Experimental approach of solid solution boundaries of the TiCxOy oxycarbide

Abstract : The establishment and modeling of the C-O-Ti ternary phase diagram rely on thermodynamic and diagrammatic data. The knowledge of the C-Ti and O-Ti 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 since TiCxOy is reported as a complete solid solution between TiCx and TiOy. In addition, the evolution of the boundaries of this complete solid solution is not well established as a function of the temperature. The two binary phases on which the complete solid solution is based exhibit a wide range of composition and crystallize under the body face centered form, where titanium atoms occupy the first sub-lattice and oxygen atoms can replace carbon atoms in the second sub-lattice within octahedral sites. To determine the boundaries of this large solid solution, two complementary approaches were undertaken during this study. A previous work allowed a better understanding of the reactional mechanisms involved during the carbothermal reduction of anatase [1]. It showed that this reaction operates through solid-gas equilibrium. Actually, a destabilization phenomenon of starting reactants (anatase and carbon black) and the nucleation of a first generation of oxycarbide phase, rich in oxygen, operate during the first step of the reaction. X-Ray diffraction patterns emphasize a second peak shifted toward low angles which can be attributed to a second generation of titanium oxycarbide. The increase of the titanium oxycarbide lattice parameter corresponds to the modification of the stoichiometry in carbon and oxygen of this phase. The observations obtained by Transmission Electron Microscopy have shown that the second oxycarbide grows by the motion of dislocations in the crystal during the last step of the reaction. Based on these results, carbon black and anatase powder were mixed in different proportions in order to check the complete solid solution between TiCx and TiOy. The mixtures were heat treated under flowing Argon in a graphite furnace during a long dwell time to make sure that the reaction is complete. Several temperatures were chosen to check the evolution of the solid solution boundaries. The characterization methods were coupled and compared so as to determine the stoichiometry in carbon and oxygen of synthesized oxycarbides (elemental analysis, wavelength dispersive spectroscopy, Le Bail refinements and nuclear microprobe). The second empirical approach is based on the possible diffusion of oxygen within the face centered lattice of titanium carbide. Sintered by SPS (Spark Plasma Sintering), titanium carbide monoliths and anatase powder are closed in a graphite die and then heated by Hot-Pressing under a load of 50 MPa during variable dwell times in order to observe an interdiffusion phenomenon between these two compounds. During the heating up, a non-reversible allotropic transition of the anatase in rutile and a densification phenomenon which reaches more than 99% before the dwell are evidenced. The characterization of the created interphase was performed by Scanning Electron Microscopy and Transmission Electron Microscopy. In fact, the reaction between TiO2 and TiC first induces the formation of Magneli type sub-oxide phases with TinO2n-1 formulae. The indexation of electron diffraction patterns permitted to identify the more reduced form of the Magneli phase. This compound then reacts with the titanium carbide to form the oxycarbide phase. The description of the interdiffusion phenomenon and the quantification of the stoichiometry (carbon and oxygen content) allowed us to determine the limit composition of the oxycarbide and to compare this composition with the results obtained during the first part of this study. These results will be useful for the optimization of adjustable variables using the CALPHAD method [2] . [1]J. David, G. Trolliard, A. Maître. Transmission electron microscopy study of the reaction mechanisms involved in the carbothermal reduction of anatase. Acta Mater. (2013), 61, 5414–5428. [2]H. L. Lukas, S. G. Fries, B. Sundman. Computational thermodynamics: the CALPHAD method; Cambridge University Press, (2007).
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Contributeur : Olivier Rapaud <>
Soumis le : samedi 4 juillet 2015 - 17:52:19
Dernière modification le : mardi 5 février 2019 - 19:50:04

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Florian Réjasse, Olivier Rapaud, Gilles Trolliard, Alexandre Maitre. Experimental approach of solid solution boundaries of the TiCxOy oxycarbide. ECERS 2015, Jun 2015, Tolède, Spain. ⟨http://www.ecers2015.org/⟩. ⟨hal-01171536⟩

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