Pyrolytic carbon

From Freepedia

Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets.

Generally it is produced by heating a hydrocarbon nearly to its breakdown temperature, and permitting the graphite to crystallise (pyrolysis). One method is to take a synthetic fiber, and heat it in a vacuum. Another method is to place seeds or a plate in the very hot gas to collect the graphite coating.

Pyrolytic carbon samples usually have a single cleavage plane, similar to mica, because the graphene sheets crystalize in a planar order (as opposed to graphite, which forms microscopic randomly-oriented zones). Because of this, it is more thermally conductive along the cleavage plane (and less against the plane) than graphite, making it one of the best thermal conductors available. It is also more diamagnetic against the cleavage plane, than along it, exhibiting the greatest diamagnetism of any room temperature solid (by weight). It is possible to levitate reasonably pure and sufficiently ordered samples over rare earth permanent magnets.

Applications

It is used unreinforced for missile nosecones, and ablative (uncooled) rocket motors.

In fiber form, it is used to reinforce plastics and metals (see Carbon fiber and Graphite-reinforced plastic).

Because blood clots do not easily form on it, it is often advisable to line a blood-contacting prosthesis with this material in order to reduce the risk of thrombosis. For example, it finds use in artificial hearts and prosthetic heart valves. Blood vessel stents, by contrast, are often lined with a polymer that has heparin as a pendant group, relying on drug action to prevent clotting. This is at least partly because of pyrolytic carbon's brittleness and the large amount of permanent deformation which a stent undergoes during expansion.