New selective solar absorbent coatings based on high entropy alloys
High entropy alloys are a newcomer to the alloy world and go beyond the concept of conventional alloy design, with more than five of their building blocks. They also have a simple solid solution structure due to their high configuration entropy.
High entropy alloys exhibit superior mechanical properties, have the ability to withstand heat and are resistant to corrosion, wear and irradiation, thus offering great potential for advancement in the renewable energy industry.
A team of researchers led by Associate Professor Xianghu Gao and Professor Gang Liu at the Lanzhou Institute of Chemical Physics in Chinese Academy of Sciencess (CAS) recently synthesized a range of selective high temperature solar absorber coatings based on high entropy alloys using the methods of component modulation, conformational entropy optimization and structural design with high entropy nitrides. high entropy.
The team designed a color selective solar absorbent coating with an infrared reflective layer of high entropy alloy nitride, aluminum, silicon dioxide and high entropy alloy nitrogen oxide. This allows a thermal emittance of less than 10% and a high solar absorption of 93.5%.
Next, they identified that a single layer of high entropy alloy nitride ceramic also exhibited optimal intrinsic absorption properties, and thus, they synthesized a coating with a simple structure.
High entropy alloy nitride was used as absorption layer and Si3NOT4 or SiO2 was used as an anti-reflective coating. Thus, a coating with an absorption of 92.8% and an emissivity of less than 7% was obtained, exhibiting ideal thermal stability under vacuum at 650 ° C for 300 hours.
The researchers further improved the absorptive capacity of the absorber by choosing high entropy low nitrogen alloy films as the main absorption layer, stainless steel as the substrate, high entropy alloy films with high nitrogen content. entropy with high nitrogen content as extinguishing interference layer and Al2O3, SiO2 or if3NOT4 as an anti-reflective coating. This led to a structure with gradually reduced optical constants from the substrate to the surface.
The researchers used a combination of optical simulation and magnetron sputtering techniques to improve the efficiency of the preparation, contributing to 96% higher solar absorption and low thermal emission of less than 10%.
Next, the researchers analyzed the mechanism of light absorption with a finite difference time domain simulation. The high entropy alloy nitride absorber still retained optimum optical properties even after annealing at 600 ° C under vacuum for 168 hours, denoting its excellent thermal stability.
The photothermal conversion efficiency of the absorber was estimated at various concentration ratios and operating temperatures, and the efficiency could reach 90.1% at an operating temperature of 550 ° C and 100 suns.
Compared with the most advanced absorber recently reported, the high entropy alloy based absorber has shown both higher photothermal conversion efficiency and thermal stability.
The absorbers have been coated on various substrate materials, retaining superior optical properties and achieving large scale preparation on aluminum foil. A study of the absorption spectra at various angles of incidence showed that the absorption coating exhibited good absorption in the range of the incident light angle of 0 ° to 60 °.
When subjected to the radiation of simulated sunlight, the surface temperature of the coating exceeded 100 ℃, showing that the material might hold promise for the field of interfacial water evaporation.
High temperature solar selective absorbent coatings with superior optical properties and high temperature resistance were prepared as part of this study, expanding the functional applications of high entropy alloys in the field of new energetic materials.
This study was funded by the Youth Innovation Promotion Association of CAS, the Regional Key Projects of the CAS Science and Technology Services Network Program, and the Major Science and Technology Projects of Gansu Province.
Guo, H.-X., et al. (2021) A new high temperature colored multilayer solar absorbent coating based on a high entropy MoNbHfZrTi alloy: optimized preparation and chromaticity study. Solar energy materials and solar cells. doi.org/10.1016/j.solmat.2020.110444.