Vaporization Thermodynamics in the Vanadium-Rich Portion of the Vanadium-Silicon System by High Temperature Knudsen Cell Mass Spectrometry

摘要

Vaporization behavior and phase equilibria in the vanadium-rich portion of the vanadium-silicon system were investigated by high temperature Knudsen cell mass spectrometry. At about 1900K and below the V3Si single-phase region was found to include congruently vaporizing compositions, and at about 200 0K and above V5Si3 was found to be congruently vaporizing. In an intermediate temperature range both the V3Si and V5Si3 regions will contain congruently vaporizing compositions, and there will exist a local maximum in the total vapor pressure in the solid two-phase region; the term euatmotic point is suggested for this maximum. The V3Si single-phase region was found to extend from 17.5 to 25.5 atomic percent silicon at 1800K. Thermodynamic activities leading to free energies of phase formation were measured by direct comparison with the solid elements. Enthalpies of phase formation at 298.15K, calculated from the experimental data in combination with published heat capacity and entropy data, are: 1/4 V3Si, -5.37 kK; 1/8 V5Si3, -6.33 kK.