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Hydrogenation properties of ball-milled MgH2–10wt%Mg17Al12 composite
International Journal of Hydrogen Energy 32 (2007) 3406 – 3410 www.elsevier.com/locate/ijhydene
Hydrogenation properties of ball-milled MgH2 –10 wt%Mg17Al12 composite X.L. Wang a , J.P. Tu a,∗ , P.L. Zhang b , X.B. Zhang a , C.P. Chen a , X.B. Zhao a a Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
Received 31 October 2006; received in revised form 2 March 2007; accepted 2 March 2007 Available online 29 June 2007
Abstract As-cast Mg17Al12 alloy was prepared by induction melting method, and then ball-milled with MgH2 for 50 h to obtain MgH2 –10 wt% Mg17Al12 composite. The structure and morphology of the composite were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). XRD results showed that MgH2 decomposed partly during ball milling. Ball-milled Mg17Al12 alloy could absorb hydrogen at the temperatures over 473 K and desorb hydrogen at 613 K. The MgH2 –10 wt% Mg17Al12 composite showed rapid H-absorption rate and higher H-capacity with good activation properties, compared with ball-milled MgH2 . With the increase in hydriding temperature, the maximum hydrogen absorption capacity of MgH2 –10 wt% Mg17Al12 composite increased from 1.12 wt% at 393 K to 6.50 wt% at 473 K. The favorable hydrogenation performance of the MgH2 –10 wt% Mg17Al12 composite was mainly attributed to the synergistic effect of the catalytic efficiency of Mg17Al12 alloy and defects on the surface and/or in the interior of Mg formed during ball milling process. 䉷 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Mg–Al alloy; MgH2 ; Hydriding kinetics; Composite; Ball milling; H-capacity
1. Introduction Among the known metals and alloys with potential use in hydrogen storage, Mg-based alloys have attracted much interest for their high hydrogen capacity and low cost [1]. Unfortunately they have poor hydrogen absorption and desorption rates, and their hydrides are stable for most of the practical applications. To improve hydriding/dehydriding kinetics and to lower the working temperature, many researches have been carried out ranging from the addition of a catalytic component [2,3], surface treatment [4,5] to the introduction of non-conventional fabrication methods [6,7]. To obtain the lightweight hydrogen storage alloy, element Al has been studied on interacting with Mg as hydrogen storage material because of its low density. Bouaricha et al. [8] reported the hydriding behavior of Mg–Al and leached Mg–Al compounds prepared by high-energy ball milling. And Andreasen et al. [9] also studied the interaction of hydrogen with an Mg–Al alloy pre-exposed to air with in situ ∗ Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856.
E-mail address: [email protected] (J.P. Tu).
time resolved X-ray powder diffraction. The results showed that the addition of Al improved the resistance towards oxygen contamination. Bououdina and Guo [10] comparatively studied (Mg + Al) and (Mg + Al + Ni) mixtures prepared by mechanical alloying. Zaluska et al. [11] reported that in the functional nano-composite of Mg and Al, the binder (aluminum) played a dual role. Mg instantly reacts with Al after releasing hydrogen and forms various Mg–Al phases, depending on the relative concentration of Mg and Al. The excellent thermal conductivity of Al improved the heat transfer so that the rates of desorption can increase. Mg-based composite prepared by high-energy ball milling has been generally accepted as a simple and effective method [12,13]. The positive effect is due, on the one hand, to the ball milling which results in formation of a reactive surface with dislocations and other defects, and, on the other, to the presence of additives along with magnesium playing the role of catalysts in the hydriding and dehydriding processes [14–16]. In this present work, a composite containing Mg17Al12 alloy and MgH2 was synthesized by ball milling. In comparison with the hydrogenation of Mg17Al12 alloy and MgH2 , the
0360-3199/$ - see front matter 䉷 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. doi:10.1016/j.ijhydene.2007.03.003
X.L. Wang et al. / International Journal of Hydrogen Energy 32 (2007) 3406 – 3410
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synergistic effect of ball-milled MgH2 and Mg17Al12 alloy on the hydrogen hydriding/dehydriding behavior of the composite was discussed. 2. Experimental details
Fig. 2. SEM image of Mg17Al12 powder after ball milling for 50 h.
