Papers by Keyword: Ab Initio

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Authors: Giovanni Alfieri, Tsunenobu Kimoto
Abstract: An annealing study, in the 100-1400 C temperature range ,was carried out on Cl-implanted n- or p-type 4H-SiC epilayers. The electrical characterization of the epilayers shows the rise of several deep levels and the role of Cl, on both carrier concentration and defects' microscopic structure, is discussed in the light of theoretical results obtained by density functional calculations performed on a 64-atom cubic SiC supercell.
Authors: Doris Vogtenhuber, Jana Houserova, Walter Wolf, Raimund Podloucky, Wolfgang Pfeiler, Wolfgang Püschl
Authors: T.B. Boo, H.H. Kwong, Yuan Ping Feng
Authors: Sara Chentouf, Jean Marc Raulot, Hafid Aourag, Thierry Grosdidier
Abstract: The formation energies of the T.M impurities Ti and Zr were calculated using DFT calculations at absolute zero and ab initio MD simulations at 300 K. We found that, with increasing temperature, Zr impurities become more stable and prefer to segregate at the interface of ∑5 (310)[001] grain boundary. In the case of Ti, the results show that it remains a stable defect when temperature increases.
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.
Authors: Vasilii Gusakov
Abstract: In the framework of a unified approach the diffusion coefficient (the prefactor and activation barrier) of an interstitial oxygen Oi, the hydrogen molecule H2, vacancy, oxygen dimer in silicon crystals and Oi in Si1-xGex solid solutions, silicon nanotubes and nanowires has been calculated. For all the above cases, the calculated values of the diffusion coefficient are in good agreement with the experimental data. The calculated equilibrium structures, electrical activity, the vibrational spectrum, the mechanism of diffusion of oxygen dimer fully describe the experimental results. Our study has revealed that the diffusivity of impurities (defects) in alloys can decrease considerably and this variation results from the fact that the prefactor depends on the concentration of component elements of the alloy.
Authors: Claude Mijoule, El Hocine Megchiche, Eric Andrieu, Daniel Monceau
Abstract: The insertion and diffusion energies of oxygen in presence of vacancies in nickel are studied by using the first-principle projector augmented waves (PAW). When the oxygen atom is located in a substitution site, the formation of a vacancy-oxygen pair is observed. Furthermore, we show that the formation of divacancies allows the oxygen atom to migrate more easily in the metal. A model for the migration process of the three-defect system is proposed. Finally, thermal expansion effects are included in our study; it is shown that temperature effects emphasize the diffusion.
Authors: Y. Inoue, Tokuteru Uesugi, Yorinobu Takigawa, Kenji Higashi
Abstract: The grain boundary structure and its energy are necessary for the fundamental understanding of the physical properties of materials. In aluminum, three distinct atomic structures of a Σ9(221)[110] tilt grain boundary have been reported in previous studies using atomistic simulations and a high-resolution transmission electron microscopy (HRTEM). In this work, we studied the atomic structure and energy of the Σ9 tilt grain boundary in aluminum using first-principles calculations. A comparison of the grain boundary energies among the three distinct Σ9 tilt grain boundaries determined through first-principles calculations allowed us to identify the most stable atomic structure of Σ9 tilt grain boundary in aluminum.
Authors: Yusuke Kinoshita, Yoshitaka Umeno, Takayuki Kitamura
Abstract: Using the first-principles calculation, the elastic constant C44 of Ag/Al multilayers with different modulation periods from 0.43 nm to 2.27 nm has been evaluated in order to examine the effect of atomic and electronic structures on it. With increasing modulation period, C44 decreases and becomes close to that obtained by the conventional mixing rule, however, the difference of 8 % still remains at the modulation period of 2.27 nm. As C44 correlates with the average interplanar spacing, the decrease of C44 can be explained by the decrease of the charge density in the stacking direction due to the increase of the average interplanar spacing. The difference in the electronic structure is included in the effect of atomic structure.
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