J.-L. Bobet et al. / Journal of Alloys and Compounds 356–357 (2003) 570–574
573
Table 2
Crystallite size of both Mg and MgH2 for different grinding duration and after hydriding at 300 8C for 2 h
RMG duration
Hydrided for 2 h
at 300 8C
2 h
3 h
4 h
5 h
L Mg (nm)
L MgH2 (nm)
21.2 (1.6)
20.2 (1.8)
20.6 (1.4)
15.0 (8.7)
22.0 (2.7)
12.9 (0.6)
22.9 (1.0)
11.4 (1.1)
24.0 (2.7)
19.9 (2.0)
rapidly for the mixture containing Cr2O3 FSC. As men-
tioned previously, the number of crystallites per particle is
higher for the mixture containing Cr2O3 FSC and then, the
limiting diffusion layer (MgH2 layer on the surface of Mg
particle) was reached more slowly. Moreover, the zeta
potential of the (Mg1(c)Cr2O3) mixture is higher than that
of (Mg1Cr2O3 FSC) (i.e. 135 and 122 mV, respectively).
As the zeta potential is mainly due to the Cr31 ion on the
surface of Mg particle, this difference can be explained by
the presence of some Cr atoms (replacing Cr31). The
presence of chromium metal can play a major role in the
improvement of the sorption properties. Some XPS experi-
ments are in progress to confirm the zeta potential mea-
surements.
the adsorption of BN layers at freshly exposed magnesium
surfaces and the reaction with oxygen species of active B
and N radicals formed as a result of B–N bond breaking
during the milling operation. Then, the impermeable
surface oxide or hydroxide layer is not re-formed and the
metallic Mg surfaces are readily accessible to hydrogen.
Then the different behaviour between BN and graphite
addition can be explained by the difference in structure
(existence of stronger interlayer bond in BN compared
with graphite) and the difference in oxidation sensitivity
[16]. A more detailed study on (Mg1BN) mixtures is
under way.
4. Conclusion
After hydridation, the crystallite size of MgH2 increases
(Table 2) and the particle size only slightly decreases (Fig.
4) so that the density of the grain boundary decreases. This
leads to a decrease in the initial hydriding rate as a
function of the number of hydriding/dehydriding cycles.
The decrease in the number of nucleation sites by sintering
can also be considered. Finally, the absorption properties
reported here are very close to that obtained by Oelerich et
al. [14] for nanocomposite MgH2 /Cr2O3 mixtures. More-
over, a significant absorption is obtained at temperatures as
low as 200 8C (i.e. almost 3 wt.% after only 1 min).
In Fig. 5, the effects of BN addition are compared with
that of graphite addition published by Bouaricha et al. [15].
The large improvement in the absorption properties of
(Mg110wt.% BN) mixture compared with pure mag-
nesium can be related (as it was for graphite addition) to
RMG is a good way to produce MgH2 ‘in situ’ and to
increase significantly the reactive surface area. Both steps
of the hydrogen reaction (i.e. nucleation and diffusion) are
improved because of both the creation of defects and the
decrease in the number of crystallites per particle for
diffusion. Various additives such as oxides, intermetallics
and metals have been tried. All additives allow one to
improve the sorption properties. Furthermore, for YNi, the
decomposition, during RMG, into yttrium hydride and Ni
induces: (i) the use of higher wt.% of YNi in the starting
mixture and (ii) the best sorption properties. Oxide addi-
tion is also effective and the electronic nature of the metal
constituent of the oxide appears to be of prime importance.
Finally, the sorption properties obtained are very similar to
those of nanocrystalline products which offers some
promise for the future use of the RMG process.
The results obtained using Cr2O3 FSC are almost
equivalent to those obtained for nanocomposite MgH2 /
Cr2O3. The variation of the number of crystallites per
particle gives a satisfactory explanation of the phenom-
enon. These results encourage us to continue this study and
to use more widely the supercritical fluid process.
Finally, BN addition has similar effect to graphite
addition. The difference in hydrogen sorption property
improvement can be correlated with the difference in
oxidation behaviour and chemical bonding between C and
BN.
References
Fig. 5. Hydrogen absorption behaviour at T5300 8C and P51.1 MPa of
Mg1BN compared with Mg1graphite [15] mixtures.
[1] I.G. Konstanchuk, E.Y. Ivanov, M. Pezat, B. Darriet, V.V. Boldyrev,
P. Hagenmuller, J. Less-Common Met. 131 (1987) 181.