- Email: [email protected]
Comparative study on the controlled hydriding combustion synthesis and the microwave synthesis to prepare Mg2Ni from micro-particles
international journal of hydrogen energy 35 (2010) 3129–3135
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Comparative study on the controlled hydriding combustion synthesis and the microwave synthesis to prepare Mg2Ni from micro-particles Qian Li a,*, Jing Liu a, Yang Liu a, Kuo-Chih Chou a,b a b
Shanghai Key Laboratory of Modern Metallurgy & Materials processing, Shanghai University, Shanghai 200072, PR China Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China
article info
abstract
Article history:
The present work is devoted to studying the influence of the application of the different
Received 8 April 2009
physical fields of magnetic field and microwave field during the preparation process of
Accepted 8 July 2009
Mg2Ni hydrogen storage alloys on its physicochemical properties including thermody-
Available online 9 October 2009
namic and kinetic characteristics, hydrogen absorption/desorption properties, phase composition and morphology. The advantage and disadvantage of the two different
Keywords:
preparation methods were discussed and the possible reasons for the effect of the external
Hydrogen storage alloys
physical field on the hydrogenation properties of the alloy were analyzed.
Physical field
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Thermodynamic properties Kinetics
1.
Introduction
In recent years, there are considerable researches on magnesium and its alloys for on-board hydrogen storage due to their high hydrogen storage capacity by weight and low cost. It is well known that the hexagonal Mg metal phase has a maximum gravimetric hydrogen capacity of 7.6 mass% (MgH2) with good reversibility [1,2]. However, the main disadvantages of Mg-based hydrogen storage materials are the relatively high working temperatures (e.g. w573 K for Mg2NiH4), slow desorption kinetics and a high reactivity toward air and oxygen [3,4]. Moreover, the problem of thermodynamics and kinetics of Mg-based metal hydrides is a significant obstacle for practical on-board application, and further improvement in performance is needed for storage media applications. A lot of research has been recently conducted on the metal hydrides for improving adsorption/ desorption properties based on hydrogen storage capacity,
kinetics, decomposition temperature, thermal properties, toxicity, cycling behavior and cost. It mainly focuses on the following aspects: (1) element substitution [5]; (2) element doping [6]; (3) preparation of composite materials [7]; (4) new production methods [8,9]; (5) surface modification [10]; (6) heat treatment or annealing [11]; (7) nanometer material or amorphous material [4]. Kinetic as well as thermodynamic properties can be affected by alloy composition, crystal structure and morphology. It is well known that preparation method is a significant factor influencing hydriding/dehydriding characteristics of hydrogen storage alloys. Since Reilly et al. [3] discovered the reversible hydrogen adsorption ability of the Mg2Ni to form the ternary hydride Mg2NiH4 in 1968, its physical and chemical properties have been studied. Many efforts has been focused on Mg-based hydrides in recent years to reduce the desorption temperature and fasten the re/dehydrogenation reactions. These can be accomplished to some
* Corresponding author. Tel.: þ86 21 56334045; fax: þ86 21 56338065. E-mail address: [email protected] (Q. Li). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.07.121
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international journal of hydrogen energy 35 (2010) 3129–3135
composition and morphology) of Mg2Ni alloy. The two different preparation methods were compared and the possible reasons for the effect of the external physical fields on the hydrogenation properties of the alloy were analyzed.
2.
Fig. 1 – The processing thermal conditions for preparation of samples A, B and C.
