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On the synthesis, characterization and hydrogenation behaviour of Mg-based composite materials Mg-x wt.% CFMmNi5 prepared through mechanical alloying
Journal of Alloys and Compounds 292 (1999) 202–211
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On the synthesis, characterization and hydrogenation behaviour of Mg-based composite materials Mg-x wt.% CFMmNi 5 prepared through mechanical alloying S.S. Sai Raman, D.J. Davidson, O.N. Srivastava* Department of Physics, Banaras Hindu University, Sampoornanand Nagar, Sigra, Varanasi 221005, India Received 11 May 1999; accepted 17 May 1999
Abstract The present study deals with the investigations on the synthesis, structural / microstructural characteristics and hydrogenation / dehydrogenation behaviour of Mg-bearing composite materials Mg-x wt.% CFMmNi 5 prepared through mechanical alloying. The composite materials Mg-x wt.% CFMmNi 5 have been successfully synthesized through ball-milling (mechanical alloying) by employing a high energy attritor mill. The mechanical alloying has been carried out in hexane medium by varying the milling parameters say, speed (revolutions per minute) and milling duration. The as-milled composite materials have been activated at 4006108C under a hydrogen pressure of |35–40 kg cm 22 . These composite materials exhibit high hydrogen storage capacity and fast absorption / desorption kinetics in comparison to the thermally melted counterparts. It has been found that the highest storage capacity material (|5.4 wt.% at 3508C) corresponds to Mg-30 wt.% CFMmNi 5 . The composite material also exhibits fast desorption kinetics (about 90 cm 3 min 21 ), which is at least two times faster than conventionally prepared (RF melting) alloys. The highest hydrogen storage capacity and fast kinetics were obtained for the mechanically alloyed samples with the optimized milling conditions, i.e. speed |400 rev min 21 and time duration of 5 h. The hydriding rate and the improved hydrogen storage capacity of these composite materials have been found to be strongly correlated with structural and microstructural characteristics as brought out through XRD and SEM techniques. The uniform particle size distribution and interfacial grain boundaries explored from the SEM investigations paves the way for better hydrogen storage capacity and fast absorption and desorption kinetics. 1999 Elsevier Science S.A. All rights reserved. Keywords: Mg-bearing composite; Hydrogen absorption; Hydrogen storage capacity; Rare earth nickel composite; Mechanical alloying
1. Introduction Owing to the vulnerability, price rise and negative impact of fossil fuels on the environment, it is imperative to usher in renewable and clean energy sources. Of all the alternative fuels, hydrogen is now known to be a potential renewable substitute for petroleum or gasoline. As hydrogen is very light (¯10 5 times lighter than petroleum), to harness hydrogen, it has to be properly stored. In the search for viable and widely acceptable means of storing hydrogen, metal hydrides are being considered as a promising technology. For an effective automotive storage system, the hydride used should satisfy certain specific criteria. For example, it must be prepared from inexpensive metals, should have a low weight density, higher storage *Corresponding author. Tel.: 191-542-361-937; fax: 191-542-317468. E-mail address: [email protected] (O.N. Srivastava)
capacity ($3.0 wt.%), moderate dissociation temperature and good kinetics. The present known hydrides, even though viable, have the perennial problems related to rather low storage capacities. After a decade of extensive research in the area of metal hydrides, it has now become clear that present state-of-the-art alloy systems (namely LaNi 5 , FeTi and Mg 2 Ni) may lead to low storage capacities. For example, at ambient conditions, the hydrogen storage capacity corresponds to LaNi 5 51.5 wt.%, FeTi5 1.78 wt.%, and Mg 2 Ni with a high storage capacity of |3.8 wt.%, but can be operated only at high temperatures of 200–3008C. The present problem in regard to the improvement in the hydrogen storage capacity and fast desorption kinetics can be solved by the following new routes of material processing: (i) surface modification and optimization of the present storage systems LaNi 5 , FeTi and high temperature hydride Mg 2 Ni, (ii) development of altogether new storage materials, e.g. transition metal complexes, composite materials, nanocrystalline materials
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00272-8
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(nano particles), new carbon variants (fullerenes C 60 and other higher versions, carbon nano-fibers (CNF) and nanotubules (CNT)); it appears at present that the later routes may have a better chance of success. In particular the composite materials seem potentially promising in regard to obtaining viable hydrogen storage systems operated at ambient as well as high temperature conditions [1]. It has been already known from previous investigations that the light-weight magnesium-based rare earth alloy composite hydrides possess reversible hydrogen storage characteristics and fast absorption–desorption kinetics as compared to MgH 2 . For example, composite materials Mg-x wt.% LaNi 5 (x510–40) prepared using thermal sintering has been found to react with hydrogen at higher temperatures and form low stability hydrides with better storage capacity and hydriding behaviour than that of individual components, without increasing weight and cost [2]. Magnesium acts as binder in the composite Laves phase alloys, i.e. Mg-x wt.% ZrFe 1.4 Cr 0.6 and Mg-x wt.% TiMn 1.5 and their hydrogenation characteristics have been reported by two groups [3–4]. Some of the new and novel composite hydrogen storage materials have been investigated and explored in terms of pressure–composition– temperature (P–C–T) isotherms and desorption kinetics [5–12]. In our earlier communication [13], we have successfully demonstrated the hydrogenation behaviour of the composite alloy Mg-x wt.% CFMmNi 5 (x520–50) synthesized through thermal (RF induction) melting technique. However, it may be pointed out that the hydrogenation characteristics of the present materials prepared through mechanical alloying are distinctly different from that of the sample prepared through thermal melting. For example, the hydrogen storage capacity of |5.4 wt.% was obtained with lower temperature (300–3508C) as compared with the thermally melted alloys which desorbs at 400–5008C. Yet another difference relates to the faster desorption kinetics (about 90 cm 3 min 21 ) which is at least two times faster than that of the corresponding thermally melted composite alloys. This paper deals with the synthesis, characterization and hydrogenation behaviour of the composite materials Mg-x wt.% CFMmNi 5 prepared through mechanical alloying. It has been shown that the material corresponding to Mg-30 wt.% CFMmNi 5 prepared through ball-milling in hexane medium for 5 h and at a speed of 400 rev min 21 has the highest storage capacity of |5.4 wt.% at 3506108C. This material also exhibits fast desorption kinetics (about 90 cm 3 min 21 ). In order to understand the curious hydrogenation behaviour of these composite materials, structural / microstructural characterization have been undertaken by employing XRD and SEM. These revealed the presence of the parent ingredients Mg, CFMmNi 5 and small amount of a free Ni phase in the as-milled materials, whereas the thermally melted alloys exhibited a multiphasic nature. After hydrogenation, in addition to Mg, CFMmNi 5 and Ni
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phases, few peaks corresponding to MgH 2 were also found.
