The development of high-speed flight technology has put forward an urgent demand for high- performance thermal structure materials. High-entropy carbides (HECs) ceramics are a fast-emerging family of materials that combine the excellent properties of high-entropy ceramics and ultra-high temperature ceramics. HECs have a broad application prospect in extreme service environments, which has received extensive attention from scholars in recent years. Compared with traditional ultra-high temperature carbides containing only one or two transition metal elements, HECs have a greater potential for development because of their improved comprehensive performance and greater designability of composition and properties. After successive exploration of HECs in recent years, researchers have obtained many interesting results, developed a variety of preparation methods, and gained comprehensive understanding of microstructure and properties. The basic theories and the laws on HECs obtained from experimental process are reviewed in this paper. Preparation methods of HECs including powders, blocks, coatings and films, as well as fiber-reinforced HECs-based composites are summarized. Research progress on the properties of HECs, such as the mechanical properties, thermal properties, and especially the oxidation and ablation resistance related to high-temperature applications, is reviewed and discussed. Finally, the scientific issues that need to be further explored in this area are emphasized, and the prospects are proposed.

Environmental barrier coating (EBC) is a key material for high power-to-weight ratio aero engine, which can provide effective protection for the hot end components of ceramic matrix composites, and prevent the erosion of gas and environmental corrosive media. At present, high entropy rare earth disilicates ((xRE_1/x)₂Si₂O₇) are the most promising next-generation environmental barrier coatings. In order to enhance the CMAS corrosion resistance of high entropy rare earth disilicates, a novel high entropy (Y_0.25Yb_0.25Er_0.25Tm_0.25)₂Si₂O₇/RE-Si-Al-O (RE=Yb, Y, and La) multiphase ceramic was designed and prepared. The results show that the RE-Si-Al-O glass phase can not only wrap the ceramic grains, but also exist at the grain boundaries. Moreover, this multiphase ceramics can promote the growth of rare earth disilicate grains, reduce the number of grain boundaries, and decrease the number of diffusion channel of CMAS melt. As the radius of rare earth ion in the RE-Si-Al-O glass phase increases, the glass phase is more prone to react with Ca²⁺ ion in the CMAS melt, generating apatite, reducing the activity of the CMAS melt, inhibiting the erosion of high entropy rare earth disilicate grains by the CMAS molten salt, and thus improving the CMAS corrosion resistance of high entropy rare earth disilicates. After corrosion at 1500 ℃ for 48 h, there is still a residual CMAS layer on the surface of (Y_0.25Yb_0.25Er_0.25Tm_0.25)₂Si₂O₇/La-Si-Al-O multiphase ceramics, indicating that the multiphase ceramics have good resistance to CMAS corrosion. In conclusion, the microstructure design of this multiphase ceramic provides a new approach to improve the long-term application of EBC materials in high-temperature CMAS environments.

High-entropy boride ceramics (HEBs) consisting of four or more principle metallic elements rapidly develop in recent years due to their outstanding unique physical properties and excellent elevated temperature properties, showing extraordinary promise as potential thermal protection materials applied in extreme environments. However, on the basis of unclear role of each element on their oxidation reaction, HEBs are generally difficult to densify because of their low self-diffusion coefficients and possible sluggish diffusion effect, resulting in limited mechanical properties and low oxidation resistance. In this work, a novel type of HEBs, (Ti_0.25Zr_0.25Hf_0.25Ta_0.25)B₂-B₄C composites, were prepared by boro/carbothermal reduction method combined with hot-pressing sintering at 1900 ℃. The effect of B₄C at the volume fractions ranging from 10% to 30% on the mechanical properties and oxidation resistance of the composites was systematically investigated. Microstructure analyses indicate that homogenously distributed B₄C can suppress grain growth of the HEBs matrix and promote toughening mechanisms such as crack deflection and crack branching, consequently resulting in strengthening and toughening composites. When the volume fraction of B₄C is 20%, the as-prepared composite shows a high relative density (96.1%) and good mechanical properties with Vickers hardness of (24.6±1.1) GPa, flexural strength of (570.0±27.6) MPa and fracture toughness of (5.58±0.36) MPa·m^1/2. In addition, exploration on the oxidation resistance of (Ti_0.25Zr_0.25Hf_0.25Ta_0.25)B₂-B₄C composites at temperatures ranging from 800 ℃ to 1400 ℃ shows that excellent oxidation resistance occurs at the chosen temperatures due to the formation of a dense and continuous oxidation scale, which acts as a barrier layer preventing oxygen inward diffusion. The main compositions of the oxide scale are TiO_x, (Zr, Hf)O₂ oxides and B₂O₃ at 800 ℃, while multicomponent oxidation products of (Zr, Hf, Ta)O_x, (Zr, Hf)O₂ and TiTaO₄ are formed in the oxide scale at 1100 ℃. As the temperature increased to 1400 ℃, thickness of the oxide layer significantly increases due to their volatilization of B₂O₃, while continuous B₂O₃ glassy phase plays a crucial role in the oxidation process of HEBs. When the B₄C volume fraction not less than 20%, TiTa₂O₇ and TiO₂ which were embedded in B₂O₃ glass, could effectively insulate inward oxygen and interfacial oxide thickness and enhance oxidation resistance of the composites. In summary, the primary work can be used as a reference to the researches relating to optimizing mechanical properties and oxidation resistance for HEBs.

