Enhanced Hydriding and Dehydriding Kinetics of as-Spun Nanocrystalline Mg2Ni-Type Alloy Substituting Ni with Cu

Rare Metal Materials and Engineering Volume 40, Issue 10, October 2011 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2011, 40(10): 1693-1698.

ARTICLE

Enhanced Hydriding and Dehydriding Kinetics of as-Spun Nanocrystalline Mg2Ni-Type Alloy Substituting Ni with Cu Zhang Yanghuan1,2,

Ma Zhihong1,3,

Zhao Dongliang1,

Ren Huiping2,

Guo Shihai1,

Wang Xinlin1 1

Central Iron and Steel Research Institute, Beijing 100081, China;

2

Inner Mongolia University of Science and Technology, Baotou 014010,

3

China; Baotou Research Institute of Rare Earths, Baotou 014010, China

Abstract: In order to improve the hydriding and dehydriding kinetics of the Mg2Ni-type alloy, Ni in the alloy was partially substituted by element Cu. The nanocrystalline Mg2Ni1-xCux (x=0, 0.1, 0.2, 0.3, 0.4) alloys were prepared by melt-spinning technology. The structures of the as-cast and spun alloys were analyzed by XRD, SEM and HRTEM. The hydriding and dehydriding kinetics of the alloys were measured by using an automatically controlled Sieverts apparatus. The results show that all the as-spun alloys hold a nanocrystalline structure. The substitution of Cu for Ni leads to form secondary phase Mg2Cu without changing of the major phase of Mg2Ni in the alloy, The hydrogen absorption capacity of the alloys first increases and then decreases with rising of Cu content, but the hydrogen desorption capacity of the alloys monotonously grows with increasing of Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and the kinetics of the alloys. Key words: Mg2Ni-type alloy; substituting Ni with Cu; melt-spinning; microstructure; hydriding and dehydriding kinetics

Mg and Mg-based alloys are regarded to be more promising materials for hydrogen storage because of their high hydrogen capacity and low production cost[1,2]. Unfortunately, some shortcomings of these kinds of metal hydrides, such as slow sorption/desorption kinetics, high thermal stability, have limited their practical applications. Therefore, finding ways of improving the hydration kinetics of Mg-based alloys has been one of the main challenges faced by researchers in this area. Various attempts, involving mechanical alloying (MA)[3], GPa hydrogen pressure method[4], melt spinning[5], gravity casting[6], hydriding combustion synthesis[7], surface modification[8], alloying with other elements[9], adding catalysts[10] etc, have been undertaken to improve the hydrogen storage properties. Zaluska et al.[11] reported that a milled mixture of Mg2NiH4 and MgH2 shows excellent absorption/desorption kinetics at 220-240 °C, yielding a maximum hydrogen concentration of

more than 5 wt%. Hanada et al. found that a hydrogen storage capacity of 6.5 wt% in a temperature range of 150-250 °C can be obtained[12] by ball milling MgH2 and nanosized-Ni. Recham et al.[13] confirmed that the hydrogen absorption property of ball-milled MgH2 can be enhanced by adding NbF5, and 5 wt% NbF5 catalyzed MgH2 composite desorbs 3 wt% H2 at 150 °C. Cui et al.[14] documented that amorphous and/or nanocrystalline Mg-Ni-based alloys possess superior hydrogen absorption and desorption performances at room temperature. Ball-milling, indubitably, is a quite powerful method for the fabrication of the nanocrystalline and amorphous Mg and Mg-based alloys. However, the milled Mg and Mg-based alloys show very poor hydrogen absorbing and desorbing stability due to the fact that the metastable structures formed by ball milling tended to vanish during multiple hydrogen absorbing and desorbing cycles[15]. Comparatively, the melt-spun technique can overcome the above mentioned shortcoming and

Received date: June 11, 2011 Foundation item: National Natural Science Foundations of China (50871050 and 50961001); Natural Science Foundation of Inner Mongolia, China (2010ZD05) and High Education Science Research Project of Inner Mongolia, China (NJzy08071) Corresponding author: Zhang Yanghuan, Professor, Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, P. R. China, Tel: 0086-10-62187570, E-mail: [email protected] Copyright © 2011, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

