Misplaced Pages

#320679

27-491: The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds . 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, including titanium carbide and tungsten carbide , are important industrially and are used to coat metals in cutting tools. The long-held view

54-539: A phenyl group ) and [Fe 6 C(CO) 6 ]. Similar species are known for the metal carbonyls and the early metal halides. A few terminal carbides have been isolated, such as [CRuCl 2 {P(C 6 H 11 ) 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: Interstitial compound In materials science , an interstitial defect

81-399: A tungsten carbide – cobalt material can be increased by adding 6–30% of titanium carbide to tungsten carbide. This forms a solid solution that is more brittle and susceptible to breakage. Titanium carbide can be etched with reactive-ion etching . Titanium carbide is used in preparation of cermets , which are frequently used to machine steel materials at high cutting speed. It

108-767: A "methanide", although this compound is often called methylsodium. See Methyl group#Methyl anion for more information about the CH − 3 anion. Several carbides are assumed to be salts of the acetylide anion C 2− 2 (also called percarbide, by analogy with peroxide ), which has a triple bond between the two carbon atoms. Alkali metals, alkaline earth metals, and lanthanoid metals form acetylides, for example, sodium carbide Na 2 C 2 , calcium carbide CaC 2 , and LaC 2 . Lanthanides also form carbides (sesquicarbides, see below) with formula M 2 C 3 . Metals from group 11 also tend to form acetylides, such as copper(I) acetylide and silver acetylide . Carbides of

135-402: A close-packed metal lattice. 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. Iron forms a number of carbides, Fe 3 C , Fe 7 C 3 and Fe 2 C . The best known is cementite , Fe 3 C, which is present in steels. These carbides are more reactive than

162-452: A crystal with elementary particles having energy above the displacement threshold for that crystal, but they may also exist in small concentrations in thermodynamic equilibrium . The presence of interstitial defects can modify the physical and chemical properties of a material. The idea of interstitial compounds was started in the late 1930s and they are often called Hagg phases after Hägg. Transition metals generally crystallise in either

189-411: A mixed titanium-tin carbide, which is a two-dimensional conductor. Carbides can be generally classified by the chemical bonds type as follows: Examples include calcium carbide (CaC 2 ), silicon carbide (SiC), tungsten carbide (WC; often called, simply, carbide when referring to machine tooling), and cementite (Fe 3 C), each used in key industrial applications. The naming of ionic carbides

216-459: A shear modulus of 188 GPa. Titanium carbide is soluble in solid titanium oxide , with a range of compositions which are collectively named "titanium oxycarbide" and created by carbothermic reduction of the oxide. Tool bits without tungsten content can be made of titanium carbide in nickel -cobalt matrix cermet , enhancing the cutting speed, precision, and smoothness of the workpiece. The resistance to wear , corrosion , and oxidation of

243-503: A split [110] structure or a tetrahedral truly interstitial one. Carbon, notably in graphite and diamond, has a number of interesting self-interstitials - recently discovered using Local-density approximation -calculations is the "spiro-interestitial" in graphite, named after spiropentane , as the interstitial carbon atom is situated between two basal planes and bonded in a geometry similar to spiropentane. Small impurity interstitial atoms are usually on true interstitial sites between

270-419: Is a type of point crystallographic defect where an atom of the same or of a different type, occupies an interstitial site in the crystal structure . When the atom is of the same type as those already present they are known as a self-interstitial defect . Alternatively, small atoms in some crystals may occupy interstitial sites, such as hydrogen in palladium . Interstitials can be produced by bombarding

297-399: Is not systematic. Salt-like carbides are composed of highly electropositive elements such as the alkali metals , alkaline earth metals , lanthanides , actinides , and group 3 metals ( scandium , yttrium , and lutetium ). Aluminium from group 13 forms carbides , but gallium , indium , and thallium do not. These materials feature isolated carbon centers, often described as "C", in

SECTION 10

#1732787654321

324-406: 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: The following table shows 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

351-457: 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. Note that methanide in this context is a trivial historical name. According to the IUPAC systematic naming conventions, a compound such as NaCH 3 would be termed

378-592: The actinide elements , which have stoichiometry MC 2 and M 2 C 3 , are also described as salt-like derivatives of C 2− 2 . The C–C triple bond length ranges from 119.2 pm in CaC 2 (similar to ethyne), to 130.3 pm in LaC 2 and 134 pm in UC 2 . The bonding in LaC 2 has been described in terms of La with the extra electron delocalised into the antibonding orbital on C 2− 2 , explaining

