Design and experimental investigation of potential low-thermal-conductivity high-entropy rare-earth zirconates

Developing new high-entropy rare-earth zirconate (HE-RE₂Zr₂O₇) ceramics with low thermal conductivity is essential for thermal barrier coating materials. In this work, the average atomic spacings, interatomic forces, and average atomic masses of 16 rare-earth elements occupying the A site of the cubic A₂B₂O₇ crystal structure were calculated by density functional theory. These three physical qualities, as vectors, characterise the corresponding rare-earth elements. The distance between the two vectors quantitatively describes the difference between two rare-earth elements. For greater difference between two rare-earth elements, the disorder degree of HE-RE₂Zr₂O₇ is higher, and therefore the thermal conductivity is lower. From theoretical calculation, the thermal conductivity of the ceramics gradually increases in the order (Sc0.2Y0.2La0.2Ho0.2Yb0.2)₂Zr₂O₇, (Sc0.2Ce0.2Nd0.2Eu0.2Gd0.2)₂Zr₂O₇, (Sc0.2Y0.2Tm0.2Yb0.2Lu0.2)₂Zr₂O₇, and (Sc0.2Er0.2Tm0.2Yb0.2Lu0.2)₂Zr₂O₇. Using the solution precursor plasma spray method and pressureless sintering method, four types of HE-RE₂Zr₂O₇ powder and bulk samples were prepared. The samples all showed a single defective fluorite structure with uniform distribution of the elements and a stable phase structure. The thermal conductivities of the sintered HE-RE₂Zr₂O₇ bulk samples were in the range 1.30–1.45 W·m⁻¹·K⁻¹ at 1400 °C, and their differences were consistent with the theoretical calculation results. Among the ceramics, (Sc0.2Y0.2La0.2Ho0.2Yb0.2)₂Zr₂O₇ showed the lowest thermal conductivity (1.30 W∙m⁻¹∙K⁻¹, 1400 °C), highest thermal expansion coefficient (10.19 × 10⁻⁶ K⁻¹, 200–1400 °C), highest fracture toughness (1.69 ± 0.28 MPa∙m1/2), and smallest brittleness index (3.03·μm−1/2). Therefore, (Sc0.2Y0.2La0.2Ho0.2Yb0.2)₂Zr₂O₇ is considered to be an ideal candidate material for next-generation thermal barrier coating applications.