Hydriding and dehydriding kinetics of melt spun nanocrystalline Mg20Ni10-xCux (x = 0-4) alloys

doi:10.4236/ns.2010.21003

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Vol.2, No.1, 18-25 (2010) Natural Science

http://dx.doi.org/10.4236/ns.2010.21003

Hydriding and dehydriding kinetics of melt spun

nanocrystalline Mg20Ni10-xCux (x = 0-4) alloys

Yang-Huan Zhang1,2,*, Dong-Liang Zhao1, Bao-Wei Li2, Hui-Ping Ren1, Shi-Hai Guo1, Xin-Lin Wang1

1Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing, China

2School of Material, Inner Mongolia University of Science and Technology, Baotou, China; zyh59@yahoo.com.cn

Received 22 October 2009; revised 13 November 2009; accepted 17 November 2009.

ABSTRACT

The nanocrystalline Mg2Ni-type electrode alloys

with nominal compositions of Mg20Ni10-xCux (x =

0, 1, 2, 3, 4) were synthesized by melt-spinning

technique. The microstructures of the alloys

were characterized by XRD, SEM and HRTEM.

The hydrogen absorption and desorption kinet-

ics of the alloys were measured using an auto-

matically controlled Sieverts apparatus. The re-

sults show that all the as-spun alloys hold ty-

pical nanocrystalline structure. The substitution

of Cu for Ni does not change the major phase

Mg2Ni but it leads to the formation of the sec-

ondary phase Mg2Cu. The hydrogen absorption

capacity of the alloys first increases and then

decreases with rising Cu content, but the hy-

drogen desorption capacity of the alloys mono-

tonously grows with increasing Cu content. The

melt spinning significantly improves the hydro-

genation and dehydrogenation capacities and

kinetics of the alloys.

Keywords: Mg2Ni-Type Alloy; Substituting Ni with

Cu; Melt Spinning; Hydriding and Dehydriding

Kinetics

1. INTRODUCTION

Mg and Mg-based alloys has been considered as poten-

tial materials for solid state hydrogen storage in the form

of metallic hydrides such as MgH2 and Mg2NiH4. The

theoretical hydrogen storage capacities of MgH2 and

Mg2NiH4 are 7.6 wt.% and 3.6 wt.% [1,2] respectively,

which is quite adequate for commercial applications as a

hydrogen fuel source [3]. Unfortunately, the slow sorp-

tion/desorption kinetics and high dissociation tempera-

ture of these kinds of metal hydrides limit their practical

application. 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)

[4], GPa hydrogen pressure method [5], melt spinning

[6], gravity casting [7], polyol reduction [8], hydriding

combustion synthesis [9], surface modification [10], al-

loying with other elements [11,12], adding catalysts [13]

etc, have been undertaken to improve the activation and

hydriding property.

