Multiphase composition design is a strategy for optimizing the microstructures and properties of ceramic materials through mutual inhibition of grain growth, complementary property improvement, or even mutually reinforcing effects. More interesting phenomena can be expected if chemical interactions between the constituent phases exist. In this study, spark plasma sintering was used to prepare fully dense dual-phase (Zr,Hf,Ta)B₂–(Zr,Hf,Ta)C ceramics from self-synthesized equimolar medium-entropy diboride and carbide powders. The obtained ceramics were composed of two distinct solid solution phases, the Zr-rich diboride phase and the Ta-rich carbide phase, indicating that metal element exchange occurred between the starting equimolar medium-entropy diboride and carbide phases during sintering. Owing to the mutual grain-boundary pinning effect, fine-grained dual-phase ceramics were obtained. The chemical driving force originating from metal element exchange during the sintering process is considered to promote the densification process of the ceramics. The metal element exchange between the medium-entropy diboride and carbide phases significantly increased the Young’s modulus of the dual-phase ceramics. The dual-phase medium-entropy 50 vol% (Zr,Hf,Ta)B₂–50 vol% (Zr,Hf,Ta)C ceramics with the smallest grain size exhibited the highest hardness of 22.4±0.2 GPa. It is inferred that optimized comprehensive properties or performance of dual-phase high-entropy or medium-entropy ceramics of diborides and carbides can be achieved by adjusting both the volume content and the metal element composition of the corresponding starting powders of diborides and carbides.

The coordinated development of new energy vehicles and the energy storage industry has become essential for reducing carbon emissions. The cathode material is the key material that determines the energy density and cost of a power battery, but currently developed and applied cathode materials cannot meet the requirements for high specific capacity, low cost, safety, and good stability. High-entropy materials (HEMs) are a new type of single-phase material composed of multiple principal elements in equimolar or near-equimolar ratios. The interaction between multiple elements can play an important role in improving the comprehensive properties of the material, which is expected to solve the limitations of battery materials in practical applications. Therefore, this review provides a comprehensive overview of the current development status and modification strategies of power batteries (lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs)), proposes a high-entropy design strategy, and analyses the structure–activity relationship between the high-entropy effects and battery performance. Finally, future research topics related to high-entropy cathode materials, including computational guide design, specific synthesis methods, high-entropy electrochemistry, and high-throughput databases, are proposed. This review aims to provide practical guidance for the development of high-entropy cathode materials for next-generation power batteries.

Rare-earth aluminates (REAlO₃) are potential thermal barrier coating (TBC) materials, but the relatively high thermal conductivity (k₀, ~13.6 W·m⁻¹·K⁻¹) and low fracture toughness (K_IC, ~1.9 MPa·m^1/2) limit their application. This work proposed a strategy to improve their properties through the synergistic effects of high-entropy engineering and particulate toughening. High-entropy (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)AlO₃ (HEAO)-based particulate composites with different contents of high-entropy (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEZO) were designed and successfully prepared by solid-state sintering. The high-entropy feature of both the matrix and secondary phases causes the strong phonon scattering and the incorporation of the HEZO secondary phase, remarkedly inhibiting the grain growth of the HEAO phase. As a result, HEAO–xHEZO (x = 0, 5%, 10%, 25%, and 50% in volume) ceramic composites show low thermal conductivity and high fracture toughness. Compared to the most commonly applied TBC material—yttria stabilized-zirconia (YSZ), the HEAO–25%HEZO particulate composite has a lower thermal conductivity of 0.96–1.17 W·m⁻¹·K⁻¹ (298–1273 K), enhanced fracture toughness of 3.94±0.35 MPa·m^1/2, and comparable linear coefficient of thermal expansion (CTE) of 10.5×10⁻⁶ K⁻¹. It is believed that the proposed strategy should be revelatory for the design of new coating materials including TBCs and environmental barrier coatings (EBCs).

It is generally reported that the grain growth in high-entropy ceramics at high temperatures is relatively slower than that in the corresponding single-component ceramics owing to the so-called sluggish diffusion effect. In this study, we report a fast grain growth phenomenon in the high-entropy ceramics (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)MgAl₁₁O₁₉ (HEMA) prepared by a conventional solid-state reaction method. The results demonstrate that the grain sizes of the as-sintered HEMA ceramics are larger than those of the corresponding five single-component ceramics prepared by the same pressureless sintering process, and the grain growth rate of HEMA ceramics is obviously higher than those of the five single-component ceramics during the subsequent heat treatment. Such fast grain growth phenomenon indicates that the sluggish diffusion effect cannot dominate the grain growth behavior of the current high-entropy ceramics. The X-ray photoelectron spectroscopy (XPS) analysis reveals that there are more oxygen vacancies (O_V) in the high-entropy ceramics than those in the single-component ceramics owing to the variable valance states of Eu ion. The high-temperature electrical conductivities of the HEMA ceramics support this analysis. It is considered that the high concentration of O_V and its high mobility in HEMA ceramics contribute to the accelerated migration and diffusion of cations and consequently increase the grain growth rate. Based on this study, it is believed that multiple intrinsic factors for the high-entropy ceramic system will simultaneously determine the grain growth behavior at high temperatures.

