Preparation and thermophysical properties of medium-entropy fluorite-structured oxide of (Y0·25Zr0.25Hf0.25Ce0.25)O2−δ

A medium-entropy fluorite-structured ceramic of (Y 0·25 Zr 0.25 Hf 0.25 Ce 0.25 )O 2−δ was prepared using a solid state reaction method combined with conventional sintering at 1600  ° C for 10 h. This ceramic has uniform elemental distribution without impurity and segregation. The annealing experiment results show a reversible transformation between single phase and double phases in (Y 0·25 Zr 0.25 Hf 0.25 Ce 0.25 )O 2−δ in 800–1600 °C. The reversible phase transformation does not display any obvious influence on the thermal expansion of this sample. The thermal expansion coefficient of (Y 0·25 Zr 0.25 Hf 0.25 Ce 0.25 )O 2−δ is 12.4 × 10 −6  K −1 in 100–1500 °C, higher than that of 8 wt% yttria-stabilized zirconia (8 YSZ). Furthermore, (Y 0·25 Zr 0.25 Hf 0.25 Ce 0.25 )O 2−δ possesses a lower thermal conductivity (1.51 W m −1  K −1 at 900  ° C) and higher oxygen barrier property compared with 8 YSZ at high temperature. The Vickers' hardness, Young's modulus and fracture toughness of (Y 0·25 Zr 0.25 Hf 0.25 Ce 0.25 )O 2−δ are 10.0 GPa, 193.9 GPa and 1.93 MPa m 1/2 , respectively. These results demonstrate that (Y 0·25 Zr 0.25 Hf 0.25 Ce 0.25 )O 2−δ can be used as a promising candidate ceramic of thermal barrier coatings.