Quadrant Model of Reality Book 18 Carbon

Chemistry chapter

Carbon chapter. Carbon has four valence electrons and therefore looks like a cross/quadrant. It is known as the miracle element because it is important in so many areas as well as being the basis of life.

Carbon nanobuds[edit]

Main article: Carbon nanobud

Nanobuds have been obtained by adding buckminsterfullerenes to carbon nanotubes.

Fullerite[edit]

The C60 fullerene in crystalline form

Fullerites are the solid-state manifestation of fullerenes and related compounds and materials.

"Ultrahard fullerite" is a coined term frequently used to describe material produced by high-pressure high-temperature (HPHT) processing of fullerite. Such treatment converts fullerite into a nanocrystalline form of diamond which has been reported to exhibit remarkable mechanical properties.[37]

Properties[edit]

For the past decade, the chemical and physical properties of fullerenes have been a hot topic in the field of research and development[according to whom?], and are likely to continue to be for a long time. Popular Science has discussedpossible uses of fullerenes (graphene) in armor.[39] In April 2003, fullerenes were under study for potential medicinal use: binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma. The October 2005 issue of Chemistry & Biology contains an article describing the use of fullerenes as light-activated antimicrobial agents.[40]

In the field of nanotechnology, heat resistance and superconductivity are some of the more heavily studied properties.

A common method used to produce fullerenes is to send a large current between two nearby graphite electrodes in an inert atmosphere. The resulting carbon plasma arc between the electrodes cools into sooty residue from which many fullerenes can be isolated.

There are many calculations that have been done using ab-initio quantum methods applied to fullerenes. By DFT and TD-DFT methods one can obtain IR, Raman and UV spectra. Results of such calculations can be compared with experimental results.

Fullerenes are sparingly soluble in many solvents. Common solvents for the fullerenes include aromatics, such as toluene, and others like carbon disulfide. Solutions of pure buckminsterfullerene have a deep purple color. Solutions of C70 are a reddish brown. The higher fullerenes C76 to C84 have a variety of colors. C76 has two optical forms, while other higher fullerenes have several structural isomers. Fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature.

Solvent C60

mg/mL

[citation needed] C70

mg/mL

[citation needed]

1-chloronaphthalene 51 ND

1-methylnaphthalene 33 ND

1,2-dichlorobenzene 24 36.2

1,2,4-trimethylbenzene 18 ND

tetrahydronaphthalene 16 ND

carbon disulfide 8 9.875

1,2,3-tribromopropane 8 ND

chlorobenzene 7 ND

p-xylene 5 3.985

bromoform 5 ND

cumene 4 ND

toluene 3 1.406

benzene 1.5 1.3

carbon tetrachloride 0.447 0.121

chloroform 0.25 ND

n-hexane 0.046 0.013

cyclohexane 0.035 0.08

tetrahydrofuran 0.006 ND

acetonitrile 0.004 ND

methanol 4.0×10−5 ND

water 1.3×10−11 ND

pentane 0.004 0.002

heptane ND 0.047

octane 0.025 0.042

isooctane 0.026 ND

decane 0.070 0.053

dodecane 0.091 0.098

tetradecane 0.126 ND

acetone ND 0.0019

isopropanol ND 0.0021

dioxane 0.0041 ND

mesitylene 0.997 1.472

dichloromethane 0.254 0.080

ND, not determined

Some fullerene structures are not soluble because they have a small band gap between the ground and excited states. These include the small fullerenes C28,[44] C36 and C50. The C72 structure is also in this class, but the endohedral version with a trapped lanthanide-group atom is soluble due to the interaction of the metal atom and the electronic states of the fullerene. Researchers had originally been puzzled by C72 being absent in fullerene plasma-generated soot extract, but found in endohedral samples. Small band gap fullerenes are highly reactive and bind to other fullerenes or to soot particles.

Solvents that are able to dissolve buckminsterfullerene (C60 and C70) are listed at left in order from highest solubility. The solubility value given is the approximate saturated concentration.[45][46][47][48][49]

Solubility of C60 in some solvents shows unusual behaviour due to existence of solvate phases (analogues of crystallohydrates). For example, solubility of C60 in benzene solution shows maximum at about 313 K. Crystallization from benzene solution at temperatures below maximum results in formation of triclinic solid solvate with four benzene molecules C60·4C6H6 which is rather unstable in air. Out of solution, this structure decomposes into usual face-centered cubic (fcc) C60 in few minutes' time. At temperatures above solubility maximum the solvate is not stable even when immersed in saturated solution and melts with formation of fcc C60. Crystallization at temperatures above the solubility maximum results in formation of pure fcc C60. Millimeter-sized crystals of C60 and C70 can be grown from solution both for solvates and for pure fullerenes.[

Quantum mechanics[edit]

In 1999, researchers from the University of Vienna demonstrated that wave-particle duality applied to molecules such as fullerene.[52]

Superconductivity[edit]

Main article: Buckminsterfullerene

Chirality[edit]

Some fullerenes (e.g. C76, C78, C80, and C84) are inherently chiral because they are D2-symmetric, and have been successfully resolved. Research efforts are ongoing to develop specific sensors for their enantiomers.

Construction[edit]

Two theories have been proposed to describe the molecular mechanisms that make fullerenes. The older, “bottom-up” theory proposes that they are built atom-by-atom. The alternative “top-down” approach claims that fullerenes form when much larger structures break into constituent parts.[53]

In 2013 researchers discovered that asymmetrical fullerenes formed from larger structures settle into stable fullerenes. The synthesized substance was a particular metallofullerene consisting of 84 carbon atoms with two additional carbon atoms and two yttrium atoms inside the cage. The process produced approximately 100 micrograms.[53]

However, they found that the asymmetrical molecule could theoretically collapse to form nearly every known fullerene and metallofullerene. Minor perturbations involving the breaking of a few molecular bonds cause the cage to become highly symmetrical and stable. This insight supports the theory that fullerenes can be formed from graphene when the appropriate molecular bonds are severed.[53][54]

Production technology[edit]

Fullerene production processes comprise the following five subprocesses: (i) synthesis of fullerenes or fullerene-containing soot; (ii) extraction; (iii) separation (purification) for each fullerene molecule, yielding pure fullerenes such as C60; (iv) synthesis of derivatives (mostly using the techniques of organic synthesis); (v) other post-processing such as dispersion into a matrix. The two synthesis methods used in practice are the arc method, and the combustion method. The latter, discovered at the Massachusetts Institute of Technology, is preferred for large scale industrial production.[55][56]

Applications[edit]

Fullerenes have been extensively used for several biomedical applications including the design of high-performance MRI contrast agents, X-Ray imaging contrast agents, photodynamic therapy and drug and gene delivery, summarized in several comprehensive reviews.[57]

Tumor research[edit]

While past cancer research has involved radiation therapy, photodynamic therapy is important to study because breakthroughs in treatments for tumor cells will give more options to patients with different conditions. More recent experiments using HeLa cells in cancer research involves the development of new photosensitizers with increased ability to be absorbed by cancer cells and still trigger cell death. It is also important that a new photosensitizer does not stay in the body for a long time to prevent unwanted cell damage.[58]

Fullerenes can be made to be absorbed by HeLa cells. The C60 derivatives can be delivered to the cells by using the functional groups L-phenylalanine, folic acid, and L-arginine among others.[59] The purpose for functionalizing the fullerenes is to increase the solubility of the molecule by the cancer cells. Cancer cells take up these molecules at an increased rate because of an upregulation of transporters in the cancer cell, in this case amino acid transporters will bring in the L-arginine and L-phenylalanine functional groups of the fullerenes.[60]

Once absorbed by the cells, the C60 derivatives would react to light radiation by turning molecular oxygen into reactive oxygen which triggers apoptosis in the HeLa cells and other cancer cells that can absorb the fullerene molecule. This research shows that a reactive substance can target cancer cells and then be triggered by light radiation, minimizing damage to surrounding tissues while undergoing treatment.[61]

When absorbed by cancer cells and exposed to light radiation, the reaction that creates reactive oxygen damages the DNA, proteins, and lipids that make up the cancer cell. This cellular damage forces the cancerous cell to go through apoptosis, which can lead to the reduction in size of a tumor. Once the light radiation treatment is finished the fullerene will reabsorb the free radicals to prevent damage of other tissues.[62] Since this treatment focuses on cancer cells, it is a good option for patients whose cancer cells are within reach of light radiation. As this research continues into the future, it will be able to penetrate deeper into the body and be absorbed by cancer cells more effectively.[58]

Popular culture[edit]

Main article: Fullerenes in popular culture

Examples of fullerenes in popular culture are numerous. Fullerenes appeared in fiction well before scientists took serious interest in them. In a humorously speculative 1966 column for New Scientist, David Jones suggested that it may be possible to create giant hollow carbon molecules by distorting a plane hexagonal net by the addition of impurity atoms.[70]

On 4 September 2010, Google used an interactively rotatable fullerene [71] C60 as the second 'o' in their logo to celebrate the 25th anniversary of the discovery of the fullerenes.[72][73]

QMREncapsulating atoms in dodecahedrane[edit]

The practice of encapsulating atoms and small molecules inside of caged structures is a practice of chemists aimed at altering the properties of the parent structure, and of testing hypothesis about chemical structure and dynamics.[citation needed] Chemists have accomplished such with dodecahedrane by shooting helium ions (He+) at a film of C20H20; Cross, Saunders and Prinzbach obtain microgram quantities of "He@C20H20"—the nomenclature for one helium atom trapped inside one molecule of dodecahedrane—a substance described as quite stable.[12] The result has been described as the world's smallest helium balloon.

Dodecahedrane is a chemical compound (C20H20) first synthesised by Leo Paquette of Ohio State University in 1982, primarily for the "aesthetically pleasing symmetry of the dodecahedral framework".[1][2]

In this molecule,[3] each vertex is a carbon atom that bonds to three neighbouring carbon atoms. The 108° angle of each regular pentagon is close to the ideal bond angle of 109.5° for an sp3 hybridised atom. Each carbon atom is bonded to a hydrogen atom as well. The molecule, like fullerene, has Ih symmetry, evidenced by its proton NMR spectrum in which all hydrogen atoms appear at a single chemical shift of 3.38 ppm. Dodecahedrane is one of the Platonic hydrocarbons, the others being cubane and tetrahedrane, and does not occur in nature.

Encapsulating atoms in dodecahedrane[edit]

The practice of encapsulating atoms and small molecules inside of caged structures is a practice of chemists aimed at altering the properties of the parent structure, and of testing hypothesis about chemical structure and dynamics.[citation needed] Chemists have accomplished such with dodecahedrane by shooting helium ions (He+) at a film of C20H20; Cross, Saunders and Prinzbach obtain microgram quantities of "He@C20H20"—the nomenclature for one helium atom trapped inside one molecule of dodecahedrane—a substance described as quite stable.[12] The result has been described as the world's smallest helium balloon.

