Recent progress in the field of hydrogen storage materials has received extensive support from mechanochemistry methods. Mechanical milling has been widely used not only to tune metal micro- structures for modifying their hydrogenation properties but also as an efficient tool for the synthesis of hydrogen storage materials. Solid/solid, solid/liquid and solid/gas reactions can be activated by mechanochemistry.
Mechanical milling acts as a combination of compression and shear on the powder between two colliding balls and between ball and container wall. Upon impact, the powder particles trapped be- tween them will first experience elastic deformations, which are reversible in the elastic region (Fig. 22). If the load increases, the material enters into plastic region and irreversible deformations oc- cur, which may be followed by breakage of the material. Therefore, collisions result in impulses of compression and shear that generate plastic deformation and fracture of the particles. Most of the en- ergy transferred to the powder is mainly used for creating new surfaces and concomitant particle refining.
Very similar mechanical effects are produced by Severe Plastic Deformation (SPD) techniques such as Equal Channel Angular Pressing (ECAP), Cold Rolling (CR), and High Pressure Torsion (HPT). There- fore, it could be expected that SPD will have similar impact on hydrogen storage behaviour than mechanical milling.
In the particular case of solid/solid reactions, while comminution is an important result of milling, agglomeration assisted by cold-welding processes and thereby the formation of active interfaces is more relevant. Solid-state reactions likely happen at the interface between solid particles of the dif- ferent reactants. The increase of the surface-area to bulk-volume ratio due to mechanically induced decreasing particle size increases the interfacial contact area between reactive compounds and there- fore also the rate of reaction. In addition, plastic deformations due to mechanical work create defects providing fast atomic diffusion pathways and contributing to the increased reaction rate. The intense mixing action provided by the random motion of milled powders and repeated particle fracture and welding favours the formation of active interfaces and ensures the chemical homogeneity of the final product at long milling time.
As concerns solid/gas reactions, particle comminution and related effects such as fresh surface gen-
eration and diffusion path reduction are determinant. Thus,Mechanical Millingunder hydrogen gas is
characterized by fast formation of hydride compounds under moderate pressure and temperature. For
instance, several days are required for the synthesis of Mg2CoH5and Mg2FeH6hydrides by sintering
methods at temperatures as high as 750 K, whereas the reaction takes place in only 3 h by reactive ball
milling under the same hydrogen pressure (9 MPa)[116,139]. Embrittlement due to hydrogen absorp-
tion favours particle refinement and fast synthesis of binary metal hydrides. From this point of view, milling energy (determined by milling process parameters such as rotation speed and ball-to-powder mass ratio) and mechanical properties of reactants (toughness, fracture limit) must play a key role. For the synthesis of ternary hydrides starting from elemental powders, the above-mentioned mechanisms of cold-welding and interface diffusion of solid reactants should be also considered.
The feasibility of hydride formation in solid/gas reactions is governed by thermodynamics (i.e. hy- dride stability under external pressure and temperature conditions). Thus, the formation of highly sta-
ble hydrides such as TiH2, ZrH2and MgH2is straightforward. In contrast, the synthesis of hydrides that
are reversible near room temperature, such as LaNi5and NaAlH4requires high pressure for the hydride
to be stable at the temperatures reached during the milling process[121]. If the pressure is not high en-
ough, only the
a
-solid solution LaNi5H0.15or the more stable Na3AlH6phase are observed, respectively.In this context, the failure to obtain the LiAlH4or ternary alkaline hexa-alanates by reactive ball milling
is not surprising since they are not stable at the usual operating pressure and temperature[121].
Therefore, in reactive ball-milling experiments, both macroscopic and local pressures as well as temperatures have to be considered in detail. As concerns pressure, one can distinguish the mechan- ical pressure in the material trapped between two colliding steel balls (internal mechanical pressure) and the gas vial pressure (external isostatic pressure). The gas pressure is not significantly affected by milling due to the high compressibility of the gas phase. Most modern equipment allows for gas pres- sures up to 15 MPa. In contrast, the internal pressure of the material at mechanical impact may well reach some GPa and is not isotropic. This would explain for instance the formation of the high pressure
c
-MgH2phase by mechanical milling, which otherwise occurs in anvil cells above 2 GPa[282]. Forma-tion of
c
-MgH2phase can be obtained by mechanical milling of thermodynamically stableb-MgH2phase under argon atmosphere, which demonstrates that its formation is related to the internal mechanical pressure.
As concerns the temperature, one can also distinguish between the temperature of the material trapped between the milling tools (material temperature at impact) and the macroscopic material temperature. For materials with high thermal conductivity (i.e. metals) or by milling under hydrogen gas (which also offers high thermal conductivity), the macroscopic material temperature is not ex- pected to differ much from the vial temperature. The temperature increase in the vial does not exceed some tens of degrees and temperatures in the range 320–350 K have been monitored by temperature gauges. This temperature increase is however non-negligible for hydrogen storage systems that are reversible near normal conditions of pressure and temperature. Several methods can be followed to circumvent this problem such as working at higher hydrogen pressures and minimizing temperature
increase by using short milling times (below10 min). Another and more elegant alternative ap-
proach is working at low temperatures, i.e. cryo-milling, though the temperature of the system has to be kept high enough to allow for hydrogen mobility in the bulk powder material.
Mechanical milling of powders can be performed under other reactive gases such as diborane and ammonia or in a liquid medium such as THF for the synthesis of hydrogen storage materials. Once again fresh surface generation induced by mechanical work is likely to be determinant to promote so- lid/gas and solid/liquid reactions. Contrary to reactive mechanical milling under hydrogen gas, these preparation methods are only recently explored in the literature. Further progress is still needed to understand the involved reaction mechanisms and the feasibility of compound formation by these no- vel routes. A wide research field remains open for the production of new hydrogen storage systems with undiscovered hydrogenation properties.
Acknowledgments
We thank V. Balema for fruitful discussions regarding this review. The work was supported in part by the Danish National Research Foundation (Center for Materials Crystallography), the Danish Stra- tegic Research Council (Center for Energy Materials and the HyFillFast project), and by the Danish Re- search Council for Nature and Universe (Danscatt). We are grateful to the Carlsberg Foundation. J.H. would like to thank the Natural Science and Engineering Council of Canada and also the Research Council of Norway for additional funding that permitted a sabbatical leave at the Institute for Energy Technology (IFE) in Norway. ML, FC and JZ would like to thank CNRS and the French agency ANR for financial supports trough research programs ALHAMO and NANOHYDLI.
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