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Structural transformations in graphite induced by magneto-mechanical-milling in hydrogen atmosphere
Journal of Alloys and Compounds 402 (2005) 256–262
Structural transformations in graphite induced by magneto-mechanical-milling in hydrogen atmosphere A. Smolira a , M. Szymanska b , E. Jartych b,∗ , A. Calka c , L. Michalak a b
a Institute of Physics, Maria Curie-Sklodowska University, Pl.M. Curie-Sklodowskiej 1, PL-20-031 Lublin, Poland Department of Experimental Physics, Institute of Physics, Technical University of Lublin, ul. Nadbystrzycka 38, PL-20-618 Lublin, Poland c Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia
Received 29 March 2005; accepted 13 April 2005 Available online 13 June 2005
Abstract Laser desorption time-of-flight mass spectrometry, X-ray diffraction and M¨ossbauer spectroscopy methods were used for characterization of phase transformations induced in graphite during controlled reactive ball milling in hydrogen atmosphere. During milling, the crystalline structure of the graphite transformed to nanostructure as proved by X-ray diffraction studies. The hydrogen storage capacity of the nanostructured graphite depends strongly on the energy of milling process and reached 2.718 wt.% in this study. The mass spectrometry method revealed in the milling products a variety of bare carbon clusters as well as hydrogenated carbon clusters which may have from one to four hydrogen atoms. Complementary measurements performed using M¨ossbauer spectroscopy allowed it to recognize the crystalline phases of iron compounds observed in the X-ray diffraction patterns. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; High-energy ball milling; X-ray diffraction; Mass spectrometry
1. Introduction Hydrogen storage materials, i.e. materials having a large capacity of hydrogen absorption are the subjects of the intensive research from the point of view of potential application of hydrogen as a fuel. Among these materials, the alloys based on Mg, Zr and Ti are the most promising because of their low cost, light weight and high capacity of hydrogen storage (of the order of 5–8 wt.%) [1–4]. Recently, reactive ball milling in hydrogen atmosphere became a well-established method for production of hydrogen storage materials. Moreover, much attention has been paid to the investigations of hydriding properties of carbon-based nanostructural materials, which possess defective structure and where defects act as hydrogen trapping sites. It has been found that nanostructured graphite has the ability to absorb hydrogen up to 7.4 wt.% during the reactive ball milling process [5]. Moreover, the hydrogen ∗
Corresponding author. Tel.: +48 81 538 1618; fax: +48 81 525 9385. E-mail address: [email protected] (E. Jartych).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.04.154
storage capacity of ball milled carbon depends on milling time, as reported in [6]. Graphite is also treated as a catalyst. Very recently, it has been shown that addition of the graphite during milling of magnesium at high temperature under hydrogen atmosphere quickly led to magnesium hydride formation [7]. Catalytic properties of a nanostructured graphite may be explained by the fact that terminating hydrogen atoms at the edges of the nano-lattice plane of graphite create new sp3 bonding, as proved by neutron scattering technique [8]. Although reactive ball milling under hydrogen atmosphere is a potential technology for preparation of materials with high capacity of hydrogen storage, some problems need basic investigations. Hydrogen in materials may exist as molecules and/or atoms, as well as in the form of hydrocarbons [5]. The nature of the absorption/desorption behaviours has not been fully recognized. One of the methods for studying hydrogen which is delivered from the material is the thermal desorption mass spectroscopy (TDS). Some TDS spectra of mechanically milled carbon-type materials (i.e.
A. Smolira et al. / Journal of Alloys and Compounds 402 (2005) 256–262
natural graphite, activated carbon fibres and activated carbon powder) have been reported earlier [5,6]. The spectra were broad with peaks that were not fully developed. Those results indicated that hydrogen in nanocarbon hydrogenated during mechanical milling might exist in the form of H2 molecules and/or in the form of atoms occupying interstitial voids. In our previous work [9], we proposed the laser desorption (LD) and the reflectron time-of-flight mass spectrometry (RTOF MS) methods for investigations of structural evolution of graphite milled in hydrogen atmosphere. The mass spectra revealed a range of peaks with high values of the mass to charge ratio m/z. However, the intensities of the peaks were relatively small and some of them were hardly separated from the background. In order to improve the intensity of the mass spectra, the linear mode of the time-of-flight mass spectrometry (LTOF MS) technique was used in studies described in this work. The aim of the present investigations was to recognize the mass peaks in detail and also to determine
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the hydrogen storage capacity of the graphite milled under hydrogen atmosphere. Moreover, M¨ossbauer spectroscopy was used as a complementary technique to reveal the possible Fe contaminations from the milling media.
2. Experimental details 2.1. Sample preparation Powder of graphite (mass 2 g) with a purity of 99.95% and the initial particle size of about 50 m was placed into the vial of a magneto-mill Uni-Ball-Mill 5. The milling modes in this mill can be easily adjusted from the low-energy shearing to the high-energy impact (for more detail see [10,11]). In the shearing mode, the balls both rotate and oscillate around an equilibrium position at the bottom of the milling cylinder in a strong magnetic field (Fig. 1). In the impact mode, the
Fig. 1. The Uni-Ball-Mill 5 with the FeNdB external magnets operating in the shearing and the impact modes of milling [10,11].