♦ ∇
∇--Mg17 Al12 Δ--Mg ♦--MgH2
(b)
Δ Δ Δ∇
Intensity
Mg17Al12 alloy was prepared by melting pure metallic magnesium and aluminum in an induction furnace under vacuum. The ingot was then mechanically pulverized into powder smaller than 100 mesh. MgH2 was prepared by hydriding Mg powder (with purity of 99.9% and 200 mesh) at 623 K under 4.0 MPa H2 for 6 h to reach complete hydrogenation. Then the mixture composed of Mg hydrides and as-cast Mg17Al12 powder was ball-milled for 50 h under argon with a ball-to-powder weight ratio of 20:1. The microstructure and morphology of the synthesized samples were examined by X-ray diffraction (XRD, Philips PW1050 diffractometer, Cu K radiation) and scanning electron microscopy (SEM, FEI, SIRION). The apparatus employed for hydriding measurements is similar to the equipment described in Ref. [17]. The vessel filled with a 3.0 g powder sample of the MgH2 –Mg17Al12 composite was evacuated to 10−2 Pa by a rotary vacuum pump and heated to 573 K for 1 h to decompose the hydrides completely. Then the temperature was set to 393 K, and hydrogen was introduced at an initial pressure of 4 MPa. After measuring the hydrogen absorbing behavior at 393 K, the vessel was evacuated to 0.1 MPa and heated to above 513 K to desorb the hydrogen. The hydrogenation procedure was then repeated at 413, 433, 453 and 473 K, respectively.
♦
∇ ♦ ∇∇ Δ
♦
♦ Δ
Δ♦ ♦ ♦ ∇♦
(a)
3. Results and discussion 3.1. Microstructure characterization
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20
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40
50 2θ
60
70
80
(721)
ball-milled Mg17Al12
as-cast Mg17Al12
10
10
Fig. 3. XRD patterns of MgH2 and ball milled MgH2 –Mg17Al12 composite: (a) Mg after hydriding at 623 K; (b) MgH2 –10 wt% Mg17Al12 composite after ball milling for 50 h.
(332) (422) (510)
Intensity
(400)
(411)
Fig. 1 shows XRD patterns of Mg17Al12 alloy before and after ball milling. There is only Mg17Al12 phase after milling,
50 2θ
60
70
80
Fig. 1. XRD patterns of Mg17Al12 alloy before and after ball milling.
which suggests that Mg17Al12 was stable during ball milling. After ball milling for 50 h, it can be seen that the diffraction peaks are broadened and the intensity decreases significantly due to the grain refinement and the introduction of strain in the alloy. Fig. 2 shows a SEM image of Mg17Al12 powder after ball milling for 50 h. It exhibits that the powder size is not uniform. From the XRD pattern of MgH2 –10 wt%Mg17Al12 composite after ball milling for 50 h, as shown in Fig. 3, there exist three phases as Mg, MgH2 and Mg17Al12 in the composite. MgH2 decomposed partly forming Mg phase and Mg17Al12 remained stable as seen in Fig. 1. It can also be seen that after ball milling the diffraction peaks are broadened, and the intensity decreases significantly. Fig. 4(a) shows a SEM micrograph of Mg–10 wt% Mg17Al12 after ball milling for 50 h. The size
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Hydrogen desorption capacity / wt.%
2.5
2.0
1.5
1.0
0.5
Ball milled Mg17Al12 alloy
0.0 0 Fig. 4. (a) SEM micrograph of MgH2 –10 wt% Mg17Al12 composite after ball milling for 50 h, (b) EDS spectrum of Al element in (a).
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20 25 Time / min
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Fig. 6. Hydrogen desorption kinetics curve of ball-milled Mg17Al12 at 613 K.
Hydrogen absorption capacity / wt.%
2.5 573K 2.0
553K 513K
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493K 1.0
473K
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1200 1600 Time / Sec
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Fig. 5. Hydrogen absorption kinetics curve of ball-milled Mg17Al12 alloy at different temperatures under H2 pressure of 4 MPa.
