Visualization of dislocation dynamics in colloidal crystals

The dominant mechanism for creating large irreversible strain in atomic crystals is the motion of dislocations, a class of line defects in the crystalline lattice. Here we show that the motion of dislocations can also be observed in strained colloidal crystals, allowing detailed investigation of their topology and propagation. We describe a laser diffraction microscopy setup used to study the growth and structure of misfit dislocations in colloidal crystalline films. Complementary microscopic information at the single-particle level is obtained with a laser scanning confocal microscope. The combination of these two techniques enables us to study dislocations over a range of length scales, allowing us to determine important parameters of misfit dislocations such as critical film thickness, dislocation density, Burgers vector, and lattice resistance to dislocation motion. We identify the observed dislocations as Shockley partials that bound stacking faults of vanishing energy. Remarkably, we find that even on the scale of a few lattice vectors, the dislocation behavior is well described by the continuum approach commonly used to describe dislocations in atomic crystals.     

Direct observation of structural defects in laser-deposited superconducting y-ba-cu-o thin films

The defect structure of in situ pulsed, laser-deposited, thin films of the high-transition temperature superconductor Y-Ba-Cu-O has been observed directly by atomic resolution electron microscopy. In a thin film with the nominal composition YBa(2)Cu(3)O(7) (123), stacking defects corresponding to the cationic stoichiometry of the 248, 247, and 224 compounds have been observed. Other defects observed include edge dislocations and antiphase boundaries. These defects, which are related to the nonequilibrium processing conditions, are likely to be responsible for the higher critical currents observed in these films as compared to single crystals.     

Geometric origin of hexagonal close packing at a grain boundary in gold

Using electron microscopy, we identify local, intergranular regions of hexagonal close-packing at a grain boundary in gold. By analyzing the topological defects that connect this layer to the adjacent face-centered cubic grains, we explain the geometric origin of this interfacial reconstruction. We extend this analysis to predict the stacking arrangements found over a range of intergranular misorientations. These results help to unify our understanding of the defects that control the behavior of polycrystalline materials by showing how line defects that are already well understood in the bulk also can determine the atomic arrangements at grain boundaries.     

The microstructure of high--critical current superconducting films

The microstructure of superconducting films that have shown high-critical current densities has been studied. The films are shown to be epitaxial and contain twins and precipitates. The main difference between these films and low current carrying samples is the absence of grain boundaries. These boundaries are therefore identified as the cause of the lower critical current in ceramic samples.     

MAGNETISM: Magnets Fast and Small

As magnetic devices become smaller and faster, it is becoming increasingly important to understand the dynamics of magnetic domains on small spatial and temporal scales. In their Perspective, Miltat and Thiaville highlight the report by Acremann et al., whose magnetooptical microscope combines state of the art space and time resolution, thus allowing the precessional motion of magnetic domains to be studied directly. The dynamics they observe are complex, indicating that in both the space and time domains, neither the magnetization nor the applied field should be viewed as uniform.     

Ultrahigh strength and high electrical conductivity in copper

Methods used to strengthen metals generally also cause a pronounced decrease in electrical conductivity, so that a tradeoff must be made between conductivity and mechanical strength. We synthesized pure copper samples with a high density of nanoscale growth twins. They showed a tensile strength about 10 times higher than that of conventional coarse-grained copper, while retaining an electrical conductivity comparable to that of pure copper. The ultrahigh strength originates from the effective blockage of dislocation motion by numerous coherent twin boundaries that possess an extremely low electrical resistivity, which is not the case for other types of grain boundaries.     

Brownian motion of an ellipsoid

We studied the Brownian motion of isolated ellipsoidal particles in water confined to two dimensions and elucidated the effects of coupling between rotational and translational motion. By using digital video microscopy, we quantified the crossover from short-time anisotropic to long-time isotropic diffusion and directly measured probability distributions functions for displacements. We confirmed and interpreted our measurements by using Langevin theory and numerical simulations. Our theory and observations provide insights into fundamental diffusive processes, which are potentially useful for understanding transport in membranes and for understanding the motions of anisotropic macromolecules.     