2. Experimental details
2.1. Synthesis of Mg-x wt.% CFMmNi5 composite materials First, the CFMmNi 5 (CFMm5cerium-free misch-metal) alloy was prepared through a solid-state diffusion process, by melting the stoichiometric mixtures of individual elements. High purity lanthanum (99.9%), neodymium (99.9%), praseodemium (99.9%) and nickel (99.5%) were taken in correct stoichiometric proportions, pressed into pellet forms (130.5 cm) to obtain an alloy with stoichiometry CFMmNi 5 . These were melted employing an RF induction furnace (18 kW) under an argon atmosphere in a double jacket silica tube with facilities for water flow in the outer jacket and argon flow in the inner jacket. The initially prepared alloy ingot was melted several times (5–6 times) to achieve homogeneity [14]. The agglomerate produced in this way was removed from the silica tube, crushed and subjected to X-ray diffraction characterization. The composite materials Mg-x wt.% CFMmNi 5 was prepared using mechanical alloying technique by taking a stoichiometric amount of magnesium (99.99% purity) and the pre-synthesized alloy CFMmNi 5 . The magnesium and alloy powders were sieved to achieve a particle size of about 100 mm. The mechanical alloying of these composite mixtures were carried out by employing a high energy ball mill (Sazegvari attritor system type 01 HD). It has a cylindrical container of inner diameter 13 cm, a volume about 1400 cc with an impellor and stainless steal balls of diameter 6 mm. In passing it should be mentioned that very recently Sapru et al. [15] reported a similar type of experiment which embodies the effect of processing parameters on Mg-based hydrogen storage materials prepared by mechanical alloying. The synthesis of composite materials Mg-x wt.% CFMmNi 5 were carried out under hexane medium by varying the concentration (x510–50) and the milling speed (100–400 rev min 21 ). The ball to powder weight ratio in all the experiments were confined to 20:1. After the mechanical alloying, the milled powder was extracted form the hexane and immediately transferred to an argon-filled glove box.
2.2. Structural, microstructural characteristics: XRD and SEM explorations The structural characterization of the as-milled composite materials and their hydrogenated versions were carried out through XRD. A Philips X-ray powder diffractometer PW-1710 equipped with graphite monochromator working ˚ was used for this with Cu Ka radiation ( l51.5418 A)
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purpose. The microstructural characteristics were examined by scanning electron microscopy employing a Philips XL-20 series SEM with 30 kV secondary electrons.
2.3. Activation procedure of Mg-x wt.% CFMmNi5 composite materials Since the as-synthesized composite materials do not desorb at room temperature, these were suitably activated. Several activation modes, including activation at high temperatures (100–4008C), were attempted for the asmilled Mg-x wt.% CFMmNi 5 composite materials. It was found that the most favourable activation process corresponded to that of an annealing treatment at 4006108C under hydrogen pressure of |35–40 kg cm 22 . A known quantity of finely granulated powder of Mg-x wt.% CFMmNi 5 composite material was placed in a reactor and was evacuated to the order of 10 24 Torr. About 40 kg cm 22 of hydrogen (purity 99.999%) was introduced form a high pressure gas cylinder. The bottom portion of the reactor was heated at a temperature 4006108C for 4–6 h continuously and then it was cooled to room temperature slowly (¯10 h). After the activation, the hydrogen was desorbed. The hydrogenation characteristics of these composite materials such as desorption kinetics and pressure–composition–temperature (P–C–T) isotherms were measured at different isothermal conditions using a Sieverts type apparatus fabricated in our laboratory and utilized earlier for hydrogenation studies of standard hydrogen storage materials [16].
3. Results and discussion
3.1. Hydrogenation /dehydrogenation characteristics The main aim of synthesizing the Mg-based composite materials, e.g. Mg-x wt.% CFMmNi 5 (x510–50) was to find out the viability and feasibility of utilizing them as high hydrogen storage capacity materials for automotive and other high temperature applications. Here we describe and discuss the hydrogenation / dehydrogenation behaviour such as pressure–composition isotherms (PCT), desorption kinetics and their correlation with structural / microstructural characteristics. The dehydriding behaviour of the composite materials Mg-x wt.% CFMmNi 5 milled under hexane medium with various milling durations (3, 5, 7 h) was evaluated and extensively analyzed for several values of x (10, 20, 30, 40, 50) at a temperature ranging from 300 to 3508C. Fig. 1 represents the desorption kinetic curves at a temperature of |3508C and at a pressure of 1–2 kg cm 22 for the composite material and Mg-x wt.% CFMmNi 5 with x510, 20, 30, 40 and 50 respectively, milled under hexane medium for 5 h duration. These results reveal that desorption rate was strongly affected by the intermetallic alloy
Fig. 1. Representative desorption kinetic curves for the composite material Mg-x wt.% CFMmNi 5 (x510, 20, 30, 40, 50) at temperature T|3508C and at a pressure P|1–2 kg cm 22 when milled under hexane medium for 5 h.
(CFMmNi 5 ) concentration. As is evident from Fig. 1, the best dehydriding kinetics and high hydrogen storage capacity belongs to the composite mixture Mg-30 wt.% CFMmNi 5 treated at 5 h time duration in an attritor ball mill. The initial release of hydrogen from the composite material was linear with time, and this decreased at longer times. From the hydrogen desorption vs. time curves (Fig. 1), it may be inferred that the initial hydriding–dehydriding rate is much faster and about two times better than the conventionally prepared (RF induction melted) Mg-x wt.% CFMmNi 5 composite alloys. It may also be noted that half of the saturation value was accomplished within 2 min. However, the total time required for complete saturation has been found to be |9–10 min. Moreover, the close observations of Fig. 1 paves the way for selecting the crucial composition in order to obtain the optimized composite material with high hydrogen storage capacity and fast kinetics. In the present investigation, the optimized composite material with high hydrogen storage capacity (|5.4 wt.% at 3508C) and faster desorption kinetics (|90 cm 3 min 21 ) corresponds to Mg-30 wt.% CFMmNi 5 , which was then further investigated in terms of different isothermal temperature conditions and milling time durations. Fig. 2 shows the representative desorption kinetic curves for the composite material Mg-30 wt.% CFMmNi 5 at three different temperatures – 300, 325 and 3508C with milling time duration of 5 h. It may be pointed out that the desorption rate strongly depends upon the
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Fig. 2. The amount of hydrogen desorbed vs. time (kinetic curves) during the dehydriding process of the composite material Mg-30 wt.% CFMmNi 5 at different isothermal temperature conditions (300, 325, 3508C) when milled under hexane medium with optimized time durations of t|5 h and at a speed of |400 rev min 21 .
