锯齿型单壁碳纳米管的广义层错能计算

基于密度泛函理论的第一性原理方法,计算了两种不同半径的锯齿型单壁碳纳米管以及单层石墨的广义层错能曲线。对小位移处广义层错能曲线进行斜率拟合所得切变模量与其它文献值一致。对三条广义层错能曲线的对比可知,锯齿型碳纳米管与单层石墨的广义层错能曲线相近,锯齿型单壁碳纳米管的曲率效应不明显。     

Atomistic simulations to study crack tip behaviour in single crystal of bcc niobium and hcp zirconium

In this article, molecular dynamics based simulations were carried out to study the fracture toughness of single crystals of niobium (Nb) and zirconium (Zr). Separate set of simulations were performed with different orientations of crack plane in Nb and Zr. In each case, efforts were made to align the crack front with principal planes of corresponding crystal structure, that was bcc for Nb and hcp for Zr. Deformation in single crystal of Nb and Zr was governed either by twinning or emergence of dislocations from the crack tip and surfaces. The deformation mechanism in single crystal of Nb and Zr also helps in governing the overall toughness of the material, while deforming via twinning leads to higher change in the crack length, whereas dislocations emerging from the crack tip blunts the opening and improves the toughness.     

First-principles study of the effects of selected interstitial atoms on the generalized stacking fault energies,strength, and ductility of Ni

We analyze the influences of interstitial atoms on the generalized stacking fault energy （GSFE）, strength, and ductility of Ni by first-principles calculations. Surface energies and GSFE curves are calculated for the （112） （111） and / 101） （ 1 1 1） systems. Because of the anisotropy of the single crystal, the addition of interstitials tends to promote the strength of Ni by slipping along the （10T） direction while facilitating plastic deformation by slipping along the （115） direction. There is a different impact on the mechanical behavior of Ni when the interstitials are located in the slip plane. The evaluation of the Rice criterion reveals that the addition of the interstitials H and O increases the brittleness in Ni and promotes the probability of cleavage fracture, while the addition of S and N tends to increase the ductility. Besides, P, H, and S have a negligible effect on the deformation tendency in Ni, while the tendency of partial dislocation is more prominent with the addition of N and O. The addition of interstitial atoms tends to increase the high-energy barrier γmax, thereby the second partial resulting from the dislocation tends to reside and move on to the next layer.     

Calculation of absolute concentrations and probability of resonant absorption for iron-bearing precipitates in zirconium alloys

In order to find the absolute concentrations and the probability of resonant absorption, the theoretical dependence of effective thickness from Mössbauer absorption line area has been obtained. Calculations of absolute concentrations of secondary phase precipitate in zirconium alloys with natural iron and with iron enriched with Mössbauer isotopes were carried out.     

第一性原理研究Nb、Sn、Cu、Fe和Cr对Zr (0001)晶面抗疖状腐蚀性能的影响

采用密度泛函理论研究Nb、Sn、Cu、Fe和Cr 5种合金元素对氧在Zr(0001)晶面吸附能的影响, 发现Nb、Sn和Cu会促进氧在Zr(0001)晶面吸附, Fe和Cr对氧吸附的影响因吸附的位置不同而存在差异。研究5种合金元素对氧化后Zr(0001)晶面化学键的破坏情况, 发现Nb对Zr(0001)晶面破坏程度最小而且可以迅速复原, Sn可以使(0001)晶面相邻的两个化学键都变长, 对晶面的破坏性大。最后讨论5种合金元素在Zr(0001)晶面的偏聚能, 发现Sn、Fe和Cr偏聚能为负值, 容易偏聚到Zr(0001)晶面, 而Nb和Cu偏聚能为正值, 不易在Zr(0001)晶面偏聚。综合以上分析, Nb可以促进氧在Zr(0001)晶面的吸附行为, 氧化后(0001)晶面可以快速复原, 从而阻碍其他氧原子进入, 抑制疖状腐蚀的发生。Sn容易偏聚到Zr(0001)晶面, 可以促进氧在Zr(0001)晶面吸附, 氧化后会造成Zr(0001)晶面较大的破坏, 促进氧进入Zr(0001)晶面, 促进疖状腐蚀的发生。     

