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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applied. The exfoliated graphite is a granular compound obtained by expansion of natural graphite flakes (see fig.1(a)) and it shows uncommon particles: these indeed assume a typical worm-like shape (or accordion-like , see fig.1(b)) that stems from the mono-directional expansion of the flakes along the crystalline c-axis (i.e. perpendicular to the crystal basal planes). Sometimes exfoliated graphite is equivalently referred to as expanded graphite . Expandable graphite instead, also referred to as graphite intercalated compound (GIC), is the stage just before the expansion in which the natural flakes have only been intercalated with apposite chemical species.

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Fig. 1: (a) Natural flakes of graphite and (b) Worm-like particles of exfoliated graphite. (Photos by courtesy of Asbury Carbons Inc.)

Di ff erent pressures of compression result on di ff erent densities of the final product: graphite compacts usually correspond to those made at lower pressures (and densities) whereas FG to higher pressures, owning a typical flat and mirror surfaces. Compressed expanded graphite) (CEG) finally includes all the compressed states. During the compression, the ”worms” arrange on a preferred orientation along the bedding plane (perpendicular to the pressure axis) retaining a certain porosity that is the fraction of voids volume with the respect to the apparent volume. The pore shape is supposed to be oblate spheroidal and the pores seem to change shape and volume during loading (Balima et al. (2013, 2014)). To control the porosity means to control the anisotropy of the CEG and thus the in-plane and out-of-plane properties such as elasticity, strength, electrical conductivity and thermal conductivity (Celzard et al. (2005); Chung (2015)). This microstructure moreover gives a compound with high resilience and viscous component (Gu et al. (2002); Luo and Chung (2000); Chen and Chung (2012)): high capability of dissipate energy both reversible and irreversible together with the chemical resistance in a large range of temperature (up to 3500 ◦ C) and low gas permeability, make such a material excellent in gaskets and sealing applications, often in sandwiched structures to gether with steel (e.g. stainless steel foils) or impregnated with yarns and textile forms. Multilayered laminates with metallic or non-metallic materials improve toughness for handling and the elastic modulus. Strength and modulus have been mainly investigated in compressive static loading at room temperature, even to observe the relation with the electrical properties as Xi and Chung (2019); Wei et al. (2010); Luo and Chung (2000). A few mechanical tests has been reported about high temperatures (Dowell and Howard (1986)) or fracture behavior (Gu et al. (2002); Leng et al. (1998) and no reports is available about fatigue damage or strain-rate sensitivity. Because of its conformability and low thermal expansion (coe ffi cient of thermal expansion in the in-plane direction equal to 0 − 1 · 10 − 6 K − 1 ), CEG are also e ff ective as thermal interface material whose main applications can be found in the LED lighting industry (Luo et al. (2002); Chung (2012); Chen and Chung (2014)). Working temperature range is from − 240 ◦ C to + 3500 ◦ C , depending on inert or oxydizing atmospheres. No oxydation appears up to approximately + 450 ◦ C on most commer cial products. Both thermal and conductivity properties are exploited in resistive heating elements (Chugh and Chung (2002)), fuel cells, batteries electrodes (Ko and Oh; Luo et al. (2002); Bhattacharya et al. (2004)) and electromagnetic field shielding (Chung (2012); Luo et al. (2002); Chung (2000); Sykam and Rao (2018); more recently Xi and Chung (2019) observed the dielectrical and piezoelectrical properties for mechanical sensing and electric powering. Other