223109-2
Isobe et al.
Appl. Phys. Lett. 96, 223109 ͑2010͒
FIG. 2. BF image ͑a͒, FFT ͑b͒, and IFFT ͑c͒ from 10 mol % catalyzed
sample at room temperature.
Fig. 1 shows the bright field ͑BF͒ images and SADP of
MgH2 catalyzed with 1 mol % Nb2O5 at RT, 150 and
200 °C. In case of RT, the diffraction rings and patterns of
MgH2 and MgO were confirmed. MgO could be considered
as an impurity in the starting material of MgH2. At 150 °C,
the diffraction ring of MgH2 became weak and the diffrac-
tion ring of Mg appeared. Therefore, it was confirmed that
the decomposition of MgH2 and the formation of Mg started
from around 150 °C. In the case of 200 °C, the diffraction
ring of MgH2 almost disappeared and the Debye ring of Mg
developed, meaning that the Mg crystal was formed with
several 10 nm in size. On the other hand, Nb2O5 was not
identified by both BF images and SADP. In BF images, the
shape of sample hardly changed while the contrast became
weak with increasing temperature, indicating that the density
of the particle decreased due to formation of voids.
In order to identify microstructure of interface between
MgH2 and Nb2O5, MgH2 catalyzed with 10 mol % Nb2O5
was observed by using high resolution microscopy with
HVEM ͑1250 kV͒. On the desorption kinetics, the dehydro-
genation characteristics of the MgH2+1 mol % Nb2O5 is su-
perior to that of MgH2+10 mol % Nb2O5 due to a larger
amount of interface volume between MgH2 and catalyst. Fig-
ure 2 shows ͑a͒ BF image, ͑b͒ FFT image, and ͑c͒ IFFT
image at RT. As shown in Fig. 2͑a͒, it was confirmed that
upper and bottom parts are MgH2 and Nb2O5, respectively.
Meanwhile Mg formation is confirmed near the interface.
Figures 2͑b͒ and 2͑c͒ are FFT and IFFT images from square
in ͑a͒, showing phases of MgH2 and Mg. It was indicated
that the initial decomposition from MgH2 to Mg occurred at
the interface between MgH2 and Nb2O5 even at RT. The
reason why the decomposition started even at RT was prob-
ably due to an effect of electron-beam irradiation. Here, the
effect of electron-beam irradiation on the decomposition
should be considered. By electron-beam irradiation, the tem-
perature would be locally increased. The decomposition
could be caused by this local heating. However, despite the
electron-beam irradiation could affect the entire sample; the
results showed that Mg phase appeared at the interface. From
this consideration, it can be recognized that the decomposi-
tion started at the interface due to the catalytic effect. The
size of Mg phase, as shown in the dottted line in Fig. 2͑a͒
was approximately 60 nm. Figure 3 shows BF image and
related FFT and IFFT images at 100 and 200 °C. The Mg
phase grew up with increasing temperature. Conclusively, we
FIG. 3. BF image ͑a͒, FFT ͑b͒, and IFFT ͑c͒ from 10 mol % catalyzed
sample heated to 100 °C BF image ͑d͒, FFT ͑e͒, and IFFT ͑f͒ from 10
mol % catalyzed sample heated to 200 °C.
interface between MgH2 and Nb2O5 and proceeded with in-
creasing temperature. Here, it should be noticed that Mg
phase appeared and grew up between MgH2 and catalyst,
indicating that MgH2 phase did not touch the catalyst any
longer. In that case, it should be considered that how the
catalyst does work. In general, surface reaction is regarded as
the rate determining step of the decomposition of MgH2. On
the surface, hydrogen molecules form with hydrogen atoms.
The catalyst should rescue the activation energy of the sur-
face reaction. So, it can be recognized that hydrogen mol-
ecules emitted at the interface. As a result, it can be sug-
gested that hydrogen atoms diffuse from MgH2 phase to the
interface through Mg phase, as shown in Fig. 4. With respect
to state of hydrogen atoms, initially hydrogen is as atoms in
successively observed the growth of Mg phase started at the
FIG. 4. Schematic of dehydrogenation process of MgH2.
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