Preparation, morphology, and microstructure of diameter-controllable vapor-grown carbon nanofibers
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Preparation, morphology, and microstructure of diameter-controllable vapor-grown carbon nanofibers Yue-Ying Fan, Feng Li, Hui-Ming Cheng,a) Ge Su, Ying-Da Yu, and Zu-Hong Shen Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110015, China (Received 24 July 1997; accepted 23 October 1997)
Pure vapor-grown carbon nanofibers (VGCNF’s) with controllable diameters of 10–200 nm were prepared by an improved floating catalyst method. Through transmission electron microscopy (TEM) observation, it was found that VGCNF’s have a duplex structure, a hollow and high-crystallinity graphite filament called primary carbon fiber surrounded by a pyrocarbon layer with low graphite crystallinity. It was observed using high-resolution TEM that VGCNF’s have excellent graphitic crystallinity with graphite layers stacked neatly parallel to fiber axis. Moreover, x-ray diffraction results showed that the graphitic crystallinity of carbon fibers became higher with decreasing diameter of carbon fibers.
I. INTRODUCTION
Vapor-grown carbon nanofibers (VGCNF’s) are one kind of pyrolytic products made by pyrolyzing hydrocarbon compounds in the presence of a transition metal catalyst in a hydrogen atmosphere. Because of their distinctive properties, such as high strength and electric conductivity and special functional properties, scientists have shown a great deal of attention to the mass production of these materials. Tibbetts et al.1 anticipated, after studying physical properties of vaporgrown carbon fibers, that vapor-grown carbon fibers with smaller diameters should be stronger than those with larger diameters, and experimental results proved this anticipation.2 Furthermore, it was reported that the diameter of the nanofibers is governed by the size of the catalyst particles,3 and for the mass production of VGCNF’s the key process is the seeding of the catalyst particles. Tibbetts and Gorkiewicz4 produced VGCNF’s with diameters of 50–100 nm successfully using a mixture of ferrocene and liquid hydrocarbon in a vertical furnace. The results show that this method provides the possibility of mass production. However, because of the difficulty of efficiently dispersing small Fe particles in spraying catalyst solution, undefined mixtures of fibers and soot were always obtained. Rodriguez5 produced carbon nanofibers with diameters of 50 –80 nm from catalytic decomposition of hydrocarbon gas with catalyst metal powder using the support catalyst method. Although the support catalyst method can produce highquality fibers, the preparation and defined dispersion of ultrafine catalyst particles are very difficult, and the yield of this method is too inefficient to be commercialized. In order to solve the above problems, Zhang and the a)
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J. Mater. Res., Vol. 13, No. 8, Aug 1998
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present authors6 make use of the combining advantages of the floating catalyst-method and support
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