of composite powder changes in a large range because there is more than one phase in the composite and the ductility of each phase is different. The full realization of the catalyst function depends not only on the catalytic characteristics of the composite phase, but also on its distribution state [11]. Fig. 4(b) presents the EDS spectrum of Al element in the MgH2 –10 wt% Mg17Al12 composite. It can be observed that Mg17Al12 phase disperses uniformly in the composite. The brittle MgH2 and the mechanical driving force result in a highly dispersive distribution of the Mg17Al12 phase in the Mg matrix. 3.2. Hydrogenation properties As comparison, the hydrogen absorption kinetics curves of ball-milled Mg17Al12 alloy at different temperatures under H2 pressure of 4 MPa are shown in Fig. 5. The as-milled Mg17Al12 alloy began to absorb hydrogen at 473 K without any activa-
tion. With the increase in absorption temperature, the maximum hydrogen absorption capacity increased from 0.85 wt% at 473 K to 2.19 wt% at 573 K in 40 min. Besides, the hydrogen absorption rates increased with the temperature. For example, the ball-milled Mg17Al12 alloy could absorb 44.62% of its full hydrogen absorption capacity at 473 K in 10 min while it could absorb 78.74% of its full hydrogen absorption capacity at 573 K in 10 min. For hydrogen desorption as shown in Fig. 6, the ball-milled Mg17Al12 alloy could desorb hydrogen at 613 K under hydrogen pressure of 0.1 MPa, and the hydrogen desorption capacity reached 2.01 wt% in 30 min. The hydriding kinetics of ball-milled MgH2 and MgH2 –10 wt% Mg17Al12 composite are shown in Fig. 7. After being dehydrided at 573 K, both the as-milled samples needed no activation for rapid H-absorption. As shown in Fig. 7, it is obvious that the MgH2 –10 wt% Mg17Al12 composite shows higher hydrogen absorption capacity especially at the temperatures below 433 K, compared to the ball-milled MgH2 . It is 1.12, 4.86 and 5.35 wt% at 393, 413 and 433 K, respectively, while that of ball-milled MgH2 is 0.80, 1.83 and 3.98 wt% corresponding to the same absorption temperatures, respectively. With increase in the hydriding temperature, the maximum hydrogen absorption capacity of MgH2 –10 wt% Mg17Al12 composite increased and reached to 6.50 wt% at 473 K. Fig. 8 shows the hydrogen desorption kinetics curves of ball-milled MgH2 –10 wt% Mg17Al12 composite under 0.1 MPa H2 after hydriding at 473 K. It indicated that the ball-milled MgH2 –10 wt% Mg17Al12 composite could desorb hydrogen at 533 K slowly. The desorbed capacity reached 5.22 wt% in 120 min. With the increase in desorption temperature, the hydrogen desorption kinetics improved obviously. The desorbed hydrogen capacity reached 5.86 wt% in 60 min and it could finish 85.49% of its full hydrogen desorption capacity in 30 min at 593 K. The improvement of the absorption and desorption kinetics was attributed to the action of the catalytic phase of Mg17Al12 alloy and mechanical driving force. It is generally accepted
X.L. Wang et al. / International Journal of Hydrogen Energy 32 (2007) 3406 – 3410
Hydrogen desorption capacity / wt.%
Hydrogen absorption capacity / wt.%
8 7 6 6
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1--393 K 2--413 K 3--433 K 4--453 K 5--473 K 6--493 K
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Fig. 8. Hydrogen desorption curves of ball-milled MgH2 –10 wt% Mg17Al12 composite after hydriding at 473 K.
Hydrogen absorption capacity / wt.%
8 7 5 6
4
3
5 2 4
1--393 K 2--413 K 3--433 K 4--453 K 5--473 K
3 2 1
mixture of the three phases during ball milling will unavoidably cause some Mg17Al12 alloy powder to become wrapped in the Mg particles. When the hydride layer formed, the Mg17Al12 alloy powders would be surrounded by MgH2 . They provide the “rapid passageway” for hydrogen atoms to pass through the hydride layer, which is similar to the reaction mechanism reported before [19]. The synergism between the ball-milled MgH2 and Mg17Al12 alloy modified the hydrogenation behavior of the MgH2 –10 wt% Mg17Al12 composite.
