Thermodynamics stabilities of intermediate phases in the Ca-Si system

Vaporization thermodynamics in the binary system calcium–silicon has been studied by Knudsen effusion-mass spectrometry and vacuum microbalance techniques. 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 enthalpy changes for the appropriate vaporization reactions were determined from the temperature dependence of the measured vapor pressures. The standard reaction enthalpy changes were also evaluated by the third-law method using 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 obtained for ΔfH°298 were the following: Ca2Si=−56.1±3.1, Ca5Si3=−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.