Effect of substitution on hysteresis in some high-pressure AB2 and AB3 metal hydrides

Ti0.95Zr0.05Cr1.2Mn0.8-xVx-H-2 (x=0, 0.05 or 0.1), Ti0.95Zr0.05Cr1.2Mn0.8-yCoy-H-2 (y=0, 0.1 or 0.2) and Ce0.8La0.2Ni5-zCoz-H-2 (z=0, 0.5, 1.0 or 1.5) systems were studied experimentally in order to (a) find suitable alloys for high-pressure applications, and (b) gain insight into the reasons for the hysteresis and its reduction due to certain partial substitutions. A special volumetric pressure-composition isotherm (PCI) measurement technique at high pressures (up to 400 bar in this case), pressure differential scanning calorimetry (PDSC), and X-ray diffractometry (XRD) of both starting alloys and SO2- (or air-) poisoned high-pressure hydrides were applied. Practically hysteresis-free high-pressure alloy-hydrogen systems with good PCI plateau properties were found, e.g. Ti0.95Zr0.05Cr1.2Mn0.75V0.05-H-2. Furthermore, it was shown that an intermediate hydride phase in the Ce0.8La0.2Ni3.5Co1.5-H-2 system exists, which explains the remarkably reduced hysteresis compared to the less or non-substituted systems. Finally, an experimental correlation between hysteresis and discrete unit cell volume expansion between coexisting phases was displayed. It suggests that the volume expansion is a major reason for the hysteresis, but more experimental data needs to be collected. (C) 1999 Elsevier Science S.A. All rights reserved.