J. Sains MIPA, Desember 2011, Vol. 17, No. 3, Hal.: 83 - 86 ISSN 1978-1873
MG CATALYZED 2wt% SiC OBTAINED BY INTENSIVE MECHANICAL ALLOYING FOR HYDROGEN STORAGE MATERIALS APPLICATION Zulkarnain Jalil1,#, Adi Rahwanto1, Bambang Soegijono2 and Azwar Manaf2 1Department 2Graduate
of Physics, Syiah Kuala University, Kopelma Darussalam, Banda Aceh School of Materials Science, University of Indonesia, Jl. Salemba Raya 4, Jakarta # E-mail:
[email protected]
ABSTRAK Penggilingan bola berenergi tinggi baru-baru ini telah berhasil digunakan untuk membuat bahan penyimpan hidrogen. Dalam penelitian ini, bahan penyimpan hidrogen berbasis logam Mg dengan komposisi Mg-2% berat SiC telah dibuat dengan metode intensif mekanis paduan untuk menghasilkan bahan nanokristal. Hasil karakterisasi struktur dengan XRD menunukkan bahwa setelah 180 jam penggilingan, ukuran kristal berkurang 10 nm. Hasil ini sangat menarik sebab metode intensif mekanis paduan menunjukkan cara yang menarik untuk mensintesis bahan penyimpan hidrogen berbasis magnesium. Gambar SEM menunjukkan bahwa setelah beberapa jam penggilingan terlihat bahwa permukaan dari kristal tidak teratur sebagai hasil pemecahan berulang selama proses penggilingan. Kata kunci: penyimpanan hidrogen, magnesium, hidrida logam, penggilingan bola
ABSTRACT Recently, the high energy ball milling was successfully introduced to prepare hydrogen storage materials. In this work, Mg-based hydrogen-storage materials with the compositions of Mg 2 wt% SiC were prepared by intensive mechanical alloying method to produce the nanocrystalline materials. As the results, structural characterization by XRD showed that after 180 hours of milling time the crystallite size decreases around 10 nm. This can be noted that the intensive mechanical alloying showed an interesting way to synthesize the magnesium based hydrogen storage material. From SEM images of the sample powder before and after several hours of milling times can be seen that the surface of the powders is very irregular, as a result of the repeated fracturing events during the milling process. Keywords: hydrogen storage, magnesium, metal hydrides, ball milling
1. INTRODUCTION Among the metal hydrides, magnesium has the theoretically highest weight capacity for hydrogen storage (7.6 wt.%), lightweight and a reasonably low cost1. However, high working temperature, slow reaction kinetics and difficult activation limit the practical application of Mg-based hydrides. Many efforts have been done to improve sorption properties and reaction kinetics such as element substitution (metal or metal oxides) as catalyst in nanometer scale and modification of ball milling technique as well2-5. Recently, the reactive ball milling under hydrogen atmosphere was also successfully introduced to prepare hydrogen storage materials6,7. Especially for searching suitable catalyst, very recently, Ranjbar et.al8 introduced a new catalyst based on silicon carbide (SiC) inserted in magnesium as host material. The sorption properties is improved and the kinetics also very fast. Motivated by this, here we report our work on Mg catalyzed with 2wt% of SiC prepared by vibration ball milling (VBM). The aim of investigations were to synthesize nanocrystalline Mg-SiC by mechanical alloying using mechanical milling method.
