Nonequilibrium molecular dynamics for bulk materials and nanostructures

We describe a method of constructing exact solutions of the equations of molecular dynamics in non-equilibrium settings. These solutions correspond to some viscometric flows, and to certain analogs of viscometric flows for fibers and membranes that have one or more dimensions of atomic scale. This work generalizes the method of objective molecular dynamics (OMD) (Dumitric? and James, 2007). It allows us to calculate viscometric properties from a molecular-level simulation in the absence of a constitutive equation, and to relate viscometric properties directly to molecular properties. The form of the solutions is partly independent of the form of the force laws between atoms, and therefore these solutions have implications for coarse-grained theories. We show that there is an exact reduction of the Boltzmann equation corresponding to one family of OMD solutions. This reduction includes most known exact solutions of the equations of the moments for special kinds of molecules and gives the form of the molecular density function corresponding to such flows. This and other consequences leads us to propose an addition to the principle of material frame indifference, a cornerstone of nonlinear continuum mechanics. The method is applied to the failure of carbon nanotubes at an imposed strain rate, using the Tersoff potential for carbon. A large set of simulations with various strain rates, initial conditions and two choices of fundamental domain (unit cell) give the following unexpected results: Stone-Wales defects play no role in the failure (though Stone-Wales partials are sometimes seen just prior to failure), a variety of failure mechanisms is observed, and most simulations give a strain at failure of 15-20%, except those done with initial temperature above about 1200 K and at the lower strain rates. The latter have a strain at failure of 1-2%.     

Morphology Development in Polymer Fibers undergoing Solvent/Non-solvent Exchange

Summary: The solvent/non-solvent interchange across the fiber surface affects the morphology of the fiber in various ways. In this paper, simulations have been performed to elucidate the diverse morphologies obtained during spinning of polymer fibers under the presence of a non-solvent. The proposed model deals with a ternary system derived from Cahn-Hilliard equation, alternatively known as Time Dependent Ginzburg Landau-TDGL model B equation, involving the spatio-temporal evolution of concentration order parameter. Depending on the coexistence region of the ternary phase diagram, various fiber morphologies including concentric bands, internal microfibrillar structures, and porous structures were discerned. It may be inferred that the formation of the aforementioned diverse morphologies is a direct consequence of the initial conditions of the starting mixtures in a manner governed by the relative rates of solvent/non-solvent exchange and the dynamics of phase separation.     

X-ray spectroscopic study of the electronic structure of visible-light responsive N-, C- and S-doped TiO2

The electronic origins of the visible-light response of N-, C- and S-doped TiO₂ have been studied using X-ray absorption, X-ray emission, and X-ray photoelectron spectroscopies. New electronic states are observed in the bulk band gap, above the valence band edge of pure TiO₂, which can be directly related to the visible-light absorption of the N-, C- and S-doped TiO₂ materials.     

AC susceptibility study in the single-phase Bi-2223 system

We have found that the composition, Bi_1.2Pb_0.33Sr_1.54Ca_2.06Cu₃O_y which is comparatively poorer in Bi and Sr with respect to 2:2:2:3, a single-phase 110 K superconductor can be obtained. Such polycrystalline samples have been synthesized by two different methods. The synthesized samples have been characterized by their X-ray diffraction patterns and temperature dependence of DC electrical resistivity and AC susceptibility down to 20 K have been carried out. The samples have been identified as having single phase with a sharp transition. The results of their AC susceptibility have been analysed using Clems critical state model. The effective volume fraction of the grains and the temperature dependence of the intergranular current density have been estimated.