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 indicat e that the variation of oxygen potential with the nonstoichiometric paramet er delta in stability domain of TiO2-delta with rutile structure can be rep resented by the relation, Delta mu o(2) = -6RT In delta - 711970(+/-1600) J /mol. The corresponding relation between non-stoichiometric parameter delta and partial pressure of oxygen across the whole stability range of TiO2-de lta at 1573 K is delta proportional to P-O2(-1/6). It is therefore evident that the oxygen deficient behavior of nonstoichiometric TiO2-delta is domin ated 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 (Ti-n O2n-1) up to n = 15. Beyond this value of n, the oxygen potential steps were too small to b e resolved. Based on composition of the Magneli phase in equilibrium with T iO2-delta, the maximum value of n is estimated to be 28. The chemical poten tial of titanium was derived as a function of composition using the Gibbs-D uhem relation. Gibbs energies of formation of the Magneli phases were deriv ed 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 i n this study refine values of -2436.2(+/-26.1) kJ/mol and -1771.3(+/-6.9) k J/mol, respectively, given in the JANAF thermochemical tables.