Experimental investigation and thermodynamic modeling of the C-Me-O systems (where Me = Hf, Zr)
Résumé
The establishment and modeling of the C-Me-O ternary phase diagram rely on thermodynamic and diagrammatic data. The knowledge of binary phase diagrams, described independently, is required. A comprehensive thermodynamic description of these binary systems is needed in order to establish the associated ternary system. The ternary solid solutions are modeled using specific models and interaction parameters assessed through experiments and available data from the literature. Starting from already published thermodynamic descriptions of the different phases and models, the ternary system could not be satisfactorily described using conventional models because the zirconium and hafnium oxycarbides are reported as partial solid solutions. In addition, the evolution of the boundaries of these solids solutions is not well established as a function of the temperature.
Based on previous works which allowed the understanding of the reactional mechanisms involved during the carbothermal reduction of group IVb transition metal dioxides [1, 2], carbon black and dioxide powders were mixed in different proportions to synthesize oxycarbides. Several high temperatures were chosen to check the evolution of the solid solutions boundaries. The characterization methods were coupled and compared so as to accurately determine the stoichiometry in carbon and oxygen of synthesized oxycarbides.
These results have been used for the optimization of adjustable variables using the CALPHAD method in order to assess the ternary phases diagrams C-Me-O.
[1] J. David, G. Trolliard, A. Maître, Acta Mater. 61 (2013) 5414–5428.
[2] F. Réjasse, G. Trolliard, O. Rapaud, A. Maître, J. David, RSC Adv. 5 (2015) 45341–45350.
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