Thesis - Oztek_Muzaffer_T_200508_MA

The average value of percent hydrogen for the sample described was calculated as
1.04% (LaNi 5 H 4.5 ). Mechanical milling, followed by a single hydriding/dehydriding cycle
resulted in the activation of LaNi 5 . This was explained by the generation of new surfaces
by cracking due to cell volume expansion upon hydriding [62]. The percent hydrogen
obtained, as opposed to the theoretical value of 1.57% (LaNi 5 H 6.7 ), was most likely due
to the application of different activation procedures and experimental parameters, such as
low H 2 pressure. It was reported previously that at high H/M ratios corresponding to the
full hydride LaNi 5 H 6.7 , reproducibility is reported to be markedly affected by extrinsic
experimental conditions and intrinsic properties, such as compositional variances, of
LaNi 5 [53].
The results of DSC analyses were dependent on several uncontrolled experimental
parameters. For instance, from the milled samples, only a portion was taken for hydrogen
uptake analysis, and the analyzed portion may not have been representative of the whole.
The size of the sample taken to the DSC was another variable, at low pressures the
amount of H 2 was not enough to fully hydride the sample. Smaller samples had higher
hydrogen capacities due to the more H 2 per unit mass of sample.
3.1.3.3. Effects of Type of Mill, Milling Speed and Milling Atmosphere
Additional LaNi 5 was milled using a Fritsch Pulverisette Planetary Mill. Milling
was done for 10 minutes, with a ball to powder ratio of 10:1. Milling speeds of 100 rpm
and 300 rpm were chosen to observe the effect of those parameters on activation. In the
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