Thermodynamics of the Phase Formation of the Titanium Silicides

Vaporization thermodynamics in the titanium−silicon binary system has been studied by a Knudsen effusion−mass spectrometer technique. The equilibrium vapor pressure of silicon gas over two-phase mixtures of TiSi2 + TiSi, TiSi + Ti5Si4, and Ti5Si4 + Ti5Si3 has been measured over a temperature range of ca. 1700−2050 K, and the vapor pressure of titanium gas was measured over a two-phase mixture of Ti5Si4 + Ti5Si3. Standard enthalpy and entropy changes for the appropriate dissociation reactions, within the temperature ranges of the experiments, were determined from the temperature dependence of the measured vapor pressures. Estimated thermal functions were combined with the vapor pressure data and used in a third-law evaluation of the standard enthalpies of formation at 298 K of the intermediate compounds. The results obtained are 1/3TiSi2, −53.5 ± 5 kJ; 1/2TiSi, −71.5 ± 5 kJ; 1/9Ti5Si4, −75.9 ± 5 kJ; 1/8Ti5Si3, −78.1 ± 5 kJ. The results obtained in this work are compared with previous estimates and measurements of the thermodynamics of phase formation of the titanium silicides.