temperature of desorption. It has been found that the composite material Mg-30 wt.% CFMmNi 5 possesses a high hydriding rate (|40–50 cm 3 min 21 ) and a high hydrogen storage capacity (|4.0 wt.%) at a relatively low temperature (3008C), which is 5–6 times faster than that of the magnesium hydride (MgH 2 ). The dependence of the desorption rate on the milling durations such as 3, 5 and 7 h are exhibited in Fig. 3. It shows that the fast desorption kinetics for the composite materials Mg-30 wt.% CFMmNi 5 were obtained at 3508C when milled under hexane medium for 5 h. It may be concluded from desorption kinetics curves (Figs. 1–3) that the rate of desorption was very much dependent on the various experimental parameters such as, (i) intermetallic alloy concentration in the composite materials, (ii) desorption temperature, and (iii) duration of milling. The representative P–C–T desorption isotherms of the as-prepared composite materials Mg-x wt.% CFMmNi 5 with x530 milled for different time durations (3, 5, 7 h) are shown in Fig. 4a–c. The P–C–T values were plotted at three different isothermal temperature conditions, e.g. 300, 325 and 3508C for each milling duration. It was observed that the hydrogen storage capacity of the composite materials mainly depends upon two factors, namely (i) temperature of desorption (range between 300 and 3508), and (ii) time duration of milling (e.g. 3, 5, 7 h). The plateau region (metal hydride phase) and the hydrogen
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Fig. 3. Desorption kinetic curves for the composite material Mg-30 wt.% CFMmNi 5 at temperature |3508C and pressure of |1–2 kg cm 22 when milled at different time durations (t51, 3, 5, 7 h) and a speed of |400 rev min 21 .
storage capacity of the P–C–T isotherms of the composite materials Mg-x wt.% CFMmNi 5 increases with increasing milling time durations, up to 5 h (e.g. t53, 5 h), thereafter it was found to decrease (e.g. t57 h) (see Fig. 5). Therefore, it is evident that the highest hydrogen storage capacity (|5.4 wt.% at 3508C) material corresponds to Mg-30 wt.% CFMmNi 5 milled under hexane medium with a speed of 400 rev min 21 and milling duration of |5 h. It may be pointed out that the optimized composite materials Mg-30 wt.% CFMmNi 5 possesses one of the highest known storage capacity which is better than that conventionally prepared (RF induction melted) alloys and also approximately matching the theoretical storage capacity (about 5.4 wt.%). There was only one plateau region as inferred from the P–C isotherms which clearly envisages that the as-milled samples possess only two phases of initial ingredients such as Mg and intermetallic phase CFMmNi 5 . The addition of the intermetallic alloy CFMmNi 5 to magnesium reduces the temperature of dissociation of hydrogen molecules on the composite metal hydride surface and tends to form the MgH 2 phase reversibly. Moreover, the P–C–T desorption isotherm curves exhibit plateaux at pressures of 1–2 kg cm 22 which clearly reveals the reversible desorption of the MgH 2 phase in the presence of the secondary alloy component (i.e.) CFMmNi 5 . In order to explore why the composition corresponding to (x530), i.e. Mg-30% CFMmNi 5 is the
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Fig. 4. Pressure composition isotherms for the mechanically alloyed composite materials Mg-30 wt.% CFMmNi 5 at various temperature (300, 325, 3508C) when milled at different time durations t of (a) 3, (b) 5, and (c) 7 h.
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Fig. 5. Variation of the hydrogen storage capacity of the composite material Mg-30 wt.% CFMmNi 5 with different milling durations and at different temperatures (300, 325, 3508C).
optimum composition in regard to high hydrogen storage capacity, we have extensively studied the behaviour of P–C isotherms by varying the alloy concentration x (e.g. 10, 20, 30, 40, 50). Fig. 6 shows the representative pressure–composition isotherm of the composite materials Mg-x wt.% CFMmNi 5 with several values of x (e.g. 10, 20 30, 40, 50) milled in an attritor mill for 5 h time durations. The P–C–T curves were plotted at 3508C similar as in Fig. 4a–c. It was clearly noticed that the hydrogen storage capacity varies with respect to the addition of CFMmNi 5 alloy to form composite materials (see Fig. 7). The optimized hydrogen storage capacity of |5.4 wt.% at
3508C corresponds to Mg-30 wt.% CFMmNi 5 , which is approximately approaching the value of MgH 2 . An important aspect of the present mechanically alloyed composite materials relates to the fact that after 10–15 hydrogenation–dehydrogenation cycles, the sample pulverized to fine particles (1–5 mm) without any deterioration in hydrogenation characteristics. Fig. 8 represents the P–C desorption isotherms of the composite materials Mg-30 wt.% CFMmNi 5 at 3508C for various numbers of hydrogenation cycles (runs) (e.g. 1, 3, 5, 7, 9). The observations revealed that the hydrogen storage capacity of |3.0 wt.% obtained during the initial cycle of hydrogenation increases
Fig. 6. Pressure–composition isotherms of the Mg-x wt.% CFMmNi 5 (x510, 20, 30, 40, 50) composite material at 3508C when milled under hexane medium for 5 h.
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3.2. Structural, microstructural characteristics: XRD and SEM explorations
Fig. 7. Hydrogen storage capacity vs. concentration of CFMmNi 5 curve of the Mg-30 wt.% CFMmNi 5 composite materials.
with increasing hydrogenation–dehydrogenation cycles and finally achieved the optimum hydrogen storage capacity (|5.4 wt.%) at the 9th cycle and remained constant even after several cycles. Henceforth it may be claimed that the composite materials Mg-x wt.% CFMmNi 5 (x510, 20, 30, 40, 50) possess good cyclic stability in regard to hydrogenation–dehydrogenation characteristics.
