Experimental and first-principles study of Ti-C-O system: Interplay of thermodynamic and structural properties

The thermodynamic properties and vacancy formation mechanism of Ti–C–O system were investigated by means of empirical calorimetry method and first-principles calculations. The heat of combustion of titanium oxycarbides (TiC1−xOx, 0≤x≤1) solid solution were first measured by burning the materials in oxygen bomb calorimeter through sophisticated design experimental environment. The mixing enthalpy of the reaction (1−x)TiC + xTiO=TiC1−xOx was further calculated based on the measured data. Mixing enthalpy was also calculated via efficient first-principles method based on the density functional theory. The vacancies in TiC1−xOx were described as two kinds of models that are disordered and segregated vacancies in titanium oxycarbides solid solution. The calculation results with segregation model show good agreement with the calorimetric results. The heat capacity values were further calculated to deduce the mixing and formation Gibbs free energy. The additive law of the heat capacity and entropy means that the structure of titanium oxycarbides tends to be in order, and this result shows a good agreement with the segregated model in TiC1−xOx. These results are explained on the basis of the interplay between thermodynamic and structural properties, which offered the important theoretic foundation for the novel titanium production process.