Dehydrogenation properties of nano-/amorphous Mg2NiHx by hydrogen induced mechanical alloying

Article abstract of DOI:10.1016/S0925-8388(00)01131-2

In recent years, nanocrystalline materials have been of interest for hydrogen storage applications. Especially, the great improvement of the hydrogenation properties of Mg alloys that can be achieved by nanocrystallization. In this study, nanocrystalline Mg₂NiH_x is fabricated from Mg and Ni chips by hydrogen induced mechanical alloying (MA) for 96 h under a high-pressure hydrogen atmosphere. The balls to chips mass ratios (BCR) are 30:1 and 66:1. The particles obtained are characterized by XRD and TEM, and absorbed hydrogen contents (AHC) were measured by TGA. For dehydrogenation kinetics, activation energies are calculated by isothermal thermogravimetry analysis (ITGA) and pressure-composition-isotherm (PCI) analysis. The results of XRD and TEM revealed that the Mg₂NiH_x peaks are broadened in the case of high BCR and the particles are composed of the nanocrystalline phases less than 10 nm with the amorphous phase. The results of ITGA and PCI analysis show that the dehydrogenation kinetics are greatly improved by nanocrystallization. The results show that BCR conditions mainly affect the size and fraction of the nanocrystalline phases, the resultant AHC and the dehydrogenation kinetics.

Full text of DOI:10.1016/S0925-8388(00)01131-2

Journal of Alloys and Compounds 312 (2000) 60–67
L
www.elsevier.com/locate/jallcom
Dehydrogenation properties of nano-/amorphous Mg2NiHx by hydrogen
induced mechanical alloying
*
Tae-Whan Hong, Shae-Kwang Kim, Young-Jig Kim
School of Metallurgical and Materials Engineering, Sung Kyun Kwan University, 300 Chunchun-dong, Jangan-gu, Suwon, Kyunggi-do 440-746,
South Korea
Received 18 April 2000; accepted 13 July 2000
Abstract
In recent years, nanocrystalline materials have been of interest for hydrogen storage applications. Especially, the great improvement of
the hydrogenation properties of Mg alloys that can be achieved by nanocrystallization. In this study, nanocrystalline Mg2NiHx is
fabricated from Mg and Ni chips by hydrogen induced mechanical alloying (MA) for 96 h under a high-pressure hydrogen atmosphere.
The balls to chips mass ratios (BCR) are 30:1 and 66:1. The particles obtained are characterized by XRD and TEM, and absorbed
hydrogen contents (AHC) were measured by TGA. For dehydrogenation kinetics, activation energies are calculated by isothermal
thermogravimetry analysis (ITGA) and pressure–composition-isotherm (PCI) analysis. The results of XRD and TEM revealed that the
Mg2NiHx peaks are broadened in the case of high BCR and the particles are composed of the nanocrystalline phases less than 10 nm with
the amorphous phase. The results of ITGA and PCI analysis show that the dehydrogenation kinetics are greatly improved by
nanocrystallization. The results show that BCR conditions mainly affect the size and fraction of the nanocrystalline phases, the resultant
AHC and the dehydrogenation kinetics.
2000 Elsevier Science B.V. All rights reserved.
Keywords: Amorphous materials; Mechanical alloying; X-ray diffraction; Hydrogen storage materials
1. Introduction
extent filled up by segregation. In this regard, nanocrystal-
line materials are one of the best candidates for hydrogen
storage [1–3].
Nanocrystalline structure was defined as a new solid
state as reported by Gleiter and co-workers. If the grain
size is reduced to 10 nm or less, the density of the grain
boundaries increases to 1018|1021 /cm3. This means that
about 50% of the atoms are located in the grain
boundaries. In other words, nanocrystalline structure can
be regarded as a structural compound, one half of which
consists of a crystalline phase and the other half of a grain
boundary phase. This state entails considerable changes in
the intrinsic material properties. In nanocrystalline materi-
als, self-diffusion is enhanced by the factor of 1019
compared with conventional polycrystalline materials. The
reason for this is the considerable density and the high
cleanliness of the grain boundaries, which is in contrast to
normal materials where the grain boundaries are to some
On the other hand, magnesium and magnesium alloys
are attractive materials for hydrogen storage applications,
but activation, hydrogenation/dehydrogenation kinetics,
thermodynamic equilibrium parameters and degradation
behaviors have to be improved for applications [4].
Recently, crystal structure modification of the nanocrystal-
line structure with amorphous phases has been considered
as one of the methods for improving the kinetic properties
of the Mg and Mg alloys which experience problems with
high desorption temperature and degradation phenomena.
Especially, in the case of Mg based hydrogen storage
intermetallic compounds, the nanocrystalline particles of
Mg2Ni with a grain size of less than 100 nm have been
synthesized by mechanical alloying (MA) for improving
the hydrogenation properties. The nanocrystalline particles
shows a much easier activation compared with their
conventional polycrystalline counterparts, mainly due to
the nanocrystalline and amorphous phases with a variety of
interstices available for hydrogen behaviors [5–10].
*Corresponding author. Tel.: 182-31-290-7357; fax: 182-31-290-
7371.
E-mail address: yjk1122@yurim.skku.ac.kr (Y.-J. Kim).
0925-8388/00/$ – see front matter
PII: S0925-8388(00)01131-2
2000 Elsevier Science B.V. All rights reserved.

