THERMODYNAMIC INVESTIGATION OF THE PD-PT-GE TERNARY ALLOYS AT HIGH-TEMPERATURE

The enthalpies of formation h(f) of the Pd-Pt-Ge alloys were measured at 1623 K as a function of composition by direct reaction calorimetry (drop method) using a high-temperature Calvet-type calorimeter. The me asurements were performed for four values (0.10, 0.25, 0.50 and 0.75) of R=x(Pt)/(x(Pt)+x(Ge)) and two values (0.60 and 0.75) of R'=x(Pd)/(x (Pd)+x(Ge)). The thermodynamic behaviour of the melts is characterized by strong negative deviations from ideality as observed for the Pd-Ge and Pt-Ge binaries. The iso-enthalpy curves at 1623 K show that the e nthalpy minimum lies near the straight line from pure Pd2Ge to pure Pt Ge as at 1269 K. The thermodynamic functions of formation of the terna ry melt were calculated within the framework of the regular associated model assuming that two kinds of associates are present in the binari es, i.e. Pd2Ge and PtGe. The values of the thermodynamic parameters us ed for this calculation were those of the two binaries with Ge assumin g ideality for the Pd-Pt system. With these assumptions, the behaviour of the ternary melts seems to be well described in terms of the entha lpy of formation. The Gibbs free energy of formation of the Pd-Pt-Ge l iquid phase was also calculated in the same way and will be compared w ith experimental results measured by Knudsen-cell mass spectrometry wh en available.