Ultrafine-grained high-entropy zirconates with superior mechanical and thermal properties

Ultrafine-grained(Sm_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Zr_(2)O_(7)high-entropy zirconates with single fluorite structure have been fabricated by high-pressure sintering of the self-synthesized nanopowders for the first time.The as-sintered samples exhibit a good microstructure with a grain size of 220 nm and a relative density of 96.8%,which yield excellent comprehensive mechanical properties with a high Vickers hardness of 12.5 GPa and a high fracture toughness of 3.4 MPa·m1/2.In addition,the as-sintered samples possess a good thermostability with the grain growth rate of 30 nm/h,and a low thermal conductivity of 1.57 W·m^(-1)·℃^(-1)at room temperature.The superior mechanical and thermal properties are primarily attributed to the“high-entropy”and grain-refinement effects and good interface bonding.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential,yet relevant studies have been rarely reported.In this work,we report nanocrystalline(La_(0.25)Ce_(0.25)Nd_(0.25)Eu_(0.25))B6 high-entropy rare-earth hexaboride(HEReB6-1)ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time.The as-fabricated samples exhibited a highly dense(96.3%)nanocrystalline(94 nm)microstructure with major(001)fiber textures and good grain boundaries without any impurities,resulting in a remarkable mechanical,electrical,and thermionic emission performance.The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures;these were greater than the average values of corresponding binary rare-earth hexaborides,such as a Vickers hardness of 23.4±0.6 GPa and a fracture toughness of 3.0±0.4 MPa•m^(1/2)at room temperature.More importantly,they showed high emission current densities at elevated temperatures,which were higher than the average values of the corresponding binary rare-earth hexaborides.For instance,the maximum emission current density reached 48.3 A•cm^(−2)at 1873 K.Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

Hard nanocrystalline gold materials prepared via high-pressure phase transformation

As one of the important materials,nanocrystalline Au(n-Au)has gained numerous interests in recent decades owing to its unique properties and promising applications.However,most of the current n-Au thin films are supported on substrates,limiting the study on their mechanical properties and applications.Therefore,it is urgently desired to develop a new strategy to prepare nAu materials with superior mechanical strength and hardness.Here,a hard n-Au material with an average grain size of~40 nm is prepared by cold-forging of the unique Au nanoribbons(NRBs)with unconventional 4H phase under high pressure.Systematic characterizations reveal the phase transformation from 4H to face-centered cubic(fcc)phase during the cold compression.Impressively,the compressive yield strength and Vickers hardness(HV)of the prepared n-Au material reach~140.2 MPa and~1.0 GPa,which are 4.2 and 2.2 times of the microcrystalline Au foil,respectively.This work demonstrates that the combination of high-pressure cold-forging and the in-situ 4H-to-fcc phase transformation can effectively inhibit the grain growth in the obtained n-Au materials,leading to the formation of novel hard n-Au materials.Our strategy opens up a new avenue for the preparation of nanocrystalline metals with superior mechanical property.

Extraordinary high-temperature mechanical properties in binder-free nanopolycrystalline WC ceramic

From the perspective of high-temperature applications,materials with excellent high-temperature mechanical properties are always desirable.The present work demonstrates that the binder-free nanopolycrystalline WC ceramic with an average grain size of 103 nm obtained by high-pressure and hightemperature sintering exhibits excellent mechanical properties at both room temperature and high temperature up to 1000℃.Specifically,the binder-free nanopolycrystalline WC ceramic still maintains a considerably high Vicker hardness H_(V)of 23.4 GPa at 1000℃,which is only 22%lower than the room temperature H_(V).This outstanding thermo-mechanical stability is superior to that of typical technical ceramics,e.g.SiC,Si_(3)N_(4),Al_(2)O_(3),etc.Nanocrystalline grains with many dislocations,numerous low-energy,highly stableΣ2 grain boundaries,and a relatively low thermal expansion coefficient,are responsible for the observed outstanding high-temperature mechanical properties.

Heat-treated glassy carbon under pressure exhibiting superior hardness,strength and elasticity

Glassy carbon(GC)is a type of non-graphitizing disordered carbon material at ambient pressure and high temperatures,which has been widely used due to its excellent mechanical properties.Here we report the changes in the microstructure and mechanical properties of GC treated at high pressures(up to 5 GPa)and high temperatures.The formation of intermediate sp2-sp3 phases is identified at moderate treatment temperatures before the complete graphitization of GC,by analyzing synchrotron X-ray diffraction,Raman spectra,and transmission electron microscopy images.The intermediate metastable carbon materials exhibit superior mechanical properties with hardness reaching up to 10 GPa and compressive strength reaching as high as 2.5 GPa,nearly doubling those of raw GC,and improving elasticity and thermal stability.The synthesis pressure used in this study can be achieved in the industry on a commercial scale,enabling the scalable synthesis of this type of strong,hard,and elastic carbon materials.

基于二十面体的新型B_(12)CN和B_(13)CN结构的第一性原理研究

以二十面体为基元的富硼化合物具有复杂多样的电子和机械性能.在本工作中,我们采用粒子群优化结构预测方法结合第一性原理计算,首次对二十面体基元的三元B_(12)CN和B_(13)CN化合物的晶体结构和性质进行了全面系统的研究.我们搜索得到了B_(13)CN和B_(12)CN化合物的新结构,其空间群均为Cmc21,与α-B的变体结构相比,新结构具有更优异的热力学稳定性.B_(12)CN的热力学稳定性和机械性能均稍逊于B_(13)CN.此外,B含量的微小差异造成了B_(12)CN与B_(13)CN两种三元化合物迥异的电学特性,即B_(13)CN具有半导体特性,而B_(12)CN具有空穴型导电特性.此外,在B_(12)CN与B_(13)CN系列新结构的拉伸过程中,由于二十面体的连续破坏和二十面体之间的连续断键造成的应力再增强、结构多级破坏和类蠕变变形等特殊变形机制也同样被揭示.

3C-SiC各向异性行为的原位纳米划痕研究

立方碳化硅(3C-SiC)是一种在极端条件下具有优异机械和物理性能的理想材料.然而,由于其生长深度有限且脆性较高,研究其材料去除和摩擦性能具有一定的挑战性.在本研究中,我们使用扫描电子显微镜对厚度约为20μm的3C-SiC单晶进行了原位纳米划痕研究,并观察了其各向异性行为.随后在(100)平面上沿[110]和[100]方向分别进行了纳米划痕实验.与[100]方向相比,[110]方向在划痕过程中表现出更高的硬度,导致材料去除率较低和摩擦系数较高.通过对划痕沟槽的原子级分辨观察,我们发现3C-SiC的塑性去除是通过位错滑移和显著的晶格畸变实现的.在塑性变形阶段,两个划痕方向的亚表面主要经历了全位错滑移.此外,强烈的应变导致了多晶化,这是3C-SiC中的一个重要变形机制.