| Due to the outstanding properties and attractive potential applications, carbon nanotubes (CNTs) have attracted great scientific interests. To obtain a large-scale, continuous production technology of CNTs and to control the growth of CNTs in diameter, layer number and chirality are the dreams of scientists. Double-walled carbon nanotubes (DWCNTs) is a critical type of one-dimensional material between single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), which is regarded as the simplest MWCNT, and have special properties than SWCNTs. So the preparation of DWCNTs has been a hot topic in CNT research field..In our experiment, filament-like DWCNTs and SWCNTs have been synthesized by the floating catalytic chemical vapor deposition method with ferrocene as catalyst precursors, xylene as carbon sources and sulfur as additive via an improved diameter-changed reactor. We also investigate the influence of temperature, carrier flow rate, S;Fe ration, reaction time and feeding speed on the growth of products. The results show that good quality DWCNTs can be synthesized under the optimized growing parameters and S:Fe ratio plays a critical role on the synthesis of SWCNTs and DWCNTs. Based on the results, we build the thermodynamic model and kinetic model of DWCNTs growth on the catalyst particles. And the results also show that the diameter-changed reactor which two different diameter quartz tubes are seamless connected, can adjust the carrier gas velocity in different parts of the reactor effectively. Under the optimized conditions, the reactor can increase pyrolysis efficiency of carbon sources and promote the synthesis of DWCNTs. The reason for the zone selectivity at different flow rate and the influence of the diameter-changed structure on the growth of DWCNTs are also preliminary discussed.It is believed that this diameter-changed reactor may become a feasible equipment for the continuous synthesis of DWCNTs in the future.
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