Analysis of sloping plateaux in alloys and intermetallic hydrides - II. Real systems

Plateau pressure regions in metal hydride systems are due to the co-existence of two solid phases, the dilute and hydride phases, and, according to the phase rule, the plateau pressures should be invariant. These plateaux often slope, however, and in intermetallics and alloys at moderate temperatures the major cause of this sloping can be attributed to compositional inhomogeneities. In this paper, such sloping plateaux in the intermetallic compounds LaNi5 and LaN4.8Sn0.2 are analyzed using the theory developed in Part I. Similarly, composition inhomogeneities which have been intentionally introduced into a Pd0.85Ni0.15 alloy are analyzed in terms of the compositional distribution of Ni. An important result is that the compositional inhomogeneities are closely Gaussian, although such a distribution has not been assumed a priori. It is shown that the "plateau" enthalpies obtained for the inhomogeneous Pd0.85Ni0.15 alloy differ dependent upon whether they are determined via calorimetry or by van't Hoff plots. The latter changes with % conversion to the hydride phase but the former is nearly constant. An explanation for this unexpected behavior is given. (C) 2004 Elsevier B.V. All rights reserved.