Pyrolytic carbon is a material similar to

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...

, but with some covalent bonding between its

graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.

sheets as a result of imperfections in its production. Pyrolytic carbon is man-made and is thought not to be found in nature.Ratner, Buddy D. (2004). Pyrolytic carbon. In
Biomaterials science: an introduction to materials in medicine
'. Academic Press. p. 171-180. . Google Book Search. Retrieved 7 July 2011. Generally it is produced by heating a

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

nearly to its

decomposition temperature Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is req ...

, and permitting the graphite to

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

ize (

pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...

). One method is to heat synthetic fibers in a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

. It is used in high temperature applications such as missile

nose cone A nose cone is the conically shaped forwardmost section of a rocket, guided missile or aircraft, designed to modulate oncoming airflow behaviors and minimize aerodynamic drag. Nose cones are also designed for submerged watercraft such as ...

s, rocket motors, heat shields, laboratory furnaces, in

graphite-reinforced plastic Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...

, coating nuclear fuel particles, and in biomedical

prostheses In medicine, a prosthesis (plural: prostheses; from grc, πρόσθεσις, prósthesis, addition, application, attachment), or a prosthetic implant, is an artificial device that replaces a missing body part, which may be lost through trau ...

Physical properties

Pyrolytic graphite samples usually have a single cleavage plane, similar to

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

, because the graphene sheets crystallize in a planar order, as opposed to pyrolytic carbon, which forms microscopic randomly oriented zones. Because of this, pyrolytic graphite exhibits several unusual

anisotropic Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...

properties. It is more thermally conductive along the cleavage plane than pyrolytic carbon, making it one of the best planar thermal conductors available. Pyrolytic graphite forms

mosaic crystal In crystallography, mosaicity is a measure of the spread of crystal plane orientations. A mosaic crystal is an idealized model of an imperfect crystal, imagined to consist of numerous small perfect crystals (crystallites) that are to some extent ra ...

s with controlled mosaicities up to a few degrees. Pyrolytic graphite is also more

diamagnetic Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...

( χ = −4×10⁻⁴) against the cleavage plane, exhibiting the greatest diamagnetism (by weight) of any room-temperature diamagnet. In comparison, pyrolytic graphite has a

relative permeability In multiphase flow in porous media, the relative permeability of a phase is a dimensionless measure of the effective permeability of that phase. It is the ratio of the effective permeability of that phase to the absolute permeability. It can be ...

of 0.9996, whereas

bismuth Bismuth is a chemical element with the Symbol (chemistry), symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental ...

has a relative permeability of 0.9998 (

table Table may refer to: * Table (furniture), a piece of furniture with a flat surface and one or more legs * Table (landform), a flat area of land * Table (information), a data arrangement with rows and columns * Table (database), how the table data ...

Magnetic levitation

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. 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 in

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

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. The carbon's

magnetic susceptibility In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...

weakens upon sufficient illumination, leading to an unbalanced

magnetization In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Movement within this field is described by direction and is either Axial or Di ...

of the material and movement when using a specific geometry.

Applications

* It is used non-reinforced for

missile In military terminology, a missile is a guided airborne ranged weapon capable of self-propelled flight usually by a jet engine or rocket motor. Missiles are thus also called guided missiles or guided rockets (when a previously unguided rocket i ...

s and

ablative In grammar, the ablative case (pronounced ; sometimes abbreviated ) is a grammatical case for nouns, pronouns, and adjectives in the grammars of various languages; it is sometimes used to express motion away from something, among other uses. T ...

(boiloff-cooled)

rocket motor A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance ...

s. * In fiber form, it is used to reinforce plastics and metals (see

Carbon fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...

and

Graphite-reinforced plastic Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...

). * Pebble-bed

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...

s use a coating of pyrolytic carbon as a

neutron moderator In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely mo ...

for the individual pebbles. * Used to coat graphite cuvettes (tubes) in

graphite furnace atomic absorption Graphite furnace atomic absorption spectroscopy (GFAAS) (also known as Electrothermal Atomic absorption spectroscopy (ETAAS)) is a type of spectrometry that uses a graphite-coated furnace to vaporize the sample. Briefly, the technique is based on ...

