ABSORPTION-DESORPTION CHARACTERISTICS OF HYDROGEN FOR MISCHMETAL-NICKEL-MANGANESE ALLOYS

Mischmetal (Mm)-nickel-manganese alloys MmNi6-xMnx (x=0.25~1.0) have been found to have the same hexagonal structure as LaNi5 and MmNi5. They react readily with hydrogen to form hydrides of MmNi4.76Mn0.25H0.5, MmNi4.5Mn0.5H6.6, MmNi4.25Mn0.75H6.5 and MmNi4.0Mn1.0H6.5 (hydrogen content l,5wt%) under 60 atm hydrogen pressure and at room temperature. Dissociation pressures of these hydrides are dependent on content of manganese, which substitutes partially for nickel, and log P decreases in proportion to x. The enthalpy change in hydride formation as determined from the dissociation isotherms (Fig. 3) for the MmNi4.5Mn0.5-H system is given-4.2kcal/mol, and the dissociation pressure at 30°C yields 3 atm. The desorption rate of hydrogen for MmNi4.5Mn0.5 is larger than those for LaNi5 and MmNi5, and a value of 5.6~6. 9kcal/mol cames out as the apparent activation energy of hydrogen desorption. For MmNi4.5Mn0.5 the hydrogen absorption-desorption cycles are repeated 30 times, but the variation in hydrogen absorption-desorption ability was hardly noticed. The hydride of MmNi4.5Mn0.5 proved very great as a hydrogen storage material. © 1979, The Chemical Society of Japan. All rights reserved.