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Fabrication and hydrogen storage property study of nanostructured Mg–Ni–B ternary alloys
Journal of Alloys and Compounds 479 (2009) 409–413
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Fabrication and hydrogen storage property study of nanostructured Mg–Ni–B ternary alloys Huaiyu Shao, Kohta Asano, Hirotoshi Enoki, Etsuo Akiba ∗ National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 10 November 2008 Received in revised form 11 December 2008 Accepted 16 December 2008 Available online 25 December 2008 Keywords: Hydrogen storage materials Nanostructures Mechanical alloying X-ray diffraction Transmission electron microscopy
a b s t r a c t Mg–Ni–B ternary alloys with nanostructure were produced by mechanical alloying method. Alloys with various structures were obtained. X-ray diffraction was used to define the structure and phase information as well as crystalline size of the alloys. Bright-field and dark-field transmission electron microscope (TEM) technique was applied to observe the morphology of the samples. Electron diffraction was used to confirm the structure of the alloys. The hydrogen storage properties of the obtained Mg–Ni–B alloys were studied by high pressure differential scanning calorimetry (DSC) and pressure–composition isotherm (PCT) methods. Body centered cubic (BCC) structure alloys show better hydrogen absorption properties than CsCl and Mg2 Ni type structure alloys in this work. Mg48 Ni48 B4 and Mg50 Ni45 B5 BCC alloys showed a hydrogen absorption content of 1.93 and 1.94 mass% at 373 K. Substitution or addition of a small amount of B to Mg50 Ni50 alloy is effective to improve hydrogen storage properties. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since Reilly and Wiswall reported Mg–Cu–H system [1] and Mg–Ni–H system [2] as hydrogen storage materials, Mg-based alloys have been widely studied. Mg-based alloys have several important advantages for potential hydrogen storage application, such as low price, large abundance in earth’s crust and high capacities of the hydrides (7.6 mass% for MgH2 , 3.6 mass% for Mg2 NiH4 , 4.5 mass% for Mg2 CoH5 and 5.5 mass% for Mg2 FeH6 ). However, Mg-based materials also face a severe obstacle for the application that Mg-based materials prepared by conventional melting method show quite poor hydrogen storage kinetics. Mg and Mg2 Ni usually need a hydrogen absorption temperature higher than 523 K. Mechanical alloying (MA) method has been widely applied to prepare Mg-based alloys. MA as a high energy operation of repeated welding, fracturing and re-welding of sample powders [3], is a novel and well-known synthesis technique to prepare nanostructured and non-equilibrium alloys. Recently, it has almost become the major preparation method to produce Mg-based hydrogen storage alloys [4–16] because the fresh surface area and defects formed during the milling process could greatly improve the hydrogen absorption properties of these Mg-based alloys. Our group has successfully obtained Mg–Co [17,18], Mg–Ti [19] and Mg–Ni [20] body centered cubic (BCC) alloys by mechanical
∗ Corresponding author. E-mail addresses: [email protected] (H. Shao), [email protected] (E. Akiba). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.067
alloying method. Compared with face centered cubic (FCC) and hexagonal close packing (HCP) lattice, BCC lattice has lower packing density and more octahedral and tetrahedral sites per atom, which means more interstitial sites are available for hydrogen occupancy in BCC lattice. This work is about the preparation and hydrogen storage study of Mg–Ni–B ternary alloys (mostly with BCC structure). The purpose is to study the effect of addition/substitution of boronthe lightest non-metal solid-state element on the preparation and hydrogen storage properties of Mg–Ni alloys. 2. Experimental details Mg–Ni–B ternary alloys were produced using a Fritsch P5 planetary ball mill. Fabrication results of some Mg–Ni alloys were used to compare with those of Mg–Ni–B ternary alloys. The detailed fabrication process of Mg–Ni binary alloys could be found here [20]. The samples are with the compositions shown in Table 1. The beginning samples were mixture of Mg (purity >99.9%, 100 mesh), Ni (purity >99.9%, 300 mesh) and B (purity >99.9%, 300 mesh) powders weighed according to the atomic ratio of x:y:(100−x–y) in Mgx Niy B100−x–y . The mixture samples of 2 g in weight and stainless-steel milling balls of 10 mm in diameter were put into the milling vials with a ball-to-powder weight ratio of 20:1. The vials were then closed with 0.1 MPa argon in a hermetic way. The whole weighting process was conducted in glove box with high purity argon as protecting atmosphere. The mechanical alloying process of these alloys was carried out with a rotation speed of 200 rpm and a milling duration of 200 h. Structure and phase analysis of the obtained alloy samples was conducted using a Rigaku RINT-2500 V diffractometer with Cu K␣ radiation at a generator voltage of 50 kV and a current of 200 mA. The diffraction data were collected at a scan rate of 0.8◦ /min with a scan step of 0.04◦ . The size distribution and morphology observation in bright-field and dark-field was carried out by transmission electron microscope (TEM) using JEOL JEM-2000FX II operating at the accelerating voltage of 200 kV. Electron diffraction technique was conducted to confirm the structure of the samples.
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H. Shao et al. / Journal of Alloys and Compounds 479 (2009) 409–413
Table 1 Structure and hydrogen storage properties of Mg–Ni–B system alloys. Sample no.