Reconstruction of Current Flow and Imaging of Current-Limiting Defects in Polycrystalline Superconducting Films

Magneto-optical imaging was used to visualize the inhomogeneous penetration of magnetic flux into polycrystalline TlBa2Ca2Cu3Ox films with high critical current densities, to reconstruct the local two-dimensional supercurrent flow patterns and to correlate inhomogeneities in this flow with the local crystallographic misorientation. The films have almost perfect c-axis alignment and considerable local a- and b-axis texture because the grains tend to form colonies with only slightly misaligned a and b axes. Current flows freely over these low-angle grain boundaries but is strongly reduced at intermittent colony boundaries of high misorientation. The local (<10-micrometer scale) critical current density Jc varies widely, being up to 10 times as great as the transport Jc (scale of approximately 1 millimeter), which itself varies by a factor of about 5 in different sections of the film. The combined experiments show that the magnitude of the transport Jc is largely determined by a few high-angle boundaries.     

Volcanism in response to plate flexure

Volcanism on Earth is known to occur in three tectonic settings: divergent plate boundaries (such as mid-ocean ridges), convergent plate boundaries (such as island arcs), and hot spots. We report volcanism on the 135 million-year-old Pacific Plate not belonging to any of these categories. Small alkalic volcanoes form from small percent melts and originate in the asthenosphere, as implied by their trace element geochemistry and noble gas isotopic compositions. We propose that these small volcanoes erupt along lithospheric fractures in response to plate flexure during subduction. Minor extents of asthenospheric melting and the volcanoes' tectonic alignment and age progression in the direction opposite to that of plate motion provide evidence for the presence of a small percent melt in the asthenosphere.     

Brownian motion of stiff filaments in a crowded environment

The thermal motion of stiff filaments in a crowded environment is highly constrained and anisotropic; it underlies the behavior of such disparate systems as polymer materials, nanocomposites, and the cell cytoskeleton. Despite decades of theoretical study, the fundamental dynamics of such systems remains a mystery. Using near-infrared video microscopy, we studied the thermal diffusion of individual single-walled carbon nanotubes (SWNTs) confined in porous agarose networks. We found that even a small bending flexibility of SWNTs strongly enhances their motion: The rotational diffusion constant is proportional to the filament-bending compliance and is independent of the network pore size. The interplay between crowding and thermal bending implies that the notion of a filament's stiffness depends on its confinement. Moreover, the mobility of SWNTs and other inclusions can be controlled by tailoring their stiffness.     

Deformation twinning in nanocrystalline aluminum

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.     

滚塑成型在小型节能船艇中的应用

描述了滚塑成型技术以及该技术的原理在国内外的发展现状,分析了国内外聚乙烯材料在小型船艇中的应用现状,滚塑成型技术应用于船艇制造的优缺点,讨论了将滚塑成型技术应用于小型游艇建造的可行性以及应用前景、社会经济效益,并指出了这种技术用于船舶建造的问题及解决方法.要充分考虑船艇的稳性富余和耐波性能,通过合适的压载提高滚塑船舶的性能,滚塑成型技术在小型船舶行业有着广阔的前景.     

Grain boundary-mediated plasticity in nanocrystalline nickel

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.     

Equatorially dominated magnetic field change at the surface of Earth's core

Slow temporal variations in Earth's magnetic field originate in the liquid outer core. We analyzed the evolution of nonaxisymmetric magnetic flux at the core surface over the past 400 years. We found that the most robust feature is westward motion at 17 kilometers per year, in a belt concentrated around the equator beneath the Atlantic hemisphere. Surprisingly, this motion is dominated by a single wavenumber and persists throughout the observation period. This phenomenon could be produced by an equatorial jet of core fluid, by hydromagnetic wave propagation, or by a combination of both. Discrimination between these mechanisms would provide useful constraints on the dynamics of Earth's core.     