Effects of stacking fault energy on defect formation process in face-centered cubic metals

To elucidate the effect of stacking fault energies (SFEs) on defect formation by the collision cascade process for face-centred cubic metals, we used six sets of interatomic potentials with different SFEs while keeping the other properties almost identical. Molecular dynamic simulations of the collision cascade were carried out using these potentials with primary knock-on atom energies (E_PKA) of 10 and 20 keV at 100 K. Neither the number of residual defects nor the size distributions for both self-interstitial atom (SIA) type and vacancy type clusters were affected by the difference in the SFE. In the case of E_PKA = 20 keV, the ratio of glissile SIA clusters increased as the SFE decreased, which was not expected by a prediction based on the classical dislocation theory. The trend did not change after annealing at 1100 K for 100 ps. For vacancy clusters, few stacking fault tetrahedrons (SFTs) formed before the annealing. However, lower SFEs tended to increase the SFT fraction after the annealing, where large vacancy clusters formed at considerable densities. The findings of this study can be used to characterise the defect formation process in low SFE metals such as austenitic stainless steels.     

Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals

ABSTRACT

Ultra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300?K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior.     

氚在锆合金包壳材料中的扩散行为模拟

基于氚扩散基本模型建立了氚扩散行为一维模拟程序,对模拟程序进行了典型实验验证,模拟结果与实验结果符合较好。分析了不同氚浓度、温度分布对锆合金包壳材料中氚的扩散行为的影响。分析结果表明：包壳-芯块间隙内氚浓度的升高会导致进出包壳的氚扩散通量提高,渗透通量增大;由于包壳氧化层相对较低的扩散系数,包壳氧化层的存在对氚渗透有较大的限制作用;温度对氚扩散速率的影响很显著,温度越高,扩散速度越快;锆合金外表面氧化层的相对低温限制了氚渗透出包壳管的速率,温度梯度导致的热致扩散有利于氚向包壳冷测扩散。     

Zr掺杂形式对钨电子发射材料组织性能的影响

采用中频感应加热烧结方法制备了W-1.5%La₂O₃-0.1%Y₂O₃-0.1%ZrO₂和W-1.5%La₂O₃-0.1%Y₂O₃-0.08%ZrH₂电子发射材料,烧结样品的致密度约为95.5%。热电子发射测试结果表明,添加氢化锆的热电子发射材料样品的零场发射电流密度大于添加氧化锆的样品,分析认为是添加的氢化锆在烧结时,发生分解,生成活性的Zr可以捕获钨晶界中的杂质氧,净化晶界,从而提高了电子发射;维氏显微硬度表明添加氢化锆样品的硬度高于添加氧化锆的样品,分析表明是氢化锆的添加有效改善了钨晶粒之间的结合性,提升了钨电子发射材料的硬度。利用SEM,EDS,XRD、金相显微镜等表面分析设备对样品进行了表征,样品结构显示添加氢化锆与添加氧化锆相比,不仅钨晶粒尺寸由13.63 μm降至11.63 μm,而且稀土相尺寸由1.87 μm降至1.66 μm,这种组织结构的变化有利于电子发射。     

Atomic simulations to evaluate effects of stacking fault energy on interactions between edge dislocation and spherical void in face-centred cubic metals

In this study, molecular dynamics simulations were performed to elucidate the effects of stacking fault energy (SFE) on the physical interactions between an edge dislocation and a spherical void in the crystal structure of face-centred cubic metals at various temperatures and for different void sizes. Four different types of interaction morphologies were observed, in which (1) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the trailing partial; (2) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the leading partial; (3) the partial dislocations detached from the void almost simultaneously without jog formation; and (4) the partial dislocations detached from the void almost simultaneously with jog formation. With an increase in void size or SFE, the interaction morphology changed in the above-mentioned order. It was observed that the magnitude of the critical resolved shear stress (CRSS) and its dependence on the SFE were determined by these interaction morphologies. The value of the CRSS in the case of interaction morphology (1) is almost equal to an analytical one based on the linear elasticity by employing the Burgers vector of a single partial dislocation. The maximum value of the CRSS is also obtained by the analytical model with the Burgers vector of the two partial dislocations.     

