不同孪晶界密度银纳米线拉伸形变行为的分子动力学模拟

采用分子动力学方法模拟了不同孪晶界密度银纳米线的拉伸形变行为,分析了孪晶界密度对多晶银纳米线屈服强度、弹性模量和塑性变形机理的影响.在弹性形变区域,孪晶界的存在对杨氏模量变化的作用不明显.在塑性形变阶段,首先从表面边缘开始产生位错成核,然后延伸并受阻于孪晶界.在进一步拉伸载荷作用下,孪晶界将作为位错源产生新的位错.模拟结果表明,银纳米线的强度与孪晶界和晶粒的尺寸有关.孪晶界密度较小(即晶粒的长径比大于1)时,此纳米线的屈服应力比单晶纳米线还要小,只有当孪晶界密度较大时(即晶粒的长径比小于1),孪晶界使得纳米线得到强化.综合分析了孪晶界和晶粒尺寸对银纳米线的影响,为构建高强度金属纳米线打下基础.最后讨论了温度和拉伸速度对孪晶纳米线屈服应力所产生的影响,随着温度的升高,孪晶纳米线与单晶纳米线的屈服应力差先升高后趋于稳定;当拉伸速度逐渐增大,孪晶纳米线与单晶纳米线的屈服应力差先稳定后增大.     

基于分子动力学方法的孪晶银纳米线拉伸形变模拟

采用分子动力学方法研究了具有不同孪晶界密度的< 111>晶向孪晶Ag纳米线在拉伸载荷下的形变行为,讨论了孪晶界对纳米线力学强度的影响,并分别阐明具有不同孪晶界密度Ag纳米线的塑性形变机理.结果表明,与单晶Ag纳米线的强度进行对比可知,基于不同的形变模式,孪晶界的引入对纳米线可以起到弱化作用或者强化作用.以孪晶片层厚度的倒数(1/TBS)作为临界值,当1/TBS小于0.2 nm^-1时,孪晶界作为位错源,表现为对纳米线的弱化作用;当1/TBS大于0.2 nm^-1时,孪晶界阻碍位错运动表现为强化作用.强化作用机理分为两种:当1/TBS介于0.2到0.5 nm^-1时,形变机理以孪晶界和位错相互作用为主,断裂开口均在纳米线内部产生,随着位错增殖形成孔洞,进而向四周蔓延;当1/TBS大于0.5 nm^-1时,孪晶界发生迁移以容纳位错活动,位错不断增殖穿过孪晶界形成剪切带,进而导致纳米线的颈缩.由孪晶界密度不同引起的强化作用和弱化作用均随温度升高而减弱.     

孔洞和孪晶界对银纳米线形变行为联合影响的分子动力学模拟（英文）

在分子动力学模拟的基础上,系统地研究了银纳米线在单轴拉伸条件下的塑性形变。本文研究了不同孔径大小的[111]晶向的孪晶纳米线的力学性能。研究发现,孔洞的存在对弹性形变阶段没有影响。孪晶界上的孔洞在塑性形变阶段主要有两个作用。在初始塑性形变阶段,当孔洞的尺寸较小时,主要是作为位错源产生新的位错;随着孔洞尺寸的增大,主要作用变为阻碍位错滑移。在塑性形变的后期,孔洞的两个作用相辅相成,但随着孔洞尺寸的增大,孔洞作为位错源产生位错的作用变得显著,最终导致纳米线的塑性减弱。     

预压缩变形Mg-Zn-Y-Zr合金的微观组织及时效硬化效应

对铸造Mg-8.14Zn-1.44Y-0.47Zr (%,质量分数)合金进行了0%～5%预压缩变形后,再经160℃进行等温时效处理。利用电子背散射衍射、高分辨率的透射电镜等分析手段研究了预变形镁合金的形变组织特征及其对后续时效组织、时效硬化效应的影响。结果表明:经预变形在镁合金基体中预置■拉伸孪晶和小角度晶界(位错),其小角度晶界所占百分比由1%预变形的2.6%增加到5%预变形的■拉伸孪晶变体数量随预变形量增加呈先增大后减少的趋势。160℃时效进程中,孪晶界成为准晶I-Mg_3Zn_6Y择优析出的有效衬底;而基体中存在的位错,提高了杆状或盘状β_1′-MgZn_2相析出密度,进而提高了镁合金时效初期析出速率及近峰值时效硬度; 5%预变形下达到近峰值时效硬度所需时间由未预变形的12 h缩短到6 h,近峰值时效硬度由未预变形的HV 81.5提高到HV 104.8。     

