Traian Dumitrica
Department of Mechanical Engineering
University of Minnesota
Abstract：Mechanics at the nanoscale is an interdisciplinary area where the traditional concepts of mechanical engineering run across the fundamentally different aspects of solid state physics. In our studies of nanoparticles and nanotubes, we have encountered situations when this interpenetration gives rise to new and useful phenomena. This talk will concentrate on two such examples:
(i) Recently, our colleagues discovered that although silicon nanoparticles are superhard, they stick to a substrate when colliding at 1-2 km/s. Molecular dynamics simulations explain this puzzling result in a surprising way: Although the contact force is relatively low by macroscopic standards, the impact pressure causes the high speed particle to change its crystalline structure and soak up so much energy that the particle can't bounce away, see figure. This understanding may help researchers who are developing wear-resistant coatings created by many such high-speed impacts.
(ii) The strength of carbon nanotubes has been of great interest but their ideal value has remained elusive experimentally. From microscopic simulations, we were able to identify the specific atomistic mechanisms of relaxation in nanotubes -- the brittle unzipping through a series of lattice-trapped states and the single bond flip-rotations -- and predict their dependence to temporal and thermal conditions. Combination of static barrier computations with the probabilistic approach of transition state theory allowed us to compare the two distinct channels of mechanical relaxation and to obtain the ideal strength of nanotubes as a function of time, symmetry, and temperature. This leads to the construction of yield-strain map for the tubes of different chirality, at various temperatures and different load rate.
The talk will conclude with a brief discussion of carbon nanotubes as torsional springs for NEMS devices, where elasticity concepts combined with a “microscopic effective strain” concept provide the key for understanding the microscopic electromechanical data.
Figure. A nanoparticle containing some 30,000 silicon atoms and moving at 900 meters per second will bounce off a surface (left sequence), but at 2,000 meters per second, it sticks (right sequence). The higher-speed impact causes two sequential changes in the crystalline structure.
Location: 606 conference room
Date and time: August 12th, 2011 10:00 A.M
