共查询到20条相似文献，搜索用时 15 毫秒
1.
利用边界积分方法及基于Peierls-Nabarro模型的位错理论,分析了理想纳米触头下位错的生成, 得到了滑移原子层上相对滑移量、位错产生的临界载荷以及激发位错所需的激发能。导出了位错产生临 界载荷的近似解析式。用于验证数值分析结果的有效性。该方法在连续力学的计算中引入了包含原子信 息的层间势能函数,为分析多尺度的力学问题提供了有效的方法。 相似文献
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Jesson DE Chen KM Pennycook SJ Thundat T Warmack RJ 《Science (New York, N.Y.)》1995,268(5214):1161-1163
With the combination of the height sensitivity of atomic force microscopy and the strain sensitivity of transmission electron microscopy, it is shown that near singular stress concentrations can develop naturally in strained epitaxial films. These crack-like instabilities are identified as the sources of dislocation nucleation and multiplication in films of high misfit. This link between morphological instability and dislocation nucleation provides a method for studying the basic micromechanisms that determine the strength and mechanical properties of materials. 相似文献
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We report transmission electron microscope observations that provide evidence of deformation twinning in plastically deformed nanocrystalline aluminum. The presence of these twins is directly related to the nanocrystalline structure, because they are not observed in coarse-grained pure aluminum. We propose a dislocation-based model to explain the preference for deformation twins and stacking faults in nanocrystalline materials. These results underscore a transition from deformation mechanisms controlled by normal slip to those controlled by partial dislocation activity when grain size decreases to tens of nanometers, and they have implications for interpreting the unusual mechanical behavior of nanocrystalline materials. 相似文献
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Heterogeneous nucleation experiments bridging the scale from molecular ion clusters to nanoparticles
Winkler PM Steiner G Vrtala A Vehkamäki H Noppel M Lehtinen KE Reischl GP Wagner PE Kulmala M 《Science (New York, N.Y.)》2008,319(5868):1374-1377
Generation, investigation, and manipulation of nanostructured materials are of fundamental and practical importance for several disciplines, including materials science and medicine. Recently, atmospheric new particle formation in the nanometer-size range has been found to be a global phenomenon. Still, its detailed mechanisms are mostly unknown, largely depending on the incapability to generate and measure nanoparticles in a controlled way. In our experiments, an organic vapor (n-propanol) condenses on molecular ions, as well as on charged and uncharged inorganic nanoparticles, via initial activation by heterogeneous nucleation. We found a smooth transition in activation behavior as a function of size and activation to occur well before the onset of homogeneous nucleation. Furthermore, nucleation enhancement for charged particles and a substantial negative sign preference were quantitatively detected. 相似文献
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目的探讨应用AO锁骨钩钢板（CHP）治疗锁骨远端骨折伴肩锁关节脱位的实用价值．方法对1999年8月-2007年4月吉林省吉林中西医结合医院收治的锁骨远端骨折伴肩锁关节脱位28例,均采用AO锁骨钩钢板（CHP）内固定治疗进行回顾性分析研究．结果全部骨性愈合,随访8—48个月,平均23个月．无钢板断裂、螺钉松动、感染及再脱位,无骨折不愈合病例,肩关节功能均恢复良好．CHP取出时间8～17个月,平均11个月．结论CHP对锁骨远端骨折并肩锁关节脱位治疗效果理想,骨折不愈合可以避免,患者可以早期进行功能练习,使骨折愈合与功能恢复同步进行,而且具有操作简单、易掌握、损伤小、疗效确切的特点,值得临床应用推广,是目前有效治疗此种损伤的最佳方法． 相似文献
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Xu G Aeppli G Bisher ME Broholm C DiTusa JF Frost CD Ito T Oka K Paul RL Takagi H Treacy MM 《Science (New York, N.Y.)》2000,289(5478):419-422
Magnetic neutron scattering provides evidence for nucleation of antiferromagnetic droplets around impurities in a doped nickel oxide-based quantum magnet. The undoped parent compound contains a spin liquid with a cooperative singlet ground state and a gap in the magnetic excitation spectrum. Calcium doping creates excitations below the gap with an incommensurate structure factor. We show that weakly interacting antiferromagnetic droplets with a central phase shift of pi and a size controlled by the correlation length of the quantum liquid can account for the data. The experiment provides a quantitative impression of the magnetic polarization cloud associated with holes in a doped transition metal oxide. 相似文献
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Shan Z Stach EA Wiezorek JM Knapp JA Follstaedt DM Mao SX 《Science (New York, N.Y.)》2004,305(5684):654-657
