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Chapter 3 – Raman Spectroscopy of Graphene and Graphene-based Strain Sensors These studies are beneficial in terms for using graphene as reinforcement in bulk composites, because, the bulk scale production of graphene indeed produces few-layer graphene onwards with various thickness and lateral dimensions.50, 118, 127 For instance, it has been shown by electrochemical synthesis FLG (with little defects) with different lateral dimensions can be produced in large scale.123 Bulk composites produced, in PP124 and PMMA125, from the 20 μm (electrochemical synthesis method) diameter flakes showed increased reinforcing ability (even at 20 wt.%) than 5 μm diameter flakes, in agreement with the above argument by Gong et al.274 Whereas the LPE method produces FLG flakes that are few microns or sub-micron in diameter, which showed very little reinforcement in polyvinyl alcohol (PVA) composite films.291 In some cases, researchers have followed G band shift with strain to study the stress transfer in bulk composites. Srivastava et al. have observed G band shift rate of -2.4 cm-1/% strain for thermally-reduced GO (TRGO)/PDMS composites compared to -0.1 cm- 1/% strain for single-walled carbon nanotubes (SWNT)/PDMS composite, suggesting enhanced load transfer effectiveness for graphene.275 While Xu et al. observed slightly higher G band shift rate of -4.2 cm-1/% strain for TRGO/PDMS composites, which was attributed to slight degree of functionalisation and doping during the sample preparation procedure.292 In case of GO, where the 2D band is absent and the G band is overlapped with D’ band, the D band could be followed for stress transfer studies in composites as described by Li et al.293 With the knowledge of the shift rates and Gruneisen parameter, effective Young’s modulus of GO was derived, and found it decreased with increased loading. Similar results were obtained when using GO as reinforcements in bulk composites294, contrary to unfunctionalised graphene produced by electrochemical methods124, 125, suggesting that increasing the concentration of GO (more than 1 wt.%) resulted in aggregation within the composites with deteriorating properties. All these studies demonstrate the unambiguous importance of Raman band shifts with strain. Most importantly all the Raman bands of graphene are sensitive to strain. Even though the 2D band exhibits a much broader line width compared to G band, its high Raman sensitivity/shift rate (Table 3-1) allows even small strain/stress variations to 90PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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