High-entropy carbide (HEC) ceramics are distinguished by their high hardness, oxidation resistance, corrosion resistance, wear resistance, and high thermal conductivity, positioning them as promising candidates for application in extreme environments. However, inherent brittleness of these high-entropy ceramics limits their further application. In order to enhance the toughness of HEC ceramics, polycarbosilane (PCS), a precursor of silicon carbide (SiC), was added into the precursor of (Zr, Hf, Nb, Ta, W)C high-entropy ceramic. The in-situ formed SiC (SiC_i) by pyrolysis of PCS can serve as reinforcement for HEC ceramics. The results demonstrate that the volume fraction of SiC in the ceramics obtained from the pyrolysis of PCS is 23.38%. The SiC phases, with an average grain size of 1.19 μm, are evenly distributed in the high-entropy ceramic matrix. The pyrolysis process of ceramic precursors was investigated, revealing that the pyrolysis products of PCS exit as amorphous O_x-Si-C_y at low pyrolysis temperature, while a crystalline SiC phase emerges when the pyrolysis temperature exceeds 1500 ℃. Bulk (Zr, Hf, Nb, Ta, W)C-SiC_i ceramic was prepared by hot-pressing of precursor-derived ceramic powders obtained through pyrolysis at 1600 ℃. Mechanical properties of (Zr, Hf, Nb, Ta, W)C-SiC_i ceramic bulk were investigated, and composite ceramic bulks toughened by commercial silicon carbide nanopowders or silicon carbide whiskers were also prepared for comparison. Compared with (Zr, Hf, Nb, Ta, W)C ceramic, all composite ceramic bulks exhibit enhanced flexural strength and toughness. Notably, the in-situ generated SiC_i via precursor-derived method shows the most significant toughening effect. Flexural strength and fracture toughness of (Zr, Hf, Nb, Ta, W)C-SiC_i ceramic are (698±9) MPa and (7.9±0.6) MPa·m^1/2, respectively, representing improvements of 17.71% and 41.07% compared to that of (Zr, Hf, Nb, Ta, W)C ceramic bulk. Taking all above data into comprehensive account, the improvement is mainly due to the small grain size and uniform distribution of SiC in the composite ceramics prepared via precursor-derived method, which enhance energy consumption and hinder crack propagation under external stress.

High-entropy transition metal carbides (HETMCs) have emerged as promising candidate materials in advanced structural application due to their superior physical and chemical properties compared to traditional carbides. Among these materials, (TiVNbMoW)C has garnered attention due to its outstanding mechanical properties and wear resistance. However, previous studies on the single-phase formation process of (TiVNbMoW)C and the effect of carbon vacancy concentration on its mechanical properties remain inadequate. In this study, TiC, VC, NbC, Mo₂C, WC, elemental W powder, and graphite powder were innovatively selected as raw materials, and (TiVNbMoW)C_x with different carbon vacancy concentrations was successfully prepared by spark plasma hot pressing sintering technology. Effects of carbon vacancies on phase composition, phase evolution, microstructure, and mechanical property of the material were systematically studied. The results show that in the Ti-V-Nb-Mo-W-C system, the carbides corresponding to Mo, Ti, Nb and V elements began to dissolve into each other at 1500 ℃, forming (MoTiNbV)C phase. With increase of temperature, W element gradually participates in solid solution, resulting in densification of the material at 1700 ℃, and formation of (TiVNbMoW)C as a high entropy single phase occurs at 1800 ℃. The mass ratio of carbon to transition metals (C/TM) has a great influence on phase structure and microstructure of the material. When the C/TM is 0.7, the W element cannot dissolve sufficiently to form a single-phase structure, resulting in a composition consisting of (MoTiNbV)C and W₂C phases. At a C/TM of 0.8, the sample exhibits a single-phase (TiVNbMoW)C structure characterized by a large number of carbon vacancies. At a C/TM of 0.9, carbon vacancies reach saturation. At a C/TM of 1.0, excessive carbon enrichment within the material results in a decreased degree of densification. An optimal concentration of carbon vacancies is beneficial for grain refinement and enhancement of the mechanical properties of materials. The sintered sample with a C/TM of 0.8 exhibits the highest hardness, elastic modulus and fracture toughness, demonstrating the most favorable integral mechanical properties. Therefore, this study provides an important basis for a comprehensive understanding of (TiVNbMoW)C_x high-entropy carbides. Future research may introduce additional elements to optimize material properties and broaden its application in high-end manufacturing and related fields.

