Interaction of point defects with an edge dislocation in the gradient theory of elasticity

Interaction between a point defect and an edge dislocation is studied in the framework of the gradient theory of elasticity. The change in the energy of the system caused by a displacement of the point defect relative to the dislocation line is calculated. The results of the theoretical analysis are used to describe edge dislocation pinning by impurity atoms.     

Effect of the gaseous medium in the process of rolling on the microhardness of aluminum and iron

The load dependence of the microhardness of polycrystalline aluminum and iron specimens produced by rolling in a nitrogen, helium, or air medium has been investigated. It has been found that nitrogen and helium have different effects on the microhardness of these metals in the low-load range. This difference is associated with the specific features in the intensity of dynamic penetration of nitrogen and helium into the surface layer of aluminum and iron, which depends on the initial defect crystal structure of the metals, as well as on the type of bonding of helium atoms and nitrogen molecules with metal atoms. It has been shown that the effect of the gaseous medium of the rolling on the microhardness manifests itself only in a very thin surface layer of metal specimens, where the microhardness exhibits a size effect, and an increase in the microhardness indentation depth remains unchanged with an increase in the load and does not depend on the gaseous medium of the prerolling of the specimens.     

Mechanism for defect dependence of damage morphology in HfO2/SiO2 high reflectivity coating under nanosecond ultraviolet laser irradiation

High reflectivity coating is designed and fabricated with HfO₂/SiO₂ stacks. The laser-induced damage experiment is prepared by a third-order harmonic generation of Nd:YAG laser (355 nm, 8 ns). Typical damage micrographs are obtained using atomic force microscope (AFM) and scanning electron microscope (SEM). A theoretical model based on the thermal transfer is tried to describe the defect dependence of damage morphology. Three situations are considered in this model: single defect, couple defects and high absorption film. The calculated results show that the second situation agrees well with the experiment.     

Determination of ratios of photoelectron mean free paths from orbital ionization cross-sections

Ratios of inelastic-scattering mean free paths of photoelectrons in carbonaceous overlayers and in pure condensates of fluorides have been determined through the use of two procedures, respectively: (1) the variation of two intensities of orbital electrons from the substrate as the carbon overlayer is removed successively by argon-ion bombardment; and (2) the ratio of two intensities and the corresponding theoretical cross-sections. The results show that the dependence of the mean free path on kinetic energy agrees with that predicted by the asymptotic form aE/lnαE for energies in the range 400—120OeV; the value of α obtained, however, is near unity rather than the theoretical value of 0.1. The derived variation of the mean free path agrees with published measurements for NaF. The relation among intensity, cross-section and mean free path applies only when the spectra considered are symmetrical.     

Ion mixing in multilayer films and the doping of the near-surface layers of polycrystalline substrates under irradiation by ion beams with a wide energy spectrum

Various factors influencing ion mixing efficiency are considered. It is established that the energy transfer from ions and primary knocked-on atom films to subsequent displacement cascades underlies the penetration of atoms from multilayer films into a polycrystalline substrate. The penetration of implanted atoms to great depths is in this case determined by the defect density, the radiation induced migration of the implanted atoms, and their interaction with the atoms of the matrix. All of these factors can be described in terms of the isotropic mixing model. It is shown that when doped by atoms from multilayer films, gradient layers form that are determined by the range of radiation in the substrate of the atoms to be implanted and their migration under irradiation by an ion beam with a wide range of energies.     

Lattice relaxation and the energy of mixing in dilute alkali-alkali alloys

The energy of solution for a homovalent impurity in a simple metal is calculated, with account taken of the static displacement field around the soluted atom. The asymptotic behaviour of the distorting force and the atomic displacements are expressed in terms of the pseudopotentials for host and impurity atoms. Numerical results for the heat of mixing in dilute alkali-alkali systems show a delicate balance between the volume misfit term on the one hand, and the electronegativity and relaxation terms on the other hand. The relaxation energy proves to be of the same order as the heat of solution itself. The values and the trends in the heat of mixing compare reasonably well with the empirical data, except for lithium-based alloys, and the predicted variation of the bulk modulus with concentration agrees with the experiment.     

