M A T E R I A L S C H A R A C T E R I Z A T I O N 6 2 ( 2 0 1 1 ) 4 4 2 – 4 5 0

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Effect of amorphous Mg50Ni50 on hydriding and dehydridingbehavior of Mg2Ni alloy

D. Guzmána,⁎, S. Ordoñezb, J.F. Fernándezc, C. Sánchezc, D. Serafinid, P.A. Rojase,C. Aguilarf, P. Tapiag

aDepartamento de Ingeniería en Metalurgia, Facultad de Ingeniería, Universidad de Atacama y Centro Regional de Investigación y DesarrolloSustentable de Atacama (CRIDESAT), Av. Copayapu 485, Copiapó, ChilebDepartamento de Ingeniería Metalúrgica, Facultad de Ingeniería, Universidad de Santiago de Chile, Av. Lib. Bernardo O'Higgins 3363,Santiago, ChilecDepartamento de Física de Materiales, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco 28049, Madrid, SpaindDepartamento de Física, Facultad de Ciencias, Universidad de Santiago de Chile and Center for Interdisciplinary Research in Materials,CIMAT, Av. Lib. Bernardo O'Higgins 3363, Santiago, ChileeEscuela de Ingeniería Mecánica, Facultad de Ingeniería, Av. Los Carrera 01567, Quilpué, Pontificia Universidad Católica de Valparaíso, PUCV,ChilefDepartamento de Ingeniería Metalúrgica y Materiales, Universidad Técnica Federico Santa María, Av. España 1680, Valparaíso, ChilegDepartamento de Ingeniería en Metalurgia, Facultad de Ingeniería, Universidad de Atacama, Av. Copayapu 485, Copiapó, Chile

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +56 52206646; faE-mail address: danny.guzman@uda.cl (D

1044-5803/$ – see front matter © 2011 Elsevdoi:10.1016/j.matchar.2011.02.007

A B S T R A C T

Article history:Received 23 September 2010Received in revised form14 February 2011Accepted 23 February 2011

Composite Mg2Ni (25 wt.%) amorphous Mg50Ni50 was prepared by mechanical millingstarting with nanocrystallineMg2Ni and amorphousMg50Ni50 powders, by using a SPEX 8000D mill. The morphological and microstructural characterization of the powders wasperformed via scanning electron microscopy and X-ray diffraction. The hydridingcharacterization of the composite was performed via a solid gas reaction method in aSievert's-type apparatus at 363 K under an initial hydrogen pressure of 2 MPa. Thedehydriding behavior was studied by differential thermogravimetry. On the basis of theresults, it is possible to conclude that amorphous Mg50Ni50 improved the hydriding anddehydriding kinetics ofMg2Ni alloy upon cycling. A tentative rationalization of experimentalobservations is proposed.

© 2011 Elsevier Inc. All rights reserved.

Keywords:Hydrogen storage materialsAmorphous materialsMechanical alloyingHydriding and dehydridingprocessesX-ray diffraction

1. Introduction

Hydrogen storage inmetal hydrides offers a high storage densityper unit volume and is relatively safer than other hydrogenstorage methods, such as the compression and liquefaction ofhydrogen gas and absorption by carbon [1]. Magnesium is a

x: +56 52206619.. Guzmán).

ier Inc. All rights reserve

suitablematerial forhydrogen storagebecause of its lightweight,high hydrogen capacity (7.6 wt.% H for pure MgH2), and low cost.Nevertheless, MgH2 is very stable (equilibrium pressure at 552 Kis 0.1 MPa), and its hydrogen absorption and desorption kineticsare considerably slow [2,3] for technological applications. Severalattempts have been made to overcome these limitations by

d.

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alloying magnesium with other elements [4–7], producingmagnesium-based composites [8–11], and modifying its mor-phology and microstructure [12].

