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Allotropes of Carbon

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Carbon is capable of forming many allotropes due to its valency. Well-known forms of carbon include diamond and graphite. In recent decades many more allotropes and forms of carbon have been discovered and researched including ball shapes such as buckminsterfullerene and sheets such as graphene.

Larger scale structures of carbon include nanotubes, nanobuds and nanoribbons. Other unusual forms of carbon exist at very high temperature or extreme pressures.

Atomic carbon is a very short-lived species and, therefore, carbon is stabilized in various multi-atomic structures with different molecular configurations called allotropes. The three relatively well-known allotropes of carbon are amorphous carbon, graphite, and diamond. Once considered exotic, fullerenes are nowadays commonly synthesized and used in research; they include buckyballs,[28][29] carbon nanotubes,[30] carbon nanobuds[31] and nanofibers.[32][33] Several other exotic allotropes have also been discovered, such as lonsdaleite,[34] glassy carbon,[35] carbon nanofoam[36] and linear acetylenic carbon (carbyne).[37]


A large sample of glassy carbon.
The amorphous form is an assortment of carbon atoms in a non-crystalline, irregular, glassy state, which is essentially graphite but not held in a crystalline macrostructure. It is present as a powder, and is the main constituent of substances such as charcoal, lampblack (soot) and activated carbon. At normal pressures carbon takes the form of graphite, in which each atom is bonded trigonally to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons.[38] The resulting network is 2-dimensional, and the resulting flat sheets are stacked and loosely bonded through weak van der Waals forces. This gives graphite its softness and its cleaving properties (the sheets slip easily past one another). Because of the delocalization of one of the outer electrons of each atom to form a π-cloud, graphite conducts electricity, but only in the plane of each covalently bonded sheet. This results in a lower bulk electrical conductivity for carbon than for most metals. The delocalization also accounts for the energetic stability of graphite over diamond at room temperature.


Some allotropes of carbon:
a) diamond;
b) graphite;
c) lonsdaleite;
d–f) fullerenes (C60, C540, C70);
g) amorphous carbon;
h) carbon nanotube.

At very high pressures carbon forms the more compact allotrope diamond, having nearly twice the density of graphite. Here, each atom is bonded tetrahedrally to four others, thus making a 3-dimensional network of puckered six-membered rings of atoms.

Diamond has the same cubic structure as silicon and germanium and because of the strength of the carbon-carbon bonds, it is the hardest naturally occurring substance in terms of resistance to scratching. Contrary to the popular belief that "diamonds are forever", they are in fact thermodynamically unstable under normal conditions and transform into graphite. However, due to a high activation energy barrier, the transition into graphite is so extremely slow at room temperature as to be unnoticeable. Under some conditions, carbon crystallizes as lonsdaleite. This form has a hexagonal crystal lattice where all atoms are covalently bonded. Therefore, all properties of lonsdaleite are close to those of diamond.

Fullerenes have a graphite-like structure, but instead of purely hexagonal packing, they also contain pentagons (or even heptagons) of carbon atoms, which bend the sheet into spheres, ellipses or cylinders. The properties of fullerenes (split into buckyballs, buckytubes and nanobuds) have not yet been fully analyzed and represent an intense area of research in nanomaterials.

The names "fullerene" and "buckyball" are given after Richard Buckminster Fuller, popularizer of geodesic domes, which resemble the structure of fullerenes. The buckyballs are fairly large molecules formed completely of carbon bonded trigonally, forming spheroids (the best-known and simplest is the soccerball-shaped C60 buckminsterfullerene). Carbon nanotubes are structurally similar to buckyballs, except that each atom is bonded trigonally in a curved sheet that forms a hollow cylinder. Nanobuds were first reported in 2007 and are hybrid bucky tube/buckyball materials (buckyballs are covalently bonded to the outer wall of a nanotube) that combine the properties of both in a single structure.

Of the other discovered allotropes, carbon nanofoam is a ferromagnetic allotrope discovered in 1997. It consists of a low-density cluster-assembly of carbon atoms strung together in a loose three-dimensional web, in which the atoms are bonded trigonally in six- and seven-membered rings. It is among the lightest known solids, with a density of about 2 kg/m3.

Similarly, glassy carbon contains a high proportion of closed porosity, but contrary to normal graphite, the graphitic layers are not stacked like pages in a book, but have a more random arrangement.

Allotropes of carbon. Diamond[edit] Diamond is one well known allotrope of carbon.

