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Hydriding and dehydriding properties of nanostructured Mg2Ni alloy prepared by the process of hydriding combustion synthesis and subsequent mechanical grinding
Journal of Alloys and Compounds 425 (2006) 235–238
Hydriding and dehydriding properties of nanostructured Mg2Ni alloy prepared by the process of hydriding combustion synthesis and subsequent mechanical grinding Xiaofeng Liu 1 , Yunfeng Zhu, Liquan Li ∗ College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China Received 25 January 2006; accepted 29 January 2006 Available online 28 February 2006
Abstract In this study, we synthesized a nanostructured Mg2 Ni alloy by means of hydriding combustion synthesis and subsequent mechanical grinding. Results on the structural change with mechanical grinding and the hydriding/dehydriding properties are presented. It was found that the nanostructured Mg2 Ni showed great improvement in hydriding rates at 313 and 373 K, and the reaction of dehydriding began at around 370 K, which is 190 K lower than that of the HCS product. Several factors were considered accounting for the enhancement in hydriding and dehydriding properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; Intermetallics; Mg-based alloys; Hydriding combustion synthesis; High-energy ball milling
1. Introduction Magnesium and magnesium-based alloys are still of particular interest for hydrogen storage because they are advantageous in specific hydrogen capacity (7.2 wt.% for pure Mg and 3.6 wt.% for Mg2 Ni), cost and environmental impact. They have been considered as potential candidates for mobile applications [1] despite their high thermodynamic stability (−75 kJ/mol H2 for pure Mg) and poor hydriding/dehydriding kinetics. To date, numerous investigations have been carried out to overcome these obstacles so as to make them meet the needs for on-board storage, and much progress has been achieved. In this paper, we report the excellent hydriding/dehydrding properties of a nanostructured Mg2 Ni synthesized by a novel approach combining the hydriding combustion synthesis (HCS) and mechanical grinding. The process of HCS, first proposed in 1997 by Yagi et al., has been regarded as an innovative method for directly synthesis of Mg2 Ni hydride by a simple process, and was well-known for its ∗ 1
Corresponding author. Tel.: +86 25 83587255. E-mail address: [email protected] (L. Li). Graduate student.
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.073
high activity of product that is free of activation [2], as well as the fast hydriding rates of its product [3]. We have already systematically investigated the reaction mechanism [4], the optimization of the synthesis conditions [5,6], the structural evolution [7] and the mechanism contributing to the high hydriding activity of the product of HCS [8]. Recently, notable improvement in hydriding and dehydriding properties of nanostructured magnesium-based alloys prepared by mechanical alloying and mechanical grinding has been reported [9–11]. The mechanical process generally involves the reduction of grain size and the formation of various defects, which shorten the diffusion length and increase the number of active sites for hydrogenation. Furthermore, metal hydrides of Mg2 NiH4 and MgH2 [12] had been found to be destabilized by mechanical grinding, resulting in a marked decrease in dehydriding temperature. In this study, in order to enrich the knowledge of HCS and to decrease the working temperature for magnesium-based hydrogen storage alloys, we investigate the effect of mechanical grinding on the hydriding and dehydriding properties of the HCS product. The results of this study are expected to provide significant information for on-board hydrogen storage for magnesium alloys.
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2. Experimental 2.1. Sample preparation Commercially available magnesium (<147 m in diameter and 99.9 wt.% in purity) and nickel (2–3 m in diameter and 99.9 wt.% in purity) powders were used for hydriding combustion synthesis. Details of the HCS process have been described elsewhere [13]. Before grinding, the as-synthesized powders were mechanically crushed into powders of less than 147 m in diameter. Mechanical grinding was performed on a planetary milling apparatus equipped with two stainless steel vials of 50 cm3 . After filling the vial with the powders of HCS, the anti-sticking agent graphite (5% in weight) and stainless steel balls (six big balls of 10 mm in diameter and 40 small balls of 6 mm in diameter, corresponding to a ball to powder ratio of 30:1), the system was evacuated for several minutes using a rotary pump. Afterwards, argon atmosphere was introduced and then the system was evacuated again to remove air from the vial. Finally, the raw materials were subjected to the mechanical grinding of 200 rpm for durations up to 40 h under 0.1 MPa argon.