Two stable intermetallics, Mg2Ni and MgNi2, have beenreported in theMg–Ni system [13]. The former absorbs hydrogenin a solid solution up toMg2NiH0.3 and forms a hydride, Mg2NiH4,at higher hydrogen concentrations. The hydride has a lowerstoragecapacity thanMgH2,but fasterhydridinganddehydridingkinetics [4,14,15]. Below 503 K, it crystallizes in amonoclinic low-temperature phase, transforming to a cubic high-temperaturephase above that temperature [16–18]. On the other hand, MgNi2does not react with hydrogen at pressures up to 2.8 MPa and atemperature of 623 K [4]. In relation to nonequilibrium hydrides,Orimo et al. [19–21] worked on the amorphous Mg50Ni50–H2

system. The results obtained in these investigations showed thatthe amorphous phase can absorb approximately 2.2 wt.% Hwithout changing its amorphous structure. On the other hand,Guzmán et al. [22] reported the formation of oversaturated solidsolution in the nanocrystalline Mg2Ni–H2 system (correspondingto Mg2NiH2.2).

The mechanical alloying (MA) process developed by Benja-min et al. [23,24] in the early 1970s has been widely used toproduce hydrogen storage materials, particularly Mg-basedalloys. Considering the large differences in melting points and

Fig. 1 – XRD patterns of Mg and Ni in 2:1 atomic proportion:(a) milled for 10 h and (b) milled as (a) and annealed at 673 Kfor 0.5 h.

vapor pressures between Mg and Ni, the production of Mg–Nibased alloys via the conventional casting method is moredifficult than theMA process [25–27]. In addition, MA producesa microstructural refinement [19,20,22,28–31] and increasesthe specific surface area [32], which improve the hydriding anddehydriding kinetics [33–35]. Thus, amorphous Mg50Ni50,produced by MA, has showed a marked improvement in thehydriding and dehydriding kinetics compared to Mg2Ni andball-milled Mg2Ni [28,35,36].

Although many researches have been performed on thehydriding and dehydriding behavior of the nanocrystalline/amorphous composites in the Mg–Ni system [19,21,22,29,30,36,37], the interaction between amorphous and nanocrys-talline phases is not yet completely understood. In this study,it is proposed to establish the effect of the amorphousMg50Ni50 on the hydriding and dehydriding properties ofnanocrystalline Mg2Ni.

2. Experimental Procedures

2.1. Preparation of Composite Mg2Ni–Amorphous Mg50Ni50

On the basis of the results obtained in previous studies [25–27],intermetallic Mg2Ni was synthesized by combining the MAand heat treatment. The MA process of Mg turnings (98 wt.%pure, particle size below 4.7 mm; Aldrich) and Ni powders

Fig. 2 – XRD patterns of Mg and Ni in 1:1 atomic proportion:(a) initial mixture and (b) milled for 20 h.

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(99.99 wt.% pure, particle size below 61 μm; Fluka) in a 2:1atomic proportion was carried out in a SPEX 8000 D mill for10 h, under a controlled atmosphere of Ar (99.999 wt.% pure)using a ball to powder weight ratio of 20:1. The heat treatmentwas accomplished under vacuum at 673 K for 0.5 h.

According to the data reported by Guzmán et al. [38],amorphous Mg50Ni50 was produced via MA of an equiatomicmixture of Mg turnings and Ni powders in a SPEX 8000 D millfor 20 h with a ball to powder weight ratio of 20:1 under inertAr atmosphere.

In order to produce the composite material, a mixtureconsisting of 75 wt.% Mg2Ni and 25 wt.% amorphous Mg50Ni50was milled for 10 min in a SPEX 8000 D mill under inert Aratmosphere using a ball to powder weight ratio of 10:1. Forcomparison, a Mg2Ni sample was obtained by the previouslymentioned procedure but without adding any amorphousphase. This sample will help to establish the effect of theamorphous phase on the hydriding and dehydriding proper-

Fig. 3 – SEMmicrographs obtained in SE mode, showing general apowders. Particle size distributions are showed in each case.

ties of Mg2Ni without any influence of the microstructuralrefinement of Mg2Ni during the composite synthesis (milling).

