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Sur l'élaboration de nouveaux carbonitrures : des variétés graphitiques aux structures peu compressibles

Abstract : In Materials Chemistry, the optimal performance research in condensed matter has always been a challenge. At the begin of the 90's LIU and COHEN have been proposed an empirical model by ab-initio method in the aim to calculate the bulk modulus in covalent materials and a new material called C3N4-β isostructural of Si3N4-βwhich would have mechanical properties similar to those of diamond. From this time, other polymorphs (four in 3D and one in 2D) were predicted for this carbon nitride with C3N4 composition. In the first step, our approach consisted to synthesize one bidimensionnal carbon nitride in order to convert it in second step to a dense phase by analogy with the diamond synthesis. For it, two processes could be used: the direct conversion (HPHT process) or the flux assisted conversion. The 2D carbon nitride was elaborated by thermal decomposition of the thiosemicarbazide under nitrogen flux. The characterization by different physico-chemical methods and several tests to modify its composition and its crystallinity led to underline a relation between the composition and the crystallinity. Thus, it was see the presence through this 2D phase of some amorphous nanoparticles into which, it was noticed the existence of some crystallized-nanodomains with cubic symmetry and composed only by nitrogen and carbon. The bulk modulus of this phase was measured thanks to the 3rd Birch-Murnaghan equation and it value has been shown equal to 76% of diamond one's and 96% of cubic boron nitride one's (second hardest material). Thereby, this study permitted to elaborate one 3D carbon nitride polymorph at ambient pressure and at moderate temperature and to confirm the low compressibility of 3D carbon nitrides.
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Denis Foy. Sur l'élaboration de nouveaux carbonitrures : des variétés graphitiques aux structures peu compressibles. Matériaux. Université Sciences et Technologies - Bordeaux I, 2007. Français. ⟨NNT : 2007BOR13499⟩. ⟨tel-00374077⟩

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