extent by changing the microstructure of the hydride through different preparation methods. For example, mechanical milling (MM) and hydriding combustion synthesis (HCS) are both well-known methods for production of magnesiumbased hydrogen storage alloys. Intermetallic Mg2Ni has been successfully formed using mainly high energy ball milling [4] or in a low energy ball milling [12]. It is important for developing an industrial process of hydriding combustion synthesis of hydrogen storage alloy Mg2NiH4 within only one temperature scanning without any activation process [8]. Mg–Ni hydrogen storage alloy films were grown by simultaneous magnetron sputter co-deposition of Mg and Ni atoms and Ar ion irradiation under negative bias voltage [13]. A novel compound synthesized in an Mg–Ni system by a high-pressure technique has a hydrogen capacity of 2.23 mass% [9]. Nanocrystalline Mg–Ni alloy powder was produced via the magnetron co-sputtering technique [14]. Mg–Ni multilayer and Ni-rich Mg thin films were deposited by electron gun and pulsed laser deposition [15]. An innovative method, isothermal evaporation casting process, is developed to produce Mg2Ni alloy for mass production [16]. On the other hand, the Mg2Ni hydrides which possess beneficial functional properties can be obtained under the physical field. For instance, Si et al. [17] adopted the laser sintering method to prepare the Mg2Ni hydrogen storage samples. Bystrzycki et al. [18] studied the hydrogen sorption properties of nanocrystalline Mg2Ni intermetallics prepared by mechanical milling under controlled shearing/impact mode in a magnetic Uni– Ball–Mill 5. It has been observed [19] that the Mg2NiH4 prepared by microwave-assisted activation synthesis possesses high hydrogen storage capacity and excellent kinetic properties. La–Mg–Ni ternary alloy and Mg2FeH6 have been successfully prepared under an external magnetic field in our group recently [20], Mg2FeH6 can absorb hydrogen as much as 6.75 mass% within 1000 s [21]. In this work, we explored the influence of different preparation methods with and without external physical fields (high magnetic field and microwave field) on the adsorption/ desorption properties (thermodynamic and kinetic characteristics, hydrogen absorption/desorption properties, phase
Experimental details
According to the stoichiometric proportion of Mg2Ni, 2:1 atomic ratio of Mg (>99.5 mass% purity, 100–200 mesh) and Ni (>99.5 mass% purity, w200 mesh) were used as raw materials, which were well-mixed by an ultrasonic homogenizer in ethanol for 45 min. After being completely dried in the cabinet, they were compressed by a uniaxial single-acting press into form pellets of 5 mm in height and 15 mm in diameter. Then, the wafers were crushed to small fragments. The fragments were put in the heart of the furnace and were synthesized by the conventional hydriding combustion synthesis (HCS) for sample A and by the controlled hydriding combustion synthesis (CHCS) for sample B, respectively. Both experiments have been conducted twice: with and without a high magnetic field. The experimental apparatus for magnet was described in our previous work [22]. The processing
Fig. 2 – XRD patterns of Mg2Ni prepared by different methods before (I) and after (II) hydriding reaction.
international journal of hydrogen energy 35 (2010) 3129–3135
Fig. 3 – SEM morphologies of Mg2Ni prepared by different methods before hydriding reaction: (a) sample A; (b) sample B; (c) sample C.
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Fig. 4 – SEM morphologies of Mg2Ni prepared by different methods after hydriding reaction: (a) sample A; (b) sample B; (c) sample C.
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thermal conditions for preparation of samples were presented as Fig. 1. As far as preparation method by microwave-assisted activation synthesis is concerned, the sample C was obtained by the same method as described in our previous paper [19]. The pressure-composition-temperature (PCT) and kinetic properties were measured by a volumetric method according to the Sievert’s law using the automatic apparatus from SUZUKI HOKAN CO., LTD. in Japan. The X-ray intensity was measured over a diffraction angle from 10 to 90 with a velocity of 0.02 per step and 2 /min. The diffraction patterns were analyzed by a whole pattern fitting procedure using the standard program diffraction files in order to determine the structural properties and the weight concentration of each crystalline phase. The crystallite size was calculated using Hall formula (detailed in [23]) and the values were retained if the difference in crystallite size determined by methods was smaller than 10%. The morphologies were studied by scanning electronic microscopy (SEM) with an energy dispersive X-ray analysis system (EDS). Phase composition and lattice parameter of samples were analyzed and calculated by software Jade 5.0.
3.
activation synthesis can be hydrogenated easily under the current conditions (573 K and 4 MPa). The temperature of the hydriding/dehydriding reaction can be reduced in a certain extent attributed to the effect of the microwave field. The synthesis temperature is 779 K holding for 15 min, which is the same as the eutectic temperature of magnesium and nickel system but lower than the synthesis temperature of the induction melting (823 K). However, it is
Results and discussion
3.1. Microstructure, composition and morphology of Mg2Ni prepared by different methods X–ray diffraction patterns of Mg2Ni prepared by different methods are given in Fig. 2, from which it can be seen that only the peaks of Mg and Ni and a trace of MgH2 exist in the sample A. It suggests that no Mg2Ni can be directly obtained from Mg and Ni at 673 K and under 4 MPaH2 without an external magnetic field, and under the same condition, a little amount of Mg can be hydrogenated into MgH2. After subsequent hydriding reaction (see Fig. 2 (II)), the structure changed to the major phase Mg2NiH4 and a little amount of unreacted Mg and Ni. The effect of the magnetic magnitude on the structure of the composite can be observed from the X-ray pattern of sample B, which is composed of MgH2 þ Mg2NiH4 þ a little of Ni þ a trace of Mg, and it indicates that the magnetic intensity of 4 T is not enough to assure the completed reaction between Ni and Mg. The sample B after hydrogenation is made of mainly the phase Mg2NiH4 plus a small amount of Mg2NiH0.3. Sample C is made of Mg and Ni after the treatment of the microwave field. After being hydrogenation activated in the PCT equipment, the major phases of sample C are Mg2NiH4 and Mg2NiH3.85, and small amount of Ni was found in sample C. According to Hall equation, the grain size is estimated to be ca. 1520, 410 and 253 nm for samples A, B and C, respectively. It can be seen from Fig. 1 that the synthesis temperature of Mg2NiH4 can be decreased to 673 K holding for 180 min under an external magnetic field. The minimum synthesis temperature of Mg2NiH4 is 753 K holding for 60–540 min using the conventional hydriding combustion synthesis reported by Li [24], who thought a slightly lower synthesis temperature than the eutectic temperature (779 K) was quite beneficial and available for hydriding combustion synthesis of Mg2NiH4. Sample C prepared by microwave-assisted
Fig. 5 – Pressure-composition isotherms at different temperatures of (a) sample A, (b) sample B and (c) sample C prepared by different synthesis approaches.
