Graphitization-induced microstructural changes in tetrahydrofuran-derived pyrolytic carbon spheres
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uo Bai and Hui-Ming Chenga) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China
Qing-Kui Cai School of Materials and Metallurgy, Northeastern University, Shenyang 110004, People’s Republic of China (Received 21 October 2005; accepted 15 February 2006)
This article is a study of monodispersed, submeter-sized solid carbon spheres having smooth surfaces and almost perfectly round shapes. These spheres were synthesized by pyrolysis of tetrahydrofuran in the absence of a catalyst. Microstructures of carbon spheres before and after graphitization were systematically investigated using electron microscopy, thermogravimetric analysis, and x-ray diffraction. The sphere is believed to consist of an underdeveloped spiral-shell core and a surface with discrete fragments of concentrically arranged graphene layers. Under lower temperature heat treatment, the underdeveloped spiral-shell structure changes to a well-developed spiral-shell structure. After graphitization, the spiral-shell core is shown to transform into continuous and closed polyhedral secondary shells, whereas the exterior discrete fragments of graphene sheets transform into discontinuous polyhedral surface shells. The mechanisms of these microstructural changes are discussed.
I. INTRODUCTION
Since the discoveries of cage-structured fullerene molecules1–3 and carbon onions,4 a renewed interest has been aroused in spherical carbons for their fundamental scientific issues as well as for various potential applications. Spherical carbons can be classified into carbon onions (2–20 nm in diameter), carbon spheres (50 nm–1 m in diameter), and carbon beads (more than 1 m in diameter) based on their size.5 Carbon onions with a wellgraphitized onion-like structure have been obtained by various methods, such as high-energy electron beam irradiation of polyhedral graphitic particles,4 heat treatment of carbon soot,6 arc discharge,7 and the plasma technique.8 For industrial applications such as anode materials in secondary lithium batteries, larger carbon spheres with diameters of several hundred nanometers may be required because they are easier to synthesize than carbon onions and have shorter diffusion paths for
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0292 2198 J. Mater. Res., Vol. 21, No. 9, Sep 2006 http://journals.cambridge.org Downloaded: 13 Mar 2015
lithium ions than carbon beads of larger diameter. Up until now, various methods have been demonstrated for the synthesis of carbon spheres such as CH4 chemical vapor deposition in the presence of a metallic5 or oxide catalyst,9 pyrolysis of the vapor of solid camphor,10 or liquid pentane11 with the assistance of an iron-containing organic compound. High-pressure reduction of carbides with metal catalysis12 and a plasma arc technique with the assistance of Ni particles13 have also been used. The methods requiring catalysts certainly are more complex, and the retained
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