In order to unravel the curious hydrogenation behaviour of Mg-30 wt.% CFMmNi 5 , structural (X-ray diffraction, XRD), microstructural (scanning electron microscopy, SEM) characteristics were carried out. It was found that the hydrogenation–dehydrogenation characteristics are closely correlated with the microstructures and types of phases present in the composite materials. Fig. 9a shows the representative X-ray diffraction pattern of the mixture Mg-30 wt.% CFMmNi 5 before mechanical alloying. The starting material embodies the individual ingredients Mg and CFMmNi 5 which are seen in the form of a mixture. A small peak corresponding to free nickel was also observed in the sample. Fig. 9b–d represents the X-ray diffractogram taken from the mechanically alloyed composite materials Mg-30 wt.% CFMmNi 5 with different milling durations of 3, 5 and 7 h. The formation of the composite material is elucidated through a comparison of Fig. 9a with Fig. 9b–d. These results reveal that the mechanically alloyed composite materials of Mg-30 wt.% CFMmNi 5 still embody the initial ingredients magnesium, cerium-free misch-metal pentanickellide and nickel. This is important since it reveals that no other intermediate alloy such as Mg 2 Ni was formed. Thus the composite materials would be capable of exhibiting the hydrogenation / dehydrogenation of Mg in addition to the intermetallic alloy CFMmNi 5 . The small excess nickel concentration plays a vital role as catalyst, which in turn improves the hydrogenation–dehydrogenation characteristics of the composite materials. The XRD peaks however, exhibit changes in intensity and peak width. Extensive analysis of these results revealed that the peaks corresponding to magnesium decrease in intensity
Fig. 8. Representative P–C–T isotherm of the Mg-30 wt.% CFMmNi 5 composite material at a temperature of |3508C for various desorption cycles.
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Fig. 10. X-ray powder diffraction patterns of the hydrogenated composite materials Mg-30 wt.% CFMmNi 5 milled with various milling durations t of (a) 3, (b) 5, and (c) 7 h.
Fig. 9. X-ray powder diffraction patterns (a) for the mixture Mg-30 wt.% CFMmNi 5 before mechanical alloying; the composite materials Mg30wt.% CFMmNi 5 milled in the hexane medium at different milling time durations t of (b) 3, (c) 5, and (d) 7 h.
(e.g. 1600, 350, 125, 100 counts) and become broadened (increased FWHM) with increased milling durations of 3, 5 and 7 h. For example, compared to the 3-h ball-milling, the FWHM for the 5-h duration has increased by about 75%. The composite material showing optimized hydrogenation behaviour obtained for the ball-milling duration of 5 h exhibited particle sizes of |5–10 mm. The XRD patterns taken from the hydrogenated materials showed features broadly similar to those exhibited by the as-milled composite materials with regard to the observed phases along with a small concentration of MgH 2 phase which are depicted in Fig. 10a–c. It is expected that an important
parameter influencing hydrogenation–dehydrogenation is the presence of free Ni on the surface. As is known, the presence of free Ni or nickel bearing phases on the surface of the samples decrease the activation energy of dissociation of hydrogen molecules. The hydrogen storage capacity and desorption kinetics are probably dependent on the free nickel and nickel containing phases (such as CFMmNi 5 ) which form sites favourable for hydrogen reaction and may also serve as a channels by which the hydrogen can enter and exit the bulk of the main phase. In order to find out the changes in the microstructural features with milling parameters, particularly the duration of ball-milling, scanning electron micrograph employing secondary electrons have been undertaken. Fig. 11a,c,e exhibits the representative microstructural details of mechanically milled composite materials Mg-30 wt.% CFMmNi 5 with milling time durations of 3, 5 and 7 h, respectively. It can be easily seen from the micrographs that for the composite material with 5 h milling (Fig. 11c), the particle size of |10 mm has become uniformly distributed in comparison with Fig. 11a and e. Another noticeable fact from the microstructural characteristics of
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Fig. 11. Representative scanning electron micrographs of the as-milled (a,c,e) and hydrogenated (b,d,f) composite materials Mg-30 wt.% CFMmNi 5 with milling durations of 3, 5, and 7 h.
the as-milled composite materials with 5 h milled duration is that the particles do not agglomerate and remain separate, as individual entities. But for both the extreme cases such as the materials with 3 and 7 h milling time durations (Fig. 11a,e), the particle size increases due to coalescence and do not remain separate. A microstructure of the composite materials Mg-30 wt.% CFMmNi 5 milled with various time durations (e.g. 3, 5, 7 h) exhibiting extensive cracking and breakage of larger grains, obtained after activation and several hydrogenation–dehydrogenation cyclings, is shown in the SEM image of Fig. 11b,d,f. It can be seen that the effect of hydrogenation produces fracture and breakage of the surface into small domains, which could apparently provide a channel for hydrogen to diffuse in and out of the lattice. Thus, the improvements in desorption kinetics and the attainment of an optimized high hydrogen storage capacity after activation and repeated cycling of the composite material Mg-30 wt.% CFMmNi 5 can be qualitatively understood in terms of the following reasons: (i) hydrogenation of Mg particles, (ii) the availability of several hydrogen diffusion channels arising from interfacial grain boundaries between magnesium and alloy
component, (iii) the resulting volume expansion-induced cracking and consequent segregation of the metal surface due to the catalytic activities of free Ni and nickel-bearing phases, such as CFMmNi 5 , which leads to the continuous exposure of fresh surface for hydrogenation.
4. Conclusion In summary, it can be said that the mechanically alloyed composite materials Mg-x wt.% CFMmNi 5 represent a viable storage system with high hydrogen storage capacity and fast desorption kinetics. The mechanically alloyed composite materials exhibit the highest hydrogen storage capacity and fast absorption / desorption kinetics in comparison with the thermally melted counterparts. The best hydrogen storage materials with a storage capacity of |5.4 wt.% at 3508C were obtained for the composition Mg-30 wt.% CFMmNi 5 with a milling speed of 400 rev min 21 for 5 h is hexane medium. This composite material also exhibits fast desorption kinetics (about 90 cm 3 min 21 ) which is at least two times faster than conventionally
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prepared (RF melting) alloys. The hydrogenation behaviour such as hydriding rate and the improved hydrogen storage capacity of these composite materials has been correlated with the structural and microstructural characteristics. For example, the higher storage capacity and fast desorption kinetics can be explained based on the uniform particle size distribution and the availability of several hydrogen diffusion channels arising from interfacial grain boundaries.
Acknowledgements The authors are grateful to Professor M. Groll (University of Stuttgart, Germany), Professor T.N. Veziroglu (President, IAHE, Florida, USA), Professor A.R. Verma, Professor Y.C. Simhadri and Dr B.A. Danassanacharya for encouragement. The financial support of the Ministry of Non-Conventional Energy Sources (MNES), Council for Scientific and Industrial Research (CSIR), UGC-DAEF and AICTE are gratefully acknowledged.
