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Hydrogen storage alloy of Mg 2 Ni produced from Mg and Ni ultrafine particles


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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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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 samples were cooled down to room temperature and taken out from the furnace.
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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 22.4 905 33

UFPs Generation speed Phase constitution Specific surface 2 (m /g) Mean particle size (nm)

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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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 LogPatm = -3360/T + 6.389 (2) (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) [2] Li, Liquan; Akiyama, Tomohiro; Yagi, Jun-ichiro, J.Alloys Comp. 308 (2000)
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[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)

Mg

Intansity (a.u.)

Ni
0 10 20 30 40 50 60 70 80

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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Fig.2 (b) TEM micrographs of the Ni particles

Mg2Ni Ni Mg unknown
(a)

Intansity (a.u.)

(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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Mg2Ni Unkown

Intensity (a.u.)

( 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 3.0

Hydrogen Pressure,P (MPa)

2.5

673K
2.0 1.5 1.0 0.5

623K

573K
0.0 0.0 0.2 0.4

523K
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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1.4 1.2 1.0 0.8

Log Patm

0.6 0.4 0.2 0.0 -0.2 -0.4 0.0015 0.0016 0.0017 0.0018
-1

Experiment Linerar Fit

0.0019

1/T (K )

Fig.6. The Log( plateau pressure) vs. the inverse temperature for Mg2Ni.

1.4

1.2

673K

1.0

623K 573K

0.8

523K

H/M

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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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。

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