Spark plasma sintering of (5RE.2)Ta3O9 high-entropy ceramics and their thermal properties

Six rare-earth tantalate high-entropy ceramics of (5RE(.2))Ta3O9 (RE represents any five elements selected from La, Ce, Nd, Sm, Eu, Gd) were designed and prepared by spark plasma sintering process at 1400 degrees C in this study. The (5RE(.2))Ta3O9 ceramics only consist of a single-phase solid solution with perovskite structure. Their relative densities are all above 90%, and the average grain size is in the range of 1.47-2.92 mu m. The thermal conductivity of (5RE(.2))Ta3O9 ceramics is in 2.24-1.90 W m(-1) K-1 (25 degrees C-500 degrees C), which is much lower than that of yttria-stabilized zirconia. In six samples, (La.2Nd.2Sm.2Gd.2Eu.2)Ta3O9 possesses a thermal conductivity of 1.90 W m(-1) K-1, a thermal expansion coefficient of 3.47 x 10(-6) K-1 (500 degrees C), a Vickers hardness of about 7.33 GPa, and a fracture toughness of about 5.20 MPa m(1/2), which are suitable for its application as thermal barrier coatings.