Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
Hydrogen storage alloy of Mg2Ni produced from Mg
and Ni ultrafine particles
The State Key Laboratory of Rare Earth Materials Chemistry and
Applications, Peking University, Beijing 100871, China
Yuejun Yu
Abstract:
Mg and Ni ultrafine particles were produced by hydrogen plasma metal
reaction method, then mixed in the molar ratio of 2:1 and heated in Ar flow
atmosphere to get Mg2Ni compound. Structure and hydrogen storage properties
of the compound were measured. There are some features about this method: (a)
The particles are mixed just by ultrasonic homogenizer, (b) The heating
temperature is lower than that in chemical literature and, (c) The process of
compound preparation is without hydrogen. This new method has advantages
such as short process time, simple equipment and high product purity. The
hydrogen absorption rate was enhanced and the plateau pressure was lowered
greatly.
Keywords: Mg2Ni, ultrafine particles, hydrogen plasma metal reaction, and
hydrogen storage alloy.
1. Introduction
Mg-based hydrogen storage alloys are always attractive to many investigators,
though the hydriding and dehydriding kinetics of Mg are slow and the hydride is
too stable for most practical applications. Mg rich intermetallic alloys are being
investigated in order to determine the optimum composition leading to the
best-reversible hydrogen absorption properties. Among them, Mg2Ni is the most
intensively studied compound due to its low specific weight and cost [1].
Several fabrication techniques have been developed to produce nano-scale
Mg2Ni, and the typical one is the energetic ball milling of hydrides. But its
shortcomings are: the impurity is high, the process time is long, and the product is
not uniformity, as a result, application of the method is limited. Nomura et al. [4]
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Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
and Reily et al. [5] investigated formation of Mg2Ni by heat-treatment of Mg and
Ni powder and found that it was possible to get the compound. However,
temperature for the treatment was must elevated up to 673 K because Mg and Ni
particles were quite large. The high temperature prevents from getting Mg2Ni
with a fine microstructure. In our previous reports, we found that plasma metal
reaction (HPMR) can be applied to producing ultrafine particles (UFPs) of various
metals or metallic alloys industrially. This makes us to think about preparation of
Mg2Ni compound by using ultrafine Mg and Ni ultrafine particles. The main
objects of this study are whether the treatment temperature can be lowered,
whether fine and homogenous microstructure can be get by using Mg and Ni
ultrafine particles and what hydrogen storage properties the compound behaves.
2. Experiment
2.1 Preparation of Ultrafine Particles
The ultrafine particle was produced through HPMR (hydrogen plasma metal
reaction). A schematic illustration for the experimental equipment of producing
UFP samples was described previously [3]. A commercial Mg ingot with a purity of
99.9% and a Ni ingot with 99.9% were chosen. Preparation conditions are
summarized in Table 1.
The structure of the samples was characterized through X-ray diffraction
(XRD) with monochromatic Cu Kα radiation The size distribution and shape of
the particles were observed by transmission electron microscopy (TEM), and the
average particle size was evaluated by means of XRD,TEM, and specific surface
area.
Table 1
Sample
Atmosphere
Pressure
Gas flow rate
Arc current
Arc voltage
Conditions for the Preparation of UFPs
Mg (99.9%)
50% H2 +50% Ar
0.1MPa
100L/min
200A
25V
Ni (99.9%)
50% H2 +50% Ar
0.1MPa
100L/min
200A
25V
2.2 Preparation of the Compound
For preparation of Mg2Ni compound, Mg and Ni ultrafine particles was mixed
in the molar ratio of 2:1 with an ultrasonic homogenizer ethanol?. After completely
dried in the air, some of the well-mixed powder was compressed at a pressure of
75MPa to form many cylinders, and each had a weight of about 0.6g and a diameter
of 13mm. Some of the mixed powder was directly used as a sample and the weight of
the sample was also 0.6g.
After the samples were put in the furnace, the system evacuated down to 10-3Pa.
Then the samples were heated up to 773K, 673K, and 623K at a heating rate of 4.4
K/s and were kept for 120min in a argon flow atmosphere of 1l/min. Finally, the
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Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
samples were cooled down to room temperature and taken out from the furnace.
2.3 PCT Experiment
Pressure-composition isotherms were measured separately at 523K, 573K, 623K,
and 673K at 3.5MPa. Before measuring the pressure-composition isotherms, the
sample should be sufficiently activated through a thermal treatment: heating at 673K
for 1 hour in primary vacuum.