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0 0
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600
900 Time / Sec
1200
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Fig. 7. Hydrogen absorption kinetics curves of ball-milled (a) MgH2 ; (b) MgH2 –10 wt% Mg17Al12 composite at different temperatures under 4 MPa H2 .
that the rate of absorption is controlled by the following factors [18]: (a) the rate of hydrogen dissociation at the surface; (b) the ability of hydrogen to penetrate the surface; (c) the rate of hydrogen diffusion into the bulk metal and through the hydride already formed. Mechanical milling can facilitate nucleation by creating many defects on the surface and/or in the interior of Mg, or by an additive acting as active sites for the nucleation, and shorten the diffusion distance by reducing the effective particle sizes of Mg. As shown in Fig. 7, the as-milled Mg17Al12 can only absorb 0.85 wt% H at 473 K. When the hydriding temperature was lower than 473 K, the contribution of Mg17Al12 alloy to the total hydrogen absorption capacity of MgH2 –10 wt% Mg17Al12 composite is insignificant. In the present work, Mg17Al12 phase was mainly acting as catalyst for the hydriding process. The cubic Mg17Al12 is brittle, while Mg is tough. Repeated welding, fracturing and rewelding of the
4. Conclusion As-cast Mg17Al12 alloy was prepared by melting the pure Mg and Al in an induction furnace under vacuum, mechanically pulverized to the powder, and then ball-milled with MgH2 for 50 h to obtain MgH2 –10 wt% Mg17Al12 composite. XRD results showed that MgH2 decomposed partly to form Mg phase during ball milling. The ball-milled Mg17Al12 alloy could absorb hydrogen at the temperatures over 473 K and could desorb hydrogen at 613 K. The ball-milled MgH2 –10 wt% Mg17Al12 composite showed rapid H-absorption rate and high H-capacity with good activation properties, compared to ball-milled MgH2 . With the increase in the hydriding temperature, the hydrogen absorption capacity of MgH2 –10 wt% Mg17Al12 composite increased from 1.12 wt% at 393 K to 6.50 wt% at 473 K. For hydrogen desorption, the ball-milled MgH2 –10 wt% Mg17Al12 composite could desorb 5.22 wt% hydrogen at 533 K in 120 min under 0.1 MPa after hydriding at 473 K. The desorbed hydrogen capacity reached 5.86 wt% in 60 min and it could finish 85.49% of its full hydrogen desorption capacity in 30 min at 593 K. The favorable hydrogenation performance was mainly attributed to the synergistic effect of the catalytic efficiency of Mg17Al12 alloy and defects on the surface and/or in the interior of Mg formed during ball milling process.
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Acknowledgment This work was supported by the Special Funds for Major States Basic Research Project (No. 2006AA11A159) of MOST, China. References [1] Reilly JJ, Wiswall RH. The reaction of hydrogen with alloys of magnesium and nickel and the formation of Mg2 NiH4 . Inorg Chem 1968;7:2254–6. [2] Bobet J-L, Akiba E, Nakamura Y, Darriet B. Study of Mg–M (M = Co, Ni and Fe) mixture elaborated by reactive mechanical alloying—hydrogen sorption properties. Int J Hydrogen Energy 2000;25:987–96. [3] Bolcich JC, Yanwny AA, Corso HL, Peretti HA, Ayala CO. Hydrogen storage employing Mg-10 wt% Ni alloy. Int J Hydrogen Energy 1994;19(7):605–9. [4] Wang W, Chen CP, Chen LX, Wang QD. Change in structure and hydrogen storage properties of La2 Mg16 Ni alloy after modification by mechanical grinding in tetrahydrofuran. J Alloys Compds 2002;339: 175–9. [5] Hampton MD, Juturu R, Lomness J. The activation of Mg2 Ni for initial hydrogen uptake by treatment with water vapor. J Hydrogen Energy 1999;24:981–8. [6] Shao HY, Xu HR, Wang YT, Li XG. Preparation and hydrogen storage properties of Mg2 Ni intermetallic nanoparticles. Nanotechnology 2004;15:269–74. [7] Mosaner P, Bazzanella N, Bonelli M, Checchetto R, Miotello A. Mg:Nb films produced by pulsed laser deposition for hydrogen storage. Mater Sci Eng 2004;B108:33–7. [8] Bouaricha S, Dodelet JP, Guay D, Huot J, Boily S, Schulz R. Hydriding behavior of Mg–Al and leached Mg–Al compounds prepared by highenergy ball-milling. J Alloys Compds 2000;297:282–93.