Effect of gamma energy, temperature and shelf life on the response of gamma and BD-100R neutron bubble detectors

The use of commercially available bubble detectors for gamma and neutron detection has been reported by several groups. We have carried out systematic investigations to study the response of gamma bubble detector as a function of energy, the effect of waiting time on sensitivity of BD-100R neutron bubble detector and the effect of shelf life on both types of bubble detectors. Our results show that the response of the gamma bubble detector is energy dependent. Waiting time of up to 260 hours for BD-100R neutron detector has no significant effect. Both the detectors have a limited shelf life. These factors must be taken into account while using these detectors.     

Effect of Cr,Mo, and Nb additions on intergranular cohesion of ferritic stainless steel: First-principles determination

Effects of Cr, Mo, and Nb on the ferritic stainless steel ]2（210） grain boundary and intragranularity are investigated using the first-principles principle. Different positions of solute atoms are considered. Structural stability is lowered by Cr doping and enhanced by Mo and Nb doping. A ranking on the effect of solute atoms enhancing the cohesive strength of the grain boundary, from the strongest to the weakest is Cr, Mo, and Nb. Cr clearly prefers to locate in the intragranular region of Fe rather than in the grain boundary, while Mo and Nb tend to segregate to the grain boundary. Solute Mo and Nb atoms possess a strong driving force for segregation to the grain boundary from the intragranular region, which increases the grain boundary embrittlement. For Mo- and Nb-doped systems, a remarkable quantity of electrons accumulate in the region close to Mo （Nb）. Therefore, the bond strength may increase. With Cr, Mo, and Nb additions, an anti-parallel island is formed around the center of the grain boundary.     

Al掺杂浓度对CrSi2电子结构及光学性质的影响

采用基于第一性原理的赝势平面波方法,对不同Al掺杂浓度CrSi2的几何结构、能带结构、态密度和光学性质进行了计算和比较。几何结构和电子结构的计算表明:Al掺杂使得CrSi2的晶格常数a和b增大,c变化不大,晶格体积增大;Cr(Si1-xAlx)2仍然是间接带隙半导体,掺杂使得费米面向价带移动,且随着掺杂量的增大而更深地嵌入价带中,费米能级附近的电子态密度主要由Cr的3d态电子贡献。光学性质计算表明,随着掺杂量的增大,Cr(Si1-xAlx)2的静态介电常数、第一介电峰、折射率n0逐渐增大,平均反射效应减弱,表明Al掺杂有效增强了CrSi2对光的吸收,能够提高其光电转换效率。计算结果为CrSi2光电材料的应用和设计提供了理论指导。     

过渡金属元素X(X=Mn,Fe,Co,Ni)掺杂对ZnO基阻变存储器性能的影响

实验表明掺杂是一种改善阻变存储器性能的有效手段,但其物理机理鲜有研究.本文采用第一性原理方法系统研究了过渡金属元素X(X=Mn,Fe,Co,Ni)掺杂对ZnO基阻变存储器中氧空位迁移势垒和形成能的影响.计算结果表明Ni掺杂可同时有效降低+1和+2价氧空位在掺杂原子附近的迁移势垒,X掺杂均减小了氧空位的形成能,特别是掺杂Ni时氧空位的形成能减小最为显著(比未掺杂时减少了64%).基于该结果制备了未掺杂和Ni掺杂ZnO阻变存储器,研究表明通过掺杂控制体系中氧空位的迁移势垒和形成能,可以有效改善器件的初始化过程、操作电压、保持性等阻变性能.研究结果有助于理解探究影响阻变的微观机制,并可为掺杂提高阻变存储器性能提供一定的理论指导.     