QMrA fullerene ligand is a type of ligand encountered in organometallic chemistry with a fullerene coordinated to a metal. Fullerenes are mostly all carbon, spherical molecules, with the most basic being C60. One of the many uses for fullerenes is their ability to be used as ligands in organometallic systems. Fullerenes were first synthesized in 1985 by Harry Kroto, Richard Smalley, et al.[1] The first use of fullerenes as ligands however did not start until 1991, where C60 is used as a ligand on platinum in the system [(Ph3)P]2Pt(η2-C60).[2][3] Since this point, there have been many different systems using fullerenes, involving different transition metals and binding modes. Most of the fullerene ligands are based on C60, though there are systems which use different sized fullerenes, such as C70 as seen with C70Rh(H)(CO)(PPh3)2.[4]

QMRExohedral fullerenes, also called exofullerenes, are fullerenes that have additional atoms, ions, or clusters attached their outer spheres, such as C50Cl10 [1] and C60H8.[2] or fullerene ligands.

QMREndohedral fullerenes, also called endofullerenes, are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C60 complex was synthesized in 1985 and called La@C60.[2] The @ (at sign) in the name reflects the notion of a small molecule trapped inside a shell. Two types of endohedral complexes exist: endohedral metallofullerenes and non-metal doped fullerenes.

Notation[edit]

In a traditional chemical formula notation, a buckminsterfullerene (C60) with an atom (M) was simply represented as MC60 regardless of whether M was inside or outside the fullerene. In order to allow for more detailed discussions with minimal loss of information, a more explicit notation was proposed in 1991,[2] where the atoms listed to the left of the @ sign is situated inside the network composed of the atoms listed to the right. The example above would then be denoted M@C60 if M were inside the carbon network. A more complex example is K2(K@C59B), which denotes "a 60-atom fullerene cage with one boron atom substituted for a carbon in the geodesic network, a single potassium trapped inside, and two potassium atoms adhering to the outside."[2]

The choice of the symbol has been explained by the authors as being concise, readily printed and transmitted electronically (the at sign is included in ASCII, which most modern character encoding schemes are based on), and the visual aspects suggesting the structure of an endohedral fullerene.

Endohedral metallofullerenes[edit]

Doping fullerenes with electropositive metals takes place in an arc reactor or via laser evaporation. The metals can be transition metals like scandium, yttrium as well as lanthanides like lanthanum and cerium. Also possible are endohedral complexes with elements of the alkaline earth metals like barium and strontium, alkali metals like potassium and tetravalent metals like uranium, zirconium and hafnium. The synthesis in the arc reactor is however unspecific. Besides unfilled fullerenes, endohedral metallofullerenes develop with different cage sizes like La@C60 or La@C82 and as different isomer cages. Aside from the dominant presence of mono-metal cages, numerous di-metal endohedral complexes and the tri-metal carbide fullerenes like Sc3C2@C80 were also isolated.

In 1999 a discovery drew large attention. With the synthesis of the Sc3N@C80 by Harry Dorn and coworkers, the inclusion of a molecule fragment in a fullerene cage had succeeded for the first time. This compound can be prepared by arc-vaporization at temperatures up to 1100 °C of graphite rods packed with scandium(III) oxide iron nitride and graphite powder in a K-H generator in a nitrogen atmosphere at 300 Torr.[3]

Endohedral metallofullerenes are characterised by the fact that electrons will transfer from the metal atom to the fullerene cage and that the metal atom takes a position off-center in the cage. The size of the charge transfer is not always simple to determine. In most cases it is between 2 and 3 charge units, in the case of the La2@C80 however it can be even about 6 electrons such as in Sc3N@C80 which is better described as [Sc3N]+6@ [C80]−6. These anionic fullerene cages are very stable molecules and do not have the reactivity associated with ordinary empty fullerenes. They are stable in air up to very high temperatures (600 to 850 °C).

The lack of reactivity in Diels-Alder reactions is utilised in a method to purify [C80]−6 compounds from a complex mixture of empty and partly filled fullerenes of different cage size.[3] In this method Merrifield resin is modified as a cyclopentadienyl resin and used as a solid phase against a mobile phase containing the complex mixture in a column chromatography operation. Only very stable fullerenes such as [Sc3N]+6@ [C80]−6 pass through the column unreacted.

In Ce2@C80 the two metal atoms exhibit a non-bonded interaction.[4] Since all the six-membered rings in C80-Ih are equal[4] the two encapsulated Ce atoms exhibit a three-dimensional random motion.[5] This is evidenced by the presence of only two signals in the 13C-NMR spectrum. It is possible to force the metal atoms to a standstill at the equator as shown by x-ray crystallography when the fullerene is exahedrally functionalized by an electron donation silyl group in a reaction of Ce2@C80 with 1,1,2,2-tetrakis(2,4,6-trimethylphenyl)-1,2-disilirane.

Non-metal doped fullerenes[edit]

Martin Saunders in 1993 produced endohedral complexes He@C60 and Ne@C60 by pressurizing C60 to ca. 3 bar in a noble-gas atmosphere.[6] Under these conditions about one out of every 650,000 C60 cages was doped with a helium atom. The formation of endohedral complexes with helium, neon, argon, krypton and xenon as well as numerous adducts of the He@C60 compound was also demonstrated[7] with pressures of 3 kbars and incorporation of up to 0.1% of the noble gases.

While noble gases are chemically very inert and commonly exist as individual atoms, this is not the case for nitrogen and phosphorus and so the formation of the endohedral complexes N@C60, N@C70 and P@C60 is more surprising. The nitrogen atom is in its electronic initial state (4S3/2) and is therefore to be highly reactive. Nevertheless N@C60 is sufficiently stable that exohedral derivatization from the mono- to the hexa adduct of the malonic acid ethyl ester is possible. In these compounds no charge transfer[disambiguation needed] of the nitrogen atom in the center to the carbon atoms of the cage takes place. Therefore 13C-couplings, which are observed very easily with the endohedral metallofullerenes, could only be observed in the case of the N@C60 in a high resolution spectrum as shoulders of the central line.

The central atom in these endohedral complexes is located in the center of the cage. While other atomic traps require complex equipment, e.g. laser cooling or magnetic traps, endohedral fullerenes represent an atomic trap that is stable at room temperature and for an arbitrarily long time. Atomic or ion traps are of great interest since particles are present free from (significant) interaction with their environment, allowing unique quantum mechanical phenomena to be explored. For example, the compression of the atomic wave function as a consequence of the packing in the cage could be observed with ENDOR spectroscopy. The nitrogen atom can be used as a probe, in order to detect the smallest changes of the electronic structure of its environment.

Contrary to the metallo endohedral compounds, these complexes cannot be produced in an arc. Atoms are implanted in the fullerene starting material using gas discharge (nitrogen and phosphorus complexes) or by direct ion implantation. Alternatively, endohedral hydrogen fullerenes can be produced by opening and closing a fullerene by organic chemistry methods. A recent example of endohedral fullerenes includes single molecules of water encapsulated in C60.[8] Water endofullerene is a young compound and has been poorly studied. The most interesting in such endofullerenes is their hypothetical ability to transform into endohedral covalent derivatives, a new class of fullerene derivatives. One of the examples of the mentioned interconversions, induced by compression, has been studied by DFT methods.[9]

According to state-of-the-art DFT calculations, noble gas endofullerenes should demonstrate unusual polarizability. Thus, calculated values of mean polarizability of Ng@C60 do not equal to the sum of polarizabilities of a fullerene cage and the trapped atom, i.e. exaltation of polarizability occurs.,.[10][11] The sign of the Δα polarizability exaltation depends on the number of atoms in a fullerene molecule: for small fullerenes (n<30), it is positive; for the larger ones (n>30), it is negative (depression of polarizability). Sabirov has proposed the following formula, describing the dependence of Δα on n: Δα = αNg(2e−0.06(n – 20)-1). It describes the DFT-calculated mean polarizabilities of Ng@C60 endofullerenes with sufficient accuracy. The calculated data allows using C60 fullerene as a Faraday cage,[12] which isolates the encapsulated atom from the external electric field. The mentioned relations should be typical for the more complicated endohedral structures (e.g., C60@C240[13] and giant fullerene-containing onions [14]).

Molecular endofullerenes[edit]

Closed fullerenes encapsulating small molecules have been synthesized by long sequences of organic reactions. Notable achievements are the synthesis of the dihydrogen endofullerene H2@C60 and the water endofullerene H2O@C60 by the groups of Komatsu [15] and Murata.[16] The encapsulated molecules display unusual physical properties which have been studied by a variety of physical methods.[17]

QMRTetrathiafulvalene (TTF) center typical in organic superconductors

A Bechgaard salt is any one of a number of organic charge-transfer complexes that exhibit superconductivity at low temperatures.[1] They are named for chemist Klaus Bechgaard, who was one of the first scientists to synthesize them and demonstrate their superconductivity with the help of physicist Denis Jérôme.[2] Most Bechgaard salt superconductors are extremely low temperature, and lose superconductivity above the 1-2 K range, although the most successful compound in this class superconducts up to almost 12 K.

All Bechgaard salts are formed using a small, planar organic molecule as an electron donor, with any of a number of electron acceptors [such as perchlorate (ClO4) or tetracyanoethylene (TCNE)]. All the organic electron donors contain multiply conjugated heterocycles with a number of properties, including planarity, low ionization potential and good orbital overlap between heteroatoms in neighboring donor molecules. These properties help the final salt conduct electrons by shuttling them through the orbital vacancies left in the donor molecules.

All Bechgaard salts have a variation on a single tetrathiafulvalene motif - different superconductors have been made with appendages to the motif, or using a tetraselenafulvalene center instead (which is a related compound), but all bear this general structural similarity.

There are a wide range of other organic superconductors including many other charge-transfer complexes.

Wudl et al. first demonstrated that the salt [TTF+

]Cl−

was a semiconductor.[3] Subsequently, Ferraris et al. showed that the charge-transfer salt [TTF]TCNQ is a narrow band gap semi-conductor.[4] X-ray diffraction studies of [TTF][TCNQ] revealed stacks of partially oxidized TTF molecules adjacent to anionic stacks of TCNQ molecules. This “segregated stack” motif was unexpected and is responsible for the distinctive electrical properties, i.e. high and anisotropic electrical conductivity. Since these early discoveries, numerous analogues of TTF have been prepared. Well studied analogues include tetramethyltetrathiafulvalene (Me4TTF), tetramethylselenafulvalenes (TMTSFs), and bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, CAS [66946-48-3]).[5] Several tetramethyltetrathiafulvalene salts (called Fabre salts) are of some relevance as organic superconductors.