2.2. Sample characterization Since both the product of HCS and the grinded product were in its hydride state, they were dehydrided prior to measurement of hydriding property. For the product of HCS, it was first dehydrided at 573 K, and then cooled to 313 K to do the absorption. Absorption at 373 K was also carried out prior to a dehydrding process at 573 K. As for the grinded product, absorption at 313 and 373 K was carried out prior to a dehydriding process by keeping the specimen in dynamic vacuum at 473 K for 120 min. The temperature of 473 K was carefully determined so as to minimize structural change such as grain growth as well as crystallization in the amorphous region [11], and the dehydriding duration of 120 min was long enough for the specimen to release all the hydrogen atoms. The dehydring property was measured by heating the specimen in the evacuated sample chamber at a rate of 20 K/min. A similar technique was used by Nick E. Tran et al. [14]. The temperature and the pressure changes in the sample chamber were digitally recorded at a regular interval of 6 s; as a result the amount of hydrogen desorbed as a function of temperature can be derived. The structural change during mechanical grinding was examined by X-ray diffraction (XRD) using Cu K␣ radiation, at 40 kV, 20 mA.
3. Results and discussion The XRD patterns of the mechanical grinded HCS products for durations of 5, 10, 20 and 40 h are shown in Fig. 1. The XRD pattern of the product of HCS is also shown in Fig. 1 marked as “0 h”. From the result of Rietveld analysis using the software Rietica, the as-synthesized product of HCS contained 79 wt.% Mg2 NiH4 and 21 wt.% Mg2 NiH0.3 . The solid solution phase Mg2 NiH0.3 is an ordinary composition of the HCS product [15]. The intensities of diffraction peaks of Mg2 NiH4 decreased markedly after only 5 h of grinding. These peaks even disappeared completely after grinding for only 10 h. However, the peak intensities of Mg2 NiH0.3 did not changed so rapidly. This result indicates that the solid state amorphization of Mg2 NiH4 may occur preferentially, due to its brittle character comparing with that of Mg2 NiH0.3 . The diffused peaks of Mg2 NiH0.3 can be unambiguously identified even after grinding for 40 h. According to Scherrer equation, the crystal size of Mg2 NiH0.3 was roughly estimated, changing from 25 nm after grinding for 5 h to 8 nm after grinding for 20 h. Therefore, the asgrinded HCS product can be described as a nanostructured composite with nanosized crystallites embedded in its amorphous matrix.
Fig. 1. XRD patterns of the mechanical grinded HCS products for durations of 5, 10, 20 and 40 h in an argon atmosphere of 0.1 MPa. The pattern shown as “0 h” is from the product of HCS.
Hydriding rates at 313 and 373 K of the HCS product and the 40 h grinded product, respectively, are shown in Fig. 2. We can see that the hydriding rates increased markedly after grinding. Only 100 s or so was required for the 40 h grinded product to achieve the saturated hydrogen capacity of 2.76 wt.% at 313 K and 2.78 wt.% at 373 K. Moreover, the temperature did not seem to be a rate delaying factor for hydriding if we comparing the shape of the hydriding curve of 313 K with that of 373 K. However, the product of HCS reacts with hydrogen at a relative low rate at both temperatures of 313 and 373 K comparing with the grinded product. The increase in hydriding rates after mechanical grinding is likely attributed to the appearance of various defects and decrease of diffusion of length because of intensive milling [16,17]. Moreover, the HCS product by itself has considerable activity actually as shown in Fig. 2. It absorbed 0.5 wt.% of hydrogen within 200 s at 373 K, due to its unique
Fig. 2. Hydriding rates of the HCS product and the mechanical grinded product at 313 and 373 K, respectively. The initial hydrogen pressure was 3.0 MPa.
