- Email: [email protected]
MgH2 as dopant for improved activation of commercial Mg ingot
Journal of Alloys and Compounds 575 (2013) 364–369
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
MgH2 as dopant for improved activation of commercial Mg ingot P. Jain a,⇑, J. Lang a, N.Y. Skryabina b, D. Fruchart c, S.F Santos d, K. Binder e, T. Klassen e, J. Huot a,⇑ a
Hydrogen Research Institute, Université du Québec à Trois-Rivières, 3351 des Forges, Trois-Rivières, Québec G9A 5H7, Canada Department of Physics, Perm State University, 15, Bukireva, 614990 Perm, Russia c Institut Néel, BP 166, 38042 Grenoble Cedex 9, France d CECS, Federal University of ABC, Rua Santa Adélia 166, Santo André, SP 09210-170, Brazil e Institute of Materials Technology, Helmut-Schmidt-University, University of the Federal Armed Forces, Holstenhofweg 85, 22043 Hamburg, Germany b
a r t i c l e
i n f o
Article history: Received 25 March 2013 Received in revised form 13 May 2013 Accepted 15 May 2013 Available online 7 June 2013 Keywords: Hydrogen storage materials Magnesium Severe plastic deformation technique MgH2 catalytic powder Activation behavior
a b s t r a c t In this paper, we propose a method to decrease the activation time (first hydrogenation) of commercial Mg. This new alternative processing route uses a combination of cold rolling and short time ball milling to obtain full hydrogen capacity quickly in the first hydrogenation. As ball milling of ductile materials leads to particle agglomeration, brittle Mg plates produced by repetitive cold rolling were used as starting material. These rolled plates were then ball milled for 30 min with and without the addition of 5 wt% Mg or MgH2 powders. All the synthesized samples were investigated for hydrogen storage, absorption– desorption behavior and microstructure using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and pressure-composition temperature (PCT) methods. Results showed slow activation behavior for cold rolled Mg plates, which was slightly improved after milling. Further improvement was obtained by adding 5 wt% of Mg powder during ball milling. In contrast, when the plates were ball milled with 5 wt% of MgH2 powder a drastic improvement in activation behavior was observed with hydrogen capacity reaching up to 6.2 wt% in comparison to 2.74 wt% for undoped and 3.57 wt% for Mg doped samples. These results reveal that ball milling with ductile Mg powder deforms only the surface of Mg plates while brittle MgH2 powders causes fracturing and cracks, increasing the surface area and generating heterogeneous nucleation sites within the bulk material. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen fuel cells and other hydrogen consuming devices require a material that can store hydrogen on demand under moderate conditions. This need has developed interest in metal hydrides owing to their high hydrogen content, compactness, safety and ease of hydrogen storage and transportation. Among metal hydrides, Mg shows promising features such as low cost, abundance along with high storage capacity of 7.6 wt% and high energy density of 2.33 kW h/kg [1,2]. However, high thermodynamic stability of magnesium hydride (MgH2) coupled with slow hydriding/ dehydriding kinetics and slow hydrogen diffusion rate in Mg matrix makes first hydrogenation, the so-called activation, very difficult. Usually, the material has to be subjected to high temperature-high pressure conditions before it starts reacting with hydrogen. Such an activation process requires relatively large expenditures in terms of time and energy, thereby increasing production cost [3]. Many studies have shown that reducing crystallite size of magnesium by using high energy ball milling technique results in ⇑ Corresponding authors. E-mail addresses: [email protected] (P. Jain), [email protected] (J. Huot). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.099
enhanced sorption kinetics [4–6]. However, use of high energy ball milling for industrial level production could be time consuming and expensive in terms of capital and operation cost as it necessitates relatively high specific energy. Also, high reactivity and low air resistance of resultant nanocrystalline powder requires a complex and expensive handling process [7]. Additionally, ball milling of ductile materials like Mg is less effective than brittle ones, as it leads to particle agglomeration, requiring use of process controlling agent [8]. The latest trend is to use severe plastic deformation (SPD) techniques, particularly cold rolling, to synthesize Mg based materials [9–15]. Ueda et al. [9] used cold rolling to prepare Mg–Ni laminated composite followed by heat treatment to synthesize Mg2Ni alloy. They showed that Mg2Ni alloy prepared by this method transformed to Mg2NiH4 upon hydrogenation. Miyamura et al. [10–12] investigated effect of cold rolling on various Mg based composites such as Mg/Cu, Mg/Al and Mg/Pd. Dufour and Huot [13] showed that Mg/Pd system prepared by cold rolling has much faster activation and better air resistance compared to ball milled sample. Combined use of cold rolling with other SPD techniques (e.g. equal channel angular pressing and ball milling) to reach a desired texture and grain structure in Mg alloys has also been demonstrated [14,15]. Recently, Amira and Huot [16] studied the effect of cold rolling on the activation behavior of as-cast and
P. Jain et al. / Journal of Alloys and Compounds 575 (2013) 364–369
die-cast Mg alloys and mentioned ball milling of rolled material with 5 wt% MgH2, as essential step for activation. In the present work we demonstrate the complete procedure with suitable explanation for doping Mg samples with MgH2 powder to obtain faster activation. From the existing literature on material modification via mechanochemical treatments, we know that – (a) presence of only two slip systems at room temperature in Mg causes drastic work hardening and stress concentration, enabling cracking and fracture during plastic deformation [17]; (b) ball milling is less effective for ductile materials like Mg due to sticking and agglomeration of powder on balls and vial surfaces [14]; (c) ball milling is more energetic than cold rolling for uniform distribution of additives in Mg based materials [18]. The aim of this study is to improve activation behavior of low cost, easily available commercial magnesium ingot by combined use of cold rolling and ball milling. More specifically, to use cold rolling for transforming ductile Mg plates into brittle pieces. This cold rolled product can then be easily ball milled to incorporate additives. The effect of these co-operative phenomena on the structure and hydrogen storage properties of commercial Mg plates is discussed in detail. Various groups have shown that by using dopants like Al, In, Fe, Nb improvements in absorption kinetics can be attained but only after few absorption/desorption cycles [19–22]. On the other hand, Knott et al. reported the auto catalytic effect of finely divided MgH2 on Mg [23]. In the present study, we investigated the addition of 5 wt% Mg and MgH2 powders for improving activation of rolled Mg plates. The idea is to avoid use of expensive catalyst or dopants while keeping the capacity and purity of the material.
