Synergistic Use of Thermogravimetric and Electrochemical Techniques for Thermodynamic Study of TiOx (1.67≤x≤2.0) at 1573 K

A thermodynamic study of the Ti–O system at 1573 K has been conducted using a combination of thermogravimetric and emf techniques. The results indicate that the variation of oxygen potential with the nonstoichiometric parameter δ in stability domain of TiO2−δ with rutile structure can be represented by the relation, ΔμO2=−6RTlnδ−711970(±1600) J/mol. The corresponding relation between non-stoichiometric parameter δ and partial pressure of oxygen across the whole stability range of TiO2−δ at 1573 K is δ ∝ PO2−1⁄6. It is therefore evident that the oxygen deficient behavior of nonstoichiometric TiO2−δ is dominated by the presence of doubly charged oxygen vacancies and free electrons. The high-precision measurements enabled the resolution of oxygen potential steps corresponding to the different Magneli phases (TinO2n−1) up to n=15. Beyond this value of n, the oxygen potential steps were too small to be resolved. Based on composition of the Magneli phase in equilibrium with TiO2−δ, the maximum value of n is estimated to be 28. The chemical potential of titanium was derived as a function of composition using the Gibbs-Duhem relation. Gibbs energies of formation of the Magneli phases were derived from the chemical potentials of oxygen and titanium. The values of −2441.8(±5.8) kJ/mol for Ti4O7 and −1775.4(±4.3) kJ/mol for Ti3O5 obtained in this study refine values of −2436.2(±26.1) kJ/mol and −1771.3(±6.9) kJ/mol, respectively, given in the JANAF thermochemical tables.