Thermodynamics of Evaporation of Liquid Magnesium - Tin Alloys

Based on the values of the partial pressures of magnesium above dimagnesium stannide and melts with tin, determined by the boiling point method (isobaric and isothermal variants, respectively) and tin, calculated by numerical integration of the Gibbs - Duhem equation in accordance with known expressions, the thermodynamic functions were determined: changes in entropy, enthalpy, and free energy of evaporation. Methods to determine the vapor pressure of isobaric and isothermal variants of the boiling point method and calculate thermodynamic values are described. The dependences of the values of partial vapor pressure of magnesium and tin were determined, based on which the energy functions were determined. The measurement error was 7.07%. Data on the change in evaporation entropies are presented graphically. An increase in the partial entropies of evaporation of magnesium and tin was noted with a decrease in their content in the alloy (each) to less than 20 at. %. Extremes are noted: a maximum for magnesium and a minimum for tin at a concentration corresponding to the stoichiometric composition of dimagnesium stannide (60 at. % Мg). The latter indicates the presence of a dissociating compound in the liquid phase that affects evaporation. The values of the change in enthalpy and Gibbs free energy of evaporation are tabulated. It was established that the values of partial and integral enthalpies and Gibbs free energy of vaporization monotonically increase from Mg to Sn in accordance with second-degree dependencies on the concentration of components in the alloy and linearly (Gibbs energy) with temperature. The very small change in the integral value of the free energy of evaporation of magnesium (0.03 ± 0.002 kJ/mol) at 1373 K (1100 °C) indicates a practical coincidence with the boiling point of magnesium. The energy functions of evaporation of magnesium-tin melts will supplement the thermodynamic database and can be used for thermal engineering calculations in the design of distillation processes and apparatus.