Novel Boron Nitride Polymorphs with Graphite-Diamond Hybrid Structure

Both boron nitride(BN)and carbon(C)have sp,sp^(2)and sp^(3)hybridization modes,thus resulting in a variety of BN and C polymorphs with similar structures,such as hexagonal BN(hBN)and graphite,cubic BN(cBN)and diamond.Here,five types of BN polymorph structures are proposed theoretically,inspired by the graphite-diamond hybrid structures discovered in a recent experiment.These BN polymorphs with graphite-diamond hybrid structures possess excellent mechanical properties with combined high hardness and high ductility,and also exhibit various electronic properties such as semi-conductivity,semi-metallicity,and even one-and two-dimensional conductivity,differing from known insulators hBN and cBN.The simulated diffraction patterns of these BN hybrid structures could account for the unsolved diffraction patterns of intermediate products composed of so-called“compressed hBN”and diamond-like BN,caused by phase transitions in previous experiments.Thus,this work provides a theoretical basis for the presence of these types of hybrid materials during phase transitions between graphite-like and diamond-like BN polymorphs.

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.

The rise of plastic deformation in boron nitride ceramics

Ceramics are bonded by ionic or covalent bonds,with very limited slip systems for dislocation nucleation and movement[1].The poor deformability and natural brittleness are the major drawbacks of ceramics,especially when compared with metals.Under stress,ceramics tend to fracture before noticeable plastic deformation takes place.Cracks occur and propagate rapidly in ceramics subjected to stress much lower than the theoretical strength[2].As a result,ceramics can only endure very small strains(<1%),absorb limited mechanical energy,and display poor toughness[3].Moreover,microstructure imperfections in ceramics may decrease the toughness even further.Due to the lack of significant plastic deformation capacity for ceramic materials,the catastrophic failures without warning are easy to happen under stress which critically increases the unreliability of ceramics in the applications as structural materials.

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.

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.