Continuous production of carbon nanotube-grafted fibres: a route to manufacture hierarchical composites
Creators: name: De Luca, Hugo
Publisher: name: Imperial College London
Year: 2021

description: The mechanical performance of structural composites depends strongly on the interface between the reinforcing fibres and the surrounding matrix. Nanoengineering this interface with carbon nanotubes (CNTs) improves its strength and toughness, yet has proved difficult to scale. Typically, the CNT coverage is heterogeneous, poorly oriented, or has an inappropriate thickness, whilst the grafting process degrades the properties of parent fibres. This thesis focuses on the development of a new process to produce CNT-grafted structural fibres continuously (1 to 5 m.h-1), reel-to-reel, on whole tows in an open chemical vapour deposition (CVD) reactor. The process operates on both carbon (AS4) and quartz (Quartzel) fibres, following a continuous bi-catalyst precursor deposition step. The CNT coating coverage and thickness was optimised, following an in-depth investigation of a range of process variables, including reactive gas composition, catalyst reduction and growth duration, growth temperature and line speed. The thickness of the CNT layer is a crucial factor; in the literature, most CNT layers are so long (compared to the primary fibre diameter) that the overall fibre volume is reduced. Here, the goal was to grow CNTs with lengths around a few hundred nanometres, to reinforce the interface without increasing the primary fibre separation. In the case of the quartz fibres, morphology improvement through fine-tuning of the catalyst deposition and CVD process variables produced a uniform CNT coating around 200 nm, throughout the fibre tow. Using single fibre pull-out tests, the coating was found to increase the interfacial shear strength (IFSS) with an archetypical epoxy to 90.3 ± 2.1 MPa (+12%) compared to commercially-sized quartz fibres. Percolation through the coating, predominantly parallel to the fibres, provided a high electrical conductivity (2.7 ± 0.3 S m1) to the otherwise insulating quartz fibre composites. This conductive path displayed a piezoresistive response that allowed monitoring of both elastic strain and the onset of fibre breakage/debonding. The ability to detect the accumulation of damage was validated by comparison to simultaneous acoustic emission measurements collected during mechanical testing. Grafting CNTs on carbon fibres is particularly challenging, as the catalyst for CNT growth intrinsically dissolves carbon and can accelerate gasification reactions; catalyst pitting is, therefore, common and typically reduces the underlying fibre mechanical properties. The application of an in-situ potential difference (300 V), between the fibres and a cylindrical graphite foil counter electrode, however, enhances the growth and significantly reduces damage to the parent fibres. The properties of the fibres were evaluated by bundle composite tensile tests. Uniform growth of small diameter ca. 10 nm and 300 nm long CNTs was found to improve the IFSS with epoxy to 96.7 MPa (+7.5%), compared to that of as-received carbon fibres. Interestingly, the interfacial strength improved further to 100.2 MPa when combined with an epoxy system reinforced with 1 wt.% CNT. This synergistic effect between the nanoengineered epoxy and the nanoengineered carbon fibres was attributed to simultaneous reinforcement of both the interphase and the surrounding matrix. The interfacial improvements measured at the microscale, by single fibre tests, were investigated at the macroscale with the manufacture of short beam shear testing coupons. Continuous, reel-to-reel, CVD is a practical and scalable route to produce hierarchical fibre feedstocks in large quantities. It provides a path towards the manufacture of large multifunctional composites for commercial applications.