2. MATERIALS AND METHODS Mg (Merck, 0.03 mm, 95%), SiC (99.9%, 400 mesh, Sigma Aldrich) powders were used. The powders were filled into a home-made hardened steel vial and sealed together with 10 balls (5.6 mm in 2011 FMIPA Universitas Lampung
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Zulkarnain Jalil et al Mg Catalyzed 2wt% SiC Obtained by Intensive Mechanical Alloying
diameter). The powders are milled in a vibratory ball mill (VBM, Kawasaki) at a rotational speed of 900 rpm (ball to powder ratio 10:1) for 180 hours. Every several hours of milling time (30, 40, 60, 80, 100, 160 and 180 hour) small amount of powder were separated into a small container for further characterization. This method called then as Intensive Mechanical Alloying (IMA). Structural changes during milling were characterised by XRD (Philips PW 3710, Co-K radiation) and a high resolution scanning electron microscopy (SEM JEOL JSM-5310LV) was used to observe the morphological changes during milling. The onset temperature (Tonset) was investigated using DTA (Shimadzu D-50) with a flow rate of 20 ml/min and heating rate from 20-450 C.
3. RESULT AND DISCUSSION 3.1. X-ray diffraction Figure 1 shows the evolution of the XRD diffraction pattern for Mg-2wt% SiC as a function of milling time and intensity. Mg-2wt% SiC, BM 180 h
160 h
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Figure 1. X-ray diffraction patterns of Mg-2wt% SiC produced via VBM At early stage of milling, the starting mixture shows the presence of microcrystalline materials. Then, it can be seen for the next milling time that the diffraction peaks broaden but no changes in the 2 position. The as-received sample composed mainly Mg. The Mg peaks locate at 2 = 37,86o; 40,56; 43,66; 56,8; 68,06; 75,28; 82,5; 84,24, and 87,14. SiC peak was not detected. Because the amount of SiC was to small (2 wt%). Thus, it difficults to detect by XRD. The same result was found after 30 h milling. When the milling time increase up to 40 hour, the XRD patterns still not change significantly. But, at 60 hour the peak broadening was start to appear. At 80 to 100 hour, the Mg peaks were broadening significantly. At 160 hour and 180 hour the peak unity in one peak and composed one phase. It can be seen that the powders already reduced into nanocrystalline. We can conclude that Mg completely decrease after milling in 180 hours. Thanks to high energy vibration ball mill. This results showed that mechanical alloying using VBM is very attractive and promising methode to synthesize nanostructure materials for solid hydrogen storage. This behaviour considering the applied mechanical deformation between ball and powders. Thus, making use of higher energy during milling promotes the formation of nanostructure of magnesium.
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3.2. Electron microscopy The SEM micrographs of Fig. 2 show secondary electron image of powders intensively milled in the VBM for 160 and 180 hours. The surface of the powder is irregular, as a result of the fracturing during the milling process. This is consistent with the structural analysis by XRD as shows in Fig. 1.
(a)
(b)
Figure 2. SEM image of Mg-2wt% SiC after milling for (a) 160 h and (b) 180 h This indicates that the powders is un-uniformly distributed on the metal surface and the grain size, calculated by Scherrer method9, reaches around 10 nm after 180h of milling. The formation of nanocrystalline material is obtained after long milling (in 180 hours). These results suggest that deformation during milling take longer time due to the ductility of magnesium. Thermal investigation for the sample after 180 miling inform us that the material decomposed at temperature 330 C (Figure . This result is still high for Mg-based hydrogen storage materials application. But, however, the IMA method show an attractive method to produce nanocrystalline powder.
DTA Scan
Mg-2wt% SiC, 180h Milling
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Figure 3. DTA scan of Mg-2wt% SiC after milling 180 h
4. CONCLUSIONS Mg catalyzed with 2 wt% of silicon carbide (SiC) and prepared via vibratory ball milling has been successfully done. This process results in high surface area powders with finely dispersed SiCparticles on the surface of Mg. The Mg-SiC material exhibits a microstructure in nanometer scale. Due to these results, nanocrystalline Mg-SiC is suggested a further work for improvement the
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sorption properties and the kinetics of Mg-based hydrides as hydrogen storage material. However, the operation temperature is still high (330 C) for the mass production. ACKNOWLEDGEMENTS A fruitful discussions with Dr Abbas Ranjbar (Wollongong University, Australia) is gratefully acknowledged.
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