Microstructural optimization of LaMg<subscript>12</subscript> alloy for hydrogen storage

Abstract: 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 20.9m/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)Å) 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 0.2–3μm depending on the RS synthesis route used to prepare the original alloy. Hydrogen absorption resulted in a two-step disproportionation process: LaMg12 +H2 →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 ∼370°C. For the alloys synthesized at 10.5m/s and 20.9m/s, it was accompanied by an extra desorption event at 415°C. This extra peak was associated with Mg-assisted low temperature hydrogen desorption from LaH2 proceeding below 450°C and leading to a recombination process to form the initial intermetallic alloy LaMg12. [Copyright &y& Elsevier]