2. Experimental Mg plates cut from commercial Norsk Hydro ingot were used as a starting material for cold rolling. Materials for ball milling were Mg (99.8% purity) and MgH2 (98% purity) powders from Alfa Aesar. Durston DRM 100 rolling mill with stainless steel rolls of 13 cm length, 7.5 cm diameter and driven by 1.1 kW electric motor was used for rolling. Mg plates with initial thickness of 2 mm were placed between two stainless steel (316) plates of 0.8 mm thickness and rolled horizontally in air. After each pass, the distance between the rolls was reduced to exert higher stress on plates for fast deformation; thereafter the plates were folded and re-rolled keeping constant distance between rolls till a final thickness of around 0.15 mm was attained. Thus, after 25 rolls brittle Mg pieces were obtained and used for investigation. These cold rolled Mg pieces were then ball milled in Ar atmosphere for 30 min, with and without additives using a Spex High Energy Ball Mill with ball to powder ratio of 10:1. In all, four different samples were prepared for present study, as described in Table 1. The hydrogen sorption properties were measured with a homemade Sieverttype apparatus. All measurements were performed at 623 K with a hydrogen pressure of 2000 kPa for absorption and 2 kPa for desorption. The sample was loaded in the reactor inside a glove box. Prior to measurement the system was hydrogen flushed and pumped. Thereafter, the argon in the reactor was pumped. Therefore, all pumping were either from hydrogen filled atmosphere or argon atmosphere. X-ray diffraction was performed using Bruker D8 Focus X-ray apparatus with Cu Ka radiation. Phase abundances, crystallite size and microstrain were evaluated from Rietveld method using Topas software [24]. Crystallite size were calculated by using the volume weighted mean column height based on integral breath while microstrain was computed using broadening modeled by a Voigt function as Table 1 Samples prepared for present investigation with notation. S. No.
Sample
Notation
1 2
25 cold rolled Mg pieces 25 cold rolled Mg pieces followed by 30 min ball milling 25 cold rolled Mg pieces +5wt%Mg powder ball milled for 30 min 25 cold rolled Mg pieces +5wt%MgH2 powder ball milled 30 min
Mg(CR) Mg(CR)
3 4
BM
Mg(CR) + Mg Mg(CR) + MgH2
BM BM
365
recommended by Balzar et al. [25]. Morphological studies were made with a Jeol JSM-5500 electron microscope. Chemical analysis was performed by using EDX on a Hitachi S-3400 microscope.
3. Result and discussion Morphology of all the samples under investigation along with as cast (ingot) Mg as reference is shown in Fig. 1. The presence of layers and pin holes on as cast magnesium reveals ductile characteristic of material with presence of oxide traces. Cold rolling causes fracturing with deformation and orientation of grains along rolling direction thereby generating a textured surface, as observed in Fig. 1b. Ball milling of cold rolled plates without or with Mg and MgH2 powders as additives (Fig. 1c–e), has similar effect on morphology. During milling, Mg plates interact with each other by successive events of cold welding and fracture, resulting in smaller particles and homogenization of the mixture. However, distribution of Mg or MgH2 powder on rolled Mg plates cannot be observed by secondary electrons due to negligible difference in electron density between these two phases. Chemical analysis of the Mg(CR) + MgH2 BM sample gave a composition by weight of Mg-96%, O-3% and Fe-1%. Assuming that oxygen is present in the form of MgO this translates to an abundance of 7.5 wt% MgO. Origin of iron is attributed to contamination during milling. Such a small amount of iron is essentially impossible to see in the X-ray powder diffraction. Rietveld refinement on X-ray diffraction data was performed to identify presence of different phases and to calculate microstrain and crystallite size. Fig. 2 shows the diffraction pattern of as cast Mg plates in comparison with the pattern of 25 cold rolled plates before and after heat treatment at 623 K. As expected, the as cast sample do not show any texture. From Rietveld refinement the crystallite size was determined to be 64 nm. Rolling induces texture with pronounced increase in intensity of (0 0 2) peak. However, an increase in crystallite size from 64 nm to 97 nm was noticed after rolling. As the crystallite size computed is in fact the volume weighted mean column height, it is somewhat understandable that if most of the crystallite are elongated along 0 0 2 direction then the mean may increase. Therefore, the observed increase in crystallite size is only representative of grain elongation along (0 0 2) direction and may not be the true reflection of actual crystallite size. Heat treatment under vacuum at 623 K for 5 h induces grain growth from 97 nm to 211 nm with no drastic change in texture. In fact, there was a slight increase in texture along (0 1 3) Bragg peak. Also, there is a slight reduction of microstrain as could be expected. Fig. 3 shows the XRD patterns of rolled plates after 30 min ball milling without (Fig. 3a) or with additives (Fig. 3b and c). In Mg(CR) + MgH2 BM sample, a small Bragg peak corresponding to MgH2 is observed. There is also loss of preferred orientation for Mg along (0 0 2) direction as could be expected because of randomization due to ball milling. Rietveld analysis gave phases abundances as 97% Mg and 3% MgH2 which are very close to nominal values. Despite the fact that rolling was done in air, no peak corresponding to oxides or hydroxides was observed in any of the samples. This may be due to very small crystallite size of these impurities making their peaks extremely broad and indistinguishable from background [18]. The crystallite size and strain of Mg as well as crystallite size of MgH2 as determined by Rietveld analysis for all the samples are shown in Table 2. The main aim of the present study is to improve the first hydrogenation or the activation process of commercial Mg ingot by combined use of two mechanical processes: cold rolling and ball milling. Therefore, we shall first investigate the activation behavior of rolled Mg plates before and after ball milling. Before taking hydrogen sorption measurements, the synthesized samples were heated at 623 K under dynamic vacuum
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Fig. 1. Scanning electron microscope (SEM) micrographs of commercial Mg ingot: (a) as-cast, (b) Mg(CR), (c) Mg(CR) – BM, (d) Mg(CR) + Mg Magnification: left column 250, right column 1000.