QMRBuckminsterfullerene (or bucky-ball) is a spherical fullerene molecule with the formula C60. It has a cage-like fused-ring structure (truncated icosahedron) which resembles a football (soccer ball), made of twenty hexagons and twelve pentagons, with a carbon atom at each vertex of each polygon and a bond along each polygon edge.

It was first generated in 1985 by Harold Kroto, James R. Heath, Sean O'Brien, Robert Curl, and Richard Smalley at Rice University.[2] Kroto, Curl and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of buckminsterfullerene and the related class of molecules, the fullerenes. The name is a reference to Buckminster Fuller, as C60 resembles his trademark geodesic domes. Buckminsterfullerene is the most common naturally occurring fullerene molecule, as it can be found in small quantities in soot.[3][4] Solid and gaseous forms of the molecule have been detected in deep space.[5]

Buckminsterfullerene is one of the largest objects to have been shown to exhibit wave–particle duality; as stated in the theory every object exhibits this behavior.[6][7] Its discovery led to the exploration of a new field of chemistry, involving the study of fullerenes.

Buckminsterfullerene derives from the name of the noted futurist and inventor Buckminster Fuller. One of his designs of a geodesic dome structure bears great resemblance to C60; as a result, the discoverers of the allotrope named the newfound molecule after him. The general public, however, sometimes refers to buckminsterfullerene, and even Mr. Fuller's dome structure, as buckyballs.[8]

Carbon has four valence electrons and looks like quadrant. It is known as the miracle element

History[edit]

Main article: Fullerene

The structure associated with fullerenes was described by Leonardo da Vinci.[9] Albrecht Dürer also reproduced a similar icosahedron containing 12 pentagonal and 20 hexagonal faces but there are no clear documentations of this.[10][11

Theoretical predictions of buckyball molecules appeared in the late 1960s – early 1970s,[12][13] but they went largely unnoticed. In the early 1970s, the chemistry of unsaturated carbon configurations was studied by a group at the University of Sussex, led by Harry Kroto and David Walton. In the 1980s a technique was developed by Richard Smalley and Bob Curl at Rice University, Texas to isolate these substances. They used laser vaporization of a suitable target to produce clusters of atoms. Kroto realized that by using a graphite target,[14] any carbon chains formed could be studied. Another interesting fact is that, at the same time, astrophysicists were working along with spectroscopists to study infrared emissions from giant red carbon stars.[11][15][16] Smalley and team were able to use a laser vaporization technique to create carbon clusters which could potentially emit infrared at the same wavelength as had been emitted by the red carbon star.[11][17] Hence, the inspiration came to Smalley and team to use the laser technique on graphite to create the first fullerene molecule.

C60 was discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley. Using laser evaporation of graphite they found Cn clusters (where n>20 and even) of which the most common were C60 and C70. A solid rotating graphite disk was used as the surface from which carbon was vaporized using a laser beam creating hot plasma that was then passed through a stream of high-density helium gas.[18] The carbon species were subsequently cooled and ionized resulting in the formation of clusters. Clusters ranged in molecular masses but Kroto and Smalley found predominance in a C60 cluster that could be enhanced further by letting the plasma react longer.[3, 6] They also discovered that the C60 molecule formed a cage-like structure, a regular truncated icosahedron.[11][18]

For this discovery they were awarded the 1996 Nobel Prize in Chemistry. The discovery of buckyballs was surprising, as the scientists aimed the experiment at producing carbon plasmas to replicate and characterize unidentified interstellar matter. Mass spectrometry analysis of the product indicated the formation of spheroidal carbon molecules.[12]

The experimental evidence, a strong peak at 720 atomic mass units, indicated that a carbon molecule with 60 carbon atoms was forming, but provided no structural information. The research group concluded after reactivity experiments, that the most likely structure was a spheroidal molecule. The idea was quickly rationalized as the basis of an icosahedral symmetry closed cage structure. Kroto mentioned geodesic dome structures of the noted futurist and inventor Buckminster Fuller as influences in the naming of this particular substance as buckminsterfullerene.[12]

Further developments[edit]

The versatility of fullerene molecules has led to a large amount of research exploring their properties. One interesting property is the relatively large volume of the internal space of the molecule. Atoms of different elements may be placed inside the molecular cage formed by the carbon atoms.[19]

Beam-experiments conducted between 1985 and 1990 provided more evidence for the stability of C60 while supporting the closed-cage structural theory and predicting some of the bulk properties such a molecule would have. Around this time, intense theoretical group theory activity also predicted that C60 should have only four IR-active vibrational bands, on account of its icosahedral symmetry.[20]

In 1989 physicists Wolfgang Krätschmer and Donald R. Huffman observed unusual optical absorptions in thin carbon films produced by arc-processed graphite rods. Among other features, the IR spectra showed FOUR discrete bands in close agreement to those proposed for C60. A paper published by the group in 1990 followed on from their thin film experiments, and detailed the extraction of a benzene soluble material from the arc-processed graphite. This extract had crystal and X-ray analysis consistent with arrays of spherical C60 molecules, approximately 0.7 nm in diameter.[20]

In 2012 a toxicity study by Tarek Baati and Fathi Moussa from the University of Paris, showed that C60 dissolved in olive oil was not toxic to rodents.[21] In a video interview with Professor Fathi Moussa regarding the study, further information was provided regarding the toxicity study, and the method of action whereby the lifespan of the rodents was increased by 90% relative to controls when the animals were dosed with C60 olive oil.

QMRIn organic chemistry, an alkene is an unsaturated hydrocarbon that contains at least one carbon–carbon double bond.[1] Alkene, olefin, and olefine are used often interchangeably (see nomenclature section below). Acyclic alkenes, with only one double bond and no other functional groups, known as mono-enes, form a homologous series of hydrocarbons with the general formula CnH2n.[2] Alkenes have two hydrogen atoms less than the corresponding alkane (with the same number of carbon atoms). The simplest alkene, ethylene (C2H4), which has the International Union of Pure and Applied Chemistry (IUPAC) name ethene is the organic compound produced on the largest scale industrially.[3] Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes.[2]

QMRCarbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1,[1] significantly larger than for any other material. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology. In particular, owing to their extraordinary thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials. For instance, nanotubes form a tiny portion of the material(s) in some (primarily carbon fiber) baseball bats, golf clubs, car parts or damascus steel.[2][3]

Nanotubes are members of the fullerene structural family. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene. These sheets are rolled at specific and discrete ("chiral") angles, and the combination of the rolling angle and radius decides the nanotube properties; for example, whether the individual nanotube shell is a metal or semiconductor. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Individual nanotubes naturally align themselves into "ropes" held together by van der Waals forces, more specifically, pi-stacking.

Applied quantum chemistry, specifically, orbital hybridization best describes chemical bonding in nanotubes. The chemical bonding of nanotubes is composed entirely of sp2 bonds, similar to those of graphite. These bonds, which are stronger than the sp3 bonds found in alkanes and diamond, provide nanotubes with their unique strength.

The first intermolecular logic gate using SWCNT FETs was made in 2001.[6] A logic gate requires both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to oxygen and n-FETs otherwise, it is possible to protect half of an SWNT from oxygen exposure, while exposing the other half to oxygen. This results in a single SWNT that acts as a not logic gate with both p and n-type FETs within the same molecule.

Single-walled nanotubes are dropping precipitously in price, from around $1500 per gram as of 2000 to retail prices of around $50 per gram of as-produced 40–60% by weight SWNTs as of March 2010.[citation needed]

SWNTs have been viewed as too expensive for widespread application but are forecast to make a large impact in electronics applications by 2020 according to The Global Market for Carbon Nanotubes report.

QMRThis literary device also dates back to ancient Sanskrit literature. In Vishnu Sarma's Panchatantra, an inter-woven series of colorful animal tales are told with one narrative opening within another, sometimes three or four layers deep, and then unexpectedly snapping shut in irregular rhythms to sustain attention. In Ugrasrava's epic Mahabharata, the Kurukshetra War is narrated by a character in Vyasa's Jaya, which itself is narrated by a character in Vaisampayana's Bharata, which itself is narrated by a character in Ugrasrava's Mahabharata.

Graphenated carbon nanotubes (g-CNTs)[edit]

SEM series of graphenated CNTs with varying foliate density

Graphenated CNTs are a relatively new hybrid that combines graphitic foliates grown along the sidewalls of multiwalled or bamboo style CNTs. Yu et al.[15] reported on "chemically bonded graphene leaves" growing along the sidewalls of CNTs. Stoner et al.[16] described these structures as "graphenated CNTs" and reported in their use for enhanced supercapacitor performance. Hsu et al. further reported on similar structures formed on carbon fiber paper, also for use in supercapacitor applications.[17] Pham et al. [18][19] also reported a similar structure, namely "graphene-carbon nanotube hybrids", grown directly onto carbon fiber paper to form an integrated, binder free, high surface area conductive catalyst support for Proton Exchange Membrane Fuel Cells electrode applications with enhanced performance and durability. The foliate density can vary as a function of deposition conditions (e.g. temperature and time) with their structure ranging from few layers of graphene (< 10) to thicker, more graphite-like.[20]

The fundamental advantage of an integrated graphene-CNT structure is the high surface area three-dimensional framework of the CNTs coupled with the high edge density of graphene. Graphene edges provide significantly higher charge density and reactivity than the basal plane, but they are difficult to arrange in a three-dimensional, high volume-density geometry. CNTs are readily aligned in a high density geometry (i.e., a vertically aligned forest)[21] but lack high charge density surfaces—the sidewalls of the CNTs are similar to the basal plane of graphene and exhibit low charge density except where edge defects exist. Depositing a high density of graphene foliates along the length of aligned CNTs can significantly increase the total charge capacity per unit of nominal area as compared to other carbon nanostructures

Peapod[edit]

A carbon peapod[30][31] is a novel hybrid carbon material which traps fullerene inside a carbon nanotube. It can possess interesting magnetic properties with heating and irradiation. It can also be applied as an oscillator during theoretical investigations and predictions.[32][33]

Cup-stacked carbon nanotubes[edit]

Cup-stacked carbon nanotubes (CSCNTs) differ from other quasi-1D carbon structures, which normally behave as quasi-metallic conductors of electrons. CSCNTs exhibit semiconducting behaviors due to the stacking microstructure of graphene layers.[34]

Extreme carbon nanotubes[edit]

Cycloparaphenylene

The observation of the longest carbon nanotubes grown so far are over 1/2 m (550 mm long) was reported in 2013.[35] These nanotubes were grown on Si substrates using an improved chemical vapor deposition (CVD) method and represent electrically uniform arrays of single-walled carbon nanotubes.[1]

The shortest carbon nanotube is the organic compound cycloparaphenylene, which was synthesized in early 2009.[36][37]

The thinnest carbon nanotube is the armchair (2,2) CNT with a diameter of 0.3 nm. This nanotube was grown inside a multi-walled carbon nanotube. Assigning of carbon nanotube type was done by a combination of high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and density functional theory (DFT) calculations.[38]

The thinnest freestanding single-walled carbon nanotube is about 0.43 nm in diameter. Researchers suggested that it can be either (5,1) or (4,2) SWCNT, but the exact type of carbon nanotube remains questionable.[39] (3,3), (4,3) and (5,1) carbon nanotubes (all about 0.4 nm in diameter) were unambiguously identified using aberration-corrected high-resolution transmission electron microscopy inside double-walled CNTs.[40]

The highest density of CNTs was achieved in 2013, grown on a conductive titanium-coated copper surface that was coated with co-catalysts cobalt and molybdenum at lower than typical temperatures of 450 °C. The tubes averaged a height of 380 nm and a mass density of 1.6 g cm−3. The material showed ohmic conductivity (lowest resistance ∼22 kΩ).[41][

Strength[edit]

See also: Mechanical properties of carbon nanotubes

Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp2 bonds formed between the individual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (9,100,000 psi).[43] (For illustration, this translates into the ability to endure tension of a weight equivalent to 6,422 kilograms-force (62,980 N; 14,160 lbf) on a cable with cross-section of 1 square millimetre (0.0016 sq in).) Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to ~100 gigapascals (15,000,000 psi), which is in agreement with quantum/atomistic models.[44] Since carbon nanotubes have a low density for a solid of 1.3 to 1.4 g/cm3,[45] its specific strength of up to 48,000 kN·m·kg−1 is the best of known materials, compared to high-carbon steel's 154 kN·m·kg−1.

Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% and can increase the maximum strain the tubes undergo before fracture by releasing strain energy.

Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multi-walled carbon nanotubes and carbon nanotube bundles down to only a few GPa.[46] This limitation has been recently addressed by applying high-energy electron irradiation, which crosslinks inner shells and tubes, and effectively increases the strength of these materials to ~60 GPa for multi-walled carbon nanotubes[44] and ~17 GPa for double-walled carbon nanotube bundles.[46]

CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional, or bending stress

Hardness[edit]

Standard single-walled carbon nanotubes can withstand a pressure up to 25 GPa without [plastic/permanent] deformation. They then undergo a transformation to superhard phase nanotubes. Maximum pressures measured using current experimental techniques are around 55 GPa. However, these new superhard phase nanotubes collapse at an even higher, albeit unknown, pressure.

The bulk modulus of superhard phase nanotubes is 462 to 546 GPa, even higher than that of diamond (420 GPa for single diamond crystal).[60]

Wettability[edit]

The surface wettability of CNT is of importance for its applications in various settings. Although the intrinsic contact angle of graphite is around 90°, the contact angles of most as-synthesized CNT arrays are over 160°, exhibiting a superhydrophobic property. By applying a low voltage as low as 1.3V, the extreme water repellant surface can be switched into superhydrophilic.[61]

Kinetic properties[edit]

Multi-walled nanotubes are multiple concentric nanotubes precisely nested within one another. These exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing. This is one of the first true examples of molecular nanotechnology, the precise positioning of atoms to create useful machines. Already, this property has been utilized to create the world's smallest rotational motor.[62] Future applications such as a gigahertz mechanical oscillator are also envisioned.

Electrical properties[edit]

Band structures computed using tight binding approximation for (6,0) CNT (zigzag, metallic), (10,2) CNT (semiconducting) and (10,10) CNT (armchair, metallic).

Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if n = m, the nanotube is metallic; if n − m is a multiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor. Thus all armchair (n = m) nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting.[63]

However, this rule has exceptions, because curvature effects in small diameter tubes can strongly influence electrical properties. Thus, a (5,0) SWCNT that should be semiconducting in fact is metallic according to the calculations. Likewise, zigzag and chiral SWCNTs with small diameters that should be metallic have a finite gap (armchair nanotubes remain metallic).[63] In theory, metallic nanotubes can carry an electric current density of 4 × 109 A/cm2, which is more than 1,000 times greater than those of metals such as copper,[64] where for copper interconnects current densities are limited by electromigration.

Because of its nanoscale cross-section, electrons propagate only along the tube's axis. As a result, carbon nanotubes are frequently referred to as one-dimensional conductors. The maximum electrical conductance of a single-walled carbon nanotube is 2G0, where G0 = 2e2/h is the conductance of a single ballistic quantum channel.[65]

Intrinsic superconductivity has been reported,[66] although other experiments found no evidence of this, leaving the claim a subject of debate.[67]

Optical properties[edit]

Main article: Optical properties of carbon nanotubes

Potential applications[edit]

Main article: Potential applications of carbon nanotubes

The strength and flexibility of carbon nanotubes makes them of potential use in controlling other nanoscale structures, which suggests they will have an important role in nanotechnology engineering. The highest tensile strength of an individual multi-walled carbon nanotube has been tested to be 63 GPa.[43] Carbon nanotubes were found in Damascus steel from the 17th century, possibly helping to account for the legendary strength of the swords made of it.[150][151] Recently, several studies have highlighted the prospect of using carbon nanotubes as building blocks to fabricate three-dimensional macroscopic (>1mm in all three dimensions) all-carbon devices. Lalwani et al. have reported a novel radical initiated thermal crosslinking method to fabricated macroscopic, free-standing, porous, all-carbon scaffolds using single- and multi-walled carbon nanotubes as building blocks.[13] These scaffolds possess macro-, micro-, and nano- structured pores and the porosity can be tailored for specific applications. These 3D all-carbon scaffolds/architectures maybe used for the fabrication of the next generation of energy storage, supercapacitors, field emission transistors, high-performance catalysis, photovoltaics, and biomedical devices and implants.

Biomedical[edit]

Researchers from Rice University and State University of New York - Stony Brook have shown that the addition of low weight % of carbon nanotubes can lead to significant improvements in the mechanical properties of biodegradable polymeric nanocomposites for applications in tissue engineering including bone,[152][153][154] cartilage,[155] muscle[156] and nerve tissue.[153][157] Dispersion of low weight % of graphene (~0.02 wt.%) results in significant increases in compressive and flexural mechanical properties of polymeric nanocomposites. Researchers at Rice University, Stony Brook University, Radboud University Nijmegen Medical Centre and University of California, Riverside have shown that carbon nanotubes and their polymer nanocomposites are suitable scaffold materials for bone tissue engineering [14][148][158] and bone formation.[159][160]

In November 2012 researchers at the American National Institute of Standards and Technology (NIST) proved that single-wall carbon nanotubes may help protect DNA molecules from damage by oxidation.[161]

A highly effective method of delivering carbon nanotubes into cells is Cell squeezing, a high-throughput vector-free microfluidic platform for intracellular delivery developed at the Massachusetts Institute of Technology in the labs of Robert S. Langer.[162]

Carbon nanotubes have furthermore been grown inside microfluidic channels for chemical analysis, based on electrochromatography. Here, the high surface-area-to-volume ratio and high hydrophobicity of CNTs are used in order to greatly decrease the analysis time of small neutral molecules that typically require large bulky equipment for analysis.[163][164]

Structural[edit]

Because of the carbon nanotube's superior mechanical properties, many structures have been proposed ranging from everyday items like clothes and sports gear to combat jackets and space elevators.[165] However, the space elevator will require further efforts in refining carbon nanotube technology, as the practical tensile strength of carbon nanotubes must be greatly improved.[45]

For perspective, outstanding breakthroughs have already been made. Pioneering work led by Ray H. Baughman at the NanoTech Institute has shown that single and multi-walled nanotubes can produce materials with toughness unmatched in the man-made and natural worlds.[166][167]

Carbon nanotubes being spun to form a yarn, CSIRO

Carbon nanotubes are also a promising material as building blocks in hierarchical composite materials given their exceptional mechanical properties (~1 TPa in modulus, and ~100 GPa in strength). Initial attempts to incorporate CNTs into hierarchical structures (such as yarns or fibres) has led to mechanical properties that were significantly lower than these potential limits. Windle et al. have used an in situ chemical vapor deposition (CVD) spinning method to produce continuous CNT yarns from CVD-grown CNT aerogels.[168][169] CNT yarns can also be manufactured by drawing out CNT bundles from a CNT forest and subsequently twisting to form the fibre (draw-twist method, see picture on right). The Windle group have fabricated CNT yarns with strengths as high as ~9 GPa at small gage lengths of ~1 mm, however, strengths of only about ~1 GPa were reported at the longer gage length of 20 mm.[170][171] The reason why fibre strengths have been low compared to the strength of individual CNTs is due to a failure to effectively transfer load to the constituent (discontinuous) CNTs within the fibre. One potential route for alleviating this problem is via irradiation (or deposition) induced covalent inter-bundle and inter-CNT cross-linking to effectively 'join up' the CNTs.[172] Espinosa et al. developed high performance DWNT-polymer composite yarns by twisting and stretching ribbons of randomly oriented bundles of DWNTs thinly coated with polymeric organic compounds. These DWNT-polymer yarns exhibited an unusually high energy to failure of ~100 J·g−1 (comparable to one of the toughest natural materials – spider silk[173]), and strength as high as ~1.4 GPa.[174] Effort is ongoing to produce CNT composites that incorporate tougher matrix materials, such as Kevlar, to further improve on the mechanical properties toward those of individual CNTs.

Because of the high mechanical strength of carbon nanotubes, research is being made into weaving them into clothes to create stab-proof and bulletproof clothing. The nanotubes would effectively stop the bullet from penetrating the body, although the bullet's kinetic energy would likely cause broken bones and internal bleeding.[175]

Electrical circuits[edit]

Nanotube-based transistors, also known as carbon nanotube field-effect transistors (CNTFETs), have been made that operate at room temperature and that are capable of digital switching using a single electron.[176] However, onemajor obstacle to realization of nanotubes has been the lack of technology for mass production. In 2001 IBM researchers demonstrated how metallic nanotubes can be destroyed, leaving semiconducting ones behind for use as transistors. Their process is called "constructive destruction," which includes the automatic destruction of defective nanotubes on the wafer.[177] This process, however, only gives control over the electrical properties on a statistical scale.