1693

Zhang Yanghuan et al. / Rare Metal Materials and Engineering, 2011, 40(10): 1693-1698

The hydrogen absorption and desorption kinetics of the as-cast and spun alloys were measured by an automatically controlled Sieverts apparatus. The hydrogen absorption was conducted at 1.5 MPa and 200 °C, and the hydrogen desorption at a pressure of 1×10-4 MPa and 200 °C.

effectively restrain the rapid degradation of the hydrogen absorbing and desorbing cycle properties of Mg and Mgbased[16]. Additionally, the melt-spinning technique is also a very impactful method to obtain a nanocrystalline structure and is very suitable for mass-production of nanocrystalline Mg-based alloys. Spassov et al [17] prepared Mg2(Ni,Y)-type Mg63Ni30Y7hydrogen storage alloy by rapid solidification, obtaining amaximum hydrogen absorption capacity (about 3.0 wt%). It was found that the hydrogenation kinetics of the melt-spun Mg2(Ni, Y) alloy exceed those of the conventionally prepared polycrystalline Mg2Ni alloys and to be comparable to the hydrogen absorption characteristics of nanocrystalline ball-milled Mg2Ni.Huang et al.[18] prepared the amorphous and nanocrystalline Mg-based alloy (Mg 60 Ni 25 ) 90 Nd 1 0 by melt-spinning, attaining a maximum hydrogen capacity of 4.2 wt% H. The objective of this work is to produce ternary Mg20Ni10-xCux (x=0-4) alloys with a nanocrystalline structure by melt spinning and to examine the influence of substituting Ni with Cu on hydrogen absorption and desorption kinetics of the as-spun nanocrystalline Mg2Ni-type alloys.

2

Results and Discussion

2.1 Microstructure characteristics The XRD profiles of the as-cast and spun (25 m/s) alloys are shown in Fig.1. It is evident that all the as-cast and spun alloys display a single phase structure. Both the substitution of Cu for Ni and the melt spinning treatment do not change the Mg2Ni major phase of the alloys. Based on the XRD data, the lattice parameters, cell volume and full width at half maximum (FWHM) values of the main diffraction peaks of the as-cast and spun (25 m/s) alloys were calculated by software of Jade 6.0. The obtained results are listed in Table 1. It is found from Table 1 that the melt spinning renders not only an evident enlargement in the lattice parameters and cell volume of the alloys, but also a visibly increase in the FWHM values of the main diffraction peaks of the alloys, which is doubtless attributed to the refined average grain size and stored stress in the grains produced by melt spinning. Based on the FWHM values of the broad diffraction peak (203) in Fig.1, the grain size D (nm) of the as-spun alloy was calculated using Scherrer’s equation. The grain sizes of the as-spun alloys are in a range of 2 to 6 nm, consistent with results reported by Friedlmeier et al[19].

1 Experiment The compositions of the experimental alloys were Mg20Ni10-xCux (x=0, 1, 2, 3, 4). For convenience, the alloys were denoted with Cu content as Cu0, Cu1, Cu2, Cu3 and Cu4, respectively. The alloy ingots were prepared using a vacuum induction furnace in a helium atmosphere at a pressure of 0.04 MPa. After induction melting, the melt was poured into a copper mould cooled by water, and a cast ingot was obtained. The part of the as-cast alloys was re-melted and spun by melt-spinning with a rotating copper roller. The spinning rate was approximately expressed by the linear velocity of the copper roller. The spinning rates used in the experiment were 15, 20, 25 and 30 m/s, respectively. The phase structures of the alloys were determined by XRD (D/max/2400). A Philips SEM (QUANTA 400) linked with an energy dispersive spectrometer (EDS) was used for morphological characterization and chemical analysis of the as-cast alloys. The thin film samples of the as-spun alloys were preparedby using ion etching method in order to observe the morphology with HRTEM (JEM-2100F), and also to determine the crystalline state of the samples with electron diffraction (ED).

a

-Mg2Ni

Intensity/a.u.

