AFM针尖纳米切削晶向和切削方向影响的分子动力学

"建立了AFM针尖切削单晶铜的三维分子动力学模型,研究了工件材料不同晶向和刀具切削方向对切削过程中工件材料变形的影响．采用EAM势计算工件原子之间的作用,采用Morse势计算刀具原子之间的作用．模拟结果表明工件材料晶向和切削方向对纳米切削过程有显著影响．沿[110]方向切削比[100]方向切削产生的切屑结合更紧密,切削工件材料(110)晶向比切削工件材料(100)晶向产生的切屑体积更小,工件材料变形区域更小．研究了工件材料晶向和切削方向组合的不同纳米切削过程中系统势能变化情况．"     

Molecular dynamics simulation of subsurface deformed layers in AFM-based nanometric cutting process

Three-dimensional molecular dynamics simulations of AFM-based nanometric cutting monocrystalline copper with pin tool radius of 0.713 nm are performed to investigate the effect of uncut chip thicknesses (0.1805 nm, 0.361 nm, 0.5415 nm, 0.722 nm, 0.9025 nm, 1.0875 nm, and 1.268 nm) on the depth of subsurface deformed layers. The EAM potential and Morse potential are utilized respectively to compute the interactions between workpiece atoms, the interactions between workpiece atoms and tool atoms. The single-atom potential energy variations of the workpiece atoms within the subsurface regions during the cutting process are obtained and analyzed through a deformation criterion to determine the deformation behaviors of subsurface atoms. The simulation results reveal that the depth of subsurface deformed layers is affected by the AFM pin tool's rake angle. At each uncut chip thickness, the AFM pin tool presents different negative rake angles, consequently different degrees of deformation in the subsurface take place.     

A new model for deformed carbon nanotubes using Green’s function

A new method for modeling and analysis of deformed carbon nanotubes (CNTs) using Green’s function, is presented in this paper, for the first time. Using the proposed method, a new circuit model is obtained for the deformation region of a deformed single-walled CNT (SWCNT), which the values of its elements depend on the type of deformation and also the deformation parameters such as the coupling matrices and the energy variations of deformation region. The comparison between the obtained results from the analysis of proposed model and the literature gives a good match which approves the accuracy and correctness of the proposed model.     

Quasicontinuum analysis of the effect of tool geometry on nanometric cutting of single crystal copper

A dynamic multiscale simulation based on quasicontinuum method (QC) has been conducted to study the effect of tool geometry in nanometric cutting process of single crystal copper. In the simulation, the many-body EAM potential is used for the interactions between copper atoms in of the workpiece. The simulation captures the atomistic behaviors of material removal mechanisms from the free surface and the mobility of dislocations and their interactions with the computational cost of local atomistic simulation method. Simulations are performed on single crystal copper to study the atomistic details of material removal, chip formation, sub-surface deformation, and machining mechanism. The simulation results demonstrate that tool edge radius has significant effect on chip formation and subsurface deformation, because the effective rake angle varies with the tool edge radius. In addition, different effective rake angles result in different stress states and smoother surface can be obtained under bigger clearance angle. The variations of tangential force, normal force as well as the ratio of normal force to tangential force are obtained to analyze the effects of tool edge radius, rake angle and clearance angle in quantitative way.     

Residual stress field in steel samples during plastic deformation and recovery processes

An X-ray diffraction method was applied to measure residual stresses and stored elastic energy in deformed and annealed polycrystalline ferritic and austenitic steel samples. The orientation distribution of plastic incompatibility second-order stresses created during elastoplastic deformation was determined and presented in Euler space. Using deformation models, these stresses were correlated with different types of intergranular interactions occurring in the studied materials. An important decrease of the first- and the second-order residual stresses was observed during recovery and recrystallisation processes. Diffraction peak widths, related to dislocation density, were studied and correlated with stress variation during annealing process. Differences in stress relaxation between ferritic and austenitic samples were explained by different values of the stacking fault energy, which influences dislocation climb and cross-slip.     

