Carbide

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

In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. [1]

Contents

Interstitial / Metallic carbides

Tungsten carbide end mills. Tungsten carbide.jpg
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. [2] These carbides have metallic properties and are refractory. Some exhibit a range of stoichiometries, being a non-stoichiometric mixture of various carbides arising due to crystal defects. Some of them e.g. titanium carbide, TiC and tungsten carbide are important industrially and are used to coat metals in cutting tools. [3]

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: [2]

The following table [2] [3] 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.

MetalStructure of pure metalMetallic
radius (pm)
MC
metal atom packing
MC structureM2C
metal atom packing
M2C structureOther carbides
titanium hcp147ccprock salt
zirconium hcp160ccprock salt
hafnium hcp159ccprock salt
vanadium bcc 134ccprock salthcph/2V4C3
niobium bcc146ccprock salthcph/2Nb4C3
tantalum bcc146ccprock salthcph/2Ta4C3
chromium bcc128Cr23C6, Cr3C,
Cr7C3, Cr3C2
molybdenum bcc139hexagonalhcph/2Mo3C2
tungsten bcc139hexagonalhcph/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. [4]

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. [2]

Some metals, such as lead and tin, are believed not to form carbides under any circumstances. [5] There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor. [6]

Chemical classification of carbides

Carbides can be generally classified by the chemical bonds type as follows: (i) salt-like (ionic), (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), [2] each used in key industrial applications. The naming of ionic carbides is not systematic.

Salt-like / saline / ionic 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 allylides. [2] 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. [7]

Methanides

Methanides are a subset of carbides distinguished by their tendency to decompose in water producing methane. Three examples are aluminium carbide Al
4
C
3
, magnesium carbide Mg
2
C
[8] 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. [9]

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. [10]

Acetylides / Ethynides

Calcium carbide. Carbid.jpg
Calcium carbide.

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. [2] 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. [2]

Allylides

The polyatomic ion C4−
3
, sometimes called allylide, is found in Li4C3 and Mg2C3. The ion is linear and is isoelectronic with CO2. [2] The C-C distance in Mg2C3 is 133.2 pm. [11] 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. [2] 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.

Molecular carbides

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

In addition to the carbides, other groups of related carbon compounds exist: [2]

See also

Related Research Articles

Alkali metal Group of highly-reactive chemical elements

The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they constitute group 1, which lies in the s-block of the periodic table. All alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having very similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour. This family of elements is also known as the lithium family after its leading element.

Hydroxide Chemical compound

Hydroxide is a diatomic anion with chemical formula OH. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. A hydroxide attached to a strongly electropositive center may itself ionize, liberating a hydrogen cation (H+), making the parent compound an acid.

Ionic bonding Chemical bonding involving attraction between ions

Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds. It is one of the main types of bonding along with covalent bonding and metallic bonding. Ions are atoms with an electrostatic charge. Atoms that gain electrons make negatively charged ions. Atoms that lose electrons make positively charged ions. This transfer of electrons is known as electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complex nature, e.g. molecular ions like NH+
4
or SO2−
4
. In simpler words, an ionic bond results from the transfer of electrons from a metal to a non-metal in order to obtain a full valence shell for both atoms.

In chemistry, a hydride is formally the anion of hydrogen, H. The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms, are called hydrides: water is a hydride of oxygen, ammonia is a hydride of nitrogen, etc. For inorganic chemists, hydrides refer to compounds and ions in which hydrogen is covalently attached to a less electronegative element. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.

Boron carbide Extremely hard ceramic compound

Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic and covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers hardness of >30 GPa, it is one of the hardest known materials, behind cubic boron nitride and diamond.

Superhard material Material with Vickers hardness exceeding 40 gigapascals

A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high bond covalency. As a result of their unique properties, these materials are of great interest in many industrial areas including, but not limited to, abrasives, polishing and cutting tools, disc brakes, and wear-resistant and protective coatings.

In chemistry, a nitride is a compound of nitrogen where nitrogen has a formal oxidation state of −3. Nitrides are a large class of compounds with a wide range of properties and applications.

Intermetallic

An intermetallic is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties. They can be classified as stoichiometric or nonstoichiometic intermetallic compounds.

Acetylide refers to chemical compounds with the chemical formulas MC≡CH and MC≡CM, where M is a metal. The term is used loosely and can refer to substituted acetylides having the general structure RC≡CM. Acetylides are reagents in organic synthesis. The calcium acetylide commonly called calcium carbide is a major compound of commerce.

