Hydriding and dehydriding kinetics of melt spun nanocrystalline Mg 20 Ni 10-xCux ( x = 0-4 ) alloys

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 kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that all the as-spun alloys hold typical nanocrystalline structure. The substitution of Cu for Ni does not change the major phase Mg2Ni but it leads to the formation of the secondary phase Mg2Cu. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys monotonously grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacities and kinetics of the alloys.


INTRODUCTION
Mg and Mg-based alloys has been considered as potential materials for solid state hydrogen storage in the form of metallic hydrides such as MgH 2 and Mg 2 NiH 4 .The theoretical hydrogen storage capacities of MgH 2 and Mg 2 NiH 4 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 sorption/desorption kinetics and high dissociation temperature 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.
Gennari et al. [14] reported that the nanocrystalline Mg 2 Ni synthesized by combined milling-annealing procedure can readily hydrogen absorption during the first cycle and show excellent absorption kinetics at 200 .℃ Muthukumar et al. [15] confirmed that a maximum hydrogen capacity of 3.67 wt.% for the Mg 2 Ni alloy prepared by mechanical alloying (MA) could be achieved for an initial absorption temperature of 300 ℃ and supply pressure of 20 bar.Recham et al. [16] found that the hydrogen absorption property of ball-milled MgH 2 can be enhanced by adding NbF 5 , and the milled MgH 2 + NbF 5 composite can desorbs 3 wt.%H 2 at 150℃.Dobrovolsky et al. [17] synthesized a MgH 2 (50 wt.%) + TiB 2 (50 wt.%) composite by intensive mechanical milling and found that TiB 2 addition decreases the dissociation temperature of the MgH 2 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 solubilize particular elements into MgH 2 or Mg 2 NiH 4 above the thermodynamic equilibrium limit, which is helpful to destabilize MgH 2 or Mg 2 NiH 4 [18].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 [19].Alternatively, melt-spun technique can overcome the above mentioned shortcoming and effective avoiding the significant degradation of hydrogen absorbing and desorbing cycle properties of Mg and Mgbased [20].Additionally, the melt-spinning technique is a very effective method to obtain a nanocrystalline structure and is very suitable for mass-production of nanocrystalline Mg-based alloys.It was also clarified that nanocrystalline alloys produced by melt-spinning could have excellent hydriding characteristics even at room temperature, similar to the alloys produced by the MA process.Spassov et al. [21] prepared Mg 2 (Ni, Y) hydrogen storage alloy with exact composition Mg 63 Ni 30 Y 7 by rapid solidification, and its maximum hydrogen absorption capacity (about 3.0 wt.%) and hydrogenation kinetics of the as-spun Mg 2 (Ni, Y) alloy were found to exceed those of the polycrystalline Mg 2 Ni alloys prepared by conventional technology and to be comparable to the hydrogen absorption characteristics of ball-milled nanocrystalline Mg 2 Ni.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 dehydriding kinetics of the Mg 2 Ni-type alloys [22].Therefore, it is very desirable to investigate the influence of substituting Ni with Cu on the hydriding and dehydriding characteristics of Mg 2 Ni-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.

EXPERIMENTAL METHODS
The alloy ingots were prepared using a vacuum induction 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 spinning rate was approximately expressed by the linear velocity of the copper roller because it is too difficult to measure a real spinning rate i.e. cooling rate of the sample during spinning.The spinning rates used in the experiment were 15, 20, 25 and 30 m/s, respectively.The nominal compositions of the experimental alloys were Mg 20 Ni 10-x Cu x (x = 0, 1, 2, 3, 4).For convenience, the alloys were denoted with Cu content as Cu 0 , Cu 1 , Cu 2 , Cu 3 and Cu 4 , 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 parameters of 160 mA, 40 kV and 10°/min respectively, was performed with CuK α1 radiation filtered by graphite.The thin film samples of the as-spun alloys were prepared by ion etching for observing the morphology with high resolution transmission electron microscope (H-RTEM) (JEM-2100F, operated at 200 kV), and for determining the crystalline state of the samples with electron 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 ℃ ption in a vacuum (1×10 -4 MPa) at 200 too.℃

