Effect of Reactive Ball Milling Time on the Hydrogenation/Dehydrogenation Properties of Nanocomposite MgH2/7TiMn1.5 Powders M. Sherif El-Eskandarany, H. Al-Matrouk, Ehab Shaban and Ahmed Al-Duweesh Nanotechnology and Advanced Materials Program, Energy and Building Research Center, Kuwait Institute for Scientific Research Safat 13109, Kuwait - State of Kuwait, msherif@kisr.edu.kw ABSTRACT Nanocrystalline MgH2 powders were prepared by high- energy reactive ball milling of pure Mg powders under 50 bar of a hydrogen gas atmosphere. The powders obtained after 200 h of milling were doped with 7 wt% of Mn3.6Ti2.4 powders and then ball milled 50 h, using a high energy ball mill. The as-milled MgH2/7Mn3.6Ti2.4 nanocomposite powders obtained after 200 h of milling were homogeneous in shape (spherical-like), particle size (~ 1.5 m in diameter) with grain size of less than 10 nm in diameter. (1.5 mm). This nanocomposite binary system enjoyed superior hydrogenation/dehydrogenation kinetics at 275 o C, as elucidated by the short time required to absorb and desorb 5.3 wt% H2 within 2 and 8 min, respectively. At this temperature the system possessed excellent absorption/desorption cyclability of 1000 complete cycles within 1400 h. Keywords: reactive ball milling, metal hydrides, nanocatalysts, nanocomposites 1 INTRODUCTION Since the beginning of 1990's reactive ball milling (RBM), which was introduced to the worldwide society of materials science and powder metallurgy by Calka [1] and El-Eskandarany et al. [2], has a well-known technique for preparing nanocrystalline metal hydrides and their composite powders [3]. However, MgH2 has received a great attention as pioneering hydrogen storage material [4], the high thermal stability of MgH2 system and its poor hydrogenation and consequence dehydrogenation kinetics, of it restric this attractive system to be utilized in real automobile applications. Mechanical alloying of MgH2 with pure metal (e.g. Ti, Fe, Ni, Nb, V) [6], intermetallic compounds (e.g. Zr100-xNix alloys) [7] and dual metal/metal oxide composite (e.g. Ni/Nb2O5) [8] powders is considered as a promising approach used for enhancing the hydrogenation/dehydrogenation kinetics of MgH2. The present study has been addressed in part to investigate the effect of ball milling time on developing homogeneous nanocomposite MgH2/7wt% TiMn1.5 powder particles with uniform composition and micro/nano- structures. In addition, the effect of grain/particle distributions of TiMn2 additive on improving the hydrogenation/dehydrogenation kinetics of MgH2 powder matrix were investigated in terms of structure, morphology, thermal stability, hydrogen storage capacity and cyclability. 2 EXPERIMENTAL PROCEDURE Pure Mg metal powders (~ 80 m, 99.8 %), TiMn1.5 shots (~  m, 99.8 %) and hydrogen gas (99.999 %) were used as starting materials. A certain amount of the Mg powders (5 g) was balanced inside a helium (He) gas atmosphere (99.99%) - glove box and then were sealed together with twenty five hardened steel balls into a hardened steel vial (220 ml in volume), using a gas- temperature-monitoring system (GST). The ball-to-powder weight ratio was 40:1. The vial was then evacuated to the level of 10 -3 bar before introducing H2 gas to fill the vial with a pressure of 50 bar. The milling process was carried out at room temperature using high energy ball mill operated at a rotation speed of 250 rpm. After 200 h of RBM the powders were discharged from the vial inside the glove box and sealed into Pyrex vails. The as-synthesized MgH2 powders were then mixed in the glove with 7 wt.% TiMn1.5 shots, using an agate mortar and pestle. Five gram of the mixed powders for each composite system were charged together with twenty five hardened steel balls into the hardened steel vial and sealed under He gas atmosphere. The vial was then filled with 50 bar of hydrogen gas atmosphere and mounted on the high energy ball mill. The milling process was interrupted after selected time (3, 6, 12.5, 25, 37.5 and 50 h) to get a small amount of the milled powders. The powders were then characterized by means of X-ray diffraction (XRD) with CuK radiation, 200 kV-field emission high resolution transmission electron microscopy/scanning transmission electron microscopy (FE-HRTEM/STEM), equipped with energy-dispersive X- ray spectroscopy (EDS), 15 kV-field emission scanning electron microscope (FE-SEM/EDS). The hydrogenation properties; including the pressure-composition-temperature (PCT) and absorption/desorption kinetics were investigated via Sievert’s method. The thermal stability of the ball milled powders indexed by the decomposition temperature of MgH2 and activation energy, for the nanocomposite powders were investigated by means of a differential scanning calorimeter (DSC) at different heating rate. 28 TechConnect Briefs 2015, TechConnect.org, ISBN 978-1-4987-4727-1