Thermodynamic Properties of a Non-IdealHe3-He4Solution Model

Deviations from ideality for ${\mathrm{He}}^3$-${\mathrm{He}}^4$ solutions are calculated on the basis of a simple real solution model. Expressions for equilibrium total pressure are derived for situations with and without superfluid ${\mathrm{He}}^4$. Effects of non-ideality on previously reported calculations of lambda-point dependence on ${\mathrm{He}}^3$ concentration are examined, and an equation representing the lambda-points of concentrated as well as dilute ${\mathrm{He}}^3$ solutions is given. The results are used in the comparison of various bases for treatment of the lambda-point shift. Assumption that the partial molal enthalpy of superfluid vanishes gives more satisfactory results than assumption of zero partial molal entropy of superfluid.