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

A series of rare-earth-tantalate high-entropy ceramics ((5RE_0.2)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 (5RE_0.2)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 (5RE_0.2)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 (Gd_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)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 (5RE_0.2)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·m^1/2). The results above show that (5RE_0.2)Ta₃O₉ ceramics could be a promising candidate for thermal barrier coatings.