Synthesis of vapor-grown carbon fibers from camphor without catalyst and their characterization
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Indrajit Mukhopadhyay Department of Material Chemistry, Shinshu University, Ueda-386, Japan (Received 28 September 2002; accepted 7 May 2003)
Vapor-grown carbon fibers and carbon micro-beads were produced in the absence of catalysts from a natural precursor, camphor, by a thermal chemical vapor deposition process, at different temperatures in an argon medium. Scanning and transmission electron microscopy, Raman spectra, and electrical conductivity studies were used to characterize these fibers. It was observed that cylindrical fibers (diameter ∼3 m) were obtained at 1033 K and rippled fibers (diameter ∼5 m) were formed at 1273 K while carbon beads (diameter ∼0.5-1 m) were formed at 1173 K. It is proposed that agglomeration of carbon beads predominate at pyrolysis temperature greater than 1173 K, resulting into rippled type fibers.
I. INTRODUCTION
Carbon fibers of diameter smaller than 10 m produced by vapor-phase reaction are generally termed vapor-grown carbon fibers (VGCFs).1 Sometimes they are also called catalytically chemical-vapor grown carbon filaments as they are largely grown in the presence of catalysts. In general, these fibers are short, but long VGCFs have also been synthesized utilizing modified techniques.2 Currently, the high cost of conventional polyacrylonitrile (PAN) and pitch-based carbon fibers restricts their use from anything other than defense, aerospace, and some sports materials. For domestic purpose, VGCF is only an alternative to PAN and pitch-based carbon fibers with its high mechanical properties to cost ratio.3 Lower costs of these fibers lie in single-step synthesis and low-cost feeding material. Although they have poorer mechanical behavior than PAN and pitchbased carbon fibers, VGCFs have higher thermal and electrical properties. Therefore, VGCFs have great potential in preparation of composite materials.4 Significant efforts were made by studying the effect of different precursors, carrier gas, flow rate, and catalysts for the production of these fibers. Numerous precursors, namely acetylene, ethylene, benzene, toluene, n-hexane, and nbutane, have been used for the production of VGCFs. Various catalysts like iron, nickel, cobalt, and their respective salt/compounds have been extensively used for this synthesis. A detailed mechanism of growth has also been elucidated elsewhere.5,6 These carbon fibers have been applied in areas like secondary lithium batteries,7,8 electromagnetic interference shielding,9 electron field emission display,10,11 etc. J. Mater. Res., Vol. 18, No. 9, Sep 2003
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In almost all cases, these vapor-grown carbon fibers have been synthesized in the presence of catalyst particles. Two types of mechanisms, tip and base mode growth, have been proposed.5,6 However, little effort has been made to understand the growth of VGCFs in the absence of any catalyst particle. In addition, these fibers have been produced from precursors derived from fossil fuels. Hence, for 21st century applications, they should be
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