Formation mechanism of MgB₂ in 2LiBH₄ + MgH₂ system for reversible hydrogen storage

KOU Hua-qin, XIAO Xue-zhang, CHEN Li-xin, LI Shou-quan, WANG Qi-dong

Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Received 16 May 2010; accepted 29 November 2010

Abstract: The formation conditions of MgB₂ in 2LiBH₄ + MgH₂ system during dehydrogenation were investigated and its mechanism was discussed. The results show that direct decomposition of LiBH₄ is suppressed under relative higher initial dehydrogenation pressure of 4.0×10⁵ Pa, wherein LiBH₄ reacts with Mg to yield MgB₂, and 9.16% (mass fraction) hydrogen is released within 9.6 h at 450 °C. However, under relatively lower initial dehydrogenation pressure of 1.0×10² Pa, LiBH₄ decomposes independently instead of reacting with Mg, resulting in no formation of MgB₂, and 7.91% hydrogen is desorbed within 5.2 h at 450 °C. It is found that the dehydrogenation of 2LiBH₄ + MgH₂ system proceeds more completely and more hydrogen desorption amount can be obtained within a definite time by forming MgB₂. Furthermore, it is proposed that the formation process of MgB₂ includes incubation period and nucleus growth process. Experimental results show that the formation process of MgB₂, especially the incubation period, is promoted by increasing initial dehydrogenation pressure at constant temperature, and the incubation period is also influenced greatly by dehydrogenation temperature.

Key words: complex hydride; LiBH₄; MgB₂; hydrogen storage; formation mechanism

1 Introduction

Hydrogen is an ideal secondary energy carrier for application because of its highly energy content and environmental harmony. However, efficient hydrogen storage and transportation is a key technical challenge in promoting its further applications. LiBH₄, which owns high gravimetric and volumetric hydrogen densities (18.5% in mass fraction and 121 kg/m³), has been regarded as one of the most promising hydrogen storage materials[1]. The hydrogen desorption reaction at elevated temperature (>400 °C) proceeds as follows[2]:

LiBH₄→LiH+B+3/2H₂                         (1)

Upon the overall dehydriding reaction, 13.5% of hydrogen can be released from LiBH₄. Unfortunately, it is experimentally shown that LiBH₄ is thermodynamically stable for practical use, and it is required for extremely rigorous reaction conditions to reverse[3-4]. VAJO et al[5-6] developed a hydrogen storage system composed of LiBH₄ and MgH₂, in which LiBH₄ was effectively destabilized and reaction reversibility was improved. The reversible dehydrogenation/rehydrogenation reaction is expressed as follows:

2LiBH₄+MgH₂?MgB₂+2LiH+4H₂               (2)

Interestingly, previous experiments show that a hydrogen atmosphere is necessary for dehydrogenation according to reaction (2). If under dynamic vacuum, however, MgB₂ is not formed, and dehydrogenation follows another reaction path expressed as[7]

2LiBH₄+MgH₂→Mg+2LiH+2B+4H₂             (3)

Meanwhile, calorimetric measurements by NAKAGAWA et al[8] showed the formation of MgB₂ unless under H₂ atmosphere rather than inert gas. PRICE et al[9-11] found that the dehydrogenation pathway was hardly affected by the stoichiometry ratio of LiBH₄ to MgH₂ and Li-Mg alloy was formed under dynamic vacuum after dehydrogenation at high temperature. PINKERTON et al[7] reported a thermodynamically and kinetically boundaries of the H₂ pressure-temperature for formation of MgB₂ in TiCl₃-catalyzed 2LiBH₄+ MgH₂ system. A wide sloping plateau was observed by B?SENBERG et al[12] during dehydrogenation of neat 2LiBH₄+MgH₂ system. However, there is still no further investigation on appearance reason of the plateau. Although the impact of hydrogen atmosphere on the appearance of MgB₂ in the product has been studied extensively, little investigation on full understand of the formation of MgB₂ has been done as yet.

Because MgB₂ plays an important role in reversible hydrogen storage, it is significant to reveal the detailed connection between comprehensive reaction conditions and formation of MgB₂. Therefore, in the present work, we investigated the intrinsic formation mechanism of MgB₂ during the dehydrogenation of 2LiBH₄+MgH₂ system.

