Synthesis of Sr2AlH7 by ball milling followed by hydrogenation

Article abstract of DOI:10.1016/j.jallcom.2004.11.006

The hydride Sr₂AlH₇ was synthesized by ball milling of the alloy consisting of Sr and Al (2:1) under hydrogen at 150 rpm for 10 h followed by hydrogenation or sintering at 533 K for 2 days. The yield of Sr ₂AlH₇ in the hydrogenated product was 66 wt.%, which is larger than that in the sintered product. However, the mass fraction of Al in the milled samples decreased largely when the Sr₂Al alloy was milled at 300 rpm for 50 h or longer. In such a case, the Sr₂AlH₇ hydride cannot be obtained during the subsequent hydrogenation.

Full text of DOI:10.1016/j.jallcom.2004.11.006

Journal of Alloys and Compounds 394 (2005) 308–311
Synthesis of Sr2AlH7 by ball milling followed by hydrogenation
Q.A. Zhang, E. Akiba∗
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan
Received 13 October 2004; received in revised form 1 November 2004; accepted 3 November 2004
Available online 10 December 2004
Abstract
The hydride Sr2AlH7 was synthesized by ball milling of the alloy consisting of Sr and Al (2:1) under hydrogen at 150 rpm for 10 h followed
by hydrogenation or sintering at 533 K for 2 days. The yield of Sr2AlH7 in the hydrogenated product was 66 wt.%, which is larger than that
in the sintered product. However, the mass fraction of Al in the milled samples decreased largely when the Sr2Al alloy was milled at 300 rpm
for 50 h or longer. In such a case, the Sr2AlH7 hydride cannot be obtained during the subsequent hydrogenation.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Hydrogen absorbing materials; Ball milling; Crystal structure; Gas–solid reaction
1. Introduction
panied by Al in the product as shown in Eq. (2), which limits
the yield of Sr2AlH7.
As reported previously [1–3], SrAl2 can be hydrogenated
toSrAl2H2 atabout463 K. Increasingthehydrogenationtem-
perature to 513 K, SrAl2H2 further absorbs hydrogen to form
Sr2AlH7 and Al. These reactions are shown below.
Ball milling has been proved to be a powerful technique to
synthesize hydrides, such as Mg2FeH6 and Mg2CoH5, which
are hard to be obtained by conventional methods [9,10]. We
have used the ball milling technique to synthesize Sr2AlH7
from the alloy of Sr and Al in a ratio of 2:1. Generally speak-
ing, two methods of ball milling were used for the direct syn-
thesis of the hydrides [11]. One is reactive milling, in which
ball milling is carried out under a hydrogen atmosphere. The
other method is milling of elemental hydrides to produce
complex hydrides. In this work, we synthesized the Sr2AlH7
hydride by ball milling of a Sr2Al alloy under hydrogen but
it was followed by further hydrogenation of different types
of samples.
SrAl2+H2→ SrAl2H2
(1)
(2)
4SrAl2H2+3H2→ 2Sr2AlH7+6Al
The hydride Sr2AlH7 decomposes to SrH2, Al and H2 when
the temperature is raised to about 563 K. Sr2AlH7 has a large
H/M (where H and M indicate hydrogen and metals, respec-
tively)ratio. Thus, itcanbeconsideredasapossiblecandidate
for hydrogen storage application.
In the binary Sr–Al system, several intermetallic com-
pounds like SrAl4, SrAl2, Sr5Al9 and Sr8Al7 were reported,
but Sr2Aldoesnotexist[4–8]. Therefore, SrAl2 hasbeenused
as the starting material to synthesize Sr2AlH7 [2]. However,
the hydrogenation reaction rate is very slow because long-
range diffusion of Al is necessary during the formation of
Sr2AlH7. Moreover, the Sr2AlH7 hydride is always accom-
2. Experimental details
The Sr2Al alloy was prepared by induction melting of Sr
and Al metals. On the basis of the stoichiometric amounts
of the starting materials, an extra 3 wt.% of Sr was added to
compensate the loss of Sr during induction melting. The alloy
prepared was used without further treatment. In order to pre-
vent exposure to air, the subsequent procedures were carried
out in a glove box under a dry argon atmosphere. The alloy
∗
Corresponding author. Tel.: +81 29 861 4541; fax: +81 29 861 4541.
E-mail address: e.akiba@aist.go.jp (E. Akiba).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.11.006

