Polymer‐derived ceramic (PDC) route has been widely used to fabricate various ceramics or ceramic‐matrix composites in recent years. However, the synthesis of high‐entropy ceramics via PDC route has ...rarely been reported until now. Herein, we successfully synthesized a class of high‐entropy carbides, namely (Hf0.25Nb0.25Zr0.25Ti0.25)C (HEC‐1), via PDC route. The polymer‐derived HEC‐1 ceramics consisted of numerous superfine particles with the average particle size ~800 nm. Meanwhile, they possessed a rock‐salt structure of metal carbides and high‐compositional uniformity from nanoscale to microscale. In addition, the as‐obtained HEC‐1 ceramics had a low oxygen impurity content of 0.51% and a low free carbon impurity content of 2.56%. This work will open up a new research field on the fabrication of high‐entropy ceramics or high‐entropy ceramic‐matrix composites via PDC route.
Powder synthesis is critical for implementing the wide applications of high‐entropy diborides (HEBs). However, the low‐temperature synthesis of HEB powders was rarely reported. Herein, the ...low‐temperature synthesis of the single‐phase HEB nanopowders via molten salt‐mediated magnesiothermic reduction (MMR) method was reported for the first time. The results showed that the as‐synthesized nanopowders consisted of the single‐phase HEBs and their average particle sizes are in the range of 28‐56 nm. Meanwhile, they possessed the good compositional homogeneity and the low‐content oxygen impurity in the range of 4.13‐6.12 at%. In addition, their formation mechanism could be well interpreted by a classical MMR growth process.
The nanocrystalline (Hf0.25Zr0.25Ta0.25Ti0.25)C high‐entropy carbide ceramics were successfully prepared through high‐pressure sintering of the self‐synthesized nanopowders for the first time. The ...results showed that the relative density, grain size, hardness, and elastic modulus of all the as‐prepared samples increased gradually with the increase of sintering temperatures from 1200 to 1600°C. The as‐fabricated samples at 1600°C possessed a relative density of 97.1% and a mean grain size of 98 ± 35 nm and simultaneously exhibited good comprehensive mechanical properties with Vickers hardness of 25.7 ± 0.6 GPa and fracture toughness of 4.3 ± 0.4 MPa·m1/2 at room temperature due to the grain refinement strengthening mechanism and microcrack deflection and bridging toughening mechanism. In addition, they showed a linear decrease in high‐temperature Vickers hardness with the increase of the temperature from room temperature to 1000°C, which was determined by the combined effects of the intrinsic softening of HEC‐1 ceramics at elevated temperature and the presence of ZrO2 impurity phase at the grain boundaries.
In this work, a novel (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)(N0.5C0.5) high‐entropy nitride‐carbide (HENC‐1) with multi‐cationic and ‐anionic sublattice structure was reported and their thermophysical and ...mechanical properties were studied for the first time. The results of the first‐principles calculations showed that HENC‐1 had the highest mixing entropy of 1.151R, which resulted in the lowest Gibbs free energy above 600 K among HENC‐1, (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)N high‐entropy nitrides (HEN‐1), and (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy carbides (HEC‐1). In this case, HENC‐1 samples were successfully fabricated by hot‐pressing sintering technique at the lowest temperature (1773 K) among HENC‐1, HEN‐1 and HEC‐1 samples. The as‐fabricated HENC‐1 samples showed a single rock‐salt structure of metal nitride‐carbides and high compositional uniformity. Meanwhile, they exhibited high microhardness of 19.5 ± 0.3 GPa at an applied load of 9.8 N and nanohardness of 33.4 ± 0.5 GPa and simultaneously possessed a high bulk modulus of 258 GPa, Young's modulus of 429 GPa, shear modulus of 176 GPa, and elastic modulus of 572 ± 7 GPa. Their hardness and modulus are the highest among HENC‐1, HEN‐1 and HEC‐1 samples, which could be attributed to the presence of mass disorder and lattice distortion from the multi‐anionic sublattice structure and small grain in HENC‐1 samples. In addition, the thermal conductivity of HENC‐1 samples was significantly lower than the average value from the “rule of mixture” between HEC‐1 and HEN‐1 samples in the range of 300‐800 K, which was due to the presence of lattice distortion from the multi‐anionic sublattice structure in HENC‐1 samples.