2 Experimental

LiBH₄ (95% in mass fraction) and MgH₂ (98% in mass fraction) were purchased from Alfa Aesar Corp. All materials were used as-received in powder form. The sample of 2LiBH₄ + MgH₂ system was mechanically milled under 1 MPa hydrogen pressure in a Planetary mill at 400 r/min for 2 h. The milling vessel and balls were made of stainless steel. The ball to powder mass ratio was in around 40:1. All sample operations were performed in a glovebox under continuous purified argon atmosphere. Hydrogen desorption behaviors of the samples were monitored with a Sievert’s type apparatus. Dehydriding performance started from a finite initial hydrogen pressure with heating to aimed temperature at a constant ramping rate of 5 °C/min. Each time, 150-250 mg sample was put into a closed large reaction sample volume (820 mL), which resulted in (0.2-0.3)×10⁵ Pa pressure change after dehydrogenation. The identification of the samples was carried out by X-ray diffractometry (XRD, X'Pert-PRO, Cu K_α radiation). To prevent H₂O and O₂ contamination during the measurements, a special sample holder was used.

3 Results and discussion

3.1  Investigation on dehydrogenation process of 2LiBH₄+MgH₂ system

The dehydriding behaviors of 2LiBH₄ + MgH₂ system were measured firstly by heating to 450 °C and under initial dehydrogenation pressure of 1.0×10² Pa and 4.0×10⁵ Pa, respectively, as shown in Fig.1. Because the pressure increase during the dehydrogenation is small, the initial dehydrogenation pressure almost presents the reaction pressure circumstance. It can be seen that the dehydriding curve under 1.0×10² Pa initial hydrogen gas back-pressure exhibits a two-step feature, whereas roughly three-step feature under 4.0×10⁵ Pa initial dehydrogenation pressure. For both samples, their first dehydrogenation steps are similar, after which about 2.5% hydrogen is released. Evident difference appears after the first dehydrogenation step. Under 1.0×10² Pa initial dehydrogenation pressure, hydrogen is released acutely as temperature increases, reaching 7.91% within 5.2 h at 450 °C. In comparison, the dehydriding curve under 4.0×10⁵ Pa initial dehydrogenation pressure exhibits a sloping plateau with slow hydrogen desorption. However, about 6 h later, hydrogen then evolves rapidly, and hydrogen desorption capacity of 9.16% is finally obtained within 9.6 h.

Fig.1 Dehydriding curves of 2LiBH₄+MgH₂ systems performed under initial dehydrogenation pressure of 1.0×10² Pa (a) and 4.0×10⁵ Pa (b) and temperature profile (c) at ramping rate of  5 °C/min

Figure 2 shows the XRD patterns of the dehydrogenated samples performed under 1.0×10² Pa and 4.0×10⁵ Pa initial dehydrogenation pressure, respectively. When an initial dehydrogenation pressure of 4.0×10⁵ Pa is applied, MgB₂, Mg and LiH are produced (Fig.2(b)). On the other hand, when an initial hydrogen pressure of 1.0×10² Pa is applied, three phases of Mg, B and LiH are produced (Fig.2(a)). Unfortunately, no diffraction peak of boron can be observed, suggesting that boron is amorphous, which agreed with other references[2-3]. These results confirm the impact of initial dehydrogenation pressure on dehydriding products of LiBH₄-MgH₂ system that the formation of MgB₂ needs hydrogen overpressure of a fraction of MPa, and relatively lower initial H₂ gas back-pressure leads to Mg and B produced instead of MgB₂. A little MgO phase may be caused by oxidation during loading. The broad peak around 2θ=15° corresponds to the thin film of sample holder.

Fig.2 XRD patterns of dehydrogenated 2LiBH₄+MgH₂ systems performed under initial dehydrogenation pressures of 1.0×10² Pa (a) and 4.0×10⁵ Pa (b)

