L Journal of Alloys and Compounds 356–357 (2003) 603–607 www.elsevier.com / locate / jallcom Synthesis of nanocrystalline hydrogen storage materials * J. Huot , M.-L. Tremblay, R. Schulz ´ HERA Hydrogen Storage Systems, 577 Le Breton, Longueuil, Quebec, J4G 1R9, Canada Received 2 September 2002; accepted 15 November 2002 Abstract In this paper, we present a new and powerful method to directly synthesize nanocrystalline metal hydrides from elementary components. This method consists of milling at high temperature under hydrogen with the addition of graphite to facilitate the first hydrogenation. We found that a synergetic effect is achieved and that it is possible to reach full hydrogenation in a very short time. This technique was tested on magnesium, which is known to be a material hard to activate. We found that full hydrogenation was realized after only about 1 h. This represents an order of magnitude reduction in processing time compared to any other methods known in the art.  2003 Elsevier B.V. All rights reserved. Keywords: Reactive milling; Activation; Nanocomposite; Metal hydrides 1. Introduction of magnesium hydride during milling and on decomposi- tion of this hydride [12]. However, the maximum yield of Synthesis of metal hydride by mechanical alloying and magnesium hydride was only 40 wt.% after 10 h of milling mechanical milling is a relatively new but well docu- [13]. Bobet et al. reported the formation of 35 wt.% of mented technique [1–5]. Usually, the milling is performed magnesium hydride when milling under 1.1 MPa of under an inert atmosphere (generally argon) and the hydrogen for 5 h without using any catalyst [14]. When Co hydrogenation is performed in a subsequent step. However, (10 wt.%) was added as a catalyst, the yield of magnesium for cost reduction and ease of formation, milling under hydride went up to 71 wt.% for 10 h milling [15]. The hydrogen (reactive milling) could be an attractive alter- hydrogenation showed a two-step process: Nucleation and native. Reactive milling has been investigated in numerous diffusion [16]. Because reactive milling could be associ- systems such as magnesium-based systems and systems ated to nucleation [17], the activation process was sup- with group IV and V transition metals [6–10]. In this pressed in the ball-milled materials. For short milling time paper, we will confine our discussion to the synthesis of under hydrogen atmosphere, a hydride layer was formed magnesium hydride by reactive milling and describe a new on every magnesium particle. This layer acted as a reactive milling technique which drastically improves the protection against oxidation. For longer milling time, hydride formation yield. decrepitation occurred which resulted in ultra-fine powder Reactive milling of magnesium has been intensively [15]. The in-situ hydrogenation during ball milling was studied by a number of researchers. Without catalyst, the probably too slow to entirely cover every particle with a formation of magnesium hydride by reactive milling is hydride layer. Therefore, the protection against oxidation quite slow. For example, Gennari et al. synthesized was not as effective in the long milling time samples thus magnesium hydride by milling for 50 h under a hydrogen reducing the hydrogenation kinetics. The authors con- pressure of 0.5 MPa [11]. However, even for such a long cluded that a short milling time is the most suitable route milling time, only 50% of the magnesium was hydrided. for producing a hydride with fast hydrogenation kinetics Various catalysts have been used to speed up the reaction. [17]. Tessier and Akiba showed that a concentration of nickel as Wang et al. used the alloy ZrFe Cr (40 wt.%) as a 1.4 0.6 low as 1 at.% has a beneficial effect on formation kinetics catalyst in reactive milling of magnesium under hydrogen pressure [18,19]. They found that the decomposition reaction of the nanostructured magnesium hydride was *Corresponding author. E-mail address: jh@herahydrogen.com (J. Huot). nucleation and growth controlled for transformation frac- 0925-8388 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00120-8