Theory, Modeling and Numerical Simulation

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Authors: Micha Sinder, Z. Burshtein, Joshua Pelleg
Abstract: In their paper, R. Merkle et al [R. Merkle, J. Maier, K.D. Becker and M. Kreye, Phys. Chem. Chem. Phys. 6, 3633 (2004)] conducted an experimental study on the chemical diffusion of oxygen in Fe-doped SrTiO3 single crystals driven by large changes in the oxygen ambient partial pressure. The stoichiometry dependence of the chemical diffusion coefficient was derived on the basis of the concept of conservative ensembles for two independent trapping reactions, which then served for calculating the evolution of vacancy profiles. The theoretical predictions were compared to the experimental results. In the framework of the same model, in the present communication, the chemical diffusion of oxygen was analyzed by the concept of a dynamic reaction front [M. Sinder, J. Pelleg, Phys. Rev. E 61, 4935 (2000); Z. Koza, Phys. Rev. E 66, 011103 (2002)]. We show, that by using a quasi-chemical reaction rate profile, it is possible to obtain information relating to the position and width of the zone where the reaction takes place. It is indicated, that the reaction rate distribution can be directly calculated from measured concentration profiles of the immobile reactant.
Authors: H.J. Dai, H.B. Dong, H.V. Atkinson, Peter D. Lee
Abstract: A coupled cellular automaton-finite difference (CA-FD) model is used to simulate the detailed dendritic structure evolution of the columnar-to-equiaxed transition (CET) for Al-Cu alloys during solidification. The effects of material properties (nucleation undercooling, density of nuclei in bulk liquid and alloy solidification range) on the CET are investigated. Simulated results reveal that: (1) equiaxed grains form at an earlier stage with a smaller critical nucleation undercooling; (2) CET is promoted if the density of nuclei in bulk liquid is increased; (3) extending the alloy solidification range promotes the CET. Finally, CET maps corresponding to different alloy concentrations are constructed, illustrating the relationship between processing conditions and the resulting grain structures for alloys with different solidification ranges.
Authors: Nasser Mohieddin Abukhdeir, Alejandro D. Rey
Abstract: A Landau-de Gennes type model for the direct isotropic/smectic A phase transition is used to study surface-enhanced smectic ordering in the stable smectic temperature regime. A unified surface-free energy functional is proposed which can be utilized for homeotropic and planar surface anchoring. The time-dependent complex Landau-Ginzburg evolution equations and boundary conditions are derived for thin-film geometry. Simulation results are presented for the two types of anchoring and compared to observations from experiments and previous simulations. Simple visualization software for smectic layering was developed and is also presented that is compatible with discretized numerical solutions of the model.
Authors: Masahiko Matsubara, Massimo Celino, Philip S. Salmon, Carlo Massobrio
Abstract: We describe two examples of application focusing on first-principles molecular dynamics as an effective tool to unravel the atomic-scale structure of condensed-matter systems. The first application is on disordered network-forming materials and the second is on silicon-doped fullerenes. We show that an accurate modelling of interatomic forces based on density functional theory, when combined with an account of the temperature evolution, is an unavoidable prerequisite for analyzing and interpreting experimental results on a quantitative basis. In the case of disordered systems, we describe the basic structural features of amorphous GeSe4 and highlight the predominant chemical order in this system. The effect of adding or removing an electron charge on the stability of Si-doped fullerenes is exemplified by considering the finite temperature evolution of heterofullerenes.
Authors: Tarik Omer Ogurtani, Oncu Akyildiz
Abstract: The morphological evolution of intragranular voids induced by the surface drift-diffusion under the action of capillary forces, electromigration (EM) forces, and thermal stress gradients (TSG) associated with steady state heat flow is investigated in passivated metallic thin films via computer simulation using the front-tracking method. As far as the device reliability is concerned, the most critical configuration for interconnect failure occurs even when thermal stresses are low if the normalized ratio of interconnect width to void radius is less than certain range of values (which indicates the onset of heat flux crowding). This regime manifests itself by the formation of two symmetrically disposed finger shape extrusions (pitchfork shape slits) on the upper and lower shoulders of the void surface on the windward side. The void growth (associated with supersaturated vacancy condensation) on the other hand inhibits anode displacement but enhances cathode and shoulder slit velocities drastically, which causes lateral spreading.
Authors: Chu Chun Fu, Estelle Meslin, Alain Barbu, F. Willaime, V. Oison
Abstract: Carbon atoms are always present in Fe-based materials, either as impurities even in high purity samples or as an intrinsic constituent in steels. Density Functional Theory calculations have been performed to study the interaction between C atoms and vacancies (V) in α-Fe. We find that the formation of VCn complexes is energetically favourable for n ≤ 3, with VC2 being the most stable one. The energy gain corresponding to the clustering reaction VCn-1 + C → VCn depends mainly on the strength of C-C covalent bonds. The vacancy diffusivity is shown to be significantly modified by the formation of vacancy-carbon complexes, exhibiting non-Arrhenius behaviour. Effective vacancy diffusion coefficients in α-Fe are calculated as a function of temperature and carbon content using a simplified thermodynamic model. The results are discussed in detail in the limiting case of excess of C with respect to vacancies.

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