Journal of Alloys and Compounds 376 (2004) 222–231 Environmental degradation by hydrolysis of nanostructured -MgH 2 hydride synthesized by controlled reactive mechanical milling (CRMM) of Mg R.A. Varin a,∗ , S. Li a,1 , A. Calka b a Department of Mechanical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 b Department of Materials Science and Engineering, University of Wollongong, Wollongong NSW 2518, Australia Received 11 December 2003; accepted 23 December 2003 Abstract Controlled reactive mechanical milling (CRMM) of Mg powder under “shearing” mode in hydrogen gas for 26.5, 50, 70 and 100 h results in the formation of nanostructured -MgH 2 hydride. Based on the comparison of integrated X-ray diffraction (XRD) intensities of Mg and -MgH 2 peaks, it is postulated that during reactive milling up to 50 h the crystalline Mg is mostly consumed to form -MgH 2 . However, from 50 to 100 h the crystalline Mg mostly forms an amorphous phase with only a small fraction of it being consumed for the -MgH 2 creation. The formation of amorphous Mg was also observed in the 2Mg–Fe mixture subjected to controlled reactive mechanical alloying (CRMA) in hydrogen. A massive formation of Mg(OH) 2 hydroxide is observed by XRD in the powders reactively milled in hydrogen and subsequently exposed to the ambient environment for about 4 months. The formation of Mg(OH) 2 occurs due to hydrolysis of nanostructured -MgH 2 . Abnormally high weight losses on the order of ∼16–24 wt.% are observed during thermogravimetric analysis (TGA) of powders containing Mg(OH) 2 which confirm the release of water from Mg(OH) 2 upon heating. Also, differential scanning calorimetry (DSC) curves show endothermic peaks corresponding to the release of water from Mg(OH) 2 which are in excellent agreement with DSC peaks corresponding to the release of water from Mg(OH) 2 in the 2Mg–Fe 10 h reactively milled and “aged” mixture. © 2004 Elsevier B.V. All rights reserved. Keywords: Nanostructured materials; Magnesium hydrides and hydroxides; Reactive mechanical milling; X-ray diffraction; Thermal analysis and calorimetry 1. Introduction There is now a strong consensus that hydrogen is consid- ered as a leading candidate for clean, sustainable energy sys- tems in the future [1,2]. One of the principal applications of hydrogen will be in the proton exchange membrane (PEM) fuel cells for various types of vehicles. However, as pointed out by Ritter et al. [2] onboard hydrogen storage is proving to be one of the most important issues and potentially biggest roadblock for the implementation of a hydrogen economy. The most common storage systems such as high pressure gas cylinders and cryogenic tanks for liquid hydrogen suf- fer from inherent safety problems and relatively low volu- metric densities (∼40 kg H 2 m -3 for gas under 80 MPa and ∗ Corresponding author. Fax: +1-519-888-6197. E-mail address: ravarin@mecheng1.uwaterloo.ca (R.A. Varin). 1 On leave of absence from Powder Metallurgy Research Academy, Central South University, Changsha 410083, PR China. ∼71 kg H 2 m -3 for liquid hydrogen) [3,4]. The highest volu- metric densities of hydrogen (80–150 kg H 2 m -3 ) are found in solid metal/intermetallic hydrides [4]. One of the most attractive metal hydrides is magnesium dihydride, MgH 2 . It is characterized by quite high volumetric and gravimet- ric hydrogen capacity of ∼110 kg H 2 m -3 and 7.6 wt.%, re- spectively [3–5]. Its additional advantage is that Mg is a low-cost material. Furthermore, it is now well established that MgH 2 can be relatively easily synthesized by reactive mechanical alloying (RMA) [6–13] in the nanostructured (nanocrystalline) form having nanometer-sized grains ran- domly distributed within the micrometer-sized powder par- ticles. Nanocrystalline MgH 2 has much improved hydrogen sorption kinetics and further improvements can be achieved by alloying nanocrystalline MgH 2 with various elements, or by adding catalysts and oxides [6–12,14–18]. However, very recently we have found that in the 2Mg–Fe powders processed by controlled reactive mechanical al- loying (CRMA) and exposed to the ambient environment 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.12.040