Thermodynamic Investigation of The Pd-Pb Binary Liquid Alloys

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Abstract
The enthalpy of formation, hf, of the Pd-Pb alloys was measured by direct reaction calorimetry (drop method) at 840 K and 1344K. At 840 K the results are in good agreeement with those obtained previously between 922 and 1154K (no temperature dependence). At 1344K, they confirm that the enthalpy of formation of the liquid phase increases with increasing temperature above 1154 K (as previously observed at 1240 K). The crystallization of Pd3Pb was observed and we can deduce its enthalpy of formation. The strong negative departures from ideality of the thermodynamic behaviour of the melt (hf min = −40 kj mol−1, Cp xs > 0 at high temperature) correspond undoubtedly to the presence of chemical short-range order. A “short-range order-disorder transition” begins between 1154 and 1240 K. In order to apply the regular associated solution model we calculated the free enthalpy of formation of the liquid phase from the phase diagram and the thermodynamic functions of formation of the solid intermediate compounds. The application of the model allowed us to explain the behaviour of the Pd-Pb melts by the existence of two kinds of associate i.e. Pd2Pb and PdPb.

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