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Proposition et modélisation ab initio de nouveaux matériaux ultra-durs dans le ternaire B C N.

Abstract : Due to the technological importance behind the possibility to discover novel classes of hard materials an enormous research effort has been directed during the last decades towards the synthesis and characterisation of promising carbon-based compounds such as carbon nitrides and boron carbon nitrides. However, despite many attempts of synthesis and the indisputable progresses made in the field, amorphous samples with unclear crystallographic data have been often obtained in many research laboratories. In particular, several problems arise from the fact that most of the samples are of polymorphic nature, thus leading to a difficult and uncertain spectroscopic characterisation. A general understanding of the relations between composition and the electronic structure properties has therefore been provided theoretically in this Thesis to get further insight into the characteristics of pure crystalline forms. As one might expect this work has suddenly been turned out into a complicate and challenging task because of the lack of reliable experimental crystal structures to be used as references for the computational inputs. Therefore it became essential to propose hypothetical bi- and three-dimensional model phases to obtain trends on the relative stability, electronic and mechanical properties of carbon and boron carbon-nitrides. So far as that is concerned, a systematic study of pure crystalline CNx (where x=0.36 and 1.33) and BC2N systems has been proposed as an important complement to the experimental knowledge. Thanks to the progress in modern computer technology it has also been possible to compute such an investigation via ab-initio (first-principles) methods by testing and probing different solid state calculational approaches. In fact, one of the first objectives of this project has been the search of a valid computational density-functional-based scheme able to reproduce and/or predict the hardness and stability of a wide variety of ultra-hard materials.Calculations of the cohesive properties and standard enthalpies of formation have been carried out to address the thermodynamic stability of different isoelectronic compositions, namely C3N4, C11N4 and BC2N. The hardness has also been studied by means of the analysis of the calculated elastic and bulk moduli. The investigation of the electronic properties has been achieved with the calculation of the density of states, band structure, electron density maps and crystal orbital overlap population analysis. For some of the studied molecular clusters, the 13C NMR shifts have been evaluated to provide a spectroscopic discrimination between systems with very similar structural characteristics. This is the case of the hexagonal and orthorhombic models of the graphitic-like C3N4 form. Finally, the determination of the electron-energy loss near edge structures of C, B and N K ionisation edges has been computed in order to provide reference spectra of pure crystalline materials, likely to allow a discrimination of polymorphic samples. Results are presented to demonstrate that carbon nitrides are ultra-hard systems with outstanding mechanical properties. In particular, the carbon rich composition, C11N4, has shown larger cohesive energies and it is generally stiffer than the electronic analogue C3N4. However, the possibility to deposit single phase samples should be highly hampered in both stoichiometries by their large positive enthalpies of formation. The introduction of boron atoms (boron carbon nitrides) has displayed a slight decreasing in the magnitudes of the elastic and bulk moduli, though the calculated values are still higher than that of cubic boron nitride (i.e. the second hardest known material). Nevertheless, three-dimensional BC2N phases have also shown exothermic enthalpies of formation which point to an easier deposition of the "BCN" materials with respect to carbon nitrides. Therefore, by considering the whole set of the investigated model phases, sp3-bonded boron carbon nitrides result as the best candidates for novel ultra-hard materials which could, in principle, be synthesised with the actual techniques. Very recent experimental results seem to support this general tendency.
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Submitted on : Tuesday, July 22, 2003 - 3:32:49 PM
Last modification on : Thursday, January 11, 2018 - 6:15:07 AM
Long-term archiving on: : Tuesday, September 11, 2012 - 9:10:51 PM

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Maurizio Mattesini. Proposition et modélisation ab initio de nouveaux matériaux ultra-durs dans le ternaire B C N.. Other. Université Sciences et Technologies - Bordeaux I, 2001. English. ⟨tel-00003143⟩

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