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Fig. 2. Linear time-of-flight mass spectrometer (LTOF MS) with laser desorption (LD) ion source.
ball movement during the milling process is confined to the vertical plane by the cell walls and is controlled by an external magnetic field (Fig. 1). In both cases, magnetic field is provided by the FeNdB magnets. The intensity and direction of magnetic field can be externally adjusted allowing the ball trajectories and impact energy to be varied in a controlled manner. High carbon steel balls and stainless-steel vial were used. The milling vial was evacuated and purged several times before the final fill up with hydrogen gas up to 500 kPa pressure. The milling time of the graphite powder in hydrogen atmosphere was 116 h in the shearing mode and 100 h in the impact mode. 2.2. Sample characterization The structure of the starting graphite and the graphite milled in hydrogen atmosphere was characterized using Xray diffraction (XRD). Measurements were performed using Philips PW 1730 diffractometer equipped with Cu K␣ radiation. The Scherrer’s formula was used to estimate the grain sizes of the graphite milled in the shearing and in the impact modes. The LTOF MS, described in detail elsewhere [12–18], was used for the mass characterization of the graphite powder milled in hydrogen atmosphere as well as for the starting graphite, which was a reference sample. The graphite sample placed on the holder was lighted by the focused pulsed laser beam (Fig. 2). The nitrogen laser LN300C (λ = 337 nm) with an output pulse length of 5 ns and a maximum power of 50 kW was used. This laser irradiation causes the absorption of the laser light energy and desorption with ionisation of the sample molecules. The generated ions are accelerated in the
electric field between the sample holder and the grounded flat electrode (Fig. 2). Next, the accelerated ion beam flows in the field free region to the detector. The signal from the detector is directed to an oscilloscope (Hewlett Packard Oscilloscope, HP54615B, 500 MHz, 1 GSa/s). The ion packet created in the ion source as a mixture of different ions is separated to series of packets in dependence on the mass to charge ratio m/z. Thus the spectrum from the oscilloscope is the dependence of the intensity of ion current on the m/z ratio. For the measurements, the sample of the graphite powder (mass 5 × 10−3 g) was mixed with methanol (10 L) and was placed on a flat stainless-steel sample holder and then dried in the air at room temperature giving a visible and apparently homogenous spot of ∼5 mm diameter. The ion acceleration voltage was 17 kV. All spectra were registered as an average of 64 laser shots. In order to estimate the hydrogen storage capacity of the investigated material carbon–hydrogen–nitrogen (CHN) gas spectroscopy was carried out using a Carlo Erba Elemental Analyser Model 1106. The analysis was performed on as received, milled, and annealed samples with mass of 1.0 g. The sample was placed in a small tin foil capsule and then combusted in the presence of oxygen. The gases given off are then swept over various packings of chromium oxide, cobalt oxides, and copper using an argon carrier gas. This is done in order to further combust the gases, remove the excess of oxygen, and to reduce the nitrogen oxides to elemental nitrogen. These gases of nitrogen, water vapour and carbon dioxide are then separated in a Poropak QS gas chromatography column and then measured at a detector. The peaks areas detected are then integrated, with elemental percentages calculated from these peak areas and the initial mass.
A. Smolira et al. / Journal of Alloys and Compounds 402 (2005) 256–262
M¨ossbauer spectroscopy measurements were carried out at room temperature in standard transmission geometry using a source of 57 Co in a rhodium matrix. A 25-m thick metallic iron foil was taken as a standard for calibration of spectrometer.
3. Results and discussion Fig. 3 presents XRD patterns measured for the starting graphite and the graphite milled in hydrogen atmosphere in the shearing and the impact modes. It may be noticed that the main diffraction peak (0 0 2) of the starting graphite (Fig. 3a),
Fig. 3. XRD patterns of (a) starting graphite, (b) graphite milled in hydrogen atmosphere for 116 h using the shearing mode and (c) graphite milled in hydrogen atmosphere for 100 h using the impact mode in the Uni-Ball-Mill 5.
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lying between 2θ angles of 20◦ and 30◦ , is significantly broadened for the graphite milled in the shearing mode for 116 h (Fig. 3b). The grain sizes estimated using Scherrer’s formula are of the order of 10–12 nm. In the case of the impact mode (Fig. 3c), the peak (0 0 2) is very broad but has not completely disappeared. Both observations indicate that during milling the crystalline structure of the graphite was transformed to nanostructure. A similar effect was observed by Orimo et al. [5] for the graphite milled in hydrogen for 80 h using a planetary ball mill. In that case, the nanostructured graphite with the grain size of 4 nm was formed. Slightly different results were obtained by Shindo et al. [6] for natural graphite milled in hydrogen for 100 h using a similar mill. The authors observed the disappearance of the peak (0 0 2) and attributed this effect to the conversion of the crystalline graphite into an amorphous phase. In both studies, the authors did not observe any diffraction peaks caused by other crystalline phases. In contrast to the results mentioned above, the XRD patterns obtained in this work reveal clearly developed crystalline peaks for 2θ angles of about 36◦ and 43.5◦ for both modes of milling (Fig. 3b and c) and 51◦ and 74◦ in the case of the impact mode (Fig. 3c). The interpretation of these crystalline peaks, which broadening gives the average grain sizes of the order of 3–7 nm, will be given later. Fig. 4 presents the positive-ion LD mass spectra of the pure graphite and the graphite milled in hydrogen atmosphere in both shearing and impact modes. Spectrum for the starting graphite (Fig. 4a) was registered up to m/z = 400. In the case of the graphite milled in hydrogen (Fig. 4b and c), there were no well-developed peaks in the mass spectra over m/z = 100 for the shearing and m/z = 140 for the impact modes, respectively. In all spectra, the peaks with m/z = 23 and 39 represent ions of Na+ and K+ , respectively. They are typical for laser desorption/ionization investigations because of their presence in the environment and on the surface of the sample holder. In spite of the fact that these ions may be treated as contaminations, the strictly determined positions of the mass peaks for Na+ and K+ ions allow it to calibrate the spectrometer. It may be noticed that in the mass spectra of the graphite milled in hydrogen (Fig. 4b and c) rather intensive peaks with m/z = 52, 56 and 59 are present. The values of m/z may indicate the presence of Cr+ , Fe+ and Ni+ ions, respectively. The metallic ions existing in the graphite powder come from the milling media. The mass spectrum of the starting graphite (Fig. 4a) revealed a wide range of peaks, that appear regularly for a mass number being total multiplicity of 12 (n = 2–22). These peaks indicate that during laser desorption/ionization process the carbon clusters Cn are released from the graphite. Similar mass spectra were observed for small fullerenes (or clusters) Cn , with n = 4–70 atoms, obtained using a standard laser evaporation source [19,20]. As reported in our earlier work [9], mass spectrometry measurements revealed hydrocarbon molecules (besides carbon clusters) in the hydrogenated nanostructured graphite. In
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Fig. 5. Positive-ion LD mass spectra obtained by LTOF MS method in the limited range of m/z for (a) graphite milled in hydrogen in the impact mode and (b) graphite milled in the shearing mode.
