The stacking structure of Nb₂CSe₂, a newly synthesized layered metal carbo-selenide, is elucidated by scanning transmission electron microscopy. Nb₂CSe₂ features Se-Nb-C-Nb-Se quintuple atomic layers. These layers stacked in Bernal mode. According to the mode, Nb₂CSe₂ crystallizes in a trigonal symmetry (space group P3(_)m1, No. 164), with lattice parameters of a = 3.33 Å, and c = 18.20 Å. Electronic structure calculations indicate that the metal carbo-selenide has Fermi energy crossing the bands where they touch to give a zero gap, showing it is an electronic conductor. As evidenced experimentally, the electrical conductivity is as high as 6.6 ´ 10⁵ S m^-1, outperforming the counterparts in the MXene family. Due to the layered structure, the bonding in Nb₂CSe₂ with an ionic formula of (Nb^1.48+)₂(C^1.74-)(Se^0.61-)₂ is highly anisotropic with metallic–covalent–ionic bonding in intralayers while weak bonding between interlayers. The layered nature is further evidenced by elastic properties, interlayer energy, and friction coefficient determination. These characteristics recognize that Nb₂CSe₂ is exactly the analogue of MoS₂ that is the typical binary van der Waals solid (vdW). Moreover, vibrational properties are reported, which may offer an optical identification standard to the new ternary vdW solid in spectroscopic studies including Raman scattering and infra-red absorption.

CrTaO₄ (or Cr_0.5Ta_0.5O₂) has been unexpectedly found to play a decisive role in improving the oxidation resistance of Cr and Ta-containing refractory high-entropy alloys (RHEAs). This rarely encountered complex oxide can effectively prevent the outward diffusion of metal cations from the RHEAs. Moreover, the oxidation kinetics of CrTaO₄-forming RHEAs is comparable to that of the well-known oxidation resistant Cr₂O₃- and Al₂O₃-forming Ni-based superalloys. However, CrTaO₄ has been ignored and its mechanical and thermal properties have yet to be studied. To fill this research gap and explore the untapped potential for its applications, here we report for the first time the microstructure, mechanical and thermal properties of CrTaO₄ prepared by hot-press sintering of solid-state reaction synthesized powders. Using the HAADF and ABF-STEM techniques, rutile crystal structure was confirmed and short range ordering was directly observed. In addition, segregation of Ta and Cr was identified. Intriguingly, CrTaO₄ exhibits elastic/mechanical properties similar to those of yttria stabilized zirconia (YSZ) with Young’s modulus, shear modulus, and bulk modulus of 268, 107, and 181 GPa, respectively, and Vickers hardness, flexural strength, and fracture toughness of 12.2±0.44 GPa, 142±14 MPa, and 1.87±0.074 MPa·m^1/2. The analogous elastic/mechanical properties of CrTaO₄ to those of YSZ has spurred inquiries to lucrative leverage it as a new thermal barrier material. The measured melting point of CrTaO₄ is 2103±20 K. The anisotropic thermal expansion coefficients are α_a = (5.68±0.10)×10⁻⁶ K⁻¹, α_c = (7.81±0.11)×10⁻⁶ K⁻¹, with an average thermal expansion coefficient of (6.39±0.11)×10⁻⁶ K⁻¹. The room temperature thermal conductivity of CrTaO₄ is 1.31 W·m⁻¹·K⁻¹ and declines to 0.66 W·m⁻¹·K⁻¹ at 1473 K, which are lower than most of the currently well-known thermal barrier materials. From the perspective of matched thermal expansion coefficient, CrTaO₄ pertains to an eligible thermal barrier material for refractory metals such as Ta, Nb, and RHEAs, and ultrahigh temperature ceramics. As such, this work not only provides fundamental microstructure, elastic/mechanical and thermal properties that are instructive for understanding the protectiveness displayed by CrTaO₄ on top of RHEAs but also outreaches its untapped potential as a new thermal barrier material.

Willemite Zn₂SiO₄ crystallizes in such a way that Zn and Si are tetrahedrally coordinated with O in an ionic-covalent manner to form ZnO₄ and SiO₄ tetrahedra as the building units. The tetrahedra are corner-sharing, of which one SiO₄ tetrahedron connects eight ZnO₄ tetrahedra, and one ZnO₄ tetrahedron links four ZnO₄ tetrahedra and four SiO₄ tetrahedra. The unique crystallographic configuration gives rise to parallel tunnels with a diameter of 5.7 Å along the c-axis direction. The tunnel structure of Zn₂SiO₄ definitely correlates with its interesting elastic and thermal properties. On the one hand, the elastic modulus, coefficient of thermal expansion (CTE), and thermal conductivity are low. Zn₂SiO₄ has low Vickers hardness of 6.6 GPa at 10 N and low thermal conductivity of 2.34 W/(m·K) at 1073 K. On the other hand, the elastic modulus and CTE along the c-axis are significantly larger than those along the a- and b-axes, showing obvious elastic and thermal expansion anisotropy. Specifically, the Young’s modulus along the z direction (E_z = 179 GPa) is almost twice those in the x and y directions (E_x = E_y = 93 GPa). The high thermal expansion anisotropy is ascribed to the empty tunnels along the c-axis, which are capable of more accommodating the thermal expansion along the a- and b-axes. The striking properties of Zn₂SiO₄ in elastic modulus, hardness, CTE, and thermal conductivity make it much useful in various fields of ceramics, such as low thermal expansion, thermal insulation, and machining tools.

MXene is a new intercalation pseudocapacitive electrode material for supercapacitor application. Intensifying fast ion diffusion is significantly essential for MXene to achieve excellent electrochemical performance. The expansion of interlayer void by traditional spontaneous species intercalation always leads to a slight increase in capacitance due to the existence of species sacrificing the smooth diffusion of electrolyte ions. Herein, an effective intercalation−deintercalation interlayer design strategy is proposed to help MXene achieve higher capacitance. Electrochemical cation intercalation leads to the expansion of interlayer space. After electrochemical cation extraction, intercalated cations are deintercalated mostly, leaving a small number of cations trapped in the interlayer silt and serving as pillars to maintain the interlayer space, offering an open, unobstructed interlayer space for better ion migration and storage. Also, a preferred surface with more −O terminations for redox reaction is created due to the reaction between cations and −OH terminations. As a result, the processed MXene delivers a much improved capacitance compared to that of the original Ti₃C₂T_x electrode (T stands for the surface termination groups, such as −OH, −F, and −O). This study demonstrates an improvement of electrochemical performance of MXene electrodes by controlling the interlayer structure and surface chemistry.

MAX phases (Ti₃SiC₂, Ti₃AlC₂, V₂AlC, Ti₄AlN₃, etc.) are layered ternary carbides/nitrides, which are generally processed and researched as structure ceramics. Selectively removing A layer from MAX phases, MXenes (Ti₃C₂, V₂C, Mo₂C, etc.) with two-dimensional (2D) structure can be prepared. The MXenes are electrically conductive and hydrophilic, which are promising as functional materials in many areas. This article reviews the milestones and the latest progress in the research of MAX phases and MXenes, from the perspective of ceramic science. Especially, this article focuses on the conversion from MAX phases to MXenes. First, we summarize the microstructure, preparation, properties, and applications of MAX phases. Among the various properties, the crack healing properties of MAX phase are highlighted. Thereafter, the critical issues on MXene research, including the preparation process, microstructure, MXene composites, and application of MXenes, are reviewed. Among the various applications, this review focuses on two selected applications: energy storage and electromagnetic interference shielding. Moreover, new research directions and future trends on MAX phases and MXenes are also discussed.