A novel high-entropy material, (Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 ((5RE0.2)2Si2O7) was prepared by the sol-gel method and investigated as a promising environmental barrier coating (EBC) for SiC-based ...composites. The results of X-ray diffraction and transmission electron microscopy indicated that rare-earth elements were distributed homogeneously in the single monoclinic phase. Moreover, it was found that the new material (5RE0.2)2Si2O7 had good phase stability, well-matched coefficient of thermal expansion with SiC-based composite, and excellent resistance to water-vapor corrosion. The water-vapor corrosion test of (5RE0.2)2Si2O7 coated Cf/SiC composites further confirmed that (5RE0.2)2Si2O7 was suitable for application as EBC material and could provide effective protection to Cf/SiC composites from water-vapor damage.
The formation possibility of a new (Zr0.25Nb0.25Ti0.25V0.25)C high-entropy ceramics (ZHC-1) was first analyzed by the first-principles calculations and thermodynamical analysis and then it was ...successfully fabricated by hot pressing sintering technique. The first-principles calculation results showed that the mixing enthalpy of ZHC-1 was 5.526 kJ/mol and the mixing entropy of ZHC-1 was in the range of 0.693R–1.040R. The thermodynamical analysis results showed that ZHC-1 was thermodynamically stable above 959 K owing to its negative mixing Gibbs free energy. The experimental results showed that the as-prepared ZHC-1 (95.1% relative density) possessed a single rock-salt crystal structure, some interesting nanoplate-like structures, and high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, compared with the initial metal carbides (ZrC, NbC, TiC and VC), it showed a relatively low thermal conductivity of 15.3 ± 0.3 W/(m⋅K) at room temperature, which was due to the presence of solid solution effects, nanoplates and porosity. Meanwhile, it exhibited the relatively high nanohardness of 30.3 ± 0.7 GPa and elastic modulus of 460.4 ± 19.2 GPa and the higher fracture toughness of 4.7 ± 0.5 MPa m1/2, which were attributed to the solid solution strengthening mechanism and nanoplate pullout and microcrack deflection toughening mechanism.
A new (Zr0.25Nb0.25Ti0.25V0.25)C high-entropy ceramic (ZHC-1) with a single rock-salt crystal structure of metal carbides, some interesting nanoplate-like structures and high compositional uniformity from nanoscale to microscale was fabricated and investigated in detailed and it exhibited the superior mechanical and thermal physical performances, which would endow ZHC-1 a promising candidate for extreme environmental applications. Display omitted
Here, we report a novel high-entropy rare earth zirconate (HE-REZ) (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Zr2O7 ceramic aerogel prepared through a sol-gel template method and high-temperature calcination ...followed by 3-D-structure reconstruction. The structural evolution and crystallisation behaviour of the prepared aerogel were characterised through scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The results indicated that the as-prepared HE-REZ ceramic aerogels had a typical nanoporous structure. The HE-REZ ceramic aerogels thermally treated at 900 °C presented an ultralow room temperature thermal conductivity of 0.031 W/(m·K), high specific surface areas of 443.26 m2/g and a relatively high strength of 12.95 MPa. The effects of different calcination temperatures on the microstructure of the samples were also investigated. Therefore, the excellent insulation performance of these unique HE-REZ ceramic aerogels indicate that they can be used as high-temperature insulators for hypersonic vehicles in the future.
