铝中氦原子行为的密度泛函研究

用密度泛函理论计算了大量He原子存在时He在金属铝中不同位置的能量,并在理论上预测了铝中的氦原子行为.结果表明,铝晶胞内He原子择优占位区是空位,而在整个晶体范围,最有利于容纳He原子的区域是晶界,其次是空位和位错.在fcc-铝的两种间隙位中,He原子优先充填四面体间隙位.间隙He原子的迁移能很小,易于通过迁移在晶内聚集,或被空位、晶界、位错等缺陷束缚.     

铝中氦原子行为的密度泛函研究

用密度泛函理论计算了大量He原子存在时He在金属铝中不同位置的能量,并在理论上预测了铝中的氦原子行为。结果表明,铝晶胞内He原子择优占位区是空位,而在整个晶体范围,最有利于容纳He原子的区域是晶界,其次是空位和位错。在fcc-铝的两种间隙位中,He原子优先充填四面体间隙位。间隙He原子的迁移能很小,易于通过迁移在晶内聚集,或被空位、晶界、位错等缺陷束缚。     

Motion of a helium atom in a quartz crystal with dislocations

The motion of a helium atom through the quartz crystal structure disturbed by dislocations has been considered. Two cases of their action on the motion of the helium atom have been discussed. (1) Dislocations, in particular, screw dislocations, can be represented as contracted or extended helical (six-, four-, three-membered) channels consisting of SiO₄ tetrahedra. In this case, the helium atom moves inside the dislocation as in the crystallographic channel with changed parameters. (2) Dislocations can cross the crystallographic channel. This leads to an excess or a deficit of oxygen atoms of SiO₄ tetrahedra in the immediate environment of the helium atom located in the channel. In both cases, the displacement of the helium atom appears as a Frenkel-Kontorova soliton. However, in the latter case, the dependence of the activation energy of this soliton on the number of defects is discontinuous and exhibits a “mobility gap” of the helium atom.     

The electrical properties of dislocations in semiconductors

According to the theory of Read, closely spaced acceptor sites occur at dangling bonds along dislocations in semiconductors and lead to the formation of space-charge tubes which scatter electrons. In order to test Read's theory, parallel arrays of dislocations have been introduced by plastic bending and extensive electrical and galvanomagnetic measurements have been made on n-type germanium crystals. The experimental results are not in agreement with Read's theory; however, it is shown that Read's basic model is applicable but it must be modified by the assumption of non-specular electron scattering at the edges of the space-charge tubes—the electrical and galvanomagnetic consequences of the theory plus modification are worked out and good agreement with experiments obtained. The following quantities are derived: conductivity (σ), Hall constant (R), and magnetoresistance (MR) for current both parallel (‖) and perpendicular (⊥) to the long axis of the tubes with magnetic field (H) at right angles to the current. The quantities R, σ_‖ and σ_⊥ were measured versus temperature from 78°K to 300°K and MR was measured at 78°K as a function of the angle between H and the normal to the neutral plane (N). It was found that the variation of the electrical properties in the sample, particularly along the N direction, is so great that to obtain valid data one must use small samples cut from bent crystals; data are presented for whole samples, as well as small samples, in order to illustrate this effect. The dislocation acceptor level in n-type germanium is found to lie at ～0.50 ev below the conduction band edge. By using the above theory one can determine values of the dislocation density (N) from the result of electrical measurements on semiconductors; this was done in order to determine, on deformed n-type germanium, variations in N from region to region of bent samples as a function of temperature and time of deformation and annealing. In addition, other experiments were performed on plastically bent indium antimonide; it was found that the electrical effects of dislocations, although considerably different from those in germanium, can also be explained by the theory presented here.     

Theory of electrical resistance of multivalley semiconductors with screw dislocations

The diffusion theory of electrical resistance in multivalley semiconductors of Si-type with screw dislocations is developed. Intervalley electron transitions are taken into account. The effect of the inhomogeneous dislocation distribution on the electrical resistance of the crystal is considered.It is shown that randomly distributed dislocations having the mean density 10¹⁰ cm–2 contribute some percents to the electrical resistance at room temperature. Intervalley transitions make this contribution much lower (by one order or more). The inhomogeneity of dislocation distribution enhances the electrical resistance several orders as compared with randomly distributed dislocations.     

