High entropy ultra-high temperature ceramic thermal insulator (Zr1/5Hf1/5Nb1/5Ta1/5Ti1/5)C with controlled microstructure and outstanding properties

• Highly porous high entropy UHTC was developed as a good thermal insulator. • High entropy samples show high porosity, high strength, and low thermal conductivity. • High entropy sample demonstrates improved oxidation resistance. Due to advancements of hypersonic vehicles, ultra-high temperature thermal insulation materials are urgently requested to shield harsh environment with superhigh heat flux. Toward this target, ultra-high temperature ceramics (UHTCs) are the only choice due to their excellent capability at ultra-high temperatures. We herein report a novel highly porous high entropy (Zr 1/5 Hf 1/5 Nb 1/5 Ta 1/5 Ti 1/5 )C fabricated by foam-gelcasting-freeze drying technology combined with in-situ pressureless reaction sintering. The porous (Zr 1/5 Hf 1/5 Nb 1/5 Ta 1/5 Ti 1/5 )C exhibited ultra-high porosity of 86.4%-95.9%, as well as high strength and low thermal conductivity of 0.70–11.77 MPa and 0.164–0.239 W/(m⋅K), respectively. Specifically, SiC sintering additive only locates at the pit of the surface of sintering neck between UHTC grains, and there is no secondary phase or intergranular film at the grain boundary. Besides, the oxidation resistance of high entropy carbide powders is greatly improved compared with that of the mixed five carbide powders. This work clearly highlights the merits of highly porous high entropy (Zr 1/5 Hf 1/5 Nb 1/5 Ta 1/5 Ti 1/5 )C as an ultra-high temperature thermal insulation material.