Structural Investigation of Graphitic Foam

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Structural and chemical characteristics of pitch-based graphitic foam have been studied using scanning electron microscopy, transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy. Chemically, the carbon atoms in these materials are found to have identical bonding states as those in pure graphite single crystals. Microstructural studies indicate that they have a cellular morphology with the cell walls made up of graphitic layers. The walls can be smooth or stepped depending upon the orientation of graphitic layers with respect to the cells. Ligaments between neighboring cells and junctions of ligaments (corners of three or more cells) distinctly show layers of graphitic planes, irregular flakes, and beam-like protruding structures made up of folded layers of graphite. The network of interconnecting pores have openings on the cell walls that have ruptured edges and sharp corners. This indicates that they were formed after hardening of the precursor, resulting in brittle fracture of the walls. The sharp cracks at pore openings may enhance mechanical failure of these structures and attempts to minimize them may improve mechanical behavior. Higher magnification studies in the TEM indicate the presence of flaky layers of graphite crystals and also long tubular structures protruding from the cell walls and ligaments. These fibrous structures (possibly rolled stacks of graphitic planes) have lengths in excess of several microns and diameters ranging between several tens of nanometers to several hundred nanometers. The walls are thick: several tens of nanometers. These structures, intermediate in size between micron size graphitic fibers and nanometer size multiwalled "nanotubes," have been discussed in light of other well investigated graphitic structures. (C) 2002 American Institute of Physics.



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