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On the synthesis, characterization and hydrogenation behaviour of Mg-based composite materials Mg-x wt.% CFMmNi 5 prepared through mechanical alloying S.S. Sai Raman, D.J. Davidson, O.N. Srivastava* Department of Physics, Banaras Hindu University, Sampoornanand Nagar, Sigra, Varanasi 221005, India Received 11 May 1999; accepted 17 May 1999
Abstract The present study deals with the investigations on the synthesis, structural / microstructural characteristics and hydrogenation / dehydrogenation behaviour of Mg-bearing composite materials Mg-x wt.% CFMmNi 5 prepared through mechanical alloying. The composite materials Mg-x wt.% CFMmNi 5 have been successfully synthesized through ball-milling (mechanical alloying) by employing a high energy attritor mill. The mechanical alloying has been carried out in hexane medium by varying the milling parameters say, speed (revolutions per minute) and milling duration. The as-milled composite materials have been activated at 4006108C under a hydrogen pressure of |35–40 kg cm 22 . These composite materials exhibit high hydrogen storage capacity and fast absorption / desorption kinetics in comparison to the thermally melted counterparts. It has been found that the highest storage capacity material (|5.4 wt.% at 3508C) corresponds to Mg-30 wt.% CFMmNi 5 . The composite material also exhibits fast desorption kinetics (about 90 cm 3 min 21 ), which is at least two times faster than conventionally prepared (RF melting) alloys. The highest hydrogen storage capacity and fast kinetics were obtained for the mechanically alloyed samples with the optimized milling conditions, i.e. speed |400 rev min 21 and time duration of 5 h. The hydriding rate and the improved hydrogen storage capacity of these composite materials have been found to be strongly correlated with structural and microstructural characteristics as brought out through XRD and SEM techniques. The uniform particle size distribution and interfacial grain boundaries explored from the SEM investigations paves the way for better hydrogen storage capacity and fast absorption and desorption kinetics. 1999 Elsevier Science S.A. All rights reserved. Keywords: Mg-bearing composite; Hydrogen absorption; Hydrogen storage capacity; Rare earth nickel composite; Mechanical alloying
1. Introduction Owing to the vulnerability, price rise and negative impact of fossil fuels on the environment, it is imperative to usher in renewable and clean energy sources. Of all the alternative fuels, hydrogen is now known to be a potential renewable substitute for petroleum or gasoline. As hydrogen is very light (¯10 5 times lighter than petroleum), to harness hydrogen, it has to be properly stored. In the search for viable and widely acceptable means of storing hydrogen, metal hydrides are being considered as a promising technology. For an effective automotive storage system, the hydride used should satisfy certain specific criteria. For example, it must be prepared from inexpensive metals, should have a low weight density, higher storage *Corresponding author. Tel.: 191-542-361-937; fax: 191-542-317468. E-mail address: [email protected] (O.N. Srivastava)
capacity ($3.0 wt.%), moderate dissociation temperature and good kinetics. The present known hydrides, even though viable, have the perennial problems related to rather low storage capacities. After a decade of extensive research in the area of metal hydrides, it has now become clear that present state-of-the-art alloy systems (namely LaNi 5 , FeTi and Mg 2 Ni) may lead to low storage capacities. For example, at ambient conditions, the hydrogen storage capacity corresponds to LaNi 5 51.5 wt.%, FeTi5 1.78 wt.%, and Mg 2 Ni with a high storage capacity of |3.8 wt.%, but can be operated only at high temperatures of 200–3008C. The present problem in regard to the improvement in the hydrogen storage capacity and fast desorption kinetics can be solved by the following new routes of material processing: (i) surface modification and optimization of the present storage systems LaNi 5 , FeTi and high temperature hydride Mg 2 Ni, (ii) development of altogether new storage materials, e.g. transition metal complexes, composite materials, nanocrystalline materials
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00272-8
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(nano particles), new carbon variants (fullerenes C 60 and other higher versions, carbon nano-fibers (CNF) and nanotubules (CNT)); it appears at present that the later routes may have a better chance of success. In particular the composite materials seem potentially promising in regard to obtaining viable hydrogen storage systems operated at ambient as well as high temperature conditions [1]. It has been already known from previous investigations that the light-weight magnesium-based rare earth alloy composite hydrides possess reversible hydrogen storage characteristics and fast absorption–desorption kinetics as compared to MgH 2 . For example, composite materials Mg-x wt.% LaNi 5 (x510–40) prepared using thermal sintering has been found to react with hydrogen at higher temperatures and form low stability hydrides with better storage capacity and hydriding behaviour than that of individual components, without increasing weight and cost [2]. Magnesium acts as binder in the composite Laves phase alloys, i.e. Mg-x wt.% ZrFe 1.4 Cr 0.6 and Mg-x wt.% TiMn 1.5 and their hydrogenation characteristics have been reported by two groups [3–4]. Some of the new and novel composite hydrogen storage materials have been investigated and explored in terms of pressure–composition– temperature (P–C–T) isotherms and desorption kinetics [5–12]. In our earlier communication [13], we have successfully demonstrated the hydrogenation behaviour of the composite alloy Mg-x wt.% CFMmNi 5 (x520–50) synthesized through thermal (RF induction) melting technique. However, it may be pointed out that the hydrogenation characteristics of the present materials prepared through mechanical alloying are distinctly different from that of the sample prepared through thermal melting. For example, the hydrogen storage capacity of |5.4 wt.% was obtained with lower temperature (300–3508C) as compared with the thermally melted alloys which desorbs at 400–5008C. Yet another difference relates to the faster desorption kinetics (about 90 cm 3 min 21 ) which is at least two times faster than that of the corresponding thermally melted composite alloys. This paper deals with the synthesis, characterization and hydrogenation behaviour of the composite materials Mg-x wt.% CFMmNi 5 prepared through mechanical alloying. It has been shown that the material corresponding to Mg-30 wt.% CFMmNi 5 prepared through ball-milling in hexane medium for 5 h and at a speed of 400 rev min 21 has the highest storage capacity of |5.4 wt.% at 3506108C. This material also exhibits fast desorption kinetics (about 90 cm 3 min 21 ). In order to understand the curious hydrogenation behaviour of these composite materials, structural / microstructural characterization have been undertaken by employing XRD and SEM. These revealed the presence of the parent ingredients Mg, CFMmNi 5 and small amount of a free Ni phase in the as-milled materials, whereas the thermally melted alloys exhibited a multiphasic nature. After hydrogenation, in addition to Mg, CFMmNi 5 and Ni
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phases, few peaks corresponding to MgH 2 were also found.
