Influence of order-disorder transition on the mechanical and thermophysical properties of A 2 B 2 O 7 high-entropy ceramics

The order-disorder transition (ODT) of A 2 B 2 O 7 compounds obtained enormous attention owing to the potential application for thermal barrier coating (TBC) design. In this work, the influence of ODT on the mechanical and thermophysical properties of dual-phase A 2 B 2 O 7 high-entropy ceramics was investigated by substituting Ce 4+ and Hf 4+ with different ionic radii on B-sites (Zr 4+ ). The X-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM) results show that r_A^3+/r_B^4+=1.47 is the critical value of ODT phase boundary with different doping B-site ion contents, and the energy dispersive spectroscopy (EDS) results further indicate the uniform distribution of elements. Interestingly, owing to the high intrinsic disorder derived from high-entropy effect, the A 2 B 2 O 7 high-entropy ceramics exhibit unreduced modulus ( E 0 ≈ 230 GPa) and enhanced mechanical properties ( HV ≈ 10 GPa, K IC ≈ 2.3 MPa·m 0.5 ). A 2 B 2 O 7 high-entropy ceramics exhibit excellent thermal stability with relatively high thermal expansion coefficients (TECs) (Hf0.25, 11.20×10 −6 K −1 , 1000 °C). Moreover, the matching calculation implied that the ODT further enhances the phonon scattering coefficient, leading to a relatively lower thermal conductivity of (La 0.25 Eu 0.25 Gd 0.25 Yb 0.25 ) 2 (Zr 0.85 Ce 0.15 ) 2 O 7 (1.48–1.51 W/(m·K), 100–500 °C) compared with other components. This present work provides a novel composition design principle for high-entropy ceramics, as well as a material selection rule for high-temperature insulation applications.