Gennari et al. [14] reported that the nanocrystalline

Mg2Ni synthesized by combined milling-annealing pro-

cedure can readily hydrogen absorption during the first

cycle and show excellent absorption kinetics at 200. ℃

Muthukumar et al. [15] confirmed that a maximum hy-

drogen capacity of 3.67 wt.% for the Mg2Ni alloy pre-

pared by mechanical alloying (MA) could be achieved

for an initial absorption temperature of 300 ℃ and sup-

ply pressure of 20 bar. Recham et al. [16] found that the

hydrogen absorption property of ball-milled MgH2 can

be enhanced by adding NbF5, and the milled MgH2+

NbF5 composite can desorbs 3 wt.% H2 at 150℃. Do-

brovolsky et al. [17] synthesized a MgH2 (50 wt.%) +

TiB2 (50 wt.%) composite by intensive mechanical mil-

ling and found that TiB2 addition decreases the dissocia-

tion temperature of the MgH2 hydride by about 50.℃

Indubitably, ball-milling is a very effective method for

the preparation of nanocrystalline and amorphous Mg

and Mg-based alloys. Particularly, it is suitable to solu-

bilize particular elements into MgH2 or Mg2NiH4 above

the thermodynamic equilibrium limit, which is helpful to

destabilize MgH2 or Mg2NiH4 [18]. However, the milled

Mg and Mg-based alloys show very poor hydrogen ab-

sorbing and desorbing stability due to the fact that the

metastable structures formed by ball milling tended to

vanish during multiple hydrogen absorbing and desorb-

ing cycles [19]. Alternatively, melt-spun technique can

overcome the above mentioned shortcoming and effec-

tive avoiding the significant degradation of hydrogen

absorbing and desorbing cycle properties of Mg and Mg-

based [20]. Additionally, the melt-spinning technique is

a very effective method to obtain a nanocrystalline struc-

ture and is very suitable for mass-production of nano-

crystalline Mg-based alloys. It was also clarified that

nanocrystalline alloys produced by melt-spinning could

Y. H. Zhang et al. / Natural Science 2 (2010) 18-25

have excellent hydriding characteristics even at room

temperature, similar to the alloys produced by the MA

process. Spassov et al. [21] prepared Mg2(Ni, Y) hydro-

gen storage alloy with exact composition Mg63Ni30Y7 by

rapid solidification, and its maximum hydrogen absorp-

tion capacity (about 3.0 wt.%) and hydrogenation kinet-

ics of the as-spun Mg2(Ni, Y) alloy were found to ex-

ceed those of the polycrystalline Mg2Ni alloys prepared

by conventional technology and to be comparable to the

hydrogen absorption characteristics of ball-milled na-

nocrystalline Mg2Ni. It was determined that the addition

of third element stabilizes the nanostructure of Mg-Ni

based alloy, which could be very important for practical

H-storage materials.

Our previous work indicated that the substitution of

Co for Ni significantly improved the hydriding and de-

hydriding kinetics of the Mg2Ni-type alloys [22]. There-

fore, it is very desirable to investigate the influence of

substituting Ni with Cu on the hydriding and dehydrid-

ing characteristics of Mg2Ni-type alloys prepared by

melt-spinning. The objective of this work is to produce

the Mg-Ni-based ternary nanocrystalline alloys by melt

spinning and to examine their structures and hydriding

and dehydriding kinetics.

2. EXPERIMENTAL METHODS

The alloy ingots were prepared using a vacuum induc-

tion furnace in a helium atmosphere at a pressure of 0.04

MPa. Part of the as-cast alloys was re-melted and spun

by melt-spinning with a rotating copper roller. The spin-

ning rate was approximately expressed by the linear ve-

locity of the copper roller because it is too difficult to

measure a real spinning rate i.e. cooling rate of the sam-

ple during spinning. The spinning rates used in the ex-

periment were 15, 20, 25 and 30 m/s, respectively. The

nominal 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 morphologies of the as-cast alloys were observed

by scanning electron microscope (SEM) (Philips Q-

UANTA 400). The phase structures of the as-cast and

spun alloys were determined by XRD diffractometer (D/

max/2400). The diffraction, with the experimental pa-

rameters of 160 mA, 40 kV and 10°/min respectively,

was performed with CuKα1 radiation filtered by graphite.

The thin film samples of the as-spun alloys were pre-

pared by ion etching for observing the morphology with

high resolution transmission electron microscope (H-

RTEM) (JEM-2100F, operated at 200 kV), and for de-

termining the crystalline state of the samples with elec-

tron diffraction (ED).

The hydrogen absorption and desorption kinetics of

the alloys were measured by an automatically controlled

Sieverts apparatus. The hydrogen absorption was con-

ducted at 1.5 MPa at 200 and the hydrogen desor℃p-

tion in a vacuum (1×10-4 MPa) at 200 too.℃

3. RESULTS AND DISCUSSIONS

3.1. Microstructure Characteristics

The SEM images of the as-cast alloy are illustrated in

Figure 1, displaying a typical dendrite structure. The

substitution of Cu for Ni does not change the morphol-

ogy of the alloys but it causes a significant refinement of

the grains. The result obtained by energy dispersive

spectrometry (EDS) indicates that the major phase of the

as-cast alloys is Mg2Ni phase (denoted as A). Some

small massive matters in the alloys containing Cu can

clearly be seen in Figure 1, which are determined by

EDS to be Mg2Cu phase (denoted as B).

