Synergistic use of thermogravimetric and electrochemical techniques for thermodynamic study of TiO_x (1.67 ≤x≤ 2.0) at 1573 K

Abstract

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 TiO_2-δ with rutile structure can be represented by the relation, ΔµO₂=-6RT ln δ -711970(± 1600) J/mol. The corresponding relation between non-stoichiometric parameter δ and partial pressure of oxygen across the whole stability range of TiO_2-δ at 1573 K is δ ∝ PO₂^-1/6. It is therefore evident that the oxygen deficient behavior of nonstoichiometric TiO_2-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 (TinO_2n-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 TiO_2-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.