多羟基化合物法制备五次孪晶银纳米线的生长机理

运用多羟基化合物方法, 在添加表面活性剂聚乙烯吡咯烷酮(PVP) K30的溶液中合成了多次孪晶银纳米颗粒和纳米线. 运用透射电子显微术(TEM)和光吸收谱, 对不同的摩尔比n(PVP):n(AgNO3)和不同的搅拌条件下制备的纳米线进行了对比研究. 结果表明, 这种方法的制备过程中不仅存在由五个{111}面包裹成的锥形生长, 而且还同时存在垂直于生长方向的{110}层状生长, 并且两者之间还存在着竞争; 另外对纳米线的弯折处进行的高分辨电子显微学研究表明, 纳米线制备过程中遭受的塑性变形在纳米线中产生了大量的层错和位错; 纳米线折断产生的新鲜断口容易成为新的晶粒形核位置.     

金属纳米线中凸凹微结构对初始形变的影响

构建了具有代表性的系列凸纳米线和凹纳米线,利用分子动力学模拟研究了2种微结构对拉伸形变的影响.结果表明,微凸纳米线与单晶纳米线表现出类似的行为,其能量和应力应变曲线等均无显著差异.改变不同的凸起高度未发现显著差别.沿z轴的应力分布分析表明凸微结构使局域应力降低,不能诱导产生初始位错滑移;微凹纳米线表现更明显的塑性形变特征,小应变时能量上升的幅度低于单晶和凸纳米线,但大应变条件下能量上升更高,微凹纳米线的第一屈服点早于单晶和凸纳米线,且其屈服应力不是最大应力,沿z轴的应力分布表明凹陷处产生增加的局域应力,凹陷附近可以诱导产生初始位错滑移.原子排布位图从微观上进一步阐述了上述形变特征.     

银纳米线的制备及电催化还原氧性能研究

分别以聚乙烯基吡咯烷酮(PVP),乙二醇作为软模板和还原剂,采用不同晶种(AgCl、Ag)快速合成了银纳米线.通过SEM和TEM表征,证明合成银纳米线材料形貌均一,颗粒含量很少.并且发现以AgCl为晶种合成的银纳米线长径比为200左右(SNWH),而以Ag为晶种合成的长径比为30左右(SNWL).以银纳米线作为电催化氧...     

不同温度下尼龙6性能和结构变化的原位研究

利用拉伸热台对尼龙6在不同温度下的拉伸性能进行了测试和分析.随着拉伸温度的升高,尼龙6样品的屈服强度和杨氏模量逐渐下降.采用了一个阿伦尼乌斯模型来描述尼龙6屈服强度与拉伸温度之间的关系,并描述了尼龙6样品黏度、玻璃化温度对屈服强度的影响.通过原位同步辐射广角衍射分析,我们发现未拉伸的尼龙6样品中的α相与γ相的相对比率随温度升高而下降.在拉伸条件下,随着拉伸温度的升高,样品的结晶部分容易产生滑移或晶区的熔融,有序程度下降,因而样品承载外力的能力下降,体现为屈服强度和杨氏模量的下降.     

CePO4纳米线的热稳定性及光学性能

以铈盐和磷酸为前驱体, 经水热合成获得了高长径比的具有六方晶型和单斜晶型独居石结构的CePO4纳米线, 采用XRD、HRTEM、SEM和荧光光度计对其晶相组成、形貌及发光性能进行了表征. 结果表明, 六方晶型CePO4纳米线直径约为40 nm, 长度约为3 μm; 单斜晶型独居石结构CePO4纳米线直径约为50 nm, 长度可达10 μm, 产物均为高纯且结晶良好的CePO4晶体. 可通过控制水热反应时间达到控制CePO4纳米线晶型的目的. 随水热反应时间的延长, 磷酸铈纳米线从六方晶型转变为单斜晶型(独居石结构). 随煅烧温度的升高, 磷酸铈纳米线直径增大, 但经1000 ℃煅烧仍具有一维线性结构, 其热稳定性高. 磷酸铈纳米线在紫外光激发下具有可见光区蓝紫发射区, 但随煅烧温度的升高, 磷酸铈纳米线的发光强度明显降低.     