Allotropes of carbon

The hardness and high dispersion of light of diamond make it useful for both industrial applications and jewelry. Diamond is the hardest known natural mineral. This makes it an excellent abrasive and makes it hold polish and luster extremely well. No known naturally occurring substance can cut (or even scratch) a diamond, except another diamond. The market for industrial-grade diamonds operates much differently from its gem-grade counterpart. With the continuing advances being made in the production of synthetic diamond, future applications are beginning to become feasible.

Diamond

Graphite. Amorphous Carbon. Buckminsterfullerenes. Glassy Carbon. Atomic and Diatomic Carbon. Carbon nanofoam. Carbide-derived Carbon. Lonsdaleite. Linear acetylenic carbon. Other possible forms. Variability of Carbon. Penta-graphene. Penta-graphene is a proposed carbon allotrope composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling.[1] Penta-graphene was proposed in 2014 on the basis of analyses and simulations.[1] Theoretical calculations showed that penta-graphene is dynamically and mechanically stable, and can withstand temperatures up to 1,000 K (730 °C; 1,340 °F).[1] Owing to its atomic configuration, penta-graphene has an unusually negative Poisson’s ratio and very high ideal strength believed to be higher than that of graphene.[1] ^ Jump up to: a b c d Zhang, S.; Zhou, J.; Wang, Q.; Chen, X.; Kawazoe, Y.; Jena, P. (2015).

Penta-graphene

"Penta-graphene: A new carbon allotrope". Proceedings of the National Academy of Sciences: 201416591. doi:10.1073/pnas.1416591112. Borophene. B 36° borophene, front and side view Borophene is a proposed crystalline allotrope of boron.

Borophene

One unit consists of 36 atoms arranged in an 2-dimensional sheet with a hexagonal hole in the middle.[1][2] Theory[edit] Computational studies suggested that extended borophene sheets with partially filled hexagonal holes are stable.[3][4] Global minimum searches for B− 36 lead to a quasiplanar structure with a central hexagonal hole. Borophene is predicted to be fully metallic.[1] Boron is adjacent to carbon in the periodic table and has similar valence orbitals. History[edit] In 2014 a research team at Brown University, led by Lai-Sheng Wang, showed that the structure of B 36 was not only possible but highly stable.[5][6][2] Photoelectron spectroscopy revealed a relatively simple spectrum, suggesting a symmetric cluster.

Graphyne. Soot. Emission of soot from a large diesel truck, without particle filters Soot /ˈsʊt/ is impure carbon particles resulting from the incomplete combustion of hydrocarbons.

Soot

It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed fuel particles such as coal, cenospheres, charred wood, petroleum coke, and so on, that may become airborne during pyrolysis and that are more properly identified as cokes or chars. Soot is theorized to be the second largest cause of global warming.[1][2] Sources[edit] Soot as an airborne contaminant in the environment has many different sources, all of which are results of some form of pyrolysis. Carbon black. The current International Agency for Research on Cancer (IARC) evaluation is that, "Carbon black is possibly carcinogenic to humans (Group 2B)".[3] Short-term exposure to high concentrations of carbon black dust may produce discomfort to the upper respiratory tract, through mechanical irritation.

Carbon black

Common uses[edit] A small mound of carbon black. Total production was around 8,100,000 metric tons (8,900,000 short tons) in 2006.[4] The most common use (70%) of carbon black is as a pigment and reinforcing phase in automobile tires. Carbon black also helps conduct heat away from the tread and belt area of the tire, reducing thermal damage and increasing tire life. Diamond-like carbon. A ta-C thin film on silicon (15 mm diameter) exhibiting regions of 40 nm and 80 nm thickness.

Diamond-like carbon

A Co-alloy valve part from a producing oil well (30 mm diameter), coated on the right side with ta-C, in order to test for added resistance to chemical and abrasive degradation in the working environment. Dome coated with DLC for optical and tribological purposes. Superdense carbon allotropes. Superdense carbon allotropes are proposed configurations of carbon atoms that result in a stable material with a higher density than diamond.

Superdense carbon allotropes

Three structures[edit] Various configurations were simulated at various temperatures and pressures. This resulted in three structures, termed hP3, tI12 and tP12, that appear to be stable enough to have the potential for fabrication. They are nearly as hard as diamonds, but denser by 3.2 per cent. These are the densest carbon allotropes yet achieved.