X. Liu et al. / Journal of Alloys and Compounds 425 (2006) 235–238
structure [7,8,13] comparing with that of the product prepared by a conventional ingot metallurgy process. This must also have contributed to the fast hydriding rates of the grinded product. However, the saturated hydrogen capacities of grinded product at 313 or 373 K were 0.6 wt.% or so lower than the theoretical value of 3.42 wt.% for Mg2 Ni-5 wt.% graphite. Our results indicated that the hydriding in higher temperatures did not show any increment in the saturated capacity. This can be explained by considering the effect of carbon, which was used as a lubricant in this study and may have dissolved, in a small amount, into the interstitial sites of the milled Mg2 Ni alloy. Funaki et al. has confirmed that carbon atoms dissolved into the sites of an amorphous MgNi alloy that otherwise would have been occupied by hydrogen, as a result the hydrogen capacity decrease with increasing carbon amount [18]. Besides the effect of carbon, we cannot exclude completely the oxidation due to the un-removed oxygen during HCS, and the contamination of iron during mechanical grinding that will also decrease the hydrogen capacity. The amounts of hydrogen desorbed as a function of the temperature during heating in vacuum of the HCS product as well as the grinded product are shown in Fig. 3. The dehydriding onset (the temperature at which hydrogen begins to release from the hydride) of the HCS product was at around 560 K and this process occurred rapidly over a narrow temperature range. The amount of hydrogen desorbed at 650 K was 2.92 wt.%, which agrees well with the result of Rietveld analysis (79 wt.% of Mg2 NiH4 and 21 wt.% of Mg2 NiH0.3 corresponding to a overall hydrogen content of 2.91%). This result confirmed the validity of the method used for measuring the dehydriding property in the present study. After grinding for 40 h, the dehydriding onset was around 370 K as shown in Fig. 3, which was 190 K lower than that of HCS product. All hydrogen atoms were released from the grinded product at round 570 K because the curve was flat after 570 K. The sample for the curve of the re-hydrogenated product
237
in Fig. 3 was obtained by dehydriding the as milled product at 473 K and subsequent re-hydrogenation at 313 K. Its dehydriding onset did not shift to higher temperatures and the dehydriding rate did not change obviously, indicating the grinded product exhibited fairy good cyclic stability. The great decrease in the dehydriding onset by mechanical grinding of HCS product suggests that the hydride was thermodynamically destabilized. From the XRD results, the diffraction intensities of the peaks of Mg2 NiH4 decreased rapidly, implying the remarkable decrease in the crystal size and the increase in the volume fraction of grain boundary region. The mobility of hydrogen atoms located at the distorted grain boundary is higher comparing with those located at the inter-grain region; this may lead to a cooperative dehydriding phenomenon between these two regions [19]. Furthermore, mechanical grinding result in completely destruction of oxide layer covered in the alloy surface. This can improve hydriding rates by removing the barriers for hydrogen diffusion on the one hand. On the other hand, the graphite particle that uniformly dispersed in the nanostructured product with grinding have been found to prevent the restoration of this layer in hydriding/dehydridng cycles and consequently accelerate back diffusion of hydrogen atoms in the process of dehydriding [20,21]. A systematic investigation concentrated on the microstructure and thermal stability of the grinded HCS product will be reported later. 4. Conclusions Mechanical grinding the product of HCS results in remarkable improvement in hydriding and dehydridng properties. The grinded product could absorb 2.76 wt.% within 100 s at 313 K. The dehydriding reaction of the grinded product and the rehydrogenated grinded product started at around 370 K, which is 190 K lower than that of the HCS product. These results are very attractive for the mobile applications of magnesium-based hydrogen storage alloys. Acknowledgements This research was supported by National Natural Science Foundation of China (Grant No. 50371037). Valuable suggestions from Dr. Dongming Liu on the experiment are gratefully acknowledged. References
Fig. 3. Plots of the temperature vs. the desorbed amount of hydrogen of the HCS product as well as the mechanical grinded products. The average heating rate was 20 K/min.
