Abstract
The enthalpy of formation h f of Au-Pb alloys was measured at 695, 871 and 1123 K by direct reaction calorimetry (drop method) with the help of a high temperature Calvet calorimeter. The measurements led to the following equations: h f(kJ mol −1)= χ Au(1-χ Au) (−12.2686 + 8.1423 χ Au) at 695 K; h f ( kJ mol−1)= χ Au(1 − χ Au ) (−6.1750 + 3.4653 χ Au) at 871 K; h f (kJ mol −1)= χ Au(1- χ Au (−4.4416 + 5.6702 χ Au) at 1123 K. We also obtained the boundary of the liquid + Au(s) region from the breaks in the h f( χ Au) curves. From our enthalpic data and those of the literature for the free enthalpy of formation we calculated at 695, 871 and 1123 K the mole fractions corresponding to the gold-rich liquidus were in good agreement with our experimental values. Our investigation allows us to state definitely that the enthalpy of formation depends strongly on the temperature. The positive excess heat capacity suggests accentuated short-range order in the melts which has to be investigated with reference to associated models.
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