[9] Andreasen A, Sorensen MB, Burkarl R, Moller B, Molenbroek AM, Pedersen AS. et al. Interaction of hydrogen with an Mg–Al alloy. J Alloys Compds 2005;404–406:323–6. [10] Bououdina M, Guo ZX. Comparative study of mechanical alloying of (Mg+Al) and (Mg+Al+Ni) mixtures for hydrogen storage. J Alloys Compds 2002;336:222–31. [11] Zaluska A, Zaluski L, Ström-Olsen JO. Structure, catalysis and atomic reactions on the nano-scale: a systematic approach to metal hydrides for hydrogen storage. Appl Phys 2001;A72:157–65. [12] Zhu M, Zhu WH, Chung CY, Che ZX, Li ZX. Microstructure and hydrogen absorption properties of nano-phase composite prepared by mechanical alloying of MmNi5−x (CoAlMn)x and Mg. J Alloys Compds 1999;293–295:531–5. [13] Liang G, Huot J, Boily S, Van Neste A, Schulz R. Hydrogen storage in mechanically milled Mg–LaNi5 and MgH2 –LaNi5 composites. J Alloys Compds 2000;297:261–5. [14] Khrussanova M, Grigorova E, Bobet J-L, Khristov M, Peshev P. Hydrogen sorption properties of the nanocomposite Mg–Mg2 Ni1−x Cox obtained by mechanical alloying. J Alloys Compds 2004;365:308–13. [15] Zhu M, Zhu WH, Gao Y, Che XZ, Ahn JH. The effect of Mg content on microstructure and hydrogen absorption properties of mechanical alloyed MmNi3.5 (CoAlMn)1.5 –Mg. Mater Sci Eng 2000;A286:130–4. [16] Sun DL, Enoki H, Bououdina M, Akiba E. Phase components and hydriding properties of the sintered Mg-x wt% LaNi5 (x = 20 − 50) composites. J Alloys Compds 1999;282:252–7. [17] Gao RG, Tu JP, Wang XL, Zhang XB, Chen CP. The absorption and desorption properties of nanocrystalline Mg2 Ni0.75 Cr0.25 alloy containing TiO2 nanoparticles. J Alloys Compds 2003;356–357:649–53. [18] Zaluski L, Zaluska A, Strom-Olsen JO. Nanocrystalline metal hydrides. J Alloys Compds 1997;253:70–6. [19] Wang P, Wang AM, Zhang HF. Hydriding properties of a mechanically milled Mg-50 wt% ZrFe1.4 Cr0.6 composite. J Alloys Compds 2000;297:240–5.
Hydrogenation properties of ball-milled MgH2 –10 wt%Mg17Al12 composite X.L. Wang a , J.P. Tu a,∗ , P.L. Zhang b , X.B. Zhang a , C.P. Chen a , X.B. Zhao a a Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
Received 31 October 2006; received in revised form 2 March 2007; accepted 2 March 2007 Available online 29 June 2007
Abstract As-cast Mg17Al12 alloy was prepared by induction melting method, and then ball-milled with MgH2 for 50 h to obtain MgH2 –10 wt% Mg17Al12 composite. The structure and morphology of the composite were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). XRD results showed that MgH2 decomposed partly during ball milling. Ball-milled Mg17Al12 alloy could absorb hydrogen at the temperatures over 473 K and desorb hydrogen at 613 K. The MgH2 –10 wt% Mg17Al12 composite showed rapid H-absorption rate and higher H-capacity with good activation properties, compared with ball-milled MgH2 . With the increase in hydriding temperature, the maximum hydrogen absorption capacity of MgH2 –10 wt% Mg17Al12 composite increased from 1.12 wt% at 393 K to 6.50 wt% at 473 K. The favorable hydrogenation performance of the MgH2 –10 wt% Mg17Al12 composite was mainly attributed to the synergistic effect of the catalytic efficiency of Mg17Al12 alloy and defects on the surface and/or in the interior of Mg formed during ball milling process. 䉷 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Mg–Al alloy; MgH2 ; Hydriding kinetics; Composite; Ball milling; H-capacity
1. Introduction Among the known metals and alloys with potential use in hydrogen storage, Mg-based alloys have attracted much interest for their high hydrogen capacity and low cost [1]. Unfortunately they have poor hydrogen absorption and desorption rates, and their hydrides are stable for most of the practical applications. To improve hydriding/dehydriding kinetics and to lower the working temperature, many researches have been carried out ranging from the addition of a catalytic component [2,3], surface treatment [4,5] to the introduction of non-conventional fabrication methods [6,7]. To obtain the lightweight hydrogen storage alloy, element Al has been studied on interacting with Mg as hydrogen storage material because of its low density. Bouaricha et al. [8] reported the hydriding behavior of Mg–Al and leached Mg–Al compounds prepared by high-energy ball milling. And Andreasen et al. [9] also studied the interaction of hydrogen with an Mg–Al alloy pre-exposed to air with in situ ∗ Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856.