Figure 2 presents the XRD profiles of the as-cast and

spun Mg20Ni10-xCux (x = 0, 1, 2, 3, 4) alloys, showing

that the substitution of Cu for Ni does not change the

phase structure. All the as-cast and spun alloys display a

single phase structure. This seems to be contrary with

the result of SEM observation. It is most probably asso-

ciated with the fact that Mg2Ni and Mg2Cu hold com-

pletely identical structure and nearly same lattice con-

stants. On the other hand, the amount of the Mg2Cu

phase is very little so that the XRD observation can not

detect its presence. Listed in Table 1 are the lattice pa-

rameters, cell volume and full width at half maximum

(FWHM) values of the main diffraction peaks of the

as-cast and spun (20 m/s) alloys which were calculated

by software of Jade 6.0. It can be derived from Table 1

that the substitution of Cu for Ni causes the FWHM

values of the main diffraction peaks of the as-cast and

spun alloys significantly increased, and it leads to the

lattice parameters and cell volume of the alloys cleverly

enlarged, which is attributed to the slightly larger atomic

radius of Cu than Ni. It can be seen from Table 1 that

melt spinning causes the FWHM values of the main dif-

fraction peaks of the alloys significantly increased which

is doubtless attributed to the refinement of the grains and

the stored stress in the grains produced by melt spinning.

Based on the FWHM values of the broad diffraction

peak (203) in Figure 2b, the crystallite size <Dhkl > (Ǻ)

of the as-spun alloy was calculated using Scherrer’s

equation. The grain sizes of the as-spun alloys are in a

range of 2-6 nm, consistent with results reported by

Friedlmeier et al. [23]. It is very important to notice that

<D> values were calculated on the same peak having

Miller indices (203) due to better possibility of mutual

comparison.

Figure 3 shows HRTEM micrographs and electron

diffraction patterns of the as-spun Cu1 and Cu3 alloys,

which reveal a nanocrystalline microstructure, with an

average crystal size of about 2-5 nm. This result agrees

very well with the XRD observation shown in Figure 2.

Y. H. Zhang et al. / Natural Science 2 (2010) 18-25

From HRTEM observations there is some evidence that

the as-spun alloys are strongly disordered and nanos-

tructured, but they are free of amorphous phase. Spassov

et al. [21] reported that Mg-based alloys with nanocrys-

talline microstructures can be obtained by controlling the

cooling rates. The crystal defects in the as-spun alloy,

stacking faults (denoted as A), twin-grain boundaries

(denoted as B), dislocations (denoted as C) and sub-

grain boundaries (denoted as D), can clearly be seen in

Figure 4.

Figure 1. SEM images of the as-cast alloys together with EDS spectra of sections A and B in

Figure 1 (b): (a) Cu0 alloy, (b) Cu4 alloy.

Figure 2. XRD profiles of the as-cast and spun alloys: (a) As-cast, (b) As-spun (20 m/s).

20 μm

Section A

0 2 4 6 8

Cu Cu

keV

Section B

02468 10

keV

20 30 40 50 60 70 80 90100

(403)

(204)

(220)

(118)

(215)

(206)

(203)

(113) (105)

(200)

(112)

△

(103)

(102)

(101) (003)

(100)

△

Cu0

2θ(de g re e )

Cu1

Cu2

Cu3

(b)

Cu4

△ -Mg2Ni

20 30 40 50 60 70 80 90100

(403)

(204)

(220)

(118)

(215)

(206)

(212)

(211)

(106) (203)

(113)(105)

(200)

(112)

(103)

(102)

(101) (003)

(100)

△

△Cu0

2θ(deg r ee)

Cu1

Cu2

Cu3

(a) △ -Mg2Ni Cu4

Y. H. Zhang et al. / Natural Science 2 (2010) 18-25

Table 1. The lattice parameters, cell volume and the FWHM values of the major diffraction peaks of the alloys.

