Mechanochemical synthesis of nanostructured chemical hydrides in hydrogen alloying mills

Article abstract of DOI:10.1016/j.jallcom.2006.08.301

Mechanical alloying of magnesium metal powders with hydrogen in specialized hydrogen ball mills can be used as a direct route for mechanochemical synthesis of emerging chemical hydrides and hydride mixtures for advanced solid-state hydrogen storage. In the 2Mg-Fe system, we have successfully synthesized the ternary complex hydride Mg₂FeH₆ in a mixture with nanometric Fe particles. The mixture of complex magnesium-iron hydride and nano-iron released 3-4 wt.%H₂ in a thermally programmed desorption experiment at the range 285-295 °C. Milling of the Mg-2Al powder mixture revealed a strong competition between formation of the Al(Mg) solid solution and the β-MgH₂ hydride. The former decomposes upon longer milling as the Mg atoms react with hydrogen to form the hydride phase, and drive the Al out of the solid solution. The mixture of magnesium dihydride and nano-aluminum released 2.1 wt.%H₂ in the temperature range 329-340 °C in the differential scanning calorimetry experiment. The formation of MgH₂ was suppressed in the Mg-B system; instead, a hydrogenated amorphous phase (Mg,B)H_x, was formed in a mixture with nanometric MgB₂. Annealing of the hydrogen-stabilized amorphous mixture produced crystalline MgB₂.

Full text of DOI:10.1016/j.jallcom.2006.08.301

Journal of Alloys and Compounds 434–435 (2007) 743–746
Mechanochemical synthesis of nanostructured chemical hydrides in
hydrogen alloying mills
Z. Wronskia,b,∗, R.A. Varinb, C. Chiub, T. Czujkob, A. Calkac
a CANMET’s Materials Technology Laboratory, Natural Resources Canada, Ottawa, Canada
b Department of Mechanical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1
c Department of Materials Science and Engineering, University of Wollongong, NSW 2518, Australia
Available online 9 October 2006
Abstract
Mechanicalalloyingofmagnesiummetalpowderswithhydrogeninspecializedhydrogenballmillscanbeusedasadirectrouteformechanochem-
ical synthesis of emerging chemical hydrides and hydride mixtures for advanced solid-state hydrogen storage. In the 2Mg–Fe system, we have
successfully synthesized the ternary complex hydride Mg2FeH6 in a mixture with nanometric Fe particles. The mixture of complex magnesium-iron
hydride and nano-iron released 3–4 wt.%H2 in a thermally programmed desorption experiment at the range 285–295 ◦C. Milling of the Mg–2Al
powder mixture revealed a strong competition between formation of the Al(Mg) solid solution and the ␤-MgH2 hydride. The former decomposes
upon longer milling as the Mg atoms react with hydrogen to form the hydride phase, and drive the Al out of the solid solution. The mixture
of magnesium dihydride and nano-aluminum released 2.1 wt.%H2 in the temperature range 329–340 ◦C in the differential scanning calorimetry
experiment. The formation of MgH2 was suppressed in the Mg–B system; instead, a hydrogenated amorphous phase (Mg,B)Hx, was formed in a
mixture with nanometric MgB2. Annealing of the hydrogen-stabilized amorphous mixture produced crystalline MgB2.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Nanostructures; Amorphous materials; Magnesium diboride; High-energy ball milling; Mechanochemical synthesis; X-ray
diffraction; Differential scanning calorimetry
1. Introduction
under hydrogen at room temperature [3,4]. However, the yield
of these reactions must still be improved. Magnesium tetrahy-
droaluminate, Mg(AlH4)2 contains 9.3 wt.%H2. However, little
is known about this alanate and its synthesis has been conducted
via an indirect reaction in organic solvents, from which it crys-
tallizes as an impure compound [5].
We have recently reported on the use of a controlled reac-
tive mechanical alloying (CRMA) route towards development
of hydrogen storage media based on Mg–Fe [6] and Mg–Al [7]
systems. In the present work, we recapitulate our findings with
these two systems, and add new results on another Mg–B system.
Mg metal exhibits a high-storage capacity by weight
(7.6 wt.%H2) in MgH2 hydride and has been considered a can-
didate for solid-state hydrogen storage in fuel cell for cars.
However, its high-thermodynamic stability, high-desorption
temperature, low-plateau pressure, and slow sorption kinet-
ics preclude its use. Nanocrystalline MgH2, prepared in lab-
oratory high-energy ball mills shows improved kinetics of
sorption–desorption, yet the storage of hydrogen in magnesium
metal still needs development of new compositions and pro-
cesses. Several studies on hydrogenation of Mg–Fe mixtures
have been reported, all of them targeting the synthesis of Mg
complex hexahydroferrate, Mg2FeH6 (containing 5.5 wt.%H2).
This hydride has usually been synthesized by mechanical activa-
tion of powders followed by sintering in a hydrogen atmosphere
[1,2], and in a single process by reactive mechanical alloying
2. Experimental
2.1. Mechanical milling in hydrogen alloying mill
The reagent grade (Alfa Aesar) fine elemental powders of Mg with purity bet-
ter than 99.8% were used for preparation of a stoichiometric 2Mg–Fe mixture for
reactive mechanical alloying with hydrogen to form Mg2FeH6 complex hydride.
The cast Mg–2Al alloy from an experimental foundry (CANMET, Ottawa) was
crushed and used for milling with intent to form Mg(AlH4)2 alanate. Mechanical
milling was carried out in a Uni-Ball Mill 5 (A.O.C. Sci. Eng. Pty, Australia)
using a controlled energy impact mode [6,7] under 3200 impacts per minute, in
∗
Corresponding author at: CANMET’s Materials Technology Laboratory,
Natural Resources Canada, Ottawa, Canada K1A 0G1. Fax: +1 613 992 8735.
E-mail address: zwronski@nrcan.gc.ca (Z. Wronski).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.301

