Nature Designs Hard and Tough Materials at the Nanoscale
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where macroscopic softening sets in, but at a much higher critical temperature. “This means that there is a temperature range, extending over almost 200°C, where the atoms still move exclusively like in a solid by local hopping over thermal barriers, although macroscopically the system already behaves like a viscous liquid. As in the glassy state, this hopping involves the coordinated motion of many atoms,” said Faupel. Faupel said that the critical temperature was found to be exactly located at the temperature Tc where the so-called modecoupling theory predicts the freezing-in of liquidlike motion upon cooling. The Tc was obtained from quasi-elastic neutron scattering by A. Meyer at the Munich University of Technology. “Moreover, our isotope-effect measurements, which are the first measurements of this kind near Tc in any material, demonstrate that even in the equilibrium melt of the novel bulk-glass-forming alloys, the atomic dynamics are highly coordinated and far away from the hydrodynamic limit of uncorrelated binary collisions,” Faupel said. “This seems to be a prerequisite of the exceptional glassforming abilities.”
Nature Designs Hard and Tough Materials at the Nanoscale The nanoscale size of mineral particles in bone, teeth, and other biological materials may have evolved to ensure optimum strength and maximize tolerance of flaws, according to a research team from the Max Planck Institute for Metals Research (MPI), the Austrian Academy of Sciences, and the University of Leoben. While it is clear that the composite character of biological materials plays an important role in determining their strength, little is known about the role of the nanometer scale of mineral particles. The research team reports in the May 13 issue of the Proceedings of the National Academy of Sciences that there exists a critical nanometer size at which the particles found in biocomposites become insensitive to flaws: They maintain strength equivalent to a perfect crystal
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despite inherent defects. This phenomenon also suggests that the engineering concept of stress concentration at flaws is no longer valid for nanoscale design. Biomaterials like bone are molecular composites of proteins and biominerals. While the stiffness of biocomposites is similar to that of the mineral, their fracture energy can be several orders of magnitude higher than the mineral. For example, the composite shell of nacre shows a fracture strength that is 3000 times higher than its mineral constituent CaCO3. Despite the complicated hierarchical structures of biocomposites, the smallest building blocks in biological materials are generally on the nanometer length scale and aligned in a generic structure of mineral platelets staggered in a protein matrix. H. Gao of MPI, I.L. Jäger of the Academy and the University of Leoben, and coworkers have found that this generic nanostructure of biomaterials may be the key to the high fracture strength of these materials. Their analysis demonstrates that the mineral crystals carry the tensile load while the protei
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