| In this thesis, previous works on crystal structures and micro-structures of hydrogen storage alloys and their hydrides have been intensively reviewed. On this basis, the X-ray diffraction micro-structures (mainly refer to nanometer crystallite) of AB5-type alloys and their hydrides were selected as the main subject of this study. Until now, although much attention has been paid to the crystal structures and the grain properties of AB5-type hydrogen storage alloys, the X-ray diffraction microstructures between them were much less studied. However, because of the importance of it in the structural conjunction between crystal structure and grain, and the determination of it on the properties of grain boundary, lattice strain and defect, it can be supposed to have many effects on the electrochemistry of AB5-type hydrogen storage alloys. Difficulty on this study is the shortage of advanced micro-structural analysis. For the AB5-type hydrogen storage alloys, it's well-known that its X-ray diffraction line broadening during or after the hydrogen-induced cycling is always anisotropic. But the classic model for the anisotropic broadening is based on the assumption that the crystallites are ellipsoid, which is far away from the reality. However, the model proposed by Popa in 1998 for the anisotropic line broadening not only can get the shape of the crystallite, but also can obtain the distribution of the crystallites with different shape. In this study, the nanometer crystallites of over-stoichiometric La(Ni, Sn)5+x (x=0.1-0.4) alloys obtained at different cooling conditions, e.g. the as-cast, the annealed and the rapidly solidified with the rate 5 m/s, 10 m/s, 15 m/s, 20 m/s, were investigated and correlated to the crystal structures and grains observed by SEM technique. Furthermore, based on the studies on phase transitions and lattice changes of MlNi3.75Coo.75Mn0.3Al0.2 commercialized Co-included alloy, MlNi4.0Fe0.6Al0.3Si0.1 Co-free alloy and over-stoichiometric LaNi4.830Sn0.314 alloy during the hydriding-dehydriding process, the micro-structural evolutions, especially the nanometer crystallite changes during the hydriding-dehydriding process were intensively studied by in-situ X-ray diffraction. At the last of this thesis, relationships between crystallite and electrochemical behaviors were also investigated.Structural analysis by Rietveld refinement indicated that over-stoichiometric La(Ni, Sn)5+x (x=0.1-0.4) alloys still keep the CaCus-type structure, while Sn only occupy 3g(0.5, 0, 0.5) sites and some of Ni replace La at la(0, 0, 0) sites in the form of Ni-Ni "dumbbells" oriented along c-axis. One of the characteristics of this "dumbbell" structure is the B33 component of thermal parameters of the atoms in la(0, 0, 0) sites is much higher than others. Another characteristic of this "dumbbell" structure is the enhancement in c-axis is much higher than that in a-axis compared to LaNi5. By calculating the crystallite sizes of the over-stoichiometric alloys prepared at different cooling conditions, it indicated that the as-cast alloy is composed by the plate-like and the columnar crystallites, while the alloy solidified at the cooling rate of 5 m/s is exclusively composed by plate-like crystallites. As the increasing of cooling rate, crystallite size along (001) plane is enhanced, while that along (110) plane almost keep untouched. For the alloys prepared at higher cooling rates(10 m/s, 15 m/s, 20 m/s), they are composed by two type of crystallites. One is columnar that grow directionally along (001) plane and the other is plate-like obtained by the side growth along (110) plane. Accordingly, pattern of as-cast alloy observed bySEM are dendritic, which mainly consists of grains grow either along or perpendicularly to the direction of heat transformation, while grains of the alloy solidified at the cooling rate of 5 m/s are exclusively columnar. For the alloys prepared at higher cooling rates(10 m/s, 15 m/s, 20 m/s), both the columnar and the dendritic grains have been found out by SEM technique. In a word, all...
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