Low energy mechanical alloying of a Mm-Ni-Cu mixture: synthesis, structure and hydriding properties of the products

P. LACOURT; M.R. ESQUIVEL; G. MEYER

Reyhavik, Iceland

Conferencia; MH2008-International Symposium on Metal-Hydrogen Systems; 2008

Hannes Jónsson-Svein Ólafsson-University of Iceland

An AB5 intermetallic was synthesized by low energy mechanical alloying of a mixture of chunks of Mm (La0.25Ce0.52Nd0.17Pr0.06) and powdered Ni and Cu metals. The four successive stages occurring during this process [1] were identified and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and differential scanning calorimetry (DSC). The Initial stage occurs up to 15 h. The cycle of fracture-cold welding is dominated by the first process. Mm, the ductile one is more affected by the process and the amount of energy impinged to the system is preferentially absorbed by this alloy. Superficial microforging on Mm particles is observed.  No homogeneous chemical composition is achieved and Mm average particle size is strongly reduced. Between 15 and 120 h, the Intermediate stage evolves. Initial Cu and Ni are still individually identified by XRD and EDS. Initial Mm-Ni-Cu structures start to grow as observed in the broad bump within the 20 -40 º 2q range of the XRD patterns obtained. At longer integrated milling times, the Final stage is reached. A homogenous chemical composition is obtained and the AB5 is formed at 120 h of processing. As milling time increases, the Completion stage appears and the equilibrium reached between cold welding and fracture leads to particles refinement, to a narrower particle size distribution and to final chemical composition. Crystallization processes were observed by XRD at temperatures higher than 400 ºC but no crystallization peaks were observed by thermal heating up to 570 ºC by DSC. The structural properties observed will be correlated to the AB5 hydrogen sorption properties and applied to a one-stage hydrogen compression system. References: [1]. L. Lu, M.O. Lai, Mechanical alloying, V. 4, Kluver Academic Publishers, Boston, 1998, p 69-153.