Lin Chen

The development of Generation IV nuclear reactors, such as the supercritical water‐cooled reactor (SCWR) and the lead‐bismuth‐cooled fast reactor (LFR), is significantly hindered by the degradation of structural materials under extreme operational conditions. Currently, widely investigated coating materials, including yttria‐stabilized zirconia (YSZ), SiC, and RE ₂ Zr ₂ O ₇ , fail to satisfy the multiple requirements imposed by extreme nuclear environments. Consequently, the development of ceramics for nuclear protective coatings is imperative. Rare‐earth tantalates RE ₃ TaO ₇ possess many desirable properties, including excellent chemical stability, high‐temperature stability, high thermal expansion coefficients (TECs = 9.0–12.0 × 10 ⁻⁶ K ⁻¹ ), relatively low thermal conductivities (1.0–2.1 W·m ⁻¹ ·K ⁻¹ ) desirable for coolant‐facing protective coatings on reactor structural components, and outstanding tolerance to irradiation‐induced amorphization, thereby rendering them promising candidates for structural component protection in extreme nuclear applications. This review comprehensively describes the failure mechanisms of structural alloys in coolant environments and defines the essential property requirements for nuclear protective coatings. We summarize the current research status of conventional coatings, including YSZ, SiC, and RE ₂ Zr ₂ O ₇ , and point out the advantages of the thermophysical properties of RE ₃ TaO ₇ in diverse environments. This work suggests that RE ₃ TaO ₇ ceramics are promising candidates for further development as nuclear protective coatings and outlines their future research directions.

Rare-earth tantalates (RETaO4) have been extensively investigated as thermal protective materials, but their application as environmental barrier coatings (EBCs) for ceramic matrix composites (CMCs) is limited by their high thermal expansion coefficients (TECs ≥ 9.0×10−6 K−1). High-entropy design provides a feasible route to tailor the thermal properties of RETaO4; however, most reports focus on equimolar RETaO4 high-entropy ceramics (HECs). In this study, a series of nonequimolar monoclinic-prime (m') RETaO4 HECs are designed and synthesized to clarify the composition-structure-property relationships. The nonequimolar design is employed to regulate configurational complexity, polyhedral distortion, and lattice strain in m'-RETaO4 within the same phase. Atomic-scale transmission electron microscopy (TEM) and geometric phase analysis (GPA) characterizations reveal the associated local distortion and lattice strain. These observations link nonequimolar composition with local structural distortion and macroscopic thermal transport behavior. The lowest thermal conductivity reaches 1.52–2.68 W·m−1·K−1 at 25–900 °C, and this is mainly attributed to RE-site disorder, lattice strain, and polyhedral distortion. Polyhedral distortion also suppresses thermal expansion by restricting atomic anharmonic vibrations. Among the designed compositions, the optimized nonequimolar m'-RETaO4 HEC (Sc0.2Y0.2Tm0.2Ho0.2Dy0.1Gd0.1)TaO4 exhibits the lowest TEC of 6.2×10−6 K−1 at 1500 °C, which is closer to that of SiC-based CMCs than the other compositions, indicating its potential as a candidate EBC material. This work proposes that polyhedral distortion and lattice strain are important structural factors for tailoring the thermal properties of nonequimolar RETaO4 HECs, providing guidance for the design of complex oxides.

Tailoring crystals and microstructures of multicomponent rare-earth tantalates RETaO₄ nano powders can promote their applications as thermal protective coatings materials because their properties are dominated by the structures. In this work, multicomponent RETaO₄ nano powders are synthesized via the chemical co-precipitation method, and their structures are tailored via changing the annealing temperatures. After annealed at 800~1500 °C, the multicomponent RETaO₄ nano powders can be crystallized into the metastable tetragonal (t'), monoclinic-prime (m'), and monoclinic (m) phases, and their particle sizes (8~652 nm) are gradually increased with the increasing temperature. The optimal annealing temperature of RETaO₄ powders is determined to be 1000 °C based on the crystallinity degree and particle sizes, which are 11.3 nm and 89.5 %, respectively. The high-resolution transmission electron microscope (HR-TEM) and corresponding EDS mapping have validated the compositional uniformity of each element in nano scale, and the interplanar spacing of different phases are corresponding to the XRD Rietveld refinements. This work demonstrates that high-temperature heat treatment can act as an effect mean to tailor the particle sizes and crystal structures of RETaO₄ ceramics, which can be further applied to synthesis nano spherical powders and coatings in the future study.