The potential of carbon nanotubes was demonstrated in 2003 when room-temperature ballistic transistors with ohmic metal contacts and high-k gate dielectric were reported, showing 20–30x higher ON current than state-of-the-art Si MOSFETs. This presented an important advance in the field as CNT was shown to potentially outperform Si. At the time, a major challenge was ohmic metal contact formation. In this regard, palladium, which is a high-work function metal was shown to exhibit Schottky barrier-free contacts to semiconducting nanotubes with diameters >1.7 nm.[178][179]

The first nanotube integrated memory circuit was made in 2004. One of the main challenges has been regulating the conductivity of nanotubes. Depending on subtle surface features a nanotube may act as a plain conductor or as a semiconductor. A fully automated method has however been developed to remove non-semiconductor tubes.[180]

Another way to make carbon nanotube transistors has been to use random networks of them.[181] By doing so one averages all of their electrical differences and one can produce devices in large scale at the wafer level.[182] This approach was first patented by Nanomix Inc.[183] (date of original application June 2002[184]). It was first published in the academic literature by the United States Naval Research Laboratory in 2003 through independent research work. This approach also enabled Nanomix to make the first transistor on a flexible and transparent substrate.[185][186]

Large structures of carbon nanotubes can be used for thermal management of electronic circuits. An approximately 1 mm–thick carbon nanotube layer was used as a special material to fabricate coolers, this material has very low density, ~20 times lower weight than a similar copper structure, while the cooling properties are similar for the two materials.[187]

In 2013, researchers demonstrated a Turing-complete prototype micrometer-scale computer.[188][189][190] Carbon nanotube transistors as logic-gate circuits with densities comparable to modern CMOS technology has not yet been demonstrated.

Electrical cables and wires[edit]

Wires for carrying electric current may be fabricated from pure nanotubes and nanotube-polymer composites. It has already been demonstrated that carbon nanotube wires can successfully be used for power or data transmission.[191] Recently small wires have been fabricated with specific conductivity exceeding copper and aluminum;[192][193] these cables are the highest conductivity carbon nanotube and also highest conductivity non-metal cables. Recently, composite of carbon nanotube and copper have been shown to exhibit nearly one hundred times higher current-carrying-capacity than pure copper or gold.[194] Significantly, the electrical conductivity of such a composite is similar to pure Cu. Thus, this Carbon nanotube-copper (CNT-Cu) composite possesses the highest observed current-carrying capacity among electrical conductors. Thus for a given cross-section of electrical conductor, the CNT-Cu composite can withstand and transport one hundred times higher current compared to metals such as copper and gold.

Actuators[edit]

Main article: Carbon nanotube actuators

The exceptional electrical and mechanical properties of carbon nanotubes have made them alternatives to the traditional electrical actuators for both microscopic and macroscopic applications. Carbon nanotubes are very good conductors of both electricity and heat, and they are also very strong and elastic molecules in certain directions.

Batteries[edit]

Carbon nanotubes' (CNTs) exciting electronic properties have shown promise in the field of batteries, where typically they are being experimented as a new electrode material, particularly the anode for lithium ion batteries. This is due to the fact that the anode requires a relatively high reversible capacity at a potential close to metallic lithium, and a moderate irreversible capacity, observed thus far only by graphite-based composites, such as CNTs. They have shown to greatly improve capacity and cyclability of lithium-ion batteries, as well as the capability to be very effective buffering components, alleviating the degradation of the batteries that is typically due to repeated charging and discharging. Further, electronic transport in the anode can be greatly improved using highly metallic CNTs.[195]

More specifically, CNTs have shown reversible capacities from 300 to 600 mAhg−1, with some treatments to them showing these numbers rise to up to 1000 mAhg−1.[196] Meanwhile, graphite, which is most widely used as an anode material for these lithium batteries, has shown capacities of only 320 mAhg−1. By creating composites out of the CNTs, scientists see much potential in taking advantage of these exceptional capacities, as well as their excellent mechanical strength, conductivities, and low densities.[195]

Paper batteries[edit]

A paper battery is a battery engineered to use a paper-thin sheet of cellulose (which is the major constituent of regular paper, among other things) infused with aligned carbon nanotubes.[197] The potential for these devices is great, as they may be manufactured via a roll-to-roll process, which would make it very low-cost, and they would be lightweight, flexible, and thin. In order to productively use paper electronics (or any thin electronic devices), the power source must be equally thin, thus indicating the need for paper batteries. Recently, it has been shown that surfaces coated with CNTs can be used to replace heavy metals in batteries.[198] More recently, functional paper batteries have been demonstrated, where a lithium-ion battery is integrated on a single sheet of paper through a lamination process as a composite with Li4Ti5O12 (LTO) or LiCoO2 (LCO). The paper substrate would function well as the separator for the battery, where the CNT films function as the current collectors for both the anode and the cathode. These rechargeable energy devices show potential in RFID tags, functional packaging, or new disposable electronic applications.[199]

Solar cells[edit]

One of the promising applications of single-walled carbon nanotubes (SWNTs) is their use in solar panels, due to their strong UV/Vis-NIR absorption characteristics. Research has shown that they can provide a sizable increase in efficiency, even at their current unoptimized state. Solar cells developed at the New Jersey Institute of Technology use a carbon nanotube complex, formed by a mixture of carbon nanotubes and carbon buckyballs (known as fullerenes) to form snake-like structures. Buckyballs trap electrons, but they can't make electrons flow.[200] Add sunlight to excite the polymers, and the buckyballs will grab the electrons. Nanotubes, behaving like copper wires, will then be able to make the electrons or current flow.[201]

Additional research has been conducted on creating SWNT hybrid solar panels to increase the efficiency further. These hybrids are created by combining SWNT's with photo-excitable electron donors to increase the number of electrons generated. It has been found that the interaction between the photo-excited porphyrin and SWNT generates electro-hole pairs at the SWNT surfaces. This phenomenon has been observed experimentally, and contributes practically to an increase in efficiency up to 8.5%.[202]

Further information: Carbon nanotubes in photovoltaics

Hydrogen storage[edit]

In addition to being able to store electrical energy, there has been some research in using carbon nanotubes to store hydrogen to be used as a fuel source. By taking advantage of the capillary effects of the small carbon nanotubes, it is possible to condense gases in high density inside single-walled nanotubes. This allows for gases, most notably hydrogen (H2), to be stored at high densities without being condensed into a liquid. Potentially, this storage method could be used on vehicles in place of gas fuel tanks for a hydrogen-powered car. A current issue regarding hydrogen-powered vehicles is the on-board storage of the fuel. Current storage methods involve cooling and condensing the H2 gas to a liquid state for storage which causes a loss of potential energy (25–45%) when compared to the energy associated with the gaseous state. Storage using SWNTs would allow one to keep the H2 in its gaseous state, thereby increasing the storage efficiency. This method allows for a volume to energy ratio slightly smaller to that of current gas powered vehicles, allowing for a slightly lower but comparable range.[203]

An area of controversy and frequent experimentation regarding the storage of hydrogen by adsorption in carbon nanotubes is the efficiency by which this process occurs. The effectiveness of hydrogen storage is integral to its use as a primary fuel source since hydrogen only contains about one fourth the energy per unit volume as gasoline. Studies however show that what is the most important is the surface area of the materials used. Hence activated carbon with surface area of 2600 m2/g can store up to 5,8% w/w. In all these carbonaceous materials, hydrogen is stored by physisorption at 70-90K.[204]

Experimental capacity[edit]

One experiment[205] sought to determine the amount of hydrogen stored in CNTs by utilizing elastic recoil detection analysis (ERDA). CNTs (primarily SWNTs) were synthesized via chemical vapor disposition (CVD) and subjected to a two-stage purification process including air oxidation and acid treatment, then formed into flat, uniform discs and exposed to pure, pressurized hydrogen at various temperatures. When the data was analyzed, it was found that the ability of CNTs to store hydrogen decreased as temperature increased. Moreover, the highest hydrogen concentration measured was ~0.18%; significantly lower than commercially viable hydrogen storage needs to be. A separate experimental work performed by using a gravimetric method also revealed the maximum hydrogen uptake capacity of CNTs to be as low as 0.2%.[206]

In another experiment,[207] CNTs were synthesized via CVD and their structure was characterized using Raman spectroscopy. Utilizing microwave digestion, the samples were exposed to different acid concentrations and different temperatures for various amounts of time in an attempt to find the optimum purification method for SWNTs of the diameter determined earlier. The purified samples were then exposed to hydrogen gas at various high pressures, and their adsorption by weight percent was plotted. The data showed that hydrogen adsorption levels of up to 3.7% are possible with a very pure sample and under the proper conditions. It is thought that microwave digestion helps improve the hydrogen adsorption capacity of the CNTs by opening up the ends, allowing access to the inner cavities of the nanotubes.

Limitations on efficient hydrogen adsorption[edit]

The biggest obstacle to efficient hydrogen storage using CNTs is the purity of the nanotubes. To achieve maximum hydrogen adsorption, there must be minimum graphene, amorphous carbon, and metallic deposits in the nanotube sample. Current methods of CNT synthesis require a purification step. However, even with pure nanotubes, the absorption capacity is only maximized under high pressures, which are undesirable in commercial fuel tanks.

Supercapacitor[edit]

Main article: Supercapacitor

MIT Research Laboratory of Electronics uses nanotubes to improve supercapacitors. The activated charcoal used in conventional ultracapacitors has many small hollow spaces of various size, which create together a large surface to store electric charge. But as charge is quantized into elementary charges, i.e. electrons, and each such elementary charge needs a minimum space, a significant fraction of the electrode surface is not available for storage because the hollow spaces are not compatible with the charge's requirements. With a nanotube electrode the spaces may be tailored to size—few too large or too small—and consequently the capacity should be increased considerably.[208]

Radar absorption[edit]

Main article: Radar-absorbent material

Radars work in the microwave frequency range, which can be absorbed by MWNTs. Applying the MWNTs to the aircraft would cause the radar to be absorbed and therefore seem to have a smaller radar cross-section. One such application could be to paint the nanotubes onto the plane. Recently there has been some work done at the University of Michigan regarding carbon nanotubes usefulness as stealth technology on aircraft. It has been found that in addition to the radar absorbing properties, the nanotubes neither reflect nor scatter visible light, making it essentially invisible at night, much like painting current stealth aircraft black except much more effective. Current limitations in manufacturing, however, mean that current production of nanotube-coated aircraft is not possible. One theory to overcome these current limitations is to cover small particles with the nanotubes and suspend the nanotube-covered particles in a medium such as paint, which can then be applied to a surface, like a stealth aircraft.[209]

Textile[edit]

The previous studies on the use of CNTs for textile functionalization were focused on fiber spinning for improving physical and mechanical properties.[210][211][212] Recently a great deal of attention has been focused on coating CNTs on textile fabrics. Various methods have been employed for modifying fabrics using CNTs. Shim et al. produced intelligent e-textiles for Human Biomonitoring using a polyelectrolyte-based coating with CNTs.[213] Additionally, Panhuis et al. dyed textile material by immersion in either a poly (2-methoxy aniline-5-sulfonic acid) PMAS polymer solution or PMAS-SWNT dispersion with enhanced conductivity and capacitance with a durable behavior.[214] In another study, Hu and coworkers coated single-walled carbon nanotubes with a simple “dipping and drying” process for wearable electronics and energy storage applications.[215] In the recent study, Li and coworkers using elastomeric separator and almost achieved a fully stretchable supercapacitor based on buckled single-walled carbon nanotube macrofilms. The electrospun polyurethane was used and provided sound mechanical stretchability and the whole cell achieve excellent charge-discharge cycling stability.[216] CNTs have an aligned nanotube structure and a negative surface charge. Therefore, they have similar structures to direct dyes, so the exhaustion method is applied for coating and absorbing CNTs on the fiber surface for preparing multifunctional fabric including antibacterial, electric conductive, flame retardant and electromagnetic absorbance properties.[217][218][219]

