PubMed · 12652664
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Alfred Forchel. 2003. Switching light with light.. https://doi.org/10.1038/nmat801
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Three techniques have been used to study dislocations in ice: etch pitting-replication, transmission electron microscopy, and X-ray topography (XT). It is shown that, because ice is a weak absorber of X-rays and can be produced with a low dislocation density, allowing relatively thick specimens to be studied, the most useful technique is XT. The observations that have been made with conventional XT are briefly outlined. However, the introduction of high-intensity synchrotron radiation, with its concomitant short exposure times, showed that images obtained through conventional XT observations were of dislocations that had undergone recovery. The important dynamic observations and measurements that have been made using synchrotron X-ray topography are presented. Dynamic synchrotron X-ray topography observations of ice single crystals undergoing deformation in situ have shown that slip mainly occurs by the movement of screw and 60 degrees (1/3) [1120] dislocations on the basal plane, although non-basal slip by edge dislocations can also occur. The operation of Frank-Read and other dislocation multiplication sources have been clearly demonstrated and dislocation velocities have been measured. In contrast, in polycrystals, dislocation generation occurred at grain boundaries where there are stress concentrations before lattice dislocation generation mechanisms operate. Faulted dislocation loops have been determined to be mainly interstitial in both polycrystals and single crystals.
Current theories of snow metamorphism indicate that sublimating snow crystals have rounded shapes, while growing crystals have shapes that depend on growth rates. At slow growth rates, crystals are rounded. At moderate rates, they have flat faces with rounded edges. At fast growth rates, crystals have flat faces with sharp edges, and they have hollow faces at very fast growth rates. The main growth/sublimation mechanism is thought to be by the homogeneous nucleation of new layers at or near crystal edges. It was also suggested that the equilibrium shape of snow crystals would be temperature dependent: rounded above -10.5 degrees C, and faceted below. To test these paradigms, we have performed SEM investigations of snow samples having undergone metamorphism under natural conditions, and of snow samples subjected to isothermal metamorphism at -4 degrees and -15 degrees C in the laboratory. In general, current theories predicting crystal shapes as a function of growth rates, and of whether crystals are growing or sublimating, are verified. However, the transition in equilibrium shapes from rounded to faceted at -10.5 degrees C is not observed in our isothermal experiments that reveal a predominance of rounded shapes after more than a month of metamorphism at -4 and -15 degrees C. Some small crystals with flat faces that also have sharp angles at -15 degrees C, are observed in our isothermal experiments. These faces are newly formed, and contradict current theory. Several hypotheses are proposed to explain their occurrence. One is that they are due to sublimation at emerging dislocations.
In this work, the influence of an internal electric field upon the crystallization of lysozyme and thaumatin is explored using a modified design of the gel-acupuncture setup. From a crystallographic point of view, the orientation of crystals that grow preferentially over different types of electrodes inside capillary tubes is also evaluated. Finally, the crystal quality and the three-dimensional structure of these proteins grown with and without the electric field influence are analyzed by means of X-ray diffraction methods.