3. Result and Discussion
3.1 Characteristics of UFPs
Figure 1 shows X-ray diffraction patterns of the original particles. The Mg
particles are of a hexagonal crystal structure, and the Ni particles are of cubic crystal
structure. TEM micrographs of the Mg particles are given in Fig. 1. The Mg particles
are of a nearly hexagonal form and Ni particles are of spherical form. The sizes of the
Mg particles range from 200nm to 500nm and the sizes of the Ni particles range from
20nm to 50nm. Results of XRD, TEM and specific surface area analysis are
summarized in Table 2.
Table 2 Characteristics of Mg and Ni UFPs
Mg
Ni
291.6(g/h)
12.4 (g/h)
hcp
Fcc
area 3.9
22.4
UFPs
Generation speed
Phase constitution
Specific
surface
(m2/g)
Mean particle size (nm)
905
33
3.2 Structure of Synthesized Mg2Ni Alloys
XRD patterns of samples after heating at different temperatures are shown in Fig
2. For the sample heated at 623 K, several weak peaks corresponding to Mg2Ni
appear around 20 degree even though most of peaks are those of Mg and Ni particles.
The weak peaks imply that Mg and Ni particles begin to combine into Mg2Ni at 623
K. After heating at 673 K, the sample completely transforms into Mg2Ni and peaks of
the original particles are not detected.
As can be seen in Fig. 2, there are still a few unknown matters. Nevertheless, the
peaks are different from those of Mg and Ni ultrafine particles. So we infer they may
not be original particles but other Mg-Ni compound. Since the peaks are too weak, we
do not know their exact composition. From intensity of the unknown peaks, amount
of the unknown phase is estimated to be smaller than 3%. The purity of Mg2Ni
prepared in this study is much high than in the ball milling method.
For knowing the compact effect, a sample without compress was heated at 673 K
and its XRD result is given in Fig. 3. The peak widths are wider and peaks of
unknown phase are stronger than those of the compacted one. The compact is helpful to the
interdiffusion of Mg and Ni, as a result, it is helpful to formation of Mg2Ni.
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Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
3.3 Hydrogen Storage Properties
As the sample synthesized at 673 K is nearly composed from Mg2Ni and has fine
microstructure, it is selected for measurement of hydrogen storage properties. The
pressure-composition isotherms (PCT) of the hydride dissociation were measured at
673K, 623K, 573K, and 523K respectively starting from 3.5 bar, and results are
shown in Fig. 4. The maximum absorption capacity for the sample is 1.1 H/M, which
is slightly less than that of the stoichiometric hydride Mg2NiH4 (H/M=1.33). Plateaus
pressure corresponding to the hydride formation clearly appeared on each isotherm
and increases with temperature.
Figure 5 shows the temperature dependence of the plateaus pressure. From the
plots, the dependence can be expressed by Van’t Hoff equation as follows:
LogPatm=-4007.8/T+7.2919.
(1)
[4]
Here P is the plateaus pressure and T is temperature. Nomura et al.
and Reily et al.
[5]
prepared Mg2Ni by ????? methods and measured its PCT curves. The dependence
in their works were described by the following equations.
LogPatm=-3245/T+ 6.265
(2)
LogPatm = -3360/T + 6.389
(3)
The plateau pressure in this study is lower and the enthalpy change is higher in
this study than their data. We presume that it is because of the fine microstructure. For
hydriting is an exothermic reaction, and high enthalpy change may be helpful to the
reaction.
Figure 7 shows the absorption rates of Mg2Ni at different temperatures after one
absorption cycle. When the sample was fully activated, it absorbs hydrogen at an
amazing speed. The hydrogen content absorbed achieves 95% of the maximum
amount within 4 min at 623K and 573K, 90% within 20 min at 673 K. However, it
took 2 hours to achieve 60% at 573 K. We know that the nucleation of the low
temperature hydride phase is the rate-limitary step for nanocrystalline Mg2Ni. But at
537K, the nucleation of the high-temperatured-Mg2NiH4 phase is so fast that it is no
longer a rate limiting step. The absorption is then only controlled by diffusion [6]. But
the counterreaction of absorption is stronger at a higher temperature. This is one
reason why all the datas were lower than the stoichiometric hydride.
4. Conclusion
A nanocrystalline Mg2Ni compound is synthesized from Mg and Ni ultrafine
particles in inert gas at a low temperature, yielding fine microstructure with the
highest purity. This compound shows more improved kinetics of absorption at low
temperatures and lower plateau pressure compared to Mg2Ni produced by ball milling.
The compound is stable during hydriding-dehydriding cycling.
