Pyrolytic carbon
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.
Generally it is produced by heating a hydrocarbon nearly to its decomposition 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.
Physical properties
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, 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. It is also more diamagnetic against the cleavage plane, exhibiting the greatest diamagnetism of any room temperature solid (by weight). It is even possible to levitate reasonably pure and sufficiently ordered samples over rare earth permanent magnets.
Applications
- It is used unreinforced for missile nosecones, and ablative (boiloff-cooled) rocket motors.
- In fiber form, it is used to reinforce plastics and metals (see Carbon fiber and Graphite-reinforced plastic).
- Pebble bed nuclear reactors use a coating of pyrolytic carbon as a neutron moderator for the individual pebbles.
- Used to coat graphite cuvettes (tubes) in Graphite Furnace Atomic Absorption furnaces to decrease heat stress, thus increasing cuvette lifetimes.
Pyrolytic carbon is used for several applications in electronic thermal management: thermal interface material, heat spreaders (sheets) and heat sinks (fins)
- It is used to fabricate grid structures in some high power vacuum tubes.
Biomedical applications
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 artificial 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.
Pyrolytic carbon is also in medical use to coat anatomically correct orthopaedic implants, a.k.a. replacement joint. In this application it is currently marketed under the name "pyrocarbon". These implants have been approved by the FDA for use in the hand for metacarpophalangeal (knuckle) replacements. They are produced by two companies: Ascension Orthopedics [1] and Nexa Orthopedics.
Footnotes
- ↑ Cook; et al. (1999). "Long-term follow-up of pyrolytic carbon metacarpophalangeal implants". J Bone Joint Surg Am. 81 (5): 635–48. PMID 10360692.