Nguyen Duc Trung Kien

Monte Carlo Study of Room-Temperature Ferromagnetism in C-Doped ZnO

The room-temperature ferromagnetism of diluted magnetic semiconductors (DMS) doped with light elements has been extensively studied for potential applications in spintronic devices. The existence and stability of ferromagnetism of DMS with low concentration regime is an interesting topic, followed by researches using both Monte Carlo (MC) simulation and first-principle calculations. Both continuous and discrete spin models were developed to describe the ferromagnetic phase but it is still unclear, which one produces a better outcome. To clarify the issue of a continuous spin model versus a discrete spin model, we have performed a large-scale MC simulation of C-doped ZnO system. We found that the discrete spin model exhibit magnetic phase transition at near room-temperature while the continuous spin model does not show phase transition in the investigated low temperature range.

Polarization versus Mori-Tanaka approximations for effective conductivity of isotropic composites

Our polarization approximations for the effective conductivity of isotropic multicomponent materials, constructed recently as approximate solutions to the minimum energy principles, are compared with the widely used Mori-Tanaka approximation, derived as an approximate solution of the field equations. The similarities and differences, advantages and disadvantages of both approaches are analysed with illustrating numerical examples.

Fast Fourier Transformation method for computing the permeability of periodic porous media

A Fourier-based method is adopted to determine the permeability of periodic porous media made up of a rigid skeleton saturated by viscous fluid. The flow, induced by a prescribed macroscopic gradient of pressure, adopts the Stokes equations with incorporating a condition of adherence at the surface of the solid. The permeability is determined by solving a linear problem on a unit cell for which we determine the local velocity fields due to a prescribed gradient of pressure. The method uses the Fourier Transformation and exact expressions of the periodic Green tensor in the Fourier space. It is shown that the resolution of the problem requires an introducing of undetermined forces acting within the solid phase.

Equivalent Inclusion Approach and Approximations for Thermal Conductivity of Composites with Fibrous Fillers

Based on the polarization approximations, the expression for the thermal conductivity of composites with randomly oriented inclusions of fiber forms is firstly derived. Equivalent inclusion approach is then developed to account for possible diversions such as non-idealistic geometric forms of the inhomogeneities, or the fact that the conductivity of the fibers is unknown, using reference conductivity data. Applications involving experimental data from the literature show the usefulness of the approach.