The Study Of Preparation, Characterization And Properties Of Helical Carbon Nanotubes

With people's knowledge to nano-structured materials enriching very fast, the study to nano-structured materials has focused from preparing nanoparticles to synthesizing nano composites with special properties and nanostructures with some peculiar properties. Colloid chemistry method has been widely used to prepare many kinds of nano-structured materials with special microstructures because its reaction is in liquid, its reaction condition is mild and uniform, and its reaction condition is controllable. Nowadays, nano composites have attracted great interest. Nano particles with core/shell structure are typical examples of these nano composites. Core/shell nano particles have aroused many interests because of their many controllable advantages. The main difficulty for the use of metals arises from their instability toward oxidation in air, which becomes easier as the size gets smaller. Both silica and carbon are inert materials. The advantage of using them as shell materials coating magnetic particles is that both of them are very stable in water. We can easy control the interparticle interactions through controlling the shell thickness, and prevent grain growth and agglomeration during heat treatment too.Otherwise, with the discovery of microwave absorption of carbon nanotubes (CNTs), the microwave absorption of CNTs has aroused great interests in this field. The HCNTs have been proved to have special properties by theory and experiments because of their special morphology and microstructure.In this thesis, we focused on the studies of the synthesis, characterization, magnetic and microwave absorption properties of helical carbon nanotubes obtained using Ni as catalyst. Moreover, carbon nanotubes decorated with Fe/SiO2 nanoparticles synthesized via sol-gel combined hydrogen reduction method and the decomposition of acetylene. Colloid chemistry method was widely used in all of these experiments of the thesis. Our main work includes:1. The preparation, characterization and properties of carbon nanotubes coated with Fe nanoparticles with silica shell.In this work, nickel chloride, ferrous chloride, citric acid and ethanol were used as raw material. The carbon nanotubes coated with Fe/SiO2 nanoparticles obtained through non-aqueous sol-gel combined hydrogen reduction method, were prepared by using decomposition of acetylene gas at the relatively low temperature 475℃. Our results show that this is a simple, low-cost and environmentally friendly route for the mass production of these kind of new carbon nanotubes coated with Fe/SiO2 nanoparticles. The XRD and Raman results reveal that the as-prepared composites are graphite with defect, polycrystalline Ni and Fe nanoparticles. The field-emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscope (HR-TEM) images show that the samples have tubular structure with many defects, and there exist a large number of pair-like structures. Moreover, the magnetic and microwave absorption properties of the as-prepared sample are also investigated. These results show that the magnetic properties of the sample are greatly increased by adding Fe nanoparticles with silica shell to carbon nanotubes compared with pure carbon nanotubes. Furthermore, the results also demonstrate that the microwave absorption of the samples at high frequencies is good.2. The preparation, characterization and properties of HCNTs with high helical purity.Over Ni nanoparticles generated by means of a sol-gel combined hydrogen reduction method, HCNTs with high purity and highly symmetric structures were synthesized in acetylene pyrolysis at relatively low temperature (425℃) using Ni as catalyst. Our results show that this is a simple, low-cost and environmentally friendly route for the mass production of HCNTs with high purity. The FE-SEM and HR-TEM images show that there are often two nanotubes with opposite handedness fused in one nanoparticle. The XRD and Raman results reveal that the composite of the as-prepared sample are graphite with defect and polycrystalline Ni. Moreover, the magnetic and microwave absorbing properties of the as-prepared sample are also investigated.