405-422: The hexagonal close packed or face centered cubic structures, both of which can be considered to be made up of layers of hexagonally close packed atoms. In both of these very similar lattices there are two sorts of interstice, or hole: It was suggested by early workers that: These were not viewed as compounds, but rather as solutions, of say carbon, in the metal lattice, with a limiting upper “concentration” of

432-411: The M 2 C 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

459-428: The [111] crowdion interstitial, which can be understood as a long chain (typically some 10–20) of atoms along the [111] lattice direction, compressed compared to the perfect lattice such that the chain contains one extra atom. In semiconductors the situation is more complex, since defects may be charged and different charge states may have different structures. For instance, in silicon, the interstitial may either have

486-723: The appearance of black powder with the sodium chloride ( face-centered cubic ) crystal structure . It occurs in nature as a form of the very rare mineral khamrabaevite ( Russian : Хамрабаевит ) - (Ti,V,Fe)C. It was discovered in 1984 on Mount Arashan in the Chatkal District , USSR (modern Kyrgyzstan ), near the Uzbek border. The mineral was named after Ibragim Khamrabaevich Khamrabaev, director of Geology and Geophysics of Tashkent , Uzbekistan . Its crystals as found in nature range in size from 0.1 to 0.3 mm. Titanium carbide has an elastic modulus of approximately 400 GPa and

513-632: The diamond structure. Boron carbide , B 4 C, 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, such as B 25 C. Metal complexes containing C are known as metal carbido complexes . Most common are carbon-centered octahedral clusters, such as [Au 6 C(P Ph 3 ) 6 ] (where "Ph" represents

540-458: The ground state interstitial structure is similarly a [110] split interstitial. These split interstitials are often called dumbbell interstitials, because plotting the two atoms forming the interstitial with two large spheres and a thick line joining them makes the structure resemble a dumbbell weight-lifting device. In other bcc metals than iron, the ground state structure is believed based on recent density-functional theory calculations to be

567-412: 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. Some metals, such as lead and tin , are believed not to form carbides under any circumstances. There exists however

SECTION 20

#1732787654321

594-415: The lattice atoms. Large impurity interstitials can also be in split interstitial configurations together with a lattice atom, similar to those of the self-interstitial atom. Interstitials modify the physical and chemical properties of materials. Titanium carbide Titanium carbide , Ti C , is an extremely hard ( Mohs 9–9.5) refractory ceramic material, similar to tungsten carbide . It has

621-442: The lattice site, and they are displaced symmetrically from it along one of the principal lattice directions . For instance, in several common face-centered cubic (fcc) metals such as copper, nickel and platinum, the ground state structure of the self-interstitial is the split [100] interstitial structure, where two atoms are displaced in a positive and negative [100] direction from the lattice site. In body-centered cubic (bcc) iron

648-752: The metallic conduction. The polyatomic ion C 4− 3 , sometimes called allylide , is found in Li 4 C 3 and Mg 2 C 3 . The ion is linear and is isoelectronic with CO 2 . The C–C distance in Mg 2 C 3 is 133.2 pm. Mg 2 C 3 yields methylacetylene , CH 3 CCH, and propadiene , CH 2 CCH 2 , on hydrolysis, which was the first indication that it contains C 4− 3 . 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

675-638: The methanides or methides; two-atom units, " C 2− 2 ", in the acetylides ; and three-atom units, " C 4− 3 ", in the allylides. The graphite intercalation compound KC 8 , prepared from vapour of potassium and graphite, and the alkali metal derivatives of C 60 are not usually classified as carbides. 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 and beryllium carbide Be 2 C . Transition metal carbides are not saline: their reaction with water

702-399: The same as those already present in the lattice. The structure of interstitial defects has been experimentally determined in some metals and semiconductors . Contrary to what one might intuitively expect, most self-interstitials in metals with a known structure have a 'split' structure, in which two atoms share the same lattice site. Typically the center of mass of the two atoms is at

729-506: The smaller atom that was determined by the number of interstices available. A more detailed knowledge of the structures of metals, and binary and ternary phases of metals and non metals shows that: One example is the solubility of carbon in iron. The form of pure iron stable between 910 °C and 1390 °C, γ-iron, forms a solid solution with carbon termed austenite which is also known as steel . Self-interstitial defects are interstitial defects which contain only atoms which are

#320679