Controllable oxidation of UHTCs thin film for high-performance solar spectral selective absorption coatings

Ultra-high temperature ceramics (UHTCs) based solar spectral selective absorption coatings (SSACs) have shown great potential in the field of solar-thermal conversion due to their excellent thermal stability and adjustable optical properties. Contrary to the conventional understanding that oxidation of UHTCs would lead to a decline in the performance of SSACs, our recent investigations have revealed that controlled oxidation of UHTCs can enhance the absorptance of SSACs. This review summarizes the oxidation behavior of UHTCs-based SSACs, analyzes their absorption mechanism, elaborates on the differences in oxidation kinetics between thin films and bulk materials, and summarizes the latest research on oxidation phenomena of UHTCs in SSACs. In this paper, we demonstrate that mild oxidation is capable of modulating the band structure of UHTCs, narrowing their band gap, and enhancing visible light absorption. Severe oxidation, on the other hand, induces the formation of a beneficial gradient optical constant structure within the coating, thereby minimizing light reflection to the maximum extent. Furthermore, we summarize and propose a pre-oxidation strategy for controllable oxidation, alongside a post-treatment method designed to inhibit continuous oxidation of the coating. Finally, we discuss the challenges associated with the precise control of the oxidation process and component distribution of UHTC-based SSACs.