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Generation of curved or closed-shell carbon nanostructures by ball-milling of graphite
Journal of Crystal Growth 218 (2000) 57}61
Generation of curved or closed-shell carbon nanostructures by ball-milling of graphite X.H. Chen *, H.S. Yang , G.T. Wu , M. Wang , F.M. Deng , X.B. Zhang, J.C. Peng, W.Z. Li Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China Department of Applied Physics, Hunan University, Changsha 410082, People's Republic of China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China Received 14 February 2000; accepted 26 April 2000 Communicated by D.T.J. Hurle
Abstract Curved or closed-shell carbon nanostructures were produced by ball-milling of graphite. A high resolution indicates that the ball-milling not only produces bend of graphite sheets, forming carbon nanoarches, but also produces closed-shell carbon nanostructures, nearly carbon &onions'. The possible formation mechanism is proposed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Graphite; Ball-milling; Nanoarches; Closed-shells nanostructures
1. Introduction There has been an increasing interest in carbon nanoclusters, which are typically in the range of a few to a few tens of nanometers. The development of a single arc-discharge method to synthesize macroscopic quantities of C and C [1,2] has stimu lated researches for other fullerene-like molecules and other novel forms of carbon. Higher fullerenes such as C , C , C , C and C can be isolated [3,4], and hollow carbon nanotubes have been discovered [5]. Furthermore, concentric shelled graphitic spheres, e.g. carbon &onion', have been produced by electron beam irradiation [6].
* Corresponding author. E-mail address: [email protected] (X.H. Chen).
Among the methods to prepare nanostructured materials, mechanical milling/alloying o!ers the possibility of producing nanocrystalline structures in di!erent alloy systems. The ball-milling technique has been used to prepare various kinds of nanostructured materials, such as pure metallic elements [7,8], solid solutions and intermetallic compounds. In addition, for carbonaceous materials, such as natural graphite, based on structural investigations by X-ray di!raction and Raman scattering, it has been found that structural transformation was obtained, and the defects were induced by ball-milling [9,10]. For example, increase in d is in the step, which is similar to graphite intercalation compound (GIC) [11]. Recently, Huang et al. reported that nanoarches or curved carbon nanostructures were formed during high-milling of graphite [12]. In this report, based on investigations by transmission electron
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 4 8 6 - 3
58
X.H. Chen et al. / Journal of Crystal Growth 218 (2000) 57}61
microscopy, we have sought to study the structural transformation in graphite produced by ball-milling. Apart from the nanoarches or highly curved carbon nanostructures, we found closed-shell carbon nanostructures, nearly, carbon &onions'. Also, the possible formation mechanism of carbon nanostructures is proposed.
2. Experimental procedure The ball-milling is carried out in a conventional planetary ball mill. A powder of pure graphite (99.9%, 325 mesh) was placed in a hard-bearing steel vial under a dry pure argon atmosphere. The steel vial is sealed with an elastomer &O' ring seal. The ball-milling is performed without interruption. The weight ratio of steel balls to graphite powder is 40 : 1, and the rotation rate of the vial is 270 rpm. Milling times of 150 and 250 h are selected. Highresolution images were taken in a side-entry JEOL 2010 transmission electron microscope with a point-to-point resolution of 0.19 nm operated at 200 kV. The specimens were dispersed in acetone by ultrasound, and then dropped on the holey carbon grids.
3. Results and discussion The HRTEM images of pristine graphite is shown in Fig. 1. The original graphite is a crystalline material, and the layers are planar with a constant interlayer spacing and without noticeable defects. After 150 h of ball-milling, the samples are very heterogeneous as displayed in Figs. 2a and b. Large stacks of graphite layers are not noticeably broken and destroyed, only an exfoliation and a crumpling of graphite sheets are generated. The crumpled layers lead to various nanostructure. Fig. 2 shows the bend or curvature of graphite sheets at various angles in the samples after 150 h of ball-milling. The bent angles of graphite sheets are in the range of a few degrees to less than 1803. Angles of about 90, 120 and 1503 are most frequently observed (arrows a, b and c in Fig. 2a). Various nanoarches with one end closed were also frequently observed in the samples after the ball-
Fig. 1. HRTEM image for pristine graphite.
