Thermodynamic properties of Gd-Bi alloys determined by emf measurements in LiCl-KCl-GdCl3 electrolyte

Abstract

Thermodynamic properties of binary Gd-Bi alloys (mole fraction, xGd = 0.02–0.35) were determined using electromotive force (emf) measurements in molten LiCl-KCl-GdCl3 electrolyte in complement with structural, microstructural, and thermal characterization. X-ray diffractometry (XRD) and scanning electron microscopy (SEM) identified the phase constituents of Gd-Bi alloys (xGd = 0.02–0.35) as Bi matrix and GdBi compound. According to differential scanning calorimetry (DSC) measurements up to 1250 K, a eutectic transition [L = Bi + GdBi] was observed at 543 K but the reported peritectic transition [L + GdBi = GdBi2] was not detected, suggesting the absence of the GdBi2 compound. Based on structural, microstructural, and thermal characterization of Gd-Bi alloys (xGd = 0.02–0.35), the GdBi2 compound was not detected, and its thermodynamic stability is questionable. The emf of Gd-Bi alloy (xGd = 0.16) at 700–1048 K relative to Gd(s) was measured by electrodepositing pure Gd metal at 25 K increments. The emf values of Gd-Bi alloys were determined via coulometric titration of Gd into liquid Bi using a two-phase [L + GdBi] alloy as a reference electrode at 773–973 K. The solubility of Gd in liquid Bi was also estimated to be 0.50 mol% (773 K), 0.99 mol% (873 K), and 2.21 mol% (973 K) and the excess partial molar Gibbs energy (∆G̅Gdex) of liquid Gd-Bi alloys was as low as 65 kJ mol–1, indicating strong chemical interactions between Gd and Bi.