Thermodynamics of the Pb–Pd system

Abstract

In the ranges 590< T/K<1170 and 0.10< x Pd<0.75, the thermodynamic properties of the palladium–lead system were obtained from electromotive force measurements ( E=f( T, x Pd) using the reversible concentration cell: −W,Pb(pure)/Pb 2+ in liquid electrolyte / Pb x Pd 1−x, W +. Activities of lead in the liquid and solid [Pb+Pd] alloys derived from the measured cell potentials allowed the molar partial (Δ mix G Pb °, Δ mix H Pb ° and Δ mix S Pb °) and integral (Δ mix G m °,xs, Δ mix H m °,xs and Δ mix S m °,xs) excess thermodynamic functions to be calculated in the one- and two-phase regions (reference state, pure liquid Pd and Pb). The integral molar excess Gibbs energy of formation of the liquid alloy has been described by the following relation: Δ mix G °,xs m =x(1−x)[∑ i i=1A i(1−2x) n−1] with x=x Pd and A i=h i−Ts i. The best description of this excess function was obtained with the following values: h 1=−16 564; h 2=13 270; h 3=−8979; h 4=2984; and s 1=1.8554; s 2=−0.7850; s 3=s 4=0. Assuming that all of the intermediate phases are stoichiometric, the excess functions of formation have been calculated. These data are as follows: Phase −Δ f G°(700 K) −Δ f H° kJ/mol kJ/mol Pb 2Pd 30.33±0.1 33.31±0.3 PbPd 42.43±0.2 42.7±0.6 Pb 9Pd 13 48.31±0.3 47.3±1.2 (γ)-Pb 3Pd 5 49.96±0.4 48.6±2 PbPd 3 55.76±0.5 55.2±4 The validity of this assumption will be discussed. Moreover, the liquidus and solidus lines of the equilibrium phase diagram, as well as the range of stoichiometry of the definite compounds, have been specified.