Composition
Structure
Lattice parameter (nm)
Hydrogen absorption content (mass%)
Hydrogen desorption content (mass%)
1 2
Mg50 Ni50 Mg60 Ni35 B5
BCC CsCl type
0.3008 0.3796
1.77 0.50
0.1 0.05
3
Mg60 Ni30 B10
Mg2 Ni type
a = 0.5206 c = 1.323
0.19
0
4 5 6 7 8 9 10 11 12 13
Mg50 Ni45 B5 Mg50 Ni40 B10 Mg50 Ni35 B15 Mg40 Ni55 B5 Mg40 Ni50 B10 Mg40 Ni45 B15 Mg48 Ni48 B4 Mg46 Ni46 B8 Mg44 Ni44 B12 Mg42 Ni42 B16
BCC BCC CsCl type BCC CsCl type CsCl type BCC BCC BCC BCC
0.3034 0.3069 0.3806 0.2958 0.3784 0.3809 0.3009 0.3018 0.3029 0.3039
1.94 1.84 0.71 1.18 1.10 0.14 1.93 1.76 1.73 1.53
0.1 0.07 0.11 0.47 0.38 0.07 0.25 0.25 0.18 0.21
Differential scanning calorimetry (DSC) measurements under hydrogen atmosphere were performed to study the thermodynamic properties of Mg–Ni–B alloys in hydrogen. DSC measurements were conducted using a Rigaku TP-8230HP apparatus under an initial hydrogen pressure of 4 MPa. The temperature was increased from room temperature to 773 K and then cooled to room temperature at a rate of 5 K/min. Pressure–composition isotherm (PCT) properties were evaluated using an automatic apparatus from Suzuki Shokan Company. The measurements were operated under a hydrogen pressure up to 7 MPa at 373 K. Before the measurements, the samples were evacuated, heated to 373 k and kept at this temperature for 2 h.
3. Results and discussion Fig. 1 presents the main structure types of Mg–Ni–B system alloys produced by mechanical alloying. The structure information is also shown in Table 1. In this figure, (a)–(d) indicates Mg60 Ni35 B5 , Mg60 Ni30 B10 , Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys after 200 h mechanical alloying process, respectively. Mg60 Ni35 B5 alloy mainly shows CsCl type structure (primitive cubic, space group: Pm3m) which is one of the typical structures for the Mg–Ni–B system alloys in this work. The lattice parameter of the cubic structure is 0.3796 nm (Table 1). From the XRD curve of Mg60 Ni35 B5 alloy, we could observe broadening of the diffraction peaks after mechanical alloying. This indicates the obtaining of very fine microstructure of the sample during the milling process. Besides the main reason of
Fig. 1. XRD patterns of Mg–Ni–B alloys with different structures: (a) Mg60 Ni35 B5 , (b) Mg60 Ni30 B10 , (c) Mg50 Ni45 B5 and (d) Mg48 Ni48 B4 .
fine crystalline size of the samples, some other factors could also cause peak broadening, such as plastic deformation and microstrain. By Scherrer equation [21], we calculated the crystalline size of CsCl-type-structure Mg60 Ni35 B5 alloy. The average crystalline size is about 10–20 nm. Mg60 Ni30 B10 alloy shows Mg2 Ni type structure (primitive hexagonal, space group: P62 22) after milling process. The lattice parameters are a = 0.5206 nm and c = 1.323 nm. We could also observe severe peak broadening in this sample. The crystalline size is calculated to be several nanometers by Scherrer equation. Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys show BCC structure (space group: Im3m) after 200 h milling. Peak broadening is quite obvious from Fig. 1(c) and (d). From the study of Mg–Ni system [20], we know that the BCC structure alloys consist of crystalline BCC phase and amorphous BCC phase. For a perfect amorphous phase, there is no certain lattice parameter in the sample. In our work, the amorphous phase shows similar local structure to the crystalline BCC phase [22,23]. The lattice parameter of Mg50 Ni45 B5 BCC alloy is 0.3034 nm and one for Mg48 Ni48 B4 BCC alloy is 0.3009 nm. These two BCC structure alloys are with a mean crystalline structure of 1–5 nm. Compared with Mg50 Ni50 , Mg50 Ni45 B5 sample could be considered as Mg50 Ni50 alloy with substitution of a certain mount of B to Ni. Mg48 Ni48 B4 sample could be taken as addition of some B to Mg50 Ni50 alloy. Fig. 1 only shows four samples with typical structures of Mg–Ni–B system alloys. The structure information of other samples is shown in Table 1. The various structures with different Mg–Ni–B compositions produced by mechanical alloying method
Fig. 2. Structure results of Mg–Ni–B system alloys with different compositions prepared by mechanical alloying.
H. Shao et al. / Journal of Alloys and Compounds 479 (2009) 409–413
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Fig. 3. Bright-field and dark-field TEM images of (a), (b) Mg60 Ni35 B5 alloy with CsCl structure; (c), (d) Mg50 Ni45 B5 BCC structure; and (e), (f) Mg48 Ni48 B4 BCC structure. Insets are the indexed electron diffraction patterns of corresponding alloys.
are presented in Fig. 2. It should be pointed out that Mg–Ni binary alloys reported elsewhere [20] were mechanical alloyed for 100 h. Sample no. 1 in Table 1 in this work was milled for 200 h. The preparation results of Mg–Ni binary alloys are adopted to make the production diagram of Fig. 2. From Fig. 2, we may see that for Mg–Ni binary alloys, which were milled for 100 h, there are three typical structures-FCC structure (Mg30 Ni70 ), BCC structure (Mg33 Ni67 , Mg40 Ni60 , Mg50 Ni50 and Mg60 Ni40 ) and Mg2 Ni type structure (Mg67 Ni33 ). We may simply conclude that for Mgx Ni100−x , when x is from 33 to 60, after 100 h milling in our work conditions, we can obtain BCC structure. From Table 1, we may see that Mg50 Ni50 after 200 h milling also shows BCC structure. For sample No. 3–Mg60 Ni30 B10 alloy, after 200 h milling, it shows Mg2 Ni type structure. Considering the fact of synthesis of the same structure in Mg67 Ni33 composition alloy, there seems to be a rule that when the atomic ratio of Mg to Ni in Mgx Niy B100−x–y composition is two, after 200 h milling, the alloy shows Mg2 Ni type structure.