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Z. Wronski et al. / Journal of Alloys and Compounds 434–435 (2007) 743–746
the mill cylinder rotating at 200 rpm with four hardened steel balls. The ball-
to-powder ratio was as 40:1 and the milling time varied from 1 to 300 h. The
pressure of the H2 gas was adjusted sequentially during the milling to maintain
a pressure of 880 kPa, after the hydrogen was absorbed in the powders. Small
powder samples were collected at various times of milling for microstructural
XRD and thermal DSC analysis. The processing details for milling Mg–2Fe and
2Mg–Al powders were given in our previous publications [6,7].
2.2. Materials characterization
The crystalline structure of as-milled and reference powders were character-
ized by Rigaku Rotaflex D/Max B rotating anode powder diffractometer using
Cu K␣1 radiation (λ = 0.15406 nm). The separation of crystallite size and strain
was obtained from broadening of Bragg peaks using Cauchy/Gaussian approxi-
mation and the linear regression plots as described in our previous studies [6,7].
The standard reference material, LaB6 (National Institute of Standards and Tech-
nology NIST-SRM # 660) was used for subtracting the instrumental broadening
of the Bragg peaks.
The thermal behaviour of the powders was studied by differential scanning
calorimetry (DSC) using a ∼20 mg powder sample, a heating rate of 4 ◦C/min
and an argon flow rate of 16 ml/min. The enthalpy change of desorption, ꢀH, was
calculated from the endothermic peak area. The yield of reactions was estimated
assuming that the enthalpy formation of ␤-MgH2 and Mg2FeH6 is ꢀHf = 74 and
77.4 kJ/mol H2, respectively [8].
3. Results
3.1. Hydrogen alloying in Mg–Fe system: direct synthesis
of ternary Mg2FeH6
Nanostructured ternary complex hydride, Mg2FeH6 was syn-
thesized after more than 100 h of reactive milling of the 2Mg–Fe
powder mixture under 880 kPa of H2 (Fig. 1). The ‘hydrogen
alloying’ reaction yielded ∼60% of the ternary complex metal
hydride. The hydride was in a mixture with a nanometric phase
of ␣-Fe, with a grain size of ∼17 nm, the latter being uni-
formly distributed in the hydride matrix, as shown in Fig. 1,
obtained with field-emission transmission electron microscope
(FE TEM). In a thermally programmed desorption (TPD) exper-
iment, reported in [6], 3–4 wt.%H2 was desorbed in less than
20 min at 285–295 ◦C temperature range, without use of addi-
tional catalyst.
Fig. 1. Development of the nanostructure in the 2Mg–Fe powder milled with
hydrogen; XRD patterns (bottom; replotted from [6]), and direct imaging of
structure in FE TEM (top); the latter shows Fe nanoparticles in a hydride matrix.
effects for various scenarios used in mechanochemical synthesis
of hydrides and hydride mixtures in this system can be found in
our previous report [7].
The hydrogenated Mg–2Al powder, when milled for 100 h,
yields approximately 2.1 wt.%H released in the DSC experi-
ment (estimated from endothermic signal) in the temperature
range of 329–340 ◦C. This hydrogen was stored in a nanometric
3.2. Hydrogen alloying in a Mg–Al system: formation of
Al(Mg) solid solution
The FCC cubic solid solution of Mg in Al, Al(Mg), is the
only product of the reaction up to 10 h of milling (Fig. 2). The
cast Mg–2Al alloy exhibited a microstructure consisting of Al
dendrites in the Al3Mg2 intermetallic matrix. Upon relatively
short milling, the Mg released from the intermetallic compound
dissolves in Al. This induces nanostructure formation in Al(Mg)
solid solution in the early stages of the milling, which is seen
as a large broadening of XRD lines in the pattern for 10 h
milling, Fig. 2. With increasing milling time, upon hydrogena-
tion, the Al(Mg) extended solid solution disproportionates with
the formation of ␤-MgH2 and Al phases. This is reflected in a
change of the lattice parameter a from 0.4038 0.0016, through
0.4114 0.0014, to 0.4034 0.0015 nm for powders milled for
1, 10 and 200 h, respectively (Table 1). Quantification of those
Fig. 2. Development of the nanostructure in the Mg–2Al powder milled with
hydrogen, as revealed by XRD (replotted from [7]).

