Preparation Of Helical Carbon Nanotubes

Helical carbon nanotubes (HCNTs) have attracted much attention because of their unique physical and chemical properties, as well as their prospective applications in various fields. Though it has achieved significant development there is still challenge in the controllable preparation of HCNTs with high yield and good purity, basing on a rational growth mechanism. This dissertation tried to take up the challenge mentioned above and promoted the yield and purity of HCNTs mildly, through optimization of experimental design.Firstly, in this thesis, a novel Nickel-based catalyst was synthesized via a sol-gel method, and used as catalyst in the synthesis of HCNTs, with acetylene as the carbon source gas under a relatively low temperature (475℃). The morphologies of HCNTs were characterized by FE-SEM, TEM, HR-TEM and the analysis hat the product is a helical and tubular structure. XRD, EDAX and Raman results indicated that the as-prepared product were some kind of graphite or carbon structures.Secondly, it was found that the particle size of the catalyst had much to do with the morphologies and yield of the product. The highest content of HCNTs and the highest yield were both obtained when the catalyst particles were about50to70nm, which is the smallest particle size in our experiments. As the catalyst particle size increasing, more flocculent or straight-like structure were obtained with a lower yield. We also researched the relationship between the reaction temperature and morphologies or yield. And a temperature of475℃was found to be the most appropriate temperature for the generation of the HCNTs. Lower temperature (<450℃) caused incomplete growth of HCNTs and short length, while higher temperature (>500℃) create straight-shaped carbon nanotubes. Reaction time was found to be another important factor that affecting the morphologies and yield. Within60min, the yield increased with reaction time and saturated after50min. While the aspect ratio increased with reaction time and the diameter stay unchanged. The growth mechanism of HCNTs were also proposed based on the unsymmetrical growth structure of HCNTs found in the experiment.Thirdly, research on the yield of product showed that7.262g product could be obtained by using0.035g catalyst precursor with a yield of210(g-HCNTs/g-catalysts) which is the highest yield ever reported. At the same time, the morphologies and structures almost stayed unchanged with a stable yield when quantity of catalyst precursor changed. This indicated that it was a feasible way for magnified preparation. Lastly, the conductive and electromagnetic properties of HCNTs were also characterized. The as-prepared HCNTs in this thesis were a good semiconducting material. It also showed better broadband microwave absorption property compared to the commercial straight carbon nanotubes.