2. Experimental details
2.1. Synthesis of Mg-x wt.% CFMmNi5 composite materials First, the CFMmNi 5 (CFMm5cerium-free misch-metal) alloy was prepared through a solid-state diffusion process, by melting the stoichiometric mixtures of individual elements. High purity lanthanum (99.9%), neodymium (99.9%), praseodemium (99.9%) and nickel (99.5%) were taken in correct stoichiometric proportions, pressed into pellet forms (130.5 cm) to obtain an alloy with stoichiometry CFMmNi 5 . These were melted employing an RF induction furnace (18 kW) under an argon atmosphere in a double jacket silica tube with facilities for water flow in the outer jacket and argon flow in the inner jacket. The initially prepared alloy ingot was melted several times (5–6 times) to achieve homogeneity [14]. The agglomerate produced in this way was removed from the silica tube, crushed and subjected to X-ray diffraction characterization. The composite materials Mg-x wt.% CFMmNi 5 was prepared using mechanical alloying technique by taking a stoichiometric amount of magnesium (99.99% purity) and the pre-synthesized alloy CFMmNi 5 . The magnesium and alloy powders were sieved to achieve a particle size of about 100 mm. The mechanical alloying of these composite mixtures were carried out by employing a high energy ball mill (Sazegvari attritor system type 01 HD). It has a cylindrical container of inner diameter 13 cm, a volume about 1400 cc with an impellor and stainless steal balls of diameter 6 mm. In passing it should be mentioned that very recently Sapru et al. [15] reported a similar type of experiment which embodies the effect of processing parameters on Mg-based hydrogen storage materials prepared by mechanical alloying. The synthesis of composite materials Mg-x wt.% CFMmNi 5 were carried out under hexane medium by varying the concentration (x510–50) and the milling speed (100–400 rev min 21 ). The ball to powder weight ratio in all the experiments were confined to 20:1. After the mechanical alloying, the milled powder was extracted form the hexane and immediately transferred to an argon-filled glove box.
2.2. Structural, microstructural characteristics: XRD and SEM explorations The structural characterization of the as-milled composite materials and their hydrogenated versions were carried out through XRD. A Philips X-ray powder diffractometer PW-1710 equipped with graphite monochromator working ˚ was used for this with Cu Ka radiation ( l51.5418 A)
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purpose. The microstructural characteristics were examined by scanning electron microscopy employing a Philips XL-20 series SEM with 30 kV secondary electrons.
2.3. Activation procedure of Mg-x wt.% CFMmNi5 composite materials Since the as-synthesized composite materials do not desorb at room temperature, these were suitably activated. Several activation modes, including activation at high temperatures (100–4008C), were attempted for the asmilled Mg-x wt.% CFMmNi 5 composite materials. It was found that the most favourable activation process corresponded to that of an annealing treatment at 4006108C under hydrogen pressure of |35–40 kg cm 22 . A known quantity of finely granulated powder of Mg-x wt.% CFMmNi 5 composite material was placed in a reactor and was evacuated to the order of 10 24 Torr. About 40 kg cm 22 of hydrogen (purity 99.999%) was introduced form a high pressure gas cylinder. The bottom portion of the reactor was heated at a temperature 4006108C for 4–6 h continuously and then it was cooled to room temperature slowly (¯10 h). After the activation, the hydrogen was desorbed. The hydrogenation characteristics of these composite materials such as desorption kinetics and pressure–composition–temperature (P–C–T) isotherms were measured at different isothermal conditions using a Sieverts type apparatus fabricated in our laboratory and utilized earlier for hydrogenation studies of standard hydrogen storage materials [16].
3. Results and discussion
3.1. Hydrogenation /dehydrogenation characteristics The main aim of synthesizing the Mg-based composite materials, e.g. Mg-x wt.% CFMmNi 5 (x510–50) was to find out the viability and feasibility of utilizing them as high hydrogen storage capacity materials for automotive and other high temperature applications. Here we describe and discuss the hydrogenation / dehydrogenation behaviour such as pressure–composition isotherms (PCT), desorption kinetics and their correlation with structural / microstructural characteristics. The dehydriding behaviour of the composite materials Mg-x wt.% CFMmNi 5 milled under hexane medium with various milling durations (3, 5, 7 h) was evaluated and extensively analyzed for several values of x (10, 20, 30, 40, 50) at a temperature ranging from 300 to 3508C. Fig. 1 represents the desorption kinetic curves at a temperature of |3508C and at a pressure of 1–2 kg cm 22 for the composite material and Mg-x wt.% CFMmNi 5 with x510, 20, 30, 40 and 50 respectively, milled under hexane medium for 5 h duration. These results reveal that desorption rate was strongly affected by the intermetallic alloy
Fig. 1. Representative desorption kinetic curves for the composite material Mg-x wt.% CFMmNi 5 (x510, 20, 30, 40, 50) at temperature T|3508C and at a pressure P|1–2 kg cm 22 when milled under hexane medium for 5 h.
(CFMmNi 5 ) concentration. As is evident from Fig. 1, the best dehydriding kinetics and high hydrogen storage capacity belongs to the composite mixture Mg-30 wt.% CFMmNi 5 treated at 5 h time duration in an attritor ball mill. The initial release of hydrogen from the composite material was linear with time, and this decreased at longer times. From the hydrogen desorption vs. time curves (Fig. 1), it may be inferred that the initial hydriding–dehydriding rate is much faster and about two times better than the conventionally prepared (RF induction melted) Mg-x wt.% CFMmNi 5 composite alloys. It may also be noted that half of the saturation value was accomplished within 2 min. However, the total time required for complete saturation has been found to be |9–10 min. Moreover, the close observations of Fig. 1 paves the way for selecting the crucial composition in order to obtain the optimized composite material with high hydrogen storage capacity and fast kinetics. In the present investigation, the optimized composite material with high hydrogen storage capacity (|5.4 wt.% at 3508C) and faster desorption kinetics (|90 cm 3 min 21 ) corresponds to Mg-30 wt.% CFMmNi 5 , which was then further investigated in terms of different isothermal temperature conditions and milling time durations. Fig. 2 shows the representative desorption kinetic curves for the composite material Mg-30 wt.% CFMmNi 5 at three different temperatures – 300, 325 and 3508C with milling time duration of 5 h. It may be pointed out that the desorption rate strongly depends upon the
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Fig. 2. The amount of hydrogen desorbed vs. time (kinetic curves) during the dehydriding process of the composite material Mg-30 wt.% CFMmNi 5 at different isothermal temperature conditions (300, 325, 3508C) when milled under hexane medium with optimized time durations of t|5 h and at a speed of |400 rev min 21 .
