Vaporization thermodynamics of the tungsten silicides

Abstract

Vaporization thermodynamics in the tungsten-silicon binary system has been studied by a Knudsen effusion-mass spectrometry technique. The equilibrium vapor pressure of silicon gas over two phase mixtures of WSi 2 + W 5Si 3 and W 5Si 3 + W has been measured over an approximate temperature range 1700–2050 K. Standard enthalpy and entropy changes for the dissociation reactions 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 thirdlaw evaluation of the standard enthalpies of formation of the intermediate compounds. The results obtained are −26.2 kJ (g-atom) −1 for WSi 2 and −17.2 kJ (g-atom) −1 for W 5Si 3. The results obtained in this work are compared with previous estimates and measurements of the thermodynamics of phase formation of the tungsten silicides.