For the formation of CsCl type structure after 200 h milling, now it is difficult for us to make any conclusion of the composition rule of how to obtain CsCl type structure in Mg–Ni–B system alloys. After 200 h milling, Mg50 Ni50 , Mg50 Ni45 B5 , Mg50 Ni40 B10 , Mg40 Ni55 B5 , Mg48 Ni48 B4 , Mg46 Ni46 B8 , Mg44 Ni44 B12 and Mg42 Ni42 B16 alloys show BCC structure. The lattice parameters are presented in Table 1. It is very interesting that when we compare these data of sample no. 1, 10–13, we find that with the increasing addition composition of B in Mg–Ni alloys, the lattice parameter is increasing. We know that the atomic size of B is much smaller than those of Mg and Ni. If B atom occupies the corner sites of Mg or Ni in the BCC lattice, the lattice parameter will be smaller when more B is added to the alloy. However, it is reverse. The only explanation we can think of is that after the mechanical alloying process, B occupies the interstitial sites while not the Mg or Ni corner sites in the BCC lattice. Fig. 3 presents TEM images of CsCl structure alloy sample (Mg60 Ni35 B5 ) and BCC alloy samples (Mg50 Ni45 B5 and
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Fig. 4. DSC curves of (a) Mg50 Ni50 BCC alloy ball milled for 100 h, (b) Mg50 Ni50 BCC alloy ball milled for 200 h, (c) Mg50 Ni45 B5 BCC alloy, (d) Mg48 Ni48 B4 BCC alloy, (e) Mg60 Ni35 B5 CsCl-type-structure alloy and (f) Mg60 Ni30 B10 Mg2 Ni-type-structure alloy.
Mg48 Ni48 B4 ). The insets are the selected area diffraction patterns for the corresponding samples. Fig. 3(a) and (b) shows the brightfield and dark-field photos of Mg60 Ni35 B5 sample. From the photos, especially the dark-field one, we may see that the crystalline size of the samples is tens of nanometers. This agrees with the size value calculated from the XRD patterns. The electron diffraction rings in the inset are well indexed to CsCl structure, which agrees with the XRD result. From Fig. 3(c)–(f), we may clearly see the crystalline structure in Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys. These alloys have a much smaller crystalline size than Mg60 Ni35 B5 CsCl structure alloys. The crystalline size for Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys is several nanometers. This size result is in good agreement with the ones calculated from the XRD curves by Scherrer equation. When we measure the radiuses of the electron diffraction rings in Fig. 3(c) and (e), we find that the radiuses (named R1 –R6 from inside to outside) are in accord with the rule expressed as: R12 : R22 : R32 : R42 : R52 : R62 = 1 : 2 : 3 : 4 : 5 : 6
(1)
This means Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys could be only indexed to BCC structure. The lattice parameter data obtained from these diffraction rings agree with the ones calculated from the XRD patterns. Fig. 4 indicates the DSC curves of the Mg–Ni–B system alloys. Samples in Fig. 4(a)–(d) are BCC structure alloys – Mg50 Ni50 BCC alloy ball milled for 100 h in Fig. 4(a); Mg50 Ni50 BCC alloy ball milled for 200 h in Fig. 4(b); Mg50 Ni45 B5 BCC alloy ball milled for 200 h in Fig. 4(c) and Mg48 Ni48 B4 BCC alloy ball milled for 200 h in Fig. 4(d). Fig. 4 (e) and (f) represent Mg60 Ni35 B5 CsCl-type-structure alloy ball milled for 200 h and Mg60 Ni30 B10 Mg2 Ni-type-structure alloy ball milled for 200 h, respectively. We have clarified the peak attributions of the five main peaks in the DSC curves of the Mg–Ni BCC alloys [20]. In Fig. 4(a), the first big exothermic peak at 432 K is due to the hydrogen absorption of the BCC alloy. After this reaction, the hydrogenated sample still keeps BCC structure. The second exothermic peak is a small one. It shows a peak temperature of 554 K. This peak is due to the hydrogen absorption again and decomposition of BCC hydride phase to Mg2 NiH4 , Ni and a small amount of MgNi2 . The endothermic peak at about 744 K in the heating process and the exothermic peak at 693 K in the cooling process are due to the sam-
ple (Mg2 NiH4 phase) desorbing hydrogen and absorbing hydrogen again. The small peak at 494 K in the cooling process is due to the transformation reaction from high temperature Mg2 NiH4 phase to low temperature one. Comparing Fig. 4(a) (Mg50 Ni50 BCC alloy ball milled for 100 h) with Fig. 4(b) (Mg50 Ni50 BCC alloy ball milled for 200 h), we may see that, 200 h-milled Mg50 Ni50 BCC alloy shows a first hydrogen absorption peak at a much earlier temperature (410 K) than 100 hmilled Mg50 Ni50 BCC alloy (432 K). The reason is thought to be the finer structure and more defects and strain produced during 100 more hours, which cause better hydrogen absorption kinetics. Mg50 Ni45 B5 and Mg48 Ni48 B4 BCC alloys show the first hydrogen absorption peak temperatures of 408 and 406 K, respectively, which are a bit lower than that of 200 h-milled Mg50 Ni50 BCC alloy. So we may say that Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys show better kinetics after the substitution of 5 at.% of B to Ni and the addition of 4 at.% of B. This is also confirmed by PCT measurement results and will be discussed later. From Fig. 4(e), we may see that there is no obvious hydrogen absorption peak before 573 K, which means CsCl-type structure Mg60 Ni35 B5 does not absorb hydrogen in this condition. For Mg60 Ni30 B10 Mg2 Ni-type-structure alloy, there is a broad but low exothermic peak at about 420 K. This peak should be attributed to hydrogen absorption of part of the Mg2 Ni-structure phase in the Mg60 Ni30 B10 alloy. From this low peak, we cannot expect fast kinetics from this sample. Fig. 5 indicates the PCT curves of five samples (Mg50 Ni50 BCC alloy milled for 200 h, Mg48 Ni48 B4 BCC alloy, Mg50 Ni45 B5 BCC alloy, Mg60 Ni35 B5 CsCl-type-structure alloy and Mg60 Ni30 B10 Mg2 Ni-type-structure alloy) at 373 K. The hydrogen absorption and desorption content values are presented in Table 1. From Fig. 5, we may see that at 373 K, Mg60 Ni35 B5 CsCl-type-structure alloy and Mg60 Ni30 B10 Mg2 Ni-type-structure alloy absorb hydrogen with a small amount. Mg60 Ni35 B5 alloy shows a hydrogen absorption content of 0.50 mass% after the hydrogenation process. Mg60 Ni30 B10 alloy shows a hydrogen absorption content of only 0.19 mass%. These results are in agreement with the DSC results. The other samples in Table 1 with CsCl type structure also cannot absorb large amount of hydrogen at 373 K in these measurement conditions. However, the BCC structure alloys all can absorb hydrogen with large content from the results in Table 1. Compared with 200 h Mg50 Ni50 BCC alloy, Mg48 Ni48 B4 and Mg50 Ni45 B5 BCC alloys present better hydrogen storage properties. After the hydrogen absorption process, Mg50 Ni50 BCC alloy absorbed hydrogen with a content of 1.77 mass%. Mg48 Ni48 B4 and Mg50 Ni45 B5 alloys showed a hydrogen absorption content of 1.93 and 1.94 mass%. We could also find that
Fig. 5. PC isotherms of different Mg–Ni–B system alloys at 373 K.