Some notes on the role of meta-stable excited state of helium atom in laser-induced helium gas breakdown spectroscopy

A study of the experimental results on the plasma emissions of water and ethanol vapor samples, induced by Nd:YAG laser in ambient helium and nitrogen gases at atmospheric pressure, is presented here. The result reveals distinct geometrical and spectral characteristics of the plasma emissions generated in the helium gas when compared to those observed from nitrogen gas plasma. Most remarkable is the narrow line width and low continuum background exhibited by emission lines of the analyte atoms from helium plasma, including the hydrogen emission line which is known to suffer from notorious broadening effects in conventional laser induced breakdown spectroscopy (LIBS). It is further shown on the basis of the measured spatial distributions and time profiles of the emission intensities, that the excellent spectral quality is attained by taking advantage of the meta-stable excited state of helium atoms for the delayed excitation of the hydrogen and other analyte atoms, this allows the detection of those atomic emissions to be performed under more favorable conditions. The result of this study has thus demonstrated the feasibility of achieving high-quality spectrochemical analysis, including hydrogen analysis with laser-induced helium gas breakdown spectroscopy. PACS 52.38.Mf     

单晶硅片中的位错在快速热处理过程中的滑移

研究了单晶硅片中维氏压痕诱生的位错在不同气氛下高温快速热处理中的滑移行为.研究表明: 在快速热处理时, 位错在压痕残余应力的弛豫过程中能发生快速滑移; 当快速热处理温度高于1100℃时, 在氮气氛下处理的硅片比在氩气氛下处理的硅片有更小的位错滑移距离. 我们认为这是由于氮气氛下的高温快速热处理在压痕处注入的氮原子钉扎了位错, 增加了位错的临界滑移应力, 从而在相当程度上抑制了位错的滑移. 可以推断氮气氛下的高温快速热处理注入的氮原子增强了硅片的机械强度. 关键词： 快速热处理 位错滑移 机械性能 单晶硅     

Microhardness of armco iron samples with different initial structures after rolling in a helium or nitrogen medium

We have analyzed the dependences of microhardness H on load P for the surface layers of Armco iron samples with different initial structures, i.e., annealed and after equal-channel angular pressing (ECAP). The microhardness was measured in air after rolling of the sample in helium or nitrogen medium. It has been found that these mediums produce different effects on the H(P) dependences for preliminarily annealed samples compared to those subjected to ECAP. This is due to the differences in their initial defects structures and in the types of binding forces of helium atoms and nitrogen molecules with dislocations, which determine the intensity of their penetration into the surface layer of the samples under investigation. We have obtained curves that describe the release of helium from samples rolled in different mediums and have measured the amount of helium.

     

High strength and high electrical conductivity bulk Cu

The achievement of both high strength and high electrical conductivity in bulk materials is challenging in the development of multifunctional materials, because the majority of the strengthening methods reduce the electrical conductivity of the materials significantly. At room temperature, dislocations have little scattering effect on conduction electrons. Thus, a high density of dislocations can strengthen conductors without significantly increasing the resistivity. However, at room temperature (RT), which is defined as 295?±?2?K in this paper, deformation can only introduce a limited number of dislocations in pure metals due to dislocation annihilation, i.e. recovery. This limitation is expanded by a well-controlled liquid nitrogen temperature (LNT), which is defined as 77?±?0.5?K in this paper, deformation process that permits accumulation of both nanotwins and a high density of dislocations accompanied by significantly less recovery than that in RT-deformed samples. The dislocations are organized into refined dislocation cells, with thicker cell boundaries in LNT-deformed samples than those deformed at RT. LNT deformation stores more energy in the material than RT deformation. LNT deformation produces bulk pure Cu with a yield strength about 1.5 times that of RT-deformed Cu. The RT resistivity increase is less than 5% compared with that of annealed Cu.     

Kinetics of the Excited States of Helium in an Atmospheric-Pressure Glow Discharge in a Helium-Nitrogen Mixture

Using an atomic-absorption spectral analysis technique, we determined the concentrations of helium atoms in states 2¹S, 2¹P, 2³S, and 2³P in an atmospheric-pressure glow discharge in helium (99.98%) and in a mixture of helium with nitrogen (99.5%He+0.5%N₂). It is shown that the population of the lower excited levels of helium atoms (n = 2) in its mixture with nitrogen is almost an order of magnitude smaller than in the case of a discharge in helium. The maximum of the concentration of excited atoms in a discharge both in helium and in its mixture with nitrogen is in the cathode region at a distance of about 0.1 mm from the cathode. The reaction of quenching of excited helium atoms by nitrogen molecules is responsible for the sharp decrease in the concentration of He(n = 2) on addition of nitrogen into helium. Presented at the 5th Symposium on Physics and Diagnostics of Laboratory and Astrophysical Plasmas, Belarus, Serbia and Montenegro, Minsk, Belarus, September 20–23, 2004; 12th International Congress on Plasma Physics, October 25–29, 2004, Nice, France. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 530–537, July–August, 2005.     