Optical power detectors[edit]

A spray-on mixture of carbon nanotubes and ceramic demonstrates unprecedented ability to resist damage while absorbing laser light. Such coatings that absorb as the energy of high-powered lasers without breaking down are essential for optical power detectors that measure the output of such lasers. These are used, for example, in military equipment for defusing unexploded mines. The composite consists of multiwall carbon nanotubes and a ceramic made of silicon, carbon and nitrogen. Including boron boosts the breakdown temperature. The nanotubes and graphene-like carbon transmit heat well, while the oxidation-resistant ceramic boosts damage resistance. Creating the coating involves dispersing the nanotubes in toluene, to which a clear liquid polymer containing boron was added. The mixture was heated to 1,100 °C (2,010 °F). The result is crushed into a fine powder, dispersed again in toluene and sprayed in a thin coat on a copper surface. The coating absorbed 97.5 percent of the light from a far-infrared laser and tolerated 15 kilowatts per square centimeter for 10 seconds. Damage tolerance is about 50 percent higher than for similar coatings, e.g., nanotubes alone and carbon paint.[220][221]

Acoustics[edit]

Carbon nanotubes have also been applied in the acoustics(such as loudspeaker and earphone). In 2008 it was shown that a sheet of nanotubes can operate as a loudspeaker if an alternating current is applied. The sound is not produced through vibration but thermoacoustically.[222][223] In 2013, a carbon nanotube (CNT) thin yarn thermoacoustic earphone together with CNT thin yarn thermoacoustic chip was demonstrated by a research group of Tsinghua-Foxconn Nanotechnology Research Center in Tsinghua University,[224] using a Si-based semi-conducting technology compatible fabrication process.

Environmental remediation[edit]

A CNT nano-structured sponge (nanosponge) containing sulfur and iron is more effective at soaking up water contaminants such as oil, fertilizers, pesticides and pharmaceuticals. Their magnetic properties make them easier to retrieve once the clean-up job is done. The sulfur and iron increases sponge size to around 2 centimetres (0.79 in). It also increases porosity due to beneficial defects, creating buoyancy and reusability. Iron, in the form of ferrocene makes the structure easier to control and enables recovery using magnets. Such nanosponges increase the absorption of the toxic organic solvent dichlorobenzene from water by 3.5 times. The sponges can absorb vegetable oil up to 150 times their initial weight and can absorb engine oil as well.[225][226]

Earlier, a magnetic boron-doped MWNT nanosponge that could absorb oil from water. The sponge was grown as a forest on a substrate via chemical vapor disposition. Boron puts kinks and elbows into the tubes as they grow and promotes the formation of covalent bonds. The nanosponges retain their elastic property after 10,000 compressions in the lab. The sponges are both superhydrophobic, forcing them to remain at the water's surface and oleophilic, drawing oil to them.[227][228]

Water treatment[edit]

It has been shown that carbon nanotubes exhibit strong adsorption affinities to a wide range of aromatic and aliphatic contaminants in water,[229][230][231] due to their large and hydrophobic surface areas. They also showed similar adsorption capacities as activated carbons in the presence of natural organic matter.[232] As a result, they have been suggested as promising adsorbents for removal of contaminant in water and wastewater treatment systems.

Moreover, membranes made out of carbon nanotube arrays have been suggested as switchable molecular sieves, with sieving and permeation features that can be dynamically activated/deactivated by either pore size distribution (passive control) or external electrostatic fields (active control).[233]

Other applications[edit]

Carbon nanotubes have been implemented in nanoelectromechanical systems, including mechanical memory elements (NRAM being developed by Nantero Inc.) and nanoscale electric motors (see Nanomotor or Nanotube nanomotor).

Carboxyl-modified single-walled carbon nanotubes (so called zig-zag, armchair type) can act as sensors of atoms and ions of alkali metals Na, Li, K.[234] In May 2005, Nanomix Inc. placed on the market a hydrogen sensor that integrated carbon nanotubes on a silicon platform. Since then, Nanomix has been patenting many such sensor applications, such as in the field of carbon dioxide, nitrous oxide, glucose, DNA detection, etc. End of 2014, Tulane University researchers have tested Nanomix's fast and fully automated point of care diagnostic system in Sierra Leone to help for rapid testing for Ebola. Nanomix announced that a product could be launched within three to six months.

Eikos Inc of Franklin, Massachusetts and Unidym Inc. of Silicon Valley, California are developing transparent, electrically conductive films of carbon nanotubes to replace indium tin oxide (ITO). Carbon nanotube films are substantially more mechanically robust than ITO films, making them ideal for high-reliability touchscreens and flexible displays. Printable water-based inks of carbon nanotubes are desired to enable the production of these films to replace ITO.[235] Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs.

A nanoradio, a radio receiver consisting of a single nanotube, was demonstrated in 2007.

A flywheel made of carbon nanotubes could be spun at extremely high velocity on a floating magnetic axis in a vacuum, and potentially store energy at a density approaching that of conventional fossil fuels. Since energy can be added to and removed from flywheels very efficiently in the form of electricity, this might offer a way of storing electricity, making the electrical grid more efficient and variable power suppliers (like wind turbines) more useful in meeting energy needs. The practicality of this depends heavily upon the cost of making massive, unbroken nanotube structures, and their failure rate under stress.

Carbon nanotube springs have the potential to indefinitely store elastic potential energy at ten times the density of lithium-ion batteries with flexible charge and discharge rates and extremely high cycling durability.

Ultra-short SWNTs (US-tubes) have been used as nanoscaled capsules for delivering MRI contrast agents in vivo.[236]

Carbon nanotubes provide a certain potential for metal-free catalysis of inorganic and organic reactions. For instance, oxygen groups attached to the surface of carbon nanotubes have the potential to catalyze oxidative dehydrogenations[237] or selective oxidations.[238] Nitrogen-doped carbon nanotubes may replace platinum catalysts used to reduce oxygen in fuel cells. A forest of vertically aligned nanotubes can reduce oxygen in alkaline solution more effectively than platinum, which has been used in such applications since the 1960s. Here, the nanotubes have the added benefit of not being subject to carbon monoxide poisoning.[239]

Wake Forest University engineers are using multiwalled carbon nanotubes to enhance the brightness of field-induced polymer electroluminescent technology, potentially offering a step forward in the search for safe, pleasing, high-efficiency lighting. In this technology, moldable polymer matrix emits light when exposed to an electric current. It could eventually yield high-efficiency lights without the mercury vapor of compact fluorescent lamps or the bluish tint of some fluorescents and LEDs, which has been linked with circadian rhythm disruption.[240]

Candida albicans has been used in combination with carbon nanotubes (CNT) to produce stable electrically conductive bio-nano-composite tissue materials that have been used as temperature sensing elements.[241]

The SWNT production company OCSiAl developed a series of masterbatches for industrial use of single-wall CNTs in multiple types of rubber blends and tires, with initial trials showing increases in hardness, viscosity, tensile strain resistance and resistance to abrasion while reducing elongation and compression[242] In tires the three primary characteristics of durability, fuel efficiency and traction were improved using SWNTs. The development of rubber masterbatches built on earlier work by the Japanese National Institute of Advanced Industrial Science & Technology showing rubber to be a viable candidate for improvement with SWNTs.[243]

Introducing MWNTs to polymers can improve flame retardancy and retard thermal degradation of polymer.[244] The results confirmed that combination of MWNTs and ammonium polyphosphates show a synergistic effect for improving flame retardancy.[245]

QMR

Carbon has four valence electrons and thus looks like a quadrant. Scientists call silicon and carbon, which both have four valence electrons and look like quadrants, "the miracle elements"

Colossal carbon tubes (CCTs) are a tubular form of carbon. In contrast to the carbon nanotubes (CNTs), colossal carbon tubes have much larger diameters ranging between 40 and 100 μm. Their walls have a corrugated structure with abundant pores, as in corrugated fiberboard, where the solid membranes have a graphite-like layered structure.

CCTs have technologically attractive properties such as ultralight weight, extremely high strength, excellent ductility and high conductivity - which make them possibly suitable for clothing. They are excellent conductors[ambiguous], are 15 times stronger than the strongest carbon fiber (T1000), have 30 times the tenacity of Kevlar and are 224 times stronger than individual cotton fibers. The tubes exhibit an ultra low density comparable to that of carbon nanofoams.

CCTs have a tensile strength of 7 GPa,[1] and a high specific strength (tensile strength per density), and a breaking length of 6,000 km.[2] This exceeds the specific strength of the strongest carbon nanotube; this strength is sufficient to support a space elevator[3] if retained in a fabricated macroscale structure.

CCTs conduct electricity and show some of the properties of semiconductors.

QMRCarbon nanofoam is an allotrope of carbon discovered in 1997 by Andrei V. Rode and co-workers at the Australian National University in Canberra.[1] It consists of a cluster-assembly of carbon atoms strung together in a loose three-dimensional web. The material is extremely light, with a density of 2–10 mg/cm3 (0.0012 lb/ft3).[1][2][3]

Each cluster is about 6 nanometers wide and consists of about 4000 carbon atoms linked in graphite-like sheets that are given negative curvature by the inclusion of heptagons among the regular hexagonal pattern. This is the opposite of what happens in the case of buckminsterfullerenes, in which carbon sheets are given positive curvature by the inclusion of pentagons.

The large-scale structure of carbon nanofoam is similar to that of an aerogel, but with 1% of the density of previously produced carbon aerogels—or only a few times the density of air at sea level. Unlike carbon aerogels, carbon nanofoam is a poor electrical conductor. The nanofoam contains numerous unpaired electrons, which Rode and colleagues propose is due to carbon atoms with only three bonds that are found at topological and bonding defects. This gives rise to what is perhaps carbon nanofoam's most unusual feature: it is attracted to magnets, and below −183 °C can itself be made magnetic.

QMRCarbon peapod is a hybrid nanomaterial consisting of spheroidal fullerenes encapsulated within a carbon nanotube. It is named due to their resemblance to the seedpod of the pea plant. Since the properties of carbon peapods differ from those of nanotubes and fullerenes, the carbon peapod can be recognized as a new type of a self-assembled graphitic structure.[4] Possible applications of nano-peapods include nanoscale lasers, single electron transistors, spin-qubit arrays for quantum computing, nanopipettes, and data storage devices thanks to the memory effects and superconductivity of nano-peapods.[5][6]

History[edit]

Single-walled nanotubes (SWNTs) were first seen in 1993 as cylinders rolled from a single graphene sheet. In 1998, the first peapod was observed by Brian Smith, Marc Monthioux and David Luzzi.[7] The idea of peapods came from the structure that was produced inside a transmission electron microscope in 2000.[4] They were first recognized in fragments obtained by a pulsed-laser vaporization synthesis followed by treatment with an acid and annealing.

