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 ≤ x_Sr ≤ 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 x_Sr = 0.35 during cooling-heating cycle. At higher mole fractions (x_Sr ≥ 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.