INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 17 (2006) 2187–2191
doi:10.1088/0957-4484/17/9/018
Spark plasma sintering assisted diamond formation from carbon nanotubes at very low pressure J Shen1,4 , F M Zhang1,2 , J F Sun1 , Y Q Zhu3 and D G McCartney3 1
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China 2 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China 3 School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK E-mail:
[email protected]
Received 26 September 2005, in final form 6 December 2005 Published 31 March 2006 Online at stacks.iop.org/Nano/17/2187 Abstract The thermal stability of multi-walled carbon nanotubes (MWCNTs) was assessed under various spark plasma sintering (SPS) conditions. Our experimental results show that the MWCNTs transform into micrometre-sized diamonds at 1500 ◦ C and only 80 MPa during SPS. The resulting diamonds are single or agglomerated crystalline particles, with diameters up to 100 µm, and are sheathed with an amorphous carbon layer and a residue of a few CNT layers. The SPS-assisted transition from MWCNTs to diamond was investigated, and a model for CNT breakage is proposed to describe the initial diamond growth. Taking into account the rapid and simple operating features of SPS, we believe that this process exhibits great potential for diamond synthesis on a large scale.
1. Introduction As self-assembling nanostructures, carbon nanotubes (CNTs) have attracted much attention as model systems for nanoscience studies and for various potential applications. Because of their exceptional mechanical properties, carbon nanotubes offer a kind of nanosized reinforcement for polymer [1], metal [2] and ceramic [3] composites. In order to apply CNTs in composites, a better in-depth understanding of their thermal stability under severe physical conditions, such as under various high temperature and pressure conditions, is important. Some reports have shown that CNTs can maintain their tubular structures at temperatures of up to 2800 ◦ C in high vacuum [4] and are stable at 2700 ◦ C during hot pressing with tens of MPa [5]. However, CNTs have also been reported to transform to carbon onions, and even diamonds at high temperatures, when accompanied by pressures of several GPa [6, 7]. Spark plasma sintering (SPS) is a newly developed pressure assisted sintering method, characterized notably by its spark plasmas created by on–off pulsed direct current. 4 Author to whom any correspondence should be addressed.
0957-4484/06/092187+05$30.00
During SPS treatment, powders of ceramics and cermets contained in a graphite die can be processed for diverse novel bulk material applications [8]. SPS differs from other conventional sintering methods, such as vacuum sintering, hot pressing and hot isostatic pressing, in its mechanisms in that the plasma plays a vital role in promoting phase changes. Our previous results have shown that CNTs cannot retain their original structures and convert to diamonds during SPS processing [9]. In the present work, the thermal stability of CNTs in SPS was evaluated following SPS processing. The mechanism whereby CNTs accomplish a phase change to diamond at such low pressure was investigated and a model was developed. Scanning electron microscopy (SEM), Raman spectroscopy, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were all employed to characterize the materials.
2. Experimental details The MWCNT starting material was obtained from Shenzhen Nanotech Port, Ltd, China and had been produced by catalytic
© 2006 IOP Publishing Ltd Printed in the UK
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Figure 1. High-resolution TEM image (a) and Raman spectrum (b) of the starting multiwall carbon nanotubes.
chemical vapour deposition (CCVD) in which CH4 or C2 H2 was converted into CNTs at 1000 ◦ C in the presence of Ni and La catalysts. The impurities of the as-prepared MWCNTs included mainly amorphous carbon and a small fraction of residual catalyst. The purity of the as-received MWCNTs was better than 95%. The SPS experiments were carried out in a Dr Sinter® model SPS-1050 spark plasma sintering system (Sumitomo Coal Mining Co., Japan) under an axial pressure of 80 MPa and at different temperatures in a vacuum (mechanical pump). The temperature was monitored with an optical pyrometer focused on a hole (2 mm in depth and 0.5 mm in diameter) in the graphite die. The heating rate was maintained at 100 K min−1 and the applied direct current was about 1000 A (voltage 1000 ◦ C) and high pressure (>1 GPa) will promote the agglomeration of single crystal diamonds [10]. In general, SPS processes are commonly used to consolidate refractory metals and functionally graded materials which are hard to densify by conventional sintering methods [11]. The present results
Spark plasma sintering assisted diamond formation from carbon nanotubes at very low pressure
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Figure 2. SEM images of the sintered nanotube compacts showing single diamond crystals (a), a large, single diamond crystal (b), an agglomerated diamond crystal (c), and a large agglomerated diamond crystal (d).
