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environment, whereas CNTs are 1d and has less surface area), 2) Higher inherent band shift per unit strain of the Raman 2D band, and 3) The highest intensity of graphene’s most strain sensitive band (2D band). FLG/epoxy composite coatings were investigated as a Raman active strain sensor by following the changes in the 2D band with applied strain. Little or no strain sensitivity was observed for coatings made from flakes having a length shorter than the critical length of graphene (i.e., < 3 μm). Whereas for flakes longer than the critical length, several factors such as the processing history, loading of graphene, defects in the graphene and alignment of flakes within the composites could influence the shift rates of the band. Graphene coatings were also prepared from both MC (top-down approach) and CVD methods (bottom-up approach) and were successfully demonstrated as Raman-based strain sensor. The CVD based coatings were found to have residual compression stresses which relaxed over the first 2 to 3 deformation cycles. These results suggest that sensors made from transferred graphene should be cycled by an external strain as a conditioning step during manufacture to ensure stable and accurate readings while in use. Moreover, a hysteresis loop (interfacial damage) and its upshift were observed in MC graphene coatings in the absence of residual strains. These MC graphene samples, also exhibited a strain hardening behaviour with a higher shift rates and thus a more effective modulus than CVD graphene. In general, the shift rates of MC graphene are at least ~ 35 % higher than the CVD graphene, making the former more sensitive to strain, which may be due to MC graphene being a large single crystal domain. The absolute accuracy and resolution of these strain sensor coatings were calculated from the knowledge of 2D band position and absolute resolution of the Raman spectrometer. In comparison to the commercially available wide-area strain sensors, CVD graphene films with a calculated absolute accuracy of ~ ± 0.01 % strain and absolute resolution of ~ 27 μ𝜀 show promise for a wide area Raman-based strain sensors. Chapter 8 – Conclusions and Future Works 238PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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