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Conception in silico d'une nouvelle phase de carbure de bore

Abstract : The goal of this work is to reinforce the boron carbide B 4 C in its use for shielding. This thesis has been devoted to the theoretical study of new boron carbides using ab initio methods based on the density functional theory (DFT) using the local density approximation (LDA) and generalized gradient approximation (GGA). These have first been validated by comparing the calculated results with the experimental data on the boron carbide B 4 C. They were then used to design new materials. The idea was to reinforce C-B-C chains with respect to vacancies formation. In the new material, (B 11 C p )C-C, chains are C-C. With the help of Wade’s ionic model, I have also found that increasing the carbon concentration in the icosaedra, (B 10 C 2 )C-C, forms another metastable phase. The concentration in the latter phase, 28.6% carbon concentra- tion, is however outside the domain of concentration were icosahedral phases are known to exist. Once the structures were designed, the second step was to chek their (meta)stability. This required the calculation of their formation energies, their elastic constants and of phonons dispersion curves. The third step was to check mechanical strengthening. To this end, I have shown that these materials are less subject to vacancy formation, and that vacancies can withstand a huge hydostatic pressure without a discontinuous change of volume, at variance with vacancies in B 4 C. The calculated elastic properties showed a large increase in the bulk modulus, Young modulus and shear modulus. I have shown that different methods can be used to distinguish materials with C-C chains, especially (B 11 C p )C-C, from B 4 C. These methods are the X-ray diffraction, Raman and infrared vibrational spectroscopy. In particular, I have identified all of the modes of (B 11 C p )C-C by projecting their eigenvectors onto those of B 4 C and I have calculated their Raman scattering spectrum in a polycrystalline average. I have also studied the Raman spectrum of B 4 C under pressure, and the results are in excellent agreement with experiment. This enables me to confirm the mode identifications made at ambiant pressure and to understand the difference between calcultions in a monoclinic symmetry and the experimental data which exhibit a rhombohedral average. In order to provide a method of synthesis, I then studied the phase transition that can occur under pressure between the material (B 11 C p )CC ( represents a vacancy) and the material (B 11 C p )C-C. A synthesis method has been elaborated in order to synthesize this last material. This method is magnesiothermic reaction under high pressure.
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Contributor : Antoine Jay Connect in order to contact the contributor
Submitted on : Friday, April 22, 2016 - 4:06:51 PM
Last modification on : Tuesday, July 6, 2021 - 3:19:19 AM
Long-term archiving on: : Saturday, July 23, 2016 - 1:32:08 PM


  • HAL Id : tel-01199235, version 2


Antoine Jay. Conception in silico d'une nouvelle phase de carbure de bore. Physique [physics]. Ecole Doctorale Polytechnique, 2015. Français. ⟨tel-01199235v2⟩



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