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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applications are plasma facing, microelectronic cooling (Song et al. (2005); Marotta et al. (2005),insulation material in crucibles and low friction applications.

2. Microstructure of flexible graphite

The name graphite is assigned to all those materials that share the same fundamental structures i.e. a series of stacked parallel layer planes made of carbon atoms bonded by strong covalent bonds; the hybridized sp 2 orbital allows a regular hexagonal structure (Pierson (1994)) as in fig.2. Among each layer weak bonds (Van der Waals) exist and allow the planes to slide one past another resulting in a preferential shear direction of deformation.

Fig. 2: Crystalline structure of graphite

Natural graphite flakes in fig.1(a) are highly crystalline and can be roughly described as flat disks with a average diameter of the order of 500 µ m and thicknesses one tenth thereof (Celzard et al. (2005)). The intercalation process includes the addition of one or more liquid-state chemical substances among the graphite layers of the singular flake and their vaporization upon heating. The high pressure delivered make the flakes to expand over 300 times the original thickness and create the accordion-like worms. The most used intercallant is H 2 S O 4 (sulphuric acid), which guaran tees an high e ffi ciency on the volume exfoliation (Yoshida et al. (1991)), but also other substances can be used as ferric cloride ( FeCl 3 ), bromine, nitric acid HNO 3 and some forms of THF (tetrahydrofuran). Before the heating, the intercallant is present among the layer in a non-uniform distribution according to the Daumas-Herold model (Daumas and Herold (1969)): it occupies the inter-layer spaces as islands in a disordered arrangement. The number of layers that occur among each island is called stage . A schematical idea is given in fig.3.

(a)

(b)

Fig. 3: (a) Layer of graphites spaced with intercallant islands (stage equal to 2),(b) cell structure and single cell wall. (Pictures by Chung (2015))

The heating can be provided by either internal sources (such as electric current) or externals (such as flame, re sistance e ff ect or microwave). Depending on the nature of the intercallant, the expansion can be reversible (bromine intercallants) or irreversible (nitric and sulphuric acid) (Chung (1987)) and leave a small amount of oxide which makes the material conductive by electron holes (Chung (1987, 2015)).