Behavior of hydrogen and defects in zirconium under irradiation

The accumulation of hydrogen and defects in the E-125 zirconium alloy (Zr-2.5% Nb) is investigated. The hydrogen concentration is maximum on the surface of zirconium alloy samples after electrolytic hydrogenation. The hydrogen concentration decreases at a depth of about 0.5 μm and then gradually grows with increasing depth. The surface of the zirconium alloy is strengthened and becomes more fragile after hydrogenation. A plastic deformation of the zirconium alloy gives rise to traps with different binding energies of hydrogen. The primary type of traps, the binding energy, and the amount of hydrogen captured by traps depend on the deformation magnitude and the sequence of deformation and hydrogenation processes. High mobility of hydrogen in plastically deformed samples is observed under bombardment of the surface of the zirconium alloy by a helium ion beam with an energy of 2.34 MeV. The variation of the hydrogen concentration in the near-surface region of zirconium under ion bombardment depends on the extent of deformation: upon bombardment by helium ions, the hydrogen concentration in the near-surface region of the metal increases for deformations from 1 to 3% and decreases for deformations of 4 and 5%.     

Resonant states of shallow acceptors in uniaxially deformed germanium

The states of shallow acceptors in uniaxially deformed germanium are studied theoretically. A non-variational numerical computational method is developed for determining the energy and wave functions of localized states of holes in the acceptor field as well as the states of the continuous spectrum (including resonant impurity states). The dependence of the energy of the lower resonant state on strain is studied. It is found that this state is formed from the excited 4Γ ₈ ⁺ state with a binding energy of 1.3 meV (in the absence of deformation) and not from the ground state. The results presented in this work may be useful in the study of the conditions for the generation of far IR radiation in deformed p-Ge, which involves optical transitions between resonant and localized acceptor states.     

Potential energy surfaces in the two-center shell model

The two-center shell model of two equal overlapping spheroids is combined with Lawrence's liquid-drop shapes. Within this framework, potential energy surfaces for nuclei from different mass regions are calculated. In particular, the transition of the ground state deformation from spherical to deformed is investigated for a sequence of ruthenium isotopes.     

Potential energy surfaces for<Emphasis Type="Italic">N</Emphasis> =<Emphasis Type="Italic">Z</Emphasis>,<Superscript>20</Superscript>Ne-<Superscript>112</Superscript>Ba nuclei

We have calculated the potential energy surfaces forN = Z,²⁰Ne-¹¹²Ba nuclei in an axially deformed relativistic mean field approach. A quadratic constraint scheme is applied to determine the complete energy surface for a wide range of the quadrupole deformation. The NL3, NL-RA1 and TM1 parameter sets are used. The phenomenon of (multiple) shape coextistence is studied and the calculated ground and excited state binding energies, quadrupole deformation parameters and root mean square (rms) charge radii are compared with the available experimental data and other theoretical predictions.     

Constrained relativistic mean-field approach with fixed configurations

A diabatic (configuration-fixed) constrained approach to calculate the potential energy surface (PES) of the nucleus is developed in the relativistic mean-field model. As an example, the potential energy surfaces of ²⁰⁸Pb obtained from both adiabatic and diabatic constrained approaches are investigated and compared. It is shown that the diabatic constrained approach enables one to decompose the segmented PES obtained in usual adiabatic approaches into separate parts uniquely characterized by different configurations, to follow the evolution of single-particle orbits till the very deformed region, and to obtain several well-defined deformed excited states which can hardly be expected from the adiabatic PESs.     

Calculated properties of superheavy nuclei

Ground-state properties of the heaviest nuclei are analyzed within a macroscopic-microscopic approach. The main attention is paid to such properties as deformation, deformation energy, energy of the first rotational state 2⁺ of a nucleus, and the branching ratio of α decay to this 2⁺ state with respect to the decay to the ground state 0⁺. The analysis concerns the problem of experimental confirmation of theoretically predicted deformed shapes of superheavy nuclei situated in the region around the nucleus ²⁷⁰Hs. A large region of even-even nuclei with proton, Z=82–128, and neutron, N=126–190, numbers is considered.     

