Materials Science and Engineering A 449–451 (2007) 1123–1126 Mechanical alloying, nitrogen storage and magnetization of Ca–Co powder H. Okumura ∗ , F.N. Ishikawa, Y. Morotomi, E. Yamasue, K.N. Ishihara Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan Received 22 August 2005; received in revised form 3 February 2006; accepted 9 February 2006 Abstract Mechanical alloying was performed on the Ca–Co system and the nitrogen storage property and magnetization were investigated. The solid solution of Co dissolving Ca is formed during MA process that has limited storage ability, while longer milling produces amorphous/nanocrystalline phase with the ability. The milled Ca–Co powder absorbed and desorbed more nitrogen than pure Ca and Co powder each separately milled and processed. The increase of Ca surface area mainly controls the storage property for short MA, while that for longer MA is ascribed to the non- equilibrium phase. The stored amount for Ca–Co powder increases with milling time, and its magnetization significantly decreased. The large reduction is explained by linear dilution of atomic moment based on the rigid band model, while the deviation from the model for longer MA is explained by formation of the non-equilibrium phase with limited magnetization but with nitrogen storage ability. © 2006 Elsevier B.V. All rights reserved. Keywords: Nitrogen storage; Mechanical alloying; Ca–Co; Non-equilibrium phase; Magnetization; Solid solution 1. Introduction Recently, nitrogen storage alloys have attracted attention as a new functional material, and 14 examples have been reported so far [1–5]. They are the alloys that can absorb nitrogen under an NH 3 or N 2 atmosphere and desorb under an H 2 atmosphere as NH 3 gas at higher temperatures (623–723 K). The Co element, either in the hcp or fcc form, dissolves Ca about 6–8 at.% for <473–873 K, while Ca does not dissolves Co, and they do not form any intermetallic compound [6]. Mechan- ical alloying (MA) and the solid state reaction on the alloy system has not been studied, as far as we know, and the MA may be suitable to develop a new nitrogen storage alloy. We previously reported that Ca–Fe powder composite prepared by ball milling (BM) has nitrogen absorption (NA) and nitrogen desorption (ND) capabilities, and that the formation of a non- equilibrium phase may be the key to the high nitrogen storage ability [5]. The cobalt element, as an iron, is known to have a catalytic ability to dissociate nitrogen sources such as N 2 or NH 3 gas [7]. Also, Ca has high affinity with nitrogen and is rel- atively abundant and inexpensive. In addition, the variation of Co magnetization alloyed by Ca has not been reported and is of interest. In this paper, thus, the nitrogen storage properties and ∗ Corresponding author. Tel.: +81 75 753 5476; fax: +81 75 753 5476. E-mail address: okumura@energy.kyoto-u.ac.jp (H. Okumura). magnetization variation of the Ca–Co powder prepared by BM are investigated, as well as the formation of a non-equilibrium phase and solid solutions, and compared with the Ca–Fe system using the model in our former paper [5]. 2. Experimental procedure The Ca granules of 99.5% purity (particle size 1–3 mm) and Co powder of 99% purity (about 100 m) were used as starting materials. The Ca and Co (10 g in total) were measured in a glove box under an Ar atmosphere to exhibit 1:1 by a molar ratio, and the mixtured sample was ball-milled with a ball/sample weight ratio of 10/1, with 8 g of toluene added as PCA. The details of the experimental procedures are the same as those applied for the Ca–Fe alloy system [5]. 3. Results and discussion Nitrogen contents and nitrogen storage abilities per 1 mol of sample after milling, absorption and desorption treatments are shown in Table 1. The definition of nitrogen storage ability (N) and the calibrations made for the minor contaminating elements are the same as described in [5]. The Ca powder milled for 50 h absorbs 0.308 mol of nitrogen (0.322–0.014) and desorbs 0.009 mol, while Co powder milled for 50 h could absorb and desorb less than 0.01 mol. The N value of the Ca–Co powder 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.312