Carbide

In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by the chemical bonds type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "intermediate" transition metal carbides. Examples include calcium carbide (CaC2), silicon carbide (SiC), tungsten carbide (WC; often called, simply, carbide when referring to machine tooling), and cementite (Fe3C),[1] each used in key industrial applications. The naming of ionic carbides is not systematic.

TiC-xtal-3D-vdW
Lattice structure of titanium carbide.

Salt-like (saline) carbides

Salt-like carbides are composed of highly electropositive elements such as the alkali metals, alkaline earth metals, and group 3 metals, including scandium, yttrium, and lanthanum. Aluminium from group 13 forms carbides, but gallium, indium, and thallium do not. These materials feature isolated carbon centers, often described as "C4−", in the methanides or methides; two-atom units, "C2−
2
", in the acetylides; and three-atom units, "C4−
3
", in the sesquicarbides.[1] The graphite intercalation compound KC8, prepared from vapour of potassium and graphite, and the alkali metal derivatives of C60 are not usually classified as carbides.[2]

Methanides

Carbides of this class decompose in water producing methane. Three such examples are aluminium carbide Al
4
C
3
, magnesium carbide Mg
2
C
[3] and beryllium carbide Be
2
C
.

Transition metal carbides are not saline carbides but their reaction with water is very slow and is usually neglected. For example, depending on surface porosity, 5–30 atomic layers of titanium carbide are hydrolyzed, forming methane within 5 minutes at ambient conditions, following by saturation of the reaction.[4]

Note that methanide in this context is a trivial historical name. According to the IUPAC systematic naming conventions, a compound such as NaCH3 would be termed a "methanide", although this compound is often called methylsodium.[5]

Acetylides

Several carbides are assumed to be salts of the acetylide anion C22– (also called percarbide), which has a triple bond between the two carbon atoms. Alkali metals, alkaline earth metals, and lanthanoid metals form acetylides, e.g., sodium carbide Na2C2, calcium carbide CaC2, and LaC2.[1] Lanthanides also form carbides (sesquicarbides, see below) with formula M2C3. Metals from group 11 also tend to form acetylides, such as copper(I) acetylide and silver acetylide. Carbides of the actinide elements, which have stoichiometry MC2 and M2C3, are also described as salt-like derivatives of C2−
2
.

The C-C triple bond length ranges from 119.2 pm in CaC2 (similar to ethyne), to 130.3 pm in LaC2 and 134 pm in UC2. The bonding in LaC2 has been described in terms of LaIII with the extra electron delocalised into the antibonding orbital on C2−
2
, explaining the metallic conduction.[1]

Sesquicarbides

The polyatomic ion C4−
3
, sometimes called sesquicarbide or allylenide, is found in Li4C3 and Mg2C3. The ion is linear and is isoelectronic with CO2.[1] The C-C distance in Mg2C3 is 133.2 pm.[6] Mg2C3 yields methylacetylene, CH3CCH, and propadiene, CH2CCH2, on hydrolysis, which was the first indication that it contains C4−
3
.

Covalent carbides

The carbides of silicon and boron are described as "covalent carbides", although virtually all compounds of carbon exhibit some covalent character. Silicon carbide has two similar crystalline forms, which are both related to the diamond structure.[1] Boron carbide, B4C, on the other hand, has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect boron carbide is similar to the boron rich borides. Both silicon carbide (also known as carborundum) and boron carbide are very hard materials and refractory. Both materials are important industrially. Boron also forms other covalent carbides, e.g. B25C.

Interstitial carbides

Tungsten carbide
Tungsten carbide end mills.

The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds.[1] These carbides have metallic properties and are refractory. Some exhibit a range of stoichiometries, e.g. titanium carbide, TiC. Titanium carbide and tungsten carbide are important industrially and are used to coat metals in cutting tools.[7]

The long-held view is that the carbon atoms fit into octahedral interstices in a close-packed metal lattice when the metal atom radius is greater than approximately 135 pm:[1]

  • When the metal atoms are cubic close-packed, (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure.
  • When the metal atoms are hexagonal close-packed, (hcp), as the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves 2:1 stoichiometry with the CdI2 structure.