Defect ensembles on the surface of loaded metals as a result of their reversible aggregation

The structures of nanodefect ensembles formed on the surfaces of copper, gold, and molybdenum under a load have been investigated. The properties of the defect ensembles are described in the framework of the reversible aggregation model. The size distribution of nanodefects is thermodynamically determined by the maximum “entropy of their mixing” with atoms of the crystal lattice. The entropy of mixing reaches a maximum value at a small concentration of defects due to a considerable difference in the sizes of defects and atoms. This concentration agrees closely with the experimental data. The reduced size distribution of defects is universal.     

质子在碳化硅中不同深度的非电离能量损失

利用蒙特卡罗方法,应用Geant4程序,模拟计算了1—500 MeV质子在碳化硅材料中的非电离能量损失,并研究了不同种类的初级反冲原子对非电离能量损失的贡献.模拟结果表明:在相同质子辐照下,碳化硅材料中的非电离能量损失要比硅、镓等半导体材料更小,说明碳化硅器件的稳定性更好,抗位移损伤能力更强;当靶材料足够厚时,在不同能量质子辐照下,材料损伤最严重的区域会随着质子入射能量的增加从质子射程末端逐渐前移到材料表面;不同种类的初级反冲原子对非电离能量损失的贡献表明,在低能质子辐照下,28Si和~(12)C是位移损伤的主要原因,而随着质子能量的增加,通过核反应等过程产生的次级离子迅速增多,并对材料浅层造成严重的位移损伤.     

Driving mechanism of neutron-irradiation-induced amorphization in silicon carbide

In this study, irradiation-induced amorphization in silicon carbide (SiC) by 1 MeV neutrons was investigated using molecular dynamics (MD) simulations. The crystalline-to-amorphous (c-a) transition occurred at 0.27 dpa with a structure relaxation of the whole lattice. Fast neutrons have produced many displacement spikes with unsaturated coordinated atoms at the center. Our results have shown that the two-coordinated Si atoms play a key role in defect accumulation and amorphization. There are two types of such defects: displaced-atom-induced (D-type) defect and vacancy-induced (V-type) defect. The D-type defect tends to form clusters and promotes the formation of C Frenkel pairs after 0.13 dpa. The V-type defect enhances the driving force of c-a transition and finally triggers amorphization at high concentration based on thermodynamics.     

氮原子修饰点缺陷石墨烯结构和性质的理论研究

氮原子掺杂石墨烯对基于石墨烯的器件和催化研究具有重要的应用价值.本文采用基于密度泛函理论的计算方法,研究了氮原子修饰的C-Bridge(碳原子吸附在石墨烯碳碳键桥位)、C-Top(碳原子吸附在石墨烯一个碳原子上方)和C7557(碳原子对吸附在石墨烯碳环上方)三种不同点缺陷类型的石墨烯物理性质.讨论不同缺陷石墨烯结构在用氮原子进行修饰前后体系的稳定性、电子结构等;计算得到了缺陷处原子的分波态密度(PDOS)图,分析了原子间的相互作用;模拟出氮原子修饰后缺陷石墨烯恒流模式的STM图像,以便和实验上得出的图像进行对比.计算结果表明,对于所选取的三种不同缺陷,氮原子能够较稳定地吸附在缺陷表面.C-Bridge和C-Top缺陷结构本身具有磁矩,经氮原子修饰后结构磁矩消失.与之相反,C7557缺陷结构本身没有磁矩,经氮原子修饰后缺陷体系带有磁矩.另外,C-Bridge和CTop两种不同缺陷结构石墨烯经过氮原子修饰后,体系几何结构变得完全一样.     

