Enthalpies of formation of liquid and solid (gallium + palladium) alloys
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I.
INTRODUCTION
T H E R E is not much information available on the palladium-gallium system. The phase diagram of Elliott, Ixl redrawn by Massalski, 121is shown in Figure 1. The crystallographic parameters and the stability range of the five compounds PdGas, Pd3GaT, PdGa, PdsGa3, and PdzGa are well known: I1,31 PdGa5 (body-centered tetragonal), Pd3Ga7 (cubic, Ir3Sn7 type), and PdsGa3 (orthorhombic, Ge3Rh5 type) melt peritectically at 473, 733, and 1278 K, respectively. GaPd (cubic, FeSi type) melts at 1318 K, and PdzGa (orthorhombic, Co2Si type) melts congruently at 1538 K. A high-temperature phase, Pdo.55Gao.45 (W type), existing above 1200 K has been confirmed by Khalaff and Schubert.J4] The existence of another high-temperature phase, Pd0.szsGa0.475, has been claimed. I31 All compounds show a very small homogeneity range except PdzGa at high temperatures. The Pd3Ga phase reported by Elliott t~J according to Schubert et al.,t31 was reinvestigated by Khalaff and Schubert, I41 and alternatively, the formation of three phases, Pd7Ga3, PdsGa2, and Pdl3Gas, has been reported. The limits of the Pd-rich terminal phase are not yet known. Pratt et al. I51 and Vogelbein et al. I61 measured enthalpies of formation of PdGa and of Pdo.ssGa012 and Pdo.vsGa0.25, respectively. No free energy measurements were apparently performed on this system. Since no enthalpy data of the liquid alloy phase were measured, the present investigation was undertaken. This research is part of a systematic study of the thermodynamic properties of liquid binary alloys with A1, Ga, and In. I7,81
DRISS EL ALLAM, formerly Ph.D. student, Universit6 de Provence, MARCELLE GAUNE-ESCARD, Director of Research, and JEANPIERRE BROS, Professor, Universit6 de Provence, are with SETT-UA 1168, Centre de Sainte Jrrrme, Av. Escadrille Normandie-Niemen, 13397 Marseille Cedex 13, France. ERHARD HAYER, Assistant Professor, University of Vienna, is with the Institute of Inorganic Chemistry, Wahringerstr. 42, 1090 Vienna, Austria. Manuscript submitted January 21, 1991. METALLURGICAL TRANSACTIONS B
II.
EXPERIMENTAL PROCEDURE
High-purity metals (99.999 wt pct Pd plate from ALFA-Ventron, Karlsruhe, Federal Republic of Germany, and 99.99 wt pct Ga rods from Koch and Light Ltd., London) were employed in this study. The palladium was rinsed with acetone pa dried, and cut into small pieces. The gallium was melted under a dilute solution of hydrochloric acid (5 pct) in warm water, divided into small droplets, and dried after rinsing with cold distilled water. Measurements at 1322 < T / K < 1760 were performed using a very high temperature (THT) calorimeter of the Eyraud-Petit type described elsewhere. E9"l~ It was equipped with an automated thermostated sample charger, t1~ The calorimeter cell employed was an alumina tube closed at the lower end, with about 0.012-m external diameter and 0.47-m height. At the closed end, it was shaped to fit the thermopile dimension. The alloying process was performed in a thin-walled graphite crucible of about 0.050-m height and 0.0065-m ins
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