Effects of element addition on the microstructure and thermophysical properties of seven-principal cations rare-earth high-entropy oxide

Three new rare-earth high-entropy oxides with compositions of (Y 1/7 La 1/7 Nd 1/7 Sm 1/7 Gd 1/7 Yb 1/7 Lu 1/7 ) 2 O 3 (7HEO), (Y 1/8 La 1/8 Nd 1/8 Sm 1/8 Gd 1/8 Yb 1/8 Lu 1/8 Ce 1/8 ) 2 O 3 (8HEO) and (Y 1/10 La 1/10 Nd 1/10 Sm 1/10 Gd 1/10 Yb 1/10 Lu 1/10 Ce 1/10 Zr 1/10 Hf 1/10 ) 2 O 3 (10HEO) were prepared using a solid-state reaction method combined with conventional sintering at 1600  ° C for 10 h 7HEO has a monoclinic structure. The addition of Ce into 7HEO not only leads to the phase transformation from monoclinic to bixbyite structure, but also contributes to the grain refinement. When the three elements of Ce, Zr and Hf are added into 7HEO, a second fluorite-structured phase forms and further promotes the grain refinement. 10HEO and 8HEO have a higher thermal conductivity compared with 7HEO in 25–600 °C. When the temperature reaches 900  ° C, the thermal conductivity of 8HEO is lower than that of 7HEO. The addition of Ce or Ce/Zr/Hf into 7HEO causes the weakening of the bond strength and the increase of the bond length of cation-O 2- in the samples, which in turn increases the thermal expansion coefficient. Moreover, 7HEO possesses a better oxygen barrier property compared with 8HEO and 10HEO. The hardness and Young's modulus of 7HEO are lower than those of 8HEO and 10HEO.