The high temperature vaporization and thermodynamics of the titanium oxides. XI. Stoichiometric titanium monoxide

Vaporization measurements by the Knudsen technique and by mass spectrometry have been performed on the incongruently vaporizing titanium monoxide at the stoichiometric composition. Over this sample at 1806 K Ti(g) has a higher pressure than TiO(g) by a factor of about 3.5. The free energy difference between TiO(g) and TiO(s) is the first primary result, and its value at 1806 K of 63.3±2.0 kcal/mole is independent of auxiliary data. The second primary result is the activity of Ti, 0.13±0.04, in nearly stoichiometric TiO at the same temperature. Over the temperature range 1694–1919 K, the pressures of the various species are given by the equation lnP=−A/RT+B/R, and the values of the parameters A in kcal/mole and B in e.u. are for Ti 116.3±3.2 and 31.8±1.8; TiO, 130.2±3.0 and 37.0±2.0; O, 175.6±4.9 and 34.0±2.7; O2, 229.4±9.8 and 36.2±5.4, respectively. The second-law enthalpy and entropy of vaporization at 298 K of TiO to form TiO(g) are 138.2±3.0 kcal/mole and 45.0±2.0 e.u., respectively. The third-law value is 141.2±0.1 kcal/mole. The results corroborate the values given in the 1975 Supplement to the JANAF Tables, but show that previously reported values of the O2 potential are too high by a factor of about 43.