temperature of desorption. It has been found that the composite material Mg-30 wt.% CFMmNi 5 possesses a high hydriding rate (|40–50 cm 3 min 21 ) and a high hydrogen storage capacity (|4.0 wt.%) at a relatively low temperature (3008C), which is 5–6 times faster than that of the magnesium hydride (MgH 2 ). The dependence of the desorption rate on the milling durations such as 3, 5 and 7 h are exhibited in Fig. 3. It shows that the fast desorption kinetics for the composite materials Mg-30 wt.% CFMmNi 5 were obtained at 3508C when milled under hexane medium for 5 h. It may be concluded from desorption kinetics curves (Figs. 1–3) that the rate of desorption was very much dependent on the various experimental parameters such as, (i) intermetallic alloy concentration in the composite materials, (ii) desorption temperature, and (iii) duration of milling. The representative P–C–T desorption isotherms of the as-prepared composite materials Mg-x wt.% CFMmNi 5 with x530 milled for different time durations (3, 5, 7 h) are shown in Fig. 4a–c. The P–C–T values were plotted at three different isothermal temperature conditions, e.g. 300, 325 and 3508C for each milling duration. It was observed that the hydrogen storage capacity of the composite materials mainly depends upon two factors, namely (i) temperature of desorption (range between 300 and 3508), and (ii) time duration of milling (e.g. 3, 5, 7 h). The plateau region (metal hydride phase) and the hydrogen
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Fig. 3. Desorption kinetic curves for the composite material Mg-30 wt.% CFMmNi 5 at temperature |3508C and pressure of |1–2 kg cm 22 when milled at different time durations (t51, 3, 5, 7 h) and a speed of |400 rev min 21 .
storage capacity of the P–C–T isotherms of the composite materials Mg-x wt.% CFMmNi 5 increases with increasing milling time durations, up to 5 h (e.g. t53, 5 h), thereafter it was found to decrease (e.g. t57 h) (see Fig. 5). Therefore, it is evident that the highest hydrogen storage capacity (|5.4 wt.% at 3508C) material corresponds to Mg-30 wt.% CFMmNi 5 milled under hexane medium with a speed of 400 rev min 21 and milling duration of |5 h. It may be pointed out that the optimized composite materials Mg-30 wt.% CFMmNi 5 possesses one of the highest known storage capacity which is better than that conventionally prepared (RF induction melted) alloys and also approximately matching the theoretical storage capacity (about 5.4 wt.%). There was only one plateau region as inferred from the P–C isotherms which clearly envisages that the as-milled samples possess only two phases of initial ingredients such as Mg and intermetallic phase CFMmNi 5 . The addition of the intermetallic alloy CFMmNi 5 to magnesium reduces the temperature of dissociation of hydrogen molecules on the composite metal hydride surface and tends to form the MgH 2 phase reversibly. Moreover, the P–C–T desorption isotherm curves exhibit plateaux at pressures of 1–2 kg cm 22 which clearly reveals the reversible desorption of the MgH 2 phase in the presence of the secondary alloy component (i.e.) CFMmNi 5 . In order to explore why the composition corresponding to (x530), i.e. Mg-30% CFMmNi 5 is the
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Fig. 4. Pressure composition isotherms for the mechanically alloyed composite materials Mg-30 wt.% CFMmNi 5 at various temperature (300, 325, 3508C) when milled at different time durations t of (a) 3, (b) 5, and (c) 7 h.
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Fig. 5. Variation of the hydrogen storage capacity of the composite material Mg-30 wt.% CFMmNi 5 with different milling durations and at different temperatures (300, 325, 3508C).
optimum composition in regard to high hydrogen storage capacity, we have extensively studied the behaviour of P–C isotherms by varying the alloy concentration x (e.g. 10, 20, 30, 40, 50). Fig. 6 shows the representative pressure–composition isotherm of the composite materials Mg-x wt.% CFMmNi 5 with several values of x (e.g. 10, 20 30, 40, 50) milled in an attritor mill for 5 h time durations. The P–C–T curves were plotted at 3508C similar as in Fig. 4a–c. It was clearly noticed that the hydrogen storage capacity varies with respect to the addition of CFMmNi 5 alloy to form composite materials (see Fig. 7). The optimized hydrogen storage capacity of |5.4 wt.% at
3508C corresponds to Mg-30 wt.% CFMmNi 5 , which is approximately approaching the value of MgH 2 . An important aspect of the present mechanically alloyed composite materials relates to the fact that after 10–15 hydrogenation–dehydrogenation cycles, the sample pulverized to fine particles (1–5 mm) without any deterioration in hydrogenation characteristics. Fig. 8 represents the P–C desorption isotherms of the composite materials Mg-30 wt.% CFMmNi 5 at 3508C for various numbers of hydrogenation cycles (runs) (e.g. 1, 3, 5, 7, 9). The observations revealed that the hydrogen storage capacity of |3.0 wt.% obtained during the initial cycle of hydrogenation increases
Fig. 6. Pressure–composition isotherms of the Mg-x wt.% CFMmNi 5 (x510, 20, 30, 40, 50) composite material at 3508C when milled under hexane medium for 5 h.
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3.2. Structural, microstructural characteristics: XRD and SEM explorations
Fig. 7. Hydrogen storage capacity vs. concentration of CFMmNi 5 curve of the Mg-30 wt.% CFMmNi 5 composite materials.
with increasing hydrogenation–dehydrogenation cycles and finally achieved the optimum hydrogen storage capacity (|5.4 wt.%) at the 9th cycle and remained constant even after several cycles. Henceforth it may be claimed that the composite materials Mg-x wt.% CFMmNi 5 (x510, 20, 30, 40, 50) possess good cyclic stability in regard to hydrogenation–dehydrogenation characteristics.
