High-entropy ceramics, as a newly discovered type of material in recent years, have attracted widespread attention due to their unique structure and high-entropy effects. However, due to the sluggish ...diffusion effects inherent in high entropy, these ceramics require prolonged sintering at high temperatures to achieve full densification, leading to long fabrication cycles and significant energy consumption. This study utilized flash sintering to successfully obtain dense single-phase high-entropy ceramics in just 60 s while lowering the furnace temperature by 400 °C. Various tests demonstrate that the application of an external electric field during flash sintering promotes material diffusion and densification processes, effectively suppressing excessive grain growth and elemental volatilization. Consequently, the high-entropy ceramics (Na0.2Bi0.2Ba0.2Sr0.2Ca0.2)TiO3 exhibit outstanding energy storage performance (Wrec = 5.8 J/cm3) under moderate electric fields. This research confirms the feasibility of flash sintering in the preparation of dielectric materials for capacitors, providing a practical and cost-effective approach for the low-cost and sustainable development of the ceramic manufacturing industry.
•High-entropy perovskite energy storage ceramics were prepared by flash sintering.•The dependence of high-entropy ceramics on high temperature is broken.•A high Wrec = 5.8 J/cm3 is obtained in high-entropy cceramics by flash sintering.
A high-entropy metal disilicide, (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2, has been successfully synthesized. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and electron backscatter ...diffraction (EBSD) collectively show the formation of a single high-entropy silicide phase. This high-entropy (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2 possesses a hexagonal C40 crystal structure with ABC stacking sequence and a space group of P6222. This discovery expands the known families of high-entropy materials from metals, oxides, borides, carbides, and nitrides to a silicide, for the first time to our knowledge, as well as demonstrating that a new, non-cubic, crystal structure (with lower symmetry) can be made into high-entropy phase. This (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2 exhibits high nanohardness of 16.7 ± 1.9 GPa and Vickers hardness of 11.6 ± 0.5 GPa. Moreover, it has a low thermal conductivity of 6.9 ± 1.1 W m−1 K−1, which is approximately one order of magnitude lower than that of the widely-used tetragonal MoSi2 and ∼1/3 of those reported values for the hexagonal NbSi2 and TaSi2 with the same crystal structure.
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•A high-entropy silicide, (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2, synthesized.•Expanding high-entropy materials to a silicide for the first time.•A new, non-cubic, high-entropy crystal structure demonstrated.•High nanohardness of 16.7 ± 1.9 GPa and Vickers hardness of 11.6 ± 0.5 GPa.•Much-reduced thermal conductivity of 6.9 ± 1.1 W m−1K−1(∼1/10 of that of MoSi2).
The sustainable development of civil nuclear energy requires the fabrication of the durable nuclear wasteforms, in particular for high-level radioactive waste, which involves the design of the ...composition and microstructure. Herein, we demonstrated that high-entropy ceramics (Eu1-xGdx)2(Ti0.2Zr0.2Hf0.2Nb0.2Ce0.2)2O7 are the potential candidate as immobilizing hosts for high-level radioactive waste. The static aqueous leaching test indicates that the normalized leaching rates for the simulated radionuclides Ce (LRCe) and Gd (LRGd) in as-prepared high-entropy ceramics are approximately 10−6~10−8 g·m−2·d−1 after 42 days testing, much lower than those reported values in doped-Gd2Zr2O7 (10−6~10−3 g·m−2·d−1). The excellent chemical durability is mainly due to the synergistic effects of the compositional complexity and severe lattice distortion. Compared to their ternary oxides, the low oxygen vacancy concentration slows down the migration and diffusion of cations. Moreover, the lattice distortion increases the lattice potential energy, also inhibiting the migration of cations. This study provides a strategy for the development and application of high-entropy ceramics as the wasteforms.
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•HEO are studied for high level waste immobilization for the first time.•Leaching rate for the Gd are over 2 orders of magnitude lower than reported values.•Low σ of the HEO implies low migration and diffusion rate of cations.•Good stability of HEO may be due to complex composition and lattice distortion.