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international journal of hydrogen energy 35 (2010) 3129–3135
Table 1 – Van’t Hoff equations of the samples A, B and C for hydrogen absorption and desorption and their corresponding calculated entropies and enthalpies.
DH (kJ/mol) DS (J/mol K) lg (P/0.1 MPa)(523 K T 623 K) DH (kJ/mol) DS (J/mol K) lg (P/0.1 MPa)(523 K T 623 K) DH (kJ/mol) DS (J/mol K) lg (Peq/0.1 MPa)(523 K T 573 K) lg (Peq/0.1 MPa) lg (Peq/0.1 MPa) lg (Peq/0.1 MPa) lg (Peq/0.1 MPa)
Absorption
Desorption
Ref.
61.1 117.9 3191/T þ 6.158 64.0 123.3 3346/T þ 6.442 60.9 5.0 118.6 10.0 3181/T þ 6.195 3211/T þ 6.222 3245/T þ 6.273 3245/T þ 6.273 3525/T þ 6.667
66.2 124.1 3458/T þ 6.480 68.4 128.7 3575/T þ 6.722 61.9 1.2 115.7 1.9 3216/T þ 6.025 3300/T þ 6.261 3360/T þ 6.383 3306/T þ 6.300 3724/T þ 6.883
Sample A
well known that without any treatment on the raw magnesium and nickel powder, a hydriding/dehydriding reaction would never be successful. The SEM morphologies of the samples A, B and C before and after hydriding are shown in Figs. 3 and 4, respectively. It is clearly seen that the surfaces of all samples are irregular. The particles of sample A and sample B before hydriding have a size of 2–10 mm and are well dispersed. There is little difference between the particle sizes of the samples A and B before hydriding. Comparing Fig. 3(a) and 3(b) with Fig. 3(c), it shows that the particles in the sample C treated by microwave field are uniformly distributed. The particle size is about 1–3 mm, and its shape is sub-polyhedral. It can be seen from Fig. 4 that all samples are well distributed. Since the sample graininess does not become obviously smaller, the pulverization-resistance ability of the material is strengthened. This ability indicates that the samples possess a long cycling life, which was confirmed in literature [19].
Sample B
Sample C
[1] [1] [1] [2]
3.3. Kinetic properties of Mg2Ni prepared by different methods Fig. 6 presents the kinetic curves of hydrogen absorption and desorption in all samples from 523 K to 573 K. As is evident from the shape of the hydriding curve, all samples exhibit
3.2. Thermodynamic properties of Mg2Ni prepared by different methods Fig. 5 gives the PCT curves of all samples in absorption and desorption of hydrogen at 573 K, 553 K and 523 K. These alloys exhibit the long, flat plateaus of PCT curves and the high quantity of desorbed hydrogen, which reaches w97% of the maximum hydrogen storage capacity. Compared Fig. 5(a) with Fig. 5(b), it is easy to see that the effect of the magnetic field on the hydrogen storage capacity is not obvious, because the reversible hydrogen contents of sample A and sample B are about 3.5 mass%. However, Fig. 5(c) shows that the reversible hydrogen content of sample C is about 3.0 mass%, which might be caused by the un-optimization preparation process for the sample C prepared under the microwave field. Van’t Hoff equations of the hydriding/dehydriding processes in all samples were calculated and the relationships between the temperature and the equilibrium plateau pressure, which were obtained from Fig. 5 at hydrogen composition of 1.75 mass%, were also analyzed. Table 1 summaries the thermodynamic information obtained from the equilibrium plateau pressure and temperature in the hydrogen absorption and desorption procedure in the present work as well as those from literatures [1,2].
Fig. 6 – The overall kinetic curves of hydrogen absorption and desorption for sample A, sample B and sample C prepared by different synthesis approaches at different temperatures.
