Lattice stability and the effect of Co and Re on the ideal strength of Ni:First-principles study of uniaxial tensile deformation

Using first-principles density functional calculations, lattice stability of γ-Ni under [001], [110], and [111] uniaxial tensions and the effect of alloying elements Co and Re on the uniaxial tensile behavior of γ-Ni were studied in this paper.With elastic constants and phonon spectra calculations, we examined the mechanical stability and phonon stability of Ni during the uniaxial tensions along the three characteristic directions. The results show that the mechanical stability and phonon stability of a lattice occurs before the maximum stress–strain point under the [001] and [111] tension, respectively.The effects of Co and Re on the ideal tensile strength of γ-Ni show a significant directivity： Co and Re have little effect on the stresses in [001] and [111] directions, but increases the ideal strength of the system in the weakest uniaxial tensile direction. Moreover, the strengthening effect of Re is significantly better than that of Co. By further analyzing electronic structure, it is found that the effect of alloying elements on the uniaxial tensile behavior of γ-Ni comes from their interactions with host atoms.

多层平板探测的X射线多能成像方法

提出一种使用多层平板探测器获取X射线多能图像的成像方法。分别给出单层、多层平板探测器结构,并介绍多层平板探测成像系统工作原理。阐明X射线成像原理,并通过仿真探究单能射线切换下的双能成像、双能减影原理。进一步分析不同kVp,滤过下X射线源的频谱变化,在kVp切换成像试验中多次曝光采集胸部体模低、中、高三能图像,发现胸模多能图像中肋骨、肺部区域存在差异,且对多能图像双能减影可实现骨骼增强、抑制。类似地仿真探究单次曝光下双能射线在双层平板中双能成像、减影原理。给出射线源在不同滤过下的能谱变化,在多层平板成像试验单次曝光中分别使第一层、第三层平板采集胸部体模低、高能图像。发现多层平板采集的低、高能图像中肋骨、肺部区域亦存在差异,对其双能减影亦可实现骨骼增强、抑制。试验表明使用多层平板探测可实现X射线多能成像。

Novel structures and mechanical properties of Zr_(2)N:Ab initio description under high pressures

With the formation of structural vacancies,zirconium nitrides(key materials for cutting coatings,super wearresistance,and thermal barrier coatings) display a variety of compositions and phases featuring both cation and nitrogen enrichment.This study presents a systematic exploration of the stable crystal structures of zirconium heminitride combining the evolutionary algorithm method and ab initio density functional theory calculations at pressures of 0 GPa,30 GPa,60 GPa,90 GPa,120 GPa,150 GPa,and 200 GPa.In addition to the previously proposed phases P42/mnm-,Pnnn-,and Cmcm-Zr2 N,five new high-pressure Zr_(2)N phases of PA/nmm,IA/mcm,P2_(1)/m,P3 m1,and C2/m are discovered.An enthalpy study of these candidate configurations reveals various structural phase transformations of Zr2 N under pressure.By calculating the elastic constants and phonon dispersion,the mechanical and dynamical stabilities of all predicted structures are examined at ambient and high pressures.To understand the structure-property relationships,the mechanical properties of all Zr_(2)N compounds are investigated,including the elastic moduli,Vickers hardness,and directional dependence of Young’s modulus.The Cmncm-Zr2 N phase is found to belong to the brittle materials and has the highest Vickers hardness(12.9 GPa) among all candidate phases,while the I4/mcm-Zr2 N phase is the most ductile and has the lowest Vickers hardness(2.1 GPa).Furthermore,the electronic mechanism underlying the diverse mechanical behaviors of Zr2 N structures is discussed by analyzing the partial density of states.

Boron at tera-Pascal pressures

The study of boron structure is fascinating because boron has various allotropes containing boron icosahedrons under pressure. Here, we propose a new boron structure(space group Fm3m) that is dynamically stable at 1.4 tera-Pascal(TPa)using density functional theory and an evolutionary algorithm. The unit cell of this structure can be viewed as a structure with a boron atom embedded in the icosahedron. This structure behaves as a metal, and cannot be stable under ambient pressure. Furthermore, we found electrons gather in lattice interstices, which is similar to that of the semiconductor Na or Ca_(2)N-Ⅱ under high pressure. The discovery of this new structure expands our comprehension of high-pressure condensed matter and contributes to the further development of high-pressure science.

