| Carbon nanotubes are proved to have good mechanical, electrical, thermal properties by both simulations and experiments. There are many problems in synthesis of carbon nanotubes for the study of practical applications of carbon nanotubes, such as: orientation, conductivity, number of walls. Some of these problems can be solved by the post-processing after production, but the ultimate solution is the controlled synthesis of the carbon nanotubes. In this paper single-walled carbon nanotubes (SWNTS), double-walled carbon nanotubes (DWNTs) and triple-walled carbon nanotubes (TWNTs) are synthesized respectively by thermal chemical vapor deposition through controlling the structure and composition of catalysts. The walls number of carbon nanotubes can be controlled and the mechanism of synthesis is discussed.In this paper, the freeze-drying method was used in the preparation of the Fe/Mo/MgO catalyst for the controlled production of carbon nanotubes with Fe(NO3)·9H2O, (NH4)6Mo7O24·4H2O and Mg(OC2H5)2 for precursors. The synthesis of carbon nanotubes was conducted in a quartz tube fixed in a tubular furnace by flowing of H2 and CH4. It was found that (a) when the mole ratio of Fe/Mo/Mg is 1:0.02:13, the reacting gas is CH4/H2=300/200 (sccm), the product mainly contained SWNTs; (b) when the mole ratio of Fe/Mo/Mg is 1:0.1:13, the reacting gas is CH4/H2=300/200 (sccm), the product mainly contained DWNTs;(c) when the mole ratio of Fe/Mo/Mg is 1:0.5:13, the reacting gas is CH4/H2=150/200 (sccm), the product mainly contained TWNTs. With the increase of mole ratio of Mo, the walls number of carbon nanotubes was increased correspondingly.HRTEM and Raman spectroscopy revealed that diameters of SWNTs were about 0.8-2.0nm; DWNTs about 2-4nm; MWNTS about 4-6nm. TGA and DTA revealed that the burning temperature of SWNTs was about 610℃ and the yield was 28.8%; then that of DWNTs was 640-660℃ and 51.5%; and that of MWNTs was 670-725℃ and 69.2%. The characters of DWNTs were similar to SWNTs.Further investigations were carried out upon carbon nanotubes synthesis under different conditions by altering reacting times and reacting gas flow rate. X-Ray Diffraction (XRD) revealed that Mo2C was the important composition through the catalyzing. The investigation of TEM and yield of carbon nanotubes suggested that the reacting gas flow rate also could change the walls number of carbon nanutubes. In this study, reacting gas decomposition rate on the face of catalysts and carbon transmission in catalysts both could affect on the walls number in the synthesis processing of carbon nanotubes...
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