Figure 3. HRTEM micrographs and ED of the as-spun alloys (30 m/s): (a) Cu1 alloy (b) Cu3 alloy.

Figure 4. Crystal defects in the as-spun (30 m/s) Cu3 alloy taken by HRTEM: (a) Stacking fault, (b) Twin grain bound-

ary, (c) Dislocations and sub-grain boundaries.

3.2. Hydriding and Dehydriding Characteristics

Figure 5 shows the hydrogen absorption capacity and

kinetics of the as-cast and spun Cu1 and Cu3 alloys. It

can be seen that all the hydriding kinetic curves of the

as-spun alloys show an initial fast hydrogen absorption

stage after which the hydrogen content is saturated at

longer hydrogenation time, indicating that the melt spin-

ning significantly improves the hydrogen absorption

property of the alloys. The hydrogen absorption capaci-

ties of the alloys increase with rising spinning rate.

When the spinning rate grows from 0 (As-cast is defined

as spinning rate of 0 m/s) to 30 m/s, the hydrogen ab-

sorption capacity of the Cu1 alloy in 10 min rises from

1.99 to 3.12 wt.%, and from 1.74 to 2.88 wt.% for the

Cu3 alloy.

The hydrogenation kinetics and storage capacity of all

the as-spun nanocrystalline Mg2Ni-type alloys studied

FWHM values Lattice parameters and cell volume

2θ(20.02°) 2θ(45.14°) a (Å) c (Å) V (Å3)

Alloys

As-cast 20m/s As-cast 20m/sAs-cast20 m/sAs-cast 20 m/s As-cast 25 m/s

Cu0 0.122 0.129 0.169 0.1735.20975.210113.244 13.258 311.29 311.67

Cu1 0.133 0.184 0.178 0.2185.21025.215413.252 13.262 311.54 312.39

Cu2 0.148 0.232 0.183 0.2235.21365.216213.283 13.307 312.67 313.55

Cu3 0.151 0.257 0.192 0.2325.21545.217113.297 13.313 313.22 313.80

Cu4 0.165 0.286 0.204 0.2525.21715.220313.302 13.317 313.54 314.28

50 nm

5 nm

Y. H. Zhang et al. / Natural Science 2 (2010) 18-25

Figure 5. Hydrogen absorption kinetic curves of the as-cast and spun alloys: (a) Cu1 alloy (b) Cu3 alloy.

are superior to those of conventional polycrystalline ma-

terials with similar composition. The enhanced hydro-

genation property by melt spinning is undoubtedly asso-

ciated with the refinement of the grains produced by

melt spinning [24]. By refining the microstructure, a lot

of new crystallites and grain boundaries are created

which can act as fast diffusion paths for hydrogen ab-

sorption. Based on the result reported by Orimo and Fu-

jii [25], the distribution of the maximum hydrogen con-

centrations in three nanometer-scale regions, i.e. grain

region and grain boundary region as well as amorphous

region, have been experimentally determined to be 0.3

wt.% H in the grain region of Mg2Ni, 4.0 wt.% H in the

grain boundary and 2.2 wt.% H in the amorphous region.

It revealed that the hydrides mainly exist in grain-

boundary region and the amorphous phase region. The

improved hydrogenation characteristics can be explained

with the enhanced hydrogen diffusivity in the nanocrys-

talline 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 ab-

sorption. It is known that the nanocrystalline micro-

structures can accommodate higher amounts of hydrogen

than polycrystalline ones. The crystalline material, when

melt spun, becomes at least partially disordered and its

structure changes to nanocrystalline. Consequently, high

densities of crystal defects such as dislocations, stacking

faults and grain boundaries are introduced. The large

number of interfaces and grain boundaries available in

the nanocrystalline materials provide easy pathways for

hydrogen diffusion and accelerates the hydrogen ab-

sorbing/desorbing process.

