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

Area of Science:

• Materials Science
• Solid State Chemistry
• Nanotechnology

Background:

• High-entropy borides offer significant potential for advanced structural and functional applications.
• Research on high-entropy borides, particularly rare-earth hexaborides, remains limited.

Purpose of the Study:

• To synthesize and characterize novel nanocrystalline high-entropy rare-earth hexaboride (HEReB₆-1) ceramics.
• To evaluate the mechanical, electrical, and thermionic emission properties of the fabricated HEReB₆-1 ceramics.

Main Methods:

• Fabrication of HEReB₆-1 ceramics via high-pressure sintering of self-synthesized nanopowders.
• Microstructural analysis using electron microscopy to assess density, grain size, and texture.
• Mechanical testing (Vickers hardness, fracture toughness) and electrical resistivity measurements.
• Thermionic emission measurements at elevated temperatures.

Main Results:

• Successfully synthesized dense (96.3%), nanocrystalline (94 nm) HEReB₆-1 ceramics with (001) fiber texture and clean grain boundaries.
• Achieved superior mechanical properties: Vickers hardness of 23.4 ± 0.6 GPa and fracture toughness of 3.0 ± 0.4 MPa•m^1/2.
• Demonstrated high electrical resistivity from room temperature to high temperatures, exceeding binary counterparts.
• Exhibited high thermionic emission current densities, reaching 48.3 A•cm^-2 at 1873 K, surpassing binary rare-earth hexaborides.

Conclusions:

• Nanocrystalline HEReB₆-1 ceramics possess outstanding comprehensive performance.
• The superior properties make these ceramics highly suitable for thermionic emission cathode applications.
• This work pioneers the development of high-performance high-entropy rare-earth hexaborides.