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 morphology 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 Mg 2 Ni 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 Mg 2 Cu phase (denoted as B).
Figure 2 presents the XRD profiles of the as-cast and spun Mg 20 Ni 10-x Cu x (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 associated with the fact that Mg 2 Ni and Mg 2 Cu hold completely identical structure and nearly same lattice constants.On the other hand, the amount of the Mg 2 Cu phase is very little so that the XRD observation can not detect its presence.Listed in Table 1 are the lattice parameters, 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 diffraction 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 <D hkl > (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 Cu 1 and Cu 3 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.
From HRTEM observations there is some evidence that the as-spun alloys are strongly disordered and nanostructured, but they are free of amorphous phase.Spassov et al. [21] reported that Mg-based alloys with nanocrystalline 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 subgrain boundaries (denoted as D), can clearly be seen in Figure 4.

Hydriding and Dehydriding Characteristics
Figure 5 shows the hydrogen absorption capacity and kinetics of the as-cast and spun Cu 1 and Cu 3 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 spinning significantly improves the hydrogen absorption property of the alloys.The hydrogen absorption capacities 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 absorption capacity of the Cu 1 alloy in 10 min rises from 1.99 to 3.12 wt.%, and from  are superior to those of conventional polycrystalline materials with similar composition.The enhanced hydrogenation property by melt spinning is undoubtedly associated 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 absorption.Based on the result reported by Orimo and Fujii [25], the distribution of the maximum hydrogen concentrations 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 Mg 2 Ni, 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 grainboundary region and the amorphous phase region.The improved hydrogenation characteristics can be explained with 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 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 absorbing/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 increases and then decreases with the variation of Cu content.The Cu 2 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 nanocrystalline structure because the high surface to volume ratios (high specific surface area) and the presence of large number of grain boundaries in nanocrystalline alloys enhance the kinetics of hydrogen absorption/ desorption.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 refined by Cu substitution.The increase of the cell volume is very helpful to hydrogen absorption capacity, and the grain boundary possesses the largest hydrogen absorption 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 hydrogen absorption capacity of the alloys.
Figure 7 shows the hydrogen desorption capacity and kinetics of the as-cast and spun Cu 1 and Cu 3 alloys, indicating 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 desorption capacity of the Cu 1 alloy in 20 min increases from 0.29 to 0.98 wt.%, and from 0.52 to 1.49 wt.% for Cu 3 alloy, respectively.The nanocrystalline Mg 2 Nibased alloys produced by melt spinning exhibit higher H-absorption capacity and faster kinetics of hydriding/ dehydriding than crystalline Mg 2 Ni.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 nanocrystalline Mg 2 Ni 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 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 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 Mg 2 Ni 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 hydogen 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 Mg 2 Ni-type alloys.Firstly, the partial substitution of element Cu for Ni in Mg 2 Ni compound decreases the stability of the hydride and makes the desorption reaction easier [29].Secondly, the presence of Mg 2 Cu 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 probably stabilizes the nanostructure of the alloy obtained by melt spinning, which could be very important for practical H-storage materials on the base of Mg 2 Ni.In multicomponent Mg-based hydrogen storage alloys surface segregation and formation of microcrack passages for Hdiffusion improve the kinetics of hydriding/dehydriding.

CONCLUSIONS
1) All the as-spun Mg 20 Ni 10-x Cu x (x = 0, 1, 2, 3, 4) alloys hold nanocrystalline structures and are free of amorphous phase.The substitution of Cu for Ni does not change the major phase of the alloy, but it leads to a significant refinement of the grains and the formation of secondary phase Mg 2 Cu 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 attributed to the formation of the nanocrystalline structure caused by melt spinning.
3) With the substitution of Cu for Ni, the hydrogen absorption of the alloys first increases and then decreases.But it clearly improves the hydrogen desorption capacity and dehydriding rate of the alloys, for which the decreased stability of the hydride by Cu substitution is mainly responsible.

Figure 1 .
Figure 1.SEM images of the as-cast alloys together with EDS spectra of sections A and B in Figure 1 (b): (a) Cu 0 alloy, (b) Cu 4 alloy.

Figure 5 .
Figure 5. Hydrogen absorption kinetic curves of the as-cast and spun alloys: (a) Cu 1 alloy (b) Cu 3 alloy.

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