T.-W. Hong et al. / Journal of Alloys and Compounds 312 (2000) 60–67
61
In previous studies [11,12], hydrogen induced mechani-
2. Experimental methods
cal alloying was proposed using Mg and Ni chips to
fabricate the nanocrystalline Mg2NiHx particles with the
amorphous phases. In this paper, the dehydrogenation
properties of the Mg2NiHx particles were characterized by
isothermal thermogravimetry analysis (ITGA) and the fully
hydrogenation behaviors by automatic pressure–composi-
tion-isotherm (PCI) analysis without activation. On the
basis of our results, we intended to optimize the MA
conditions.
2.1. Fabrication and structure analysis of Mg2NiHx
Mg and Ni chips were prepared by drilling, without
cutting oil, Mg (99.93%) and Ni (99.92%) ingots [Fig.
1(a)]. The initial chips (Mg–55 mass%Ni, 6 g) were
charged in an AISI 304 jar, designed for high pressure
operation and equipped with needle valves for evacuating
air and introducing hydrogen gas. The jar of 450 cc
Fig. 1. The shape of Mg and Ni chips (a) and schematic diagram of Sievert’s type automatic pressure–composition-isotherm apparatus (b).

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T.-W. Hong et al. / Journal of Alloys and Compounds 312 (2000) 60–67
volume and 0.5 inch Cr steel balls (AKS Co. Ltd.) were
used, and the balls to chips mass ratios (BCR’s) were 30:1
and 66:1. After evacuating to 6.7 Pa by rotary pump, the
chips were processed by reactive mechanical alloying for
96 h under 2 MPa of high purity hydrogen gas
(99.9999%). It was performed by 200 rpm at the ambient
temperature using a planetary ball mill (FRITSCH,
pulverisette-5).
For the calculation of the dehydrogenation reaction
enthalpy on synthesized Mg2NiHx, isothermal thermog-
ravimetry analysis (ITGA) was carried out under a pure
argon atmosphere at 453, 463, 473, 483, 493, 503 and 513
K with holding time for 1 h using TGA-50H (SHIMADZU
Co.). Using a Sievert’s type automatic PCI apparatus, the
hydrogenation/dehydrogenation properties were observed
at 393, 423, 453, 483, 513, 543 and 573 K. The schematic
diagram of the automatic PCI apparatus was shown in Fig.
1(b).
The MA particles were characterized by X-ray diffrac-
tion patterns using a M18XHF-SRA diffractometer (Mac-
˚
science Co., 1.5405 A CuKa, 2 deg/min). The morpholo-
gy of the particles were observed using SEM (JSM-35CF,
JEOL Co.) and TEM (HITACHI Co. H-9000-NA).
3. Results and discussion
2.2. Evaluation of the dehydrogenation properties
3.1. The dehydrogenation properties in 30:1 BCR
In order to find the dehydrogenation starting temperature
and the phase transition phenomena, thermogravimetry
analysis (TGA) was carried out under a pure argon
atmosphere with heating to 773 K by 1 K/min. Differen-
tial thermal analysis (DTA, SHIMADZU Co. DTA-50)
also was carried out from 1 K to 1273 K with the same
heating rate as for TGA.
Fig. 2(a) shows the results of the DTA profiles in 30:1
BCR. The endothermic reactions in stage 1 and 2 are
mainly due to the dehydrogenation phenomena and the
phase transition, respectively. The hydrogen desorption
takes place around 450 K (onset temperature) in stage 1
and the crystal growth and the phase separation proceeds
Fig. 2. The results of DTA on MA particles in 30:1 BCR (a), 66:1 BCR (b) and TGA (c). In (c), the arrows indicate the dehydrogenation starting
temperature (initial point) about 453, 480 K, respectively.