Effect of the saturation conditions and structure on the retention of helium in structural materials

A comparison between the kinetics of helium desorption upon linear heating of samples saturated using various regimes is performed, and the effect of dislocations on the retention of helium in materials is estimated. In order to investigate the effect of the conditions of saturation of materials with helium on its retention, samples of austenitic stainless steel 0Kh16N15M3B saturated using various methods were studied, namely, helium irradiation in a cyclotron, in a magnetic mass-separation setup, inside IRT-2000 and BOR-60 reactors, and using the so-called “tritium trick” technique. The investigations show that when saturation of the samples with helium is accompanied by the introduction of radiation defects (in wide limits of helium concentrations and radiation damage), the kinetics of helium evolution from samples of this type is adequate to the kinetics of its evolution from samples irradiated in a reactor. The investigation of the kinetics of helium evolution from the samples of 0Kh16N15M3B steel both after a preliminary deformation and in the process of deformation showed that, in the process of heating, the helium atoms can migrate along dislocation pipes, resulting in a significant effect on the release of helium and its redistribution in the volume of the material. The activation energy for helium pipe diffusion in austenitic steel 0Kh16N15M3B is about 0.7 eV. Mobile dislocations favor the ejection of helium onto the surface of the material, to grain boundaries, interphase interfaces, etc.     

氦泡在bcc钨中晶界处成核长大的分子动力学模拟

钨(W)是潜在的聚变堆面向等离子体材料.聚变反应中产生的氦(He)不溶于金属W,并在其中易聚集形成He泡,使W基体发生脆化,从而导致W基体的性能发生退化.在前人工作的基础上,本文采用分子动力学研究了He泡在单晶bcc-W中以及bcc-W中∑3[211](110)和∑9[110](411)晶界处He泡形核长大初期的演化过程.结果发现,晶界处He泡的长大机制和单晶W中有所不同.单晶W中He泡通过挤出位错环促进长大.而He泡在∑3[211](110)晶界处的长大机制为:首先挤出并发射少量自间隙W原子,而后挤出1/2⟨111⟩位错线,随后,该位错线会沿晶界面上[111]方向迁移出去;在∑9[110](411)晶界处,He泡在我们的模拟时间尺度范围内没有观察到W自间隙子的发射和位错的挤出.     

Ordered growth of germanium dots induced by the strain field of tilt dislocations in molecular bonded silicon (0 0 1) thin films

Germanium dots have been grown on high twist angle (twist angle as high as 20°) molecular bonded silicon (0 0 1) substrates. We show that, depending on the thickness of the silicon film, the strain field generated by an ordered array of mixed edge interfacial tilt (miss-cut) dislocations may induce an ordered growth of germanium dots. We also show that in order to observe an influence of the mixed edge interfacial dislocations on the growth of germanium dots, the thickness of the film has to be much lower that the period of the mixed edge dislocations array. Germanium dots grown by molecular beam epitaxy on 10-15 nm thick silicon films with the period of tilt dislocation array of 43 nm show a high degree of self-ordering.     

不同加载条件下位错和溶质原子交互作用的数值模拟

动态应变时效,即位错和溶质原子的动态交互作用,对合金材料的力学性质产生重要影响. 本文基于蒙特卡罗方法,建立了“多位错-溶质原子” 二维动力学模型,分别模拟了单位错-恒定应力率、多位错-无应力、多位错-恒定应力和多位错-恒定应力率四种条件下位错和溶质原子的演化过程. 单位错-恒定应力率情况下,低应力率时位错被溶质原子钉扎而无法脱钉,高应力率时位错未被钉扎而一直运动,只有在适当应力率范围内,位错才呈现出反复的钉扎和脱钉;多位错-无应力时,溶质原子向正/负位错的下/上方偏聚;多位错-恒定应力时,位错运动受溶质原子钉扎的影响随应力增大而减小;多位错-恒定应力率时,集群化的钉扎和脱钉过程导致了位错总位移呈现阶梯状的演化. 模拟结果表明：“位错-溶质原子”尺度上呈现了动态应变时效微观过程,与其理论描述相一致. 关键词： 动态应变时效 动力学模型 位错运动     

