Raman spectroscopic characterization of submicron vapor-grown carbon fibers and carbon nanofibers obtained by pyrolyzing
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Raman spectroscopic characterization of submicron vapor-grown carbon fibers and carbon nanofibers obtained by pyrolyzing hydrocarbons M. Endo, K. Nishimura, Y.A. Kim, K. Hakamada, and T. Matushita Faculty of Engineering, Shinshu University, 500 Wakasato, Nagano 380, Japan
M.S. Dresselhaus and G. Dresselhaus Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 8 March 1999; accepted 23 September 1999)
Variations of the properties of submicron vapor-grown carbon fibers (VGCFs) and nanofibers, with diameters around 0.1–0.2 m and 80–100 nm, respectively, are observed by Raman spectroscopy as a function of heat-treatment temperature. The microstructural evolution strongly depends on the original properties of the material, such that the main transition temperatures associated with the onset for establishing two-dimensional graphene ordering are defined below 1500 °C for the nanofibers and 2000 °C for the submicron VGCFs, respectively. The relative intensities (ID/IG) of the as-grown phase for submicron VGCFs and nanofibers are 3.44 and 1.35, while those for the corresponding graphitized samples are 0.393 and 0.497, respectively. Vapor-grown carbon fibers (VGCFs) can be successfully obtained by the decomposition of hydrocarbons, over substrate-seeded ultrafine catalyst particles and also over floating ultrafine catalyst particles of transition metals, under a hydrogen atmosphere.1–6 The VGCF material has shown promise for potential applications as a functional filler for various kinds of composite materials because of its unique properties, such as high thermal and electrical conductivity, and excellent mechanical properties.7 In terms of morphology, it is reported that VGCFs consist of a duplex structure due to different growth processes. The core part, made of long, straight, and parallel carbon layers, is primarily formed by catalytic effects, while the sheath part corresponds to pyrolytic carbon deposited during the secondary thickening growth process, resulting in an annular ring structure.8 Therefore, it is very important to understand both parts of the microstructure for optimizing the VGCFs properties in relation to their manufacturing conditions. Recently, we produced two kinds of VGCFs using the same apparatus with different processing conditions, such as temperature, time, and benzene partial pressure.9 The type with diameters in the range 80–100 nm is labeled as “nanofibers,” whereas the other type with diameters in the range 0.1–0.2 m is labeled as “submicron VGCFs,” hereafter denoted as s-VGCFs. On the basis of the nanostructure observed by a transmission electron microscopy (TEM) study,9 it is assumed that the former corresponds to almost the pure core part of the VGCFs, whereas the latter corresponds to very thin VGCFs, which grow by the same process as the thicker ones with diameters of ∼10 m. These s-VGCFs and nanofibers could be very promising as materials with novel func4474
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J. Mater. Res., Vol
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