the figures. Besides the peaks attributed to bare Cn clusters (m/z = 60, 72, 84, 96, 108, 120 and 132), also species with one to four additional hydrogen atoms are prominent. The possible interpretation of some peaks is given in Table 1. Our mass spectra for the hydrogenated graphite are similar to those for hydrogenated carbon clusters which were emitted during impact of highly charged Xe on C84 target [21]. The authors observed small Cn Hx clusters with n = 2–22 and concluded that for a given n, most clusters contain less than x = 4
Fig. 4. Positive-ion LD mass spectra obtained by LTOF MS method for (a) starting graphite, (b) graphite milled in hydrogen atmosphere for 116 h using the shearing mode and (c) graphite milled in hydrogen atmosphere for 100 h using the impact mode in the Uni-Ball-Mill 5.
order to discuss this problem in detail, the mass spectra for the graphite milled in hydrogen atmosphere in both shearing and impact modes were additionally measured in the limited range of m/z values (i.e. over 60). They are shown in Fig. 5. The most intensive peaks are marked by the m/z values inside
Table 1 The possible hydrogenated carbon clusters observed in the mass spectra of the graphite milled under hydrogen atmosphere in the impact and the shearing modes m/z
Cn Hx impact mode
Cn Hx shearing mode
64 85 86 87 97 99
C5 H4 C7 H1 C7 H2 C7 H3 C8 H1 C8 H3
C5 H4 C7 H2
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hydrogen atoms. Moreover, for odd n values (and n ≤ 10) the monohydro species (Cn H1 ) are the most prominent, while for even n values (and n ≤ 10) the dihydro products (Cn H2 ) are dominant. In our investigations, the first effect is observed only for n = 7, the peak with m/z = 85 for C7 H1 has the highest intensity in the spectrum for the graphite milled in the impact mode (Fig. 5a). Differences between our results and those reported in [21] are obviously connected with the various processes of hydrogenation, sample preparation, etc. However, information obtained from ref. [21] confirms the interpretation of our results given in Table 1. Another effect may be observed in this study if we compare the mass spectra for the graphite milled under hydrogen in the shearing and the impact modes (Fig. 5 and Table 1). The milling process in the impact mode gives much more bare and hydrogenated carbon clusters as compared to the shearing mode. This fact may be a result of the difference in the energy of milling in both modes. Moreover, two peaks with m/z = 113 and 115 are visible in the mass spectrum for the graphite milled in the impact mode (Fig. 5a). They may be attributed to the fragments of hydrocarbons with n = 9 carbon atoms. The primary purpose of the combustion analysis CHN was to determine the amount of the hydrogen absorbed during milling. The hydrogen storage capacity of the graphite investigated in this work reached 0.613 and 2.718 wt.% for the shearing and impact modes, respectively. It is clear that from the application point of view, the milling process performed in the impact mode is more profitable. In this case, the energy of impacts is higher than in the shearing mode and the milled graphite has more defects acting as trapping sites for hydrogen. M¨ossbauer spectroscopy based on the 57 Fe isotope allows it to investigate hyperfine interactions in any materials containing iron. Because mass spectrometry results suggested iron contaminations from the milling media, M¨ossbauer spectroscopy was used to confirm this statement. Measurements performed for the starting graphite excluded the presence of iron in the powder. Fig. 6 presents the M¨ossbauer spectra registered for the graphite milled under hydrogen in the shearing
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Fig. 6. Room-temperature M¨ossbauer spectra for the graphite milled using the impact and the shearing modes; open circles: experimental data; solid thick line: theoretical fit; solid thin line: singlet arising from the Fe–Cr alloy; dotted line: doublet arising from the Fe–Cr–Ni or Fe–Cr alloy.
and impact modes. The obtained spectra proved that during milling not only a little amount of pure iron contaminated the graphite samples, but also that the iron formed some paramagnetic compounds. Each spectrum is a superposition of some components. In the case of the graphite milled in the shearing mode, the spectrum consists of two dominating components (the single line and the doublet marked in Fig. 6) and a very weak component arising from ␣-Fe (the six-line pattern not marked in Fig. 6). In the spectrum for the graphite milled in the impact mode, the ␣-Fe sextet is more visible (small lines for velocities of −5.39, −3.12, −0.90, 0.76, 2.98 and 5.15 mm s−1 that are characteristic for ␣-iron), however its contribution to the whole spectrum is still small. The main components are similar as in the case of the spectrum for the graphite milled in the shearing mode. Numerical fitting of the spectra allowed it to determine the hyperfine interactions parameters for the phases existing in the samples, i.e. the isomer shift relative to ␣-iron, δ, the quadrupole splitting, ∆, and hyperfine magnetic field, Bhf . In Table 2, the hyperfine
Table 2 Hyperfine interaction parameters obtained from the numerical fitting of the M¨ossbauer spectra δ (mm s−1 ) −0.11 0.31 0.00 −0.14 0.34 0.00 −0.105 −0.15 −0.11 −0.07/−0.10 −0.03/0.12 0.35
∆ (mm s−1 )
Bhf (T)
0.46 0.01
32.95
0.45 0.01
32.83
0.55/0.85 0.38
Component
A (%)
Identified compounds
Ref.
