| dc.description.abstract | A systematic investigation of the structural stability, evolution and hydrogenstorage
properties of Mg-based hydrides was carried out, involving mechanical milling
and chemical alloying. The effects of milling on particle size, lattice parameter,
microstructure, and phase composition of the powder mixtures were characterised using
SEM, X-Ray diffraction and quantitative Rietveld analyses. Mechanical milling was
shown to be an effective method of refining the particle size, particularly when MgH2 is
involved. The influences of the selected chemical elements, including transition metals,
graphite carbon and rare-earth metals, on hydrogen desorption/absorption of various
milled mixtures were clearly identified using coupled Thermogravimetry (TG) and
Differential Scanning Calorimetry (DSC). The as-received MgH2 shows an onset
desorption temperature of 420°C. Mechanical milling reduces the onset temperature to
330°C. Chemical alloying, via surface catalysis and/or solid-solutioning, further
increases the desorption kinetics and reduces the desorption temperature down to
250°C. The degree of such effect decreases from Ni, Al, Fe, Nb, Ti, to Cu. Further
comparison of desorption characteristics of MgH2 mixed and mechanically alloyed with
Ni clearly shows that the kinetic improvement and the effective reduction of the
desorption temperature is mainly due to a catalytic effect, rather than solid-solutioning
of Ni. Although posing little influence on desorption characteristics, graphite improves
the absorption behaviour of MgH2. The rare earth metals, Y and Ce, do not seem to
influence hydrogen desorption of MgH2 due to the formation of stable hydride phases,
but CeO2 in the (MgH2+Ce) mixture provides a beneficial effect on desorption kinetics.
Multi-component mixtures of (MgH2+15Fe+5Ce) and (MgH2+Al+Ni+Y+Ce) exhibit
relatively fast desorption kinetics and the lowest desorption temperature at about 240°C
and 220°C, respectively. Finally, mechanical alloying of the non-stoichiometric
compositions of (3MgH2+Fe) and (4MgH2+Fe) effectively generated a new ternary
hydride, Mg2FeH6, with a very high yield of about 80wt% from the (3MgH2+Fe)
mixture, which is a promising candidate for heat-storage. The research findings laid a
clear and valuable foundation for future development of new and cost-effective Mgbased
hydrogen storage materials with a high capacity, a low desorption temperature
and rapid kinetics. | en_US |