for 5 h. As seen previously, this step did not change the sample texture but increased the crystallite size. Thereafter hydrogen absorption was performed at 623 K under 2000 kPa of hydrogen. Fig. 4a
BM, (e) Mg(CR) + MgH2
BM.
shows the activation curve for Mg(CR) plates. It is observed that cold rolled plates react very slowly with hydrogen with the sample absorbing less than 1 wt% H2 in 13 h. After ball milling the rolled
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P. Jain et al. / Journal of Alloys and Compounds 575 (2013) 364–369
Fig. 2. X-ray diffraction patterns of Mg ingot (a) as cast, (b) after 25 cold rolls and (c) after 5 h vacuum annealing at 623 K of 25 cold rolled plates.
plates for 30 min and under the same condition of activation, hydrogen absorption increases to 2.7 wt% H2 in 13 h. The slight improvement in kinetic is probably due to the formation of surface defects and increase in surface area during ball milling. Still the amount of hydrogen absorbed by this sample is significantly low, making it necessary to use dopants or extend the activation time for further increase the sorption capacity. The activation curves of rolled Mg plates ball milled with 5 wt% Mg and MgH2 powders are shown in Fig. 4c and d. The sample with Mg powder absorbs 3.6 wt% H2 in 13 h which is slightly better than sample without additive. Also the shape of the two curves is similar, showing existence of long incubation time before hydrogen absorption. When Mg powder is replaced by same amount of MgH2 powder, a drastic improvement in activation time and capacity is observed, 6.2 wt% H2 absorption in 13 h. To verify the hydrogenation results, diffraction profile of Mg(CR) + MgH2 BM sample in hydride state was obtained by quenching the sample upon complete hydrogenation at 623 K under 2000 kPa H2 pressure. In Fig. 5 the diffraction pattern of hydride state (Fig. 5b) is compared with as cast (Fig. 5a) and dehydrided state (Fig. 5c). The pattern of as cast sample contains peaks corresponding to MgH2 and Mg with slight loss in texture. However, hydrogenation induces a complete loss in texture as observed in Fig. 5b and c. Rietveld refinement done on hydride state, showed 77% MgH2, 19% Mg and 4% MgO phases, thereby verifying the obtained capacity. Upon desorption, complete transformation of MgH2 to Mg is observed with 6% MgO. The amount of MgO phase determined by Rietveld analysis is slightly lower than the one measured by EDX. This may be due to the absorption coefficient of MgO as well as very broad peak of MgO due to its very small crystallite size which makes this peak difficult to distinguish from the
Fig. 3. X-ray diffraction patterns of rolled Mg plates after 30 min ball milling (a) without catalyst, (b) with 5 wt% Mg powder and (c) with 5 wt% MgH2 powder.
background. The small crystallite size may explain the absence of peak corresponding to MgO in as cast sample. It is known that at room temperature MgO crystallize in very small crystallites (1.4 nm) and that the crystallite size slowly increases with temperature up to 4.3 nm at 623 K [26]. This may explain why in X-ray patterns the MgO phase is not seen in samples prepared at room temperature and visible in samples that have been heated. Seeing that the addition of MgH2 to cold rolled Mg improves the activation kinetic we investigated the reduction of activation temperature by performing activation on Mg(CR) + MgH2 BM sample at 573 K under 2000 kPa hydrogen pressure. The activation kinetic was found to be much slower. A capacity of 5.7 wt% was reached after 40 h. Therefore, the technique presented here is useful to improve the kinetic but do not drastically lower the operation temperature. This result is understandable because the thermodynamic of magnesium hydrogenation could not be changed by cold rolling or adding magnesium hydride to magnesium. Fig. 6 shows the hydrogenation characteristics of samples after initial hydrogenation (activation). Remarkably, kinetics of all the samples is similar, thereby confirming that difficulties observed in initial hydrogenation of magnesium do not generally apply to subsequent cycles and also the fact that no catalyst has been added. We assume that presence of magnesium hydride formed during initial hydrogenation is responsible for faster kinetics in higher cycles. Thus, all samples showed similar hydrogen absorption kinetics. The only difference being total capacity which reflects the activation effectiveness. Among the samples investigated in present study, the cold rolled Mg plate’s ball milled with 5 wt% MgH2 additive showed faster activation behavior than the sample with same amount of Mg powder. However, after 5 h of heat treatment at 623 K under vac-
Table 2 Crystallite size and microstrain of the Mg phase and phase abundances in wt% as evaluated from Rietveld refinement of all the samples. For crystallite size and microstrain values in parenthesis are uncertainties on the last significant digit. Sample
Mg ingot Mg (CR) Mg(CR) VA Mg(CR) BM Mg(CR) + Mg BM Mg(CR) + MgH2 BM Mg(CR) + MgH2 BM (after H-absorption) Mg(CR) + MgH2 BM (after H-desorption)
Mg plates
MgH2
Size (nm)
Strain (%)
Size (nm)
Strain (%)
64(2) 97(1) 211(7) 47(1) 46(1) 51(1) 105(9) 46(1)
0.096(2) 0.084(1) 0.035(1) 0.162(2) 0.135(1) 0.134(1) 0.114(4) 0.070(2)
– –
– –
– – – 66(1) –
– – – 0.067(1) –
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Fig. 4. Activation (first hydrogenation) at 623 K, 2000 kPa H2 pressure for (a) Mg(CR); (b) Mg(CR) BM; (c) Mg(CR) + Mg-30 min BM and (d) Mg(CR) + MgH230 min BM.