Singlet Doublet Sextet Singlet Doublet Sextet Singlet Singlet Singlet Singlet Doublet Doublet
12 87 1 26 68 6
Cr–Fe Fe–Ni–Cr or Fe–Cr ␣-Fe Cr–Fe Fe–Ni–Cr or Fe–Cr ␣-Fe Cr–Fe arc melted Fe–Ni mechanically alloyed Fe–Ni–Cr arc melted Fe–Ni–Cr electrodeposited Fe–Ni–Cr electrodeposited Fe–Cr chemically precipitated
This work Shearing Mode This work Impact Mode [22] [24] [23] [25] [25] [26]
δ: isomer shift relative to ␣-iron; ∆: quadrupole splitting; Bhf : hyperfine magnetic field; A: relative contribution of the component. The error for δ and ∆ is equal to ± 0.01 mm s−1 , and for Bhf is ± 0.05 T.
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interactions parameters are listed together with the relative contribution of the component that was estimated from the area of the spectral lines. It may be seen that the relative contribution of the sextet in the spectrum for the graphite milled in the impact mode is larger than that of the shearing mode. Higher energy of milling process involves more contamination by iron, and presumably also by Cr and Ni from the milling media. The comparison of the hyperfine interaction parameters with the literature data allowed it to recognize compounds which iron formed during milling process. The singlet and the doublet observed in the spectra may be attributed to Cr–Fe, or Fe–Ni–Cr, or Fe–Ni alloys. Similar parameters (added in Table 2) were observed for alloys prepared by arc melting [22,23], mechanical alloying [24], electrodeposition [25] and chemical precipitation [26]. In the light of the M¨ossbauer spectroscopy results, the peaks of crystalline phases visible in the XRD patterns may be now recognized. The most intensive diffraction peak lying at 2θ angle of 43.5◦ may originate from the Cr–Fe alloy [22] and that at about 51◦ from the Fe–Ni–Cr alloy [25]. However, two peaks at 36◦ and 74◦ remain still unrecognised. These peaks may correspond to a mixture of a range of hydrocarbons.
4. Conclusions The structural evolution of graphite ball milled under hydrogen atmosphere in the Uni-Ball-Mill 5 were examined using X-ray diffraction and linear time-of-flight mass spectrometry. The milling process leads to the formation of hydrogenated nanocrystalline graphite which coexists with iron-based paramagnetic compounds. As the mass spectrometry showed, the milling product consists of small (n < 10) bare carbon clusters Cn and hydrogenated carbon clusters Cn Hx with x = 1–4 hydrogen atoms. The energy of the milling process is the important parameter. High-energy milling (impact mode) leads to the formation of more structural defects as compared to low-energy milling (shearing mode). Thus, graphite milled in the impact mode has higher ability of hydrogen absorption than that milled in the shearing mode. On the other hand, this mode of milling involves relatively more contaminations from the milling media, as revealed by M¨ossbauer spectroscopy.
References [1] J.L. Bobet, B. Chevalier, Book of Abstracts of Fourth International Conference on Mechanochemistry and Mechanical Alloying INCOME 2003, Braunschweig, Germany, September 7–11, 2003, p. 56.
[2] S. Doppiu, P. Solsona, T. Spassov, S. Surinach, M.D. Baro, Book of Abstracts of Fourth International Conference on Mechanochemistry and Mechanical Alloying INCOME, Braunschweig, Germany, September 7–11, 2003, p. 57. [3] G. Mulas, L. Schiffini, G. Cocco, Book of Abstracts of Fourth International Conference on Mechanochemistry and Mechanical Alloying INCOME 2003, Braunschweig, Germany, September 7–11, 2003, p. 58. [4] Ch. Borchers, T.I. Khomenko, A.V. Leonov, O.S. Morozova, Book of Abstracts of Fourth International Conference on Mechanochemistry and Mechanical Alloying INCOME 2003, Braunschweig, Germany, September 7–11, 2003, p. 59. [5] S. Orimo, T. Matsushima, H. Fujii, T. Fukunaga, G. Majer, J. Appl. Phys. 90 (2001) 1545. [6] K. Shindo, T. Kondo, Y. Sakurai, J. Alloys Compd. 372 (2004) 201. [7] J. Huot, M.-L. Tremblay, R. Schulz, J. Alloys Compd. 356–357 (2003) 603. [8] K. Itoh, Y. Miyahara, S. Orimo, H. Fujii, T. Kamiyama, T. Fukunaga, J. Alloys Compd. 356–357 (2003) 608. [9] E. Jartych, A. Calka, A. Smolira, L. Michalak, Vacuum 78 (2005) 347. [10] A. Calka, A.P. Radlinski, Mater. Sci. Eng. A A134 (1991) 1350. [11] A. Calka, R.A. Varin, in: T.S. Srivastan, R.A. Varin, M. Khor (Eds.), Proceedings of the International Symposium on Processing and Fabrication of Advanced Materials IX (PFAM-IX), ASM International, Materials Park, OH, 2001, p. 263. [12] A. Bajuk, K. Gluch, L. Michalak, Rapid Commun. Mass Spectrom. 15 (2001) 2383. [13] A. Bajuk, L. Michalak, Rapid Commun. Mass Spectrom. 16 (2002) 951. [14] S. Ptasinska, A. Bajuk, L. Michalak, Vacuum 70 (2003) 439. [15] S. Ptasinska, L. Michalak, M. Smolira, Rapid Commun. Mass Spectrom. 17 (2003) 917. [16] M.C. Prieto, V.V. Kovtoun, R.J. Cotter, J. Mass Spectrom. 37 (2002) 1158. [17] M. Guilhaus, J. Mass Spectrom. 30 (1995) 1519. [18] U. Boesl, R. Weinkauf, C. Weickhardt, E.W. Schlag, Int. J. Mass Spectrom. Ion Process. 