milling of 150 h (arrow a in Fig. 2a and arrows a and b in Fig. 2b). Some nanoarches, with bent angle less than 453, are similar to the nanotube-like morphology (arrows a and b in Fig. 2b). The formation of nanoarches or highly curved nanostructures indicate that the graphite sheets have high #exibility and tenacity. Apart from the morphology of the above nanostructures, the closed-shells graphite particles, which are of polyhedral or spherical shapes, are observed after 150 h of ball-milling. Fig. 2a shows the polyhedral shapes of the particles or elongated particles with a large hollow, which is similar to that obtained by arc-discharge [13]. The inner loops are not circular but consist of several segments of straight lines. The shapes of the capping graphite sheets is quite similar to the structure of ends of nanotubes reported by Iijima et al. [14]. From Fig. 2a, it can also be seen that the elongated particles are not completely separated from the graphitic stacks. The image of closed-shell carbon particles obtained by 150 h milling is shown in Fig. 3. Fig. 3a shows the image of a quasi-spherical graphitic particle, which is similar to that obtained by electron irradiation except having a larger central hollow core. The particles were made up of concentric graphite sheets, which are related to the so-called giant fullerenes, nearly carbon &onion'. Fig. 3b shows another onion-like nanoparticle which consists of two components, an inner metal particle and outer graphitic shells. The metal particle
X.H. Chen et al. / Journal of Crystal Growth 218 (2000) 57}61
59
Fig. 2. HRTEM image showing the high-curved graphite sheets or nanoarches produced by ball-milling for 150 h. (a) The di!erent bending angles of the graphite sheets are showed in arrow a (903), arrow b (1203) and arrow c (1503), respectively. Two polyhedral shape of carbon nanostructures are also showed in the image. (b) Various carbon nanoarches (arrow a, b, c and d), the &kink' in graphite sheet (arrow e) and broken graphite sheets (arrow f) are seen.
Fig. 3. HRTEM images of the closed-shells nanostructures with (a) a hollow core, (b) an inner metal particle.
wrapped in graphitic shell may be introduced by ball-milling as impurity [15]. The outer graphitic layers tightly surround the metal particle without any gap and bent to follow the curvature of the surface of the metal particle. Although onion-like carbon particles have been produced previously under extreme conditions, such as very high-temperature (in the case of arc-discharge) [13] or highenergy electron irradiation [6], the formation of
onion-like nanoparticle by ball-milling of graphite, to our knowledge, has not been reported before. It is evident that these closed-shell carbon particles are formed by bending of the sp sheets under the heavy mechanical deformation. Fig. 4 shows the images observed in the sample obtained by milling for 250 h. We can see that the sample is strongly disordered, and the crumpled layers crisscross, leading to a microporous
60
X.H. Chen et al. / Journal of Crystal Growth 218 (2000) 57}61
Fig. 4. HRTEM image for graphite ground 250 h of milling time.
