Thesis - Oztek_Muzaffer_T_200508_MA

One of the limiting factors for hydrogen storage capacity and uptake rate is the
reluctance of hydrogen to occupy all available interstices in the metal atom network; thus,
detailed structural data are needed, if possible, to understand this phenomenon [54].
Mostly, the determination of hydrogen positions has been accomplished by neutron
vibrational spectroscopy [54-56], which is not considered in this study.
The addition of different elements also affects the crystal structure. There are
different types of imperfections in crystalline structures. A vacancy is a point defect,
which is the absence of an atom from an atomic site. Dislocations are line defects that
cause lattice distortion centered around a line [57].
In the case of LaNi 5 , lattice constants are reported to increase with the addition of
Al [40] and crystalline imperfections occur. With the substitution of Ni with Al, the
dislocation density and vacancy concentration induced by hydrogenation are reported to
decrease [58].
In another study, mechanically alloyed LaNi 4.2 Al 0.8 (from its corresponding pure
elements milled for 30 hours in a Spex 8000 Mixer Mill using a round bottom stainless
steel vial) was reported to form an amorphous phase directly from the starting mixture
without formation of another phase. The La(Ni,Al) 5 alloy with a hexagonal CaCu 5 phase
was formed only after annealing at 750 ºC for 0.5 hour [59]. For the samples prepared
with that sequence, the surface segregation of La was more pronounced than that of
polycrystalline powders from arc-melted ingots. Thus, an increment in solid state
solubility was observed [60]. This was explained by the increased area of Ni clusters,
which play an important role in the adsorption of H 2 to the surface.
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