Thermodynamics of Fe—Sm, Fe—H, and H—Sm Systems and Its Application to the Hydrogen—Disproportionation—Desorption—Recombination (HDDR) Process for the System Fe17Sm2—H2.

Abstract Thermodynamic properties of the Fe–Sm, Fe–H, and H–Sm systems have been analyzed by means of the CALPHAD method. Thermodynamic models have been defined to describe the Gibbs energy of the individual phases and the model parameters have been optimized using different experimental information: phase diagram data, calorimetric data, and equilibrium partial pressures of hydrogen. The entropy of Fe17Sm2 has been derived from experimental data on low-temperature heat capacity. The thermodynamic parameters for Fe17Sm2 have been evaluated using binary (Fe–Sm) and ternary (Fe17Sm2–H2) data simultaneously. The calculated phase diagrams and thermodynamic properties are in agreement with experiments. The thermodynamics of hydrogen in the interstitial solid solution Fe17Sm2Hδ have been described by compound energy formalism. Combining the obtained thermodynamic descriptions of the individual phases, the equilibrium temperatures of the recombination reaction: Fe+SmH2±x⇒Fe17Sm2+H2, which is an important part of the HDDR process, have been calculated as a function of hydrogen pressure.