Three-Dimensional Metallic Boron Carbide: Stability and Properties

The design of novel materials through the strategic modification of their structural building blocks represents a powerful approach to achieving significant advancements in materials science. This study thoroughly examines the structural, mechanical, electronic, acoustic, and thermodynamic properties of a three-dimensional monoclinic boron carbide (3D m-B₈C₈) structure using first-principles methods based on density functional theory (DFT). We introduce a unique cage-based 3D monoclinic boron carbide structure, constructed from 4-, 5-, and 6-membered rings, which demonstrates remarkable dynamic, thermal, and mechanical stability. Our advanced first-principles calculations reveal that this architecture exhibits metallic characteristics, as confirmed by both GGA-PBE and HSE06 hybrid functionals. In contrast to the ductile and low Vickers hardness 3D-B₆C₆, the 3D m-B₈C₈ displays significant brittleness, a high Vickers hardness of 45.40 GPa (32.36 GPa), a low Poisson's ratio of 0.188, and a universal anisotropic index of 0.903. When compared to established thermal coating (TBC) materials such as yttria-stabilized zirconia (YSZ), which has a fracture toughness range of 2.0 to 2.3 MPa m^1/2 and a minimum thermal conductivity of 2.20 W m⁻¹ K⁻¹, the 3D m-B₈C₈ demonstrates superior fracture toughness of 5.336 MPa m^1/2 and a minimum thermal conductivity of 3.773 W m⁻¹ K⁻¹. These exceptional characteristics suggest that 3D m-B₈C₈ could serve as a compelling candidate for applications in environmental protection, thermal barriers, and oxygen-resistant coatings. The material exhibits a Debye temperature of 1524.15 K, an acoustic Grüneisen constant of 1.240, and a phonon thermal conductivity of 85.52 W m⁻¹ K⁻¹ at 300 K. Its melting temperature is 3311.94 K, with a thermal expansion coefficient of 7.337 μK⁻¹ and notable phonon inelastic scattering. These findings expand the range of boron carbide materials with new properties, presenting exciting prospects for advanced engineering applications and encouraging further experimental synthesis efforts.