The thermodynamics of the ErFe 2H system using differential heat conduction calorimetry

Abstract

A differential twin-cell heat flow calorimeter has been constructed for the measurement of the enthalpy changes of the reaction of hydrogen gas with intermetallic compounds as a function of their hydrogen contents. It was designed especially for use with those intermetallic compounds which form very stable hydride phases; therefore prior to the calorimetric studies it must be possible to evacuate the sample in situ at elevated temperatures to remove all of the hydrogen resulting from the activation procedure. The calorimeter was employed in this research for the determination of the thermodynamics of the reaction of H 2 with ErFe 2H x over the range of hydrogen contents x = 0.4. ErFe 2H is a multiplateau system; the enthalpy change for hydride formation corresponding to the plateau near x = 3 is more exothermic than that corresponding to the preceding plateau near x = 2. In addition to the calorimetric determination of the enthalpies of reaction, the free energies were determined simultaneously from the equilibrium hydrogen pressures. Using these free energy and enthalpy data the entropies were also obtained over a range of hydrogen contents.