Storing hydrogen in materials is based on the observation that metals can reversibly absorb hydrogen. However, the practical application of such a finding is found to be rather challenging especially for vehicular applications. The ideal material should reversibly store a significant amount of hydrogen under moderate conditions of pressures and temperatures. To date, such a material does not exist, and the high expectations of achieving the scientific discovery of a suitable material simultaneously with engineering innovations are out of reach. Of course, major breakthroughs have been achieved in the field, but the most promising materials still bind hydrogen too strongly and often suffer from poor hydrogen kinetics and/or lack of reversibility. Clearly, new approaches have to be explored, and the knowledge gained with high-energy ball milling needs to be exploited, i.e. size does matter! Herein, progress made towards the practical use of magnesium as a hydrogen store and the barriers still remaining are reviewed. In this context, the new approach of tailoring the properties of metal hydrides through size restriction at the nanoscale is discussed. Such an approach already shows great promise in leading to further breakthroughs because both thermodynamics and kinetics can be effectively controlled at molecular levels.

Hydrogen in magnesium: new perspectives toward functional stores

The low or lack of solubility of fullerenes, carbon nanotubes and graphene/graphite in organic solvents and water severely hampers the study of their chemical functionalizations and practical applications. Covalent and noncovalent functionalizations of fullerenes and related materials via mechanochemistry seem appealing to tackle these problems. In this review article, we provide a comprehensive coverage on the mechanochemical reactions of fullerenes, carbon nanotubes and graphite, including dimerizations and trimerizations, nucleophilic additions, 1,3-dipolar cycloadditions, Diels–Alder reactions, [2 + 1] cycloadditions of carbenes and nitrenes, radical additions, oxidations, etc. It is intriguing to find that some reactions of fullerenes can only proceed under solvent-free conditions or undergo different reaction pathways from those of the liquid-phase counterparts to generate completely different products. We also present the application of the mechanical milling technique to complex formation, nanocomposite formation and enhanced hydrogen storage of carbon-related materials.

Mechanochemistry of fullerenes and related materials

The problem of solid state hydrogen storage

In order to obtain faster hydrogen sorption kinetics, MgH2-Fe nanocomposites were prepared by high-energy ball milling. The MgH2 decomposition was studied in samples obtained by changing in a systematic way both the catalyst amount and the degree of microstructural refinement. To this purpose, blends containing increasing Fe concentration have been ball milled in processing conditions able to impart different amount of structural defects. The resulting samples have been characterized by X-ray diffraction to investigate the microstructural features and the phase composition, while the powder morphology and the degree of catalyst dispersion were analyzed by scanning electron microscopy. Differential scanning calorimetry was carried out to characterize the hydrogen desorption behavior of these nanocomposites. Experimental results clearly show that the characteristics of the desorption process are dominated, among other factors, by the morphology of the catalyst dispersion, which in turns depends on the processing conditions and blend composition. In order to achieve low desorption temperatures the homogeneous catalyst dispersion in micron-size particles throughout the structure is required. This condition can be achieved by suitable tuning of the milling conditions and of the catalyst amount.

Hydrogen desorption from ball milled MgH2 catalyzed with Fe

Hydrogen storage in Mg–Ni–Fe compounds prepared by melt spinning and ball milling

Magnesium hydride–graphite composites produced via ball milling exhibit fast H2 desorption/absorption kinetics; moreover, their activation behaviour is particularly attractive if compared with MgH2-based materials obtained so far. In this work, we report the phase analysis (by X-ray diffraction) and investigate the crystal structure and the desorption behaviour (using a home-made Sievert apparatus) of MgH2–graphite composites produced by ball milling. Milling with graphite is found to improve the desorption rate of MgH2 changing the reaction mechanism.

Hydrogen desorption from magnesium hydride-graphite nanocomposites produced by ball milling

Defect structures in FeAl B2 Alloys

A powder mixture of Al65Cu23Fe12 composition milled for four hours in a planetary ball mill was used as a starting material for quasicrystalline structure formation. Phase analysis of as-milled samples shows a mixture of intermetallic phases with preponderance of A2 and DO3 structures. Also the presence of residual Al and Fe was detected. Annealing the milled samples gives rise to complex solid-state transformations. Mossbauer spectroscopy shows that the residual Fe amount decreases gradually with annealing temperature and that starting from the temperature of 440†C only an asymmetrical doublet is present. The formation of a structure almost totally icosahedral was observed in the XRD patterns of samples annealed at 600†C and above. The qualitative differences in the Mossbauer spectra observed for these samples may be associated with the presence of “approximant” phases to quasicrystalline structure.

Mössbauer and X-Ray Diffraction Study of the Phase and Structure Transformations During Annealing of Mechanically Alloyed Al65Cu23Fe12

High-energy ball milling was used to promote the solubilization of iron into NiAl powder for an iron concentration range of 10–30 wt.%. The microstructural evolution induced by the intense mechanical deformations, under different milling conditions, was followed by X-ray diffraction and Mossbauer spectroscopy. The Mossbauer spectra are dominated by a magnetic sextet of about 33 T. Increasing the time and the speed of milling gives rise to a non-resolved doublet, having parameters typical of a NiAl compound with Fe atoms in solution. At the same time a reduction of lattice parameter occurs, which can be correlated to composition variations and partial disordering of the NiAl structure. Subsequent annealing modifies the Mossbauer spectra noticeably. In particular, the nonmagnetic component becomes a broad singlet. Both diffraction analysis and Mossbauer spectroscopy indicate that a fee Ni(Al,Fe) solid solution is forming in samples milled in agate. It is observed that the grain size of the milled products remains in the nanometric range even after thermal treatment, which adds interest to possible applications.

High-Energy Ball Milling of NiAl(Fe) System

http://www.wien2k.at/papers/intermetallics.pdf

Defective structure of the ThFe0.2Sn2 and PrFe0.4Sn2 compounds

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