Nuclear deformations in the pairing-plus-quadrupole model (III). Static nuclear shapes in the rare-earth region

The potential energy of deformation (β, γ), is calculated with the pairing-plus-quadrupole model for nuclei with N=82–126, Z=50–82. There is a sudden onset of deformation in the N=86–90 region, and the static nuclear shape, the lowest minimum of the potential function, changes from spherical to prolate. The disappearance of deformation in the Z=74–80 region is more gradual, and the static shape changes from prolate to asymmetric to oblate to spherical. The energy of zero-point motion is calculated, and it is concluded that all the stable deformed shapes of the region are prolate. Proton and neutron energy gaps, intrinsic quadrupole moments, moments of inertia and gyromagnetic ratios of the doubly even nuclei of the rare-earth region are calculated and compared with the experimental data.     

The breaking of intrinsic reflection symmetry in nuclear ground states

Negative-parity excited states of doubly even nuclei have earlier been attributed to vibrational excitations. This paper shows that an interpretation starting from a reflection asymmetric intrinsic state is more appropriate for certain nuclei in the radium region. Theoretical evidence for stable octupole deformation comes from a deformed shell-model calculation in which we use a single-particle potential with a realistic radial shape and a finite-range interaction for the surface energy. The octupole effect systematically improves the agreement between theoretical and experimental masses. The low-lying O⁺ excitations observed in experiment are compatible with the calculated collective octupole potentials. The possibility of obtaining further evidence from the spectroscopy of odd-mass nuclei is considered in an exactly solvable model, which shows that the smaller energy splitting observed in odd-A parity doublets mainly reflects single-particle fragmentation of the collective mode. The systematics of theoretical shell structure and experimental spectroscopy suggests the presence of other regions of octupole collectivity near the limits of stability.     

Influencing range of vacancy defects in zigzag single-walled carbon nanotubes

The influencing range of a vacancy defect in a zigzag single-walled nanotube is characterized with both structural deformation and variation in bandstructure. This paper proposes a microscopic explanation to relate the structural deformation to the bandstructure variation. With an increasing defect density, the nanotubes become oblate and the energy gap between the deep localized gap state and the conducting band minimum state decreases. Theoretical results shed some light on the local energy gap engineering via vacancy density for future potential applications.     

A molecular dynamics study of phase transformations in mono-crystalline Si under nanoindentation

A molecular dynamics (MD) simulation is adopted to examine the deformation behavior and phase transformation of mono-crystalline Si in nanoindentation with a spherical indenter. The techniques of coordination number and radial distribution function are used to monitor and elucidate the detailed mechanism of the phase transformation throughout the whole process in which the evolution of structural phase change and the relevant distributions of bonding length can be traced and exhibited. In this article, the phases of BC8 and R8, which have the same coordinate number as the phase Si-I and were difficult to distinguish from each other in previous studies, are successfully identified and extracted from the deformed region during unloading. Moreover, the effect of the indenter-radius size on the structural phase transformation of mono-crystalline Si for three different crystallographically oriented surfaces is investigated. It is found that the onset of the plastic deformation tends to take place only as the ratio of the indentation depth to the tip radius is larger than 0.7. Under this condition the structural phase transformation can be easily observed in the residual deformed region after unloading.     

Elasticity and shape equation of a liquid membrane

The density of the elastic energy of a deformed membrane in a liquid state is calculated. The thermodynamic equilibrium of its different parts is taken into account. The shape equation of a closed membrane is deduced. The quantity which keeps its value, when the variations of the energy of the system are calculated, is not the area of the deformed membrane, but its area in the flat tension free state. Because of this, additional terms appear in the second variation around the stable state. The case of a lipid bilayer and its fluctuations is examined for both free and blocked exchange of molecules between the monolayers, comprising the bilayer. Received 4 February 2002 / Received in final form 15 April 2002 Published online 2 October 2002 RID="a" ID="a"e-mail: bivas@issp.bas.bg     