To understand the formation process of MgB₂ and the effectiveness of the plateau in the dehydriding curve under 4.0×10⁵ Pa initial dehydrogenation pressure, we performed XRD phase analysis at the different dehydriding stages (I, II, III and IV points in Fig.1(b)) in the dehydrogenation process, as shown in Fig.3. Figure 3(a) shows the XRD pattern of 2LiBH₄+MgH₂ mixture prepared by mechanical milling. After dehydrogenation proceeding for 1.7 h, the XRD pattern in Fig.3(b) corresponds to LiBH₄ and Mg metal, indicating that MgH₂ has decomposed into Mg and H₂ relative to the first dehydrogenation step, described as reaction (4). For XRD pattern in Fig.3(c) corresponding to dehydriding for 3.5 h, there is a little LiH besides LiBH₄ and Mg, but no MgB₂ appears, which can be seen from the illustrated pattern. This indicates that a small amount of LiBH₄ has decomposed. Then, nearly no changes display in Fig.3(d) after dehydriding for 5 h, which is in accordance with the appearance of plateau in Fig.1(b). However, strong peaks of MgB₂ arise when the reaction duration extends to 6.8 h, as shown in Fig.3(e). Meanwhile, some LiBH₄ and Mg are detected still. After the overall dehydrogenation completed about 9.6 h later, massive MgB₂ has formed and a small quantity of Mg is obtained, and no LiBH₄ can be observed simultaneously (Fig.2(b)). This suggests that MgB₂ is formed from the reaction of LiBH₄ and Mg, which can be described as reaction (5). Combined with the dehydriding curve of Fig.1(b), the dehydriding process of 2LiBH₄ + MgH₂ system under 4.0×10⁵ Pa initial dehydrogenation pressure can be described as follows: along with temperature increasing to 450 °C, MgH₂ is firstly decomposed, forming Mg and releasing H₂; when temperature holding at 450 °C, a small amount of LiBH₄ is slowly decomposed; over 6 h extending, MgB₂ is uninterruptedly produced accompanying with a large amount of H₂ evolved. Thus, the dehydrogenation process of 2LiBH₄+MgH₂ at 450 °C under 4.0×10⁵ Pa initial dehydrogenation pressure can be summarized as three steps: 1) decomposition of MgH₂ to form Mg and H₂; 2) decomposition of a small amount of LiBH₄; 3) fast dehydrogenation of LiBH₄ to react with Mg and form MgB₂. Obviously, steps 2) and 3) present two dehydrogenation pathways of LiBH₄-MgH₂ system under relatively higher initial dehydrogenation pressure and relatively lower initial dehydrogenation pressure, respectively.

MgH₂→Mg+H₂                              (4)

Mg+2LiBH₄→MgB₂+2LiH+3H₂                 (5)

Fig.3 XRD patterns of 2LiBH₄+MgH₂ systems at different dehydriding stages performed under 4.0×10⁵ Pa initial dehydrogenation pressure before dehydrogenation (a) and after dehydriding for 1.7 h (b), 3.5 h (c), 5 h (d) and 6.8 h (e) (Peak identifications of LiBH₄ are originated from Ref.[3].)

It can be found that dehydrogenation pathway under higher initial pressure (recorded as DP(1)) is the sequence of reaction (4) and reaction (5). However, dehydrogenation pathway under lower initial pressure (recorded as DP(2)) is nothing but the result of physical stacking of reaction (4) and reaction (1). The key difference between DP(1) and DP(2) originates from the second dehydrogenation reaction. Under relatively higher initial dehydrogenation pressure, direct decomposition of LiBH₄ is suppressed and LiBH₄ can react with Mg to yield MgB₂. Conversely, LiBH₄ decomposes independently under relatively lower initial dehydrogenation pressure. So, the dehydrogenation pathway is greatly dependence of reaction hydrogen pressure.

3.2 Investigation on influence of initial dehydro- genation pressure

Figure 4 shows dehydriding behaviors of 2LiBH₄ + MgH₂ system applied to different initial hydrogen pressures from room temperature to 450 °C. The dehydriding curves under 1.0×10⁵ Pa and 2.0×10⁵ Pa initial hydrogen pressure demonstrate a two-step dehydrogenation. For both of them, the first step is similar, with desorbed hydrogen of about 2.5%. However, the second hydrogen desorption step under 2.0×10⁵ Pa initial hydrogen pressure is slower than that under 1.0×10⁵ Pa. After holding at 450 °C for 13 h, desorption of 8.05% and 8.12% hydrogen are released under 1.0×10⁵ Pa and 2.0×10⁵ Pa initial hydrogen pressure, respectively. Three-step dehydrogenation is obviously observed under 3.0×10⁵ Pa and 4.0×10⁵ Pa together with 4.8×10⁵ Pa initial hydrogen pressure, in all which dehydriding plateau and fast releasing of hydrogen after the plateau can be identified. Furthermore, it can be found that the hydrogen desorption rate of the second step corresponding to decomposition of LiBH₄ and dehydriding plateau become slower and shorter with increasing initial pressure. These plateaus consume approximately are 7.5, 4 and 3 h for initial hydrogen pressure of 3.0×10⁵, 4.0×10⁵, 4.8×10⁵ Pa, respectively. Simultaneously, the hydrogen desorption rate in the third step increases gradually with increasing initial hydrogen pressure. As a result, 9.1% and 9.0% hydrogen have been released within 9 h under 4.0×10⁵ Pa and 4.8×10⁵ Pa initial hydrogen pressure, while 7.9% hydrogen is released within even 13 h under 3.0×10⁵ Pa initial hydrogen pressure. It can be concluded that more complete dehydrogenation of 2LiBH₄+MgH₂ occurs and more hydrogen desorption amount can be obtained within a definite time with increasing initial dehydrogenation pressure.

