High-entropy ceramics (HECs), defined as single-phase inorganic solid solutions comprising five or more principal elements in equimolar or near-equimolar ratios, have emerged as a frontier and hotspot in materials science over the past decade. Their expansive compositional space and diverse crystal structures open up new avenues for the design and performance regulation of ceramic materials. Initially focused on proving the feasibility of entropy-stabilized phases, the field rapidly expanded into a vast, complex landscape of non-equimolar, multi-anionic, and medium-entropy compositions. This exploratory “great chaos” successfully validated the concept across diverse ceramic families—oxides, carbides, borides, nitrides, silicides—and unlocked extraordinary properties, including ultra-high temperature stability, exceptional radiation tolerance, ultralow thermal conductivity, and superior energy storage density. The realization of performance-tailored HECs fundamentally depends on rational compositional design and precise control of preparation processes—core challenges that remain at the heart of current research. However, a clear “scissors gap” has emerged between the rapid accumulation of experimental data and the lag in theoretical frameworks and data comparability. This review synthesizes a decade of research to chart a crucial transition “from chaos to order.” It formulates emerging design paradigms for targeted applications such as oxidation-resistant ultra-high temperature ceramics, thermal barrier coatings, durable nuclear materials, and high-performance energy storage and conversation materials. The analysis highlights the shift from discovery to quantitative efforts integrating computational thermodynamics, advanced characterization, and machine learning. Despite remarkable progress, significant bottlenecks persist in processing, standardized characterization, and scaling from powder to component. The future roadmap emphasizes establishing robust structure-property relationships, fostering community-wide data standards, and advancing rational, physics- and AI-guided design to systematically realize the immense technological potential of HECs.

Ultrasonic-assisted hot pressing (UAHP) has shown significant potential in enhancing both the densification and mechanical performance of metallic materials. However, the poor high-temperature stability of ultrasonic systems severely limits its application in the fabrication of high-melting-point materials. To fill this gap, UAHP was operated at temperatures exceeding 2000 °C and employed in the preparation of monolithic boron carbide (B₄C) ceramics for the first time. The densification behavior, microstructure evolution, and mechanical properties of B₄C fabricated via UAHP were systematically investigated and compared with those prepared by conventional hot pressing (HP). It was demonstrated that the introduction of high-frequency ultrasonic vibration in UAHP can not only accelerate the densification rate but also reduce the densification temperature and enhance the mechanical properties of B₄C. Specifically, the relative density of B₄C increased from 90.90% to 97.22% at 1900 °C under UAHP, which was comparable to that achieved by HP at 1950 °C, indicating a 50 °C reduction in densification temperature. In addition, a significant increase in densification efficiency by reducing the densification time during UAHP endowed B₄C with both near-full density and superior mechanical properties. The B₄C ceramics prepared by UAHP at 1950 °C for 20 min and at 2050 °C for 5 min exhibited flexural strengths of 669.3±19.4 and 688.3±32.5 MPa, respectively, and fracture toughnesses of 4.37±0.23 and 4.22±0.29 MPa·m^1/2, respectively. These results suggest that UAHP is a promising strategy for efficient densification and optimization of the mechanical properties of B₄C ceramic and opens a new avenue for the preparation of difficult sintering ceramics.

Oxide scales grown on carbides or borides based ultrahigh thermal protection materials during service play crucial roles in the safe operation of the systems in extreme environments, where advancing technologies are pushing temperature limits beyond 3000 °C, exceeding the melting points of all known nonradioactive oxides. Although cationic solid solutions offer a pathway to modulate melting behavior, conventional phase diagrams show that most solid solutions exhibit lower melting points than their parent components. The mechanisms underlying melting point elevation in oxides have remained unclear. Here, we demonstrate a cationic design strategy for ultrahigh melting point oxides based on simultaneous control of the valence electron concentration, cation size, orbital overlap, coordination number and crystallographic symmetry. Using this approach, we developed a Ta-doped HfO₂ solid solution with a melting point of 3006 °C, the highest reported nonradioactive oxide, which represents an increase of nearly 150 °C over the parent oxide. This approach should be universally applicable to designing various ceramics with high or ultrahigh melting points.

