Microstructural optimization of LaMg12 alloy for hydrogen storage

A Rapid Solidification technique was applied to a LaMg12 alloy in order to achieve refinement of the grain size. Thin ribbons were produced by solidifying the melt on a spinning copper wheel in an argon atmosphere using three different rotations speeds, 3.1, 10.5 and 20.9 m/s. The ribbons were analyzed by synchrotron X-ray diffraction (SR XRD), electron probe microanalysis (EPMA), and TEM, and they were subjected to hydrogen absorption-desorption cycling and to thermal desorption spectroscopy (TDS) characterization. SR XRD and EPMA revealed formation of two phases, LaMg12-x and Mg. From SR XRD it was found that, depending on the cooling rate, the LaMg12-x alloy crystallized with three different structural modifications, hexagonal TbCu7 (highest cooling rate), tetragonal ThMn12 (medium cooling rate) and orthorhombic LaMg11 type (lowest cooling rate). A metastable TbCu7-type structure (sp. gr. P6/mmm; a = 5.9617(3); c = 5.2153(5) angstrom) was not known from the earlier performed research and is reported for the La-Mg system for the first time. From the scanning electron microscopy (SEM) studies, RS was found to cause a significant grain refinement and an amorphisation for the highest cooling rate. The particle size of the formed hydride phases varied in the range 0.2-3 mu m depending on the RS synthesis route used to prepare the original alloy. Hydrogen absorption resulted in a two-step disproportionation process: LaMg12 + H-2 -> LaH3 + Mg -> LaH3 + MgH2. A decrease in the grain size improved the hydrogenation kinetics. Hydrogen desorption studied by TDS and in situ SR XRD showed a major peak of hydrogen evolution at similar to 370 degrees C. For the alloys synthesized at 10.5 m/s and 20.9 m/s, it was accompanied by an extra desorption event at 415 degrees C. This extra peak was associated with Mg-assisted low temperature hydrogen desorption from LaH2 proceeding below 450 degrees C and leading to a recombination process to form the initial intermetallic alloy LaMg12. (C) 2010 Elsevier B.V. All rights reserved.