Mass spectrometric determination of activity coefficients of dilute aluminium and calcium in liquid silicon and ferrosilicon alloys

The vaporisation of liquid Fe-Si-Al-Ca alloys was studied with a Knudsen-cell mass spectrometric technique. A single graphite effusion cell was used and silicon was chosen as thermodynamic reference. In the studied ranges of temperature [1523-1923K] and composition (wt%Fe 0-55; wt%Al 0-0,5; wt%Ca 0-0,5), activities of Al and Ca were derived from measured Al+, Ca+, Si+ ionic intensities and from Si activity in the melt. The results in terms of activity coefficients at infinite dilution are : gamma(Al)(proportional to) = 0,45 + 10(-1) [T-1687] and ln(gamma(Ca)(proportional to)) = 1,52 - 15634/T in liquid Si and gamma(Al)(proportional to) = 0,45 + 1,03[1-X-si] and ln(gamma(Ca)(proportional to)) = 1,167 - 8,807x(si) in liquid Fe-Si alloys at 1723K (T: degrees K, X-Si : Si molar fraction). A good agreement is found with previous results derived from metal/slag equilibria.