Low‐temperature synthesis of high‐entropy (Hf_0.2Ti_0.2Mo_0.2Ta_0.2Nb_0.2)B₂ powders combined with theoretical forecast of its elastic and thermal properties

Abstract

A theoretical calculation combined with experiment was used to study highentropy (Hf 0.2 Ti 0.2 Mo 0.2 Ta 0.2 Nb 0.2 )B 2 (HEB-HfTiMoTaNb). The theoretical calculation suggested HEB-HfTiMoTaNb could be stable over a wide temperature range. Then, a novel solvothermal/molten salt-assisted borothermal reduction method was proposed to efficiently pre-disperse transitional metal atoms in a precursor and synthesize (Hf 0.2 Ti 0.2 Mo 0.2 Ta 0.2 Nb 0.2 )B 2 nanoscale powders at 1573 K for 6 h, which is nearly 300 K lower than previous reports. The characterization results indicated that the as-synthesized nanoscale HEB-HfTiMoTaNb powder was hexagonal single-phase with homogeneous elements distribution and uniform size, and the oxygen content of the particles is 0.97 wt%. Simultaneously, the mechanical properties, anisotropic nature, and thermal properties of HEB-HfTiMoTaNb were investigated by density functional theory (DFT) calculations. The Cannikin's law was adopted to explain the improvement of comprehensive mechanical properties. In addition, a significant reduction of thermal conductivity was observed for HEB-HfTiMoTaNb and it only was 1/15 of the value of HfB 2 . This work suggests a reliable technique for synthesis of nanosized HEB powders and discovery of high-entropy materials under the guidance of first-principle theory.

Low temperature controllable synthesis of transition metal and rare earth borides mediated by molten salt

“…212 Recently, Gao et al synthesised (Hf 0.2 Ti 0.2 Mo 0.2 Ta 0.2 Nb 0.2 )B 2 nanopowder via MSM-BRS in KCl at 1300 °C for 6 h, and predicted its properties such as hardness and thermal conductivity, based on the density functional theory (DFT) calculations. 213 While in many cases single-phase high entropy boride powder could be synthesised directly at a relatively low temperature via one of the molten salt mediated routes discussed above, in some cases, a mixture of multiple phases was obtained. Even so, owing to its good quality such as homogeneous structure and composition, high surface area and high reactivity/sinterablity, it could still be readily converted into the corresponding single phase high entropy boride powder or bulk ceramic, via post-synthesis heattreatment or densification, under appropriate conditions, as illustrated by Guo et al 214 Despite the fact that the product powder resultant from MSM-BRS in NaCl-KCl at 1100 °C comprised two boride phases, (Hf,Zr)B 2 and (Ta,Nb,Ti) B 2 , after 10 min spark plasma sintering (SPS) at 1900 °C and 50 MPa, the originally desired single-phase bulk (Zr 0.2 Hf 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )B 2 high-entropy ceramic was still obtained.…”

“…212 Recently, Gao et al synthesised (Hf 0.2 Ti 0.2 Mo 0.2 Ta 0.2 Nb 0.2 )B 2 nanopowder via MSM-BRS in KCl at 1300 °C for 6 h, and predicted its properties such as hardness and thermal conductivity, based on the density functional theory (DFT) calculations. 213…”