In order to unravel the curious hydrogenation behaviour of Mg-30 wt.% CFMmNi 5 , structural (X-ray diffraction, XRD), microstructural (scanning electron microscopy, SEM) characteristics were carried out. It was found that the hydrogenation–dehydrogenation characteristics are closely correlated with the microstructures and types of phases present in the composite materials. Fig. 9a shows the representative X-ray diffraction pattern of the mixture Mg-30 wt.% CFMmNi 5 before mechanical alloying. The starting material embodies the individual ingredients Mg and CFMmNi 5 which are seen in the form of a mixture. A small peak corresponding to free nickel was also observed in the sample. Fig. 9b–d represents the X-ray diffractogram taken from the mechanically alloyed composite materials Mg-30 wt.% CFMmNi 5 with different milling durations of 3, 5 and 7 h. The formation of the composite material is elucidated through a comparison of Fig. 9a with Fig. 9b–d. These results reveal that the mechanically alloyed composite materials of Mg-30 wt.% CFMmNi 5 still embody the initial ingredients magnesium, cerium-free misch-metal pentanickellide and nickel. This is important since it reveals that no other intermediate alloy such as Mg 2 Ni was formed. Thus the composite materials would be capable of exhibiting the hydrogenation / dehydrogenation of Mg in addition to the intermetallic alloy CFMmNi 5 . The small excess nickel concentration plays a vital role as catalyst, which in turn improves the hydrogenation–dehydrogenation characteristics of the composite materials. The XRD peaks however, exhibit changes in intensity and peak width. Extensive analysis of these results revealed that the peaks corresponding to magnesium decrease in intensity
Fig. 8. Representative P–C–T isotherm of the Mg-30 wt.% CFMmNi 5 composite material at a temperature of |3508C for various desorption cycles.
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Fig. 10. X-ray powder diffraction patterns of the hydrogenated composite materials Mg-30 wt.% CFMmNi 5 milled with various milling durations t of (a) 3, (b) 5, and (c) 7 h.
Fig. 9. X-ray powder diffraction patterns (a) for the mixture Mg-30 wt.% CFMmNi 5 before mechanical alloying; the composite materials Mg30wt.% CFMmNi 5 milled in the hexane medium at different milling time durations t of (b) 3, (c) 5, and (d) 7 h.
(e.g. 1600, 350, 125, 100 counts) and become broadened (increased FWHM) with increased milling durations of 3, 5 and 7 h. For example, compared to the 3-h ball-milling, the FWHM for the 5-h duration has increased by about 75%. The composite material showing optimized hydrogenation behaviour obtained for the ball-milling duration of 5 h exhibited particle sizes of |5–10 mm. The XRD patterns taken from the hydrogenated materials showed features broadly similar to those exhibited by the as-milled composite materials with regard to the observed phases along with a small concentration of MgH 2 phase which are depicted in Fig. 10a–c. It is expected that an important
parameter influencing hydrogenation–dehydrogenation is the presence of free Ni on the surface. As is known, the presence of free Ni or nickel bearing phases on the surface of the samples decrease the activation energy of dissociation of hydrogen molecules. The hydrogen storage capacity and desorption kinetics are probably dependent on the free nickel and nickel containing phases (such as CFMmNi 5 ) which form sites favourable for hydrogen reaction and may also serve as a channels by which the hydrogen can enter and exit the bulk of the main phase. In order to find out the changes in the microstructural features with milling parameters, particularly the duration of ball-milling, scanning electron micrograph employing secondary electrons have been undertaken. Fig. 11a,c,e exhibits the representative microstructural details of mechanically milled composite materials Mg-30 wt.% CFMmNi 5 with milling time durations of 3, 5 and 7 h, respectively. It can be easily seen from the micrographs that for the composite material with 5 h milling (Fig. 11c), the particle size of |10 mm has become uniformly distributed in comparison with Fig. 11a and e. Another noticeable fact from the microstructural characteristics of
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Fig. 11. Representative scanning electron micrographs of the as-milled (a,c,e) and hydrogenated (b,d,f) composite materials Mg-30 wt.% CFMmNi 5 with milling durations of 3, 5, and 7 h.
the as-milled composite materials with 5 h milled duration is that the particles do not agglomerate and remain separate, as individual entities. But for both the extreme cases such as the materials with 3 and 7 h milling time durations (Fig. 11a,e), the particle size increases due to coalescence and do not remain separate. A microstructure of the composite materials Mg-30 wt.% CFMmNi 5 milled with various time durations (e.g. 3, 5, 7 h) exhibiting extensive cracking and breakage of larger grains, obtained after activation and several hydrogenation–dehydrogenation cyclings, is shown in the SEM image of Fig. 11b,d,f. It can be seen that the effect of hydrogenation produces fracture and breakage of the surface into small domains, which could apparently provide a channel for hydrogen to diffuse in and out of the lattice. Thus, the improvements in desorption kinetics and the attainment of an optimized high hydrogen storage capacity after activation and repeated cycling of the composite material Mg-30 wt.% CFMmNi 5 can be qualitatively understood in terms of the following reasons: (i) hydrogenation of Mg particles, (ii) the availability of several hydrogen diffusion channels arising from interfacial grain boundaries between magnesium and alloy
component, (iii) the resulting volume expansion-induced cracking and consequent segregation of the metal surface due to the catalytic activities of free Ni and nickel-bearing phases, such as CFMmNi 5 , which leads to the continuous exposure of fresh surface for hydrogenation.
4. Conclusion In summary, it can be said that the mechanically alloyed composite materials Mg-x wt.% CFMmNi 5 represent a viable storage system with high hydrogen storage capacity and fast desorption kinetics. The mechanically alloyed composite materials exhibit the highest hydrogen storage capacity and fast absorption / desorption kinetics in comparison with the thermally melted counterparts. The best hydrogen storage materials with a storage capacity of |5.4 wt.% at 3508C were obtained for the composition Mg-30 wt.% CFMmNi 5 with a milling speed of 400 rev min 21 for 5 h is hexane medium. This composite material also exhibits fast desorption kinetics (about 90 cm 3 min 21 ) which is at least two times faster than conventionally
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prepared (RF melting) alloys. The hydrogenation behaviour such as hydriding rate and the improved hydrogen storage capacity of these composite materials has been correlated with the structural and microstructural characteristics. For example, the higher storage capacity and fast desorption kinetics can be explained based on the uniform particle size distribution and the availability of several hydrogen diffusion channels arising from interfacial grain boundaries.
Acknowledgements The authors are grateful to Professor M. Groll (University of Stuttgart, Germany), Professor T.N. Veziroglu (President, IAHE, Florida, USA), Professor A.R. Verma, Professor Y.C. Simhadri and Dr B.A. Danassanacharya for encouragement. The financial support of the Ministry of Non-Conventional Energy Sources (MNES), Council for Scientific and Industrial Research (CSIR), UGC-DAEF and AICTE are gratefully acknowledged.
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