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Chapter 7 β Application of Graphene composites: Raman Strain Sensors strain.270 For the uniaxial strain the Gruneisen parameter of 2D band (πΎ2π·) could be defined as, πΎ =β 1 π₯π2π· Equation7.1 2π· π0 π (1βπ) where, βπ βΙ is the shift rate of 2D band, π0 is initial position of 2D band 2π· 2π· before straining and π is the Poissonβs ratio. In the literature, various values of Gruneisen parameters are reported and it is still a matter of argument.267, 447 The Poissonβs ratio of the polymer coatings was used 0.33 as the graphene exhibited elastic deformation indicating good adhesion between graphene and the substrates once the residual strain had been annealed. This ratio was taken as 0.33, as the Poisson ratio of PMMA is 0.3-0.35 and SU-8 is a transversely isotropic material with an in-plane Poisson ratio of 0.33. The Gruneisen parameter value of 2.7 was used as calculated by the first-principle calculations of Mohiuddin et al.256 near ideal shift rates of -62 cm-1/% strain (Figure 7.12c,f) were experimentally obtained for both MC and CVD graphene which is similar to the value (~ - 60 cm-1/% strain) reported in the published literature.256, 269, 273, 274 The shift rates of the loading and unloading curves of MC graphene increases with the deformation cycles and reached the theoretical ideal shift rate (Figure 7.12c). This behaviour could be related to the strain hardening of the graphene, meaning graphene exhibits more resistance to the deformation in the subsequent deformation cycles. The reason for this behaviour is unclear at this point, but the flattening of the ripples present in the graphene during the deformation could be one plausible explanation. However, strain hardening behaviour was not observed for MC graphene with pre-existing residual strains (from fabrication step), undergoing cyclic loadings. As explained in 7.3.2.4, exfoliated graphene was under residual tension before the cyclic deformation. Upon cyclic deformation, the residual strain relaxed and graphene exhibited elastic deformation. Whereas for the CVD graphene, the shift rate is approximately half of the theoretical ideal shift rate ~ -30 cm-1/% strain. This shift rate however is lower than the results of -72 cm-1/% strain from stretching CVD graphene on PDMS. It should be noted that the work of Bissett et al. used a more deformable substrate (PDMS) and hence the 2π· 215PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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