2.2. Characterization

The powders produced weremicrostructurally characterized byX-ray diffraction (XRD) in a Shimadzu XRD-600 diffractometerusingCuKα radiation.Theaveragecrystallite sizewascalculatedusing the Scherrer method [39]. In order to obtain the full widthat half maximum peak intensity, the experimental peaks werefitted using a Lorentz function. The sizes, morphologies, andphasedistributionsof thepowderswere studiedusinga Jeol 5410scanning electronmicroscope (SEM) equippedwithX-ray energydispersive spectroscopy (EDS). In order to determine the particlesize distribution, four micrographswere recorded using approx-imately 400 measurements in each analysis.

Hydrogen absorption of the composite was investigated ina Sievert's-type apparatus at 363 K and an initial hydrogen

nd magnified views of (a) Mg2Ni and (b) amorphous Mg50Ni50

445M A T E R I A L S C H A R A C T E R I Z A T I O N 6 2 ( 2 0 1 1 ) 4 4 2 – 4 5 0

pressure of 2 MPa, whereas hydrogen desorption was inves-tigated by thermogravimetry under flowing N2, using aheating rate of 5 K min−1 in a SDT 2960 TA instrument. Thus,the apparent activation energy for the hydrogen desorptionprocess was obtained via the Kissinger method [40] usingheating rates of 5, 10, 15, and 20 K min−1.

Fig. 4 – XRD patterns of (a) Mg2Ni (25 wt.%) amorphousMg50Ni50 composite and (b) milled Mg2Ni.

3. Results and Discussion

3.1. Microstructural and Morphological Characterization

Fig. 1 (a) shows the XRD pattern of a mixture of Mg and Ni in a2:1 atomic proportion milled for 10 h in a SPEX mill. It can beseen that the sample consisted of a mixture of Mg2Ni andresidual Ni with an average crystallite size of 11 and 15 nm,respectively. Additional Mg needed to maintain the millingstoichiometry formed an amorphous phase, which could besubsequently transformed in a mechanical and/or thermalmanner into Mg2Ni [25–27].

Fig. 1 (b) shows the XRD pattern of a sample annealed at673 K for 0.5 h under vacuum. As can be seen, the sampleconsistedmainly ofMg2Ni and a small amount of Ni because ofthe loss of Mg by the MgO formation. The reason for thedecrease of the Ni content can be understood by consideringthat the amorphous transformation into Mg2Ni (423 K) andthe solid reaction between Mg and residual Ni to form Mg2Ni(473–523 K) occur below 673 K [25–27]. After annealing (673 K),the Mg2Ni crystallite size increased to 30 nm. Further detailsabout theMg2Ni productionviaMAandMAandheat treatmentcan be found in Ref. [27].

Fig. 2 (a) shows the XRD pattern of the initial mixture of Mgand Ni in a 1:1 atomic proportion, whereas Fig. 2 (b) shows theXRD pattern of the powders after 20 h of milling. The millingconditions employed were sufficient to produce an amor-phous Mg50Ni50 alloy. The latter result is in agreement with aprevious report [38] in which more information about phaseevolution during the amorphization process can be obtained.

Fig. 3 shows SEM images of Mg2Ni and amorphousMg50Ni50obtained in the secondary electron mode (SE). It can be seenthat Mg2Ni was composed of large irregular agglomerates withan apparent diameter of approximately 30 μm and severalsmaller particles with flake-like shapes around 1.5 μm weldedtogether (Fig. 3 (a)). On the other hand, the amorphousMg50Ni50 was composed of large irregular agglomerates withamaximum apparent size of 500 μmand small round particleswith an apparent size of 3 μm (Fig. 3 (b)).

A previous work [29] confirmed that the microstructuralrefinement in the nanometric range of Mg2Ni by millingincreases its hydriding and dehydriding kinetics comparedwith crystalline Mg2Ni. The latter behavior can be understoodby considering the decrease in the diffusion path and theincrease in the inter-grain region because of the appearance ofthe nanocrystalline structure.