E-mail address: [email protected] (J.P. Tu).
time resolved X-ray powder diffraction. The results showed that the addition of Al improved the resistance towards oxygen contamination. Bououdina and Guo [10] comparatively studied (Mg + Al) and (Mg + Al + Ni) mixtures prepared by mechanical alloying. Zaluska et al. [11] reported that in the functional nano-composite of Mg and Al, the binder (aluminum) played a dual role. Mg instantly reacts with Al after releasing hydrogen and forms various Mg–Al phases, depending on the relative concentration of Mg and Al. The excellent thermal conductivity of Al improved the heat transfer so that the rates of desorption can increase. Mg-based composite prepared by high-energy ball milling has been generally accepted as a simple and effective method [12,13]. The positive effect is due, on the one hand, to the ball milling which results in formation of a reactive surface with dislocations and other defects, and, on the other, to the presence of additives along with magnesium playing the role of catalysts in the hydriding and dehydriding processes [14–16]. In this present work, a composite containing Mg17Al12 alloy and MgH2 was synthesized by ball milling. In comparison with the hydrogenation of Mg17Al12 alloy and MgH2 , the
0360-3199/$ - see front matter 䉷 2007 Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy. doi:10.1016/j.ijhydene.2007.03.003
X.L. Wang et al. / International Journal of Hydrogen Energy 32 (2007) 3406 – 3410
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synergistic effect of ball-milled MgH2 and Mg17Al12 alloy on the hydrogen hydriding/dehydriding behavior of the composite was discussed. 2. Experimental details
Fig. 2. SEM image of Mg17Al12 powder after ball milling for 50 h.
♦ ∇
∇--Mg17 Al12 Δ--Mg ♦--MgH2
(b)
Δ Δ Δ∇
Intensity
Mg17Al12 alloy was prepared by melting pure metallic magnesium and aluminum in an induction furnace under vacuum. The ingot was then mechanically pulverized into powder smaller than 100 mesh. MgH2 was prepared by hydriding Mg powder (with purity of 99.9% and 200 mesh) at 623 K under 4.0 MPa H2 for 6 h to reach complete hydrogenation. Then the mixture composed of Mg hydrides and as-cast Mg17Al12 powder was ball-milled for 50 h under argon with a ball-to-powder weight ratio of 20:1. The microstructure and morphology of the synthesized samples were examined by X-ray diffraction (XRD, Philips PW1050 diffractometer, Cu K radiation) and scanning electron microscopy (SEM, FEI, SIRION). The apparatus employed for hydriding measurements is similar to the equipment described in Ref. [17]. The vessel filled with a 3.0 g powder sample of the MgH2 –Mg17Al12 composite was evacuated to 10−2 Pa by a rotary vacuum pump and heated to 573 K for 1 h to decompose the hydrides completely. Then the temperature was set to 393 K, and hydrogen was introduced at an initial pressure of 4 MPa. After measuring the hydrogen absorbing behavior at 393 K, the vessel was evacuated to 0.1 MPa and heated to above 513 K to desorb the hydrogen. The hydrogenation procedure was then repeated at 413, 433, 453 and 473 K, respectively.
♦
∇ ♦ ∇∇ Δ
♦
♦ Δ
Δ♦ ♦ ♦ ∇♦
(a)
3. Results and discussion 3.1. Microstructure characterization
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50 2θ
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as-cast Mg17Al12
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Fig. 3. XRD patterns of MgH2 and ball milled MgH2 –Mg17Al12 composite: (a) Mg after hydriding at 623 K; (b) MgH2 –10 wt% Mg17Al12 composite after ball milling for 50 h.