Rare-earth zirconates (REZs) have attracted attention in the field of thermal barrier materials because they are more resistant to calcium-magnesium-aluminum-silicon oxide (CMAS) corrosion than yttria stabilized zirconia (YSZ). High-entropy design of zirconates is an effective method to enhance CMAS corrosion resistance, but currently the ability of its corrosion resistance still does not meet the growing requirement. In this work, a solid-state reaction technique was used to synthesize high-entropy rare-earth zirconate (HE-REZ) (Y_0.2Gd_0.2Er_0.2Yb_0.2Lu_0.2)₂Zr₂O₇ powder with a single-phased defect fluorite structure, and pressureless sintering (PLS) combined with cold isostatic pressing (CIP) technique was used to efficiently prepare bulk samples. The phase composition, microstructure, element distribution, thermal and mechanical properties were studied, focusing on the CMAS corrosion resistance. According to the results, under the same CMAS corrosion environment at 1300 ℃, the corrosion depth of HE-REZ with a relative density of 98.6% is only 2.6% of 7YSZ and 22.6% of Gd₂Zr₂O₇ (GZO). The synergistic effect of zirconates' chemical inertness and high-entropy materials' sluggish diffusion accounts for this exceptional corrosion resistance. The obtained HE-REZ shows higher hardness and Young's modulus, larger coefficient of linear expansion, and lower thermal conductivity than ever, making its mechanical and thermal properties superior to GZO. All these outcomes demonstrate the good application potential of (Y_0.2Gd_0.2Er_0.2Yb_0.2Lu_0.2)₂Zr₂O₇ in the field of thermal barrier materials.

Microstructural design is a promising strategy to enhance the toughness and plasticity of structural ceramics while maintaining their inherently excellent hardness, which can facilitate their applications in extreme environments. In this work, the possibility of establishing a symbiotic structure with metal atomic-layer phase-separation (MALPS) in carbide structural ceramics was investigated. The carbide ceramic samples were synthesized from raw materials comprising transition metals with different component numbers, graphite powders, and a small amount of aluminum by spark plasma sintering at 1900 ℃ and under a pressure of 30 MPa. It was found that Al-MALPS structure was observed exclusively in the high-entropy (TiZrHfNbTa)C ceramic, which was not a MAX phase with long-range-order but rather a composite featuring a non-periodic cross-stacking of single metal atomic layers within the carbide matrix. Characterization by spherical aberration correction transmission electron microscopy and energy dispersive spectroscopy from nanometer to atomic scales revealed that the single Al atomic layers were sparsely embedded onto the {111} planes of the carbide face-centered cubic structure. Combined with the first-principles calculations, the formation of MALPS structure was found to be driven by thermodynamic stability, lattice distortion, and sluggish-diffusion effect of high entropy, rather than the differential diffusion of Al in various carbide lattices. This work could promote the design and regulation of atomic-scale microstructures in structural ceramics, aiming for high performance with synergetic high hardness-strength-toughness.

In the process of electrolyzing water to produce hydrogen, the sluggish electrocatalytic kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) limit the energy conversion efficiency. High-entropy materials have been considered as potential catalysts due to their unique structural features and excellent performance, which could potentially replace traditional metal oxides and precious metals for energy conversion and water electrolysis. Due to the incompatibility between different metals and non-metals, there have been few reports on the synthesis of high-entropy compounds, especially high-entropy metal phosphides. In this study, a series of carbon-based high-entropy alloy phosphide nanoparticles were synthesized using citric acid as complexing agent and ammonium dihydrogen phosphate as phosphorus source via a low-temperature Sol-Gel method with different elemental metals. In 1 mol·L^-1 KOH solution, FeCoNiMoCeP/C exhibited good water electrolysis performance at a current density of 10 mA·cm^-2, with overpotentials of 119 and 240 mV for the HER and OER, respectively. Similarly, in overall water splitting studies, FeCoNiMoCeP/C also showed excellent catalytic activity. When operating at a current density of 10 mA·cm^-2, FeCoNiMoCeP/C required only 1.53 V as the combined anode and cathode voltage for electrolyzing water. This is due to the synergistic effects among the atoms of high-entropy phosphide catalysts which provide more reaction sites to increase reaction activity and selectivity. This study is expected to expand the potential applications of high-entropy alloys in the field of electrocatalysis.

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.

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.

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.

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.