Tailoring the structure and properties of materials using the high‐entropy (HE) effect is of significant interest in the fields of environmental and thermal barrier coatings (TBCs). In this work, a ...new class of dense HE rare‐earth niobates was successfully prepared by a solid‐phase reaction method, including (Sm1/5Dy1/5Ho1/5Er1/5Yb1/5)NbO4 (5HERN), (Sm1/6Dy1/6Ho1/6Er1/6Yb1/6Lu1/6)NbO4 (6HERN), (Sm1/7Dy1/7Ho1/7Er1/7Yb1/7Lu1/7Gd1/7)NbO4 (7HERN), and (Sm1/8Dy1/8Ho1/8Er1/8Yb1/8Lu1/8Gd1/8Tm1/8)NbO4 (8HERN), along with eight single rare‐earth niobates (RENbO4, RE = Sm, Dy, Ho, Er, Yb, Lu, Gd, and Tm). X‐ray diffraction analysis showed that 5–8HERN are single‐phase solid solutions with a monoclinic structure (space group C12/c1). The thermal expansion coefficients of 7HERN and 8HERN exceed 11 × 10−6 K−1 at 1200°C and are much higher than those of the RENbO4 compositions (10.13–10.74 × 10−6 K−1) and other some HE rare‐earth oxides (10.27–10.87 × 10−6 K−1). Importantly, 5–8HERN have lower oxygen‐ion conductivity and higher activation energy than yttrium‐stabilized zirconia (YSZ) and the RENbO4 compositions. The oxygen‐ion conductivity of 5HERN (7.52 × 10−7 S cm−1, 900°C) was 105 times lower than that of YSZ (0.01 S cm−1, 750°C). The hardness of 5–8HERN is ∼7.81–8.46 GPa and these compositions have low intrinsic lattice thermal conductivity at high temperature (1.28–1.69 W m−1 K−1 at 900°C). The mechanism by which the HE effect improved the material properties was elucidated. Young's modulus, hardness, thermal expansion coefficient, and intrinsic lattice thermal conductivity are linearly related to the mass, size, and distortion degree of samples. In contrast, the oxygen‐ion conductivity depends on both the degrees of disorder and distortion and the oxygen‐ion vacancy concentration. Based on their overall performance, especially their high thermal expansion coefficients and excellent oxygen‐barrier performance, HE rare‐earth niobates show potential for further development as TBC materials.
TiSnNbTaGa2O12 (TSNTGO) high–entropy dielectric ceramic was successful synthesized using the solid-state reaction method. X–ray diffraction (XRD), Rietveld refinement and transmission electron ...microscopy (TEM) were used to verify that all samples have rutile structure of P42–mnm space group. The analysis of Raman spectra proved that the εr exhibited a contrary changing trend as that of Raman shift near 822 cm−1, and the same similar relationship occurred between Q×f values and full width at half maximum (FWHM). The exploration of complex chemical bonding theory (P–V–L theory) for TSNTGO ceramics suggested that the bond ionicity played a dominant role in εr, while Q×f and τf were primarily determined by lattice energy and bond energy, respectively. Notably, the TSNTGO ceramics sintered at 1275 ℃ demonstrated optimal microwave dielectric properties of εr = 33.62 ± 0.03, Q×f = 51,832 ± 1000 GHz (f0 = 6.4 GHz), and τf = –0.38 ± 0.16 ppm/℃.
The reaction sequence and mechanical properties were studied for (Mg,Co,Ni,Cu,Zn)O high‐entropy ceramics that were synthesized using field‐assisted sintering technology. The evolution from binary ...oxide starting powders to a single‐phase rock salt structure exhibited a distinct incorporation sequence. For the rock salt oxides, MgO and CoO had the lowest vacancy formation energies and were the first to be incorporated into the high‐entropy ceramic followed by NiO, which had a higher vacancy formation energy. Both CuO and ZnO had different crystal structures, and were incorporated into the single phase structure after the rock salt oxides due to the additional energy barrier associated with the transformations from their original structures to the rock salt structure. Distinctive morphological features including Cu‐rich regions and lattice distortion were observed in the high entropy ceramic. In addition, a trade‐off between densification and grain growth resulted in a maximum in strength (323 MPa) and elastic modulus (108 GPa) after densification at 900°C. This study has revealed new information that can be used to design other high‐entropy ceramics including selection criteria for constituent compounds based on crystal structure and defect formation energy as well as the effects of grain size and porosity on control strength and elastic modulus.
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