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Predicting the impact of cross-links on the mechanics of carbon nanotube-based materials is a challenging endeavor, as the micro- and nanostructure is composed of continuous nanofibers, discontinuous interfaces, and covalent bridges. Here we demonstrate a new modeling solution in the context of the distinct element method (DEM). By representing nanotubes as bonded cylinder segments undergoing van der Waals adhesion, viscous friction, and contact bonding, we are able to simulate how cross-linking transforms a soft bundle into a strong one. We predict that the sp3-sp cross-links formed by interstitial carbon atoms can improve the tensile strength by an order of magnitude, in agreement with experiment and molecular dynamics simulations. The DEM methodology allows performing the multiscale simulation needed for developing strategies to further enhance the mechanical performance.
Using the technique of tight-binding electron–ion dynamics,
we have calculated the response of crystalline GaAs when a
femtosecond laser pulse excites 1–20% of the valence
electrons. Above a threshold fluence, which corresponds to
promotion of about 12% of the valence electrons to the conduction
band, the lattice is destabilized and the band gap collapses
to zero. This result supports the conclusion that structural
changes on a subpicosecond time scale observed in pump-probe
experiments are of a nonthermal nature.
We report new theoretical studies of the electronic and structural response of materials to fast intense laser pulses, with durations ∼ 10–100 feintoseconds and intensities up to ∼10 terawatts/cm2. The results provide still stronger evidence that GaAs undergoes a true nonthermal phase transition as the intensity is varied at constant pulse duration. For C60, there are also different regimes of behavior as a function of pulse intensity. At low intensity, various optically-active modes are observed. At high intensity, the breathing moode is by far the most dominant. At still higher intensities, there is photofragmentation, with the evolution of dinmers and other products. These results were all obtained in simulation-. using tight-binding electron-ion dyanamics (TED).
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