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international journal of hydrogen energy 35 (2010) 3129–3135
excellent kinetic properties at 523 K and under 3 MPa of H2 or 4 MPa of H2 and reach 90% of their maximum capacity within 20–60 s. It took about 20 s is needed to reach 90% of the maximum absorption capacity for sample B and sample C and about 60 s for the same capacity in sample A. As mentioned above, after the physical field was introduced into the preparation process of Mg2Ni hydrogen storage alloys, it can impose positive effect on their hydriding/dehydriding kinetic properties. The same effect appears in the dehydriding process. However, it is still unclear that is the origin of the effect induced by the physical field on the kinetic property of hydrogen storage alloys thoroughly. In this particular case, a possible explanation for the improvement of kinetics could be the formation of new structure and the decrease of reaction activation energy due to the transmission of energy into materials from the physical field. On the other hand, the reason that the kinetic properties of the sample prepared by microwave-assisted activation synthesis are significantly improved is the activation effect of the microwave field, which might decrease the reaction activation energy.
4.
Conclusions
A technique saving time and energy has been developed for preparing Mg2Ni hydrogen storage alloys and the influence of the physical fields on the physicochemical properties of Mg2Ni is analyzed and some more important conclusions from this research are listed below. (a) From the comparison of the hydrogen absorption/ desorption capacities and the relationships between the temperature and the equilibrium plateau pressure in all samples, it can be seen that magnetic field and microwave field have influence on the thermodynamic properties as well as kinetic properties of Mg2Ni. (b) The DH and DS in the hydrogen absorption and desorption reactions of Mg2Ni prepared by different synthesis approaches are calculated from the Van’t Hoff equation with the least-squares fitting of the PCT experimental data. The relationships between equilibrium plateau pressure and temperature in the hydrogen absorption and desorption procedure are then derived. (c) The reason why the magnetic field and microwave field impose positive effect on their hydriding/dehydriding properties can be preliminarily discussed from microstructure, composition and morphology of Mg2Ni prepared by different methods.
Acknowledgements The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (2007AA05Z118), the National Natural Science Foundation of China (50804029), a Foundation for the Author of National Excellent Doctoral Dissertation of China (200746), the Program for Changjiang Scholars and Innovative Research Team in University (IRT0739) and the Innovation Fund for Graduate Students of Shanghai University.
references
[1] Song MY, Park HR. Pressure–composition isotherms in the Mg2Ni–H2 system. J Alloys Compd 1998;270:164–7. [2] Li L, Akiyama T, Yagi J. Reaction mechanism of hydriding combustion synthesis of Mg2NiH4. Intermetallics 1999;7: 671–7. [3] Reilly JJ, Wiswall RH. The reaction of hydrogen with alloys of magnesium and nickel and the formation of Mg2NiH4. J Inorg Chem 1968;7:2254–6. [4] Zaluska A, Zaluski L, Stro¨m-Olsen JO. Nanocrystalline magnesium for hydrogen storage. J Alloys Compd 1999;288: 217–25. [5] Grigorova E, Khristov M, Khrussanova M, Bobet J-L, Peshev P. Effect of additives on the hydrogen sorption properties of mechanically alloyed composites based on Mg and Mg2Ni. Int J Hydrogen Energy 2005;30:1099–105. [6] Zhang YS, Yang HB, Yuan HT, Yang ED, Zhou ZX, Song DY. Dehydriding properties of ternary Mg2Ni1xZrx hydrides synthesized by ball milling and annealing. J Alloys Compd 1998;269:278–83. [7] Barkhordarian G, Klassen T, Bormann R. Effect of Nb2O5 content on hydrogen reaction kinetics of Mg. J Alloys Compd 2004;364:242–6. [8] Li L, Akiyama T, Yagi J. Activation behaviors of Mg2NiH4 at different hydrogen pressure in hydriding combustion synthesis. Int J Hydrogen Energy 2001;26:1035–40. [9] Kamegawa