[1] L. Schlapbach, A. Z¨uttel, Nature 414 (2001) 353. [2] T. Akiyama, L. Li, J. Yagi, Proceedings of the Fourth Pacific Rim International Conference on advanced Materials and Processing (PRICM4), Hawaii, 2001, p. 469. [3] X. Liu, L. Li, Mater. Sci. Forum 488/489 (2005) 901. [4] L. Li, T. Akiyama, J. Yagi, Intermetallics 7 (1999) 671. [5] L. Li, T. Akiyama, J. Yagi, J. Alloys Compd. 308 (2000) 98. [6] L. Li, T. Akiyama, T. Kabutomori, J. Yagi, Mater. Trans. 40 (1999) 1079. [7] L. Li, I. Saita, T. Akiyama, Intermetallics 13 (2005) 662. [8] D. Liu, L. Li, Int. J. Hydrogen Energy (in press). [9] T. Hong, S. Kim, Y. Kim, J. Alloys Compd. 312 (2000) 60.
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[10] R. Janot, L. Aymard, A. Rougier, G.A. Nazri, J.M. Tarascon, J. Phys. Chem. Solid 65 (2004) 529. [11] S. Orimo, H. Fujii, K. Ikeda, Acta Mater. 45 (1997) 331. [12] A. Zaluska, L. Zaluski, J.O. Str¨om-Olsen, J. Alloys Compd. 289 (1999) 197. [13] I. Saita, L. Li, K. Saito, T. Akiyama, J. Alloys Compd. 356/357 (2003) 490. [14] N.E. Tran, M.A. Imam, C.R. Feng, J. Alloys Compd. 359 (2003) 225. [15] L. Li, I. Saita, K. Saito, T. Akiyama, J. Alloys Compd. 345 (2002) 189.
[16] C. Iwakura, S. Nohara, S.G. Zhang, H. Inoue, J. Alloys Compd. 345 (1999) 246. [17] H. Inoue, T. Ueda, S. Nohara, N. Fujita, C. Iwakura, et al., Electrochim. Acta 3 (1998) 2215. [18] K. Funaki, S. Orimo, H. Fujii, H. Sumida, J. Alloys Compd. 270 (1998) 160. [19] H. Fujii, S. Orimo, K. Ikeda, J. Alloys Compd. 253/254 (1997) 80. [20] J.-L. Bobet, E. Grigorova, M. Khrussanova, J. Alloys Compd. 366 (2004) 298. [21] S. Bouaricha, J.-P. Dodelet, D. Guay, J. Huot, R. Schulz, J. Mater. Res. 16 (2001) 2893.
Hydriding and dehydriding properties of nanostructured Mg2Ni alloy prepared by the process of hydriding combustion synthesis and subsequent mechanical grinding Xiaofeng Liu 1 , Yunfeng Zhu, Liquan Li ∗ College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China Received 25 January 2006; accepted 29 January 2006 Available online 28 February 2006
Abstract In this study, we synthesized a nanostructured Mg2 Ni alloy by means of hydriding combustion synthesis and subsequent mechanical grinding. Results on the structural change with mechanical grinding and the hydriding/dehydriding properties are presented. It was found that the nanostructured Mg2 Ni showed great improvement in hydriding rates at 313 and 373 K, and the reaction of dehydriding began at around 370 K, which is 190 K lower than that of the HCS product. Several factors were considered accounting for the enhancement in hydriding and dehydriding properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; Intermetallics; Mg-based alloys; Hydriding combustion synthesis; High-energy ball milling
1. Introduction Magnesium and magnesium-based alloys are still of particular interest for hydrogen storage because they are advantageous in specific hydrogen capacity (7.2 wt.% for pure Mg and 3.6 wt.% for Mg2 Ni), cost and environmental impact. They have been considered as potential candidates for mobile applications [1] despite their high thermodynamic stability (−75 kJ/mol H2 for pure Mg) and poor hydriding/dehydriding kinetics. To date, numerous investigations have been carried out to overcome these obstacles so as to make them meet the needs for on-board storage, and much progress has been achieved. In this paper, we report the excellent hydriding/dehydrding properties of a nanostructured Mg2 Ni synthesized by a novel approach combining the hydriding combustion synthesis (HCS) and mechanical grinding. The process of HCS, first proposed in 1997 by Yagi et al., has been regarded as an innovative method for directly synthesis of Mg2 Ni hydride by a simple process, and was well-known for its ∗ 1
Corresponding author. Tel.: +86 25 83587255. E-mail address: [email protected] (L. Li). Graduate student.