Fig.4 Dehydriding curves of 2LiBH₄ + MgH₂ systems performed under different initial dehydrogenation pressures and temperature profile at temperature ramping rate of 5 °C/min

The XRD patterns of the dehydrogenated 2LiBH₄+MgH₂ samples applied to different initial hydrogen pressures are shown in Fig.5. All dehydriding products of different initial hydrogen pressures consist of Mg, MgB₂ and LiH. Nevertheless, the peak intensity of Mg appears much weaker along with increasing hydrogen pressure, while the peak intensity of MgB₂ appears much stronger. Compared the products of reaction (2) with reaction (3), it is found that LiH exists in both reactions. Simultaneously, it is noteworthy that boron produced in reaction (3) cannot be characterized by XRD[2-3]. Therefore, the dehydrated product, Mg or MgB₂, is the signal of each dehydrogenation pathway. MgB₂ represents DP(1), while the presence of Mg metal in the products represents DP(2). Due to the LiBH₄ to Mg molar ratio of 2:1, the relative content of MgB₂ to Mg in the products implies that the relative proportion of LiBH₄ that reacts with Mg or decomposes independently. In other words, the relative diffraction intensity of MgB₂ to Mg metal in the XRD patterns implies the occurrence rate of DP(1) or DP(2) in the overall dehydrogenation. If only MgB₂ phase exists in the products, it means that the whole dehydrogenation reaction proceeds as DP(1). The dehydrogenation reaction entirely follows DP(2), in contrast, only when Mg metal phase exists in the products. According to Fig. 5, we can conclude that both DP(1) and DP(2) appear in the whole dehydrogenation reaction under various initial hydrogen pressures. However, with increasing the initial dehydrogenation pressure, dehydrogenation prefers to follow DP(1).

Fig.5 XRD patterns of dehydrogenated 2LiBH₄ + MgH₂ systems performed under initial dehydrogenation pressures of 1.0×10⁵ Pa (a), 2.0×10⁵ Pa (b), 3.0×10⁵ Pa (c), 4.0×10⁵ Pa (d) and 4.8×10⁵ Pa (e)

3.3 Investigation on influence of dehydrogenation temperature

Figure 6 shows the dehydriding curve of 2LiBH₄+ MgH₂ system from room temperature to 500 °C under 4.0×10⁵ Pa initial hydrogen pressure. It can be seen that no dehydriding plateau appears after the first desorption step, and hydrogen is rapidly released along with temperature rising. Finally, a dehydriding capacity of 8.49% is obtained within 5 h. The XRD pattern of the dehydrogenated product is shown in Fig.7(a). Unfortunately, a large amount of unexpected Mg metal remained, besides some MgB₂ formed. From the relative diffraction intensity of MgB₂ to Mg metal in the XRD pattern, we infer that the main proportion of LiBH₄ is decomposed independently, and a small amount of LiBH₄ retains for reacting with Mg to produce MgB₂. It suggests that, although the dehydriding rate is improved, the temperature of 500 °C is too high to suppressing direct decomposition of LiBH₄ under 4.0×10⁵ Pa initial hydrogen pressure. The presence of MgH₂ in the products probably originated from rehydrogenation of Mg metal during air cooling from 500 °C to room temperature under the hydrogen pressure of 4.0×10⁵ Pa.