131 (1994) 87. [19] H. Handschuh, G. Gantef¨or, B. Kessler, P.S. Bechthold, W. Eberhardt, Phys. Rev. Lett. 74 (1995) 1095. [20] H. Kietzmann, R. Rochow, G. Gantef¨or, W. Eberhardt, K. Vietze, G. Seifert, P.W. Fowler, Phys. Rev. Lett. 81 (1998) 5378. [21] T. Schlath¨olter, M.W. Newman, T.R. Niedermayr, G.A. Machicoane, J.W. McDonald, T. Schenkel, R. Hoekstra, A.V. Hamza, Eur. Phys. J. D 12 (2000) 323. [22] S.K. Xia, E. Baggio-Saitovitch, F.C. Rizzo Assuncao, V.A. Pena Rodriguez, J. Phys. Condens. Matter 5 (1993) 2729. [23] S. Willgeroth, H. Ullrich, J. Hesse, J. Phys. Met. Phys. 14 (1984) 387. [24] R.B. Scorzelli, Hyp. Inter. 110 (1997) 143. [25] E. Kuzmann, M. El-Sharif, C.U. Chisholm, G. Principi, C. Tosello, K. Havancsak, A. Vertes, K. Nomura, V.K. Garg, L. Takacs, in: M. Miglierini, D. Petridis (Eds.), M¨ossbauer Spectroscopy in Materials Science, NATO Science Series—High Technology, vol. 66, Kluwer Academic Publishers, Dordrecht/Boston/London, 1999, p. 203. [26] E.G. Caspary, T. Grygar, in: M. Miglierini, D. Petridis (Eds.), M¨ossbauer Spectroscopy in Materials Science, NATO Science Series—High Technology, vol. 66, Kluwer Academic Publishers, Dordrecht/Boston/London, 1999, p. 57.
Structural transformations in graphite induced by magneto-mechanical-milling in hydrogen atmosphere A. Smolira a , M. Szymanska b , E. Jartych b,∗ , A. Calka c , L. Michalak a b
a Institute of Physics, Maria Curie-Sklodowska University, Pl.M. Curie-Sklodowskiej 1, PL-20-031 Lublin, Poland Department of Experimental Physics, Institute of Physics, Technical University of Lublin, ul. Nadbystrzycka 38, PL-20-618 Lublin, Poland c Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia
Received 29 March 2005; accepted 13 April 2005 Available online 13 June 2005
Abstract Laser desorption time-of-flight mass spectrometry, X-ray diffraction and M¨ossbauer spectroscopy methods were used for characterization of phase transformations induced in graphite during controlled reactive ball milling in hydrogen atmosphere. During milling, the crystalline structure of the graphite transformed to nanostructure as proved by X-ray diffraction studies. The hydrogen storage capacity of the nanostructured graphite depends strongly on the energy of milling process and reached 2.718 wt.% in this study. The mass spectrometry method revealed in the milling products a variety of bare carbon clusters as well as hydrogenated carbon clusters which may have from one to four hydrogen atoms. Complementary measurements performed using M¨ossbauer spectroscopy allowed it to recognize the crystalline phases of iron compounds observed in the X-ray diffraction patterns. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; High-energy ball milling; X-ray diffraction; Mass spectrometry
1. Introduction Hydrogen storage materials, i.e. materials having a large capacity of hydrogen absorption are the subjects of the intensive research from the point of view of potential application of hydrogen as a fuel. Among these materials, the alloys based on Mg, Zr and Ti are the most promising because of their low cost, light weight and high capacity of hydrogen storage (of the order of 5–8 wt.%) [1–4]. Recently, reactive ball milling in hydrogen atmosphere became a well-established method for production of hydrogen storage materials. Moreover, much attention has been paid to the investigations of hydriding properties of carbon-based nanostructural materials, which possess defective structure and where defects act as hydrogen trapping sites. It has been found that nanostructured graphite has the ability to absorb hydrogen up to 7.4 wt.% during the reactive ball milling process [5]. Moreover, the hydrogen ∗
Corresponding author. Tel.: +48 81 538 1618; fax: +48 81 525 9385. E-mail address: [email protected] (E. Jartych).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.04.154
storage capacity of ball milled carbon depends on milling time, as reported in [6]. Graphite is also treated as a catalyst. Very recently, it has been shown that addition of the graphite during milling of magnesium at high temperature under hydrogen atmosphere quickly led to magnesium hydride formation [7]. Catalytic properties of a nanostructured graphite may be explained by the fact that terminating hydrogen atoms at the edges of the nano-lattice plane of graphite create new sp3 bonding, as proved by neutron scattering technique [8]. Although reactive ball milling under hydrogen atmosphere is a potential technology for preparation of materials with high capacity of hydrogen storage, some problems need basic investigations. Hydrogen in materials may exist as molecules and/or atoms, as well as in the form of hydrocarbons [5]. The nature of the absorption/desorption behaviours has not been fully recognized. One of the methods for studying hydrogen which is delivered from the material is the thermal desorption mass spectroscopy (TDS). Some TDS spectra of mechanically milled carbon-type materials (i.e.
