PubMed · 14593386
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2003. Let there be light!. https://doi.org/10.1038/nmat1015
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The shapes of two-dimensional (2D) nanostructures on surfaces are determined by their boundary energies as well as by long-range elastic, electrostatic or magnetic interactions. Although it is well known that long-range interactions can give rise to shape bifurcation-an abrupt change in shape symmetry at a critical size-a general description of the evolution of shape with size, systematically incorporating both the azimuthal dependence of the boundary energy and long-range interactions, has been lacking. Here we show that unconstrained shape relaxation, including previously ignored boundary curvature, leads to a novel, continuous shape change from convex at small size to concave at large size. In addition to demonstrating a method to quantitatively determine the azimuthal dependence of the boundary energy, we show that the energy gain associated with boundary curvature relaxation is a key factor in stabilizing surface nanostructures. For 7 x 7 domains on Si(111), boundary curvature reduces the formation free-energy by up to 50%.
Despite of the widespread and increasing use of digitized signals, the ultrasonic testing community has not realized yet the full potential of the electronic processing. The performance of an ultrasonic flaw detection method is evaluated by the success of distinguishing the flaw echoes from those scattered by microstructures. So, de-noising of ultrasonic signals is extremely important as to correctly identify smaller defects, because the probability of detection usually decreases as the defect size decreases, while the probability of false call does increase. In this paper, the wavelet transform has been successfully experimented to suppress noise and to enhance flaw location from ultrasonic signal, with a good defect localization. The obtained result is then directed to an automatic Artificial Neuronal Networks classification and learning algorithm of defects from A-scan data. Since there is some uncertainty connected with the testing technique, the system needs a numerical modelling. So, knowing the technical characteristics of the transducer, we can preview which are the defects that experimental inspection should find. Indeed, the system performs simulation of the ultrasonic wave propagation in the material, and gives a very helpful tool to get information and physical phenomena understanding, which can help to a suitable prediction of the service life of the component.
The characteristic ability of FIB (focused ion beam) fabrication to remove materials from a very small and/or precisely located slab using an accelerated Ga ion beam were employed to prepare cross-sectional thin films of zinc-coated steel sheets composed of Fe-Zn intermetallic compounds. A few defects observed frequently on the galvannealed (GA) coating surface were analyzed. Streaky marks indicated the rich concentration of oxides at the interface between the galvannealed coating and the steel substrate. The annealing experiment indicated the existence of Mn and Si oxides on the steel substrate. The grain size of ferrite in the extreme surface of the substrate was smaller as compared with conventional IF (Interstitial Free) steels. Furthermore, some superlattice spots were observed at the fine ferrite grain. Defects that are termed 'wavy pattern' and 'dross' were also analyzed by the combination of the FIB and TEM (Transmission Electron Microscope) techniques.