Solution Thermodynamics and Aseotropic Mixtures in the Melts of Lead-Calcium System

The partial and integral thermodynamic functions of mixing and evaporation for the melts of Pb-Ca system have been calculated from the vapor pressure data determined by boiling point method with involvement of published data. The formation of alloys containing <66.67 at % Ca is endothermic and accompanied by disorder growth, but it is exothermic at >66.67 at % Ca and leads to ordering in the system. Melting enthalpy of Ca2Pb of 13 +/- 1.6 kJ/mol was found from the difference in sublimation and evaporation enthalpies. Based on the data on vapor pressure for the components, Pb-Ca constitution diagram has been supplemented by the fields of liquid and vapor coexistence under atmospheric pressure and vacuum of 1.33 and 0.7 kPa. The existence of two azeotropic mixtures with calcium concentration of 37.6 and 81.3 at % and boiling points of 2217 and 1635 degrees C (2490 and 1908 K), respectively, has been revealed. When pressure (boiling point) decreases, the composition of azeotropic mixtures change according to Vrevskii laws. The position of field borders of vapor-liquid equilibrium indicates the impossibility of separation of Pb-Ca system into metals by distillation.