Letters to the Editor

trans-hinged phase, the notion of coordination statistics 16. in paracrystal theory would likewise apply.

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Rich, A. and Crick, R.H.C., J. Mol. Biol., 1961, 3,

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REFERENCES 19.

Sladkov, A.M., Kasatochkin, V.I., Korshak, V.V. and 20. Kudryavtsev, Yu.P., Inventor’s Certificate, No. 107 21. (7 Dec. 1971). (Priority date 4 Nov. 1960). Sladkov, A.M. and Kudtyavtsev, Yu.P., Priroda, 1969, 22. 5, 37 (in Russian). Sladkov,A.M., Sov.Sci.Rev., 1981, B3,75. 23. Heimann, R.B., Evsyukov, SE. and Koga, Y., Carbon, 1997, 35 (in press). 24. Kudryavtsev, Yu.P., Heimann, R.B. and Evsyukov, SE., J.Mater.Sci., 1996, 31, 5557. Kavan, L., Carbon, 1994,32, 1533. 25. Kavan, L., in Chemistry and Physics of Carbon, P.A.Thrower, Ed. (Dekker, New York, 1991), vol. 23, 26. p. 69. Whittaker, A.G., Science, 1978, 200, 763. Heimann, R.B., Kleiman, J. and Salansky, N.M., 27. Naiure, 1983, 306, 164. Heimann, R.B., Kleiman, J. and Salansky, N.M., 28. Carbon, 1984.22, 147. Lindenmeyer, P.H. and Hosemann, R., J. Appl. Phys., 1963, 34, 42. Natta, G., Bassi, I.W. and Fagherazzi, G., Eur. Polym. J., 1969, 5, 239.

29. 30.

Pereeo. G. and Bassi. I.W., Makromolek. Chem.. 1963, 31.

483. Hess, K. and Klessig, H., Na&rwiss., 1943,31, 171. Hosemann, R. and Bagchl, S.N., Direct Analysis of Diffraction by Matter. North Holland. Amsterdam, 1962. Hosemann, R., J. Appl. Phys., 1963, 34,25. Stratton, W.O., J. Polymer Sci., 1959,41, 143. AMO,T. and Coulson, C.A., Proc. Roy. Sot., 1961, A264, 165. Evsyukov, S.E., Paasch, S., Ihomas, B. and Heimann, R.B., Ber. Bunsenges.Phys.Chem., 1997, 101, 837. Shlmoyama, M., Niino, H. and Yabe, A., Mukromol. Chem., 1991,193,569. Bochvar, D.A., Stankevich, I.V., Gal’pern, E.G. and Bakuradse, R.Sh., Zhurn. Srrukt. Khim., 1987, 28(4), 23 (in Russian). Kjiima, M., Toyabe, T. and Shlrakawa, H., Chem. L4&ers, 1995,553. Udod, LA., Shchurik, V.I., Bulychev, B.M., Sirotinkin, S.P., Guseva, M.B., Babev, V.G., Kudryavtsev, Yu.P. and Evsyukov, S.E., J. Mater. Chem., 1993.3, 413. Whittaker, A.G. and Wolten, G.M., Science, 1972, 178, 54. Whlttaker, A.G., Neudorffer, M.E. and Watts, E.J., Carbon, 1983, 21, 597. Whlttaker, A.G., Carbon, 1979,17, 21. Kasatochkin, V.I., Korshak, V.V., Kudryavtsev, Yu.P., Sladkov, A.M. and Sterenberg, L.E., Carbon, 1973, 11 7n __, ._. Korshak, V.V., Kudryavtsev, Yu.P., Khvostov, V.V., Guseva. M.B.. Babaev. V.G. and Rvlova. O.Yu.. 61, T9i. ’ ,

Reneker, D.H., J. Polym. Sci., 1962.59, S39. Carbon; 1987, i5, 735. Pechhold, W. and Blasenbrey, S., Kolloidz. u. 2. 32. Baughman, R.H., Galvao, D.S., Cui, C. and Dantas, Polym., 1967, 216, 235. S.O., Chem. Phys. L&f., 1997,269, 356.

Thermal stability of carbon nanotubes under 5SGPa

MING ZHANG+~, D.W.HE~, X.Y.ZHANG~, L.JP, B.Q.WEPD.H.WU* F.X.ZHANG~, Y.F.Xuband W.K.WANG~

aDepartment of Mechanic Engineering, Tsinghua University, Beijing, 100083 bInstitute of Physics, Chinese Academy of Sciences, Beijing 100080 China

(Received 20 June 1997; accepted in revised form 2 September 1997)

Key words - A. Nanotubes, C. x-ray diffraction, TEM, D. phase transition

Since the initial discovery [l], carbon nanotubes have attracted much interest because of their properties and potential applications in high-performance nanoscale materials and electronic devices [2-121. The thermal stability of carbon nanotubes has been studied by a few researchers. Ajayan et al. [13] indicated that the entire sample disappeared when weighed amounts of carbon nanotube samples produced by the arc-discharge method were heated in air at 850°C for 15 minutes. de Heer and Ugarte [ 141 investigated the heat treatment of carbon soot produced by the carbon arc method in high vacuum, and found that hollow nanometric carbon onions and nanotubes are formed when the annealing temperature up to 24OO”C, which indicates that the structure of carbon nanotubes will be maintained up to 2400°C. Monthioux [ 151 also predict that the structure of nanotubes may not be lost under 3OOO’C.

