Pyrolytic carbon

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Sheets of pyrolytic carbon

Pyrolytic carbon levitating over permanent magnets

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 not found in nature.^[1] 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 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 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.

It is also more diamagnetic (-400x10^-6) against the cleavage plane, exhibiting the greatest diamagnetism (by weight) of any room temperature diamagnet. It is even possible to levitate reasonably pure and sufficiently ordered samples over rare earth permanent magnets.

Applications

• It is used nonreinforced for missile nose cones, 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.
• It is used as a monochromator for neutron and x-ray scattering studies.
• Radial Head Prosthesis
• It is also used in Automotive industries where a desired amount of friction is required between two components
• Highly Ordered Pyrolytic Graphite (HOPG) is used as the dispersive element in HOPG spectrometers which are used for X-ray spectrometry.

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 joints. In this application it is currently marketed under the name "PyroCarbon". These implants have been approved by the U.S. Food and Drug Administration for use in the hand for metacarpophalangeal (knuckle) replacements. They are produced by two companies: Tornier (BioProfile) and Ascension Orthopedics.^[2] (On September 23, 2011, Integra LifeSciences acquired Ascension Orthopedics.) The FDA has also approved PyroCarbon interphalangeal joint replacements under the Humanitarian Device Exemption.^[3]

Footnotes

External links

Biomedical applications

• Ascension Orthopedics
• Pyrolytic Carbon Manufacturing
• Pyrolytic Carbon for Biomedical Applications