镁铁双羟基复合金属氧化物的可控合成及晶面生长特征研究

采用成核/晶化隔离法合成镁铁双羟基复合金属氧化物MgFe-LDH,考察了Mg ∶ Fe摩尔比对MgFe-LDH晶形的影响,探讨了晶化温度及晶化时间对晶面生长选择性及晶粒尺寸的影响规律.结果表明,随Mg ∶ Fe摩尔比增大,层板阳离子排列更为规整.晶化温度对晶粒尺寸的影响显著大于晶化时间的影响.晶化温度相同,随晶化时间延长, MgFe-LDH的晶体结构趋于完整,晶粒尺寸变化不大;晶化时间相同,随晶化温度升高,晶体结构趋于完整,晶粒尺寸明显增大.所得到的MgFe-LDH沿a轴方向的晶粒尺寸对晶化温度变化的敏感程度远大于对晶化时间变化的敏感程度,但总是沿a轴方向的晶粒尺寸大于沿c轴方向的尺寸,即[110]晶面的生长速率比[002]晶面的生长速率快.     

孔洞和孪晶界对银纳米线形变行为联合影响的分子动力学模拟

Based on molecular dynamics simulations, the plastic deformation of silver nanowires under uniaxial tension has been studied systematically. In this paper, the mechanical properties of [111]-oriented twin nanowires with different hole sizes have been studied. The existence of holes has no effect on the elastic deformation stage. The hole on the twin boundary has two main roles in the plastic deformation stage. During the initial stages of plastic deformation, the main function of the hole is to produce new dislocations as dislocation sources at small hole sizes.Upon increasing the hole size, the main effect changes to stop dislocation slip. During the late stages of plastic deformation, the two functions of the hole complement each other, upon increasing the hole size, the function of the hole as dislocation sources becomes obvious, leading to weakening of the plasticity of the nanowires.     

硅通孔内铜电沉积填充机理研究进展

上海交通大学多元兼容集成制造技术团队针对TSV互连的深孔填充电镀难题, 借助有限元软件和任意拉格朗日-欧拉算法, 完成了方程组的数值解算, 实现了TSV填充模式的数值仿真。利用有限元和任意拉格朗日-欧拉算法分析了盲孔的填充机制, 通孔的蝴蝶形式的电镀填充过程, 以及不同深宽比孔的同时填充模式,并利用仿真数据进行了样品的研制及参数优化。分析了电镀的电流密度和热处理温度对电镀填充TSV-Cu的力学属性的影响。通过原位压缩试验研究了电流密度对TSV-Cu的力学性能和显微组织的影响。利用单轴薄膜拉伸试验分析了热处理工艺对TSV-Cu材料属性的影响。结果表明, 随着热处理温度的升高, TSV-Cu的断裂强度和屈服强度明显下降, 杨氏模量呈波纹状变化但变化趋势缓慢。基于上述研究结果, 研究了热失配应力所导致的互连结构热变形机制, 通过自主搭建的原位测试系统,实时观测TSV-Cu随温度变化而产生的变形大小,以研究影响TSV-Cu互连热应力应变的规律。 结果表明, TSV-Cu 的热变形过程分为弹性变形阶段、类塑性强化阶段以及塑性变形阶段。     

Deformation and microstructure of extended-chain polydiacetylene crystals

The mechanical properties (Young's modulus, ultimate tensile strength, deformation processes) of extended-chain polydiacetylene crystals are investigated. The properties observed are similar to those of metal and ceramic whiskers. The elastic modulus is strain-dependent and the ultimate tensile strength increases with decreasing crystal size. The maximum tensile strength observed was 1700 Nmm^?2. The ultimate tensile strength seems to be controlled by the presence of a small number of defects near the surface at which fracture nucleates. Irreversible deformation of the crystals was observed to occur by crack propagation normal and parallel to the direction of the macromolecules. The observed mechanical behavior corresponds to exceptionally high per-chain properties. The per-chain modulus obtained for these crystals is nearly as high as that of diamond. A chain-aligned polyethylene fiber with the same per-chain mechanical properties would have an ultimate strength as high as 0.9 × 10⁴ Nmm^?2.     

