Novel Method for Control of Quantum Entanglement Developed
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cability to the study of fundamental biological questions, by providing a direct means to study coupling between biochemical and biomechanical reaction cycles.” STEVEN TROHALAKI
High-Velocity Ballistic Impact with Boron Carbide Produces Localized Amorphization Localized amorphization induced by shock has been observed in crystalline boron carbide. Mingwei Chen and Kevin Hemker of Johns Hopkins University and James McCauley of the U.S. Army Research Laboratory at Aberdeen Proving Ground have demonstrated that extremely high-rate deformation can initiate formation of amorphized bands in the rhombohedral crystal structure of B4C. Boron carbide is commonly used as ballistic armor. Although the material is effective for protection against low-energy projectiles such as bullets from handguns, it is less effective upon more powerful impacts. As reported in the March 7 issue of Science, Chen, McCauley, and Hemker found that the measured drop in the impact resistance of boron carbide at impact pressures in the range of 20–23 GPa could be attributed to the formation of nanoscale intragranular amorphous bands in the crystal. The researchers observed the amorphous bands by high-resolution electron microscopy (HREM). They used transmission electron microscope electron energy-loss spectroscopy to compare the chemical composition of the amorphous regions to that of the crystalline B 4C. These spectra allowed the researchers to rule out the possibility of a pressureinduced decomposition or chemical reaction since there was no detectable difference in chemical composition. The researchers also determined the HREM images were not consistent with rebonding of two cracked surfaces or melting. Accordingly, they believe that the amorphization is a solid-state transformation instigated by the ballistic event. Recent diamond anvil studies demonstrated a phase transition at a pressure around 20 GPa, and previous nanoindentation experiments at even higher pressures suggested amorphization, but this speculation was discounted based on Raman spectra. The researchers in this study suggest that their results warrant renewed scrutiny of the previous interpretations of the diamond anvil and nanoindentation results. The drop in performance of boron carbide when exposed to high-impact pressures has been known for years. The curMRS BULLETIN/MAY 2003
rent research provides a microscopic explanation for this highly unfavorable property for a material employed as ballistic armor. The researchers do not propose a means to avoid formation of the amorphization regions that appear to be responsible for the drop in strength of crystalline boron carbide. Nevertheless, they speculate that this improved understanding of how shock can alter materials properties and permit the synthesis of novel structures could further expand the realm of possibilities for innovative materials synthesis. Chen said, “It’s like having a sturdy table and suddenly kicking the legs out from underneath it.” “This discovery was very enlightening, because it tells us that under extre
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