Structural, thermophysical, mechanical and electrical properties of equimolar five-principal element fluorite-structured high-entropy oxides

Four kinds of high-entropy oxides (HEOs) with compositions of Y 0.2 Zr 0.2 Hf 0.2 Ce 0.2 X 0.2 O 2−δ (X = La, Nd, Gd, Dy, abbreviated as HEO-La, Nd, Gd, Dy, respectively) are synthesized by solid-state-reaction method combined with conventional sintering in air atmosphere at 1600 °C for 10 h. These HEOs have a fluorite-structured phase with homogeneous element distribution. The thermal conductivity ( κ ) of these four samples is in 1.68–2.18 W m −1 K −1 in 25–900 °C. At 800 °C and 900 °C, the κ value of HEO-Gd is lower than those of the HEO-La, Nd, Dy, yttria-stabilized zirconia (YSZ) and some other HEOs with various crystal structures ((Gd 0.2 Dy 0.2 Ho 0.2 Er 0.2 Tm 0.2 )Ta 3 O 9 , (La 0.25 Eu 0.25 Gd 0.25 Yb 0.25 ) 2 Zr 2 O 7 , (Sm 0.2 Dy 0.2 Ho 0.2 Er 0.2 Yb 0.2 )NbO 4 , Hf 0.25 Zr 0.25 Ce 0.25 Gd 0.125 Ca 0.125 O 2-δ and Sr(Y 0.2 Sm 0.2 Gd 0.2 Dy 0.2 Yb 0.2 ) 2 O 4 ). The sample of HEO-Gd has the highest thermal expansion coefficient (CTE) (12.1 × 10 −6 K −1 ) and fracture toughness ( K IC ) (2.5 MPa m 1/2 ) among HEO-La, Nd, Gd, Dy. Its K IC value is higher than those of other HEOs mentioned above. In addition, HEO-Gd exhibits a better oxygen barrier property compared with YSZ in 450–700 °C. These results indicate that HEO-Gd is a promising candidate as high-temperature thermal insulation ceramic.