Spectrally selective response of ZrO<inf>x</inf>/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 ~1.0-2.5 μm wavelength range, together with other intrinsic absorptions, providing enhanced solar absorption in 0.3-2.5 μm 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 ~12.5 sccm nitrogen flow rate, exhibit the best solar thermal response among investigated structures with absorptance α ~ 0.88 and 0.85 and emittance ε27°C ~ 0.04 and 0.1 on stainless steel and copper substrates. Thermal studies showed high temperature stability at ~700 °C and 600 °C in vacuum for these solar selective coatings on SS and Cu substrates, whereas at or below 200 °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.