Calorimetry within hysteresis loops: Application to LaNi 5H

Abstract

Calorimetric data are reported at 298 K for H 2 absorption within the hysteresis gap for the LaNi 5H system. Relative partial molar enthalpies for H 2 absorption were determined along an absorption hysteresis scan commencing from the desorption pressure plateau and ending near the absorption plateau. These enthalpic data can be explained if H 2 absorption consists of two processes: reversible H 2 solution into the two coexisting solid phases and formation of some hydride phase. The contribution of the latter to a hysteresis scan increases from essentially zero adjacent to the desorption plateau to a large fraction near the absorption plateau. The enthalpies for H 2 solution adjacent to the desorption plateau, the reversible region of the scans, as a function of the overall hydrogen content can provide a useful method for the determination of the relative partial enthalpies for H 2 solution in the coexisting single phases. The enthalpies for the reversible solution of H 2 must approach the values for the coexisting single phases at the phase boundaries. This method gives for the relative partial molar enthalpies of the single phases values of −13.5 kJ (mol H) −1 and − 16.8 kJ mol −1 for the hydrogen-saturated solid and the hydride phase respectively. Since the enthalpy change for the reaction 1 2 H 2( g) + α → β is −14.8 kJ mol −1, it is clear that small discontinuities in the thermodynamic parameters exist at the phase boundaries. Previously direct measurements in the single-phase regions did not detect these small differences, caused in part by the difficulty in locating the hydride phase boundary. Remeasurement of the partial enthalpies in the primary solubility region (α phase) support the present result from hysteresis scan calorimetry. It is shown that the method of hysteresis scan calorimetry is useful for the determination of thermodynamic parameters for coexisting single phases in structural transformation systems such as LaNi 5H.