A. Smolira et al. / Journal of Alloys and Compounds 402 (2005) 256–262
natural graphite, activated carbon fibres and activated carbon powder) have been reported earlier [5,6]. The spectra were broad with peaks that were not fully developed. Those results indicated that hydrogen in nanocarbon hydrogenated during mechanical milling might exist in the form of H2 molecules and/or in the form of atoms occupying interstitial voids. In our previous work [9], we proposed the laser desorption (LD) and the reflectron time-of-flight mass spectrometry (RTOF MS) methods for investigations of structural evolution of graphite milled in hydrogen atmosphere. The mass spectra revealed a range of peaks with high values of the mass to charge ratio m/z. However, the intensities of the peaks were relatively small and some of them were hardly separated from the background. In order to improve the intensity of the mass spectra, the linear mode of the time-of-flight mass spectrometry (LTOF MS) technique was used in studies described in this work. The aim of the present investigations was to recognize the mass peaks in detail and also to determine
257
the hydrogen storage capacity of the graphite milled under hydrogen atmosphere. Moreover, M¨ossbauer spectroscopy was used as a complementary technique to reveal the possible Fe contaminations from the milling media.
2. Experimental details 2.1. Sample preparation Powder of graphite (mass 2 g) with a purity of 99.95% and the initial particle size of about 50 m was placed into the vial of a magneto-mill Uni-Ball-Mill 5. The milling modes in this mill can be easily adjusted from the low-energy shearing to the high-energy impact (for more detail see [10,11]). In the shearing mode, the balls both rotate and oscillate around an equilibrium position at the bottom of the milling cylinder in a strong magnetic field (Fig. 1). In the impact mode, the
Fig. 1. The Uni-Ball-Mill 5 with the FeNdB external magnets operating in the shearing and the impact modes of milling [10,11].
A. Smolira et al. / Journal of Alloys and Compounds 402 (2005) 256–262
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Fig. 2. Linear time-of-flight mass spectrometer (LTOF MS) with laser desorption (LD) ion source.
ball movement during the milling process is confined to the vertical plane by the cell walls and is controlled by an external magnetic field (Fig. 1). In both cases, magnetic field is provided by the FeNdB magnets. The intensity and direction of magnetic field can be externally adjusted allowing the ball trajectories and impact energy to be varied in a controlled manner. High carbon steel balls and stainless-steel vial were used. The milling vial was evacuated and purged several times before the final fill up with hydrogen gas up to 500 kPa pressure. The milling time of the graphite powder in hydrogen atmosphere was 116 h in the shearing mode and 100 h in the impact mode. 2.2. Sample characterization The structure of the starting graphite and the graphite milled in hydrogen atmosphere was characterized using Xray diffraction (XRD). Measurements were performed using Philips PW 1730 diffractometer equipped with Cu K␣ radiation. The Scherrer’s formula was used to estimate the grain sizes of the graphite milled in the shearing and in the impact modes. The LTOF MS, described in detail elsewhere [12–18], was used for the mass characterization of the graphite powder milled in hydrogen atmosphere as well as for the starting graphite, which was a reference sample. The graphite sample placed on the holder was lighted by the focused pulsed laser beam (Fig. 2). The nitrogen laser LN300C (λ = 337 nm) with an output pulse length of 5 ns and a maximum power of 50 kW was used. This laser irradiation causes the absorption of the laser light energy and desorption with ionisation of the sample molecules. The generated ions are accelerated in the
electric field between the sample holder and the grounded flat electrode (Fig. 2). Next, the accelerated ion beam flows in the field free region to the detector. The signal from the detector is directed to an oscilloscope (Hewlett Packard Oscilloscope, HP54615B, 500 MHz, 1 GSa/s). The ion packet created in the ion source as a mixture of different ions is separated to series of packets in dependence on the mass to charge ratio m/z. Thus the spectrum from the oscilloscope is the dependence of the intensity of ion current on the m/z ratio. For the measurements, the sample of the graphite powder (mass 5 × 10−3 g) was mixed with methanol (10 L) and was placed on a flat stainless-steel sample holder and then dried in the air at room temperature giving a visible and apparently homogenous spot of ∼5 mm diameter. The ion acceleration voltage was 17 kV. All spectra were registered as an average of 64 laser shots. In order to estimate the hydrogen storage capacity of the investigated material carbon–hydrogen–nitrogen (CHN) gas spectroscopy was carried out using a Carlo Erba Elemental Analyser Model 1106. The analysis was performed on as received, milled, and annealed samples with mass of 1.0 g. The sample was placed in a small tin foil capsule and then combusted in the presence of oxygen. The gases given off are then swept over various packings of chromium oxide, cobalt oxides, and copper using an argon carrier gas. This is done in order to further combust the gases, remove the excess of oxygen, and to reduce the nitrogen oxides to elemental nitrogen. These gases of nitrogen, water vapour and carbon dioxide are then separated in a Poropak QS gas chromatography column and then measured at a detector. The peaks areas detected are then integrated, with elemental percentages calculated from these peak areas and the initial mass.
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M¨ossbauer spectroscopy measurements were carried out at room temperature in standard transmission geometry using a source of 57 Co in a rhodium matrix. A 25-m thick metallic iron foil was taken as a standard for calibration of spectrometer.
3. Results and discussion Fig. 3 presents XRD patterns measured for the starting graphite and the graphite milled in hydrogen atmosphere in the shearing and the impact modes. It may be noticed that the main diffraction peak (0 0 2) of the starting graphite (Fig. 3a),
Fig. 3. XRD patterns of (a) starting graphite, (b) graphite milled in hydrogen atmosphere for 116 h using the shearing mode and (c) graphite milled in hydrogen atmosphere for 100 h using the impact mode in the Uni-Ball-Mill 5.
