Synthesis, characterization and macroscopic manipulation of carbon nanotubes
This thesis describes the synthesis, characterisation and macroscopic manipulation of carbon nanotubes. Two interconnected aspects were investigated: (i) growth of carbon nanotubes by catalytic chemical vapour deposition of acetylene and (ii) assembly and reinforcement of macroscopic carbon nanotube fibres. The first part describes a systematic study of the growth of carbon nanotubes by chemical vapour deposition. In particular, the effect of the growth parameters, such as catalyst composition, catalyst carrier, temperature, and retention time, on the yield and quality of CNTs was investigated. It was found that Fe2Co is the most active catalyst in the bimetallic family of Fe1-ξCoξ, whereas deviation from this ideal compound was identified as poisoning the catalytic reaction. Furthermore, the CaCO3 support was found to be an additional source of carbon. This chemical reaction was explained by the decomposition of CaCO3, which produces CO2 groups that react with acetylene to result in carbon for the growth of CNTs. Hence, it was discovered that carbonate supports are not only acting as an inert support to avoid the coarsening of catalyst particles but as an additional source of reactant during the growth. In the second part, macroscopic, oriented fibres were assembled from CNTs in order to exploit their excellent mechanical properties. Electrical as well as mechanical properties were measured. Although both semiconducting and metallic CNTs are present in the starting material, electrical measurements on the macroscopic ropes of CNTs revealed a hopping-like conductance of the fibres. Measurements of mechanical properties have pointed out that these fibres lack the strength of an individual nanotube. The main problem is the weak van der Waals interaction between the tubes which lead the tubes to slide easily along their axes. This problem was addressed by irradiation experiments: Pure covalent carbon-carbon bonds were established in CNT fibres by a low dose of electron and ultraviolet irradiation resulting in an enhancement of the shear modulus between the tubes. Moreover, electrical measurements have demonstrated that a low dose of electron irradiation can significantly increase the shear modulus via cross-linking of CNTs in the fibre, but a high electron dose disorders the graphitic structure and results in a loss of the intrinsically high Young's modulus of the individual tubes. In contrast to electrons, the ultraviolet photons do not displace carbon atoms in the nanotube, therefore the reinforcement can be achieved by keeping the graphitic structure of the tubes intact.