h up to 700 degrees C, and the selectivity (alpha/epsilon) remains above 6. The present work shows the possibility of exploring NbB2-based tandem absorber coatings for CSP applications.