The plastic behavior of crystalline materials is mainly controlled by the nucleation and motion of lattice dislocations. We report in situ dynamic transmission electron microscope observations of nanocrystalline nickel films with an average grain size of about 10 nanometers, which show that grain boundary-mediated processes have become a prominent deformation mode. Additionally, trapped lattice dislocations are observed in individual grains following deformation. This change in the deformation mode arises from the grain size-dependent competition between the deformation controlled by nucleation and motion of dislocations and the deformation controlled by diffusion-assisted grain boundary processes. 相似文献
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Micropatterning of single crystals for technological applications is a complex, multistep process. Nature provides alternative fabrication strategies, when crystals with exquisite micro-ornamentation directly develop within preorganized frameworks. We report a bio-inspired approach to growing large micropatterned single crystals. Micropatterned templates organically modified to induce the formation of metastable amorphous calcium carbonate were imprinted with calcite nucleation sites. The template-directed deposition and crystallization of the amorphous phase resulted in the fabrication of millimeter-sized single calcite crystals with sub-10-micron patterns and controlled crystallographic orientation. We suggest that in addition to regulating the shape, micropatterned frameworks act as sites for stress and impurity release during the amorphous-to-crystalline transition. The proposed mechanisms may have direct biological relevance and broad implications in materials synthesis. 相似文献
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The spontaneous fracture of polymer gels was studied. Contrary to crystalline solids, where fracture usually happens instantaneously at a well-defined breaking strength, the fracture of a polymer gel can occur with a delay. When a constant force was applied, the cracks nucleated and started to propagate after a delay that can be as long as 15 minutes, depending on the force. This phenomenon can be understood by calculating the activation energy for crack nucleation in arbitrary dimension and accounting for the inhomogeneity of the gel network in terms of its fractal dimension. 相似文献
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A specially designed solid-state deuterium nuclear magnetic resonance probe was used to examine the effect of uniaxial elongation on the chain mobility in the amorphous region of semicrystalline nylon 6. In measurements conducted near the glass transition temperature, there was measurable deformation-induced enhancement of the mobility of the amorphous chains up to the yield point. This enhanced mobility decayed once deformation was stopped. Enhanced mobility was not observed in deformation at room temperature. The mechanics of deformation can be explained by the Robertson model for glassy polymers near the glass transition temperature, which states that applied stress induces liquid-like behavior in the polymer chains. 相似文献
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Isolated single crystals of polyethylene have been crystallized from the glassy state by annealing uniformly thick films of amorphous linear polyethylene just above their glass transition temperature. In agreement with previous results for polycarbonate, substantial molecular mobility at the glass transition temperature is. 相似文献
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Jacobs K Zaziski D Scher EC Herhold AB Paul Alivisatos A 《Science (New York, N.Y.)》2001,293(5536):1803-1806
The transition between four- and six-coordinate structures in CdSe nanocrystals displays simple transition kinetics as compared with the extended solid, and we determined activation volumes from the pressure dependence of the relaxation times. Our measurements indicate that the transformation takes place by a nucleation mechanism and place strong constraints on the type of microscopic motions that lead to the transformation. The type of analysis presented here is difficult for extended solids, which transform by complicated kinetics and involve ill-defined domain volumes. Solids patterned on the nanoscale may prove to be powerful models for the general study of structural transitions in small systems, as well as in extended solids. 相似文献
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Pyzalla A Camin B Buslaps T Di Michiel M Kaminski H Kottar A Pernack A Reimers W 《Science (New York, N.Y.)》2005,308(5718):92-95