In order to eliminate the influence of the microstructuralrefinement of Mg2Ni during the composite synthesis (milling)and establish the effects of the amorphous phase on thehydriding and dehydriding properties of Mg2Ni, the resultsobtained from the composite were compared with thoseobtained from a Mg2Ni sample milled for 10 min.

Fig. 4 (a) shows theXRDpatternof compositeMg2Ni (25 wt.%)amorphous Mg50Ni50 produced by milling. The amorphousphase was detected in the XRD pattern due to an increase inthediffraction intensity over thebackground level in the regionsclose to 2θ≈20, 40, and 65° [38]. The average crystallite size ofMg2Ni in the composite was 25 nm. On the other hand, Fig. 4 (b)shows theXRDpatternof theMg2Ni prepared for comparison. Inthis case, the calculated crystallite size of Mg2Ni was 23 nm. Onthe basis of the results, it is possible to conclude that Mg2Ni inthe composite and the milled Mg2Ni presented similar micro-structural refinements; therefore, any difference in the hydrid-ing and dehydriding properties between the milled Mg2Ni andthe composite material might be related to the presence of theamorphous Mg50Ni50 phase.

Fig. 5 (a) shows the SEM image of the composite materialobtained in the SE mode. It can be seen that the particle sizedistribution of the composite is similar to that of theMg2Ni alloy(Fig. 3 (a)). EDSanalysis revealed that the brighterparticles in thebackscattered electronview (BSE) image (Fig. 5 (b)) correspond toamorphous Mg50Ni50 and the darker areas to the Mg2Ni matrix(Fig. 5 (c)). On the basis of the results, it is possible to concludethat the milling time employed (10 min) in the compositesynthesis was sufficient to produce an intimate contactbetween Mg2Ni and the amorphous Mg50Ni50 particles. Finally,another important conclusion from the XRD (Fig. 4) and SEM(Fig. 5) results is that no alloy formation (betweenMg2Ni and theamorphous phase) occurred during milling.

Fig. 6 – Hydriding kinetics of (a) composite Mg2Ni (25 wt.%)amorphous Mg50Ni50 and (b) milled Mg2Ni at 363 K under aninitial hydrogen pressure of 2 MPa.

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3.2. Hydriding and Dehydriding Characterization

The hydriding and dehydriding characteristics of the preparedcomposite were studied and compared to that of the milledMg2Ni to establish the possible utility of the composite forhydrogen storage. The samples were activated after onehydriding (initial hydrogen pressure of 2 MPa at 363 K) anddehydriding (under vacuum of 27 Pa at 453 K) cycle. Thedehydriding temperature was selected to avoid the crystalli-zation process of amorphous Mg50Ni50 that starts at approx-imately 573 K [41]. The low equilibrium H pressure of Mg2Nibelow 473 K [42] precludes the ability to obtain the desorptiondata by the volumetric method. In this first hydriding cycle,the Mg2Ni sample absorbed approximately 1.60 wt.% H,whereas the composite material only absorbed 1.47 wt.% H.

Fig. 6 shows the hydrogen absorption curves of thecomposite and Mg2Ni samples during four hydriding cycles.

Fig. 5 – SEM micrographs showing (a) SE view and (b) BSE ofcomposite material (arrows indicate the position ofamorphous Mg50Ni50). (c) Elemental EDS analysis patterns ofthe selected area in (b).

