PSI - Issue 25

E. Solfiti et al. / Procedia Structural Integrity 25 (2020) 420 – 429 E. Solfiti and F. Berto / Structural Integrity Procedia 00 (2019) 000–000

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The expansion is basically a thermal shock that forces the islands to expand like ”baloons” along the c-axis (per pendicular to the basal planes). Higher the rate of thermal expansion, higher will be the volume swept by the gas. Conversely, slower the expansion and easier the possibility for the gas to percolate through the pores, resulting in a lower expanded volume. Also the flake sizes influences the e ff ectiveness of expansion both of the singular worm and in the overall amount of exfoliated volume. Such conclusion might be better understood by modeling the flakes as cylinders with a certain thickness and radius. As the radius increases, the external surface area increases slower than the internal volume and a larger part of the gaseous content should cross a longer path to percolate out of the layers (Reynolds and Greinke (2001)). The GIC obtained after heating therefore shows a cellulars structure in the microscale with disordered ”baloons” that stretched the layers and make them to bend. The cell walls are typically made up to about 60 layers as in fig.3(b). Such a structure is a natural honeycombs structure and might give a negative Poisson ratio at early stage of compression as explained by Celzard et al. (2005). Since no binder is used, during compaction the worms interlock only mechanically in their contact points and even though their identity is retained (Dowell and Howard (1986)), the constitutive cells deform and flatten out. Kobayashi et al. (2012) and Krzesin´ska et al. (2001) observed the presence of structural flat units that deform uniformly during compression. Their sizes correspond to the exfoliated graphite worms that have been compressed and they probably keep the orientation of the original basal planes along the worm backbone; henceforth they will be referred to as deformation units . The constitutive properties are thus influenced by the relative misalignment of such deformation units and their interlocking e ff ectiveness. At densities approximately ranging from 0.047 to 0.67 g / cm 3 , graphite compacts are developed (Chung (2015)) and can be classified as heterogenous disordered materials. The interlocking is still low enough to allow a good sliding among layers but giving a good visco-elastic behavior (and elastomeric as described in Chen and Chung (2015)). All of the commercial flexible graphite sheets or foils fall approximately in a range 0.7-1.36 g / cm 3 of density (being 2.26 g / cm 3 the theoretical density of the crystalline graphite) and 0.076-3 mm of thicknesses (including only unreinforced prod ucts). Thicker or more dense product have been also produced for academic purposes and tested as in Wei et al. (2010). FG is the most anisotropic type of CEG. Indeed, looking at fig.4 from Celzard et al. (2005): as the density increases, the orientation of the deformation units change and tend to rotate parallel to the bedding plane.

Fig. 4: Observed relation among density and anisotropy. Pictures by Celzard et al. (2005).

At low density and high porosity, the compound can be considered as isotropic and the deformation units randomly oriented. Their orientation strongly vary at early stages of compression but after a certain density value of approx. 0.5 0.6 g / cm 3 , such variation is no more significant and the anisotropy, or more precisely, transversal isotropy is reached (Krzesin´ska et al. (2001); Celzard et al. (2005). In-plane properties are retained in the in-plane directions but strongly di ff erentiate in the perpendicular direction. Since random misaligned particles are anyway retained and can influence that properties such as modulus and conductivity, Celzard et al. (2002) specifies that anisotropy is only influenced but not exclusively determined by the overall units orientation.

3. Mechanical properties

Almost all mechanical properties of FG depend on the following parameters: original flake sizes, ash content, chemical intercalant species, exfoliated volume, density and thickness of the final products. Their influence will be shown in the next subsections.