1 Proceedings International Hydrogen Energy Congress and Exhibition IHEC 2005 Istanbul, Turkey, 13-15 July 2005 Hydrogen Storage Characteristics of Mg 17 Al 12 Compound and Mg-10%Al Composition Prepared By Mechanical Alloying R. Vijay a , R. Sundaresan a , G.V.Narasimaha Rao, M. P. Maiya b and S. Srinivasa Murthy b a Int. Advanced Research Centre for Powder Metallurgy and New Materials, Balapur, Hyderabad – 500 005, India. b Refrigeration and Air-conditioning Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai-600 036, India. Email: ssmurthy@iitm.ac.in , Fax: +91 44 2257 0545 ABSTRACT Intermetallic Mg17Al12 compound and a substoichiometric alloy Mg – 10%Al were prepared by mechanical alloying (MA) of appropriate quantities of elemental magnesium and aluminium powders. Milling was carried out in a Fritsch high energy planetary mill with ball to powder ratio of 15:1 under wet milling condition for periods ranging up to 48 h. The milled powders were characterised for phase content by XRD. Intermetallic compound Mg 17 Al 12 started forming from 8 h and 16 h of milling in stoichiometric and substoichiometric compositions respectively. In the stoichiometric composition 48 h milling was sufficient for complete transformation to Mg17Al12. In Mg – 10%Al composition, in addition to Mg 17 Al 12 , MgH 2 also started forming after 16 h of milling. After 48 h milling, the sample showed no residual Al, indicating complete transformation to Mg 17 Al 12 ; in addition Mg and MgH 2 were also present. Grain sizes decreased with milling time, that of the compound in stoichiometric composition reaching 12 nm after 48 h, and that of Mg in the substoichiometric alloy reaching a limiting value 24 nm after 24 h of milling. The absorption and desorption characteristics were evaluated at 100, 200 and 300°C. Sub stoichiometric Mg – 10Al alloy absorbed 5.5 wt% (80% of theoretical capacity) hydrogen in 3 minutes at 300°C. The absorption capacity and rate of absorption decreased with decrease in temperature to 200°C and it took 50 minutes to absorb 4 wt% (58% of theoretical capacity) hydrogen. There was no absorption of hydrogen at 100°C. The desorption kinetics at 300°C is very slow and it desorbed only 2.5 wt% (< 50% of the quantity absorbed) hydrogen in 200 minutes. There was no desorption of hydrogen at 200°C. In contrast the stoichiometric Mg 17 Al 12 compound did not absorb hydrogen at any of these temperatures. Keywords: Metal hydrides, hydrogen storage, mechanical alloying 1. INTRODUCTION The properties required by hydrogen storage materials for commercial applications are high reversible storage capacity, low sorption temperatures, faster hydriding/dehydriding rates, good cyclic stability, low density and low cost. Magnesium based hydrogen storage systems are potential candidates for hydrogen storage because of high storage capacity (up to 7.6 wt%), low density, low cost and availability. However, because of its high hydriding/dehydriding temperature (at least 300°C) and relatively poor kinetics [1], it is not suitable for practical hydrogen storage applications. For improving the storage capacity and kinetics, various attempts have been made by grain refinement [2], addition of metallic catalysts such as V, Ti, Ni, Fe, Mn, Cu and Ag [3-5] or metal oxides catalysts like TiO 2 [6], Cr 2 O 3 , Al 2 O 3 and Ce 2 O 3 [7], formation of intermetallic compounds such as Mg 2 Ni [8-16] and Mg 17 Al 12 [17], addition of intermetallic compounds like LaNi 5 [18-21], MmNi 5-x M x (Mm = mischmetal, M = Co, Al, Fe) [22-23], FeTi [24], Fe 0.92TiMn0.08 [25], FeTi1.2 [26-37] and ZrFe 1.4Cr0.6 [28]. Bouaricha et al. [20] investigated the hydrogen storage characteristics of Mg-Al alloy. and found formation of Mg 17 Al 12 compound. The hydrogen absorption and desorption studies were made at