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The reversible hydrogen storage capacity and hydridingkinetics of the composite material were greater than those ofMg2Ni. The initial hydrogen absorption rate for both samplesshowed no obvious dependence on the hydriding cycles. Thus,the average initial rate of hydrogen absorption for thecomposite material (0.60±0.06 wt.%H min−1) was approxi-mately four times higher than that for the Mg2Ni sample(0.15±0.02 wt.%Hmin−1). On the basis of the results, it ispossible to conclude that the amorphous phase improved thehydriding kinetics and hydrogen storage capacity of Mg2Niupon cycling under the experimental conditions. The latterbehavior could be attributed to two factors. First, it has beenreported that on the surface of amorphous Mg50Ni50, Mg iscombined with oxygen whereas Ni is present in metallic state[43,44]. This Ni, which formed enriched layers, served as aneffective catalyst for dissociating hydrogen molecules. Sec-ond, Mg–Ni based amorphous alloys showed superior oxida-tion resistances to their crystalline counterparts [45], which,added to the existence of metallic Ni underneath the topsurface, provided “passageways” to the composite for diffus-ing hydrogen atoms in atmospheres containing oxygen.

Fig. 7 shows XRD patterns of the composite and Mg2Nisamples after five hydriding cycles. It is possible to observe thatboth materials consisted of a mixture of Mg2NiH4, Mg2NiH0.3,residual Ni, andMgO. The only difference between the two XRDpatterns was the increase in the diffraction intensity over the

Fig. 7 – XRD patterns of hydrogenated (a) composite Mg2Ni(25 wt.%) amorphous Mg50Ni50 and (b) milled Mg2Ni.

background level in the regions close to 2θ≈20, 40, and 65° in thecomposite pattern (Fig. 7 (a)), which proved the presence of thehydrogenated amorphous phase [46].

Dehydrogenation of the samples was studied by thermo-gravimetric analysis after the last hydriding cycle. Fig. 8 (a)shows the results. The total hydrogen mass losses for themilledMg2Ni (1.6 wt.%) and composite (1.4 wt.%) samplesweresimilar to the amount of hydrogen absorbed during the firsthydriding cycle, demonstrating that only partial dehydroge-nation was achieved in the volumetric experiments under theexperimental conditions. A slight rise of the thermogravi-metric curves in the high temperature region was due to theoxidation of the alloys by the residual oxygen absorbed on thealloy surfaces. Iwakura et al. previously reported the latterbehavior in Mg–Ni based alloys [37].

After five cycles, the hydrogen storage capacities of themilled Mg2Ni and composite samples were, respectively, 47%and 73% of their initial capacities. Considering that bothsamples were subjected to some desorption conditions andthat the capacity loss could be attributed principally to partial

Fig. 8 – (a) Thermogravimetric and (b) derivativethermogravimetric profiles of the hydrogenated samplesobtained at 5 K/min under flowing N2.

Fig. 9 – Plot of ln(β TP−2) versus 1000 TP

−1 for dehydriding ofmilledMg2Ni and composite Mg2Ni (25 wt.%) amorphous Mg50Ni50.

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dehydrogenation, it is possible to conclude that the amorphousphase improved thedesorptionkinetics of hydrogenatedMg2Ni.

Hydrogen absorption can produce nanocrystallization inthe amorphous phase [47,48]. This fact could worsen thehydriding and dehydriding kinetics of the composite. Adetailed investigation of how the hydrogen absorption processaffects amorphous Mg50Ni50 stability is beyond the scope ofthis work. However, on the basis of the results of XRD of thehydrogenated composite (Fig. 7(a)), it is possible to establishthat during the hydriding and dehydriding cycles, a completecrystallization of the amorphous phase did not occur.

Fig. 8 (b) shows thederivative thermogravimetric curvesof themilled Mg2Ni and composite materials. Instead of two separatedesorption steps (desorption of the hydrogenated amorphousphase andMg2NiH4), the entire composite releasedhydrogen in asingle principal process. The temperature of the maximumhydrogen desorption rate of the composite material (481 K) waslower than that of the milled Mg2Ni (525 K). Furthermore, thestarting temperature for the desorption of the composite (375 K)was similar to those reported by Orimo et al. [20] (373 K) andGuzmán et al. [46] (373 K) for the desorption of hydrogenatedamorphous Mg50Ni50. On the basis of the results, it is possible toconclude that during the desorption process of the compositematerial, a catalytic effect of hydrogenated amorphous Mg50Ni50on thehydrogendesorption ofMg2NiH4was observed. Zaluska etal. [49] previously observed a similar behavior in the MgH2–Mg2NiH4 system. Catalytic desorption could be explained owingto the interaction between neighboring particles of Mg2NiH4 andhydrogenated amorphous Mg50Ni50. Thus, when hydrogenatedamorphous Mg50Ni50 begins to desorb hydrogen, it undergoes acontraction that applies a strain to the attached Mg2NiH4 andfacilitates its desorption.