The hydrogen absorption kinetic curves of the as-spun

alloys are plotted in Figure 6. It can be seen that the

hydrogen absorption capacity of as-spun alloys first in-

creases and then decreases with the variation of Cu con-

tent. The Cu2 alloy shows a maximum hydrogen absorp-

tion capacity at 200℃. The kinetics of hydrogenation

was extremely fast so that the alloys absorbed more than

95% of their hydrogen capacities within the first 5 min.

The excellent hydriding kinetics is ascribed to the na-

nocrystalline structure because the high surface to vol-

ume ratios (high specific surface area) and the presence

of large number of grain boundaries in nanocrystalline

alloys enhance the kinetics of hydrogen absorption/ de-

sorption. The positive function of Cu substitution on the

hydrogen absorption capacity and kinetics of the alloy is

attributed to the increased cell volume and the grain re-

fined by Cu substitution. The increase of the cell volume

is very helpful to hydrogen absorption capacity, and the

grain boundary possesses the largest hydrogen absorp-

tion capability [25]. It was well known that the catalytic

action of Ni on hydriding is stronger than Cu. Therefore,

it is understandable that a superfluous amount of Cu

substitution (x>2) must lead to a decrease of the hydro-

gen absorption capacity of the alloys.

Figure 7 shows the hydrogen desorption capacity and

kinetics of the as-cast and spun Cu1 and Cu3 alloys, in-

dicating that the dehydriding capability of the alloys

obviously meliorates with rising spinning rate. When the

spinning rate grows from 0 to 30 m/s, the hydrogen de-

sorption capacity of the Cu1 alloy in 20 min increases

from 0.29 to 0.98 wt.%, and from 0.52 to 1.49 wt.% for

Cu3 alloy, respectively. The nanocrystalline Mg2Ni-

based alloys produced by melt spinning exhibit higher

H-absorption capacity and faster kinetics of hydriding/

dehydriding than crystalline Mg2Ni. A similar result was

reported by Spassov et al. [21]. The specific capacity

and hydriding/dehydriding kinetics of hydride materials

depend on their chemical composition and crystalline

structure [26]. The observed essential differences in the

hydriding/dehydriding kinetics of the melt-spun nano-

crystalline Mg2Ni type alloys studied most probably

have to be associated with the composition of the alloy

as well as with the differences in their microstructure

due to the different spinning rates. It was reported that

0 20406080100120

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(a)

Hydrogen absorptiom (wt.%)

Time (min)

As-cast

15 m/s

20 m/s

25 m/s

30 m/s

0 20406080100120

0.0

0.5

1.0

1.5

2.0

2.5

3.0 (b)

Hydrogen absorption (wt.%)

Time (min)

As-cast

15 m/s

20 m/s

25 m/s

30 m/s

Y. H. Zhang et al. / Natural Science 2 (2010) 18-25

the high surface to volume ratios, i.e. high specific sur-

face area, and the presence of large number of grain

boundaries in nanocrystalline alloys enhance the kinetics

of hydrogen absorption/desorption [21]. Zaluski et al.

[27] and Orimo et al. [28] confirmed that the hydriding/

dehydriding characteristics at low temperatures (lower

than 200℃) of nanocrystalline Mg2Ni alloys prepared by

mechanical alloying can be improved by reducing the

grain size (20-30 nm), due to hydrogen occupation in the

disordered interface phase.

The hydrogen desorption kinetic curves of the as-spun

alloys are plotted in Figure 8. An important feature of

the dehydrogenation process in the alloys is very slow

initial hydrogen desorption, followed by slack increase

in the amount of hydrogen desorbed. Figure 8 indicates

that the Cu substitution significantly improves the hy-

dogen desorption capacity and kinetics of the alloys.

