Pyrolytic graphite
Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets as a result of imperfections in its production.
Pyrolytic carbon is man-made and is not thought to be found in nature. Generally it is produced by heating a hydrocarbon nearly to its decomposition temperature, and permitting the graphite to crystallise (pyrolysis). One method is to heat synthetic fibers in a vacuum. Another method is to place seeds on a plate in the very hot gas to collect the graphite coating. It is used in high temperature applications such as missile nose cones, rocket motors, heat shields, laboratory furnaces, in graphite-reinforced plastic, and in biomedical prostheses.
Pyrolytic carbon samples usually have a single cleavage plane, similar to mica, because the graphene sheets crystallize in a planar order, as opposed to graphite, which forms microscopic randomly oriented zones. Because of this, pyrolytic carbon exhibits several unusual anisotropic properties. It is more thermally conductive along the cleavage plane than graphite, making it one of the best planar thermal conductors available.
Pyrolitic graphite forms mosaic crystals with controlled mosaicities up to a few degrees.
It is also more diamagnetic (χ = −4×10−4) against the cleavage plane, exhibiting the greatest diamagnetism (by weight) of any room-temperature diamagnet.
Few materials can be made to magnetically levitate stably above the magnetic field from a permanent magnet. Although magnetic repulsion is obviously and easily achieved between any two magnets, the shape of the field causes the upper magnet to push off sideways, rather than remaining supported, rendering stable levitation impossible for magnetic objects (see Earnshaw's Theorem). Strongly diamagnetic materials, however, can levitate above powerful magnets.
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