△

b

△ -Mg2Ni

Cu4

Cu4

Cu3

Cu3

Cu2

Cu2

(203)

△△

△

△

(403)

△

(220)

△△△

(204)

△

△

(215) (118)

△△ △

△

(206)

Cu0

(112) (200) (105) (113)

(003)

△

(100) (101) (102)

△ △

(103)

△△

(403)

△

(220)

△△

(204)

△△ △ △ △

(206)

△

Cu1 △

(215) (118)

(103)

△ △ △

(112) (200) (105) (113) (203)

△

△

(106) (211) (212)

(100) (003) (101) (102)

Cu1

△

Cu0

20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 100 2θ/(°) 2θ/(°) Fig.1 XRD patterns of the as-cast and spun alloys: (a) as-cast; (b) as-spun (25 m/s)

Table 1 The lattice parameters, cell volume and the FWHM values of the major diffraction peaks of the alloys FWHM values Alloys Cu0 Cu1 Cu2 Cu3 Cu4

2θ (20.02°)

Lattice parameters and cell Volume

2θ (45.14°)

a/nm

c/nm

V/nm3

As-cast

25 m/s

As-cast

25 m/s

As-cast

25 m/s

As-cast

25 m/s

As-cast

25 m/s

0.122 0.133 0.148 0.151 0.165

0.131 0.241 0.258 0.282 0.292

0.169 0.178 0.183 0.192 0.204

0.179 0.237 0.242 0.259 0.273

0.520 97 0.521 02 0.521 36 0.521 54 0.521 71

0.521 05 0.521 58 0.521 72 0.521 85 0.522 10

1.324 4 1.325 2 1.328 3 1.329 7 1.330 2

1.326 5 1.327 7 1.331 1 1.331 9 1.332 3

0.311 29 0.311 54 0.312 67 0.313 22 0.313 54

0.311 88 0.312 79 0.313 76 0.314 11 0.314 50

1694

Zhang Yanghuan et al. / Rare Metal Materials and Engineering, 2011, 40(10): 1693-1698

The SEM images of the as-cast alloy are presented in Fig.2, displaying a typical dendrite structure. The substitution of Cu for Ni renders an evident refinement of the grains instead of changing the morphology of the alloys. The result obtained by EDS indicates that the major phase of the as-cast alloys is Mg2Ni phase (denoted as A). Some small massive matters in the alloys substituted by Cu can clearly be seen in Fig.1, which are determined by EDS to be Mg2Cu phase (denoted as B). This apparently is contrary with the result of XRD observation. It is most probably associated with the fact that the amount of the Mg2Cu phase is very little so that the XRD observation can not detect the presence of the Mg2Cu phase. The HRTEM micrographs and electron diffraction patterns of the as-spun (30 m/s) Cu2 and Cu4 alloys are illustrated in Fig.3, exhibiting a nanocrystalline microstructure with an average crystal size of about 2 nm to 5 nm. It is found from HRTEM observations that the as-spun alloys are strongly disordered a

b A

A

B

20 μm Intensity/a.u.

Mg

Mg Section A

Section B

Cu

Ni Ni

Ni

Cu Cu 0

2

4

6

8

Cu 0

E/keV

2

4

6

8

E/keV

Fig.2 SEM images of the as-cast alloys together with typical EDS spectra of sections A and B in Fig.2b: (a) Cu0 alloy and (b) Cu3 alloy a

10

a

b

100 nm Fig.3 HRTEM images and ED patterns of the as-spun (30 m/s) alloys: (a) Cu2 alloy, (b) Cu4 alloy

and nanostructured, but no amorphous phase is detected, which is consistent with the XRD observation. The crystal defects in the as-spun alloy, stacking faults (denoted as A), twin-grain boundary (denoted as B), dislocations (denoted as C) and sub-grain boundary (denoted as D), are clearly viewable in Fig.4. 2.2 Hydriding and dehydriding kinetics The hydriding process was carried out at a hydrogen pressure of 1.5 MPa (in fact, this is initial pressure of hydriding process) at 200 °C, and dehydriding process was carried out in a vacuum (1×10-4 MPa) at 200 °C, too. The hydrogen absorption kinetic curves of the as-cast and spun (25 m/s) alloys are plotted in Fig.5. It is quite evident that the hydrogen absorption capacity of the alloys is visibly enhanced by substituting Ni with Cu. It is noteworthy that, as the amount of Cu substitution is increased to 0.3, such substitution causes a decline of hydrogen absorption capacity, which is primarily attributed to the increase of the MgCu2 phase due to the fact the hydrogen absorption capability of the MgCu2 phase is very low. It is viewed from Fig.5a that the substitution of Cu for Ni markedly ameliorates the hydrogen absorption kinetics of the as-cast alloys, to be ascribed to the increased cell volume and the refined grain caused by Cu substitution by virtue of the grain boundary possesses the largest hydrogen absorption capability[20]. b