Production and structure[edit]

Carbon peapods can be naturally produced during carbon nanotube synthesis by pulsed laser vaporization. C60 fullerene impurities are formed during the annealing treatment and acid purification, and enter the nanotubes through defects or vapor-phase diffusion.[11] Fullerenes within a nanotube are only stabilized at a diameter difference of 0.34 nm or less, and when the diameters are nearly identical, the interacting energy heightens to such a degree (comparable to 0.1 GPa) that the fullerenes become unable to be extracted from the SWNT even under high vacuum.[4] The encapsulated fullerenes have diameters close to that of C60 and form a chain inside the tube. Controlled production of carbon peapods allow for greater variety in both the nanotube structure and the fullerene composition. Varying elements can be incorporated into a carbon peapod through doping and will dramatically affect the resulting thermal and electrical conductivity properties.

Chemical properties[edit]

The existence of carbon peapods demonstrates further properties of carbon nanotubes, such as potential to be a stringently controlled environment for reactions. C60 molecules normally form amorphous carbon when heated to 1000–1200 °C under ambient conditions; when heated to such a high temperature within a carbon nanotube, they instead merge in an ordered manner to form another SWNT, thus creating a double-wall carbon nanotube.[4] Owing to the ease with which fullerenes can encapsulate or be doped with other molecules and the transparency of nanotubes to electron beams, carbon peapods can also serve as nano-scale test tubes. After fullerenes containing reactants diffuse into an SWNT, a high-energy electron beam can be used to displace carbon atoms and induce high reactivity, thus triggering formation of C60 dimers and merging of their contents.[12] Additionally, due to the enclosed fullerenes being limited to only a one-dimensional degree of mobility, phenomena such as diffusion or phase transformations can easily be studied.[11]

Electronic properties[edit]

The diameter of carbon peapods range from ca. 1 to 50 nanometers. Various combinations of fullerene C60 sizes and nanotube structures can lead to various electric conductivity property of carbon peapods due to orientation of rotations. For example, the C60 @ (10,10) is a good superconductor and the C60 @ (17,0) peapod is a semiconductor. The calculated band gap of C60 @ (17,0) equals 0.1 eV.[13] Research into their potential as semiconductors is still ongoing. Although both the doped fullerides and ropes of SWNTs are superconductors, unfortunately, the critical temperatures for the superconducting phase transition in these materials are low. There are hopes that carbon nano-peapods could be superconducting at room temperature.[14]

With chemical doping, the electronic characteristics of peapods can be further adjusted. When carbon peapod is doped with alkali metal atoms like potassium, the dopants will react with the C60 molecules inside the SWNT. It forms a negatively charged C606− covalently bound, one-dimensional polymer chain with metallic conductivity. Overall, the doping of SWNTs and peapods by alkali metal atoms actively enhances the conductivity of the molecule since the charge is relocated from the metal ions to the nanotubes.[15] Doping carbon nanotubes with oxidized metal is another way to adjust conductivity. It creates a very interesting high temperature superconducting state as the Fermi level is significantly reduced. A good application would be the introduction of silicon dioxide to carbon nanotubes. It constructs memory effect as some research group has invented ways to create memory devices based on carbon peapods grown on Si/SiO2 surfaces.[

QMRGraphite /ˈɡræfaɪt/, archaically referred to as Plumbago, is a crystalline form of carbon, a semimetal, a native element mineral, and one of the allotropes of carbon. Graphite is the most stable form of carbon under standard conditions. Therefore, it is used in thermochemistry as the standard state for defining the heat of formation of carbon compounds. Graphite may be considered the highest grade of coal, just above anthracite and alternatively called meta-anthracite, although it is not normally used as fuel because it is difficult to ignite.

Graphie is made up of carbon. Carbon is the other miracle element according to chemists. It also has four valence electrons and resembles a quadrant

Types and varieties[edit]

There are three principal types of natural graphite, each occurring in different types of ore deposit:

Crystalline flake graphite (or flake graphite for short) occurs as isolated, flat, plate-like particles with hexagonal edges if unbroken and when broken the edges can be irregular or angular;

Amorphous graphite: very fine flake graphite is sometimes called amorphous in the trade;[5]

Lump graphite (also called vein graphite) occurs in fissure veins or fractures and appears as massive platy intergrowths of fibrous or acicular crystalline aggregates, and is probably hydrothermal in origin.[6]

Highly ordered pyrolytic graphite or more correctly highly oriented pyrolytic graphite (HOPG) refers to graphite with an angular spread between the graphite sheets of less than 1°.[7]

The name "graphite fiber" is also sometimes used to refer to carbon fiber or carbon fiber-reinforced polymer.

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.[4] Minerals associated with graphite include quartz, calcite, micas and tourmaline. In meteorites it occurs with troilite and silicate minerals.[4] Small graphitic crystals in meteoritic iron are called cliftonite.[6]

According to the United States Geological Survey (USGS), world production of natural graphite in 2012 was 1,100,000 tonnes, of which the following major exporters are: China (750 kt), India (150 kt), Brazil (75 kt), North Korea (30 kt) and Canada (26 kt). Graphite is not mined in the United States, but U.S. production of synthetic graphite in 2010 was 134 kt valued at $1.07 billion.

Structure[edit]

Graphite has a layered, planar structure. In each layer, the carbon atoms are arranged in a honeycomb lattice with separation of 0.142 nm, and the distance between planes is 0.335 nm.[9] Atoms in the plane are bonded covalently, with only three of the four potential bonding sites satisfied. The fourth electron is free to migrate in the plane, making graphite electrically conductive. However, it does not conduct in a direction at right angles to the plane. Bonding between layers is via weak van der Waals bonds, which allows layers of graphite to be easily separated, or to slide past each other.

The two known forms of graphite, alpha (hexagonal) and beta (rhombohedral), have very similar physical properties, except the graphene layers stack slightly differently.[10] The hexagonal graphite may be either flat or buckled.[11] The alpha form can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated above 1300 °C.[12]

History of natural graphite use[edit]

In the 4th millennium B.C., during the Neolithic Age in southeastern Europe, the Mariţa culture used graphite in a ceramic paint for decorating pottery.[22]

Some time before 1565 (some sources say as early as 1500), an enormous deposit of graphite was discovered on the approach to Grey Knotts from the hamlet of Seathwaite in Borrowdale parish, Cumbria, England, which the locals found very useful for marking sheep.[23][24] During the reign of Elizabeth I (1533–1603), Borrowdale graphite was used as a refractory material to line moulds for cannonballs, resulting in rounder, smoother balls that could be fired farther, contributing to the strength of the English navy. This particular deposit of graphite was extremely pure and soft, and could easily be broken into sticks. Because of its military importance, this unique mine and its production were strictly controlled by the Crown.[25]

Other names [edit]

Historically, graphite was called black lead or plumbago.[6][26] Plumbago was commonly used in its massive mineral form. Both of these names arise from confusion with the similar-appearing lead ores, particularly galena. The Latin word for lead, plumbum, gave its name to the English term for this grey metallic-sheened mineral and even to the leadworts or plumbagos, plants with flowers that resemble this colour.

The term black lead usually refers to a powdered or processed graphite, matte black in color.

Abraham Gottlob Werner coined the name graphite ("writing stone") in 1789. He attempted to clear up the confusion between molybdena, plumbago and blacklead after Carl Wilhelm Scheele in 1778 proved that there are at least three different minerals. Scheele's analysis showed that the chemical compounds molybdenum sulfide (molybdenite), lead(II) sulfide (galena) and graphite were three different soft black minerals.[27][28][29]

Uses of natural graphite[edit]

Natural graphite is mostly consumed for refractories, batteries, steelmaking, expanded graphite, brake linings, foundry facings and lubricants.[8] Graphene, which occurs naturally in graphite, has unique physical properties and is among the strongest substances known. However, the process of separating it from graphite will require more technological development.

Refractories[edit]

This end-use began before 1900 with the graphite crucible used to hold molten metal; this is now a minor part of refractories. In the mid-1980s, the carbon-magnesite brick became important, and a bit later the alumina-graphite shape. Currently the order of importance is alumina-graphite shapes, carbon-magnesite brick, monolithics (gunning and ramming mixes), and then crucibles.

Crucibles began using very large flake graphite, and carbon-magnesite brick requiring not quite so large flake graphite; for these and others there is now much more flexibility in size of flake required, and amorphous graphite is no longer restricted to low-end refractories. Alumina-graphite shapes are used as continuous casting ware, such as nozzles and troughs, to convey the molten steel from ladle to mold, and carbon magnesite bricks line steel converters and electric arc furnaces to withstand extreme temperatures. Graphite Blocks are also used in parts of blast furnace linings where the high thermal conductivity of the graphite is critical. High-purity monolithics are often used as a continuous furnace lining instead of the carbon-magnesite bricks.

The US and European refractories industry had a crisis in 2000–2003, with an indifferent market for steel and a declining refractory consumption per tonne of steel underlying firm buyouts and many plant closures. Many of the plant closures resulted from the acquisition of Harbison-Walker Refractories by RHI AG and some plants had their equipment auctioned off. Since much of the lost capacity was for carbon-magnesite brick, graphite consumption within refractories area moved towards alumina-graphite shapes and monolithics, and away from the brick. The major source of carbon-magnesite brick is now imports from China. Almost all of the above refractories are used to make steel and account for 75% of refractory consumption; the rest is used by a variety of industries, such as cement.

According to the USGS, US natural graphite consumption in refractories was 12,500 tonnes in 2010.[8]

Batteries

Batteries[edit]

The use of graphite in batteries has been increasing in the last 30 years. Natural and synthetic graphite are used to construct the anode of all major battery technologies.[8] The lithium-ion battery utilizes roughly twice the amount of graphite than lithium carbonate.[30]

The demand for batteries, primarily nickel-metal-hydride and lithium-ion batteries, has caused a growth in graphite demand in the late 1980s and early 1990s. This growth was driven by portable electronics, such as portable CD players and power tools. Laptops, mobile phones, tablet, and smartphone products have increased the demand for batteries. Electric vehicle batteries are anticipated to increase graphite demand. As an example, a lithium-ion battery in a fully electric Nissan Leaf contains nearly 40 kg of graphite.

