Thermodynamic Properties of Strontium-Bismuth Alloys Determined by Electromotive Force Measurements

The thermodynamic properties of Sr-Bi alloys were determined by electromotive force (emf) measurements to evaluate the viability of liquid bismuth metal as a medium for separating alkali/alkaline-earth fission products from molten salt electrolyte. A Sr(s)|CaF2-SrF2|Sr(in Bi) cell was used to measure emf values at 748-1023 K for thirteen Sr-Bi alloys at mole fractions 0.05 ≤ xSr ≤ 0.75. Activity values of strontium in bismuth were determined at 788 K, 888 K, and 988 K as well as the partial molar entropy and enthalpy at each composition. Reproducible emf values within ± 5 mV were obtained up to xSr = 0.35 during cooling-heating cycle. At higher mole fractions (xSr ≥ 0.40), the emf values exhibited increased hysteresis during the thermal cycles due to the strong tendency of the alloys to form meta-stable phases. The non-equilibrium phase behavior of Sr-Bi alloys was verified by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and microstructural analyses. Compared to the existing equilibrium phase diagram, two additional phases of Sr2Bi3 and Sr4Bi3 were identified and discussed. Liquid-state solubility of Sr was 15-40 mol% at 788-988 K and the activity values were as low as 10−13 at 788 K, implying strong chemical interactions between Sr and Bi.