Porous medium-entropy (Zr_0.25Ce_0.25Hf_0.25Y_0.25)O_2-δ ceramic with low thermal conductivity: Using aerogel structure as the precursor

The amorphous aerogel precursor with a BET-specific surface area of 203.58 m²/g was synthesized by sol-gel and CO₂ supercritical drying using metal chlorides as cation source. It was then calcined at 950 °C to obtain a monolithic defective fluorite (Zr_0.25Ce_0.25Hf_0.25Y_0.25)O_2-δ ceramic aerogel with a grain size of 20 nm and an ultra-low thermal conductivity of 0.054 W m⁻¹ K⁻¹ at room temperature. Bulk (Zr_0.25Ce_0.25Hf_0.25Y_0.25)O_2-δ with porosity of 62.24–66.11% and flexural strength of 2.359–4.487 MPa were synthesized by pressing and sintering. The sintered porous (Zr_0.25Ce_0.25Hf_0.25Y_0.25)O_2-δ ceramic has a low thermal diffusivity of 0.152–0.207 mm²/s and a thermal conductivity of 0.138–0.201 W m⁻¹ K⁻¹ with a thermal transfer behavior distinct from that of micro-grain ceramics, which is caused by the severe lattice distortion, porous structure, and nanoscale phonon boundary scattering.