Fig. 6. Second (after initial activation) hydrogen absorption curves at 623 K, 2000 kPa H2 pressure for (a) Mg(CR); (b) Mg(CR) BM; (c) Mg(CR) + Mg BM and (d) Mg(CR) + MgH2 BM.
Fig. 5. X-ray diffraction patterns of rolled Mg plate’s ball milling for 30 min with 5 wt% MgH2 powder in (a) as cast state; (b) after absorption under 2000 kPa H2 and (c) after desorption till 2 kPa at 623 K.
Fig. 7. Activation behavior of Mg(CR) + MgH2 catalytic powder after heating at 623 K (a) under vacuum for 1 h; (b) under vacuum for 2 h; (c) under vacuum for 5 h and (d) under 2000 kPa H2 pressure.
uum it is expected that MgH2 additive is desorbed to pure Mg. Therefore, the starting material for first hydrogenation (activation) measurements should be the same for both Mg(CR) + Mg BM and Mg(CR) + MgH2 BM samples. Thus, in principle, activation should be same for these two samples which is obviously not the case. To get the better understanding of the mechanism involved, we measured the activation behavior of Mg(CR) + MgH2 BM sample under different conditions: (a) pre-heating the sample at 623 K under dynamic vacuum for much smaller times i.e. 1 h and 2 h in comparison to 5 h – to have partial amount of MgH2 and thereafter introducing 2000 kPa H2 for absorption measurements and (b) heating to 623 K under 2000 kPa hydrogen pressure – to restrict desorption of MgH2. Fig. 7 shows the effect of annealing time and atmosphere on activation behavior. It is clear that all curves have similar shape and the only discrepancy is the total capacity. However, we see a clear trend that the total capacity increases with annealing time while the smallest capacity is for the sample annealed under hydrogen pressure. This means that prior complete dehydrogenation of MgH2 is mandatory for maximum results. The different results obtained by ball milling the Mg plates with Mg powder and with MgH2 powder can be better understood by
taking into account that the mechanisms of microstructure evolution during ball-milling are different in a ductile–ductile system (Mg plates and Mg powder) than in a ductile-fragile one (in the case of Mg plates and MgH2). This matter is discussed in several papers of ball-milling technique and the interested reader could obtain detailed information in the classical review paper of Suryanarayana [27]. We propose the following model to explain the possible interaction during milling of Mg plates produced by work hardening with two catalytic powders-ductile Mg powder and brittle MgH2 powder. During milling, the energy transfer between Mg plates covered with ductile Mg powder is sufficient only to cause breaking of particles into smaller particles with Mg powder deforming the surface of plates, thereby increasing the hydrogen capacity slightly more in comparison to plates milled without Mg powder. In the case of MgH2 additive, a much higher impact occurs on collision between plates covered with brittle powder, which results not only in reducing the particle size but also incorporating MgH2 powder in Mg matrix producing cracks, kissing bonds and others defect sites along the grain boundaries, thereby providing
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dopants and catalytic powder by MgH2 powder. In this work we used ball milling technique to add dopants. Future work is aimed at using cold rolling in inert atmosphere to incorporate MgH2 into Mg matrix. This method would enable us to have further reduction in production time and cost. Acknowledgements This work was supported by grant from Natural Science and Engineering Council of Canada. N.Y.S., D.F., and J.H. would like to thanks the Ministry of Education of Perm Region (Russia) for funding project C-26/211. P.J. is thankful to Hydro-Quebec for her Post-Doctoral fellowship. References
Fig. 8. Second (after initial activation) hydrogen absorption curves at 623 K, 2000 kPa H2 pressure for Mg(CR) + MgH2 catalytic powder after heating at 623 K, (a) under vacuum for 1 h; (b) under vacuum for 2 h; (c) under vacuum for 5 h and (d) under 2000 kPa H2 pressure.