双层平板探测的双能成像检测系统

本文提出一种使用双层平板探测器单次曝光获取X射线双能图像,并实现双能减影的成像检测方法。介绍了单层平板、双层平板探测器结构以及双层平板探测成像系统工作原理。仿真分析不同、相同管电压在不同滤过下X射线的能谱变化,进一步通过试验分别探究使用kVp切换,双层平板方案时胸部体模双能成像特性。研究发现kVp切换、双层平板方案采集的双能图像皆可表征人体软组织、骨骼对低能、高能射线的衰减差异,双能平板方案较kVp切换还具有无运动伪影,剂量较低的优势。提出了一种适用于双层平板的双能图像配准方案,配准后的低、高能图像经双能减影后得到骨骼增强、骨骼抑制图像。试验结果表明双层平板获取的双能减影图像可提升医生诊断时的视觉效果,减影图像对比度优于kVp切换方案。

Dependence of mechanical properties on the site occupancy of ternary alloying elements in γ'-Ni3Al: Ab initio description for shear and tensile deformation

The site occupancy behavior of ternary alloying elements inγ'-Ni3Al(a key strengthening phase of commercial Ni-based single-crystal superalloys)can change with temperature and alloy composition owing to the effect of entropy.Using a total-energy method based on density functional theory,the dependence of tensile and shear behaviors on the site preference of alloying elements inγ'-Ni3Al were investigated in detail.Our results demonstrate that Fe,Ru,and Ir can significantly improve the ideal tensile and shear strength of theγ'phase when occupying the Al site,with Ru resulting in the strongest enhancement.In contrast,elements with fully filled d orbitals(i.e.,Cu,Zn,Ag,and Cd)are expected to reduce the ideal tensile and shear strength.The calculated stress-strain relationships of Ni3Al alloys indicate that none of the alloying elements can simultaneously increase the ideal strength of theγ'phase for both Ni1-site and Ni2-site substitutions.In addition,the charge redistribution and the bond length of the alloying elements and host atoms during the tensile and shear processes are analyzed to unveil the underlying electronic mechanisms.

应变和电场调控HfSe_(2)/PtSe_(2)异质结的电子结构

本团队通过基于密度泛函理论的第一性原理方法,系统地研究了HfSe2/PtSe2范德瓦尔斯异质结(van der Waals heterostructures,vdWHs)的电子性质,包括堆垛方式、层间耦合、应变和外电场的影响.发现堆垛方式可以调节能带对齐类型--AA,AB′和AC′堆垛时为Ⅱ型,AB,AC,AA′则为Ⅰ型.在六种堆垛方式中,AA堆垛是最稳定的,其层间距为2.87A,带隙为1.0 eV,Ⅱ型的能带对齐方式有利于电子-空穴载流子的分离.进一步的计算表明,HfSe_(2)/PtSe_(2)异质结的电子性质可以通过垂直应变和双轴面内应变有效调节:在施加应变或改变层间距后,可以在HfSe_(2)/PtSe_(2)异质结中观察到从Ⅱ型到Ⅰ型能带对齐类型的转变;不仅如此,压缩应变和层间耦合还可以有效调控异质结的带隙大小.本研究将为未来HfSe_(2)/PtSe_(2)异质结在纳米电子及光电设备中的应用提供理论基础.

超柔二维氮化碳和可调分子筛的预测

基于进化算法和第一性原理计算,我们对二维C3N4体系进行了全面的结构搜索,发现了3种比graphitic-C3N4更稳定的二维结构:P6/m-C3N4,P2/m-C3N4和P62m-C3N4.更重要的是,通过形变测试,发现3种结构均表现出超柔特性.它们的柔软性远好于graphitic-C3N4,能量领域具有非常宽的极小值.当晶格常数变化±5%时,P6/mC3N4,P2/m-C3N4,P62m-C3N4体系的能量(应力)变化仅分别是±23.52,±42.21和±3.41 meV/atom(±0.33,±0.53和±0.07 GPa),远远小于graphitic-C3N4的平均值125.24 meV/atom(3.27 GPa)(应力可比graphitic-C3N4小46倍).这种极其微小的变化主要归因于连接C3N3环的N–C–N键具有高度的灵活性.此外,连接起来的C3N3环分别形成了六边形(128?2)、四边形(112?2)和三边形(108?2)的孔状结构,且孔径可调,预测这3种新结构在可调分子筛有广泛的应用前景.最后,我们分析了新结构在形变过程中电子结构的变化规律.