The following table[1][7] shows actual structures of the metals and their carbides. (N.B. the body centered cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close-packed lattice.) The notation "h/2" refers to the M2C type structure described above, which is only an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms can be seen to be untrue as the packing of the metal atom lattice in the carbides is different from the packing in the pure metal, although it is technically correct that the carbon atoms fit into the octahedral interstices of a close-packed metal lattice.

Metal Structure of pure metal Metallic
radius (pm)
MC
metal atom packing
MC structure M2C
metal atom packing
M2C structure Other carbides
titanium hcp 147 ccp rock salt
zirconium hcp 160 ccp rock salt
hafnium hcp 159 ccp rock salt
vanadium bcc 134 ccp rock salt hcp h/2 V4C3
niobium bcc 146 ccp rock salt hcp h/2 Nb4C3
tantalum bcc 146 ccp rock salt hcp h/2 Ta4C3
chromium bcc 128 Cr23C6, Cr3C,
Cr7C3, Cr3C2
molybdenum bcc 139 hexagonal hcp h/2 Mo3C2
tungsten bcc 139 hexagonal hcp h/2

For a long time the non-stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected.[8]

Intermediate transition metal carbides

In these carbides, the transition metal ion is smaller than the critical 135 pm, and the structures are not interstitial but are more complex. Multiple stoichiometries are common; for example, iron forms a number of carbides, Fe3C, Fe7C3 and Fe2C. The best known is cementite, Fe3C, which is present in steels. These carbides are more reactive than the interstitial carbides; for example, the carbides of Cr, Mn, Fe, Co and Ni are all hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons. These compounds share features with both the inert interstitials and the more reactive salt-like carbides.[1]

Molecular carbides

Au6C(PPh3)6
The complex [Au6C(PPh3)6]2+, containing a carbon-gold core.

Metal complexes containing C are known as metal carbido complexes. Most common are carbon-centered octahedral clusters, such as [Au6C(PPh3)6]2+ and [Fe6C(CO)6]2−. Similar species are known for the metal carbonyls and the early metal halides. A few terminal carbides have been isolated, e.g., [CRuCl2{P(C6H11)3}2].

Metallocarbohedrynes (or "met-cars") are stable clusters with the general formula M
8
C
12
where M is a transition metal (Ti, Zr, V, etc.).

Impossible carbides

Some metals, such as lead and tin, are believed not to form carbides under any circumstances.[9] There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor.[10] (In 2007, there were two reports of a lead carbide PbC2, apparently of the acetylide type; but these claims have yet to be published in reviewed journals.)

Related materials

In addition to the carbides, other groups of related carbon compounds exist:[1]

References

  1. ^ a b c d e f g h i j k Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 318–22. ISBN 978-0-08-022057-4.
  2. ^ Shriver and Atkins — Inorganic Chemistry
  3. ^ O.O. Kurakevych; T.A. Strobel; D.Y. Kim; G.D. Cody (2013). "Synthesis of Mg2C: A Magnesium Methanide". Angewandte Chemie International Edition. 52 (34): 8930–8933. doi:10.1002/anie.201303463. PMID 23824698.
  4. ^ A. I. Avgustinik; G. V. Drozdetskaya; S. S. Ordan'yan (1967). "Reaction of titanium carbide with water". Powder Metallurgy and Metal Ceramics. 6 (6): 470–473. doi:10.1007/BF00780135 (inactive 2019-08-20).
  5. ^ Weiss, Erwin; Corbelin, Siegfried; Cockcroft, Jeremy Karl; Fitch, Andrew Nicholas (1990). "Über Metallalkyl- und -aryl-Verbindungen, 44 Darstellung und Struktur von Methylnatrium. Strukturbestimmung an NaCD3-Pulvern bei 1.5 und 300 K durch Neutronen- und Synchrotronstrahlenbeugung". Chemische Berichte. 123 (8): 1629–1634. doi:10.1002/cber.19901230807. ISSN 0009-2940.
  6. ^ Fjellvag H.; Pavel K. (1992). "Crystal Structure of Magnesium Sesquicarbide". Inorg. Chem. 31 (15): 3260. doi:10.1021/ic00041a018.
  7. ^ a b Peter Ettmayer; Walter Lengauer (1994). "Carbides: transition metal solid state chemistry". In R. Bruce King (ed.). Encyclopedia of Inorganic Chemistry. John Wiley & Sons. ISBN 978-0-471-93620-6.
  8. ^ C.H. de Novion; J.P. Landesman (1985). "Order and disorder in transition metal carbides and nitrides: experimental and theoretical aspects". Pure Appl. Chem. 57 (10): 1391. doi:10.1351/pac198557101391.
  9. ^ John Percy (1870). The Metallurgy of Lead, including Desiverization and Cupellation. London: J. Murray. p. 67. Retrieved 2013-04-06.
  10. ^ Y. C. Zhou; H. Y. Dong; B. H. Yu (2000). "Development of two-dimensional titanium tin carbide (Ti2SnC) plates based on the electronic structure investigation". Materials Research Innovations. 4 (1): 36–41. doi:10.1007/s100190000065.
Acetylene