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lying between 2θ angles of 20◦ and 30◦ , is significantly broadened for the graphite milled in the shearing mode for 116 h (Fig. 3b). The grain sizes estimated using Scherrer’s formula are of the order of 10–12 nm. In the case of the impact mode (Fig. 3c), the peak (0 0 2) is very broad but has not completely disappeared. Both observations indicate that during milling the crystalline structure of the graphite was transformed to nanostructure. A similar effect was observed by Orimo et al. [5] for the graphite milled in hydrogen for 80 h using a planetary ball mill. In that case, the nanostructured graphite with the grain size of 4 nm was formed. Slightly different results were obtained by Shindo et al. [6] for natural graphite milled in hydrogen for 100 h using a similar mill. The authors observed the disappearance of the peak (0 0 2) and attributed this effect to the conversion of the crystalline graphite into an amorphous phase. In both studies, the authors did not observe any diffraction peaks caused by other crystalline phases. In contrast to the results mentioned above, the XRD patterns obtained in this work reveal clearly developed crystalline peaks for 2θ angles of about 36◦ and 43.5◦ for both modes of milling (Fig. 3b and c) and 51◦ and 74◦ in the case of the impact mode (Fig. 3c). The interpretation of these crystalline peaks, which broadening gives the average grain sizes of the order of 3–7 nm, will be given later. Fig. 4 presents the positive-ion LD mass spectra of the pure graphite and the graphite milled in hydrogen atmosphere in both shearing and impact modes. Spectrum for the starting graphite (Fig. 4a) was registered up to m/z = 400. In the case of the graphite milled in hydrogen (Fig. 4b and c), there were no well-developed peaks in the mass spectra over m/z = 100 for the shearing and m/z = 140 for the impact modes, respectively. In all spectra, the peaks with m/z = 23 and 39 represent ions of Na+ and K+ , respectively. They are typical for laser desorption/ionization investigations because of their presence in the environment and on the surface of the sample holder. In spite of the fact that these ions may be treated as contaminations, the strictly determined positions of the mass peaks for Na+ and K+ ions allow it to calibrate the spectrometer. It may be noticed that in the mass spectra of the graphite milled in hydrogen (Fig. 4b and c) rather intensive peaks with m/z = 52, 56 and 59 are present. The values of m/z may indicate the presence of Cr+ , Fe+ and Ni+ ions, respectively. The metallic ions existing in the graphite powder come from the milling media. The mass spectrum of the starting graphite (Fig. 4a) revealed a wide range of peaks, that appear regularly for a mass number being total multiplicity of 12 (n = 2–22). These peaks indicate that during laser desorption/ionization process the carbon clusters Cn are released from the graphite. Similar mass spectra were observed for small fullerenes (or clusters) Cn , with n = 4–70 atoms, obtained using a standard laser evaporation source [19,20]. As reported in our earlier work [9], mass spectrometry measurements revealed hydrocarbon molecules (besides carbon clusters) in the hydrogenated nanostructured graphite. In
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Fig. 5. Positive-ion LD mass spectra obtained by LTOF MS method in the limited range of m/z for (a) graphite milled in hydrogen in the impact mode and (b) graphite milled in the shearing mode.
the figures. Besides the peaks attributed to bare Cn clusters (m/z = 60, 72, 84, 96, 108, 120 and 132), also species with one to four additional hydrogen atoms are prominent. The possible interpretation of some peaks is given in Table 1. Our mass spectra for the hydrogenated graphite are similar to those for hydrogenated carbon clusters which were emitted during impact of highly charged Xe on C84 target [21]. The authors observed small Cn Hx clusters with n = 2–22 and concluded that for a given n, most clusters contain less than x = 4
Fig. 4. Positive-ion LD mass spectra obtained by LTOF MS method for (a) starting graphite, (b) graphite milled in hydrogen atmosphere for 116 h using the shearing mode and (c) graphite milled in hydrogen atmosphere for 100 h using the impact mode in the Uni-Ball-Mill 5.
order to discuss this problem in detail, the mass spectra for the graphite milled in hydrogen atmosphere in both shearing and impact modes were additionally measured in the limited range of m/z values (i.e. over 60). They are shown in Fig. 5. The most intensive peaks are marked by the m/z values inside
Table 1 The possible hydrogenated carbon clusters observed in the mass spectra of the graphite milled under hydrogen atmosphere in the impact and the shearing modes m/z
Cn Hx impact mode
Cn Hx shearing mode
64 85 86 87 97 99
C5 H4 C7 H1 C7 H2 C7 H3 C8 H1 C8 H3
C5 H4 C7 H2
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hydrogen atoms. Moreover, for odd n values (and n ≤ 10) the monohydro species (Cn H1 ) are the most prominent, while for even n values (and n ≤ 10) the dihydro products (Cn H2 ) are dominant. In our investigations, the first effect is observed only for n = 7, the peak with m/z = 85 for C7 H1 has the highest intensity in the spectrum for the graphite milled in the impact mode (Fig. 5a). Differences between our results and those reported in [21] are obviously connected with the various processes of hydrogenation, sample preparation, etc. However, information obtained from ref. [21] confirms the interpretation of our results given in Table 1. Another effect may be observed in this study if we compare the mass spectra for the graphite milled under hydrogen in the shearing and the impact modes (Fig. 5 and Table 1). The milling process in the impact mode gives much more bare and hydrogenated carbon clusters as compared to the shearing mode. This fact may be a result of the difference in the energy of milling in both modes. Moreover, two peaks with m/z = 113 and 115 are visible in the mass spectrum for the graphite milled in the impact mode (Fig. 5a). They may be attributed to the fragments of hydrocarbons with n = 9 carbon atoms. The primary purpose of the combustion analysis CHN was to determine the amount of the hydrogen absorbed during milling. The hydrogen storage capacity of the graphite investigated in this work reached 0.613 and 2.718 wt.% for the shearing and impact modes, respectively. It is clear that from the application point of view, the milling process performed in the impact mode is more profitable. In this case, the energy of impacts is higher than in the shearing mode and the milled graphite has more defects acting as trapping sites for hydrogen. M¨ossbauer spectroscopy based on the 57 Fe isotope allows it to investigate hyperfine interactions in any materials containing iron. Because mass spectrometry results suggested iron contaminations from the milling media, M¨ossbauer spectroscopy was used to confirm this statement. Measurements performed for the starting graphite excluded the presence of iron in the powder. Fig. 6 presents the M¨ossbauer spectra registered for the graphite milled under hydrogen in the shearing
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Fig. 6. Room-temperature M¨ossbauer spectra for the graphite milled using the impact and the shearing modes; open circles: experimental data; solid thick line: theoretical fit; solid thin line: singlet arising from the Fe–Cr alloy; dotted line: doublet arising from the Fe–Cr–Ni or Fe–Cr alloy.