H. Shao et al. / Journal of Alloys and Compounds 479 (2009) 409–413
Mg48 Ni48 B4 and Mg50 Ni45 B5 alloys show much lower equilibrium hydrogen pressure than Mg50 Ni50 BCC alloy, which means these two alloys have better hydrogen absorption kinetics than Mg50 Ni50 alloy. This is also confirmed from the DSC measurement results. So we may conclude that substitution or addition of certain amount of B is effective to improve hydrogen storage properties. By ball milling, a small amount of Fe impurity from the milling balls usually could be detected in the produced samples. But according to our results, there is no obvious effect of this amount of Fe on the hydrogen storage properties of Mg–Ni–B BCC alloys. In Table 1, we should notice that if much more B is added or substituted to the Mg50 Ni50 alloy, the hydrogen absorption content will decrease. Mg50 Ni40 B10 BCC alloy shows a hydrogen absorption content of 1.84%, although it is still higher than 1.77% for Mg50 Ni50 alloy, but it is much lower than Mg50 Ni45 B5 alloy. Similar situations happened to Mg46 Ni46 B8 , Mg44 Ni44 B12 and Mg42 Ni42 B16 BCC alloys. The excellent hydrogen storage properties of these Mg–Ni–B BCC alloys at 373 K are great improvements compared with those Mg–Ni alloys prepared by conventional melting ways, which usually need hydrogen absorption temperature more than 500 K. The excellent properties of our Mg–Ni–B BCC alloys are thought to be due to the BCC structure, the nanostructure of the alloys and more fresh surface area and defects created during mechanical alloying process. 4. Conclusions The main results are summarized as follows: 1. Mg–Ni–B ternary alloys with different compositions were prepared by mechanical alloying method. Three types of structures – BCC structure, CsCl type structure and Mg2 Ni type structure were obtained in these alloys. The BCC structure and CsCl structure are also confirmed by electron diffraction rings. 2. The average crystalline size of CsCl-type-structure Mg60 Ni35 B5 alloy is 10–20 nm. The one of Mg2 Ni-type-structure Mg60 Ni30 B10 alloy. The BCC alloys are several nanometers in crystalline size. 3. The Mg–Ni–B alloys with CsCl structure or Mg2 Ni structure cannot absorb large amount of hydrogen at 373 K, while Mg–Ni–B BCC alloys show much better hydrogen absorption properties at this temperature. Mg48 Ni48 B4 and Mg50 Ni45 B5 BCC alloys showed a hydrogen absorption content of 1.93 and 1.94 mass%. Substitution or addition of a small amount of B to Mg50 Ni50 alloy is effective to improve hydrogen storage properties. 4. The enhanced hydrogen absorption properties of Mg–Ni–B BCC alloys at 373 K are due to the BCC structure, the nanostructure of the alloys and more fresh surface area and defects.
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Acknowledgements This research was supported by New Energy and Industrial Technology Development Organization (NEDO, Japan) under “Basic Technology Development Project for Hydrogen Safety and Utilization” and subsequently Japan Society for the Promotion of Science (JSPS) fellowship. References [1] J.J. Reilly, R.H. Hiswall, Inorganic Chemistry 6 (1967) 2220–2223. [2] J.J. Reilly, R.H. Wiswall, Inorganic Chemistry 7 (1968) 2254–2256. [3] F.H. Froes, C. Suryanarayana, K. Russell, C.G. Li, Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing 193 (1995) 612–623. [4] Y.Q. Lei, Y.M. Wu, Q.M. Yang, J. Wu, Q.D. Wang, Zeitschrift Fur Physikalische Chemie-International Journal of Research in Physical Chemistry & Chemical Physics 183 (1994) 379–384. [5] M.Y. Song, Journal of Materials Science 30 (1995) 1343–1351. [6] L. Zaluski, A. Zaluska, J.O. Stromolsen, Journal of Alloys and Compounds 217 (1995) 245–249. [7] S. Orimo, K. Ikeda, H. Fujii, Y. Fujikawa, Y. Kitano, K. Yamamoto, Acta Materialia 45 (1997) 2271–2278. [8] G. Liang, S. Boily, J. Huot, A. Van Neste, R. Schulz, Mechanically Alloyed, Metastable and Nanocrystalline Materials, Part 2 269–2 (1998) 1049– 1053. [9] J.L. Bobet, E. Akiba, Y. Nakamura, B. Darriet, International Journal of Hydrogen Energy 25 (2000) 987–996. [10] J. Chen, H.T. Takeshita, D. Chartouni, N. Kuriyama, T. Sakai, Journal of Materials Science 36 (2001) 5829–5834. [11] A. Abdellaoui, S. Mokbli, F. Cuevas, A. Latroche, A. Percheron-Guegan, H. Zarrouk, Journal of Alloys and Compounds 356 (2003) 557–561. [12] M. Au, Materials Science and Engineering B-Solid State Materials for Advanced Technology 117 (2005) 37–44. [13] Y. Feng, L.F. Hao, H.T. Yuan, M. Zhao, International Journal of Hydrogen Energy 32 (2007) 1701–1706. [14] G. He, L.F. Jiao, H.T. Yuan, Y.Y. Zhang, Y.J. Wang, Journal of Alloys and Compounds 450 (2008) 375–379. [15] S. Rousselot, M.P. Bichat, D. Guay, L. Roue, Journal of Power Sources 175 (2008) 621–624. [16] Y.H. Zhang, X.Y. Han, B.W. Li, H.P. Ren, X.P. Dong, X.L. Wang, Journal of Alloys and Compounds 450 (2008) 208–214. [17] Y. Zhang, Y. Tsushio, H. Enoki, E. Akiba, Journal of Alloys and Compounds 393 (2005) 147–153. [18] Y. Zhang, Y. Tsushio, H. Enoki, E. Akiba, Journal of Alloys and Compounds 393 (2005) 185–193. [19] K. Asano, H. Enoki, E. Akiba, Materials Transactions 48 (2007) 121–126. [20] H. Shao, K. Asano, H. Enoki and E. Akiba, Journal of Alloys and Compounds (2008), doi:10.1016/j.jallcom.2008.11.004, in press. [21] A.T. Dagostino, Analytica Chimica Acta 262 (1992) 269–275. [22] J. Matsuda, H. Shao, Y. Nakamura and E. Akiba, “Nano-structure and hydrogenation reaction of Mg50Co50 BCC alloy prepared by ball-milling”, Nanotechnology (2009), in press. [23] J. Nakamura, H. Shao, J. Matsuda, T. Proffen, Y. Nakamura, E. Akiba, Poster for ACNS2008, Santa Fe, America, 2008.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Fabrication and hydrogen storage property study of nanostructured Mg–Ni–B ternary alloys Huaiyu Shao, Kohta Asano, Hirotoshi Enoki, Etsuo Akiba ∗ National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 10 November 2008 Received in revised form 11 December 2008 Accepted 16 December 2008 Available online 25 December 2008 Keywords: Hydrogen storage materials Nanostructures Mechanical alloying X-ray diffraction Transmission electron microscopy