Acetylene (systematic name: ethyne) is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure form and thus is usually handled as a solution. Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities.As an alkyne, acetylene is unsaturated because its two carbon atoms are bonded together in a triple bond. The carbon–carbon triple bond places all four atoms in the same straight line, with CCH bond angles of 180°.

Bhopal disaster

The Bhopal disaster, also referred to as the Bhopal gas tragedy, was a gas leak incident on the night of 2–3 December 1984 at the Union Carbide India Limited (UCIL) pesticide plant in Bhopal, Madhya Pradesh, India. It is considered to be the world's worst industrial disaster. Over 500,000 people were exposed to methyl isocyanate (MIC) gas. The highly toxic substance made its way into and around the small towns located near the plant.Estimates vary on the death toll. The official immediate death toll was 2,259. The government of Madhya Pradesh confirmed a total of 3,787 deaths related to the gas release. A government affidavit in 2006 stated that the leak caused 558,125 injuries, including 38,478 temporary partial injuries and approximately 3,900 severely and permanently disabling injuries. Others estimate that 8,000 died within two weeks, and another 8,000 or more have since died from gas-related diseases. The cause of the disaster remains under debate. The Indian government and local activists argue that slack management and deferred maintenance created a situation where routine pipe maintenance caused a backflow of water into a MIC tank, triggering the disaster. Union Carbide Corporation (UCC) argues water entered the tank through an act of sabotage.

The owner of the factory, UCIL, was majority owned by UCC, with Indian Government-controlled banks and the Indian public holding a 49.1 percent stake. In 1989, UCC paid $470 million (equivalent to $845 million in 2018) to settle litigation stemming from the disaster. In 1994, UCC sold its stake in UCIL to Eveready Industries India Limited (EIIL), which subsequently merged with McLeod Russel (India) Ltd. Eveready ended clean-up on the site in 1998, when it terminated its 99-year lease and turned over control of the site to the state government of Madhya Pradesh. Dow Chemical Company purchased UCC in 2001, seventeen years after the disaster.

Civil and criminal cases filed in the United States against UCC and Warren Anderson, UCC CEO at the time of the disaster, were dismissed and redirected to Indian courts on multiple occasions between 1986 and 2012, as the US courts focused on UCIL being a standalone entity of India. Civil and criminal cases were also filed in the District Court of Bhopal, India, involving UCC, UCIL and UCC CEO Anderson. In June 2010, seven Indian nationals who were UCIL employees in 1984, including the former UCIL chairman, were convicted in Bhopal of causing death by negligence and sentenced to two years imprisonment and a fine of about $2,000 each, the maximum punishment allowed by Indian law. All were released on bail shortly after the verdict. An eighth former employee was also convicted, but died before the judgement was passed.

Calcium carbide

Calcium carbide, also known as calcium acetylide, is a chemical compound with the chemical formula of CaC2. Its main use industrially is in the production of acetylene and calcium cyanamide.The pure material is colorless, however pieces of technical-grade calcium carbide are grey or brown and consist of about 80–85% of CaC2 (the rest is CaO (calcium oxide), Ca3P2 (calcium phosphide), CaS (calcium sulfide), Ca3N2 (calcium nitride), SiC (silicon carbide), etc.). In the presence of trace moisture, technical-grade calcium carbide emits an unpleasant odor reminiscent of garlic.Applications of calcium carbide include manufacture of acetylene gas, and for generation of acetylene in carbide lamps; manufacture of chemicals for fertilizer; and in steelmaking.