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Figure 3. Raman spectrum of CNTs which were spark plasma sintered at 1500 ◦ C for 30 min.
thus demonstrate that SPS can also be applied to synthesize diamond crystals from carbon nanotubes. Microscopic confocal Raman spectra were obtained using a He–Ne laser beam excited at 632.8 nm. The results reveal that the starting CNTs had the D band peak at 1320 cm−1 and the G band peak at 1575 cm−1 . After sintering at 1500 ◦ C for 30 min, the samples exhibited a strong D band peak at 1332 cm−1
(figure 3), corresponding to the characteristic Raman shift of the cubic diamond phase. The G band peak of the Raman shift, relating to the variation of sp2 bonded carbon vibrations in graphite layers, also increased to 1587 cm−1 accordingly. The Raman results agree well with the SEM observations (figure 2) that diamond was directly transformed from the carbon nanotubes during SPS. A TEM image of the sintered nanotubes ground from the compacts is presented in figure 4. Some polygonal diamond crystals with a diameter 200 to 700 nm on the unconverted CNT frameworks can be clearly observed. The selected area electron diffraction (SAED) pattern (inset) is indexed as the fcc diamond [110] zone axis. A high-resolution TEM image is displayed in figure 5. The enlarged image (inset) shows the lattice spacing between neighbouring planes of the crystal to be about 0.21 nm, very close to the (111) plane separation of fcc diamond (0.206 nm). This HRTEM observation is in good agreement with the above Raman results, confirming the fcc diamond formation from CNTs under SPS rather than hexagonal diamond, whose Raman spectra features a peak at 1150 cm−1 [12]. Figure 5 also shows a diamond crystal sheathed with amorphous carbon. The inner crystalline diamond core and the outer amorphous carbon shell are connected by an atomic interface, indicated by the arrows. This atomic interface appears to be the remaining layers of the CNTs and thus provides vital evidence for diamond growth in CNTs. Sun et al 2189
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Figure 4. TEM image and corresponding SAED pattern (inset) of a diamond crystal.
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Figure 5. HRTEM image of the diamond crystal core and amorphous carbon sheath; the inset at bottom right corner shows the (111) planes with a spacing of 0.21 nm.
also reported a similar core–sheath phenomenon in their diamond rod [13] and nanocrystalline diamond [14] growth from CNTs by the hydrogen plasma post-treatment method. A mechanism that involves a carbon nanotube breaking into nano-onions and then transforming to diamond has previously been established by Wei et al [15]. Our observations reveal that the formation of such intermediate onion-like structures does indeed occur during the carbon nanotube to diamond transformation process in SPS, as shown in figure 6. A similar intermediate phase was also previously reported, in which much higher pressures were applied [6, 7]. Therefore 2190
Figure 6. HRTEM image of the carbon onions as the intermediate phase for diamond formation, showing the amorphous carbon sheath.
