Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes.

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (LaCeNdEu)B high-entropy rare-earth hexaboride (HEReB-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm at 1873 K. Such superior performance makes the nanocrystalline HEReB-1 ceramics highly suitable for potential applications in thermionic emission cathodes.