Porous (Y0.25Ho0.25Yb0.25Lu0.25)₂Si₂O₇: A novel high‐entropy ceramic with high porosity and excellent thermal stability

With the rapid development of hypersonic vehicles, the thermal protection systems face great challenges due to the severe aerodynamic heat. As a result, conventional fiber insulation tiles would experience severe shrinkage at high temperatures (>1500°C), endangering the safety of the aircraft. To solve the problem, a high‐entropy effect was introduced to increase the high‐temperature resistance of thermal insulation materials, and highly porous (Y 0.25 Ho 0.25 Yb 0.25 Lu 0.25 ) 2 Si 2 O 7 was prepared by in situ reaction sintering process in this work. The fabricated porous samples present excellent overall properties, including superhigh porosity (95.09%–93.10%), lightweight (0.29–0.41 g/cm 3 ), and very low thermal conductivity (0.066–0.085 W/(m·K)). More importantly, porous (Y 0.25 Ho 0.25 Yb 0.25 Lu 0.25 ) 2 Si 2 O 7 has outstanding high‐temperature dimensional stability. Although the porosity of the sample is higher than 90%, its linear shrinkage at 1550°C for 2 h is less than 1%. The results demonstrate that porous high‐entropy (Y 0.25 Ho 0.25 Yb 0.25 Lu 0.25 ) 2 Si 2 O 7 is a promising thermal insulation material in extreme high‐temperature environments.