Two High-entropy ceramics, (La0.2Nd0.2Yb0.2Y0.2Sm0.2)2Ce2O7 (LNYSC) and (La0.2Nd0.2Yb0.2Y0.2 Lu0.2)2Ce2 O7 (LNYLC) were synthesized using a sol-gel technique and sintering at high temperatures. The ...lattice type, micro-morphology, and thermophysical and mechanical properties of these oxides were investigated. The results showed that the obtained high-entropy powders and bulk samples have single fluorite lattice, dense microstructure, and clear grain boundary. These oxides have less thermal conductivities than (7YSZ), and the thermal conductivity of LNYLC is lower than that of LNYSC. The thermal expansion coefficients of these high-entropy oxides are larger than that of 7YSZ. Owing to the relative less negative electrical difference in LNYLC that in LNYSC, the thermal expansion coefficient of LNYLC is larger than that of LNYSC. Further, these oxides exhibit outstanding lattice stability up to 1200 °C, which can help to improve the durability of thermal barrier coatings. However, these oxides have lower mechanical properties compared to 7YSZ.
This research revealed the mechanisms of irradiation damage in the novel high entropy ceramic materials. (Zr0.25Ta0.25Nb0.25Ti0.25)C high-entropy carbide ceramics (HECC) with a single-phase rock-salt ...structure was synthesized by spark plasma sintering, which was irradiated by 3 MeV Zr ions to 20 dpa at 25, 300, and 500 °C. X-ray diffraction analysis showed that (Zr0.25Ta0.25Nb0.25Ti0.25)C maintained a high phase stability without phase transformation after irradiation. About 0.2% lattice parameter expansion was revealed. The irradiation-induced microstructures were comprised of defect clusters with diameters of several nanometers, without void formation or radiation-induced segregation. The defect clusters were characterized by transmission electron microscopy as two types of dislocation loops, including perfect loops with Burgers vectors of b = a/2<1 1 0> and faulted Frank loops with Burgers vectors of b = a/3<1 1 1>. The growth of dislocation loops may be suppressed by the strong local lattice distortion. Nanoindentation tests showed irradiation-induced hardness increase, which was possibly caused by dislocation loops and lattice strain. Overall, the high irradiation resistance, along with other excellent physical properties makes HECC promising structural materials for advanced reactor designs.
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In this work, a novel high-entropy n-type thermoelectric material Sr0.9La0.1(Zr0.25Sn0.25Ti0.25Hf0.25)O3 with pure perovskite phase was prepared using a conventional solid state processing route. The ...results of TEM and XPS show that various types of crystal defects and lattice distortions, such as oxygen vacancies, edge dislocations, in-phase rotations of octahedron and antiparallel cation displacements coexist in this high-entropy ceramic. At 873 K, the high-entropy ceramics showed both a low thermal conductivity (1.89 W/m/K) and a high Seebeck coefficient (393 μV/K). This work highlights a way to obtain high-performance perovskite-type oxide thermoelectric materials through high-entropy composition design.
Herein, the phase evolution, densification and grain growth process of the high entropy ceramics during flash sintering were systematically characterized and quantified to understand the ...microstructural evolution for the first time. It was demonstrated that the densification rate of (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 by flash sintering in this work was generally around 60 times that of conventional sintering at 1600 °C, while the grain growth rate by flash sintering was only around 1.5–6 times that of conventional sintering, indicating that grain growth was suppressed during flash sintering. The grain growth mechanisms by flash sintering and conventional sintering could be both attributed to surface diffusion and volume diffusion. In addition, the flash sintered high-entropy ceramics as promising immobilization materials for high-level radioactive waste (HLW) exhibited excellent aqueous durability with normalized leaching rates of Nd, Gd and Zr approximately 10−6∼10−7 g m−2 d−1 after 42 days, which were much lower than most reported pyrochlore materials.