Thermal barrier coatings (TBCs) with high working temperatures and long service life are indispensable for the hot-end components of gas turbines and aircraft engines. This work designs and verifies that tantalate high-entropy ceramics (HECs) coatings are excellent TBCs with working temperatures reaching 1500 °C. We reveal the structural evolutions and failure mechanisms of tantalate HECs coatings synthesized via the air plasma spraying (APS), after they are subjected to thermal shock at 1500 ºC for 614 times, thermal fatigues at 1150 ºC for 12830 times, and annealing at 1100 ºC for 384 h. The thermal stress caused by temperature gradient, differences in thermal expansion coefficients (TECs) and mechanical properties between ceramic coatings and bond coat (BC) lead to the spalling of coatings during thermal shock, while the effects of BC oxidization are limited. In thermal fatigues, the accumulative thermal stress between BC and thermal growth oxides (TGO) is higher than the fracture resistance when the h/R ratio is higher than 0.32 (h and R are the TGO thickness and undulation radius, respectively), which mainly leads to spalling of coatings. Additionally, the effects of coatings sintering and stiffness are also considered, which lead to their surficial spalling during the high-temperature service. Two different failure mechanisms are proposed based on their microstructural evolutions, and the synthesized tantalate HECs coatings can be used at temperatures up to 1500 ºC, which further promote the designs and applications of high-performance TBCs.

The trade‐off relation between thermal conductivity and fracture toughness limits applications of brittle ceramic thermal insulation materials, and we propose that the high‐density dislocation engineering acts as an effective strategy to synergistically reduce thermal conductivity and enhance toughness. The spark plasma sintering (SPS) and heat treatments introduce high‐density dislocations (10 ⁸ ∼10 ¹⁰ mm ⁻² ) into the ferroelastic YTaO ₄ /Y ₃ TaO ₇ ceramic composites as thermal insulation materials. The effects of high‐density dislocations on reducing thermal conductivity and enhancing toughness are elucidated from the phonon relaxation time and crack propagation behaviors, respectively. The high‐density dislocations produce large lattice strains to reduce phonon relaxation time, and the lowest thermal conductivity reaches 1.32 W·m ⁻¹ ·K ⁻¹ . The interfacial enhancements, ferroelastic domains, and high‐density dislocations synergistically boost the toughness to 5.0 MPa·m ^1/2 , and the increment is higher than 50%. The effects of high‐density dislocations on toughness and thermal conductivity are revealed from an atomic scale, and the proposed high‐density dislocation strategy breaks the trade‐off relationship between thermal conductivity and toughness for brittle ceramic thermal insulation materials.

Directional optimization of properties is of significant interest for various protective coatings materials. This study investigates the effects of SiO₂ doping on AlTaO₄ ceramics as environmental barrier coatings (EBCs), and the changes in crystal structure and comprehensive properties are thoroughly investigated. The x mol% SiO₂–AlTaO₄ (SATO, x = 0, 3, 6, 9, 12, 15) ceramics are fabricated via a solid‐state sintering process, and their structures, as well as thermo‐mechanical properties, are systematically analyzed. The results reveal that SiO₂ doping introduces point defects and lattice distortions, which significantly reduce the thermal conductivity by 44% at 900°C, and effectively suppresses the increase in thermal conductivity at elevated temperatures. Furthermore, the fracture toughness is improved from 3.01 to 3.38 MPa·m^1/2, demonstrating superior mechanical properties compared with other EBC materials. The thermal expansion coefficients (TECs) are reduced to 5.71 × 10⁻⁶ K⁻¹ at 1200°C, which match with those of SiC ceramic matrix composites (TECs = 5.50 × 10⁻⁶ K⁻¹). These findings highlight that SATO ceramics are promising candidates as EBC materials, and SiO₂ doping can further advance its high‐temperature applications.

