Dual-phase rare-earth-zirconate high-entropy ceramics with glass-like thermal conductivity

A series of rare earth zirconates (RE₂Zr₂O₇) high-entropy ceramics with single- and dual-phase structure were prepared. Compared with La₂Zr₂O₇ and Yb₂Zr₂O₇, the smaller “rattling” ions (Yb³⁺, Er³⁺, Y³⁺) have been incorporated into pyrochlore lattice in (La_0.2Nd_0.2Y_0.2Er_0.2Yb_0.2)₂Zr₂O₇ (LNYEY) while larger ions (La³⁺, Nd³⁺, Sm³⁺, Eu³⁺) incorporated into fluorite lattice in (La_0.2Nd_0.2Sm_0.2Gd_0.2Yb_0.2)₂Zr₂O₇ (LNSGY). Due to high-entropy lattice distortion and resonant scattering derived from smaller ions Yb³⁺, Er³⁺, and Y³⁺, LNYEY shows a lower glass-like thermal conductivity (1.62-1.59 W m^-1 K^-1, 100-600℃) than LNSGY (1.74-1.75 W m^-1 K^-1, 100-600℃). Moreover, LNYEY and LNSGY exhibit enhanced Vickers’ hardness (LNYEY, H_v = 11.47 ± 0.41 GPa; LNSGY, H_v = 10.96 ± 0.26 GPa) and thermal expansion coefficients (LNYEY, 10.45 × 10^-6 K^-1, 1000℃; LNSGY, 11.02 × 10^-6 K^-1, 1000℃). These results indicate that dual-phase rare-earth-zirconate high-entropy ceramics could be desirable for thermal barrier coatings.     

High-entropy ferroelastic rare-earth tantalite ceramic: (Y0.2Ce0.2Sm0.2Gd0.2Dy0.2)TaO4

In this study, high-entropy rare-earth tantalate ceramics (Y_0.2Ce_0.2Sm_0.2Gd_0.2Dy_0.2)TaO₄ ((5RE_0.2)TaO₄) have been successfully fabricated. The possibility of formation of (5RE_0.2)TaO₄ was verified via first-principles calculations. In addition, the phase structure, ferroelastic toughening mechanism, thermophysical, and mechanical properties were systematically investigated. The (5RE_0.2)TaO₄ ceramics have lower phonon thermal conductivity (1.2–2.6 W·m^–1·K^–1) in the entire temperature range than that of RETaO₄ and YSZ. (5RE_0.2)TaO₄ has a higher fracture toughness and lower brittleness index than YSZ. The thermal expansion coefficients of (5RE_0.2)TaO₄ are as high as 10.3 × 10^-6 K^–1 at 1200°C and Young's modulus is 66–189 GPa, and thus, (5RE_0.2)TaO₄ possesses great potential for application in thermal barrier coatings (TBCs).     

Surface energies in high-entropy carbides with variable carbon stoichiometry

The carbon vacancy in high-entropy carbides (HECs) has a significant impact on their physical and chemical properties, yet relevant studies have still been relatively few. In this study, we investigate the surface energies of HECs with variable carbon vacancies through first-principles calculations. The results show that the surface energy of the (1 0 0) surface of the stoichiometric HECs is significantly lower than that of (1 1 1) surface. With the decrease in carbon stoichiometry, the surface energies of both (1 0 0) and (1 1 1) surfaces increase gradually, which is mainly due to the weakening of covalent bonding and the decrease of metal Hirshfeld-I (HI) charges. However, the surface energy of (1 0 0) surface increases more quickly than that of (1 1 1) surface and will exceed that of (1 1 1) surface when the carbon stoichiometry decreases to a certain extent, which is primarily attributed to the greater decrease rate of metal HI charges of (1 0 0) surface.     

高熵氧化物的制备及应用研究进展

高熵氧化物作为近几年发展起来的新型氧化物体系，打破了传统掺杂氧化物的设计理念，由五种及以上氧化物以等摩尔或近等摩尔构成，因其具有简单的结构和优异的性能等受到国内外研究人员的广泛关注。高熵氧化物由于多主元且主元之间混乱排列，易形成岩盐型、氟化钙型、尖晶石型或钙钛矿等固溶体结构，从而表现出优异的性能，尤其在能源存储材料和磁性材料方面有十分广阔的应用前景，但目前对高熵氧化物应用研究较少。本工作介绍了国内外高熵氧化物的制备方法，主要包括固相法、热解法、共沉淀法、水热合成法和液相燃烧合成法等，比较了各方法的优缺点和发展前景；归纳了高熵氧化物作为锂离子电极材料、巨介电材料、磁性材料和催化材料等方面的应用；指出了高熵氧化物目前研究存在的问题，讨论了解决措施，展望了高熵氧化物未来的发展趋势。     

Synthesis of high-entropy diboride nanopowders via molten salt-mediated magnesiothermic reduction

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.     

Thermophysical and mechanical properties of novel high-entropy metal nitride-carbides

In this work, a novel (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)(N_0.5C_0.5) 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, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)N high-entropy nitrides (HEN-1), and (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)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.     

Deformation Behaviors of an Additive-Manufactured Ni32Co30Cr10Fe10Al18 Eutectic High Entropy Alloy at Ambient and Elevated Temperatures

Eutectic high-entropy alloys (EHEAs) that have superior formability are attractive for direct laser deposition technology. In this study, a regular-shaped bulk Ni₃₂Co₃₀Cr₁₀Fe₁₀Al₁₈ EHEA without apparent pores and micro-cracks was successfully fabricated by direct laser deposition. The as-deposited alloy showed a high tensile strength of 1.3 GPa with a ductility of 35% at ambient temperature and a tensile strength of 320 MPa at 760 °C. The deformation mechanisms of the as-deposited alloy at ambient and elevated temperatures were investigated by coupling the in-situ tensile test with a scanning electron microscope. It is revealed that the excellent combination of strength and ductility originated from the synergic effects of the FCC and B2 phases in eutectic lamellae. And the generation of cracks along phase boundaries restricted its high-temperature strength above 760 °C.     

