Thermodynamic stabilities of intermediate phases in the Ca-Si system

Vaporization thermodynamics in the binary system calcium-silicon has been s tudied by Knudsen effusion-mass spectrometry and vacuum microbalance techni ques. The equilibrium partial pressure of Ca(g) over the two-phase regions in the composition range 20-75at.% Si has been measured and the standard en thalpy changes for the appropriate vaporization reactions were determined f rom the temperature dependence of the measured vapor pressures. The standar d reaction enthalpy changes were also evaluated by the third-law method usi ng the pressure data in conjunction with estimated Gibbs energy functions. Standard enthalpies of formation of the calcium silicides were derived from the standard reaction enthalpy values at room temperature. The results obt ained for Delta H-f degrees (298) were the following: Ca2Si=-56 +/-3.1, Ca5 Si3=-55.3 +/-3.5, CaSi=-49.6 +/-2.2, Ca3Si4=-40.6 +/-1.5, Ca14Si19=-44.4 +/ -2.3, CaSi2= -37.8 +/-1.6 all in kJ/mol atoms. The results for Ca2Si, CaSi and CaSi2 may be compared with previous measurements, all other results are first determinations. (C) 2001 Elsevier Science B.V. All rights reserved.