单晶硅微纳构件加工表面性能的时变性研究

采用蒙特卡罗方法和分子动力学方法相结合, 模拟单晶硅微纳构件加工表面的时效过程, 研究其对加工表面质量和构件力学性能的影响. 模拟结果表明: 在时效过程中, 单晶硅微纳构件加工变质层的有序度显著提高, 残余应力大幅降低, 表面粗糙度略有增加, 此外还发现加工变质层中非晶硅原子在时效过程中大幅减少, 部分非晶硅出现了再结晶现象, 其中部分BCT5-Si以及金属相(Si-Ⅱ)结构原子转化为金刚石结构(Si-I). 时效作用对加工后单晶硅微纳构件表面性能具有重要的影响, 同时可以提高微纳构件的拉伸力学性能. 关键词： 蒙特卡罗方法 纳米加工 表面性能 时变性     

On the shell model and its application to the deformation energy of heavy nuclei

The technical realisation of the shell model with arbitrary fields is presented in detail, with special emphasis of the unusual and large deformations of the nuclear shape as they may occur in the fission process. We discuss how realistic parametrisations of the nuclear shape and the potential well can be developed and how the parameters of the average fields can be determined. We restrict ourselves to wells with a Woods-Saxon distribution in the radial coordinate. By means of Strutinsky's shell correction approach, the single particle energies deserve to calculate the potential part of a collective Hamiltonian. Its behaviour with varying deformation is discussed both qualitatively and quantitatively. Considered are the deformation types of elongation (1), necking (2), reflection (3) and axial (4) asymmetry of the nuclear shape. Evidence is given for geometrical symmetries which can be correlated to normal modes in finite nuclei. The transition from spherical to deformed nuclei is presented in detail for the radium isotopes, revealing the importance of hexadecapole deformations. Finally, we give an extensive and systematic presentation of the energies and of the deformations at the various stationary points of the deformation energy for the nuclei in the actinide region and for the hypothetical superheavy nuclei.     

Chain conformations on the surface of polyethylene single crystals

New conformations for the shortest folds of polyethylene single crystals were determined from the principles: (1) the conformations of polymethylene chains are basically determined by the rotational isomeric approximation, (2) the crystal part is deformed where it is connected with the fold part, and (3) the conformation of the fold must be determined as the minimum of surface energy, not of conformation energy. It was shown that the surface energy, assuming the crystal state as the reference state, was composed of four terms: (i) the deformation energy of the crystal part, (ii) the sublimation energy of the fold part, (iii) the conformation energy of the fold part, and (iv) the interaction energy between folds. The conformations of the shortest fold were basically (GTGGTGGG) for (110)-folds and (G'G'TTGG) for (200)-folds. The setting angles of all the chains in the crystal part are rotated from their normal angle (41°) to 41 + a° at the boundary between the two parts. The fold part is deformed so as to fit in with the deformed boundary of the crystal part. The conformations with minimum surface energy were obtained at a = 30° for both (110)-and (200)-folds. Their surface energies were about 16 kcal/mole of fold (300 erg/cm2). The surface energies, assuming the liquid state as the reference state, were about 7 kcal/mole of fold (130 erg/cm²).     

丰中子A~100质量区内的扁椭球高K同核异能态

远离核素图上“稳定谷”的丰中子核一直是核物理学研究的热点。作为形变丰中子核的一种特殊的亚稳定激发态,高K同核异能态的形状大多数为长椭球,扁椭球的高K同核异能态非常少见。近期的一项实验认为丰中子核94Se上的$ {K}^{\mathrm{\pi }}={7}^{-} $两准粒子态为扁椭球。这是形变原子核上存在扁椭球高K同核异能态的第一个实验证据。结合相关实验,我们猜测可能有其它尚未发现的扁椭球高K同核异能态存在于丰中子A~100质量区。利用组态限制势能面计算方法,本文对丰中子A~100质量区内的$ {K}^{\mathrm{\pi }}={9}^{-} $和$ {K}^{\mathrm{\pi }}={7}^{-} $两准粒子态进行了研究,并预言了此质量区内扁椭球高K同核异能态的可能位置。根据Nilsson模型,扁椭球高K同核异能态的存在与费米能级附近的高Ω单粒子轨道有关。这些高Ω单粒子轨道来源于高j闯入态在扁椭球形变时的退简并。扁椭球高K同核异能态是研究丰中子核形变参数、激发能等物理性质的理想对象,有助于加深我们对于形变原子核能级结构的理解。