a b s t r a c t Mg–Ni–B ternary alloys with nanostructure were produced by mechanical alloying method. Alloys with various structures were obtained. X-ray diffraction was used to define the structure and phase information as well as crystalline size of the alloys. Bright-field and dark-field transmission electron microscope (TEM) technique was applied to observe the morphology of the samples. Electron diffraction was used to confirm the structure of the alloys. The hydrogen storage properties of the obtained Mg–Ni–B alloys were studied by high pressure differential scanning calorimetry (DSC) and pressure–composition isotherm (PCT) methods. Body centered cubic (BCC) structure alloys show better hydrogen absorption properties than CsCl and Mg2 Ni type structure alloys in this work. Mg48 Ni48 B4 and Mg50 Ni45 B5 BCC alloys showed a hydrogen absorption content of 1.93 and 1.94 mass% at 373 K. Substitution or addition of a small amount of B to Mg50 Ni50 alloy is effective to improve hydrogen storage properties. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since Reilly and Wiswall reported Mg–Cu–H system [1] and Mg–Ni–H system [2] as hydrogen storage materials, Mg-based alloys have been widely studied. Mg-based alloys have several important advantages for potential hydrogen storage application, such as low price, large abundance in earth’s crust and high capacities of the hydrides (7.6 mass% for MgH2 , 3.6 mass% for Mg2 NiH4 , 4.5 mass% for Mg2 CoH5 and 5.5 mass% for Mg2 FeH6 ). However, Mg-based materials also face a severe obstacle for the application that Mg-based materials prepared by conventional melting method show quite poor hydrogen storage kinetics. Mg and Mg2 Ni usually need a hydrogen absorption temperature higher than 523 K. Mechanical alloying (MA) method has been widely applied to prepare Mg-based alloys. MA as a high energy operation of repeated welding, fracturing and re-welding of sample powders [3], is a novel and well-known synthesis technique to prepare nanostructured and non-equilibrium alloys. Recently, it has almost become the major preparation method to produce Mg-based hydrogen storage alloys [4–16] because the fresh surface area and defects formed during the milling process could greatly improve the hydrogen absorption properties of these Mg-based alloys. Our group has successfully obtained Mg–Co [17,18], Mg–Ti [19] and Mg–Ni [20] body centered cubic (BCC) alloys by mechanical
∗ Corresponding author. E-mail addresses: [email protected] (H. Shao), [email protected] (E. Akiba). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.067
alloying method. Compared with face centered cubic (FCC) and hexagonal close packing (HCP) lattice, BCC lattice has lower packing density and more octahedral and tetrahedral sites per atom, which means more interstitial sites are available for hydrogen occupancy in BCC lattice. This work is about the preparation and hydrogen storage study of Mg–Ni–B ternary alloys (mostly with BCC structure). The purpose is to study the effect of addition/substitution of boronthe lightest non-metal solid-state element on the preparation and hydrogen storage properties of Mg–Ni alloys. 2. Experimental details Mg–Ni–B ternary alloys were produced using a Fritsch P5 planetary ball mill. Fabrication results of some Mg–Ni alloys were used to compare with those of Mg–Ni–B ternary alloys. The detailed fabrication process of Mg–Ni binary alloys could be found here [20]. The samples are with the compositions shown in Table 1. The beginning samples were mixture of Mg (purity >99.9%, 100 mesh), Ni (purity >99.9%, 300 mesh) and B (purity >99.9%, 300 mesh) powders weighed according to the atomic ratio of x:y:(100−x–y) in Mgx Niy B100−x–y . The mixture samples of 2 g in weight and stainless-steel milling balls of 10 mm in diameter were put into the milling vials with a ball-to-powder weight ratio of 20:1. The vials were then closed with 0.1 MPa argon in a hermetic way. The whole weighting process was conducted in glove box with high purity argon as protecting atmosphere. The mechanical alloying process of these alloys was carried out with a rotation speed of 200 rpm and a milling duration of 200 h. Structure and phase analysis of the obtained alloy samples was conducted using a Rigaku RINT-2500 V diffractometer with Cu K␣ radiation at a generator voltage of 50 kV and a current of 200 mA. The diffraction data were collected at a scan rate of 0.8◦ /min with a scan step of 0.04◦ . The size distribution and morphology observation in bright-field and dark-field was carried out by transmission electron microscope (TEM) using JEOL JEM-2000FX II operating at the accelerating voltage of 200 kV. Electron diffraction technique was conducted to confirm the structure of the samples.