microtexture. From Fig. 4, it can also be seen that the large unbroken aromatic layers are still remaining, despite strong exfoliation and crumpling of the graphite sheets. Compared with the sample obtained by milling for 150 h, the sample obtained by milling for 250 h do not possess distinguishable carbon nanostructures. The strongly disorganized structure of crumpling aromatic layers is similar to the annealed carbon soot at 17003C prepared by carbon arc method [16]. About the mechanism of formation, it is quite noteworthy that nanoparticles are obtained here without the help of intense electron or ionic beams, as reported in Ref. [6]. It is claimed that the high energy of the incident projectile creates many defects and provides enough structural #uidity to the graphitic structure to allow a closure of the network. For the condition of arc-discharge, the formation of the curvature of graphite sheets happened during the cooling of quasi-liquid carbon. In the case of high-temperature-treated or electronirradiated samples, the curved or closed carbon nanostructures is thought to minimize the energy of the sp sheets by eliminating dangling bonds at the end of the layer [6]. In the case of ball-milling, it is obvious that the curved or closed carbon nanostructures are generated by bending of graphite sheets. According to a report by Huang [12], the nanoarches were formed by direct curling or bending of the #at sp sheets. However, based on the investigation of high-resolution TEM image in Fig. 2b, another possible formation mechanism of
nanoarch is presented. Fig. 2b shows the various nanoarches marked by arrowheads a, b, c and d, respectively. Nanoarches a and b consist of multilayers of curved graphitic sheets, which can be thought to be formed by direct bending of the #at sp sheets, while the nanoarches c and d may be formed by another way. By careful observation of nanoarches c or d, we can see that it is the smallest nanoarch generated by only two neighboring graphite sheets linked together. We may conclude that they could not be produced by direct bending of #at graphite sheets. In fact, in the milling process, high-frequent collisions and the high velocity of steel balls make the local pressure very high (2}6 GPa), and long duration ball-milling produced a su$ciently high temperature. On the other hand, the characteristic structure of the graphite, which consists of hexagonal linked layers bonded through localized in-plane p(sp) hybrids and delocalized out-of-plane p orbits, gives it a high in-plane strength while there is a relatively weak out-ofplane interaction. Therefore the ball-milling process induces a high defect density. Arrow e in Fig. 2b indicates a typical &kink', which may be introduced with weak deformation of graphite sheets by ball-milling. When the energy generated by ballmilling is high enough to break the covalent bonds within the graphite layers, vacancies including dangling bonds are formed (arrow f in Fig. 2b). It is likely that the dangling bonds are eliminated by the high temperature produced by ball-milling. So, when two ruptured graphite sheets are adjacent, they tend to quench by producing C}C bonds to eliminate the dangling bonds, and two adjacent sheets linked together, result in the formation of nanoarch (arrows c and d in Fig. 2b). Therefore, we believe that these nanoarches are produced not only by direct bending of graphite sheets, but also by regraphization of local broken graphite sheets. As for the formation of onion-like nanoparticles by ball-milling, a two-stage model is proposed. First, the #at graphite sheets are bent or curved by ball-milling, and the graphite sheets are imperfect and include many defects and dangling bonds. Second, when the ends of the two curved graphite sheets are very close to each other, the high-energy ball-milling provides enough energy to graphitize for imperfect graphitic structure and to allow
X.H. Chen et al. / Journal of Crystal Growth 218 (2000) 57}61
a closure of the network. From an energetic point of view, the quasi-spherical carbon &onions' are the most stable form of carbon particles [17]. When the highly curved graphitic stacks with one end opened are obtained, they are in a metastable state in the light of the theory of minimization of surface area, so the need to minimize the energy drives this curling up to spherical forms like onions. Upon increasing milling, the cumulated mechanical energy produces a high density of defects into the graphene planes, provoking an intense stripping and folding of the aromatic layers, and the lamellar microtexture is progressively destroyed. The typical carbon nanostructures obtained by milling for 150 h have been lost in the sample obtained by milling for 250 h (Fig. 4). So, the results of the ball-milling not only relate to ball to power ratio, the rotation rate of the vial and medium in the vial, but also relate to the milling time. With increasing ball-milling time, di!erent carbon structure and microtexture are obtained. The curved or closed-shells carbon nanostructures are produced only in the intermediate stage of milling.
4. Conclusion Highly curved carbon nanostructures or closedshell graphite particles are formed by ball-milling of graphite. Their formation is explained by bending of the #at sp graphite layers directly, as well as by linking of broken graphite sheets under mechanical deformation. The results of ball-milling processing of graphite supply a new method to produce carbon nanostructures, especially onion-like carbon nanoparticles.
61
Acknowledgements The authors acknowledge the "nancial support from the Natural Science Foundation of China (Grant no. 59972031).
