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Chapter 7 – Application of Graphene composites: Raman Strain Sensors 7.3.2.5. Discussion: Comparison of micromechanics of exfoliated and CVD graphene In the current study, MC and CVD graphene coatings (in-house produced) were subjected to various cyclic deformation sequences to understand their deformation behaviour. It can be seen from the results that, the residual pre-strains (mainly from the transfer process) present in the graphene flakes play a major role in their mechanical behaviour as identified previously by other research groups.256, 269, 270, 272, 449, 450 For example, Fu et al. reported a slight decrease in the electrical resistance in initial stretching cycles followed by an increase in resistance in the subsequent cycles which they attributed to the relaxation of residual strains in the transferred CVD/PDMS film.309 In general, in all the experiments residual pre-compression was observed in CVD graphene which relaxes over the initial few deformation cycles. Whereas, the MC flakes initially has relatively little pre-strain. However, in general, the shift rates of MC graphene were at least ~ 35 % higher than the CVD graphene. In order to understand this difference, the structure of the two materials needs to be considered. MC graphene is a large single crystal and hence it exhibits higher stiffness and modulus and hence the shift rates. In contrast, the CVD graphene exists as a continuous single-layer film with large number of smaller domains approximately 7 μm in diameter. The domain boundaries act as a defect in the crystal structure142, 277 potentially meaning that each domains act as an individual flake. Hence shear lag theory would predict that there would be a lower strain at the edges of the domains, based upon similar arguments as that for the solvent exfoliated flakes (as explained in 7.2.3). Furthermore, some domains could be small enough for edge effects to dominate so that full stress transfer does not occur within the flake, lowering the band shift rate. Another plausible reason for incomplete transfer of stress to the graphene CVD may be the poor interface being formed or wrinkling of the PMMA/graphene film taking place during the transfer process. 223PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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