THERMODYNAMIC CHARACTERIZATION OF THE ZRNI-H SYSTEM BY REACTION CALORIMETRY AND P-C-T MEASUREMENTS

In this research we have characterized the thermodynamics of hydrogen absorption in the range H ZrNi = n = 0 to n ≈ 1.0; this region has been previously neglected because the equilibrium hydrogen pressures are outside the ranges of the gauges which are usually employed for metal-hydrogen research, e.g. about 10-5 Pa at 323 K. With the aid of reaction calorimetry and X-ray diffraction there is shown to be a two-solid phase plateau region from n ≈ 0.2 to n = 0.65 (323 K). A technique consisting of heat treating the hydrogen-containing sample in order to attain internal equilibrium was employed between each calorimetric determination in this region of low hydrogen content. For the hydride formation reaction corresponding to this low pressure plateau ΔHf = -47.5 ± 1 kJ (mol H)-1. For larger hydrogen contents, i.e. the two solid phase plateau region, which extends from n ≈ 1.0 to n = 2.3, the enthalpy for hydride formation for samples which had been hydrided and then evacuated and annealed at 623 K prior to the run was determined calorimetrically to be -35.0 kJ (mol H)-1. Enthalpies for this plateau for both hydride formation and decomposition have also been determined from van't Hoff plots of pf and pd. The pressures for this plateau exhibit a relatively large hysteresis and the enthalpy values determined from the van't Hoff plots reflect this. © 1990.