necessary nucleation sites for hydrogen absorption. Further penetration of cracks into bulk of magnesium is obtained by volume contraction when doped MgH2 is desorbed prior to activation. As shown in Fig. 8, after activation step all samples have same kinetics and capacity. Thus, confirming that first absorption or activation of commercial Mg is an important step needed for improving commercialization of the material for hydrogen storage applications. 4. Conclusion
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
A magnesium–magnesium hydride composite was successfully prepared by combined use of cold rolling and ball milling. Addition of 5 wt% MgH2 via short milling to rolled Mg plates resulted in significant improvement in activation time and capacity with absorption of 6.2 wt% H2 in 13 h at 623 K under 2000 kPa hydrogen pressure. Structural and morphological results showed complete loss in preferred orientation after one hydrogenation/dehydrogenation cycle. The present work demonstrates a simple method to reduce production cost of light weight hydrogen storage materials without compromising on chemical purity and hydrogen capacity by using (a) cheaper Mg ingot rather than expensive MgH2 as starting material, (b) reducing milling time and particle agglomeration by performing cold rolling prior to ball milling, (c) replacing expensive
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
MgH2 as dopant for improved activation of commercial Mg ingot P. Jain a,⇑, J. Lang a, N.Y. Skryabina b, D. Fruchart c, S.F Santos d, K. Binder e, T. Klassen e, J. Huot a,⇑ a
Hydrogen Research Institute, Université du Québec à Trois-Rivières, 3351 des Forges, Trois-Rivières, Québec G9A 5H7, Canada Department of Physics, Perm State University, 15, Bukireva, 614990 Perm, Russia c Institut Néel, BP 166, 38042 Grenoble Cedex 9, France d CECS, Federal University of ABC, Rua Santa Adélia 166, Santo André, SP 09210-170, Brazil e Institute of Materials Technology, Helmut-Schmidt-University, University of the Federal Armed Forces, Holstenhofweg 85, 22043 Hamburg, Germany b
a r t i c l e
i n f o
Article history: Received 25 March 2013 Received in revised form 13 May 2013 Accepted 15 May 2013 Available online 7 June 2013 Keywords: Hydrogen storage materials Magnesium Severe plastic deformation technique MgH2 catalytic powder Activation behavior
a b s t r a c t In this paper, we propose a method to decrease the activation time (first hydrogenation) of commercial Mg. This new alternative processing route uses a combination of cold rolling and short time ball milling to obtain full hydrogen capacity quickly in the first hydrogenation. As ball milling of ductile materials leads to particle agglomeration, brittle Mg plates produced by repetitive cold rolling were used as starting material. These rolled plates were then ball milled for 30 min with and without the addition of 5 wt% Mg or MgH2 powders. All the synthesized samples were investigated for hydrogen storage, absorption– desorption behavior and microstructure using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and pressure-composition temperature (PCT) methods. Results showed slow activation behavior for cold rolled Mg plates, which was slightly improved after milling. Further improvement was obtained by adding 5 wt% of Mg powder during ball milling. In contrast, when the plates were ball milled with 5 wt% of MgH2 powder a drastic improvement in activation behavior was observed with hydrogen capacity reaching up to 6.2 wt% in comparison to 2.74 wt% for undoped and 3.57 wt% for Mg doped samples. These results reveal that ball milling with ductile Mg powder deforms only the surface of Mg plates while brittle MgH2 powders causes fracturing and cracks, increasing the surface area and generating heterogeneous nucleation sites within the bulk material. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen fuel cells and other hydrogen consuming devices require a material that can store hydrogen on demand under moderate conditions. This need has developed interest in metal hydrides owing to their high hydrogen content, compactness, safety and ease of hydrogen storage and transportation. Among metal hydrides, Mg shows promising features such as low cost, abundance along with high storage capacity of 7.6 wt% and high energy density of 2.33 kW h/kg [1,2]. However, high thermodynamic stability of magnesium hydride (MgH2) coupled with slow hydriding/ dehydriding kinetics and slow hydrogen diffusion rate in Mg matrix makes first hydrogenation, the so-called activation, very difficult. Usually, the material has to be subjected to high temperature-high pressure conditions before it starts reacting with hydrogen. Such an activation process requires relatively large expenditures in terms of time and energy, thereby increasing production cost [3]. Many studies have shown that reducing crystallite size of magnesium by using high energy ball milling technique results in ⇑ Corresponding authors. E-mail addresses: [email protected] (P. Jain), [email protected] (J. Huot). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.099
enhanced sorption kinetics [4–6]. However, use of high energy ball milling for industrial level production could be time consuming and expensive in terms of capital and operation cost as it necessitates relatively high specific energy. Also, high reactivity and low air resistance of resultant nanocrystalline powder requires a complex and expensive handling process [7]. Additionally, ball milling of ductile materials like Mg is less effective than brittle ones, as it leads to particle agglomeration, requiring use of process controlling agent [8]. The latest trend is to use severe plastic deformation (SPD) techniques, particularly cold rolling, to synthesize Mg based materials [9–15]. Ueda et al. [9] used cold rolling to prepare Mg–Ni laminated composite followed by heat treatment to synthesize Mg2Ni alloy. They showed that Mg2Ni alloy prepared by this method transformed to Mg2NiH4 upon hydrogenation. Miyamura et al. [10–12] investigated effect of cold rolling on various Mg based composites such as Mg/Cu, Mg/Al and Mg/Pd. Dufour and Huot [13] showed that Mg/Pd system prepared by cold rolling has much faster activation and better air resistance compared to ball milled sample. Combined use of cold rolling with other SPD techniques (e.g. equal channel angular pressing and ball milling) to reach a desired texture and grain structure in Mg alloys has also been demonstrated [14,15]. Recently, Amira and Huot [16] studied the effect of cold rolling on the activation behavior of as-cast and
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die-cast Mg alloys and mentioned ball milling of rolled material with 5 wt% MgH2, as essential step for activation. In the present work we demonstrate the complete procedure with suitable explanation for doping Mg samples with MgH2 powder to obtain faster activation. From the existing literature on material modification via mechanochemical treatments, we know that – (a) presence of only two slip systems at room temperature in Mg causes drastic work hardening and stress concentration, enabling cracking and fracture during plastic deformation [17]; (b) ball milling is less effective for ductile materials like Mg due to sticking and agglomeration of powder on balls and vial surfaces [14]; (c) ball milling is more energetic than cold rolling for uniform distribution of additives in Mg based materials [18]. The aim of this study is to improve activation behavior of low cost, easily available commercial magnesium ingot by combined use of cold rolling and ball milling. More specifically, to use cold rolling for transforming ductile Mg plates into brittle pieces. This cold rolled product can then be easily ball milled to incorporate additives. The effect of these co-operative phenomena on the structure and hydrogen storage properties of commercial Mg plates is discussed in detail. Various groups have shown that by using dopants like Al, In, Fe, Nb improvements in absorption kinetics can be attained but only after few absorption/desorption cycles [19–22]. On the other hand, Knott et al. reported the auto catalytic effect of finely divided MgH2 on Mg [23]. In the present study, we investigated the addition of 5 wt% Mg and MgH2 powders for improving activation of rolled Mg plates. The idea is to avoid use of expensive catalyst or dopants while keeping the capacity and purity of the material.
