We have obtained bulk samples of the graphite intercalation compound, CaC6, by a novel method of synthesis from highly oriented pyrolytic graphite. The crystal structure has been completely determined showing that it is the only member of the MC6, metal-graphite compounds that has rhombohedral symmetry. We have clearly shown the occurrence of superconductivity in the bulk sample at 11.5 K, using magnetization measurements.

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The electrical conductivity of cubic samples (monoliths) made of moderately compressed expanded graphite has been measured along two orthogonal directions, namely parallel and perpendicular to the pressing force. It is found that these highly porous materials exhibit a percolation behaviour, with a transition from insulator to conductor occurring at a very low critical density. Besides, the monoliths are found to be quite isotropic as their porosity is high; conversely, increasing their apparent density makes them become more and more anisotropic thanks to the orientation of their constitutive graphite particles. Both percolation and effective media theories are shown to accurately fit the conductivity data as far as isotropic materials are concerned. Microstructural parameters derived from the fits are discussed and found to be in fair agreement with the expected description of compressed expanded graphite, i.e. insulating voids surrounded by thin conducting flakes.

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