Graphite () is a crystalline allotrope (form) of the element carbon. It consists of many stacked layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on a large scale (1.3 million metric tons per year in 2022) for uses in many critical industries including refractories (50%), lithium-ion batteries (18%), foundries (10%), and lubricants (5%), among others (17%). Graphite converts to diamond under extremely high pressure and temperature. Graphite's low cost, thermal and chemical inertness and characteristic conductivity of heat and electricity finds numerous applications in high energy and high temperature processes.
Types and varieties Graphite can occur naturally or be produced synthetically. Natural graphite is obtained from naturally occurring geologic deposits and synthetic graphite is produced through human activity.
Natural Graphite occurs naturally in ores that can be classified as either amorphous (microcrystalline) or crystalline (flake or lump/chip) which is determined by the ore morphology, crystallinity, and grain size. All naturally occurring graphite deposits are formed from the metamorphism of carbonaceous sedimentary rocks, and the ore type is due to its geologic setting. Coal that has been thermally metamorphosed is the typical source of amorphous graphite. Crystalline flake graphite is mined from carbonaceous metamorphic rocks, while lump or chip graphite is mined from veins which occur in high-grade metamorphic regions. There are serious negative environmental impacts to graphite mining.
Synthetic Synthetic graphite has high purity and is usually produced by the thermal graphitization of hydrocarbon materials at temperatures in excess of 2,100 °C, most commonly through the Acheson process. The high temperatures are maintained for weeks, and are required not only to form the graphite from the precursor carbons but also to vaporize any impurities that may be present, including hydrogen, nitrogen, sulfur, organics, and metals. The resulting synthetic graphite is highly pure—in excess of 99.9% C purity—but typically has lower density, conductivity and a higher porosity than its natural equivalent. Synthetic graphite can be formed into very large (centimeter-scale) flakes while maintaining its high purity, unlike almost all sources of natural graphite. Synthetic graphite can also be formed by other methods including by chemical vapor deposition from hydrocarbons at temperatures above 2,500 K (2,230 °C), by decomposition of thermally unstable carbides, or by crystallization from metal melts supersaturated with carbon.
Research Research and development efforts continue into new methods for the industrial production of graphite for a variety of applications, including lithium-ion batteries, refractories, and foundries, among others. Significant work has been done on graphitizing of traditionally non-graphitizable carbons. A company in New Zealand utilizes forestry waste to produce what they have termed 'biographite' through a process referred to as thermo-catalytic graphitization. Another group in the United States uses a method referred to as photocatalytic graphitization to produce highly crystalline highly pure graphite for lithium-ion batteries and other applications from a variety of carbon sources.
Natural
Occurrence Graphite occurs in metamorphic rocks as a result of the reduction of sedimentary carbon compounds during metamorphism. It also occurs in igneous rocks and in meteorites. Minerals associated with graphite include quartz, calcite, micas and tourmaline. The principal export sources of mined graphite are, in order of tonnage, China, Mexico, Canada, Brazil, and Madagascar. Significant unexploited graphite resources also exist in Colombia's Cordillera Central in the form of graphite-bearing schists. In meteorites, graphite occurs with troilite and silicate minerals. Small graphitic crystals in meteoritic iron are called cliftonite. Some microscopic grains have distinctive isotopic compositions, indicating that they were formed before the Solar System. They are one of about 12 known types of minerals that predate the Solar System and have also been detected in molecular clouds. These minerals were formed in the ejecta when supernovae exploded or low to intermediate-sized stars expelled their outer envelopes late in their lives. Graphite may be the second or third oldest mineral in the Universe.
Structure Graphite consists of sheets of trigonal planar carbon. The individual layers are called graphene. In each layer, each carbon atom is bonded to three other atoms forming a continuous layer of sp2 bonded carbon hexagons, like a honeycomb lattice with a bond length of 0.142 nm, and the distance between planes is 0.335 nm. Bonding between layers is relatively weak van der Waals bonds, which allows the graphene-like layers to be easily separated and to glide past each other. Electrical conductivity perpendicular to the layers is consequently about 1000 times lower. There are two allotropic forms called alpha (hexagonal) and beta (rhombohedral), differing in terms of the stacking of the graphene layers: stacking in alpha graphite is ABA, as opposed to ABC stacking in the energetically less stable beta graphite. Rhombohedral graphite cannot occur in pure form. Natural graphite, or commercial natural graphite, contains 5 to 15% rhombohedral graphite and this may be due to intensive milling. The alpha form can be converted to the beta form through shear forces, and the beta form reverts to the alpha form when it is heated to 1300 °C for four hours.
Thermodynamics
The equilibrium pressure and temperature conditions for a transition between graphite and diamond is well established theoretically and experimentally. The pressure changes linearly between 1.7 GPa at 0 K and 12 GPa at 5000 K (the diamond/graphite/liquid triple point). However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20 °C (293 K) and 1 standard atmosphere (0.10 MPa), the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible. However, at temperatures above about 4500 K, diamond rapidly converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa is needed.
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