Solar Energy, Vol.134, 353-365, 2016
Spectrally selective response of ZrOx/ZrC-ZrN/Zr absorber-reflector tandem structures on stainless steel and copper substrates for high temperature solar thermal applications
ZrOx/ZrC-ZrN/Zr absorber-reflector tandem layered structures were fabricated on stainless steel (SS) and copper (Cu) substrates using DC/RF magnetron sputtering system. ZrC-ZrN absorber layer was grown on the Zr infrared reflector, in conjunction with the top ZrOx anti-reflecting layer. Absorbing properties, of ZrC-ZrN absorber layer, were optimized by varying nitrogen flow during deposition of this layer. The optimized ZrN fraction in a ZrC-ZrN layer showed additional plasmonic absorption in similar to 1.0-2.5 mu m wavelength range, together with other intrinsic absorptions, providing enhanced solar absorption in 0.3-2.5 mu m wavelength range. The detailed structural, micro-structural, surface and optical characterization showed the strong structure - solar thermal property correlation. We observed that absorber-reflector tandem structures, fabricated at similar to 12.5 sccm nitrogen flow rate, exhibit the best solar thermal response among investigated structures with absorptance alpha similar to 0.88 and 0.85 and emittance epsilon(27 degrees C) similar to 0.04 and 0.1 on stainless steel and copper substrates. Thermal studies showed high temperature stability at similar to 700 degrees C and 600 degrees C in vacuum for these solar selective coatings on SS and Cu substrates, whereas at or below 200 degrees C in the air. These studies suggest that ZrOx/ZrC-ZrN/Zr absorber-reflector tandem structures may be a good choice for high temperature applications under vacuum/inert conditions. (C) 2016 Elsevier Ltd. All rights reserved.
Keywords:High temperature solar selective coatings;Absorber-reflector tandem;Sputtering;Optical properties;Structural properties;Thermal stability