High‐entropy metal diborides (Hf0.2Zr0.2Ta0.2M0.2Ti0.2)B2 (M = Nb, Mo, Cr) are a new class of ultrahigh temperature ceramics. The novel structural, mechanical, and electronic properties of ...(Hf0.2Zr0.2Ta0.2M0.2Ti0.2)B2 (M = Nb, Mo, Cr) are studied within density functional theory, with the compositional disorder being treated using the special quasi‐random structures technique. The elastic stiffness constants Cij, as well as bulk, shear and Young's modulus of (Hf0.2Zr0.2Ta0.2M0.2Ti0.2)B2 (M = Nb, Mo, Cr) are studied to estimate the mechanical properties. The ductility is also investigated from several criterions, showing that (Hf0.2Zr0.2Ta0.2M0.2Ti0.2)B2 (M = Nb, Mo, Cr) is still intrinsically brittle. The high Debye temperature of (Hf0.2Zr0.2Ta0.2M0.2Ti0.2)B2 (M = Nb, Mo, Cr) indicates the strong interatomic interactions and good thermal conductivity. The detailed investigations of electronic structure reveal that covalent and metallic bonds are respectively formed in the boron layer and the metal layer, mixed ionic and covalent bonds are formed between the metals and boron interlayer.
The high‐entropy ceramics (Hf0.2Zr0.2Ta0.2M0.2Ti0.2)B2 (M = Nb, Mo, Cr) are stable thermodynamically and mechanically. The elastic modulus follows the mixing rule. (Hf0.2Zr0.2Ta0.2M0.2Ti0.2)B2 (M = Nb, Mo, Cr) possess higher Debye temperature. They exhibit a combination of metallic, covalent and ionic bonding. Mechanical properties of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 are more excellent than (Hf0.2Zr0.2Ta0.2M0.2Ti0.2)B2 (M = Mo, Cr).
•Nine compositionally-complex fluorite oxides (CCFOs) are made and investigated.•CCFOs exhibit reduced thermal conductivity and increased cubic phase stability.•Lower thermal conductivity is achieved ...in medium-entropy non-equimolar CCFOs.•High modulus and hardness retain in CCFOs with reduced thermal conductivity.•Non-equimolar CCFOs exhibit amorphous-like T-dependent thermal conductivity.
Using fluorite oxides as an example, this study broadens high-entropy ceramics (HECs) to compositionally-complex ceramics (CCCs) or multi-principal cation ceramics (MPCCs) to include medium-entropy and/or non-equimolar compositions. Nine compositions of compositionally-complex fluorite oxides (CCFOs) with the general formula of (Hf1/3Zr1/3Ce1/3)1-x(Y1/2X1/2)xO2-δ (X = Yb, Ca, and Gd; x = 0.4, 0.148, and 0.058) are fabricated. The phase stability, mechanical properties, and thermal conductivities are measured. Compared with yttria-stabilized zirconia, these CCFOs exhibit increased cubic phase stability and reduced thermal conductivity, while retaining high Young’s modulus (∼210 GPa) and nanohardness (∼18 GPa). Moreover, the temperature-dependent thermal conductivity in the non-equimolar CCFOs shows an amorphous-like behavior. In comparison with their equimolar high-entropy counterparts, the medium-entropy non-equimolar CCFOs exhibit even lower thermal conductivity (k) while maintaining high modulus (E), thereby achieving higher E/k ratios. These results suggest a new direction to achieve thermally-insulative yet stiff CCCs (MPCCs) via exploring non-equimolar and/or medium-entropy compositions.
Four high-entropy metal diborides have been synthesized and densified by borocarbothermal reduction of metal oxides with boron carbide and graphite and subsequent spark plasma sintering. Three of ...them, (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B2, (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B2, and (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B2, possess single high-entropy phases and have been sintered to >99% of the theoretical densities. The fourth (Hf0.2Zr0.2Ti0.2Mo0.2W0.2)B2 specimen contained a Ti–Mo–W rich secondary phase in addition to the primary metal diboride phase. The specimens made by borocarbothermal reduction exhibit improved hardnesses in comparison with those samples previously fabricated via high energy ball milling and spark plasma sintering. Interestingly, the single-phase (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B2 and (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B2 (both of which have Vickers hardness values of ~25 GPa) are substantially harder than (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B2 (20.5 GPa), despite MoB2 and CrB2 being typically considered as softer components. These single-phase high-entropy metal diborides were found to have low thermal conductivities of 12–25 W/mK, which are ~1/10 to ~1/5 of the reported values of HfB2 and ZrB2.
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