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 (Sc_0.2Y_0.2La_0.2Ho_0.2Yb_0.2)₂Zr₂O₇, (Sc_0.2Ce_0.2Nd_0.2Eu_0.2Gd_0.2)₂Zr₂O₇, (Sc_0.2Y_0.2Tm_0.2Yb_0.2Lu_0.2)₂Zr₂O₇, and (Sc_0.2Er_0.2Tm_0.2Yb_0.2Lu_0.2)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, (Sc_0.2Y_0.2La_0.2Ho_0.2Yb_0.2)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∙m^1/2), and smallest brittleness index (3.03·μm^−1/2). Therefore, (Sc_0.2Y_0.2La_0.2Ho_0.2Yb_0.2)₂Zr₂O₇ is considered to be an ideal candidate material for next-generation thermal barrier coating applications.

The present work attempted to produce thick zirconia coatings formed by micro-arc oxidation as a hydrogen permeation barrier on zirconium hydride alloy. A novel multiphase zirconia coating was achieved, exhibiting superior hydrogen permeation barrier performance. The growth dynamics, formation mechanism, and phase evolution behavior of thick zirconia coatings were explored, and the hydrogen permeation barrier performance was evaluated by means of vacuum dehydrogenation experiment. The hydrogen desorption quantity was monitored by analyzing pressure changes with a quadruple mass spectrometer (QMS). Experimental results show that the multiphase coatings were composed of monoclinic ZrO₂ (m-ZrO₂), tetragonal ZrO₂ (t-ZrO₂), and a trace of cubic ZrO₂ (c-ZrO₂). The coatings were generally divided into a dense and uniform inner, intermediate layer, and a porous top layer. The quantitative analysis indicates an increased amount of m-ZrO₂ toward the coating surface and an increased amount of t-ZrO₂ toward the oxide/metal interface. This novel multiphase thick zirconia coating can noticeably improve hydrogen permeation resistance, and the permeation reduction factor (PRF) value is improved by nearly 13 times compared with bare zirconium hydride. It is demonstrated that hydrogen desorption is retarded to some extent in the presence of thick zirconia coating. Hydrogen desorption of the sample with ceramic coating started at 660 °C, which was apparently higher than that of the sample without coating.