Thermodynamic properties of calcium–bismuth alloys determined by emf measurements

The thermodynamic properties of Ca–Bi alloys were determined by electromotive force (emf) measurements to assess the suitability of Ca–Bi electrodes for electrochemical energy storage applications. Emf was measured at ambient pressure as a function of temperature between 723 K and 1173 K using a Ca(s)|CaF2(s)|Ca(in Bi) cell for twenty different Ca–Bi alloys spanning the entire range of composition from xCa = 0 to 1. Reported are the temperature-independent partial molar entropy and enthalpy of calcium for each Ca–Bi alloy. Also given are the measured activities of calcium, the excess partial molar Gibbs energy of bismuth estimated from the Gibbs–Duhem equation, and the integral change in Gibbs energy for each Ca–Bi alloy at 873 K, 973 K, and 1073 K. Calcium activities at 973 K were found to be nearly constant at a value of aCa = 1 × 10−8 over the composition range xCa = 0.32–0.56, yielding an emf of ∼0.77 V. Above xCa = 0.62 and coincident with Ca5Bi3 formation, the calcium activity approached unity. The Ca–Bi system was also characterized by differential scanning calorimetry over the entire range of composition. Based upon these data along with the emf measurements, a revised Ca–Bi binary phase diagram is proposed.