Form of liquidus curves of binary systems

Summary

• 1. The form of liquidus curves in binary systems of the eutectic type having nondissociating components was investigated by methods of mathematical analysis.

• 2. It was shown that in ideal systems for which ΔCP = 0 the form of the liquidus curves is determined completely by the entropy change in the passage of the component from the crystalline state to the solution.

• 3. For ΔC_P ≠ 0, the form of the liquidus curves of ideal systems depends substantially on the value and sign of the ratio$\frac{{\Delta C_p }}{{\Delta S_i }}$.

• 4. It was shown that linearity of the liquidus curve is not a criterion of nonideality of the system.

• 5. In real systems the form of the liquidus curve depends greatly on the type of departure from ideality and the character of the relation of activity coefficient to temperature and concentration.

• 6. When the departures of mixtures from Raoult's law are not too small, the form of liquidus curves at Ni = 1.0-0.8 is associated unequivocally with the type of departure from Raoult's law, so that for γ_i > 1 the liquidus curves are convex toward the composition axis and for γ_i < 1 they are convex in the direction away from the composition axis. The proportion of exceptions to this rule should not be more than about 5%

• 7. The conclusions concerning the form of liquidus curves of eutectic systems were extended to binary systems with layer separation in the liquid phase ( γ_i > 1) and with compound formation γ_i < 1).

• 8. All the theoretical conclusions were checked against experimental data and were found to be in good agreement with them.