Porous high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2: A novel strategy towards making ultrahigh temperature ceramics thermal insulating

Transition metal diborides based ultrahigh temperature ceramics (UHTCs) are characterized by high melting point, high strength and hardness, and high electrical and thermal conductivity. The high thermal conductivity arises from both electronic and phonon contributions. Thus electronic and phonon contributions must be controlled simultaneously in reducing the thermal conductivity of transition metal diborides. In high entropy (HE) materials, both electrons and phonons are scattered such that the thermal conductivity can significantly be reduced, which opens a new window to design novel insulating materials. Inspired by the high entropy effect, porous HE (Zr0.2Hf0.2Nb0.2Ta0.2Ti0.2)B2 is designed in this work as a new thermal insulting ultrahigh temperature material and is synthesized by an in-situ thermal borocarbon reduction/partial sintering process. The porous HE (Zr0.2Hf0.2Nb0.2Ta0.2Ti0.2)B2 possesses high porosity of 75.67%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low room temperature thermal diffusivity and thermal conductivity of 0.74 mm2 s−1 and 0.51 W m−1 K−1, respectively. In addition, it exhibits high compressive strength of 3.93 MPa. The combination of these properties indicates that exploring porous high entropy ceramics such as porous HE (Zr0.2Hf0.2Nb0.2Ta0.2Ti0.2)B2 is a novel strategy in making UHTCs thermal insulating.