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K2.02 - Unprecedented Solid-State Viscoelasticity Discovered in Exfoliated Graphite 
December 1, 2014   11:00am - 11:15am

Viscoelastic solids are valuable for passive vibration damping. Unprecedented solid-state viscoelasticity has been discovered in exfoliated graphite. The highly viscous behavior is due to the friction at the interface between the carbon layers in the graphite. The interfacial mechanism of mechanical energy dissipation is in contrast to the bulk viscous deformation mechanism, which is the case for polymers such as rubber. The loss tangent of the graphite reaches 35, compared to 0.7 for rubber. The incorporation of this graphite in a cement matrix results in microscale constrained-layer damping, with the loss modulus reaching the exceptionally high value of 7.5 GPa. In order for exfoliation to occur, the graphite layers that make up the wall of an intercalate island must be able to stretch greatly. The stretching of the wall enables an intercalate island to expand like a balloon. A wall consists of multiple layers of graphite, such that each layer does not necessarily extend all the way across the length of an island. There are about 60 graphite layers (on the average) in the cell wall of the exfoliated graphite used. The stretching of a wall is made possible by the sliding of the graphite layers with respect to one another within the wall. This sliding requires the overcoming of the van der Waals’ forces between the graphite layers. The vapor-related driving force for exfoliation is adequate for overcoming these forces. For an irreversibly exfoliated graphite, the tremendous sliding of the graphite layers has already occurred during the completed exfoliation, so no further tremendous sliding occurs upon subsequent vibration. Nevertheless, the exfoliation process has irreversibly loosened the binding of the graphite layers and, as a consequence, a degree of sliding between the layers can easily occur upon subsequent vibration. This looseness is consistent with a very low modulus (~110 kPa) in the direction perpendicular to the wall, as shown by nanoindentation testing. The smoothness of the load vs. displacement curve during nanoindentation indicates that the indentation mechanism involves the stretching of the cell walls rather than the breakthrough of the walls. The deformation is mostly reversible upon unloading. With the displacement attributed to the sliding between the graphite layers in a cell wall (width ~20 nm), the maximum shear strain in the cell wall is ~39. This indicates elastomeric deformation, which has been previously reported in polymers only. The reversibility of the sliding is probably made possible by the ease of sliding of the graphite layers and the cellular structure, in which the extremities of a cell serve as pinning points that effectively link the graphite layers. Such links are akin to the crosslinks in rubber. Without exfoliation, the sliding is relatively difficult, due to the strong binding between the layers.

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