High-entropy transition metal nitrides (HENs) are renowned for their thermal stability, corrosion and oxidation resistance, and exceptional mechanical properties, endowing them suitable for use as surface protection films for structural and moving components. However, mapping relationship between broadly adjustable metal components and mechanical properties of HENs is quite complex due to their diversity of HENs components. Taking (NbMoTaW)N_x thin film as the research object, this study prepared (NbMoTaW)N_x (x = 0, 0.59, 0.80, 0.95) thin films with different nitrogen contents by regulating nitrogen flow velocity during the film growth process based on the magnetron sputtering technique. Following analysis of (NbMoTaW)N_x thin films' composition, structure, morphology, and performance, the primary influence mechanism that govern their mechanical properties were explored. The findings revealed that by manipulating nitrogen vacancy, coordinated regulation over the lattice distortions of the nitrogen and metal sublattices was achieved. Due to high degree of the nitrogen and metal sublattice distortions, the (NbMoTaW)N_0.80 sample demonstrated the highest hardness and best wear resistance performance. After excluding factors such as electronic structure, residual stress, and grain size that affect mechanical properties, a direct relationship between lattice distortions and mechanical properties of HENs films was confirmed. In summary, this research has unearthed a straightforward strategy for controlling the lattice distortions, offering a novel approach to adjust and optimize the performance of nitride films, and ultimately providing a more effective solution to address the mechanical damage issues that arise in the context of complex service environments.

With improvement in service temperature of thermal structural components for the new generation hypersonic aircraft, higher requirements are put forward for the phase stability and ablation resistance of the thermal protection coatings (TPCs). Carrying out high-entropy design for traditional transition metal oxide ZrO₂ and HfO₂ coatings, solid-phase reaction and supersonic atmosphere plasma spraying (SAPS) were applied to prepare (Hf_0.125Zr_0.125Sm_0.25Er_0.25Y_0.25)O_2-δ (M1R3O), (Hf_0.2Zr_0.2Sm_0.2Er_0.2Y_0.2)O_2-δ (M2R3O), (Hf_0.25Zr_0.25Sm_0.167Er_0.167Y_0.167)O_2-δ (M3R3O) high-entropy oxide (HEO) coatings. The effects of rare earth content on phase structure evolution, phase stability and ablative resistance of HEO coatings were investigated. M2R3O coating and M3R3O coating possessed excellent phase stability and ablation resistance, which maintained stable phase structure after ablation by oxygen-acetylene flame with heat flux density of 2.38-2.40 MW/m², without decomposition of solid solution and precipitation of rare earth components. Mass ablation rate and linear ablation rate of M2R3O coating after cyclic ablation for 180 s are 0.01 mg/s and -1.16 μm/s, respectively. Compared with M1R3O coating (0.09 mg/s, -1.34 μm/s) and M3R3O coating (0.02 mg/s, -4.51 μm/s), the reductions of ablation rate are 88.9%, 13.4%, respectively, and 50.0%, 74.3% for M2R3O coatings, respectively, presenting the best ablation resistance. M2R3O coating exhibits excellent ablation resistance due to its high melting point (>2200 ℃) and low thermal conductivity ((1.07±0.09) W/(m·K)), which effectively protects the internal SiC transition layer and C/C composites from oxidation damage, avoiding interface cracking caused by the formation of SiO₂ phase.