References [1] W. Kratschmer, L.D. Lamb, K. Fostiropoulos, D.R. Hu!man, Nature 347 (1990) 354. [2] R.E. Hau#er, Y. Chai, L.P.F. Chibante, J. Conceicao, C. Jin, L.-S. Wang, S. Maruyama, R.E. Smalley, Mater. Res. Soc. Symp. Proc. 206 (1991) 627. [3] F. Diederich, R. Ettl, Y. Rubin, R.L. Whtten, R. Beck, M. Arvarez, S. Anz, D. Sensharuma, F. Wudl, K.C. Khemani, A. Koch, Science 252 (1991) 548. [4] K. Kikuchi, N. Nakahara, T. Wakkabayashi, M. Honda, H. Matsumiya, T. Moriwaki, Y. Achiba, Chem. Phys. Lett. 188 (1992) 177. [5] S. Iijima, Nature 354 (1991) 56. [6] H.W. Kroto, K.G. McKay, Nature 331 (1988) 328. [7] H.J. Fecht, E. Hellstem, Z. Fu, W.L. Johnon, Metall. Trans. A 21 (1990) 2333. [8] J. Eckert, J.C. Holzer, C.E. Krill, W.L. Johnon, J. Mater. Res. 7 (1992) 1751. [9] M. Nakamizo, H. Honda, M. Inagaki, Carbon 16 (1978) 281. [10] L. Nikiel, P.W. Jagodzinski, Carbon 31 (1993) 1313. [11] J.B. Aladekoma, R.H. Bragg, Carbon 28 (1990) 897. [12] J.Y. Huang, H. Yasuda, H. Mori, Chem. Phys. Lett. 303 (1999) 130. [13] W. Kratschmer, L.D. Lamb, K. Foristopoulos, D.R. Hu!man, Nature 347 (1990) 354. [14] S. Iijima, T. Ichihashi, Y. Ando, Nature 356 (1992) 776. [15] M. Tidjani, Jalil Lachter, T.S. Kabre, R.H. Bragg, Carbon 24 (1986) 447. [16] W.S. Bacsa, W.A. De Heer, D. Ugarte, A. Chatelain, Chem. Phys. Lett. 211 (1993) 346. [17] D. Ugarte, Carbon 33 (1995) 989.
Generation of curved or closed-shell carbon nanostructures by ball-milling of graphite X.H. Chen *, H.S. Yang , G.T. Wu , M. Wang , F.M. Deng , X.B. Zhang, J.C. Peng, W.Z. Li Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China Department of Applied Physics, Hunan University, Changsha 410082, People's Republic of China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China Received 14 February 2000; accepted 26 April 2000 Communicated by D.T.J. Hurle
Abstract Curved or closed-shell carbon nanostructures were produced by ball-milling of graphite. A high resolution indicates that the ball-milling not only produces bend of graphite sheets, forming carbon nanoarches, but also produces closed-shell carbon nanostructures, nearly carbon &onions'. The possible formation mechanism is proposed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Graphite; Ball-milling; Nanoarches; Closed-shells nanostructures
1. Introduction There has been an increasing interest in carbon nanoclusters, which are typically in the range of a few to a few tens of nanometers. The development of a single arc-discharge method to synthesize macroscopic quantities of C and C [1,2] has stimu lated researches for other fullerene-like molecules and other novel forms of carbon. Higher fullerenes such as C , C , C , C and C can be isolated [3,4], and hollow carbon nanotubes have been discovered [5]. Furthermore, concentric shelled graphitic spheres, e.g. carbon &onion', have been produced by electron beam irradiation [6].
* Corresponding author. E-mail address: [email protected] (X.H. Chen).