非故意掺杂碳对ZnMnO:N磁性影响的实验与理论研究

利用金属有机源化学气相沉积技术, 通过改变受主掺杂源和导入氢气并提高生长压力来逐步抑制C的办法, 在蓝宝石上外延了Mn, N共掺ZnO薄膜. X射线衍射显示所有样品都具有良好的单轴取向. ZnMnO:N样品的Raman光谱中C元素相关的振动模明显消失. 同时van der Pauw法Hall效应测量表明, 通过逐步对C的抑制, 样品由n型导电转变成p型导电, 这主要是由于C与N形成复合体取代O位(CN)O, 具有最低形成能且充当浅施主. 对N, Mn共掺ZnO晶体的第一性原理模拟计算显示了N, Mn共掺ZnO的态密度在Fermi能级处存在较强的自旋极化, 表明N 2p电子与Mn 3d电子之间存在较强的p-d相互作用, 形成磁性束缚激子产生磁矩. 一旦引入C后, C, N形成复合体取代O位, 导致体系磁性减弱或者消失. 模拟计算结果与实验表征分析结果一致表明: 对于Mn, N共掺ZnO薄膜样品, 引入C与N形成复合体取代O位, Mn, N共掺ZnO薄膜磁性减弱或消失. 因此, Mn 3d电子与N 2p局域束缚的电子形成的磁性束缚激子决定了Mn, N共掺ZnO薄膜室温铁磁信号的产生.     

Nb在Ni3Al中取代行为及合金化效应的第一性原理研究

采用基于第一性原理的平面波赝势方法,研究了Nb原子在Ni₃Al中的格点取代行为及合金化效应.通过对不同原子被置换后体系的形成热、结合能及电子态密度的计算和比较,发现Nb原子倾向于取代Ni₃Al中的Al原子,其取代行为主要由系统的电子结构决定,计算结果与实验相符.为了进一步研究Nb原子的取代行为,对Nb原子占据的格点以松散或紧凑分布下体系的总能、形成热、结合能以及电子态密度进行了计算,结果表明Nb原子占据的格点更倾向于紧凑分布.为了研究Nb对Ni_{3关键词： 第一性原理 3}Al合金')" href="#">Ni₃Al合金 电子结构 合金化效应     

Effects of soft beam energy on the microstructure of Pb37Sn, Au20Sn, and Sn3.5Ag0.5Cu solder joints in lensed-SM-fiber to laser-diode-affixing application

This paper reports on the effectiveness of soft beam energy as a heat source to form an optimum solder joint to fix a lensed fiber permanently on a Ni/Au-plated substrate. Solders, i.e., Pb37Sn, Au20Sn, and Sn3.5Ag0.5Cu (SAC) [wt%] were evaluated for this fluxless application. The microstructures of the solder joints have been examined using scanning electron microscopy (SEM), in order to understand the response of these solder materials to the focussed white light. Obviously, the exposure time has a greater effect on the soldering temperature before reaching the peak temperature, which is determined by the power. A power setting of 40 W can reach approximately 340 °C, 30 W can reach about 310 °C while 25 W can easily reach 260 °C. In general, a higher soldering temperature than the melting temperature is required to form good wetting solder joints for fluxless applications. However, too high an input thermal energy may result in premature aging for the cases of Pb37Sn and SAC, and lateral cracks for the case of Au20Sn. The thermal cracks and voids observed in Au20Sn solder joint were attributed to the fact that the soft beam heating profile does not suit the AuSn preform. Out of these three solder types, SAC demonstrated just the right response to the soft beam, i.e., good wetting, fine and homogeneous structure, and no cracks or other visible failures.     

Effect of adhesion force between crack planes on subharmonic and DC responses in nonlinear ultrasound

Subharmonic and DC responses in nonlinear ultrasound have been expected as a possible means of detecting closed cracks. Recently, it has been reported that subharmonics in a closed crack markedly increases above a certain input wave amplitude. Such a phenomenon is called "threshold behavior". However, the mechanism of threshold behavior has yet to be elucidated. To clarify this, we introduced adhesion force as a short-range force into the previous analytical model, which expresses the nonlinear contact vibrations of crack planes with intense ultrasound and provides a DC displacement as an approximation of the subharmonic response. Consequently, upward convex curves of displacement against input wave amplitude above the threshold were reproduced for the first time. The validity of the derived analytical solution is discussed by comparison with experimentally observed subharmonics.