Mechanical properties of carbon nanotube and graphene based yarns

Carbon allotropes including graphene and carbon nanotubes (CNTs) have superior mechanical, electrical and thermal properties. Both in academia and industry, these nanomaterials offer a wide range of application areas. Transferring their superior material performance at the nanomaterial level to the macro scale applications is quite challenging which also includes material, and process related problems. To overcome aforementioned problems, the "fiber" macroscopic form is an important structural assembly that can convey the nano-scale properties of graphene and CNTs to much larger scale. This thesis traces the fiber manufacturing of carbon nanomaterials such as graphene and CNTs via traditional spinning approaches by following the methods below: • First focus is on the wet-spinning of graphene fibers. This method started with the graphene oxide (GO) synthesis. Then, custom-made wet spinning line was used to spin GO fibers continuously. Consecutively, GO fibers were reduced to obtain graphene fibers using chemical reduction methods. • Second objective of this thesis is to spin pure CNTs fibers from CNTs arrays. Hence, dry spinning method was selected for fiber formation. CNTs production was optimized to have spinnable CNTs arrays. First, this thesis aimed to produce flexible and strong graphene fibers. GO nanosheets were chemically exfoliated by using the modified Hummer's method. This approach enabled to achieve highly oriented graphene oxide liquid crystals (GOLCs), where GO nanosheets were dispersed in aqueous solutions. The physical properties of GO nanosheets were confirmed by X-ray diffraction (XRD) and Raman spectroscopy, which also pointed out their potential use as of spinning solution. Fiber spinning procedure was carried out by varying GO concentrations and coagulation baths. 30 and 40 mg ml-1 GO concentrations enabled continuous GO fiber spinning, where coagulation baths with composition of 3-5% NaOH / Ethanol and 3-5% CaCl2 / Ethanol: Deionized water (3: 1) assisted the structural integrity of fibers throughout spinning. Further washing step with methanol was added to remove coagulation bath residues on GO fibers. To gain electrical conductivity, GO reduction was carried out by using Hydroiodic Acid (HI), Ascorbic Acid (AA) and Sodium borohydride (NaBH4) chemicals. Subsequent NaBH4 chemical reduction gave graphene fibers with highest electrical conductivity of 214 S cm-1. The results showed that as-spun GO fibers with specific ultimate tensile strength of 7 N/tex show strain to failure (%) of 10%, which pointed out much higher ductility compared to studies in literature. Second objective of this thesis was to explore CNTs fiber production from vertically aligned carbon nanotubes (VA-CNTs) by the implementation of dry spinning method. First, multi-walled CNTs were produced on a coated silicon bed by chemical vapor deposition (CVD) method, which also enables high purity and ease in scalable manufacturing. Several material and process parameters such as coating material, CVD production temperature and process time, nature of carbon source were systematically studied by monitoring the changes in length of individual CNTs and their associated spinnability. Four different types of wafers with different compositions such as Fe (~ 2nm), SiO2 (~ 300 nm), Al2O3 (~10 nm) and Fe (~ 2nm), Al2O3 (~10 nm) and Fe (~ 2nm), Al2O3 (~10 nm) and Fe (~ 6nm) were used in CVD system where helium/hydrogen and ethylene gases were used as carrier gases and carbon source, respectively. The effect of water vapor on the spinnability of CNTs were also investigated. Each CNTs arrays was indexed based on their length and spinnability. Apart from their ease in fiber spinning, the intensity ratio of D and G peak (ID/IG) were examined by Raman spectroscopy. The length of CNTs arrays were measured by image processing from optical microscope images. Custom-made dry spinning device, which included a motor enabling x-axis movement, another motor for twisting and a mounted stage for CNTs arrays, was built.