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.073
high activity of product that is free of activation [2], as well as the fast hydriding rates of its product [3]. We have already systematically investigated the reaction mechanism [4], the optimization of the synthesis conditions [5,6], the structural evolution [7] and the mechanism contributing to the high hydriding activity of the product of HCS [8]. Recently, notable improvement in hydriding and dehydriding properties of nanostructured magnesium-based alloys prepared by mechanical alloying and mechanical grinding has been reported [9–11]. The mechanical process generally involves the reduction of grain size and the formation of various defects, which shorten the diffusion length and increase the number of active sites for hydrogenation. Furthermore, metal hydrides of Mg2 NiH4 and MgH2 [12] had been found to be destabilized by mechanical grinding, resulting in a marked decrease in dehydriding temperature. In this study, in order to enrich the knowledge of HCS and to decrease the working temperature for magnesium-based hydrogen storage alloys, we investigate the effect of mechanical grinding on the hydriding and dehydriding properties of the HCS product. The results of this study are expected to provide significant information for on-board hydrogen storage for magnesium alloys.
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2. Experimental 2.1. Sample preparation Commercially available magnesium (<147 m in diameter and 99.9 wt.% in purity) and nickel (2–3 m in diameter and 99.9 wt.% in purity) powders were used for hydriding combustion synthesis. Details of the HCS process have been described elsewhere [13]. Before grinding, the as-synthesized powders were mechanically crushed into powders of less than 147 m in diameter. Mechanical grinding was performed on a planetary milling apparatus equipped with two stainless steel vials of 50 cm3 . After filling the vial with the powders of HCS, the anti-sticking agent graphite (5% in weight) and stainless steel balls (six big balls of 10 mm in diameter and 40 small balls of 6 mm in diameter, corresponding to a ball to powder ratio of 30:1), the system was evacuated for several minutes using a rotary pump. Afterwards, argon atmosphere was introduced and then the system was evacuated again to remove air from the vial. Finally, the raw materials were subjected to the mechanical grinding of 200 rpm for durations up to 40 h under 0.1 MPa argon.
2.2. Sample characterization Since both the product of HCS and the grinded product were in its hydride state, they were dehydrided prior to measurement of hydriding property. For the product of HCS, it was first dehydrided at 573 K, and then cooled to 313 K to do the absorption. Absorption at 373 K was also carried out prior to a dehydrding process at 573 K. As for the grinded product, absorption at 313 and 373 K was carried out prior to a dehydriding process by keeping the specimen in dynamic vacuum at 473 K for 120 min. The temperature of 473 K was carefully determined so as to minimize structural change such as grain growth as well as crystallization in the amorphous region [11], and the dehydriding duration of 120 min was long enough for the specimen to release all the hydrogen atoms. The dehydring property was measured by heating the specimen in the evacuated sample chamber at a rate of 20 K/min. A similar technique was used by Nick E. Tran et al. [14]. The temperature and the pressure changes in the sample chamber were digitally recorded at a regular interval of 6 s; as a result the amount of hydrogen desorbed as a function of temperature can be derived. The structural change during mechanical grinding was examined by X-ray diffraction (XRD) using Cu K␣ radiation, at 40 kV, 20 mA.
3. Results and discussion The XRD patterns of the mechanical grinded HCS products for durations of 5, 10, 20 and 40 h are shown in Fig. 1. The XRD pattern of the product of HCS is also shown in Fig. 1 marked as “0 h”. From the result of Rietveld analysis using the software Rietica, the as-synthesized product of HCS contained 79 wt.% Mg2 NiH4 and 21 wt.% Mg2 NiH0.3 . The solid solution phase Mg2 NiH0.3 is an ordinary composition of the HCS product [15]. The intensities of diffraction peaks of Mg2 NiH4 decreased markedly after only 5 h of grinding. These peaks even disappeared completely after grinding for only 10 h. However, the peak intensities of Mg2 NiH0.3 did not changed so rapidly. This result indicates that the solid state amorphization of Mg2 NiH4 may occur preferentially, due to its brittle character comparing with that of Mg2 NiH0.3 . The diffused peaks of Mg2 NiH0.3 can be unambiguously identified even after grinding for 40 h. According to Scherrer equation, the crystal size of Mg2 NiH0.3 was roughly estimated, changing from 25 nm after grinding for 5 h to 8 nm after grinding for 20 h. Therefore, the asgrinded HCS product can be described as a nanostructured composite with nanosized crystallites embedded in its amorphous matrix.
