Ultrafine-grained (Sm_0.2Gd_0.2Dy_0.2Er_0.2Yb_0.2)₂Zr₂O₇ high-entropy zirconates with single fluorite structure have been fabricated by high-pressure sintering of the self-synthesized nanopowders for the first time. The as-sintered samples exhibit a good microstructure with a grain size of 220 nm and a relative density of 96.8%, which yield excellent comprehensive mechanical properties with a high Vickers hardness of 12.5 GPa and a high fracture toughness of 3.4 MPa·m^1/2. In addition, the as-sintered samples possess a good thermostability with the grain growth rate of 30 nm/h, and a low thermal conductivity of 1.57 W·m⁻¹·℃⁻¹ at room temperature. The superior mechanical and thermal properties are primarily attributed to the “high-entropy” and grain-refinement effects and good interface bonding.

The high-purity and superfine high-entropy zirconate nanopowders, namely (Y_0.25La_0.25Sm_0.25Eu_0.25)₂Zr₂O₇ nanopowders, without agglomeration, were successfully synthesized via polymerized complex method at low temperatures for the first time. The results showed that the crystallinity degree, lattice strain, and particle size of the as-synthesized powders were gradually enhanced with the increase of the synthesis temperature from 800 to 1300 ℃. The as-synthesized powders involved fluorite phase in the range of 800-1200 ℃ while they underwent the phase evolution from fluorite to pyrochlore at 1300 ℃. It is worth mentioning that the as-synthesized powders at 900 ℃ are of the highest quality among all the as-synthesized powders, which is due to the fact that they not only possess the particle size of 11 nm without agglomeration, but also show high purity and good compositional uniformity.

High-entropy nanomaterials have been arousing considerable interest in recent years due to their huge composition space, unique microstructure, and adjustable properties. Previous studies focused mainly on high-entropy nanoparticles, while other high-entropy nanomaterials were rarely reported. Herein, we reported a new class of high-entropy nanomaterials, namely (Ta_0.2Nb_0.2Ti_0.2W_0.2Mo_0.2)B₂ high-entropy diboride (HEB-1) nanoflowers, for the first time. Formation possibility of HEB-1 was first theoretically analyzed from two aspects of lattice size difference and chemical reaction thermodynamics. We then successfully synthesized HEB-1 nanoflowers by a facile molten salt synthesis method at 1423 K. The as-synthesized HEB-1 nanoflowers showed an interesting chrysanthemum-like morphology assembled from numerous well-aligned nanorods with diameters of 20-30 nm and lengths of 100-200 nm. Meanwhile, these nanorods possessed a single-crystalline hexagonal structure of metal diborides and highly compositional uniformity from nanoscale to microscale. In addition, the formation of the as-synthesized HEB-1 nanoflowers could be well interpreted by a classical surface-controlled crystal growth theory. This work not only enriches the categories of high-entropy nanomaterials but also opens up a new research field on high-entropy diboride nanomaterials.

Nanocrystalline HfB₂ powders were successfully synthesized by molten salt synthesis technique at 1373 K using B and HfO₂ as precursors within KCl/NaCl molten salts. The results showed that the as-synthesized powders exhibited an irregular polyhedral morphology with the average particle size of 155 nm and possessed a single-crystalline structure. From a fundamental aspect, we demonstrated the molten-salt assisted formation mechanism that the molten salts could accelerate the diffusion rate of the reactants and improve the chemical reaction rate of the reactants in the system to induce the synthesis of the high-purity nanocrystalline powders. Thermogravimetric analysis showed that the oxidation of the as-synthesized HfB₂ powders at 773–1073 K in air was the weight gain process and the corresponding oxidation behavior followed parabolic kinetics governed by the diffusion of oxygen in the oxide layer.