Displacement damage and transmutations in metals under neutron and proton irradiation

The knowledge of the defect and impurity generation rates, as well as the defect spatial distribution, is the corner stone for the understanding of the evolution of material properties under irradiation. This knowledge is also an essential element for comprehensive experimental simulations of the behavior of irradiated materials.In this article the interaction of neutron and proton irradiation with metals is discussed with respect to displacement damage production. Charged particle irradiation is also briefly illustrated. After discussion of the primary interaction of projectiles (neutrons, charged particles in general, and protons in particular) with target atoms/nuclei, we describe the interaction of a recoil atom with other target atoms resulting in the slowing down of the projectile, displacement damage, impurity atom production due to nuclear reactions, and the creation of atomic displacement cascades. Then the further evolution of defect structure is discussed. The next section, devoted to subcascade formation, is divided into two parts. The first experimental evidence of subcascade formation under neutron and charged particle irradiation is presented. Then the models of subcascade formation are described. Finally we review the models for the calculation of displacement damage and show how these models can be applied to displacement damage calculation under neutron irradiation with a demonstration of a real application of the methods discussed to several nuclear facilities. To cite this article: P. Vladimirov, S. Bouffard, C. R. Physique 9 (2008).     

Control of defect concentrations within a semiconductor through adsorption

The technologically useful properties of a crystalline solid depend upon the concentration of defects it contains. Here we show that defect concentrations as deep as 0.5 microm within a semiconductor can be profoundly influenced by gas adsorption. Self-diffusion rates within silicon show that nitrogen atoms adsorbed at less than 1% of a monolayer lead to defect concentrations that vary controllably over several orders of magnitude. The results show that previous measurements of diffusion and defect thermodynamics in semiconductors may have suffered from neglect of adsorption effects.     

Numerical simulation of the modulation to incident laser by the repaired damage site in a fused silica subsurface

One of the main factors of laser induced damage is the modulation to incident laser which is caused by the defect in the subsurface of the fused silica.In this work,the repaired damage site irradiated by CO 2 laser is simplified to a Gaussian rotation according to the corresponding experimental results.Then,the three-dimensional finite-difference time-domain method is employed to simulate the electric field intensity distribution in the vicinity of this kind of defect in fused silica front subsurface.The simulated results show that the modulation is notable,the E max is about 2.6 times the irradiated electric field intensity in the fused silica with the damage site (the width is 1.5 μm and depth is 2.3 μm) though the damage site is repaired by CO 2 laser.The phenomenon and the theoretical result of the annular laser enhancement existed on the rear surface are first verified effectively,which agrees well with the corresponding experimental results.The relations between the maximal electric field intensity in fused silica with defect depth and width are given respectively.Meanwhile,the corresponding physical mechanism is analysed theoretically in detail.     

飞秒激光惰性气体Ar原子多次电离过程的空间分辨研究

使用100飞秒、1014 W/cm2的强激光与惰性气体Ar原子相互作用,我们利用飞行时间质谱仪结合Z扫描技术获得了各次电离的Ar原子在激光焦点附近不同位置处强度分布.测量得到的各个不同电离态Ar n+(n=1～4)的产额随激光焦点位置变化关系与不同电离态的空间分布理论预言相一致.     

Synthesis and characterization of a single diamond crystal with a high nitrogen concentration

In this paper,we explore diamond synthesis with a series of experiments using an Fe-Ni catalyst and a P3N5 additive in the temperature range of 1250-1550 ℃ and the pressure range of 5.0-6.3 GPa.We also investigate the influence of nitrogen on diamond crystallization.Our results show that the synthesis conditions(temperature and pressure) increase with the amount of P3N5 additive increasing.The nitrogen impurity can significantly influence the diamond morphology.The diamonds stably grow into strip and lamellar shapes in the nitrogen-rich environment.The Fourier-transform infrared spectrum shows that the nitrogen concentration increases rapidly with the content of P3N5 additive increasing.By spectrum analysis,we find that with the increase of the nitrogen concentration,the Ib-type nitrogen atoms can aggregate in the A-centre form.The highest A-centre nitrogen concentration is approximately 840 ppm.     

