Ab Initio Prediction of Mechanical and Electronic Properties of Ultrahigh Temperature High‐Entropy Ceramics (Hf_0.2Zr_0.2Ta_0.2M_0.2Ti_0.2)B₂ (M = Nb, Mo, Cr)

Abstract

High-entropy metal diborides (Hf 0.2 Zr 0.2 Ta 0.2 M 0.2 Ti 0.2 )B 2 (M ¼ Nb, Mo, Cr) are a new class of ultrahigh temperature ceramics. The novel structural, mechanical, and electronic properties of (Hf 0.2 Zr 0.2 Ta 0.2 M 0.2 Ti 0.2 )B 2 (M ¼ Nb, Mo, Cr) are studied within density functional theory, with the compositional disorder being treated using the special quasi-random structures technique. The elastic stiffness constants C ij , as well as bulk, shear and Young's modulus of (Hf 0.2 Zr 0.2 Ta 0.2 M 0.2 Ti 0.2 )B 2 (M ¼ Nb, Mo, Cr) are studied to estimate the mechanical properties. The ductility is also investigated from several criterions, showing that (Hf 0.2 Zr 0.2 Ta 0.2 M 0.2 Ti 0.2 )B 2 (M ¼ Nb, Mo, Cr) is still intrinsically brittle. The high Debye temperature of (Hf 0.2 Zr 0.2 Ta 0.2 M 0.2 Ti 0.2 )B 2 (M ¼ Nb, Mo, Cr) indicates the strong interatomic interactions and good thermal conductivity. The detailed investigations of electronic structure reveal that covalent and metallic bonds are respectively formed in the boron layer and the metal layer, mixed ionic and covalent bonds are formed between the metals and boron interlayer.

Two‐step synthesis process for high‐entropy diboride powders

“… and a = 0.3103(3) nm and c = 0.3367(9) nm from Ref. ), but lower than values of a = 0.3116 nm and c = 0.3391 nm predicted in a previous computational study . The larger lattice parameters measured in the present study indicated that more of the larger transition metal species (eg, a = 0.317 nm and c = 0.354 nm for ZrB 2 ) were incorporated into the lattice of the HEB powders.…”

“…12 ), but lower than values of a = 0.3116 nm and c = 0.3391 nm predicted in a previous computational study. 13 The larger lattice parameters measured in the present study indicated that more of the larger transition metal species (eg, a = 0.317 nm and c = 0.354 nm for ZrB 2 ) were incorporated into the lattice of the HEB powders. These results indicated that the two step synthesis process used in the present study was better at producing uniform solid solutions than other processes.…”

“…1-3 ZrB 2 and HfB 2 are the most studied boride UHTCs due to their melting temperatures in excess of 3000°C, thermal conductivities that can be over 100 W/m K at 25°C, superior mechanical properties, and good chemical stability. [4][5][6] Recently, high-entropy ultra-high temperature ceramics (HE-UHTCs), including high-entropy boride (HEB), [7][8][9][10][11][12][13] high-entropy carbide (HEC), [14][15][16][17][18] and high-entropy nitride (HEN) 19 ceramics have been reported. HEB materials are solid solutions of individual transition metal diborides that have attracted increasing interest due to the high thermal and electrical conductivities, and chemical stability.…”

A step forward from high-entropy ceramics to compositionally complex ceramics: a new perspective

“…Although the initial work on high-entropy borides resulted in relatively low densities (~ 92%) due to significant oxide contamination from high-entropy ball milling, several subsequent studies quickly improved powder synthesis and fabrication methods that enhanced relative densities and properties [46,47,[49][50][51][52][53][54][55][56]. In 2018 and 2019, several groups [21,22,[57][58][59][60][61][62][63][64][65][66] also independently reported the fabrications of high-entropy carbides as another subclass of high-entropy UHTCs.…”

“…Additionally, modeling has revealed the importance of size and interatomic force constant disorder resulting in thermally-insulative HECs [78,[108][109][110]. Various other modeling studies have been conducted [60,[111][112][113][114][115]. A complete review and critical assessment of the modeling of HECs is beyond the scope of this perspective article.…”