Steelmaking[edit]

Natural graphite in this end use mostly goes into carbon raising in molten steel, although it can be used to lubricate the dies used to extrude hot steel. Supplying carbon raisers is very competitive, therefore subject to cut-throat pricing from alternatives such as synthetic graphite powder, petroleum coke, and other forms of carbon. A carbon raiser is added to increase the carbon content of the steel to the specified level. An estimate based on USGS US graphite consumption statistics indicates that 10,500 tonnes were used in this fashion in 2005.[8]

Brake linings[edit]

Natural amorphous and fine flake graphite are used in brake linings or brake shoes for heavier (nonautomotive) vehicles, and became important with the need to substitute for asbestos. This use has been important for quite some time, but nonasbestos organic (NAO) compositions are beginning to reduce graphite's market share. A brake-lining industry shake-out with some plant closures has not been beneficial, nor has an indifferent automotive market. According to the USGS, US natural graphite consumption in brake linings was 6,510 tonnes in 2005.[8]

Foundry facings and lubricants[edit]

A foundry facing mold wash is a water-based paint of amorphous or fine flake graphite. Painting the inside of a mold with it and letting it dry leaves a fine graphite coat that will ease separation of the object cast after the hot metal has cooled. Graphite lubricants are specialty items for use at very high or very low temperatures, as forging die lubricant, an antiseize agent, a gear lubricant for mining machinery, and to lubricate locks. Having low-grit graphite, or even better no-grit graphite (ultra high purity), is highly desirable. It can be used as a dry powder, in water or oil, or as colloidal graphite (a permanent suspension in a liquid). An estimate based on USGS graphite consumption statistics indicates that 2,200 tonnes was used in this fashion in 2005.[8]

Pencils[edit]

Graphite pencils

Graphite pencils

The ability to leave marks on paper and other objects gave graphite its name, given in 1789 by German mineralogist Abraham Gottlob Werner. It stems from graphein, meaning to write/draw in Ancient Greek.[6][31]

From the 16th Century, pencils were made with leads of English natural graphite, but modern pencil lead is most commonly a mix of powdered graphite and clay; it was invented by Nicolas-Jacques Conté in 1795.[32][33] It is chemically unrelated to the metal lead, whose ores had a similar appearance, hence the continuation of the name. Plumbago is another older term for natural graphite used for drawing, typically as a lump of the mineral without a wood casing. The term plumbago drawing is normally restricted to 17th and 18th century works, mostly portraits.

Today, pencils are still a small but significant market for natural graphite. Around 7% of the 1.1 million tonnes produced in 2011 was used to make pencils.[30] Low-quality amorphous graphite is used and sourced mainly from China.[8]

Other uses[edit]

Natural graphite has found uses in zinc-carbon batteries, in electric motor brushes, and various specialized applications. Graphite of various hardness or softness results in different qualities and tones when used as an artistic medium.[34] Railroads would often mix powdered graphite with waste oil or linseed oil to create a heat resistant protective coating for the exposed portions of a steam locomotive's boiler, such as the smokebox or lower part of the firebox.[

Expanded graphite[edit]

Expanded graphite is made by immersing natural flake graphite in a bath of chromic acid, then concentrated sulfuric acid, which forces the crystal lattice planes apart, thus expanding the graphite. The expanded graphite can be used to make graphite foil or used directly as "hot top" compound to insulate molten metal in a ladle or red-hot steel ingots and decrease heat loss, or as firestops fitted around a fire door or in sheet metal collars surrounding plastic pipe (during a fire, the graphite expands and chars to resist fire penetration and spread), or to make high-performance gasket material for high-temperature use. After being made into graphite foil, the foil is machined and assembled into the bipolar plates in fuel cells. The foil is made into heat sinks for laptop computers which keeps them cool while saving weight, and is made into a foil laminate that can be used in valve packings or made into gaskets. Old-style packings are now a minor member of this grouping: fine flake graphite in oils or greases for uses requiring heat resistance. A GAN estimate of current US natural graphite consumption in this end use is 7,500 tonnes.[8]

Intercalated graphite[edit]

Main article: Graphite intercalation compound

Structure of CaC6

Graphite forms intercalation compounds with some metals and small molecules. In these compounds, the host molecule or atom gets "sandwiched" between the graphite layers, resulting in a type of compounds with variable stoichiometry. A prominent example of an intercalation compound is potassium graphite, denoted by the formula KC8. Graphite intercalation compounds are superconductors. The highest transition temperature (by June 2009) Tc = 11.5 K is achieved in CaC6, and it further increases under applied pressure (15.1 K at 8 GPa).[

Uses of synthetic graphite[edit]

Invention of a process to produce synthetic graphite[edit]

A process to make synthetic graphite was invented by Edward Goodrich Acheson (1856–1931). In the mid-1890s, Acheson discovered that overheating carborundum, which he is also credited with discovering, produced almost pure graphite. While studying the effects of high temperature on carborundum, he had found that silicon vaporizes at about 4,150 °C (7,500 °F), leaving behind graphitic carbon. This graphite was another major discovery for him, and it became extremely valuable and helpful as a lubricant.[6]

In 1896 Acheson received a patent for his method of synthesizing graphite,[37] and in 1897 started commercial production.[6] The Acheson Graphite Co. was formed in 1899. In 1928 this company was merged with National Carbon Company (now GrafTech International). Acheson also developed a variety of colloidal graphite products including Oildag and Aquadag. These were later manufactured by the Acheson Colloids Co. (now Acheson Industries, a unit of Henkel AG).

Uses of synthetic graphite[edit]

Invention of a process to produce synthetic graphite[edit]

A process to make synthetic graphite was invented by Edward Goodrich Acheson (1856–1931). In the mid-1890s, Acheson discovered that overheating carborundum, which he is also credited with discovering, produced almost pure graphite. While studying the effects of high temperature on carborundum, he had found that silicon vaporizes at about 4,150 °C (7,500 °F), leaving behind graphitic carbon. This graphite was another major discovery for him, and it became extremely valuable and helpful as a lubricant.[6]

In 1896 Acheson received a patent for his method of synthesizing graphite,[37] and in 1897 started commercial production.[6] The Acheson Graphite Co. was formed in 1899. In 1928 this company was merged with National Carbon Company (now GrafTech International). Acheson also developed a variety of colloidal graphite products including Oildag and Aquadag. These were later manufactured by the Acheson Colloids Co. (now Acheson Industries, a unit of Henkel AG).

Scientific research[edit]

Highly oriented pyrolytic graphite (HOPG) is the highest-quality synthetic form of graphite. It is used in scientific research, in particular, as a length standard for scanner calibration of scanning probe microscope.[38][39]

Electrodes[edit]

Graphite electrodes carry the electricity that melts scrap iron and steel (and sometimes direct-reduced iron: DRI) in electric arc furnaces, which are the vast majority of steel furnaces. They are made from petroleum coke after it is mixed with coal tar pitch. They are then extruded and shaped, baked to carbonize the binder (pitch), and finally graphitized by heating it to temperatures approaching 3000 °C, at which the carbon atoms arrange into graphite. They can vary in size up to 11 ft. long and 30 in. in diameter. An increasing proportion of global steel is made using electric arc furnaces, and the electric arc furnace itself is getting more efficient, making more steel per tonne of electrode. An estimate based on USGS data indicates that graphite electrode consumption was 197,000 tonnes in 2005.[8]

On a much smaller scale, graphite is also used for making electrodes for electrical discharge machining (EDM), commonly used to make plastic injection molds.

Powder and scrap[edit]

The powder is made by heating powdered petroleum coke above the temperature of graphitization, sometimes with minor modifications. The graphite scrap comes from pieces of unusable electrode material (in the manufacturing stage or after use) and lathe turnings, usually after crushing and sizing. Most synthetic graphite powder goes to carbon raising in steel (competing with natural graphite), with some used in batteries and brake linings. According to the USGS, US synthetic graphite powder and scrap production was 95,000 tonnes in 2001 (latest data).[8]

Neutron moderator[edit]

Main article: Nuclear graphite

Special grades of synthetic graphite also find use as a matrix and neutron moderator within nuclear reactors. Its low neutron cross-section also recommends it for use in proposed fusion reactors. Care must be taken that reactor-grade graphite is free of neutron absorbing materials such as boron, widely used as the seed electrode in commercial graphite deposition systems—this caused the failure of the Germans' World War II graphite-based nuclear reactors. Since they could not isolate the difficulty they were forced to use far more expensive heavy water moderators. Graphite used for nuclear reactors is often referred to as nuclear graphite.

Other uses[edit]

Graphite (carbon) fiber and carbon nanotubes are also used in carbon fiber reinforced plastics, and in heat-resistant composites such as reinforced carbon-carbon (RCC). Commercial structures made from carbon fiber graphite composites include fishing rods, golf club shafts, bicycle frames, sports car body panels, the fuselage of the Boeing 787 Dreamliner and pool cue sticks and have been successfully employed in reinforced concrete, The mechanical properties of carbon fiber graphite-reinforced plastic composites and grey cast iron are strongly influenced by the role of graphite in these materials. In this context, the term "(100%) graphite" is often loosely used to refer to a pure mixture of carbon reinforcement and resin, while the term "composite" is used for composite materials with additional ingredients.[40]

Modern smokeless powder is coated in graphite to prevent the buildup of static charge.

Graphite has been used in at least three radar absorbent materials. It was mixed with rubber in Sumpf and Schornsteinfeger, which were used on U-boat snorkels to reduce their radar cross section. It was also used in tiles on early F-117 Nighthawk (1983)s.

Graphite mining, beneficiation, and milling[edit]

Large graphite specimen. Naturalis Biodiversity Center

Graphite is mined by both open pit and underground methods. Graphite usually needs beneficiation. This may be carried out by hand-picking the pieces of gangue (rock) and hand-screening the product or by crushing the rock and floating out the graphite. Beneficiation by flotation encounters the difficulty that graphite is very soft and "marks" (coats) the particles of gangue. This makes the "marked" gangue particles float off with the graphite, yielding impure concentrate. There are two ways of obtaining a commercial concentrate or product: repeated regrinding and floating (up to seven times) to purify the concentrate, or by acid leaching (dissolving) the gangue with hydrofluoric acid (for a silicate gangue) or hydrochloric acid (for a carbonate gangue).

In milling, the incoming graphite products and concentrates can be ground before being classified (sized or screened), with the coarser flake size fractions (below 8 mesh, 8–20 mesh, 20–50 mesh) carefully preserved, and then the carbon contents are determined. Some standard blends can be prepared from the different fractions, each with a certain flake size distribution and carbon content. Custom blends can also be made for individual customers who want a certain flake size distribution and carbon content. If flake size is unimportant, the concentrate can be ground more freely. Typical end products include a fine powder for use as a slurry in oil drilling and coatings for foundry molds, carbon raiser in the steel industry (Synthetic graphite powder and powdered petroleum coke can also be used as carbon raiser). Environmental impacts from graphite mills consist of air pollution including fine particulate exposure of workers and also soil contamination from powder spillages leading to heavy metals contaminations of soil.