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MATERIALS RESEARCH

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Behavior of carbon nanotubes under high pressure and high temperature D.S. Tang, L.C. Chen, L.J. Wang, L.F. Sun, Z.Q. Liu, G. Wang, W.Y. Zhou, and S.S. Xiea) Institute of Physics & Center of Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, People’s Republic of China (Received 2 July 1999; accepted 8 November 1999)

The structural changes of carbon nanotubes induced by high pressure and high temperature were investigated by means of x-ray diffraction, Raman scattering, scanning electron microscopy, and transmission electron microscopy. It is shown that, with increasing pressure and temperature, the lattice constant d002 of tubes shortens, and then tubes collapse into tapelike ones; at the same time the C–C bonds at high curvature break, which lead the tapelike tubes to break into graphite sheets as diamond crystallization centers. Compared with graphite, the diamond particles from carbon nanotubes have many defects as the trace of tubes.

I. INTRODUCTION 1

Since their discovery in 1991, carbon nanotubes have attracted more and more interest for their unique morphology and exceptionally physical and chemical properties.2–6 Theoretical studies of carbon nanotubes suggest that their properties depend sensitively on their morphology and structure.7–9 Therefore high pressure, which can vary the interatomic distance of substance and thus change its phase transition, was employed as an important probe of the relation between the structure and the property of carbon nanotubes. On the basis of graphite, the transformation from carbon nanotubes to diamond under high pressure and high temperature was realized already,10,11 but the behavior of carbon nanotubes under high pressure and high temperature is not clear yet even though much experimental work has been done in this aspect. For example, Zhou et al.12 studied defects in carbon nanotubes through the average radial compressibility calculated from the pressure-induced shift in d002. Zhang et al.13 investigated the thermal stability of carbon nanotubes under 5.5 GPa. Zhu et al.14 investigated the structural change of carbon nanotubes under shock waves. In this paper, we investigate the behavior of carbon nanotubes under high pressure (up to 7.0 GPa) and high temperature (up to 1800 °C) by means of x-ray diffraction, Raman scattering, scanning electron microscopy, and transmission electron microscopy. The structural change of the sample induced by high pressure and

high temperature reveals that the tubes first become tapes and then graphitelike sheets as nuclei of diamond growing into diamond particles. II. EXPERIMENTAL

In our experiments, multiwalled carbon nanotubes, loosely entangled and most highly graphitized, were prepared by modified arc discharge.15 The subsequent purification by the oxidizing process made the carbon nanotubes free of contaminating nanoparticles and amorphous carbon [Fig. 3(a)]. The high pressure was created by a 600-ton oil pressure machine, and the pressuretransmittin