Intermediate-temperature solid oxide fuel cell (IT-SOFC) is promising for carbon neutrality, but its cathode is limited by the contradiction between thermal compatibility and catalytic activity. Herein, we propose a high-entropy double perovskite cathode material, GdBa(Fe_0.2Mn_0.2Co_0.2Ni_0.2Cu_0.2)₂O_5+δ (HE-GBO) with improved compatibility and activity, in view of the high-entropy strategy by multi-elemental coupling, which possesses double perovskite structure and excellent chemical compatibility with state-of-the-art Gd_0.1Ce_0.9O_2-δ (GDC). The polarization resistance (R_p) of the symmetrical cells with HE-GBO cathode is 1.68 Ω·cm² at 800 ℃, and the corresponding R_p of HE-GBO-GDC (mass ratio 7:3) composite cathode can be greatly reduced (0.23 Ω·cm² at 800 ℃) by introducing GDC. Dendritic microchannels anode-supported single cells with HE-GBO and HE-GBO-GDC cathodes realize maximum power densities of 972.12 and 1057.06 mW/cm² at 800 ℃, respectively, indicating that cell performance can be enhanced by high-entropy cathodes. The results demonstrate that high-entropy double perovskite cathode material HE-GBO has a high potantial to solve the conflict problem of thermal compatibility and catalytic activity in IT-SOFCs.

Flash sintering is a sintering technology coupled with temperature field and electric field, with characteristics of rapid mass transfer at low temperature, showing significant advantages in the synthesis of high entropy ceramics. In this study, relatively dense high entropy oxide ceramic (MgCoNiCuZn)O was synthesized by flash sintering, which properties were compared with those of conventional sintered samples. Under flash sintering condition of room temperature, the electric field intensity of 50 V/cm and the current density of 300 mA/mm², the time of phase transformation is only 10 s. The maximum relative density of flash sintered sample is 94%, which is 22.8% higher than that of conventional sintered sample. The maximum hardness of flash sintered sample is 5.05 GPa, which is 3.95 GPa higher than that of conventional sintered sample. When the frequency is lower than 2 Hz, the dielectric constant of flash sintering sample is one order of magnitude higher than that of conventional sintered sample. The property improvement of flash sintered samples is attributed to the acceleration of mass transfer by the critical electric field to increase the material density, and the extra defects introduced by the critical electric field.

Flash sintering is an electric field assisted sintering technology which has attracted much attention in recent years. This review introduces its origin, development, and basic characteristics. In the study of flash incubation and initiation process, the nonlinear conductivity characteristics and electrochemical blackening phenomenon are narrated, and the defect mechanism dominated by oxygen vacancy is recounted. As for rapid densification during flash sintering, it is proposed that the generation and movement of defects caused by electric field produce Coulomb force between powder particles, which is conducive to density in the early stage of flash sintering. Meanwhile, the densification process is accompanied by the rapid movement of metal cations. In terms of grain growth and microstructure evolution during the flash sintering, the sample temperature is asymmetrically distributed along the current direction, and the internal grain boundary mobility in the sample is significantly improved. During this stage, electrochemical defects exert a significant impact on the microstructure. Based on the above researches, we developed ceramic flash joining technology by using phenomenon of low-temperature rapid mass transfer under electric field, and realized rapid joining between similar kind of ceramics/ceramics, ceramics/metals, and even dissimilar ceramics/ceramics. A new ultrafast ceramic synthesis technology by flash sintering was developed, which not only realized the rapid synthesis of typical oxide ceramics, but also realized the rapid synthesis of high entropy ceramics and oxide ceramics with eutectic morphology. An electroplastic forming technology of oxide ceramics was developed, and a rapid tensile and bending deformation of zirconia ceramics at low temperature and low stress was preliminarily realized. Finally, this review summarizes the challenges in the field of flash sintering mechanism, and looks forward to the development direction of flash sintering from two aspects of Joule heating effect and nonthermal effect, aiming to be beneficial to the development of flash sintering technology in China.

The thermoelectric properties of ZrNiSn-based half-Heusler materials were hindered due to their high thermal conductivity. In order to reduce the lattice thermal conductivity, the high-entropy alloys ZrNiSn and Zr_0.5Hf_0.5Ni_1-xPt_xSn (x=0, 0.1, 0.15, 0.2, 0.25, 0.3) were prepared by levitation melting and spark plasma sintering. Configurational entropy of the alloys was manipulated by Hf substitution for Zr and Pt substitution for Ni. Effects of configuration entropy on the thermoelectric properties were investigated. The reslults showed that the minimum sum of lattice thermal conductivity and bipolar thermal conductivity (κ_l+κ_b) at 673 K for Zr_0.5Hf_0.5Ni_0.85Pt_0.15Sn was optimized at 2.1 W·m^-1·K^-1, which was significantly reduced by about 58% when compared with ZrNiSn. This finding provides an effective strategy for reducing lattice thermal conductivity of ZrNiSn-based alloy to offer great potential for further improvement of thermoelectrics.