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H. Shao et al. / Journal of Alloys and Compounds 479 (2009) 409–413
Table 1 Structure and hydrogen storage properties of Mg–Ni–B system alloys. Sample no.
Composition
Structure
Lattice parameter (nm)
Hydrogen absorption content (mass%)
Hydrogen desorption content (mass%)
1 2
Mg50 Ni50 Mg60 Ni35 B5
BCC CsCl type
0.3008 0.3796
1.77 0.50
0.1 0.05
3
Mg60 Ni30 B10
Mg2 Ni type
a = 0.5206 c = 1.323
0.19
0
4 5 6 7 8 9 10 11 12 13
Mg50 Ni45 B5 Mg50 Ni40 B10 Mg50 Ni35 B15 Mg40 Ni55 B5 Mg40 Ni50 B10 Mg40 Ni45 B15 Mg48 Ni48 B4 Mg46 Ni46 B8 Mg44 Ni44 B12 Mg42 Ni42 B16
BCC BCC CsCl type BCC CsCl type CsCl type BCC BCC BCC BCC
0.3034 0.3069 0.3806 0.2958 0.3784 0.3809 0.3009 0.3018 0.3029 0.3039
1.94 1.84 0.71 1.18 1.10 0.14 1.93 1.76 1.73 1.53
0.1 0.07 0.11 0.47 0.38 0.07 0.25 0.25 0.18 0.21
Differential scanning calorimetry (DSC) measurements under hydrogen atmosphere were performed to study the thermodynamic properties of Mg–Ni–B alloys in hydrogen. DSC measurements were conducted using a Rigaku TP-8230HP apparatus under an initial hydrogen pressure of 4 MPa. The temperature was increased from room temperature to 773 K and then cooled to room temperature at a rate of 5 K/min. Pressure–composition isotherm (PCT) properties were evaluated using an automatic apparatus from Suzuki Shokan Company. The measurements were operated under a hydrogen pressure up to 7 MPa at 373 K. Before the measurements, the samples were evacuated, heated to 373 k and kept at this temperature for 2 h.
3. Results and discussion Fig. 1 presents the main structure types of Mg–Ni–B system alloys produced by mechanical alloying. The structure information is also shown in Table 1. In this figure, (a)–(d) indicates Mg60 Ni35 B5 , Mg60 Ni30 B10 , Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys after 200 h mechanical alloying process, respectively. Mg60 Ni35 B5 alloy mainly shows CsCl type structure (primitive cubic, space group: Pm3m) which is one of the typical structures for the Mg–Ni–B system alloys in this work. The lattice parameter of the cubic structure is 0.3796 nm (Table 1). From the XRD curve of Mg60 Ni35 B5 alloy, we could observe broadening of the diffraction peaks after mechanical alloying. This indicates the obtaining of very fine microstructure of the sample during the milling process. Besides the main reason of
Fig. 1. XRD patterns of Mg–Ni–B alloys with different structures: (a) Mg60 Ni35 B5 , (b) Mg60 Ni30 B10 , (c) Mg50 Ni45 B5 and (d) Mg48 Ni48 B4 .
fine crystalline size of the samples, some other factors could also cause peak broadening, such as plastic deformation and microstrain. By Scherrer equation [21], we calculated the crystalline size of CsCl-type-structure Mg60 Ni35 B5 alloy. The average crystalline size is about 10–20 nm. Mg60 Ni30 B10 alloy shows Mg2 Ni type structure (primitive hexagonal, space group: P62 22) after milling process. The lattice parameters are a = 0.5206 nm and c = 1.323 nm. We could also observe severe peak broadening in this sample. The crystalline size is calculated to be several nanometers by Scherrer equation. Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys show BCC structure (space group: Im3m) after 200 h milling. Peak broadening is quite obvious from Fig. 1(c) and (d). From the study of Mg–Ni system [20], we know that the BCC structure alloys consist of crystalline BCC phase and amorphous BCC phase. For a perfect amorphous phase, there is no certain lattice parameter in the sample. In our work, the amorphous phase shows similar local structure to the crystalline BCC phase [22,23]. The lattice parameter of Mg50 Ni45 B5 BCC alloy is 0.3034 nm and one for Mg48 Ni48 B4 BCC alloy is 0.3009 nm. These two BCC structure alloys are with a mean crystalline structure of 1–5 nm. Compared with Mg50 Ni50 , Mg50 Ni45 B5 sample could be considered as Mg50 Ni50 alloy with substitution of a certain mount of B to Ni. Mg48 Ni48 B4 sample could be taken as addition of some B to Mg50 Ni50 alloy. Fig. 1 only shows four samples with typical structures of Mg–Ni–B system alloys. The structure information of other samples is shown in Table 1. The various structures with different Mg–Ni–B compositions produced by mechanical alloying method
Fig. 2. Structure results of Mg–Ni–B system alloys with different compositions prepared by mechanical alloying.
