L Journal of Alloys and Compounds 327 (2001) 275–280 www.elsevier.com / locate / jallcom Structural and hydriding properties of (Mg Ca )Ni Laves phase alloys 1 2x x 2 a, a a b * N. Terashita , K. Kobayashi , T. Sasai , E. Akiba a Tsukuba Research Lab., Japan Metals & Chemicals Co. Ltd,5-9-6 Tokodai, Tsukuba, Ibaraki 300-2635, Japan b National Institute of Advanced Industrial Science and Technology, AIST Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 11 April 2001; accepted 30 April 2001 Abstract Novel Mg-based pseudo-binary alloys (Mg Ca )Ni which desorb hydrogen at room temperature were successfully developed. The 12x x 2 alloys can be synthesized by a conventional melting method and annealing technique. The crystal structure of annealed (Mg Ca )Ni 12x x 2 (0.32#x#1.0) was the C15 Laves phase type and in this composition range the alloys were single phase. Flat plateaus in the pressure–composition isotherms were observed at ambient temperatures. The maximum hydrogen content of the present study is 1.4 mass% (H/M¯0.7) under hydrogen pressure of 3.5 MPa at 313 K. The effects of substitution of Ca for Mg on the lattice parameter, hydrogen equilibrium pressure and phase changes with hydrogenation are discussed.  2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Energy storage materials; Gas–solid reactions; X-ray diffraction 1. Introduction Laves compound MgYNi reported by Orimo et al. [6] and 4 RMg Ni (R5rare earth) reported by Kadir et al. [7–9] 2 9 Pure magnesium metal absorbs hydrogen to form MgH and, Kohno et al. [10]. 2 and its hydrogen contents is as high as 7.6 mass% [1]. In In this study, we focused on a ternary magnesium addition, it is rich in natural resources and less expensive calcium nickel system especially on the compositions of than other hydrogen absorbing metals. Therefore mag- (Mg Ca )Ni , which are pseudo-binary alloys based on 12x x 2 nesium-based hydrogen absorbing alloys have a great MgNi (C36 Laves phase) and CaNi (C15 Laves phase). 2 2 potential for applications such as hydrogen transportation This system has been reported by Oesterreicher et al. [11], and storage. Especially, the intermetallic compound Mg Ni but they did not report any hydrogen absorbing phase. 2 reversibly forms Mg NiH and its hydrogen contents is as We report here on preparation of ternary magnesium– 2 4 high as 3.6 mass% [2], which is one of the highest values calcium–nickel alloys using a conventional induction among hydrogen absorbing intermetallic compounds. melting and annealing technique, change in morphology, However, the hydrogen desorption temperature of Mg Ni crystalline structure of each phase, and hydriding / dehy- 2 or other magnesium-based alloys is above 550 K for 0.1 driding properties. MPa hydrogen desorption pressure [2]. Furthermore, al- most all the magnesium-based alloys except Mg Ni dis- 2 proportionate to form MgH and intermetallic compounds 2. Experimental procedures 2 which do not absorb hydrogen upon hydriding [3]. There- fore, they are unfavorable for most of the applications to 2.1. Sample preparation be used in practical hydrogen energy systems. Recently, a few studies on new hydrogen absorbing Pseudo-binary intermetallic compounds (Mg Ca )Ni 12x x 2 intermetallic compounds containing magnesium have been (where x50, 0.17, 0.33, 0.5, 0.67, 0.83, 1) were melted in found in the literature. For example, these are the AB alumina crucible using a vacuum high frequency induction 2 Laves phase compounds MgNi M (M5Cr, Mn, Fe) furnace and were cast into a water-cooling board mold 1.9 0.1 reported by Tsushio and Akiba [4,5], the C15-b type AB made of iron under 0.03 MPa argon atmosphere. The 2 starting elements were commercial grade Mg, Ca and Ni. The shape of the cast alloys was plate-like and the *Corresponding author. E-mail address: terasitan@jmc.co.jp (N. Terashita). thickness approximately 2 cm. The weight of each cast 0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388(01)01563-8