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These parameters are generally interdependent. For example, the optimum milling<br />
time is affected by the type of mill, temperature, etc. Ball to powder ratio (BPR) and time<br />
of milling for a desired product are dependent on the capacity of the mill. For small<br />
capacity mills, BPR of 10:1 is reported to be the commonly used [47].<br />
Ball milling has been reported to activate FeTi, Mg 2 Ni and LaNi 5 , which are inert<br />
to hydrogen prior to milling. For FeTi, milling is believed to give rise to Fe rich clusters<br />
on the surface and the generation of new surfaces by cracking [49]. LaNi 5 has been<br />
synthesized from elemental La and Ni via milling at room temperature and, using a<br />
similar method, the partially substituted La 0.5 Zr 0.5 Ni 5 has been synthesized from an<br />
equimolar mixture of LaNi 5 and ZrNi 5 [48].<br />
1.5. Crystallography Of Hydrides<br />
The understanding of the crystallographic aspect of hydrides and their precursors<br />
is important in terms of materials characterization. The positions of the host metal before,<br />
during and after hydride formation have been reported. The orientation of hydrogen<br />
within the material and the physical and chemical changes experienced by it has been<br />
investigated to justify the limits of hydriding reactions from a structural point of view.<br />
These parameters affect phase changes and physical properties, and change with<br />
composition or outside conditions. Therefore material characterization is a necessity for<br />
each different sample under study.<br />
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