Rare-earth-tantalate high-entropy ceramics with sluggish grain growth and low thermal conductivity

A series of rare-earth-tantalate high-entropy ceramics ((5RE0.2)Ta3O9, where RE = five elements chosen from La, Ce, Nd, Sm, Eu and Gd) were prepared by conventional sintering in air at 1500 °C for 10 h. The (5RE0.2)Ta3O9 high-entropy ceramics exhibit an orthogonal structure and sluggish grain growth. No phase transition occurs in the test temperature of 25–1200 °C. The thermal conductivities of all (5RE0.2)Ta3O9 ceramics are in the range of 1.14–1.98 W m−1 K−1 at a test temperature of 25–500 °C, approximately half of that of YSZ. The sample of (Gd0.2Ce0.2Nd0.2Sm0.2Eu0.2)Ta3O9 exhibits a low glass-like thermal conductivity with a value of 1.14 W m−1 K−1 at 25 °C. The thermal expansion coefficient of (5RE0.2)Ta3O9 ceramics ranges from 5.6 × 10−6 to 7.8 × 10−6 K−1 at 25–800 °C, and their fracture toughness is high (3.09–6.78 MPa·m1/2). The results above show that (5RE0.2)Ta3O9 ceramics could be a promising candidate for thermal barrier coatings.