CrB₂ crystallizes in an AlB₂-type crystal structure, and the chemical bonding in CrB₂ includes B sp²‒B sp² covalent bonds in the graphite-analogous six-numbered B ring, B p_z‒Cr 3d covalent‒ionic bonds, and Cr‒Cr metallic bonds from theoretical calculations. However, the crystal structure and chemical bonding properties have not been experimentally validated. To fill this research gap, herein, the crystal structure and chemical bonding of CrB₂ were evaluated for the first time via aberration-corrected transmission electron microscopy (AC-TEM) coupled with electron energy loss spectroscopy (EELS). Combined with first-principles calculations based on density functional theory (DFT), CrB₂ is confirmed to have an AlB₂-type structure, where Cr bonds to each other in the (001) plane via metallic bonding and where B bonds in the form of a graphite-like six-membered ring in the (002) plane through sp² hybridization, whereas Cr‒B ionic‒covalent bonding is formed in the (110) plane. A detailed analysis of the experimental and calculated results of the EELS of CrB₂ shows that the hybridization of Cr 3d and B has a significant effect on the EELS of transition metal borides (TMB₂). In addition, the hysteresis loop of CrB₂ was tested for the first time on the basis of theoretical calculations, and the molar susceptibility of CrB₂ was approximately 5.77×10⁻⁴ emu/mol. The present work is helpful for understanding the structure‒property relationships, which are essential for tailoring the properties from a crystal structure and chemical bonding point of view and promoting the practical application of TMB₂ in extreme aerospace environments.

Cr–Nb-containing refractory high-entropy alloys (RHEAs) have high strength above 1200 °C but low density close to that of Ti-based alloys, which makes them promising for application in aero engines. However, oxidation is the bottleneck that limits their practical application. Recently, CrNbO₄ has been found to effectively protect them from oxidation. Nevertheless, little is known about this oxide. To elucidate the protection mechanism of CrNbO₄ and explore its properties, we report for the first time the microstructure, mechanical, and thermal properties of CrNbO₄. Using atomic-resolution high-annular dark field (HAADF) and annular bright field (ABF) techniques, we confirmed the rutile-type structure of CrNbO₄, identified the precipitation of Cr₂O₃, and observed Cr segregation at the interface boundary between CrNbO₄ and Cr₂O₃. The Young’s modulus (E), shear modulus (G), and bulk modulus (B) of CrNbO₄ are 253, 100, and 180 GPa, respectively, whereas the Vickers hardness (H_V), flexural strength (σ_f), and fracture toughness (K_IC) of CrNbO₄ are 10.2±0.58 GPa, 205±8 MPa, and 1.54±0.12 MPa·m^1/2, respectively. The measured melting point of CrNbO₄ is 2053±20 K. The anisotropic thermal expansion coefficient (TEC) is α_a = (5.38±0.09)×10⁻⁶ K⁻¹, α_c = (7.44±0.14)×10⁻⁶ K⁻¹, and the average TEC is (6.07±0.12)×10⁻⁶ K⁻¹, which is close to that of refractory metals and RHEAs. Interestingly, the room temperature thermal conductivity of CrNbO₄ is 1.09 W·m⁻¹·K⁻¹ and decreases to 0.45 W·m⁻¹·K⁻¹ at 1473 K, which is lower than that of most of the currently well-known thermal insulation materials. Consequently, CrNbO₄ can be considered a novel dual-functional scale on top of RHEAs to protect them from oxidation and thermal attack.

Carbon fiber composites hold significant promise as electromagnetic wave (EMW)-absorbing materials. However, balancing lightweight materials with excellent mechanical properties, low thermal conductivity, and EMW absorption for multifunctional applications remains challenging. Herein, a novel hydrothermal carbon (HC)-coated three-dimensional (3D) needled carbon fiber-reinforced silicon–boron carbonitride (C_f/HC–SiBCN) composite was developed via an optimized precursor infiltration and pyrolysis (PIP) process combined with impregnation–filtration. By adjusting the precursor concentration and number of impregnation‒filtration cycles, a hierarchical C_f/HC–SiBCN composite with the density of 0.32 g·cm⁻³ was obtained, which exhibited remarkable mechanical properties, including flexural strengths of 14.75±0.43 MPa (xy-direction) and 14.45±0.66 MPa (z-direction), along with a compressive strength of 9.36±0.20 MPa (z-direction). It also demonstrated low thermal conductivity (0.145 W·m⁻¹·K⁻¹) and exceptional EMW absorption, with a minimum reflection loss (RL_min) of −58.13 dB and an effective absorption bandwidth (EAB) of 7.38 GHz. Owing to their combination of lightweight, enhanced mechanical properties, low thermal conductivity, and superior EMW absorption capabilities, C_f/HC–SiBCN composites are highly suitable for multifunctional applications.

