Destabilization of High Desorption Temperature Hydrides by Nano Compositing

5 Destabilization of High Desorption Temperature Hydrides by (Nano)Compositing 

The desorption peak temperature maxima for the MgH constituent in a composite are plotted in Fig. 3.31 as a function of the content of LiAlH. It is seen that the ROM for the MgH desorption temperature in the (MgH -i- LiAlH ) nanocomposite 

As mentioned before in order to determine whether or not the free A1 formed upon decomposition of LiAlH /LijAlH in the composite could act as a catalyst, we also prepared composites with the content of A1 equivalent to the content of A1 in the Awt%LiAlH. Their DSC desorption peak temperature maxima are also plotted in Fig. 3.31. The composites with the equivalent content of A1 do not seem to follow the ROM behavior. Therefore, one can tentatively conclude that the underlying physical mechanism for the ROM behavior is not related to the catalytic effect of free Al. However, this possibility, however remote, cannot be completely ruled out of hand because the particle size of free Al formed upon decomposition might be much smaller than that obtained by ball milling of Al metal powder added to MgH powder. Nanosized free Al could aquire catalytic behavior. However, at the moment we do not have any evidence for that. 

Nonetheless, we have ealculated the lattice parameters and t e unit ce vo ume of free Mg which is formed from the decomposition of MgH an oun decreases with increasing content of LiAlH in a composite up to 30 wt c an t en more or less saturates as shown in Fig. 3.33. Such lattice shrinkage is most i e y 

Based on the DSC cnrve in Fig. 3.9b a pnre LiAlH milled for 20 h shonld decompose at 300°C throngh the reactions (Rib) and (R2) of (3.12) and (3.13), respectively, to LiH and A1 and in doing so, desorb a theoretical purity-corrected (97%) amount of eqnal to 7.66 wt%. However, it is seen in Fig. 3.34 that a pure LiAlH desorbs about 7 wt%H at 300°C. This amount is 0.7 wt% deficient with respect to the theoretical valne. This missing amount of H is lost, most likely, owing to the partial decomposition of LiAlH occurring dnring milling as evidenced by the presence of the 

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