MAX/MAB phases are a series of non-van der Waals ternary layered ceramic materials with a hexagonal structure, rich in elemental composition and crystal structure, and embody physical properties of both ceramics and metals. They exhibit great potential for applications in extreme environments such as high temperature, strong corrosion, and irradiation. In recent years, two-dimensional (2D) materials derived from the MAX/MAB phase (MXene and MBene) have attracted enormous interest in the fields of materials physics and materials chemistry and become a new 2D van der Waals material after graphene and transition metal dichalcogenides. Therefore, structural modulation of MAX/MAB phase materials is essential for understanding the intrinsic properties of this broad class of layered ceramics and for investigating the functional properties of their derived structures. In this paper, we summarize new developments in MAX/MAB phases in recent years in terms of structural modulation, theoretical calculation, and fundamental application research and provide an outlook on the key challenges and prospects for the future development of these layered materials.

Silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) composites have become the preferred candidate for structural applications in advanced nuclear energy systems, because of their low neutron toxicity, neutron irradiation tolerance and high-temperature oxidation resistance. In recent years, both academia and industry either domestic or abroad have carried out a lot of researches on SiC_f/SiC composites for nuclear application, and numerous important achievements have been made. This paper summarized and analysed some critical directions of SiC_f/SiC composites for nuclear applications, including nuclear-grade SiC fibers, fibre/matrix interfaces, composite processing, modeling and simulation, corrosion behavior and surface protection, joining technology, as well as radiation damage. The key issues and potential solutions of SiC_f/SiC composites for nuclear applications have been pointed out in account to the requirements, anticipating to be beneficial to promoting further researches and final applications.

Garnet is considered as a potential matrix for immobilizing high-level radioactive waste (HLW) because of its large actinide package capacity and chemical flexibility. Y_3-xNd_xFe₅O₁₂ (0≤x≤2) series yttrium iron garnet (YIG) samples was successfully synthesized by solid-state sintering method using Nd³⁺ to simulate trivalent actinides. The solubility limit of Nd in the YIG and the influence of Nd doping amount on the phase and microstructure of ceramics was studied. Then the chemical durability of Nd-doped yttrium iron garnet solidified body under different pH conditions was evaluated. The results show that the ceramics with x≤1.7 show homogeneous single YIG, but the ceramics with x≥1.8 exhibit coexistence of three crystal phases (YIG, NdFeO₃ and Fe₂O₃). The solubility limit of Nd³⁺ in the pure YIG is about 29.5% (in mass). As the amount of Nd doping increases, the density of the ceramics increases, the volume decreases, and the porosity decreases. The leaching experiment results show that the normalized leaching rate of the elements (LR_i) is 10^-6~10^-5g·m^-2·d^-1. LR_Y is smaller than LR_Nd, and the normalized leaching rate of the elements in acid leachate is slightly higher than those in neutral and alkaline leachates. The results indicate that YIG ceramics are a potential candidate form for trivalent actinides.

A₂B₂O₇ pyrochlore is considered as a candidate host matrix for high-level radioactive wastes due to its incorporation and physicochemical stability. Nd_1.8Th_0.2Zr₂O₇ and Nd₂Zr_1.8Th_0.2O₇ pyrochlore samples doped with 20% thorium (in molar) were successfully prepared by using spray-pyrolysis and high temperature sintering method. The structures of the synthesized immobilization were characterized, and the chemical durability tests were investigated by the MCC-1 method. Structural analyses show that samples Nd_1.8Th_0.2Zr₂O₇ and Nd₂Zr_1.8Th_0.2O₇ exhibit pure single pyrochlore structure. Rietveld refinement analyses show that the value of 48f oxygen site parameter of 20% thorium-doped Nd₂Zr₂O₇ pyrochlore increases, which suggests that the structure is transforming from ordered pyrochlore to disordered structure, as compared with Nd₂Zr₂O₇. The Nd_1.8Th_0.2Zr₂O₇ pyrochlore structure evolution results from the distortion of the AO₈ hexahedral structure, while the B-site substitution leads to partial deformation of the BO₆ octahedron. The leaching experiment results show that the normalized leaching rate of thorium is as low as 10^-5g·m^-2·d^-1 after 42 d. Thorium can be well incorporated at the A and B cation sites of the Nd₂Zr₂O₇ pyrochlore structure of which the immobilization exhibits the excellent physical and chemical properties.

In contrast to conventional solid-phase sintering, molten salt method can provide a fast mass transfer and nucleation process at lower temperatures which has potential to synthesize the ceramic solid solution for immobilization of high-level nuclear waste (HLW). In this work, Nd-doped zircon (ZrSiO₄) ceramics (Zr_1-xNd_xSiO_4-x/2 (0≤x≤0.1)) were prepared by the molten salt synthesis(MSS) at different sintering temperatures (1100, 1200, 1300, 1400, 1500 ℃) for different sintering time (3, 6, 9, 12, and 15 h). Chemical stability of Nd-doped zircon ceramics in simulated geological disposal environment was studied by static leaching test (PCT). Zr_1-xNd_xSiO_4-x/2 was synthesized by the molten salt method under the optimum molar ratio of molten salt to oxide at 10:1, sintering temperature at 1200 ℃ and sintering time of 6 h with the solid solution of Nd in ZrSiO₄ being increased to 8% (in mol). The MSS can reduce the synthetic temperature, shorten the sintering time and save the solid solution. The immobilizing mechanism of ZrSiO₄ ceramics for trivalent actinide nuclides is lattice immobilizing. Experimental results show that the normalized leaching rate (LR_Nd) of Nd is as low as ~10^-5 g·m^-2·d^-1. ZrSiO₄ ceramics have no phase evolution before and after leaching, suggesting good structural stability. TLeaching model consolidates that Nd leaching is due to dissolution of the ceramic surface layer. Data from this study show that MSS is a promising method to synthesize ceramics solid solution.

Perovskite-structured praseodymium aluminum oxide (PrAlO₃) exhibits high stability which has a site that can be doped with other rare earth ions, enabling it a promising new neutron-absorbing material matrix. However, the current research on PrAlO₃ mainly focuses on the preparation methods of single crystal materials and their optical and magnetic property. Here, we firstly prepared a high-density perovskite phase PrAlO₃ ceramics by solid-phase reaction synthesis using tetraethyl orthosilicate (TEOS) as a liquid phase sintering aid, and then studied its microstructure and thermal property by XRD, SEM, push-rod technique, and laser flash method. The results showed that, by pre-synthesizing PrAlO₃ powder at 1200 ℃ and adding 0.4%-1.0%(in mass) TEOS as a liquid phase sintering aid, PrAlO₃ ceramic with a relative density higher than 99% could be obtained at around 1500 ℃, while the relative density of the product without sintering aids was only 96%. The thermal conductivity of PrAlO₃ ceramic at a subcritical temperature of 360 ℃ was 4.99 W·m^-1·K^-1, superior to those of Dy₂TiO₅ and GdAlO₃ ceramics, and its linear thermal expansion coefficient from room temperature to 800 ℃ was only 10.2×10^-6 K^-1. Moreover, the bending strength and Vickers hardness of PrAlO₃ ceramics reached 95.55 MPa and 7.95 GPa, respectively, and the fluorescence spectrum exhibited characteristic emission peaks of Pr³⁺. This study shows that high-density perovskite phase PrAlO₃ ceramics can be prepared by a convenient method with good thermophysical property and mechanical property. They exhibit good application prospects as a rare earth-based neutron-absorbing nuclear material.