Dislocation injection, reconstruction, and atomic transport on {001} Au terraces

High-resolution electron microscopy investigations of Au films show that adatoms on (100) surfaces insert into the underlying terrace to form surface dislocations. This injection readily occurs when the number of adatoms on a terrace is approximately 20 atoms or less. The surface dislocation glides along the terrace, but is repelled from the edges. The dislocation escapes by squeezing out in the dislocation line direction (not gliding out the terrace edge). Atomistic simulations confirm the dislocation stability, easy glide along the terrace and trapping at the terrace edge. These results have profound implications for film growth.     

Specific features in the generation and motion of dislocations in silicon single crystals doped with nitrogen

The specific features in the generation and motion of dislocations are investigated in Si: N single crystals grown by the Czochralski method. The motion of dislocation loops is analyzed by the four-point bending technique in the temperature range 500–800°C. The dislocation loops are preliminarily introduced into the samples with the use of a Knoopp indenter at room temperature. It is found that doping with nitrogen leads to a considerable increase in the critical stress of the onset of dislocation motion from surface sources (indentations) and in the stress of the generation of dislocations from internal sources. The velocity of dislocation motion in Si: N crystals is less than that in undoped crystals (under comparable loads). The hardening effect of nitrogen is explained by the fact that nitrogen promotes the decomposition of a solid solution of oxygen in silicon during postcrystallization cooling.     

冲击加载下铝中氦泡和孔洞的塑性变形特征研究

通过分子动力学模拟研究了在相同冲击加载强度下单晶铝中氦泡和孔洞的塑性变形特征,结果发现氦泡和孔洞的塌缩是由发射剪切型位错环引起的,而没有观测到棱锥型位错环发射. 氦泡和孔洞周围的位错优先成核位置基本一致,但是氦泡周围发射的位错环数目比孔洞多,位错环发射速度明显比孔洞快. 且氦泡和孔洞被冲击波先扫过部分比后扫过部分发射位错困难. 通过滑移面上的分解应力分析发现,氦泡和孔洞周围塑性特征的差别是由于氦泡内压引起最大分解应力分布改变造成的. 氦泡和孔洞被冲击波先后扫过部分塑性不对称是因为冲击波扫过时引起形状变化, 关键词： 分子动力学 冲击波 氦泡 孔洞     

Electrical conductivity of germanium with dislocation grids

Samples of n-type germanium with a donor concentration N _d=2.4×10¹⁶ cm⁻³ are plastically deformed to a degree of strain equal to 18–40% to detect static conduction by electrons trapped on dislocations in a system of dislocation grids. In samples with 20%<δ<31%, which retain an electronic type of conductivity, the conductivity for T<8 K, which is weakly temperature-dependent, is associated with conduction by electrons trapped on dislocations. The nonmonotonic dependence of the conductivity at 4.2 K on the degree of strain as the latter increases from 18% to 40% attests to the existence of an energy gap between the donor and acceptor dislocation states in strongly plastically deformed germanium. Zh. éksp. Teor. Fiz. 115, 115–125 (January 1999)     

The effect of hydrogen disorder on dislocation movement and plastic deformation of ice

In the structure of ice at low pressures (ice I-h) the oxygen atoms are crystallographically arranged but the hydrogen atoms are believed to be randomly arranged consistent with the so-called Bernal-Fowler rules. The effect of this randomness is to make it impossible for a dislocation to move through the ice lattice without creating point defects (breaches of the Bernal-Fowler rules). A calculation of the energy of the defects that would have to be created gives a value so large that it would require a stress of about one tenth of the shear modulus of ice to push the dislocation through. It therefore seems likely that dislocations could not move through ice unless the hydrogen atoms are reoriented by thermally activated point defects ahead of the dislocation. This necessity will greatly slow down dislocations in ice and provides an explanation for the observed behaviour of ice single crystals in creep and constant strain-rate tests, and of the softening of ice at low temperatures produced by small concentrations of dissolved fluoride ions.     

Influence of Dislocation Pinning on the Creep Rate in Aluminum

The influence of the dislocation mobility on the creep rate in aluminum has been estimated. In a steady state of creep, the dislocation mobility is varied by pinning dislocations using impurity atoms during heating. It has been shown that the change in the creep rate is proportional to the fraction of impurity atoms that migrate from the solid solution of deformed aluminum toward the dislocations.