A kinetic study on the hydrogen induced amorphization in ErNi 2 laves phase

Abstract

Structural changes of the ErNi 2 Laves phase during the hydrogen absorption and desorption process are investigated by X-ray diffraction measurements and differential scanning calorimetry. In order to investigate the mechanism of the hydrogen induced amorphization in ErNi 2, a kinetic studies on the crystalline to glass transition are performed by measuring X-ray intensities at different states of the transition and interpreted using the Johnson-Mehl-Avrami equation. The transformation from the crystalline into the amorphous state is confirmed in the temperature range of 463–573 K at 50 atm H 2. The amorphous phase gradually grows after hydrogen absorption is finished. The structure of the hydrogenated sample, which is examined just after hydrogen absorption, is found to be a tetragonal structure. The activation energy and rate constant exponent, n, for the amorphization is 17 kcal/mol and 0.5, respectively. The hydrogenated amorphous ErNi 2 returns to the original Laves structure after hydrogen desorption is accomplished. From the measured activation energy and rate constant exponent, it is believed that the mechanism of the hydrogen induced amorphization is not related to the motion of hydrogen atom but to the lattice distortion by the motion of Ni atoms so that the surroundings of hydrogen atoms in the amorphous state is similar to those of hydrogen atoms in ErH 2.