Journal of Alloys and Compounds 395 (2005) 159–165 Mechanical alloying and nitrogen storage properties of Ca–Fe powder F.N. Ishikawa ∗ , K. Irie, E. Yamasue, K.N. Ishihara, H. Okumura Department of Energy Science, Kyoto University, Yoshida-honmachi Sakyo-ku, Kyoto city, Kyoto 606-8501, Japan Received 6 September 2004; accepted 12 October 2004 Available online 7 January 2005 Abstract Nitrogen absorption and desorption properties as well as possible formation of a non-equilibrium phase were investigated on Ca–Fe powder prepared by ball milling (BM) method. The milled Ca–Fe powder absorbed and desorbed more amount of nitrogen than pure Ca powder and pure Fe powder when each was separately milled and processed. The amount of nitrogen absorbed and desorbed by the Ca–Fe powder is increased with milling time and its magnetization significantly decreased. The large reduction in the magnetization is explained by formation of the non-equilibrium alloy phase in the Ca–Fe powder system, where the new phase is capable of absorbing and desorbing nitrogen and has its magnetization significantly less than pure Fe. © 2004 Elsevier B.V. All rights reserved. Keywords: Nitrogen storage; Mechanical alloying; Ca–Fe; Non-equilibrium phase; Magnetization 1. Introduction Recently, nitrogen storage alloys have attracted attention as a new functional material [1–3]. They are the alloys that can reversibly absorb and desorb nitrogen, and 13 examples have been reported so far [1–3], where the nitrogen is absorbed under an NH 3 or N 2 atmosphere at high temperatures and desorbed under an H 2 atmosphere as NH 3 gas. The nitrogen storage capacities per unit volume of these alloys are superior to conventional high pressure containers of nitrogen stored at 15 MPa. The nitrogen storage alloys generally consist of both a metal having high affinity with nitrogen and another metal that can dissociate the nitrogen sources such as NH 3 and N 2 gases. Rare earth metals and Fe have been used as the for- mer and the latter, respectively, but the resource amount of the former is limited and the price is quite high. One aim of this study is thus to develop a nitrogen storage alloy using calcium (Ca) instead of rare earth metals, since Ca has high ∗ Corresponding author. Present address: University of Southern Califor- nia, Department of Materials Science and Engineering, Los Angeles, CA 90089-0482, USA. Tel.: +1 81 75 753 5476; fax: +1 81 75 753 5476. E-mail address: fishikaw@usc.edu (F.N. Ishikawa). affinity with nitrogen and is relatively abundant and inexpen- sive, possibly leading to the alloy cost reduction. Another aim of this study is to research and develop a new and non- equilibrium state in the Ca–Fe alloy system through a solid state reaction such as mechanical alloying (MA). As illustrated in Fig. 1, Ca and Fe atoms are immiscible and do not form intermetallic compounds at any composition and temperature. The solid state reaction on the alloy system has not been studied, as far as we know, and the MA method may be suitable to develop a new nitrogen storage alloy with the non-equilibrium state. In the MA process, which was orig- inally developed in 1970 [4] to achieve a particle dispersion strengthening superalloy, the repeated addition of mechan- ical power can finely mix and alloy two or more elements through a solid state reaction and, in recent years, much atten- tion is paid as one of the methods to develop non-equilibrium materials [5,6]. Non-equilibrium states are generally repre- sented by the supersaturated solid solution, metastable phase, nanocrystal and amorphous, etc., and it is frequently reported that, due to the nano-order mixing, partial amorphization or extension of solid solubility can be observed when using the MA method for various systems, which do not form either a solid solution or an intermetallic compound as an equilib- rium phase [5,6]. Therefore, in this study, the MA method is 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.10.068