Int. J. Hydrogen Energy, Vol. 7, No. 1, pp. 8%94, 1982. Printed in Great Britain. 0360-3199/82/010089--06 $03.00/0 Pergamon Press Ltd. International Association for Hydrogen Energy. THE INFLUENCE OF ALUMINIUM ON THE PROPERTIES OF THE Mg2Cu-H2 SYSTEM A, BIRIS, D. LuPu, R. V. BUCUR, E. INDREA, G. BORODI and M. BOGDAN Institute of Isotopic and Molecular Technology, P.O. Box 243, R-3400 Cluj-Napoca, Romania (Received for publication 5 February 1981) Abstract The addition of aluminium to a mixture of Mg-Cu (2:1 atomic ratio) leads by melting to the formation of increasing amounts of Mg(Cu,AI)2 where aluminium substitutes for copper atoms in the MgCu2 lattice, raising the lattice parameter. The hydrogen absorbed in this phase is strongly bounded, as proved by desorption isotherms and nuclear magnetic resonance data. INTRODUCTION THE SEARCHfor light and cheap aluminium-based alloys with aluminium playing an active part in hydrogen absorption is a very attractive idea proposed also by other authors [1]. However, aluminium is a metal in which the solubility of hydrogen is very low [2] and although aluminium trihydride has been known for a long time, it cannot be prepared directly from aluminium and hydrogen [3] and only a surface reaction could be obtained, even using atomic hydrogen. Our studies on the absorption of hydrogen in Mg-AI-Ti alloys are now in progress [4]. The participation of aluminium in the hydrogen absorption was reported in the literature [5] in a study on some Ce-Mg-AI alloys. On the other hand, many binary [6] intermetallic compounds containing aluminium show lower ability to absorb hydrogen with respect to the other component taken alone. Thus it seems that the vicinity of aluminium in an intermetallic compound or alloy is determinant for blocking or stimulating the hydrogen absorption; like, for example, magnesium in Mg2Cu absorbs hydrogen while in MgCu2 it does not [7]. Following this idea we studied the hydrogen absorption in magnesium-copper alloys (at constant atomic ratio Mg/Cu = 2 : 1) to which different amounts of aluminium are added. RESULTS AND DISCUSSION The alloys were obtained from pure metals (99.99%) melted in a graphite crucible under argon atmosphere. A mechanical stirrer of stainless steel was used in order to get a homogeneous melt. The absorption-desorption studies were performed as described elsewhere [8]. The samples were saturated with hydrogen at high pressures after which the desorption was started. Usually 2-3 days were needed to reach the equilibrium in the Mg2Cu region (see below). X-Ray experiments were performed on a horizontal powder diffractometer with Cu K~radiation (). = 1.54178/~) Ni filtered. The diffracted profiles were measured with a proportional gas detector, single-channel pulse-height discrimination and standard associated counting circuitry. (a) Quantitative phase composition analysis Semiquantitative analysis was achieved using well homogenized standard samples of known chemical composition. The phase composition of standards was calculated from the Cu-Mg phase diagram. For the quantitative analysis of the standard sample it is assumed to be a mixture of a component i with mass absorption coefficient/~i and matrix with mass absorption coefficient /~m. The mass absorption coefficient of the matrix was determined through semiquantitative analysis. The weight fraction xi of the component i is given by Klug and Alexander [9]. [i/loi = Xi~i xe(~ - ~m) + ~m 89