Toward high-temperature thermal tolerance in solar selective absorber coatings: choosing high entropy ceramic HfNbTaTiZrN

The most recent advances in high-entropy materials provide impetus for the development of high-performance materials, simultaneously providing high-temperature robustness and excellent functional properties owing to the high configurational entropy and distorted lattices. Thus, in this work, double-layer high-entropy alloy nitride HfNbTaTiZrN with a well-designed metal content gradient is employed to fabricate a solar selective absorber coating (SSAC). We extensively investigated whether the coating meets the definition of a high-entropy material from the points of elemental analysis and phase structure, combined with thermodynamic calculation. Remarkably, the new, tailored SSAC exhibits an exceptionally high solar absorptance (α = 96%) and a suppressed thermal emittance (e = 8.2%) at 82 °C and omnidirectional absorption. Investigations of the long-term thermal stability indicate that the HfNbTaTiZrN-based SSAC due to the entropy-driven structural stabilization could endure heat treatment at 600 °C for 168 h, retaining a performance criterion (PC) value less than 0.05, which implies the feasibility of practical applications. More importantly, the photothermal conversion efficiency (η), which is utilized to quantitatively evaluate the performance at elevated temperatures, reaches 90.1% at a working temperature of 550 °C under 100 suns, and even after annealing at 600 °C for 168 h, the efficiency drop is still less than 5%. Overall, the combination of thermal robustness and photothermal conversion efficiency at working temperatures less than 600 °C provides significant potential to maximize solar energy harvesting and pioneers an opportunity to explore simultaneous multifunctional applications of high-entropy alloys.