Spectrally selective response of ZrOx/ZrC–ZrN/Zr absorber–reflector tandem structures on stainless steel and copper substrates for high temperature solar thermal applications

•Design & development of ZrOx/ZrC–ZrN/Zr tandem absorber–reflector layer structure and structure–property correlation.•Effect of electronic contribution in the enhanced absorption and its optimization using different synthesis parameters.•Microsturctural properties (especially after heat treatment) and their impact on solar performance.•High temperature stability ∼700 °C in vacuum and ∼200 °C in open air.

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.