Ab initio investigation on mechanical properties of copper

Employing the ab initio total energy method based on the density functional theory with the generalized gradient approximation, we have investigated the theoretical mechanical properties of copper (Cu) systematically. The theoretical tensile strengths are calculated to be 25.3 GPa, 5.9 GPa, and 37.6 GPa for the fcc Cu single crystal in the [001], [110], and [111] directions, respectively. Among the three directions, the [110] direction is the weakest one due to the occurrence of structure transition at the lower strain and the weakest interaction of atoms between the (110) planes, while the [111] direction is the strongest direction because of the strongest interaction of atoms between the (111) planes. In terms of the elastic constants of Cu single crystal, we also estimate some mechanical quantities of polycrystalline Cu, including bulk modulus B, shear modulus G, Young's modulus E_p, and Poisson's ratio ν.     

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.     

Study of theoretical tensile strength of Fe by afirst-principles computational tensile test

This paper employs a first-principles total-energy method to investigate the theoretical tensile strengths of bcc and fcc Fe systemically. It indicates that the theoretical tensile strengths are shown to be 12.4, 32.7, 27.5~GPa for bcc Fe, and 48.1, 34.6, 51.2~GPa for fcc Fe in the [001], [110] and [111] directions, respectively. For bcc Fe, the [001] direction is shown to be the weakest direction due to the occurrence of a phase transition from ferromagnetic bcc Fe to high spin ferromagnetic fcc Fe. For fcc Fe, the [110] direction is the weakest direction due to the formation of an instable saddle-point `bct structure' in the tensile process. Furthermore, it demonstrates that a magnetic instability will occur under a tensile strain of 14%, characterized by the transition of ferromagnetic bcc Fe to paramagnetic fcc Fe. The results provide a good reference to understand the intrinsic mechanical properties of Fe as a potential structural material in the nuclear fusion Tokamak.     

Orientation dependence of structural transition in fcc Al driven under uniaxial compression by atomistic simulations

By molecular dynamics simulations employing an embedded atom method potential,we have investigated structural transformations in single crystal Al caused by uniaxial strain loading along the [001],[011] and [111] directions. We find that the structural transition is strongly dependent on the crystal orientations. The entire structure phase transition only occurs when loading along the [001] direction,and the increased amplitude of temperature for [001] loading is evidently lower than that for other orientations. The morphology evolutions of the structural transition for [011] and [111] loadings are analysed in detail. The results indicate that only 20% of atoms transit to the hcp phase for [011] and [111] loadings,and the appearance of the hcp phase is due to the partial dislocation moving forward on {111} fcc family. For [011] loading,the hcp phase grows to form laminar morphology in four planes,which belong to the {111} fcc family; while for [111] loading,the hcp phase grows into a laminar structure in three planes,which belong to the {111} fcc family except for the (111) plane. In addition,the phase transition is evaluated by using the radial distribution functions.     

Strain-induced changes to the electronic structure of germanium

Density functional theory calculations (DFT) are used to investigate the strain-induced changes to the electronic structure of biaxially strained (parallel to the (001), (110) and (111) planes) and uniaxially strained (along the [001], [110] and [111] directions) germanium (Ge). It is calculated that a moderate uniaxial strain parallel to the [111] direction can efficiently transform Ge to a direct bandgap material with a bandgap energy useful for technological applications.     

Theory of surface atom mean square displacements in chromium

The temperature dependence of the mean square displacements for atoms on the (100) and (110) surfaces of chromium has been studied theoretically using the harmonic approximation. Calculations were carried out for crystals with periodic boundary conditions in two directions and free boundary conditions in the third direction using a Born-van Karman model with central interactions up to third nearest neighbors and non-central angular stiffness interaction between nearest and next nearest neighbors. The values of the force constants were chosen to give good fits to the elastic constants and to the bulk phonon dispersion curves in the three major directions [100], [110] and [111]. The ratio of the mean-square displacements at a surface to that in the bulk was calculated as a function of temperature for both the (100) and (110) surfaces. The theoretical, results are compared with the available experimental data from low-energy electron diffraction.     

Temperature effect on the elastic properties of yttrium garnet ferrite Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript>

The Young’s moduli along the [100] and [110] crystallographic directions and the shear modulus along the [100] direction in a high-purity yttrium garnet ferrite single crystal are measured in the temperature range from 20 to 600°C. All the independent elastic constants are calculated for this temperature range. The behavior of the elastic moduli and elastic anisotropy factor is analyzed in the vicinity of the critical temperature of the magnetic phase transition.     

Stress wave propagation in A1 single crystals

Transmitted wave profiles are presented for A1 single crystals that were loaded along crystallographic directions [100], [110] and [111] by a stress wave of the amplitude 1·85 GPa. The experiments were carried out with the Hopkinson Split Bar Technique. The observed wave profiles strongly depend on the specimen length. The constitutive relation of elastic-plastic-relaxing solid was proposed for the interpretation of obtained results. Mechanical properties can then be inferred by comparing observed rates of decay with theoretical models of stress relaxation.     