A, Goto Y, Kataoka R, Takamura H, Okada M. High-pressure synthesis of novel compounds in an Mg–Ni system. Renewable Energy 2008;33:221–5. [10] Kim JS, Lee CR, Choi JW, Kang SG. Effects of F-treatment on degradation of Mg2Ni electrode fabricated by mechanical alloying. J Power Sources 2002;104:201–7. [11] Spassov T, Solsona P, Surinˇach S, Baro´ MD. Optimization of the ball-milling and heat treatment parameters for synthesis of amorphous and nanocrystalline Mg2Ni-based alloys. J Alloys Compd 2003;349:242–54. [12] Gennari FC, Esquivel MR. Structural characterization and hydrogen sorption properties of nanocrystalline Mg2Ni. J Alloys Compd 2008;459:425–32. [13] Wirth E, Milcius D, Pranevicius LL, Noreusc D, Sato T, Templier C. Influence of ion irradiation effects on the hydriding behavior of nanocrystalline Mg–Ni films. Vacuum 2007;81:1224–8. [14] Wirth E, Milcius D, Filiou C, Nore´us D. Exploring the hydrogen sorption capacity of Mg–Ni powders produced by the vapour deposition technique. Int J Hydrogen Energy 2008; 33:3122–7. [15] Patel N, Kale A, Mosaner P, Checchettoa R, Miotello A, Das G. Deuterium thermal desorption from Ni-rich deuterated Mg thin films. Renewable Energy 2008;33:232–6. [16] Hsu CW, Lee SL, Jeng RR, Lin JC. Mass production of Mg2Ni alloy bulk by isothermal evaporation casting process. Int J Hydrogen Energy 2007;32:4907–11. [17] Si TZ, Liu DM, Zhang QA. Microstructure and hydrogen storage properties of the laser sintered Mg2Ni alloy. Int J Hydrogen Energy 2007;32:4912–6. [18] Bystrzycki J, Czujko T, Varin RA, Mizera J. The effect of milling mode on the hydriding properties of nanocrystalline Mg2Ni. Solid State Phenomena 2004; 99–100:137–40. [19] Liu Y, Li Q, Lin GW, Chou KC, Xu KD. Properties of Mg2NiH4 prepared by microwave-assisted activation synthesis from micro-particles. J Alloys Compd 2009;468: 455–61. [20] Li Q, Lu XG, Chou KC, Xu KD, Zhang JY, Chen SL. Feasibility study on the controlled hydriding combustion synthesis of
international journal of hydrogen energy 35 (2010) 3129–3135
Mg–La–Ni ternary hydrogen storage composite. Int J Hydrogen Energy 2007;32:1875–84. [21] Li Q, Liu J, Chou KC, Lin GW, Xu KD. Synthesis and dehydrogenation behavior of Mg–Fe–H system prepared under an external magnetic field. J Alloys Compd 2008;466: 146–52. [22] Li Q, Xu KD, Chou KC, Lu XG, Zhang JY, Lin GW. Synthesis and hydrogenation properties of Mg–3mol%LaNi3 composite
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prepared under an external magnetic field. Intermetallics 2007;15:61–8. [23] Li Q, Lin Q, Chou KC, Jiang LJ, Zhan F. Hydrogen storage properties of mechanically alloyed Mg-8mol%LaNi0.5 composite. J Mater Res 2004;19:2871–6. [24] Li LQ, Saita I, Saito K, Akiyama T. Effect of synthesis temperature on the purity of product in hydriding combustion synthesis of Mg2NiH4. J Alloys Compd 2002;345:189–95.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Comparative study on the controlled hydriding combustion synthesis and the microwave synthesis to prepare Mg2Ni from micro-particles Qian Li a,*, Jing Liu a, Yang Liu a, Kuo-Chih Chou a,b a b
Shanghai Key Laboratory of Modern Metallurgy & Materials processing, Shanghai University, Shanghai 200072, PR China Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China
article info
abstract
Article history:
The present work is devoted to studying the influence of the application of the different
Received 8 April 2009
physical fields of magnetic field and microwave field during the preparation process of
Accepted 8 July 2009
Mg2Ni hydrogen storage alloys on its physicochemical properties including thermody-
Available online 9 October 2009
namic and kinetic characteristics, hydrogen absorption/desorption properties, phase composition and morphology. The advantage and disadvantage of the two different
Keywords:
preparation methods were discussed and the possible reasons for the effect of the external
Hydrogen storage alloys
physical field on the hydrogenation properties of the alloy were analyzed.
Physical field
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Thermodynamic properties Kinetics
1.
