Carbon covalent carbides

Carbon is the only Group 14/IV element that forms both monatomic and polyatomic anions. There are three classes of carbides saline carbides (saltlike carbides), covalent carbides, and interstitial carbides. The heavier elements in Group 14/IV form polyatomic anions, such as Si44 and Sn52, in which the atoms form a tetrahedron and trigonal bipyramid, respectively. 

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

In carbides, carbon is bound to elements with lower or similar EN-values. We distinguish three types of carbides. The salt-like carbides with elements from groups 1, 2 and 3 are decomposed by water A14C3 +12 H20 — 4 Al(OH)3 + 3 CH4. In addition, there are the covalent carbides like SiC and B4C and a intermediate group with most transition metals. In the intermediate group C atoms are located in the octahedral cavities of metal close packings. The melting points vary from 3000 to some extreme values of about 4800 °C and their hardness lies between 7 and 10 on the Mohs scale. Furthermore, the... 

As with the hydrides (Chap. 2), the carbides are divided into three classes—the covalent, the saltlike, and the metallic (or interstitial). The volatile covalent carbides (for example, CC14, (CN)2, CH4, and CS2) are discussed elsewhere of the nonvolatile covalent carbides, silicon carbide (carborundum, SiC), is by far the most important. Although there are three known crystal forms of this compound, we may, for simplification, imagine it as a diamond structure in which every alternate carbon atom is replaced by a silicon atom. Thus it is not surprising that this compound is almost as hard and chemically inert as is diamond itself. 

Carbides can be covalent or metal-like, the most important of the covalent carbides being SiC which like carbon crystallizes in both hexagonal and cubic structures. However, contrary to carbon, the basal planes in the hexagonal structure and the (111) faces of the cubic variant are linked by chemical bonds, so the corresponding surface energies should be about 103 rather than 102 mJ/m2. Estimated values for the surface energy of both faces are close to 1500 mJ/m2 (Appendix F). 

Covalent Carbides. Although other carbides (e.g., Be2C) are at least partially covalent, the two elements that approach carbon closely in size and electro-... 

A present-day definition of organic chemistry would be one that includes the study of carbon compounds, and in particular those compounds that possess covalent carbon/carbon and carbon/hydrogen bonds. This definition is very general, and as such there are many exceptions to it. For example, carbonates and carbides are normally considered under the chemistry of the cation with which they are associated. This is because, even though they contain covalent bonds, they are often ionic solids with high melting points, and these are characteristic properties of inorganic compounds. 

Silicon carbide, SiC carborundum), an extremely hard covalent carbide and an excellent abrasive, is produced by the reaction of silicon dioxide with carbon at 2000°C ... 

Covalent carbides, which have giant-molecular structures, as in silicon carbide (SiC) and boron carbide (B4C3). These are hard high-melting solids. Other covalent compounds of carbon (CO2, CS2, CH4, etc.) have covalent molecules. 

The difference in electronegativity between carbon and the other element forming a carbide is an important factor in determining the nature of the compound. As shown in Table 2.1, that difference in the interstitial carbides is large (Box A) while it is much less pronounced in the covalent carbides (Box B). 

The difference in electronegativity between the two elements of the covalent carbides is small. The carbon atom is only slightly smaller than the other atom. The bonding is essentially covalent.1 1 Only two covalent carbides, silicon carbide and boron carbide, fully meet the refractory criteria. Other carbides such as beryllium carbide, Be2C, are only partially covalent and, while they have a high melting point, are generally not chemically stable and are not considered here. 

Covalence. Carbon bonding is covalent, that is, the atoms share a pair of electrons. Such covalent bonds are strong since the carbon atom is small and four of its six electrons (the four sp valence electrons) form bonds. This is the case for the two covalent carbides, silicon carbide and boron carbide (see Ch. 7). The bonding in interstitial carbides is not as straightforward and is a combination of covalent, metallic, and ionic bonding as reviewed in Sec. 6.0. 

The atomic and crystalline structures of covalent carbides are less complex and generally better understood and characterized than those of interstitial carbides. Bonding is essentially covalent where the carbon atoms bond to the silicon or boron atoms by sharing a pair of electrons and, like all covalent bonds, these atoms form definite bond angles. The bonding is achieved by the hybridization of the valence electrons of the respective atoms. 

In a semiconductor material, the forbidden-energy gap is such that electrons in usable quantities are able to jump across it from the filled valence band to the empty eonduction band.1 1 The three elements that form the covalent carbides, i.e., boron, silicon, and carbon (in the form of doped diamond) are semiconductors and one would expect to find semiconductor properties in their compounds. 

Covalent Carbides Compounds composed of carbon and low-electronegativity nonmetals or metalloids are covalent carbides. The most important covalent carbide is silicon carbide (SiC), a very hard material. Over 500,000 tons of silicon carbide are produced annually, mostly for use as an abrasive material in the cutting and polishing of metals. In a process analogous to the formation of calcium carbide, silicon carbide forms by the reaction of silicon oxide with coke at high temperatures. 

Descaibe the difference between an ionic carbide and a covalent carbide. Which types of atoms will form these carbides with carbon ... 

Ionic carbides are composed of carbon, generally in the form of the carbide ion, 2 , and low-electronegativity metals, such as the alkali and alkaline earth metals. Covalent carbides are composed of carbon and low-electronegativity nonmetals or metalloids, such as silicon. 

Despite the low-intrinsic sinterability, which is attributed to highly covalent bonds and low volume and grain boundary diffusion rates, significant advances in pressureless sintering of UHTCs have been made. Various additives have been incorporated to improve the densification of ZrB and HfB. Metals, carbon, carbides and silicides have been used to enhance the densification. Addition of carbon and carbides (B C SiC, TiC, WC) assists in densification of borides to a density of more than 95 % at temperature around 2000-2100 C. These additives react with surface... 

Only the carbon atom can gain four electrons this only happens when it is combined with extremely electropositive elements and this state may be regarded as exceptional. Bonding in carbides is almost invariably predominantly covalent. 

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

Carbides, which are binary compounds containing anionic carbon, occur as covalent and as salt-like compounds. The salt-like carbides are water-reactive and, upon hydrolysis, yield flammable hydrocarbons. Typical hydrolysis reactions include ... 

The most important of the extrinsic factors that affect the hardnesses of the transition metals are covalent chemical bonds scattered throughout their microstructures. These bonds are found between solute atoms and solvent atoms in alloys. Also, they lie within precipitates both internally and at precipitate interfaces with the matrix metal. In steel, for example, there are both carbon solutes and carbide precipitates. These effects are ubiquitous, but there... 

The carbides with the NaCl structure may be considered to consist of alternating layers of metal atoms and layers of semiconductor atoms where the planes are octahedral ones of the cubic symmetry system. (Figure 10.1). In TiC, for example, the carbon atoms lie 3.06A apart which is about twice the covalent bond length of 1.54 A, so the carbon atoms are not covalently bonded, but they may transfer some charge to the metal layers, and they do increase the valence electron density. 

---