(332) (422) (510)
Intensity
(400)
(411)
Fig. 1 shows XRD patterns of Mg17Al12 alloy before and after ball milling. There is only Mg17Al12 phase after milling,
50 2θ
60
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Fig. 1. XRD patterns of Mg17Al12 alloy before and after ball milling.
which suggests that Mg17Al12 was stable during ball milling. After ball milling for 50 h, it can be seen that the diffraction peaks are broadened and the intensity decreases significantly due to the grain refinement and the introduction of strain in the alloy. Fig. 2 shows a SEM image of Mg17Al12 powder after ball milling for 50 h. It exhibits that the powder size is not uniform. From the XRD pattern of MgH2 –10 wt%Mg17Al12 composite after ball milling for 50 h, as shown in Fig. 3, there exist three phases as Mg, MgH2 and Mg17Al12 in the composite. MgH2 decomposed partly forming Mg phase and Mg17Al12 remained stable as seen in Fig. 1. It can also be seen that after ball milling the diffraction peaks are broadened, and the intensity decreases significantly. Fig. 4(a) shows a SEM micrograph of Mg–10 wt% Mg17Al12 after ball milling for 50 h. The size
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X.L. Wang et al. / International Journal of Hydrogen Energy 32 (2007) 3406 – 3410
Hydrogen desorption capacity / wt.%
2.5
2.0
1.5
1.0
0.5
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0.0 0 Fig. 4. (a) SEM micrograph of MgH2 –10 wt% Mg17Al12 composite after ball milling for 50 h, (b) EDS spectrum of Al element in (a).
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Fig. 6. Hydrogen desorption kinetics curve of ball-milled Mg17Al12 at 613 K.
Hydrogen absorption capacity / wt.%
2.5 573K 2.0
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493K 1.0
473K
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Fig. 5. Hydrogen absorption kinetics curve of ball-milled Mg17Al12 alloy at different temperatures under H2 pressure of 4 MPa.
of composite powder changes in a large range because there is more than one phase in the composite and the ductility of each phase is different. The full realization of the catalyst function depends not only on the catalytic characteristics of the composite phase, but also on its distribution state [11]. Fig. 4(b) presents the EDS spectrum of Al element in the MgH2 –10 wt% Mg17Al12 composite. It can be observed that Mg17Al12 phase disperses uniformly in the composite. The brittle MgH2 and the mechanical driving force result in a highly dispersive distribution of the Mg17Al12 phase in the Mg matrix. 3.2. Hydrogenation properties As comparison, the hydrogen absorption kinetics curves of ball-milled Mg17Al12 alloy at different temperatures under H2 pressure of 4 MPa are shown in Fig. 5. The as-milled Mg17Al12 alloy began to absorb hydrogen at 473 K without any activa-
tion. With the increase in absorption temperature, the maximum hydrogen absorption capacity increased from 0.85 wt% at 473 K to 2.19 wt% at 573 K in 40 min. Besides, the hydrogen absorption rates increased with the temperature. For example, the ball-milled Mg17Al12 alloy could absorb 44.62% of its full hydrogen absorption capacity at 473 K in 10 min while it could absorb 78.74% of its full hydrogen absorption capacity at 573 K in 10 min. For hydrogen desorption as shown in Fig. 6, the ball-milled Mg17Al12 alloy could desorb hydrogen at 613 K under hydrogen pressure of 0.1 MPa, and the hydrogen desorption capacity reached 2.01 wt% in 30 min. The hydriding kinetics of ball-milled MgH2 and MgH2 –10 wt% Mg17Al12 composite are shown in Fig. 7. After being dehydrided at 573 K, both the as-milled samples needed no activation for rapid H-absorption. As shown in Fig. 7, it is obvious that the MgH2 –10 wt% Mg17Al12 composite shows higher hydrogen absorption capacity especially at the temperatures below 433 K, compared to the ball-milled MgH2 . It is 1.12, 4.86 and 5.35 wt% at 393, 413 and 433 K, respectively, while that of ball-milled MgH2 is 0.80, 1.83 and 3.98 wt% corresponding to the same absorption temperatures, respectively. With increase in the hydriding temperature, the maximum hydrogen absorption capacity of MgH2 –10 wt% Mg17Al12 composite increased and reached to 6.50 wt% at 473 K. Fig. 8 shows the hydrogen desorption kinetics curves of ball-milled MgH2 –10 wt% Mg17Al12 composite under 0.1 MPa H2 after hydriding at 473 K. It indicated that the ball-milled MgH2 –10 wt% Mg17Al12 composite could desorb hydrogen at 533 K slowly. The desorbed capacity reached 5.22 wt% in 120 min. With the increase in desorption temperature, the hydrogen desorption kinetics improved obviously. The desorbed hydrogen capacity reached 5.86 wt% in 60 min and it could finish 85.49% of its full hydrogen desorption capacity in 30 min at 593 K. The improvement of the absorption and desorption kinetics was attributed to the action of the catalytic phase of Mg17Al12 alloy and mechanical driving force. It is generally accepted
X.L. Wang et al. / International Journal of Hydrogen Energy 32 (2007) 3406 – 3410
Hydrogen desorption capacity / wt.%
Hydrogen absorption capacity / wt.%
8 7 6 6
4
5
1--393 K 2--413 K 3--433 K 4--453 K 5--473 K 6--493 K
5 4 3
3
2
2
1
1
6 5 593 K 4 3
533 K
2 1 0 0
20
0 0
300
600
900 1200 Time / Sec
3409
1500
1800
40
60 Time / min
80
100
120
Fig. 8. Hydrogen desorption curves of ball-milled MgH2 –10 wt% Mg17Al12 composite after hydriding at 473 K.