2. Experimental Mg plates cut from commercial Norsk Hydro ingot were used as a starting material for cold rolling. Materials for ball milling were Mg (99.8% purity) and MgH2 (98% purity) powders from Alfa Aesar. Durston DRM 100 rolling mill with stainless steel rolls of 13 cm length, 7.5 cm diameter and driven by 1.1 kW electric motor was used for rolling. Mg plates with initial thickness of 2 mm were placed between two stainless steel (316) plates of 0.8 mm thickness and rolled horizontally in air. After each pass, the distance between the rolls was reduced to exert higher stress on plates for fast deformation; thereafter the plates were folded and re-rolled keeping constant distance between rolls till a final thickness of around 0.15 mm was attained. Thus, after 25 rolls brittle Mg pieces were obtained and used for investigation. These cold rolled Mg pieces were then ball milled in Ar atmosphere for 30 min, with and without additives using a Spex High Energy Ball Mill with ball to powder ratio of 10:1. In all, four different samples were prepared for present study, as described in Table 1. The hydrogen sorption properties were measured with a homemade Sieverttype apparatus. All measurements were performed at 623 K with a hydrogen pressure of 2000 kPa for absorption and 2 kPa for desorption. The sample was loaded in the reactor inside a glove box. Prior to measurement the system was hydrogen flushed and pumped. Thereafter, the argon in the reactor was pumped. Therefore, all pumping were either from hydrogen filled atmosphere or argon atmosphere. X-ray diffraction was performed using Bruker D8 Focus X-ray apparatus with Cu Ka radiation. Phase abundances, crystallite size and microstrain were evaluated from Rietveld method using Topas software [24]. Crystallite size were calculated by using the volume weighted mean column height based on integral breath while microstrain was computed using broadening modeled by a Voigt function as Table 1 Samples prepared for present investigation with notation. S. No.
Sample
Notation
1 2
25 cold rolled Mg pieces 25 cold rolled Mg pieces followed by 30 min ball milling 25 cold rolled Mg pieces +5wt%Mg powder ball milled for 30 min 25 cold rolled Mg pieces +5wt%MgH2 powder ball milled 30 min
Mg(CR) Mg(CR)
3 4
BM
Mg(CR) + Mg Mg(CR) + MgH2
BM BM
365
recommended by Balzar et al. [25]. Morphological studies were made with a Jeol JSM-5500 electron microscope. Chemical analysis was performed by using EDX on a Hitachi S-3400 microscope.
3. Result and discussion Morphology of all the samples under investigation along with as cast (ingot) Mg as reference is shown in Fig. 1. The presence of layers and pin holes on as cast magnesium reveals ductile characteristic of material with presence of oxide traces. Cold rolling causes fracturing with deformation and orientation of grains along rolling direction thereby generating a textured surface, as observed in Fig. 1b. Ball milling of cold rolled plates without or with Mg and MgH2 powders as additives (Fig. 1c–e), has similar effect on morphology. During milling, Mg plates interact with each other by successive events of cold welding and fracture, resulting in smaller particles and homogenization of the mixture. However, distribution of Mg or MgH2 powder on rolled Mg plates cannot be observed by secondary electrons due to negligible difference in electron density between these two phases. Chemical analysis of the Mg(CR) + MgH2 BM sample gave a composition by weight of Mg-96%, O-3% and Fe-1%. Assuming that oxygen is present in the form of MgO this translates to an abundance of 7.5 wt% MgO. Origin of iron is attributed to contamination during milling. Such a small amount of iron is essentially impossible to see in the X-ray powder diffraction. Rietveld refinement on X-ray diffraction data was performed to identify presence of different phases and to calculate microstrain and crystallite size. Fig. 2 shows the diffraction pattern of as cast Mg plates in comparison with the pattern of 25 cold rolled plates before and after heat treatment at 623 K. As expected, the as cast sample do not show any texture. From Rietveld refinement the crystallite size was determined to be 64 nm. Rolling induces texture with pronounced increase in intensity of (0 0 2) peak. However, an increase in crystallite size from 64 nm to 97 nm was noticed after rolling. As the crystallite size computed is in fact the volume weighted mean column height, it is somewhat understandable that if most of the crystallite are elongated along 0 0 2 direction then the mean may increase. Therefore, the observed increase in crystallite size is only representative of grain elongation along (0 0 2) direction and may not be the true reflection of actual crystallite size. Heat treatment under vacuum at 623 K for 5 h induces grain growth from 97 nm to 211 nm with no drastic change in texture. In fact, there was a slight increase in texture along (0 1 3) Bragg peak. Also, there is a slight reduction of microstrain as could be expected. Fig. 3 shows the XRD patterns of rolled plates after 30 min ball milling without (Fig. 3a) or with additives (Fig. 3b and c). In Mg(CR) + MgH2 BM sample, a small Bragg peak corresponding to MgH2 is observed. There is also loss of preferred orientation for Mg along (0 0 2) direction as could be expected because of randomization due to ball milling. Rietveld analysis gave phases abundances as 97% Mg and 3% MgH2 which are very close to nominal values. Despite the fact that rolling was done in air, no peak corresponding to oxides or hydroxides was observed in any of the samples. This may be due to very small crystallite size of these impurities making their peaks extremely broad and indistinguishable from background [18]. The crystallite size and strain of Mg as well as crystallite size of MgH2 as determined by Rietveld analysis for all the samples are shown in Table 2. The main aim of the present study is to improve the first hydrogenation or the activation process of commercial Mg ingot by combined use of two mechanical processes: cold rolling and ball milling. Therefore, we shall first investigate the activation behavior of rolled Mg plates before and after ball milling. Before taking hydrogen sorption measurements, the synthesized samples were heated at 623 K under dynamic vacuum
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Fig. 1. Scanning electron microscope (SEM) micrographs of commercial Mg ingot: (a) as-cast, (b) Mg(CR), (c) Mg(CR) – BM, (d) Mg(CR) + Mg Magnification: left column 250, right column 1000.