Z. Wronski et al. / Journal of Alloys and Compounds 434–435 (2007) 743–746
745
Table 1
4. Discussion
Al grain size, lattice parameter a, strain, and H desorbed from Mg–2Al powder
mixture
Several concurrent mechanisms may occur during milling:
(i) reaction of hydrogen with milled Mg to form the most stable
phase of MgH2; (ii) dissolution of the alloying atoms of the sec-
ond metal into the crystalline lattice of host metal that lowers the
strength of the Mg–H bond, destabilizes the MgH2, thus promot-
ing the formation of a ternary complex hydride; (iii) enhanced
interdiffusion of the atoms of Mg and the second metal that leads
to the formation of an extended nanocrystalline solid solution,
followed by disproportionation of the solid-state solution that is
driven by its reaction with the hydrogen; this results in the forma-
tion of a nanometric mixture of MgH2 with the second metal; (iv)
mixing of elements in an amorphous phase containing hydrogen
and formation of ‘amorphous hydride phase’, am-Mg(M)Hx.
The ‘alloying’ of MgH2 and the mixing of elements can be effec-
tively promoted by proliferation of microdefects in ball-milled
powders.
Milling
time (h)
Grain size
(nm)
a (nm)
Strain
H2 (wt.%)
10
50
100
200
7a
73
77
50
0.4114 0.0014a 0(10−3
0.4036 0.0015
0.4035 0.0014
0.4034 0.0015
)
0.0
1.9
2.1
2.0
1.1 × 10−3
1.1 × 10−3
1.4 × 10−3
The a for Al metal in unmilled powder was 0.4038 0.0016.
a
Solid solution.
magnesium dihydride/Al metal mixture. No formation of ternary
hydride Mg(AlH4)2 was observed.
3.3. Hydrogen alloying in Mg–B system: formation of
amorphous Mg–B–H phase
Amorphous boron batches were mixed with elemental Mg in
the atomic ratio 1:2 with the intent to mechanically synthesize
the Mg(BH4)2 complex hydride, however, no formation of
this ternary, complex hydride was observed. The reaction
product after short time milling of powders was a mixture of
MgH2 and MgO (the latter forming during manipulation in
the X-ray diffractometer). However, the XRD pattern at the
bottom of Fig. 3, for the powders milled for 200 h, demonstrates
absence of the MgH2 phase. Instead, the peaks rising above the
background can be attributed to MgB2. The prominent feature
is the broad background, which rises between 25◦ and 45◦ and
is centered roughly at 2θ ∼ 35◦. The annealing of this powder
in the DSC produced a single endothermic heat event at 358 ◦C.
We are inclined to attribute it to the hydrogen evolution. The
XRD pattern for the annealed powder is shown in Fig. 3 (top
pattern). The pattern exhibits strong peaks from the MgB2
phase, and the elevated background disappears. Therefore, the
XRD pattern in Fig. 3 can be understood if assuming further
MgB2 phase forms during the decomposition of the amorphous