The elastic properties and energy characteristics of Au nanowires: an atomistic simulation study

This paper have performed molecular static calculations with the quantum corrected Sutten Chen type many body potential to study size effects on the elastic modulus of Au nanowires with [100], [110] and [111] crystallographic directions, and to explore the preferential growth orientation of Au nanowires. The main focus of this work is the size effects on their surface characteristics. Using the common neighbour analysis, this paper deduces that surface region approximately consists of two layer atoms. Further, it extracts the elastic modulus of surface, and calculate surface energy of nanowire. The results show that for all three directions the Young＇s modulus of nanowire increases as the diameter increases. Similar trend has been observed for the Young＇s modulus of surface. However, the atomic average potential energy of nanowire shows an opposite change. Both the potential and surface energy of [110] nanowire are the lowest among all three orlentational nanowires, which helps to explain why Au nanowires possess a [110] preferred orientation during the experimental growth proceeds.     

单轴[110]应力硅电子迁移率

基于k·p微扰法研究单轴[110]应力作用下硅的导带结构,获得单轴[110]应力硅的导带底能量及电子有效质量.在此基础上,考虑电子谷间、谷内及电离杂质散射,采用弛豫时间近似计算单轴[110]应力硅沿不同晶向的电子迁移率.结果表明：单轴[110]应力作用下硅的电子迁移率具有明显的各向异性.在[001]、[110]及[110]输运晶向中,张应力作用下电子沿[110]晶向输运时迁移率有较大的增强,由未受应力时的1 450 cm²·Vs^-1提高到2 GPa应力作用下的2 500 cm²·Vs^-1.迁移率增强的主要原因是电子有效质量的减小,而应力作用下硅导带能谷分裂导致的谷间散射几率的减小对电子迁移率的影响并不显著.     

Molecular dynamics simulation of the plastic behavior anisotropy of shock-compressed monocrystal nickel

Molecular dynamics simulations were used to study the plastic behavior of monocrystalline nickel under shock compression along the [100] and [110] orientations. The shock Hugoniot relation, local stress curve, and process of microstructure development were determined. Results showed the apparent anisotropic behavior of monocrystalline nickel under shock compression. The separation of elastic and plastic waves was also obvious. Plastic deformation was more severely altered along the [110] direction than the [100] direction. The main microstructure phase transformed from face-centered cubic to body-centered cubic and generated a large-scale and low-density stacking fault along the family of { 111 } crystal planes under shock compression along the [100] direction. By contrast, the main mechanism of plastic deformation in the [110] direction was the nucleation of the hexagonal, close-packed phase, which generated a high density of stacking faults along the [110] and[1?10] directions.     

基于HfO2的阻变存储器中Ag导电细丝方向和浓度的第一性原理研究

采用基于密度泛函理论的第一性原理方法, 研究了基于HfO₂的阻变存储器中Ag 导电细丝浓度以及方向性. 通过计算Ag杂质5种方向模型的分波电荷态密度等势面图、形成能、 迁移势垒和分波电荷态密度最高等势面值, 发现[-111]方向最有利于Ag导电细丝的形成, 这对器件的开启电压、形成电压和开关比有很大影响. 本文基于最佳的[-111]导电细丝方向, 设计了4 种Ag 浓度结构. 计算4种Ag浓度结构的分波电荷态密度等势面图, 得出Ag浓度低于4.00 at.% 时晶胞结构中无导电细丝形成且无阻变现象. 当Ag浓度从4.00 at.%增加到4.95 at.% 时, 晶胞结构中发现有导电细丝形成, 表明Ag浓度高于4.00 at.%时, 晶胞中可以发生阻变现象. 然而, 通过进一步对比计算这两种晶胞结构中Ag的形成能、分波电荷态密度最高等势面值、总态密度与Ag的投影态密度发现, Ag浓度越大, 导电细丝却不稳定, 并且不利于提高阻变存储器的开关比. 本文的研究结果可为改善基于HfO₂的阻变存储器的性能提供一定理论指导.     

铁冲击相变的晶向效应

采用基于火炮加载的三样品精细波剖面对比测量,研究了晶向效应对铁弹-塑性转变及体心立方结构(bcc,α相)至六角密排结构(hcp,ε相)相变特性的影响.观测到单晶铁异常的弹-塑性转变行为,这与基于位错密度描述的黏塑性本构模型计算结果相符,对应的Hugoniot弹性极限δ_(HEL)均大于6 GPa,且具有晶向相关性,即δ(111)/(HEL)δ(110)/(HEL)δ(100)/(HEL);系统获取了相变起始压力P_(PT)晶向相关性的实验数据,[100],[110]和[111]晶向的PPT实测值分别为13.89±0.57 GPa,14.53±0.53 GPa,16.05±0.67 GPa,其变化规律与非平衡分子动力学计算结果相符.上述结果揭示出冲击压缩下单晶铁存在塑性与相变微观机理的强耦合,为完善用于冲击实验描述的相场动力学模型提供了重要的实验支撑.     

