HYDROGEN ABSORPTION CHARACTERISTICS OF THE ZR1-XHOXCO2 SYSTEM IN THE PRESSURE RANGE 0-40 BAR

Hydrogen absorption pressure-composition isotherms were determined for the intermetallic compounds Zr1-xHoxCo2 (x = 0.4, 0.6 and 0.8) belonging to the C-15-type Laves phase structure in the temperature range 50-200-degrees-C at pressures up to 40 bar. The maximum hydrogen concentration (number of hydrogen atoms per formula unit) increases from 0.1 to 3.8 as the holmium concentration (x) is increased from 0 to 1. The ratio of the number density of hydrogen atoms in the alloy to the density of liquid hydrogen lies in the range 0.1-1.24. The absorption of hydrogen is accompanied by a maximum of 19% expansion in the unit cell volume of the host without any change in the C-15-type Laves phase structure. For higher values of x the materials are highly distorted when the hydrogen concentration exceeds about 3.0 and tend towards the amorphous state. The relative partial molar enthalpy (DELTAH(H)BAR) and entroPY (DELTAS(H)BAR) of hydrogen absorption in these materials are found to lie in the range 7-43 kJ (mol H)-1 and 10-115 J K-1 (mol H)-1 respectively. The hydrogen absorption kinetics recorded at different temperatures for all the compositions indicate that the kinetics are fast for lower x values.