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Fig. 3. Bright-field and dark-field TEM images of (a), (b) Mg60 Ni35 B5 alloy with CsCl structure; (c), (d) Mg50 Ni45 B5 BCC structure; and (e), (f) Mg48 Ni48 B4 BCC structure. Insets are the indexed electron diffraction patterns of corresponding alloys.
are presented in Fig. 2. It should be pointed out that Mg–Ni binary alloys reported elsewhere [20] were mechanical alloyed for 100 h. Sample no. 1 in Table 1 in this work was milled for 200 h. The preparation results of Mg–Ni binary alloys are adopted to make the production diagram of Fig. 2. From Fig. 2, we may see that for Mg–Ni binary alloys, which were milled for 100 h, there are three typical structures-FCC structure (Mg30 Ni70 ), BCC structure (Mg33 Ni67 , Mg40 Ni60 , Mg50 Ni50 and Mg60 Ni40 ) and Mg2 Ni type structure (Mg67 Ni33 ). We may simply conclude that for Mgx Ni100−x , when x is from 33 to 60, after 100 h milling in our work conditions, we can obtain BCC structure. From Table 1, we may see that Mg50 Ni50 after 200 h milling also shows BCC structure. For sample No. 3–Mg60 Ni30 B10 alloy, after 200 h milling, it shows Mg2 Ni type structure. Considering the fact of synthesis of the same structure in Mg67 Ni33 composition alloy, there seems to be a rule that when the atomic ratio of Mg to Ni in Mgx Niy B100−x–y composition is two, after 200 h milling, the alloy shows Mg2 Ni type structure.
For the formation of CsCl type structure after 200 h milling, now it is difficult for us to make any conclusion of the composition rule of how to obtain CsCl type structure in Mg–Ni–B system alloys. After 200 h milling, Mg50 Ni50 , Mg50 Ni45 B5 , Mg50 Ni40 B10 , Mg40 Ni55 B5 , Mg48 Ni48 B4 , Mg46 Ni46 B8 , Mg44 Ni44 B12 and Mg42 Ni42 B16 alloys show BCC structure. The lattice parameters are presented in Table 1. It is very interesting that when we compare these data of sample no. 1, 10–13, we find that with the increasing addition composition of B in Mg–Ni alloys, the lattice parameter is increasing. We know that the atomic size of B is much smaller than those of Mg and Ni. If B atom occupies the corner sites of Mg or Ni in the BCC lattice, the lattice parameter will be smaller when more B is added to the alloy. However, it is reverse. The only explanation we can think of is that after the mechanical alloying process, B occupies the interstitial sites while not the Mg or Ni corner sites in the BCC lattice. Fig. 3 presents TEM images of CsCl structure alloy sample (Mg60 Ni35 B5 ) and BCC alloy samples (Mg50 Ni45 B5 and
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Fig. 4. DSC curves of (a) Mg50 Ni50 BCC alloy ball milled for 100 h, (b) Mg50 Ni50 BCC alloy ball milled for 200 h, (c) Mg50 Ni45 B5 BCC alloy, (d) Mg48 Ni48 B4 BCC alloy, (e) Mg60 Ni35 B5 CsCl-type-structure alloy and (f) Mg60 Ni30 B10 Mg2 Ni-type-structure alloy.
Mg48 Ni48 B4 ). The insets are the selected area diffraction patterns for the corresponding samples. Fig. 3(a) and (b) shows the brightfield and dark-field photos of Mg60 Ni35 B5 sample. From the photos, especially the dark-field one, we may see that the crystalline size of the samples is tens of nanometers. This agrees with the size value calculated from the XRD patterns. The electron diffraction rings in the inset are well indexed to CsCl structure, which agrees with the XRD result. From Fig. 3(c)–(f), we may clearly see the crystalline structure in Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys. These alloys have a much smaller crystalline size than Mg60 Ni35 B5 CsCl structure alloys. The crystalline size for Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys is several nanometers. This size result is in good agreement with the ones calculated from the XRD curves by Scherrer equation. When we measure the radiuses of the electron diffraction rings in Fig. 3(c) and (e), we find that the radiuses (named R1 –R6 from inside to outside) are in accord with the rule expressed as: R12 : R22 : R32 : R42 : R52 : R62 = 1 : 2 : 3 : 4 : 5 : 6
(1)
This means Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys could be only indexed to BCC structure. The lattice parameter data obtained from these diffraction rings agree with the ones calculated from the XRD patterns. Fig. 4 indicates the DSC curves of the Mg–Ni–B system alloys. Samples in Fig. 4(a)–(d) are BCC structure alloys – Mg50 Ni50 BCC alloy ball milled for 100 h in Fig. 4(a); Mg50 Ni50 BCC alloy ball milled for 200 h in Fig. 4(b); Mg50 Ni45 B5 BCC alloy ball milled for 200 h in Fig. 4(c) and Mg48 Ni48 B4 BCC alloy ball milled for 200 h in Fig. 4(d). Fig. 4 (e) and (f) represent Mg60 Ni35 B5 CsCl-type-structure alloy ball milled for 200 h and Mg60 Ni30 B10 Mg2 Ni-type-structure alloy ball milled for 200 h, respectively. We have clarified the peak attributions of the five main peaks in the DSC curves of the Mg–Ni BCC alloys [20]. In Fig. 4(a), the first big exothermic peak at 432 K is due to the hydrogen absorption of the BCC alloy. After this reaction, the hydrogenated sample still keeps BCC structure. The second exothermic peak is a small one. It shows a peak temperature of 554 K. This peak is due to the hydrogen absorption again and decomposition of BCC hydride phase to Mg2 NiH4 , Ni and a small amount of MgNi2 . The endothermic peak at about 744 K in the heating process and the exothermic peak at 693 K in the cooling process are due to the sam-