The phase transformation of a van der Waals compound under high pressure has been of considerable technical interest because graphite can be converted to diamond by the application of high pressure and high temperature. Recently many reports on fullerenes at high

pressure have been published. These studies aimed at probing the cage structure stability and looking for new structures [16-191. In particular, it has been shown that Ceo could be transformed under high pressure into the other carbon polymorphs, diamond and graphite [ 171. It has also been shown [20] that when carbon onions formed by electron irradiation of soot are heated to 7OO’C and irradiated with electrons, their cores can be transformed to diamond. The authors indicated that carbon onions act as nanoscopic pressure cells for diamond formation. Up to now, a detailed study of the thermal stability and structural transformation of carbon nanotubes under high pressure has not been carried out. In fact, the study of thermal stability and microstructural changes of carbon nanotubes under high pressure is necessary for further understanding the structure and properties of carbon nanotubes and their applications in nanocomposites.

Carbon nanotubes typically grow in arc discharge at a temperature of 3OOOK; however, the ac-grown method has not yielded large quantities of carbon nanotubes uncontaminated by other forms of carbon,

1672 Letters to the Editor

which are required for high pressure study. To date, the catalytic decomposition of hydrocarbons is regarded as an effective method of producing large quantities of nanotubes [S].

In this paper, the thermal stability of carbon nanotubes under high pressure are studied. X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used as the main methods to monitor structural changes of carbon nanotubes.

Carbon nanotubes were produced by catalytic chemical vapor deposition (CCVD), which involves the decomposition of hydrocarbons over metal catalysts at a temperature of 900K. After purification, the content of carbon nanotubes in the obtained sample is over 90%. The cylindrical shape of the sample was prepared by hydrostatic pressure, and then each sample (about 5 mg of carbon nanotubes) was pressed into a 5 mm-diameter cylinder before being placed in the high pressure cavity. The hydrostatic pressure of the Belt-type apparatus was created by a 320-ton oil press machine. A NiCr-NiAl thermocouple was brought into the pressurized zone through a pyrophyllite cylindrical gasket, graphite furnace and BN sample cell to measure the sample temperature which was controlled by a “Eurotherm” temperature controller. The temperature fluctuation was less than S.5”C. A schematic diagram of the assembly part of the sample in our high pressure experiments is shown in [21]. The samples had been annealed under 5.5GPa for 25 min. Subsequently, the temperatures were quenched to room temperature in about 1 min. followed by a gradual pressure release over 1 h. The x- ray diffraction measurement experiments of the samples were performed on M03XHF with CuKa radiation and 3 KW. All TEM observations were carried out on a H9OOONAR TEM operating at 300 kV. The samples for TEM were dispersed in ethanol, and then placed on a holey carbon microgrid.

Fig. 1 shows powder XRD patterns of as-produced carbon nanotubes and carbon nanotubes annealed at different temperatures under 5.5 GPa. The pattern for carbon nanotubes is similar to the typical powder pattern given in 122). The broadened peaks indicate that the nanotubes are in the nanometer range (Fig.la). We find that the carbon nanotubes begin

i cd

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1 A _A

(c) J

JL (b) - -

(a) I I I I

20 40 60 80

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Fig. 1. X-ray diffraction patterns of (a) as-produced carbon nanotubes ; (b) carbon nanotubes annealed under 5.5 GPa. at 950°C and (c) 115O’C.

Fig. 2. TEM images of carbon nanotubes.

transforming when the annealing temperature approaches 950°C under 5.5 GPa (Fig.lb); and are mostly transformed into graphite when the annealing temperature is 1150°C (Fig. lc). These results can also be obtained from TEM observations. The x-ray diffraction pattern of carbon nanotubes can be indexed on the basis of the hexagonal “graphite” unit cell with a larger c-axis parameter and the same C-C bond length. From these oatterns it is found that with increasing annealing temperature the interlayer spacing decreases from 0.345 to 0.334nm and the FWHM (002) decreases from 1.36” to 0.41’.

In order to further investigate the exact structural changes of these carbon nanotubes, TEM is employed. Figure 2 gives the TEM patterns of carbon nanotubes. The TEM image reveals that the carbon nanotubes have a morphology similar to that of typical carbon nanotubes, i.e., a hollow core and multiple layers of graphitic carbon arranged concentrically around the tube axis. The diameters of the nanotubes in our sample are remarkably uniform of about 30-40 nm, whereas the length is several micrometers. The nanotubes are uncontaminated by other forms of carbon except for small amount of catalyst residue. No ribbon-like and onion-like structures are found in the sample. Figure 3 shows the TEM image of carbon nanotubes annealed at 950°C

Fig. 3. TEM image of carbon nanotubes annealed at 950°C under 5.5 GPa..

