Medium- and High-Entropy Rare Earth Hexaborides with Enhanced Solar Energy Absorption and Infrared Emissivity

The development of a new generation of solid particle solar receivers (SPSRs) with high solar absorptivity (0.28-2.5 μm) and high infrared emissivity (1-22 μm) is crucial and has attracted much attention for the attainment of the goals of "peak carbon" and "carbon neutrality". To achieve the modulation of infrared emission and solar absorptivity, two types of medium- and high-entropy rare-earth hexaboride (ME/HEREB₆) ceramics, (La_0.25Sm_0.25Ce_0.25Eu_0.25)B₆ (MEREB₆) and (La_0.2Sm_0.2Ce_0.2Eu_0.2Ba_0.2)B₆ (HEREB₆), with severe lattice distortions were synthesized using a high-temperature solid-phase method. Compared to single-phase lanthanum hexaboride (LaB₆), HEREB₆ ceramics show an increase in solar absorptivity from 54.06% to 87.75% in the range of 0.28-2.5 μm and an increase in infrared emissivity from 76.19% to 89.96% in the 1-22 μm wavelength range. On the one hand, decreasing the free electron concentration and the plasma frequency reduces the reflection and ultimately increases the solar absorptivity. On the other hand, the lattice distortion induces changes in the B-B bond length, leading to significant changes in the Raman scattering spectrum, which affects the damping constant and ultimately increases the infrared emissivity. In conclusion, the multicomponent design can effectively improve the solar energy absorption and heat transfer capacity of ME/HEREB₆, thus providing a new avenue for the development of solid particles.