for 5 h. As seen previously, this step did not change the sample texture but increased the crystallite size. Thereafter hydrogen absorption was performed at 623 K under 2000 kPa of hydrogen. Fig. 4a
BM, (e) Mg(CR) + MgH2
BM.
shows the activation curve for Mg(CR) plates. It is observed that cold rolled plates react very slowly with hydrogen with the sample absorbing less than 1 wt% H2 in 13 h. After ball milling the rolled
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Fig. 2. X-ray diffraction patterns of Mg ingot (a) as cast, (b) after 25 cold rolls and (c) after 5 h vacuum annealing at 623 K of 25 cold rolled plates.
plates for 30 min and under the same condition of activation, hydrogen absorption increases to 2.7 wt% H2 in 13 h. The slight improvement in kinetic is probably due to the formation of surface defects and increase in surface area during ball milling. Still the amount of hydrogen absorbed by this sample is significantly low, making it necessary to use dopants or extend the activation time for further increase the sorption capacity. The activation curves of rolled Mg plates ball milled with 5 wt% Mg and MgH2 powders are shown in Fig. 4c and d. The sample with Mg powder absorbs 3.6 wt% H2 in 13 h which is slightly better than sample without additive. Also the shape of the two curves is similar, showing existence of long incubation time before hydrogen absorption. When Mg powder is replaced by same amount of MgH2 powder, a drastic improvement in activation time and capacity is observed, 6.2 wt% H2 absorption in 13 h. To verify the hydrogenation results, diffraction profile of Mg(CR) + MgH2 BM sample in hydride state was obtained by quenching the sample upon complete hydrogenation at 623 K under 2000 kPa H2 pressure. In Fig. 5 the diffraction pattern of hydride state (Fig. 5b) is compared with as cast (Fig. 5a) and dehydrided state (Fig. 5c). The pattern of as cast sample contains peaks corresponding to MgH2 and Mg with slight loss in texture. However, hydrogenation induces a complete loss in texture as observed in Fig. 5b and c. Rietveld refinement done on hydride state, showed 77% MgH2, 19% Mg and 4% MgO phases, thereby verifying the obtained capacity. Upon desorption, complete transformation of MgH2 to Mg is observed with 6% MgO. The amount of MgO phase determined by Rietveld analysis is slightly lower than the one measured by EDX. This may be due to the absorption coefficient of MgO as well as very broad peak of MgO due to its very small crystallite size which makes this peak difficult to distinguish from the
Fig. 3. X-ray diffraction patterns of rolled Mg plates after 30 min ball milling (a) without catalyst, (b) with 5 wt% Mg powder and (c) with 5 wt% MgH2 powder.
background. The small crystallite size may explain the absence of peak corresponding to MgO in as cast sample. It is known that at room temperature MgO crystallize in very small crystallites (1.4 nm) and that the crystallite size slowly increases with temperature up to 4.3 nm at 623 K [26]. This may explain why in X-ray patterns the MgO phase is not seen in samples prepared at room temperature and visible in samples that have been heated. Seeing that the addition of MgH2 to cold rolled Mg improves the activation kinetic we investigated the reduction of activation temperature by performing activation on Mg(CR) + MgH2 BM sample at 573 K under 2000 kPa hydrogen pressure. The activation kinetic was found to be much slower. A capacity of 5.7 wt% was reached after 40 h. Therefore, the technique presented here is useful to improve the kinetic but do not drastically lower the operation temperature. This result is understandable because the thermodynamic of magnesium hydrogenation could not be changed by cold rolling or adding magnesium hydride to magnesium. Fig. 6 shows the hydrogenation characteristics of samples after initial hydrogenation (activation). Remarkably, kinetics of all the samples is similar, thereby confirming that difficulties observed in initial hydrogenation of magnesium do not generally apply to subsequent cycles and also the fact that no catalyst has been added. We assume that presence of magnesium hydride formed during initial hydrogenation is responsible for faster kinetics in higher cycles. Thus, all samples showed similar hydrogen absorption kinetics. The only difference being total capacity which reflects the activation effectiveness. Among the samples investigated in present study, the cold rolled Mg plate’s ball milled with 5 wt% MgH2 additive showed faster activation behavior than the sample with same amount of Mg powder. However, after 5 h of heat treatment at 623 K under vac-
Table 2 Crystallite size and microstrain of the Mg phase and phase abundances in wt% as evaluated from Rietveld refinement of all the samples. For crystallite size and microstrain values in parenthesis are uncertainties on the last significant digit. Sample
Mg ingot Mg (CR) Mg(CR) VA Mg(CR) BM Mg(CR) + Mg BM Mg(CR) + MgH2 BM Mg(CR) + MgH2 BM (after H-absorption) Mg(CR) + MgH2 BM (after H-desorption)
Mg plates
MgH2
Size (nm)
Strain (%)
Size (nm)
Strain (%)
64(2) 97(1) 211(7) 47(1) 46(1) 51(1) 105(9) 46(1)
0.096(2) 0.084(1) 0.035(1) 0.162(2) 0.135(1) 0.134(1) 0.114(4) 0.070(2)
– –
– –
– – – 66(1) –
– – – 0.067(1) –
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Fig. 4. Activation (first hydrogenation) at 623 K, 2000 kPa H2 pressure for (a) Mg(CR); (b) Mg(CR) BM; (c) Mg(CR) + Mg-30 min BM and (d) Mg(CR) + MgH230 min BM.