Fig.6 Dehydriding curve of 2LiBH₄ + MgH₂ system from room temperature to 500 °C under 4.0×10⁵ Pa initial hydrogen pressure and temperature ramping rate of 5 °C/min

Fig.7 XRD patterns of dehydrogenated 2LiBH₄ + MgH₂ systems performed under different conditions: (a) RT -500 °C for 5 h; (b) RT -400 °C for 15 h, then increased to 450 °C for 13 h

Figure 8 shows the dehydriding curve of 2LiBH₄+ MgH₂ system from room temperature to 400 °C then increased to 450 °C under 4.0×10⁵ Pa initial hydrogen pressure. It is found that the dehydriding plateau at 400 °C is quite flat, suggesting that the decomposition of LiBH₄ is suppressed significantly. At this time, the amount of hydrogen desorbed almost maintains at 2.8% and no trace of the formation of MgB₂ (massive hydrogen is released abruptly) appears until temperature increases to 450 °C, even though dehydriding plateau extends to 15 h at 400 °C. The phase composition after dehydrogenation is given in Fig.7(b). The relative diffraction intensity of MgB₂ in the final product, which is similar to that in Fig.2(b), is in agreement with the dehydriding behavior shown in Fig.8. This indicates that the temperature of 400 °C is not high enough to facilitate the formation of MgB₂ in 2LiBH₄+MgH₂ system, though the direct decomposition of LiBH₄ is suppressed effectively. As a result, it can be concluded that the temperature of 450 °C is proper for the formation of MgB₂ in the 2LiBH₄+MgH₂ system under 4.0×10⁵ Pa initial dehydrogenation pressure.

Fig.8 Dehydriding curve of 2LiBH₄ + MgH₂ system from room temperature to 400 °C and then increased to 450 °C under 4.0×10⁵ Pa initial hydrogen pressure (at 400 °C for 15 h and at 450 °C for 13 h, temperature ramping rate of 5 °C/min)

On the basis of above results, it is found that the dehydrogenation of 2LiBH₄+MgH₂ system proceeds more completely and more hydrogen desorption capacity can be obtained within a definite time by forming MgB₂ than separated decomposition of LiBH₄ and MgH₂. Actually, the formation process of MgB₂ obeys the general features of nucleation, in particular, the effect of supercool or superheat and component concentration on the potency of nucleation: 1) the increase of the supercool degree or superheat degree can enhance the nucleation; 2) the nucleation potency improves with the component concentration of the reactants close to stoichiometric ratio of the product[13-14].

In general, the nucleation rate increases dramatically along with superheat rising, resulting in that the incubation is shortened significantly[15]. According to the above experiment results, it is found that the formation of MgB₂ indeed requires an incubation period, exhibiting as a plateau in the dehydriding curve, which is greatly affected by reaction temperature. It shows that elevated temperature promotes the incubation of MgB₂, and the ability of incubation is deteriorated at the decreased temperature. Furthermore, it shows that the component concentration of LiBH₄ to Mg is maintained more closer to 2:1 during the dehydriding plateau, the incubation process for MgB₂ is more favorable, and vice versa. Under the lower hydrogen gas back-pressure, LiBH₄ is decomposed quickly, resulting in little LiBH₄ remained for the incubation of MgB₂. Due to the relatively higher hydrogen gas back-pressure inhibiting the decomposition of LiBH₄, there is sufficient LiBH₄ for incubation of MgB₂. As a result, the component concentration of LiBH₄ to Mg is maintained more closer to 2:1 during the dehydriding plateau under relatively higher hydrogen gas back-pressure than the lower hydrogen gas back-pressure. So, it can be seen from Fig.4 that the incubation period is shortened by increasing hydrogen gas back-pressure. Consequently, it is inferred that the incubation period of 1.0×10⁵ Pa or 2.0×10⁵ Pa initial dehydrogenation pressure is much longer than that of 3.0×10⁵ Pa. The reason that the incubation plateau was not observed clearly was that the majority of LiBH₄ decomposed independently and only a small amount of LiBH₄ reacted with Mg to produce MgB₂, which was good consistent with the XRD reflection result. On the basis of above analysis, it is suggested that the plateau of the dehydriding curve relates to the incubation period for nucleation of MgB₂, after which it should be the rapid growth of nucleus accompanying with a large amount of H₂ released.