it is believed that the mechanism of diamond transformation from CNTs in SPS involves the conversion of nanotubes to intermediate nano-onions then to diamond. A carbon onion consists of nanoscopic pressure cells, so it favours diamond formation [16]. It is interesting that the carbon onions are also surrounded by an amorphous carbon sheath with a thickness of 2–10 nm, as indicated by the arrows in figure 6. In order to elucidate the transformation from CNTs to diamond, a model, as illustrated in figure 7, is proposed by referring to the previous model proposed by Wei et al [15]. SPS is a field activated sintering technique based on the electrical spark discharge phenomenon. A high-energy, lowvoltage sparking pulse-current momentarily generates a spark plasma at high localized temperatures. The DC pulses also result in several effects, namely, spark plasma, spark impact pressure, Joule heating, and electrical field diffusion [9]. At high temperature, and in the presence of plasmas during SPS, the CNTs (figure 7(a)) will absorb energy, leading to the breakage of some C–C bonds with high defect density and high energy, and thus the tubular structure will collapse at one point (figure 7(b)). To reduce the surface energy, the broken carbon domains tend to close from inside to outside, resulting in the formation of carbon nano-onions. Due to the smaller radius and lower energy level contained at the outer layers, these broken carbon domains cannot be converted into an associated crystal form, leaving the carbon onions sheathed by an amorphous layer, as shown in figures 7(c) and (d). Furthermore, a high internal pressure will build up due to the significant change in the lattice spacing of the graphitic layers of the carbon onions, promoting diamond nucleation within the onion cores (figure 7(e)). With the assistance of energy from SPS, the diamond nucleus will grow continuously until a specific limit, such as the termination of the reaction or the overall volume energy becoming excessive, and thus the
Spark plasma sintering assisted diamond formation from carbon nanotubes at very low pressure
SPS technique appears to be one which is highly efficient and energy saving for diamond synthesis.
4. Conclusions
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Under SPS, MWCNTs are unstable, transforming to diamond in the absence of any additional catalyst. Well-defined fcc diamond crystals, both single and agglomerated, and up to 100 µm in size, are obtained at 1500 ◦ C, with a soaking time of 30 min and an axial pressure 80 MPa. The resulting diamond is sheathed by amorphous carbon which is separated by an atomic interface from the remaining MWCNT layers. Spark plasmas are believed to play a key role for the transition of MWCNTs to diamond and the mechanism involves the breakage of some C–C bonds, the formation of carbon nano-onions and the nucleation and growth of the diamond phase within the onion cores. Finally, SPS is shown to provide an alternative method for high-efficiency diamond generation on a large scale.
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Acknowledgments We thank the National Nature Science Foundation of China (Grant No. 50374035) and the EPSRC (UK) for financial support. This work was also supported by the Program for New Century Excellent Talents in University (China).
Figure 7. Schematic illustration of the transformation model of carbon nanotubes to diamond in SPS.
References
observed diamond is generally surrounded by an amorphous carbon layer (figure 7(f)). The relatively low pressure of 80 MPa applied in the present SPS-assisted diamond generation is rather interesting when compared with other processes employed to convert CNTs to diamond, such as laser irradiation [17], shock wave synthesis [18], chemical vapour deposition by nanotube coating [19], high-pressure and/or high-temperature (HPHT) treatment [20, 21], and hydrogen plasma post-treatment [14]. Generally, pressures greater than several GPa and high temperatures are needed. In addition, catalysts such as Ni, Co, and other metals/alloys are often necessary for a significant yield of diamond. In this context, it is believed that the spark plasma plays the key role in the conversion of CNTs to diamond. Diamond forms when localized highly energetic conditions arise. Recently, a debate about whether or not spark plasmas are actually generated during the SPS process has arisen. However, in this study, the SPS-assisted diamond formation under such low pressure has provided, indirectly, some supporting evidence for the presence of spark plasmas in SPS. This is because the significant pressure reduction from the GPa to the MPa level for diamond formation suggests such plasma effects. SPS also possesses several other advantages, namely, a wide range of sintering temperatures from a few hundred to 2000 ◦ C; controllable heating rates which can be set to several hundred degrees per minute for extremely rapid processing, as well as a capacity to process large samples, for example 50 mm in diameter and 10 mm thick. Therefore, the
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