High-entropy nanolaminated materials, referred to as MAX phases, have exceptional potential in various fields, including physics, mechanics, and energy storage, owing to their diverse compositions and outstanding properties. However, synthesizing stable high-entropy phases presents significant challenges because of the considerable differences in the physical and chemical properties of complex elements. In this study, we added low-melting-point metal tin (Sn) as an additive to facilitate the formation of solid solutions. The cohesion energy and formation enthalpy of the Sn-containing system are negative, which maintains the thermodynamic stability of the system, and the incorporation of Sn decreases the mixing enthalpy of the target high-entropy MAX phase and inhibits the formation of competing phases. The addition of Sn increases the lattice parameter and improves the structural stability by increasing the lattice distortion of octahedral M₆X and prism M₆A, which facilitates the successful synthesis of single-phase high-entropy MAX bulk materials. In addition, the high-entropy MAX phases with added Sn retain good mechanical and physical properties. This study provides a novel approach for the synthesis and application of high-entropy MAX phase materials, which has the potential to contribute to advancements in multiple technological fields.

The stacking structure of Nb₂CSe₂, a newly synthesized layered metal carbo-selenide, was elucidated by scanning transmission electron microscopy. Nb₂CSe₂ features Se−Nb−C−Nb−Se quintuple atomic layers. These layers are stacked in Bernal mode. In this mode, Nb₂CSe₂ crystallizes in a trigonal symmetry (space group P $\overline{3}$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 it touches to give a zero gap, indicating that 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. Owing 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 and weak bonding between interlayers. The layered nature is further evidenced by elastic properties, interlayer energy, and friction coefficient determination. These characteristics indicate that Nb₂CSe₂ is an analog of molybdenum disulfide (MoS₂), which is a typical binary van der Waals (vdW) solid. Moreover, vibrational properties are reported, which may offer an optical identification standard for new ternary vdW solids in spectroscopic studies, including Raman scattering and infrared absorption.

In response to the development of the concepts of “carbon neutrality” and “carbon peak”, it is critical to developing materials with high near-infrared (NIR) solar reflectivity and high emissivity in the atmospheric transparency window (ATW; 8–13 μm) to advance zero energy consumption radiative cooling technology. To regulate emission and reflection properties, a series of high-entropy rare earth stannate ceramics (HE-RE₂Sn₂O₇: (Y_0.2La_0.2Nd_0.2Eu_0.2Gd_0.2)₂Sn₂O₇, (Y_0.2La_0.2Sm_0.2Eu_0.2Lu_0.2)₂Sn₂O₇, and (Y_0.2La_0.2Gd_0.2Yb_0.2Lu_0.2)₂Sn₂O₇) with severe lattice distortion were prepared using a solid phase reaction followed by a pressureless sintering method for the first time. Lattice distortion is accomplished by introducing rare earth elements with different cation radii and mass. The as-synthesized HE-RE₂Sn₂O₇ ceramics possess high ATW emissivity (91.38%–95.41%), high NIR solar reflectivity (92.74%–97.62%), low thermal conductivity (1.080–1.619 W·m⁻¹·K⁻¹), and excellent chemical stability. On the one hand, the lattice distortion intensifies the asymmetry of the structural unit to cause a notable alteration in the electric dipole moment, ultimately enlarging the ATW emissivity. On the other hand, by selecting difficult excitation elements, HE-RE₂Sn₂O₇, which has a wide band gap (E_g), exhibits high NIR solar reflectivity. Hence, the multi-component design can effectively enhance radiative cooling ability of HE-RE₂Sn₂O₇ and provide a novel strategy for developing radiative cooling materials.

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.