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Pyrolytic carbon is a material similar tographite, but with somecovalent bonding between itsgraphene sheets as a result of imperfections in its production.
Pyrolytic carbon is man-made and is thought not to be found in nature.[1] Generally it is produced by heating ahydrocarbon nearly to itsdecomposition temperature, and permitting the graphite tocrystalize (pyrolysis).
One method is to heatsynthetic fibers in avacuum, producingcarbon fibers.
It is used in high temperature applications such as missilenose cones, rocket motors, heat shields, laboratory furnaces, ingraphite-reinforced plastic, coating nuclear fuel particles, and in biomedicalprostheses.
It was developed in the late 1950s as an extension of the work on refractoryvapor deposition of metals.[2]
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Pyrolytic graphite samples usually have a singlecleavage plane, similar tomica, because the graphene sheets crystallize in a planar order, as opposed to[clarification needed] pyrolytic carbon, which forms microscopic randomly oriented zones. Because of this, pyrolytic graphite exhibits several unusualanisotropic properties. It is morethermally conductive along the cleavage plane than pyrolytic carbon, making it one of the best planar thermal conductors available.
Pyrolytic graphite formsmosaic crystals with controlled mosaicities up to a few degrees.
Pyrolytic graphite is also morediamagnetic (χ = −4×10−4) against the cleavage plane, exhibiting the greatest diamagnetism (by weight) of any room-temperature diamagnet. In comparison[dubious –discuss], pyrolytic graphite has arelative permeability of 0.9996, whereasbismuth has a relative permeability of 0.9998 (table).
Few materials can be made tomagnetically 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 (seeEarnshaw's theorem). Strongly diamagnetic materials, however, can levitate above powerful magnets.
With the easy availability of rare-earth permanent magnets developed in the 1970s and 1980s, the strong diamagnetism of pyrolytic graphite makes it a convenient demonstration material for this effect.
In 2012, a research group inJapan demonstrated that pyrolytic graphite can respond to laser light or sufficiently powerful natural sunlight by spinning or moving in the direction of the field gradient.[3][4] The carbon'smagnetic susceptibility weakens upon sufficient illumination, leading to an unbalancedmagnetization of the material and movement when using a specific geometry.
Recently, it has been suggested that pyrolytic carbon may possibly be the explanation for the mysterious 'spokes' in Saturn's rings. Due to the process of Chemical Vapor Deposition methane gas at high temperatures (1400K) may have been converted to pyrolytic carbon. The abundant silicates in Saturn's B ring may have acted as a substrate for the pyrolytic carbon to be deposited on. Since pyrolytic carbon is highly diamagnetic the silicate grains coated in pyrolytic carbon can levitate above and below the ring plane due to Saturn's equatorial magnetic field. When sunlight hits these pyrolytic carbon-coated grains they lose electrons due to the photoelectric effect and become paramagnetic and are pulled back to the main ring structure as they are now attracted to Saturn's equatorial magnetic field. The visibility of the 'spokes' is dependent on the angle of the sunlight hitting the rings and the angle the observer is observing the rings. ( Referencehttps://arxiv.org/abs/2303.07197 ).
Because blood clots do not easily form on it, it is often advisable to line a blood-contactingprosthesis with this material in order to reduce the risk ofthrombosis. For example, it finds use inartificial hearts andartificial heart valves.Blood vesselstents, by contrast, are often lined with a polymer that hasheparin as a pendant group, relying on drug action to prevent clotting. This is at least partly because of pyrolytic carbon'sbrittleness and the large amount ofpermanent deformation, which a stent undergoes during expansion.
Pyrolytic carbon is also in medical use to coat anatomically correct orthopedic 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.[7] On September 23, 2011,Integra LifeSciences acquired Ascension Orthopedics. The company's pyrolytic carbon implants have been used to treat patients with different forms of osteoarthritis.[8][9] In January 2021, Integra LifeSciences sold its orthopedics company toSmith+Nephew for $240 million.[10]
The FDA has also approved PyroCarbon interphalangeal joint replacements under theHumanitarian Device Exemption.[11]