Tantalum carbide Chemical compound

Tantalum carbides (TaC)form a family of binary chemical compounds of tantalum and carbon with the empirical formula TaCx, where x usually varies between 0.4 and 1. They are extremely hard, brittle, refractory ceramic materials with metallic electrical conductivity. They appear as brown-gray powders, which are usually processed by sintering.

Cottrell atmosphere

In materials science, the concept of the Cottrell atmosphere was introduced by A. H. Cottrell and B. A. Bilby in 1949 to explain how dislocations are pinned in some metals by boron, carbon, or nitrogen interstitials.

Octahedral molecular geometry Molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The octahedron has eight faces, hence the prefix octa. The octahedron is one of the Platonic solids, although octahedral molecules typically have an atom in their centre and no bonds between the ligand atoms. A perfect octahedron belongs to the point group Oh. Examples of octahedral compounds are sulfur hexafluoride SF6 and molybdenum hexacarbonyl Mo(CO)6. The term "octahedral" is used somewhat loosely by chemists, focusing on the geometry of the bonds to the central atom and not considering differences among the ligands themselves. For example, [Co(NH3)6]3+, which is not octahedral in the mathematical sense due to the orientation of the N-H bonds, is referred to as octahedral.

Chalcogenide

A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term chalcogenide is more commonly reserved for sulfides, selenides, tellurides, and polonides, rather than oxides. Many metal ores exist as chalcogenides. Photoconductive chalcogenide glasses are used in xerography. Some pigments and catalysts are also based on chalcogenides. The metal dichalcogenide MoS2 is a common solid lubricant.

Graphite intercalation compound

Graphite intercalation compounds (GICs) are complex materials having a formula CXm where the ion Xn+ or Xn− is inserted (intercalated) between the oppositely charged carbon layers. Typically m is much less than 1. These materials are deeply colored solids that exhibit a range of electrical and redox properties of potential applications.

In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central ion/molecule/atom is called a ligand. This number is determined somewhat differently for molecules than for crystals.

An interstitial compound, or interstitial alloy, is a compound that is formed when an atom with a small enough radius sits in an interstitial “hole” in a metal lattice. Examples of small atoms are hydrogen, boron, carbon and nitrogen. The compounds are industrially important.They are neither typically ionic nor covalent.

Crystal structure of boron-rich metal borides Boron chemical complexes

Metals, and specifically rare-earth elements, form numerous chemical complexes with boron. Their crystal structure and chemical bonding depend strongly on the metal element M and on its atomic ratio to boron. When B/M ratio exceeds 12, boron atoms form B12 icosahedra which are linked into a three-dimensional boron framework, and the metal atoms reside in the voids of this framework. Those icosahedra are basic structural units of most allotropes of boron and boron-rich rare-earth borides. In such borides, metal atoms donate electrons to the boron polyhedra, and thus these compounds are regarded as electron-deficient solids.

Metal halides

Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. Covalently bonded metal halides may be discrete molecules, such as uranium hexafluoride, or they may form polymeric structures, such as palladium chloride.

Titanium diselenide Chemical compound

Titanium diselenide (TiSe2) also known as titanium(IV) selenide, is an inorganic compound of titanium and selenium. In this material selenium is viewed as selenide (Se2−) which requires that titanium exists as Ti4+. Titanium diselenide is a member of metal dichalcogenides, compounds that consist of a metal and an element of the chalcogen column within the periodic table. Many exhibit properties of potential value in battery technology, such as intercalation and electrical conductivity, although most applications focus on the less toxic and lighter disulfides, e.g. TiS2.

Carbohydrides are solid compounds in one phase composed of a metal with carbon and hydrogen in the form of carbide and hydride ions. The term carbohydride can also refer to a hydrocarbon.

References

  1. Kunst, Helmut; Haase, Brigitte; Malloy, James C.; Wittel, Klaus; Nestler, Montia C.; Nicoll, Andrew R.; Erning, Ulrich; Rauscher, Gerhard (2006). "Metals, Surface Treatment". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a16_403.pub2.
  2. 1 2 3 4 5 6 7 8 9 10 11 Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 318–22. ISBN   978-0-08-022057-4.
  3. 1 2 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.
  4. 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. S2CID   59467042.
  5. John Percy (1870). The Metallurgy of Lead, including Desiverization and Cupellation. London: J. Murray. p. 67. Retrieved 2013-04-06.
  6. 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. S2CID   135756713.
  7. Shriver and Atkins — Inorganic Chemistry
  8. 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.
  9. 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 31 May 2021).CS1 maint: DOI inactive as of May 2021 (link)
  10. 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.
  11. Fjellvag H.; Pavel K. (1992). "Crystal Structure of Magnesium Sesquicarbide". Inorg. Chem. 31 (15): 3260. doi:10.1021/ic00041a018.