On Activities of Coexisting Elements in Molten Iron. II : The Activity of Silicon in Molten Fe-Si System

Constructing the following electrode concentration cell, the authors measured electromotive force corresponding to the change of silicon content in iron by potentiometer. Fe-Si/silicate slag/pure Si The temperature range of the experiment was 1,520～1,540℃ and for the measurement of temperature Pt-Pt-Ph thermocouple was used. As preliminary experiments, the reversibility and thermoelectromotive force were investigated. The theoretical relation between electromotive force E and the activity of silicon a_ is as follows : E=RT/nF×lna^i_/a_-RT/nF×ln a^i_/a\u27_……(1) where a^i_ : the activity of silicon ion in the molten slag. a\u27_ : the activity of pure silicon. n : the number of Faradays required for the cell reaction. Because pure silicon is selected as a standard state, then the following equation is obtained from eqn. (1) : E≒- 0.0002T/n×log a_ ……(2) n is approximately determined as n=4 by the calculation from authors\u27 data. The activity of silicon has been obtained by substituting the values E, n and T in eqn. (2) Judging from this result, Fe-Si binary solution is recognized to be semi-regular solution and obeys Henry\u27s law up to about N_=0.08, where N_ represents the mole fraction of silicon in Fe-Si binary solution. Furthermore, the free energy change ΔG^0_ of the following reaction at 1,530℃ has been calculated from authors\u27 data, Si (liquid)=Si (in Fe)……(3) ΔG^0_ represents the free energy of solution of silicon at unit activity on a scale in which the activity is equal to percentage at infinite dilution. To convert this to activity on a mole fraction scale which the activity of pure liquid silicon is unity requires the following eqn. (4) : ΔG^0_=μ^0_ (dil. soln.)-μ^0_ (pure)=RTlna_ (dil. soln.)/a_ (pure)……(4) From the relation between loga_/N_=γ_ and square of the mole fraction of iron N^2Fe, activity coefficient γ^0_ which represents r_ at N_=0 is obtained as 0.013 and the mole fraction of silicon in dilute solutions is 0.0199 times its weight percentage. From these two values, the ratio of the activities of silicon on the two scales is obtained as 0.00026. Substituting this value to eqn. (4), authors have been obtained the following value : ΔG^0_=-29,700 cal