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 sink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, th ...

s (fins). * It is occasionally used to make

tobacco pipe A tobacco pipe, often called simply a pipe, is a device specifically made to smoke tobacco. It comprises a chamber (the bowl) for the tobacco from which a thin hollow stem (shank) emerges, ending in a mouthpiece. Pipes can range from very simp ...

s. * It is used to fabricate grid structures in some high-power

vacuum tubes A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as a ...

. * It is used as a

monochromator A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots ''mono-'', "s ...

for neutron and X-ray scattering studies. * Prosthetic heart valves * Radial head prosthesis * It is also used in automotive industries where a desired amount of friction is required between two components. *

Highly oriented pyrolytic graphite Highly oriented pyrolytic graphite (HOPG) is a highly pure and ordered form of synthetic graphite. It is characterised by a low mosaic spread angle, meaning that the individual graphite crystallites are well aligned with each other. The best HOPG sa ...

(HOPG) is used as the dispersive element in HOPG spectrometers, which are used for X-ray spectrometry. *It is used in personal protective gear.

Biomedical applications

Because blood clots do not easily form on it, it is often advisable to line a blood-contacting

prosthesis In medicine, a prosthesis (plural: prostheses; from grc, πρόσθεσις, prósthesis, addition, application, attachment), or a prosthetic implant, is an artificial device that replaces a missing body part, which may be lost through trau ...

with this material in order to reduce the risk of

thrombosis Thrombosis (from Ancient Greek "clotting") is the formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system. When a blood vessel (a vein or an artery) is injured, the body uses platelets (thro ...

. For example, it finds use in

artificial heart An artificial heart is a device that replaces the heart. Artificial hearts are typically used to bridge the time to heart transplantation, or to permanently replace the heart in the case that a heart transplant (from a deceased human or, experi ...

s and

artificial heart valve An artificial heart valve is a one-way valve implanted into a person's heart to replace a heart valve that is not functioning properly (valvular heart disease). Artificial heart valves can be separated into three broad classes: mechanical hear ...

Blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away ...

stent In medicine, a stent is a metal or plastic tube inserted into the lumen of an anatomic vessel or duct to keep the passageway open, and stenting is the placement of a stent. A wide variety of stents are used for different purposes, from expandab ...

s, by contrast, are often lined with a polymer that has

heparin Heparin, also known as unfractionated heparin (UFH), is a medication and naturally occurring glycosaminoglycan. Since heparins depend on the activity of antithrombin, they are considered anticoagulants. Specifically it is also used in the treatm ...

as a pendant group, relying on drug action to prevent clotting. This is at least partly because of pyrolytic carbon's

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...

ness and the large amount of permanent deformation, which a stent undergoes during expansion. Pyrolytic carbon is also in medical use to coat anatomically correct orthopedic implants, a.k.a.

replacement joint Replacement arthroplasty (from Greek ''arthron'', joint, limb, articulate, + ''plassein'', to form, mould, forge, feign, make an image of), or joint replacement surgery, is a procedure of orthopedic surgery in which an arthritic or dysfunctional ...

s. In this application it is currently marketed under the name "PyroCarbon". These implants have been approved by the U.S.

Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a List of United States federal agencies, federal agency of the United States Department of Health and Human Services, Department of Health and Human Services. The FDA is respon ...

for use in the hand for metacarpophalangeal (knuckle) replacements. They are produced by two companies: Tornier (BioProfile) and Ascension Orthopedics. On September 23, 2011,

Integra LifeSciences Integra LifeSciences is a global medical device manufacturing company headquartered in Princeton, New Jersey. Founded in 1989, the company manufactures products for skin regeneration, neurosurgery, reconstructive and general surgery. Integra art ...

acquired Ascension Orthopedics. The company’s pyrolytic carbon implants have been used to treat patients with different forms of osteoarthritis. In January 2021, Integra LifeSciences sold its orthopedics company to Smith+Nephew for $240 million. The FDA has also approved PyroCarbon interphalangeal joint replacements under the Humanitarian Device Exemption.

Footnotes

{{reflist Allotropes of carbon Magnetic levitation Refractory materials