Computer-aided thermodynamic study of solid CoPd alloys by Knudsen cell mass spectrometry, and calculation of the phase diagram

Abstract

F.c.c. solid CoPd alloys have been investigated thermodynamically by means of computer-aided Knudsen cell mass spectrometry. Thermodynamic evaluation has been performed by applying the “digital intensity ratio” method. The thermodynamic excess properties can be described algebraically by means of thermodynamically adapted power series with two adjustable parameters, i.e. C 1 G (−20 810 + 9.608T) J mol −1) and C 2 G (−30 720 + 6.78T) J mol −1). At 1470 K, f.c.c. solid CoPd alloys are characterized by negative molar excess Gibbs energies G E, exothermic molar heats of mixing ( H E) and small negative molar excess entropies S E. At 1470 K, the minimum G E value is −4600 J mol −1 (61.9 at.% Pd), the minimum H E value is −9400 J mol −1 (59.5 at.% Pd) and the minimum S E value is −3.3 J mol −1 K −1 (55.9 at.% Pd). The thermodynamic activities of Co show small positive deviations from the ideal case for the Co-rich alloys ( x Pd < 0.34), and negative deviations from Raoults' law for alloys with higher Pd contents. The Pd activities a Pd show negative deviations from the ideal case for all compositions. The phase diagram has been computed by means of a generally applicable procedure for the calculation of the equilibrium compositions of coexisting phases. This was achieved using the results of this work, thermodynamic data from earlier mass spectrometric studies on the liquid phase, and literature data for the heat capacities and enthalpies of Co and Pd.