Stages of mechanical alloying during the synthesis of intermetallics applied to the thermal compression of hydrogen

N.M. CERÓN-HURTADO; M.R. ESQUIVEL

Ciudad de San Juan, San Juan, Argentina

Congreso; Hyfusen 2009 – Tercer Congreso Nacional –Segundo Congreso Iberoamericano Hidrógeno y Fuentes Sustentables de Energía; 2009

Instituto de Energia y Desarrollo Sustentable (IEDS)-CNEA

The stages and processes governing the mechanical alloying of a mixture of La0,25Ce0,52Nd0,17Pr0,06, Ni and Sn are discussed in this work. Four stages are identified and characterized during the evolution of the process. The morphology, surface characteristics and particle size distribution is analyzed by a combination of Scanning Electron Microscopy (MEB) and an ad-hoc Imaging analyzer software. The chemical homogeneity is analyzed by Energy Dispersive Spectroscopy (EDS). The presence, identity and determination of the different crystalline structures are characterized by X-ray Diffraction (XRD). The crystallite size and strain produced by milling are quantified using this technique. The amount of the different phases present during milling is studied using Rietveld refinement measurements. The Initial stage occurs up to 30 h of integrated milling time. The stage is governed by fracture of the bigger particles of Sn and La0,25Ce0,52Nd0,17Pr0,06. No chemical homogeneity is observed and no phases other than Sn, Ni and La0,25Ce0,52Nd0,17Pr0,06 are detected. The Intermediate stage is observed between 30 h and 50 h. At this stage, the particle size is diminished and the surface of the metals becomes more reactive. The diffusion effects are enhanced and  cold welding affects the evolution of the process. Between 50 h and 70 h, cold welding and fracture reached equilibrium and the final stage is obtained. The chemical composition is no longer changed and the La0,25Ce0,52Nd0,17Pr0,06Ni4,7Sn0,3 intermetallic is obtained. The intermetallic is assigned to Space Group P6/mmm The Lanthanides occupy the Wyckoff positions (1a) while Sn and Ni are distributed among the  (2c, 3g) ones. The cell parameters found are a = 4,91(2) Å and c =  4,06(7) Å.  At longer milling times, the Completion stage is reached and the refinement of the structure occurred. The correlation between the final structure characteristics and the dynamic hydrogen measurements are also presented in this work.