Among the methods to prepare nanostructured materials, mechanical milling/alloying o!ers the possibility of producing nanocrystalline structures in di!erent alloy systems. The ball-milling technique has been used to prepare various kinds of nanostructured materials, such as pure metallic elements [7,8], solid solutions and intermetallic compounds. In addition, for carbonaceous materials, such as natural graphite, based on structural investigations by X-ray di!raction and Raman scattering, it has been found that structural transformation was obtained, and the defects were induced by ball-milling [9,10]. For example, increase in d is in the step, which is similar to graphite intercalation compound (GIC) [11]. Recently, Huang et al. reported that nanoarches or curved carbon nanostructures were formed during high-milling of graphite [12]. In this report, based on investigations by transmission electron
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 4 8 6 - 3
58
X.H. Chen et al. / Journal of Crystal Growth 218 (2000) 57}61
microscopy, we have sought to study the structural transformation in graphite produced by ball-milling. Apart from the nanoarches or highly curved carbon nanostructures, we found closed-shell carbon nanostructures, nearly, carbon &onions'. Also, the possible formation mechanism of carbon nanostructures is proposed.
2. Experimental procedure The ball-milling is carried out in a conventional planetary ball mill. A powder of pure graphite (99.9%, 325 mesh) was placed in a hard-bearing steel vial under a dry pure argon atmosphere. The steel vial is sealed with an elastomer &O' ring seal. The ball-milling is performed without interruption. The weight ratio of steel balls to graphite powder is 40 : 1, and the rotation rate of the vial is 270 rpm. Milling times of 150 and 250 h are selected. Highresolution images were taken in a side-entry JEOL 2010 transmission electron microscope with a point-to-point resolution of 0.19 nm operated at 200 kV. The specimens were dispersed in acetone by ultrasound, and then dropped on the holey carbon grids.
3. Results and discussion The HRTEM images of pristine graphite is shown in Fig. 1. The original graphite is a crystalline material, and the layers are planar with a constant interlayer spacing and without noticeable defects. After 150 h of ball-milling, the samples are very heterogeneous as displayed in Figs. 2a and b. Large stacks of graphite layers are not noticeably broken and destroyed, only an exfoliation and a crumpling of graphite sheets are generated. The crumpled layers lead to various nanostructure. Fig. 2 shows the bend or curvature of graphite sheets at various angles in the samples after 150 h of ball-milling. The bent angles of graphite sheets are in the range of a few degrees to less than 1803. Angles of about 90, 120 and 1503 are most frequently observed (arrows a, b and c in Fig. 2a). Various nanoarches with one end closed were also frequently observed in the samples after the ball-
Fig. 1. HRTEM image for pristine graphite.
milling of 150 h (arrow a in Fig. 2a and arrows a and b in Fig. 2b). Some nanoarches, with bent angle less than 453, are similar to the nanotube-like morphology (arrows a and b in Fig. 2b). The formation of nanoarches or highly curved nanostructures indicate that the graphite sheets have high #exibility and tenacity. Apart from the morphology of the above nanostructures, the closed-shells graphite particles, which are of polyhedral or spherical shapes, are observed after 150 h of ball-milling. Fig. 2a shows the polyhedral shapes of the particles or elongated particles with a large hollow, which is similar to that obtained by arc-discharge [13]. The inner loops are not circular but consist of several segments of straight lines. The shapes of the capping graphite sheets is quite similar to the structure of ends of nanotubes reported by Iijima et al. [14]. From Fig. 2a, it can also be seen that the elongated particles are not completely separated from the graphitic stacks. The image of closed-shell carbon particles obtained by 150 h milling is shown in Fig. 3. Fig. 3a shows the image of a quasi-spherical graphitic particle, which is similar to that obtained by electron irradiation except having a larger central hollow core. The particles were made up of concentric graphite sheets, which are related to the so-called giant fullerenes, nearly carbon &onion'. Fig. 3b shows another onion-like nanoparticle which consists of two components, an inner metal particle and outer graphitic shells. The metal particle
X.H. Chen et al. / Journal of Crystal Growth 218 (2000) 57}61
59
Fig. 2. HRTEM image showing the high-curved graphite sheets or nanoarches produced by ball-milling for 150 h. (a) The di!erent bending angles of the graphite sheets are showed in arrow a (903), arrow b (1203) and arrow c (1503), respectively. Two polyhedral shape of carbon nanostructures are also showed in the image. (b) Various carbon nanoarches (arrow a, b, c and d), the &kink' in graphite sheet (arrow e) and broken graphite sheets (arrow f) are seen.