Letters to the Editor 1673

a barrier (smaller than that under ambient pressure) respectively, the carbon nanotubes will be transformed into the more stable state - graphite. In other words, the high pressure can promote carbon nanotubes to transform into graphite and reduce the transition temperature, which may explain why thermal stability of carbon nanotubes decreases under high pressure.

In summary, the thermal stabiiity of carbon nanotubes under high pressure was investigated by XRD and TEM. The structure of carbon nanotubes is found to be transformed into ribbon-like and onion-like structures at only 95O”C, and forms graphite at 115O’C under 55GPa. This transformation may be induced by the high pressure, which may therefore play an important role in microstructural changes of carbon nanotubes, and decrease their thermal stability.

Fig. 4. TEM image of carbon nanotubes annealed at 1150°C under 5.5 GPa..

under 5.5 GPa. The TEM image shows that many ribbon-like and onion-like graphitic structures are found when the carbon nanotubes are annealed at 95O’C. The characteristics of hollow cored nanotubes disappear. The TEM image of the carbon nanotubes annealed at 1150°C is shown in Fig.4, the typical graphitic TEM image shows that carbon nanotubes are mostly transformed to graphite.

As we know, high pressure can provide a high vacuum condition in cylinder, thus the effects of air on the transformation can be neglected in our study. According to previous research results, under ambient pressure as well as argon protection or high vacuum, the structure of carbon nanotubes may be kept under 3000K. However, in our study, under 5.5 GPa, it begins to be changed only at 950°C and is mostly transformed into graphite at 1150°C. Although the exact mechanisms have not been understood, it may be generally believed that high pressure takes an important role in the structural transformations of carbon nanotubes here. For graphite, the interaction between the graphite layers is of van der Waals type, whereas the bonding between the neighboring carbon atoms in a graphene layer is covalent and stiff. Carbon nanotubes are nothing but the cylindrical form of graphitic sheets. There should not be much deviation in the carbon-carbon bond distance within a graphitic layer whether it is a tubule or a graphitic crystal. However, the large interlayer spacing in a &bon nanotube can give rise to uncorrelated atomic positions between carbon atoms on two adjacent cylindrical shells, and the loosely connected parallel turbostratic cylinders make it easier to bend and twist the nanotubes compared with graphite. Therefore, when carbon nanotubes are treated under high pressure, the interlayer spacing may be reduced. If the pressure and annealing temperature are high enough to make the mterlayer spacing approach a critical value and overcome

Acknowledgment - This work was supported by Post- doctor Foundation of China and National Natural Science Foundation of China. Natura. The authors would like to thank Dr. C.L. Xu for useful discussion and D.H.Peng for providing carbon nanotubes.

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REFERENCES

Iiiima. S.. Nature. 1991. 354. 56. Smalley, ‘R.E. Thess, A. and

‘Lee Mater. R., Science, Sci. E&. 1996, B, 1993, 273,483 19, 1.

Amaratunga, G.A., Chowalla, M., Kiely, C.J., Alexandrou, I., Aharonov, R. and Devenish, R.M. Nature, 1996, 383, 321. Li, W.Z., Xie S.S. and Wang, G., Science, 1996, 274, 1701 -I”-.

Charlier, J.-C., De Vita, A., Blase, X. and Car, R., Science 1996, 275, 646. Ebbesen, T.W., Phys. Today, 1996, 49, 26. Niu, C., Sichel, E.K., Hoch, R., Moy, D. and TeMent, H., Appl. Phys. Lett., 1997, 70(11), 1480. Dai, H.J., Moy, E., Lu, Y.Z., Fan, S. and Lieber, C.M., Nafure, 1995, 375, 769. Bockram, M. and Cobden, D.H., Science, 1995, 275, 1922. Carroll, D.L., Redlinch, P. and Ajayan, P.M., Phys. Rev. Let?., 1997, 78(14), 2811. Ball, P., Nature, 1996, 382, 207. Ajayan, P.M., Nature, 1993, 362, 520. Heer W.A. and Ugarte, D., Chem. Phys. Lett., 1993, 207, 480 Monthioux, M. and Lavin, J.G., Carbon, 1994, 32, 335. Samara, G.A., Hansen, L.V., Morosin, B. and Schirber, J.E., Phys. Rev. B, 1996,53, 5211. Marques, L., Hodeau, J.-L., Nunez-Regueiro, M. and Perroux, M., Phys. Rev. B, 1996.54, 12633. Ywasa .Y., Science, 1994,264, 1570. Nunez-Regueiro, M., Marques, L., Hodeau, J.-L., Bethoux, 0. and Penoux, M., Phys. Rev. L&t., 1995, 74, 278. Banhart F. and Ajayan, P.M., Nature, 1996, 382, 433. Zhang F.X. and Wang, W.K., Phys. Rev. B, 1995, 52, 3113. Rao, C.N., Seshadri, R. Govindaraj, A. and Sen, R., Mater. Sci. Eng., 1995. 15, 209.