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Chapter 7 – Application of Graphene composites: Raman Strain Sensors sensing ability was calculated at 0.1 % and 0.3 % strain levels as indicated. b,e) The shift of 2D band vs. loading strain. Boxes at 0 % strain indicate the behaviour of the residual stresses at the end of the deformation cycles. c,f) Shift rate of 2D band, obtained from slopes of (b) and (e). The dashed horizontal line indicates the ideal shift rates calculated by using the Gruneisen parameters (𝛾=2.7). 7.3.2.3. Sequence III: Mapping of strain over a wide-area of CVD graphene In order to demonstrate both the spatial strain mapping ability and life time of graphene sensors, a CVD graphene coating was deformed for 25 cycles using the sequence shown in Figure 7.15a. The average 2D position within the 100 square micron region of interest is shown in Figure 7.15b. As seen in previous samples, the film was initially in compression, with this compression being relaxed over the first few cycles of deformation. The strain response of the film was then found to be stable for the rest of the experiment where the maximum and minimum peak positions were constant within error. The 2D band position was also manually mapped at the start (0 % strain), mid-point (0.3 % strain), and end (0 % strain) of the initial, 6th, and 25th cycles (Figure 7.15c). The mapping was taken at step intervals of 10 ± 2 μm and confirms that the residual pre- compression relaxes over the first few deformation cycles to give a stable system. In particular, the Raman maps for cycles 6 and 25 are very similar confirming that the strain behaviour of graphene film stabilises. As noted earlier, the grain size (wrinkles) of graphene film is approximately 7 μm (Figure 7.10), hence the mapped region corresponds to approximately 14 randomly orientated grains in each direction. Therefore the relative uniformity of the Raman band position strongly suggests that individual grain orientation relative to the direction of deformation is not significant factor in the strain response. 218PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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