c

B

A

B C D 5 nm

Fig.4 Crystal defects in the as-spun (30 m/s) Cu4 alloy taken by HRTEM: (a) stacking fault, (b) twin-grain boundary, and (c) dislocations and sub-grain boundaries

1695

Zhang Yanghuan et al. / Rare Metal Materials and Engineering, 2011, 40(10): 1693-1698

Fig.5 Hydrogen absorption kinetic curves of the as-cast (a) and as-spun (b) (25 m/s) alloys

The hydrogen absorption capacity and kinetics of the as-cast and spun Cu1 and Cu3 alloys are shown in Fig.6. It is viewable that the melt spinning evidently improves the hydrogen absorption property of the alloys. The hydrogen absorption capacity in 10 min is increased from 1.99 to 3.12 wt% for Cu1 alloy, and from 1.74 to 2.88 wt% for the Cu3 alloy by increasing spinning rate form 0 (As-cast was defined as spinning rate of 0 m/s) to 30 m/s. It is very evident that the hydrogen absorption capacity kinetics of all the as-spun nanocrystalline and amorphous Mg2Ni-type alloys studied are superior to those of conventional polycrystalline materials with the same composition. The improved hydrogenation characteristics is attributed to the enhanced hydrogen diffusivity in the nanocrystalline microstructure as the nanocrystalline leads to an easier access of hydrogen to the nanograins, avoiding the long-range diffusion of hydrogen through an already formed hydride, which is often the slowest stage of absorption. It is known that the nanocrystalline microstructures can accommodate higher amounts of hydrogen than polycrystalline ones. The large number of interfaces and grain boundaries available in the nanocrystalline materials provide easy pathways for hydrogen diffusion and promote the absorption of hydrogen. Fig.7 describes the influence of substituting Ni with Cu on the hydrogen desorption kinetics of the as-cast and spun alloys. It is observable that both the hydrogen desorption capacity and the kinetics of the alloys increase with growing of the amount of Cu substitution. With the increase in Cu content from 0 to

Fig.6 Hydrogen absorption kinetic curves of the as-cast and spun alloys: (a) Cu1 alloy; (b) Cu3 alloy

0.4, the hydrogen desorption capacity in 10 min rises from 0.11 to 0.56 wt% for the as-cast alloy, and from 0.56 to 1.31 wt% for the as-spun (25 m/s) alloy. The enhanced hydrogen desorption kinetics can be ascribed to the fact that the substitution of Cu for Ni in the Mg2Ni compound decreases the stability of the hydride and makes the desorption reaction easier[21,22]. A similar result was reported by Simićić et al[2]. The hydrogen desorption kinetics curves of the as-cast and spun alloys are shown in Fig.8. The figures display that the dehydriding capability of the alloys notably meliorates with rising of spinning rate. The hydrogen desorption capacity in 10 min is increased from 0.21 to 0.83 wt% for the Cu1 alloy, and from 44 to 1.29 wt% for Cu3 alloy by enhancing of spinning rate from 0 to 30 m/s. It is found that the as-spun nanocrystalline Mg2Ni alloy exhibits a superior hydrogen desorption kinetics to the crystalline Mg2Ni, which is consistent with the result reported by Spassov et al [17]. The observed essential differences in the hydriding/dehydriding kinetics of the melt-spun nanocrystalline Mg2Ni type alloys studied the most probably have to be associated with the composition of the alloys as well as with the differences in their microstructure due to the different spinning rates. It was reported that the high surface to volume ratios, i.e. high specific surface area, and the presence of large number of grain boundaries in nanocrystalline alloys enhance the hydriding and dehydriding kinetics[17]. Zaluski et al.[23] and Orimo et al.[24] validated that the hydriding/dehydriding kinetics of the milled nanocrystalline Mg2Ni alloys at low temperatures (lower than 200 °C) can be

