COMPUTER-AIDED THERMODYNAMIC STUDY OF SOLID CO-PD ALLOYS BY KNUDSEN CELL MASS-SPECTROMETRY, AND CALCULATION OF THE PHASE-DIAGRAM

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 rati o'' method. The thermodynamic excess properties can be described algeb raically by means of thermodynamically adapted power series with two a djustable 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 alloy s are characterized by negative molar excess Gibbs energies G(E), exot hermic molar heats of mixing (H-E) and small negative molar excess ent ropies 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 therm odynamic activities of Co show small positive deviations from the idea l case for the Co-rich alloys (X(Pd) < 0.34), and negative deviations from Raoults' law for alloys with higher Pd contents. Th Pd activities a(Pd) show negative deviations from the ideal case for all compositio ns. The,phase diagram has been computed by means of a generally applic able procedure for the calculation of the equilibrium compositions of coexisting phases. This was achieved using the results of this work, t hermodynamic data from earlier mass spectrometric studies on the liqui d phase, and literature data for the heat capacities and enthalpies of Co and Pd.