Shock-Wave Modulation of the Dielectric Constant of Photonic Crystals Produces Optical Phenomena
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Review Letters direct experimental proof of the hypothesis. They used x-ray scattering to study a deeply undercooled TiZrNi liquid to demonstrate the correlation between the nucleation barrier and growing ISRO with decreasing temperature. Beamline electrostatic levitation (BESL), a technique recently developed by Kelton, made the experiments possible, allowing in situ x-ray diffraction studies on electrostatically levitated droplets. The researchers used containerless processing to study 2.3–2.5-mm-diameter TiZrNi droplets, which were melted using a YAG laser and charged positively by using ultraviolet light followed by levitation in vacuum (10 -7 Torr) between electrostatic plates. Decoupling between heating and positioning using electrostatic levitation showed an improvement over electromagnetic levitation techniques, allowing undercooling studies on a wider range of materials. In the case of the TiZrNi system investigated, the researchers found that the
increasing ISRO is responsible for the nucleation of the metastable icosahedral i-phase formed in the first of two recalescence events (a 105 K temperature rise), instead of the thermodynamically stable polytetrahedral C14 Laves phase, which forms just a few seconds later, recognized by a second recalescence (a 25 K temperature rise). Recalescence is the process whereby the liberated heat of fusion causes a temperature rise during solidification. Since the driving free energy for the formation of the C14 phase is larger, the preferred nucleation of the i-phase indicates a smaller nucleation barrier, consistent with the x-ray evidence that the short-range order in the supercooled liquid is closer to that of the i-phase than to that of the C14 structure. A better understanding of the physics of undercooled liquids is of technological importance, particularly the elucidation of the reasons for the unusual stability of undercooled liquids against crystallization in order to gain better control of their
Geometry Can Provide Interlocking of Protective Tiles for the Space Shuttle One of the critical components on the space shuttle is its thermal protection system (TPS). The TPS consists of various materials applied externally, primarily in the form of tiles, to the outer structural skin of aluminum and graphite epoxy. During reentry of the shuttle from earth orbit, the TPS protects the skin from overheating and failure due to the frictional heating of high-speed contact with the earth’s atmosphere. A weakness of the TPS is that mechanically it is not a cohesive system. Each of the tiles is an independent unit, and damage to or failure of one tile can cascade into overall catastrophic failure. Yuri Estrin (Technical University of Clausthal, Germany) along with A.V. Dyskin, E. Pasternak, H.C. Khor from the University of Western Australia and A.J. Kanel-Belov from the International University of Bremen, Germany have proposed a new concept for the design of the tile-covering, based on topological interlocking of the tiles. The basis of their proposal, as published in the
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