Thermochemistry of binary liquid gold alloys: The systems gold-copper and gold-silver at 1379 K
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AHmix = Xa,Xcu(-28,821 - 2,468 XAu + 9,541 X2u)J mol -~
Au-Ag:
AHmix = XAuXAg(-- 15,820 - 529 XAu)J mol-1
The results are compared with excess Gibbs energy data from the published literature to yield approximate excess entropies of mixing. For the gold-copper system the observed parabolic dependence of the enthalpy interaction parameter on composition is well accounted for by the quasi-chemical theory.
I.
INTRODUCTION
DURINGrecent years there has
been a greatly increased interest in the theoretical aspects of the thermochemistry of alloys. Initially this interest was stimulated by the semiempirical theory of Miedema et al. ,~-4which drew the attention of metal physicists to the many challenging theoretical problems in alloy chemistry. As a result a number of theoretical papers has appeared in this field, notably by Pettifor,5'6 by Watson and Bennett, 7'8 by Williams, Gelatt, and Moruzzi,9'~~ by Chelikowsky, 12'13 and others. These theoretical investigations have, in turn, drawn attention to the fact that reliable thermochemical data for alloys are still very scarce. This is true, in particular, for alloys of the early transition metals, which are highly reactive and/or very refractory. Recognizing this lack of data we have recently initiated in our laboratory a systematic thermochemical study of the binary alloys between transition metals and noble metals. In this area we have so far completed thermochemical studies of the liquid alloys of copper with Mn,~4Fe, Co, Ni, ~5 Ti, Zr, H f , 16'17 and Sc, Y, La, Lu, V, Cr. ~8 Very recently these investigations have been extended to the alloys of the first row transition metals with liquid goldfl In the course of our work on the transition metal-noble metal systems we came to realize that there still exist important discrepancies in the published thermochemical data for the liquid alloys Cu-Ag, Cu-Au, and Ag-Au. This led us to reinvestigate these systems, ahd our new calorimetric results for the system Cu-Ag already have been published. ~7In the present communication we report new enthalpy of mixing data for the liquid alloys of gold with copper and silver.
A. Atmosphere and Calorimetric Cell
The measurerrtents were carried out at 1379 --- 1 K in an atmosphere of argon which was purified by passing the gas through concentrated H 2 5 0 4 , o v e r activated copper at 500 K, and finally over titanium powder at 920 K. The gas flows through a fused silica "liner" which provides a complete envelope for the calorimetric cell and for the final gettering system. The layout of the liner assembly is shown schematically in Figure 1. The calorimeter cell consists of a 12 mm diameter cylindrical silica crucible, about 60 mm long, which is contained within a slightly larger alumina crucible and inserted into a thin-walled stainless steel tube about 20 mm in diameter and 300 mm long, one end closed. This tube fits snugly inside the fused silica liner and protects the bottom part of the liner against the contents of the calorimeter cell. It also, to some extent, contributes to th
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