Introduction
In recent years, there are considerable researches on magnesium and its alloys for on-board hydrogen storage due to their high hydrogen storage capacity by weight and low cost. It is well known that the hexagonal Mg metal phase has a maximum gravimetric hydrogen capacity of 7.6 mass% (MgH2) with good reversibility [1,2]. However, the main disadvantages of Mg-based hydrogen storage materials are the relatively high working temperatures (e.g. w573 K for Mg2NiH4), slow desorption kinetics and a high reactivity toward air and oxygen [3,4]. Moreover, the problem of thermodynamics and kinetics of Mg-based metal hydrides is a significant obstacle for practical on-board application, and further improvement in performance is needed for storage media applications. A lot of research has been recently conducted on the metal hydrides for improving adsorption/ desorption properties based on hydrogen storage capacity,
kinetics, decomposition temperature, thermal properties, toxicity, cycling behavior and cost. It mainly focuses on the following aspects: (1) element substitution [5]; (2) element doping [6]; (3) preparation of composite materials [7]; (4) new production methods [8,9]; (5) surface modification [10]; (6) heat treatment or annealing [11]; (7) nanometer material or amorphous material [4]. Kinetic as well as thermodynamic properties can be affected by alloy composition, crystal structure and morphology. It is well known that preparation method is a significant factor influencing hydriding/dehydriding characteristics of hydrogen storage alloys. Since Reilly et al. [3] discovered the reversible hydrogen adsorption ability of the Mg2Ni to form the ternary hydride Mg2NiH4 in 1968, its physical and chemical properties have been studied. Many efforts has been focused on Mg-based hydrides in recent years to reduce the desorption temperature and fasten the re/dehydrogenation reactions. These can be accomplished to some
* Corresponding author. Tel.: þ86 21 56334045; fax: þ86 21 56338065. E-mail address: [email protected] (Q. Li). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.07.121
3130
international journal of hydrogen energy 35 (2010) 3129–3135
composition and morphology) of Mg2Ni alloy. The two different preparation methods were compared and the possible reasons for the effect of the external physical fields on the hydrogenation properties of the alloy were analyzed.
2.
Fig. 1 – The processing thermal conditions for preparation of samples A, B and C.
extent by changing the microstructure of the hydride through different preparation methods. For example, mechanical milling (MM) and hydriding combustion synthesis (HCS) are both well-known methods for production of magnesiumbased hydrogen storage alloys. Intermetallic Mg2Ni has been successfully formed using mainly high energy ball milling [4] or in a low energy ball milling [12]. It is important for developing an industrial process of hydriding combustion synthesis of hydrogen storage alloy Mg2NiH4 within only one temperature scanning without any activation process [8]. Mg–Ni hydrogen storage alloy films were grown by simultaneous magnetron sputter co-deposition of Mg and Ni atoms and Ar ion irradiation under negative bias voltage [13]. A novel compound synthesized in an Mg–Ni system by a high-pressure technique has a hydrogen capacity of 2.23 mass% [9]. Nanocrystalline Mg–Ni alloy powder was produced via the magnetron co-sputtering technique [14]. Mg–Ni multilayer and Ni-rich Mg thin films were deposited by electron gun and pulsed laser deposition [15]. An innovative method, isothermal evaporation casting process, is developed to produce Mg2Ni alloy for mass production [16]. On the other hand, the Mg2Ni hydrides which possess beneficial functional properties can be obtained under the physical field. For instance, Si et al. [17] adopted the laser sintering method to prepare the Mg2Ni hydrogen storage samples. Bystrzycki et al. [18] studied the hydrogen sorption properties of nanocrystalline Mg2Ni intermetallics prepared by mechanical milling under controlled shearing/impact mode in a magnetic Uni– Ball–Mill 5. It has been observed [19] that the Mg2NiH4 prepared by microwave-assisted activation synthesis possesses high hydrogen storage capacity and excellent kinetic properties. La–Mg–Ni ternary alloy and Mg2FeH6 have been successfully prepared under an external magnetic field in our group recently [20], Mg2FeH6 can absorb hydrogen as much as 6.75 mass% within 1000 s [21]. In this work, we explored the influence of different preparation methods with and without external physical fields (high magnetic field and microwave field) on the adsorption/ desorption properties (thermodynamic and kinetic characteristics, hydrogen absorption/desorption properties, phase
Experimental details
According to the stoichiometric proportion of Mg2Ni, 2:1 atomic ratio of Mg (>99.5 mass% purity, 100–200 mesh) and Ni (>99.5 mass% purity, w200 mesh) were used as raw materials, which were well-mixed by an ultrasonic homogenizer in ethanol for 45 min. After being completely dried in the cabinet, they were compressed by a uniaxial single-acting press into form pellets of 5 mm in height and 15 mm in diameter. Then, the wafers were crushed to small fragments. The fragments were put in the heart of the furnace and were synthesized by the conventional hydriding combustion synthesis (HCS) for sample A and by the controlled hydriding combustion synthesis (CHCS) for sample B, respectively. Both experiments have been conducted twice: with and without a high magnetic field. The experimental apparatus for magnet was described in our previous work [22]. The processing
Fig. 2 – XRD patterns of Mg2Ni prepared by different methods before (I) and after (II) hydriding reaction.
international journal of hydrogen energy 35 (2010) 3129–3135
Fig. 3 – SEM morphologies of Mg2Ni prepared by different methods before hydriding reaction: (a) sample A; (b) sample B; (c) sample C.
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Fig. 4 – SEM morphologies of Mg2Ni prepared by different methods after hydriding reaction: (a) sample A; (b) sample B; (c) sample C.
