Nanocrystalline β-γ-β cyclic phase transformation in reacted ball milled MgH2 powders

Pure magnesium powders were ball milled under a hydrogen pressure of 50 bar at room temperature, using reactive ball milling (RBM) approach. The results have shown that a single stable phase of beta-MgH2 is obtained upon RBM for 25 h. Increasing the RBM time leads to a significant decreasing on the grain size and an increase in the iron contamination that were introduced to the powders upon using hard steel milling tools. Remarkable changes in the transformed mass fractions of beta-MgH2 phase to a metastable gamma-MgH2 phase with increasing the REM time could be detected. Cyclic beta-gamma-beta phase transformations were observed several times upon changing the REM time. After 200 h of RBM time, the decomposition temperature and activation energy were recorded to be 399 degrees C and 131 kJ/mol, respectively. Moreover, the times required for complete absorption and desorption of 7 wt.% of hydrogen at 250 degrees C were recorded to be 3140 s and 35,207 s under 10 and 0 bar, respectively. At 300 degrees C, the powders that were obtained upon RBM time for 200 h possess excellent hydrogenation properties for any pure MgH2 system, indexed by high hydrogen storage capacity (7.54 wt.%) with complete 600 absorption/desorption cycles. Improvements of hydrogenation/dehydrogenation kinetics are attributed to the presence of gamma-phase, the existence of Fe contamination and the nanocrystallinity of the ball milled powders. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.