Creep damage by void nucleation and growth limits the lifetime of components subjected to loading at high temperatures. We report a combined tomography and diffraction experiment using high-energy synchrotron radiation that permitted us to follow in situ void growth and microstructure development in bulk samples. The results reveal that void growth versus time follows an exponential growth law. The formation of large void volumes coincides with texture evolution and dislocation density, reaching a steady state. Creep damage during a large proportion of sample creep life is homogeneous before damage localization occurs, which leads to rapid failure. The in situ determination of void evolution in bulk samples should allow for the assessment of creep damage in metallic materials and subsequently for lifetime predictions about samples and components that are subject to high-temperature loading. 相似文献
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Sample dimensions influence strength and crystal plasticity 总被引：1，自引：0，他引：1
When a crystal deforms plastically, phenomena such as dislocation storage, multiplication, motion, pinning, and nucleation occur over the submicron-to-nanometer scale. Here we report measurements of plastic yielding for single crystals of micrometer-sized dimensions for three different types of metals. We find that within the tests, the overall sample dimensions artificially limit the length scales available for plastic processes. The results show dramatic size effects at surprisingly large sample dimensions. These results emphasize that at the micrometer scale, one must define both the external geometry and internal structure to characterize the strength of a material. 相似文献
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Rheology of the upper mantle: a synthesis 总被引：4，自引：0，他引：4
Rheological properties of the upper mantle of the Earth play an important role in the dynamics of the lithosphere and asthenosphere. However, such fundamental issues as the dominant mechanisms of flow have not been well resolved. A synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the Earth's upper mantle. The hot and shallow upper mantle flows by dislocation creep, whereas cold and shallow or deep upper mantle may flow by diffusion creep. When the stress increases, grain size is reduced and the upper mantle near the transition between these two regimes is weakened. Consequently, deformation is localized and the upper mantle is decoupled mechanically near these depths. 相似文献
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寰枢关节在发育解剖上具有独特性,一定方式的外力损伤可造成寰枢椎骨折和脱位,其诊断很大程度取决于影像学.X线,CT,MRI3种方法是最常用的诊断方法．X线检查简单、价廉,对寰枢关节半脱位具有一定的诊断能力,但诊断阳性率低,可疑诊断较多;CT显示断面解剖,部分脱位征象显示清楚,诊断准确率较X线检查高;MRI能直观显示关节囊、韧带及颈髓情况,在显示枕大孔区韧带结构较CT有优势,因而在寰枢区疾病诊断中具有重要价值． 相似文献
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Zheng H Rivest JB Miller TA Sadtler B Lindenberg A Toney MF Wang LW Kisielowski C Alivisatos AP 《Science (New York, N.Y.)》2011,333(6039):206-209
The study of first-order structural transformations has been of great interest to scientists in many disciplines. Expectations from phase-transition theory are that the system fluctuates between two equilibrium structures near the transition point and that the region of transition broadens in small crystals. We report the direct observation of structural fluctuations within a single nanocrystal using transmission electron microscopy. We observed trajectories of structural transformations in individual nanocrystals with atomic resolution, which reveal details of the fluctuation dynamics, including nucleation, phase propagation, and pinning of structural domains by defects. Such observations provide crucial insight for the understanding of microscopic pathways of phase transitions. 相似文献
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Lifshitz Y Köhler T Frauenheim T Guzmann I Hoffman A Zhang RQ Zhou XT Lee ST 《Science (New York, N.Y.)》2002,297(5586):1531-1533
A model for diamond nucleation by energetic species (for example, bias-enhanced nucleation) is proposed. It involves spontaneous bulk nucleation of a diamond embryo cluster in a dense, amorphous carbon hydrogenated matrix; stabilization of the cluster by favorable boundary conditions of nucleation sites and hydrogen termination; and ion bombardment-induced growth through a preferential displacement mechanism. The model is substantiated by density functional tight-binding molecular dynamics simulations and an experimental study of the structure of bias-enhanced and ion beam-nucleated films. The model is also applicable to the nucleation of other materials by energetic species, such as cubic boron nitride. 相似文献