Control rods are essential components in nuclear reactors, used to maintain the desired state of fission reactions. Among potential materials for this application, RE ₂ Hf ₂ O ₇ compounds exhibit exceptional neutron absorption capacity and structural stability under prolonged radiation exposure, minimizing the risk of reduced neutron absorption efficiency. Herein, dense RE ₂ Hf ₂ O ₇ ceramics (RE = Eu, Gd, Tb, Dy, Tm) were synthesized via vacuum solid‐state reactive sintering. Structural analysis revealed distinct phase formations: Eu ₂ Hf ₂ O ₇ , Gd ₂ Hf ₂ O ₇ , and Tb ₂ Hf ₂ O ₇ crystallized in the pyrochlore structure, while Dy ₂ Hf ₂ O ₇ and Tm ₂ Hf ₂ O ₇ adopted a defective fluorite configuration. The thermophysical properties of these ceramics were systematically evaluated. RE ₂ Hf ₂ O ₇ demonstrated superior thermal conductivity (1.4‐2.4 W·m ⁻¹ ·K ⁻¹ , 298–1073 K) compared to the conventional material Dy ₂ TiO ₅ . Furthermore, their thermal expansion coefficients (6.0–10.5 × 10 ⁻⁶ K ⁻¹ , 350–1100 K) closely matched those of Dy ₂ TiO ₅ (7.5–10.5 × 10 ⁻⁶ K ⁻¹ ), ensuring compatibility in reactor environments. Mechanical testing indicated robust performance, with Vickers hardness values ranging from 12.6 to 14.9 GPa, attributed to high bulk density and enhanced atomic bond strength. This work highlights the feasibility of tailoring RE ₂ Hf ₂ O ₇ ceramics through rare‐earth and hafnium composition adjustments, offering a pathway to durable, high‐performance neutron absorbers. The combined thermal, mechanical, and structural advantages position these materials as promising candidates for next‐generation nuclear reactor control rods.

High-entropy ceramics (HECs) are widely studied to boost the properties of various materials, based on the high-entropy effects and high disorders in ionic radius and atomic weights. To further optimize the properties of RETaO₄ (RE is rare-earth) as thermal barrier coatings (TBCs), a diatomic site high-entropy engineering is applied to increase their disorders, subsequently boosting their thermophysical properties. Four RETaO₄ HECs are designed and synthesized in this work, including (Gd[ ]Dy[ ]Ho[ ]Er[ ]Y[ ](Ta[ ]Nb[ ]O₄, (5HECs-1), (Gd[ ]Dy[ ]Ho[ ]Er[ ]Yb[ ](Ta[ ]Nb[ ]O₄, (5HECs-2), (Gd[ ]Dy[ ]Ho[ ]Er[ ]Yb[ ]Y[ ](Ta[ ]Nb[ ]O₄, (6HECs-1), and (Gd[ ]Dy[ ]Ho[ ]Er[ ]Yb[ ]Lu[ ](Ta[ ]Nb[ ]O₄, (6HECs-2). It shows that the thermal conductivity (1.41–1.59 W[ ]m[ ]K[ ], [ ]C) is reduced, while thermal expansion coefficients are improved ([ ]–[ ] [ ]K[ ] at [ ]C), which are better than their single-RE ceramics. Furthermore, Young’s modulus is proportional to their A-site ionic radius, and RETaO₄ HECs have higher modulus than single-RE RETaO₄ and RENbO₄ ceramics. Accordingly, the diatomic site high-entropy engineering is effective in boosting the thermophysical properties of RETaO₄, and this strategy can be tried in various oxide ceramics.