Fig. 3. HRTEM images of the closed-shells nanostructures with (a) a hollow core, (b) an inner metal particle.
wrapped in graphitic shell may be introduced by ball-milling as impurity [15]. The outer graphitic layers tightly surround the metal particle without any gap and bent to follow the curvature of the surface of the metal particle. Although onion-like carbon particles have been produced previously under extreme conditions, such as very high-temperature (in the case of arc-discharge) [13] or highenergy electron irradiation [6], the formation of
onion-like nanoparticle by ball-milling of graphite, to our knowledge, has not been reported before. It is evident that these closed-shell carbon particles are formed by bending of the sp sheets under the heavy mechanical deformation. Fig. 4 shows the images observed in the sample obtained by milling for 250 h. We can see that the sample is strongly disordered, and the crumpled layers crisscross, leading to a microporous
60
X.H. Chen et al. / Journal of Crystal Growth 218 (2000) 57}61
Fig. 4. HRTEM image for graphite ground 250 h of milling time.
microtexture. From Fig. 4, it can also be seen that the large unbroken aromatic layers are still remaining, despite strong exfoliation and crumpling of the graphite sheets. Compared with the sample obtained by milling for 150 h, the sample obtained by milling for 250 h do not possess distinguishable carbon nanostructures. The strongly disorganized structure of crumpling aromatic layers is similar to the annealed carbon soot at 17003C prepared by carbon arc method [16]. About the mechanism of formation, it is quite noteworthy that nanoparticles are obtained here without the help of intense electron or ionic beams, as reported in Ref. [6]. It is claimed that the high energy of the incident projectile creates many defects and provides enough structural #uidity to the graphitic structure to allow a closure of the network. For the condition of arc-discharge, the formation of the curvature of graphite sheets happened during the cooling of quasi-liquid carbon. In the case of high-temperature-treated or electronirradiated samples, the curved or closed carbon nanostructures is thought to minimize the energy of the sp sheets by eliminating dangling bonds at the end of the layer [6]. In the case of ball-milling, it is obvious that the curved or closed carbon nanostructures are generated by bending of graphite sheets. According to a report by Huang [12], the nanoarches were formed by direct curling or bending of the #at sp sheets. However, based on the investigation of high-resolution TEM image in Fig. 2b, another possible formation mechanism of
nanoarch is presented. Fig. 2b shows the various nanoarches marked by arrowheads a, b, c and d, respectively. Nanoarches a and b consist of multilayers of curved graphitic sheets, which can be thought to be formed by direct bending of the #at sp sheets, while the nanoarches c and d may be formed by another way. By careful observation of nanoarches c or d, we can see that it is the smallest nanoarch generated by only two neighboring graphite sheets linked together. We may conclude that they could not be produced by direct bending of #at graphite sheets. In fact, in the milling process, high-frequent collisions and the high velocity of steel balls make the local pressure very high (2}6 GPa), and long duration ball-milling produced a su$ciently high temperature. On the other hand, the characteristic structure of the graphite, which consists of hexagonal linked layers bonded through localized in-plane p(sp) hybrids and delocalized out-of-plane p orbits, gives it a high in-plane strength while there is a relatively weak out-ofplane interaction. Therefore the ball-milling process induces a high defect density. Arrow e in Fig. 2b indicates a typical &kink', which may be introduced with weak deformation of graphite sheets by ball-milling. When the energy generated by ballmilling is high enough to break the covalent bonds within the graphite layers, vacancies including dangling bonds are formed (arrow f in