Synthesis and dispersion of isolated high aspect ratio gold nanowires

We report the assembly properties of high density and high aspect ratio metal nanowire arrays (Au, Cu and Ag with diameters ranging from 40 to 250 nm) after release from the anodic alumina oxide (AAO) templates. Individual Ag and Cu nanowires were observed following release from the template, however, in the case of gold nanowires, the dispersion was dependent on size and aspect ratio. 40-100 nm gold wires aggregated to form bundles or disordered mats. We show that a simple cyanide-mediated release from the AAO template, results in isolated dispersion of wires even for the smallest wire diameters. Possible stabilising mechanisms for observed tendency of nanowires dispersion are discussed.     

Lattice defects in decahedral multiply twinned particles of palladium

Multiply twinned decahedral particles of palladium have been studied by high-resolution electron microscopy in order to obtain evidence of lattice defects resulting from stress-relief processes due to internal elastic strains. This characterization revealed the presence of stacking faults parallel and adjacent to twin boundaries as well as inclined to twin boundaries. Additionally, for the first time stacking fault tetrahedra have been observed in the bulk of tetrahedral subunits. At the surface of the particles a pronounced micro-facetting has been shown to occur.     

Nucleation of filaments of the TGBA phase in free-standing films

A model of filament nucleation in the twist grain boundary A (TGBA) phase in free-standing smectic films is proposed. It is based on a concept of finite blocks of parallel smectic layers forming a helical structure in the interior of a film. In our model, the blocks resemble twin-like domains in smectic-A liquid crystal. The blocks are mediated to surrounding smectic layers in film either continuously (coherent twin boundary) or discontinuously via dislocation loops wrapping up blocks. Edge components of dislocation loops form incoherent twin boundary. Screw components forming twist grain boundary connect neighbour blocks among themselves. Nucleation of TGBA filament in a film is treated using a simplified model based on isotropic smectic-A elastic theory. Discussion showed that filaments can be nucleated just below the transition temperature from the isotropic to the TGBA phase under slow temperature decrease. The nucleation of dislocation loops is possible due to low compression modulus of material which can be expected just below the transition temperature.     

TEXTURE AND CRYSTALLINITY EVOLUTION IN ISOTACTIC POLYPROPYLENE INDUCED BY ROLLING AND THEIR INFLUENCE ON MECHANICAL PROPERTIES

The orientation and crystallinity evolution of isotactic polypropylene （iPP） induced by rolling were studied using wide angle X-ray scattering with an area detector. The tensile mechanical properties of rolled isotactic polypropylene sheets were also measured in this work. The texture component method was used to analyze the rolling texture. The rolling texture consists mainly of （010）[001], （130）[001] and [001]//RD fiber components in the sample with a rolling true strain of 1.5. The results reveal that crystallinity drastically decreases during rolling. It is suggested that amorphization is a deformation mechanism which takes place as an alternative to crystallographic intralamellar slip depending on the orientation of the lamellae. Both the orientation and crystallinity affect the tensile mechanical properties of rolled polypropylene. Crystallinity influences the elastic modulus on both directions and yield strength on transverse direction at the first stage of deformation. Orientation is the main reason for the changes of mechanical properties, especially at the latter part of deformation. The changes of both tensile strength and elongation percentage on rolling direction are larger than those on transverse direction, which results from the orientation. At last, the anisotropic mechanical properties occur on the rolling and transverse direction： high tensile strength with low elongation percentage on rolling direction and low tensile strength with high elongation percentage on transverse direction.     

Plastic deformation of polyethylene II. Change of mechanical properties during drawing

The crystallinity, elastic modulus, and tensile strength of samples of various draw ratios together with the true stress—strain curves of high-density polyethylene were determined to establish correlations with morphological changes occurring during deformation. Changes of crystallinity at draw ratios below 5, i.e., constancy during drawing of quenched film and a decrease during drawing of annealed film, are explained by the formation of microfibrils with crystallinity independent of the thermal history of the film. The microfibrils slide past each other at higher draw ratios, generating an increasing number of interfibrillar tie molecules, which is reflected in the increasing number of interfibrillar tie molecules, which is reflected in the increase of crystallinity, elastic modulus, and tensile strength. From the true stress—strain curves, the differential work density for the deformation of the volume element was calculated as a function of the draw ratio. It contains two components which reflect two different mechanisms of deformation. The first component, decreasing with increasing draw ratio, can be associated with the destruction of the original microspherulitic structure; the second one, increasing with increasing draw ratio, can be associated with the deformation of the new fiber structure, i.e., with the sliding motion of the microfibrils formed during the first deformation step.