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Chapter 7 – Application of Graphene composites: Raman Strain Sensors shows the relaxation of residual pre-compression of CVD graphene in the initial two cycles. The strain sensing ability was calculated at 0.1 % and 0.3 % strain levels as indicated. b,e) The shift of 2D band vs. loading strain. The hysteresis loop observed in the exfoliated graphene indicates the interfacial damage. 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 theoretical ideal shift rate calculated by using the Gruneisen parameter (𝛾=2.7). The initial 2D band position was significantly higher in the CVD coating (~2660 cm- 1) compared to exfoliated graphene (~2640 cm-1), indicating that the CVD graphene was under residual compression. This residual compression was found throughout the sample, with the average band position being ~2659.7 ± 2.5 cm-1 over 200 random spots. The residual strain is believed to be developed from the transfer process and drying of the PMMA top coat.270 The initial two deformation cycles had allowed the graphene to relax this compression (Figure 7.12d,e). From the third cycle onwards, approximately elastic deformation occurs, indicating good stress transfer between the graphene and matrix. Unlike in the exfoliated graphene, the 2D band position at 0 % strain is constant at the end of the final three deformation cycles, indicating no further significant relaxation of residual strains. Figure 7.13: Shift of Raman 2D band of MC graphene composite coating for cyclic deformation with same maximum loading strain. Elastic deformation was observed in the cycle 1 and the development of hysteresis loop and its upshift as a result of interfacial deformation is clearly observed in cycle 4. The theoretical ideal shift rate for graphene is calculated by using the Gruneisen parameter, 𝛾, which is a measure of rate of change of phonon frequency in a crystal with 214PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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