Investigations on the system sodium-gold
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I.
INTRODUCTION
T H E phase diagram for the Na-Au system has been determined mainly by Mathewson I'l and Kienast and Verma. 121 Compilations by Hansen, 131 Elliot, lal and Massalski tSj also give the phase diagram. Existence of the intermetallic compounds NaAu2, NaAu, and Na2Au is well established, but uncertainty remains concerning their range of homogeneity. No information is available on the thermodynamic properties of this binary system. The coulometric titration technique using solid electrolytes was first employed by Wagner 161to study Ag2S. It has since been used in cells to study chalcogenides 17-~2j where one of the components has a high diffusion coefficient. It has also been used to examine alloy systems in the liquid state t~3,14t as well as alloys with slow component diffusion and is made possible by the use of thinfilm electrodes. 1~S1 Molten salt electrolytes have been used for titrations to study Li alloys. It61 Many other examples exist in the literature. In the present work, coulometric titrations are carried out on thin-film electrodes of Na-Au alloys in conjunction with galvanic cells employing CaF2 solid electrolytes at 775 ~ Information is obtained on Gibbs energies of mixing in the system and the range of homogeneity of NaAu2 along with the type and degree of disorder of this compound. Cells of the following types were constructed: Ca-Sn (1) ] CaF2 (s) I NaF (s), Na-Au (s, 1) Xca = 0.1
[I]
Ca-Sn (l) I CaF2 (s) [ NaF (s), Na (l) [II]
XCa = 0.1
Running current through cell I or cell II produces Na at the right-hand electrode, and with the help of a coulometer, the composition of the Na-Au alloy is controlled very accurately. The Na in the right-hand electrode of cell II is generated in situ. The relative partial molar Gibbs energy of Na in the Na-Au alloy is given by GNMa =
-F(Ez - E,)
[ 1]
R. ALQASMI, formerly with the Department of Applied Science, Brookhaven National Laboratory, is Vice Dean, Faculty of Science, United Arab Emirates University, Alain, UAE. J.J. EGAN, formerly Senior Scientist, Department of Applied Science, Brookhaven National Laboratory, Upton, NY, is retired. Manuscript submitted October 25, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
where El and E, are the electromotive forces (EMFs) of cells I and II. The activity of Na in Na-Au is given by aN =exp(GNMa~
\RT/
[2]
The relative integral molar Gibbs energy is given by G M = (1
-
XNa )
- X N a ) 2 dx f0 ~N"(1 -G~a
[3]
where XNa is the mol fraction of Na in Na-Au. These quantities are often useful when dealing with intermetallic compounds. The integrals were obtained by graphical integration. Cells of the type Na (1), NaF (s) I CaF2 (s) I NaF (s), Na-Au (s, 1)
[III]
would yield the same result but were not operated since the Ca-Sn reference electrodes were found much easier to use experimentally than Na electrodes.
II.
EXPERIMENTAL
The experimental arrangement of cell I is shown in Figure 1. The two electrodes and the CaF2 crystal along with an iron cylinder were supported in a holder that was con
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