Characterisation and development of carbon nanotube macrostructures
An investigation into the properties of CNT fibres as they relate to their microstructures has been carried out. Firstly, an overview of the current literature about CNTs and CNT materials has been set out. The properties of individual CNTs have been reviewed, and then expanded to CNT bundles and networks to explain the significant loss in properties between CNTs and the final materials. The Floating Catalyst Chemical Vapour Deposition (FC-CVD) route in particular has been reviewed in detail as the production method for CNT materials investigated in this work. The different processing paths that these materials have undergone post-production has then been explained at length. The mechanical and electrical properties of current CNT materials have been explored. In particular, it has been explained how as fibres with high linear densities are produced, the specific properties of the fibre can decline by over an order of magnitude, despite minimal changes in the CNT components of that material. This has been done via the comparison of experimental results to a finite element model (FEM) of the most common CNT microstructures (chapter 3). This model successfully described the relationship between the decline in electrical conductivity as fibre diameter increases, as well as looking at the parameter space for CNT materials to suggest what improvements can be made to prevent this effect. Mechanical testing of microfibres has been performed to observe how the mechanical response of these fibres is linked to the structure of the CNTs in the fibre (chapter 4). The fracture zone of microfibres displays similarities to fibrous composites, with nanofibrils extending from each side of the fracture. In some microfibres, the stress-strain behaviour shows evidence of this as a “tail" artefact after fracture. One microfibre was studied in detail, showing a tail length of about 100 μm, indicating a lower-bound estimate for CNT fibril length. The mechanical response of these fibres was then compared to that of thicker fibres tested, again observing the decrease in specific properties as fibre diameter increases. A new method of CNT fibre treatment has been described which lays out how to produce novel microstructures for thick CNT fibres that minimise the loss of specific material properties as the thickness of the fibre increases (chapter 5). This was achieved via a CNT fibre-twisting process in conjunction with superacid (acids with pKa <-10) treatments. The resultant fibres showed up to a 386% increase in specific electrical conductivity and up to a 106% increase in specific strength compared to the base material, which is comparable to the best acid stretching treatments. Finally, instantaneous solvation of CNTs (required for many treatments of CNT fibres) has been demonstrated for sub-superacid strength acid for the first time ever (chapter 6). This was done by applying a potential difference of about 0.7 V to CNTs submerged in concentrated sulphuric acid, driving the repulsion of CNTs from each other. This effect will enable acid treatments of CNT fibres to take place in safer, weaker acids which can be more easily used in industrial settings.
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