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international journal of hydrogen energy 35 (2010) 3129–3135
thermal conditions for preparation of samples were presented as Fig. 1. As far as preparation method by microwave-assisted activation synthesis is concerned, the sample C was obtained by the same method as described in our previous paper [19]. The pressure-composition-temperature (PCT) and kinetic properties were measured by a volumetric method according to the Sievert’s law using the automatic apparatus from SUZUKI HOKAN CO., LTD. in Japan. The X-ray intensity was measured over a diffraction angle from 10 to 90 with a velocity of 0.02 per step and 2 /min. The diffraction patterns were analyzed by a whole pattern fitting procedure using the standard program diffraction files in order to determine the structural properties and the weight concentration of each crystalline phase. The crystallite size was calculated using Hall formula (detailed in [23]) and the values were retained if the difference in crystallite size determined by methods was smaller than 10%. The morphologies were studied by scanning electronic microscopy (SEM) with an energy dispersive X-ray analysis system (EDS). Phase composition and lattice parameter of samples were analyzed and calculated by software Jade 5.0.
3.
activation synthesis can be hydrogenated easily under the current conditions (573 K and 4 MPa). The temperature of the hydriding/dehydriding reaction can be reduced in a certain extent attributed to the effect of the microwave field. The synthesis temperature is 779 K holding for 15 min, which is the same as the eutectic temperature of magnesium and nickel system but lower than the synthesis temperature of the induction melting (823 K). However, it is
Results and discussion
3.1. Microstructure, composition and morphology of Mg2Ni prepared by different methods X–ray diffraction patterns of Mg2Ni prepared by different methods are given in Fig. 2, from which it can be seen that only the peaks of Mg and Ni and a trace of MgH2 exist in the sample A. It suggests that no Mg2Ni can be directly obtained from Mg and Ni at 673 K and under 4 MPaH2 without an external magnetic field, and under the same condition, a little amount of Mg can be hydrogenated into MgH2. After subsequent hydriding reaction (see Fig. 2 (II)), the structure changed to the major phase Mg2NiH4 and a little amount of unreacted Mg and Ni. The effect of the magnetic magnitude on the structure of the composite can be observed from the X-ray pattern of sample B, which is composed of MgH2 þ Mg2NiH4 þ a little of Ni þ a trace of Mg, and it indicates that the magnetic intensity of 4 T is not enough to assure the completed reaction between Ni and Mg. The sample B after hydrogenation is made of mainly the phase Mg2NiH4 plus a small amount of Mg2NiH0.3. Sample C is made of Mg and Ni after the treatment of the microwave field. After being hydrogenation activated in the PCT equipment, the major phases of sample C are Mg2NiH4 and Mg2NiH3.85, and small amount of Ni was found in sample C. According to Hall equation, the grain size is estimated to be ca. 1520, 410 and 253 nm for samples A, B and C, respectively. It can be seen from Fig. 1 that the synthesis temperature of Mg2NiH4 can be decreased to 673 K holding for 180 min under an external magnetic field. The minimum synthesis temperature of Mg2NiH4 is 753 K holding for 60–540 min using the conventional hydriding combustion synthesis reported by Li [24], who thought a slightly lower synthesis temperature than the eutectic temperature (779 K) was quite beneficial and available for hydriding combustion synthesis of Mg2NiH4. Sample C prepared by microwave-assisted
Fig. 5 – Pressure-composition isotherms at different temperatures of (a) sample A, (b) sample B and (c) sample C prepared by different synthesis approaches.
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Table 1 – Van’t Hoff equations of the samples A, B and C for hydrogen absorption and desorption and their corresponding calculated entropies and enthalpies.
DH (kJ/mol) DS (J/mol K) lg (P/0.1 MPa)(523 K T 623 K) DH (kJ/mol) DS (J/mol K) lg (P/0.1 MPa)(523 K T 623 K) DH (kJ/mol) DS (J/mol K) lg (Peq/0.1 MPa)(523 K T 573 K) lg (Peq/0.1 MPa) lg (Peq/0.1 MPa) lg (Peq/0.1 MPa) lg (Peq/0.1 MPa)
Absorption
Desorption
Ref.