The preparation of high-entropy (HE) ceramics with designed composition is essential for verifying the formability models and evaluating the properties of the ceramics. However, inevitable oxygen contamination in non-oxide ceramics will result in the formation of metal oxide impurity phases remaining in the specimen or even escaping from the specimen during the sintering process, making the elemental compositions of the HE phase deviated from the designed ones. In this work, the preparation and thermodynamic analysis during the processing of equiatomic 9-cation HE carbide (HEC9) ceramics of the IVB, VB, and VIB groups were studied focusing on the removing of the inevitable oxygen impurity existed in the starting carbide powders and the oxygen contamination during the powder mixing processing. The results demonstrate that densification by spark plasma sintering (SPS) by directly using the mixed powders of the corresponding single-component carbides will inhibit the oxygen-removing carbothermal reduction reactions, and most of the oxide impurities will remain in the sample as (Zr,Hf)O₂ phase. Pretreatment of the mixed powders at high temperatures in vacuum will remove most part of the oxygen impurity but result in a remarkable escape of gaseous Cr owing to the oxygen-removing reaction between Cr₃C₂ and various oxide impurities. It is found that graphite addition enhances the oxygen-removing effect and simultaneously prevents the escape of gaseous Cr. On the other hand, although WC, VC, and Mo₂C can also act as oxygen-removing agents, there is no metal-containing gaseous substance formation in the temperature range of this study. By using the heat-treated powders with added graphite, equiatomic HEC9 ceramics were successfully prepared by SPS.

Ultra-high temperature ceramics (UHTCs) are generally referred to the carbides, nitrides, and borides of the transition metals, with the Group IVB compounds (Zr & Hf) and TaC as the main focus. The UHTCs are endowed with ultra-high melting points, excellent mechanical properties, and ablation resistance at elevated temperatures. These unique combinations of properties make them promising materials for extremely environmental structural applications in rocket and hypersonic vehicles, particularly nozzles, leading edges, and engine components, etc. In addition to bulk UHTCs, UHTC coatings and fiber reinforced UHTC composites are extensively developed and applied to avoid the intrinsic brittleness and poor thermal shock resistance of bulk ceramics. Recently, high- entropy UHTCs are developed rapidly and attract a lot of attention as an emerging direction for ultra-high temperature materials. This review presents the state of the art of processing approaches, microstructure design and properties of UHTCs from bulk materials to composites and coatings, as well as the future directions.

High-entropy ceramics attract more and more attention in recent years. However, mechanical properties especially strength and fracture toughness for high-entropy ceramics and their composites have not been comprehensively reported. In this work, high-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ (HEB) monolithic and its composite containing 20 vol% SiC (HEB-20SiC) are prepared by hot pressing. The addition of SiC not only accelerates the densification process but also refines the microstructure of HEB, resulting in improved mechanical properties. The obtained dense HEB and HEB-20SiC ceramics hot pressed at 1800 ℃ exhibit four-point flexural strength of 339±17 MPa and 447±45 MPa, and fracture toughness of 3.81±0.40 MPa·m^1/2 and 4.85±0.33 MPa·m^1/2 measured by single-edge notched beam (SENB) technique. Crack deflection and branching by SiC particles is considered to be the main toughening mechanisms for the HEB-20SiC composite. The hardness Hv_0.2 of the sintered HEB and HEB-20SiC ceramics is 23.7±0.7 GPa and 24.8±1.2 GPa, respectively. With the increase of indentation load, the hardness of the sintered ceramics decreases rapidly until the load reaches about 49 N, due to the indentation size effect. Based on the current experimental investigation it can be seen that the room temperature bending strength and fracture toughness of the high-entropy diboride ceramics are within ranges commonly observed in structure ceramics.

Ultra-high temperature ceramics (UHTCs) are considered as a family of nonmetallic and inorganic materials that have melting point over 3000 ℃. Chemically, nearly all UHTCs are borides, carbides, and nitrides of early transition metals (e.g., Zr, Hf, Nb, Ta). Within the last two decades, except for the great achievements in the densification, microstructure tailoring, and mechanical property improvements of UHTCs, many methods have been established for the preparation of porous UHTCs, aiming to develop high-temperature resistant, sintering resistant, and lightweight materials that will withstand temperatures as high as 2000 ℃ for long periods of time. Amongst the synthesis methods for porous UHTCs, sol–gel methods enable the preparation of porous UHTCs with pore sizes from 1 to 500 μm and porosity within the range of 60%–95% at relatively low temperature. In this article, we review the currently available sol–gel methods for the preparation of porous UHTCs. Templating, foaming, and solvent evaporation methods are described and compared in terms of processing–microstructure relations. The properties and high temperature resistance of sol–gel derived porous UHTCs are discussed. Finally, directions to future investigations on the processing and applications of porous UHTCs are proposed.

High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides (La₂O₃, Nd₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, and Y₂O₃) and ZrO₂ are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO₂ are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores (5RE_1/5)₂Zr₂O₇ have been formed after heated at 1000 ℃. The (5RE_1/5)₂Zr₂O₇ are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the (5RE_1/5)₂Zr₂O₇ high-entropy ceramics are below 1 W·m^-1·K^-1 in the temperature range of 300-1200 ℃. The (5RE_1/5)₂Zr₂O₇ can be potential thermal barrier coating materials.

A high-entropy silicide (HES), (Ti_0.2Zr_0.2Nb_0.2Mo_0.2W_0.2)Si₂ with close-packed hexagonal structure is successfully manufactured through reactive spark plasma sintering at 1300 ℃ for 15 min. The elements in this HES are uniformly distributed in the specimen based on the energy dispersive spectrometer analysis except a small amount of zirconium that is combined with oxygen as impurity particles. The Young’s modulus, Poisson’s ratio, and Vickers hardness of the obtained (Ti_0.2Zr_0.2Nb_0.2Mo_0.2W_0.2)Si₂ are also measured.