ple (Mg2 NiH4 phase) desorbing hydrogen and absorbing hydrogen again. The small peak at 494 K in the cooling process is due to the transformation reaction from high temperature Mg2 NiH4 phase to low temperature one. Comparing Fig. 4(a) (Mg50 Ni50 BCC alloy ball milled for 100 h) with Fig. 4(b) (Mg50 Ni50 BCC alloy ball milled for 200 h), we may see that, 200 h-milled Mg50 Ni50 BCC alloy shows a first hydrogen absorption peak at a much earlier temperature (410 K) than 100 hmilled Mg50 Ni50 BCC alloy (432 K). The reason is thought to be the finer structure and more defects and strain produced during 100 more hours, which cause better hydrogen absorption kinetics. Mg50 Ni45 B5 and Mg48 Ni48 B4 BCC alloys show the first hydrogen absorption peak temperatures of 408 and 406 K, respectively, which are a bit lower than that of 200 h-milled Mg50 Ni50 BCC alloy. So we may say that Mg50 Ni45 B5 and Mg48 Ni48 B4 alloys show better kinetics after the substitution of 5 at.% of B to Ni and the addition of 4 at.% of B. This is also confirmed by PCT measurement results and will be discussed later. From Fig. 4(e), we may see that there is no obvious hydrogen absorption peak before 573 K, which means CsCl-type structure Mg60 Ni35 B5 does not absorb hydrogen in this condition. For Mg60 Ni30 B10 Mg2 Ni-type-structure alloy, there is a broad but low exothermic peak at about 420 K. This peak should be attributed to hydrogen absorption of part of the Mg2 Ni-structure phase in the Mg60 Ni30 B10 alloy. From this low peak, we cannot expect fast kinetics from this sample. Fig. 5 indicates the PCT curves of five samples (Mg50 Ni50 BCC alloy milled for 200 h, Mg48 Ni48 B4 BCC alloy, Mg50 Ni45 B5 BCC alloy, Mg60 Ni35 B5 CsCl-type-structure alloy and Mg60 Ni30 B10 Mg2 Ni-type-structure alloy) at 373 K. The hydrogen absorption and desorption content values are presented in Table 1. From Fig. 5, we may see that at 373 K, Mg60 Ni35 B5 CsCl-type-structure alloy and Mg60 Ni30 B10 Mg2 Ni-type-structure alloy absorb hydrogen with a small amount. Mg60 Ni35 B5 alloy shows a hydrogen absorption content of 0.50 mass% after the hydrogenation process. Mg60 Ni30 B10 alloy shows a hydrogen absorption content of only 0.19 mass%. These results are in agreement with the DSC results. The other samples in Table 1 with CsCl type structure also cannot absorb large amount of hydrogen at 373 K in these measurement conditions. However, the BCC structure alloys all can absorb hydrogen with large content from the results in Table 1. Compared with 200 h Mg50 Ni50 BCC alloy, Mg48 Ni48 B4 and Mg50 Ni45 B5 BCC alloys present better hydrogen storage properties. After the hydrogen absorption process, Mg50 Ni50 BCC alloy absorbed hydrogen with a content of 1.77 mass%. Mg48 Ni48 B4 and Mg50 Ni45 B5 alloys showed a hydrogen absorption content of 1.93 and 1.94 mass%. We could also find that
Fig. 5. PC isotherms of different Mg–Ni–B system alloys at 373 K.
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Mg48 Ni48 B4 and Mg50 Ni45 B5 alloys show much lower equilibrium hydrogen pressure than Mg50 Ni50 BCC alloy, which means these two alloys have better hydrogen absorption kinetics than Mg50 Ni50 alloy. This is also confirmed from the DSC measurement results. So we may conclude that substitution or addition of certain amount of B is effective to improve hydrogen storage properties. By ball milling, a small amount of Fe impurity from the milling balls usually could be detected in the produced samples. But according to our results, there is no obvious effect of this amount of Fe on the hydrogen storage properties of Mg–Ni–B BCC alloys. In Table 1, we should notice that if much more B is added or substituted to the Mg50 Ni50 alloy, the hydrogen absorption content will decrease. Mg50 Ni40 B10 BCC alloy shows a hydrogen absorption content of 1.84%, although it is still higher than 1.77% for Mg50 Ni50 alloy, but it is much lower than Mg50 Ni45 B5 alloy. Similar situations happened to Mg46 Ni46 B8 , Mg44 Ni44 B12 and Mg42 Ni42 B16 BCC alloys. The excellent hydrogen storage properties of these Mg–Ni–B BCC alloys at 373 K are great improvements compared with those Mg–Ni alloys prepared by conventional melting ways, which usually need hydrogen absorption temperature more than 500 K. The excellent properties of our Mg–Ni–B BCC alloys are thought to be due to the BCC structure, the nanostructure of the alloys and more fresh surface area and defects created during mechanical alloying process. 4. Conclusions The main results are summarized as follows: 1. Mg–Ni–B ternary alloys with different compositions were prepared by mechanical alloying method. Three types of structures – BCC structure, CsCl type structure and Mg2 Ni type structure were obtained in these alloys. The BCC structure and CsCl structure are also confirmed by electron diffraction rings. 2. The average crystalline size of CsCl-type-structure Mg60 Ni35 B5 alloy is 10–20 nm. The one of Mg2 Ni-type-structure Mg60 Ni30 B10 alloy. The BCC alloys are several nanometers in crystalline size. 3. The Mg–Ni–B alloys with CsCl structure or Mg2 Ni structure cannot absorb large amount of hydrogen at 373 K, while Mg–Ni–B BCC alloys show much better hydrogen absorption properties at this temperature. Mg48 Ni48 B4 and Mg50 Ni45 B5 BCC alloys showed a hydrogen absorption content of 1.93 and 1.94 mass%. Substitution or addition of a small amount of B to Mg50 Ni50 alloy is effective to improve hydrogen storage properties. 4. The enhanced hydrogen absorption properties of Mg–Ni–B BCC alloys at 373 K are due to the BCC structure, the nanostructure of the alloys and more fresh surface area and defects.
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