At the same time, the results show that none of the involved experiments in this work entirely followed DP(1) to produce only MgB₂ rather than Mg. Because MgB₂ plays a key role in the reversibility of LiBH₄-MgH₂ system, the full formation of MgB₂ is necessary during the dehydrogenation of 2LiBH₄+MgH₂ system. In fact, there are two ways to make the dehydrogenation of 2LiBH₄+MgH₂ system just only follow DP(1): first one is to violently suppress the direct decomposition of LiBH₄. Generally, increasing initial dehydrogenation pressure can improve the ability in suppressing the decomposition of LiBH₄ at constant temperature. However, excess high pressure will inevitably lead to the decomposition of MgH₂ in harsh condition. Thus, the hydrogen pressure applied in the decomposition of LiBH₄ should be as low as possible. In this case, it shows that an applied hydrogen pressure of at least 4.0×10⁵ Pa is potentially appropriate to obtain comprehensive ability in yielding MgB₂ at 450 °C in 2LiBH₄+MgH₂ system. Nevertheless, a small sealed reaction volume may be used for dehydrogenation of 2LiBH₄+MgH₂ system under relative lower initial dehydrogenation pressure or vacuum, through which not only MgH₂ decomposes readily but also there is sufficient hydrogen pressure to suppress the direct decomposition of LiBH₄ after the decomposition of MgH₂. The other way for the full formation of MgB₂ is to make the incubation period very short so that the reaction of LiBH₄ with Mg occurs quickly before the separated decomposition of LiBH₄. Adding nucleating agent or catalysts may be a useful method for that purpose. So, it seems that using a proper small sealed reaction volume with nucleating agent or catalyst is a potentially effective strategy for improving the comprehensive properties of 2LiBH₄+MgH₂ system, which is being investigated currently.

4 Conclusions

1) The dehydrogenation pathway of 2LiBH₄+MgH₂ system was investigated carefully. It is found that the dehydrogenation pathway is determined by initial dehydrogenation pressure, which suppresses the direct decomposition of LiBH₄ or not.

2) Under relatively higher initial dehydrogenation pressure, LiBH₄ reacts with Mg to produce MgB_2. However, under relatively lower initial dehydrogenation pressure, LiBH₄ is decomposed independently, resulting in no formation of MgB₂.

3) The dehydrogenation of 2LiBH₄+MgH₂ system proceeds more completely and more hydrogen desorption amount can be obtained within a definite time by forming MgB₂.

4) The formation process of MgB₂ consisting of incubation period and nucleus growth process is proposed. The results show that the formation process of MgB₂ is enhanced by increasing initial dehydrogenation pressure at constant temperature. Additionally, elevated temperature could significantly reduce incubation period, while the ability to suppress the decomposition of LiBH₄ is deteriorated under a finite hydrogen gas back-pressure.

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2LiBH₄+MgH₂体系放氢过程中MgB₂的形成机理

寇化秦，肖学章，陈立新，李寿权，王启东

浙江大学 材料科学与工程学系，杭州 310027

摘  要：对2LiBH₄+MgH₂体系放氢过程中MgB₂的形成条件及机理进行研究。结果表明：在较高的4.0×10⁵ Pa初始氢背压下放氢时，会抑制2LiBH₄+MgH₂体系中LiBH₄的自行分解，进而使其与MgH₂分解放氢后生成的Mg发生反应生成MgB₂，同时在450 °C、9.6 h内释放出9.16%(质量分数)的氢气；而在较低的1.0×10² Pa初始氢背压下放氢时，体系中LiBH₄会先行发生自行分解，从而不能与Mg发生反应生成MgB₂，在450 °C、5.2 h内只能放出7.91%的氢气。2LiBH₄+MgH₂体系放氢生成MgB₂可以使放氢反应进行得更彻底，并释放出更多的氢气。2LiBH₄+MgH₂放氢时MgB₂的形成过程是一个孕育-长大的过程，随着氢背压的增高，孕育期缩短；而随着反应温度的降低，孕育期延长。

关键词：配位氢化物；LiBH₄；MgB₂；储氢；形成机理

(Edited by YANG Hua)

Foundation item: Project (2010CB631300) supported by the National Basic Research Program of China; Project (50631020) supported by the National Natural Science Foundation of China; Project (NCET-07-0741) supported by the Program for New Century Excellent Talents in Universities, China; Project (20090101110050) supported by the University Doctoral Foundation of the Ministry of Education, China

Corresponding author: CHEN Li-xin; Tel/Fax: +86-571-87951152; E-mail: lxchen@zju.edu.cn

DOI: 10.1016/S1003-6326(11)60819-4