钛合金表面抗高温氧化涂层的研究进展

钛合金在制造航空航天发动机等关键部件方面具有重要的应用价值，但高温性能不稳定是制约其发展前景的主要原因。采用激光熔覆技术在钛合金表面制备耐高温涂层，是在不改变钛合金材料整体性能的前提下赋予材料表面特殊性能的重要途径。首先介绍了钛合金的氧化行为，并简要分析了钛合金在氧化过程中的氧化特点及失效形式，指明其改善途径。随后总结分析了目前常用的镍基高温涂层、TiAl系高温涂层和高熵合金高温涂层的研究现状，其中镍基合金涂层具有较高的结合强度、良好的耐磨性和优异的耐蚀性，但由于涂层与基体中元素的扩散速率不同导致的差异，造成柯肯达尔空洞的产生，涂层变得不稳定，涂层与基体的结合强度降低。TiAl基合金的高温性能与镍基高温合金相近，且密度小，有代替镍基合金的发展趋势，其涂层表面可以生成均匀致密的Al2O3氧化膜，并且与钛合金基体间的化学成分差异小，基本不发生互扩散现象。但二元TiAl系涂层对于Al的用量有严格的要求，当其使用温度超过850 ℃时，抗氧化性能也会严重降低。因此Ti-Al-X系涂层中X元素（例如Cr、Si、Ni等元素）的添加，可以适当地降低Al含量，促进均匀致密的Al2O3氧化膜的形成，有效地阻止氧元素向基体的扩散，并且比二元TiAl涂层的脆性要低、塑性更好。高熵合金高温涂层具有许多优异的性能，调整其中某一种或者某几种元素的含量都可以进一步优化性能，因此应用前景极为广阔，但其还处于实验室研究阶段，元素配比的不合理，基体元素对熔覆层的反作用，都会使高熵合金的脆性和力学性能达不到理论效果，不能进入真正的应用阶段。最后展望了激光熔覆技术在钛合金表面制备高温涂层的发展趋势。     

高熵合金AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7扩散阻挡层的制备与性能研究

目的 验证15 nm厚度AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7的势垒层热稳定性和扩散阻挡性能。方法 采用直流磁控溅射技术在n型Si(111)基片上真空溅射沉积15 nm的AlCrTaTiZrRu(3 nm)/(AlCrTaTiZrRu)N0.7 (12 nm)双层阻挡层，随后在双层AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7薄膜的顶部沉积50 nm厚的Cu膜，最终制得Cu/AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7/Si复合薄膜试样。将样品在真空退火炉中分别进行600~900 ℃高温退火30 min，以模拟最恶劣的应用环境。用场发射扫描电镜(FE-SEM)、X射线衍射仪(XRD)、能谱分析仪(EDS)、四探针电阻测试仪(FPP)以及原子力显微镜(AFM)对试样的表面形貌、物相组成、化学成分、方块电阻和粗糙度进行表征分析。结果 沉积态AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7薄膜呈现非晶结构，与Cu膜和Si衬底的结合良好。在800 ℃退火后，Cu/AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7/Si薄膜系统结构完整，膜层结构界面之间未出现分层现象，表面Cu颗粒团聚现象加剧，Si衬底和Cu膜表面未发现Cu-Si化合物生成，薄膜方阻保持在较低的0.070 ?/sq;900 ℃退火后，薄膜系统未出现层间分离和空洞现象，Cu膜表面形成孤立的大颗粒Cu-Si化合物，薄膜电阻率大幅上升。结论AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7双层结构在800 ℃退火后仍能有效抑制Cu与Si相互扩散，其非晶结构增强了Cu/HEA/HEAN0.7/Si体系的热稳定性和扩散阻挡性。     

Nanoporous Surface High-Entropy Alloys as Highly Efficient Multisite Electrocatalysts for Nonacidic Hydrogen Evolution Reaction

Electrocatalytic hydrogen evolution in alkaline and neutral media offers the possibility of adopting platinum-free electrocatalysts for large-scale electrochemical production of pure hydrogen fuel, but most state-of-the-art electrocatalytic materials based on nonprecious transition metals operate at high overpotentials. Here, a monolithic nanoporous multielemental CuAlNiMoFe electrode with electroactive high-entropy CuNiMoFe surface is reported to hold great promise as cost-effective electrocatalyst for hydrogen evolution reaction (HER) in alkaline and neutral media. By virtue of a surface high-entropy alloy composed of dissimilar Cu, Ni, Mo, and Fe metals offering bifunctional electrocatalytic sites with enhanced kinetics for water dissociation and adsorption/desorption of reactive hydrogen intermediates, and hierarchical nanoporous Cu scaffold facilitating electron transfer/mass transport, the nanoporous CuAlNiMoFe electrode exhibits superior nonacidic HER electrocatalysis. It only takes overpotentials as low as ≈240 and ≈183 mV to reach current densities of ≈1840 and ≈100 mA cm⁻² in 1 m  KOH and pH 7 buffer electrolytes, respectively; ≈46- and ≈14-fold higher than those of ternary CuAlNi electrode with bimetallic Cu–Ni surface alloy. The outstanding electrocatalytic properties make nonprecious multielemental alloys attractive candidates as high-performance nonacidic HER electrocatalytic electrodes in water electrolysis.