Hydrogen absorption capacity / wt.%
8 7 5 6
4
3
5 2 4
1--393 K 2--413 K 3--433 K 4--453 K 5--473 K
3 2 1
mixture of the three phases during ball milling will unavoidably cause some Mg17Al12 alloy powder to become wrapped in the Mg particles. When the hydride layer formed, the Mg17Al12 alloy powders would be surrounded by MgH2 . They provide the “rapid passageway” for hydrogen atoms to pass through the hydride layer, which is similar to the reaction mechanism reported before [19]. The synergism between the ball-milled MgH2 and Mg17Al12 alloy modified the hydrogenation behavior of the MgH2 –10 wt% Mg17Al12 composite.
1
0 0
300
600
900 Time / Sec
1200
1500
1800
Fig. 7. Hydrogen absorption kinetics curves of ball-milled (a) MgH2 ; (b) MgH2 –10 wt% Mg17Al12 composite at different temperatures under 4 MPa H2 .
that the rate of absorption is controlled by the following factors [18]: (a) the rate of hydrogen dissociation at the surface; (b) the ability of hydrogen to penetrate the surface; (c) the rate of hydrogen diffusion into the bulk metal and through the hydride already formed. Mechanical milling can facilitate nucleation by creating many defects on the surface and/or in the interior of Mg, or by an additive acting as active sites for the nucleation, and shorten the diffusion distance by reducing the effective particle sizes of Mg. As shown in Fig. 7, the as-milled Mg17Al12 can only absorb 0.85 wt% H at 473 K. When the hydriding temperature was lower than 473 K, the contribution of Mg17Al12 alloy to the total hydrogen absorption capacity of MgH2 –10 wt% Mg17Al12 composite is insignificant. In the present work, Mg17Al12 phase was mainly acting as catalyst for the hydriding process. The cubic Mg17Al12 is brittle, while Mg is tough. Repeated welding, fracturing and rewelding of the
4. Conclusion As-cast Mg17Al12 alloy was prepared by melting the pure Mg and Al in an induction furnace under vacuum, mechanically pulverized to the powder, and then ball-milled with MgH2 for 50 h to obtain MgH2 –10 wt% Mg17Al12 composite. XRD results showed that MgH2 decomposed partly to form Mg phase during ball milling. The ball-milled Mg17Al12 alloy could absorb hydrogen at the temperatures over 473 K and could desorb hydrogen at 613 K. The ball-milled MgH2 –10 wt% Mg17Al12 composite showed rapid H-absorption rate and high H-capacity with good activation properties, compared to ball-milled MgH2 . With the increase in the hydriding temperature, the hydrogen absorption capacity of MgH2 –10 wt% Mg17Al12 composite increased from 1.12 wt% at 393 K to 6.50 wt% at 473 K. For hydrogen desorption, the ball-milled MgH2 –10 wt% Mg17Al12 composite could desorb 5.22 wt% hydrogen at 533 K in 120 min under 0.1 MPa after hydriding at 473 K. The desorbed hydrogen capacity reached 5.86 wt% in 60 min and it could finish 85.49% of its full hydrogen desorption capacity in 30 min at 593 K. The favorable hydrogenation performance was mainly attributed to the synergistic effect of the catalytic efficiency of Mg17Al12 alloy and defects on the surface and/or in the interior of Mg formed during ball milling process.
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