1696

Zhang Yanghuan et al. / Rare Metal Materials and Engineering, 2011, 40(10): 1693-1698

the secondary phase Mg2Cu in the as-cast alloys instead of changing of the Mg2Ni-type major phase in the alloy. Additionally, Cu substitution visibly refines the grains of the as-cast alloy, whereas it causes an imperceptible impact on the glass forming ability of the alloy. 2) With the increase in the amount of Cu substitution, the hydrogen absorption capacity of the as-cast Mg2Ni-type alloys first increases and then decreases. But it markedly improves the hydrogen desorption capability of the as-cast and spun alloys. 3) Melt spinning evidently promotes the hydriding and dehydriding performances of the Mg2Ni-type alloys. Hydriding and dehydriding capacities and rates of the alloys markedly rise with increasing of spinning rate.

References 1 Schlapbach L, Zütyel A. Nature[J], 2001, 414: 353 2 Simićić M V, Zdujić M, Dimitrijević R et al. J Power Sources[J], 2006, 158: 730 3 Ebrahimi-Purkani A, Kashani-Bozorg S F. J Alloy Compd[J], Fig.7 Hydrogen desorption kinetic curves of the as-cast (a) and as-spun (b) (25 m/s) alloys

2008, 456: 211 4 Kyoi D, Sakai T, Kitamura N et al. J Alloy Compd[J], 2008, 463: 306 5 Palade P, Sartori S, Maddalena A et al. J Alloy Compd[J], 2006, 415: 170 6 Song M Y, Yim C D, Bae J S et al. J Alloy Compd[J], 2008, 463: 143 7 Liu X F, Zhu Y F, Li L Q. J Alloy Compd[J], 2008, 455: 197 8 Liu F J, Suda S. J Alloy Compd[J], 1995, 231: 742 9 Czujko T, Varin R A, Chiu C et al. J Alloy Compd[J], 2006, 414: 240 10 Sakintuna B, Lamari-dirkkim F, Hirscher M. Int J Hydrogen Energy[J], 2007, 32: 1121 11 Zaluska A, Zaluski L, Stroem-Olsen J O. J Alloy Compd[J], 1999, 289: 197 12 Hanada N, Ichikawa T, Fujii H. J Phys Chem B[J], 2005, 109: 7188 13 Recham N, Bhat V V, Kandavel M et al. J Alloy Compd[J], 2008, 464: 377 14 Cui N, Luan B, Zhao H J et al. J Alloy Compd[J], 1996, 233: 236 15 Song M Y, Kwon S N, Bae J S et al. Int J Hydrogen Energy[J], 2008, 33: 1711 16 Savyak M, Hirnyj S, Bauer H D et al. J Alloy Compd[J], 2004,

Fig.8 Hydrogen desorption kinetic curves of the as-cast and spun alloys: (a) Cu1 alloy (b) Cu3 alloy

364: 229 17 Spassov T, Köster U. J Alloy Compd[J], 1998, 279: 279 18 Huang L J, Liang G Y, Sun Z B et al. J Power Sources[J], 2006,

improved by reducing the grain size, due to the fact hydrogen atoms mainly occupied in the disordered interface phase and the grain boundary.

3 Conclusions 1) The substitution of Cu for Ni leads to the formation of

160: 684 19 Friedlmeier G, Arakawa M, Hiraia T et al. J Alloy Compd[J], 1999, 292: 107 20 Orimo S, Fujii H. Appl Phys A[J], 2001, 72: 167 21 Woo J H, Lee K S. J Electrochem Soc[J], 1999, 146(3): 819 22 Takahashi Y, Yukawa H, Morinaga M. J Alloy Compd[J], 1996,

1697

Zhang Yanghuan et al. / Rare Metal Materials and Engineering, 2011, 40(10): 1693-1698

242: 98 23 Zaluski L, Zaluska A, Strön-Olsen J O. J Alloy Compd[J], 1997,

253-254: 70 24 Orimo S, Fujii H, Ikeda K. Acta Mater[J], 1997, 45: 331

1698