carbides

Carbides are binary compounds in which carbon is combined with elements of similar or lower electronegativity, mostly metals. Almost any carbide can be prepared by one of several general methods. Three broad classifications arise from general trends in their properties. The most electropositive metals form ionic or saltlike carbides. Nonmetals of electronegativity similar to that of carbon form covalent (diamond-like) or molecular carbides. The transition metals tend to form what are called interstitial carbides. Metals of groups 1, 2, and 3 of the periodic system commonly form ionic carbides, pure samples of which are transparent solids that are poor conductors of electricity; treatment with acids—or, in certain cases, even water—decomposes them into hydrocarbons and metal hydroxides. Ionic carbides have discrete carbon anions of the forms C4-, sometimes called methanides because they can be viewed as being derived from methane, (CH4); C22-, called acetylides and derived from acetylene (C2H2); and C34-, derived from allene (C3H4). The best-characterized methanides are probably beryllium carbide (Be2C) and aluminum carbide (Al4C3). Beryllium oxide (BeO) and carbon react at 2,000° C to produce the brick-red beryllium carbide. Pale yellow aluminum carbide, prepared from aluminum and carbon in a furnace, reacts as a typical methanide with water to produce methane. In addition to the many well known and well characterized acetylides of the alkali metals and the alkaline earth metals, lanthanum (La) forms two different acetylides, and copper (Cu), silver (Ag), and gold (Au) form explosive acetylides. Zinc (Zn), cadmium (Cd), and (Hg) form acetylides as well, although they are not as well characterized. Calcium carbide, CaC2, the most important of these compounds, is chiefly employed as a source of acetylene for use in the chemical industry. Only two carbides are considered completely covalent: these are formed with boron (B) and silicon (Si), the two elements that are most similar to carbon in size and electronegativity. Silicon carbide (SiC), known as carborundum, is prepared by the reduction of silicon dioxide (SiO2) with elemental carbon in an electric furnace. Like diamond, carborundum is extremely hard and is used industrially as an abrasive. Chemically inert, it has a diamond structure in which each silicon atom and each carbon atom are surrounded tetrahedrally by four atoms of the other type. Boron carbide (B4C), which has similar properties of inertness and extreme hardness, is prepared by the reduction of boron oxide (B2O3) with carbon in an electric furnace. In the structure of B4C, the boron atoms occur in icosahedral groups of 12, and the carbon atoms occur in linear chains of three. Another boron carbide (BC3), produced from the reaction of benzene (C6H6) and boron trichloride (BCl3) at 800° C, has a graphitelike structure. Interstitial (metallic) carbides, which most often are formed from metals of groups 4, 5, 6, 7, 8, 9, and 10, are very hard and electrically conductive. (The metalloid selenium also forms an interstitial carbide.) Several of them, especially tungsten carbide and the carbides of titanium, tantalum, and niobium, are important components of the composite materials called cermets. Interstitial carbides are derived primarily from relatively large transition metals acting as a host lattice and the small carbon atoms occupying interstices of the close-packed metal atoms, are characterized by both extreme hardness and extreme brittleness. Possessing very high melting points (typically about 3,000° to 4,000° C), they retain many of the properties associated with the metal itself, such as high conductivity of heat and electricity as well as metallic luster. At elevated temperatures, some interstitial carbides retain the mechanical properties of metals, such as malleability. The radii of several of the early transition metals are large enough to form interstitial monocarbides, MC, but most transition metals form interstitial carbides of several stoichiometries. Manganese (Mn), for example, is known to form at least five different interstitial carbides. In contrast to the saltlike carbides, most interstitial carbides do not react with water and are chemically inert. Those of industrial importance include the extremely hard tungsten carbide (WC) and tantalum carbide (TaC), used as high-speed cutting tools. Iron carbide (cementite), Fe3C, is an important component in steel. Rhenium is the only refractory metal that does not form carbides.