When Cu content x increases from 0 to 4, the hydrogen

desorption capacity of the as-spun (20 m/s) alloy in 20

min rises from 0.62 to 1.43 wt.%, and from 0.89 to 1.69

wt.% for the as-spun (30 m/s) alloy. Several possibilities

can be considered as the reasons why the substitution of

Cu for Ni enhances the hydrogen desorption kinetics of

Mg2Ni-type alloys. Firstly, the partial substitution of

element Cu for Ni in Mg2Ni compound decreases the

stability of the hydride and makes the desorption reac-

tion easier [29]. Secondly, the presence of Mg2Cu phase

apparently has catalytic effects for the hydriding and

dehydriding reactions of Mg and Mg-based alloys [19].

Additionally, the addition of the third element Cu proba-

bly stabilizes the nanostructure of the alloy obtained by

melt spinning, which could be very important for practi-

cal H-storage materials on the base of Mg2Ni. In multi-

component Mg-based hydrogen storage alloys surface

segregation and formation of microcrack passages for H-

diffusion improve the kinetics of hydriding/dehydriding.

Figure 6. Hydrogen absorption kinetic curves of the as-spun alloys: (a) 20 m/s, (b) 30 m/s.

Figure 7. Hydrogen desorption kinetic curves of the as-cast and spun alloys: (a) Cu1 alloy (b) Cu3 alloy.

0 20406080100120

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(b)

Hydrogen absorption (wt.%)

Time (min)

Cu0

Cu1

Cu2

Cu3

Cu4

020406080100120

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(a)

Hydrogen absorption (wt.%)

Time (min)

Cu0

Cu1

Cu2

Cu3

Cu4

020 40 60 80100120

0.0

0.2

0.4

0.6

0.8

1.0

1.2 (a)

Hydrogen desorption (wt.%)

Time (min)

As-cast 25 m/s

15 m/s 30 m/s

20 m/s

020 40 60 80100120

0.0

0.4

0.8

1.2

1.6

2.0 (b)

Hydrogen desorption (wt.%)

Time (min)

As-cast 25 m/s

15 m/s 30 m/s

20 m/s

Y. H. Zhang et al. / Natural Science 2 (2010) 18-25

Figure 8. Hydrogen desorption kinetic curves of the as-spun alloys: (a) 20 m/s, (b) 30 m/s.

4. CONCLUSIONS

1) All the as-spun Mg20Ni10-xCux (x = 0, 1, 2, 3, 4) alloys

hold nanocrystalline structures and are free of amor-

phous phase. The substitution of Cu for Ni does not

change the major phase of the alloy, but it leads to a sig-

nificant refinement of the grains and the formation of

secondary phase Mg2Cu in the as-cast alloys.

2) Melt spinning significantly improves the hydriding

and dehydriding properties of the alloys. Hydriding and

dehydriding capacities and rates of the alloy markedly

rise with increasing spinning rate, which is mainly at-

tributed to the formation of the nanocrystalline structure

caused by melt spinning.

3) With the substitution of Cu for Ni, the hydrogen

absorption capacity of the alloys first increases and then

decreases. But it clearly improves the hydrogen desorp-

tion capacity and dehydriding rate of the alloys, for

which the decreased stability of the hydride by Cu sub-

stitution is mainly responsible.

5. ACKNOWLEDGEMENTS

This work is supported by Hi-Tech Research and Development Pro-

gram of China (2007AA03Z227), National Natural Science Founda-

tions of China (50871050 and 50701011), Natural Science Foundation

of Inner Mongolia, China (200711020703) and Higher Education Sci-

ence Research Project of Inner Mongolia, China (NJzy08071).

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020 40 60 80100120

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1.6

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(a)

Hydrogen desorption (wt.%)

Time (min)

Cu0 Cu3

Cu1 Cu4

Cu2

0 20406080100120

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1.6

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Cu1 Cu4

Cu2

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