Carbide

In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by the chemical bonds type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "intermediate" transition metal carbides. Examples include calcium carbide (CaC2), silicon carbide (SiC), tungsten carbide (WC; often called, simply, carbide when referring to machine tooling), and cementite (Fe3C), each used in key industrial applications. The naming of ionic carbides is not systematic.

Carbide lamp

Carbide lamps, or acetylene gas lamps, are simple lamps that produce and burn acetylene (C2H2) which is created by the reaction of calcium carbide (CaC2) with water (H2O).Acetylene gas lamps were used to illuminate buildings, as lighthouse beacons, and as headlights on motor-cars and bicycles. Portable acetylene gas lamps, worn on the hat or carried by hand, were widely used in mining in the early twentieth century. They are still employed by cavers, hunters, and cataphiles. Small carbide lamps called "carbide candles" or "smokers" are used for blackening rifle sights to reduce glare. They are used because of the sooty flame produced by acetylene.

Cementite

Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, and is a frequently found and important constituent in ferrous metallurgy. While cementite is present in most steels and cast irons, it is produced as a raw material in the iron carbide process, which belongs to the family of alternative ironmaking technologies. The name cementite originated from the research of Floris Osmond and J. Werth, where the structure of solidified steel consists of a kind of cellular tissue in theory, with ferrite as the nucleus and Fe3C the envelope of the cells. The carbide therefore cemented the iron.

Schottky diode

The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes.

When sufficient forward voltage is applied, a current flows in the forward direction. A silicon diode has a typical forward voltage of 600–700 mV, while the Schottky's forward voltage is 150–450 mV. This lower forward voltage requirement allows higher switching speeds and better system efficiency.

Silicon carbide

Silicon carbide (SiC), also known as carborundum , is a semiconductor containing silicon and carbon. It occurs in nature as the extremely rare mineral moissanite. Synthetic SiC powder has been mass-produced since 1893 for use as an abrasive. Grains of silicon carbide can be bonded together by sintering to form very hard ceramics that are widely used in applications requiring high endurance, such as car brakes, car clutches and ceramic plates in bulletproof vests. Electronic applications of silicon carbide such as light-emitting diodes (LEDs) and detectors in early radios were first demonstrated around 1907. SiC is used in semiconductor electronics devices that operate at high temperatures or high voltages, or both. Large single crystals of silicon carbide can be grown by the Lely method and they can be cut into gems known as synthetic moissanite.

Tungsten carbide

Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes through a process called sintering for use in industrial machinery, cutting tools, abrasives, armor-piercing rounds, other tools and instruments, and jewelry.

Tungsten carbide is approximately twice as stiff as steel, with a Young's modulus of approximately 530–700 GPa (77,000 to 102,000 ksi), and is double the density of steel—nearly midway between that of lead and gold. It is comparable with corundum (α-Al2O3) in hardness and can only be polished and finished with abrasives of superior hardness such as cubic boron nitride and diamond powder, wheels, and compounds.

Union Carbide

Union Carbide Corporation is a wholly owned subsidiary (since 2001) of Dow Chemical Company. It currently employs more than 2,400 people. Union Carbide produces chemicals and polymers that undergo one or more further conversions by customers before reaching consumers. Some are high-volume commodities and others are specialty products meeting the needs of smaller markets. Markets served include paints and coatings, packaging, wire and cable, household products, personal care, pharmaceuticals, automotive, textiles, agriculture, and oil and gas. The company is a former component of the Dow Jones Industrial Average. Union Carbide was 50.9% stakeholder in Union Carbide India Limited, the company responsible for the Bhopal disaster.Founded in 1917 as the Union Carbide and Carbon Corporation, from a merger with National Carbon Company, the company's researchers developed an economical way to make ethylene from natural gas liquids, such as ethane and propane, giving birth to the modern petrochemical industry. Before divesting them, the chemical giant owned consumer products Eveready and Energizer batteries, Glad bags and wraps, Simoniz car wax, and Prestone antifreeze. The company divested other businesses before being acquired by Dow Chemical on February 6, 2001, including electronic chemicals, polyurethane intermediates, industrial gases and carbon products.

Compounds
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