Fig. 1. XRD patterns of the mechanical grinded HCS products for durations of 5, 10, 20 and 40 h in an argon atmosphere of 0.1 MPa. The pattern shown as “0 h” is from the product of HCS.
Hydriding rates at 313 and 373 K of the HCS product and the 40 h grinded product, respectively, are shown in Fig. 2. We can see that the hydriding rates increased markedly after grinding. Only 100 s or so was required for the 40 h grinded product to achieve the saturated hydrogen capacity of 2.76 wt.% at 313 K and 2.78 wt.% at 373 K. Moreover, the temperature did not seem to be a rate delaying factor for hydriding if we comparing the shape of the hydriding curve of 313 K with that of 373 K. However, the product of HCS reacts with hydrogen at a relative low rate at both temperatures of 313 and 373 K comparing with the grinded product. The increase in hydriding rates after mechanical grinding is likely attributed to the appearance of various defects and decrease of diffusion of length because of intensive milling [16,17]. Moreover, the HCS product by itself has considerable activity actually as shown in Fig. 2. It absorbed 0.5 wt.% of hydrogen within 200 s at 373 K, due to its unique
Fig. 2. Hydriding rates of the HCS product and the mechanical grinded product at 313 and 373 K, respectively. The initial hydrogen pressure was 3.0 MPa.
X. Liu et al. / Journal of Alloys and Compounds 425 (2006) 235–238
structure [7,8,13] comparing with that of the product prepared by a conventional ingot metallurgy process. This must also have contributed to the fast hydriding rates of the grinded product. However, the saturated hydrogen capacities of grinded product at 313 or 373 K were 0.6 wt.% or so lower than the theoretical value of 3.42 wt.% for Mg2 Ni-5 wt.% graphite. Our results indicated that the hydriding in higher temperatures did not show any increment in the saturated capacity. This can be explained by considering the effect of carbon, which was used as a lubricant in this study and may have dissolved, in a small amount, into the interstitial sites of the milled Mg2 Ni alloy. Funaki et al. has confirmed that carbon atoms dissolved into the sites of an amorphous MgNi alloy that otherwise would have been occupied by hydrogen, as a result the hydrogen capacity decrease with increasing carbon amount [18]. Besides the effect of carbon, we cannot exclude completely the oxidation due to the un-removed oxygen during HCS, and the contamination of iron during mechanical grinding that will also decrease the hydrogen capacity. The amounts of hydrogen desorbed as a function of the temperature during heating in vacuum of the HCS product as well as the grinded product are shown in Fig. 3. The dehydriding onset (the temperature at which hydrogen begins to release from the hydride) of the HCS product was at around 560 K and this process occurred rapidly over a narrow temperature range. The amount of hydrogen desorbed at 650 K was 2.92 wt.%, which agrees well with the result of Rietveld analysis (79 wt.% of Mg2 NiH4 and 21 wt.% of Mg2 NiH0.3 corresponding to a overall hydrogen content of 2.91%). This result confirmed the validity of the method used for measuring the dehydriding property in the present study. After grinding for 40 h, the dehydriding onset was around 370 K as shown in Fig. 3, which was 190 K lower than that of HCS product. All hydrogen atoms were released from the grinded product at round 570 K because the curve was flat after 570 K. The sample for the curve of the re-hydrogenated product
237