Fig. 2b). It is likely that the dangling bonds are eliminated by the high temperature produced by ball-milling. So, when two ruptured graphite sheets are adjacent, they tend to quench by producing C}C bonds to eliminate the dangling bonds, and two adjacent sheets linked together, result in the formation of nanoarch (arrows c and d in Fig. 2b). Therefore, we believe that these nanoarches are produced not only by direct bending of graphite sheets, but also by regraphization of local broken graphite sheets. As for the formation of onion-like nanoparticles by ball-milling, a two-stage model is proposed. First, the #at graphite sheets are bent or curved by ball-milling, and the graphite sheets are imperfect and include many defects and dangling bonds. Second, when the ends of the two curved graphite sheets are very close to each other, the high-energy ball-milling provides enough energy to graphitize for imperfect graphitic structure and to allow
X.H. Chen et al. / Journal of Crystal Growth 218 (2000) 57}61
a closure of the network. From an energetic point of view, the quasi-spherical carbon &onions' are the most stable form of carbon particles [17]. When the highly curved graphitic stacks with one end opened are obtained, they are in a metastable state in the light of the theory of minimization of surface area, so the need to minimize the energy drives this curling up to spherical forms like onions. Upon increasing milling, the cumulated mechanical energy produces a high density of defects into the graphene planes, provoking an intense stripping and folding of the aromatic layers, and the lamellar microtexture is progressively destroyed. The typical carbon nanostructures obtained by milling for 150 h have been lost in the sample obtained by milling for 250 h (Fig. 4). So, the results of the ball-milling not only relate to ball to power ratio, the rotation rate of the vial and medium in the vial, but also relate to the milling time. With increasing ball-milling time, di!erent carbon structure and microtexture are obtained. The curved or closed-shells carbon nanostructures are produced only in the intermediate stage of milling.
4. Conclusion Highly curved carbon nanostructures or closedshell graphite particles are formed by ball-milling of graphite. Their formation is explained by bending of the #at sp graphite layers directly, as well as by linking of broken graphite sheets under mechanical deformation. The results of ball-milling processing of graphite supply a new method to produce carbon nanostructures, especially onion-like carbon nanoparticles.
61
Acknowledgements The authors acknowledge the "nancial support from the Natural Science Foundation of China (Grant no. 59972031).
References [1] W. Kratschmer, L.D. Lamb, K. Fostiropoulos, D.R. Hu!man, Nature 347 (1990) 354. [2] R.E. Hau#er, Y. Chai, L.P.F. Chibante, J. Conceicao, C. Jin, L.-S. Wang, S. Maruyama, R.E. Smalley, Mater. Res. Soc. Symp. Proc. 206 (1991) 627. [3] F. Diederich, R. Ettl, Y. Rubin, R.L. Whtten, R. Beck, M. Arvarez, S. Anz, D. Sensharuma, F. Wudl, K.C. Khemani, A. Koch, Science 252 (1991) 548. [4] K. Kikuchi, N. Nakahara, T. Wakkabayashi, M. Honda, H. Matsumiya, T. Moriwaki, Y. Achiba, Chem. Phys. Lett. 188 (1992) 177. [5] S. Iijima, Nature 354 (1991) 56. [6] H.W. Kroto, K.G. McKay, Nature 331 (1988) 328. [7] H.J. Fecht, E. Hellstem, Z. Fu, W.L. Johnon, Metall. Trans. A 21 (1990) 2333. [8] J. Eckert, J.C. Holzer, C.E. Krill, W.L. Johnon, J. Mater. Res. 7 (1992) 1751. [9] M. Nakamizo, H. Honda, M. Inagaki, Carbon 16 (1978) 281. [10] L. Nikiel, P.W. Jagodzinski, Carbon 31 (1993) 1313. [11] J.B. Aladekoma, R.H. Bragg, Carbon 28 (1990) 897. [12] J.Y. Huang, H. Yasuda, H. Mori, Chem. Phys. Lett. 303 (1999) 130. [13] W. Kratschmer, L.D. Lamb, K. Foristopoulos, D.R. Hu!man, Nature 347 (1990) 354. [14] S. Iijima, T. Ichihashi, Y. Ando, Nature 356 (1992) 776. [15] M. Tidjani, Jalil Lachter, T.S. Kabre, R.H. Bragg, Carbon 24 (1986) 447. [16] W.S. Bacsa, W.A. De Heer, D. Ugarte, A. Chatelain, Chem. Phys. Lett. 211 (1993) 346. [17] D. Ugarte, Carbon 33 (1995) 989.