T.-W. Hong et al. / Journal of Alloys and Compounds 312 (2000) 60–67
63
in stage 2 [13]. On the other hand, the exothermic reaction
in stage 3 seems to occur by the degradation of the
particles. The result of TGA in Fig. 2(c) shows that the
dehydrogenation temperature starts at about 480 K (initial
point) and absorbed hydrogen capacity (AHC) is around
2.25 mass%. The results show that the dehydrogenation
temperature is lowered to around 90 K, whereas the value
of AHC is decreased by about 1.4 mass% as compared
with the fully hydrogenated capacity of polycrystalline
Mg2Ni (3.6 mass% at 572 K) [14]. Fig. 3 shows that the
morphology of the particles and the XRD pattern in 30:1
BCR. The fine particles, less than 1 mm, with some porous
surface are agglomerated and are composed of a mixture of
Mg2NiHx, MgHx and Ni. The retained Ni, which is seen in
the early MA stage, is caused by the formation of MgHx
[15]. From the typical bright field image and the corre-
sponding selected area diffraction (SAD) pattern of
Mg2NiHx (Fig. 4), reveal that the average crystal size is
less than 50 nm and the nanocrystalline and amorphous
phases coexist. In Fig. 5, the result of ITGA and the
calculated Arrhenius plot indicate that the hydrogenation
reaction dose not proceed for 1 h below 453 K, but the
dehydrogenation proceeds over 503 K, and the activation
energy is estimated to be 51.662.1 kJ/mol H2 by the
linear fit of the Arrhenius plot within the range from 0.5 to
1 ks. The dehydrogenation behaviors of PCI and the
corresponding van’t Hoff plot in Fig. 6 show that the
calculated DrH0 is about 81.160.2 kJ/mol H2 except the
case of the dehydrogenation reactions at 483 K.
Fig. 3. The morphology of the particles (a) and the XRD patterns (b) in
30:1 BCR. (s: MgHx, d: Ni, .: Mg2NiHx).
Fig. 4. The SAD pattern (a) and the bright field image (b) of the Mg2NiHx particles after 96 h MA in 30:1 BCR.