phase containing hydrogen. Since the enthalpy of decompo-
sition of the amorphous phase is unknown, we are not able
to evaluate the amount of hydrogen released from this phase
using DSC.
In this study, we observed all the mechanisms described
above. The formation of Mg2FeH6 complex hydride in the
2Mg–Fe system (Fig. 1) can be explained by the enthalpy
of formation of the ternary hydride that differs only slightly
from that for MgH2 [8]. On the other hand, the observed
disproportionation of Mg–2Al alloys can be explained by a
relatively high-thermodynamic stability of MgH2 compared to
that of Mg–Al–H, compounds [9], combined with a relatively
low stability of Mg(AlH4)2 complex hydride. Finally, the
ultimate mixing of elements was observed in the Mg–B–H
system. Apparently, the metastable amorphous phase is the
preferred phase when compared to the unstable Mg(BH)2
complex and the stable magnesium dihydride.
5. Conclusions
Mechanical alloying of magnesium metal powders with
hydrogen in specialized hydrogen ball mills can provide a direct
route for mechanochemical synthesis of complex chemical
(complex) hydrides and hydrides mixtures for advanced solid-
state hydrogen storage. Chemical (complex) hydride mixed with
crystalline extended solid solutions, metal nanoparticles, and
amorphousphasescanbeproduceddependingonchemicalcom-
position and process conditions.
Acknowledgements
This work was partially funded from NRCan Climate Change
Technology & Innovation funds and was also supported by
a grant from the Natural Sciences and Engineering Research
Council of Canada, which are greatly acknowledged. The
authors thank Dr. John Wilson from the XRD laboratory in the
CANMET-MMSL for providing XRD patterns and Dr. Graham
Carpenter for the TEM picture.
References
Fig. 3. XRD patterns of Mg–2B powders milled with hydrogen as-milled (bot-
tom) and the annealed (top) powders.
[1] J. Huot, H. Hayakawa, E. Akiba, J. Alloys Compd. 248 (1997) 164–167.
[2] I. Saita, K. Saito, T. Akiyama, J. Alloys Compd. 390 (2005) 265–269.

746
Z. Wronski et al. / Journal of Alloys and Compounds 434–435 (2007) 743–746
[3] J. Huot, S. Boily, E. Akiba, R. Schulz, J. Alloys Compd. 280 (1998) 306–309.
[4] F.C. Gennari, F.J. Castro, J.J.A. Gamboa, J. Alloys Compd. 339 (2002)
261–267.
[5] M. Fichtner, O. Fuhr, J. Alloys Compd. 345 (2002) 286–296.
[6] R.A. Varin, S. Li, Z. Wronski, O. Morozowa, T. Khomenko, J. AlloysCompd.
390 (2005) 282–296.
[7] R.A. Varin, Ch. Chiu, T. Czujko, Z. Wronski, Nanotechnology 16 (2005)
2261–2274.
[8] B. Bogdanovic´, A. Reiser, K. Schlichte, B. Spliethoff, B. Tesche, J. Alloys
Compd. 345 (2002) 77–89.
[9] C.X. Shang, M. Bououdina, Y. Song, Z.X. Guo, Int. J. Hydrogen Energy 29
(2004) 73–80.