Enrichment of the nitrogen atomic component with the <Superscript>15</Superscript>N isotope in a post-discharge zone

Isotope atomic composition in the post-discharge zone of a pulsed discharge in a nitrogen flow was studied by electron paramagnetic resonance. It is shown that, while the atomic concentration in the post-discharge zone decreases, the relative content of the ¹⁵N isotope increases more than 30 times compared to its natural abundance. Such a high isotope enrichment exceeds more than fourfold the corresponding maximal value that is predicted for the nitrogen atoms by the existing theoretical model. Analysis of the experimental results shows that the isotope-selective dissociation of nitrogen molecules proceeds in the post-discharge zone after the most part of atoms created in the discharge zone had recombined at the tube surface. A mechanism that explains that a nitrogen isotope enrichment as high as that is quite possible is proposed.     

First-principles study of plasmons in doped graphene nanostructures

The operating frequencies of surface plasmons in pristine graphene lie in the terahertz and infrared spectral range,which limits their utilization. Here, the high-frequency plasmons in doped graphene nanostructures are studied by the timedependent density functional theory. The doping atoms include boron, nitrogen, aluminum, silicon, phosphorus, and sulfur atoms. The influences of the position and concentration of nitrogen dopants on the collective stimulation are investigated,and the effects of different types of doping atoms on the plasmonic stimulation are discussed. For different positions of nitrogen dopants, it is found that a higher degree of symmetry destruction is correlated with weaker optical absorption. In contrast, a higher concentration of nitrogen dopants is not correlated with a stronger absorption. Regarding different doping atoms, atoms similar to carbon atom in size, such as boron atom and nitrogen atom, result in less spectral attenuation. In systems with other doping atoms, the absorption is significantly weakened compared with the absorption of the pristine graphene nanostructure. Plasmon energy resonance dots of doped graphene lie in the visible and ultraviolet spectral range.The doped graphene nanostructure presents a promising material for nanoscaled plasmonic devices with effective absorption in the visible and ultraviolet range.     

Interaction of plasma facing materials for fusion devices with low energy hydrogen and helium particles

Abstract

Radiation damage formed in metal specimens exposed to long pulse tokamak plasmas of TRIAM-1M was examined by transmission electron microscopy. By comparing these results with those of low energy hydrogen ion irradiation it was concluded that the charge exchange energetic neutrals of hydrogen emitted from the core plasma caused remarkable displacement damage. The flux of the neutrals in the energy range of 0.5–3 keV which were responsible for displacement damage, was estimated to be about 1.5–3 × 10¹⁸ H/m²/s. These energetic neutrals cause not only material degradation at the sub-surface region but also change bulk properties of plasma facing components in a plasma confinement device. Effect of helium plasma was also discussed with emphasis on very strong effects on damage accumulation. Damage by He is a serious issue of plasma facing materials.     

Hollow cathode discharge (HCD) dark space diagnostics with laser photoionization and galvanic detection

We report on a new technique for measuring the cathode dark space width and the variation of ground state atomic density within it by measuring the ionization current generated by laser photoionization of neutral ground state metallic atoms. The technique is supported by a theoretical model of charge displacement in the dark space based on the assumption of a Lorentzian gas and thus on Langevin equations. After verifying the applicability of the theoretical approach, measurements of dark space width with respect to pressure, current and nature of the buffer gas are presented for an uranium HCD. Results of variation in density of ground state neutral uranium, in the dark space, versus current and pressure in Xe are also given. These last results are of interest when using photoionization currents in the HCD dark space for laser spectroscopy, such as photoionization studies of elements like uranium.     

分子动力学模拟BCC铁中级联碰撞产生的缺陷结构及其演变(英文)

用分子动力学模拟BCC铁中级联碰撞原子的演变过程.结果显示,级联碰撞的原子不仅可以临时地产生自间隙原子、空位、以及<100>,<110>和<111>哑铃型缺陷,也可以产生<110>和<111>,以及<110>和<110>哑铃型缺陷的复合缺陷.模拟显示在模拟的能量范围内,如果初级碰撞原子的能量越大,该原子引起的级联碰撞覆盖区域会越大.发现具有几千电子伏特初级碰撞能量的原子引起的级联碰撞覆盖区域大约为11~15个晶格常数大小.同时还发现在级联碰撞的初始阶段(0.0~0.75 ps)不仅会有自间隙原子与空位的生成,同时也会有少量的自间隙原子与空位缺陷复合而消失.