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T.-W. Hong et al. / Journal of Alloys and Compounds 312 (2000) 60–67
Fig. 5. The profiles of ITGA (a) and the Arrhenius plot (b) from 0.5 to 1.0 ks in 30:1 BCR.
3.2. The dehydrogenation properties in 66:1 BCR
Mg2NiH1.8 is affected by the dehydriding temperature and
the AHC. Fig. 7 shows the morphology of the particles and
the XRD patterns in 66:1 BCR. The Mg2NiH4, a little Ni
is detected with small MgHx, and nanocrystalline particles,
less than 1 mm, with a high porous surface are observed in
comparison with those of 30:1 BCR. Fig. 8 shows the
microstructure of the particles after 96 h MA in 66:1 BCR.
The average grain size is measured at less than 10 nm from
the bright field image and the particles are composed of the
nanocrystalline and amorphous phases. The corresponding
SAD patterns of the Mg2NiHx show the continuous and
broaden ring pattern, which indicates the coexistence of the
homogeneously distributed amorphous and the fine
nanocrystalline phases. According to Fujii et al. [18] and
Yamamoto et al. [19], these phases was considered to be
affirmative factors of dehydrogenation reaction kinetics
such as ‘cooperative dehydriding phenomena’. Fig. 9
shows the results of ITGA and the Arrhenius plot in 66:1
Fig. 2(b) shows the results of the DTA profiles in 66:1
BCR. The endothermic reactions in stage 1 and 2 are by
the dehydrogenation reactions (onset temperature around
440 K) and the coarsening of the nanocrystalline Mg2Ni,
respectively. The endothermic reaction in stage 3 is caused
by the phase separation of the Mg2Ni and the hydrogen
desorption initial temperature indicated at around 453 K in
Fig. 2(c). These results demonstrate that the Mg2NiHx
particles synthesized in 66:1 BCR show more stable
behaviors compared with those in 30:1 BCR. Even though
the dehydrogenation starting temperature is poorer, about
13 K, than Orimo et al. [16], the value of AHC, around
2.80 mass%, is larger than their achievement, 1.6 mass%
[16,17]. These results are caused by the synthesized phase
of various hydrides via MA. In other words, the difference
of hydrogen capacity between LT–Mg2NiH4 and
Fig. 6. The PCI profiles of the dehydriding reactions (a) and the van’t Hoff plot (b) from 513 to 573 K in 30:1 BCR.

T.-W. Hong et al. / Journal of Alloys and Compounds 312 (2000) 60–67
65
Fig. 7. The morphology of the particles (a) and the XRD patterns (b) in 66:1 BCR. (s: MgHx, d: Ni, ,: Mg2NiH4).
Fig. 8. The brightfield image of the Mg2NiHx particles after 96 h MA in 66:1 BCR (a) and the SAD pattern (b) shows the continuous and broaden ring
patterns.

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T.-W. Hong et al. / Journal of Alloys and Compounds 312 (2000) 60–67
Fig. 9. The profiles of ITGA (a) and the Arrhenius plot (b) from 0.5 to 1.0 ks in 66:1 BCR.
BCR. The dehydrogenation reaction proceeds for 1 h at
453 K slowly, but at over 473 K the dehydrogenation
reaction is completely established. The activation energy is
estimated to be 43.463.6 kJ/mol H2 by the linear fit of the
Arrhenius plot with the range from 0.5 to 1 ks. Fig. 10
shows the data of PCI on the Mg2Ni, which treated of
initial dehydrogenation without activation. The PCI curve
at 483 K shows the dehydrogenation plateau pressure at
0.01 MPa without the entrapped hydrogen. The plateau
slope of about 0.05, the hysteresis of about 0.94, the
hydrogen capacity of about 2.25 mass% and the reversible
capacity of 1.08 mass% are observed by the PCI curves.
From the van’t Hoff plot, the calculated dehydrogenation
reaction enthalpy, DrH0 is about 91.460.9 kJ/mol H2. By
comparing the properties between 30:1 and 66:1 BCR, the
Mg2Ni particles in 66:1 BCR might be better candidate
materials for hydrogen storage applications.
4. Conclusions
Mg2NiHx composed of the nanocrystalline and the
amorphous phases are synthesized successfully by hydro-
gen induced mechanical alloying from Mg and Ni chips.
The BCR conditions mainly affect the hydrogenation
properties due to the formation of the Mg2NiHx. The
maximum AHC is 2.8 mass% after 96 h MA in 66:1 BCR
and the hydrogen capacity of the nanocrystalline and
amorphous phases is about 2.25 mass% at 483 K from PCI
analysis. The dehydrogenation properties are mainly de-
pendent on the microstructure of the particles. In such a
way that the properties were better in the case of 66:1
BCR. The crystal structure modification to nanocrystalline
with the amorphous phase for controlling Mg2NiHx by
hydrogen induced mechanical alloying might be one of the
reasonable solutions for manufacturing Mg2NiHx hydrogen
Fig. 10. The PCI profiles of the dehydriding reactions (a) and the van’t Hoff plot (b) from 483 to 573 K in 66:1 BCR.

T.-W. Hong et al. / Journal of Alloys and Compounds 312 (2000) 60–67
67
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