New Synthetic Methods And Properties Of Carbon Nanotubes And Their Composites

Carbon nanotubes (CNTs) have attracted great attention due to their unique one-dimensional tubular structure, excellent physical and chemical properties, and their fascinating potential application. Moreover, because of the quantum confinement effect, the nanostructure-filled CNTs composites not only optimize the properties of CNTs, but also modulate the properties of the filling materials (nanostructures). In this paper, CNTs were prepared by CVD process, and the influences of the experimental parameters on the yield and morphology of CNTs growth were investigated; Furthermore, in-situ synthesis of nanostructure-filled CNTs and their properties were investigated, and the growth mechanism was discussed. The main results in the thesis are summarized as follows:1. CNTs were prepared by CVD process in which dimethyl sulfide was used as carbon source and Fe/MgO was used as the catalyst. The morphology and microstructure of the synthesized products were examined using SEM, Raman spectroscopy, XRD, and TEM. The results demonstrate that the appropriate temperature for the pyrolysis of C2H6S is 1000℃, and the optimum Ar flow rate for CNTs growth is around 1200～1800 sccm. There were many Y-junction CNTs in the products, possessing great potential applications in the field of nano-devices. Furthermore, the growth mechanism of the Y-junction CNTs was discussed and a simple growth model was proposed. 2. CNTs were prepared by CVD process in which ethanol was used as carbon source, and ferrocene was used as the catalyst precursor. The morphology and microstructure of the synthesized products were examined using SEM, XRD, and HRTEM. The results demonstrate that the appropriate temperature for the pyrolysis of ethanol is 860℃, and the electromagnetic properties of the products grown at different Ar flow rate and ferrocene content were studied, and the relationship bwetten reflectivity and frequency was investigated. Furthermore, the microwave absorbing properties of the samples were studied. The results show that when the thickness of the CNTs coating is 2 mm, there is a maximal reflection loss of-24.58 dB. The peak of the reflection loss shifts to lower frequency when the thickness increases.3. CNTs were prepared by CVD process in which ethanol was used as carbon source, ferrocene was used as the catalyst precursor and dichlorobenzene was used as the additive. The morphology and microstructure of the synthesized products were examined using SEM, XRD, and HRTEM. The results demonstrate that dichlorobenzene is critical for the filling of CNTs, and the filling ratio of Fe/Fe3C nanowires is high. The electromagnetic properties of the products were studied, and the relationship between reflectivity and frequency was investigated. Furthermore, the microwave absorbing properties of the samples were studied. The results are that when the thickness of the CNTs coating is 2 mm, there is a peak of reflection loss at 4.5GHz. The peak of the reflection loss shifts to lower frequency when the thickness increases, showing that the as-grown CNTs is promising as effective absorber in low frequency.4. We present a method to synthesize the novel metal sulfide nanowires-filled CNTs by a CVD process directly on stainless steel (SS) substrates, involving the pyrolysis of dimethyl sulfide on stainless steel substrate without other catalyst. In this method, dimethyl sulfide was used as carbon source for CNTs growth and sulfur source for metal sulfide nanowires, so the dimethyl sulfide concentration is the critical for the in-situ growth of metal sulfide nanowires filled CNTs. The results demonstrate that the optimum dimethyl sulfide vapor concentration for in-situ metal sulfide filled CNTs growth is around 3.36～5.48%. Moreover, the growth mechanism of the metal sulfide nanowires-filled CNTs is proposed, and the CNTs are filled with metal sulfide nanowires through capillary action. The electromagnetic and the microwave absorbing properties of the samples were studied. The results are that when the thickness of the CNTs coating is 2 mm, the peak of reflection loss is 29.58 dB at 14.80 GHz. The peak of the reflection loss shifts to lower frequency when the thickness increases. Furthermore, multiple absorbing peaks appeared, which helps to broaden microwave absorbing bandwidth, and provides larger application fields for applications in radar materials and stealth materials.