Fig. 6. Second (after initial activation) hydrogen absorption curves at 623 K, 2000 kPa H2 pressure for (a) Mg(CR); (b) Mg(CR) BM; (c) Mg(CR) + Mg BM and (d) Mg(CR) + MgH2 BM.
Fig. 5. X-ray diffraction patterns of rolled Mg plate’s ball milling for 30 min with 5 wt% MgH2 powder in (a) as cast state; (b) after absorption under 2000 kPa H2 and (c) after desorption till 2 kPa at 623 K.
Fig. 7. Activation behavior of Mg(CR) + MgH2 catalytic powder after heating at 623 K (a) under vacuum for 1 h; (b) under vacuum for 2 h; (c) under vacuum for 5 h and (d) under 2000 kPa H2 pressure.
uum it is expected that MgH2 additive is desorbed to pure Mg. Therefore, the starting material for first hydrogenation (activation) measurements should be the same for both Mg(CR) + Mg BM and Mg(CR) + MgH2 BM samples. Thus, in principle, activation should be same for these two samples which is obviously not the case. To get the better understanding of the mechanism involved, we measured the activation behavior of Mg(CR) + MgH2 BM sample under different conditions: (a) pre-heating the sample at 623 K under dynamic vacuum for much smaller times i.e. 1 h and 2 h in comparison to 5 h – to have partial amount of MgH2 and thereafter introducing 2000 kPa H2 for absorption measurements and (b) heating to 623 K under 2000 kPa hydrogen pressure – to restrict desorption of MgH2. Fig. 7 shows the effect of annealing time and atmosphere on activation behavior. It is clear that all curves have similar shape and the only discrepancy is the total capacity. However, we see a clear trend that the total capacity increases with annealing time while the smallest capacity is for the sample annealed under hydrogen pressure. This means that prior complete dehydrogenation of MgH2 is mandatory for maximum results. The different results obtained by ball milling the Mg plates with Mg powder and with MgH2 powder can be better understood by
taking into account that the mechanisms of microstructure evolution during ball-milling are different in a ductile–ductile system (Mg plates and Mg powder) than in a ductile-fragile one (in the case of Mg plates and MgH2). This matter is discussed in several papers of ball-milling technique and the interested reader could obtain detailed information in the classical review paper of Suryanarayana [27]. We propose the following model to explain the possible interaction during milling of Mg plates produced by work hardening with two catalytic powders-ductile Mg powder and brittle MgH2 powder. During milling, the energy transfer between Mg plates covered with ductile Mg powder is sufficient only to cause breaking of particles into smaller particles with Mg powder deforming the surface of plates, thereby increasing the hydrogen capacity slightly more in comparison to plates milled without Mg powder. In the case of MgH2 additive, a much higher impact occurs on collision between plates covered with brittle powder, which results not only in reducing the particle size but also incorporating MgH2 powder in Mg matrix producing cracks, kissing bonds and others defect sites along the grain boundaries, thereby providing
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dopants and catalytic powder by MgH2 powder. In this work we used ball milling technique to add dopants. Future work is aimed at using cold rolling in inert atmosphere to incorporate MgH2 into Mg matrix. This method would enable us to have further reduction in production time and cost. Acknowledgements This work was supported by grant from Natural Science and Engineering Council of Canada. N.Y.S., D.F., and J.H. would like to thanks the Ministry of Education of Perm Region (Russia) for funding project C-26/211. P.J. is thankful to Hydro-Quebec for her Post-Doctoral fellowship. References
Fig. 8. Second (after initial activation) hydrogen absorption curves at 623 K, 2000 kPa H2 pressure for Mg(CR) + MgH2 catalytic powder after heating at 623 K, (a) under vacuum for 1 h; (b) under vacuum for 2 h; (c) under vacuum for 5 h and (d) under 2000 kPa H2 pressure.
necessary nucleation sites for hydrogen absorption. Further penetration of cracks into bulk of magnesium is obtained by volume contraction when doped MgH2 is desorbed prior to activation. As shown in Fig. 8, after activation step all samples have same kinetics and capacity. Thus, confirming that first absorption or activation of commercial Mg is an important step needed for improving commercialization of the material for hydrogen storage applications. 4. Conclusion
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
A magnesium–magnesium hydride composite was successfully prepared by combined use of cold rolling and ball milling. Addition of 5 wt% MgH2 via short milling to rolled Mg plates resulted in significant improvement in activation time and capacity with absorption of 6.2 wt% H2 in 13 h at 623 K under 2000 kPa hydrogen pressure. Structural and morphological results showed complete loss in preferred orientation after one hydrogenation/dehydrogenation cycle. The present work demonstrates a simple method to reduce production cost of light weight hydrogen storage materials without compromising on chemical purity and hydrogen capacity by using (a) cheaper Mg ingot rather than expensive MgH2 as starting material, (b) reducing milling time and particle agglomeration by performing cold rolling prior to ball milling, (c) replacing expensive
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