In addition, other dehydriding reactions could be observedin the derivative thermogravimetric curves of Mg2Ni and thecomposite material around 585 K (Fig. 8 (b)). This reaction wasunaffected by the existence of the amorphous phase. Accord-ing to Selvan et al. [50,51], this desorption process corre-sponded to the dehydriding of Mg2NiH, which is obtainedaccording to the following equations:

Mg2NiH4→Mg2NiH þ 1:5 H2 ðaround 540–570 KÞ

Mg2NiH→Mg2NiH0:3 þ 0:35 H2 ðaround 560–590 KÞ:

The apparent activation energy for the principal dehydrid-ing process was estimated with the Kissinger method [40].According to the Kissinger theory, the slope of the linear plotof ln(β TP

−2) versus TP−1 is the apparent activation energy of

the process, where β is the heating rate and TP is the peaktemperature.

Fig. 9 shows the Kissinger plot of the dehydriding process ofMg2Ni and the composite materials. The apparent activationenergy for the hydrogen desorption of the composite material(48±6 kJ mol−1) was lower than the calculated value for thedehydriding process of Mg2Ni (85±4 kJ mol−1). The latter resultsconfirmed the existence of a catalytic effect during thecomposite desorption. On the other hand, the apparentactivation energies for hydrogen desorption of the compositeand Mg2Ni samples were lower than that reported by Selvan etal. [50] for the desorption of Mg2NiH4 (183 kJ mol−1). The

discrepancy between the values reported by Selvan et al. [50]and those obtained in this study can be understood consideringthedifferences in the experimental procedures. Selvan et al. [50]employed thermogravimetric analysis under a hydrogen pres-sure of 101.3 kPa, whereas in this study, the thermogravimetricanalyses were carried out under flowing N2.

4. Conclusions

In order to improve the hydriding and dehydriding propertiesof Mg2Ni, a composite of Mg2Ni with 25 wt.% of amorphousMg50Ni50 was prepared via mechanical milling. On the basis ofthe results, it is possible to conclude that amorphous Mg50Ni50improved the hydrogen storage capacity and hydridingkinetics of Mg2Ni alloy upon cycling. In addition, a catalyticeffect of amorphous Mg50Ni50 on the hydrogen desorption ofMg2NiH4 was observed. Thus, it is proposed that whenhydrogenated amorphous Mg50Ni50 begins to desorb hydro-gen, it undergoes a contraction that applies a strain to theattached Mg2NiH4 and facilitates its desorption.

Finally, although the addition of amorphous Mg50Ni50improved the hydrogen absorption and desorption kineticsof Mg2Ni and its cyclability at low temperatures, the technicaltargets established by the U.S. Department of Energy (DOE) [52]for transport applications have not been achieved. The mainproblem of the composites based onMg2Ni is the high stabilityof Mg2NiH4, which has an equilibrium H pressure of 91 Pa at358 K (maximum delivery temperature recommended byDOE), which is very low for mobile applications.

Acknowledgments

The authors gratefully acknowledge “Fondo Nacional Desar-rollo Científico y Tecnológico de Chile”, FONDECYT, proyectNo. 1070085 and “Comisión Nacional de Investigación Cientí-fica y Tecnológica de Chile”, CONICYT, for their economicsupport granted to realize this work. Some authors thank the

449M A T E R I A L S C H A R A C T E R I Z A T I O N 6 2 ( 2 0 1 1 ) 4 4 2 – 4 5 0

Spanish Ministry of Education and Science, MEC, for itsfinancial support under contract No. MAT2008-06547-C02-01.

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