and impact modes. The obtained spectra proved that during milling not only a little amount of pure iron contaminated the graphite samples, but also that the iron formed some paramagnetic compounds. Each spectrum is a superposition of some components. In the case of the graphite milled in the shearing mode, the spectrum consists of two dominating components (the single line and the doublet marked in Fig. 6) and a very weak component arising from ␣-Fe (the six-line pattern not marked in Fig. 6). In the spectrum for the graphite milled in the impact mode, the ␣-Fe sextet is more visible (small lines for velocities of −5.39, −3.12, −0.90, 0.76, 2.98 and 5.15 mm s−1 that are characteristic for ␣-iron), however its contribution to the whole spectrum is still small. The main components are similar as in the case of the spectrum for the graphite milled in the shearing mode. Numerical fitting of the spectra allowed it to determine the hyperfine interactions parameters for the phases existing in the samples, i.e. the isomer shift relative to ␣-iron, δ, the quadrupole splitting, ∆, and hyperfine magnetic field, Bhf . In Table 2, the hyperfine
Table 2 Hyperfine interaction parameters obtained from the numerical fitting of the M¨ossbauer spectra δ (mm s−1 ) −0.11 0.31 0.00 −0.14 0.34 0.00 −0.105 −0.15 −0.11 −0.07/−0.10 −0.03/0.12 0.35
∆ (mm s−1 )
Bhf (T)
0.46 0.01
32.95
0.45 0.01
32.83
0.55/0.85 0.38
Component
A (%)
Identified compounds
Ref.
Singlet Doublet Sextet Singlet Doublet Sextet Singlet Singlet Singlet Singlet Doublet Doublet
12 87 1 26 68 6
Cr–Fe Fe–Ni–Cr or Fe–Cr ␣-Fe Cr–Fe Fe–Ni–Cr or Fe–Cr ␣-Fe Cr–Fe arc melted Fe–Ni mechanically alloyed Fe–Ni–Cr arc melted Fe–Ni–Cr electrodeposited Fe–Ni–Cr electrodeposited Fe–Cr chemically precipitated
This work Shearing Mode This work Impact Mode [22] [24] [23] [25] [25] [26]
δ: isomer shift relative to ␣-iron; ∆: quadrupole splitting; Bhf : hyperfine magnetic field; A: relative contribution of the component. The error for δ and ∆ is equal to ± 0.01 mm s−1 , and for Bhf is ± 0.05 T.
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interactions parameters are listed together with the relative contribution of the component that was estimated from the area of the spectral lines. It may be seen that the relative contribution of the sextet in the spectrum for the graphite milled in the impact mode is larger than that of the shearing mode. Higher energy of milling process involves more contamination by iron, and presumably also by Cr and Ni from the milling media. The comparison of the hyperfine interaction parameters with the literature data allowed it to recognize compounds which iron formed during milling process. The singlet and the doublet observed in the spectra may be attributed to Cr–Fe, or Fe–Ni–Cr, or Fe–Ni alloys. Similar parameters (added in Table 2) were observed for alloys prepared by arc melting [22,23], mechanical alloying [24], electrodeposition [25] and chemical precipitation [26]. In the light of the M¨ossbauer spectroscopy results, the peaks of crystalline phases visible in the XRD patterns may be now recognized. The most intensive diffraction peak lying at 2θ angle of 43.5◦ may originate from the Cr–Fe alloy [22] and that at about 51◦ from the Fe–Ni–Cr alloy [25]. However, two peaks at 36◦ and 74◦ remain still unrecognised. These peaks may correspond to a mixture of a range of hydrocarbons.
4. Conclusions The structural evolution of graphite ball milled under hydrogen atmosphere in the Uni-Ball-Mill 5 were examined using X-ray diffraction and linear time-of-flight mass spectrometry. The milling process leads to the formation of hydrogenated nanocrystalline graphite which coexists with iron-based paramagnetic compounds. As the mass spectrometry showed, the milling product consists of small (n < 10) bare carbon clusters Cn and hydrogenated carbon clusters Cn Hx with x = 1–4 hydrogen atoms. The energy of the milling process is the important parameter. High-energy milling (impact mode) leads to the formation of more structural defects as compared to low-energy milling (shearing mode). Thus, graphite milled in the impact mode has higher ability of hydrogen absorption than that milled in the shearing mode. On the other hand, this mode of milling involves relatively more contaminations from the milling media, as revealed by M¨ossbauer spectroscopy.
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