PSI - Issue 26

E. Solfiti et al. / Procedia Structural Integrity 26 (2020) 187–198 E. Solfiti and F. Berto / Structural Integrity Procedia 00 (2019) 000–000

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compression of expanded particles can be applied giving lower density material which is often referred to as graphite compact. Finally, one can point at all the density range material as compressed expanded graphite. It is clarified right now that in the following review, concern will be about sheets, foils and compacts and not about di ff erent arrangements of compressed expanded graphite. The exfoliated graphite, i.e. the expanded particles, is instead a granular compound obtained by expansion of natural graphite flakes (purely crystalline, figure 1a) which show uncommon worm-like or accordion-like shape once expanded upon heating (figure 1b). These stem from the mono-directional expansion of flakes along the crystalline c-axis, perpendicular to the crystal basal planes. Because of that, sometimes the terms expanded and exfoliated graphite are used in interchangeable manner. The exfoliated graphite itself is sometimes used in industrial application as fire extinguisher, thermal insulator,lubricant addition or as a blending material in polimeric composites [Chung (1987)]. Expandable or intercalated graphite instead represent the stage just before the expansion in which the natural flakes have only been intercalated with the proper chemical specie. The ”worms” deform under

(a)

(b)

Fig. 1: (a) SEM pictures of natural graphite flakes and (b) worm-like particles of exfoliated graphite. [By courtesy of Asbury Carbons Inc.]

compression and they are mainly arranged on the direction parallel to the bedding plane (perpendicular to the pressure axis), retaining a certain content of air voids which decrease along with the increase of compaction pressure. In the early stage of compression, the overall material can be defined as a heterogeneous disordered material with an higher degree of inherent isotropy [Celzard et al. (2005)] whereas, after a certain value of density, the anisotropy comes out to a large extent a ff ecting the elasticity, strength, electrical conductivity, thermal conductivity and so on. Porosity and resistance to chemical agent are involved in applications such as fuel cells and batteries electrodes [Luo et al. (2002); Bhattacharya et al. (2004)] and working temperature range goes from -240 ◦ C to + 2500 ◦ C [Sigraflex ® ], depending on inert or oxidizing atmospheres: oxidation appears up to approximately + 450 ◦ C on most commercial products [Sigraflex ® and Grafoil ® ]. From a mechanical point of view, such a microstructure results also on high resilience and viscous response [Gu et al. (2002) and Luo and Chung (2000)]: graphite layers indeed show a typical hexagonal lattice that is retained after expansion in the form of somewhat regular honeycomb cells which allow a certain amount of sliding among layers and hence friction dissipative phenomena [Chen and Chung (2012)]. This capability of dissipate energy together with the chemical resistance in a large range of temperature (up to beyond 2500 ◦ C) and low gas permeability, make FG excellent in gaskets and sealing applications, often in sandwiched structures together with stainless steel foils or as impregnated yarns. Because of that, the research trend often moved towards the investigation in static compression and recovery loading [Toda et al. (2013), Wang et al. (2015) and Kobayashi et al. (2012)], often at room temperature or even in order to observe the relation between the electrical properties variation as in Xi and Chung (2019), Wei et al. (2010) and Luo and Chung (2000). A few mechanical tests have been reported about high temperatures [Dowell and Howard (1986)] or fracture behavior [Gu et al. (2002) and Leng et al. (1998)] and no data are available about fatigue life and damage mechanism. Because of its conformability (high capability to conform to rough surface), FG is e ff ective as thermal interface material for cooling and insulation [Marotta et al. (2005)] and furthermore, having thermal and electrical conductivity comparable to or higher than metals such as copper and