Observation of superflow in solid helium

We report on the observation of nonclassical rotational inertia in solid helium-4 confined to an annular channel in a sample cell under torsional motion, demonstrating superfluid behavior. The effect shows up as a drop in the resonant oscillation period as the sample cell is cooled below 230 millikelvin. Measurement of 17 solid samples allows us to map out the boundary of this superfluid-like solid or supersolid phase from the melting line up to 66 bars. This experiment indicates that superfluid behavior is found in all three phases of matter.     

Molecular loops in the galactic center: evidence for magnetic flotation

The central few hundred parsecs of the Milky Way host a massive black hole and exhibit very violent gas motion and high temperatures in molecular gas. The origin of these properties has been a mystery for the past four decades. Wide-field imaging of the (12)CO (rotational quantum number J = 1 to 0) 2.6-millimeter spectrum has revealed huge loops of dense molecular gas with strong velocity dispersions in the galactic center. We present a magnetic flotation model to explain that the formation of the loops is due to magnetic buoyancy caused by the Parker instability. The model has the potential to offer a coherent explanation for the origin of the violent motion and extensive heating of the molecular gas in the galactic center.     

Curvilinear, three-dimensional motion of chromatin domains and nucleoli in neuronal interphase nuclei

The term "nuclear rotation" refers to a motion of nucleoli within interphase nuclei of several cell types. No mechanism or function has been ascribed to this phenomenon, and it was unknown whether nuclear structures in addition to nucleoli participate in this motion. Moreover, it was unclear whether nuclear rotation occurs independent of concurrent motion of juxtanuclear cytoplasm. The work reported here presents quantitative evidence, for three-dimensional intranuclear, tandem motion of fluorescently labeled chromatin domains associated with nucleoli and those remote from nucleoli. The results show that such motion is curvilinear, that it is not restricted to nucleoli, and, moreover, that it occurs independently of motion of juxtanuclear, cytoplasmic structures. These results suggest that this motion represents karyoplasmic streaming and its function is to transpose to nuclear pores those chromatin domains actively transcribed.     

Experimental coarsening of antiphase domains in a silicate mineral

Combined annealing experiments and observations by transmission electron microscopy show that in natural pigeonite crystals antiphase domains coarsen approximately according to a rate law in which the tenth power of the average domain size is proportional to time. This result suggests that certain cations (possibly Ca(2+)) were segregated preferentially onto the antiphase boundaries. The domain size in samples quenched from above the high-low transformation temperature is large and apparently independent of annealing time and temperature. It appears that large domains can be generated either by very fast or by very slow cooling; thus the estimation of geological cooling rates from the sizes of antiphase domains in natural samples becomes rather difficult.     

Ultralow thermal conductivity in disordered, layered WSe2 crystals

The cross-plane thermal conductivity of thin films of WSe2 grown from alternating W and Se layers is as small as 0.05 watts per meter per degree kelvin at room temperature, 30 times smaller than the c-axis thermal conductivity of single-crystal WSe2 and a factor of 6 smaller than the predicted minimum thermal conductivity for this material. We attribute the ultralow thermal conductivity of these disordered, layered crystals to the localization of lattice vibrations induced by the random stacking of two-dimensional crystalline WSe2 sheets. Disordering of the layered structure by ion bombardment increases the thermal conductivity.     

Anomalous strength characteristics of tilt grain boundaries in graphene

Graphene in its pristine form is one of the strongest materials tested, but defects influence its strength. Using atomistic calculations, we find that, counter to standard reasoning, graphene sheets with large-angle tilt boundaries that have a high density of defects are as strong as the pristine material and, unexpectedly, are much stronger than those with low-angle boundaries having fewer defects. We show that this trend is not explained by continuum fracture models but can be understood by considering the critical bonds in the strained seven-membered carbon rings that lead to failure; the large-angle boundaries are stronger because they are able to better accommodate these strained rings. Our results provide guidelines for designing growth methods to obtain sheets with strengths close to that of pristine graphene.