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.