Reference
[1] X..-L. Wang, N. Haraikawa, S. Suda, J.Alloys Comp. 231 (1995)
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Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
[2] Li, Liquan; Akiyama, Tomohiro; Yagi, Jun-ichiro, J.Alloys Comp. 308 (2000)
[3] X.G.Li, A.Chiba, and S.Takahashi, J.Magn. Magn. Mater. 170,339 (1997)
[4] K. Nomura, E. Akiba, and S. Ono, Int. J. Hydrogen Energy 6 (3), 295 (1981)
[5] J. J. Reilly and R. H. Wiswall, Inorg. Chem. 7, 2254 (1968)
[6] G.Liang, J.Huot, S.Boily J. Alloys Comp. 282 (1999) 286-290
[7] R.L.Holtz, M.A.Imam, J. Materials Science 2267-2274 (1997)
Intansity (a.u.)
Mg
Ni
0
10
20
30
40
50
60
70
2 Theta (deg.)
Fig .1. X-ray diffraction patterns of the original particles
Fig. 2. (a) TEM micrographs of the Mg particles
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Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
Fig.2 (b) TEM micrographs of the Ni particles
Mg2Ni
Ni
Mg
unknown
Intansity (a.u.)
(a)
(b)
(c)
(d)
0
10
20
30
40
50
60
70
80
2 Theta (deg.)
Fig. 3.XRD patterns of (a) 2Mg+Ni UFPs ; (b) crystallized at 623K ; (c) crystallized
at 673K ; (d) crystallized at 773K.
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Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
Intensity (a.u.)
Mg2Ni
Unkown
（ a）
(b)
0
10
20
30
40
50
60
70
80
2 Theta (deg.)
Fig. 4. XRD patterns of crystallized at 673K. A: without compacted, B: (a) without
compacted;(b)compacted
3.5
Hydrogen Pressure,P (MPa)
3.0
2.5
673K
2.0
1.5
623K
1.0
0.5
573K
0.0
523K
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Hydrogen Concentration, H/M
Fig.5 Pressure-composition isotherms at different temperature of the Mg2Ni prepared
by HPMR
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Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
1.4
1.2
1.0
Log Patm
0.8
0.6
0.4
0.2
Experiment
Linerar Fit
0.0
-0.2
-0.4
0.0015
0.0016
0.0017
0.0018
0.0019
-1
1/T (K )
Fig.6. The Log( plateau pressure) vs. the inverse temperature for Mg2Ni.
1.4
673K
1.2
1.0
623K
573K
0.8
H/M
523K
0.6
0.4
0.2
0.0
-20
0
20
40
60
80
100
120
140
160
180
hydrogenation time (min)
Fig. 7.Rate of hydrogen for Mg2Ni (crystallized at 673K) at different temperature.
Under 3.0MPa
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Series of Selected Papers from Chun-Tsung Scholars,Peking University (2002)
致 谢
我要特别感谢我的导师李国星教授和实验室的学长们，感谢他们给我的无限
鼓励和支持。
作者简介：余玥君,化学系99级本科生，北京人，生性活泼，热爱“冒险”。高中
时代表中国参加在韩国汉城召开的第一届APEC青年科学节，并在会上发表论文一
篇。99年考入北大化学系，继续“冒险”之旅。“ 政基金”的获得便是成果之
一，也因此被引入科学的森林，在其中乐此不疲。
感悟与寄语：
不知道是不是所有人都在摸索。我相信科学便是在黑暗中寻找一缕阳光。在
刚开始实验的时候，一切都是新鲜的，仿佛在洞口，还有鲜花和鸟鸣。渐渐地，
我走进洞里，依仗着手中的火把。再后来，火把也灭了。我只有扶着岩壁前行，
期望哪个松动的石缝能带给我一线的光亮。很多时候，我感到彷徨和恐惧。在科
学面前，我看到自己的渺小。
1 年半的时间，很惭愧没有做出什么很出色的成绩，无以回报我的导师和那
些帮助过我的人。然而对我自己，却是一笔财富。令我更加成熟和自信。从此，
我不再惧怕黑暗，我也将用黑色的眼睛去寻找光明。
指导教师简介：李星国教授，男，1957 年 9 月生，湖北省孝感市人。1982 年获
华中科技大学理学学士，1987 年获日本岩手大学工学硕士，1990 年获日本东北
大学工学博士。2000 年到北京大学化学与分子工程学院任教，并获 2000 年度国
家杰出青年科学基金。主要从事纳米材料的制备、性能和应用方面的研究，在国
内外学术杂志上发表论文 100 余篇。
401