in Fig. 3 was obtained by dehydriding the as milled product at 473 K and subsequent re-hydrogenation at 313 K. Its dehydriding onset did not shift to higher temperatures and the dehydriding rate did not change obviously, indicating the grinded product exhibited fairy good cyclic stability. The great decrease in the dehydriding onset by mechanical grinding of HCS product suggests that the hydride was thermodynamically destabilized. From the XRD results, the diffraction intensities of the peaks of Mg2 NiH4 decreased rapidly, implying the remarkable decrease in the crystal size and the increase in the volume fraction of grain boundary region. The mobility of hydrogen atoms located at the distorted grain boundary is higher comparing with those located at the inter-grain region; this may lead to a cooperative dehydriding phenomenon between these two regions [19]. Furthermore, mechanical grinding result in completely destruction of oxide layer covered in the alloy surface. This can improve hydriding rates by removing the barriers for hydrogen diffusion on the one hand. On the other hand, the graphite particle that uniformly dispersed in the nanostructured product with grinding have been found to prevent the restoration of this layer in hydriding/dehydridng cycles and consequently accelerate back diffusion of hydrogen atoms in the process of dehydriding [20,21]. A systematic investigation concentrated on the microstructure and thermal stability of the grinded HCS product will be reported later. 4. Conclusions Mechanical grinding the product of HCS results in remarkable improvement in hydriding and dehydridng properties. The grinded product could absorb 2.76 wt.% within 100 s at 313 K. The dehydriding reaction of the grinded product and the rehydrogenated grinded product started at around 370 K, which is 190 K lower than that of the HCS product. These results are very attractive for the mobile applications of magnesium-based hydrogen storage alloys. Acknowledgements This research was supported by National Natural Science Foundation of China (Grant No. 50371037). Valuable suggestions from Dr. Dongming Liu on the experiment are gratefully acknowledged. References
Fig. 3. Plots of the temperature vs. the desorbed amount of hydrogen of the HCS product as well as the mechanical grinded products. The average heating rate was 20 K/min.
[1] L. Schlapbach, A. Z¨uttel, Nature 414 (2001) 353. [2] T. Akiyama, L. Li, J. Yagi, Proceedings of the Fourth Pacific Rim International Conference on advanced Materials and Processing (PRICM4), Hawaii, 2001, p. 469. [3] X. Liu, L. Li, Mater. Sci. Forum 488/489 (2005) 901. [4] L. Li, T. Akiyama, J. Yagi, Intermetallics 7 (1999) 671. [5] L. Li, T. Akiyama, J. Yagi, J. Alloys Compd. 308 (2000) 98. [6] L. Li, T. Akiyama, T. Kabutomori, J. Yagi, Mater. Trans. 40 (1999) 1079. [7] L. Li, I. Saita, T. Akiyama, Intermetallics 13 (2005) 662. [8] D. Liu, L. Li, Int. J. Hydrogen Energy (in press). [9] T. Hong, S. Kim, Y. Kim, J. Alloys Compd. 312 (2000) 60.
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[10] R. Janot, L. Aymard, A. Rougier, G.A. Nazri, J.M. Tarascon, J. Phys. Chem. Solid 65 (2004) 529. [11] S. Orimo, H. Fujii, K. Ikeda, Acta Mater. 45 (1997) 331. [12] A. Zaluska, L. Zaluski, J.O. Str¨om-Olsen, J. Alloys Compd. 289 (1999) 197. [13] I. Saita, L. Li, K. Saito, T. Akiyama, J. Alloys Compd. 356/357 (2003) 490. [14] N.E. Tran, M.A. Imam, C.R. Feng, J. Alloys Compd. 359 (2003) 225. [15] L. Li, I. Saita, K. Saito, T. Akiyama, J. Alloys Compd. 345 (2002) 189.
[16] C. Iwakura, S. Nohara, S.G. Zhang, H. Inoue, J. Alloys Compd. 345 (1999) 246. [17] H. Inoue, T. Ueda, S. Nohara, N. Fujita, C. Iwakura, et al., Electrochim. Acta 3 (1998) 2215. [18] K. Funaki, S. Orimo, H. Fujii, H. Sumida, J. Alloys Compd. 270 (1998) 160. [19] H. Fujii, S. Orimo, K. Ikeda, J. Alloys Compd. 253/254 (1997) 80. [20] J.-L. Bobet, E. Grigorova, M. Khrussanova, J. Alloys Compd. 366 (2004) 298. [21] S. Bouaricha, J.-P. Dodelet, D. Guay, J. Huot, R. Schulz, J. Mater. Res. 16 (2001) 2893.