Q.A. Zhang, E. Akiba / Journal of Alloys and Compounds 394 (2005) 308–311
309
ingot was ground to powders with particle size smaller than
300 ␮m for subsequent ball milling. The milling was made
using a Fritsch Pulverisette 5 planetary mill under hydrogen
(0.6 MPa). Stainless steel bowls and balls (the diameter was
10 mm) were used. The weight of the powder was 2 g and the
ball to powder weight ratio was 20:1.
After ball milling, the milled samples were loaded into
stainless steel containers and placed in stainless steel au-
toclaves. Hydrogenation reactions were carried out under a
hydrogen pressure of 7 MPa at 533 K for 2 days. A part of
the milled samples was pressed under 20 MPa into a pel-
let of 5 mm radius. The pellet was sintered under hydrogen
(7 MPa) at 533 K for 2 days. To evaluate the phase structures
of the samples, XRD measurements were performed using
a Rigaku RINT2500V diffractometer with Cu K␣ radiation
at 50 kV and 200 mA. The XRD profiles were analyzed with
the Rietveld refinement program RIETAN-2000 [12].
Fig. 2. XRD patterns for the samples milled under hydrogen at a rotation
speed of 300 rpm for (a) 50 h; (b) 100 h; (c) 150 h.
3. Results and discussion
during the milling. However, Sr2AlH7 cannot be synthesized
by prolonging the milling time. Even though the milled prod-
ucts were further hydrogenated, there was no evidence of the
formation of Sr2AlH7 (as shown in Fig. 3).
Fig. 1 shows the result of the Rietveld analysis for the
Sr2Al alloy. Three phases Sr8Al7, hexagonal Sr and fcc Sr
were found in the alloy. For the Rietveld refinement, the struc-
ture models for the phases were taken from the reported struc-
tures [8,13,14]. As shown in the figure, the diffraction pattern
calculated from the structure model is in good agreement with
that measured. The amounts of the Sr8Al7, hexagonal Sr and
fcc Sr phases were calculated to be 65, 31 and 4 wt.%, re-
spectively. In fact, the Sr2Al alloy should consist of Sr8Al7
and fcc Sr under the equilibrium condition according to the
phase diagram [4]. However, the hexagonal Sr was also found
in this case.
Fig. 2 shows the XRD patterns for the samples milled un-
der hydrogen at a rotation speed of 300 rpm for 50, 100 and
150 h, respectively. It can be seen that the samples consist
of SrH2 and Al with a small amount of impurity Fe. This
means that the Sr absorbed hydrogen to form SrH2; while
the Sr8Al7 phase reacted with hydrogen to form SrH2 and Al
In order to understand the reason why Sr2AlH7 did not
form in the above experimental conditions, the XRD pat-
terns of the samples milled at 300 rpm for 50, 100 and 150 h
were refined by the Rietveld method. The phase abundance
calculated is listed in Table 1. The amount of Al decreased
with prolonged milling time. The original mass fraction of
Al in the as-cast alloy was 13 wt.%. However, the mass frac-
tion of Al decreased to 6 wt.% after ball milling for 50 h.
A similar phenomenon was also reported by Huot et al. [9].
They found that the mass fraction of Co in the milled mixture
Fig. 1. Rietveld refinement of the observed XRD pattern for Sr2Al alloy.
Reflection markers are for Sr8Al7, hexagonal Sr and fcc Sr phases (from
above), respectively.
Fig. 3. XRD patterns for the hydrogenated samples with a prior ball milling
under hydrogen at 300 rpm for (a) 50 h; (b) 100 h; (c) 150 h.