Electron density distribution in Si and Ge using multipole, maximum entropy method and pair distribution function analysis

The local, average and electronic structure of the semiconducting materials Si and Ge has been studied using multipole, maximum entropy method (MEM) and pair distribution function (PDF) analyses, using X-ray powder data. The covalent nature of bonding and the interaction between the atoms are clearly revealed by the two-dimensional MEM maps plotted on (100) and (110) planes and one-dimensional density along [100], [110] and [111] directions. The mid-bond electron densities between the atoms are 0.554 e/?³ and 0.187 e/?³ for Si and Ge respectively. In this work, the local structural information has also been obtained by analyzing the atomic pair distribution function. An attempt has been made in the present work to utilize the X-ray powder data sets to refine the structure and electron density distribution using the currently available versatile methods, MEM, multipole analysis and determination of pair distribution function for these two systems.      

Experimental determination of third-order elastic constants of diamond

To determine the nonlinear elastic response of diamond, single crystals were shock compressed along the [100], [110], and [111] orientations to 120 GPa peak elastic stresses. Particle velocity histories and elastic wave velocities were measured by using laser interferometry. The measured elastic wave profiles were used, in combination with published acoustic measurements, to determine the complete set of third-order elastic constants. These constants represent the first experimental determination, and several differ significantly from those calculated by using theoretical models.     

Temperature dependence of the magnetoresistance of single crystal aluminium

The magnetoresistance of single crystal aluminium was measured with the magnetic field in the three main crystallographic directions [100], [110] and [111], in the temperature range 4.2–20 K and in magnetic fields up to 7 T. The exponent of a Tⁿ-law is shown to be decreasing with increasing fields.     

Directional anisotropy of surface self-diffusion on Pt(110)

The rate of surface self-diffusion on a Pt(110) single crystal in the [1$\text{1}$0] and [001] directions was measured at 1200–1750 K by monitoring the decay of a sinusoidal surface profile. The surface diffusion rate in the [1$\text{1}$o] direction was much faster than in the [001] direction. The activation energy of surface self-diffusion was 1.70 and 3.16 eV for the [1$\text{1}$0] and [001] directions, respectively, in good agreement with theoretical estimates. For large amplitudes of the profile the decay rate for the [001] direction was also dependent on the amplitude. This behavior can be explained by the appearance of (111) facets on the profile, which cause a retardation of the profile decay.     

Pressure-induced phase transitions in the Cd-Yb periodic approximant to a quasicrystal

The phase study of a Cd-Yb 1/1 approximant crystal over a wide pressure and temperature range is crucial for the comparison study between periodic and quasiperiodic crystals. The Cd(4) tetrahedra, the most inner part of the atomic clusters, exhibited various structural ordering in the orientation sensitive to pressure and temperature. Five ordered phases appeared in a P-T span up to 5.2 GPa and down to 10 K. The propagation direction of ordering alternated from [110] to <111> to at about 1.0 GPa and again to [110] at 3.5-4.3 GPa. The primarily ordered phases that appeared by cooling to 210-250 K between 1.0-5.2 GPa further transformed to finely ordered ones at 120-155 K. Besides the original short-range type interaction, a long-range type interaction was likely developed under pressure to lead to the primary ordering of Cd(4) tetrahedra. Coexistence of these interactions is responsible for the complicated phase behavior.     

面心立方单晶镍纳米线稳定性及磁性的第一性原理计算

本文采用第一性原理方法,研究了轴向为低指数晶向的面心立方(fcc)单晶镍纳米线的稳定性和磁性.计算表明,[110] 是fcc镍纳米线最容易出现的取向,[111] 取向次之,而 [001] 取向则很难出现,这一结果与实验事实符合.镍纳米线按照原子位置和磁性强弱的不同,可以分成简单的芯-壳结构,在纳米线芯部,原子的磁矩大小与块体基本一致.在纳米线表面,镍原子的磁矩比芯部原子有所增加.表面原子磁矩与轴向的取向相关,[110] 为轴向的纳米线表面原子磁矩最低,而[001] 为轴向的纳米线表面原子磁矩最高. 关键词： 镍 纳米线 第一性原理 原子磁矩     

Effects of growth direction on lasing performance in GaAs---AlxGa1−xAs quantum wells

The properties of GaAs---AlGaAs quantum well lasers are studied theoretically as a function of the crystallographic growth direction. The growth directions considered are [001] [111], [110], [310], [311] and [211]. The electronic dispersion is obtained using an 8×8 k·p Hamiltonian which couples the electron, heavy-hole, light-hole and spin-orbit split-off bands. We calculate the threshold current for single quantum well lasers and determine the lowest threshold current for the growth directions considered. It is seen that for some growth directions the threshold current can be less than that previously calculated for a strained-layer quantum well laser. The results also differ from a previous model which completely decoupled the valence and conduction bands.