Improved spectral emissivity and mechanical properties of high-entropy (Ti0.2Zr0.2Nb0.2Mo0.2Hf0.2)B2 derived from boro/carbothermal reduction

High-purity, nano-sized (Ti0.2Zr0.2Nb0.2Mo0.2Hf0.2)B2 (TZNMH) diboride powders were successfully synthesized via a combined flocculation precipitation and boro/carbothermal reduction method. Then, the solid solution formation, thermal radiation performance and mechanical properties of TZNMH were investigated. It is found that high entropy effect endows TZNMH ceramic with outstanding near-blackbody radiation performance in the UV-Vis-NIR region, with a thermal emissivity ≥ 95%. This is because high entropy induces robust intraband transition. A dense and single-phase TZNMH ceramic was obtained by spark plasma sintering with high hardness (HV0.1 =31.6 GPa) which remained above 30 GPa even after 60 h exposure at 1800 °C in Ar environment. This high hardness of TZNMH ceramic is believed to arise from the strong hybridization between metals and boron, as revealed by density functional theory calculations. This study demonstrates that high-entropy boride ceramics can be used as ideal high-temperature structural component with high thermal emissivity.