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Journal of Alloys and Compounds 580 (2013) S163–S166
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Journal of Alloys and Compounds
Dehydrogenation process of AlH3 observed by TEM
Yuki Nakagawa a, , Shigehito Isobe a,b, Yongming Wang a, Naoyuki Hashimoto a, Somei Ohnuki a,
⇑
Liang Zeng c, Shusheng Liu c, Takayuki Ichikawa c, Yoshitsugu Kojima c
a Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8278, Japan
b Creative Research Institution, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
c Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 27 February 2013
Dehydrogenation processes of
a- and c-AlH3 were investigated by in situ transmission electron
microscopy observations. The relationship between Al2O3 thickness and dehydrogenation kinetics was
also clarified. The initial shape of -AlH3 particle was cubic and that of -AlH3 particle was rod-shaped.
The process of -AlH3 was quite similar with -AlH3. The precipitation and growth of Al was observed in
a
c
Keywords:
Hydrogen storage materials
AlH3
TEM
c
a
both processes. The dehydrogenation kinetics did not depend on Al2O3 thickness. It was found that
milling effect on the dehydrogenation kinetics was larger than doping effect. The dehydrogenation
process was discussed in terms of both microscopic and kinetic studies.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
process of a-AlH3 were performed by some groups. Ikeda et al.
observed the precipitation and growth of Al particles inside the
AlH3 particles [8]. Muto et al. determined the surface films on
AlH3 particles as amorphous Al2O3 with a thickness of 3–5 nm by
means of electron energy-loss spectroscopy [9]. However, the
AlH3 is a promising hydrogen storage material because of its
high gravimetric and volumetric densities (10.1 mass% and
149 kg H2/m3, respectively) [1]. It also desorbs hydrogen in a rela-
tively low temperature range (100–200 °C) following the simple
dehydrogenation process of
microscopic studies. Dehydrogenation property of
studied by some groups [10–12]. Graetz et al. reported the faster
dehydrogenation kinetics of -AlH3 than that of -AlH3 in the
low temperature range (ꢀ100 °C) [11]. In this paper, our purpose
is to investigate the dehydrogenation processes of - and -AlH3
c-AlH3 was not discussed in these
reaction (AlH3 ? Al + 3/2 H2).
a-AlH3 is the most stable phase
c-AlH3 was
among the various crystalline structures of AlH3 [2]. Baranowski
et al. reported an estimated equilibrium H2 pressure of 1 GPa at
25 °C according to the thermodynamic values taken from the
c
a
report by Sinke et al. [3,4]. This value indicates that
a
-AlH3 can
a
c
spontaneously desorb hydrogen at room temperature. However,
it has been suggested that the Al2O3 films on surface of AlH3 parti-
cles inhibit the spontaneous dehydrogenation at room tempera-
ture [5]. Graetz et al. suggested that the enhanced stabilization of
in nanoscale by means of in situ transmission electron microscopy
(TEM) observations. From the microscopic observations and kinet-
ics studies, the relationship between Al2O3 thickness and dehydro-
genation kinetics was also clarified.
Dow’s
[6]. They also stated its stabilization mechanism remains un-
known. Kato et al. investigated the surface change of -AlH3 during
a-AlH3 was primarily attributed to a thick surface oxide film
2. Experimental procedures
a
the dehydrogenation by means of in situ X-ray photoelectron spec-
troscopy [7]. They proposed the dehydrogenation mechanism
including the effect of Al2O3 film. They claimed that dehydrogena-
tion only starts when the oxide film breaks up due to thermal
expansion of the bulk AlH3. In this way, Al2O3 film seems to play
an important role in the kinetics. However, its exact role has not
been clarified yet. Microscopic studies about the dehydrogenation
AlH3 was prepared by the chemical reaction between LiAlH4 and AlCl3 in ether
solution [2]. Hand-milled sample was prepared by grinding AlH3 for 5 min in an
agate mortar under Ar atmosphere. Ball-milled and doped samples were prepared
by ball-milling under 0.1 MPa Ar for 1 h. Ball-milling processes were performed
by using a planetary ball-mill apparatus (Fritsch Pulverisette 7 at 200 rpm), with
18 stainless steel balls (7 mm in diameter) and 300 mg samples (ball:powder ra-
tio = 64:1, by mass). Powder X-ray diffraction (XRD, Philips, X’Pert-Pro) was per-
formed with Cu K
a radiation. Thermogravimetry and differential thermal analysis
(TG-DTA, Bruker, 2000SA) was performed with a heating rate of 2 °C/min under he-
lium gas flow rate of 300 mL/min. Transmission electron microscopy (TEM, JEOL,
JEM-2010) observations were carried out at 200 kV. The samples were dispersed
on a molybdenum microgrid mesh for in situ TEM observations. A heating holder
was required to heat samples in the TEM column.
⇑
Corresponding author. Tel./fax: +81 11 706 6772.
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.