310
Q.A. Zhang, E. Akiba / Journal of Alloys and Compounds 394 (2005) 308–311
Table 1
Phase abundance (wt.%) of the products milled under hydrogen at a rotation
speed of 300 rpm for 50, 100 and 150 h
Phase
50 h
100 h
150 h
Al
SrH2
Fe
6
93
1
5
94
1
2
96
2
Fig. 5. Calculated and observed XRD patterns for the sample milled under
hydrogen at a rotation speed of 150 rpm for 10 h followed by hydrogenation
at 533 K for 2 days. Reflection markers are for Sr2AlH7, Al, SrH2, SrO and
Fe (from above), respectively.
dride is obviously present. This means that the milled prod-
uct consisting of SrH2 and Al reacted with hydrogen to form
Sr2AlH7, which can be expressed as
Fig. 4. Calculated and observed X-ray diffraction patterns for the sample
milled under hydrogen at a rotation speed of 150 rpm for 10 h. Reflection
markers are for Al, SrH2 and Fe (from above), respectively.
4SrH2+2Al + 3H2→ 2Sr2AlH7
(3)
Table 2 compares the phase abundance of the products before
and after hydrogenation. The hydrogenated sample contains
66 wt.% of Sr2AlH7, 31 wt.% of Sr2H, 1 wt.% of Al, SrO and
Fe. The remained SrH2 was caused by the loss of Al during
ball milling, which has a great effect on the yield of Sr2AlH7.
The milled sample was also sintered in hydrogen at 533 K
for 2 days. Fig. 6 shows the Rietveld refinement of the XRD
pattern of the sintered sample. We found that the result in the
sintered sample is similar to that in the hydrogenated sample.
However, the yield of Sr2AlH7 is 54 wt.%, which is lower
than that in the hydrogenated sample as shown in Table 2.
Actually, the difference between the two treatment methods
is only the distinct shapes of the samples. The hydrogenated
sample was treated in a powder form but the sintered one was
in a pellet form. The lower yield of Sr2AlH7 in the sintered
sample might be caused by the lower surface area for reaction.
of 2Mg + Co was 15–20 wt.% but it was originally 55 wt.%.
However, they also found that the mass fraction of Fe in the
milled mixture of 2Mg + Fe was close to the starting ratio.
Moreover, we found that the lattice parameter of the impu-
˚
rity Fe in the milled products was 3.717(2) A, which is larger
˚
than the reported value 3.6599 A [15]. This indicates that the
impurity Fe is probably a (Fe, Al) solid solution. We noted
that the mass fraction of Al in the Sr2AlH7 hydride is around
13 wt.%. Thus the low mass fraction of Al in the above milled
products might be the reason for the absence of Sr2AlH7.
In order to decrease the loss of Al during ball milling, the
Sr2Al alloy was milled under hydrogen at a rotation speed of
150 rpm for 10 h. Fig. 4 shows the calculated and observed
XRD patterns for the milled sample. The sample consisted
of SrH2 and Al with a small amount of impurity Fe (1 wt.%).
The mass fraction of Al was calculated to be 11 wt.%, as
shown in Table 2. Thus, the mixture of SrH2 and Al with a
small loss of Al can be easily produced by milling of the alloy
under hydrogen at 150 rpm for 10 h.
The milled product was further hydrogenated at 533 K for
2 days. The calculated and observed XRD patterns of the
hydrogenated sample are shown in Fig. 5. The Sr2AlH7 hy-
Table 2
Phase abundance (wt.%) of the products milled under hydrogen at 150 rpm
for 10 h followed by hydrogenation or sintering
Phase
Milling
Hydrogenation
Sintering
Sr2AlH7
Al
SrH2
Fe
–
11
88
1
66
1
31
1
54
2
42
1
Fig. 6. Calculated and observed XRD patterns for the sample milled under
hydrogen at a rotation speed of 150 rpm for 10 h followed by sintering at
533 K for 2 days. Reflection markers are for Sr2AlH7, Al, SrH2, SrO and Fe
(from above), respectively.
SrO
–
1
1

Q.A. Zhang, E. Akiba / Journal of Alloys and Compounds 394 (2005) 308–311
311
Zhu and Akiba reported that Sr2AlH7 which was prepared
by hydrogenation of SrAl2 reversibly dehydrogenated and
hydrogenated in a commercially available pct apparatus with-
out a catalyst at 543 K [16]. This reaction is expressed by Eq.
(4) being the same as Eq. (3) obtained in the present work.
under “Development of fundamental technologies in the safe
utilization of hydrogen”. The authors thank Dr. H. Enoki of
the National Institute of Advanced Industrial Science and
Technology of Japan for technical assistance.
2Sr2AlH7↔ 4SrH2+2Al + 3H2
(4)
References
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4. Conclusions
The hydride Sr2AlH7 was successfully synthesized by ball
milling of Sr2Al alloy under hydrogen at 150 rpm for 10 h fol-
lowed by hydrogenation or sintering at 533 K for 2 days. The
hydrogenated product had a higher yield of Sr2AlH7 than the
sintered product. During ball milling, the mass fraction of Al
decreased, leading to the formation of some SrH2. However,
if the Sr2Al alloy is milled at 300 rpm for 50 h or longer,
the mass fraction of Al decreases strongly. As a result, the
Sr2AlH7 hydride cannot be obtained during a subsequent hy-
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Acknowledgments
This work has been supported by New Energy and
Industrial Technology Development Organization (NEDO)
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