61.1 117.9 3191/T þ 6.158 64.0 123.3 3346/T þ 6.442 60.9 5.0 118.6 10.0 3181/T þ 6.195 3211/T þ 6.222 3245/T þ 6.273 3245/T þ 6.273 3525/T þ 6.667
66.2 124.1 3458/T þ 6.480 68.4 128.7 3575/T þ 6.722 61.9 1.2 115.7 1.9 3216/T þ 6.025 3300/T þ 6.261 3360/T þ 6.383 3306/T þ 6.300 3724/T þ 6.883
Sample A
well known that without any treatment on the raw magnesium and nickel powder, a hydriding/dehydriding reaction would never be successful. The SEM morphologies of the samples A, B and C before and after hydriding are shown in Figs. 3 and 4, respectively. It is clearly seen that the surfaces of all samples are irregular. The particles of sample A and sample B before hydriding have a size of 2–10 mm and are well dispersed. There is little difference between the particle sizes of the samples A and B before hydriding. Comparing Fig. 3(a) and 3(b) with Fig. 3(c), it shows that the particles in the sample C treated by microwave field are uniformly distributed. The particle size is about 1–3 mm, and its shape is sub-polyhedral. It can be seen from Fig. 4 that all samples are well distributed. Since the sample graininess does not become obviously smaller, the pulverization-resistance ability of the material is strengthened. This ability indicates that the samples possess a long cycling life, which was confirmed in literature [19].
Sample B
Sample C
[1] [1] [1] [2]
3.3. Kinetic properties of Mg2Ni prepared by different methods Fig. 6 presents the kinetic curves of hydrogen absorption and desorption in all samples from 523 K to 573 K. As is evident from the shape of the hydriding curve, all samples exhibit
3.2. Thermodynamic properties of Mg2Ni prepared by different methods Fig. 5 gives the PCT curves of all samples in absorption and desorption of hydrogen at 573 K, 553 K and 523 K. These alloys exhibit the long, flat plateaus of PCT curves and the high quantity of desorbed hydrogen, which reaches w97% of the maximum hydrogen storage capacity. Compared Fig. 5(a) with Fig. 5(b), it is easy to see that the effect of the magnetic field on the hydrogen storage capacity is not obvious, because the reversible hydrogen contents of sample A and sample B are about 3.5 mass%. However, Fig. 5(c) shows that the reversible hydrogen content of sample C is about 3.0 mass%, which might be caused by the un-optimization preparation process for the sample C prepared under the microwave field. Van’t Hoff equations of the hydriding/dehydriding processes in all samples were calculated and the relationships between the temperature and the equilibrium plateau pressure, which were obtained from Fig. 5 at hydrogen composition of 1.75 mass%, were also analyzed. Table 1 summaries the thermodynamic information obtained from the equilibrium plateau pressure and temperature in the hydrogen absorption and desorption procedure in the present work as well as those from literatures [1,2].
Fig. 6 – The overall kinetic curves of hydrogen absorption and desorption for sample A, sample B and sample C prepared by different synthesis approaches at different temperatures.
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excellent kinetic properties at 523 K and under 3 MPa of H2 or 4 MPa of H2 and reach 90% of their maximum capacity within 20–60 s. It took about 20 s is needed to reach 90% of the maximum absorption capacity for sample B and sample C and about 60 s for the same capacity in sample A. As mentioned above, after the physical field was introduced into the preparation process of Mg2Ni hydrogen storage alloys, it can impose positive effect on their hydriding/dehydriding kinetic properties. The same effect appears in the dehydriding process. However, it is still unclear that is the origin of the effect induced by the physical field on the kinetic property of hydrogen storage alloys thoroughly. In this particular case, a possible explanation for the improvement of kinetics could be the formation of new structure and the decrease of reaction activation energy due to the transmission of energy into materials from the physical field. On the other hand, the reason that the kinetic properties of the sample prepared by microwave-assisted activation synthesis are significantly improved is the activation effect of the microwave field, which might decrease the reaction activation energy.
4.
Conclusions
A technique saving time and energy has been developed for preparing Mg2Ni hydrogen storage alloys and the influence of the physical fields on the physicochemical properties of Mg2Ni is analyzed and some more important conclusions from this research are listed below. (a) From the comparison of the hydrogen absorption/ desorption capacities and the relationships between the temperature and the equilibrium plateau pressure in all samples, it can be seen that magnetic field and microwave field have influence on the thermodynamic properties as well as kinetic properties of Mg2Ni. (b) The DH and DS in the hydrogen absorption and desorption reactions of Mg2Ni prepared by different synthesis approaches are calculated from the Van’t Hoff equation with the least-squares fitting of the PCT experimental data. The relationships between equilibrium plateau pressure and temperature in the hydrogen absorption and desorption procedure are then derived. (c) The reason why the magnetic field and microwave field impose positive effect on their hydriding/dehydriding properties can be preliminarily discussed from microstructure, composition and morphology of Mg2Ni prepared by different methods.
Acknowledgements The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (2007AA05Z118), the National Natural Science Foundation of China (50804029), a Foundation for the Author of National Excellent Doctoral Dissertation of China (200746), the Program for Changjiang Scholars and Innovative Research Team in University (IRT0739) and the Innovation Fund for Graduate Students of Shanghai University.
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